Search for ferromagnetic ordering in Pd doped wide band gap semiconductors GaN and ZnO

GaN and ZnO are possible candidates for dilute magnetic semiconductors with Curie temperatures above room temperature. Doping with transition metals like Co, Mn or Fe could be a simple way to create such systems. The perturbed angular correlation (PAC) probe ¹⁰⁰Pd/¹⁰⁰Rh is isoelectronic to cobalt and therefore a perfect tool to investigate the incorporation of transition metals into these compounds as well as the influence of other impurities on internal magnetic fields. The (0001) and (10 $\bar{1}$ 10) surfaces of ZnO single crystals, freestanding GaN films, and GaN thin films (6 μm) on sapphire substrates were recoil-implanted with the ¹⁰⁰Pd/¹⁰⁰Rh probe. The probe was produced using the fusion evaporation reaction ⁹²Zr(¹²C, 4n)¹⁰⁰Pd at a beam energy of 69 MeV. Subsequently, the incorporation of the probe was studied by PAC spectroscopy during an isochronal annealing program. First results without and with an applied external magnetic field are indicative of a strongly disturbed lattice vicinity of Pd impurities in both hosts. No signs of spontaneous ferromagnetic ordering were observed.     

Photoacoustic determination of energy band gap of semiconductors

Semiconducting materials are employed in the fabrication of a number of semiconductor devices and opto-electronic detectors etc depending on their properties, state of purity and perfection and energy band gap values. In the present study, a latest and novel photoacoustic spectroscopic technique has been employed for the determination of energy band gap of some semiconductors namely CdS, CdSe, CdTe, ZnS, ZnO, Se and Si in the powder form. Values obtained have been compared with those reported by conventional methods.     

二维组合宽带隙材料的研究

运用FDTD(时域有限差分法)研究了弹性波/声波通过二维正方形排列的铁/水声子晶体的传播特性.发现铁/水声子晶体在高频范围存在完全带隙，软包层铁/水声子晶体在低频范围出现共振带隙，它们分别对应于不同的带隙形成机理.通过铁/水声子晶体和软包层铁/水声子晶体的组合，可以得到从高频到低频的组合宽带隙结构，从而达到在更宽频范围内控制弹性波传播的目的；而且通过调整包层的厚度、铁芯的大小、以及填充率等结构参数，可以有效地调节组合宽带隙的频率结构.     

宽带隙半导体CdAl2S4电子结构、弹性和光学性质的研究

采用基于密度泛函理论(DFT)的第一性原理计算方法,对宽带隙半导体CdAl2S4的晶格结构、电学、弹性和光学性能进行了系统的研究. 研究结果表明：CdAl2S4为直接带隙的宽带隙半导体材料;是弹性稳定的具有各向异性的延展性材料;该晶体的光学性质在中能区(3.5 ~12.5 eV)具有较强的各向异性,其强反射峰处于紫外能量区域,因此其可用作紫外光探测或屏蔽材料.     

宽带隙半导体CdAl_2S_4电子结构、弹性和光学性质的研究

采用基于密度泛函理论(DFT)的第一性原理计算方法,对宽带隙半导体Cd Al_2S_4的晶格结构、电学、弹性和光学性能进行了系统的研究.研究结果表明:Cd Al2S4为直接带隙的宽带隙半导体材料;是弹性稳定的具有各向异性的延展性材料;该晶体的光学性质在中能区(3.5~12.5 e V)具有较强的各向异性,其强反射峰处于紫外能量区域,因此其可用作紫外光探测或屏蔽材料.     

Surface and interface issues in wide band gap semiconductor electronics

Wide band gap (WGB) materials are the most promising semiconductors for future electronic devices, and are candidates to replace the conventional materials (Si, GaAs, …) that are approaching their physical limits. Among WBG materials, silicon carbide (SiC) and gallium nitride (GaN) have achieved the largest advancements with respect to their material quality and device processing. Clearly, the devices performances depend on several surface and interface properties, which in turn are often crucially determined by the quality of the available material, as well as by the device processing maturity. In this paper, some surface and interface issues related to SiC and GaN devices processing are reviewed. First, the control of metal/SiC barrier uniformity and surface preparation will be discussed with respect to the performance of Schottky-based devices. Moreover, the impact of high-temperature annealing required for high-voltage Schottky diodes and MOSFETs fabrication, on the surface morphology and device performances will also be briefly presented. In the second part, it will be shown that for GaN the material quality is still the main concern, since dislocations have a severe influence on the current transport and barrier homogeneity of metal/GaN interfaces. Other practical implications of thermal annealing and surface passivation during GaN-based devices fabrication will also be addressed.     

