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Effect of epitaxial layer thickness on the deep level defects in MBE grown n-type Al0.33Ga0.67As
Abstract
The effect of the epitaxial layer thickness on the deep level defects in MBE grown n-Al0.33Ga0.67As is investigated for the first time using current-voltage-temperature (I-V-T), deep level transient spectroscopy (DLTS) and Laplace DLTS techniques. The epitaxial layer thickness is found to have profound effects on the room temperature I-V characteristics, the number of defects detected by DLTS and their concentrations. In this investigation we compare n-Al0.33Ga0.67As samples having epitaxial layer thicknesses of 2 µm and 1.5 µm. Our results reveal that by increasing the layer thickness (1) the reverse current increases; (2) the number of electrically active deep defects increases from two to six; (3) the concentration of the traps increases.
The I-V-T and DLTS measurements carried out from 20 K to 300 K show that there is a dominant trap with an activation energy of ∼0.48 eV in both samples but with different concentrations (8.59 × 1014cm-3 and 1.48 × 1014 cm-3 for 2 µm and 1.5 µm samples, respectively). The reduction in the reverse current is directly related to the number of defects and their concentration. Our findings have important implications on the performances of electronic and optoelectronic devices. An attempt to explain the effect of the layer thickness on the deep traps in n-Al0.33Ga0.67As will be given (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
... Henisch [8] stated 51 that the fluctuations in SBHs are unavoidable as they exist even 52 in the most carefully processed devices. 53 Although AlGaAs Schottky diodes have been investigated for 54 more than four decades, it seems that the only study that explored 55 the influence of epitaxial layer thickness in AlGaAs Schottky diodes 56 was that conveyed by Mari et al. [9] in which they searched for 57 dependence on deep level defects at low temperatures. In this 58 study we report the effect of epitaxial layer thickness on the dependence has been noticed before in AlGaAs Schottky diodes 101 [11] and in some other V-III compounds such as AlGaN [12,13]. ...
... Thus, we only compared our result to a part of the 166 extracted line between the values measured at 300 K and 573 K. 167 The change of barrier height with temperature for our two samples 168 and the data from Zhang [3] is illustrated in Fig. 3. 169 It is clear that the device with a thinner epitaxial layer has a 170 higher barrier height, and as the thickness reduces to 1 lm, the 171 value of SBH at room temperature was increased to 0.87 eV. This 172 degradation of the electrical performance of the MBE grown 173 AlGaAs with increasing the layer thickness has been observed 174 before [9] and correlated to creation of deep level defects in GaAs. 175 It is well known that silicon (Si), and other commonly used donors 176 in AlGaAs, give rise to a large concentration of traps in the materi-177 als, often referred to as DX centers [15]. ...
... 175 It is well known that silicon (Si), and other commonly used donors 176 in AlGaAs, give rise to a large concentration of traps in the materi-177 als, often referred to as DX centers [15]. Recently, Mari et al. [9] 178 investigated the presence of those centers into the epitaxial Si: and allow the creation of other types of defects [9]. We believe that Dashed line represents data obtained from Ref. [3]. ...
The effect of epitaxial layer thickness on electrical characteristics of two Ti/n-Al0.33Ga0.67As Schottky barrier diodes was studied in the temperature range of 300–420K. Comparing the current–voltage (I–V) characteristics of two samples with epitaxial layer thicknesses of 2μm and 1.5μm discloses that the device with a thinner epitaxial layer has a higher barrier height and hence a lower reverse current. Specifically, we found that increasing the Al0.33Ga0.67As thickness from 1.5μm to 2μm would lower the value of the barrier height by ∼12% at 300K. We associated such retrogression of the electrical quality to the presence of deep level traps in the Si:AlxGa1−xAs layer. For both samples we found that the effective barrier height decreases with increasing the annealing temperature. Yet, the sample with a thinner layer showed more stability and less temperature dependence.
High electric field transport in n-Ga1−xAlxAs semiconductor alloys for composition x range from 0.0 to 0.5 is studied theoretically. The calculations are based on a shifted Maxwellian approach for the three-valley conduction-band model. An expression for the alloy scattering probability involving no fitting parameters is used in these calculations. The calculations predict a close to linear decrease of peak velocity with an increase in the alloy composition x. The absence of the onset of negative differential mobility at x≥0.35 is found.
A quantitative improvement in deep‐level transient spectroscopy (DLTS) resolution has been demonstrated by using Laplace transform method for the emission rate analysis. Numerous tests performed on the software used for the calculations as well as on the experimental setup clearly demonstrated that in this way the resolution of the method can be increased by more than an order of magnitude. Considerable confidence in this approach was gained through measurements of a selection of well‐characterized point defects in various semiconductors. The results for platinum in silicon and EL2 in GaAs are presented. For each of these cases conventional DLTS give broad featureless lines, while Laplace DLTS reveals a fine structure in the emission process producing the spectra.
