Selective Area Metalorganic Vapor Research Articles

Nucleus and spiral growth mechanisms of nitride semiconductors in metalorganic vapor phase epitaxy

Nucleus and spiral growth mechanisms of GaN and InN are investigated using the selective-area metalorganic vapor phase epitaxy (SA-MOVPE) technique on GaN bulk substrates. Nucleus growth of GaN occurs within selective areas having no screw-type dislocations, while spiral growth occurs within selective areas having screw-type dislocations. These growth modes are simultaneously observed on a single substrate in a single growth run. The nucleus and spiral growths of GaN result in the formation of step-free surfaces and growth spirals, respectively, wherein the interstep distance of growth spirals enables us to estimate the degree of surface supersaturation (σ). The σ dependences of nucleus and spiral growth rates of GaN are experimentally investigated. We found that these dependences are well explained by the classical crystal growth theories advocated by Burton, Cabrera, and Frank. We also investigate nucleation of InN using step-free GaN surfaces as an ideal platform.

(Invited) Selective-Area Metal Organic Vapor-Phase Epitaxy of III-V on Si: What About Defect Density?

Driven by fabrication cost reduction and device performance improvement, the Silicon semiconductor industry continues its never-ending pursuit of new approaches for fabricating integrated circuits. The heterogeneous integration of III-V high mobility semiconductors on a silicon platform in a 3D architecture such as Fin-Field-Effect-Transistor is a winning combination to further boost the performance of future CMOS devices.Direct heteroepitaxy of III-V compound semiconductors on Si has traditionally represented a formidable challenge, due to the extensive lattice mismatch of 8% between the Si substrate and high mobility III-V compounds, such as In0.53Ga0.47As/InP heterostructure. The defect confinement technique has recently attracted great interest due to the possibility of obtaining high quality III-V based active layers on Si. Using this approach we integrate III-V materials monolithically on Si through the epitaxial growth of III-V materials into a pre-patterned structure by Selective Area Metal-Organic Vapor Phase Epitaxy.We report here on the growth of InP on STI patterned Si wafers using the defect confinement technique while focusing on scaled trench widths (W < 50 nm). We demonstrate the impact of the crystalline alignment of the InP layer with the underlying substrate by exploring as starting geometry at the bottom in between the STI either a rounded etch pit covered with a Ge buffer layer versus a crystalline <111> V-groove structure in Si. We show the large impact of the main layer growth temperature and the growth pressure on the trench filling, the growth uniformity and the crystal quality which we could correlate with changes in resistivity and the presence of impurity diffusion in the III-V layer. Moreover, we derived a fundamental understanding and theoretical modeling of the growth mechanisms in STI trenches and the determining role of the nucleation layer. This lead to a strong enhancement of the crystalline quality and growth uniformity of the InP semiconductor.As a conclusion, this study of the InP selective area growth brings some elements for the optimization of the heteroepitaxy of III-V compounds on patterned Si substrates. The demonstration of a clear reduction in defect density along the trench orientation is an original achievement and confirms the potential of heterogeneous integration approach on a common Si platform.

Bottom-Up Formation of Vertical Free-Standing Semiconductor Nanowires Hybridized with Ferromagnetic Nanoclusters

The author introduces and summarizes the results on bottom-up formation and structural characterizations obtained so far for the MnAs nanoclusters and MnAs/semiconductor nanowire hybrids. First, MnAs nanoclusters were grown by selective-area metal-organic vapor phase epitaxy. They had a hexagonal NiAs-type crystal structure. Their &lt;00(0)1&gt; direction was parallel to &lt;111&gt;B direction of zinc-blende-type GaAs substrates. Hybrid MnAs/GaAs nanowires, subsequently, were fabricated by combining selective-area metal-organic vapor phase epitaxy of GaAs nanowire templates and endotaxial MnAs nanoclustering on them. MnAs nanoclusters ordered at six ridges of hexagonal GaAs nanowires were formed possibly owing to more atomic steps between {0-11} crystal facets. In the case of hybrid MnA/InAs nanowires, MnAs nanoclusters were not formed only on the {0-11} side-walls, and/or ridges between them, but on the top {111}B crystal facets of hexagonal InAs nanowires. MnAs nanoclusters were formed much deeper into the InAs nanowires than into the GaAs nanowires. These facts are possibly due to the InAs nanowires are thermally less stable than the GaAs nanowires. Some of the hybrid MnA/InAs nanowires were bent at the parts where the MnAs nanoclusters were grown into the host nanowires mainly owing to the strain effects.

