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The Effect of AlN Buffer Layer on AlGaN/GaN/AlN Double‐Heterostructure HEMT
Abstract
We investigated the effect of crystal quality of AlN buffer layer on AlGaN/GaN/AlN double‐heterostructure high electron mobility transistor (DH‐HEMT). The material quality of the GaN channel and the AlGaN barrier such as the dislocation density and the interface roughness deteriorated and the two‐dimensional electron gas (2DEG) mobility decreased as the threading dislocation density (TDD) of the AlN buffer increased. It was also revealed that the thickness and the Al mole fraction of the AlGaN barrier were affected by the strain variation of the GaN channel depending on the TDD of the AlN buffer. The variation of the compressive strain of the GaN channel was responsible for the 2DEG density change by affecting the barrier condition and the piezoelectric polarization charge. Low‐temperature Hall effect measurement revealed that the interface roughness scattering was dominant factor for the mobility of the DH‐HEMT, which was approximately 2‐6 × 103 cm2 / V·s. This article is protected by copyright. All rights reserved.
... We followed the study by Freedsman et al. 22 and optimized the nucleation conditions for sample B in terms of V-pit formation. The intention was to provoke different microstructural properties in all layers above the nucleation layer up to the AlGaN barrier 23,24 . As we want to focus on dislocations in the AlGaN barriers of both samples, we need to find a way to characterize them electrically. ...
... One potential mechanism is the Al pulling effect similar to the In pulling effect in InGaN layers grown on GaN [28][29][30] . Different nucleation conditions as used in our work were shown to induce different depth profiles of the grown-in V-pit density, which in turn has an influence on stress relaxation and thus on Al incorporation 3,24,30,31 . ...
GaN epitaxially grown on Si is a material for power electronics that intrinsically shows a high density of dislocations. We show by Conductive Atomic Force Microscopy (C-AFM) and Defect Selective Etching that even for materials with similar total dislocation densities substantially different subsets of dislocations with screw component act as current leakage paths within the AlGaN barrier under forward bias. Potential reasons are discussed and it will be directly shown by an innovative experiment that current voltage forward characteristics of AlGaN/GaN Schottky diodes shift to lower absolute voltages when such dislocations are present within the device. A local lowering of the Schottky barrier height around conductive dislocations is identified and impurity segregation is assumed as responsible root cause. While dislocation related leakage current under low reverse bias could not be resolved, breakdown of AlGaN/GaN Schottky diodes under high reverse bias correlates well with observed conductive dislocations as measured by C-AFM. If such dislocations are located near the drain side of the gate edge, failure of the gate in terms of breakdown or formation of percolation paths is observed for AlGaN/GaN high electron mobility transistors.
... In fact, according to the literature, replication of the buffer layer roughness towards the surface is common in several superlattices systems such as Si/Ge, 61 GaAs/AlAs 62 and GaN/AlN. [63][64] The high level of alignment between the atoms in both ZnO and Zn 1-x Mg x O layers as well as the individual layer thicknesses are concluded from the high-resolution STEM image of figure 5a 2 ). The total thickness of the superlattice is 158514 Å while the average thicknesses and respective standard deviations considering the 50 ZnO and Zn 1-x Mg x O layers are found to be 171 Å and 141 Å, respectively. ...
New software for the simulation and fitting of the specular X-ray reflectivity (XRR) measurements has been developed. The novel software employs the recursive formalism of the dynamical theory of X-ray...
... Hierbei beeinflusst der Spannungszustand der GaN-Schicht die Effektivität des Einbaus von In in InGaN. Ähnlich kann im vorliegenden Fall die unterschiedliche Nukleation zu unterschiedlichen Spannungszuständen der GaN-Kanalschicht führen, welche wiederum den Al-Einbau in die AlGaN-Barriere beeinflusst [Bes20b,Che09,Cho20,He16]. Inwiefern der unterschiedliche Al-Gehalt die elektrischen Versetzungseigenschaften beeinflusst, ist nicht eindeutig beantwortbar. ...
