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inorganic heterostructure films for hybrid light emitting diode
Applied Surface Science 254 (2007) 295–298 www.elsevier.com/locate/apsusc
Synthesis and characterization of organic/inorganic heterostructure films for hybrid light emitting diode Toshihiko Toyama *, Tokuyuki Ichihara, Daisuke Yamaguchi, Hiroaki Okamoto Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Available online 12 July 2007
Abstract Thin-film light emitting devices based on organic materials have been gathering attentions for applying a flat-panel display and a solid-state lighting. Alternatively, inorganic technologies such as Si-based thin-film technology have been growing almost independently. It is then expected that combining the Si-based thin-film technology with the organic light emitting diode (OLED) technology will develop innovative devices. Here, we report syntheses of the hybrid light emitting diode (LED) with a heterostructure consisting of p-type SiCx and tris-(8-hydroxyquinoline) aluminum films and characterization for the hybrid LEDs. We present the energy diagram of the heterostructure, and describe that the use of high dark conductivities of the p-type SiCx as well as inserting wide-gap intrinsic a-SiCx at the p-type SiCx/Alq interface are effective for improving device performance. # 2007 Elsevier B.V. All rights reserved. PACS : 78.60.Fi; 73.21.Ac; 73.40.Lq; 72.80.Le; 79.60.I; 79.60.Jv Keywords: Light emitting diode; Thin film; Silicon carbide; Organic semiconductor; Electronic structure; Trapped-charge-limited current
1. Introduction Thin-film light emitting devices have been gathering attentions for applying a flat-panel display and a solid-state lighting [1,2]. An organic light emitting diode (OLED) is one of the most promising candidates because of high luminance, excellent visibility, lightweight and mass-producibility [1,2]. Alternatively, Si-based thin-film technology has also excellent mass-producibility as evidenced by productions of thin-film transistor and thin-film solar cells [3]. Furthermore, we investigated thin-film light emitting devices based on silicon carbide (SiCx) alloys very previously [4]. So far, the organic and inorganic technologies have been growing almost independently, however, it is expected that combining the Si-based thinfilm technology with the organic light emitting diode technology will develop innovative devices. For instance, compared to the organic materials, Si-based thin-films usually have excellent durability against humidity, thus improving the
* Corresponding author. Tel.: +81 66 850 6317; fax: +81 66 850 6316. E-mail address: [email protected] (T. Toyama). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.071
lifetime of OLED is to be possible due to the combined technology. Here, we describe syntheses of the hybrid light emitting diode (LED) based on the organic/inorganic heterostructure consisting of p-type SiCx and tris-(8-hydroxyquinoline) aluminum (Alq) films, and we also mention the energy diagram of the heterostructure as well as current transport properties of the hybrid LED. 2. Experimental details The structure of the hybrid LED was glass/transparent conductive oxide (TCO)/p-type SiCx/Alq/Al. Regarding the TCO layer, SnO2:F/indium tin oxide (ITO) double-coated film was used [4]. The p-type SiCx layer was deposited on the TCO layer by plasma enhanced chemical vapor deposition (PECVD) from a mixture of H2, SiH4, C2H6, and B2H6 gases at 230 8C. The excitation frequency was 40.68 MHz [5,6]. The other deposition conditions are described elsewhere in detail [5–7]. Amorphous (a-) or microcrystalline (mc-) SiCx films were prepared for the p-type SiCx. After short air breakage for transferring the sample from the PECVD apparatus, Alq (Kodak) and Al were deposited on the p-type SiCx sequentially
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by thermal evaporation in high vacuum (<3 106 Torr). The area of the Al electrode was 0.033 cm2. Typical thicknesses of p-type SiCx and Alq layers were 15 and 70 nm, respectively. A conventional OLED with a hole-transport material of 4,40 bis[N-(1-naphthyl)-N-(2-naphthyl)amino]biphenyl (TNB) (Kodak) instead of p-type SiCx was also fabricated. Ultraviolet-ray photoyield spectroscopy (UV-PYS) is often used for identifying the band profile. An UV-PYS system consisting of a vacuum monochromator and a channeltron was used [8]. Electrons excited by the monochromatic light are emitted from surfaces, and detected by the channeltron. Device characterization was performed with Keithley 2400 current source/voltmeter (or voltage source/ammeter), Topcom BM-7fast luminance meter, and Ocean Optics USB2000 spectrometer. The LEDs were set in vacuum in an optical dewar, and the light emission was monitored through a glass window. All measurements were carried out at room temperature.
