Electron transport within the wurtzite and zinc-blende phases of gallium nitride and indium nitride

Wide energy gap semiconductors are broadly recognized as promising materials for novel electronic and opto-electronic device applications. As informed device design requires a firm grasp on the material properties of the underlying electronic materials, the electron transport that occurs within the wide energy gap semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving and burgeoning field of research, we review analyzes of the electron transport within some wide energy gap semiconductors of current interest in this paper. In order to narrow the scope of this review, we will primarily focus on the electron transport that occurs within the wurtzite and zinc-blende phases of gallium nitride and indium nitride in this review, these materials being of great current interest to the wide energy gap semiconductor community; indium nitride, while not a wide energy gap semiconductor in of itself, is included as it is often alloyed with other wide energy gap semiconductors, the resultant alloy often being a wide energy gap semiconductor itself. The electron transport that occurs within zinc-blende gallium arsenide will also be considered, albeit primarily for bench-marking purposes. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art wide energy gap semiconductor orthodoxy. A brief tutorial on the Monte Carlo electron transport simulation approach, this approach being used to generate the results presented herein, will also be provided. Steady-state and transient electron transport results are presented. The evolution of the field, a survey of the current literature, and some applications for the results presented herein, will also be featured. We conclude our review by presenting some recent developments on the electron transport within these materials. This review is the latest in a series of reviews that have been published on the electron transport processes that occur within the class of wide energy semiconductor materials. The results and references have been updated to include the latest developments in this rapidly evolving field of study.     

Optical electronegativity, bulk modulus and electronic polarizability of materials

Correlations between the energy gap, optical electronegativity and bulk modulus are given for compound and ternary chalcopyrite semiconductors. Bulk modulus and electronic polarizability values have been evaluated from the optical electronegativity. The estimated values of these parameters are in good agreement with the available experimental data.     

On the relationship between gap edge and specific heat of a superconductor

We critically examine three empirical procedures that have been proposed and used in the literature to extract, from electronic specific heat curves in the superconducting state, a value for the zero temperature energy gap. We do this by calculating, from Eliashberg theory, the electronic specific heat for many superconductors for which the electron-phonon spectral densities are known. Model spectral densities are also considered so as to test, in more extreme cases, the proposed procedures. We conclude that only one case, a modification of an empirical method, yields quantitatively reliable results.     

Steady-state and transient electron transport within the wide energy gap compound semiconductors gallium nitride and zinc oxide: an updated and critical review

The wide energy gap compound semiconductors, gallium nitride and zinc oxide, are widely recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these wide energy gap compound semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyzes of the electron transport within the wide energy gap compound semiconductors, gallium nitride and zinc oxide. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, zinc-blende gallium nitride, and wurtzite zinc oxide. The electron transport that occurs within bulk zinc-blende gallium arsenide will also be considered, albeit primarily for bench-marking purposes. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art wide energy gap compound semiconductor orthodoxy. A brief tutorial on the Monte Carlo electron transport simulation approach, this approach being used to generate the results presented herein, will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials. The wurtzite gallium nitride and zinc-blende gallium arsenide results, being presented in a previous review article of ours (O’Leary et al. in J Mater Sci Mater Electron 17:87, 2006), are also presented herein for the sake of completeness.     

Quantum computation of photoelastic properties of ionic crystals

A general scheme for computation of photoelastic properties has been proposed using the well-established quantum ion dependent theory. The families under consideration are I–VII type ionic crystals and AB ₂ type cross compounds. The calculated values of strain polarisability constant (λ) for the crystals in these families are compared with the predicted values by some other workers as well as with the available experimental ones. The results of the present work are in much better agreement with the available experimental data than the earlier estimated ones. This formulation is proposed to extend for other families of similar nature and theoretical analysis of the properties of nano materials. The industrial applications are also suggested.     

Model for calculating the refractive index of a III–V semiconductor

An empirical relationship modeled by a theoretical numerical model has been presented for estimating the refractive indices of semiconductors, especially the III–V semiconductors, relative to their energy gaps.

This model is based on the fact that there is a strong correlation between the energy gap and the refractive index of a given material. This physical relationship remains strictly intrinsic and specific to the material considered.

The performance of this model is compared with that of some other numerical models established by other authors. An analysis based on calculations of the errors between this model and the experimental data has also been carried out.

For the first time, the present model is applicable to the whole range of energy gaps, taking into account the fact that the refractive index for an infinite energy gap is equal to one.

Good agreement is observed between the computed values and the refractive indices reported in the literature for well-known semiconductors.     

Structural and Electronic Properties of SrS,SrSe, and SrTe Under Pressure

The electronic band structure and the total energy of SrX (X=S, Se, Te) in NaCl-type and CsCl-type structures were studied using the tight binding linear muffin-tin orbital method. The calculated ground state properties such as lattice constant and bulk modulus are in agreement with the experimental values. The transition pressures and volumes also agree well with the experimental results. The energy gap at ambient conditions were calculated. The metallization pressures and volumes have also been estimated.     

