- Email: [email protected]
Investigation of Fermi level pinning at semipolar (11–22) p-type GaN surfaces
Accepted Manuscript Investigation of Fermi level pinning at semipolar (11-22) p-type GaN surfaces Young-Yun Choi, Seongjun Kim, Munsik Oh, Hyunsoo Kim, Tae-Yeon Seong PII: DOI: Reference:
S0749-6036(14)00401-7 http://dx.doi.org/10.1016/j.spmi.2014.10.031 YSPMI 3470
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
25 July 2014 21 October 2014 26 October 2014
Please cite this article as: Y-Y. Choi, S. Kim, M. Oh, H. Kim, T-Y. Seong, Investigation of Fermi level pinning at semipolar (11-22) p-type GaN surfaces, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/ j.spmi.2014.10.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of Fermi level pinning at semipolar (11-22) p-type GaN surfaces Young-Yun Choi,1 Seongjun Kim,2 Munsik Oh,2 Hyunsoo Kim,2∗ and Tae-Yeon Seong1∗ 1
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea 2
School of Semiconductor and Chemical Engineering, Semiconductor Physics
Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea
Schottky barrier height (SBH; ΦB) and their dependence on the work function of metals (ΦM) at semipolar (11-22) p-GaN surfaces were investigated using Schottky diodes fabricated with different metals. The SBH increased with temperature, whereas the ideality factor decreased. This behavior was explained by means of the barrier inhomogeneity model, giving the mean barrier heights of 1.93 – 2.05 eV for different metals. The S-parameter (dΦB/dΦM) was obtained to be 0.04. This small S-parameter implies that the surface Fermi level is nearly perfectly pinned at deep-level states (caused by vacancy-related and/or Mg-induced defects) located at 1.98 eV above the valence band. This finding indicates that the surface modification is essentially required for the formation of high-quality ohmic and/or Schottky contacts.
∗ ∗
E-mail: [email protected] E-mail: [email protected] 1
Keywords: Schottky contacts, semipolar GaN, barrier inhomogeneity model, surface states 1. Introduction III-nitride semiconducting materials are technologically important because of their applications in high-efficiency optoelectronic and electronic devices, such as light-emitting diodes (LEDs), Schottky barrier photodetectors, and metal-semiconductor field effect transistors (MESFETs) [1-4]. In particular, LEDs grown on c-plane (polar) substrates suffer from a serious decrease in the internal quantum efficiency (IQE) of InGaN/GaN LEDs due to the quantum-confined Stark effect, which is induced by the spontaneous and piezoelectric polarization fields [5,6]. Thus, to resolve this problem, nonpolar or semipolar substrates, such as [11-20], [1-100] and [11-22] planes, were used for LEDs, whose optical performance was significantly improved as compared to conventional LEDs [7-9]. Despite such advantages, however, nonpolar or semipolar GaN-based optical and electronic devices have not been extensively investigated. This is in part due to difficulties in developing high-quality metal contacts on nonpolar or semipolar GaN surfaces [10]. Until now, research performed on metal contacts has been only limited to n-type semipolar GaN [11,12]. For example, Jung et al. [11] investigated the electrical characteristics of metal contacts with different work functions on semipolar n-type GaN and reported that thermionic field emission theory produced a relatively low S-parameter (defined as dΦB/dΦM) of 0.26, indicating a density of surface states as high as 3.2 × 1013 states/cm2/eV and a bare surface barrier height of 1.57 eV. These high surface states and barrier height were suggested to be related to the pinning of the Fermi level at the semipolar n-GaN surface. Jung et al.[12], investigating the electrical properties of Ti/Al ohmic contacts to (11-22) semipolar n-GaN (n = 3.6 × 1018 cm–3), showed that annealing caused the formation of ohmic contacts with a specific contact resistance of 3.2 × 10–4 Ωcm2. The ohmic behavior was attributed to a high interfacial carrier of 9.2 × 1018 cm–3. However, a study of the electrical and optical properties of metal contacts on semipolar 2
(11-22) p-GaN has been hardly carried out [13]. In fact, no work on Schottky contacts to semipolar p-GaN has been hitherto performed. Thus, it is essential to get a full understanding of the electrical properties of metal contacts on semipolar GaN to develop high-performance optoelectronic and electronic devices [14]. In this work, we investigated the Schottky barrier heights and S-parameter at semipolar (11-22) p-GaN surfaces. To understand the Schottky characteristics, different types of models, such as thermionic emission (TE) and barrier inhomogeneity models, are used.
