- Email: [email protected]
p-CuGaS2 Schottky diode
Journal of Physics and Chemistry of Solids 64 (2003) 1787–1790 www.elsevier.com/locate/jpcs
Interface Fermi level pinning in a Cu/p-CuGaS2 Schottky diode M. Sugiyamaa, R. Nakaib, H. Nakanishib, Sf. Chichibua,b,* a
Institute of Applied Physics and Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan b Department of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan
Abstract A Schottky contact to p-type CuGaS2 that showed the highest rectification ratio of approximately 500 ever reported was realized using a Cu electrode on a HF/HNO3-treated surface, as well as an excellent Au ohmic contact on a HF-etched surface. The effective Schottky barrier height of 0.9 eV was obtained from the current– voltage and capacitance – voltage characteristics. The value was smaller by 1.1 eV than that calculated from the values of the work function of Cu and electron affinity of CuGaS2. The results indicated a surface pinning of the Fermi level to certain acceptor-type gap states below the midgap. q 2003 Elsevier Ltd. All rights reserved.
1. Introduction Ternary Cu(Al,Ga,In)(S,Se)2 chalcopyrite (Ch) semiconductors [1] have direct band gap energies (Eg) ranging from 3.49 eV (CuAlS2) to 1.05 eV (CuInSe2). In contrast to the wurtzite (WZ) structure materials, their piezoelectric polar axis dose not coincide with the c-axis, although Ch structure has a crystal anisotropy. Therefore, Ch compounds are possible candidates for light-emitting media for pure blue and green spectral ranges, since it is still difficult to realize cw laser diodes (LDs) whose wavelength is longer than 470 nm using the WZ structure InGaN materials due to the presence of a polarization-induced internal electric field up to MV/cm. Good quality ohmic and Schottky electrodes are indispensable to realize semiconductor optoelectronic and electronic devices. However, there have been few reported experimental results on metal-semiconductor (MS) contacts to Ch semiconductors [2 – 6]. For example, Kobayashi et al. [3] have observed an electro luminescence peak at 2.41 eV from the CuGaS2 single crystal forming an Al Schottky diode. Sato and co-workers have investigated a change in optical absorption spectra of CuAlS2 applying an electric field using an Al Schottky diode [5,6]. However, ideal * Corresponding author. Tel.: þ81-298-53-5022; fax: þ 81-29853-5205. E-mail address: [email protected] (Sf. Chichibu).
Schottky contact exhibiting negligible leakage current has not yet been realized. One of the reasons is obviously a lack of good quality single crystals. Moreover, the MS interface has not yet been controlled properly. In this conference, effects of surface treatments and metal species on the performance of ohmic and Schottky contacts to p-type CuGaS2 are discussed. A schematic band diagram of Cu/CuGaS2 Schottky diode is drawn from the results of current – voltage ðI – VÞ and capacitance –voltage ðC – VÞ measurements taking the surface Fermi level pinning into account.
2. Experiment Single-phase, Ch structure CuGaS2 single crystals were grown by the closed-tube vapor transport technique using iodine as a transport element [7]. The furnace temperatures of the source and crystallization zones were 800 and 700 8C, respectively. Typical sample size was approximately 2 £ 1 £ 0.5 mm3. All the samples showed high resistivity p-type conductivity. Also, the resistivity value is estimated to be 75 kV cm. Electrode metals were deposited on well-developed {112} surface by the thermal evaporation technique. Surface treatment conditions will be described later. I – V and C – V measurements were carried out on the as-deposited MS
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00144-6
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M. Sugiyama et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1787–1790
contacts using a picoammetere (Kethley 487) and a capacitance meter (Boonton 72B). The area of Schottky electrode was 8.5 £ 1023 cm2.
