L.S. Tan

Infrared and x-ray photoelectron spectroscopy studies of as-prepared and furnace-annealed radio-frequency sputtered amorphous silicon carbide films

The effects of annealing on the structural properties of radio-frequency sputtered amorphous silicon carbide films prepared under different hydrogen partial pressures (PH) were investigated. Infrared (IR) results of the as-prepared films suggest that as PH increases, more hydrogen is incorporated into the film to form the Si–H and C–H bonds and less silicon and carbon atoms are available to form the Si–C bonds. X-ray photoelectron spectroscopy (XPS) results of the as-prepared films agree with the IR results in that the percent of Si–C decreases and the percent of Si–H and C–H increases as PH increases. IR and XPS results of the annealed films suggest that as the annealing temperature increases, the dangling Si and C bonds will combine to form the Si–C bonds for the unhydrogenated samples. The increase in Si–C bonds for the hydrogenated samples is more likely to be due to the formation of Si–C bonds from the breaking up of the Si–H and C–H bonds.

Enhanced device performance of AlGaN/GaN HEMTs using HfO2 high-k dielectric for surface passivation and gate oxide

AlGaN/GaN high electron mobility transistors (HEMTs) using HfO2 as a surface passivation layer and metal–oxide–semiconductor HEMTs (MOS-HEMTs) using HfO2 as gate oxide have been investigated and compared with the regular HEMTs. In MOS-HEMTs, the HfO2 gate dielectric is also used for passivation simultaneously. Our measurements have shown that both passivated HEMTs and MOS-HEMTs outperformed the regular HEMTs in dc, high-frequency and pulsed-mode operations, with MOS-HEMTs exhibiting the best characteristics, including the highest drain current, the lowest gate leakage current, the largest gate voltage swing, the highest cut-off frequencies and the best immunity to current collapse. In addition, the decrease in transconductance of MOS-HEMTs relative to HEMTs is as low as 8.7%, most probably a consequence of the high-k value of HfO2. Our results thus indicate the great potential of HfO2/AlGaN/GaN MOS-HEMTs for high-frequency and high-power applications.

Ultrafast-laser-induced parallel phase-change nanolithography

A phase-change nanolithography technique is developed to fabricate up to millions of two-/three-dimensional nanostructures (∼50nm) over a large area at a high speed by combining femtosecond laser, microlens array, and wet etching process. Near-field scanning optical microscopy, electrical force microscopy, and atomic force microscopy were used to characterize optical and electrical properties of crystalline and amorphous states, respectively. Different reactions of both amorphous and crystalline areas in phase-change film to alkaline solution are demonstrated. Multiphoton absorption and ultrashort pulse contribute to nanostructure generation. This method opens up a route for nanodevice fabrication with phase-change material.

Bimetallic structure fabricated by laser interference lithography for tuning surface plasmon resonance

Tuning of surface plasmon resonance by gold and silver bimetallic thin film and bimetallic dot array is investigated. Laser interference lithography is applied to fabricate the nanostructures. A bimetallic dot structure is obtained by a lift-off procedure after gold and silver thin film deposition by an electron beam evaporator. Surface plasmon behaviors of these films and nanostructures are studied using UV-Vis spectroscopy. It is observed that for gold thin film on quartz substrate, the optical spectral peak is blue shifted when a silver thin film is coated over it. Compared to the plasmon band in single metal gold dot array, the bimetallic nanodot array shows a similar blue shift in its spectral peak. These shifts are both attributed to the interaction between gold and silver atoms. Electromagnetic interaction between gold and silver nanostructures is discussed using a simplified spring model.

Dopant Loss Mechanism in n + ∕ p Germanium Junctions during Rapid Thermal Annealing

The phenomenon of severe dopant loss during rapid thermal annealing of phosphorus-implanted germanium has been investigated. Dopant activation improves for temperatures above 500°C and reaches 100% activation for samples annealed at 600°C. However, a heavily defective junction with approximately 50% dopant loss is recorded. Although surface passivation of the implanted germanium using plasma-enhanced chemical vapor deposited silicon dioxide did not prevent the dose loss, it assisted in the achievement of defect-free, single-crystal germanium with improved electrical characteristics at a reduced thermal budget. Phosphorus introduced into germanium via solid-state diffusion from phosphosilicate glass did not exhibit dose loss upon rapid thermal annealing, suggesting that dose loss could be an effect of implant damage.

Pulsed laser annealing of Be-implanted GaN

Postimplantation thermal processing of Be in molecular-beam-epitaxy-grown GaN by rapid thermal annealing (RTA) and pulsed laser annealing (PLA) was investigated. It has been found that the activation of Be dopants and the repair of implantation-induced defects in GaN films cannot be achieved efficiently by conventional RTA alone. On the other hand, good dopant activation and surface morphology and quality were obtained when the Be-implanted GaN film was annealed by PLA with a 248nm KrF excimer laser. However, observations of off-resonant micro-Raman and high-resolution x-ray-diffraction spectra indicated that crystal defects and strain resulting from Be implantation were still existent after PLA, which probably degraded the carrier mobility and limited the activation efficiency to some extent. This can be attributed to the shallow penetration depth of the 248nm laser in GaN, which only repaired the crystal defects in a thin near-surface layer, while the deeper defects were not annealed out well. This situ...

