晶格匹配InAlN/GaN和InAlN/AlN/GaN材料二维电子气输运特性研究

文章研究了InAlN/GaN和引入AlN界面插入层形成的InAlN/AlN/GaN材料的输运性质. 样品均在蓝宝石上以脉冲金属有机物化学气相淀积法生长,霍尔迁移率变温特性具有典型的二维电子气(2DEG)特征. 综合各种散射机理包括声学形变势散射、压电散射、极性光学声子散射、位错散射、合金无序散射和界面粗糙度散射,理论分析了温度对迁移率的影响,发现室温下两种材料中2DEG支配性的散射机理都是极性光学波散射和界面粗糙度散射;AlN插入层对InAlN/GaN材料迁移率的改善作用一方面是免除2DEG的合金无序散射,另外还显著改善异质界面,抑制了界面粗糙度散射. 考虑到2DEG密度也是影响其迁移率的重要因素,结合实验数据给出了晶格匹配InAlN/GaN和InAlN/AlN/GaN材料的2DEG迁移率随电子密度变化的理论上限. 关键词： InAlN/GaN 二维电子气 迁移率     

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。     

Al组分对MOCVD制备的AlxGa1-xN/AlN/GaN HEMT电学和结构性质的影响

采用金属有机化合物化学气相沉积（MOCVD）方法制备了不同Al组分（x=0.19,0.22,0.25,0.32）的Al_xGa_1-xN/AlN/GaN 结构的高电子迁移率晶体管（HEMT）材料。研究了Al_xGa_1-xN势垒层中Al组分对HEMT材料电学性质和结构性质的影响。研究结果表明,在一定的Al组分范围内,二维电子气（2DEG）浓度和迁移率随着Al组分的升高而增大。然而,过高的Al组分导致HEMT材料表面粗糙度增大,2DEG迁移率降低,该实验现象在另一方面得到了原子力显微镜测试结果的验证。在最佳Al组分（25%）范围内,获得的 HEMT材料的2DEG浓度和室温迁移率分别达到1.2×10¹³ cm^-2和1 680 cm²/（V·s）,方块电阻低至310 Ω/□。     

AlN/β-Ga2O3异质结电子输运机制

β-Ga₂O₃具有禁带宽度大、击穿电场强的优点,在射频及功率器件领域具有广阔的应用前景.β-Ga₂O₃(201)晶面和AlN(0002)晶面较小的晶格失配和较大的导带阶表明二者具有结合为异质结并形成二维电子气(twodimensional electron gas,2DEG)的理论基础,引起了众多研究者关注.本文利用AlN的表面态假设,通过求解薛定谔-泊松方程组计算了AlN/β-Ga₂O₃异质结导带形状和2DEG面密度,并将结果应用于玻尔兹曼输运理论,计算了离化杂质散射、界面粗糙散射、声学形变势散射、极性光学声子散射等主要散射机制限制的迁移率,评估了不同散射机制的相对重要性.结果表明,2DEG面密度随AlN厚度的增加而增加,当AlN厚度为6 nm,2DEG面密度可达1.0×10¹³ cm^-2,室温迁移率为368.6 cm²/(V.s).在T<184 K的中低温区域,界面粗糙散射是限制2...     

AlN插入层对AlxGa1-xN/GaN界面电子散射的影响

本文研究AlN作为Al_xGa_1-xN/GaN插入层引起的电子输运性质的变化, 考虑了Al_xGa_1-xN和AlN势垒层的自发极化、压电极化对Al_xGa_1-xN/AlN/GaN双异质结高电子迁移率晶体管(HEMT)中极化电荷面密度、二维电子气(2DEG) 浓度的影响, 分析了AlN厚度与界面粗糙度散射和合金无序散射的关系; 结果表明, 2DEG 浓度、界面粗糙度散射和合金无序散射依赖于AlN层厚度, 插入一层1–3 nm薄的AlN层, 可以明显提高电子迁移率.     