Das Materialsystem GaN auf Si verspricht die Verknüpfung der Vorteile eines Wide Bandgap Halbleiters mit den niedrigen Herstellungskosten von Si. So können High Electron Mobility Transistoren zum Beispiel auf Basis einer AlGaN/GaN-Heterostruktur als Ausgangspunkt kosteneffektiver und zugleich sehr energieeffizienter Leistungsschalter dienen. Die heteroepitaktische Herstellung bedingt jedoch hohe Materialdefektdichten, welche sich insbesondere in Form von Versetzungen und inversen Pyramidendefekten (V-pits) äußern. Der Einfluss dieser Defekte auf Bauelemente ist auf Grund der experimentell schwierigen Untersuchung jedoch nur sehr unzureichend bekannt und verstanden. In dieser Arbeit wurde eine strukturelle und elektrische Charakterisierungsmethodik von Versetzungen und V-pits in GaN auf Si entwickelt und systematisch auf unterschiedliche Proben angewendet. Der Fokus lag dabei auf der direkten Korrelation mikrostruktureller und elektrischer Eigenschaften bei gleichzeitiger Gewährung einer statistischen Relevanz der Ergebnisse. Dabei gelang es, charakteristische Versetzungsstrukturen und V-pits als Leckstrompfade durch das Material zu identifizieren und deren Auftreten mit Herstellungsparametern der Proben zu untersuchen. So stellte sich heraus, dass letztere eine essentielle Rolle hinsichtlich der elektrischen Aktivität von Versetzungen und des Auftretens von V-pits spielen. Zusätzlich wurden die gewonnenen Erkenntnisse bezüglich ihrer Relevanz für Bauelemente untersucht. Dabei konnte erstmalig der direkte Einfluss einzelner, elektrisch aktiver Versetzungen in GaN auf Si nachgewiesen werden. So bedingen sie eine Verschiebung der Einschaltspannung von Schottky-Dioden zu niedrigeren Werten, während V-pits eine starke Degradation der kritischen elektrischen Feldstärke des Materials nach sich ziehen. Die Arbeit leistet damit einen wichtigen Beitrag zum Verständnis der Eigenschaften und Rolle dieser Defekte und damit zur technologischen Optimierung des Materialsystems.
... The sample structure shown in figure 1 was grown on AlN-template on double-side-polished sapphire substrate by high temperature metal-organic chemical vapor deposition. An additional ∼3000 nm thick AlN buffer layer was grown on the AlN-template to further improve the crystalline quality before the growth of device structure [29]. The device began with a 400 nm thick Al 0.35 Ga 0.65 N bottom contact layer Si-doped to 2 × 10 18 cm −3 . ...
Semiconductor-based avalanche photodiodes (APDs) have the advantages of lower power and simpler fabrication of arrays compared with photomultiplier tubes. It is critical for weak-light imaging that the APD is operated under back illumination and with high linear gain. However, linear gain is fairly low for conventional PIN APDs. This paper presents a back-illuminated APD, whose multiplication layer is designed to be a GaN/AlN periodically stacked structure. The GaN/AlN heterostructure is introduced to enhance the ratio of ionization coefficient between electrons and holes, which proves to be helpful for increasing the linear gain. Under back illumination, the prototype device demonstrates a record-high external quantum efficiency (reflecting linear gain) of ∼5 × 10 ² at the reverse bias of 66 V. The device response spectra were also studied to further confirm that only the photo-excited electrons can trigger the collision ionization in the multiplication layer.
... It is mentioned in Refs. [42][43][44] that interface roughness (IFR) scattering has a stronger influence than the other scattering processes. The IFR scattering is closely related to the surface morphology. ...
AlGaN/GaN high electron mobility transistors (HEMTs) on Silicon (111) substrates are grown by
using multi-stacked strained superlattices layer structures. Atomic force microscopy, X-ray
diffraction and cross-sectional transmission electron microscopy are employed to investigate the
structural properties of these AlGaN/GaN HEMT structures. The insertion of AlGaN/AlN strained
superlattices layer modified with different numbers of GaN layers is revealed to reduce the pit
density and threading dislocation density, resulting in the improvement of epilayer quality. The
temperature dependent Hall effect measurements show that the two-dimensional electron gas
characteristics are greatly improved by the employed modified superlattices layers in AlGaN/GaN
HEMT structures throughout the whole measured temperature range. The superlattices layer
engineering is shown to be important for achieving higher carrier mobility (>2340 cm2/V) and
sheet carrier density (>8.22 × 1012 cm 2) at room temperature in AlGaN/GaN HEMTs.
To enhance the electron mobility in quantum-well high-electron-mobility transistors (QW HEMTs), we investigate the transport properties in AlN/GaN/AlN heterostructures on Al-polar single-crystal AlN substrates. Theoretical modeling combined with experiment shows that interface roughness scattering due to high electric field in the quantum well limits mobility. Increasing the width of the quantum well to its relaxed form reduces the internal electric field and scattering, resulting in a binary QW HEMT with a high two-dimensional electron gas (2DEG) density of 3.68 × 10 13 cm–2, a mobility of 823 cm²/Vs, and a record-low room temperature (RT) sheet resistance of 206 Ω / □. Further reduction of the quantum well electric field yields a 2DEG density of 2.53 × 10 13 cm–2 and RT mobility > 1000 cm²/V s. These findings will enable future developments in high-voltage and high-power microwave applications on the ultrawide bandgap AlN substrate platform.
