Session B51: Materials: Synthesis, Growth, and Processing

The impact of neutral impurity concentration on charge drift mobility

High-purity germanium crystals are being grown using the Czochralski technique at the University of South Dakota. The carrier concentration, mobility and resistivity are measured by Hall Effect system. Many factors contribute to the overall mobility. We investigated the impact of neutral impurity concentration on charge drift mobility. Several samples with measured mobility lager than 35000 cm\textasciicircum 2/Vs from the grown crystals were used for this investigation. With the measured mobility and the ionized impurity concentration, we were able to calculate the neutral impurity concentration by the Matthiessen's rule. The correlations between the neutral impurity concentrations with the radius of the crystals were studied. We report that the concentration of neutral impurity constrains charge draft mobility for high-purity germanium crystals and the non-uniform distribution of neutral impurity could result in an anisotropy of draft time distribution in a given germanium detector.   

Influences of solid/liquid boundary layer thickness and tilting angle on zone-refinement of germanium crystals

In zone-refining of metals, solid/liquid (S/L) boundary layer thickness has an influence on segregation coefficient of impurity atoms. Additionally, the segregation of impurity elements during zone refining can be maximized by adjusting the zone refinement tube with a proper angle. In this paper, we report the influences of S/L boundary layer thickness on the segregation coefficients of boron, phosphor, aluminum and gallium, which have been identified as four main impurities in germanium crystal by Photothermal Ionization Spectroscopy (PTIS). The thickness of S/L boundary layer was found by using a well-known model to fit the experimental data. The optimized segregation coefficients have been used to calculate the impurity distribution along the purified ingot. In addition, we have also optimized the tilting angle of the germanium ingot to investigate the impact on the segregation. This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota.   

The Electronic Properties of Nanoscale Meta-lattice Made by High Pressure CVD

Meta-lattice can be defined as an artificial 3D superlattice with periodic structural modulation occurred at ~10nm scale. One viable route to synthesize can be as follows: A template is first prepared by close-packed nanometer-sized silica spheres, then Si/Ge or a binary semiconductor is infiltrated into voids by high pressure chemical vapor deposition (CVD). Later silica spheres can be removed by chemical method, and voids in the inverse meta-latice offer the opportunity for a second infiltration. Due to the characteristic length of voids, meta-lattice provides a platform to test novel mesoscopic electronic and thermal phenomena. A meta-lattice solid can show novel physical properties that each constituent infiltrate material does not have. Since a significan portion of atoms are located on the surface, the interface structure details are expected to play a critical role. Here we investigate Si/Ge inverse meta-lattices with or without silica template present. Tight-binding, DFT and GW/BSE techniques are employed to look into the electronic and optical properties.   

Aluminum Nitride Grown by Atomic Layer Epitaxy Characterized with Real-Time Grazing Incidence Small Angle X-ray Scattering

Aluminum nitride, gallium nitride, and indium nitride are being considered for many applications, and are currently being used commercially for LEDs. These III-nitride films are conventionally deposited by metalorganic chemical vapor deposition and molecular beam epitaxy. Research into depositing III-nitrides with atomic layer epitaxy (ALE) is underway as it is a fabrication friendly technique for thin films at lower temperatures.\\ AlN deposited with ALE at 500$^{\circ}$C have been shown to have good crystallinity, but relatively high carbon and oxygen impurities, and understanding the film deposition mechanism is an ongoing project.\footnote{N. Nepal et al., \textbf{Appl. Phys. Lett. 103} 082110 (2013)} Grazing incidence small angle x-ray scattering (GISAXS) is sensitive to surface features, making it useful for real time monitoring of deposition processes. AlN was monitored by GISAXS while being deposited with ALE using trimethylaluminum and hydrogen/nitrogen plasma at the Brookhaven National Synchrotron Light Source and the Cornell High Energy Synchrotron Source. The GISAXS of AlN ALE at nominally 400$^{\circ}$C, 450$^{\circ}$C, and 500$^{\circ}$C was compared to ex situ characterization with XPS and AFM.   

Parallel Stitching of Two-Dimensional Materials

Large scale integration of atomically thin metals (e.g. graphene), semiconductors (e.g. transition metal dichalcogenides (TMDs)), and insulators (e.g. hexagonal boron nitride) is critical for constructing the building blocks for future nanoelectronics and nanophotonics. However, the construction of in-plane heterostructures, especially between two atomic layers with large lattice mismatch, could be extremely difficult due to the strict requirement of spatial precision and the lack of a selective etching method. Here, we developed a general synthesis methodology to achieve both vertical and in-plane ``parallel stitched'' heterostructures between a two-dimensional (2D) and TMD materials, which enables both multifunctional electronic/optoelectronic devices and their large scale integration. This is achieved via selective ``sowing'' of aromatic molecule seeds during the chemical vapor deposition growth. MoS$_{2}$ is used as a model system to form heterostructures with diverse other 2D materials. Direct and controllable synthesis of large-scale parallel stitched graphene-MoS$_{2}$ heterostructures was further investigated. Unique nanometer overlapped junctions were obtained at the parallel stitched interface, which are highly desirable both as metal-semiconductor contact and functional devices/systems, such as for use in logical integrated circuits (ICs) and broadband photodetectors.   

