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The properties of excitons are examined in two model II-VI heterostructures: the Type-I CdTe/ZnTe system with small valence band offset and the Type-II ZnTe/ZnSe system. The strain induced band offset in the CdTe/ZnTe heterojunction is included to provide the confinement for the heavy hole state. It is found that the large enhancements of exciton binding energy and oscillator strength in the CdTe/ZnTe system are similar to what one finds in systems with a much larger valence band offset. For the ZnTe/ZnSe system, it is found that strong confinement of electrons and holes by the large band offsets can give rise to a very large exciton binding energy for thin heterojunction layers. Also, the mismatch in dielectric constants induces an image charge at the interface, which modifies significantly the exciton Hamiltonian in an asymmetric structure.

We report the experimental observation of negative differential resistance (NDR) at room temperature from a structure consisting of a single InAs(n)/GaSb(p) interface. The peak current densities ranged from 4.2×104 to 8.0×104 A/cm2 depending on how the structure is doped. The mechanism that causes the NDR is similar to that of an Esaki tunnel diode. We have also observed NDR at room temperature in a second class of novel devices. These structures consist of a thin layer of AlSb displaced from a single InAs(n)/GaSb(p) interface. NDR with peak current densities greater than 1.6×105 A/cm2 is seen in these structures. We attribute the increase in peak current densities with the addition of the AlSb barrier to the formation of a quasi-bound state between the AlSb layer and the InAs/GaSb interface. This quasi-bound state forms either in the conduction band of InAs or the valence band of GaSb, depending on where the AlSb barrier is placed and leads to a resonate enhancement of the current in the structures.

We describe a novel technique to improve doping in semiconductors, by the application of external electric fields during crystal growth. The dopants are modeled as charged, mobile species that are free to diffuse and drift under electric fields. In the case of molecular beam expitaxial (MBE) growth, we solve for the steady state of these species in a moving coordinate frame that travels with the growth front. We have specifically applied our analysis to Li donors in n-type ZnTe. Our results indicate that excellent improvements in the doping concentrations could be obtained under normal MBE growth conditions, with the applications, with the application of substantial electric fields.

A two-band transfer matrix method has been developed to study tunneling currents in narrow gap and interband tunnel structures. This relatively simple model gives good agreement with recently reported experimental results for InAs/AlSb/InAs/AlSb/InAs double-barrier heterostructures and InAs/AlSb/GaSb/AlSb/InAs resonant interband tunneling devices, and should be useful in the design of new interband tunneling devices.

There is still an undervalued need for quantitation in the biological, biochemical, and the molecular genetic sciences. Quantitation of electroblotted materials such as DNA, RNA, and proteins and the quantitation of molecular weights in electrophoresed gels is often necessary to an undistorted analysis. Quantitation would ensure, for instance, that the quantities of the substances being assayed fall within the linear range of that method by which they are being studied, once the linear range has been determined by a linearity study: the common phenomenon of saturation and its reverse, insufficient samples, could be easily detected. A biological data representation program is discussed that converts the results of gels and blots to numerical values and presents these values in a graphical display that is amenable to meaningful comparisons. The main goal of this work is to be able to quantitate electrophoresed and blotted molecular and biochemical results while insuring that the samples processed are within the linear range of the methods employed. This study demonstrates errors that can occur when a linearity study is not conducted. Versatile, easy-to-use quantitative graphics programs, such as the BandScan program, can further pipe numerical output to spreadsheets or statistical packages and provide the bridge between modern computation and traditional wet lab techniques. This analytical bioinformatics program that allows for statistical analysis is also a valuable tool for teaching purposes in Bioinformatics and Molecular Biology. BandScan is available free for academic institutions and class room work from the corresponding author.

We have measured the valence band offset in a cubic CdSe/ZnTe (100) heterojunction by X-ray photoelectron spectroscopy (XPS). Our preliminary result, based on analysis of one heterojunction, is 0.65±0.08 eV. The electrical characteristics of a doped n-CdSe/p-ZnTe heterojunction appear to be dominated by traps. We have attempted to verify the band offset through capacitance-voltage measurements, but have been unable to extract a consistent value for the flat band voltage from scans taken at different temperatures and frequencies.

We have observed negative differential resistance at room temperature from devices consisting of a single interface between n-type InAs and p-type GaSb. InAs and GaSb have a type II staggered band alignment; hence, the negative differential resistance arises from the same mechanism as in a p+-n+ tunnel diode. Room-temperature peak current densities of 8.2×10^4 A/cm^2 and 4.2×10^4 A/cm^2 were measured for structures with and without undoped spacer layers at the heterointerface, respectively.

