Deposition And Sn Doping Of Amorphous GaN Thin Films At Low Temperature

Recently, GaN has been considered to be the most promising optoelectronic material for many applications, such as short-wave optoelectronic devices, full color light emitting displays, high electron mobility transistors and high power electronic devices. Up to now, it is still very difficult to grow large sized GaN bulk crystals, so that GaN epitaxial layers are usually grown on sapphire or SiC by MOCVD and their diameters are less than 2 inches. On the other hand, amorphous gallium nitride a-GaN, which has the advantage of not requiring lattice-matched substrates due to its structural flexibility, could be applied to making large-area display devices.As an efficient method for large-area deposition, Magnetron Sputtering(MS) has been used to prepare GaN thin films since last decade. However, there are no reports concerning the doping of GaN by MS because it's quite difficult to make SiGa alloy target for MS deposition technically, due to the high melting point of Si and its small solubility in Ga.In this paper, we first presented a comprehensive review of the research history and current status of preparation, doping and device processing for GaN material. To improve the compatibility of MS processing, Sn, a low-melting-point metal, was chosen as a new dopant for the n-type doping of GaN.The ionization energy of Sn in GaN was quantitatively calculated to be 31 meV by the first-principle calculations based on density functional theory (DFT). The results of DOS, PDOS and Charge density difference contour map analysis also showed Sn behaved as a shallow donor in GaN, which confirmed the feasibility of Sn-doped n-type GaN thin films theoretically. Then, pure GaN and Sn-doped GaN thin films were prepared by DC reactive-MS, using pure Ga and SnGa alloy as the target, respectively. The experimental results showed that the optical band gap of GaN thin film was about 3.30eV when the substrate temperature was above 400℃ and the flow ratio of N₂ was fixed at 40 sccm. After Sn doping, the optical band gap decreased slightly. Hall effect measurement showed the Sn-doped GaN thin film wasn-type semiconductor with carrier concentration and electron mobility about 10¹⁵ cm^-3 and 10 cm²V^-1s^-1, respectively. Although the carrier concentration was low, the electron mobility was one order of magnitude larger than a-Si. As a result, Sn-doped amorphous GaN thin films were expected to be put into practice in some special fields.