Correlations of Photoluminescence and Size Evolution of Si Quantum Dots in Amorphous Silicon Carbide

Rare Metal Materials and Engineering Volume 44, Issue 12, December 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(12): 3023-3026.

ARTICLE

Correlations of Photoluminescence and Size Evolution of Si Quantum Dots in Amorphous Silicon Carbide Chang Gengrong1, 1

Ma Fei2,

Xi’an University, Xi’an 710065, China;

2

Fu Fuxing1,

Xu Kewei1,2

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an

710049, China

Abstract: Silicon quantum dots (Si QDs) with dot density up to 9×1013 cm-2 in amorphous SixC (x>1) thin films were obtained by magnetron sputtering deposition and post-annealing process at 1100 ºC. Photoluminescence measurement indicates a multi-band configuration in the range from ultraviolet to green (2.3~3.5 eV). The analysis of Stokes shift and HRTEM demonstrates that there exist two kinds of Si QDs embedded in silicon carbide dielectric matrix: α-Si QDs and c-Si QDs which show the multi-band characteristics. The photoluminescence is closely related with the microstructure size distribution of Si QDs from 1.0 to 4.0 nm. Moreover, the density and the size distribution of Si QDs can be improved further by optimizing the ratio of Si/C atoms as well as annealing parameters. This opens a route to fabricate all-Si tandem solar cell. Key words: silicon quantum dot; silicon carbide; photoluminescence; microstructure

Silicon-based materials still hold outstanding promise as the basis of renewable tandem photovoltaic solar cell due to its abundance and nontoxicity. Thereinto, silicon quantum dots (Si QDs) embedded in silicide dielectric matrix present tantalizing prospects as components in third-generation photovoltaic devices [1-3] due to its tunable band gap, which can utilize a wider range of solar spectrum and thus have higher efficiencies. Previous theoretical and experimental results[4-7] confirmed that the band gap can be modified by varying the size and the existing chemical environment of QDs. Both wide band gap and sufficient carrier mobility can be achieved by optimizing the separation and barrier height between Si QDs [8]. Both photoluminescence and high resolution transmission electron microscopy have been used to demonstrate the correlation between the optical band gap and discrete sizes of Si QDs, according to quantum confinement model [9, 10]. Previous investigations mainly focused on Si QDs embedded in silicon oxide or silicon nitride matrix [11-13], but not on those embedded in silicon carbide. However, silicon carbide (SiC) has the lower barrier height (~2.5 eV) than those of SiO2 (~9 eV) and Si3N4 (~5.3 eV). In such a case, the transport pos-

sibility of carriers among adjacent Si QDs can be increased due to the lower tunneling barriers for electrons and holes [14]. Thus it may be a good candidate matrix for photovoltaic solar cells. From this point of view, Si-rich SixC thin films can be used as the precursors to produce higher density of Si QDs with broader bands and higher mobility for all silicon tandem solar cell applications [15]. D. Y. Song et al. and K. Ostrikov et al. investigated Raman scattering and Fourier transform infrared absorption of Si QDs in α-SiC prepared by plasma-enhanced chemical vapor deposition (PECVD) without post-annealing[8,16,17]. The formation mechanism of these nano-objects, however, remains controversy, and Si clusters may be grown in the special reactive atmosphere depending on plasma conditions [16]. So it is still a great challenge to control the size and the density of Si QDs embedded in α-SiC synthesized by PECVD. Therefore, much more works are required to produce high-quality Si QDs, and to experimentally study the quantum confinement effect (QCE) of Si QDs. In the present letter, we reported on the preparation of Si QDs in α-SiC dielectric matrix through high-temperature annealing of non-stoichiometry SixC (x>1) thin films. Photolu-

Received date: April 15, 2015 Foundation item: Xi’An Science and Technology Plan Projects (CX12189WL37, CXY1352WL08); National Natural Science Foundation of China (51271139, 51171145); National Key Basic Research Development Program of China (“973” Program) (2010CB631002) Corresponding author: Xu Kewei, Professor, Xi’an University, Xi’an 710065, P. R. China, Tel: 0086-29-88258551, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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minescence (PL) measurement indicated obvious multi-band characteristics, and the optical band gap could be tuned from 2.36 to 3.47 eV by varying the atom ratio of silicon and carbon (RSi/C). This would be related with the size distribution of Si QDs as a result of quantum confinement effect, which will be discussed in hereinafter in details.

