Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

图形衬底量子线生长制备与荧光特性研究

报道了在V型槽图形衬底上利用分子束外延技术外延生长的GaAs/AlGaAs量子线.外延截面在扫描电子显微镜下可以看到在V型槽底部形成了弯月型量子线结构,量子线尺寸约为底边60 nm高14 nm的近三角形.低温87 K下光致发光谱测试在793.7和799.5 nm处出现峰值,验证了量子线的存在.理论近似计算结果显示,相比等宽度量子阱有8 meV的蓝移正是由于横向量子限制引起的. 关键词： V型槽图形衬底 量子线 GaAs     

Optical emission from the one-dimensional electron gas in narrow modulation-doped GaAs/(InGa)As/(AlGa)As quantum wires fabricated by lateral top barrier modulation

We report on the fabrication and photoluminescence characterisation of n-type doped quantum wires, which are based on a modulation-doped GaAs/(InGa)As/(AlGa)As quantum well structure, as used in inverted high electron mobility transistors. Lateral patterning was performed by electron beam lithography followed by a selective wet etch process to remove the n-type doped GaAs top barrier between the wire regions. The removal of the top barrier was verified by micro-Raman spectroscopy. Spatially indirect emission from the one-dimensional (ID) electron gas formed in the quantum wires is observed in low-temperature photoluminescence, even for the narrowest geometrical wire width of 23 nm. The emission shows a blue-shift for wire widths below 100 nm, which amounts to up to 60 meV for the narrowest wires.PACS: 78.66.Fd, 73.20.Dx, 78.55.Cr     

Fabrication of quantum wires by selective intermixing induced in GaAs/AlGaAs quantum well heterostructures by SiO2 capping and subsequent annealing

Results are presented demonstrating that selective intermixing of GaAs/AlGaAs quantum well heterostructures by SiO₂ capping and subsequent annealing can be spatially localized with a length scale compatible with the observation of lateral quantum confinement effects. Patterning of a 400 nm-thick SiO₂ encapsulation layer deposited by rapid thermal chemical vapor deposition into arrays of wires was performed using high resolution electron beam lithography and subsequent reactive ion etching. After high temperature (850°C) annealing, photoluminescence experiments indicate the creation of double barrier quantum wires when small trenches (< 100 nm) are etched in the SiO₂ film at a period greater than 800 nm. Signatures of the formation of one-dimensional subbands are observed both in photoluminescence excitation spectroscopy and linear polarization anisotropy analysis. A mechanism involving the ability of the stress field generated during annealing at the SiO₂ film edges to pilot the diffusion of the excess gallium vacancies which are responsible for the enhanced interdiffusion under SiO₂ is suggested to account for the high lateral selectivity achievable with this novel process.     

Strain and optical transitions in InAs quantum dots on (001) GaAs

To investigate the strain characteristics of InAs quantum dots grown on (001) GaAs by solid source molecular beam epitaxy we have compared calculated transition energies with those obtained from photoluminescence measurements. Atomic force microscopy shows the typical lateral size of the quantum dots as 20–22 nm with a height of 10–12 nm, and photoluminescence spectra show strong emission at 1.26 μ m when the sample is capped with a GaAs layer. The luminescence peak wavelength is red-shifted to 1.33 μ m when the dots are capped by an In_0.4Ga_0.6As layer. Excluding the strain it is shown that the theoretical expectation of the ground-state optical transition energy is only 0.566 eV (2.19 μ m), whereas a model with three-dimensionally-distributed strain results in a transition energy of 0.989 eV (1.25 μ m). It has thus been concluded that the InAs quantum dot is spatially strained. The InGaAs capping layer reduces the effective barrier height so that the transition energy becomes red-shifted.     

Nanoscale patterning using nanochannel glass replica films

Nanochannel glass replica films have been used as masks for thein-situparallel patterning of GaAs/AlGaAs single quantum wells during growth by MBE. Quantum well pyramids with lateral dimensions as small as 500 nm exhibit low temperature (5 K) photoluminescence associated with the 50 Å well thickness of the structure. By control of the replica membrane properties and deposition conditions, we demonstrate that it may be possible to reduce the lateral dimensions of layered structures beyond what would normally be expected from the starting apertures.     

Effect of barrier thickness on strain uniformity of wire in laterally aligned InGaAs/GaAs quantum wire nanostructures

The effect of barrier thickness on strain uniformity of a laterally aligned array of InGaAs quantum wire in GaAs matrix has been investigated with the finite elements method. A decrease in GaAs barrier thickness was predicted to assist the InGaAs wire to maintain its strain state in the central region up to a longer distance towards the edge of the wire along the width direction. It is suggested that, by reducing the spacing between the quantum wires, it is possible to improve uniformity of strains within the wire, thereby yielding more uniform opto-electronic properties such as sharp and narrow peaks in photoluminescence spectra.     

