Self-organized InAs quantum dots (QDs) were grown in the Stranski-Krastanov regime, by gas-source molecular beam epitaxy (GSMBE), on (100) GaAs substrates. Two important parameters have been optimized in order to grow high quality QDs with a very good reproducibility: InAs growth rate and GaAs cap layer deposition rate. Atomic force microscopy (AFM) analysis shows a unimodal QD distribution and the room temperature photoluminescence (RTPL) spectrum of the optimized sample reveals a 1.3 μm emission with a 19 meV full width at half maximum (FWHM). Photoluminescence (PL) measurements versus excitation power density and photoluminescence excitation (PLE) measurements clearly show multi-component PL emission from transitions associated with fundamental and related excited states of QDs. Furthermore a good growth reproducibility is observed. The results are promising for further work which will lead to laser fabrication.

1.5 micron range emission has been realized using the InAs quantum dots embedded into the metamorphic InGaAs layer containing 20% of InAs grown by MBE on a GaAs substrate. Growth regimes were optimized to reduce significantly the density of dislocations propagating into the active layer from the lattice mismatched interface. 2 mm long InGaAs/InGaAlAs lasers with 10 planes of quantum dots in the active region showed threshold current density about 1.4 kA/cm2 with the external differential efficiency as high as 38%. Lasing wavelength depends on the optical loss being in the 1.44–1.49 micron range at room temperature. On increasing the temperature the wavelength reaches 1.515 micron at 85C while the threshold current characteristic temperature of 55–60K was estimated. High internal quantum efficiency (η>60%)and low internal losses (α=3–4 cm ) were realized. Maximum room temperature output power in pulsed regime as high as 5.5 W for 100 micron wide stripe was demonstrated. Using the same concept 1.3 micron InGaAs/InGaAlAs quantum well lasers were fabricated. The active region contained quantum wells with high (∼40%) indium content which was possible due to the intermediate InGaAs strain relaxation layer. 1 mm stripe lasers showed room temperature threshold current densities about 3.3 kA/cm (λ=1.29 micron) and 400 A/cm2 at 85K. Thus, the use of metamorphic InGaAs layers on GaAs substrate is a very promising approach for increasing the emission wavelength of GaAs based lasers.

For multilayer semiconductor films comprising various material layers, the coupling of elastic states in different layers as well as the nonequilibrium nature of the growing process are essential in understanding the surface and interface morphological instability and hence the growth mechanisms of nanostructures in the overall film. We present the theoretical work on the stress-driven instabilities during the heteroepitaxial growth of multilayers, based on the elastic analysis and the continuous nonequilibrium model. We develop a general theory which determines the morphological evolution of surface profile of the multilayer system, and then apply the results to two types of periodic structures that are being actively investigated: alternating tensile/compressive and strained/spacer multilayers. The wetting effect, which arises from the material properties changing across layer-layer interfaces, is incorporated. It exhibits a significant influence of stabilization on film morphology, particularly for the short-period superlattices. Our results are consistent with the experimental observations in AlAs/InAs/InP(001) and Ge/Si(001) multilayer structures.

Progress in self-organized epitaxy of quantum dots lead to fabrication of edge- and surface-emitting lasers featuring excellent device characteristics. Exceedingly low lasing threshold, an internal quantum efficiency near unity, high characteristic temperature was proved, and high power operation was demonstrated. We present recent achievements on lasers and optical amplifiers based on InGaAs quantum dots in GaAs matrix.

Strained epitaxial growth of Ge on Si(001) produces self-assembled, nanometer scale islands, or quantum dots. We study this growth by atomistic simulation, computing the energy of island structures to determine when and how islanding occurs. The distribution of island sizes on a surface is determined by the relation of island energy to size. Applying the calculated chemical potential to the Boltzmann-Gibbs distribution, we predict size distributions as functions of coverage and temperature. The peak populations around 80 000 atoms (35 nm wide) compare favorably with experiment.

