Herein, we report the effect of passivation layer composition on thermal stability as measured by Raman spectra of a phosphorene/Al-doped ZnO (AZO) heterostructure. Few-layer phosphorene was formed using micromechanical exfoliation on an AZO film/Si substrate. Samples were passivated by Al2O3, Si3N4, or an Al2O3/Si3N4 stack. Both films were deposited by atomic layer deposition. When these samples were annealed at temperatures in the range from 100 to 550 °C, the nitride-only structure was found to have highest thermal sustainability, demonstrating phonon peaks of out-of-plane (Ag1) and in-plane (B2g and Ag2) modes up to the highest temperature tested (550 °C). This was 100 °C higher than that for the samples with the conventionally used Al2O3 coating. An oxide/nitride stack permitted anneals up to 500 °C. For anneals up to 200 °C, the nitride-only passivated phosphorene film was found to have a significant red shift in all the three peaks. This is attributed to the effects of tensile strain. At annealing temperatures higher than 200 °C, the spectra blue shifted, attaining values close to the bulk peak position; this suggested strain relaxation in the film at higher temperatures. This relaxation was also evident from a reduction in the full-width at half maximum of the Raman phonon peaks at higher annealing temperatures. Further study suggests that this shift may be due to a reduction in the stress of the passivation layer. The improvement in thermal stability is believed to be due to silicon nitride acting as a diffusion barrier, blocking phosphorus out-diffusion. To demonstrate the utility of higher temperature annealing, a phosphorene/AZO heterojunction diode was fabricated with a Si3N4 passivation layer. The passivated device shows a significant improvement of the diode ideality factor with an increased annealing temperature. The improvement of the diode ideality factor is due to the significant reduction of defect concentration at high annealing temperatures. These findings will promote the development of high-performance phosphorene-based heterojunction devices.
Bibliographical noteFunding Information:
Part of this work was carried out at the Nano Center of University of Minnesota, which was supported by the National Science Foundation through NNCI (Award ECCS-1542202). SKP and NI also received support through this award. We are also grateful to the atomic force microscopy (AFM) and Raman spectroscopy facilities equipped at the Characterization Facility at the University of Minnesota.