Search

In this paper, ab initio calculations are used to determine polarization difference in zinc blende (ZB), hexagonal (H) and wurtzite (WZ) AlN-GaN and GaN-InN superlattices. It is shown that a polarization difference exists between WZ nitride compounds, while for H and ZB lattices the results are consistent with zero polarization difference. It is therefore proven that the difference in Berry phase spontaneous polarization for bulk nitrides (AlN, GaN and InN) obtained by Bernardini et al. and Dreyer et al. was not caused by the different reference phase. These models provided absolute values of the polarization that differed by more than one order of magnitude for the same material, but they provided similar polarization differences between binary compounds, which agree also with our ab initio calculations. In multi-quantum wells (MQWs), the electric fields are generated by the well-barrier polarization difference, hence, the calculated electric fields are similar for the three models, both for GaN/AlN and InN/GaN structures. Including piezoelectric effect, which can account for 50% of the total polarization difference, these theoretical data are in satisfactory agreement with photoluminescence measurements in GaN/AlN MQWs. Therefore, the three models considered above are equivalent in the treatment of III-nitride MQWs and can be equally used for the description of the electric properties of active layers in nitride-based optoelectronic devices.

The light emission properties of color centers emitting in 3.3-4 eV region are investigated for hydrostatic pressures ranging up to 5GPa at liquid helium temperature. The light emission energy decreases with pressure less sensitively than the bandgap. This behavior at variance from the shift of the bandgap is typical of deep traps. Interestingly, hydrostatic pressure reveals the existence of levels that vary differently under pressure (smaller increase of the emission wavelength compared to the rest of the levels in this energy region or even decrease of it) with pressure. This discovery enriches the physics of the color centers operating in the UV in hBN., Comment: 16 pages, 3 figures

International audience; Quantum well systems based on semiconductors with the wurtzite crystalline structure have found widespread applications in photonics and optoelectronic devices, such as light-emitting diodes, laser diodes, or single-photon emitters. In these structures, the radiative recombination processes can be affected by (i) the presence of strain and polarization-induced electric fields, (ii) quantum well thickness fluctuations and blurring of a well–barrier interface, and (iii) the presence of dislocations and native point defects (intentional and unintentional impurities). A separate investigation of these phenomena is not straightforward since they give rise to similar effects, such as a decrease of luminescence efficiency and decay rate, enhancement of the Stokes shift, and strong blueshift of the emission with increasing pump intensity. In this Perspective article, we review the usefulness of measurements of the quantum well luminescence as a function of the hydrostatic pressure for both scientific research and the development of light-emitting technologies. The results presented here show that high-pressure investigations combined with ab initio calculations can identify the nature of optical transitions and the main physical factors affecting the radiative efficiency in quantum well systems. Finally, we will discuss an outlook to the further possibilities to gain new knowledge about the nature of recombination processes in quantum wells using high-pressure spectroscopy.

Studies of internal electric fields in GaN/AlxGa1-xN (3 nm/4 nm) multi-quantum-wells (MQWs) with x = 0.25, 0.5, and 1 are presented. The structures were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction, transmission electron microscopy, and secondary ion mass spectroscopy. Optical properties of these structures were investigated by ambient and high-pressure photoluminescence (PL) measurements. They were strongly affected by polarization-induced electric fields giving rise to a quantum confined Stark effect, depending on the composition of the AlGaN barrier. The optical emission energy redshifts by > 100 meV when the Al content in the barrier increases from x = 0.25 up to x = 1, while the pressure coefficients of the PL energy are significantly reduced in comparison with bulk GaN. The transition energies and their pressure dependencies are modelled for tetragonally strained structures with the same geometry using a full tensorial representation of the strain in the MQWs under external pressure. The same MQWs are also simulated using density functional theory calculations. Additionally, influence of blurring of well-barrier interface on transition energy has been modelled. A good agreement between experimental results and theoretical analysis indicates that nonlinear effects induced by the tetragonal strain due to the lattice mismatch between the substrates and the polar MQWs are responsible for a drastic decrease of the pressure coefficients of PL energy, and that these effects are well described by ab initio calculation procedures. Our results reflect the role of composition and geometry on the basic optical properties of nitride quantum wells.

Results of X-ray diffraction and low temperature optical absorption measurements of cerium doped gadolinium gallium garnet single crystals and epitaxial layers are reported. In the region of intra-configurational 4f–4f transitions the spectra of the bulk crystals exhibit the signatures of several different Ce 3+ related centers. Apart from the dominant center, associated with Ce substituting gadolinium, at least three other centers are found, some of them attributed to the so-called antisite locations of rare-earth ions in the garnet host, i.e., in the Ga positions. X-ray diffraction data prove lattice expansion of bulk GGG crystals due to the presence of rare-earth antisites. The concentration of the additional Ce-related centers in epitaxial layers is much lower than in the bulk crystals. However, the Ce-doped layers incorporate a large amount of Pb from flux, which is the most probable source of nonradiative quenching of Ce luminescence, not observed in crystals grown by the Czochralski method.

