Study of depth-dependent tetragonal distortion of quaternary AIInGaN epilayer by Rutherford backscattering/channeling

<正>A 240-nm thick Al_0.4In_0.02Ga_0.58N layer is grown by metal organic chemical vapour deposition,with an over 1-μm thick GaN layer used as a buffer layer on a substrate of sapphire（0001）.Rutherford backscattering and channeling are used to characterize the microstructure of AlInGaN.The results show a good crystalline quality of AlInGaN(x_min= 1.5%) with GaN buffer layer.The channeling angular scan around an off-normal（1213） axis in the {1010} plane of the AlInGaN layer is used to determine tetragonal distortion e_T,which is caused by the elastic strain in the AlInGaN.The resulting AlInGaN is subjected to an elastic strain at interfacial layer,and the strain decreases gradually towards the near-surface layer.It is expected that an epitaxial AlInGaN thin film with a thickness of 850 nm will be fully relaxed （e_T=0）.     

Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.     

Effect of Nonradiative Recombination on Carrier Dynamics in GaInNAs/GaAs Quantum Wells

The nonradiative recombination effect on carrier dynamics in GalnNAs/GaAs quantum wells is studied by timeresolved photoluminescence （TRPL） and polarization-dependent TRPL at various excitation intensities. It is found that both recombination dynamics and spin relaxation dynamics strongly depend on the excitation intensity. Under moderate excitation intensities the PL decay curves exhibit unusual non-exponential behaviour. This result is well simulated by a rate equation involving both the radiative and non-radiative recombinations via the introduction of a new parameter of the effective concentration of nonradiative recombination centres in the rate equation. In the spin dynamics study, the spin relaxation also shows strong excitation power dependence. Under the high excitation power an increase of spin polarization degree with time is observed. This new finding provides a useful hint that the spin process can be controlled by excitation power in GaInNAs systems.     

Time-Resolved Photoluminescence Studies of Indium-Rich InGaN Alloys

Time-resolved photoluminescence (PL) spectroscopy has be used to investigate indium-rich InGaN Mloys grown on sapphire substrates by metal organic chemical vapor deposition. Photoluminescence measurement indicates two dominant emission lines originating from phase-separated high- and low-indium-content regions. Temperature and excitation intensity dependence of the two main emission lines in these InGaN alloys have been measured.Temperature and energy dependence of PL decay lifetime show clearly different decay behaviour for the two main lines. Our results show that photo-excited carriers are deeply localized in the high indium regions while photo-excited carriers can be transferred within the low-indium-content regions as well as to high-content regions.     

Photoluminescence of ZnSe—ZnS Single Quantum Wells Grown by Vapour Phase Epitaxy

We study the photoluminescence (PL) of ultra thin layer ZnSe quantum Wells in ZnS barriers.Samples with different well widths are grown by vapour phase epitaxy and the PL spectra of these samples are measured by the excitation of a 500W Hg lamp.The peak positions of the bands coming from the excitonic luminescence show a larger blueschift with respect to the energy of free excitons in the ZnSe bulk material.The observed variation of the full width at half maximum and peak position of the bands in the spectra with the well width are interpreted to the formation of the ZnSxSe1-x alloy layer due to the interdiffusion in the interfaces between ZnSe and ZnS.According to the behaviour of the excitons in the smaller conduction band offset,the exciton binding energy is estimated from the dependence of the PL intensity on the temperature.from this result,excitons seem to show nearly three-dimensional characteristics.     

