Surface State‐Induced Anomalous Negative Thermal Quenching of Multiferroic BiFeO3 Nanowires

Wide‐bandgap semiconductor nanowires with surface defect emission centers have the potential to be used as sensitive thermometers and optical probes. Here, we show that the green luminescence of multiferroic BiFeO₃ (BFO) nanowires shows an anomalous negative thermal quenching (NTQ) with increasing temperatures. The release of trapped carriers from localized surface defect states is suggested as the possible mechanism for the increased green luminescence which was experimentally observed at elevated temperatures. A reasonable interpretation of the photoluminescence (PL) processes in BFO nanowires is achieved, and the activation energies of the PL quenching and thermal hopping are deduced. Negative thermal quenching of BFO nanowires provides a new strategy for optical thermometry at higher temperatures.     

Optical properties of 1.3 μm room temperature emitting InAs quantum dots covered by In0.4Ga0.6As/GaAs hetero-capping layer

Room temperature 1.3 μm emitting InAs quantum dots (QDs) covered by an In_0.4Ga_0.6As/GaAs strain reducing layer (SRL) have been fabricated by solid source molecular beam epitaxy (SSMBE) using the Stranski–Krastanov growth mode. The sample used has been investigated by temperature and excitation power dependent photoluminescence (PL), photoluminescence excitation (PLE), and time resolved photoluminescence (TRPL) experiments. Three emission peaks are apparent in the low temperature PL spectrum. We have found, through PLE measurement, a single quantum dot ground state and the corresponding first excited state with relatively large energy spacing. This attribute has been confirmed by TRPL measurements which allow comparison of the dynamics of the ground state with that of the excited states. Optical transitions related to the InGaAs quantum well have been also identified. Over the whole temperature range, the PL intensity is found to exhibit an anomalous increase with increasing temperatures up to 100 K and then followed by a drop by three orders of magnitude. Carrier’s activation energy out of the quantum dots is found to be close to the energy difference between each two subsequent transition energies. PACS 68.65.Ac; 68.65.Hb; 78.67.Hc     

Energy sublevels in an Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As quantum wire

We report the successful fabrication of a V-grooveAl_0.5Ga_0.5As/GaAs/Al_0.5Ga_0.5As quantum wire system and the temperature-dependent photoluminescence (PL) measurement. The PL spectra are dominated by four features at 681, 642, 635 and 621 nm attributed to the luminescences from quantum wire, top, vertical and side-wall well regions by micro-PL measurements. By the calculations of the energy structure, discrete states (localized sublevels) in the quantum wire region and continuum states (extended along the side-wall and vertical quantum wells) in side-wall and vertical quantum wells have been obtained in both the conduction and valence bands. The calculated excitation energies explain very well the peak positions and their temperature dependence in the photoluminescence measurements.     

Clustering effects in optical properties of BGaAs/GaAs epilayers

We report on the further investigation of the effect of boron incorporation on GaAs grown at 580 °C temperature on GaAs (0 0 1) substrate by metal organic chemical vapor deposition (MOCVD). High-resolution X-ray diffraction (HRXRD) has been used to determine the lattice mismatch and to estimate the boron concentration. Temperature-dependent photoluminescence has been carried out to investigate B_xGa_1−xAs/GaAs epilayers with varied boron composition (x=1.64% and 3.04%). Low temperature (10 K) PL study has shown an asymmetric and broad PL band around 1.3 eV of the emission energies with a decrease of the PL intensity with increasing boron composition. The evolution of the emission energies with temperature can be described by Varshni law for the high temperature range (T?120 and 80 K) for boron composition x=1.64% and 3.04%, respectively, while a relative discrepancy has been found to occur at low temperature. Moreover, depending on the temperature range, the PL intensity quenching is found to be thermally ensured by three activation energies. These results are attributed to the localized states induced by the non-uniform insertion of boron and the clustering of the boron atom in BGaAs bulk.     

