Nature of the band discontinuities at semiconductor heterojunction interfaces

A clear correlation was found between experimental heterojunction valence-band discontinuities locally measured by photoemission spectroscopy and the LCAO results of the Harrison model. In particular, we found that the theoretical discontinuities are accurate within 0.1–0.15 eV for lattice-matched interfaces. Empirical corrections for the bond-length difference in lattice-mismatched interfaces generally improve the agreement between theory and experiment.     

Microscopic investigation of the band discontinuities at the silicon-germanium heterojunction interface

The formation of interface energy band discontinuities has been directly monitored on Ge-covered Si(111) surfaces with photoemission spectroscopy using synchrotron radiation. The final magnitude of the band discontinuities is not consistent with the linear combination of dipole-layers which leads to the “electron affinity rule”. Strong modifications of the local density of states occur during the formation of the heterojunction and the experiments indicate that the interface is abrupt on a microscopic scale.     

Heterojunction band discontinuities of quaternary semiconductor alloys

A simple sp³ hybrid-orbital model for predicting valence-band discontinuities is proposed and applied to quaternaries. The effects of anion and cation disorder are taken into account explicitly within the coherent-potential approximation. Results for In_1?xGa_xP_1?yAs_y, In_1?xGa_xAs_1?ySb_y, Ga_1?xAl_xAs_1?ySb_y and In_1?xAl_xP_1?ySb_y lattice-matched to some binaries are shown. The broadening of valence-band-top levels due to alloy scattering is found negligible.     

Ka/Q band GaAs IMPATT amplifier technology

GaAs IMPATT diodes are capable of generating 2 to 3 W simultaneously with 18–22% efficiency in the 33 GHz to 46 GHz frequency band. The design of the amplifier circuits which utilize these devices is discussed. The circuit design is based on a 3-step closed form algorithm. The first step is a passive circuit characterization with an automatic network analyzer. In the second step, a computer is used to generate diode device lines. The third step is load line synthesis for predictable operation. The resulting performance is described. 2 W over a 2GHz bandwidth was achieved simultaneously with a minimum gain of 12 dB.     

Transient saturation absorption spectroscopy excited near the band gap at high excitation carrier density in GaAs

Transient saturation absorption spectroscopy in GaAs thin films was investigated using femtosecond pump and supercontinuum probe technique at excitation densities higher than 1×10^{19}cm^{-3}. The Coulomb enhancement factor of the electron－hole plasma results in a spectrum hole at the pump wavelength. Two distinct transmission peaks at two sides of the pump wavelength are observed, arising from the bleaching of transitions from the heavy- and light-hole bands to the conduction band. The dynamic process of the transient saturation absorption is fitted using a bi-exponential function. The fast decay process is dominated by the carrier-phonon scattering and the slow process may be attributed to the electron－hole recombination.     

Nonparabolicity and warping in the conduction band of GaAs

The dispersion of the conduction band in GaAs is calculated using k·p models which in different ways take into account the coupling to the p-bonding and p-antibonding states. Nonparabolicity, warping and spin-splitting are accurately described up to energies about 50 meV above the conduction band minimum by the 8×8 Kane model. For higher energies a 14×14 matrix is required.     

Model for degradation of band gap photo-luminescence in GaAs

Summary Degradation of near band gap photo-luminescence emission in GaAs with time of exposure to low power, c.w. laser excitation at room temperature is quantitatively described by a model based on defect reactions that are promoted by trapping and recombination of excess carriers at nonradiative recombination sites. The proposed model accurately describes the observed degradation rate, its power and temperature dependence, as well as the absence of degradation at a surface with shallow ion implantation. Summer student from the Materials Sciences Department at Massachusetts Institute of Technology, Cambridge, Mass., USA.     

Experimental tests of the dielectric electronegativity approach to the problem of heterojunction band discontinuities

We tested Van Vechten's recent suggestion for improving the accuracy in predicting heterojunction band discontinuities from semiempirical ionization potential formulae. The results provide some interesting suggestions on the dependence of the ionization potential on the lattice parameter.     

Optical band gap width in GaAs in megagauss magnetic fields

The band gap width in GaAs in magnetic fields of up to 10 MG is calculated using a five-band kp model. The selection rules for interband electron transitions in strong magnetic fields are found, and the dependences of the interband transition probabilities on a magnetic field are calculated. The electronic spectra calculated in the five-band model are compared with those calculated in the Kane model and in the tight-coupling approximation. The calculations are shown to agree with experimental data if the contribution from the density-of-states tails and excitonic effects to light absorption is taken into account.     

Polarization conversion at the discontinuities in semiconductor opto-electronic systems

The origin of unexpected polarization conversion at the junctions between two semiconductor rib waveguides is demonstrated by using full-vectorial numerical approaches.     

Optical properties and band structure of short-period GaAs/AlAs superlattices

We have studied six GaAs/AlAs superlattices with periods ranging from 18 to 60 Å and different average aluminum composition. Three of these samples are shown to be direct bandgap materials whose band structure differs strongly from that of the corresponding alloy, but is correctly described by an envelope function calculation. The three remaining samples are shown to be indirect both in real and reciprocal space. The lowest energy transitions are found to arise from an exciton involving a heavy hole state mostly confined in the GaAs layer and at the Brillouin zone center (Λ), and an electronic state of X character confined in the AlAs layers. Analysis of the time decay of the luminescence shows that this is a momentum-forbidden exciton made allowed by disorder scattering, which leads to a luminescence efficiency comparable to that of the direct bandgap samples. Partial lifting of the degeneracy of the three X orbitals by the superlattice potential is also observed. Finally, we take advantage of the strong dependence of these indirect transition energies on the band discontinuities to estimate the valence band offset to be about 550 meV in this system.