Doping Experiments in ZNO
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DOPING EXPERIMENTS IN ZNO R.E. STAUBER**, P.A. PARILLA*, J.D. PERKINS*, J.A. DEL CUETO*, D.S. GINLEY* * National Renewable Energy Lab, Golden CO 80401 ** University of Colorado, Boulder CO 80304, [email protected]
ABSTRACT This paper reports on attempts to fabricate transparent electrically conductive p-type ZnO by pulsed laser deposition (PLD) and sputtering using N2, N2O and NO gases. Expanding on the work of Kawai and coworkers [1,2], we used an ion source, rather than an ECR source in the PLD chamber to dissociate N2O gas, and explored the use of aluminum in addition to gallium as potential co-dopants. The most promising results have been obtained with DC reactive sputtering of gallium-doped zinc metal targets. A three to six order of magnitude reduction in n-type carrier density was observed when 2% of the argon sputtering gas was replaced with NO.
INTRODUCTION ZnO is a wide bandgap semiconductor that is used as a transparent conducting electrode material in devices such as flat panel displays and solar cells. It can easily be made n-type by doping with gallium or aluminum, for example. If it could reliably be made p-type, it could open the way to a whole family of novel ZnO based electronics (UV LEDs, transparent transistors, etc). P-type ZnO was proposed in a theoretical paper by Yamamoto et al [3] where they suggested the simultaneous doping (co-doping) of Group 3 elements (Ga, Al, In) with nitrogen in order to increase the solubility of the acceptor dopant and to lower its energy level with respect to the valence band maximum. Their calculated Madelung energies (Table 1) for a crystal doped with a hypothetical acceptor-donor-acceptor complex like N-GaN were several eV lower than for an undoped crystal, implying it might be possible to have a net p-type doping effect that is still energetically favorable. Nitrogen by itself is not easily incorporated into a ZnO crystal lattice because its substitution on an oxygen site results in higher crystal binding energies. Doping Type Madelung energy relative A few months later, a group at to the undoped ZnO (eV) the same university reported p-type ZnO ZnO:Al -6.44 films made with a gallium-doped target by introducing N2O through an ECR ZnO:Ga -13.72 source into the pulsed-laser deposition ZnO:In -9.73 (PLD) chamber during growth [1]. With ZnO:N +.079 Ga present, p-type material with carrier ZnO:2N +.091 densities to 4*1019 were reported (Table ZnO:(2N+Al) -4.74 2), but with no Ga in the target, they ZnO:(2N+Ga) -12.06 produced a p-type film with very low ZnO:(2N+In) -7.79 carrier density (1010 cm-3) – an apparent Table 1: Madelung energies for selected confirmation of the co-doping theory. dopants in ZnO as calculated by KatayamaYoshida. [3]
F2.8.1
P (mbar)
N µ ρ -3 2 ) (cm (cm /Vs) (Ω-cm) -3 -2 20 N2O No No 1x10 4.3x10 4.5x10 0.31 N2O No Yes 1x10-3 5x105 2.0x1010 1.9x103 N2O Yes Yes 1x10-3 2.0 4x1019 0.07 N2 No Yes 1x10-3 0.17 1.3x1019 1.3 Table 1: p-type doping in ZnO as reported by Josephs [1]. Dopant
Ga
ECR
Carrier type n p p n
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