S. Subbanna

Molecular‐beam‐epitaxial growth and selected properties of GaAs layers and GaAs/(Al,Ga)As superlattices with the (211) orientation

We present the first detailed study of the molecular‐beam‐epitaxial growth and the properties of GaAs layers and GaAs/(Al,Ga)As superlattices with the (211)A and (211)B orientations rather than the (100) orientation. There are significant differences in growth morphology of the two (inequivalent) (211) surfaces of GaAs, as well as in their low‐temperature photoluminescence. Electron diffraction patterns taken during the growth indicate some submicroscopic faceting on (211)B growth, but much less on (211)A. The visual microscopic morphology of both orientations is improved drastically by first growing a GaAs/(Al,Ga)As superlattice buffer layer, especially for the (211)A growth, which becomes essentially featureless. Doping studies show that silicon acts as an essentially uncompensated donor on GaAs (211)B layers, down to the lowest As:Ga flux ratios, but on (211)A layers, it is amphoteric: At low As:Ga flux ratios, it behaves as a partially compensated acceptor, at high flux ratios, as a partially compensa...

Staggered‐lineup heterojunctions as sources of tunable below‐gap radiation: Experimental verification

We report experimental verification of the prediction of widely bias‐tunable below‐gap luminescence, from lattice‐matched (p) (Al,In)As/ (n) InP heterojunctions, a system that has been predicted to have staggered lineup. The diodes, grown by molecular beam epitaxy, exhibit strong luminescence at 1.4 K, with a peak energy that shifts from 0.97 to 1.04 eV as the (pulsed) current density is increased from 4.5 to 40 A/cm2. Nonshifting injection luminescence at 1.4 eV, due to hole injection into the n‐InP substrate, was also present, but appreciably weaker (<25%) than the interface luminescence. The spectra indicate that the band lineup in the (Al,In)As/InP system is indeed staggered, with a residual gap at the interface close to 0.96 eV. The corresponding conduction and valence‐band offsets are 0.52 and 0.40 eV.

Excitonic optical nonlinearity induced by internal field screening in (211) oriented strained‐layer superlattices

The nonlinear optical properties of a new class of strained‐layer superlattices (intrinsic Stark effect superlattices) have been investigated. Specifically, we have compared the nonlinear transmission of Ga1−xInxAs‐GaAs strained‐layer superlattices grown along the (211) axis to identical superlattices grown along the (100) axis, and found that the optical nonlinearity in the (211) sample is about one order of magnitude greater than in the (100) sample. A blue shift of the exciton resonance and an increase in the exciton absorption strength in the (211) sample with increasing light intensity was observed (attributed to screening of the intrinsic Stark effect fields by photogenerated carriers), resulting in the stronger optical nonlinearity. The maximum of the nonlinear absorption index, ‖α2‖, in the (211) sample was 54 cm/W (‖Im χ3‖=0.33 esu) whereas in the (100) sample the maximum of ‖α2‖ was 6.9 cm/W (‖Im χ3‖=0.042 esu). The measured carrier recovery time in both samples was 2 ns.

Electrooptic modulation in polar growth axis InGaAs/GaAs multiple quantum wells

We present electrooptic modulation results on [100], [211]A and [211]B oriented InGaAs/GaAs multiple quantum wells. Internal electric fields are generated by a combination of strain, due to lattice mismatch, and the piezoelectric properties of III‐V semiconductors in the [211] structures. These fields have opposite orientation in the [211]A and [211]B samples. They do not occur in the [100] samples. The total electric field is a superposition of the strain‐generated field, the built‐in field from the p‐n junction and any externally applied field. We show that whereas in the conventional [100] structures the exciton energy is a quadratic function of applied field, the strain‐generated fields cause a linear shift in the exciton resonance with applied field in the [211] structures. In addition, the direction of the excitonic shift is opposite in the [211]A and [211]B samples, because the sign of the strain‐generated fields are opposite for these samples.

Size-induced direct-to-indirect gap transition in GaSb/AlSb multiple quantum well structures

Using excitation spectroscopy we have investigated the size-induced cross-over from direct to indirect bandstructure in GaSb/AlSb quantum wells. In samples with Lz < 30 A we observe in particular the emission from the indirect and the direct energy gap of the wells, whereas for larger Lz only the direct transition occurs. Time-resolved measurements yield a change of the carrier lifetime by about a factor of 100 at the cross-over.

Direct-indirect band gap crossover in two-dimensional GaSb/AlSb-quantum-well-structures

Abstract Using excitation and time-resolved spectroscopy we have investigated the size-dependent change from direct to indirect band structure in two-dimensional GaSb/AlSb structures. In the indirect regime (L z ⩽38 A) we observe L- and Λ-point transitions, whereas in the direct-gap samples only the Λ-point emission occurs. Direct evidence for the crossover is provided by the increase of the carrier life-time from less than 1 ns in direct-gap samples to more than 100 ns in indirect-gap samples.

Verification of Direct-Indirect Cross-Over in GaSb/AlSb MQW's by Time Resolved Spectroscopy

In GaSb/AlSb multi quantum well structures (MQW) we previously observed a quantum size induced direct to indirect band structure transition at well widths around 40 A. Using time resolved optical spectroscopy, we investigated the time dependence of the quantum well emission as a function of the well width. The life time of the quantum well emission changes by two orders of magnitude if the well width of 40 A is exceeded. This verifies the direct to indirect cross-over in the GaSb/AlSb system

Recombination in GaSbAlSb multiple QWS under high excitation conditions

Abstract We have studied the recombination dynamics in GaSb AlSb MQW structures under high excitation conditions. Time-resolved measurements were performed using the up-conversion technique. The carrier dynamics in these structures is strongly influenced by the small energetic separation between the Γ-valley and the L-valleys even in the direct material with Lz ≥ 40 A. At room-temperature we find that the Auger recombination with its coefficient C = 4 · 10−28cm6s−1 is the dominant recombination mechanism at high carrier densities. The Auger coefficient C shows a monotonic increase with temperature up to 400 K in contrast to theory.

Optical spectroscopy on Eo+Δo transitions in GaSbAlSb quantum wells

Abstract We have investigated optical transitions between the lowest conduction subband (Γ6) and the split-off valence band (Γ7) in GaSb AlSb multi quantum wells with well widths between 40A and 120A. The corresponding emission lines at an energy Eo + Δ0 were observed in all samples and can be clearly associated with this transition by the variation with well width LZ. Temperature dependent investigations show a cross-over of the Δo and Eo energies in a temperature range between 280K and 700K which is strongly dependent on the well widths.

E0+Δ0 transitions in GaSb/AlSb quantum wells

We have observed optical transitions between the first subbands of the conduction band and the split‐off valence band in GaSb/AlSb quantum well structures. The well width dependence of the emission energies is traced to quantization in the conduction band and in the split‐off band. By comparison with data for the band‐edge transitions the effective Δ0 gaps is the quantum wells are determined. Contrary to previous calculations the Δ0 gap energies are almost independent of the well width.