Search

Fully microscopic many-body models based on inputs from first principle density functional theory are used to calculate the carrier losses due to radiative- and Auger-recombinations in bulk tellurium. It is shown that Auger processes dominate the losses for carrier densities in the range typical for applications as lasers. The Auger loss depends crucially on the energetic position of the $H_6$ valence bands. At cryogenic temperatures of 50$\,$K (100$\,$K) the Auger coefficient, $C$, varies by about six (three) orders of magnitude within the range of published distances between these bands and the valence bandedge. Values for $C$ at the high and low end of these ranges are found if the distance is smaller or larger than the bandgap, respectively. At room temperature the sensitivity is reduced to about a factor of four with $C$ values ranging between $0.4$ and $1.6\times 10^{-27}$cm$^6$s$^{-1}$. Here, radiative losses dominate for carrier densities up to about $10^{16}/$cm$^3$ with a loss coefficient $B\approx 10^{-11}$cm$^3$s$^{-1}$. The radiative losses are about two to three times lower than in typical bulk III-V materials for comparable wavelengths., Comment: 5 pages, 4 figures. Submitted to Applied Physics Letters on 07/27/2022

The influence of the growth conditions as well as the device design on the device performance of (GaIn)As/Ga(AsSb)/(GaIn)As "W"-quantum well lasers is investigated. To this purpose, the epitaxy process is scaled to full two inch substrates for improved homogeneity while the growth process is carried out in a single run for an improved quality. Furthermore, the optical confinement factor is increased by increasing the aluminum concentration within the cladding layers to a value of 65%. The procedure is carried out for devices with emission wavelengths of 1.26 micrometer as well as 1.30 micrometer. Differential efficiencies as high as 58% and threshold current densities as low as 0.16 kA/cm^2 are observed in case of devices emitting at 1.26 micrometer at room temperature. Furthermore, excellent characteristic temperatures of T_0=(72 plus minus 5)K and T_1=(293 plus minus 16) K are recorded in the temperature range between 10 degree Celsius and 100 degree Celsius. Devices emitting at 1.30 micrometer exhibit differential efficiencies of 31% and threshold current densities of 0.50 kA/cm^2 at room temperature. Further improvements of these properties and wavelength extension schemes are briefly discssused.

This paper discusses the temperature-dependent properties of (GaIn)As/Ga(AsSb)/(GaIn)As W-quantum well heterostructures for laser applications based on theoretical modeling as well as experimental findings. A microscopic theory is applied to discuss band bending effects giving rise to the characteristic blue shift with increasing charge carrier density observed in type-II heterostructures. Furthermore, gain spectra for a W-quantum well heterostructure are calculated up to high charge carrier densities. At these high charge carrier densities, the interplay between multiple type-II transitions results in broad and flat gain spectra with a spectral width of approximately 160 nm. Furthermore, the temperature-dependent properties of broad-area edge-emitting lasers are analyzed using electroluminescence as well as laser characteristic measurements. A first indication for the theoretically predicted broad gain spectra is presented and the interplay between the temperature-dependent red shift and the charge carrier density-dependent blue shift is discussed. A combination of these effects results in a significant reduction of the temperature-induced red shift of the emission wavelengths and even negative shift rates of (-0.10 plusminus 0.04) nm/K are achieved.

The LDA-1/2 method is employed in density functional theory calculations for the electronic structure of III-V dilute bismide systems. For the representative example of Ga(SbBi) with Bi concentrations below $10 \%$, it is shown that this method works very efficiently, especially due to its reasonably low demand on computer memory. The resulting bandstructure and wavefunctions are used to compute the interaction matrix elements that serve as input to microscopic calculations of the optical properties and intrinsic losses relevant for optoelectronic applications of dilute bismides.

The dynamics of band-gap renormalization and gain build-up in monolayer MoTe$_2$ is investigated by evaluating the non-equilibrium Dirac-Bloch equations with the incoherent carrier-carrier and carrier-phonon scattering treated via quantum-Boltzmann type scattering equations. For the case where an approximately $300$ fs-long high intensity optical pulse generates charge-carrier densities in the gain regime, the strong Coulomb coupling leads to a relaxation of excited carriers on a few fs time scale. The pump-pulse generation of excited carriers induces a large band-gap renormalization during the time scale of the pulse. Efficient phonon coupling leads to a subsequent carrier thermalization within a few ps, which defines the time scale for the optical gain build-up energetically close to the low-density exciton resonance., Comment: This is a post-peer-review version of an article published in Physical Review B

For opto-electronic and photo-voltaic applications of perovskites, it is essential to know the optical properties and intrinsic losses of the used materials. A systematic microscopic analysis is presented for the example of methylammonium lead iodide where density functional theory is used to calculate the electronic band structure as well as the dipole and Coulomb matrix elements. These results serve as input for a many-body quantum approach used to compute the absorption, photoluminescence, and the optical and Auger losses for a wide range of application conditions. To illustrate the theory, the excitonic properties of the material system are investigated and numerical results are presented for typical photo-voltaic operation conditions and for the elevated carrier densities needed for laser operation., Comment: 6 pages, 5 figures

The electronic structure of bulk GaAs$_{1-x}$Bi$_x$ systems for different atomic configurations and Bi concentrations is calculated using density functional theory. The results show a Bi-induced splitting between the light-hole and heavy-hole bands at the $\Gamma$-point. We find a good agreement between our calculated splittings and experimental data. The magnitude of the splitting strongly depends on the local arrangement of the Bi atoms but not on the uni-directional lattice constant of the supercell. The additional influence of external strain due to epitaxial growth on GaAs substrates is studied by fixing the in-plane lattice constants.

Using optical pump-white light probe spectroscopy the gain dynamics is investigated for a VECSEL chip which is based on a type-II heterostructure. The active region the chip consists of a GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs multiple quantum well. For this structure, a fully microscopic theory predicts a modal room temperature gain at a wavelength of 1170 nm, which is confirmed by experimental spectra. The results show a gain buildup on the type-II chip which is delayed relative to that of a type-I chip. This slower gain dynamics is attributed to a diminished cooling rate arising from reduced electron-hole scattering., Comment: 4 pages, 4 figures