Semiconductor Single Crystals Research Articles

Sub-5 μm-thick spalled single crystal Si foils by decoupling crack initiation and propagation

Spalling of a brittle semiconductor substrate is an attractive method for fabricating sub-50 μm thick single crystal semiconductor wafers in large area. Here, a spalling process to fabricate single crystal Si foils with controlled thicknesses ranging from sub-5 to 38 μm is demonstrated using electroplated Ni stressor layers on Si substrates. In this study, the thickness profile of the Ni stressor layer was varied by changing the Ni electroplating current density, and the effect of thickness variation on the crack initiation and propagation in the underlying Si substrate was evaluated. The critical Ni thickness for crack initiation in our Ni/Si bilayer system was determined and we also show that the crack propagation depth, i.e., spalled Si thickness, is proportional to the thickness of the Ni layer at the central area of the Si wafer. Ni layer plated using 40 mA/cm2 resulted in sub-5 μm thick Si foil. In addition, the spalling mechanics of a Ni/Si bilayer system was analytically explored to give insight into the requirements of the Ni stressor layer for crack initiation.

Dense Assembly of Soluble Acene Crystal Ribbons and Its Application to Organic Transistors.

The preparation of uniform large-area highly crystalline organic semiconductor single crystals remains a challenge in the field of organic field-effect transistors (OFETs). Crystal densities in the channel regions of OFETs have not yet reached sufficiently high values to provide efficient charge transport, and improving channel crystal densities remains an important research area. Herein we fabricated densely well-aligned single crystal arrays of the 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS_PEN) semiconductor using a straightforward scooping-up (SU) methodology to quickly produce a large-area self-assembled semiconductor crystal layer. The resulting crystalline TIPS_PEN strip arrays obtained using the SU method revealed a packing density that was 2.76 times the value obtained from the dip-coated channel, and the mean interspatial distance between the crystal strips decreased from 21.5 to 7.8 μm. The higher crystal packing density provided efficient charge transport in the FET devices and directly yielded field-effect mobilities as high as 2.16 cm(2)/(V s). These field-effect mobilities were more than three times the values obtained from the OFETs prepared using dip-coated channels. Furthermore, the contact resistance between the source/drain electrodes and the TIPS_PEN crystals decreased by a factor of 2. These contributions represent a significant step forward in improving semiconductor crystal alignment for the fabrication of large-area high-performance organic electronics.

Growth and Etch Forms of Germanium Microcrystals on a Silicon Oxide Substrate

In the fabrication of electronic and photonic devices, it is often necessary to control the geometric forms of single-crystal semiconductors—such as silicon or germanium—to optimize the device performance. This work presents a series of geometric forms of germanium microcrystals after either germanium growth or etch, which can change the size and shape of an existing crystal and bring it to a distinctive polyhedral form. This form is usually different from the equilibrium form, which minimizes the sum of surface free energy and strain energy for a given crystal. Germanium crystals are first formed on a substrate of thermal silicon oxide in a Centura® 300 mm RP Epi chamber from Applied Materials. Germane (GeH4) is used as the germanium source. These germanium crystals are then thermally annealed. After anneal, the crystals are approximately 5 to 10 μm in diameter. Fig. 1 shows the scanning electron micrograph of (a) a small and (b) a large germanium crystal after a 850°C anneal. After anneal the crystals are near equilibrium, and certain low-index faces can be seen. The largest appear to be {113} faces, while {111} and {110} faces can also be resolved. The surface between these faces is rounded. The orientation of the visible faces with respect to the substrate appears to be random. Only the part of the crystal surface not in contact with the substrate, as shown in Fig. 1, will be discussed. Further growth of germanium on a crystal in Fig. 1(a) makes certain faces larger and the rounded surface smaller. Fig. 2 shows the scanning electron micrograph of a Fig. 1(a) crystal with more germanium growth at 700°C. The majority of the surface is covered by {113}, {111}, {110}, and {100} faces. The crystal in Fig. 2 may experience more change in shape and size if the germanium growth were to be continued. Fig. 3(a) shows a germanium crystal in Fig. 1(b) after a 30 sec. etch in chlorine at 330°C. {113} faces are dominant as in Fig. 2. {111} and {110} faces have mostly disappeared. Between neighboring (113), (131), and (311) faces are sharp edges. These edges can still be seen, as in Fig. 3(b), after a 180 sec. etch. This etch form is clearly different from both the equilibrium form in Fig. 1 and the growth form in Fig. 2. The etch form of a germanium crystal can depend on the etch conditions. A 560°C etch of a crystal in Fig. 1(b) in hydrogen chloride is shown in Fig. 4. After a 15 sec. etch, the surface is mostly covered by faces that are close to {103} (Fig. 4(a)). These faces are generally rough, somewhat rounded, and partly covered by shallow triangular pyramids. The surface becomes even rougher after a 30 sec. etch (Fig. 4(b)), although the nearly {103} oriented faces and the edges between them can still be resolved. The crystal forms shown in this work are determined by the atomic processes of growth and etch as well as the diamond structure of germanium. These forms therefore provide important information about the site-specific reactivity on germanium surfaces in growth and etch. Figure 1

