Nonalloyed Al ohmic contacts to MgxZn1−xO

Abstract

INTRODUCTIONZinc oxide (ZnO) is a wide energy bandgap semiconductor that has potential applications for photodetectors, 1,2 light-emitting diodes, lasers, 3,4 and modulators 5 operating in the ultraviolet range. The direct bandgap of ZnO can be tuned from 3.3 eV to 4.0 eV by alloying ZnO with MgO to form the ternary compound, magnesium zinc oxide (Mg x Zn 1Ϫx O). The ZnO/Mg x Zn 1Ϫx O heterostructure is promising for a wide range of electronic and photonic devices. 6,7 Metal/Mg x Zn 1Ϫx O contacts, including ohmic and Schottky types, play important roles in device performance. In the case of ohmic-contact technology, nonalloyed ohmic contact with low specific contact resistance is preferred, particularly for shallow junction and low voltage devices, as it provides smooth metal-semiconductor interfaces resulting from the limited interface reaction. In this paper, we report the results of Al nonalloyed ohmic contacts on epitaxial Mg x Zn 1Ϫx O (0 Յ ϫ Յ 0.34) thin films. EXPERIMENTALZnO and Mg x Zn 1Ϫx O films (200-500 nm) were grown on R-plane sapphire (R-Al 2 O 3 ) substrates using metal organic chemical vapor deposition (MOCVD). The R-plane sapphire was chosen as a substrate instead of the commonly used C-plane sapphire as it offers two advantages: (1) the c-axis of ZnO lies in the surface plane, with the lattice mismatch parallel to the c-axis of ZnO being lower than the mismatch perpendicular to the c-axis (1.53% versus 18.3%), resulting in overall less mismatch; and (2) in-plane anisotropy in the ZnO/R-Al 2 O 3 system can be used to make novel optical, electrical, and piezoelectric devices. 8 The composition of the Mg x Zn 1Ϫx O films were evaluated using the Rutherford backscattering spectroscopy (RBS) technique. The surface morphology was inspected using fieldemission scanning electron microscopy (FESEM). Xray diffraction (XRD) was used to analyze the crystallinity of the material. After the growth of the Mg x Zn 1Ϫx O films, metal layers (Al/Au: 100 nm/100 nm) were deposited using an electron-beam evaporator to form the nonalloyed ohmic contacts. Aluminum was chosen as the ohmic contact metal because of its low barrier height on ZnO. 9 Gold was used as a cover layer to prevent the surface oxidation after the contact formation, as well as for bonding. The transmission line method (TLM) was used to measure the specific contact resistances. The linear TLM metallization pattern consists of seven square contact pads (area 80 ϫ 80 m 2 ), separated by different spacing, Al nonalloyed ohmic contacts were fabricated and characterized on Mg x Zn 1-x O (0 Յ ϫ Յ 0.34) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). Specific contact resistances were evaluated by the transmission line method (TLM). A specific contact resistance of 2.5 ϫ 10 Ϫ5 ⍀cm 2 was obtained for Al contact to ZnO with an electron concentration of 1.6 ϫ 10 17 cm Ϫ3 . The current flow mechanism was studied by investigating the dependence of specific contact resistances on electron concentra...

Contacts to ZnO

“…The achievement of acceptable ZnO device characteristics relies heavily on developing low specific contact resistance Ohmic metallization schemes [15][16][17][18][19][20][21][22][23][24]. 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].…”

Al ohmic contacts to HCI-treated MgxZn1−xO

“…For comparison, Au ohmic contacts to as-grown and HCl-treated ZnO and Mg 0.1 Zn 0.9 O films were also investigated. We previously reported that a n ϩ layer forms at the Al-ZnO interface, 15 leading to a lower contact resistance. However, such a heavily doped thin interface layer does not form for Au contacts to Mg x Zn 1-x O.…”

“…For ZnO samples, a n ϩ layer was formed at the interface of Al and ZnO even without HCl surface treatment. 15 HCl treatment reduces the sheet resistance of ZnO; however, the heavily n-type doped thin layer formed at the interface dominates the ohmic behavior, as Al is a shallow donor in ZnO 23 and it reacts actively with oxygen in ZnO. 20 An oxygen plasma treatment study was conducted to further investigate the effect of oxygen vacancies on ohmic contact formation to Mg 0.1 Zn 0.9 O thin films.…”

“…There have been several reports on ohmic contacts to ZnO. [12][13][14][15][16][17][18] However, little information on ohmic contacts to Mg x Zn 1-x O films is available in the literature. We previously reported Al nonalloyed ohmic contacts to Mg x Zn 1-x O (0.1 Յ x Յ 0.34) with relatively high specific contact resistance (ϳ10 Ϫ3 Ω cm 2 ).…”

“…We previously reported Al nonalloyed ohmic contacts to Mg x Zn 1-x O (0.1 Յ x Յ 0.34) with relatively high specific contact resistance (ϳ10 Ϫ3 Ω cm 2 ). 15 In this paper, we report the results of Al nonalloyed ohmic contacts to HCl-treated Mg x Zn 1-x O (0 Յ x Յ 0.2) epitaxial thin films with improved ohmic characteristics.…”