Effect of substrate temperature on microstructural and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on n-Si （111） at various substrate temperatures by pulsed laser deposition （PLD）. X-ray diffraction （XRD）, photoluminescence （PL）, Fourier transform infrared spectrophotometer （FTIR）, and scanning electron microscopy （SEM） were used to analyze the structure, morphology, and optical property of the ZnO thin films. An optimal crystallized ZnO thin film was obtained at the substrate temperature of 600℃. A blue shift was found in PL spectra due to size confinement effect as the grain sizes decreased. The surfaces of the ZnO thin films were more planar and compact as the substrate temperature increased.     

Polarization, piezoelectric constants, and elastic constants of ZnO, MgO, and CdO

We report first-principles density functional calculations of the polarizations, piezoelectric stress constants, and elastic constants for the II-VI oxides MgO, ZnO, and CdO in the wurtzite structure. Using our pseudopotential self-interaction corrected implementation of density functional theory, we obtain polarization values of −0.060, −0.022, and −0.10 C/m², and piezoelectric constants, e₃₃ (e₃₁) of 1.64 (−0.58), 1.34 (−0.57), and 1.67 (−0.48) C/m² for structurally relaxed MgO (with its in-plane lattice parameter fixed to that calculated for ZnO), ZnO, and CdO, respectively. The large polarization gradients between the end-point compounds in the MgO-ZnO-CdO system augur well for the production of large internal fields in ZnO-based polarization field effect transistors.     

Structural and Room‐Temperature Transport Properties of Zinc Blende and Wurtzite InAs Nanowires

Here, direct correlation between the microstructure of InAs nanowires (NWs) and their electronic transport behavior at room temperature is reported. Pure zinc blende (ZB) InAs NWs grown on SiO₂/Si substrates are characterized by a rotational twin along their growth‐direction axis while wurtzite (WZ) InAs NWs grown on InAs (111)B substrates have numerous stacking faults perpendicular to their growth‐direction axis with small ZB segments. In transport measurements on back‐gate field‐effect transistors (FETs) fabricated from both types of NWs, significantly distinct subthreshold characteristics are observed (I_on/I_off ～ 2 for ZB NWs and ～10⁴ for WZ NWs) despite only a slight difference in their transport coefficients. This difference is attributed to spontaneous polarization charges at the WZ/ZB interfaces, which suppress carrier accumulation at the NW surface, thus enabling full depletion of the WZ NW FET channel. 2D Silvaco‐Atlas simulations are used for ZB and WZ channels to analyze subthreshold current flow, and it is found that a polarization charge density of ≥10¹³ cm^?2 leads to good agreement with experimentally observed subthreshold characteristics for a WZ InAs NW given surface‐state densities in the 5 × 10¹¹–5 × 10¹² cm^?2 range.     

纤锌矿GaN/AlxGa1-xN量子阱材料中极化子效应

采用Lee-Low-Pines(LLP)变分法研究纤锌矿量子阱材料中电子-声子相互作用有关问题,给出纤锌矿GaN/AlxGa1-xN和InxGa1-xN/GaN量子阱材料中极化子基态能量和第一激发态到基态的跃迁能量随量子阱宽度和材料组分的变化关系.研究中考虑了纤锌矿量子阱材料中光学声子模各向异性以及声子频率随波矢的变化关系.同时还对运用LLP变分方法计算极化子能量时所采用的几种不同处理方法进行了分析和讨论,并给出相应的数值计算结果.     

Enargite Cu3PS4: A Cu–S‐Based Thermoelectric Material with a Wurtzite‐Derivative Structure

Compound semiconductors derived from ZnS (zincblende and wurtzite) with tetrahedral framework structures have functions for various applications. Examples of such materials include Cu–S‐based materials with zincblende‐derivative structures, which have attracted attention as thermoelectric (TE) materials over the past decade. This study illuminates superior TE performance in polycrystalline samples of enargite Cu₃P_1?xGe_xS₄ with a wurtzite‐derivative structure. The substitution of Ge for P dopes holes into the top of the valence band composed of Cu‐3d and S‐3p, whereby its multiband characteristic leads to a high TE power factor. Furthermore, a reduction in the grain size to 50–300 nm can effectively decrease phonon mean free paths, leading to low thermal conductivity. These features result in a dimensionless TE figure of merit ZT of 0.5 at 673 K for the x = 0.2 sample. Environmentally benign and low‐cost characteristics of the constituent elements of Cu₃PS₄, as well as its high‐performance thermoelectricity, make it a promising candidate for large‐scale TE applications. Furthermore, this finding extends the development field of Cu–S‐based TE materials to those with wurtzite‐derivative structures.     

Evaluation of diffusion mechanism and structural characterizations of ZnS compound during chemical reaction of Zn and S in liquid phase

The present study investigates the diffusion mechanism, morphology and structural characteristics of crystalline zinc sulfide (ZnS) during the reactive diffusion process. The samples with nominal composition of Zn₅₀S₅₀ were prepared via capsulation of high purity of zinc and sulfur followed by the annealing process at various reaction temperatures. The prepared samples were characterized using X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. The structural measurements confirm the formation of zinc sulfide with wurtzite structure during the annealing process at 550°C. The wurtzite allotrope of ZnS is not stable at a high annealing temperature and is transformed to cubic zinc-blende structure. During the annealing process, a continuous layer of ZnS compound forms at the Zn/S interface. Both Zn and S diffuse into the formed ZnS layer and the growth of that occurs mainly toward the Zn side. Under this condition, Kirkendall voids form and accumulate near the ZnS/S interface.     

MgZnO和ZnO晶体薄膜紫外发光特性比较

用电子束蒸发反应沉积在Si(111)衬底上低温生长了立方MgZnO薄膜和高度C—轴取向的ZnO薄膜，X-射线光电子能谱(XPS)结果表明，立方MgZnO薄膜中的Mg含量比靶源中的高，紫外光致荧光谱(UVPL)测试显示，与ZnO相比MgZnO的荧光峰从393nm蓝移至373nm，这可能与MgZnO的带隙变宽有关，对ZnO薄膜的研究还发现，生长过程中充O2与否对ZnO发光特性的影响显著，不充O2时样品的紫外荧光峰较之充O2条件下制得的样品发生红移。     

Large-Signal Characterizations of DDR IMPATT Devices Based on Group III-V Semiconductors at Millimeter-Wave and Terahertz Frequencies

Large-signal (L-S) characterizations of double-drift region (DDR) impact avalanche transit time (IMPATT) devices based on group III-V semiconductors such as wurtzite (Wz) GaN, GaAs and InP have been carried out at both millimeter-wave (mm-wave) and terahertz (THz) frequency bands. A L-S simulation technique based on a non-sinusoidal voltage excitation (NSVE) model developed by the authors has been used to obtain the high frequency properties of the above mentioned devices. The effect of band-to-band tunneling on the L-S properties of the device at different mm-wave and THz frequencies are also investigated. Similar studies are also carried out for DDR IMPATTs based on the most popular semiconductor material, i.e. Si, for the sake of comparison. A comparative study of the devices based on conventional semiconductor materials (i.e. GaAs, InP and Si) with those based on Wz-GaN shows significantly better performance capabilities of the latter at both mm-wave and THz frequencies.