3. Results and discussion First, formation of non-alloyed ohmic contact is attempted, since there has been no reported result on the good performance ohmic contact to p-type CuGaS2. Several metal species having different work function ðfM Þ such as AuðfAu ¼ 5:1 eVÞ; CuðfCu ¼ 4:6 eVÞ; AlðfAl ¼ 4:3 eVÞ; and InðfIn ¼ 4:1 eVÞ were examined using a couple of surface electrodes. As expected from the large fAu of 5.1 eV, Au electrode gives the best result though other metals often show an ohmic property. However, the I – V curve sometimes shows asymmetric rectifying property when the metals are deposited on as-grown surfaces or on the surface cleaned only by organic solvents. The reliability and reproducibility of the ohmic property are greatly improved by etching the surface using a dilute HF (DHF) solution at RT for 30 s. Typical I – V curve of Au/pCuGaS2/Au structure formed after the etching is shown in the left inset of Fig. 1. Indeed, the performance of ohmic contacts depends strongly on the surface treatment conditions rather than the metal species. The slope parameter ðSÞ of CuGaS2 is calculated to be approximately 0.32, which is an index of interface behavior and is defined as S ¼ 2dfB =dfM [8] and 1=S 2 1 ¼ 0:1ð11 2 1Þ2 ; [9] where d represents the derivative, fB is
the Schottky barrier height, and 11 ¼ 5:6 [10] is the dielectric constant. This small S value means that most of Ch compounds including CuGaS2 approach the Bardeen limit [11] due to their covalent nature. Indeed, the S values of Cu(Al,Ga,In)(S,Se)2 materials are calculated to be as small as 0.18 – 0.36. Therefore, fB would decrease slightly with increasing fM ; and fabrication of ohmic/Schottky contact is considered to be difficult. Nevertheless, Au is exclusively used as an ohmic electrode in the following experiments, since Au/p-CuGaS2 gives the best ohmic property. To make Schottky contact to p-type semiconductors, metals having small fM are preferable to use [12]. Therefore, CuðfCu ¼ 4:6 eVÞ; AlðfAu ¼ 4:3 eVÞ; and InðfIn ¼ 4:1 eVÞ are examined. The best results are obtained by a removal of surface oxides such as Cu – O and Ga– O from well-developed {112} mirror surfaces using a solution composed of HF:HNO3:H2O ¼ 1:1:4. As shown in the right inset of Fig. 1, Cu and Al exhibit either Schottky or rectifying property. However, In dose not form an ideal Schottky barrier though fIn is the smallest. This inconsistency is similar to that of the ohmic contact study; fB dose not strongly depend on fM and the Schottky performance is affected by the surface treatment procedure. Current density –voltage ðJ – VÞ characteristics of the Cu/ CuGaS2/Au Schottky diode is shown in Fig. 1. The forward current density increases exponentially with increasing V from 0.25 to 0.45 V, and the slope gives the ideality factor n ¼ 1:5 in J ¼ J0 ½expðqV=nkB TÞ 2 1;
where T is the temperature, q is the electron charge and kB is the Boltzman constant. J0 is the saturated reverse current density and is given by J0 ¼ Ap T 2 exp½2ðfB 2 DfB Þ=kB T; p
Fig. 1. Current density – voltage ðJ – VÞ characteristics of the Cu/CuGaS2/Au Schottky diode at 300 K. The left inset shows the current– voltage ðI – VÞ characteristics of Au ohmic contacts. The right inset shows linear I – V characteristics of Cu, Al, and In/CuGaS2/Au diodes.
