A method to estimate the strain state of SiGe/Si by measuring the bandgap

sing the result of model-solid theory, we have obtained the relationship between bandgap and strain of Si1-x Ge-x alloy on Si (100) substrate with x < 0.85. It was shown that the deviation between the bandgap of strained SiGe and relaxed SiGe is proportional to the strain. According to the theoretical result, a novel method was suggested to determine the strain state of SiGe/ Si through measuring the bandgap. The strain in the SiGe/Si multi-quantum wells was measured using the new method and the results had good agreement with that from XRD measurement.

Silicon membrane resonant-cavity-enhanced photodetector

A Si resonant-cavity-enhanced (RCE) photodiode was fabricated on a silicon membrane. The Si membrane was formed by etching from the back side of the silicon-on-insulator substrate with the buried SiO2 layer as etch-stop layer. A gold layer was deposited serving as an electrode layer and bottom mirror of the RCE photodiode. The photodiode had an external quantum efficiency of 33.8% at the resonant wavelength of 848 nm and a full width at half maximum (FWHM) of 17 nm. The responsivity was 4.6 times that of a conventional Si p-i-n photodiode with the same absorption layer thickness. (c) 2005 American Institute of Physics.

Growth of Ge quantum dot mediated by boron on Ge wetting layer

We, report on the influence of boron on the formation of Ge quantum dots. The investigated structure consists of a Ge wetting layer, on which a sub-monolayer boron is deposited and subsequently a Ge top layer. For sufficiently thin Ge top layers, the strain field induced by boron on Ge wetting layer destabilizes the Ge top layer and causes the formation of small Ge quantum dots. However, for thicker Ge top layers, boron on the Ge wetting layer diffuses into Ge layers, compensates partly the strain and delays the evolution of Ge quantum dots. By this method, small Ge quantum dots with high density as well as size uniformity can be formed by optimizing the growth condition. (c) 2005 Elsevier B.V. All rights reserved.

Demonstration of low-cost Si-based tunable long-wavelength resonant-cavity-enhanced photodetectors

A simple process for fabricating low-cost Si-based continuously tunable long-wavelength resonant-cavity-enhanced (RCE) photodetectors has been investigated. High-contrast SiO2/Si(Deltan similar to2) was employed as mirrors to eliminate the need to grow thick epitaxial distributed Bragg reflectors. Such high-reflectivity SiO2/Si mirrors were deposited on the as-grown InGaAs epitaxy layers, and then were bonded to silicon substrates at a low temperature of 350 C without any special treatment on bonding surfaces, employing silicate gel as the bonding medium. The cost is thus decreased. A thermally tunable Si-based InGaAs RCE photodetector operating at 1.3-1.6 mum was obtained, with a quantum efficiency of about 44% at the resonant wavelength of 1476 nm and a tuning range of 14.5 nm. It demonstrates a great potential for industry processes. (C) 2005 American Institute of Physics.

Thermally tunable optical filter with crystalline silicon as cavity

A novel method to fabricate a thermally tunable filter with a tuning range of 26 nm from 1.504 to 1.530 mum is reported. The high-reffectivity bottom mirror is deposited in the hole formed by anisotropically etching in the basic solution from the backside of the slice with the buried SiO2 layer in silicon-on-insulator substrate as the etching-stop layer. Because of the formation of the mesa and the removing of the substrate of the hole, the power from the metal heater can be more effectively consumed in the crystalline silicon cavity. So it lowers the power consumption and the filter has a higher tuning range. (C) 2004 Elsevier B.V. All rights reserved.

MOCVD-grown high-mobility Al0.3Ga0.7N/AlN/GaN HEMT structure on sapphire substrate

High-mobility Al0.3Ga0.7N/AlN/GaN high electron mobility transistors (HEMT) structure has been grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrate. Electron mobility of 2185 cm(2)/V s at room temperature and 15,400 cm(2)/V s at 80 K with 2DEG density of 1.1 X 10(13) cm(-2) are achieved. The corresponding sheet resistance of the HEMT wafer is 258.7 Omega/sq. The AlN interfacial layer between the GaN buffer and the AlGaN barrier layer reduces the alloy disorder scattering. X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements have been conducted, and confirmed that the wafer has a high crystal quality. (c) 2006 Elsevier B.V. All rights reserved.

