Showing papers on "Band gap published in 1975"

Model for the Electronic Structure of Amorphous Semiconductors

Abstract: It is pointed out that a model which agrees well with the observed properties of semiconducting glasses is an attractive Hubbard model of localized states. Such a model has no gap for two-electron excitations but an energy gap for one-electron ones. The suggested physical model for a two-electron excitation is a new covalent bond in the structure, which is severely localized. It is also proposed that the one-electron excitation spectrum is wholly, or almost wholly, extended, and all observed gaps are identical with the mobility gap.

The Standard Thermodynamic Functions for the Formation of Electrons and Holes in Ge, Si, GaAs , and GaP

Abstract: The forbidden energy gaps of Ge, Si, , and have been used to obtain the standard Gibbs energy, enthalpy and entropy of formation of electrons and holes for each semiconductor up to the melting points. The forbidden energy gap is the standard Gibbs energy of formation of electrons and holes and the enthalpy and entropy have been obtained from the energy gap as a function of temperature and familiar thermodynamic relationships. Energy gaps as a function of temperature, available in the literature, have been fit to the semiempirical equation of Varshni and used to extrapolate the energy gaps and thereby the three thermodynamic functions to the melting points. It is well known that the energy gaps, i.e., the Gibbs energies, decrease with increasing temperature but it is not well known that the enthalpy of formation increases with temperature and that it is proportional to the slope of the familiar logarithmic plot of the intrinsic carrier concentration over vs. . Examples of the utility of the enthalpy function are given. It is the entropy that leads to the decrease in energy gap with increasing temperature and its magnitude is large near the respective melting points (10–13 cals/deg, i.e.,) arising from the interactions of electrons and holes with the lattice. The intrinsic carrier concentrations were calculated from the forbidden energy gaps and the average effective masses which were estimated for the higher temperatures.

Preparation and optical properties of Ga1−xInxN thin films

Abstract: By the use of the electron beam plasma technique, it has been found that the solid solutions of Ga1−xInxN can be synthesized over the entire composition region. From the optical measurements, the direct energy gap at 78 °K was determined to be 3.46 eV for GaN and 2.11 eV for InN. Also its composition dependence was found to deviate downward from linearity. From the infrared reflectivity measurement and the resultant K‐K dispersion analysis, the transverse optical frequencies for long‐wavelength phonons of GaN and InN were 563 and 478 cm−1, respectively. The optical phonons in this quasibinary system were concluded to exhibit a one‐mode–type behavior. The Brout sum rule was discussed for a large number of the diatomic crystals of ANB8−N type and its relation with respect to the reduced mass was derived as Σiω2i(k=0) =A μ−1.5. According to this relation, the longitudinal optical frequency of InN was deduced to be 694 cm−1. On the other hand, from the result of the annealing treatment for the solid‐solution ...

High-field transport in wide-band-gap semiconductors

Abstract: The drift velocity in high electric fields is calculated for several wide-band-gap semiconductors. Saturated velocities above ${10}^{7}$ cm/sec are found for several and SiC, diamond, and GaN hold promise for values above 2\ifmmode\times\else\texttimes\fi{}${10}^{7}$ cm/sec.

Electron-Hole-Pair Creation Energies in Semiconductors

Abstract: The rule that the electron-hole-pair creation energy is 3 times the semiconductor band gap is extended to include a large group of insulators. It is used, together with the free-particle approximation, to describe cathodoluminescent phosphor efficiencies and the escape probabilities of secondary electrons.

Hole drift velocity in silicon

Abstract: Drift velocities for holes in high-purity Si were measured for fields between about 3 and 5\ifmmode\times\else\texttimes\fi{}${10}^{4}$ V/cm and temperatures between 6 and 300\ifmmode^\circ\else\textdegree\fi{}K for the crystallogrphic directions $〈100〉$, $〈110〉$, and $〈111〉$. The Ohmic mobility is theoretically interpreted on the basis of a two-band model consisting of a spherical parabolic and a spherical nonparabolic band, and the relaxation-time approximation. The low-temperature Ohmic mobility is strongly influenced by the nonparabolicity of the heavy-hole band. The high-field region ($E\ensuremath{\ge}{10}^{3}$ V/cm) was analyzed using a single warped heavy-hole band model and a Monte Carlo technique. Anisotropy of hot-hole drift velocity is associated with warping of the valence band. Optical- and acoustic-scattering mechanisms are found to be of comparable strength.

