Picosecond Optical Determination of Carrier Lifetime in Polysilicon Films
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PICOSECOND OPTICAL DETERMINATION OF CARRIER LIFETIME IN POLYSILICON FILMS
N.K. BAMBHA-, W.L. NIGHAN*, JOHNSON"
I.H. CAMPBELL-,
P.M. FAUCHET*,
and N.M.
*Princeton Laboratory for Ultrafast Spectroscopy, Department of Electrical Engineering, Princeton University, Princeton NJ 08544 **Xerox Palo Alto Research Center, 3333 Coyotte Hill Road, Palo Alto CA 94304 ABSTRACT
The lifetime of optically injected carriers is determined in polySi/SiO 2/Si structures grown by LPCVD at 625 C. These samples are as-grown, have undergone H diffusion or have been implanted by phosphorous ions, followed by various annealing schedules. The lifetime measurement is done in an all-optical, contactless fashion, using the tools of picosecond time-resolved spectroscopy. We find that the lifetime due to recombination at the grain boundaries (trapping time) increases after implantation only if subsequent annealing increases the grain size. The trapping time also increases after hydrogen diffusion. After trapping in relatively shallow grain boundary states, thermalization into deeper lying states is slow on a subnanosecond time scale. Introduction Polycrystalline silicon is a useful material for a variety of applications. In thin film form, it can lead to a novel transistor technology [1] and ultimately to three-dimensional integration of circuits. It is also a convenient material for the photoconductive switch technology [21, which is employed to generate picosecond electrical pulses. In all these applications, the carrier lifetime is a crucial parameter. Trapping occurs at the grain boundaries, where defects may act as efficient capture centers. To decrease or increase the carrier lifetime, it seems logical either to change the grain size (thereby modifying the defect density) or to alter the electronic properties of the grain boundaries. In this report, we investigate the effect of annealing (which may change the grain size) and of doping or hydrogen diffusion (which may alter the electronic properties of the boundaries) on the trapping time in polycrystalline silicon films with small grain size. Samples The samples are grown at 625 °C by low pressure chemical vapor deposition (LPCVD). The polySi layer is nominally- 500 nm thick and is isolated from the Si wafer by a 500 nm thick SiO 2 layer. The polySi grains are columnar [3] with typical size estimated from Raman measurements [41 to be less than 50 nm. Hydrogen is diffused at 400' C for 60 minutes in some samples. The other samples are implanted with phosphorous ions (5 1015 cm- 2 at 150 keV). Most of the samples are subsequently annealed in a N2 atmosphere at 900° C or at 1150' C for 60 minutes. The implanted, non-annealed samples are made of a mixture of microcrystalline and amorphous regions. The samples annealed at 900 °C are polycrystalline with approximately the same grain size as the as-grown samples or the H diffused samples. The samples annealed at 1150 *C have larger grains. In Figure 1, the Raman spectra of the five types of samples and of single crystal Si are compared. Expe
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