Experimental proof for nanoparticle origin of photoluminescence in poroussilicon layersMargit Koós, István Pócsik, and Éva B. Vázsonyi Citation: Applied Physics Letters 62, 1797 (1993); doi: 10.1063/1.109553 View online: http://dx.doi.org/10.1063/1.109553 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/62/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photoluminescence origins of the porous silicon nanowire arrays J. Appl. Phys. 110, 073109 (2011); 10.1063/1.3645049 Anisotropic photoluminescence from porous silicon layers made under polarized illumination: Origin ofcontradictory experimental observations J. Appl. Phys. 93, 2410 (2003); 10.1063/1.1540746 Photoluminescence stabilization of anodically-oxidized porous silicon layers by chemicalfunctionalization Appl. Phys. Lett. 81, 601 (2002); 10.1063/1.1492306 On the origin of photoluminescence in sparkeroded (porous) silicon Appl. Phys. Lett. 63, 2771 (1993); 10.1063/1.110792 An oligosilane bridge model for the origin of the intense visible photoluminescence of porous silicon J. Appl. Phys. 73, 1924 (1993); 10.1063/1.353182

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

132.174.255.116 On: Sat, 20 Dec 2014 16:11:30

Experimental proof for nanoparticle origin of photoluminescence in porous silicon layers

Margit Ko6s and lsthn P6csik Research Institute for Solid State Physics, H-1525 Budapest, I? 0. Box 49, Hungary

ha B. Wzsonyi Research Institute for Material Sciences, H-1525 Budapest, P. 0. Box 49, Hungary

(Received 10 September 1992; accepted for publication 14 January 1993)

A series of nonlinear phenomena in the excitation intensity dependence of photoluminescence (PL) was observed in porous silicon (PS) at room temperature. From a low level of excitation, the blue shift of the PL spectra was detected followed by complete saturation of the integrated PL intensity, without detectable change in the spectral position. This behavior may well be experimental proof of the nonparticle origin of PS light emission.

Arguably the most intensively investigated phenome- non in the last two years in semiconductor physics is the effective visible luminescence in porous silicon (PS), dis- covered by Canham.’ This luminescence transition was at- tributed to silicon quantum wires, which have a larger band gap than bulk crystalline Si (c-Si) because of the two-dimensional quantum confinement of the carriers.* This observation was soon confirmed by other laborato- ries,z14 and visible electroluminescence from a porous Si np heterojunction has demonstrated the real importance of the phenomenon. 5

In spite of the efforts devoted to clarifying the origin of this highly efficient form of photoluminescence it is still far from being fully understood. Light emission from PS at energies larger than the band gap in c-Si, 1.1 eV, even at room temperature, indicates significant widening of the band gap compared with the crystalline one, and increased probability of radiative transition-possibly due to some size effect, which results in optical spectra remarkably dif- ferent from the spectra of the bulk material. The blue shift of the luminescence peak wavelength with thinning of the pore walls of PS in HF acid solution’A represents an indi- rect observation of size dependence. In addition to the ob- servation of visible light emission from PS it has been shown from optical absorption measurements that PS is highly transparent above 700 nm, thereby supporting a band gap considerably larger than the crystalline one.6*7 This gap broadening may be a consequence of two- dimensional quantum confinement in the very narrow pore walls, as has been postulated.6

Although many experimental data are in good agree- ment with a quantum size effect, thermal annealing exper- iments have directed attention to surface properties. It has been shown that heat treatment of PS at temperatures above 300 “6 causes a dramatic drop in luminescence in- tensity together with red shift of the PL peak,**’ indicating the importance of Si-H species in promoting the efficiency of luminescence transition in porous silicon. Some similar- ities between the luminescence properties of PS and amor- phous silicon have led to an alternative suggestion concern- ing the fine structure of the photoluminescence region.4p10

To clarify the still confused picture, well directed ex- perimental investigations are needed. In this letter the ex-

citation intensity dependence of PL properties is examined in porous Si layers formed on nondegenerate, p-type wafers in order to show that the dominant feature of PL is the size effect and the carrier confinement is three-dimensional rather than two. Our results indicate that the blue shift of the PL spectra and saturation of PL intensity with increas- ing excitation level are in keeping with the nanoparticle system, rather than relating to quantum wires.

The energy level structure in nanoparticulate semicon- ductors is entirely discrete, similarly to molecules, because of the three-dimensional spatial confinement, and these or- bitals are localized in real space within the volume of the nanoparticles. ‘**12 This means that the continuous band of energies of the whole sample does not exist in the usual way. The molecule-like orbitals of the different nanoparti- cles are closely spaced in energy, so the independent par- allel absorptions form a single, bandlike absorption struc- ture. This disintegration of the cooperative band due to the localization can easily be studied by intensity dependence of PL.

