In this paper we describe the use of an electrochemical sulphur cell to grow ZnSxSe1−x alloys on GaAs substrates. The cell uses the ionic transport of Ag ions in Ag2S to produce a sulphur flux which depends on the applied voltage. The advantages and disadvantages of this type of source will be discussed. We also describe the versatility of the cell for fabricating ZnSxSe1−x/ZnSySe1−y multilayers as well as more complex profiles.

II-VI semiconductor surface passivants, insulators, and epitaxial films have been deposited onto selective surface areas by employing a new masking and lift-off technique. The II-VI layers were grown by either conventional or photoassisted molecular beam epitaxy (MBE). CdTe has been selectively deposited onto HgCdTe epitaxial layers as a surface passivant. Selective-area deposition of ZnS has been used in metal-insulator-semiconductor (MIS) structures. Low resistance ohmic contacts to p-type CdTe:As have also been realized through the use of selectively-placed thin films of the semi-metal HgTe followed by a thermal evaporation of In. Epitaxial layers of HgTe, HgCdTe, and HgTe-CdTe superlattices have also been grown in selective areas on CdZnTe substrates, exhibiting specular morphologies and double-crystal x-ray diffraction rocking curves (DCXD) with full widths at half maximum (FWHMs) as narrow as 140 arcseconds.

A brief summary is given of the characterization of an optically addressed Spatial Light Modulator (SLM) that uses a thin film of CdS as the hhotosensor and a liquid crystal as the electro-optic material. The required ghdark impedance of the CdS thin film is achieved by a depletion region created by surface acceptor states. Analysis of the SLM complex impedance as a function of frequency and illumination indicates that the voltage switching is due to the reduction in impedance with illumination caused by the increased capacitance of the CdS thin film. The impedance measurements as well as photoluminescence data were consistent with an acceptor surface state model accounting for the depletion region of the CdS thin fim. Additionally the SLM is shown to exhibit photoelectrochemical cell characteristics such as dark rectification, and a photocurrent and photovoltage proportional to the illumination intensity. The SLM performance characteristics of good resolution and sensitivity is attributed to the surface state induced depletion region.

The photoluminescence properties of MgxZn1−xSe solid solution grown on ZnSe substrate by a closed solid-state diffusion technique have been investigated. The depth profiles of the diffusion region shows distinguished surface layer and thick graded layer. The PL. spectra of MgxZn1−xSe side of the sample excited with a wide range of excitation energy shift the edge emission band toward longer wavelengths with decreasing excitation energy. The edge emission spectrum was found to shift toward the shorter wavelength with electric field applied parallel to the compositional gradient ofMgxZn1−xSe-ZnSe heterostructure.

ZnSe, ZnTe and ZnSe-ZnTe strained-layer superlattices (SLS's) have been successfully grown by atomic layer epitaxy (ALE) using molecular beam epitaxy (MBE-ALE). The ideal ALE growth, i.e., one monolayer per cycle of opening and closing the shutters of the constituent elements, was obtained for ZnSe in the substrate temperature range of 250-350° C. However, for ZnTe, precise control of the Te beam intensity is needed to obtain the ALE growth. Optical properties of the (ZnSe)l-(ZnTe) 1 SLS were evaluated by photoluminesence. ZnSe films were also grown by ALE using metalorganic molecular beam epitaxy (MOMBE-ALE). Diethylzinc (DEZn), diethylsulfur (DES) and diethylselenium (DESe) were used as source gases for Zn, S and Se, respectively. The ALE growth of ZnSe was achieved at substrate temperature between 250 and 300° C which is about 150° C lower than that for the conventional MOMBE.

This paper describes the results of the first attempt to reduce misfit dislocations in epilayers of a wide bandgap II-VI semiconductor on GaAs substrates by utilizing the ZnSe-ZnS strained-layer superlattice (SLS) structure. From a theoretical calculation, SLSs consisting of a 200A-ZnSe and a IOA-ZnS layer in one period can be grown as lattice-matched films to GaAs substrates. It has been found from the photoluminescence measurements and electron-beam-induced-current (EBIC) image observations that the generation of misfit dislocations can be markedly reduced, as expected.

