Indian Journal of Chemistry Vol. 44A, May 2005, pp. 1001-1008

Kelvin probe force microscopy study on anodization-related variations of porous silicon nanostructures combined with photoluminescence and Raman scattering

Ping He l & Jinghong Li 1.2. * IS tate Key Laboratory of Electroanalytica l Chemistry, Changchun Institute of Applied Chemistry

Chinese Academy of Sciences, Changchun, Ji lin 130022, P R China 20epartment of Chemistry , T singhua Uni versity , Beijing 100084. P R China

Email : jhli @mail.ts inghua.edu .cn

Received 15 Delober 2004

Microscopic structure o f as-anodi zed porous silicon obtained at different current densities and anodi zation time has been studi ed using photoluminescence, Kel vin probe force mi croscopy and Raman scattering. The shallow hillocks form a cluster-like surface under small current density as well as anodi zation time. As the current density and anodi zation time increase, the surface layer of the porous silicon and the corresponding surface potential beco me rougher and larger. respective ly. Also, the stronger reproducible photoluminescence and the down shift as we ll as asymmetric broadening of the Raman lines are observed . A blue-shift of the photoluminescence band of as-anodi zed porou s silicon is observed with increasing the current density while a red-shift is observed with increasing the anodiza tion time in the electrochemi ca l etching process .

IPC Code: lnt. Cl. 7 B82B; C25B ; C25F3/02

The promotion of silicon from being the key material for microelectronics to an interesting material for photonic or photoelectronic applications is a consequence of the possibility to reduce its dimensionality by cheap and easy technologies '. Electrochemical etching of silicon wafers under controlled conditions leads to the formation of porous silicon (PS)2.C" generating disordered array of nanoscale holes or pores perpendicular to the surface of the substrate, where quantum confinement effect of photo-excited caniers7 yields a band gap open ing and an increased radiative transition rate. This leads to drastic changes in the optical , electrical, thermal and chemical properties of single crystal silicon.

Until now, a variet.y of methods have been used to investigate and understand the microstructure and the interesting physical properties of PS . Among these, atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmiSSIOn electron microscopy (TEM) are often used to investigate the surface (or cross-section) topography of the PS8. Raman scattering spectroscopy is usually employed to correlate the photoluminescence (PL) of PS with the conesponding microstructure. Other techniques such as FT-IR, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are often used to investigate the surface species of PS, which could be attributed to the photoluminescence.

It is well known that surface potential is an important property in many scientific di sciplinesl)·I3. Being a very sensitive parameter, the surface potential can reflect imperceptible structural variation, surface modification , contamination and other surface-rel ated processes. Although surface potential can be estimated from field-emission spectra using FowJer­Nordheim model, the resulting values are not reli able because of uncertainty in the parameters employed and nonlinear behavior of the spectra. In recent years, scanning probe microscopy (SPM) techniques have proved to be informative in nanoscale studies of domain structures 14 . Among these techniques, Kelvin probe force microscopy (KPFM), a surface potentiometry based on AFM, is frequently used to provide information on the high-resolution lateral distribution of the local surface potential over the material surface 1S

-21

. Both the surface potential and the topographic image can be measured simultaneously without contacting the sample surface. Until now, KPFM has been successfully used to investigate the electric properties and the surface potential distributions on surfaces of many materials , including SAMs22

, semiconductors23-2s

, etc. Microscopic structure of the as-anodized PS

obtained at different current densities and anodization time employing PL, KPFM and Raman scattering spectra are reported here. As the anodization time and

1002 INDIAN J CHEM, SEC A, MAY 2005

current density increased in the electrochemical etching process, the PS surface layer and the corresponding surface potential became rougher and larger, respectively . Also the stronger reproducible PL and the downshift as well as asymmetric broadening of the Raman line were observed.

Materials and Methods Specimens were fabricated from a lightly boron­

doped p-type (100) silicon wafer with resistivity of 8-11 Q cm. Prior to electrochemical etching, atomically smooth hydrogen-terminated silicon surface was prepared26

.30

. The PS surfaces were formed by standard electrochemical anodization in a 1: 1 solution of 42% HF : 99.9% ethanol at room temperature. Current densities were varied from 10 to 40 mA/cm2 and anodization time from 10 to 20 min . The entire anodization process was controlled by a DC power supply . After constant current etching, the PS samples were rinsed with ethanol and dried under a gentle stream of nitrogen. The surface of the etched samples looked brown. The presence of PS nanocrystallites was confirmed immediately after etching by using a hand-held UV lamp to induce red PL, which can be seen with naked eye.