Size and spatial distribution homogeneity of nanostructures is greatly improved by making stacks of nanostructures separated by thin spacers. In this work we present in situ and in real time stress measurements and reflection high energy electron diffraction (RHEED) observations and ex situ transmission electron microscopy (TEM) characterization of stacked layers of InAs quantum wires (QWr) separated by InP spacer layers, d(InP), of thickness between 3 and 20 nm. For d(InP) < 20 nm, the amount of InAs involved in the newly created QWr from the 2nd stack layer on, exceeds that provided by the In cell. Our results suggest that in those cases InAs 3D islands formation starts at the P/As switching and lasts during further InAs deposition. We propose an explanation for this process that is strongly supported on TEM observations. The results obtained in this work imply that concepts like the existence of a critical thickness for 2D-3D growth mode transition should be revised in correlated QWr stacks of layers.

The influence of a nanoengineering procedure on the properties of single- and multi-layer self-assembled InAs quantum dot (QDs) has been studied using photoluminescence, transmission electron microscopy (TEM), and electroluminescence. Optical properties of QDs were optimized by shape engineering via adjustment of a GaAs overlayer thickness prior to a heating (truncation) step. TEM micrographs have confirmed that the employed growth procedure results in truncated pyramidal QDs covered entirely by AlAs with smooth top interfaces. Truncated QDs with two-monolayer-thick AlAs capping have demonstrated a strong blue shift of ground state (GS) energies and up to 10 meV larger separation between GS and first excited state energy levels as compared to non-capped QDs. We believe that AlAs capping, in combination with truncation procedure, results in significant suppression of carrier transport between QDs within the same layer as well as between QD layers. A record high characteristic temperature for GS lasing threshold, T0 = 380 K up to 55 °C, as well as a maximum saturated gain of 16 (5.3 per layer) cm−1, were measured for a 1.22 μm edge-emitting lasers with this truncated triple layer QD gain medium. A maximum saturated gain, 27 (3.9 per layer) cm−1, and T0 = 110 K have been demonstrated in a 1.19 μm lasers with truncated seven layer QD medium.

We present the fabrication of a Quantum Dot Microcavity Light Emitting Diode (QD-MCLED) emitting at 1.3 μm at room temperature. The long wavelength emission is achieved by using for the first time InGaAs QDs directly grown on GaAs, by metal organic chemical vapour deposition. Electroluminescence bright emission, peaked at 1298 nm with a FWHM of 6.5 meV and a line shape independent on the applied bias is found.

One-dimensional (In,Ga)As quantum dot (QD) arrays are formed on planar singular and shallow-patterned (mesa gratings) GaAs (100) substrates by self-organized anisotropic strain engineering of an (In,Ga)As/GaAs quantum wire (QWR) superlattice (SL) template in molecular beam epitaxy. On planar singular substrates, highly uniform one-dimensional single QD arrays, which are extended over 10 μm length, are realized with efficient photoluminescence. The shallow mesa gratings along [0–11] and [011] induce two different types of steps which differently affect the surface migration processes crucial for QWR template development, i.e., strain driven In adatom migration along [011] and surface reconstruction induced adatom migration along [0–11]. While type-A steps along [0–11] have no significant effect on the adatom migration along [011] and [0–11], type-B steps along [011] hinder the surface reconstruction induced migration along [0–11] to prevent formation of QWR and ordered QD arrays.

Formation of the CuPt ordered structure during the organometallic vapor phase epitaxial (OMVPE) growth of GaInP has significant effects on the electrical and optical properties, necessitating control of ordering. Formation of the CuPt structure is thermodynamically driven by the surface structure during growth. This has led to the study of the use of surfactants as a way of controlling ordering. To date, these studies have centered on the group V elements isoelectronic with P. Sb and Bi are both larger than P. This leads to less ordering. N, on the other hand, is smaller than P, which makes it potentially interesting. The other disordering mechanism, observed for Te as a surfactant in GaInP, is kinetic. Rapid step propagation leads to a reduction in ordering. This suggests another potentially interesting class of surfactants, the group VII elements, such as Br, that have been reported to increase the step velocity during OMVPE growth. This paper describes the results of the addition of the surfactants N and Br during the OMVPE growth of GaInP. Addition of N (DMHy) results in a clear reduction in order parameter, due to thermodynamic effects. The addition of Br (CBr4) is also observed to systematically decrease the amount of CuPt ordering and is found to correspond to a significant roughening of the surface, which is postulated to be the origin of the reduction in order parameter.

