Search

Wide-bandgap semiconductors such as GaN/AlGaN and ZnO/MgZnO quantum wells are promising for improving the spectral reach and high-temperature performance of terahertz quantum cascade lasers, due to their characteristically large optical phonon energies. Here, a particle-based Monte Carlo model is developed and used to quantify the potential of terahertz sources based on these materials relative to existing devices based on GaAs/AlGaAs quantum wells. Specifically, three otherwise identical quantum cascade structures based on GaN/AlGaN, ZnO/MgZnO, and GaAs/AlGaAs quantum wells are designed, and their steady-state carrier distributions are then computed as a function of temperature. The simulation results show that the larger the optical phonon energies (as in going from the AlGaAs to the MgZnO to the AlGaN materials system), the weaker the temperature dependence of the population inversion. In particular, as the temperature is increased from 10 to 300 K, the population inversions are found to decrease by factors of 4.48, 1.50, and 1.25 for the AlGaAs, MgZnO, and AlGaN structure, respectively. Based on these results, the AlGaN and MgZnO devices are then predicted to be in principle capable of laser action without cryogenic cooling. [ABSTRACT FROM AUTHOR]

The heat capacity of conduction electrons obtained from the Sommerfeld expansion is shown to be tunable via the Rashba and Dresselhaus spin-orbit coupling parameters. Using AlInSb/InSb/AlInSb as a representative heterostructure with alterable well and asymmetric barrier regions, the heat capacity is higher for the spin-down electrons and suffers a reduction for wider wells. A further lowering is obtained through the application of a uniaxial strain. Finally, we suggest a method to determine the spin lifetimes for spin relaxing via the D'yakonov-Perel' mechanism from experimental estimates of thermodynamic potentials such as the Helmholtz free energy and the heat capacity. [ABSTRACT FROM AUTHOR]

A microscopic model, based on a full-Brillouin-zone description of the electronic structure, is used to investigate Auger transitions in InGaN/GaN quantum wells. The lack of momentum conservation along the confining direction enhances direct (i.e., phononless) Auger transitions, leading to Auger coefficients in the range of those predicted for phonon-dressed processes in bulk InGaN. The dependence of Auger coefficients on temperature and quantum well thickness is analyzed. The limitations of conventional multiband models, based on zone-center approximations of the band structure, are discussed. [ABSTRACT FROM AUTHOR]

2-D numerical simulation is employed to assess a number of possible design approaches aimed at optimizing the internal quantum efficiency (IQE) of ZnO-based light-emitting diodes (LEDs) grown along the c-axis. First, the relative performance of similar ZnO-based and GaN-based LED structures is compared and discussed. Second, the effects on IQE of thickness, doping, and alloy composition of the MgZnO electron blocking layer (EBL) is studied in order to maximize the carrier confinement in the active region. The optimum number of quantum wells is also addressed, and different strategies for barrier doping are considered, showing that, if the EBL is doped p-type, a similar doping in the barriers is not required to compensate for the spontaneous and piezoelectric interface charges and to enhance hole transport. Different choices of the geometrical and doping parameters of the n-type access region are considered, and the impact of different values of the electron mobility is determined. Finally, the analysis of a ZnO/BeZnO LED structure suggests that the incorporation of BeZnO layers does not provide significant advantages. [ABSTRACT FROM PUBLISHER]

As a preparatory step toward establishing reliable numerical design tools for ZnO-based optoelectronic devices, we have reassessed the available information on material parameters relevant for the simulation of light-emitting diodes (LEDs) with active regions including ZnO, MgZnO, and BeZnO layers. The impact of different approximations for the electronic structure and the interface polarization charge on the optical properties of bulk ZnO and ZnO/MgZnO quantum wells has been evaluated, and a consistent set of parameters has been used not only for systematic comparison of ZnO/MgZnO and ZnO/BeZnO single quantum well structures but also for the first simulation of a realistic ZnO/BeZnO multiple quantum well LED. [ABSTRACT FROM AUTHOR]

In this work, a particle-based Monte Carlo model is used to quantify the potential of terahertz sources based on the ZnO-based material system relative to existing devices based on GaAs/AlGaAs quantum wells. Specifically, two otherwise identical quantum cascade structures based on ZnO/MgZnO and GaAs/AlGaAs quantum wells are designed, and their non- equilibrium carrier distributions are then computed as a function of temperature. The simulation results show that, because of their larger optical phonon energy, ZnO/MgZnO quantum cascade laser structures exhibit weaker temperature dependence of the population inversion than in the case of similar structures made of GaAs/AlGaAs. In particular, as the temperature is increased from 10 K to 300 K, population inversion is found to decrease by a factor of 4.48 and 1.50 for the AlGaAs and MgZnO structure, respectively. Based on these results, the MgZnO devices are then predicted to be, in principle, capable of laser action without cryogenic cooling. [ABSTRACT FROM AUTHOR]

We discuss, through numerical device simulation, a number of possible design approaches intended for optimizing internal quantum efficiency (IQE) of light-emitting diodes based on InGaN quantum wells (QWs) grown along the c-axis emitting in the near-ultraviolet region. We study the effects on IQE thickness, doping, and alloy composition of the electron and hole blocking layers in order to maximize the confinement of both carrier species in the active region. We discuss the selection of the number of QWs to be employed in the active region and their optimum width, and we show the comparatively minor effects of the thickness of the barrier layers. We also compare different strategies for barrier duping, confirming that a p-type doping in all barriers helps to compensate the spontaneous and piezoelectric surface charges and to enhance hole transport. Finally, we evaluate the impact of Auger recombination on IQE and its role the experimentally observed efficiency droop. Whenever possible, we suggest practical design criteria and provide technologically feasible sets of design parameters. [ABSTRACT FROM AUTHOR]

Due to their large optical phonon energies, nitride semiconductors are promising for the development of terahertz quantum cascade lasers with dramatically improved high-temperature performance relative to existing GaAs devices. Here, we present a rigorous Monte Carlo study of carrier dynamics in two structures based on the same design scheme for emission at 2 THz, consisting of GaN/AlGaN or GaAs/AlGaAs quantum wells. The population inversion and hence the gain coefficient of the nitride device are found to exhibit a much weaker (by a factor of over 3) temperature dependence and to remain large enough for laser action even without cryogenic cooling. [ABSTRACT FROM AUTHOR]