Spontaneous and piezoelectric polarization in the nitrides is analyzed. The slab model was designed and proved to be appropriate to obtain the spontaneous polarization in AlN, GaN and InN. The spontaneous polarization and polarization related electric fields in AlN, GaN and InN were determined using DFT slab calculations. The procedure generates single value of spontaneous polarization in the nitrides. It was shown that Berry phase polarization may be applied to determination of spontaneous polarization by appropriate addition of polarization induced electric fields. The electric fields obtained from slab model are consistent with the Berry phase results of Bernardini et al. The obtained spontaneous polarization values are: 8.69*10-3 C/m2, 1.88*10-3 C/m2, and 1.96*10-3 C/m2 for AlN, GaN and InN respectively. The related Berry phase polarization values are 8.69*10-2 C/m2, 1.92*10-2 C/m2, and 2.86*10-2 C/m2, for these three compounds, respectively. The GaN/AlN multiquantum wells (MQWs) were simulated using ab intio calculations. The obtained electric fields are in good agreement with those derived from bulk polarization values. GaN/AlN MQWs structures, obtained by MBE growth were characterized by TEM and X-ray measurements. Time dependent photoluminescence measurements were used to determine optical transition energies in these structures. The PL obtained energies are in good agreement with ab initio data confirming overall agreement between theoretical and experimental data.

Low temperature, infrared transmission spectra of lutetium aluminum garnet (LuAG) bulk crystals and epitaxial layers doped with Ce are presented. In the region of intra-configurational 4f–4f transitions the spectra of the bulk LuAG crystal exhibit the signatures of several different Ce3+ related centers. Apart from the dominant center, associated with Ce substituting lutetium, at least six other centers are found, some of them attributed to so-called antisite locations of rare-earth ions in the garnet host, i.e., ions in the Al positions. X-ray diffraction data prove lattice expansion of bulk LuAG crystals due presence of rare-earth antisites.

Luminescence of CdTe quantum dots embedded in ZnTe is quenched at pressure of about 4.5 GPa in the high-pressure experiments. This pressure-induced quenching is attributed to the “zinc-blende–cinnabar” phase transition in CdTe, which was confirmed by the first-principles calculations. Theoretical analysis of the pressure at which the phase transition occurs for CdTe was performed using the CASTEP module of Materials Studio package with both generalized gradient approximation (GGA) and local density approximation (LDA). The calculated phase transition pressures are equal to about 4.4 GPa and 2.6 GPa, according to the GGA and LDA calculations, respectively, which is in a good agreement with the experimental value. Theoretically estimated value of the pressure coefficient of the band-gap luminescence in zinc-blende structure is in very good agreement with that recently measured in the QDs structures. The calculated Debye temperature, elastic constants and specific heat capacity for the zinc-blend structure agree well with the experimental data; the data for the cinnabar phase are reported here for the first time to the best of the authors' knowledge.

Studies of ambient-pressure and high-pressure behavior of photoluminescence (PL) for series of InxAl1-xN layers are presented. The measured evolution of PL energy (E-PL) with x is characterized by a clear decrease of E-PL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band-gap energy changes E-G with x. However, values of E-PL are clearly lower than E-G, for 0

The results of comprehensive theoretical and experimental study of binary GaN/AlN multi-quantum well (MQW) systems oriented along polar c-direction of their wurtzite structure are presented. A series of structures with quantum wells and barriers of various thicknesses were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction and transmission electron microscopy. It was shown that in general the structures of good quality were obtained, with the defect density decreasing with increasing quantum well thickness. The optical transition energies in these structures were investigated comparing experimental measurements with ab initio calculations of the entire GaN/AlN MQW structure depending on the QW widths and strains, allowing for direct determination of the energies of optical transitions and the electric fields in wells/barriers by electric potential double averaging procedure. Photoluminescence (PL) measurements revealed that the emission efficiency as well as the shape o...

The influence of the hydrostatic pressure on the radiative intra-configurational 4f→4f transitions of several Yb3+ ions doped dielectrics and semiconductors with different energy gaps and crystal structures is presented. A thorough analysis of ambient pressure spectra and the pressure behavior of the Yb3+ luminescence lines in InP, GaN, LiNbO3, YPO4, GdPO4 and Gd3Ga5O12 allowed to determine the ambient pressure and pressure dependence of the Yb3+ energy level positions in crystal fields of different symmetries. The comparison of the Yb3+ luminescence decay times in different crystal hosts and their pressure dependencies have also been carried out. The results revealed the significant effect of interaction between the band states and the dopant states, as well as the local symmetry of ytterbium dopant on its radiative transition rate, a relevant parameter when considering the potential applications of ytterbium as optically active ion in different crystal hosts.

