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SiC heterodiodes
Materials Science and Engineering B61 – 62 (1999) 320 – 324
Simulation and electrical characterization of GaN/SiC and AlGaN/SiC heterodiodes E. Danielsson a,*, C.-M. Zetterling a, M. O8 stling a, B. Breitholtz a, K. Linthicum b, D.B. Thomson b, O.-H. Nam b, R.F. Davis b b
a KTH, Department of Electronics, P.O. Box Electrum 229, S-164 40, Kista, Sweden Department of Materials Science & Engineering, North Carolina State Uni6ersity, NC, USA
Abstract Heterojunctions on SiC is an area in rapid development, especially GaN/SiC and AlGaN/SiC heterojunctions. The heterojunction can improve the performance considerably for BJTs and FETs. In this work heterojunction diodes have been manufactured and characterized. The structure was a GaN or AlGaN n-type region on top of a 6H-SiC p-type substrate. Two different approaches of growing the n-type region were tested. The GaN was grown with the MBE technique using a polycrystalline GaN buffer, whereas the AlGaN was grown with CVD and an AlN buffer. The AlGaN had an aluminum mole fraction of around 0.1. Mesa structures were formed using Cl2 RIE of GaN/AlGaN, which showed good selectivity on 6H-SiC (about 1:6). A Ti metallization with subsequent RTA was used as contact to GaN and AlGaN, and the contact to 6H-SiC was liquid InGa. Both I –V and C–V measurements were performed on the heterojunction diode. The ideality factor of the diodes, doping concentration of the SiC, and the band alignment of the heterojunction were extracted. © 1999 Elsevier Science S.A. All rights reserved.
1. Introduction SiC has several properties that makes it a promising candidate for future power devices. One interesting possibility with the SiC crystal structure is that it can be used as a substrate for GaN or AlGaN epitaxial growth. This makes it possible to create heterojunctions in SiC technology, which is a very powerful tool when designing devices. Several reports have covered this subject recently [1 – 4]. The devices manufactured are usually heterojunction bipolar transistors (HBTs) and heterostructure field effect transistors (HFETs). The HBTs are often made with a SiC base and collector, with the wider bandgap nitride as the emitter. In the HFETs SiC is only used as an inactive substrate to grow the structure on, and various compositions of Alx Ga1 − x N is used to create the heterostructures. GaN is unintentionally n-type doped during growth, which is believed to originate from the N-vacancy [5] or be due to silicon or oxygen impurities [6,7]. Since it is
* Corresponding author.
difficult to reduce the background doping it is preferable to use GaN or AlGaN as the n-type region in SiC heterojunction design. The carrier concentration of unintentionally doped AlGaN decreases with aluminum content [8], but silicon can be used to intentionally dope the AlGaN to the desired level. It is difficult to grow GaN and AlGaN on SiC due to the 2–3% lattice mismatch, so it is common to use a buffer layer to relax the interface and nucleate the growth. The lattice match improves with aluminum content, and for AlN the lattice mismatch is only 1%. In this work two approaches have been tested for the buffer layer. The first, tested with GaN, was a polycrystalline GaN layer, and the second approach, tested with AlGaN, was an AlN buffer layer [9,10].
2. Experimental The starting material for the heterodiodes was p-type SiC substrates with a 4 mm thick p-type epitaxial layer with doping around 1016 cm − 3. The first step was to grow the GaN and AlGaN epitaxially on the substrates
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E. Danielsson et al. / Materials Science and Engineering B61–62 (1999) 320–324
using molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) respectively. The GaN layer was undoped, with an unintentional doping causing a carrier concentration in the 1018 cm − 3 region. The epi-layer was approximately 0.5 mm thick. Between the GaN layer and the SiC there was 20 nm thick polycrystalline GaN buffer grown at low temperature to relax the interface. The AlGaN was Si doped slightly above 1018 cm − 3, and the aluminum mole fraction was around 0.1. The epi-layer was approximately 0.5 mm thick, with a 12 nm AlN buffer layer between the AlGaN and the SiC. The AlN only has a 1% lattice mismatch on SiC, but the wide bandgap of AlN (: 6.2 eV) [11] will produce a rather high barrier for the carriers. The AlGaN sample was relaxed and had a fine pattern of cracks in the epi-layer. Mesa structures were formed using a reactive ion etch (RIE) with Cl2 as active gas. The etch rate was around 150 nm min − 1 for GaN and AlGaN. On SiC the etch rate was only around 25 nm min − 1, which corresponds to a selectivity of 1:6 for this etch. The chamber pressure was 100 mTorr and the RF-power was set to 200 W, and photoresist was used as etch mask. Sputtering of titanium was used to deposit a TiN/Ti structure as metallization on the GaN and AlGaN. Between the first and second titanium layer the samples were annealed by rapid thermal annealing (RTA) at 950°C for 30 s in a N2 ambient to form the TiN. The first layer was made thin (around 50 nm) to limit the reaction with GaN. Recent reports of titanium as contact to GaN has shown low contact resistivity after RTA [12,13], and TLM measurements on both the GaN and the AlGaN samples showed ohmic behavior with low contact resistance (rC :5·10 − 6 Vcm2). The low contact resistance is probably due to the formation of TiN at the Ti/GaN interface which creates N-vacancies, and the highly doped region of the GaN near the interface will produce a very thin tunneling barrier for carriers [12,13]. To check how deep the reaction is under the metal contact the metal was etched off with a mixture of HNO3 and HF (1:1) and the structure was raster scanned with a stylus profilometer. This measurement did not show any large consumption of the GaN under the contact. The backside contact to the p-type 6H-SiC was liquid InGa, and since the doping of the SiC substrate was rather low ( :1018 cm − 3) the contact characteristics and the series resistance of the substrate results in an early current saturation of the heterodiodes. Both C–V and I – V measurements were performed on the heterodiodes at elevated temperatures, ranging from room temperature up to 300°C.