ð1Þ
ð2Þ
where A is the effective Richardson constant and DfB is the amount of Schottky barrier lowering, which is due to the image charge on the metal for ideal case. J0 is estimated to be 8.2 £ 1029 A/cm2 from the extrapolation of the exponentially increasing region of the J – V plot onto the y-axis in Fig. 1. As shown, the reverse current density is far large than J0 ; and n ¼ 1:5 implies that nearly half of the current is due to generation-recombination current caused by the tunneling, thermoionic field emission, and a part of fB lowering. The effective Schottky barrier height fB 2 DfB is thus estimated to be kB T lnðAp T 2 =J0 Þ ¼ 0:9 eV: It is also noted that the diode has fairly large series resistance, which is partly due to low carrier density in the bulk that will be estimated later. Nevertheless, Cu/CuGaS2/Au Schottky diode shows the rectification ratio, which is defined as the forward current over the reverse current, of approximately 500 at 0.8 V. The value is the highest one ever reported for CuGaS2 [3]. The 1=C2 – V curve of the Cu/CuGaS2/Au Schottky diode is shown in Fig. 2. From the linear approximation of the data between the reverse bias ðVR Þ of 1.5 and 4 V,
M. Sugiyama et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1787–1790
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Fig. 2. Capacitance – voltage ðC – VÞ characteristics of the Cu/CuGaS2/Au Schottky diode at 300 K. Vertical axis represents 1=C2 :
the diffusion potential ðVD Þ and the ionized acceptor minus donor density ðNA 2 ND Þ in the bulk are estimated to be 0.6 V and 2.0 £ 1014 cm23, respectively. This small NA 2 ND may be due to heavy compensation of carriers. The Fermi level position, EF 2 EV ; in the bulk is estimated to be 0.3 eV under the assumption that all donors and acceptors are ionized, using the values of NA 2 ND and the effective density of states NV ; which is calculated from the value of the hole effective mass [14] of 0:69m0 ; where m0 is the electron mass. In p-type semiconductor, conductivity ðsÞ is given by s ¼ qmp p; where mp is the hole mobility and p is the hole density. If mp is in the range between 0.1 and 1 cm2 V21 s21, the corresponding p is 8.3 £ 1014 – 8.3 £ 1013 cm23. These small p values are comparable to NA 2 ND estimated from the C – V measurement. Therefore, effective barrier height, fB;eff is estimated to be ðEF 2 EV Þ þ qVD ¼ 0:9 eV: The value agrees with that obtained from the J – V curve; ðfB 2 DfB ¼ 0:9 eVÞ: The large 1=C 2 values for 0 , VR , 1:5 V are considered to be due either to the increases of NA 2 ND in the surface region or the Schottky effect. The depth and density of the high acceptor density region are estimated from the C – V depth profiling method [13] to be 200 nm and 1 £ 1016 cm23, respectively. The presence of this defective region might lower the fB;eff : Using the values of fCu ¼ 4:6 eV and electron affinity ðxÞ of CuGaS2(4.1 eV), [15] schematic band diagram of the Cu/CuGaS2 MS junction is drawn in Fig. 3. It is evident that fB;eff ¼ 0:9 eV obtained for the experiment is smaller by DfB ¼ 1:1 eV than ideal fB of 2 eV calculated from the values of fCu ; x; and Eg. Since our Schottky junction is not ideal, the amount of total barrier lowering including the image charge effect is shown by DfB : The lowering of fB and increase of NA 2 ND are thus
Fig. 3. Schematic drawing of the energy diagram of the Cu/CuGaS2 junction.
considered to be due to the Fermi level pinning at the acceptor-type defective gap states at the interface, which sometimes happens in the materials approaching the Bardeen limit [11]. The origin of the gap state is unclear. However, it may be related to the interface charge modulation due to the presence of cation sub-oxides, stress, and vacancies. It should be noted that the universal defect level [16] lies in the lower half of the band gap (acceptor-type) in CuGaS2, because the valence band energies are upshifted with respect to the binary analogue compound ZnS due to the proximity of Cu-d levels [1].
4. Conclusion In summary, a Cu/p-CuGaS2/Au Schottky diode exhibiting the highest rectification ratio of 500 was realized by the means of appropriate surface treatments of CuGaS2. The performance of both ohmic and Schottky junctions were affected by the MS interface conditions rather than fM of the metals. It is necessary to reduce the number of interface pinning states to obtain better Schottky performance. pCuGaS2 was shown to be a promising medium as a hole injector, since good ohmic contact can be formed even on the high-resistivity sample.
Acknowledgements The authors are grateful to Dr. Alex Zunger, Dr. S.-H. Wei, and Prof. Sho Shirakata for stimulating discussions.
