掺铒GaN薄膜光致发光的研究

采用傅立叶变换红外光谱（FT－IR）研究了掺铒GaN薄膜光致发光特性，光致发光谱（PL）的测量结果表明：选用退火时间长的电阻加热退火护退火，有利于薄膜中晶格损伤的恢复，MOCVD，MBF两种方法制备的GaN薄膜，注入铒，退火后的PL谱形状基本一样，薄膜中O，C的含量越大，可能导致1539nm处的PL强度越强，不同衬底对掺铒GaN薄膜的红外光致发光影响很大，在Si衬底上外延生长的GaN样品峰值在1539nm处的PL积分强度只有Al2O3(0001)衬底上外延生长GaN样品的30％，MBE生长的GaN/Al2O3样品，注入铒，退火后，当测量温度从15K变化到300K时，样品发光的温度猝灭是30％。     

InxGa1-xAs盖层InAs量子点的发光性能研究

用分子束外延系统在GaAs(001)衬底上生长InAs量子点,在InAs量子点上插入3 nm的In_0.4Ga_0.6As层,可将量子点发射波长调谐到1 300 nm附近.对样品进行氢等离子处理,研究处理前后样品的InAs量子点光致发光(PL)强度的变化.结果表明,在InAs量子点与相邻层的界面上以及GaAs层中存在界面缺陷,采用氢等离子处理可有效地抑制界面缺陷,大幅度地提高发光效率.     

退火温度对PLD法制备ZnO:Eu3+,Li+薄膜结构和发光性质的影响

采用脉冲激光沉积(PLD)法在Si(111)衬底上制备了Eu3+,Li+共掺杂的ZnO薄膜,分别在450,500,550和600℃条件下进行退火,退火气氛为真空。利用X射线衍射(XRD)仪和荧光分光光度计研究了退火温度对薄膜结构和光致发光(PL)的影响。研究结果表明,Eu3+,Li+共掺杂的ZnO薄膜具有c轴择优取向,Eu3+,Li+没有单独形成结晶的氧化物,均以离子形式掺入ZnO晶格中。PL谱中有较宽的ZnO基质缺陷发光,ZnO基质与稀土Eu3+之间存在能量传递,但没有有效的能量传递。随着退火温度的增加,薄膜发光先增强后减弱,退火温度为550℃时发光最强。当用395 nm的激发光激发样品时,仅观察到稀土Eu3+在594 nm附近的特征发光峰,但发光强度随退火温度变化不明显。     

退火时间对6H-SiC(0001)表面外延石墨烯形貌和结构的影响

在分子束外延(MBE)设备中,采用高温退火的方法在6H-SiC(0001)表面外延石墨烯,并研究了退火时间对外延石墨烯形貌和结构的影响.利用反射式高能电子衍射(RHEED)、原子力显微镜(AFM)、激光拉曼光谱(Raman)和近边X射线吸收精细结构(NEXAFS)等实验技术对制备的样品进行了表征.RHEED结果发现,不同退火时间的样品在SiC衍射条纹的外侧都出现了石墨烯的衍射条纹.AFM测试表明,外延石墨烯的厚度随退火时间增加而增大,且样品孔洞减少、表面更加平整.Raman测试表明,外延石墨烯拉曼谱中2D峰和G峰的位置相对高定向热解石墨(HOPG)中2D峰和G峰的位置蓝移,且退火时间增加,峰的蓝移量减小.对样品中碳原子K边NEXAFS谱测量显示,样品中存在sp2杂化的碳原子,退火时间增加使碳原子的1s→π以及1s→σ吸收的强度增大,且1s电子到π态的吸收峰相对HOPG的向高能偏移.     

Sn掺杂ZnO纳米针的结构及其生长机制(英文)

利用包括磁控溅射和热氧化的两步法在Si(111)衬底上制备了Sn掺杂ZnO纳米针.首先用磁控溅射法在Si(111)衬底上制备Sn:Zn薄膜,然后在650℃的Ar气氛中对薄膜进行热氧化,制备出Sn掺杂ZnO纳米针.样品的结构、成分和光学性质采用X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、高分辨透射电子显微镜(HRTEM)、能量散射X射线(EDX)谱和光致发光(PL)光谱等技术手段进行分析.结果表明,制备的样品为具有六方纤锌矿结构的单晶Sn掺杂ZnO纳米针,Sn掺杂量为2.5%(x,原子比),底部和头部直径分别为200-500 nm和40 nm,长度为1-3μm,结晶质量较高.室温光致发光光谱显示紫外发光峰比纯ZnO的发光峰稍有蓝移,这可归因于能谱分析中探测到的Sn的影响.基于本实验的实际条件,简单探讨了Sn掺杂ZnO纳米针的生长机制.     

