Superlattices and Microstructures, Vol. 12, No. 1, 1992 77

PHOTOLUMINESCENCE AND RAMAN SCATTERING SPECTRA FROM POROUS SILICON

Koichi Inoue, Kenzo Maehashi, and Hisao Nakashima The Institute of Scientific and Industrial Research, Osaka University

8-1 Mihogaoka, Ibaraki, Osaka 567, Japan.

Osamu Matsuda and Kazuo Murase Department of Physics, Faculty of Science, Osaka University

1-l Machikaneyama, Toyonaka, Osaka 560, Japan.

(Received 4 August 1992)

Depth profiles of photoluminescence (PL) and Raman scattering properties of an- odized porous Si have been studied by micro-measurement techniques. The depth- structural-inhomogeneity strongly affects the optical properties of porous Si. The Si 2p absorption edge of porous Si has been examined with synchrotron radiation. It is concluded that the PL blue-shift correlates with the low-energy shift of the Raman peak and with the high-energy shift of the Si 2p absorption edge.

The visible photoluminescence (PL) from porous sil- icon has attracted much attention because of the possi- bility of an application to optoelectronic devices.’ There are, however, two completely different interpretations for the origin of the PL. The one is that the PL comes from the columnar structure of porous Si, and the blue shift of the PL is due to quantum size effects in the wire- like structures’ The other is that the PL comes from Si compounds such as siloxene,’ which are expected to ex- ist on large surfaces of porous Si. Many experiments have been carried out to clarify the origin of the PL. However, there are some controversies in experimental results. One of the experimental difficulties may be re- lating to the inhomogeneous porous structures, particu- larly to the depth inhomogeneity.

In this paper, the depth inhomogeneity of the porous silicon is studied by micro-photoluminescence and micro-Raman scattering measurements, and fur- ther, the Si 2p-core-level absorption spectra of porous silicon is examined to clarify the electronic structures of this material.

Porous silicon layers were prepared by anodizing n-(5 - 10 Rem) Si(lO0) wafers in the solution of hydro fluoride acid for 0.5-10 min with a current density of 20- 200 mA/cm’ under a white light exposure from a 300 W Tungsten lump. We prepared samples also from p+(O.Ol Rem) Si without the light exposure. From scanning- electron-microscope observations of cleaved surfaces, it

was found that large pores reach a depth of over 70 pm, while finer pores are observed particularly near the sur- face in the n-samples.

Excitation photon-energy dependence of the PL was determined with a scanning monochromator for fluores- cence analysis with the resolution of 10 nm. Micro-PL and micro-Raman experiments were performed in air by focusing 476.5 nm Ar ion laser light on the cleaved cross section of sample wafers. The excitation spotsize was about 10 pm, and the excitation power was 1 - 2 mW on the sample. The Raman scattering light and the PL was detected in the backscattering configuration.

Total yield spectra of photoelectrons in Si 2p ab- sorption region were taken with synchrotron radiation from SOR-RING, the 0.4.GeV electron-storage ring at the Institute for Solid State Physics of the University of Tokyo.

Figure 1 shows the typical PL spectra. A broad peak is observed approximately with a single Gaussian line- shape. The peak energy depends on the sample prepa- ration conditions. For example, the solid line in Fig. 1 shows the spectrum of the sample anodized for 2 min with a current density of 20 mA/cm’, while the dashed line, the spectrum of a dark portion of the sample an- odized for 0.5 min with 80 mA/cm’. During the an- odization, both samples were exposed with a white light, from the 300 W rump, whose intensity affected the PL spectra very much.

0 1992 Academic Press Limited

78

I I I I I I

1.6 1.8 2 PHOTON ENERGY (eV)

Fig.1. Photoluminescence spectra of porous Si. Solid line (# 1): a n- Si sample anodized for 2 min with 20 mA/cm’, and dashed line (# 2): a dark portion of the sample anodized for 0.5 min with 80 mA/cm2.

2l------

s 2% t

0

0 0 0

$ 1.8

2 0

0

0

0

0

A------ ‘2 3 EXCITATION PHOTON EN&GY (eV)

Fig.2. Excitation energy dependence of PL peak po- sition of porous Si. Open and closed circles show for two different samples. For both samples the PL peak noticeably shifts to low-energy as the excitation energy decreases.

Superlattices and Microstructures, Vol. 72, No. I, 7 992

‘1’111~ IWdk r’rrergy of t,lw 1’1. rLIs0 drpmls or, t hr e*c I tatioil energy as shown in Fig. L’. These two curves WIVY~ taken for two different samples. The peak shifts toward lower energy as the excitatiou energy decreases. ‘I’hi:. tendency is commonly observed for t,hese two samples. but t,hc dependence is very different. The reason of t,his dependence will be discussed later

1

RAMAN SHIFT (cm-‘)

Fig.3. Typical Raman spectrum of porous Si. Dashed line shows the spectrum of bulk Si (c-Si) for comparison.

