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Etch pit investigation of free electron concentration controlled 4H-SiC
Journal of Crystal Growth 369 (2013) 38–42
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Etch pit investigation of free electron concentration controlled 4H-SiC Hong-Yeol Kim a, Yun Ji Shin b, Jung Gon Kim c, Hiroshi Harima c, Jihyun Kim a, Wook Bahng b,n a b c
Department of Chemical and Biological Engineering, Korea University, Seoul 136-701, Republic of Korea Power Semiconductor Research Center, Korea Electrotechnology Research Institute, Changwon, Gyeongsangnam-do 642-120, Republic of Korea Department of Electronics, Kyoto Institute of Technology, Kyoto 606-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2012 Received in revised form 29 January 2013 Accepted 30 January 2013 Communicated by: M. Skowronski Available online 8 February 2013
Etch pits were investigated using the molten KOH selective etching method to examine dependence of etch pit shape and size on free electron concentration. The free electron concentrations of highly doped 4H-silicon carbide (SiC) were controlled by proton irradiation and thermal annealing, which was confirmed by a frequency shift in the LO-phonon–plasmon-coupled (LOPC) mode on micro-Raman spectroscopy. The proton irradiated sample with 5 1015 cm 2 fluence and an intrinsic semi-insulating sample showed clearly classified etch pits but different ratios of threading screw dislocation (TSD) and threading edge dislocation (TED) sizes. Easily classified TEDs and TSDs on proton irradiated 4H-SiC were restored as highly doped 4H-SiC after thermal annealing due to the recovered carrier concentrations. The etched surface of proton irradiated 4H-SiC and boron implanted SiC showed different surface conditions after activation. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Doping A1. Etching A1. Line defects B2. Semiconducting materials
1. Introduction Silicon carbide (SiC) has high potential for high-temperature, high-power, and high-frequency devices due to its superior physical and chemical stability, thermal conductivity, and wide bandgap. However, defects formed during growth are the reason for the degraded properties of the devices under a harsh environment [1,2]. Characterizations of defects are essential to reduce defect density and enhance the electrical properties of semiconductor materials. One of the characterization methods is classification of each defect by chemical selective etching using molten potassium hydroxide (KOH). Chemically etched SiC shows various defects such as threading screw dislocation (TSD), basal plane dislocation (BPD), threading edge dislocation (TED), and complexes [3–5]. BPDs are easy to distinguish from other dislocations due to their seashell-shaped etch features and different location of the pit core. Although etch pits of TSD and TED have a six-fold symmetry structure, screw dislocations have a larger pit size than edge dislocations. However, discriminating TSD and TED is difficult on highly doped n þ -type SiC, whereas the two types of low doped SiC are clearly defined [3–6]. The cause of this phenomenon is not fully understood. Sakwe et al. reported that different Burgers vectors with different strain fields at each dislocation directly affect etch rate [4]. Wu found that impurities and dopant atoms of highly doped SiC modify etch rate [6]. In addition, Zhang et al. reported that Fermi level is an important
n
Corresponding author. Tel.: þ82 55 280 1621. E-mail address: [email protected] (W. Bahng).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.01.047
factor to selectively etch dislocations by comparing dislocation etch pits on n-type SiC and those on p-type SiC that were converted from nitrogen doped n-type SiC by high-temperature boron diffusion [3]. However, boron atoms were the other dopants, so the total dopant concentration was very high, which could affect dislocation etch rate. In this study, we focused on the dependence of etch pit shape and size on free electron concentration that was controlled by proton irradiation and thermal annealing. High-energy proton irradiation is an effective method to reduce the free carrier concentration at a specific location by creating point and point-like defects [7,8]. Moreover, hydrogen atoms in SiC were easily movable and diffused. Therefore, the shape of etched surfaces with different free electron concentrations could be compared regardless of the total dopant concentrations.
