间歇供氨高温AlN缓冲层对AlGaN/GaN异质结的影响

研究了以间歇供氨的方法直接高温生长的氮化铝为缓冲层的AlGaN/GaN材料,高温氮化铝的应用可以有效的提高晶体质量和表面形貌,并且二维电子气的浓度和迁移率也得到改善。     

Effect of a high temperature AIN buffer layer grown by initially alternating supply of ammonia on AIGaN/GaN heterostuctures

morphology of GaN. The mobility and concentration of 2DEG of A1GaN/GaN heterostuctures was also ameliorated.     

Role of barrier and buffer layer defect states in AlGaN/GaN HEMT structures

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures to correlate their effect on two-dimensional electron gas (2DEG) confinement. Also, we have used Auger electron spectroscopy (AES) to detect the chemical composition as a function of lateral position on a growth wafer and to correlate chemical effects with electronic properties. We investigated several high-quality AlGaN/GaN heterostructures of varying electrical properties using incident electron beam energies of 0.5–15 keV to probe electronic state transitions within each of the heterostructure layers. The LEEN depth profiles reveal differences between sucessful and failed structures and highlight the importance of acceptor deep defect levels in the near 2DEG region. Variations in the GaN and AlGaN band edge emissions, as well as the yellow defect emission across an AlGaN/GaN heterostructure growth wafer have been observed. AES and LEEN spectroscopy of the growth wafer suggest that variation in the cation concentration may play a role in the mechanism responsible for the deep aceceptor level emission in the AlGaN barrier layer.     

Enhancement of the Schottky Barrier Height using a Nitrogen-Rich Tungsten Nitride Thin Film for the Schottky Contacts on AlGaN/GaN Heterostructures

Tungsten, stoichiometric W₂N, and nitrogen-rich W₂N films were used as Schottky contacts on AlGaN/GaN heterostructures. The nitrogen content in the film was controlled by varying the nitrogen-to-argon gas flow ratio during the reactive sputter deposition. The diode with the nitrogen-rich film exhibited a higher Schottky barrier height and the leakage current was comparable to that of the Ni/Au Schottky contact. Analysis suggested that this was due to the increase of the tungsten nitride work function as the result of higher nitrogen incorporation. Furthermore, after 600°C thermal annealing, the diode was stable and showed no change in the leakage current.     

Effect of AlN buffer layer thickness on the properties of GaN films grown by pulsed laser deposition

The GaN films are grown by pulsed laser deposition (PLD) on sapphire, AlN(30 nm)/Al₂O₃ and AlN(150 nm)/Al₂O₃, respectively. The effect of AlN buffer layer thickness on the properties of GaN films grown by PLD is investigated systematically. The characterizations reveal that as AlN buffer layer thickness increases, the surface root-mean-square (RMS) roughness of GaN film decreases from 11.5 nm to 2.3 nm, while the FWHM value of GaN film rises up from 20.28 arcmin to 84.6 arcmin and then drops to 31.8 arcmin. These results are different from the GaN films deposited by metal organic chemical vapor deposition (MOCVD) with AlN buffer layers, which shows the improvement of crystalline qualities and surface morphologies with the thickening of AlN buffer layer. The mechanism of the effect of AlN buffer layer on the growth of GaN films by PLD is hence proposed.     

Hydrogen sensors based on AlGaN/AlN/GaN HEMT

Pt/AlGaN/AlN/GaN high electron mobility transistors (HEMT) were fabricated and characterized for hydrogen sensing. Pt and Ti/Al/Ni/Au metals were evaporated to form the Schottky contact and the ohmic contact, respectively. The sensors can be operated in either the field effect transistor (FET) mode or the Schottky diode mode. Current changes and time dependence of the sensors under the FET and diode modes were compared. When the sensor was operated in the FET mode, the sensor can have larger current change of 8 mA, but its sensitivity is only about 0.2. In the diode mode, the current change was very small under the reverse bias but it increased greatly and gradually saturated at 0.8 mA under the forward bias. The sensor had much higher sensitivity when operated in the diode mode than in the FET mode. The oxygen in the air could accelerate the desorption of the hydrogen and the recovery of the sensor.     

蓝宝石衬底上AIGaN／GaN HEMT自热效应研究

建立了包含“自热效应”的AIGaN／GaN HEMT（高电子迁移率晶体管）直流I-V特性解析模型。从理论的角度分析了自热效应对AlGaN／GaN HEMT器件的影响，并同已有的实验结果进行了对比，符合较好。证明基于这种模型的理论分析适于AIGaN／GaN HEMT器件测试及应用的实际情况。     

Effects of rapid thermal annealing on ohmic contact of AIGaN/GaN HEMTs

Ohmic contacts with Ti/Al/Ti/Au source and drain electrodes on A1GaN/GaN high electron mobility transistors （HEMTs） were fabricated and subjected to rapid thermal annealing （RTA） in flowing N2. The wafer was divided into 5 parts and three of them were annealed for 30 s at 700, 750, and 800 ℃, respectively, the others were annealed at 750 ℃ for 25 and 40 s. Due to the RTA, a change from Schottky contact to Ohmic contact has been obtained between the electrode layer and the A1GaN/GaN heterojunction layer. We have achieved a low specific contact resistance of 7.41 × 10-6Ω cm2 and contact resistance of 0.54 Ω.mm measured by transmission line mode （TLM）, and good surface morphology and edge acuity are also desirable by annealing at 750 ℃ for 30 s. The experiments also indicate that the performance of ohmic contact is first improved, then it reaches a peak, finally degrading with annealing temperature or annealing time rising.     

