Two-dimensional electron transport in AlGaN/GaN heterostructures

We present a theoretical study of electron transport properties of two-dimensional electron gas in AlGaN/GaN heterostructures. By assuming a drifted Fermi–Dirac distribution and taking into account all major scattering mechanisms, including polar optical and acoustic phonons, background impurities, dislocation and interface roughness, the momentum- and energy-balance equations derived from Boltzmann equation are solved self-consistently. The dependence of the electron drift velocity and electron temperature as a function of the applied electric field are obtained and discussed.     

Magneto-gyrotropic photogalvanic effects in GaN/AlGaN two-dimensional systems

Magneto-gyrotropic photogalvanic and spin-galvanic effects are observed in (0001)-oriented GaN/AlGaN heterojunctions excited by terahertz radiation. We show that free-carrier absorption of linearly or circularly polarized terahertz radiation in low-dimensional structures causes an electric photocurrent in the presence of an in-plane magnetic field. Microscopic mechanisms of these photocurrents based on spin-related phenomena are discussed. Properties of the magneto-gyrotropic and spin-galvanic effects specific for hexagonal heterostructures are analyzed.     

High field transport properties in a GaN/AlGaN heterostructure

The drift velocity, electron temperature, electron energy and momentum loss rates of a two-dimensional electron gas are calculated in a GaN/AlGaN heterojunction (HJ) at high electric fields employing the energy and momentum balance technique, assuming the drifted Fermi–Dirac (F–D) distribution function for electrons. Besides the conventional scattering mechanisms, roughness induced new scattering mechanisms such as misfit piezoelectric and misfit deformation potential scatterings are considered in momentum relaxation. Energy loss rates due to acoustic phonons and polar optical phonon scattering with hot phonon effect are considered. The calculated drift velocity, electron temperature and energy loss rate are compared with the experimental data and a good agreement is obtained. The hot phonon effect is found to reduce the drift velocity, energy and momentum loss rates, whereas it enhances the electron temperature. Also the effect of using drifted F–D distribution, due to high carrier density in GaN/AlGaN HJs, contrary to the drifted Maxwellian distribution function used in the earlier calculations, is brought out.     

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.     

Heat transport in low-dimensional systems

Recent results on theoretical studies of heat conduction in low-dimensional systems are presented. These studies are on simple, yet non-trivial, models. Most of these are classical systems, but some quantum-mechanical work is also reported. Much of the work has been on lattice models corresponding to phononic systems, and some on hard-particle and hard-disc systems. A recently developed approach, using generalized Langevin equations and phonon Green's functions, is explained and several applications to harmonic systems are given. For interacting systems, various analytic approaches based on the Green–Kubo formula are described, and their predictions are compared with the latest results from simulation. These results indicate that for momentum-conserving systems, transport is anomalous in one and two dimensions, and the thermal conductivity κ diverges with system size L as κ ～ L ^α. For one-dimensional interacting systems there is strong numerical evidence for a universal exponent α = 1/3, but there is no exact proof for this so far. A brief discussion of some of the experiments on heat conduction in nanowires and nanotubes is also given.     

Carrier transport mechanism of strained AlGaN/GaN Schottky contacts

Using polarization field effect-based thermionic field emission (PFE-TFE) model based on current–voltage–temperature data, possible carrier transport mechanisms for Pt/Au and Cr/Pd Schottky contacts to Al_0.25Ga_0.75N/GaN layers were investigated. Thermionic emission (TE) model was also investigated to compare to the PFE-TFE. It was shown that Schottky barrier heights (SBHs) are significantly affected by a polarization field-induced carrier density of the AlGaN layer. In addition, relatively little temperature dependence on the leakage current density of both contacts was found, which is in good agreement with the PFE-TFE model. The results indicate that the TFE is responsible for the current flow across the metal/AlGaN–GaN interface at T ≥ 293 K.     

AlGaN/GaN抛物量子点的尺寸相关三阶非线性极化率研究

在有效质量近似框架下,通过求解AlxGa1－xN/GaN自组织圆柱形量子点的薛定谔方程,计算了量子点中导带电子在两个正交方向：Z‖和Z⊥方向上产生的三阶非线性极化率.计算中,电子的运动受到自组织量子点的抛物型束缚势和内建压电场的影响.计算表明,量子点的三阶非线性极化率的数量级达到了10－14m2/V2.进一步在Z‖和Z⊥方向上,考察了三阶非线性极化率与两个方向上的抛物束缚势的频率ωp、ωz,量子点的高度L,半径R,Al的含量x之间的关系.     

