Graduate Thesis Or Dissertation

Recombination and generation lifetime characterization of p/p⁺ epitaxial silicon wafers

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  • The refresh times of all dynamic charge storage devices, best characterized by the generation lifetime at roan temperature and the recombination lifetime at higher device operating temperatures (T>70°C), strongly influence the efficient and successful operation of dynamic charge storage devices such as DRAM's and CCD's. Both recombination and generation lifetime characteristics of p/p⁺ epitaxial silicon wafers are investigated. Extremely high generation lifetimes on the order of 20-30 msec are observed on p-type epitaxial layers which are correlated by DLTS measurements where no significant trap levels with concentrations higher than 3x10¹¹cm⁻³ are observed. Schroder's [1] simple technique for the determination of recombination lifetimes of materials using a pulsed MOS capacitor technique at elevated temperatures ignores the lateral quasi-neutral bulk generation for short base width devices such as thin epitaxial layers. Consequently, calculations using Schroder's technique indicate that the recombination lifetime of a given material is a function of MOS capacitor diameter. A simple one dimensional approach was developed [2] for the measurement of recombination lifetimes in which quasi-neutral bulk generation in the lateral area of MOS capacitors and the time dependence of the width of space-charge region are taken into consideration for short-base-width devices. The apparent recombination lifetime of thin p-type epitaxial layers were limited to 2 psec by the lower lifetime of the epitaxial layer - p⁺ substrate interface. Investigation of wafers with various epitaxial layer thickness revealed that the actual recombination lifetime of the bulk epitaxial material is an order of magnitude higher than the apparent values. Precipitates at the epitaxial layer - substrate interface were suspected as the main limiting factor causing a localized degradation of recombination lifetimes at the epitaxial layer - substrate interface in p/p⁺ epitaxial wafers. Application of an intrinsic gettering cycle and G4JS simulation heat treatments suggests up to three fold improvement of apparent recombination lifetimes.
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