Herein, a 3.5 GHz (S‐band) power performance is reported on an AlGaN/GaN high‐electron‐mobility transistor (HEMT) with AlN buffer on SiC substrate. A 4 inch epi‐wafer is grown by high‐temperature metal–organic chemical vapor deposition. The fabricated devices with a 400 nm gate and direct current and radio‐frequency (RF) characteristics are examined. These device shows a transconductance of 233 mS mm ⁻¹ and a maximum drain current of 820 mA mm ⁻¹ . The pulsed current–voltage ( I–V ) characteristic shows a low slump ratio with a 0.36% and 2.2% for Z 1 and Z 2 , respectively. The power performance shows output power = 5.5 W mm ⁻¹ with 54.3% power added efficiency, and V DS was 50 V. The potential of an AlN buffer HEMT is demonstrated by the results for use in next‐generation high‐power RF devices.
GaN-based high-electron mobility transistors (HEMTs) based power amplifiers (PA) have enjoyed commercial success in the microwave frequency spectrum (3–30 GHz) competing with silicon and silicon carbide-based transistors. However, low thermal conductivity of GaN limits the HEMT performance in the higher frequencies of millimetre-wave regime (30–100 GHz). This chapter uses the 2DEG in the AlN/GaN/AlN heterostructure as a channel in AlN HEMT-based mm-wave PAs. The high thermal conductivity of the AlN buffer layer, coupled with the highly quantum-confined, high-density 2DEG in this heterostructure, provide an advantage for high-power, mm-wave signal amplification. The state-of-art scaled AlN HEMTs demonstrate output powers of ∼2 W/mm at operating frequencies upto 94 GHz. The output powers in these HEMTs in these first-generation devices are limited by ∼ 20% dispersion due to the presence of carrier traps on the epitaxial surface. A path to higher output powers is presented through the demonstration of a unique in-situ AlN passivation technique which drastically reduces the RF-DC dispersion to 2–3%. Furthermore, enhancement-mode operation is demonstrated for the first time in AlN metal-oxide-semiconductor (MOS)-HEMTs for enabling complementary logic and RF circuits on the AlN platform. This is achieved by scaling the GaN channel layer of the AlN/GaN/AlN heterostructure down to 3 nm which results in a unique operation where just a 2DHG is present in the as-grown structure, but upon application of a positive gate bias a 2DEG is induced, which then acts as a channel for E-mode HEMTs. Future directions are provided to improve the performance of current AlN/GaN/AlN HEMTs state-of-art, such as incorporation of scandium aluminum nitride (ScAlN) as a high-K dielectric, AlGaN channels for higher breakdown voltages and utilizing the 2DHG as a backgate.KeywordsMBEGallium nitrideGaNAluminum nitrideAlNn-channelHEMTsElectrons2DEGPolarization-inducedmm-wavePower amplifiersPassivationRF dispersionE-modeD-modeT-gate
Presence of mobile carrier layers in close proximity, such as in electron-hole/electron-electron bilayers, have been studied in material systems such as GaAs and have led to the observation of scientifically interesting physical phenomena. However, the lack of efficient p-type doping has limited such studies in III-nitride semiconductors. Recent discovery of the purely polarization-induced undoped 2DHG in metal-polar GaN channel on AlN now opens the door to several technologically and scientifically interesting III-nitride semiconductor heterostructure designs. By growing a thin AlN layer on top of the metal-polar GaN/AlN heterostructure, a parallel high-density 2D electron gas (2DEG) is induced a few-10s of nm above the 2DHG. This chapter studies this metal-polar AlN/GaN/AlN heterostructure containing a 2DHG-2DHG bilayer in detail. Electro-luminescence experiment shows emission of photons with energy close to the GaN bandedge, providing proof of existence of the polarization-induced 2DEG-2DHG bilayer. This also represents first-of-its-kind light emission from a completely undoped III-nitride heterostructure. The transport properties of the 2DEG with very high densities of 2 − 3 × 1013 cm−2 are then studied in the regimes of low and high electric fields and magnetic fields. Shubhinokov de Haas oscillations are observed for the first time in AlN/GaN/AlN 2DEGs at magnetic fields > 25 T. The highly quantum confined high-density 2DEGs provides access to GaN conduction band electron states higher than previously possible in magnetotransport measurements, through which electron effective mass of 0.3 m0 is extracted at densities of 2 − 3 ×1013 cm−2. Directions are provided to guide future experiments exploring exciting scientific phenomena such as 2DHG-2DEG coulomb drag in III-nitride semiconductors.KeywordsMBEGallium nitrideGaNAluminum nitrideAlNp-channel2DHGHolesn-channelHEMTsElectrons2DEGSdHShubhinokov de Haas oscillationsEffective massHall transportSaturation velocity2DEG-2DHG bilayerPolarization-induced
Defects (VAl/VN) and unintentional impurities (C/O/Si/Hi) will inevitably exist during the preparation of AlN by metal-organic chemical vapor deposition and physical vapor transport. In particular, Hi is very difficult to remove in a vacuum environment. First-principle calculations were used to research the mechanical properties, thermal conductivity, and optical properties of AlN and its defect-containing systems. Results showed that the deformation resistance of Al36SiN35 and Al35ON36 systems was relatively poor, and the hardness was relatively small. When the impurity C or O located in the N position, the effect on the anisotropy of AlN was relatively large. O impurities and Al vacancies had relatively large effects on the thermal conductivity of AlN lattice. When the O impurity was located at the N position and the Si impurity was located at the Al position, the influence on the optical properties of AlN was relatively large, which would help improve the polarization ability, refractive index, reflectivity, and absorption of the visible light of AlN.