2. Experimental Three sets of 350 mm thick 41 off-axis n þ -type 4H-SiC samples with electron concentrations of 2.6 1018 cm 3 were prepared. Two sets of samples (set A and B) were irradiated with highenergy protons as described below. One set of samples (set A) was investigated as-irradiated state. The other set of samples (set B) was thermally annealed at 1700 1C for 90 min to restore free electron concentration. Then, both samples underwent KOH etching and were compared with the etched surface of semiinsulating 4H-SiC. The last set (set C) was implanted with boron to investigate the shapes of the p-type sample dislocation etch
H.-Y. Kim et al. / Journal of Crystal Growth 369 (2013) 38–42
pits. Table 1 shows the tested samples and processes for each set. Proton irradiation was performed on the backside of the SiC (carbon face) to generate lower free-electron-concentration region near the Si face using the MC-50 cyclotron at the Korea Institute of Radiological and Medical Science. Proton energy and fluence bombarded on the 4H-SiC surface were 8.5 MeV and 5 1015 cm 2, respectively. Boron doping was performed on n þ -SiC, where a total dose and implanted energies were 7.5 1014 cm 2 and 30–260 keV, respectively. Then, the boronimplanted SiC was thermally activated at 1700 1C for 30 min. Each set of samples was chemically etched in molten KOH at 530 1C for 3 min. The etched surfaces (Si faces) were investigated with a Nomarski microscope. The free electron concentrations and the experimental proton penetration depth were examined using micro-Raman spectroscopy with a 532 nm wavelength laser line. Micro-Raman scattering was measured at room temperature by back scattering geometry, which was zðxy,xyÞz and xðyz,yzÞx, where the z direction was parallel to the c-axis and the x and y directions were perpendicular to the c-axis. The irradiation direction of the protons paralleled the c-axis of the samples (Fig. 1).
3. Results and discussion Previous studies demonstrated irradiation induced damage and reduced carrier concentrations. Castaldini et al. reported that 6.5 MeV proton irradiation on n-type 4H-SiC creates various carrier traps whose activation energies (EC–ET) are 0.2–1.09 eV [9]. Kozlovski et al. showed carrier removal rates with different subjected electron and proton fluences [10]. The longitudinal optical (LO) phonon mode shifts toward higher frequency and its intensity decreases with increasing carrier concentration due to LO-phonon–plasmon-coupling (LOPC) [11–13]. The LO frequency measured at xðyz,yzÞx (cross section) is called the quasi-LO mode due to the combined frequency of A1(LO) and E1(LO) [14], which also varies by free electron concentration [7]. A Raman spectroscopy line-scan on the cross section of the sample was performed from the edge to 350 mm of the sample to assess the penetration depth of 8.5 MeV protons in SiC. The location that showed minimum quasi-LO frequency was a penetration depth of 8.5 MeV protons because damage was
39
concentrated by the increased collision between the proton and lattice atoms. The experimentally examined penetration depth of protons was approximately 320–330 mm, which was similar to the Stopping and Range of Ions in Matter (SRIM) calculated penetration depth (364 mm), as shown in Fig. 1. The difference between the calculated penetration depth and experimentally examined depth was o45 mm, which was attributed to non-uniformly generated proton energy. Because protons did not sufficiently reach the surface, the Si face was mechanically polished by Raman experimentally assessed penetration depth before KOH etching. The E1(LO) mode is inactive on zðxy,xyÞz geometry, whereas the A1(LO) mode is active and affected by the free electron concentrations in this geometry. The shift of the A1(LO) mode toward a higher frequency with increased doping level is contributed by the LOPC mode [11]. The observed LOPC mode shifts toward lower frequency after proton irradiation because the free electron concentration decreased due to irradiationinduced damage. The peak shift of the LOPC mode resulting from proton irradiation, KOH etching, and thermal annealing is shown in Fig. 2. It can be seen that the LOPC mode of set A was shifted lower frequency after proton irradiation and KOH etching even though the location of the peak intensity was lower and peak location was at a little higher frequency that right after proton irradiation (blue line and diamond shape in Fig. 2). The LOPC mode of set B also had been shifted toward lower frequency after proton irradiation. However, its location and line width were returned to the state of pre-irradiation (dark yellow and circle shape in Fig. 2). Free carrier concentrations can be estimated by the peak LOPC frequency mode using the dielectric function, which is given by the contributions from phonons and plasmons under phonon and plasmon damping [11–13]. " # o2 o2 o2P eðoÞ ¼ e1 1 þ 2 LO 2 TO , oTO o ioG oðo þigÞ where oP is the plasma frequency given by sffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4pne2 oP ¼ e1 mn where mn is the effective electron mass and n is the free electron concentration. In the above equations, oLO and oTO denote the longitudinal optical and transverse optical phonon frequencies,
Table 1 Processes for each sample set. Set
A
B
C
Process
Proton irradiation KOH etching
Proton irradiation 1700 1C annealing for 90 min KOH etching
Boron implantation 1700 1C annealing for 30 min KOH etching
Fig. 1. Schematic diagram of proton irradiation and Raman measurement direction and numerically and experimentally assessed proton penetration depths.