Study of depth-dependent tetragonal distortion of quaternary AlInGaN epilayer by Rutherford backscattering/channeling

A 240-nm thick Al_0.4In_0.02Ga_0.58N layer is grown by metal organic chemical vapour deposition, with an over 1-μ m thick GaN layer used as a buffer layer on a substrate of sapphire (0001). Rutherford backscattering and channeling are used to characterize the microstructure of AlInGaN. The results show a good crystalline quality of AlInGaN (χ_min=1.5%) with GaN buffer layer. The channeling angular scan around an off-normal <12^{-13> axis in the {101-0} plane of the AlInGaN layer is used to determine tetragonal distortion e_T, which is caused by the elastic strain in the AlInGaN. The resulting AlInGaN is subjected to an elastic strain at interfacial layer, and the strain decreases gradually towards the near-surface layer. It is expected that an epitaxial AlInGaN thin film with a thickness of 850 nm will be fully relaxed (e_T = 0).}     

Comparison between photoluminescence spectroscopy and photoreflectance spectroscopy in CuGaSe2 epilayer

Photoluminescence （PL） spectroscopy and photoreflectance （PR） spectroscopy are very useful techniques for studying the properties of materials. In this paper, the same material of Cu-rich metal-organic vapour phase epitaxy （MOVPE） grown CuGaSe2 layer is investigated in a temperature range from 20 to 300 K to compare these two techniques. Both PL and PR spectra appear red shifted, less intense and broadened. The temperature dependence of interband transitions is studied by using the Manoogian Leclerc equation. The values of the band gap energy at T=0K and the effective phonon temperature are estimated. The temperature dependences of intensities and broadenings of PL and PR spectral lines are also analysed. Based on the results of the comparison, the features and applications of the PL and PR can be shown in detail.     

Growth and Optical Properties of Double Heterostructure GaN／InGaN／GaN Films with Large Composition

The double heterostructure GaN/InGaN/GaN films with different thicknesses of the InGaN layer were grown at 780℃ or 800℃ by metal-organic chemical vapour deposition.The samples were investigated using x-ray diffraction (XRD),room-temperature photoluminescence (PL) and Raman scattering.The dependences of the samples on both the growth temperature and the thickness of the InGaN layer were studied.The composition of InGaN was determined by the results of XRD,and the bowing parameter of InGaN was calculated in terms of the PL spectra.When the thickness of the InGaN layer was reduced,the phase separation of InGaN was found in some samples.The raman frequency of the A1(LO) and E2(low) modes in all the samples shifted and did not agree with Vegard‘s law.     

Graphene films grown on sapphire substrates via solid source molecular beam epitaxy

A method for growing graphene on a sapphire substrate by depositing an SiC buffer layer and then annealing at high temperature in solid source molecular beam epitaxy (SSMBE) equipment was presented. The structural and electronic properties of the samples were characterized by reflection high energy diffraction (RHEED), X-ray diffraction Φ scans, Raman spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results of the RHEED and Φ scan, as well as the Raman spectra, showed that an epitaxial hexagonal α-SiC layer was grown on the sapphire substrate. The results of the Raman and NEXAFS spectra revealed that the graphene films with the AB Bernal stacking structure were formed on the sapphire substrate after annealing. The layer number of the graphene was between four and five, and the thickness of the unreacted SiC layer was about 1--1.5 nm.     

Growth and characterization of InAsSb alloys by metalorganic chemical vapour deposition

Low pressure metalorganic chemical vapour deposition (LP-MOCVD) growth and characteristics of InAsSb on (100) GaSb substrates are investigated. Mirror-like surfaces with a minimum lattice mismatch are obtained. The samples are studied by photoluminescence spectra, and the output is 3.17μm in wavelength. The surface of InAsSb epilayer shows that its morphological feature is dependent on buffer layer. With an InAs buffer layer used, the best surface is obtained. The InAsSb film shows to be of n-type conduction with an electron concentration of 8.52×10¹⁶cm^-3.     

Effects of growth temperature and metamorphic buffer on electron mobility of InAs film grown on Si substrate by molecular beam epitaxy

The growth of the InAs film directly on the Si substrate deflected from the plane(100) at 4° towards(110) has been performed using a two-step procedure. The effect of the growth and annealing temperature on the electron mobility and surface topography has been investigated for a set of samples. The results show that the highest electron mobility is4640 cm~2/V·s in the sample, in which the 10-nm InAs nucleation layer is grown at a low temperature of 320 ℃ followed by ramping up to 560 ℃, and the nucleation layer was annealed for 15 min and the second layer of InAs is grown at 520 ℃.The influence of different buffer layers on the electron mobility of the samples has also been investigated, which shows that the highest electron mobility of 9222 cm~2/V·s at 300 K is obtained in the sample grown on a thick and linearly graded InGaAlAs metamorphic buffer layer deposited at 420 ℃.     

