Session W25: Semiconductor Growths and Wide Bandgap Semiconductors

Studies on single-phase, indium-rich In1-xGaxN epilayers grown by high-pressure CVD

While significant progress in the growth of high quality group III-nitride epitaxial layers has been made during the last decades, the growth of indium rich InGaN epilayers at processing conditions compatible with wide band gap group III-nitrides is still very challenging, especially under low-pressure MOCVD growth conditions. Utilization of elevated pressures to stabilize the growth surface lead to elevated growth temperatures and opened a window for the growth of single-phase material with indium concentrations well above 30{\%}. We present growth conditions and results on structural quality and optical properties of single-phase indium-rich In$_{1-x}$Ga$_{x}$N epilayers. The layers were studied by X-ray diffraction, Raman spectroscopy, atomic force microscopy, optical absorption and IR reflectance spectroscopy, and photoluminescence spectroscopy.   

Growths of Lattice-Matched AlInN Alloys on GaN

III-Nitride alloys have tremendous importance for solid state lighting, power electronics, visible lasers, and thermoelectric applications. Lattice-matched AlInN material has the potential to realize large bandgap material for achieving optimized heterostucture design for nitride-based devices. In this work, the growths of AlInN alloys with different In-contents were performed. The growths were performed by employing metalorganic chemical vapor deposition. The AlInN alloys were grown on 2.7 $\mu $m undoped GaN template grown on sapphires. The growth temperatures were investigated from 750 $^{\circ}$C up to 860 $^{\circ}$C. From our experiments, the In-content from 0.367{\%} up to 22.8{\%} were obtained from AlInN alloy as the growth temperature were reduced from 860 $^{\circ}$C down to 750 $^{\circ}$C. The crystal quality and In-content of the AlInN film were characterized by high-resolution X-ray diffraction measurements. Scanning electron microscopy measurements and atomic force microscopy were carried out to characterize the surface morphology of the film. The optimized growth condition for the lattice-matched AlInN film was achieved by employing growth temperature of 780 $^{\circ}$C at growth pressure of 20 Torr, with growth rate of 0.15 $\mu $m/hr.   

Physical and electrical interface structure of crystalline oxides on GaN

High electron mobilities at semiconductor-oxide interfaces are important for heterostructure field effect transistor applications, depending critically on the electronic and physical structure of the interfaces. Here, we report on the growth and characterization of crystalline oxides of (Mg,Ca)O deposited epitaxially on GaN using molecular beam epitaxy. By carefully engineering the MgO-GaN interface through a combination of precise layer-by-layer control of the deposition and lattice matching to the GaN substrate through alloying of the MgO with CaO to reduce and eliminate dislocations, a high breakdown field can be achieved. In addition, careful substrate surface preparation is a key element in producing high quality films. We discuss an approach for generating carbon-free surfaces for epitaxy and show surface and interface characterization of (Mg,Ca)O/GaN films using capacitance-voltage measurements, atomic force microscopy, and x-ray scattering.   

MOCVD Epitaxy of GaN by Employing SiO$_{2}$ Colloidal Microspheres Templates

Nitride semiconductors play an important role for light-emitting diodes (LEDs) and lasers in visible spectrum. Low dislocation density GaN is of great importance in achieving high efficiency nitride LEDs and reliable lasers Here, we investigate the growth mechanism of GaN by employing SiO$_{2}$ colloidal microspheres as a mechanism to reduce the dislocation density of the GaN template growths. Very few studies have been carried out on the influence of implementing the colloidal microspheres as mechanism to reduce dislocation density in GaN epitaxy. The monolayer of silica microspheres arrays is deposited on an existing n-type GaN template, and we performed two-step epitaxy processes to achieve good coalescence of the grown film by covering the microspheres. The characterization by employing atomic force microscopy and scanning electron microscopy of optimized film indicated high quality GaN templates. The growths of InGaN-based quantum wells LEDs were performed on the template, and the electroluminescence shows these devices exhibited enhanced luminescence efficiency in comparison to the conventional devices.   