40
H.-Y. Kim et al. / Journal of Crystal Growth 369 (2013) 38–42
respectively. G is the phonon damping constant, g is the plasmon damping constant, and ep is the high-frequency dielectric constant. The LOPC mode peak frequency of pre-irradiated n þ -SiC was 977.3 cm 1 which represented free electron concentrations of 2.6 1018 cm 3 approximated by LOPC fitting analysis. In addition, the LOPC frequency shifted to 964.5 cm 1 ( o5 1016 cm 3 free electrons) after proton irradiation, which was the same as the A1(LO) mode of semi-insulating 4H-SiC, as denoted in Table 2. Fig. 3 shows the etch pits of intrinsic semi-insulating 4H-SiC and proton irradiated 4H-SiC. BPDs, TEDs, and TSDs were clearly distinguished in both samples, as shown in Fig. 3a and b. However, the etch pits were slightly different. TSDs in the semi-insulating 4H-SiC showed 1.5–2 times larger etch pits than TEDs in length. Although the LOPC mode of the proton-irradiated sample was shifted slightly toward higher frequency due to annealing effect by 530 1C KOH etching, the etch pits of TSDs had a more clear hexagonal feature and
Fig. 2. LO-phonon–plasmon-coupled (LOPC) mode shift after each process. Inset shows the Raman measurement geometry.
Table 2 Peak position of LO-phonon–plasmon-coupled (LOPC) mode and estimated free electron concentrations of 4H-silicon carbide (SiC) after different processes. Sample
LOPC mode (cm 1) Free electron concentration (cm 3)
Semi-insulating 964.5 Pre-irradiation (n þ ) 977.3 Proton-irradiation 964.5 Irradiation & KOH etching (Set A) 965.7 1700 1C annealing (Set B) 976.7
o5 1016 2.6 1018 o5 1016 2.5 1017 2.5 1018
were much longer (9–10 times) than those of TEDs. The different ratios of TSD and TED on both samples indicated that the difference in growth conditions and irradiation damage were important factors to investigate etch pits. Furthermore, hydrogen atoms penetrated due to irradiation as well as non-uniformly distributed nitrogen atoms in SiC and acted as impurities. Wu suggested that impurities and stress are related to the difficulty in classifying dislocations in SiC [6]. However this failed to explain our case because classifying the two kinds of dislocation was easier after proton irradiation, as hydrogen ions acted as impurities. Proton irradiation-induced damage was partially restored by high temperature thermal annealing because most of the defect complexes in the proton-irradiated SiC are annealed at 41000– 1100 1C [15,16], indicating that reduced carrier concentrations due to irradiation damage could be recovered by high temperature annealing. The Raman spectra LOPC mode on thermally annealed samples shifted toward a lower frequency similar to the level of pre-irradiated SiC (Fig. 2), which demonstrated that most of the free electrons increased as much as in the initial state. The LOPC mode shift and the free carrier concentrations of 4HSiCs before and after proton irradiation and thermal annealing are shown in Table 2. Fig. 4 a–c shows the KOH-etched Nomarski images of the pre-irradiated (a), proton irradiated (b), thermally annealed (c), and boron implanted 4H-SiC surfaces (d), respectively. The large and hexagonal TSDs on the proton-irradiated SiC surface became smaller and round shaped after thermal annealing. Although the electron concentrations were not fully restored, the 1700 1C thermally annealed 4H-SiC showed isotropically etched dislocation with an irregular size and shape, which was similar with pre-irradiated n þ -type 4H-SiC (Fig. 4 a and c). The above proton irradiated and annealed results clearly confirmed that free electrons significantly affected the selective dislocation etch rate. Boron implanted 4H-SiC also demonstrated the dependence of etch pit shape and size on free electron concentrations. Boron implanted and thermally activated samples showed easily distinguishable TEDs and TSDs (Fig. 4d). The highly nitrogen doped n þ type 4H-SiC was converted to p-type SiC after boron implantation because the extra holes in the p-type dopant compensated for the free electron. The surface characteristics that show distinguishable dislocations on p-type SiC were consistent with a previous study [3]. However, Zhang et al. concluded that the dislocation etch rate on p-type SiC is much faster than that of n-type SiC. This contradicted our results that the dislocation etch rate on proton-irradiated SiC was faster than that of boron-implanted SiC under the same etching conditions. This discrepancy was
Fig. 3. Etch pits of semi-insulating (a) and proton irradiated (b) 4H-silicon carbide (SiC). The insets shows magnified area with red lines that include threading screw dislocation (TSD) and threading edge dislocation (TED). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
H.-Y. Kim et al. / Journal of Crystal Growth 369 (2013) 38–42
41
Fig. 4. Etch pits of dislocations in n þ -type (a), proton irradiated (b), 1700 1C annealed (c), and boron implanted (d) 4H-silicon carbide (SiC).
