Effect of Al Distribution on Carrier Generation of Atomic Layer Deposited Al-Doped ZnO Films Do-Joong Lee, a Jang-Yeon Kwon, a Soo-Hyun Kim, b Hyun-Mi Kim, a and Ki-Bum Kim a,c,z a Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea b School of Materials Science and Engineering, Yeungnam University, Gyeongsan-si, Gyeongsangbuk-do 712-749, Korea c WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea The effect of the Al distribution on the electrical properties of Al-doped ZnO (AZO) films deposited by atomic layer deposition (ALD) is investigated. In order to control the Al distribution, the pulsing time of trimethylaluminum (TMA) is varied from 2 (within an ALD window) to 0.1 s. As a result, the areal density of Al atoms incorporated in a single dopant layer decreases from 3.3  10 14 to 1.2  10 14 cm 2 . Hall measurements reveal that the minimum resistivity of the ALD-AZO films is decreased from 3.2  10 3 to 1.7  10 3 X cm as a result of reducing the TMA pulsing time from 2 to 0.1s. This decrease is due to the obvious increase of the carrier concentration from 1.4  10 20 to 4.7  10 20 cm 3 . It is suggested that both the improved doping efficiency (from 13 to 58%) and the insertion of more dopant layers within the ZnO matrix are responsible for the increase of the carrier concentration. V C 2011 The Electrochemical Society. [DOI: 10.1149/1.3568881] All rights reserved. Manuscript submitted December 28, 2010; revised manuscript received February 25, 2011. Published March 22, 2011. Transparent conducting oxides (TCOs) are one of key compo- nent materials for photovoltaics, transparent electronics, and flat panel displays. 1–3 Typically, oxide-matrix materials are doped with foreign elements, as in the cases of Sn-doped In 2 O 3 (ITO) and Al- doped ZnO (AZO), to reduce the film resistivity down to 10 4 X cm. 4,5 TCOs have been deposited using various methods including sputtering, chemical vapor deposition, pulsed laser deposition (PLD), and spray pyrolysis. 4–7 Recently, there have been several attempts to deposit TCO materials by atomic layer deposition (ALD). 8–10 Due to the surface-saturated and self-limiting reaction mechanism, those films have been utilized in various devices such as nanostructured solar cells and organic light emitting diodes which require either complex structures or low processing temperatures. 11– 13 Because of these emerging applications, the structural and electri- cal properties of ALD-TCO films have been studied by several groups. 14–16 Recently, we investigated the evolution of microstructure as well as a doping mechanism in AZO films deposited by ALD. 17 From the scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) analysis, it was confirmed that ALD-AZO films have a layer-by-layer structure consisting of a ZnO matrix and dopant layers. That is, dopants are non-uniformly distributed. In addi- tion, the doping efficiency (ratio of contributed electrons to an actual doping concentration) of those films was only 13%, which is much lower than the resulting efficiencies from the other methods (for instance, 60% for sputtering and 86–118% for CVD). 7,18 Namely, not all Al atoms act as dopants. This poor doping efficiency is likely due to the incorporation of excess Al atoms within a single dopant layer (3.3  10 14 cm 2 ), which is controlled by the surface-satura- tion of the precursor. That areal density is about three times higher compared to the areal density from bulk incorporation, which is typi- cally around 10 14 cm 2 (doping concentration of 10 21 cm 3 ). 19 There have been also several reports mentioning an optimum doping concentration (2–4 atom % of Al) of AZO films for obtaining the maximum carrier concentration. 5,6,20,21 If an Al content is larger than the optimum value, the carrier concentration is reduced. This decrease has generally been attributed to either the formation of Zn vacancies (V Zn 2 ) as acceptors or the formation of a (ZnO) m (Al 2 O 3 ) homologous phase. 22,23 In addition, stoichiometric Al 2 O 3 might be formed during the incorporation of a single cycle AlO x in ALD-AZO films. This would also result in a poor doping efficiency because Al in the stoichiometric Al 2 O 3 are incapable of donating free electrons via either substitution (Al Zn þ ) or through the release of oxygen vacan- cies (V o 2þ ) / zinc interstitials (Zn i 2þ ) by capturing oxygen from the ZnO matrix. 6,17 From these possibilities, it is speculated that the incorporation of excess Al atoms in a single dopant layer is responsi- ble for a low carrier concentration in the ALD-AZO films (<2  10 20 cm 3 ). 9,16,17 Therefore, to improve the doping efficiency of those films, it is necessary to reduce the amount of Al atoms incor- porated in a single dopant layer. In this article, we investigated effects of Al distribution on the electrical properties of ALD-AZO films. In order to control the amount of Al atoms incorporated in a single dopant layer, the pro- cess parameter was varied during the deposition. The electrical properties including resistivity, Hall mobility, and carrier concentra- tion were investigated using the Hall measurement. Specifically, these properties were correlated with the carrier generation behavior and the distribution of Al atoms within a ZnO matrix. Experimental AZO films were deposited on SiO 2 (100 nm)/Si substrates using a showerhead-type GENI-MP1000 ALD system (ASM-Genitech, Inc.) at a deposition temperature of 200  C and a working pressure of 3 Torr. Diethylzinc (DEZ) and trimethylaluminum (TMA) were used as precursors for the deposition of ZnO and Al 2 O 3 and water vapor was used as a reactant. For the delivery of the DEZ and TMA molecules into a chamber, Ar was used as a carrier gas with a flow rate of 5 s ccm (standard cubic centimeter per minute). Pulsing times of DEZ and water vapor were fixed to 2 s, while the pulsing time of TMA was varied from 0.1 to 3 s. For instance, the Langmuir exposure of the TMA was 0.1 Torr s for 2 s pulsing and 0.005 Torr s for 0.1 s pulsing, respectively. The Ar purge gas was flowed for 5 s beginning right after the injection of the precursor and the water vapor. The dop- ing concentration of the AZO films were varied by repeating various numbers of ZnO ALD cycles (R ALD ) and 1 ALD cycle of Al 2 O 3 for a total of 200 ALD cycles. Film thickness was measured using an ellipsometry (k: 632.8 nm). Film composition was analyzed by Auger electron spectroscopy (AES, Perkin-Elmer/PHI 660). Resistivity, Hall mobility, and carrier concentration were measured using a Hall mea- surement system (BIO-RAD, HL5500PC). Results Control of the amount of Al atoms in a single dopant layer.— As suggested above, it is necessary to reduce the areal density of Al atoms in a single dopant layer in order to improve the doping effi- ciency of the ALD-AZO films. This can be achieved by reducing the surface coverage of the precursor. From the Langmuir isotherm, the surface coverage of the precursor is dependent on both the reaction time and partial pressure. 24,25 Therefore, in this study, we varied the pulsing time of the TMA molecules (t TMA ) to control the areal z E-mail: kibum@snu.ac.kr Journal of The Electrochemical Society, 158 (5) D277-D281 (2011) 0013-4651/2011/158(5)/D277/5/$28.00 V C The Electrochemical Society D277 Downloaded 06 Apr 2011 to 147.46.133.122. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp