M. Gladysiewicz

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...

The surface boundary conditions in GaN/AlGaN/GaN transistor heterostructures

The distribution of electric field in GaN(cap)/AlGaN/GaN(buffer) transistor heterostructures with various AlGaN layer thicknesses (10, 20, and 30 nm) has been studied by contactless electroreflectance and compared with theoretical calculations performed for various positions of the Fermi-level on GaN surface. For the three samples the best agreement between experimental data and theoretical calculations has been found at the same position of the Fermi-level on GaN surface (i.e., 0.55±0.05 eV below the conduction band). It means that the Fermi-level is pinned on GaN surface and this pinning can be treated as the boundary condition for the distribution of polarization-related fields in this heterostructure.

Contactless electroreflectance studies of Fermi level position on c-plane GaN surface grown by molecular beam epitaxy and metalorganic vapor phase epitaxy

Contactless electroreflectance (CER) has been applied to study the Fermi-level position on c-plane GaN surface in Van Hoof structures grown by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE). A clear CER resonance followed by strong Franz-Keldysh oscillation (FKO) of various periods was clearly observed for the series of samples of different thicknesses (30, 50, and 70 nm) of undoped GaN layer. The built-in electric field in this layer has been determined from the period of GaN-related FKO. A good agreement between the calculated and measured electric fields has been found for the Fermi-level located ∼0.4 and ∼0.3 eV below the conduction band for the MBE and MOVPE samples, respectively.

Contactless electroreflectance of GaInNAsSb/GaAs single quantum wells with indium content of 8%-32%

Interband transitions in GaInNAsSb∕GaAs single quantum wells (SQWs) with nominally identical nitrogen and antimony concentrations (2.5% N and 7% Sb) and varying indium concentrations (from 8% to 32%) have been investigated by contactless electroreflectance (CER). CER features related to optical transitions between the ground and excited states have been clearly observed. Energies of the QW transitions extracted from CER measurements have been matched with those obtained from theoretical calculations performed within the effective mass approximation for various conduction-band offsets (QC) and various electron effective masses. It has been found that the QC increases from 40% to 80% with the rise of the indium content from 8% to 32% and the electron effective mass is close to 0.09m0. The results show that the band gap discontinuity in GaInNAsSb∕GaAs SQWs can be broadly tuned with a change in the indium concentration.

Band gap discontinuity in Ga0.9In0.1N0.027As0.973−xSbx∕GaAs single quantum wells with 0⩽x<0.06 studied by contactless electroreflectance spectroscopy

Contactless electroreflectance (CER) spectroscopy has been applied to study optical transitions in Ga0.9In0.1N0.027As0.973−xSbx∕GaAs single quantum well (QW) with antimony content varying from 0% to 5.4%. CER features related to optical transitions between the ground and excited states have been clearly observed. Energies of the QW transitions have been matched with those obtained from theoretical calculations. It has been determined that the conduction band offset decreases from ∼55% to ∼45% with the increase in Sb content from 0% to 5.4%. This result demonstrates that the band gap discontinuity for Ga0.9In0.1N0.027As0.973−xSbx∕GaAs system can be simply tuned by a change in antimony content.

Band structure and the optical gain of GaInNAs/GaAs quantum wells modeled within 10-band and 8-band kp model

The band structure and optical gain have been calculated for GaInNAs/GaAs quantum wells (QWs) with various nitrogen concentrations within the 10-band and 8-band kp models. Two approaches to calculate optical properties of GaInNAs/GaAs QWs have been compared and discussed in the context of available material parameters for dilute nitrides and the conduction band nonparabolicity due to the band anti-crossing (BAC) interaction between the N-related resonant level and the conduction band of a host material. It has been clearly shown that this nonparabolicity can be neglected in optical gain calculations since the dispersion of conduction band up to the Femi level is very close to parabolic for carrier concentrations typical for laser operation, i.e., 5 × 1018 cm−3. This means that the 8-band kp model when used to calculate the optical gain is very realistic and much easier to apply in QWs containing new dilute nitrides for which the BAC parameters are unknown. In such an approach, the energy gap and electron effective mass for N-containing materials are needed, instead of BAC parameters. These parameters are available experimentally much easier than BAC parameters.The band structure and optical gain have been calculated for GaInNAs/GaAs quantum wells (QWs) with various nitrogen concentrations within the 10-band and 8-band kp models. Two approaches to calculate optical properties of GaInNAs/GaAs QWs have been compared and discussed in the context of available material parameters for dilute nitrides and the conduction band nonparabolicity due to the band anti-crossing (BAC) interaction between the N-related resonant level and the conduction band of a host material. It has been clearly shown that this nonparabolicity can be neglected in optical gain calculations since the dispersion of conduction band up to the Femi level is very close to parabolic for carrier concentrations typical for laser operation, i.e., 5 × 1018 cm−3. This means that the 8-band kp model when used to calculate the optical gain is very realistic and much easier to apply in QWs containing new dilute nitrides for which the BAC parameters are unknown. In such an approach, the energy gap and electron ...

