Crystalline Semiconductor Materials Research Articles

Solution-Processable liquid crystalline organic semiconductors based on 6-(Thiophen-2-yl)benzo[b]thiophene for organic thin-film transistors

Liquid crystalline organic semiconductors have garnered significant attention in the realm of organic thin-film transistors (OTFTs) due to their inherent controllability of phase and electrical properties with temperature variations. Typically, liquid crystalline semiconductors possess distinct structures comprising rigid anisotropic cores and flexible chains. Here, we sought to identify liquid crystalline semiconductor materials by appending a thiophene ring and octyl chain to the anisotropic 6-(thiophen-2-yl)benzo[b]thiophene (TBT) backbone. Among the investigated TBT derivatives, the compound featuring additional thiophene ring and octyl chains on both sides exhibited pronounced liquid crystalline characteristics, specifically the SmE phase. The optical, thermal, and electrical characteristics as well as thin film properties of the TBT derivatives were investigated. The TBT compound was employed through solution-shearing as an active layer for top-contact/bottom-gate OTFTs. The liquid-crystalline TBT (2,6-bis(5-octylthiophen-2-yl)benzo[b]thiophene) thin film annealed at low temperature (30 °C) showed hole mobility as high as 0.025 cm2/Vs and current on/off ratio of > 106. This results in a significant enhancement by an order of magnitude compared to the sample annealed at 70 °C in the SmE phase, and nearly a two-fold improvement compared to the performance observed in the sample annealed at 130 °C in the transition phase. The utilization of the TBT derivative as a p-channel material represents a significant step toward the development of novel liquid crystalline semiconductors.

Development of Large‐Diameter and Very High Purity Ge Crystal Growth Technology for Devices

AbstractHigh‐purity germanium (HPGe) single crystals find their applications as radiation detectors especially for spectroscopy of high‐energy photons and particles in nuclear physics. Growing “detector‐grade” HPGe single crystals, with tailored structural defects and controlled impurity content, uniform throughout the crystal is a challenging task in this type of crystalline semiconductor material. All different process steps for HPGe, namely, i) reducing germanium dioxide powder into Ge metal; ii) zone refining of intrinsic source material (6 N) up to an ultrahigh purity (13 N); iii) Czochralski growth of high‐purity single crystals, are developed in house. A specific preparatory coating process for zone refining is entrenched as part of this work. Gallium, a dominant impurity usually found in HPGe, can be avoided. Using these advanced process know‐hows, a technology to grow volume Ge single crystals of large diameter (3 in.) with very high purity (net carrier concentration ≈ 5 × 1010 cm−3 or equivalent to 1 part per trillion level) is successfully developed. The zone‐refined Ge bars and the crystals show p‐type conductivity. Some crystals exhibit n‐type conductivity, and n–p or p–n transition is encountered in a single growth experiment. Besides the first results of encompassing very high‐purity crystals with uniform diameter, a controlled dislocation density (≈104 cm−2) can be realized.

Direct Observation of Size-Dependent Phase Transition in Methylammonium Lead Bromide Perovskite Microcrystals and Nanocrystals.

Methylammonium (MA) lead halide perovskites have been widely studied as active materials for advanced optoelectronics. As crystalline semiconductor materials, their properties are strongly affected by their crystal structure. Depending on their applications, the size of MA lead halide perovskite crystals varies by several orders of magnitude. The particle size can lead to different structural phase transitions and optoelectronic properties. Herein, we investigate the size effect for phase transition of MA lead bromide (MAPbBr3) by comparing the temperature-dependent neutron powder diffraction patterns of microcrystals and nanocrystals. The orthorhombic-to-tetragonal phase transition occurs in MAPbBr3 microcrystals within the temperature range from 100 to 310 K. However, the phase transition is absent in nanocrystals in this temperature range. In this work, we offer a persuasive and direct evidence of the relationship between the particle size and the phase transition in perovskite crystals.