a

1 Experiment Si-rich SixC thin films were prepared on single-crystal (100)-oriented silicon wafers by magnetron co-sputtering under high vacuum of 3.4×10-6 Pa using separated silicon and carbon targets (2 in. diameter, purity of 99.99%). The supply power at targets and the deposition time were adjusted to obtain a series of samples with different RSi/C but the same film thickness of 120 nm. Post-annealing was then carried out in N2 atmosphere at 1100 oC for 1 h, the temperature rise/drop rate was kept at 25 o C/min. RSi/C was determined by the in-situ X-ray photoelectron spectroscopy of SPECS XRC1000 with Al Kα X-ray source (1486.5 eV) and a hemispherical energy analyzer. The size and microstructures were characterized by high resolution transmission electron microscopy (HRTEM) using a JEOL JEM-2010 electron microscopy with a 200 kV field emission gun. PL measurements were carried out at room temperature using a spectra analyzer system (RF-5301, Photo Research Co.) with excitation wavelength of 320 nm (3.87 eV) which was adequate for band-to-band luminescence excitation.

2

5 nm

b

5 nm c 5 nm

5 nm

3 nm

3 nm

Results and Discussion

Five samples (R1, R2, R3, R4 and R5) were involved in this study, corresponding to different RSi/C of 2.0, 3.0, 3.8, 4.3 and 5.2, respectively. Taking sample R4 for an example, Fig.1a shows HRTEM image of the as-deposited SixC thin films, where there is no Si QDs, it looks like homogeneous. Whereas, a great many of well-separated Si QDs appear in amorphous dielectric matrix after post-annealing, as shown in Fig.1b. Si QDs can exist in crystal state (c-Si QDs) or amorphous state (infancy of c-Si QDs, α-Si QDs), as indicated by SAED in the inset of Fig.1b. The density may be up to the maximum of 9×1013 cm-2 or so for sample R4. As the RSi/C is increased further, the density of Si QDs decreases gradually and reaches a value of about 5×1013 cm-2 in sample R5. High-density-dot samples are made up of Si QDs which can be divided into a series of size ranges, like, ~1.0, 1.0~2.0, 2.0~3.0 and 4.0 nm in size, as shown in Fig.1c. Fig.2 shows the statistical histogram of size distribution of Si QDs. In sample R4, more than 87% of the QDs fall within a narrow size range of 0.5~2 nm, as shown in Fig.2a, while there are two peaks at average sizes of 1.45 nm (60%) and 3.63 nm (30%) in sample R2, as shown in Fig.2b. This size distribution may generate multi-band PL spectrum with FWHM (full width at half maximum) of 0.77 eV, which will be addressed in following sections. Fig.3a shows the PL spectra of the five samples (R1~R5). It is worth noting that sample R4 has the widest PL individual

Fig.1

7 nm

HRTEM images of Si QDs embedded in silicon carbide thin films of sample R4 (a), sample R4 after post annealing (b), and sample R5 (c)

components (sub-bands) in a spectral range of 357.3~523.2 nm (2.37~3.47 eV). Taking sample R4 for the typical example, the deconvoluted spectrum is shown in the inset of Fig.3b. As RSi/C is increased from 2.0 (R1) to 4.3 (R4), the number of PL sub-bands increase from 4 to 7 and then decrease to 5 of sample R5 with RSi/C of 5 (R5), as shown in Fig.3b, which correspond well to the size distribution in Fig.2. Simultaneously, the total PL intensity increases gradually when RSi/C is increased from 2 to 4.3 and then is decreased, as shown in Fig.3c. Higher PL intensity can be attributed to the increase of the density of Si QDs in a particular size range, since larger RSi/C is favorable for the nucleation site of Si QDs. On the other hand, nitrogen atoms may passivate the dangling bonds of Si aotms produced by phase separation, resulting in ultra-small Si QDs. Optimized PL intensity and spectrum range are obtained in sample R4 with RSi/C of 4.3. As for the samples with RSi/C larger than 4.3, reaction heat overcomes the potential barrier at the Si-QD boundary and coalesces, or Ostwald

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PL Intensity/a.u.

