meV Kbar Research Articles

Current- and capacitance-voltage characteristics of Cd/p-GaTe Schottky barrier diodes under hydrostatic pressure

Schottky barrier diodes (SBDs) on p-type GaTe have been fabricated by Cd metallization and characterized by current–voltage (I–V) and capacitance–voltage (C–V) techniques as a function of hydrostatic pressure (0.0–7.0 kbar). The evaluation of the experimental data reveals a decrease of barrier height (Φb), ideality factor (n) and series resistance (Rs) with an increase in the hydrostatic pressure. The zero-bias barrier height, ideality factor and series resistance values for the Cd/p-GaTe SBD by I–V measurements have been in the range of 0.743–0.682 eV, 1.246–1.219 and 30.5–16.4 Ω for the 0.0–7.0 kbar pressure interval, respectively. C–V measurements at 1.0 MHz have resulted in higher barrier heights than those obtained from I–V measurements. The discrepancy between Schottky barrier heights (SBHs) obtained from I–V and C–V measurements is explained by the introduction of a spatial distribution of SBHs due to barrier height inhomogeneities that prevail in the metal/GaTe interface. The change of barrier height values (Φb(0) − Φb(P)) obtained from the I–V and C–V techniques turns out to have a mean linear pressure coefficient of −8.77 ± 0.10 meV kbar−1 (=−87.7 ± 1.0 eV GPa−1), approximately equal to that found for the band gap of GaTe. We have concluded that the variation of the barrier height due to the applied pressure should follow precisely the variation of the semiconductor band gap, accepting that the Fermi level is a reference level which is pinned to the conduction-band minimum (CBM) as a function of the pressure.

Effects of hydrostatic pressure on Raman scattering in Ge quantum dot superlattices

Self-organized Ge/Si quantum dots (QD's) in strained Si/Ge short-period superlattices are studied by Raman scattering under hydrostatic pressure excited in-resonance and off-resonance with the confined Ge-like E 1 transition of Ge using 488 and 514.5 nm lines from an argon-ion laser and 632.8 nm line from a He–Ne laser. The Raman spectra of Ge–Ge, Si–Ge and Si-2TA modes of the QD's were obtained as a function of pressure in the range of 1–70 kbar. Our results show that the mode Grüneisen parameter of the Ge–Ge phonon mode in QD's is found to be γ=0.81±0.01, which is smaller than that of the bulk Ge. We observe resonance effects with the confined Ge-like E 1 transition and the pressure coefficient of this resonating electronic transition obtained is ∼5±1 meV kbar −1.

Combined Raman and transmission spectroscopy of ZnTe under pressure

We report measurements of both Raman and transmission spectroscopy in the same samples, under high pressure. The combination of two spectroscopies is shown to improve the reliability of the data. The phase transition is found at 940 ± 0.2 kbar. The pressure dependence of the band gap is described by a density deformation potential of 4.90 ± 0.15 eV, or a linear pressure coefficient of 10.2 ± 0.3 meV kbar −1 at ambient pressure. We find that the shifts of the LO and TO phonon energies are linear with the shift of the band gap and therefore fit to a linear dependence on density with corresponding deformation potentials of 223 ± 3 cm −1 (LO) and 290 ± 3 cm −1 (TO).

Polarized optical studies of ordered InGaP 2 under pressure

In this paper, we discuss polarized photomodulated reflectivity (PR) and photoluminescence (PL) studies of MOCVD grown InGaP 2 epilayers lattice-matched to a GaAs substrate. These structures were grown on a (001) face with a misorientation of two degrees along <110>. The PR spectra of the ordered sample show a strong polarization dependence. For the electric field E parallel to [1̄10] two features in the PR spectra are seen; for E∥[110], however, additional features are observed. A comparison with the spectra of disordered samples of the same alloy composition has enabled a determination of the band gap reduction due to ordering. The linewidths of the PR peaks are approximately 5-10 meV which has enabled us to study them in detail as a function of hydrostatic pressure at cryogenic temperatures. The pressure dependence is slightly sublinear with the first order term of 8-9 meV kbar -1 for pressures well below the Γ-X crossover. Also observed is the indirect level crossing which occurs under pressure at about 40-kbar. An analysis of PR lineshapes at 1-bar is also presented at several commonly used experimental temperatures.

