We fabricated InAs self-assembled quantum dots (QDs) on strained layer using molecular beam epitaxy. The strained layer consisted of InAs/GaAs superlattice(SL) and GaAs barrier layer on (001) GaAs substrate. Through controlling thickness of the strained layer, we formed two-dimensional alignments of QDs on misfit dislocation arrays along <110> directions made by strained layer. The increase of the strained layer thickness resulted in a stronger alignment of QDs, which were observed by atomic force microscopy studies. The aligned QDs were confirmed to confine carriers well and have different size distributions by photoluminescence measurement.

Gallium nitride films were grown by rf-plasma assisted molecular beam epitaxy. A small indium flux was used as surfactant during the growth. The optical and electrical properties of the films grown with and without In surfactant were characterized by investigating the photoluminescence (PL), high resolution x-ray diffraction (HRXRD) and low-frequency noise power spectra. The sample grown in the presence of In surfactant showed a suppressed yellow luminescence (YL) compared to the one grown without In surfactant. Significant reduction in the full width at half maximum of the GaN (0002) x-ray diffraction peak, indicating a better film quality, was obtained when In surfactant was used during growth. Atomic force microscopy studies show that the root mean squared surface roughness for films grown with and without the In surfactant are 5.86 and 6.99 nm respectively indicating significant improvement in surface morphology. This is attributed to the enhanced 2-dimensional growth by In surfactant. A smaller Hooge parameter was obtained from the low-frequency noise measurement for the sample grown with In surfactant indicating that application of In surfactant led to significant reduction in the trap density of the material.

The shape and the positions of self-organized PbSe quantum dots embedded in PbEuTe are studied by means of grazing-incidence small angle x-ray scattering and x-ray diffraction. Using a detailed numerical analysis of the measured x-ray data, we have determined a truncated pyramidal shape of the free-standing dots. The type of the lateral dot ordering depends substantially on the period of the PbSe/PbEuTe superlattice. In the case of smaller periods, the lateral ordering of the dots obeys a short-range order model and we have determined the statistical properties of the dot arrangement. For intermediate spacer thicknesses the dots form a hexagonal lattice. For larger periods, the dots are completely uncorrelated and their distribution can be described as a two-dimensional ideal gas. From coplanar x-ray diffraction we have determined additionally the vertical correlation of the dot positions changes from a mere vertical correlation for small superlattice periods, to a distorted trigonal lattice for the intermediate periods. For the largest period, no vertical correlation of dot positions is observed.

Cathodoluminescence wavelength imaging (CLWI) of InAs/GaAs self-assembled quantum dots (SAQDs) was performed to study the spatial variation in the spectral lineshape of the broadened quantum dot (QD) ensemble. The lineshape was found to vary on a scale of ∼μm, revealing attendant variations in the size distribution of SAQD clusters on this spatial scale. Energy variations in clusters of SAQDs are found to exhibit a spatial correlation with the efficiency of luminescence and the activation energy for thermal re-emission of carriers. A reduction in the energy variation of the QD clusters occurs when the thickness of the spacer layers in vertically self-organized samples is reduced or the number of stacks is increased. SAQDs were also prepared by punctuated island growth (PIG), in which deposition of the total desired amount is broken into two or more stages each separated by time delays. CLWI reveals a reduced variation in the energy of the dominant CL emission on a ∼μm spatial scale, correlating with a narrower size distribution of larger QDs for PIG, as measured in atomic force microscopy.

The effect of thermal treatment during and after growth of InGaAs/GaAs quantum dot (QD) structures was studied. Transmission electron microscopy and atomic force microscopy confirmed the presence of interacting QDs, as was expected from analysis of temperature dependence of QD photoluminescence (PL) peak. The results indicate that the effect of post-growth annealing can be similar to the effect of elevated temperature of capping layer growth. Both, these thermal treatments can lead to a similar In and Ga interdiffiusion resulting in a similar blue-shift of QD PL peak.

A quantitative model of surface segregation free from adjustable parameters is suggested for the growth of ternary III-V compounds. In contrast to previous approaches, the model considers the dynamics of surface population by the three elements producing the ternary alloy. The underlying assumption is that the atoms in the adsorption layer are in equilibrium with the crystal bulk. Elastic strain arising in the epitaxial layer due to the lattice constant mismatch with the substrate is found to be one of the key factors affecting segregation. Along with growth temperature, it controls the segregation efficiency and the composition profile evolution in a growing heterostructure. The effect of the V/III ratio, growth rate and other parameters is accounted for. Here, we apply the model to analyze the InGaAs growth by molecular beam epitaxy owing to the vast experimental data available for the model verification. The theoretical predictions show a good agreement with the experimental observations

