The luminescence behaviour of porous silicon layers

Solid State Communications, Vol. 85, No. 4, pp. 347-350, 1993. Printed in Great Britain.

0038-1098/93 $6.00 + .00 Pergamon Press Ltd

THE LUMINESCENCE BEHAVIOUR OF POROUS SILICON LAYERS J. Oswald, M. Nikl and J. Pangrhc Institute of Physics, Czechoslovak Academy of Sciences, Cukrovarnick~i 10, 16200 Prague, Czechoslovakia

(Received 15 June 1992; in revised form 18 September 1992 by T.P. Martin) The excitation and emission spectra together with decay kinetics and luminescence degradation curves have been measured in porous silicon layers prepared by electrochemical anodization. The degradation of luminescence depending on wavelength and excitation light intensity was observed for uncovered porous silicon layers. The decay of luminescence is non-exponential with the mean lifetime of about 3 #s. Strong luminescence in the visible range of spectra at room temperature was obtained in powder and scale-like samples arising during the preparation of porous silicon layers. 1. INTRODUCTION THE RECENT demonstration of efficient photoluminescence of porous silicon [1, 2] (PS) in the visible range of spectra at room temperature has raised a number of questions on its origin, behaviour and possible explanation. Mechanism of luminescence of PS has not been clarified yet. The photoluminescence of PS layers was initially attributed to a quantum size effects within pure crystalline material [1], but later the speculation appeared that PS is an amorphous phase in its as-anodized state [3]. Diffraction patterns of TEM obtained with the PS layer showed, however, the crystalline character of PS [4]. Another possible mechanism was proposed by Brandt et al. [5], where the origin of strong room temperature luminescence in PS was ascribed to siloxene derivates, present in PS. The first observation of electroluminescence [6] in the visible range and photolithographic preparation of micron-dimension PS structures [7] exhibiting visible luminescence demonstrated the real chance to use this material for fabrication of a new type of devices. In this paper the results of measurements of the luminescence excitation spectra and kinetics of PS layers are presented and luminescence spectra of PS layers are compared with those of powder and scalelike samples prepared in the same way. 2. EXPERIMENTAL P-type ( 1 0 0) silicon wafers of resistivity 2.1-3.9 f~cm were used for the preparation of PS films. The PS layers were formed by electrochemical anodization in

water solution of HF (40% HF + DI H20 1:1) at a constant current density of-,~ 10 mA cm -2. Besides PS layers on the silicon substrate, white or yellow powder also grew on the surface of the PS films. The diameter of the powder particles was of the order of microns. The powder samples were obtained by this method when a mixture of saturated water solution of NaF and 40% HF was used as an electrolyte and the current density was -,~ 1 mA cm -2 with a preparation time of the order of hours. The colour of the prepared powder depends on the mixture ratio of NaF to HF solutions, usually, the ratio 2 : 1 was used. Under the same preparation conditions small scales of the PS layer (scale-like samples) peeled off from the substrate. The typical dimensions of the scale-like samples were of the order of a few mm 2 and the thickness was approximately 1 #m. The surface of the PS layers was analysed by XPS method and three main elements were found out namely oxygen, silicon and hydrogen [8]. All measurements of photoluminescence excitation spectra and decay kinetics were performed by means of an Edinburgh Instrument Model 199S Spectrofluorometer. Emission was excited by a hydrogen filled continuous or coaxial flash lamp (a full width at half maximum (FWHM) of light pulses was ,-~ 1 ns) and a xenon high pressure flash lamp (FWHM ,,~ 2#s). Excitation and emission wavelengths were selected using single grating monochromators (dispersion 5.2 nm mm -1). Luminescence was detected at right angles to the excitation beam with a Mullard XP2233 photomultiplier (PMT). Decay curves were measured by a single photon counting method. Emission spectra were corrected for the monochromator and PMT spectral response, excitation ones for the spectral dependence of

347

LUMINESCENCE BEHAVIOUR OF POROUS SILICON LAYERS

348

excitation light energy. As the spectral measurements on the above spectrofluorometer could be performed only upto 780 nm, they were completed by measurements of photoluminescence in the range 600-950 nm detected by a PMT with S1 photocathode, using CW Ar or HeNe lasers for excitation. These spectra were not corrected for the spectral response of the apparatus.

