Emission mechanisms in stabilized iron-passivated porous silicon: Temperature and laser power dependences

Physica B 407 (2012) 472–476

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Emission mechanisms in stabilized iron-passivated porous silicon: Temperature and laser power dependences M. Rahmani a, A. Moadhen a,n, A. Mabrouk Kamkoum a, M.-A. Zaı¨bi a,b, R. Chtourou c, L. Haji d, M. Oueslati a a

Unite´ de Spectroscopie Raman, Faculte´ des Sciences de Tunis, Universite´ de Tunis El Manar, 2092 El Manar, Tunis, Tunisia Ecole Supe´rieure des Sciences et Techniques de Tunis, Universite´ de Tunis, 5 Av. Taha Hussein, 1008 Montfleury, Tunis, Tunisia c Laboratoire de Photovoltaı¨que et de Semiconducteurs, Centre de Recherche et de Technologie de l’Energie, BP 95, Hammam-Lif 2050, Tunisia d Universite´ Europe´enne de Bretagne, CNRS FOTON-UMR 6082, 6 rue de Ke´rampont, BP 80518, 22305 Lannion Cedex, France b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 June 2011 Received in revised form 11 November 2011 Accepted 12 November 2011 Available online 18 November 2011

Photoluminescence (PL) measurements of porous silicon (PS) and iron-porous silicon nanocomposites (PS/Fe) with stable optical properties versus temperature and laser power density have been investigated. The presence of iron in PS matrix is confirmed by Raman spectroscopy. The PL intensity of PS and PS/Fe increases at low temperature, the evolution of integrated PL intensity follows the modified Arrhenius model. The incorporation of iron in PS matrix reduces the activation energy traducing the existence of shallow levels related to iron atoms. Also, the temperature dependence of the porous silicon PL peak position follows a linear evolution at high temperature and a quadratic one at low temperature. Such evolution is due to the thermal carriers’ redistribution and an energy transfer. Similarly, we have compared the laser power dependence of the PL in PS and PS/Fe layers. The results prove that the recombination process in PS is realised through the lower energy traps localised in the electronic gap. However, the observed emission in PS/Fe is essentially due to direct transitions. So, we can conclude that the presence of iron in PS matrix induces a strong modification of the PL mechanisms. & 2011 Elsevier B.V. All rights reserved.

Keywords: Porous silicon Iron Photoluminescence Energy traps

1. Introduction Porous silicon (PS), obtained by electrochemical etching in HF solution, has optical properties that make it an interesting material for various applications in optoelectronic devices like light emitting diodes (LED). This material is still investigated as some means of improving the light emission properties of silicon [1]. However, PS suffers from instability problem due to its large specific surface [2,3]. To resolve this problem it is necessary to modify the PS surface structure. In our last work [4], we have found that iron can be easily introduced inside the silicon pores by the impregnation method. Then, an enhancement and a stabilisation of the PS photoluminescence (PL) has been found, we attributed this result to the large ironpassivation of Si nanocrystallites (nc-Si). Many research efforts have investigated the optical properties of iron-porous silicon nanocomposites (PS/Fe) [4–6]. Nevertheless, in most of these works, the effect of temperature and laser irradiation on the PL behaviour has been ignored. Le´tant and Vial [7] have investigated the luminescence versus temperature of fresh and oxidised porous silicon layers under different atmospheres. They have shown a modification of the continuous and pulsed PL versus temperature for fresh porous

n

Corresponding author. Te´l.: þ21671872600; fax: þ 21671885073. E-mail address: [email protected] (A. Moadhen).

0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.11.018

silicon layer compared to oxidised ones. The interpretation of these results is based on the energy transfer from carrier’s photocreated in the crystallites to Si–H vibrations on the surface. In another work, the origin of emission from PS is studied by Zou et al. [8]. The authors have examined the temperature dependent photoluminescence spectra of fresh and six month-aged PS, they found that aging and oxidation of the PS surface led to a blueshift in the excitation spectrum and the PL intensity decreases with increasing temperature. From those results, Zou et al. proposed a model in which the emission occurs from excitons trapped by surface states, whose nature is dependent on the chemical composition on the surface. The present paper reported on the temperature and the laser power dependences of the PL from PS and PS/Fe nanocomposites. The aim of this work is to establish the mechanisms of emission in iron-passivated porous silicon in order to fully characterize and optimise this material in the pursuit of obtaining novel optoelectronic devices.

