Annealing effect on InAs islands on GaAs(0 0 1) substrates studied by scanning tunneling microscopy

Post-annealing effects on InAs islands grown on GaAs(0 0 1) surfaces have been investigated by scanning tunneling microscopy (STM) connected to molecular beam epitaxy (MBE). It is found that for islands grown by 1.6 ML InAs deposition at 450 °C, post-annealing at 450 °C in an As₄ atmosphere causes dissolving of the InAs islands. In contrast, for larger islands obtained by 2.0 ML InAs deposition at 450 °C, the post-annealing leads to coarsening of the islands. The result can be explained in terms of a critical nucleus in heterogeneous nucleation.     

Ferromagnetic GaSb/Mn digital alloys

In order to realize spintronic devices in narrow-gap semiconductors, we have carried out studies on the well-known InAs/GaSb-based materials and structures. As a key component to such devices, GaSb/Mn digital alloys were successfully grown by molecular beam epitaxy. Good crystal quality was observed with transmission electron microscopy showing well-resolved Mn-containing layers and no evidence of 3D MnSb precipitates in as-grown samples. Ferromagnetism was observed in GaSb/Mn digital alloys with temperature-dependent hysteresis loops in magnetization up to 400 K (limited by the experimental setup). Magnetotransport studies were also carried out, both in the conventional Hall-bar configuration, and on gated Hall-bar structures. Both anomalous Hall effect and tunable ferromagnetism with applied gate bias were investigated. Annealing studies of the digital alloys reveal evidence of migration of Mn atoms at elevated temperatures.     

Interfaces in InAs/GaSb Superlattices Grown by Molecular Beam Epitaxy

Short period InAs（4ML）/GaSb（SML） superlattices （SLs） with InSb- and mixed-like （or Ga1-xInxAs1-ySby- like） interfaces （IFs） are grown by molecular-beam epitaxy （MBE） on （001） GaSh substrates at optimized growth temperature. Raman scattering reveals that two kinds of IFs can be formed by controlling shutter sequences. X-ray diffraction （XRD） and atomic force microscopy （AFM） demonstrate that SLs with mixed-like IFs are more sensitive to growth temperature than that with InSb-like IFs. The photoluminescence （PL） spectra of SLs with mixed-like IFs show a stronger intensity and narrower line width than with InSb-like IFs. It is concluded that InAs/GaSb SLs with mixed-like IFs have better crystalline and optical properties.     

InAs wetting layer evolution on GaAs(0 0 1)

The evolution of two-dimensional (2D) strained InAs wetting layers on GaAs(0 0 1), grown at different temperatures by molecular beam epitaxy, was studied by in situ high-resolution scanning tunneling microscopy. At low growth temperature (400 °C), the substrate exhibits a well-defined GaAs(0 0 1)-c(4 × 4) structure. For a disorientation of 0.7°, InAs grows in the step-flow mode and forms an unalloyed wetting layer mainly along steps, but also in part on the terrace. The wetting layer displays some local c(4 × 6) reconstruction, for which a model is proposed. 1.2 monolayer (ML) InAs deposition induces the formation of 3D islands. At a higher temperature (460 °C), the wetting layer is obviously alloyed even at low InAs coverage. The critical thickness of the wetting layer for the 2D-to-3D transition is shifted to 1.50 ML in this case presumably since the strain is reduced by alloying.     

Investigation of the effect of varying growth pauses on the structural and optical properties of InAs/GaAs quantum dot heterostructures

Effect of growth pause or ripening time on structural and optical properties of self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE ) technique with two different growth rates of InAs (0.032 MLs⁻¹ and 0.197 MLs⁻¹) has been investigated. The QD heterostructures were grown at 520 ^°C with InAs monolayer coverage of 2.7 ML. The results were explained on the basis of high angle annular dark field scanning transmission electron microscope (HAADF-STEM), scanning electron microscope (SEM) and photoluminescence (PL) measurements. Introduction of growth pause leads the QD system towards a thermodynamic equilibrium state which in turn makes interesting changes on the morphology of the samples. Coagulation of some smaller dots occurs because of ripening to produce evolved QDs and the dot density reduces with growth pause.     

Extremely Low Density InAs Quantum Dots with No Wetting Layer

Extremely low density InAs quantum dots （QDs） are grown by molecular beam droplet epitaxy. The gallium deposition amount is optimized to saturate exactly the excess arsenic atoms present on the GaAs substrate surface during growth, and low density InAs/GaAs QDs （4× 10^6 cm^-2） are formed by depositing 0.65 monolayers （MLs） of indium. This is much less than the critical deposition thickness （1.7 ML）, which is necessary to form InAs/GaAs QDs with the conventional Stranski-Krastanov growth mode. The narrow photoluminescence linewidth of about 24 meV is insensitive to cryostat temperatures from IO K to 250K. All measurements indicate that there is no wetting layer connecting the QDs.     