In this study, we investigate the effects of Si-doped GaN insert layer on the structural and electrical characteristics of AlGaN/GaN/GaN:Si/AlN double-heterostructure high-electron-mobility transistor (DH-HEMT). Compared with an AlGaN/GaN/AlN HEMT without a Si-doped GaN layer, the AlGaN/GaN/GaN:Si/AlN DH-HEMT showed an improved root-mean-square roughness of 0.57 nm and a decreased GaN channel compressive stress of –1.44 GPa. Also, an increased two-dimensional electron gas mobility of 1845 cm²/V∙s and a reduced specific contact resistance of 1.49 × 10⁻⁶ Ω∙cm² were obtained. The direct-current output results showed an increased maximum drain current of 600 mA/mm and transconductance of 130 mS/mm, which was attributed to the enhanced 2DEG mobility and reduced contact resistance by the insertion of Si-doped GaN layer in the DH-HEMT.
To improve the electron mobility of quantum well (QW) gallium nitride (GaN) high electron mobility transistors (HEMT), we investigated QW and conventional AlGaN/GaN heterostructures grown by metal organic vapor phase epitaxy. Using calculation and experimental results, we revealed that the primary reason for the reduced electron mobility of the QW GaN-HEMT is the increase in intrasubband scattering events because of excessive electron confinement, which is caused by a strong polarization electric field. A strained Al
0.30
Ga
0.70
N/Al
0.86
Ga
0.14
N buffer structure was applied to alleviate the electric field in the GaN channel while maintaining a strong electron confinement. It enables to increase the piezo polarization of Al
0.30
Ga
0.70
N and thus, reduces that of the GaN channel. Consequently, the electron mobility improved to 1420 cm
2
/Vs for the QW GaN-HEMT structure with the Al
0.30
Ga
0.70
N/Al
0.86
Ga
0.14
N buffer from 1100 cm
2
/Vs for the common QW GaN-HEMT structure. To the best of our knowledge, 1420 cm
2
/Vs at a two-dimensional electron gas density of
cm
−2
is the highest electron mobility in the QW double heterostructure.
The recent demonstration of ∼2 W/mm output power at 94 GHz in AlN/GaN/AlN high electron mobility transistors (HEMTs) establishes AlN as a promising platform for millimeter-wave high frequency electronics. The current state-of-art AlN HEMTs using ex-situ deposited silicon nitride (SiN) passivation layers suffer from soft gain compression due to trapping of carriers by surface states. Reducing surface state dispersion in these devices is thus desired to access higher output powers. This work provides a potential solution by using a novel in-situ crystalline AlN passivation layer. A thick, 30+ nm top AlN passivation layer moves the as-grown surface away from the 2DEG channel and reduces its effect on the device. Through a series of metal-polar AlN/GaN/AlN heterostructures growths, it is found that pseudomorphically-strained ≤15 nm thin GaN channels are crucial to be able to grow thick AlN barriers without cracking. The fabricated recessed-gate HEMTs on an optimized heterostructure with 50 nm AlN barrier layer and 15 nm GaN channel layer shows reduction in DC-RF dispersion down to 2−6% compared to 20% in current state-of-art ex-situ PECVD SiN passivated HEMTs. These results demonstrate the efficacy of this unique in-situ crystalline AlN passivation technique, and should unlock higher mm-wave powers in next-generation AlN RF HEMTs.
This article is protected by copyright. All rights reserved.
Gallium nitride high-electron-mobility transistors (GaN HEMTs) are at a point of rapid growth in defense (radar, SATCOM) and commercial (5G and beyond) industries. This growth also comes at a point at which the standard GaN heterostructures remain unoptimized for maximum performance. For this reason, we propose the shift to the aluminum nitride (AlN) platform. AlN allows for smarter, highly-scaled heterostructure design that will improve the output power and thermal management of III-nitride amplifiers. Beyond improvements over the incumbent amplifier technology, AlN will allow for a level of integration previously unachievable with GaN electronics. State-of-the-art high-current p-channel FETs, mature filter technology, and advanced waveguides, all monolithically integrated with an AlN/GaN/AlN HEMT, is made possible with AlN. It is on this new AlN platform that nitride electronics may maximize their full high-power, high-speed potential for mm-wave communication and high-power logic applications.
