B. Lunn

Spin injection into semiconductors

The injection of spin-polarized electrons is presently one of the major challenges in semiconductor spin electronics. We propose and demonstrate a most efficient spin injection using diluted magnetic semiconductors as spin aligners. Time-resolved photoluminescence with a Cd0.98Mn0.02Te/CdTe structure proves the feasibility of the spin-alignment mechanism.

PHOTOLUMINESCENCE AND P-TYPE CONDUCTIVITY IN CDTE:N GROWN BY MOLECULAR BEAM EPITAXY

An rf nitrogen (N) plasma source has been used to achieve p‐type conductivity in molecular beam epitaxy CdTe layers grown with a Cd overpressure. Photoluminescence and secondary ion mass spectrometry measurements have confirmed the incorporation of the N species, and evidence for the resulting p‐type conductivity has been obtained using capacitance‐voltage and current‐voltage techniques. Net hole concentrations as high as 2×1017 cm−3 have so far been achieved, which contrasts with the normally n‐type nature of our undoped CdTe layers.

Experimental and theoretical studies of the low-temperature growth of chromia and alumina

Abstract This paper describes the results of experimental and theoretical studies on the role of phase-stabilization and point defects in the growth of α-Al 2 O 3 alumina and eskolaite Cr 2 O 3 chromia. The experimental approach has been to utilize the similarity in the structure of α-Al 2 O 3 and Cr 2 O 3 , so that the chromia acts as a ‘template’ to the growth of the α phase. The aluminium and chromium oxides (and their alloys) are grown in the temperature range of 300–500°C from elemental beams of chromium and/or aluminium and an oxygen plasma source. The theoretical studies have utilized Molecular Dynamic and Monte Carlo methods to investigate several important factors such as native defects (e.g. vacancies and interstitials), impurities (e.g. Fe) and diffusion barriers, which significantly affect the phase stability and the manner in which α-Al 2 O 3 thin films form.

In-situ X-ray imaging of III–V strained-layer relaxation processes

Abstract The important value of the X-ray topography (XRT) technique for the investigation of III–V strained-layer relaxation processes is described. In addition to post-growth ex-situ XRT studies, a unique combined XRT/MBE growth facility has been constructed which allows the generation, motion and interaction of misfit dislocations to be monitored in-situ during epilayer growth, for the first time. The in-situ data already obtained for (100) InGaAs strained-layer growth on both Czochralski- and vertical-gradient freeze-grown GaAs substrates indicates technologically important differences in the initial relaxation process, including, in the latter case, the observation of a previously unreported secondary relaxation phase. Initial results relating to the influence of both post-growth annealing and the subsequent cool-down process are also described.

Optically pumped stimulated emission in ZnS/ZnCdS multiple quantum-wells, MBE-grown on GaP

Abstract Optically pumped stimulated emission is observed in a series of ZnS Zn 1−x Cd x S multiple quantum-well (MQW) structures, grown on GaP substrates by MBE. We report lasing from a ZnS Zn 0.97 Cd 0.03 S MQW at wavelengths as low as 333nm, the shortest yet reported in a semiconductor heterostructure. The lasing threshold decreases for deeper wells and reaches 6.5 kW.cm−2 at 8K and 80 kW.cm−2 at 300K for the MWQs with a Cd-composition of 0.2. The results are compared to known values for similar structures grown on GaAs by MOCVD, which reveals the potential of the GaP-substrate MBE technology for the widest bandgap II–VI heterostructures incorporating ZnS-based ternary alloys.

Magnetic tuning of the exciton binding energy and band-offsets determination in a CdTe/Cd1-xMnxTe superlattice

Abstract Magnetic tuning of the electronic properties of heterostructures incorporating diluted magnetic semiconductors offers various opportunities which are well illustrated in this optical study of a Cd1-xMnxTe superlattice (x = 6.6%). Due to the exceptional quality of this structure grown on a InSb substrate both the 1S and 2S states of the heavy hole exciton are observed, which allows the determination of the valence band discontinuity at the interface and offers the unique opportunity to study the variation of the exciton binding energy with the quantum well depth. The partition ratio of the band gap discontinuity at the interface between the conduction and valence band is found to be close to 60/40 ie smaller than earlier values obtained from quantum wells with larger values of x but in good agreement with a theoretical estimation.