Self-organized growth of strained semiconductor heterostructures has enabled the realization of ordered arrays of quantum dots that can be incorporated into the active region of electronic and optoelectronic devices. Highly uniform In(Ga)As/Ga(Al)As with greatly reduced photoluminescence linewidths (FWHM=19 meV, T=17K) have been grown and characterized. Various aspects of carrier dynamics in these dots, such as measurement of carrier relaxation times and the modulation bandwidths of quantum dot lasers, estimation of the tunneling time in vertically coupled dots along with tuning of the intersubband electronic energy levels have also been studied. The favorable relaxation times can be exploited to realize far infrared emission and detection based on intersubband transitions in the dots. The optoelectronic properties of the dots and the dynamics of carriers therein are also extremely attractive for high-speed wavelength switching and the design of electro-optic modulators. The electro-optic coefficients in the quantum dots have been measured and the linear E-O coefficient, r1 =2.6 ×10−11m/V, is found to be comparable to that in LiNbO3

We present low-temperature near-field scanning optical microscopy (NSOM) measurements of self-organized InP quantum dots (QD) embedded in a GaInP layer. We observed an anomalously strong increase in the emission energy (∼270 meV) of a single QD by changing the tip-to-surface distance over ∼100 nm in the near-field region. The effect indicates the formation of a near-field nanocavity having extremely high photon-exciton mixing and can be interpreted as the observation of the coupled photon-exciton pair

A GaAs/A1GaAs laser structure with multi-quantum-well cladding layer has been employed to investigate the group III interdiffusion. With this structure the photoluminescence (PL) signals from both the top C doped cladding layers and the laser active region quantum wells can be observed simultaneously. In contrast to the depth dependence of the group III vacancy enhanced interdiffusion, the quantum wells in the thick top cladding layer showed a uniform layer intermixing and the extent of intermixing was less than that observed in the quantum wells of the laser active region. The Al-Ga interdiffusion coefficient, DAl-Ga, of the cladding layer is about 4 times less than that of the active region. A more efficient carrier transport from the cladding layer to the active region in the intermixed sample was deduced from the temperature varied PL spectra. The built-in electric field is proposed to enhance the vacancy diffusion in the active region

The band offsets and band gap are the most important parameters that determine the electrical and optical behavior of a heterojunction. In situscanning tunneling spectroscopy (STS) was employed to measure the valence band offset of strained Si1−xGex-on-Si (100) for the first time. The valence band offsets of strained Si0.77Ge0.23and Si0.59Ge0.41on Si(100) are found to be 0.21eV and 0.36eV, respectively. The results are in good agreement with theory and with results from other experimental methods. Due to band bending and surface states, it is difficult to determine the conduction band edge at the interface of Si1−xGex/Si exactly, but the conduction band offset is found to be much smaller than the valence band offset

The decay of the oscillation of the intensity of the specular beam of the reflection high-energy electron diffraction pattern is analyzed during the molecular beam epitaxial growth of strained InGaAs/GaAs heteroepitaxial structures. The oscillations' amplitude was found to decrease exponentially versustime during the InGaAs growth. Further, the decay time constant decreases with InAs mole fraction, indicating that the lattice strain increases islanding during growth. A simple semi-quantitative model based on the growth front roughening is formulated to explain the results. Assuming that the oscillation decay is related partly to the strain and partly due to kinetic effects during growth, a decay component that is solely due to strain can be separated; we find that the onset of increased roughening due to misfit strain component roughly corresponds to the equilibrium critical layer thickness

A stress-induced kinetically-driven morphological instability is of general applicability to driven systems. The effect of stress on the reaction mobility for incorporation into the growing solid couples to stress variations along a perturbed planar growth front, resulting in amplification or decay of the perturbation depending on the sign of the stress. Experimentally we studied a model system in which stress is applied externally to a chemically pure substance, permitting us to isolate the effect of strain from any possible effects of composition, and found that the new kinetically-driven effect dominates the behavior for solid phase epitaxial growth (SPEG) of Si(001). A linear stability analysis of a sinusoidally perturbed planar growth front, incorporating both the kinetically and the energetically driven effects, has been performed. Stability maps are developed, indicating parameter ranges under which the morphological evolution is dominated by the energetically-driven instability, the kinetically-driven instability, and the kinetically-driven stabilization. Numerical values of the key dimensionless parameters for SPEG and for SiGe/Si(001) Molecular Beam Epitaxy (MBE) are very similar in cases where they are known, indicating that the kinetically-driven effect may be important in determining morphological evolution related to quantum dot formation in MBE as well

We present the real-time pseudodielectric function <ε(E)> of low-temperature-grown GaAs (LT-GaAs) thin films during the growth as a function of growth temperature Tg and thickness. We obtained accurate measurements of the real-time <εc(E)> by using in situspectroscopic ellipsometry (SE) in conjunction with active feedback control of the substrate temperature using diffuse reflectance spectroscopy. We show that for epitaxial LT-GaAs layers, the peak in the imaginary pseudodielectric function <ε2(E)> decreases in amplitude and sharpness systematically with decreasing Tg. We also revealed an abrupt change in <εc(E)> near the critical epitaxial thickness hepi, the value of which decreases with decreasing Tg. Above hepi, the LT-GaAs grows polycrystalline (amorphous) above (below) Tg ∼ 190°C. We also simultaneously monitored the surface roughness and crystallinity by using real-time reflection high-energy electron diffraction (RHEED). These results represent progress in obtaining real-time control over the composition and morphology of LT-GaAs

