Study of the optical and structural properties of multi quantum well structures grown on high-index surfaces

The MOVPE overgrowth of high [0 1¯ 1]-oriented ridges confined at sides by facets related to {n 1 1} crystallographic planes is reported. We studied the influence of the side tilt on the thickness of the AlGaAs and GaAs epitaxial layers grown under the condition of the kinetic growth mode. The multi quantum well (MQW) structures were prepared on the sides of ridges tilted at 54.7°, 45° and 30° to (1 0 0). The sidewall surface morphologies before and after epitaxial growth were evaluated and compared. We observed no tendency towards planarization towards a neighbouring high-index crystallographic plane, such as (2 1 1) and (3 1 1). We also showed that the quantum wells of the MQW structure make a smooth transition over the edge between the top surface and the facet as both AlGaAs and GaAs grew at similar rates on the surfaces.     

Optical and structural properties of ingaasp miscibility-gap solid solutions grown by MOVPE on GaAs(001) substrates

Optical and structural properties of InGaAsP solid solutions grown by MOVPE at 600°C on GaAs(001) substrates are studied. The photoluminescence spectra of InGaAsP solid solutions with a composition corresponding to the miscibility gap contain a main band and an additional auxiliary band. It is established that both bands are related to band-to-band radiative transitions; i. e., the studied layer includes two solid solutions with different compositions and different band gaps. It is shown that the observed high-energy shift of the additional band with an increasing level of excitation is governed by the nanometer size of domains in the corresponding solid solution. This conclusion is consistent with the results of TEM study, which revealed the presence of a periodic structure comprised of alternating domains with different compositions. This structure of alternating domains extends along the [100] and [010] directions with a characteristic period of 10 nm.     

The kinetics of the growth of nitrogen-doped ZnSe grown by photo-assisted MOVPE

Detailed growth kinetic studies of the photo-assisted growth of ZnSe from the precursors dimethylzinc triethylamine adduct (DMZn.TEN) with either dimethylselenium (DMSe) or diethylselenium (DESe) have shown that there are two regimes of growth temperature, low temperature growth dominated by site blocking from unreacted precursors and a high temperature regime dominated by precursor desorption. The proposed growth mechanism is based on a surface bimolecular reaction mediated by hydrogen radicals which is initiated by the decomposition of surface DMZn. The same mechanism has been used to explain the residual hydrogen concentration in the DMSe grown layers of 1 × 10¹⁷ cm⁻³ and the enhancement in hydrogen incorporation that occurs with DESe by deviating from 1:1 precursor ratio. Nitrogen doping using trimethylsilylazide (TMSiN₃) in combination with DESe has achieved nitrogen incorporation up to 1 × 10²⁰ cm⁻³ but the incorporation of hydrogen is also observed to increase and the growth rate decrease. This has been explained using a Langmuir Hinshelwood model causing a disruption of the surface bimolecular reaction and bonding hydrogen radicals to surface nitrogen.     

the properties of long wavelength strained layer superlattice lasers grown by MOVPE

The strain generated in strained layer superlattices (SLS’s) gives rise to a range of novel electronic and structural properties. These electronic properties could be used to either improve the performance of existing optoelectronic components or be exploited to create new devices. One such application is to enhance the efficiency and reliability of long wavelength lasers used in optical telecommunication systems. Such lasers are currently based on InP/InGaAsP lattice matched materials systems which emit at either 1.3 or 1.55 µm. It is believed that Auger recombination and inter-valence band absorption constitute the major mechanisms which limit the overall efficiency and determine threshold current of such devices. Biaxial strain, incorporated into strained layer su-perlattices, causes electronic band structure modifications which may suppress these non radiative loss mechanisms. Atmospheric pressure MOVPE growth of a long wave-length laser structure is described. The structure consists of an In_xGa_1?xAs/In_yGa_1?yAs SLS active region with InP cladding layers grown on InP (001). Structural analyses by transmission electron microscopy and x-ray double crystal diffraction have demon-strated that it is possible to grow high quality lasers based on this strained layer sys-tem. A range of structures which emitted at µ1.55 µm were grown and lased, at room temperature, under photoinjection. Equivalent threshold current densities, J_th, were typically in the range 1.5-6 kA/cm² and high slope efficiencies, η, were achieved.     

