Walter R. L. Lambrecht

Valence‐band discontinuity between GaN and AlN measured by x‐ray photoemission spectroscopy

The valence‐band discontinuity at a wurtzite GaN/AlN(0001) heterojunction is measured by x‐ray photoemission spectroscopy. The method first measures the core level binding energies with respect to the valence‐band maximum in both GaN and AlN bulk films. The precise location of the valence‐band maximum is determined by aligning prominent features in the valence‐band spectrum with calculated densities of states. Tables of core level binding energies relative to the valence‐band maximum are reported for both GaN and AlN. Subsequent measurements of separations between Ga and Al core levels for thin overlayers of GaN film grown on AlN and vice versa yield a valence‐band discontinuity of ΔEV=0.8±0.3 eV in the standard type I heterojunction alignment.

High field electron transport properties of bulk ZnO

The Monte Carlo method is used to simulate electron transport for electric field strengths up to 350 kV/cm in bulk, wurtzite structure ZnO. The relevant parts of the conduction bands of a first-principles band structure are approximated by spherically symmetric, nonparabolic valleys located at the Γ and Umin symmetry points of the Brillouin zone. It is shown that the analytic expressions represent the band structure and the density of states well over a range of nearly 5 eV from the bottom of the conduction band. The simulated electron steady-state drift velocity versus electric field characteristics are calculated for lattice temperatures of 300, 450, and 600 K. For room temperature, drift velocities higher than 3×107 cm/s are reached at fields near 250 kV/cm. Examination of the electron energy distributions shows that the strong decrease of the differential mobility with increasing electric field in the field range studied is to be associated with the pronounced nonparabolicity of the central valley and...

Stacking fault band structure in 4H–SiC and its impact on electronic devices

First principles calculations of the stacking fault (SF) in 4H–SiC indicate the occurrence of an interface band in the gap with maximum depth of 0.2–0.3 eV below the conduction band minimum at the M point. The energy of formation of SFs in 3C–, 4H–, and 6H–SiC on the other hand is found to be of order a few meV/pair. Thus, there is a thermodynamic driving force promoting growth of SF area in an n-type sample. Radiationless recombination of electrons trapped at the SF with holes is proposed to provide sufficient energy to overcome the partial dislocation motion barriers towards formation of additional SF area in a device under forward bias.

Electronic band structure of SiC polytypes : A discussion of theory and experiment

After a brief discussion of the origin of polytypes and a few general remarks on the relationship between polytypism and properties of SiC, we focus on a comparative study of their electronic band structures. We first explain how the different band structures can be put on the same footing by examining Brillouin zone folding effects. Then we discuss the dependency of some of the important eigenvalues on hexagonality. Next, we discuss some of the available spectroscopic information on the band structures. Finally, we examine some of the details near the gaps in further detail such as the location of the conduction-band minima, the effective masses and the crystal field splittings and masses of the upper valence bands. Open questions and areas where experimental verification is needed are pointed out.

Total energy differences between SiC polytypes revisited

The total energy differences between various SiC polytypes (3 C ,6 H ,4 H ,2 H ,1 5 R, and 9R) were calculated using the full-potential linear muffin-tin orbital method using the Perdew-Wang generalized gradient approximation @I. P. Perdew, in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrich ~Akademie-Verlag, Berlin, 1991! ,p . 11 #to the exchange-correlation functional in the density-functional method. Numerical convergence versus k-point sampling and basis-set completeness are demonstrated to be better than 0.5 meV/atom. The parameters of several generalized anisotropic next-nearest-neighbor Ising models are extracted and their significance and consequences for epitaxial growth are discussed. @S0163-1829~98!01019-4# I. INTRODUCTION Despite many years of study, the origin of polytypism in SiC is still not completely understood. A much debated question is whether polytypism is a manifestation of kinetic factors during growth or whether polytypes should be viewed as distinct ~possibly metastable! thermodynamic phases with a specific stability range of external parameters ~such as pressure and temperature!. In a thermodynamic approach to the problem, the most important quantities are the total freeenergy differences between the various polytypes. A major contribution to the latter is the energy difference at zero temperature. Vibrational entropy contributions at higher temperature were discussed by Heine et al. 1,2 and Zywietz et al. 3 Several groups have performed first-principles local-densityfunctional calculations of these energy differences using the norm-conserving pseudopotential plane-wave method. 4‐8 However, there are significant discrepancies between the results of various calculations for these energy differences, which are of order of a few meV/atom or less. More seriously, the three more recent calculations appear to invalidate some of the important conclusions drawn from these calcu

Ferromagnetic redshift of the optical gap in GdN

We report measurements of the optical gap in a GdN film at temperatures from

Rare-earth mononitrides

300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}6\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Band-structure analysis of the conduction-band mass anisotropy in 6H and 4H SiC.

, covering both the paramagnetic and ferromagnetic phases. The gap is

Theoretical study of the band offsets at GaN/AlN interfaces

1.31\phantom{\rule{0.3em}{0ex}}\mathrm{eV}

Optical reflectivity of 3C and 4H-SiC polytypes: Theory and experiment

in the paramagnetic phase and redshifts to