Epitaxial Si/InAs/Si heterostructure grown on (001) Si substrate by molecular beam epitaxy (MBE) and annealed at 800°C, and 880°C were investigated by High Resolution Transmission Electron Microscopy (HRTEM). Extensive interdiffusion at 800°C leads to the formation of an InAs solid solution as well as InAs-enriched regions with extensions of ∼6nm, which exhibit two kinds of ordering. The ordering of InAs molecules occurred, respectively, in {110} planes inclined and parallel to the [001] growth direction. It is attributed to the energy gain from the reduced number of mixed Si-As and Si-In bonds. The sample grown at 800°C shows photoluminescence in the 1.3.µm region, which is tentatively attributed to the recombination of excitons localised in the ordered regions

Effect of copper-induced recrystallization on the piezoresistivity of germanium films is reported. SEM and XRD are used to study the crystalline structure of the films, while RBS, PIXE, and EDX are utilized to investigate their composition. Significant grain growth is observed in Ge films, upon annealing a Cu/Ge bilayer at 400°C for 30 minutes. Also a dominant orientation of Ge(111) is observable when amorphous precursor films are used. Hall measurements and cantilever beam bending show a considerable increase in hole mobility and longitudinal gauge factor, respectively

In this work, In0.5Ga0.5P layers were grown by Chemical Beam Epitaxy on GaAs (001) substrates. A set of samples was grown with temperatures kept in the range of 500°C to 560°C with V/III ratio 15. Another set was grown at 560°C with V/II ratio varied in the range 15 to 35. The evolution of ordering as function of growth temperature and V/III ratio was evaluated by photoluminescence measurements at 77K, Transmission Electron Diffraction (TED) and images using Transmission Electron Microscopy (TEM)-Dark Field. A 48meV reduction in the band gap energy was measured by photoluminescence measurements at 77 K when growth temperature was increased. This result is associated to the occurrence of CuPtB ordering in the InGaP layers observed by TED. The TEM-Dark field examination shows that the ordered domains are larger for samples grown at higher temperatures

A small reduction in band gap, from 1.915eV to 1.902eV, occurs when the V/III ratio is increased from 15 to 35. The TED patterns present diffuse scattering for all samples. For those grown with higher V/III ratio, spots are also observed. TEM-dark field images show that the ordered regions become larger, elongated and inclined; some of them exhibit long range ordering

Silicon-germanium heterostructures incorporating virtual substrates are successfully used for both microelectronic and optoelectronic applications. However their use may be limited by the rough surface morphology and defect content, especially threading dislocations. High quality silicon-germanium heterostructures incorporating virtual substrates have been grown epitaxially using different techniques. Also various methods for improving the surface roughness and threading dislocation density have been reported. This study reports comparatively the effects of two different types of grading (linear and step wise) of the germanium concentration in the virtual substrate on the surface morphology and defect content of silicon-germanium heterostructures grown by Low Pressure Chemical Vapour Deposition (LPCVD) in the Southampton University Microelectronics Centre. Step-grading is demonstrated to produce superior virtual substrates

We proposed a concise and novel scheme to determine the crystallographic misorientation of heteroepitaxial structures. In addition to subtle high-resolution transmission electron microscope images, the information revealed from selected-area diffraction patterns at the interfaces offers another path to determine the angles of misorientations. The principle is to extract the basically three-dimensional misorientation information from a two-dimensional selected-area diffraction pattern through the employment of the Laue circle

GaNP bulk layers with different N concentrations and GaN0.025P0.975/GaP multiple quantum wells (MQWs) with various well thicknesses were grown on (100) GaP substrates by gas-source molecular beam epitaxy with a RF nitrogen radical beam source. Using high-resolution X-ray rocking curves, photoluminescence (PL) and absorption measurements, we have shown that incorporation of N in GaNxP1−x, alloys (x ≥0.43%) leads to a direct bandgap behavior of GaNP, and yields strong room-temperature PL from the epilayers. A large Stokes shift of 200 meV is found between the PL peak energy and the absorption edge for the GaNP epilayers, indicating a very strong carrier localization. From the PL peaks of a series of GaN0.025P0.975/GaP MQWs with different well thicknesses grown at the same growth condition, a large conduction-band effective mass mc* − 0.9 me has been obtained for the GaN0.025P0.975alloy, indicating a mixing of Γ and X band wave functions in the conduction band

The growth of thin films has been categorized into three types namely the Frank-van der Merwe (FM) mode, the Stranski-Krastanov (SK) mode, and the Volmer-Weber (VW) mode. The strain, surface, and interfacial energies were calculated for the three types of growth mode of the GaInN/GaN and GaPSb/GaP systems by considering the effects of the dislocation density and reconstruction of dangling bonds. The phase diagrams of the growth modes were determined for the GaInN/GaN and GaPSb/GaP systems. In the GaInN/GaN system, the VW mode appears in the most region of the phase diagram. The region of the VW mode becomes smaller as the dislocation density increases. In the GaPSb/GaP system, the SK and VW modes appear and the region of these modes becomes larger as the reconstruction ratio increases