Band Gap Of CdTe Research Articles

Photoluminescence of CdTe under hydrostatic pressure

We have determined the pressure dependence of the band gap of CdTe at 80 K by measuring the photoluminescence under hydrostatic pressures up to the phase transition of CdTe at ∼ 39 kbar. For pressures below ≈ 20 kbar the band gap increases linearly with a pressure coefficient of 7.9±0.2 meV/kbar. At higher pressures a substantial nonlinearity is observed. Photoluminescence is not observed after the phase transition and is not recovered when the pressure is lowered well below the transition pressure.

Implantation effects on resonant Raman scattering in CdTe and Cd 0.23Hg 0.77Te

We have studied In + implanted CdTe and Cd 0.23Hg 0.77Te by resonant Raman scattering. The laser excitation was in resonance with the E O + Δ O band gap in CdTe or the E 1 gap in Cd 0.23Hg 0.77Te. Under these conditions dipole forbidden but defect ind scattering by one longitudinal optical (LO) phonon as well as Fröhlich-induced two-LO phonon scattering is observed. In both cases scattering is found to be strongly affected by ion implantation. In + was implanted at an ion energy of 350 keV with doses ranging from 10 11 to 5×10 14 ions/cm 2. The intensity ratio of the one-LO phonon lines is found to be a quantitative measure of the implantation damage in CdTe and Cd 0.23Hg 0.77Te even for doses as low as 10 11 ions/cm 2. It is shown that the observed effects of implantation damage on resonant Raman scattering by LO phonons are due to a broadening and an energy shift of the corresponding resonances in the Raman scattering efficiency.

Radiative recombination across the E0+ Delta 0 band gap in CdTe.

Resonantly excited photoluminescence spectroscopy has been used to investigate radiative recombination across the ${E}_{0}$+${\ensuremath{\Delta}}_{0}$ band gap in undoped bulk CdTe. For excitation with photon energies slightly higher than the ${E}_{0}$+${\ensuremath{\Delta}}_{0}$ band-gap energy, luminescence is observed at this gap energy. This emission is interpreted as the radiative recombination of photocreated electrons and holes localized in the lowest conduction band and in the spin-orbit-split-off ${\ensuremath{\Gamma}}_{7}$ valence band, respectively. Similar photoluminescence transitions across the ${E}_{0}$+${\ensuremath{\Delta}}_{0}$ gap are also observed in Cd-rich ${\mathrm{Cd}}_{\mathrm{x}}$${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Te.

Fermi level pinning in the middle of the band gap in CdTe: Cl crystals: Role of deep localized states

The level distribution in semi-insulating CdTe crystals doped with Cl was revealed by two independent methods, i.e., by studying (1) the equilibrium Hall effect and (2) the spectral and intensity dependences of the photo-Hall effect. The pinning of the Fermi level near the gap midpoint is due to a considerable concentration (≈ 1015 cm−3) of the Ev + 0.95 eV levels, which are described to an intrinsic acceptor lattice defect — a Cd vacancy. Furthermore, there is a quasicontinuous set of levels in the energy regions Ev + (0.8-0.5) eV; the density of these levels increases exponentially toward the valence band. It is supposed that these states are connected with donor-Cd vacancy pairs. The distance between the pair components is of the order of (1−2)a, where a is the lattice constant.

Hydrostatic pressure dependence of CdTe

The pressure dependence of the direct band gap of CdTe has been measured by observing the direct exciton photoluminescence and donor-acceptor pair photoluminescence under hydrostatic pressures up to 35 kbar. A value of 6.5+or-0.2 meV kbar-1 was obtained corresponding to a hydrostatic deformation potential a=-2.74+or-0.09 eV. From a tight-binding calculation the authors estimate the valence and conduction band contributions to a.

Hydrostatic and uniaxial pressure coefficients of CdTe.