n recent years, ultrawide bandgap semiconductor materials represented by aluminum nitride (AlN) are attracting worldwide attention due to their excellent high-frequency power characteristics, stable high-temperature performance, low energy loss, and good ultraviolet (UV) transmittance. It has great application prospects in the fields of high-efficiency optoelectronic devices, high-power and high-frequency electronic devices, ultra-high voltage power electronic devices, deep UV warning and guidance, and deep UV-LED disinfection. The physical vapor transport (PVT) method has the advantages of simple growth process, fast growth rate, and high crystal integrity, and has gradually become one of the most effective methods for growing bulk AlN crystals. This review systematically summarizes the latest research progress in AlN crystals grown by PVT method in recent years, and introduces its applications in deep UV-LEDs, UV lasers and Shottky barrier diodes (SBD). Finally, the challenges and application prospects of AlN crystals are discussed. As an important new type of direct band gap ultrawide bandgap semiconductor material, AlN crystal has shown extremely important strategic application value. The output power of deep UV-LED devices meets practical requirements, and high-power electronic power devices are still in the verification stage. From the perspective of material superiority, it has considerable development potential.
A high mobility of 2250 cm2/V·s of a two-dimensional electron gas
(2DEG) in a metalorganic chemical vapor deposition-grown AlGaN/GaN heterostructure was demonstrated. The mobility enhancement was a result of better electron confinement due to a sharp AlGaN/GaN interface, as confirmed by scanning transmission electron microscopy analysis, not owing to the formation of a traditional thin AlN exclusion layer. Moreover, we found that the electron mobility in the sharp-interface heterostructures can sustain above 2000 cm2/V·s for a wide range of 2DEG densities. Finally, it is promising that the sharp-interface AlGaN/GaN heterostructure would enable low contact resistance fabrication, less impurity-related scattering, and trapping than the AlGaN/AlN/GaN heterostructure, as the high-impurity-contained AlN is removed.
Models for calculating the sheet densities of two-dimensional electron gas (2DEG) induced by spontaneous and piezoelectric polarization in AlGaN/GaN, AlGaN/AlN/GaN, and GaN/AlGaN/GaN heterostructures are provided. The detailed derivation process of the expression of 2DEG sheet density is given. A longstanding confusion in a very widely cited formula is pointed out and its correct expression is analyzed in detail.
Two dimensional electron gases in AlxGa1−xN/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance–voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined self-consistently from a coupled Schrödinger and Poisson equation solver for pseudomorphically and partially relaxed barriers with different alloy compositions. By comparison of theoretical and experimental results, we demonstrate that the formation of two dimensional electron gases in undoped and doped AlGaN/GaN structures rely both on piezoelectric and spontaneous polarization induced effects. In addition, mechanisms reducing the sheet carrier concentrations like nonabrupt interfaces, dislocations, and the possible influence of surface states on the two dimensional electron gases will be discussed briefly. © 2000 American Institute of Physics.
AlGaN/GaN/AlN quantum-well field-effect transistors have been demonstrated. By replacing a semi-insulating GaN epilayer with a highly resistive AlN epilayer in the device structure, parasitic conduction in the GaN epilayer, leakage current through the GaN epilayer, and the channel electrons spillover into the GaN epilayer have been completely eliminated and the drain current collapse has been reduced. The fabricated devices on sapphire substrate with 1 μm gate length show a high saturation current (>1 A/mm), and excellent gate control capability with a pinch-off voltage of −6 V. Even without passivation, the devices exhibit small drain current collapse (<10%) under 1 μs pulse gate driving.
The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c -plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm 2/ V s along with 8.44×1012 cm -2 electron density, which resulted in a two-dimensional sheet resistance of 435 Ω/◻ for the Al 0.2 Ga 0.8 N / AlN / GaN heterostructure. The sample designed with an Al 0.88 In 0.12 N barrier exhibited very high sheet electron density of 4.23×1013 cm -2 with a corresponding electron mobility of 812 cm 2/ V s at room temperature. A record two-dimensional sheet resistance of 182 Ω/◻ was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al 0.88 In 0.12 N / AlN / GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fiel-
ds that exist in the sample with an Al 0.88 In 0.12 N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors (HEMTs), after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs.
Transmission electron microscopy has been used to observe Al segregation around the threading dislocations in Al 0.1 Ga 0.9 N and Al 0.3 Ga 0.7 N grown by metalorganic chemical vapor deposition on 6H–SiC. Dislocation lines were found to have up to 70% more Al concentration than those regions free of dislocations in the matrix. The Al-depleted regions around the dislocations are shown to be within a few nanometers from the dislocation lines. The results also show that more Al segregate to edge dislocations than to screw ones. © 2001 American Institute of Physics.
