GaNAs/GaAs量子阱的激子局域化以及退局域化

我们利用光荧光(PL)以及时间分辨光谱(TRPL)研究了用MBE生长在GaAs衬底上的GaNAs/GaAs量子阱的激子局域化以及退局域化。研究发现,在低温下用连续光(CW)激发,由于GaNAs中势振荡所产生的局域激子发光是所测量到光谱的主要发光来源。然而在脉冲激发下,情况完全不同。在高载流子密度激发或者高温下GaNAs/GaAs量子阱中例外,一个高能端的PL峰成为了主要的发光来源。通过研究,我们将这个新的发光峰指认为量子阱中非局域激子复合的PL峰。这个发光峰在温度和激发强度的变化过程中与局域激子相互竞争。我们相信这一过程也是许多文献所报道的在InGaN和AlGaN等氮化物中经常观测到的发光峰位随温度“S”形变化的主要根源。     

GaNAs/GaAs量子阱的激子局域化以及退局域化

我们利用光荧光(PL)以及时间分辨光谱(TRPL)研究了用MBE生长在GaAs衬底上的GaNAs/GaAs量子阱的激子局域化以及退局域化.研究发现,在低温下用连续光(Cw)激发,由于GaNAs中势振荡所产生的局域激子发光是所测量到光谱的主要发光来源.然而在脉冲激发下,情况完全不同.在高载流子密度激发或者高温下GaNAs/GaAs量子阱中例外,一个高能端的PL峰成为了主要的发光来源.通过研究,我们将这个新的发光峰指认为量子阱中非局域激子复合的PL峰.这个发光峰在温度和激发强度的变化过程中与局域激子相互竞争.我们相信这一过程也是许多文献所报道的在InGaN和AlGaN等氮化物中经常观测到的发光峰位随温度"S"形变化的主要根源.     

Long wavelength InAs self-assembled quantum dots embedded in GaNAs strain-compensating layers

We investigated the effect of GaNAs strain-compensating layers (SCLs) on the properties of InAs self-assembled quantum dots (QDs) grown on GaAs (0 0 1) substrates. The GaNAs material can be used as SCL thereby minimizing the net strain, and thus is advantageous for multi-stacking of InAs QDs structures and achieving long wavelength emission. The emission wavelength of InAs QDs can be tuned by changing the nitrogen (N) composition in GaNAs SCLs due to both effects of strain compensation and lowering of potential barrier height. A photoluminescence emission at 77 K was clearly observed for sample with GaN_0.024As_0.976 SCL. Further, we observed an improvement of optical properties of InAs QDs by replacing the more popular GaAs embedding layers with GaNAs SCLs, which is a result of decreasing non-radiative defects owing to minimizing the total net strain.     

1 eV吸收带边GaInAs/GaNAs超晶格太阳能电池的阱层设计

使用In, N分离的GaInAs/GaNAs超晶格作为有源区是实现高质量1eV带隙 GaInNAs基太阳能电池的重要方案之一. 为在实验上生长出高质量相应吸收带边的超晶格结构, 本文采用计算超晶格电子态常用的Kronig-Penney模型比较了不同阱层材料选择下, 吸收带边为1 eV的GaInAs/GaNAs超晶格相关参数的对应关系以及超晶格应变状态. 结果表明: GaNAs与GaInAs作为超晶格阱层材料在实现1 eV的吸收带边时具有不同的考虑和要求; 在固定1 eV的吸收带边时, GaNAs材料作为阱层可获得较好的超晶格应变补偿, 将有利于生长高质量且充分吸收的太阳能电池有源区. 关键词： GaInAs/GaNAs超晶格 Kronig-Penney模型 太阳能电池     

Optical anisotropy of GaNAs grown on GaAs(0 0 1) substrate

In this paper GaN_xAs_1−x surfaces during growth are observed using reflectance difference or reflectance anisotropy spectroscopy (RDS or RAS). The epi-layer was grown by solid-source molecular beam epitaxy (MBE) system with a RF nitrogen prasma source. RD spectra showed broader structure and reduced amplitude compared to those of GaAs surfaces; GaAs(2 × 4)-like features were still observed with weak and blue-shifted peaks. In the low growth temperature region, an extra structure was also observed around 3.02 eV. We proposed that GaN_xAs_1−x surface can be classified into three types of the surface.     

