Generation-recombination noise in pseudomorphic Modulation-doped Al/sub 0.2/Ga/sub 0.8/As/In/sub 0.1/Ga/sub 0.9/As/GaAs micro-Hall devices

The noise spectrum in micro-Hall devices based on pseudomorphic Al/sub 0.2/Ga/sub 0.8/As/In/sub 0.1/Ga/sub 0.9/As/GaAs modulation-doped heterostructures was measured between 4 Hz and 65 kHz, allowing components due to thermal, 1/f, and generation-recombination to be characterized. Applying deep level noise spectroscopy (DLNS) in the temperature range of 77-300 K to analyze the generation-recombination part of the spectrum, two electron traps contributing to noise density were identified. An emission activation energy of 474 meV was measured for the dominant trap, corresponding to the well-known DX center originating from the AlGaAs barrier. The other deep level, with an emission activation energy of 242 meV, is probably related to defects in the InGaAs layer. The structures under investigation resulted in high-performance micro-Hall devices: a supply-current-related sensitivity up to 725 V/spl middot/A/sup -1//spl middot/T/sup -1/ at 77 K independent of bias current, a signal-to-noise sensitivity of 155 dB/spl middot/T/sup -1/ and a detection limit of 340 pT/spl middot/mm/spl middot/Hz/sup -1/2/ at 77 K were measured.     

InAs quantum dots capped by GaAs,In0.4Ga0.6As dots,and In0.2Ga0.8As well

We have fabricated and characterized three types of InAs quantum dots (QDs) with different InxGa1-xAs capping layers. Post-growth atomic force microscopy measurements show that the In0.2Ga0.8As/InAs structure has a smooth surface (dot-in-well structure), whereas the In0.4Ga0.6As/InAs structure revealed large QDs with a density similar to that underneath InAs QDs on GaAs (dot-in-dot). With increasing In mole fraction of the capping layer and increasing In0.4Ga0.6As thickness, the energy position of the room-temperature photoluminescence (PL) peak is red-shifted. The quantum dot-in-dot structure emits stronger room-temperature PL than does the quantum dot-in-well structure. With a spatially distributed strain in the InAs quantum dot, we have solved the three-dimensional Schr?dinger equation by the Green's function theory for the eigenvalues and eigen wave functions. It is concluded that the ground state increases its wave function penetration into the low-barrier InxGa1-xAs capping layer so that its energy position is red-shifted. The reduced PL peak intensity of the dot-in-well (compared with GaAs covered dots) is due to the reduced overlapping between the ground state and the extended states above the GaAs barrier. The overlapping reduction in the dot-in-dot is over compensated for by the reduced relaxation energy (full width at half-maximum), indicating the importance of the sample quality in determining the PL intensity.     

Al0.15Ga0.85As／GaAs与In0.15Ga0.85As／GaAs甚长波QWIPs响应率的仿真对比

对In0.15Ga0.85As／GaAs和Al0.15Ga0.85As／GaAs两种甚长波段量子阱红外探测器（QwIP）响应率进行计算。采用物理模型与等效电路模型，结合Crosslight和Spice等软件详细表征了这两种QWIPs的吸收系数、暗电流、响应率、量子效率等物理特性。结果表明随外加偏压的升高QWIP的响应率增加，T=40K时，In0.15Ga0.85As／GaAs QwIP的响应率明显比Al0.15Ga0.85As／GaAs QWIP高出2倍以上，通过对量子效率的对比，使仿真结果得到验证。     

Photoluminescence of heterostructures with highly strained GaAs quantum wells in Al0.48In0.52As and Ga0.47In0.53As layers

Heterostructures comprising highly strained GaAs quantum wells in Al_0.48In_0.52As and Ga_0.47In_0.53As layers have been grown by the metalorganic chemical vapor deposition method on InP(100) substrates. The photoluminescence spectra of these structures have been studied, and it is established that GaAs quantum wells form type-I heterojunctions with Al_0.48In_0.52As layers and type-II heterojunctions with Ga_0.47In_0.53As layers.     

Application of selective implantation in Al0.5Ga0.5As/In0.25Ga0.75As/GaAs pseudomorphic single quantum wire structures

A pseudomorphic Al0.5Ga0.5As/In0.25Ga0.75As/GaAs asymmetric quantum wire (QWR) structure was grown on GaAs V-grooved substrate by low pressure metal organic vapor phase epitaxy. The formation of crescent shaped QWRs at the bottom of the V-grooves was confirmed by both transmission electron microscopy and photoluminescence (PL) spectra. The temperature dependence of PL spectra demonstrated a fast decrease of the sidewall quantum well PL intensity with increasing temperature, which originates from relaxation of carriers from well to wire region. The self-aligned dual implantation technique was successfully used to selectively disable the adjacent quantum structures. Decrease of the PL intensity of QWR at 8 K was observed after selective implantation, which resulted from a decreased number of carriers relaxed from adjacent quantum structures.     

Positive delayed photoconductivity in double heterostructures Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As of the p-type

Positive delayed photoconductivity was observed for the first time in double p-type heterostructures Al_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5 As upon exposure to the radiation of a red light-emitting diode. In this state, the concentration and mobility of two-dimensional holes are increased 1.5 and 1.7 times, respectively, as compared to the initial dark values. The delayed photoconductivity can be explained by the presence of deep electron traps located above the Fermi level at the inverted heterointerface.     

