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Physica E 27 (2005) 121–123

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Defects in porous Si investigated by a temperature variation ofphotoluminescence spectra

J.I. Yu, In-Ho Bae�

Department of Physics, Yeungnam University, Kyongsan 712-749, Republic of Korea

Received 15 September 2004; accepted 25 October 2004

Abstract

Photoluminescence (PL) measurements on porous Si (PS) are carried out to investigate the inter defects and to

determine the activation energies in PS. The activation energies of the electrons confined in PS A peak and B peak, as

obtained from the temperature-dependent PL spectra, were 67 and 61meV, respectively.

r 2004 Elsevier B.V. All rights reserved.

PACS: 78.

Keywords: Porous Si; Photoluminescence; Silicon

1. Introduction

The realization of efficient photoluminescence(PL) from porous Si(PS) requires that not onlythese mechanisms be known but also processingmethods are developed to produce stable PLproperties [1]. The recombination processes innanocrystals require, on the contrary, detailedinformation on the nature of the surfaces, theirpoint and extended defects, the amount of hydro-gen passivation and their oxidation state. The

e front matter r 2004 Elsevier B.V. All rights reserve

yse.2004.10.010

ng author. Tel.: +8219 669 7749;

0749.

ss: ihbae@yumail.ac.kr (I.-H. Bae).

determination of these properties for layers withtypical thicknesses of some micrometer is adifficult task and only limited information isavailable up to now. The experimental techniqueemployed for surface studies in porous silicon aremainly electron microscopy, infrared studies (IR)and X-ray photoemission spectroscopy (XPS),which give information on the monocrystallinecharacter, the existence of SiH, SiH2 and SiOconfigurations, as well as the charge states of thesilicon atoms on the surfaces and interfaces [2,3].In this paper we report on the strong red PL of

PS formed on Si substrate. PS samples areprepared by a conventional chemical anodizingof Si substrates. Temperature-dependence PL

d.

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"A" peak Porous Si : Exp. : Fit

J.I. Yu, I.-H. Bae / Physica E 27 (2005) 121–123122

measurements were carried out in order toinvestigate the interband transitions and to deter-mine the activation energies for the PS.

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

"B" peak

PL

Int

ensi

ty [

arb.

uni

ts]

Photon energy [eV]

2. Experimental details

A sample is used with a boron-doped p-typeSi(1 0 0) wafer with the resistivity of 5–10O cm. Anelectrochemical anodizing was proceeded to formPS layer on the Si substrate in the solution ofHF:H2O:C2H5OH (1:1:2) under the illuminationof halogen lamp with 250W. The anodizingcurrent density was 65mA/cm2 and the etchingtime was 6min for Si wafer. In PL measurement,He–Cd laser with a wavelength of 442 nm wasused, as an excitation source. The luminescencelight from the sample was focused with collectionlenses, dispersed by a 0.75m SPEX single gratingmonochromator and detected by a water-cooledphotomultiplier tube using lock-in technique.

Fig. 1. Spectra of PL PS anodized on p-type Si wafer at current

density of 65mA/cm2 for 6min at room temperature.

2.62.42.22.01.6 1.81.41.2

Porous Si

Step : 30 K

30 K

18 K

300 K

PL

Int

ensi

ty [

arb.

uni

ts]

Photon energy [eV]

Fig. 2. PL spectra measured at several temperatures for the PS.

3. Results and discussions

Fig. 1 shows the PL spectra of PS at 300K. Itcan be seen that Fig. 1 shows two red PL peaks at1.601 and 1.788 eV. The one peak A have strongintensity and the other peak B have weak intensity.The A peak has the origin from the localizedexciton at the interface region between nanocrys-talline Si and SiO2 layer of the surface [4], the Bpeak corresponding to the defect energy state,where SiO2 involving defects may have PL in thevisible band. Peak B is the defect called non-bridging oxygen hole center, which has a wave-length of about 640–680 nm [5].The PL spectra measured at several tempera-

tures for the PS is shown Fig. 2. As thetemperature is increased, the integrated PL in-tensity corresponding to the bound electron peakis given by [6]

I ¼I0

1þ C expð�DEA=kBTÞ; (1)

where I0 is the integrated PL intensity at 0K, C isthe ratio of the thermal escape rate to the radiationrecombination rate, DEA is the activation energy,

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0.00 0.01 0.02 0.03 0.04 0.05 0.06

Porous Si(a) ; "A" peak

Inte

grat

ed P

hoto

lum

ines

cenc

e In

tens

ity

[arb

. uni

ts]

0.00 0.01 0.02 0.03 0.04 0.05 0.06

1/T (K-1)

(b) ; "B" peak

Inte

grat

ed P

hoto

lum

ines

cenc

e In

tens

ity

[arb

. uni

ts]

Fig. 3. Integrated PL intensity as a function of the reciprocal

temperature for PS.

J.I. Yu, I.-H. Bae / Physica E 27 (2005) 121–123 123

and kB is the Boltzmann constant. Using Eq. (1),the activation energies DEA of the confinedelectrons in PS, as determined from the solid linesof Figs. 3(a) and (b) are 6772.1 and 6172.8meV,respectively.

4. Conclusion

A photoluminescent PS layer is formed on aboron-doped p-type Si(1 0 0) wafer by anodizationat a current density of 65mA/cm2 at roomtemperature. The PS layer is correlated to the redPL. The activation energy of the electrons confinedin PS A peak and B peak were 67 and 61meV,respectively.

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