InGaAsN量子阱的光致发光谱研究(英文)

我们测量了低N组分的InGaAsN/InGaAs/GaAs量子阱材料的光致发光(PL)谱,测量温度范围从13K到300K。实验结果显示,InGaAsN的PL谱的主峰值的能量位置随温度的变化呈现出反常的S型温度依赖关系。用Varshni经验公式对实验数据进行拟合之后,发现在低温下InGaAsN量子阱中的载流子是处于局域态的。此外,我们还测量了样品在不同的温度、不同的能量位置的瞬态谱,结果进一步证实了:在低温下,InGaAsN的PL谱谱峰主要是局域态激子的复合发光占据主导地位,而且InGaAsN中的载流子局域态主要是由N等电子缺陷造成的涨落势引起的。     

Preparation and luminescent characteristic of Li3NbO4 nanophosphor

Synthesis and luminescence properties of Li₃NbO₄ oxides by the sol-gel process were investigated. The products were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy and absorption spectra. The PL spectra excited at 247 nm have a broad and strong blue emission band maximum at 376 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO₆]⁷⁻. The optical absorption spectra of the samples sintered at temperatures of 600 and 700 °C exhibited the band-gap energies of 4.0 and 4.08 eV.     

Luminescent properties of GaN films grown on porous silicon substrate

GaN films have been grown on porous silicon at high temperatures (800-1050 °C) by metal organic vapor phase epitaxy. The optical properties of GaN layers were investigated by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. PL spectra recorded at 5 K exhibit excitonic emissions around 3.36-3.501 eV and a broad yellow luminescence at 2.2 eV. CL analysis at different electron excitation conditions shows spatial non-uniformity in-depth of the yellow and the band-edge emissions. These bands of luminescence are broadened and red- or blue-shifted as the electron beam penetrates in the sample. These behaviors are explained by a change of the fundamental band gap due to residual strain and the local thermal effect. It was found that the use of AlN buffer layer improves the crystalline quality and the luminescence property of GaN.     

Optical properties of GaAs1−x N x alloys grown by molecular beam epitaxy

Optical properties of GaAs_1?x N _x alloys grown by molecular beam epitaxy using GaAs (001) as the substrate have been studied. These include photoluminescence (PL), cathodoluminescence (CL), photocurrent and photomemory effects. The low-temperature (77?K) PL characteristics were measured on samples with 0–0.105% N content. The wide emission band indicates the defective nature of the materials. The widening of the band for materials with increasing nitrogen concentration also suggested that the concentration of defect states in these materials dramatically increased with increasing nitrogen content. The PL and CL spectra for GaAs_1?x N _x layer 1854 did not show identical characteristics. Some layers showed a very sharp fall in photocurrent at low temperatures, indicating a very sharp photoquenching and an interaction between antisite, interstitial and vacancy defects. The photomemory effect, which causes photoquenching and the transition from the EL2 to the EL2? metastable state, was strongly influenced by the optical exposure and thermal history of the sample.     

Temperature dependent photoluminescence of PECVD a-SiOx:H (x<2)

The temperature dependent visible photoluminescence (PL) property of a-SiO_x:H (x<2) samples prepared in a PECVD system by using SiH₄+CO₂ gas mixture is investigated at a temperature range of 20 K-400 K. One of the two explicitly distinguished PL bands, with varying peak photon energies between 1.70 and 2.05 eV, can be detected at only low temperatures below 200 K, which is attributed to tail-to-tail radiative recombination. Thermal quenching parameter (T_L) of the tail-to-tail PL band is calculated as varying between 120 and 280 K as the atomic oxygen concentration ([O]at.%) of the samples increases. Stokes shift (ΔE_Stokes) of the tail-to-tail PL band is found to change from 85 meV to 420 meV due to band tail widening. The other PL band emerges at 2.1 eV and can be detected at higher temperatures with thermal activation behavior. The activation energies calculated about room temperature vary in the range of 8 meV-50 meV with oxygen concentration. Thermal activation of the 2.1 eV PL band is attributed to the behavior of thermally activated incoherent hopping migration of electrons. These electrons combine with self trapped holes (STHs) to form self trapped excitons (STEs). STEs are localized at intrinsic defects of SiO₂ structure such as oxygen vacancies (E′ centers) and non-bridging oxygen hole centers (NBOHC).     

Photoluminescence of CdGeP2 and (Cd,Mn)GeP2

Photoluminescence of CdGeP₂ (112) single crystal and CdGeP₂ epitaxial film grown on GaAs (001) substrate have been studied and their spectral similarity found. Spectral bands associated with donor/acceptor transitions peak at close energies for both substances and all are lower than the energy gap of the chalcopyrite crystal.On the other hand, the growth of (Cd,Mn)GeP₂ ferromagnetic layer on CdGeP₂ (112) single crystal was performed to make it possible observation of PL from both the ferromagnetic layer and substrate. The green laser excitation (514, 532 nm) produces a proper photoluminescence similar to that in the undoped CdGeP₂ crystal and film. An extra emission from the ferromagnetic-nonmagnetic heterojunction occurs to extend up to photon energies exceeding E_g of the host semiconductor. The short wavelength photoluminescence is to be due to (Cd,Mn)GeP₂ dilute magnetic semiconductor (DMS). This fact states that Mn-doped II-IV-V₂ chalcopyrites are closer to II-VI DMS than to another group III-V DMS, where the heavy Mn-doping suppresses photoluminescence at all. Features of the observed short wavelength emission are discussing based on the temperature and spectral analyses.     

