Absorption, photoluminescence, photoreflectance and transmission electron microscopy measurements have been performed on 2 and 3-monolayer thin InAs quantum wells(QW) (with 250Å wide GaAs barriers) grown on (511) GaAs substrate. For comparison, similar samples grown on (100) substrates have also been studied. (511)-grown thin quantum wells may have possible quantum wire configuration. However, polarization studies show a small anisotropic absorption from the (511) sample, which indicates that the optical property of the (511)-quantum well is different than either that of a quantum wire or that of a (l00)-grown quantum well structures. A theoretical calculation, making a unitary transformation of the valence band k•P Hamiltonian matrix to the (511) base, and using a perturbation method to determine the new wavefunctions, yield an anisotropic absorption comparable with the experimental result. We have also compared the transition energies from PL data with the calculated one using the conventional effective mass approximation.

Raman Scattering by coupled longitudinal optic phonons and two-dimensional electron gas electrons in In0.52Al0.48As-In0.53Ga0.47As δ-doped heterostructures provides a powerful probe of electronic properties in these In-based structures. The two highest frequency modes, of the three coupled electron-phonon modes expected in this system, were observed, with the highest frequency mode being identified for the first time in InGaAs-based systems. The large dispersion of this mode makes it a particularly sensitive probe for changes in such properties as carrier concentration and subband energy. For structures with higher carrier concentrations coupling of the longitudinal optic phonon to multiple electron intersubband transitions is resolved. These measurements are particularly useful for heavily-doped structures for which room-temperature Hall measurements cannot distinguish channel electrons from those in parallel conduction paths. In addition HBr-based reactive ion etching has been performed on these structures, and the effects have been correlated with etch time and bias voltage for structures with different cap layer thicknesses.

Discrete peaks, due to the excited 2S state of the n=l symmetric heavy hole (hh) exciton, are observed in the photoluminescence excitation spectra of GaAs/AlGaAs coupled double quantum well (CDQW) samples. The observations of the 2S state allow a precise determination of 1S-2S splitting, and the experimental results show a good agreement with a simple calculation which is based on a model of fractional-dimension of the exciton state. The obtained values for the 1S-2S splitting are significantly reduced compared with the corresponding single (or multiple) quantum wells (QW). Central acceptors confined in CDQWs have also been studied. Due to built in electric fields, interwell and intrawell excitons bound to neutral acceptors are simultaneously observed in weakly coupled doped double QWs. The diamagnetic shift and binding energy of the bound excitons are similar to results of the single QW’s, while the diamagnetic shift of the free exciton differs from single QW’s.

A direct comparison of admittance spectroscopy and DLTS on p-type Si/Si1-xGex/Si quantum wells with x = 0.17 was performed using n+p mesa diodes. The temperature dependence of potential barriers at the QW and of the Fermi level determines the activation energy Ea of the conductance across the QW. From Ea a band offset of ΔEV = 0.16 eV was estimated. DLTS data suggest that hole emission from the QW was observed with an activation energy in correspondence with ΔEV.

The relation of relaxation to surface morphology in linearly-graded InAlAs was examined as a function of growth temperature using x-ray diffraction and atomic force microscopy (AFM). Samples were grown at temperatures ranging from 370°C to 550°C. Weak diffraction features for samples grown at 370°C and 420°C limited determination of the extent of their relaxation . The fractional relaxation of samples grown between 470°C to 550°C was essentially identical (~77%) and symmetric in orthogonal [110] directions. The character of the surface morphology changed from random small scale roughness to a roughness more periodic in nature as the growth temperature increased. Although no asymmetry was observed in the relaxation, the roughness developed an asymmetry at higher growth temperatures with more prominent ridges along [110]-[110].

Quantum wires which form at the T-shaped intersection of two 7 nm wide quantum wells have been embedded in the active region of AlGaAs/GaAs lasers. The quantum wires whose dimensions can be precisely controlled on a monolayer length scale have been prepared by cleaved edge overgrowth, a molecular beam growth technique which involves regrowth on the cleavage plane of a previously grown multilayer structure. The quantum wire emission wavelength is found to be nearly independent of the optical excitation level. This absence of band-gap renormalization effects at all pump intensities implies that the gain mechanism in our laser is excitonic, and further indicates a marked increase in the stability of the excitonic gas phase in one dimension. This is consistent with the observed increase in the exciton binding energy by more than 50% compared to the two-dimensional case.