Fig. 2. Light emission spectra of hybrid LED (solid) and of OLED (dotted), respectively.
3. Results and discussion Fig. 1 shows the UV photoyield spectrum of the a-SiCx film. The photoemission from surfaces, Y, can be expressed as Y (E Eth)5/2 where Eth denotes the emission threshold energy correspond to the excitation energy from the Fermi level to the vacuum level, and E the photon energy being in accordance with an indirect optical excitation equation [9]. Eth was estimated as around 5.3 eV, which is in good agreement with the Eth of a-SiCx with low carbon contents reported by Brown et al. [9]. On the other hand, the temperature dependence of a-SiCx indicated the activation energy of 0.6 eV, and the activation energy of a p-type amorphous silicon alloy corresponds to the Fermi level above the valence band top [3]. The optical bandgap of a-SiCx of 1.9 eV was estimated from the transmittance spectrum in accordance with the Tauc plot [3]. Based on the experimental results, the energy diagram
of the hybrid LED is depicted in the inset of Fig. 1. The energy structure including the energy levels of the normal lowest unoccupied molecular orbital (LUMO) and highest unoccupied molecular orbital (HOMO) of Alq is referred to Ref. [10]. From the hybrid LED, yellowish-green light emission was successfully observed. Fig. 2 shows a typical emission spectrum of the hybrid LED compared to that of OLED. The emission peaking at 540 nm of the hybrid LED arises from the Alq layer. The difference in the wavelengths of 600–800 nm would originate from the interference fringe due to the refractive index of p-type SiCx being higher than that of TNB. Fig. 3 displays a typical current–voltage (J–V) characteristic of the hybrid LED. A clear rectification is confirmed. Inset shows the double logarithmic plot of the J–V curve at forward voltages. At low voltages below 8 V, the J–V curve show a power law, J–Vm+1 with an exponent m of 1.7 close to m of
Fig. 1. UV photoyield spectrum of p-type a-SiCx. Inset shows energy diagram of hybrid LED based on p-type a-SiCx/Alq heterojunction.
Fig. 3. J–V characteristic of hybrid LED with p-type a-SiCx. The hybrid LED was driven by voltages. Inset shows log–log plot of the J–V characteristic for forward voltages.
T. Toyama et al. / Applied Surface Science 254 (2007) 295–298
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Fig. 4. L–J characteristics of hybrid LEDs with p-type mc-SiCx (solid) and aSiCx (dotted), respectively.
trap-free space-charge-limited current (=1) [11]. Between 8 and 10 V, an apparent negative resistance behavior appears, suggesting that a hole tunneling process occurs maybe at the potential height valence band top between SiCx and HOMO level of Alq as shown in Fig. 1 because light emission starts at around 10 V. At high voltages over 15 V, another power law with m = 8.9 appears, indicating that the trapped-chargelimited current (TCLC) should dominate as the J–V characteristics of OLED in the light emitting region [10]. Fig. 4 shows the luminance–current (L–J) characteristics of the hybrid LEDs with p-type a- and mc-SiCx. Fig. 4 clearly indicates that the use of mc-SiCx is effective for decreasing operation current as well as for increasing maximum luminance. The dark conductivity of a-SiCx was 5.3 107 S cm1, while that of mc-SiCx was 1.1 102 S cm1. High hole concentration and high hole mobility are usually responsible for the large dark conductivity of mc-SiCx [3]. Alternatively, concerning the optical absorption coefficient, a, at a wavelength of 540 nm, a of a-SiCx was 3 104 cm1, while a of mc-SiCx was 4 104 cm1. Therefore, it is inferred that the dominant mechanism for improving device characteristics is improving the hole injection due to the high hole concentration and/or improving hole transport due to the increased hole mobility rather than the slight increase in the optical transmittance through the p-type SiCx. Besides, inserting highly resistive wide-gap a-SiCx with an optical bandgap of 3.3 eV into the interface between the p-type SiCx and Alq is effective for further improving L–J characteristics, being similar to the a-SiCx-based p–i–n LEDs [4]. In Fig. 5, the effects of wide-gap a-SiCx are demonstrated; current–voltage (J–V) and L–J characteristics are depicted as a function of thickness of the wide-gap a-SiCx layer. Due to inserting the wide-gap a-SiCx layer, the operation current decreased, and the exponent of the power law for TCLC also decreased. The exponent, m, varied with the thickness of the wide-gap a-SiCx, d; m = 13, 10, 7.2, and 7.6 were estimated for
Fig. 5. J–V (a) and L–J (b) characteristics of hybrid LEDs with a structure of ptype mc-SiCx/wide-gap a-SiCx/Alq. The hybrid LEDs were driven by current. The thicknesses of wide-gap a-SiCx are varied [without wide-gap a-SiCx; (*), 2 nm; (&) 3 nm; (~) 5 nm (^)]. Discontinues in luminance occur in changing measurement ranges.
