A direct digital frequency synthesizer with fourth-order phase domain Delta Sigma noise shaper and 12-bit current-steering DAC

This paper presents a direct digital frequency synthesizer (DDFS) with a 16-bit accumulator, a fourth-order phase domain single-stage Delta Sigma interpolator, and a 300-MS/s 12-bit current-steering DAC based on the Q(2) Random Walk switching scheme. The Delta Sigma interpolator is used to reduce the phase truncation error and the ROM size. The implemented fourth-order single-stage Delta Sigma noise shaper reduces the effective phase bits by four and reduces the ROM size by 16 times. The DDFS prototype is fabricated in a 0.35-mu m CMOS technology with active area of 1.11 mm(2) including a 12-bit DAC. The measured DDFS spurious-free dynamic range (SFDR) is greater than 78 dB using a reduced ROM with 8-bit phase, 12-bit amplitude resolution and a size of 0.09 mm(2). The total power consumption of the DDFS is 200)mW with a 3.3-V power supply.

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.

High-temperature characteristics of GaInNAs/GaAs single-quantum-well lasers grown by plasma-assisted molecular beam epitaxy

GaInNAs/GaAs single-quantum-well (SQW) lasers have been grown by solid-source molecular beam epitaxy. N is introduced by a home-made de-active plasma source. Incorporation of N into InGaAs decreases the bandgap significantly. The highest N concentration of 2.6% in a GaInNAs/GaAs QW is obtained, corresponding to the photoluminescence (PL) peak wavelength of 1.57 mum at 10 K. The PL peak intensity decreases rapidly and the PL full width at half maximum increases with the increasing N concentrations. Rapid thermal annealing at 850 degrees C could significantly improve the crystal quality of the QWs. An optimum annealing time of 5s at 850 degrees C was obtained. The GalnNAs/GaAs SQW laser emitting at 1.2 mum exhibits a high characteristic temperature of 115 K in the temperature range of 20 degrees C- 75 degrees C.

Phonon-induced Raman scattering in GaNAs

The Raman scattering spectra of MBE-grown GaNAs epilayers were investigated. The resonant enhancement of Raman scattering due to the E+ states in the conduction band was observed and the Raman peaks related to the phonons at non-Gamma points of the Brillouin Zone were detected. II was clearly seen that the local vibrational mode induced by nitrogen impurities evolves to the GaN-like lattice phonon mode when the nitrogen content increases. By comparing the Raman spectra measured before and after 850 degreesC rapid thermal annealing, it was tentatively suggested that two weak peaks were induced by the pairing or clustering effect of nitrogen.

The formation and stability of Si1-xCx alloys in Si implanted with carbon

Si1-xCx alloys of carbon (C) concentration between 0.6%-1.0% were grown in Si by ion implantation and high temperature annealing. The formation of Si1-xCx alloys under different ion doses and their stability during annealing were studied. If the implanted dose was less than that for amorphizing Si crystals, the implanted C atoms would like to combine with defects produced during implantation and it was difficult to form Si1-xCx alloys after being annealed at 850 degreesC. With the increment of implanted C ion doses, the lattice damage increased and it was easier to form Si1-xCx alloys. But the lattice strain would become saturate and only part of implanted carbon atoms would occupy the substitutional positions to form Si1-xCx alloys as the implanted carbon dose increased to a certain degree. Once Si1-xCx alloys were formed, they were stable at 950 degreesC, but part of their strain would release as the annealing temperature increased to 1 000 degreesC. Stability of the alloys became worse with the increment of carbon concentration in the alloys.

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.

The formation and characteristics of Si1-xCx alloys in Si crystals by means of implantation of cions with different doses

Carbon ions with concentration of (0.6-1.5)% were implanted into silicon crystals at room temperature and Si1-xCx alloys were grown by solid phase epitaxy with high temperature annealing. The formation and characteristics of Si1-xCx alloys under different implanted carbon doses were studied. If the implanted carbon atom concentration was less than 0.6%, carbon atoms would tend to combine with the defects produced during implantation and it was difficult for Si1-xCx alloys to form during annealing at 850-950 degreesC. With the increase of implanted C concentration, almost all implanted carbon atoms would occupy substitution positions to form Si1-xCx alloys, but only part of implanted carbon atoms would occupy the substitution position to form Si1-xCx alloys as the implanted dose increased to 1.5 %. Most Si1-xCx alloy phases would vanish as the annealing temperature was increased higher.

Flip-chip bonded hybrid CMOS/SEED optoelectronic smart pixels

Hybrid integration of GaAs/AlGaAs multiple quantum well self electro-optic effect device (SEED) arrays are demonstrated flip-chip bonded directly onto 1 mu m silicon CMOS circuits. The GaAs/AlGaAs MQW devices are designed for 850 nm operation. Some devices are used as input light detectors and others serve as output light modulators. The measurement results under applied biases show good optoelectronic characteristics of elements in SEED arrays. Nearly the same reflection spectrum is obtained for the different devices at an array and the contrast ratio is more than 1.2:1 after flip-chip bonding and packaging. The transimpedance receiver-transmitter circuit can be operated at a frequency of 300 MHz.