Theoretical study of quantum-confined band-edge shifts and radiative lifetimes in oxidized Si(001) quantum films: comparison with experiment for Si/SiO2 quantum wells

The electronic structure and radiative lifetimes of Si(001) quantum films terminated by SiO₄ tetrahedra, which simulate Si/SiO₂ quantum wells (QWs), are calculated by the extended Hückel-type non-orthogonal tight-binding method. It is found that calculated band-gap widenings and radiative lifetimes account for band-edge shifts and photoluminescence (PL) peak shifts and lifetimes measured in amorphous-Si/SiO₂ QWs, suggesting that quantum confinement effects on the extended band-edge states in the amorphous-Si layer are responsible for the observed results. However, it is shown that band-edge shifts and PL energies and lifetimes observed in crystalline-Si/SiO₂ QWs cannot be reproduced properly by the interface model proposed in this study, implying that further studies are needed on the atomic structure of the crystalline-Si/SiO₂ interface.     

Phonon spectra of stoichiometric rare-earth compound of EuGa2S4

Investigations of EuGa₂S₄ have been done on the photoluminescence (PL) related to the transition between 4f⁶5d and 4f⁷ configuration of the Eu²⁺ ion and its excitation (PLE) spectra, Raman scattering and infrared absorption. The energies of phonons coupled to the ground and the excited states of the transition are analyzed to be 34 and 19 meV from the shapes of the PL and PLE bands, respectively. The former corresponds to the energy of the Raman line showing the highest intensity. The latter is close to the value obtained from analysis of the temperature dependence of the half width of the PL band. These correspondences indicate that the relevant emission of EuGa₂S₄ surely has phonon-terminated character.     

Growth and photoluminescence properties of inclined ZnO and ZnCoO thin films on SrTiO_3（110） substrates

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO 3（STO）（110） substrates.Inclined ZnO and cobalt-doped ZnO（ZnCoO） thin films are grown on unetched STO（110） substrates using oxygen plasma assisted molecular beam epitaxy,with the c-axis 42 inclined from the normal STO（110） surface.The growth geometries are ZnCoO[100]//STO[110] and ZnCoO[111]//STO[001].The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D 0 X emissions associated with A 0 X emissions,and the characteristic emissions for the 2 E（2G）→ 4A2（4F） transition of Co 2＋ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film,indicating the incorporation of Co 2＋ ions at the lattice positions of the Zn 2＋ ions.The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature.     

Characteristics of high Al content AlGaN grown by pulsed atomic layer epitaxy

Al_0.91Ga_0.09N epilayers have been obtained by pulsed atomic layer epitaxy (PALE) technique on sapphire (0 0 0 1) substrates. Deep ultraviolet (DUV) photoluminescence (PL) spectroscopy and Raman scattering spectrum have been employed to study the optical transitions in Al_0.91Ga_0.09N epilayers. We found the exciton-phonon interaction by fitting the asymmetric PL peak, in which the transverse optical phonon (TO) and the longitudinal optical (LO) phonon are the main contributor. The abnormal S-shaped temperature dependence of the PL band peak is less pronounced or has disappeared. Further analysis shows that there possibly exists a high density of deeper localized state (∼90 meV) in Al_0.91Ga_0.09N. The formation of these localized states provides a favorable condition for efficient light emission.     

Evaluation of optical quality and properties of Ga0.64In0.36N0.006As0.994 lattice matched to GaAs by using photoluminescence spectroscopy

We have investigated optical properties of Ga_0.64In_0.36N_0.006As_0.994/GaAs single quantum-well structures using photoluminescence technique. We have found that nitrogen creates potential fluctuations in the InGaNAs structures, so it is the cause of trap centres in these structures and leads to localized excitons recombination dynamics. The near-band edge PL at 2 K exhibited a blueshift with an increase in excitation intensity of a sample but there is not such a shift in the PL peak position energy of same sample at 150 K. It has been found that PL spectra have a large full width at half maximum (FWHM) value at 2 K. These results are discussed in terms of carrier localization. Additionally, our results suggest decreasing PL integrated intensity in this structure, possibly due to non-radiative recombination. It has been shown that thermal annealing reduces the local strain created by nitrogen. By annealing process, a blue shifted emission can be observed.     

