Preparation and characterization of erbium doped sol-gel silica glasses

Erbium-doped silica glasses were made by sol-gel process. Intensive photoluminescence (PL) spectra from the Er-doped silica glasses at room temperature were measured. A broadband peak at 1535 ma, corresponding to the I-4(13/2)-I-4(15/2) transition, its full width at half-maximum (FWHM) of 10 nm, and a shoulder at 1546 nm in the PL spectra were observed. At lower temperatures, main line of 1535 nm and another line of 1552 Mn instead of 1546 nm appear. So two types of luminescence centers must exist in the samples at different temperature. The intensity of main line does not decrease obviously with increasing temperature. By varying the Er ion concentration in the range of 0.2 wt% - 5wt%, the highest photoluminescence intensity was obtained at 0.2wt% erbium doped concentration. Luminescence intensity decreases with increasing erbium concentration. Cooperative upconversion was used to explain the concentration quenching of luminescence from silica glass with high erbium concentration. Extended X-ray absorption fine structure measurements were carried out. It was found that the majority of the erbium impurities in the glasses have a local structure of eight first neighbor oxygen atoms at a mean distance of 0.255 nm, which is consistent with the typical coordination structure of rare earth ion.

A study on GaP/Si heterostructures grown by GS-MBE

GaP/Si is a promoting heterostructure for Si-based optoelectronic devices since lattice constants of GaP and Si are so closed that they can match with each other. GaP was successfully grow on (100) Si subtracts by Gas-Source Molecular Bean Epitaxy (GS-MBE) in the study. The GaP/Si heterostructure was characterized by X-ray double crystal diffraction, Anger electron spectrograph, X-ray photonic spectrograph and photoluminescence (PL) measurements. The results showed that the epitaxial GaP layers are single crystalline, in which a parallel to and a (perpendicular to)are 0.54322 and 0.54625 nm, respectively. The peaks in PL spectra of GaP epitaxial layer grown on Si are 650, 627 and 640 nm, respectively. The study demonstrated that GaP/Si is a kind of lattice matched heterostructures and will be a promoting materials for future integrated photonics.

The effect of low temperature GaAs nucleation on the growth of GaN on Silicon (001) during MOVPE process

High quality cubic GaN was grown on Silicon (001) by metalorganic vapor phase epitaxy (MOVPE) using a GaAs nucleation layer grown at low temperature. The influence of various nucleation conditions on the GaN epilayers' quality was investigated. We found that the GaAs nucleation layer grown by atomic layer epitaxy (ALE) could improve the quality of GaN films by depressing the formation of mixed phase. Photoluminescence (PL) and X-ray diffraction were used to characterize the properties of GaN epilayers. High quality GaN epilayers with PL full width at half maximum (FWHM) of 130meV at room temperature and X-ray FWHM of 70 arc-min were obtained by using 10-20nm GaAs nucleation layer grown by ALE.

Study of infrared luminescence from Er-implanted GaN films

In this study, we report the dependences of infrared luminescence properties of Er-implanted GaN thin films (GaN:Er) on the kinds of substrates used to grow GaN, the growth techniques of GaN, the implantation parameters and annealing procedures. The experimental results showed that the photoluminescence (PL) intensity at 1.54 mum was severely influenced by different kinds of substrates. The integrated PL peak intensity from GaN:Er /Al2O3 (00001) was three and five times stronger than that from GaN:Er /Si (111) grown by molecular beam epitaxy (MBE) and by metalorganic chemical vapor deposition (MOCVD), respectively. The PL spectra observed from GaN:Er/Al2O3 (0001) grown by MOCVD and by MBE displayed a similar feature, but those samples grown by MOCVD exhibited a stronger 1.54 mum PL. It was also found that there was a strong correlation between the PL intensity with ion implantation parameters and annealing procedures. Ion implantation induced damage in host material could be only partly recovered by an appropriate annealing temperature procedure. The thermal quenching of PL from 15 to 300 K was also estimated. In comparison with the integrated PL intensity at 15 K, it is reduced by only about 30 % when going up to 300 K for GaN:Er/Al2O3 sample grown by MOCVD. Our results also show that the strongest PL intensity comes from GaN:Er grown on Al2O3 substrate by MOCVD. (C) 2004 Elsevier B.V. All rights reserved.

Correlation between Er3+ emission and the microstructure of a-SiOx : H < Er > films

Photoluminescence (PL) from Er-implanted hydrogenated amorphous silicon suboxide (a-SiOX:H(x<2.0)) films was measured. Two luminescence bands with maxima at lambda congruent to 750 nm and lambda congruent to 1.54mum, ascribed to the a-SiOX:H intrinsic emission and Er3+ emission, were observed. Peak intensities of the two bands follow the same trend as a function of annealing temperature from 300 to 1000degreesC. Micro-Raman results indicate that the a-SiOX:H films are a mixture of two phases, an amorphous SiOX matrix and amorphous silicon (a-Si) domains embedded there in. FTIR spectra confirm that hydrogen effusion from a-SiOX:H films occurs during annealing. Hydrogen effusion leads to a reconstruction of the microstructure of a-Si domains, thus having a strong influence on Er3+ emission. Our study emphasizes the role of a-Si domains on Er3+ emission in a-SiOX:H films.