The effect of pressure on the luminescence from GaAs/AlGaAs quantum wells

Photoluminescence (PL) from GaAs quantum wells with widths from 50 AA to 300 AA and AlxGa1-xAs barriers (x=0.3 and 1) was studied under pressure up to 35 kbar at two temperatures (300 K and 77 K). We used (and compared) three types of pressure devices: gas cells, liquid cells and the diamond-anvil cell. Accurate values for the pressure variation of the PL energy were obtained. They reveal the small dependence on the parameters of the well, in agreement with the envelope-function calculation. Pressure shifts of the PL lines is the same at 77 K and at 300 K. In several samples we found the change of the pressure coefficient of the direct ( Gamma ) line at the Gamma -X crossover pressure. We interpret this as the resonance effect due to the mixing of the Gamma state in the well with the X continuum in the barriers. This means that the pressure dependence of the quantum-well lines should not be fitted with a single curve below and above the Gamma -X crossover pressure. From our results we obtain the linear pressure coefficient of the GaAs energy gap equal to 11.6 meV kbar-1. The deformation potential of the gap seems to be almost independent of pressure up to 15 kbar.

Large pressure effect on photoluminescence lines in 6 H SiC:Ti crystal

The effect of hydrostatic pressure up to 7.5 kbar on the so-called ABC lines in the photoluminescence spectrum of hexagonal 6 H SiC crystals with traces of Ti atoms has been studied at 77 K. Linear pressure shifts of these lines were found to be rather large (1.08–1.25 meV kbar −1) considering low compressibility of the crystal and the expected very small pressure coefficient for the band gap in 6 H SiC. The recorded pressure shifts exceed by more than 10 times those of the well-known R luminescence lines in ruby, Al 2O 3:Cr 3+, which has roughly the same compressibility as SiC. Fairly high sensitivity of ABC lines to the applied pressure is interpreted in terms of a ligand field model for a quasimolecular impurity centre where Ti atom substitutes an about 20% smaller Si atom in SiC crystal.

Unusual behaviour of the Ge DX centre in GaAs: coexistence of two localized donor states

Hall measurements have been performed as a function of hydrostatic pressure and temperature to study the effect of deep donor levels on the electron concentration and mobility in bulk GaAs heavily doped with germanium. For the first time, the coexistence of the metastable DX centre and the nonmetastable localized deep A1 level has been unambiguously observed in GaAs. Furthermore, the authors have determined the energy positions and pressure dependences of both resonant levels (0.066 eV and -6.9 meV kbar-1 for the A1 level and 0.105 eV and -10 meV kbar-1 for the DX centre) and the energy barrier for electron emission from the DX centre (0.285 eV). The latter appeared to be pressure independent. From the mobility behaviour during depopulation of DX centres, an elegant proof for the negative-U character of DX centres and the positive-U character of deep A1 levels has been obtained.

Pressure dependence of infrared photoluminescence spectra from HgTe/CdTe superlattices

The authors have investigated the pressure dependence of the infrared photoluminescence (PL) of HgTe/CdTe superlattices (SLS) at liquid nitrogen temperature up to 30 kbar. They observed several PL peaks with energies ranging from approximately 130 meV to approximately 700 meV. The most prominent peak at approximately 130 meV, which has been attributed to the recombination across the SL bandgap, moves higher in energy with a pressure coefficient of <or approximately=1 meV kbar-1. Other peaks at higher energies have similar pressure coefficients of 0-2 meV kbar-1. A calculation based on the envelope-function approximation gives a pressure coefficient of at least approximately 6.5 meV kbar-1, which is far outside the error bars of the measured pressure dependence of the main peak, <or approximately=1 meV kbar-1. Possible explanations, including a modification of the current model of HgTe/CdTe SL bands and a reinterpretation of the PL peaks, are examined.

