Long-wavelength light emission from InAs quantum dots covered by GaAsSb grown on GaAs substrates

We fabricated InAs quantum dots (QDs) with a GaAsSb strain-reducing layer (SRL) on a GaAs(0 0 1) substrate. The wavelength of emission from InAs QD is shown to be controllable by changing the composition and thickness of the SRL. An increase in photoluminescence intensity with increasing compositions of Sb and thickness of the GaAsSb SRL is also seen. The efficiency of radiative recombination was improved under both conditions because the InAs/GaAsSb/GaAs hetero-interface band structure more effectively suppressed carrier escape from the InAs QDs.     

Strong photoluminescence and laser operation of InAs quantum dots covered by a GaAsSb strain-reducing layer

GaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs is reduced due to the tensile strain induced by the GaAsSb SRL, resulting in a redshift of photoluminescence (PL) peaks of the InAs QDs. A strong PL signal around a wavelength of 1.3 μm was observed even at room temperature. A laser diode containing InAs QDs with GaAsSb SRLs in the active region was fabricated, which exhibits laser oscillation in pulsed operation at room temperature. These results indicate that GaAsSb SRLs have a high potential for fabricating high efficient InAs QDs laser diodes operating at long-wavelength regimes.     

Sb/As intermixing in self-assembled GaSb/GaAs type II quantum dot systems and control of their photoluminescence spectra

The intermixing of Sb and As atoms induced by rapid thermal annealing (RTA) was investigated for type II GaSb/GaAs self-assembled quantum dots (QD) formed by molecular beam epitaxy growth. Just as in InAs/GaAs QD systems, the intermixing induces a remarkable blueshift of the photoluminescence (PL) peak of QDs and reduces the inhomogeneous broadening of PL peaks for both QD ensemble and wetting layer (WL) as consequences of the weakening of quantum confinement. Contrary to InAs/GaAs QDs systems, however, the intermixing has led to a pronounced exponential increase in PL intensity for GaSb QDs with annealing temperature up to 875 °C. By analyzing the temperature dependence of PL for QDs annealed at 700, 750 and 800 °C, activation energies of PL quenching from QDs at high temperatures are 176.4, 146 and 73.9 meV. The decrease of QD activation energy with annealing temperatures indicates the reduction of hole localization energy in type II QDs due to the Sb/As intermixing. The activation energy for the WL PL was found to drastically decrease when annealed at 800 °C where the QD PL intensity surpassed WL.     

Raman study of InAs/GaAs quantum dot solar cells

We report polarized and resonant Raman study of InAs/GaAs quantum dot solar cell (QDSC) structures. Raman spectra obtained from the top surfaces of the samples suggested that the formation of InAs QDs induced tensile strain in the overgrown GaAs layers. Furthermore, a longitudinal optical phonon-plasmon (LPP) coupled modes were observed in the p-type GaAs layers. The tensile strain was increased with an increase in the QD size. The hole concentrations estimated by fitting the individual LPP coupled modes were in the range of 2.4–3.5 × 10¹⁸ cm^?3. Resonant Raman spectra obtained from the cleaved sides, where the QDs were located, showed a 225 cm^?1 mode in parallel polarization configurations. Based on accurate analysis, this mode was identified as the LA(X) phonon of GaAs.     

InAs量子点引入应力对调制掺杂结构中二维电子气输运特性的影响

系统研究了在调制掺杂AlGaAs/GaAs异质结中嵌入InAs量子点后对二维电子气输运特性的影响。使用分子束外延设备生长了量子点层与二维电子气沟道距离（T_ch）不同的3个样品,霍尔测试结果表明,二维电子气的电子迁移率和载流子浓度都随T_ch的减小而降低。基于几何相位分析算法对部分样品的高分辨透射电镜图像进行了处理,得到了其应变分布图。结果表明,应变主要分布在量子点的周围,并延伸到了量子点的上方。该不均匀的应力场可能是除库伦散射外影响电子迁移率降低的另一个重要因素。     