Investigation of optical quenching of photoconductivity in high-resistivity GaN epilayer

In undoped high-resistivity GaN epilayers grown by metalorganic chemical vapor deposition (MOCVD) on sapphire, deep levels are investigated by persistent photoconductivity (PPC) and optical quenching (OQ) of photoconductivity (PC) measurements. The PPC and OQ are studied by exciting the samples with two beams of radiation of various wavelengths and intensities. When the light wavelengths of 300 and 340 nm radiate the GaN epilayer, the photocurrent without any quenching effect is rapidly increased because the band gap transition only occurs. If the background light is 340 nm and the quenching light is 564 or 828 nm, the quenching of a small photocurrent generates but clearly. Two broad quenching bands that extend from 385 to 716 nm and from 723 to 1000 nm with a maximum at approximately 2.2 eV (566 nm) is observed. These quenching bands are attributed to hole trap level's existence in the GaN epilayer. We point out that the origin of the defects responsible for the optical quenching can be attributed to nitrogen antisite and/or gallium vacancy. (c) 2006 Elsevier B.V. All rights reserved.

Growth and characterization of semi-insulating GaN films grown by MOCVD

High resistivity unintentionally doped GaN films were grown on (0001) sapphire substrates by metalorganic chemical vapor deposition. The surface morphology of the layer was measured by both atomic force microscopy and scanning electron microscopy. The results show that the films have mirror-like surface morphology with root mean square of 0.3 nm. The full width at half maximum of double crystal X-ray diffraction rocking curve for (0002) GaN is about 5.22 arc-min, indicative of high crystal quality. The resistivity of the GaN epilayers at room temperature and at 250 degrees C was measured to be approximate 10(9) and 10(6) Omega(.)cm respectively, by variable temperature Hall measurement. Deep level traps in the GaN epilayers were investigated by thermally stimulated current and resistivity measurements.

Influence of the cavity on the low-temperature photoluminescence of SiGe/Si multiquantum wells grown on a silicon-on-insulator substrate

The influences of the cavity on the low-temperature photoluminescence of Si0.59Ge0.41/Si multiquantum wells grown on silicon-on-insulator substrates are discussed. The positions of the modulated photoluminescence (PL) peaks not only relate to the nature of SiGe/Si multiquantum wells, but also relate to the characteristic of the cavity. With increasing temperature, a redshift of the modulated PL peak originating from the thermo-optical effect of the cavity is observed.

Growth and characterization of 0.8-mu m gate length AlGaN/GaN HEMTs on sapphire substrates

AlGaN/GaN high electron mobility transistor (HEMT) structures were grown on 2 inch sapphire substrates by MOCVD, and 0.8-mu m gate length devices were fabricated and measured. It is shown by resistance mapping that the HEMT structures have an average sheet resistance of approximately 380 Omega/sq with a uniformity of more than 96%. The 1-mm gate width devices using the materials yielded a pulsed drain current of 784 mA/mm at V-gs=0.5 V and V-ds=7 V with an extrinsic transconductance of 200 mS/mm. A 20-GHz unity current gain cutoff frequency (f(T)) and a 28-GHz maximum oscillation frequency (f(max)) were obtained. The device with a 0.6-mm gate width yielded a total output power of 2.0 W/mm (power density of 3.33 W/mm) with 41% power added efficiency (PAE) at 4 GHz.

Study of GaN thin films grown on vicinal SiC (0001) substrates by molecular beam epitaxy