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M. Sugiyama et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1787–1790
This work was supported in part by the 21st Century COE program ‘Promotion of Creative Interdisciplinary Materials Science for Novel Functions’ under Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties, and Applications, Pergamon Press, Oxford, 1975. [2] J. Stankiewicz, W. Giriat, Appl. Phys. Lett. 35 (1979) 70. [3] S. Kobayashi, Y. Momiyama, F. Kaneko, Jpn. J. Appl. Phys., Part 2 31 (1992) L1606. [4] K. Tanaka, Y. Kimura, S. Okamoto, Y. Inoue, K. Sato, Jpn. J. Appl. Phys., Part 1 37 (1998) 3350. [5] N. Ishibashi, T. Nishi, N. Hayashi, C. Furuhashi, K. Sato, Jpn. J. Appl. Phys., Part 2 38 (1999) L626.
[6] T. Nishi, N. Ishibashi, N. Hayashi, C. Furuhashi, K. Sato, Jpn. J. Appl. Phys. Suppl. 39-1 (2000) 296. [7] The growth method was very similar to.S. Chichibu, M. Shishikura, J. Ino, S. Matsumoto, J. Appl. Phys. 70 (1991) 1648. [8] S. Kurtin, T.C. McGill, C.A. Mead, Phys. Rev. Lett. 22 (1969) 1433. [9] W. Mo¨nch, Appl. Surf. Sci. 92 (1996) 367. [10] R. Marquez, C. Rincon, Phys. Status Solidi b 191 (1995) 115. [11] J. Bardeen, Phys. Rev. 71 (1947) 717. [12] W. Schottky, Z. Phys. 15 (1914) 872. [13] C.P. Wu, E.C. Douglas, C.W. Mueller, Trans. IEEE Electron. Devices ED-22 (1975) 319. [14] P.W. Yu, D.L. Dowing, Y.S. Park, J. Appl. Phys. 45 (1974) 5283. [15] M. Sugiyama, H. Nakanishi, S.F. Chichibu, Jpn. J. Appl. Phys., Part 2 40 (2001) L428. [16] W.E. Spicer, I. Lindau, P. Skeath, C.Y. Su, J. Vac. Sci. Technol. 17 (1980) 1019.
Interface Fermi level pinning in a Cu/p-CuGaS2 Schottky diode M. Sugiyamaa, R. Nakaib, H. Nakanishib, Sf. Chichibua,b,* a
Institute of Applied Physics and Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan b Department of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan
Abstract A Schottky contact to p-type CuGaS2 that showed the highest rectification ratio of approximately 500 ever reported was realized using a Cu electrode on a HF/HNO3-treated surface, as well as an excellent Au ohmic contact on a HF-etched surface. The effective Schottky barrier height of 0.9 eV was obtained from the current– voltage and capacitance – voltage characteristics. The value was smaller by 1.1 eV than that calculated from the values of the work function of Cu and electron affinity of CuGaS2. The results indicated a surface pinning of the Fermi level to certain acceptor-type gap states below the midgap. q 2003 Elsevier Ltd. All rights reserved.
1. Introduction Ternary Cu(Al,Ga,In)(S,Se)2 chalcopyrite (Ch) semiconductors [1] have direct band gap energies (Eg) ranging from 3.49 eV (CuAlS2) to 1.05 eV (CuInSe2). In contrast to the wurtzite (WZ) structure materials, their piezoelectric polar axis dose not coincide with the c-axis, although Ch structure has a crystal anisotropy. Therefore, Ch compounds are possible candidates for light-emitting media for pure blue and green spectral ranges, since it is still difficult to realize cw laser diodes (LDs) whose wavelength is longer than 470 nm using the WZ structure InGaN materials due to the presence of a polarization-induced internal electric field up to MV/cm. Good quality ohmic and Schottky electrodes are indispensable to realize semiconductor optoelectronic and electronic devices. However, there have been few reported experimental results on metal-semiconductor (MS) contacts to Ch semiconductors [2 – 6]. For example, Kobayashi et al. [3] have observed an electro luminescence peak at 2.41 eV from the CuGaS2 single crystal forming an Al Schottky diode. Sato and co-workers have investigated a change in optical absorption spectra of CuAlS2 applying an electric field using an Al Schottky diode [5,6]. However, ideal * Corresponding author. Tel.: þ81-298-53-5022; fax: þ 81-29853-5205. E-mail address: [email protected] (Sf. Chichibu).
