Properties of AlyGa1-yN/AlxGa1-xN/AlN/GaN Double-Barrier High Electron Mobility Transistor Structure

Electrical properties of Al_yGa_1-yN/Al_xGa_1-xN/AlN/GaN structure are investigated by solving coupled Schrödinger and Poisson equation self-consistently. Our calculations show that the two-dimensional electron gas (2DEG) density will decrease with the thickness of the second barrier (Al_yGa_1-yN) once the AlN content of the second barrier is smaller than a critical value y_c, and will increase with the thickness of the second barrier (Al_yGa_1-yN) when the critical AlN content of the second barrier y_c is exceeded. Our calculations also show that the critical AlN content of the second barrier y_c will increase with the AlN content and the thickness of the first barrier layer (Al_xGa_1-xN).     

纤锌矿AlGaN/AlN/GaN异质结构中光学声子散射影响的电子迁移率

对含有AlN插入层纤锌矿Al_xGa_1-xN/AlN/GaN异质结构,考虑有限厚势垒和导带弯曲的实际 异质结势,同时计入自发极化和压电极化效应产生的内建电场作用,采用数值自洽求解薛定谔方程和泊松方程, 获得二维电子气(2DEG)中电子的本征态和本征能级.依据介电连续模型和Loudon单轴晶体模型, 用转移矩阵法分析该体系中可能存在的光学声子模及三元混晶效应.进一步, 在室温下计及各种可能存在的光学声子散射,推广雷-丁平衡方程方法,讨论2DEG分布及二维电子迁移率的 尺寸效应和三元混晶效应.结果显示: AlN插入层厚度和Al_xGa_1-xN势垒层中Al组分的增加均会 增强GaN层中的内建电场强度,致使2DEG的分布更靠近异质结界面,使界面光学声子强于其他类型的 光学声子对电子的散射作用而成为影响电子迁移率的主导因素.适当调整AlN插入层的厚度和Al组分, 可获得较高的电子迁移率.     

N极性GaN/AlGaN异质结二维电子气模拟

通过自洽求解薛定谔方程和泊松方程,较系统地研究了GaN沟道层、AlGaN背势垒层、Si掺杂和AlN插入层对N极性GaN/AlGaN异质结中二维电子气(2DEG)的影响,分析表明,GaN沟道层厚度、AlGaN背势垒层厚度及Al组分变大都能一定程度上提高二维电子气面密度,AlGaN背势垒层的厚度和Al组分变大也可提高二维电子气限阈性,且不同的Si掺杂形式对二维电子气的影响也有差异,而AlN插入层在提高器件二维电子气面密度、限阈性等方面表现都较为突出,在模拟中GaN沟道层厚度小于5nm时无法形成二维电子气,超过20nm后二维电子气面密度趋于饱和,而AlGaN背势垒厚度超过40nm后二维电子气也有饱和趋势,对均匀掺杂和delta掺杂而言AlGaN背势垒层Si掺杂浓度超过5×10~(19)cm~(-3)后2DEG面密度开始饱和,而厚度为2nmAlN插入层的引入会使2DEG面密度从无AlN插入层时的0.93×10~(13)cm~(-2)提高到1.17×10~(13)cm~(-2)。     

The effects of GaN capping layer thickness on two-dimensional electron mobility in GaN/AlGaN/GaN heterostructures

In this paper we present a study of the effect of GaN capping layer thickness on the two-dimensional (2D)-electron mobility and the two-dimensional electron gas (2DEG) sheet density which is formed near the AlGaN barrier/buffer GaN layer. This study is undertaken using a fully numerical calculation for GaN/Al_xGa_1−xN/GaN heterostructures with different Al mole fraction in the Al_xGa_1−xN barrier, and for various values of barrier layer thickness. The results of our analysis clearly indicate that increasing the GaN capping layer thickness leads to a decrease in the 2DEG density. Furthermore, it is found that the room-temperature 2D-electron mobility reaches a maximum value of approximately 1.8×10³ cm² /Vs⁻¹ for GaN capping layer thickness grater than 100 Å with an Al_0.32Ga_0.68N barrier layer of 200 Å thick. In contrast, for same structure, the 2DEG density decreases monotonically with GaN capping layer thickness, and eventually saturates at approximately 6×10¹² cm⁻² for capping layer thickness greater than 500 Å. A comparison between our calculated results with published experimental data is shown to be in good agreement for GaN capping layers up to 500 Å thickness.     

