Formation of Low Resistance Nonalloyed Ti∕Au Ohmic Contacts to n-Type ZnO by KrF Excimer Laser Irradiation
Electrochemical and solid-state letters
We report on the formation of high-quality nonalloyed Ti ͑30 nm͒/Au ͑50 nm͒ ohmic contacts to n-type ZnO:Al using laser irradiation treatment. The electrical characteristics of the Ti/Au contacts are considerably improved when laser-irradiated. The specific contact resistances are measured to be ϳ10 −4 ⍀ cm 2 when the ZnO layers were laser irradiated, which is about two orders of magnitude lower than that of the as-grown sample. Based on electrical and X-ray photoelectron spectroscopy results,
... ctroscopy results, a possible mechanism for the laser-irradiation-induced improvement is described. ZnO is a wide bandgap semiconductor material that has a variety of applications, e.g., transparent conducting electrodes, window materials in displays and solar cells, varistors, piezoelectric transducers, and surface-acoustic-wave devices. In addition, ZnO has a lowpower threshold for optical pumping at room temperature and a large exciton binding energy of 60 meV at room temperature.      Recently, owing to the growth of p-type ZnO, great attention has been paid to ZnO for its potential for replacing GaN, 6, 7 and this is of great technological importance for the fabrication of optoelectronic devices. To realize high-performance ZnO-based optoelectronic devices, the formation of high-quality ohmic and Schottky contacts are essential. Studies have been limited mostly to n-type contacts. This is mainly because the growth of p-type ZnO layers is extremely difficult to achieve. Thus, various types of n-type contacts to ZnO, such as Al-, Ti-, Ta-, and Re-based contacts,     have been extensively investigated so far. It was shown that these contacts produced specific contact resistance in the range of ϳ10 −4 to ϳ10 −5 ⍀ cm 2 upon annealing. Such a postdeposition annealing at high temperatures during a device process has been widely used to improve ohmic contact characteristics. However, thermal annealing at high temperatures may cause surface roughening, surface decomposition, and spiky interfaces, resulting in the deterioration of device performance and hence device reliability. 12 To improve thermal degradation, Kim et al. 11 introduced a refractory Re layer between Ti/Au and ZnO layers. In addition, other groups made efforts to obtain nonalloyed ohmic contacts using various surface treatment techniques prior to metal deposition. For example, Lee et al. 13 showed that plasma-treatment was effective in forming nonalloyed Ti/Au ohmic contacts on n-ZnO ͑n d = ϳ7 ϫ 10 17 cm −3 ͒ with contact resistivity of 4.3 ϫ 10 −5 ⍀ cm 2 . Akane et al. 14 used KrF excimer laser treatment to form nonalloyed In ohmic contacts to n-ZnO. However, their samples produced relatively high specific contact resistance of 7 ϫ 10 −1 ⍀ cm 2 . In this work, we have investigated KrF laser irradiation effects ͑in different gas ambients͒ on the electrical properties of nonalloyed Ti/Au ohmic contacts to n-ZnO. The results show that the electrical behaviors of the Ti/Au contacts are influenced by the gas ambients used. For example, the contacts give specific contact resistances of 3.22 ϫ 10 −4 and 1.82 ϫ 10 −4 ⍀ cm 2 when laserirradiated in O 2 and N 2 ambient, respectively. The 0.7 m thick Al-doped n-type ZnO layers were grown on ͑0001͒ sapphire substrates at 800°C by a rf sputtering system using a ZnO target containing 0.01 wt % Al 2 O 3 . The rf power was 100 W with a frequency of 13.56 MHz, and the working pressure was 10 mTorr with Ar/O 2 gas ratio of 1:3. The carrier concentration and * Electrochemical Society Active Member.