Session A37: Semiconductors I: Growth and Processing

Single phase In$_{x}$Ga$_{1-x}$N (0.25 $\le $ x $\le $ 0.63) alloys synthesized by MOCVD.

In$_{x}$Ga$_{1-x}$N alloys have received much attention due to their applications in optoelectronic devices operating in the near infrared region to the near UV region as the band gap of In$_{x}$Ga$_{1-x}$N alloys can be continuously tuned from $\sim $ 0.7 eV (InN) to 3.4 eV (GaN). Recently, it has been suggested that high quality In-rich InGaN alloys offer great potential applications in many important areas as follows; (1) high efficiency multijunction solar cell, (2) high efficiency photoelectronchemical (PEC) cell, and (3) thermoelectric (TE) devices. Our recent experimental results show that In-rich InGaN alloys could be as good as SiGe alloys in terms of figure of merit (ZT) for TE applications. However many experimental techniques have proven that growth of In-rich InGaN alloys is extremely challenging due to the solid phase miscibility gap between InN and GaN. Here we present the growth of single phase InGaN alloys with high In-contents by metal organic chemical vapor deposition on AlN/Al$_{2}$O$_{3}$ and/or GaN/Al$_{2}$O$_{3}$ templates. X-ray diffraction was employed to determine indium content. Single peak of wide range theta-2theta scan of (002) plane confirmed that there is no evidence of phase separation. Optical and electrical properties and surface morphology were also studied by photoluminescence, Hall-effect and atomic force microscopy measurements, respectively.   

High Mobility InN epilayers grown on AlN templates by MOCVD

Among III-nitrides, InN has the smallest electron effective mass, the largest mobility and smallest direct band gap. These distinguished properties make InN an interesting material for the applications in high speed electronic devices and full color displays. However, obtaining InN and In-rich InGaN epilayers with controllable conductivity is still a challenging task. We report here on the growth and transport property studies of InN epilayers on AlN templates, as compared to GaN templates. Significant improvements in the electrical and optical properties of InN epilayers were observed on AlN templates. A hall mobility of 1390 cm$^{2}$/Vs with a carrier concentration of 1x10$^{19}$cm$^{-3}$ at room temperature was observed, which is highest value reported for MOCVD grown InN epilayers. The photoluminescence emission spectra revealed band to band emission peak at $\sim $0.75 eV. The effects of post growth annealing on transport, structural and optical properties of undoped and Mg-doped InN epilayers will also be presented.   

Growth of the BN -- nano - structured materials using borazine decomposition by Laser Chemical Vapor Synthesis

We describe BN nanostructured materials growth by Laser Chemical Vapor Synthesis (LCVS) using the precursor borazine B$_{3}$N$_{3}$H$_{6}$. As result due to laser induced creation of the active chemical intermediates in the bulk volume, and further development of the ``dark'' chain processes in the borazine vapor with formation of the BN- nano-tubes and hydrogen gas. The phenomenological model qualitatively describing the observed phenomenon was developed and applied to explanation of the studied effects. The variation of radiation density (J/cm$^{2}$) for both harmonics and pressure is used for optimized the amount obtained.   

Anomalous Coherent Bragg Rod Analysis Studies of GaAs/InGaAs

A considerable amount of work has been carried out recently in correlating growth conditions with electronically observed properties in Group III-V systems using a variety of characterization techniques. Ambiguity in interpretation of most characterization techniques arises due to difficulties in separating roughness effects from segregation and inter-diffusion of atomic species. We apply x-ray resonant techniques to the Coherent Bragg Rod Analysis (COBRA) phase retrieval procedure to produce high resolution electron density maps from bragg rod scans to determine with a high degree of accuracy, the relative concentrations of In and Ga in a system comprising of 1ML of GaAs on InGaAs.   

Growth and Characterization of Non-Polar, A-Plane ZnO Thin Films

The growth and characterizations of non-polar a-axis-oriented epitaxial films of ZnO deposited by magnetron sputtering were investigated in comparison with the polar c-axis-oriented counterparts. The single-phase a-axis-oriented films were obtained at 700-degree C substrate temperature, whereas both a- and c-axis orientations were observed when deposited at lower temperatures ranging from 400 degree C to 600 degree C. The structural, morphological and temperature effects on photoluminescence (PL) behaviors have been studied. X-ray rocking curves show that the FWHM of (11-20) peak increases with temperature up to 600 degree C but start to decreases beyond 700C. Atomic force microscopy reveals significant changes in surface morphology as the growth temperature varies. Optical properties of the differently oriented films will be presented. PL emission in UV range was observed for the a-plane ZnO films which has been attributed to neutral donor bound exciton. The effect of Al-doping will also be discussed. *Also with NSYSU.   

