M. J. Ashwin

Growth and properties of GaSbBi alloys

Molecular-beam epitaxy has been used to grow GaSb 1− x Bi x alloys with x up to 0.05. The Bi content, lattice expansion, and film thickness were determined by Rutherford backscattering and x-ray diffraction, which also indicate high crystallinity and that >98% of the Bi atoms are substitutional. The observed Bi-induced lattice dilation is consistent with density functional theory calculations. Optical absorption measurements and valence band anticrossing modeling indicate that the room temperature band gap varies from 720u2009meV for GaSb to 540u2009meV for GaSb 0.95Bi0.05, corresponding to a reduction of 36u2009meV/%Bi or 210u2009meV per 0.01u2009A change in lattice constant.

High Bi content GaSbBi alloys

The epitaxial growth, structural, and optical properties of GaSb 1– x Bi x alloys have been investigated. The Bi incorporation into GaSb is varied in the range 0u2009<u2009xu2009≤u20099.6% by varying the growth rate (0.31–1.33u2009μm h−1) at two growth temperatures (250 and 275u2009°C). The Bi content is inversely proportional to the growth rate, but with higher Bi contents achieved at 250 than at 275u2009°C. A maximum Bi content of xu2009=u20099.6% is achieved with the Bi greater than 99% substitutional. Extrapolating the linear variation of lattice parameter with Bi content in the GaSbBi films enabled a zinc blende GaBi lattice parameter to be estimated of 6.272u2009A. The band gap at 300u2009K of the GaSbBi epitaxial layers decreases linearly with increasing Bi content down to 410u2009±u200940u2009meV (3u2009μm) for xu2009=u20099.6%, corresponding to a reduction of ∼35u2009meV/%Bi. Photoluminescence indicates a band gap of 490 ± 5u2009meV at 15u2009K for xu2009=u20099.6%.

Theoretical and experimental studies of electronic band structure for GaSb1−xBix in the dilute Bi regime

Photoreflectance (PR) spectroscopy was applied to study the band gap in GaSb1−xBix alloys with Bi < 5%. Obtained results have been interpreted in the context of ab initio electronic band structure calculations in which the supercell (SC) based calculations are joined with the alchemical mixing (AM) approximation applied to a single atom in the cell. This approach, which we call SC-AM, allows on the one hand to study alloys with a very small Bi content, and on the other hand to avoid limitations characteristic of a pure AM approximation. It has been shown that the pure AM does not reproduce the GaSb1−xBix band gap determined from PR while the agreement between experimental data and the ab initio calculations of the band gap obtained within the SC-AM approach is excellent. These calculations show that the incorporation of Bi atoms into the GaSb host modifies both the conduction and the valence band. The shift rates found in this work are respectively −26.0 meV per % Bi for the conduction band and 9.6 meV per % Bi for the valence band that consequently leads to a reduction in the band gap by 35.6 meV per % Bi. The shifts found for the conduction and valence band give a ~27% (73%) valence (conduction) band offset between GaSb1−xBix and GaSb. The rate of the Bi-related shift for the split-off band is −7.0 meV per % Bi and the respective increase in the spin–orbit split-off is 16.6 meV per % Bi.

Temperature dependence of the band gap of GaSb1−xBix alloys with 0 < x ≤ 0.042 determined by photoreflectance

GaSb1−xBix layers with 0u2009<u2009xu2009≤u20090.042 have been studied by photoreflectance in 15–290u2009K temperature range. We found that due to the incorporation of Bi atoms into the GaSb host, the E0 band gap-related transition redshifts (∼30u2009meV per 1% Bi) and significantly broadens. The shift of the E0 transition in the temperature range 10–270u2009K has been found to be ∼70u2009meV, very similar to the energy shift in GaSb over the same temperature range. We analyzed the energy and broadening of the E0 transition using the Varshni and Bose-Einstein formulas and found that the Varshni and Bose-Einstein parameters of GaSb1−xBix are similar to those of GaSb. Moreover we concluded that the inhomogeneities in GaSb1−xBix alloys is less important than in dilute bismide arsenides since Bi atoms are more similar to Sb atoms (in electronegativities and ionic sizes).

