Many III – V semiconductor alloys exhibit spontaneous (111) alternate monolayer ordering when grown from the vapor phase. We have studied theoretically the ordering induced changes in the optical properties of the semiconductor alloys. We describe (i) the change of the band gap ΔE9, and the valence band splitting ΔE12 as functions of the long range order parameter η, (ii) the consequence of coexistence of (001) epitaxial strain and (111) chemical ordering on the optical properties, (iii) optical anisotropy and spin polarization effects in the ordered alloy, (iv) theory of reflectance-difference spectroscopy in ordered alloy, and (v) the ordering-induced changes on high energy E1 and E2 transitions. Specific, experimentally testable predictions are listed in the summary section.

We use the information from low temperature PLE spectra measured on a set of samples with different degree of order for an investigation of the correlation between the order dependent valence band splitting and band gap reduction. The experimental values are compared with predictions from recent ab initio band structure calculations.

Moreover, we show how the low temperature optical properties of ordered GaInP2 are related to the microstructure of the material. We find a close correlation between the size of the ordered domains and the relative intensity of the intrinsic PL-peak from excitonic recombination compared with the rapidly shifting, below band-gap luminescence emission.

Atomic ordering in InGaAs, InAlAs, and GaAsSb crystals grown on (110) InP substrates by molecular beam epitaxy, has been studied by transmission electron microscopy. In the electron diffraction pattern from these crystals, superstructure spots associated with CuAu-I type ordered structure are found. When the tilting angle of the substrates increases, the ordering becomes stronger. The ordering is also stronger in crystals grown on substrates tilted toward the <001> or the <001> direction than those on substrates tilted toward the <110> direction. From these results, one can conclude that atomic steps on the growth surface play an important role in the formation of ordered structures. The ordering becomes stronger when the growth temperature increases. In high resolution images of the crystal, doubling in periodicity of 220 and 200 lattice fringes is found, which is associated with CuAu-I type ordered structure. Moreover, anti-phase boundaries are very often observed in the ordered regions. It is also found that ordering is not perfect, and that ordered regions are plate-like microdomains lying on planes slightly tilted from the (110) plane. On the basis of these results together with considerations of the atomic arrangement of the ordered structure and surface reconstruction on the (110) plane, we propose four possible models for the ordering. Through precise evaluation of these models, two models are selected as the most probable ones: these involve formation of the ordered structures on surfaces dominated by two monolayer steps. These models have been experimentally proven by the analyses of electron diffraction patterns from different InGaAs crystals grown on different growth surfaces. We have fabricated InGaAs/N-InAlAs heterostructures with a strongly ordered InGaAs channel layer. The measured two-dimensional electron gas mobilities from these structures are found to be 100,000 cm2/Vs in the <110> direction and 161,000 cm2/Vs in the <110> direction with a sheet electron concentration (Ns) of 9.5 × 1011 cm−2 at 6 K. The latter mobility is much higher than both calculated alloy scattering limited mobility and the best experimental results for lattice-matched InGaAs/N-InA]As systems. The mobility enhancement in the <110> direction is considered to be achieved by the suppression of alloy scattering due to the occurrence of ordering in the InGaAs channel layer.

We identify via thermodynamic energy minimization the role of subsurface strain (caused by surface reconstruction and dimerization) in the ordering of Ga0.51n0.5P alloys. Depending on the growth conditions, the alloy surfaces can have either β2(2×4), c(4×4) or c(8×6) reconstructions, with characteristic 2×1, 1×2 and 2×3 RHEED patterns. We show that (i) the 1×2 reconstruction will lead to a CUPtA surface ordering, (ii) a 2×1 reconstruction will lead to a CuPtB ordering, (iii) a 2×3 reconstruction will lead to a 3-period ordering, and (iv) single (double) bilayer steps are stable at low (high) anion chemical potential. These results are in good agreement with recent experimental observations.

Ga0.54In0.46As0.12P0.88 lattice matched to GaAs and grown by metal organic vapor phase epitaxy (MOVPE) shows an anomalous temperature behaviour of its cathodoluminescence (CL) emission. Using high resolution x-ray diffraction (HRXRD) and transmission electron diffraction (TED), ordering in this quarternary alloy can be identified as the reason for this behaviour. The ordering follows the same trends with respect to misorientation that are known for InGaP. In addition to ordering, compositional fluctuations related to a miscibility gap are found in this material. In contrast, layers with a higher As-content (y=0.5; y=0.76) do not show properties related to ordering.

