V. A. Wilkinson

Role of radiative and nonradiative processes on the temperature sensitivity of strained and unstrained 1.5 μm InGaAs(P) quantum well lasers

By measuring the spontaneous emission from strained and unstrained 1.5 μm InGaAs quantum well lasers as a function of temperature we deduce the temperature dependence of the radiative current density at threshold corresponds to a characteristic temperature T0≊300 K, close to that expected from theory, whereas T0 of the threshold current is around 60 K. We conclude from our analysis that the large temperature dependence of long wavelength lasers is due to Auger recombination.

Elimination of wavy layer growth phenomena in strain-compensated GaInAsP/GaInAsP multiple quantum well stacks

Abstract A fifty period multiple quantum well stack composed of alternating layers of GaInAsP (100 A, + 1% lattice mismatch) and GaInAsP (100 A, -1% lattice mismatch) has been grown with a minimum of strain relaxation. The strain-compensated structure was over 1 μm thick, exhibited an excellent high resolution X-ray diffraction rocking curve and had a low temperature photoluminescence linewidth of only 6 meV. The growth of this structure was made possible by suppressing the wavy layer growth phenomena. The onset of wavy layer growth can be delayed by using a high V/III ratio in the gas phase and by choosing a growth temperature high enough for the barrier composition to lie outside the Ga-In-As-P miscibility limit isotherm.

Evidence for strain relaxation via composition fluctuations in strained quaternary / quaternary and quaternary / ternary multiple quantum well structures

Abstract The occurence of large-scale contrast modulations is discussed, which have been observed in the transmission electron microscope in cross-sectional samples containing Ga x In 1− x As y P 1− y with a number of different x and y values. 90°-wedge samples were used because of their known geometry, in which thin-foil contrast effects can be excluded. The samples exhibiting large-scale modulations of a period greater than 100 nm all contained quaternary / quaternary or quaternary / ternary multiple quantum well stacks. Indications for a correlation between the onset, strength and frequency of the modulations with the well/barrier composition and its proximity to the centre of the miscibility gap are presented. The contrast is also found to be influenced by the stack thickness and strain. In large stacks, the modulations are related to waviness in epi-layer growth. The contrast is discussed in terms of composition modulation patterns connected with strain fluctuations, which are set up in interfacial regions and which evolve into wavy layer growth as means of misfit strain relief.

Investigation of the band structure of the strained systems InGaAs/GaAs and InGaAs by high-pressure photoluminescence

InxGa1-xAs quantum wells grown pseudomorphically in GaAs and AlGaAs with values ofx up to 0.25 have been studied by photoluminescence under high hydrostatic pressure. We concentrate here on the pressure range where the emissions quench and take on the characteristics of theX-minima. In the InGaAs/GaAs structures, these transitions display an unexpected pressure coefficient, -2.6 meV/kbar, twice that of theX minima in GaAs. We assign these transitions to theX minima in the wells, and therefore make a direct measurement of the strainedX positions as a function of composition. In the InGaAs/AIGaAs structures the crossovers occur against theX-minima in the barriers and these crossovers yield an accurate value for the band offset ratio for InGaAs/GaAs heterojunctions which is found to be 60:40 (CB:VB).

Growth and characterization of relaxed epilayers of InGaAs on GaAs

Abstract We report the growth of 3 μm thick relaxed layers of In x Ga 1− x As on GaAs substrates, with x from 0.1 to 1, and give some results of compositional, optical, structural and electrical characterisation. Compositions were determined by several techniques, with results which agreed to within a Δx of ±0.02. For x above 0.2 the crystal quality is very poor. Some layers have been grown with stepped compositions up to x = 0.4 and this improves the crystal quality. We show that in the InGaAs/GaAs system there is a threshold strain, rather than threshold composition change, above which crystal quality is degraded.

The pressure dependence of the photoluminescence intensity in hydrogenated amorphous silicon

Abstract The hydrostatic pressure dependence of the photoluminescence (PL) intensity from hydrogenated amorphous silicon has been measured in a diamond anvil cell at 80 K. The PL intensity is not quenched significantly by pressures up to ∼90 kbar, but in the pressure range ∼90–130 kbar quenching is observed. The luminescence displays a red shift of 1.9±0.1 meV kbar−1, with a hysteresis of ∼35meV at the highest pressures.

Miniature cryogenic diamond anvil cell

Abstract The miniature cryogenic diamond anvil described previously [D.J. Dunstan and W. Scherrer, Rev. Sci. Instrum. 59, 627 (1988)] has been modified to facilitate its use, and has been taken to 26GPa. The modifications are described here, together with some details of operation.

Laminated gaskets for absorption and electrical measurements in the diamond anvil cell

We have developed gaskets laminated from two layers of steel both for optical absorption and for electrical measurements in the diamond anvil cell. For absorption, these gaskets provide masking around the sample, avoiding the need for imaging of the gasket hole. For electrical measurements, laminated gaskets avoid the need for specially prepared diamonds as the feedthrough wires are not in contact with the anvils. Using argon as a pressure medium, both techniques have been used up to about 50 kbar at 300 and at 10 K.

The pressure dependence of the valence band discontinuity in quantum well structures

The use of hydrostatic pressure to determine the valence and conduction band offsets and their pressure dependence in both Type I and Type II quantum well structures will be described. Recent exper...

Magneto-optical studies of CdTe/CdMnTe semimagnetic semiconductor superlattices under high pressure

Abstract The photoluminescence (PL) of a CdTe/CdMnTe superlattice has been studied at pressures up to 4.1GPa, where the phase transition occurs. PL is observed up to this pressure, and it moves to higher energy with pressure at 66meV/GPa. This result is consistent with theory. Magnetic fields decrease the band-gap of the semimagnetic CdMnTe barriers and this reduces the PL energy. The pressure dependence of this effect is expected to provide a stringent test of the theory of semimagnetic materials and of superlattices.