M. van Schilfgaarde

Theory of AlN, GaN, InN and their alloys

Abstract This review focuses on the fundamental properties of III–V compound semiconductors from a theoretical or computational standpoint. Its purpose is to summarize the contributions of electronic structure theory to the present context and to provide some foundations for the modern techniques. This will enable one to assess the limitations of the techniques employed previously.

InTlP — a proposed infrared detector material

In1−xTlxP is proposed as a promising material for infrared detectors. A number of key optical and structural properties are studied within local density‐functional theory. In1−xTlxP at x=0.67 and In1−xTlxAs at x=0.15 are estimated to have a gap of 0.1 eV. Their binding energies are larger than that of InSb, and they are found to form stable zinc‐blende alloys for all x. In1−xTlxP nearly lattice matches to InP, and offers the potential to integrate detector array and read‐out circuit.

Growth of Tl-containing III-V materials by gas-source molecular beam epitaxy

Abstract Synthesis of the Tl–V binaries and In–Tl–V alloys has been investigated using gas-source molecular beam epitaxy. With this approach, neither the binary nor ternary Tl-containing phosphides could be attained, with virtually all of the Tl present in the form of metallic droplets. While the Tl–As system was found to produce a compound, this phase was Tl rich with a Tl/As ratio ranging between 6/1 and 9/1. This phase was always accompanied by metallic Tl which oxidized rapidly upon exposure to air. Zincblende InTlAs was synthesized, as determined by Auger electron spectroscopy, but was always accompanied by metallic Tl droplets. The formation of both the Tl–As and In–Tl–As phases was found to be strongly dependent upon growth temperature. Both the TlAs x and the InTlAs phases were found to oxidize in air. The Tl–Sb and In–Tl–Sb system also produced a Tl rich phase, with a composition of Sb 2 Tl 7 , as measured by electron microprobe analysis. Unlike the arsenides and phosphides, the Tl–Sb compound could be synthesized even at growth temperatures of 375°C. This phase was found to exist in a CsCl structure and was accompanied by metallic Sb rather than elemental Tl.

Deactivation in heavily arsenic-doped silicon

We have combined ab initio calculations with a general statistical theory to predict the properties of heavily arsenic-doped silicon. Although we find that a lattice vacancy surrounded by four arsenic (VAs4) is the dominant deactivating complex at high arsenic concentrations in equilibrium, vacancy clusters with fewer arsenic neighbors are present in significant quantities. These smaller complexes are essential not only to the establishment of equilibrium, since SiAs4 clusters are extremely rare, but can also explain deactivation even if VAs4 formation is kinetically inhibited. This suggests that materials with similar arsenic concentration and deactivation fractions can have different microscopic states, and therefore behave differently in subsequent processing. Good agreement is found between theory and experiment for the electronic concentration as a function of temperature and total arsenic concentration. We also show that for low arsenic concentrations, full activation is the equilibrium condition.

Defect modeling studies in HgCdTe and CdTe

We have used a quasichemical formalism to calculate the native point defect densities in x = 0.22 Hg1−xCdxTe and CdTe. The linearized muffin-tin orbital method, based on the local density approximation and including gradient corrections, has been used to calculate the electronic contribution to the defect reaction free energies, and a valence force field model has been used to calculate the changes to the vibration free energy when a defect is created. We find the double acceptor mercury vacancy is the dominant defect, in agreement with previous interpretations of experiments. The tellurium antisite, which is a donor, is also found to be an important defect in this material. The mercury vacancy tellurium antisite pair is predicted to be well bound and is expected to be important for tellurium antisite diffusion. We consider the possibilities that the tellurium antisite is the residual donor and a Shockley-Read recombination center in HgCdTe and suggestions for further experimental work are made. We predict that the cadmium vacancy, a double acceptor, is the dominant defect for low cadmium pressures, while the cadmium interstitial, a double donor, dominates at high cadmium pressures.

Temperature dependence of band gaps in HgCdTe and other semiconductors

Band-edge shifts induced by the electron-phonon interaction are calculated for HgCdTe alloys and various semiconductor compounds starting from accurate zero-temperature band structures. The calculated temperature variation of gaps agrees with experiments to better than 10% in all materials except InAs and InSb where the deviation is about 50%. While the simple picture that the intra (inter)-band transitions reduce (increase) the gap still holds, we show that both the conduction band edge Ec and valence band edge Ev move down in energy. These shifts in Ev affect the valence band offsets in heterojunctions at finite temperature. The temperature variations of valence band offset and the electron effective mass are also reported.

