The Einstein relation in nanostructured materials: simplified theory and suggestion for experimental determination

An attempt is made to study the Einstein relation for the diffusivity–mobility ratio (DMR) in quantum wells (QWs) and quantum well wires (QWWs) of tetragonal compounds on the basis of a newly formulated electron energy spectrum taking into account the combined influences of the anisotropies in effective electron mass, the spin–orbit splitting, and the presence of crystal field splitting, respectively. The results for quantum-confined III–V compounds form a special case of our generalized analysis. The DMR has also been studied for QWs and QWWs of II–VI and IV–VI materials. Taking QWs and QWWs of CdGeAs₂, InAs, CdS and PbSe as examples, it was found that the DMR increases with increasing carrier statistics and decreasing film thickness respectively in various oscillatory manners emphasizing the influence of dimensional quantizations and the energy band constants in different cases. An experimental method of determining the DMR in nanostructures with arbitrary dispersion laws has also been suggested and the present simplified analysis is in agreement with the suggested relationship. The well-known results for nanostructures with parabolic energy bands have also been obtained as special cases from this generalized analysis under certain limiting conditions.     

Quantum automata for some multiperiodic languages

We exhibit small size measure-once one-way quantum finite automata (mo-1qfa’s) inducing multiperiodic stochastic events. Moreover, for certain classes of multiperiodic languages, we exhibit: (i) isolated cut point mo-1qfa’s whose size logarithmically depends on the periods; (ii) Monte Carlo mo-1qfa’s whose size logarithmically depends on the periods and polynomially on the inverse of the error probability.     

Impact of forward bias on electroluminescence efficiency in blue and green InGaN quantum well diodes: A comparative study

Electroluminescence (EL) spectral intensity in the high-brightness blue and green InGaN single-quantum-well (SQW) diodes has been comparatively studied over a wide temperature range and as a function of injection current. When the necessary forward bias conditions to get a certain current level are different, it is found that the anomalous EL quenching previously observed below 100 K for the SQW diodes strongly changes and shows a striking difference between the blue and green SQW diodes. This unusual EL evolution pattern is attributed to both internal and external fields, suggesting the importance of the internal piezoelectric field effects on the efficient carrier capture processes by localized tail states within the SQW under the presence of high-density misfit dislocations.     

Improvement of emission efficiency in polymer light-emitting devices based on phosphorescent polymers

We report improvement of emission efficiency in polymer light-emitting devices (PLEDs) employing phosphorescent polymers. A hole-blocking layer was inserted between the emissive layer and the cathode to enhance recombination efficiency for the injected holes and electrons. Aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolato (BAlq) was used for the hole-blocking layer. The resultant PLEDs exhibited significant improvement of emission efficiency. The respective external quantum efficiencies for red, green and blue PLEDs were 6.6, 11 and 6.9%. These values are very high compared with those based on conventional fluorescent polymers.     

连续输出20WAlGaAs／GaAs量子阱激光二极管线列阵

采用ＭＯＣＶＤ实现了ＡｌＧａＡｓ／ＧａＡｓ量子阱结构，获得了连续输出２０Ｗ激光二极管线列阵，线列阵长度１．０ｃｍ，激射波长８０８±４ｎｍ。     

在（311）面上所生长的Al_xGa_（1－x）As／GaAs单量子阱特性

文章报道了用分子束外延（ＭＢＥ）法在６００℃和６５０℃下，在Ｓｉ掺杂的ＧａＡｓ衬底的（３１１）Ａ和（３１１）Ｂ面上成功地生长了高质量的ＡｌｘＧａ１－Ａｓ／ＧａＡｓ单量子阱材料。计算了光荧光（ＰＬ）峰值能量，并与实验作了比较。讨论了（３１１）Ａ和（３１１）Ｂ面上的不同生长特性。     

多孔硅发光机制的分析

从量子力学的基本理论出发讨论了量子限制效应，推导出多孔硅有效禁带宽度增量并用量子限制效应和表面态及其物质在发光中作用的理论解释了ＰＳ光致发光的实验现象。     

Shallow hydrogen-induced donor in monocrystalline silicon and quantum wires

Electrical and optical properties of bistable shallow donors in monocrystalline silicon, which are introduced by proton implantation followed by annealing at 450 °C, have been studied. The temperature dependences of equilibrium and non-equilibrium carrier concentration and relaxation kinetics were investigated. IR absorption lines of bistable shallow donor electronic excitations were detected. The obtained experimental data demonstrate that the bistable shallow donors can be identified as quantum wire defect nanoclusters.     

Impurity induced disordering of GaInAs quantum wells with barriers of AlGaInAs or of GaInAsP

Impurity induced disordering of GaInAs quantum well structures with barriers of AlGaInAs and of GaInAsP has been investigated using boron and fluorine. The impurities were introduced by ion implantation followed by thermal annealing. Annealing unimplanted P-based quaternary material at temperatures greater than 500° C caused a blue shift of the exciton peak. At annealing temperatures greater than 650° C red shifts in the exciton peak of unimplanted Al-based quaternary material were observed. Boron implantation caused small blue shifts of the exciton peak in both material systems at low annealing temperatures. Much larger blue shifts were observed in the fluorine implanted samples.     

Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).