Growth, structural and optical properties of ternary InGaN nanorods prepared by selective-area metalorganic chemical vapor deposition

Ternary InGaN nanorods were prepared on dielectric-masked nano-holes with selective area metalorganic chemical vapor deposition. To overcome the tendency for random nucleation of GaN at low temperatures, a pulsed growth procedure was introduced to enhance the diffusion length of Ga adatoms on SiO2, resulting in good selectivity at typical temperature ranges for InGaN. Photoluminescence from the InGaN nanorods can be tuned from near ultraviolet (400 nm) to blue-green (∼500 nm). Microstructural properties were characterized by transmission electron microscopy; threading dislocations from the underlying GaN template were terminated at the nanorod/template interface, resulting in dislocation-free nanorods. The height of dislocation-free InGaN nanorods is about 150 nm, which is much larger than the critical thickness for the onset of misfit dislocations in planar InGaN growth with typical thickness of less than 10 nm for an indium composition between 10 and 20%. The composition profile of In along the growth direction was examined by energy dispersive x-ray spectroscopic mapping and line scan. Oscillations of In composition along the growth direction were observed and are likely due to the kinetic competition between In and Ga adatoms. These InGaN nanorods are expected to be useful as templates for growing higher In composition nano-light-emitting diodes.

Growth kinetics and mass transport mechanisms of GaN columns by selective area metal organic vapor phase epitaxy

Three-dimensional GaN columns recently have attracted a lot of attention as the potential basis for core-shell light emitting diodes for future solid state lighting. In this study, the fundamental insights into growth kinetics and mass transport mechanisms of N-polar GaN columns during selective area metal organic vapor phase epitaxy on patterned SiOx/sapphire templates are systematically investigated using various pitch of apertures, growth time, and silane flow. Species impingement fluxes on the top surface of columns Jtop and on their sidewall Jsw, as well as, the diffusion flux from the substrate Jsub contribute to the growth of the GaN columns. The vertical and lateral growth rates devoted by Jtop, Jsw and Jsub are estimated quantitatively. The diffusion length of species on the SiOx mask surface λsub as well as on the sidewall surfaces of the 3D columns λsw are determined. The influences of silane on the growth kinetics are discussed. A growth model is developed for this selective area metal organic vapor phase epitaxy processing.

(Invited) Selective-Area Metal Organic Vapor-Phase Epitaxy of InGaAs/InP Heterostrucures on Si for Advanced CMOS Devices

Driven by fabrication cost reduction and device performance improvement, the Silicon semiconductor industry continues its never-ending pursuit of new approaches for fabricating integrated circuits. The heterogeneous integration of III-V high mobility semiconductors on a silicon platform in a 3D architecture such as Fin-Field-Effect-Transistor (FinFET) is a winning combination to further boost the performance of future CMOS devices.Direct heteroepitaxy of III-V compound semiconductors on Si has traditionally represented a formidable challenge, due to the extensive lattice mismatch of 8% between the Si substrate and high mobility III-V compounds, such as In0.53Ga0.47As/InP heterostructure. The defect confinement technique has recently attracted great interest due to the possibility of obtaining high quality III-V based active regions, using nanometer scale trenches with high aspect ratio (>3) in order to eliminate (111)-oriented defects (such as threading dislocations, twins or stacking faults) on the sidewalls. Using this approach we integrate III-V materials monolithically on Si through the epitaxial growth of III-V materials into a pre-patterned structure by Selective Area Metal-Organic Vapor Phase Epitaxy (SA-MOVPE). Silicon 300mm wafers substrates patterned with ~300nm deep Shallow Trench Isolation (STI) structures are used with trenches having typical widths ranging from 500 nm down to ~20nm.We report here on the SA-MOVPE growth of InP on STI patterned Si wafers using the defect confinement technique while focusing on scaled trench widths (W < 50 nm). We demonstrate the impact of the crystalline alignment of the InP layer with the underlying substrate by exploring as starting geometry at the bottom in between the STI either a rounded etch pit covered with a Ge buffer layer versus a crystalline <111> V-groove structure in Si. The structures are fully characterized by Scanning and Transmission Electron Microscopy, Atomic Force Microscopy, X-Ray Diffraction, Secondary Ion Mass Spectroscopy and Scanning Spreading Resistance Microscopy. We demonstrate the dependence of the material quality and trench filling upon different sets of growth conditions.We show the large impact of the main layer growth temperature and the growth pressure on the trench filling, the growth uniformity and the crystal quality which we could correlate with changes in resistivity and the presence of impurity diffusion in the III-V layer. Moreover, we derived a fundamental understanding and theoretical modeling of the growth mechanisms in STI trenches and the determining role of the nucleation layer. This lead to a strong enhancement of the crystalline quality and growth uniformity of the InP semiconductor.As a conclusion, this study of the InP selective area growth brings some elements for the optimization of the heteroepitaxy of III-V compounds on patterned Si substrates. The demonstration of a clear reduction in defect density along the trench orientation is an original achievement and confirms the potential of heterogeneous integration approach for advanced III-V based logic devices or III-V based photonic applications on a common Si platform.

