Semiconductor Single Crystals Research Articles

Perovskite CsPbBr3 Single Crystal Detector for High Flux X-Ray Photon Counting.

X-ray photon-counting detectors capable of resolving the energies of single X-ray photons are critical in medical imaging, and a high count rate is essential for photon-counting detectors. Here, we report the performance of the perovskite CsPbBr3 single-crystal semiconductor detector for X-ray photon counting. The CsPbBr3 detector noise floor, energy response linearity, energy resolution, count rate, and polarization were evaluated. By fine-tuning the detector working conditions, our CsPbBr3 detector with a planar electrode can count an incident X-ray photon rate of ~0.099 and ~0.336 Mcps/pixel at 10% and 30% deadtime loss, respectively, with corresponding energy resolutions of ~18% at 59.5 keV and ~12% at 122 keV at same pulse processing conditions. We also demonstrated that our CsPbBr3 detectors show negligible polarization under an X-ray flux of ~0.45 M photons/s/mm2 for the typical timescale of multiple clinical X-ray scans, such as 1 s- 100 s. Our evaluation demonstrates the high potential of CsPbBr3 detectors for X-ray photon-counting applications in medical and industrial diagnostics with a lower cost than the current state-of-the-art.

'Molecular Beam Epitaxy' on Organic Semiconductor Single Crystals: Characterization of Well-Defined Molecular Interfaces by Synchrotron Radiation X-ray Diffraction Techniques.

Epitaxial growth, often termed “epitaxy”, is one of the most essential techniques underpinning semiconductor electronics, because crystallinities of the materials seriously dominate operation efficiencies of the electronic devices such as power gain/consumption, response speed, heat loss, and so on. In contrast to already well-established epitaxial growth methodologies for inorganic (covalent or ionic) semiconductors, studies on inter-molecular (van der Waals) epitaxy for organic semiconductors is still in the initial stage. In the present review paper, we briefly summarize recent works on the epitaxial inter-molecular junctions built on organic semiconductor single-crystal surfaces, particularly on single crystals of pentacene and rubrene. Experimental methodologies applicable for the determination of crystal structures of such organic single-crystal-based molecular junctions are also illustrated.

Organic Semiconductor Single‐Crystal Light‐Emitting Transistors

AbstractOrganic semiconductor single crystals, with the unique advantages of high carrier transport properties, high optical quality, and low defect density, are ideal candidates for constructing high‐performance organic light‐emitting transistors (OLETs) and advancing their related potential applications, including smart displays, micro‐nano light sources, organic electrically pumped lasers, and integrated photonic circuits. Recently, significant progress has been made in organic semiconductor single‐crystal light‐emitting transistors (OSSCLETs), particularly high‐mobility emissive organic semiconductor single crystals, high‐performance light‐emitting transistors, and color‐tunable organic semiconductor single crystals and OLETs. In this review, the exciting advances in OSSCLETs are summarized in terms of advanced organic semiconductor materials, high‐quality organic semiconductor single crystals, advanced device technologies, and high‐performance light‐emitting transistors. Finally, the challenges and prospects of OSSCLETs as well as promising research directions in this field are briefly summarized.

Persulfate activation boosted highly efficient photodegradation of norfloxacin catalyzed by Pt selectively loading LaOCl (001)

The spatial loading of redox cocatalysts on different faces of semiconductor single crystals is an effective approach to construct efficient photocatalysts. This work selectively deposits Pt nanoparticles on the electron-rich (001) face of LaOCl catalyst (Pt/LaOCl-P). The Pt/LaOCl-P catalyst with selective deposition on (001) facet exhibited higher photocatalytic activity for NOR degradation than the catalyst with random deposition on the entire surface (Pt/LaOCl-R). It indicates that the separation of facet charges upon photoexcitation may be a unique pathway for efficient photocatalysts. After the addition of persulfate, the generated sulfate radicals have a synergistic effect with the original hydroxyl radicals and superoxide radicals, which further improves the photodegradation performance. In addition, the effects of catalyst dosage and pH on photodegradation were discussed and the recycling performance of the materials was tested. The possible degradation pathways and mechanism of NOR were degraded by DFT calculation and LC-MS analysis.