AlxGa1-xN/GaN异质结构中Al组分对二维电子气性质的影响

通过用数值计算方法自洽求解薛定谔方程和泊松方程，研究了Al组分对Al_xGa1-xN/GaN异质结构二维电子气性质的影响，给出了Al_xGa_{1-x< /sub>N/GaN异质结构二维电子气分布和面密度，导带能带偏移以及子带中电子分布随AlxGa 1-xN势垒层中Al组分的变化关系，并用AlxGa1-xN/GaN 异质结构自发极化与压电极化机理和能 关键词： x}Ga_1-xN/GaN异质结构')" href="#">Al_xGa_1-xN/GaN异质结构 二维电子气 自发极化 压电极化     

用肖特基电容电压特性数值模拟法确定调制掺杂AlxGa1-xN/GaN异质结中的极化电荷

研究发展了用肖特基电容电压特性数值模拟确定调制掺杂Al_xGa_1-xN/GaN异质结中极化电荷的方法.在调制掺杂的Al_0.22Ga_0.78N/GaN异质结上制备了Pt肖特基接触,并对其进行了C-V测量.采用三维费米模型对调制掺杂的Al_0.22Ga_0.78N/GaN异质结上肖特基接触的C-V特性进行了数值模拟,分析了改变样品参数对C-V特性的影响.利用改变极化电荷、n-AlGaN 关键词： xGa_1-xN/GaN异质结')" href="#">Al_xGa_1-xN/GaN异质结 极化电荷 电容电压特性 数值模拟     

High temperature electron transport properties of AlGaN/GaN heterostructures with different Al-contents

Electron transport properties in AlGaN/GaN heterostructures with different Al-contents have been investigated from room temperature up to 680 K. The temperature dependencies of electron mobility have been systematically measured for the samples. The electron mobility at 680 K were measured as 154 and 182 cm²/V·s for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively. It was found that the electron mobility of low Al-content Al_0.15Ga_0.85N/GaN heterostructure was less than that of high Al-content Al_0.40Ga_0.60N/GaN heterostructure at high temperature of 680 K, which is different from that at room temperature. Detailed analysis showed that electron occupations in the first subband were 75% and 82% at 700 K for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively, and the two dimensional gas (2DEG) ratios in the whole electron system were 30% and near 60%, respectively. That indicated the 2DEG was better confined in the well, and was still dominant in the whole electron system for higher Al-content AlGaN/GaN heterostructure at 700 K, while lower one was not. Thus it had a higher electron mobility. So a higher Al-content AlGaN/GaN heterostructure is more suitable for high-temperature applications.     

Effects of GaN cap layer thickness on an AlN/GaN heterostructure

In this study, we investigate the effects of GaN cap layer thickness on the two-dimensional electron gas (2DEG) electron density and 2DEG electron mobility of AlN/GaN heterostructures by using the temperature-dependent Hall measurement and theoretical fitting method. The results of our analysis clearly indicate that the GaN cap layer thickness of an AlN/GaN heterostructure has influences on the 2DEG electron density and the electron mobility. For the AlN/GaN heterostructures with a 3-nm AlN barrier layer, the optimized thickness of the GaN cap layer is around 4 nm and the strained a-axis lattice constant of the AlN barrier layer is less than that of GaN.     