Ga2Te3 and tellurium interfacial layers in ZnTe/GaSb heterostructures studied by Raman scattering

Raman spectroscopy of ZnTe layers grown by molecular beam epitaxy on (100) GaSb is reported. When the laser excitation is above the band gap of the ZnTe, scattering is observed only from the ZnTe LO mode and overtones. With excitation below the ZnTe band gap, a series of low frequency peaks is observed. By comparison with bulk data these peaks are identified as originating from Ga2Te3 and Te present at the GaSb/ZnTe interface. We conclude that the presence of this interface material may degrade the layer quality and give rise to the anomalously large strain previously reported for such epilayers.

In situ X-ray topography studies during the molecular beam epitaxy growth of InGaAs on (001) GaAs: effects of substrate dislocation distribution on strain relaxation

We report results from a novel facility constructed to enable in situ X-ray diffraction studies during the molecular beam epitaxy growth of Ill-V strained layer device structures on 50 mm diameter substrates. This new facility, used in conjunction with the Daresbury synchrotron source, permits X-ray topographic imaging of individual misfit dislocations formed during the molecular beam epitaxy growth process. The misfit dislocation growth and interactions can be imaged as a function of layer thickness, strain, growth and post-growth conditions. Our recent results show that the nature and distribution of dislocations threading up from the substrate are crucial in determining the initial pattern of epilayer relaxation. Under certain growth conditions and substrate dislocation distributions, large areas of the epilayer remain free of misfit dislocations at epilayer thicknesses significantly higher than the measured initial critical thickness tc1. We have observed in situ for the first time a second critical thickness tc2 (under certain conditions tc2>2tc1) at which there is a rapid increase in misfit dislocation density as a second misfit dislocation source(s) becomes active.

An ordered Ga2Te3 phase in the ZnTe/GaSb interface

A Ga2Te3 interfacial phase has been observed in a ZnTe/(001)GaSb heterostructure by high resolution electron microscopy under special imaging conditions. This phase exists in domains 5–10 nm in size on the ZnTe side of, and usually 2–4 nm away from, the interface. A structural model has been proposed for this phase that is derived from the sphalerite cell with cation sites occupied either fully (occupancy 1) or partially (occupancy 5/9) by Ga atoms. The fully occupied Ga sites form a regular array of uninterrupted chains along the [110] direction of the sphalerite unit cell. The partially occupied Ga sites can also be considered as forming chains containing both Ga atoms and vacancies along the [110] direction. Within these chains vacancies are highly mobile, resulting in an average Ga occupancy of 5/9. The unit cell of Ga2Te3 is orthorhombic with the space group Amm2. The lattice parameters of the unit cell have been derived from electron diffraction data.

Electrical and structural assessment of CdTe and CdMnTe layers grown by MBE on InSb substrates

Abstract Single layers and multiple quantum wells (MQWs) of CdTe and CdMnTe have been grown by MBE on InSb substrates. These systems have been analysed using double crystal X-ray diffraction (DCXRD). The single layers have been shown to be of good structural quality while the MQW systems yield rocking curves with well resolved diffraction satellites (up to the seventh order) indicating good quality and periodicity. All undoped layers have been found to be n-type with free carrier concentrations in the range 10 14 -10 15 cm -3 but the densities for CdMnTe layers are generallyless than for CdTe layers grown under identical conditions. When doped with indium, both types of layers have exhibited a saturation in carrier concentration as the In atom concentration was increased beyond 10 18 cm -3 probably as a result of the generation of compensating acceptor states.