Papers

ICMCTF2002 Session C5-2: p-type and n-type Semiconducting and TCO Films

Thursday, April 25, 2002 1:30 PM in Room Sunset

Thursday Afternoon

Start	Invited?	Item
1:30 PM		C5-2-1 Co-doping for the Fabrication of p-type ZnO T. Yamamoto (Kochi University of Technology, Japan) Conductive n-type zinc oxide (ZnO) films have been investigated more recently, and is very promising as a transparent conducting oxide (TCO) thin film. Developing alternatives to indium tin oxide (In₂O₃:Sn, ITO) is desirable because of the high cost and scarcity of indium. ZnO is lower in cost and also easier to etch than ITO is, so it may replace ITO as a front electrode in some future displays, such as flat-panel displays. The theoretical prediction for the realization of p-type ZnO by codoping Ga and N in the ratios of N:Ga=2:1 was proposed by us ¹. The deliberate codoping of the donors, Ga species, is essential for the delocalization of the impurity states at the acceptors and the stabilization of the ionic charge distributions in p-type highly doped ZnO. The donor is not the p-type killer but a good by-player who activates acceptors, i.e., the reactive codopant. Subsequent confirmation of the applicability of the codoping to produce p-type ZnO was conducted by Osaka`s group ². The fabrication of p-type ZnO with a direct band gap of 3.3 eV is driving the development of ZnO technology; ZnO will be an important material in short-wavelength light emitting devices because ZnO is lower in cost and can be deposited successfully at low temperatures (typically ≤ 200 degree C). We will discuss what causes the difficulty to be doped as p-type for ZnO based on the electronic structures and the Madelung energy calculated by ab-initio electronic-band-structure calculations, and the thermodynamic parameter which is the formation enthalpy of the solid compounds, Zn₃N₂. Then we will propose materials design to fabricate low-resistivity p-type ZnO by the codoping. ¹ ¹T. Yamamoto and H. Katayama-Yoshida: Jpn. J. Appl. Phys. 38 (1999) L166, T. Yamamoto and H. Katayama-Yoshida: Mater. Res. Soc. Proc. 623 (2000)223. ² M. Joseph, H. Tabata and T. Kawai: Jpn. J. Appl. Phys. 38 (1999) L1205.
2:10 PM		C5-2-3 Preparation of PN Junctions Using Oxide Semiconductor Thin Films Deposited by Magnetron Sputtering T. Minami, S. Suzuki, T. Miyata (Kanazawa Institute of Technology, Japan) In 1993, we reported the first fabrication of PN and PIN junction diodes consisting of N-type ZnO and P-type NiO thin films, which are wide band-gap oxide semiconductors[1]. From the purpose of developing oxide thin-film solar cells, this paper describes the fabrication of PN heterojunction thin-film diodes using polycrystalline oxide semiconducting thin films with various band-gap energies. Oxide thin films were prepared on glass substrates by r.f. magnetron sputtering using a powder target. For example, Al-doped ZnO (AZO) and Mg-doped CuCrO₂ thin films were used as N- and P-type oxide semiconducting layers, respectively. AZO and CuCrO₂:Mg thin films were deposited on substrates at 250-300°C in a pure Ar gas atmosphere at a pressure of 1 Pa with an r.f. power of 80 W. In P-type CuCrO₂:Mg film depositions, a mixture of Cu₂O, Cr₂O₃ and dopant MgO powders calcined at 1000$B!n(B in air for 1 h was used as the target. The resulting CuCrO₂:Mg thin films with a resistivity of 1-4X10^?2Ωcm, were found to be P-type, or positive hole conduction, as evidenced from both Hall effect and Seebeck effect measurements. The voltage-current (V-I) characteristic of a PN junction fabricated using CuCrO₂:Mg and AZO thin films exhibited an ohmic, or linear, relationship. This V-I characteristic is attributed to a PN junction consisting of degenerated semiconducting layers[1]. In order to obtain high resistivity P- and N-type oxide thin-film layers, undoped CuCrO₂ and ZnO thin films were deposited in an O₂ gas atmosphere. As a result, highly resistive or insulating CuCrO₂ and ZnO thin films were obtained. A PIN junction diode was fabricated by depositing high resistivity CuCrO₂ and ZnO thin-film layers, or I-layers, between the P-type CuCrO₂:Mg and the N-type AZO thin-film layers. The resulting PIN junction diode exhibited a rectifying V-I characteristic and a photovoltage when illuminated. [1]H.Sato, T.Minami, S.Takata and T.Yamada, Thin Solid Films, 236 (1993) p.27.
