Area Metalorganic Vapor Phase Research Articles

INGAN/GAN based semipolar green converters

In order to achieve highly efficient green light emission, we are investigating the realization of InGaN-based luminescence conversion structures optically pumped by a blue LED. Using selective area metalorganic vapor phase epitaxy, we have grown inverted pyramid structures. On the side facets of these structures, semipolar InGaN/GaN multi-quantum wells can be deposited which may have promising characteristics for high luminescence conversion efficiencies. In order to enhance the green emission intensity, both the absorption of the blue excitation light and the conversion to green light must be optimized. By varying the growth parameters, we could stabilize the formation of {101¯1} semipolar facets resulting in better quantum well morphology and hence better green light conversion. Moreover, the QW number is optimized to make a balance between thermal load – decreasing the quality of the early grown quantum wells – and the lower quality of the quantum wells grown on top of many others. The thermal load has been identified as a very critical parameter for such structures emitting green light at about 505nm.

Fabrication of ridge‐shaped AlGaInP/GaInP quantum well structure for observation of evanescent wave coupling effect

AbstractA ridge‐shaped AlGaInP/GaInP quantum well (QW) structure with a ridge top width in the sub‐wavelength region (500–600 nm) is successfully fabricated by selective area metal organic vapor phase epitaxial growth on patterned GaAs substrates, as the first step towards the realization of high‐efficiency light‐emitting diodes (LEDs) utilizing the evanescent wave coupling effect. It is confirmed by an X‐ray diffraction study that AlGaInP layers grown on all the different facets of the ridge structure satisfy the lattice matching requirements for high‐efficiency LED applications. A considerable enhancement of the light‐extraction efficiency in the fabricated ridge‐shaped QW structure is demonstrated by means of a photoluminescence analysis. The realization of the evanescent wave coupling effect is suggested by theoretical simulation. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Morphology evolution and optical properties of GaN nano‐pyramids grown by selective area MOVPE

AbstractThe evolution of GaN nano‐pyramid growth by selective area metalorganic vapor phase epitaxy (SA‐MOVPE) on GaN/c‐plane sapphire templates with extremely low pattern fill factor and large inter‐hole spacing was studied with scanning electron microscopy (SEM). The optical nanostructure characteristics were investigated by micro‐photoluminescence spectroscopy at room temperature. It was found that sub‐100 nm structures can be fabricated by controlling the growth time and mask opening size. Single nanostructures exhibit more intense luminescence and a red shift of the band edge transition compared to the GaN layer. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of InAs nanowire conductivity: influence of growth method and structure

AbstractThe conductivity and crystal structure of nominally undoped InAs nanowires deposited by three different methods – 1. selective area metal organic vapor phase epitaxy (SA MOVPE), 2. gold assisted vapor liquid solid (VLS) MOVPE and 3. extrinsic catalyst free VLS molecular beam epitaxy (MBE) – is investigated. The influence on conductivity by stacking faults and different growth conditions is analyzed to determine the main impact. It is found that in terms of crystal structure, nanowires deposited by VLS MOVPE and VLS MBE behave similarly showing a zinc blende (ZB) phase while nanowires deposited by SA MOVPE feature a high density of stacking faults and a tendency to higher amounts of wurtzite (WZ) when grown with a decreased growth rate. However, the conductivity of wires deposited by VLS MOVPE is found to be much higher and statistically less dispersive compared to the other two wire types. An electrical similarity between nominally undoped wires in VLS MOVPE and previously reported intentionally doped wires in SA MOVPE is observed and discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A growth model of cubic GaN microstripes grown by MOVPE: Vapour phase diffusion model including surface migration effects

AbstractGrowth features of c‐GaN stripes grown by selective area metalorganic vapour phase epitaxy (SA‐MOVPE) on GaAs (001) substrates with stripe patterns aligned along [110] direction were analysed by vapour phase diffusion (VPD) model including surface migration effects from (111) facets. An addition of surface migration effects was found to improve the correlation between the simulation and experiment for the fill factors smaller than 0.5. The effects of the surface migration length (L) and the weight of surface migration contribution on the surface profile and the growth rate of the c‐GaN stripes were also analysed. An increase of L was found to strongly enhance the growth rate at the centre of c‐GaN stripes, while increasing the weight of contribution leads to the growth rate enhancement at the edges of the stripes. Our simulation demonstrated that the most probable values of L and the weight of contribution were 0.8 µm and 0.13, respectively. According to our simulation, mechanism of cubic‐to‐hexagonal structural phase transition is suggested as an incentive of the surface concentration profiles.

