Superlattices and Microstructures 45 (2009) 343–348

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Superlattices and Microstructures

journal homepage: www.elsevier.com/locate/superlattices

Formation of Ge nanocrystals and evolution of the oxidematrix in as-deposited and annealed LPCVD SiGeO filmsA. Rodríguez a,∗, B. Morana a, J. Sangrador a, T. Rodríguez a, A. Kling b,M.I. Ortiz c, C. Ballesteros ca Dpto. Tecnología Electrónica, E.T.S.I. de Telecomunicación, Universidad Politécnica de Madrid, 28040 Madrid, Spainb Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugalc Dpto. Física, E.P.S., Universidad Carlos III, 28911 Leganés (Madrid), Spain

a r t i c l e i n f o

Article history:Available online 9 December 2008

Keywords:Semiconductor nanocrystalsDielectric matrixLPCVDTransmission electron microscopyFourier-transform infrared spectroscopy

a b s t r a c t

SiGeO films were deposited by LPCVD using Si2H6, GeH4 and O2as reactive gases and furnace annealed to segregate the possibleexcess of Si and Ge in the form of nanocrystals embedded inan oxide matrix. For low GeH4:Si2H6 flow ratios and depositiontemperatures of 450 ◦C or lower, the deposited film consists of aSiO2 matrix incorporating Ge. No Ge oxides and no nanocrystalsare detected. After annealing of the samples with SiO2 matricesat temperatures of 600 ◦C or higher, quasi-spherical isolatedGe nanocrystals with diameters ranging from 4.5 to 9 nm andhomogeneously distributed throughout the whole film thicknessare formed. In the samples deposited with low GeH4:Si2H6 flowratios, the original SiO2 matrix holds its composition.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Group IV nanocrystals embedded in a dielectric matrix have applications in Si-based optoelectron-ics [1] as well as in non-volatile memories [2,3]. The fabrication of these nanostructures incorporatingSi, Ge or SiGe nanocrystals and exhibiting luminescence emission has been undertaken by differentmethods [4–8]. In this work, the fabrication of structures of this kind by LPCVD of SiGeO films andsubsequent annealing to segregate the excess of Si and/or Ge in the form of nanocrystals embeddedin an oxide matrix has been studied. The presence of nanocrystals and the nature and evolution of theoxide matrix have been analyzed by several experimental techniques as a function of the depositionand annealing conditions.

∗ Corresponding author. Tel.: +34 91 336 73 66.E-mail address: andres.rodriguez.dominguez@upm.es (A. Rodríguez).

0749-6036/$ – see front matter© 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.spmi.2008.10.037

344 A. Rodríguez et al. / Superlattices and Microstructures 45 (2009) 343–348

2. Experimental procedures

The deposition of the SiGeO filmswas carried out on Si wafers using Si2H6, GeH4 and O2 as reactantgases. The pressure was set at 240 mTorr, the deposition temperature was varied in the 400 to 500 ◦Crange and several GeH4:Si2H6:O2 gas flow ratioswere selected in the 0:10:2 to 20:10:2 interval, whichcorrespond to GeH4/Si2H6 flow ratios (F ) ranging from F = 0 to F = 2. Growth rates from 20 to 40nm/h are obtained. Selected as-deposited sampleswere annealed at temperatures from600 to 1100 ◦Cfor 1 h in a high purity N2 (N55) atmosphere.The composition of the as-deposited and annealed oxide matrices was analyzed by Fourier

Transform Infrared spectroscopy (FTIR). High Resolution Transmission Electron Microscopy (HRTEM)with Energy Dispersive X-Ray (EDX) analysis and Z-contrast imaging in Scanning TransmissionElectron Microscopy (STEM) mode was used to observe the presence of nanoparticles, their size andareal density and to analyze their degree of crystallization. Crystallization studies were carried out byelectron diffraction pattern simulations using Fast Fourier Transform (FFT) of the HRTEM images andsubsequent filtering to improve the contrast. To avoid or minimize electron beam irradiation effectsthe analyses were carried out using a current density below 7 A cm−2.

