Photovoltage and luminescence study of stacked InAs/GaAs self-organized quantum dots

The ground and excited state excitonic transitions of stacked InAs self-organized quantum dots (QDs) in a laser diode structure are studied. The interband absorption transitions of QDs are investigated by non-destructive PV spectra, indicating that the strongest absorption is related to the excited states with a high density and coincides with the photon energy of lasing emission. The temperature and excitation (electric injection) intensity dependences of photoluminescence and electroluminescence indicate the influence of state filling effect on the luminescence of threefold stacked QDs. The results indicate that different coupling channels exist between electronic states in both vertical and lateral directions.

Optical transitions and type-II band lineup of MBE-grown GaNAs/GaAs single-quantum-well structures

We have investigated transitions above and below band edge of GaNAs/GaAs and InGaNAs/GaAs single quantum wells (QWs) by photoluminescence (PL) as well as by absorption spectra via photovoltaic effects. The interband PL peak is observed to be dominant under high excitation intensity and at low temperature. The broad luminescence band below band edge due to the nitrogen-related potential fluctuations can be effectively suppressed by increasing indium incorporation into InGaNAs. In contrast to InGaNAs/GaAs QWs, the measured interband transition energy of GaNAs/GaAs QWs can be well fitted to the theoretical calculations if a type-II band lineup is assumed. (C) 2001 Elsevier Science B.V. All rights reserved.

Static and dynamic electric field domain formation in a doped GaAs/AlAs superlattice

We have observed the transition from static to dynamic electric field domain formation induced by a transverse magnetic field and the sample temperature in a doped GaAs/AlAs superlattice. The observations can be very well explained by a general analysis of instabilities and oscillations of the sequential tunnelling current in superlattices based solely on the magnitude of the negative differential resistance region in the tunnelling characteristic of a single barrier. Both increasing magnetic field and sample temperature change the negative differential resistance and cause the transition between static and dynamic electric field domain formation. (C) 2000 Elsevier Science B.V. All rights reserved.

Methods to tune the electronic states of self-organized InAs/GaAs quantum dots

After capping InAs islands with a thin enough GaAs layer, growth interruption has been introduced. Ejected energy of self-organized InAs/GaAs quantum dots has been successfully tuned in a controlled manner by changing the thickness of GaAs capping layer and the time of growth interruption and InAs layer thickness. The photoluminescence (PL) spectra showing the shift of the peak position reveals the tuning of the electronic states of the QD system. Enhanced uniformity of Quantum dots is observed judging from the decrease of full width at half maximum of FL. Injection InAs/GaAs quantum dot lasers have been fabricated and performed on various frequencies. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Self-sustained oscillations caused by magnetic field in a weakly-coupled GaAs/AlAs superlattice

We have investigated the influence of transverse magnetic field B up to 14 T at 1.6 K on the tunneling processes of electric field domains in doped weakly coupled GaAs/AlAs superlattices. Three regimes, i.e, stable field domains, current self-sustained oscillations and averaged field distribution are successively observed with increasing B. The mechanisms of switching-over among these regimes are due to B-induced modification of the dependence of the effective electron drift velocity on electric field. The simulated calculation gives a good agreement with the observed experimental results. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Photocurrent derivative spectra of ZnCdSe-ZnSe double multi-quantum wells

Photocurrent (PC) spectra of ZnCdSe-ZnSe double multi-quantum wells are measured at different temperature. Its corresponding photocurrent derivative (PCD) spectra are obtained by computing, and the PCD spectra have greatly enhanced the sensitivity of the relative weak PC signals. The polarization dependence of the PC spectra shows that the transitions observed in the PC spectra are heavy-hole related, and the transition energy coincide well with the results obtained by envelope function approximation including strain. The temperature dependence of the photocurrent curves indicates that the thermal activation is the dominant transport mechanism of the carriers in our samples. The concept of saturation temperature region is introduced to explain why the PC spectra have different temperature dependence in the samples with different structure parameters. It is found to be very useful in designing photovoltaic devices.