Formation process for and strain effect in InAs quantum dots grown on GaAs substrates by using molecular beam epitaxy

Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements were used to investigate the dependences of the formation process and the strain on the As/In ratio and the substrate temperature of InAs quantum dots (QDs) grown on GaAs substrates by using molecular beam epitaxy. The thickness of the InAs wetting layer and the shape and the size of the InAs QDs were significantly affected by the As/In ratio and the substrate temperature. The strains in the InAs layer and the GaAs substrate were studied by using RHEED patterns. The magnitude in strain of the InAs QDs formed at a low substrate temperature was larger than that in InAs QDs grown at high substrate temperature. The present results can help to improve the understanding of the formation process and the strain effect in InAs QDs.     

Numerical simulation of a coupling effect on electronic states in quantum dots

Lasers operating at 1.3 μm have attracted considerable attention owing to their potential to provide efficient light sources for next-generation high-speed communication systems. InAs/GaAs quantum dots (QDs) were pointed out as a reliable low-cost way to attain this goal. However, due to the lattice mismatch, the accumulation of strain by stacking the QDs can cause dislocations that significantly degrade the performance of the lasers. In order to reduce this strain, a promising method is the use of InAs QDs embedded in InGaAs layers. The capping of the QD layer with InGaAs is able to tune the emission toward longer and controllable wave-lengths between 1.1 and 1.5 μm. In this work, using the effective-mass envelope-function theory, we investigated theoretically the optical properties of coupled InAs/GaAs strained QDs based structures emitting around 1.33 μm. The calculation was performed by the resolution of the 3D Schrödinger equation. The energy levels of confined carriers and the optical transition energy have been investigated. The oscillator strengths of this transition have been studied with and without taking into account the strain effect in the calculations. The information derived from the present study shows that the InGaAs capping layer may have profound consequences as regards the performance of an InAs/GaAs QD based laser. Based on the present results, we hope that the present work make a contribution to experimental studies of InAs/GaAs QD based structures, namely the optoelectronic applications concerning infrared and mid-infrared spectral regions as well as the solar cells.     

Interband transitions and electronic properties of InAs/GaAs quantum dots embedded in AlxGa1−xAs/GaAs modulation-doped heterostructures

Reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and double-crystal X-ray curves showed that high-quality InAs quantum dot (QD) arrays inserted into GaAs barriers were embedded in an Al_0.3Ga_0.7As/GaAs heterostructure. The temperature-dependent photoluminescence (PL) spectra of the InAs/GaAs QDs showed that the exciton peak corresponding interband transition from the ground electronic subband to the ground heavy-hole subband (E₁-HH₁) was dominantly observed and that the peak position and the full width at half maximum corresponding to the interband transitions of the PL spectrum were dependent on the temperature. The activation energy of the electrons confined in the InAs/GaAs QDs was 115 meV. The electronic subband energy and the energy wave function of the Al_0.3Ga_0.7As/GaAs heterostructures were calculated by using a self-consistent method. The electronic subband energies in the InAs/GaAs QDs were calculated by using a three-dimensional spatial plane wave method, and the value of the calculated (E₁-HH₁) transition in the InAs/GaAs QDs was in reasonable agreement with that obtained from the PL measurement.     

Effect of thermal annealing in the microstructural and the optical properties of uncapped InAs quantum dots grown on GaAs buffer layers

The effect of thermal annealing on self-assembled uncapped InAs/GaAs quantum dots (QDs) has been investigated using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images showed that the lateral sizes and densities of the InAs QDs were not changed significantly up to 650 °C. When the InAs/GaAs QDs were annealed at 700 °C, while the lateral size of the InAs QDs increased, their density decreased. The InAs QDs disappeared at 800 °C. PL spectra showed that the peaks corresponding to the interband transitions of the InAs QDs shifted slightly toward the high-energy side, and the PL intensity decreased with increasing annealing temperature. These results indicate that the microstructural and the optical properties of self-assembled uncapped InAs/GaAs can be modified due to postgrowth thermal annealing.     

