R. C. Miller

Impurity trapping, interface structure, and luminescence of GaAs quantum wells grown by molecular beam epitaxy

A correlation has been demonstrated between impurity trapping and the interface structure in GaAs quantum well (QW) superlattices and single quantum well structures grown by molecular beam epitaxy (MBE) on (100) GaAs. Using low‐temperature cathodoluminescence, photoluminescence, and transmission electron miscroscopy, we have shown that interface roughness in QW superlattices is related to trapped impurities at interfaces. We have observed that impurities originate from either the substrate or from the GaAlAs MBE layers. A new getter smoothing effect associated with QW structures is shown to produce efficient impurity trapping and yield higher quality GaAs QW with atomically smooth interfaces.

Laser oscillation from quantum states in very thin GaAs−Al0.2Ga0.8As multilayer structures

We report optically pumped laser oscillation from multilayer heterostructures consisting of alternating layers of GaAs and Al0.2Ga0.8As. Very thin GaAs layers (50−500 A) exhibit one−dimensional bound states above the band gap of bulk GaAs. The laser oscillation occurs at energies which are slightly below the exciton associated with the lowest energy n=1 bound state.

TEMPERATURE DEPENDENCE OF THE OPTICAL PROPERTIES OF FERROELECTRIC LiNbO3 AND LiTaO3

Linear and nonlinear optical investigations of LiNbO3 and LiTaO3 from room temperature to temperatures above their ferroelectric phase transitions are described. The measurements include studies of second harmonic generation with an Nd laser (1.06 μ) and determinations of the refractive indices as well as the birefringence at 0.6328 μ. All the optical data exhibit distinct changes at the Curie temperatures, Tc = 1195°C ± 15°C for LiNbO3, and Tc = 610°C ± 10°C for LiTaO3. In each case the phase change has the characteristics of a second‐order ferroelectric transition. The uniaxial paraelectric phases are concluded to be centrosymmetric — probably space group R3c.

Motion of 180° Domain Walls in Metal Electroded Barium Titanate Crystals as a Function of Electric Field and Sample Thickness

The domain dynamics of polarization reversal in metal electroded BaTiO3 single crystals have not been adequately described in the literature. The present paper contains quantitative measurements of the velocity of the sidewise motion of 180° domain‐walls in BaTiO3 crystals, most of which were electroded with 200‐A thick films of Au. The wall velocity has been measured as a function of several variables including the applied electric field, the crystal thickness and the impurity content added to the crystal growth melts.The wall velocity as a function of the field E is described by v∞ exp (−δ/E) where δ and v∞ are constants over several decades of velocity. The quantities δ and v∞ are the activation field and extrapolated wall velocity for E=∞, respectively. This field dependence of the wall velocity is the same as that reported previously for liquid electroded specimens. The thickness dependence of the wall velocity, which is through δ, is very pronounced and similar to that observed in measurements of ot...

Current injection GaAs‐AlxGa1−xAs multi‐quantum‐well heterostructure lasers prepared by molecular beam epitaxy

Low‐current‐threshold room‐temperature injection GaAs‐AlxGa1−xAs multi‐quantum‐well (MQW) lasers have been prepared by molecular beam epitaxy. Under pulsed current injection, lasing emission attributed to the n=1 electron‐to‐light‐hole (1 e‐lh) confined‐particle transition was observed at threshold. Above threshold, lasing emission involving the n=1 electron‐to‐heavy‐hole transition (1 e−hh) became dominant. Single longitudinal mode operation has also been observed for these broad‐area MQW lasers. For heat‐sink temperatures between 8 and 100 °C, the lasing current threshold Ith for the 1 e−hh transition has an exponential variation with temperature of the form Ith∝ exp(T/T0), where T0=230 °K.

Extrinsic layer at AlxGa1−xAs‐GaAs interfaces

Photoluminescence studies of GaAs‐AlxGa1−xAs heterostructures grown by molecular beam epitaxy (MBE) which contain one and two GaAs quantum wells suggest that in some of the samples the first well grown usually exhibits a ’’rough’’ (6 A≲δL≲15 A) first interface and an acceptorlike luminescing impurity in the first few tens of angstroms of GaAs grown. The luminescence may arise from carbon which accumulates on the AlxGa1−xAs ‐vacuum interface during growth and is then deposited in the first few unit cells of GaAs. The phenomenon discussed may be relevant to the problem of growing high electron mobility field‐effect transistor heterostructures by MBE with GaAs on the top.

Optical processes of 2D electron plasma in GaAs-(AlGa)As heterostructures

Abstract We observed the characteristic behavior of a 2D electron plasma in optical emission and absorption across the energy gaps in modulation-doped GaAs-(AlGa)As quantum-well heterostructures. A renormalization of the energy gap with magnitude comparable to the Hartree energy is found in emission spectra. Large breakdown of the parity selection rule of the optical matrix element in quantum-wells is also observed. In absorption there is evidence of final-state electron-hole interactions even at plasma densities n≳ 4 × 1011 cm−2.

Asymmetric Hysteresis Loops and the Pyroelectric Effect in Barium Titanate

Dynamic pyroelectric techniques have been used to study single crystal BaTiO3 hysteresis loops which have been reported in several recent investigations to be asymmetric with respect to both the polarization and electric field axes. It is shown that an apparent polarization bias occurs because of electrode‐edge effects and that this bias may be shifted with an appropriate application of a dc field which serves to alter the direction of the polarization in fringe regions which contribute to the observed pyroelectric signal. The pyroelectric hysteresis loops are symmetric with respect to the polarization when electrode‐edge effects are eliminated. These data strongly suggest that asymmetric remanent polarizations reported in several piezoelectric and pyroelectric investigations may have been due to edge effects. The pyroelectric hysteresis loops are found to be biased with respect to the electric field axis in the same manner as that reported in a recent piezoelectric study. Small residual pyroelectric sign...

Properties of (Al,Ga)As/GaAs heterostructures grown by molecular beam epitaxy with growth interruption

Abstract Very high-quality GaAs/Al 0.37 Ga 0.63 As single quantum wells with extended monolayer-flat regions have been grown by molecular beam epitaxy with growth interruption at hetero-interfaces. Low-temperature photoluminescence and excitation spectroscopy have been used to study these samples. The spectra exhibit well-resolved multiple sharp peaks which are due to changes in well thickness of one monolayer (ML). These peaks are the sharpest and best resolved to date for narrow wells. For example, the full widths at half maximum for single wells of width 57 A (20 ML), 28 A (10 ML), and 17 A (5 ML) are 1.0, 1.7, and 6.0 meV, respectively. Although growth interruption introduces neutral acceptors, presumably carbon, they do not affect the 77 K mobilities of selectively doped heterostructures with thin spacers.

Staggered band alignments in AlGaAs heterojunctions and the determination of valence‐band offsets

Photoluminescence spectra from Al0.37Ga0.63As/AlAs multiple quantum well structures with staggered band alignments are presented which provide the first direct optical measure of the valence‐band offset at a semiconductor heterojunction. The experiment takes advantage of the crossover occurring at a critical aluminum concentration above which the indirect X minima in the AlAs become the lowest energy conduction bands in the system, and recombination occurs across the interface. The resulting emission fixes the valence‐band offset at ΔEv =342±4 meV for this structure.