2. Experimental The starting material was a boron-doped p-type Si (1 0 0) substrate having 1–4 O cm resistivity. The PS layers were formed using electrochemical anodisation in a solution of HF (40%)/C2H5OH/H2O

M. Rahmani et al. / Physica B 407 (2012) 472–476

3. Results and discussion Under temperature variation, the PL behaviour is dependent on the chemical composition on the surface and is related to the storing time of the PS [8]. We notice that the two types of sample analysed in this work are four months aged, so we believed that they have stable optical properties [4]. Furthermore, we point out the fact that they were prepared in the same conditions. Fig. 1 presents Raman spectra of PS and PS/Fe nanocomposites. Three prominent features are depicted from PS spectrum. The main longitudinal optical phonon mode (LO) of Si at 520 cm  1 shows a broadening, in around 480–500 cm  1, and an asymmetric shape. These observations are related to amorphous Si [9]. In fact, our PS is four months aged: then, the oxidation induces the amorphization of Si nanocrystallites (nc-Si). As well as the LO peak, two smaller broad peaks are recorded at 624 cm  1 and 300 cm  1. These peaks are already reported by some authors [10–13]. The peak at 624 cm  1 is due to multiphonon processes (TA þLO), which appears when increasing the surface of PS relative to the volume. The second broad peak at 300 cm  1 is attributed to multiphonon process (2TA) [10–13]. The Raman spectrum of PS/Fe shows the LO phonon mode, which is less broadened and less intense than that in PS indicating that the most of nc-Si are well protected by iron against oxidation. Nevertheless, two new Raman bands appeared at 211 cm  1 and 278 cm  1. It has reported that Raman spectroscopy measurements of semiconducting FeSi2 layers on silicon substrates show

many Raman shifts between 150 cm  1 and 550 cm  1 and which are assigned to Raman-active modes of b-FeSi2 [14–16]. For example, Lekfi et al. [15] have investigated theoretically and experimentally the different active modes of b-FeSi2 in infrared (IR) reflectivity and Raman spectroscopy. These authors have reported that Raman and IR peaks do not occur at the same frequencies because the b-FeSi2 has a symmetric centre and then the group theory forbids the IR-active mode to be Raman-active mode and conversely. Accordingly, the two bands observed in our present study can be also attributed to b-FeSi2 Raman-active modes in PS/Fe. The positions of the two peaks are higher than those of other reports [14–16]. This fact may be understood to the oscillator strength of the Si–Fe bonds, leading in our case to a blueshifted Raman lines. Fig. 2(a) gives the temperature-dependent photoluminescence (PL) of PS layer. The temperature increase induces a decrease of the emission intensity and slightly redshift ( E40 meV) of the maximum of the PL band. The PL intensity behaviour is due to the reduction, at low temperature, of the extension of carrier’s wave function, which lowered the probability of nonradiative recombination [1]. The aging PS surface contains different chemical bonds such as Si–O–H and O–Si–O. Any chemical bond induces a trapping state in the electronic gap and a possible energy transfer between the different states is produced under the temperature effect [8,17]. In our case, the presence of oxygen, hydroxyl and

100

1.873 eV

60

40

20 1.831 eV 0 1.6

1.8 Energy (eV)

PL intensity (arb. unit.)

Raman intensity (arb. unit.)

16000

624 cm-1

(a) 300 cm-1

8K 30 K 50 K 100 K 140 K 180 K 220 K 260 K 300 K

80 PL intensity (arb. unit.)

(2:1:1). The current density was 10 mA/cm2 and the etching duration was equal to 10 min. Then, the PS layer was impregnated, during 10 min, in 0.3 M Fe(NO3)3 aqueous solution, maintained at room temperature. The solution solubility was total, i.e. no iron clusters were in suspension. The impregnation was applied without any stirring of the solution in order to avoid any turbulence effect during the reaction. To eliminate the residual molecules into PS, the samples were washed in de-ionised water and were dried by nitrogen gas. This type of sample is labelled PS/Fe. Laser Raman spectra of the samples were recorded at room temperature using a micro-Raman spectrometer (Jobin-Yvon confocal micro-Raman T64000) with a resolution 0.5 cm  1 and the recording time was set equal to 60 s. A triple monochromator and a GaAs photomultiplier were used to record the PL spectra. The excitation source used in this work is an Ar þ laser (488 nm). The experiments were carried out in ambient air. The samples were mounted in a helium cryostat for temperaturedependent studies in the range 8–300 K.

473

2.0

2.2

8K 30 K 100 K 140 K 180 K 220 K 260 K 300 K

1.727 eV

12000

8000

4000 211

cm-1

278 cm-1

(b)

0 200

400 Raman shift (cm-1)

600

Fig. 1. Raman spectra of PS and PS/Fe.