In an effort to develop materials that are sensitive to mid and far infrared radiation, we examine InAs quantum dot/GaAs matrix multilayer structures grown by molecular beam epitaxy (MBE). Customized electrical and optical properties result from nanoscale-level manipulation of the dots' physical dimensions. The MBE growth temperature can be set to yield dots having the desired lateral dimension; however this leads to dots of insufficient vertical height. It is therefore necessary to grow the dots in a manner that allows independent control of the lateral and vertical dimensions. In this experiment, the vertical dimension is controlled by growing the dots in a multilayer structure with GaAs matrix layers. An initial layer of InAs quantum dots was grown on top of GaAs, followed by a few seconds short growth of GaAs, and then followed by the growth of another layer of InAs dots. The GaAs laterally surrounds, but does not bury, the InAs quantum dots. When the second layer of InAs dots is grown, they tend to self-organize directly on top of the exposed first layer of dots. We then grew a third layer of dots in the same manner. This effectively results in a pseudo-single layer of dots of the desired height which is then completely buried in GaAs. The goal is to develop structures that can be integrated into high operating temperature quantum dot infrared detectors (QDIPs) that have maximum sensitivity, robustness, and portability.

We propose a new nano-probe-assisted technique which enables the formation of site-controlled InAs quantum dots (QDs). High-density two-dimensional indium (In) nano-dot arrays on a GaAs substrate were fabricated by using a specially designed atomic-force-microscope (AFM) probe (the Nano-Jet Probe). This developed probe has a hollow pyramidal tip with a sub-micron size aperture on the apex and an In-reservoir tank within the stylus. By applying a voltage pulse between the pyramidal tip and the sample, In clusters were extracted from the reservoir tank within the stylus through the aperture, resulting in the In nano-dot formation. These In nano-dots can be directly converted to InAs QD arrays by subsequent irradiation of arsenic flux.

We studied electron resonant tunneling and Coulomb blockade in nanocrystalline Si (nc-Si) with double SiO2 barriers, which is fabricated in-situ in a plasma enhanced chemical vapor deposition system. In capacitance-voltage measurements, discrete capacitance peaks due to electron resonant tunneling into energy levels of nc-Si dots and Coulomb blockade charging in nc-Si dots have been observed at room temperature. For smaller dots, capacitance peaks are more distinct due to the stronger Coulomb blockade effect. Meanwhile, conductance plateau in the region of capacitance peaks are also observed due to the charging effect in nc-Si dots. Experimental results are in agreement with theoretical evaluation based on the model of Coulomb blockade.

The optical properties of quantum wires (QWRs) grown using lateral composition modulation (LCM) were studied by photoluminescence (PL) measurement as a function cryostat temperature (Tcr). 3 stacked arrays of QWRs were formed by sequential growth of ∼ 180 Å-thick LCM layers (lateral period: ∼ 90 Å) induced by (InP)1/(GaP)1 short-period superlattices, and 200 Å-thick InGaP spacers at the growth temperature of 490 °C. The formation of QWRs was confirmed by a transmission electron microscopy measurement. By the analysis of the dependence of PL intensity and peak energy of the QWRs on Tcr, the origin of higher energy peak (H) and lower energy peak (L) were investigated. While behavior of the H peak is similar to that of an ordered InGaP, the L peak shows the insensitivity of PL peak energy to Tcr. This is attributed to compensation of the bandgap by competition of strain in the QWR region and indicates the L peak is related to the QWRs. Strong dependence of the L peak on the position of polarizer also supports this. Additionally, the PL peak intensity of the L peak has the maximum value not at the lowest Tcr (10 K) but at 50 K, while the H peak decrease continuously as T increases. We introduced the idea of compensation of the thermal expansion coefficient to explain this phenomenon.