Effect of thermal annealing on r.f. sputtering-deposited nanocrystalline GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> thin films

GaNAs thin films were deposited on Corning glass substrates by radio frequency (r.f.) sputtering in molecular nitrogen ambient. The stoichiometry in the GaNAs alloy was controlled by changing the nitrogen incorporation in the film during the growth process, through the variation of the r.f. power in the range 30–80 watts which produced films with N concentrations in the range: x = 0.85–0.90. The structural and optical properties of the GaNAs thin films were studied by X-ray diffraction (XRD), photoacoustic (PA) and photoluminescence (PL) spectroscopies. XRD measurements show a broad diffraction band with a peak close to the (002) diffraction line of the GaN hexagonal phase, and a slight shoulder at the position corresponding to the (111) GaAs cubic phase. The PA absorption spectra showed a remarkable shift to higher energies of the absorption edge as the r.f. power decreases corresponding to the films with higher N concentrations. Thermal annealing of the GaNAs films at temperatures of 450 °C produced a GaAs nanocrystalline phase with grain sizes in the range 10–13 nm, as confirmed by the XRD measurements that showed a well-defined peak in the (111) GaAs direction, and also by the PA spectra which showed an absorption band at energies around 1.45 eV due to the quantum confinement effects. PL spectra of thermal-annealed GaNAs films showed a very intense emission at 1.5 eV which we have associated to transitions between the first electron excited level and acceptor states in the GaAs nanocrystallites.     

Reduction of dislocation density and improvement of optical quality in ZnO layers by MgO-buffer annealing

Optical properties of ZnO thin films with/without MgO-buffer annealing were investigated by low and room temperature photoluminescence measurements. The ZnO films were grown on c-sapphire substrates by plasma-assisted molecular-beam epitaxy employing a thin MgO-buffer layer. Dislocation density of ZnO layer was reduced from 5.3 × 10⁹ to 1.9 × 10⁹ cm⁻² by annealing MgO-buffer prior to the growth of ZnO. The intensity of free exciton emission from the sample with MgO-buffer annealing was almost twice of that from the sample without annealing, while the deep level emission from the sample with MgO-buffer annealing was about 1/3 of that without annealing. The MgO-buffer annealing improves optical quality of overgrown ZnO films.     

Effects of hydrogen annealing on the room temperature ferromagnetism and optical properties of Cr-doped ZnO nanoparticles

We explore the effects of hydrogen annealing on the room temperature ferromagnetism and optical properties of Cr-doped ZnO nanoparticles synthesized by the sol-gel method. X-ray diffraction and x-ray photoelectron spectroscopy data show evidence that Cr has been incorporated into the wurtzite ZnO lattice as Cr²⁺ ions substituting for Zn²⁺ ions without any detectable secondary phase in as-synthesized Zn_0.97Cr_0.03O nanopowders. The room temperature magnetization measurements reveal a large enhancement of saturation magnetization M_s as well as an increase of coercivity of H₂-annealed Zn_0.97Cr_0.03O:H samples. It is found that the field-cooled magnetization curves as a function of temperature from 40 to 400 K can be well fitted by a combination of a standard Bloch spin-wave model and Curie–Weiss law. The values of the fitted parameters of the ferromagnetic exchange interaction constant a and the Curie constant C of H₂-annealed Zn_0.97Cr_0.03O:H nanoparticles are almost doubled upon H₂-annealing. Photoluminescence measurements show evidence that the shallow donor defect or/and defect complexes such as hydrogen occupying an oxygen vacancy H_o may play an important role in the origin of H₂-annealing induced enhancement of ferromagnetism in Cr-H codoped ZnO nanoparticles.     

