Fermi edge singularity evidence from photoluminescence spectroscopy of AlGaAs/InGaAs/GaAs pseudomorphic HEMTs grown on (3 1 1)A GaAs substrates

InGaAs/AlGaAs/GaAs pseudomorphic high electron mobility transistor (P-HEMT) structures were grown by Molecular Beam Epitaxy (MBE) on (3 1 1)A GaAs substrates with different well widths, and studied by photoluminescence (PL) spectroscopy as a function of temperature and excitation density.The PL spectra are dominated by one or two spectral bands, corresponding, respectively, to one or two populated electron sub-bands in the InGaAs quantum well. An enhancement of PL intensity at the Fermi level energy (E_F) in the high-energy tail of the PL peak is clearly observed and associated with the Fermi edge singularity (FES). This is practically detected at the same energy for all samples, in contrast with energy transitions in the InGaAs channel, which are shifted to lower energy with increasing channel thickness. PL spectra at low temperature and low excitation density are used to optically determine the density of the two-dimensional electron gas (2DEG) in the InGaAs channel for different thicknesses. The results show an enhancement of the 2DEG density when the well width increases, in good agreement with our previous theoretical study.     

Optical and transport properties of δ-doped pseudomorphic AlGaAs/InGaAs/GaAs structures

We investigate the effects of spacer layer thickness on the optical and transport properties of the n-type-doped pseudomorphic Al_0.30Ga_0.70As/In_0.15Ga_0.85As / GaAs structures. A-doped AlGaAs/InGaAs/GaAs structure with a 6nm spacer layer yields a sheet carrier concentration of 1.5×10¹² cm^–2 at 77K with electron mobility of 6.4×10³ cm²/Vs, 3.11×10⁴ cm²/Vs, and 3.45×10⁴ cm²/Vs at room temperature, 77 and 20K, respectively. The effects of the different scattering mechanisms on luminescence linewidth and electron mobility have also been discussed.     

Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures

An original time resolved cathodoluminescence set up has been used to investigate the optical properties and the carrier transport in quantum structures located in InGaAs/AlGaAs tetrahedral pyramids. An InGaAs quantum dot formed just below the top of the pyramid is connected to four types of low-dimensional barriers: InGaAs quantum wires on the edges of the pyramid, InGaAs quantum wells on the (111)A facets and segregated AlGaAs vertical quantum wire and AlGaAs vertical quantum wells formed at the centre and at the pyramid edges. Experiments were performed at a temperature of 92 K, an accelerating voltage of 10 kV and a beam probe current of 10 pA. The cathodoluminescence spectrum shows five luminescence peaks. Rise and decay times for the different emission wavelengths provide a clear confirmation of the peak attribution (previously done with other techniques) to the different nanostructures grown in a pyramid. Moreover, experimental results suggest a scenario where carriers diffuse from the lateral quantum structures towards the central structures (the InGaAs quantum dot and the segregated AlGaAs vertical quantum wire) via the InGaAs quantum wires on the edges of the pyramid. According to this hypothesis, we have modeled the carrier diffusion along these quantum wires. An ambipolar carrier mobility of 1400 cm²/V s allows to obtain a good fit to all temporal dependences. PACS 78.67.-n     

Oscillatory behavior of magneto-optical interband emissions in asymmetric quantum well structures

We report on the oscillatory behavior of the photoluminescence intensity from asymmetric AlGaAs/InGaAs/GaAs quantum well structures in the presence of a perpendicular magnetic field. Two distinct photoluminescence peaks originating from transitions from the ground (e₁) and the first excited (e₂) electronic states to the heavy hole state (hh₁) are observed. The opposite phase of the oscillations shows clearly the competitive process between the transitions from the ground and first excited states. Electron transfer mechanisms cannot explain the origin of these oscillations. The optical oscillations emerge from changes in the effective electron–hole interaction.     

