J.E. Nicholls

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.

CdS/CdSe intrinsic Stark superlattices

Strained layer superlattices of wurtzite CdS/CdSe have been grown on (111)A GaAs substrates by metalorganic chemical vapor deposition and their optical properties studied by photoluminescence spectroscopy. It is shown that the superlattice layers contain giant strain‐induced piezoelectric fields exceeding 2×108 V m−1. These fields are similar to those reported for (111) orientated III–V superlattices, but an order of magnitude greater. The recombination energies from a series of samples provide evidence for a type II conduction band offset of 0.23±0.10 eV (the electron wells being in the CdS), with the band structure heavily modified by the internal electric fields. In addition, the photoluminescence peak emission energy shows a strong dependence on the excitation power. This is interpreted as further evidence for the effect of internal fields. We conclude that this system shows new effects not previously observed in II–VI compound superlattices. The large band‐gap tunability and the space‐charge effects ...

Growth and assessment of CdS and CdSe layers produced on GaAs by metalorganic chemical vapour deposition

Abstract CdS and CdSe layers have been grown successfully on GaAs by MOCVD. The surface morphology, RHEED patterns and photo-luminescence properties of the layers were studied for different substrate orientations. It was found that the CdS grew hexagonally on the (111)A substrate face, whilst the layers produced on (100), (110) and (111)B faces were a mixture of cubic and hexagonal phases. Layers of CdSe grown on GaAs were also a mixture of cubic and hexagonal phases for substrate faces other than (111)A, for which the layer was predominantly cubic. The change in photoluminescence with growth temperature was investigated and for CdS the best growth temperature, as measured by the width of the bound exciton emission, was 350 ° C. No evidence of large scale diffusion of Ga or As from the substrate was found.

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.

Variational methods for calculating exciton binding energies in quantum well structures

A new and powerful mathematical technique is described for evaluating the binding energies of excitons within the framework of the variational method. The technique is applied to infinite wells and the binding energies of 1s- and 2s-type excitons calculated as a function of well width.

Direct optically detected magnetic resonance observation of a copper centre associated with the green emission in ZnSe

The green emission at 530 nm in zinc selenide has been studied by means of optically-detected magnetic resonance (ODMR). Observation of hyperfine structure shows unequivocally that copper ions are involved with the emission. The copper ions lie in C2v symmetry with gX=1.951+or-0.004, gY=2.214+or-0.004, gZ=2.042+or-0.004, mod AX mod <10*10-4 cm-1, mod AY mod =(155+or-5)*10-4 cm-1 and mod AZ mod <10*10-4 cm-1, with X, Y and Z respectively parallel to (110), (110) and (001) or equivalent directions. The spectrum is attributed to Cu2+ either at the zinc substitutional site or at the interstitial site that is surrounded by four selenium ions. The lowering of the symmetry from Td to C2v is attributed to the presence of another ion in an (001) type direction relative to the copper. The results of previous workers suggest that this second ion is itself copper, so that the centre would be a copper-substitutional-copper-interstitial pair. The ODMR results show surprisingly that the emission is due to donor-acceptor recombination, the copper centre acting as a deep acceptor.

CdS/CdSe strained layer superlattices grown by MOCVD

The authors report the first growth of superlattices in the wurtzite CdS/CdSe system. The superlattices were grown by metal-organic chemical vapour deposition on GaAs (111) substrates. Transmission electron microscopy shows the superlattices to be of good quality with abrupt interfaces. The layers were purely hexagonal with no twinning. Low-temperature photoluminescence of the superlattices was studied for various layer thicknesses. All the structures showed a broad intense near-infrared emission which shifts to higher energy for the thinnest layers. In addition, thicker layers showed a narrower red emission at 690 nm. The photoluminescence spectra provide preliminary evidence that the structure is of type II.

Photoluminescence studies of CdS/CdSe wurtzite superlattices; Evidence for large piezoelectric effects

Abstract Strained-layer superlattices of wurtzite CdS/CdSe were grown on (111)A GaAs. The photoluminescence shows a single intense emission band which shifts to lower energy in longer period structures. This shift, which cannot be accounted for by the usual Kronig-Penney model, provides direct evidence for the presence of piezoelectric fields in the structures.