Luminescence properties of CdTe and CdSe quantum dots have been studied under high hydrostatic pressure. The luminescence pressure coefficients of the II–VI quantum dots appear to be very similar to the pressure coefficients of the band-gap of bulk CdTe and CdSe, respectively. In contrary to that, the luminescence pressure coefficients of the III–V quantum dots are significantly lower than pressure coefficients of energy gaps of the appropriate dot materials. The discrepancy can be explained by the theoretical model, which takes into account effects of strain on pressure coefficients in thin strained layers. The experimentally observed pressure-induced quenching of the QDs luminescence is attributed to the “zinc-blende–cinnabar” phase transition in CdTe QDs and to the “zinc-blende–rock-salt” phase transition in CdSe QDs.

The single-phase perovskite structure of the Ba0.95Pb0.05TiO3+0.1%Co2O3 ceramics was confirmed by the X-ray diffraction method. Microstructural studies revealed that the samples were of good quality and chemically homogeneous. The thermal behaviour of ceramics was studied using the in situ high-temperature X-ray synchrotron powder diffraction investigation. The energy gap of about 3.2 eV was estimated using a reflectance spectroscopy. Measurements showed the influence of Pb and Co on the character of phase transition in the BaTiO3 structure. The results were compared to the ones obtained for pure BaTiO3.

Pressure-dependent transport and photoluminescence (PL) measurements were performed on InN. In the transport measurements on the sample with a low electron concentration (ne lower than about 1018 cm–3) a Localized Donor State (LDS) was seen to cross the Fermi level at elevated pressure magnitude. It was accompanied by the transfer of electrons from the Conduction Band to the LDS and a significant increase in electron mobility. In a sample with a higher concentration of electrons no transport anomaly was observed. Application of pressure helps to confirm that the disputed photoluminescence mechanism in InN consists in band-to-band recombination. No evidence of the participation of this LDS in radiative recombination processes was found in the pressure-dependent PL measurements. On the contrary, the dependence of the PL intensity on excitation power suggested the involvement of the LDS in nonradiative recombination processes. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reported hydrostatic pressure measurements of Hall electron concentration and Hall mobility of InN suggest the presence of a localized donor state in the conduction band. Since localized-state emission is rather inefficient, the involvement of such a localized state would be highly inadvantageous for device applications. Pressure-dependent photoluminescence (PL) experiments of InN epilayers show PL pressure coefficients dE E /dp of 30.7 +2.0 meV/GPa and 21.4 ± 3.0 meV/GPa for InN epilayers with electron concentration of ∼5 x 10 18 cm -3 and ∼5 x 10 17 cm -3 , respectively. These values are comparable with the InN band gap pressure coefficient dE G /dp of ∼25 -30 meV/GPa (indicating regular band-to-band emission), and differ significantly from the value of 5-10 meV/GPa that should be expected in the case of localized-state emission. Therefore, in the presented experiments no evidence of the involvement of localized donor states in radiative recombination processes in InN is found.

Detailed investigations of the photoluminescence properties of Mn4+ (3d3) ions in YAlO3 have been performed in the temperature range 10–600 K. The luminescence of Mn4+ ions due to the transition consists of two zero-phonon lines (R lines) at 691.3 and 692.7 nm, which became visible only at low temperature, and their vibronic sidebands that cover the range of 660–740 nm. The thermal quenching of the luminescence intensity due to the non-radiative decay occurs at temperatures above 420 K. The temperature dependence of the luminescence decay time shows a quasi-linear decrease from τ = 4.9 to 1.6 ms in the temperature range from 90 to 420 K (with a temperature coefficient −0.01 ms K−1) that makes YAlO3:Mn a potentially good phosphor for a fibre optic fluorescence thermometer in this temperature range. The high-pressure low-temperature luminescence measurements in a diamond-anvil cell reveal similar pressure coefficients for Mn4+ and Cr3+ dopant ions in YAlO3, equal to 1.16 cm−1 kbar−1 and 1.08 cm−1 kbar−1, respectively.

Localization phenomena related to the fluctuating band potential profile of the InGaN alloy were investigated via hydrostatic pressure dependent measurements of luminescence of an InGaN quantum structure. We examine the suitability of the hydrostatic pressure coefficient as a tool for evaluation of the degree of localization of recombining carriers in the InGaN alloy. This is done by experimental investigation of the correlation between the pressure coefficient of the luminescence peak and the Stokes shift between luminescence and the absorption edge, which constitutes a measure of the degree of localization of recombining carriers in the InGaN alloy.

