A. D. Pitt

Investigation of InGaAs-InP quantum wells by optical spectroscopy

A detailed study of the optical properties of InGaAs-InP single quantum wells (QWS) grown by atmospheric-pressure metal-organic chemical vapour deposition is described. Photoluminescence (PL), photoluminescence excitation (PLE), photoconductivity (PC) and electroreflectance (ER) are employed to study both undoped and modulation-doped quantum wells. The role of extrinsic processes in determining the low-temperature PL spectra is demonstrated from the variation of peak position and linewidth with temperature. The best PL linewidth obtained for a 150 AA well is 5.3 meV, fairly close to the limit imposed by alloy fluctuations in the InGaAs. The role of free carriers in the undoped QWS in determining the energy threshold for optical absorption is demonstrated from a comparison of PLE and PC spectra. Lineshape fitting of the PL spectra is described, and it is deduced that at 160K recombination processes in both doped and undoped QWS proceed with wavevector conservation, although at lower temperatures highly anomalous lineshapes are found in modulation-doped samples. The observation of a threshold in PC spectra under forward bias is interpreted as a transition from the valence-band well to the top of the conduction well. The ratio of the conduction- to valence-band discontinuities is deduced to be approximately 0.4:0.6.

InGaAs‐InP multiple quantum wells grown by atmospheric pressure metalorganic chemical vapor deposition

The optical and structural properties of multiple quantum wells of InGaAs‐InP grown by atmospheric pressure metalorganic chemical vapor deposition are reported. Room‐temperature excitons are resolved for well widths from 50 to 200 A. Below 50 K, exciton linewidths, in absorption, of less than 10 meV are obtained. Absorbances for allowed valence to conduction subband transitions are found to be independent of well width, as expected in the two‐dimensional limit. A lower bound for the conduction‐band discontinuity of 235±20 meV is obtained.

Photoinduced quenching of infrared absorption nonuniformities of large diameter GaAs crystals

Low‐temperature quenching of inhomogeneities in two‐dimensional infrared images of large diameter liquid encapsulated Czochralski (LEC) crystals of semi‐insulating GaAs is reported. The temperature and wavelength dependence of the ‘‘fatiguing’’ phenomena have the same characteristics as those reported for the deep donor EL2. These results demonstrate in a clear way that the dominant nonuniformities of the infrared (0.7–1.4 eV) transmission of LEC GaAs are due to fluctuations in the concentration of neutral EL2. In the quenched state, weak nonuniformities with reverse contrast to EL2 still remain. These may be due to a photoelastic effect in the strain fields of dislocations, or to the presence of another center whose spatial fluctuations are the reverse of those of EL2.

The growth by MOCVD using new group VI sources and assessment by HRTEM and Cl of Zn-based II–VI single crystal layers

Abstract The structural quality, as determined by cross sectional high resolution transmission electron microscopy, and cathodo-luminescence data is reported for zinc sulphide and zinc selenide single crystal layers grown heteroepitaxially on to gallium arsenide by metalorganic chemical vapour deposition using reactions between dimethyl zinc and either alkyl or heterocyclic compounds containing the group VI elements. Comparison is made with layers grown using conventional hydrides as the group VI sources.

Optical properties of InGaAs‐InP single quantum wells grown by atmospheric pressure metalorganic chemical vapor deposition

Low‐temperature photoluminescence and photoconductivity studies of high quality InGaAs‐InP quantum wells grown by metalorganic chemical vapor deposition at atmospheric pressure are reported. The best luminescence linewidth obtained for a 100‐A well is found to be 7.9 meV. The residual line broadening is discussed in terms of interface fluctuations, and is compared directly with structural properties as determined by high resolution lattice imaging transmission electron microscopy.

Effect of growth temperature on the optical, electrical and crystallographic properties of epitaxial indium gallium arsenide grown by MOCVD in an atmospheric pressure reactor

Indium gallium arsenide was grown epitaxially by metalorganic chemical vapour deposition (MOCVD) on (100) indium phosphide using trimethylindium. At an optimum growth temperature of 680°C, material of very good properties can be grown. The carrier concentration is 1x1015 cm-3 n-type with a mobility 80000 cm2 s-1 V-1 at 7 K. Double crystal X-ray diffraction rocking curve widths are <30 arc seconds using the (400) reflection with Cu radiation and the non-dispersive mode. The peak width in photoluminescence spectra at 2 K is < 2 meV. At lower growth temperatures the photoluminescence deteriorates dramatically. In previous work this has been attributed to enhanced carbon incorporation. In this work X-ray diffraction and transmission electron microscopy (TEM) measurements indicate that the deterioration is structural in origin and possibly due to the onset of longwavelength spinodal decomposition. TEM studies show that the interface between alloy and substrate is essentially defect free for optimally grown near lattice matched layers. Interfaces showing 2D electron gas effects and quantum wells can readily be grown. For wells, pauses in the growth can be used to sharpen interfaces and produce quantum wells of high photoluminescence efficiency.

Demonstration of quantum confinement in InSb‐In1−xAlxSb multiquantum wells using photoluminescence spectroscopy

We report the observation of quantum confinement, using photoluminescence, in InSb‐In1−xAlxSb (0.08≤x≤0.23) multiquantum well samples grown by molecular beam epitaxy. A series of samples were studied with different well widths and varying concentration of aluminum in the barriers. The upshifted luminescence energies behave qualitatively as expected due to changes in confinement, and are in good quantitative agreement with calculated upshifts taking into account strain in the barriers. These results demonstrate that good quality heterostructures can be obtained in this material system and show its potential for device applications.

Mbe Growth of Strained-Layer InSb/InAlSb Structures

The MBE growth and related materials characterisation of InSb/InAlSb strained-layer structures is described. Band-gap considerations and critical thickness calculations are presented and indicate that this material system should offer considerable device potential. Detailed structural studies, performed using both transmission electron microscopy and X-ray diffraction, confirm the growth of high quality multiple quantum-wells, and 2K photoluminescence has shown corresponding energy upshifted transitions.

Characterization of alloy composition in Ga1-xAlxAs/GaAs structures: comparison of photovoltage, X-ray, SIMS and RHEED techniques

Abstract A comparative study of techniques for the characterization of alloy composition in Ga 1- x Al x As/GaAs single and multilayer structures has been carried out. Good agreement ( x ± 0.015) is demonstrated between double-crystal X-ray diffraction and photovoltage spectroscopy measurements using the generally accepted calibration of Casey and Panish. A slightly modified form of the band gap versus composition curve is presented which further improves the agreement to x ±0.005. Profiling conditions achieving linear SIMS Al profiles are reported, yielding compositions in agreement with PVS and X-ray within x +-0.01 (when calibrated at a single composition). A systematic discrepancy between results from the above techniques and in-situ RHEED oscillation measurements is reported.

Photoluminescence and photoluminescence excitation studies on GaAs/AlAs short period superlattices near the direct/indirect crossover

A photoluminescence (PL) and PL excitation (PLE) investigation of GaAs/AlAs superlattices near the direct to indirect gap, type I to type II superlattice crossover is reported. The nature of the phonon coupling, the magnitude of the Stokes’ shifts between PL and PLE, and the dependence of the PL spectra on hydrostatic pressure are studied. From these results a change of the GaAs width (d) by just 1 monolayer (ML) from 12 to 11 ML is shown to cover the range from near coincidence of the GaAs (Γ) and AlAs (X) conduction band minima, but with type I, direct gap character dominant at d=12 ML, to clearly indirect gap, type II behavior at d=11 ML.