Linear Pressure Coefficient Research Articles

Pressure dependence of intersubband transitions in HgTe/Hg0.3Cd0.7Te superlattices

The optical absorption coefficient of HgTe/Hg0.3Cd0.7Te superlattices (SLs) and its pressure dependence has been investigated at hydrostatic pressures up to 30 kbar at room temperature. The corresponding intersubband transition energies result from a comparison of experimental and theoretical absorption coefficients. The latter is based on the band structure, which is calculated using Kane’s four-band (8×8 k · p) model together with the envelope function approximation. The experimental linear pressure coefficients of the H1 — E1 and H1 — L1 intersubband transitions are in good agreement with the theoretical values, e.g., 7.15±0.3 meV/kbar and 6.2±0.3 meV/kbar compared to 7.4 and 6.4 meV/kbar, respectively. this is in stark contrast to the pressure dependence of ≤1 meV/kbar of the photoluminescence (PL) peaks of a similar SL reported in the literature. Consequently, we conclude that the reported PL peaks are not due to intersubband transitions and that the k · p model correctly reproduces the electronic band structure and its pressure dependence of HgTe/Hg1−xCdxTe SLs.

Raman scattering in InN films and nanostructures

Studies of lattice dynamics devoted to wurtzite InN are presented. Raman scattering experiments on both InN thin films and nanometric islands grown by Metal–Organic Vapor Phase Epitaxy (MOVPE) were performed at room temperature. From the Raman spectra recorded from InN films under hydrostatic pressure up to 13 GPa, linear pressure coefficients and the corresponding Grüneisen parameters for both E 2 and A 1 (LO) phonons were extracted for the wurtzite structure up to 11 GPa, close to the starting pressure of the hexagonal to rock-salt phase transition of InN. Spectra at higher pressure suggest that InN undergoes a gradual phase transition, and the reverse transition exhibits a strong hysteresis effect during the downstroke. Then, we discuss recent results on large single InN islands grown on GaN buffer layers, obtained by spatially resolved micro-Raman measurements. The magnitude of the residual strain is estimated, using a recent determination of phonon deformation potentials. It is found to vary linearly as a function of island height.

Raman scattering in hexagonal InN under high pressure

The behavior of the ${E}_{2}$ and ${A}_{1}(\mathrm{LO})$ optical phonons of hexagonal indium nitride under hydrostatic pressure was studied using Raman spectroscopy. Linear pressure coefficients and the corresponding Gr\uneisen parameters for both modes were determined for the wurtzite structure up to $11.6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, close to the starting pressure of the hexagonal to rocksalt phase transition of InN. Raman spectra acquired within the $11.6\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}13.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ pressure range suggest that wurtzite InN undergoes a gradual phase transition, and the reverse transformation exhibits a strong hysteresis effect during the downstroke.

Structural, electronic, and optical properties of beryllium monochalcogenides

The results of first-principles theoretical study of the structural, electronic and optical properties of beryllium monochalcogenides BeTe, BeSe and BeS, performed using the full potential linearized augmented plane wave (FP-LAPW) method are presented. The calculated structural parameters and band gaps compare very well with previous theoretical results. The trends of the band gap pressure coefficients and volume deformation potentials for these II-VI compounds are investigated. The linear pressure coefficients for the \(\Gamma - \)X and \(\Gamma - \Gamma\) band gaps increase with decrease in anion atomic weight. The dependence of the direct and indirect band gaps on the relative change of lattice constant are found to follow almost the same type of trends in each of these compounds. The volume deformation potential (\(a_{v}^{\alpha}\)) for the direct (\(\alpha = \Gamma - \Gamma\)) and indirect (\(\alpha = \Gamma - L\)) gaps are positive, but negative for the indirect (\(\Gamma - {\rm X}\)) gap. Furthermore, \(a_{v}^{\alpha}\), for \(\alpha = \Gamma - {\rm X}\) transitions decreases with increase in anion atomic number whereas \(a_{v}^{\Gamma-\Gamma}\), increases. The optical properties have also been calculated. From the reflectivity spectra, the compounds will be useful for optical applications. The variation of the band gaps with respect to the application of pressure and the origin of some of the peaks in the optical spectra are discussed in terms the calculated electronic structure.

