Inelastic light scattering from electronic excitations in deep-etched quantum dots and wires

Resonant Raman spectroscopy of modulation-doped GaAs/AlGaAs multiple quantum well dots and wires is reported. Deep etching with a SiCl₄ reactive ion etching process achieved an excellent aspect ratio (>10:1) and low surface damage for dots and wires of sizes in the range 60–250 nm. A rich spectrum of single particle excitations was observed at Raman shifts in the range 1–35 meV for both dots and wires. Sharp resonances were found for the Raman intensities. The electronic scattering in wires exhibits distinct polarization properties in agreement with theoretical predictions and the spin density excitation energies are in reasonable agreement with Hartree approximation calculations. The dispersion of the intrasubband plasmon collective mode in 60 nm wires has been determined. The excitations in dots show a systematic shift to higher energy with decreasing dot diameter consistent with increased confinement. Magneto-Raman scattering from dot samples was also investigated at magnetic fields up to 12 T and the excitation spectra show level splitting, level crossing and mode softening with increasing magnetic field.     

Oxygen-doped ZnTe phosphors for synchrotron X-ray imaging detectors

ZnTe:O powder phosphors were successfully prepared by a dry synthesis process using gaseous doping and etching media. It was found that dry doping by O₂ through ball-milling was an effective way to synthesize ZnTe:O powder phosphors and produced a red emission centered at 680 nm with a decay time of 1.1 μs. The emission intensity of dry O₂-doped samples was three times more intense than from ZnO-doped samples, possibly due to a more uniform distribution of oxygen substitution on tellurium sites. The samples annealed in a 95% N₂/5% H₂ forming gas atmosphere exhibited a x-ray luminescent efficiency five times higher than did powders annealed in vacuum or N₂ atmosphere. This enhancement was attributed to the removal of surface tellurium oxides. ZnTe:O phosphor screens were prepared with x-ray luminescence efficiencies equivalent to 56% of ZnSe:Cu,Ce,Cl and 76% of Gd₂O₂S:Tb screens under 17-keV radiation. An x-ray imaging resolution of 2.5 lines/mm was resolved, the same as that measured for commercial ZnSe:Cu,Ce,Cl and Gd₂O₂S:Tb screens. These results indicate that ZnTe:O is a promising phosphor candidate for synchrotron x-ray imaging applications.     

［（Cdse）_m（ZnSe）_n］_p－Znse多量子阱中的多声子散射

对用原子层外延方法，在［００１］晶向ＧａＡｓ衬底上生长的［（Ｃｄｓｅ）ｍ（Ｚｎｓｅ）ｎ］ｐ－ＺｎＳｅ应变量子阱结构，在１０～３００Ｋ温度范围内测量了喇曼散射光谱，观察到两种类ＺｎＳｅＬＯ声子限制模．利用改变样品温度和入射光能量实现了共振喇曼散射，观察到高达７阶的类ＺｎＳｅＬＯ声子模．并讨论了多声子喇曼散射和热萤光过程的区别．     

Synthesis and Optical Properties of Si-Rich Nitride Containing Silicon Quantum Dots

Hydrogenated silicon-rich nitride films were deposited by plasma-enhanced chemical vapor deposition using NH₃ and SiH₄. As-deposited samples were thermally annealed under different conditions in argon ambient. Fourier-transform infrared spectroscopy was carried out to investigate the bonding configurations, and Raman scattering spectroscopy was used to study the microstructures and confirm the presence of Si quantum dots (QDs). We found that a near-stoichiometric silicon nitride matrix was formed after high-temperature processing. When the annealing temperature reached 1100°C, the degree of crystallinity (X _c) increased to 51.6% for the 60-min sample compared with 46.1% for the 30-min sample. Red-light and yellow-light emission were obtained from the samples annealed at 1100°C for 30 min and 60 min, respectively. The emission mechanism is dominated by excitons confined within the Si QDs. The ultra-nanocrystals play an important role in the luminescence blue-shift. We measured the bandgap values from optical absorption studies. The increase of the optical bandgap from 1.80 eV to 1.90 eV with increase of the annealing temperature from 950°C to 1100°C is ascribed to the silicon clusters and nitride matrix.     

