Measurement of small-signal and large-signal responses of packaged laser modules at high temperature

In this paper, the pulsed injection method is extended to measure the chip temperature of various packaged laser modules, such as the DFB laser modules, the FP laser modules, and the EML laser modules. An optimal injection condition is obtained by investigating the dependence of the lasing wavelength on the width and period of the injection pulse in a relatively wide temperature range. The small-signal frequency responses and large-signal performances of packaged laser modules at different chip temperature are measured. The adiabatic small-signal modulation characteristics of packaged LD are first extracted. In the large-signal measurement, the effects of chip temperature, bias current and driving signal on the performances of the laser modules are discussed. It has been found that the large-signal performances of the EML modules depend on the different red-shift speeds of the DFB and EAM sections as chip temperature varying, and the optimal characteristics may be achieved at higher temperature.

Visible blind p(+)-pi-n(-)-n(+) ultraviolet photodetectors based on 4H-SiC homoepilayers

p(+)-pi-n(-)-n(+) ultraviolet photodetectors based on 4H-SiC homoepilayers have been presented. The growth of the 4H-SiC homoepilayers was carried out in a LPCVD system. The size of the active area of the photodetectors was 300 x 300 mu m(2). The dark and illuminated I-V characteristics had been measured at reverse biases form 0 to 20 V at room temperature, and the illuminated current was at least two orders of magnitude than that of dark current below 13 V bias. The peak value zones of the photoresponse were located at 280-310 nm at different reverse biases, and the peak value located at 300 nm was 100 times greater than the cut-off response value in 380 nm at a bias of 10V, which showed the device had good visible blind performance. A small red-shift about 5 nm on the peak responsivity occurred when reverse bias increased from 5 to 15 V. (c) 2006 Elsevier Ltd. All rights reserved.

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51 % and a stabilized efficiency of 8.01% (AM 1.5, 100 mw/cm(2)) at room temperature. (c) 2006 Published by Elsevier B.V.

Metal film effect on performance of very-small-aperture lasers

Special characteristics of very-small-aperture lasers are observed, including threshold current change, red shift of the spectral position, and short lifetime at low drive current. Physical mechanisms that underlie these special characteristics are analyzed: we find that optical feedback caused by a metal film and heat accumulation inside the laser diode lead to the special characteristics of VSALs, such as threshold current change, red shift of the spectral position, and short lifetime at low drive current, etc. Theoretical simulation is in good agreement with the experimental results.

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.

Structural and photoluminescent properties of ternary Zn1-xCdxO crystal films grown on Si(111) substrates

The ternary Zn1-xCdxO (0less than or equal toxless than or equal to0.6) alloying films with highly c-axis orientation have been deposited on Si(111) substrates by direct current reactive magnetron sputtering method. X-ray diffraction measurement indicates that the wurtzite-type structure of ZnO can be stabilized up to nominal Cd content x similar to 0.6 without cubic CdO phase separation. The lattice parameter c of Zn1-xCdxO increases almost linearly from 5.229 Angstrom (x = 0) to 5.247 Angstrom (x = 0.6), indicating that Cd substitution takes place on the Zn lattice sites. The photoluminescence spectra of the Zn1-xCdxO thin films measured at 12 K display a substantial red shift (similar to0.3 eV) in the near-band-edges (NBEs) emission of ZnO: from 3.39 eV of ZnO to 3.00 eV of Zn0.4Cd0.6O. The direct modulation of band gap caused by Zn/Cd substitution is responsible for the red shift effect in NBE emission of ZnO. (C) 2003 Elsevier Science B.V. All rights reserved.

Localized exciton dynamics in AlInGaN alloy

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved PL (TRPL) at various temperatures. The fast red-shift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/,tau)13], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed QDs or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent beta on temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered QDs. Furthermore, the localized states are found to have 0D density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.

Temperature behaviour of the orange and blue emissions in ZnS : Mn nanoparticles

The temperature dependences of the orange and blue emissions in 10, 4.5, and 3 nm ZnS:Mn nanoparticles were investigated. The orange emission is from the T-4(1)-(6)A(1) transition of Mn2+ ions and the blue emission is related to the donor-acceptor recombination in the ZnS host. With increasing temperature, the blue emission has a red-shift. On the other hand, the peak energy of the orange emission is only weakly dependent on temperature. The luminescence intensity of the orange emission decreases rapidly from 110 to 300 K for the 10 nm sample but increases obviously for the 3 nm sample, whereas the emission intensity is nearly, independent of temperature for the 4.5 nm sample. A thermally activated carrier-transfer model has been proposed to explain the observed abnormal temperature behaviour of the orange emission in ZnS:Mn nanoparticles.

Temperature dependence of photoluminescence of flat and undulated SiGe/Si multiple quantum wells

Photoluminescence (PL) of strained SiGe/Si multiple quantum wells (MQW) with flat and undulated SiGe well layers was studied at different temperature. With elevated temperature from 10K, the no-phonon (NP) peak of the SiGe layers in the flat sample has firstly a blue shift due to the dominant transition converting from bound excitons (BE) to free excitons (FE), and then has a red shift when the temperature is higher than 30K because of the narrowing of the band gap. In the undulated sample, however, monotonous blue shift was observed as the temperature was elevated from 10 K to 287 K. The thermally activated electrons, confined in Si due to type-II band alignment, leak into the SiGe crest regions, and the leakage is enhanced with the elevated temperature. It results in a blue shift of the SiGe luminescence spectra.