Bi-induced band gap reduction in epitaxial InSbBi alloys

The properties of molecular beam epitaxy-grown InSb 1− x Bi x alloys are investigated. Rutherford backscattering spectrometry shows that the Bi content increases from 0.6% for growth at 350u2009°C to 2.4% at 200u2009°C. X-ray diffraction indicates Bi-induced lattice dilation and suggests a zinc-blende InBi lattice parameter of 6.626u2009A. Scanning electron microscopy reveals surface InSbBi nanostructures on the InSbBi films for the lowest growth temperatures, Bi droplets at intermediate temperatures, and smooth surfaces for the highest temperature. The room temperature optical absorption edge was found to change from 172u2009meV (7.2u2009μm) for InSb to ∼88u2009meV (14.1u2009μm) for InSb 0.976Bi0.024, a reduction of ∼35u2009meV/%Bi.

N incorporation in GaInNSb alloys and lattice matching to GaSb

The incorporation of N into MBE grown GaNSb and GaInNSb is investigated. Measurements of the N fraction in GaNSb show the familiar linear dependence on inverse growth rate, followed by a departure from this at low growth rates; a similar behaviour is observed for GaInNSb. Unexpectedly, the point at which there is a departure from this linear behaviour is found to be extended to lower growth rates by the addition of small amounts of In. These results are compared to a kinetic theory-based model from which it is postulated that the change in behaviour can be attributed to an In-induced change in the characteristic surface residence lifetime of the N atoms. In addition, a method is demonstrated for growing GaInNSb lattice-matched to GaSb(001) for compositions with band gaps covering the 2–5u2009μm region.

Optical absorption by dilute GaNSb alloys: Influence of N pair states

The optical properties of GaNSb alloys with N contents of up to 2.5% have been investigated at room temperature using infrared absorption spectroscopy. The evolution of the absorption onsets with N content has been described using a three level band anticrossing model of the N localized states interactions with the GaSb conduction band. This approach includes the effect of N pair states, which is critical to reproduce the observed optical properties. This confirms theoretical predictions that N pair states have a more pronounced effect on the band dispersion in GaNSb than in GaNAs.

Controlled nitrogen incorporation in GaNSb alloys

The incorporation of N in molecular-beam epitaxy of GaNxSb1−x alloys with x ⩽ 0.022 has been investigated as a function of temperature (325–400°C) and growth rate 0.25–1.6 μmh−1. At fixed growth rate, the incorporated N fraction increases as the temperature is reduced until a maximum N content for the particular growth rate reached. At each temperature, there is a range of growth rates over which the N content is inversely proportional to the growth rate; the results are understood in terms of a kinetic model. The systematic growth rate- and temperature-dependence enables the N content and resulting band gap to be controlled.

Low- and high-energy photoluminescence from GaSb1−xBixwith 0 <x≤ 0.042

Two photoluminescence (PL) peaks were observed in temperature-dependent PL spectra of GaSb1−xBix layers with 0 < x ≤ 0.042. The high-energy (HE) peak was found to be associated with the bandgap-related emission in GaSb1−xBix, since its energy corresponds to the bandgap determined from photoreflectance measurements. The low-energy (LE) peak was attributed to the optical transition between the conduction band and native acceptor states, and was observed at low temperatures where acceptor states are not occupied by electrons. With increasing temperature, the intensity of the LE peak is quenched with the activation energy corresponding to the energy difference between HE and LE peaks.

Molecular-beam epitaxy and lattice parameter of GaNxSb1−x: deviation from Vegard's law for x > 0.02

The N content of a series of GaNSb samples grown by molecular-beam epitaxy is investigated using high-resolution x-ray diffraction (HRXRD) and secondary-ion mass spectrometry. The N contents determined by the two methods agree well at lower N compositions (x 0.02). Analysis of the HRXRD-determined lattice constant using Vegards law, underestimates the N content for high N compositions. The underestimation is found to be up to 24% for x = 0.03. The variation of the lattice parameter with N content is modelled by considering the influence of the density of the interstitial N-Sb complex rising with increasing N content.