GaInAsP-GaAs buried ridge structure lasers emitting at 0.8μm have been fabricated on material grown in two steps by low pressure metalorganic chemical vapor deposition and gas-source molecular beam epitaxy. Preliminary laser results are reported, and the feasibility of using gas-source molecular beam epitaxy for the regrowth of sophisticated laser system is demonstrated.

Rode pointed out that periodically-disturbed surface morphology can occur on epilayers grown on slightly misoriented substrates. Johnson and Legg investigated Rode's prediction in Metalorganic Vapour Phase Epitaxy (MOVPE) of AIGaAs at fixed V/III ratio, but no report is available on the faceting as a function of V/III ratio in epilayers of GaAs grown by MOVPE. We have therefore investigated faceting, as function of V/III ratio, during the growth of GaAs by MOVPE at atmospheric pressure, and have found that faceting in epilayers, grown by MOVPE, is not only dependent on misorientation angle but also on the V/III ratio. These layers have characterized by optical microscopy, SEM, photoluminescence(PL), and Hall ineasuremwnts. Direct correlation between surface morphology and near-band-edge PL has been observed. Mirror-like smooth morphology was obtained at a relatively low V/Ill ratio (≈ 15); faceting occur as a ratio is gradually is increased to about 25 and beyond. On the basis of our study, we believe the dimensions of faceting on micrometer scale can be tailored by varying the V/III ratio. These patterns can be utilized in the fabrication of optoelectronic and transferred electron devices, large scale integration of the later being possible.

We have studied excitation transfer from the host quantum well to strain-confined quantum dots. We find that there is a long-lived intermediate state acting as a reservoir, holding the excitation before it is thermally activated over a barrier to reach the quantum dot. The barrier height increases monotonically with dot size.

The photomodulated transmission spectrum of a single Ge layer grown at 500°C on the Si(1 11) surface by molecular beam epitaxy is reported. The nominal Ge layer thickness was 50 nm. The modulation spectrum of thicker layers is dominated by a threelobed structure centered 70–80 meV above the bulk direct band edge. This structure is ascribed to excitation-induced broadening of the lowest direct exciton.

Coherently strained CdSe quantum structures are fabricated under varying dynamical growth conditions during the epitaxy of cubic CdSe on (100) ZnSe. Reflection high energy electron diffraction (RHEED) is employed to monitor the growth mode (2D vs. 3D). Conventional photoluminescence (PL) shows that both growth modes yield quantum structures with high PL efficiencies in which excitons are strongly localized by interface fluctuations at varying length scales. Spatially-resolved, near-field PL from quantum structures formed during 3D growth reveals reproducible fine structure in the PL spectrum attributed to emission from excitons laterally confined to quantum dot-like regions. Transmission electron microscopy (TEM) studies suggest that these observations result from a combination of island growth and strain-driven interdiffusion.

Near-edge and extended x-ray absorption fine structure (NEXAFS and EXAFS) measurements have been made as a function of Pb dopant concentration to study the changes in local geometric and electronic structure in several heavy-metal fluoride glasses (Zr/Ba/La/Al/Na, or ZBLAN). A strong correlation is found between the observed glass instability for Pb concentrations > 10% and the structural changes that are measured exclusively around the Ba atoms. Implications of these novel findings are examined in light of the present understanding of these complex glass systems.

The photovoltaic p-n heterojunction based on thin layer of 10 nim size copper sulfide nanocrystals and evaporated cadmium sulfide film was prepared. The partial oxidation of copper sulfide nanocrystals prior the fabrication of the heterostructure results in the formation of additional absorption band in near-IR region beyond the fundamental absorption of bulk copper sulfide and cadmium sulfide. This band can be attributed to the intervalence charge transfer between copper ions in the core and in the oxidized shell. The photovoltaic effect observed in the near-IR region at ITO/nano-CuS/CdS/Au heterojunction is due to the generation of the electron-hole pairs in the CuS nanocrystalline film followed by the separation of charge carriers at the CuS/CdS junction.

Silicon (oxy)nitride thin films were deposited on (100) 4″ silicon substrates in a conventional LPCVD reactor at 812°C and 230 mTorr, using mixtures of SiCl2H2 -NH3-N2O of variable composition in order to evaluate their optical properties for potential applications in SOI based Si technology. The depositions were carried out keeping the flow of SiCl2H2 and NH3 constant at 20 and 60 sccm respectively while that of N2O was varying between 0 and 180 sccm. Optical reflection measurements within the range 190 to 860 nm, performed on our films were analyzed within the Forouhi-Bloomer model (Phys. Rev. B34, 7018 (1986)) for amorphous semiconductors. From the analysis of the optical spectra, the constants of the model were derived. It was shown that the refractive index of the films varies between the limits set by those of silicon nitride and silicon dioxide, the increase of the N2O flow implying a decrease of the refractive index of the films. The film thicknesses were also derived from the analysis of the optical spectra. Capacitance - voltage measurements made on Metal-Insulator-Semiconductor (MIS) structures using our films as the insulator, have shown that, the electric charge density within the dielectric initially increases and then decreases with increasing the amount of oxygen into the deposition ambiance. Our films are suitable for optoelectronic applications in SOI structures compatible with C-MOS technology.