Hg 0.8 Cd 0.2 Te native defects: densities and dopant properties

We examine the native defect equilibrium in HgCdTe, including cation and anion vacancies, interstitials, and antisites in the analysis. A gradient correction to the local density functional has been added to the defect formation enthalpies calculated within the local density approximation, and preliminary predictions of the dominant ionization states are made. Temperature-dependent defect formation entropies and the temperature dependence of the pre-exponentials are incorporated into the calculation of the defect densities. Degenerate Fermi-Dirac statistics are used for the electronic equilibration, and the intrinsic reaction constant as a function of composition and temperature is calculated. We theoretically substantiate the doubly ionized mercury vacancy as the dominant defect in HgCdTe, and expect the doubly ionized mercury vacancy densities to be comparable in HgZnTe. We predict that tellurium antisites are donors and will be present for some annealing conditions in sufficient quantities to be measured and possibly to affect device performance.

How dislocations affect transport

Dislocations crossing a junction in HgCdTe have little effect on detector responsivity, but are known to reduce the zero bias impedance RoA. and increase the leakage current, especially at low temperatures where RoA is dominated by tunneling and generation/recombination processes. We have calculated the Coulomb and piezoelectric fields associated with dislocations in an attempt to interpret their effect on the junction’s transport properties. Dislocation electric fields can affect transport since they are superimposed on the built-in and applied junction fields which control the currents. The screening of the fields in the neutral region is consistent with the dislocations’ small effect on responsivity. Their impact in the space charge region is found to be significant and consistent with the nonlinear dependence of performance on dislocation density. The piezoelectric potential of the typical 60° dislocation in a sphalerite crystal, and the Coulomb potential of a dislocation crossing the junction plane other than normally, are angularly varying in the junction plane. Angular variation of the potentials can be qualitatively interpreted as an angular modulation of the potential barrier. Because of the nonlinear dependence of junction currents on the barrier (or the junction potential), the angular variation of the currents does not vanish upon averaging. We find that the range of the Coulomb potential is too small to account for a major portion of the experimentally reported performance degradation but may be responsible for the reduction of RoA at cryogenic temperatures and low dislocation density, and that the longer range piezoelectric potential may be important. We also find that superposing the potentials of neighboring dislocations, because of the nonlinear dependence of junction leakage currents on junction potentials may account for the observed nonlinearity of performance degradation with dislocation density as measured by etch pit density.

Magnetic phase stability of 3d-metals and plutonium

Abstract We have performed self-consistent spin-polarized calculations for the magnetic structure and exchange parameters of pure 3d-metals and Pu. Local field effects and the strength of non-Heisenberg interactions are estimated. For α-Mn and α-Pu antiferromagnetic structures are dominant, and a complexe magnetic ordering was found for the low temperature structures. An analytical expression for the magnetic torque and a non-collinear low energy magnetic state of γ-Fe are presented.

Comparison of In 1− x Tl x Sb and Hg 1− x Cd x Te as long wavelength infrared materials

Cohesive energies, elastic constants, band structures, and phase diagram are calculated to evaluate the In1−xTlxSb alloy (ITA) as a long-wavelength infrared (LWIR) material compared to Hg1−xCdxTe (MCT). To obtain a 0.1 eV gap at zero temperature, the x value for ITA is estimated to be x=0.083 as compared to x=0.222 for MCT. At this gap, ITA is more robust than MCT because the cohesive energies order as InSb>TlSb>CdTe>HgTe, and ITA has the stronger bonding InSb as the majority component. Although TlSb is found to favor the CsCl structure, ITA is a stable alloy in the zincblende structure for low x values. However, our phase diagram indicates that it is difficult to grow the 0.1 eV gap ITA from the melt, because above the eutectic the liquidus curve is flat, and the solidus drops rapidly. Moreover, the width of the stable concentration range of the zincblende solid phase shrinks at low temperatures due to the presence of the CsCl structure.