Electrical transport and optical properties of Cd_3As_2 thin films

Cd_3As_2, as a three-dimensional(3D) topological Dirac semimetal, has attracted wide attention due to its unique physical properties originating from the 3D massless Dirac fermions. While many efforts have been devoted to the exploration of novel physical phenomena such as chiral anomaly and phase transitions by using bulk crystals, the development of high-quality and large-scale thin films becomes necessary for practical electronic and optical applications. Here, we report our recent progress in developing single-crystalline thin films with improved quality and their optical devices including Cd_3As_2-based heterojunctions and ultrafast optical switches. We find that a post-annealing process can significantly enhance the crystallinity of Cd_3As_2 in both intrinsic and Zn-doped thin films. With excellent characteristics of high mobility and linear band dispersion, Cd_3As_2 exhibits a good optical response in the visible-to-mid-infrared range due to an advantageous optical absorption, which is reminiscent of 3D graphene. It also behaves as an excellent saturable absorber in the mid-infrared regime. Through the delicate doping process in this material system, it may further open up the long-sought parameter space crucial for the development of compact and high-performance mid-infrared ultrafast sources.     

Quantum transport properties of the three-dimensional Dirac semimetal Cd_3As_2 single crystals

The discovery of the three-dimensional Dirac semimetals have expanded the family of topological materials,and attracted massive attentions in recent few years.In this short review,we briefly overview the quantum transport properties of a well-studied three-dimensional Dirac semimetal,Cd_3As_2.These unusual transport phenomena include the unexpected ultra-high charge mobility,large linear magnetoresistivity,remarkable Shubnikov-de Hass oscillations,and the evolution of the nontrivial Berry's phase.These quantum transport properties not only reflect the novel electronic structure of Dirac semimetals,but also give the possibilities for their future device applications.     

Coherent Acoustic Phonon and Its Chirping in Dirac Semimetal Cd_3As_2

Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_(2 )is carried out.An acoustic phonon(AP)mode with central frequency f=0.037 THz(i.e.,1.23 cm~(-1)or 0.153 meV)is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping(i.e.,variation of the phonon frequency)is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.     

Large and anisotropic linear magnetoresistance in bulk stoichiometric Cd_3As_2 crystals

Cd3As2 was recently identified as a novel three-dimensional(3D)topological semimetal hosting the long-pursuing 3D Dirac Fermion.Crystals of Cd3As2 grown preferentially along the[100]and[112]directions were obtained through the modified chemical vapor transfer growth method,thus allowing the examination of transport anisotropy.The resistivity and magnetoresistance(MR)are basically linear with respect to magnetic field(H)in the measured temperature range of 2–300 K irrespective of the directions.The linear resistivity and MR are significantly anisotropic not only along[100]and[112]directions but also with respect to tilt angle between the growth directions and H,thus providing transport signatures of the 3D Dirac Fermion as well as the possible linear and anisotropic change of Weyl Fermi surface in H.Very large MR along the[100]direction is observed,even approaching 3100%at 2 K and 14 k Oe(1 Oe=79.5775 A m-1).The results would be helpful in renewing interest in studying emergent phenomena arising from bulk 3D Dirac Fermion as well as in paving the way for Cd3As2 to be used in magnetoelectronic sensors.     

Unexpected low thermal conductivity and large power factor in Dirac semimetal Cd_3As_2

Thermoelectrics has long been considered as a promising way of power generation for the next decades. So far,extensive efforts have been devoted to the search of ideal thermoelectric materials, which require both high electrical conductivity and low thermal conductivity. Recently, the emerging Dirac semimetal Cd3As2, a three-dimensional analogue of graphene, has been reported to host ultra-high mobility and good electrical conductivity as metals. Here, we report the observation of unexpected low thermal conductivity in Cd3As2, one order of magnitude lower than the conventional metals or semimetals with a similar electrical conductivity, despite the semimetal band structure and high electron mobility. The power factor also reaches a large value of 1.58 m W·m-1·K-2at room temperature and remains non-saturated up to 400 K.Corroborating with the first-principles calculations, we find that the thermoelectric performance can be well-modulated by the carrier concentration in a wide range. This work demonstrates the Dirac semimetal Cd3As2 as a potential candidate of thermoelectric materials.     