Pressure dependence of the polarized reflectance spectrum of a solid charge-transfer complex, perylene-hexacyanobutadiene (HCBD)

The polarized reflectance spectrum of the perylene-HCBD complex has been measured at high pressures up to 26 kbar. Extraordinary intensity of the charge-transfer band is observed at high pressures, which cannot be reproduced by a conventional analytical model such as a dimer or trimer model. It is shown that this intensity can be naturally explained by applying the results of diagrammatic valence-bond calculations for an infinite-chain model, which has been reported by Painelli et al. [J. Chem. Phys. 87 (1987) 1705]. The degree of charge transfer ϱ, the transfer integral t, and the site-energy difference from the donor to acceptor Δ at each pressure are estimated, and the pressure dependence of t and Δ is found to be ∂ ln t/∂ P = +2.9% kbar −1 and ∂Δ/∂ P = −9.6 meV kbar −1, respectively.

Pressure effects in organopolysilane

Structure and electronic properties in phenyl-methyl polysilane films have been investigated as a function of pressure up to 50 kbar. At pressures lower than 10 kbar, a drastic reduction in the interchain separation is induced, whereas the change in the bandgap energy is small, −1 meV kbar −1. These results are discussed on the basis of the σ-electron conjugated model.

DLTS investigation of deep levels in bulk GaAs under uniaxial stress

This work reports uniaxial stress DLTS measurements on the EL2, EL3 and EL6 levels in n-type bulk GaAs. The stress was parallel to the (111) axis. The defect state EL6 seems unaffected by stresses up to 1.5 kbar. EL2 exhibits a pressure coefficient of 1.3 meV kbar-1. EL3 shows a low-stress (2.8 meV kbar-1) and a high-stress (4.6 meV kbar-1) pressure coefficient. These results are discussed in connection with previously reported data published in the literature. An anomalous electric field dependence of the emission rate of the EL3 level is also reported.

Investigation of the electronic structures of (GaAs) n(AlAs) n short period superlattices by photoluminescence spectroscopy under hydrostatic pressure

The photoluminescence of (GaAs) n (AlAs) n short-period superlattices (SLs) with different period lengths ( n=4-17) is studied under hydrostatic pressure in the range of 0–50 kbar at both 300 and 77 K. The dependence of the energy separation between the conduction band Γ-like and X-like levels on the SL periodicity is obtained independent of which level is higher. The type I-type II transition is observed at atmospheric pressure and at both 300 and 77 K in a (GaAs) 11(AlAs) 11 SL. It is found that the pressure coefficient of X-like states decreases with decreasing period length of the SL, changing from -2.2 meV kbar for (GaAs) 17(AlAs) 17 to -1.2 meV kbar . for (GaAs) 4(AlAs) 4. The Γ-X mixing potential V mix obtained from the investigation of the pressure dependence of luminescence intensities at 77 K is 4, 14, 7, 1.4 meV for n=6, 8, 11, 17 samples, respectively. These results demonstrate that the state mixing due to the zone folding effect is quite small though the mixing potential increases slightly with the decrease of the period length of the SL.

Pressure dependence of the polarized reflectance spectrum of the solid charge-transfer complex, perylene–TCNQ: Estimation of microscopic parameters

The polarized reflectance spectrum of the perylene–TCNQ complex has been measured at high pressure up to 27 kbar using a diamond anvil cell incorporated into a microspectrophotometer. The degree of charge transfer (ρ), the transfer integral (t), and the site-energy difference (Δ) are estimated at each pressure from analysis of the charge-transfer band. The pressure dependences of t and Δ are found to be ∂ ln t/∂P=+2.3% kbar−1 and ∂Δ/∂P=−6.4 MeV kbar−1, respectively. It is shown that the observed pressure dependence of the stabilization energy of the charge-transfer excited state is mainly attributable to the change in the electrostatic energy accompanying the lattice contraction.

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.

The pressure dependence of the photoluminescence intensity in hydrogenated amorphous silicon

Abstract The hydrostatic pressure dependence of the photoluminescence (PL) intensity from hydrogenated amorphous silicon has been measured in a diamond anvil cell at 80 K. The PL intensity is not quenched significantly by pressures up to ∼90 kbar, but in the pressure range ∼90–130 kbar quenching is observed. The luminescence displays a red shift of 1.9±0.1 meV kbar−1, with a hysteresis of ∼35meV at the highest pressures.

