Photoconductivity of Gallium Selenide Crystals

Abstract

Single crystals of GaSe have been prepared by reaction of the elements, followed by gradient freeze crystallization. Crystals as grown were $p$-type, probably because of Cu acceptors, with hole mobility of 15 ${\mathrm{cm}}^{2}$/volt sec, hole concentration of ${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and acceptor ionization energy of 0.12 ev. In many ways the properties of GaSe crystals are very similar to those of ZnTe crystals. The absorption edge of GaSe is at 6310 A, corresponding to a band gap of 1.97 ev; the temperature coefficient of band gap is about -4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ev/deg. Insulating and photosensitive GaSe crystals can be prepared by compensating the acceptor impurities by incorporated donors from Groups VII or IV. Rectification tests indicate $p$-type photoconductivity. Thermal quenching of photoconductivity corresponds to an ionization energy of about 0.5 ev, whereas optical quenching corresponds to an ionization energy of about 1.0 ev; thus a large Franck-Condon shift is indicated. Below the temperature at which thermal quenching of photoconductivity occurs, the sensitivity is within the range of sensitive CdS or CdSe crystals.