probably caused by irradiation-induced damage. As the mass and size of boron ions are much larger than those of hydrogen ions, the damage effect was more serious, which might change SiC surface states. The boron implanted surface showed many small dots even after thermal activation (Fig. 4), which cannot be investigated on boron-doped 4H-SiC using the thermal diffusion method [3]. This kind of damage might affect the selective etch rate. Similar dots were observed on proton-irradiated samples, as shown in Fig. 4c, although the number of dots was much smaller, which was evidence of relatively small damage. Small dots were rarely observed on thermally annealed proton irradiated SiC, because most of the defect complexes in the proton-irradiated SiC were annealed at 1700 1C [15,16]. Therefore, the surface state and the free electron concentrations were recovered through high temperature thermal annealing.
4. Conclusion Proton irradiated and thermally annealed n þ -type 4H-SiCs showed different etch pit sizes and shapes of TSD and TED. These results indicate that the size and shape of etch pits were mainly affected from the free electron concentrations controlled by proton irradiation and thermal annealing. Proton irradiation damaged the surface by creating small dots that could be observed on the boron implanted surface. However, the damage from proton irradiation was annealed at high temperature. Proton irradiation onto the SiC is a more effective method to investigate the relationship between carrier concentrations and etch pit shape than that of the diffusion method, as the effect from the impurities can be reduced.
Acknowledgement This study was supported by ‘‘World Premier Materials’’ project of Korea Ministry of Knowledge Economy.
References [1] J.P. Bergman, H. Lendenmann, P.A. Nilsson, U. Lindenfelt, P. Skytt, Crystal defects as source of anomalous forward voltage increase of 4H-SiC diodes, Materials Science Forum 353–356 (2000) 299. [2] J. Takahashi, N. Ohtani, M. Matsumoto, S. Shinoyama, Sublimation growth of 6H- and 4H-SiC single crystals in the½1100 and ½1120 directions, Journal of Crystal Growth 181 (1997) 229. [3] Z. Zhang, Y. Gao, T. Sudarshan, Delineating structural defects in highly doped n-type 4H-SiC substrates using a combination of thermal diffusion and molten KOH etching, Electrochemical and Solid-State Letters 7 (2004) G264. ¨ [4] S.A. Sakwe, R. Muller, P.J. Wellmann, Optimization of KOH etching parameters for quantitative defect recognition in n- and p-type doped SiC, Journal of Crystal Growth 289 (2006) 520. ¨ [5] B. Kallinger, S. Polster, P. Berwian, J. Friedrich, G. Muller, A.N. Danilewsky, A. Wehrhahn, A.-D. Weber, Threading dislocations in n- and p-type 4H-SiC material analyzed by etching and synchrotron X-ray topography, Journal of Crystal Growth 314 (2011) 21. [6] P. Wu, Etching study of dislocations in heavily nitrogen doped SiC crystals, Journal of Crystal Growth 312 (2010) 1193. [7] H.-Y. Kim, J.A. Freitas Jr., J. Kim, Penetration effects of high-energy protons in GaN: a micro-Raman spectroscopy study, Electrochemical and Solid-State Letters 14 (2011) H5. [8] H.-Y. Kim, J.A. Freitas Jr., J. Kim, Probing proton irradiation effects in GaN by micro-Raman spectroscopy, Europhysics Letters 96 (2011) 26004. [9] A. Castaldini, A. Cavallini, L. Rigutti, F. Nava, S. Ferrero, F. Giorgis, Deep levels by proton and electron irradiation in 4H-SiC, Journal of Applied Physics 98 (2005) 053706. [10] V.V. Kozlovski, V.V. Emtsev, A.M. Ivanov, A.A. Lebedev, G.A. Oganesyan, D.S. Poloskin, N.B. Strokan, Charge carrier removal rates in n-type silicon and silicon carbide subjected to electron and proton irradiation, Physica B: Condensed Matter 404 (2009) 4752.
