Time-resolved photoluminescence spectra of self-assembled InAs/GaAs quantum dots

Two types of InAs self-assembled Quantum dots (QDs) were prepared by Molecular beam epitaxy. Atomic force microscopy (AFM) measurements showed that, compared to QDs grown on GaAs substrate, QDs grown on InGaAs layer has a significantly enhanced density. The short spacing (several nanometer) among QDs stimulates strong coupling and leads to a large red-shift of the 1.3 mu m photoluminescence (PL) peak. We study systematically the dependence of PL lifetime on the QDs size, density and temperature (1). We found that, below 50 K, the PL lifetime is insensitive to temperature, which is interpreted from the localization effects. As T increases, the PL lifetime increases, which can be explained from the competition between the carrier redistribution and thermal emission at higher temperature. The increase of carriers in QDs migrated from barriers and wetting layer (WL), and the redistribution of carriers among QDs enhance the PL lifetime as T increases. The thermal emission and non-radiative recombination have effects to reduce the PL lifetime at higher T. As a result, the radiative recombination lifetime is determined by the wave function overlapping of electrons and holes in QDs, and QDs with different densities have different PL lifetime dependence on the QDs size. (c) 2005 Elsevier B.V. All rights reserved.

Magnetism and photoluminescence in manganese-gallium oxide nanowires with monoclinic and spinel structures

Manganese-gallium oxide nanowires were synthesized via in situ Mn doping during nanowire growth using a vapor phase evaporation method. The microstructure and composition of the products were characterized via transmission electron microscopy (TEM), field emission scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The field and temperature dependence of the magnetization reveal the obvious hysteresis loop and large magnitude of Curie-Weiss temperature. The photoluminescence of the manganese-gallium oxide nanowires were studied in a temperature range between 10 and 300 K. A broad green emission band was observed which is attributed to the T-4(1)-(6)A(1) transition in Mn2+ (3d(5)) ions. (c) 2005 Elsevier B.V. All rights reserved.

Photoluminescence of self-assembled InAs/GaAs quantum dots covered by InAlAs and InGaAs combination strain-reducing layer

Self-assembled InAs/GaAs quantum dots covered by the 1-nm InxAl(1-x)As (x = 0.2,0.3) and 3-nm In0.2Ga0.8As combination strain-reducing layer are fabricated, whose height can take up to 30-46 nm. The luminescence emission at a long-wavelength of 1.33 mum and the energy separation between the ground and the first-excited state of 86 meV are observed at room temperature. Furthermore, comparative study proves that the energy separation can increase to 91 meV by multiple stacking.

Study on Er3+ emission from the erbium-doped hydrogenated amorphous silicon suboxide film

The erbium-doped hydrogenated amorphous silicon suboxide films containing amorphous silicon clusters were prepared. The samples exhibited photoluminescence peaks at around 750 nm and 1.54 mum, which could be assigned to the electron-hole recombination in amorphous silicon clusters and the intra-4f transition in Er3+, respectively. Correlations between the intensities of these two photoluminescence peaks and oxidation and dehydrogenation of the films during annealing were studied. It was found that the oxidation is triggered by dehydrogenation of the films even at low annealing temperatures, which decisively changes the intensities of the two photoluminescence peaks. On the other hand, the increase of Er content in the erbium-doped hydrogenated amorphous silicon suboxide film will enhance Er3+ emission at 1.54 mum, while quench amorphous silicon cluster emission at 750 nm, such a competitive relationship, was also observed in the erbium-doped silicon nanocrystals embedded in SiO2 matrix. Moreover, we found that Er3+ emission is not sensitive to whether silicon clusters are crystalline or amorphous. The amorphous silicon clusters can be as sensitizer on Er3+ emission as that of silicon nanocrystals. (C) 2003 American Institute of Physics.