Pinpoint Growth Mechanism Of ZnO Nanoprisms

We investigate the growth mechanism of ZnO nanoprisms synthesized by thermal evaporation method. Temperature is tuned to control the growth driving force while other conditions are fixed. Classical nucleation theory and growth dynamics are used to analyze the competition between growth in lateral and vertical directions. Interfacial diffusion properties, step edge diffusion barrier and several other factors affecting the growth of nanostructures are taken into account. Based on these considerations we have established a model which suggests a quantitative relation between temperature and the size of nanoprisms. Programmed cooling processes are introduced into the thermal evaporation to verify the theoretical expectations. It is also demonstrated that a morphology-controllable hierarchical prisms, which is expected from our theoretical model, can be easily achieved by tuning the temperature.   

Titanium oxide nano-clusters prepared via amorphous solid water: Improved overlap with solar spectrum

The need to develop reliable and cost effective alternative energy sources is rapidly growing. Solar light is among the most important and promising, therefore new materials are searched for to effectively overlap and harvest the solar spectrum. Here we describe the preparation of titanium oxide nano-clusters utilizing amorphous solid water (ASW) as a reactive buffer layer to assist the growth of titanium oxide clusters. These clusters were grown in-vacuum on top of ASW layers at 120K. Upon evaporation of the water layer, seed Ti(OH)$_{n}$ clusters polymerize via a solid state ``Sol-Gel-like'' mechanism. Self termination process, dictated by the evaporated water vapor, results in hemispherical clusters 5-7 nm in diameter. The clusters were analyzed by XPS at 300K. The clusters grown this way were studied by employing 4K scanning tunneling spectroscopy, revealing a typical diode-like I-V profile. An apparent band gap of 2.3$\pm $0.5 eV was obtained, significantly narrower than the bulk value of TiO$_{2}$ crystal (3.2 eV). Thermal stability of these defect rich clusters need to be studied, since this may prove important for photo-catalysis and photovoltaic applications.   

On the logarithmic-normal distribution in nucleation and growth processes

The logarithmic-normal (lognormal) distribution is one of the most frequently observed distributions in nature and describes a large number of physical, biological and even sociological phenomena. However, a derivation of this distribution from first principles is lacking. We propose a differential equation governing the time development of grain size distribution in random nucleation and growth processes. The solution of this equation provides an analytical derivation of size distributions that has a form of the lognormal type. The resulting expression is used to discuss the grain size distribution of solid phase crystallized Si-films.   

Si nanomembranes with mixed crystal orientations

Higher-carrier-mobility CMOS devices enhance processor speed. Carrier mobility can be optimized by fabricating mixed regions of Si(110) (high hole mobility) and Si(001) (high electron mobility) on a single substrate, so-called hybrid-orientation technology (HOT). We fabricate a mixed-crystal-orientation material using Si nanomembrane (SiNM) transfer and overgrowth. The top Si layer of SOI(110) is patterned with an array of holes and removed from its handle substrate, creating a Si(110)NM, which is then bonded to Si(001). We deposit Si over the structure with CVD, which is much faster on Si(001) than on Si(110), allowing planarization of the surface (i.e., hole filling), to produce a flat mesh of Si(001) and Si(110) regions. We characterize the mesh with XRD, SEM, and AFM. We can fabricate HOT membranes for transfer to various (including flexible) substrates, and can incorporate strain to tune mobilities. With strained Si(110)NMs, we expect a hole mobility enhancement of $\sim $70{\%} over Si(001) while maintaining the high electron mobility of Si(001), thereby dramatically reducing the mobility imbalance between n and p-type devices.   

Strain Relaxation in Elastically Strain-Sharing Silicon (110) Nanomembranes

Tensilely strained Si(110) has potential for advanced CMOS and p-MOS devices because, depending on strain and current direction, hole mobility can be increased to 150{\%} of that of unstrained Si(001) and an electron mobility nearly equal. Previous efforts to strain Si(110) tensilely have relied on the formation of partial dislocations for strain relaxation, resulting in large asymmetric components of the in-plane strain and high threading dislocation densities, which alter device performance. We use thin tri-layer heterostructures of Si/SiGe/Si(110) grown with MBE on Si(110)-on-insulator that elastically strain share when released from the handling substrate. Besides their very high flexibility, these nanomembranes (NMs) are virtually dislocation free and exhibit a lower degree of asymmetric in-plane strain relaxation than achieved with threading dislocation relaxation; the NMs rely on elastic strain transfer rather than partial dislocation propagation. We use AFM, XRD, and Raman to characterize growth, strain transfer, and strain anisotropy.   