2:30 PM		C5-2-4 Highly Oriented Transparent Conducting Oxide Films of In₂O₃:Sn and ZnO:Al for Organic Light Emitting Devices H. Kim, J.S. Horwitz, W.H. Kim, Z.H. Kafafi, D.B. Chrisey (US Naval Research Laboratory) Transparent conducting oxide thin films such as indium tin oxide (ITO) and aluminum doped zinc oxide (AZO) were grown by pulsed laser deposition (PLD) on single crystals of yttria-stabilized zirconia (YSZ), sapphire (Al₂O₃), and magnesium oxide (MgO). The structural, electrical and optical properties of these films were investigated as a function of the deposition conditions. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at fluences of 1 - 2 J/cm² as function of substrate temperature (300 °C to 600 °C) and oxygen pressure (1 to 100 mTorr). X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the structure and morphology of the deposited films. UV/VIS/NIR spectroscopy and Hall effect measurements were used to characterize the optical and electrical properties. Highly oriented ITO films (300 nm thick), grown on single crystalline YSZ substrates at 300 °C and 10 mTorr of oxygen, show a resistivity as low as 1.4 x 10^-4 Ω-cm with a visible transmittance of 90%. Resistivities of ~ 4 x 10^-4 Ω-cm were achieved for AZO films (300 nm thick) grown on single crystal sapphire substrate at 300 °C and 10 mTorr of oxygen. The properties of these films grown on various single crystal substrates will be compared and the use of these films as transparent anode electrodes for organic light-emitting diodes will be discussed.
2:50 PM		C5-2-5 Characterization of the Interface Junction in n-ZnO/p-Si Photodiodes J.Y. Lee, Y.S. Choi, W.H. Choi, H.W. Yeom, S. Im (Yonsei University, South Korea) N-ZnO films (with a thickness of ~100 nm) were deposited on p-Si substrates at various temperatures of 300, 400, 480, and 550°C with two Ar/O₂ gas ratios (6/1, 4/1) by RF magnetron sputtering to form n-p heterojunction photodiodes. The best photodiode was obtained at 480°C with the gas ratio of 6/1, even though the n-ZnO deposited at 550°C showed the best film qualities as characterized by x-ray diffraction (XRD) and photoluminescence (PL) experiments. Large current leakage was observed from the diodes prepared at 550°C . X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were performed on ultra-thin ZnO films, which were deposited on p-Si for 3 min. at the same temperatures as for the thick film deposition. According to XPS and AFM analysis, the interfacial thin n-ZnO becomes O-rich, containing a thick SiO₂ layer underneath and the interface becomes less smooth as the deposition temperature and the O₂ ratio increase. It is concluded that the O-rich ZnO with the SiO₂ layer and the rough interface are main sources for the high junction leakage observed from the n-ZnO/p-Si photodiodes prepared at 550°C .
3:10 PM		C5-2-6 DC sputtered In₂O₃ Thin Films P. Malar, S. Kasiviswanathan, V. Damadara Das (Indian Institute of Technology Madras, India) Indium oxide is a n-type semiconductor with a wide bandgap of ~3.7 eV. The interesting characteristics of higher transmittance and lower resistivity of In₂O₃ make it suitable for application as a transparent conducting electrode in solar cells. Thin films of In₂O₃ were deposited by DC reactive sputtering of In target in Ar and O₂ gas mixture onto different substrates. Deposition was done at a constant power level of ~100W and a final pressure of 510^-2 torr. The selected area diffraction pattern suggested that the films were polycrystalline and single-phase in nature. The lattice spacings 'd' values calculated from the diffraction pattern were found to match well with the standard ASTM values. Optical absorption measurement near the fundamental edge showed the films to be highly transparent with an average transmittance of ~85%. The films showed electrical resistivity of about 510^-4 (cm, which is comparable to the lowest resistivity values reported in the literature.