氧分压对ZnO:Eu~(3+),Li~+薄膜结构和光学性质的影响

利用脉冲激光沉积(PLD)法在Si(111)衬底上制备了ZnO:Eu~(3+),Li+薄膜,分别通过XRD谱和光致发光(PL)谱测试研究了氧分压不同时薄膜结构和发光性质。XRD谱研究表明,所有样品均仅出现ZnO基质的(002)衍射峰,说明Eu~(3+)已进入ZnO基质晶格,没有单独形成结晶氧化物。PL谱研究表明,当用325 nm的光激发样品时,样品的发光仅由ZnO基质的紫外发光和绿光发射组成,没有出现稀土Eu~(3+)的特征发光峰。当用395 nm的光激发样品时,在594和613 nm处出现两个明显的Eu~(3+)特征发光峰,分别属于Eu~(3+)的磁偶极跃迁(~5D_0→~7F_1)和电偶极跃迁(~5D_0→~7F_2),而且随着氧压的增加,594 nm处的发光峰强度变化不大,但613 nm处的发光峰强度明显增强。     

具有SiC缓冲层的Si衬底上直接沉积碳原子生长石墨烯

石墨烯是近年发现的一种新型多功能材料.在合适的衬底上制备石墨烯成为目前材料制备的一大挑战.本文利用分子束外延(MBE)设备,在Si 衬底上生长高质量的SiC 缓冲层,然后利用直接沉积C原子的方法生长石墨烯,并通过反射式高能电子衍射(RHEED)、拉曼(Raman)光谱和近边X 射线吸收精细结构谱(NEXAFS)等实验技术对不同衬底温度(800、900、1000、1100 °C)生长的薄膜进行结构表征.实验结果表明,在以上衬底温度下都能生长出具有乱层堆垛结构的石墨烯薄膜.当衬底温度升高时,碳原子的活性增强,其成键的能力也增大,从而使形成的石墨烯结晶质量提高.衬底温度为1000 °C时结晶质量最好.其原因可能是当衬底温度较低时,碳原子活性太低不足以形成有序的六方C-sp2环.但过高的衬底温度会使SiC 缓冲层的孔洞缺陷增加,衬底的Si 原子有可能获得足够的能量穿过SiC薄膜的孔洞扩散到衬底表面,与沉积的碳原子反应生成无序的SiC,这一方面会减弱石墨烯的生长,另一方面也会使石墨烯的结晶质量变差.     

PbTe(111)薄膜的分子束外延生长及其表面结构特性

采用分子束外延(MBE)方法在Ba F_2(111)衬底上直接外延生长了Pb Te薄膜。反射高能电子衍射(RHEED)实时监控的衍射图样揭示了Pb Te在Ba F_2(111)表面由三维生长向二维生长的变化过程。转动对称性的研究结合第一性原理密度泛函理论(DFT)的计算揭示了在富Pb及衬底温度(Tsub)为350°C的生长条件下,得到的Pb Te(111)薄膜具有稳定的(2×1)重构表面。Pb Te(111)-(2×1)表面覆盖Te膜后,通过300°C的退火处理,重构表面可完全复原,这为大气环境下Pb Te薄膜表面结构的保护提供了有效的方法。     

YSi_2纳米颗粒的制备及光学、电学性能研究

采用脉冲激光烧蚀高纯YSi2靶,在n型Si(100)单晶衬底上制备YSi2纳米颗粒。原子力显微镜(AFM)观察样品表面颗粒尺寸约40~50 nm。X射线光电子能谱(XPS)测试结果表明,YSi2纳米颗粒成分为Y-O-Si。室温下对样品的光致发光(PL)性能进行测试,在500 nm处有一个较大的宽峰,409 nm附近出现强度较弱的发光峰。前者与样品中Y-O-Si电荷迁移带有关,后者为衬底表面纳米尺寸SiOx复合中心离子发光。室温下,对原位制备的薄膜电学(I-V/C-V)性能进行测试,结果表明薄膜的介电常数约为13.6。     

