Photocarrier Radiometric Lifetime Measurements of Intrinsic Amorphous-Crystalline Silicon Heterostructure
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0910-A06-03
Photocarrier Radiometric Lifetime Measurements of Intrinsic Amorphous-Crystalline Silicon Heterostructure Keith R Leong1, Andreas Mandelis1,2, Nazir P Kherani1, and Stefan Zukotynski1 1 Electrical and Computer Engineering, University of Toronto, 10 Kings College Road, Toronto, Ontario, M5S 3G4, Canada 2 Center for Advanced Diffusion–Wave Fields Technologies, Mechanical and Industrial Engineering, University of Toronto, Toronto, M5S 3G8, Canada ABSTRACT Intrinsic hydrogenated amorphous silicon films were deposited by the DC saddle field system on crystalline silicon wafers. The transport parameters of the amorphous-crystalline silicon heterostructures were evaluated by Photocarrier Radiometric (PCR) lifetime measurements. We present the influence of surface pre-treatments on the interface passivation and determine that the standard wafer cleaning treatment SPM/SC1/SC2 yields the highest effective lifetime.
INTRODUCTION In amorphous-crystalline silicon heterojunctions, minimization of impurities and interfacial defects is important for optimum device performance. Cleaning and etching of the native oxide are important steps before device fabrication. The major effect of impurities and defects at a heterojunction interface is the increase in the surface/interface recombination velocity, which reduces the effective carrier lifetime and thus device efficiency [1, 2]. Photocarrier Radiometry (PCR) [3, 4, 5] is an all optical technique that affords the simultaneous extraction of all four electronic transport parameters: lifetime, diffusivity, and back and front surface recombination velocities (SRV) of crystalline silicon. PCR measures the radiative recombination (conduction band to mid-gap defect levels) of modulated photoexcited carriers within crystalline silicon. Crystalline silicon (c-Si) substrates were subjected to various cleaning and etching solutions. The DC saddle field glow discharge PECVD system [6] was employed to deposit hydrogenated amorphous silicon (a-Si:H) thin films on crystalline silicon (c-Si) substrates after the cleaning and etching treatments, creating an amorphous-crystalline heterostructure. This structure was evaluated using the PCR system. We present the correlation between PCR lifetime improvements and surface cleaning and etching pretreatments
EXPERIMENTAL DETAILS N-type Czochralski grown crystalline silicon (c-Si) wafers of 350 µm thickness and of 1 to 2 Ω cm resistivity, were cleaned and etched with a variety of combinations of solutions. Table I lists the sequence of cleaning and etching steps employed on each sample. The following steps were carried out for 10 minutes under ultrasonic agitation at 80oC: acetone, isopropyl alcohol
(IPA), ammonium hydroxide/hydrogen peroxide/DI-water mixture in the ratio of 1:2:10 (SC1 or RCA1), hydrochloric acid/hydrogen peroxide/DI-water in the ratio 1:1:5 (SC2 or RCA2). A sulfuric acid/hydrogen peroxide mixture in the ratio 1:4 (SPM or piranha), and sulfuric acid/hydrogen peroxide/hydrofluoric acid mixture in the ratio of 1:4:0.01% of HF
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