Large excitation-power dependence of pressure coefficients of InxGa1-xN/InyGa1-yN quantum wells

Excitation-power dependence of hydrostatic pressure coefficients (dE/dP) of InxGa1-xN/InyGa1-yN multiple quantum wells is reported. When the excitation power increases from 1.0 to 33 mW, dE/dP increases from 26.9 to 33.8 meV/GPa, which is an increase by 25%. A saturation behavior of dE/dP with the excitation power is observed. The increment of dE/dP with increasing carrier density is explained by an reduction of the internal piezoelectric field due to an efficient screening effect of the free carriers on the field.

An investigation on the pressure-induced phase transition of nanocrystalline ZnS

An in situ energy dispersive x-ray diffraction study on nanocrystalline ZnS was carried out under high pressure up to 30.8 GPa by using a diamond anvil cell. The phase transition from the wurtzite to the zinc-blende structure occurred at 11.5 GPa, and another obvious transition to a new phase with rock-salt structure also appeared at 16.0 GPa-which was higher than the value for the bulk material. The bulk modulus and the pressure derivative of nanocrystalline ZnS were derived by fitting the Birch-Murnaghan equation. The resulting modulus was higher than that of the corresponding bulk material, indicating that the nanomaterial has higher hardness than the bulk material.

Pressure behavior of Te isoelectronic centers in ZnS : Te

ZnS:Te epilayers with Te concentration from 0.5% to 3.1% were studied by photoluminescence under hydrostatic pressure at 15 K. Two emission bands related to the isolated Te-1 and Te-2 pair isoelectronic centers were observed in the samples with Te concentrations of 0.5% and 0.65%. For the samples with Te concentrations of 1.4% and 3.1%, only the Te-2-related peak was observed. The pressure coefficients of all the Te-1-related bands were found to be unexpectedly much larger than that of the ZnS band gap. The pressure coefficients for all the Te-2-related bands are, however, rather smaller than that of ZnS band gap as usually observed. Analysis based on a Koster-Slater model indicates that an increase of the valence bandwidth with pressure is the main reason for the faster pressure shift of the Te-1 centers, and the huge difference in the pressure behavior of the Te-1 and Te-2 centers is due mainly to the difference in the pressure-induced enhancement of the impurity potential on the Te-1 and Te-2 centers. (C) 2002 American Institute of Physics.

Photoluminescence from ZnS1-xTex alloys under hydrostatic pressure

ZnS1-xTex (0.02less than or equal toxless than or equal to0.3) alloys are studied by photoluminescence under hydrostatic pressure at room temperature. Only a wide emission band is observed for each sample. Its peak energy is much lower than the corresponding band gap of alloys. These bands are ascribed to the radiative annihilation of excitons bound at Te-n(ngreater than or equal to2) isoelectronic centers. The pressure coefficients of the emission bands are smaller than those of alloy band gaps from 48% to 7%. The difference of the pressure coefficient of the emission band and the band gap increases when the binding energy of Te-n centers decreases. It seems contrary to our expectation and needs further analysis. The integrated intensities of emission bands decrease with increasing pressure due to the decreasing of the absorption coefficient associated with the Te-n centers under pressure. According to this model the Stokes shifts between the emission and absorption bands of the Te-n centers are calculated, which decrease with the increasing Te composition in alloys.

Temperature and pressure dependences of the Mn2+ and donor-acceptor emissions in ZnS : Mn2+ nanoparticles

Temperature and pressure dependent measurements have been performed on 3.5 nm ZnS:Mn2+ nanoparticles. As temperature increases, the donor-acceptor (DA) emission of ZnS:Mn2+ nanoparticles at 440 nm shifts to longer wavelengths while the Mn2+ emission (T-4(1)-(6)A(1)) shifts to shorter wavelengths. Both the DA and Mn2+ emission intensities decrease with temperature with the intensity decrease of the DA emission being much more pronounced. The intensity decreases are fit well with the theory of thermal quenching. As pressure increases, the Mn2+ emission shifts to longer wavelengths while the DA emission wavelength remains almost constant. The pressure coefficient of the DA emission in ZnS:Mn2+ nanoparticles is approximately -3.2 meV/GPa, which is significantly smaller than that measured for bulk materials. The relatively weak pressure dependence of the DA emission is attributed to the increase of the binding energies and the localization of the defect wave functions in nanoparticles. The pressure coefficient of Mn2+ emission in ZnS:Mn2+ nanoparticles is roughly -34.3 meV/GPa, consistent with crystal field theory. The results indicate that the energy transfer from the ZnS host to Mn2+ ions is mainly from the recombination of carriers localized at Mn2+ ions. (C) 2002 American Institute of Physics.