2. Experimental procedure Mg-doped semipolar (11-22) p-GaN wafers were grown on m-plane sapphire substrates by a metal organic chemical vapor deposition system. Hall-effect measurements showed that the semipolar samples had a carrier concentration of 3.36 × 1017 cm–3 and a mobility of 7.67 cm2V–1s–1. The inset of Fig. 1 shows Schottky diodes which were prepared using a standard photolithographic technique, where metals were deposited by an e-beam evaporation. Prior to metal deposition, the p-GaN samples surface was cleaned by a buffered oxide etchant (BOE) solution and deionized water. First of all, a Zn/Ag(10 nm/200 nm) scheme was deposited on the semipolar p-GaN surface and was rapid-thermal annealed at 300 °C for 1 min in an oxygen atmosphere to form an ohmic contact [13]. After that, 100-nm-thick Pt, Ni, Cu, and Ti layers as Schottky contact schemes were deposited onto the circle region (50 µm in diameter) (inset in Fig. 1). The electrical characteristics of the Schottky diodes were measured in the temperature range of the 300 – 500 K by a parameter analyzer (HP4156A).
3. Results and discussion Figure 1 exhibits the forward I-V characteristics of Schottky diodes fabricated with Pt, Ni, Cu and Ti. The inset shows the optical microscopic image of Schottky diode with Cu contact. 3
The saturation current densities were relatively large and varied from ~10–5 to ~10–7 A/cm2 depending on the Schottky metals, being indicative of a considerable leakage component. To evaluate the Schottky characteristics, use of a proper conduction model is required. The metal-semiconductor contact theory implies that the current flow mechanisms are dependent on the tunneling parameter (E00) [15-17], defined as E00=(qh/4π)(N/εsm*)½, where q is the electron charge, N is carrier concentration, h is the Planck constant, εs is the dielectric constant of GaN (εs=8.9ε0), and m* is the hole effective mass (m*=0.8me). For instance, the dominant transport mechanism is thermionic emission (TE) for kT/qE00 ≫ 1, thermionic field emission (TFE) for kT/qE00 ≈1, and field emission (FE) for kT/qE00 ≪ 1 [15-17], where k is the Boltzmann constant and T is the temperature. kT/qE00 value was estimated to be 6.41 and indicates that for our semipolar p-GaN, TE is dominant. The I-V characteristics of TE model can be given as,
ಳ
1,
(1)
where A is the contact area, A** is the Richardson constant (103.8 A/cm2K2), ΦB is the effective Schottky barrier height (SBH), and n is the ideality factor. The effective SBHs obtained using Eq. (1) were 0.75, 0.78, 0.68, and 0.70 eV for Ti, Cu, Ni, and Pt contacts, respectively. The ideality factors were estimated to be 3.70, 3.43, 3.73, and 4.01 for Ti, Cu, Ni, and Pt contacts, respectively. The ideality factors are much larger than unity, which is consistent with large saturation current densities. This implies that the TE model is not appropriate to describe the carrier transport in our semipolar p-GaN samples. To obtain more accurate Schottky parameters, I-V-T measurements were performed on all of the Schottky diodes in the range of 300 – 500 K (not shown).
4
Figure 2 shows SBHs and ideality factors as a function of temperature. The SBH increases with increasing temperature, whereas the ideality factor decreases. This behavior cannot be explained by the TE model, in which the SBH and ideality factor should be independent of temperature. In polar GaN Schottky diodes, this unusual behavior was frequently observed [18-22], as it is related to the inhomogeneity of the contact [23,24]. The barrier heights were assumed to have a Gaussian distribution with a mean SBH (ΦB,m) and a standard deviation (σ) in the barrier inhomogeneity model, as described below [16,24], Φ Φ , T 0
σమ
(2)
Figure 3 illustrates plot of ΦB vs. 1/T, and 1/n – 1 vs. 1/T for a Ni contact, where the straight line confirms the Gaussian distribution. These results show that the standard deviation (σ) is independent of the temperature. The linear fit obtained using Eq. (2) showed that the mean barrier heights (ΦB,m) at 0 K are 1.92, 1.93, 2.05, and 1.97 eV for the Ti, Cu, Ni, and Pt contacts, respectively. The standard deviations (σ) were also measured to be 0.25, 0.24, 0.27, and 0.26 eV for the Ti, Cu, Ni, and Pt contacts, respectively. The ideality factor (n) can be explained as a function of the temperature, as described below [24],
್
1
(3)
where ρa and ρb are the voltage coefficients explained as follows : ΦB,m = ΦB0,m + ρaV and σ2 = σ02 + ρbV, where the subscript “0” denotes a zero bias. In this model, ρa and ρb should be independent of the temperature, resulting in a straight line as shown in Fig. 3. For all of the samples, ρa and ρb measured from the linear fit were almost the same, i.e., ρa = ~0.3 V and ρb = ~–0.02 V. This indicates that ΦB,m and n are dependent on positive and negative forward bias, respectively, illustrating the homogenization of potential fluctuations with increasing forward bias [24].