Low contact-resistivity and high-uniformity Ni/Au ohmic contacts on Si nanomembranes grafted to Si substrates via low-temperature rapid thermal annealing

Grafted lattice-mismatched semiconductor heterostructures have the potential to simultaneously achieve low on-resistance and high breakdown performance. These novel semiconductor devices are typically fabricated by grafting single-crystal semiconductor nanomembrane (NM) layers onto a foreign substrate. The optimal temperature for grafting often prevents the use of high temperatures to achieve optimal ohmic contacts. Herein, we present a method to form low temperature Ni/Au ohmic contacts for the anode electrode to a p + Si NM that is grafted to a Si substrate. Ohmic contact resistivity (ρc) and its device-to-device uniformity are characterized against different annealing temperatures and are also compared against contact resistivity of p + SOI structure. Low resistivity ohmic contacts (ρc = 5.92 × 10-6 Ω cm2) and high uniformity were achieved on grafted NM structure. Demonstration of low ρc for NM-transferred layers via a low-temperature metallization scheme achieves an important step towards further developing high performance lattice-mismatched heterostructure-based devices.

New Lead-free Organic-Inorganic Hybrid Semiconductor Single Crystals for a UV-Vis-NIR Broadband Photodetector.

Organic-inorganic hybrid semiconducting (OIHS) materials, which can detect broader spectral regions, are highly desired in several applications including biomedical imaging, night vision, and optical communications. Although lead (Pb)-halide perovskites have reached a mature research stage, high toxicity of Pb hinders their large-scale viability. Tin (Sn)-based perovskites are the most common OIHS broadband light absorbers that replace toxic Pb; however, they are extremely unstable due to the notorious Sn2+ oxidation. Herein, a novel, non-toxic, and solution-processed millimeter-sized OIHS single crystal [Ga(C3H7NO)6](I3)3 has been grown at room temperature. Both the absorption measurement and density functional theory calculations have confirmed a narrow indirect band gap of 1.32 eV. The corresponding photodetector based on this single crystal demonstrated excellent performance including an ultraviolet-visible-near infrared (UV-vis-NIR) response between 325 and 1064 nm, fast response time (trise/tdecay = 3.8 ms/5.4 ms), and profound air storage stability (41 h), thus outperforming most common photodetectors based on Sn-based perovskites. This work not only provides a profound understanding of this novel organic-inorganic single-crystal material but also demonstrates its great potential to realize the high-performance UV-vis-NIR broadband photodetectors.

(Invited, Digital Presentation) Epitaxial Organic Molecular Interfaces As Well-Ordered Model Systems for Molecular Semiconductor p-n Junctions for Optoelectronic Applications

Donor-acceptor molecular interfaces are nothing but p-n junctions for organic optoelectronic devices such as organic light emitting diodes and organic solar cells where an exciton forms or dissociates. Understanding about dominating factors determining the intermolecular assembly and charge carrier exchange processes are highly anticipated for the establishment of smart design strategies of practically efficient devices. Molecular heterojunctions built on single crystal organic semiconductors provide well-ordered model systems disentangling complex intermolecular contacts in real devices. Moreover, the p-n junctions of highly ordered molecular assembly lead to the enhancement of the mobility of charge carriers and excitons through the inter-molecular delocalization of the electronic states, which potentially opens a new route for efficient optoelectronic applications of molecular semiconductors. In this contribution, crystallographic and electronic structures of well-ordered molecular semiconductor homo- and hetero-epitaxial junctions are overviewed.One representative system is a “complementary” p-n junction of perfluoropentacene (C22F14) crystalline ad-layers formed on the single crystal pentacene (C22H14) [1]. Grazing-incidence X-ray diffraction (GIXD) analyses revealed hetero-epitaxial growth of perfluoropentacene in a uniform crystallographic orientation with respect to the surface lattice of the single crystal pentacene. The energy-momentum dispersions of inter-molecular electronic bands were successfully demonstrated by angle-resolved photoelectron spectroscopy (ARPES) for the hetero-epitaxial perfluoropentacene as well as for the single crystal pentacene, which strongly suggests realization of the delocalization of electrons and holes across this well-ordered molecular p-n junction.To further improve the crystallinity of the epitaxial molecular heterojunctions, the authors’ group proposed a new concept of “quasi-homoepitaxial organic semiconductor junctions”, at which a molecular species with nearly identical in-plane lattice constants to those of the substrate molecular single crystal surface is stacked [2]. Actually, quasi-homoepitaxial crystallites of bis(trifluoromethyl)dimethylrubrene was revealed to form highly-ordered interface of much improved mean crystallite size on the single crystal surface of (unsubstituted) rubrene by means of high-resolution GIXD. The electronic structures exhibiting spontaneous electron transfer at this inter-molecular contact and possible opto-electronic applications of this quasi-homoepitaxial junction are also discussed in this presentation.This contribution is supported by Grant-in Aids for Transformative Research Areas (A) “Dynamic Exciton: Emerging Science and Innovation” (JSPS-KAKENHI Grant Number JP21H05405).[1] Y. Nakayama, et al., J. Phys. Chem. Lett. 10 (2019) 1312.[2] K. Takahashi, et al., J. Phys. Chem. Lett. 12 (2021) 11430.