3. Results and discussion

3.1. As-deposited samples

The samples deposited at with F ≤ 1 at all the temperatures considered lookmirror-like and showan excellent uniformity in the whole area. However, the sample deposited at 450 ◦Cwith F = 2, looksmatt due to its rough surface, and exhibits some lack of homogeneity throughout the wafer.The FTIR spectra of the samples deposited at 450 ◦C with different values of F are displayed in

Fig. 1(a). In the sample deposited with F = 0, absorption bands were found at around 463, 814 and1070 cm−1, all of them related to Si–O–Si bonds. Another band appears at around 875 cm−1, which isattributed to the bendingmode of Si–H bonds and to the SiH2 scissors vibration [9,10]. The presence ofH bonded to Si indicates that there are Si atoms are not bonded to four oxygen atoms, thus constitutingan excess of Si in the oxide film [11]. As F increases at a constant temperature of 450 ◦C, the intensityof the Si–H band decreases significantly (relative to the intensity of the Si–O–Si one), and finally itvanishes for F ≥ 1, indicating that the presence of Ge reduces or almost inhibits the incorporation ofSi in excess into the films. For F ≥ 1, another band starts to appear at 995 cm−1, which is related toSi–O–Ge bonds.The spectra of the samples deposited with F = 0.2 at different temperatures from 400 to 500 ◦C

are displayed in Fig. 1(b). For deposition temperatures of 400 and 450 ◦C, the spectra exhibit the bandsassociated to Si–O–Si bonds aswell as the Si–H related one, but not the 995 cm−1 band, so the presenceof Si–O–Ge bonds is discarded. The spectrum of the sample deposited with F = 0.2 at 500 ◦C shows astrong absorption band at around 870–880 cm−1 attributed to the existence of Ge–O–Ge bonds. Theband at 995 cm−1, related to Si–O–Ge bonds, is also present and it is very intense in relation with theother bands existing in the spectrum.Fig. 2 shows cross-sectional HRTEM images taken along the 〈110〉 direction of the Si-substrate as

well as Z-contrast STEM images of several as-deposited samples deposited at 450 ◦C with differentvalues of F . For F = 0.2, some dark regions in the HRTEM image suggest that the film is nothomogeneous, although it is fully amorphous and no nanocrystals are observed. The bright regionsof the STEM image are caused by the presence of a high Z element, and therefore indicate that Ge isincorporated to the film as elemental Ge. For F = 1 and F = 2, the contrast in the HRTEM imagesof the layers are characterized by a distribution of amorphous nanoparticles (grey areas) and somenanocrystals (black dots). The STEM imaging combined with EDX compositional analysis (not shown)carried out in very thin areas of the samples, indicate that the grey areas as well as the nanocrystalsare very rich in Ge, or even pure Ge, in both cases. With regard to the nanocrystals, HRTEMand FFT patterns indicate, using the (110) Si substrate as an internal calibration, that the averagerelationship between the interplanar spacings dhkl (nanocrystal)/dhkl (Si-substrate)= 1.04, whichwiththe resolution achieved is the one expected for dhkl (bulk Ge)/dhkl (bulk Si), thus supporting that the

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Fig. 1. (a) FTIR spectra of as-deposited samples deposited at 450 ◦C with different values of F from F = 0 (no Ge) to F = 2. (b)FTIR spectra of as-deposited samples deposited with F = 0.2 at different temperatures in the 400–500 ◦C interval.

nanocrystals are of Ge. In the F = 2 sample, incomplete crystallization of some nanocrystals wasobserved. Table 1 summarizes the interval of diameters measured for the amorphous nanoparticles,using the STEM images, and nanocrystals, using the HRTEM micrographs, as well as an estimation ofthe number of them by unit area of the sample (areal density).

3.2. Annealed samples

The samples selected for the annealing experiments are those deposited at a temperature of 450 ◦Cor lower with F ≤ 1, which show a pure SiO2 matrix with Ge incorporated, in some cases in the formof nanoparticles.Fig. 3(a) displays selected FTIR spectra of the samples deposited at 450 ◦Cwith F = 0.2 and F = 0.5

and annealed at 600, 800 and 1000 ◦C. For F = 0.2, the spectrum of the sample annealed at 600 ◦Cshows the same bands than the one of the as-deposited sample and with almost the same intensities.Annealing at 800 ◦C causes a slight increase in the intensity of the 870–890 cm−1 band and the band at995 cm−1 also starts to appear. Considering now the sample of F = 0.5, it is found that the intensitiesof the 870–890 cm−1 and 995 cm−1 bands, attributed to the presence of Ge–O–Ge and Si–O–Ge bondsrespectively, increase substantially after annealing at a temperature of 600 ◦C or higher.

