纳米硅结构薄膜光致发光的温度依赖特性

综合考虑纳米硅结构薄膜的特殊性质,如量子限制效应、光学带隙和光跃迁振子强度对纳米硅粒径的依赖特性以及光学带隙和光辐射的温度依赖特性等,给出了一个解析表达式来分析具有一定粒径分布的纳米硅结构薄膜的光致发光(PL)强度分布,其中选取了两种纳米硅的粒径分布,即高斯分布和对数正态分布。结果表明,随着平均粒径和粒径分布偏差的减小,纳米硅薄膜的PL谱峰蓝移。随着环境温度的升高,纳米硅结构薄膜的PL谱峰红移且相对发光强度减弱。纳米硅结构薄膜光辐射拟合的结果与实验数据的比较分析表明,该模型能够很好地解释纳米硅结构薄膜在不同温度下的PL特性。     

Photoluminescence and magneto-optical properties of multilayered type-II ZnTe/ZnSe quantum dots

Multilayered Zn–Se–Te structures grown by migration enhanced epitaxy are studied by temperature- and excitation-dependent photoluminescence (PL) as well as magneto-PL. The PL consists of two bands: a blue band, overlaid with band edge sharp lines, dominant at low temperatures and high excitation, and a green band, which appears at elevated temperature and low excitation. Upon varying excitation intensity by four orders of magnitude, the green band peak energy shifts by ∼60 meV, indicating recombination of excitons in type-II quantum dots (QDs); no significant shift is observed for the blue band. Therefore, the green emission is attributed to ZnTe/ZnSe type-II QDs, which co-exist with isoelectronic centers, responsible for the blue and band edge emissions. The existence of type-II ZnTe/ZnSe QDs is further confirmed by magneto-PL, for which the observed oscillations in the PL intensity as a function of magnetic field is explained in terms of the optical Aharonov–Bohm effect.     

V形GaAs/AlGaAs量子线结构的微区光致发光谱研究

在室温下用显微光致发光的方法对单根V形GaAs/AlGaAs量子线进行了沿垂直于量子线方向的 空间分辨扫描测试,观察到各种量子结构的光致发光谱随空间位置的变化.在量子线区域附 近观察到来自量子线(QWR)、颈部量子阱(NQWL)和垂直量子阱(VQWL)等各种结构的发光,而 在距离量子线约1μm以远的发光光谱表现出侧面量子阱(SQWL)的发光.对全部发光光谱用高 斯线形进行了拟合,发现QWR和SQWL的发光包含了两个荧光峰,将它们分别归诸为电子到轻 、重空穴的跃迁.拟合后发光强度的空间变化直接确定了与量子线 关键词： V形GaAs/AlGaAs量子线 显微光致发光 空间分辨扫描     

Time-resolved photoluminescence spectra of high-density InGaAs/AlGaAs quantum wire structures

We investigated the temperature dependence of the time-resolved photoluminescence (PL) spectra of high-density InGaAs/AlGaAs quantum wire (QWR) distributed-feedback laser structure on a submicron grating. A red-shift of peak in the time-resolved PL after photo-excitation was observed due to the relaxation of the photo-generated excitons from the entire QWR to localized centers at 10 K. On the other hand, at 60 K, no red-shift in the time-resolved PL spectra was observed since the localization centers are thermally activated and the excitons are delocalized.     

Growth conditions effects on optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs (1 1 3)A substrate

We have investigated the optical properties of InAs/GaAs (1 1 3)A quantum dots grown by molecular beam epitaxy (MBE) with different growth rates by photoluminescence spectroscopy (PL) as a function of the excitation density and the sample temperature (10–300 K). Reflection high-energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A redshift of the InAs QDs PL band emission was observed when the growth rate was increased. This result was explained by the increase of the InAs quantum dot size with increasing growth rate. A significant redshift was observed when the arsenic flux was decreased. The evolution of the PL peak energy with increasing temperature has showed an S-shaped form due to the localization effects and is attributed to the efficient relaxation process of carriers in different InAs quantum dots and to the exciton transfer localized at the wetting layer.     

Luminescence of color centers formed in alkali-earth-doped yttrium orthoaluminate crystals

Alkali-earth-doped yttrium orthoaluminate crystals grown in a reducing atmosphere are found to show bright photoluminescence (PL) in visible wavelength regions under the excitation by UV light source. From the results of transmission, PL, PL excitation and time-resolved PL spectra for the samples with different types and concentrations of dopants and the comparison to the results for the samples grown under different conditions, the origin of principal emission is determined to be color centers stabilized by heterovalent ions. The observed fast lifetime and high quantum yield of the luminescence can be explained by dipole-allowed transition between the levels localized in a vacancy. Comparing the effects on the optical properties from several types of dopant ions and taking influence from the UV irradiation into account, a model for the structure of emission centers is proposed.     