Laser oscillation from quantum states in very thin GaAs−Al0.2Ga0.8As multilayer structures

Abstract: We report optically pumped laser oscillation from multilayer heterostructures consisting of alternating layers of GaAs and Al0.2Ga0.8As. Very thin GaAs layers (50−500 A) exhibit one−dimensional bound states above the band gap of bulk GaAs. The laser oscillation occurs at energies which are slightly below the exciton associated with the lowest energy n=1 bound state.

The low-energy absorption edge in 2H-MoS2 and 2H-MoSe2

Abstract: The optical transmission of thick samples of 2H-MoS2 and 2H-MoSe2 has been measured below the A exciton energy for each material. The absorption coefficients have been calculated, and the low absorption starting at 1·1–1·2 eV has been attributed to an indirect d-d transition for both materials. There appears to be considerable d-p valence band overlap giving rise to further higher absorption before the strong A, B excitons, which have been associated with direct transitions from a p-like valence band. An energy band scheme has been sketched on the basis of these interpretations.

Comparison of interface-state generation by 25-keV electron beam irradiation in p-type and n-type MOS capacitors

Abstract: The interface state induced by 25‐keV electron beam irradiation in MOS capacitors having p‐ and n‐type substrates with several different doping concentrations have been studied. For radiation dosage on or above the order of 1×10−5 C/cm2, all of the radiation‐induced interface‐state distributions tend to have a similar shape which is asymmetrical about the midgap, independent of the type and concentration of the silicon dopants, and independent of the initial interface‐state distributions. The states in the upper half of the silicon band gap are acceptor type which peak around 0.2 eV from the midgap, whereas the states in the lower half of the band gap are donor type with a lower density. For radiation dosage below 1×10−7 C/cm2 the postradiation interface states are proportional to their initial values. An explanation based on the broken bond model is presented to account for the observations.

The optical and electrical effects of high concentrations of defects in irradiated crystalline gallium arsenide

Abstract: The damage produced by fast neutron irradiation of gallium arsenide has been studied by a number of techniques. The electrical resistivity, which increases with dose at low doses to semi-insulating values, shows a remarkable, specimen-independent decrease for doses greater than 1017 n cm-2 from values of ca. 109 Ω cm to 3 Ω cm for the highest dose of 1.5 × 1020 n cm-2. In this high dose region the temperature dependence of the resistivity at low temperatures is given by exp [b/T 1/4] and it is suggested that in this highly disordered state conduction occurs by tunnel-assisted hopping between defect levels in the band gap. The presence of such levels is indicated by the strong optical absorption tail which is produced from 0.1 eV to the crystalline edge at 1.5 eV. Although at the highest doses the samples contain a high degree of disorder, X-ray diffraction shows that they are basically crystalline. Lattice parameter determinations show that it increases with dose, linearly at first, then tending ...

The electrical properties and the magnitude of the indirect gap in the semiconducting transition metal dichalcogenide layer crystals

Abstract: Room-temperature electrical resistivity and Hall effect measurements as a function of pressure are reported on p-type MoS2 and on n-type MoS2, MoSe2 and MoTe2. In each case, the resistivity decreases under pressure, due to an increase in the carrier concentration. The Hall mobility is relatively pressure-independent. The data are consistent with the predominance of extrinsic conduction in these semiconductors until well above room temperature. The impurity activation energy and its pressure dependence are given, together with estimates of the intrinsic indirect bandgap obtained from high temperature conductivity measurements, photoemission studies and band structure calculations.

