High-Temperature Annealing Induced He Bubble Evolution in Low Energy He Ion Implanted 6H-SiC

Bubble evolution in low energy and high dose He-implanted 6 H-SiC upon thermal annealing is studied. The(0001)-oriented 6 H-SiC wafers are implanted with 15 keV helium ions at a dose of 1×10~(17) cm~(-2) at room temperature. The samples with post-implantation are annealed at temperatures of 1073, 1173, 1273, and 1473 K for30 min. He bubbles in the wafers are examined via cross-sectional transmission electron microscopy(XTEM)analysis. The results present that nanoscale bubbles are almost homogeneously distributed in the damaged layer of the as-implanted sample, and no significant change is observed in the He-implanted sample after 1073 K annealing. Upon 1193 K annealing, almost full recrystallization of He-implantation-induced amorphization in 6 H-SiC is observed. In addition, the diameters of He bubbles increase obviously. With continually increasing temperatures to 1273 K and 1473 K, the diameters of He bubbles increase and the number density of lattice defects decreases.The growth of He bubbles after high temperature annealing abides by the Ostwald ripening mechanism. The mean diameter of He bubbles located at depths of 120-135 nm as a function of annealing temperature is fitted in terms of a thermal activated process which yields an activation energy of 1.914+0.236 eV.     

Structural evolution of a-Si:H/SiO2 multilayers upon step by step thermal annealing

a-Si:H/SiO_2 multilayers were prepared by alternatively changing plasma enhanced chemical vapour deposition of a-Si:H layers and in situ plasma oxidation process. Subsequently, as-grown samples were annealed at temperatures from 350℃ to 1100℃ in N_2 ambient with an increment of 100℃. The evolution of bonding configurations and structures with annealing treatments was systematically investigated by Fourier-transform infrared spectroscopy. The peak position of Si－O stretching vibration of SiO_2 layers shift to 1087cm^{-1} after annealing at 1100℃, which demonstrates that the SiO_2 films fabricated by plasma oxidation after high temperature annealing can have similar properties to the thermal grown ones. A Si－O vibration from interfacial SiO_x was identified: the value x was found to increase as increasing the annealing temperature, which is ascribed to the cooperation of hydrogen effusion and reordering of the oxygen bond in SiO_x networks. The H-related bonds were observed in the form of H－Si－O_3 and H－Si－Si_{3-n}O_n (n=1－2) configurations, which are supposed to be present in SiO_2 and interfacial SiO_x layers, respectively. The H atoms bonded in different bonding configurations effuse at different temperatures due to their different desorption energies.     

Strain Effect on Photoluminescences from InGaN MQWs with Different Barriers Grown by MOCVD

InGaN/GaN MQWs, InGaN/AlGaN MQWs and InGaN/AlInGaN MQWs are grown on （0001） sapphire substrates by MOCVD. Membrane samples are fabricated by laser lift-off technology. The photoluminescence spec-ra of membranes show a blue shift of peak positions in InGaN/GaN MQWs, a red shift of peak positions in InGaN/AlGaN MQWs and no shift of peak positions in InGaN/AIlnGaN MQWs from those of samples with substrates. Different changes in Raman scattering spectra and HR-XRD （0002） profile of InGaN/AlInGaN MQWs, from those of InGaN/GaN MQWs and InGaN/AlGaN MQWs, are observed. The fact that the strain changes differently among InGaN MQWs with different barriers is confirmed. The AIlnGaN barrier could adjust the residual stress for the least strain-induced electric field in InGaN/AIlnGaN quantum wells.     

Effect of Microstructure of TiO2 Thin Films on Optical Band Gap Energy

TiO2 coatings are prepared on fused silica with conventional electron beam evaporation deposition. After annealed at different temperatures for four hours, the spectra and XRD patterns of TiO2 thin film are obtained. XRD patterns reveal that only anatase phase can be observed in TiO2 coatings regardless of the different annealing temperatures, and with the increasing annealing temperature, the grain size gradually increases. The relationship between the energy gap and microstructure of anatase is determined and discussed. The quantum confinement effect is observed that with the increasing grain size of TiO2 thin film, the band gap energy shifts from 3.4 eV to3.21 eV. Moreover, other possible influence of the TiO2 thin-film microstructure, such as surface roughness and thin film absorption, on band gap energy is also expected.     

Annealing temperature influence on the degree of inhomogeneity of the Schottky barrier in Ti/4H–SiC contacts

Tung＇s model was used to analyze anomalies observed in Ti/Si C Schottky contacts. The degree of the inhomogeneous Schottky barrier after annealing at different temperatures is characterized by the ‘T0anomaly＇ and the difference（△Φ）between the uniformly high barrier height（Φ0B） and the effective barrier height（Φeff B）. Those two parameters of Ti Schottky contacts on 4H–Si C were deduced from I–V measurements in the temperature range of 298 K–503 K. The increase in Schottky barrier（SB） height（ΦB） and decrease in the ideality factor（n） with an increase measurement temperature indicate the presence of an inhomogeneous SB. The degree of inhomogeneity of the Schottky barrier depends on the annealing temperature, and it is at its lowest for 500-°C thermal treatment. The degree of inhomogeneity of the SB could reveal effects of thermal treatments on Schottky contacts in other aspects.     

