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White light luminescence from nano-ZnS doped porous silicon K. W. Cheaha, #, Ling Xua, b and Xinfan Huangb a

Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong

b

State key Laboratory of Solid State Microstructures and Department of Physics,

Nanjing University, Nanjing, Nanjing, P.R.China Nano-ZnS was deposited into porous silicon. By varying the concentration of Zn2+ ion solution during nano-ZnS formation, the amount of nano-ZnS in porous silicon host can be controlled. The doped porous silicon exhibited a gradual shift in its photoluminescence peak from red to blue as a function of the nano-ZnS coverage. At an optimum doping, white light photoluminescence was obtained. A study in the luminescence lifetime showed that the radiative recombination at the blue end of the visible spectrum was due to nano-ZnS, whereas, luminescence emission at the red end of the visible spectrum came from porous silicon. The latter luminescence was due to in part tunneling of excited electrons from nanoZnS into porous silicon and in part direct excitation of porous silicon layer. Time-resolved photoluminescence also showed that radiative recombination was effectively dominated by the nano-ZnS. Photoluminescence excitation result revealed the presence of two excitation levels; one belonged to nano-ZnS at near uv region, and another at about 520 nm from the surface states of porous silicon and nano-ZnS. The doping of nano-ZnS into porous silicon demonstrates that luminescence color tuning is possible when an appropriate functional material is introduced into porous silicon.

# to whom all correspondence should be addressed to.

A8.3.1

Porous silicon (PS) has been the subject of intense research since the report by Canham [1] that strong photoluminescence can be observed from it. Considerable effect has been spent in producing photoluminescence (PL) spanning the visible range [2]. However, it is noted that green and blue emissions are neither strong nor efficient. Recent works in doping PS with organic materials have met with some success in tuning the color of the luminescence [3, 4]. Nevertheless, the potential of such doping process is yet to be realized. ZnS is a wide bandgap semiconductor that is extensively studied as a phosphor [5-8]. Its wide bandgap allows for a variety of luminescence materials to be doped and therefore producing color tunable phosphor. However, the wide bandgap of ZnS prevents it being used as an electrically driven luminescence device. Binh and Adessi [9] reported that by coating a nano-layer of wide bandgap semiconductor onto metal its surface potential can be modified. The principle involves electron injection into the nano-layer which becomes a reservoir for the injected charge. This charge accumulation changes the surface potential, and thus, modifies the surface property e.g. emission property. It is possible to extend this principle to semiconductor heterojunction in which one of the layers is in nano-dimension. The effects of quantum confinement and surface charge effect ca