Quantum corrections to the electrical conduction in an AlGaN/GaN heterostructure

A new method for magneto-transport characterisation of semiconductor heterostructures is presented. The classical model of mixed conduction, modified by corrections resulting from quantum effects, has been used in the analysis of the conductivity-tensor components, magnetoresistance, and Hall coefficient in n-type Al_0.85Ga_0.15N/GaN in magnetic fields up to 12 T, in the temperature range from 2 to 295 K. The mixed conduction is due to high-mobility carriers in the conduction band in the interface and to low-mobility carriers in the conduction band in the GaN layer and in an impurity band. The corrections to the conduction of high-mobility carriers result from quantum effects: negative magnetoresistance, extraordinary Hall effect, and freeze-out of electrons. Negative magnetoresistance is due to localisation of electrons and to increasing tunnel coupling between electron states in different minima of a random potential, due to interface roughness. The extraordinary Hall effect has been explained by interaction of electrons with magnetic moments of dislocations in the interface. Decreasing concentration of electrons is probably due to Landau quantisation of the conduction band in the interface of the heterostructure. Received: 27 November 2000 / Accepted: 18 December 2000 / Published online: 3 April 2001     

Quantum confined carrier transition in a GaN/InGaN/GaN single quantum well bounded by AlGaN barriers

We investigated the carrier transition properties of the GaN/InGaN/GaN single quantum well bounded by AlGaN barriers. In order to confirm the carrier transition coming from the single quantum well, the single quantum well layer was etched by reactive ion etching method. The structural property of the samples was characterized by high resolution X-ray diffraction measurements. In micro-photoluminescence measurements, it is clearly shown that the donor bound exciton transition of the single quantum well sample was redshifted compared to the etched one due to strain. Moreover, a lot of peaks were observed below the GaN band gap energy due to carrier localization in the InGaN/GaN single quantum well, including carrier localization center and quantum confined states. The excitation power dependence and time resolved photoluminescence spectra were investigated to characterize the optical transition of the single quantum well.     

Temperature dependence of electron transport in GaN/AlGaN quantum cascade detectors

In this work we investigate the influence of extractor design and temperature on transport properties of quantum cascade detector. For this purpose we realize numerical calculation of electron lifetimes considering electron–phonon and electron impurities scattering. Electron–phonon interactions are treated using Fermi Golden Rule which allows to calculate lifetime of carriers with temperature and structure design taking into account. Transport characteristics of the quantum cascade detectors have been computed using density matrix theory. As a result, we have obtained the system of ordinary differential equations describing dynamics of electron distribution functions and intersubband correlations. Managing carrier lifetime in quantum wells gives us possibility to make device response faster. Also carrier lifetime is the relevant characteristic, allows us to calculate a lot of parameters such as quantum efficiency and photocurrent.     

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.     

影响AlGaN/GaN量子级联激光器性能的因素研究

在已有理论基础之上,采用严格的计算方法对激光器实现太赫兹(THz)波的辐射进行了可能性分析。利用传递矩阵法,通过Matlab软件计算了基于AlGaN/GaN材料体系的三能级量子级联激光器导带子能级与电子波函数的分布,详细分析了由该材料特有的极化效应所产生的极化场,得出了在近共振条件下偶极跃迁元、外加电场、垒层Al组分及导带子能级能级差之间的关系,并研究了它们对激光器性能的影响。分析结果表明,实现受激辐射的条件非常严格,Al组分取0.15或0.16时较为适宜,同时外加电场需大于63kV/cm,但不能过大,这样才能满足近共振条件,实现粒子数反转达到太赫兹量子级联激射。在Al组分为0.15,外加电场为69.0kV/cm时激光器的偶极跃迁元最大,表明跃迁几率也最大,对激光器的性能有利,可以为量子级联激光器构造较好的有源区。     

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.     