Influences of initial nitridation process on the optical and structural characterization of GaN layer grown on sapphire (0001) by metalorganic chemical vapor deposition

In this paper. we investigate the influences of the initial nitridation of sapphire substrates on the optical and structural characterizations in GaN films. Two GaN samples with and without 3 min nitridation process were investigated by photoluminescence (PL) spectroscopy in the temperature range of 12-300 K and double-crystal X-ray diffraction (XRD). In the 12 K PL spectra of the GaN sample without nitridation, four dominant peaks at 3.476, 3.409 3.362 and 3.308 eV were observed, which were assigned to donor bound exciton, excitons bound to stacking faults and extended structural defects. In the sample with nitridation, three peaks at 3.453, 3.365. and 3.308 eV were observed at 12 K, no peak related to stacking faults. XRD results at different reflections showed that there are more stacking faults in the samples without nitridation.

Orientation relationship between hexagonal inclusions and cubic GaN grown on GaAs(001) substrates

Cubic GaN/GaAs(0 0 1) epilayers and hexagonal inclusions are characterized by X-ray diffraction (XRD), Photoluminescence (PL), Raman spectroscopy, and transmission electron microscopy (TEM). The X-ray {0 0 0 2} and (1 0 (1) over bar 0) pole figures show that the orientation relationships between cubic GaN and hexagonal inclusions are (1 1 1)//(0 0 0 1), <1 1 2 >//<1 0 (1) over bar 0 >. The distribution of hexagonal inclusions mainly results from the interfacial bonding disorder in the grain boundaries parallel to hexagonal <0 0 0 1 > directions and the lattice mismatch in <0 0 0 1 > directions on {1 0 (1) over bar 0} planes. In order to reduce the energy increase in cubic epilayers, hexagonal lamellas with smaller sizes in <0 0 0 1 > directions often nucleate inside the buffer layer or near the interface between the buffer layer and the epitaxial layer, and penetrate through the whole epitaxial layer with this orientation relationship. (C) 2001 Elsevier Science B.V. All rights reserved.

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.

Growth and characterization of GaInNAs/GaAs by plasma-assisted molecular beam epitaxy

We have studied the growth of GaInNAs/GaAs quantum well (QW) by molecular beam epitaxy using a DC plasma as the N sourer. The N concentration was independent of the As pressure and the In concentration, but inversely proportional to the growth rate. It was almost independent of T, over the range of 400-500 degreesC, but dropped rapidly when T-g exceeded 500 degreesC. Thermally-activated N surface segregation is considered to account for the strong falloff of the N concentration. As increasing N concentration, the steep absorption edge of the photovoltage spectra of GaInNAs/GaAs QW became gentle, the full-width at half-maximum of the photoluminescence (PL) peal; increased rapidly, and a so-called S-shaped temperature dependence of PL peak energy showed up. All these were attributed to the increasing localized state as N concentration. Ion-induced damage was one of the origins of the localized state. A rapid thermal annealing procedure could effectively remote the localized state. (C) 2001 Elsevier Science D.V. All rights reserved.

Quality improvement of GaInNAs/GaAs quantum wells grown by plasma-assisted molecular beam epitaxy

The optimum growth condition of GaInNAs/GaAs quantum wells (QWs) by plasma-assisted molecular beam epitaxy was investigated. High-resolution X-ray diffraction and photoluminescence (PL) measurements showed that ion damage drastically degraded the quality of GaNAs and GaInNAs QWs and that ion removal magnets can effectively remove the excess ion damage. Remarkable improvement of PL intensity and obvious appearance of pendellosung fringes were observed by removing the N ions produced in the plasma cell. When the growth rate increased from 0.73 to 1.2 ML/s, the optimum growth temperature was raised from 460 degreesC to 480 degreesC and PL peak intensity increased two times. Although the N composition decreased with increasing growth rate, degradation of optical properties of GaInNAs QWs was observed when the growth rate was over 0.92 ML/s. Due to low-temperature growth of GaInNAs QWs, a distinctive reflection high-energy electron diffraction pattern was observed only when the GaAs barrier was grown under lower As-4 pressure. The samples with GaAs barriers grown under lower As-4 pressure (V/III ratio about 24) exhibited seven times increase in PL peak intensity compared with those grown under higher As-4 pressure (V/III ratio about 50). (C) 2001 Elsevier Science B,V. All rights reserved.