5
Figure 4 exhibits a variation of the mean barrier heights as a function of the metal work functions. For comparison, the mean barrier heights from polar p-GaN samples were also plotted. For the polar p-GaN, the barrier heights decrease with increasing the work function of the metals, whereas for the semipolar samples, the barrier heights insignificantly vary with the work function. Consequently, the S-parameter of semipolar p-GaN is estimated to be 0.04, which is much smaller than that of polar p-GaN (S = 0.27)[20]. This implies that the surface Fermi level can be pinned at surface states located 1.98 eV above the valance band, as will be shown later. So the Schottky-Mott model cannot be used to describe the barrier height characteristics. The density of surface states (Ds) can be estimated by the Cowley-Sze model, as described below [25],
(4)
మ
where εi is the permittivity and ti is the thickness of the interfacial layer. It is, however, noted that it is not easy to obtain both of the parameters (εi and ti). Thus, following the treatments by Cowley and Sze [25], Schmitz et al.[26], and Arulkumaran et al.[27] who used εi, the permittivity of free space, and ti, a thickness of 5 Å, in calculating Ds, we also employed these values in this work. The Ds was measured to be 2.92 × 1014 states/cm2/eV. On the other hand, those obtained from polar p-GaN samples were 3.04 × 1013 states/cm2/eV, respectively [20]. A comparison shows that for the semipolar p-GaN surface, the Fermi level has a much stronger tendency to get pinned. To evaluate the dependence of surface states on the surface band bending, the bare surface barrier height (qΦs) was estimated using the relation given below [28],
ಳ ಾ
(5)
where χ is the electron affinity of semipolar p-GaN (4.1 eV), qΦB = 1.92 eV, and work function (qΦM) = 4.33 eV for a Ti contact. The qΦS was measured to be 1.98 eV. The qΦS of 6
the polar p-GaN was also obtained using qΦB = 1.48 eV, S-parameter = 0.27, and qΦM = 4.33 eV [20]. A comparison shows that the qΦS of the semipolar p-GaN is larger than that (1.80 eV) of the polar p-GaN. This implies that the semipolar plane experiences the downward surface band-bending more seriously, as shown in the inset in Fig. 4. The exact reason why the polarization filed-minimized semipolar p-GaN samples undergo more serious bandbending is not clearly understood at this moment. However, the mechanism may be explained as follows. The semipolar p-GaN may contain a high density of crystallographic defects, such as threading dislocations, vacancies, stacking faults, and hexagonal shaped pits [29,30]. As described previously, the surface Fermi level was pinned at the surface states located at 1.98 eV above the valence band, which is comparable to the energy level of vacancy-related defects, such as deep NGa donors [31], VN-related deep donors [32], or VGa-ON (Ga vacancy and O on N sites) [33]. This indicates that the deep level states may be caused by the presence of such vacancy-related and/or antisite defects. Moreover, different types of crystallographic defects, such as threading dislocations and stacking faults [29,30], may also generate deep-level states, causing the Fermi level pinning.
4. Summary and conclusion We investigated the carrier transport characteristics and surface states at semipolar (11-22) p-GaN using Schottky diodes fabricated with Ti, Cu, Ni, and Pt. Measurements showed that the SBHs increased with increasing temperature, whose behavior was explained by means of the barrier inhomogeneity model. The S-parameter of semipolar p-GaN was estimated to be close to zero, indicating that the surface Fermi level is almost perfectly pinned due to the presence of a high density of deep-level defects. This finding could provide an important clue about how to design contact schemes because the control of the surface states is crucial for
7
the development of high-quality ohmic and Schottky contacts for high-efficiency semipolar GaN-based LEDs and MESFETs.
Acknowledgments This work was supported by the Industrial Strategic Technology Development Program, 10041878, Development of WPE 75% LED Device Process and Standard Evaluation Technology funded by the MKE, Korea, and Priority Research Center Program through the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea (2011-0027956).
