Ming-Zhu Huang

A minimal basis semi-ab initio approach to the band structures of semiconductors

Abstract The band structures of 32 of the most important semiconductor crystals are calculated using an efficient, minimal basis, orthogonalized LCAO method. These include the diamond structure of C, Si, Ge, α-Sn; the zinc blende structure of β-SiC, BN, BP, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, β-ZnS, ZnSe, ZnTe, CdS, CdTe; the wurtzite structure of AlN, GaN, ZnO, α-ZnS, CdS, CdSe; the sodium chloride structure of CdO, GeTe, SnTe and trigonal Se and Te. The calculations, which involve diagonalizations of small size matrix equations yield results having the following characteristics: (1) satisfactory valence bands and lower conduction bands and bulk densities of states; (2) the gap sizes and the locations of valence band maximum and conduction band minimum in agreement with experiment; (3) reasonable values of fractional ionicity and electron and hole effective masses. These are achieved by fine-tuning the exchange parameters in the construction of the potentials. Application of this approach to the study of the electronic structures of disordered and other complex semiconductor systems is also discussed.

Effects of Al substitution in Nd2Fe17 studied by first‐principles calculations

We have studied the effect of Al substitution in Nd2Fe17 compound by means of first‐principles calculations. We first obtain the site‐decomposed potentials for Fe from self‐consistent calculation on Y2Fe17 and the atomiclike potentials in the crystalline environment for Al and Nd. Calculations are carried out for a single Al substituting one Fe at four different Fe sites (6c), (9d), (18f ), and (18h), two Al substituting two Fe (18h), and four Al substituting three Fe (18h) and one Fe (18f ). Our results show that the Al moment is oppositely polarized to Fe. The average moment per Fe atom actually increases for Al substituting Fe (18h) and Fe (18f ) is about the same for Al substituting Fe (6c), and is drastically reduced when replacing Fe (9d). Experimentally, Al is shown to be excluded from the (9d) sites because of the small Wigner–Seitz volume. When two Fe atoms are replaced by two Al atoms, the total moment is only slightly less than when only one Fe atom is replaced, and the Ms per Fe site actually ...

First‐principles calculation of the electronic and magnetic properties of Nd2Fe17−xMx (M=Si, Ga) solid solutions

We have studied the electronic and magnetic properties of the Si and Ga substituted Nd2Fe17 compounds by means of first‐principles calculations. The results show that for the Nd2Fe17−xSix (x=1,2,3, and 4) series, the total Fe moments per cell decrease as x increases but the average local Fe moments actually increase up to x=3, except for the 18h site which is the most preferable site for substitution. For Nd2Fe15Ga2, the moments on every Fe site increase. These results are also compared with the earlier calculation on Nd2Fe17−xAlx (x=1,2, and 4). From the spin‐polarized density of states (DOS) and local DOS for Si, Al, and Ga, it is shown that different substituting elements can change the electronic structure of Nd2Fe17 crystal substantially because of the slightly different interactions with the Fe atoms, and the change in the lattice constants. Estimation of Curie temperatures based on the spin‐fluctuation model shows qualitative agreement with experiments.

Band theoretical investigation of the Curie temperatures of modified R2Fe17‐based intermetallic compounds

The Curie temperatures of several Nd2Fe17‐based intermetallic compounds are calculated based on the spin‐fluctuation model of Mohn and Wohlfarth and the band structure results calculated using the self‐consistent orthogonalized linear combinations of the atomic orbitals method in the local spin density approximation. It is shown that the calculated Tc can qualitatively account for the observed Tc enhancement in this class of modified compounds by interstitial doping of N or C, or by elemental substitution of Fe by Si, Al or Ga. A simple explanation of Tc enhancement based purely on the magnetovolume effect is not justified. It is also shown that neither the Stoner–Curie temperature nor the characteristic spin‐fluctuation temperature is adequate to explain the experimentally observed Tc enhancement in these compounds.