高阻GaN的MOCVD外延生长

利用金属有机化合物气相沉积(MOCVD)在蓝宝石衬底上生长了高阻GaN薄膜。对GaN成核层生长的反应室压力、生长时间和载气类型对GaN缓冲层电学特性的影响进行了分析。实验结果表明,延长GaN成核层的生长时间,降低成核层生长时的反应室压力,载气由H₂换为N₂都会得到高阻的GaN缓冲层。样品的方块电阻R_s最高为2.49×10¹¹ Ω/□。以高阻GaN样品为衬底制备了AlGaN/AlN/GaN结构HEMT器件,迁移率最高达1 230 cm²/(V·s)。     

金属有机物化学气相沉积生长GaN薄膜的室温热电特性研究

采用金属有机物化学气相沉积技术生长了不同掺杂浓度的GaN薄膜, 并且通过霍尔效应测试和塞贝克效应测试, 表征了室温下GaN薄膜的载流子浓度、迁移率和塞贝克系数. 在实验测试的基础上, 计算了GaN薄膜的热电功率因子, 并且结合理论热导率确定了室温条件下GaN薄膜的热电优值(ZT). 研究结果表明: GaN薄膜的迁移率随着载流子浓度的增加而减小, 电导率随着载流子浓度的增加而增加; GaN 薄膜材料的塞贝克系数随载流子浓度的增加而降低, 其数量级在100–500 μV/K范围内; GaN薄膜材料在载流子浓度为1.60×10¹⁸ cm^-3时, 热电功率因子出现极大值4.72×10^-4 W/mK²; 由于Si杂质浓度的增加, 增强了GaN薄膜中的声子散射, 使得GaN薄膜的热导率随着载流子浓度的增加而降低. GaN薄膜的载流子浓度为1.60×10¹⁸ cm^-3时, 室温ZT达到极大值0.0025.     

蓝宝石衬底上RF-MBE生长的GaN中的极性控制和螺旋位错的降低

近年来人们报道了用MBE方法生长GaN的飞速进展,利用RF-MBE方法可以获得高的GAN生长速率和高的电子迁移率.本文讨论了用RF-MBE方法在蓝宝石衬底上生长GaN过程中的极性控制和螺旋位错的降低.在充分氮化的蓝宝石衬底上直接生长GaN,使GaN的极性控制为N-极性,并用高温生长的AlN核化层实现GaN的Ga-极性.对于N-和Ga-极性的GaN这两种情况,高温生长的AlN中间迭层的引入,可以有效地抑制螺旋位错的扩散.位错的降低使GaN的室温电子迁移率得到提高,对于Ga-极性的GaN,其值为332cm²/V·s;而对于N-极性的GaN,其值为688cm²/V·s.     

Ta2O5-PMMA复合栅绝缘层对OFETs性能的影响

选用五氧化二钽（Ta₂O₅）-聚甲基丙烯酸甲酯（PMMA）复合材料作为栅绝缘层制备了并五苯有机场效应晶体管（OFETs）。通过在Ta₂O₅表面旋涂一层PMMA可以降低栅绝缘层的表面粗糙度,增大其场效应晶体管的迁移率。研究了厚度在20~60 nm范围内的PMMA对复合绝缘层表面形貌、粗糙度以及器件电学性能的影响。结果表明,当PMMA厚度为40 nm时,器件的电学性能最佳。与单一的Ta₂O₅栅绝缘层器件相比,其场效迁移率由4.2×10^-2 cm²/（V·s）提高到0.31 cm²/（V·s）;栅电压增加到-20 V时,开关电流比由2.9×10²增大到2.9×10⁵。     

高灵敏度Sb基量子阱2DEG的霍尔器件

用分子束外延技术将高灵敏度的InAs/AlSb量子阱结构的Hall器件赝配生长在GaAs衬底上。设计了由双δ掺杂构成的Hall器件的新结构,有效地提高了器件的面电子浓度。与传统的没有掺杂的InAs/AlSb量子阱结构的Hall器件相比,室温下器件电子迁移率从15 000 cm²·V^-1·s^-1 提高到16 000 cm²·V^-1·s^-1。AFM测试表明材料有好的表面形态和结晶质量。从77 K 到300 K对Hall器件进行霍尔测试,结果显示器件不同温度范围有不同散射机构。双δ掺杂结构形成高灵敏度、高二维电子气（2DEG）浓度的InAs/AlSb异质结Hall器件具有广阔的应用前景。     

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.     