Thermal stability of C-doped GaAs/AlAs DBR structures

Thermal stability of heavily carbon-doped and undoped DBRs has been investigated by reflectivity measurements and Raman spectroscopy. These analytical techniques are used to study the effect of heavy C-doping on Al–Ga interdiffusion during subsequent high-temperature anneals. Reflectivity spectra around the DBR stop-band wavelength clearly show that the growth-rate is reduced due to etching associated with the CBr₄ precursor used, but they also indicate that no Al–Ga interdiffusion that could significantly degrade the DBR performance takes place for any samples during annealing. The results are supported by Raman spectra, which indicate the positions of the LO and LOPC modes do not change when the DBRs are annealed, whether the DBRs are doped or not. Simulations of Al–Ga interdiffusion at GaAs/AlAs DBR interfaces indicates that intermixing up to ～15 nm on either side of each interface will not affect the reflectivity of the DBR stack significantly. The observed small changes in the stop-band central wavelength and peak reflectivity due to annealing is most likely a consequence of increased surface roughness resulted from annealing.     

ZnSe单晶的热壁外延生长(英文)

We have successfully grown a ZnSe single crystal film on GaAs(100) by hoi wall epitaxy. We confirmed that the epilayer is ZnSe single crystal from the analysis of scanning eletron microscopy and X-ray diffraction. The strong near-band-edge emission is found in the PL spectra and the Es-band related to free exiton is also very strong. They are much stronger than the deep center band emission, which shows the perfection of the epilayer. We have also studied the ZnSe/GaAs interface by AES and XPS.     

Luminescent Properties of Nanometer Si with Embedded Structure

Blue luminescence at about 431nm is obtained from epitaxial silicon after C^ implantation,annealing in hydrogen ambience and chemical etching sequentially. When annealed in nitrogen ambience and etched accordingly, there is a much narrower peak at about 430nm. During C^ implantation,C=O compounds are introduced into and embedded in the surface of nanometer Si formed during annealing,at last, nanometer silicon with embedded structure is formed,which contributes to the blue emission.     

Germanium quantum dots in an unstrained GaAs/ZnSe/Ge/ZnSe heterosystem

Arrays of Ge quantum dots in unstrained GaAs/ZnSe/Ge heterostructures were obtained by molecular-beam epitaxy for the first time. Their spatial parameters are examined by scanning tunneling microscopy, and their electronic structure is studied by Raman spectroscopy.     

Au点阵模板控制生长ZnO堆垒单晶棒

采用磁控溅射技术在Si(111)衬底上溅射Au薄膜,900℃退火生成Au点阵模板,在Au点阵模板上溅射ZnO薄膜,O2气氛下1 000℃退火制备了ZnO堆垒单晶棒。研究了不同直径Au点阵模板对ZnO单晶棒结构性能的影响。采用扫描电镜(SEM)、高分辨透射电子显微镜(HRTEM)、X射线衍射(XRD)、X射线光电子能谱(XPS)对样品结构形貌进行了分析。结果表明,生成有序排列的ZnO棒均由诸多六方纤锌矿单晶堆垒而成,较小Au点阵生成单晶棒的直径约为100nm。室温光致发光PL谱表明在376nm出现一个较强近紫外发射,在488nm附近出现一个较宽的深能级绿光发射,说明所制备样品具有良好的发光特性。     

Cathodoluminescence spectroscopy of deep defect levels at the ZnSe/GaAs interface with a composition-control interface layer

In this work we investigate ZnSe/GaAs heterostructures with an additional 2 nm controlled interfacial layer (CIL) of Se- or Zn-rich composition to modify the band offset. The samples are analyzed as a function of annealing temperature by cathodoluminescence spectroscopy. The as-prepared samples show defect luminescence at ∼ 0.9 eV. With staged annealing at increasing temperatures, both the Zn-rich as well as the Se-rich interfacial layer exhibits luminescence at ∼ 1.9 eV, indicative of defect formation with an onset temperature of ∼400°C. Excitation-dependent spectroscopy provides evidence for defect formation near the interface, which extends into the ZnSe epilayer at higher temperatures. Compared to earlier work, where the threshold temperature for defect formation in bulk samples fabricated under Se-rich growth conditions occurs at temperatures as low as 325°C, the resistance to defect formation has now been improved to that of stoichiometric ZnSe. These results demonstrate that epitaxially grown CILs provide a means to alter ZnSe/GaAs band offsets without degrading the heterojunction’s resistance to defect formation at elevated temperatures.     

