Chemically sprayed CdO: Cr thin films for formaldehyde gas detection and optoelectronic applications

Area of Science:

• Materials Science
• Thin Film Technology
• Semiconductor Physics

Background:

• Cadmium Oxide (CdO) is a promising n-type semiconductor with applications in transparent conductive films and gas sensors.
• Doping CdO with transition metals can tune its optoelectronic properties for specific applications.
• Investigating Chromium (Cr) doping in CdO is crucial for understanding its potential in advanced material applications.

Purpose of the Study:

• To synthesize and characterize Chromium (Cr) doped Cadmium Oxide (CdO) thin films.
• To investigate the influence of Cr doping on the structural, optical, electrical, and microstructural properties of CdO films.
• To evaluate the gas sensing performance of Cr-doped CdO films, specifically for formaldehyde detection.

Main Methods:

• Chemical spray pyrolysis for thin film deposition.
• Spectroscopic ellipsometry for film thickness measurement.
• Powder X-ray diffraction (XRD) for structural analysis.
• Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM) for microstructural and surface morphology analysis.
• Energy Dispersive Spectroscopy (EDS) for elemental composition.
• Micro-Raman spectroscopy for bond vibration analysis.
• UV-Vis-NIR spectrophotometry for optical properties and band gap estimation.
• Hall effect measurements for electrical characterization.
• Gas sensing measurements for formaldehyde detection.

Main Results:

• The films exhibited a cubic crystal structure with preferential growth along the (111) plane.
• Cr doping led to a minimal solubility of approximately 0.75 wt% in CdO.
• Surface roughness varied with Cr concentration, and FESEM revealed a smooth microstructure.
• High optical transmittance (>75%) was observed in the visible-near-infrared (vis-NIR) region.
• The optical band gap increased to a maximum of 2.35 eV with 1.0 wt% Cr doping.
• Electrical measurements confirmed n-type semiconductivity with increased carrier density, mobility, and dc-conductivity upon Cr doping.
• A high carrier mobility of 85 cm^2V^-1s^-1 was achieved at 0.75 wt% Cr doping.
• The 0.75 wt% Cr-doped CdO film demonstrated a significant response of 74.39% to formaldehyde gas.

Conclusions:

• Chromium doping effectively modifies the structural, optical, and electrical properties of CdO thin films.
• The optimal Cr doping concentration for high carrier mobility and gas sensing performance appears to be around 0.75 wt%.
• Cr-doped CdO films show potential for applications as sensitive and selective formaldehyde gas sensors.