Nonlinear relaxation of a sharp concentration profile typical in layered semiconductor junctions is investigated using the Path Probability Method (PPM) of irreversible statistical mechanics. We employ the vacancy mechanism for atomic migration and the pair approximation for the statistical treatment. The PPM is a microscopic method, from which we can derive macroscopic parameters. Our results show that at the initial stage of the relaxation of a sharp concentration profile, atoms near the junction may diffuse up against the concentration gradient. More surprisingly, our numerical examples convincingly demonstrate that the atom flux goes up against the local chemical potential gradient near a sharp profile, indicating that in such highly nonlinear regime the usual linear diffusion theory in which the atom flux is linearly proportional to the chemical potential gradient breaks down. It is shown that the cause of the uphill diffusion is the repulsion among different species, which is also the physical origin of the square gradient term.

Currently, the materials with the highest thermoelectric figure of merit (ZT) are one-band materials. The presence of both electrons and holes lowers ZT, so two-band materials such as semimetals are not useful thermoelectric materials. However, by preparing these materials in the form of two-dimensional quantum-well superlattices, it is possible to separate the two bands and transform the material to an effectively one-carrier system. We have investigated theoretically the effect of such an approach and our results indicate that a significant increase in ZT may be achieved. We have also evaluated the possibility of using intercalation as a means to achieve an increase in ZT. Our results allow the possibility of using new types of materials as thermoelectric refrigeration elements.

The eigenstates of electrons and holes confined in cylindrical quantum wires with infinite potential barrier are found. The degree of linear polarization of luminescence is calculated.

Raman scattering from a (CdSe)m(ZnSe)n-ZnSe strained quantum well structure, composed of (CdSe)m(ZnSe)n as well layers and thick ZnSe as barrier layers, is presented in this report. Two kinds of ZnSe-like LO phonon confined modes were observed by Raman scattering at 10-300 K. Resonance Raman scattering was accomplished by changing sample temperature and incident photon energy and as a results, up to 7 orders of ZnSe-like LO multi-phonon modes were observed. In the discussion, we suggest that anti-Stokes Raman scattering measurements can provide a meaningful way to distinguish the multi-phonon Raman scattering from hot luminescence.

The realisation of resonant tunneling transistors with a quantum-well base layer requires good Ohmic contacts on a two-dimensional electron gas. Both a good base-emitter separation and a good base-collector separation without the depletion of the base layer are essential requirements. The depletion of the base layer would prevent the injection of base current. Also the penetration of the contact metal needs to be restricted to prevent destruction of the two-dimensional electron gas layer. A 100 nm base-emitter separation can be obtained using a self-aligned base etching combined with the PdGe non-alloyed contact technology. A 10 nm wide quantum well spacer layer prevents the quantum-well consumption by the PdGe regrowth and allows the charging of the base layer.

Cyclotron resonance (CR) of electrons confined in an InAsZAlo.5Gao.5Sb quantum well has been used to investigate the negative persistent photo-effect (NPP) at low temperatures. After an in situ cross-gap light illumination, the electron density is reduced 28%, and the CR effective mass decreases from (0.0342±0.0002)mo to (0.0322±0.0002)mo. The response time of the NPP build-up transient has been studied via the photon-dose technique, and it is on the order of 10 msec with an illumination power flux of ~ 10mW/cm2.

A survey of the most common and useful interband optical techniques for characterizing semiconductor quantum well structures is presented. These are divided into three sections; ground state spectroscopy which deals with the transition between the lowest energy state of the conduction band well and the highest energy state of the valence band well; density-of-states spectroscopy which probes higher energy transitions between excited states in the conduction and valence band wells; and other spectroscopies which entails more sophisticated experiments which don’t fit conveniently into the first two categories. Examples are given of each in order to demonstrate the power of optical spectroscopy to elucidate the properties of semiconductor quantum wells.

[EuTe(ξ)/PbTe(η)]n short period superlattices (SLs) were grown by MBE along the [111] growth direction, with ξ denoting the number of monolayers of EuTe separated by η monolayers of nonmagnetic PbTe, whereby 2 ≤ ξ ≤5 EuTe layers alternate with 6 ≤ξ≤ 15 PbTe layers, such that η= 3ξ.We find that SLs with f = 4 show an antiferromagnetic-like phase transition; those with ξ = 3 and 5 show a ferrimagnetic-like phase transition, while bulk EuTe is a simple two-sublattice antiferromagnet. The transition temperatures Tc for these short period SLs are determined by temperature-dependent magnetization (i.e., M(T)) measurements with field cooling (FC) and zero field cooling (ZFC) cycles. The magnetic ordering temperatures quickly converge from below to the bulk phase Neél temperature 9.6 K, as ξ increases from 2 to 5. We propose a canted antiferromagnetism model to describe M(T) for SLs with ξ = 2 and ξ = 3, consistent with a mean-field analysis of Tc. We also discuss why the magnetic character of EuTe can be modulated from antiferromagnetic to ferrimagnetic.

