Self-assembled quantum dots, wires and quantum-dot lasers

Molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs and InAs/InAlAs/InP quantum dots (QDs) and quantum wires (QWRs) have been studied. By adjusting growth conditions, surprising alignment. preferential elongation, and pronounced sequential coalescence of dots and wires under specific condition are realized. The lateral ordering of QDs and the vertical anti-correlation of QWRs are theoretically discussed. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 3.6 W from both uncoated facets is achieved fi-om vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). A RT CW output power of 0.6 W/facet ensures at least 3570 h lasing (only drops 0.83 dB). (C) 2001 Elsevier Science B.V, All rights reserved.

High power 980 nm Al-free strained quantum-well lasers for erbium-doped fiber amplifiers

In this paper, we reported on the fabrication of 980 nm InGaAs/InGaAsP strained quantum-well (QW) lasers with broad waveguide. The laser structure was grown by low-pressure metalorganic chemical vapor deposition on a n(+)- GaAs substrate. For 3 mu m stripe ridge waveguide lasers, the threshold current is 30 mA and the maximum output power and the output power operating in fundamental mode are 350 mW and 200 mW, respectively. The output power from the single mode fiber is up to 100 mW, the coupling efficiency is 50%. We also fabricated 100 mu m broad stripe coated lasers with cavity length of 800 mu m, a threshold current density of 170 A/cm(2), a high slope efficiency of 1.03 W/A and a far-field pattern of 40 x 6 degrees are obtained. The maximum output power of 3.5 W is also obtained for 100 mu m wide coated lasers. (C) 2000 Elsevier Science B.V. All rights reserved.

Mechanism on exciton-mediated energy transfer in erbium-doped silicon

Exciton-mediated energy transfer model in Er-doped silicon was presented. The emission intensity is related to optically active Er concentration, lifetime of excited Er3+ ion and spontaneous emission. The thermal quenching of the Er luminescence in Si is caused by thermal ionization of Er-bound exciton complex and nonradiative energy back-transfer processes, which correspond to the activation energy of 6.6 and 47.4 meV, respectively. Er doping in silicon introduces donor states, a large enhancement in the electrical activation of Er (up to two orders of magnitude) is obtained by co-implanting Er with O. It appears that the donor states are the gateway to the optically active Er. (C) 2000 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.

Strain relaxation of GeSi alloy with low dislocation density grown on low-temperature Si buffers

We have developed a low-temperature (LT) growth technique. Even with Ge fraction x upto 90%, the total thickness of fully relaxed GexSi1-x buffers can he reduced to 1.7 mu m with dislocation density lower than 5 x 10(6) cm(-2). The surface roughness is no more than 6 nm. The strain relaxation is quite inhomogeneous From the beginning. Stacking faults generate and form the mismatch dislocations in the interface of GeSi/LT-Si. (C) 1999 Elsevier Science B.V. All rights reserved.

High-field cyclotron resonance and electron-phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B > similar to 23 T), the three samples behave very differently. For the most lightly doped sample (3 x 10(11) cm(-2)) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6 x 10(11) cm(-2)) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2 x 10(12) cm(-2)) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR. (C) 1998 Elsevier Science B.V. All rights reserved.

2-C-支链-1,6-脱水吡喃葡萄糖的开环反应及吡喃烯糖的合成研究

在糖化学合成中，1,6-脱水吡喃糖不仅是合成具有生物活性低聚糖、糖共体、抗原、抗体以及天然产物等化合物重要原料，而且还是许多具有生物活性的天然产物的结构单元。同时，它还具有[3,2,1]的双环缩醛结构，使其在糖化学合成中具有高的立体选择性和区域选择性，同时减少了C-1 和C-6 位的保护和去保护的优点。此外，环内的缩醛开环后，又可以相应地在C-1 和C-6 位进行官能团转化以及糖苷化反应。 本文报道了一种新的1,6-脱水吡喃糖的合成方法，并设计合成了2-C-支链-1,6-脱水吡喃葡萄糖1-195、1-197、1-198 以及2-C-支链-6-硫代1,6-脱水吡喃葡萄糖1-225。到目前为止，1,6-脱水糖开环并进行糖苷化反应，存在选择性较差、产率低的缺点。我们发现，在乙腈做溶剂的条件下，NiCl5 能高立体选择性高产率地催化化合物1-195、1-197、1-198 开环并与ROH、RSH 发生糖苷化反应。在NiCl5-乙腈条件下，合成了一系列2-C-支链-α-糖苷和2-C-支链-β-硫代糖苷，并对2-C-支链1,6-脱水吡喃葡萄糖的生成机理以及开环机理进行了探讨。 烯糖在糖化学合成中是重要的起始原料，从Fischer 首次合成烯糖至今，一直不断地有新的合成方法出现。但目前文献报道的方法存在所用试剂有毒、价格贵和操作繁琐等缺点。我们对Fischer-Zach 方法进行了改进， 发现Zn-NaH2PO4-H2O 和Zn-PEG600-H2O 体系都能很好地合成烯糖。该方法具有条件温和、绿色环保、操作简单的优点。在Zn-NaH2PO4 溶液或Zn-PEG600 条件下，以溴代糖为原料，高产率地合成一系列的烯糖。 The 1,6-anhydrohexopyranoses are crucial subunits of myriad bioactive nature products, as well as important syntons of carbohydrate chemistry which have been extensively used to prepare the biologically potential oligosaccharides, glycoconjugates, antibiotics, and structurally varied nature products. Their particular [3.2.1] bicyclic skeleton makes them have high regio- and stereo-control in a variety of reactions, and such structure avoids protecting hydroxyl groups at C1 and C6.Additionally, the cleavage of the internal acetal under acidic conditions could be beneficial for further transformations of functional group and glycosylation of the corresponding pyranosyl sugar at the C6 or C1 site. Herein we developed a novel approach to prepare the 1,6-anhydrohexopyranose, and synthesized the 2-C-branched-1,6-anhydrohexopyranose 1-195, 1-197, 1-198 and 2-C-branched-6-thio-1,6-anhydrohexopyranose 1-225. Until now, glycosylation of 1,6-anhydrohexopyranoses has been limited because of the low yields and low stereoselectivity. In this paper, we found that NiCl5-MeCN system could selectively cleave the ring of 1,6-anhydrohexopyranoses with alcohols and thiols at room temperature in high yields. A series of 2-C-branched-α-glycosides and 2-C-branched-β-thioglycosides have been synthesized via NiCl5-catalyzed. Furthermore, we investigated the formation and ring-opening mechanism of 2-C-acetylmethyl-1,6-anhydrohexopyranose. Glycals are significant starting material in carbohydrate chemistry. After the Fischer-Zach method for forming glucal was reported for the first time, the numerous synthetic methods for glycals have been explored. However, there are several drawbacks in the existing methods, such as the usage of very expensive and toxic reagents, intricate operation, and the influence of acid-sensitive and base-sensitive functional group. We improved the Fischer-Zach method and developed a facile, mild and environmentally benign methodology towards the synthesis of the glycals in Zn-NaH2PO4-H2O or Zn-PEG600-H2O system. Our method involves the treatment of glycosyl bromides with Zn in NaH2PO4 aqueous solution or PEG600-H2O at room temperature, affording various glycals in excellent yields.