We report Raman studies of the transformations between the zinc-blende (alpha) and high-pressure (beta) phases of bulk GaAs and AlAs epitaxial films under increasing and decreasing hydrostatic pressure using a 300-K diamond-anvil press. The forward alpha-beta thresholds, as measured by the simultaneous onset of opacity and loss of Raman signal, are P(a)t = 12.4 +/- 0.4 GPa for AlAs and P(g)t = 17.3 +/- 0.4 GPa for GaAs. On decompression from 20 GPa or less, reversal to the zinc-blende state occurs in both materials with a hysteresis of 6-8 GPa; otherwise, GaAs enters a metastable phase. After reversal, the returning optical-phonon peaks exhibit asymmetric broadening and negative frequency shifts. Analogy to ion-bombarded GaAs shows that postreversal material is comprised of zinc-blende microcrystallites with diameters approximately 65 angstrom and approximately 175 angstrom in GaAs and AlAs, respectively. Thermodynamic considerations based on the hysteresis and microcrystallite size suggest that the surface energy per unit area for a beta-nucleus in a pure a matrix is approximately 0.04-0.15 eV/angstrom2, in rough agreement with previous microscopic calculations for a rocksalt-zinc-blende AlAs/GaAs heterointerface. We propose that the kinetic homointerface in the bulk nucleation transitions is similar to the static sixfold-fourfold heterointerface involved in the superlattice phase changes discussed in the second paper.

The pressure-induced alpha-beta structural transitions in [001]-oriented AlAs/GaAs superlattices (SL's) are studied by Raman scattering using a 300-K diamond-anvil press. The threshold pressures of the forward and reverse transitions in the AlAs and GaAs constituents of each SL are accurately measured for layer thicknesses in the range 300-20 angstrom, and comparison is made to the bulk transitions reported in the preceding paper. We obtain direct microscopic evidence that (i) the SL constituents transform separately (first AlAs, then GaAs) or simultaneously, depending on whether the AlAs layers are thicker or thinner than approximately 50 angstrom; (ii) overpressing of zinc-blende AlAs by approximately 5 GPa above its bulk stability limit is not matched by GaAs underpressing: (iii) the postreversal condition of the SL's is marked by increasing signs of bulk and interface disorder as the constituent layer thickness decreases. The AlAs overpressing shows that the effective polymorphic stability of these SL's is GaAs controlled over a wide layer-thickness range. The thermodynamics of high-pressure SL transitions is discussed. We find that the beta-AlAs/alpha-GaAs sixfold-fourfold interface encountered at high pressure has the empirical energy density sigma(beta-alpha) = 0.12 +/- 0.02 eV/angstrom2, and is best described by a disordered-interface model. Comparison to microscopic theory for a pseudomorphic sixfold-fourfold interface shows that the calculated geometry probably does not occur at the static interfaces of AlAs/GaAs SL's, but might exist during transformation at the kinetic boundary of small beta-nuclei forming within an alpha-matrix. Alloylike pressure stability is predicted when the SL periods are substantially thinner than the smallest beta-nuclei.

Hydrostatic pressure (P) is an important tool for the study of semiconductor heterostructures. It can tune their electronic energy bands, and as a consequence, has been used to study the band offset at heterointerfaces,[1] the possibility of tunable quantum well lasers,[2] and the indirect-gap related DX-defects.[3] The phase stability of heterostructures can be studied under high pressure more conveniently than by modifying chemical composition, and novel superpressing phenomena have been observed.[4] Phase stability is discussed elsewhere in this proceedings.[5] External hydrostatic pressure can also affect the mechanical stability of heterostructures by tuning lattice mismatch.[6] This tuning is of interest and importance for strained-layer heterostructures now finding wide device applications, since lattice-mismatch strain can strongly influence electronic properties.[7]

The pressure dependence of the Raman spectrum of epitaxial zinc-blende GaAs has been investigated at room temperature up to 22 GPa. A phase transition to an opaque state is clearly detected at 17.3 +/- 0.4 GPa by the disappearance of the zinc-blende phonons. No detectable Raman signal is found in the high-pressure phase. The 1-atm state of GaAs obtained upon reducing the pressure after the phase transition depends on the maximum pressure (P(max)) reached. When P(max) is less than approximately 20 GPa, the zinc-blende phonons reappear, and the observed Raman spectrum indicates an orientationally disordered crystalline phase containing zinc-blende microcrystals approximately 70 angstrom in size. When P(max) is greater than approximately 20 GPa, an opaque metastable crystalline phase exhibiting a previously unreported cinnabarlike Raman spectrum is recovered at 1 atm. Possible candidate structures for this metastable phase are discussed in light of the available data.

We report the observation of a new trapping center in Al0.3Ga0.7As. The center becomes active under a hydrostatic pressure of approximately 45 kbar, and has an unusually deep emission barrier. It quenches all radiative transitions and causes a hysteresis in the photoluminescence intensity, which we interpret via a lattice relaxation model. It is neither the DX nor the SD center, and is probably related to a donor.

We have studied by electrolyte electroreflectance and photoluminescence a GaAs/AlGaAs resonant tunneling structure (RTS) with a highly n-doped GaAs cap, before and after hydrogenation. We measured the amount of passivation of shallow donor states and of deep traps in the cap and found the approximate pinning levels and interace charges of the RTS.

Epitaxial heterostructures configured as single or repeated layers, and, more recently, as islands, have become ubiquitous within semiconductor physics because of the diverse phenomena and applications made possible through their tailored growth.[1] The practical limits of heterostructure tailoring are defined by a variety of instabilities related to bulk, interface, and local bonding properties, and the study of these instabilities offers fundamental insight into the competition between mechanical and chemical forces at each structural level. Despite the importance of such issues, most experimental and theoretical work on heterostructure stability has been limited to external environments compatible with growth, and, hence, has not considered the effects of extreme hydrostatic pressure.[2–4] Although this is understandable in terms of current interest trends, it overlooks the nature of pressure as a thermodynamic parameter capable of shifting heterostructures into regions of phase space which ordinarily are either inaccessible, or only attained in chemically different materials systems.