Vol. 85, No. 4 (a)

(b)

E

3. RESULTS AND DISCUSSION The dependence of the shape and the position of the emission band on the excitation wavelength is shown in Fig. 1. The position of maxima of luminescence intensity is shifted to the high energy side for higher energy excitation. The shape of emission spectra for Aex = 458 nm can be fitted by the Gaussian curve, however, for Aex = 632.8nm some structure of the emission spectrum was observed (Fig. 1). For uncovered samples and laser excitation the degradation of luminescence was observed and after covering the surface of the sample by a thin film of paraffin the degradation was stopped (see Fig. 2). The degradation increases with the increase in the laser excitation intensity or with the decrease in the excitation light wavelength. The excitation spectra differ for different emission

I

/At'-',

0

I

I

50

100

150

T i m e (s)

Fig. 2. Degradation of luminescence under Ar 458 nm line excitation (a) uncovered, (b) covered by paraffin. wavelengths (Fig. 3). For increasing emission wavelengths the maximum of the luminescence excitation spectrum was shifted toward the higher wavelength side. The luminescence decay curve of PS can be seen in Fig. 4. It is a strongly non-exponential curve with the mean lifetime Tmean

_

_

x l(t) dt = 3.01 #s. f~o I( t) dt

f0°° t

(1)

This value is in good agreement with the lifetime presented by other authors for PS [5]. (eV)

PS Xex . 4 5 8 n m ---PS )~,, , 6 3 2 . 8 n m . . . . ScaLe-Like )~,, -458nm . . . . Powder ),ex =632.8nm

i i

55 i

i

i

;

:

4.5 :

:

:

i

55 i

i

i

LO.

%':. p. °- %~

a

I //\ ", ,/.,..i \ "',, ii/1:<' \ ,,,

08

• n°

o

n

o

•

b

0 ° a

o ° Qo • °'~2

•

:°-

*•

~.

o

.

:-'.

• ~

n-

~-:

•

06

.

o

.,

O 4

..

:

i'

° t~ a'~==

.

a

t

02

L ....

600

i

~". . . . .

700 800 WaveLength (nm)

..

#~

:

0e

B

"~--

900

Fig. l. Comparison of the luminescence spectra of a PS layer, scale-like and powder samples prepared by electrochemical anodization and the dependence of luminescence spectra of the PS layer on the excitation wavelength.

~/J.._;..o_°o°"" i

200

250

500

i

350

!

400

WaveLength (nm)

Fig. 3. Excitation spectra for emission wavelengths (a) Aem = 570 nm and (b) ,~e,~= 700 nm.

LUMINESCENCE BEHAVIOUR OF POROUS SILICON LAYERS

Vol. 85, No. 4

band v

Time tats} 465

'

9 3

1395

'

'

--2

I(t)~(n+v)

104

xv,.~t

for a long enough time, or more exactly [10] const I(t) = --------~" (to + t)

103

u~ 10 2

", ,..~::~

--:.: .::.

-.

i01

..

:

0

i

[00

*

t

200

!

349

t

300

i

1

i

400

i

500

Chonnet number

Fig. 4. Decay curve of luminescence at room temperature for Aex = 355nm and )~em 580nm. The filled function is of the form I ( t ) = (0.507+0.0114 x T) -25, time t is given in #s. The origin of the function is explained in the text. =

To explain the shape of the decay kinetics of PS, the results of already reported photosensitivity measurements [9] can be used. It is evident from these measurements that free electrons and holes are created under visible light excitation in the conduction and valence bands, respectively. Further, it was shown from the temperature dependence of photocurrent [9] that electrical conduction behaviour of PS is very similar to that of amorphous hydrogenated Si and that localized states acting as carrier traps exist in the bandgap. In the case of the recombination luminescence where electron trap states with the only activation energy Ea in the band exist and their cross-section is the same as that of the holes for the capture of electrons, it is derived [10] that concentrations of both electrons trapped at localized states n(t) and electrons moving in the conduction band v(t) are proportional to t -l. If it is further supposed that the mean free path of electrons As is sufficiently large to allow the recombination of electrons with a large number of holes (As > 10-Tm), the intensity of emission is proportional to the product of the hole concentration (n + v) and electron concentration in the conduction