Photoluminescence behaviors from stoichiometric gadolinium oxide films

Stoichiometric gadolinium oxide thin films have been grown on silicon (100) substrates with a low-energy dual ion-beam epitaxial technique. Gadolinium oxide shares Gd2O3 structures although the ratio of gadolinium and oxygen in the film is about 2:1 and a lot of oxygen deficiencies exist. Photoluminescence (PL) measurements have been carried out within a temperature range of 5-300 K. The detailed characters of the PL emission integrated intensity, peak position, and peak width at different temperature were reported and an anomalous photoluminescence behavior was observed. The character of PL emission integrated intensity is similar to that of some other materials such as porous silicon and silicon nanocrystals in silicon dioxide. Four peaks relative to alpha band and beta band were observed also. Therefore we suggest that the nanoclusters with the oxygen deficiencies contribute to the PL emission and the model of singlet-triplet exchange splitting of exciton was employed for discussion. (C) 2003 American Institute of Physics.

Violet/blue photoluminescence from CeO2 thin film

CeO2 thin film was fabricated by dual ion beam epitaxial technique. The violet/blue PL at room temperature and lower temperature was observed from the CeO2 thin film. After the analysis of crystal structure and valence in the compound was carried out by the XRD and XPS technique, it was inferred that the origin of CeO2 PL was due to the electrons transition from Ce4f band to O2p band and the defect level to O2p band. And these defects levels were located in the range of 1 eV around Ce4f band.

Role of amorphous silicon domains on Er3+ emission in the Er-doped hydrogenated amorphous silicon suboxide film

An investigation on the correlation between amorphous Si (a-Si) domains and Er3+ emission in the Er-doped hydrogenated amorphous silicon suboxide (a-Si:O:H) film is presented. On one hand, a-Si domains provide sufficient carriers for Er3+ carrier-mediated excitation which has been proved to be the highest excitation path for Er3+ ion; on the other hand, hydrogen diffusion from a-Si domains to amorphous silicon oxide (a-SiOx) matrix during annealing has been found and this possibly decreases the number of nonradiative centres around Er3+ ions. This study provides a better understanding of the role of a-Si domains on Er3+ emission in a-Si:O:H films.

Influence of coupling between Er3+, nc-Si and nonradiative centers on photoluminescence from Er3+-doped nc-Si/SiO2 films

Con-elation between nc-Si, Er3+ and nonradiative defects in Er-doped nc-Si/SiO2 films is studied. Upon the 514.5 run laser excitation, the samples exhibit a nanocrystal-related spectrum centered at around 750 nm and an Er3+ luminescence line at 1.54mum. With increasing Er3+ content in the films,the Er3+ emission becomes intense while the photoluminescence at 750 nm decreases. Hydrogen passivation of the samples is shown to result in increases of the two luminescence peaks. However, the effect of hydrogen treatment is different for the samples annealed at different temperatures. The experimental results show that the coupling between Er3+, nc-Si and noradiative centers has a great influence on photoluminescence from nc-Si/SiO2 < Er > films.

Mode splitting in photoluminescence spectra of a quantum-dot-embedded microcavity

A microcavity structure, containing self-assembled InGaAs quantum dots, is studied by angle-resolved photoluminescence (PL) spectroscopy. A doublet with the splitting energy of 0.5-1.5 nm appears when the detection angle is larger than 35degrees. This doublet is identified as mode splitting (not the Rabi splitting) by polarization measurements. We find that it is the considerable deviation of the cavity-mode frequency from the central frequency of the stop band that makes the TE and TM cavity modes split more discernibly. The inhomogeneous broadening of quantum dots gives the TE and TM cavity modes a chance to show up simultaneously in the PL spectra. (C) 2003 American Institute of Physics.

Correlation between Er3+ emission and the microstructure of a-SiOx : H < Er > films

Photoluminescence (PL) from Er-implanted hydrogenated amorphous silicon suboxide (a-SiOX:H(x<2.0)) films was measured. Two luminescence bands with maxima at lambda congruent to 750 nm and lambda congruent to 1.54mum, ascribed to the a-SiOX:H intrinsic emission and Er3+ emission, were observed. Peak intensities of the two bands follow the same trend as a function of annealing temperature from 300 to 1000degreesC. Micro-Raman results indicate that the a-SiOX:H films are a mixture of two phases, an amorphous SiOX matrix and amorphous silicon (a-Si) domains embedded there in. FTIR spectra confirm that hydrogen effusion from a-SiOX:H films occurs during annealing. Hydrogen effusion leads to a reconstruction of the microstructure of a-Si domains, thus having a strong influence on Er3+ emission. Our study emphasizes the role of a-Si domains on Er3+ emission in a-SiOX:H films.

