CHROMATICITY AND ELECTROLUMINESCENT EFFICIENCY OF ATOMIC LAYER EPITAXY SRS-CE THIN-FILMS

We have studied various factors that control the luminous properties o f atomic layer epitaxy (ALE) SrS:Ce electroluminescent (EL) devices by using electron paramagnetic resonance, photoluminescent decay, x-ray diffraction, and charge-voltage characterization. EL and PL light is g reen shifted in ALE SrS:Ce films, compared to powders and thin films p repared with some other techniques. The origin of the green shift is d iscussed. Moderate heat treatments of as-deposited SrS:Ce devices were found to dramatically improve the EL efficiency. This is due to the r emoval of defects leading to nonradiative Ce3+ decay and to reduction of Ce4+ to radiative Ce3+. Different heat treatments were found to inf luence the transferred charge, so that there is an optimal heat treatm ent temperature between 500 and 750 degrees C. Heat treatments also in duce macroscopic stress in these SrS:Ce films. This was seen to induce a local stress around Ce3+ sites. However, the stresses were not foun d to directly correlate with the luminous efficiency, The SrS:Ge growt h temperature was found to affect the Ce3+ excitation efficiency in EL . Electronic traps in SrS:Ce films are discussed. We suggest that some traps can be beneficial in providing necessary charge for the Ce3+ ex citation process and others can be deleterious in that they suppress r adiative Ce3+ decay events. To determine the effects of Ce doping, und oped SrS films were also deposited and fully characterized. Finally, w e compare the properties of films prepared by ALE to those deposited v ia rf sputtering. (C) 1998 American Institute of Physics.