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the AlGaN/AlN/GaN in an SiC heterostructure, which demonstrates the existence of the high-quality two-dimensional electron gas (2DGE) at the AlN/GaN interface. While the carrier concentration reaches 1.32 × 10¹³ cm ^{- 2} and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21 × 10⁴ cm²/(V·s) at 2 K. The Shubnikov—de Haas (SdH) oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEG is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.     

AlGaN插入层对InAlN/AlGaN/GaN异质结散射机制的影响

InAlN/AlN/GaN异质结中,名义上的AlN插入层实为Ga含量很高的AlGaN层, Al, Ga摩尔百分比决定了电子波函数与隧穿几率,因此影响与InAlN/AlGaN势垒层有关的散射机制.本文通过求解薛定谔-泊松方程与输运方程,研究了AlGaN层Al摩尔百分含量对InAlN组分不均匀导致的子带能级波动散射、导带波动散射以及合金无序散射三种散射机制的影响.结果显示:当Al含量由0增大到1,子带能级波动散射强度与合金无序散射强度先增大后减小,导带波动散射强度单调减小;在Al含量为0.1附近的小组分范围内,合金无序散射是限制迁移率的主要散射机制,该组分范围之外,子带能级波动散射是限制迁移率的主要散射机制;当Al摩尔百分含量超过0.52,三种散射机制共同限制的迁移率超过无插入层结构的迁移率, AlGaN层显示出对迁移率的提升作用.     

AlGaN/AlN/GaN结构中二维电子气的输运特性

对使用金属有机物汽相沉积法生长的AlGaN/AlN/GaN结构进行的变温霍尔测量，测量结果指出在AlN/GaN界面处有二维电子气存在且迁移率和浓度在2K时分别达到了1.4×10⁴cm²·V^-1·s^-1和9.3×10¹²cm^-2，且在200K到2K范围内二维电子气的浓度基本不变，变磁场霍尔测量发现只有一种载流子(电子)参与导电.在2K温度下，观察到量子霍尔效应，Shubnikov-de Haas (SdH) 振荡在磁场约为3T时出现，证明了此结构呈现了典型的二维电子气行为.通过实验数据对二维电子气散射过程的半定量分析，推出量子散射时间为0.23ps，比以往报道的AlGaN/GaN结构中的散射时间长，说明引入AlN层可以有效减小合金散射，进一步的推断分析发现低温下以小角度散射占主导地位.     

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.     

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.     

The effect of interface roughness scattering on low field mobility of 2D electron gas in GaN/AlGaN heterostructure

We report the results of our experimental and theoretical studies concerning the temperature dependence of electron mobility in a two dimensional electron gas (2DEG) confined at the GaN/AlGaN interface. Experimental mobility of about at 3.8 K remains almost constant up to lattice temperature , it then decreases rapidly down to about at . The results are discussed using a theoretical model that takes into account the most important scattering mechanisms contributing to determine the mobility of electrons in 2DEG. We show that the polar optical phonon scattering is the dominant mechanism at high temperatures and the acoustic deformation potential and piezoelectric scatterings are dominant at the intermediate temperatures. At low temperatures, the Hall mobility is confined by both the interface roughness (IFR) and ionised impurity scattering. The correlation length (Λ) and lateral size (Δ) of roughness at the GaN/AlGaN heterointerface have been determined by fitting best to our low-temperature experimental data.     

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.