Edge emission in cadmium sulphide crystals

The green and blue edge emission excited by 3650 Å mercury radiation has been measured at 4·2°K for a number of CdS crystals grown under controlled partial pressures of cadmium and sulphur. The I 1 and I 2 bound exciton lines which dominate the blue emission are associated with exciton recombination at acceptors (cadmium vacancies) and donors (sulphur vacancies) respectively. The two phonon-assisted series which constitute the green emission give an indication of the relative concentrations of the same donors and acceptors which are involved in the blue emission. The relative intensities of the two green series and the I 1 and I 2 lines are correlated with the conditions under which the crystals were grown. It is shown that growth in excess pressures of cadmium vapour leads to a high donor content and n-type semiconducting samples. Growth in high pressures of sulphur vapour leads to a high acceptor content but the acceptors are compensated by a comparable concentration of shallow donors. Such samples are insulating (ρ similar 10 10 Ω cm) with poor photosensitivity.