Ohmic contacts are optimized on δ-doped lattice-matched (x=0.53) and pseudomorphic (x=0.80) lno.53Gao.47As/Alo.52Ino.48As/InxGa1-xAs/AI0.52ln0.48As layers grown on InP by molecular beam epitaxy (MBE). The layers are characterized by temperature-dependent Hall measurements (4-300 K) and Shubnikov-de Haas measurements at 1.2 K. Lowest specific transfer resistance (< 0.1 O.mm) is obtained with a Ni/AuGe/Ni/Au metal sequence alloyed at 280 °C for 1 min. The influence of the Ni content in the metallization is studied and AuGe layers evaporated from the eutectic composition (Au:Ge, 88:12 weight %) are compared with successively evaporated Au and Ge layers. It is shown that the resulting contact resistance is not very sensitive to changes in these parameters. The solid state reactions involved are studied by sputter Auger depth profiling.

Schottky characteristics of electrical contacts to complex heterostructures consisting of intermediate semiconductor layers with various thicknesses and underlying bulk layers have been studied. For Au/InAlAs/InP structures, I-V characteristics with thin InAlAs layers are found to be similar to those of a MIS tunneling diode, while those with thick InAlAs layers are found to be close to that of Au/InAlAs. For Au/InP/InAlAs structures, a substantial current barrier are found to be formed not at Au/InP but at InP/InAlAs heterojunction. Schottky barrier height in the case of strained intermediate layers of In1-xGaxP is also discussed.

We have measured the static I-V characteristics of pseudomorphic conventional HEMTs and FETs based on parallel array of quantum wires (UNIFETs) at ambient, 77, and 1.5K, respectively. These measurements show that at ambient temperature the saturation-regime extrinsic transconductance (gm) of the UNIFETs is about 80% higher than that of the conventional HEMTs. The gm of both types of devices increases as the temperature is lowered. The rate of increase is higher for the UNIFETs due to the one-dimensional character of electron transport. FETs based on array of parallel quantum wires are well-suited for high-frequency applications because of their high transconductance resulting from size quantization and lateral confinement of the carriers.

We present comparisons of photoluminescence (PL) data for various vertical-cavity surface-emitting laser (VCSEL) and distributed quantum well structures taken with the pump beam (and the collection path) in two different configurations: normal to the surface of the sample; and perpendicular to a cross-section of the epitaxial layers. We demonstrate that the cross-sectional PL (XPL) technique can resolve individual features in the structures, and that the surface-normal PL (NPL) spectra are perturbed by the multilayer mirrors in the VCSELs. We elucidate a potential method for transforming between the NPL spectra and the non-perturbed XPL spectra and evaluate the sensitivity of this method to various measurement as well as material parameters. This simulation technique is well-suited to wide parametric variations of the dispersion curves for the complex dielectric constant of the materials, the pump field distribution, and the depth profile of the gain medium.

For the thermal quenching of quantum well (QW) photoluminescence (PL) controversal de-activation energies have been reported. Values range from the total exciton (or electron-hole) binding energy Ex to half of Ex or to the binding energy of the shallower bound particle. We resolve this controversy by performing steady-state measurements both under high and extremely low (Iex≈l μW/mm2) excitation conditions on a series of multiple QW structures of the material systems InxGa1-xAs/GaAs and GaAs/AlyGa1-yAs. For high and low excitation, we find that the final de-activation step is associated with Ex. In an intermediate temperature range, the first decade of PL quenching is governed by a de-activation energy close to the binding energy of the shallower bound particle.

Also performed were temperature dependent photoluminescence excitation (PLE) measurements under high injection in InxGa1-xAs/GaAs double QWs with additional AlyGa1-yAs cladding barriers. Vertical transport between the QWs is observed and the underlying de-activation energies are found to be equal to the total exciton binding energies Ex We discuss our findings in the frame of a simple model for the thermalization of electrons and holes.

We present the results of luminescence studies for free-standing CdZnS/ZnS strained layer multi-quantum wells (MQWs). PL measurements of the CdZnS/ZnS MQW at 1.4 K show intense, sharp excitonic emission in blue-ultraviolet spectral region. The band discontinuity is estimated by an analysis of the luminescence results in terms of a finite-potential well model. A free-standing MQW is assumed. The calculated zero-stress band discontinuity is in extremely good agreement with the value from Harrison’s model.