Perspective:optically-pumped Ⅲ–Ⅴ quantum dot microcavity lasers via CMOS compatible patterned Si (001) substrates

Direct epitaxial growthⅢ–Ⅴquantum dot(QD)structures on CMOS-compatible silicon substrates is considered as one of the most promising approaches to achieve low-cost and high-yield Si-based lasers for silicon photonic integration.However,epitaxial growth ofⅢ–Ⅴmaterials on Si encounters the following three major challenges:high density of threading dislocations,antiphase boundaries and thermal cracks,which significantly degrade the crystal quality and potential device performance.In this review,we will focus on some recent results related to InAs/GaAs quantum dot lasers on Si(001)substrates byⅢ–Ⅴ/Ⅳhybrid epitaxial growth via(111)-faceted Si hollow structures.Moreover,by using the step-graded epitaxial growth process the emission wavelength of InAs QDs can be extended from O-band to C/L-band.High-performance InAs/GaAs QD microdisk lasers with sub-milliwatts threshold on Si(001)substrates are fabricated and characterized.The above results pave a promising path towards the on-chip lasers for optical interconnect applications.     

InAs/GaAs系列量子点研究

为了获得波长长、均匀性好和发光效率高的量子点,采用分子束外延(MBE)技术和S-K应变自组装模式,在GaAs(100)衬底上研究生长了三种InAs量子点。采用MBE配备的RHEED确定了工艺参数:As压维持在1.33×10-5Pa;InAs量子点和In0.2Ga0.8As的生长温度为500℃;565℃生长50nmGaAs覆盖层。生长了垂直耦合量子点(InAs1.8ML/GaAs5nm/InAs1.8ML)、阱内量子点(In0.2Ga0.8As5nm/InAs2.4ML/In0.2Ga0.8As5nm)和柱状岛量子点(InAs分别生长1.9、1.7、1.5ML,停顿20s后,生长间隔层GaAs2nm)。测得对应的室温光致发光(PL)谱峰值波长分别为1.038、1.201、1.087μm,半峰宽为119.6、128.0、72.2nm、相对发光强度为0.034、0.153、0.29。根据PL谱的峰位、半峰宽和相对发光强与量子点波长、均匀性和发光效率的对应关系,可知量子点波长有不同程度的增加、均匀性越来越好、发光效率显著增强。     

Transition Mechanism of InAs Quantum Dot to Quantum Ring Revealed by Photoluminescence Spectra

The transition mechanism of InAs quantum dot (QD) to quantum ring (QR) was investigated. After the growth of InAs QDs, a thin layer of GaAs was overgrown on the InAs QD and the sample was annealed at the same temperature for a period of time. It was found that the central part of the InAs islands started to out diffuse and formed ring shape only after a deposition of a critical thickness (1 ~ 2 nm) of GaAs capped layer depending on the size of InAs QDs. This phenomenon was revealed by photoluminescence measurement and atomic force microscopy image. It is suggested that the strain energy provided by the GaAs overgrown layer is responsible for the InAs to diffuse out of the island to form QR.     

Functionalized Self‐Assembled InAs/GaAs Quantum‐Dot Structures Hybridized with Organic Molecules

Low‐dimensional III–V semiconductors have many advantages over other semiconductors; however, they are not particularly stable under physiological conditions. Hybridizing biocompatible organic molecules with advanced optical and electronic semiconductor devices based on quantum dots (QDs) and quantum wires could provide an efficient solution to realize stress‐free and nontoxic interfaces to attach larger functional biomolecules. Monitoring the modifications of the optical properties of the hybrid molecule–QD systems by grafting various types of air‐stable diazonium salts onto the QD structures surfaces provides a direct approach to prove the above concepts. The InAs/GaAs QD structures used in this work consist of a layer of surface InAs QDs and a layer of buried InAs QDs embedded in a wider‐bandgap GaAs matrix. An enhancement in photoluminescence intensity by a factor of 3.3 from the buried QDs is achieved owing to the efficient elimination of the dangling bonds on the surface of the structures and to the decrease in non‐radiative recombination caused by their surface states. Furthermore, a narrow photoluminescence band peaking at 1620 nm with a linewidth of 49 meV corresponding to the eigenstates interband transition of the surface InAs QDs is for the first time clearly observed at room temperature, which is something that has rarely been achieved without the use of such engineered surfaces. The experimental results demonstrate that the hybrid molecule–QD systems possess a high stability, and both the surface and buried QDs are very sensitive to changes in their surficial conditions, indicating that they are excellent candidates as basic sensing elements for novel biosensor applications.     

