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In single GaAs quantum wells sandwiched between relatively thick AlxGa1−xAs, only weak photoluminescence which is dominated by extrinsic processes is observed. This is attributed to a degraded interface at the bottom AlGaAs/GaAs heterojunction. The situation worsens when the thickness of the bottom AlGaAs layer is increased, the GaAs thickness is reduced, or the Al mole fraction is increased. Inserting a graded three‐period superlattice between the AlGaAs and GaAs quantum well at the bottom interface causes the intensity of the photoluminescence associated with exciton recombination to increase to more than 160 times that in the conventional quantum well. Furthermore, the impurity related photoluminescence from the improved quantum well is almost entirely absent; the photoluminescence is dominated by the recombination of the ground state n=1 heavy‐hole free exciton. This dramatic improvement is indicative of the improvement of the interface resulting from the insertion of a thin superlattice.

Doping with silicon donors and beryllium acceptors has been studied in MBE grown AlxGa1−xAs covering the entire composition range from GaAs to AlAs. The Si donor binding energy and the persistent photoconductivity were observed to depend on the alloy composition. In addition to the dominant deep donor level, a shallow donor level is observed which is postulated to be tied to the Γ minimum in the 0.25

We report a detailed x‐ray scattering study of GaAs grown on Si and Ge substrates by molecular beam epitaxy. We find that the Q dependence of the peak widths can be completely understood in terms of residual strains in the epitaxial film. This result places lower limits on the domain size and antiphase domain size (if they exist) of 6000 and 4000 A, respectively. In addition, we find that the GaAs lattice is commensurate with that of the Ge substrate, but that the GaAs lattice is incommensurate with the Si substrate; however, in both cases the GaAs lattice has tetragonal symmetry. We show that the measured lattice parameters and thus the tetragonal distortion can be explained by differential thermal expansion for the Si substrate sample.

The luminescent properties of thin single quantum well AlxGa1−xAs/GaAs structures grown by molecular beam epitaxy were investigated. Since the photoluminescence response of these structures is dominated by the quality of the heterointerfaces, and particularly the inverted heterointerface, the effect of using several different types of interfaces was investigated. In structures with conventional abrupt heterointerfaces, only weak luminescence dominated by extrinsic processes was observed. By grading the Al mole fraction over ∼150 A region at the heterointerface, an eightfold improvement in the excitonic photoluminescent intensity was obtained. By inserting a three‐period superlattice of graded average composition, a 160‐fold improvement in the excitonic photoluminescent intensity over the abrupt interface structure was obtained. The effect of using dimeric arsenic was also investigated. Since these structures provide feedback as to the quality of the heterointerfaces they can be used as a means of finding ...

High‐resolution photoluminescence measurements performed on several GaAs/Al0.25Ga0.75As multiquantum well structures revealed extremely sharp excitonic transitions separated in energies corresponding to roughly half‐monolayer fluctuations in the well sizes. To understand these observations, a model for the interface in molecular beam epitaxy (MBE) is developed using studies of crystal growth. Results of this structural model are coupled with the theory of the effects of interface roughness on photoluminescence linewidth. A comparison of the experimental and theoretical studies shows that an interface formed under optimized growth conditions can be described by an interfacial layer made up of GaAs and AlGaAs islands. The sizes and the relative concentrations of these islands depend upon the precise instant at which the interface is formed. Consequences of this model for photoluminescence spectroscopy are discussed.

We report on the elimination of collapse of drain I/V characteristics in modulation-doped field-effect transistors at 77 K by replacing the doped AlGaAs with a thin GaAs/AlGaAs superlattice where only the GaAs is doped. Such thin barriers (10 A/15 A) are transparent to the electrons making the electron transfer into the bulk GaAs (undoped) possible. Room-temperature transconductances of 180 mS/mm which increased to 210 mS/mm at 77 K under both dark and light conditions were obtained. Furthermore, the threshold voltage of these devices did not shift appreciably on cooling (+0.12 V), and no noticeable light sensitivity at 77 K was observed for this device structure.

