Photoreflectance study of electric field distributions in semiconductors heterostructures grown on semi-insulating substrates

We have studied the photoref lectance (PR) spectra from a MBE grown heterostructure consisting of 200 nm of Ga_0.83Al_0.17As, a 800 nm GaAs buffer layer on a semi-insulating (100) LEC GaAs substrate. By varying both the pump beam wavelength and modulation frequency (up to 100 kHz) we are able to identify the component layers, their quality and the properties of the various interfaces. In this study we find evidence for a low density of interface states between the GaAs buffer layer and GaAlAs layer and a relatively large density of interface states between the substrate and buffer regions. These states, previously observed by Deep Level Transient Spectroscopy of doped structures, are presumably associated with the interface produced by MBE growth on etched and air exposed substrates. However, in our material, since the substrate is semi-insulating and the buffer layer is undoped, it is difficult to resolve these states spatially by C-V techniques. Our results show that the PR technique can be used to characterize low conductivity or semi-insulating structures such as enhancement mode MESFET and HEMT type devices and it may be useful for the in-situ characterization of epigrown surfaces and interfaces     

Defects and defect reduction processing in semiconductor heterostructures

Defects such as dislocations and interfaces play a crucial role in the performance of heterostructure devices. The full potential of GaAs on Si heterostructures can only be realized by controlling the defect density. The nature of misfit dislocations at the heterointerface has been studied and a mechanism for the formation of 60° and 90° misfit dislocations has been proposed. Threading dislocations in the epilayer are the most prominent defects and their density has to be controlled to fully utilize the properties of semiconductor heterostructures. Various processes to reduce defect densities in the epilayers have been discussed and in particular, the use of strained layer superlattices to reduce the threading dislocation density has been presented in this paper. Several superlattice structures have been used to reduce the density of threading dislocations in the GaAs epilayer. In this study, we have optimized the use of strained layer superlattices with respect to the position, period and number to reduce and control the dislocation density. The use of strained layer superlattices in conjunction with rapid thermal annealing was found to be the most effective method in reducing threading dislocation density. Transmission electron microscopy has been used to study the dislocation density reduction and the interaction of threading dislocations with the strained layers.     

Fe/(Ga,Mn)As异质结的界面结构和磁性质

Fe/（Ga,Mn）As heterostructures were fabricated by all molecular-beam epitaxy.Double-crystal X-ray diffraction and high-resolution cross-sectional transmission electron micrographs show that the Fe layer has a well ordered crystal orientation and an abrupt interface.The different magnetic behavior between the Fe layer and（Ga, Mn）As layer is observed by superconducting quantum interference device magnetometry.X-ray photoelectron spectroscopy measurements indicate no Fe₂As and Fe-Ga-As compounds,i.e.,no dead magnetic layer at the interface, which strongly affects the magnetic proximity and the polarization of the Mn ion in a thin（Ga,Mn）As region near the interface of the Fe/（Ga,Mn）As heterostructure.     

Defect Engineering in Oxide Heterostructures by Enhanced Oxygen Surface Exchange

The synthesis of materials with well‐controlled composition and structure improves our understanding of their intrinsic electrical transport properties. Recent developments in atomically controlled growth have been shown to be crucial in enabling the study of new physical phenomena in epitaxial oxide heterostructures. Nevertheless, these phenomena can be influenced by the presence of defects that act as extrinsic sources of both doping and impurity scattering. Control over the nature and density of such defects is therefore necessary to fully understand the intrinsic materials properties and exploit them in future device technologies. Here, it is shown that incorporation of a strontium copper oxide nano‐layer strongly reduces the impurity scattering at conducting interfaces in oxide LaAlO₃–SrTiO₃(001) heterostructures, opening the door to high carrier mobility materials. It is proposed that this remote cuprate layer facilitates enhanced suppression of oxygen defects by reducing the kinetic barrier for oxygen exchange in the hetero‐interfacial film system. This design concept of controlled defect engineering can be of significant importance in applications in which enhanced oxygen surface exchange plays a crucial role.     