Si molecular-beam epitaxy (MBE) on smooth Si(100) surfaces is shown to occur at room temperature. We demonstrate for the first time that Si deposition becomes amorphous after growth of a limiting epitaxial thickness (hepi). hepi is ≊10–30 Å at room temperature and increases rapidly at higher temperatures with a rate-dependent activation energy in the range 0.4–1.5 eV. The effect is tentatively linked to surface roughening during growth at low temperatures, and is probably general in MBE, also occurring for Si/Si(111), Ge/Si(100, and GaAs/GaAs(100).
The effect of group V/III flux ratio Γ on deep electron traps in AlxGa1−xAs (x=0.7) grown by molecular beam epitaxy at 720 °C has been studied by deep level transient spectroscopy. The photoluminescence characteristics of a GaAs single quantum well sandwiched by Al0.7Ga0.3As are determined by the electron traps denoted as E4–E6(E6’) in Al0.7Ga0.3As with the activation energies of 0.77 eV (E4), 0.72 eV (E5), 0.90 eV (E6), and 1.00 eV (E6’). The concentrations of these traps are minimized to the order of 1013 cm−3 at Γ∼2 in spite of high Al content.
Five electron traps were detected successfully in heavily Si-doped GaAs and AlxGa1−xAs of low Al content with a Si concentration of above 1×1019 cm−3 using deep level transient spectroscopy. The junctions were grown by liquid phase epitaxy and were strongly compensated. The traps were investigated for functions of the Si concentration and the AlAs mole fraction. The traps are discussed in terms using their spectra and concentration as opposed to the previous results which used point defects in the GaAs and AlGaAs. The traps show distinctive features, which can be attributed to strongly Si-compensated crystals. Three traps among them were confirmed to be DX centers. © 2002 American Institute of Physics.
A preliminary study of growth interruption effects on the GaAs/AlGaAs layers is reported. Deep level transient spectroscopy (DLTS) is performed on Al Schottky barrier devices fabricated on Si-doped, isotype GaAs/Al 0.78 Ga 0.22 As heterostructures grown by molecular beam epitaxy (MBE), both with (200 s) and without, growth interruption. Our spectra extending from $15 to 300 K show four peaks corresponding to deep level defects with thermal activation energies (E A): 0.09, 0.32, 0.42 and 0.52 eV, which are detected in both types of samples (close to and) above liquid nitrogen temperature, although with different relative concentrations. Comparison with reported work on GaAs/Al x Ga 1Àx As samples with different compositions (x) shows that latter two of these deep levels are counterparts of the levels, P3–P4, earlier observed in MBE grown Al x Ga 1Àx As materials with Al content different than that used here. However, the deep level spectra of our samples show dramatic differences for the growth interrupted and uninterrupted samples in the temperature range below 77 K. Whereas the samples prepared without growth interruption show a prominent, hitherto unobserved, positive (majority carrier emission) peak, P0 (E A ¼ 0.029 eV), with shoulders on both higher and lower temperature flanks, the samples with growth interruption show a pronounced negative peak, apparently corresponding to a deep level with E A ¼ 0.031 eV, in the same temperature region, under similar measurement conditions. The deep-level characteristics, including emission rate signatures and apparent capture cross-sections, are determined for the observed features using the thermal emission rate data. Some interesting additional features related to depth profiles of deep levels, bias effects and charge-induced metastability effects are also reported.
A new technique, deep‐level transient spectroscopy (DLTS), is introduced. This is a high‐frequency capacitance transient thermal scanning method useful for observing a wide variety of traps in semiconductors. The technique is capable of displaying the spectrum of traps in a crystal as positive and negative peaks on a flat baseline as a function of temperature. It is sensitive, rapid, and easy to analyze. The sign of the peak indicates whether the trap is near the conduction or valence band, the height of the peak is proportional to the trap concentration, and the position, in temperature, of the peak is uniquely determined by the thermal emission properties of the trap. In addition, one can measure the activation energy, concentration profile, and electron‐ and hole‐capture cross sections for each trap. The technique is presented with a simple theoretical analysis for the case of exponential capacitance transients. Various traps in GaAs are used as examples to illustrate certain features of the DLTS technique. Finally, a critical comparison is made with other recent capacitance techniques.