Raman scattering investigation of aGe/SiO2/Sinanocrystal system under hydrostatic pressure

We have studied the hydrostatic pressure dependence of Ge nanocrystals embedded in a ${\mathrm{SiO}}_{2}$ matrix on a Si substrate by Raman scattering at room temperature. During the first cycle of increasing pressure, we observed a step change in the Ge Raman mode from 310.4 to $313.8{\mathrm{cm}}^{\ensuremath{-}1}$ at $\ensuremath{\sim}23\mathrm{kbar}.$ The linear pressure coefficients \ensuremath{\alpha} obtained before and after the step change at $\ensuremath{\sim}23\mathrm{kbar}$ are 0.42 and $0.64{\mathrm{cm}}^{\ensuremath{-}1}{\mathrm{kbar}}^{\ensuremath{-}1},$ respectively. Upon decreasing pressure, the Ge mode follows a single slope of pressure coefficient $\ensuremath{\alpha}=0.64{\mathrm{cm}}^{\ensuremath{-}1}{\mathrm{kbar}}^{\ensuremath{-}1}.$ A finite-element analysis was carried out to investigate the elastic-field distribution in the ${\mathrm{G}\mathrm{e}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}/\mathrm{Si}$ nanocrystal system, where the discontinuity of the specific geometric configuration with different elastic constants causes local areas of stress concentration around the interface. The step change of the Raman shifts with pressure at $\ensuremath{\sim}23\mathrm{kbar}$ was attributed to complete delamination between the ${\mathrm{SiO}}_{2}$ film and the Si substrate.

1H-NMR parameters of common amino acid residues measured in aqueous solutions of the linear tetrapeptides Gly-Gly-X-Ala at pressures between 0.1 and 200 MPa

For the interpretation of chemical shift changes induced by pressure in proteins, a comparison with random-coil data is important. For providing such a data basis, the pressure dependence of the 1H-NMR chemical shifts of the amino acids X in the random-coil model peptides Gly-Gly- X-Ala were studied for the 20 common amino acids at two pH values (pH 5.0 and 5.4) at 305 K, in the pressure range from 0.1 to 200 MPa. The largest shift changes Δδ with pressure p can be observed for the backbone amide protons. The average linear pressure coefficient δ Δp is 0.38 ppm GPa −1, with a root mean square deviation of 0.2 ppm GPa −1. In contrast to the downfield shift typical for amide protons, the H α-resonances typically shift upfield, with a pressure coefficient of −0.025 ppm GPa −1 and a root mean square deviation of 0.05 ppm GPa −1. The side chain resonances are only weakly influenced by pressure, on average they are shifted by 0.014 ppm GPa −1 , with a root mean square deviation of 0.14 ppm GPa −1. The exceptions are the side chain amide protons of asparagine and glutamine. Here, values of 0.214 (Asn H δ21), 0.417 (Asn H δ22), 0.260 (Gln H ε21) and 0.395 (Gln H ε22) ppm GPa −1 can be observed. In both cases, the pressure dependent shift is larger for the pro-E proton than for the pro-Z proton. Within the limits of error the equilibrium constant for the trans- and cis-conformers at the proline peptide bond is independent of pressure in the pressure range studied.

Acceptor–hydrogen complexes in semiconductors under pressure

The structure of acceptor–hydrogen complexes is a subject of fundamental and technological interest. To probe the interactions between hydrogen, the acceptor, and the surrounding host atoms, hydrostatic pressure may be applied over a wide range. Using infrared spectroscopy at liquid-helium temperatures, we have observed carbon and carbon–hydrogen local vibrational mode (LVMs) in InP at hydrostatic pressures as high as 5.5 GPa. For pressures beyond 4.5 GPa, the carbon–hydrogen mode was not observed, perhaps as a result of a transformation of the complex into a different configuration. The LVM arising from carbon substitutional impurities varies linearly with pressure, whereas the shift of the carbon–hydrogen mode has a positive curvature. Both of these observations are in qualitative agreement with the pressure dependence of LVMs in GaAs. While the substitutional carbon impurities show very similar pressure shifts in the two materials, the linear pressure coefficient of the carbon–hydrogen stretch mode in InP is nearly three times that in GaAs.

Pressure dependence of photoluminescence of ZnTe/Zn1−xCdxTe strained‐layer superlattice

AbstractPhotoluminescence (PL) spectra of ZnTe/Zn1−xCdxTe (x = 0.23) strained‐layer superlattice (SLS) samples were studied at hydrostatic pressures up to 4.8 GPa at room temperature. A sublinear pressure dependence of the PL peaks for both the heavy‐hole exciton and the donor–acceptor pair was observed. The first‐ and second‐order pressure coefficients of the PL peaks were obtained by a least‐squares fit. An analytical expression for the heavy‐hole exciton was also derived. The pressure coefficients obtained from the analytical expression are 104 meV GPa−1 and −4.44 meV GPa−2, in excellent agreement with the experimental values of 103.9 meV GPa−1 and −4.5 meV GPa−2. It is shown that the contribution to the linear pressure coefficient comes exclusively from the hydrostatic component. Both hydrostatic and biaxial terms are responsible for the second‐order pressure coefficient, with a negative contribution of −6.07 meV GPa−2 and a positive contribution of 1.63 meV GPa−2, respectively. Copyright © 2001 John Wiley & Sons, Ltd.

