Molecular beam epitaxial growth of site-controlled InAs quantum dots on pre-patterned GaAs substrates

Ex situ electron-beam lithography followed by conventional wet etching has been used to pattern small holes 60–150 nm wide, 13 nm deep in GaAs substrates. These holes act as preferential nucleation sites for InAs dot growth during subsequent overgrowth. By varying either the InAs deposition amount or the thickness of a GaAs buffer layer, the occupancy over the patterned sites can be controlled. Comparison with growth on a planar substrate shows that preferential nucleation occurs due to a reduction in the apparent critical thickness above the pattern site; the magnitude of this reduction is dependent on the dimensions of the initial pattern.     

InGaAs/GaAs量子点红外探测器

与量子阱红外探测器相比,量子点红外探测器具有不制作表面光栅就能在垂直入射红外光照射下工作以及工作温度更高等优势。然而,目前阻碍量子点红外探测器性能提高的技术瓶颈主要来自组装量子点较差的大小均匀性、较低的量子点密度以及垂直入射下子带跃迁吸收效率低等原因。利用分子束外延技术研究了如何从量子点材料生长和器件设计两方面来克服这些困难,并且制作了几种不同结构的InGaAs/GaAs量子点红外探测器。 在77 K时,这些器件在垂直入射条件下观察到了很强的光电流信号。     

Improvements of stacked self-assembled InAs/GaAs quantum dot structures for 1.3 μm applications

We propose the growth of thick ‘spacer’ layers (d) for high-quality 10-stack InAs/GaAs quantum dots (QDs) emitting at 1.23 μm without the use of strain reduction layers (SRLs). All samples were grown using molecular beam epitaxy (MBE) and extensively characterised using X-ray diffraction, optical spectroscopy and microscopy techniques. We demonstrate that for d<50 nm, large ‘volcano-like’ defects are formed at the top of the stacked structure, while for d=50 nm, these features were not observed. The process of suppressing these abnormal defects has resulted in significant photoluminescence (PL) enhancement, paving the way for the realisation of defect-free QD laser devices.     

Investigation of the effect of growth interruption on the formation of InAs/GaAs quantum dot superlattice near the InAs critical thickness

Two kinds of superlattices (i) with and (ii) without growth interrupt (GI) after deposition of 1.77 monolayers (ML) of InAs on GaAs (0 0 1) were grown by solid-source molecular beam epitaxy (MBE) and assessed by transmission electron microscopy (TEM) techniques, double crystal X-ray diffraction (DCXRD) and photoluminescence (PL) measurements in order to gain an understanding of the structural and compositional properties. In case (i) formation of coherent dislocation free self-organized quantum dots (SOQDs) with 2.8-3.2 nm height and 13-16 nm lateral size was observed, whereas in case (ii) no quantum dots had formed. In order to better understand the implication of growth interruption for the formation mechanism, highly localised assessment of the composition of the QD was carried out via atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS).     

Advances in self-assembled semiconductor quantum dot lasers

In the past 20 years the semiconductor laser has become a key device in optical electronics because of its pure output spectrum and high quantum efficiency. As the capabilities of laser diodes have grown, so has the range of applications contemplated for them. A great success in semiconductor lasers has been brought by the ability to artificially structure new materials on an atomic scale by using advanced crystal growth methods such as MBE and MOVPE. The laser performance successes gained using quantum wells in optoelectronic devices can be extended by adopting quantum wire and quantum dot structures. There have been several reports of successful lasing action in semiconductor dot structures within the past few years. In this article I will briefly review the recent progress in the development of quantum dot lasers.     

Pulsed laser annealing of self-organized InAs/GaAs quantum dots

The effect of pulsed laser annealing (PLA), using an excimer laser, on the luminescence efficiency of self-organized InAs/GaAs and In_0.4Ga_0.6As/GaAs quantum dots has been investigated. It is found that such annealing can enhance both the peak and integrated photoluminescence (PL) efficiency of the dots, up to a factor of 5–10 compared to as-grown samples, without any spectral shift of the luminescence spectrum. The improved luminescence is attributed to the annealing of nonradiative point and extended defects in and around the dots.     

