- Email: [email protected]
InXGa1-XN films deposited by reactive RF-sputtering
Journal of Non-Crystalline Solids 358 (2012) 2362–2365
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
InXGa1-XN films deposited by reactive RF-sputtering Takashi Itoh a,⁎, Shun Hibino a, Tatsuro Sahashi a, Yoshinori Kato a, Sunao Koiso a, Fumitaka Ohashi b, Shuichi Nonomura b a b
Department of Electrical and Electronic Engineering, Gifu University, Japan Environment and Renewable Energy Systems, Gifu University, Japan
a r t i c l e
i n f o
Article history: Received 17 August 2011 Received in revised form 30 December 2011 Available online 2 February 2012 Keywords: InXGa1-XN; Photo-absorption layer material; Low temperature deposition; Simultaneous reactive rf- magnetron sputtering; Photosensitivity
a b s t r a c t The low temperature deposition of photoconductive InXGa1-XN films was carried out by simultaneous reactive rf- magnetron sputtering. In the films deposited using Ga and In targets, XRD peaks corresponding to wurtzite InXGa1-XN were observed. The optical band gap energy decreased from 3.27 eV to 1.63 eV with increasing the In composition ratio X = In/(Ga + In) from 0 to 0.58. The N composition ratio N/(Ga + In) and photosensitivity of the film with the optical band gap energy of 2.09 eV were 0.68 and 1.89, respectively. In order to improve the photosensitivity, GaN and InN targets were used instead of Ga and In targets. The N/(Ga + In) and photosensitivity of the film with the optical band gap energy of 2.08 eV deposited using GaN and InN targets were 0.86 and 252, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Multi-junction is one of techniques to improve the conversion efficiency of thin film solar cells. In multi-junction thin film solar cells, photo-absorption layer materials with various band gap energies are required. Especially, materials with band gap energy from 2 eV to 2.5 eV are necessary as a photo-absorption layer material for top cell. Since band gap energy of almost photo-absorption layer materials used in thin film solar cells is less than 2.0 eV. The band gap energy of crystalline Indium gallium nitride (InXGa1-XN) is tunable from 0.6 eV to 3.4 eV depending on the In composition ratio [1]. Therefore, InXGa1-XN is considered as a promising material in multi-junction thin film solar cells with a variation of the band gap energy [2,3]. Conventional deposition process for the crystalline InXGa1-XN is metalorganic vapor phase epitaxy (MOVPE) at substrate temperature over 750 °C [3]. In multi-junction thin film solar cells, however, low temperature deposition is required. Low temperature deposition of amorphous InXGa1-XN (a-InXGa1-XN) and microcrystalline InXGa1-XN (μc-InX Ga1-XN) films would be possible. Since a- and μc-GaN films can be deposited at low substrate temperature [4]. In a-GaN films, photoconductivity is observed [4]. Therefore, a- or μc-InXGa1-XN films are expected as photo-absorption layer material with the band gap energy from 2 to 2.5 eV for top cell in multi-junction thin film solar cells. However, the deposition and properties of a- and μc-InXGa1-XN alloy films have not been known.
⁎ Corresponding author. Tel./fax: + 81 58 293 2680. E-mail address: [email protected] (T. Itoh). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.01.023
In this work, InXGa1-XN films with various In composition ratios have been deposited at low substrate temperature by simultaneous reactive rf- magnetron sputtering. The structural, optical and electrical properties of the InXGa1-XN films have been studied also.
2. Experimental InXGa1-XN films were deposited on Corning 7059 glass and quartz glass substrates at room temperature (around 30 °C) by simultaneous reactive rf- magnetron sputtering. Ga and In metals were used as source targets. Mixture of Ar and N2 gases were used for the sputtering and N source gases and the partial pressures of Ar and N2 gases were 1.5 Pa and 3.5 Pa, respectively. The rf-power (13.56 MHz) inputted into the Ga target PGa was kept at 40 W. In order to control the In composition ratio X = In/(Ga + In) of the films, the rf-power inputted into the In target PIn was varied from 0 W to 40 W. The film thickness was from 0.30 to 0.76 μm. The In composition ratio X = In/(Ga + In) and N composition ratio N/(Ga + In) were obtained from X-ray photoelectron spectroscopy (XPS) using Quantera-SXM (ULVAC). Structure of the samples was investigated by X-ray Diffraction (XRD). The optical absorption coefficient spectra for the films were obtained from the optical transmittance spectrum (HITACHI U-4000S). The optical band gap energy EO was defined as the photon energy at the absorption coefficient of 5000 cm− 1. Finally, the dark conductivity σd and photoconductivity σp of the films were measured at room temperature (around 30 °C) in air. An Al gap electrode was evaporated on the films. In the σp measurement, AM-1.5 light (100 mW/cm2) was used as irradiation light.
