Growth of vanadium dioxide thin film on fused silica glass by controlling discharge voltage in reactive sputtering

抄録

Introduction

Vanadium dioxide (VO₂) attracts attention due to its reversible insulator-metal transition (IMT) characteristics.¹⁾ VO₂ shows abrupt resistance changes of 3-4 orders of magnitude accompanying a structural phase transition from a low-temperature monoclinic structure to a high-temperature tetragonal structure at around 68°C. The IMT of VO₂ can be achieved by electrical and optical excitation, making it a promising material for application in various fields.²⁾ However, selective growth of VO₂ is a critical issue because of the presence of various oxide phases in the V-O phase diagram. In addition, it is also known that the VO₂ crystal phase showing IMT on an amorphous glass substrate is hard to prepare. In this study, we attempted to fabricate a single-phase VO₂(M1) film on the fused silica substrate with IMT by monitoring the discharge voltage and controlling the process over time.

Experiment

VO₂ thin film was deposited on fused silica glass substrates using a reactive direct current magnetron sputtering (r-DCMS) system. A DC power of 50 W was applied to the vanadium metal (φ50 mm, 99.9%) target. The Ar gas flow rate and pressure were set to 5.0 sccm and 1.0 Pa. Before the VO₂ deposition, the hysteresis behavior of the r-DCMS system was measured by increasing/decreasing the O₂ flow rate while monitoring the discharge voltage. Fig. 1 (a) shows the measured hysteresis curve. The deposition of VO₂ films was performed in the transition region of the O₂ flow rate decreasing profile. The deposition was conducted in two types of operations. One is a fixed discharge voltage operation at 380 V (O₂ 0.28 sccm) and 370 V (O₂ 0.32 sccm). Second was a transient voltage change operation: the discharge voltage was initially set at 390 V and waited for 20 seconds. Then, the O₂ flow rate was reduced to change the voltage to 360 V, 350 V, 340 V, and 320 V and waited for another 20 seconds. These operations were repeated during the deposition. The substrate temperature and the deposition time were 400 ± 5℃ and 10 minutes. The crystallinity of the prepared films was evaluated by X-ray diffraction (XRD). The resistance changes against the temperature (R-T characteristics) were investigated by the two-probe method.

Result and discussion

Fig. 1 (b) shows the XRD patterns for the deposited samples. The diffraction peak from VO₂(M) (011) plane at 2θ = 27.99° was observed only in the 390/350 V sample, indicating the crystal growth of VO₂ thin films on a fused silica glass substrate.

Fig. 1 (c) shows the R-T characteristics of the prepared samples. In all samples except 390/350 V, the IMT was not observed. In the 390/350 V sample, one order of magnitude change in resistance was achieved. These results correspond with the XRD results. Single-phase VO₂ (M1) is successfully obtained by controlling the discharge voltage though VO₂ was deposited on the fused silica glass substrate. The present results will contribute to the deposition of selective single-phase VO₂ (M1) films by reactive sputtering.

Refference

1) C. H. Griffiths, and H. K. Eastwood, J. Appl. Phys 45, 2201 (1974).

2) F. J. Morin, Appl. Phys. Lett., 3, 34 (1959)