Schottky contact exhibiting negligible leakage current has not yet been realized. One of the reasons is obviously a lack of good quality single crystals. Moreover, the MS interface has not yet been controlled properly. In this conference, effects of surface treatments and metal species on the performance of ohmic and Schottky contacts to p-type CuGaS2 are discussed. A schematic band diagram of Cu/CuGaS2 Schottky diode is drawn from the results of current – voltage ðI – VÞ and capacitance –voltage ðC – VÞ measurements taking the surface Fermi level pinning into account.
2. Experiment Single-phase, Ch structure CuGaS2 single crystals were grown by the closed-tube vapor transport technique using iodine as a transport element [7]. The furnace temperatures of the source and crystallization zones were 800 and 700 8C, respectively. Typical sample size was approximately 2 £ 1 £ 0.5 mm3. All the samples showed high resistivity p-type conductivity. Also, the resistivity value is estimated to be 75 kV cm. Electrode metals were deposited on well-developed {112} surface by the thermal evaporation technique. Surface treatment conditions will be described later. I – V and C – V measurements were carried out on the as-deposited MS
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00144-6
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M. Sugiyama et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1787–1790
contacts using a picoammetere (Kethley 487) and a capacitance meter (Boonton 72B). The area of Schottky electrode was 8.5 £ 1023 cm2.
3. Results and discussion First, formation of non-alloyed ohmic contact is attempted, since there has been no reported result on the good performance ohmic contact to p-type CuGaS2. Several metal species having different work function ðfM Þ such as AuðfAu ¼ 5:1 eVÞ; CuðfCu ¼ 4:6 eVÞ; AlðfAl ¼ 4:3 eVÞ; and InðfIn ¼ 4:1 eVÞ were examined using a couple of surface electrodes. As expected from the large fAu of 5.1 eV, Au electrode gives the best result though other metals often show an ohmic property. However, the I – V curve sometimes shows asymmetric rectifying property when the metals are deposited on as-grown surfaces or on the surface cleaned only by organic solvents. The reliability and reproducibility of the ohmic property are greatly improved by etching the surface using a dilute HF (DHF) solution at RT for 30 s. Typical I – V curve of Au/pCuGaS2/Au structure formed after the etching is shown in the left inset of Fig. 1. Indeed, the performance of ohmic contacts depends strongly on the surface treatment conditions rather than the metal species. The slope parameter ðSÞ of CuGaS2 is calculated to be approximately 0.32, which is an index of interface behavior and is defined as S ¼ 2dfB =dfM [8] and 1=S 2 1 ¼ 0:1ð11 2 1Þ2 ; [9] where d represents the derivative, fB is
the Schottky barrier height, and 11 ¼ 5:6 [10] is the dielectric constant. This small S value means that most of Ch compounds including CuGaS2 approach the Bardeen limit [11] due to their covalent nature. Indeed, the S values of Cu(Al,Ga,In)(S,Se)2 materials are calculated to be as small as 0.18 – 0.36. Therefore, fB would decrease slightly with increasing fM ; and fabrication of ohmic/Schottky contact is considered to be difficult. Nevertheless, Au is exclusively used as an ohmic electrode in the following experiments, since Au/p-CuGaS2 gives the best ohmic property. To make Schottky contact to p-type semiconductors, metals having small fM are preferable to use [12]. Therefore, CuðfCu ¼ 4:6 eVÞ; AlðfAu ¼ 4:3 eVÞ; and InðfIn ¼ 4:1 eVÞ are examined. The best results are obtained by a removal of surface oxides such as Cu – O and Ga– O from well-developed {112} mirror surfaces using a solution composed of HF:HNO3:H2O ¼ 1:1:4. As shown in the right inset of Fig. 1, Cu and Al exhibit either Schottky or rectifying property. However, In dose not form an ideal Schottky barrier though fIn is the smallest. This inconsistency is similar to that of the ohmic contact study; fB dose not strongly depend on fM and the Schottky performance is affected by the surface treatment procedure. Current density –voltage ðJ – VÞ characteristics of the Cu/ CuGaS2/Au Schottky diode is shown in Fig. 1. The forward current density increases exponentially with increasing V from 0.25 to 0.45 V, and the slope gives the ideality factor n ¼ 1:5 in J ¼ J0 ½expðqV=nkB TÞ 2 1;
where T is the temperature, q is the electron charge and kB is the Boltzman constant. J0 is the saturated reverse current density and is given by J0 ¼ Ap T 2 exp½2ðfB 2 DfB Þ=kB T; p
Fig. 1. Current density – voltage ðJ – VÞ characteristics of the Cu/CuGaS2/Au Schottky diode at 300 K. The left inset shows the current– voltage ðI – VÞ characteristics of Au ohmic contacts. The right inset shows linear I – V characteristics of Cu, Al, and In/CuGaS2/Au diodes.