T. Itoh et al. / Journal of Non-Crystalline Solids 358 (2012) 2362–2365
0.8 Deposition rate of InN films (nm/min)
3
X=In/(Ga+In)
0.6
2
(103)
(101) Intensity (a.u.)
0.18 0.28
30
40
50
60
70
2θ (Deg.)
B
70
(103)
60
50
(102) 40
(101)
1 0
0.4
X 0.08
0.57
2θ (Deg.)
Fig. 1(A) shows the dependence of In composition ratio X = In/ (Ga + In) on rf-power imputed into the In target PIn. The X increased from 0 to around 0.57 with increasing the PIn from 0 W to 40 W. Inset in the Fig. 1(A) shows the dependence of deposition rate for InN films on the PIn. Here, the deposition of the InN films was carried out. The deposition conditions were the PIn from 5 W to 40 W, substrate temperature of room temperature (around 30), the partial pressures of Ar and N2 gases of 1.5 Pa and 3.5 Pa, respectively. The deposition rate for the InN films increased linearly with increasing the PIn from 5 W to 40 W. This result indicates that the X is controllable by the PIn. Fig. 1(B) shows the dependence of the N composition ratio N/ (Ga + In) on the X in InXGa1-XN films. The N/(Ga + In) decreased from 0.67 to 0.38 with increasing the X from 0 to 0.57. The N/(Ga + In) of the films was smaller than that of stoichiometoric InXGa1-XN. Fig. 2(A) shows the XRD patterns for the films with various X. In the XRD patterns for the films, (101), (102) and (103) XRD peaks corresponding to wurtzite InXGa1-XN were observed. The positions of the XRD peaks shifted to smaller angle with increasing the X. Fig. 2(B) shows the variation of the positions for XRD peaks as a function of the X. In Fig. 2(B), the dashed lines express positions of XRD peaks varied depending on the theoretical calculations of Vegard's low with lattice constants of wurtzite GaN and InN. Here, the lattice
(102)
A
3. Results
A
2363
0
10
20 30 PIn (W)
30
40
0
0.2
0.4
0.6
0.8
1
X=In/(Ga+In) Fig. 2. (A) XRD patterns for the films with various In composition ratio X = In/(Ga + In) deposited using Ga and In targets and (B) dependence of peak positions for XRD peaks on the X. The dashed lines express positions for XRD peaks varied depending on the theoretical calculations of Vegard's low with lattice constants of wurtzite GaN and InN.
0.2
0 0
10
20
30
40
PIn (W)
B
1.1
GaN & InN
1
N/(Ga+In)
0.9 0.8 0.7 0.6 0.5
Ga & In
0.4 0.3
0
0.1
0.2
0.3
0.4
0.5
0.6
X=In/(Ga+In) Fig. 1. (A) Dependence of In composition ratio X = In/(Ga + In) on rf-power imputed into the In target PIn. Inset shows the dependence of deposition rate of InN films on the PIn. (B) Dependence of N composition ratio N/Ga + In) on the X. Close circles are the N/(Ga + In) of the film deposited using Ga and In targets and open circles are those deposited using GaN and InN targets.
constants of wurtzite GaN are a of 3.18 Å and c of 5.19 Å and those of wurtzite InN are a of 3.54 Å and c of 5.71 Å [5]. The experimental and calculated results show good match in variation of positions for XRD peaks as a function of X. The crystallite size obtained from XRD peaks using Scherrer's formula was some ten nm. These results indicate that the deposited films are microcrystalline wurtzite InXGa1-XN alloys. Fig. 3 shows the dependence of optical band gap energy EO on the X. Here, Tauc's gap is generally used as the EO in amorphous materials. However, the films were μc-InXGa1-XN films. It is not known that Tauc's gap is used as EO of microcrystalline materials, because microcrystalline materials have hetero-structure. In amorphous materials, photon energy at absorption coefficient of 5000 cm − 1 is also used as EO. Therefore, the EO was defined as the photon energy at the absorption coefficient of 5000 cm − 1. The EO decreased from 3.27 eV to 1.63 eV with increasing the X from 0 to 0.58. Similar tendency in crystalline InXGa1-XN is also reported [2]. However, the EO at 2.5 eV for the films with the In/(Ga + In) of 10% was smaller than that of epitaxial InXGa1-XN. The N/(Ga + In) of the films was smaller than that of stoichiometric InXGa1-XN and the N/(Ga + In) decreased with increasing the In/(Ga + In). Therefore, one of possibilities might be small N/(Ga + In). However, the detail was not understood yet and further investigation was needed. This result indicates that the EO can be controlled from 2 eV to 2.5 eV by the X from 0.27 to 0.08 in μc-InXGa1-XN films.