分子束外延GaAs/Si应变层的光致发光和光反射谱研究

研究了分子束外延GaAs/Si光致发光谱的激发强度和温度依赖关系。确定出2个本征发光峰,分别对应于导带至m_J=±3/2和m_J=±1/2价带的复合。这种价带的移动和分裂归因于由GaAs和Si的热膨胀系数不同所引起的GaAs层双轴张应力。还观测到4个非本征发光峰,分别为导带至m_J=±1/2碳受主态发光、可能与缺陷有关的发光以及可能由Mn和Cu受主杂质引起的发光。室温下将GaAs/Si和GaAs/GaAs材料的光反射谱进行比较,前者明显向低能移动约8meV,观测到3个特征谱结构,与光致发光结果相一致。     

Mobility of modulation doped AlGaAs/low-temperature MBE-grown GaAs heterostructures

A study of the mobility of a novel modulation doped heterostructure in which the channel region is made of low-temperature molecular beam epitaxially grown GaAs (LT-GaAs) and all other layers are grown at normal temperatures is presented for the first time. The resistivity of the as-grown samples(in- situ annealed) is very high, as is that of single layers of bulk LT-GaAs. However, in the presence of light, the resistivity of the LT-GaAs modulation-doped field effect transistor (MODFET) is significantly lower, facilitating reliable Hall measurements. We speculate that the observed decrease in resistivity of the LT-GaAs MODFET is due to the formation of a two-dimensional electron gas (2DEG) at the heterointerface under illumination. A number of samples grown under different growth conditions were investigated. Mobilities for these samples were found to be in the range of 250 to 750 cm²Vs at 300K and ∼3000 to 5500 cm²Vs at 77K. A first-order computer simulation was implemented to calculate the mobility of the 2DEG using the relaxation-time approximation to solve the Boltzmann equation, taking into account different scattering mechanisms. Scattering by the arsenic clusters and by ionized impurities in the LT-GaAs MODFET channel are found to be the two dominant mechanisms limiting the mobility of the LT-GaAs MODFET samples. Experimental values are in good agreement with theoretical results.     

准一维半导体纳米结构的电子显微学研究

本文总结了近年来我们在功能准一维纳米结构材料研究方面所获得的一些有意义的结果。借助于现代电子显微镜技术，不仅研究了硅、氮化稼、氧化锌等一维纳米材料的形貌和显微结构，还研究了其一维择优生长机理及小尺度效应。尤其是利用高能量分辨电子能量损失谱、高角环形暗场探头等先进技术，解决了一个传统X－光等结构分析手段所不能解决的难题，分析了一种SiOx/SiC复合纳米电缆的成份与结构。     

On the band-structure lineup at Schottky contacts and semiconductor heterostructures

The band-structure lineup at semiconductor interfaces is explained by the intrinsic interface-induced gap states (IFIGS) that derive from the complex band structures of the semiconductors. The barrier heights of metal–semiconductor or Schottky contacts as well as the band-edge offsets of semiconductor heterostructures are composed of a zero-charge-transfer term plus an electrostatic-dipole contribution which are determined by the IFIGS branch-point energies of the semiconductors and the electronegativity difference of the two materials in contact, respectively. This concept will be illustrated by experimental core-level shifts induced by metal adatoms on group-IV semiconductor surfaces. Choosing Si and SiO₂ Schottky contacts and heterostructures as typical examples, it will be demonstrated that the IFIGS-and-electronegativity concept self-consistently explains the barrier heights of Schottky contacts and the valence-band offsets of heterostructures. The IFIGS-and-electronegativity concept also resolves the alleviation of the Fermi-level pinning by ultra-thin insulator interlayers in Schottky contacts. Finally, the modification of Schottky barriers by atomic interlayers will be discussed.     