The surface topographies and surface potentials of the PS samples were investigated using an optical­beam-deflection commercial KPFM system (SPA400 multifunction unitlSP13800 probe station, Seiko Instruments Inc. , Japan) operated in the ambient atmosphere. A rectangular gold-coated silicon cantilever was employed . The cantilever was vibrated at a frequency of 24.76 kHz, slightly higher than the resonant frequency of the cantilever. A modulation a.c. voltage of 6 V (peak-to-peak) was applied between the probe and the sample at a frequency of 22.28 kHz, a little more than 2 kHz below the cantilever vibration frequency in order to avoid crosstalk between the potential and topographic images. Surface potentials and topographic images of the samples were acquired simultaneously at a probe scan rate of 0.5 Hz.

PL spectra and Raman spectra were recorded at room temperature via a JY T64000 micro-Raman and photoluminescence system (JY Corporation, France) . The 488 nm line from an argon ion laser was used as an excitation source. The power of incident laser light on the sample surface in the PL experiments was about 10 mW in a spot with a diameter of 100 ~m. In the Raman scattering measureme'~ts , the power was limited to about I m W in order to avoid thermal

heating effece '. The instrumental line width obtained by using 100 ~m slits was around 0.5 cm· '.

Results and Discussion

Photoluminescence It is known that the production of PS in HF acid is

limited to a low potential region. In this work, the current density was precisely controlled, which was smaller than the critical current densi ty, so as to avoid the occurrence of the electrochemical polishing.

Bulk crystalline silicon has a conduction band minimum at the X (2n/a), (1 , 0, 0) point along the L1(I , 0, 0) direction of the Brillouin zone (BZ), while the valence band maximum is at the zone center of the BZ. Because of this, oJ;tical transitions must conserve momentum in crystalline solid, as indirect transitions in k space utilize phonons to conserve momentum'2 . However, interaction of optical phonons with incident photons is limited to the center of the BZ, and both optical absorption and emission are very weak in comparison to direct gap materials such as GaAs . Therefore, bulk crystalline silicon is extremely inefficient in emitting radiation in the visible spectrum under optical or electrical excitations"' . After PS was prepared, however, the PL or EL phenomena could be observed because of the band gap opening and the increased radiative transiti on rate.

During the present investigations, PL images of as­prepared PS at different current densities and anodization time were recorded. As shown in Fig. 1, with increasing current density and/or anodization time, the reproducible PL intensity became stronger and stronger. Compared with that of PS etched at the current density of 10 mA/cm2, the PL intensity at the current density of 40 mA/cm2 increased almost 5-6 times. Furthermore, a blue-shift band was observed in the PL with increasing current density. For the etching time of 10 min. the PL peak shifted from 705 to 689 nm as the current density changed from 10 to 40 mA/cm2 (Fig. lA). Similarly, the PL peak shifted from 720 to 698 nm when the current density changed from 10 to 40 mA/cm2 at the constant etching time of 20 min. (Fig. I B) .

PS was typically formed in a columnar structure on the (100) face of the silicon substrate. Thi s represented a typical two-dimensional quantum confinement. Whilst testing the quantum confinement theory, Wilson et a/.f> and Schuppler el (il .34

demonstrated that the emission energy of oca].;

HE & Ll: MICROSCOPIC STRUCTURE OF AS-ANODIZED POROUS SILICON 1003

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Fig. I- PL of the as-anodized PS form ed o n lightly boron-doped p -type (100) orientation silicon wafer [res istivity 8-11 Q cm].

luminescence increased with decreasing the average particle size. When the size of nanocrystals was comparable to Bohr' s exciton radius aB (5 nm for bulk silicon), one would expect the ' quantum confinement effect which would lead to band-gap widening between electronic levels 7.35.36.

In fact, increasing the cunent density induces higher porosity and the formation of smaller nanocrystals37

.38

. Intuitively, this observation makes sense as electrochemical polishing can be envisaged as the point at which the size of the nanocrystals shrinks to zero 1.39. Accordingly. the blue-shift of the PL with increasing cunent density observed in this work is consistent with the quantum confinement effect, and the estimated size of the microstructure for p-type PS lay in the characteristic 2-3.5 nm range3

.1,39.