Empirical performance of polycrystalline silicon pixel circuit components for monolithic, large-area photon counting arrays

Photon counting detectors (PCDs) offer several advantages for medical x-ray imaging compared to conventional imagers. However, current PCDs, whose circuits are fabricated using crystalline silicon semiconductor material, are not well-suited for large-area imaging applications such as breast CT and kilovoltage cone-beam CT (kV CBCT) in radiotherapy. To address this challenge, prototype PCDs based on polycrystalline silicon (a semiconductor better suited for manufacture of large-area devices) were created and, in this paper, an empirical determination of the maximum count rate of individual pixel circuit components (amplifier, comparator, clock generator and counter) corresponding to those prototypes is reported. For each circuit component, test input pulses (which were generated so as to approximate those in a complete pixel circuit) were used to obtain output response waveforms from which count rate was determined. The maximum count rate (in units of counts per second, cps) for the amplifier was determined to be 20.8 kcps while that of the comparator, clock generator and counter components were determined to be 1.2 Mcps, 98 kcps and 4.9 Mcps, respectively. The comparator and counter components provide count rates beyond that required for breast CT (∼108 kcps at a pixel pitch of 330 μm) while those components as well as the clock generator exceed the rate required for kV CBCT in radiotherapy (∼72 kcps at a pitch of 400 μm). It is anticipated that new polycrystalline silicon circuit designs for the amplifier and clock generator could provide count rates sufficient for both applications.

Towards the fabrication of third generation solar cells on amorphous, flexible and transparent substrates with well-ordered and disordered Si-nanowires/pillars

Abstract Si Nanowires (NWs) are typically synthesized on limited substrates that lack essential characteristics, such as mechanical flexibility and optical transparency. Most of the synthesis processes are also costly and inhibit widespread applications enabled by NWs. Throughout this study, therefore, we have shown that ordered and disordered single crystalline silicon nanowires can be grown and transferred from Si wafer to a broad range of foreign substrates while maintaining their original order on the mother substrates. Vertically-aligned Si NWs have been successfully transferred to Ag-pre-coated glasses, transparent-conductive-oxides and metal foils that ensure ohmic contacts between Si NWs and the transferred substrates, which are critical for scalable manufacturing of electronics and opto-electronic devices. This strategy presents an opportunity to develop low-cost device manufacturing with highly crystalline semiconductor materials, and is a critical leap towards the next generation high performance core-shell Si-NWs solar cells. In order to demonstrate devices based on the transferred NWs, the NWs are coated with a thin layer of CZTS to fabricate a third generation solar cell. The devices were characterized and exhibited the highest power conversion efficiency of 1.31% reported to date, for such transferred NWs and material combinations.

Thermal conductivity of silicon at elevated temperature: Role of four-phonon scattering and electronic heat conduction

While using first-principles-based Boltzmann transport equation approach to predict the thermal conductivity of crystalline semiconductor materials has been a routine, the validity of the approach is seldom tested for high-temperature conditions. Most previous studies only focused on the phononic contribution, and neglected the electronic part. In this paper, we present a Boltzmann transport equation study on high-temperature thermal conduction in bulk silicon by considering the effects of both phonons and electrons. Both polar and bipolar contributions to the electronic thermal conductivity are calculated from first-principles. More than 25% of heat is shown to be conducted by electrons at 1500 K. The computed total thermal conductivity of silicon faithfully reproduces the measured data. The approach presented in this paper is expected to be applied to other high-temperature functional materials, and the results could serve as benchmarks and help to explain the high-temperature phonon and electron transport phenomena.

Enhanced Reduction of Thermal Conductivity in Amorphous Silicon Nitride-Containing Phononic Crystals Fabricated Using Directed Self-Assembly of Block Copolymers.

Studies have demonstrated that the thermal conductivity (κ) of crystalline semiconductor materials can be reduced by phonon scattering in periodic nanostructures formed using templates fabricated from self-assembled block copolymers (BCPs). Compared to crystalline materials, the heat transport mechanisms in amorphous inorganic materials differ significantly and have been explored far less extensively. However, thermal management of amorphous inorganic solids is crucial for a broad range of semiconductor devices. Here we present the fabrication of freestanding amorphous silicon nitride (SiNx) membranes for studying κ in an amorphous solid. To form a periodic nanostructure, directed self-assembly of cylinder-forming BCPs is used to pattern in the SiNx highly ordered, hexagonally close packed nanopores with pitch and neck width down to 37.5 and 12 nm, respectively. The κ of the nanoporous SiNx membranes is 60% smaller than the classically predicted value based on just the membrane porosity. In comparison, holes with much larger neck widths and pitches patterned by e-beam lithography lead to only a slight reduction in κ, which is closer to the classical porosity-based prediction. These results demonstrate that κ of amorphous SiNx can be reduced by introducing periodic nanostructures that behave as a phononic crystal, where the relationship between the smallest dimension of the nanostructure and the length scale of the mean-free paths of the dominant, heat-carrying phonons is critical. Additionally, changing the orientation of the hexagonal array of nanopores relative to the primary direction of heat flow has a smaller impact on amorphous SiNx than was previously observed in silicon.

Fabrication of solar cell using titanium, zinc, carbon and gold nanoparticles

Gold, titanium dioxide, zinc oxide, and carbon nanoparticles each have characteristics that can be used as the active, electron transport, and hole transport layer in a solar cell. Nanoparticle based solar cells have emerged as a good substitute for silicon solar cells. They cost lower to manufacture, as compared to producing solar cells with crystalline semiconductor material. This is because the nanoparticles can be synthesised at room temperature, while silicon solar cells need to be made using high temperature vacuum deposition method. In this research, the nanoparticles will be synthesised using submerged glow discharge plasma. After synthesis, the nanoparticle products were analysed using scanning electron microscopy to determine their size and shape. The solar cell will then be fabricated in multiple configurations. Testing on these multiple solar cells will be done under direct sunlight to determine if the cells work and how each material affects the cell.

Defect-induced Stress Imaging in Single and Multi-crystalline Semiconductor Materials

Residual stress is a key feature which has to be taken into account in all steps of fabrication and processing of semiconductor materials and devices. The photo-elastic microscope SIREX (Scanning Infrared Stress Explorer) measures the depolarization of the light of an infrared laser probe caused by stress-induced birefringence in the sample. Subsequently, a map visualizes the in-plane optical anisotropy induced by stress. Hence, the system is applied as non-destructive tool to map stress distributions with high lateral resolution (3µm) and highest possible stress sensitivity (0.1 kPa). The so-called multi-polarization analysis results in maps of the in-plane maximum shear stress magnitude Δσ and the direction of the principal stress components. These maps can reveal the local stress fields of buried defects and their alignment in distinctive crystallographic directions. The fields are usually at least one order of magnitude larger in extension than the related defects themselves. Examples of extended defect structures in different crystalline semiconductor materials are presented in this paper. The emphasis has been put on line-shaped structures such as slip lines and twins which are partly accompanied by either tensile or compressive point-like stress sources. These structures are showing model characteristics for developing formal procedures of data and image processing under industrial conditions.

Inorganic light‐emitting diode displays using micro‐transfer printing

AbstractLarge quantities of microscopic red, green, and blue light‐emitting diodes (LEDs) made of crystalline inorganic semiconductor materials micro‐transfer printed in large quantities onto rigid or flexible substrates form monochrome and color displays having a wide range of sizes and interesting properties. Transfer‐printed micro‐LED displays promise excellent environmental robustness, brightness, spatial resolution, and efficiency. Passive‐matrix and active‐matrix inorganic LED displays were constructed, operated, and their attributes measured. Tests demonstrate that inorganic micro‐LED displays have outstanding color, viewing angle, and transparency. Yield improvement techniques include redundancy, physical repair, and electronic correction. Micro‐transfer printing enables revolutionary manufacturing strategies in which microscale LEDs are first assembled into miniaturized micro‐system “light engines,” and then micro‐transfer printed and interconnected directly to metallized large‐format panels. This paper reviews micro‐transfer printing technology for micro‐LED displays.

(Invited) Gallium Nitride on Silicon

Beginning with the first GaN-based high electron mobility transistor (HEMT) fabricated in 1993, the most noteworthy advantage of III-nitride materials over other wide bandgap semiconductors is the implementation of heterostructures. The commercially employed AlGaN/GaN heterostructure exhibits a sheet charge as a result of the large spontaneous and piezoelectric polarization-induced field and large conduction band offset. This built-in field induces a two-dimensional electron gas (2DEG) that is linearly proportional to the Al-mole fraction. A 2DEG density greater than 1013 cm-2 represents a 5–10x gain above typical GaAs or InP pHEMTs. The associated mobility at this high current density is 1,300–2,000 cm2/Vs. Remarkable progress has been made in high-performance GaN HEMTs grown on a variety of substrates. In fact, the commercial success of the GaN HEMT is directly attributable to the ability to deposit high-quality thin films on non-GaN substrates. There are principally three substrates that have been utilized with heteroepitaxial GaN-based devices: sapphire, SiC, and silicon. While several other materials have been used as substrates for depositing III-Nitride thin films, many with good crystalline quality, the vast majority of research and development efforts have focused on the aforementioned substrate materials. GaN HEMT device performance design poses multiple, often divergent, challenges to the optimal substrate material selection. In other words, comparing the various substrate options, there is not one perfect material choice. The unique requirements of each application dictates the decision regarding the best substrate. When it comes to substrate size and cost, undoubtedly, silicon is best. Therefore, GaN-on-Si heteroepitaxy has leveraged the development and refinement of this near-perfect crystalline semiconductor material. Furthermore, over many decades, the silicon industry has refined a wide range of substrate electrical resistivities, both n- and p-type. While the high-quality, low-cost, and large-diameter of Si substrates are well understood, other inherent advantages of GaN-on-Si include the ability to leverage established Si processes for wafer grinding and polishing, via-hole formation, and AuSi eutectic die attach. GaN-on-Si HEMTs have employed substrates with the resistivity tailored to the application; particularly p-type for power switching and intrinsic (float-zone, highly resistive) for RF power amplifiers. Historically, the lack of early adoption of GaN-on-Si was attributable to the technical challenges presented by the ~17% lattice mismatch and ~56% thermal expansion coefficient mismatch between GaN and Si (111). To address these, development efforts led to stress-accommodating nucleation and transition layer schemes, allowing growth of crack-free and low warp (Al,Ga,In)N layers on high quality, low-cost, large-diameter Si substrates. Si has a reasonable thermal conductivity that is similar to bulk crystalline GaN, but inferior to competing substrates; principally SiC-based. This is a key design consideration for exploitation of the exceptional power density offered by GaN HEMTs. The inferior Si substrate thermal conductivity can limit device scaling, which is a factor in applications where the junction temperature rise is limited. These mismatches have been researched extensively and several techniques to overcome their impact on epitaxial crystal quality have been developed, and will be discussed. Silicon’s “thermal stability,” meaning its relative inertness at typical GaN growth temperatures, greater than 1000 °C, is also inferior to SiC and sapphire. This can result in auto-doping of the Si substrate during nucleation of the GaN epitaxy. In particular, the Al and Ga, whether introduced in the growth chamber intentionally or unintentionally, can diffuse into the silicon from the gas phase creating a p-doped layer at the surface of the substrate. While considerable work has been done to mitigate the auto-doping effect for RF amplifier applications at higher frequencies (above ~ 6GHz) the capacitive coupling of this conductive layer can limit the dc-to-RF conversion efficiency of the device. These efforts will be reviewed and compared. Finally, the generally applied principle for fabricating GaN-on-Si devices has been to utilize traditional III-V processes. However, recent efforts have also incorporated more “Si-like” processing techniques to leverage existing silicon fab excess capacity. The heteroepitaxial growth and processing techniques developed for GaN-on-Si HFET fabrication will be reviewed, and future directions will be offered.

(Invited) Direct Electrodeposition of Crystalline III-V Semiconductor Films

This presentation will describe our latest results on the concept of using liquid metal electrodes as platforms for direct electrodeposition of crystalline inorganic semiconductor materials at comparatively low temperatures. Large scale production of crystalline thin films of III-V semiconductors is challenging in two ways. First, the majority of synthetic methods are energy- and resource-intensive. For example, vapor phase crystal growth techniques such as metal-organic vapor phase epitaxy and molecular beam epitaxy involve ultra-high vacuum (UHV) equipment and temperatures above 400 °C. Similarly, liquid phase epitaxy requires sustained temperatures in excess of 750 °C and pressurized furnaces. Second, these same fabrication methods can be difficult to incorporate directly into device fabrication processes. For example, high temperatures and corrosive reagents can damage delicate electronic device architectures (e.g. complementary metal-oxide-semiconductor (CMOS)) and sensitive platforms (e.g. plastics). As a result, cumbersome and costly transfer and integration steps are necessary. Electrodeposition of III-V semiconductors has been extensively investigated since electrochemistry offers precise control of (heterogeneous) reaction rates and simple process electronics and equipment (i.e. a current or potential source and a beaker). However, stoichiometry, purity, and crystallinity have proven difficult to control. Here, experimental data will be shown for a new tactic that offers the purity and quality of aforementioned high temperature, vapor-phase syntheses with the simplicity and affordability of conventional electrodeposition. Specifically, data will be presented that illustrates the preparation of polycrystalline films of GaAs, GaSb, InAs, InSb, and alloys thereof through an electrochemical liquid-liquid-solid (ec-LLS) process. Chronoamperometic and voltammetric responses recorded during ec-LLS will be shown. Materials characterization data in the form of electron & X-ray diffraction, X-ray photoelectron & Raman spectroscopies, and optical & electron microscopies will be also be discussed. Comparisons and distinctions with other, previously demonstrated electrochemical, chemical vapor deposition, and chemical metallurgical processes will be presented. The prospects for making high quality (epitaxial) III-V films and for seemless incorporation of III-V films on sensitive device platforms through ec-LLS will be discussed.