Measurements were carried out on PS layers prepared on ( 100) oriented p-type silicon wafer with 0.8-1.2 $2 cm resistivity. The polished surface was cleaned and etched before being anodized at a current density of 5-20 mA/cm’ in 20% HF acid solution followed by etching in 40% HF. Photoluminescence was excited by an unfocused ( 1.3 mm beam diameter) Arf laser using various wavelengths and intensities. Luminescence was dispersed by a grating monochromator of 1 m optical path, and the signal was detected by a photomultiplier with a CsAgO photocathode utilizing the standard lock-in technique. Appropriate filter- ing was applied to prevent scattering of the laser light. Measurements were performed at room temperature and at 80 K in reflexion geometry. Because of the degradation of PL intensity, the samples were first excited by the highest excitation power used in the measurements until reaching stabilization.

In Fig. 1, the typical changes of PL spectra with exci- tation intensity are shown for samples anodized at 5 mA/ cm2 for 60 min. With increasing laser power, the PL spec- tra shift to higher energies accompanied by a small change of the spectral shape, Blue shift of the PL emission similar to Fig. 1 was observed at various excited wavelengths from

1797 Appl. Phys. Lett. 62 (15), 12 April 1993 0003-6951/93/151797-03$06.00 0 1993 American Institute of Physics 1797 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

132.174.255.116 On: Sat, 20 Dec 2014 16:11:30

1.0

5

25

2 0.5 h.2 n

Ei k

z

0.0

9 130 m W = 488 nm

11 4 * I I a I4 I r 1 t r 17 ! t 4 1 t b 500 600 700 800 900 1000

Wavelength [nm]

PIG. 1. Blue shift of room-temperature PL spectra with excitation inten- sity on PS sample of 1.2 CI cm and 5 &cm*, excited by the 488 nm laser line. Every twentieth point is enlarged for the sake of clarity and easy assignment.

458 to 514 run, however it was smaller when excitation intensities lower than that shown in Fig. 1 were applied. This nonlinearity has also been observed in samples anod- ized by greater current densities.

The blue shift in PL finished when the excitation be- comes close to the saturation region. The shapes of the PL spectra are shown in Fig. 2 on an accurate relative intensity scale. The three spectra belonging to 78, 130, and 180 m W exciting power have almost the same peak energy and spec- tral shape although small enhancement at shorter wave- length can be seen on the spectrum excited by larger in- tensity. On further increasing the excitation, unusual behavior of light’ emission was observed, viz., the PL in- tensity at every emitted frequency decreased. Similar changes of PL spectra as depicted in Fig. 2 were also found in the samples anodized by greater current densities.

The integrated emission intensities are shown in the

5

9 h .5 ii 2

ti

0.0

= 78 m W . - .

600 700 800 900 1000 111

Wavelength [nm]

PIG. 2. Room-temperature PL spectra at excitation intensities in the saturation range on PS sample of 1.2 Clcm and 5 mA/cm’. The PL intensities of the different spectra are shown on a comparable scale. Every twentieth point is enlarged for the sake of clarity and easy assignment. Insert shows the integrated PL intensity as a function of excitation inten- sity at room temperature.

insert of Fig. 2 as a function of excitation intensity at dif- ferent wavelengths. Linear dependence can be found at moderate excitation levels followed by saturation at higher ones. At shorter wavelengths the saturation takes place at lower exciting power. Besides the saturation, decreasing light emission in spite of increasing excitation power indi- cates strong nonlinearity in the luminescence process.

The real novelties of these experimental results are blue shift and saturation of photoluminescence with in- creasing excitation intensity. Both of these properties indi- cate nonlinear optical response in porous silicon, similar to that calculated for quantum confined structures.i2 The mechanism responsible for our finding is believed to be the luminescence from a highly conlined structure. Raman scattering measurements have also been reported13 provid- ing stronger support for the nanoparticle structure than for the quantum wires.

These results can be explained by a nanoparticle model which supposes independent absorption and luminescence of the different particles. The PL transition that takes place between the discrete orbitals of these nanoparticles de- pends on particle size. The blue shift of the PL spectra with increasing excitation level can be described by supposing that the greater the energy of the emitted photon, the shorter the lifetime of its excited state. This is a not too rough condition because with decreasing particle size the gap broadens, and the decreasing size also keeps the ex- cited electron and its hole nearer to each other, so the recombination process goes faster; this was measured pre- cisely in the picosecond domain by .Matsumoto et al. I4 If the excitation intensity is increased the low energy part of the PL spectrum reaches its saturation level first, while the high energy part of the spectrum is still in the linear range, and the peak of the PL spectrum shifts to higher energies. On further increasing the excitation level the high energy part of the PL spectrum also reaches its saturation level, and blue shift is no longer experienced.