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[11] H. Harima, S.-I. Nakashima, T. Uemura, Raman scattering from anisotropic LO-phonon–plasmon-coupled mode in n-type 4H- and 6H-SiC, Journal of Applied Physics 78 (1995) 1996. [12] M. Chafai, A. Jaouhari, A. Torres, R. Anto´n, E. Martı´n, J. Jime´nez, W.C. Mitchel, Raman scattering from LO phonon–plasmon coupled modes and Hall-effect in n-type silicon carbide 4H-SiC, Journal of Applied Physics 90 (2001) 5211. [13] J.C. Burton, L. Sun, M. Pophristic, S.J. Lukacs, F.H. Long, Z.C. Feng, I.T. Ferguson, Spatial characterization of doped SiC wafers by Raman spectroscopy, Journal of Applied Physics 84 (1998) 6268. [14] C.A. Arguello, D.L. Rousseau, S.P.S. Porto, First-order Raman effect in Wurtzite-type crystals, Physical Review 181 (1969) 1351.
[15] H.J. von Bardeleben, J.L. Cantin, I. Vickridg, G. Battistig, Proton-implantationinduced defects in n-type 6H- and 4H-SiC: an electron paramagnetic resonance study, Physical Review B: Condensed Matter 62 (2000) 10126. [16] Y. Koshka, J.B. Dufrene, J.B. Casady, Hydrogen interaction with defects and impurities in 6H-SiC, Journal of Electronic Materials 32 (2003) 423.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Etch pit investigation of free electron concentration controlled 4H-SiC Hong-Yeol Kim a, Yun Ji Shin b, Jung Gon Kim c, Hiroshi Harima c, Jihyun Kim a, Wook Bahng b,n a b c
Department of Chemical and Biological Engineering, Korea University, Seoul 136-701, Republic of Korea Power Semiconductor Research Center, Korea Electrotechnology Research Institute, Changwon, Gyeongsangnam-do 642-120, Republic of Korea Department of Electronics, Kyoto Institute of Technology, Kyoto 606-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2012 Received in revised form 29 January 2013 Accepted 30 January 2013 Communicated by: M. Skowronski Available online 8 February 2013
Etch pits were investigated using the molten KOH selective etching method to examine dependence of etch pit shape and size on free electron concentration. The free electron concentrations of highly doped 4H-silicon carbide (SiC) were controlled by proton irradiation and thermal annealing, which was confirmed by a frequency shift in the LO-phonon–plasmon-coupled (LOPC) mode on micro-Raman spectroscopy. The proton irradiated sample with 5 1015 cm 2 fluence and an intrinsic semi-insulating sample showed clearly classified etch pits but different ratios of threading screw dislocation (TSD) and threading edge dislocation (TED) sizes. Easily classified TEDs and TSDs on proton irradiated 4H-SiC were restored as highly doped 4H-SiC after thermal annealing due to the recovered carrier concentrations. The etched surface of proton irradiated 4H-SiC and boron implanted SiC showed different surface conditions after activation. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Doping A1. Etching A1. Line defects B2. Semiconducting materials
1. Introduction Silicon carbide (SiC) has high potential for high-temperature, high-power, and high-frequency devices due to its superior physical and chemical stability, thermal conductivity, and wide bandgap. However, defects formed during growth are the reason for the degraded properties of the devices under a harsh environment [1,2]. Characterizations of defects are essential to reduce defect density and enhance the electrical properties of semiconductor materials. One of the characterization methods is classification of each defect by chemical selective etching using molten potassium hydroxide (KOH). Chemically etched SiC shows various defects such as threading screw dislocation (TSD), basal plane dislocation (BPD), threading edge dislocation (TED), and complexes [3–5]. BPDs