Ga(In)N Photonic Crystal Light Emitters with Semipolar Quantum Wells

We present directional photonic crystal light emitters produced as periodic semipolar GaInN quantum wells, grown by selective area metal organic vapour phase epitaxy. The emitted angle-dependent modal structure for sub-micrometer stripes and embedded photonic crystal structures is analyzed experimentally in detail, and the introduction of an Al0.12Ga0.88N cladding layer is investigated. We provide a complete simulation based on the finite-difference time-domain method, which allows to identify all leaky modes as well as their spectral and angular dependence.

GaAs/InGaP Core–Multishell Nanowire-Array-Based Solar Cells

Semiconductor nanowires (NWs) are good candidate for light-absorbing material in next generation photovoltaic and III–V NW-based multi-heterojunction solar cells using lattice-mismatched material system are expected as high energy-conversion efficiencies under concentrated light. Here we demonstrate core–shell GaAs NW arrays by using catalyst-free selective-area metal organic vapor phase epitaxy (SA-MOVPE) as a basis for multijunction solar cells. The reflectance of the NW array without any anti-reflection coating showed much lower reflection than that of a planar wafer. Next we then fabricated core–shell GaAs NW array solar cells with radial p–n junction. Despite the low reflectance, the energy-conversion efficiency was 0.71% since a high surface recombination rate of photo-generated carriers and poor ohmic contact between the GaAs and transparent indium–tin-oxide (ITO) electrode. To avoid these degradations, we introduced an InGaP layer and a Ti/ITO electrode. As a result, we obtained a short-circuit current of 12.7 mA cm-2, an open-circuit voltage of 0.5 V, and a fill factor of 0.65 for an overall efficiency of 4.01%.

Growth rate and composition of InGaN during InGaN/GaN quantum wells selective area metal-organic vapor phase epitaxy considering surface diffusion

Film growth rate and composition variation were numerically analyzed during the selective area growth of InGaN on both GaN triangular stripe and hexagonal pyramid microfacets. To obtain the In composition of the film, concentration of In and Ga atoms due to the surface diffusion was added to the concentration determined from the Laplace equation that governs the gas phase diffusion. When the surface diffusion is considered, the In composition and resulting wavelength increased compared to the results obtained from the vapor phase diffusion model. The In content also increased according to the increasing mask width.

Indium-Rich InGaP Nanowires Formed on InP (111)A Substrates by Selective-Area Metal Organic Vapor Phase Epitaxy

We studied the growth of indium-rich InGaP nanowires (NWs) on InP (111)A substrates by selective-area metal organic vapor phase epitaxy (SA-MOVPE). We obtained vertically aligned InGaP NWs by optimizing growth conditions, such as group III supply ratio and V/III ratio. We found that the height, diameter, shape, and composition of InGaP NWs depended significantly on the supply ratios of trimethylgallium (TMGa) and trimethylindium (TMIn). As the supply ratio of TMGa was increased, the lateral growth was drastically enhanced, and the uniformity of NWs deteriorated. Furthermore, the sidewall facets of NWs changed from {211} to {110} as the supply ratio of TMGa was increased, indicating the possibility of structural transition from wurtzite (WZ) to zinc blende (ZB). We propose a possible growth model for such lateral growth, uniformity, and structural transition. Photoluminescence (PL) measurements revealed that the Ga compositions ranged approximately from 0 to 15%. Our results show that highly uniform InGaP NWs can be grown by controlling the growth conditions.

