1.3-μm InAs/GaAs quantum dots grown on Si substrates

We compare the effect of InGaAs/GaAs strained-layer superlattice(SLS) with that of GaAs thick buffer layer(TBL)serving as a dislocation filter layer. The InGaAs/GaAs SLS is found to be more effective than GaAs TBL in blocking the propagation of threading dislocations, which are generated at the interface between the GaAs buffer layer and the Si substrate. Through testing and analysis, we conclude that the weaker photoluminescence for quantum dots(QDs) on Si substrate is caused by the quality of capping In_(0.15)Ga_(0.85)As and upper GaAs. We also find that the periodic misfits at the interface are related to the initial stress release of GaAs islands, which guarantees that the upper layers are stress-free.     

Electrical properties of MOCVD-grown GaN on Si （111） substrates with low-temperature AIN interlayers

The electrical properties of the structure of GaN grown on an Si （111） substrate with low-temperature （LT） A1N interlayers by metal-organic chemical-vapour deposition are investigated. An abnormal P-type conduction is observed in our GaN-on-Si structure by Hall effect measurement, which is mainly due to the A1 atom diffusing into the Si substrate and acting as an acceptor dopant. Meanwhile, a constant n-type conduction channel is observed in LT-A1N, which causes a conduction-type conversion at low temperature （50 K） and may further influence the electrical behavior of this structure.     

Intense terahertz emission from undoped GaAs/n-type GaAs and?InAs/AlSb structures grown on Si substrates in the transmission-geometry excitation

Intense terahertz radiation was generated from femtosecond laser-irradiated InAs and GaAs layers on Si substrates. Results show that InAs/Si and GaAs/Si films can be excited in reflection and transmission geometries. The InAs/Si film exhibited weaker emission for both excitation cases but it will be more feasible as a spectroscopic THz source due to the absence of complex spectral features in its emission spectrum. The GaAs/Si emission is characterized by Fabry?CPerot oscillations but it is 90% of that of p-InAs bulk crystal emission intensity in the reflection geometry. Excitation fluence measurements showed that the InAs/Si film saturates easily due to the laser??s shallow penetration depth in InAs.     

Studies on the nucleation of MBE grown Ⅲ-nitride nanowires on Si

GaN and AlN nanowires(NWs) have attracted great interests for the fabrication of novel nano-sized devices. In this paper, the nucleation processes of GaN and AlN NWs grown on Si substrates by molecular beam epitaxy(MBE)are investigated. It is found that GaN NWs nucleated on in-situ formed Si_3N_4 fully release the stress upon the interface between GaN NW and amorphous Si_3N_4 layer, while AlN NWs nucleated by aluminization process gradually release the stress during growth. Depending on the strain status as well as the migration ability of Ⅲ group adatoms, the different growth kinetics of GaN and AlN NWs result in different NW morphologies, i.e., GaN NWs with uniform radii and AlN NWs with tapered bases.     

Nonlinear electron transport in Si δ-doped GaAs

In this paper we report abnormal non-ohmic behaviour observed in Si δ-doped GaAs. We have performed measurements in Hall bar and Van der Pauw geometries in which electron transport phenomena can be studiekd by looking into the dependence of the device resistance R_xx on the applied current I or input power per electron, P. Transport of electrons in single Si δ-doped layers in GaAs, grown by MOCVD, in the linear and non-linear response regimes was studied. Our experimental results show that: i) R_xx dramatically decreases with increasingI (P) when sample current (input power) is larger than a critical value I_c (P_c); ii) I_c increases with electron density (Si δ-doping concentration); and iii) the decrease in R_xx with increasing I in the non-linear response regime can be observed both at zero and high magnetic fields over a wide temperature range. When R_xx is plotted as a function of the power loss rate per electron (P) the values of the critical power (P_c) for different samples are roughly the same. We suggest that redistribution of the ionised donors in the sample occurring when I >I_c takes the major responsibility for the observed abnormal phenomena.     

Effect of surface segregation on the sharpness of heteroboundaries in multilayered Si(Ge)/Si1−x Gex structures grown from atomic beams in vacuum

The main reasons for composition intermixing in the vicinity of heteroboundaries in an Si(Ge)/Si_1?x Ge_x heterosystem grown by the molecular beam epitaxy method are considered. The proposed model explains all the experimentally observed peculiarities, such as the clearly manifested asymmetry of the solid solution profile in layers, and demonstrates the noticeable erosion of the composition profile at the boundaries even in the absence of the surface segregation effect. It is shown that the surface segregation in the Ge/Si_1?x Ge_x system may play a positive role, leading to an increase in the profile steepness of the Si_1?x Ge_x layer near the boundaries.     