We investigated the growth rate enhancement in selective area metalorganic vapor phase epitaxy with a new surface boundary condition for the vapor phase concentration of source materials. In our model, the growth rate is proportional to the vapor phase concentration at a distance of the vapor phase mean-free path of source materials from the substrates, and the vapor phase concentration is laterally uniform at the top of the stagnant layer. This model predicts that the growth rate modulation produced by mask-patterning disappears at a specific pressure at which the vapor phase mean-free path reaches the stagnant layer thickness. We controlled the vapor phase mean-free path of trimethylindium in InP growth by varying the growth pressure from 6 to 180 Torr with a fixed total inlet gas flow rate to the reactor. Experiments showed a great reduction of the growth rate enhancement with the decrease in the growth pressure and the growth rate enhancement virtually disappeared at the lowest pressure. We also investigated the dependence of the stagnant layer thickness on the mask design. The experimental stagnant layer thickness related to the typical surface length, such as the period of mask-patterning.

We study the growth of InAs/AlxGa1-xAs/GaAs heterostructures as well as their structural and optical properties. Structurally coherent InAs islands with a narrow size distribution are found to be formed only in a very narrow range of InAs coverage. In striking contrast, the photoluminescence band in the 1.15–1.35 eV spectral range, which is commonly attributed to the emission from InAs quantum dots, is present for all of our structures, regardless the presence or absence of InAs islands and their strain state. Moreover, for constant InAs coverage this PL band follows not the Γ gap but the L gap of the AlxGa1-xAs barrier. This latter result is in disagreement with effective mass calculations for three-dimensionally confined excitons.

Compositional ordering is observed in essentially all IIIN semiconductor alloys. The mechanism leading to the specific ordered structures formed is only now beginning to be understood. It is intimately related to the atomic-scale physical processes occurring on the surface during epitaxial growth, specifically surface reconstruction and the attachment of atoms at steps. This paper will emphasize the use of surface analytic techniques to better understand the ordering processes during organometallic vapor phase epitaxial growth of GalnP. Surface photo absorption has been used to determine, for the first time, that the surface reconstruction is (2×4)-like during growth of GalnP with Cu-Pt ordering. By increasing the growth temperature and/or decreasing the V/Ill ratio the concentration of phosphorus dimers characteristic of the (2×4) structure is observed to decrease in tandem with a decrease in the degree of order measured in the epitaxial layers. This suggests that formation of the (2×4) reconstruction is necessary for formation of the Cu-Pt ordered structure.

Atomic force microscopy was used to examine the step structure. For exactly (001)-oriented substrates, growth at low V/Ill ratios produces islands surrounded by single monolayer steps. Increasing the V/Ill ratio leads to the formation of bilayer steps approximately 6 Å high. A similar change in the structure of steps is observed in the vicinal regions for substrates misoriented to produce [110] steps. This phenomenon is interpreted in terms of the stabilization, at high V/Ill ratios, of the (2×2) reconstruction on the (111)B regions of the bilayer steps. This may also have an effect on formation of the Cu-Pt ordered structure.

By changing the substrate temperature or the input partial pressure of the phosphorus precursor, the degree of order and, hence, the bandgap energy of GalnP, can be modulated during growth. This has allowed the production of a variety of single and double heterostructures. The heterostructures produced by changing the temperature from 740 to 620°C and from 620 to 520°C are abrupt. The heterostructures formed by changing the phosphine flow rate are not. The ordered structure in the lower layer persists into the upper layer. This may be an indication of the importance of the step structure.

The growth of InAs on GaAs self assembled quantum dots and their electronic properties is studied. The limit for coherent island growth is calculated with a simple model and compared to the experimentally observed values. Cross section transmission electron micrography and atomic force micrography are used to investigate this limit. The electronic properties studies involved the use of capacitance spectroscopy to map out the density of states in these structures as well as to probe the temperature dependence of the Coulomb Blockade effect in this system. It was found that Coulomb Blockade could be observed at temperatures in excess of 77K, representing an attractive approach for single electronics operating at high temperatures.

In order to understand Ge island nucleation and evolution, we have studied strain relaxation and clustering of Ge grown on Si(111) by molecular beam epitaxy (MBE) with in situ reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), and Rutherford backscattering spectrometry (RBS). Our goal is to tailor the size and density of the nanocrystals by controlling thermodynamics and kinetics. At low temperature (∼ 450°C), we observe a sharp 2D–3D growth mode transition after 2.5ML ±0.1ML (we define a thickness of 1ML to be one-third the length of the body diagonal of the Ge conventional unit cell), when transmission diffraction features appear in RHEED and the surface lattice constant begins to relax. The mechanisms of island growth and strain relaxation change with growth temperature. At ∼ 700°C, transmission diffraction spots never appear in RHEED for Ge/Si(111) and strain relaxation occurs gradually. After 37ML of growth, the apparent in-plane lattice parameter increases only 1.5% over that of the Si substrate. This behavior is explained by the different manner in which islands initially nucleate and grow in the two temperature regimes. At low temperature, small islands nucleate and grow on a relatively rough wetting layer (which itself provides preferential sites for dislocation introduction). The areal density of the small islands is relatively high. At high temperature, a small number of islands grow very large from the outset. A general model indicates how, at low temperature. The relative difficulty of overcoming the barrier to dislocation formation actually results in an apparent larger degree of strain relief than at high temperature.

