Photoluminescence from ion-beam cosputtered SiSiO2 thin films

Solid State Communications, Vol. 100, No. 6, pp. 403-406, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 00X-1098/96 $12.00+.00

Pergamon

PII: SOO38-1098(96)00422-X

PHOTOLUMINESCENCE M. Allegrini,” “Dipartimento

FROM ION-BEAM

C. Ciofi,b A. Diligenti,b

COSPUTI’ERED

F. FUSO,~A. Nannini,b

Si/SiO* THIN FILMS

V. Pellegrinid

and G. Pennellib

di Fisica della Materiale

Tecnologie Fisiche Avanzate, Universiti di Messina, Salita Sperone 31, I-98166, Sant’Agata, Italy bDipartimento di Ingegneria dell’Informazione: Elettronica, Informatica, Telecommunicazioni Universita di Pisa, via Diotisalvi 2, I-56126 Pisa, Italy ‘Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica Universita di Pisa, Piazza Torricelli 2, I-56126 Pisa, Italy ‘Scuola Normale Superiore and Istituto Nazionale per la Fisica della Materia, Piazza dei Cavalieri 7, I-56126, Pisa, Italy (Received

30 May 1996; accepted 28 June 1996 by A. Pinczuk)

We report the observation of visible and stable room temperature photoluminescence (PL) from thin composite Si/SiOz films deposited onto various substrates (Si, SiOz, Al and C) by means of an ion-beam sputtering system. Transmission electron microscopy (TEM) observations of the films deposited on carbon substrates reveal the presence of filamentary structures, with nanometric dimensions, of crystalline silicon embedded in an amorphous matrix. While changes of film composition do not influence the main features of the PL, we found that the PL shape and intensity is strongly dependent on the nature of the substrate. The necessary combination of Si and SiOz for producing the light-emitting material and the TEM results suggest that quantum confinement of carriers in the silicon nanocrystals is a possible origin of the observed PL. Copyright 0 1996 Elsevier Science Ltd

In recent years an intense research effort has been devoted to the realization and the experimental analysis of semiconductor nanostructures and quantum dots [l, 21. This interest stems primarily from the possibility of enhancing the material optical properties owing to a very large confinement of carriers and giant excitonic effects. The observation of strong photoluminescence (PL) from porous silicon (PS) [3] and its interpretation on the basis of a quantum confinement model [4, 51 has stimulated a great deal of theoretical and experimental work pursued by the need of all-silicon based optical and optoelectronic devices. Apart from PS, light emission from silicon-based structures has been obtained mainly by ion-implantations [6-81, by molecular-beam epitaxy grown Si/Si02 superlattices [9] and by embedding silicon nanocrystals in amorphous or Si02 matrices. From a technical point of view, silicon nanocrystals can be fabricated by means of various methods: chemical 403

vapor deposition [lo-121, sputtering [13, 141, evaporation [15], laser ablation [16], and aerosol processes [17, 181. In many cases, the correlation between the observed PL and the distribution in sizes of the silicon nanocrystals supported the conclusion of the quantum-confinement nature of the optical recombination process. Here we report the observation of a strong and stable photoluminescence from Si/Si02 composite thin films produced through a technique (namely ion-beam sputtering) originally proposed by some of us a few years ago [19]. As schematically depicted in the inset of Fig. 1, two collimated beams of neutralized argon ions (accelerating voltage = 6 kV) impinge onto the rotating target at a point halfway between the center and the periphery; the substrate is put at a distance of 3.3 cm from the target. The target is made of alternate sectors of SiOZ and crystalline Si. The composition of the deposited films was changed by changing 8,13being the ratio between the angular amplitude of the Si sectors with respect to 2x. If one assumes

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Fig. 1. Photoluminescence spectra under C.W. excitation at 457.9 nm from films deposited on different substrates. In the inset: sketch of the sputtering system.