Pressure dependence of local vibrational modes in InP

Using infrared spectroscopy and a diamond-anvil cell, we have observed carbon and carbon-hydrogen local vibrational modes (LVM's) in InP at hydrostatic pressures as high as 5.5 GPa at liquid-helium temperatures. For pressures beyond 4.5 GPa, the carbon-hydrogen mode was not observed, perhaps as a result of a transformation of the complex into a different configuration. The LVM arising from carbon substitutional impurities varies linearly with pressure, whereas the shift of the carbon-hydrogen mode has a positive curvature. Both of these observations are in qualitative agreement with the pressure dependence of LVM's in GaAs. While the substitutional carbon impurities show very similar pressure shifts in the two materials, the linear pressure coefficient of the carbon-hydrogen stretch mode in InP is nearly three times that in GaAs. For all the measured modes, the Gr\"uneisen parameters increase with pressure.

Effects of hydrostatic pressure on Raman scattering in Ge quantum dots

Raman scattering under hydrostatic pressure is used to investigate the phonon modes of self-organized Ge quantum dots (QD's) grown by solid source molecular-beam epitaxy. The pressure dependence of Ge-Ge phonon and Si acoustical-overtone (2TA) modes are studied from 0 to 67 kbar at room temperature. Our results show that the overlapping spectra of the Ge-Ge phonon and Si 2TA modes occur around $303 {\mathrm{cm}}_{\ensuremath{-}1}$ at ambient pressure which can be resolved at relatively low pressure of about 3 kbar. The linear pressure coefficient of Ge-Ge phonon mode in QD's is found to be $0.29 {\mathrm{cm}}_{\ensuremath{-}1}/\mathrm{k}\mathrm{b}\mathrm{a}\mathrm{r},$ which is slightly smaller than the corresponding quantity in bulk Ge.

Dominant role of the piezoelectric field in the pressure behavior of InGaN/GaN quantum wells

We show that the emission characteristics of InGaN/GaN quantum wells under hydrostatic pressure are strongly influenced by the built-in piezoelectric field. The dominant role of the piezoelectric field is established from the dramatic increase of the photoluminescence decay time with pressure and the dependence of the linear pressure coefficient of the photoluminescence peak energy on Si doping in the barriers and excitation intensity. A nonlinear increase of the piezoelectric field with hydrostatic pressure determined from these experiments is explained as being due to a significant dependence of the InGaN piezoelectric constants with strain.

Effects of Confinement on the Coupling between Nitrogen and Band States in InGaAs1?xNx/GaAs (x ? 0.025) Structures: Pressure and Temperature Studies

We report photo-modulated reflectance studies under applied pressure and variable temperature, and related calculations, that probe the influence of N-related resonant anti-bonding states on the electronic structure of dilute III–N–V quantum wells (QWs). Three InyGa1—yAs1—xNx/GaAs multiple QW samples with N contents of 0–2.5% are investigated up to 85 kbar at 300 K, and for 300–10 K at 1 atm. While the temperature dependence is only minimally affected by the N content, the pressure shifts of the intersubband transition energies depend significantly on the percentage of N. The linear pressure coefficients are much smaller than those of the InGaAs band gap. A ten-band k · p model is in broad accord with the observed pressure dependence; the predicted non-linear shift is somewhat larger than measured.

Different character of the donor-acceptor pair-related 3.27 eV band and blue photoluminescence in Mg-doped GaN. Hydrostatic pressure studies

High-pressure measurements were employed to study mechanisms of (i) the 3.27 eV emission band (and phonon replicas) and (ii) 2.9--3.1 eV blue photoluminescence in different types of GaN:Mg samples. GaN layers with low Mg doping, grown on bulk GaN crystals and on sapphire substrates, were used to study the 3.27 eV uv band. This band results from donor-acceptor pair (DAP) recombination (with both impurities of shallow character). For high Mg doping resulting in blue light emission at about 3 eV, a bulk GaN crystal and a layer grown on sapphire were employed. For the all samples the pressure coefficient of the corresponding luminescence bands was measured at 77 K by means of the diamond anvil technique. The energy shift of the 3.27 eV band under hydrostatic pressure is used to determine the linear pressure coefficient characteristic for (shallow) DAP recombination. It has a value of about 35 meV/GPa, close to the pressure shift of the GaN band gap (about 40 meV/GPa). The 2.9--3.1 eV blue luminescence band exhibits much lower pressure coefficients of about 23 meV/GPa. It shows that the initial state of the blue emission has localized character in both types of GaN material and supplies a strong argument against models of radiative recombination in highly-Mg-doped/compensated GaN material that postulate an involvement of delocalized (shallow) electronic states as the initial states.

