六角氮化硼片能带结构的应变调控

采用第一性原理研究了应变对单层、双层和三层BN片能带结构的影响.研究表明,随着张应变的增大,BN片的带隙线性变小,且变化的斜率与层数以及多层BN片的堆垛方式无关.     

立方氮化硼薄膜的光学带隙

用射频溅射法在p型Si(100)衬底上沉积立方氮化硼(c-BN)薄膜,薄膜的成分由傅里叶变换红外谱标识,用紫外-可见分光光度计测量了c-BN薄膜的反射光谱,利用K-K(Kramers-Kroning)关系从反射谱计算出c-BN薄膜的光吸收系数,进而确定c-BN薄膜的光学带隙.对于立方相含量为55.4%的c-BN薄膜,光学带隙为5.38eV.关键词：立方氮化硼薄膜光学带隙K-K关系     

掺杂菱形BN片的石墨烯纳米带的电子特性

采用基于密度泛函理论的第一性原理方法,系统研究掺杂菱形BN片的石墨烯纳米带的电子特性.掺杂使扶手椅型石墨烯纳米带（AGNRs）的带隙增大,不同位置掺杂AGNRs的带隙大小略有差异.在无磁性态,无论是否掺杂,锯齿型石墨烯纳米带（ZGNRs）都为金属.在铁磁态,掺杂使ZGNRs由金属转变为半导体.而处于反铁磁态时,无论是否掺杂,ZGNRs都为半导体,掺杂使其带隙发生改变.掺杂的AGNRs和ZGNRs的结构稳定,掺杂ZGNRs的基态为反铁磁态.掺杂菱形BN片可以有效调控GNRs的电子特性.     

Temperature dependence of the fundamental band gap parameters in cadmium-rich Zn<Subscript>x</Subscript>Cd<Subscript>1-x</Subscript>Se using photoluminescence spectroscopy

Thin films of ternary Zn_xCd_1-xSe were deposited on GaAs (100) substrate using metalorganic-chemical-vapour-deposition (MOCVD) technique. Temperature dependence of the nearband-edge emission from these Cd-rich Zn_xCd_1-xSe (forx = 0025, 0.045) films has been studied using photoluminescence spectroscopy. Relevant parameters that describe temperature variation of the energy and broadening of the fundamental band gap have been evaluated using various models including the two-oscillator model, the Bose-Einstein model and the Varshni model. While all these models suffice to explain spectra at higher temperatures, the two-oscillator model not only explains low temperature spectra adequately but also provides additional information concerning phonon dispersion in these materials.     

A Novel Approach for the Evaluation of Band Gap Energy in Semiconductors

On the basis of absorption nature of semiconductors, we present a novel and simple method to determine the band gap energies of semiconductors directly from their absorption spectra at any temperatures, without any fitting processes and restrictions of sample thickness. The key point of the approach is the different dependence of the absorption coefficient derivative on the photon energy at different absorption regions in semiconductors. We first demonstrate and verify the approach by detailed temperature-dependent absorption measurements, combined with photoluminescence measurements and empirical band gap equations for the direct band gap of uniform InAs films, and then extend successfully to the indirect band gap of elemental Ge and to the ternary HgCdTe alloys with composition gradient. Furthermore, we have also shown that our approach can not only evaluate the average band gap energy for ternary semiconductor alloys, but also estimate their composition uniformity to monitor the material quality.     

Advanced optical spectroscopy in nitrides and oxides: Some of the progress made during the past ten years

I review the recent progress made concerning the optical properties of (In,Al,Ga)N-based and (Zn,Mg)O-based epilayers and low dimensional systems like quantum wells, superlattices and microcavities.     

立方氮化硼单晶紫外吸收光谱的研究

对于在高温、高压下合成的人工立方氮化硼(cBN)片状单晶进行了紫外吸收光谱和第一性原理的能带结构研究。实验中采用了UV WINLAB光谱分析仪,数据分析由MOLECULAR SPECTROSCOPY软件进行拟合运算,通过特殊的石英夹具对样品的测试表明cBN的紫外吸收波长限为198 nm,带隙为6.26 eV。结合第一性原理计算的cBN的能带结构和电子态密度的计算,可以证实导致紫外光吸收的过程是价带电子吸收光子到导带的间接跃迁。文章实验结果与目前报道的cBN能带结构中禁带宽度的吻合较好,表明cBN具有良好的紫外特性,是一种具有发展前景的紫外光电和高温半导体器件材料。     

An analysis of the effect of nitrogen and a screened (by free carriers) Coulomb field on the binding energy of hydrogenic shallow donors in GaInAsN

The effect of nitrogen concentration on the screening with free carriers and binding energy of hydrogenic shallow donors in GaInAsN alloys is investigated. The binding energy is calculated using a novel algebraic model which was proposed recently by Gönül et al. (2006) [16], in order to find an analytical solution to the screened Coulomb potential. The results show that the nitrogen concentration is a strong factor in producing a screening field of free carriers and in affecting the binding energy of hydrogenic shallow donors.     

Effect of triangle vacancy on thermal transport in boron nitride nanoribbons

Recently, triangle vacancy in hexagonal boron nitride is observed experimentally. Using nonequilibrium Green’s function method, we investigate thermal transport properties of boron nitride nanoribbons (BNNRs) with a triangle vacancy. The effect of triangle vacancy on the phonon transmission of zigzag-edged BNNRs (Z-BNNRs) is different from that of armchair-edged BNNRs (A-BNNRs). The triangle vacancy induces antiresonant dips in the spectrum of Z-BNNRs. Moreover, the boron-terminated triangle vacancy causes antiresonant zero-transmission dip and the number of the zero-transmission dip increases with the geometrical size of triangle vacancy. For the A-BNNRs with triangle vacancy, except some antiresonant dips, a resonant peak is found in the transmission. The antiresonant and resonant phenomena are explained by analyzing local density of states and local thermal currents. Although the antiresonant dip and the resonant peak are both originated from quasibound states, their distributions of local thermal currents are distinct, which leads to the transport discrepancy. In addition, the thermal conductance of BNNRs decreases linearly with increasing the vacancy size.     

Electronic properties of boron nanotubes with axial strain

The electronic properties of boron nanotubes with axial strain are investigated by first principle calculations. The band gaps of the (3, 3) and (5, 0) boron nanotubes are found to be modified by axial strain significantly. We find that the semiconductor-metal transition occurs for the (3, 3) boron nanotubes with both compressive and tensile strain. While for the (5, 0) boron nanotubes, only the tensile strain induces the semiconductor-metal transition. These boron nanotubes have the largest gaps under compressive strain.      

Surface order dependent magnetic thin film growth: Fe on GaN(0 0 0 1)

X-ray photoemission spectroscopy and X-ray magnetic circular dichroism have been used to study the growth process, chemical composition and magnetic character of iron deposited on ordered and disordered GaN(0 0 0 1) surfaces. On the (1 × 1) ordered surface the Fe grows uniformly but with disruption to the substrate surface, subsequently nitrogen desorbs from the surface, some of which diffuses into the Fe overlayer. The film is magnetically fractured, with high magnetic coercivity and broad switching fields. Conversely, the gallium rich disordered surface protects the underlying substrate from any disruption and initially induces non-uniform growth, the Fe clusters coalesce at ∼12 Å, to produce a uniform film with desirable magnetic characteristics. Films beyond this point (>12 Å) indicate sharp hysteresis loops with low coercivities. For the resultant film (36 Å) we measure a magnetic moment of 2.02 μ_B, in agreement with bulk bcc iron (2.068 μ_B.)