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.     

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.     

不同淀积厚度InAs量子点的喇曼散射

利用喇曼散射方法在77K温度下对不同淀积厚度的InAs/GaAs量子点材料进行了研究.在高于InAs体材料LO模的频率范围内观察到了量子点的喇曼特征峰,分析表明应变效应是影响QD声子频率的主要因素.实验显示,随着量子点层淀积厚度L的增加,InAs量子点的声子频率由于应变释放发生红移.在加入InAlAs应变缓冲层的样品中,类AlAs声子峰随L增大发生了蓝移,从侧面证实了InAs量子点层的应变释放过程.     

OMVPE of InAs quantum dots on an InGaP surface

The organometallic vapor phase epitaxy of InAs quantum dots has been investigated by comparing the effect the underlying surface has on the quantum dot physical characteristics. Atomic force microscopy measurements were used to identify the InAs QDs coalesce to significantly larger size when deposited on an InGaP surface compared to a GaAs surface. Quantitative assessment of the total QD volume on different surfaces such as GaAs, InGaP, and GaAsP implicates the role of indium in the underlying surface for the increase in QD size on InGaP surfaces.     

Wavelength controlled InAs/InP quantum dots for telecom laser applications

This article reviews the recent progress in the growth and device applications of InAs/InP quantum dots (QDs) for telecom applications. Wavelength tuning of the metalorganic vapor-phase epitaxy grown single layer and stacked InAs QDs embedded in InGaAsP/InP (1 0 0) over the 1.55-μm region at room temperature (RT) is achieved using ultra-thin GaAs interlayers underneath the QDs. The GaAs interlayers, together with reduced growth temperature and V/III ratio, and extended growth interruption suppress As/P exchange to reduce the QD height in a controlled way. Device quality of the QDs is demonstrated by temperature-dependent photoluminescence (PL) measurements, revealing zero-dimensional carrier confinement and defect-free InAs QDs, and is highlighted by continuous-wave ground-state lasing at RT of narrow ridge-waveguide QD lasers, exhibiting a broad gain spectrum. Unpolarized PL from the cleaved side, important for realization of polarization insensitive semiconductor optical amplifiers, is obtained from closely stacked QDs due to vertical electronic coupling.     

The engineering and properties of InAs quantum dot molecules in a GaAs matrix

Arrays of InAs quantum dot (QD) molecules in the GaAs matrix, which consist of pairs of vertically aligned InAs QDs, have been synthesized by molecular beam epitaxy. A study of the resulting structures by transmission electron microscopy demonstrated that the vertically aligned QDs are equal in size. Photoluminescence measurements revealed that the spectra of the samples under study contain bands corresponding to electronic states in QD molecules.     

