Cyrus E. Dreyer

Gallium vacancy complexes as a cause of Shockley-Read-Hall recombination in III-nitride light emitters

We describe a mechanism by which complexes between gallium vacancies and oxygen and/or hydrogen act as efficient channels for nonradiative recombination in InGaN alloys. Our identification is based on first-principles calculations of defect formation energies, charge-state transition levels, and nonradiative capture coefficients for electrons and holes. The dependence of these quantities on alloy composition is analyzed. We find that modest concentrations of the proposed defect complexes (∼1016 cm−3) can give rise to Shockley-Read-Hall coefficients A=(107−109) s−1. The resulting nonradiative recombination would significantly reduce the internal quantum efficiency of optoelectronic devices.

Effects of strain on the electron effective mass in GaN and AlN

Stress is known to strongly alter the effective mass in semiconductors, changing the mobility of carriers. Transport measurements on AlGaN/GaN heterostructures indicated a large increase in mobility under tensile strain [M. Azize and T. Palacios, J. Appl. Phys. 108, 023707 (2010)]. Using first-principles methods, we calculate the variation of electron effective mass in GaN and AlN under hydrostatic and biaxial stress. Unexpected trends are found, which are explained within k·p theory through a variation of the interband momentum matrix elements. The magnitude of the effective-mass reduction is too small to explain the experimentally reported increase in mobility.

Band alignments and polarization properties of BN polymorphs

Boron nitride can occur in the zincblende, wurtzite, and layered hexagonal phases. We use first-principles techniques based on hybrid density functional theory to study the electronic structure and polarization properties of these polymorphs. We find from the band structures that they have indirect band gaps. We report the band offsets between the polymorphs and with respect to GaN and AlN; a very small conduction-band offset between wurtzite BN and AlN is found. The piezoelectric polarization coefficients of wurtzite BN are opposite in sign to those in the other nitrides, and the magnitude of the spontaneous polarization is significantly larger.

Iron as a source of efficient Shockley-Read-Hall recombination in GaN

Transition metal impurities are known to adversely affect the efficiency of electronic and optoelectronic devices by introducing midgap defect levels that can act as efficient Shockley-Read-Hall centers. Iron impurities in GaN do not follow this pattern: their defect level is close to the conduction band and hence far from midgap. Using hybrid functional first-principles calculations, we uncover the electronic properties of Fe and we demonstrate that its high efficiency as a nonradiative center is due to a recombination cycle involving excited states. Unintentional incorporation of iron impurities at modest concentrations (1015 cm–3) leads to nanosecond nonradiative recombination lifetimes, which can be detrimental for the efficiency of electronic and optoelectronic devices.

Role of excited states in Shockley-Read-Hall recombination in wide band-gap semiconductors

Defect-assisted nonradiative Shockley-Read-Hall recombination is an important process in wide-band-gap semiconductors. However, nonradiative capture rates decrease exponentially with the energy of the transition, and therefore the mechanisms by which such recombination can take place are unclear. The authors show here that excited states of defects play a key role in turning specific defects into efficient nonradiative centers. The wider the band gap of the material, the greater a role these excited states are likely to play. Specifically, the authors can explain why certain gallium vacancy complexes are efficient nonradiative centers in group-III nitrides, the key materials for solid-state lighting.

Correct implementation of polarization constants in wurtzite materials and impact on III-nitrides

The intrinsic electric field that exists in certain classes of materials plays a key role in many fields of electronics. Researchers reveal and correct a shortcoming in the way that the electric polarization has been modeled in a technologically important class of these materials.

Calcium as a nonradiative recombination center in InGaN

Calcium can be unintentionally incorporated during the growth of semiconductor devices. Using hybrid functional first-principles calculations, we assess the role of Ca impurities in GaN. Ca substituted on the cation site acts as a deep acceptor with a level ~1 eV above the GaN valence-band maximum. We find that for Ca concentrations of 1017 cm−3, the Shockley–Read–Hall recombination coefficient, A, of InGaN exceeds 106 s−1 for band gaps less than 2.5 eV. A values of this magnitude can lead to significant reductions in the efficiency of light-emitting diodes.

Brittle fracture toughnesses of GaN and AlN from first-principles surface-energy calculations

Cracks are a major limitation for the growth of III-nitride bulk crystals and thick epitaxial films. The propensity for a crack to propagate on a given plane is determined by the anisotropic fracture toughness, Γ. Using first-principles surface-energy calculations, we have determined toughnesses for brittle fracture (Γb) in GaN and AlN, for cracks on the nonpolar {101¯0} m-and {112¯0} a planes, and the polar (0001)+(0001¯) c plane. For both materials, Γb values for cracks on the nonpolar planes are similar, and significantly smaller than for the c plane. Calculated critical thicknesses for AlGaN grown on GaN correctly predict the fracture planes in polar and nonpolar growth.

Growth of coherent BGaN films using BBr3 gas as a boron source in plasma assisted molecular beam epitaxy

Incorporating boron into gallium nitride to make BxGa1-xN solid solutions would create an avenue for extreme alloys due to the fact that wurtzite phase BN has a larger band gap and smaller lattice parameters compared to GaN. In this paper, the authors report the growth of high crystal quality, random alloy BxGa1-xN thin films with x up to 3.04% grown on (0001) Ga-face GaN on sapphire substrates using plasma assisted molecular beam epitaxy and BBr3 gas as a B source. High resolution x-ray diffraction was used to measure both the c plane spacing and the strain state of the films. It was determined that the films were fully coherent to the GaN substrate. Elastic stress-strain relations and Vegards law were used to calculate the composition. Atom probe tomography was used to confirm that the BxGa1-xN films were random alloys. In addition to demonstrating a growth technique for high crystal quality BxGa1-xN thin films, this paper demonstrated the use of BBr3 as a novel B source in plasma assisted molecular be...

Electric field dependence of optical phonon frequencies in wurtzite GaN observed in GaN high electron mobility transistors

Due to the high dissipated power densities in gallium nitride (GaN) high electron mobility transistors (HEMTs), temperature measurement techniques with high spatial resolution, such as micro-Raman thermography, are critical for ensuring device reliability. However, accurately determining the temperature rise in the ON state of a transistor from shifts in the Raman peak positions requires careful decoupling of the simultaneous effects of temperature, stress, strain, and electric field on the optical phonon frequencies. Although it is well-known that the vertical electric field in the GaN epilayers can shift the Raman peak positions through the strain and/or stress induced by the inverse piezoelectric (IPE) effect, previous studies have not shown quantitative agreement between the strain and/or stress components derived from micro-Raman measurements and those predicted by electro-mechanical models. We attribute this discrepancy to the fact that previous studies have not considered the impact of the electric...