AlN/InN和AlN/GaN超晶格能带结构研究

利用Krönig-Penney 模型和形变势理论,从理论上探讨了纤锌矿型AlN/InN和AlN/GaN超晶格系统的能带结构及不同应变模式对能带结构的影响,计算得到了能带结构随各亚层参量变化的一般性规律、超晶格的能量色散关系、应变造成的影响以及不同亚层厚度的系统禁带宽度和导带第一子禁带宽度.研究发现,通过改变亚层厚度可以从不同形式设计能带结构,应变会改变系统禁带宽度,使带阶和子能带明显窄化,价带结构趋于复杂甚至生成准能带结构.与实验结果对比后发现,该模型适于模拟窄势阱结构超晶格,而对于宽势阱则必须 关键词： AlN/InN和AlN/GaN超晶格 Krönig-Penney模型 应变 子能带     

Effect of electron–phonon interaction on surface states in wurtzite nitride semiconductors under pressure

A variational theory is proposed to study the electronic surface states in semi-infinite wurtzite nitride semiconductors under the hydrostatic pressure. The electronic surface state energy level is calculated, by taking the effects of the electron–Surface–Optical–phonon interaction, structural anisotropy and the hydrostatic pressure into account. The numerical computation has been performed for the electronic surface state energy levels, coupling constants and the average penetrating depths of the electronic surface state wave functions under the hydrostatic pressure for wurtzite GaN, AlN and InN, respectively. The results show that electron–Surface–Optical–phonon interaction lowers the electronic surface state energy levels. It is also found that the electronic surface state energy levels decrease with the hydrostatic pressure in wurtzite GaN and AlN. But for wurtzite InN, the case is contrary. It is shown that the hydrostatic pressure raised the influence of electron–phonon interaction on the electronic surface states obviously. The effect of electron–Surface–Optical–phonon interaction under the hydrostatic pressure on the electronic surface states cannot be neglected, in specially, for materials with strong electron–phonon coupling and wide band gap.     

The influence of AlGaN/GaN superlattices as electron blocking layers on the performance of blue InGaN light-emitting diodes

P-AlGaN/P-GaN superlattices are investigated in blue InGaN light-emitting diodes as electron blocking layers.The simulation results show that efficiency droop is markedly improved due to two reasons:(i) enhanced hole concentration and hole carrier transport efficiency in AlGaN/GaN superlattices,and(ii) enhanced blocking of electron overflow between multiple quantum-wells and AlGaN/GaN superlattices.     

Simple theoretical analysis of the Einstein’s photoemission from quantum confined superlattices

In this paper, we study the Einstein’s photoemission from III–V, II–VI, IV–VI and HgTe/CdTe quantum well superlattices (QWSLs) with graded interfaces and quantum well effective mass superlattices in the presence of a quantizing magnetic field on the basis of newly formulated dispersion relations in the respective cases. Besides, the same has been studied from the afore-mentioned quantum dot superlattices and it appears that the photoemission oscillates with increasing carrier degeneracy and quantizing magnetic field in different manners. In addition, the photoemission oscillates with film thickness and increasing photon energy in quantum steps together with the fact that the solution of the Boltzmann transport equation will introduce new physical ideas and new experimental findings under different external conditions. The influence of band structure is apparent from all the figures and we have suggested three applications of the analyses of this paper in the fields of superlattices and microstructures.     

Enhancement of electrical conductivity by Al-doped ZnO ceramic varistors

Zn_1−xAl_xO ceramic samples with various x values (0.00≤x≤0.20) are sintered in air at temperatures of 850 °C for 10 h and then quenched to room temperature. Structural, surface morphology and I-V characteristics of the samples are investigated using X-ray diffractometer (XRD), scanning electron microscope (SEM) and dc electrical measurements. It is found that addition of Al up to 0.05 does not influence the well-known peaks related to wurtzite structure of ZnO ceramics, and other unknown peaks could be formed above 0.05 of Al. The cell parameters of Al-doped samples are a little shorter than the undoped ZnO, and also the shape and size of grains are clearly affected. Average crystalline diameters, deduced from XRD analysis, are between 39.90 and 47.18 nm, which are 25 times lower than those obtained from SEM micrographs. Although breakdown field, nonlinear coefficient and barrier height are generally decreased by Al addition, the electrical conductivity is improved. These results are discussed in terms of the interaction mechanism between atoms of Al and Zn in both under and over-doped regions.     

