No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effect of the AIN nucleation layer growth on AlN material quality
Abstract
AlN layers were grown by metalorganic vapor-phase epitaxy at high temperatures up to 1500°C. Nucleation layer growth parameters and flow conditions before nucleation were changed and the effect on the AlN layer grown on top was studied. Structural analysis performed by high-resolution X-ray diffractometry and transmission electron microscopy showed that pregrowth conditions affect the material quality drastically. The best structural quality as indicated by a screw (including mixed) dislocation density of 8×108cm−2, together with smooth surface morphology was found to result from simultaneous switching on of ammonia and TMAl at the beginning of nucleation layer growth.
... Moreover, it has been reported that the pretreatment of sapphire substrates either by ammonia (NH 3 ) or trimethylaluminum (TMAl) is strongly correlated with the surface morphology [13,14], polarity [15], and crystal quality [16]. Researchers demonstrated that atomically smooth Al-polar AlN films were obtained without nitridation [16]. ...
... Moreover, it has been reported that the pretreatment of sapphire substrates either by ammonia (NH 3 ) or trimethylaluminum (TMAl) is strongly correlated with the surface morphology [13,14], polarity [15], and crystal quality [16]. Researchers demonstrated that atomically smooth Al-polar AlN films were obtained without nitridation [16]. Other research group have shown that mixed polarity by pregrowth nitridation of the sapphire substrate results in rough film surfaces because of the different growth rates of N-polar and Al-polar AlN films [8,17]. ...
... The topic of defective and roughsurface AlN/sapphire templates resulting from the TMAl pretreatment has been addressed before [21,22]. Reentila et al have argued that neither nitridation nor alumination gave the best structural quality [16]. In contrary, studies have shown that better AlN and GaN crystal quality can be achieved after pretreatment using TMAl [23,24]. ...
The growth of high quality AlN epitaxial films relies on precise control of the initial growth stages. In this work, we examined the influence of the trimethylaluminum (TMAl) pretreatment of sapphire substrates on the structural properties, crystal quality and growth modes of heteroepitaxial AlN films on (0001) sapphire substrates. Without the pretreatment, the AlN films nucleated on the smooth surface but exhibited mixed crystallographic Al- (N-) polarity, resulting in rough AlN film surfaces. With increasing the pretreatment time from 1 to 5 s, the N-polarity started to be impeded. However, small islands were formed on sapphire surface due to the decompostion of TMAl. As a result, small voids became noticeable at the nucleation layer (NL) because the growth started as quasi three-dimensional (3D) but transformed to 2D mode as the film grew thicker and got coalesced, leading to smoother and Al-polar films. On the other hand, longer pretreatment time of 40 s formed large 3D islands on sapphire, and thus initiated a 3D-growth mode of the AlN film, generating Al-polar AlN nanocolumns with different facets, which resulted into rougher film surfaces. The epitaxial growth modes and their correlation with the AlN film crystal quality under different TMAl pretreatments are also discussed.
... For Samples A-E (all similar to Sample C as shown in Fig. 1(a)), the average reflectance value stays constant at high temperature, indicating that the expectant layer-by-layer growth mode dominates the HT-AlN growth. While for prolonged nitrided Sample F, the damping reflectance in Fig. 1b) suggests rough surface morphology 12,13 , which may be caused by island growth of the HT-AlN epilayer. ...
... It can be found that when prolonging the sapphire nitridation time from 7 s (Sample C) to 100 s (Sample E), the (0002) FWHM values increase, while (10-12) values decreases. The minimum FWHM values of (0002) and (10)(11)(12), 55 and 734 arcsec, are obtained for Sample C and E, respectively. Similar variation trend was also reported in GaN epilayers 16 , though less related physical mechanism has been put forward so far. ...
... The surface morphology were characterized by a Bruker Dimension ICON-PT atomic force microscopy (AFM). The symmetric (0002) and asymmetric (10)(11)(12) -scan curves of all samples were measured by a Bruker AXS D8 Discover HRXRD. ...
Influence of sapphire pretreatment conditions on crystalline quality of AlN epilayers has been investigated by metal organic chemical vapor deposition (MOCVD). Compared to alumination treatment, it is found that appropriate sapphire nitridation significantly straightens the surface atomic terraces and decreases the X-ray diffraction (0002) full width at half maximum (FWHM) to a minimum of 55 arcsec, indicating a great improvement of the tilting feature of the grain structures in the AlN epilayer. More importantly, there is no inversion domains (IDs) found in the AlN epilayers, which clarifies that optimal sapphire nitridation is promising in the growth of high quality AlN. It is deduced that the different interfacial atomic structures caused by various pretreatment conditions influence the orientation of the AlN nucleation layer grains, which eventually determines the tilting features of the AlN epilayers.
... AlN (aluminum nitride) has direct wide bandgap (6.2 eV), high resistance (10 11 -10 13 X cm), high piezoelectric coefficient (d 33 = 5.56), high hardness (12 GPa for the (0001) plane), and good thermal conductivity (285 W / mK) [1][2][3][4][5]. Since AlN has unique properties, it has many different application areas, for example, ultraviolet photodetectors, lightemitting diodes (LED), quantum cascade lasers (QCL), radio frequency filters, micro-electro-mechanical systems, and solar cells [6][7][8][9][10][11][12][13][14][15]. Moreover, AIN is used as a buffer layer for GaN (the most studied extensively III-nitride semiconductor) growth on Si. ...
... The in situ reflectance's maximum and minimum values are represented by the max/ min values. Surface roughness is determined by the difference between the in situ reflectance's starting and minimum points [9]. It is expected that the max/ min value is high, which is the determination of optical quality while the start-min value is low, which is the determination of surface roughness. ...
High crystalline and optical quality aluminum nitride (AlN) films with thin thickness have been grown on Al2O3 by MOVPE (metal-organic vapor phase epitaxy) and the NH3 flow rate has been changed to improve the morphology and quality of the films. Some characterization types of equipment such as atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), and Raman spectroscopy have been carried out to investigate the effect of different NH3 flow rates on surface morphology, roughness, and crystal quality of AlN, respectively. Unlike in the literature, in situ optical reflectance measurements have been given depending on NH3 flow rate and optical characterization has been performed by UV–VIS–NIR spectrophotometry. The well-defined interference patterns in the optical transmittance graph report a sharp interface between AlN and Al2O3. Also, all obtained samples have a sharp absorption edge that shows the quality of the films, but Sample B with 900 sccm NH3 flow has the sharpest absorption edge because it has high optical quality and low defect. The RMS (root mean square), DS (screw-type dislocation density), and DE (edge-type dislocation density) values of AlN with 900 sccm NH3 flow are 0.22 nm, 7.86 х 10⁷ , and 1.68 х 10¹⁰ cm⁻², respectively. The results obtained are comparable to the literature.
... These phenomena are a result of crystal defect due to too much indium alloyed at these low temperature, that leads to deep band recombination [18,19]. These problems degraded some roles of MQW such as increasing indium content within the LED and increase its brightness [20,21]. ...
... From the calculation, we obtain the dislocation of the crystal was ρ = 1.17 × 10 5 cm −2 . This value proves that the samples have a very low defect compared to [21] and [32], even with the thickness that is bulkier, which is in micron order. We can deduce the existence of high-quality single-crystalline within the sample. ...
To produce a deep green (530 nm–570 nm) LED, the suitable indium (In) composition in the InxGa1-xN/GaN multi-quantum well (MQW) structure is crucial because a lower indium composition will shift the wavelength of emission towards the ultraviolet region. In this paper, we clarify the effects of an indium-rich layer to suppress such blue shifting, especially after the annealing process. According to characterizations by the uses of XRD and TEM, narrowing of the MQW layer was observed by the indium capping, while without the capping, the annealing results in a slight narrowing of MQW on the nearest layer to the p-type layer. By adding an indium capping layer, the blue shift of the photoluminescence was also suppressed and a slight red shift to keep green emission was observed. Such photoluminescence properties were consistent with the tiny change of the MQW as seen in the XRD and TEM characterizations.