Tuning of exciton-photon coupling in a planar semiconductor microcavity by applying hydrostatic pressure

By means of hydrostatic pressure tuning, we have observed the strong-coupling exciton-polariton mode in a planar microcavity with an InGaAs/GaAs quantum well embedded in it, over a pressure range from 0.37 to 0.41 GPa. The experimental data can be fitted very well to a corresponding theoretical formula with a unique value of the vacuum Rabi splitting equal to 6.0 meV. A comparison between pressure tuning and other tuning methods is made as regards to what extent the intrinsic features of the exciton and cavity will be influenced during the tuning procedure.

Pressure-induced phase transition of nanocrystalline zinc sulfide

In situ energy dispersive X-ray diffraction measurements on nanocrystalline zinc sulfide have been performed by using diamond anvil cell with synchrotron radiation. There is a phase transition which the ultimate structure is rocksalt when the pressure is up to 16.0GPa. Comparing the structure of body materials, the pressure of the phase transition of nano zinc sulfide is high. We fit the: Birch-Murnaghan equation of state and obtained its ambient pressure bulk modulus and its pressure derivative. The bulk modulus of nanocrystalline zinc sulfide is higher than that of body materials, it indicate that the rigidity of nanocrystalline zinc sulfide is high.

Very low-pressure VLP-CVD growth of high quality gamma-Al2O3 films on silicon by multi-step process

gamma-Al2O3 films were grown on Si (10 0) substrates using the sources of TMA (AI(CH3)(3)) and O-2 by very low-pressure chemical vapor deposition. The effects of temperature control on the crystalline quality, surface morphology, uniformity and dielectricity were investigated. It has been found that the,gamma-Al2O3 film prepared at a temperature of 1000degreesC has a good crystalline quality, but the surface morphology, uniformity and dielectricity were poor due to the etching reaction between 0, and Si substrate in the initial growth stage. However, under a temperature-varied multi-step process the properties Of gamma-Al2O3 film were improved. The films have a mirror-like surface and the dielectricity was superior to that grown under a single-step process. The uniformity of gamma-Al2O3 films for 2-in epi-wafer was <5%, it is better than that disclosed elsewhere. In order to improve the crystalline quality, the gamma-Al2O3 films were annealed for I h in O-2 atmosphere. (C) 2002 Elsevier Science B.V. All rights reserved.

Photoluminescence of Te isoelectronic traps in ZnSeTe/ZnSe quantum wells under hydrostatic pressure

The photoluminescence (PL) of ZnSe0.92TeD0.08/ZnSe superlattice quantum wells at 77K under hydrostatic pressure up to 7.8 GPa was studied. Strong PL peaks from excitons trapped in isoelectronic traps in ZnSe0.92Te0.08 were observed. It was found that the pressure coefficients of the PL, peaks from Te traps are about half of that of ZnSe. It demonstrates the localized characteristic of the potential of Te isoelectronic. traps. The excitons transition between Te traps in ZnSe1 Te-- x(x) and (CdSe)(1) /(ZnSe)(3) superlattice was also investigated.

