Influence of the quantum-confined Stark effect on the temperature-induced photoluminescence blueshift of In GaN/GaN quantum wells in laser diode structures

Measurements of the excitation power-dependence and temperature-dependence photoluminescence(PL) are performed to investigate the emission mechanisms of In Ga N/Ga N quantum wells(QWs) in laser diode structures. The PL spectral peak is blueshifted with increasing temperature over a certain temperature range. It is found that the blueshift range was larger when the PL excitation power is smaller. This particular behavior indicates that carriers are thermally activated from localized states and partially screen the piezoelectric field present in the QWs. The small blueshift range corresponds to a weak quantum-confined Stark effect(QCSE) and a relatively high internal quantum efficiency(IQE) of the QWs.     

Theoretical study of quantum-confined band-edge shifts and radiative lifetimes in oxidized Si(001) quantum films: comparison with experiment for Si/SiO2 quantum wells

The electronic structure and radiative lifetimes of Si(001) quantum films terminated by SiO₄ tetrahedra, which simulate Si/SiO₂ quantum wells (QWs), are calculated by the extended Hückel-type non-orthogonal tight-binding method. It is found that calculated band-gap widenings and radiative lifetimes account for band-edge shifts and photoluminescence (PL) peak shifts and lifetimes measured in amorphous-Si/SiO₂ QWs, suggesting that quantum confinement effects on the extended band-edge states in the amorphous-Si layer are responsible for the observed results. However, it is shown that band-edge shifts and PL energies and lifetimes observed in crystalline-Si/SiO₂ QWs cannot be reproduced properly by the interface model proposed in this study, implying that further studies are needed on the atomic structure of the crystalline-Si/SiO₂ interface.     

InGaAsN量子阱的光致发光谱研究(英文)

我们测量了低N组分的InGaAsN/InGaAs/GaAs量子阱材料的光致发光(PL)谱,测量温度范围从13K到300K。实验结果显示,InGaAsN的PL谱的主峰值的能量位置随温度的变化呈现出反常的S型温度依赖关系。用Varshni经验公式对实验数据进行拟合之后,发现在低温下InGaAsN量子阱中的载流子是处于局域态的。此外,我们还测量了样品在不同的温度、不同的能量位置的瞬态谱,结果进一步证实了:在低温下,InGaAsN的PL谱谱峰主要是局域态激子的复合发光占据主导地位,而且InGaAsN中的载流子局域态主要是由N等电子缺陷造成的涨落势引起的。     

Exciton diffusion dynamics in quantum nanostructures on V-groove patterned substrates

Time-resolved photoluminescence (PL) measurements were performed for two different quantum nanostructures on V-groove patterned substrates; SiGe/Si quantum wells (QWs) on V-grooved Si substrates and AlGaAs spontaneous vertical quantum wells on V-grooved GaAs substrates. Anomalous behaviours of the PL, such as the decrease of the decay time of the SiGe (111) QWs and the (111)A AlGaAs layer, were fully explained by taking the exciton diffusion towards the bottom of the V-groove into account, showing that the exciton diffusion driven by the spatial nonuniformity of the alloy compositions and/or geometry of the substrates is a key to controlling the PL properties of the nanostructures.     

Exciton capture and thermal escape in InAs dot-in-a-well laser structures

The photoluminescence (PL), its temperature and power dependences have been studied in InAs quantum dots (QDs) embedded in asymmetric In_xGa_1?y As/GaAs quantum wells (QWs) with variable In_xGa_1?x As compositions in the capping layer. Three stages for thermally activated decay of QD PL intensity have been revealed. A set of rate equations for exciton dynamics (relaxation into QWs and QDs, and thermal escape) are solved to analyze the mechanism of PL thermal decay. The variety of PL intensities and peak positions, as well as the activation energies of PL intensity decay in DWELL structures with different compositions of a capping layer are discussed.     

Quantum dots fabricated by twofold cleaved edge overgrowth

Cleaved edge overgrowth (CEO) has proven to be a powerful technique for the fabrication of atomic scale T-shaped quantum wires (QWRs) which form at the intersection of two quantum wells (QWs). Here we report on the first experimental demonstration of quantum dots (QDs) which result when three QWs intersect each other at right angles. Optical emission from zero-dimensional (0D) states in these QDs which were fabricated by a twofold CEO technique is clearly identified by means of micro-photoluminescence ( PL) and PL excitation ( PLE) spectroscopy. In contrast to the inhomogeneously broadened QW and QWR signals originating from the complex sample structure, the QD response, which is characterised by sharp lines (FWHM 70 eV), is strongly spatially localised at a position where the QWs meet.     

Detecting spatially localized excitons in InGaN quantum well structures with a micro-photoluminescence technique

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photoluminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 μm spatial resolution. A sharp PL line (linewidth of <0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions.     