改善塑料光学零件成型质量的几个问题

从改进注射成型技术和模具制造技术的角度介绍了提高塑料光学零件的质量所采取的措施。在抑制变形的成型技术中所采用的注射压缩成型方法中，用机械的方法对熔融塑料均匀加压以减小塑件的变形及残余应力。也可采用适当的补偿方法修正模具以补偿塑件的变形。在模具制造中可以在模芯表面镀一层非电镀镍，然后用金刚石车削机床加工，而获得很好的表面粗糙度和表面面形精度。     

Role of vacancies in the structural relaxation of Pd-Mo alloys saturated with hydrogen

The presence of an anomalously large number of vacancies in Pd-Mo alloys subjected to hydrogenation is revealed using precise x-ray diffractometry. These alloys are found to undergo nonmonotonic structural evolution during long-term relaxation. The evolution is characterized by aperiodic time variations in the number of coexisting phases, in the volume of each of them, and in the defect structure and by the cooperative motion of vacancies (as well as hydrogen in the early stage) between the matrix and defect regions. The key features of this evolution are an anomalously high concentration of not only hydrogen but also vacancies and a high concentration of defect regions, which causes thermodynamic instability of the system. The structural evolution has an oscillating character, because the maxima of thermodynamic instability of the matrix and of an ensemble of defect regions are separated in time.     

Photoluminescence study of hydrogen donors in ZnO

A photoluminescence study of hydrogenated ZnO bulk crystals is presented. Two excitonic recombination lines at 3362.8 and 3360.1 meV are assigned to hydrogen shallow donors. Experimental evidence is presented that the corresponding donor to the line at 3362.8 meV, previously labeled I₄, originates from hydrogen trapped within the oxygen vacancy, H_O. The line at 3360.1 meV was found to be due to hydrogen located at the bond-centered lattice site, H_BC. The corresponding shallow donor has an ionization energy of 53 meV.     

Time-resolved photoluminescence study of the red emission in nanoporous SiGe alloys

Porous Si_1−xGe_x (PSiGe) layers with efficient room temperature visible photoluminescence (PL) were elaborated by anodical etching from p-type doped epitaxial layers with Ge contents from 5 to 30%. The luminescence is characterised by a broad PL band centred at 1.8 eV. Time resolved photoluminescence decay is studied in porous silicon germanium as a function of germanium content, temperature, emission energies and surface passivation. The PL decay line shape is well described by a stretched exponential in all cases. The effective lifetime at low temperature in as prepared porous Si_1−xGe_x is 400 μs, i.e. an order of magnitude less than in porous silicon. After the formation of a 20 Å thick oxide surface layer we observe a decrease of the effective lifetime to 20 μs at T=4 K.     

Clustering effects in optical properties of BGaAs/GaAs epilayers

We report on the further investigation of the effect of boron incorporation on GaAs grown at 580 °C temperature on GaAs (0 0 1) substrate by metal organic chemical vapor deposition (MOCVD). High-resolution X-ray diffraction (HRXRD) has been used to determine the lattice mismatch and to estimate the boron concentration. Temperature-dependent photoluminescence has been carried out to investigate B_xGa_1−xAs/GaAs epilayers with varied boron composition (x=1.64% and 3.04%). Low temperature (10 K) PL study has shown an asymmetric and broad PL band around 1.3 eV of the emission energies with a decrease of the PL intensity with increasing boron composition. The evolution of the emission energies with temperature can be described by Varshni law for the high temperature range (T?120 and 80 K) for boron composition x=1.64% and 3.04%, respectively, while a relative discrepancy has been found to occur at low temperature. Moreover, depending on the temperature range, the PL intensity quenching is found to be thermally ensured by three activation energies. These results are attributed to the localized states induced by the non-uniform insertion of boron and the clustering of the boron atom in BGaAs bulk.     

Direct observation of variations of optical properties in single quantum dots by using time-resolved near-field scanning optical microscope

We study the variations of optical properties of self-assembled In_0.5Ga_0.5As single quantum dots (QDs) in the spatial and time domains by combining a near-field scanning optical microscope with an ultrafast pulsed laser. Through the examinations of several tens of QDs, we find that the variations of photoluminescence (PL) intensity strongly depend on the condition of the initial carrier creation. The differences in quantum efficiency and those in the carrier flow rate into QDs cause the large distribution of PL intensity when the carriers are excited in the barrier layers. From the results of time-resolved PL decay measurements, we find that there are two types of QDs exhibiting quite different PL decay profiles.     

