Electron mobility in two-dimensional electron gas in AIGaN/GaN heterostructures and in bulk GaN

We report on temperature dependencies of the electron mobility in the two-dimensional electron gas (2DEG) in AIGaN/GaN heterostructures and in doped bulk GaN. Calculations and experimental data show that the polar optical scattering and ionized impurity scattering are the two dominant scattering mechanisms in bulk GaN for temperatures between 77 and 500K. In the 2DEG in AIGaN/GaN heterostructures, the piezoelectric scattering also plays an important role. Even for doped GaN, with a significant concentration of ionized impurities, a large volume electron concentration in the 2DEG significantly enhances the electron mobility, and the mobility values close to 1700 cm²/Vs may be obtained in the GaN 2DEG at room temperature. The maximum measured Hall mobility at 80K is nearly 5000 cm²/Vs compared to approximately 1200 cm²/Vs in a bulk GaN layer. With a change in temperature from 300 to 80K, the 2DEG in our samples changes from nondegenerate and weakly degenerate to degenerate. Therefore, in order to interpret the experimental data, we propose a new interpolation formula for low field mobility limited by the ionized impurity scattering. This formula is valid for an arbitrary degree of the electron gas degeneracy. Based on our theory, we show that the mobility enhancement in the 2DEG is related to a much higher volume electron concentration in the 2DEG, and, hence, to a more effective screening.     

Steady-State Electron Transport and Low-Field Mobility of Wurtzite Bulk ZnO and Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O

Steady-state electron transport and low-field electron mobility characteristics of wurtzite ZnO and Zn_1−x Mg _x O are examined using the ensemble Monte Carlo model. The Monte Carlo calculations are carried out using a three-valley model for the systems under consideration. Acoustic and optical phonon scattering, intervalley (equivalent and nonequivalent) scattering, ionized impurity scattering, and alloy disorder scattering are used in the Monte Carlo simulations. Steady-state electron transport is analyzed, and the population of valleys is also obtained as a function of applied electric field and ionized impurity concentrations. The negative differential mobility phenomena is clearly observed and seems compatible with the occupancy and effective nonparabolicity factors of the valleys in bulk ZnO and in Zn_1−x Mg _x O with low Mg content. The low-field mobilities are obtained as a function of temperature and ionized impurity concentrations from the slope of the linear part of each velocity–field curve. It is seen that mobilities begin to be significantly affected for ionized impurity concentrations above 5 × 10¹⁵/cm³. The calculated Monte Carlo simulation results for low-field electron mobilities are found to be consistent with published data.     

Synthesis of wurtzite GaN thin film via spin coating method

In this research, hexagonal wurtzite structure gallium nitride (GaN) thin film was grown on silicon (Si) substrate by using spin coating deposition method. Simple ethanol-based precursor with the addition of diethanolamine solution was used. High resolution X-ray diffraction results revealed that wurtzite structure GaN thin film with (002) preferred orientation was deposited on Si substrate. Flied-emission scanning electron microscopy and atomic force microscopy results showed that crack free GaN thin film with uniform and dense grains of GaN was formed. Finally, lattice vibrational characterization by p-polarized infrared reflectance technique revealed a strong reststrahlen feature of crystalline wurtzite GaN, and the transverse and longitudinal phonon modes of wurtzite GaN were clearly identified.     

Hot electron transport in wurtzite-GaN: effects of temperature and doping concentration

The hot electron transport in wurtzite phase gallium nitride (Wz-GaN) has been studied in this paper.An analytical expression of electron drift velocity under the condition of impact ionization has been developed by considering all major scattering mechanisms such as deformation potential acoustic phonon scattering,piezoelectric acoustic phonon scattering,optical phonon scattering,electron-electron scattering and ionizing scattering.Numerical calculations show that electron drift velocity in Wz-GaN saturates at 1.44 x 105 m/s at room temperature for the electron concentration of 1022 m-3.The effects of temperature and doping concentration on the hot electron drift velocity in Wz-GaN have also been studied.Results show that the saturation electron drift velocity varies from 1.91 × 105-0.77 × 105 m/s for the change in temperature within the range of 10--1000 K,for the electron concentration of 1022 m-3;whereas the same varies from 1.44 x 105-0.91 × 105 m/s at 300 K for the variation in the electron concentration within the range of 1022-1025 m-3.The numerically calculated results have been compared with the Monte Carlo simulated results and experimental data reported earlier,and those are found to be in good agreement.     