Quantum Interference above 4.2 K in a GaAl0.3As0.7/GaAs Aharonov-Bohm Ring

Magneto-conductance measurements of a micron sized GaAI_0.3As_0.7/GaAs Aharonov–Bohm devicehas been performed at temperatures above T=4.2 K, inthe regime where only a few transverse modes are occupied. Wefind that the Aharonov–Bohm oscillations are still visible atthese relatively high temperatures. The electron density ofthe Aharonov–Bohm device was during the measurementscontrolled via a gate voltage applied to a small stub whichwas attached to one arm of the ring. Due to this highlyasymmetric gate configuration the electron density in the ringis strongly asymmetric, and hence an oscillating conductanceas a function of gate voltage is expected—in analogy withthe Mach–Zender interferometer.     

Lasing characteristics of lasers with a vertical cavity based on In0.2Ga0.8As quantum wells

Semiconductor lasers with a vertical cavity with a high external quantum efficiency and high radiation power have been developed and constructed. Powers up to 10 W at T=300 K and 20 W at T=250 K have been obtained for 500 μm aperture lasers operating in the pulsed regime. Pis’ma Zh. Tekh. Fiz. 25, 40–44 (October 12, 1999)     

Surface morphology of Al0.3Ga0.7N/Al2O3-high electron mobility transistor structure

We present surface properties of buffer films (AIN and GaN) and Al0.3Gao.zN/Al2O3-High Electron Mobility Transistor (HEMT) structures with/without AIN interlayer grown on High Temperature (HT)-AIN buffer/Al2O3 substrate and Al2O3 substrate. We have found that the GaN surface morphology is step-flow in character and the density of dislocations was about 10(8)-10(9) cm(-2). The AFM measurements also exhibited that the presence of atomic steps with large lateral step dimension and the surface of samples was smooth. The lateral step sizes are in the range of 100-250 nm. The typical rms values of HEMT structures were found as 0.27, 0.30, and 0.70 nm. HT-AIN buffer layer can have a significant impact on the surface morphology of Al0.3Ga0.7N/Al2O3-HEMT structures.     

Excellent compositional homogeneity in In0.3Ga0.7As crystals grown by the traveling liquiduszone (TLZ) method

The influence of convection in a melt on the compositional homogeneity of the TLZ-grown In_0.3Ga_0.7As crystals has been investigated by growing crystals with various dimensions on the ground. Excellent compositional homogeneity such as 0.3 plus or minus 0.01 in InAs mole fraction for a distance of 25 mm was obtained when the melt diameter was limited to 2 mm and convective flow in the melt was suppressed. On the other hand, when the crystal diameter was increased to 10 mm, both axial and radial compositional homogeneity was deteriorated due to convection in the melt. Comparing with the numerical simulation, convective flow velocity less than 1.4 mm/h may be sufficient for growing homogeneous crystals and it is not so difficult to suppress convective flow velocity below 1.4 mm/h for 10 mm diameter crystals in microgravity. Therefore, larger homogeneous In_0.3Ga_0.7As crystals are expected to be grown by the TLZ method on board the International Space Station.     

Structural Control of Rashba Spin–Orbit Coupling in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As Quantum Wells

We investigated the correlation between the Rashba spin–orbit coefficient α and potential shape of the quantum wells (QW), where α values are experimentally deduced from the weak antilocalization analysis. We studied the gate I–V properties of the QW samples and have obtained results consistent with the potential shapes predicted for these QWs.     

Catalyst-free synthesis of well-aligned ZnO nanowires on In0.2Ga0.8N, GaN, and Al0.25Ga0.75N substrates

Well-aligned ZnO nanowires have been synthesized vertically on In0.2Ga0.8N, GaN, and Al0.25Ga0.75N substrates, using a catalyst-free carbon thermal-reduction vapor phase deposition method for the first time. The as-synthesized nanowires are single crystalline wurtzite structure, and have a growth direction of [0001]. Each nanowire has a smooth surface, and uniform diameter along the growth direction. The average diameter and length of these nanowires are 120-150 nm, and 3-10 )m, respectively. We suggest that the growth mechanism follow a self-catalyzing growth model. Excitonic emission peaked around 385 nm dominates the room-temperature photoluminescence spectra of these nanowires. The room-temperature photoluminescence and Raman scattering spectra show that these nanowires have good optical quality with very less structural defects.     

Polaronic relaxation in La0.8Bi0.2Fe0.7Mn0.3O3

In this work, the dielectric properties of La_0.8Bi_0.2Fe_0.7Mn_0.3O₃ ceramics have been investigated in a temperature range of 76–320 K and a frequency range of 300 Hz–10 MHz. Two thermally activated dielectric relaxations were observed with the activation energy around 0.283 ± 0.007 eV for the low-temperature relaxation and 0.268 ± 0.007 eV for the high-temperature relaxation. Annealing in N₂ and O₂ can destroy and create the high-temperature relaxation, respectively. But the treatments have no significant influence on the low-temperature relaxation. The low-temperature relaxation was found to be bulk effect related to the dipolar effect due to the hopping polarons, and the high-temperature relaxation was associated with the Maxwell–Wagner relaxation due to surface-layer effect caused by hopping polarons blocked and trapped at the surfaces of the sample.     

Magneto-electric coupling in multiferroic La0.8Bi0.2Fe0.7Mn0.3O3 ceramic

We present here the structural, electrical, magnetic and magneto-electric properties of La_0.8Bi_0.2Fe_0.7Mn_0.3O₃ multiferroic sample synthesized by conventional solid state reaction route. X-ray diffraction (XRD) data show single phase character of the sample. Temperature (150-475 K) variation of dielectric constant exhibits step like escalation which consists of two transitions corresponding to the respective peaks in temperature vs. loss tangent curve. Slight shift in peak positions to higher temperatures with the increase of frequency indicates the presence of relaxor behavior. Magnetic field dependent magnetization (M-H) curve at 300 K demonstrates weak ferromagnetic conduct. Coupling between the electric and magnetic orders has been explored by means of magneto-capacitance measurement. Presence of magneto-electric (ME) coupling ascertains the candidature of this material for device application.