# Photoconductive Semiconductor Switches: Materials, Physics, and Applications
## 4 Diamond PPSS Devices
Initially, an electron in the triplet ground state of \({}^{3}\)A\({}_{2}\) in the \(V_{\rm N}\) center is excited to the triplet excited state \({}^{3}\)E by absorbing the first photon. Subsequently, a second photon ionizes the electron into the conduction band, causing a charge state conversion from negatively charged \(V_{\rm N}\) centers (\(V_{\rm N}^{0}\)) to neutral \(V_{\rm N}\) centers (\(V_{\rm N}^{0}\)). In the third step, an electron in the \(V_{\rm N}^{0}\) state is excited from the \({}^{2}\)E to the \({}^{2}\)A\({}_{1}\) level, and finally, an electron is excited from the valence band to the vacant state of \(V_{\rm N}^{0}\), reverting the charge state back to \(V_{\rm N}^{-}\) as described in Refs. [28; 52; 54]. The free electrons and holes generated during the photoionization process contribute to the photocurrent, enabling continuous photocurrent detection. Interestingly, under higher illumination, the increase in photocurrent exhibited a linear trend rather than a quadratic one. This behavior could be attributed to two potential factors: the saturation of the transition from \({}^{3}\)A\({}_{2}\) to \({}^{3}\)E, and background signals arising from other defects, such as isolated substitutional N atoms, also known as P1 centers. An investigation into three grades of synthetic diamond [52]--high-pressure high-temperature (HPHT) belt-press, HPHT split-sphere, and chemical vapor deposition (CVD) general--revealed that N impurities can act as efficient recombination centers, leading to sub-ns response times. The HPHT Belt-Press diamond PCSS demonstrated responsivity at 532 nm in the range of 2-7 \(\times\) 10\({}^{-5}\) A/W under a moderate electric field of 2 kV/cm. In comparison, Schottky diode diamond photoconductive detectors exhibited a responsivity of only 10\({}^{-6}\) A/W at a few volts of reverse bias, with response times no faster than 100 ns. Non-epitaxial single-crystal diamond detectors, biased at 50 V, were unable to achieve responsivity greater than 10\({}^{-7}\) A/W in the visible spectrum without the use of defect passivation treatments. Natural diamond PCSS devices, biased at 7 kV/cm, achieved a responsivity of 5 \(\times\) 10\({}^{-4}\) A/W at an above-bandgap wavelength of 222 nm. For comparison,  
Figure 7: A simplified electronic energy level diagram (not to scale) represents (**a**) the energy states in HPHT diamond and (**b**–**e**) the \(V_{\rm N}^{-}\) center at room temperature. In (**b**), the absorption of a first photon excites an electron from the \({}^{3}\)A\({}_{2}\) ground state to the 3\({}_{\rm E}\) excited state of the \(V_{\rm N}^{-}\) center. In (**c**), \({}^{3}\)E electrons can either radiatively decay back to the \({}^{3}\)A\({}_{2}\) ground state or be further excited into the diamond conduction band (CB) by absorbing a second photon. The spin-selective non-radiative decay of electrons to the singlet state \({}^{1}\)A\({}_{1}\) (transition 3), followed by a transition to the metastable \({}^{1}\)E state, enables photo-detected magnetic resonance (PDMR) and optically detected magnetic resonance (ODMR). In (**d**), the two-photon ionization of the \(V_{\rm N}\) center results in the formation of an \(V_{\rm N}^{0}\) center and a free electron in the CB. Through a subsequent two-photon process, in which the \(V_{\rm N}^{0}\) center is first excited and then an electron from the valence band (VB) is promoted into the vacated orbital of the \(V_{\rm N}^{0}\) center, (**e**) the \(V_{\rm N}\) center can be converted back to its negatively charged state (\(V_{\rm N}^{-}\)) (re-generated from Ref. [54]). a GaN PCSS biased at 24.5 kV/cm demonstrated a responsivity of 3.5 \(\times\) 10\({}^{-5}\) A/W at 532 nm with 2 mJ/pulse, while a SiC PCSS, biased at 18.7 kV/cm, achieved a responsivity of 2.5 \(\times\) 10\({}^{-5}\) A/W at 532 nm with 4 mJ/pulse. To gain a deeper understanding of the characteristics of synthesized diamond for high-power optoelectrical applications, various diamond structures for extrinsic PCSSs were explored [45].