This report describes work performed to develop and optimize the process of radio frequency (RF) sputtering for the fabrication of thin-film solar cells on glass. The emphasis is on CdTe-related materials including CdTe, CdS, ZnTe, and ternary alloy semiconductors. Pulsed laser physical vapor deposition (LPVD) was used for exploratory work on these materials, especially where alloying or doping are involved, and for the deposition of cadmium chloride layers. For the sputtering work, a two-gun sputtering chamber was implemented, with optical access for monitoring temperature and growth rate. We studied the optical and electrical properties of the plasmas produced by two different kinds of planar magnetron sputter guns with different magnetic field configurations and strengths. Using LPVD, we studied alloy semiconductors such as CdZnTe and heavily doped semiconductors such as ZnTe:Cu for possible incorporation into graded band gap CdTe-based photovoltaic devices.

We report the successful growth of ZnSe and ZnSe/ZnS superlattices on GaAs by pulsed laser deposition. An XeCl excimer laser operated at 308 nm was used to ablate/evaporate II‐VI bulk targets in an ultra high vacuum chamber. These superlattices are important optoelectronic materials that have major applications in the field of blue light emitting diodes and semiconductor lasers. For typical growth temperatures in the range 350–450 °C we obtained epitaxial layers with excellent optical properties. The laser power and fluence were varied to produce growth rates in the range 0.1–1 A/pulse. The layer thicknesses were determined by optical reflection spectroscopy. the photoluminescence of the pulsed laser deposited ZnSe layers showed dominant bound and free exciton features. The superlattice samples showed large blue shifts (∼400 meV) in the photoluminescence as the layer thicknesses were varied. These results are consistent with strong quantum confinement of the heavy holes in the ZnSe layers (valence band of...

We have investigated the pulsed laser evaporation of elemental Cd targets, with the aim of understanding the velocity distributions in the plumes and the changes which occur under moderate electrical bias. We report detailed kinetic energy distributions of the species in the laser evaporated plumes. In these experiments, frequency doubled, Q-switched pulses of a Nd:YAG laser were used at a 10 Hz repetition rate to generate the plumes. The velocity distributions of individual atomic species were determined by time-of-flight (TOF) light emission spectroscopy, while the time resolved ion/atom currents were measured with a collector above the target. We have simultaneously measured the dependence of the time resolved optical and electrical signals on the electrical bias applied between target and collector. We find that the typical kinetic energies in the plume are on the order of 10-200 eV, while the ionized species travel two to three times faster than the neutral particles. These results provide fundamental information about the physics of the pulsed laser evaporation process, and subsequent evolution of the plume.

We report the successful growth of ZnSe and ZnSe/ZnS superlattices on GaAs by pulsed laser deposition. An XeCl excimer laser operated at 308 nm was used to ablate/evaporate Il-VI bulk targets in an ultra high vacuum enclosure. For typical growth temperatures in the range 350°–450°C we obtained epitaxial layers with excellent optical properties. The laser power and fluence were varied to produce growth rates in the range 0.1–1 Å/pulse. The photoluminescence of the pulsed laser deposited ZnSe layers showed dominant bound and free exciton features. The superlattice samples showed large blue shifts (∼400 meV) in the photoluminescence as the layer thicknesses were varied. These results are consistent with strong quantum confinement of the heavy holes in the ZnSe layers (valence band offset for ZnSe/ZnS ∼ 850±100 meV), while the electrons are not confined to either layer because of very small conduction band offsets Strong exciton photoluminescence exhibited by our samples indicates that pulsed laser deposition is a promising growth technique for the fabrication of iI-VI epitaxial layers and strained layer superlattices for visible light emitter applications.

We report the successful growth of ZnTe and ZnSe epitaxial layers on GaAs by pulsed laser deposition. A frequency doubled Nd:YAG laser was used to ablate/evaporate II-VI bulk targets and pressed powder targets in an ultra high vacuum enclosure. For typical growth temperatures in the range 200°-400°C x-ray analysis of the layers revealed sharp peaks with no evidence of growth in other orientations. Polarization dependent Raman spectroscopy was also used to further characterize the epitaxial layers, by verifying the selection rules for backscattering from oriented films. The low temperature photoluminescence spectra show distinct near-band-edge features indicating high crystalline quality. The photoluminescence of the films grown from bulk targets was superior to that of films grown from pressed powder targets, indicating that the use of high purity bulk targets is critical. Our results indicate that pulsed laser deposition is a promising new growth technique for the fabrication of II-VI epitaxial layers with unique advantages.