Effect on the optical properties and surface morphology of cubic GaN grown by metalorganic chemical vapor deposition using isoelectronic indium surfactant doping

The surfactant effect of isoelectronic indium doping during metalorganic chemical vapor deposition growth of cubic GaN on GaAs (1 0 0) substrates was studied. Its influence on the optical properties and surface morphology was investigated by using room-temperature photoluminescence (PL) and atomic force microscopy. It is shown that the sample with small amount of In-doping has a narrower PL linewidth, and a smoother surface than undoped cubic GaN layers. A slight red shift of the near-band-edge emission peak was observed. These results revealed that, for small TMIn flow rates, indium played the role of the surfactant doping and effectively improved the cubic GaN film quality; for large TMIn flow rates, the alloying formation of Ga1-xInxN might have occurred. (C) 2002 Elsevier Science B.V. All rights reserved.

Photoluminescence properties of self-organized InGaAs/GaAs quantum dot structures

Optical properties of InGaAs/GaAs self-organized quantum dots (QDs) structures covered by InxGa1-x As capping layers with different In contents chi ranging from 0. 0 (i.e., GaAs) to 0. 3 were investigated systematically by photoluminescence (PL) measurements. Red-shift of the PL peak energies of the InAs QDs covered by InxGa1-xAs layers with narrower linewidth and less shifts of the PL emissions via variations of the measurement temperatures were observed compared with that covered by GaAs layers. Calculation and structural measurements confirm that the red-shift of the PL peaks are mainly due to strain reduction and suppression of the In/Ga intermixing due to the InxGa1-xAs cover layer, leading to better size uniformity and thus narrowing the PL linewidth of the QDs. 1. 3 mum wavelength emission with very narrow linewidth of only 19. 2 meV at room temperature was successfully obtained from the In0.5Ga0.5As/GaAs QDs covered by the In0.2Ga0.8As layer.

Influence of InxGa1-xAs (0 <= x <= 0.3) cap layer on structural and optical properties of self-assembled InAs/GaAs quantum dots

Optical and structural properties of self-organized InAs/GaAs quantum dots (QDs) with InxGa1-xAs or GaAs cover layers grown by molecular beam epitaxy (MBE) have been characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL) measurements. The TEM and AFM images show that the surface stress of the InAs QDs was suppressed by overgrowth of a InxGa1-xAs covering layer on the top of the QDs and the uniformity of the QDs preserved. PL measurements reveal that red shifts of the PL emission due to the reduction of the surface strain of the InAs islands was observed and the temperature sensitivity of the PL emission energy was suppressed by overgrowth of InxGa1-xAs layers compared to that by overgrowth of GaAs layers.

Photoluminescence studies of type-II self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots grown on (311)A GaAs substrate

We investigated the photoluminescence (PL) of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots (QDs) grown on (311)A GaAs substrate. The PL peak at 10 K shifts to lower energy by about 30 meV when the excitation power decreases by two orders of magnitude. It has a red-shift under pressure, that is the character of X-like transition. Moreover, its peak energy is smaller than the indirect gap of bulk Al0.5Ga0.5As and In0.55Al0.45As. We then attribute that peak to the type-II transition between electrons in X valley of Al0.5Ga0.5As and heavy holes in In0.55Al0.45As QDs. A new peak appears at the higher energy when temperature is increased above 70 K. It shifts to higher energy with increasing pressure, corresponding to the transition from conduction Gamma band to valence band in QDs. The measurements demonstrate that our In0.55Al0.45As/Al0.5Ga0.5As quantum dots are type-II QDs with X-like conduction-band minimum. To interpret the second X-related peak emerged under pressure, we discuss the X-valley split in QDs briefly. (C) 2000 American Institute of Physics. [S0003-6951(00)04622-2].

Raman scattering of nanocrystalline silicon embedded in SiO2

Raman scattering of nanocrystalline silicon embedded in SiO2 matrix is systematically investigated. It is found that the Raman spectra can be well fitted by 5 Lorentzian lines in the Raman shift range of 100-600 cm(-1). The two-phonon scattering is also observed in the range of 600-1100 cm(-1) The experimental results indicate that the silicon crystallites in the films consist of nanocrystalline phase and amorphous phase; both can contribute to the Raman scattering. Besides the red-shift of the first order optical phonon modes with the decreasing size of silicon nanocrystallites, we have also found an enhancement effect on the second order Raman scattering, and the size effect on their Raman shift.

Optical properties of nanometer silicon prepared by silicon ion implanted into SiO2 layers

The samples of silicon nanocrystals (nc-Si) were prepared by Si ion implanted into SiO2 layers. Photoluminescence spectra were measured at room temperature and their dependence on thermal annealing was investigated. The experimental results show that PL peaks originate from the defects in SiO2 layers caused by ion implantation when the thermal annealing temperature is lower than 800 C. The PL peak from nc-Si was observed when the thermal annealing temperature was higher than 900 C, and PL intensity reached its maximum at the thermal annealing temperature of 1100 C. As the annealing temperature increases the red shift of PL peak from nc-Si shows the quantum size effect. The characterized Raman scattering peak of nc-Si was observed at the right angle scattering configuration for the first time. It provides further support for the PL measurements.