Semiconductor Energy Gap Research Articles

Light-induced shifts in the electronic and shallow-donor states in GaAs-(Ga,Al)As quantum dots

We treat the interaction of light with a spherical GaAs-(Ga,Al)As quantum dot within a dressed-band approach. The Kane band-structure scheme is used to model the GaAs bulk semiconductor and the interaction with the laser field is treated through the renormalization of the semiconductor energy gap and conduction/valence effective masses. This approach, valid far from resonances, is used to investigate the light shifts induced in the electronic and shallow on-center donor states in semiconductor quantum dots, which are shown to be quite considerable. This model calculation may be extended to include magnetic-field effects, and it is suggested that the strong localization of the electronic and impurity states due to the quantum dot and enhanced by laser confinement may prove useful for manipulation of electronic and donor states in some proposed solid-state-based quantum computers.

Dressed-band approach to laser-field effects in semiconductors and quantum-confined heterostructures

A theoretical study of the effects of a laser field on the electronic and optical properties of GaAs-(Ga,Al)As heterostructures is presented by using a Kane model for the GaAs bulk semiconductor and working within an extended dressed-atom approach. For a laser tuned far below any resonances, the effects of the laser-semiconductor interaction correspond to a renormalization of the semiconductor energy gap and conduction/valence effective masses. This renormalized one-body approach may be used to give a qualitative indication of the laser effects on a variety of optoelectronic phenomena in semiconductor heterostructures for which the effective-mass approximation provides a good physical description. As a test, the exciton Stark shift in quantum wells is calculated and the effects due to the band-structure laser dressing are found to be of the same order of magnitude as those obtained from many-body diagrammatic techniques. We have also analyzed the effects of laser dressing on the shallow-donor peak energies in quantum wells, and found them comparable with those produced by a magnetic field of a few teslas.

The effective density of induced interface states of ionized cluster beam deposited Ag/n-Si Schottky junctions

The series of Ag/n-Si(1 1 1) Schottky junctions, prepared by the ionized cluster beam deposition method for an Ag ion acceleration voltage of U a=0 V, 300 V, 600 V and 1 kV, exhibits strong excess capacitances, the peaks of which gradually shift from reverse to forward bias voltages. These C– U measurements, the first such reported, are interpreted on the basis of the existence of external bias-dependent induced interfacial net charge density. It has been found that either reverse or forward excess capacitance of pertinent samples of the series is directly related to the induced interfacial charges above. From the above data an effective density of interface states is extracted which is characterized by the bias-dependent sharp spikes distributed along the semiconductor energy gap.

Laser effects in semiconductor heterostructures within an extended dressed-atom approach

We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system. The semiconductor is modeled via a simple two-band isotropic scheme and the interaction with the laser field is incorporated through the renormalization of the semiconductor energy gap and valence and conduction effective masses. We calculate the effects originated by the laser dressing on the donor and exciton peak energies in quantum-well heterostructures and show that the laser dressing may be quite considerable and readily observable.

Laser dressing effects in low-dimensional semiconductor systems

A study of the effects of a laser field on the energy spectra of low-dimensional GaAs–(Ga,Al)As semiconductor systems is presented by using a Kane band-structure model for the GaAs bulk semiconductor. For a laser tuned far below any resonances, the effects of the laser–semiconductor interaction correspond to a renormalization or dressing of the semiconductor energy gap and conduction/valence effective masses. This renormalized approach may be used to give an adequate indication of the laser effects on any semiconductor heterostructures for which the effective-mass approximation provides a good physical description. As an application, it is shown that dressing effects on the donor and exciton peak energies in quantum-well heterostructures may be quite considerable and readily observable.

The Temperature Dependence of the Energy Gap of CdGeP2 Semiconductor

The theoretkal method of Fan and shocklyaitardeen is used to cakulate the temperature dependence of the energy gap in the temperature range (300 — 425 Ki for the ternary semiconducting compound CdGeri2. the results are (L63-1 .71 )100.01 eV in the temperature -range mentioned above and they are in good agreement with those of the reported experimental work The temperature variation of AEg and Eg were discussed in terms of the effects of the electron = phonon interaction and the lattice dilation The effect of the lattice expansion is larger than that of the lattice vibration, but both effecis „ for this compound ,. are found to be linear with T for the rtempitattirre make investigateci The empirical relation of ( Prarshni„ Ravin.. dra Srivastava and Seehra Sehra) is also wed to calcultikie Eg versus T It gives good results in comparison with the reported experimental work „ as well as with the calculated results.