High-Performance In0.5Ga0.5 As/GaAs Quantum-Dot Lasers on Silicon With Multiple-Layer Quantum-Dot Dislocation Filters

Compound-semiconductor-based lasers grown directly on silicon substrates would constitute an important technology for the realization of on-chip optical interconnects. The characteristics of GaAs-or InP-based devices on silicon can be degraded by the large density of propagating dislocations resulting from the large lattice mismatch (> 4%). The use of multiple layers of self-organized In(Ga, Al)As/GaAs quantum dots (QDs) as a 3D dislocation filter to impede the propagation of dislocations and to reduce dislocation density in GaAs/Si lattice-mismatched heterostructures has been investigated. The effectiveness of this technique, depending on QD composition, size, areal density, and number of dot layers, is analyzed by a quasi-3D model of strain-dislocation interaction. It is found that ten layers of InAs QDs of size ~20-30 nm constitute the most effective dislocation filter. This is experimentally verified by cross-sectional transmission electron microscopy, photoluminescence, and performance characteristics of In_0.5Ga_0.5As/GaAs QD separate confinement heterostructure lasers on Si. The lasers exhibit J_th~900 A/cm² at 273 K, the large characteristic temperature (T₀=278 K) is in the temperature range of 5degC-85degC, and the output slope efficiency (~0.4 W/A) is independent of temperature in the range of 5degC-50degC.     

1.3μm自组织InGaAs/InAs/GaAs量子点激光器分子束外延生长

报道了分子束外延生长的1.3μm多层InGaAs/InAs/GaAs自组织量子点及其室温连续激射激光器.室温带边发射峰的半高宽小于35meV,表明量子点大小比较均匀.原子力显微镜图像显示,量子点密度可以控制在(1～7)×1010cm-2范围之内,而面密度处于4×1010cm-2时有良好的光致发光谱性能.含有三到五层1. 3μm量子点的激光器成功实现了室温连续激射.     

Influence of bismuth doping of InAs quantum-dot layer on the morphology and photoelectronic properties of Gas/InAs heterostructures grown by metal-organic chemical vapor deposition

GaAs/InAs quantum dot (QD) heterostructures prepared by metalloorganic chemical vapor deposition (MOCVD) are investigated. It is established that the introduction of isovalent bismuth doping during the growth of InAs QD layer results in the suppression of the nanocluster coalescence and favors the formation of more uniform QDs. Bismuth itself is virtually not incorporated into the dots, its role being mainly in limiting the migration mobility of atoms at the surface of the growing layer. A method for investigating the morphology of buried layers of InAs QDs in GaAs matrix by atomic-force microscopy is developed; it relies on the removal of the cap layer by selective chemical etching. The photoluminescence (PL) and photoelectric sensitivity spectra of the fabricated heterostructures and their relation to the morphology of the QD layer are studied. In doped structures, PL and selective photosensitivity owing to the QDs are observed at a wavelength of 1.41 µm with the linewidth of 43 meV at room temperature. Some of the morphological features and photoelectronic properties of the MOCVD-grown heterostructures are related to the formation of a transitional layer at the GaAs/InAs QD interface due to the diffusion-induced mixing of the components.     

Intraband photoconductivity induced by interband illumination in InAs/GaAs heterostructures with quantum dots

The effect of lateral intraband photoconductivity in undoped InAs/GaAs heterostructures with quantum dots (QDs) has been studied, with QD levels populated with carriers by means of interband optical excitation of varied power at different wavelengths. In the absence of interband illumination, no photoconductivity is observed in the mid-IR spectral range. At the same time, additional exposure of the structures to visible or near-IR light gives rise to a strong photoconductivity signal in the mid-IR spectral range (3?C5 ??m), associated with intraband transitions in QDs. The signal is observed up to a temperature of ??200 K. Use of interband optical pumping makes the intraband photoconductivity signal stronger, compared with similar structures in which doping serves to populate QD levels.     

Room-temperature optical absorption in the InAs/GaAs quantum-dot superlattice under an electric field

Electroluminescence and absorption spectra of a ten-layer InAs/GaAs quantum dot (QD) superlattice built in a two-section laser with sections of equal length is experimentally studied at room temperature. The thickness of the GaAs spacer layer between InAs QD layers, determined by transmission electron microscopy, is ∼6 nm. In contrast to tunnel-coupled QDs, QD superlattices amplify the optical polarization intensity and waveguide absorption of the TM mode in comparison with the TE mode. It is found that variations in the multimodal periodic spectrum of differential absorption of the QD superlattice structure are strongly linearly dependent on the applied electric field. Differential absorption spectra exhibit the Wannier-Stark effect in the InAs/GaAs QD superlattice, in which, in the presence of an external electric field, coupling of wave functions of miniband electron states is suppressed and a series of discrete levels called the Wannier-Stark ladder states are formed.     