The pressure dependence of direct band-gap CdTe has been measured by observing the direct exciton photoluminescence and donor-acceptor pair photoluminescence under hydrostatic pressures up to 35 kbar. A value of 6.5\ifmmode\pm\else\textpm\fi{}0.2 meV ${\mathrm{kbar}}^{\ensuremath{-}1}$ was obtained corresponding to a hydrostatic deformation potential $\ensuremath{\Xi}=2.74\ifmmode\pm\else\textpm\fi{}0.09$ eV. Using this value, the uniaxial stress data of Thomas J. Appl. Phys. 32, 2298 (1961) is reassessed to determine a best value of $\ensuremath{\Delta}=8.7\ifmmode\pm\else\textpm\fi{}0.3$ meV ${\mathrm{kbar}}^{\ensuremath{-}1}$, corresponding to ${D}_{u}=1.11$ eV and ${D}_{{u}^{\ensuremath{'}}}=2.57$ eV.

The effects of a high-temperature anneal on the electrical and optical properties of bulk CdTe:In

The electrical and optical properties of bulk CdTe:In have been studied by means of van der Pauw-Hall-effect measurements and low-temperature photoluminescence (PL). As-grown, the bulk CdTe:In samples exhibited high resistivity (ρ>108 Ω cm). Following a high-temperature anneal at 900 °C in a Cd overpressure, the CdTe samples became n-type (n∼1016 −1017 cm−3 at 300 K). The carrier concentration was found to depend on the anneal time. The PL spectra from as-grown samples are dominated by edge emission at 1.583–1.584 eV, the nature of which is determined from the optical measurements. As a result of the anneal, PL emission above 1.606 eV (the band gap of CdTe at 4.5 K) is observed. Excitation PL measurements, along with studies of the temperature and power dependencies of the PL emissions from the annealed samples, suggest an increase in band gap to 1.632 eV has occurred, which we believe is due to the incorporation of interstitial Cd into the lattice. Our study provides a correlation between the electrical and optical properties of bulk CdTe:In which, upon annealing, may be used as a conducting II-VI substrate in epitaxial film growth.

Fermi Level Pinning in the Middle of the Band Gap in CdTe:Cl Crystals‐Role of Deep Localized States

Fermi Level Pinning in the Middle of the Band Gap in CdTe:Cl Crystals‐Role of Deep Localized States

Preparation of CdTe thin films by laser irradiation of electrodeposits

We describe here the first attempts to electrodeposit cadmium and tellurium layers onto a glass substrate covered with indium tin oxide with the intention of synthesizing CdTe by laser irradiation of the deposit. We successively deposited tellurium and cadmium cathodically in aqueous solutions of H 2SO 4 and TeO 2 or of CdSO 4 respectively. The deposits were then laser irradiated and CdTe was formed. The as-formed CdTe, of cubic crystallographic structure, exhibits a band edge for direct transitions of about 1.5 eV in agreement with the band gap of CdTe.

Photoelectric study of the Ni- and Ni-TeO2-electrodeposited n-type CdTe interfaces

The barrier heights of the electrodeposited n-type CdTe thin-film metal semiconductor (MS) and metal-thin insulating layer-semiconductor (MIS) devices have been measured by internal photoemission. The data for both MS and MIS devices can be fit with the square root of the photocurrent per absorbed photon dependence consistent with the Fowler theory. The barrier height of Ni-CdTe is 0.75 eV at room temperature, and its temperature dependence is about the same as that of the CdTe band gap. This suggests that the barrier height is pinned with respect to the CdTe valence band edge. Oxidation in air at 300 °C for an hour produces a layer of about 25 Å or TeO2 on the CdTe surface. Thicker oxide can be grown for longer oxidation time. Negative oxide charges are found in some devices. At room temperature, the barrier height of Ni-TeO2-CdTe is 0.92 eV. As temperature is decreased, the barrier height increases and its rate of change with temperature is less than that of the CdTe band gap. Samples stored in room atmosphere show aging effects, one of which is the increase in surface-state density. The dips in the curve of the spectral dependence of the relative photon-induced current are caused by electrons which drift to the metal after being photoexcited from occupied surface states below the Fermi level. These dips correspond to peaks in the energy distribution of the surface state density. At zero bias and 170 °K, four peaks at 1.02, 1.06, 1.18, and 1.26 eV are observed for the MIS devices. For the MS devices zero biased at room temperature, four peaks at 0.85, 0.94, 1.02, and 1.05 eV are observed. The magnitudes of the dips at 0.85, 0.94, 1.18, and 1.26 eV vary with bulk-defect density and these levels shift towards higher energy as temperature is decreased. On the other hand, the levels at 1.02 and 1.05 eV for the MS devices and 1.02 and 1.06 eV for the MIS devices are independent of temperature and bulk defect density.

