Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition

Abstract

The simultaneous co-doping of ZnO nanowires grown by chemical bath deposition is of high interest for a large number of engineering devices, but the process conditions required and the resulting physicochemical processes are still largely unknown. Herein, we show that the simultaneous co-doping of ZnO nanowires with Al and Ga following the addition of Al­(NO3)3 and Ga­(NO3)3 in the chemical bath operates in a narrow range of conditions in the high-pH region, where the adsorption processes of respective Al­(OH)4 – and Ga­(OH4)− complexes on the positively charged m-plane sidewalls are driven by attractive electrostatic forces. The structural morphology and properties of ZnO nanowires are significantly affected by the co-doping and mainly governed by the effect of Al­(III) species. The incorporation processes of Al and Ga dopants are characterized by significant interplay effects, and the amount of incorporated Ga dopants into ZnO nanowires is found to be larger than the amount of incorporated Al dopants owing to energetic considerations. The Al and Ga dopants are located in the bulk of ZnO nanowires, but a part of Al and Ga lies on their surfaces, their incorporation processes in the bulk being enhanced by thermal annealing under oxygen atmosphere. Eventually, the Al and Ga dopants directly affect the incorporation of hydrogen-related defects, notably by annihilating the formation of VZn-nH defect complexes. These findings present an efficient strategy to proceed with the co-doping of ZnO nanowires grown by chemical bath deposition, opening perspectives to control their electronic structure properties with a higher precision.

Single and Co-Doping of ZnO Nanowires with Al and Cl Using One Precursor by Chemical Bath Deposition

“…The adsorption process of ion complexes containing the dopant on the charged surfaces of ZnO NWs during CBD is required for the incorporation of dopants, but its nature and magnitude strongly depend on the pH value and dopant. When using column-IIIA dopants including Al and Ga, electrostatic forces dominate the adsorption process such that the incorporation process operates in the high-pH region, where the nonpolar m -plane sidewalls of ZnO NWs are positively charged. , The co-doping of ZnO NWs with Al and Ga was found to incorporate a higher concentration of Ga dopants as Ga Zn than of Al dopants as Al Zn both of them acting as shallow donors owing to energetic considerations . However, this also showed that competitive physicochemical processes can proceed on the nonpolar m -plane sidewalls of ZnO NWs, favoring the incorporation of a given dopant at the expense of the other dopant.…”

“…However, it is worth noticing that although Cl(I) species reduce the incorporation of Al dopants, it is not fully hampered as Ga(III) species can do. 50 In addition, the Al/Zn atomic ratios before and after thermal annealing are kept constant such that the amount of Al dopants is not affected.…”

“…The present decrease may come from the band gap renormalization usually reported in degenerate ntype semiconductors, 76 and more specifically in the case of Gadoped ZnO NWs. 50 Interestingly, the use of AlCl − complexes predominantly act on their nonpolar m-plane sidewalls as compared to Cl − ions through competitive adsorption processes. Nevertheless, Cl − ions also play a significant role besides the major role of Al(OH) 4 − complexes, in contrast to Al-and Ga-codoped ZnO NWs for which the role of Ga(OH) 4…”

“…− complexes is much less pronounced. 50 Before thermal annealing as presented in Figure 10b, the incorporation of Al and Cl dopants within a respective cationic and anionic doping approach operates. When using Al(NO 3 ) 3 as a chemical additive, one-sixth of the initial Al(III)/Zn(II) species ratio in aqueous solution is incorporated into Al-doped ZnO NWs.…”

“…Recently, the use of Al(NO 3 ) 3 and Ga(NO 3 ) 3 in the same chemical bath has shown great promise to incorporate both dopants into ZnO NWs. 50 Alternatively, a promising and yet unexplored approach consists in developing the co-doping of ZnO NWs by using one single metallic salt incorporating two different dopants substituting for the zinc and oxygen sites, respectively. Among the possible candidates, the chemical additives incorporating Al and Cl are of high potential because the doping of ZnO NWs grown with Al by CBD is well documented.…”