Contactless electroreflectance studies of surface potential barrier for N- and Ga-face epilayers grown by molecular beam epitaxy

Two series of N- and Ga-face GaN Van Hoof structures were grown by plasma-assisted molecular beam epitaxy to study the surface potential barrier by contactless electroreflectance (CER). A clear CER resonance followed by strong Franz-Keldysh oscillation of period varying with the thickness of undoped GaN layer was observed for these structures. This period was much shorter for N-polar structures that means smaller surface potential barrier in these structures than in Ga-polar structures. From the analysis of built-in electric field it was determined that the Fermi-level is located 0.27 ± 0.05 and 0.60 ± 0.05 eV below the conduction band for N- and Ga-face GaN surface, respectively.

Electronic band structure of compressively strained Ge1−xSnx with x < 0.11 studied by contactless electroreflectance

Contactless electroreflectance is applied to study direct optical transitions from the heavy hole, light hole, and spin-orbit split-off band to the conduction band in compressively strained Ge1−xSnx layers of various Sn concentrations at room temperature. It is shown that the energies of these transitions are in very good agreement with theoretical predictions, which take into account non-linear variation of bandgap and spin-orbit splitting plus the strain-related shifts obtained from the Bir-Pikus theory. The bowing parameter for the direct bandgap has been determined to be 1.8 ± 0.2 eV and agree with this one obtained within ab initio calculations, which is 1.97 eV (for indirect bandgap the bowing parameter is 0.26 eV).

Ground and excited state transitions in as-grown Ga0.64In0.36N0.046As0.954 quantum wells studied by contactless electroreflectance

The optical transitions of as-grown Ga0.64In0.36N0.046As0.954 multiple quantum wells grown at the low temperature of 375°C were studied by contactless electroreflectance (CER). The investigation was carried out at room temperature for a set of samples having quantum well (QW) widths ranging from 3.9to8.1nm. The ground and the excited state transitions were clearly observed in CER spectra (the ground state transition was observed at the wavelength of 1.9μm for the 8.1nm wide QW). The experimental QW transition energies were compared with theoretical predictions based on an effective mass formalism model. Good agreement between experimental data and theoretical calculations has been obtained assuming that the conduction band offset for GaInNAs∕GaAs interface is 80% and the electron effective mass is 0.09m0.

Electronic band structure and material gain of III-V-Bi quantum wells grown on GaSb substrate and dedicated for mid-infrared spectral range

The 8-band kp Hamiltonian is applied to calculate electronic band structure and material gain in III-V-Bi quantum wells (QWs) grown on GaSb substrates. We analyzed three Bi-containing QWs (GaSbBi, GaInSbBi, and GaInAsSbBi) and different Bi-free barriers (GaSb and AlGaInAsSb), lattice matched to GaSb. Bi-related changes in the electronic band structure of III-V host incorporated into our formalism are based on recent ab-initio calculations for ternary alloys (III-Ga-Bi and III-In-Bi) [Polak et al., Semicond. Sci. Technol. 30, 094001 (2015)]. When compared to Bi-free QWs, the analyzed Bi-containing structures show much better quantum confinement in the valence band and also larger redshift of material gain peak per percent of compressive strain. For 8 nm thick GaInSb/GaSb QWs, material gain of the transverse electric (TE) mode is predicted at 2.1 μm for the compressive strain of e = 2% (32% In). The gain peak of the TE mode in 8 nm thick GaSbBi/GaSb QW reaches this wavelength for compressive strain of 0.15%...