ECR plasma in growth of cubic GaN by low pressure MOCVD

To heteroepitaxally grow the crystalline cubic-GaN (c-GaN) film on the substrates with large lattice mismatch is basically important for fabricating the blue or ultraviolet laser diodes based on cubic group III nitride materials. We have obtained the crystalline c-GaN film and the heteroepitaxial interface between c-Gan and GaAs (001) substrate by the ECR Plasma-Assisted Metal Organic Chemical Vapor Deposition (PA-MOCVD) under low-pressure and low-temperature (similar to600degreesC) on a homemade ECR-plasma Semiconductor Processing Device (ESPD). In order to decrease the growth temperature, the ECR plasma source was adopted as the activated nitrogen source, therefore the working pressure of MOCVD was decreased down to the region less than 1 Pa. To eliminate the damages from energetic ions of current plasma source, a Multi-cusp cavity,coupling ECR Plasma source (MEP) was selected to use in our experiment. To decrease the strain and dislocations induced from the large lattice mismatch between c-GaN and GaAs substrate, the plasma pretreatment procedure i.e., the initial growth technique was investigated The experiment arrangements, the characteristics of plasma and the growth procedure, the characteristics on-GaN film and interface between c-GaN and GaAs(001), and the roles of ECR plasma are described in this contribution.

Hydrostatic pressure effect on photoluminescence from a GaN0.015As0.985/GaAs quantum well

Photoluminescence from a GaN0.015As0.985/GaAs quantum well has been measured at 15 K under hydrostatic pressure up to 9 GPa. Both the emissions from the GaNAs well and GaAs barrier are observed. The GaNAs-related peak shows a much weaker pressure dependence compared to that of the GaAs band gap. A group of new peaks appear in the spectra when the pressure is beyond 2.5 GPa, which is attributed to the emissions from the N isoelectronic traps in GaAs. The pressure dependence of the GaNAs-related peaks was calculated using the two-level model with the measured pressure coefficients of the GaAs band gap and N level as fitting parameters. It is found that the calculated results deviate seriously from the experimental data. An increasing of the emission intensity and the linewidth of the GaNAs-related peaks was also observed and briefly discussed. (C) 2001 American Institute of Physics.

Optical spectra of CdSe nanocrystals under hydrostatic pressure

Optical spectra of CdSe nanocrystals are measured at room temperature under pressure ranging from 0 to 5.2 GPa. The exciton energies shift linearly with pressure below 5.2 GPa. The pressure coefficient is 27 meV GPa(-1) for small CdSe nanocrystals with the radius of 2.4 nm. With the approximation of a rigid-atomic pseudopotential, the pressure coefficients of the energy band are calculated. By using the hole effective-mass Hamiltonian for the semiconductors with wurtzite structure under various pressures, we study the exciton states and optical spectra for CdSe nanocrystals under hydrostatic pressure in detail. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbit coupling on the hole states are investigated. The Coulomb interaction of the exciton states is also taken into account. It is found that the theoretical results are in good agreement with the experimental values.

Pressure dependence of Mn2+ fluorescence in ZnS : Mn2+ nanoparticles

The photoluminescence of Mn2+ in ZnS:Mn2+ nanoparticles with an average size of 4.5 nm has been measured under hydrostatic pressure from 0 to 6 GPa. The emission position is red-shifted at a rate of -33.3+/-0.6meV/GPa, which is in good agreement with the calculated value of -30.4meV/GPa using the crystal field theory. (C) 2000 Elsevier Science B.V. All rights reserved.

Pressure behaviour of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots with multi-modal distribution in size

The pressure behaviour of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots (QDs) has been studied at 15 K in the pressure range of 0-1.3 GPa. The atomic force microscopy image shows that the QDs have a multi-modal distribution in size. Three emission peaks were observed in the photoluminescence (PL) spectra, corresponding to the different QD families. The measured pressure coefficients are 82, 93 and 98 meV GPa(-1) for QDs with average lateral size of 26, 52 and 62 nm, respectively. The pressure coefficient of small QDs is about 17% smaller than that of bulk In0.55Al0.45As An envelope-function calculation was used to analyse the effect of pressure-induced change of barrier height, effective mass and dot size on the pressure coefficients of QDs. The Gamma-X state mixing was also included in the evaluation of the reduction of the pressure coefficients. The results indicate that both the pressure-induced increase of effective mass and Gamma-X mixing respond to the decrease of pressure coefficients, and the Gamma-X mixing is more important for small dots. The calculated Gamma-X interaction potentials are 15 and 10 meV for QDs with lateral size of 26 and 52 nm, respectively. A type-II alignment for the X conduction band is suggested according to the pressure dependence of the PL intensities. The valence-band offset was then estimated as 0.15 +/- 0.02.