are easy to distinguish from other dislocations due to their seashell-shaped etch features and different location of the pit core. Although etch pits of TSD and TED have a six-fold symmetry structure, screw dislocations have a larger pit size than edge dislocations. However, discriminating TSD and TED is difficult on highly doped n þ -type SiC, whereas the two types of low doped SiC are clearly defined [3–6]. The cause of this phenomenon is not fully understood. Sakwe et al. reported that different Burgers vectors with different strain fields at each dislocation directly affect etch rate [4]. Wu found that impurities and dopant atoms of highly doped SiC modify etch rate [6]. In addition, Zhang et al. reported that Fermi level is an important
n
Corresponding author. Tel.: þ82 55 280 1621. E-mail address: [email protected] (W. Bahng).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.01.047
factor to selectively etch dislocations by comparing dislocation etch pits on n-type SiC and those on p-type SiC that were converted from nitrogen doped n-type SiC by high-temperature boron diffusion [3]. However, boron atoms were the other dopants, so the total dopant concentration was very high, which could affect dislocation etch rate. In this study, we focused on the dependence of etch pit shape and size on free electron concentration that was controlled by proton irradiation and thermal annealing. High-energy proton irradiation is an effective method to reduce the free carrier concentration at a specific location by creating point and point-like defects [7,8]. Moreover, hydrogen atoms in SiC were easily movable and diffused. Therefore, the shape of etched surfaces with different free electron concentrations could be compared regardless of the total dopant concentrations.
2. Experimental Three sets of 350 mm thick 41 off-axis n þ -type 4H-SiC samples with electron concentrations of 2.6 1018 cm 3 were prepared. Two sets of samples (set A and B) were irradiated with highenergy protons as described below. One set of samples (set A) was investigated as-irradiated state. The other set of samples (set B) was thermally annealed at 1700 1C for 90 min to restore free electron concentration. Then, both samples underwent KOH etching and were compared with the etched surface of semiinsulating 4H-SiC. The last set (set C) was implanted with boron to investigate the shapes of the p-type sample dislocation etch
H.-Y. Kim et al. / Journal of Crystal Growth 369 (2013) 38–42
pits. Table 1 shows the tested samples and processes for each set. Proton irradiation was performed on the backside of the SiC (carbon face) to generate lower free-electron-concentration region near the Si face using the MC-50 cyclotron at the Korea Institute of Radiological and Medical Science. Proton energy and fluence bombarded on the 4H-SiC surface were 8.5 MeV and 5 1015 cm 2, respectively. Boron doping was performed on n þ -SiC, where a total dose and implanted energies were 7.5 1014 cm 2 and 30–260 keV, respectively. Then, the boronimplanted SiC was thermally activated at 1700 1C for 30 min. Each set of samples was chemically etched in molten KOH at 530 1C for 3 min. The etched surfaces (Si faces) were investigated with a Nomarski microscope. The free electron concentrations and the experimental proton penetration depth were examined using micro-Raman spectroscopy with a 532 nm wavelength laser line. Micro-Raman scattering was measured at room temperature by back scattering geometry, which was zðxy,xyÞz and xðyz,yzÞx, where the z direction was parallel to the c-axis and the x and y directions were perpendicular to the c-axis. The irradiation direction of the protons paralleled the c-axis of the samples (Fig. 1).