Getting high-efficiency photoluminescence from Si nanocrystals in SiO2 matrix

Silicon nanocrystals in SiO2 matrix are fabricated by plasma enhanced chemical vapor deposition followed by thermal annealing. The structure and photoluminescence (PL) of the resulting films is investigated as a function of deposition temperature. Drastic improvement of PL efficiency up to 12% is achieved when the deposition temperature is reduced from 250 degreesC to room temperature. Low-temperature deposition is found to result in a high quality final structure of the films in which the silicon nanocrystals are nearly strain-free, and the Si/SiO2 interface sharp. The demonstration of the superior structural and optical properties of the films represents an important step towards the development of silicon-based light emitters. (C) 2002 American Institute of Physics.

Selectively excited photoluminescence of GaAs1-xSbx/GaAs single quantum wells

GaAsSb/GaAs single quantum wells grown by molecular-beam epitaxy are studied by selectively excited photoluminescence measurements. We have simultaneously observed the photoluminescence (PL) from both type-I and type-II transitions in the samples. The two transitions exhibit different PL behavior under different excitation energies. As expected, the peak energy of the type-I emission remains constant in the entire excitation energy range we used, while the type-II transition shows a significant blueshift with increasing excitation energy. The observed blueshift can be well explained by an electron-hole charge separation model at interface. This result, along with the excitation-power-dependent PL and the measured longer carrier decay time, provides more direct information on the type-II nature of the band alignment in GaAsSb/GaAs quantum well structures. (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.

Anomalous temperature dependence of photoluminescence in GaInNAs/GaAs multiple quantum wells

Photoluminescence (PL) spectra of GaInNAs/GaAs multiple quantum wells grown on a GaAs substrate by molecular beam epitaxy are measured in a range of temperatures and excitation power densities. The energy position of the dominant PL peak shows an anomalous S-shape temperature dependence instead of the Varshni relation. By careful inspection, especially for the PL under lower excitation power density, two near bandedge peaks are well identified. These are assigned to carriers localized in nitrogen-induced bound states and interband excitonic recombinations, respectively. It is suggested that the temperature-induced switch of such two luminescence peaks in relative intensity causes a significant mechanism responsible for the S-shape shift observed in GaInNAs. A quantitative model based on the thermal depopulation of carriers is used to explain the temperature dependence of the PL peak related to N-induced bound states.

Effect of rapid thermal annealing and hydrogen plasma treatment on the microstructure and light-emission of silicon-rich oxide film

Silicon-rich silicon oxide (SRSO) films are prepared by plasma-enhanced chemical vapor deposition method at the substrate temperature of 200degreesC. The effect of rapid thermal annealing and hydrogen plasma treatment on tire microstructure and light-emission of SRSO films are investigated in detail using micro-Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and photoluminescence (PL) spectra. It is found that the phase-separation degree of the films decreases with increasing annealing temperature from 300 to 600degreesC, while it increases with increasing annealing temperature from 600 to 900degreesC. The light-emission of the films are enhanced with increasing annealing temperature up to 500degreesC, while it is rapidly reduced when the annealing temperature exceeds 600degreesC. The peak position of the PL spectrum blueshifts by annealing at the temperature of 300degreesC, then it red-shifts with further raising annealing temperature. The following hydrogen plasma treatment results in a disproportionate increase of the PL intensity and a blueshift or redshift of the peak positions, depending on the pristine annealing temperature. It is thought that the size of amorphous silicon clusters, surface structure of the clusters and the distribution of hydrogen in the films can be changed during the annealing procedure. The results indicate that not only cluster size but also surface state of the clusters plays an important role in the determination of electronic structure of the amorphous silicon cluster and recombination process of light-generated carriers.