8
References [1] S. Y. Lee, K. K. Choi, H-H Jeong, H. S. Choi, T.-H. Oh, J. O Song, T.-Y. Seong, Semicond. Sci. Technol. 24, 092001 (2009). [2] E. J. Tarsa, P. Kozodoy, J. Ibbetson, and B. P. Keller, G. Parish and U. Mishra, Appl. Phys. Lett. 77, 316 (2000). [3] K. A. McIntosh, R. J. Molnar, L. J. Mahoney, K. M. Molvar, N. Efremow, Jr., and S. Verghese, Appl. Phys. Lett. 76, 3938 (2000). [4] M. A. Khan, J. N. Kuznia, A. R. Bhattarai, and D. T. Olson, Appl. Phys. Lett. 62, 1786 (1993). [5] F. Bernardini, V. Fiorentini, and D. Vanderbilt, Phys. Rev. B 56, R10024 (1997). [6] J.-H. Ryou, P. D. Yoder, J. P. Liu, Z. Lochner, H. Kim, S. Choi, H. J. Kim, and R. D. Dupuis, IEEE J. Select. Topic. Quantum Electron. 15, 1080 (2009). [7] M. D. Craven, P. Waltereit, J. S. Speck, and S. P. DenBaars, Appl. Phys. Lett. 84, 496 (2004). [8] A. Chitnis, C. Chen, V. Adivarahan, M. Shatalov, E. Kuokstis, V. Mandavilli, J. Yang, and M. A. Khan, Appl. Phys. Lett. 84, 3663 (2004). [9] F. Scholz, T. Wunderer, M. Feneberg, K. Thonke, A. Chuvilin, U. Kaiser, S. Metzner, F. Bertram, J. Christen, Phys. Stat. Solidi A 207, 1407 (2010). 9
[10] H. Kim, S.-N. Lee, Y. Park, J. S. Kwak, T.-Y. Seong, Appl. Phys. Lett. 93, 032105 (2008). [11] S. Jung, S.-N. Lee, and H. Kim Appl. Phys. Lett. 102, 151603 (2013). [12] S. Jung, S.-N. Lee, K.-S. Ahn, and H. Kim, Jpn. J. Appl. Phys. 51, 061001 (2012). [13] J.-S.Park, J. Han and T.-Y.Seong (unpublished). [14] J.-S. Jang, D. Kim, T.-Y. Seong, J. Appl. Phys. 99, 073704 (2006). [15] A. Y. C. Yu, Solid-State Electron. 13, 239 (1970). [16] J.H. Werner and H.H. Guttler, J. Appl. Phys. 69, 1522 (1991). [17] J. H. Werner and H.H. Guttler, Phys. ScriptaT39, 258 (1991). [18] T. Sawada, Y. Izumi, N. Kimura, K. Suzuki, K. Imai, S.-W. Kim and T. Suzuki, Appl. Surf. Sci. 216, 192 (2003). [19] C. K. Tan, A. A. Aziz, and F. K. Yan, Appl. Surf. Sci. 252, 5930 (2006). [20] Y. Park, K.-S. Ahn, and H. Kim, Jpn. J. Appl. Phys. 51, 09MK01 (2012). [21] Y.-J Lin, Appl. Phys. Lett. 86, 122109 (2005). [22] F. Lucolano, F. Roccaforte, F. Giannazzo, and V. Raineri, J. Appl. Phys. 102, 113701 (2007). [23] F. A. Padovani and R. Stratton, Solid-State Electron. 9, 695 (1966). [24] E. H. Rhoderick, IEE Proc. 129, 1 (1982). [25] A. M. Cowley and S. M. Sze, J. Appl. Phys. 36, 3212 (1965). [26] A. C. Schmitz, A. T. Ping, M. A. Khan, Q. Chen, J. W. Yang, and I. Adesida, J. electron Mater. 27, 255 (1998). [27] S. Arulkumaran, T. Egawa, G.-Y. Zhao, H. Ishikawa, T. Jimbo, and M. Umeno, Jpn. J. Appl. Phys. 39, L351 (2000). [28] V. M. Bermudez J. Appl. Phys. 86, 1170 (1999). [29] C. F. Johnston, M. A. Moram, M. J. Kappers, C. J. Humphreys, Appl. Phys. Lett. 94, 10
161109 (2009). [30] S. Marcinkevicius, Y. Zhao, K. M. Kelchner, S. Nakamura, S. P. DenBaars, J. S. Speck,Appl. Phys. Lett. 103, 131116 (2013). [31] G. Salviati, N. Armani, C. Zanotti-Fregonara, E. Gombia, M. Albrecht, H. Strunk, M. Mayer, M. Kamp, and A. Gasparotto, MRS Internet J. Nitride Semicond. Res. 5S1, W11.50 (2000). [32] B. Han, M. P. Ulmer, and B. W. Wessels, Physica B 340-342, 470 (2003). [33] H. Yu, H. Chen, D. Li, Y. Han, X. Zheng, Q. Huang, and J. Zhou, J. Cryst. Growth 263, 94 (2004).
11
Figure captions
Fig. 1
The typical forward I-V characteristics of Schottky diodes fabricated with Ti, Cu, Ni, and Pt. The inset shows an optical microscopy image obtained from a Schottky diode.
Fig. 2
(a) SBHs and (b) ideality factors as a function of the temperature.
Fig. 3
plots of ΦB vs. 1/T, and 1/n – 1 vs. 1/T for a Ni Schottky contact.
Fig. 4
Variation of the mean barrier height (ΦB,m) as a function of the metal work functions (ΦM).