Donor impurity states in zinc-blende GaN/AlGaN quantum well: Quantum confinement and laser-dressed effects

Within the framework of effective-mass approximation, the effects of a laser field on the ground-state donor binding energy in zinc-blende (ZB) GaN/AlGaN quantum well (QW) have been investigated variationally. Numerical results show that the donor binding energy is highly dependent on QW structure parameters and Al composition in ZB GaN/AlGaN QW. The laser field effects are more noticeable on the donor binding energy of an impurity localized inside the QW with small well width and low Al composition. However, for the impurity located in the vicinity of the well edge of the QW, the donor binding energy is insensible to the variation of Al composition, well width and laser field intensity in ZB GaN/AlGaN QW. In particular, the competition effects between laser field and quantum confinement on impurity states have also been investigated in this paper.     

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.     

Photoluminescence enhancement by localized surface plasmons in AlGaN/GaN/AlGaN double heterostructures

Double heterostructures AlGaN/GaN/AlGaN grown by hydride vapor phase epitaxy and designed for use as light emitting diodes for 360 nm wavelength were patterned by shallow nanoholes and injected with Ag/SiO₂ or Al nanoparticles. A 1.8 times increase in the photoluminescence and microcathodoluminescence signal from the GaN active region was observed for 100 nm diameter Al nanoparticles, the efficiency decreased compared to the reference planar samples for small Al nanoparticles of 30–40 nm diameter, and a moderate increase of 1.2 times was detected for Ag/SiO₂ nanoparticles. The observed phenomena are explained by the GaN emitter coupling with localized surface plasmons produced by metallic nanoparticles. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Impact of pressure on transport properties of AlGaN/GaN high electron mobility transistor

The properties of Al_xGa_1−xN/GaN high electron mobility transistor (HEMT) impacted by pressure are characterized quantitatively. The results indicate that the dislocation density increases as the critical thickness decreases with increasing pressure. The two-dimensional electron gas density was found to be linearly changeable with the pressure. A simulation has been completed to verify the influence of electron mobility. The results show that the misfit dislocation scattering induced by the pressure is a major limiting factor for the properties of HEMT.     

Schottky forward current transport mechanisms in AlGaN/GaN HEMTs over a wide temperature range

The behavior of Schottky contacts in AlGaN/GaN high electron mobility transistors(HEMTs) is investigated by temperature-dependent current–voltage(T –I–V) measurements from 300 K to 473 K. The ideality factor and barrier height determined based on the thermionic emission(TE) theory are found to be strong functions of temperature, while present a great deviation from the theoretical value, which can be expounded by the barrier height inhomogeneities. In order to determine the forward current transport mechanisms, the experimental data are analyzed using numerical fitting method,considering the temperature-dependent series resistance. It is observed that the current flow at room temperature can be attributed to the tunneling mechanism, while thermionic emission current gains a growing proportion with an increase in temperature. Finally, the effective barrier height is derived based on the extracted thermionic emission component, and an evaluation of the density of dislocations is made from the I–V characteristics, giving a value of 1.49 × 107cm-2.     

Correlation-induced memory effects in transport properties of low-dimensional systems

We demonstrate the remnant presence of initial correlations in the steady-state electrical current jS flowing between low-dimensional interacting leads. The leads are described as Luttinger liquids and electrons can tunnel via a quantum point contact. We derive an analytic result for the time-dependent current and show that ground-state correlations have a large impact on the relaxation and long-time behavior. In particular, the I-V characteristic is not reproduced by quenching the interaction in time. We further present a universal formula of jS for an arbitrary sequence of interaction quenches and it is established that jS is history dependent for nonsmooth switching process.     

1.8-kV circular AlGaN/GaN/AlGaN double-heterostructure high electron mobility transistor

In this paper, we present a 1.8-kV circular AlGaN/GaN/AlGaN double-heterostructure high electron mobility transistor(DH HEMT) with a gate-drain spacing L_(GD)= 18.8 μm. Compared with the regular DH HEMT, our circular structure has a high average breakdown electric-field strength that increases from 0.42 MV/cm to 0.96 MV/cm. The power figure of meritV_(BR)~2/RON for the circular HEMT is as high as 1.03 ×10~9 V~2·Ω~(-1)·cm~(-2). The divergence of electric field lines at the gate edge and no edge effect account for the breakdown enhancement capability of the circular structure. Experiments and analysis indicate that the circular structure is an effective method to modulate the electric field.