Structure, magnetization, and low-temperature spin dynamic behavior of zincblende Mn-rich Mn(Ga)As nanoclusters embedded in GaAs

We have fabricated a set of samples of zincblende Mn-rich Mn(Ga)As clusters embedded in GaAs matrices by annealing (Ga,Mn)As films with different nominal Mn content at 650 degrees C. For the samples with Mn content no more than 4.5%, the Curie temperature reaches nearly 360 K. However, when Mn content is higher than 5.4%, the samples exhibit a spin-glass-like behavior. We suggest that these different magnetic properties are caused by the competing result of dipolar and Ruderman-Kittel-Kasuya-Yosida interaction among clusters. The low-temperature spin dynamic behavior, especially the relaxation effect, shows the extreme creeping effect which is reflected by the time constant tau of similar to 10(11) s at 10 K. We explain this phenomenon by the hierarchical model based on the mean-field approach. We also explain the memory effect by the relationship between the correlation function and the susceptibility.

Enhancement of the far-field output power and the properties of the very-small-aperture lasers

We report on a VSAL structure fabricated by a 650 nm edge emitting laser diode with an Au-coated facet and an aperture size of 250 x 500 nm. The far field output power can maintain at 1 mW and the power density is 7.5 mW/mu m(2). Some properties of the VSAL including the threshold current change, the red-shift of the spectral position, and the strong relative-intensity-noise are presented. The physical mechanisms responsible for these phenomena are also discussed, which may contribute to the understanding and application of the potential device for near-field optics.

Effect of the variation of temperature on the structural and optical properties of ZnO thin films prepared on Si (111) substrates using PLD

ZnO thin films were prepared on Si (1 11) substrates at various temperatures from 250 to 700 degrees C using pulsed laser deposition (PLD) technique in order to investigate the structural and optical properties of the films. The structural and morphological properties of the films were investigated by XRD and SEM measurements, respectively. The quality of the films was improved with the increase of the temperature. By XRD patterns the FWHMs of the (0 0 2) peaks of the ZnO films became narrower when the temperatures were above 500 degrees C. The FWHMs of the peaks of (0 0 2) of the films were as narrow as about 0. 19 degrees when films were grown at 650 and 700 degrees C. This indicates the superior crystallinity of the films. The optical properties of the films were studied by photoluminescence spectra using a 325 nm He-Cd laser. The two strongest UV peaks were found at 377.9 nm from ZnO films grown at 650 and 700 degrees C. This result is consistent with that of the XRD investigation. Broad bands in visible region from 450 to 550 nm were also observed. Our works suggest that UV emissions have close relations with not only the crystallinity but also the stoichiometry of the ZnO films. (c) 2005 Elsevier Ltd. All rights reserved.

Memory effect in a system of zincblende Mn-rich Mn(Ga)As nanoclusters embedded in GaAs

Zincblende Mn-rich Mn(Ga)As nanoclusters embedded in GaAs matrices are fabricated by in situ postgrowth annealing diluted magnetic semiconductor (Ga,Mn)As films with Mn concentration ranging from 2.6% to 8% at 650 degrees C. Magnetization measurements show that memory effect and slow magnetic relaxation, the typical characteristics of the spin-glass-like phase, occur below the blocking temperature of 45 K in samples with high Mn concentration, while for samples with low Mn concentration, ferromagnetic order remains up to 360 K. The behavior of low-temperature spin dynamics can be explained by the hierarchical model. (c) 2007 American Institute of Physics.

Improved field emission properties from metal-coated diamond films

Diamond films were prepared by microwave plasma chemical vapor deposition (MWPCVD). In order to obtain better field emission properties, the samples coated with different metals were prepared. The results showed that the field emission properties of diamond coated with metals could be greatly improved in comparison to pure diamond film and the different kinds of coated metals have different influences on the field emission properties. The possible reasons of effects on the field emission properties are discussed, which were probably due to the reduced effective surface work function by metal coatings; but the detail of the mechanism should be studied further. The surface morphology and microstructure of the sample were characterized by Atomic Force Microscope (AFM), X-ray photoelectron spectroscopy (XPS), X-ray Diffraction (XRD) and Raman spectrum tests. (c) 2006 Elsevier B.V. All rights reserved.

Fabrication process and power and lifetime characteristics of very-small-aperture laser

High output power very-small-aperture laser has been created on 650 nm edge emitting laser diodes. The far-field output power is 0.4 mW at the 25 mA driving current, and the highest output power exceeds 1 mW. The special fabrication process is described and the failure mechanism leading to the short lifetime of the devices is discussed.