Structural and photoelectric studies on double barrier quantum well infrared detectors

GaAs/AlAs/GaAlAs double barrier quantum well (DBQW) structures are employed for making the 3 similar to 5 mu m photovoltaic infrared (IR) detectors with a peak detectivity of 5x10(11) cmHz(1/2)/W at 80K. The double crystal x-ray diffraction is combined with synchrotron radiation x-ray analysis to determine the exact thickness of GaAs, AlAs and GaAlAs sublayers. The interband photovoltaic (PV) spect ra of the DBQW sample and the spectral response of the IR photocurrent of the devices are measured directly by edge excitation method, providing the information about spatial separation processes of photogenerated carriers in the multiquantum wells and the distribution of built-in field in the active region.

Formation mechanism of electric field domains

Hydrostatic pressure measurements are used to investigate the formation mechanism of electric field domains in doped weakly-coupled GaAs/AlAs superlattices. For the first plateau-like region in the I-V curve, two kinds of sequential resonant tunnelling are observed. For P<2 kbar the high-field domain is formed by the Gamma-Gamma process, while for P>2 kbar the high-field domain is formed by the T-X process. For the second plateau-libe region, the high-field domain is attributed to Gamma-X sequential resonant tunnelling. (C) 1998 Elsevier Science B.V. All rights reserved.

Electronic and vibrational Raman scattering in resonance with yellow luminescence transitions in GaN on sapphire substrate

We analyze low-temperature Raman and photoluminescence spectra of MBE-grown GaN layers on sapphire. Strong and sharp Raman peaks are observed in the low frequency region. These peaks, which are enhanced by excitation in resonance with yellow luminescence transitions, are attributed to electronic transitions related to shallow donor levels in hexagonal GaN. It is proposed that a low frequency Raman peak at 11.7 meV is caused by a pseudo-local vibration mode related to defects involved in yellow luminescence transitions. The dependence of the photoluminescence spectra on temperature gives additional information about the residual impurities in these GaN layers.