Metal-nonmetal transition in vo2 - x-ray photoemission and resistivity measurements

Abstract: The semiconductor-metal transition in VO2 has been investigated by means of XPS measurements. The band gap in the semiconducting region is found to be temperature-dependent with a room temperature value of Eg=0.3 eV, in agreement with resistivity measurements. The core lines, which are asymmetric below Tnu broaden and become symmetric above Tt. A model is discussed to explain the broadening in the metallic region, which requires that the core hole-valence electron interaction is comparable to the bandwidth. The relative shift of the O 1s and V 2p peak positions below and above Tt indicates that the charges on the ions are highest in the metallic phase, suggesting a more ionic bond in this region.

Energy Levels of Electron and Hole Traps in the Band Gap of Doped Anthracene Crystals

Abstract: It is discussed under which conditions electron and hole trap levels in the band gap of doped molecular crystals may be described by a simple polarization model taking into account the shift of electron and hole levels (positive and negative ionic states) of the free dopant molecule during its incorporation into a polarizable crystal-continuum. Measurements are presented for anthracene doped with tetracene (T), acridine (Ac), and phenazine (Ph). The trap levels obtained are: E = 0.42 eV; E = 0.12 to 0.17 eV (depending on the orientation); E = 0 eV; E = 0.21 eV, E = 0 eV; E = 0.54 eV for holes (p) and electrons (n), respectively.

Band structure of semiconductor superlattices

Abstract: A theory of the band structure of semiconductor superlattices has been developed for both the direct-band-gap and indirect-band-gap barrier layers taking into account the multivalley and nonparabolic band structure of the materials forming the superlattice. For direct-band-gap barrier layers the nonparabolicity in the band structure may alter the electronic energy levels measured from the bottom of the potential wells by as much as 26%. On the other hand for indirect-band-gap barrier layers the alteration due to the nonparabolicity is about 14%. It is also found that even for indirect-band-gap barrier layers the band structure is mainly determined by the states corresponding to the direct-gap minimum. Energy levels calculated on the basis of the theory presented are also found to agree with those obtained in recent experiments with double-barrier heterostructures.

The standard thermodynamic functions for the formation of electrons and holes in ge, si, gaas, and gap

Abstract: The forbidden energy gaps of Ge, Si, , and have been used to obtain the standard Gibbs energy, enthalpy and entropy of formation of electrons and holes for each semiconductor up to the melting points. The forbidden energy gap is the standard Gibbs energy of formation of electrons and holes and the enthalpy and entropy have been obtained from the energy gap as a function of temperature and familiar thermodynamic relationships. Energy gaps as a function of temperature, available in the literature, have been fit to the semiempirical equation of Varshni and used to extrapolate the energy gaps and thereby the three thermodynamic functions to the melting points. It is well known that the energy gaps, i.e., the Gibbs energies, decrease with increasing temperature but it is not well known that the enthalpy of formation increases with temperature and that it is proportional to the slope of the familiar logarithmic plot of the intrinsic carrier concentration over vs. . Examples of the utility of the enthalpy function are given. It is the entropy that leads to the decrease in energy gap with increasing temperature and its magnitude is large near the respective melting points (10–13 cals/deg, i.e.,) arising from the interactions of electrons and holes with the lattice. The intrinsic carrier concentrations were calculated from the forbidden energy gaps and the average effective masses which were estimated for the higher temperatures.

Temperature dependence of the band gap and comparison with the threshold frequency of pure GaAs lasers

Abstract: Recent measurements of the lasing energy as a function of temperature in high‐purity GaAs lasers have attempted to investigate the relation between this energy and the one‐electron band gap. Because of a lack of precision in the position of the band gap at high temperature, these measurements show strongly conflicting results. In this work, we report two sets of differential reflectivity measurements (electroreflectivity and piezoreflectivity) performed on GaAs under high‐resolution conditions. Both series of results give for the excitonic absorption edge at room temperature a value E0=1.424±0.002 eV which is about 20 meV higher than the lasing energy reported for the highest‐purity GaAs samples. This result confirms that the lasing energy in GaAs is well below the one electron band gap. In addition, we show that this energy separation is an increasing function of temperature. Finally we discuss a simple model of band‐to‐band recombination including electron‐electron interaction effects and we show that i...