Wide-gap layered oxychalcogenide semiconductors: Materials, electronic structures and optoelectronic properties

Applying the concept of materials design for transparent conductive oxides to layered oxychalcogenides, several p-type and n-type layered oxychalcogenides were proposed as wide-gap semiconductors and their basic optical and electrical properties were examined. The layered oxychalcogenides are composed of ionic oxide layers and covalent chalcogenide layers, which bring wide-gap and conductive properties to these materials, respectively. The electronic structures of the materials were examined by normal/inverse photoemission spectroscopy and energy band calculations. The results of the examinations suggested that these materials possess unique features more than simple wide-gap semiconductors. Namely, the layered oxychalcogenides are considered to be extremely thin quantum wells composed of the oxide and chalcogenide layers or 2D chalcogenide crystals/molecules embedded in an oxide matrix. Observation of step-like absorption edges, large band gap energy and large exciton binding energy demonstrated these features originating from 2D density of states and quantum size effects in these layered materials.     

First-principles elastic constants and electronic structure of beryllium chalcogenides BeS, BeSe and BeTe

A theoretical study of structural, elastic and electronic properties of BeS, BeSe and BeTe is presented using the full-potential augmented plane-waves plus local orbitals (APW + lo) within density-functional theory (DFT). Results are obtained using both the local-density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation potentials. The ground-state properties, like lattice constant, bulk modulus and its first derivative obtained from our calculations agree very well with experimental and other theoretical calculations. Band structures, and total valence charge densities including spin–orbit interaction are analyzed in great detail. The calculated values of the energy gaps, bandwidths, and spin–orbit splittings and the correct band degeneracies are compared to experimental and/or ab initio results. The calculated energy gap for the series of beryllium chalcogenides BeS, BeSe and BeTe is found to be indirect (Γ–X) and underestimated by about 40% for both LDA and PBE-GGA compared to experiment. We have also reported the elastic constants of these materials; the elastic constants have been derived by the stress–strain relation.     

The variation of valence and conduction band edges charge densities with lattice constant in GaP and InP

Valence and conduction band edge charge densities for direct band gap InP and indirect band gap semiconductors GaP are calculated for different values of the lattice constant. These variations affect the bonding properties of these semiconductors and the effect of interstitial impurities.     

Structural, electronic and optical properties of (Sc,Y)N solid solutions

The refractory nitrides ScN and YN are candidate semiconductors for optoelectronic and DMS applications. Studies have included alloying ScN with other III-V semiconductors such as GaN to engineer the electronic, optical and structural properties. We suggest that ScN-YN and higher order alloys will lead to further control of these properties. We demonstrate complete solid solubility in the ScN-YN system through analysis of sputter deposited composition-spread thin films. Characterization of electrical, optical and structural properties is presented as a function of composition, demonstrating excellent epitaxy, high mobility, high absorptivity and high energy optical gap across the entire composition range.     

Electronegativity estimation of electronic polarizabilities of semiconductors

On the basis of the viewpoint of structure-property relationship in solid state matters, we proposed some useful relations to quantitatively calculate the electronic polarizabilities of binary and ternary chalcopyrite semiconductors, by using electronegativity and principal quantum number. The calculated electronic polarizabilities are in good agreement with reported values in the literature. Both electronegativity and principal quantum number can effectively reflect the detailed chemical bonding behaviors of constituent atoms in these semiconductors, which determines the magnitude of their electronic polarizabilities. The present work provides a useful guide to compositionally design novel semiconductor materials, and further explore advanced electro-optic devices.     

Hydrogen-related defects in crystalline semiconductors: a theorist''s perspective

Hydrogen is a common impurity in all semiconductors. Although it is sometimes deliberately introduced, hydrogen often penetrates into the crystal during device processing. It interacts with broken or weak covalent bonds, such as those found at extended and localized defect centers. The main results of these covalent interactions are shifts of energy levels out of (or into) the gap and new optical activity (infrared absorption and Raman scattering). The shifts in energy levels lead to the passivation (or activation) of the electrical activity of various centers. Hydrogen can also interact with the perfect crystal and with itself, sometimes leading to the formation of extended structures known as platelets. Finally, H also acts as a catalyst, dramatically enhancing the diffusivity of interstitial oxygen in Si. The consequences of these interactions are substantial changes in the electrical and optical properties of the crystal, and in the lifetime of charge carriers. The thermal stability of the complexes containing hydrogen varies from room temperature up to several hundreds of degrees Celsius, and the diffusion of H is trap-limited up to rather high temperatures. Hydrogen normally exists in more than one configuration and charge state in semiconductors. A range of experimental and theoretical techniques have been used to investigate the rich properties of hydrogen in semiconductors, and several extensive reviews focusing mostly on the experimental side of these issues have been published in the past five years. The present review focuses mostly on the theoretical work performed in this field. However, the most recent experimental results are also discussed, and the current understanding of hydrogen interactions in semiconductors summarized.     