Substrate-induced stress in silicon nanocrystal/SiO₂ multilayer structures

A Raman frequency upshift in the nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in the Si nanocrystals. The 10-period amorphous-Si(3 nm)/amorphous-SiO2 (3 nm) layers are deposited by high-vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N2 atmosphere at 1100°C for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high growth temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in the thermal expansion coefficient between the substrate and the Si/SiO2 SL film. Such a substrate-induced stress indicates a new method for tuning the optical and electronic properties of Si nanocrystals for strained engineering.     

Effect of Quenching on the Grain Boundary Relaxation in PM2000 ODS Alloy

Grain boundary relaxation in a Fe-based ODS alloy is studied by internal friction measurements. It is found that a grain-boundary peak appears at a lower temperature in the quenched specimens than that in the annealed specimens. The activation energy of the peak is H=2.82±0.11 eV for the former while H = 2.53±0.08 eV for the latter. In addition, a new relaxation peak is observed at the high temperature side of the grain boundary peak in the quenched specimens with an activation energy of 4.41±0.25 eV. The height of the peak increases with increasing quenching temperature. The results suggest that both the shift of the grain-boundary peak and the appearance of the new peak are due to increasing vacancies by quenching that are favourable for the motion of the grain boundaries.     

Temperature－Induced Switching－Over of the Luminescence Transitions in GaInNAs／GaAs Quantum Wells

Photoluminescence (PL) spectra of the GaInNAs/GaAs single quantum well (SQW) with different N compositions are carefully studied in a range of temperatures and excitation power densities. The anomalous S-shape temperature dependence of the PL peak is analysed based on the competition and switching-over between the peaks related to N-induced localized states and the peak related to interband excitonic recombination. It is found that with increasing N composition, the localized energy increases and the turning point of the S-shape temperature dependence occurs at higher temperature, where the localized carriers in the handrail states obtain enough thermal activation energy to be dissociated and delocalized. The rapid thermal annealing (RTA)effectively reduces the localized energy and causes a decrease of the switching-over temperature.     

Optical and structural properties of Ge-ion-implanted fused silica after annealing in different ambient conditions

Ge+ ions are implanted into fused silica glass at room temperature and a fluence of 1×10 17 cm-2 . The as-implanted samples are annealed in O2, N2 and Ar atmospheres separately. Ge0 , GeO and GeO2 coexist in the as-implanted and annealed samples. Annealing in different atmospheres at 600℃ leads each composite to change its content. After annealing at 1000℃, there remains some amount of Ge 0 in the substrates. However, the content of Ge decreases due to out-diffusion. After annealing in N2 , Si–N composite is formed. The absorption peak of GeO appears at 240 nm after annealing in O2 atmosphere, and a new absorption peak occurs at 418 nm after annealing in N2 atmosphere, which is attributed to the Si–N composite. There is no absorption peak appearing after annealing in Ar atmosphere. Transmission electron microscopic images confirm the formation of Ge nanoparticles in the as-implanted sample and GeO 2 nanoparticles in the annealed sample. In the present study, the GeO content and the GeO2 content depend on annealing temperature and atmosphere. Three photoluminescence emission band peaks at 290, 385 and 415 nm appear after ion implantation and they become strong with the increase of annealing temperature below 700℃, and their photoluminescences recover to the values of as-grown samples after annealing at 700℃. Optical absorption and photoluminescence depend on the annealing temperature and atmosphere.     

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.     

Time—Resolved Photoluminescence Studies of AlInGaN Alloys

We study the two samples of AlInGaN,i.e.,1-μm Gan grown at 1030℃ on the buffer and followed by a 0.6μm-thick epilayer of AlInGaN under the low pressure of 76 Torr and the AlInGaN layer deposited diectly on the buffer layer without the high-temperature GaN layer,by temperature-dependent photoluminescence(PL) spectroscopy and picosecond time-resolved photoluminescence(TRPL) spectroscopy.The TRPL signals of both the samples were fitted well as a stretched exponential decay at all temperatures,indicating significant disorder in the material.We attribute the disorder to nanoscale quantum dots or discs of high indium concentration.Temperature dependence of dispersive exponent β shows that the stretched exponential decay of the two samples comes from dfferent mechanisms.The different depths of the localization potential account for the difference,which is illustrated by the results of temperature dependence of radiative recombination lifetime and PL peak energy.     

Electrical and optical characteristics of vanadium in 4H-SiC

A semi-insulating layer is obtained in n-type 4H-SiC by vanadium-ion implantation. A little higher resistivity is obtained by increasing the annealing temperature from 1450 to 1650℃. The resistivity at room temperature is as high as 7.6×10⁶\Omega .cm. Significant redistribution of vanadium is not observed even after 1650℃ annealing. Temperature-dependent resistivity and optical absorption of V-implanted samples are measured. The activation energy of vanadium acceptor level is observed to be at about E_C-1.1eV.     