We studied influence of annealing in water vapour and proton exchange on high-pressure low-temperature luminescence spectra of near stoichiometric LN : Cr, MgO crystals, containing both Cr Li and Cr Nb centers by using diamond-anvil cell technique. The observed changes of the spectra of Cr 3+ ions are associated with the larger inhomogeneous broadening and increase of the splitting of the 2 E level of the so called δ Cr Nb center. This shows that neither increase of hydrogen concentration in LN by almost two orders of magnitude (from 10 18 to almost 10 20 cm -3 ) nor proton-exchange procedure do not create any new optically active Cr 3+ centers in those crystals. This does not support the hypothesis about the close compensation of Cr 3+ Nb sites in LiNbO 3 crystals by hydrogen ions.

The model based on Harrison theory of bonding is used to explain quantitatively the nephelauxetic effect in LiNbO 3 and Y 3 Al 5 O 12 crystals doped with chromium, studied by high-hydrostatic pressure spectroscopy. The model uses only one adjustable parameter and can also be used for other dopant–ligands systems in different compounds. We show that not only ligands, but also the second neighbors of the dopant, influence the nephelauxetic effect.

Effective acceptor doping of wide-band-gap semiconductors is still an outstanding problem. Beryllium has been suggested as a shallow acceptor in GaN, but despite sporadic announcements, Be-induced p-type doping has never been practically realized. Be-doped GaN possesses two luminescence bands; one at 3.38 eV and a second near 2.2 eV at an energy close to that of the parasitic yellow luminescence often found in undoped GaN crystals. We have performed high hydrostatic pressure studies of bulk, Be-doped gallium nitride crystals using the diamond anvil cell technique. We observed a splitting of the yellow luminescence line under hydrostatic pressure into two components, one which is strongly dependent on applied pressure and another whose pressure dependence is more modest. Together with hybrid functional calculations, we attribute the strongly-varying component to the beryllium–oxygen complex. The second component of the yellow luminescence possesses very similar pressure behavior to the yellow luminescence observed in undoped samples grown by the same method, behavior which we find consistent with the CN acceptor. At higher pressure, we observe the vanishing of yellow luminescence and a rapid increase in luminescence intensity of the UV line. We explain this as the pressure-induced transformation of the Be–O complex from a highly localized state with large lattice relaxation to a delocalized state with limited lattice relaxation.

Electronic band structure calculations of InAlN and InGaN under pressure are presented for two different arrangements of the In atoms, uniform and clustered. The band gap pressure coefficients exhibit strong bowing, and the effect is especially large when indium atoms are clustered. The theoretical results are compared with the results of photoluminescence measurements performed at high hydrostatic pressures on InAlN and InGaN quasi-bulk epilayers. We discuss the modification of the uppermost valence band due to formation of In clusters which, together with the related lattice relaxations, may be a possible source of the observed phenomena.

Results of EPR and optical studies of chromium ions substituting niobium that undergoes photo-bleaching in LiNbO3:Cr,MgO crystals are reported. Two-component kinetics of photo-bleaching are observed in EPR and optical measurements. The different CrNb centers exhibit very similar EPR signals in lithium niobate crystals. The possible mechanisms of different photo-bleaching kinetics are proposed.

Low temperature, infrared absorption spectra of gadolinium gallium garnet crystals doped with Ce are presented. In the region of intraconfigurational 4$f$-4$f$ transitions the spectra exhibit existence of at least two different, major Ce${}^{3+}$ related centers in the GGG crystals and also some other centers at lower concentration. The spectrum of 4$f$-4$f$ intrashell transitions of Ce${}^{3+}$ ions extends up to about 3700 cm${}^{\ensuremath{-}1}$ due to the large splitting of the ${}^{2}$${F}_{7/2}$ excited state. In the visible region the absorption spectrum shows influence of symmetry-related selection rules. The absorption coefficient changes in the region of 4${f}^{1}$-5${d}^{1}$ transitions due to thermal population of the second level, belonging to the ${}^{2}$${F}_{5/2}$ ground state. This suggests that the symmetry of the site occupied by Ce${}^{3+}$ ions, which substitute Gd${}^{3+}$, is higher than ${D}_{2}$ expected for garnet hosts.

High-pressure and time-resolved studies of the optical emission from n-type doped GaN/AlN multi-quantum-wells (MQWs) with various well thicknesses are analysed in comparison with ab initio calculations of the electronic (band structure, density of states) and optical (emission energies and their pressure derivatives, oscillator strength) properties. The optical properties of GaN/AlN MQWs are strongly affected by quantum confinement and polarization-induced electric fields. Thus, the photoluminescence (PL) peak energy decreases by over 1 eV with quantum well (QW) thicknesses increasing from 1 to 6 nm. Furthermore, the respective PL decay times increased from about 1 ns up to 10 μs, due to the strong built-in electric field. It was also shown that the band gap pressure coefficients are significantly reduced in MQWs as compared to bulk AlN and GaN crystals. Such coefficients are strongly dependent on the geometric factors such as the thickness of the wells and barriers. The transition energies, their oscilla...