Self-assembled GaN hexagonal disks and pyramids grown on $\gamma $-LiAlO$_{2}$ by plasma-assisted molecular-beam epitaxy

The self-assembled GaN pyramid, disk, and pillar have been grown on $\gamma $-LiAlO$_{2 }$substrate by plasma-assisted molecular-beam epitaxy. We observed the largest GaN disk is 4.48 $\mu $m. Furthermore, a model was developed to demonstrate that the GaN disk was established due to Nitrogen atoms were captured by most-outside Gallium atoms, and pyramid was obtained by the missing of most-outside Nitrogen atoms. Transmission electron microscopy measurement shows that the epilayers of GaN disk were grown with a tilt angle of 28 degree. It conforms to the prediction of the model. The properties of luminescence measured by photo-luminescence and cathode-luminescence were under investigation. Base on the result, we proposed a GaN hexagonal disk p-n junction for high efficiency of optoelectronic device. * Published in Applied Physics Letters 94, 062105 (2009).   

Effect of Nitrogen on electrical properties of Titanium Aluminum Nitride

Titanium aluminum nitride thin film by DC magnetron sputtering was deposited on Si and glass substrate. Surface electrical conductivity variation of titanium aluminum nitride for different Nitrogen and Argon gas composition was investigated using four point probe measurements. The morphology of titanium aluminum nitride films was characterized using atomic force microscopy AFM. Experimental results show that increasing the nitrogen gas in the mixture makes the film less conductive. Formation of titanium aluminum nitride structures for different gas mixtures was investigated using XRD.   

Lattice Parameter Variation in ScGaN Alloy Thin Films on MgO(001) Grown by RF Plasma Molecular Beam Epitaxy

We present the structural and surface characterization of the alloy formation of scandium gallium nitride Sc$_{x}$Ga$_{1-x}$N(001)/MgO(001) grown by radio-frequency molecular beam epitaxy over the Sc range of x = 0-100{\%}. In-plane diffraction measurements show a clear face-centered cubic surface structure with single-crystalline epitaxial type of growth mode for all x; a diffuse/distinct transition in the surface structure occurs at near x = 0.5. This is consistent with out-of-plane diffraction measurements which show a linear variation of perpendicular lattice constant a$_{\bot }$ for x = 0 to 0.5, after which a$_{\bot }$ becomes approximately constant. The x = 0.5 transition is interpreted as being related to the cross-over from zinc-blende to rock-salt structure. This work has been supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No. 0304314). Equipment support from ONR is also acknowledged.   

Catalyst-free Indium Nitride Nanostructures Grown by Plasma-Assisted MOCVD

The growth of Indium Nitride nanostructures directly on n-type Si (111) substrate was achieved without ammonia using nitrogen plasma as sole atomic nitrogen contribution. In this study we determine the growth conditions for optimal structural characteristics adjusting plasma generation power, substrate temperature, and III/V ratio. The structures formed nucleation sites directly interspersed on Si substrate without the use of hetero catalysis. SEM observations show that structurally the nanostructures range from narrow and horizontal with a high 1-dimensional consistency in width and length, to vertical and conical. Effects of the growth parameters along with the growth mechanism of nanostructures will be discussed.   

Dielectrophoretic Alignment of ZnO Nanowires

Work on individually constructed devices has demonstrated that nanowires (NWs) offer great promise for applications such as sensing and optoelectronics. Despite this work, reliable large scale alignment and integration of these individual nanostructures into a lithographically defined process remains a challenge. Dielectrophoresis (DEP) is a promising alignment method in which a nonuniform electric field is used to exert force on and manipulate NWs in solution. DEP offers the possibility of rapid, large area room-temperature assembly of NWs across opposing electrodes. DEP structures were fabricated on Si substrates and consisted of pairs of parallel Al electrodes on a 600nm insulating SiO2 film. ZnO NWs were suspended in isopropyl alcohol (IPA) and flowed across the electrodes. Alignment yield was investigated as a function of electrode spacing (2 micrometer, 5 micrometer, 8 micrometer, 11 micrometer), DEP voltage, DEP frequency, IPA flow rate, and NW length. The electrical properties of the formed ZnO NW devices will also be reported.   

Synthesis of Zinc Oxide Nanowires by Chemical Vapor Deposition

One-dimensional nanostructures of zinc oxide (ZnO) are promising structures for future nano-optoelectronic devices and applications ranging from solid-state lighting to photovoltaics. They can also potentially serve as template matrices for nanoscale sensors. Realizing well aligned nanostructures has remained challenging. We present here the growth of ZnO nanowires by chemical vapor deposition using a variety of metal and non-metallic catalysts, as well as using a catalyst-free approach. The resulting nanowires were characterized through electron microscopy, optical absorption, photoluminescence, and confocal Raman spectroscopy.   