The Electrodeposition of Crystalline Gallium Antimonide Using Electrochemical Liquid-Liquid Solid Deposition (ec-LLS)

The growth of crystalline III-V semiconductor material from aqueous solutions has maintained a challenging task in the field of semiconductor electrodeposition. Electrochemical liquid-liquid solid deposition (ec-LLS) is a method in which an applied potential is used to grow crystalline semiconductors from a liquid metal electrode/solvent.1 In this study crystalline gallium antimonide (c-GaSb) films were electrodeposited from both Sb­2O3 and SbCl3 precursors in aqueous electrolyte via ec-LLS at mild temperatures (40o-90oC) and ambient pressure. The electrodeposition of c-GaSb films has been previously attempted2,3 but the deposits contained large amounts of excess antimony and required post-deposition annealing steps. However, in the present study c-GaSb films with stoichiometric amounts of Ga and Sb were obtained without any post-deposition annealing. In addition, the growth conditions were varied and the effects on the grain size, texture and composition of the resultant films were examined using X-ray diffraction, energy-dispersive X-ray spectroscopy and Raman spectroscopy. The results showed that the temperature, precursor concentration and pH were the most significant parameters that influenced the crystalline quality and composition of the as-grown films.

Determining Microwire Morphology and Current Transient Trends from Single Microwire Electrodeposition

This presentation describes work on understanding and controlling the electrochemical liquid-liquid-solid (ec-LLS) process for the preparation of crystalline semiconductor materials. In this work, data are presented to test the hypothesis that the current-time transient collected during a potentiostatic ec-LLS deposition of Ge microwires contains useful details about the crystal growth. A photoresist pattern with a single hole was fabricated on top of a conductive support to make an ultramicroelectrode (UME) with a defined diameter ranging between 10 and 35 microns. These single-hole UMEs were filled with a liquid metal which acted as the electrode and crystal solvent in ec-LLS. Individual germanium microwires were then electrodeposited onto the liquid metal by chronoamperometry from an aqueous solution using a variety of different potential, temperature, and liquid metal conditions. Current transients will be discussed in relation to their correlation with morphological and crystallographic features of as-prepared Ge microwires. The prospects for real-time adjustment of microwire growth during an ec-LLS process will be presented.

Comparison of defects in crystalline oxide semiconductor materials by electron spin resonance

Defects in crystalline InGaZnO4 (IGZO) induced by plasma were investigated using electron spin resonance (ESR). Thermal stabilities and ɡ factors of two ESR signals (A and B observed at ɡ = 1.939 and 2.003, respectively) in IGZO were different from those of the ESR signals observed in component materials such as Ga2O3 (signal observed at ɡ = 1.969), In2O3 (no signal), and ZnO (signal observed at ɡ = 1.957). Signal A in IGZO increased upon annealing at 300 °C for 1 h, but decreased when annealing was continued for more than 2 h. On the other hand, signal B decreased upon annealing at 300 °C for 1 h. The ESR signal in ZnO decayed in accordance with a second-order decay model with a rate constant of 2.1 × 10−4 s−1; however, this phenomenon was not observed in other materials. This difference might have been due to randomly formed IGZO lattices such as asymmetrical (Ga, Zn)O and In-O layers. Defects in signals A and B in IGZO were formed in trap states (at the deep level) and tail states, respectively.

