In situ low temperature growth of poly-crystalline germanium thin film on glass by RF magnetron sputtering

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 1501–1505

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

In situ low temperature growth of poly-crystalline germanium thin film on glass by RF magnetron sputtering ¨ Chao-Yang Tsao a,n, Jurgen W. Weber a, Patrick Campbell a, Gavin Conibeer a, b Dengyuan Song , Martin A. Green a a b

ARC Photovoltaics Centre of Excellence, University of New South Wales, Sydney, NSW 2052, Australia Yingli Green Energy Holding Co. Ltd., 071051 Baoding, China

a r t i c l e in fo

abstract

Article history: Received 2 July 2009 Received in revised form 22 September 2009 Accepted 21 February 2010 Available online 15 March 2010

Structural and optical properties of germanium thin films deposited on silicon nitride coated glass are investigated with the aim to develop a material for the bottom cells of low cost monolithic tandem solar cells. The films were deposited by radio-frequency magnetron sputtering at various substrate temperatures (Ts) r 450 1C. X-ray diffraction spectra reveal the structural evolution from amorphous to crystalline phase with increasing Ts We find that the film sputtered at 450 1C is poly-crystalline with strong (1 1 1) preferential orientation, confirmed by cross-sectional transmission electron microscopy. Optical band gaps of these films derived from Tauc plots, using absorption coefficient values derived from both reflectance/transmittance measurements and spectroscopic ellipsometry data, are in a reasonable agreement. Optical band gap values decrease from  0.88 to 0.68 eV over the transition from the amorphous to poly-crystalline phase. The absorption coefficient of the poly-crystalline Ge film is higher than that of bulk Ge over a wide waveband and exhibits an absorption tail. The optical properties upon substrate temperature are correlated with the structural properties of Ge films. & 2010 Elsevier B.V. All rights reserved.

Keywords: Poly-crystalline germanium Thin film RF sputtering Glass

1. Introduction Thanks to its narrow band gap of 0.67 eV, crystalline germanium (c-Ge) has been successfully used in photovoltaic applications, such as thermo-photovoltaics [1] and bottom cells of tandem and multijunction cells [2] to convert near infrared radiation into electrical energy. Despite the high cell performance achieved, these approaches require using expensive germanium (Ge) wafers, resulting in very high cell fabrication costs. Substrate cost is also presently holding back the use of Si and Si–Ge alloy material in these structures, while all are restricted to wafer size only. In this paper we investigate fabricating poly-crystalline Ge (poly-Ge) thin films on glass to avoid these problems and provide scope for monolithic large area solar cell modules. We choose poly-Ge material for its inherent advantages over amorphous and intermediate phases such as its low sheet resistance, which avoids the need for using transparent conductive oxide as a back contact layer, and its long term ultraviolet stability.

n

Corresponding author. Tel.: + 61 2 9666 5286; fax: + 61 2 9666 5203. E-mail addresses: [email protected], [email protected] (C.-Y. Tsao). 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.02.035

To produce high quality poly-Ge thin films, various techniques have been investigated by other groups [3–5]. However, with most of these approaches the film quality reported was poor unless either high substrate temperature (Ts) or an ex situ crystallization process was employed. The high Ts excludes the use of low cost substrates such as glass, while the extra crystallization process required increasing the fabrication cost. In this paper, 300 nm poly-Ge thin films on silicon nitride (SiNx) coated glass are grown in situ at low Ts r450 1C by RF magnetron sputtering, an inexpensive, non-ultra high vacuum deposition technique capable of fabricating large area films. Our previous work [6] has explored the structural properties of the films and found that the film as sputtered at 450 1C was polycrystalline with strong (1 1 1) preferential orientation. In this paper, we further examine the film structure by cross-sectional transmission electron microscopy (XTEM) and extend the research scope into the optical properties of the films to assist in solar cell design. Reflectance and transmittance spectra are used to determine the absorption coefficient, from which we derive optical band gaps using the Tauc method. These results are checked against spectroscopic ellipsometry (SE) derived values. We present the influence of how variation of Ts affects optical properties. The correlation between the structural and optical properties is also studied.

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2. Experimental details

(111)

3. Results and discussion

Fig. 1 depicts the typical room temperature XRD spectra of the Ge films sputtered at various Ts. We attribute a hump at around 221 to the glass substrate. As shown, the transformation of the diffraction peaks appearing at around 2y ¼27.31, 45.31, and 53.71, which correspond to the (1 1 1), (2 2 0), and (3 1 1) crystal planes of Ge, reveals that the structure of the films evolved from amorphous, microcrystalline to poly-crystalline phase [6]. The film sputtered at 255 1C is amorphous while the grain sizes (g) of the other films were estimated from the full width at half maximum (FWHM) of XRD characteristic peaks using a modified version of the Scherrer formula 0:9l ; ½Dð2yÞdcos y

(311) 450°C

400°C 370°C 280°C 255°C 20

30

40

50

60

2θ (degree) Fig. 1. XRD spectra of Ge thin films on SiNx-coated glass sputtered at various temperatures. A hump at around 221 is attributed to the glass substrate.