Atomic structures of two-dimensional strained InAs epitaxial layers on a GaAs(001) surface: in situ observation of quantum dot growth

Scanning tunneling microscopy and reflection high-energy electron diffraction under ultrahigh vacuum conditions were used to make an in situ study of atomic structures at the surface of an InAs/GaAs heterostructure grown by molecular-beam epitaxy. It was observed that the deposition of approximately 0.3 ML of indium on an arsenic-enriched GaAs(001)-2 × 4 surface leads to the formation of the 4 × 2 phase while the deposition of 0.6 ML indium leads to the appearance of a new 6 × 2 reconstruction. It is shown that layer-by-layer two-dimensional epitaxial growth of InAs on GaAs(001) as far as 13 monolayers can only be achieved if the growth front reproduces the 4 × 2 or 6 × 2 symmetry of the substrate and models of 4 × 2 and 6 × 2 reconstructions are proposed. Atomic-resolution images of faceted planes on the surface of three-dimensional islands in an InAs/GaAs(001) system were obtained for the first time and structural models of these were developed.     

Bandgap engineering of self-assembled InAs quantum dots with a thin AlAs barrier

We investigate the effects of a thin AlAs layer with different position and thickness on the optical properties of InAs quantum dots (QDs) by using transmission electron microscopy and photoluminescence (PL). The energy level shift of InAs QD samples is observed by introducing the thin AlAs layer without any significant loss of the QD qualities. The emission peak from InAs QDs directly grown on the 4 monolayer (ML) AlAs layer is blueshifted from that of reference sample by 219 meV with a little increase in FWHM from 42–47 meV for ground state. In contrast, InAs QDs grown under the 4 ML AlAs layer have PL peak a little redshifted to lower energy by 17 meV. This result is related to the interdiffusion of Al atom at the InAs QDs caused by the annealing effect during growing of InAs QDs on AlAs layer.     

Optimization of InAs/GaSb type-II superlattices for high performance of photodetectors

The optimum growth conditions and strain balancing processes have been studied using molecular beam epitaxy (MBE) grown 51 Å InAs/40 Å GaSb type-II superlattices (SLs) designed to have cut-off wavelength of 10 μm. The most dominant factor in reducing the defect level in the SL structure was buffer growth temperature evidenced by transmission electron microscopy. In the study of the strain balancing process, the SLs could be lattice matched to the GaSb substrate by increasing the thickness of the InSb interfaces (IFs) from a nominal value of 1.0 to 1.4 ML, however, the structural quality degraded dramatically when the thickness of IFs reached beyond 1.0 ML. By optimizing the growth condition and MBE shutter sequences, micron thick InAs/GaSb SLs with a reduced lattice mismatch were routinely obtained with the full-width half-maximum of 18 arcsec, and the root mean square values of surface roughness of 2 Å in 5 μm area scan of atomic force microscopy demonstrating high quality. Correlation between material quality and photoresponse signal strength in photoconductivity measurements was made on SL samples with cut-off wavelength on the order of 10 μm.     

Optimization of InAs/GaAs quantum-dot structures and application to 1.3-μm mode-locked laser diodes

The self-assembled growth of InAs/GaAs quantum dots by molecular beam epitaxy is conducted by optimizing several growth parameters, using a one-step interruption method after island formation. The dependence of photoluminescence on areal quantum-dot density is systematically investigated as a function of InAs deposition, growth temperature and arsenic pressure. The results of this investigation along with time-resolved photoluminescence measurements show that the com- bination of a growth temperature of 490℃, with a deposition rate of 0.02 ML/s, under an arsenic pressure of 1×10^-6 Torr （1 Torr = 1.33322×10^2 Pa）, provides the best compromise between high density and the photoluminescence of quantum dot structure, with a radiative lifetime of 780 ps. The applicability of this 5-layer quantum dot structure to high-repetition-rate pulsed lasers is demonstrated with the fabrication and characterization of a monolithic InAs/GaAs quantum-dot passively mode-locked laser operating at nearly 1300 nm. Picosecond pulse generation is achieved from a two-section laser, with a 19.7-GHz repetition rate.     

