Formation mechanism and optical properties of InAs quantum dots on the surface of GaAs nanowires

Formation mechanism and optical properties of InAs quantum dots (QDs) on the surface of GaAs nanowires (NWs) were investigated. This NW-QDs hybrid structure was fabricated by Au-catalyzed metal organic chemical vapor deposition. We found that the formation and distribution of QDs were strongly influenced by the deposition time of InAs as well as the diameter of GaAs NWs. A model based on the adatom diffusion mechanism was proposed to describe the evolution process of the QDs. Photoluminescence emission from the InAs QDs with a peak wavelength of 940 nm was observed at room temperature. The structure also exhibits a decoupling feature that QDs act as gain medium, while NW acts as Fabry-Perot cavity. This hybrid structure could serve as an important element in high-performance NW-based optoelectronic devices, such as near-infrared lasers, optical detectors, and solar cells.     

Deep levels induced by InAs/GaAs quantum dots

Self-organised InAs/GaAs quantum dots (QDs) were formed by molecular beam epitaxy using the Stranski–Krastanov growth mode. Deep-level transient spectroscopy as well as secondary ion mass spectrometry have been used to characterise structures containing the QDs. DLTS depth profiling procedures indicate that deep level-related defects are localised in GaAs in the vicinity of the QD plane. For the first time, we report the presence of a deep level-related trap with an extremely high thermal activation energy of E_c − 1.03 eV. An electron trap at E_c − 0.78 eV can be identified as the well-known level related to the EL2 family. We conclude that a third trap revealed at E_c −0.57 eV is the familiar PL killer related to the intrinsic point defect-oxygen complex. The latter is confirmed by results of the SIMS study, which indicates that the amount of oxygen accumulated at the InAs/GaAs heterointerface is increased. This paper demonstrates that the EL2 and oxygen-related deep-level centers occur by the presence of InAs/GaAs QDs. We present the hypothesis that deep states could be a factor limiting the efficiency of QD-based devices.     

Luminescence tuning of amorphous Si quantum dots prepared by plasma-enhanced chemical vapor deposition

Amorphous Si (a-Si) quantum dots (QDs) embedded in a silicon nitride film were prepared by a plasma-enhanced chemical vapor deposition (PECVD) technique using gaseous mixtures of silane, hydrogen and nitrogen. We observed that the Si QDs had an amorphous structure from the Raman spectroscopy measurement. The Fourier transform infrared (FTIR) spectra showed that the relative transmittance of the SiH bands decreased, but that of the NH bands increased, with increasing nitrogen flow rate. During the deposition of SiNx, the number of dangling bonds of silicon acting as nucleation sites increased. As the hydrogen flow rate increased the growth rate decreased, due to the reduction in the hydrogen partial pressure. The hydrogen and nitrogen gas flow rates were found to be important parameters for determining the size of the a-Si QDs. In addition, we observed that the PL peak shifted toward a higher energy with increasing hydrogen and nitrogen gas flow rates, which was attributed to the increase in the quantum confinement effect in the a-Si QDs.     

Scanned probe imaging of quantum dots inside InAs nanowires

We show how a scanning probe microscope (SPM) can be used to image electron flow through InAs nanowires, elucidating the physics of nanowire devices on a local scale. A charged SPM tip is used as a movable gate. Images of nanowire conductance versus tip position spatially map the conductance of InAs nanowires at liquid-He temperatures. Plots of conductance versus backgate voltage without the tip present show complex patterns of Coulomb-blockade peaks. Images of nanowire conductance identify their source as multiple quantum dots formed by disorder along the nanowire--each dot is surrounded by a series of concentric rings corresponding to Coulomb blockade peaks. An SPM image locates the dots and provides information about their size. In this way, SPM images can be used to understand the features that control transport through nanowires. The nanowires were grown from metal catalyst particles and have diameters approximately 80 nm and lengths 2-3 microm.     

Growth of InAs quantum dots on focussed ion beam implanted GaAs(100)

The growth of self-assembled InAs quantum dots on implantation doped GaAs was studied. Be and Si ions were implanted in a combined ion implantation/molecular beam epitaxy process to generate p- and n-type GaAs, respectively. The quality of the InAs quantum dots was investigated by photoluminescence spectroscopy and scanning electron microscopy. By employing an in situ annealing step before re-growth it was possible to fabricate high quality InAs quantum dots on ion doped GaAs for Be doses up to 1.4×10¹⁴ cm⁻². The sheet resistance of the Be doped GaAs was as low as 1 kΩ at 300 K and 0.6 kΩ at 4.2 K, respectively. Only for rather low Si doses up to 5×10¹³ cm⁻² acceptable photoluminescence could be detected. The sheet resistance for these doses was 1 kΩ at 300 K and 1.7 kΩ at 4.2K.     