Invited: Metal-Semiconductor-Metal Photocurrent Chip for Hydrogen Peroxide and Biomolecular Sensing with Chemiluminescence

Photodetectors are generally the most promising, representative and widely-used type of detectors, and the applications of photodetectors are quite extensively[1-3]. Basically photodetectors are made of photosensitive elements which are able to turn the light signal into electrical signal. Nowadays, the detecting wavelength region is very broad and can be classified into infrared, visible light and ultraviolet zone; furthermore, the applications of each region are manifold especially in visible light region. Additionally owing to the nature of fast responding speed, it promises photodetectors an advantageous future in biomedical sensing territory[4-5]. This study is focus on fabrication method of the flexible[6-8] photodetectors and application to biomolecule sensing through luminescence. The photosensitive element is based on single crystalline silicon semiconductor material which is treated with chemical etching process to achieve pliable characteristic[9]. Then, the metal-semiconductor-metal[10-14]structural flexible photodetectors are achieved after integrated with simple fabricating processes. In detail, the accomplished devices are handled with immobilization steps to fix the target enzyme which is able to catalyze the luminescent reaction (Fig.1). Owing to only extremely stable sensor for biomedical use, it is necessary to ensure the operation function at any crucial conditions. In this study, we provide a portable biomolecule sensor with outstanding flexible characteristic and high sensitivity and it exhibits illustrious stable consequence in various situations. Figure 1.Schematic diagram of immobilization process flow and luminescence detection.A pliable silicon chip measured under fifty micrometers in thickness is achieved through a simple method via alkaline etchant. The silicon chip with designed thickness is featured of great flexibility and relatively little fragility. A surface modification is applied to the flexible silicon chip with non-toxic and easy-arranged texturing solution from texturization process. Owing to anisotropic etching, the three-dimensional structure of pyramidal shape is constructed onto the chip surface. The reflectivity decreases advantageously in virtue of increasing roughness, and the final reflectivity is only one-fourth in comparison with original smooth chip surface due to formation of surface microstructure. The flexible MSM photodetectors are accomplished after mantling metal electrode with well-designed pattern. The semiconductor material we used is featured of high natural abundance and the manufacturing processes are considerably uncomplicated.In taking advantage of immobilization technique, the targeting enzyme is bound to photodetector devices and as-prepared devices are employed in optical sensing of luminescence which is catalyzed by the immobilized enzyme. This optical sensor exhibits great flexibility and remarkable durability and shows marvelous stability even in various stretching conditions. Furthermore, the photodetector devices are featured portable and cost-efficient. A novel application of this detector in healthcare field which is a significant issue nowadays is performed on available hair dye products. The examination is taken into consideration owing to the potential damage caused by hydrogen peroxide of which the main ingredient in hair dye products (Fig.2). For this interdisciplinary-technology-based device, the flexible optical sensor can be utilized in multiple regions and it can be integrated with promising instruments in virtue of great mechanical property. Figure 2.Hydrogen peroxide detections for hair dye samples through luminescence sensing by flexible MSM photodetectors.

Low Temperature Geometrically Confined Growth of Pseudo Single Crystalline GeSn on Amorphous Layers for Advanced Optoelectronics

The geometrically confined growth of highly crystalline semiconductor materials like Ge on amorphous layers (without epitaxial template) has been a challenge in materials science. The success of this technology, especially at low fabrication temperature can enable 3D Si photonics by building photonic devices on metal/interconnect layers, and enable inexpensive, high-throughput fabrication of devices such as tandem solar cell and thin-film transistor.Here we report that, by incorporating Sn into Ge, we are able to obtain highly crystalline GeSn thin film on amorphous substrates at low crystallization temperatures ranging from 340 to 464 ºC through enhanced crystallization from the Ge-Sn euetectic system, and are able to transform the material toward a direct band-gap material through band structure modification. A highly textured Ge0.91Sn0.09 is obtained from the sample with 9.5 at. % as-deposited Sn composition, which shows grain sizes up to tens of microns, as revealed by electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and Raman spectroscopy. EBSD confirms that most grain boundaries are either twin or low angle boundaries, which are benign to optoelectronic properties. Strikingly, the nucleation center distances range from 0.1 to 1 mm, orders of magnitude larger than common solid state crystallization, which indicates an exceedingly high grain growth rate vs. a low nucleation rate. As a result of this feature, we are able to fabricate geometrically confined pseudo single crystalline GeSn grain using patterning techniques. Another remarkable result is that 9 at.% Sn is incorporated substitutionally into Ge, far exceeding the equilibrium solubility limit of ~1%. The high Sn composition, with the addition of ~0.24% tensile strain in the film, shifts the direct band gap to 0.5 eV (2500nm), right on the verge of indirect-to-direct band gap transition. This transition will greatly enhance the material’s optoelectronic performance as a photonic device, and will also broaden the absorption spectrum for a tandem solar cell.

