Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrierconfinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at ~3.8 eV or ~4.0 eV, GaN band edge emission at ~3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region     

Influence of annealed ohmic contact metals on electron mobility of strained AlGaN/GaN heterostructures

The influence of annealed ohmic contact metals on the electron mobility of a two dimensional electron gas (2DEG) is investigated on ungated AlGaN/GaN heterostructures and AlGaN/GaN heterostructure field effect transistors (AlGaN/GaN HFETs). Current-voltage (I-V) characteristics for ungated AlGaN/GaN heterostructures and capacitance-voltage (C-V) characteristics for AlGaN/GaN HFETs are obtained, and the electron mobility for the ungated AlGaN/GaN heterostructure is calculated. It is found that the electron mobility of the 2DEG for the ungated AlGaN/GaN heterostructure is decreased by more than 50% compared with the electron mobility of Hall measurements. We propose that defects are introduced into the AlGaN barrier layer and the strain of the AlGaN barrier layer is changed during the annealing process of the source and drain, causing the decrease in the electron mobility.     

高击穿电压AlGaN/GaN HEMT电力开关器件研究进展

作为第三代宽禁带半导体材料的典型代表,GaN材料在各个应用领域的研究工作都受到了高度的重视。概述了基于AlGaN/GaN HEMT结构的新型高压、高频、低损耗电力开关器件的最新研究进展。从器件的结构特征入手,详细介绍了改善器件击穿特性的途径、高频开关特性的研究情况、Si衬底上AlGaN/GaN HEMT结构材料的生长、增强型器件的制备技术和功率集成电路的研究等几个国际上的热点问题。最后,对该项研究面临的问题及未来的发展趋势做了展望。     

effect of the parameters of AlN/GaN/AlGaN and AlN/GaN/InAlN heterostructures with a two-dimensional electron gas on their electrical properties and the characteristics of transistors on their basis

The effect of the layer thickness and composition in AlGaN/AlN/GaN and InAlN/AlN/GaN transistor heterostructures with a two-dimensional electron gas on their electrical and the static parameters of test transistors fabricated from such heterostructures are experimentally and theoretically studied. It is shown that the use of an InAlN barrier layer instead of AlGaN results in a more than twofold increase in the carrier concentration in the channel, which leads to a corresponding increase in the saturation current. In situ dielectric-coating deposition on the InAlN/AlN/GaN heterostructure surface during growth process allows an increase in the maximum saturation current and breakdown voltages while retaining high transconductance.     

Low- and high-frequency capacitance of aluminum gallium nitride/gallium nitride heterostructures with interface traps

We have studied frequency dependence of capacitance properties of aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructures with interface traps at the AlGaN/GaN interface. We have shown that ability of charge in interface traps to respond to external measuring signal is responsible for the frequency dispersion of capacitance curves. The difference between capacitance curves measured at low and high frequencies in experimental structures is similar to the difference between simulated capacitances of the heterostructures with interface traps measured at low and high frequencies. At a high frequency when the charge in interface traps does not follow the measuring signal, the capacitance curves are only shifted in voltage compared to the curve of the structure without interface traps. But for low frequency a capacitance peak is observed. Interface traps hence contribute to experimentally observed capacitance dispersion.     

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。     

Elevated-Temperature Annealing Effects on AlGaN/GaN Heterostructures

The effect of high-temperature annealing of undoped AlGaN/GaN heterostructures on different substrates was systematically studied between 1100°C and 1230°C. An AlN spacer layer was found to add stability to structures on sapphire substrates. AlGaN/GaN heterostructures on SiC substrates demonstrated excellent robustness for the temperature range studied, maintaining their mobility, sheet resistance, and sheet concentration values, even after annealing. A silicon nitride, SiN _x , capping layer was found to assist in minimizing surface roughness during annealing and maintaining the electrical characteristics of the heterostructures. AlGaN/GaN heterostructures on SiC substrates showed a 20% decrease in mobility for uncapped samples compared with SiN _x -capped samples.     

