High Current Density Diamond Photoconductive Semiconductor Switches with a Buried, Metallic Conductive Channel

Laterally configured diamond photoconductive semiconductor switches (PCSS) with a buried, metallic p+ current channel are reported. Above bandgap ( λ≤226 nm) optical triggering enables responsivity of over 130 mA/W. The use of low-impurity semi-insulating diamond as an active absorption layer enables fast rise and fall times (~2 ns) and on/off ratios greater than 10¹¹. The PCSS excited with a laser energy of 20 nJ per pulse passes a high current density (44 A/cm) under a DC bias of 60 V, thanks to the buried metallic p+ current channel. The reported devices promise high current carrying capacity without the need for filamenting while leveraging the excellent optical, electronic, and thermal properties of diamond.<br/>In this work, diamond PCSS structures with electrode spacings of 8 μm, 50 μm, and 100 μm were fabricated. The lateral PCSS devices were based on a 500 μm thick, 4 × 4 mm²Type IIa high-pressure, high-temperature (HPHT) diamond substrate. A 500 nm thick layer of heavily boron-doped p+ diamond with an atomic doping concentration of 5 × 10²⁰ cm^-3 was grown on which a 1.5 μm thick layer of unintentionally doped layer was grown using microwave plasma enhanced chemical vapor deposition (MPCVD). The p+ diamond layer has a sheet resistance of 41.4 Ω/square and serves as the low-resistivity channel between electrodes. Rectangular ohmic metal contacts composed of Ti (30 nm) / Pt (30 nm) / Au (100 nm) were deposited by e-beam evaporation, followed by thermal annealing at 450 ^oC in an ambient of argon gas. All three PCSS devices have the same contact width of 150 μm.<br/>Photoconductive measurements of the diamond PCSS devices were carried out using a tunable optical parametric oscillator (OPO) laser in the spectral range of 210-226 nm, with a laser pulse width of 5 ns. The laser spot size was fixed to a diameter of 2 mm using an aperture to keep the laser power density uniform, and the laser spectral width was below 0.1 nm. The optical power output was fixed to 40 μJ/pulse, which corresponds to a laser power density of 252 kW/cm² and total energy of 10 nJ, 127 nJ, and 191 nJ for the 8 μm, 50 μm 100 μm PCSS. The oscilloscope was triggered by a silicon photodiode that detected the scattered light of the laser.<br/>In conclusion, buried channel diamond PCSS devices fabricated on Type IIa diamond showed large on/off ratios, fast rise-time, and high current densities when excited with an above bandgap laser source. The PCSS devices exhibit linear current-voltage characteristics up to a DC bias of +/−60 V, implying that even higher current densities are possible before carrier velocity saturation occurs. TCAD simulation and experimental data predict that over 90% of current conduction is through the buried, metallic conductive channel. The buried channel design enables higher photocurrents during the ON-state utilizing the high conductivity p⁺ channel, while still maintaining large OFF-state resistances. This study’s results advocate new designs for PCSS to realize higher ON-state current and efficiency.