Growth and optical properties of ZnO microwells by chemical vapor deposition method

The well-like ZnO nanostructures were obtained by chemical vapor deposition method. The uniform and dense ZnO slim nano-columns were grown along the circle to form a microwell. The growth mechanisms, such as 1D linear, 2D screw dislocation and step growth are discussed. These observations provide some insight into the growth kinetics in vapor-solid growth process. The fabrication of ZnO microwell morphology provided a direct experimental evidence for explaining the 1D growth mechanism based on the axial screw dislocation. Photoluminescence (PL) microscopy showed the surface-related optical properties. The green light emission enhancement revealed that the ZnO microwells have waveguide properties. The abnormal enhancement of integrated PL intensity of deep-level emission with temperature increase showed abundant surface state existence.     

Giant optical anisotropy in single InAs quantum dots

We present the experimental evidence of giant optical anisotropy in single InAs QDs. Polarization-resolved photoluminescence spectroscopy in single QDs reveals a linear polarization ratio which fluctuates, from one dot to another, in sign and in magnitude with absolute values up to 82%. We do not observe any dependence of the linear polarization on incident power and temperature.     

Synthesis and optical properties of amorphous C-Si-O particles

Amorphous C-Si-O particles, prepared by pyrolyzing PDMS in a horizontal furnace using the carrier gas N₂/H₂ at 900 °C, were characterized by electron microscopy, X-ray diffraction, FTIR spectroscopy, X-ray photoelectron spectroscopy and fluorescence spectroscopy. The particles possess four luminescence peaks at 440, 465, 533 and 620 nm, and produce stable red, green or blue light emissions at room temperature when irradiated with appropriate wavelengths, the four peaks attribute to different defect centers in the particles. And the particles exhibit the highest photoluminescence intensity when annealed at 600 °C.     

Statistical analysis of micro-luminescence maps of implanted optical centers in diamond

We report on an approach to determine the number of optical emitters based on establishing the proper relationship between the statistics of implanted ions and the signal from optical centers (e.g. photoluminescence) collected at different points in the sample. Knowing the number of implanted ions, one can estimate the probability for an ion to create an optical center—the conversion efficiency. The micro-luminescence mapping and statistical analyses were performed on a model Xe optical center in diamond and the conversion efficiency was estimated to be about 30%.     

Syntheses and optical properties of triphenylene-containing conjugated polymers

In this paper, we report the detailed synthesis and optical properties of three new conjugated polymers containing triphenylene units in the backbone. Polymer PTPT exhibits strong folding propensity and forms foldamers in both polar and nonpolar solvents. PTPA, with two long alkyl chains attached to the bridging phenyl ring, exhibits mainly as interchain aggregates in “poor” solvents (DMSO and acetonitrile), but adopts a folding conformation in solvent mixtures with a high poor solvent content. PTPV, on the other hand, adopts a random nonfolding conformation in both polar and nonpolar solvents. The low folding propensity of PTPV is likely due to the added geometrical flexibility of the vinyl bonds. Among the three polymers, PTPV is most fluorescent with a fluorescence quantum yield as high as 0.87, suggesting its potential applications as light-emitting materials or fluorescence-based sensors. PTPT, on the other hand, with its strong folding property, may find applications as efficient charge-transporting materials.     

Multi-peak behavior of photoluminescence of silica particles heat-treated in hydrogen at elevated temperature

Pure nano-partical silica was prepared by sol-gel method, and then was treated in a H₂ ambient at different temperatures. The surface structure and valence bonding of samples were analyzed with IR spectrum and X-ray induced photoelectron spectroscopy, respectively. Its photoluminescence property was studied with fluorescence spectroscopy. The results showed that only one luminescence band at 344 nm exists for silica heat-treated at 450 °C. Silica samples heat-treated at higher temperatures showed completely different luminescence comparing with the samples heat-treated at lower temperatures. Multi-peaks were found in the SiO₂ samples heat-treated at temperatures higher than 700 °C, in which the luminescence peaks at about 379 and 392 nm are similar with the β bands in silica glass originated from the same defect center of two-fold coordinated silicon atoms, and the luminescence bands at about 493 and 528 nm are few reported. The intensity of the luminescence bands increase with the increase of heat treatment temperature from 700 to 900 °C.