The temperature dependence of the photoluminescence from overgrown GaAs/Al0.3Ga0.7As quantum dots and wires

Photoluminescence measurements on GaAs/Al_0.3Ga_0.7As quantum dots and wires fabricated using electron bears lithography and reactive ion etching are reported both before and after regrowth with a layer of Al_0.4Ga_0.6As. Dots exhibit little change in luminescence efficiency from the bulk with a reduction in diameter either before or after regrowth. Surface recombination therefore appears to be suppressed. In wires, however, luminescence intensity is very sensitive to wire width, decreasing rapidly with this parameter, but recovers and becomes independent of size after overgrowth. The temperature dependence of the photoluminescence from the dots and wires showed that dots and wires less than 150nm in width luminesced to higher temperatures than the larger diameter structures and dots liminesced to higher temperatures than wires of comparable width. This suggests that there is a finite coherence area effect which increases the radiative lifetimes of excitons in the quantum structures due to the geometric constraint, in the lateral direction in the wires and in all three directions in the dots. Below 20K bound exciton luminescence dominates in the dots but not in the wires. In wires it is still possible for the excitons to diffuse to nonradiative sites within the exciton lifetime. Regrowth at 750°C causes migration of aluminium into the quantum well and causes the shape of the well to become parabolic resulting shifts in the exciton emission to shorter wavelengths, making it difficult to separate the effect of processing from those due to quantum confinement.     

Insight into optical properties of strain-free quantum dot pairs

Self-assembled GaAs/AlGaAs quantum dot pairs (QDPs) are grown by molecular beam epitaxy using high temperature droplet epitaxy technique. A typical QDP consists of dual-size quantum dots as observed based on atomic force microscopy image. The average height of quantum dot is 5.7 nm for the large quantum dots and 4.6 nm for the small ones. The average peak-to-peak distance of the two dots is about 75 nm. The optical properties of GaAs QDPs are studied by measuring excitation power-dependent and temperature-dependent photoluminescence. Unique photoluminescence properties have been observed from both excitation power-dependent and temperature-dependent measurements. Excitation power-dependent as well as temperature-dependent PL measurements have suggested lateral exciton transfer in the QDPs.     

Growth of quantum wire arrays by MBE on nonplanar substrates

With single-step molecular beam epitaxy growth, GaAs/AlGaAs quantum wire (QW wire) arrays were fabricated over mesas an GaAs nonplanar substrates patterned by conventional photolithography and wet chemical etching. Faceting and surface migration of atoms during crystal growth resulted in lateral variation in the quantum well (QW) layer thickness on different facet planes. This caused the tops of the mesas to be sharp enough to provide lateral quantum-size-effects (QSEs). In conventional photoluminescence (PL), PL with a micro-optical-system, and photoreflectance measurements under different conditions, a large blue shift was observed in the energy level positions for electronic transitions corresponding to QWs at the tops of mesas compared with those corresponding to QWs on nonpatterned areas of the same substrate. The blue shift was in contradiction with the fact that the GaAs QW layers at the tops of mesas were thicker than those on nanpatterned areas, and illustrated the realization of QW wires at the tops of mesas. Calculations also proved that there was a lateral QSE at the tops of mesas and this was further proof for the formation of QW wires there.     

InGaAs/GaAs应变脊形量子线分子束外延非平面生长研究

提出了新型InGaAs/GaAs应变脊形量子线结构．这种应变脊形量子线结合了非平面应变外延层中沿不同晶向能带带隙的变化、非平面生长应变层In组分的变化，以及非平面外延层厚度的变化等三方面共同形成的横向量子限制效应的综合作用．在非平面GaAs衬底上用分子束外延生长了侧面取向为（113）的脊形AlAs/In GaAs/AlAs应变量子线．用10K光致荧光谱测试了其发光性质．用Kronig-Penney模型近似计算了这种应变脊形结构所具有的横向量子限制效应，发现其光致荧光谱峰位的测试结果，与计算结果相比，有10meV的“蓝移”．认为这一跃迁能量的“蓝移”是上述三方面横向量子限制效应综合作用的结果 关键词：     

Plasmons in the modulated and confined 2DEG: A Raman scattering study

Lateral wire arrays have been fabricated from a single modulation-doped GaAs quantum well employing reactive ion etching. Depending on the etch depth, the two-dimensional electron gas (2DEG) in the well acquires different degrees of modulation up to complete confinement. The different regimes are identified by their unique photoluminescence and Raman spectra. Deep-etched wires show plasmon resonances down to a width of 100nm.     

Selective growth of GaAs quantum dots and vertical quantum wires in two-dimensional V-grooves

We propose and demonstrate a novel technique for the fabrication of quantum dot (QD) structures using metal organic chemical vapor deposition (MOCVD). The GaAs quantum dots are grown at the bottom of the two-dimensional V-groove (2DVG) structures which are composed of (1 1 1)A and (1 1 1)B-facets on GaAs(1 0 0). The 2DVG is formed by MOCVD selective growth on a SiO₂ patterned substrate. It should be noted that the 2DVGs cannot be formed by a chemical wet etching technique because the facet's anisotropy of etching ratios are different. By changing the growth condition, we can obtain GaAs QD structures which have a size of less than 10 nm, and vertical GaAs quantum wires (V-QWRs) in 2DVGs. We have observed photoluminescence from each structure. We have also demonstrated stacking of GaAs QDs in the 2DVG on GaAs (1 0 0).     