for Si and SiOZ the same sputtering yield and sticking coefficient, 0 represents the fraction of Si in the film. During samples fabricat@ the deposition rate of both Si and SiOZ was about 15OAih-‘, and consequently the growth of a typical film (thickness of about 5OOA) required at least three hours at a residual pressure in the vacuum chamber of 5 x low5 mbar. All the depositions were performed with the substrate at room temperature. The substratts were silicon oxidized wafers (oxide thickness -5400 A) whose surface was divided in two parts; on one of them an Al film was evaporated before the Si/SiOZ sputtering, to provide an electrical contact for conductivity measurements across the Si/SiOZ film. In addition, a standard holy carbon grid (diameter = 3 mm) was fixed on the substrate to allow a subsequent transmission electron microscopy (TEM) analysis. In this way, from the same deposition run three films on different substrates, with different nucleation properties, were obtained. The optical analysis was performed exploiting both continuous wave (c.w.) and pulsed laser excitation, provided by an Arf laser (457.9nm single-line emission, power = 100 mW) and a XeCl excimer laser (wavelength 308 nm, pulse duration ~16 ns FWHM, pulse energy ~0.3 mJ), respectively. The use of the pulsed excitation scheme allowed us to get informations on the dynamical properties of the PL emission. On the other hand, the UV excitation provided by the excimer laser enabled PL investigations in the blue region of the spectrum. The PL emission was collected in a backscattering geometry and analyzed by a monochromator (Jobin-Yvon Ramanor U 1000, and HR640, for C.W. and pulsed excitation experiments, respectively). PL detection in C.W. excitation was accomplished by a GaAs photomultiplier, whereas in pulsed excitation experiments an intensified optical multichannel analyzer (OMA PAR IRY lOOO/BRG), with adjustable gate width, rR, and gate delay, Td, with respect

to the arrival of the laser pulse on the sample, was employed. Figure 1 shows typical results obtained for films deposited on SiOZ, Al and C under C.W. excitation at X = 457.9 nm. In this case the ratio f3 was 0.5. The spectrum of the film deposited onto Si02 shows two distinct peaks at 5100 and 63OOA (emitted photon energy ~2.4 and ~1.9 eV, respectively). As can be noted, the shape and the maximum intensity of the spectra depend on the nature of the substrate. After the optical analysis, in order to perform the electrical characterization, Al dots (diameter = 2 mm) were evaporated onto the film deposited onto Al, obtaining Al/composite material/Al structures. The d.c. I-V measurements, performed with a semiconductor parameter analyzer, both across and along the film plane (in this last case by contacting two adjacent dots) showed that the film was insulating, at least at room temperature. The films deposited on Si02 were subjected to an annealing process (900 “C for 1 h in N2 atmosphere) to check whether it was possible to modify their transport properties. While the insulating behavior of the films was not strongly modified by the annealing, we did observe heavy modifications of the optical spectra, as shown in Fig. 2: the higher energy peak disappeared after the annealing and the other one shifted to a lower energy. Figure 3 displays changes in the optical spectra as a function of film composition (0.28 5 8 5 0.61) for films deposited onto Si02 substrates. Results do not show any correlation between the optical emission and 8; apart from the intensity of the low-energy emission for 0 = 0.39, the shape and intensity of the spectra are almost unchanged. Similar results have been observed for films deposited on Al. The PL stability as a function of time on a long time scale was also investigated. PL measurements performed

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Energy (meV) Fig. 2. Photoluminescence spectra under C.W. excitation for films deposited onto Si02 with different postdeposition processes.

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Energy (meV) Fig. 3. Photoluminescence spectra under C.W. excitation for different compositions of the Si/SiOz film: 8 is the silicon fraction of the target. on films stored in air at room temperature for 30 days did not reveal any detectable change in the emission spectra. Besides the PL bands observed under C.W. excitation at =1.9eV and =2.4eV, strong PL emission was also present in the blue range, peaked around 2.7-2.9eV. Figure 4 shows the PL spectrum (solid line) of a sample deposited onto SiO;! with 19= 0.5, obtained under pulsed at X = 308nm (photon energy laser excitation hv = 4 eV) and using Td = 5 ns and rg = 10 ps. Spectra acquired at different 7d and rg settings suggest that this blue band has a relatively fast dynamics with typical time scale of -100 ns. A rough estimation of the PL efficiency was derived by measuring the intensity ratio of the PL emission and of the excitation radiation. Taking into account the different photon energy for emission and excitation, we found efficiency for the blue-band of 8-12% under