Impact of base size and shape on formation control of multifaceted InP nanopyramids by selective area metal organic vapor phase epitaxy

We report the impact of base size and shape on the evolution control of multifaceted InP (100) nanopyramids grown by selective area metal organic vapor phase epitaxy. The pyramid top surfaces are composed of a (100) center facet surrounded by high-index {103} and {115} facets. Their arrangement and (relative) size depend on the size and shape of the pyramid top area. For a certain shape, only the (100) facet remains below a critical size of the top area. The arrangement and (relative) size of the top facets in turn are governed by the {110} and {111} side facets whose area (ratio) depends on the pyramid base size and shape. This self-consistently determines the ratio of the (100) top facet area and the sum of the {110} and {111} side facet areas as well as the height of the pyramids.

MOVPE of n-doped GaAs and modulation doped GaAs/AlGaAs nanowires

Two different fabrication approaches were compared to obtain conductive GaAs nanowires: on one hand by modulation doping of GaAs/AlGaAs core/shell nanowires, on the other hand by Si-doping of GaAs nanowires. Modulation doped GaAs/AlGaAs core–shell nanowires were grown by selective area metal organic vapor phase epitaxy (MOVPE) on GaAs (1 1 1) substrates. The influences of growth parameters and mask design on aspect ratio of the core structures were investigated. The specialty of this study was that the growth mode was switched from vertical GaAs wire growth to the AlGaAs conformal shell overgrowth by intentionally changing the growth chemistry from the use of a more stable group III source – trimethylgallium (TMGa) to more easily decomposed sources – the alternative sources triethylgallium (TEGa) and dimethylethylaminealane (DMEAAl). It was found that the diameter and length of the core structures strongly depend on the arsenic partial pressure and growth temperature as well as mask design. The uniformity of shell growth is also influenced by the mask design. Additionally an alternative approach for the production of conductive GaAs nanowires was under study. To this end, the influence of Si-doping on GaAs core growth was investigated. It was found that doping has a detrimental impact on growth morphology leading to an undesirable enhanced growth rate on the nanowire side facets.

1.55 μm InAs quantum dot distribution on truncated InP pyramids and regrowth by selective area epitaxy

Position and distribution control of 1.55-μm InAs quantum dots (QDs) on truncated InP pyramids grown by selective area Metal Organic Vapor Phase Epitaxy (MOVPE) is reported. The arrangement of the {103}, {115}, and (100) facets on the pyramid top surface is governed by the shape of the pyramid base and top surface area. This allows the precise position and distribution control of the QDs due to preferential nucleation on the {103} and {115} facets. With shrinking QD number upon reduced top surface area, sharp emission from single QDs is observed at 1.55 μm. Regrowth of a passive InP structure around the pyramids establishes submicrometer-scale active-passive integration for efficient microcavity QD nanolasers and single photon sources operating in the 1.55-μm telecom wavelength region and their implementation in photonic integrated circuits.

Selective Area Metal–Organic Vapor Phase Epitaxy of Nitride Semiconductors for Multicolor Emission

Selective area metal-organic vapor phase epitaxy (SA-MOVPE) allows in-plane control of emission wavelength by tailored width of masks. For InGaN/GaN multiple quantum wells (MQWs), modulation of luminescence wavelength was achieved based on a balance between vapor-phase diffusion of group-III precursors and their surface incorporation. For the basic understanding of the SA-MOVPE of nitride semiconductors, thickness profiles of GaN, InN, AlN, and InGaN layers around relatively wide (>10 mum) masks were investigated. The effective lateral diffusion length D/k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> , which is the ratio of the vapor-phase mass diffusivity of a precursor to its surface incorporation rate constant, was extracted for GaN and InN. The value was much larger for InN due to smaller surface incorporation rate. In the SA-MOVPE of InGaN bulk layer at around 800degC, indium incorporation rate seems to be limited by the surface flux of a gallium precursor, resulting in no variation in the indium content. Varied width of the InGaN wells by the existence of masks seems to govern the shift in the luminescence wavelength from InGaN/GaN MQWs. Therefore, design of the thickness distribution of GaN based on the quantitative model is essential to the controlled in-plane color modulation of solid-state lighting devices using SA-MOVPE.

Longitudinal bandgap modulated broadband (&gt;150 nm) InGaAs/GaAs MQWs superluminescent diodes by selective area MOVPE growth

The characteristics of broadband superluminescent diodes (SLDs) are presented. The longitudinal bandgap modulated InGaAs/GaAs multiple quantum wells of broadband SLDs are grown by selective area metal organic vapour phase epitaxy (MOVPE) growth. The centre wavelength was 1060 nm. The 3 dB bandwidth was 130 nm and the 10 dB bandwidth was 174 nm. The output power was 0.3 mW.

