Germanium nanoclusters in silica thin films

Materials Science and Engineering B69 – 70 (2000) 468 – 473 www.elsevier.com/locate/mseb

Germanium nanoclusters in silica thin films M. Stoiber a,*, S. Schiestel b, C.A. Carosella b, R.M. Stroud b, K.S. Grabowski b a

The Uni6ersity of Leoben, Franz-Josef Strasse 18, A-8700 Leoben, Austria b Na6al Research Laboratory, Washington, DC, USA

Abstract We have characterized the properties of Ge nanoclusters in silica films. The films are grown by physical vapor deposition (PVD) or ion-beam-assisted deposition (IBAD): by co-deposition of Ge and SiO2 with and without the presence of an argon ion beam. The IBAD process affects the development and ultimate morphology of the nanoclusters. Rutherford backscattering (RBS) and index of refraction measurements give the volume fraction of Ge nanoclusters in the silica films and quantify the effects of sputtering on the ultimate film composition. At a Ge/SiO2 arrival rate greater than or equal to 0.8, SiO2 is preferentially sputtered; at lower arrival rates Ge is preferentially sputtered. Absorption measurements are used to deduce the effective band gap of the Ge nanoclusters and the growth of the nanoclusters with annealing. X-ray diffraction studies and transmission electron microscopy confirm that the IBAD processing accelerates the growth of the Ge nanoclusters at low annealing temperatures and inhibits the ultimate size of the nanoclusters, as compared to clusters in those films grown by PVD. Photoluminescence of the Ge-silica films is examined for the IBAD and PVD cases. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Germanium nanoclusters; Silica thin films; Physical vapor deposition; Ion-beam-assisted deposition

1. Introduction

2. Experimental

Ion-beam-assisted deposition (IBAD) is a technique that has gained a foothold in the optical film industry because it produces films that are denser and more uniform than evaporated or sputtered films. We have explored the use of IBAD to produce nanocomposite structures [1]. Both metal and semiconductor nanoclusters (nc’s) have been produced by IBAD [2,3]. The goal is to make nc’s that are uniform in size and density. Sizes from 1 to 20 nm are interesting, so that one can examine the effects of quantum confinement on, for example, photoluminescence (PL) and non-linear optical properties. In this paper, we examine the production of Ge nc’s in silica thin films prepared by IBAD and physical vapor deposition (PVD). Films are characterized by various optical techniques, by X-ray diffraction (XRD), by Rutherford backscattering (RBS) and by transmission electron microscopy (TEM). The effects of film preparation and annealing on PL are also illustrated.

Ge nanocluster containing thin silica films were deposited in an ultrahigh vacuum chamber (5·10 − 9 torr) both by electron beam evaporation of silica and Ge (PVD) and simultaneous ion bombardment (IBAD). The Ge/SiO2 arrival rate was varied between 0.2 and 1.6. An argon ion beam was used for the IBAD films with an energy of 100 eV and a current of 35 mA/cm2. Films were annealed in vacuum or in a H2/He mixture. Reflectivity and transmissivity data were taken with an optical spectrophotometer. These measurements, along with film thickness measurements, were employed to find the index of refraction and the absorption coefficient of the films. A Rigaku rotating anode X-ray source was used to detect the presence of crystalline Ge in the films. RBS quantified the elemental composition of the films, and TEM identified the size and size distribution of the Ge nc’s in a limited number of samples. Finally, PL was excited with an Ar+-ion laser at 458 nm and measured with a liquid nitrogen cooled CCD detector array.

* Corresponding author. E-mail address: [email protected] (M. Stoiber)

0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 4 1 3 - 4

M. Stoiber et al. / Materials Science and Engineering B69–70 (2000) 468–473

3. Optical characterization Fig. 1 presents the refractive indices of the films for the various Ge/SiO2 arrival rates, with and without an ion beam. The measurements will lie between the index of Ge (4.6) and that of silica ( 1.47). Volume fractions of Ge in the films are calculated using the data of Fig. 1 and Maxwell – Garnet (MG) theory oeff =om

1+ 2·fy ·(oe −om)/(oe +2·om) 1− fy ·(oe −om)/(oe +2·om)

neff =(oeff)1/2

(1)

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where neff and oeff are the index of refraction and the dielectric constant of the mixture, om the dielectric constant of the matrix; oe the dielectric constant of the cluster element; and fn the volume fraction of the clusters [4]. Fig. 1 clearly shows the trend to higher indices of refraction for the IBAD films compared to the PVD films, when the Ge/SiO2 arrival rate is greater than or equal to 0.8. There is also a slight decrease in index of refraction for the 0.2 and 0.4 arrival rate films when the same comparison is made. The effect is due to arrival rate dependent sputtering effects in the thin films; for low arrival rates Ge is preferentially removed,

Fig. 1. Refractive indices for (a) PVD and (b) IBAD films with various Ge/SiO2 arrival rates. The preferential sputtering of silica at an arrival rate ] 0.8 increases the indices for the IBAD films.

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Table 1 A comparison of the Ge volume fraction, fn calculated from MG theory using the data of Fig. 1, and the results obtained by RBS Ge/SiO2 arrival rate

0.2

0.4

0.8

1.2

1.6

MG Ge fn IBAD PVD

0.046 0.070

0.10 0.12

0.21 0.20

0.33 0.28

0.39 0.31

RBS Ge fn IBAD PVD

0.068 0.068

0.10 0.10

0.24 0.16

0.48 0.25

0.54 0.41

for high arrival rates SiO2 is preferentially removed. Since an arrival rate of 0.8 is close to the Ge percola-

tion value, we surmize that Ge atom bonding causes the observed arrival rate-dependent sputtering. Volume fractions of Ge, calculated as described above, are compared with volume fractions measured with RBS in Table 1. Since the MG approximation is only good for small volume fractions, we do not expect the calculations to agree with measurements at the higher volume fractions. Optical spectrometry is used to measure the absorption coefficient of the Ge-silica thin films. Theory predicts that the energy gap (Eg) for Ge should become larger as the nc’s become smaller, due to quantum confinement effects [5]. The energy gap of the films is found by the so-called Tauc’s plot technique, that uti-

Fig. 2. The shift of energy gap with annealing for (a) the PVD and (b) IBAD films with various Ge/SiO2 arrival rates. The change of energy gap with annealing temperature for a given film indicates that the cluster sizes are changing.