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Fig. 2. Cross-sectional bright field HRTEM (upper part) and Z-contrast STEM (lower part) micrographs of samples depositedat 450 ◦C with different values of F .

Table 1Size and density of nanoparticles and nanocrystals in as-deposited samples with different values of F (results obtained fromthe HRTEM and Z-contrast STEM images).

Flow ratio F 0.2 1 2

Ge-amorphous nanoparticles

Diameter – 5.0–10.7 nm 3.9–11.5 nmAreal density – 7.3× 1011 cm−2 2.3× 1012 cm−2

Ge-nanocrystals

Diameter – 3.0–13.2 nm 2.5–9.1 nm

Fig. 3(b) shows the spectra of samples depositedwith F = 0.2 at 400 ◦C and subsequently annealedat temperatures up to 1000 ◦C. The spectra don’t shownoticeable bands at 870–890 cm−1 or 995 cm−1after annealing of the samples at 600 or 800 ◦C, so Ge–O–Ge and Si–O–Ge bonds are not formed. Thetemperature must be increased up to 1000 ◦C to observe only a slight shoulder at 995 cm−1, while noband corresponding to Ge–O–Ge bonds appears.Cross-sectional HRTEM and STEM images taken along the 〈110〉 direction of the Si-substrate of

samples deposited at 450 ◦C with F = 0.2 and annealed at 600 ◦C and 800 ◦C are shown in Fig. 4.Spherical nanocrystals are visible in the sample annealed at 600 ◦C. As in the as-deposited samples,FFT patterns and EDX spectra confirm that the nanocrystals are of pure Ge. At 800 ◦C, Ge nanocrystalsare also observed. After annealing at 1000 ◦C, Ge nanoparticles are not visible in the HRTEM images(not included). Z-contrast imaging in STEMmode of this sample confirms that Ge agglomerates of anykind are not present in the sample. Table 2 summarizes the interval of diameters of the nanocrystalsmeasured in the HRTEM images of the samples in which they are visible, as well as an estimation ofthe areal density of them. It should be noted that in all cases the nanocrystals are homogeneouslydistributed in the volume of the layers.

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Fig. 3. (a) FTIR spectra of samples deposited at 450 ◦Cwith different values of F and annealed at several temperatures. (b) FTIRspectra of samples deposited with F = 0.2 at 400 ◦C and annealed at different temperatures.

Table 2Size and density of nanocrystals in the sample depositedwith F = 0.2 and annealed at different temperatures (results obtainedfrom the HRTEM images).

Temperature 600 ◦C 800 ◦C 1000 ◦C

Ge-nanocrystals

Diameter 4.9–9 nm 4.5–7.9 nm –Areal density 7.4× 1011 cm−2 6.7× 1011 cm−2 –

4. Conclusions

The fabrication of nanostructures with nanocrystals embedded in an oxide matrix by LPCVD ofSiGeO films and annealing has been demonstrated. At a deposition temperature of 450 ◦C with F < 1and also at 400 ◦C with F = 0.2, the as-deposited material consists of almost stoichiometric Si oxidewith an amount of Ge atoms incorporated to the film in the form of amorphous and crystalline Genanoparticles and no evidences of the formation of Ge oxides are found. At 450 ◦C and with F ≥ 1the deposited material consists of a matrix formed by a mixture of Si and SiGe oxides of unknown

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Fig. 4. Cross-sectional bright field HRTEM images of samples depositedwith F = 0.2 at 450 ◦C and annealed at 600 and 800 ◦C.Z-contrast STEM image of the sample annealed at 1000 ◦C.

proportions, also incorporating amorphous and crystalline Ge. The sample deposited at 450 ◦C withF = 0.2 holds its structure, consisting of a matrix of Si oxide with embedded Ge nanocrystals, afterannealing at 600 ◦C. No evidences of the formation of Ge oxides are found. This sample is consideredthe most appropriate one for optoelectronic applications.

Acknowledgment

This work was funded by the Spanish CICYT Project MAT2004-04580.

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