Synthesis and structural properties of Ge nanocrystals ...

Synthesis and structural properties of Ge nanocrystals in multilayer superlattice structure B. Zhang*, S. Shrestha, P. Aliberti, M. A. Green and G. Conibeer ARC Photovoltaics Centre of Excellence, University of New South Wales Sydney, New South Wales 2052, Australia ABSTRACT Ge nanocrystals (Ge NCs) were grown in a multilayered superlattice using magnetron co-sputtering and subsequent thermal annealing. The purpose is to produce a material in which the band gap can be controlled by controlling the Ge NC size and to investigate the potential of this material for use in tandem solar cells. The presence of size-controlled Ge NCs was revealed by Raman spectroscopy, glancing incidence X-ray diffraction (GIXRD) and Transmission Electron Microscope (TEM), and this was supplemented by the observation of blue shifts in the absorption and photoluminescence (PL) properties. Raman spectra showed Ge-Ge active phonon modes at around 300 cm-1 implying the formation of high quality Ge NCs. With increasing annealing temperature and duration, more Ge precipitate changed from a non-crystalline phase to a crystalline phase. However, calculation of degree of crystallinity indicated that a considerable amount of non-crystalline Ge remained at our chosen annealing conditions. GIXRD measurements exhibited three Bragg peaks associated with crystalline Ge. TEM images showed direct evidence of the crystal lattice of the Ge NCs. The size of nanocrystals increased with annealing duration indicating nanocrystal growth by diffusion. The growth of nanocrystals was found to be confined by the GeO2/SiO2 spacing layers, and the average crystallite size was determined by the thickness of the GeRO layers. However, enhanced interdiffusion at elevated annealing temperature weakened the size confinement effect of the multilayer structure. Hence an optimum annealing condition is needed to produce high quality and reproducible Ge NCs. Our preliminary work indicates that it may be promising to use Ge NCs as absorber materials in tandem solar cells. Keywords: Germanium, nanocrystals, superlattice, size control, tandem solar cells

1. INTRODUCTION During the last few decades, the quantum confinement effect in nanostructures has been extensively studied in order to explore new properties of materials and their potential applications in optoelectronic and electronic devices. Recently, this effect has been suggested to be used to fabricate absorber materials in thin film tandem solar cells 1. In such a tandem structure, different bandgaps of the sub-cells can be realized by engineering the degree of quantum confinement. The all-silicon tandem solar cell using silicon nanocrystals (Si NCs) sandwiched between layers of a silicon-based dielectric has been demonstrated recently 2-4. As an alternative to silicon, Germanium nanocrystals (Ge NCs) also attract a lot of research interest. Ge NCs have more prominent quantum size effects than Si NCs because of their larger excitonic Bohr radius, and hence it is easier to modulate the electronic structure of Ge around the bandgap. Moreover, the lower melting point of Ge at 938.3 oC indicates that Ge NCs can form at lower temperatures than Si NCs. This is indeed a significant advantage both for processing compatibility and for long term process costs. So far, Ge NCs randomly dispersed in a thick SiO2 layer have been realized by various technologies including cosputtering, hydrothermal oxidation, chemical vapor deposition and implantation 5-9. However, these approaches suffer from the poor control of size, position, shape and density of nanocrystals. The fluctuation of structural properties may lead to difficulties in characterization and poor performance in eventual devices. Recently, a superlattice structure, consisting of alternate layers of nanocrystals rich films and spacing films, has been proposed to improve the uniformity *[email protected]; phone 61 2 9385-6782; fax 61 2 9385-5104

Nanoscale Photonic and Cell Technologies for Photovoltaics II, edited by Loucas Tsakalakos, Proc. of SPIE Vol. 7411, 741103 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.825583

Proc. of SPIE Vol. 7411 741103-1

of size and distribution of nanocrystals 10. Although it has been widely adopted in forming Si NCs in SiO2 matrix, there is still a much scope for systematic studies of this structure for Ge NCs 11. In this paper, we present the synthesis and characterizations of Ge NCs in a superlattice structure, consisting of alternate layers of Ge-rich SiO2 (GeRSiO) and a mixture of the nominally stoichiometric oxides GeO2/SiO2 formed by magnetron co-sputtering and subsequent thermal annealing. Both structural and optical properties of Ge NCs and multilayered films were studied, and the growth mechanism of the nanocrystals was also discussed.