... AlN, one of the III-nitride semiconductor materials, has the widest energy bandgap in the III-nitride semiconductors with an effective mechanical strength, and high chemical and thermal stability [4][5][6][7][8]. These extraordinary properties provide wide application areas in optoelectronic and electronic devices such as UV photodetectors-light emitting diodes, quantum cascade lasers, missile-warning systems, etc. [9][10][11][12][13][14][15][16][17][18]. Moreover, the AlN buffer layer has played a crucial role in GaN/Si high-power/high-frequency devices in recent years to be used in electric vehicles, wind tribunes, fast-chargers, etc. ...
... The reactor was then cooled down to 850 • C and the growth was initiated by first feeding TMAl into the reactor for 5 s before introducing NH 3 ; this technique assures that an aluminum-rich layer is initially nucleated, preventing the formation of the amorphous SiN x interlayer and involving an Al face growth front [30]. It was demonstrated that the duration and flow amount of TMAl precursor was crucial to keep step flow growth mode and to get a high-quality AlN template before the main AlN layer [14,38,[43][44][45]. Following, a thick AlN nucleation layer (~20-30 nm) was grown at a low temperature. ...
In the present study, the PALE-AlN (pulsed atomic layer epitaxy) epilayers were grown on the Si (111) substrates at different growth temperatures by metal organic vapor phase epitaxy (MOVPE) technique. The oxygen (O) and silicon (Si) concentrations of grown PALE-AlN epilayers and interface between epilayer and substrate were investigated by secondary ion mass spectroscopy (SIMS). It was observed that O and Si concentration change with growth temperature of epilayers as well as the interface significantly. HRXRD (high-resolution x-ray diffraction) analyses showed that the highest growth temperature results with the lowest full width at half maximum (FWHM) value for both ɷ scans. Scanning electron microscope (SEM) and atomic force microscopy (AFM) analyses indicated that relatively low growth temperature grown samples gave rise to 2D-like growth mode with openings while increased growth temperature resulted in change the growth mode to a columnar mode with increasing V-shape pits because of the increasing diffusion coefficient of O impurities and Si atoms in AlN epilayers.
... Templates with 1500 nm thick AlN were grown at elevated temperature [14,15] in a multiwafer reactor on a 430 µm thick sapphire substrate off-oriented by 0.2°and a 0.5°to sapphire mplane. [16] These AlN base layers show typical full width at half maximum of X-ray rocking curves (XRC-FWHM) of ≈ (60 ± 5) arcsec for the (00.2) reflection for both off-cut angles. ...
... The near-surface region of the AlN template, the homoepitaxial AlN, the AlN/GaN SL, and the bottom of the Al 0.75 Ga 0.25 N buffer layer are visible. This ADF STEM image was obtained in the [11][12][13][14][15][16][17][18][19][20] AlN viewing direction and shows all dislocation types formed in the sample. ...
Strain relaxation mechanisms in AlGaN based light emitting diodes emitting in the ultraviolet B spectral range (UVB‐LEDs) grown on different AlN/sapphire templates are analyzed by combining in situ reflectivity and curvature data with transmission electron microscopy. In particular, the impact of dislocation density, surface morphology, and lattice constant of the AlN/sapphire templates is studied. For nonannealed AlN/templates with threading dislocation densities (TDDs) of 4 × 109 and 3 × 109 cm−2 and different surface morphologies strain relaxation takes place mostly by conventional ways, such as inclination of threading dislocation lines and formation of horizontal dislocation bands. In contrast, a TDD reduction down to 1 × 109 cm−2 as well as a reduction of the lattice constant of high temperature annealed AlN template leads to drastic changes in the structure of subsequently grown AlGaN layers, e.g., to transformation to helical dislocations and enhanced surface enlargement by formation of macrofacets. For the growth of strongly compressively strained AlGaN layers for UVB‐LEDs the relaxation mechanism is strongly influenced by the absolute values of TDD and the lattice constant of the AlN templates and is less influenced by their surface morphology. UVB‐LED structures grown on different AlN/sapphire templates are analyzed by in situ reflectivity, curvature data, and transmission electron microscopy. This analysis suggests that for the growth of strongly compressively strained Al0.5Ga0.5N the relaxation mechanism is mostly influenced by the absolute values of threading dislocation density and the lattice constant of the AlN templates and is less influenced by their surface morphology.
... The N-polarity growth of AlN films was often observed after the nitridation. 3,7 Also, the decomposition of sapphire amid the hydrogen ambient causes the creation of the Npolarity of the film. 8 Oxygen (O) released from the sapphire decomposition was found to play a significant role in the formation of N-polar AlN films. ...
... 6,9 As the N-polar AlN films generally have rough surfaces, the suppression of the Al x O y N z formation is crucial for the MOCVD growth of UV devices. 7,9 A few approaches have been reported to suppress the N-polarity and to promote the Al-polarity amid the nitridation. Mohn et al. showed that a well-designed annealing process of the nucleation layer (NL) was necessary to suppress the N-polarity. ...
The trimethylaluminum (TMAl) preflow process has been widely applied on sapphire substrates prior to growing Al-polar AlN films by metalorganic chemical vapor deposition. However, it has been unclear how the TMAl preflow process really works. In this letter, we reported on carbon's significance in the polarity and growth mode of AlN films due to the TMAl preflow. Without the preflow, no trace of carbon was found at the AlN/sapphire interface and the films possessed mixed Al- and N-polarity. With the 5 s preflow, carbon started to precipitate due to the decomposition of TMAl, forming scattered carbon-rich clusters which were graphitic carbon. It was discovered that the carbon attracted surrounding oxygen impurity atoms and consequently suppressed the formation of AlxOyNz and thus N-polarity. With the 40 s preflow, the significant presence of carbon clusters at the AlN/sapphire interface was observed. While still attracting oxygen and preventing the N-polarity, the carbon clusters served as randomly distributed masks to further induce a 3D growth mode for the AlN growth. The corresponding epitaxial growth mode change is discussed.
... Nucleation conditions of NLs are crucial for acquisition of low-defect-density AlN epitaxial films [61][62][63][64] . Balaji et al. investigated the effect of NL nucleation temperature on the crystalline quality of AlN epitaxial films in Fig. 3 [65] . ...
Solar-blind ultraviolet photodetectors (SBPDs) have attracted tremendous attention in the environmental, industrial, military, and biological fields. Aluminum gallium nitride (AlGaN), a kind of representative III-nitride semiconductor, has promising prospects in solar-blind photodetection owing to its tunable wide bandgap and industrial feasibility. Considering the high defect density in the AlGaN epilayer directly grown on a sapphire substrate, employing an AlN/sapphire template turns out to be an effective method to achieve a high-quality AlGaN epilayer, thereby enhancing the SBPD performances. In recent years, a variety of remarkable breakthroughs have been achieved in the SBPDs. In this paper, the progress on photovoltaic AlGaN-based SBPDs is reviewed. First, the basic physical properties of AlGaN are introduced. Then, fabrication methods and defect annihilation of the AlN/sapphire template are discussed. Various photovoltaic SBPDs are further summarized, including Schottky barrier, metal-semiconductor-metal, p-n/p-i-n and avalanche photodiodes. Furthermore, surface modification and photoelectrochemical cell techniques are introduced. Benefitting from the development of fabrication techniques and optoelectronic devices, photovoltaic AlGaN photodiodes exhibit a promising prospect in solar-blind ultraviolet photodetection.