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.     

初始层生长条件和Mg活性剂对MOVPE方法生长GaN薄膜质量的影响

The initial growth conditions of a 100 nm thick GaN layer and Mg-surfactant on the quality of the GaN epilayer grown on a 6H-SiC substrate by metal-organic vapor phase epitaxy have been investigated in this research. Experimental results have shown that a high Ⅴ/Ⅲ ratio and the initially low growth rate of the GaN layer are favorable for two-dimension growth and surface morphology of GaN and the formation of a smoother growth surface. Mg-surfactant occurring during GaN growth can reduce the dislocations density of the GaN epilayer but increase the surface RMS, which are attributed to the change of growth mode.     

Hall analysis of the electrical properties of n-type AlxGa1-xAs grown by MOVPE

Variable temperature Hall measurements were used to study the electrical properties of undoped and Se-doped Al_xGa_1-xAs (0 <x < 0.4) layers grown by metalorganic vapour phase epitaxy (MOVPE). It is shown that the deep donor activation energy measured in undoped AlGaAs exhibits a similar dependency upon composition as that reported for Si-doped AlGaAs grown by MBE. For Al_xGa_1-xAs, doping with selenium is found to reduce the activation energy from 66 meV (forn = 4.1 x 10¹⁶/cm³), to 9 meV (forn = 1.6 × 10¹⁸/cm³).     

Wide-gap semiconductor InGaN and InGaAln grown by MOVPE

We have achieved InGaN growth on sapphire substrates at temperatures substantially higher than conventional growth temperatures for InGaN. When the growth temperature was changed from 500 (conventional) to 800° C (this work) in InGaN, the x-ray diffraction line width (full width at half maximum) decreased from 100 to 30 min. At 77 K, edge emission was observed in PL. In order to further improve crystalline quality, we have investigated ZnO as a lattice-matching substrate. First, the surface treatment and the resistance to the reducing atmosphere at high temperatures was briefly investigated. We report the first successful lattice-matched growth of InGaN. The x-ray diffraction line width of InGaN grown on ZnO was about 20% smaller than that of films grown on sapphire substrates, thus using lattice-matched substrates was shown to have an effect on improving the crystalline quality of InGaN. Single crystal InGaAlN has been also realized on sapphire substrates. The indium, gallium and aluminum contents were 2.2, 22.5 and 74.3%, respectively. The optical transmission characteristic of this InGaAlN was measured.     

Optical and structural properties of MOVPE grown GaxIn1−xAs/InP strained multiple quantum well atructures

This paper presents a study of the structural and optical properties of strained GaInAs/ InP multiple quantum well (MQW) structures fabricated by LP-MOVPE. The composition of the Ga _x In_1−x As films ranged fromx = 0.17 tox = 1.0 and was determined by sputtered neutral mass spectrometry (SNMS) on thick layers. The structures of the MQW samples with well widths from 1.5 to 5 nm were investigated by high resolution x-ray diffraction (HR-XRD). Simulations of the diffraction patterns showed that transition layers of approximately 2 monolayer (ML) thickness with high lattice mismatch exist at the interfaces. Photoluminescence (PL) measurements indicate well widths of a multiple of a monolayer with local variations of one monolayer. The PL peak energies vary smoothly with the Ga concentration. These results were confirmed by optical absorption measurements.     

Recombination emission from InAs quantum dots grown on vicinal GaAs surfaces

Photoluminescence (PL) spectra of InAs/GaAs heteroepitaxial structures with quantum dots (QDs) have been studied. The structures were grown by submonolayer migration-enhanced epitaxy on vicinal substrates with the amount of deposited InAs close to the critical value of 1.8 monolayer (ML). The origin and evolution of the structure of PL spectra were studied in relation to the direction and angle of misorientation, temperature, and power density and spectrum of the exciting radiation. A blue shift and narrowing of the PL band with increasing misorientation angle was established experimentally. The fact that QDs become smaller and more uniform in size is explained in terms of a lateral confinement of QDs on terraces with account taken of the step bunching effect. The temperature dependences of the positions and full widths at half-maximum (FWHM) of PL bands are fundamentally different for isolated and associated QDs. The exciton ground states contribute to all low-temperature spectral components. The excited exciton state contributes to the recombination emission from QDs, as evidenced by the temperature dependence of the integrated intensity of the PL bands. A quantitative estimate is given of the electronic structure of different families of InAs QDs grown on GaAs substrates misoriented by 7° in the [001] direction.     