We used high hydrostatic pressure to perform photoluminescence measurements on polar ZnO/ZnMgO quantum well structures. Our structure oriented along the c-direction (polar direction) was grown by plasma-assisted molecular beam epitaxy on a-plane sapphire. Due to the intrinsic electric field, which exists in polar wurtzite structure at ambient pressure, we observed a red shift of the emission related to the quantum-confined Stark effect. In the high hydrostatic pressure experiment, we observed a strong decrease of the quantum well pressure coefficients with increased thickness of the quantum wells. Generally, a narrower quantum well gave a higher pressure coefficient, closer to the band-gap pressure coefficient of bulk material 20 meV/GPa for ZnO, while for wider quantum wells it is much lower. We observed a pressure coefficient of 19.4 meV/GPa for a 1.5 nm quantum well, while for an 8 nm quantum well the pressure coefficient was equal to 8.9 meV/GPa only. This is explained by taking into account the press...

It is a well-established experimental fact that exciton-phonon coupling is very efficient in ZnO. The intensities of the phonon-replicas in ZnO/(ZnMg)O quantum structures strongly depend on the internal electric field. We performed high-pressure measurements on the single ZnO/(ZnMg)O quantum well. We observed a strong increase of the intensity of the phonon-replicas relative to the zero phonon line. In our opinion this effect is related to pressure induced increase of the strain in quantum structure. As a consequence, an increase of the piezoelectric component of the electric field is observed which leads to an increase of the intensity of the phonon-replicas.

Studies of the spectroscopic properties of Ce${}^{3+}$ dopant in bulk Gd${}_{3}$Ga${}_{5}$O${}_{12}$:Ce crystal under pressure are presented. In spite of strong intershell $4f\ensuremath{\rightarrow}5d$ absorption bands at ambient pressure, the cerium luminescence in Gd${}_{3}$Ga${}_{5}$O${}_{12}$ is entirely quenched even at low temperature. It has been shown that applying pressure allows for recovery of the $5d\ensuremath{\rightarrow}4f$ radiative transitions. Further increase of pressure improves the emission efficiency. This effect is analyzed in terms of two possible phenomena: (i) by pressure-induced electronic crossover of the excited 5$d$ energy level of the Ce${}^{3+}$ with the conduction band bottom of the host crystal, and (ii) by decrease of electron-lattice coupling with increasing pressure, resulting in reduction of the Stokes shift and nonradiative transitions between the low vibrational levels of the 5$d$ state and high vibrational levels of the ground 4$f$ state. The results of high-pressure absorption and luminescence measurements point out that the ambient-pressure luminescence quenching is caused by the donor-like charge transfer processes due to the resonant location of the Ce${}^{3+}$ 5$d$ electronic levels with respect to the host conduction band. In such a situation, the ionization of Ce${}^{3+}$ to Ce${}^{4+}$ occurs, accompanied by large lattice relaxation, which enables the nonradiative recombination to the Ce 4$f$ state. The pressure-induced approach of the conduction-band bottom of the host crystal by the excited 5$d$ energy level of the Ce${}^{3+}$ results in mixing between conduction band states and the 5$d$ state, and the broadband luminescence from this mixed state is observed at pressures from 30 up to about 60 kbar. Then, for pressures exceeding 60 kbar, this luminescence is replaced by the classic ${\mathrm{Ce}}^{3+}\phantom{\rule{0.16em}{0ex}}5d\ensuremath{\rightarrow}4f$ transitions. Using a simple anticrossing model and configuration coordinate model in the appropriate pressure range, quantitative description of the system has been carried out, and the pressure-induced decrease of electron-lattice coupling has been shown.

A model, based on the Harrison theory of bonding is developed for quantitative explanation of the nephelauxetic effect, i.e., a reduction of interelectronic crystal field Racah repulsion parameters B and C due to the covalency of bonds. The results are used for a description of the effect in chromium-doped lithium niobate crystals. The model can be also applied to other systems.

The optical spectroscopy measurements of gadolinium gallium garnet (GGG) crystals doped with Yb show evidence of the presence of non-equivalent optical centers with very similar radiative decay rates. The energy level schemes of those centers have been determined on the basis of optical absorption, luminescence and Raman experiments. Crystal field fitting resulted in two sets of slightly different crystal field parameters for two non-equivalent Yb centers. Both sets of parameters describe perfectly the experimentally detected Y b3 + energy levels. Correlation between systematic trends in the experimental energy level schemes and crystal field parameters is discussed.

The pressure-induced changes in the electronic band structures of In-containing nitride alloys, InxGa1-xN and InxAl1-xN are examined experimentally as well as by ab initio calculations. It is found that the band gap pressure coefficients, dEg/dp, exhibit very large bowing with x, and calculations with simulation of clustered distributions of the In atoms over the cation sites show a strong enhancement of this effect. This relates well to the experimental data obtained from pressure dependent photoluminescence measurements for InxGa1-xN and InxAl1-xN layers, performed in this work, and combined with existing data for InxGa1-xN layers. We discuss possible explanations of the anomalously large magnitude of the dEg/dp bowing in these nitride alloys.