Effect of hydrostatic pressure on the characteristic parameters of Au/n-GaAs Schottky-barrier diodes

$\mathrm{Au}/n\ensuremath{-}\mathrm{GaAs}$ Schottky barrier diodes SBDs have been fabricated. Schottky diode parameters such as the ideality factor, the series resistance, and the Schottky barrier height (SBH), ${\ensuremath{\Phi}}_{b},$ have been measured as a function of hydrostatic pressure using the current-voltage $(I\ensuremath{-}V)$ technique. We have seen that the SBH has a linear pressure coefficient of 11.21 meV/kbar (=112.1 meV/GPa). Also, the series resistance value increases with increasing pressure. 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 valance-band maximum as a function of the pressure. That is, we have concluded that the experimental results is in agreement with the model that the pressure coefficient is caused by the pressure coefficient of the direct midgap level.

Reduction of the energy gap pressure coefficient of GaN due to the constraining presence of the sapphire substrate

We have performed a detailed investigation of the photoluminescence pressure dependence of heteroepitaxial GaN thin films on sapphire substrates. A comparison between as grown GaN on sapphire and free-standing GaN membranes, created using a laser assisted substrate liftoff process, revealed that the presence of the sapphire substrate leads to an energy gap pressure coefficient reduction of approximately 5%. This result agrees with the numerical simulations presented in this article. We established that the linear pressure coefficient of free-standing GaN is 41.4±0.2 meV/GPa, and that the deformation potential of the energy gap is −9.36±0.04 eV. Our results also suggest a new, lower value of the pressure derivative for the bulk modulus of GaN (B′=3.5).

High Pressure Luminescence of Zincblende and Wurtzite GaN

At hydrostatic pressures up to 12 GPa, photoluminescence experiments were performed simultaneously on zincblende (cubic) and wurtzite (hexagonal) gallium nitride. The linear band gap pressure coefficient of hexagonal GaN was determined to be (0.94 ± 0.04) meV/GPa larger than the one of cubic GaN.

Pressure study of structural properties and phonons in YBa{sub 2}Cu{sub 3}O{sub 7}

The pressure dependence of crystal structure and vibrational properties of YBa{sub 2}Cu{sub 3}O{sub 7} is investigated by first-principles calculations based on density functional theory. They have determined the optimized geometry for different unit cell volumes by relaxing the atoms to their force-free positions. The tendency of altered bond strength with pressure is consistent with experimental investigations and previous c-axis strain calculations. The pressure effect on the three A{sub 1g} phonon frequencies dominated by oxygen vibrations is in very good agreement with experimental investigations showing constant linear pressure coefficients d{omega}/dp over a wide range of pressure values.

Pressure Dependence of Band Gaps and Deep Centers in Chalcopyrite Semiconductors

We present a review of the effect of high pressure on the electronic, optical and defect properties in chalcopyrite semiconductors with the formula I–III–VI2. We show that the linear pressure coefficients of the band gap in these materials are not constant. While the bulk moduli of this family of semiconductors are rather constant, the dependence of the band gap on volume exhibits systematic trends when the group III cations are varied. We also found that there are deep center emissions whose pressure coefficients are larger than those of the band gap. We propose that the origin of these deep emissions are deep acceptors and their pressure coefficients can be understood in terms of p–d hybridization in the valence bands.

Pressure Dependence of Band Gaps and Deep Centers in Chalcopyrite Semiconductors

We present a review of the effect of high pressure on the electronic, optical and defect properties in chalcopyrite semiconductors with the formula I–III–VI2. We show that the linear pressure coefficients of the band gap in these materials are not constant. While the bulk moduli of this family of semiconductors are rather constant, the dependence of the band gap on volume exhibits systematic trends when the group III cations are varied. We also found that there are deep center emissions whose pressure coefficients are larger than those of the band gap. We propose that the origin of these deep emissions are deep acceptors and their pressure coefficients can be understood in terms of p–d hybridization in the valence bands.

Pressure dependence of the direct band gap in tetrahedral semiconductors

The sublinearity of the pressure dependence of the band gap of tetrahedral semiconductors is not well represented by the usual quadratic fit to pressure, nor by the linear fit to lattice constant, and this is responsible for much of the scatter in reported results. Linear fitting to the change in density eliminates this scatter. Thus high-pressure experimentation is shown to be much more accurate than is supposed. We propose that the linear pressure coefficient should be defined and reported as the rate of change of band gap with pressure at zero pressure, obtained using a suitable equation of state and performing a linear fit for the density deformation potential. Data for III-V and II-VI semiconductors are reassessed accordingly and recommended linear pressure coefficients are given.