Metallorganic chemical vapor deposition of InAs/GaSb superlattices on GaAs substrates using a two-step InAs buffer layer

InAs/GaSb type II superlattices (T2SLs) were grown heteroepitaxially, via metallorganic chemical vapor deposition (MOCVD), on GaAs substrates. The 7% lattice mismatch between the T2SL and GaAs substrate was accommodated through the use of a two-step InAs buffer layer. The periodicity of the grown structures was confirmed by X-ray diffraction (XRD). FTIR measurements of the structures yielded cutoff wavelength values from 4.9 μm to 8.7 μm, which, as expected, scaled with thickness of the InAs in the SL structure. Kronig-Penny modeling of the structures produced the general trend of the experimental data.     

Morphology and composition of InAs/GaAs quantum dots

Surface compositional maps of self-organized InAs/GaAs quantum dots were obtained with laterally resolved photoemission spectroscopy. We found a surface In concentration of about 0.85 at the center of the islands which decreases to 0.75 on the wetting layer. Comparison with concentration values found in the core of similar dots suggests a strong In segregation on the topmost surface layers of the dots and on the surrounding wetting layer. Furthermore, the morphological properties of the dots such as size and density have been measured with plan-view transmission electron microscopy and low energy electron microscopy.     

爆炸辅助气相沉积法制备炭纳米线的研究

根据爆炸辅助气相沉积法生长碳纳米管的机理,设计了两种制备炭纳米线的方案:(1)使用低活性铁-镍二元金属催化剂;(2)对钴催化剂作用下碳纳米管的生长实施冷冻。透射电子显微镜显示这两种方法制备的炭纳米线均为纳米颗粒组装而成,具有非常粗糙的表面。其中,使用铁-镍二元催化剂所制炭纳米线直径分布不均匀,黏结情况严重;而在冷冻钴催化剂作用下炭纳米管生长过程所得的炭纳米线直径分布比较均匀,黏结情况也大为减少。这两种纳米线的差别与金属催化剂的活性有关。光催化降解亚甲基蓝实验表明:冷冻碳纳米管生长所得炭纳米线具有良好的催化辅助功能,可以提高ZnS纳米晶的光催化活性。     

Tunable double quantum dots in InAs nanowires defined by local gate electrodes

We report on low-temperature transport measurements on single and double quantum dots defined using local gates to electrostatically deplete InAs nanowires grown by chemical beam epitaxy. This technique allows us to define multiple quantum dots along a semiconducting nanowire and tune the coupling between them.     

Heteroepitaxial growth of vertical GaAs nanowires on Si(111) substrates by metal-organic chemical vapor deposition

Epitaxial growth of vertical GaAs nanowires on Si(111) substrates is demonstrated by metal-organic chemical vapor deposition via a vapor-liquid-solid growth mechanism. Systematic experiments indicate that substrate pretreatment, pregrowth alloying temperature, and growth temperature are all crucial to vertical epitaxial growth. Nanowire growth rate and morphology can be well controlled by the growth temperature, the metal-organic precursor molar fraction, and the molar V/III ratio. The as-grown GaAs nanowires have a predominantly zinc-blende crystal structure along a <111> direction. Crystallographic {111} stacking faults found perpendicular to the growth axis could be almost eliminated via growth at high V/III ratio and low temperature. Single nanowire field effect transistors based on unintentionally doped GaAs nanowires were fabricated and found to display a strong effect of surface states on their transport properties.     

Growth and properties of vertically well-aligned GaN nanowires by thermal chemical vapor deposition process

The vertically well-aligned GaN nanowires on c-Al₂O₃ substrates were grown via a vapor-liquid-solid mechanism. X-ray diffraction indicated the GaN nanowires to have epitaxial and homogeneous in-plan alignment with the c-Al₂O₃ substrates and a strong preferred orientation along the c-axis. The GaN nanowires had a single-crystalline hexagonal structure and c-axis orientation, as confirmed by high resolution transmission electron microscopy.     

InAs/GaSb superlattices fabricated by metalorganic chemical vapor deposition

The possibility of fabricating InAs/GaSb strained-layer superlattices by metalorganic chemical vapor deposition has been experimentally demonstrated. The results of transmission electron microscopy and photoluminescence spectroscopy investigations showed that the obtained structures comprise an InAs?GaSb superlattice on a GaSb substrate consisting of 2-nm-thick InAs and 3.3-nm-thick GaSb layers.

     

Photoluminescence studies of InAs/GaAs quantum dots covered by InGaAs layers

Photoluminescence (PL), PL excitation (PLE), and time-resolved PL were used to study effects of InGaAs layers on the optical properties of InAs/GaAs quantum dots (QDs). A rich fine structure in the excited states of confined excitons (up to n = 4 quantum states) was observed, providing useful information to study the quantum states in the InAs/GaAs QDs. A significant redshift of the PL peak energy for the QDs covered by InGaAs layers was observed, attributing to the decrease of the QD strain and the lowing of the quantum confinement.     