Photoluminescence of CdTe: A comparison of bulk and epitaxial material

Photoluminescence (PL) studies of bulk and epitaxial CdTe samples obtained from several sources are discussed. Steady state PL measurements were carried out at temperatures ranging from 16–300 K. The effects of surface preparation, substrate temperature, and film thickness were studied in detail for homoepitaxial films grown on the (111)A and (100) planes of CdTe. PL studies of epitaxial CdTe films grown on (0001) sapphire by molecular beam epitaxy (MBE), by hot wall MBE, and by metal-organic chemical vapor deposition (MOCVD), and on the (111)B and (100) planes of GaAs by MBE have also been completed. The CdTe epilayers on sapphire and GaAs substrates typically display a bright PL spectrum dominated by the near edge peak at 1.58 eV (77 K). In addition, a number of films exhibit a near edge peak at 1.503 eV at 300 K, which is indicative of high quality epitaxy and which allowed direct measurement of the room temperature band gap of CdTe. PL studies of epitaxial Cd1−x MnxTe films grown by MBE on 5.0 μm thick CdTe buffer layers on GaAs substrates reveal shifts of the band gap into the visible spectral region with increasing x accompanied by a significant increase in edge peak magnitude.

Excitation spectra of exciton luminescence in CdTe

AbstractMeasurements of the excitation spectra of the photoluminescence lines below the band gap of CdTe led to the identification of emissions originating from excitons bound to neutral donors, to ionized donors, and to neutral acceptors, as well as from two‐electron transitions. Oscillatory variations of the free and bound exciton emission intensity with the energy change of the exciting light above the band gap and their changes with temperature indicated details of the exciton relaxation mechanisms.

Temperature dependance of the fundamental absorption edge in CdTe

Optical measurements made on CdTe put in light an extrinsic transition previously used to determine the interband edge of this compound. The donor level involved has a depth of 30 meV inside the bandgap at 77°K and of 60meV at 300°K. The value obtained for its temperature coefficient, suggests an association of this level with the L minimum of the conduction band. Also, determined is the true optical bandgap of CdTe between many conflicting results. One obtains: E g = 1.529 meV with a temperature coefficient of −3.10 −4eV/°K.

Thin film CdS-CdTe heterojunction diodes

It is demonstrated that a layer of CdTe of the order of 100 Å in thickness is formed at the CdS-Te interface during the fabrication of thin film, Au-CdS-Te diodes by standard vapor deposition procedures. The existence of the interfacial layer is confirmed using X-ray diffraction studies, and substantiated by diode photocurrent measurements which show a threshold photon energy that corresponds to the bandgap of CdTe. The presence of the CdTe layer can explain the excellent rectification ratios and high reverse breakdown voltages achieved in vapor-deposited Au-CdS-Te diodes. The observed photoresponse, current vs. voltage, and capacitance vs. voltage characteristics are shown to be consistent with those predicted by a semiconductor heterojunction model of the CdS-CdTe interface.

Temperature dependence of the band gap in CdTe

The temperature dependence of the direct band gap in CdTe is investigated theoretically. The available experimental data shows that the band edges vary as T 2 at low temperatures. The calculations indicate the band shifts are due to hole interactions with acoustical phonons; the temperature dependent shifts in the conduction band are negligible. Good agreement between predicted and measured band edge shifts are obtained when the degeneracy and p-symmetry of the valence band are considered. This allows the influence of TA phonons to be included, which provide the major contribution to the hole self energies.

Temperature dependence of the band gap in CdTe

Abstract The temperature dependence of the direct band gap in CdTe is investigated theoretically. The available experimental data shows that the band edges vary as T 2 at low temperatures. The calculations indicate the band shifts are due to hole interactions with acoustical phonons; the temperature dependent shifts in the conduction band are negligible. Good agreement between predicted and measured band edge shifts are obtained when the degeneracy and p -symmetry of the valence band are considered. This allows the influence of TA phonons to be included, which provide the major contribution to the hole self energies.