... Bulk AlN substrates with a low dislocation density are the ideal candidate [19][20][21][22][23][24]; however, they are currently very expensive and only available in small sizes [25,26]. When AlGaN is grown on foreign substrates such as silicon, sapphire, and silicon carbide, the lattice mismatch usually leads to films with a relatively high density of threading dislocations [27,28]. This is the main cause for non-radiative recombination, leading to the deterioration of the optical and electrical performance of AlGaN UV optoelectronic devices [1,4,27]. ...
Fully relaxed, crack free, smooth AlxGa1−xN layers with up to 50% Al composition were demonstrated on pseudo-substrates composed of dense arrays of 10 × 10 µm2 compliant porous GaN-on-porous-GaN tiles. The AlGaN layers were grown in steps for a total of 1.3 µm. The growth conditions necessary to demonstrate high quality films at higher Al compositions also suppressed any sidewall growth.
... 11,14,15 Thereby, pretreatment of the substrate surface and growth conditions during AlN nucleation have a strong effect on the quality of the overlying AlN layers. 16,17 The growth mode is commonly modified by simultaneous temperature and V/III ratio modulation. A growth pressure of 100 mbar, low temperatures, and high V/III ratios are associated with the 3D growth mode and used for the nucleation layers (NL), while high temperatures and low V/III ratios are associated with 2D layer-by-layer growth used to deposit thick AlN layers. ...
The epitaxial growth of high-quality AlN on sapphire substrates is challenging due to high lattice and thermal mismatch and low Al-adatom mobility, which cause high dislocation density and rough surface morphology. High-temperature AlN (deposited at 1130 [Formula: see text]C) was grown on low-temperature AlN nucleation layers (880 [Formula: see text]C) with different V/III ratios and reactor pressures by metal-organic chemical vapor deposition. Surface and crystal quality was optimized using high V/III ratios. Thereby, slow layer-by-layer growth at high V/III laterally overgrows the 3D nucleation layer reducing the dislocation density, twist, and tilt in the crystal. This was as effective as multistep growth with increasing V/III. At high pressure of 95 mbar, step-bunching occurred. This indicates low surface supersaturation due to parasitic reactions in the gas phase. This was suppressed by low growth pressure of 50 mbar, while the crystal quality worsened.
... AlGaN-based LEDs operating in the deep ultraviolet (DUV) spectral range are usually grown on AlN template deposited on c-plane sapphire substrate. However, the AlN layers grown on sapphire substrate usually exhibit a large number of threading dislocations (TDs) [8], which extend into the AlGaN epi-layer deposited on the top. In order to overcome this difficult, many approaches have been explored for the growth of AlGaN [9,10]. ...
High Al-content AlGaN epilayers were grown on AlN template by using indium (In) surfactant with plasma-assisted molecular beam epitaxy (PA-MBE), and deep ultraviolet emission at 235 nm was obtained at room temperature. The effects and mechanisms of In-surfactant on the crystalline quality and optical properties of AlGaN were investigated. It was found that In-surfactant could facilitate two-dimensional AlGaN growth by reducing activation barrier for Al/Ga atoms to cross steps and effectively increasing the migration rate on the growth surface, and thus improve surface morphology and decrease defect density. The photoluminescence measurements revealed that the optical properties were remarkably improved by adopting In as surfactant, and phase separation was also effectively eliminated. Furthermore, the concentration of impurities including oxygen and silicon was decreased, which is attributed to higher defects formation energy for these impurities with In-surfactant assisted epitaxy growth.
... This phenomena is believed resulted by the crystal defect caused by the dislocation in InxGa1−xN from the presence of a lot of indium [5]. The quantum well (QW) was introduced to counter this problem, and it tackled the double peak problem [2,6]. Contrary, the very thin InxGa1−xN layer in the QW causes the indium content escaped from the layer when heated. ...
The indium composition in InxGa1-xN/GaN multi-quantum well structure (MQW) is crucial because lower indium composition will shift the wavelength towards ultraviolet region. Nevertheless, at certain indium content in MQW, it will out diffuse from the MQW resulting in the wavelength shift from green to much shorter wavelength, after the annealing process for p-type activation. In this study, we had grown a full Light Emitting Diode device with the MQW
layer at a relative high temperature for green LED with indium pre-flow at the top of n-type layer just beneath the MQW using Metal Organic Chemical Vapor Deposition (MOCVD). Transmission Electron Microscopy (TEM) image of the MQW prior and post the activation of
p-type had been observed, which resulted in good contrast, showing the abruptness of the MQW layer of the device. Homogenous layers of InxGa1-xN/GaN has been observed. We also managed to reduce the wavelength shift of the device significantly. The optical, crystal properties of grown devices had been studied.
... The LED heterostructures were grown by metalorganic vapor phase epitaxy with standard precursors on 2 inch (0001) oriented sapphire substrates. Templates with 1500 nm AlN grown simultaneously at elevated temperature 31,32) in a multi-wafer reactor were employed to ensure similar dislocation densities. These AlN base layers show typical full width at half maximum of X-ray rocking curves (measured with a Panalytical X'PertPro) of 60 arcsec for the (002) reflection and 550 arcsec for the (102) reflection. ...
The influence of the n-AlGaN contact layer thickness and doping profile on the efficiency, operating voltage and lifetime of 310 nm LEDs has been investigated. Increasing the n-contact layer thickness reduces the operation voltage of the LEDs and increases the emission power slightly. Optimizing the n-doping profile yielded enhanced conductivity and reduced operation voltage with a simultaneous output power enhancement of the LEDs. Lifetime measurements have shown that even though the output power of the LEDs was enhanced the lifetimes were not negatively affected. Room temperature photoluminescence indicates a low concentration of point defects in the n-doping region yielding minimum AlGaN resistivity.
... The investigated (In x )Al y Ga 1−x−y N layers were grown by metalorganic vapor phase epitaxy in a close coupled showerhead reactor. AlN/sapphire templates 13) are used as substrates with a subsequently grown AlN/AlGaN-superlattice (SL) for strain management and a relaxed Al 0.5 Ga 0.5 N buffer layer acting as a quasi-substrate 14,15) for further growth with a threading dislocation density of about 2 × 10 9 cm −2 . The (In x )Al y Ga 1−x−y N layers were grown with N 2 as carrier gas under a pressure of 400 mbar. ...
Consistent studies of the quaternary composition are rare as it is impossible to fully determine the quaternary composition by X-ray diffraction or deduce it from that of ternary alloys. In this paper we determined the quaternary composition by wavelength dispersive X-ray spectroscopy of In xAl y layers grown by metal organic vapor phase epitaxy. Further insights explaining the peculiarities of In xAl yGa1−x−y N growth in a showerhead reactor were gained by simulations of the precursor decomposition, gas phase adduct formation and indium incorporation including desorption. The measurements and simulations agree very well showing that the indium incorporation in a range from 0% to 2% is limited by desorption which is enhanced by the compressive strain to the relaxed Al0.5Ga0.5N buffer layer as well as indium incorporation into AlN particles forming in the gas phase. Utilizing In xAl yGa1−x−y N layers containing 2% of indium for multiple quantum wells (MQWs), it was possible to show an almost five times higher photoluminescence intensity of InAlGaN MQWs in comparison to AlGaN MQWs.
... For growth, firstly a 50 nm thick standard nucleation layer was deposited on the sapphire at process temperature T proc = 980°C and an input group V to group III (V/III) ratio of 4000. 22) Nucleation was followed by AlN growth at temperatures above T proc = 1140°C and V/III ratios between 30 and 540 by changing the NH 3 flow from 250 to 4250 sccm at constant TMAl flow of 400 sccm. HTA was done in a sintering oven in N 2 atmosphere at a total pressure of 1000 mbar in faceto-face configuration of the samples following. ...