ð1Þ
ð2Þ
where A is the effective Richardson constant and DfB is the amount of Schottky barrier lowering, which is due to the image charge on the metal for ideal case. J0 is estimated to be 8.2 £ 1029 A/cm2 from the extrapolation of the exponentially increasing region of the J – V plot onto the y-axis in Fig. 1. As shown, the reverse current density is far large than J0 ; and n ¼ 1:5 implies that nearly half of the current is due to generation-recombination current caused by the tunneling, thermoionic field emission, and a part of fB lowering. The effective Schottky barrier height fB 2 DfB is thus estimated to be kB T lnðAp T 2 =J0 Þ ¼ 0:9 eV: It is also noted that the diode has fairly large series resistance, which is partly due to low carrier density in the bulk that will be estimated later. Nevertheless, Cu/CuGaS2/Au Schottky diode shows the rectification ratio, which is defined as the forward current over the reverse current, of approximately 500 at 0.8 V. The value is the highest one ever reported for CuGaS2 [3]. The 1=C2 – V curve of the Cu/CuGaS2/Au Schottky diode is shown in Fig. 2. From the linear approximation of the data between the reverse bias ðVR Þ of 1.5 and 4 V,
M. Sugiyama et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1787–1790
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Fig. 2. Capacitance – voltage ðC – VÞ characteristics of the Cu/CuGaS2/Au Schottky diode at 300 K. Vertical axis represents 1=C2 :
the diffusion potential ðVD Þ and the ionized acceptor minus donor density ðNA 2 ND Þ in the bulk are estimated to be 0.6 V and 2.0 £ 1014 cm23, respectively. This small NA 2 ND may be due to heavy compensation of carriers. The Fermi level position, EF 2 EV ; in the bulk is estimated to be 0.3 eV under the assumption that all donors and acceptors are ionized, using the values of NA 2 ND and the effective density of states NV ; which is calculated from the value of the hole effective mass [14] of 0:69m0 ; where m0 is the electron mass. In p-type semiconductor, conductivity ðsÞ is given by s ¼ qmp p; where mp is the hole mobility and p is the hole density. If mp is in the range between 0.1 and 1 cm2 V21 s21, the corresponding p is 8.3 £ 1014 – 8.3 £ 1013 cm23. These small p values are comparable to NA 2 ND estimated from the C – V measurement. Therefore, effective barrier height, fB;eff is estimated to be ðEF 2 EV Þ þ qVD ¼ 0:9 eV: The value agrees with that obtained from the J – V curve; ðfB 2 DfB ¼ 0:9 eVÞ: The large 1=C 2 values for 0 , VR , 1:5 V are considered to be due either to the increases of NA 2 ND in the surface region or the Schottky effect. The depth and density of the high acceptor density region are estimated from the C – V depth profiling method [13] to be 200 nm and 1 £ 1016 cm23, respectively. The presence of this defective region might lower the fB;eff : Using the values of fCu ¼ 4:6 eV and electron affinity ðxÞ of CuGaS2(4.1 eV), [15] schematic band diagram of the Cu/CuGaS2 MS junction is drawn in Fig. 3. It is evident that fB;eff ¼ 0:9 eV obtained for the experiment is smaller by DfB ¼ 1:1 eV than ideal fB of 2 eV calculated from the values of fCu ; x; and Eg. Since our Schottky junction is not ideal, the amount of total barrier lowering including the image charge effect is shown by DfB : The lowering of fB and increase of NA 2 ND are thus
Fig. 3. Schematic drawing of the energy diagram of the Cu/CuGaS2 junction.
