Mg:TiO2 alloy thin films based MOS capacitors grown on GaAs substrates

Abstract

Electron beam evaporation technique is employed to synthesise TiO2, Mg0.1Ti0.9O2 and Mg0.2Ti0.8O2 thin films (TFs) grown on (100) n-type GaAs substrates. Field emission gun-scanning electron microscope (FEG-SEM) results show that the TFs have a thickness of ~225 nm. The non- contact atomic force microscopy (NC-AFM) images shows the pore volume of the TiO2, Mg0.1Ti0.9O2 and Mg0.2Ti0.8O2 TFs enhanced gradually. UV-Vis absorption measurements are performed on the samples to determine the main bandgap and defect level transition of the material. A unique modified Urbach theoretical model has been introduced to simulate the experimental absorption spectrum. The main bandgap energy of the TiO2, Mg0.1Ti0.9O2 and Mg0.2Ti0.8O2 samples are calculated to be ~3.45 eV, 3.85 eV and 4.30 eV respectively. A gradual enhancement in main bandgap transition probability and decrease in defect level transition of the material has been observed with enhanced incorporation of Mg into the TiO2 host material. X-ray diffraction (XRD) is performed, which shows a continuous change in lattice constant of TiO2 with Mg. Current (I)-voltage (V) characteristics of the TiO2, Mg0.1Ti0.9O2 and Mg0.2Ti0.8O2 Schottky devices revealed that the leakage current at −1 V was 1.28 × 10−6 A, 1.46 × 10−9 A and 2.44 × 10−10 A respectively. Capacitance (C)–voltage (V) measurements are performed on the devices at different frequencies. A theoretical simulation has been adopted by amending the delta depletion model at 1 MHz. The dielectric constant and the flat band voltage of the TiO2, Mg0.1Ti0.9O2 and Mg0.2Ti0.8O2 devices are found to be 100, 120 and 160 and 16.1 V, 14.7 V and 9.7 V respectively. Hill-Coleman’s method shows a gradual enhancement of the density of interface states (Dit) with Mg concentration. The calculated Dit value of the TiO2, Mg0.1Ti0.9O2 and Mg0.2Ti0.8O2 TF devices are ~6.16 × 1010 eV−1 cm−2, 6.44 × 1010 eV−1 cm−2 and 1.11 × 1011 eV−1 cm−2 respectively. The observed C-V hysteresis confirms an enhancement in the charge retention into the film with increasing Mg concentration, which in turn improves the memory window (MW) from ~0.36 V (at±7 V) to ~0.67 V (at±7 V) and ~0.87 V (at±10 V) to ~1.0 V (at±10 V) with sweeping voltage.