Influence of nanomechanical force on the electronic structure of InAs/GaAs quantum dots

We show nanomechanical force is useful to dynamically control the optical response of self-assembled quantum dots, giving a method to shift electron and heavy hole levels, interval of electron and heavy hole energy levels, and the emission wavelength of quantum dots (QDs). The strain, the electron energy levels, and heavy hole energy levels of InAs/GaAs(001) quantum dots with vertical nanomechanical force are investigated. Both the lattice mismatch and nanomechanical force are considered at the same time. The results show that the hydrostatic and the biaxial strains inside the QDs subjected to nanomechanical force vary with nanomechanical force. That gives the control for tailoring band gaps and optical response. Moreover, due to strain-modified energy, the band edge is also influenced by nanomechanical force. The nanomechanical force is shown to influence the band edge. As is well known, the band offset affects the electronic structure, which shows that the nanomechanical force is proven to be useful to tailor the emission wavelength of QDs. Our research helps to better understand how the nanomechanical force can be used to dynamically control the optics of quantum dots.     

Raman study of InAs quantum dots on GaAs/InP grown by low pressure metal-organic chemical vapor deposition

Raman spectra of InAs quantum dots (QDs) on InP substrate were investigated. Both longitudinal-optic (LO) and transverse-optic (TO) frequency of InAs QDs showed a large blue-shift comparing to its bulk due to the compressive strain in InAs QDs. Raman scattering of InAs QDs with a thin GaAs interlayer was studied. We obtained that the peak position of LO and TO mode of InAs QDs became larger blue-shifted when we inserted the GaAs layer. At the same time, we found a red-shift of the frequency of GaAs LO mode because of tensile strain. Theoretical calculation was performed and its prediction coincided with our experiment results well. They both showed that strain played an important role in formation of InAs QDs.     

Effects of growth conditions on the formation of self-assembled InAs quantum dots grown on (1 1 5)A GaAs substrate

Effects of growth conditions on the formation of InAs quantum dots (QDs) grown on GaAs (1 1 5)A substrate were investigated by using the reflection high-energy electron diffraction (RHEED) and photoluminescence spectroscopy (PL). An anomalous evolution of wetting layer was observed when increasing the As/In flux ratio. This is attributed to a change in the surface reconstruction. PL measurements show that QDs emission was strongly affected by the InAs deposited amount. No obvious signature of PL emission QDs appears for sample with 2.2 ML InAs coverage. Furthermore, carrier tunneling from the dots to the non-radiative centers via the inclination continuum band is found to be the dominant mechanism for the InAs amount deposition up to 4.2 MLs.     

Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE

Self-assembled InAs quantum dots (QDs) with high-density were grown on GaAs(0 0 1) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many two-dimensional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.     

Controlled stacking growth of uniform InAs quantum dots by molecular beam epitaxy

Double-stacked InAs quantum dots (QDs) were grown by molecular beam epitaxy via Stranski–Krastanov growth mode. Transition of the facet formation from {1 3 6} plane to {1 1 0} plane was observed during the stacking growth of InAs QDs by reflection high-energy electron-beam diffraction. The enhanced growth rate and the different facet formation in the stacking growth were caused by tensile strain of the GaAs underlying layer. Low arsenic pressure and low growth rate conditions played an important role for a perfect coupling and uniformity in the size of the stacked QDs. The narrow photoluminescence line width of 17.6 meV was successfully obtained from the stacked InAs QDs.     

Temperature behavior of Franz-Keldysh oscillation and evaluation of interface electric fields attributed to strain effect of InAs/GaAs quantum dot

In this work, the electric field-induced Franz-Keldysh effect was used to investigate the localized electric fields in GaAs interfaces attributed to strain effect of InAs/GaAs quantum dots (QD). The electric fields were investigated by photoreflectance spectroscopy (PR). PR spectra of the InAs/GaAs QDs showed complex Franz-Keldysh oscillations (FKOs) with various temperatures. It is suggested that the FKOs originated from the interface electric fields predominately caused by the strain-induced polarization at GaAs interface near the InAs QDs. The InAs/GaAs QDs have a broad range of interface electric fields from ～10⁴ V/cm to ～2х10⁵ V/cm. Temperature behavior of FKO amplitude distribution is explained by temperature dependent carrier confinement effect.     

