Boris Gelmont

Monte Carlo simulation of electron transport in gallium nitride

The results of an ensemble Monte Carlo simulation of the electron transport in gallium nitride (GaN) are presented. The calculation shows that intervalley electron transfer plays a dominant role in GaN in high electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. An analytic expression for the polar optical momentum relaxation time for phonon energies larger than the thermal energy is also derived. This expression applies to many wide‐gap semiconductors, such as GaN and SiC, at room temperature since these semiconductors have large polar optical‐phonon energies (on the order of 100 meV). The calculated mobility agrees well with the results of the Monte Carlo calculation.

Elastic strain relaxation and piezoeffect in GaN-AlN, GaN-AlGaN and GaN-InGaN superlattices

We calculated the elastic strain relaxation in (GaN)n-(AlN)n, (GaN)n(AlxGa1−xN)n and (GaN)n(InxGa1−xN)n superlattices where n is the number of layers in the superlattice cell. This calculation and a similar calculation for a semiconductor–insulator–semiconductor structure allowed us to determine the lower and upper bounds for the elastic strain relaxation in (GaN)m(AlN)n superlattices with arbitrary n/m ratios, i.e., we determine a full range of the critical thicknesses for GaNm(AlN)n superlattices. The obtained theoretical results can also be applied to other superlattices based on III nitrides and their solid solutions. Our theory agrees with the experimental data for GaN-AlN superlattices. Also, we show that the piezoelectric effect may cause a large shift of the absorption edge in defect-free GaNm(AlxGa1−xN)n superlattices.

The influence of the strain-induced electric field on the charge distribution in GaN-AlN-GaN structure

We show that strongly pronounced piezoelectric properties play a key role in GaN‐AlN‐GaN semiconductor‐insulator‐semiconductor (SIS) and related structures. In sufficiently thin AlN layers, the lattice constant mismatch is accommodated by internal strains rather than by the formation of misfit dislocations. These lattice‐mismatch‐induced strains generate polarization fields. We demonstrate that, in a GaN‐AlN‐GaN SIS structure with the growth axis along a (0001) crystallographic direction, the strain‐induced electric fields can shift the flat band voltage and produce an accumulation region on one side and a depletion region on the other side of the AlN insulator. On which side of the insulator the accumulation region is produced depends on the type of atomic plane at the heterointerface (Ga or N). The surface charge density caused by the piezoeffect is on the order of 1012 cm−2. As a consequence of the asymmetry in the space charge distribution, the capacitance‐voltage (C‐V) characteristics of the SIS stru...

Electron mobility in two-dimensional electron gas in AlGaN/GaN heterostructures and in bulk GaN

We report on temperature dependencies of the electron mobility in the two-dimensional electron gas (2DEG) in AIGaN/GaN heterostructures and in doped bulk GaN. Calculations and experimental data show that the polar optical scattering and ionized impurity scattering are the two dominant scattering mechanisms in bulk GaN for temperatures between 77 and 500K. In the 2DEG in AIGaN/GaN heterostructures, the piezoelectric scattering also plays an important role. Even for doped GaN, with a significant concentration of ionized impurities, a large volume electron concentration in the 2DEG significantly enhances the electron mobility, and the mobility values close to 1700 cm2/Vs may be obtained in the GaN 2DEG at room temperature. The maximum measured Hall mobility at 80K is nearly 5000 cm2/Vs compared to approximately 1200 cm2/Vs in a bulk GaN layer. With a change in temperature from 300 to 80K, the 2DEG in our samples changes from nondegenerate and weakly degenerate to degenerate. Therefore, in order to interpret the experimental data, we propose a new interpolation formula for low field mobility limited by the ionized impurity scattering. This formula is valid for an arbitrary degree of the electron gas degeneracy. Based on our theory, we show that the mobility enhancement in the 2DEG is related to a much higher volume electron concentration in the 2DEG, and, hence, to a more effective screening.

THz-Spectroscopy of Biological Molecules.

The terahertz frequency absorption spectraof DNA molecules reflect low-frequencyinternal helical vibrations involvingrigidly bound subgroups that are connectedby the weakest bonds, including thehydrogen bonds of the DNA base pairs,and/or non-bonded interactions. Althoughnumerous difficulties make the directidentification of terahertz phonon modes inbiological materials very challenging, ourresearch has shown that such measurementsare both possible and fruitful. Spectra ofdifferent DNA samples reveal a large numberof modes and a reasonable level ofsequence-specific uniqueness. In an attemptto show that the long wavelength absorptionfeatures are intrinsic properties ofbiological materials determined by phononmodes, a normal mode analysis has been usedto predict the absorption spectra ofpolynucleotide RNA Poly[G]-Poly[C]. Directcomparison demonstrated a correlationbetween calculated and experimentallyobserved spectra of the RNA polymers, thusconfirming that the fundamental physicalnature of the observed resonance structureis caused by the internal vibration modesin the macromolecules.In this work we demonstrate results fromFourier-Transform Infrared (FTIR)spectroscopy of DNA macromolecules andrelated biological materials in theterahertz frequency range. Carefulattention was paid to the possibility ofinterference or etalon effects in thesamples, and phenomena were clearlydifferentiated from the actual phononmodes. In addition, we studied thedependence of transmission spectra ofaligned DNA and polynucleotide film sampleson molecule orientation relative to theelectromagnetic field, showing the expectedchange in mode strength as a function ofsample orientation. Further, the absorptioncharacteristics were extracted from thetransmission data using the interferencespectroscopy technique, and a stronganisotropy of terahertz characteristics wasdemonstrated.