12
13
14
16
Highlights >>Dependence of SBH on metal work functions at semipolar (11-22) p-GaN surfaces is studied >>SBH increases with temperature, whereas the ideality factor decreases >>Temperature dependence of SBH and ideality factor explained by the barrier inhomogeneity model >>The surface Fermi level is almost perfectly pinned at deep-level states
17
S0749-6036(14)00401-7 http://dx.doi.org/10.1016/j.spmi.2014.10.031 YSPMI 3470
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
25 July 2014 21 October 2014 26 October 2014
Please cite this article as: Y-Y. Choi, S. Kim, M. Oh, H. Kim, T-Y. Seong, Investigation of Fermi level pinning at semipolar (11-22) p-type GaN surfaces, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/ j.spmi.2014.10.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of Fermi level pinning at semipolar (11-22) p-type GaN surfaces Young-Yun Choi,1 Seongjun Kim,2 Munsik Oh,2 Hyunsoo Kim,2∗ and Tae-Yeon Seong1∗ 1
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea 2
School of Semiconductor and Chemical Engineering, Semiconductor Physics
Research Center, Chonbuk National University, Jeonju 561-756, Republic of Korea
Schottky barrier height (SBH; ΦB) and their dependence on the work function of metals (ΦM) at semipolar (11-22) p-GaN surfaces were investigated using Schottky diodes fabricated with different metals. The SBH increased with temperature, whereas the ideality factor decreased. This behavior was explained by means of the barrier inhomogeneity model, giving the mean barrier heights of 1.93 – 2.05 eV for different metals. The S-parameter (dΦB/dΦM) was obtained to be 0.04. This small S-parameter implies that the surface Fermi level is nearly perfectly pinned at deep-level states (caused by vacancy-related and/or Mg-induced defects) located at 1.98 eV above the valence band. This finding indicates that the surface modification is essentially required for the formation of high-quality ohmic and/or Schottky contacts.
∗ ∗
E-mail: [email protected] E-mail: [email protected] 1
Keywords: Schottky contacts, semipolar GaN, barrier inhomogeneity model, surface states 1. Introduction III-nitride semiconducting materials are technologically important because of their applications in high-efficiency optoelectronic and electronic devices, such as light-emitting diodes (LEDs), Schottky barrier photodetectors, and metal-semiconductor field effect transistors (MESFETs) [1-4]. In particular, LEDs grown on c-plane (polar) substrates suffer from a serious decrease in the internal quantum efficiency (IQE) of InGaN/GaN LEDs due to the quantum-confined Stark effect, which is induced by the spontaneous and piezoelectric polarization fields [5,6]. Thus, to resolve this problem, nonpolar or semipolar substrates, such as [11-20], [1-100] and [11-22] planes, were used for LEDs, whose optical performance was significantly improved as compared to conventional LEDs [7-9]. Despite such advantages, however, nonpolar or semipolar GaN-based optical and electronic devices have not been extensively investigated. This is in part due to difficulties in developing high-quality metal contacts on nonpolar or semipolar GaN surfaces [10]. Until now, research performed on metal contacts has been only limited to n-type semipolar GaN [11,12]. For example, Jung et al. [11] investigated the electrical characteristics of metal contacts with different work functions on semipolar n-type GaN and reported that thermionic field emission theory produced a relatively low S-parameter (defined as dΦB/dΦM) of 0.26, indicating a density of surface states as high as 3.2 × 1013 states/cm2/eV and a bare surface barrier height of 1.57 eV. These high surface states and barrier height were suggested to be related to the pinning of the Fermi level at the semipolar n-GaN surface. Jung et al.[12], investigating the electrical properties of Ti/Al ohmic contacts to (11-22) semipolar n-GaN (n = 3.6 × 1018 cm–3), showed that annealing caused the formation of ohmic contacts with a specific contact resistance of 3.2 × 10–4 Ωcm2. The ohmic behavior was attributed to a high interfacial carrier of 9.2 × 1018 cm–3. However, a study of the electrical and optical properties of metal contacts on semipolar 2
(11-22) p-GaN has been hardly carried out [13]. In fact, no work on Schottky contacts to semipolar p-GaN has been hitherto performed. Thus, it is essential to get a full understanding of the electrical properties of metal contacts on semipolar GaN to develop high-performance optoelectronic and electronic devices [14]. In this work, we investigated the Schottky barrier heights and S-parameter at semipolar (11-22) p-GaN surfaces. To understand the Schottky characteristics, different types of models, such as thermionic emission (TE) and barrier inhomogeneity models, are used.