321
Fig. 1. Results from I– V measurements for heterodiodes on the MBE-sample plus one diode on the CVD sample at different temperatures. Note that the measured voltage is the cathode voltage, so the forward voltage is negative.
3. Results
3.1. I–V measurements The I–V measurements were performed with a HP4156 parameter analyzer on a probe station, and the temperature was controlled using a hot chuck. The measurements were made on circular diodes with diameters of 50, 100 and 200 mm, and at temperatures of 25 (RT), 100, 150, 200 and 300°C. Representative results from the measurements are shown in Fig. 1 and Fig. 2. From the measurements the ideality factor in the linear region is extracted and compared for the different temperatures. The ‘turn-on’ voltage for all of the heterodiodes were lower than expected for wide bandgap junctions, only :1.2 V when it should be around 2.5 V. This could be due to a defect level in the p-type SiC 1.25 eV above the valence band. The defect level acts as a recombination center when electrons
Fig. 2. Results from I – V measurements for different diameters of heterodiodes on the MBE-sample.
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determine the band alignment Eq. (1) is used, where VD is determined from the C–V measurement as the intercept with the x-axis.
2 1 2 = + (VD − V) 2 qoGaNNGaN qoSiCNSiC CD
(1)
When VD is known the band offset at the conductance band is calculated using Eq. (2). The doping is extracted from the slope, but it can only be determined if the doping on one side is known or much larger than the other side [15]. In the calculations the doping of the GaN layer was assumed to be 5×1018 cm − 3, but since the dependence is logarithmic this value is not so crucial. Fig. 3. Results from C–V measurements of heterodiodes from the CVD-sample. This shows better linear dependence than the MBEsample. The band lineup in the inset is exaggerated.
from the GaN conductance band to tunnel to this level at forward bias [14]. The MBE sample with the polyGaN buffer has a lower forward voltage drop at high currents than the CVD sample with the AlN buffer, so the I– V measurement analysis is mainly performed on the MBE sample. In Fig. 1 results from the MBE sample at different temperatures is presented, and one result from the CVD sample is also included as comparison. The CVD sample has : 50 times lower current density at 5 V compared to the MBE sample, which is probably due to the extra barrier for carriers that is introduced with the AlN buffer, see inset of Fig. 3. The influence from the cracks in the AlGaN epi-layer is uncertain. The MBE sample has a linear region around −1.7 V where the current scales with area. This is the region where the curves overlap in Fig. 2, since the y-axis is current density. The smaller area diodes has a longer linear region, which probably is due to smaller leakage through the poly-GaN since the grain boundary area is smaller and the probability of defects is lower. The temperature measurement shows the expected overall increase in current at elevated temperatures, see Fig. 1. The ideality factors for different temperatures are summarized in Table 1, which have mainly been extracted from the 50 mm diodes, since the linear regions were often too short for the other diameters. The same problem occurs at 300°C, since the leakage current at low voltages shortens the linear region.