Superconductivity at the Normal Metal/Dirac Semimetal Cd_3As_2 Interface

We investigate the interface between a three-dimensional Dirac semimetal Cd₃As₂ and a normal metal via softpoint contact spectroscopy measurement.The superconducting gap features were detected below 3.8 K and 7.1 K in the case of Cd₃As₂ single crystals sputter-coated with the Pt and Au films,respectively,in the differential conductance dI/dV-V plots of the point contacts.As the applied magnetic field increased,the drop in the zerobias contact resistanc...     

ZnO纳米线紫外探测器的制备和快速响应性能的研究

一维ZnO纳米结构由于具有比表面积大、室温下具有大激子结合能等特点而受到广泛关注. 但是如何实现纳米结构的器件一直是目前研究的一个挑战. 文章通过水热方法, 在玻璃衬底上实现了ZnO纳米线横向生长, 并制备出基于ZnO纳米线的金属-半导体-金属紫外探测器. 测量结果显示器件在365 nm处探测器的响应度达到5 A/W, 并且制备的探测器在空气中对紫外光照具有快速的响应, 其上升时间约4 s, 下降时间约5 s, 这与ZnO纳米线中的氧空位吸附和脱附水分子相关.     

Optoelectronic properties of single-crystalline GaInAsSb quaternary alloy nanowires

Bandgap engineering of semiconductor nanomaterials is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here we report, for the first time, the growth of single-crystalline quaternary alloyed Ga_(0.75)In_(0.25)As_(0.49)Sb_(0.51) nanowires via a chemical-vapor-deposition method. The synthesized nanowires have a uniform composition distribution along the growth direction, with a zinc-blende structure. In the photoluminescence investigation,these quaternary alloyed semiconductor nanowires show a strong band edge light emission at 1950 nm(0.636 e V). Photodetectors based on these alloy nanowires show a strong light response in the near-infrared region(980 nm) with the external quantum efficiency of 2.0 × 10~4% and the responsivity of 158 A/W. These novel near-infrared photodetectors may find promising applications in integrated infrared photodetection, information communication, and processing.     

Structure and electrical properties of p-type twin ZnTe nanowires

Resonant tunneling is firstly found in twin p-type ZnTe nanowire field-effect transistors. The twin ZnTe nanowires are synthesized via the thermal evaporation process. X-ray diffraction and high-resolution transmission electron microscopy characterization indicate that the as-grown twin nanowire has a zinc-blende crystal structure with an integrated growth direction of [11-1]. The twin plane is (11-1) and the angle between the mirror symmetrical planes is 141°. The formation of twins is attributed to the surface tension from the eutectic liquid droplet. Field-effect transistors based on single ZnTe twin nanowire are constructed, the corresponding electrical measurements demonstrate that the twin nanowires have a p-type conductivity with a mobility (μ _h ) of 0.11 cm² V⁻¹ S⁻¹, and a carrier concentration (n _h ) of 1.1×10¹⁷ cm⁻³. Significantly, the negative differential resistance with a peak-to-valley current ratio of about 1.3 is observed in p-type twin ZnTe nanowire field-effect transistors at room temperature. As the periodic barriers produced in the periodic twin interfaces can form multi-barrier and multi-well along one-dimensional direction. The multibarrier can be modulated under external electrical field. When the resonant condition is met, the space charge will be enhanced with the inherent feedback mechanism, and the resonant tunneling will occur.     

Non-Monotonic Evolution of Carrier Density and Mobility under Thermal Cycling Treatments in Dirac Semimetal Cd_3As_2 Microbelts

Tunable carrier density plays a key role in the investigation of novel transport properties in three-dimensional topological semimetals.We demonstrate that the carrier density,as well as the mobility,of Dirac semimetal Cd_3As_2 nanoplates can be effectively tuned via in situ thermal treatment at 350 K for one hour,resulting in non-monotonic evolution by virtue of the thermal cycling treatments.The upward shift of Fermi level relative to the Dirac nodes blurs the surface Fermi-arc states,accompanied by an anomalous phase shift in the oscillations of bulk states,due to a change in the topology of the electrons.Meanwhile,the oscillation peaks of bulk longitudinal magnetoresistivity shift at high fields,due to their coupling to the oscillations of the surface Fermi-arc states.Our work provides a thermal control mechanism for the manipulation of quantum states in Dirac semimetal Cd_3As_2 at high temperatures,via their carrier density.     