2. Experimental procedure Mg-doped semipolar (11-22) p-GaN wafers were grown on m-plane sapphire substrates by a metal organic chemical vapor deposition system. Hall-effect measurements showed that the semipolar samples had a carrier concentration of 3.36 × 1017 cm–3 and a mobility of 7.67 cm2V–1s–1. The inset of Fig. 1 shows Schottky diodes which were prepared using a standard photolithographic technique, where metals were deposited by an e-beam evaporation. Prior to metal deposition, the p-GaN samples surface was cleaned by a buffered oxide etchant (BOE) solution and deionized water. First of all, a Zn/Ag(10 nm/200 nm) scheme was deposited on the semipolar p-GaN surface and was rapid-thermal annealed at 300 °C for 1 min in an oxygen atmosphere to form an ohmic contact [13]. After that, 100-nm-thick Pt, Ni, Cu, and Ti layers as Schottky contact schemes were deposited onto the circle region (50 µm in diameter) (inset in Fig. 1). The electrical characteristics of the Schottky diodes were measured in the temperature range of the 300 – 500 K by a parameter analyzer (HP4156A).
3. Results and discussion Figure 1 exhibits the forward I-V characteristics of Schottky diodes fabricated with Pt, Ni, Cu and Ti. The inset shows the optical microscopic image of Schottky diode with Cu contact. 3
The saturation current densities were relatively large and varied from ~10–5 to ~10–7 A/cm2 depending on the Schottky metals, being indicative of a considerable leakage component. To evaluate the Schottky characteristics, use of a proper conduction model is required. The metal-semiconductor contact theory implies that the current flow mechanisms are dependent on the tunneling parameter (E00) [15-17], defined as E00=(qh/4π)(N/εsm*)½, where q is the electron charge, N is carrier concentration, h is the Planck constant, εs is the dielectric constant of GaN (εs=8.9ε0), and m* is the hole effective mass (m*=0.8me). For instance, the dominant transport mechanism is thermionic emission (TE) for kT/qE00 ≫ 1, thermionic field emission (TFE) for kT/qE00 ≈1, and field emission (FE) for kT/qE00 ≪ 1 [15-17], where k is the Boltzmann constant and T is the temperature. kT/qE00 value was estimated to be 6.41 and indicates that for our semipolar p-GaN, TE is dominant. The I-V characteristics of TE model can be given as,
ಳ
1,
(1)
where A is the contact area, A** is the Richardson constant (103.8 A/cm2K2), ΦB is the effective Schottky barrier height (SBH), and n is the ideality factor. The effective SBHs obtained using Eq. (1) were 0.75, 0.78, 0.68, and 0.70 eV for Ti, Cu, Ni, and Pt contacts, respectively. The ideality factors were estimated to be 3.70, 3.43, 3.73, and 4.01 for Ti, Cu, Ni, and Pt contacts, respectively. The ideality factors are much larger than unity, which is consistent with large saturation current densities. This implies that the TE model is not appropriate to describe the carrier transport in our semipolar p-GaN samples. To obtain more accurate Schottky parameters, I-V-T measurements were performed on all of the Schottky diodes in the range of 300 – 500 K (not shown).
4
Figure 2 shows SBHs and ideality factors as a function of temperature. The SBH increases with increasing temperature, whereas the ideality factor decreases. This behavior cannot be explained by the TE model, in which the SBH and ideality factor should be independent of temperature. In polar GaN Schottky diodes, this unusual behavior was frequently observed [18-22], as it is related to the inhomogeneity of the contact [23,24]. The barrier heights were assumed to have a Gaussian distribution with a mean SBH (ΦB,m) and a standard deviation (σ) in the barrier inhomogeneity model, as described below [16,24], Φ Φ , T 0
σమ
(2)
Figure 3 illustrates plot of ΦB vs. 1/T, and 1/n – 1 vs. 1/T for a Ni contact, where the straight line confirms the Gaussian distribution. These results show that the standard deviation (σ) is independent of the temperature. The linear fit obtained using Eq. (2) showed that the mean barrier heights (ΦB,m) at 0 K are 1.92, 1.93, 2.05, and 1.97 eV for the Ti, Cu, Ni, and Pt contacts, respectively. The standard deviations (σ) were also measured to be 0.25, 0.24, 0.27, and 0.26 eV for the Ti, Cu, Ni, and Pt contacts, respectively. The ideality factor (n) can be explained as a function of the temperature, as described below [24],
್
1
(3)
where ρa and ρb are the voltage coefficients explained as follows : ΦB,m = ΦB0,m + ρaV and σ2 = σ02 + ρbV, where the subscript “0” denotes a zero bias. In this model, ρa and ρb should be independent of the temperature, resulting in a straight line as shown in Fig. 3. For all of the samples, ρa and ρb measured from the linear fit were almost the same, i.e., ρa = ~0.3 V and ρb = ~–0.02 V. This indicates that ΦB,m and n are dependent on positive and negative forward bias, respectively, illustrating the homogenization of potential fluctuations with increasing forward bias [24].