氮化对蓝宝石衬底上AlGaN/GaN异质结

研究了不同氮化条件对蓝宝石衬底上生长的AlGaN/GaN异质结材料特性的影响。研究表明,随着氮化过程NH₃流量的增大,GaN外延层的晶体质量得到了改善,GaN内应力释放,但是AlGaN/GaN异质结构中二维电子气的迁移率有所恶化。讨论了上述现象出现的原因。     

Near-Infrared Absorption in Lattice-Matched AlInN/GaN and Strained AlGaN/GaN Heterostructures Grown by MBE on Low-Defect GaN Substrates

We have investigated near-infrared absorption and photocurrent in lattice-matched AlInN/GaN and strained AlGaN/GaN heterostructures grown by molecular-beam epitaxy (MBE) on low-defect GaN substrates for infrared device applications. The AlGaN/GaN heterostructures were grown under Ga-rich conditions at 745°C. Material characterization via atomic force microscopy and high-resolution x-ray diffraction indicates that the AlGaN/GaN heterostructures have smooth and well-defined interfaces. A minimum full-width at half-maximum of 92 meV was obtained for the width of the intersubband absorption peak at 675 meV of a 13.7 Å GaN/27.5 Å Al_0.47Ga_0.53N superlattice. The variation of the intersubband absorption energy across a 1 cm × 1 cm wafer was ±1%. An AlGaN/GaN-based electromodulated absorption device and a quantum well infrared detector were also fabricated. Using electromodulated absorption spectroscopy, the full-width at half-maximum of the absorption peak was reduced by 33% compared with the direct absorption measurement. This demonstrates the suitability of the electromodulated absorption technique for determining the intrinsic width of intersubband transitions. The detector displayed a peak responsivity of 195 μA/W at 614 meV (2.02 μm) without bias. Optimal MBE growth conditions for lattice-matched AlInN on low-defect GaN substrates were also studied as a function of total metal flux and growth temperature. A maximum growth rate of 3.8 nm/min was achieved while maintaining a high level of material quality. Intersubband absorption in AlInN/GaN superlattices was observed at 430 meV with full-width at half-maximum of 142 meV. Theoretical calculations of the intersubband absorption energies were found to be in agreement with the experimental results for both AlGaN/GaN and AlInN/GaN heterostructures.     

Electron mobility in an AlGaN/GaN two-dimensional electron gas. I. Carrier concentration dependent mobility

The transport properties of two-dimensional electron gas (2-DEG) at the AlGaN/GaN interface were studied by characterizing the 2-DEG mobility dependence on carrier concentration, n/sub s/, and temperature. High-quality AlGaN/GaN heterostructures were grown, and heterostructure field effect transistors (HFETs) using a Fat FET geometry were fabricated. Measurements of 2-DEG mobility were performed by magnetoresistance and capacitance-conductance. In order to understand the dominant transport factors, the mobility was modeled using different scattering mechanisms and compared to our results. It is found that mobility dependence on n/sub s/ shows a bell-shape behavior over the whole temperature range. For low n/sub s/ the mobility is dominated by Coulomb interaction from interface charge, and at high n/sub s/ the mobility is dominated by interface roughness. Using previously reported experimental values of interface charge and interface roughness in our modeling, we show good agreement with mobility measurement results. Scattering from interface states in AlGaN/GaN heterostructures, seems to be related to the high polarization field in the heterointerface. At temperatures higher than 200K polar optical phonon scattering dominates the transport, yet both interface charge and roughness affect the mobility at the low and high n/sub s/, respectively.     

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.     

微波功率AlGaN/GaN HFET的二维能带和异质结构设计

在综述微波功率AlGaN/GaN HFET技术发展趋势基础上,提出了二维异质结能带优化设计的新课题.从自洽求解薛定谔方程和泊松方程出发研究了利用异质界面上的极化电荷来剪裁异质结能带.用极化电荷设计近矩形前势垒能增大高能热电子的隧穿势垒宽度,抑制电流崩塌.背势垒中的极化电荷强化了沟道阱的量子限制,减弱了沟道中的强场峰,能提高击穿电压和抑制电流崩塌.薄势垒层中的极化电荷强化了沟道阱的结构,降低势垒高度后能产生高密度的电子气.优化设计二维异质结构能抑制沟道中的强场峰和电流崩塌,提高击穿电压和大漏压下的输出功率.     

Microwave field-effect transistors based on group-III nitrides

A concept for the design of experimental AlGaN/GaN/AlGaN double heterostructures with a two-dimensional electron channel are discussed, together with their main properties. The structures were formed by ammonia molecular-beam epitaxy on sapphire substrates. The foundations for postgrowth technology are developed for microwave field-effect transistors based on Group-III nitrides, including the formation of a mesa isolation and the preparation stage of nonrectifying contacts and the Schottky barrier. The first field-effect transistors fabricated based on the above heterostructures have a complete set of static characteristics and can operate in a mode of weak microwave signals at a frequency of 8.15 GHz.     