From conformal overgrowth to lateral growth of indium arsenide nano structures on silicon substrates by MOVPE

A methodology for the deposition of lateral InAs nanostructures on silicon by selective area metal organic vapor phase epitaxy (SA-MOVPE) is presented. Growth parameters which are optimal for the SA-MOVPE of conformal InAs overgrowth on GaAs nanowires were transferred to the lateral SA growth of InAs structures on patterned silicon substrates. The substrate pretreatment conditions and growth parameters were further optimized with respect to selectivity and nanostructure morphology. It is found that lateral growth of InAs nano structures can be achieved on patterned Si(110) as well as on patterned silicon on insulator (SOI) substrates. An investigation of the laterally grown InAs/Si nanowires' crystal structure revealed a faceted but nevertheless abrupt Si–InAs interface on the Si(110) substrate as well as relaxation and a high crystallinity of the deposited InAs on both Si template types. The morphology and crystallinity of laterally grown structures are discussed in detail and compared to that of vertical shell/core InAs/GaAs nanowires.

Selective-Area Growth and Electrical Characterization of Hybrid Structures between Semiconducting GaAs Nanowires and Ferromagnetic MnAs Nanoclusters

We report on the structural and electrical characterizations of MnAs/GaAs hybrid nanowires fabricated by combining selective-area metal–organic vapor phase epitaxy of undoped GaAs nanowires and endotaxial nanoclustering of MnAs. As a result of endotaxy, MnAs nanoclusters are typically embedded in the six ridges of hexagonal GaAs nanowires. However, the MnAs nanoclusters are formed not only at the six ridges, but on the surfaces of six 01̄1 facets of hexagonal GaAs nanowires, when we decrease the growth temperature from 600 to 400 °C. From the cross-sectional characterizations by transmission electron microscopy, the size and density of the MnAs nanoclusters formed at the top part of the GaAs nanowires are much larger than those formed at the bottom part of the nanowires. Current and voltage characteristics of MnAs/GaAs hybrid nanowires are investigated using two-terminal device structures of free-standing hybrid nanowires. The hybrid nanowires formed on Zn-doped p-type GaAs (111)B substrates show ohmic characteristics, while those formed on Si-doped n-type GaAs (111)B substrates show clear rectifying characteristics. The hybrid nanowires show p-type conductivity possibly owing to the formation of p-type GaAs layers near the nanowire surfaces caused by the thermal diffusion of Mn atoms into GaAs nanowires during the endotaxial nanoclustering of MnAs.

Selective-Area Growth and Electrical Characterization of Hybrid Structures between Semiconducting GaAs Nanowires and Ferromagnetic MnAs Nanoclusters

We report on the structural and electrical characterizations of MnAs/GaAs hybrid nanowires fabricated by combining selective-area metal–organic vapor phase epitaxy of undoped GaAs nanowires and endotaxial nanoclustering of MnAs. As a result of endotaxy, MnAs nanoclusters are typically embedded in the six ridges of hexagonal GaAs nanowires. However, the MnAs nanoclusters are formed not only at the six ridges, but on the surfaces of six 01 facets of hexagonal GaAs nanowires, when we decrease the growth temperature from 600 to 400 °C. From the cross-sectional characterizations by transmission electron microscopy, the size and density of the MnAs nanoclusters formed at the top part of the GaAs nanowires are much larger than those formed at the bottom part of the nanowires. Current and voltage characteristics of MnAs/GaAs hybrid nanowires are investigated using two-terminal device structures of free-standing hybrid nanowires. The hybrid nanowires formed on Zn-doped p-type GaAs (111)B substrates show ohmic characteristics, while those formed on Si-doped n-type GaAs (111)B substrates show clear rectifying characteristics. The hybrid nanowires show p-type conductivity possibly owing to the formation of p-type GaAs layers near the nanowire surfaces caused by the thermal diffusion of Mn atoms into GaAs nanowires during the endotaxial nanoclustering of MnAs.