垂直电场作用下的钎锌矿相GaN电子输运特性的Monte Carlo研究

在GaN NMOSFET中,沟道电子由于受垂直于其运动方向电场的作用而产生界面散射,从而影响MOSFET特性.研究采用Monte Carlo体模拟方法计算钎锌矿相GaN材料在界面散射下的电子输运特性.模拟中在电子漂移方向加一个水平电场,同时在与其垂直的方向加另外一个电场,在垂直电场作用下,电子发生界面散射.采用基于指数...     

Negative electron diffusivity in high electric fields

The prediction that the electron diffusivity may be negative in high electric fields under some conditions has been examined starting from the Boltzmann equation and assuming a Maxwellian distribution function. It is found that the diffusion constant is positive for predominant acoustic phonon, polar optical phonon or impurity atom scattering. But the constant may be negative when effects of nonparabolicity are important and energy relaxation is limited by non-polar optical phonon scattering but the momentum relaxation is dominated by impurity atom scattering. Calculations with the parameter values of InSb, silicon and germanium show that only in materials like germanium at low temperatures of about 27 K the diffusion constant may be negative for impurity concentrations of 10¹⁷–10¹⁸ cm^?3.     

High-temperature transport properties of 2DEG in AlGaN/GaN heterostructures

Transport properties of the two-dimensional electron gas (2DEG) in fully strained and partially strain-relaxed Al_0.22Ga_0.78N/GaN heterostructures at temperatures from 300 to 680 K have been investigated by Hall effect measurements. The 2DEG mobility was found to decrease rapidly with increasing temperature at the initial stage and then decrease slowly as temperature is further increased. Those features indicate strongly that the 2DEG mobility is primarily limited by LO phonon scattering processes at high temperatures. Meanwhile, the calculated results show that more electrons transfer to the higher-order sub-bands with increasing temperature, and hence the effect of screening on LO phonon scattering is weakened and the alloy scattering of the AlGaN layer on the 2DEG becomes stronger. Thus variation of 2DEG occupation in different sub-bands with increasing temperature also decreases mobility of the 2DEG.     

Optical properties of gallium nitride bulk crystals grown by chloride vapor phase epitaxy

The optical properties of bulk crystals of gallium nitride grown by chloride vapor-phase epitaxy are investigated. It is shown that these crystals exhibit exciton luminescence bands. Analysis of the energy positions of the band maxima imply certain conclusions about the presence or absence of mechanical stresses in the bulk crystals of GaN obtained. Analysis of the luminescence spectra also reveals that the temperature dependence of the width of the GaN band gap E _g in the temperature range T=6–600 K is well described by the expression E _g (T)=3.51−7.4×10⁻⁴ T ²(T+600)⁻¹ eV. It is estimated that values of the free electron concentration in these crystals do not exceed 10¹⁸ cm⁻³. The optical characteristics of the bulk GaN crystals are compared analytically with literature data on bulk crystals and epitaxial layers of GaN grown by various methods. Fiz. Tekh. Poluprovodn. 33, 1173–1178 (October 1999)     

AlxGa1-xAs/GaAs异质结中电子迁移率的压力效应

对Al_xGa_1-xAs/GaAs半导体单异质结系统,引入有限高势垒与考虑导带弯曲的真实势，同时计入电子对异质结势垒的隧穿，利用变分法和记忆函数方法讨论在界面光学声子和体纵光学声子的散射下，异质结界面附近电子迁移率随温度的变化关系及其压力效应.结果显示：电子迁移率随温度、压力的增加而减小；且两种声子的散射作用均随压力增强，界面光学声子的变化幅度更大.因此，在讨论压力的情形下，界面光学声子的作用不容忽略.     

