İsmail H. Oğuzman

Electronic transport studies of bulk zincblende and wurtzite phases of GaN based on an ensemble Monte Carlo calculation including a full zone band structure

The ensemble Monte Carlo technique including the details of the first four conduction bands within the full Brillouin zone is used to calculate the basic electronic transport properties for both zincblende and wurtzite crystal phases of bulk gallium nitride. The band structure throughout the Brillouin zone is determined using the empirical pseudopotential method. Calculations of the electron steady‐state drift velocity, average energy, valley occupancy and band occupancy in the range of electric fields up to 500 kV/cm are presented. It is found that the threshold electric field for intervalley transfer is greater and that the second conduction band is more readily occupied in wurtzite than in zincblende GaN over the range of electric fields examined here.

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 ...

Monte Carlo calculation of electron initiated impact ionization in bulk zinc-blende and wurtzite GaN

Calculations of the high-field electronic transport properties of bulk zinc-blende and wurtzite phase gallium nitride are presented focusing particularly on the electron initiated impact ionization rate. The calculations are performed using ensemble Monte Carlo simulations, which include the full details of the band structure derived from an empirical pseudopotential method. The model also includes the numerically generated electron impact ionization transition rate, calculated based on the pseudopotential band structures for both crystallographic phases. The electron initiated impact ionization coefficients are calculated as a function of the applied electric field. The electron distribution is found to be cooler and the ionization coefficients are calculated to be lower in the wurtzite phase as compared to zinc-blende gallium nitride at compatable electric-field strengths. The higher electron energies and the resulting larger impact ionization coefficients in zinc-blende gallium nitride are believed to ...

Hole transport properties of bulk zinc‐blende and wurtzite phases of GaN based on an ensemble Monte Carlo calculation including a full zone band structure

In this paper, we present calculations of the hole transport properties of bulk zinc‐blende and wurtzite phase GaN at field strengths at which impact ionization does not occur significantly. The calculations are made using an ensemble Monte Carlo simulator, including the full details of the band structure and a numerically determined phonon scattering rate based on an empirical pseudopotential method. Band intersection points—including band crossings and band mixings—are treated by carefully evaluating the overlap integral between the initial and possible final drift states. In this way, the hole trajectories in phase space can be accurately traced. It is found that the average hole energies are significantly lower than the corresponding electron energies for the field strengths examined. This result is most probably due to the drastic difference in curvature between the uppermost valence bands and the lowest conduction band. The relatively flat valence bands impede hole‐heating, leading to low average hole energy.

DIELECTRIC PROPERTIES OF WURTZITE AND ZINCBLENDE STRUCTURE GALLIUM NITRIDE

Abstract We present calculated results for the wavevector and frequency dependent dielectric functions of zincblende and wurtzite GaN based on empirical pseudopotential band structures. The q → -dependent static dielectric functions, ϵ∞( q → ), are found to be similar for the two crystal modifications. The optical dielectric functions, ϵ∞(ω), however, are different in the range 6 eV

Calculation of the wave‐vector‐dependent interband impact‐ionization transition rate in wurtzite and zinc‐blende phases of bulk GaN

We present calculations of the wave‐vector‐dependent interband impact‐ionization transition rate in wurtzite and zinc‐blende phases of bulk GaN. The transition rate is determined by integrating Fermi’s golden rule for a two‐body, screened Coulomb interaction over the possible final states using a numerically generated dielectric function and pseudowavefunctions. The full details of all relevant conduction and valence bands in zinc‐blende and wurtzite GaN are included from an empirical pseudopotential calculation. It is found that the transition rate is consistent with a relatively ‘‘soft’’ threshold energy.

Theoretical study of hole initiated impact ionization in bulk silicon and GaAs using a wave‐vector‐dependent numerical transition rate formulation within an ensemble Monte Carlo calculation

In this paper, calculations of the hole initiated interband impact ionization rate in bulk silicon and GaAs are presented based on an ensemble Monte Carlo simulation with the inclusion of a wave‐vector‐dependent numerical transition rate formulation. The ionization transition rate is determined for each of the three valence bands, heavy, light, and split‐off, using Fermi’s golden rule with a two‐body, screened Coulomb interaction. The dielectric function used within the calculation is assumed to be wave‐vector‐dependent. Calculations of the field‐dependent impact ionization rate as well as the quantum yield are presented. It is found from both the quantum yield results and examination of the hole distribution function that the effective threshold energy for hole initiated impact ionization is relatively soft, similar to that predicted for the corresponding electron initiated ionization rate threshold in both GaAs and silicon. It is further found that light‐hole initiated ionization events occur more frequ...

Theoretical investigation of wave-vector-dependent analytical and numerical formulations of the interband impact-ionization transition rate for electrons in bulk silicon and GaAs

The electron interband impact‐ionization rate for both silicon and gallium arsenide is calculated using an ensemble Monte Carlo simulation with the expressed purpose of comparing different formulations of the interband ionization transition rate. Specifically, three different treatments of the transition rate are examined: the traditional Keldysh formula, a new k‐dependent analytical formulation first derived by W. Quade, E. Scholl, and M. Rudan [Solid State Electron. 36, 1493 (1993)], and a more exact, numerical method of Y. Wang and K. F. Brennan [J. Appl. Phys. 75, 313 (1994)]. Although the completely numerical formulation contains no adjustable parameters and as such provides a very reliable result, it is highly computationally intensive. Alternatively, the Keldysh formula, although inherently simple and computationally efficient, fails to include the k dependence as well as the details of the energy band structure. The k‐dependent analytical formulation of Quade and co‐workers overcomes the limitatio...

Effect of doping on the reliability of GaAs multiple quantum well avalanche photodiodes

The effect of various doping methods on the reliability of gallium arsenide/aluminum gallium (GaAs/AlGaAs) multiple quantum well (MQW) photodiode (APD) structures fabricated by molecular beam epitaxy is investigated. Reliability is examined by accelerated life tests by monitoring dark current and breakdown voltage. Median device lifetime and the activation energy of the degradation mechanism are computed for undoped, doped-barrier, and doped-well APD structures. Lifetimes for each device structure are examined via a statistically designed experiment. Analysis of variance (ANOVA) shows that dark current is affected primarily by device diameter, temperature and stressing time, and breakdown voltage depends on the diameter, stressing time, and APD type. It is concluded that the undoped APD has the highest reliability, followed by the doped-well and doped-barrier devices, respectively. To determine the source of the degradation mechanism for each device structure, failure analysis using the electron-beam induced current method is performed. This analysis reveals some degree of device degradation caused by ionic impurities in the passivation layer, and energy-dispersive spectrometry subsequently verifies the presence of ionic sodium as the primary contaminant. However since all device structures are similarly passivated, sodium contamination alone does not account for the observed variation between the differently doped APDs. This effect is explained by dopant migration during stressing, which is verified by free carrier concentration measurements using the capacitance-voltage (C-V) technique.

Dielectric Functions of Wurtzite and Zincblende Structure GaN

The authors present calculated longitudinal frequency and wavevector dependent dielectric functions of zincblende and wurtzite structure GaN using band energies and wavefunctions generated in the framework of the empirical pseudopotential method. They discuss the anisotropy of the static dielectric function and find that the results are in satisfactory agreement with experimental data for {vert_bar}{rvec q}{vert_bar} {yields} 0.