香根草生态学特性及其应用研究

用百喜草(Paspalum notatum)和水花(Alternanthera philoxeroides)等草本植物作对比，通过叶片离体实验、温室盆栽实验、田间小区实验和大田推广实验，并结合室内分析，较系统地研究了香根草(Vetiveria zizanioides)的抗热、抗冻和抗盐特性和对污水的净化效果。此外，还探讨了矮化香根草和促进香根草分蘖的可能性；提出了香根草生态工程的思想，阐述了香根草生态工程推广应用的价值与效果；通过对国内野生香根草资源的调查和标本鉴定,指出了过去对这个种的错误描述，提出了保护野生香根草资源和香根草湿地的紧迫性与重要意义。主要研究结果如下： 1.离体叶片电解质渗漏的测定结果表明，3种植物对高温、冻害、干旱的抵抗力依次为香根草 > 百喜草 > 水花生，特别是在胁迫程度高、幅度大或胁迫时间长时，香根草所表现出的强抗逆性更加明显。香根草有比百喜草和水花生更强的适应能力。2.叶片电导率的实验结果显示，香根草、水花生和百喜草对盐胁迫的反应相差不明显，但盆栽实验则显示出较大差异，抗盐能力明显表现为香根草 > 水花生 > 百喜草。导致3种植物生物量下降10％的EC_(se)值分别为6.4、5.1和4.3 dS m~(-1)；导致生物量下降50％的EC_(se)分别为20.9、16.2和11.5 dS m~(-1)。在抗盐等级划分上，香根草和水花生属抗盐植物种类，百喜草属中度抗盐种类。总的来说，3种植物都表现出了相对较强的抗盐特性。以上结果还表明考察植物的抗盐性时必须用整株做为实验材料，离体叶片或组织培养所得到的结果不能作为植物抗盐能力的标准。3.植物体内Na~+和Cl~-含量随土壤盐度水平升高而升高，而茎叶中的Na~+和Cl~-又直接影响到植物的生物量、叶面积、株高等。这表明盐度对3种植物生长发育的影响相当明显，且植物体的Na~+和Cl~-很可能都是影响植物生长发育的直接因子。然而，叶片光合色素受Na~+或Cl~-的影响不甚明显，而且在Na~+和Cl~-的浓度不高时色素含量还随茎叶中Na~+和Cl~-的浓度升高而显现增加趋势，即使Na~+和Cl~-的浓度较高时，光合色素下降的幅度也远不及生物量的大。这可能由于光合作用是被“反馈抑制”，而不是直接被Na~+或Cl~-抑制。4.3种植物表现出完全不同抗盐机理。香根草表现出拒盐特性，而且将所吸收的盐分的大部分(尤其是Na~+)滞留在根系内：水花生表现为稀盐特性，即当植株体内含盐量增加时，其水分含量亦增加，并趋向肉质化；百喜草有较强的吸盐能力，并可能通过盐腺泌出体内高浓度的盐分且通过加速老叶死亡来排除过多的盐。5.广州市李坑垃圾卫生填埋场的渗滤污水含有高浓度的污染物，经过人工净化后仍未 达到排放标准，对植物产生毒害并带来严重的环境污染，须作进一步净化，并建议开展生物净化来辅助物理和化学净化。6.在被观测的4种植物中，风眼莲(Eichhornia crassipes)在高浓度和低浓度2种污水中均被毒害致死；百喜草在高浓度的渗滤液中不能存活，在低浓度中受严重伤害；水花生在高浓度污水中受较重伤害，但在低浓度形成庞大生物体，这可能是污水的“富营养化”所致；香根草在这2种污水中亦受伤害，但受害程度为4种植物中最轻。总的来说，它们的抗污能力和去污效果表现为香根草 > 水花生 > 百喜草 > 风眼莲。7.在污水中表现相对较好的2种植物中，水花生对低浓度污水的净化效果总体上好于香根草，尤其对总氮和硝态氮的净化效果明显好于香根草，但香根草对高浓度污水中的7种“污物”的净化效果均优于水花生，且对低浓度污水中的磷与COD的净化亦优于水花生。在被观测的7种水质指标中，植物对氨氮的净化效果最好，净化率在77％～91％之间。此外，香根草对磷也表现出了较强的净化能力，净化率高达70％以上。香根草和水花生可分别作为高、低2种浓度垃圾污水的净化植物来辅助净化污水。8.香根草虽不是水生植物，但能在水中生长。用香根草作为污水的净化植物时须用支撑物将其固定；香根草和水花生都生长迅速，生物体高大，因此用它们净化被污染的环境时，还应适时对其地上部分进行剪割以保证新生的植株体能不断地吸收水中“污物”，使它们都能成为“超级生物累积器”。