Structure and electronic properties of nitrogen-containing carbon nanotubes

The structure and electronic properties of nitrogen-containing carbon nanotubes are investigated using self-consistent field crystal orbital method. The models of these tubes are constructed by substitution of some carbon atoms with nitrogen atoms in the armchair carbon nanotube. The calculated band structures show that these nanotubes can be metals or semiconductors depending on the relative position of nitrogen and carbon atoms. The most stable non-metallic tubes are narrow energy gap (EG) semiconductors. Metallic tubes have two overlapped partially filled bands. The influence of Fermi vector positions on metal–insulator phase transition is discussed in the metallic NC tubes.

Direct evidence for the inverted band structure of HgTe

Angular-resolved photoemission measurements of the nonpolar (110)-cleavage face of HgTe single crystals have been performed along the \ensuremath{\Sigma} line to determine details of the band structure near the valence band maximum (VBM). Three bands are observed between VBM and 1 eV binding energy, instead of the two observed for a positive energy gap semiconductor CdTe. Their energy separations and positions relative to the Fermi energy are investigated at the \ensuremath{\Gamma} point and at slightly off-normal emission, applying room and low temperature of 40 K. In contrast to the heavily debated results of HgSe [K.-U. Gawlik et al., Phys. Rev. Lett. 78, 3165 (1997)] the clear observations for HgTe are consistent with the model of an inverted band structure, reflecting a semiconductor with a negative band gap.

Basic Model Relations for Temperature Dependencies of Fundamental Energy Gaps in Semiconductors

Novel analytical models describing the temperature dependencies of fundamental energy gaps in semiconductors are shown to be consistent with basic equations due to the electron–phonon interaction mechanism. The principal deficiencies of three-parameter models and the inevitability of using more elaborate four-parameter representations are illustrated by alternative fittings of high-precision Eg(T)-data given for GaAs by Grilli et al. Peculiarities of parameter constellations revealed by measured Eg(T)-dependencies in wide-gap versus medium-gap materials are briefly discussed.

Optical Absorption and Raman Scattering Measurements in CuAlSe2 at High Pressure

AbstractThe ternary compound CuAlSe2 is a direct energy gap semiconductor (Eg = 2.6 eV at 300 K) crystallizing in the tetragonal chalcopyrite structure. In this work the optical absorption edge and the Raman active modes of CuAlSe2 were measured as a function of pressure up to 30 GPa. The measurements were performed in a membrane diamond‐anvil cell at ambient temperature using neon gas as pressure transmitting medium. The direct energy gap (Γ → Γ) increases linearly with pressure at the rate of 4.7 × 10−2 eV GPa−1. At 6.7 GPa the character of the fundamental gap changes to pseudodirect (Γ → Γ). This gap decreases with pressure at a rate of −2.9 × 10−2 eV GPa−1. The effect of pressure on the phonon frequencies is discussed in terms of the Grüneisen parameters. A first‐order structural phase transition was observed at 12 GPa in the upstroke and at 2 GPa in the downstroke.

Determination of recombination and photogeneration parameters of a-Si:H using photoconductivity measurements

The non-linear dependence of photoconductivity (PC) on wavelength and intensity of illumination was used to evaluate the lifetime and quantum efficiency coefficient of carrier photogeneration in RF sputtered thin films of a-Si:H. The change of spatial distribution of radiation intensity over the sample thickness with the change of photon energy has been taken into account. Due to this, for the photon energies greater than the semiconductor energy gap, the decay of PC with an increase of absorption coefficient of radiation could be described using only one recombination parameter: the lifetime factor proportional to carrier lifetime. The comparison of the values of absorption coefficient obtained from the measurements of optical transmittance, CPM and the quantum efficiency coefficient is presented. The determination of carrier lifetime as a function of energy of photons that generate the free carriers is discussed.

Anharmonic effects in light scattering due to optical phonons in CuGaS2.

The ternary compound CuGaS2 is a direct energy gap semiconductor ~Eg;2.50 eV at 300 K! that crystallizes in the tetragonal chalcopyrite (I 42d) structure and its isoelectronic cubic analog is ZnS. The temperature dependence ~10–300 K! of the Raman active phonons has been studied up to 18 GPa in a membrane diamond anvil cell, using helium gas as a pressure transmitting medium. These measurements allowed us to determine the temperature dependence of the mode Gruneisen parameters and to separate the isobaric temperature dependence of the optical modes into pure-volume and pure-temperature contributions. By this procedure, the cubic and quartic anharmonicities responsible for the pure-temperature contributions to the mode frequencies were determined. The mode Gruneisen parameters of the center-of-zone G5 1 mode ~which corresponds to the edge-of-zone TA in ZnS! is negative throughout the whole temperature range, reaching 21.12 at 77 K. At low temperatures, this provides a possible driving mechanism for the negative values of the thermal expansion coefficient in semiconductors. @S0163-1829~96!07428-0#