Editorial: Is Zirconia the Holy Grail of All-Ceramic Restorations?

Editorial: Is Zirconia the Holy Grail of All-Ceramic Restorations?

Intrinsic Linear Dichroism of Organic Single Crystals toward High‐Performance Polarization‐Sensitive Photodetectors (Adv. Mater. 22/2022)

Polarization-Sensitive Photodetectors High-performance polarization-sensitive organic photodetectors with a linear dichroic ratio as high as 1.9 are successfully developed by Huanli Dong, Zhongming Wei, Liqiang Li, Wenping Hu, and co-workers, as described in article number 2105665, utilizing the intrinsic anisotropic nature of organic semiconductor single crystals. These devices show new opportunities for using organic semiconductors to develop highly compact polarization-sensitive photodetectors in diverse applications, and open up a new area for the study of organic semiconductors.

Systematic investigations on doping dependent thermal transport properties of single crystal silicon by time-domain thermoreflectance measurements

Doped silicon (Si) is the key material in semiconductor industries. It is important to fully understand the doping effect on thermal transport properties of single-crystal Si, especially for the thermal management of Si-based electronic devices. In this work, we used a femtosecond-laser time-domain thermoreflectance method to characterize the thermal transport properties of Si with different doping concentrations (1013-1019 cm−3) and doping elements, boron (B) and phosphorus (P). Results show that the thermal conductivity of heavy doped Si (about 2 × 1019 cm−3) by both B and P decreases by about 22% compared with their pure counterparts. Theoretical calculations based on the Boltzmann transport equation were also carried out for comparison with measurement results, and different scattering terms were discussed to find out the main factors for suppressing the thermal transport in doped Si within different doping concentration ranges. Combining with a cryogenic system, the thermal conductivity of doped Si samples and interfacial thermal conductance between Si and aluminum thin films were also measured under low-temperature conditions (down to 160 K). Our systematic studies on the doping effect in the thermal transport of single-crystal semiconductors not only facilitate the understanding of the microscale thermal transport, but also provide references in industrial applications of doped semiconductors.

Luminescent High‐Mobility 2D Organic Semiconductor Single Crystals

Abstract2D organic semiconductor single crystals comprising one or a few molecular layers of macroscopic lateral sizes are ideal materials for ultrathin, flexible, and transparent field‐effect devices—a platform for transistors and sensors. In recent years, these 2D materials have demonstrated high performance not inferior to their 3D counterparts. However, light emissive properties of 2D organic semiconductor single crystals have not yet been reported, and a combination of high charge‐carrier mobility and bright luminescence in one material is still a challenge for 2D organic optoelectronics. Emissive high‐mobility 2D organic semiconductor based on a [1]benzothieno[3,2‐b]benzothiophene (BTBT)‐derivative, 2,7‐bis(4‐decylphenyl)[1]benzothieno[3,2‐b][1]benzothiophene (DPBTBT), is presented here. DPBTBT molecules self‐organize in large‐area ultrathin single‐crystalline films consisting of one or a few molecular layers. These 2D single crystals perfectly suit as an active layer of organic field‐effect transistors in full accordance with Shockley's model and uniquely combine the high charge‐carrier mobility reaching 7.5 cm2 V–1 s–1 with prominent light emissive properties, which allow a demonstration of the first 2D organic light‐emitting transistor. The high charge‐carrier mobility and thermal stability of the crystalline phases, pronounced luminescence, and good shelf‐life stability suggest that emissive BTBT‐type molecules are a promising avenue for 2D organic optoelectronics.

