Hot electron cooling in parabolic and modulation doped quantum wells and doped superlattices

Hot electron cooling in variously structured and doped quantum wells and superlattices has been studied by low temperature steady-state photoluminescence. A parabolic quantum well realized by thickness grading of Al_0.3Ga_0.7As and GaAs epitaxial layers deposited by molecular beam epitaxy with electron level spacings of ∼25 meV did not show increased electron plasma temperatures compared to thick epitaxially deposited GaAs or square quantum wells with electron level spacings greater than the LO phonon energy of GaAs; this implies that mechanisms involving intersubband Δk ≠ 0 transitions and interfacial recombination are dominant in the parabolic structure. Investigations as a function of carrier concentration in modulation-doped quantum wells and n-type superlattices with strong miniband formation indicate that increasing the carrier concentration in either structure above ∼ 5 × 10¹⁷ cm^-3 significantly increases the electron plasma temperatures, even under low light excitation, suggesting that such structures may be suited for high efficiency hot electron photovoltaic and photoelectrochemical cells.     

AlGaAs/InGaAs/GaAs赝配高电子迁移晶体管的kink效应研究

利用二维器件模拟软件MEDICI对AlGaAs/InGaAs/GaAs赝配高电子迁移晶体管器(PHEMT)件进行了仿真，研究了PHEMT器件的掺杂浓度与电子浓度分布，PHEMT器件内部的电流走向及传输特性，重点研究了不同温度和不同势垒层浓度情况下PHEMT器件的kink效应.研究结果表明：kink效应主要与处于高层深能级中的陷阱俘获/反俘获过程有关，而不是只与碰撞电离有关. 关键词： 高电子迁移率晶体管 kink效应 二维电子气     

Unconventional Fractional Quantum Hall States in a Wide Quantum Well

JETP Letters - A bilayer electron system that is formed in a 60-nm-wide GaAs quantum well and has a large difference of the electron densities in the layers has been studied. It has been found...     

Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Spontaneous emission of terahertz radiation from structures with GaAs/AlGaAs quantum wells in a longitudinal magnetic field has been studied. It is shown that some bands in the emission spectrum can be related to radiative electron transitions between resonant and localized impurity states, as well as to the transitions with participation of subband states. The temperature dependence of the equilibrium intraband absorption of terahertz radiation and its modulation in a longitudinal electric field in GaAs/AlGaAs quantum wells has been investigated.     

Microwave-induced magnetic field state with zero conductivity in GaAs/AlAs Corbino disks and hall bars

The microwave photoconductivity of the 2D electron gas in GaAs/AlAs heterostructures has been investigated at a temperature of 4.2 K in magnetic fields up to 1.5 T. It has been found that the magnetic field state with zero conductivity appears in GaAs/AlAs Corbino disks irradiated by 130.70-GHz microwave radiation. This state was previously observed only in GaAs/AlGaAs Corbino disks with much higher electron mobility and lower density. It has been shown that the microwave photoconductivity measured in high magnetic fields on Corbino disks can significantly differ from the value calculated from the results of the measurements on Hall bars. This difference is explained by the fact that the conditions of the appearing magnetoplasmons that affect the magnitude and character of the microwave photoconductivity (photoresistance) in the Corbino disks are nonequivalent to those in the Hall bars.     

Silicon migration during MBE growth of doped (A1, Ga)As films

Silicon migration during MBE growth of (Al, Ga)As and (Al, Ga)As/GaAs or AlAs/GaAs superlattices has been studied by electrochemical C-V and secondary ion mass spectrometry (SIMS) concentration-depth profiling. It is found to be concentration dependent, with no preferential migration towards or away from the growth front. At high concentrations, superlattice disordering during growth is observed using photovoltage and transmission electron microscopy (TEM) techniques. On the basis of C-V, SIMS and TEM data we propose that silicon migration occurs as the result of a concentration-dependent diffusion process. This is substantiated by measurements of the two-dimensional electron-gas mobility in selectively doped heterojunctions as a function of growth temperature and silicon concentration.     

高温Cs激活GaAs光电阴极表面机理研究

研究了高温Cs激活过程中,GaAs光电阴极表面势垒的变化机理。在考虑GaAs材料体内负电性p型掺杂杂质与材料表面正电性Cs+所形成的偶极子对表面势垒的作用后,通过求解均匀掺杂阴极中电子所遵循的一维连续性方程,得到了反射式均匀掺杂阴极的量子效率公式,通过求解薛定谔方程得到了到达阴极表面的光电子的逸出概率公式,利用公式对GaAs光电阴极的Cs激活过程进行了分析。分析发现,激活过程中GaAs光电阴极的量子效率和光电子的逸出概率正比于偶极子层的电场强度。     

Coherent spin dynamics of different density high mobility two-dimensional electron gas in a GaAs quantum well

We have addressed the dependence of quasi-two-dimensional electron spin dephasing time on the electron gas density in a 17-nm GaAs quantum well using the time-resolved magneto-optical Kerr effect. A superlinear increase in the electron dephasing time with decreasing electron density has been found. The degree of electron spin relaxation anisotropy has been measured and the dependence of spin-orbit splitting on electron gas density has been determined.     

Cylindrical GaAs quantum wires incorporated within chrysotile asbestos nanotubes: fabrication and polarized optical absorption spectra

GaAs has been injected into chrysotile asbestos channels. GaAs quantum wires (nanocylinders) with diameters ∼ 6 nm have been observed in the channels by means of transmittance electron microscopy. Polarized optical absorption spectra of asb-GaAs (chrysotile asbestos containing GaAs wires) have been studied. A high anisotropy of the absorption has been observed, and intersubband transitions in the visible light region due to a strong quantum size effect have been found.     