Study of GaAs layers grown on Ge substrates by MOVPE and in situ monitored by laser reflectometry

Metalorganic vapor phase epitaxy (MOVPE) of GaAs on Ge substrate has been studied by laser reflectometry (LR) with 632.8 nm laser beam. The layers were grown by varying substrate temperature and the V/III ratio. The relative difference between refractive index of film and substrate results in pronounced easily detected interference oscillations in the reflected beam intensity. The oscillations period provides an accurate and immediate measure of growth rate. In addition, the variations of extrema of oscillations and the average value of reflectance provide an estimate of the quality and surface roughness. A procedure for quantifying the roughening observed during the in situ LR monitoring of growth was presented. Furthermore, the measured interference effects were analyzed by calculating the reflectance. In order to explain the reflectometry behavior, the film was also characterized by X-ray diffraction and scanning electronic microscopy.     

MOVPE growth of GaAsN: surface study by AFM and optical properties

We report on the growth of GaAs_1−xN_x thin films on GaAs substrates (2° off) by metalorganic vapor-phase epitaxy, in the temperature range 500–600°C. A mixture of N₂ and H₂ was used as the carrier gas. Using dimethylhydrazine as nitrogen source, we incorporated up to 3.5% of nitrogen, at 530°C. The growth condition dependence of nitrogen content was studied, and it reveals a distribution coefficient 350 times lower for nitrogen than for arsine at 530°C. Nitrogen incorporation is controlled by surface kinetics. The evolution of surface morphology has been investigated by atomic force microscopy as a function of the nitrogen composition and of growth temperature. For nitrogen content up to 2%, the GaAsN vicinal surface is characterized by a step–terrace structure with bunched steps, and the step edges straighten when increasing the growth temperature. For higher nitrogen content terraces are no longer observed and, above 3%, widely-spaced cross-hatch lines, characteristic of a partial relaxation of strain in the epilayers, appear. Optical properties were studied by low (7 K) and room-temperature photoluminescence and photoreflectance. As usual for this material, a degradation of optical characteristics is observed with increasing N content along with the evolution of surface morphology.     

Optical and morphological properties of In(Ga)As/GaAs quantum dots grown on novel index surfaces

Optical and morphological properties of self assembled In(Ga)As/GaAs quantum dot systems, grown on high index (N11) substrates, for a wide range of orientations, coverages and compositions, are presented and reviewed. The use of high Miller index substrate orientations permits to intervene on major quantum dots properties such as shape, size distribution, transition energy and emission polarisation, thus opening a wide range of device design opportunities.     

MOVPE growth and optical properties of gan deposited on c-plane sapphire

We address combined utilization of temperature dependent reflectance, photo-luminescence, and Raman spectroscopy measurements to optimize the struc-tural and electronic properties of GaN epilayers deposited on sapphire. Last, we study residual strain fields in such epilayers.     

GaN growth on porous silicon by MOVPE

GaN films have been grown at 1050 °C on porous silicon (PS) substrates by metalorganic vapour phase epitaxy. The annealing phase of PS has been studied in temperature range from 300 to 1000 °C during 10 min under a mixture of ammonia (NH₃) and hydrogen (H₂). The PS samples were characterized after annealing by scanning electronic microscope (SEM). We observed that the annealing under the GaN growth conditions does not affect the porous structure.For the growth of the active GaN layer we used a thin AlN layer in order to improve wetting between GaN and PS/Si substrate. The growth of AlN and GaN films was controlled by laser-reflectometry. We estimated the porosity of PS samples from the evolution of the reflectivity signal during the AlN growth. The crystalline quality and surface morphology of GaN films were determined by X-ray diffraction and SEM, respectively. Preferential growth of hexagonal GaN with (0002) direction is observed and is clearly improved when the thickness of AlN layer increases. Epitaxial GaN layers were characterized by photoluminescence.     

