Jan Kopaczek

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 0 < x ≤ 9.6% by varying the growth rate (0.31–1.33 μm h−1) at two growth temperatures (250 and 275 °C). The Bi content is inversely proportional to the growth rate, but with higher Bi contents achieved at 250 than at 275 °C. A maximum Bi content of x = 9.6% 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 6.272 A. The band gap at 300 K of the GaSbBi epitaxial layers decreases linearly with increasing Bi content down to 410 ± 40 meV (3 μm) for x = 9.6%, corresponding to a reduction of ∼35 meV/%Bi. Photoluminescence indicates a band gap of 490 ± 5 meV at 15 K for x = 9.6%.

Experimental and theoretical studies of band gap alignment in GaAs1−xBix/GaAs quantum wells

Band gap alignment in GaAs1−xBix/GaAs quantum wells (QWs) was studied experimentally by photoreflectance (PR) and theoretically, ab initio, within the density functional theory in which the supercell based calculations are combined with the alchemical mixing approximation applied to a single atom in a supercell. In PR spectra, the optical transitions related to the excited states in the QW (i.e., the transition between the second heavy-hole and the second electron subband) were clearly observed in addition to the ground state QW transition and the GaAs barrier transition. This observation is clear experimental evidence that this is a type I QW with a deep quantum confinement in the conduction and valence bands. From the comparison of PR data with calculations of optical transitions in GaAs1−xBix/GaAs QW performed for various band gap alignments, the best agreement between experimental data and theoretical calculations has been found for the valence band offset of 52 ± 5%. A very similar valence band offse...

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 0 < x ≤ 0.042 have been studied by photoreflectance in 15–290 K temperature range. We found that due to the incorporation of Bi atoms into the GaSb host, the E0 band gap-related transition redshifts (∼30 meV per 1% Bi) and significantly broadens. The shift of the E0 transition in the temperature range 10–270 K has been found to be ∼70 meV, 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).

Contactless electroreflectance and theoretical studies of band gap and spin-orbit splitting in InP1−xBix dilute bismide with x ≤ 0.034

Contactless electroreflectance is applied to study the band gap (E-0) and spin-orbit splitting (Delta(SO)) in InP1-xBix alloys with 0 < x <= 0.034. The E-0 transition shifts to longer wavelengths very significantly (-83 meV/% Bi), while the E0 + Delta(SO) transition shifts very weakly (-13 meV/% Bi) with the rise of Bi concentration. These changes in energies of optical transitions are discussed in the context of the valence band anticrossing model and ab initio calculations. Shifts of E-0 and E-0 + Delta(SO) transitions, obtained within ab-initio calculations, are -106 and -20 meV per % Bi, respectively, which is in a good agreement with experimental results.

Pressure coefficients for direct optical transitions in MoS2, MoSe2, WS2, and WSe2 crystals and semiconductor to metal transitions.

The electronic band structure of MoS2, MoSe2, WS2, and WSe2, crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT). In the PR spectra direct optical transitions (A and B) have been clearly observed and pressure coefficients have been determined for these transitions to be: αA = 2.0 ± 0.1 and αB = 3.6 ± 0.1 meV/kbar for MoS2, αA = 2.3 ± 0.1 and αB = 4.0 ± 0.1 meV/kbar for MoSe2, αA = 2.6 ± 0.1 and αB = 4.1 ± 0.1 meV/kbar for WS2, αA = 3.4 ± 0.1 and αB = 5.0 ± 0.5 meV/kbar for WSe2. It has been found that these coefficients are in an excellent agreement with theoretical predictions. In addition, a comparative study of different computational DFT approaches has been performed and analyzed. For indirect gap the pressure coefficient have been determined theoretically to be −7.9, −5.51, −6.11, and −3.79, meV/kbar for MoS2, MoSe2, WS2, and WSe2, respectively. The negative values of this coefficients imply a narrowing of the fundamental band gap with the increase in hydrostatic pressure and a semiconductor to metal transition for MoS2, MoSe2, WS2, and WSe2, crystals at around 140, 180, 190, and 240 kbar, respectively.

Direct optical transitions at K- and H-point of Brillouin zone in bulk MoS2, MoSe2, WS2, and WSe2

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS2, MoSe2, WS2, and WSe2. In order to interpret optical transitions observed in CER, PR, and PzR spectra, the electronic band structure for the four crystals has been calculated from the first principles within the density functional theory for various points of Brillouin zone including K and H points. It is clearly shown that the electronic band structure at H point of Brillouin zone is very symmetric and similar to the electronic band structure at K point, and therefore, direct optical transitions at H point should be expected in modulated reflectance spectra besides the direct optical transitions at the K point of Brillouin zone. This prediction is confirmed by experimental studies of the electronic band structure of MoS2, MoSe2, WS2, and WSe2 crystals by CER, PR, and PzR spectroscopy, i.e., techniques which are very sensitive to critical points of Brillouin zone. For the four crystals besides the A transition at K point, an AH transition at H point has been observed in CER, PR, and PzR spectra a few tens of meV above the A transition. The spectral difference between A and AH transition has been found to be in a very good agreement with theoretical predictions. The second transition at the H point of Brillouin zone (BH transition) overlaps spectrally with the B transition at K point because of small energy differences in the valence (conduction) band positions at H and K points. Therefore, an extra resonance which could be related to the BH transition is not resolved in modulated reflectance spectra at room temperature for the four crystals.

Optical properties of GaAsBi/GaAs quantum wells: Photoreflectance, photoluminescence and time-resolved photoluminescence study

Photoreflectance (PR), photoluminescence (PL) and time-resolved PL were applied to study the optical properties, particularly the localized and delocalized states and carrier dynamics, in GaAs1−xBix/GaAs quantum wells. With increasing Bi concentration the ground state transition (i.e., the transition between the first heavy hole and the first electron subband) red shifts due to Bi-related reduction of the GaAs1−xBix energy gap. Additionally, the transition related to the excited states in the quantum wells is clearly observed for the sample with high Bi concentration of 5.6%, confirming these quantum wells are type I. The PL measurements show the S-shape behavior and indicate the strong localization effect below 150 K for all measured samples, while the PL emission above 150 K is related to delocalized states. The localized character of emission at low temperatures is confirmed by time-resolved PL studies. At 10 K the decay time has strong spectral dispersion (i.e. the decay time increases from ~10 ns to ~400 ns going from the high to low energy side of the PL peak). This dispersion disappears above 190 K. At room temperature the decay time is in the order of a few ns.

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.

Photoacoustic spectroscopy of absorption edge for GaAsBi/GaAs nanowires grown on Si substrate

GaAsBi/GaAs nanowires (NWs) grown on Si substrate and proper reference samples have been studied by photoacoustic (PA) spectroscopy. It has been shown that PA signal originating from NWs is quite strong and can be easily identified in the PA spectra, as well as distinguished from the signal originating from the Si substrate. The absorption edge of GaAsBi/GaAs and GaAs NWs has been determined from the analysis of amplitude PA spectra to be 1.26 eV and 1.42 eV, respectively. These values are consistent with the band gap reduction resulting from the introduction of ∼2% Bi in bulk GaAsBi alloy. The presented results prove that, despite light scattering, which is typical for NWs, PA spectroscopy is an excellent tool to study the absorption edge in semiconductor NWs.