800

1.4

1.6

1.8 Energy (eV)

2.0

Fig. 2. Temperature dependence photoluminescence spectra of PS (a) and PS/Fe (b).

M. Rahmani et al. / Physica B 407 (2012) 472–476

2a EðTÞ ¼ Eð0Þ y=T e 1

ð1Þ

E(0) is an energy at T¼0 K, a is an empirical parameter representing the coupling of excitons with the phonon and y is an effective temperature, which takes into account the average energy of phonons. The best values of a and y, with their corresponding uncertainties, are reported in Fig. 3. Besides on PS/Fe, the PL peak position is unchanged with increasing temperature. In Fig. 4 the FWHM variation of the PL curves of PS and PS/Fe with increasing temperature is reported. The variation corresponding to PS/Fe is more important than that of PS, which is quite constant. For PS/Fe sample, the FWHM of the PL curves increases linearly from 8 K up to 100 K, then it decreases exponentially. Some authors have examined the evolution of the FWHM of the PL spectra, versus temperature, for the confined excitons in ZnSe/ZnS quantum dots (QD) or in InAs/GaAs quantum dots [20,21]. They have reported an opposite behaviour to our case in PS/Fe. At low temperature, the authors have attributed the reduction of the FWHM to the population of large QDs by thermal carriers, but at high temperature the electron–phonon scattering increases then the FWHM of the PL spectrum grows.

1.0

FWHM (eV)

defects in aging PS surface induces energy levels in PS electronic gap and some energy transfers are produced between these levels. Hence, the rate of energy transfer to lower energy traps increase slightly with increasing temperature from 100 to 300 K, which explain the redshift of the emission maximum. Regarding the PS/Fe, the shape of the PL and its maximum emission does not present a significant shift with increasing temperature (Fig. 2b). The deposition of iron in PS changes the surface termination, it results in a change of the recombination process after the elimination of the most defects and consequently the disappearance of energy transfer from carriers localised in the electronic gap. However, the PL intensity decreases from 8 to 300 K, indicating the increasing of the ratio of nonradiative processes due to coupling with phonons. These results of PS/Fe confirm our previous studies [4,18] and prove that the deposition of iron, by the impregnation method, passivates the nc-Si and induces an improvement of PS optical properties. The effect of the temperature variation on the PL peak position of porous silicon layer is presented in Fig. 3. The results show a linear dependence of the PL peak position at high temperature (T4150 K) and a quadratic one at low temperature, which suggest a thermal carriers redistribution and an energy transfer occurrence. Adding that the PL peak position dependence on temperature is well described by Bose–Einstein equation [19]:

0.6

0.4

0

50

100 150 200 Temperature (K)

250

300

Fig. 4. The FWHM evolution of PS and PS/Fe versus temperature. The solid lines are to guide the eyes.

2600

2400

PS/Fe

2200

2000 PS 1800

1600 0

200

400

600 800 1000 1/(KBT) (eV)

1200

1400

Fig. 5. Integrated PL intensity versus inverse temperature of PS and PS/Fe. The solid lines are the best fit using Eq. (2).

However, at our knowledge, the variation of the FWHM versus temperature has not been studied in the case of PS. In fact, the FWHM of the PL spectrum of PS traduces the gap distribution [1] and is related to carrier’s population. In our previous work, we have found that the presence of iron in porous silicon matrix induces two shallow levels localised in the PS electronic gap [22]. Basing on these results, we can deduce that the carriers in PS/Fe populate the nc-Si and the shallow levels correspond to Fe atoms at temperatures lower than 100 K. Then at high temperature, the FWHM of the PL spectrum is related only to the nc-Si. Fig. 5 exhibits the temperature variation of the integrated PL intensity (IPL) of PS and PS/Fe. The theoretical model of Arrhenius [23] does not fit correctly the experimental data, then, we conducted to fit the temperature variation of the integrated PL intensity with two nonradiative thermal activation energies, using the following equation [23]: IPL ðTÞ ¼

Fig. 3. Dependence on temperature of porous silicon PL peak position. The solid line correspond to the best fit using Eq. (1).

PS/Fe

0.8

PS

Integrated PL intensity (arb. unit.)

474

I0 1 þa1 expðEa1 =K B TÞ þ a2 expðEa2 =K B TÞ

ð2Þ

I0 is the integrated intensity of PL at 0 K, KB Boltzmann constant, a1 and a2 are the process rate parameter, Ea1 is the activation energy dominant at lower temperature (T r 100 K) and Ea2 is the activation energy at upper temperature range. Table 1

M. Rahmani et al. / Physica B 407 (2012) 472–476

PL intensity (arb. unit.)

