Photoluminescence enhancement and degradation in porous silicon: Evidence for nonconventional photoinduced defects

JOURNAL OF

LUMINESCENCE ELSEVIER

Journal

of Luminescence

71 (1997) 77-82

Photoluminescence enhancement and degradation in porous silicon: Evidence for nonconventional photoinduced defects H. El Houichet”, *, M. Oueslati”, B. BessaYsb,H. Ezzaouiab aLaboratoire de Spectroscopic Raman, Dipartement blnstitut National de Recherche

Scientifique

de Physique, Fact&

et Technique,

BP 95, 2050 Hammam-Lif Received

des Sciences de Tunis, le Belvhdkre

USI, Laboratoire

de PhotovoltaiQue

et des Mattriaux

1060, Tunis, Tunisie Semiconducteurs,

Tunisie

26 April 1996; revised 8 July 1996; accepted

25 July 1996

Abstract

We report results concerning photoluminescence (PL) enhancement and degradation in fresh and oxidised porous silicon (PS). The PL evolution is explained by being based on three dissociated phenomena such as hydrogen photodesorption, photooxidation and nonconventional photocreated defects. It was found that time evolution of PL spectra depends on ambient atmosphere and PS porosity. An increase of the PL intensity followed by a degradation occurs after a short interruption of laser excitation, depending on temperature. This PL behaviour is attributed to relaxation of nonradiative recombination centres like photoinduced defects other than Si dangling bonds. It was pointed out that these photoinduced defects have a density and a relaxation time depending on laser power and temperature. Keywords:

Porous silicon; Photoluminescence;

Photoinduced

1. Introduction Porous silicon (PS) attracts much attention owing to its strong and visible photoluminescence (PL) at room temperature. Up to now, this PL has been very controversial. In some works PL was attributed to surface states such as silicon complexes like siloxene [l], hydride species SiH, [a], polysilanes [S] and hydrogenated amorphous silicon [4]. In other works, this PL was explained by quantum confinement of electrons and holes into Si nanostructures [S]. In most experiments [6], the maximum of the PL band blueshifts at low temper-

* Corresponding

author.

0022-2313/97/$17.00 Copyright PII SOO22-2313(96)00101-9

0

1997 Elsevier

defects; Relaxation

ature, proving that the confined states are joined to silicon band gap. The PL behaviour within excitation was studied by many authors [7-lo]. They have observed a PL degradation depending on ambient atmosphere (O,, HZ, Nz, air, etc.), temperature and laser line excitation. In some works, this degradation was attributed to the nonradiative recombination centres in PS surface which results from oxygen adsorption due to laser heating C9,lOl. In our experiments, it has been found that during laser excitation, PL can increase or decrease according to PS porosity and its oxidation degree. In this paper, we report results concerning PL behaviour of freshly prepared and oxidised PS as

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a function of time. The origins and mechanisms governing the PL behaviour are discussed.

2. Experimental procedure

8

PS was prepared by the electrochemical anodisation method at a constant current density in a solution formed by a mixture of HF and deionised water. The substrate used is p-type Si (100) with a resistivity of l-l.5 R cm. Current density and HF concentrations are chosen in order to prepare PS with a porosity of 80% and 40%. Some PS samples of 80% porosity were stored in ambient air for six months so as to obtain highly oxidised PS. PL was measured by a set-up consisting of a “Dilor” triple monochromator, a photomultiplicator with a GaAs photocathode and a photon counter. The 514.5 nm line of a continuous Ar+ laser was used to excite samples.

01

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Time (s) Fig. 1. Time evolution in air, at T = 300 K, of the PL intensity: (a) PS of 80% porosity (the spectrometer is positioned at a frequency of 14 350 l/cm); (b) PS of 40% porosity (the spectrometer is positioned at a frequency of 13 300 l/cm). The laser power is 4 W/cm’.

3. Results and discussion 3. I. Freshly prepared PS Fig. 1 shows time evolution

in air and at

T = 300 K of the PL intensity of two PS samples of

40% and 80% porosity irradiated with a laser power of 4 W/cm’. The spectrometer is fixed at the frequencies 14350 and 13300 l/cm, respectively, for PS of 80% and 40% porosity. These frequencies are closed to the peak positions of PL spectra measured under vacuum (10m6 Torr), where no evolution of the PL peak frequency was observed. After a rapid degradation (Fig. l), the PL intensity increases and tends to stabilise. The period during which the PL intensity stabilises depends on PS porosity. The stability is attained in 500 s for a porosity of 80% and after 2000s for a porosity of 40%. The PL degradation observed at the beginning of the laser excitation has been reported elsewhere [ 111. In order to understand the origin of this PL photodegradation, we have studied the PL behaviour under controlled vacuum (10e6 Torr) at T = 300 K. Within excitation in vacuum, only a PL degradation is observed followed by a stabilisation

a”

=1 6; 2 B ._E

. 4-

(b)

11.

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300

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500

Time (s) Fig. 2. Time evolution of the PL intensity in vacuum (1O-6 Torr): (a) PS of 80% porosity (the spectrometer is positioned at a frequency of 14 350 l/cm); (b) PS of 40% porosity (the spectrometer is positioned at a frequency of 13 300 l/cm). The laser power is 4 W/cm’.