但是，任何单一植物对污水或污物的净化能力都是有限的，要想对垃圾污水产生好的净化效果，最好的办法也许是将香根草、水花生等多种有良好净化效果的植物有机结合起来，同时种植在污水中。9.植物通过吸收来去除“污物”的能力是很有限的。香根草和水花生对污水中N、P、Cl~-的吸收量只占净化量的一部分，和原液中N、P、Cl~-的含量相比，吸收量所占的比例更低。植物净化系统中的去污方式除根系的吸收作用外，还有根系的吸附、元素的沉降、固结和挥发、水体中微小动物和微生物的作用等，即是通过根系微生态系统的综合作用来达到净化目的。10.植物生长延缓剂对香根草的株高生长、分蘖速度和抽穗开花等方面都产生影响，但不同的药剂种类和浓度所产生的影响相差甚远。低浓度的延缓剂不仅不抑制香根草的株高生长，而且还有显著地促进作用，高浓度虽有抑制作用，但这种抑制也只是短期的，一般不超过2个月。延缓剂对分蘖的形成具有较好的促进作用，而且这种促进作用持续3个月左右。3种延缓剂中，以低浓度的B9促进分蘖的效果最显著，比对照高出50％。不同的延缓剂对香根草抽穗扬花的影响表现不同，PP333具有一定的促进效果，而CCC和B9-尤其是B9-表现出明显的抑制作用。总的来说，B9对香根草矮化、促进分蘖和抑制开花方面产生的效果要好于CCC和PP333。11.植物生长延缓剂对香根草的作用效果与对一般作物的作用效果相差较远，这可能是香根草的抗性较强，延缓剂对它的影响不明显所致。总的来说，植物生长延缓剂对香根草株高生长的抑制效果不理想。有关这方面的研究还有待进一步开展。12.公路滑坡不仅带来严重的交通阻塞，而且危及到行车与行人的安全。常规治理公路滑坡的手段是采取砌石墙的工程措施，这不仅费用昂贵，而且缺乏生态效益，也难以达到根治效果。应用香根草生态工程(The Vetiver Eco-engineering)对滑坡路段进行综合治理,能产生良好护坡固土效果，而且所需成本仅为机械工程的12％～18％。香根草生态工程是指以香根草为主要或核心草种，同时与一些适合当地生长的植物—包括乔木、灌木、草本、藤本一进行有机搭配，并在需要时辅以一定的工程措施，用于水土流失治理和退化生态系统的恢复的植物生态工程措施。13.为了使香根草生态工程尽快生效，应采取一定的栽培管理措施。香根草应等高密植，注意施肥，前期不要被遮光；所搭配的植物应该是抗逆性强，乔、灌、草、藤结合起来，并注意在香根草带中间适当种植一些生长迅速、抗瘠能力强、覆盖效果好的草本植物，这不仅能使坡面更稳固，而且能变得更美观。总之，香根草生态工程在公路护坡方面所产生的经济效益、社会效益和生态效益都是相当明显的，在我国南方地区有着广阔的应用前景。14.一般认为，香根草原产印度，然而，中国亦有天然的香根草群落分布。早在70多年前就在海南发现野生的香根草种，1957年又在广东吴川发现一片面积达6000多hm~2的湿生性的天然香根草群落。40年来，这片珍贵的热带湿地遭到严重破坏，香根草的面积和密度都大幅度下降。对此，笔者建议尽快在当地划一块自然保护区，这不仅是为了保护珍贵的野生香根草资源，保护当地的生物多样性和生态环境，而且能为研究热带湿地和草原提供理想基地，并为研究香根草的起源、系统演化和分类提供理想素材。15.过去的文献一直把广东吴川这片湿地中的野生香根草种认为是V. nigritana,但本文的标本鉴定认为它并不是V. nigritana或其它种，而是V. zizanioides。不过，引种栽培观测和RAPD技术研究都表明，这一野生种和普通的栽培种属于同一个种的不同生态型或基因型，而导致形成这种差异的原因是由于野生种生长在长年渍水或干湿交替的环境所致。16.目前对香根草的起源和系统演化知之甚少，香根草属的系统分类还很不完善，有 关香根草对重金属、污染物的抗性机理等诸多方面都还不太清楚或很不清楚。因此，对 香根草这一神奇植物的研究还有待继续深入地开展下去。总之，香根草这一多年生的禾本科植物对盐、碱、酸、瘦、冷、热、早、涝都表现出了较强抗性，在水土流失防治、退化生境恢复、污染环境净化、贫瘠土壤改良、农田小气候改善等方面都表现出了较好效果。香根草无愧是一种神奇之草。然而，要真正将香根草生态工程在我国南方地区大规模推广开来，还需要做较多的工作，这包括更深一步的科学研究、更广泛的宣传、培训与示范、更充分的资源利用与经济创收等。