Synthesis of Core-Shell BiFeO3/ZnO nanostructures

We will discuss the synthesis of core-shell BiFeO3/ZnO nanoparticles, layer-by-layer using a combination solvothermal/hydrothermal synthetic method. The goal of this research was to synthesize novel magnetoelectric/semiconducting core-shell nanoparticles for building multifunctional devices. Furthermore, we will discuss (1) development of the combination synthetic method; (2) the effects of reactant concentration, pH, or time on the morphology of the particles; (3) the incorporation of dopants such as Mn into the ZnO shell; and (4) magnetic properties of the core-shell nanoparticles.   

Surface Reconstructions and Bi Incorporation in GaAs1-xBix Alloys

Incorporation of Bi into GaAs allows for a large reduction of the GaAs bandgap per percent incorporation (7x greater than In, with modest increase in lattice size) and shows strong electro- and photoluminescence (PL). This will allow for longer wavelength devices to be grown on GaAs substrates, than is currently possible with pseudomorphic InGaAs on GaAs. GaAsBi growth is challenging as Bi has a strong tendency to surface segregate. Careful control of growth parameters, especially the As:Ga flux ratio is required, as Bi only incorporates when this ratio is close to unity. The low As:Ga flux ratio also makes Ga droplet formation a problem. Reflection high-energy electron diffraction (RHEED) is used as a crucial tool in locating the optimum growth conditions. As the As:Ga ratio is lowered to between 2 and 1 the RHEED pattern shifts from a 1x3 to a 2x1 reconstruction([1-10]x[110]), which corresponds to a surprising abrupt increase in bismuth incorporation. RHEED phase maps will be presented along with PL spectra and high-resolution x-ray diffraction rocking curves of grown samples.   

Photoluminescence and structural characterization of InAsSb/InAs strain balanced SLS grown on GaSb for infrared detector applications

InAs$_{0.91}$Sb$_{0.09}$, epitaxially grown on GaSb, has received steady attention in the past few years for optical detectors in the 3-5micron range. Attempts to increase the detection wavelength by increasing the Sb mole fraction have been hindered by the lack of lattice-matched substrates. In this work we report the growth of strain balanced InAs/InAsSb superlattice structures (SLS) strain-balanced to GaSb for potential application in photodetectors beyond 5 microns. The strain balanced method permits the incorporation of larger Sb mole fractions in the Sb layers, considerably extending the absorption cutoff. We find the PL-energy of the InAsSb/InAs MQW stack to depend linearly on the Sb mole fraction for samples with Sb compositions ranging from 14{\%} to 27{\%}. For the latter composition, a PL energy corresponding to a wavelength of 10 $\mu $m is detected. This can be explained by a type II band-alignment. The optical results are compared to the expectations from energy band simulation software. The simulations take into account the band offsets according to the solid model theory, energy band shifts due to strain and quantization energies.   

Evolution of heteroepitaxial (001) diamond films on (001) Ir/YSZ/Si substrates

We present new results on the heteroepitaxial diamond growth on Ir/YSZ/Si multilayer substrate assembly using microwave plasma chemical vapor deposition. A 150 nm thick epitaxial yittria stabilized zirconia (YSZ) is grown on (001) Si using pulsed laser deposition followed by 150 nm of Ir using e-beam evaporation. Methane in hydrogen was used as the carbon source. Nucleation of epitaxial diamond crystals is induced by bias enhanced nucleation (BEN). The effects of bias voltage, bias time, and methane concentration on the nucleation density and the Ir surface structure are studied systematically using SEM, AFM, and high resolution TEM. An epitaxial diamond nucleation density higher than 10$^{11}$ cm$^{-2}_{ }$has been demonstrated with optimized BEN. At these high nucleation densities diamond crystals start coalescing after only a few minutes of growth. Continuous, uniform thickness diamond films, without visible coalescence boundaries, have been grown in just 20 minutes. TEM images of thicker films produced by continual growth show a defect band on the order of 1 micrometer localized at the diamond/Ir interface. Past this thickness the defects dramatically decrease in density, resulting in single crystal diamond that is of higher crystalline order than the underlying Ir as confirmed by X-ray diffraction.