A detailed study of the characteristics of undoped GaN films, grown on either vicinal or nominal flat SiC (0001) substrates by molecular beam epitaxy, has been carried out using photoluminescence and Raman scattering techniques. The I I K photoluminescence spectra of the GaN film grown on the vicinal SiC (0001) substrate show a strong and sharp near-bandgap peak (full width at half maximum (FWHM) similar to 16 meV). This feature contrasts with that of the GaN film grown on the nominal flat SiC (0001) substrate where the I I K photoluminescence spectra exhibit the near-bandgap peak (FWHM similar to 25 meV) and the intensity is approximately seven times weaker than that of the vicinal film sample. The redshift of the near-bandgap peak associated with excitons bound to shallow donors is related to the stress caused by both the lattice mismatch and the thermal expansion coefficient difference between GaN and SiC substrates. The measured thermal activation energy of the shallow donor of 33.4 meV is determined by using an Arrhenius plot of the near-bandgap luminescence versus I IT from the slope of the graph at high temperature. The temperature dependence of the FWHM of the near-bandgap luminescence has also been studied. The Raman scattering measurements from the vicinal film reveal that the E-2 phonon peak is strengthened and the A(1)(LO) phonon peak is shifted towards the low-frequency side with enhanced intensity, in comparison to that from the nominal flat film, suggesting a reduction in the density of defects and a lower free carrier concentration in the vicinal GaN film.

Photoluminescence enhancement of porous silicon by organic cyano compounds

A pronounced photoluminescence enhancement on chemically oxidized porous silicon was induced by a series of organic cyano compounds including 1,2-dicyanoethylene (CE), 1,3-dicyanobenzene (1,3-CB), 1,4-dicyanobenzene (1,4-CB), 1-cyanonaphthalene (1-CN), and 9-cyanoanthracene (9-CA). Photoluminescence enhancement effects were reversible for all compounds studies in this work. A dependence of photoluminescence enhancement on the steric effect and the electronic characteristics of these compounds and the structure of the porous silicon substrates were analyzed in terms of the photoluminescence enhancing factors. Surface chemical composition examined by Fourier transform infrared (FTIR) spectra demonstrated that the surface Si-H bonds were not changed and no new luminescent compounds were formed on porous silicon surface during adsorption of cyano compounds. A mechanism based on induced surface states acting as radiative recombination centers by cyano compounds adsorption was suggested.

Hydrogen sensors based on Pt-AlGaN/GaN back-to-back Schottky diode

In this paper, platinum (Pt) with a thickness of 45 nm was sputtered on the surface of AlGaN/GaN heterostructure to form the Schottky contact and the back-to-back Schottky diodes were characterized for H-2 sensing at room temperature. Both the forward and reverse current of the devices increased with exposure to H-2 gas, which was attributed to Schottky barrier height reduction caused by hydrogen absorption in the catalytic metals. A shift of 0.7 V at 297 K was obtained at a fixed forward current of 0.1 mA after switching from N-2 to 40% H-2 in N-2. The sensor's responses under different concentrations from 2500 ppm H-2 to 40% H-2 in N-2 at 297 K were investigated. Time response of the sensor at a fixed bias of 1 V was given. Finally, the decrease of the Schottky barrier height and the sensitivity of the sensor were calculated. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Hydrogen sensors based on Pt-AlGN/AIN/GaN Schottky diode - art. no. 68291R

Pt/AlGaN/AlN/GaN Schottky diodes have been fabricated and characterized for H-2 sensing. Platinum (Pt) with a thickness of 20nm was evaporated on the sample to form the Schottky contact. The ohmic contact, formed by evaporated Ti/Al/Ni/Au metals, was subsequently annealed by a rapid thermal treatment at 860 degrees C for 30 s in N-2 ambience. Both the forward and reverse current of the device increased greatly when exposed to H-2 gas. The sensor's responses under different hydrogen concentrations from 500ppm to 10% H-2 in N-2 at 300K were investigated. A shift of 0.45V at 297K is obtained at a fixed forward current for switching from N-2 to 10% H-2 in N-2. Time response of the sensor at a fixed bias of 0.5 V was also measured. The turn-on response of the device was rapid, while the recovery of the sensor at N-2 atmosphere was rather slow. But it recovered quickly when the device was exposed to the air. The decrease in the barrier height of the diode was calculated to be about 160meV upon introduction of 10% H-2 into the ambient. The sensitivity of the sensor is also calculated. Some thermodynamics analyses have been done according to the Langmuir isotherm equation.

SHicon-based resonant-cavity-enhanced photodetectors

Silicon-based resonant-cavity-enhanced photodetectors (RCE-PD) with Si, Ge islands and InGaAs as absorption materials were introduced, respectively. The Ge islands and Si RCE-PD had a membrane structure and the Si-based InGaAs RCE-PDs were fabricated by bonding technology.