To quantitatively measure the order parameter in ordered III-V materials we have developed two techniques based on electron microscopy. The first technique calculates the diffraction pattern of ordered material quantitatively and fits the calculated data to experimentally obtained data. A test sample, specifically grown for this technique, shows that the method can determine the average order parameter with high accuracy. We found an order parameter of 0.34, which coincides with the value found by piezo-reflectance measurements. To measure the order parameter on a smaller length scale, we use a technique based on image processing of high-resolution TEM images. By comparing ordered and disordered parts of the image, we calculate the relative order parameter on an atomic length scale. Analyzing the data provides information about the development of the ordering in the crystal.

Optical polarization in ordered GaInP2 has been studied by low-temperature photoluminescence. By comparing our experimental results to the theoretical predictions, we show that to obtain quantitative information about ordering from the polarization of near-band-gap transitions, the effects of lattice mismatch, substrate misorientation, and excitonic transitions should be taken into account.

Lateral order/disorder structures have been fabricated out of ordered GaInP quantum wells by ion implantation.

Using high resolution electron beam lithography for definition of implantation masks, we achieve wire structures down to 35nm width. Implantation was carried out at energies down to 25keV and doses from 3e10cm−2 to 6e13cm−2. The so created vacancies lead to a reduction of the disordering temperature threshold of about 850°C for mere thermal disordering. Annealing those structures at 800°C results in selective disordering in the unmasked regions. We have ordered wire structures within disordered barriers. Even for the smallest mask widths of 35nm, the PL emission of wires and barriers are well resolved.

Photoluminescence measurements on ordered InGaP2 were studied as a function of temperature, laser power density, and magnetic field. The temperature varied between 1.4 and 300 K, the laser power densities ranged from 10 nW/cm2 to 20 W/cm2, and the maximum magnetic field was 13.6 T. The data show two spectra, excitonic and band-to-band transitions, depending upon the incident laser power density. A consistent interpretation of the band-to-band spectrum leads to a spatially separated electron-hole transitions between the ordered domains. Three different mechanisms for spatially indirect transitions are presented. An analysis of the linear data also allows a determination of the valence-band mass in the 100 direction of mv ∼0.25 m0.

Ballistic-electron-emission microscopy (BEEM) has been used to study band-offsets in n-and p-type GaInP/GaAs heterostructures. We determine room temperature offsets of 30 meV and 350 meV in the conduction and valence bands, respectively, for thin GaInP layers grown by metal-organic chemical vapor deposition (MOCVD) at 610°C. Low temperature (77 K) measurements also indicate at least 90% of the band discontinuity lies in the valence band for these ordered GaInP samples.

Efficient, low-temperature luminescence at energies far above that of the exciting cw-laser is reported at junctions of GaAs-GaInP2 and GaAs-AlxGa1−x.InP2. The signal originates from the high-band gap layers and disappears only if the excitation energy is tuned below the GaAs band gap, as monitored by up-converted photoluminescence excitation spectroscopy. This shows that the non-linear process is induced by the generation of electrons and holes in the GaAs. Furthermore, it is found that the up-conversion is only observed if the (A1)GaInP2 layers are CuPtB long-range ordered. The reason for this is the inherent presence of metastable states in these ordered alloys. It is argued that cold Auger processes cause the nonlinear effect at these type I interfaces.

Comparative study on the formation of the three types of ordered structures (TP-A,CuPt-A, and CuPt-B) on group III sublattice in III-III-V type alloys are made for four kinds of alloys of AlInAs, GaInAs, AlInP, and GaInP grown by gas source molecular beam epitaxy. The bonding energy difference between the constituent binaries of a IIIA - IIIB-V type alloy is demonstrated to be an important factor for the ordered structure formation. The implications of the result for the formation mechanism is discussed.

Ternary semiconductors like GaInP under certain growth conditions exhibit a (partial) chemical ordering in form of a superlattice of alternate Ga-rich and In-rich planes in (111) direction. We have performed measurements of the polarization properties of light propagating in ordered GaInP/AlGaInP quantum well waveguide structures with various amounts of strain and observed a mode conversion between transverse electric (TE) and transverse magnetic (TM) modes for light propagating along (110). Lasers built of ordered material with the cavity in this direction show a distorted polarization of the laser light which depends on ordering and strain. We show that these effects are caused by an optical birefringence due to the reduced symmetry of the ordered material which leads to a coupling of the TE and TM modes. Only a new linear combination of TE and TM modes, the “super-modes”, can propagate in the waveguide without change. Within this simple model the polarization behavior of the light in the waveguide and in lasers can be explained very well.