Carrier Dynamics Determined by Carrier-Phonon Coupling in InGaN/GaN Multiple Quantum Well Blue Light Emitting Diodes

Phonon sidebands in the electrolumiescence(EL) spectra of InGaN/GaN multiple quantum well blue light emitting diodes are investigated. S-shaped injection current dependence of the energy spacing(ES) between the zero-phonon and first-order phonon-assisted luminescence lines is observed in a temperature range of 100–150 K.The S-shape is suppressed with increasing temperature from 100 to 150 K, and vanishes at temperature above200 K. The S-shaped injection dependence of ES at low temperatures could be explained by the three stages of carrier dynamics related to localization states:(i) carrier relaxation from shallow into deep localization states,(ii) band filling of shallow and deep localization states, and(iii) carrier overflow from deep to shallow localization states and to higher energy states. The three stages show strong temperature dependence. It is proposed that the fast change of the carrier lifetime with temperature is responsible for the suppression of S-shaped feature.The proposed mechanisms reveal carrier recombination dynamics in the EL of InGaN/GaN MQWs at various injection current densities and temperatures.     

Substrate-induced stress in silicon nanocrystal/SiO2 multilayer structure

A Raman frequency upshift of nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in Si nanocrystals. The 10-period amorphous-Si(3 nm)/amorphous-SiO₂ (3 nm) layers are deposited by high vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N₂ atmosphere at 1100 ℃ for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high grown temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in thermal expansion coefficient between the substrate and the Si/SiO₂ SL film. Such a substrate-induced stress indicates a new method to tune the optical and the electronic properties of Si nanocrystals for strained engineering.     

Effects of Substrate Temperature and Vacuum Annealing on Properties of ITO Films Prepared by Radio-Frquency Magnetron Sputtering

Indium tin oxide （ITO） films were prepared by rf magnetron sputtering under two conditions： （i） at substrate temperature Ts from room temperature （RT） to 350℃ （ii） with additional post-annealing in vacuum at 400℃ for 30 min in comparison of their crystalline structures, and electrical-optical properties of the films deposited. From the experimental results, it is found that, under the first condition, the crystalline structures and the electrical-optical properties of the films are improved with the increasing Ts. Under the other condition, i.e. with the additional post-annealing, the films exhibit higher degree of crystallinities and better electrical-optical properties. Under the two deposition conditions, inter-relation between electrical-optical properties and the crystalline structure is observed clearly. However, even under the same annealing condition, it is observed that improved properties of the films are different, depending on their deposition temperatures, which implies that an initial stage of the ITO film before annealing is an important factor for the film＇s properties improved after annealing. The resistivity of 2.33 × 10-4 Ω·cm can be achieved at Ts of 350℃after annealing.     

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.     

Effect of high temperature annealing on strain and band gap of GaN nanoparticles

Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200 oC for 30 min. XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200 oC, and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions. If the as-synthesised GaN nanoparticles at 950 oC are regarded as standard, the thermal expansion changes nonlinearly with temperature and is anisotropic; the expansion below 1000oC is smaller than that above 1000 oC. This study provides an experimental demonstration for selecting the proper annealing temperature of GaN. In addition, a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200 oC is observed, which can be ascribed to the dominant transitions from the C(Γ₇) with the peak energy at 3.532 eV.     

Various Trap States at SiGe-SiO2 Interface Formed by a Pulsed Laser

We report fabrication of low-dimensional structures in air by a pulsed laser on SiGe alloy samples in which different oxide structures are formed by laser irradiation and annealing treatment. The micro-structures on SiGe are more complex than those on Si. A series of photolumineseence （PL） emission is observed due to various trap states at the SiGe-SiO2 interface formed under different preparing conditions. The peak centre of PL emission exhibits red=shift from Si to SiGe because of narrower gap. A model for explaining the PL emission is proposed in which the trap states of the interface between some oxide and SiGe play an important role.     

Anomalous Pressure Behavior of N-Cluster Emissions in GaAs0.973Sb0.022N0.005

Photoluminescence of GaAs0.973Sb0.022N0.005 is investigated at different temperatures and pressures. Both the alloy band edge and the N-related emissions, which show different temperature and pressure dependences, are observed. The pressure coefficients obtained in the pressure range 0-1.4GPa for the band edge and N-related emissions are 67 and 45meV/GPa, respectively. The N-related emissions shift to a higher energy in the lower pressure range and then begin to redshift at about 8.5GPa. This redshift is possibly caused by the increase of the X-valley component in the N-related states with increasing pressure.     

Effect of high temperature band gap of GaN annealing on strain and nanoparticles

<正>Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200℃for 30 min.XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200℃,and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions.If the as-synthesised GaN nanoparticles at 950℃are regarded as standard,the thermal expansion changes nonlinearly with temperature and is anisotropic;the expansion below 1000℃is smaller than that above 1000℃.This study provides an experimental demonstration for selecting the proper annealing temperature of GaN.In addition,a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200℃is observed,which can be ascribed to the dominant transitions from the C（Γ₇） with the peak energy at 3.532 eV.