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.     

基于GaN/AlGaN高电子迁移率场效应晶体管的室温太赫兹探测器

本文主要论述一种带平板天线,基于GaN/AlGaN高电子迁移率场效应晶体管的室温太赫兹探测器。太赫兹辐射下,由于天线的作用会在栅下感应出平行沟道和垂直沟道的太赫兹电场,由此在源漏产生强烈依赖于栅压的直流光电流。尽管栅极远离源漏两端,平行沟道和垂直沟道的太赫兹电场依然很强。探测器可以用自混频理论很好的描述。在室温下,探测器的响应度和噪声等效功率分别为100 nW/sqrt（Hz）和 3 mA/W。探测器具有很高的响应速度,在5 KHz的调制下,光电流没有衰减。如果缩少栅极和源漏的距离将进一步提高探测灵敏度。     

纳米半导体材料的制备技术

介绍了纳米半导体材料的定义、性质及其在未来信息技术中的地位,讨论了纳米半导体材料的制备方法。     

Characterization of a room temperature terahertz detector based on a GaN/AlGaN HEMT

We report on the characterization of a room temperature terahertz detector based on a GaN/AlGaN high electron mobility transistor integrated with three patch antennas.Experimental results prove that both horizontal and perpendicular electric fields are induced in the electron channel.A photocurrent is generated when the electron channel is strongly modulated by the gate voltage.Despite the large channel length and gate-source/drain distance, significant horizontal and perpendicular fields are achieved.The device is well described by the self-mixing of terahertz fields in the electron channel.The noise-equivalent power and responsivity are estimated to be 100 nW/（Hz）^1/2 and 3 mA/ W at 292 K,respectively.No decrease in responsivity is observed up to a modulation frequency of 5 kHz. The detector performance can be further improved by engineering the source-gate-drain geometry to enhance the nonlinearity.     

Structural,electrical and optical characterization of singlecrystal ErAs layers grown on GaAs by MBE

A detailed study is presented of the structural, electrical, and optical properties of ErAs films grown on GaAs by molecular beam epitaxy (MBE). ErAs layers 1500Å thick were grown successfully over a relatively wide range of substrate temperatures (420-580° C), although overgrowth of GaAs on ErAs was found to be difficult. In-situ reflection highenergy electron diffraction (RHEED), x-ray diffraction, and Rutherford backscattering (RBS) measurements all indicate single crystal growth. Analysis of X-ray rocking curves reveals that, over the range of substrate temperatures studied, strain due to the lattice mismatch between ErAs and GaAs is completely inelastically relieved in the 1500Å thick ErAs layers. Variable-temperature Hall measurements reveal metallic behaviour in all samples, with no pronounced dependence on substrate temperature. Spectrally narrow (0.6 meV) intra 4f-shell transitions of Er³⁺ (4f¹¹), at 1.54 μm, have been observed in ErAs epitaxial layers both in absorption (by Fourier transform infra-red spectroscopy, FTIR) and in emission (by cathodoluminescence). The crystal-field splittings observed in the FTIR spectra are consistent with the cubic(O _h)symmetry expected for the Er lattice site in unstrained ErAs, in good agreement with the x-ray analyses.     

高功率半导体激光器评述

作者综合评述了高功率半导体激光器的发展现状,对高功率半导体阵列器件结构进行了分析,介绍了设计高功率半导体激光器所涉及的关键技术。     

A new LPE growth method of semiconductor heterostructures with thickness profile variation of epitaxial layers

We have investigated and developed a method for the LPE growth of layers with approximately parabolic cross section. The channels were created during the growth process by modulating the liquid phase thickness with W or Mo wires parallel to the substrate. The main parameters of the channel can be controlled by changing the wire’s diameter and its distance from the substrate. This method can be incorporated directly into the growth process of a laser structure with an unstable resonator without the need of additional treatments as chemical etching, to produce the channel structure.     