Synthesis of a New Modification of Lithium Chloride Confirming Theoretical Predictions

In accordance with prior calculations, the new polymorph β‐LiCl (wurtzite structure type) has been synthesised, by the low‐temperature atomic‐beam‐deposition (LT–ABD) technique, in a mixture with α‐LiCl (rock salt structure type) by depositing LiCl vapour (2 to 5.3 × 10^–4 mbar) onto a cooled substrate (–30 to –60 °C). The maximum β‐LiCl fraction of 53 % was obtained using a sapphire (0001) substrate at –50 °C and 3.7 × 10^–4 mbar LiCl vapour pressure. The proportion of the new polymorph contained in the bulk sample decreases as temperature or vapour pressure deviate from these values, until finally the rock salt type LiCl is found exclusively. When the samples are warmed up to room temperature, β‐LiCl irreversibly transforms to α‐LiCl. The X‐ray diffraction pattern of the two phase LiCl sample measured at –50 °C has been indexed and refined based on a hexagonal unit cell for β‐LiCl with the lattice constants a = 3.852(1) Å and c = 6.118(1) Å and a cubic unit cell for α‐LiCl with the lattice constant a = 5.0630(8) Å. By Rietveld refinement the wurtzite type ofstructure (P6₃mc, No. 186) was suggested for the new hexagonal modification of LiCl with the Li–Cl distances (2.32 and 2.34 Å) being 8 % smaller than those of α‐LiCl. Moreover, the cell volume decreases as much as 16 % during the transition from β‐LiCl to α‐LiCl. Both the shifts in bond lengths and volume correspond well with the situation encountered for LiBr and LiI. Besides the variation of LiCl vapour pressure and substrate temperature, also different substrate materials were employed for testing their influence on formation of the β‐LiCl polymorph.     

Optical transmission through basic-structural-unit superlattices

In this paper, by the transparent-component-decimation (TCD) method we obtain three kinds of new basic- components (BCs) through simplifying and decomposing the BCs of three-component Thue-Morse (3CTM) sequence. Based on these new BCs we propose a type of basic-structural-units (BSUs) and investigate the optical transmission of the one-dimensional (1D) superlattices composed of these BSUs. It is found that if the substrates of the 1D BSU superlattices are certain, the optical transmission at the central wavelength (CW) will be determined completely by the number and the type of BSUs and has nothing to do with the marshalling sequence. In particular, if the substrates are identical, the numbers of different types of BSUs are all the same and the middle two elements of BSUs constitute a cycle, then no matter whether the system is periodic, or quasiperiodic, or aperiodic, or unordered, or even random, it will be transparent at the CW. The conclusion is confirmed by the numerical results. Similar to the even layers of neighbourhood identical elements in TCD method, such a kind of optical BSU subsystem can also be decimated from the chain in the process of transmission investigation. There would be a potential application in the designing of some interesting optical devices.     

Dependence of electron dynamics on magnetic fields in semiconductor superlattices

Numerical simulation results are presented for a drift-diffusion rate equation model which describes electronic transport due to sequential tunneling between adjacent quantum wells in weakly coupled semiconductor superlattices （SLs）. The electron dynamics is dependent on the external magnetic field perpendicular to the electron motion direction, and a detailed explanation is given. Using different parameters, the system shows different dynamic behaviors, and three distinct phenomena are observed and controlled by increasing magnetic field. （i） For a lower doping density, the system state transfers from stable state to oscillationary state. （ii） An opposite result is obtained to that in the case （i） for an intermediate value of the doping density, and the state changes from oscillationary to stationary. （iii） The state varies between oscillationary and stationary when doping density is large. Then, a detailed theoretical analysis is given to explain these surprise phenomena. The distribution of the electric-field domain along the SLs is plotted. We find the structure of the domain is almost uniform for a lower doping density, and no domain occurs in the SLs. By adding an external ac signal, complex nonlinear behaviors are observed from the Poincaré map and the corresponding phase diagrams when the driving frequency changes.