Nitrogen grain-boundary passivation of In-doped ZnO transparent conducting oxide

Abstract

We have investigated the properties and conduction limitations of spray pyrolysis grown, low-cost transparent conducting oxide ZnO thin films doped with indium. We analyze the optical, electrical, and crystallographic properties as functions of In content with a specific focus on postgrowth heat treatment of these thin films at 320 • C in an inert, nitrogen atmosphere, which improves the films electrical properties considerably. The effect was found to be dominated by nitrogen-induced grain-boundary passivation, identified by a combined study using in situ resistance measurement upon annealing, x-ray photoelectron spectroscopy, photoluminescence, and x-ray diffraction studies. We also highlight the chemical mechanism of morphologic and crystallographic changes found in films with high indium content. By optimizing growth conditions according to these findings, ZnO:In with a resistivity as low as 2 × 10 −3 cm, high optical quality (T ≈ 90%), and sheet resistance of 32 / has been obtained without any need for postgrowth treatments.

Low-Cost, High-Performance Spray Pyrolysis-Grown Amorphous Zinc Tin Oxide: The Challenge of a Complex Growth Process

“…The latter leads to a large number of trap states at the interface between grains, accompanied with a depletion layer within the grain which becomes wider the lower the carrier concentration. 37,[39][40][41] We suspect a similar mechanism to be responsible for the SP grown a-ZTO.…”

“…As with ZnO, the improvement is larger for samples with lower carrier concentrations, indicating the same mechanism. 37 The improvement also scales with the rms roughness, showing that more granular films with a higher surface area are more dramatically changed by the annealing. Independent of the discussed changes in stoichiometry and morphology, all samples remain highly transparent.…”

“…The film resistivity was recorded during a 50 °C → 290 °C → 50 °C annealing cycle with a total process time of 15 min. 37 To evaluate the stoichiometry of the films, X-ray photoelectron spectroscopy (XPS) was used (Omicron Multiprobe XPS; monochromized Al Kα source (XM 1000, 1486.7eV); EA125 U5 analyser). Prior to measurements, all films were sonicated in acetone and isopropanol with further Ar-ion etching (750 V; sputter current ≈6 μA; pressure 1.5 × 10 −3 Pa; 10 min).…”

“…The thin-film resistivity was measured in situ by four-point probe measurements using spring-loaded gold contacts in linear configuration with a Keithley 2400 source meter. The film resistivity was recorded during a 50 °C → 290 °C → 50 °C annealing cycle with a total process time of 15 min …”

Raman investigation of N‐implanted ZnO: Defects, disorder and recovery

“…To get rid of unwanted interstitials (I Zn s) and to use left aside V Zn s purposefully, N codoped with Li, Mg, or P is found to be effective for activating shallow acceptors. In case of granular In‐doped ZnO, N can be used to passivate grain boundary defects and a significant decrease in resistivity (~2 × 10 −3 Ω·cm) has been observed . Altogether, it can be understood that nature of energy‐favored nitrogen‐native defect complex in ZnO depends critically on the residual defects in the vicinity.…”