Polycrystalline iron sulphide based semiconductors for solar energy conversion

Potentialities of iron sulphide semiconductors for solar energy conversion have been outlined. Iron sulphide semiconductors have been characterized with respect to their bulk (XRD, EDAX), surface (XPS) and electronic (UV-Vis reflectance spectroscopy) structure. Materials have been formed into electrodes by screenprinting and have been thermally activated under different conditions.Synthetic polycrystalline pyrite activated in air-hydrogen atmosphere showed the highest efficiency for solar energy conversion in polyiodide electrolyte. Distinguishing characteristics of these materials are due to the optimal absorption and charge transfer properties at the electrode-electrolyte interface. From parallel experiments carried out on pyrrhotite and natural pyrite-based semiconductors, it was derived that the latter characteristics are related to favourable substoichiometric surface sites and to the formation of an iron oxide bulk phase in the semiconductors.     

Ab initio and DFT study on 1,4-dinitroglycoluril configurational isomers: cis-DINGU and trans-DINGU

cis-1,4-Dinitroglycoluril (1,4-dinitrotetrahydroimidazo[4,5-d]imidazole-2,5-(1H,3H)-dione, C(4)H(4)N(6)O(6), known as DINGU in the literature) is an important explosive regarded as one of the potential ingredients for LOVA (low vulnerability ammunition) applications. However, there is no study concerning trans-DINGU, to the best of our knowledge. Thus, in the present study the structural and electronic properties of the configurational isomers of DINGU (cis- and trans-isomers) have been investigated by performing density functional theory (DFT) calculations at B3LYP/6-31G(d,p), UB3LYP/6-31G(d,p) levels and also ab initio calculations at RHF/6-31G(d,p) and UHF/6-31G(d,p) levels. The optimized geometries, vibrational frequencies, electronic structures, and some thermodynamical values for the presently considered DINGU isomers have been obtained in their ground states. Comparing the calculated energy data (total electronic energy, heat of formation values and homolytic bond dissociation values of N-NO(2)) and the calculated bond lengths, cis-DINGU was found to be a more stable molecule than the trans-isomer.     

Plasmon Modes Induced by Anisotropic Gap Opening in Au@Cu2O Nanorods

Integration of semiconductors with noble metals to form heteronanostructures can give rise to many interesting plasmonic and electronic properties. A number of such heteronanostructures have been demonstrated comprising noble metals and n‐type semiconductors, such as TiO₂, ZnO, SnO₂, Fe₃O₄, and CuO. In contrast, reports on heteronanostructures made of noble metals and p‐type semiconductors are scarce. Cu₂O is an unintentional p‐type semiconductor with unique properties. Here, the uniform coating of Cu₂O on two types of Au nanorods and systematic studies of the plasmonic properties of the resultant core–shell heteronanostructures are reported. One type of Au nanorods is prepared by seed‐mediated growth, and the other is obtained by oxidation of the as‐prepared Au nanorods. The (Au nanorod)@Cu₂O nanostructures produced from the as‐prepared nanorods exhibit two transverse plasmon peaks, whereas those derived from the oxidized nanorods display only one transverse plasmon peak. Through electrodynamic simulations the additional transverse plasmon peak is found to originate from a discontinuous gap formed at the side of the as‐prepared nanorods. The existence of the gap is verified and its formation mechanism is unraveled with additional experiments. The results will be useful for designing metal–semiconductor heteronanostructures with desired plasmonic properties and therefore also for exploring plasmon‐enhanced applications in photocatalysis, solar‐energy harvesting, and biotechnologies.     

Optical and thermal band gap energy of chemically deposited bismuth(III) selenide thin films

The band gap energy of bismuth(III) selenide in thin-film form was determined using the optical and thermal methods. The optical band gap energy of 0.35 eV was calculated on the basis of the recorded optical spectra in the near-infrared region, within the framework of a parabolic approximation for the dispersion relation, using the equations which arise from Fermi’s golden rule for electronic transitions from valence to conduction band. From the temperature dependence of the dark electrical resistance of the bismuth(III) selenide thin films in the region of intrinsic and extrinsic conduction, a thermal band gap energy of 0.37 eV and an ionization energy of the donor impurity level of 0.13 eV were calculated. The thermal, as well as the optical band gap energy are in excellent agreement with a literature value for bulk bismuth(III) selenide. On the basis of these data, several conclusions on the film microstructure (nanocrystalline versus glassy) are derived and also an estimation of the higher bound to the Bohr’s excitonic radius for bulk Bi₂Se₃ is given.     

Universal energy-level alignment of molecules on metal oxides

Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.