纤锌矿GaN/AlxGa1-xN量子阱中激子能量

考虑了纤锌矿GaN/Al<,x>Ga<,1-x>N量子阱(QW)材料中空穴带质量和光学声子模的各向异性以及声子频率随波矢变化的效应,采用改进的LLP变分法计算了纤锌矿氮化物QW中激子的基态能量和结合能.给出了激子的基态能量和结合能随着QW宽度和Al组分变化的函数关系,并对闪锌矿和纤锌矿GaN/Al<0.3>Ga<,0....     

纤锌矿GaN/AlxGa1-xN量子阱中自由极化子能级

采用Lee-Low-Pines (LLP)变分法研究了纤锌矿GaN/Al0.3Ga0.7N量子阱中自由极化子能量和电子-声子相互作用对自由极化子能量的影响.理论计算中考虑了量子阱中定域声子模(Confined phonon modes)和半空间声子模(Half-space phonon modes)的影响以及电子有效质...     

Structural study of GaN grown on (001) GaAs by organometallic vapor phase epitaxy

Detailed transmission electron microscope (TEM) and transmission electron diffraction (TED) examination has been performed on organometallic vapor phase epitaxial GaN layers grown on (001) GaAs substrate to investigate microstructures and phase stability. TED and TEM results exhibit the occurrence of a mixed phase of GaN. The wurtzite (α) phase grains are embedded in the zinc-blende (β) phase matrix. It is shown that there are two types of the wurtzite GaN phase, namely, the epitaxial wurtzite and the tilted wurtzite. The tilted wurtzite grains are rotated some degrees ranging from ∼5° to ∼35° regarding the GaAs substrate. A simple model is presented to describe the occurrence of the mixed phases and the two types of the wurtzite phase.     

AlGaN/GaN异质结构中二维电子气的高温输运性质

采用高温Hall测量仪对一个全应变和一个部分应变弛豫的AlGaN/GaN异质结构中2DEG的高温输运特性进行了研究,温度变化范围从室温到680K.研究结果表明:在高温段2DEG的迁移率主要受LO声子散射限制; 在室温,异质界面处的非均匀压电极化场对2DEG迁移率的散射也是一个主要的散射机制.同时,计算结果显示,随着温度升高,更多的电子跃迁到更高的子带,在更高的子带,其波函数逐渐扩展到AlGaN层内部以及GaN体内更深的位置,导致LO声子散射的屏蔽效应减弱且来自AlGaN层内的合金无序散射增强.     

The influence of OMVPE growth pressure on the morphology, compensation, and doping of GaN and related alloys

Growth pressure has a dramatic influence on the grain size, transport characteristics, optical recombination processes, and alloy composition of GaN and AlGaN films. We report on systematic studies which have been performed in a close spaced showerhead reactor and a vertical quartz tube reactor, which demonstrate increased grain size with increased growth pressure. Data suggesting the compensating nature of grain boundaries in GaN films is presented, and the impact of grain size on high mobility silicon-doped GaN and highly resistive unintentionally doped GaN films is discussed. We detail the influence of pressure on AlGaN film growth, and show how AlGaN must be grown at pressures which are lower than those used for the growth of optimized GaN films. By controlling growth pressure, we have grown high electron mobility transistor (HEMT) device structures having highly resistive (10⁵ Ω-cm) isolation layers, room temperature sheet carrier concentrations of 1.2×10¹³ cm⁻² and mobilities of 1500 cm²/Vs, and reduced trapping effects in fabricated devices.     

