Stephen K. O’Leary

TRANSIENT ELECTRON TRANSPORT IN WURTZITE GAN, INN, AND ALN

Transient electron transport and velocity overshoot in wurtzite GaN, InN, and AlN are examined and compared with that which occurs in GaAs. For all materials, we find that electron velocity overshoot only occurs when the electric field is increased to a value above a certain critical field, unique to each material. This critical field is strongly dependent on the material, about 4 kV/cm for the case of GaAs but much higher for the III–nitride semiconductors: 140 kV/cm for GaN, 65 kV/cm for InN, and 450 kV/cm for AlN. We find that InN exhibits the highest peak overshoot velocity and that this velocity overshoot lasts over the longest distances when compared with GaN and AlN. Finally, using a one-dimensional energy–momentum balance approach, a simple model is used to estimate the cutoff frequency performance of nitride based heterojunction field effect transistors (HFETs) and a comparison is made to recently fabricated AlGaN/GaN HFETs.

Steady-state and transient electron transport within bulk wurtzite indium nitride: An updated semiclassical three-valley Monte Carlo simulation analysis

Recent experimentation, performed on bulk wurtzite InN, suggests that the energy gap, the effective mass of the electrons in the lowest-energy valley, and the nonparabolicity coefficient of the lowest-energy valley are not as originally believed for this material. Using a semiclassical three-valley Monte Carlo simulation approach, we analyze the steady-state and transient electron transport that occurs within bulk wurtzite InN using a revised set of material parameters, this revised set of parameters taking into account this recently observed phenomenology. We find that the peak electron drift velocity is considerably greater than that found previously. The impact that this revised set of parameters has upon the transient electron transport is also found to be significant.

Potential performance of indium-nitride-based devices

We study how electrons, initially in thermal equilibrium, drift under the action of an applied electric field within bulk wurtzite indium nitride. We find that the optimal cutoff frequency for an ideal indium-nitride-based device ranges from around 10GHz when the device thickness is set to 10μm to about 2.5THz when the device thickness is set to 0.1μm. We thus suggest that indium nitride offers great promise for future high-speed device applications.

The dependence of the Tauc and Cody optical gaps associated with hydrogenated amorphous silicon on the film thickness: αl Experimental limitations and the impact of curvature in the Tauc and Cody plots

Using a model for the optical spectrum associated with hydrogenated amorphous silicon, explicitly taking into account fundamental experimental limitations encountered, we theoretically determine the dependence of the Tauc and Cody optical gaps associated with hydrogenated amorphous silicon on the thickness of the film. We compare these results with that obtained from experiment. We find that the curvature in the Tauc plot plays a significant role in influencing the determination of the Tauc optical gap associated with hydrogenated amorphous silicon, thus affirming an earlier hypothesis of Cody et al. We also find that the spectral dependence of the refractive index plays an important role in influencing the determination of the Cody optical gap. It is thus clear that care must be exercised when drawing conclusions from the dependence of the Tauc and Cody optical gaps associated with hydrogenated amorphous silicon on the thickness of the film.

Recombination of drifting holes with trapped electrons in stabilized a-Se photoconductors: Langevin recombination

Stabilized amorphous selenium (a-Se) is one of the x-ray photoconductors that is currently used in recently developed direct conversion flat panel x-ray image detectors. We have studied the recombination of free holes with trapped electrons in stabilized a-Se. Electrons were deeply trapped in a-Se by carrying out repetitive electron time-of-flight (TOF) transient photoconductivity experiments. By using conventional and interrupted field hole time-of-flight (IFTOF) transient photoconductivity techniques in a TOF, IFTOF, TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient between free holes and trapped electrons. We find that the capture process of holes by trapped electrons closely follows the Langevin recombination mechanism.

On defining the optical gap of an amorphous semiconductor: an empirical calibration for the case of hydrogenated amorphous silicon

It is pointed out that there are a number of different means whereby the optical gap of an amorphous semiconductor may be defined. We analyze some hydrogenated amorphous silicon data with respect to a number of these empirical measures for the optical gap. By plotting these gap measures as a function of the breadth of the optical absorption tail, we provide a means of relating these disparate measures of the optical gap.

A simplified joint density of states analysis of hydrogenated amorphous silicon

We propose a simplified empirical model for the density of state functions of hydrogenated amorphous silicon that neglects the conduction band tail electronic states. The corresponding joint density of states function is then computed. We find, while this analysis is considerably simplified, that the resultant joint density of states function compares favorably with that determined from an empirical model for the density of states functions with the conduction band tail taken into account. Analytical and asymptotic results, relating the parameters characterizing the underlying density of states functions with the joint density of states function, are developed. The density of states parameters corresponding to hydrogenated amorphous silicon are then determined through an analysis of some hydrogenated amorphous silicon joint density of states experimental data. It is suggested that this simplified empirical model for the density of states functions will prove of greater utility to the experimentalist.

The dependence of the Fermi level on temperature, doping concentration, and disorder in disordered semiconductors

We employ an elementary model for the distribution of electronic states to develop a quantitative theory of equilibrium occupation statistics in disordered semiconductors. In particular, assuming Fermi–Dirac statistics and charge neutrality, we determine how the Fermi level position varies with temperature for various amounts of disorder and various dopant concentration levels, disorder being represented by the breadth of the tails in the conduction band and valence band distributions of electronic states. We find that as the disorder is increased the Fermi level is pulled towards the intrinsic Fermi level. An explanation for this result is provided.

Steady-state and transient electron transport within the wide energy gap compound semiconductors gallium nitride and zinc oxide: an updated and critical review

The wide energy gap compound semiconductors, gallium nitride and zinc oxide, are widely recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these wide energy gap compound semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyzes of the electron transport within the wide energy gap compound semiconductors, gallium nitride and zinc oxide. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, zinc-blende gallium nitride, and wurtzite zinc oxide. The electron transport that occurs within bulk zinc-blende gallium arsenide will also be considered, albeit primarily for bench-marking purposes. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art wide energy gap compound semiconductor orthodoxy. A brief tutorial on the Monte Carlo electron transport simulation approach, this approach being used to generate the results presented herein, will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials. The wurtzite gallium nitride and zinc-blende gallium arsenide results, being presented in a previous review article of ours (O’Leary et al. in J Mater Sci Mater Electron 17:87, 2006), are also presented herein for the sake of completeness.

Electron mobility limited by scattering from screened positively charged dislocation lines within indium nitride

In the present work, we address the open question of the contribution from threading dislocations to the problem of unintentional n-type conductivity exhibited by indium nitride through an examination of the effect that positively charged dislocation lines have on the transverse electron mobility within this material. Assuming that the threading dislocation lines within indium nitride act as a source for free electrons, the screening associated with the positively charged threading dislocation lines is evaluated. The impact this screening has on the dislocation limited electron mobility within this material is then considered. Our results indicate that one of the implications of attributing a donor character to the threading dislocation lines within indium nitride would be a strong non-uniformity in the free electron concentration in the plane of growth of this semiconductor. This contrasts dramatically with the case of gallium nitride.