2. EXPERIMENTAL APPROACH The multilayered films were fabricated on both silicon and quartz substrates by RF-magnetron co-sputtering. The sputtering target was a circular fused quartz plate partially covered by high purity (99.9999%) Ge strips. Schematic diagram of the sputtering machine and the image of the assembled target are shown in Fig.1. The bass pressure of the chamber was pumped down to 3.0x10-4 Pa and then the thin films were deposited at 25W RF power. Alternate layers of GeRSiO and GeO2/SiO2 were deposited by Ar sputtering and by reactive sputtering with oxygen, respectively, resulting in a multi-bilayer structure. In situ cleaning of the target using Ar plasma was performed before deposition of GeRSiO layers in order to ensure the removal of any GeOx formed on the strip surface during reactive sputtering. A thick GeO2/SiO2 capping layer was grown on the top of the structure to prevent possible oxidation of Ge during thermal annealing or penetration of moisture. Post deposition thermal annealing in vacuum was then used to form Ge NCs in situ in the sputtering machine.

Fig.1 The schematic diagram of the sputtering machine (left) and the target setting (right): a 4 inch fused quartz plate amounted uniformly with six Ge strips.

Various technologies have been used to characterize the structural and optical properties of the samples. TEM specimens were prepared by a dual beam high resolution focused ion beam (FIB) and the images were obtained at 200 kV. Raman scattering spectra were obtained using micro-Raman spectroscopy in a backscattering configuration. The beam was excited by the 514.6 nm line of an Ar laser with a spectral resolution of 1~2 cm-1. GIXRD and X-ray reflection (XRR) using CuKα radiation (λ=0.154nm) were operated at a voltage of 45 kV and a current of 40 mA. The glancing angle between the incident X-ray beam and the sample surface for GIXRD was set at 0.30 degree, very close to the critical angle for total external reflection. Optical reflection and transmission were measured by a double beam UV-visible-NIR spectrophotometer attached with an integrating sphere for extraction of absorption properties of Ge NCs. In addition, PL spectra were recorded at room temperature in the range of 450nm-1080nm, using a 422 nm blue excitation light source. Samples deposited on silicon wafers were used for Raman, TEM, XRD and XRR measurements, whilst samples deposited on quartz substrates were used for optical characterization.


3. RESULTS AND DISCUSSION Fig. 2(a) shows the Raman spectra of the as-deposited and the annealed samples, including those for bulk Ge as a reference. For these samples, the sputtering times for each GeRSiO layer and GeO2/SiO2 layer were 8 mins and 6 mins, respectively, and the post deposition annealing was performed at 685 oC for 40 mins. The Raman results indicate a clear transition to the crystalline Ge with annealing. A broad hump at around 270 cm-1 is observed in the spectra of the asdeposited film which is attributed to the non-crystalline Ge phase. But in the annealed film, it is replaced by a sharp peak at 300.5 cm-1, which is very close to the Ge-Ge optical phonon mode for bulk Ge (300.2 cm-1), indicating the formation of Ge NCs with good crystallinity. Peak broadening and an asymmetric shoulder on the lower frequency side can be interpreted by the model of the optical phonon confinement effect in nanocrystals 12-14. However, it seems that in our Ge system the high frequency side of the Raman peak does not show a shift to lower phonon frequencies which is common in usual nanocrystals. The reason for this lack of a peak red shift is being studied.

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Fig. 2 (a) Raman spectra and (b) GIXRD image of the as-deposited and annealed films. The sputtering times of each GeRSiO layer and GeO2/SiO2 layer were 8 mins and 6 mins, respectively.