˚ of the X-ray, y is the angle where l is the wavelength (1.5418 A) satisfying Bragg’s law, D(2y) is the FWHM in radian and d corrects for instrumental broadening [6]. The grain sizes were estimated to be 20 nm (280 1C),  88 nm (370 1C), 113 nm (400 1C), and  132 nm (450 1C). Based on the strong (1 1 1) preferential orientation observed, the film deposited at 450 1C is concluded to be poly-crystalline. This temperature is much lower than that reported elsewhere for RF sputtered Ge [4]. Detailed measurements and discussion of XRD can be found from the authors’ previous publication [6]. To further confirm the crystallinity of the film, XTEM measurement was performed. Fig. 2 shows cross-sectional bright-field and dark-field TEM images of this sample. The film structure is identified as being columnar directly from the substrate interface. Also observed are upside-down cone shape grains, typically appearing with (1 1 1) orientation, at part of regions in the images, confirming the XRD result shown in Fig. 1. A part of the grains nucleates from the bottom surface and grows to the top surface while others terminate during thin film growth. The surface roughness of the film is also visible in the bright-field image. 3.2. Optical properties

3.1. Structural properties

g¼

(220)

Intensity (arb.units)

Ge thin films were deposited with an AJA ATC2200 RF magnetron sputtering system equipped with a substrate holder heated by a quartz halogen lamp heater. Films were sputtered onto glass using a 4 inch undoped Ge target (99.999% purity) with an argon process pressure of 0.13 Pa (1 mTorr), a plasma excitation frequency of 13.56 MHz, and a deposition rate of  4.9 nm/min. The Ge deposition rate was monitored using a quartz crystal deposition rate monitor, whose tooling factor was calibrated using stylus profiler data (Dektak IIA). SiNx-coated borosilicate glass panes (Borofloat33, 5  5 cm2, 3 mm thick) were used as substrates. SiNx films of 75 nm thickness acting as barrier layers were deposited by plasma-enhanced chemical vapour deposition. We chose this thickness for minimum reflectance; it has also been found to suffice as a barrier layer with Si films undergoing thermal processes such as solid phase crystallization and rapid thermal annealing. Nevertheless, we have not investigated whether its thickness affects the Ge layer morphology. The SiNx-coated glass panes were cleaned with de-ionized water in an ultrasonic bath and then heated at 20 1C in a nitrogen atmosphere for 10 min to remove any residual moisture prior to deposition. The films were deposited at different Ts, specifically 255, 280, 370, 400, and 450 1C. Temperature calibration data using a thermocoupleprobed Si wafer were supplied by AJA. XTEM (JEOL-3000F operated at 300 kV) was performed to confirm the crystallinity of grown Ge thin films. The samples cross-sections were cut with a diamond saw, glued, and then thinned with a standard mechanical polishing technique followed by argon ion milling at 3 kV. A double-beam UV/visible/IR spectrophotometer (Varian Cary 5 G) and an attached integrating sphere (Labsphere, RSA-CA-50) were used to measure transmission and reflection spectra in the wavelength range of 210–2300 nm. SE measurements (Woollam Co. M-2000 variable angle spectroscopic ellipsometer equipped with WVASE32s and CompleteEASEs software) were performed ex situ over 190–1690 nm (0.73–6.5 eV) at 651, 701, and 751 incidence. To remove the light reflected from the rear surface, all the rear surfaces of the glass sample substrates were abraded, to scatter most light away from the detector. Optical properties of each material were determined employing various fitting models—Cauchy (glass), Cody–Lorentz oscillator (SiNx), and a Kramers–Kronig consistent B-spline line (Ge) [7]. In addition, to model both the roughness and oxidation effects on the surface, a surface roughness layer was added and for very high roughness a separate EMA layer was used [7].