d = 0 nm (or without the wide-gap a-SiCx), 2, 3, and 5 nm, respectively. Therefore, as shown in Fig. 5(b), the maximum luminance is obtained from the hybrid LED with the 3-nm thick wide-gap a-SiCx of which exponent for TCLC is the minimum among the samples, indicating a decrease in the trap density at p-type SiCx/Alq interface. After further optimizations for the thicknesses of p-type SiCx and Alq, the maximum luminance of 170 cd/m2 has been achieved. Finally, a primitive lifetime test was carried out. The lifetime of the hybrid LED until the half to the initial luminance was measured with the following conditions: constant operating current, in air atmosphere, and without passivation against humidity. The lifetime of the hybrid LED was still as low as 220 min, however, it was almost equivalent or slightly longer than the lifetime of OLED with a structure of ITO/SnO2/TNB/ Alq/Al.
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4. Conclusions We have investigated on the hybrid LEDs for developing stable flat-panel light emitting devices. The structure of the hybrid LED was glass/ITO/SnO2/p-type SiCx/Alq/Al. The UVPYS and the activation energy of the dark conductivity revealed the energy structure of p-type SiCx/Alq heterojunction. Highly conductive mc-SiCx is effective for decreasing operation current as well as for increasing maximum luminance. Besides, inserting highly resistive wide-gap a-SiCx with an optical bandgap of 3.3 eV into the interface between the p-type SiCx and Alq is effective for further improving L–J characteristics, indicating that the p-type SiCx/Alq interface plays a crucial role in determining the device performance, and that the exponent of the power law at the TCLC region is a good quality factor for characterizing the p-type SiCx/Alq interface. Finally, the maximum luminance of 170 cd/m2 has been achieved employing the p-type SiCx/wide-gap a-SiCx/Alq structure. Acknowledgements The authors would like to thank to Dr. Takahashi for her kind advices on UV-PYS measurements. The author (TT) would like to thank to Ms. Shinohara of Kodak Japan Ltd. for providing
organic materials. This work was partially supported by Grantin Aids of New Energy and Industrial Technology Development Organization. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] J. Shinar, Organic Light-Emitting Devices, Springer, Berlin, 2003. [3] K. Tanaka, E. Maruyama, T. Shimada, H. Okamoto, Amorphous Silicon, John Wiley & Sons, New York, 1999. [4] Y. Hamakawa, D. Kruangam, T. Toyama, M. Yoshimi, S.M. Paasche, H. Okamoto, Optoelectron. Device Technol. 4 (1989) 281. [5] T. Toyama, Y. Nakano, T. Ichihara, H. Okamoto, J. Non-Cryst. Solids 338– 340 (2004) 106. [6] T. Harano, T. Toyama, H. Okamoto, in: Ye. Qinghao (Ed.), Technical Digest of 15th International Photovoltaic Science and Engineering Conference (PVSEC-15), Shanghai Scientific & Technical Publishers, Shanghai, 2005, pp. 1130–1131. [7] M. Takahashi, K. Kodama, M. Noda, P. Hedblom, A. Grishin, M. Okuyama, Jpn. J. Appl. Phys. 41 (2002) 6797. [8] E.O. Kane, Phys. Rev. 127 (1962) 131. [9] T.M. Brown, C. Bittencourt, M. Sebastiani, F. Evangelisti, Phys. Rev. B 55 (1997) 9004. [10] S.T. Lee, X.Y. Hou, M.G. Mason, C.W. Tang, Appl. Phys. Lett. 72 (1998) 1593. [11] M.A. Lampert, P. Mark, Current Injection in Solids, Academic, New York, 1970.