GIXRD patterns of this sample are shown in Fig. 2(b). In the as-deposited film there are no obvious peaks but only two broad bands at around 2θ = 26 o and 2θ = 49 o due to the absence of the crystalline phases. After annealing, the sample shows three sharp peaks at 27.34 o, 45.60 o and 53.88 o, corresponding to the groups of planes {111}, {220} and {311} of crystalline Ge, respectively. This observation confirms the good crystallinity of the Ge phase in the films and agrees well with our Raman results. The full width at half maximum (FWHM) of the peaks has been estimated after the background signal subtraction and curve fitting. Then the average size of the Ge NCs has been calculated from the {111} peak broadening using the Scherrer equation15:

g=

Kλ Δ (2θ ) cos(θ )

(1)

where λ is the wavelength of the X-ray, θ is the Bragg diffraction angle at the peak position, Δ(2θ) is the integral breadth in radians, and K = 0.9 is a correction factor. Hence the average nanocrystal size is calculated to be about 5.2 nm.


Fig. 3 Cross-section HRTEM image of Ge NCs in annealed film. The sputtering times of each GeRSiO layer and each GeO2/SiO2 layer were 8 mins and 6 mins, respectively.

The size and crystal structure of the Ge NCs in the same sample were also investigated using high resolution TEM (HRTEM, as shown in Fig. 3). The TEM evidence shows that the nanocrystals tend to be spherical in structure. The Ge NCs size is about 5.5 nm ± 0.4 nm and this is slightly larger than that determined by XRD. A possible explanation for the deviation is the spatial nonuniformity of the Ge NCs size. The TEM measurement probes a much smaller sample region compared with XRD measurement. In addition, the penetration depth of incident X-ray source is larger than the thickness of the film in our XRD measurements, so information measured by XRD is averaged throughout the whole film. Therefore, the XRD results give spatially averaged information about nanocrystal size but TEM does not. The clear lattice observed in the TEM image gives direct evidence of the formation of the Ge NCs. It should be noted that the distance between the lattice fringes is about 0.33 nm, which is the lattice spacing of the {111} planes of the Ge diamond structure.

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Raman Shift (cm-1) Fig. 4 Raman spectra of the samples annealed at 685 oC for different duration.


The underlying growth process of Ge NCs in the superlattice structure was investigated in detail. For all the samples used in this study, the sputtering times of each GeRSiO layer and GeO2/SiO2 layer were both 6 mins. Fig. 4 shows the Raman spectra of the samples annealed at 685 oC for different duration. The crystallization of Ge is found to take place within the first 6 mins of annealing. However, the noticeable broad hump, which is attributed to small nanocrystals, suggests an early stage of the crystallization process at this annealing time. As annealing duration increases, Raman peaks become sharper and narrower as the small nanocrystals grow in size. It is also seen that the peaks show negligible difference for annealing durations longer than 10 mins. In fact, it is not very surprising to see that the Ge crystallinity increases with the annealing duration, if we realize that nanocrystals or clusters in our samples grow by diffusion of Ge atoms or clusters in the SiO2 matrix. The nanocrystal sizes for different annealing duration were calculated from XRD data, as shown in Fig. 5. It is noticed that the nanocrystal size increases with annealing duration. This agrees well with the Raman results and the growth dynamics by diffusion of neighboring Ge atoms as mentioned earlier. Most importantly, the size increase begins to level off after 15 mins annealing, indicating that the nanocrystals approach an upper limit of size. A similar size confinement effect was also observed in the growth of Si NCs in SiO2 matrix 2,10. In our superlattice structure, the nanocrystals are confined within the GeRSiO layers separated by the GeO2/SiO2 layers. The GeO2/SiO2 spacing layers work as barrier layers to the crystal growth in the direction of multilayer growth because they do not contain any Ge atoms which could diffuse to the growing nanocrystals. In such situations, Ge tends to precipitate nanocrystals of a diameter approximately equal to the original thickness of the GeRO layers. This control mechanism is more significant for thin layers with thickness of a few nanometers, within which there is a 2D rather than a 3D diffusion process for migration of Ge atoms to nucleating sites. This size control effect is described in more detail elsewhere and the results show an increase of nanocrystal size with the sputtering time (proportional to the layer thickness) of the GeRSiO layers 16.

