Alexei Bykhovski

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...

Piezoresistive effect in wurtzite n‐type GaN

We report on the measurements of the piezoresistive effect in the n‐type wurtzite GaN films. The 3–5 μm thick GaN layers were deposited slightly off axis over basal plane sapphire substrates. The static and dynamic gauge factor (GF) of these structures was measured at room temperature for both longitudinal and transverse configurations. The dynamic effect is related to a strong piezoeffect in GaN. The maximum dynamic GF observed was ∼130 (approximately four times larger than for SiC).

Quantum shift of band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes

We report on the band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes with varying widths and barrier thicknesses of the quantum wells. In these devices, we observe that the stimulated emission peak wavelength shifts to shorter values with decreasing well thickness. From the comparison of the results of the quantum mechanical calculations of the subbands energies with the measured data, we estimate the effective conduction- and valence-band discontinuities at the GaN–In0.13Ga0.87N heterointerface to be approximately 130–155 and 245–220 meV, respectively. We also discuss the effect of stress on the estimated values of band discontinuities.

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.

THE INFLUENCE OF THE DEFORMATION ON THE TWO-DIMENSIONAL ELECTRON GAS DENSITY IN GAN-ALGAN HETEROSTRUCTURES

We report on the effect of external strain on the two-dimensional electron gas density in AlGaN/GaN heterostructures grown on sapphire by low pressure metalorganic chemical vapor deposition. The electron sheet concentration in the studied samples was 4×1012–2×1013 cm−2 and decreased with compressive strain. Lower doped heterostructures had a higher sensitivity to applied strain. The comparison between the experimental data and our model shows that the GaN layers are primarily nitrogen terminated at the heterointerface.

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.

Piezoelectric doping and elastic strain relaxation in AlGaN–GaN heterostructure field effect transistors

We calculate the sheet electron density induced by the piezoelectric effect in AlxGa1−xN–GaN heterostructure field effect transistors. This density is limited by the elastic strain relaxation, which depends on AlGaN barrier layer thickness and on the Al molar fraction in the barrier layer. Piezoelectric doping is more important in structures with larger Al content and thinner barrier layers. These results agree with our experimental data.

Strain and charge distribution in GaN‐AlN‐GaN semiconductor‐insulator‐semiconductor structure for arbitrary growth orientation

We demonstrate that, in a GaN‐AlN‐GaN semiconductor‐insulator‐semiconductor structure, the strain‐induced electric fields across the interface depend on the angle, θ, between the c axis and the growth direction. The magnitude of the strain induced polarization has a maximum in (0001) crystallographic direction (θ=0°) and a subsidiary maximum near θ=70°. This angular dependence is a unique feature of wurtzite‐type structures. Considering θ as an independent parameter for device design, one can obtain structures with flat band voltage shift from 0 to 1.5 V for 30 A AlN film, with different positions of accumulation‐depletion regions, and with electron (hole) charge varying from 0 to more than 1012 cm−2.

PIEZOEFFECT AND GATE CURRENT IN ALGAN/GAN HIGH ELECTRON MOBILITY TRANSISTORS

We report on a high positive turn-on voltage (close to 2.5 V) of the gate-source leakage current in AlGaN/GaN high electron mobility transistors (HEMTs). The piezoeffect, the barrier, and channel doping result in the electron sheet concentration as high as 1013 cm−2. A larger conduction band discontinuity and a larger electron effective mass (compared to AlGaAs/GaAs HEMTs) lead to a lower gate current and to a higher turn-on voltage. This means that AlGaN/GaN technology can be suitable for applications in digital and mixed-mode integrated circuits.