40 30 20

0

1

2

5

6

Sample R2 RSi/C=3

30

Frequency/%

4

b

35

shift

2.7 3.0 Energy/eV

3.3

3.6

3 4 Si/C Ratio

5

2 0

3.6 2.1 2.4 2.4 2.7 2.7 3.0 3.0 3.3 3.6 Energy (eV) Energy/eV

2.1 2.4 2.7 3.0 3.3 3.6 3.9 Energy/eV

c

2

4

2.0

E(eV)=1.56+2.40d

1.8

2

N. M. Park et a This work

d [10]

1.6 1.4 1.2 1.0 2.1 2.4 2.7 3.0 3.3 3.6 3.9 Energy/eV

RSi/C (R1~R5); (b) sub-bands of Lorentzian-fitted PL spectra of

10

different samples and one typical deconvoluted spectrum of

5

samples R4 in the inset; (c) integrated PL intensity; (d) meas0

1

2 3 4 5 6 Si QD Size/nm

7

ured (filled stars) and fitted (dash line) sizes of Si QDs

8

ripening occurs among adjacent dots[18], which leads to reducing the density of Si QDs and narrow size distribution. To demonstrate this issue, the sizes of QDs can be calculated by the measured PL sub-band energy according to the empirical equation of E(eV)=1.56+2.40/d2, where E is energy gap of the three-dimensionally confined Si QD, d is the dot size. The calculated QD sizes were plotted in Fig.3d, which is in good agreement with the fitted result according to QCE model[10]. It is clear that carriers are confined in Si QDs and the multi-band PL spectrum origins from QCE of Si QDs. In addition, Stokes shift, the difference between luminescence energy and absorption energy, was also analyzed, and the results are listed in Table 1. Lockwood[19] and Allan[20] reported that a nanocrystal Si dot with a size of about 1.5 nm corresponds to a Stokes shift of about 1 eV, while the Stokes shift is smaller than 1 eV for α-Si QDs. Therefore, there exist two kinds of Si QDs embedded in silicon carbide dielectric matrix: α-Si QDs (3.1~3.5 eV) and c-Si QDs (2.3~2.95 eV). This multi-band configuration is extremely advantageous to improve the photoelectric conversion efficiency of photovoltaic solar cell.

Stokes

8 7 6 5 4 3 2 1 0

2.4

6

Fig.3 (a) Room temperature PL spectra of SixC films with different

and sample R2 (b) (solid curves represent the fitting using a

gap

R1

b

8

15

Gaussian function)

Sub-band

R2

9 8 7 6 5 4 3 2

20

Fraction histograms of the sizes of Si QDs in sample R4 (a)

Table 1

R3

25

0

Fig.2

3

Integ. PL Intensity/a.u.

0 40

R4

2.1

10

a

R5

Quantum-dot Size/nm

Frequency/%

50

8 7 6 5 4 3 2 1 0

Intensity/a.u. (a.u.) PLPLintensity

a Sample R4 RSi/C=4.3

Si/C Ratio

60

Stokes shift of the measured sub-bands gap (eV) 2.32

2.54

2.66

2.75

2.94

3.14

3.36

1.46

1.24

1.12

1.03

0.84

0.64

0.42

3 Conclusions 1) The size-dependent optical properties of Si QDs embedded in silicon carbide can be demonstrated by PL measurement and high resolution transmission electron microscopy. 2) Si QDs in SixC thin films can be divided into a series of size ranges, including ~1.0, 1.0~2.0, 2.0~3.0 and 4.0 nm in sizes. PL spectrum is in the range from green to ultraviolet (2.3~3.5 eV), and can also be tuned by changing RSi/C. 3) Stokes shift indicates that there exist α-Si QDs and c-Si QDs. This multi-band configuration is closely related with the size distribution of Si QDs.

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