Session A24: Materials: Synthesis, Growth and Processing (Bullk and Films)

Epitaxial Growth of Zinc Oxide on Single Crystalline Gold Plates

Although metal-oxide interfaces are the critical components of many electronic and optical devices, it is rare to find epitaxial metal-oxide structures. We demonstrate for the first time, a method for the low temperature, epitaxial growth of zinc oxide (ZnO) on single crystalline gold plates. The gold plates, up to 100$\mu$m in width, are grown from a gold-surfactant complex. Even with the large lattice mismatch between (111) gold and (0001) ZnO, we are able to form epitaxial zinc oxide at 90$^\circ$C on top of the single crystal gold plates. This epitaxial growth is confirmed using transmission electron microscopy, electron diffraction, and electron backscatterer diffraction. Micro-photoluminescence is also performed to investigate the optical properties of the epitaxial zinc oxide. We remove the grown ZnO membranes from the gold plates using a stamping and etching process. These membranes can potentially be used to fabricate high quality microdisks and photonic crystals. The metal-oxide interfaces that we have fabricated may have the ability to be used in a number of technologically important applications, including as better electrical contacts and for improved light extraction from planar LED structures.   

Forward Raman scattering in ZnO: observation of phonon polaritons

Single crystal wurtzite ZnO samples have been studied by forward Raman scattering. Spectra were measured at 295 and 75 K using 442 and 448 nm laser lines for excitation below the fundamental absorption edge. Measuring spectra in forward geometry at varied scattering angles allowed achieving small and variable magnitudes of phonon wave vector necessary to observe a polatiton effect. The observed frequencies of the polaritons formed by A$_{1}$ TO phonons will be compared to the calculated dispersion of phonon polaritons in ZnO.   

SiGe Nanomembranes: Defect-Free Single-Crystalline Substrates for Growth of Strained Si/SiGe Heterostructures

Silicon-Germanium semiconductor alloys play a pivotal role in the strain engineering of heterostructures for microelectronic devices; however, high quality, single crystalline, defect free films with more than minimal Ge concentration do not exist. The lattice mismatch between Si and bulk SiGe results in biaxial tensile strain in thin Si films and leads to electronic band offsets that allow for the confinement of electrons in the strained Si layer, i.e., a two-dimensional electron gas. Many of the current techniques used to create relaxed SiGe rely on plastic relaxation of the alloy, which impose strain variations and inject crystalline defects into all epitaxial layers grown on top. These defects can significantly degrade the performance of any device in the active layer. We demonstrate the fabrication of SiGe nanomembranes (NM): fully elastically relaxed, smooth, single-crystalline sheets of SiGe alloy. These SiGe NMs can be transferred to new handling substrates, bonded, and used as templates for growth of new defect-free materials. We compare the material quality of strained Si/SiGe heterostructures grown on SiGe NMs with those created on SiGe substrates relaxed via dislocations.   

XRD analysis of high purity germanium single crystals grown by Czochralsk method

Two high purity germanium (HPGe) crystals were grown in argon and hydrogen atmosphere by Czochralsk method, respectively. Both XRD 2$\theta $ scanning and rocking curve ($\omega $-scanning) were used to investigate the quality of the grown crystals. XRD 2$\theta $ scanning results show that an extremely strong (400) peak at 2$\theta $ of 66.15$^{\circ}$ and a very feeble (200) peak at 31.56$^{\circ}$ were observed in three samples cut from the crystal grown in Ar atmosphere, indicating the crystals with $<$100$>$ orientation. However, there is an obvious split on (400) peak for three samples, which could be attributed to defects in the crystal. Additionally, according to the reflection and extinction law of germanium crystal with face-centered cubic lattice, the reflection of 200 should be forbidden. The presence of (200) peak at 31.56$^{\circ}$ could be resulted from XRD multiple-beams scattering. For the crystal grown in hydrogen atmosphere, only very strong and non-split (400) peak at 2$\theta $ of 66.15$^{o}$ was observed in all three samples. The $\omega $-scanning results of two crystals at the fixed 2$\theta $ of 66.15$^{\circ}$ show that the crystal grown in hydrogen atmosphere has much more highly symmetric rocking curve with narrower FWHM, which exhibits that the crystal grown in hydrogen atmosphere has very high quality. This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota.   

