Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273-343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using alpha = alpha(max) + (alpha(min) - alpha(max))/[1 + exp 2(E - E-g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E-g(T) = E-g(273 K) - (8.8 x 10(-4)T(2))/(483 + T) + 0.088 (Varshni empirical formula) and E-g(T) = E-g(273 K)-0.231447/[exp(362/T)-1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor similar to 0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.

Analysis of temperature dependence of silicon-on-insulator thermo-optic attenuator

The temperature dependence of silicon-on-insulator thermo-optic attenuators is analysed, which originates from the temperature dependence of characteristics of multimode interference. The attenuator depth and power consumption are independent of temperature while the insertion loss depends on the temperature heavily. The variation of the insertion loss decreases from 4.3 dB to 1 dB as the temperature increases from 273 K to 343 K.

Relaxation of carriers in terbium-doped ZnO nanoparticles

Terbium ions were successfully incorporated in nano-sized zinc oxide particles with a doping concentration up to 3% by using a wet chemical route. Four narrow emission peaks of Tb3+ ions and a broad emission band of the surface states on ZnO nano-hosts were observed for all Tb-doped nanoparticles. Relaxation of carriers from excited states of ZnO hosts to rare earth (RE) dopants is disclosed by the fact that the emission intensity of Tb3+ centers increases with increased Tb content at the expense of the emission from surface defect states in ZnO matrix. (C) 2001 Elsevier Science B.V. All rights reserved.

Study of rapid carrier capture and relaxation in InAs/GaAs heterostructures

The rapid carrier capture and relaxation processes in InAs/GaAs quantum dots were studied at 77K by using a simple degenerate pump-probe technique. A rising process was observed in the transient reflectivity, following the initial fast relaxation associated with GaAs bulk matrix, and this rising process was assigned to be related to the carrier capture from the GaAs barriers to InAs layers. The assignment was modeled using Kramers-Kronig relation. By analyzing the rising process observed in the transient reflectivity, the carrier capture time constants were obtained. The measured capture times decrease with the increase of carrier concentration.

New annealing processes and explanation for novel silicon pn junctions formed by proton implantation

Proton-implanted n-type Si wafers were annealed at 950 degrees C to achieve novel pn junctions. The novel pn junctions are explained by the combined use of four models. The background (e.g. oxygen impurity) of an Si wafer is suggested to play a key role in creating the novel pn junction.

Optical anisotropy of InAs submonolayer quantum wells in a (311) GaAs matrix

The steplike density of states obtained from reflectance-difference spectroscopy demonstrates that ultrathin InAs layers should be regarded as two-dimensional quantum wells rather than isolated clusters, even for the sample with only 1/3 monolayer InAs in (311)-oriented GaAs. The degree of anisotropy is within the intrinsic anisotropy of (311)-oriented ultrathin quantum wells, indicating that there is little structural or strain anisotropy in the InAs islands. (C) 1998 Elsevier Science B.V.

环戊二烯基轻稀土烷基配合物的合成、表征和反应性能的研究

本文通过LnCl_3·2LiCl (Ln=La, Nd)和等摩尔的CH_3C_5H_4Na在四氢呋喃中于室温反应，得到了一种新的阴离子型配合物[Li(THF)_2]_2 (M-Cl)_4[(η~5-CH_3C_5H_4)Ln·THF] (Ln=La, Nd)，对其进行了元素分析，红外光谱和核磁谱的鉴定，并测得了[Li(THF)_2]_2 (M-Cl)_4[(η~5-CH_3C_5H_4)Nd·THF]的X-光晶体结构。晶体属单斜晶系，空间群为P_(21/n)，晶脆参数为a=12.130(5)，b=17.343(5)，c=17.016(5)A，β=108.54(3)°，V=3393.87A~3，E=4，R=0.0505，中心钕原子分别与CH_3Cp~-。THF和四个桥氯配位，形成稳定的八配位的八面体结构。通过[Li(THF)_2]_2(M-Cl)_4[(η~5-CH_3C_5H_4)Nd·THF]与2摩尔的t-BuLi在四氢呋喃和戊烷的混和溶液中反应，分离得到一种新的阴离子型配合物晶体，经元素分析，红外光谱，核磁和水解色质谱的鉴定，证明为[Li(DME)_3][(η~5-CH_3C_5H_4)Nd(t-Bu)_3]，此配合物不仅对空气和水汽极为敏感，而且对温度也很敏感，即使在-5 ℃下放置也会逐渐分解，发生β-H消除。我们进一步研究了LnCl_3·2LiCl与2摩尔的C_5H_5Na的四氢呋喃反应液在低温下与等摩尔的CH_3Li乙醚溶液反应，从中分离得到一种新的阴离子型烷基配合物晶体[Li(DME)_3][(η~5-C_5H_5)_3LnCH_3] (Ln=La, Nd)对其进行了元素分析，红外光谱，核磁和水解色质谱的鉴定。此外，从这一反应中还得到另一种副产物，经X-光结构鉴定为[Li(DME)_3]~+{[(η~5-C_5H_5)Nd(M-Cl)_2Nd(η~5-C_5H_5)](M-Cl)_4(M_4-D)[(η~5-C_5H_5)Nd(M-CH_3)_2Nd(η~5-C_5H_5)]}~(2-)[Li(DME)_3]~+。本文还研究了NdCl_3·2LiCl与2倍摩尔的CH_3C_5H_4Na的四氢呋喃反应液和等摩CH_3Li乙醚溶液于-78 ℃下反应，从中分离得到另一种新的阴离子型配合物[Li(DME)_3][(η~5-CH_3C_5H_4)_3NdCH_3]并对其进行了元素分析，红外光谱的鉴定。我们还研究了配合物[Li(DME)_3][(η~5-CH_3C_5H_4)Nd(t-Bu)_3]对苯乙烯的催化聚合活性，发现它可以单独引发苯乙烯聚合，得无规聚苯乙烯。