We have measured and analyzed the photoluminescence spectra from MBE grown CdxZn1−xTe/ZnTe strained layer superlattices under high pressure. In these superlattices, the electrons and heavy holes are confined in the CdxZn1−xTe layer (type I) while the light holes are confined in the ZnTe layer (type II). The low temperature photoluminescence is typically dominated by a strong feature due to the decay of the light-hole exciton, and weaker features due to the decay of the heavy-hole exciton and due to luminescence from an excited quantum well state. We report the pressure dependence of the photoluminescence up to 6.8 GPa. Our results are consistent with a small valence band offset between CdTe and ZnTe. We have measured the difference of the hydrostatic valence band deformation potentials.

The nearly lattice—matched InAs/GaSb/AlSb system offers tremendous flexibility in designing novel heterostructures due to the wide range of available band alignments. We have recently exploited this advantage to demonstrate several different devices exhibiting negative differential resistance (NDR) based on interband tunneling. These devices show a wide range of different characteristics including very high peak current densities (1.6 × 105 A/cm2) or peak to valley current ratios (20:1 at 300K and 88:1 at 77K). We have also studied “traditional” double barrier (resonant) tunneling in the InAs/GaSb/AlSb system. In particular, extremely high peak current densities in InAs/AlSb double barrier devices have been exploited to fabricate oscillators operating at the highest frequencies yet reported. Two and three terminal tunneling devices in this material system show great promise for use in high frequency analog and digital applications.

We present photoluminescence spectra from CdZnj_Te /ZnTe and ZnSe,,Tei_ /ZnTe strained layer superlattices grown by MBE, and analyze the band alignments and strain effects. Our results are based on fitting the dominant photoluminescence peaks to the superlattice band structure obtamed by k •theory. We find that the valence band offset of the CdZniTe /ZnTe system is quite small. On the other hand, the photoluminescence data from the ZnSeTei_ /ZnTe superlattices suggest that the band alignment is type II, with a large valence band offset. We also investigate the band gap bowing in the ZnSeTej_ alloys, and determine the individual components of the bowing in valence and conduction bands. Based on our results for band alignments, we evaluate the prospects for minority carrier injection in wide bandgap heterostructures based on ZnSe, ZnTe, and CdTe.

We have calculated the absorption strengths for intersubband transitions in n-type Si1–xGex/Si superlattices. These transitions can be used for the detection of long-wavelength infrared radiation. A significant advantage in Si1–xGex/Si superlattice detectors is the ability to detect normally incident light; in Ga1–xAlxAs/GaAs superlattices intersubband absorption is possible only if the incident light contains a polarization component in the growth direction of the superlattice. We present detailed calculations of absorption coefficients, and peak absorption wavelengths for [100], [111], and [110] Si1–xGex/Si superlattices. Peak absorption strengths of about 2000–6000 cm^–1 were obtained for typical sheet doping concentrations ([approximately-equal-to]10^12 cm^–2). Absorption comparable to that in Ga1–xAlxAs/GaAs superlattice detectors, compatibility with existing Si technology, and the ability to detect normally incident light make these devices promising for future applications.

Little has been published about Te-rich ZnSexTel−x grown at low temperatures, in spite of some successes in the fabrication of wide band gap light emitting devices from ZnSeTe alloys grown at higher temperatures. We present x-ray diffraction and photoluminescence (PL) spectra for ZnSeTe epilayers and ZnSeTe/ZnTe superlattices grown by molecular beam epitaxy (MBE). These we compare with measurements on ZnTe, ZnSe and CdZnTe epilayers and on CdZnTe/ZnTe superlattices grown under similar conditions and also with data published for ZnSeTe alloys grown at high temperatures. Equilibrium phase diagrams for the ZnSeTe alloy system suggest a large miscibility gap at MBE growth temperatures; this may account for some unusual features in the (PL) spectra and for large line widths in the x-ray data. In spite of these possible miscibility problems, we find that ZnSeTe alloys luminesce brightly.