Dependency of photoluminescence from SiO_2 thin films containing Si_(1-x)Ge_x quantum dots on Ge/Si doping ratio

SiO2 thin films containing Si1-xGex quantum dots （QDs） are prepared by ion implantation and annealing treatment. The photoluminescence （PL） and microstructural properties of thin films are investigated. The samples exhibit strong PL in the wavelength range of 400-470 nm and relatively weak PL peaks at 730 and 780 nm at room temperature. Blue shift is found for the 400-nm PL peak, and the intensity increases initially and then decreases with the increase of Ge-doping dose. We propose that the 400-470 nm PL band originates from multiple luminescence centers, and the 730- and 780-nm PL peaks are ascribed to the Si=O and GeO luminescence centers.     

Temperature dependent growth of InGaN/GaN single quantum well

InGaN/GaN single quantum well (SQW) structures under various InGaN growth temperatures have been grown by metal organic chemical vapor deposition (MOCVD), the surface morphologies and optical properties are investigated. The radius of the typical V-pits on the SQW surface is affected by the InGaN well-temperature, and the surface roughness decreased as the well-temperature reduced. Room-temperature photoluminescence (PL) and cathode luminescence (CL) shows the quantum well and quantum dot (QD)-like localized state light emission of the SQWs grown at 700 and 690 °C, respectively, whereas the samples grown at 670 and 650 °C present hybrid emission peaks. Excitation power dependent PL spectra indicates the QD-like localized state emission dominates at low excitation power and the quantum well emission starts to take over at high excitation power.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.     

Effect of localized-exciton energy relaxation on the emission spectrum of ZnS-ZnSe single quantum wells

A study is reported on the dependence of exciton photoluminescence spectra of ZnS-ZnSe quantum wells with different localization-center concentrations on the excitation intensity and temperature. The shape of the experimental low-temperature photoluminescence band is shown to agree well with that calculated in a model of exciton hopping to the nearest localization center and in one that takes into account transitions of a localized exciton to all centers in its local environment. The parameters characterizing localized excitons in these quantum structures of a submonolayer thickness have been determined.     

Luminescence study of Si/Ge quantum dots

We present a photoluminescence (PL) study of Ge quantum dots embedded in Si. Two different types of recombination processes related to the Ge quantum dots are observed in temperature-dependent PL measurements. The Ge dot-related luminescence peak near 0.80 eV is ascribed to the spatially indirect recombination in the type-II band lineup, while a high-energy peak near 0.85 eV has its origin in the spatially direct recombination. A transition from the spatially indirect to the spatially direct recombination is observed as the temperature is increased. The PL dependence of the excitation power shows an upshift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is ascribed to band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperatures are observed in contrast to the capped samples.     

Deuterium in In-based quantum wells

The photoluminescence (PL) of high quality InGaAs/GaAs typically shows one strong intrinsic band, due to the heavy-bole free-exciton (HHFE) recombination. After sample irradiation with deuterium, two bands appear at energies below that of HHFE: a deeper band, D, due to radiative recombination at a deuterium-related site, and a shallower band, (D,X), attributed to an exciton bound to the same state. A maximum of the binding energy has been observed as a function of the well width for both states. As the indium molar fraction x increases, the strength of the D band decreases, together with its binding energy, until the band becomes no more detectable at x=0.37. The (D,X) band cannot be resolved for x ≥ 0.19. Deuterium diffusion has no beneficial effect on the PL intensity of InGaAs/GaAs quantum wells. Different is the case of InAs/GaAs quantum wells having well width below 2 monolayers. Although no strain relaxation occurs in the samples, the PL of the virgin samples is typical of structures with a high number of defects. After deuterium diffusion, the PL intensity increases by one to three orders of magnitude, depending whether non-radiative centers or thermal escape of carriers from the well rule the PL efficiency.     

用巯基乙酸作稳定剂制备CdSe纳米晶的光学性质

以巯基乙酸为稳定剂制备了CdSe纳米晶，通过尺寸选择沉淀得到2nm到3nm之间不同尺寸的纳米晶，利用室温光吸收，光致发光（PL）和光致发光激发（PLE）谱来研究了CdSe纳米团簇的光学性质。紫外－可见吸收谱给了具有清晰激光特征的尖锐吸收边，这表明样品的尺寸分布很窄。光致发光研究表明，样品有两个发射带，一个具有较高能量位于吸收边，来自电子－空穴对从最低激发态能级弛豫后的辐射复合，另一个低能发射带归属于基质与纳米晶界面存在的俘获中心。PLE谱中有2个吸收带，分别是S－S和P－P跃迁。最后还给出了不同激发能量下的发光特性。     

Low-temperature time-resolved vacuum ultraviolet luminescent spectroscopy of KH2PO4 crystals with defects

Low-temperature photoluminescence (PL) of unactivated KDP crystals under selective synchrotron excitation is for the first time measured with subnanosecond time resolution. Time-resolved PL (2–6 eV) and PL excitation (4–35 eV) spectra, as well as PL kinetics, are measured at 7 K. From the acquired experimental data, luminescent bands related to intrinsic defects of the KDP lattice are identified; in particular, the long-wave band at 2.6 eV is assigned to L defects, and the band at 3.5–3.6 eV is attributed to D defects. An efficient energy transfer over the hydrogen sublattice is shown to take place in KDP at low temperatures. It results in the efficient excitation of L and D center photoluminescence in the fundamental absorption region, at electron transitions to the bottom levels of the conduction band, corresponding to the states of the hydrogen atom. The band gap E _g is evaluated to be 8.0–8.8 eV.     