Interestingly. we observed the red-shift of PL band with increasing the anodization time in the case of

constant cun'ent density . As shown in Figs 1 A and IB, the PL peak of as-anodized PS shifted from 705 to 720 nm as the etching time increased from 10 to 20 min at the cunent density of 10 mA/cm2

• from 692 ro 714 nm at 20 mA/cm2, from 690 to 707 nm at 30 mA/cm2 and from 689 to 698 nm at 40 mA/cm2

. This behavior was somewhat far from the well known 'porosity-PL peak relationship', which could be due to the inhomogeneous anodization in ethanollHF solution. Generally speaking, the anodization process involved the diffusion of HF (or F) and the formation of hydrogen bubble at the PS surface. Longer anodization time favored the formation of hydrogen bubbles in the etching process . The hydrogen bubbles restricted the infiltration of the electrolyte in the deeper parts of the PS. Consequently, the deep local structure remained relatively large as the PS layer grew, while the local current density increased near the surface where the interconnected structure was

4 0 4 1 . more open ' . This favoured the removal of subsurfacic small nanocrystallites or allowed them to reach a certain size, leading to inhomogeneous

h· 37 etc Ing .

Surface topography and surface potential The morphology structure of the PS surface has

been studied extensively. Cullis and Canham42, and

Lehmann and Gosele4 3 have reported the ' sponge­like ' microstructure of p-type PS with particle sizes on the nanometer scale. Patel et al. 35 have reported the wire- or dot-like morphology of the PS microstructure with sizes of these wires (dots) being in the range of a few nanometers and isolated from each other. Thus, the morphology of PS is strongly dependent on doping, conductivity and on the anodization condition such as cunent density, etching time, irradiation of the light, etc. Because surface potential can reflect imperceptible structural variation, it is significant to investigate the con'elation of surface potential and anodization condition and, furthermore, other surface­related processes.

Figure 2 shows some representational three­dimensional KPFM pictures of as-anodized PS obtained at cunent densities of 20 and 40 mA/cm2

.

Figures 2a-2d provide the topographical images while Figs 2a' -2d' show the surface potential images. The surface of single crystal silicon (Fig. 2a) is very smooth, with maximum difference of height of only 0 .6 1. As for surface potential (Fig. 2a'), a very small value (5.90 mY) of maximum difference is observed. The data show the homogeneities of the surface of

1004 INDIAN J CHEM. SEC A. MAY 2005

Fig. 2-Three-dimensional KPFM images of the as-anodized PS surface formed on lightly boron-doped p-type (100) orientation silicon wafer [resistivity 8-11 Q cm; scan area 500 nm x 500 nm. a: Single crystalline silicon sample and b-d: PS samples. (b: 20 mA/cm2@ 10 min ; c: 40 mA/cm2@ 10 min; and d: 40 mAlcm2@20 min)]. (contd).

HE & LI: MICROSCOPIC STRUCTURE OF AS-ANODIZED POROUS SIliCON 1005

Fig. 2 (contd.)- Three-dimensional KPFM images of the as-anodi zed PS surface formed on lightly boron-doped p-type (100) orientation silicon wafer [resistivity 8-11 Q cm; scan area SOO nm x SOO nm. a: Single crystalline silicon sample and b-d: PS samples. (b: 20 mA/cm2@ 10 min; c: 40 mA/cm2@ 10 min; and d: 40 mA/cm2@20 min)].

single crystal silicon. After the formation of PS, the surface topography and surface potential change greatly. Under the small current density , the shallow hillocks just form a cluster-like surface. With increasing the current density and the anodization time, the surface layer of PS and the corresponding surface potential become rougher and larger, respectively. The corresponding data of maximum difference of morphological inhomogeneities and surface potential changes are provided in Fig. 3, all data being obtained by SPA400 multifunction unitlSPI3800 probe station . These data show that the microstructure of PS including surface potential is strongly dependent on the anodization condition44

.

Thus, the ·PS skeleton is often modeled as sponge structure, which consists of spherically shaped clusters with all the silicon ato ~ l1s located at the sites of a diamond lattice. Also, the 'iclminescent PS layer consists of highly packed, isolated and/or interconnected network of silicon nanocrystallites (wires or holes)7.

The influence on the morphology is due to the electrochemical etching reactions in which the corrosion 'and diffusion of species such as HF, F and Hz take place between the PS surface layer and the electrolyte. Other transport processes such as convection as well as charge migration in bulk also have great impact on the texture of the PS layer. It is reasonable that with increasing current density and anodization time, all of the transport and corrosion

processes bring about increasing morphological inhomogeneities.

On the other hand, the synthesis of PS under anodization bias resulted in both the silicon dissolution and the formation of nanoscale SiH and Si02 at the interface of PS/HF electrolyte45

, which in tum was expected to generate surface traps of different types with the wide band gap of PS46. The dangling bonds of atoms in the outer shells of the cluster are usually saturated with the hydrogen atoms47, which were produced in the etching process of silicon. The hydrogenation of the silicon surface led to additional electronic states, which partly overlapped with bulk band structure and partly lay within forbidden regions, resulting in the higher positive potential defect48,49. Using the same cun'ent density, increasing the etching time would facilitate the production of more nanoscale Si02, which in tum bound to more hydrogen atoms, leading to higher positive potential. Similarly, on the condition of same etching time, with increasing CUITent density, the quantity of nanoscale Si02 increased. Accordingly, binding to more hydrogen atoms gave the PS a more positive surface potential. It is reasonable, therefore, that the surface potential of PS was correlated to the etching condition and the physical properties such as PL and Raman scattering should be related to the surface potential.