The effect of pseudomorphic constraints on phonon pressure response in 2-constituent semiconductor heterostructures is investigated. Several hundred pressure-Raman measurements on different superlattices and bulk samples belonging to the GaAs/AlAs system are reported. Calculations of the internal superlattice strain in each constituent as a function of applied pressure are used to predict the induced shift of the LO( Gamma ) phonons. In agreement with theory, no statistically significant substrate or layer-thickness related differences could be measured for the LO( Gamma ) pressure coefficients in GaAs/AlAs heterostructures. Surprisingly, however, differences are observed for the 2TA(X) peak. The calculations indicate that internal-strain effects can be an order of magnitude larger in other common epitaxial systems.

We present a study of the radiative transitions in Al0.3Ga0.7As under hydrostatic pressure in the range 0-70 kbar using photoluminescence at 15 to 125 K. A new trapping center is reported. The center forms an efficient carrier trap, and produces a pressure-induced hysteresis in the intensity of the radiative transitions. A generic large-lattice-relaxation model with an unusually large emission barrier is proposed to understand the strong hysteresis. We postulate that the center is higher than the X conduction band at ambient pressures, and present arguments to show that it is indeed a different center, not caused by either the DX or the shallow-donor centers. We have also obtained pressure coefficients of several direct and indirect transitions. The activation energies of various radiative transitions and an understanding of the scattering processes at chosen pressures is obtained from the temperature dependence of the luminescence intensities.

Signatures associated with electronic states confined in a parabolic quantum well in a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterostructure have been observed in its piezomodulated reflectivity spectrum. The spectra also exhibit electronic transitions with states confined to the spin-orbit-split valence band as the initial states. A comparison of the relative intensity of 11H and 11L signatures in the piezomodulated and those in the photomodulated reflectivity spectrum emphasizes the contribution of the strain dependence of the energies of the confined states.

Intersubband transitions induced in a simple Faraday transmission geometry by metallic grating couplers have been studied in several GaAs/Al0.3Ga0.7As MQW samples lightly doped with donors and with well widths between 210 Å and 320 Å. A sensitive pumping and probing technique was employed in which chopped visible pump light and spatially modulated infrared radiation are transmitted simultaneously through the MQW. The excess free electron density in the well created by < 100 microW/cm2 of red light is estimated to be ≅ 109cm−2 per well. The measured E1 – E0 energies are in good agreement with a simple ID model calculatio found that the grating coupling efficiency drops from 15% to 5% when the ratio of the transition wavelength to the grating period is increased from 1.5 to 3.6.

Measurements of intersubband absorption and cyclotron resonance on lightly-doped GaAs/AlGaAs quantum-well structures with a sensitive optical pumping technique show that the observed intersubband transitions are due to free electrons and that the large photo-generated excess free electron density is due to compensation by acceptors.

The authors present a study of the deep and shallow donor levels under hydrostatic pressure. The shallow levels follow the conduction bands, while the deep levels are strongly sublinear with pressure. The temperature dependence of the intensities and energies is used to obtain an energy level diagram of the deep levels at high pressures.

The number of usable devices that can be obtained from an epitaxially-grown structure is often dependent on the uniformity of the growth on the wafer. Thus the spatial uniformity of an epitaxial growth and its relationship to growth conditions have been issues of continuing interest. Here, an apparatus for measuring the photoluminescence over an entire wafer is described. A specific example, the use of this data to relate indium coverage on the back surface of MBE-grown material to the properties of the epitaxial layers, is then discussed. A clear correlation between the details of the indium coverage and the local growth rate on the front surface of the wafer is found. >

Beryllium acceptors doped in the centers of GaAs/AlGaAs quantum wells with widths between 300 angstrom and 100 angstrom, as well as in a "bulk" epitaxial layer of GaAs for comparison have been studied by far infrared magnetospectroscopy. Results clearly show the effects of confinement on the acceptor, and the observed increase in transition energy is in qualitiative agreement with recent calculations. For the bulk acceptor the observed splittings in a magnetic field cannot be explained with calculated g-values.

A detailed study of the photoreflectance spectra of a GaAs-${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As superlattice as a function of temperature has revealed the temperature coefficients of the quantum-well transitions associated with the direct $\ensuremath{\Gamma}$ conduction band (CB) of GaAs and the staggered transitions from the $X\ensuremath{-}\mathrm{CB}$ of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ to the valence band of GaAs. The data have been fitted to Varshni's equation. We have also observed the evolution of the excitonic transitions, especially for lower quantized states, as the temperature is decreased yielding the binding energies.

We present a cryogenic temperature study of confined transitions and deep impurity levels in a GaAs-Ga1−xAlxAs multiple quantum well under hydrostatic pressure. Photoreflectance at 80 to 300K was used to study quantized states upto n=7. A sublinear pressure behavior was found, with pressure coefficients that decreased with increasing n. Indirect transitions from L and X conduction bands were also observed. Photoluminescence at 18K was used to observe a deep level, and its phonon replicas, that is resonant below 24 and above 80kbar. The level is observed both in bulk GaAs and the quantum well, and maybe due to Si impurities.

Far infrared transmission has been used to study beryllium acceptors in AlGaAsGaAs quantum wells with widths between 200A and 100A. A bulk sample of GaAs was also studied and the binding energy was found to be 28.5meV. Our results clearly show the effects of confinement on the acceptor by an increase in binding energy in qualitative agreement with recent calculations.