(2)

(3)

However, it was not possible to fit the decay in Fig. 4 by formula (3) successfully. It was shown [10] that if the hypothesis about equality of cross-sections of trap centres at and holes ah for electrons capture is abandoned, it is no longer possible to use the expression (3) for the description of decay. In such a case only approximate solution exists const I(t) - (t o + 0------7, (4) where p is the real number related to the ratio of the above mentioned cross-sections. The best fit of decay time was obtained for p = 2.5 in Fig. 4, which gives 7 = at/ah = 1/3 using the results of Curie [10]. Because good agreement between the fit and the experimental curve was achieved (see Fig. 4), it is reasonable to suppose that the recombination of electrons and holes with the participation of trap states in the bandgap is the leading process responsible for the shape of the luminescence decay of PS at room temperature. The difference in the shape of the approximate function and the experimental dots in the initial (-~ 0.5/zs) part of the decay is very frequent in the case of recombination kinetics and can be influenced by several factors: (i) Attractive Coulombic interaction between electrons and holes enhances the recombination process in the case of nearby lying pairs. Such an interaction is not considered in the simple recombination kinetics (2). (ii) Small dimensions of quantum wires may strongly restrict the motion of created electrons and holes in the bulk, which could lead (in combination with (i)) to an "overcritical" concentration of electrons and holes at the initial stage of the recombination, enhancing in such a way the recombination. The surface states could also act as the centres of radiative recombination (by catching electrons or holes), enhancing in such a way the quantum efficiency of the emission (the fraction of the "surface" and "bulk" Si atoms is strongly increased here in comparison with bulk Si sample). It agrees with the strong dependence of the luminescence intensity on the environment and it can explain the

350

LUMINESCENCE BEHAVIOUR OF POROUS SILICON LAYERS

degradation of luminescence by the change of surface states and the disappearance of this effect after covering the surface. Very intensive luminescence in the visible range of spectra was also observed for powder and scalelike samples, Fig. 1. The maximum of luminescence intensity was shifted to the high energy side in comparison with the PS film (Fig. 1). The nature of a strong visible luminescence at room temperature of the powder samples is not known. We assume that powder samples are formed by some silicon compound with strong luminescence, but it has to be verified. 4. CONCLUSION We have compared the luminescence properties of porous silicon layers prepared by electrochemical anodization with those of scale-like and powder samples manufactured at low current density in a different electrolyte. Their luminescence properties are very similar. We assume that at the high current density and the short time of sample preparation quantum "wires" are created and luminescence can be explained by the confinement effect. For the powder

Vol. 85, .No. 4

samples growing on the surface of the PS layers at the low current density and in the special electrolyte we supposed the formation of some silicon compound, but this hypothesis has to be verified. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

L.T. Canham, AppL Phys. Lett. 57, 1046 (1990). L.T. Canham, K.J. Marsh & D. Brumhead, Electron Times 590, 1 (1991 ). S.R. Goodes, T.E. Jenkins, M.I.J. Beale, J.D. Benjamin & C. Picketing, Semicond. Sci. TechnoL 3, 483 (1988). A.G. Cullis & L.T. Canham, Nature 353, 335 (1992). M.S. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber & M. Cardona, Solid State Commun. 81, 307 (1992). N. Koshida & H. Koyama, AppL Phys. Lett. 60, 347 (1991). V.V. Doan & M.J. Sailor, AppL Phys. Lett. 60, 619 (1992). J. Oswald & J. Zemek (to be published). N. Koshida, Y. Kiuchi & S. Yoshimura, Proc. lOth Symposium on Photoelectronic Image Devices, London (1991). D. Curie, Luminescence Crystalline, pp. 128134. Dunod, Paris (1960).