High-precision transfer-printing and integration of vertically oriented semiconductor arrays for flexible device fabrication

Flexible electronics utilizing single crystalline semiconductors typically require post-growth processes to assemble and incorporate the crystalline materials onto flexible substrates. Here we present a high-precision transfer-printing method for vertical arrays of single crystalline semiconductor materials with widely varying aspect ratios and densities enabling the assembly of arrays on flexible substrates in a vertical fashion. Complementary fabrication processes for integrating transferred arrays into flexible devices are also presented and characterized. Robust contacts to transferred silicon wire arrays are demonstrated and shown to be stable under flexing stress down to bending radii of 20 mm. The fabricated devices exhibit a reversible tactile response enabling silicon based, nonpiezoelectric, and flexible tactile sensors. The presented system leads the way towards high-throughput, manufacturable, and scalable fabrication of flexible devices.

Structural Dynamics of GaN Microcrystals in Evolutionary Selection Selective Area Growth probed by X-ray Microdiffraction

A method to grow high quality, single crystalline semiconductor material irrespective of the substrate would allow a cost-effective improvement to functionality and performance of optoelectronic devices. Recently, a novel type of substrate-insensitive growth process called Evolutionary Selection Selective Area Growth (ES-SAG) has been proposed. Here we report the use of X-ray microdiffraction to study the structural properties of GaN microcrystals grown by ES-SAG. Utilizing high resolution in both direct and reciprocal spaces, we have unraveled structural dynamics of GaN microcrystals in growth structures of different dimensions. It has been found that the geometric proportions of the growth constrictions play an important role: 2.6 μm and 4.5 μm wide growth tunnels favor the evolutionary selection mechanism, contrary to the case of 8.6 μm growth tunnels. It was also found that GaN microcrystal ensembles are dominated by slight tensile strain irrespective of growth tunnel shape.

Organo‐Gallium/Indium Chalcogenide Complexes of Copper(I): Molecular Structures and Thermal Decomposition to Ternary Semiconductors

Several organo-gallium/indium chalcogenide complexes of copper(I), stabilized by trialkylphosphines, were isolated, structurally characterized by using single-crystal X-ray diffraction, and investigated in thermolysis experiments. The syntheses with [E(Me3Si)2] (E=S, Se) as a starting material and a chalcogen source involve the elimination of volatile silyl acetate, silyl ethers, and methane from copper(I) acetate, and Group 13 metal trimethyl compounds, respectively. Chalcogenide complexes, according to the general formulas [(R3PCu)4(MeM)4E6] (1-6) and [(R3PCu)6(MeM)4M4S13] (7-9; with R=alkyl and M=Ga, In), and mixed chalcogenide-phenylchalcogenolate complexes [(iPr3PCuEPh)3(MeGaE)4] (10, 11) were isolated. The heavy atom cores of 1-6 consist of an octahedron of chalcogen atoms, interpenetrated by a cube of metal atoms. Depending on the steric demand of the phosphine ligands, two constitutions are observed; the metal atoms of the same element either forming tetrahedra, or parallelograms, respectively. This constitutional isomerism is further investigated by quantum chemical calculations. Complexes 7-9 contain a central sulfur atom, surrounded by two interpenetrating tetrahedra of Group 13 metal atoms, an octahedron of copper atoms, and an icosahedron of twelve outer sulfur atoms; the heavy atom framework of 10 and 11 is a "cut-out" of this structure. Thermolysis experiments include thermogravimetry measurements and subsequent Rietveld phase analysis of the residues by using powder X-ray diffraction. The homologous compounds 1, 3, 4, and 6 yield the respective crystalline ternary semiconductor material CuME2 at temperatures below 300 °C. Partial release of Me3 M during the thermolysis process results in excess copper in the residue and therefore in small amounts of additional binary copper chalcogenide phases or metallic CuM alloys. Compound 8 produces nanocrystalline CuGaS2 at about 300 °C.