Schottky Contact Behaviour as a Function of Metal and ZnO Surface Polarity

Transcription

1 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society 0957-K09-03 Schottky Contact Behaviour as a Function of Metal and ZnO Surface Polarity M.W. Allen 1, P. Miller 2, J.B. Metson 3, R.J. Reeves 2, M.M. Alkaisi 1, and S.M. Durbin 1 1 Dept. of Electrical & Computer Eng, University of Canterbury, Christchurch, 8020, New Zealand 2 Dept. of Physics, University of Canterbury, Christchurch, 8020, New Zealand 3 Dept. of Chemistry, University of Auckland, Auckland, New Zealand ABSTRACT Hall effect, photoluminescence (PL) and Schottky diode measurements were made on the Zn-polar ( 0001), O-polar ( 0001) and m-plane ( 1100) faces of hydrothermally grown, bulk ZnO. Several polarity related differences were observed in the PL spectra. The most noticeable was increased emission from free excitons and from a triplet of emissions at ev on the Zn-polar face. Polarity effects were also observed in the properties of highly rectifying, planar, silver oxide diodes fabricated by RF sputtering using an Ag target and an Ar/O 2 plasma. The most significant of these was a consistent 130 mev higher barrier height for silver oxide diodes on the Zn-polar face compared to the O-polar face. These polarity effects are thought to result from the internal compensation of bound spontaneous polarization charges at the Zn-polar and O-polar faces. In addition, Au and Ag Schottky diodes with image-force-controlled ideality factors were achieved on the Zn-polar face of bulk ZnO without any special surface treatments. INTRODUCTION The ability to fabricate high quality ohmic and Schottky contacts is a key step towards the manufacture of ZnO based opto-electronic devices. While good progress has been made in the fabrication of low-resistivity ohmic contacts, the production of Schottky contacts with reproducible properties is proving more challenging [1], and there is a need for further study of the role the ZnO surface plays in determining contact properties. The availability of bulk, single crystal ZnO provides material of consistent quality [2] in which the surface factors influencing the performance of Schottky contacts can be investigated. One such factor is the polarity of the ZnO surface. ZnO has a high ionicity and a wurtzite structure with a lack of inversion symmetry along the c-axis. This produces a large spontaneous polarization ( Cm -2 [3]) with bound sheet charges of opposite sign occurring at the Zn-polar and O-polar faces. These bound polarization charges may have a significant impact on the morphology, chemical activity and electrical properties of the surface, all of which are known to influence Schottky contact formation. In this paper, we compare the rectifying performance of three common Schottky metals (Au, Ag & Pd), investigate the influence of surface polarity on electrical and optical properties, and report on a reliable and reproducible method of fabricating high quality Schottky contacts using silver oxide.

2 EXPERIMENT Three double-sided polished, undoped, c-axis wafers and one m-plane wafer were purchased from Tokyo Denpa Co. Ltd. (Japan). These wafers were sliced and chemomechanically polished from bulk, single ZnO crystals grown using the hydrothermal method at temperatures relatively low compared to other techniques [2]. No etching, annealing or surface treatment of the as-received wafers was performed, apart from ultrasonic cleaning in acetone, methanol and isopropyl alcohol. Each double-sided polished c-axis wafer was cut to provide Znand O-polar pieces for Hall effect measurement, PL, and for fabricating Schottky diode arrays. Single-field Hall effect measurements Table I shows the results of single field (0.51 T) Hall effect measurements taken at 300 K using the van der Pauw geometry. These measurements indicate that the bulk carrier concentration of Table I. Summary of Hall effect measurements on c-axis oriented ZnO samples. Wafer n (cm -3 ) µ (cm 2 V -1 s -1 ) ρ (Ωcm) Wafer A1 (c-axis) Wafer A2 (c-axis) Wafer A3 (c-axis) hydrothermally grown, c-axis ZnO wafers may be less than the concentration of bound spontaneous polarization charges at the Zn-polar and O-polar surfaces. In this case, there will be insufficient bulk carriers to compensate the bound spontaneous polarization charges and strong band bending may occur [4], resulting in an inversion layer of degenerate holes and a depletion region (containing positive space charge due to ionized donors) near the Zn-polar surface, and an electron accumulation layer near the O-polar surface, as shown in Figure 1. This assumes that these surfaces are clean of adsorbates and are unreconstructed. However, it is possible that the degenerate charge carriers in the inversion and accumulation layers have contributed to the Hall effect measurements. Zn-polar face accumulation (electrons) -Q SP inversion (holes) Q SP depletion (ionized donors) E C E F E V ionized acceptors O-polar face Figure 1. Band banding and internal charge compensation at the Zn-polar and O-polar faces of bulk, c-axis ZnO in response to the bound spontaneous polarization charges Q SP and -Q SP (Q SP = cm -2 [3]). (0001) (0001)