以CdS-ZnS核-壳量子点/聚酰胺-胺(PAMAM)树形分子纳米复合材料自组装制备发光超薄膜

制备了表面带负电荷的CdS-ZnS核-壳结构量子点/聚酰胺-胺(PAMAM)树形分子纳米复合材料, 并以其为阴离子, 以聚二甲基二丙烯基氯化铵(PDDA)为聚阳离子自组装制备了光致发光(PL)超薄膜. 对膜的生长均匀性、表面平整性及发光性能进行表征, 发现膜的UV-Vis吸收强度和PL强度随层数增加呈线性增大, 表明每层膜均匀生长; AFM照片表明单个复合层膜非常平整, 10层膜仍具有良好的平整度, 并发出明亮的蓝绿色光.     

Fabrication and Luminescent Properties of 6H-SiC/3C-SiC/6H-SiC Quantum Well Structure

Quantum well structure film of 6H-SiC/3C-SiC/6H-SiC was fabricated on 6H-SiC (0001) with the substrate temperature of 1350 K by solid source molecular beam epitaxy (SSMBE) through the variation of Si flux rate. The crystal polytypes and luminescent properties of the film were characterized by reflection high energy electron diffraction (RHEED) and photoluminescence (PL), respectively. The results of RHEED indicated that the film was 6H-SiC/3C-SiC/6H-SiC with the quantum well structure. The results of PL excited by He-Gd laser at room temperature showed that there were intense emissions in the range of 480–600 nm, which could not be observed from the substrate. The fitting peaks were consistent with the results calculated from the model of quantum well structure, which showed that such intense emissions were probably from the quantum wells with different widths.     

Growth of 3C-SiC Films on On-axis 6H-SiC Substrates by LPCVD

Cubic SiC (3C-SiC) films were deposited on on-axis 6H-SiC (0001) substrates by low-pressure chemical vapor deposition (LPCVD). The result of X-ray diffraction patterns shows that the 3C-SiC films were of good crystalline quality. The influence of the growth parameters (flow rates of the gas sources and growth temperature) on the growth rate of the SiC films is discussed. The results show that the transport of silane or its reaction products is the limiting factor for the growth. The surface morphology of the SiC films was observed by atomic force microscope imaging. From these results it can be concluded that the growth of the films is in agreement with a Stranski-Krastanov growth mode.     

Defect Characterization of 6H-SiC Studied by Slow Positron Beam

The defect formation and annealing behavior in as-grown and electron-irradiated 6H-SiC wafers were investi-gated by variable-energy slow positron beam. For the n-type as-grown samples, it was found that annealing decreased the defect concentration due to recombination with interstitial, and when it was annealed at 1400 ±C for 30 min in vacuum, a 20 nm thick Si layer was found on the top of SiC substrate, which is a direct proof of the Si atom diffusing to the surface when annealed at the high temperature stages. During the high temperature annealing stage, we found an obvious surface effect occurred that induced the higher S parameter close to the surface. This may be caused by the diffusion of the Si atoms to the surface during annealing. After 10 MeV electron irradiation of the n-type 6H-SiC, the positron effective diffusion length decreased from 86.2 nm to 39.1 nm. This shows that there are some defects created in n-type 6H-SiC. But in the p-type 6H-SiC irradiated by 10 MeV electrons, the change is very small. This may be because of the opposite charge of the vacancy defects. The same annealing behavior as that of as-grown 6H-SiC samples was also observed for the 1.8 MeV electron-irradiated 6H-SiC samples except that after being annealed at 300 ±C, its defect concentration increased. This may be explained as the generation of carbon vacancies, due to either the recombination between divacancies and silicon interstitial, or the charge of the charge states.     

Ni/4H-SiC 肖特基势垒磁场下输运性质的分析

Ni/4H-SiC Schottky contacts with good characteristics were fabricated using electron beam evaporation to deposit Ni on 4H-SiC（（0001）Si face）. Current-voltage（I-V）characteristics of Ni / 4H-SiC Schottky barrier have been studied in the temperature range from 160 K to 300 K in magnetic fields（B）up to 10 T. The thermionic emission theory and relaxation time approximation Boltzmann eqation were employed to calculate the I - V characteristics,and it is found that the change of current shows a linear relation with B2 and V,and is inversely proportional to the temperature,which well agrees with experimental results.     