AlN阻挡层对AlGaN/GaNHEMT器件的影响

利用低压MOCVD技术在蓝宝石衬底上生长了AlGaN/GaN异质结和AlGaN/AlN/GaN异质结二维电子气材料,采用相同器件工艺制造出了AlGaN/GaN HEMT器件和AlGaN/AlN/GaN HEMT器件.通过对两种不同器件的比较和讨论,研究了AlN阻挡层的增加对AlGaN/GaN HEMT器件性能的影响.     

Impact of UV/ozone surface treatment on AIGaN/GaN HEMTs

Surface treatment plays an important role in the process of making high performance AlGaN/GaN HEMTs. A clean surface is critical for enhancing device performance and long-term reliability. By experimenting with different surface treatment methods, we find that using UV/ozone treatment significantly influences the electrical properties of Ohmic contacts and Schottky contacts. According to these experimental phenomena and Xray photoelectron spectroscopy surface analysis results, the effect of the UV/ozone treatment and the reason that it influences the Ohmic/Schottky contact characteristics of AlGaN/GaN HEMTs is investigated.     

AlN成核层厚度对Si上外延GaN的影响

采用低温AlN成核层,在Si(111)衬底上,用金属有机化学气相沉积(MOCVD)法生长了GaN薄膜。采用高分辨X射线衍射(XRD)、椭圆偏振光谱仪和原子力显微镜(AFM)研究了AlN成核层的厚度对GaN外延层的影响。对AlN的测试表明,AlN的表面粗糙度(RMS)随着厚度增加而变大。对GaN的测试表明,所有GaN样品在垂直方向处于压应变状态,并且随AlN厚度增加而略有减弱。GaN的(0002)_ω扫描的峰值半宽(FWHM)随着AlN成核层厚度增加而略有升高,GaN(10-12)_ω扫描的FWHM随着厚度增加而有所下降。(10-12)_ω扫描的FWHM与GaN的刃型穿透位错密度相关,A1N成核层的厚度较大时会降低刃型穿透位错密度,并减弱c轴方向的压应变状态。     

界面极化效应对AIxGa1-xN／GaN异质结pin探测器光电响应的影响

设计了正面入射的探测波长范围限制在326～365nm的AlxGa1-χN／GaN异质结pin光电探测器，利用自洽求解薛定谔-泊松方程计算了A1xGa1-xN／GaN异质结在无极化、完全极化和部分极化的能带图，结合光电响应谱的模拟，分析了界面极化效应对AlxGa1-xN／GaN异质结pin紫外光电探测器响应特性的影响并提出了改善方法。     

Local electronic properties of AlGaN/GaN heterostructures probed by scanning capacitance microscopy

Local electronic properties in Al_xGa_1−xN/GaN heterostructure field-effect transistor epitaxial layer structures are probed using scanning capacitance microscopy. Acquisition of scanning capacitance images over a wide range of bias voltages combined with theoretical analysis and numerical simulation allows the presence, detailed nature, and possible structural origins of nanometer- to micronscale inhomogeneities in electronic structure to be elucidated. Substantial lateral variations in local threshold voltages for transistor channel formation are observed, at length scales ranging from submicron to >2 μm, and found to arise primarily from local variations in Al_xGa_1−xN layer thickness. Features in electronic structure are also observed that are consistent with the existence of networks of negatively charged threading edge dislocations, as might be formed at island coalescence boundaries during epitaxial growth. The negative charge associated with these structures appears to lead to local depletion of carriers from the channel in the Al_xGa_1−xN/GaN transistor epitaxial layer structure.     

Unpassivated GaN/AlGaN/GaN power high electron mobility transistors with dispersion controlled by epitaxial layer design

In this paper, a novel GaN/AlGaN/GaN high electron mobility transistor (HEMT) is discussed. The device uses a thick GaN-cap layer (∼250 nm) to reduce the effect of surface potential fluctuations on device performance. Devices without Si₃N₄ passivation showed no dispersion with 200-ns-pulse-width gate-lag measurements. Saturated output-power density of 3.4 W/mm and peak power-added efficiency (PAE) of 32% at 10 GHz (V_DS=+15 V) were achieved from unpassivated devices on sapphire substrates. Large gate-leakage current and low breakdown voltage prevented higher drain-bias operation and are currently under investigation.     

