Semiconductor to metal transition in degenerate ZnO: Al films and the impact on its carrier scattering mechanisms and bandgap for OLED applications Jitendra Kumar Jha • Reinaldo Santos-Ortiz • Jincheng Du • Nigel D. Shepherd Received: 28 November 2013 / Accepted: 22 January 2014 / Published online: 31 January 2014 Ó Springer Science+Business Media New York 2014 Abstract Temperature dependent Hall measurements revealed that ionized impurity scattering was the dominant mechanism in sputter deposited, degenerate, aluminum doped zinc oxide (AZO) films up to *530 nm thickness, and a semiconductor to metal transition was observed when thickness was further increased. With the increase in film thickness, the mobility and conductivity also increased from 6.70 to 18.7 cm 2 V -1 s -1 and 1.83 9 10 2 –8.28 9 10 2 (X cm) -1 , respectively. However, this was accompanied by a larger than 0.2 eV Burstein–Moss blue-shift of the inter- band absorption edge determined from absorption spectra. The movement of the Fermi level further into the conduc- tion band that accompanies the Burstein–Moss shift results in a corresponding workfunction decrease of the films. This means that the interface barrier for hole injection in anode applications such as organic light emitting diodes (OLEDs) becomes larger, which translates into higher turn-on volt- ages and lower current and power efficiencies compared to indium tin oxide anodes. It is suggested that improving conductivity through mobility increases, and increasing workfunction through surface functionalization may improve the prospects of AZO films in OLEDs and other applications where in addition to conductivity and trans- parency, workfunction is also critical. 1 Introduction Zinc oxide (ZnO) is a direct, wide bandgap, transparent semiconductor that is being vigorously studied for a variety of optoelectronic applications including use as the trans- parent conducting oxide (TCO) electrode in organic light emitting diodes (OLEDs) and solar cells [1–3]. The principal economic drivers for the development of alternatives to indium tin oxide (ITO), the current standard, relates to Earth’s limited reserve of indium, and consequently, its high cost [4]. ZnO doped with Aluminum (AZO) is a good potential replacement for ITO due to an electrical conduc- tivity that is close to that of ITO, and higher transmissivity [5]. The electrical conductivity and optical transmissivity of AZO as a function of sputter deposition substrate tempera- tures, power densities, working pressures and film thickness [6–9] have been reported by various groups. For example, Huang et al. [6] have reported reduced transmissivity with increases in deposition pressure, or with decreases in sub- strate temperature. Park et al. [7] obtained maximum elec- trical conductivity at 150 °C substrate temperature, whereas Chun et al. [8] reported maximum electrical conductivity for films deposited with 160 W of power applied to a 2 in. target. Others have studied the effect of deposition pressure on electrical conductivity [9] and report an optimal working pressure of 3.75 9 10 -3 Torr. Electrical conductivity as a function of Al concentration in sputtered films showed the highest electrical conductivity at 4 % Al concentration [7, 11]. AZO has also been grown by pulsed laser deposition where an electrical resistivity of 1.8 9 10 -4 X cm -1 and transmissivity of *90 % were measured for 580 nm thick films [12]. Al concentrations of 2 % produced the minimum electrical resistivity for pulsed laser deposited films in a different study [13]. Chemical vapor deposited films exhibited a minimum electrical resistivity of 7 9 10 -4 X cm -1 with a working pressure of 1.125 Torr [14], and a resistivity of 2.45 9 10 -2 X cm -1 and trans- mittance of 94 % have been reported for AZO films deposited by chemical spray deposition [15]. J. K. Jha  R. Santos-Ortiz  J. Du  N. D. Shepherd (&) Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203-5017, USA e-mail: Nigel.shepherd@unt.edu 123 J Mater Sci: Mater Electron (2014) 25:1492–1498 DOI 10.1007/s10854-014-1758-9