Electroluminescence from n-n isotype heterostructures of graded-band-gap ZnMgO: Al and ZnO films on platinized Si

We report on room temperature electroluminescence (EL) from n-n isotype heterostructures composed of Al-doped graded-band-gap Zn_1-xMg _xO (g-ZnMgO : Al) and ZnO films fabricated on Pt/Ti/SiO ₂/Si substrates. The isotype heterostructure device generated EL emission at operation voltages as low as 3-5 V, whose emission spectra covered visible and near infrared regions under the unipolar operation condition, with g-ZnMgO : Al as positive. The intensity of light emission increased nonlinearly and short-wavelength emissions in the visible region became appreciable at high injection currents. We also observed negative capacitance at high forward bias above the onset voltage, implying generation of holes by hot electrons. We discussed impact ionization as a possible origin of the EL from the heterostructure and the effect of quasi-electric field generated in the graded-band-gap layer in conjunction with the apparent upconversion EL by considering the electric field needed for impact ionization.