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Energy (meV) Fig. 4. Comparison between photoluminescence spectra from a porous silicon sample (dotted line) and the composite material (solid line) under pulsed laser excitation at 308nm (pulse duration of 16ns, pulse energy 0.3 m.l).

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pulsed excitation. The large uncertainty in the avaluation of the collection region of the apparatus, entering the determination of the efficiency, prevented us to derive a more accurate value. In order to confirm this result, we carried out a comparison of the blue PL emission of thin films with that of conventional PS layers. For these samples PL efficiencies are typically of the order of few %. PS samples were produced as follows: (i) starting material: n+ wafers (ND = 5 x 1018 cmp3); (ii) anodization in an electrochemical cell (Si as anode and Pt as cathode) filled with a solution of HF (50%) and ethanol (502) at a current density of 3.7 x 10-3-1.5 x lo-’ A/ under illumination of a 300 W tungsten lamp Fistance between the lamp and the Si wafer = 25 cm); (iii) anodization time: 15 min; (iv) rinsing in deionized water [20]. A representative PL spectrum of a PS sample is shown in Fig. 4 (dashed line). While the two red bands of the thin Si/Si02 film have emission intensities (and therefore PL efficiencies) less than those of PS samples excited in the same experimental conditions, the peak intensity of the blue-emission band reported in Fig. 4 is much larger for the composite thin film. It must be noted that the thickness is very different in tht two cases: some tens of microns for PS and only 500A for the Si/Si02 films. This fact provides a further strengthening of the PL efficiency in thin film composite films. A last important question regards the origin of the observed emissions in the Si/SiO* composite thin films. Since it is known from the literature that PL can occur also in amorphous silicon [21] (or in Si02 [16]), several tests were carried out in order to relate the observed PL with the necessary combination of both Si and SiO*. To this end, Si alone was sputtered on crystalline silicon and SiOZ substrates; the same was done for SiO,? alone. The PL of the resulting films was investigated by means of the procedure described before. The result was that no detectable PL was found in the samples made of Si or SiOz alone on either Si or Si02 substrates. A microstructural analysis of the films was carried out by TEM observations on films deposited on the holy carbon grids. The structure of the films appeared to be amorphous but for the presence of small clusters containing crystalline silicon filaments. Some of these clusters resembled tangled skeins of twisted crystalline silicon filaments and had an extension of a few hundreds of nm. In some other cases, the clusters had the characteristic shape shown in Fig. 5, where a bent crystalline wire encircled an amorphous egg-shaped particle. In both cases, and in all the observed films, the crystalline filaments had a thickness ranging from less than 20nm up to 60 nm and a variable length, up to several hundreds of nm. The density of the clusters was rather low in all the observed films. Typically, only a few clusters could be found in an area of 10 pm*.

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and on various substrates, and to verify the above presented hypothesis on the PL origin. Finally, the material and the technique are compatible with the integrated circuit technology. Films with SiOz and other insulating materials are being investigated and attempts are being made to obtain conductive electroluminescent films.

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11. 12. 13. 14. 15. Fig. 5. Transmission electron microscopy images of crystalline silicon twisted filaments (a) and egg-shaped particles (b).

16.

17. In conclusion, a composite Si/SiO* photoluminescent material has been deposited by means of a rather simple technique, an Ar ion-beam sputtering, and the critical influence of the substrate on the photoluminescence has been shown. TEM observations of crystalline silicon structures suggest that PL could be produced by them. In any case, both Si and SiO;? must be present to obtain a photoluminescent film. Further work is in progress to characterize by TEM analysis the nanostructure of the films produced under different experimental conditions

18. 19. 20.

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