A bistable, self-latching inverter by the monolithic integration of resonant tunnelling diode and high electron mobility transistor

This paper reports that the structures of AlGaAs/InGaAs high electron mobility transistor (HEMT) and AlAs/GaAs resonant tunnelling diode (RTD) are epitaxially grown by molecular beam epitaxy (MBE) in turn on a GaAs substrate. An Al_0.24Ga_0.76As chair barrier layer, which is grown adjacent to the top AlAs barrier, helps to reduce the valley current of RTD. The peak-to-valley current ratio of fabricated RTD is 4.8 and the transconductance for the 1-μm gate HEMT is 125mS/mm. A static inverter which consists of two RTDs and a HEMT is designed and fabricated. Unlike a conventional CMOS inverter, the novel inverter exhibits self-latching property.     

Metamorphic growth of InAlAs/InGaAs MQW and InAs HEMT structures on GaAs

The large electron mobility at room temperature and the absence of Schottky barrier to metals make InAs two-dimensional electron gas (2DEG) a good candidate for SPIN-FET (FET—field-effect-transistor) applications. So far the growth was done either on the InAlAs epilayers compositionally matched to InP substrates, or on Sb-based compound semiconductors. Here we aim to grow InAs 2DEG on GaAs substrates by using a strain-relaxing buffer layer. We introduce In_0.4Al_0.6As glue layer to the metamorphic structure and investigate the physical properties of InAlAs/InGaAs multi-quantum-well (MQW) structures via X-ray diffraction, transmission electron microscopy, and photoluminescence. We find that the use of As dimer instead of tetramer and the choice of proper growth temperature are essential for successful growth. InAs-inserted-channel InAlAs/InGaAs inverted high-electron-mobility transistor (HEMT) structures show promising results for SPIN-FET application.     

Quantum wire heterostructure for optoelectronic applications

Quantum wire (QWR) heterostructures suitable for optoelectronic applications should meet a number of requirements, including defect free interfaces, large subband separation, long carrier lifetime, efficient carrier capture. The structural and opticl properties of GaAs/AlGaAs and InGaAs/GaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on nonplanr substrates, which satisfy many of these criteria, are described. These crescent-shaped QWRs are formed in situ during epitaxial growth resulting in virtually defect free interfaces. Effective wire widths as small as 10nm have been achieved, corresponding to electron subband separations greater than K_BT at room temperature. The enhanced density of states at the QWR subbands manifests itself in higher optical absorption and emission as visualized in photoluminescence (PL), PL excitation, amplified spontaneous emission and lasing spectra of these structures. Effective carrier capture into the wires via connected quantum well regions, which is important for enhancing the otherwise extremely small capture cross section of these wires, has also been observed. Room temperature operation of GaAs/AlGaAs and strained InGaAs/GaAs QWR lasers with threshold currents as low as 0.6mA has been demonstrated.     

Si-delta doping and spacer thickness effects on the electronic properties in Si-delta-doped AlGaAs/GaAs HEMT structures

In this work, we investigate the effect of the δ-Si doping on the barrier and the spacer thickness on the electronic properties of AlGaAs/GaAs HEMTs structures grown by molecular beam epitaxy on (1 0 0) oriented GaAs substrates. Photoluminescence measurements as function of the temperature are used to determine the relaxation processes of the electron and the hole in the channel. The photoluminescence characterizations of Si-delta-doped AlGaAs/GaAs HEMTs structures have been studied in the 10–300 K temperature range. Low temperature PL spectra show the optical transition (E_e–h) that occurs between the fundamental states of electrons to holes in the GaAs channel. Increase of the Si-δ-doping density and decrease of the spacer width improve the two-dimensional electron gas confinement and decrease defects densities in the canal. The band structure of Si-delta-doped AlGaAs/GaAs HEMTs structures at T = 10 K has been studied theoretically using the finite differences method to self-consistently and simultaneously solve Schrödinger and Poisson equations written within the Hartree approximation.     

