Morphological, Optical, and Electrical Properties of a MOS Capacitor Based on Rare Earth Oxide and p-Porous GaAs

Abstract

This paper reports the structural, optical, and electrical properties of dysprosium oxide (Dy₂O₃) deposited by electron beam deposition under ultra-vacuum on porous p-type GaAs. A porous GaAs layer was produced by electrochemical anodic etching of a (100)-heavily doped p-type GaAs substrate in hydrofluoric acid (HF) and ethanol C₂H₅OH solution. The surface topography of the elaborated Dy₂O₃ layer was determined based on atomic force microscopy (AFM) images. AFM studies showed that the structure and roughness of the Dy₂O₃ layer were strongly dependent on the roughness and surface of porous GaAs. Dy₂O₃ is polycrystalline and exhibits a cubic crystalline structure, as confirmed by x-ray diffraction (XRD) analysis. The optical properties of Dy₂O₃/p-porous GaAs were analyzed using various techniques including ellipsometry and photoluminescence (PL) to obtain information on surface and interface quality, bandgap, optical constants, dielectric constant, and thickness. The photoluminescence (PL) spectra revealed an intense peak at 835 nm and additional weak emission peaks at 473 nm and 540 nm, respectively. The observed intense peak can be directly attributed to the interband recombination process of free carriers in the direct bandgap of p-GaAs, while the weak emission peaks at 473 nm and 540 nm correspond to 4F9/2-6H15/2 and 4F9/2-6H13/2 transitions, respectively. In the spectral region of 350 nm to 500 nm, the average thickness of the Dy₂O₃ layer was determined to be 11 nm. The electrical properties of the (Co/Au)/Dy₂O₃/p-porous GaAs metal–oxide–semiconductor (MOS) capacitor were investigated via capacitance–voltage (C–V) and conductance–voltage (G/ω–V) measurements in the temperature range of 100–400 K and frequency range of 50 Hz to 1 MHz, respectively. The experiments demonstrated that both capacitance and conductance were influenced by temperature and frequency. Additionally, the effect of temperature on interface state density (N_ss) was studied, which showed that an increase in temperature led to a decrease in the interface state density (N_ss) of the (Co/Au)/Dy₂O₃/p-porous GaAs (MOS) capacitor, as calculated by the Hill–Coleman method. The mean values of N_ss for the (Co/Au)/Dy₂O₃/p-porous GaAs (MOS) capacitor were determined to be approximately 10¹² eV⁻¹ cm⁻², making it suitable for electronic device applications. The lower values of N_ss can be attributed to a low amount of local defect microstructure at the Dy₂O₃/p-porous GaAs interface due to the incorporation of the Dy₂O₃ layer into the porous GaAs. The electrical conductivity of the (Co/Au)/Dy₂O₃/p-porous GaAs (MOS) capacitor was studied using impedance spectroscopy in the frequency range from 50 Hz to 1 MHz at temperatures ranging from 80 K to 450 K. At low frequencies, the conductivity of alternating current (σ_AC) remained nearly constant, whereas at high frequencies, it increased rapidly, representing σ_DC and σ_AC, respectively. The Arrhenius plot of σ_AC shows two distinct slopes corresponding to two activation energies, 35 MeV and 10 MeV, in the chosen temperature range.