Alloy disorder scattering limited mobility of two-dimensional electron gas in the quaternary AlInGaN/GaN heterojunctions

Nitride heterojunction field effect transistors (HFETs) with quaternary AlInGaN barrier layers have achieved remarkable successes in recent years based on highly improved mobility of the two-dimensional electron gases (2DEGs) and greatly changed AlInGaN compositions. To investigate the influence of the AlInGaN composition on the 2DEG mobility, the quaternary alloy disorder (ADO) scattering to 2DEGs in AlInGaN/GaN heterojunctions is modeled using virtual crystal approximation. The calculated mobility as a function of AlInGaN alloy composition is shown to be a triangular-scarf-like curved surface for both cases of fixed thickness of AlInGaN layer and fixed 2DEG density. Though the two mobility surfaces are quite different in shape, both of them manifest the smooth transition of the strength of ADO scattering from quaternary AlInGaN to ternary AlGaN or AlInN. Some useful principles to estimate the mobility change with the Al(In,Ga)N composition in Al(In,Ga)N/GaN heterojunctions with a fixed 2DEG density are given. The comparison between some highest Hall mobility data reported for Al_xGa_1−xN/GaN heterojunctions (x=0.06~0.2) at very low temperature (0.3~13 K) and the calculated 2DEG mobility considering ADO scattering and interface roughness scattering verifies the influence of ADO scattering. Moreover, the room temperature Hall mobility data of Al(In,Ga)N/AlN/GaN heterojunctions with ADO scattering eliminated are summarized from literatures. The data show continuous dependence on Hall electron density but independence of the Al(In,Ga)N composition, which also supports our theoretical results. The feasibility of quaternary AlInGaN barrier layer in high conductivity nitride HFET structures is demonstrated.     

Influence of AlN interfacial layer on electrical properties of high-Al-content Al0.45Ga0.55N/GaN HEMT structure

Unintentionally doped high-Al-content Al_0.45Ga_0.55N/GaN high electron mobility transistor (HEMT) structures with and without AlN interfacial layer were grown by metal-organic chemical vapor deposition (MOCVD) on two-inch sapphire substrates. The effects of AlN interfacial layer on the electrical properties were investigated. At 300 K, high two-dimensional electron gas (2DEG) density of 1.66 × 10¹³ cm⁻² and high electron mobility of 1346 cm² V⁻¹ s⁻¹ were obtained for the high Al content HEMT structure with a 1 nm AlN interfacial layer, consistent with the low average sheet resistance of 287 Ω/sq. The comparison of HEMT wafers with and without AlN interfacial layer shows that high Al content AlGaN/AlN/GaN heterostructures are potential in improving the electrical properties of HEMT structures and the device performances.     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.     

AlGaN/GaN heterostructure-based surface acoustic wave-structures for chemical sensors

We present a new approach in forming of interdigital surface acoustic wave-structures on AlGaN/GaN heterostructure to be applied in chemical sensors technology. This approach uses a selective self-aligned SF₆ plasma treatment of the AlGaN/GaN barrier layer to modify 2DEG density and surface field distribution in the range of interdigital transducers (IDTs) thus enabling SAW excitation. Secondary ion mass spectroscopy was applied to explain the modification of 2DEG density in the plasma treated AlGaN/GaN heterostructure. The initial results in the process technology and characterization are presented.     

AlGaN barrier thickness dependent surface and interface trapping characteristics of AlGaN/GaN heterostructure

The aluminium gallium nitride (AlGaN) barrier thickness dependent trapping characteristic of AlGaN/GaN heterostructure is investigated in detail by frequency dependent conductance measurements. The conductance measurementsin the depletion region biases (−4.8 V to −3.2 V) shows that the Al_0.3Ga_0.7N(18 nm)/GaN structure suffers from both the surface (the metal/AlGaN interface of the gate region) and interface (the AlGaN/GaN interface of the channel region) trapping states, whereas the AlGaN/GaN structure with a thicker AlGaN barrier (25 nm) layer suffers from only interface (the channel region of AlGaN/GaN) trap energy states in the bias region (−6 V to −4.2). The two extracted time constants of the trap levels are (2.6–4.59) μs (surface) and (113.4–33.8) μs (interface) for the Al_0.3Ga_0.7N(18 nm)/GaN structure in the depletion region of biases (−4.8 V to −3.2 V), whereas the Al_0.3Ga_0.7N (25 nm)/GaN structure yields only interface trap states with time constants of (86.8–33.3) μs in the voltage bias range of −6.0 V to −4.2 V. The extracted surface trapping time constants are found to be very muchless in the Al_0.3Ga_0.7N(18 nm)/GaN heterostructure compared to that of the interface trap states. The higher electric field formation across the AlGaN barrier causes de-trapping of the surface trapped electron through a tunnelling process for the Al_0.3Ga_0.7N(18 nm)/GaN structure, and hence the time constants of the surface trap are less.     

