Bangzhi Liu

Oxidation of silicon nanowires

Silicon nanowires have received attention for nanoscale electronic devices and chemical and biological sensors. The thermal oxide grown on the silicon nanowires could be used in a variety of devices, so the oxidation of the silicon nanowires is investigated in this work. Silicon nanowires with an average radius of 37nm were grown for these experiments using the vapor-liquid-solid technique with Au to mediate the growth. Etching of the Au tips from the silicon nanowires was performed prior to oxidation to avoid local accelerated oxidation at the nanowire tip. Oxidation was performed at 700°C for 1–121h and at 650 and 750°C for 4h in O2, and the oxidized nanowires were examined by transmission electron microscopy. Depending on the conditions for oxidation, an oxide shell as thin as 6nm was observed, or the entire nanowire was oxidized. The kinetics of oxidation differ from those of a planar silicon wafer and are discussed in this work.

Orientation dependence of nickel silicide formation in contacts to silicon nanowires

The orientation dependence of Ni silicide phase formation in the silicidation of silicon nanowires (SiNWs) by Ni has been studied. SiNWs with a [112] growth direction contacted by Ni pads form θ-Ni2Si for annealing conditions from 350 to 700 °C for 2 min. The θ-Ni2Si has an epitaxial orientation of θ-Ni2Si[001]∥Si[111¯] and θ-Ni2Si(100)∥Si(112) with the SiNW. On the other hand, SiNWs with a [111] growth direction react with Ni pads to form NiSi2 with an epitaxial orientation of NiSi2[11¯0]∥Si[11¯0] and NiSi2(111)∥Si(111) after annealing at 450 °C for 2 min. The [111] SiNWs were also silicided at 700 °C for 2 min, forming the low-resistivity NiSi phase. The epitaxial phases identified in the reactions of Ni films with SiNWs suggest that lattice matching at both the silicide/Si growth front and the surface of the original SiNW may play a significant role in determining the first silicide segment to grow.The orientation dependence of Ni silicide phase formation in the silicidation of silicon nanowires (SiNWs) by Ni has been studied. SiNWs with a [112] growth direction contacted by Ni pads form θ-Ni2Si for annealing conditions from 350 to 700 °C for 2 min. The θ-Ni2Si has an epitaxial orientation of θ-Ni2Si[001]∥Si[111¯] and θ-Ni2Si(100)∥Si(112) with the SiNW. On the other hand, SiNWs with a [111] growth direction react with Ni pads to form NiSi2 with an epitaxial orientation of NiSi2[11¯0]∥Si[11¯0] and NiSi2(111)∥Si(111) after annealing at 450 °C for 2 min. The [111] SiNWs were also silicided at 700 °C for 2 min, forming the low-resistivity NiSi phase. The epitaxial phases identified in the reactions of Ni films with SiNWs suggest that lattice matching at both the silicide/Si growth front and the surface of the original SiNW may play a significant role in determining the first silicide segment to grow.

Oxidation of silicon nanowires for top-gated field effect transistors

The oxidation of unintentionally doped p-type silicon nanowires grown by the vapor-liquid-solid (VLS) method and their integration into top-gated field effect transistors is reported. Dry thermal oxidation of as-grown silicon nanowires with diameters ranging from 20to400nm was carried out at 700 and 900°C with or without the addition of a chlorinated gas source. The oxidation rate was strongly dependent on the as-grown nanowire diameter, with the large-diameter nanowires oxidizing up to five times faster than the smallest nanowires at 900°C. At each diameter, the addition of trichloroethane (TCA) enhanced the rate compared to oxidation in pure O2. Top-gated field effect transistors fabricated from nanowires oxidized at 700°C had significantly less hysteresis in their subthreshold properties when TCA was added, but oxidation at 900°C with or without TCA provided hysteresis-free devices with improved subthreshold slope. Such enhancements in the electrical properties are expected based on advances in planar ...

Nickel and nickel silicide Schottky barrier contacts to n-type silicon nanowires

Schottky contacts to n-type silicon nanowires were fabricated using Ni or nickel silicide contacts in a wraparound or end contact geometry, respectively. Series resistance in the test structures was reduced by heavily doping the opposite end of the silicon nanowire, facilitating Ohmic contact formation and reducing the resistance of the nanowire itself. The effective Schottky barrier height is reported as a function of nanowire doping, ambient, and applied back gate bias, highlighting some of the important variables affecting current transport in Schottky contacts to semiconductor nanowires. For the silicide contact to the most lightly doped silicon nanowire, measurements in N2 showed that the effective barrier height without a back gate bias was 0.69 eV, and the ideality factor was 1.1.

In Situ Axially Doped n-Channel Silicon Nanowire Field-Effect Transistors

Axially doped (n+-p--n+) silicon nanowires were synthesized using the vapor-liquid-solid technique by sequentially modulating the introduction of phosphine to the inlet gas stream during growth from a silane source gas. Top-gate and wrap-around-gate metal oxide semiconductor field-effect transistors that were fabricated after thermal oxidation of the silicon nanowires operate by electron inversion of the p- body segment and have significantly higher on-state current and on-to-off state current ratios than do uniformly p- -doped nanowire field-effect devices. The effective electron mobility of the devices was estimated using a four-point top-gate structure that excludes the source and drain contact resistance and was found to follow the expected universal inversion layer mobility versus effective electric field trend. The field-effect properties of wrap-around-gate devices are less sensitive to global-back-gate bias and thus provide better electrostatic control of the nanowire channel. These results demonstrate the ability to tailor the axial doping profile of silicon nanowires for future planar and vertical nanoelectronic applications.

