B. Segall

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.

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.

Spin-dependent resonant tunneling through semimetallic ErAs quantum wells in a magnetic field.

Resonant tunneling through semimetallic ErAs quantum wells embedded in GaAs structures with AlAs barriers was recently found to exhibit an intriguing behavior in magnetic fields which is explained in terms of tunneling selection rules and the spin-polarized band structure including spin-orbit coupling.

Electronic and optical properties of the group-III nitrides, their heterostructures and alloys

Various aspects of the electronic structure of the group III nitrides are discussed. The relation between band structures and optical response in the vacuum ultraviolet is analyzed for zincblende and wurtzite GaN and for wurtzite AlN and compared with available experimental data obtained from reflectivity and spectroscopic ellipsometry. The spin-orbit and crystal field splittings of the valence band edges and their relations to exciton fine structure are discussed including substrate induced biaxial strain effects. The band-offsets between the III-nitrides and some relevant semiconductor substrates obtained within the dielectric midgap energy model are presented and strain effects which may alter these values are discussed. The importance of lattice mismatch in bandgap bowing is exemplified by comparing Al{sub x}Ga{sub 1{minus}x}N and In{sub x}Ga{sub 1{minus}x}N.

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

The valence-band discontinuity at a wurtzite GaN/AIN (0001) heterojunction is measured by means of x-ray photoemission spectroscopy. The method first measures the core level binding energies with respect to the valence-band maximum in both GaN and A1N bulk films. The precise location of the valenceb and maximum is determined by aligning prominent features in the valenceb and spectrum with calculated densities of states. Subsequent measurements of separations between Ga and Al core levels for thin overlayers of GaN film grown on A1N and vice versa yield a valence-band discontinuity of ΔEv = 0.8+- 0.3 eV in the standard Type I heterojunction alignment.

Elastic Constants and Related Properties of the Group III-Nitrides

The elastic constants of the Group-III nitrides, c-BN, AlN and GaN were calculated from first-principles using the full-potential linear muffin-tin orbital method and local density approximation. The relation between the elatic constants in zincblende and wurtzite is studied by means of a tensor coordinate transformation approach. The latter combined with a correction for the internal displacement of the rotated tetrahedra is found to provide good results for the C h 11 C h 12 and C h 44 but not for C h 13 and C h 33 . These two require explicit calculations involving distortions along the c-axis. The calculations also provide information on the transverse optical phonons. By deriving Keating model parameters we show that BN is much stiffer against bond-angle distortions than the other nitrides.

Theoretical Studies of ZnO and Related Mg x Zn 1-x O Alloy Band Structures

First principles calculations are carried out for ZnO, MgO and ZnMgO2 in various crystal structures. The nature of the valence band ordering in ZnO is shown to depend strongly on the Zn3d band position. MgO in wurtzitic form is found to gave an unusual 5-fold coordiated structure. The band gap dependence in the alloy system is found to be in fair agreement with experimental data and the band-offset is predicted to be type I.

Theoretical Studies of ZnO and Related Mg x Zn1−xO Alloy Band Structures

First principles calculations are carried out for ZnO, MgO and ZnMgO2 in various crystal structures. The nature of the valence band ordering in ZnO is shown to depend strongly on the Zn3d band position. MgO in the wurtzitic form is found to gave an unusual 5-fold coordiated structure. The band gap dependence in the alloy system is found to be in fair agreement with experimental data and the band-offset is predicted to be type

Theoretical study of group-III nitride alloys

Band gap bowing, structural relaxations, and energies of formation were calculated for the three pseudobinary nitride zincblende alloy systems Al-Ga, In-Ga and In-Al using the full-potential linearized muffin-tin orbital method. The cluster expansion and Connolly-Williams approaches were used to relate calculated band structures and energies of formation of ordered compounds to the behavior of disordered alloys. Effects of bond length and volume variation on those properties are discussed. An interpolation formula for the gap of the full pseudoternary Al{sub x}Ga{sub y}In{sub z}N system is proposed and tested by separate calculations. Extension of the results to the wurtzite alloys is discussed.

Electronic Structure and Related Properties of Tetrahedrally Bonded Wide-Band-Gap Materials Containing Early Elements of the Periodic Table

The tetrahedrally bonded materials involving early elements of the periodic table, such as Be, B, C, and N have rather extreme properties compared to the conventional tetrahedrally bonded semiconductors. These unique properties make them interesting for a variety of applications, including their use as hard coatings for mechanical tools, metal/ceramic composites, heat-sinks, and electronics. The latter are the most demanding type of applications because they require semiconductor grade purity single crystalline material in order to realize the superior performance these materials promise to have. But their extreme properties also makes them difficult materials to grow in the form of pure single crystals. Their properties are thus not yet very well known. In order to make a rational choice of material for specific applications, it is necessary to understand the interrelationships between the intrinsic materials properties and the trends in these properties with atomic number, crystal structure, and so on. Virtually all materials properties can in ultimate instance be related to the underlying electronic structure. The purpose of this paper is to describe the basic trends in electronic structure of these materials and some of their related properties. The results presented here are based on a systematic study of the electronic structure of these materials carried out over a number of years in our research group. Although many of the results discussed here were presented elsewhere they were not included in an overall description of the trends as presented here. Some new results are included as well. Extensive references are provided as a guide to our previous work not covered here in detail.