H. El Houichet et al. 1 Journal of Luminescence

1.4

1.5

1.6

1.7

1.8

1.9

2.0

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Energy (eV) Fig. 3. PL spectra for PS of 40% porosity: (a) at the stabilisation of the PL intensity under vacuum (10m6 Torr), T = 300 K; (b) at the stabilisation of the PL intensity in air, T = 300 K. The laser power is 4 W/cm*.

(Fig. 2). At the stabilisation, a short interruption of the laser excitation produces a partial recovery of the PL intensity immediately followed by a degradation. After laser light interruption, the partial recovery of the PL intensity cannot be due to passivation of photocreated Si dangling bonds, since the PS sample is in vacuum. Thereby, apart from the fact that hydrogen desorption occurs at the beginning of the excitation (leading to the creation of Si dangling bonds), one could guess an instantaneous creation of defects, during laser irradiation, which relax during the excitation interruption. Within laser excitation in air (Fig. l), an oxide layer begins to be formed. Indeed, one may remark (Fig. 3) that, for PS of 40% porosity the PL peak energy of the spectrum measured in vacuum (after stabilisation of the PL intensity) blueshifts toward high energy when it was measured in air (after stabilisation of the PL intensity). This blueshift is due to size reduction of the Si nanocrystallites from an oxidation process. Thus, the increase of the PL intensity (Fig. 1) during laser light exposure in air is due to the formation of an oxide layer which decreases at one and the same time the size of the Si crystallites and the nonradiative recombination centres such as photocreated Si dangling bonds (by

71 (1997) 77-82

19

passivating the surface). The reduction of the crystallite size induces a more important confinement of the charge carriers in the PS nanostructures. Most of authors [7-lo] have not observed any PL enhancement when PS is irradiated in air. These contradictory results seem to be related to the various PS preparation and experimental conditions applied. Within time, the formed oxide layer thickness increases, blocking the diffusion of oxygen and consequently the oxidation; then a stabilisation of the PL intensity is observed (Fig. 1). In highly porous silicon, the Si nanocrystallites and the specific surface area are small [12]. Thus the oxide thickness stabilising the PL intensity is rapidly attained. This can explain the fact that, for large PS porosity, PL intensity stabilisation is made faster (Fig. 1). 3.2. Oxidised PS Fig. 4 shows time evolution under vacuum (10m6 Torr) at T = 300 K (Fig. 4(a)) and at T = 20 K (Fig. 4(b)) of the PL intensity for highly oxidised PS when it is exposed to a laser power of 4 W/cm’ and the spectrometer positioned at a frequency of 15000 l/cm (which corresponds to the PL peak frequency under vacuum at T = 300 K). The behaviours at T = 300 K and T = 20 K are similar. However, PL degradation is more important and more rapid at T = 20 K than at T = 300 K. On the other hand, after an interruption of the laser excitation, the PL behaviour at T = 300 K and T = 20 K are quite different. A partial recovery was observed at T = 300 K; the PL tends to recover its initial intensity (t = 0) when the period of interruption increases. At T = 20 K, no notable recovery of the PL intensity was observed even after a relatively long period of interruption. To interpret these results (Figs. 4(a) and (b)), we consider that the PS layer is formed by Si nanocrystallites enveloped in an amorphous SiOZ layer. If PL degradation in vacuum (Fig. 4) only results from photocreated Si dangling bonds due to desorption of hydrogen located at the SiOJSi interface, then degradation should be more important at T = 300 K than at T = 20 K. On the other hand, the partial recovery of the PL intensity in vacuum at T = 300 K, after laser light interruption, cannot

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H. El Houichet et al. /Journal

of Luminescence

,

0.80.6c s

0.40.2-

I

I

h

0

50

100

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250

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800

lOO()

Time(s)

1.0

0.8 0.6 s s 0.4 0.2 0.0 0

200

400

600

Time(s) Fig. 4. Time evolution of the PL intensity for oxidised PS (80% porosity) in vacuum (10m6 Torr): (a) T = 300 K; (b) T = 20 K. The laser power is 4 W/cm2 and the spectrometer is fixed at a frequency of 15 000 l/cm.

be explained by Si dangling bonds effect. This leads to the fact that other photoinduced defects take place at the SiOJSi interface under laser light exposure. These defects would relax during laser light interruption leading to the partial recovery of the PL intensity. Thus, during PL degradation, Si dangling bonds effects due to hydrogen desorption from the SiO@i interface exist, but relaxation related to photoinduced defects should be absolutely

