Search

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 ambient-pressure and high-pressure behaviour of photoluminescence (PL) as well as the phase transition pressure from wurtzite to cubic structure for a series of Zn 1−x Mg x O alloys as a function of the Mg-content x (0 ≤ x ≤ 0.34) are presented. The measured PL peak energy is shown to increase approximately linearly with increasing magnesium content from 3.36 eV to 4.11 eV, and the phase transition pressure from wurtzite to cubic structure decreases with increasing x from 12.6 GPa to 5.9 GPa. The phase transition is accompanied by a strong decrease of the near band-gap PL intensity. The value of the PL peak pressure coefficient d E PL /d p changes non-monotonically. It varies between 18.6 meV/GPa and 25.2 meV/GPa, and seems to depend on the layer thickness rather than on the layer composition, exhibiting strong scattering for several alloys with similar magnesium content. This behaviour can be due to the stabilizing influence of sapphire substrate, as is indicated both by the pressure coefficients and by the values of pressure of phase transition from wurtzite to rock salt structure in Zn 1−x Mg x O layers with the same or similar composition and different layer thicknesses.

The temperature dependence of the band-gap energy affects the evaluation of the energy positions of rare-earth 4f levels in relation to the vacuum level. Neglecting this dependence may lead to strongly distorted results.

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 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.

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.

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.

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.

Results of high-pressure luminescence spectroscopy of Cr 3+ doped MgO–2.5A1 2 O 3 non-stoichiometric green spinel under pressures up to 130 kbar at temperature T =10 K are reported. It is shown that Cr 3+ ions in this crystal form four major groups of centers. The broadband infrared luminescence is a result of the very large inhomogeneous broadening, which affects the intermediate strength-crystal field centers. Due to this broadening some of the Cr 3+ ions experience a very broad spectrum of the crystal field strength from a weak to strong crystal field scheme.

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.

The results of high-pressure spectroscopic studies of near-stoichiometric ${\mathrm{LiNbO}}_{3}:\mathrm{C}\mathrm{r}(0.1%)$ and ${\mathrm{LiNbO}}_{3}:\mathrm{M}\mathrm{g}\mathrm{O},\mathrm{C}\mathrm{r}(0.1%)$ crystals are reported. Much lower inhomogeneous broadening of electronic transitions in near-stoichiometric crystals than in congruent ones allows us to obtain better resolution of the spectroscopic measurements. The results of measurements show that ${\mathrm{Cr}}^{3+}$ ions occupy mostly the same centers in near-stoichiometric crystals and in the congruent ones. In near-stoichiometric crystals doped only with chromium, the ${\mathrm{Cr}}^{3+}$ ions substitute the ${\mathrm{Li}}^{+}$ sites in the ${\mathrm{LiNbO}}_{3}$ host forming one major center (denoted as center \ensuremath{\gamma}) and two lower concentration centers, denoted by \ensuremath{\alpha} and \ensuremath{\beta}. In near-stoichiometric crystals doped additionally with a proper amount of MgO, the additional low-strength crystal-field center \ensuremath{\delta} dominates the spectrum. This center corresponds to the ${\mathrm{Cr}}^{3+}$ ions in ${\mathrm{Nb}}^{5+}$ sites. The hydrostatic pressure above 75 kbar transforms this center into the high-strength crystal-field center. Due to better resolution of measurements, it was possible to determine the splitting of the ${}^{2}E$ level for the \ensuremath{\delta} center, which is equal to about 17 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Estimated values of the energy difference between the position of the ${}^{4}{T}_{2}$ and the ${}^{2}E$ levels for centers \ensuremath{\beta}, \ensuremath{\gamma}, and \ensuremath{\delta} in these near-stoichiometric ${\mathrm{LiNbO}}_{3}$ crystals are equal to -20, -242, and more than -1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively.

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...

Studies of the spectroscopic properties of Ce3+ dopant in bulk Gd 3 Ga 5 O 12 :Ce crystal under pressure are presented. In spite of strong inter-shell 4f → 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 recovering the 5d → 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 5d energy level of the Ce3+ with the conduction band bottom of host crystal, and (ii) by decrease of electron-lattice coupling with increasing pressure, resulting in reducing of Stokes shift and non-radiative transitions between the low vibrational levels of the 5d state and high vibrational levels of the ground 4f state.

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.

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)

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

In nitride heterostructures and devices, effects related to (i) polarization induced electric fields (PIEFs) and (ii) spatial segregation of indium leading to exciton/carrier localization are of major significance. However, separate investigation of these effects is not straightforward since they give rise to identical observations, such as a Stokes shift of the luminescence with respect to absorption and a blue shift of luminescence with increasing pump intensity. In this work, we review the usefulness of measurements of the hydrostatic pressure dependence of InGaN luminescence for the verification of the presence of PIEFs in quantum structures and light emitting devices. Additionally, we show that the pressure coefficient is not or only slightly sensitive to the degree of localization in the InGaN alloy. Thus, the variation of the luminescence pressure coefficient in different quantum structures can be almost entirely assigned to changes in the magnitude of internal electric field. Using this knowledge, we demonstrate how the degree of PIEF screening in InGaN based LDs can be evaluated.

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.

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]

The hydrostatic pressure dependence of photoluminescence, dEPL/dp, of InxGa1−xN epilayers has been measured in the full composition range 0 0.4 and a relatively steep dependence for x

The authors studied the hydrostatic pressure dependence of photoluminescence (PL) from In-rich InxGa1−xN with In contents x between 0.58 and 1.00. The observed PL pressure coefficients of 20–25meV∕GPa agree well with previously reported experimental and theoretical values of the band gap pressure coefficient, from which they conclude that band-to-band recombination is responsible for PL emission. This contrasts with earlier reports, where relatively low PL pressure coefficients were interpreted as evidence of the involvement of strongly localized states in the PL emission. The reported observation of band-to-band recombination in In-rich InGaN is encouraging from the point of view of the construction of light emitters, since band-to-band recombination is more efficient than recombination via localized states. Furthermore, significant bowing of the band gap pressure coefficient in In-rich InxGa1−xN, as predicted by theory, is confirmed.