[en] An efficient model is proposed to evaluate the impact of interface grading on the properties of semiconductor heterostructures. In the plane-wave approximation, the interface grading is taken into account by simply multiplying the Fourier components of the potential by a Gaussian function, which results only in a very small increase of the computation time. We show that the interface grading may affect the transition energies, the field strength for resonant coupling of subbands, and even the miniband formation in complex systems such as quantum-cascade lasers. This model provides a convenient tool for the incorporation of interface grading into the design of heterostructures.

[en] We present scattering rates for electrons at longitudinal optical phonons within a model completely formulated in the Fourier domain. The total intersubband scattering rates are obtained by averaging over the intrasubband electron distributions. The rates consist of the Fourier components of the electron wave functions and a contribution depending only on the intersubband energies and the intrasubband carrier distributions. The energy-dependent part can be reproduced by a rational function, which allows for the separation of the scattering rates into a dipole-like contribution, an overlap-like contribution, and a contribution which can be neglected for low and intermediate carrier densities of the initial subband. For a balance between accuracy and computation time, the number of Fourier components can be adjusted. This approach facilitates an efficient design of complex heterostructures with realistic, temperature- and carrier density-dependent rates.

[en] Light emitting diodes for the visible spectral region based on planar (In,Ga)N/GaN heterostructures suffer from a high dislocation density and lack of suitable substrates. An alternative is the integration of these heterostructures into nanowires (NWs) grown on Si by molecular beam epitaxy. Indications for the absence of the quantum-confined Stark effect (QCSE) have been reported in the literature, when the piezoelectric polarization is reduced due to an efficient strain relaxation in the NW geometry. In order to elucidate the origin of the observed luminescence centered at 2.4 eV, we combine transmission electron microscopy, cathodoluminescence and micro-photoluminescence (μ-PL) spectroscopy on single NWs. The μ-PL spectra contain a combination of two types of transitions: (i) several sharp lines from localization centers, which are not affected by the excitation power and (ii) a broader band that blueshifts with higher excitation powers. The former are probably related to composition fluctuations in the (In,Ga)N, while the latter is attributed to an inter-well transition between the two 11 nm thick (In,Ga)N insertions separated by an only 2 to 3 nm thick barrier layer. The blueshift under high excitation evidences a screening of the polarization field. Thus, the QCSE appears to be present in these NW heterostructures in contrast to previous reports.

[en] We investigate the effect of post-growth rapid thermal annealing (RTA) on the transport and lasing characteristics of terahertz quantum-cascade lasers (THz QCLs) operating in a frequency range between 4.88 and 4.94 THz. The emission frequencies are blue shifted by about 80 GHz after RTA, which is attributed to a shift of the gain maximum to higher frequencies due to composition grading at the interfaces between the quantum wells and barriers of the annealed wafer pieces. The optical output power of the annealed THz QCLs is reduced, which is explained by a broadening of the levels due to the annealing process. (paper)

[en] We present a self-consistent model for carrier transport in periodic semiconductor heterostructures completely formulated in the Fourier domain. In addition to the Hamiltonian for the layer system, all expressions for the scattering rates, the applied electric field, and the carrier distribution are treated in reciprocal space. In particular, for slowly converging cases of the self-consistent solution of the Schrödinger and Poisson equations, numerous transformations between real and reciprocal space during the iterations can be avoided by using the presented method, which results in a significant reduction of computation time. Therefore, it is a promising tool for the simulation and efficient design of complex heterostructures such as terahertz quantum-cascade lasers

[en] We have realized GaAs/AlAs quantum-cascade lasers operating at 4.75 THz exhibiting more than three times higher wall plug efficiencies than GaAs/Al_0_._2_5Ga_0_._7_5As lasers with an almost identical design. At the same time, the threshold current density at 10 K is reduced from about 350 A/cm"2 for the GaAs/Al_0_._2_5Ga_0_._7_5As laser to about 120 A/cm"2 for the GaAs/AlAs laser. Substituting AlAs for Al_0_._2_5Ga_0_._7_5As barriers leads to a larger energy separation between the subbands reducing the probability for leakage currents through parasitic states and for reabsorption of the laser light. The higher barriers allow for a shift of the quasi-continuum of states to much higher energies. The use of a binary barrier material may also reduce detrimental effects due to the expected composition fluctuations in ternary alloys.

[en] We present a compact model for the efficient simulation of the gain characteristics in THz quantum-cascade lasers (QCLs) based on the self-consistent solution of the Schrödinger and Poisson equations in the framework of a one-dimensional scattering-rate approach. The total intersubband scattering rates are factorized into the squared modulus of the respective dipole matrix elements and an energy-dependent factor, which we use as an approximation for the various scattering processes. Intended for designing THz QCLs, this model allows the efficient calculation of the gain characteristics and current densities due to a significantly reduced numerical effort compared to a full quantum transport theory. In view of the large parameter space, which causes even small variations of the parameters to have a complex influence on the lasing properties, the accuracy of the predicted results appears to be sufficient. We explore the benefits and limits of this approach by applying it to various THz QCLs based on either the bound-to-continuum design or the resonant-longitudinal-optical-phonon design. A remarkable agreement between the numerical and experimental results, in particular for the energy position of the gain maximum and lasing energy, is found. Finally, we use our model to design a THz QCL operating at a voltage close to the lower theoretical limit in order to reduce excess heating

[en] Vertical stacks of (In, Ga)N insertions in GaN nanowires are grown by molecular beam epitaxy. The chemical composition and strain within the structure are probed by a combination of high-resolution x-ray diffraction, transmission electron microscopy, and geometrical phase analysis. The (In, Ga)N insertions are coherently strained. Finite-element simulations strongly support an effective strain relaxation mechanism of the surrounding GaN matrix due to the nanowire geometry, leading to high-quality (In, Ga)N/GaN nanowire heterostructures. An intense green photoluminescence emission is observed and attributed to an inter-well transition between the stacked (In, Ga)N insertions.

[en] The far-field distribution of the emission intensity of terahertz (THz) quantum-cascade lasers (QCLs) frequently exhibits multiple lobes instead of a single-lobed Gaussian distribution. We show that such multiple lobes can result from self-interference related to the typically large beam divergence of THz QCLs and the presence of an inevitable cryogenic operation environment including optical windows. We develop a quantitative model to reproduce the multiple lobes. We also demonstrate how a single-lobed far-field distribution can be achieved.

[en] Local oscillators in terahertz heterodyne spectrometers have to be operated in continuous-wave mode at precisely defined target frequencies. In particular, for advanced airborne instruments, several specifications such as operating temperature and cooling requirements are necessary to be considered. We have developed a quantum-cascade laser (QCL) applicable as a local oscillator for heterodyne spectroscopy of the OI line at 4.745 THz, which is of particular interest for astronomy. We demonstrate a distributed-feedback QCL operating in continuous-wave mode up to about 60 K, which can be tuned precisely to the target frequency when operated in a mechanical cooler. (paper)