Distribution control of 1.55μm InAs quantum dots down to small numbers on truncated InP pyramids grown by selective area metal organic vapor phase epitaxy

Distribution control of InAs quantum dots (QDs) on truncated InP pyramids by selective area growth is reported. The top surface of the pyramids is composed of a (100) facet and high-index facets aside. The arrangement of the facets is governed by the shape of the pyramid base and top surface area. The QDs preferentially nucleate on the high-index facets determining position and distribution. The QD number is reduced with shrinking top surface size. Positioning of four, three, two, and single QDs is realized depending on the top surface’s shape and size. Emission from single QDs is observed at 1.55μm.

Nonlinear Kinetic Analysis of InP and InAs Metal Organic Vapor Phase Epitaxy by Selective Area Growth Technique

Kinetic analysis based on the combination of selective area metal organic vapor phase epitaxy (SA-MOVPE) and two-dimensional numerical simulation can be used to extract real surface kinetics, which usually remain unknown because the growth rate of deposited film is limited by diffusional mass-transfer rate in SA-MOVPE. To explain previously observed concentration dependency on surface reaction rate constant and to establish a more reliable model for the surface process, non-linear surface kinetics were used in this study to analyze SA-MOVPE process for InP and InAs. Surface reaction rate constant (ksn) and adsorption equilibrium constant (K) of a film precursor in a nonlinear model was successfully extracted in SA-MOVPE for InP and InAs. Results show that a nonlinear model that takes adsorption–desorption equilibrium into account can well explain the concentration dependency and can be potentially used in predicting growth behavior regardless of mask width or partial pressure influence.

In xGa 1−xAs/InP selective area metal-organic vapor phase epitaxy for non-magnetic semiconductor spintronics

We carried out selective area metal-organic vapor phase epitaxy (SA-MOVPE) of In-rich In x Ga 1− x As/InP modulation-doped heterostructures in an N 2 atmosphere. The aim is to obtain wire structures which exhibit strong spin–orbit coupling without using etching of the heterostructures for their fabrication. The wire geometry was studied as a function of mask opening width. The transport properties were determined at 0.5 K and up to 10 T magnetic field. A clear growth enhancement was confirmed as the initial opening width decreases; thus, the InGaAs as well as the total thicknesses became larger. Moreover, we confirmed the top width saturates in some narrower wires due to geometrically limited growth by facets. Some narrower wires showed high resistivity, which might originate from structural deterioration in In 0.77Ga 0.23As due to the growth enhancement and/or non-uniformity of the parallel wires. On the other hand, wider wires showed Shubnikov–de Haas (SdH) oscillations, which exhibited SdH beating patterns. All in all it is possible to achieve strong spin–orbit coupling in In x Ga 1− x As/InP wires produced by SA-MOVPE.

Non-linear Surface Reaction Kinetics of GaAs MOVPE Explored by Selective Area Growth

Two-dimensional or three-dimensional numerical simulation on growth rate non-uniformity of selective area metalorganic vapor phase epitaxy in sub-millimeter scale can extract real surface kinetics, which is normally hindered by mass transport rate of film precursors. Non-linear surface kinetics is introduced to analyse group-III precursor concentration dependency for the first time and surface reaction rate constant (ksn) of adsorbed species and adsorption equilibrium constant (K) are extracted from GaAs-MOVPE at 575 C. The effect of misorientation angle of GaAs (100) substrate on these kinetic parameters is examined. It is found that ksn is about 3.4×10-5 mol/m2/s independent of the angle, whereas K is 6.9-12×10-5 m3/mol dependent on the angle. The obtained value of ksn can be converted to lifetime of adsorbed species on GaAs surface and it is 0.3 sec. This is mostly the same with gas-phase decomposition rate of trimethylgallium and supports the accuracy of our non-linear kinetic analysis.

Photonic crystal slabs with hexagonal air holes fabricated by selective area metal organic vapor phase epitaxy

It is shown that the photonic crystal slab (PCS) with hexagonal air holes has band gaps in the guided mode spectrum, which can be compared to that of the PCS with circular air holes, thus it is also a good candidate to be used for the PC devices. The PC with hexagonal air holes and a = 0.5 μm and r = 0.15 μm was fabricated successfully by selective area metal organic vapor phase epitaxy (SA-MOVPE). The vertical and smooth sidewalls are formed and the uniformity is very good. The same process was also used to fabricate a hexagonal air hole array with the width of 0.1 μm successfully. The air-bridge PCS with hexagonal air holes and a = 0.3 μm and r = 0.09 μm was also fabricated successfully by SA-MOVPE. Further optimization of the growth conditions for the sacrificial layer and the selective etching of the GaAs cap layer is also needed. Our experimental results indicate that SA-MOVPE is a promising method for fabricating PC devices and photonic nanostructures.

