P. Dawson

Silicon doping of MBE-grown GaAs films

Two concentration ranges of silicon doping in MBE-grown GaAs films have been investigated in some detail. In lightly doped films, with a free-electron concentration of ≈1016cm−3, low-temperature photoluminescence spectra have been analysed to develop a model to account for spectral features previously attributed to Ge and Si acceptor levels.In heavily doped films, a maximum free-electron concentration of ≈7×1018 cm−3 has been obtained, which is only rather weakly dependent on growth conditions and the nature of the arsenic species (As2 or As4). Transmission electron microscopy has shown that no significant precipitation effects occur when higher Si fluxes are used but there is evidence for autocompensation. The maximum PL intensity (300 K) is found at a lower free electron concentration then with Sn-doped films, and is more sharply peaked, but there is no evidence for an anomalous Moss-Burstein shift.

Photoluminescence Studies Of Type II GaAs/AlAs Quantum Wells Grown By MBE

It is now commonly accepted that the fractional valence band offset in the GaAs/ AlxGal-xAs material system lies in the region 0.3-0.4. This has important consequences for quantum well structures in which the bar Kier material is indirect (x > 0.45). If the GaAs width L is sufficiently small (Lz < 30Å) then for GaAs/AlAs quantum wells the lowest confined electron state of the system is at the X minima of the AlAs. This leads to so-called type II recombination involving electrons in the AlAs and holes in the GaAs. We have performed an extensive investigation of such phenomena on a number of GaAs/AlAs multiple quantum well structures where Lz < 30Å, using the techniques of photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE). In the PL spectrum we observe both type I recombination; that is recombination of electrons and holes confined in the GaAs, and also a series of lines associated with the type II process. Using PLE we have identified the lowest confined exciton state of the type II system and combining this information with the temperature dependence of the type II PL spectra we ascribe the highest energy emission at 6K as due to the recombination of localised excitons. Further-more, absorption edges associated with higher lying (n>1) electron states confined at the X minima in the AlAs are observed in the PLE spectrum. These have relative strength and position which are in good agreement with theoretical predictions. Combining these results with a calculation of the type II exciton binding energy has led us to determine the fractional valence band offset to lie in the range 0.33-0.34.

LOW-POWER, ALL-OPTICAL NONLINEAR ABSORPTION IN ASYMMETRIC DOUBLE QUANTUM-WELLS

We demonstrate all‐optical nonlinearities at low cw pump powers (50 W/cm2) using a novel heterostructure design which spatially separates photoexcited electron‐hole pairs. Theoretical calculations based on many‐body interactions are in good agreement with the measured spectra.

Influence of growth interruption on inverted interface quality in single AlAs‐GaAs quantum wells grown by molecular beam epitaxy

Three techniques have been combined to correlate interface morphology and optical properties of single AlAs‐GaAs quantum wells grown by molecular beam epitaxy (MBE) with and without growth interruption at the inverted (GaAs on AlAs) interface. Surface recovery and interface formation were monitored in situ by reflection high energy electron diffraction, optical properties were assessed by photoluminescence excitation (PLE) spectroscopy and the results compared with a Monte Carlo simulation of MBE growth, extended to evaluate PLE linewidths. Criteria for linewidth reduction have been established and interface morphology described.

Dynamic RHEED Techniques and Interface Quality in MBE-Grown GaAs/(Al,Ga)As Structures

The RHEED intensity oscillation technique has received widespread attention in the study of MBE growth and is, in principle, capable of providing detailed information on growth dynamics. There are, however, some rather serious complications associated with diffraction behaviour which must be resolved before its full potential can be realised.

Optical nonlinearities due to long-lived electron-hole plasmas

By choice of appropriate layer thicknesses it is possible to produce GaAs/AlAs quantum well structures in which the lowest energy electron and hole wavefunctions are localised in spatially separate layers. As the low temperature radiative lifetime ol such a structure is long (ps) it is possible to observe cw optical nonlinearities at relatively low power densities. In this paper results of optical pump/probe experiments will be presented from a range of standard type II structures along with results from a new type of structure that enhances the spatial separation of the electron/hole population.

Studies of compositional variations in germanium quantum dots grown on silicon

The authors report optical and scanning TEM studies of Ge dots grown on Si. In an attempt to control the self-organized growth process and promote dot size uniformity the dot layers were grown on a 4.5 nm Si0.6Ge0.4 alloy template layer. Photoluminescence results indicate the formation of carrier confining Ge rich islands, while Raman scattering results indicate an alloy throughout the structures formed. The samples were studied in the UK high resoln. scanning TEM facility at Liverpool, UK. Energy dispersive anal. of individual line scans through the sample show that the structures are composed of an alloy throughout with an asym. distribution of Ge in the dots and in the wetting layer close to the dots. The authors discuss the results in the light of the proposed growth mode for these dots and conclude that attempts to manipulate the compn. of these dots during growth may be problematic due to the self-organized nature of their formation. [on SciFinder (R)]

Effects Of Electronic Coupling On The Band Alignment Of GaAs-AlAs Quantum Well Structures

We report the results of a systematic investigation of the effects of decreasing the AlAs layer thickness from 41Å to 5Å on the band alignment of GaAs-AlAs quantum wells in which the GaAs thickness was kept constant at nominally 25Å. Combining the techniques of photoluminescence and photoluminescence excitation spectroscopy we have mapped out both the direct r-related bandgap and the x-r bandgap as a function of AlAs thickness. We observe a reversal of the band alignment from the type II to the type I arrangement when the AlAs thickness is reduced below ~13Å In addition, we present further evidence which confirms that the type II emission process is related to the Xz-r pseudo-direct bandgap. In the structures with very thin (<10Å) AlAs layers we note a significant modification of the type I excitation spectra where the n=1 exciton peak can be hundreds of times stronger than the apparent absorption in the continuum region.