The results of high-pressure low-temperature optical measurements in diamond-anvil cell of bulk indium phosphide crystals doped with Yb are reported. The observed initial increase in energy of $f\text{\ensuremath{-}}f$ intrashell transitions between the ${^{2}F}_{5/2}$ excited and ${^{2}F}_{7/2}$ ground levels at pressure up to 6 GPa is interpreted as a result of a linear increase in the spin-orbit parameter of ${\text{Yb}}^{3+}$ ions with pressure coefficient equal to $0.0059\text{ }{\text{cm}}^{\ensuremath{-}1}/\text{GPa}$ due to the covalency effects and mixing of the ${\text{Yb}}^{3+}$ and phosphorus wave functions. Above 6 GPa the pressure dependences of the peak positions of all the luminescence lines flatten out and the values of pressure coefficients strongly decrease. A tentative explanation is provided in terms of the $f$ levels of ${\text{Yb}}^{3+}$ approaching of the top of the valence band of InP crystal host, which limits further increase in $f\text{\ensuremath{-}}f$ intrashell transitions energies above 6 GPa pressure.

The results of high-pressure luminescence measurements of congruent LiNbO 3 : Mg,Cr(0.2%) crystals doped with two concentrations of magnesium (2% and 5.5%) reveal new information about the electronic structure of different Cr 3+ centers in this material. Pressure dependencies of the luminescence decays of major Cr 3+ centers have been analyzed using seven- level model. The results of the analysis allow to establish the energy differences between the 4 T 2 and 2 E excited levels for these centers. No R-lines luminescence, associated with the additional center, that is present in crystals doped with 5.5% of magnesium, has been observed even at a pressure of 154 kbar.

High quality GaN/AlGaN multiquantum well (QW) structures were grown by ammonia molecular beam epitaxy along the (0001) polar and (11 (2) over bar0) nonpolar directions. Each sample contains three QWs with thicknesses of 2, 3, and 4 nm as well as 10 nm Al0.30Ga0.70N barriers. The measured photoluminescence (PL) spectrum consists of three peaks originating from the radiative recombination of excitons in individual QWs. In the nonpolar sample, the energy positions (E-PL) of the observed peaks are separated because of the quantum confinement effect, whereas in the polar sample an additional redshift is induced by the quantum confined Stark effect. The dependence of E-PL on QW width was used to estimate the built-in electric field magnitude in the latter sample to be about 2 MV/cm. Hydrostatic pressure studies of the PL in both samples gave qualitatively different results. In the polar sample, the pressure shift of E-PL, dE(PL)/dp decreases significantly with QW width. The important finding is derived from the observation of a QW width independent dE(PL)/dp in the nonpolar sample. It shows that for GaN/Al0.30Ga0.70N, the quantum confinement remains practically independent of the applied hydrostatic pressure. This result reveals that in the polar sample, the variation in dE(PL)/dp with the QW width is due to the pressure-induced increase in the built-in electric field F-int. Thus, a more quantitative analysis of the latter effect becomes justified. We found that the F-int increases with pressure with a rate of about 80 kV(cmGPa)(-1). (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3043888]

We demonstrate the effect of conduction band shape evolution of InGaN with increasing In content and applying hydrostatic pressure. The influence of conduction band filling on the hydrostatic pressure dependence of photoluminescence in In0.7Ga0.3N and InN is investigated. It is found that the PL pressure coefficient dEPL/dp of InN changes from ∼27 meV/GPa to ∼21 meV/GPa when the electron concentration increases from 3.6×1017 cm–3 to 1.1×1019 cm–3. In contrast, no significant change of dEPL/dp with electron concentration was observed for In0.7Ga0.3N. We conclude that the pressure sensitivity of the Fermi level, which is responsible for the lowering of dEPL/dp with respect to dEG/dp in InN, is much less prominent in In0.7Ga0.3N than in InN. We attribute this difference to the larger band gap of In0.7Ga0.3N, which lowers the pressure sensitivity of m*. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we present spectroscopic data clarifying the nature of the ${\mathrm{Cr}}^{3+}$ centers and ${\mathrm{Cr}}^{3+}$-related defect in the $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}:\mathrm{Cr},\mathrm{Mg}$ crystal containing an excess of ${\mathrm{Li}}^{+}$ ions. Apart from quite well known lines associated with the $^{4}T_{2}\ensuremath{\rightarrow}^{4}A_{2}$ and $^{2}E\ensuremath{\rightarrow}^{4}A_{2}$ transitions of the ${\mathrm{Cr}}^{3+}$ ions, the luminescence spectrum of this crystal reveals some additional relatively narrow lines in the range of $750--860\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. We carried out high-pressure and magneto-optical measurements of the crystallographically oriented samples of this crystal at temperature of $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The measured spectra, decay times, pressure coefficients, and the dependence of the luminescence on high magnetic field show that the luminescence lines observed in lithium-rich $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}:\mathrm{Cr},\mathrm{Mg}$ originate from both ${\mathrm{Cr}}^{3+}$ ions and strongly antiferromagnetically coupled ${\mathrm{Cr}}^{3+}\text{\ensuremath{-}}{\mathrm{Cr}}^{3+}$ first nearest-neighbor pairs. The ground-state levels of the pairs can be described by the isotropic exchange interaction $\mathcal{H}=J{\mathbit{S}}_{1}∙{\mathbit{S}}_{2}+j{({\mathbit{S}}_{1}∙{\mathbit{S}}_{2})}^{2}$, where ${\mathbit{S}}_{1}$ and ${\mathbit{S}}_{2}$ are the total spins of each of the two chromium ions. The obtained results enabled us to determine the ground-state spin-spin exchange constants $J$ and $j$ as well as the excited-state zero field splitting constant $D$.

