The role of beryllium in the band structure of MgZnO: Lifting the valence band maximum

摘要

We investigate the effect of Be on the valence band maximum (VBM) of MgZnO by measuring the band offsets of Mg_xZn_(1-x)O/Be_xMg_yZn_(1-x-y)O heterojunctions using X-ray photoelectron spectroscopy measurements. Mg_xZn_(1-x)O/Be_xMg_yZn_(1-x-y)O films have been grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The valence band offset (ΔE_v) of Mg_(0.15)Zn_(0.85)O (E_g = 3.62 eV)/Be_(0.005)Mg_(0.19)Zn_(0.805)O (E_g = 3.73 eV) heterojunction is 0.01 eV and Be_(0.005)Mg_(0.19)Zn_(0.805)O has a lower VBM. The increased Mg composition is the main factor for the reduction of VBM. The VBM of Mg_xZn_(1-x)O is lower by 0.03 eV with the enlargement of E_g from 3.62 eV to 3.73 eV by increasing Mg composition. Considering the effect of increased Mg composition, it is concluded that the little amount of Be makes the VBM go up by 0.02 eV when the E_g of the alloy is 3.73 eV. The ΔE_v of Mg_(0.11)Zn_(0.89)O (E_g = 3.56 eV)/Be_(0.007)Mg_(0.12)Zn_(0.873)O (E_g = 3.56 eV) heterojunction is calculated to be 0.03 eV and Be_(0.007)Mg_(0.12)Zn_(0.873)O has a higher VBM than Mg_(0.11)Zn_(0.89)O, which means that a little amount Be lifts the VBM by 0.03 eV when the E_g of the alloy is 3.56 eV. The experimental measurements have offered a strong support for the theoretical research that alloying Be in Mg_xZn_(1-x)O alloys is hopeful to form a higher VBM and to enhance the p-type dopability of MgZnO.