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PHOTOCHEMICAL ETCHING EFFECTS ON OPTICAL PROPERTIES OF POROUS SILICON

Y. KANEMITSU', H. UTO', Y. MASUMOTO', T. MATSUMOTO-, T. FUTAGI", and H. MIMURA_ Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan "Electronics Research Laboratories, Nippon Steel Corporation, Sagamihara, Kanagawa 229, Japan

ABSTRACT

We have studied effects of hydrogen-termination on optical properties of porous Si. The hydrogen concentration in the samples was controlled by photochemical etching after the electrochemical anodization. Transmission electron microscopy examination shows that the size of Si crystallites in porous silicon ranges from 3 to 10 nm and no significant size changes occur by the photochemical etching process. The PL spectra, the PL excitation spectra, and the picosecond PL decay rate were sensitive to the photochemical etching process. Spectroscopic analysis suggests that the radiative recombination occurs at the near-surface region of the Si crystallites.

INTRODUCTION

Very recently, a great deal of research effort has focused on nanometer-size crystallites made from indirect-gap semiconductors such as Si [1] or Ge [2]. Especially, the discovery of the strong luminescence from Si nanostructures fabricated by electrochemical anodization, often called porous Si, is an extremely important scientific breakthrough with enormous technological implications. However, the origin and mechanism of strong visible luminescence in porous Si still remain unclear. It has been suggested that the strong visible photoluminescence (PL) is attributable to the quantum-confinement effects of electrons and holes in nanometer-size Si crystallites [3]. Incontrast, it has been pointed out that with a large surface-to-volume ratio inthe highly porous structure, the influence of surface effect on luminescence processes is enhanced and the surface of porous Si is responsible for the origin of luminescence [4-6].

In this paper, we show that optical properties such as the PL spectrum, the PL intensity, the PL excitation spectrum, and the picosecond PL decay rate are sensitive to the hydrogen termination of nanocrystallite surfaces inporous Si. It was concluded that the luminescence results from the recombination of electrons and holes at the near-surface region while the inside of Si crystallites plays an important role in both the photocarrier generation process and the transfer process from the inside to the near-surface region.

Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society

222

EXPERIMENT

Porous silicon layers were formed on p-type (100) silicon wafers with - 4 Qcm resistivity. Thin Al films were evaporated on the back of the wafers. The anodization was carried out in HF-ethanol solution (HF:H 20:C 2H5 OH=1:1:2) at a constant current density of 10 mA/cm 2 for 5 min. The thickness of the porous Si layer was about 6 Pm. Furthermore, photochemical etching of the wafer was carried out for 1-5 min in HFethanol solution under illumination with a 500 W tungsten lamp from a distance of 20 cm. After the ph