ð1Þ

3.2.1. Reflectance and transmittance Fig. 3 shows the dependence of the optical reflectance (R) and transmittance (T) on Ts. Arrows across the curves indicate the trend with increasing Ts. For the reflectance spectra, the curves can be separated roughly into two regions of interest: wavelengths l o600 nm (region 1) and l 4600 nm (region 2). In region 2, the curves exhibit interference fringes, a sign of incomplete absorption. In a tandem structured solar cell also involving indirect band gap material such as Si, the Ge layer would be expected to share a light trapping scheme. With decreasing wavelength the envelopes bounding interference extrema decay with the onset of stronger absorption. In region 1, reflection spectra are strongly dependent on the near-surface crystallinity [8]. The curves indicate that with increasing Ts the films change gradually from the amorphous to poly-crystalline

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1503

100 glue

80

R (%)

air

Ge

60

255°C 280°C 370°C 400°C 450°C region 1 region 2

40 SiNx

20 glass

Ge/SiNx interface SiNx/glass interface

0 400

800

1200

1600

2000

2400

2000

2400

Wavelength (nm) grains

100 255°C 280°C 370°C 400°C 450°C

T (%)

80

~300 nm ~75 nm

60

40

Fig. 2. Cross-sectional TEM image of the Ge film sputtered at 450 1C: (a) brightfield, and (b) dark-field. Surface roughness is visible.

20

0 phase [6]. Also noted is that the increasing Ts leads to apparent reflectance reduction in this region (see Fig. 3(b)). This is not interpreted as a reduction in crystal quality, but is attributed to a graded effective refractive index because of the monotonically increasing sub-wavelength scale surface roughness, up to 10 nm, as revealed by our atomic force microscopy (AFM) measurements shown earlier [6]. We note how the wavelength at near-zero transmittance varies from  800 nm (sputtered at 255 1C) to 600 nm (sputtered at 450 1C). This blue-shift in absorption edge and the increase of absorption in the reflectance spectra suggest bonding changes in the Ge films with accompanied Ts [9]. Eqs. (2) and (3) [10,11] given below were used to calculate the Ge absorption coefficient a deduced from R and T. This simple method can be applied because with near-normal incidence all back reflections are negligible T ¼ ð1RÞexpðatÞ;

ð2Þ

  1 1R ln ; t T

ð3Þ

a¼

where t is the thickness of the film. Eq. (3) uses T to account for absorption and R to correct for uncoupled light and is only valid where T is nonzero. Subsequently, the optical band gap energy (Eg) was determined by Tauc’s equation [10,12], given as ðahnÞ1=n ¼ BðhnEg Þ;

ð4Þ

where hn is the photon energy, B is the edge width parameter dependent on structural disorder of films, and the exponent n depends on the type of transition. For a direct-allowed transition, n¼1/2, for an indirect-allowed transition, n¼2, and for a direct-forbidden transition, n¼3/2 [13]. For Ge, known as an

400

800

1200

1600

Wavelength (nm) Fig. 3. The reflectance (R) and the transmission (T) spectra of Ge films sputtered at various substrate temperatures: (a) R and (b) T. Arrows across the curves indicate the increasing substrate temperature.

indirect semiconductor, n was chosen as 2. Although Eq. (4) was originally developed and is most suitable for extracting the band gap for amorphous materials, it has been used to determine the band gap of nano-crystalline materials [10,13] and poly-Ge films [14]. Here it was used to roughly estimate the band gap shift of our films. Fig. 4 (a) shows a Tauc plot of (ahn)1/2 versus photon energy hn of Ge films sputtered at various Ts based on the a values obtained from R and T. The linear portion was extrapolated to yield the optical band gap Eg at (ahn)1/2 ¼0. As demonstrated in the figure, there is a general trend for the band gaps to decrease with increase in Ts while the change from a knee shape curve (a-Ge) to a straight line (c-Ge) indicates the increase in crystallinity [15]. The band gap of the sample sputtered at 255 1C is  0.88 eV in agreement with the literature for a-Ge [12], while that of the film sputtered at 4501 is  0.68 eV, approaching that of c-Ge. A direct correlation between crystallized degree and band gap is apparent. As mentioned, the above method is limited to wavelengths where T40 (see Fig. 3(b)), while poor detector sensitivity over 1600 nm in R (while inline T measurement was possible, we had to use an integrating sphere, which has low throughput efficiency, for R scans) leads to high noise (see Fig. 3(a)). We have used SE to explore outside this range, as well as to independently confirm the above findings.