InGaAs/GaAs quantum dot heterostructures for 3–5 μm IR detectors

Specific features in the formation of InAs quantum dots (QD) by MOCVD were studied in relation to the growing time or equivalent thickness of the InAs layer. TEM and photoluminescence studies have shown that, as the growing time of QDs in a GaAs matrix becomes longer, both the size and shape of the QDs are modified; namely, the aspect ratio increases. Selectively doped multilayer InGaAs/GaAs QD structures were fabricated, and photoconductivity in the IR range was studied for lateral and vertical electron transport. Under a normal incidence of light, intraband photoconductivity in the mid-IR range, 2.5–5 μm, was observed at temperatures of up to 110 K.     

Below‐bandgap absorption in InAs/GaAs self‐assembled quantum dot solar cells

A new approach to derive the below‐bandgap absorption in InAs/GaAs self‐assembled quantum dot (QD) devices using room temperature external quantum efficiency measurement results is presented. The significance of incorporating an extended Urbach tail absorption in analyzing QD devices is demonstrated. This tail is used to evaluate the improvement in the photo‐generated current. The wetting layer and QD absorption contributions are separated from the tail absorption. Several absorption peaks due to QD excited states and potentially different size QDs are observed. An inhomogeneous broadening of 25 meV arising from the variance in the size of QDs is derived. Copyright © 2014 John Wiley & Sons, Ltd.     

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.     

Characteristics of InGaAs Submonolayer Quantum-Dot and InAs Quantum-Dot Photonic-Crystal Vertical-Cavity Surface-Emitting Lasers

We have made InGaAs submonolayer (SML) quantum-dot (QD) and InAs QD photonic-crystal vertical-cavity surface-emitting lasers (PhC-VCSELs) for fiber-optic communications in the 990 and 1300 nm ranges, respectively. The active region of the InGaAs SML QD PhC-VCSEL contains three InGaAs SML QD layers, with each of the SML QD layer formed by alternating depositions of InAs and GaAs. The active region of the InAs QD PhC-VCSEL contains 17 undoped InAs-InGaAs QD layers. Both types of QD PhC-VCSELs exhibit single-mode characteristics throughout the current range, with side-mode suppression ratio (SMSR) larger than 35 dB. A maximum output power of 5.7 mW has been achieved for the InGaAs SML QD PhC-VCSEL. The near-field image study of the QD PhC-VCSELs indicates that the laser beam is well confined by the photonic-crystal structure of the device.     

(113)B和(100)面InAs/GaAs量子点光致发光光谱特性研究

利用分子束外延技术在(100)和(113)B GaAs衬底上进行了有/无AlAs盖帽层量子点的生长,测量了其在4～100 K温度区间的PL光谱。通过对PL光谱的积分强度、峰值能量和半高宽进行分析进而研究载流子的热传输特性。无AlAs盖帽层的(113)B面量子点的PL光谱的热淬灭现象可以由载流子极易从量子点向浸润层逃逸来解释。然而,有AlAs盖帽层的(113)B量子点的PL热淬灭主要是由于载流子进入了量子点与势垒或者浸润层界面中的非辐射中心引起的。并且其PL的温度依存性与利用Varshni定律计算的体材料InAs的温度依存性吻合很好,表明载流子通过浸润层进行传输受到了抑制,由于AlAs引起的相分离机制(113)B量子点的浸润层已经消失或者减小了。(100)面有AlAs盖帽层的PL半高宽的温度依存性与无AlAs盖帽层的量子点大致相同,表明在相同外延条件下相分离机制在(100)面上不如(113)B面显著。