Fabrication and characterization of all oxide P-N junction diodes

Abstract

We have grown ZnO and MgO mixed oxide thin films by pulsed laser deposition. Most other similar work reported in the open literature so far involved preparing wurtzite structured MgxZn₁-xO (w-MZO) films on sapphire substrates with x<0.4. We argue that the cubic form of ZnO, including cubic MgxZn₁-xO (c-MZO) could be useful as transparent conducting oxide (TCO), n-type or p-type doped semiconducting oxide, wide bandgap UV detector and dilute magnetic semiconductor (DMS) that can be matched with and grown on many other cubic oxides and perovskites to form interesting devices. By the use of single crystal substrate of matched cubic structures, such as MgO and LaAlO₃ (LAO), we have demonstrated that cubic MgxZn₁-xO thin films even with x=0.57, representing maximum reported solubility of Zn in MgO, can be stabilized under normal ambient. High quality homoepitaxially and heteroepitaxially grown c-MZO films on MgO and LAO respectively have also been obtained. MZO is a wide bandgap material. Its absorption band edge shifts deep into UV with increased MgO content. Indium doped c-MZO (In-MZO) thin films show typical n-type semiconducting properties. Electrical conductivity at room temperature as high as 10² S cm⁻¹ has been achieved for samples with In 8 at.% doping. Yet they remain optically transparent over the whole visible spectrum. Thus they become desirable n-type TCO. Li-doped NiO (Li₀.₁₅Ni₀.₈₅O, LNO), on the other hand, is a p-type conductor with good optical transmittance. We have fabricated a number of optical transparent all-oxide p-n junctions based on p-LNO/n-Nb doped SrTiO3 single crystal substrate, p-LNO/n-ZnO(wurtzite) and p-LNO/n-In-MZO. Their structural qualities have been studied and excellent heteroepitaxial relationships have been demonstrated. The results of our present investigation provide affirmative evidence for using all-oxide to develop p-n diode junction devices. In particular the advantages of utilizing the diversified properties of oxide such as wide bandgap, good visible/UV transmittance, potential DMS and stability at high temperature can be realized in these junction diodes. We have also illustrated that c-MZO is a very good base material for fabricating all-oxide p-n junction diodes.