2364
T. Itoh et al. / Journal of Non-Crystalline Solids 358 (2012) 2362–2365
4
EO (eV)
3
GaN & InN 2
Ga & In
1 0
0.1
0.2
0.3
0.4
0.5
0.6
X=In/(Ga+In) Fig. 3. Dependence of optical band gap energy EO on In composition ratio X = In/(Ga+ In). Closed circles are the EO of the film deposited using Ga and In targets and open circles are those deposited using GaN and InN targets.
The dark conductivity σd and photoconductivity σp of the films with the EO (X) of 2.20 eV (0.18) and 2.09 eV (0.29) were obtained from dark current and photocurrent — voltage characteristics. The obtained σd and σp were shown in Fig. 4. The closed circles and triangles are the σd and σp, respectively. The σd of the films with the EO of 2.20 eV is smaller than that with the EO of 2.09 eV. In both films, the σp was larger than the σd. This result suggests that the films are photoconductive. However, the photosensitivities σp/σd of the films with the EO of 2.20 eV and 2.09 eV were 6.51 and 1.89, respectively and these values were very small. 4. Discussion The σp/σd of the films deposited using Ga and In targets was small. The small σp/σd would be caused by the small N/(Ga + In). It is reported that shallow donor band is probably formed from Nvacancy in crystalline GaN [6]. The N/(Ga + In) of the films with the EO of 2.20 eV and 2.09 eV were smaller than that of stoichiometric InXGa1-XN as shown in Fig. 1(B). N-vacancies exist as a large density in the films, which resulted large density of the native donor. The large density of the native donor induced the large σd. As the result, the σp/σd is small. Based on above discussion, it is expected that the
10-2 p
Conductivity (S/cm)
10-3 d
(Ga & In)
(Ga & In)
10-4 10-5
p
(GaN & InN)
10-6 10-7 d
(GaN & InN)
10-8 2
2.1
2.2
2.3
EO (eV) Fig. 4. Variation of dark conductivity σd and photoconductivity σp as a function of optical band gap energy EO. Closed circles and triangles are σd and σp of the films deposited using Ga and In targets, respectively. Open circles and triangles are σd and σp of the film deposited using GaN and InN targets, respectively.
increase of the N/Ga + In) led small σd and large σp/σd. To realize the large σp/σd with small density of N-vacancy, therefore, we have to discuss in the small N/(Ga + In). One possibility as causes of small N/(Ga + In) in the films is small density of N radical in the process of sputtering. In MOCVD process of GaN, NH3 is generally used as N source [2]. In this reactive rf-sputtering process of InXGa1-XN, however, N2 gas was used as nitrogen source. The decomposition energy of N2 is larger than that of NH3 due to the difference in the binding energy of N-N bond (945.33 kJ/mol) and N-H bond (less than 339 kJ/mol) [7]. Therefore, use of N2 as nitrogen source gas led the small density of N radical in the reactive rf-sputtering process. As the results, the N/(Ga + In) is small in the films. In order to increase the N/(Ga + N) and the σp/σd, the deposition of InXGa1-XN films was conducted using GaN and InN targets instead of Ga and In targets. The rf-power imputed into GaN target was kept at 40 W and that into InN target was varied from 15 W to 20 W. Other deposition conditions were the same as deposition using Ga and In targets. The film thickness was around 240 nm. The EO of the film with the X of 0.28 and 0.35 deposited using GaN and InN targets were 2.21 eV and 2.08 eV, respectively. As shown in Fig. 3, the effect of the difference in targets would be small on the EO and the EO would depend on the X. The N/(Ga + In) of the films deposited using GaN and InN targets was larger than that deposited using Ga and In targets as shown in Fig. 1(B). This result indicates that the N/(Ga + In) was improved by using GaN and InN targets instead of Ga and In targets. In the process using Ga and In targets, the nitriding for Ga and In would occur after sputtered out from the targets. On the other hand, there is possibility that Ga and In combined with N are sputtered out from targets in the process using GaN and InN targets. The density of N radical in the process using GaN and InN targets would also be larger than that using Ga and In targets. Therefore, the N/(Ga + In) was improved by using GaN and InN targets instead of Ga and In targets. The dark conductivity σd and photoconductivity σp of the films with the EO (X) of 2.21 eV (0.28) and 2.08 eV (0.35) were obtained from dark current and photocurrent – voltage characteristics. The obtained σd and σp were also shown in Fig. 4. The open circles and triangles are the σd and σp, respectively. The variation of σd and σp in the films deposited using GaN and InN targets showed similar tendency that deposited using Ga and In targets. As shown in Fig. 4, the σd of the film deposited using GaN and InN targets was smaller than that deposited using Ga and In targets, compared with the film with almost the same EO. The σp/σd of the films deposited using GaN and InN targets was larger than that deposited using Ga and In targets. There would be two reasons why the σd decreased and the σp/σd increased. One is the improvement of N/(Ga + N). The N/(Ga + N) increased by using GaN and InN targets. This improvement of N/(Ga + In) would led the decrease of density of N-vacancy i.e. native donor. The other is the change in the structure. The structure of the films deposited using Ga and In targets was microcrystalline. On the other hand, the structure of the film deposited using GaN and InN targets would be amorphous. Since no XRD peaks were observed in the XRD pattern for the film deposited using GaN and InN targets. The doping efficiency in amorphous materials is smaller than that in microcrystalline materials. Therefore, the doping efficiency in the film deposited using GaN and InN targets would be smaller than that deposited using Ga and In targets. As the results, the σd decreased and the σp/σd increased by using GaN and InN targets. In this work, the films were deposited at room temperature. However, the substrate temperature of room temperature would not be best for the deposition of the films. We will report the substrate temperature dependence and the thermal annealing effect in a future publication. Based on above results, the photoconductive a-InXGa1-XN with large σp/σd can be deposited by using GaN and InN targets at low substrate temperature. The a-InXGa1-XN films have a possibility to use as photo-absorption layer material of top cell in multi-junction thin film
T. Itoh et al. / Journal of Non-Crystalline Solids 358 (2012) 2362–2365
solar cells. However, the N/(Ga + In) was still smaller than that of stoichiometric InXGa1-XN. Therefore, further improvement of σp/σd is expected by increasing N/(Ga + In) more.
2365
Acknowledgement This work was partially founded by the International Research Center for Innovative Solar Cell Program from NEDO.
5. Summary InXGa1-XN films were deposited at room temperature by simultaneous reactive rf- magnetron sputtering using Ga and In targets. The deposited films were microcrystalline wurtzite InXGa1-XN. The EO can be controlled from 3.27 eV to 1.63 eV by the X = In/(Ga + In) from 0 to 0.58. The films were photoconductive. However, the σp/σd of 1.89 was small in the film with the EO of 2.09 eV and the N/(Ga + In) of 0.68. The σp/σd of 252 in the film with the EO of 2.08 eV was improved by using GaN and InN targets instead of Ga and In targets. This improvement would be caused by the increase of N/(Ga + In) and the structural change, because the N/(Ga + In) of the film deposited using GaN and InN targets was 0.86 and the film would be amorphous InXGa1-XN. Based on above results, the photoconductive a-InXGa1-XN with large σp/σd deposited by using GaN and InN targets have a possibility to use as photo-absorption layer material of top cell in multi-junction thin film solar cells.
References [1] V.Y. Davydov, A.A. Klochikhin, V.V. Emtsev, D.A. Kurdyukov, S.V. Ivanov, V.A. Vekshin, F. Bechstedt, J. Furthmuller, J. Aderhold, J. Graul, A.V. Mudryi, H. Harima, A. Hashimoto, A. Yamamoto, E.E. Haller, Phys. Status Solidi B 234 (2002) 787–796. [2] O. Jani, I. Ferguson, C. Honsberg, S. Kurtz, Appl. Phys. Lett. 91 (2007) 132117–132119. [3] M. Horie, K. Sugita, A. Hashimoto, A. Yamamoto, Sol. Energy Mater. Sol. Cells 93 (2009) 1013. [4] S. Nonomura, S. Kobayashi, T. Gotoh, S. Hirata, T. Ohmori, T. Itoh, S. Nitta, K. Morigaki, J. Non-Cryst. Solids 198–200 (1996) 174–177. [5] I. Akasaki, H. Amano, Jpn. J. Appl. Phys. 36 (1997) 5393–5408. [6] D.C. Look, D.C. Reynolds, W. Kim, O. Aktas, A. Botchkarev, A. Salvador, H. Morkoc, J. Appl. Phys. 80 (1996) 2960–2963. [7] J.A. Kerr, Strength of chemical bonds, in: D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 1996, pp. 9-51–9-62.