Sizes calculated from XRD peaks of {111} planes

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Fig. 6 shows the Raman spectra of the samples annealed at different temperatures for 40 mins. According to our earlier discussion, this annealing duration was enough to complete the growth process. As expected, while increasing annealing temperature the small nanocrystalline asymmetric hump is reduced as in the case of increasing annealing duration. The results also illustrate the formation of Ge NCs in SiO2 matrix at annealing temperature down to 620 oC. More


experiments are in progress in order to identify the threshold temperature needed for activating the crystallization process in the superlattice structure. To quantitatively understand the film composition, the Raman spectra are decomposed into three Gaussian peaks corresponding to the crystalline peak at ~ 300.5 cm-1, the intermediate peak at ~ 292 cm-1, and the non-crystalline peak at ~ 270 cm-1. The intermediate peak is usually attributed to small nanocrystalline particles of size less than 3 nm 17.

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Fig. 6 Raman spectra of the samples annealed at different temperatures for 40 mins.

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Raman Shift (cm-1) Fig. 7 Raman spectrum of the sample annealed at 720 oC for 40 mins and the spectrum is fitted with three Gaussian peaks.


The fitted Raman spectrum for the sample annealed at 720 oC is shown in Fig. 7. It can be seen that the peak is well fitted with these three components. The volume fraction of crystallinity Fc can be simply calculated using the equation 18:

Fc =

Ic + Im I c + I m + β I nc

(2)

where Ic, Inc, and Im are the integrated intensities of the crystalline, non-crystalline, and intermediate (or nanocrystalline) components, respectively. β is the ratio of the cross section of the non-crystalline phase to crystalline phase. The β value is unity in our calculation which is appropriate for crystallites of a few nm in size 19. Therefore, the calculated crystallinity for the sample in Fig.7 is about 74%. This low volume fraction of crystallinity implies that a significant amount of excess non-crystalline Ge co-exists with Ge NCs in the GeRSiO layers even at the highest annealing temperature among our chosen conditions. A few experiments are currently in progress to study the effect of Ge precipitate density on the growth of Ge NCs and their properties.

Fig. 8 Cross-section TEM image of the multilayer film. The thicknesses of GeRSiO and GeO2/SiO2 layers are ~ 3.6 ± 0.2 nm and 7.8 ± 0.2 nm, respectively.

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Fig. 9 XRR patterns of (a) the samples annealed at 620 oC and 720 oC for 40 mins, and (b) the samples annealed at 685 oC for 6 mins and 25 mins.