Hartree-Fock calculations at semiempirical level, using the Extended Debey-Hückel Approximation, and LDA with pseudopotentials are used to investigate the properties of homoepitaxial islands formed by Cu and Ag. The islands have a three-dimensional shape and the dimensions of their height and basis are in the range of the bulk Fermi wavelength. Linear chains of Ag and Cu atoms are deposited on the island steps. The calculations describe the binding action of the island on the chain as well as the allowed energies and the density of states of the complete system. The results indicate that, due to the corrugation of the potential at the island-substrate interface, the more stable chain configurations are the ones near the island top rather than at its basis. Furthermore at these small sizes the island steps act as weak and permeable barriers. In fact, the electronic charge retains bulk-like features, though traces of quantum confinement can be identified.

A Transmission Electron Microscopy analysis of misfit stress based on the Stoney formula is proposed. It is applied to the GaInAs/(001)InP system to determine the residual stress for layers thicker than the critical thickness. The measured stress corresponds well to the value calculated using the elasticity theory and the nominal lattice mismatch: the structure is always nearly fully strained even for an epilayer two times larger than the critical thickness. Very few segments of misfit dislocation are observed indicating that the plastic relaxation is not active yet. A tentative explanation, based on the difficulty to create the core of a misfit dislocation, is explored.

Self-assembled Ga1−xInxNyAs1-y quantum dots were grown on GaAs by solid source molecular beam epitaxy (SSMBE). Introduction of N was achieved by a RF Nitrogen plasma source. Formation of quantum dots by S-K growth mode is confirmed by observation of standard 2D-3D RHEED pattern transition. Atomic force microscopy (AFM) and photoluminescence (PL) measurements were used to characterize the structure and optical properties of GaInNAs quantum dots. High GaInNAs quantum dot density (1010∼1011cm−2) was obtained for different In and N composition (0.3≤ x ≤1, y≤0.01). The effect of surface coverage on dot density, dot size, and optical properties was studied in detail. Adjusting the bandgap confinement by incorporating a GaNAs strain-reduction layer into the GaInNAs dot layer was found to extend the emission wavelength by 170nm. Room temperature pulsed operation is demonstrated for a Ga0.5In0.5N0.01As0.99 quantum dot laser emitting at ∼1.1μm.

Experimental and theoretical studies are reported for a quadruple period ordering found in GaAsSb alloy layers grown by molecular beam epitaxy at high growth temperatures. We propose a growth model that accounts for the observed three-dimensional (3D) ordered structure. It is shown that the already-ordered material in the previously grown layer affects the reconstruction of the growth front with respect to the underlying alloy template resulting in the correct stacking of the individual 2D ordered layers into the observed 3D ordered structure.

The magnetic properties of films of diluted magnetic semiconductors (DMS) such as (Ga,Mn)As, as well as bulk grown crystals of similar materials, have been found to be extremely sensitive to growth conditions, both in terms of the ferromagnetic transition temperature, and the details of their magnetization curves. We study an impurity band model for carriers in Mn-doped DMS applicable in the low carrier density regime, and discuss the effects of clustering on the magnetic properties of DMS, using both numerical mean field and Monte Carlo simulations. In addition, we study the effects of dimensionality on the transition temperature and other magnetic behaviour, and compare our results with experimental data.

Pit nucleation has been observed in a variety of semiconductor thin films. We present a model in which pit nucleation is considered to arise from a near-equilibrium nucleation process in which the adatom concentration plays an important role. Although pits relieve elastic energy more efficiently than islands, pit nucleation is prevented if the adatom concentration is too high. Inhomogeneities in the adatom density on the surface due to three-dimensional islands enhance pit nucleation. Thermodynamic considerations predict several different growth regimes in which pits may nucleate at different stages of growth depending on the materials system and growth conditions. However kinetics must be taken into account to make direct comparisons to experimental observations. These comparisons show good agreement given the uncertainties in quantifying experimental parameters such as the surface energy.