In this work, we investigate AlN/sapphire templates grown by metalorganic vapor phase epitaxy (MOVPE) subjected to an intermediate high temperature annealing (HTA) step at different AlN film thicknesses. To avoid cracking of the MOVPE layers during HTA at 1700 °C these layers have to be grown under low incorporation of tensile strain. For each sample the growth is stopped at a certain thickness between 230 nm and 1.2 μm followed by HTA. X-ray rocking curve FWHM of 0002 and 10–12 reflections lead to an estimation of threading dislocation densities (TDD) as low as cm⁻² for the thickest AlN layers of 880 nm and 1.2 μm after annealing. For all layer thicknesses MOVPE growth is subsequently continued to a total AlN layer thickness of 1.5 μm to reach for a smooth surface. The change of the AlN strain state after HTA leads to an increased wafer curvature at room temperature and compressive strain in the subsequently grown AlN layers increasing the wafer bow at room temperature. Hence, to serve as a base layer, e.g. for ultraviolet light emitting diodes, a trade-off between low TDD, surface smoothness and wafer bow has to be found.
... Pulsed direct current (DC) reactive sputtering has been used in our laboratory to deposit AlN films on a nitrided sapphire substrate because a higher deposition rate of dense AlN films can be obtained that suppresses arcing (unusual charged atom in the plasma) by interrupting the negative voltage and momentarily applying a positive voltage (bipolar pulsed DC) at the cathode (Al target). 1 A metal (+c)-polarity of the III-N film is necessary to improve the III-N structure since it has a higher surface diffusivity that leads to a step-flow growth compared with nitrogen (c)-polarity. 2 Therefore, various methods have been attempted to control the polarity of the AlN film on a sapphire substrate, [2][3][4][5] which include modifying the initial growth conditions such as by using a very low V/III ratio, Al-source/N-source pulsed flow, or Al-source pre-flow in the metal organic vapor phase epitaxy (MOVPE) method. Polarity inversion of AlN has also been accomplished by varying the oxygen partial pressure in liquid phase epitaxy (LPE) with the Ga-Al flux method. ...
The effect of oxygen partial pressure (PO2) on polarity and crystalline quality of AlN films grown on nitrided a-plane sapphire substrates by pulsed direct current (DC) reactive sputtering was investigated as a fundamental study. The polarity inversion of AlN from nitrogen (−c)-polarity to aluminum (+c)-polarity occurred during growth at a high PO2 of 9.4×10³ Pa owing to Al-O octahedral formation at the interface of nitrided layer and AlN sputtered film which reset the polarity of AlN. The top part of the 1300 nm-thick AlN film sputtered at the high PO2 was polycrystallized. The crystalline quality was improved owing to the high kinetic energy of Al sputtered atom in the sputtering phenomena. Thinner AlN films were also fabricated at the high PO2 to eliminate the polycrystallization. For the 200 nm-thick AlN film sputtered at the high PO2, the full width at half-maximum values of the AlN (0002) and (10−12) X-ray diffraction rocking curves were 47 and 637 arcsec, respectively.
... It has been shown that high dislocation density in the epilayers is the primary reason for the deteriorating leakage current in AlGaN-based photodiodes. Therefore, several methods have been used to improve the AlN quality by enhancing migration during deposition and reducing the gas phase reactions between trimethylaluminum (TMA) and ammonia (NH3) [3,4], such as changing growth temperature, using epitaxial lateral overgrowth (ELO) on patterned sapphire [5], and adopting a lowpressure flow-modulated MOCVD method [6]. Among them, ELO of AlN on patterned sapphire substrates in combination with the recently emergent technology of growing thick AlN layers (≥1μm) is an effective approach for the reduction of the TDD down to the low 10 8 cm −2 range [7][8][9][10], and provide a preferred solution for obtaining high quality AlGaN epilayers and high performance visible-blind UV photodetectors [11]. ...
We introduced the use of solution-processed few-layer hexagonal boron nitride (h-BN) stripe patterns embedded in the lateral epitaxial overgrowth (LEO) of AlN grown on sapphire substrates using high temperature metal organic chemical vapor deposition (MOCVD). This straightforward usage of h-BN film contributes to reducing the lattice mismatch and almost entirely terminates the threading dislocations originating from the AlN/sapphire interface, which results in a low pit density and the absence of air-voids in the AlN template. Compared with AlN templates grown on conventional sapphire substrates, the full width at half maximum of the AlN template grown on the h-BN pattern in the (0002) and (10-12) planes decreased from 376 arcsec to 227 arcsec and from 495 arcsec to 398 arcsec, respectively. For device applications, AlGaN-based visible-blind UV photodetectors fabricated using the as-obtained high quality AlN templates show one order of magnitude reduction of the dark leakage current and 50% increase in the responsivity. Our results suggest that the h-BN pattern plays a promising role in the growth of a high quality AlN template, leading to the improvement of performance of AlGaN-based optoelectronic devices.
... A higher off-cut value may supply an additional inclination component to the dislocation lines favoring their mutual annihilation. A comparison with TDD values obtained by TEM showed that the TDDs estimated from the XRC scans are usually a bit higher than the TDDs measured by TEM [14]. The diffraction volume of the X-ray beam is much higher than the layer thickness and a big part of the signal stems from the first grown 100 nm of the AlN layer with a very high TDD (Fig. 1). ...
The authors would like to correct Fig. 3 of their article in Phys. Status Solidi B 253, No. 5 (2016). Due to a regrettable error, the published Fig. 3 is the duplicate of Fig. 2 which was overlooked during proof reading.
... The best crystalline quality and surface morphology can only be achieved when neither Al nor N atoms dominate at the surface. 25,26 To optimize the integrated HEMT-LED performance, we have developed a GaN/AlN buffer platform featuring high buffer resistivity and excellent crystalline quality simultaneously. The influence of the growth temperature and the V/III ratio of the AlN layer on the crystalline quality and surface morphology of the upper AlGaN/GaN heterostructures has been investigated. ...
For the development of a metal-interconnection-free integration scheme for monolithic integration of InGaN/GaN light-emitting diodes (LEDs) and AlGaN/GaN high-electron-mobility transistors (HEMTs), a common buffer to achieve high brightness, low leakage current, and high breakdown in the integrated HEMT–LED device is essential. Different buffer structures have been investigated, and their impacts upon both the LED and HEMT parts of the HEMT–LED device have been analyzed. Results indicated that a GaN/AlN buffer structure is the most ideal to serve as a common buffer platform, offering both the excellent crystalline quality and superior buffer resistivity required by the HEMT–LED device. Growth of the AlN layer was particularly crucial for engineering the dislocation density, surface morphology, as well as resistivity of the buffer layer. Using the optimized GaN/AlN buffer structure, the LED part of the HEMT–LED device was improved, showing greatly enhanced light output power and suppressed reverse leakage current, while the breakdown characteristics of the HEMT part were also improved.
... First, about 0.5 μm thick smooth AlN layers with a root mean square roughness value of about 0.5 nm were grown on planar c-plane sapphire. Cross-sectional TEM as well as x-ray rocking curve measurements suggested a total TDD of about 10 10 cm −2 in these layers [10]. Then, a SiN x mask was deposited onto the AlN/sapphire templates. ...
This study reports on defect distribution and compositional homogeneity of Alx
Ga1−x
N layers with a nominal composition x of 0.5 grown on AlN by metal organic vapor phase epitaxy. The AlN layers with a low threading dislocation density (TDD) of a few 108 cm−2 were obtained by ELO and showed periodic surface macrosteps. Alx
Ga1−x
N growth on these AlN surfaces results in inhomogeneous Ga distribution due to enhanced Ga incorporation on the macrostep sidewalls. Variation of AlGaN deposition rate strongly influences the Ga distribution as well as the defect structure in the layers. Low growth rates (0.2 μm h−1) result in an inhomogeneous TD distribution with formation of alternating stripes with lower and higher defect densities. Additionally, self-organized formation of additional Ga-rich areas at the top edge of the steps is observed. In contrast, at a higher growth rate of 1 μm h−1 the formation of additional Ga-rich areas can be completely suppressed, but the defect density increases. This leads to an optimum growth rate to minimize the TDD.
