Single Quantum Well Structures Research Articles

Growth of high quality strained AlxGa1−xAs/In0.26Ga0.74As/AlzGa1−zAs quantum wells and the effect of silicon nitride encapsulation and rapid thermal annealing

We report on the realization of high quality single quantum wells (SQWs) in the highly strained AlxGa1−xAs/In0.26Ga0.74As/AlzGa1−zAs system with 0≤(x, z)≤1.0. Photoluminescence (PL) linewidths of 5.5±0.4 meV for 0.30≤(x and z)≤0.70 have been achieved. Use of both alloys and short period multiple layer structures as the well and/or barrier layers has been examined. The silicon nitride encapsulation as well as rapid thermal annealing (RTA) induced changes in the PL properties of the as-grown SQW structures have been examined. Deposition of the nitride is found to induce a blue shift in the exciton peak. RTA induces a further blue shift, though not as large as that induced by RTA of unencapsulated (i.e., as-grown) structures. The RTA induced changes indicate interdiffusion of the group III atoms at the GaAs/InGaAs and InGaAs/AlGaAs interfaces.

In situ patterning and overgrowth for the formation of buried GaAs/AlGaAs single quantum-well structures

Buried GaAs/AlGaAs single quantum-well structures have been fabricated for the first time by in situ electron beam (EB) lithography. The process includes the molecular beam epitaxy of a GaAs/AlGaAs single quantum well, electron-beam direct writing, Cl2 gas etching, and overgrowth of an AlGaAs layer. A thin GaAs oxide layer was used as the etching mask, which was selectively formed on a clean GaAs surface by EB irradiation under an O2 ambient. Subsequent Cl2 gas etching resulted in the formation of isolated quantum wells. Prior to the overgrowth, thermal cleaning with atomic hydrogen was employed for removing the oxide mask. The cathodoluminescence image of the buried quantum well demonstrates the high quality of the resultant structure formed by in situ EB lithography.

Study of high-quality ZnSe/GaAs/ZnSe single quantum well and ZnSe/GaAs heterostructures

We have used the migration-enhanced epitaxy (MEE) technique to grow high-quality ZnSe/GaAs/ZnSe single quantum well (SQW) structures and ZnSe/GaAs heterostructures. The atomic layer controlled epitaxy of ZnSe-on-GaAs and the question of growth-rate/MEE cycle for ZnSe growth was also investigated. The first observation of luminescence from ZnSe/GaAs/ZnSe SQW structures is reported.

Quantum-well structures of InAlP/InGaP grown by gas-source molecular-beam epitaxy

Both single quantum-well (SQW) and multiple quantum-well (MQW) structures have been produced using the technique of gas-source molecular-beam epitaxy to grow the two wide band-gap ternary alloys, InAlP and InGaP. SQWs as narrow as two monolayers observed by bright field Transmission Electron Microscopy were found to be laterally uniform with abrupt InAlP/InGaP interfaces. Photoluminescence of SQWs of differing thickness produced a larger quantum confinement energy shift than expected, with emission at 570 nm for an InGaP well of 3.0 nm in thickness. The number and amplitude of peaks detected in double-crystal x-ray diffraction (DCXR) measurements of the MQW samples matched, to within the limit of the dynamic range of the DCXR system, the peaks calculated in a periodic two-layer dynamical simulation of the x-ray rocking curve.

Single and multiple AlGaAs quantum-well structures grown by liquid-phase epitaxy

The Al0.05Ga0.95As/Al0.35Ga0.65As single- and multiple-quantum-well structures with well widths less than 20 Å have been successfully fabricated by liquid-phase epitaxy using low-temperature, two-phase, and dummy wafer methods. The transmission electron microscope cross-section technique is applied to study the thickness variation of quantum well and the transition width at the heterojunction interface. An interesting phenomenon observed during liquid-phase epitaxy growth is that the interface of Al0.05Ga0.95As grown on Al0.35Ga0.65As is rather flat and sharp whereas the opposite growth sequence gives rise to a wavy interface. It is believed that the flat Al0.05Ga0.95As/Al0.35Ga0.65As interface is due to a tendency to deposit the epilayer when the Al0.05Ga0.95As solution is in contact with the Al0.35Ga0.65As substrate and the wavy Al0.35Ga0.65As/ Al0.05Ga0.95As interface is due to a tendency to dissolve the substrate when the Al0.35Ga0.65As solution is in contact with the Al0.05Ga0.95As substrate. The transition width is less than 10 Å at the Al0.05Ga0.95As/Al0.35Ga0.65As interface but is about 30–50 Å at the inverted Al0.35Ga0.65As/ Al0.06Ga0.95As interface. These values are about the same as those reported in the recent literature. The peak energy of the photoluminescence spectrum reveals the quantum size effect.

