Electroluminescence from $n-n$ isotype heterojunctions of graded-band-gap ZnMgO:Al and ZnO films*

We report room temperature electroluminescence (EL) from $n-n$ isotype heterojunction composed of Al-doped graded-band-gap Zn$_{1-x}$Mg$_{x}$O ($g$-ZnMgO:Al) and ZnO films fabricated on Pt/Ti/SiO$_{2}$/Si substrates. The graded-band-gap of $g$-ZnMgO:Al film was investigated by spectroscopic ellipsometry and found to change contiuously from 3.22 to 3.56 eV. The EL emission spectra covered visible and near infrared regions under unipolar operation condition, with $g$-ZnMgO:Al as positive, at the operation voltages as low as 3-5 V. Impact ionization/excitation process in a narrow region of the graded layer was suggested as a possible origin of the EL. We discussed multistep excitation process mediated by defect-related deep levels and the effect of quasi-electric field in the graded-band-gap layer in conjunction with the apparent upconversion EL in the heterojuncition device.