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。     

Reversible Carriers Tunnelling in Asymmetric Coupled InGaN/GaN Quantum Wells

Temperature-dependent photoluminescence （PL） and time resolved photoluminescence （TRPL） are performed to study the PL characteristics and carrier transfer mechanism in asymmetric coupled InGaN/GaN multiple quantum wells （AS-QWs）. Our results reveal that abnormal carrier tunnelling from the wide quantum well （WQW） to the narrow quantum well （NQW） is observed at temperature higher than about lOOK, while a normal carrier tunnelling from the NQW to the WQW is observed at temperature lower than 100 K. The reversible carrier tunnelling between the two Q Ws makes it possible to explore new types of temperature sensitive emission devices. It is shown that PL internal quantum efficiency （IQE） of the NQW is enhanced to about 46% due to the assistant of the abnormal carrier tunnelling.     

GaAs surface passivation by ultra-thin epitaxial GaP layer and surface As-P exchange

The GaAs surface passivation effects of epitaxially grown ultra-thin GaP layers and surface As-P exchange have been investigated. Optical properties of passivated and unpassivated InGaAs/GaAs near-surface quantum wells (QWs) grown by metal organic vapor phase epitaxy (MOVPE) are studied by low-temperature continuous-wave and time-resolved photoluminescence (PL). By optimizing the growth conditions, smooth surface morphologies and significant improvement of optical properties were observed for both passivation methods. Passivation improved the PL intensity more than two orders of magnitude and notably increased the PL decay time.     

Emission and strain in InGaAs/GaAs quantum wells with InAs quantum dots obtained at different temperatures

The photoluminescence (PL), its temperature dependence and X ray diffraction (XRD) have been studied in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with embedded InAs quantum dots (QDs), obtained with the variation of QD growth temperatures (470–535 °C). The increase of QD growth temperatures is accompanied by the enlargement of QD lateral sizes (from 12 up to 28 nm) and by the shift non monotonously of PL peak positions. The fitting procedure has been applied for the analysis of the temperature dependence of PL peaks. The obtained fitting parameters testify that in studied QD structures the process of In/Ga interdiffusion between QDs and capping/buffer layers takes place partially. However this process cannot explain the difference in PL peak positions.     

Photoluminescence linewidth broadening due to alloy/thickness fluctuation of CdxZn1 − xSe/ZnSe triple quantum wells

Photoluminescence (PL) linewidth broadening of Cd_xZn_{1 − x}Se/ZnSe triple quantum wells, grown on GaAs substrates by molecular beam epitaxy (MBE), has been investigated. Various quantum well (QW) samples have been prepared with different QW thickness and composition (Cd-composition). Measured and calculated PL linewidth are compared. Both composition and thickness fluctuations are considered for the calculation with the parameters such as the volume of exciton, nominal thickness and composition of QWs. Surface roughness measured by atomic force microscopy (AFM) is used to estimate the interface roughness. Results show that when Cd-composition increases additional linewidth broadening due to Zn/Cd interdiffusion is enhanced.     

图形化蓝宝石衬底上InGaN/GaN多量子阱发光二极管的光谱特性研究

运用电致发光(EL)和光致发光(PL)实验,分析了图形化蓝宝石衬底(PSSLEDs)和常规平面蓝宝石衬底(C-LEDs)InGaN/GaN多量子阱发光二极管的光谱特性。对比EL谱,发现PSSLEDs拥有更强的光功率和更窄的半峰宽(FWHM),说明PSSLEDs具有较高的晶体质量。其次,PSSLEDs的EL谱半峰宽随电流增加出现了更快的展宽,而这两种LED样品的PL谱半峰宽随激光功率增加呈现了基本相同的展宽变化,说明在相同电流下,PSSLEDs量子阱中载流子浓度更高,能带填充效应更强。另外,随着电流的增加,PSSLEDs和C-LEDs的峰值波长都发生蓝移,且前者的蓝移程度较小,结合半峰宽的对比分析,说明PSSLEDs量子阱中的极化电场较小。最后,对比了PSSLEDs和C-LEDs的外量子效率随电流的变化,发现PSSLEDs拥有更严重的efficiency droop,说明量子阱中极化电场不是导致efficiency droop的主要原因。     

Magneto-optical properties and exciton dynamics in diluted magnetic semiconductor nanostructures