Prompt quality monitoring of InSe and GaSe semiconductor crystals by the nuclear quadrupole resonance technique

To determine the quality of layer-structured semiconductor single crystals, the nuclear quadrupole resonance technique with successive scanning of the entire sample volume and assessment of the structural quality on the basis of the observed spectra is used. The proposed method is appropriate in the case of InSe, GaSe and GaAs ingots grown in evacuated cells by the Bridgman technique and can be used repeatedly in subsequent technological procedures with no operator access to the material. To provide scanning within a limited region of the sample and to ensure efficient interaction of the high-frequency field with the crystal, excitation and detection of the nuclear-spin-induction signal is implemented by the saddle-shaped transceiver coil of the nuclear quadrupole resonance spectrometer.

Vertical Single-Crystalline Organic Nanowires on Graphene: Solution-Phase Epitaxy and Optical Microcavities.

Vertically aligned nanowires (NWs) of single crystal semiconductors have attracted a great deal of interest in the past few years. They have strong potential to be used in device structures with high density and with intriguing optoelectronic properties. However, fabricating such nanowire structures using organic semiconducting materials remains technically challenging. Here we report a simple procedure for the synthesis of crystalline 9,10-bis(phenylethynyl) anthracene (BPEA) NWs on a graphene surface utilizing a solution-phase van der Waals (vdW) epitaxial strategy. The wires are found to grow preferentially in a vertical direction on the surface of graphene. Structural characterization and first-principles ab initio simulations were performed to investigate the epitaxial growth and the molecular orientation of the BPEA molecules on graphene was studied, revealing the role of interactions at the graphene-BPEA interface in determining the molecular orientation. These free-standing NWs showed not only efficient optical waveguiding with low loss along the NW but also confinement of light between the two end facets of the NW forming a microcavity Fabry-Pérot resonator. From an analysis of the optical dispersion within such NW microcavities, we observed strong slowing of the waveguided light with a group velocity reduced to one-tenth the speed of light. Applications of the vertical single-crystalline organic NWs grown on graphene will benefit from a combination of the unique electronic properties and flexibility of graphene and the tunable optical and electronic properties of organic NWs. Therefore, these vertical organic NW arrays on graphene offer the potential for realizing future on-chip light sources.

Nanosecond X-ray detector based on high resistivity ZnO single crystal semiconductor

The pulse radiation detectors are sorely needed in the fields of nuclear reaction monitoring, material analysis, astronomy study, spacecraft navigation, and space communication. In this work, we demonstrate a nanosecond X-ray detector based on ZnO single crystal semiconductor, which emerges as a promising compound-semiconductor radiation detection material for its high radiation tolerance and advanced large-size bulk crystal growth technique. The resistivity of the ZnO single crystal is as high as 1013 Ω cm due to the compensation of the donor defects (VO) and acceptor defects (VZn and Oi) after high temperature annealing in oxygen. The photoconductive X-ray detector was fabricated using the high resistivity ZnO single crystal. The rise time and fall time of the detector to a 10 ps pulse electron beam are 0.8 ns and 3.3 ns, respectively, indicating great potential for ultrafast X-ray detection applications.

Suppressing molecular vibrations in organic semiconductors by inducing strain.

Organic molecular semiconductors are solution processable, enabling the growth of large-area single-crystal semiconductors. Improving the performance of organic semiconductor devices by increasing the charge mobility is an ongoing quest, which calls for novel molecular and material design, and improved processing conditions. Here we show a method to increase the charge mobility in organic single-crystal field-effect transistors, by taking advantage of the inherent softness of organic semiconductors. We compress the crystal lattice uniaxially by bending the flexible devices, leading to an improved charge transport. The mobility increases from 9.7 to 16.5 cm2 V−1 s−1 by 70% under 3% strain. In-depth analysis indicates that compressing the crystal structure directly restricts the vibration of the molecules, thus suppresses dynamic disorder, a unique mechanism in organic semiconductors. Since strain can be easily induced during the fabrication process, we expect our method to be exploited to build high-performance organic devices.