Carrier transport and photoresponse for heterojunction diodes based on the reduced graphene oxide-based TiO2 composite and p-type Si

The present work reports the fabrication and detailed electrical properties of heterojunction diodes based on p-type Si and the reduced graphene oxide-based TiO₂ (TiO₂:RGO) composite. The enhanced dark conductivity was observed for TiO₂:RGO composite films. The improved electrical conductivity is considered to mainly come from the mobility enhancement. The TiO₂/p-type Si diode shows a poor rectifying behavior and low photoresponse. This is because of the dominance of electron traps in TiO₂. However, the TiO₂:RGO/p-type Si diode shows a good rectifying behavior and high photoresponse, which is attributed to high-mobility electron transport combined with the reduced number of electron traps.     

Electrical properties of p-type ZnGeP2 irradiated by H+ ions

The properties of p-type ZnGeP₂ [p₀=(5–10)·10¹⁰ cm^–3, ₀=(2–5)·10^–7 (·cm)^–1], irradiated with H⁺ ions [E=5 MeV, T_irr=300 K, D=(1·10¹²–1.7·10¹⁶) cm^–2] are studied. An increase in the resistivity (to gr_max - 5·10¹¹ ·cm) and subsequent reduction in for large currents of H⁺ ions ( - 9·10⁸ ·cm for D - 1.7·10¹⁶ cm^–2), is observed in irradiated crystals. The resistivity of irradiated p-type ZnGeP₂ is found to be very sensitive to hydrostatic pressure [(4–5)·10^–5 bar^–1]. The annealing of radiation defects in the temperature interval (20–600) °C is examined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 91–93, October, 1991.     

UV photoresponse of single ZnO nanowires

ZnO nanowires grown by site-selected molecular beam epitaxy (MBE) were contacted at both ends by Al/Pt/Au ohmic electrodes. The current–voltage (I–V) and photoresponse characteristics were obtained both in the dark and with ultraviolet (UV, 254 or 366 nm) illumination. The I–V characteristics are ohmic under all conditions, with nanowire conductivity under UV exposure of 0.2 cm. The photoresponse showed only a minor component with long decay times (tens of seconds) thought to originate from surface states. The results show the high quality of material prepared by MBE and the promise of using ZnO nanowire structures for solar-blind UV detection. PACS 81.05.Dz; 73.61.Ga; 72.80.Ey     

Electronic transport properties of lead nanowires

Lead nanowire occupies a very important position in an electronic device. In this study, a genetic algorithm(GA)method has been used to simulate the Pb nanowire. The result shows that Pb nanowires are a multishell cylinder. Each shell consists of atomic rows wound up helically side by side. The quantum electron transport properties of these structures are calculated based on the non-equilibrium Green function(NEGF) combined with the density functional theory(DFT),which indicate that electronic transport ability increases gradually with the atomic number increase. In addition, the thickest nanowire shows excellent electron transport performance. It possesses great transmission at the Fermi level due to the strongest delocalization of the electronic state. The results provide valuable information on the relationship between the transport properties of nanowires and their diameter.     

Thermal annealing effects on the dynamic photoresponse properties of Al-doped ZnO nanowires network

In this study, UV photodetectors based on a network of aluminium-doped zinc oxide (AZO) nanowires were manufactured at a low cost; for this purpose, a fast and simple fabrication process that involved dropping nanowires dispersion solution was employed instead of the conventional e-beam lithography process that is used to manufacture single nanowire–based UV photodetectors. It was demonstrated that nanowire network–based UV photodetectors provide a much faster UV photoresponse than conventional single nanowire–based UV photodetectors. The fast UV photoresponse of the fabricated UV photodetector can be attributed to the fact that the potential barriers formed in the nanowire network junctions effectively block the flow of electrons during the process of photocurrent decay. Furthermore, the UV photoresponse under illumination by a 254 nm UV source was studied as a function of the annealing temperature of the AZO nanowires network at a bias of 5 V. The fabricated UV photodetector showed the fastest response of 2 s to UV illumination in air when the sample was annealed in air for 1 h at 300 °C.     