Nanostructures of diluted magnetic semiconductors were fabricated to study novel magneto-optical properties that are derived from quantum confined band electrons interacting with magnetic ions. Quantum dots (QDs) of Cd_0.97Mn_0.03Se were grown by the self-organization on a ZnSe substrate layer. QDs of Zn_0.69Cd_0.23Mn_0.08Se and quantum wires (QWRs) of Cd_0.92Mn_0.08Se and Zn_0.69Cd_0.23Mn_0.08Se were fabricated by the electron beam lithography. A single quantum well (QW) of ZnTe/Zn_0.97Mn_0.03Te and double QWs of Cd_0.95Mn_0.05Te–CdTe were grown by molecular beam epitaxy. Magneto-optical properties and the formation and relaxation dynamics of excitons were investigated by ultrafast time-resolved photoluminescence (PL) spectroscopy. Excitons in these nanostructures were affected by the low-dimensional confinement effects and the interaction with magnetic ion spins. The exciton luminescence of the Cd_0.97Mn_0.03Se QDs shows the confined exciton energy due to the dot size of 4–6 nm and also shows marked increase of the exciton lifetime with increasing the magnetic field. The QDs of Zn_0.69Cd_0.23Mn_0.08Se fabricated by the electron beam lithography display narrow exciton PL spectrum due to the uniform shape of the dots. The exciton luminescence from the QWRs of Cd_0.92Mn_0.08Se and Zn_0.69Cd_0.23Mn_0.08Se shows the influence of the one-dimensional confinement effect for the exciton energy and the luminescence is linearly polarized parallel to the wire direction. The transient PL from the ZnTe/Zn_0.97Mn_0.03Te QWs displays, by the magnetic field, the level crossing of the exciton spin states of the nonmagnetic and magnetic layers and the spatial spin separation for the excitons. Cd_0.95Mn_0.05Te–CdTe double QWs show the injection of the spin polarized excitons from the magnetic well to the nonmagnetic QW.     

Acoustically induced dynamic potential dots

Mobile potential dots (dynamic dots, DDs) formed by surface acoustic waves (SAWs) are used to transport photogenerated electrons and holes in GaAs quantum wells (QWs). We investigate the interaction between the transported carriers and microscopic trap centers in the QW plane using spatially and time-resolved photoluminescence (PL) spectroscopy. The carriers recombine at the trap site emitting short (width0.6 ns) light pulses at a repetition rate corresponding to the SAW frequency. The dependence of the PL intensity from the traps on the number of carriers transported per DD n exhibits a well-defined, distinct plateau for n in the range from 5–20, which is attributed to the emission of a well-defined number of photons.     

Properties of InGaN/GaN quantum wells and blue light emitting diodes

We have reported the effects of growth interruption time on the optical and structural properties of high indium content In_xGa_1−xN/GaN (x>0.2) multilayer quantum wells (QWs). The InGaN/GaN QWs were grown on c-plane sapphire by metal organic chemical vapor deposition. The interruption was carried out by closing the group-III metal organic sources before and after the growth of the InGaN QW layers. The transmission electron microscopy (TEM) images show that with increasing interruption time, the quantum-dot-like region and well thickness decreases due to indium reevaporation or the thermal etching effect. As a result the photoluminescence (PL) peak position was blue-shifted and the intensity was reduced. The sizes and number of V-defects did not differ with the interruption time. The interruption time is not directly related to the formation of defects. The V-defect originates at threading dislocations and inversion domain boundaries due to higher misfit strain. Temperature dependent PL spectra support the results of TEM measurements. Also, the electroluminescence spectra of light-emitting diode show that dominant mechanism in InGaN/GaN QWs is a localized effect in the quantum-dot-like regions.     

Magneto-optical study of nonmagnetic quantum dots coupled to a magnetic semiconductor quantum well

We have investigated a series of double-layer structures consisting of a layer of self-assembled non-magnetic CdSe quantum dots (QDs) separated by a thin ZnSe barrier from a ZnCdMnSe diluted magnetic semiconductor (DMSs) quantum well (QW). In the series, the thickness of the ZnSe barrier ranged between 12 and 40 nm. We observe two clearly defined photoluminescence (PL) peaks in all samples, corresponding to the CdSe QDs and the ZnCdMnSe QW, respectively. The PL intensity of the QW peak is observed to decrease systematically relative to the QD peak as the thickness of the ZnSe barrier decreases, indicating a corresponding increase in carrier tunneling from the QW to the QDs. Furthermore, polarization-selective PL measurements reveal that the degree of polarization of the PL emitted by the CdSe QDs increases with decreasing thickness of the ZnSe barriers. The observed behavior is discussed in terms of anti-parallel spin interaction between carriers localized in the non-magnetic QDs and in the magnetic QWs.     

Coulomb effects in spatially separated electron and hole layers in coupled quantum wells

We report on the (magneto-) optical study of many-body effects in spatially separated electron and hole layers in GaAs/Al_xGa_1?x As coupled quantum wells (CQWs) at low temperatures (T = 1.4 K) for a broad range of electron-hole (e-h) densities. Coulomb effects were found to result in an enhancement of the indirect (interwell) photoluminescence (PL) energy with increasing the e-h density both for a zero magnetic field and at high fields for all Landau level transitions; this is in contrast to the electron-hole systems in single QWs where the main features are explained by the band-gap renormalization resulting in a reduction of the PL energy. The observed enhancement of the ground state energy of the system of the spatially separated electron and hole layers with increasing the e-h density indicates that the real space condensation to droplets is energetically unfavorable. At high densities of separated electrons and holes, a new direct (intrawell) PL line has been observed: its relative intensity increased both in PL and in absorption (measured by indirect PL excitation) with increasing density; its energy separation from the direct exciton line fits well to the X ^? and X ⁺ binding energies previously measured in single QWs. The line is therefore attributed to direct multiparticle complexes.