Hydrogen related defects in neutron-irradiated silicon grown in hydrogen ambient

Isochronal thermal-annealing behavior of NTD floating-zone silicon grown in hydrogen ambient (called NTD FZ(H) Si) is presented. The dependencies of resistivity and carrier mobility on annealing temperature are determined by room-temperature Hall electrical measurements. Using infrared absorption spectroscopy, hydrogen-related infrared absorption bands evolution for NTD FZ(H) Si were measured in detail. It is demonstrated that compared with NTD FZ(Ar) Si, NTD FZ(H) Si exhibits the striking features upon isochronal annealing in temperature range of 150 similar to 650 degreesC: there appears the formation of an excessive shallow donor at annealing temperature of 500 degreesC. It is shown that the annealing behavior is directly related to the reaction of hydrogen and irradiation-induced defects. The evolution of infrared absorption bands upon temperature reflects a series of complex reaction process: irradiation-induced defects decomposition, breaking of Si-H bonds, migration and aggregation of atomic hydrogen, and formation of the secondary defects. (C) 2002 Elsevier Science B.V. All rights reserved.

X-ray diffraction analysis on gallium-indium interdiffusion in quantum dot superlattices

Thermal-induced interdiffusion in InAs/GaAs quantum dot superlattices is studied by high-resolution x-ray diffraction rocking curve and photoluminescence techniques. With increasing annealing temperatures, up to 300 meV a blueshift of the emission peak position and down to 16.6 meV a narrowing of the line width are found in the photoluminescence spectra, and respective intensity of the higher-order satellite peaks to lower-order ones in the x-ray rocking curves decreases. Dynamical theory is employed to simulate the measured x-ray diffraction data. Excellent agreement between the experimental curves and the simulations is achieved when the composition, thickness and stress variations caused by interdiffusion are taken into account. It is found that the significant In-Ga intermixing occurs even in the as-grown InAs/GaAs quantum dots. The estimated diffusion coefficient is 1.8 x 10(-17) cm(2) (.) s(-1) at 650 degreesC, 3.2 x 10(-17) cm(2 .) s(-1) at 750 degreesC, and 1.2 x 10(-14) cm(2 .) s(-1) at 850 degreesC.

Using photoluminescence as optimization criterion to achieve high-quality InGaAs/AlGaAs pHEMT structure

The single delta -doped InGaAs/AlGaAs pseudomorphic HEMT structure materials were grown by molecular beam epitaxy. The photoluminescence spectra of the materials were studied. There are two peaks in the photoluminescence spectra of the materials, corresponding to two sub energy levels of InGaAs quantum well. The ratio of the two peak's intensity was used as criterion to optimize the layer structures of the materials. The material with optimized layer ;tructures exhibits the 77 It mobility and two-dimensional electron gas density of 16 500 cm(2)/Vs and 2.58 x 10(12) cm(-2) respectively, and the 300 K mobility and two-dimensional electron gas density of 6800 cm(2)/Vs and 2.55 x 10(12) cm(-2) respectively. The pseudomorphic HEMT devices with gate length of 0.2 mum were fabricated using this material. The maximum transconductance of 650 mS/mm and the cut-off frequency of 81 GHz were achieved. (C) 2001 Elsevier Science B.V. All rights reserved.

The keys to get high transconductance of AlGaAs/InGaAs/GaAs pseudomorphic HEMTs devices

There are two key points to get high transconductance of pseudomorphic HEMTS (pHEMTs) devices. From the point view of materials, the transfer efficiency of the electrons from the delta -doped AlGaAs layer to the InGaAs channel must be high. From the point view of device processing, the gate recess depth must be carefully controlled. In the present work, AlGaAs/InGaAs/GaAs pHEMTs structures were grown by molecular beam epitaxy. Layer structures of the pHEMTs were optimized to get high transfer efficiency of the electrons. Gate recess depth was also optimized. A 0.2 mum pHEMT was fabricated on the materials with optimized layer structure using the optimized gate recess depth. The maximum transconductance of 650 mS/mm and the cut-off frequency of 81 GHz were achieved. (C) 2001 published by Elsevier Science Ltd.