Investigation of Si/SiGe heterostructures patterned by reactive ion etching

We present a modified pattern technique for fabrication of nanometer structures in the Si/SiGe heterosystem. A special multilayer-resist system is developed for pattern transfer by electron-beam lithography and anisotropic SF₆/O₂ dry etching. Photoluminescence measurements are carried out on homogeneously etched samples to determine the influence of the RIE process on the optical properties. Etching induced damages are reduced by a low-temperature post-annealing step. Additionally, surface contaminations are investigated using laser desorption mass spectrometry. SiGe wires with lateral widths from 4 μm down to 25 nm have been fabricated, but photoluminescence has been observed for structures down to 600 nm lateral width, only. Further improvement has been obtained by sidewall-passivation of the nanostructures with a low temperature plasma enhanced-chemical-vapour-deposited oxide. Up to now, clear excitonic emission is detected for wires as small as 250 nm.     

Defect-related visible luminescence of ZnO nanorods annealed in oxygen ambient

ZnO nanorods prepared by a solution-phase method are annealed at different temperatures in oxygen ambient.The luminescence properties of the samples are investigated.In the same excitation condition,the photoluminescence(PL) spectra of all samples show an ultraviolet(UV) emission and a broad strong visible emission band.The asymmetric visible emis-sion band of annealed samples has a red-shift as the annealing temperature increasing from 200 ℃ to 600 ℃ and it can be deconvoluted into two subband emissions centered at 535 nm(green emission) and 611 nm(orange-red emission) by Gaussian-fitting analysis.Analyses of PL excitation(PLE) spectra and PL spectra at different excitation wavelengths reveal that the green emission and the orange-red emission have a uniform initial state,which can be attributed to the electron transition from Zn interstitial(Zni) to oxygen vacancy(Vo) and oxygen interstitial(Oi),respectively.     

静压下ZnS0.02Te0.98混晶的共振喇曼散射研究

在１５Ｋ和１～３ＧＰａ静压范围内研究了ＺｎＳ０．０２Ｔｅ０．９８混晶的共振喇曼散射，样品用ＭＢＥ方法生长在（００１）晶向的半绝缘ＧａＡｓ衬底上，利用静压调制带隙实现也４８８．０ｎｍ线的共振喇曼散射，观察到类ＺｎＴｅ和类ＺｎＳ两类ＬＯ声子模以及它们的倍频模和组合模－测得类ＺｎＴｅ的ＬＯ声子模的压力系数为４．５ｃｍ＾－１／ＧＰａ。     

ZnO/ZnSe/ZnTe Heterojunctions for ZnTe-Based Solar Cells

ZnO and ZnSe are proposed as n-type layers in ZnTe heterojunction diodes to overcome problems associated with the n-type doping of ZnTe. The structural properties and electrical characteristics of ZnO/ZnTe and ZnO/ZnSe/ZnTe heterojunctions grown by molecular beam epitaxy on (001) GaAs substrates are presented. ZnO shows a strong preference for c-plane (0001) orientation resulting in a nonepitaxial relationship and high density of rotational domains for growth on ZnTe (001). ZnSe/ZnTe structures demonstrate a (001) epitaxial relationship with high density of {111} stacking faults originating at the heterojunction interface. ZnO/ZnSe/ZnTe heterojunction diodes show excellent diode rectification and clear photovoltaic response with open-circuit voltage of V _OC = 0.8 V.     

Optical properties of porous nanosized GaAs

The optical properties of porous GaAs layers obtained by electrochemical etching of single-crystal n-and p-GaAs(100) wafers are studied. It is shown that the shape of the nanocrystals, their mean diameter, and their surface states depend on the conductivity type of the initial crystal. A shift of the peaks corresponding to the main optical phonons to lower frequencies in the Raman spectra is observed. Surface-phonon frequencies determined from the Raman spectra coincide with those determined from the reflection spectra in the infrared spectral region. The forms of the spectral dependences of the reflection coefficient in the phonon-resonance region in bulk GaAs differ from those in porous GaAs. This is caused by the appearance of additional oscillators related to spatially confined lattice vibrations in GaAs nanocrystals. Atomic-force microscopy is used to study the surface morphology of porous GaAs samples formed on the n-GaAs substrates, and a nanosized surface profile is observed. Estimations made for the mean diameter of GaAs nanocrystals based on data from the Raman scattering, infrared spectroscopy, photoluminescence, and atomic-force microscopy yield results that are in good agreement with each other.     

利用大周期光子晶体结构增强InAs/GaAs量子点的激发态发光

在该研究中,通过激光全息和湿法腐蚀的方法在InAs/GaAs量子点材料上制备光子晶体,研究了由激光二极管激发制备了光子晶体的InAs / GaAs量子点材料的光致发光光谱.发现具有光子晶体的量子点材料的光谱显示出多峰结构,光子晶体对短波长部分的发光增强和调制比对长波长部分的增强和调制更明显.InAs / GaAs量子点的光致发光光谱通过刻蚀形成的光子晶体结构得到了调控,并且量子点的激发态发光得到了明显增强.     