Self-organization of nanostructures on planar and patterned high-index semiconductor surfaces

In order to directly control the size and in particular the position of nanostructures naturally formed on high-index semiconductor surfaces during molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE), the growth on patterned high-index GaAs substrates is investigated. During MBE of (AlGa)As on patterned GaAs (311)A substrates, a new phenomenon in the selectivity of growth has been found to form a fast growing sidewall on one side of mesa stripes oriented along the [01-1] direction. Preferential migration of Ga adatoms from the mesa top as well as the mesa bottom toward the sidewall develops a smooth convex curved surface profile without facets. This unique growth mode that does not occur for the perpendicular stripe orientation nor on other patterned GaAs (n11)A and B substrates is stable for step heights down to the quantum size regime to produce lateral quantum wires on patterned GaAs (311)A substrates. Quantum confinement of excitons in the wires has been demonstrated by the transition from two-dimensional to magnetic confinement with increasing magnetic field. For device applications it is important that the wires can be stacked in the growth direction without any increase in interface roughness or wire size fluctuations, indicating a self-limiting lateral growth mechanism. Finally, in strained layer epitaxy, (InGa)As islands can be selectively placed on the mesa top and bottom leaving entirely free the curved part along the fast growing side-wall.     

Structural and optical characterization of self-assembled InAs-GaAs quantum dots grown on high index surfaces

The structural and the optical properties of InAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (QDs) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QD coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) spectra of GaAs (100) and (311) samples show typical QD features with PL peaks in the energy range 1.15-1.35 eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70K was observed. These results suggest that the high index substrates are promising candidates for production of high quality self-assembled QD materials for application to photonics.     

Effect of multicomponent InAsSbP matrix surface on formation of InSb quantum dots at MOVPE growth

Indium-antimonide quantum dots (7–9 × 10⁹ cm²) are produced on an InAs(001) substrate by metal-organic vapor-phase epitaxy at a temperature of T = 440°C. Epitaxial deposition occurred simultaneously onto an InAs binary matrix and an InAsSbP quaternary alloy matrix layer lattice-matched to the InAs substrate in terms of the lattice parameter. Transformation of the quantum-dot shape and size is studied in relation to the chemical composition of the working matrix surface, onto which the quantum dots are deposited. The use of a multicomponent layer makes it possible to control the lattice parameter of the matrix and the strains produced in the system during the formation of self-assembled quantum dots.     

Preliminary comparison of ballistic electron emission spectroscopy measurements on InAs quantum dots in a GaAs/AlGaAs heterostructure grown by MBE and MOVPE

Self-assembled InAs quantum dots (SAQDs) in GaAs/GaAlAs structures grown by molecular beam epitaxy (MBE) and metal-organic vapour phase epitaxy (MOVPE) of similar size was examined by ballistic electron emission spectroscopy. Ballistic current-voltage characteristics through the QD in the voltage range from 0.55 to 0.9 V (range where the presence of resonance states of QD is expected) with its derivative (the derivation of the spectroscopic characteristics represents quantum levels in the QD) are given. Differences in the intensities and sharpnesses of the QD levels for MBE and MOVPE grown QDs are observed.     

Structural and optical properties of InAs quantum dots grown by molecular beam epitaxy

In this work, we have studied the dependence of the size and luminescence of self-assembled InAs quantum dots (SAQDs) on the growth conditions. The SAQDs were grown on GaAs (1 0 0) substrates by molecular beam epitaxy (MBE). Their structural and optical properties were studied by atomic force microscopy (AFM), and photoluminescence spectroscopy (PL). The growth of the InAs SAQDs was in situ monitored by reflection high-energy electron diffraction (RHEED). The shape and size of the InAs SAQDs were significantly affected by the growth temperature and the arsenic over-pressure. We observe a decrease of the SAQDS density and an increase in their height by increasing the growth temperature, and/or decreasing the arsenic over-pressure. This is accompanied by a remarkable red-shift of the PL emission energy from 1.3 to .