Scanning Ion Probe Studies of Silicon Implantation Profiles in AlGaN/GaN HEMT Heterostructures

This paper reports results of scanning ion probe studies of silicon implantation profiles in source and drain regions of AlGaN/GaN high-electron-mobility transistor (HEMT) heterostructures. It is shown that both the undoped channel length and the transition region between implanted and non- implanted regions become wider with increasing depth in the structure. These results may explain the previously reported existence of resistance associated with the transition region between implanted and non-implanted semiconductor regions in AlGaN/GaN HEMT heterostructures with non-alloyed Si-implanted source and drain ohmic contact regions.     

原位钝化和催化剂钝化沟道阱能带计算

把Si3N4/AlGaN界面看成新的异质结,自洽求解薛定谔方程和泊松方程,建立起原位钝化和CATCVD钝化异质结构的新能带模型.运用这一模型,研究了原位钝化和CATCVD钝化异质结沟道阱的电子气密度、能带结构和输运性能,解释了相应的实验结果.在此基础上研究了这些新钝化异质结沟道阱能带的优化设计.优化设计的钝化复合势垒沟...     

Thermal effects in AlGaN/GaN/Si high electron mobility transistors

Group III-nitride compounds are of increasing interest for designing high power and high temperature transistors. A considerable progress in the growth and process technology of these devices has been achieved. However, there are still limitations concerning particularly the lack of native substrates. Comparison of the AlGaN/GaN high electron mobility transistors investigated favours the SiC substrate. Recently, encouraging results have been reported for AlGaN/GaN/Si. The crucial problem found in AlGaN/GaN transistors operating at high biases is the self-heating induced by high power dissipation in the active zone. The present work reports on a study of the self-heating in AlGaN/GaN HEMTs grown on Si(1 1 1). The electron-band parameters of the heterostructures have been calculated self-consistently by taking into account the piezoelectric and spontaneous polarizations. As an experiment support, direct-current characteristics of AlGaN/GaN/Si HEMTs have been used to derive the drain voltage-dependent temperature rise in the conductive channel. As has been found, the self-heating is relatively weak. An improvement in the electron transport is achieved by optimizing the epilayers and adjusting the electrode sizes at output of the transistors investigated.     

蓝宝石衬底上生长AlGaN/AlN结构的应变分析

采用低温和高温AlN复合缓冲层的方法在蓝宝石衬底上外延生长AlGaN/AlN结构,并进行了应变分析.通过X射线双晶衍射ω-2θ扫描曲线和拟合曲线分析发现,AlN是由两个不同弛豫度的应变缓冲层组成,弛豫度分别为96%和97.2%.AlN缓冲层在低温下不完全弛豫,导致高温下存在一个继续弛豫过程.X射线双晶衍射和透射光谱分析发现,AlGaN层Al组分由于未知的AlGaN弛豫度而无法确定.对AlGaN带隙公式和实验结果进行拟合,推算出弯曲系数为1.05 eV,这与已有文献相吻合.     

Study of the polarizations of (Al,Ga,AlGa)N nitride compounds and the charge density of various interfaces based on them

The results of a theoretical study based on ab initio calculations of the polarization properties of AlN, GaN, and AlGaN semiconductors with the wurtzite structure are presented. The values of the spontaneous and piezoelectric polarizations, as well as the piezoelectric constants, are calculated for these nitride compounds. With the aim of further considering prospective heterostructures based on (Al,Ga,AlGa)N compounds, the charge densities at the AlN/GaN, AlGaN/AlN, and AlGaN/GaN interfaces and carrier concentration at the AlGaN/GaN heterointerface is estimated and compared with the experimental data.     

SAW Filters Composed of Interdigital Schottky and Ohmic Contacts on AlGaN/GaN Heterostructures

We proposed surface acoustic wave (SAW) filters composed of interdigital Schottky and ohmic contacts on AlGaN/GaN heterostructures. The contribution of the SAWs appeared in the radio frequency characteristics of the filters when the Schottky contacts were reverse biased. Onsets of the SAW signals and the threshold voltage of simultaneously fabricated high-electron mobility transistors were found to almost agree with one another. We also obtained an isolation of >40 dB. These results suggest that SAW-based functional devices are likely to be realized using AlGaN/GaN heterostructures with interdigital Schottky and ohmic contacts     

Electromagnetic Modeling,Technology, and Production of Microwave C<Superscript>3</Superscript>MOSHFET Switches on AlGaN/GaN Heterostructures

The features of the electromagnetic modeling of a microwave switch for the frequency range of 1–20 GHz, which is then produced by C³MOSHFET technology on AlGaN/GaN heterostructures using high-K dielectrics and contacts with capacitive coupling, are considered.