Piezoresistance behaviors of p-type 6H-SiC nanowires

We reported, for the first time, the piezoresistance behaviors of single p-type 6H-SiC nanowires. The results suggest that present p-type 6H-SiC nanowires could be an excellent candidate for the fabrication of robust and reliable stress sensors.     

Photocatalytic reduction of carbon dioxide to methanol by Cu2O/SiC nanocrystallite under visible light irradiation

The Cu2O/SiC photocatalyst was obtained from SiC nanoparticles (NPs) modified by Cu2O. Their photocatalytic activities for reducing CO2 to CH3OH under visible light irradiation have been investigated. The results indicated that besides a small quantity of 6H-SiC, SiC NPs mainly consisted of 3C-SiC. The band gaps of SiC and Cu2O were estimated to be about 1.95 and 2.23 eV from UV-Vis spectra, respectively. The Cu2O modification can enhance the photocatalytic performance of SiC NPs, and the largest yields of methanol on SiC, Cu2O and Cu2O/SiC photocatalysts under visible light irradiation were 153, 104 and 191μmol/g, respectively.     

Optical Absorption Measurements on Nitrogen-doped 6H-SiC Single Crystals

6H-SiC bulk crystals have been prepared by sublimation method in an indu-ctively heated growth reactor. The effect of nitrogen doping on absorption for 6H-SiC was investigated. The absorption measurement based on optical method is a non-destructive and non-contact method. The band-gap narrowing with higher doping concentration was observed. For n-type doping below band-gap absorption band at 623 nm for 6H-SiC was observed. The peak intensity of the absorption band increased with increasing charge carrier concentration obtained from Hall measurements. It is also found that the nitrogen doping level decreased in the radial direction and it was the highest at the beginning of growth.     

6MAP, a fluorescent adenine analogue, is a probe of base flipping by DNA photolyase

Cyclobutylpyrimidine dimers (CPDs) are formed between adjacent pyrimidines in DNA when it absorbs ultraviolet light. CPDs can be directly repaired by DNA photolyase (PL) in the presence of visible light. How PL recognizes and binds its substrate is still not well understood. Fluorescent nucleic acid base analogues are powerful probes of DNA structure. We have used the fluorescent adenine analogue 6MAP, a pteridone, to probe the local double helical structure of the CPD substrate when bound by photolyase. Duplex melting temperatures were obtained by both UV-vis absorption and fluorescence spectroscopies to ascertain the effect of the probe and the CPD on DNA stability. Steady-state fluorescence measurements of 6MAP-containing single-stranded and doubled-stranded oligos with and without protein show that the local region around the CPD is significantly disrupted. 6MAP shows a different quenching pattern compared to 2-aminopurine, another important adenine analogue, although both probes show that the structure of the complementary strand opposing the 5'-side of the CPD lesion is more destacked than that opposing the 3'-side in substrate/protein complexes. We also show that 6MAP/CPD duplexes are substrates for PL. Vertical excitation energies and transition dipole moment directions for 6MAP were calculated using time-dependent density functional theory. Using these results, the F?rster resonance energy transfer efficiency between the individual adenine analogues and the oxidized flavin cofactor was calculated to account for the observed intensity pattern. These calculations suggest that energy transfer is highly efficient for the 6MAP probe and less so for the 2Ap probe. However, no experimental evidence for this process was observed in the steady-state emission spectra.     

Transparent polymeric hybrid film of ZnO nanoparticle quantum dots and PMMA with high luminescence and tunable emission color

ZnO nanoparticle quantum dots (QDs)/poly(methyl methacrylate) (PMMA) composites are synthesized by conventional radical polymerization in the presence of 3-(trimethoxysilyl)propylmethacrylate (TPM)-modified ZnO nanoparticle QDs. Although unmodified ZnO nanoparticle QDs were precipitated in tetrahydrofuran (THF) and show only weak emissions under UV irradiation, ZnO nanoparticle QDs/PMMA composite is well dispersed in THF and shows high emissions. TPM acts as the stabilizer and promotes the compatibility between the ZnO nanoparticle QDs and the PMMA matrix. After evaporation of THF from the ZnO nanoparticle QDs/PMMA composite solution, transparent polymeric hybrid films of ZnO nanoparticle QDs and PMMA are obtained. These polymeric hybrid films are characterized by photoluminescence (PL) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and thermogravimetric analysis. The hybrid film exhibited a high quantum yield and PL emission under ultraviolet excitation. PL emission has been successfully tuned from blue to yellow.