Ultra‐Thin Layered Ternary Single Crystals [Sn(SxSe1−x)2] with Bandgap Engineering for High Performance Phototransistors on Versatile Substrates

2D ternary semiconductor single crystals, an emerging class of new materials, have attracted significant interest recently owing to their great potential for academic interest and practical application. In addition to other types of metal dichalcogenides, 2D tin dichalcogenides are also important layered compounds with similar capabilities. Yet, multi‐elemental single crystals enable to assist multiple degrees of freedom for dominant physical properties via ratio alteration. This study reports the growth of single crystals Se‐doped SnS2 or SnSSe alloys, and demonstrates their capability for the fabrication of phototransistors with high performance. Based on exfoliation from bulk high quality single crystals, this study establishes the characteristics of few‐layered SnSSe in structural, optical, and electrical properties. Moreover, few‐layered SnSSe phototransistors are fabricated on both rigid (SiO2/Si) and versatile polyethylene terephthalate substrates and their optoelectronic properties are examined. SnSSe as a phototransistor is demonstrated to exhibit a high photoresponsivity of about 6000 A W−1 with ultra‐high photogain ≈8.8 × 105, fast response time ≈9 ms, and specific detectivity (D*) ≈8.2 × 1012 J. These unique features are much higher than those of recently published phototransistors configured with other few‐layered 2D single crystals, making ultrathin SnSSe a highly qualified candidate for next‐generation optoelectronic applications.

Self-Aligned Growth of Organic Semiconductor Single Crystals by Electric Field.

We proposed a novel but facile method for growing organic semiconductor single-crystals via solvent vapor annealing (SVA) under electric field. In the conventional SVA growth process, nuclei of crystals appeared anywhere on the substrate and their crystallographic axes were randomly distributed. We applied electric field during the SVA growth of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) on the SiO2/Si substrate on which a pair of electrodes had been deposited beforehand. Real-time observation of the SVA process revealed that rodlike single crystals grew with their long axes parallel to the electric field and bridged the prepatterned electrodes. As a result, C8-BTBT crystals automatically formed a field effect transistor (FET) structure and the mobility reached 1.9 cm(2)/(V s). Electric-field-assisted SVA proved a promising method for constructing high-mobility single-crystal FETs at the desired position by a low-cost solution process.

ChemInform Abstract: Single‐Crystal Semiconductors with Narrow Band Gaps for Solar Water Splitting

AbstractReview: [emphasis on Si‐ and InP‐based photocathodes; 53 refs.

Mesoporous TiO2 Mesocrystals: Remarkable Defects-Induced Crystallite-Interface Reactivity and Their in Situ Conversion to Single Crystals.

Oriented self-assembly between inorganic nanocrystals and surfactants is emerging as a route for obtaining new mesocrystalline semiconductors. However, the actual synthesis of mesoporous semiconductor mesocrystals with abundant surface sites is extremely difficult, and the corresponding new physical and chemical properties arising from such an intrinsic porous mesocrystalline nature, which is of fundamental importance for designing high-efficiency nanostructured devices, have been rarely explored and poorly understood. Herein, we report a simple evaporation-driven oriented assembly method to grow unprecedented olive-shaped mesoporous TiO2 mesocrystals (FDU-19) self-organized by ultrathin flake-like anatase nanocrystals (∼8 nm in thickness). The mesoporous mesocrystals FDU-19 exhibit an ultrahigh surface area (∼189 m2/g), large internal pore volume (0.56 cm3/g), and abundant defects (oxygen vacancies or unsaturated Ti3+ sites), inducing remarkable crystallite-interface reactivity. It is found that the mesocrystals FDU-19 can be easily fused in situ into mesoporous anatase single crystals (SC-FDU-19) by annealing in air. More significantly, by annealing in a vacuum (∼4.0 × 10–5 Pa), the mesocrystals experience an abrupt three-dimensional to two-dimensional structural transformation to form ultrathin anatase single-crystal nanosheets (NS-FDU-19, ∼8 nm in thickness) dominated by nearly 90% exposed reactive (001) facets. The balance between attraction and electrostatic repulsion is proposed to determine the resulting geometry and dimensionality. Dye-sensitized solar cells based on FDU-19 and SC-FDU-19 samples show ultrahigh photoconversion efficiencies of up to 11.6% and 11.3%, respectively, which are largely attributed to their intrinsic single-crystal nature as well as high porosity. This work gives new understanding of physical and chemical properties of mesoporous semiconductor mesocrystals and opens up a new pathway for designing various single-crystal semiconductors with desired mesostructures for applications in catalysis, sensors, drug delivery, optical devices, etc.