Statistics of electron emission from InAs/GaAs quantum dots

A theoretical treatment for thermal and tunneling emission of electrons from InAs/GaAs quantum dots is performed to achieve “effective emission rates” corresponding to experimentally obtained quantities. From these results, Arrhenius graphs are calculated using parameter values for quantum dots with 20/10 nm base/height dimension. Emission from the electron s shell as direct transitions, as two-step transitions from the s to the p shell, as thermal transitions from s to p followed by tunneling and as direct tunneling from the s and the p shell to the GaAs conduction band is taken into account. Due to the varying emission possibilities, Arrhenius graphs appear with complicated shapes depending on quantities originating from structural and electronic properties of the quantum dots.     

Growing InAs/GaAs quantum dots by droplet epitaxy under MOVPE conditions

Technical Physics Letters - Results of studying the formation of InAs quantum dots (QDs) on GaAs(100) substrates by droplet epitaxy using trimethylindium and arsine (AsH3) as precursors are...     

Synthesis of indium-catalyzed Si nanowires by hot-wire chemical vapor deposition

Indium (In) catalyzed silicon nanowires (SiNWs) were synthesized by using hot-wire chemical vapor deposition (HWCVD) technique. Indium droplets were deposited on Si substrates by hot-wire evaporation of In wire, which was immediately followed by the growth of SiNWs from the droplets. Three sets of samples were prepared by varying the length of In wires, l, as 3, 1 and 0.5 mm. The sizes of In catalyst droplets decreased from 271.4 ± 66.8 to 67.4 ± 16.6 nm when the l was reduced from 3 to 0.5 mm. Larger size of In droplets (271.4 ± 66.8 nm) was found to induce the growth of worm-like NWs. The decrease in size of In catalyst droplets induced the formation of aligned and tapered NWs with smaller tips. The smallest value of tapering parameter, T_p of 40.5 nm/μm is correlated to the SiNWs induced by the smallest size of In droplets (67.4 ± 16.6 nm). The as-grown SiNWs showed high purity and good crystalline structure.     

Defects in nanostructures with ripened InAs/GaAs quantum dots

InAs/GaAs quantum dot (QD) structures were grown by molecular beam epitaxy (MBE) with InAs coverages θ continuously graded from 1.5 ML to 2.9 ML. A critical coverage of 2.23 ML is found, above which the islands undergo ripening, which causes a fraction of quantum dots to increase in size and to eventually relax through the formation of pure, edge-type misfit dislocations which propagate towards the surface in the form of V-shaped defects. Concomitant with ripening, extended-defect related traps with activation energies of 0.52 and 0.84 eV were observed, and regarded as the cause of the significant worsening of the optical and electrical properties in high coverage structures. Their relationship with the observed dislocations is discussed.     

热化学气相沉积法在硅纳米丝上合成碳纳米管

利用热化学气相沉积法在负载不同厚度催化剂的硅纳米丝(SiNW)表面生长碳纳米管(CNTs),探讨了生长条件对所合成SiNW-CNT的结构和场发射特性的影响.这种类似树状的三维结构具有较高碳纳米管表面密度及降低的电场筛除效应等潜在优势.使用拉曼光谱( Raman)、电子显微镜(SEM)、透射电子显微镜(TEM)、能量扩散分光仪(EDS)分析了碳纳米管的结构性质,并在高真空下施加电场测得碳纳米管的场发射特性.结果表明:随硅纳米丝上负载催化剂镍膜厚度的变化,所合成碳纳米管的表面特性、结晶结构及功函数改变,导致电子发射难易程度的改变,进一步影响碳纳米管的场发射特性.     

Phase composition of mixed ZnS-EuS thin films grown by metal organic chemical vapor deposition

The phase composition of the mixed ZnS-EuS films deposited from volatile dithiocarbamates has been studied using differential dissolution technique (chemical method of the phase analysis) and electron microscopy. Phase composition was found to depend on the Eu content in the films, that in turn depends on a flow density ratio of the Eu and Zn volatile precursors. A single-phase solid solution, Zn_0.998Eu_≤0.002S, was observed only for films with Eu content≤1 mol%, other films were found to be two-phase. For films with the Eu content between 2 and 16% and above 80%, impurity phases, EuS and ZnS, respectively, were detected by differential dissolution technique. They evolved as low-sized sulfide precipitates encapsulated in an organic coat. No impurity phases in the films of the same Eu content were noticed by X-ray technique and Raman spectroscopy. For the films with the Eu content between 16 and 80%, sulfide phases, ZnS and EuS, were found to be free from any organic coat, and structural methods as with differential dissolution technique were also capable of observing the phases. Conditions are given to prepare Eu doped ZnS films of good quality by MOCVD technique.