Study of the Electrical and Structural Characteristics of Al/Pt Ohmic Contacts on n-Type ZnO Epitaxial Layer

Abstract

We have investigated Al/Pt ͑20/50 nm͒ ohmic contacts on a n-type zinc oxide ͑ZnO:Al͒ epitaxial layer. The samples were annealed at temperatures of 300°C and 600°C for 1 min in nitrogen ambient. Current-voltage measurements indicated that the as-deposited sample was ohmic with a specific contact resistivity of 1.25 (Ϯ0.05) ϫ 10 Ϫ5 ⍀ cm 2 . However, the annealing at 300°C resulted in a significantly better ohmic behavior, with a contact resistivity of 2 (Ϯ0.25) ϫ 10 Ϫ6 ⍀ cm 2 . A further increase in the annealing temperature to 600°C led to a decrease in specific contact resistivity due to extensive interfacial reactions between Al and ZnO. Both Auger electron spectroscopy and glancing angle X-ray diffraction were employed to investigate the nature of the interfacial reaction between the Al/Pt and ZnO layer with increasing annealing temperature. Possible explanation is given to describe the temperature dependence of the specific contact resistivity.ZnO and related oxide semiconductors, such as MgZnO and CdZnO, have acquired considerable importance as the basis for a new generation of optoelectronic devices due to their wide direct bandgap (E g ϭ 3.37 eV), and large exciton binding energy of 60 meV at room temperature, which is much higher than that of GaN ͑25 meV͒ and ZnSe ͑18 meV͒. 1-4 ZnO is an alternate candidate for optoelectronic applications, for use in the short wavelength range ͑green, blue, UV͒. 5 Although great progress has been achieved in the area of ZnO-based oxide semiconductors, many problems remain, such as the difficulty of p-ZnO growth and lack of high quality ohmic or Schottky contacts for n, p-ZnO. In particular, to realize commercialized ZnO-based optoelectronic devices, high quality ohmic and Schottky contacts are required. Several groups have reported ohmic contact schemes, however, most work is the initial stages. Akane et al. investigating In ohmic contacts to undoped n-ZnO, demonstrated that a postannealing treatment for 1 min at 300°C resulted in ohmic contact with a specific contact resistivity of 7 ϫ 10 Ϫ1 ⍀ cm 2 . 6 Kudo et al. fabricating transparent n-ZnO/p-SrCu 2 O 2 junction diode, employed n ϩ -ZnO and ITO as n-type and p-type electrodes, respectively. 2 In our previous studies, we reported on Au, Ti/Au, and Ru ohmic contact scheme with specific contact resistivity of 10 Ϫ3 -10 Ϫ5 ⍀ cm 2 . 7-9 Recently, an Al ohmic scheme was used for a metal-semiconductor-metal device structure and p-n homojunction diodes. 4,10 Although the Al have been used as n-type ohmic contact metal, there are still lack of detailed interfacial reaction study between Al and ZnO. In addition, the detailed electrical and structural examinations of the Al/Pt ohmic schemes have not been hitherto performed.In this work, we reported on electrical and structural examination of Al/Pt multilayer on Al-doped ZnO epitaxy layer with increasing annealing temperature. Using glancing angle X-ray diffraction ͑GXRD͒ and Auger electron spectroscopy ͑AES͒ depth profiles analysis, we suggested possible mechanism to describe tem...

Contacts to ZnO

“…The usual approaches involve surface cleaning to reduce barrier height or increase of the effective carrier concentration of the surface through preferential loss of oxygen [19,20]. Specific contact resistances of $3 Â 10 À4 O cm À3 were reported for Pt-Ga contacts on n-ZnO epitaxial layers [19,21], 2 Â 10 À4 O cm À3 for Ti/Au on Al-doped epitaxial layers [25,26], 0.7 O cm À3 for non-alloyed In on laserprocessed n-ZnO substrates [27], 2.5 Â 10 À5 O cm À3 for non-alloyed Al on epitaxial n-type ZnO [28] 4.3 Â 10 À5 O cm À3 for Ti/Au on plasma exposed, Al-doped n-type epitaxial ZnO [29,30] and 9 Â 10 À7 O cm À3 for Ti/Al on n + -epitaxial ZnO [31,32]. Several points are clear from the past works, namely that the minimum contact resistance generally occurs for post-deposition annealing temperatures of 200-300 1C on doped samples which must be treated so as to further increase the near-surface carrier concentration.…”

Al/Au ohmic contact to n-ZnO by dc sputtering

“…A lot of metallization schemes, such as Ti/Au, Al/Pt, Ti/Al/Pt/Au, and Al, were investigated as ohmic contact materials for n-ZnO [9][10][11][12]. These contact materials were mainly deposited by using an electron beam method [8][9][10][11][12]. The minimum contact resistance was generally obtained after annealing at 200-300 • C, which increased the carrier concentration near the surface of n-ZnO.…”

NUMERICAL SIMULATION OF OPTICAL CONFINEMENT IN ZnO BASED HETEROSTRUCTURES AT 375-NANOMETER WAVELENGTH

“…The interest in ZnO [8][9][10][11][12] is gained by its prospects in optoelectronics applications owing to its direct wide-band gap of 3.37 eV at 300 K. The large exciton binding energy of 60 meV of ZnO leads to an intense near-band edge excitonic emission at room and higher temperatures, making it suitable for space applications. Despite these advantages, ZnO is easily etched in all acids and alkalis, and this provides an opportunity for the fabrication of small-size devices.…”