3. Results and discussion Previous studies demonstrated irradiation induced damage and reduced carrier concentrations. Castaldini et al. reported that 6.5 MeV proton irradiation on n-type 4H-SiC creates various carrier traps whose activation energies (EC–ET) are 0.2–1.09 eV [9]. Kozlovski et al. showed carrier removal rates with different subjected electron and proton fluences [10]. The longitudinal optical (LO) phonon mode shifts toward higher frequency and its intensity decreases with increasing carrier concentration due to LO-phonon–plasmon-coupling (LOPC) [11–13]. The LO frequency measured at xðyz,yzÞx (cross section) is called the quasi-LO mode due to the combined frequency of A1(LO) and E1(LO) [14], which also varies by free electron concentration [7]. A Raman spectroscopy line-scan on the cross section of the sample was performed from the edge to 350 mm of the sample to assess the penetration depth of 8.5 MeV protons in SiC. The location that showed minimum quasi-LO frequency was a penetration depth of 8.5 MeV protons because damage was
39
concentrated by the increased collision between the proton and lattice atoms. The experimentally examined penetration depth of protons was approximately 320–330 mm, which was similar to the Stopping and Range of Ions in Matter (SRIM) calculated penetration depth (364 mm), as shown in Fig. 1. The difference between the calculated penetration depth and experimentally examined depth was o45 mm, which was attributed to non-uniformly generated proton energy. Because protons did not sufficiently reach the surface, the Si face was mechanically polished by Raman experimentally assessed penetration depth before KOH etching. The E1(LO) mode is inactive on zðxy,xyÞz geometry, whereas the A1(LO) mode is active and affected by the free electron concentrations in this geometry. The shift of the A1(LO) mode toward a higher frequency with increased doping level is contributed by the LOPC mode [11]. The observed LOPC mode shifts toward lower frequency after proton irradiation because the free electron concentration decreased due to irradiationinduced damage. The peak shift of the LOPC mode resulting from proton irradiation, KOH etching, and thermal annealing is shown in Fig. 2. It can be seen that the LOPC mode of set A was shifted lower frequency after proton irradiation and KOH etching even though the location of the peak intensity was lower and peak location was at a little higher frequency that right after proton irradiation (blue line and diamond shape in Fig. 2). The LOPC mode of set B also had been shifted toward lower frequency after proton irradiation. However, its location and line width were returned to the state of pre-irradiation (dark yellow and circle shape in Fig. 2). Free carrier concentrations can be estimated by the peak LOPC frequency mode using the dielectric function, which is given by the contributions from phonons and plasmons under phonon and plasmon damping [11–13]. " # o2 o2 o2P eðoÞ ¼ e1 1 þ 2 LO 2 TO , oTO o ioG oðo þigÞ where oP is the plasma frequency given by sffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4pne2 oP ¼ e1 mn where mn is the effective electron mass and n is the free electron concentration. In the above equations, oLO and oTO denote the longitudinal optical and transverse optical phonon frequencies,
Table 1 Processes for each sample set. Set
A
B
C
Process
Proton irradiation KOH etching
Proton irradiation 1700 1C annealing for 90 min KOH etching
Boron implantation 1700 1C annealing for 30 min KOH etching
Fig. 1. Schematic diagram of proton irradiation and Raman measurement direction and numerically and experimentally assessed proton penetration depths.
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H.-Y. Kim et al. / Journal of Crystal Growth 369 (2013) 38–42
respectively. G is the phonon damping constant, g is the plasmon damping constant, and ep is the high-frequency dielectric constant. The LOPC mode peak frequency of pre-irradiated n þ -SiC was 977.3 cm 1 which represented free electron concentrations of 2.6 1018 cm 3 approximated by LOPC fitting analysis. In addition, the LOPC frequency shifted to 964.5 cm 1 ( o5 1016 cm 3 free electrons) after proton irradiation, which was the same as the A1(LO) mode of semi-insulating 4H-SiC, as denoted in Table 2. Fig. 3 shows the etch pits of intrinsic semi-insulating 4H-SiC and proton irradiated 4H-SiC. BPDs, TEDs, and TSDs were clearly distinguished in both samples, as shown in Fig. 3a and b. However, the etch pits were slightly different. TSDs in the semi-insulating 4H-SiC showed 1.5–2 times larger etch pits than TEDs in length. Although the LOPC mode of the proton-irradiated sample was shifted slightly toward higher frequency due to annealing effect by 530 1C KOH etching, the etch pits of TSDs had a more clear hexagonal feature and
Fig. 2. LO-phonon–plasmon-coupled (LOPC) mode shift after each process. Inset shows the Raman measurement geometry.
