B. E. Foutz

Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

Two dimensional electron gases in Al x Ga 12x N/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance‐voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined

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.

Electron transport in wurtzite indium nitride

We present the velocity-field characteristics of wurtzite indium nitride, determined using an ensemble Monte Carlo approach. It is found that indium nitride exhibits an extremely high peak drift velocity at room temperature, 4.3×107 cm/s, at a doping concentration of 1.0×1017 cm−3. We also demonstrate that the saturation drift velocity of indium nitride, 2.5×107 cm/s, is comparable to that of gallium nitride, and much larger than that of gallium arsenide. Our results suggest that the transport characteristics of indium nitride are superior to those of gallium nitride and gallium arsenide, over a wide range of temperatures, from 150 to 500 K, and doping concentrations, up to 1.0×1019 cm−3. Hence, indium nitride has considerable potential for device applications.

Comparison of high field electron transport in GaN and GaAs

An ensemble Monte Carlo simulation is used to compare high field electron transport in bulk GaN and GaAs. In particular, velocity overshoot and electron transit times are examined. In GaN, we find the steady state velocity of the electrons is the most important factor determining transit time over distances longer than 0.2 μm. Over shorter distances velocity overshoot effects in GaN at high fields are comparable to those in GaAs. We estimate the minimum transit time across a 1 μm GaN sample to be about 3.0 ps. Similar calculations for GaAs yield 5.4 ps.

Role of Spontaneous and Piezoelectric Polarization Induced Effects in Group-III Nitride Based Heterostructures and Devices

The wurzite group-III nitrides InN, GaN, and AlN are tetrahedrally coordinated direct band gap semiconductors having a hexagonal Bravais lattice with four atoms per unit cell. As a consequence of the noncentrosymmetry of the wurzite structure and the large ionicity factor of the covalent metal–nitrogen bond, a large spontaneous polarization oriented along the hexagonal c-axis is predicted. In addition, group-III nitrides are highly piezoelectric. The strain induced piezoelectric as well as the spontaneous polarizations are expected to be present and to govern the optical and electrical properties of GaN based heterostructures to a certain extent, due to the huge polarization constants which are one of the most fascinating aspects of the nitrides. In this paper we will present theoretical and experimental results demonstrating how polarization induced electric fields and bound interface charges in AlGaN/GaN, InGaN/GaN and AlInN/GaN heterostructures lead to the formation of two-dimensional carrier gases suitable for the fabrication of high power microwave frequency transistors.

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.

Monte Carlo simulation of electron transport in wurtzite aluminum nitride

Abstract We present the steady-state, velocity-field characteristics of wurtzite aluminum nitride, determined using an ensemble Monte Carlo approach. A three valley model for the conduction band is employed and ionized impurity, polar optical phonon, piezoelectric, deformation potential and intervalley scattering mechanisms are considered. We find that aluminum nitride exhibits peak and saturation drift velocities of 1.7 × 10 7 cm s −1 and 1.4 × 10 7 cm s −1 , respectively, at a temperature of 300 K and a doping concentration of 1.0 × 10 17 cm −3 . The sensitivity of these steady-state results to variations in temperature and doping concentration is examined and is found to be much less than that of gallium arsenide. We also explore the sensitivity of our results to variations in the piezoelectric constant and demonstrate that piezoelectric scattering plays a significant role in determining the form of the wurtzite aluminum nitride velocity-field characteristic.

Low-frequency noise and mobility fluctuations in AlGaN/GaN heterostructure field-effect transistors

The 1/f low-frequency noise characteristics of AlGaN/GaN heterostructure field-effect transistors, grown on sapphire and SiC substrates by molecular beam epitaxy and organometallic vapor phase epitaxy, are reported. The Hooge parameter is deduced taking into account the effect of the contact noise and the noise originating in the ungated regions. A strong dependence between the Hooge parameter and the sheet carrier density is obtained, and it is explained using a model in which mobility fluctuations are produced by dislocations. A Hooge parameter as low as αCH≈8×10−5 is determined for devices grown on SiC substrates.

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.