Recent Advances in Atmospheric Vapor‐Phase Deposition of Transparent and Conductive Zinc Oxide

Abstract

The industrial need for high-throughput and low-cost ZnO deposition processes has triggered the development of atmospheric vapor-phase deposition techniques which can be easily applied to continuous, in-line manufacturing. While atmospheric CVD is a mature technology, new processes for the growth of transparent conductive oxides on thermally sensitive materials or flexible substrates are being developed, such as atmospheric plasma-enhanced (PE)-CVD and atmospheric spatial atomic layer deposition (ALD). In this article, the challenges and recent results on the growth of ZnO under atmospheric pressure by CVD, PE-CVD, and spatial ALD are reviewed and the use of these films as transparent electrodes in thin film solar cells are presented.

Research Update: Atmospheric pressure spatial atomic layer deposition of ZnO thin films: Reactors, doping, and devices

“…31 By controlling the substrate velocity, rapid ALD synthesis of ZnO can occur (up to 2 nm s −1 ), which is an order of magnitude higher than the growth rate of conventional ALD (Table I). 9,14,40 The range in growth rates shown is due to this parameter being a function of deposition temperature and residence time, which is controlled by pulse duration (conventional ALD) 15 or substrate velocity (AP-SALD). 9,40 In Table I, the growth rate of 2 nm s −1 using the AP-SALD head designed by the Eastman Kodak Company was obtained using an exposure time of 25 ms (100 ms for each cycle) at 200…”

“…9,26,27,29,36,40 Nitrogen-doping of ZnO reduces its carrier concentration (thus increasing resistivity, as shown in Table II), 9,26 and it has been claimed that it can make ZnO p-type with post-deposition treatment (Table II). 26,57 The nitrogen dopant is introduced by mixing ammonia with the water oxidant precursor, and this is the method used for both conventional and spatial ALD ZnO:N. 26,58 The non-pyrophoric and low-cost nature of the ammonia precursor is in contrast to the expensive, pyrophoric organometallic materials that are often used as ALD and AP-SALD precursors, 5,14 and hence ammonia has been widely studied in both ALD and AP-SALD ZnO. 26,36,[57][58][59] It is suspected that nitrogen doping reduces the carrier concentration because it is an acceptor dopant that compensates the intrinsic donors.…”

“…14,61,62 In conventional ALD, changing the Al precursor from TMA to DMAI also results in a wider dispersion of the Al dopant, leading to the doping efficiency increasing from 10% to 60%. 61 This is a similar doping efficiency as AP-SALD AZO made using the co-injection method, 27 further indicating that this method is successful in producing a uniform distribution of the dopant in ZnO films.…”

“…This limits its growth rate to typically ∼10 −2 -10 −1 nm s −1 . 6,8,12,14,15 In SALD processes, by contrast, the precursors are separated spatially (hence spatial ALD) and the substrate moves between the different precursor zones ( Fig. 1(b)).…”

“…30 in device synthesis is highly preferable for their industrial application. 5,14,17,18 Detailed reviews of closed chamber and atmospheric pressure spatial ALD reactors and their development can be found in Refs. 5 and 6.…”