We have analyzed photoluminescence spectra from CdxZnl−xTe /ZnTe and ZnSexTel−x/ZnTe strained layer superlattices grown by MBE, and obtained the band offsets by fitting to theory. We find that the valence band offset of the CdTe/ZnTe system is quite small (-50± 160 meV). In CdxZnl−xTe /ZnTe superlattices, the electrons and heavy holes are confined in the CdxZn1−xTe layers (type I), while the light holes are confined in the ZnTe layers (type II). On the other hand, the photoluminescence data from the ZnSexTe1−x /ZnTe superlattices suggest that the band alignment is type II, with a large valence band offset (−907 ± 120 meV). We also investigated the band bowing in the ZnSexTel−x alloys by optical spectroscopy, and found that there is only a small component of bowing in the valence band, while most of the bowing occurs in the conduction band. Based on our results for band alignments, we evaluate the prospects for minority carrier injection in wide bandgap heterostructures based on ZnSe, ZnTe, and CdTe.

A novel technique is described to enhance the doping in semiconductors by the application of external electric fields during crystal growth. We show that this technique can enhance the doping efficiency, and suppress self‐compensation processes in novel growth techniques such as molecular‐beam epitaxy (MBE). An obvious application of this technique is to enhance the doping of wide band gap II–VI semiconductors, where doping in both n‐ and p‐types is usually not possible to achieve because of extensive self‐compensation. The physics of the electric field assisted doping process can be described in two parts. First, the external electric field produces a change in the band bending at the growth surface and alters the carrier concentrations near the surface region. This influences doping near the surface region. Second, this enhanced surface doping concentration can be kinetically buried by low temperature growth processes. In our calculations, the dopants are modeled as charged, mobile species that are free...

We report the band structure and optical properties of Si-Si1-xGex superlattices calculated by k⋅p theory using the envelope-function approximation. In this paper we have demonstrated that direct-band-gap Si-Si1-xGex superlattices can be achieved by a suitable choice of layer thicknesses. We have presented detailed results for Si-Si0.5Ge0.5 superlattices grown on Si0.75Ge0.25 buffer layers with layer thicknesses in the range from 4 to 24 monolayers. Our calculations indicate that the optical absorption strengths can vary by 3–4 orders of magnitude even for layer thickness variations as small as 1–2 monolayers. Thus, it is important to control the layer thicknesses to a monolayer accuracy to obtain the enhanced optical absorption strengths. Although these optical absorption strengths are 3–4 orders of magnitude larger than bulk Si or Ge, they are still 3 orders of magnitude smaller than the absorption strengths due to direct transitions in materials such as GaAs.

We report the first study of the band structure of ZnSe-ZnTe superlattices. Our calculations are based on second order k⋅p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. Since the valence-band offset for this system is not well known, we have investigated the dependence of the superlattice band gap on the valence-band offset. Based on the assumption that the photoluminescence from the superlattice is due to an isoelectronic Te1-bound exciton in ZnSe, we have fitted the experimental photoluminescence data with k⋅p theory to obtain the best value of the valence-band offset. The value we find is 0.975±0.098 eV. Alternatively, assuming that the photoluminescence is due to band-to-band transitions we obtain a valence-band offset of 1.196±0.134 eV. We have also calculated the superlattice band gap as a function of the constituent-material-layer thicknesses for the first valence-band offset quoted. We expect these results to be important in gaining an understanding of the value of the band offset, and the nature of the photoluminescence from this system.

We report the first study of n‐type Si‐Si1−xGex resonant tunnel structures. Strain effects in these structures induce splittings of the sixfold conduction bands into twofold and fourfold states, and change the band‐edge profiles considerably. We demonstrate that resonant tunneling due to twofold, fourfold, or twofold and fourfold electrons can be selectively achieved by a proper choice of the layer thicknesses and alloy concentrations in the barrier layers. The possibilities for using these phenomena for making electron filters and making accurate determinations of the band offsets are discussed.

The ability to fabricate epitaxial layers of II–VI semiconductors has made it possible to circumvent the doping difficulties of II–VI semiconductors and to fabricate visible light emitters. Heteroep it axial structures are one of the promising approaches. The more interesting cases involve heavy strain and require particular values of the band offsets. We review recent work on CdTe/ZnTe and ZnSe/ZnTe multiquantum well structures.