调制掺杂GaAs/AlGaAs 2DEG材料持久光电导及子带电子特性研究

在掺Si的GaAs/AlGaAs二维电子气(2DEG)结构中，得到μ_2K=1.78×10^{6cm2/(V·s)的高迁移率.在低温(2K)和高磁场(6T)的条件下，对样品进行红 光辐照，观察到持久光电导(PPC)效应，电子浓度在光照后显著增加.通过整数量子霍尔效应 (IQHE)和Shubnikov-de Haas (SdH)振荡的测量，研究了2DEG的子带电子特性.样品在低温光 照后2DEG中第一子带和第二子带的电子浓度同时随电子总浓度的增加而增加；而且电子迁移 率也明显提高.同时，通过整数霍尔平台的宽度对光照前后电子的量子寿命变短现象作了理 论分析. 关键词： 二维电子气 量子霍尔效应 SdH振荡 持久光电导效应}     

Surface monitoring of HEMT structures

The data of Atomic Force Microscopy (AFM) surface monitoring and of mobility investigations in the HEMT structures based on the AlGaAs/InGaAs/GaAs heterostructures with pseudomorphic InGaAs channel are presented. The level of HEMT nanomaterial self-organization (LNSO) were obtained using the treatment the AFM data by the method of multifractal analysis. Direct correlation between the mobility values and LNSO was found for the most of studied HEMT structures that reflect the close relation between the LNSO values and the HEMT bulk properties. In the structures with LNSO changing from 1.13 to 1.45 the 1.5 time mobility decrease at 300 K and 4 times at 77 K was observed. The results obtained allow to suppose that the LNSO calculation based on surface monitoring of HEMT structures can be used for the optimization of their parameters and of their formation process.     

Enhanced photoluminescence intensity of 1.3-μm multi-layer InAs/InGaAs dots-in-well structure using the high growth temperature spacer layer step

The effects of growth temperature of the GaAs spacer layers (SPLs) on the photoluminescence (PL) efficiency of multi-layer GaAs-based 1.3-μm InAs/InGaAs dots-in-well (DWELL) structures have been investigated. It is found that the PL intensity of DWELLs is enhanced by incorporating a high growth temperature step for GaAs SPLs. This improved PL efficiency could be understood in term of reducing the non-radiative recombination centers. An extremely low continuous-wave room-temperature threshold current density of 35 A/cm² is achieved for an as-cleaved 5-layer device with emission at 1.31 μm by using this growth technique.     

Study of two-dimensional hole gas concentration and hole mobility in zinc delta-doped GaAs and pseudomorphic GaAs/In0.2Ga0.8As heterostructures

Zinc delta-doped GaAs and pseudomorphic GaAs/In}_0.2Ga_0.8As heterostructures grown by low-pressure metalorganic chemical vapour deposition have been demonstrated. The influence of delta-doping period and spacer thickness on two-dimensional hole gas concentrations and hole mobility was studied. From secondary-ion mass spectroscopy and Hall measurement, we conclude that zinc delta-doping can form an excellent abrupt profile (full-width at half maximum is of 10 nm) and offer a high two-dimensional hole gas sheet density (as high as 1 × 10¹³cm⁻²) By adopting a strained InGaAs material as the active channel and by carefully modulating the spacer layer thickness, one can obtain a significantly enhanced hole mobility.     

Catastrophic degradation of pulsed lasers based on AlGaAs/InGaAs/GaAs heterostructures with electron-beam pumping

Catastrophic degradation of pulsed lasers based on InGaAs/AlGaAs/GaAs structures with different design of the active domain with transverse pumping by the electron beam at T = 300 K is studied. In lasers based on structures with a InGaAs single quantum well and with seven quantum wells, the maximal values of pulsed power are 70–90 and 10–20 W, respectively.     