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255 C o 280 C o 370 C o 400 C o 450 C

1/2

5

5

4 4 o

3

255 C o 370 C o 450 C c-Ge

n

1/2

(αhν) (eV/μm)

6

o

6

3

2 2

1 1

0 0.6

0.8

1.0

1.2

1.4

1.6

1.8

hν (eV)

200

400

600

800

1000

1200

1400

1600

1800

1600

1800

Wavelength (nm)

Fig. 4. Tauc plot of (ahn)1/2 versus photon energy hn of Ge films sputtered at various substrate temperatures based on the a values obtained from R and T. 5

4pk

l

:

4

3

2

1

ð5Þ

Fig. 6 illustrates the absorption coefficients of the films calculated with Eq. (5). Bulk Ge [18] is included for reference. As shown in the figure, the spectra gradually evolve toward that of the bulk value as Ts increases. Below  1200 nm, the curve corresponding to 450 1C approaches that of the reference, indicating its higher crystallinity. However, it is noted that the reference absorption coefficient abruptly drops at around 1500 nm while those of all the sputtered films have absorption tails. The absorption tail of the film sputtered at 255 1C may be attributed to disorder of amorphous tissue and incorporated defect states existing in the energy gap due to the lower deposition temperature. The absorption tails of the other two are very likely attributed to small grains and hence a large density of (amorphous) grain boundaries. Based on the a values obtained from SE, the band gaps of the Ge films were estimated using a Tauc plot again and are shown in Fig. 7. Compared to Fig. 4, the trends of both figures are in a reasonable agreement. Table 1 summaries the band gaps of the films obtained by Tauc plots from these two different optical measurements as a function of substrate temperature.

0 200

400

600

800

1000

1200

1400

Wavelength (nm) Fig. 5. Refractive index n and extinction coefficient k spectra of Ge films sputtered at various substrate temperatures: (a) n and (b) k. Arrows across the curves indicate the trend with increasing Ts. The black solid line indicates these data for cGe from literature [16] for reference.

10

Absorption coefficient (cm-1)

a¼

o

255 C o 370 C o 450 C c-Ge

k

3.2.2. SE measurements Fig. 5 shows dispersion in refractive index and extinction coefficient of our Ge films over a variety of Ts. The data of the films sputtered at 280 and 370 1C, as well as 400 and 450 1C, are very close. For clarity, only three out of five spectra are shown accompanied with the reference n and k reported in the literature [16]. It can be seen from the figure that the n and k values of Ge films are, as expected, strongly dependent on substrate temperature. The evolution of the n and k spectra is similar to that of poly-crystalline silicon [17]. The absorption coefficient a was then calculated according to the well-known equation

10

10

6

o

255 C 370 o C 450 o C c-Ge

5

4

4. Conclusions 10

Structural and optical properties of germanium (Ge) thin films sputtered on silicon nitride coated glass at low substrate temperatures have been studied. Our study reveals that the structure evolves from amorphous to poly-crystalline phase as Ts increases from 255 to 450 1C. XTEM provided evidence that the

3

200

400

600

800

1000

1200

1400

1600

1800

Wavelength (nm) Fig. 6. The absorption coefficient of Ge films sputtered at various substrate temperatures. The arrow across the curves indicates the trend with increasing Ts. The dot line indicates that of bulk Ge [18] for reference.

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255 C o 280 C o 370 C o 400 C o 450 C

1/2

5

1/2

(αhν) (eV/μm)

Taiwan for helpful discussions about the XTEM results, and Dr. James N. Hilfiker from J.A. Woollam Co., Inc. for the spectroscopic ellipsometry measurements and data analysis. C.T. thanks Professor Armin G. Aberle for his supervision over 2007–2008 and acknowledges sponsorship from Taiwan Power Company.

o

6

4

References 3

2

1

0 0.6

0.8

1.0

1.2

1.4

1.6

1.8

hν (eV) Fig. 7. Tauc plot of (ahn)1/2 versus photon energy hn of Ge films sputtered at various substrate temperatures based on the a values obtained from SE.

Table 1 Summary of the band gaps of the films obtained by Tauc plots from two different optical measurements as a function of substrate temperature. Ts (1C) a

Eg (eV) Eg (eV)b a b

1505

255

280

370

400

450

0.88 0.89

0.86 0.87

0.83 0.85

0.74 0.73

0.68 0.68

Based on the a values obtained from R and T. Based on the a values obtained from SE.

as-deposited Ge film is poly-crystalline and of strong (1 1 1) preferential orientation, as can be concluded from XRD measurements. Band gaps estimated by Tauc plots based on both reflectance/transmittance data and on SE analyzed results have the same trend that band gaps decrease with increase in Ts. The refractive index, extinction coefficient, and absorption coefficient of the films determined by SE change toward those of c-Ge as Ts increases. The poly-Ge film produced at a low Ts of 450 1C, with a band gap of 0.608 eV, suggests our poly-Ge films could be promising for use in thin film solar cells on glass.

Acknowledgments The authors thank Dr. Per I. Widenborg and Dr. Sergey Varlamov for their help and support to this project. The authors are grateful to Yidan Huang for the XTEM measurements, Associate Professor Hsi-Lien Hsiao from Tunghai University in

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