RF-MBE生长AlGaN/GaN极化感应二维电子气材料

用射频等离子体辅助分子束外延技术 (RF- MBE)在 C面蓝宝石衬底上外延了高质量的 Ga N膜以及 Al Ga N/Ga N极化感应二维电子气材料 .所外延的 Ga N膜室温背景电子浓度为 2× 10 1 7cm- 3 ,相应的电子迁移率为 177cm2 /(V· s) ;Ga N (0 0 0 2 ) X射线衍射摇摆曲线半高宽 (FWHM)为 6′;Al Ga N/Ga N极化感应二维电子气材料的室温电子迁移率为 730 cm2 /(V· s) ,相应的电子气面密度为 7.6× 10 1 2 cm- 2 ;用此二维电子气材料制作的异质结场效应晶体管 (HFET)室温跨导达 5 0 m S/mm (栅长 1μm) ,截止频率达 13GHz(栅长 0 .5 μm)     

Electrical properties and structural optimization of GaN/InGaN/GaN tunnel junctions grown by molecular beam epitaxy

The InGaN films and GaN/InGaN/GaN tunnel junctions (TJs) were grown on GaN templates with plasma-assisted molecular beam epitaxy. As the In content increases, the quality of InGaN films grown on GaN templates decreases and the surface roughness of the samples increases. V-pits and trench defects were not found in the AFM images. p⁺⁺-GaN/InGaN/n⁺⁺-GaN TJs were investigated for various In content, InGaN thicknesses and doping concentration in the InGaN insert layer. The InGaN insert layer can promote good interband tunneling in GaN/InGaN/GaN TJ and significantly reduce operating voltage when doping is sufficiently high. The current density increases with increasing In content for the 3 nm InGaN insert layer, which is achieved by reducing the depletion zone width and the height of the potential barrier. At a forward current density of 500 A/cm², the measured voltage was 4.31 V and the differential resistance was measured to be 3.75 × 10⁻³ Ω·cm² for the device with a 3 nm p⁺⁺-In_0.35Ga_0.65N insert layer. When the thickness of the In_0.35Ga_0.65N layer is closer to the “balanced” thickness, the TJ current density is higher. If the thickness is too high or too low, the width of the depletion zone will increase and the current density will decrease. The undoped InGaN layer has a better performance than n-type doping in the TJ. Polarization-engineered tunnel junctions can enhance the functionality and performance of electronic and optoelectronic devices.     

The influence of composition and unintentional doping on the two-dimensional electron gas density in AlGaN/GaN heterostructures

We have studied the influence of Al content, AlGaN layer thickness, and unintentional background doping by oxygen on the two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures. Hall measurements were made on samples grown with molecular beam epitaxy. The 2DEG densities in the range 2–3×10¹³ cm^?2 were measured. A one-dimensional Schrödinger-Poisson model was used to describe the heterostructure. The calculations gave two-dimensional electron densities in accordance with measured values. The electron density is very sensitive to the Al concentration in the AlGaN layer, whereas the sensitivity to layer thickness is small. Our simulations also showed that the two-dimensional concentration increased 50% when the free-carrier concentration changed from 10¹⁵ cm^?3 to 10¹⁸ cm^?3. The relation between donor concentration and free-carrier concentration was found to agree when using oxygen ionization energy as a parameter.     

AlGaN/GaN high electron mobility transistors with selective area grown p-GaN gates

We report a selective area growth (SAG) method to define the p-GaN gate of AlGaN/GaN high electron mobility transistors (HEMTs) by metal-organic chemical vapor deposition. Compared with Schottky gate HEMTs, the SAG p-GaN gate HEMTs show more positive threshold voltage (V_th) and better gate control ability. The influence of Cp₂Mg flux of SAG p-GaN gate on the AlGaN/GaN HEMTs has also been studied. With the increasing Cp₂Mg from 0.16 μmol/min to 0.20 μmol/min, the V_th raises from -67 V to -37 V. The maximum transconductance of the SAG HEMT at a drain voltage of 10 V is 113.9 mS/mm while that value of the Schottky HEMT is 51.6 mS/mm. The SAG method paves a promising way for achieving p-GaN gate normally-off AlGaN/GaN HEMTs without dry etching damage.     

成核层形貌对AlGaN/GaN异质结晶体质量，表面形貌以及电学特性的影响

Nucleation layer formation is a key factor for high quality gallium nitride（GaN）growth on a sapphire substrate.We found that the growth rate substantially affected the nucleation layer morphology,thereby having a great impact on the crystal quality,surface morphology and electrical properties of AlGaN/GaN heterostructures on sapphire substrates.A nucleation layer with a low growth rate of 2.5 nm/min is larger and has better coalescence than one grown at a high growth rate of 5 nm/min.AlGaN/GaN heterostructures on a nucleation layer with low growth rate have better crystal quality,surface morphology and electrical properties.