Molecular Beam Epitaxy Growth and Photoluminescence of Type-Ⅱ （GaAs1-xSbx／InyGa1-yAs）／GaAs Bilayer Quantum Well

We report a systematical study on the molecular beam epitaxy growth and optical property of (GaAs1-xSbx/InyGa1-yAs)/GaAs bilayer quantum we]] (BQW) structures. It is shown that the growth temperature of the wells and the sequence of layer growth have significant influence on the interface quality and the subsequent photoluminescence (PL) spectra. Under optimized growth conditions, three high-quality (GaAsSb0.29/In0.4 GaAs)/GaAs BQWs are successfully fabricated and a room temperature PL at 1314 nm is observed. The transition mechanism in the BQW is also discussed by photoluminescence and photoreflectance measurements. The results confirm experimentally a type-Ⅱ band alignment of the interface between the GaAsSb and InGaAs layers.     

Structural and band tail state photoluminescence properties of amorphous SiC films with different amounts of carbon

Amorphous silicon carbide films are deposited by the plasma enhanced chemical vapour deposition technique,and optical emissions from the near-infrared to the visible are obtained.The optical band gap of the films increases from 1.91 eV to 2.92 eV by increasing the carbon content,and the photoluminescence(PL) peak shifts from 1.51 eV to 2.16 eV.The band tail state PL mechanism is confirmed by analysing the optical band gap,PL intensity,the Stocks shift of the PL,and the Urbach energy of the film.The PL decay times of the samples are in the nanosecond scale,and the dependence of the PL lifetime on the emission energy also supports that the optical emission is related to the radiative recombination in the band tail state.     

Anomalous temperature dependence of photoluminescence spectra from InAs/GaAs quantum dots grown by formation–dissolution–regrowth method

Two kinds of InAs/GaAs quantum dot(QD) structures are grown by molecular beam epitaxy in formation–dissolution–regrowth method with different in-situ annealing and regrowth processes. The densities and sizes of quantum dots are different for the two samples. The variation tendencies of PL peak energy, integrated intensity, and full width at half maximum versus temperature for the two samples are analyzed, respectively. We find the anomalous temperature dependence of the InAs/GaAs quantum dots and compare it with other previous reports. We propose a new energy band model to explain the phenomenon. We obtain the activation energy of the carrier through the linear fitting of the Arrhenius curve in a high temperature range. It is found that the Ga As barrier layer is the major quenching channel if there is no defect in the material. Otherwise, the defects become the major quenching channel when some defects exist around the QDs.     

Structure and luminescence of Ca2Si5N8：Eu^2＋ phosphor for warm white light-emitting diodes

We have synthesized Ca 2 Si 5 N 8:Eu 2+ phosphor through a solid-state reaction and investigated its structural and luminescent properties.Our Rietveld refinement of the crystal structure of Ca 1.9 Eu 0.1 Si 5 N 8 reveals that Eu atoms substituting for Ca atoms occupy two crystallographic positions.Between 10 K and 300 K,Ca 2 Si 5 N 8:Eu 2+ phosphor shows a broad red emission band centred at ～1.97 eV-2.01 eV.The gravity centre of the excitation band is located at 3.0 eV-3.31 eV.The centroid shift of the 5d levels of Eu 2+ is determined to be ～1.17 eV,and the red-shift of the lowest absorption band to be ～ 0.54 eV due to the crystal field splitting.We have analysed the temperature dependence of PL by using a configuration coordinate model.The Huang-Rhys parameter S=6.0,the phonon energy ν=52 meV,and the Stokes shift S=0.57 eV are obtained.The emission intensity maximum occurring at ～200 K can be explained by a trapping effect.Both photoluminescence (PL) emission intensity and decay time decrease with temperature increasing beyond 200 K due to the non-radiative process.     