We have investigated the photoluminescence emission energy of InP dots as a function of cap layer thickness. We find a strong blue-shift with increasing cap layer thickness. The strain tensor in the dot as well as in the surrounding matrix has been modelled using finite element methods and the band-gap has been calculated using deformation potential theory. We find good agreement between calculation and experiment. For uncapped dots we find that the emission energy is lower than for biaxially strained InP, and is indeed close to unstrained InP.

GaxIn1−xAs and GaxIn1−xP quantum wire (QWR) arrays were grown by a single-step molecular beam epitaxy. Lateral Ga(In composition modulation perpendicular to the growth direction occurs spontaneously during the growth of (GaAs)m/(InAs)n or (GaP)m/(InP)n shortperiod superlattices on InP or GaAs substrates, respectively, by the strain-induced lateral-layer ordering (SILO) process, producing lateral quantum wells. This straightforward method employs standard on-axis (001)-oriented substrates, and requires no pre-growth substrate patterning, nor does it involve post-growth processing for the formation of QWRs. Both GaxIn1−xAs infrared (1.7 μm) lasers and GaxIn1−xP visible (0.7 μm) lasers with QWR active regions have been successfully fabricated. These lasers showed many unique features that have never been observed in quantum well lasers.

High-quality (ZnS)n(ZnSe)12n and (ZnSe)n(ZnTe)11n (n=1∼4) crystals were grown at a low temperature of 200°C by hydrogen radical-enhanced chemical vapor deposition. From satellite peaks in x-ray diffraction spectra, these periodic structure crystals were confirmed to be grown coherently on substrates, in spite of large lattice mismatches between the grown layers and the substrates (͛=4∼7%). In photoluminescence (PL) spectra of these films, strong band-edge emissions were predominantly observed, resulting from a suppression of deep-level emissions. We found that the PL peak energy of (ZnSe)n(ZnTe)11n shifts systematically to lower energy by 200 meV with changes in the number of ZnSe layers (n), while relatively small shift of 13 meV was observed in (ZnS)n(ZnSe)12n. These discrepancy can be attributed to the difference of band-lineups or chemical natures of constituent atoms in these crystals.

A new method for calculation the effects of composition modulation upon the dielectric properties of semiconductors is proposed. Specific consideration is carried out for ultrathin [001] oriented (AIIICV)m(BIIICV)n superlattices (SL). These SL are formally considered to result from complete A(B) atoms orderiig over a-sublattice in a hypothetical initial structure –bulk mixed crystal (AxB1−x)C with zincblende lattice and x = m/(m + n). From this point of view a variation in dielectric constants tensor (DCT) induced by the ordering can be calculated nearly in a spirit of the phenomenological Landau theory of second-order phase transitions. It is shown that the changes in spatial dispersion of the DCT are expressed through the coefficients at invariants in the expansion of SL thermodynamic potential in a power series of the order parameter components. The way is described for numerical estimation of the thermodynamic potential coefficients from microscopic model of the initial alloy which take account of spatial correlations in A(B) atoms distribution.

Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) have been used to analyze the spontaneous appearance of lateral composition modulations in InyAl1−yAs (yIn.≅ 50%) buffer layers of single quantum well structures grown by molecular beam epitaxy on exact and vicinal (100) InP substrates, at growth temperatures in the range of 530°C–580°C. The influence of the growth temperature, substrate misorientation and epilayer mismatch on the InAlAs lateral modulation is discussed. The development of a self-induced quantum-wire like morphology in the In0.53Ga0.47As single quantum wells grown over the modulated buffers is also commented on.

At a low MBE growth temperature (400°C), an anisotropic composition modulation has been observed by Transmission Electron Microscopy (TEM) on AIlnAs layers. Optical measurements have been performed on these samples and compared with classical AllnAs grown at 530°C. We show the clustering “organization” on the low temperature layers and we propose some hypothesis to explain the composition modulation effects on the AlInAs optical properties.

Polarization independent quantum well (QW) materials operating under electroabsorption effect in optical switching and modulation devices are of intense interest recently. This is a theoretical analysis of the optical properties of strained InGaAs/InP QWs. The method of composition modification based on interdiffusion will be introduced to merge the heavy- and light- hole states in order to achieve polarization insensitivity. Results presented here show that the diffused QWs with and without as-growth tensile strain can both serve in polarization independent electro-absorption requirements. With a suitable design in the interdiffused QW materials, the optical polarization independence can operate from 1.465 to 1.540 μm (tunability of 75 nm) with a maximum absorption change of 2000 cm−1. In the case studied here, over 75% reduction in the required as-growth tensile strain is achieved as compared with the conventional rectangular QWs. This provides us with a simpler way to achieve high strain optical polarization independence through interdiffusion.