Article pubs.acs.org/JPCC
Low Temperature Aqueous Solution Route to Reliable p‑Type Doping in ZnO with K: Growth Chemistry, Doping Mechanism, and Thermal Stability Chuan Beng Tay,*,† Jie Tang,† Xuan Sang Nguyen,‡ Xiao Hu Huang,† Jian Wei Chai,§ Venky T. Venkatesan,† and Soo Jin Chua*,†,‡,§ †
NUSNNI-Nanocore, National University of Singapore, 117576 Singapore Singapore-MIT Alliance, E4-04-10, 4 Engineering Drive 3, 117576 Singapore § Institute of Materials and Research Engineering, 3 Research Link, 117602 Singapore ‡
ABSTRACT: In this paper, we identified how the growth environment chemistry can critically influence the type and nature of the incorporated K defect in ZnO films grown using the aqueous solution route, which explains the switching between p- and ntype conductivities under different doping or thermal annealing conditions. This was achieved by relating the growth environment to the structural, optical, and electrical characteristics of the films. The thermal behavior of these defects up to 700 °C confirms the proposed doping mechanism. It is found that the best route to realizing p-type conductivity is through minimizing the amount of Ki and KZn−Ki complexes because films with high concentrations of Ki have a slow p-type recovery caused by the slow out-diffusion of Ki. The highest hole concentrations for as-grown films and those that were annealed at 700 °C for 30 min were 2.6 × 1016 and 3.2 × 1017 cm−3, respectively. The upper limit for p-type doping using this route appears to be about mid-1017 cm−3. Our results show that the low temperature aqueous solution synthesis route of ZnO:K is a promising solution toward reliable p-type conductivity for future device applications. complexes.4 Taking into consideration that potassium (K) has a lower acceptor energy level compared to N, which is the best p-dopant from group V, the thermal instability of Ki relative to KZn, and the likelihood of Ki substituting Zn to form KZn, Huang et al. proposed K as the best candidate for p-type doping.5 To date, there are several reports on doping with K by various growth methods such as aqueous solution,6 radio frequency magnetron sputtering,7 and sol−gel.8 Among these methods, the aqueous solution method stands out to be well suited for p-type doping with group I elements because the aqueous environment is naturally oxygen rich due to the chemisorption of water molecules onto the oxide surfaces, thus providing a favorable chemical environment for the formation of p-type films.9 This is further complemented by the
I. INTRODUCTION Zinc oxide (ZnO) is a promising multifunctional material, with potential applications in optoelectronics, piezoelectric power generation, and dilute magnetism. In the area of optoelectronics, enormous efforts have been made to achieve reproducible and stable p-type doping. Although group I elements have shallower acceptor energy levels than group V elements1 and high dopant solubilities in the range of 1019− 1020 cm−3 through the formation of neutral complexes,2 group V elements have however dominated the early successful reports in p-type doping.3 Among the group I elements, lithium (Li) was the most investigated element because of its lowest acceptor energy level and its matching atomic radius with zinc (Zn). However, a majority of films grown with Li were highly insulating or n-type. Difficulties in obtaining p-type conductivity can be attributed to formation of interstitials,1 thermally stable Lii relative to LiZn, and the high dissociation energy of Li-based neutral © 2012 American Chemical Society
Received: July 17, 2012 Revised: October 22, 2012 Published: October 29, 2012 24239
dx.doi.org/10.1021/jp3070757 | J. Phys. Chem. C 2012, 116, 24239−24247
The Journal of Physical Chemistry C
Article
Figure 1. (a) Speciation plot of major components in the aqueous solution containing a fixed concentration of 0.3 M ZnAc2 and a variable initial concentration of KAc, ranging from 0 to 0.3 M. The plots of zinc hydroxide complexes were omitted because their concentrations were less than 10−11 over the entire concentration range of KAc. (b) Plot of solubility of zinc (SZn), pH, and concentration ratio of K+/Zn2+ against the concentration of KAc. The concentration ratios of K+/Zn2+ for samples A, B, C, D, and E, which correspond to 0, 0.03, 0.05, 0.13, and 0.18 M KAc, are marked accordingly in the plot.