Absorption and photoluminescence studies of the temperature dependence of exciton life time in lattice-matched and strained quantum well systems

We present systematic studies of the temperature dependence of linewidths and lifetimes of excitonic transitions in quantum wells grown by molecular beam epitaxy using both photoluminescence(PL) and optical absorption. The temperature ranged from 6K to room temperature. Samples under investigation were lattice-matched GaAs/AlGaAs and InGaAs/InAlAs, and strained InGaAs/GaAs and InGaAs/AlGaAs quantum wellssystems. In addition, the effects of well-size variations in GaAs/AlGaAs quantum wells were measured and analyzed. In all cases we were able to observe the excitonic transitions up to room temperature. By a careful fitting of the experimental data we separated the exciton transitions from band-to-band transitions. By deconvoluting the excitonic transitions we obtained the homogeneous and inhomogeneous linewidths. The homogeneous linewidths were used to calculate the exciton lifetimes as a function of temperature using the Heisenberg uncertainty principle. We found the lifetime decreases significantly with temperature and increases with increasing well size. These results are interpreted in terms of the exciton-phonon interaction and are expected to be very useful for the design of semiconductor optical devices operating at different temperatures.     

Improvement of the optical property and uniformity of self-assembled InAs/InGaAs quantum dots by layer-by-layer temperature and substrate rotation

The influence of layer-by-layer temperature and substrate rotation on the optical property and uniformity of self-assembled InAs/In_0.2Ga_0.8As/GaAs quantum dots (QDs) gown with an As₂ source was investigated. An improvement in the optical property of QDs was obtained by the precise control and optimization of growth temperature utilized for each layer, i.e., InAs QDs, InGaAs quantum wells, GaAs barriers and AlGaAs layers, respectively. By using a substrate rotation, the QD density increased from ∼1.4×10¹⁰ to ∼3.2×10¹⁰ cm⁻² and its size also slightly increased, indicating a good quality of QDs. It is found that the use of an appropriate substrate rotation during growth improves the room-temperature (RT) optical property and uniformity of QDs across the wafer. For the QD sample with a substrate rotation of 6 rpm, the RT photoluminescence (PL) intensity is much higher and the standard deviation of RT-PL full-width at half-maximum is decreased by 35% compared to that grown without substrate rotation.     

Mobility enhancement in MBE-grown InxGa1−xAs/In0.52Al0.48As modulation-doped heterostructures

Pseudomorphic In_xGa_1−xAs/In_0.52Al_0.48As modulation-doped heterostructures were grown by molecular beam epitaxy (MBE) on InP (100) substrates over a range of indium compositions fromx=0.53 to 0.75. Low temperature photoluminescence (PL) measurements show a prominent reduction in the InGaAs linewidth due to the quantum-size effect as the indium composition is increased from its lattice-matched value of 0.53. The lowest linewidth of 6.8 meV was achieved at an indium composition of 0.65, above which an increase in the linewidth was observed due to the overwhelming effects of interfacial strain. The Hall mobilities at 300 K and 77 K increase in correspondence to the PL linewidth reduction as the indium composition is increased. Although initial signs of mobility saturation can be seen at an indium composition of 0.65, the peak mobility at 77 K of 8.9×10⁴cm²V s⁻¹was achieved at an indium composition of 0.70. There is experimental evidence to indicate that the mobility enhancement at increasing indium composition is due to an effect of a reduction in the alloy scattering and in the effective mass of the carriers. It was found that the insertion of an additional In_0.53Ga_0.47As interface smoothing layer between the strained InGaAs channel and the In_0.52Al_0.48As spacer layer did not have a significant effect on the mobility enhancement in the heterostructures.     