Synthesis and characterization of organic/inorganic heterostructure films for hybrid light emitting diode Toshihiko Toyama *, Tokuyuki Ichihara, Daisuke Yamaguchi, Hiroaki Okamoto Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Available online 12 July 2007
Abstract Thin-film light emitting devices based on organic materials have been gathering attentions for applying a flat-panel display and a solid-state lighting. Alternatively, inorganic technologies such as Si-based thin-film technology have been growing almost independently. It is then expected that combining the Si-based thin-film technology with the organic light emitting diode (OLED) technology will develop innovative devices. Here, we report syntheses of the hybrid light emitting diode (LED) with a heterostructure consisting of p-type SiCx and tris-(8-hydroxyquinoline) aluminum films and characterization for the hybrid LEDs. We present the energy diagram of the heterostructure, and describe that the use of high dark conductivities of the p-type SiCx as well as inserting wide-gap intrinsic a-SiCx at the p-type SiCx/Alq interface are effective for improving device performance. # 2007 Elsevier B.V. All rights reserved. PACS : 78.60.Fi; 73.21.Ac; 73.40.Lq; 72.80.Le; 79.60.I; 79.60.Jv Keywords: Light emitting diode; Thin film; Silicon carbide; Organic semiconductor; Electronic structure; Trapped-charge-limited current
1. Introduction Thin-film light emitting devices have been gathering attentions for applying a flat-panel display and a solid-state lighting [1,2]. An organic light emitting diode (OLED) is one of the most promising candidates because of high luminance, excellent visibility, lightweight and mass-producibility [1,2]. Alternatively, Si-based thin-film technology has also excellent mass-producibility as evidenced by productions of thin-film transistor and thin-film solar cells [3]. Furthermore, we investigated thin-film light emitting devices based on silicon carbide (SiCx) alloys very previously [4]. So far, the organic and inorganic technologies have been growing almost independently, however, it is expected that combining the Si-based thinfilm technology with the organic light emitting diode technology will develop innovative devices. For instance, compared to the organic materials, Si-based thin-films usually have excellent durability against humidity, thus improving the
* Corresponding author. Tel.: +81 66 850 6317; fax: +81 66 850 6316. E-mail address: [email protected] (T. Toyama). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.071
lifetime of OLED is to be possible due to the combined technology. Here, we describe syntheses of the hybrid light emitting diode (LED) based on the organic/inorganic heterostructure consisting of p-type SiCx and tris-(8-hydroxyquinoline) aluminum (Alq) films, and we also mention the energy diagram of the heterostructure as well as current transport properties of the hybrid LED. 2. Experimental details The structure of the hybrid LED was glass/transparent conductive oxide (TCO)/p-type SiCx/Alq/Al. Regarding the TCO layer, SnO2:F/indium tin oxide (ITO) double-coated film was used [4]. The p-type SiCx layer was deposited on the TCO layer by plasma enhanced chemical vapor deposition (PECVD) from a mixture of H2, SiH4, C2H6, and B2H6 gases at 230 8C. The excitation frequency was 40.68 MHz [5,6]. The other deposition conditions are described elsewhere in detail [5–7]. Amorphous (a-) or microcrystalline (mc-) SiCx films were prepared for the p-type SiCx. After short air breakage for transferring the sample from the PECVD apparatus, Alq (Kodak) and Al were deposited on the p-type SiCx sequentially
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by thermal evaporation in high vacuum (<3 106 Torr). The area of the Al electrode was 0.033 cm2. Typical thicknesses of p-type SiCx and Alq layers were 15 and 70 nm, respectively. A conventional OLED with a hole-transport material of 4,40 bis[N-(1-naphthyl)-N-(2-naphthyl)amino]biphenyl (TNB) (Kodak) instead of p-type SiCx was also fabricated. Ultraviolet-ray photoyield spectroscopy (UV-PYS) is often used for identifying the band profile. An UV-PYS system consisting of a vacuum monochromator and a channeltron was used [8]. Electrons excited by the monochromatic light are emitted from surfaces, and detected by the channeltron. Device characterization was performed with Keithley 2400 current source/voltmeter (or voltage source/ammeter), Topcom BM-7fast luminance meter, and Ocean Optics USB2000 spectrometer. The LEDs were set in vacuum in an optical dewar, and the light emission was monitored through a glass window. All measurements were carried out at room temperature.