Electric field effect in the spin dynamics of self-assembled InAs/GaAs quantum dots

We identify fundamental mechanisms of electron and hole dynamics in self-organized InAs/GaAs quantum dots (QDs) subject to vertical electric fields by photocurrent investigations. We propose a spin–flip mechanism involving a spin exchange between neighboring QDs. The spin–flip process is revealed in the photocurrent dynamics when the exciton population increases unexpectedly with reverse bias.     

Entropies associated with electron emission from InAs/GaAs quantum dots

Entropies associated with the transition of electrons into and out of InAs/GaAs quantum dots (QDs) are calculated by considering the temperature dependence of energy eigenvalues due to strain and energy band offset variations. It is found that, for InAs/GaAs quantum dots with base/height dimensions of 20/10 nm, the contribution from the surrounding lattice to entropy is smaller than for the temperature region below 100 K, where most measurements of thermal emission rates are performed. Including the electron degeneracy, the total entropy change has an upper limit of when releasing the first electron from the s-shell, while the second released s-electron is connected with an entropy change not larger than the absolute value of .     

The improvement of InAs/GaAs quantum dot properties capped by Graphene

InAs self‐assembled quantum dots (QDs) were grown by molecular beam epitaxy on (001) GaAs substrate. Uncapped and capped QDs with GaAs and graphene layers were studied using atomic force microscopy and Raman spectroscopy. Graphene multi‐layer was grown by chemical vapor deposition and transferred on InAs/GaAs QDs. It is well known that the presence of a cap layer modifies the size, shape, and density of the QDs. According to the atomic force microscopy study, in contrast to the GaAs capped sample, which induce a dramatic decrease of the density and height of dots, graphene cap layer sample presents a slight influence on the surface morphology and the density of the islands compared with the uncapped one. The difference shown in the Raman spectra of the samples is due to change of strain and alloy disorder effects on the QDs. Residuals strain and the relaxation coefficients have been investigated. All results confirm the best crystalline quality of the graphene cap layer dots sample relative to the GaAs capped one. So graphene can be used to replace GaAs in capping InAs/GaAs dots. To our knowledge, such study has not been carried out until now. Copyright © 2013 John Wiley & Sons, Ltd.     

Formation of InAs quantum dots and wetting layers in GaAs and AlAs analyzed by cross-sectional scanning tunneling microscopy

We have used cross-sectional scanning-tunneling microscopy (X-STM) to compare the formation of self-assembled InAs quantum dots (QDs) and wetting layers on AlAs (1 0 0) and GaAs (1 0 0) surfaces. On AlAs we find a larger QD density and smaller QD size than for QDs grown on GaAs under the same growth conditions (500 °C substrate temperature and 1.9 ML indium deposition). The QDs grown on GaAs show both a normal and a lateral gradient in the indium distribution whereas the QDs grown on AlAs show only a normal gradient. The wetting layers on GaAs and AlAs do not show significant differences in their composition profiles. We suggest that the segregation of the wetting layer is mainly strain-driven, whereas the formation of the QDs is also determined by growth kinetics. We have determined the indium composition of the QDs by fitting it to the measured outward relaxation and lattice constant profile of the cleaved surface using a three-dimensional finite element calculation based on elasticity theory.     

Analysis of room temperature PL spectra of InAs/GaAs/InP and InAs/InP self-assembled QDs: A five-band study

In this paper, room temperature PL spectra of InAs self-assembled dots grown on GaAs/InP and InP substrate are presented. For analyzing different positions of the PL peaks, we examine the strain tensor in these quantum dots (QDs) using a valence force field model, and use a five-band k·p formalism to find the electronic spectra. We find that the GaAs tensile-stained layer affects the position of room temperature PL peak. The redshift of PL peak of InAs/GaAs/InP QDs compared to that of InAs/InP QDs is explained theoretically.