Nuclear magnetic resonance study of a Bi2Te3 topological insulator

The results of the nuclear magnetic resonance (NMR) study of a grown high-quality Bi2Te3 semiconductor single crystal have been presented. Signals from the 125Te isotope were detected by the spin echo method in the range from 10 K to room temperature. It was found that the NMR spectrum consists of two lines. The line with a positive shift of the resonance frequency corresponded to the bulk of the sample. The line with a negative shift was interpreted as a signal from the surface of the single crystal. The temperature and orientational dependences of the positions of the NMR line of nuclei in the bulk of the crystal were studied. It was shown that the shifts are mainly determined by the Knight shift due to the interaction with mobile charge carriers. The thermoactivation character of the concentration of mobile charges in the crystal under study, which corresponds to the intrinsic conductance, was proved, and the energy parameters of the thermoactivation processes were calculated.

Two-Photon Absorption in Organometallic Bromide Perovskites.

Organometallic trihalide perovskites are solution-processed semiconductors that have made great strides in third-generation thin film light-harvesting and light-emitting optoelectronic devices. Recently, it has been demonstrated that large, high-purity single crystals of these perovskites can be synthesized from the solution phase. These crystals' large dimensions, clean bandgap, and solid-state order have provided us with a suitable medium to observe and quantify two-photon absorption in perovskites. When CH3NH3PbBr3 single crystals are pumped with intense 800 nm light, we observe band-to-band photoluminescence at 572 nm, indicative of two-photon absorption. We report the nonlinear absorption coefficient of CH3NH3PbBr3 perovskites to be 8.6 cm GW(-1) at 800 nm, comparable to epitaxial single-crystal semiconductors of similar bandgap. We have leveraged this nonlinear process to electrically autocorrelate a 100 fs pulsed laser using a two-photon perovskite photodetector. This work demonstrates the viability of organometallic trihalide perovskites as a convenient and low-cost nonlinear absorber for applications in ultrafast photonics.

Single-Crystal Semiconductors with Narrow Band Gaps for Solar Water Splitting.

Solar water splitting provides a clean and renewable approach to produce hydrogen energy. In recent years, single-crystal semiconductors such as Si and InP with narrow band gaps have demonstrated excellent performance to drive the half reactions of water splitting through visible light due to their suitable band gaps and low bulk recombination. This Minireview describes recent research advances that successfully overcome the primary obstacles in using these semiconductors as photoelectrodes, including photocorrosion, sluggish reaction kinetics, low photovoltage, and unfavorable planar substrate surface. Surface modification strategies, such as surface protection, cocatalyst loading, surface energetics tuning, and surface texturization are highlighted as the solutions.

Thin-Film Vapor-Liquid-Solid Growth of Compound Semiconductors on Heteroepitaxial and Amorphous Substrates

We demonstrate the versatility of a previously developed low-cost thin-film vapor-liquid-solid (TF-VLS) compound semiconductor growth technique. Specifically, we show that the quality of compound semiconductors grown via this technique enables a variety of optoelectronic and energy devices with geometries and material combinations unavailable to traditional vapor-phase growth techniques. Critically, this is enabled by the confinement of a liquid metal solvent layer, enabling precise control over the position and shape of nuclei on non-epitaxial and hetero-epitaxial substrates simply by controlling the shape of the solvent. Unlike vapor-phase processes, in which control over the shape of a growing nuclei is extremely challenging and primarily defined by the growth rates of various crystal facets, the TF-VLS process enables full control over nuclei, enabling fabrication of single crystal and ultra-large grain polycrystalline compound semiconductors regardless of substrate microstructure. This control over the material is then utilized to demonstrate compound semiconductor devices for energy harvesting, photon generation, and photon detection.