Ce travail de thèse porte sur l’amélioration des performances des LEDs émettant dans la gamme des UVC. Celles-ci attirent de plus en plus l’attention, notamment depuis 2 ans avec l’apparition du COVID-19, grâce au pouvoir germicide du rayonnement ultraviolet. Cependant, les LEDs UV connaissent une forte chute de performancesà mesure que leur longueur d’onde d’émission diminue pour atteindre la gamme des UV-C. Pour cela, une étude détaillée est proposée dans la première partie de cette thèse, visant à améliorer la morphologie de surface et la qualité cristalline des couches d’AlGaN à fort taux d’Al (supérieur à 70%) à travers une étude paramétrique. Ensuite, des multi puits quantiques (MQWs) sont épitaxiés sur cette couche d’AlGaN et une première optimisation des conditions de croissances des MQWs a conduit à un IQE de 20% à 268 nm. Néanmoins, l’efficacité radiative des puits quantiquesAlGaN peut encore être améliorée en réduisant la densité de dislocations provenant des templates AlN sur saphir utilisés. Une des manières pour contourner certaines des problématiques des LEDs UV est la réalisation de l’ensemble de la structure de la LED UV sur des nanopyramides AlGaN par croissance sélective. Cette approche est abordée dans la seconde partie de ce manuscrit.Grâce à la croissance sélective de nanopyramides d’AlGaN, les dislocations provenant du substrat peuvent être filtrées à travers le masque diélectrique et le champ électrique interne est réduit dans les puits quantiques crus sur les facettes semi-polaires. Une étude détaillée est présentée dans les deux derniers chapitres portant sur la croissance sélective de nanopyramides d’AlGaN, mettant en évidence l’influence du design du masque de croissance sur la morphologie des pyramides ainsi que le dépôt à la surface du masque, dû à la faible longueur de diffusion des atomesd’Al. En utilisant un mode de croissance conventionnel, les pyramides tronquées obtenues ont une composition très faible en Al en leur coeur, les atomes d’Al se concentrant sur le pourtour des pyramides. En revanche, dans le cas d’un mode de croissance pulsé, l’incorporation d’Al débute dès lors que la croissance commence : ces résultats peuvent être expliqués par la gravure du GaN par l’hydrogène lors des arrêts de croissance. Des nanopyramides 100% AlGaN ont ainsi pu être obtenues. Des premiers tests de MQWs AlGaN/AlGaN crus sur ces nanopyramides présentent une émission à 285 nm.
Aluminum nitride (AlN) has been attracting increasing research attention and significant progress has been taking place due to their wide applications in ultraviolet light emitting diodes, laser diodes, surface acoustic wave devices and so on. Wherein the quality of AlN films is the key factor to realize high-performance devices and various techniques have been developed for AlN epitaxy. In this chapter, we will discuss the substrate selection for AlN epitaxy, introduce the popular AlN film growth methods including Molecular Beam Epitaxy, Metalorganic Vapor Phase Epitaxy, Physical Vapor Deposition and summarize recent progresses made with these growth methods.
Crack-free 1.3 μm thick elastically relaxed AlxGa1−xN layers were demonstrated on compliant high fill-factor 10 × 10 μm² tile patterned GaN-on-porous-GaN pseudo-substrates (PSs). Porous GaN was utilized as a semi-flexible underlayer. The AlxGa1−xN layer was grown in steps of 200, 300, 400, and 400 nm. While the AlxGa1−xN layer regrown on a co-loaded GaN-on-sapphire template cracked after deposition of only 200 nm AlGaN, on the GaN-on-porous GaN patterned substrates, no cracks were observed, and smooth films were achieved. In addition, an enhanced aluminum uptake was observed as the AlGaN growth progressed on the GaN-on-porous-GaN PS, owing to the composition pulling effect, until the point when the tiles started to coalesce. Upon further regrowth on these coalesced tiles, the aluminum uptake saturated, while the surface remained crack-free and smooth.
Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.
Developing efficient active region structures with low sensitivity to threading dislocation density (TDD) is of great technical significance for AlGaN-based deep ultraviolet (DUV) light-emitting devices. Here, we propose an active region strategy by introducing bunching effect to form self-assembled sidewall quantum wells (SQWs) with much stronger carrier confinement, resulting in a significant enhancement of internal quantum efficiency (from 46% to 59%) compared to the commonly adopted multiple quantum wells (MQWs) due to the lower sensitivity to TDD. As a demo, an AlGaN-based DUV light-emitting diode (LED) with the proposed active region involving both SQWs and MQWs presents dual-band emission and a consequent 68% enhancement in light output power compared to the DUV-LED with only MQWs as the active region, suggesting that the proposed architecture is fully suitable for the development of high performance DUV light-emitting devices even based on poor or medium quality materials.
We report on the growth of AlN epilayers at reasonably low temperatures of 1050–1110 °C on non-miscut c-plane sapphire by metal organic chemical vapor deposition (MOCVD). A systematic study of growth parameters revealed that the thickness of the low temperature (LT) nucleation layer (NL) plays a critical role in improving the screw and edge dislocation densities and surface morphology of the AlN epilayer. A surface kinetics based physico-chemical model is proposed to optimize the crystalline quality and is found to correlate well to the experimental observations. Using a 7 nm nominally-thick LT NL, a 0.5 μm thick AlN epilayer with an rms roughness of 0.15 nm, and (002) and (102) omega scan widths of 18 arc sec and 970 arc sec, respectively, was realized. A grain coalescence model for stress generation is used to correlate the change in AlN growth stress with variation in the NL thickness, and it is shown that in-situ stress measurement can be employed as an early signature for reproducibility of the crystalline quality. This study suggests that AlN/sapphire templates can potentially be realized for a reasonably low thickness (0.5 μm) and at temperatures as low as 1050 °C, which is accessible by most of the III-nitride MOCVD systems.
We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviolet B (UVB) at 311 nm, where substrate removal has been achieved by electrochemical etching of a sacrificial Al0.37Ga0.63N layer. The electroluminescence spectrum of the TFFC LED corresponds well to the as-grown LED structure, showing no sign of degradation of structural and optical properties by electrochemical etching. This is achieved by a proper epitaxial design of the sacrificial layer and the etch stop layers in relation to the LED structure and the electrochemical etch conditions. Enabling a TFFC UV LED is an important step toward improving the light extraction efficiency that limits the power conversion efficiency in AlGaN-based LEDs.
Recent progress in the development of deep ultraviolet lasers is reviewed as well as challenges for the heterostructure design and epitaxial growth for AlGaN-based laser diodes are discussed. The growth of AlN on sapphire and AlGaN heterostructures is reviewed and its impact on the performance characteristics of lasers in the UVC spectral range is presented. We achieve optically pumped AlGaN multiple quantum well (MQW) lasers near 270 nm with threshold power densities of less than 800 kW/cm² and optically pumped AlGaN MQW lasers emitting at record shortest wavelength of 237 nm. We discuss critical stepping stones towards the development of current injection deep UV laser diodes including studies on Si- and Mg-doping of AlGaN with high aluminum mole fractions. n-Al0.8Ga0.2N cladding layers with resistances as low as 0.026 Ω cm are realized and UVC-transparent p-AlGaN cladding layers are developed. Finally, electroluminescence from current injection broad area UV laser diodes is demonstrated with maximum current densities of 4.7 kA/cm². At the end, we provide an outlook of future prospect for deep UV laser diodes and discuss alternative approaches, e.g., electron beam pumping.
