Effects of microstructures on microwave properties in Y-Ba-Cu-O microstrip-resonators

Abstract

Absmct---We examined the microwave properties of YBa,Cu,O, "s with different microstructures at 5.7GHz using a microstrip resonator technique. The "s were prepared with several types of substrate temperature profiles. Films with small grains (-100nm) and larger critical current density show fairly small rf magnetic field dependence of surface resistance and resonance frequency up to about 500e. On the other hand, "s with large grains (-200nm) and smaller critical current density show significant changes in these properties. These results suggest that the coupling strength at the grain boundaries mainly dominates the power handling capability, according to a qualitative explanation based on the coupled-grain model. I. INTRODUCI~ONPassive microwave components such as resonators [l], [2] and filters [3], [4] are promising applications of high-Tc superconductors (HTS) because they have significantly lower microwave surface resistance (Rs) than normal metals. In practical applications, the power handling capability of HTS films is also important, as well as with low loss.Some researchers have already pointed out that undesirable increases in Rs and penetration depth (A), accompanied by the appearance of nonlinear behavior, easily occur when input power level is increased in stripline [5] or microstrip [6] resonators using YBa,Cu,O, (YBCO) films. It is very important to determine whether this power dependence originates from intrinsic properties of the YBCO superconducting material or extrinsic film properties such as grain boundaries, second phases, and so on. Among these possibilities, the effect of the grain boundaries is considered to be the most probable one. For example, Nguyen et. al. tried to quantitatively explain the power dependence of Rs and h by a coupled-grain model, which treated the YBCO films as a network of superconducting grains connected by grain boundaries acting as Josephson weak links [7]. According to this model, the power dependence of the YBCO films was mainly affected by the junction critical current density (Jc) and the grain size. However, they did not discuss the details of the microstructures in their films. This paper presents the relationship between the microwave power dependence and microstructures for YBCO films fabricated with different substrate temperature profiles, Manuscript received October 17, 1994. and qualitatively discusses the effect of the grain boundaries on microwave properties based on the coupled-grain model. EXPERIMENTAL TECHNIQUESThe YBCO films were deposited on both sides of 0. 5"-thick (100)MgO substrates by pulsed laser ablation using a KrF excimer laser [4], [8]. Oxygen pressure was maintained at 200mTorr during film growth; after deposition, the films were cooled in latm of 0,. Typical film thickness was about 800nm. Four types of substrate temperature profiles were used to prepare YBCO films with different microstructures. Film I was grown at a constant substrate temperature of 705°C. The growth of the other three films was started at 630°C for an initial 6nm...