Solar-blind UV detection by ultra-wide-bandgap 4HCB organic single crystal semiconductor

In this work, the solar-blind ultraviolet (UV) detection performance of organic single crystals 4-hydroxycyanobenzene (4HCB) is demonstrated. The ultra-wide bandgap and low dark current make 4HCB an important candidate for this application. Detectors with two electrode configurations, i.e., sandwiched electrode (SWE) and interdigital electrode (IDE), are fabricated based on 4HCB single crystals and measured under the illumination of 254 nm-UV light. Apparently, the IDE detector exhibits a responsivity R of 14 000 μA W−1 at a bias voltage of 1000 V, which is 2000 times higher than that of the SWE detector, due to its enhanced photoconductive gain by the surface layer edge states. To explore the possibility for the space UV detection applications in the radiation environment, the effect of neutron radiation on 4HCB detector performance is revealed. The point defects introduced by fast neutrons, mainly H vacancies, dominate the variation of the Fermi energy level and electric properties; however, this effect on photodetection is limited when the neutron flux is below 1013 n cm−2.

The prospects of organic semiconductor single crystals for spintronic applications

Spintronic applications of OSSCs are discussed from the aspects of molecular structure, stacking structure, OSSC-based spintronic device fabrication, and multifunctional applications.

Combined in Situ Photoluminescence and X-ray Scattering Reveals Defect Formation in Lead-Halide Perovskite Films.

Lead-halide perovskites have established a firm foothold in photovoltaics and optoelectronics due to their steadily increasing power conversion efficiencies approaching conventional inorganic single-crystal semiconductors. However, further performance improvement requires reducing defect-assisted, nonradiative recombination of charge carriers in the perovskite layers. A deeper understanding of perovskite formation and associated process control is a prerequisite for effective defect reduction. In this study, we analyze the crystallization kinetics of the lead-halide perovskite MAPbI3-xClx during thermal annealing, employing in situ photoluminescence (PL) spectroscopy complemented by lab-based grazing-incidence wide-angle X-ray scattering (GIWAXS). In situ GIWAXS measurements are used to quantify the transition from a crystalline precursor to the perovskite structure. We show that the nonmonotonous character of PL intensity development reflects the perovskite phase volume, as well as the occurrence of the defects states at the perovskite layer surface and grain boundaries. The combined characterization approach enables easy determination of defect kinetics during perovskite formation in real-time.

Coexistence of large positive and negative magnetoresistance in Cr2Si2Te6 ferromagnetic semiconductor

Magnetoresistance (MR) phenomenon couples the electron transport with magnetic field, which has been at the forefront of condensed matter physics and materials science. Large-MR behaviors are of particularly importance for magnetic sensor and information memory applications, and their scarcity has aroused intensive research. Moreover, due to the different physical origins, combination of large positive and negative MR (pMR and nMR) in one single compound has rarely been reported. In present work, we achieved a coexistence of large pMR and nMR in Cr2Si2Te6 ferromagnetic semiconductor single crystal with different field configurations. Specifically, a large nMR of about −60% was obtained under the in-plane field, while a large pMR higher than 1000% took over in the out-of-plane direction. We attribute this field direction-sensitive dualistic large MR behavior to the competition and cooperation effect from the ferromagnetic interaction, orbital scattering and electronic correlation that coexist in Cr2Si2Te6, which contribute to nMR, pMR, and nMR, respectively, in dominated temperature and field ranges, and show different weights under different field directions. The elucidated multiple MR mechanism in this ferromagnetic semiconductor will shed light on the pursuit of coexistence of large pMR and nMR for field-sensitive device applications.

Investigation on dark current and photoresponsivity of flexible single-crystal semiconductor photodetectors on plastic substrates

High-performance flexible photodetectors have drawn increasing attentions, and the dark current and photoresponsivity are the major performance parameters. Investigations on the influence factors and operation mechanisms of the dark current and photoresponsivity for the flexible photodetectors are highly desirable. In this paper, flexible single-crystal Si and Ge photodetectors on polyethylene terephthalate (PET) substrates were fabricated and characterized. The transferred single-crystal Si and Ge nanomembranes on PET substrates showed good crystal quality by x-ray diffraction and Raman spectroscopy. Based on the experimental and modeling results, the performance dependences on semiconductor materials, doping concentrations, dimensions of the intrinsic region, incident light wavelength and power were presented. The influential factors and the operation mechanisms for the performance dependence were discussed by analyzing the electric field distribution, energy band distribution, recombination rate, absorption coefficient, penetration depth and photogeneration rate based on the device model. The results provided useful guidelines to design high performance flexible single-crystal semiconductor photodetectors.