Growth and characterization of room temperature antiferromagnetic I-Mn-V semiconductors

The integration of ferromagnetism and semiconductors has been studied extensively, but devices operate well below room temperature. Recent theoretical and experimental works have opened a new route for spintronics based on antiferromagnets. Remarkably, high-temperature antiferromagnetic order is much more compatible with semiconductors than the ferromagnetic order. In our work we focus on the family of I-Mn-V antiferromagnetic semiconductor. We report on our synthesis of bulk and thin-film epilayers of the I-Mn-V compounds and on their basic electrical and magnetic properties. We will discuss the utility of these materials for designing antiferromagnetic semiconductor spintronic devices. \\[4pt] [1] Phys. Rev. B 83, 035321 (2010) \\[0pt] [2] http://arxiv.org/abs/1102.5373   

Nonequlibrium growth of GaInAsSb on GaSb across the immiscibility region by molecular beam epitaxy for midinfrared materials

GaInAsSb is a potentially important mid-infrared material, because alloys can in theory be grown with cut-off wavelengths from 1.7 to 4.9 ums. That potential has been hampered in the past by the present of large alloy immiscibility regions. In this work, 2 $\mu$m bulk GaInAsSb layers lattice-matched to GaSb substrates were grown across the entire compositional range, including the immiscibility region, by molecular beam epitaxy. Lower than typical growth temperatures (410-450 C) were used to limit the adatom diffusion length and move growth conditions further from equilibrium. Using a variety of techniques to characterize the optical, structural, and morphological quality of films, no phase separation was observed to occur for any alloy concentration. High resolution X-ray scans showed one narrow peak, and a single bright photoluminescence peak was observed for all samples. Smooth GaInAsSb surfaces were observed for all samples by AFM measurements, and XTEM images also show GaInAsSb layers to be homogeneous and defect free, with no sign of phase separation. EDS studies have been done, and results show low alloy scatter.   

Electronic and Structural Properties of InGaZnO Thin Films

We examine the effects of oxygen partial pressure during deposition and the structural changes resulting from a post-deposition anneal on the transport properties of InGaZnO. Amorphous oxygen-deficient samples sputter-deposited from a target with an atomic ratio of 2:2:1:7 at 50 watts DC in a 3 mTorr argon atmosphere have a resistivity of 0.16 ohm-centimeters. Amorphous oxygen-rich samples deposited similarly, except for a 10{\%} oxygen partial pressure, are insulating. For both samples, the as-deposited surfaces show a consistent grain size of approximately 30 nm. A subsequent rapid thermal anneal at 600C for 10 seconds leads to the coalescing and vertical growth of the grains with a resultant thinning of the background matrix. After anneal, the resistivity of the oxygen-deficient sample is decreased to 0.003 ohm-centimeters and 0.005 ohm-centimeters for the oxygen-rich sample. X-ray diffraction, scanning electron microscopy, atomic force microscopy and x-ray photoelectron spectroscopy data are presented to explain these changes and suggest possible methods of tuning the properties of InGaZnO for future use in thin film transistors, flexible electronics, transparent conductors and thermoelectric materials.   

Low temperature atomic layer deposition of $\alpha$-Fe$_2$O$_3$

There is significant interest in the use of $\alpha$-Fe$_2$O$_3$ (hematite) as a semiconducting thin film in a variety of applications including solar energy conversion, water oxidation, and gas sensing. In many such applications, devices may depend on non-planar geometries where traditional thin film deposition techniques are limited by line-of-sight constraints. Atomic layer deposition (ALD) is a gas-phase synthesis technique utilizing sequential self-saturating surface chemical reactions to produce uniform coatings with atomic scale control on substrates with arbitrary shape. However, ALD processes explored for Fe$_2$O$_3$ to date generally suffer from either extremely low growth rates, narrow temperature windows for self-saturating growth, or precursors with limited reactivity. In this respect, we will present a detailed study of a new, previously unexplored process for ALD of $\alpha$-Fe$_2$O$_3$ at technologically relevant temperatures between 200-300$^{\circ}$C. Self-limiting growth at $\sim$0.7 \AA/cycle was confirmed via situ quartz crystal microbalance. The results of in situ process characterization and ex situ analysis of film structure, morphology, composition, and electrical properties will be presented.   