Structural and optical properties of a catalyst-free GaAs/AlGaAs core–shell nano/microwire grown on (1 1 1)Si substrate

Heterostructure in the catalyst-free GaAs nanowire grown on the Si substrate was studied for the application of optical devices in the next generation. We fabricated AlGaAs/GaAs/AlGaAs quantum well (QW) structure on the side facet of the catalyst-free GaAs nanowire grown by molecular beam epitaxy (MBE). The cathode luminescence (CL) measurement showed that the uniform GaAs quantum well was formed between AlGaAs shell layers. On the basis of this structure, we also grew the thick AlGaAs shell layers (∼700 nm) on GaAs nanowires, and observed whispering gallery mode (WGM) resonant in the thick AlGaAs hexagonal structure.     

Structural and optical properties of InxGa1−xAs strained layers grown on GaAs substrates by MOVPE

In_xGa_1−xAs/GaAs pseudomorphic structures were grown by metalorganic vapor phase epitaxy. Reciprocal space mapping were recorded in the vicinity of (0 0 4) and (1 1 5) nodes using high resolution X-ray diffraction (HRXRD) in order to determine strain tensor components, indium compositions and thicknesses of alloys. Near-infrared photoluminescence (PL) was performed at 10 K. The impact of strain on PL response was revealed by peak energy positions and line width. In addition, valence-band splitting (VBS) and the shift of the heavy-hole were measured. Besides, photoreflectance (PR) at room temperature was useful to establish experimentally the dependence of VBS and band energy shifts (E₀ and E₀+∆₀) on elastic strain due to lattice mismatches. Other parameters such as the internal electric-field and the electro-optical energy were determined from Franz–Keldysh oscillations analysis. Good correlation between the results obtained from all investigated techniques and theoretical predictions was confirmed.     

High-performance InAs/GaAs quantum dot laser with dot layers grown at 425℃

We investigate InAs/GaAs quantum dot(QD) lasers grown by gas source molecular beam epitaxy with different growth temperatures for InAs dot layers.The same laser structures are grown,but the growth temperatures of InAs dot layers are set as 425 and 500 ℃,respectively.Ridge waveguide laser diodes are fabricated,and the characteristics of the QD lasers are systematically studied.The laser diodes with QDs grown at 425℃ show better performance,such as threshold current density,output power,internal quantum efficiency,and characteristic temperature,than those with QDs grown at 500 C.This finding is ascribed to the higher QD density and more uniform size distribution of QDs achieved at 425℃.     

Ni/Ag Schottky contacts on Al0.11Ga0.89N grown on Si (1 1 1) substrate by plasma-assisted MBE

Al_0.11Ga_0.89N/GaN samples are grown by plasma-assisted molecular beam epitaxy method on (1 1 1) silicon substrates. High purity gallium (7N) and aluminum (6N5) were used to grow Al_0.11Ga_0.89N, GaN, and AlN, respectively. The surface morphology, structural and optical properties of the sample has been investigated by scanning electron microscope (SEM), and high-resolution X-ray diffraction (HR-XRD), respectively. HR-XRD measurement showed that the sample has a typical diffraction pattern of hexagonal Al_0.11Ga_0.89N/GaN heterostructures. Ni/Ag bilayers are deposited on Al_0.11Ga_0.89N as the Schottky contacts. The effect of annealing in oxygen ambient on the electrical properties of Ni/Ag/Al_0.11Ga_0.89N is studied by current–voltage measurement. The annealing at a temperature of 700 °C for 10?min results in an increase in the ideality factor from 1.033 to 1.042 and an increase in the Schottky barrier height from 0.708 to 0.811 eV. Furthermore, the annealing in oxygen ambient also leads to an increase in the surface roughness of the contacts from 0.0098 to 0.1360 μm which is in agreement with the SEM results.     

