Atomic layer deposition of SiO2 to cap SiO2: Al stacks for modulation acceptor doping of silicon

Modulation acceptor doping (MAD) of silicon through Al-induced acceptor states in SiO2 enables high hole densities without introducing impurities into the Si lattice, but the required uncapped Al-doped SiO2 stack degrades rapidly in air. We demonstrate that an atomic-layer-deposited (ALD) silicon dioxide capping layer can prevent degradation without adding significant levels of parasitic charge. SiO2 was grown from tris(dimethylamino)silane using either ozone for thermal ALD or O2-plasma for plasma-enhanced ALD (PEALD) at different deposition temperatures, followed by rapid thermal annealing (RTA). Film quality was characterized by the refractive index, the wet-etch rate, Fourier-transform infrared spectroscopy (FTIR), and high-frequency capacitance-voltage (C-V) measurements on metal–oxide–semiconductor (MOS) capacitors. FTIR revealed Si–OH bending and O–H stretching bands in the as-deposited films that both vanished after RTA, consistent with hydrogen effusion and the observed twofold reduction in the buffered hydrofluoric acid etch rate. The optimum process, i.e., PEALD at 200 °C, produced after RTA an SiO2 thin film with a wet-etch rate of 0.4 nm s−1, the lowest fixed-charge density (Qfix ≈ + 1 × 1012 cm−2), and negligible C-V hysteresis. This optimized SiO2 cap was integrated into MAD MOS capacitors made of SiO2:Al. The protected devices retained a negative Qfix of −4 × 1012 cm−2, identical to previous reports, demonstrating no interference with MAD mechanism. These results establish PEALD SiO2 as a conformal, charge-lean capping layer that preserves the electrical properties of SiO2:Al modulation acceptor doping and paves the way for a novel alternative doping concept for silicon nanostructures.