5
Figure 4 exhibits a variation of the mean barrier heights as a function of the metal work functions. For comparison, the mean barrier heights from polar p-GaN samples were also plotted. For the polar p-GaN, the barrier heights decrease with increasing the work function of the metals, whereas for the semipolar samples, the barrier heights insignificantly vary with the work function. Consequently, the S-parameter of semipolar p-GaN is estimated to be 0.04, which is much smaller than that of polar p-GaN (S = 0.27)[20]. This implies that the surface Fermi level can be pinned at surface states located 1.98 eV above the valance band, as will be shown later. So the Schottky-Mott model cannot be used to describe the barrier height characteristics. The density of surface states (Ds) can be estimated by the Cowley-Sze model, as described below [25],
(4)
మ
where εi is the permittivity and ti is the thickness of the interfacial layer. It is, however, noted that it is not easy to obtain both of the parameters (εi and ti). Thus, following the treatments by Cowley and Sze [25], Schmitz et al.[26], and Arulkumaran et al.[27] who used εi, the permittivity of free space, and ti, a thickness of 5 Å, in calculating Ds, we also employed these values in this work. The Ds was measured to be 2.92 × 1014 states/cm2/eV. On the other hand, those obtained from polar p-GaN samples were 3.04 × 1013 states/cm2/eV, respectively [20]. A comparison shows that for the semipolar p-GaN surface, the Fermi level has a much stronger tendency to get pinned. To evaluate the dependence of surface states on the surface band bending, the bare surface barrier height (qΦs) was estimated using the relation given below [28],
ಳ ಾ
(5)
where χ is the electron affinity of semipolar p-GaN (4.1 eV), qΦB = 1.92 eV, and work function (qΦM) = 4.33 eV for a Ti contact. The qΦS was measured to be 1.98 eV. The qΦS of 6
the polar p-GaN was also obtained using qΦB = 1.48 eV, S-parameter = 0.27, and qΦM = 4.33 eV [20]. A comparison shows that the qΦS of the semipolar p-GaN is larger than that (1.80 eV) of the polar p-GaN. This implies that the semipolar plane experiences the downward surface band-bending more seriously, as shown in the inset in Fig. 4. The exact reason why the polarization filed-minimized semipolar p-GaN samples undergo more serious bandbending is not clearly understood at this moment. However, the mechanism may be explained as follows. The semipolar p-GaN may contain a high density of crystallographic defects, such as threading dislocations, vacancies, stacking faults, and hexagonal shaped pits [29,30]. As described previously, the surface Fermi level was pinned at the surface states located at 1.98 eV above the valence band, which is comparable to the energy level of vacancy-related defects, such as deep NGa donors [31], VN-related deep donors [32], or VGa-ON (Ga vacancy and O on N sites) [33]. This indicates that the deep level states may be caused by the presence of such vacancy-related and/or antisite defects. Moreover, different types of crystallographic defects, such as threading dislocations and stacking faults [29,30], may also generate deep-level states, causing the Fermi level pinning.
4. Summary and conclusion We investigated the carrier transport characteristics and surface states at semipolar (11-22) p-GaN using Schottky diodes fabricated with Ti, Cu, Ni, and Pt. Measurements showed that the SBHs increased with increasing temperature, whose behavior was explained by means of the barrier inhomogeneity model. The S-parameter of semipolar p-GaN was estimated to be close to zero, indicating that the surface Fermi level is almost perfectly pinned due to the presence of a high density of deep-level defects. This finding could provide an important clue about how to design contact schemes because the control of the surface states is crucial for
7
the development of high-quality ohmic and Schottky contacts for high-efficiency semipolar GaN-based LEDs and MESFETs.
Acknowledgments This work was supported by the Industrial Strategic Technology Development Program, 10041878, Development of WPE 75% LED Device Process and Standard Evaluation Technology funded by the MKE, Korea, and Priority Research Center Program through the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea (2011-0027956).