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
InXGa1-XN films deposited by reactive RF-sputtering Takashi Itoh a,⁎, Shun Hibino a, Tatsuro Sahashi a, Yoshinori Kato a, Sunao Koiso a, Fumitaka Ohashi b, Shuichi Nonomura b a b
Department of Electrical and Electronic Engineering, Gifu University, Japan Environment and Renewable Energy Systems, Gifu University, Japan
a r t i c l e
i n f o
Article history: Received 17 August 2011 Received in revised form 30 December 2011 Available online 2 February 2012 Keywords: InXGa1-XN; Photo-absorption layer material; Low temperature deposition; Simultaneous reactive rf- magnetron sputtering; Photosensitivity
a b s t r a c t The low temperature deposition of photoconductive InXGa1-XN films was carried out by simultaneous reactive rf- magnetron sputtering. In the films deposited using Ga and In targets, XRD peaks corresponding to wurtzite InXGa1-XN were observed. The optical band gap energy decreased from 3.27 eV to 1.63 eV with increasing the In composition ratio X = In/(Ga + In) from 0 to 0.58. The N composition ratio N/(Ga + In) and photosensitivity of the film with the optical band gap energy of 2.09 eV were 0.68 and 1.89, respectively. In order to improve the photosensitivity, GaN and InN targets were used instead of Ga and In targets. The N/(Ga + In) and photosensitivity of the film with the optical band gap energy of 2.08 eV deposited using GaN and InN targets were 0.86 and 252, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Multi-junction is one of techniques to improve the conversion efficiency of thin film solar cells. In multi-junction thin film solar cells, photo-absorption layer materials with various band gap energies are required. Especially, materials with band gap energy from 2 eV to 2.5 eV are necessary as a photo-absorption layer material for top cell. Since band gap energy of almost photo-absorption layer materials used in thin film solar cells is less than 2.0 eV. The band gap energy of crystalline Indium gallium nitride (InXGa1-XN) is tunable from 0.6 eV to 3.4 eV depending on the In composition ratio [1]. Therefore, InXGa1-XN is considered as a promising material in multi-junction thin film solar cells with a variation of the band gap energy [2,3]. Conventional deposition process for the crystalline InXGa1-XN is metalorganic vapor phase epitaxy (MOVPE) at substrate temperature over 750 °C [3]. In multi-junction thin film solar cells, however, low temperature deposition is required. Low temperature deposition of amorphous InXGa1-XN (a-InXGa1-XN) and microcrystalline InXGa1-XN (μc-InX Ga1-XN) films would be possible. Since a- and μc-GaN films can be deposited at low substrate temperature [4]. In a-GaN films, photoconductivity is observed [4]. Therefore, a- or μc-InXGa1-XN films are expected as photo-absorption layer material with the band gap energy from 2 to 2.5 eV for top cell in multi-junction thin film solar cells. However, the deposition and properties of a- and μc-InXGa1-XN alloy films have not been known.
⁎ Corresponding author. Tel./fax: + 81 58 293 2680. E-mail address: [email protected] (T. Itoh). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2012.01.023
In this work, InXGa1-XN films with various In composition ratios have been deposited at low substrate temperature by simultaneous reactive rf- magnetron sputtering. The structural, optical and electrical properties of the InXGa1-XN films have been studied also.
2. Experimental InXGa1-XN films were deposited on Corning 7059 glass and quartz glass substrates at room temperature (around 30 °C) by simultaneous reactive rf- magnetron sputtering. Ga and In metals were used as source targets. Mixture of Ar and N2 gases were used for the sputtering and N source gases and the partial pressures of Ar and N2 gases were 1.5 Pa and 3.5 Pa, respectively. The rf-power (13.56 MHz) inputted into the Ga target PGa was kept at 40 W. In order to control the In composition ratio X = In/(Ga + In) of the films, the rf-power inputted into the In target PIn was varied from 0 W to 40 W. The film thickness was from 0.30 to 0.76 μm. The In composition ratio X = In/(Ga + In) and N composition ratio N/(Ga + In) were obtained from X-ray photoelectron spectroscopy (XPS) using Quantera-SXM (ULVAC). Structure of the samples was investigated by X-ray Diffraction (XRD). The optical absorption coefficient spectra for the films were obtained from the optical transmittance spectrum (HITACHI U-4000S). The optical band gap energy EO was defined as the photon energy at the absorption coefficient of 5000 cm− 1. Finally, the dark conductivity σd and photoconductivity σp of the films were measured at room temperature (around 30 °C) in air. An Al gap electrode was evaporated on the films. In the σp measurement, AM-1.5 light (100 mW/cm2) was used as irradiation light.