Lift-off and re-growth of ZnSSe epilayer

We try to study the epitaxial ZnSe re-growth on lift-off ZnSSe epilayer. First, the ZnSSe lift-off technique from ZnSSe/GaAs substrate was investigated. The ZnSSe/GaAs wafer was glued on an indium/glass substrate and an etching solution of NH₄OH (30%):H₂O₂ (30%)=9:1 was used for the lift-off of ZnSSe epilayer. From photoluminescence examination, the intensities of the near-band and broad-band emission of ZnSSe epilayer increased from the reflection of In mirror after the lift-off. Re-growth of ZnSSe epilayers on ZnSSe/indium/glass substrate at 220°C was studied by metalorganic vapor-phase epitaxy. The near-band emission is stronger and the broad-band emission decreases after re-growth. It shows a near-band emission emission at 438 nm with an FWHM of 26.8 meV at 77 K. It means that a high-quality re-grown ZnSe epilayer can be obtained.     

半导体量子点最低导带态的量子限制效应

采用最新计算方法和半导体体材料传统量子计算结果,系统研究了14种半导体(Si,Ge,Sn,AlSb,GaP,GaAs,GaSb,InP,InAs,InSb,ZnS,ZnSe,ZnTe,CdTe)的立方量子点,得到了最低导带态的量子限制效应结果,我们把量子点对尺寸的依赖关系分为三类并详细讨论了它们的差别。     

Growth of Nanocrystalline ZnSe:N Films by Pulsed Laser Deposition

Nanocrystalline zinc-blende-structured ZnSe:N films have been deposited on GaAs(100) substrates by pulsed laser deposition (PLD). The growth of the nanocrystalline ZnSe:N films is found to be greatly affected by the pressure of ambient N₂. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) results show that the morphologies of the as-grown films are sensitive to the ambient pressure at a fixed substrate temperature of 300 °C, and the sizes of the as-grown ZnSe:N nanocrystals increase as the ambient pressure increases from 0.1 Pa to 100 Pa. The average sizes of the as-grown nanocrystals are estimated to be about 19 nm, 29 nm, and 71 nm for 0.1 Pa, 1 Pa, and 100 Pa ambient N₂ pressure, respectively. X-ray photoelectron spectroscopy analyses show that the N-doping concentration in the as-grown film is over 10²¹ cm⁻³. Raman spectra demonstrate the broadening of the longitudinal optical (LO) phonon and transverse optical (TO) phonon modes of the ZnSe nanocrystals. Based on these analyses, the mechanism of the formation of ZnSe:N nanocrystals is discussed.     

Green synthesis of yellow emitting PMMA–CdSe/ZnS quantum dots nanophosphors

We herein report the fabrication of highly fluorescent yellow emitting nanophosphors using CdSe/ZnS quantum dots (QDs) dispersed in polymethyl methacrylate (PMMA). The QDs were synthesised via a simple, non-phosphine and one pot synthetic method in the absence of an inert atmosphere. The as-prepared nanocrystallites were characterised by Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) and photoluminescence spectroscopy, energy-dispersive spectroscopy (EDS), Raman spectroscopy, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) microscopy. Optical analysis confirmed that the as-synthesised CdSe/ZnS QDs were of high quality with sharp absorption peaks, bright luminescence, narrow emission width and high PL quantum yield (up to 74%). The electron microscope images showed that the QDs are small and spherical in shape with narrow size distributions while the HRTEM micrograph confirmed the high crystallinity of the material. The Raman analysis of the QDs revealed the formation of core–shell structure and the energy dispersive spectroscopy confirmed the presence of the corresponding elements (i.e., Cd, Se, Zn and S). The dispersion of the core–shell QDs in PMMA matrix led to the red-shifting of the emission position from 393 nm in the neat PMMA to 592 nm in the nanocomposite. The fabricated highly fluorescent yellow emitting PMMA–CdSe/ZnS core–shell QDs polymer nanocomposite film display excellent optical properties without loss of luminescence. Furthermore, the as-synthesised organic soluble CdSe/ZnS QDs were successfully converted into highly water soluble QDs after ligand exchange with mercaptoundecanoic acid (MUA) without the loss of their emission properties. The simplicity of the method and the quality of the as-synthesised nanocomposite make it a promising material for the large scale fabrication of diverse optical devices.