AlxGa1-x N/GaN调制掺杂异质结构的子带性质研究

通过低温和强磁场下的磁输运测量研究了Al_0.22Ga_0.78N/GaN调制掺杂异质结构中2DEG的子带占据性质和子带输运性质.在该异质结构的磁阻振荡中观察到了双子带占据现象，并发现2DEG的总浓度随第二子带浓度的变化呈线性关系.得到了该异质结构中第二子带被2DEG占据的阈值电子浓度为7.3×10¹²cm^-2.采用迁移率谱技术得到了不同样品的分别对应于第一和第二子带的输运迁移率.发现当样品产生应变弛豫时第一子带的电子迁移 关键词： AlGaN/GaN异质结 二维电子气 子带占据 输运迁移率     

Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD

We investigate the structural and electrical properties of Al_xIn_1–xN/AlN/GaN heterostructures with AlGaN buffers grown by MOCVD, which can be used as an alternative to AlInN HEMT structures with GaN buffer. The effects of the GaN channel thickness and the addition of a content graded AlGaN layer to the structural and electrical characteristics were studied through variable temperature Hall effect measurements, high resolution XRD, and AFM measurements. Enhancement in electron mobility was observed in two of the suggested Al_xIn_1?xN/AlN/GaN/Al_0.04Ga_0.96N heterostructures when compared to the standard Al_xIn_1–xN/AlN/GaN heterostructure. This improvement was attributed to better electron confinement in the channel due to electric field arising from piezoelectric polarization charge at the Al_0.04Ga_0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same interface. If the growth conditions and design parameters of the Al_xIn_1?xN HEMT structures with AlGaN buffers can be modified further, the electron spillover from the GaN channel can be significantly limited and even higher electron mobilities, which result in lower two-dimensional sheet resistances, would be possible.     

Influence of Schottky drain contacts on the strained AlGaN barrier layer of AlGaN/AlN/GaN heterostructure field-effect transistors

Rectangular Schottky drain AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with different gate contact areas and conventional AlGaN/AlN/GaN HFETs as control were both fabricated with same size. It was found there is a significant difference between Schottky drain AlGaN/AlN/GaN HFETs and the control group both in drain series resistance and in two-dimensional electron gas (2DEG) electron mobility in the gate-drain channel. We attribute this to the different influence of Ohmic drain contacts and Schottky drain contacts on the strained AlGaN barrier layer. For conventional AlGaN/AlN/GaN HFETs, annealing drain Ohmic contacts gives rise to a strain variation in the AlGaN barrier layer between the gate contacts and the drain contacts, and results in strong polarization Coulomb field scattering in this region. In Schottky drain AlGaN/AlN/GaN HFETs, the strain in the AlGaN barrier layer is distributed more regularly.     

