<title>Non-steady-state photoelectromotive force in layered BN crystal</title>

Abstract

We report the excitation of the non-steady-state photoelectromotive force (photo-EMF) in the layered boron nitride crystal. Boron nitride crystals are characterized by an unique combination of physicochemical properties that advance its wide application in various areas of science and technology such as vacuum technology, production of the microelectronic devices, x-ray lithography. The main goal of our work is to present possibility to expand the non-steady-state photo-EMF technique for the new class of diamond related materials with extremely wide band gap (Eg = 5.67 eV for BN). The investigated pyrolytic BN crystal was grown using the chemical vapor deposition method. It consists of the monocrystal layers of the rhombohedral modification with thickness of 2000 A separated by thin (~100 A) layers of the hexagonal modification. The photo- EMF signal is generated by two coherent laser beams (λ = 532 nm) one of which is phase modulated. Both the diffusion regime of signal excitation and the excitation in an external sinusoidal electric field are investigated. The measurements performed using standard diffusion regime of photo-EMF generation reveals rather low hole photoconductivity σ₀ = (0.9-2.4)x10^{- 10}Ω^-1 cm^-1 at light intensities I₀ = 0.22-0.86 W/cm² and small diffusion length L_D = 35 nm of photoholes. We also measure the μτ- product and diffusion length using novel technique of signal generation in an external ac field: μτ= 1.0 x 10^-9 cm²/V, L_D = 50 nm. The influence of the layered structure of the investigated BN crystal on the effect of the non- steady-state photo-EMF is discussed.