In this paper a systematic study of the morphology and local defect distribution in epitaxial laterally overgrown (ELO) AlN on c‐plane sapphire substrates with different off‐cut angles ranging from 0.08° to 0.23° is presented. Precise measurements of the off‐cut angle a, using a combination of optical alignment and X‐ray diffraction with an accuracy of ±5° for the off‐cut direction and ±0.015° for the off‐cut angle were performed. For ELO AlN growth a transition from step flow growth at a < 0.14° with height undulations on the surface to step bunching with step heights up to 20 nm for a > 0.14° was observed. Furthermore, the terraces of the step bunched surface exhibit curved steps. An analysis of the local defect distribution by scanning transmission electron microscopy and a comparison with atomic force microscopy reveals a bunching of defects in line with the ELO pattern and a roughening of step edges in highly defective regions. In addition, a reduction of the threshold excitation power density for optically pumped UVC lasers with smooth surface morphologies was observed. This article is protected by copyright. All rights reserved.
Evolution of crystalline quality of AlN via high-temperature (HT) annealing induced by different sapphire pretreatments is investigated. It is found that after HT annealing at 1700 ℃ for one hour, AlN film grown on nitridation treated sapphire substrate presents much lower threading dislocation density (TDD) than that grown on alumination treated one, indicating that the combination of nitridation and HT annealing is a more effective approach to achieve high quality AlN. It is verified that the much greater grain density of the nucleation layer induced by nitridation can produce more columns with much smaller size so that they can more easily rotate during the HT annealing to reduce the TDD more effectively. A deep ultraviolet light-emitting diode (285 nm) with output power over 10 mW has been demonstrated on a HT annealed AlN template with sapphire substrate nitridation pretreatment, showing a great potential for applications.
In an effort to successfully fabricate InGaN-based for green emitting devices on patterned sapphire substrate, the indium composition in In x Ga 1-x N/GaN multi-quantum well structure is crucial because lower indium composition will shift the wavelength towards ultraviolet region. In this study, 5 micrometre of undoped GaN epilayer was deposited as a buffer layer prior to the growth structure. 6 pairs of InGaN/GaN multi-quantum well structure grown by metal organic chemical vapour deposition (MOCVD). In this research, the indium to gallium composition ratio was 9:1. The crystal and optical properties of the samples were characterized using field effect atomic force microscopy, high resolution x-ray diffraction, and photoluminescence spectroscopy.
By alloying GaN with AlN the emission of AlGaN light-emitting diodes can be tuned to cover almost the entire ultraviolet spectral range (210–400 nm), making ultraviolet light-emitting diodes perfectly suited to applications across a wide number of fields, whether biological, environmental, industrial or medical. However, technical developments notwithstanding, deep-ultraviolet light-emitting diodes still exhibit relatively low external quantum efficiencies because of properties intrinsic to aluminium-rich group III nitride materials. Here, we review recent progress in the development of AlGaN-based deep-ultraviolet light-emitting devices. We also describe the key obstacles to enhancing their efficiency and how to improve their performance in terms of defect density, carrier-injection efficiency, light extraction efficiency and heat dissipation.
The annealing of amorphous AlN and AlN/Al2O3 seed layers from atomic layer deposition (ALD) on sapphire substrates and their application as starting layers for metal-organic vapor phase epitaxial growth on sapphire was investigated. During annealing in hydrogen, the amorphous ALD layers become crystalline with epitaxial relation to the underlying sapphire substrate. In contrast to the pure AlN ALD seed layers, mixed layers containing Al2O3 help to avoid the formation of polycrystalline material. Additionally, such mixed ALD seeds support void formation at the AlN/sapphire interface resulting in the formation of a smooth AlN surface. This void formation can be seen in situ during AlN growth in the reflectivity at 405 nm. The tilt and twist component of the AlN grains could be decreased by increasing the annealing time at 1290 °C from 1.5 to 40 min.
MOVPE growth of AlN layers on bulk AlN substrates with low threading dislocation density (<10⁵ cm⁻²) can result in enhanced defect formation. Chemo-mechanical polishing (CMP) of bulk AlN using colloidal silica sol can produce locally disturbed surface with SiOx residuals. These surface disturbances lead to generation of threading dislocations (∼10⁸ cm⁻²) in homoepitaxially grown AlN layers. These dislocations show a tendency to form nano- and even micropipes, which may be associated with oxygen accumulation along the dislocation lines. As result, the subsequently grown AlGaN-based layer structures exhibit a high number of v-pits and micropipes. Inductively coupled plasma etching of AlN substrate surface prior to MOVPE growth results in clean AlN surfaces and improves the AlGaN layer quality.
In order to realize UVB LEDs with high wall-plug efficiencies, the light extraction efficiency from the LED heterostructure must be maximized and operating voltages reduced. In this study, we investigate the effect of the GaN:Mg contact layer thickness on the light-output and current-voltage characteristics of UVB LEDs. AlGaN-based LED heterostructures, that are fully transparent for UVB emission except for the GaN:Mg contact layer are grown by metal organic vapor phase epitaxy on c-plane sapphire substrates. From transfer line measurements, it is found that the p-contact resistivity increases rapidly with decreasing GaN:Mg thickness and exhibits a pronounced Schottky behavior for layer thicknesses below 40 nm. At the same time, the emission power increases from 0.1 to 1.5 mW at 20 mA with decreasing GaN:Mg thickness. Ray tracing simulations of the light extraction efficiency of the UVB LEDs show that absorption in the GaN:Mg layer leads to lower emission powers for thicker GaN:Mg layers. Furthermore, with increasing GaN:Mg thickness additional losses occur due to a decrease of the internal quantum efficiency. The electro-optical and the structural properties of the devices show that a 40 nm thick GaN:Mg contact layer is the best compromise due to the low p-contact resistivity (0.01 Ω cm²) and at the same time still sufficient UVB-transmission resulting in UVB LEDs with external quantum efficiencies of more than 1% and a wall plug efficiency of 0.4% (at 20 mA), measured on-wafer.
High-quality AlN layers with low-density threading dislocations are indispensable for high-efficiency deep ultraviolet light-emitting diodes (UV-LEDs). In this work, a high-temperature AlN epitaxial layer was grown on sputtered AlN layer (used as nucleation layer, SNL) by a high-yield industrial metalorganic vapor phase epitaxy (MOVPE). The full width half maximum (FWHM) of the rocking curve shows that the AlN epitaxial layer with SNL has good crystal quality. Furthermore, the relationships between the thickness of SNL and the FWHM values of (002) and (102) peaks were also studied. Finally, utilizing an SNL to enhance the quality of the epitaxial layer, deep UV-LEDs at 282 nm were successfully realized on sapphire substrate by the high-yield industrial MOVPE. The light-output power (LOP) of a deep UV-LED reaches 1.65 mW at 20 mA with external quantum efficiency of 1.87%. In addition, the saturation LOP of the deep UV-LED is 4.31 mW at an injection current of 60 mA. Hence, our studies supply a possible process to grow commercial deep UV-LEDs in high throughput industrial MOVPE, which can increase yield, at lower cost.
In this study we show how to realize a self-assembled nano-patterned sapphire surface on 2 inch diameter epi-ready wafer and the subsequent AlN overgrowth both in the same metal-organic vapor phase epitaxial process. For this purpose in-situ annealing in H2 environment was applied prior to AlN growth to thermally decompose the c-plane oriented sapphire surface. By proper AlN overgrowth management misoriented grains that start to grow on non c-plane oriented facets of the roughened sapphire surface could be overcome. We achieved crack-free, atomically flat AlN layers of 3.5 µm thickness. The layers show excellent material quality homogeneously over the whole wafer as proved by the full width at half maximum of X-ray measured ω-rocking curves of 120 arcsec to 160 arcsec for the 002 reflection and 440 arcsec to 550 arcsec for the 302 reflection. The threading dislocation density is 2 ∗ 10⁹ cm⁻² which shows that the annealing and overgrowth process investigated in this work leads to cost-efficient AlN templates for UV LED devices.