Because of the relatively large excitation intensities the heating effect should be taken into consideration as another possible source of the observed spectral changes. In order to clarify the role of sample heating we repeated the mea- surements at low temperatures. The cooling of the sample to 80 K caused blue shift of PL peak position typically of 20 nm, compared with its room-temperature position at equal excitation power, with no detectable change of the spectral line shape. The possibility of laser heating is now excluded because the measured shifting direction of PL maximum with increasing excitation power is opposite to that which heating would produce. The excitation intensity dependence of the PL peak position at 80 K showed the same blue shift-within error bars-as was experienced at room temperature. These results also support the nanopar- title interpretation proposed by us.

Nonradiative recombination should also be considered as a means of explaining such phenomenon, like the max- imum in PL integral intensity in excitation intensity depen- dence (see insert in Fig. 2) and which is absent from the same curve measured at 80 K. Various mechanisms are

1798 Appl. Phys. Letl., Vol. 62, No. 15, 12 April 1993 Ko~s, Wzsonyi, and Pbcsik 1798 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

132.174.255.116 On: Sat, 20 Dec 2014 16:11:30

able to cause a significant decrease in luminescence effi- ciency, Auger recombination and stimulated emission be- ing possible candidates. However, the temperature depen- dence of the stimulated emission disagrees with the measured data.

At high excitation intensities various Auger processes have a large enough transition rate to dominate the radia- tive recombination and to quench the PL. The radiative recombination of electron hole pairs confined within a nan- oparticle causes a different excitation intensity dependence in Auger recombination rate, than what can be experienced in delocalized case. For the nanoparticles the Auger re- combination requires more than one electron-hole pair on one particle and its probability depends on the overlap of the pair wave functions. The excitation power applied by us seems to be high enough for Auger recombination to take place.

The low-temperature result, that the quenching of PL intensity by high excitation powers was not observed at 80 K gives further support to this suggestion. Since Auger transition probability decreases with decreasing tempera- ture,r5 the PL intensity quenching rate caused by these nonradiative processes should also be decreased, what was exactly detected.

Summarizing the results: (i) well-defined PL blue shift was observed in PS with increasing excitation intensities at 80 and 300 K, ‘(ii) further increasing the excitation inten- sity the PL can be saturated, but the blue shift was not observable in this range, (iii) such behavior is explainable

by a nanoparticle model in which the absorption and also the PL take place in the independent individual particles.

The authors gratefully acknowledge the stimulus and advice of Professor J. Gyulai, and the vital support of the Hungarian Science Foundation under Contract Nos.: OTKA-T1975 and OTKA-T4224.

‘L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990). ‘A. Bsiesy, J. C. Vial, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain, A. Wasiela, A. Halimaoui, and G. Bomchil, Surf. Sci. 254, 195 (1991).

3N. Koshida and H. Koyama, Jpn. I. Appl. Phys. 30, L1221 (1991). 4S. Gardelis, J. S. Rimmer, P. Dawson, B. Hamilton, R. A. Kubiak, T.

E. Whall, and E. H. C. Parker, Appl. Phys. Lett. 59, 2118 (1991). “F. Namavar, H. P. Marushka, and N. M. Kalkhoram, Appl. Phys. L&t.

60, 2514 (1992). 6V. Lehman and U. Goesele, Appl. Phys. Lett. 58, 856 (1991). ‘H. Koyama, M. Araki, Y. Yamamoto, and N. Koshida, Jpn. J. Appl. Phys. 30, 3606 (1991).

‘C. Tsai, K. H. Li, J. Sarathy, S. Shih, J. C. Cambdl, B. K. Hance, and J. M. White, Appl. Phys. L&t. 59, 2814 (1991).

9S. M. Prokes, J. A. Freitas, Jr., and P. C. Searson, Appl. Phys. Let. 60, 3295 (1991).

‘ON. Noguchi, I. Suemune, M. Yamanishi, G. C. Hua, and N. Otsuka, Jpn. J. Appl. Phys. 31, L490 (1992).

“L. Brus, J. Phys. Chem. 90, 2555 (1986). “S. Schmitt-Rink, D. A. B. Miller, and D. S. Chemla, Phys. Rev. B 35,

8113 (1987). 13Z. Sui, P. P. Leong, I. P. Herman, G. S. Higashi, and H. Temkin, Appl.

Phys. Lett. 60, 2086 (1992). 14T. Matsumoto, M. Daimon, T. Futagi, and H. Mimura, Jpn. J. Appl.

Phys. 31, L619 (1992). I5 Shyh Wang, Fundamentals of Semiconductor Theory and Device Physics

(Prentice-Hall, London, 1989).

1799 Appt. Phys. Let, Vol. 62, No. 15, 12 April 1993 Kob, Wzsonyi, and P6csik 1799 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

132.174.255.116 On: Sat, 20 Dec 2014 16:11:30