We present an experimental and theoretical study of the role of band filling effects in the hydrostatic pressure dependence of photoluminescence (PL) from InN. The PL peak pressure coefficient $d{E}_{\mathrm{PL}}∕dp$ is shown to decrease from $27.3\ifmmode\pm\else\textpm\fi{}1.1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\mathrm{GPa}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}20.8\ifmmode\pm\else\textpm\fi{}0.8\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\mathrm{GPa}$ when the electron concentration increases from $3.6\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We argue that this decrease is caused by the pressure sensitivity of the Fermi level in InN, which induces a lowering of $d{E}_{\mathrm{PL}}∕dp$ with respect to the band gap pressure coefficient $d{E}_{G}∕dp$. $d{E}_{\mathrm{PL}}∕dp$ is shown to depend on the electron concentration in accordance with predictions based on ab initio calculations, taking into account the influence of conduction-band nonparabolicity.

We show that the probability of $f\text{\ensuremath{-}}f$ radiative transitions $(^{2}F_{7∕2}\ensuremath{\leftrightarrow}^{2}F_{5∕2})$ of ${\mathrm{Yb}}^{3+}$ ions in gadolinium gallium garnet (GGG) crystals is sensitive to hydrostatic pressure application. The changes of the transition energy with pressure are very small, almost negligible. However, we observed much more pronounced variations of the transition probability, which are ascribed to a coupling between the $4{f}^{13}$ with the $4{f}^{12}5{d}^{1}$ electronic configurations and variations of the coupling coefficient with pressure. The theoretical model, explaining this behavior, assumes nonmonotonic changes of the odd-parity crystal-field terms associated with lattice distortions induced by pressure. The magnitude of these distortions correlates well with the magnitude of the distortion from the noncubic symmetry of the dodecahedral crystallographic sites in GGG host, monitored by the pressure dependence of the splitting of the $^{4}F_{3∕2}$ level of ${\mathrm{Nd}}^{3+}$ ions.

We discuss the influence of indium segregation-induced disorder effects in In x Ga 1-x N alloys. Changes of the transport mechanism between InN and In x Ga 1-x N with x=0.58 were demonstrated by means of temperature-dependent conductivity measurements. Furthermore, an increase of (i) full width at half maximum of photoluminescence (PL) and (ii) the Stokes shift between PL and absorption was seen for samples approachi ng an In content of 0.5, which can also be attributed to growing disorder. Hydrostatic pressure dependent PL measurements of In-rich InGaN alloys are diacussed. Due to the fact that PL in InGaN originates from regions with higher-than-average In content, the luminescence pressure coefficient dE E /dp should not be associated with the average In content, but with the In content which is in accordance with the energy of the photon emission. This correction leads to a reduction of the large bowing of dE E /dp (associated with the band gap) which was reported earlier. Furthermore, it is shown that the electron concentration in InN has a significant influence on the measured value of dE

The radiative deexcitation probability of the 2 F5/2→ 2 F7/2 transitions within the 4f electronic shell of the Yb 3+ ion in LiNbO3 exhibits at low temperatures a strong increase with increase of hydrostatic pressure up to 250 kbar. At the same time only a very small, almost negligible, decrease of the transition energy with pressure is observed. These results are explained by coupling of the 4f 13 with the 4f 12 5d 1 electronic configurations and increase of the coupling coefficient with pressure. The mechanism of this process is discussed.

International audience; Measurements of (i) the time-resolved photoluminescence and (ii) pressure dependence of the emitted light energy in two series of quaternary (AlInGa)N multiquantum wells have been used to point out the small magnitude of internal electric field as promoting highly efficient luminescence in these specially designed heterostructures. Findings of the experimental examination give the magnitude of built-in electric field few times smaller than that predicted theoretically. Electrical transport measurements support the explanation of the obtained results consisting of the screening of the built-in electric field by means of unintentional doping.