During the thermal annealing, interdiffusion takes place across the layer interface between the GeRSiO and GeO2/SiO2 layers. In Fig. 8 we can clearly see the vague interfaces that resulted from the interdiffusion. Significant interdiffusion can dramatically blur the interface and deteriorate the superlattice structure, hence reducing the controllability of crystallite size. XRR characterization was used to investigate the interdiffusion effect in two groups of samples. Fig. 9(a) and Fig. 9(b) show the measurement results of the samples annealed at different temperatures and those of samples annealed for different duration, respectively. In Fig. 9(a), it is noticed that Bragg peaks become weaker and the peaks labeled in circles disappear at higher annealing temperature of 720 oC, indicating a degradation of the roughness at the interface of successive layers. On the other hand, patterns in Fig. 9(b) display almost the same features for both short and long annealing duration, and indicate that the layer interface remains almost unchanged for extended annealing duration. Therefore, it can be concluded that in our superlattice structure increasing annealing temperature will more effectively cause interdiffusion between GeRSiO and GeO2/SiO2 spacing layers whereas increasing annealing duration will not. This finding provides very useful information for the determination of appropriate annealing conditions which are rather important for realizing good nanocrystals without degrading the quality of the multilayer structure. In addition, we also looked into the optical properties of the Ge NCs including optical absorption and photoluminescence (PL). A sample with the same process sequence as those for structural characterizations was used for optical measurements, except that it was deposited on quartz substrate – necessary because of its transparency. The sputtering time of both GeRSiO layer and GeO2/SiO2 layer in this sample was 6 mins. Fig. 10(a) shows the absorption spectra of the multilayer film and the single thick layer of GeO2/SiO2. The multilayer film shows negligible absorption in the nearinfrared range but significant absorption at visible wavelengths. This strong absorption can be attributed to the GeRSiO layers that contain Ge NCs, since the GeO2/SiO2 film is almost transparent over all wavelengths. Fig. 10(b) shows the room-temperature PL spectrum of the multilayer film. The spectrum consists of a single broad band centred at 1.77 eV (corresponding to a wavelength of 700 nm) which can be fitted with three Gaussian distributions. Since there was no observable PL from the GeO2/SiO2 film and quartz substrate (not shown) in the range of wavelengths concerned, the PL should come from the GeRSiO layers. Furthermore, it should be noted that there are blue shifts in both the absorption edge and the PL peak energy. We tentatively consider that this is due to the bandgap increase induced by the quantum confinement effect in the Ge NCs. However, more systematic work is needed to completely understand the origin of the visible PL, since our structure has a complex ternary element system with competing phases and complicated interface conditions.

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Fig. 10 (a) Absorption spectra of the annealed multilayer film and single thick layer of GeO2/SiO2, and (b) roomtemperature PL of the annealed multilayer film. The films were deposited on quartz substrates.


2.2

4. CONCLUSION Ge NCs embedded in SiO2 matrix were prepared in a multilayer superlattice structure using magnetron co-sputtering followed by vacuum thermal annealing. The formation of Ge NCs was strongly confirmed by the evidence from Raman spectroscopy, XRD and TEM measurements. Studies on the growth mechanisms revealed that the 2D diffusion process of Ge atoms was responsible for the nanocrystal growth in the GeRSiO layers. The growth of Ge NCs was confined by the multilayer structure and the nanocrystal size was controlled by the thickness of GeRSiO layers. The results also showed that more Ge changed from a non-crystalline phase to a nanocrystalline phase with increasing annealing temperature and duration. The crystallinity calculated from Raman spectra indicated that a significant amount of noncrystalline Ge remained in the annealed films for our chosen annealing conditions. Interdiffusion was observed at the interface between the GeRSiO and GeO2/SiO2 layers. Enhanced interdiffusion would result in a rougher layer interface and hence weaken the size controllability of the superlattice structure. It is known that thermal annealing contributes to the interdiffusion process, so it is important to choose an optimized annealing condition to retain good multilayer structure without sacrificing the nanocrystal quality. Some results of the optical characterization were also reported. The blue shifts in both the optical absorption edge and PL peak energy were tentatively attributed to the quantum confinement effect in Ge NCs. However, future work is needed to completely understand the emission mechanisms. Our initial experimental results demonstrate that Ge NCs in multilayer superlattice structures may be a promising candidate for the fabrication of tandem solar cell applicable absorber films containing nanocrystal materials with engineered bandgaps. The results also suggest that optimization of the process conditions is exceptionally important for fabricating high quality and reproducible Ge NCs for future study on the electrical characteristics and device applications.

5. ACKNOWLEDGEMENTS This work was supported by the Australian Research Council (ARC) via its Centres of Excellence scheme and by the Global Climate and Energy Project (GCEP) administered by Stanford University. The authors thank the members of the Third Generation Group at the ARC Photovoltaics Centre of Excellence for their contributions to this work. Bo Zhang also thanks the Asia-Pacific Partnership on Clean Development and Climate, and IDP Education Australia for supporting his study in Australia.

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