71 (1997) 77-82

taken into account. In the following we call photoinduced defects, those which relax. At T = 300 K (Fig. 4(a)), the partial recovery of the PL intensity after laser light interruption would be due to short relaxation time of these photoinduced defects at this temperature. The PL intensity stabilises when hydrogen stops to be desorbed from the SiOJSi interface and an equilibrium between generation and relaxation of photoinduced defects is attained. However, at T = 20 K, no recovery of the PL intensity was observed. This is probably due to quasifixed photoinduced defects, hence to very long relaxation time at this temperature (no relaxation, the number of these defects becomes important). One would predict that during laser light exposure, there are simultaneously generation and relaxation of photoinduced defects. As relaxation time is much longer at T = 20 I(, so more photoinduced defects exist at this temperature, leading to more important PL degradation as shown in Fig. 4(b). Furthermore, at T = 300 K, one may notice (Fig. 4(a)) that a PL intensity recovery of about 80% was obtained after laser time interruption of 60 s. This would indicate that the photoinduced defects have a larger contribution in PL degradation (of oxidised PS) than hydrogen desorption. This is also confirmed by comparing Figs. 2(a) and 4(a), where PL degradation is respectively about 35% and 60%. Indeed, if PL degradation in highly oxidised PS (Fig. 4(a)) is mainly due to hydrogen desorption, then it should be more important in freshly PS (Fig. 2(a), where hydrogen content is higher. Now, it is the contrary to what has been observed. This is another evidence of the key role of photoinduced defects related relaxation in PL degradation. Now, it would be interesting to show directly the existence of photoinduced defects related relaxation, in vacuum (10d6 Torr) at T = 20 K and T = 300 K, by performing a cycle consisting of varying laser power from 64 W/cm2 (high level) to 16 W/cm2 (low level) and vice versa. The cycle was started from the high-level laser power (64 W/cm’), after PL intensity was made stable. At T = 20 K (Fig. 5(b)), one may observe during laser power variation, solely a decrease or an increase of the PL intensity without any notable phenomenon.

81

H. El Houichet et al. I Journal of Luminescence 71 (1997) 77-82

10 ’

0

1000 1500 2000 2500

500

Time (s)

I 0

I 300

1 600

I 900

I 1200

1 1500

Time (s) Fig. 6. Time evolution of the PL intensity versus laser power, under vacuum (1Om6 Torr) at T = 300 K, for oxidised PS. (a) 4 W/cm*, (b) 16 W/cm2, (c) 40 W/cm’, (d) 56 W/cm2 and (e) 72 W/cm2. The spectrometer is positioned at a frequency of 15000 l/cm.

12 10

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Time (s) Fig. 5. Effect of Laser power versa) on the PL intensity vacuum (10m6 Torr) at T = spectrometer is positioned at

variation (16-64 W/cm’ and vice behaviour of oxidised PS under 300 K (a) and T = 20 K (b). The a frequency of 15 000 l/cm.

However, at T = 300 K (Fig. 5(a)), one may notice, within time, a relaxation phenomenon at every laser power decrease, and a PL enhancement followed by a degradation at every laser power increase. At the start of the experiment (laser power is 64 W/cm’), one may consider that we are in the presence of a given density of photoinduced defects. At T = 300 K, by turning laser power to low level (16 W/cm’), photoinduced defects relax within time, inducing the observed slight PL intensity increase (Fig. 5(a)). When laser power is turned again

to high level (64 W/cm’), one observe a PL intensity enhancement, confirming that during exposure to low laser power (16 W/cm2), defects relaxation occurred. In fact, the PL intensity enhancement due to instantaneous laser power switching from low to high level depends on the time during which photoinduced defects relax (i.e., time exposure at low laser power). Also, the PL intensity degradation following the laser power switch from low to high level is due to instantaneous photocreation of defects. Thereby, when laser power is turned from high to low level, one may observe photoinduced defects relaxation, while when it is turned from low to high level, defects photocreation occurs. This proves that the density of photoinduced defects depends on laser power. One should notice that the process is reproducible since one observes the same PL intensity enhancement when laser power is switched from low to high level; PL intensity is more enhanced when much more defects relax (i.e., long time exposure to low laser power). To sum up, the density of photoinduced defects and their relaxation time depend respectively on laser power and temperature. On the other hand, at T = 300 K, the PL intensity is not proportional to the laser excitation

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power for oxidised PS (Fig. 6). Indeed, starting from a certain laser power, the PL intensity attains saturation. It seems that the densities of photoinduced defects and Si dangling bonds saturate when laser power increases, since PL degradation weakens (Fig. 6). This saturation could be explained by the fact that a part of the incident energy transforms in nonradiative process such as thermic phonon and Auger electron transitions.

of Luminescence

71 (1997) 77-82

Acknowledgement This work is supported by the Secretariat d’Etat a la Recherche Scientifique et a la Technologie (PNM 92).

References Cl1 MS. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber and M.

4. Conclusion In this work, we have pointed out photoinduced defects other than conventional Si dangling bonds, which relax during laser light interruption. Their density and relaxation time depend on laser power and temperature, respectively. It has been shown that low temperature has a tendency to fix the photoinduced defects, while they relax (at least partially) at rather high temperature (7’ = 300 K) when laser power is switched off. These defects are mainly created at the surface for freshly prepared PS and at the SiOJSi interface, formed essentially by amorphous Si, for oxidised PS. The determination of the real nature of these defects and their relaxation mechanism could permit a better control of the PL stability.

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