Impact of atmosphere on HPGe crystal growth

The growth of high-purity germanium crystals for radiation detectors is being developed at the University of South Dakota. High-purity germanium crystals were grown in argon and hydrogen atmosphere, individually. The growth parameters were compared and analyzed. The relationship between thermal field and crystal quality was discussed. Based on the thermal properties of argon and hydrogen gases, different thermal fields were designed to grow lower dislocation density of high-purity germanium crystals.   

Performance optimization of diffused Li on Ga/In eutectic, In/Sn solder and eutectic Ga/In Ohmic contacts to n-high purity-crystalline (100) Ge

Performance optimization study of novel contacts such as diffused lithium on Ga/In eutectic (75.5/24.5 wt{\%}), In/Sn solder (95.0/5.0 wt{\%}) and Ga/In eutectic (75.5/24.5 wt{\%}) to n-high purity-crystalline $<$100$>$ Ge (HP-SC-Ge) has been presented. Ultrasonically clean samples taken from same substrate were used to process the contacts followed by their characterization utilizing current--voltage (I--V), Hall-effect and AFM measurements. Extreme care was introduced to minimize the effect of parasitic oxide layers. Contacts such as diffused Li on eutectic Ga/In and In/Sn solder were processed in an inert glove box and characterized at 305 K (RT) and 77 K (LN) respectively. Comparative study revealed that Ga/In eutectics contacts behave throughout linear and stable, showing strong hall-effect to that of its counter parts. This was attributed due to the high adsorption behavior of anions at liquid (Ga--In) contacts and improved wettability. Whereas, for In/Sn solder case, the contacts processing considerations were substantially different, mainly because of its poor solder flow, excessive void formation, and heterogeneous phase distribution responsible for process yield loss. For diffused Li on Ga/In eutectic contacts, the linearity of the obtained Ohmic profiles was not consistent due to the high reactivity of the Li with HP-SC-Ge substrate. This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota.   

Crystal-growth Underground Breeding Extra-sensitive Detectors

CUBED (Center for Ultra-Low Background Experiments at DUSEL) collaborators from USD, SDSMT, SDSU, Sanford Lab, and Lawrence Berkeley National Laboratory are working on the development of techniques to manufacture crystals with unprecedented purity levels in an underground environment that may be used by experiments proposed for DUSEL. The collaboration continues to make significant progress toward its goal of producing high purity germanium crystals. High quality crystals are being pulled on a weekly basis at the temporary surface growth facility located on the USD campus. The characterization of the grown crystals demonstrates that the impurity levels are nearly in the range of the needed impurity level for detector-grade crystals. Currently, the crystals are being grown in high-purity hydrogen atmosphere. With an increase in purity due to the zone refining, the group expects to grow high-purity crystals by the end of 2011. The one third of the grown crystals will be manufactured to be detectors; the remaining will be fabricated in to wafers that have large applications in electro and optical devices as well as solar panels. This would allow the research to be connected to market and create more than 30 jobs and multi millions revenues in a few years.   