The hydrostatic pressure dependence of the band-edge photoluminescence of GaInAs

The hydrostatic pressure dependence of the band gap of a GaInAs epilayer has been measured by photoluminescence in a diamond anvil cell at room temperature and 80 K. The initial pressure coefficient of the band-gap is found within experimental error to be independent of temperature with a value of 10.95 ± 0.1 meV kbar -1. The dependence of the band gap on lattice constant is found to be linear, described by a deformation potential of -8.25 ± 0.1 eV for a bulk modulus of 760kbar.

Pressure studies of quasi-one dimensional amorphous Si:H films

Pressure dependences of the optical absorption edge and the linear compressibility have been investigated up to ∼ 100 kbar for quasi-one dimensional α-Si:H films prepared by glow-discharge decomposition of disilane. The dependences behave in a different way in the high and low pressure regions. When pressure is lower than ∼ 60 kbar, the material is compressed greatly and the pressure coefficient of the energy gap is − 1 meV kbar −1. Above this pressure, the material becomes more rigid and the coefficient is − 8 meV kbar −1. These results are discussed on the basis of a structural model which assumes a film structure consisting of polysilane molecules and α-Si clusters.

Photo-emission and transport studies of HfxTi1-xSe2alloys

The valence bands of HfxTi1-xSe2 have been studied by angle-resolved photoelectron spectroscopy for x=0.3 and 0.5. These results indicate the existence of small band gaps between valence and conduction bands in both these alloys. The authors interpret existing Hall effect data, which show evidence for high-mobility holes for the Hf0.3TiO.7Se2 sample, in terms of thermal excitations across the small band gap at high temperatures. They model the temperature and pressure dependence quantitatively, and determine the band gap of Hf0.3Ti0.7Se2 to be 104 meV at atmospheric pressure with a pressure derivative of -6 meV kbar-1. The new model is relevant also to TiSe2, but the conclusion that this material is semi-metallic in its high-temperature phase is unaffected.

Effect of pressure on the optical absorption in aluminium

The optical reflectivity of Al below 3 eV has been investigated in the 300 kbar pressure range (22% volume change) at T=300K. The dominant optical interband absorption at h(cross) omega =1.5 eV associated with the splitting between parallel bands shifts to higher proton energy at an initial rate of 4.35+or-0.3 meV kbar-1. The volume dependence of the corresponding pseudopotential Fourier coefficients is dU(200)/d ln V=-1.58 eV. The second parallel band absorption feature at energy 2U(111) ( approximately=0.4 eV at 1 bar) exhibits a similar pressure shift and becomes clearly observable in high-pressure reflection spectra. The optical response of Al under pressure basically follow the predictions derived from an empty-core pseudopotential in combination with the Ashcroft-Sturm expression for optical interband absorption in polyvalent nearly-free-electron metals, but with a significantly larger width of the low-energy absorption band at energy 2U(111). X-ray diffraction studies support the stability of the FCC phase up to 300 kbar.

Hydrostatic pressure control of the carrier density in GaAs / GaAlAs heterostructures; Role of the metastable deep levels

Abstract Hydrostatic pressure up to 18 kbar has been used to characterize GaAs/GaAlAs heterostructures manufactured using MOCVD and MBE techniques. The pressure dependence of the carrier concentration ns has been deduced from Hall measurements between 4.2 and 300 K and Shubnikov-De Haas experiments have been performed at helium temperature. The main result we have observed is a linear decrease of the carrier concentration as the pressure increases. We explain this behaviour taking into account the deep character of the impurity level in GaAlAs connected with the X minimum whose variation is found to be 11 meV kbar . When the Fermi energy is large enough compared to the difference of affinities ΔEc, a transition between a metallic to an insulating behaviour of the heterojunction is induced. Moreover, the metastable character of this impurity level, clearly pointed out by changing the cooling conditions must be taken into account. In conclusion, this kind of experiment, in which the carrier concentration may be varied, appears as a powerful tool to investigate the properties of the heterostructures. In the particular case of GaAs/GaAlAs the role of the metastable state is predominant in all the investigations.