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.

Position-Controlled III–V Compound Semiconductor Nanowire Solar Cells by Selective-Area Metal–Organic Vapor Phase Epitaxy

We demonstrate position-controlled III-V semiconductor nanowires (NWs) by using selective-area metal-organic vapor phase epitaxy and their application to solar cells. Efficiency of 4.23% is achieved for InP core-shell NW solar cells. We form a 'flexible NW array' without a substrate, which has the advantage of saving natural resources over conventional thin film photovoltaic devices. Four junction NW solar cells with over 50% efficiency are proposed and discussed.

Enhancement of light extraction efficiency by evanescent wave coupling effect in ridge-shaped AlGaInP/GaInP quantum wells

The optical properties of a sub-wavelength-sized ridge-shaped AlGaInP/GaInP quantum well structure fabricated by selective-area metal organic vapor phase epitaxy are investigated. The ridge structure exhibits an 8.5 times stronger photoluminescence intensity at room temperature than a planar structure, corresponding to an enhancement of at least 13.6 times in the light extraction efficiency of the ridge sample with respect to the planar sample. Both emission pattern measurements and theoretical simulation demonstrate that the high light extraction efficiency originates from the evanescent wave coupling effect in a narrow ridge structure, which can directly extract the light outside the escape cones to air.

Growth and Characterization of MnAs Nanoclusters Embedded in GaAs Nanowires by Metal–Organic Vapor Phase Epitaxy

The authors report on the fabrication of MnAs/GaAs hybrid nanowires by combining selective-area metal–organic vapor phase epitaxy of GaAs nanowires and “endotaxy” of MnAs nanoclusters. MnAs nanoclusters are embedded in the six ridges of hexagonal GaAs nanowires as a result of endotaxy. From the cross-sectional characterizations by transmission electron microscopy, the average width of MnAs nanoclusters with the hexagonal NiAs-type crystal structure and the average depth in GaAs nanowires are estimated to be about 10 and 8 nm, respectively. The magnetic responses detected from the reference samples grown on planar GaAs (111)B layers show that ferromagnetic MnAs nanoclusters are formed. The diameter of nanoclusters grown in GaAs nanowires increases with decreasing growth temperature and/or increasing distance between the GaAs nanowires, while the density of nanoclusters increases with increasing growth temperature. It is found that the diameter and density of nanoclusters are strongly influenced by the gas supplied during the decrease in temperature after the nanocluster growth.

Fabrication and Characterization of InP Nanowire Light-Emitting Diodes

We fabricated nanowire light-emitting diodes (LEDs) using InP nanowires (NWs). Indium phosphide NWs with axial p–n junction were grown by selective-area metalorganic vapor phase epitaxy. The results of secondary-electron-microscopy (SEM) observation and photoluminescence measurement showed the formation of wurtzite InP NWs with some mixture of zincblende crystal phase, as expected from the used growth conditions. NW-LEDs were fabricated by sputtering indium tin oxide (ITO) after a planarization process for the top contact and AuZn evaporation for the backside contact. Current–voltage characterisitics showed clear rectifying characteristics with a small leakage current, and fairly linear current–light output characteristics were observed. By designing the pitch of the NW array, emission from individual NWs was confirmed, which opens the possibility for realizing a single NW-LED applicable to single-photon emitters.

Fabrication and Characterization of InP Nanowire Light-Emitting Diodes

We fabricated nanowire light-emitting diodes (LEDs) using InP nanowires (NWs). Indium phosphide NWs with axial p–n junction were grown by selective-area metalorganic vapor phase epitaxy. The results of secondary-electron-microscopy (SEM) observation and photoluminescence measurement showed the formation of wurtzite InP NWs with some mixture of zincblende crystal phase, as expected from the used growth conditions. NW-LEDs were fabricated by sputtering indium tin oxide (ITO) after a planarization process for the top contact and AuZn evaporation for the backside contact. Current–voltage characterisitics showed clear rectifying characteristics with a small leakage current, and fairly linear current–light output characteristics were observed. By designing the pitch of the NW array, emission from individual NWs was confirmed, which opens the possibility for realizing a single NW-LED applicable to single-photon emitters.

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 &amp; Co. KGaA, Weinheim)