Hitesh A. Basantani

Enhanced electrical and noise properties of nanocomposite vanadium oxide thin films by reactive pulsed-dc magnetron sputtering

Thin films of VOx (1.3 ≤ x ≤ 2) were deposited by reactive pulsed-dc magnetron sputtering of a vanadium metal target while RF-biasing the substrate. Rutherford back scattering, glancing angle x-ray, and cross-sectional transmission electron microscopy measurements revealed the formation of nanocolumns with nanotwins within VOx samples. The resistivity of nanotwinned VOx films ranged from 4 mΩ·cm to 0.6 Ω·cm and corresponding temperature coefficient of resistance between −0.1% and −2.6% per K, respectively. The 1/f electrical noise was analyzed in these VOx samples using the Hooge-Vandamme relation. These VOx films are comparable or surpass commercial VOx films deposited by ion beam sputtering.

Radiation-Hard ZnO Thin Film Transistors

We report effects for up to 100 Mrad (SiO2) gamma-ray exposure on polycrystalline ZnO thin film transistors (TFTs) deposited by two different techniques. The radiation related TFT changes, either with or without electrical bias during irradiation, are primarily a negative VON shift and a smaller VT shift (ΔVON ~ - 2.5 V and ΔVT ~ - 1.5 V for 100 Mrad (SiO2) exposure). Field-effect mobility remains nearly unchanged. Both, VON and VT shifts are nearly completely removed by annealing at 200°C for 1 minute and some recovery is seen even at room temperature. We find that our ZnO TFTs are insensitive to electrical bias during irradiation; that is, unbiased measurements are useful worst case test results. To the best of our knowledge, these are the most radiation-hard thin film transistors reported to date.

Comparison of ion beam and magnetron sputtered vanadium oxide thin films for uncooled IR imaging

Uncooled Infrared (IR) focal plane arrays are an enabling technology for both military and commercial high sensitivity night vision cameras. IR imaging is accomplished using MEMS microbolometers fabricated on read-out integrated circuits and depends critically on the material used to absorb the incoming IR radiation. Suitable detector materials must exhibit a large temperature coefficient of resistance (TCR) and low noise characteristics to efficiently detect IR photons while also maintaining compatibility with standard integrated circuit (IC) processing. The most commonly used material in uncooled infrared imaging detectors is vanadium oxide deposited by reactive ion beam sputtering. Here we present a comparison of vanadium oxide thin films grown via commercial reactive ion beam sputtering to films grown using reactive pulsed DC magnetron sputtering. Films deposited using both methods were optically and structurally characterized using Raman spectroscopy, transmission electron microscopy, atomic force microscopy and grazing incidence X-ray diffraction. The measured electrical properties of the films were found to be very sensitive to the deposition conditions used. The ion beam sputtered films contained twinned FCC VOx nanocrystals with sub-nanometer twin spacing, in the form of large 10-20 nm wide columnar/conical grains. In contrast, the un-biased magnetron sputtered films consisted of equiax grains of FCC VOx (5-10 nm) encapsulated in an amorphous matrix. However, applying an RF bias to the sample substrate during the magnetron sputtering process, resulted in films that are similar in structure to ion beam deposited VOx. These differences in microstructure and composition were then correlated to the measured resistivities and TCRs of the films.

Vertically integrated pixel microbolometers for IR imaging using high-resistivity VOx

Uncooled IR bolometers form an integral part of thermal imaging cameras. Vanadium oxide material currently used for IR imaging has a resistivity between 0.1 and 1 ohm-cm and a temperature coefficient of resistance (TCR) between -1.4%K-1 to -2.4%K-1. Higher TCR materials are desired, however, such materials inevitably have higher resistivity and therefore higher electrical resistance in a lateral resistor configuration. A high resistance leads to an increase in the Johnson-Nyquist noise of the bias-induced current, thereby limiting the performance of bolometers using high resistivity material. In this work, we demonstrate high resistivity, high TCR VOx and propose the use of a vertically integrated resistor configuration an alternate pixel structure design with lower Johnson noise when compared with the conventional lateral pixel design. Biased Target Ion Beam Deposition was used to deposit high resistivity vanadium oxide thin-films (~85 nm thick). Electrical characterization of lateral resistor structures showed resistivities ranging from 2 ⨯ 103 ohm-cm to 2.1 ⨯ 104 ohm-cm, TCR varying from -2.6%K-1 to -5%K-1, Johnson noise (pixel resistance of 1.3GΩ) of 4.7 to 6μV/√Hz and 1/f noise (normalized Hooge’s parameter (α/n)) of 5 ⨯ 10-21 to 5 ⨯ 10-18 cm-3. In contrast, the through-film resistor structures showed significantly higher resistivities at 3 ⨯ 104 Ohm-cm to 1.55 ⨯ 105 Ohm-cm, TCR similar to lateral resistive structure between -2.6%K-1 to -5.1%K-1, immeasurably low Johnson noise (pixel resistance of 48KΩ) and normalized Hooge’s parameter ranging from to 5⨯10-21 to 1⨯10-18 cm-3. These results indicate the possible use of through-film resistors as an alternative to the conventional lateral-resistor design currently used in uncooled imaging microbolometers.