The Polycrystalline Thin Film Solar Cells Program, part of the United States National Photovoltaic Program, performs R&D on copper indium diselenide and cadmium telluride thin films. The objective of the Program is to support research to develop cells and modules that meet the U.S. Department of Energy's long-term goals by achieving high efficiencies (15% - 20%), low-cost ($50/M2), and long-time reliability (30 years). The importance of work in this area is due to the fact that the polycrystalline thin-film CuInSe2 and CdTe solar cells and modules have made rapid advances. They have become the leading thin films for PV in terms of efficiency and stability. The U.S. Department of Energy has increased its funding through an initiative through the Solar Energy Research Institute in CuInSe2and CdTe with subcontracts to start in Spring 1990.

Excitonic properties of ZnSe-ZnS strained-layer quantum wells (SLQWs) with type I band lineups are reviewed on the basis of our recent results of temperature- and strain-dependent photoluminescence and absorption spectra. In order to estimate the conduction and valence band offsets as a function of ZnSe well thickness, we have modified the “model-solid” theory in which the valence bands (heavy-hole band in ZnSe and light-hole band in ZnS) are relatively moved with strains. Temperature and high excitation dependent studies of the n=1 heavy-hole excitons suggest a localization of excitons and reveals the important evidence on scatterings of excitons with acoustic and optical phonons. The thermal quenching of the exciton emission is caused by thermal dissociation of quasi-two-dimensional excitons through electrons and holes, from which the activation energy for this dissociation is 4 times larger than Ea.3D (a binding energy of bulk exciton) of ZnSe. A new superlattice structure with a quasiperiodic crystal which is derived from a finite Fibonacci sequence, has been fabricated by a low-pressure MOCVD method and its photoluminescence properties are for the first time introduced.

The Cd0.3 Zn0.7S-ZnS strained-layer superlattices has for the first time been fabricated on (100)GaAs substrates by a low-pressure MOCVD method. Exciton luminescence properties were investigated by the photoluminescence spectroscopies which show that the quantum confinement effect of excitons occurs in the CdZnS alloy layer. Temperature dependence of the exciton properties reveals a dominant scattering process which originates from exciton-phonon interaction(Γ10 =68meV). Nevertheless, the large exciton binding energy of about 166meV makes it possible to produce the exciton peak at room temperature. The effect of external electric field on the exciton intensity and its peak position is found and is tentatively interpreted in terms of a quantum confined Stark effect.

A system has been developed for accurately measuring lifetimes for photo-induced excess current carriers in semiconductors using the transient photoconductivity decay method. The specifications of state-of-the-art equipment, considerations peculiar to the capture of fast transient pulses, and sophisticated statistical data analysis techniques are discussed. Experimental results are presented to demonstrate the capability of the system (a) to measure lifetimes in the 40-ns - 75-µs range for temperatures varying from 77K to 300K with 10% accuracy for single lifetime decays and 30% accuracy for individual effective lifetimes in a multi-component decay, and (b) to use a 300-ns lifetime photoconductor as a detector to measure nanosecond-time-scale structure of laser pulses. The predominant excess carrier lifetimes of HgCdTe samples grown at NCSU by photoassisted molecular beam epitaxy (PAMBE) ranged from 46 ns at 300K to 341 ns at 77K. CdTe samples and CdMnTe-CdTe superlattices exhibited a multi-component decay with the two longest components having effective lifetimes of 26 µs and 4 µs for CdTe and 75 µs and 10 µs for CdMnTe-CdTe. These values were relatively insensitive to temperature variation.

This paper will review recent progress in the growth of II-VI semiconductors at NCSU by photoassisted molecular beam epitaxy, an energy-assisted growth technique in which the substrate is illuminated during the entire growth process. The materials focused on here include undoped and As-doped CdTe, As-doped HgCdTe, and In-doped HgCdTe. These materials were grown as single epilayers, as well as incorporated in other structures including heterojunctions and modulation-doped superlattices. The photo-assist during growth has allowed a substantial reduction in substrate growth temperatures, as well as an enhanced activation of n-type and p-type dopant species. The structural, electrical, and optical properties of the samples will be discussed below.

Hg1−xAxTe materials where A= {Be, Mg, Ba, Sr, Ca} have been synthesized by the vertical Bridgman technique. Hydrostatic density measurements showing segregation are presented and for HgMgTe an effective segregation coefficient is obtained. For HgMgTe a relationship between bandgap E and composition x, is developed based on FTIR measurements of cut-on wavelengths. The variation of energy gap with composition is found to be comparable to that of HgMnTe and twice as rapid as that of HgCdTe. Carrier concentration and mobility data from room temperature and 77K Hall measurements are presented for samples annealed in a saturated overpressure of Mercury.