Dichroism in transmission of light by an array of self-assembled GaAs quantum wires on a nanofaceted A(311) surface

Dichroism in the transmission of light (the dependence of the transmittance on the direction of polarization of light) is revealed in corrugated GaAs/AlAs superlattices grown on a nanofaceted A(311) surface. It is assumed that the observed effect is associated with the structural anisotropy, i.e., with the formation of an array of GaAs quantum wires. This inference is confirmed by high-resolution electron microscopy. The GaAs/AlAs superlattices containing quantum wires also exhibit polarization anisotropy of the photoluminescence observed in the yellow-red spectral range.     

Optical and electronic properties of single modulation doped GaAs/AlGaAs V-grooved quantum wire modified by ion implantation

1 Introduction Recently, there is considerable interest in the fabrication and study of quasi-one di-mensional quantum wires (QWRs) due to their potential application for novel optoelec-tronic devices such as QWR laser array [1,2] etc. Among the various techniques devel-oped for producing quasi-one dimensional (quasi-1D) QWRs, the self-organized growth on patterned substrates has been proven to be one of the most promising methods, due to the simplicity of fabrication[3―5]. The QWR is fa…     

Time-resolved screening of the piezoelectric field in InGaAs/GaAs V-shaped quantum wires of variable profile

InGaAs/GaAs V-shaped quantum wires grown in grooves with either (111) or (411) sidewalls have been studied by ps-transient photoluminescence as a function of the excitation intensity. The optical nonlinearity associated with the screening of the internal piezoelectric field is temporally monitored by the blue shift of the spectrally resolved photoluminescence, occurring in the first 150 ps after the laser pulse, followed by a red shift at longer delays. Such an energy shift strongly depends on the photoexcited carrier density and reaches a maximum value of about 14 meV in the (411) wires. Despite their larger piezoelectric field, we observe a smaller energy shift in wires with (111) sidewalls, due to the enhanced confinement which localizes the wire wavefunction at the bottom of the groove. The observed energy shifts are consistent with the theoretical calculation of the polarization charge density induced by the strain via the piezoelectric effect.     

Structure and photoluminescence study of type-II GaAs quantum wires and dots grown on nano-faceted (3 1 1)A surface

(3 1 1)A GaAs/AlAs corrugated superlattices (CSLs) and satellite (3 1 1)B and (1 0 0) SLs were studied using Raman spectroscopy, high-resolution transmittance electron microscopy (HRTEM) and photoluminescence (PL). The thickness of GaAs layers was varied from 1 monolayer (ML) to 10 ML, the thickness of AlAs barriers was 10 ML in (3 1 1) direction. The strongest modification of the Raman spectra is found for the case of partial (<1 nm) GaAs filling of the AlAs surface. The calculated and experimental Raman spectra demonstrated a good agreement for both complete (1 nm) and partial (<1 nm) GaAs filling of the AlAs surface. According to Raman and HRTEM data, in the case of partial filling of (3 1 1)A AlAs surface, GaAs forms quantum well wires of finite length (quantum dots). A drastic difference of PL from grown side-by-side (3 1 1)A and (3 1 1)B SLs was observed. A strong room temperature PL in the green–yellow spectral region was observed in GaAs/AlAs (3 1 1)A CSLs containing GaAs type-II quantum dots.     

Cylindrical GaAs quantum wires incorporated within chrysotile asbestos nanotubes: fabrication and polarized optical absorption spectra

GaAs has been injected into chrysotile asbestos channels. GaAs quantum wires (nanocylinders) with diameters ∼ 6 nm have been observed in the channels by means of transmittance electron microscopy. Polarized optical absorption spectra of asb-GaAs (chrysotile asbestos containing GaAs wires) have been studied. A high anisotropy of the absorption has been observed, and intersubband transitions in the visible light region due to a strong quantum size effect have been found.     

Photoluminescence of single quantum wires and quantum dots

The results of a study into the photoluminescence spectra of a set of quantum dots based on GaAs enclosed in AlGaAs nanowires are presented. The steady state and time resolved spectra of photoluminescence under optical excitation both from an array of quantum wires/dots and a single quantum wire/dot have been measured. In the photoluminescence spectra of single quantum dots, emission lines of excitons, biexcitons and tritons have been found. The binding energy of the biexciton in the studied structures was deduced to be 8 meV.     

Type II GaSb/GaAs quantum dot/ring stacks with extended photoresponse for efficient solar cells

We report on the fabrication of GaAs based p–i–n solar cells containing 5 and 10 layers of type II GaSb quantum rings grown by molecular beam epitaxy. Solar cells containing quantum rings show improved efficiency at longer wavelengths into the near-IR extending up to 1500 nm and show enhanced short-circuit current under 1 sun illumination compared to a GaAs control cell. A reduction in the open-circuit voltage is observed due to the build-up of internal strain. The MBE growth, formation and photoluminescence of single and stacked layers of GaSb/GaAs quantum rings are also presented.