Synthesis and Raman spectra of hammer-shaped ZnO nanostructures via thermal evaporation growth

Hammer-shaped ZnO nanostructures were synthesized on silicon substrate via a simple thermal evaporation process without catalysts or additives. Scanning electron microscopy results shows that ordered ZnO nanohammers grow from the Si substrate. Transmission electron microscopy and selected area electron diffraction analysis indicate that a single nanohammer is a single crystal and grows along (0001) direction. X-ray diffraction patterns for prepared samples are consistent with a wurtzite ZnO structure. The effect of temperature on Raman scattering of single crystal ZnO nanohammers in the temperature range from 83 to 523 K was determined. Temperature-dependent Raman spectra of E₂(high frequency or hf) exhibit phonon frequency redshift and linewidth broadening with increasing temperature, which can be explained by a model taking into account contributions of thermal expansion and anharmonic phonon processes. Results show that decay into three phonons is the probable channel for the E₂(hf) mode.     

Electrons and phonons in quantum wells

The modulation of electron and polar optical phonon states in an AlGaAs/GaAs/AlGaAs quantum well (QW) with an inserted thin AlAs barrier is considered. The OW width dependence of electron-phonon scattering rates are estimated. The large contribution to the change of the electron subband population, the photovoltaic effect, and the electron mobility in the QW accounts for the resonant intersubband scattering of electrons by interface phonons. The decrease of electron mobility limited by polar optical phonon scattering with increasing carrier concentration in the QW is established. The conditions for the increase of mobility in the QW by inserting the AlAs barrier are found. Fiz. Tekh. Poluprovodn. 33, 1049–1053 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

退火温度对硅基GaN薄膜质量的影响

采用电泳沉积法在Si(111)衬底上制备GaN薄膜,并研究退火温度对GaN薄膜晶体质量、表面形貌和发光特性的影响。傅立叶红外吸收谱(FTIR)、X射线衍射(XRD)和扫描电镜(SEM)的测试结果表明所得样品为六方纤锌矿结构的GaN多晶薄膜,随退火温度的升高,晶粒尺寸增大,结晶化程度提高。室温下光致发光谱的测试发现了位于367 nm处的强发光峰和437 nm处的弱发光峰,其发光强度随退火温度的升高而增强,但发光峰的位置并不发生移动。     

The impact of nitridation and nucleation layer process conditions on morphology and electron transport in GaN epitaxial films

A systematic study has been performed to determine the characteristics of an optimized nucleation layer for GaN growth on sapphire. The films were grown during GaN process development in a vertical close-spaced showerhead metalorganic chemical vapor deposition reactor. The relationship between growth process parameters and the resultant properties of low temperature GaN nucleation layers and high temperature epitaxial GaN films is detailed. In particular, we discuss the combined influence of nitridation conditions, V/III ratio, temperature and pressure on optimized nucleation layer formation required to achieve reproducible high mobility GaN epitaxy in this reactor geometry. Atomic force microscopy and transmission electron microscopy have been used to study improvements in grain size and orientation of initial epitaxial film growth as a function of varied nitridation and nucleation layer process parameters. Improvements in film morphology and structure are directly related to Hall transport measurements of silicon-doped GaN films. Reproducible growth of silicon-doped GaN films having mobilities of 550 cm²/Vs with electron concentrations of 3 × 10¹⁷ cm⁻³, and defect densities less than 10⁸ cm⁻² is reported. These represent the best reported results to date for GaN growth using a standard two-step process in this reactor geometry.     

GaN n-MOSFET反型沟道电子迁移率模型

在器件物理的基础上,提出了一种半经验的GaN n-MOSFET反型沟道电子迁移率模型.该模型考虑了位错、界面态、光学声子、离化杂质、表面粗糙、声学声子,以及高场对迁移率的影响.模拟结果表明,界面态和位错是影响沟道迁移率的主要因素,尤其是界面态,它决定了迁移率的最大值,而位错密度的增加使迁移率减小.此外,表面粗糙散射和高场散射主要影响高场下载流子迁移率.由此可见,GaN n-MOSFET沟道迁移率的提高依赖于晶体质量和界面质量的提高.