Effects of the strain relaxation of an AlGaN barrier layer induced by various cap layers on the transport properties in AlGaN/GaN heterostructures

The strain relaxation of an AlGaN barrier layer may be influenced by a thin cap layer above, and affects the transport properties of AlGaN/GaN heterostructures. Compared with the slight strain relaxation found in AlGaN barrier layer without cap layer, it is found that a thin cap layer can induce considerable changes of strain state in the AlGaN barrier layer. The degree of relaxation of the AlGaN layer significantly influences the transport properties of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. It is observed that electron mobility decreases with the increasing degree of relaxation of the AlGaN barrier, which is believed to be the main cause of the deterioration of crystalline quality and morphology on the AlGaN/GaN interface. On the other hand, both GaN and AlN cap layers lead to a decrease in 2DEG density. The reduction of 2DEG caused by the GaN cap layer may be attributed to the additional negative polarization charges formed at the interface between GaN and AlGaN, while the reduction of the piezoelectric effect in the AlGaN layer results in the decrease of 2DEG density in the case of AlN cap layer.     

The influence of AlN interlayers on the microstructural and electrical properties of p-type AlGaN/GaN superlattices grown on GaN/sapphire templates

AlN with different thicknesses were grown as interlayers (ILs) between GaN and p-type Al_0.15Ga_0.85N/GaN superlattices (SLs) by metal organic vapor phase epitaxy (MOVPE). It was found that the edge-type threading dislocation density (TDD) increased gradually from the minimum of 2.5×10⁹?cm^?2 without AlN IL to the maximum of 1×10¹⁰?cm^?2 at an AlN thickness of 20 nm, while the screw-type TDD remained almost unchanged due to the interface-related TD suppression and regeneration mechanism. We obtained that the edge-type dislocations acted as acceptors in p-type Al _x Ga_1?x N/GaN SLs, through the comparison of the edge-type TDD and hole concentration with different thicknesses of AlN IL. The Mg activation energy was significantly decreased from 153 to 70?meV with a?10-nm AlN IL, which was attributed to the strain modulation between AlGaN barrier and GaN well. The large activation efficiency, together with the TDs, led to the enhanced hole concentration. The variation trend of Hall mobility was also observed, which originated from the scattering at TDs.     

Effects of donor density and temperature on electron systems in AlGaN/AlN/GaN and AlGaN/GaN structures

It was reported by Shen et al that the two-dimensional electron gas (2DEG) in an AlGaN/AlN/GaN structure showed high density and improved mobility compared with an AlGaN/GaN structure, but the potential of the AlGaN/AlN/GaN structure needs further exploration. By the self-consistent solving of one-dimensional Schr\"{o}dinger--Poisson equations, theoretical investigation is carried out about the effects of donor density (0--1\times 10¹⁹cm^-3 and temperature (50--500K) on the electron systems in the AlGaN/AlN/GaN and AlGaN/GaN structures. It is found that in the former structure, since the effective \Delta E_c is larger, the efficiency with which the 2DEG absorbs the electrons originating from donor ionization is higher, the resistance to parallel conduction is stronger, and the deterioration of 2DEG mobility is slower as the donor density rises. When temperature rises, the three-dimensional properties of the whole electron system become prominent for both of the structures, but the stability of 2DEG is higher in the former structure, which is also ascribed to the larger effective \Delta E_c. The Capacitance--Voltage (C-V) carrier density profiles at different temperatures are measured for two Schottky diodes on the considered heterostructure samples separately, showing obviously different 2DEG densities. And the temperature-dependent tendency of the experimental curves agrees well with our calculations.     

AlN隔离层生长时间对AlGaN/AlN/GaN HEMT材料电学性能的影响

利用金属有机化学气相沉积（MOCVD）设备,在蓝宝石（0001）面上外延不同生长时间AlN隔离层的Al_xGa_1-xN/AlN/GaN结构的高电子迁移率的晶体管（HEMT）,研究了AlN隔离层厚度对HEMT材料电学性能的影响。研究发现采用脉冲法外延（PALE）技术生长AlN隔离层的时间为12 s（1 nm左右）时,HEMT材料的方块电阻最小,电子迁移率为1 500 cm²·V^-1·s^-1,二维电子气（2DEG）浓度为1.16×10¹³ cm^-2。AFM测试结果表明,一定厚度范围内的AlN隔离层并不会对材料的表面形貌产生重大的影响。HRXRD测试结果表明,AlGaN/AlN/GaN具有好的异质结界面。