Electronic structure of SixGe1−x semiconductor solid solutions

Abstract The electronic structure of SixGe1−x solid solutions is studied by means of a semi-ab initio method without involving the virtual crystal approximation. The calculations are performed on large cubic cells of the diamond lattice which contain a total of 64 Si and Ge atoms homogeneously distributed in different compositional ratios. The basis functions and the potentials used in the calculation give good band structures of elemental Si and Ge respectively. The calculated variation of band gap with x is in agreement with optical experiment: the two linear curves of gap vs concentration have different slopes at high and low x values with a crossing-over at about x=0.28. The band gaps are direct for xe (0, 0.28) and indirect for x ≥ 0.28. The density of states (DOS) of the solid solution can be very well approximated by the weighted average of the bulk Si and Ge DOS. There is a very slight charge transfer of about 0.05 electron per atom from Si to Ge.

Effect of partial site occupation on the magnetic properties of Nd2Fe17−xSix by supercell calculation

The magnetic properties of the Nd2Fe17−xSix intermetallic compounds are studied by means of spin-polarized supercell calculations in which the selected sites of substitution are close to the situations in real samples. It is shown that the average Fe moment increases with x and saturates near x=3. This correlates quite well with the experimental dependence of Tc on x. The difference between supercell and unit cell calculations are pointed out and the influence of Si atoms on the density of states of the nearby Fe atoms is emphasized.

Theoretical analysis of the spin‐density distributions in Y2Fe17N3 and Y2Fe17C3

The electronic structures and spin‐density distributions in Y2Fe17N3 and Y2Fe17C3 are calculated using the self‐consistent spin‐polarized orthogonalized‐linear‐combination‐of‐atomic‐orbitals method. The N or C atoms are assumed to occupy the (9e) sites in the rhombohedral structure. The Fe (18f) and N or C at the (9e) site form covalent bonds which result in a very nonspherically symmetric spin‐density distribution. The calculation shows a reduced moment for the Fe (18f) sites due to doping. However, the moments at other sites are increased due to lattice expansion. There are some differences in the spin‐density distribution between N and C at the (9e) site. By comparing with the results of calculation on the pure Y2Fe17 at different volumes, changes in moment enhancement and density of states at the Fermi level due to lattice expansion alone and that due to the chemical effect of introducing N or C are separated.

Magnetic structure and Fe moment distribution in Nd3Fe28Ti and Nd3Fe29 compounds

We have carried out spin-polarized calculations for the Nd3Fe28Ti and the hypothetical Nd3Fe29 (3–29 phase) compounds using the first-principles OLCAO method. Because the 3–29 phase has eleven distinctively different Fe sites of varying local coordination and nearest-neighbor distances, it is an ideal case to study the distribution and correlation of the Fe moments in relation to their local environments. We find the Ti moment to be oppositely polarized to the Fe moment, and its substitution of Fe in the pure 3–29 phase actually increases the moment per Fe site from 1.72 μB to 1.78 μB. In both cases, the Fe moments have a wide range of distribution. For Nd3Fe28Ti, the largest Fe moment of 2.77 μB is found at the Fe6 (4i) site and the smallest Fe moment of 1.40 μB is at the Fe2 (4e) site. Our results are in reasonable agreement with neutron diffraction data.

Electronic structure of Si (111) interface between diamond and hexagonal phases

Abstract The electronic structure of the Si (111) interface between the diamond and hexagonal wurtzite structure is studied by first-principles LCAO method. No gap interfacial states or resonances are found; instead, the calculated density of states curve shows an oscillatory redistribution of electron states and a reduction of band gap of about 10%. It is concluded that the electronic structure of this interface can be approximated as the average of that of the bulk diamond and the hexagonal Si.

ELECTRONIC STRUCTURE AND OPTICAL PROPERTIES OF CRYSTALLINE C60

The electronic structure and optical properties of crystalline C60 and their pressure dependence have been studied by first-principles local density calculations. It is shown that fcc C60 has a low dielectric constant and an optical spectrum rich in structures. The spectrum shows five disconnected absorption bands in the 1.4 to 7.0 eV region with sharp structures in each band that can be attributed to critical point transitions. This is a manifestation of the localized molecular structure coupled with long range crystalline order unique to the C60 crystal. At a sufficient high pressure, the structures in the optical spectrum start to merge due to the merging of the bands. These results are in good agreement with some recent experimental measurements.