We observed (0001) AlN wurzite surfaces by atomic force microscopy after 500 nm regrowth in metal-organic vapor phase epitaxy. The steps changed from double to single height with decreasing V/III ratio. The single height step edges were alternating smooth and rough due to the two different step types on (0001) wurzite surfaces. By reducing the V/III ratio, the widths equalize for terraces with smooth and rough edges, until Al terminated steps start to dominate, and thus promote again double height steps. Using in situ ellipsometry at λ=400nm under static conditions, we could directly identify three different surface reconstructions at high NH3, low NH3, and without NH3 which correlates with the different step terminations.
We systematically investigated metalorganic vapor phase epitaxy (MOVPE) growths of AlN layers with trimethylgallium (TMGa) supply on sapphire substrates at 1100-1250 °C. We found that Ga incorporations into the AlN layers contributed to smooth surfaces covered with step terraces at the early stage of the Al(Ga)N growth. In addition, a GaN mole fraction leading to the smooth surfaces was found to be around 2-3% at the beginning of growth. The Ga supply during the AlN layer growth at 1150 °C provided very smooth Al0.99Ga0.01N layers on sapphire substrates.
To the end of improvement of layer morphology and crystalline perfection of thick AlGaN layers grown by hydride vapor phase epitaxy (HVPE) as well as for the suppression of crystallite formation during growth, the impact of AlN buffer layer properties on AlGaN growth was investigated. While the surface morphology of 500 nm thick AlN layer improves with higher V/III ratio toward 50 and lower growth temperatures of 1020 °C, its use as a buffer layer for thick AlGaN layers leads to strong degradation of AlGaN surface and even to formation of different crystallites during growth. Best surface morphology and crystalline perfection of thick AlGaN layers was observed for the growth on an AlN buffer layer with a higher crystal quality disregarding its 3D morphology and high surface roughness. Best results were achieved at medium V/III ratio of about 10–20 and AlN buffer growth temperature of 1060 °C.
We presented a study on the effects of low-temperature (LT) AlN buffer growth time and surface morphology on the properties of high-temperature (HT) AlN epitaxial films using metal-organic chemical vapor deposition. Atomic force microscopy, Double crystal X-ray diffraction, Raman test and optical transmission measurement were employed to characterize the four HT AlN samples with different LT AlN buffer growth time. The results demonstrate that the LT AlN buffer growth time and surface morphology are key parameters for the quality of the HT AlN films. With an optimized LT AlN buffer growth time of 6 minutes, a 1-μm-thick high-quality and crack-free AlN film was obtained. The full width at half-maximum of X-ray diffraction (0002) and (1012) rocking curves of the AlN film are 42 and 530 arcsec, respectively. The AFM measurement shows an atomically flat surface with a root mean square roughness of 0.149 nm and a step-flow growth mode. The transmittance spectrum presents a sharp absorption band gap at wavelength 203 nm.
C‐plane‐oriented sapphire substrates that were patterned on the nanoscale were overgrown by AlN using metal‐organic vapor phase epitaxy. The occurrence of undesired misaligned AlN growth was detected. We found that this misaligned growth can be overcome by a proper choice of growth temperature and V/III ratio. Up to 8 μm thick c‐plane‐oriented AlN with a coalesced surface was obtained. An effective dislocation reduction was found due to bending of threading dislocation lines toward free surfaces during lateral growth. The distribution of crystal defects suggests that step bunching in AlN is accompanied by dislocation accumulation. Furthermore, nearly defect‐free AlN crystallites with a hexagonal shape and a size of about 2 μm were observed.
Schematic cross‐section representation of AlN grown on nano‐patterned sapphire. Different AlN crystal orientations (arrows) and dislocations (solid black lines) are indicated.
X-ray diffraction and TEM investigations of MOVPE grown AlN on sapphire with small off-cuts to a- and m-plane reveal the influence of the off-cut direction and angle on the reduction of threading dislocation density by annihilation during growth. Higher off-cut angles as well as off-cut to a-plane seem to facilitate the annihilation, with the main reduction taking place within the first 300nm layer thickness. On planar substrate the thickness is limited by cracking to below 2μm which also limits the ability to further reduce the defect density. By epitaxial lateral overgrowth on stripe patterned substrates the crack-free thickness is increased and further reduction of the defect density is possible. This process is effective up to 3-5μm layer thickness. Using templates with off-cuts ≥0.2° to m-plane, step bunching perpendicular to the stripe direction occurs and bends the vertically directed threading dislocations into inclined grain boundaries starting from the point of coalescence. These partially block/incline threading dislocations over the ridge areas and thus further reduce the dislocation density. The dislocations are concentrated in stripes over the ridges and the coalescence areas. For smaller off-cut to m or especially for off-cut to a-plane, the dislocation distribution is more homogeneous but nevertheless stripe-like with alternating densities of low 108cm-2 in the laterally overgrown areas and low 109cm-2 in the areas over the ridges and the coalescence lines.
This chapter provides a brief introduction to group III-nitride ultraviolet light emitting diode
(LED) technologies and an overview of a number of key application areas for UV-LEDs. It covers the state of the art of UV-LEDs as well a survey of novel approaches for the development of high performance UV light emitters.
UV transparent lattice matched substrates, e.g., AlN and AlGaN, with low dislocation densities are required for the epitaxial growth of UV-LED structures. In the absence of inexpensive high-quality UV transparent AlN bulk substrates, the heteroepitaxial growth of AlN base layers on relatively cheap and readily available sapphire substrates is the commonly used approach for most UV-B and UV-C LEDs. This chapter provides an insight into growth, strain management, and dislocation reduction techniques in metalorganic vapor phase epitaxy (MOVPE) of AlN and hydride vapor phase epitaxy (HVPE) of AlGaN layers. For both MOVPE and HVPE epitaxial lateral overgrowth of patterned substrate surfaces is an important technique to enhance the thickness of crack-free layers. This opens the route to UV-LEDs with improved performance.
The effects of the aluminum content x and the magnesium doping concentration in the AlxGa1-xN:Mg electron blocking layer on the emission characteristics of ultraviolet light-emitting diodes has been investigated. The carrier injection in the light-emitting diodes is simulated and compared with electroluminescence measurements. The light output power depends strongly on the aluminum mole fraction x as well as on the magnesium supply in the vapor phase during the growth of the AlxGa1-xN:Mg electron blocking layer. The highest output power has been found for an aluminum content x of around 44% and an Mg/III-ratio of 3.0% for light-emitting diodes with an emission wavelength near 320 nm. This effect can be attributed to an improved carrier injection and confinement preventing electron leakage into the p-doped layers of the light-emitting diode and an effective hole injection into the active region.
The polarity of AlN epitaxial layers grown on (0001) sapphire, SiC, and nitrided sapphire substrates was examined by convergent beam electron diffraction, and the morphology and microstructure were characterized by atomic force microscopy and scattering contrast transmission electron microscopy (TEM). The AlN films grown on sapphire and SiC without a buffer layer or nitridation of the substrate were flat and had Al polarity. From TEM studies in both cross section and plan view, it was found that the threading dislocation (TD) density was ~1×108 and 2×1010 cm-2 for screw-component and pure edge component dislocations, respectively. N-face AlN was realized by pregrowth nitridation of sapphire substrates, but these films contained a small volume fraction of Al-face inversion domains which were related to hexagonal pyramids defined by {1102} facets. The density of screw-component TDs was significantly reduced due to nitridation. Cross-sectional TEM showed that the film grown on nitrided sapphire was almost free of screw-component TDs and that the density of pure edge TDs was ~4×109 cm-2.
