Enrico Bellotti

Monte Carlo simulation of electron transport in the III-nitride wurtzite phase materials system: binaries and ternaries

We present a comprehensive study of the transport dynamics of electrons in the ternary compounds, Al/sub x/Ga/sub 1-x/N and In/sub x/Ga/sub 1-x/N. Calculations are made using a nonparabolic effective mass energy band model. Monte Carlo simulation that includes all of the major scattering mechanisms. The band parameters used in the simulation are extracted from optimized pseudopotential band calculations to ensure excellent agreement with experimental information and ab initio band models. The effects of alloy scattering on the electron transport physics are examined. The steady state velocity field curves and low field mobilities are calculated for representative compositions of these alloys at different temperatures and ionized impurity concentrations. A field dependent mobility model is provided for both ternary compounds AlGaN and InGaN. The parameters for the low and high field mobility models for these ternary compounds are extracted and presented. The mobility models can be employed in simulations of devices that incorporate the ternary III-nitrides.

Ensemble Monte Carlo study of electron transport in wurtzite InN

Electronic transport in wurtzite phase InN is studied using an ensemble Monte Carlo method. The model includes the full details of the first five conduction bands derived from the pseudopotential method and a numerically calculated impact ionization transition rate using a wave-vector- dependent dielectric function. Calculated results for electron transport at both low and high electric field are presented and compared with available results from simpler methods. The dependence of the relevant transport properties on the parameters is discussed, in particular in regards to the uncertainties in the band structure and coupling constants. It is found that at a field of 65 kV/cm that the peak electron drift velocity is 4.2×107 cm/s. The peak velocity in InN is substantially higher than in GaN. The velocity field curve presents a noticeable anisotropy with respect to field direction. The peak velocity decreases to 3.4×107 cm/s for a field of 70 kV/cm in the direction perpendicular to the basal plane. The elect...

Theory of hole initiated impact ionization in bulk zincblende and wurtzite GaN

In this article, the first calculations of hole initiated interband impact ionization in bulk zincblende and wurtzite phase GaN are presented. The calculations are made using an ensemble Monte Carlo simulation including the full details of all of the relevant valence bands, derived from an empirical pseudopotential approach, for each crystal type. The model also includes numerically generated hole initiated impact ionization transition rates, calculated based on the pseudopotential band structure. The calculations predict that both the average hole energies and ionization coefficients are substantially higher in the zincblende phase than in the wurtzite phase. This difference is attributed to the higher valence band effective masses and equivalently higher effective density of states found in the wurtzite polytype. Furthermore, the hole ionization coefficient is found to be comparable to the previously calculated electron ionization coefficient in zincblende GaN at an applied electric field strength of 3 ...

Band structure nonlocal pseudopotential calculation of the III-nitride wurtzite phase materials system. Part I. Binary compounds GaN, AlN, and InN

This work presents nonlocal pseudopotential calculations based on realistic, effective atomic potentials of the wurtzite phase of GaN, InN, and AlN. A formulation formulation for the model effective atomic potentials has been introduced. For each of the constitutive atoms in these materials, the form of the effective potentials is optimized through an iterative scheme in which the band structures are recursively calculated and selected features are compared to experimental and/or ab initio results. The optimized forms of the effective atomic potentials are used to calculate the band structures of the binary compounds, GaN, InN, and AlN. The calculated band structures are in excellent overall agreement with the experimental/ab initio values, i.e., the energy gaps at high-symmetry points, valence-band ordering, and effective masses for electrons match to within 3%, with a few values within 5%. The values of the energy separation, effective masses, and nonparabolicity coefficients for several secondary valle...

Monte Carlo study of GaN versus GaAs terahertz quantum cascade structures

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 2THz, 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.

A numerical study of Auger recombination in bulk InGaN

Direct interband and intraband Auger recombination due to electron-electron-hole and hole-hole-electron transitions in bulk InGaN is investigated by first-order perturbation theory including Fermi statistics, realistic electronic structures obtained by nonlocal empirical pseudopotential calculations, and their corresponding wavevector-dependent dielectric functions. Our results confirm that the intraband Auger coefficient is negligible in alloy compositions relevant for solid-state lighting and indicate that the resonant enhancement associated with interband transitions for wavelengths ranging from blue to green cannot account for the efficiency droop experimentally observed in GaN-based light emitting diodes.

Numerical analysis of HgCdTe simultaneous two-color photovoltaic infrared detectors

In this paper, we present a physics-based full three-dimensional (3-D) numerical simulation of simultaneous two-color medium-wave infrared long-wave infrared (MWIR-LWIR) and LWIR-very-long-wave infrared (VLWIR) detectors. The present approach avoids geometrical simplifications typical of one- or two-dimensional models that can introduce errors which are difficult to quantify. We include all the relevant material physics and the drift-diffusion equations are solved on a 3-D finite element grid. We simulate device structures that have been fabricated and characterized for operation in the MWIR-LWIR spectral regions and compare the numerical results with the measured values. Furthermore, we apply the same model to predict the performance of similar detector structures intended for operation in the LWIR-VLWIR spectral regions.

Monte Carlo simulation of terahertz quantum cascade laser structures based on wide-bandgap semiconductors

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 10to300K, the population inversions are found to decrease by facto...

Theory of high field carrier transport and impact ionization in wurtzite GaN. Part I: A full band Monte Carlo model

High field electron and hole transport in wurtzite phase GaN is studied using an ensemble Monte Carlo method. The model includes the details of the full band structure derived from nonlocal empirical pseudopotential calculations. The nonpolar carrier-phonon interaction is treated within the framework of the rigid pseudoion approximation using ab initio techniques to determine the phonon dispersion relation. The calculated carrier-phonon scattering rates are consistent with the electronic structure and the phonon dispersion relation thus removing adjustable parameters such as deformation potential coefficients. The impact ionization transition rate is computed based on the calculated electronic structure and the corresponding wave-vector dependent dielectric function. The complex band structure of wurtzite GaN requires the inclusion of band-to-band tunneling effects that are critical at high electric fields. The electric-field-induced interband transitions are investigated by the direct solution of the time dependent multiband Schrodinger equation. The multiband description of the transport predicts a considerable increase in the impact ionization coefficients compared to the case in which tunneling is not considered. In the second part of this work it will be shown that the proposed numerical model correctly predicts the carrier multiplication gain and breakdown voltage of a variety of GaN avalanche photodetectors that have been recently fabricated by several research groups.

Numerical analysis of indirect Auger transitions in InGaN

Indirect phonon-assisted Auger recombination mechanisms in bulk InGaN are investigated in the framework of perturbation theory, using first-principles phonon spectral density functions and electronic structures obtained by nonlocal empirical pseudopotential calculations. Nonpolar carrier-phonon interactions are treated within the rigid pseudoion framework, thus avoiding the introduction of empirical deformation potentials. The calculated indirect Auger coefficients exhibit a weak temperature dependence and dominate over direct processes for alloy compositions corresponding to the entire visible spectrum. The present results suggest that indirect Auger processes may be relevant in the operation of InGaN-based light-emitting diodes and lasers, at least in the yellow-green spectral region.