Scanning tunneling microscopy of InAs/GaSb superlattices with various growth conditions

Cross-sectional scanning tunneling microscopy (STM) is used to characterize InAs/GaSb superlattices, grown by molecular beam epitaxy (MBE). Previous STM studies have found an interface asymmetry, with the interfaces of InAs grown on GaSb being rougher than those of GaSb on InAs. In the present work, a comparison is made between samples grown under various conditions, using elevated growth temperature or using atomic layer epitaxy (ALE). In both cases, the interface asymmetry is found to decrease compared to MBE growth at 380°C. In addition, Sb incorporation in the InAs layers is observed directly in the STM images. It is argued that the additional roughness seen at the InAs on GaSb interfaces arises in part from incorporation of excess Sb into the growing InAs layers. Elevated temperatures reduces this incorporation, and hence produces smoother interfaces.     

Time-resolved photoluminescence spectroscopy of InAs quantum dots on InP with various InAlGaAs barrier thicknesses

The Optical characteristics of InAs quantum dots (QDs) embeded in InAlGaAs on InP have been investigated by photoluminescence (PL) spectroscopy and time-resolved PL. Four different QD samples are grown by using molecular beam epitaxy, and all the QD samples have five-stacked InAs quantum dot layers with a different InAlGaAs barrier thickness. The PL yield from InAs QDs was increased with an increase in the thickness of the InAlGaAs barrier, and the emission peak positions of all InAs QD samples were measured around 1.5 μm at room temperature. The decay time of the carrier in InAs QDs is decreased abruptly in the QD sample with the 5 nm InAlGaAs barrier. This feature is explained by the tunneling and coupling effect in the vertical direction and probably defect generation.     

Thermal activation and thermal transfer of localized excitons in InAs self-organized quantum dots

We have investigated the temperature dependence of photoluminescence (PL) properties of a number of InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 monolayer (ML) to 3 ML. The temperature dependence of the InAs exciton energy and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML, indicating spontaneous formation of quantum dots (QDs). A model, involving exciton recombination and thermal activation and transfer, is proposed to explain the experimental data. In the PL thermal quenching study, the measured thermal activation energies of different samples demonstrate that the InAs wetting layer may act as a barrier for thermionic emission of carriers in high quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to thermally escape from the localized states.     

Interface effect on superlattice quality and optical properties of InAs/GaSb type-II superlattices grown by molecular beam epitaxy

We systematically investigate the influence of InSb interface (IF) engineering on the crystal quality and optical properties of strain-balanced InAs/GaSb type-II superlattices (T2SLs). The type-II superlattice structure is 120 periods InAs (8 ML)/GaSb (6 ML) with different thicknesses of InSb interface grown by molecular beam epitaxy (MBE). The high-resolution x-ray diffraction (XRD) curves display sharp satellite peaks, and the narrow full width at half maximum (FWHM) of the 0th is only 30-39 arcsec. From high-resolution cross-sectional transmission electron microscopy (HRTEM) characterization, the InSb heterointerfaces and the clear spatial separation between the InAs and GaSb layers can be more intuitively distinguished. As the InSb interface thickness increases, the compressive strain increases, and the surface "bright spots" appear to be more apparent from the atomic force microscopy (AFM) results. Also, photoluminescence (PL) measurements verify that, with the increase in the strain, the bandgap of the superlattice narrows. By optimizing the InSb interface, a high-quality crystal with a well-defined surface and interface is obtained with a PL wavelength of 4.78 μ, which can be used for mid-wave infrared (MWIR) detection.     

Properties of GaAs/InGaAs quantum-size structures containing δ〈Mn〉-doped layers

Properties of structures containing an InGaAs quantum well (QW) or InAs quantum dots (QDs) as well as δ〈C〉-and δ〈Mn〉-doped layers are investigated. Most of these structures are fabricated by the MOCVD epitaxy method; δ〈Mn〉-doped layers are obtained by low-temperature laser deposition directly in the epitaxial reactor. The structures under study exhibit three conduction channels: the δ〈C〉-doped layer, δ〈Mn〉-doped layer, and QW or wetting layer in the case of QDs. The contribution of each channel into the total conduction depend on the measurement temperature. Photoluminescence of the structures as a function of thickness of δ〈Mn〉-doped layer is studied.     