The surface diffusion of Ga on an AlGaN/GaN stripe structure in the selective MOVPE

AbstractThe growth of GaN with an AlGaN/GaN heterostructure was attempted on a (111) Si substrate by selective area metal organic vapor phase epitaxy. The GaN obtained was a trapezoidal stripe with (1$ \bar 1 $01) facets on the sides and a (0001) facet on the top. The distribution of the Al composition on the facets was investigated by cathodeluminescence analyses as a function of the position. The surface diffusion of Ga on the (0001) AlGaN facet was studied under various growth conditions. It was found that the diffu‐ sion length is enhanced by elevating the growth temperature or lowering the Al composition. By the temperature dependence of the diffusion length, the activation energy of the Ga species was estimated. The results depended on the Al composition x; 1.23 ± 0.44 eV (x = 0.05), 1.84 ± 0.58 eV (x = 0.10) and 2.72 ± 0.73 eV (x = 0.15). (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Fabrication of InP∕InAs∕InP core-multishell heterostructure nanowires by selective area metalorganic vapor phase epitaxy

We report the growth of InP∕InAs∕InP core-multishell nanowire arrays by selective area metalorganic vapor phase epitaxy. The core-multishell nanowires were designed to accommodate a strained InAs quantum well layer in a higher band gap InP nanowire. The precise control over nanowire growth direction and heterojunction formation enabled the successful fabrication of the nanostructure in which all three layers were epitaxially grown without the assistance of any catalyst. The grown nanowires were highly uniform, vertically oriented, and periodically aligned with controllable dimensions. 4K photoluminescence measurements confirmed the formation of strained InAs quantum well on InP (110) sidewalls and the well widths corresponding to the photoluminescence peaks were in good agreement with calculated values.

Realization of conductive InAs nanotubes based on lattice-mismatched InP∕InAs core-shell nanowires

We report the realization of ordered arrays of single-crystalline InAs nanotubes by a simple pure-eptiaxial approach. The process involved the fabrication of lattice-mismatched InP∕InAs core-shell nanowires using selective area metalorganic vapor phase epitaxy on InP (111)A substrates. The subsequent removal of the InP core resulted in vertically aligned InAs nanotubes which were highly uniform with well-defined features and controllable dimensions. Transmission electron microscopy studies confirmed that the nanotubes were single-crystalline with wurtzite crystal structure and temperature-dependent transport measurements revealed that they were conductive without any intentional doping. The realization of such conductive InAs nanotubes opens up new possibilities for both fundamental studies and future device applications.

Role of surface diffusion during selective area MOVPE growth of InP

Selective Area Metalorganic Vapor Phase Epitaxy (SA-MOVPE) growth of compound semiconductor, such as In 1– x Ga x As y P 1– y , is an indispensable technology to realize one-step fabrication of monolithic optical integrated circuits (OEICs). In SA-MOVPE, the growth rate of unmasked region is enhanced, because un-reacted group III precursors on the mask region diffuse into the epitaxial growth region. This growth rate enhancement is mainly caused by the gas-phase diffusion of precursors, but the contribution of surface diffusion becomes larger as the unmasked stripe becomes narrower. Moreover, in the SA-MOVPE growth of InP, the abnormal growth near the mask edge and the formation of tear-drop-like hillocks, which is considered to be originated from surface diffusion, are observed. In this work, the role of surface diffusion during SA-MOVPE growth of InP was investigated by measuring the cross-sectional area of abnormal growth and surface roughness. By changing the growth temperature from 793 K to 883 K, the area of abnormal growth increased and took maxima at 823 K, and decreased again at higher temperature. The concentration of group III precursor enhanced the abnormal growth and the roughness. The concentration effect was larger than the first order, and its relation to the surface diffusion length was suggested. This information on surface diffusion during SA-MOVPE is a key factor for the precise control of SA-MOVPE growth.

Controlled growth of highly uniform, axial/radial direction-defined, individuallyaddressable InP nanowire arrays

We report on the systematically controlled growth of InP nanowire arrays by catalyst-freeselective area metalorganic vapour phase epitaxy on partially masked InP(111)Asubstrates. The length, diameter, shape and position of the nanowires were preciselycontrolled by careful choice of the growth conditions and mask patterning. Manipulation ofthe growth conditions also enabled us to deliberately define the nanowire growth alongeither the axial or the radial direction, which has significant potential for the realization ofnovel nanostructures. Transmission electron microscopy studies revealed thatthe InP nanowires grown were single-crystalline with wurtzite crystal structureand the photoluminescence studies carried out at 4 K on InP nanowire arraysrevealed a single intense emission peak with a significant blueshift. The controlledfabrication thus enabled the nanowires to be realized in a highly uniform manner asreproducibly identical structures and with perfect positioning in predeterminedconfigurations, making them highly suitable for practical integration into nanodevices.