Experimental studies of misfit dependence of critical layer thickness in pseudomorphic InGaAs single-strained quantum-well structures

The dependence of critical layer thickness on a misfit in pseudomorphic GaAs/InGaAs/GaAs single quantum-well structures grown by molecular-beam epitaxy is studied using a phase-contrast optical microscope and photoluminescence spectroscopy. The results show that the critical layer thickness as a function of misfit follows more closely with Matthews and Blakeslee’s mechanical equilibrium model [J. Cryst. Growth 27, 118 (1974)] than People and Bean’s energy balance model [Appl. Phys. Lett. 49, 229 (1986)]. However, the observed critical layer thickness is slightly less than that predicted by the mechanical equilibrium model for single quantum-well structures.

Regrowth on molecular-beam epitaxial layers by transferring in ultrahigh vacuum between growth chambers: An assessment of the interface quality

The effect of transferring in ultrahigh vacuum conditions between molecular‐beam epitaxy chambers on the quality of regrown interfaces is addressed. In particular, the Al0.35Ga0.65As/GaAs heterointerface is probed by low‐temperature photoluminescence using a single quantum well (SQW) configuration, where the growth of the second interface has been interrupted and resumed after taking the sample back from the UHV transferring modules. Comparison of the emission characteristics of these SQW structures to others grown uninterruptedly indicates the feasibility of the UHV multichamber configuration for the realization of high‐quality heterostructures involving incompatible materials or different processing steps.

Growth of (AlIn)P/GaAs single quantum well structure on (001) GaAs by gas source MBE using AsH 3 and PH 3 gas

Observation of photoluminescence at 4.2 K from a lattice-matched (AlIn)P/GaAs single quantum well (SQW) structure grown by gas source MBE was reported for the first time. In this MBE system, a high pressure gas cell of AsH 3 and PH 3 was introduced as a gaseous source for group V. A procedure for switching the group III and V materials is attempted to make the designed SQW structure. As a result, each distinct luminescence peak from all the wells has been observed.

Design of quantum well AlGaAs-GaAs stripe lasers for minimization of threshold current-application to ridge structures

The gain saturation effect and the various leakage currents related to a strip structure (spreading current, lateral diffusion current, optical cavity recombination current, and Auger recombination current) are considered. The minimum threshold current (3.3 mA) is obtained with a multiquantum-well (MQW) structure (5*80 AA) at a cavity length of 80 mu m. At these values, the lateral leakage current (spreading and lateral diffusion currents) represents about 50% of the threshold current. It is shown that for short-cavity lasers, the MQW is preferred to the SQW (single-quantum-well) structure. However, the well number in the active layer of a ridge structure is limited by the decrease of the parallel confinement and the increase of the lateral leakage current. Finally, good agreement was obtained in comparing the calculations with experimental results for GRINSCH (graded-index separate-confinement heterojunction) SQW lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Migration-enhanced epitaxy

In an As-free or low As pressure atmosphere, surface migration of Ga or Al atoms on a GaAs substrate is greatly enhanced. Utilizing this fact, atomically flat GaAs-AlGaAs heterointerfaces were grown at a lower than usual substrate temperature by alternately supplying Ga and/or Al and As to the substrate surface. The layers grown by this method at very low substrate temperatures have good optical and electrical characteristics. In particular GaAs-AlGaAs single quantum-well structures grown by this method at 300 ° C have excellent photoluminescence characteristics.