Modification of emission wavelength of self-assembled In(Ga)As/GaAs quantum dots covered by InxGa1-xAs(0 <= x <= 0.3) layer

Red shifts of emission wavelength of self-organized In(Cla)As/GaAs quantum dots (QDs) covered by 3 nm thick InxGa1-xAs layer with three different In mole fractions (x = 0.1, 0.2 and 0.3, respectively) have been observed. Transmission electron microscopy images demonstrate that the stress along growth direction in the InAs dots was reduced due to introducing the InxGa1-xAs (x = 0.1, 0.2 and 0.3) covering layer instead of GaAs layer. Atomic force microscopy pictures show a smoother surface of InAs islands covered by an In0.2Ga0.8As layer. It is explained by the calculations that the redshifts of the photoluminescence (PL) spectra from the QDs covered by the InxGa1-xAs (x greater than or equal to 0.1) layers were mainly due to the reducing of the strain other than the InAs/GaAs intermixing in the InAs QDs. The temperature dependent PL spectra further confirm that the InGaAs covering layer can effectively suppress the temperature sensitivity of PL emissions. 1.3 mum emission wavelength with a very narrow linewidth of 19.2 mcV at room temperature has been obtained successfully from In,In0.5Ga0.5As/GaAs self-assembled QDs covered by a 3-nm In0.2Ga0.2As strain reducing layer. (C) 2001 Elsevier Science B.V. All rights reserved.

Type I-type II transition of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots

The photoluminescence (PL) of In0.55Al0.45As/Al0.5Ga0.5As self-assembled quantum dots has been measured at 15 and 80 K under hydrostatic pressure. The lateral size of the dots ranges from 7 to 62 nm. The emissions from the dots with 26, 52 and 62 nm size have a blue shift under pressure, indicating that these quantum dots have the normal type-I structure with lowest conduction band at the Gamma -valley. However, the PL peak of dots with 7 nm diameter moves to lower energy with increasing pressure. It is a typical character for the X-related transition. Then these small dots have a type-II structure with the X-valley as the lowest conduction level. An envelope-function calculation confirms that the Gamma -like exciton transition energy will rise above the X-like transition energy in the In0.55Al0.45As/Al0.5Ga0.5As structure if the dot size is small enough.

Optical properties of self-assembled ternary In(GA/Al)As quantum dots on (100) and (N 1 1)B InP substrates

In this paper, we investigated the self-assembled quantum dots formed on (100) and (N11)B (N = 2, 3, 4, 5) InP substrates by molecular beam epitaxy (MBE). Two kinds of ternary QDs (In0.9Ga0.1As and In0.9Al0.1As QDs) are grown on the above substrates; Transmission electron microscopy (TEM) and photoluminescence (PL) results confirm QDs formation for all samples. The PL spectra reveal obvious differences in integral luminescence, peak position, full-width at half-maximum and peak shape between different oriented surfaces. Highest PL integral intensity is observed from QDs on (411)B surfaces, which shows a potential for improving the optical properties of QDs by using high-index surface. (C) 2000 Elsevier Science B.V. All rights reserved.

Characterization of CdSe and CdSe/CdS core/shell nanoclusters synthesized in aqueous solution

CdSe nanoclusters overcoated with CdS shell were prepared with macapoacetic acid as stabilizer. The optical properties of CdSe nanoclusters and the influence of CdS shell on the electronic structures of CdSe cores were studied by optical absorption, photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Based on PL and PLE results and the theoretical calculation on fine structure of bandedge exciton, a model of formation of excimer within the small clusters was proposed to explain the large Stokes shift of luminescence from absorption edge observed in PL results. (C) 2000 Elsevier Science B.V. All rights reserved.

Epitaxial growth of GaNAs/GaAs heterostructure materials

A series of systematic experiments on the growth of high quality GaNAs strained layers on GaAs (001) substrate have been carried out by using DC active Nz plasma, assisted molecular beam epitaxy. The samples of GaNAs between 3 and 200 nm thick were evaluated by double crystal X-ray diffraction (XRD) and photoluminescence (PL) measurements. PL and XRD measurements for these samples are in good agreement. Some material growth and structure parameters affecting the properties of GaNAs/GaAs heterostructure were studied; they were: (1) growth temperature of GaNAs epilayer; (2) electrical current of active N-2 plasma; (3) Nz flow rate; (4) GaNAs growth rate; (5) the thickness of GaNAs strained layer. XRD and PL measurements showed that superlattice with distinct satellite peaks up to two orders and quantum well structure with intensity at 22 meV Fourier transform infrared spectroscopy (FWHM) can be achieved in molecular beam epitaxy (MBE) system. (C) 2000 Published by Elsevier Science S.A. All rights reserved.