The control of size and areal density of InAs self-assembled quantum dots in selective area molecular beam epitaxy on GaAs (0 0 1) surface

The growth of InAs quantum dots (QDs) on GaAs (0 0 1) substrates by selective area molecular beam epitaxy (SA-MBE) with dielectric mask is investigated. The GaAs polycrystals on the mask, which is formed during growth due to low GaAs selectivity between dielectric mask and epitaxial region in MBE, strongly affect the distribution of InAs QDs on the neighbouring epitaxial regions. It is found that the GaAs polycrystalline regions strongly absorb indium during QD growth, confirmed by microscopic and optical studies. GaAs polycrystalline deposit can be reduced under low growth rate and high-temperature growth conditions. Almost no reduction in QD areal density is observed when there is minimal polycrystalline coverage of the mask.     

Surfactant-mediated growth of InAs-GaAs superlattices and quantum dot structures grown at different temperatures

The structural and optical qualities of superlattice InAs-GaAs structures and quantum dots (QDs), grown by molecular beam epitaxy (MBE) at low (250 °C) and normal (∼450 °C) growth temperatures, have been investigated. The InAs layers (3 monolayers) were grown under conditions where only the indium beam impinged upon the growth surface (surfactant growth mode). This growth mode still resulted in the formation of QDs at normal growth temperatures, but with dot sizes that were much smaller than those for “normal” growth of 3 ML InAs-GaAs QD structures. In addition, at low temperature under such “arsenic-free” conditions a very high quality InAs-GaAs superlattice structure with 3 ML of InAs was formed, as demonstrated by transmission electron microscopy (TEM). This is a direct confirmation that the critical thickness of InAs can be extended well beyond the 1.7 ML limit seen at higher growth temperatures.     

Heteroepitaxial growth of InAs on GaAs(0 0 1) by in situ STM located inside MBE growth chamber

The growth of InAs on GaAs(0 0 1) is of great interest primarily due to the self-assembly of arrays of quantum dots (QDs) with excellent opto-electronic properties. However, a basic understanding of their spontaneous formation is lacking. Advanced experimental methods are required to probe these nanostructures dynamically in order to elucidate their growth mechanism. Scanning tunneling microscopy (STM) has been successfully applied to many GaAs-based materials grown by molecular beam epitaxy (MBE). Typical STM–MBE experiments involve quenching the sample and transferring it to a remote STM chamber under arsenic-free ultra-high vacuum. In the case of GaAs-based materials grown at substrate temperatures of 400–600 °C, operating the STM at room temperature ensures that the surface is essentially static on the time scale of STM imaging. To attempt dynamic experiments requires a system in which STM and MBE are incorporated into one unit in order to scan in situ during growth. Here, we discuss in situ STM results from just such a system, covering both QDs and the dynamics of the wetting layer.     

Influence of crystallinity of as-deposited Ge film on formation of quantum dot in carbon-mediated solid-phase epitaxy

The influence of crystallinity of as-deposited Ge films on Ge quantum dot (QD) formation via carbon (C)-mediated solid-phase epitaxy (SPE) was investigated. The samples were fabricated by solid-source molecular beam epitaxy (MBE). Ge/C/Si structure was formed by sequential deposition of C and Ge at deposition temperature (T_D) of 150–400 °C, and it was heat-treated in the MBE chamber at 650 °C. In the case of amorphous or a mixture of amorphous and nano-crystalline Ge film grown for T_D ≤250 °C, density of QDs increased with increasing T_D due to the increase of C-Ge bonds in Ge layer. Ge QDs with diameter of 9.2±2.1 nm were formed in the highest density of 8.3×10¹¹ cm⁻² for T_D =250 °C. On the contrary, in the case of polycrystalline Ge film for T_D ≥300 °C, density of QDs decreased slightly. This is because C incorporation into Ge layer during SPE was suppressed due to the as-crystallized columnar grains. These results suggest that as-deposited Ge film in a mixture of amorphous and nano-crystalline state is suitable to form small and dense Ge QDs via C-mediated SPE.     