SrSi2O2N2oxynitride crystallizes in the P1 space group [1]. This compound is an excellent host for phosphors, due to its superior thermal and chemical stability and large energy bandgap. Eu-doped SrSi2O2N2can be used in green LEDs, and as a green component in phosphor mixtures; it is suitable, in particular, for white LEDs. For application in LEDs, the most important feature of SrSi2O2N2:Eu2+is the intense broadband luminescence at about 530 nm. In this study, high-pressure X-ray powder diffraction (XRD) experiments are used in order to experimentally determine, for the first time, the equation of state (EOS) of SrSi2O2N2:Eu2+. The studied sample, with Eu content of 2% was prepared by solid state reaction. The in situ XRD experiment was performed at the I711 beamline of MAXII synchrotron (Lund, Sweden) for a sample mounted in a diamond anvil cell, using hydrostatic compression conditions. The applied X-ray wavelength was 0.9917 Å. In the pressure range studied (up to 9.6 GPa) the triclinic structure is found to be conserved. Lattice parameters a, b and c decrease smoothly but slightly anisotropically as a function of applied pressure, whereas the angles α, β and γ vary marginally. The material is the most compressible in the b direction and the least compressible in the a direction. Angles α and γ are almost constant whereas the value of β angle slightly increases with rising pressure. The variation of unit cell volume with pressure served for determination of the Birch-Murnaghan EOS: the resulting bulk modulus value is 103(5) GPa. The present bulk modulus value is by 22% smaller than those reported for other oxonitridosilicates such as SrSiAl2O3N2and Ce4[Si4O4N6]O (131.9(1) GPa and 131(2) GPa, respectively) [2].

The results of high-pressure low-temperature optical measurements in a diamond-anvil cell of bulk gallium nitride crystals implanted with ytterbium are reported in combination with crystal field calculations of the Yb(3+) energy levels. Crystal field analysis of splitting of the (2)F(7/2) and (2)F(5/2) states has been performed, with the aim of assigning all features of the experimental luminescence spectra. A thorough analysis of the pressure behavior of the Yb(3+) luminescence lines in GaN allowed the determination of the ambient-pressure positions and pressure dependence of the Yb(3+) energy levels in the trigonal crystal field as well as the pressure-induced changes of the spin-orbit coupling coefficient.

Results of ab initio calculations of structural, electronic, optical, and elastic properties of pure and Yb-doped InP at pressures between ambient and 10 GPa in the density functional theory framework, using the CASTEP module of Materials Studio 4.0, are reported. It is assumed (in agreement with the experiment) that in this range of pressures pure InP and InP:Yb crystallizes in space group F4¯3m with four formula unit in a unit cell. Obtained values of lattice constant, bulk modulus, and its pressure derivative are in good agreement with experimental values reported by different authors. Additional calculations of the overlap integrals between the ytterbium and phosphorus wave functions in the InP host confirm that the observed previously [Kaminska et al., Phys. Rev. B 81, 165209 (2010)] at pressure of above 6 GPa effect of limiting of increase in f–f intrashell transition energies with increased pressure is associated with the top of the valence band of InP crystal host approaching the energies of the f...

Studies of absorption and luminescence spectra, and luminescence decay times of the intrashell f-f transitions of Y b(³+) ions in bulk orthophosphates at different temperatures and at high hydrostatic pressures are presented. The spectroscopic properties of Y₀.₉₉Y b₀.₀₁PO₄ crystal with a tetragonal zircon structure and Gd₀.₉₉Y b₀.₀₁PO₄ and La₀.₉₅Y b₀.₀₅PO₄ crystals with a monoclinic monazite structure are compared. Experiments at ambient pressure were performed at temperatures from 10 to 295 K whereas measurements at high pressure were performed in a diamond-anvil cell at 10 K with a pressure up to 160 kbar. The results reveal a high thermal and pressure stability of the optical properties of the examined crystals as well as the dependence of Y b(³+) emission properties on the local symmetry of the site occupied by the ytterbium dopant.

Near-lattice-matched GaN/AlInN multiple quantum wells (MQWs) are investigated by means of the diamond anvil cell high-pressure technique. The hydrostatic pressure dependence of the photoluminescence dE(PL)/dp and the variation in the PL peak energy with the QW width for different pressures were measured. Taking into account the influence of a large Stokes shift on the correct determination of the built-in electric field value, we find that the electric field, equal to 4.2 +/- 1.1 MV/cm at ambient pressure, increases with pressure at a rate of 0.29 MV/(cm GPa). This value is in reasonable agreement with the theoretically predicted value, based on nonlinear elasticity, of 0.17 MV/(cm GPa). Interestingly, the observed behavior is very similar to strongly mismatched GaN/AlGaN QWs with a similar band offset, indicating that in GaN/AlInN QWs there is still a pressure dependence of piezoelectric effects, in spite of ambient-pressure lattice matching. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2977608]