High-quality p- Ga N /i- In 0.1 Ga 0.9 N /n- Ga N heterojunctional epilayers are grown on (0001)-oriented sapphire substrates by metal organic chemical vapor deposition. The Pendellösung fringes around the InGaN peak in high-resolution x-ray diffraction (HRXRD) confirm a sharp interface between InGaN and GaN films. The corresponding HRXRD and photoluminescence measurements demonstrate that there is no observable phase separation. The improvement in crystal quality yields high-performance photovoltaic cells with open-circuit voltage of around 2.1 eV and fill factor up to 81% under standard AM 1.5 condition. The dark current-voltage measurements show very large shunt resistance, implying an insignificant leakage current in the devices and therefore achieving the high fill factor in the illuminated case.
The mobility of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures changes significantly with Al content
in the AlGaN barrier layer, while few mechanism analyses focus on it. Theoretical calculation and analysis of the 2DEG mobility
in AlGaN/GaN heterostructures with varied Al content are carried out based on the recently reported experimental data. The
2DEG mobility is modeled analytically as the total effects of the scattering mechanisms including acoustic deformation-potential,
piezoelectric, polar optic phonon, alloy disorder, interface roughness, dislocation and remote modulation doping scattering.
We show that the increase of the 2DEG density, caused by the ascension of the Al content in the barrier layer, is a dominant
factor that leads to the changes of the individual scattering processes. The change of the 2DEG mobility with Al content are
mainly determined by the interface roughness scattering and the alloy disorder scattering at 77 K, and the polar optic phonon
scattering and the interface roughness scattering at the room temperature. The calculated function of the interface roughness
parameters on the Al content shows that the stress caused AlGaN/GaN interface degradation at higher Al content is an important
factor in the limitation of the interface roughness scattering on the 2DEG mobility in AlGaN/GaN heterostructures with high
Al content.
Silicon offers multiple advantages to power circuit designers, but at the same time suffers from limitations that are inherent to silicon material properties, such as low bandgap energy, low thermal conductivity, and switching frequency limitations. Wide bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), provide larger bandgaps, higher breakdown electric field, and higher thermal conductivity. Power semiconductor devices made with SiC and GaN are capable of higher blocking voltages, higher switching frequencies, and higher junction temperatures than silicon devices. SiC is by far the most advanced material and, hence, is the subject of attention from power electronics and systems designers. This paper looks at the benefits of using SiC in power electronics applications, reviews the current state of the art, and shows how SiC can be a strong and viable candidate for future power electronics and systems applications.
In evaluating GaN high-electron mobility transistors (HEMTs) for high-power applications, it is crucial to consider the device-level breakdown characteristics. This letter replaces the conventional AlGaN barrier and common AlGaN backbarrier with unstrained AlN, and it assesses the breakdown voltage of AlN/GaN/AlN quantum well HEMTs for gate-drain spacings in the range of 0.27-5.1 μm. The results are highlighted by a high breakdown voltage of 78 V for a gate-drain spacing of 390 nm, among the best reported for submicron-channel devices. In addition, small-signal RF measurements showed record performance for HEMTs on the AlN platform, with f
t
/f
max
= 161/70 GHz. The cut-off frequency and corresponding drain bias are benchmarked against the state-of-the-art GaN HEMTs using the Johnson figure of merit, with measured devices highlighted by a JFoM value of 2.2 THz · V. These results illustrate the potential for AlN/GaN/AlN quantum well HEMTs as a future platform for high-power RF transistors.
Boron dipyrromethene (BODIPY) is a class of important emerging fluorescent dyes. Due to their unique chemical and optical properties, near infrared (NIR)‐emitting BODIPY dyes containing nanoparticles have recently been developed for a wide array of cutting‐edge cancer optotheranostic applications. These nanoparticles not only have robust photostability and tunable photophysical properties, but they can also be flexibly tailored to a multitude of functional uses. Based on these outstanding characteristics, such nanoparticles have shown great promise in diagnosis as biological sensors, as well as in their utilization in advanced imaging and photomedicine for cancer treatment. In particular, here, this study first discusses their use as photoswitchable fluorescence probes toward in vitro single‐molecule imaging. Second, this study takes a look at their opportunities for photoacoustic imaging utilization. Third, approaches are discussed to construct new NIR‐absorbing BODIPY nanoparticles for photodynamic therapy (PDT). Fourth, this study delves into the new approach to use such nanoparticles as an emerging version of triplet–triplet annihilation upconversion (TTA‐UC) and their biological uses, such as their photoactivation prodrug therapy (PAPT) for cancer. Finally, new biological sensors based on NIR BODIPY nanoparticles are introduced. Near‐infrared boron dipyrromethene nanoparticles provide a versatile organic platform in optotheranostics. These biocompatible materials possess robust photostability and chemical stability, as well as having easily tailorable optical properties, which make them ideal for advanced biological applications such as sensing, advanced imaging, and phototherapy.