Effects of gamma-ray irradiation and electrical stress on ZnO thin film transistors

Radiation tolerance is of interest in electronic applications such as radiation sensors, nuclear reactors, x-ray imagers, and high-energy particle accelerators. While properly designed Si MOSFETS are usefully radiation resistant, most thin-film transistors (TFTs), including polysilicon and a-Si:H, are severely degraded by relatively low irradiation dose (typically <;1 Mrad) [1, 2]. We previously reported gamma ray radiation exposure results for unbiased ZnO TFTs and circuits and found only small electrical changes for doses up to 100 Mrad [3]. For applications with TFTs operating in harsh radiation environments, the effects of simultaneous electrical stress and radiation exposure are important. We report here the effects of 60Co gamma irradiation and electrical stress on the characteristics of ZnO TFTs with active and dielectric layers deposited by weak-oxidant plasma enhanced atomic layer deposition (PEALD).

Microstructure of vanadium oxide used in microbolometers

Reactive pulsed DC sputtering was used to grow a systematic series of films with resistivity ranging from 1 × 10-3 to 6.8 × 104 Ohm cm and TCR varying from 0 to -4% K-1. Throughout the parameter space studied a transition from amorphous to nano-crystalline growth was observed. Films in the resistivity range of interest for microbolometers contained a FCC VOx (0.8 < x < 1.3) phase. Altering the sputtering energetics via substrate biasing resulted in highlycolumnar, nano-twinned grains of FCC VOx, providing a microstructure reminiscent of ion beam sputtered bolometer material. Electron diffraction in the TEM confirmed the presence of a secondary, oxygen-rich amorphous phase. Micro- Raman spectroscopy, which was also found to be sensitive to the secondary amorphous phase, was used to probe the chemical composition and morphology of VOx thin films. Raman spectra from high resistivity amorphous films show a broad feature around ~890 cm-1, while spectra from lower resistivity nano-crystalline films exhibit this same amorphous feature and a second broad feature at ~320 cm-1. The resulting microstructure can be described as a nano-composite material composed of a low-resistivity crystalline phase embedded in a high-resistivity amorphous matrix. Our results suggest that both phases are required to achieve a high TCR, low resistivity material.

A Flexible Vanadium Oxide Thermistor Array for Localized Temperature Field Measurements in Brain

A prototype neural probe designed for chronic measurements and recordings of localized temperature field in brain has been fabricated and tested. An array of through-film vanadium oxide thermistors has been deposited onto glass and flexible 20-μm polyimide substrates. High resistivity vanadium oxide was deposited using bias target ion-beam deposition. Functioning sensors were 10 μm × 10 μm × 85-nm thick. Tested samples on glass showed a mean thermistor temperature coefficient of resistance of -4.060 ± 0.004%/°C and nominal resistance R₀ of 1.3 MΩ.

Evaluation of 1/f noise in prospective IR imaging thin films

Vanadium oxide (VOx) and hydrogenated silicon germanium (SixGe1-x) are the two predominant thin film material systems used as the active layer in resistive infrared imaging. Thin films of VOx used in microbolometers have a resistivity typically between 0.1 and 1 Ω-cm with a temperature coefficient of resistance, |TCR| between 1.4%/K to 2.4%/K, while SixGe1-x:H thin films have a resistivity between 200-4,000 Ω-cm with a |TCR| between 2.9%/K to 3.9%/K. Future devices may require higher TCR materials, however, higher TCR is loosely associated with higher resistivity and therefore also with high noise. This work compares 1/f noise of high resistivity VOx and Ge:H thin films having |TCR| < 3.6%/K. The high TCR thin films of VOx were found to be amorphous while, depending on the deposition conditions, the Ge:H thin films were either amorphous or mixed phase of amorphous + nanocrystalline. Evaluation of these VOx and Ge:H thin films indicates a prospects for a superior process-property relation of 1/f noise in Ge:H thin films in comparison with thin films of VOx.