Abstract

The photoconductive properties of diamond optical switch were investigated. A gap was made on the chemical vapor deposited (CVD) diamond film by photolithographic process. It was put on a high frequency microstrip line and switched with ultra short pulse UV laser. Theoretically, the limit of the rise time is determined by the propagation time over a gap distance, t_r = 1.5ϵ^1/2d/c where ϵ is dielectric constant, d is gap length and c is speed of light. For shortening the rise time the dielectric constant should be smaller. The breakdown voltage should be higher from the view of high output voltage. Among the various semiconductors, diamond has the highest breakdown voltage (E_br = 10⁷ V/cm), the high resistivity (ρ = 10¹⁶Ω cm), and small dielectric constant (ϵ_r = 5.5). The diamond switch has a potential of high voltage of 1 kV over the narrow gap of 1 µm so that it is expected that the rise time can be decreased to two orders of magnitude smaller than one of GaAs. Concerning about the fall time the diamond switch has been considered to be slow in comparison with GaAs one because the natural diamond has longer carrier lifetime. In the CVD polycrystalline diamond, however, it is possible to decrease the carrier lifetime by controlling the grain boundary and impurity condition. In this paper, the photoconductive parameters of the CVD diamond were measured with various grain size samples. New configuration of the diamond gap was proposed to reduce the surface leakage current and avoid surface flashover. This technology made it possible to apply static high electric field up to 2 × 10⁶ V/cm.¹ The dependence of the mobility (µ) and lifetime (τ) on the grain size was measured for a wide range of electric field. The lifetime decreased as the grain size became smaller and 40 ps lifetime was obtained in the 0.1 µm grain size sample, while it was about 200 ps in the natural single crystal (Fig. 1). The collection distance (µτE) vs electric field was shown in Fig. 2. It increased to be linearly proportional to electric field, and no saturation was observed for every grain size sample even at a high electric field of E = 3 × 10⁵ V/cm. These results denoted that the grain size dependence attributed to the decreasing of the mobility and the lifetime inside the grain not due to the increasing recombination ratio at the grain boundary in smaller grain size samples. At higher electric field, collection distance will become the same order of the grain size and rapid decreasing of the life time will be occurred. From Fig. 2, the ratio of the collection distance and grain size increased as the grain size decreased. Application of higher voltage on the smaller grain size sample make it possible to achieve the fastest switching with diamond.

© 1995 IEEE