We report the first realization of molecular beam epitaxy grown strained GaN quantum well field-effect transistors on single-crystal bulk AlN substrates. The fabricated double heterostructure FETs exhibit a two- dimensional electron gas (2DEG) density in excess of 2x10^13/cm2. Ohmic contacts to the 2DEG channel were formed by n+ GaN MBE regrowth process, with a contact resistance of 0.13 Ohm-mm. Raman spectroscopy using the quantum well as an optical marker reveals the strain in the quantum well, and strain relaxation in the regrown GaN contacts. A 65-nm-long rectangular-gate device showed a record high DC drain current drive of 2.0 A/mm and peak extrinsic transconductance of 250 mS/mm. Small-signal RF performance of the device achieved current gain cutoff frequency fT~120 GHz. The DC and RF performance demonstrate that bulk AlN substrates offer an attractive alternative platform for strained quantum well nitride transistors for future high-voltage and high-power microwave applications.
The current instabilities of high electron mobility transistors (HEMTs), based on thin double AlN/GaN/AlN heterostructures (~0.5 μm total thickness), directly grown on sapphire substrates, have been analyzed and compared for different AlN top barrier thicknesses. The structures were capped by 1 nm GaN and non-passivated 1 μm gate-length devices were processed. Pulsed I–V measurements resulted in a maximum cold pulsed saturation current of 1.4 A mm−1 at a gate-source voltage of +3 V for 3.7 nm AlN thickness. The measured gate and drain lag for 500 ns pulse-width varied between 6%–12% and 10%–18%, respectively. Furthermore, a small increase in the threshold voltage was observed for all the devices, possibly due to the trapping of electrons under the gate contact. The off-state breakdown voltage of V
br = 70 V, for gate-drain spacing of 2 μm, was approximately double the value measured for a single AlN/GaN HEMT structure grown on a thick GaN buffer layer. The results suggest that the double AlN/GaN/AlN heterostructures may offer intrinsic advantages for the breakdown and current stability characteristics of high current HEMTs.
The self-compensation effect in Si-doped Al0.55Ga0.45N layers was investigated using different SiH4/III ratios. The degree of compressive strain changed with SiH4 flow rate during growth. With a low SiH4/III ratio of 2.46 × 10−6, compressive strain was increased in comparison with the un-doped case. However, above this SiH4/III ratio, compressive strain decreased from εxx = −5.07 × 10−3 to 4.28 × 10−3 when the ratio was increased to 4.1 × 10−5. For higher SiH4/III ratios, the compressive strain again increased, which is attributed to the self-compensation effect of Si atoms. A similar tendency was observed in Photo-luminescence (PL) results. While the UV-to-violet ratio (IUV/IVL) of room-temperature PL remained to be almost constant for SiH4/III ratios below 8.2 × 10−5, IUV/IVL decreased rapidly above this value, as a result of self-compensation of Si atoms. These results were in good agreement with the Hall effect measurements.
We report a novel and facile method for the fabrication of various AlN nanostructures with Al polarity using polarity control and selective etching without a mask or metal catalyst. To investigate the polarity transitions of the AlN layers obtained with different growth parameters, AlN layers were grown by high-temperature metalorganic chemical vapor deposition with varying growth temperatures and trimethylaluminum (TMAl) preflow rates. The growth of Al-polar AlN was clearly supported by a lower growth temperatures and higher TMAl preflow rates. Transmission electron microscopy showed that the threading dislocations (TDs) generated at the AlN-sapphire interface were bent toward the boundary of the N-polar grain because of the three-dimensional growth mode of the mixed-polarity AlN layer. Finally, defect-free nanopillars, nanorods, nanofurrows, and nanowalls were fabricated by etching mixed-polarity AlN layers with an aqueous KOH solution.
Double heterostructures of strained GaN quantum wells (QWs) sandwiched between relaxed AlN layers provide a platform to investigate the quantum-confined electronic and optical properties of the wells. The growth of AlN/GaN/AlN heterostructures with varying GaN quantum well thicknesses on AlN by plasma molecular beam epitaxy (MBE) is reported. Photoluminescence spectra provide the optical signature of the thin GaN QWs. Reciprocal space mapping in X-ray diffraction shows that a GaN layer as thick as ∼28 nm is compressively strained to the AlN layer underneath. The density of the polarization-induced two-dimensional electron gas (2DEG) in the undoped heterostructures increases with the GaN QW thickness, reaching ∼2.5 × 1013/cm2. This provides a way to tune the 2DEG channel density without changing the thickness of the top barrier layer. Electron mobilities less than ∼400 cm2/Vs are observed, leaving ample room for improvement. Nevertheless, owing to the high 2DEG density, strained GaN QW field-effect transistors with MBE regrown ohmic contacts exhibit an on-current density ∼1.4 A/mm, a transconductance ∼280 mS/mm, and a cut off frequency fT∼104 GHz for a 100-nm-gate-length device. These observations indicate high potential for high-speed radio frequency and high voltage applications that stand to benefit from the extreme-bandgap and high thermal conductivity of AlN.