Tunneling through a narrow-gap semiconductor with different conduction- and valence-band effective masses

We have calculated tunneling conductance in metal–narrow-gap-semiconductor (NGS)–metal tunnel junctions. Flietner’s two-band model is used to describe the dispersion relation within the energy gap in an isotropic NGS with different conduction- and valence-band edge effective masses. The results are compared with the tunneling conductance calculated by Kane’s two-band model, which has been commonly used to describe the tunneling characteristics through the energy gap in semiconductors. These results propose that the tunneling conductance in the tunnel junctions in which a narrow gap semiconductor of largely different conduction- and valence-band effective masses is used as a tunneling barrier can exhibit quite a different behavior, especially in the region of the midgap, from the tunneling conductance described by Kane’s two-band model.

NO<sub>2</sub>无Doppler增宽的磁场调制激光光谱

As a wide energy gap semiconductor, diamond is expected to play a role in UV light-emitting diode or laser. However, the impurities and vacancies in diamond exert a great influence on optical properties of the crystals. It is important, therefore, to study the relation between impurities and the optical properties, and furthermore, to study the control of the impurities.

Tunneling through narrow-gap semiconductor Sb2Te3 barrier

Tunnel experiments have been performed on Au/Sb2Te3/Al tunnel junctions to study elastic interelectrode tunneling through the small energy gap of a narrow-gap semiconductor. Tunnel conductance exhibited narrow width conductance peak at zero bias voltage. This behaviour is in accordance with the result of the theoretically calculated tunnel conductance, in which the nonparabolic dispersion relation within the energy gap of the narrow-gap semiconductor used as a tunnel barrier in a metal/narrow-gap semiconductor/metal tunnel structure is included. And some interesting structures are also observed in the conductance curves.

Pressure and temperature dependences of the raman-active phonons in CuGaS 2

The ternary compound, CuGaS 2 is a direct energy gap semiconductor ( E g ~ 2.50 eV at 300 K) that crystallizes in the tetragonal chalcopyrite ( I 4 ̄ 2d ) structure and its isoelectronic cubic analog is ZnS. The temperature dependence (10–300 K) of the Raman active phonons has been studied up to 18 GPa in a membrane diamond anvil cell, using helium gas as the pressure transmitting medium. These measurements allowed us to determine the temperature dependence of the mode Grüneisen parameters and to separate the isobaric temperature dependence of the optical modes into pure-volume and pure-temperature contributions. By this procedure, the cubic and quartic anharmonicities responsible for the pure-temperature contributions to the mode frequencies were determined. The mode Grüneisen parameters of the center-of-the-zone Γ 5 1 mode (which corresponds to the edge-of-the-zone TA in ZnS) is negative throughout the whole temperature range, reaching − 0.64 at 77 K. At low temperatures, this provides a possible driving mechanism for the negative values of the thermal expansion coefficient in semiconductors.

Photoelectrochemical Characteristics of n-InSe Single Crystals

The photoelectrochemical behaviour of n-InSe single crystals in aqueous solutions has been studied in the presence and absence of several redox systems. Among them, Ce3+/4+ and KI/I2 yielded the highest photocurrents, photovoltages and show the highest stability. The semiconductor energy gap Eg = 1.2 eV, has been calculated from transmittance measurements. The flat band potential UFB = -0.40 V vs NHE and the donor density have been calculated from differential capacity measurements.

Spectroscopic measurement of the semiconductor energy gap

A homemade grating spectrometer and simple electronics allow measurements of the energy gap of semiconductor samples in introductory laboratory courses. Both transmission spectra and ‘‘photoconductive effect’’ spectra may be obtained. Results for Ge, Si, GaAs, and InP are reported.

Semiconductor energy gaps in the average Fock approximation.

Semiconductor energy gaps in the average Fock approximation.

A novel temperature-stable light-emitting diode

A new type of light-emitting diode (LED) has been developed, the light intensity of which is stable during temperature change. This device consists of a multilayer dielectric optical filter on the surface of a conventional LED. The transmissivity of the optical filter is low for the short wavelength region of the LED's light spectrum and high for the long wavelength region. When the LED's temperature increases, the spectrum of LED light shifts toward the long wavelength side because of the shrinkage of the energy gap of the compound semiconductor. The shift increases the total transmission of the light because the transmissivity is high for the long wavelength region. This increase compensates for the decrease of the LED light intensity, which is caused by the decrease of the internal quantum efficiency. The effect of this filter is confirmed by both calculation and experiment.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>