Variations in the confining potential of doped AlGaAs/GaAs quantum wells with the photon energy of excitation

A series of doped GaAs/AlGaAs single quantum well samples has been investigated using photoluminescence (PL) and PL excitation (PLE) spectroscopy. The samples are of 100 Å well width and are doped in the central 50 Å with Si in the range 5×10⁸ to 2×10¹² cm⁻². A strong dependence of the optical spectra on the photon energy of excitation is observed. This dependence is interpreted in terms of a charge transfer process between the AlGaAs barrier and the quantum well (QW), which determines the charge and potential distribution in the QW.     

Decay processes of photoluminescence in a nanocrystalline SiC thin film

Nanocrystalline (nc)-SiC film has been deposited by helicon wave plasma enhanced chemical vapor deposition technique and intense blue-white light emission is obtained. Microstructure analyses show that the 3C–SiC particles are embed in amorphous SiC matrix, and the average size of the nc-SiC is 3.96 nm. The photon energy of the main photoluminescence (PL) band is higher than the band gap of bulk SiC, which indicates that the optical emission mainly occurs in quantum states of 3C–SiC nanocrystals. In addition, the band tail states of amorphous SiC also contribute to the optical emission. Three decay processes are obtained from time-resolved PL spectra by deconvolution treatment, and the decay components correspond to the quantum confinement effect (QCE), surface states of nc-SiC particles, and band tail of amorphous SiC, respectively. The fractional integrated PL intensity of QCE related decay process decreases dramatically in the lower PL photon energy, indicating that the QCE mainly contributes to the short wavelength optical emission.     

Photoluminescence due to exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We have investigated photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well at 10 K under intense excitation conditions. It has been found that a PL band due to exciton–exciton scattering, the so-called P emission, is observed in addition to the biexciton PL under an excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical phonon. On the other hand, the P band could never be observed at an excitation energy much higher than the exciton energy, where a band-filling phenomenon appears in the PL spectrum. Furthermore, we confirmed the existence of optical gain leading to stimulated emission in the energy region of the P band using a variable-stripe-length method.     

图形化Silicon-on-Insulator衬底上分子束外延生长可动GaN微光栅的研究

GaN材料作为第三代半导体材料,具有宽禁带、直接带隙、耐腐蚀等优点,是一种非常有前景的MOEMS材料。由于GaN的刻蚀目前尚未成熟,因此图形化外延生长法是一种较好的选择。本文基于SOI(silicon-on-insulator)基片,利用硅的微加工技术和图形化GaN分子束外延生长工艺,设计并加工了工作在太赫兹波段的、可以在二维方向上运动的SOI基GaN光栅。光栅周期为16μm,光栅宽度为6μm,峰值位置为25.901μm。通过仿真优化,设计的微驱动器在水平电压220V时,水平方向上的位移为±7.26μm;垂直方向加200V电压时,垂直位移2.5μm。为了研究在图形化SOI衬底上外延生长的InGaN/GaN量子阱薄膜的光学性能,用激光拉曼光谱仪对薄膜进行了光致发光光谱实验。实验结果表明,InGaN/GaN量子阱薄膜具有良好的发光性能,其发光范围为350~500nm,覆盖了紫外光到黄绿光。由于局域态效应与禁带收缩的作用,随着环境温度由10K升高至室温,薄膜的PL光谱的峰位呈现"S"形变化趋势。     

Competition in the carrier capture between InGaAs/AlGaAs quantum dots and deep point defects

The presence of an extrinsic photoluminescence (PL) band peaked at 1.356 eV at low temperature is observed, on a large number of self-assembled InAs and In_0.5Ga_0.5As quantum dot (QD) structures, when exciting just below the GaAs absorption edge. A detailed optical characterization allows us to attribute the 1.356 eV PL band to the radiative transition between the conduction band and the doubly ionized Cu _Ga acceptor in GaAs. A striking common feature is observed in all investigated samples, namely a resonant quenching of the QD-PL when exciting on the excited level of this deep defect. Moreover, the photoluminescence excitation (PLE) spectrum of the 1.356 eV emission turns out to be almost specular to the QD PLE. This correlation between the PL efficiency of the QDs and the Cu centers evidences a competition in the carrier capture arising from a resonant coupling between the excited level of the defect and the electronic states of the wetting layer on which the QDs nucleate. The estimated Cu concentration is compatible with a contamination during the epitaxial growth. Received 13 November 2001 / Received in final form 28 May 2002 Published online 19 July 2002