Photopumped single quantum-well laser structures

Amplified spontaneous emission involving the n=1 and n=2 confined states is observed at room temperature in GaAs graded-barrier single quantum-well laser material grown by metalorganic chemical vapor deposition. These no-phonon, parity-allowed emission lines correlate well with computed transition energies.The duality of lines, their strength and sharpness, and the absence of undesirable substrate luminescence permit a specification of the quantum-well width. This validation, in conjunction with other wafer-level evaluations, provides a more complete characterization of laser material intended for fabrication.

Migration-Enhanced Epitaxy of GaAs and AlGaAs

Surface migration is effectively enhanced by evaporating Ga or Al atoms onto a clean GaAs surface under an As-free or low As pressure atmosphere. This characteristic was utilized by alternately supplying Ga and/or Al and AS4 to the substrate surface for growing atomically-flat GaAs-AlGaAs heterointerfaces, and also for growing high-quality GaAs and AlGaAs layers at very low substrate temperatures. The migration characteristics of surface adatoms have been investigated through reflection high-energy electron diffraction measurements. It was found that different growth mechanisms are operative in this method at both high and low temperatures. Both these mechanisms are expected to yield flat heterojunction interfaces. By applying this method, GaAs layers and GaAs-AlGaAs single quantum-well structures with excellent photoluminescence were grown at substrate temperatures of 200 and 300degC, respectively.

Determination of heterojunction mobilities using a novel magnetic field-dependent hall technique

Carrier transport properties of molecular-beam-epitaxy (MBE-) grown heterojunctions can be obtained by a novel technique, whereby the concentration and mobility of carriers in different subbands and/or different quantum wells are extracted using a magnetic field-dependent Hall measurement at field strengths commonly available with laboratory non-superconducting magnets. The technique generates a “mobility spectrum”, in which the maximum density of carriers is presented as a continuous function of mobility (as opposed to previous 2- or 3-carrier iterative fitting methods). The uncertainty in the mobility is seen directly as a finite width in the corresponding peak in the mobility spectrum. This paper is a report on the application of the technique to two types of MBE-grown heterostructures. For single quantum-well structures, the mobilities and carrier densities of the first and second subbands were determined. For a 5-quantum-well structure containing normal and inverted heterojunctions, the mobilities and carrier densities of the different quantum wells were determined.

Growth kinetics of molecular beam epitaxially grown GaAs/Al0.3Ga0.7As (100) normal and inverted interfaces in thin single quantum well structures examined via photoluminescence studies

The role of the relative surface kinetics of Ga and Al in determining the nature of normal and inverted interfaces defining GaAs/Al0.3Ga0.7As thin single quantum well (SQW) structures is examined via photoluminescence and excitation spectra studies on SQW structures grown under conditions determined by reflection high-energy electron diffraction intensity dynamics to shed specific light on this issue. Results are found to be in conformity with the role of surface kinetics exemplified by computer simulations of growth and underscore the critical importance of controlling both the structural and chemical nature of interfaces via choice of optimized growth conditions.

Theory of gain, modulation response, and spectral linewidth in AlGaAs quantum well lasers

We investigate theoretically a number of important issues related to the performance of AlGaAs quantum well (QW) semiconductor lasers. These include a basic derivation of the laser gain, the linewidth enhancement factor α, and the differential gain constant in single and multiple QW structures. The results reveal the existence of gain saturation with current in structures with a small number of wells. They also point to a possible two-fold increase in modulation bandwidth and a ten-fold decrease in the spectral laser linewidth in a thin QW laser compared to a conventional double heterostructure laser.

Carrier concentration in modulation-doped AlGaAs-GaAs heterostructures

Single-interface modulation-doped AlGaAs-GaAs heterostructures have very high mobilities if thick undoped spacers are introduced between the Si donors and the twodimensional electron gas. Electron densities are limited to values below 1012 cm−2. Higher channel densities are desirable for device applications and can be obtained by confining the electrons in quantum wells doped from both sides. Single quantum-well structures have been grown with sheet carrier densities exceeding 3×1012 cm−2 at 300 K and 77 K mobilities of 54,000 cm2/Vs. Single quantum wells doped from one side only with low electron concentrations of 2×1011 cm−2 have 4.2 K mobilities of 200,000cm2/Vs.