Blue light emission in forward-biased ZnS MIS diodes

Abstract

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.

Blue electroluminescence in forward-biased ZnS diodes with (SN)xelectrodes

“…The free face was then repolished and re-etched in the same manner as described above. MIS devices were fabricated using a layer of ZnS as the insulating region of the barrier (Lawther and Woods 1978) and this was deposited by electron beam evaporation from a source of Cerac, ZnS granules. The thickness (-400 A) and rate of deposition (-1 8, S") of this film were measured using a calibrated quartz crystal monitor.…”

“…Although the provision of ohmic contacts to semiconducting ZnS is a major difficulty in the production of LED, very few authors have discussed the problem in detail and even fewer have tried to relate the properties of such contacts to the EL emission obtained from the finished devices. By using an indium-cadmium alloy, Lawther and Woods (1978) were able to obtain contacts with linear Z-V characteristics in both polarities of voltage. We have been able…”

“…The electroluminescence excited from the die with two amalgam contacts showed an identical shift to lower photon energies when the temperature was reduced to 85 K. The forward-bias electroluminescence from the device with the smallest barrier height also displayed a secondary emission peak with its maximum in the orange-red at 1.9 eV (curve B). and Woods (1978) have proposed a model to explain the electroluminescence emission from forward-biased MS and MIS barriers on ZnS in terms of the replenishment of holes at the insulator-semiconductor interface by an Auger mechanism in the metal (figure 5 ) . The energy of the hot electrons entering the metal is dissipated in creating energetic holes which have sufficient energy to surmount the hole barrier and reach the semiconductor, provided the electron barrier height exceeds € , / 2 .…”

“…Attempts to produce light-emitting diodes on single crystals of these materials have recently been reviewed by one of us (Woods 1981). Since p n homojunctions have not been sucessfully fabricated on ZnSe or ZnS, other techniques have had to be utilised to excite electroluminescence, and in particular MIS diodes on ZnS operating in forward bias have been investigated as blue-light-emitting diodes (Katayama eta1 1975, Lukyanchikova et a1 1977, Lawther and Woods 1978, Lawther et a1 1980. External quantum efficiences as high as 5 X lo-* % have been claimed.…”

“…The most comprehensive description of the fabrication and properties of these devices has been provided by Lawther and Woods (1978). The essential feature of the detailed model which they suggested, is the injection of minority carriers at the insulator-semiconductor interface by the migration of hot holes from the metal via the valence band of the insulator.…”