Various procedures, such as using a buffer layer or multi-growth mode modification, were investigated for the growth of AlN layers on sapphire substrate at high temperatures by metalorganic vapor phase epitaxy. Even though the top AlN layers were grown under the same conditions, the each crystalline quality was different. There is a clear relationship between the strain during growth and the quality of AlN films. AlN grown under the least strain shows the highest quality. Furthermore, it was found that cracks were suppressed by multi-growth mode modification.
Epitaxial AlN films were deposited on (0001) sapphire using a new metalorganic chemical vapor deposition process in which optimal substrate nitridation is combined with modulated ammonia flow and a growth pressure reduction from 150 torr to 40 torr after the first stage of growth. A significant improvement in the full width half maximum (FWHM) of the rocking curves was obtained. The best layers had FWHM of 330 arc sec/650 arc sec for the (0002)/(10\bar{1}2) reflections, respectively.
Metalorganic chemical vapor deposition (MOCVD) growth of AlN, GaN and AlGaN on sapphire substrates was investigated, with the aim of realizing high-quality heterostructures with atomically smooth interfaces. AlN with surfaces constructed of single-atomic-layer steps was grown by the two-step growth technique, in which the thin first layer was grown at a low temperature (1250° C) and then the second layer was grown at a high temperature (1350° C). GaN with surfaces consisting of simple atomic-layer steps were successively grown on the AlN layer by optimizing the growth conditions. Marked degradation of surface flatness was not observed for Al0.2Ga0.8N grown on GaN.
Epitaxial films having a large lattice mismatch with their substrate invariably form a mosaic structure of slightly misoriented sub-grains. The mosaic structure is usually characterized by its x-ray rocking curve on a surface normal reflection but this is limited to the out-of-plane component unless off-axis or transmission experiments are performed. A method is presented by which the in-plane component of the mosaic misorientation can be determined from the rocking curves of substrate normal and off-axis reflections. Results are presented for two crystallographically distinct heteroepitaxial systems, ZnO, AlN, and GaN (wurtzite crystal structure) on c-plane sapphire and MgO (rock salt crystal structure) on (001) GaAs. The differences in the mosaic structure of these films are attributed to the crystallographic nature of their lattice dislocations. © 1997 American Institute of Physics.
High-resolution x-ray diffraction has been used to analyze the type and density of threading dislocations in (001)-oriented GaN epitaxial layers. For this, (00l) and (hkl) Bragg reflections with h or k nonzero were studied, the latter one measured in skew symmetric diffraction geometry. The defect analysis was applied to a variety of GaN layers grown by molecular-beam epitaxy under very different conditions. The outcome is a fundamental correlation between the densities of edge- and screw-type dislocations. © 2000 American Institute of Physics.
A high-quality thick AlN layer without cracks was grown on sapphire by the combination of the high-temperature metal-organic vapor phase epitaxial growth and epitaxial lateral overgrowth methods. The dislocation behavior and growth mode of AlN are investigated in detail. The dislocation density of the AlN layer thus grown was less than 107 cm−2.
Light-emitting diodes with emission wavelengths less than 400 nm have been developed using the AlInGaN material system. For devices operating at shorter wavelengths, alloy compositions with a greater aluminium content are required. The material properties of these materials lie on the border between conventional semiconductors and insulators, which adds a degree of complexity to the development of efficient light-emitting devices. A number of technical developments have enabled the fabrication of LEDs based on group three nitrides (III-nitrides) that emit in the UV part of the spectrum, providing useful tools for a wealth of applications in optoelectronic systems.
High-quality AlN layers with atomically flat surface were grown on a c-plane sapphire substrate by high-temperature metal-organic vapor phase epitaxy (HT-MOVPE). Controlling V/III ratio during growth led to change the growth rate for each facet resulting in the change of the macroscopic form of grain at the transition V/III ratio. The threading dislocations were annihilated with the formation of dislocation loops at the changing of grain form. AlN crystallinity was improved due to the reason that small AlN grains were incorporated by the big AlN grains during growth. These phenomena were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM).
We report on the growth of high quality AlN films on sapphire by MOVPE in an AIX2400G3-HT planetary reactor. Specific reactor hardware modifications were conducted to facilitate growth temperatures of up to 1600 °C and to obtain reduced parasitic gas phase reactions. Growth was optimized regarding growth rate and surface morphology as well as optical and structural properties of the AlN layers on sapphire. With increasing growth temperature we observe a transition from an AlN surface with a high density of large pits to a smooth pit-free morphology. The improvement in material quality with growth temperature is confirmed by X-ray diffraction, AFM, SIMS and Raman measurements. The impact of residual or intentionally introduced Ga during growth on AlN material properties is discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
In this letter we report the deposition of high quality single‐crystal films of AIN over basal plane sapphire substrates. A conventional low pressure metalorganic chemical vapor deposition (LPMOCVD) system was used for all the growths reported here. We present the results of conventional and switched atomic layer epitaxial (SALE) depositions. Conventional LPMOCVD yielded single‐crystal AIN films at temperatures in excess of 750 °C. The ALE process in contrast produced extremely smooth single‐crystal AIN layers at temperatures as low as 450 °C. To the best of our knowledge this is the lowest ever reported for chemical vapor deposition of single‐crystal AIN. X‐ray and optical characterization data are presented to compare the quality of the material resulting from the two deposition techniques.
- N Okada
- N Kato
- S Sato
- T Sumii
- T Nagai
- N Fujimoto
- M Imura
- K Balakrishnan
- M Iwaya
- S Kamiyama
- H Amano
- I Akasaki
- H Maruyama
- T Tagaki
- T Noro
- A Bandoh
N. Okada, N. Kato, S. Sato, T. Sumii, T. Nagai, N. Fujimoto, M. Imura,
K. Balakrishnan, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki, H. Maruyama,
T. Tagaki, T. Noro, A. Bandoh, J. Crystal Growth 298 (2007) 349.
- M Khan
- K Balakrishnan
- T Katona
M. Asif Khan, K. Balakrishnan, T. Katona, Nat. Photonics 2 (2008) 77.
- M Imura
- N Fujimoto
- N Okada
- K Balakrishnan
- M Iwaya
- S Kamiyama
- H Amano
- I Akasaki
- T Noro
- T Tagaki
M. Imura, N. Fujimoto, N. Okada, K. Balakrishnan, M. Iwaya, S. Kamiyama,
H. Amano, I. Akasaki, T. Noro, T. Tagaki, A. Bandoh, J. Crystal Growth 300
(2007) 136.
- F Brunner
- H Protzmann
- M Heuken
- A Knauer
- M Weyers
- M Kneissl
F. Brunner, H. Protzmann, M. Heuken, A. Knauer, M. Weyers, M. Kneissl, Phys.
Stat. Sol. c 5 (2008) 1799.
- M Asif Khan
- J N Kuznia
- R A Skogman
- D T Olson
- M Mac Millan
- W J Choyke
M. Asif Khan, J.N. Kuznia, R.A. Skogman, D.T. Olson, M. Mac Millan,
W.J. Choyke, Appl. Phys. Lett. 61 (1992) 2539.
- M Imura
- N Fujimoto
- N Okada
- K Balakrishnan
- M Iwaya
- S Kamiyama
- H Amano
- I Akasaki
- T Noro
- T Tagaki
- A Bandoh
M. Imura, N. Fujimoto, N. Okada, K. Balakrishnan, M. Iwaya, S. Kamiyama,
H. Amano, I. Akasaki, T. Noro, T. Tagaki, A. Bandoh, J. Crystal Growth 300
(2007) 136.