Low-temperature epitaxial growth of the quaternary wide band gap semiconductor SiCAlN

Two compounds SiC and AlN, normally insoluble in each other below approximately 2000 degrees C, are synthesized as a single-phase solid-solution thin film by molecular beam epitaxy at 750 degrees C. The growth of epitaxial SiCAlN films with hexagonal structure takes place on 6H-SiC(0001) substrates. Two structural models for the hexagonal SiCAlN films are constructed based on first-principles total-energy density functional theory calculations, each showing agreement with the experimental microstructures observed in cross-sectional transmission electron microscopy images. The predicted fundamental band gap is 3.2 eV for the stoichiometric SiCAlN film.     

扶手椅型氮化镓纳米管电子结构与传输特性的研究

本文采用密度泛函理论和非平衡格林函数对扶手椅型氮化镓纳米管（n,n）（2≤ n ≤10）的电子结构和输运性质进行了研究。结果表明,所有的扶手椅型氮化镓纳米管都是间接带隙半导体,带隙随着纳米管直径的增加而增加,并且得到了两极体系下氮化镓纳米管的电流-电压曲线。氮化镓纳米管的半导体特性随着纳米管直径的增加越来越明显,电子态密度和电子透射光谱都具有脉冲型尖峰并且最大峰值随着n的增加而增加。这说明电子态密度和电子透射光谱峰在能量范围内,有较好的对应关系。     

Optical spectroscopy of AlGaInP based wide band gap quantum wells

An optical spectroscopic study of wide band gap AlGaInP based quantum wells is reported. GaInP quantum wells with AIGaInP barriers of two different Al compositions are studied. Type-II behaviour for narrow quantum wells with AlInP barriers is demonstrated. The onset of this behaviour allows limits to be placed on the size of the valence band offset. The first three orders of LO-phonon satellites are observed in quantum well PL spectra and their relative intensities are compared with theoretical predictions.     

Fundamental band gap and alignment of two-dimensional semiconductors explored by machine learning

Two-dimensional(2D)semiconductors isoelectronic to phosphorene have been drawing much attention recently due to their promising applications for next-generation(opt)electronics.This family of 2D materials contains more than 400members,including(a)elemental group-V materials,(b)binary III–VII and IV–VI compounds,(c)ternary III–VI–VII and IV–V–VII compounds,making materials design with targeted functionality unprecedentedly rich and extremely challenging.To shed light on rational functionality design with this family of materials,we systemically explore their fundamental band gaps and alignments using hybrid density functional theory(DFT)in combination with machine learning.First,calculations are performed using both the Perdew–Burke–Ernzerhof exchange–correlation functional within the generalgradient-density approximation(GGA-PBE)and Heyd–Scuseria–Ernzerhof hybrid functional(HSE)as a reference.We find this family of materials share similar crystalline structures,but possess largely distributed band-gap values ranging approximately from 0 eV to 8 eV.Then,we apply machine learning methods,including linear regression(LR),random forest regression(RFR),and support vector machine regression(SVR),to build models for the prediction of electronic properties.Among these models,SVR is found to have the best performance,yielding the root mean square error(RMSE)less than 0.15 eV for the predicted band gaps,valence-band maximums(VBMs),and conduction-band minimums(CBMs)when both PBE results and elemental information are used as features.Thus,we demonstrate that the machine learning models are universally suitable for screening 2D isoelectronic systems with targeted functionality,and especially valuable for the design of alloys and heterogeneous systems.     