Preliminary comparison of ballistic electron emission spectroscopy measurements on InAs quantum dots in a GaAs/AlGaAs heterostructure grown by MBE and MOVPE

Self-assembled InAs quantum dots (SAQDs) in GaAs/GaAlAs structures grown by molecular beam epitaxy (MBE) and metal-organic vapour phase epitaxy (MOVPE) of similar size was examined by ballistic electron emission spectroscopy. Ballistic current-voltage characteristics through the QD in the voltage range from 0.55 to 0.9 V (range where the presence of resonance states of QD is expected) with its derivative (the derivation of the spectroscopic characteristics represents quantum levels in the QD) are given. Differences in the intensities and sharpnesses of the QD levels for MBE and MOVPE grown QDs are observed.     

Investigation into the InAs/GaAs quantum dot material epitaxially grown on silicon for O band lasers

InAs/GaAs quantum dot(QD)lasers were grown on silicon substrates using a thin Ge buffer and three-step growth method in the molecular beam epitaxy(MBE)system.In addition,strained superlattices were used to prevent threading disloca-tions from propagating to the active region of the laser.The as-grown material quality was characterized by the transmission electron microscope,scanning electron microscope,X-ray diffraction,atomic force microscope,and photoluminescence spectro-scopy.The results show that a high-quality GaAs buffer with few dislocations was obtained by the growth scheme we de-veloped.A broad-area edge-emitting laser was also fabricated.The O-band laser exhibited a threshold current density of 540 A/cm² at room temperature under continuous wave conditions.This work demonstrates the potential of large-scale and low-cost manufacturing of the O-band InAs/GaAs quantum dot lasers on silicon substrates.     

Fabrication and analysis of multijunction solar cells with a quantum dot (In)GaAs junction

InAs quantum dots (QDs) have been incorporated to bandgap engineer the (In)GaAs junction of (In)GaAs/Ge double‐junction solar cells and InGaP/(In)GaAs/Ge triple‐junction solar cells on 4‐in. wafers. One sun AM0 current–voltage measurement shows consistent performance across the wafer. Quantum efficiency analysis shows similar aforementioned bandgap performance of baseline and QD solar cells, whereas integrated sub‐band gap current of 10 InAs QD layers shows a gain of 0.20 mA/cm². Comparing QD double‐junction solar cells and QD triple‐junction solar cells to baseline structures shows that the (In)GaAs junction has a V_oc loss of 50 mV and the InGaP 70 mV. Transmission electron microscopy imaging does not reveal defective material and shows a buried QD density of 10¹¹ cm⁻², which is consistent with the density of QDs measured on the surface of a test structure. Although slightly lower in efficiency, the QD solar cells have uniform performance across 4‐in. wafers. Copyright © 2013 John Wiley & Sons, Ltd.     

（英）利用分子束外延在GaAs基上生长高特征温度的InAs量子点激光器

通过MBE外延系统生长了1.3 μm的GaAs基InAs量子点激光器.为了获得更好的器件性能,InAs量子点的最优生长温度被标定为520 ℃,并且在有源区中引入Be掺杂.制备了脊宽100 μm,腔长2 mm的激光器单管器件,在未镀膜的情况下,达到了峰值功率1.008 W的室温连续工作,阈值电流密度为110 A/cm^-2,在80℃下仍然可以实现连续工作,在50 ℃以下范围内,特征温度达到405 K.     

Issues of practical realization of a quantum dot register for quantum computing

We present a quantum dot structure fabricated by the lithographic positioning, which can be used as a prototype of the quantum dot register for quantum computing. Using simple model calculations we show that parameters of our quantum dot structure are very close to the ones required for two possible embodiments of a quantum computer. Results of numerical simulation of the quantum dot register, as well as discussion of materials and technological issues of fabrication of quantum logic gates are also presented.     

