Bi2ZnTiO6 thin films for next-generation photovoltaics: Study of material properties characterization and deposition conditions optimization

Abstract

The perovskite-like oxide Bi2ZnTiO6 (BZT) combines a narrow band gap with significant ferroelectric polarization, key parameters for the development of next-generation photovoltaic devices. Nevertheless, BZT remains a relatively unexplored material due to its difficult synthesis. The present experimental work unfolds and optimizes the deposition of BZT phase thin films by rf-sputtering method on standard Pt(111)/TiO2/SiO2/Si(001) substrates. The series of thin films explores different deposition conditions: Substrate temperature (350º-600ºC), RF power (50–80 W), exposure time (30–120 min), O2:Ar ratio (25–50%) and thermal treatments (air-O2). Thin films deposited between 350 and 475ºC are mainly amorphous. Above 490–550ºC the films show incipient crystallization. Depositions at temperatures above 600ºC and heat treatments resulted in partial recrystallization of spurious Bi- or Zn-rich oxides and aurivillius or pyrochlore-like phases. At 550 ºC, the films are dominated by a polycrystalline BZT phase in P4mm tetragonal structure, and the increase of the O2:Ar ratio contributes to improve the B-site order. Not least, no significant structural changes were observed after ∼3 years ageing. Diffuse reflectance measurements show relatively narrow optical band gaps between 1.4–1.9 eV within expected values for this system. Piezo-force microscopy revealed domains contrast and hysteresis loops correlated with the quality of the film crystallized BZT phase.