Reduction in the electronic band gap of titanium oxide nanotubes

The structural and electronic properties of individual titanium oxide nanotubes have been studied using both empirical and ab initio calculations. Two different types of titanium oxide nanotubes (A-nanotube and B-nanotube) have been constructed and energy-minimized by molecular mechanics calculations. We found that the A-nanotubes are energetically more favorable than the B-nanotubes. The electronic band structure of the titanium oxide nanotubes was also calculated with respect to the tubule diameter and the tubule type using the ab initio method. The band gap of the A-nanotube was reduced by up to 60% as the tubule diameter decreases from 1.2 nm to 0.5 nm.     

NaB6Si: A novel wide band gap semiconductor with great hardness

Using first-principles calculations, a novel B₁₂-based ternary compound, NaB₆Si structure is proposed in this work. This structure is confirmed to be dynamically, thermodynamically, and mechanically stable at ambient pressure. The formation energy of NaB₆Si is lower than that of experimentally synthesized Na₈B_74.5Si_17.5. The Vickers hardness is calculated to be 30.2 GPa, indicating NaB₆Si is a promising hard material. The band structure and density of states reveal that NaB₆Si is a wide band gap semiconductor with a band gap approximately 2.88 eV. This study provides a fundamental understanding of the structural, mechanical, and electronic properties in NaB₆Si.     

A minimal basis semi-ab initio approach to the band structures of semiconductors

The band structures of 32 of the most important semiconductor crystals are calculated using an efficient, minimal basis, orthogonalized LCAO method. These include the diamond structure of C, Si, Ge, α-Sn; the zinc blende structure of β-SiC, BN, BP, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, β-ZnS, ZnSe, ZnTe, CdS, CdTe; the wurtzite structure of AlN, GaN, ZnO, α-ZnS, CdS, CdSe; the sodium chloride structure of CdO, GeTe, SnTe and trigonal Se and Te. The calculations, which involve diagonalizations of small size matrix equations yield results having the following characteristics: (1) satisfactory valence bands and lower conduction bands and bulk densities of states; (2) the gap sizes and the locations of valence band maximum and conduction band minimum in agreement with experiment; (3) reasonable values of fractional ionicity and electron and hole effective masses. These are achieved by fine-tuning the exchange parameters in the construction of the potentials. Application of this approach to the study of the electronic structures of disordered and other complex semiconductor systems is also discussed.     

A nanoindentation analysis of the influence of lattice mismatch on some wide band gap semiconductor films

Nanoindentation studies are carried out on epitaxial ZnO and GaN thin films on (0 0 0 1) sapphire and silicon substrates, respectively. A single discontinuity (‘pop-in’) in the load-indentation depth curve is observed for ZnO and GaN films at a specific depths of 13-16 and 23-26 nm, respectively. The physical mechanism responsible for the ‘pop-in’ event in these epitaxial films may be due to the interaction behavior of the indenter tip with the pre-existing threading dislocations present in the films during mechanical deformation. It is observed that the ‘pop-in’ depth is dependent on lattice mismatch of the epitaxial thin film with the substrate, the higher the lattice mismatch the shallower the critical ‘pop-in’ depth.     

Effect of high temperature band gap of GaN annealing on strain and nanoparticles

<正>Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200℃for 30 min.XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200℃,and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions.If the as-synthesised GaN nanoparticles at 950℃are regarded as standard,the thermal expansion changes nonlinearly with temperature and is anisotropic;the expansion below 1000℃is smaller than that above 1000℃.This study provides an experimental demonstration for selecting the proper annealing temperature of GaN.In addition,a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200℃is observed,which can be ascribed to the dominant transitions from the C（Γ₇） with the peak energy at 3.532 eV.     

Effect of high temperature annealing on strain and band gap of GaN nanoparticles

Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200 oC for 30 min. XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200 oC, and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions. If the as-synthesised GaN nanoparticles at 950 oC are regarded as standard, the thermal expansion changes nonlinearly with temperature and is anisotropic; the expansion below 1000oC is smaller than that above 1000 oC. This study provides an experimental demonstration for selecting the proper annealing temperature of GaN. In addition, a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200 oC is observed, which can be ascribed to the dominant transitions from the C(Γ₇) with the peak energy at 3.532 eV.