3 Photoluminescence spectroscopy PL spectra were measured at 4 K using a liquid helium cooled, cold finger cryostat and the 325 nm line of a He-Cd laser. Figure 2(a) shows a comparison of non-normalized PL from the Zn-polar and O-polar faces of wafer A1, using the same laser excitation intensity on both faces. The dominant emission is from neutral donor bound excitons (denoted D O X) between ev and their two-electron satellites (TES) and longitudinal phonon replicas (energy separation 72 mev). Figure 2(b) shows the bound and free exciton regions in more detail, with the most prominent features labeled using line positions after Meyer et al. [5]. Figure 2. Non-normalized, 4 K photoluminescence spectra taken on the Zn-polar and O-polar faces of undoped, bulk, c-axis wafer A1. PL emissions above ev, which include the longitudinal and transverse A-excitons and a triplet of peaks at approximately (I 0 ), and ev, are significantly stronger on the Zn-polar face. This stronger free exciton emission from the Zn-polar face is consistent with the presence of an inversion layer of holes which may form extra free excitons with deep donor electrons ionized by the He-Cd laser. It is tempting to speculate that the triplet of peaks near I 0 are ionized versions of the neutral donor bound excitons I 6a, I 5 and I 4, the respective pairs of emissions each have an energy separation of 12.1 mev [6], alternatively they may be due to B- excitons bound to the same neutral donors. An increased density of ionized donors on the Znpolar face would be consistent with the situation described in Figure 1. Conversely, emissions between ev, which include the I 3a line (currently un-assigned), are more intense on the O-polar face. Similar polarity-related differences were observed in the non-normalized PL spectra of wafers A2 and A3 (Figure 3(a), 3(b)). Figure 3(b) also shows that the PL emissions from m- plane wafer A4, obtained using the same laser excitation intensity, were an order of magnitude less intense, perhaps a result of the spontaneous polarization vector lying parallel to the m-plane surface. The effect of annealing wafers A2 and A4 at 750 o C in 1 atm. O 2 for 90 minutes is shown in Figure 3(c), 3(d). The intensity of the emission at ev (I 4 ), attributed to shallow hydrogen donors [5], decreased dramatically on annealing. In addition, the polarity related differences in wafer A2 almost completely disappeared. Hall effect measurements on wafer A2 and A4 after annealing showed that their resistivity had increased by over 4 orders of magnitude.

4 Figure 3. 4 K photoluminescence spectra taken on (a) c-axis wafer A2; (b) c-axis wafer A3 and m-plane wafer A4; (c) c-axis wafer A2 after annealing at 750 o C in O 2 for 90 minutes; and (d) m- plane wafer A4 before and after annealing at 750 o C in O 2 for 90 minutes. Schottky diode fabrication Arrays of planar Au, Ag and Pd Schottky diodes were fabricated on the Zn-polar face of a piece of wafer A2. Each diode consisted of an annular Ti/Al/Pt (25/75/25 nm) ohmic ring surrounding a circular 300 µm diameter Schottky contact. The wafer piece was pre-cleaned using simple organic solvents and de-ionized water. The diodes were characterized by currentvoltage (I-V) measurements using an HP 4155A parameter analyzer; all measurements were carried out at room temperature and in dark conditions. Figure 4(a) shows the I-V characteristics of the best Au, Ag and Pd diodes. Where possible, the effective barrier height (Φ B ) and ideality factor (n) were calculated for each diode using thermionic emission theory and plotted against each other in Figure 4(b). These Φ B vs n plots show that the best Au and Ag diodes have very low ideality factors, close to the imageforce-controlled ideality factor n if which typically ranges between ev, depending on the bulk doping level and interface band bending [7]. Diodes with ideality factors close to n if are considered to have laterally homogeneous Schottky contacts. The I-V characteristics of the best Au diodes were re-measured 1 month after fabrication. An age related increase in barrier height was found, which has been observed previously [8,9]. Ag contacts have previously shown significant rectifying improvement with age [8] possibly due to a chemical (oxidation) reaction at the Schottky interface.