4. C–V measurements C–V measurements on heterojunctions can be used to extract band alignment and doping of the lower doped side (in this case the p-type SiC region). To
DEC = qVD − EG (SiC)+dSiC + dGaN dSiC = kT ln
NSiC NGaN , dGaN = kT ln NV (SiC) NC (GaN)
(2)
The C–V measurements were made using a HP4280 capacitance meter. The measurements were performed at elevated temperatures, but only on 200 mm diodes since the resolution of the capacitance meter was limited. At RT the C–V measurements had resolution problems and these values are unreliable for analysis. The aim was to estimate the band alignment, but on the MBE sample with the poly-GaN buffer this is a very uncertain measurement since the band alignment measurement is sensitive to interface states and fixed charge at the heterojunction. The heterojunction on the CVD sample should have less interface states and fixed charge than the MBE-sample, since this sample has an AlN buffer. However, the cracks in the epi-layer may introduce some interface states. The AlN buffer can also interfere with the band alignment measurement, because the band alignment is calculated from the built in potential, which is dependent on the heterojunction dipole charge. However, the AlN buffer layer influences two heterojunction charges, one between the SiC and AlN and the other between AlN and AlGaN. The dipole charge at these heterojunctions should cancel out, so that the extracted potential is the difference in conduction band between AlGaN and 6H-SiC. With Table 1 Ideality factors from the I–V measurements at different temperatures Sample
Diameter (mm)
Temperature (°C)
Ideality factor, h
MBE MBE MBE MBE MBE MBE MBE
50 50 100 50 100 50 100
25 100 100 150 150 200 200
1.28 1.24 1.22 1.23 1.20 1.20 1.19
E. Danielsson et al. / Materials Science and Engineering B61–62 (1999) 320–324
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Table 2 C–V measurements result for the CVD-samplea Temperature (°C)
NA (6H-SiC) (cm−3)
VD (V)
dVD (50%) (V)
DEC (eV)
Diode id.
25 25 100 100
1.9×1016 1.8×1016 2.2×1016 1.9×1016
2.82 2.26 2.24 2.33
0.3 0.3 0.09 0.1
0.11 −0.46 −0.37 −0.28
1 2 1 2
a
The variable dVD is the deviation from the fitted line where 50% of the measured data is located.
Table 3 C–V measurements result for the MBE-samples, the diameter is 200 mma Temperature (°C)
NA (6H-SiC) (cm−3)
VD (V)
dVD (50%) (V)
DEC (eV)
Diode id.
25 25 100 100 150 150 200 200 300 300
8.9×1015 8.3×1015 9.6×1015 9.8×1015 1.0×1016 1.1×1016 1.1×1016 1.0×1016 1.0×1016 1.0×1016
5.57 4.94 4.40 3.90 4.17 4.92 4.44 5.32 3.66 4.20
1.5 0.7 0.4 0.3 0.4 0.5 0.4 0.5 0.3 0.3
2.88 2.24 1.81 1.31 1.65 2.40 1.99 2.88 1.37 1.92
Unknown Unknown 1 2 1 2 1 2 1 2
a
The variable dVD is the deviation from the fitted line where 50% of the measured data is located.
this in mind the results for the CVD sample is summarized in Table 2, and the results for the MBE-sample are summarized in Table 3. The extracted values for DEC for the MBE samples are clearly unrealistic. Representative C–V measurements on the MBE-sample and CVD-sample at 100°C are shown in Fig. 3 and Fig. 4 respectively. The results from the MBE-sample clearly show that there is a large spread in the built in potential, VD, and this most likely due to the poly-GaN interface. The CVD-sample shows a much smaller spread in VD, and this is probably due to a much better interface than the MBE-sample regarding interface states and fixed charge. It is also interesting to note that the calculated DEC is positive for the MBE-samples and negative for the CVD-samples. In the calculation a positive sign of DEC means that the SiC conduction band has lower electron energy than the GaN or AlGaN conduction band, see the insets in Fig. 3 and Fig. 4 where DEC is shown with positive sign. No temperature dependence can be seen from these measurements regarding VD.
cially the poly-GaN buffer layer. The low ‘turn-on’ voltage of the measured diodes [14] has not been successfully implemented in the simulations. The simulations have been used to check the qualitative effect on I–V measurements of different heights of the AlN barrier in the conduction band, i.e. altering the electron affinity for the AlN, see Fig. 5. Simulations of C–V measurements on the AlGaN/SiC heterodiodes resulted in a DEC of − 0.34 eV, which in good agreement with the corresponding measurement of the CVD sample. The electron affinities used in the simulations were 4.12
5. Simulations Simulations were performed with the MEDICI program, with models developed for GaN and AlGaN. The SiC model is based on [16], and the GaN and AlGaN models are presented in [17]. The buffer layers of the heterodiodes are very difficult to simulate, espe-
Fig. 4. Results from C – V measurements of heterodiodes from the MBE-sample. The distortion of the curve is probably from the poly-GaN buffer layer. The band lineup in the inset is exaggerated.
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It was impossible to determine any band alignment information from C–V measurements of the MBEsample due to the poly-GaN buffer, since the measured built in potential showed a large spread between different diodes. The CVD-sample was more successful and an estimate of the conduction band offset from the 100°C data is −0.32 eV. The cracks in the CVD epi-layer could introduce some uncertainty to this measurement, but the spread in measured built in potential is small so the influence is believed to be low.