Solution-Processed hybrid Sb2S3 planar heterojunction solar cell

Thin-film solar cells based on inorganic absorbers permit a high efficiency and stability. Among or those absorber candidates, recently Sb2S3 has attracted extensive attention because of its suitable band gap (1.5eV \textasciitilde 1.7 eV) , strong optical absorption, low-cost and earth-abundant constituents. Currently high-efficiency Sb2S3 solar cells have absorber layer deposited on nanostructured TiO2 electrodes in combination with organic hole transport material (HTM) on top. However it's challenging to fill the nanostructured TiO2 layer with Sb2S3 and subsequently by HTM, this leads to uncovered surface permits charge recombination. And the existing of Sb2S3/TiO2/HTM triple interface will enhance the recombination due to the surface trap state. Therefore, a planar junction cell would not only have simpler structure with less steps to fabricate but also ideally also have a higher open circuit voltage because of less interface carrier recombination. By far there is limited research focusing on planar Sb2S3 solar cell, so the feasibility is still unclear. Here, we developed a low-toxic solution method to fabricate Sb2S3 thin film solar cell, then we studied the morphology of the Sb2S3 layer and its impact to the device performance. The best device with a structure of FTO/TiO2/Sb2S3/P3HT/Ag has PCE over 5{\%} which is similar or higher than yet the best nanostructure devices with the same HTM. Furthermore, based on solution engineering and surface modification, we improved the Sb2S3 film quality and achieved a record PCE. .   

Visible Aligned Carbon Nanotube-MoS2 Hybrids

Single-walled carbon nanotubes (SWNTs) have gained great interest due to their excellent electrical, mechanical and thermal properties. Recent progress in two-dimensional (2D) materials has opened up new horizons in the realm of physics and engineering that could lead to the revolution of future electronics and optoelectronics. Various hybrid structures have been developed for different applications. Here we report a facile method to synthesize ultrathin 2D hybrids between horizontally-aligned SWNT and monolayer molybdenum sulfide (MoS2) through chemical vapor deposition (CVD). These hybrid structures can be imaged under an optical microscope; and their Raman mapping indicates that MoS2 flakes are partially grown on top of SWNTs. Moreover, strong photocurrent signals have been observed in SWNT-MoS2 hybrids through scanning photocurrent measurements. These fundamental studies may provide a new way to fabricate 2D hybrids for future electronics and optoelectronics.   

Growth and Characterization of TMDs (MoS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, WSe$_{\mathrm{2}}$ {\&} MoTe$_{\mathrm{2}})$ and Their Alloys on Various Substrates

Transition Metal Dichalcogenides (TMDs) have been of interest over the past years due to their exciting semiconducting properties. In the bulk, TMDs possess a native indirect bandgap and transition to a direct bandgap as they approach the monolayer limit. The bandgaps range from 1.15 eV to 1.95 eV depending on composition. Using organic liquids and/or inorganic powders as precursors, CVD growth has been realized for MX$_{\mathrm{2}}$ TMDs (M $=$ Mo, W; X $=$ S, Se) and their alloys at tunable compositions. I will present the effect of tuning parameters such as temperature, gas flow, time of heat and hold on the resultant single-layer films. Different precursors can lead to different overall film structures and enable different growth conditions. The films can either be made homogeneous in bandgap or gradients of material/bandgap can be grown. The use of different substrates (dielectric, ferroelectric, piezoelectric, semicoundcing , insulating, patterned) allows an additional degree of freedom and sets the stage for subsequent experiments. I will talk about preparation methods tailored toward direct applicability of surface acoustic spectroscopy, scanning photocurrent microscopy, and ferroelectric gating of the single-layer films.   

Interfacial reaction between metal-insulator transition material NbO$_2$ thin film and wide band gap semiconductor GaN

Materials that undergo a metal-insulator transition (MIT) are potentially useful for a wide variety of applications including electronic and opto-electronic switches, memristors, sensors, and coatings. In most such materials, the MIT is driven by temperature. In one such material, NbO$_2$, the MIT mechanism is primarily of the Peierls-type, in which the dimerization of the Nb atoms without electron correlation causes the transition from metallic to semiconducting. We describe our initial work at combining NbO$_2$ and GaN in epitaxial form, which could be potentially useful in resistive switching devices operating at very high temperatures. We grow NbO2 films on GaN(0001)/Si(111) substrates using reactive molecular beam epitaxy from a metal evaporation source and molecular oxygen. X-ray diffraction shows that the films are found to grow with a single out of plane orientation but with three symmetry-related orientation domains in the plane. In situ x-ray photoelectron spectroscopy confirms that the phase pure NbO$_2$ is formed but that a chemical reaction occurs between the GaN and NbO$_2$ during the growth forming a polycrystalline interfacial layer. We perform STEM-EELS analysis of the film and the interface to further elucidate their chemical and structural properties.   

