D.E. Ashenford

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.

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.

Photoluminescence of CdTe/CdMnTe multiple quantum wells excited near the Mott transition

Abstract Using pulses from a 6 ns dye laser, the photoluminescence from CdTe/CdMnTe quantum wells is investigated as a function of carrier density. The spectra broaden with density but many sharp features remain. A biexciton is identified with a transition energy of 3.8meV less than that of the free exciton. The Mott transition for the el-hl bandedge is approached at an excitation density of 2.2 × 10 12 cm −2 . Photoluminescence from the el-h3 exciton is observed as a sharp peak at high densities. Its transition energy does not change and its intensity increases linearly with carrier density.

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.

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.

MBE growth of CdTe and Cd1−xMnxTe layers and multilayers on InSb substrates

Abstract Single and multiple layers of CdTe and CdMnTe, including MQW structures with well and barrier thicknesses down to 20 A, have been grown by MBE on (001) InSb. The broadening of the excitonic photoluminescence lines in single layers is consistent with current theories of statistical alloy fluctuations. For multiple thin layer structures the PL spectra show quantum confinement effects. The DCXRD rocking curve data show that both the single and multiple layers are of high structural quality.

Structure of CdTe‐Cd1−xMnxTe multiple quantum wells grown on (001) InSb substrates by molecular beam epitaxy

Molecular beam epitaxy has been used to prepare multiple quantum well structures of CdTe/Cd1−xMnxTe on (001)u2009InSb substrates. The growth of such a system on InSb allows the use of particularly low growth temperatures, hence minimizing interdiffusion effects. This study presents the first transmission electron microscope investigation of this multilayer system grown on InSb. The work clearly demonstrates that multiple quantum wells of high structural quality can be grown reproducibly over a wide range of layer thicknesses. The importance of efficient substrate surface cleaning prior to growth is demonstrated. In order to grow high structural quality multilayers, the choice of buffer layer is also important and a possible explanation for this observation is given.Molecular beam epitaxy has been used to prepare multiple quantum well structures of CdTe/Cd1−xMnxTe on (001)u2009InSb substrates. The growth of such a system on InSb allows the use of particularly low growth temperatures, hence minimizing interdiffusion effects. This study presents the first transmission electron microscope investigation of this multilayer system grown on InSb. The work clearly demonstrates that multiple quantum wells of high structural quality can be grown reproducibly over a wide range of layer thicknesses. The importance of efficient substrate surface cleaning prior to growth is demonstrated. In order to grow high structural quality multilayers, the choice of buffer layer is also important and a possible explanation for this observation is given.

Giant specular inverse Faraday effect in Cd0.6Mn0.4Te

Observation of the non-resonant Specular Inverse Faraday Effect (SIFE) in Cd0.6Mn0.4Te is reported. With picosecond circularly polarised pump and linearly polarised probe pulses at 532nm the pump-induced rotation of the reflected probe polarization plane was found to be approximately one order of magnitude stronger than in GaAs and reaches 3.5x10-2 deg with a pump intensity of 5 MW cm-2.