M. Stoiber et al. / Materials Science and Engineering B69–70 (2000) 468–473

Fig. 3. The % crystallinity of the Ge in the Ge/SiO2 films for different arrival rates and annealing temperatures. The IBAD processing accelerates the Ge amorphous-to-crystalline transition at low annealing temperatures. The large uncertainty of the 1.6 ib annealed at 900°C is due to film delamination.

lizes the relationship between absorption coefficient a and Eg [6]. a·'v = c·('v − Eg)2

(2)

The data of Eg for the Ge-silica films are shown in Fig. 2. The films with the highest arrival rates have an Eg close to bulk Ge, 0.67 eV. The negative slope with increasing annealing temperature suggests particle growth. The data indicates that the IBAD film Ge nc’s do not grow as much as the PVD film Ge nc’s.

3.1. XRD and TEM results The films are amorphous in an as-deposited state, which was confirmed with XRD. After annealing, however, diffraction peaks from Ge nc’s began to appear. Integrated line intensity values of the diffraction peaks were used for the analysis of the relative amount of crystallinity in the various films. (The intensity of the diffraction peaks is first normalized for film thickness and Ge concentration.) The expression ln

n

I sin2u = − 2B· 2 +ln K I0 l

(3)

is used. Here I and I0 are the intensities of a particular Ge diffraction peak (I = I0 for 100% crystallinity). B is a radiation induced Debye – Waller factor; u and l are the diffraction angle and the X-ray wavelength; and K is a proportionality constant. Reflections from the three strongest diffraction peaks of Ge, the (111), (220) and (311) planes were plotted. Eq. (3) has the form of a straight line if ln(I/I0) is plotted versus sin2u/l 2. Such a plot yields the amount of amorphous Ge in the film; this is the so-called modified Wilson plot [7]. Fig. 3 summarizes the results from the Wilson plots. The samples prepared with the higher Ge contents were

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chosen for these analyses because of the stronger peak intensities. The PVD sample with Ge/SiO2 of 1.6, annealed at 900°C, had the highest intensity and was assumed to have 100% Ge crystallinity. A clear pattern emerges from the data; the IBAD promotes early onset of crystallization at relatively low annealing temperatures. For example, for Ge/SiO2 = 1.2, the Ge in the IBAD film is 18% crystallized at 700°C, while the Ge in the PVD film is still amorphous. Recent XRD work on smaller clusters has shown that the Ge nc’s are much larger in the IBAD compared to the PVD films when Ge/SiO2 = 0.8 [8]. This suggests that the effect of higher crystallinity in the IBAD may also result from percolation of the Ge atoms, as was previously described. TEM on samples with both low (Ge/SiO2 =0.4) and high (Ge/SiO2 = 1.6) arrival rates reveal pictorially the size and the size distribution of the nc’s. At an arrival rate of 0.4 and a 900°C anneal, the IBAD film shows a high density of Ge nc’s of 3.89 0.5 nm; the PVD film has much larger nc’s with a broad size distribution (1396 nm). We have recently confirmed these results with XRD [8]. At an arrival rate of 1.6, TEM shows huge nc’s, with sizes ranging from 50 to 100 nm; both PVD and IBAD produce these large particles. But once again the IBAD nc’s are more uniform as well as more densely packed.

4. Photoluminescence results PL was examined for the Ge-silica films to look for process-dependent effects. The peaks were generally very broad. We can identify peaks that are probably associated with defects in the films as well as peaks possibly associated with the nc’s. Other workers have tried to sort out the origin of the PL peaks in the Ge-silica system [9]. Fig. 4 shows one set of data for the PL of the Ge/SiO2 = 1.2 films, for IBAD and PVD. We often see in the IBAD films a large peak at about 650 nm after vacuum annealing. Subsequent annealing in an H2/He mixture removes this peak and leaves behind a peak at shorter wavelengths. We associate the large peak in the IBAD films with a defect structure, possibly a nonbridging oxygen hole centre [3]. The PL of the PVD film shows an interesting variation of peak position with annealing. The shift in peak position with annealing temperature is consistent with the growth in size of the nc’s, although no independent measurements of nc-sizes were made at this time.

5. Conclusions IBAD alters the size and size distribution of Ge nc’s in the Ge-silica films when compared to PVD. There is

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M. Stoiber et al. / Materials Science and Engineering B69–70 (2000) 468–473

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Fig. 4. The photoluminescence from (a) IBAD and (b) PVD films with a Ge/SiO2 arrival rate of 1.2, at different annealing temperatures. The large peak in (a) at 650 nm is thought to be associated with a defect center in the silica.

overwhelming evidence that arrival-rate dependent sputtering plays a key role in the ultimate concentration of Ge nc’s in the IBAD films. The onset of Ge percolation may account for the observed sputtering phenomena as well as for the relatively low temperature of Ge crystallization for the IBAD films. We have seen evidence of quantum confinement effects for the Ge nc’s, both in the shift of the energy gap, Eg and the shift of the peak of PL with annealing. TEM has given direct evidence that IBAD can produce smaller, more uniformly distributed Ge nc’s.

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