Electrical properties of p-type GaP schottky barrier under stress

The effect of stress on a Schottky barrier height at a metal-semiconductor interface is investigated for metal-p-type GaP contacts. The diodes are fabricated by evaporating metals (Ag, Au) on polar (111)Ga and (1̄1̄1̄)P surfaces. Stress is applied to the diodes by bending the crystal wafers attached to the cantilever. The variation of the barrier height with stress is determined from the measurements of the current-voltage characteristics under stress. The barrier height decreases under compressive stress parallel to the interface and increases under tensile stress. The change in barrier height on the (111)Ga surface is greater than that on the (1̄1̄1̄)P surface. These experimental results are discussed from the point of both the piezoelectricity and the change in band gap caused by stress.     

NMR and NQR studies on transition-metal arsenide superconductors LaRu_2As_2,KCa_2Fe_4As_4F_2,and A_2Cr_3As_3

We report ~(75)As-nuclear magnetic resonance(NMR) and nuclear quadrupole resonance(NQR) measurements on transition-metal arsenides LaRu_2As_2, KCa_2Fe_4As_4F_2, and A_2Cr_3As_3. In the superconducting state of La Ru_2As_2, a Hebel–Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T1 just below Tc, which indicates that LaRu_2As_2 is a full-gap superperconducor. For KCa_2Fe_4As_4F_2, antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate RAF= Tc1/Tab1 is small and temperature independent,implying that the low energy spin fluctuations are isotropic in spin space. Our results indicate that KCa_2Fe_4As_4F_2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition. For A_2Cr_3As_3(A = Na, K, Rb, Cs), we calculate the electric field gradient by first-principle method and assign the ~(75)As-NQR peaks to two crystallographically different As sites, paving the way for further NMR investigation.     

Synthesis and red-shifted photoluminescence of single-crystalline ZnO nanowires

Local-oriented single-crystalline ZnO nanowires have been synthesized in large scale by a simple microemulsion method in the presence of sulfonate-polystyrene (S-PS) and dodecyl benzene sulfonic acid sodium salt (DBS). The as-prepared product is characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), infrared (IR) spectra and photoluminescence (PL) spectrum. The nanowires exhibit a local congregation and preferentially grow along the [0 0 2] facet. FT-IR spectrum indicates that S-PS is adsorbed on the surface of ZnO nanowires. The PL spectrum shows evidently red-shifted ultraviolet (UV) emission.     

Optical,photoelectric and electric properties of single-crystalline Sb2Se3

Measurements of absorption coefficient in the region of the absorption edge, of spectral distribution of photoconductivity and dependence of electrical conductivity upon temperature on Sb₂Se₃ single crystals are given. The absorption of light was proved to correspond to indirect forbidden transitions. The value of optical gapE _g ^opt =(1·11±0·02) eV forE a andE c was determined. From photoconductivity and conductivity measurements the values of the gaps areE _g ^opt =1·11 eV andE _g ^el =1·04 eV. The anisotropy of the electrical conductivity parallel and perpendicular to the cleavage plane is 2·2.     

Low-temperature transport properties of individual SnO2 nanowires

Stannic oxide (SnO₂) nanowires have been prepared by Chemical vapor deposition (CVD). The low-temperature transport properties of a single SnO₂ nanowire have been studied. It is found that the transport of the electrons in the nanowires is dominated by the Efros-Shklovskii variable-range hopping (ES-VRH) process due to the enhanced Coulomb interaction in this semiconducting nanowire. The temperature dependence of the resistance follows the relation lnR∼T^−1/2. On the I-V and dI/dV curves of the nanowire a Coulomb gap-like structure at low temperatures appears.