Photoemission from InAs/GaAs quantum dots decorated with cesium adatoms

An array of non-overgrown InAs/GaAs quantum dots has been decorated with adsorbed metal atoms in situ in ultrahigh vacuum. Their electron and photoemission properties have been studied. The radical modification of the spectra of the threshold emission from the quantum dots with increasing cesium coating has been found. Two photoemission channels have been established; they are characterized by considerably different intensities, spectral locations, and widths of the selective bands. It has been shown that the decoration of the quantum dots makes it possible to control the electronic structure and quantum yield of photoemission, the nature of which is related to the excitation of the electronic states of the GaAs substrate and InAs/GaAs quantum dots.     

Magnetoresistance oscillations due to Zener tunneling and microwave radiation in a 2D electron gas in GaAs quantum well with AlAs/GaAs superlattices barriers

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces magnetoresistance oscillations periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the covered frequency range depends on the microwave radiation power.     

Gigantic exchange enhancement of spin g-factor for two-dimensional electron gas in GaAs

Gigantic, oscillatory values of the spin g-factor for two-dimensional electron gas in GaAs have been measured, using the Shubnikov-deHaas effect in GaAs - Al_0.3Ga_0.7As heterojunctions. The effect has been attributed to the exchange interaction and compared with the theory of Ando and Uemura. The observed g-factor enhancement is, together with the fractional quantum Hall effect, the strongest evidence for the decisive role of many-body interactions in the 2D electron gas.     

Oscillations of the magnetoresistance of a two-dimensional electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers in a microwave field

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces oscillations of this magnetoresistance, which are periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the frequency range under study depends on the microwave radiation power.     

Tuneable photoluminescence,raman spectroscopy and electrical properties of GaAs doping superlattices

Optical properties of three GaAs doping superlattices (SLs) were studied by low and high temperature photoluminescence (PL) as well as inelastic light scattering. A new optically active transition has been observed at 1.48 eV and the high temperature behaviour of the main e–(Zn)h band has been studied. Electrical properties were monitored by current-voltage, resistivity and Hall measurements. This showed a strong variation of carrier concentration in the p-layers associated with mixed electron–hole conduction. The influence of α-particle irradiation on the superlattice structures was also investigated.     

Negative magnetoresistance of a high-mobility two-dimensional electron gas in a nonlinear regime

The effect of the measuring current I_dc on the magnetoresistance (MR) of a high-mobility two-dimensional electron gas (2DEG) in a GaAs quantum well with AlAs/GaAs superlattice barriers has been studied. It has been found that, as I_dc increases, the MR of the 2DEG in the studied structures becomes negative in the range of classically strong magnetic fields. It has been shown that the observed negative MR is due to the transport of the 2DEG in the nonlinear regime.     

The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed.     

The Effect of Point Defects on the Activation Processes in Semiinsulating GaAs Irradiated by Si28 Ions

The layered concentration, concentration profile, and mobility of electrons in the Si²⁸ ion-implanted layers (IIL) of semiinsulating GaAs are investigated. The specific resistance of the latter is also studied upon radiation annealing (RA) in the temperature range 590–800 °C using electron energies higher than the threshold energy of defect formation. The IIL are shown to form during RA at much lower temperatures. The layers exhibit high electrical activation of Si²⁸ ions, with the electron-concentration profile corresponding to the calculated one, and a low concentration of residual defects limiting the electron mobility. The radiation annealing increases the resistance of semiinsulating GaAs. The calculations show that these effects are due to the Frenkel pairs (FP) generated by radiation. A high degree of ionization of GaAs atoms significantly reduces energies of potential barriers of diffusion, FP recombination, and electrical impurity activation.     

Anisotropy of magnetic transport and self-organization of corrugated heterointerfaces in selectively doped structures on GaAs(100) substrates

The anisotropy of the longitudinal magnetoresistance has been found for a two-dimensional electron gas with a high mobility and concentration in GaAs quantum wells grown by molecular beam epitaxy on GaAs(100) substrates. The experimental data obtained are explained by the self-organization of spatially modulated heterointerfaces and are in agreement with the results of studying growth surfaces by atomic force microscopy.     

砷化镓光阴极直流高压注入器研究进展

主要介绍自由电子激光相干强太赫兹源(FEL-THz)装置上的砷化镓光阴极直流高压注入器的研究进展,并讨论其驱动未来高重复频率短波长自由电子激光器的差距。通过综合砷化镓阴极寿命的三大影响因素,提出了其工作寿命的定性物理模型;通过该模型对阴极和注入器进行优化,在直流高压电子枪上得到了5mA,32min的连续稳定输出;测量了电子束在4.8mA下归一化发射度约为4.0πmm·mrad,阴极热发射度约为0.6πmm·mrad,电子束本征横向能量约为92meV,250keV电子束在距离阴极90.6cm处纵向均方根长度约为11.5ps。这一束流状态已经基本满足FEL-THz需求。     

砷化镓光阴极直流高压注入器研究进展

主要介绍自由电子激光相干强太赫兹源（FEL-THz）装置上的砷化镓光阴极直流高压注入器的研究进展,并讨论其驱动未来高重复频率短波长自由电子激光器的差距。通过综合砷化镓阴极寿命的三大影响因素,提出了其工作寿命的定性物理模型;通过该模型对阴极和注入器进行优化,在直流高压电子枪上得到了5 mA, 32 min的连续稳定输出;测量了电子束在4.8 mA下归一化发射度约为4.0 mmmrad,阴极热发射度约为0.6 mmmrad,电子束本征横向能量约为92 meV,250 keV电子束在距离阴极90.6 cm处纵向均方根长度约为11.5 ps。这一束流状态已经基本满足FEL-THz需求。