High Efficiency Millimeter-Scale Crystalline Perovskite Solar Cells

Current state-of-the-art photovoltaics utilize high purity single-crystal semiconductors grown by sophisticated, high temperature crystal-growth processes. Alternative emerging solar cell technologies based on solution-processed materials promise to deliver power at lower cost, but so far have had limited success due to the presence of defects in the bulk and at the grain boundaries. Improving the crystal size and quality of solution-processed films could lead to unprecedented improvements in photovoltaic performance. However, large-scale crystal growth is challenging in solution-based deposition where molecular kinetics are limited by the low processing temperature. Here, for the first time we demonstrate organometallic perovskites thin-films composed of extra-ordinarily large crystalline grains (1-2 mm), fabricated using a new processing technique. The increase in the grain-size directly correlates to a dramatic increase in the efficiency, as a direct consequence of increased charge carrier mobility and reduction in defect densities. Our measured efficiency values approach ~18%, which is among the highest reported. These devices are extremely robust, exhibiting no degradation with voltage sweep direction or the rate at which the voltage was scanned. We anticipate that this technique for growing high quality large-area crystals at low temperature with low defect concentration will be universally applicable to a wide variety of other material systems thus providing a solution to a long-standing scientific challenge of overcoming polydispersity, defects and grain boundary recombination in solution-processed thin-films. From the perspective of the global photovoltaics community, these results are expected to lead the field towards the synthesis of wafer-scale crystalline perovskites necessary for the fabrication of high-efficiency single-junction and hybrid (semiconductor and perovskite) tandem planar cells. In addition, I will also discuss design principles that govern the photostability in these systems.

Direct Imaging of Highly Anisotropic Photogenerated Charge Separations on Different Facets of a Single BiVO4 Photocatalyst.

Spatially resolved surface photovoltage spectroscopy (SRSPS) was employed to obtain direct evidence for highly anisotropic photogenerated charge separation on different facets of a single BiVO4 photocatalyst. Through the controlled synthesis of a single crystal with preferentially exposed {010} facets, highly anisotropic photogenerated hole transfer to the {011} facet of single BiVO4 crystals was observed. The surface photovoltage signal intensity on the {011} facet was 70 times stronger than that on the {010} facets. The influence of the built-in electric field in the space charge region of different facets on the anisotropic photoinduced charge transfer in a single semiconductor crystal is revealed.

Oriented mesostructure for energy conversion

Semiconductors The orientation-controlled growth of mesoporous channels in single-crystal semiconductors is extremely difficult but highly desirable in optical devices. Zhao et al. used a simple evaporation process to synthesize three-dimensional open mesoporous titanium dioxide microspheres with uniform size (~800 nm), well-controlled radially oriented mesoporous channels, and “single-crystal”–like walls. Dye-sensitized solar cells using such microspheres showed good power conversion efficiency up to 12%. This evaporation-driven approach may provide a path for tailoring the orientation and self-assembly of other mesoporous metallic oxides and open the door for their potential use in optoelectronic devices. Sci. Adv. 10.1126/sciadv.1500166 (2015).

(Invited) Use of the Doping Dependence of Collection Efficiencies for Dye Sensitized Photocurrents to Demonstrate Physical Models for Charge Transfer at Single Crystal Oxide Semiconductor Surfaces

The different physical limits of an Onsager-based model for charge collection at a semiconductor electrode are revealed through an experimental examination of the doping density dependence of photocurrents at single crystal ruitile TiO2 electrodes sensitized with the N3 chromophore and a trimethine cyanine dye. As the doping density of the electrodes was varied from 1015 cm-3 to 1020 cm-3, three different regimes of behavior were observed for the magnitude and shape of the dye sensitized current-voltage curves. Low-doped crystals produced current-voltage curves with a slow rise of photocurrent with potential. At intermediate doping levels, Schottky barrier behavior was observed producing a photocurrent plateau at an electrode bias in the depletion region. At highly doped electrodes tunneling currents played a significant role, especially in the recombination processes. The fitting revealed the role of the various physical parameters that govern photoinduced charge collection in sensitized systems.

Correlation between thermal fluctuation effects and phase coherence factor in carrier transport of single-crystal organic semiconductors

We find that the phase coherence factor derived from Hall effect measurements of single-crystal thin-film field-effect transistors of pentacene, which relates the intrinsic charge transport with the phase coherence, has a strong correlation with the thermal fluctuations of transfer energies between neighboring molecules. This observation also holds true for other organic semiconductors such as tetracene, dianthrathiophene (DAT)-V, and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT). This gives us clues for constructing flexible molecular systems with high carrier mobility.