96

a1

Ea1 (eV)

a2

Ea2 (eV)

1965.47754 2455.64549

0.0066 0.10147

0.00181 0.00945

619.97368 11.87471

0.20249 0.10368

PS/Fe 920 mW

80 720 mW

PL intensity (arb. unit.)

PS PS/Fe

I0

24 PS 20

720 mW

16

360 mW

12

x2 x2 x3

8 4 1.5

64

960 mW

200 mW 40 mW 1.78 eV

1.7 1.9 Energy (eV)

x2

105

m1 = 0.41 m = 0.94 PS/Fe

104

200 mW

λExcitation: 488nm

x3

32

PS

2.1

360 mW

48

m2 = 0.72 Integrated PL intensity (arb. unit.)

Table 1 Values of the parameters a1, Ea1, a2 and Ea2 obtained by fitting the experimental data versus temperature to Eq. (2).

40 mW

475

102 Power density of laser irradiation (W.cm-2)

103

Fig. 7. Integrated PL intensity of PS and PS/Fe as function of the power density of laser irradiation. The solid lines give the corresponding fit.

Temperature: 300K

x3

16

1.74 eV

1.4

1.6

1.8 Energy (eV)

2.0

2.2

Fig. 6. Photoluminescence spectra at room temperature of iron-passivated porous silicon for different laser powers. The inset shows the PL intensity of PS following laser power.

The laser excitation leads to the excitons formation in the nc-Si of the two layers (PS and PS/Fe). But in PS and from 180 K, the excitons are trapped by energy levels. At temperature more than 180 K, the majority of transitions in PS go through an intermediate states localised in the electronic gap. However, in PS/Fe the most dominant transitions are the direct ones.

4. Conclusion summarises the best values of fitting parameters. We found that either PS or PS/Fe, Ea1 is smaller than Ea2, which is in agreement with literature [23]. The presence of iron in PS matrix reduces Ea2 traducing the existence of shallow levels related to iron atoms, such levels were highlighted by time-resolved photoluminescence and electrical studies [4,18]. To confirm the previous interpretations (identification of the underlying recombination process), laser power-dependent PL measurements were carried out. Fig. 6 illustrates the PL spectra of ironpassivated porous silicon layer for different laser powers and the inset shows the evolution of PL intensity of PS with laser power. All spectra are recorded at room temperature. From these evolutions, we notice that the PL peak position of PS and PS/Fe does not present a significant shift. So, we can deduce that the laser irradiation does not affect the structure of the porous layer. Nevertheless, the PL intensity of PS and PS/Fe increases with laser power. The variation of integrated PL intensity (IPL) with the power density of laser irradiation is shown in Fig. 7. As the power density is increased the IPL of PS and PS/Fe increases but differently. We have demonstrated in our last work [4] that the Si–Hn bonds are substituted by Si-(iron-oxide) ones. This change of surface bonds can explain the difference of the PL behaviour versus laser power density. The experimental curves in Fig. 7 are fitted by a simple power law IPL ¼Pm, where P is the power density of the laser excitation and m is an exponent that indicates the recombination type [24]. In the case of PS, m is 0.41 and 0.72 at low and high power, respectively, while the iron-passivated porous silicon is characterised by a single slope; m is 0.94. For PS, the m values are much lower than unity indicating that the recombination process in PS through the lower energy traps localised in the electronic gap without neglecting the presence of nonradiative centres, especially for low power (Po360 mW). However in the case of PS/Fe, the m value is close to unity then the emission in this layer is essentially due to direct transitions. Hence, the laser powerdependent PL study confirms the results found by the temperature investigations.

The PL spectra of PS and iron-passivated PS recorded at different temperatures and laser powers are examined and compared. Energy traps, related essentially to impurities, are localised in PS electronic gap and an energy transfer to these levels can occur. The presence of iron in PS matrix minimises and compensates these levels and a possible carrier’s transfer can occur versus temperature. The laser power dependent PL measurements prove that the radiative transitions in PS proceed through the lower energy traps localised in the electronic gap, while in PS/Fe nanocomposites, the majority of transitions are a direct interband ones. These observations indicate that the incorporation of iron in PS matrix modify the recombination mechanisms, so this study proves that surface chemistry plays an important role in forming a species that give rise to the visible emission of PS and PS/Fe.

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