Substrate removal for improved performance of QWIP–LED devices grown on GaAs substrates

Pixelless infrared imaging can be accomplished by epitaxially integrating a light emitting diode (LED) with a quantum well infrared photodetector (QWIP) large area device. The device acts as an infrared image converter by detecting a mid-to-far infrared (M/FIR) signal using the QWIP and outputting a near-infrared (NIR) signal via the integrated LED. By removing the device substrate, the detector performance can be significantly improved. This paper describes a substrate removal process, which uses a combination of mechanical polishing and chemical selective wet-etch. The choice of a suitable optical adhesive and the control of carrier/device parallelism are the two most important factors determining the success of the process.     

Temperature tuning of exciton–photon coupling in a microcavity grown on a (311)A GaAs substrate

We report on the observation of photon-exciton coupling in a MQW microcavity grown on a (311)A GaAs substrate by photoluminescence. This coupling or Rabi splitting was observed by tuning the heavy hole exciton line and the cavity mode by varying the temperature in the range 80–200 K.     

Effects of high temperature rapid thermal annealing on Ge films grown on Si（001） substrate

Tensile strain,crystal quality,and surface morphology of 500 nm thick Ge films were improved after rapid thermal annealing at 900 C for a short period(20 s).The films were grown on Si(001)substrates by ultra-high vacuum chemical vapor deposition.These improvements are attributed to relaxation and defect annihilation in the Ge films.However,after prolonged(20 s)rapid thermal annealing,tensile strain and crystal quality degenerated.This phenomenon results from intensive Si–Ge mixing at high temperature.     

Strain relaxation in epitaxial GaAs/Si (0 0 1) nanostructures

Abstract

Crystal defects, present in ~100 nm GaAs nanocrystals grown by metal organic vapour phase epitaxy on top of (0 0 1)-oriented Si nanotips (with a tip opening 50–90 nm), have been studied by means of high-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The role of 60° perfect, 30° and 90° Shockley partial misfit dislocations (MDs) in the plastic strain relaxation of GaAs on Si is discussed. Formation conditions of stair-rod dislocations and coherent twin boundaries in the GaAs nanocrystals are explained. Also, although stacking faults are commonly observed, we show here that synthesis of GaAs nanocrystals with a minimum number of these defects is possible. On the other hand, from the number of MDs, we have to conclude that the GaAs nanoparticles are fully relaxed plastically, such that for the present tip sizes no substrate compliance can be observed.     

The effect of single A1GaN interlayer on the structural properties of GaN epilayers grown on Si （111） substrates

High-quality and nearly crack-free GaN epitaxial layer was obtained by inserting a single AlGaN interlayer between GaN epilayer and high-temperature AlN buffer layer on Si (111) substrate by metalorganic chemical vapor deposition. This paper investigates the effect of AlGaN interlayer on the structural properties of the resulting GaN epilayer. It confirms from the optical microscopy and Raman scattering spectroscopy that the AlGaN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and transmission electron microscopy analysis reveal that a significant reduction in both screw and edge threading dislocations is achieved in GaN epilayer by the insertion of AlGaN interlayer. The process of threading dislocation reduction in both AlGaN interlayer and GaN epilayer is demonstrated.     

The A 1Π ∼ D 1Δ rotation-electronic interaction in CO

All available experimental data pertinent to the A ¹Π (v = 0-4) and D ¹Δ state of CO have been used to derive five independent values for the rotation-electronic interaction parameter b(A ～D). The experimental values concern a narrow R centroïd region (1·21 < R < 1·32 Å). The mean value b(A ～D) = 0·12 ± 0·01 is much smaller than the value (0·48) predicted by ab initio SCF-CI calculations which have been carried out for several internuclear distances. The experimental and calculated values for the A ¹Π ～I ¹Σ^- and a ³Π ～ a′³Σ⁺, e ³Σ^-, d ³Δ rotation-electronic perturbations available in the literature are also discussed.     

H adsorption at Ag/Si interfaces in epitaxially grown Ag(1 1 1) films on Si(1 1 1)7 × 7 substrates

The interaction of atomic H with Ag(1 1 1)/Si(1 1 1)7 × 7 surfaces was studied by thermal desorption (TD) spectroscopy and scanning tunneling microscopy (STM) at room temperature. TD spectroscopy revealed an intense peak from mono H–Si bonds, even though the Si surface was covered by the Ag atoms. This peak was not observed from Ag-coated SiO₂/Si substrates. STM observation showed no clear change of the Ag surface morphology resulting from H exposure. All these results indicate that the atomic H adsorbs at neither the Ag surfaces nor Ag bulk sites, but at the Ag/Si interface by diffusing through the Ag film.