considered to be due to the Fermi level pinning at the acceptor-type defective gap states at the interface, which sometimes happens in the materials approaching the Bardeen limit [11]. The origin of the gap state is unclear. However, it may be related to the interface charge modulation due to the presence of cation sub-oxides, stress, and vacancies. It should be noted that the universal defect level [16] lies in the lower half of the band gap (acceptor-type) in CuGaS2, because the valence band energies are upshifted with respect to the binary analogue compound ZnS due to the proximity of Cu-d levels [1].
4. Conclusion In summary, a Cu/p-CuGaS2/Au Schottky diode exhibiting the highest rectification ratio of 500 was realized by the means of appropriate surface treatments of CuGaS2. The performance of both ohmic and Schottky junctions were affected by the MS interface conditions rather than fM of the metals. It is necessary to reduce the number of interface pinning states to obtain better Schottky performance. pCuGaS2 was shown to be a promising medium as a hole injector, since good ohmic contact can be formed even on the high-resistivity sample.
Acknowledgements The authors are grateful to Dr. Alex Zunger, Dr. S.-H. Wei, and Prof. Sho Shirakata for stimulating discussions.
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M. Sugiyama et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1787–1790
This work was supported in part by the 21st Century COE program ‘Promotion of Creative Interdisciplinary Materials Science for Novel Functions’ under Ministry of Education, Culture, Sports, Science and Technology of Japan.
References [1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties, and Applications, Pergamon Press, Oxford, 1975. [2] J. Stankiewicz, W. Giriat, Appl. Phys. Lett. 35 (1979) 70. [3] S. Kobayashi, Y. Momiyama, F. Kaneko, Jpn. J. Appl. Phys., Part 2 31 (1992) L1606. [4] K. Tanaka, Y. Kimura, S. Okamoto, Y. Inoue, K. Sato, Jpn. J. Appl. Phys., Part 1 37 (1998) 3350. [5] N. Ishibashi, T. Nishi, N. Hayashi, C. Furuhashi, K. Sato, Jpn. J. Appl. Phys., Part 2 38 (1999) L626.
[6] T. Nishi, N. Ishibashi, N. Hayashi, C. Furuhashi, K. Sato, Jpn. J. Appl. Phys. Suppl. 39-1 (2000) 296. [7] The growth method was very similar to.S. Chichibu, M. Shishikura, J. Ino, S. Matsumoto, J. Appl. Phys. 70 (1991) 1648. [8] S. Kurtin, T.C. McGill, C.A. Mead, Phys. Rev. Lett. 22 (1969) 1433. [9] W. Mo¨nch, Appl. Surf. Sci. 92 (1996) 367. [10] R. Marquez, C. Rincon, Phys. Status Solidi b 191 (1995) 115. [11] J. Bardeen, Phys. Rev. 71 (1947) 717. [12] W. Schottky, Z. Phys. 15 (1914) 872. [13] C.P. Wu, E.C. Douglas, C.W. Mueller, Trans. IEEE Electron. Devices ED-22 (1975) 319. [14] P.W. Yu, D.L. Dowing, Y.S. Park, J. Appl. Phys. 45 (1974) 5283. [15] M. Sugiyama, H. Nakanishi, S.F. Chichibu, Jpn. J. Appl. Phys., Part 2 40 (2001) L428. [16] W.E. Spicer, I. Lindau, P. Skeath, C.Y. Su, J. Vac. Sci. Technol. 17 (1980) 1019.