Raman scattering Raman spectra were recorded to further elucidate

the properties of the PS. As could be seen in Fig. 4a, Raman spectrum from reference crystalline silicon

1006 INDIAN J CHEM, SEC A, MAY 2005

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was narrow and symmetric, centered at 520 cm- I. The

full width at half maximum (FWHM) was about 3.5 cm-I

. As for the PS, a downshift as well as asymmetric broadening of the Raman spectrum with an extended tail at low frequency appeared. Overall, Raman intensity from PS was about 5-8 times stronger than that from crystalline silicon, perhaps due to the surface enhancement or resonance effect.

The characteristic asymmetric broadenings of the Raman lines and the observed red shifts of their maxima were present in all the PS samples, but their values differed . No peak at 480 cm- I was observed, which indicates no substantial contribution from amorphous regions5o. The phonon line width changed from 3.5 (bulk crystal silicon) to 6.0, 7.5, 10 and 12 cm-I for samples prepared in the case of etching current densities of 10, 20, .30 and 40 mNcm2

,

respectively, suggesting thereby that the Raman

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Fig. 4--Raman scattering spectra of as-anodized PS produced on lightly boron-doped p-type (100) orientation silicon wafer with a resistivity of 8-11 Q cm (b-e). [Narrow Raman line at 520 CI11- 1

corresponding to the crystal s ilicon (a) is shown for comparison; current density: 10-40 I11A/cm2

; anodization time: 10 min].

contribution was exclusively due to the PS layer and not due to bulk silicon. The general trend was that increasing the cunent density induced higher porosity , leading to a broadening of the Raman line accompanied with a shift towards lower frequencies. This behavior could be explained by the decrease of the crystallite sizes37

.

Tsu et al. 33 established a conelation of Raman scattering and PL spectra and showed that the observed PL originated from extremely small microstructures and, as the luminescent peak increased in photon energy, the Raman feature shifted to lower energy. In bulk silicon, interaction of optical phonons with incident photons is limited to the center of the BZ. A finite particle size, l, corresponded to a momentum q = ± 2 n / I. Therefore, the position of the Raman peak reflected the position of the phonon dispersion at q = ± 2 n / l . For larger structures, since the optical phonons of silicon were degenerated at q=O, only the zone center phonons were allowed in Raman scattering and, consequently, only one peak at 520 cm- I was seen in the Raman spectrum for crystalline silicon33

. In PS, the siiicon crystal as a whole may be considered to be divided spatially by a high density of pores in the silicon network. When the nanocrystallite size becomes smaller than 3.0 nm. the motion of the caniers was confined in either one or zero dimension, and phonons were localized spatially within the small crystallites . The momentum conservation at the BZ center was relaxed, allowing non-zero k phonons to contribute to the observed one-

HE & Ll: MICROSCOPIC STRUCTURE OF AS-ANODIZED POROUS SILICON 1007

phonon Raman process. The participation of the non­zero k phonons shifted the dominant Raman peak to lower energy with an asymmetrical broadening51

-53

.

Conclusions The microscopic structure of the as-anodized PS

obtained at different current densities and anodization • time was studied by PL, KPFM and Raman scattering. KPFM was used to probe the information of the local surface potential of PS. As the current density and anodization time increased, the PS surface layer and the cOITesponding surface potential became rougher and larger, respectively. The influence on the morphology was due to the electrochemical etching reactions and a series of transport processes, including diffusion betwe~n the PS surface layer and the electrolyte, convection and charge migration in bulk Si. The binding of hydrogen atoms to the PS surface led to additional electronic states, which was expected to generate surface traps of different types with wide band gap, leading to the change of the surface potential of PS. As current density and anodization time increased in the electrochemical etching process, the stronger reproducible PL and the downshift as well as asymmetric broadening of the Raman line were observed. Also, blue-shift of the PL band of as­anodized PS was observed while increasing the current density. Interestingly, a red-shift of the PL band was observed upon increasing the anodization time in the etching process.

Acknowledgement The financial support by Outstanding Youth Fund

(No. 20125513) from the National Natural Science Foundation of China, and for the 100 People Plan Project from Chinese Academy of Sciences is sincerely acknowledged.

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