Growth of 100 mm indium antimonide single crystals by modified Czochralski technique

Currently there is a worldwide trend to increase the diameter of crystals grown from elemental semiconductors and semiconductor compounds. According to literary data the diameter of 3–5 semiconductor single crystals grown nowadays is 4 to 6 inches. So far up to 75 mm indium antimonide single crystals have been grown in Russia. Indium antimonide is the element base for the widest field of solid state electronics, i.e., optoelectronics. Indium antimonide is used for the fabrication of 3–5 mm range linear photodetectors and photodetector arrays used as light-sensitive material in heat vision systems. Growth heat conditions have been selected and 100 mm [100] indium antimonide single crystals have been grown using the modified two-stage Czochralski technique. The graphite heating unit has been oversized to accommodate a 150 mm crucible and a 4.5–5 kg load. The results of the work have provided for a substantial increase in the yield of photodetectors. The electrophysical properties of the as-grown single crystals have been studied using the Van der Pau method and proved to be in agreement with the standard parameters of undoped indium antimonide. Using the 9-field etch method of pit counting under an optical microscope the dislocation density in the 100 mm single crystals has been measured to be ≤ 100 cm-2which is similar to that for 50 mm single crystals.

Non-Debye relaxation of AgBiSe2 single crystal featuring flip-flop jumps in Bi valence state

The ternary chalcogenide based pristine AgBiSe2 semiconductor single crystal was studied for the temperature dependence dielectric measurement. The anomalous behaviour of dielectric constant and dielectric loss at higher frequency were discussed in detail. The activation energy due to the dc conductivity as well as hopping frequency were extracted from the Arrhenius equation. Moreover, the charge transport of Ag+ ions as well as the flip-flop between the pairs of Bi4+ and Bi3+ ions is responsible for various electrical properties in this crystal.

Modulated-photocurrent spectroscopy of single-crystal organic semiconductor rubrene with pristine and trap-dominated surfaces

High-performance, benchmark organic semiconductor rubrene is investigated by modulated-photocurrent (MPC) spectroscopy. In this technique, periodically modulated and steady-state photocarrier populations are simultaneously generated in the sample by two independent light beams, with the resultant photocurrent measured by a lock-in amplifier. The technique allows identifying a bandlike carrier motion (as opposed to incoherent hopping) as the dominant charge transport mechanism in this highly crystalline molecular semiconductor, in agreement with prior studies, yet without the need for fabrication of complex transistor devices. Moreover, MPC spectroscopy is used to determine the important parameters, such as the density of states for traps, their carrier capture coefficient, and attempt to escape frequency, as well as the quantum efficiency of photocarrier generation and the photocarrier mobility. The MPC spectroscopy is also shown to be able to discriminate charge transport at the surface of the crystal from that occurring in the bulk. The wealth of the thus obtained information is essential for our better understanding of the microscopic mechanisms governing photoconductivity in this class of materials.

Consequences of frequency and temperature on the ac-conductivity in ε-GaSe semiconductor single crystal

The frequency and temperature dependence of ac conductivity in a ε-GaSe single crystal was studied by assessing the dielectric constant (εr) and loss tangent (tanδ) over a broad frequency range. GaSe as a dielectric exhibited the typical effect of negative dielectric constant (NDC) at room temperature. A possible mechanism for understanding the reason behind the presence of NDC has been demonstrated on the basis of Drude model. In addition, the crystal as a dielectric material portrayed a remarkable shiftin the εr value from negative to positive at an increase of ~25 oc from room temperature. This clearly demonstrates the phase shift in the circuit behaviour from inductive to capacitive. Overall, the value of εr, tanδ, and σac was found to increase with temperature and frequency. This is primarily a result of the impact of space charge polarization, and frequency power law (σac∝ωn). The presented result thus advocates that the as grown ε-GaSe single crystal could be used as a smart and efficient dielectric material in electronic devices.