POLAR OPTICAL-PHONON SCATTERING IN THREE- AND TWO-DIMENSIONAL ELECTRON GASES

The concept of the polar optical momentum relaxation time for phonon energies larger than the thermal energy is applied to the analytical calculations of the electron mobility in bulk GaAs and in a two‐dimensional (2D) electron gas. The theory also accounts for nonparabolicity. For the bulk material, the analytical formula is in good agreement with experimental data even at elevated temperatures where it can be used as an extrapolation formula. The comparison with numerical simulations for the 2D gas shows that the analytical formula for the 2D case applies when intersubband scattering is unimportant. When many subbands are involved in scattering processes, the polar optical mobility can be estimated using the analytical formula for the bulk case. Hence the results provide convenient analytical equations for polar optical mobility which can be used in device simulators.

Current‐voltage characteristics of strained piezoelectric structures

Experimental and theoretical studies are presented of the current‐voltage characteristics of symmetrically doped n‐type GaN‐AlN‐GaN semiconductor‐insulator‐semiconductor (SIS) structures. The asymmetry caused by the strain‐induced electric field leads to the depletion layer barrier in addition to the barrier presented by a thin insulating layer of AlN. It is shown that the tunnel current depends on the degree of the elastic strain relaxation which, in turn, is related to the AlN film thickness. This dependence provides quantitative information about the film relaxation. This characterization technique is compared with the capacitance‐voltage characterization of the SIS structures. The data indicate that the low bound of the conduction‐band offset for the AlN/GaN heterointerface is close to 1 eV.

Two‐dimensional electron gas in GaN–AlGaN heterostructures deposited using trimethylamine‐alane as the aluminum source in low pressure metalorganic chemical vapor deposition

In this letter, we report the fabrication of high quality GaN–Al0.1Ga0.9N heterostructures using trimethylamine‐alane as the aluminum source. The two‐dimensional electron gas (2DEG) with mobility values as high as 5000 cm2/V s at 150 K is demonstrated. We also present the results of our calculations of the 2DEG mobility at the GaN–Al0.1Ga0.9N heterointerface. Our calculations show that the mobility enhancement in the 2DEG is related to a much larger volume carrier concentration (compared to bulk GaN) and, therefore, to a larger Fermi energy and to a more effective screening of impurity and piezoelectric scattering.

Terahertz Fourier transform characterization of biological materials in a liquid phase

Significant progress has been achieved during the last several years relating to experimental and theoretical aspects of terahertz (or submillimetre wave) Fourier transform spectroscopy of biological macromolecules. However, previous research in this spectral range has been focused on bio-materials in solid state since it was common opinion that high water absorption will obscure the spectral signatures of the bio-molecules in solutions. At the same time, the biological functions of DNA and proteins take place in water solutions. In this work, the spectra of DNA samples have been measured in liquid phase (gel) over the spectral range 10–25 cm−1 and compared with spectra obtained from solid films. The results demonstrate that there is very little interference between the spectral features of the material under test and the water background except for the band around 18.6 cm−1. Multiple resonances due to low frequency vibrational modes within biological macromolecules in solutions are unambiguously demonstrated. Higher level of sensitivity and higher sharpness of vibrational modes are observed in the liquid environment in comparison with the solid phase, with the width of spectral lines 0.3–0.5 cm−1. Gel sample spectra are found to be polarization-dependent. The ability of THz spectroscopy to characterize samples in liquid phase could be very important since it permits examination of DNA interactions in real (wet) samples. One demonstrated example of practical importance is the ability to discriminate between spectral patterns for native and denaturated DNA.

Elastic strain relaxation in GaN–AlN–GaN semiconductor–insulator–semiconductor structures

We calculated the elastic strain relaxation in wurtzite GaN–AlN–GaN semiconductor–insulator–semiconductor (SIS) structures. Elastic strain tensor components, elastic energy, the density of the misfit dislocations, and the other parameters of the system were obtained as functions of the AlN layer thickness. Theoretical values of the elastic strain relaxation are in satisfactory agreement with experimental data extracted from the capacitance‐voltage (C‐V) characteristics of GaN–AlN–GaN SIS structures. Our results confirm that the gradual relaxation process starts from 30 A AlN film thickness. The uniform contributions to the elastic strain tensor components decrease by approximately an order of magnitude when the film thickness increases from 30 to 100 A. Commensurate with this decrease is an increase in a nonuniform contribution of the misfit dislocations. The dislocation interactions lead to redistribution of dislocations within the 30–60 A range of AlN film thicknesses.