Surface Morphology and Photoluminescence of 1.3μm Wavelength In（Ga） As／GaAs Quantum Dots

Self-organized In0.5 Ga0.5As/GaAs quantum island structure emitting at 1.35 μm at room temperature has been successfully fabricated by molecular beam epitaxy(MBE) via cycled(InAs)1/(GaAs)1 monolayer deposition method.Photoluminescence(PL) measurement shows that very narrow PL linewidth of 19.2 meV at 300 K has been reached for the first time,indicating effective suppression of inhomogeneous broadening of optical emission from the In0.5Ga0.5As islands structure.Our results provide important information for optimizing the epitaxial structures of 1.3μm wavelength quantum dot （QD） devices.     

InSb quantum dot LEDs grown by molecular beam epitaxy for mid-infrared applications

We report the molecular beam epitaxial growth of InSb quantum dots (QD) inserted as sub-monolayers in an InAs matrix which exhibit intense mid-infrared photoluminescence up to room temperature. The InSb QD sheets were formed by briefly exposing the surface to an antimony flux (Sb₂) exploiting an As-Sb anion exchange reaction. Light emitting diodes were fabricated using 10 InSb QD sheets and were found to exhibit bright electroluminescence with a single peak at 3.8 μm at room temperature.     

InAs/GaAs多层堆垛量子点激光器的激射特性

利用固态源分子束外延技术，按S-K模式生长出五层堆垛InAs/GaAs量子点（QD）微结构材料. 用这种QD材料制成的激光器，内光学损耗为2.1cm-1，透明电流密度为15±10 A/cm2. 对于条宽100μm，腔长2.4mm的激光器（腔面未经镀膜处理），室温下基态激射的波长为108μm，阈值电流密度为144A/cm2，连续波光功率输出达2.67W（双面），外量子效率为63%，特征温度为320K. 研究了QD激光器翟激射特性，并对结果作了讨论.     

Self-organized GaAs patterns on misoriented GaAs (1 1 1)B substrates using dilute nitrides by molecular beam epitaxy

Recently, the growth of patterned surfaces is being used to demonstrate the site control of the three-dimensional nanostructures, and in particular quantum dots. Nevertheless the pre-patterning techniques show some disadvantages. In this work, we report a novel in situ hole-patterning technique which consists of growing by molecular beam epitaxy a dilute nitride GaAsN layer on 1° and 2° towards [2¯ 1 1] misoriented GaAs(1 1 1)B substrates. Later, we carry out a regrowth of GaAs layers on this patterned surfaces in order to improve the surface quality and the homogeneity of the characteristics of holes (size, depth, etc.). Consecutively, we use these patterned surfaces to grow InAs quantum dots, whose growth on these misorientations results in a greater difficulty. A structural characterization of the resulting samples, both hole-patterns and quantum dots, has been performed. Besides, we have realized studies of the dependence of the surface morphology on some important parameters (including substrate misorientation, thicknesses of the GaAsN and GaAs layers grown and growth conditions).     

Effects of carbon coverage on Ge quantum dots formation on Si(100) using C-Si reaction and transition of Ge growth mode

Effects of carbon (C) coverage on C-mediated Ge quantum dots (QDs) formation on a Si(100) substrate changing a state of surface reconstruction were investigated by using solid-source molecular beam epitaxy. For C=0–2.0 monolayers (MLs), the Ge QD scaled down and its density increased with C coverage. In addition, growth mode of Ge QDs changed from Volmer-Weber (VW) mode without a wetting layer to Stranski-Krastanov (SK) mode with the wetting layer for C=0.50–0.75 ML. This transition was induced by decrease in interfacial energy between Ge and Si surface due to the formation of C-Ge bonds near the Ge/Si interface. For C≥2.5 MLs, the Ge QD enlarged slightly and its density decreased with increasing C coverage, and he Ge growth mode went back to the VW mode. The Raman spectroscopy and X-ray photoelectron spectroscopy revealed the formation of a mixture of amorphous C and nano-crystalline graphite on the Si surface. Thus, the formation of a large amount of C–C (sp²) bonds induced the growth transition of QDs from the SK mode to the VW mode due to the decrease in surface energy of C.