A detailed analysis of Hall data obtained at temperatures between 20K and 300K has been made for n-type HgCdTe epilayers prepared by molecular beam epitaxy (MBE). Based on a theoretical calculation for intrinsic concentrations that takes into account the non-parabolicity of the conduction band, the electron concentrations are shown to follow nicely the model for semiconductors in their intrinsic and extrinsic ranges. The scattering mechanisms in these materials are studied through the analysis of electron mobility. Polar mode phonon scattering, acoustic mode via deformation-potential coupling, acoustic mode via piezoelectric coupling, and impurity scattering are included in the mobility calculation. The results indicate that these mechanisms are insufficient to explain the measured mobility.

The effects of alloy fluctuations, defect densities, and short-range clustering on the resonance behavior of first and second order LO and TO Raman scattering are studied in Hg1−xCdxTe (MCT). X-values between 0.20 and 0.32 and photon energies from 2.35 to 2.7 eV were used with samples prepared by a wide range of techniques--LPE, MOCVD, MBE, bulk growth, and pulsed laser annealing. We examine the resonance behavior of the HgTe-like TO mode at ∼120 cm−1 and the mode at 133 cm−1, which has been identified as originating from the preferential clustering of 3 Hg and 1 Cd about the Te. We find that the intensity of this peak for various bulk and epitaxially grown samples is unusually large only near the E, resonance. Pulsed laser annealing with a Nd:YAG-pumped dye laser strongly suppresses this mode in all samples suggesting that extremely rapid epitaxial regrowth may inhibit the 3:1 cluster formation. In addition, the resonance-enhanced LO overtones are suppressed by the pulsed laser anneal.

The optical absorption of a series of HgTe/Cd0.85 Hg0. 15 Te superlattices (SLs) have been measured in the spectral region from 2 to 12 µm at temperatures from 300 K down to 4.2 K. Several subband transitions were identified and their transition energies were compared with theoretical calculations, with the valence band offset δEv between HgTe and CdTe as a fitting parameter. It is found that at a given temperature, a value of δEv = 420 ± 100 meV fits the results of all the SLs, and δEv does not depend on temperature to within ±100 meV.

Photoassisted molecular beam epitaxy has been employed to successfully prepare p-type and n-type modulation-doped HgCdTe superlattices. The samples were grown at 170°C. In this paper, we report details of the MBE growth experiments and describe the optical and electrical properties that these new multilayered quantum well structures of HgCdTe possess.

The presence of low mobility surface electrons on n-Hg1−1CdxTe is generally not apparent in the temperature dependence of the Hall Coefficient (RH) or the Hall mobility (µH) of the layer. However, its influence is clearly seen in the magnetic field (B-field) dependence of RH. The B-field dependence of RH can be analyzed to extract the bulk and surface carrier concentrations and their respective mobilities.

This diagnostic technique has been used for evaluating epitaxial Hg1−xCdxTe layers grown by organometallic vapor phase epitaxy (OMVPE), which have been converted to n-type by annealing in Hg overpressure. In addition, the effect of anodic sulfide passivation on the B-field dependence of Hall coefficient is also outlined.

Anodic oxide passivation of p-type HgCdTe generates an inversion layer. Extremely high Hall mobility data for electrons in this layer indicated the presence of a two-dimensional electron gas. This is verified by use of the Shubnikov-de Haas effect from 1.45-4.15K. Data is extracted utilizing a numerical second derivative of DC measurement. Three sub-bands are detected. Their relative occupancies are in excellent agreement with theory and with experimental results obtained on anodic oxide as accumulation layers of n-type HgCdTe. The effective mass derived is comparable to expected.

The results of electrical characterisation of a series of MOVPE layers of CdxHg1−xTe (CMT) grown by the interdiffused multilayer process (IMP) are reported. It is shown that the properties of the CMT layers when grown as a “sandwich” between CdTe buffer and cap layers display classical p-type Hall effect curves. These have been modelled and the sensitivity of the fitting parameters are reported. It is shown that there is evidence of complex (“anomalous”) two-layer like behaviour in low x material which is not attributable to inversion behaviour.

A single-and multilayer growth model is presented. Surface order-disorder transitions are studied with the entropy calculated in the Bragg-William approximation and in the quasi-chemical approximation. A plausible explanation for high-quality growth obtained with energy-assistance is given. The model has been extended to study low-temperature epitaxial growth of HgTe and CdTe on different surfaces. The relevant surface energies are evaluated in a Green's function approach.