5 Figure 4. Au, Ag and Pd planar Schottky diodes on the Zn-polar face of bulk ZnO wafer A2. (a) Typical I-V characteristics of the best Au, Ag and Pd Schottky diodes; and (b) effective barrier height vs ideality factor plots (all measurements made at RT and in dark conditions). Further arrays of planar, silver oxide Schottky diodes were fabricated on the Zn-polar and O-polar faces of wafer A1 and on a piece of m-plane wafer A4. The silver oxide contacts were deposited by reactive RF sputtering using a high purity Ag target and a 50 W Ar/O 2 plasma. These silver oxide contacts were examined by x-ray photoelectron spectroscopy and were found to contain both metallic silver and silver oxide species. Figure 5(a) shows the typical I-V characteristics of the best silver oxide diodes on the Zn-polar and O-polar faces of wafer A1. Figure 5. Planar, silver oxide Schottky diodes on the Zn-polar and O-polar faces of c-axis wafer A1 and on m-plane ZnO wafer A4. (a) Typical I-V characteristics of the best silver oxide diodes; (b) effective barrier height vs ideality factor plots; and (c) C-V characteristics of the best silver oxide diodes at 20 khz using a Philips PM6304 RCL meter (all measurements made at RT and in dark conditions).

6 Very high rectifying ratios (in excess of 8 orders of magnitude at ±1 V) were measured, with higher rectification obtained on the Zn-polar face. Figure 5(b) shows that silver oxide diodes fabricated on the Zn-polar face consistently have ~130 mev higher effective barrier heights than those on the O-polar face. Silver oxide diodes fabricated on the non-polar, m-plane face have intermediate barrier heights compared to those on the polar faces. Capacitance-voltage (C-V) characteristics of the lowest ideality factor silver oxide diodes (Figure 5(c)) confirm the presence of higher built-in barrier heights on the Zn-polar face. CONCLUSIONS High quality Au and Ag Schottky diodes with ideality factors close to the image-forcecontrolled limit were fabricated on the Zn-polar face of hydrothermally grown, bulk ZnO without the need for special surface treatments. These Au and Ag diodes showed age-related improvements in barrier height, possibly due to oxidation reactions taking place at the Schottky interface. Silver oxide contacts deliberately fabricated by reactive RF sputtering (Ag target Ar/O 2 plasma) consistently produced very high rectifying barriers on Zn-polar, O-polar and m-plane bulk ZnO. A polarity- related effect was observed, in that barrier heights on the Zn-polar face were consistently 130 mev higher than on the O-polar face and 30 mev higher than on m-plane ZnO. Significant polarity- related effects were also observed in the 4 K PL spectra of bulk ZnO, in particular increased free exciton emission from the Zn-polar face. These polarity- related effects are consistent with the anticipated band bending near the polar surfaces of low carrier concentration wafers required to internally compensate the spontaneous polarization field. ACKNOWLEDGMENTS We acknowledge the assistance of C. Kendrick, C. Swartz, W. Lee, C. Doyle, G. Turner and H. Devereux. This work was supported by the MacDiarmid Institute for Advanced Materials and Nanotechnology, The Marsden Fund (06UOC022) and the University of Canterbury. REFERENCES 1. U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S, Dogan, V. Avrutin, S.J. Cho and H. Morkoc, J. Appl. Phys. 98, (2005). 2. K. Maeda, M. Sato, I. Niikura and T. Fukuda, Semicond. Sci. Technol. 20, S49 (2005). 3. H.F. Bernardini, V. Fiorentini and D. Vanderbilt, Phys. Rev. B. 56, R10 024, (1997). 4. J.J. Harris, K.J. Lee, J.B. Webb, H. Tang, I. Harrison, L.B. Flannery, T.S. Cheng and C.T. Foxon, Semicond. Sci. Technol. 15, 413 (2000). 5. B.K. Meyer, H. Alves, D.M. Hofmann, W. Kriegseis, D. Forster, F. Bertram, J. Christen, A. Hoffmann, M. Strassburg, M. Dworzak, U, Haboeck and A.V. Rodina, Phys. Stat. Sol. (b) 241, 231 (2004). 6. M.R. Wagner, U. Haboeck, R. McKenna, A. Hoffmann, A. Rodina, S. Lautenschlager, J. Sann and B.K. Meyer, P235, 4th Intl. Workshop on ZnO and Related Materials (2006). 7. W. Mönch, J. Vac. Sci. Technol. B 17(4), 1867 (1999). 8. M.W. Allen, M.M. Alkaisi and S.M. Durbin, Appl. Phys. Lett. 89, (2006). 9. V.A. Coleman, C. Jagadish, unpublished.