Table 2 Peak position of LO-phonon–plasmon-coupled (LOPC) mode and estimated free electron concentrations of 4H-silicon carbide (SiC) after different processes. Sample
LOPC mode (cm 1) Free electron concentration (cm 3)
Semi-insulating 964.5 Pre-irradiation (n þ ) 977.3 Proton-irradiation 964.5 Irradiation & KOH etching (Set A) 965.7 1700 1C annealing (Set B) 976.7
o5 1016 2.6 1018 o5 1016 2.5 1017 2.5 1018
were much longer (9–10 times) than those of TEDs. The different ratios of TSD and TED on both samples indicated that the difference in growth conditions and irradiation damage were important factors to investigate etch pits. Furthermore, hydrogen atoms penetrated due to irradiation as well as non-uniformly distributed nitrogen atoms in SiC and acted as impurities. Wu suggested that impurities and stress are related to the difficulty in classifying dislocations in SiC [6]. However this failed to explain our case because classifying the two kinds of dislocation was easier after proton irradiation, as hydrogen ions acted as impurities. Proton irradiation-induced damage was partially restored by high temperature thermal annealing because most of the defect complexes in the proton-irradiated SiC are annealed at 41000– 1100 1C [15,16], indicating that reduced carrier concentrations due to irradiation damage could be recovered by high temperature annealing. The Raman spectra LOPC mode on thermally annealed samples shifted toward a lower frequency similar to the level of pre-irradiated SiC (Fig. 2), which demonstrated that most of the free electrons increased as much as in the initial state. The LOPC mode shift and the free carrier concentrations of 4HSiCs before and after proton irradiation and thermal annealing are shown in Table 2. Fig. 4 a–c shows the KOH-etched Nomarski images of the pre-irradiated (a), proton irradiated (b), thermally annealed (c), and boron implanted 4H-SiC surfaces (d), respectively. The large and hexagonal TSDs on the proton-irradiated SiC surface became smaller and round shaped after thermal annealing. Although the electron concentrations were not fully restored, the 1700 1C thermally annealed 4H-SiC showed isotropically etched dislocation with an irregular size and shape, which was similar with pre-irradiated n þ -type 4H-SiC (Fig. 4 a and c). The above proton irradiated and annealed results clearly confirmed that free electrons significantly affected the selective dislocation etch rate. Boron implanted 4H-SiC also demonstrated the dependence of etch pit shape and size on free electron concentrations. Boron implanted and thermally activated samples showed easily distinguishable TEDs and TSDs (Fig. 4d). The highly nitrogen doped n þ type 4H-SiC was converted to p-type SiC after boron implantation because the extra holes in the p-type dopant compensated for the free electron. The surface characteristics that show distinguishable dislocations on p-type SiC were consistent with a previous study [3]. However, Zhang et al. concluded that the dislocation etch rate on p-type SiC is much faster than that of n-type SiC. This contradicted our results that the dislocation etch rate on proton-irradiated SiC was faster than that of boron-implanted SiC under the same etching conditions. This discrepancy was
Fig. 3. Etch pits of semi-insulating (a) and proton irradiated (b) 4H-silicon carbide (SiC). The insets shows magnified area with red lines that include threading screw dislocation (TSD) and threading edge dislocation (TED). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
H.-Y. Kim et al. / Journal of Crystal Growth 369 (2013) 38–42
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Fig. 4. Etch pits of dislocations in n þ -type (a), proton irradiated (b), 1700 1C annealed (c), and boron implanted (d) 4H-silicon carbide (SiC).
probably caused by irradiation-induced damage. As the mass and size of boron ions are much larger than those of hydrogen ions, the damage effect was more serious, which might change SiC surface states. The boron implanted surface showed many small dots even after thermal activation (Fig. 4), which cannot be investigated on boron-doped 4H-SiC using the thermal diffusion method [3]. This kind of damage might affect the selective etch rate. Similar dots were observed on proton-irradiated samples, as shown in Fig. 4c, although the number of dots was much smaller, which was evidence of relatively small damage. Small dots were rarely observed on thermally annealed proton irradiated SiC, because most of the defect complexes in the proton-irradiated SiC were annealed at 1700 1C [15,16]. Therefore, the surface state and the free electron concentrations were recovered through high temperature thermal annealing.
4. Conclusion Proton irradiated and thermally annealed n þ -type 4H-SiCs showed different etch pit sizes and shapes of TSD and TED. These results indicate that the size and shape of etch pits were mainly affected from the free electron concentrations controlled by proton irradiation and thermal annealing. Proton irradiation damaged the surface by creating small dots that could be observed on the boron implanted surface. However, the damage from proton irradiation was annealed at high temperature. Proton irradiation onto the SiC is a more effective method to investigate the relationship between carrier concentrations and etch pit shape than that of the diffusion method, as the effect from the impurities can be reduced.
Acknowledgement This study was supported by ‘‘World Premier Materials’’ project of Korea Ministry of Knowledge Economy.
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