Weiqiang Sun

An Extremely Simple Thermocouple Made of a Single Layer of Metal

A novel temperature sensor consisting of a single layer of metal (Ni, Pd, W, or Pt) is constructed. Its configuration challenges a long-established concept and may lead to development of a new category of devices. Reliable two-dimensional mapping of local temperatures is demonstrated using an array of these sensors. These single-metal thermocouples (SMTCs) can be readily applied on flexible substrates or at high temperatures.

Thin-Film Thermocouple Array for Time-Resolved Local Temperature Mapping

This letter reports on a technique to obtain time-resolved 2-D mapping of local temperatures on a solid substrate by employing an array of Cr-Ni thin-film thermocouples (TFTCs). Using 100-nm-thick Cr-Ni TFTCs, a stable and reproducible thermopower of 26.2 ± 1.5 μV/K with temperature resolutions better than 0.1 K was obtained. Investigation showed that the thermopower of these thermocouples had no obvious dependence on their junction area down to 3 × 3 μm2. In an array of 18 individual thermocouples, a relative standard deviation of 0.31% was measured for the thermopower. This technique may have applications in, e.g., thermal failure analysis, real-time and on-site thermal sensing for integrated circuits, and other modern electronic, optical, and thermal devices.

Unexpected size effect in the thermopower of thin-film stripes

For low-dimensional materials, size effect of a physical property is usually expected to occur when one (or more) of the dimension sizes decreases to that comparable to or smaller than one of the intrinsic characteristic lengths, e.g., the mean free path. We report here an unexpected size effect, that in centimeter-long stripes of 100-nm-thick metallic thin films, a reduction of the absolute value of thermopower occurs when the stripe width is in the order of 30-50 μm, which is 100–1000 times larger than the intrinsic mean free path of the material. When the stripe width is reduced to 1.5 μm, a relative reduction of thermopower up to 35% is measured in some metals. We suggest that the sidewall scattering due to rough edges of these stripes may be the origin of this unexpected phenomenon. The results may be applied to construct novel thermoelectric devices, such as thermocouples made from a single metal film.

A Nano-Stripe Based Sensor for Temperature Measurement at the Submicrometer and Nano Scales

Submicrometer dual-stripe temperature sensors made from a single piece of metal thin film (e.g., Pd) are developed. With the narrowest sensor being 900 nm in width, they show sensitivity of 1-2 μV/K for the full range of 10-300 K. The results confirm the size effect in Seebeck coefficient previously observed in microstripe sensors of the same device configuration.

Penetrative imaging of sub-surface microstructures with a near-field microwave microscope

Microwaves have the capability of penetrating through materials with low permittivity. By means of a near-field scanning microwave microscope system, we obtained two-dimensional maps of the incident microwaves reflection coefficient intensity and frequency shift, which correspond well to the spatial distribution and electrical conductance of fluids and metallic thin film structures hidden underneath 15–170 μm thick dielectric covers. The lateral resolution and sensitivity to conductivity for the target samples were found closely related to the thickness of the cover layer. The technique offers a real-time, in-situ, and a non-invasive approach to monitor the local chemical reactions, the motion of fluids, and the distribution or concentrations of ions or bio-materials in lab-on-a-chip systems. This technique also has the potential to be developed for the detection of live cells and tissues.

Towards on-chip time-resolved thermal mapping with micro-/nanosensor arrays

In recent years, thin-film thermocouple (TFTC) array emerged as a versatile candidate in micro-/nanoscale local temperature sensing for its high resolution, passive working mode, and easy fabrication. However, some key issues need to be taken into consideration before real instrumentation and industrial applications of TFTC array. In this work, we will demonstrate that TFTC array can be highly scalable from micrometers to nanometers and that there are potential applications of TFTC array in integrated circuits, including time-resolvable two-dimensional thermal mapping and tracing the heat source of a device. Some potential problems and relevant solutions from a view of industrial applications will be discussed in terms of material selection, multiplexer reading, pattern designing, and cold-junction compensation. We show that the TFTC array is a powerful tool for research fields such as chip thermal management, lab-on-a-chip, and other novel electrical, optical, or thermal devices.

Using Fourier transform infrared grazing incidence reflectivity to study local vibrational modes in GaN

Both Fourier transform infrared (FTIR) grazing incidence reflectivity and FTIR transmission methods have been used to study GaN films grown on alpha-Al2O3 (0001) substrates by atmospheric pressure metal-organic chemical vapor deposition and low pressure metal-organic chemical vapor deposition. The results show that in the frequency range from 400 to 3500 cm(-1) the signal-to-noise ratio of the FTIR grazing incidence measurement is far higher than that of the FTIR transmission measurement. Some new vibrational structures appearing in the former measurement have been discussed. The features around 1460 and 1300 cm(-1) are tentatively assigned to scissoring and wagging local vibrational modes of CH2 in GaN, respectively

Electrical characteristics for solid C60/GaN heterojunctions

Solid C60/n-GaN heterojunctions have been fabricated and their electrical properties have been studied. It has been found that the heterojunction is a strongly rectifying contact with a rectification ratio greater than 106 and with an ideality factor close to 1. The current–temperature measurement shows an exponential decrease of current with increasing reciprocal temperature, from which the effective barrier height is determined to be 0.535 eV. The series resistance measured decreases with increasing forward voltage and finally tends to be constant.

Gamma-ray irradiation effects on Fourier transform infrared grazing incidence reflection-absorption spectra of GaN films

We have studied the effects of gamma-ray irradiation on the Fourier transform infrared grazing incidence reflection-absorption spectra of GaN films grown on alpha -Al2O3 (0001) substrates using metal-organic chemical vapour deposition at atmospheric pressure. After irradiation, the spectral intensity increased markedly and a series of new absorption features appeared, among which those at 3440, 3355, 3174, 3088, 2819, 2712, 2100, 1426 and 894 cm(-1) are discussed and identified. The reason why gamma-ray irradiation can enhance the intensity and give rise to these new features is also discussed.

High-resolution transmission microscope observation of both nanometer crystalline diamond and graphite in C-implanted GaN

Using high-resolution transmission electron microscopy we have studied C-implanted GaN after a post-implantation annealing treatment. A strong evidence of existence of nanometer crystalline diamond and graphite in C-implanted GaN was given.