Polycrystalline silicon films prepared by improved pulsed rapid thermal annealing

An improved pulsed rapid thermal annealing (PRTA) has been used for the solid-phase crystallization (SPC) of a-Si films prepared by PECVD. The SPC can be completed with time-temperature budgets such as 10 cycles of 60-s 550 degrees C thermal bias/1-s 850 degrees C thermal pulse. The microstructure and surface morphology of the crystallized films are investigated by X-ray diffraction (XRD). The results indicate that this PRTA is a suitable post-crystallization technique for fabricating large-area poly-Si films on low-cost substrate. (C) 2000 Elsevier Science B.V. All rights reserved.

Annealing effect on the surface morphology and photoluminescence of InGaAs/GaAs quantum dots grown by molecular beam epitaxy

Postgrowth rapid thermal annealing was performed on InGaAs/GaAs quantum dots grown by molecular beam epitaxy. The blue shift of the emission peak and the narrowing of the luminescence line width are observed at lower annealing temperature. However, when the annealing temperature is increased to 850 degrees C, the emission line width becomes larger. The TEM image of this sample shows that the surface becomes rough, and some large clusters are formed, which is due to the interdiffusion of In, Ga atoms at the InGaAs/GaAs interface and to the strain relaxation. The material is found to degrade dramatically when the annealing temperature is further increased to 900 degrees C, while emission from quantum dots can still be detected, along with the appearance of the emission from excited state. (C) 2000 Elsevier Science B.V. All rights reserved.

Relaxed GexSi1-x layer formation with reduced dislocation density grown by ultrahigh vacuum chemical vapor deposition

A new alternative method to grow the relaxed Ge0.24Si0.76 layer with a reduced dislocation density by ultrahigh vacuum chemical vapor deposition is reported in this paper. A 1000-Angstrom Ge0.24Si0.76 layer was first grown on a Si(100) substrate. Then a 500-Angstrom Si layer and a subsequent 5000-Angstrom Ge0.24Si0.76 overlayer followed. All these three layers were grown at 600 degrees C. After being removed from the growth system to air, the sample was first annealed at 850 degrees C for 30 min, and then was investigated by cross-sectional transmission electron microscopy and Rutherford backscattering spectroscopy. It is shown that the 5000-Angstrom Ge0.24Si0.76 thick over layer is perfect, and most of the threading dislocations are located in the embedded thin Si layer and the lower 1000-Angstrom Ge0.24Si0.76 layer. The relaxation ratio of the over layer is deduced to be 0.8 from Raman spectroscopy.

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].

Microstructure and photoluminescence of a-SiOx : H

Two strong photoluminescence (PL) bands in the spectral range of 550-900 nm have been observed at room temperature from a series of a-SiOx:H films fabricated by plasma-enhanced chemical vapor deposition (PECVD) technique. One is composed of a main band in the red-light region and a shoulder; the other is located at about 850 nm, only found after 1170 degrees C annealing in N-2 atmosphere. In conjunction with infrared (IR) and micro-Raman spectra, it is thought that the two PL bands are associated with a-Si clusters in the SiOx network and nanocrystalline silicon in SiO2, respectively.

Solid-phase crystallization and dopant activation of amorphous silicon films by pulsed rapid thermal annealing

An improved pulsed rapid thermal annealing method has been used to crystallize amorphous silicon films prepared by PECVD. The solid-phase crystallization and dopant activation process can be completed with time-temperature budgets such as 10 cycles of 60-s 550 degrees C thermal bias/l-s 850 degrees C thermal pulse. A mean grain size more than 1000 Angstrom and a Hall mobility of 24.9 cm(2)/V s are obtained in the crystallized films. The results indicate that this annealing method possesses the potential for fabricating large-area and good-quality polycrystalline silicon films on low-cost glass substrate. (C) 1998 Elsevier Science B.V. All rights reserved.

A study of strong photoluminescence of SiOx : H films

We have examined photoluminescence (PL), IR absorption and Raman spectra of a series of hydrogenated amorphous silicon oxide (a-SiOx:H, (0 < x < 2)) films fabricated by plasma enhanced chemical vapor deposition (PECVD). Two strong luminescence bands were observed at room temperature, one is a broad envelope comprising a main peak around 670 nm and a shoulder at 835 nm, and the other, peaked around 850 nm; is found only after being annealed up to 1170 degrees C in N-2 environment. In conjunction with IR and Raman spectra, the origins of the two luminescent bands and their annealing behaviors are discussed on the basis of quantum confinement effects.