Theory of the photovoltage at semiconductor surfaces and its application to diffusion length measurements

Abstract This paper presents an analytical theory of the steady-state surface photovoltage, which takes into account recombination in the surface space charge region and at surface states, as well as bulk diffusion in the semiconductor. For a given wavelength of light used in the photo-excitation, the theory is able to predict the photon flux required to yield a specified surface photovoltage. The validity of the theory has been established by means of detailed comparison with exact numerical solutions. It is shown that for an Si specimen space charge recombination plays an important role in determining the surface photovoltage, particularly at photovoltages of the order 0.1 times the thermal voltage or less. The theory is applied to a rigorous examination of the validity of two standard test methods of the American Society for Testing and Materials for measuring the minority carrier diffusion length, which are based on the surface photovoltage. It is found that while the method due to Goodman works well in general and even in the presence of large surface recombination, the method due to Quilliet and Gosar does not always give the correct value of diffusion length, because of the importance of recombination in the space charge region and at the surface states—it does so only under certain restrictive conditions, i.e. the material must have a doping concentration greater than 1015 cm−3 and a long minority carrier lifetime (> 10 μs), with a surface in depletion (but not in inversion) at equilibrium and a very low surface recombination velocity.

The role of source boundary condition in spreading resistance calculations

Abstract Solutions are presented for the current density distribution at an equipotential disc electrode in contact with a slab backed by a perfect conductor. These exact solutions provide a basis for testing the validity of the two forms of source current density distribution assumed in approximate calculations of spreading resistance correction factors, viz. a uniform distribution and the distribution given by the classical solution for the infinitely thick slab. By using the latter distribution and the power loss definition for spreading resistance, a new correction factor integral has been obtained. Correction factors have been calculated by using this integral and those given by Schumann and Gardner, by Lee and by assuming a uniform current distribution. Except for Schumann and Gardners method, all the methods yield results consistent with those obtained for the current density distributions. In the case of Schumann and Gardners method, the correction factors obtained for thin slabs agree closely with those given by the exact method, despite the fact that the assumed source current distribution is in gross disagreement with the exact distribution. The close agreement in correction factors is fortuitous and is a consequence of the definition that Schumann and Gardner used for the spreading resistance. For a slab with a perfectly insulating substrate, exact solutions are not available. A comparative study has therefore been made in this case between the correction factors obtained by the four approximate methods themselves. The overall conclusion is that of the approximate methods, the uniform current density method is the most satisfactory from the point of view of self-consistency and overall accuracy.

Boron Profile Narrowing in Laser-Processed Silicon after Rapid Thermal Anneal

The narrowing or broadening of the boron profile during annealing of laser-processed samples is observed to occur depending on which of two competing mechanisms, uphill diffusion of boron due to a highly defective single-crystal layer near the surface or transient-enhanced diffusion due to end-of-range defects, dominates during the post-laser processing anneal. The results show that uphill diffusion of boron is found to dominate during annealing of a single-pulse laser-processed sample because the defects near the surface cannot be efficiently removed with a single laser pulse adjusted to a value that can melt the amorphous silicon but not the underlying crystalline substrate. Junctions thus become shallower with the post-laser processing anneal. However, with successive laser pulses, the dopants are observed to move deeper into the silicon with subsequent rapid thermal annealing cycles. This could be due to the reduced contribution of uphill boron diffusion when the defects near the surface decrease with successive pulses. The end-of-range defects, which cannot be sufficiently annealed because the melt depth is not beyond the amorphous layer, thus play the key role in broadening the boron concentration profile for multiple-pulse laser-annealed silicon.

Structural and electrical characterizations of the pulsed-laser-deposition-grown Sc2O3∕GaN heterostructure

Single-crystalline Sc2O3 was grown on GaN/sapphire template using pulsed laser deposition and the interface characteristics of the Sc2O3∕GaN heterostructure were investigated. An epitaxial relationship of [112]Sc2O3‖[213¯0]GaN and (222)Sc2O3‖(0002)GaN was revealed by x-ray diffraction and cross-sectional transmission electron microscopy. A valence band offset of 0.84eV was obtained by x-ray photoelectron spectroscopy, indicating a conduction band offset of 2.04eV across the Sc2O3∕GaN heterointerface. In addition, a low interface state density of 4×1011eV−1cm−2 was estimated from capacitance-voltage measurements. The epitaxial nature with good interface characteristics has rendered a substantially low leakage current of 1μA∕cm2 at a reverse gate bias of 30V in the Sc2O3∕GaN metal-oxide-semiconductor structures.