Effects of interdiffusion on the luminescence of InAs/GaAs quantum dots covered by InGaAs overgrowth layer

We have studied the effects of postgrowth rapid thermal annealing on the optical properties of 3-nm-height InAs/GaAs quantum dots covered by 3-nm-thick InxGa1-xAs (x = 0, 0.1, and 0.2) overgrowth layer. At higher annealing temperature (T greater than or equal to 750 degreesC), the photoluminescence peak of InGaAs layer has been observed at lower-energy side of the InAs quantum-dot peak. In addition, the blueshift in photoluminescence (PL) emission energy is found to he similar for all samples with increasing the annealing temperature from 650 to 850 degreesC. However, the trend of narrowing of photoluminescence linewidth is significantly different for InAs quantum dots with different In mole fractions in InGaAs overgrowth layer. These results suggest that the intermixing in the lateral direction plays an important role in helping to understand the modification of optical properties induced by rapid thermal annealing. (C) 2000 Elsevier Science B.V. All rights reserved.

Effect of rapid thermal annealing on GaInNAs/GaAs quantum wells grown by plasma-assisted molecular-beam epitaxy

We have studied the effect of rapid thermal annealing (RTA) on GaInNAs/GaAs quantum wells (QWs) grown by molecular-beam epitaxy using a dc plasma as the N source. It was found that RTA at low temperature (LT, 650 degrees C) and high temperature (HT, 900 degrees C) could both improve the QW quality significantly. To clarify the mechanism of quality improvement by RTA, a magnetic field perpendicular to the path of the N plasma flux was applied during the growth of the GaInNAs layers for the sake of comparison. It was found that LT-RTA mainly removed dislocations at interfaces related to the ion bombardment, whereas, HT-RTA further removed dislocations originating from the growth. LT-RTA caused only a slight blueshift of photoluminescence peak wavelength, probably due to defect-assisted interdiffusion of In-Ga at the QW interfaces. The blueshift caused by HT-RTA, on the other hand, was much larger. It is suggested that this is due to the fast defect-assisted diffusion of N-As at the QW interfaces. As defects are removed by annealing, the diffusion of In-Ga at interfaces would be predominant. (C) 2000 American Institute of Physics. [S0003- 6951(00)01535-7].

Effects of rapid thermal annealing on the optical properties of GaNxAs1-x/GaAs single quantum well structure grown by molecular beam epitaxy

The effect of rapid thermal annealing (RTA) on the optical properties of GaNxAs1-x/GaAs strained single quantum well (SQW) was studied by low-temperature photoluminescence (PL). The GaNxAs1-x/GaAs SQW structures were prepared by dc active nitrogen plasma assisted molecular beam epitaxy. PL measurements on a series of samples with different well widths and nitrogen compositions were used to evaluate the effects of RTA. The annealing temperature and time were varied from 650 to 850 degrees C and 30 s to 15 min, respectively. Remarkable improvements of the optical properties of the samples were observed after RTA under optimum conditions. The interdiffusion constants have been calculated by taking into account error function diffusion and solving the Schrodinger equation. The estimated interdiffusion constants D are 10(-17)-10(-16) cm(2)/s for the earlier annealing conditions. Activation energies of 6-7 eV are obtained by fitting the temperature dependence of the interdiffusion constants. (C) 2000 American Institute of Physics. [S0021-8979(00)10401-3].

AlGaInP visible quantum well lasers for information technology

650 nm-range AlGaInP multi-quantum well (MQW) laser diodes grown by low pressure metal organic chemical vapor deposition (LP-MOCVD) have been studied and the results are presented in this paper. Threshold current density of broad area contact laser diodes can be as low as 350 A/cm(2). Laser diodes with buried-ridge strip waveguide structures were made, threshold currents and differential efficiencies are (22-40) mA and (0.2-0.7) mW/mA, respectively. Typical output power for the laser diodes is 5 mW, maximum output power of 15 mW has been obtained. Their operation temperature can be up to 90 degrees C under power of 5 mW. After operating under 90 degrees C and 5 mW for 72 hrs, the average increments for the threshold currents of the lasers at 25 degrees C and the operation currents at 5 mW (at 25 degrees C) are (2-3) mA and (3-5) mA, respectively. Reliability tests showed that no obvious degradation was observed after 1400 hours of CW operation under 50 degrees C and 2.5 mW.

Gallium diffusion through cubic GaN films grown on GaAs(100) at high-temperature using low-pressure MOVPE

Cubic GaN films were grown on GaAs(1 0 0) substrates by low-pressure metalorganic vapor-phase epitaxy at high temperature. We have found a nonlinear relation between GaN film thickness and growth timer and this nonlinearity becomes more obvious with increasing growth temperature. We assumed it was because of Ga diffusion through the GaN film, and developed a model which agrees well with the experimental results. These results raise questions concerning the role of Ga diffusion through the GaN film, which may affect the electrical and optical properties of the material. (C) 1998 Published by Elsevier Science B.V. All rights reserved.