AlGaN/GaN heterostructure was grown on semi-insulating 6H–SiC substrate. The effect of the thickness of the initial AlN buffer layer on the crystalline quality and the stress of the grown GaN layer were investigated. The semi-insulating characteristic of the undoped GaN layer, which is very important for obtaining low device leakage current, was analyzed by photoluminescence measurement. Modulation doping of Si during the growth of AlGaN barrier layer was also introduced to increase the concentration of the two-dimensional electron gas density and hence to improve the device performance. The fabricated AlGaN/GaN heterostructure field effect transistor with gate length of 0.2 μm and SiO2 passiviation layer exhibited good small-signal characteristics such as current gain cut-off frequency of 47 GHz and the maximum oscillation frequency of 121 GHz.
The impact of AlN nucleation layers on the strain evolution in a subsequent GaN layer was investigated by in-situ wafer curvature measurement. It is shown that growth temperature and thickness of the AlN nucleation layer strongly influence the built-in strain and crystalline quality of GaN. A two-step AlN growth procedure leads to a decrease of compressive strain as well as the reduction of the total dislocation density (6–9×108 cm−2) in the overgrown GaN layer. TEM analysis reveals different relaxation mechanisms of GaN in v-pits and on flat surfaces of AlN. With a two-step AlN nucleation layer a low wafer bow can be achieved together with a low dislocation density.
Field-effect transistors (FETs) were grown on both GaN and AlGaN buffers. X-ray reciprocal space mapping and omega-2theta scans showed that the AlGaN barriers grown on these two buffers had different Al compositions and growth rates, which was attributed to the compositional pulling effect. AlGaN/GaN/AlGaN double heterojunction FETs exhibited lower output conductance and better pinch-off due to the improved electron confinement resulting from the increase in the effective back-side barrier height. Thus, this device is promising for highly scaled transistors. This device also demonstrated a state-of-the-art power added efficiency of 53.5% and an associated power gain of 9.1 dB at a drain bias of 20 V at 30 GHz.
Enhancement mode AlN/GaN high electron mobility transistors (HEMTs) were fabricated from originally depletion-mode structures using oxygen plasma treatment on the gate area prior to the gate metallization. Starting with a depletion mode AlN/GaN HEMT, the threshold voltage of the HEMT could be shifted from −3.2 to 1 V depending on the oxygen plasma treatment time to partially convert the AlN barrier layer into Al oxide. The gate current was reduced and the current-voltage curves show metal-oxide semiconductor diodelike characteristics after oxygen plasma treatment.
Elastic constants for zinc-blende and wurtzite AlN, GaN, and InN are obtained from density-functional-theory calculations utilizing ab initio pseudopotentials and plane-wave expansions. Detailed comparisons are made with the available measured values and with results obtained in previous theoretical studies. These comparisons reveal clear discrepancies between the different sets of elastic constants which are further highlighted by examining derived quantities such as the perpendicular strain in a lattice-mismatched epitaxial film and the change in the wurtzite c/a ratio under hydrostatic pressure. Trends among results for the three compounds are also examined as well as differences between results for the zinc-blende and wurtzite phases. © 1997 American Institute of Physics.
An x-ray diffraction technique is described which, by careful choice of the x-ray reflection used, minimizes errors in composition measurements resulting from strain and uncertainties in the elastic constants of a material. The method is applied to the AlGaN system, which shows a wide range of values for Poisson’s ratio in the literature and significant variation in strain state due to the high dislocation content and large thermal expansion mismatch with the substrate. It is demonstrated that accurate composition measurements of partially relaxed AlxGa1−xN layers (x<0.3) with thickness >20 nm can be made from a single measurement.
In this work strain evolution during MOVPE of AlGaN/GaN HFET structures on 3 inch SiC substrates is investigated in-situ and related to properties of the initial AlN wetting layer. It is shown that the density of pits on the AlN surface depending on growth conditions influences GaN nucleation and consequential grain coalescence and stress incorporation. Optimization of the strain and material quality of the GaN layer is demonstrated without degrading the semi-insulating properties of the HFET buffer. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Molecular-beam epitaxy grown Al x Ga 1-x N alloys covering the entire range of alloy compositions, 0≤x≤1, have been used to determine the alloy band gap dependence on its composition. The Al chemical composition was deduced from secondary ion mass spectroscopy and Rutherford backscattering. The composition was also inferred from x-ray diffraction. The band gap of the alloy was extracted from low temperature optical reflectance measurements which are relatively more accurate than photoluminescence. Fitting of the band gap data resulted in a bowing parameter of b=1.0 eV over the entire composition range. The improved accuracy of the composition and band gap determination and the largest range of the Al composition over which our study has been conducted increase our confidence in this bowing parameter. © 2002 American Institute of Physics.
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