T. Itoh et al. / Journal of Non-Crystalline Solids 358 (2012) 2362–2365
0.8 Deposition rate of InN films (nm/min)
3
X=In/(Ga+In)
0.6
2
(103)
(101) Intensity (a.u.)
0.18 0.28
30
40
50
60
70
2θ (Deg.)
B
70
(103)
60
50
(102) 40
(101)
1 0
0.4
X 0.08
0.57
2θ (Deg.)
Fig. 1(A) shows the dependence of In composition ratio X = In/ (Ga + In) on rf-power imputed into the In target PIn. The X increased from 0 to around 0.57 with increasing the PIn from 0 W to 40 W. Inset in the Fig. 1(A) shows the dependence of deposition rate for InN films on the PIn. Here, the deposition of the InN films was carried out. The deposition conditions were the PIn from 5 W to 40 W, substrate temperature of room temperature (around 30), the partial pressures of Ar and N2 gases of 1.5 Pa and 3.5 Pa, respectively. The deposition rate for the InN films increased linearly with increasing the PIn from 5 W to 40 W. This result indicates that the X is controllable by the PIn. Fig. 1(B) shows the dependence of the N composition ratio N/ (Ga + In) on the X in InXGa1-XN films. The N/(Ga + In) decreased from 0.67 to 0.38 with increasing the X from 0 to 0.57. The N/(Ga + In) of the films was smaller than that of stoichiometoric InXGa1-XN. Fig. 2(A) shows the XRD patterns for the films with various X. In the XRD patterns for the films, (101), (102) and (103) XRD peaks corresponding to wurtzite InXGa1-XN were observed. The positions of the XRD peaks shifted to smaller angle with increasing the X. Fig. 2(B) shows the variation of the positions for XRD peaks as a function of the X. In Fig. 2(B), the dashed lines express positions of XRD peaks varied depending on the theoretical calculations of Vegard's low with lattice constants of wurtzite GaN and InN. Here, the lattice
(102)
A
3. Results
A
2363
0
10
20 30 PIn (W)
30
40
0
0.2
0.4
0.6
0.8
1
X=In/(Ga+In) Fig. 2. (A) XRD patterns for the films with various In composition ratio X = In/(Ga + In) deposited using Ga and In targets and (B) dependence of peak positions for XRD peaks on the X. The dashed lines express positions for XRD peaks varied depending on the theoretical calculations of Vegard's low with lattice constants of wurtzite GaN and InN.
0.2
0 0
10
20
30
40
PIn (W)
B
1.1
GaN & InN
1
N/(Ga+In)
0.9 0.8 0.7 0.6 0.5
Ga & In
0.4 0.3
0
0.1
0.2
0.3
0.4
0.5
0.6
X=In/(Ga+In) Fig. 1. (A) Dependence of In composition ratio X = In/(Ga + In) on rf-power imputed into the In target PIn. Inset shows the dependence of deposition rate of InN films on the PIn. (B) Dependence of N composition ratio N/Ga + In) on the X. Close circles are the N/(Ga + In) of the film deposited using Ga and In targets and open circles are those deposited using GaN and InN targets.
constants of wurtzite GaN are a of 3.18 Å and c of 5.19 Å and those of wurtzite InN are a of 3.54 Å and c of 5.71 Å [5]. The experimental and calculated results show good match in variation of positions for XRD peaks as a function of X. The crystallite size obtained from XRD peaks using Scherrer's formula was some ten nm. These results indicate that the deposited films are microcrystalline wurtzite InXGa1-XN alloys. Fig. 3 shows the dependence of optical band gap energy EO on the X. Here, Tauc's gap is generally used as the EO in amorphous materials. However, the films were μc-InXGa1-XN films. It is not known that Tauc's gap is used as EO of microcrystalline materials, because microcrystalline materials have hetero-structure. In amorphous materials, photon energy at absorption coefficient of 5000 cm − 1 is also used as EO. Therefore, the EO was defined as the photon energy at the absorption coefficient of 5000 cm − 1. The EO decreased from 3.27 eV to 1.63 eV with increasing the X from 0 to 0.58. Similar tendency in crystalline InXGa1-XN is also reported [2]. However, the EO at 2.5 eV for the films with the In/(Ga + In) of 10% was smaller than that of epitaxial InXGa1-XN. The N/(Ga + In) of the films was smaller than that of stoichiometric InXGa1-XN and the N/(Ga + In) decreased with increasing the In/(Ga + In). Therefore, one of possibilities might be small N/(Ga + In). However, the detail was not understood yet and further investigation was needed. This result indicates that the EO can be controlled from 2 eV to 2.5 eV by the X from 0.27 to 0.08 in μc-InXGa1-XN films.