Impact of Crystalline Structure on the Temperature Dependence of Resistivity

Since HPGe radiation detectors work under cryogenic temperature, the electrical properties at low temperature are essential for the detector performance. In this study, the resistivity of two types of HPGe, i.e. single crystal from Czochralski growth and poly-crystal from zone refining, was investigated in the temperature range from 4.2 to 100K. It was found that there was a turning point on the resistivity vs temperature curves for both types of crystals. However, the turning points for them were significantly different: 30K for single crystalline while 60K for polycrystalline. In order to explore the reason, microstructures of both types of crystals were investigated by optical microscopy. The results showed a very good agreement between electrical properties and microstructures.   

Atomistic simulations of activated processes in nanoparticles synthesis

Core-shell and Janus nanopartices are promising building blocks for new, highly efficient solar cells. One of the most common synthetic pathways to produce such nanostructures is the use of cation exchange reactions. Although widely used, these procedures are not completely understood. We employed classical Molecular Dynamics and Monte Carlo simulations to understand these transformation at the molecular level; in particular we investigated the conversion from CdSe (sphalerite) to PbSe (rocksalt) NPs with 2-3 nm diameter. In order to recover the equilibrium free energy surfaces we used state of the art enhanced sampling techniques, including Metadynamics. The formation of hybrid core-shell structures resulted to be an activated process, where the limiting step is the transition of a sphalerite to a rocksalt PbSe nucleus. We found that the barrier height and the stability of the two phases depend on the size of the PbSe nucleus, suggesting that the process could proceed via a two step mechanism, where a small sphalerite nucleus is formed first, and it then transforms to a rocksalt nucleus. Our results give insight into possible manipulation processes at the molecular scale, which could be used to stabilize metastable NPs and tune their physical and chemical properties.   

3D Functional Elements Deep Inside Silicon with Nonlinear Laser Lithography

Functional optical and electrical elements fabricated on silicon (Si) constitute fundamental building blocks of electronics and Si-photonics. However, since the highly successful established lithography are geared towards surface processing, elements embedded inside Si simply do not exist. Here, we present a novel direct-laser writing method for positioning buried functional elements inside Si wafers. This new phenomenon is distinct from previous work, in that the surface of Si is not modified. By exploiting nonlinear interactions of a focused laser, permanent refractive index changes are induced inside Si. The imprinted index contrast is then used to demonstrate a plethora of functional elements and capabilities embedded inside Si[1]. In particular, we demonstrate the first functional optical element inside Si, the first information-storage capability inside Si, creation of high-resolution subsurface holograms, buried multilevel structures, and complex 3D architectures in Si, none of which is currently possible with other methods. This new approach complements available techniques by taking advantage of the real estate under Si, and therefore can pave the way for creating entirely new multilevel devices through electronic-photonic integration. [1]Tokel,O.,arxiv.org/abs/1409.28   

Circular photogalvanic effect in silicon nanowires

Circular photogalvanic effect (CPGE), the generation of a photocurrent whose magnitude and polarity depends on chirality of optical excitation, is demonstrated in the visible optical range in silicon nanowires, a bulk non-gyrotropic material with weak spin-orbit coupling. CPGE, which is absent in bulk Si is found to arise from interband transitions only at the metal-semiconductor contacts to Si nanowires where inversion symmetry is broken by a Schottky electric field. Furthermore, by applying a bias voltage that modulates this field, the sign and magnitude of the CPGE can be controlled. From excitation energy dependent measurements and symmetry considerations, it is argued that the [1\={1}0] surface states due to Si chains that are not aligned with the nanowire growth direction and the Schottky field produce an artificial gyrotropic optical medium that supports CPGE. This work reveals the role of the surface states in the generation of chirality-dependent photocurrents in silicon with a purely orbital-based mechanism, and also opens up new possibilities of engineering new functionalities in Si that can be integrated with conventional electronics.   

Fabrication of self-forming silver network as transparent conductive electrode with photoresist

It has been reported that a metal wire network, obtained by sputtering with a self-cracking gel film mask, can function as a TCO replacement, perhaps reducing end device cost [1]. Toward further process simplification and cost reduction, we are investigating various electroless deposition schemes to template a wire network electrode. We report here that a conventional photoresist film can be prepared with a network of microcracks and can be used as a mask to electrolessly deposit metal, e.g. silver. With this method, no vacuum chambers are required, and undeposited metal can even be recycled for additional depositions. [1] B. Han , K. Pei , Y. Huang , X. Zhang , Q. Rong , Q. Lin , Y. Guo , T. Sun , C. Guo , D. Carnahan , M. Giersig , Y. Wang , J. Gao , Z. Ren , and K. Kempa , Adv. Mater. 26, 873 (2014).