We report the first study of single‐crystalline GaAs grown by molecular beam epitaxy (MBE) on a nonpolar substrate. In this study, GaAs was grown under various conditions on Ge(100). Surface morphologies were specular and quite strongly dependent on surface preparation. Photoluminescence spectra from such GaAs is quite intense and is dominated by donor–acceptor recombination at 1.479 eV which appears to be Ge0Ga−Ge0As. We also observe its phonon replica at 1.442 eV and the recombination of bound excitons at 1.514 eV. This and capacitance–voltage measurements indicate incorporation of Ge from the substrate into the GaAs. Photoluminescence from GaAs/AlGaAs multiple quantum well structures originates from the same processes as in bulk GaAs; in quantum wells, however, the transitions occur at higher energies due to the enhancement of the GaAs band gap. The observed energies are in good agreement with theoretical predictions. We have also studied the growth of GaAs on Si(100) and on Ge films on Si(100).

Effects of an electric field on the low temperature (4.2 K) photoluminescence spectra from undoped GaAs–Al0.25Ga0.75As multiple quantum well samples grown by molecular beam epitaxy have been investigated. The photoluminescence in the semitransparent Schottky‐barrier configuration is dominated by the transitions of the n=1 light‐hole free exciton, the heavy‐hole free exciton, the light and heavy hole excitons bound to a neutral donor, and the heavy hole‐to‐neutral donor. As the electric field is increased the photoluminescence intensity is shifted from the neutral donor bound heavy‐hole exciton peak to the heavy hole‐to‐neutral donor peak and to the n=1 electron‐to‐neutral acceptor peak. More than two orders of magnitude change in intensity of the neutral donor bound heavy‐hole exciton peak is observed in the electric field range studied. No discernible shift in the energy of any of the intrinsic and near‐intrinsic emission is observed. However, some shift is observed for the n=1 electron‐to‐neutral accept...

A detailed characterization of the optical properties of GaAs/AlGaAs multiple quantum well heterostructures has been performed at superfluid helium temperatures using photoluminescence, optical transmission, and reflectance techniques yielding complementary information. We present the first report of experimentally determined absolute absorption spectra obtained from optical transmission spectra. Exciton absorption features become strongly enhanced as the wells get narrower. Theoretical calculations of the quantum energy levels correlate well with experimental observations. Reflectance spectra revealing structure of greater detail than that observed in luminescence have permitted the observation of free exciton splitting attributed to submonolayer well width fluctuations.

The 77 K operation of AlGaAs/InGaAs MODFETs has been investigated. The structures, grown by MBE, make use of a 200 A undoped In(0.15)Ga(0.85)As quantum well for electron confinement and an Si-doped Al(0.15)Ga(0.85)As top barrier. The MODFETs with 1 micron gate lengths exhibit extrinsic transconductances of 360 mS/mm and maximum currents of 310 mA/mm at 77 K. The use of a low Al mole fraction AlGaAs/InGaAs heterojunction makes it possible to avoid the persistent trapping effects encountered in AlGaAs/GaAs MODFETs without sacrificing device performance.

We have used the variational method to calculate the acceptor binding energies in GaAs–AlxGa1−xAs quantum wells. The calculation includes the coupling of the top four valence bands of both materials in the multiband effective mass approximation. To ensure the convergence of the calculation, a large number of basis functions which are made up of the s‐like or d‐like spatial states multiplied by j=3/2 spinors are used for the expansion of the acceptor wave function. Because the quantum well potential reduces the symmetry from Td to D2d, the bulk Γ8 acceptor ground state splits into Γ6 and Γ7 states. The Γ6 state is predominantly derived from the heavy‐hole subband and the Γ7 state is predominantly derived from the light‐hole subband. We have calculated the binding energies of the Γ6 state (measured from the top of the heavy‐hole subband) and the Γ7 state (measured from the top of the light‐hole subband) for both center doped and edged doped quantum wells for various barrier heights as functions of well widt...