Lattice matched quaternary alloy, BGaAsBi: growth and characterisation

The ternary semiconductor alloy GaAs$_{1\rm -x}$Bi$_{\rm x}$ has been been the focus of many recent studies due to the large decrease in the fundamental bang gap, $\Delta E_g~\simeq~-85~meV/$\% for small incorporated amounts. However, the large size of the bismuth atom relative to the arsenic it replaces results in significant lattice mismatch to GaAs substrates. We now report on the lattice matched quaternary alloy, B$_{\rm y}$Ga$_{1\rm -y}$As$_{1\rm -x}$Bi$_{\rm x}$. Incorporating a smaller atom (boron) along with the larger atom (bismuth) allows for a reduction of the epi-layer strain and lattice matched compositions, [B]:[Bi]$\simeq1$. The benefit of the choice of boron is that it does not effect the band structure of the host GaAs; no change in the band gap is observed with increasing boron content. Samples were grown by molecular beam epitaxy under conditions conducive to bismuth incorporation: low growth temperatures and low V:III ratios. Both high-resolution X-ray diffraction (XRD) and secondary ion mass spectroscopy were used to verify material composition and photoluminescence used to measure the band gap, and these results will be presented. The increasing observation of a distribution of shallow, in-gap boron related defects will also be discussed.   

Optimization of high purity germanium (HPGe) crystals growth rate through the simulation and modeling of growth system geometry

The growth rate and quality of high-purity germanium (HPGe) single crystals depend largely on the control of the thermal field such as the temperature profile and heat transfer. The control parameters of the thermal field can only be regulated externally through the growth system geometry, hydrogen and argon gas pressure, flow rate, pulling rate, and power and frequency of a RF heater. Since quantitative determination of the control parameters is exceptionally challenging and expensive, computer modeling and simulation of C$_{Z}$ growth processes play an imperative role in the advances of innovative pulling procedures and augmentation of Ge crystal quality. We present a detailed modeling and simulation study of radial and vertical temperature gradient, radial and vertical heat flux, temperature profile, thermo-elastic stresses, and defect density analysis for different crystal positions and diverse growth system geometry. We also virtually studied the consequences of targeted growth rate on temperature gradient and induction heating. A comparative analysis of simulated and available experimental results is also presented. In this effort, we have demonstrated the importance of simulation and modeling as it helps reducing the number of growth experiments significantly for the optimization of crystal quality and targeted growth rate.   

Passivation Layer by SiC Thin Film Deposition for High Efficiency Solar Cells

Deposition of a polymer derived SiC thin film as a novel, chemically and physically stable passivation layer to enhance the efficiency of solar cells by way of reducing surface recombination was studied. Starfire Matrix Polymer number 10 (SMP-10) is used to produce thin films of SiC on ion implanted silicon wafers. To ascertain the best method to deposit, three methods were tested: spin coating, spray coating, and dip coating are used. Various concentrations of SMP-10 diluted in xylene as an appropriate solvent are examined. To test the films, a contactless inductive coupling method is used. The thinner layers of SiC are grown by a lower percentage of SMP-10 (5{\%}), higher spin speed in spin coating (3000 RPM), and lower pulling out speed in dip coating (50 mm/minute). All of the methods yield controllable, repeatable, and uniform thin films. Although eliminating oxygen as an impurity in the passivation layer remains a challenge, the described approach has promise as a simple, low-cost passivation layers for higher efficiency solar cells.   

Growth of ZnO Nanowire Arrays for Advanced Ultraviolet Detectors

Zinc oxide (ZnO) provides a unique wide bandgap biocompatible material system exhibiting both semiconducting and piezoelectric properties. Bulk ZnO has a bandgap of 3.37 eV that corresponds to emissions in the solar blind ultraviolet (UV) spectral band (240-280 nm). We have grown highly ordered vertical arrays of ZnO nanowires using the metal organic chemical vapor deposition (MOCVD) technique on Si, silicon dioxide, c-plane sapphire, and GaN epitaxial substrates. UV detectors based on ZnO nanowires offer the highest UV sensitivity and lowest visible sensitivity for applications such as missile plume detection and threat warning. The development of UV detectors based on vertical nanowire arrays requires an innovative fabrication approach involving precise deposition of metal contacts, where UV sensor performance depends to a large extent on the growth conditions as well as on the substrate used. We will present experimental results on the structural, electrical, and optical properties of ZnO nanowires grown for UV sensing applications.