Emission and strain in InGaAs/GaAs quantum wells with InAs quantum dots obtained at different temperatures

The photoluminescence (PL), its temperature dependence and X ray diffraction (XRD) have been studied in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with embedded InAs quantum dots (QDs), obtained with the variation of QD growth temperatures (470–535 °C). The increase of QD growth temperatures is accompanied by the enlargement of QD lateral sizes (from 12 up to 28 nm) and by the shift non monotonously of PL peak positions. The fitting procedure has been applied for the analysis of the temperature dependence of PL peaks. The obtained fitting parameters testify that in studied QD structures the process of In/Ga interdiffusion between QDs and capping/buffer layers takes place partially. However this process cannot explain the difference in PL peak positions.     

Photoluminescence study of Si doped and undoped Chalcopyrite CuGaSe2 thin films

Photoluminescence (PL) characteristics have been studied on undoped and Si-doped CuGaSe₂ single crystal thin films grown on GaAs (001) substrate by migration-enhanced epitaxy. Room temperature PL spectrum of an undoped layer clearly shows free excitonic emission bands related to the minimum band-edge and to the split-off valence band, but no discernible emission has been observed in the low energy area. At 4.2 K, the excitonic emission due to the split-off valence band disappears. Instead, two additional emissions appear at 1.68 and 1.715 eV which are attributed to the bound exciton and band-to-acceptor transition. The Si doping to CuGaSe₂ produces two additional PL bands around 1.61 and 1.64 eV. These PL bands are attributed to the donor acceptor pair emissions due to the doped Si impurity which probably occupies Cu or Ga sites and intrinsic Cu vacancy.     

m面蓝宝石上ZnO/ZnMgO多量子阱的制备及发光特性研究

利用等离子体辅助分子束外延设备(P-MBE)在m面的蓝宝石(m-Al₂O₃)衬底上制备了ZnO/Zn_0.85Mg_0.15O多量子阱.反射式高能电子衍射谱(RHEED)图样的原位观察表明,多量子阱结构是以二维模式生长的.从光致发光谱中可以看到ZnO/Zn_0.85Mg_0.15O多量子阱在室温仍具有明显的量子限域效应.在290 K时阱宽为3 nm的ZnO/Zn_{0.85关键词： 等离子体辅助分子束外延 ZnO多量子阱 光致发光}     

Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots

Self-assembled InAs/GaAs (001) quantum dots (QDs) were grown by molecular beam epitaxy using ultra low-growth rate. A typical dot diameter of around 28 ± 2 nm and a typical height of 5 ± 1 nm are observed based on atomic force microscopy image. The photoluminescence (PL) spectra, their power and temperature dependences have been studied for ground (GS) and three excited states (1–3ES) in InAs QDs. By changing the excitation power density, we can significantly influence the distribution of excitons within the QD ensemble. The PL peak energy positions of GS and ES emissions bands depend on an excitation light power. With increasing excitation power, the GS emission energy was red-shifted, while the 1–3ES emission energies were blue-shifted. It is found that the full width at half maximum of the PL spectra has unusual relationship with increasing temperature from 9 to 300 K. The temperature dependence of QD PL spectra shown the existence of two stages of PL thermal quenching and two distinct activation energies corresponding to the temperature ranges I (9–100 K) and II (100–300 K).     

Photoluminescence evolution in self-ion-implanted and annealed silicon

Si⁺ ion-implanted silicon wafers are annealed at different temperatures from room temperature to 950~℃ and then characterized by using the photoluminescence (PL) technique at different recorded temperatures (RETs). Plentiful optical features are observed and identified clearly in these PL curves. The PL spectra of these samples annealed in different temperature ranges are correspondingly dominated by different emission peaks. Several characteristic features, such as an R line, S bands, a W line, the phonon-assistant W^\rm TA and Si^\rm TO peaks, can be detected in the PL spectra of samples annealed at different temperatures. For the samples annealed at 800~\du, emission peaks from the dislocations bounded at the deep energy levels of the forbidden band, such as D_1 and D₂ bands, can be observed at a temperature as high as 280~K. These data strongly indicate that a severe transformation of defect structures could be manipulated by the annealing and recorded temperatures. The deactivation energies of the main optical features are extracted from the PL data at different temperatures.