高灵敏度Sb基量子阱2DEG的霍尔器件

用分子束外延技术将高灵敏度的InAs/AlSb量子阱结构的Hall器件赝配生长在GaAs衬底上。设计了由双δ掺杂构成的Hall器件的新结构,有效地提高了器件的面电子浓度。与传统的没有掺杂的InAs/AlSb量子阱结构的Hall器件相比,室温下器件电子迁移率从15 000 cm²·V^-1·s^-1 提高到16 000 cm²·V^-1·s^-1。AFM测试表明材料有好的表面形态和结晶质量。从77 K 到300 K对Hall器件进行霍尔测试,结果显示器件不同温度范围有不同散射机构。双δ掺杂结构形成高灵敏度、高二维电子气（2DEG）浓度的InAs/AlSb异质结Hall器件具有广阔的应用前景。     

脉冲激光原位辐照对InAs/GaAs(001)量子点生长的影响

在InAs/GaAs(001)量子点生长过程中, 当InAs沉积量为0.9 ML时, 利用紫外纳秒脉冲激光辐照浸润层表面, 由于高温下In原子的不稳定性, 激光诱导的原子脱附效应被放大, 样品表面出现了原子层移除和纳米孔. 原子力显微镜测试表明纳米孔呈现以[110]方向为长轴(尺寸: 20-50 nm)、[110]方向为短轴(尺寸: 15-40 nm)的表面椭圆开口形状, 孔的深度为0.5-3 nm. 纳米孔的密度与脉冲激光的能量密度正相关. 脉冲激光的辐照对量子点生长产生了显著的影响: 一方面由于纳米孔的表面自由能低, 沉积的InAs优先迁移到孔内, 纳米孔成为量子点优先成核的位置; 另一方面, 孔外的区域因为In原子的脱附, 量子点的成核被抑制. 由于带有纳米孔的浸润层表面具有类似于传统微纳加工技术制备的图形衬底对量子点选择性生长的功能, 该研究为量子点的可控生长提供了一种新的思路.     

Optical transitions of InAs/In0.36Ga0.64As/GaAs(311B) surface quantum dots clearly identified by the piezoreflectance technique

The bilayer InAs/In_0.36Ga_0.64As/GaAs(311B) quantum dots (QDs), including one InAs buried quantum dot (BQD) layer and the other InAs surface quantum dot (SQD) layer, have been grown by molecular beam epitaxy (MBE). The optical properties of these three samples have been studied by the piezoreflectance (PzR) spectroscopy. The PzR spectra do not exhibit only the optical transitions originated from the InAs BQDs, but the features originated from the InAs SQDs. After the InAs SQDs have been removed chemically, those optical transitions from InAs SQDs have been demonstrated clearly by investigating the PzR spectra of the residual InAs BQDs in these samples. The great redshift of these interband transitions of InAs SQDs has been well discussed. Due to the suitable InAs SQD sizes and the thickness of In_0.36Ga_0.64As layer, the interband transition of InAs SQDs has been shifted to ∼1.55 μm at 77 K.     

Preparation,characterization and magneto-optical investigations of electrodeposited Co/Au films

Au/Co(4–8 ML)/Au single magnetic layers and Au(8 ML)/Co(4 ML)/Au(8 ML)/Co(8 ML)/Au bilayer were sequentially grown by electrodeposition on an Au(1 1 1) buffer layer electrodeposited on Si(1 1 1). The technique used in this work provides full control on the structure and the chemical composition of the different layers (no alloying) as well as on the chemistry at interfaces. scanning tunneling microscopy (STM) and atomic force microscopy (AFM) imaging and X-ray diffraction measurements show that atomically flat continuous Co(0 0 0 1) layers (4–8 ML) can be grown in epitaxy with the Au(1 1 1) substrate and that the 2 nm-thick spacer is also a continuous Au(1 1 1) layer. The Co ultrathin layers (4 and 8 ML) exhibit perpendicular magnetic anisotropy. The lateral magnetic homogeneity and magnetization reversal process have been investigated by scanning magneto-optical Kerr effect (MOKE) magnetometry and global Kerr microscopy. The correlation between magnetization switching behaviour in each layer of the Co-bilayer stack has been evidenced from in-depth sensitive MOKE measurements and microscopy. The strong coupling observed between the two Co layers is attributed to magnetostatic interaction at domain wall boundaries.     

Mn-including InAs quantum dots fabricated by Mn implantation

Mn-including InAs quantum dots (QDs) were fabricated by Mn-ion implantation and subsequent annealing. The optical, compositional, and structural properties of the treated samples were analyzed by photoluminescence (PL) and microscopy. Energy dispersive X-ray (EDX) results indicate that Mn ions diffused from the bulk GaAs into the InAs QDs during annealing, and the diffusion appears to be driven by the strain in the InAs QDs. The temperature dependence of the PL of Mn-including InAs QD samples exhibits QDs PL characteristics. At the same time, the heavy Mn-including InAs QD samples have ferromagnetic properties and high T_c.