Using ab initio all-electron methods, we investigate the evolution of effective masses, deformation potentials and pressure coefficients with the degree of long-range order in spontaneously ordered Ga0.5In0.5P alloys. We find that (i) the electron effective mass in the ordering direction increases significantly with the degree of order, while the effective mass in the perpendicular direction decreases; this produces a strong anisotropy of the electron effective mass which is not present in the disordered alloy; (ii) the band-gap deformation potential for (001) epitaxial strain decreases with increasing degree of order, reflecting the extent of the ordering-induced Γ - L coupling; (iii) the band-gap pressure coefficient decreases from 8.4 meV/kbar in the disordered alloy to 6.6 meV/kbar in the ordered CuPt structure; interpolation to partial degrees of order leads to good agreement with recent high-pressure experiments.

Self-assembled quantum dots of InP in In0.48Ga0.52P can be formed by molecular beam epitaxy (MBE) due to the strong lattice mismatch of 3.7 %. This is a promising method to realize small quantum dots which exhibit strong quantization effects and represent real 0D systems. Lasers with quantum dots in the active region should exhibit very low thresholds due to the low discrete density of states in quantum dots. On the other hand the self organized dots exhibit a considerable size fluctuation leading to an inhomogeneous broadening of the gain spectrum. We have measured the optical gain in quantum dots at room temperature and found two gain peaks which are identified as being due to the wetting layer and the quantum dots by their polarization properties. As expected from the results of the gain measuremlents, cleaved samples under optical pumping show lasing at room temperature of either the quantum dots or of the wetting layer, depending on the experimental conditions.

We report vertically-aligned InAs islands separated by GaAs barriers thin enough for electronic coupling. Thinner barriers reduced the InAs critical-thickness for island formation. Transmission electron microscopy revealed well-aligned islands with all detected islands in complete stacks. Atomic force microscopy showed the top islands of uncapped stacks are fully formed. The photoluminescence peak was sharper and shifted to lower energy compared to a single-layer growth. We attribute this shift to island-to-island electronic coupling and to the smaller compressive strain in the center of the composite structure.

A new process, named Aerotaxy, has been developed for growth of quantum dots (QDs) with identical sizes and properties. Self-assembly and intrinsic control of the nanocrystal properties is obtained by (i) an initial selection of spherical droplets of gallium (Ga), all having identical sizes within a few %, employing standard aerosol technology. In a second processing stage (ii) these droplets of gallium are allowed to react with arsine (AsH3), by which the metallic Ga droplets are completely converted into a monodisperse ensemble of galliumarsenide (GaAs) nanocrystals at temperatures as low as 260°C. GaAs nanocrystals, of approximate diameter 10 nm, have been produced and deposited on various substrates. The good crystallinity and stoichiometry of the formed particles are confirmed by transmission electron microscopy studies of individual nanocrystals.

GaAs quantum dots were formed in a near surface quantum well (QW) by producing lateral confinement with self-organized InP stressors grown in situ by metal organic chemical vapor deposition (MOCVD). We report here the influence of growth conditions on InP island formation on AlGaAs/GaAs single QW structures and also the influence of the QW structure on the optical properties of the GaAs quantum dots. We observe strong photoluminscence up to room temperature from the strain-induced quantum dots with energy redshifts of 70 meV below the QW peak.

We have shown that two-dimensional layers of arsenic nano-clusters separated by cluster-free GaAs matrix can be formed using indium delta-doping of GaAs films grown by molecular beam epitaxy at low (200°C) temperature. Spatially ordered structures of As clusters have been obtained in the epitaxial LT GaAs films doped with Si donors, Be acceptors and undoped as well.

Vertically coupled InAs quantum dots have been synthesized by MBE through successive deposition of InAs dot sheets and thin GaAs spacers. The energy of ground state transition in PL spectra has been found to depend on a number of dot sheets and the spacer width. Injection laser based on vertically coupled quantum dots demonstrated lasing via the ground state of quantum dots in the entire 80K-300K temperature range. Lower threshold current density and wider range of the thermal stability of threshold current density as compared to the single sheet quantum dot laser have been observed.