Al_xGa_(1-x)N/GaN异质结构中二维电子气的自旋性质

Ⅲ族氮化物材料有很长的电子自旋弛豫时间以及很高的居里温度,成为近年来半导体自旋电子学研究的重要材料体系之一。介绍了目前两种最主要的研究AlxGa1-xN/GaN异质结构中二维电子气(2DEG)自旋性质的物理效应:磁电阻的舒伯尼科夫-德哈斯拍频振荡和弱反局域效应,回顾了AlxGa1-xN/GaN异质结构中2DEG自旋性质的研究进展。AlxGa1-xN/GaN异质结构材料中有很强的极化电场,诱导产生很高浓度的2DEG,能够产生相当大能量的自旋分裂,并且这种自旋分裂可以被栅压所调控,因此在自旋场效应晶体管方面有很好的应用前景。然而要实现GaN基自旋电子学器件的应用,GaN中自旋注入效率是目前所面临的问题。     

异质结NIPI结构的光调制反射光谱

本文首次报道了异质结NIPI结构的室温光调制反射光谱及其随调制光强的变化,并对调制机制进行了讨论.最后,通过比较理论与实验结果,对所观测到的跃迁过程进行了指派.     

Synthesis of organic semiconductor hexadecachloro zinc phthalocyanine and its gas sensitivity

Due to the difficulty in synthesizing perhalogenated metallophthalocyanine, the method of ammonium molybdate solid phase catalysis was introduced, and by using tetrachlorophthalic anhydride and urea as the raw materials, hexadecachloro zinc phthalocyanine (ZnPcCl<,16>) was synthesized. Components of the composite were analyzed by energy spectrum, and its functional group structures and absorption peaks were characterized by IR and UV-vis spectroscopy. The thin films of gas sensors were prepared in a vacuum evaporation system and evaporated onto SiO2 substrates, where sensing electrodes were made by MEMS micromachining. The optimal conditions for the films are: substrate temperature of 150 ℃ evaporation current of 95 A and film thickness of 50 nm. The result showed that the sensors were ideally sensitive to C12 gas and could detect the minimum concentration of 0.3 ppm.     

十六氯酞菁锌有机半导体材料的合成及其气敏性研究

Due to the difficulty in synthesizing perhalogenated metallophthalocyanine, the method of ammonium molybdate solid phase catalysis was introduced, and by using tetrachlorophthalic anhydride and urea as the raw materials, hexadecachloro zinc phthalocyanine （ZnPcCl16） was synthesized. Components of the composite were analyzed by energy spectrum, and its functional group structures and absorption peaks were characterized by IR and UV-vis spectroscopy. The thin films of gas sensors were prepared in a vacuum evaporation system and evaporated onto SiO2 substrates, where sensing electrodes were made by MEMS micromachining. The optimal conditions for the films are： substrate temperature of 150 ℃ evaporation current of 95 A and film thickness of 50 nm. The result showed that the sensors were ideally sensitive to Cl2 gas and could detect the minimum concentration of 0.3 ppm.     

Photoreflectance and electrical characterization of Si-implanted GaAs

Si-implanted semi-insulating GaAs was studied by the photoreflectance (PR) technique and electrical analysis. Different energies and doses were used for Si implantation in two groups of samples. Subsequently, different rapid thermal annealing (RTA) conditions were utilized for one group of samples to optimize the damage removal and impurity activation. A significant annealing temperature dependence of the electrical activation and damage removal was observed. The 120 keV Si implantation introduced two impurity-like transitions in the PR spectra at photon energies about 40 meV and 100 meV from the band-edge. The 180 keV implantation introduced two impurity-like signals with energies about 50 meV and 65 meV from the band-edge which could be removed by RTA. Particularly, the 950‡ C, 10s RTA can totally remove the 65 meV signal caused by the 180 keV implantation. Samples exhibiting good PR signal also produced diodes from which electrical data was superior to the other cases.