advantages of the aqueous solution method being a low temperature, green, nontoxic, low cost, and scalable process. The potential of both, K as a dopant as well as aqueous solution growth method as a viable growth and doping method, has just begun to be recognized. More theoretical and experimental work is needed to understand how to produce p-type films that are reliable and thermally stable for fabrication of devices. In this work, we identified the major species in the aqueous solution and the variation of pH over a range of precursor concentrations by computing the ionic equilibrium of the aqueous precursor solution. This speciation information, supplemented by the structural, optical, and electrical information of the corresponding films which were measured using the Hall effect, resonant Raman scattering spectroscopy, X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectroscopy (SIMS) measurements, demonstrated the influence of the growth environment chemistry on the type and nature of the incorporated K defect. The effect of thermal annealing on the optical and electrical properties of the film, from 200 to 700 °C, gave a good agreement with the proposed growth and doping mechanisms.
growth pH of 10−1110 was deliberate, as it has been shown in our earlier work11 that this range of pH produces a slow growth rate of ZnO that results in a lesser concentration of native defects and better optical properties. In order to determine the type of majority species in the growth solution, the ionic equilibrium of the ZnAc2−KAc solution was computed for a range of KAc concentrations using published temperaturedependent equilibrium constants shown in eqs 1−9.12
II. SOLUTION CHEMISTRY OF THE ZNAC2−KAC SYSTEM As described in detail in the Experimental Section, the ZnO films in this work were grown in two phases. The first growth phase produced a thin layer of nanorods on the substrate, typically with lengths of about 400 to 500 nm and diameters of about 80 to 150 nm. This thin layer of nanorods formed an important “seed layer” that allowed heterogeneous growth to continue and eventually form the bulk of the coalesced film, typically 1−2 μm thick, in the second growth phase. The precursor solution that was used in the second growth phase consisted of only zinc acetate (ZnAc2) and potassium acetate (KAc) with a typical pH of less than 7, depending on the relative amounts of ZnAc2 and KAc that were added. The choice of a relatively low pH compared to the “standard”
(7)
HAc ↔ H+ + Ac− +
KAc ↔ K + Ac
−
log K1 = 4.756
(1)
log K 2 = 6.10
(2)
Zn 2 + + Ac− ↔ Zn(Ac)+
log K3 = − 1.3
(3)
ZnAc+ + Ac− ↔ Zn(Ac)2
log K4 = − 0.8
(4)
ZnO + H+ ↔ Zn(OH)+
log K5 = −7.83
(5)
ZnO + H 2O ↔ Zn(OH)2
log K 6 = −10.09
ZnO + 2H 2O ↔ Zn(OH)3− + H+
(6)
log K 7 = −9.81
ZnO + 3H 2O ↔ Zn(OH)4 2 − + 2H+ log K8 = −12.78 ZnO + 2H+ ↔ Zn 2 + + H 2O
(8)
log K 9 = 11.173
(9)
The speciation plot of the major components in the aqueous solution for a fixed concentration of 0.03 M ZnAc2 and a variable initial concentration of KAc, ranging from 0 to 0.3 M, is shown in Figure 1 (a). The concentration of K+ is almost equal to the amount of KAc that was added as a result of high dissociation of KAc. On the other hand, partial dissociation of ZnAc2 led to the dissolved zinc acetate complexes, Zn(Ac)+ and ZnAc2, forming the major components of zinc and serving as a zinc reservoir during growth. Growth units for ZnO are 24240
dx.doi.org/10.1021/jp3070757 | J. Phys. Chem. C 2012, 116, 24239−24247
The Journal of Physical Chemistry C
Article
predominantly Zn2+ with concentrations ranging from 10−3 to 10−4 M, which are several orders of magnitude larger than any of the other zinc hydroxide complexes (
