The quantum-confined Stark effect (QCSE) has been studied in electroluminescence (EL) spectra by using GaAs/AlQ 3GaQ 7As symmetric coupled double-quantum-well (CDQW) structures under a "forward" electric field. The Stark shift for the transition between the ground electron state and heavy-hole state was successfully observed, and was found to be symmetrical with regard to the flat-band bias for EL and photoluminescence (PL), in which the electric field was applied in the forward and reverse directions, respectively. Circularly polarized photoluminescence excitation (CPPLE) spectroscopy was also employed to clarify the origins of optical transitions related to either heavy-hole or light-hole resonance. Each transition between electrons and light- and heavy-holes in the CDQWs was clearly identified by PL and by polarization of CPPLE spectra.

A number of recent studies of GaAs/AlxGa(1-x)As quantum wells using low temperature photoluminescence have demonstrated the formation of bi-excitons. It is generally assumed that bi-excitons will only be formed in samples with low impurity concentration, we demonstrate in this work that bi-excitons can be observed in 150 Å quantum well samples with an acceptor doping density as high as 3x1017 cm-3. We discuss how bi-exciton formation is limited by impurity capture and show how this influence can partly account for the less than quadratic intensity dependence typically observed.

This paper reviews the use of modulation spectroscopy for the characterization of a wide variety of semiconductor heterostructures and devices. Some systems that will be discussed include pseudomorphic GaAlAs/InGaAs/GaAs modulation-doped quantum well high electron mobility transistors (including the 300K determination of the two-dimensional electron gas density), GaAlAs/GaAs, InP/InGaAs, InGaP/GaAsInAlAs/InGaAs and InGaAs/GaAs heterojunction bipolar transistors (including the determination of the built-in fields/doping levels in the emitter and collector regions), GaAs/GaAlAs quantum well infrared detectors, quantum well lasers and GaAs/GaAlAs quantum dots. Particular attention will be paid to non-destructive, contactless techniques such as photoreflectance, contactless electroreflectance and differential reflectometry that can be performed on entire wafers.

We present electric-field dependent electroreflectance and photocurrent spectra of visible-bandgap Inx(AlyGa1-y)1-xP/Inx’(Aly’Ga1-y’)1-x’P multiple-quantum-well (MQW) structures. These structures, grown by metal-organic vapor phase epitaxy on 6°-misoriented (100) GaAs substrates, have undoped MQWs sandwiched between doped Ino 5AI0.5P layers, forming p-i-n diodes. Quantum-well compositions in the range 0.46≤x≤0.52 and 0≤y≤0.4, corresponding to bandgaps in the red to yellow range, were used. The Stark shifts in these various samples were measured and found to depend on the details of the Mg p-type doping profile, confirming important diffusion effects, in agreement with secondary ion mass spectrometry and capacitance-voltage data. Our results show that these new materials are promising for visible-wavelength optical modulator applications.

Temporally and spatially resolved photoluminescence has been used to study patterned structures prepared by focused ion beam (FIB) implantation of multiple quantum wells (MQWs) followed by rapid thermal annealing (RTA). Exciton lifetimes at different positions across the interface between implanted and unimplanted regions suggest that the interface between these two regions is of good quality. Anisotropic exciton diffusion is also observed, suggesting that these structures provide lateral confinement for excitons.

We have performed low temperature (5K) photoluminescence (PL) measurements in order to study the crystalline quality of pseudomorphic modulation-doped field effect transistors (MODFET’s) grown by MBE. MODFET’s based on GaAs/AlGaAs with either an InGaAs or a (GaAs)n(InAs)m short period superlattice (SPS) channel have been studied. On modulation doped structures, the presence of free carriers into the channel strongly affects the PL emission. In this paper we present a study of PL line shape of MODFET structures with different channel thicknesses and growth conditions. A correlation between the PL line shape and these parameters is clearly observed.

Ga0.5In0.5P has been investigated using optically detected cyclotron resonance and photoluminescence techniques. For the disordered alloy, the electron mass is determined to m*=0.092 m0, and for ordered material (band gap reduction ~ 50 meV) the value m*=0.088 m0 is found. The experimentally deduced values are compared with those obtained from a five-band kp calculation.

Optical properties of quantum well structures, specifically (Al,Ga)As/GaAs heterojunction structures, have been studied using transient photoluminescence spectroscopy. The (Al,Ga)As/GaAS heterojunction structures that have been studied are a Ga0.7Al0.3As/GaAs single quantum well and a device structure which consists of both a thin single quantum well and a buried superlattice structure. Transient photoluminescence spectroscopy has been done using a pulsed laser system and luminescence spectra were obtained as a function of laser wavelength and the incident angle. The results show that PL from the single well occurs at the GaAs band gap energy with a wavelength at 860 nm while the buried superlattice emits at 785nm. The relative intensity of these two signals is dependent on the layer thicknesses, excitation wavelength and reflectivity as a function of incident angle. The absorption coefficient of the composite structure can be determined from these measurements.