Influence of oxygen treatment and temperature on electrical properties of the epitaxial Nb-doped SrTiO3 films on silicon

We have fabricated the epitaxial Nb-doped SrTiO3(NbSTO) thin films on Si substrates using a TiN film as the buffer layer.The oxygen-treatment and temperature dependence of electrical properties has been investigated.Oxygen treatment showed the surface change of NbSTO films has immense influence on the resistance switching.The resistance ratio of two resistance states decreased after oxygen treatment.With tested-temperature rising,the resistance and resistance ratio of two resistance states increased.The resistance switching of Pt/NbSTO junction as a function of oxygen-treatment and temperature can be explained by the charge trapping and detrapping process in the Pt/NbSTO interface,which will help understand the resistance switching mechanism of oxides.     

Phase Transition of a Distance-Dependent Ising Model on the Barabasi-Albert Network

We report our investigation on the behaviour of distance-dependent Ising models, which are located on the BA model network. The interaction strength between two nodes （the spins） is considered to obey an exponential decay dependence on the geometrical distance. The Monte Carlo simulation shows a phase transition from ferromagnetism to paramagnetism, and the critical temperature approaches a constant temperature as the interaction decaying exponent increases.     

Enhanced Photoluminescence of InGaN/GaN Green Light-Emitting Diodes Grown on Patterned Sapphire Substrate

Green InGaN/GaN based light-emitting diodes （LEDs） are fabricated both on planar and wet-etched patterned sapphire substrates by metalorganic vapour phase epitaxy （MOVPE）. Their photoluminescence （PL） properties of the two samples are studied. The results indicate that the PL integral intensity of the green LED on the patterned substrate is nearly two times of that on the planar one within the whole measured temperature range. The enhanced PL intensity in the green LED on the patterned substrate is shown completely contributed from the extraction efficiency, but not from the internal quantum efficiency. The conclusion is supported by temperature-dependent PL analysis on the two samples, and the mechanisms axe discussed.     

The role of microstructure in luminescent properties of Er-doped nanocrystalline Si thin films

In this contribution, we present a structural and photoluminescence (PL) analysis of Er-doped nanocrystalline silicon thin films produced by rf magnetron sputtering method. We show the strong influence of the presence of nanocrystalline fraction in films on their luminescence efficiency at 1.54 μm studied on a series of specially prepared samples with different crystallinity, i.e., percentage and sizes of Si nanocrystals. A strong increase, by about two orders of magnitude, of Er-related PL intensity in these samples with lowering of the Si nanocrystal sizes from 7.9 to about 1.5 nm is observed. The results are discussed in terms of the sensitization effect of Si nanocrystals on Er ions. From Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 114–118. Original English Text Copyright ? 2004 by Stepikhova, Cerqueira, Losurdo, Giangregorio, Alves, Monteiro, Soares. This article was submitted by the authors in English.     

Growth of Semi-Insulating GaN by Using Two-Step AIN Buffer Layer

Semi-insulating GaN is grown by using a two-step A1N buffer layer by metalorganic chemical vapour deposition. The sheet resistance of as-grown semi-insulating GaN is dramatically increased to 10^13 Ω/sq by using two-step A1N buffer instead of the traditional low-temperature GaN buffer. The high sheet resistance of as-grown GaN over 10^13 Ω/sq is due to inserting an insulating buffer layer （two-step A1N buffer） between the high-temperature GaN layer and a sapphire substrate which blocks diffusion of oxygen and overcomes the weakness of generating high density carrier near interface of GaN and sapphire when a low-temperature GaN buffer is used. The result suggests that the high conductive feature of unintentionally doped GaN is mainly contributed from the highly conductive channel near interface between GaN and the sapphire substrate, which is indirectly manifested by room-temperature photoluminescence excited by an incident laser beam radiating on growth surface and on the substrate. The functions of the two-step A1N buffer layer in reducing screw dislocation and improving crystal quality of GaN are also discussed.