BLUE-LIGHT EMISSION IN FORWARD-BIASED ZNS MIS DIODES

MIS devices are prepared by depositing Au, Pt, or Ag electrodes onto semi‐insulating layers of ZnS, which are in turn evaporated on to semiconducting ZnS substrates. The resultant diodes, which exhibit forward‐bias electroluminescence in the blue at 445 to 460 nm, are studied extensively. The photoelectric barrier heights of diodes with optimum quantum efficiency (i.e. with I‐layers 350 to 500 Å thick) are large (≈ 2.4 eV), and indicate that the semiconductor is heavily inverted at the interface. Replenishment of the minority carriers occurs by the migration of hot holes from the metal via the valence band of the semi‐insulator. Plasma radiation, emitted when hot electrons enter the metal with energies corresponding to the barrier height, is identified in inefficient MS diodes. This supports the notion that the minority carriers are produced as hot holes in the metal when the energetic electrons are de‐excited. A brightness‐current‐density law of the form B ∼ Jn, where n varies between 2.0 and about 1.5, is observed over six orders of magnitude above a threshold voltage of some 1.85 V. The limitation to the efficiency at the highest drive currents appears to be an insufficient supply of minority carriers. The maximum available current density in these devices is in the region of 10 to 20 mA mm−2, limited by a fairly high series resistance of 200 to 500 Ω. Copyright © 1978 WILEY‐VCH Verlag GmbH & Co. KGaA