8
References [1] S. Y. Lee, K. K. Choi, H-H Jeong, H. S. Choi, T.-H. Oh, J. O Song, T.-Y. Seong, Semicond. Sci. Technol. 24, 092001 (2009). [2] E. J. Tarsa, P. Kozodoy, J. Ibbetson, and B. P. Keller, G. Parish and U. Mishra, Appl. Phys. Lett. 77, 316 (2000). [3] K. A. McIntosh, R. J. Molnar, L. J. Mahoney, K. M. Molvar, N. Efremow, Jr., and S. Verghese, Appl. Phys. Lett. 76, 3938 (2000). [4] M. A. Khan, J. N. Kuznia, A. R. Bhattarai, and D. T. Olson, Appl. Phys. Lett. 62, 1786 (1993). [5] F. Bernardini, V. Fiorentini, and D. Vanderbilt, Phys. Rev. B 56, R10024 (1997). [6] J.-H. Ryou, P. D. Yoder, J. P. Liu, Z. Lochner, H. Kim, S. Choi, H. J. Kim, and R. D. Dupuis, IEEE J. Select. Topic. Quantum Electron. 15, 1080 (2009). [7] M. D. Craven, P. Waltereit, J. S. Speck, and S. P. DenBaars, Appl. Phys. Lett. 84, 496 (2004). [8] A. Chitnis, C. Chen, V. Adivarahan, M. Shatalov, E. Kuokstis, V. Mandavilli, J. Yang, and M. A. Khan, Appl. Phys. Lett. 84, 3663 (2004). [9] F. Scholz, T. Wunderer, M. Feneberg, K. Thonke, A. Chuvilin, U. Kaiser, S. Metzner, F. Bertram, J. Christen, Phys. Stat. Solidi A 207, 1407 (2010). 9
[10] H. Kim, S.-N. Lee, Y. Park, J. S. Kwak, T.-Y. Seong, Appl. Phys. Lett. 93, 032105 (2008). [11] S. Jung, S.-N. Lee, and H. Kim Appl. Phys. Lett. 102, 151603 (2013). [12] S. Jung, S.-N. Lee, K.-S. Ahn, and H. Kim, Jpn. J. Appl. Phys. 51, 061001 (2012). [13] J.-S.Park, J. Han and T.-Y.Seong (unpublished). [14] J.-S. Jang, D. Kim, T.-Y. Seong, J. Appl. Phys. 99, 073704 (2006). [15] A. Y. C. Yu, Solid-State Electron. 13, 239 (1970). [16] J.H. Werner and H.H. Guttler, J. Appl. Phys. 69, 1522 (1991). [17] J. H. Werner and H.H. Guttler, Phys. ScriptaT39, 258 (1991). [18] T. Sawada, Y. Izumi, N. Kimura, K. Suzuki, K. Imai, S.-W. Kim and T. Suzuki, Appl. Surf. Sci. 216, 192 (2003). [19] C. K. Tan, A. A. Aziz, and F. K. Yan, Appl. Surf. Sci. 252, 5930 (2006). [20] Y. Park, K.-S. Ahn, and H. Kim, Jpn. J. Appl. Phys. 51, 09MK01 (2012). [21] Y.-J Lin, Appl. Phys. Lett. 86, 122109 (2005). [22] F. Lucolano, F. Roccaforte, F. Giannazzo, and V. Raineri, J. Appl. Phys. 102, 113701 (2007). [23] F. A. Padovani and R. Stratton, Solid-State Electron. 9, 695 (1966). [24] E. H. Rhoderick, IEE Proc. 129, 1 (1982). [25] A. M. Cowley and S. M. Sze, J. Appl. Phys. 36, 3212 (1965). [26] A. C. Schmitz, A. T. Ping, M. A. Khan, Q. Chen, J. W. Yang, and I. Adesida, J. electron Mater. 27, 255 (1998). [27] S. Arulkumaran, T. Egawa, G.-Y. Zhao, H. Ishikawa, T. Jimbo, and M. Umeno, Jpn. J. Appl. Phys. 39, L351 (2000). [28] V. M. Bermudez J. Appl. Phys. 86, 1170 (1999). [29] C. F. Johnston, M. A. Moram, M. J. Kappers, C. J. Humphreys, Appl. Phys. Lett. 94, 10
161109 (2009). [30] S. Marcinkevicius, Y. Zhao, K. M. Kelchner, S. Nakamura, S. P. DenBaars, J. S. Speck,Appl. Phys. Lett. 103, 131116 (2013). [31] G. Salviati, N. Armani, C. Zanotti-Fregonara, E. Gombia, M. Albrecht, H. Strunk, M. Mayer, M. Kamp, and A. Gasparotto, MRS Internet J. Nitride Semicond. Res. 5S1, W11.50 (2000). [32] B. Han, M. P. Ulmer, and B. W. Wessels, Physica B 340-342, 470 (2003). [33] H. Yu, H. Chen, D. Li, Y. Han, X. Zheng, Q. Huang, and J. Zhou, J. Cryst. Growth 263, 94 (2004).
11
Figure captions
Fig. 1
The typical forward I-V characteristics of Schottky diodes fabricated with Ti, Cu, Ni, and Pt. The inset shows an optical microscopy image obtained from a Schottky diode.
Fig. 2
(a) SBHs and (b) ideality factors as a function of the temperature.
Fig. 3
plots of ΦB vs. 1/T, and 1/n – 1 vs. 1/T for a Ni Schottky contact.
Fig. 4
Variation of the mean barrier height (ΦB,m) as a function of the metal work functions (ΦM).
12
13
14
16
Highlights >>Dependence of SBH on metal work functions at semipolar (11-22) p-GaN surfaces is studied >>SBH increases with temperature, whereas the ideality factor decreases >>Temperature dependence of SBH and ideality factor explained by the barrier inhomogeneity model >>The surface Fermi level is almost perfectly pinned at deep-level states
17