Growth of Magneto-optically Active (Zn,Mn)Se Nanowires

We describe the growth of Zn(1-x)Mn(x)Se nanowires in ultrahigh vacuum seeded by Au nanodroplets. Electron microscopy reveals the formation of single-crystal c-axis wurtzite nanowires (typically 1-3 microm long) with Mn concentrations up to x approximately 0.6, accompanied by a dense horizontal undergrowth of shorter, crooked nanowires. Magnetophotoluminescence measurements show evidence for sp-d exchange effects in a reduced symmetry environment. We find that the optical emission is surprisingly dominated by the undergrowth of crooked nanowires.

Thermal stability of Pd/Pt/Au Ohmic contacts to InAlSb/InAs heterostructures for high electron mobility transistors

We report the thermal stability of Pd/Pt/Au Ohmic contacts to InAlSb/InAs high electron mobility transistors. An initial drop in contact resistance correlates with consumption of the InAs electron channel through reaction of both Pd and Pt with the semiconductor heterostructure during a 3 h 175 °C anneal, as determined using transmission electron microscopy. Voids form in the unreacted Pt layer after samples are aged for 1 week at 175 °C, and they grow larger when the samples are aged for 1 week at 200 °C. The contact resistance increases by more than a factor of 2 after samples are aged for 1 week at 225 °C. We discuss the degradation of the contact resistance in light of the interfacial reactions that occur during aging.

Lithography-free synthesis of freestanding gold nanoparticle arrays encapsulated within dielectric nanowires

A lithography-free method for producing freestanding one-dimensional gold nanoparticle arrays encapsulated within silicon dioxide nanowires is reported. Silicon nanowires grown by the vapor-liquid-solid technique with diameters ranging from 20 nm to 50 nm were used as the synthesis template. The gold nanoparticle arrays were obtained by coating the surface of the silicon nanowires with a 10 nm gold film, followed by thermal oxidation in an oxygen ambient. It was found that the thermal oxidation rate of the silicon nanowires was significantly enhanced by the presence of the gold thin film, which fully converted the silicon into silicon dioxide. The gold-enhanced oxidation process forced the gold into the core of the wire, forming a solid gold nanowire core surrounded by a silicon dioxide shell. Subsequent thermal treatment resulted in the fragmentation of the gold nanowire into a uniformly spaced array of gold nanoparticles encapsulated by a silicon dioxide shell, which was observed by in situ annealing in transmission electron microscopy. Analysis of many different silicon nanowire diameters shows that the diameter and spacing of the gold nanopaticles follows the Rayleigh instability, which confirms this is the mechanism responsible for formation of the nanoparticle array.

Oxidation of RuAl and NiAl Thin Films: Evolution of Surface Morphology and Electrical Resistance

RuAl and NiAl thin films on SiO2/Si were oxidized, and the results were compared to those from aluminum, ruthenium, and nickel films. Both aluminides are more oxidation resistant than nickel, aluminum, and ruthenium, and they form an outer layer of alumina after oxidation to 850 °C. The depth profiles differ for NiAl and RuAl, with alternating layers of alumina and a Ru-rich phase forming on RuAl, while a more complex structure forms on NiAl due to reaction with the substrate. The surface of RuAl after oxidation remains fairly smooth and reflective, whereas NiAl has a hazy appearance. However, the surface morphology changes at a slightly lower temperature in the case of RuAl (~ 500°C) . Both films remain conductive even after the surface begins to show signs of oxidation, with the NiAl remaining conductive to a higher temperature (after 1 h at 850 °C) than RuAl. The results show that NiAl and RuAl films can be used in an oxidizing atmosphere up to ~ 500°C (at least 1 h) for applications requiring a smooth reflective surface and to higher temperatures when the surface quality is less important but conductivity needs to be maintained (~ 800°C for RuAl and ~ 850°C for NiAl).

Composite Ohmic Contacts to SiC

Composite ohmic contacts designed for SiC devices operating in air at 350°C have been studied. Ohmic contacts to n- and p-4H-SiC were protected against inter-diffusion and oxidation by Ta-Si-N layers obtained by sputter deposition from a TaSi2 target in a mixture of Ar and N2. Platinum was sputter-deposited at 250°C to promote adhesion between the Ta-Si- N barrier layer and a thick Au cap layer. Platinum also acts as a barrier to the diffusion of Au. The electrical and mechanical characteristics of the composite contacts were stable after hundreds of hours of annealing in air at 350°C. We report the effects of thermal aging on the specific contact resistance and the semiconductor sheet resistance, and the results of wire bond pull and shear tests following aging for Ta-Si-N / Pt / Au stacks deposited on both SiO2 dielectric layers and the ohmic contact layers.