2364
T. Itoh et al. / Journal of Non-Crystalline Solids 358 (2012) 2362–2365
4
EO (eV)
3
GaN & InN 2
Ga & In
1 0
0.1
0.2
0.3
0.4
0.5
0.6
X=In/(Ga+In) Fig. 3. Dependence of optical band gap energy EO on In composition ratio X = In/(Ga+ In). Closed circles are the EO of the film deposited using Ga and In targets and open circles are those deposited using GaN and InN targets.
The dark conductivity σd and photoconductivity σp of the films with the EO (X) of 2.20 eV (0.18) and 2.09 eV (0.29) were obtained from dark current and photocurrent — voltage characteristics. The obtained σd and σp were shown in Fig. 4. The closed circles and triangles are the σd and σp, respectively. The σd of the films with the EO of 2.20 eV is smaller than that with the EO of 2.09 eV. In both films, the σp was larger than the σd. This result suggests that the films are photoconductive. However, the photosensitivities σp/σd of the films with the EO of 2.20 eV and 2.09 eV were 6.51 and 1.89, respectively and these values were very small. 4. Discussion The σp/σd of the films deposited using Ga and In targets was small. The small σp/σd would be caused by the small N/(Ga + In). It is reported that shallow donor band is probably formed from Nvacancy in crystalline GaN [6]. The N/(Ga + In) of the films with the EO of 2.20 eV and 2.09 eV were smaller than that of stoichiometric InXGa1-XN as shown in Fig. 1(B). N-vacancies exist as a large density in the films, which resulted large density of the native donor. The large density of the native donor induced the large σd. As the result, the σp/σd is small. Based on above discussion, it is expected that the
10-2 p
Conductivity (S/cm)
10-3 d
(Ga & In)
(Ga & In)
10-4 10-5
p
(GaN & InN)
10-6 10-7 d
(GaN & InN)
10-8 2
2.1
2.2
2.3
EO (eV) Fig. 4. Variation of dark conductivity σd and photoconductivity σp as a function of optical band gap energy EO. Closed circles and triangles are σd and σp of the films deposited using Ga and In targets, respectively. Open circles and triangles are σd and σp of the film deposited using GaN and InN targets, respectively.
increase of the N/Ga + In) led small σd and large σp/σd. To realize the large σp/σd with small density of N-vacancy, therefore, we have to discuss in the small N/(Ga + In). One possibility as causes of small N/(Ga + In) in the films is small density of N radical in the process of sputtering. In MOCVD process of GaN, NH3 is generally used as N source [2]. In this reactive rf-sputtering process of InXGa1-XN, however, N2 gas was used as nitrogen source. The decomposition energy of N2 is larger than that of NH3 due to the difference in the binding energy of N-N bond (945.33 kJ/mol) and N-H bond (less than 339 kJ/mol) [7]. Therefore, use of N2 as nitrogen source gas led the small density of N radical in the reactive rf-sputtering process. As the results, the N/(Ga + In) is small in the films. In order to increase the N/(Ga + N) and the σp/σd, the deposition of InXGa1-XN films was conducted using GaN and InN targets instead of Ga and In targets. The rf-power imputed into GaN target was kept at 40 W and that into InN target was varied from 15 W to 20 W. Other deposition conditions were the same as deposition using Ga and In targets. The film thickness was around 240 nm. The EO of the film with the X of 0.28 and 0.35 deposited using GaN and InN targets were 2.21 eV and 2.08 eV, respectively. As shown in Fig. 3, the effect of the difference in targets would be small on the EO and the EO would depend on the X. The N/(Ga + In) of the films deposited using GaN and InN targets was larger than that deposited using Ga and In targets as shown in Fig. 1(B). This result indicates that the N/(Ga + In) was improved by using GaN and InN targets instead of Ga and In targets. In the process using Ga and In targets, the nitriding for Ga and In would occur after sputtered out from the targets. On the other hand, there is possibility that Ga and In combined with N are sputtered out from targets in the process using GaN and InN targets. The density of N radical in the process using GaN and InN targets would also be larger than that using Ga and In targets. Therefore, the N/(Ga + In) was improved by using GaN and InN targets instead of Ga and In targets. The dark conductivity σd and photoconductivity σp of the films with the EO (X) of 2.21 eV (0.28) and 2.08 eV (0.35) were obtained from dark current and photocurrent – voltage characteristics. The obtained σd and σp were also shown in Fig. 4. The open circles and triangles are the σd and σp, respectively. The variation of σd and σp in the films deposited using GaN and InN targets showed similar tendency that deposited using Ga and In targets. As shown in Fig. 4, the σd of the film deposited using GaN and InN targets was smaller than that deposited using Ga and In targets, compared with the film with almost the same EO. The σp/σd of the films deposited using GaN and InN targets was larger than that deposited using Ga and In targets. There would be two reasons why the σd decreased and the σp/σd increased. One is the improvement of N/(Ga + N). The N/(Ga + N) increased by using GaN and InN targets. This improvement of N/(Ga + In) would led the decrease of density of N-vacancy i.e. native donor. The other is the change in the structure. The structure of the films deposited using Ga and In targets was microcrystalline. On the other hand, the structure of the film deposited using GaN and InN targets would be amorphous. Since no XRD peaks were observed in the XRD pattern for the film deposited using GaN and InN targets. The doping efficiency in amorphous materials is smaller than that in microcrystalline materials. Therefore, the doping efficiency in the film deposited using GaN and InN targets would be smaller than that deposited using Ga and In targets. As the results, the σd decreased and the σp/σd increased by using GaN and InN targets. In this work, the films were deposited at room temperature. However, the substrate temperature of room temperature would not be best for the deposition of the films. We will report the substrate temperature dependence and the thermal annealing effect in a future publication. Based on above results, the photoconductive a-InXGa1-XN with large σp/σd can be deposited by using GaN and InN targets at low substrate temperature. The a-InXGa1-XN films have a possibility to use as photo-absorption layer material of top cell in multi-junction thin film
T. Itoh et al. / Journal of Non-Crystalline Solids 358 (2012) 2362–2365
solar cells. However, the N/(Ga + In) was still smaller than that of stoichiometric InXGa1-XN. Therefore, further improvement of σp/σd is expected by increasing N/(Ga + In) more.
2365
Acknowledgement This work was partially founded by the International Research Center for Innovative Solar Cell Program from NEDO.
5. Summary InXGa1-XN films were deposited at room temperature by simultaneous reactive rf- magnetron sputtering using Ga and In targets. The deposited films were microcrystalline wurtzite InXGa1-XN. The EO can be controlled from 3.27 eV to 1.63 eV by the X = In/(Ga + In) from 0 to 0.58. The films were photoconductive. However, the σp/σd of 1.89 was small in the film with the EO of 2.09 eV and the N/(Ga + In) of 0.68. The σp/σd of 252 in the film with the EO of 2.08 eV was improved by using GaN and InN targets instead of Ga and In targets. This improvement would be caused by the increase of N/(Ga + In) and the structural change, because the N/(Ga + In) of the film deposited using GaN and InN targets was 0.86 and the film would be amorphous InXGa1-XN. Based on above results, the photoconductive a-InXGa1-XN with large σp/σd deposited by using GaN and InN targets have a possibility to use as photo-absorption layer material of top cell in multi-junction thin film solar cells.
References [1] V.Y. Davydov, A.A. Klochikhin, V.V. Emtsev, D.A. Kurdyukov, S.V. Ivanov, V.A. Vekshin, F. Bechstedt, J. Furthmuller, J. Aderhold, J. Graul, A.V. Mudryi, H. Harima, A. Hashimoto, A. Yamamoto, E.E. Haller, Phys. Status Solidi B 234 (2002) 787–796. [2] O. Jani, I. Ferguson, C. Honsberg, S. Kurtz, Appl. Phys. Lett. 91 (2007) 132117–132119. [3] M. Horie, K. Sugita, A. Hashimoto, A. Yamamoto, Sol. Energy Mater. Sol. Cells 93 (2009) 1013. [4] S. Nonomura, S. Kobayashi, T. Gotoh, S. Hirata, T. Ohmori, T. Itoh, S. Nitta, K. Morigaki, J. Non-Cryst. Solids 198–200 (1996) 174–177. [5] I. Akasaki, H. Amano, Jpn. J. Appl. Phys. 36 (1997) 5393–5408. [6] D.C. Look, D.C. Reynolds, W. Kim, O. Aktas, A. Botchkarev, A. Salvador, H. Morkoc, J. Appl. Phys. 80 (1996) 2960–2963. [7] J.A. Kerr, Strength of chemical bonds, in: D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 1996, pp. 9-51–9-62.