Fig. 2. Light emission spectra of hybrid LED (solid) and of OLED (dotted), respectively.
3. Results and discussion Fig. 1 shows the UV photoyield spectrum of the a-SiCx film. The photoemission from surfaces, Y, can be expressed as Y (E Eth)5/2 where Eth denotes the emission threshold energy correspond to the excitation energy from the Fermi level to the vacuum level, and E the photon energy being in accordance with an indirect optical excitation equation [9]. Eth was estimated as around 5.3 eV, which is in good agreement with the Eth of a-SiCx with low carbon contents reported by Brown et al. [9]. On the other hand, the temperature dependence of a-SiCx indicated the activation energy of 0.6 eV, and the activation energy of a p-type amorphous silicon alloy corresponds to the Fermi level above the valence band top [3]. The optical bandgap of a-SiCx of 1.9 eV was estimated from the transmittance spectrum in accordance with the Tauc plot [3]. Based on the experimental results, the energy diagram
of the hybrid LED is depicted in the inset of Fig. 1. The energy structure including the energy levels of the normal lowest unoccupied molecular orbital (LUMO) and highest unoccupied molecular orbital (HOMO) of Alq is referred to Ref. [10]. From the hybrid LED, yellowish-green light emission was successfully observed. Fig. 2 shows a typical emission spectrum of the hybrid LED compared to that of OLED. The emission peaking at 540 nm of the hybrid LED arises from the Alq layer. The difference in the wavelengths of 600–800 nm would originate from the interference fringe due to the refractive index of p-type SiCx being higher than that of TNB. Fig. 3 displays a typical current–voltage (J–V) characteristic of the hybrid LED. A clear rectification is confirmed. Inset shows the double logarithmic plot of the J–V curve at forward voltages. At low voltages below 8 V, the J–V curve show a power law, J–Vm+1 with an exponent m of 1.7 close to m of
Fig. 1. UV photoyield spectrum of p-type a-SiCx. Inset shows energy diagram of hybrid LED based on p-type a-SiCx/Alq heterojunction.
Fig. 3. J–V characteristic of hybrid LED with p-type a-SiCx. The hybrid LED was driven by voltages. Inset shows log–log plot of the J–V characteristic for forward voltages.
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Fig. 4. L–J characteristics of hybrid LEDs with p-type mc-SiCx (solid) and aSiCx (dotted), respectively.
trap-free space-charge-limited current (=1) [11]. Between 8 and 10 V, an apparent negative resistance behavior appears, suggesting that a hole tunneling process occurs maybe at the potential height valence band top between SiCx and HOMO level of Alq as shown in Fig. 1 because light emission starts at around 10 V. At high voltages over 15 V, another power law with m = 8.9 appears, indicating that the trapped-chargelimited current (TCLC) should dominate as the J–V characteristics of OLED in the light emitting region [10]. Fig. 4 shows the luminance–current (L–J) characteristics of the hybrid LEDs with p-type a- and mc-SiCx. Fig. 4 clearly indicates that the use of mc-SiCx is effective for decreasing operation current as well as for increasing maximum luminance. The dark conductivity of a-SiCx was 5.3 107 S cm1, while that of mc-SiCx was 1.1 102 S cm1. High hole concentration and high hole mobility are usually responsible for the large dark conductivity of mc-SiCx [3]. Alternatively, concerning the optical absorption coefficient, a, at a wavelength of 540 nm, a of a-SiCx was 3 104 cm1, while a of mc-SiCx was 4 104 cm1. Therefore, it is inferred that the dominant mechanism for improving device characteristics is improving the hole injection due to the high hole concentration and/or improving hole transport due to the increased hole mobility rather than the slight increase in the optical transmittance through the p-type SiCx. Besides, inserting highly resistive wide-gap a-SiCx with an optical bandgap of 3.3 eV into the interface between the p-type SiCx and Alq is effective for further improving L–J characteristics, being similar to the a-SiCx-based p–i–n LEDs [4]. In Fig. 5, the effects of wide-gap a-SiCx are demonstrated; current–voltage (J–V) and L–J characteristics are depicted as a function of thickness of the wide-gap a-SiCx layer. Due to inserting the wide-gap a-SiCx layer, the operation current decreased, and the exponent of the power law for TCLC also decreased. The exponent, m, varied with the thickness of the wide-gap a-SiCx, d; m = 13, 10, 7.2, and 7.6 were estimated for
Fig. 5. J–V (a) and L–J (b) characteristics of hybrid LEDs with a structure of ptype mc-SiCx/wide-gap a-SiCx/Alq. The hybrid LEDs were driven by current. The thicknesses of wide-gap a-SiCx are varied [without wide-gap a-SiCx; (*), 2 nm; (&) 3 nm; (~) 5 nm (^)]. Discontinues in luminance occur in changing measurement ranges.