A many-body theory is presented for the interband absorption in a pseudomorphic GaAlAs/InGaAs/GaAs modulation doped quantum well. The electron-electron interaction in a degenerate Fermi sea is calculated in the self-consistent Hartree approximation. In addition, the binding energy within an electron-hole pair is included in the ladder approximation as a vertex correction to the response function. In our theory, the coupling between excitons is calculated in the generalized random-phase approximation (RPA). Our numerical results for the temperature-dependence of the absorption peaks at the Fermi edge (low temperature) and the band edge (room temperature) compare well with available experimental data obtained using contactless photoreflectance techniques. At low temperatures, our theory shows that there are important modifications to the single-particle model. We extract the Fermi energy by fitting our calculated results to the thermally broadened lineshape of the absorption spectrum. As a consequence, the electron density of the quantum wells can be accurately determined by means of this contactless, non-destructive, rapid and simple characterization method. Moreover, information on the alloy composition, built-in electric field and the interface stress can also be obtained.

Selective-area epitaxy was investigated by Cl-transport VPE growth of GaxIn1-xAs in features etched in a (001) InP wafer through windows of an SiCO2 mask. Growth was conducted simultaneously on masked samples with grooves oriented along the [110] and the [110] directions and a planar InP wafer oriented 3° off (001). The structures were studied with optical and cross-sectional transmission electron microscopy (TEM), X-ray diffraction, photoluminescence (PL), and cathodoluminescence (CL). Excellent surface morphology, luminescence, and near-lattice-matching were simultaneously achieved for all samples. TEM cross-sections demonstrate that the [110] and even the [110] dove-tail grooves are filled in a wellordered manner without visible defects or undue strain. CL imaging in crosssection and planview demonstrates the trend in both samples for lower bandgap material to be found in the upper center portion of the groove.

Performance of many III-V compound heterostructure devices relies on exact control of dopant placement and concentration. This requires the dopant source to be responsive and reproducible. We report a novel electron beam evaporation source for both carbon and silicon dopant incorporation in MBE growth of III-V compounds. Carbon in particular has attracted much interest as a replacement of beryllium for p-type doping in III-V compounds. Not only high doping levels are achievable, its low diffusion coefficient makes abrupt interface possible even after high temperature processing and operation. This source provides a convenient means for generating high carbon flux. It is equipped with a compact flux monitor such that the dopant levels can be adjusted in real time. The GaAs samples show excellent photoluminescence peaks and mobility results. Good carbon doping for GaSb was reported for the first time.

We have performed second harmonic generation (SHG) measurements in the 3-5 μm region on p-type stepped quantum wells (QWs) using the tunable, high peak power pulses generated by a free electron laser. The samples were grown by MBE on (100) GaAs wafers. The asymmetric QWs are made of m monolayers of GaAs and n monolayers of Al0.5Ga0.5As sandwiched between AlAs barriers. The QWs were characterized by x-ray diffraction and room temperature photoluminescence (PL). We measured an order of magnitude enhancement of the second order susceptibility over bulk GaAs. In contrast to n-type QWs, the dominant component is the χ(2)xyz component. The results are explained by a full pseudopotential band structure calculation of χ(2).

Shallow symmetric and asymmetric GaAs/InGaAs/(Al)GaAs single quantum wells with an indium content of 5% and less, grown by MBE, show striking differences in their photoluminescence spectra. Unlike the symmetric case, the asymmetric quantum wells are found to have no bound ground state below a certain thickness, which depends on the indium content of the quantum well and the aluminium content of the barrier. For a set of asymmetric samples investigated here, the bound state vanishes at about 60Å.

Photoluminescence excitation measurements on shallow symmetric and asymmetric wells show a series of narrow lines with oscillator strengths many times exceeding those of the band to band transition. These lines can be identified with excited excitonic transitions including heavy and light holes.

The band structures of three group-V semimetals As, Sb, and Bi with rhombohedral A7 symmetry are studied using a second-neighbor tight-binding model including spin-orbit interaction with an sp3s* basis. Then the bulk tight-binding parameters are used to investigate the electronic properties of semimetal-semiconductor superlattices made of alternating (111) layers of Sb and GaSb or AlSb. It is found that the band gap can be adjustable depending primarily on the thickness of the Sb layers. An interface state is observed in the region of the gap.

The recent demonstration of PL (Photoluminescence) and EL (Electroluminescence) in porous silicon has been a tremendous advance toward the integration of optoelectronics and Si digital electronics. It has come to be understood that the primary source of luminescence is quantized structures of Si or quantum nanocrvstals (QNCs). We have demonstrated IR, visible and UV photoluminescence from QNCs formed of Si and of Ge in a homogeneous SiCO2 matrix. The nanocrystal dimensions and resulting PL indicate that the active mechanism is 3-dimensional confinement of carriers in the QNCs within the wider bandgap SiCO2. Raman analysis confirms the presence of QNCs in the ~20 to 100 A range. SIMS analysis was used to gauge compositional effects on luminescence wavelength. The QNC matrix concept is immediately extendable to several additional material systems. The stable nature of QNCs embedded within a matrix offer further advances toward the integration of optoelectronics with Si devices.