退火及铟含量对InGaAs量子点光学特性的影响

 研究了退火条件和In组份对分子束外延生长的InGaAs量子点（分别 以GaAs或AlG aAs为基体）光学特性的影响。表明：量子点中In含量的增加将导致载流子的定域能增加和基态与激发态之间的能量间隔增大。采用垂直耦合的量子点及宽能带的AlGaAs基体可增 强材料的热稳定性。以AlGaAs为基体的InGaAs量子点,高温后退火工艺 (T= 830℃)可改善低温生长的AlGaAs层的质量,从而改善量子点激光器材料的质量。     

GaAs(111)A/B surface orientation effects on electron density in normal and inverted pseudomorphic HEMTs

We present calculations of the two-dimensional electron density in a Si -doped AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (P-HEMT), at low temperature, for three different growth directions (001) and (111)A/B. The calculations are made using a self-consistent resolution of Schrödinger and Poisson equations. The presence of a strong built-in piezoelectric field in (111)A/B growth directions causes changes of the confining potential shape and the carrier distribution in the InGaAs channel. We discuss the influence of GaAs substrate orientation on the conduction-band structure and thereafter on the two-dimensional electron gas (2DEG) concentration in the channel. Our results show that the calculated 2DEG concentration in the normal P-HEMT structure grown on a (111)A GaAs substrate is significantly higher than those grown on (001) and (111)B GaAs substrates. We also note an increase of the average separation between the ionized donors and the carriers. On the other hand, the GaAs (111)B substrate orientation appears as inadequate for the type of structure (normal P-HEMT) on account of the charge-transfer reduction in the channel compared with the (001) orientation. In contrast, we demonstrate that the calculated 2DEG Si -doped GaAs/InGaAs/AlGaAs pseudomorphic inverted high electron mobility transistor (PI-HEMT) grown on aGaAs (111)B substrate is appreciably higher than that grown on (001) and afterwards an enhancement of the spatial separation between confined electrons in the channel and ionized dopants occurs. These effects might result in considerably improved devices of great interest regarding high electron mobility. PACS 73.20.Dx; 73.20.At; 73.90.+f; 73.63.Hs; 72.20.Dp     

Direct measurements of energy relaxation times on an AlGaAs/GaAs heterointerface in the range 4.2–50 K

The temperature dependence of the energy relaxation time τ_e (T) of a two-dimensional electron gas at an AlGaAs/GaAs heterointerface is measured under quasiequilibrium conditions in the region of the transition from scattering by acoustic phonons to scattering with the participation of optical phonons. The temperature interval of constant τ_e, where scattering by the deformation potential predominates, is determined. In the preceding, low-temperature region, where piezoacoustic and deformation-potential-induced scattering processes coexist, τ _e decreases slowly with increasing temperature. Optical phonons start to participate in the scattering processes at T∼25 K (the characteristic phonon lifetime was equal to τ_LOτ4.5 ps). The energy losses calculated from the τ_e data in a model with an effective nonequilibrium electron temperature agree with the published data obtained under strong heating conditions. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 371–375 (10 September 1996)     

Quantum point contacts on InGaAs/InP heterostructures

For the first time we have observed quantized conductance in a split gate quantum point contact prepared in a strained In_0.77Ga_0.23As/InP two-dimensional electron gas (2DEG). Although quantization effects in gated two-dimensional semiconductor structures are theoretically well known and proven in various experiments on AlGaAs/GaAs and also on In_0.04Ga_0.96As/GaAs, no quantum point contact has been presented in the InGaAs/InP material with an indium fraction as high as 77% so far. The major problem is the comparatively low Schottky barrier of the InGaAs (φ_B≈ 0.2 eV) making leakage-free gate structures difficult to obtain. Nevertheless this heterostructure—especially with the highest possible indium content—has remarkable properties concerning quantum interference devices and semiconductor/superconductor hybrid devices because of its large phase coherence length and the small depletion zone, respectively. In order to produce leakage-free split gate point contacts the samples were covered with an insulating SiO₂layer prior to metal deposition. The gate geometry was defined by electron-beam lithography. In this paper we present first measurements of a point contact on an In_0.77Ga_0.23As/InP 2DEG clearly showing quantized conductance.