d = 0 nm (or without the wide-gap a-SiCx), 2, 3, and 5 nm, respectively. Therefore, as shown in Fig. 5(b), the maximum luminance is obtained from the hybrid LED with the 3-nm thick wide-gap a-SiCx of which exponent for TCLC is the minimum among the samples, indicating a decrease in the trap density at p-type SiCx/Alq interface. After further optimizations for the thicknesses of p-type SiCx and Alq, the maximum luminance of 170 cd/m2 has been achieved. Finally, a primitive lifetime test was carried out. The lifetime of the hybrid LED until the half to the initial luminance was measured with the following conditions: constant operating current, in air atmosphere, and without passivation against humidity. The lifetime of the hybrid LED was still as low as 220 min, however, it was almost equivalent or slightly longer than the lifetime of OLED with a structure of ITO/SnO2/TNB/ Alq/Al.
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4. Conclusions We have investigated on the hybrid LEDs for developing stable flat-panel light emitting devices. The structure of the hybrid LED was glass/ITO/SnO2/p-type SiCx/Alq/Al. The UVPYS and the activation energy of the dark conductivity revealed the energy structure of p-type SiCx/Alq heterojunction. Highly conductive mc-SiCx is effective for decreasing operation current as well as for increasing maximum luminance. Besides, inserting highly resistive wide-gap a-SiCx with an optical bandgap of 3.3 eV into the interface between the p-type SiCx and Alq is effective for further improving L–J characteristics, indicating that the p-type SiCx/Alq interface plays a crucial role in determining the device performance, and that the exponent of the power law at the TCLC region is a good quality factor for characterizing the p-type SiCx/Alq interface. Finally, the maximum luminance of 170 cd/m2 has been achieved employing the p-type SiCx/wide-gap a-SiCx/Alq structure. Acknowledgements The authors would like to thank to Dr. Takahashi for her kind advices on UV-PYS measurements. The author (TT) would like to thank to Ms. Shinohara of Kodak Japan Ltd. for providing
organic materials. This work was partially supported by Grantin Aids of New Energy and Industrial Technology Development Organization. References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] J. Shinar, Organic Light-Emitting Devices, Springer, Berlin, 2003. [3] K. Tanaka, E. Maruyama, T. Shimada, H. Okamoto, Amorphous Silicon, John Wiley & Sons, New York, 1999. [4] Y. Hamakawa, D. Kruangam, T. Toyama, M. Yoshimi, S.M. Paasche, H. Okamoto, Optoelectron. Device Technol. 4 (1989) 281. [5] T. Toyama, Y. Nakano, T. Ichihara, H. Okamoto, J. Non-Cryst. Solids 338– 340 (2004) 106. [6] T. Harano, T. Toyama, H. Okamoto, in: Ye. Qinghao (Ed.), Technical Digest of 15th International Photovoltaic Science and Engineering Conference (PVSEC-15), Shanghai Scientific & Technical Publishers, Shanghai, 2005, pp. 1130–1131. [7] M. Takahashi, K. Kodama, M. Noda, P. Hedblom, A. Grishin, M. Okuyama, Jpn. J. Appl. Phys. 41 (2002) 6797. [8] E.O. Kane, Phys. Rev. 127 (1962) 131. [9] T.M. Brown, C. Bittencourt, M. Sebastiani, F. Evangelisti, Phys. Rev. B 55 (1997) 9004. [10] S.T. Lee, X.Y. Hou, M.G. Mason, C.W. Tang, Appl. Phys. Lett. 72 (1998) 1593. [11] M.A. Lampert, P. Mark, Current Injection in Solids, Academic, New York, 1970.