ZnSe-ZnTe superlattices have generated a great deal of interest in the optoelectonics arena particularly as visible light emitters. There is a large research effort particularly by the Japanese to make blue light emitting diodes (LEDs) with ZnSe-ZnTe superlattices. Successful growth of these superlattices has been achieved with molecular beam epitaxy (MBE). Photoluminescence from this system suggests that the valence band offset between ZnSe and ZnTe is about 1.0 eV. We have performed band structure calculations based on k•p theory to study the dependence of the valence band offset and the band gap on strain. Based on the assumption that the photoluminescence from the superlattice is due to the emission from a Tel bound exciton in ZnSe, we have fit the experimental photoluminescence data with k•p theory to obtain the best value of the valence band offset. The value we find is 0.97 ± 0.10 eV. Alternatively, assuming that the photoluminescence is due to band to band transitions, we obtain a valence band offset of 1.20 ± 0.13 eV. We have also calculated the superlattice band gap as a function of the constituent material layer thicknesses for the valence band offsets quoted. We expect these calculations to play an important role in gaining an understanding of the ZnSe-ZnTe superlattices.

We report the successful growth of ZnTe on nearly lattice-matched III-V buffer layers of InAs (0.75%), GaSb (0.15%), and on GaAs and ZnTe by molecular beam epitaxy. In situ reflection high-energy electron diffraction measurements showed the characteristic streak patterns indicative of two-dimensional growth. Photoluminescence measurements on these films show strong and sharp features near the band edge with no detectable luminescence at longer wavelengths. The integrated photoluminescence intensity from the ZnTe layers increased with better lattice match to the buffer layer. The ZnTe epilayers grown on high-purity ZnTe substrates exhibited stronger luminescence than the substrates. We observe narrow luminescence linewidths (full width at half maximum ~ 1–2 A) indicative of uniform high quality growth. Secondary-ion mass spectroscopy and electron microprobe measurements, however, reveal substantial outdiffusion of Ga and In for growths on the III-V buffer layers.

We report the first study of the self-consistent electronic wave functions at a heavily doped GaAs–AlxGa1–xAs interface. The doping densities in the GaAs are between 10^17 and 10^19 cm^–3. These doping densities are characteristic of tunnel structure devices. The Schrödinger equation is solved self-consistently in the Hartree approximation. Some of the more important results of our calculations include the fact that there is only a single subband level in the well for a wide range of biases. This level is also fairly loosely bound (bound by a few meV) in spite of the fact that the notch at the interface is on the order of a 100 meV. In accumulation layers, the potential at the interface is somewhat similar to the non-self-consistent one. However, in depletion layers the self-consistent potential can be substantially different from the one obtained in the Thomas–Fermi approximation.

We demonstrate that quasi direct band gap Si1–xGex/Si superlattices can be obtained by suitable choices of layer thicknesses. We calculate strain dependent conduction-band offsets as functions of the substrate alloy concentration, and of the epilayer alloy concentration. Optical matrix elements are computed for Si0.5Ge0.5/Si superlattices grown on Si0.75Ge0.25 buffer layers with superlattice layer thicknesses of 4 to 24 monolayers. We find that optical absorption and emission strengths can vary by three to four orders of magnitude for layer thickness variations as small as 1–2 monolayers, suggesting that layer thicknesses must be controlled to within one monolayer to obtain enhanced optical properties. Typical optical matrix elements calculated for these Si1–xGex/Si superlattices are three to four orders of magnitude larger than for bulk Si or Ge, but, are still three orders of magnitude smaller than for direct band gap materials such as GaAs.

The ZnSe–ZnTe superlattices have attracted considerable attention as possible blue/green light emitters. Although these superlattices have been successfully fabricated and show intense photoluminescence, there are many basic issues about this system which still remain unresolved. The most important of them is the value of the valence band offset between ZnSe and ZnTe. We have studied the band structure of ZnSe–ZnTe superlattices. Our calculations are based on second order k·p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. We have investigated the dependence of the superlattice band gap on the valence band offset. Based on the assumption that the photoluminescence from the superlattice corresponds to a bound exciton at a Te1 isoelectronic center in ZnSe, we have fit the experimental photoluminescence data with k·p theory to obtain the best value of the valence band offset. The value we find is 0.97±0.10 eV. Alternatively, assuming that the photoluminescence was due to band-to-band transitions we obtain a valence band offset of 1.20±0.13 eV. We have also calculated the superlattice band gap as a function of the constituent material layer thicknesses for the first valence band offset quoted. We expect these results to be important in gaining an understanding of the value of the valence band offset, and the nature of the photoluminescence from this system.