Static technique for precise measurements of surface potential and interface state density in MOS structures

Abstract: A new static technique for a very accurate measurement of charge, surface potential, low−frequency capacitance, interface charge, and interface state density in MOS structures is presented. A comparison with the conductance method is made. For a 〈111〉−oriented n−type silicon wafer, oxidized in dry oxygen and annealed in hydrogen, the distribution of the interface state density is shown over an energy range of 0.9 eV within the energy gap.

Dielectric instability and breakdown in wide bandgap insulators

Abstract: An impact−ionization model predicts a negative resistance type of dielectric instability and breakdown in wide bandgap insulators. The instability develops by the injection of electrons from the cathode, impact ionization of the lattice, and a distortion of the electric field which leads to a further increase in impact ionization. For the case of thin films, it is necessary to invoke a nonlocal ionization rate in order to obtain a negative resistance instability. O’Dwyer’s avalanche theory of breakdown, which applies well to semiconductors, cannot be used to predict a negative resistance in the small multiplication limit appropriate to wide bandgap insulators. The impact−ionization model presented here depends upon two critical parameters: the ionization bandgap of the material and the electron−LO phonon scattering length. Evaluated for the exemplary case of SiO2, the model predicts a current density and an average field at breakdown which increase rapidly for film thickness below 200 A.

Growth and characterization of liquid−phase epitaxial InxGa1−xAs

Abstract: The liquid−phase epitaxial growth of InxGa1−xAs (0

Theory of electronic band structure in intense laser fields

Abstract: The nonlinear interaction of laser radiation with crystalline solids results in a modified band structure which is a natural extension of the usual Bloch picture. A theoretical discussion is presented in which the existence of field-induced gaps is predicted. Applications to model systems such as the Kronig-Penney model are presented. Examination of realistic systems such as the narrow band gap semiconductors suggest that the effect is large and should be readily observable with currently available laser sources.

Graded‐band‐gap pGa1−xAlxAs‐nGaAs heterojunction solar cells

Abstract: The theoretical spectral response of the graded‐band‐gap pGa1−xAlxAs‐nGaAs solar cell is calculated by solving the continuity equations. The electric fields produce a collection efficiency two times larger than that of the abrupt heterojunction diodes at wavelengths less than 700 nm for surface recombination velocities of 106–107 cm/sec. The graded‐band‐gap diode can be operated at high frequencies because of the reduced transit time of minority carriers in the p‐type layer. A graded‐band‐gap pGa1−xAlxAs‐nGaAs solar cell is fabricated by utilizing the diffusion of Al in the Ga melt. The gradient of the energy gap is evaluated by photoresponse and photoluminescence measurements. The photoluminescence shows a broad emission peak from 700 to 800 nm at 77 K.

Band-gap shrinkage of semiconductors

Abstract: Direct experimental evidence of band-gap shrinkage due to the presence of impurities in semiconductors is given. Experimental results are obtained using a piezomodulation-spectroscopy technique over the (4-300)\ifmmode^\circ\else\textdegree\fi{}K temperature range. They are interpreted in the linear-screened-potential and effective-mass approximations. Coupling spectroscopic and helium-temperature magnetotransport measurements allowed us to obtain in this model a good description of the transition edges over the entire temperaure range. We studied different GaSb samples with impurity concentration allowing us to observe band-gap shrinkage with one and two types of carriers. This is easily obtained in the GaSb case whose band structure presents $\ensuremath{\Gamma}$ and $L$ minima located very near in energy. On heavily doped samples, the low-temperature values of the band-gap shrinkage are used to obtain further informations concerning subsidiary minima. On lightly doped semiconductors, non-$k$ -conserving transitions initiating on the residual acceptor level are clearly shown.