DX centers, deep levels associated with donors in III‐V semiconductors, have been extensively studied, not only because of their peculiar and interesting properties, but also because an understanding of the physics of these deep levels is necessary in order to determine the usefulness of III‐V semiconductors for heterojunction device structures. Much progress has been made in our understanding of the electrical and optical characteristics of DX centers as well as their effects on the behavior of various device structures through systematic studies in alloys of various composition and with applied hydrostatic pressure. It is now generally believed that the DX level is a state of the isolated substitutional donor atom. The variation of the transport properties and capture and emission kinetics of the DX level with the conduction‐band structure is now well understood. It has been found that the properties of the deep level when it is resonant with the conduction band, and is thus a metastable state, are similar to its characteristics when it is the stable state of the donor. And it has been consistently found that there is a large energy difference between the optical and thermal ionization energies, implying that this deep state is strongly coupled to the crystal lattice. The shifts in the emission kinetics due to the variation in the local environment of the donor atom suggest that the lattice relaxation involves the motion of an atom (the donor or a neighboring atom) from the group‐III lattice site toward the interstitial site. Total energy calculations show that such a configuration is stable provided that the donor traps two electrons, i.e., has negative U. Verification of the charge state of the occupied DX level is needed as well as direct evidence for its microscopic structure.
Using deep‐level transient capacitance spectroscopy we have investigated deep electron traps in n‐AlGaAs grown by molecular‐beam epitaxy (MBE). The thermal activation energies of seven traps, labeled ME1–ME7, observed in this study increase with increasing Al content(x) up to the direct‐indirect crossover point (x∼0.42), but show only a small change with further increases in Al content. Traps ME4–ME7 are dominant in samples with x≤0.2. Traps ME4–ME6 strongly depend on the growth ambient. The concentration of ME7 is almost independent of the ambient in the growth chamber but decreases rapidly with increasing growth temperature. ME7 is a native defect and can almost certainly be identified with the trap EL2 observed in bulk and vapor‐phase epitaxially grown GaAs. Traps ME4–ME6 are probably formed by impurities involving oxygen such as CO, H 2 O, and AsO in the growth ambient. All of the traps, ME5–ME7 are clearly responsible for a decrease in the photoluminescence intensity of MBE grown AlGaAs.
Uniformly Si doped GaAs/Al 0.33 Ga 0.67 As multilayer structures have been studied by deep level transient spectroscopy (DLTS) and photocapacitance measurements. DLTS spectra showed five peaks which are related to defects in the GaAs layers. The concentration of these defects decreased with increasing layer thickness. An additional peak, which has been observed with forward bias filling pulses, is suggested to be related to defects near the surface, most probably due to defect accumulation in multilayers. Their emission and capture properties as well as photoionization cross sections have been studied. Evidence is provided that the emission and filling processes of these deep levels are modified due to the energy quantization in the conduction band and the carrier transport through the quantum structures. No DX center related DLTS peaks or other features like persistent photoconductivity effects have been observed in any of our samples. © 1996 American Institute of Physics.
The low‐temperature limit to GaAs molecular beam epitaxy (MBE) is studied at temperatures from 250 °C to room temperature. Using transmission electron microscopy of layers grown under a variety of conditions we show that, as for Si MBE, there is an epitaxial thickness h epi at which a growing epitaxial layer becomes amorphous. The temperature, growth rate, composition, and defect density all appear to be constant during growth of the epitaxial layer, and (in analogy with Si MBE) we tentatively associate the breakdown of epitaxy at h epi with roughening of the growth surface. We demonstrate that h epi depends strongly on composition, increasing rapidly with Ga/As ratio at fixed temperature. At fixed Ga/As ratio, h epi shows an abrupt increase from ≪200 to ≳5000 Å at 210 °C. The results have implications for the growth of GaAs/GaAs for high‐speed photodetectors, as well as possible applications to GaAs/Si heteroepitaxy.
The ambipolar diffusion length and carrier lifetime are measured in Al x Ga 1-x As for several mole fractions in the interval 0≪x≪0.38. These parameters are found to have significantly higher values in the higher mole fraction samples. These increases are attributed to occupation of states in the indirect valleys, and supporting calculations are presented.
Electrical properties and the donor energy level in Se‐doped n‐Ga 1-x Al x As (0⩽x⩽0.82) prepared by metalorganic chemical vapor deposition have been investigated. The van der Pauw technique was used to measure the electrical properties of n‐Ga 1-x Al x As. The resistivity and electron concentration of Se‐doped Ga 1-x Al x As were found to be strongly affected by the donor energy level for Se. The donor energy levels E D of Se in Ga 1-x Al x As was found to remain constant at 0.003 eV for x≪0.25. For x≳0.25, E D takes the form of an inverted V with a maximum at the direct‐indirect band crossover.
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