Given that the silicon bulk lifetime is sensitive to high temperatures, ALD Al2O3 has a natural advantage over thermal SiO2 in terms of integration into industrial cell processes. Extensive experiments on Al2O3 film applications in photovoltaics have demonstrated that Al2O3 can passivate both low-doped n- and p-type silicons. ALD Al2O3 also exerts a better passivation effect on p+-type emitters than other dielectric layers. Very recently, Hoex et al. [4] found that Al2O3 can also enable high-surface
passivation for n+-type emitters within find more the range of 10 to 100 Ω/sq. Low SRVs for dielectric passivation are attributed to two passivation mechanisms: chemical passivation and field-effect passivation [5, 6]. Chemical passivation (e.g., thermal SiO2 films) decreases the interface defect density (D it). In dielectric layers such as SiN x and Al2O3, a high fixed charge density (Q f) near the silicon surface
generates an electric field, repelling electrons or holes to reduce carrier recombination on the surface. Thermal ALD Al2O3 reportedly acquires a negative Q f as high as 1013 cm-2 with sufficiently low D it (about 1011 eV-1 cm-2) after annealing [7, 8]. Experiments have shown that the fixed charge located near the Al2O3/Si interface is related to some types of defect proposed as Al vacancies, interstitial O, and interstitial H in Al2O3 film or at the interface [5]. Positron annihilation is a useful GDC-0941 molecular weight technique for vacancy-type defect investigation. Edwardson et al. [9] performed Doppler broadening of annihilation radiation (DBAR) studies and found an interface that traps positrons in an ALD Al2O3 sample, which significantly differed from the S-W result of DBAR in the current work. The discrepancy can be attributed to the different annealing conditions. In the present study, the effect of annealing temperature
on the surface passivation characteristics of Al2O3 films was investigated. Corona charging experiments were performed to distinguish between chemical and field-effect passivation mechanisms. Slow positron beam DBAR measurements were performed to probe the defects in Al2O3 films annealed at 300°C, 500°C, and 750°C. Methods Experimental Aluminum oxide films were deposited onto a 1 to 10 Ωcm p-type Czochralski Inositol oxygenase Si (100) substrate using the thermal ALD method. The 420-μm-thick double-sided polished wafers were cleaned using the RCA standard method and dipped in 1% hydrofluoric acid for 1 min before deposition to remove the native oxide layer on the surface. Thermal ALD Al2O3 films about 23 nm thick were prepared with Al(CH3)3 and H2O as reactants at 250°C. The optimum deposition temperature that led to the highest as-deposited effective lifetime was determined to be 250°C. Double faces were deposited to prepare symmetrical Al2O3/Si/Al2O3. After deposition, the samples were annealed at different temperatures (300°C to 750°C) for 10 min in air.
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