A Novel Route for the Synthesis of Graphene by Microwave Plasma Enhanced Chemical Vapor Deposition

A novel route for the synthesis of graphene by means of microwave plasma enhanced chemical vapor deposition is presented. This technique outclasses its competitors in many ways since it is less elaborate and better reproducible than micromechanical cleavage of graphite and less expensive than thermal decomposition of silicon carbide wafers. Methane diluted with hydrogen is decomposed in a high power microwave plasma and the resulting carbon radicals recombine on the surface of any substrate that withstands temperatures up to 700\r{ }C. A broad range of substrates were successfully tested including silicon, quartz, stainless steel and many metals. The resulting carbon nanostructures are freestanding graphene flakes, only four to six atomic layers thick but up to several micrometers wide and high. The flakes are perpendicular aligned to the substrate surface. Thorough qualitative analysis lead to the conclusion that the flakes are highly crystalline sp2 carbon nanostructures with few defects or impurities. A possible growth scheme is proposed and field emission measurements of as grown flakes reveal a low turn on voltage of only 3V/$\mu $m which is a promising value for possible future applications.   

Structure of Iron Silicon Germanide and Osmium Silicide Epitaxial Films Measured by X-Ray Absorption Spectroscopy

Some of the iron- and osmium-based metal silicide and germinide phases have been predicted to be direct band gap semiconductors. Therefore, they show promise for use as optoelectronic materials. We have used synchrotron-based x-ray absorption spectroscopy to study the structure of iron silicon germanide and osmium silicide films grown by molecular beam epitaxy. Osmium silicide films which are primarily in the Os$_{2}$Si$_{3}$ phase and a series of Fe(Si$_{1-x}$Ge$_{x})_{2}$ films with a nominal Ge concentration of up to x = 0.04 have been grown. X-ray absorption near edge structure (XANES) measurements on both the iron silicon germinide and osmium silicide films has been performed. An absorption edge shift of 0.9 eV is observed for the osmium silicide films; however, no shift was observed for the iron silicon germinide films. Extended x-ray absorption fine structure (EXAFS) measurements have also been performed on the iron silicon germinide films. The nearest neighbor coordination corresponding to the $\beta $-FeSi phase of iron silicide provides the best fit with the EXAFS data.   

Studies of the Ge(100) Surface Using a Low Energy Positron Beam: The Effects of Surface Reconstructions on Positron Trapping and Annihilation Characteristics

Positron annihilation induced Auger electron spectroscopy (PAES) has been applied to study the Ge(100) surface. The PAES spectrum from the Ge(100) surface displays several strong Auger peaks corresponding to M4,5N1N2,3 , M2,3M4,5M4,5 , M2,3M4,5V, and M1M4,5M4,5 Auger transitions. The integrated peak intensities of Auger transitions are used to obtain experimental annihilation probabilities for the Ge 3d and 3p core level electrons. The experimental results are analyzed by performing calculations of positron surface states and annihilation characteristics of surface trapped positrons with relevant Ge core-level electrons for the reconstructed Ge(100)-p(2x1), Ge(100)-p(2x2), and Ge(100)-c(4x2) surfaces. Estimates of positron binding energy, work function, and annihilation characteristics reveal their sensitivity to surface reconstruction of the topmost layers of clean Ge(100). These results are compared to the ones obtained for the reconstructed Si(100)-(2x1) and Si(100)-p(2x2) surfaces. A comparison with PAES data reveals an agreement with theoretical core annihilation probabilities for the Auger transitions considered.   

X-ray Photoelectron Spectroscopic Investigation of Oxidation of Hafnium

X-ray photoelectron spectroscopy has been employed to investigate the oxidation of hafnium. Thin films (20 {\AA}) of elemental hafnium were deposited on silicon substrates using e-beam technique. Two types of samples were investigated. In one type, the substrate was annealed at the desired temperature after the deposition. In the other type, the substrate was kept at the desired temperature during the deposition. The substrate temperatures were kept at 100, 200, 300, 400, 500, and 600$^{\circ}$C. Hafnium is observed to get deposited mostly as hafnium dioxide with some suboxide. The amount of the suboxide is found to vary with the processing conditions. The concentration of the dioxide and the suboxide were determined by curve fitting the spectra. The fitting was performed using the parameters determined from fitting pure elemental hafnium spectrum and pure hafnium dioxide spectrum.