Figure 3 shows the Raman scattering spectra of porous Si and of bulk Si. It is notable that an intense peak at about 517 cm-‘of porous Si clearly shifts toward lower energy with somewhat broad width accompanied by a very long low-energy tail as compared with that of bulk Si. These features are explained basically in terms of phonon confinement in the small structures of porous Si, where the momentum selection rule will be partially broken and the higher wave-number phonons will con- tribute to the Raman spectra.4s5 In this explanation, the amount of low-energy shift correlates with the smallness of the crystal size.

Figure 4 shows the depth profiles of the Raman scat- tering and the PL. In the depth profile of the Raman peak position, a low-energy shift is remarkable near the surface. This shift suggests that the crystal size near the surface is less than a few nanometers. Since the peak position changes continuously from lower to higher with the depth from the surface, it is concluded that mean crystal size changes from smaller to larger with the depth.

Superlattices and Microstructures, Vol. 72, No. 7, 1992

I 1 I I

(a) Raman Shift 0 O O 0 0

0

0 . (b) PL Intensity 0 + 0

?? 0 . 0

1.. (c) PL Peak Energy ?? ??

?? w t w I I I I

0 0.05 0.1 0.15 DEPTH (mm)

Fig.4. Depth profile of Raman scattering and PL spectra of porous Si.

In the PL depth profiles in Fig. 4, the intensity and peak position also changes with the depth. That is, the intensity becomes more intense and the peak position shifts toward higher energy as the measured point ap- proaches the surface. Then, it should be emphasized that the PL blue-shift correlates with the low-energy shift of the Raman peak in depth profiles.

The excitation energy dependence of the PL peak position shown in Fig. 2 can be explained by consid- ering the depth inhomogeneity. When the excitation energy is high, ultra violet or violet, only the surface re- gion of porous silicon is excited, because the penetration depth of the excitation light is small. From the PL depth profile the peak position shifts toward higher energy in the surface region. When the excitation energy becomes lower, the light penetrates into deeper region, where the PL peak position shifts toward lower energy. The differ- ence between the dependence for the two samples in Fig. 2 will be due to the difference in the depth profile of the porous structures. It will be concluded that structural depth inhomogeneity is important in the optical exper- iments in which excitation penetration depth is longer than the scale of the sample inhomogeneity.

Since the photoelectron escape depth is much smaller than the light penetration depth, it may be possible to avoid depth inhomogeneity by detecting photoelectrons. We took Si 2p core level absorption spectra of porous

100 105 1 PHOTON ENERGY (eV)

79

0

Fig.5. Total yield spectra of photoelectrons from porous Si. The PL spectra of samples # 1 and # 2 are shown in Fig. 1.

silicon by use of the photoelectron-total-yield technique with synchrotron radiation to know the electronic struc- tures of porous silicon.

Figure 5 shows photoelectron-total-yield spectra in Si 2p core level absorption region. It is generally known that the yield spectrum in a core excitation region well coincides with the absorption spectrum practically. These spectra were taken (a) for a hydro fluoride treated n- Si bulk crystal for comparison, (b) for an anodized p+ Si without the light illumination for 10 min with 20 mA/cm’, (c) for a dark portion of the sample anodized for 0.5 min with 80 mA/cm2 with the light illumination, and (d) for a sample anodized for 2 min with 20 mA/cmZ with the light illumination. The PL spectra for the last two samples (c) and (d) are shown in Fig. 1.

The onsets of the Si 2p absorption are located (a) at 98.80 eV, (b) at 98.95 eV, (c) at 99.02 eV, and (d) at 99.15 eV. The spectral change and high-energy shift are more clear in visible luminescent n- porous layers.

Since Si 2p absorption spectra result from the tran- sition from Si 2p to conduction band, the high-energy shift of the onset indicates that the conduction band shifts toward higher energy and the density of states of this band changes in porous silicon. It gives the strong evidence that the quantum size effects play a dominant role in porous silicon. The high-energy shift approxi- mately correlates with the blue-shift of the PL.

80 Superiattlces and Microstructures, Vol. 12, No. 7. 7 992

In above descriptions, it is concluded t,hat, the PL blue-shift approximately correlates with the low-energy shift of the Raman peak, and with the high-energy shift of Si 2p absorption edge. This is consistent with the hy- pothesis that the origin of the visible PL from porous Si is the quantum size effects, although other possibilities are not ruled out entirely.

In summary, We clarified the depth inhomogeneity of porous silicon by micro-PL and micro-Raman scattering measurement,s. This depth-structural-inhomogeneity explains the excitation energy dependence of photolu- minescence. It is concluded that the PL blue-shift ap- proximately correlates with the low-energy shift, of the Raman peak, and with the high-energy shift of Si 2p absorption edge.

Acknowledgements- The authors would like to thank S. Nakashima and H. Oe for their cooperation in the ex-

pcrimcnrh. They wish to thank t lw statrh o1’SHI, of IShI’ for their hc,lp in t hex nleasurrmcnt with synchrotron ra diation. This work is partially supported 1)~ the Krlrat d Research (+rant, by t hr Murata Science Foundation, an(l by Iketani Science alld Technology t:orlndation. ‘l’he cx- citation energy dependence was t,aken at the Material Analysis C’cntFr of ISIR of Osaka 1rnivrrsit.v.

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