References
Fig. 5. Results from I–V simulations for a 1D heterodiode with an AlN buffer layer and different AlN affinities.
V for 6H-SiC and 4.32 V for GaN (assumes the same for AlGaN) [17,18]. All the simulations have been simulated at elevated temperature (600 K), since the simulation of wide bandgap materials and heterojunctions introduces numerical problems with low leakage currents and discontinuities in the energy bands respectively. The simulations show that a high barrier in the conduction band from the AlN buffer results in a high forward voltage drop for the heterodiode, just as seen when comparing measurements on diodes from the CVD-sample (with the AlN buffer) and the MBE-sample (with poly-GaN buffer).
6. Conclusions Heterodiodes have successfully been manufactured on 6H-SiC. MBE grown GaN with a poly-GaN buffer layer showed better forward characteristics than the CVD grown AlGaN with an AlN buffer. This is probably due to a high barrier in the conduction band from the AlN buffer. All the diodes showed a low ‘turn on’ voltage, which is believed to be caused by a trap level in 6H-SiC. The ideality factor was between 1.20 and 1.30 in the linear region of the I– V characteristics.
.
[1] A.T. Ping, Q. Chen, J.W. Yang, M.A. Khan, I. Adesida, Proceedings of the IEDM, Washington DC, 1997, pp. 561–564. [2] J.I. Pankove, M. Leksono, S.S. Chang, C. Walker, B.V. Zeghbroeck, MIJ-NSR, 1 http://nsr.mihj.org/1/39/complete.html (Article 39), 1996. [3] R. Gaska, J. Yang, A. Osinsky, M.A. Khan, M.S. Shur, Proceedings of the IEDM, Washington DC, 1997, pp. 565–568. [4] S.C. Binari, J.M. Redwing, G. Kelner, W. Kruppa, Electron. Lett. 33 (1997) 242 – 243. [5] S.C. Strite, Group III Nitrides, EMIS Data Reviews No. 11, INSPEC, London, 1994. [6] W. Kim, A.E. Botchkarev, A. Salvador, G. Popovici, H. Tang, H. Morkoc¸, J. Appl. Phys. 82 (1997) 219 – 226. [7] W. Seifert, R. Franzheld, E. Buttler, H. Sobotta, V. Riede, Crystal Res. Technol. 18 (1983) 383. [8] S. Yoshida, S. Misawa, S. Gonda, J. Appl. Phys. 53 (1982) 6844 – 6848. [9] C.F. Lin, G.C. Chi, M.S. Feng, J.D. Guo, J.S. Tsang, J.M. Hong, Appl. Phys. Lett. 68 (1996) 3758 – 3760. [10] W. Shan, A.J. Fischer, J.J. Song, G.E. Bulman, H.S. Kong, M.T. Leonard, W.G. Perry, M.D. Bremser, R.F. Davis, Appl. Phys. Lett. 69 (1996) 740 – 742. [11] S. Strite, H. Morkoc¸, J. Vac. Sci. Tech. B 10 (1992) 1237–1266. [12] S.J. Pearton, F. Ren, J.C. Zolper, R.J. Shul, Mat. Res. Soc. Symp. Proc. 482 (1997) 961 – 972. [13] Y.-F. Wu, W.-N. Jiang, B.P. Keller, S. Keller, D. Kapolnek, S.P. Denbaars, U.K. Mishra, B. Wilson, Solid State Electron. 41 (1997) 165 – 168. [14] J.T. Torvik, C.-H. Qiu, M. Leksono, J.I. Pankove, Appl. Phys. Lett. 72 (1998) 945 – 947. [15] S.R. Forrest, Heterojunction Band Discontinuities, 1st edn, Elsevier, New York, 1987. [16] M. Bakowski, U. Gustavsson, U. Lindefelt, Phys. Stat. Sol. B 162 (1997) 421 – 440. [17] E. Danielsson, B. Breitholtz, C.-M. Zetterling, M. O8 stling, Phys. Scripta (1998) in press. [18] R.T. Webster, A.F.M. Anwar, Mat. Res. Soc. Symp. Proc. 482 (1997) 929 – 934.
Simulation and electrical characterization of GaN/SiC and AlGaN/SiC heterodiodes E. Danielsson a,*, C.-M. Zetterling a, M. O8 stling a, B. Breitholtz a, K. Linthicum b, D.B. Thomson b, O.-H. Nam b, R.F. Davis b b
a KTH, Department of Electronics, P.O. Box Electrum 229, S-164 40, Kista, Sweden Department of Materials Science & Engineering, North Carolina State Uni6ersity, NC, USA
Abstract Heterojunctions on SiC is an area in rapid development, especially GaN/SiC and AlGaN/SiC heterojunctions. The heterojunction can improve the performance considerably for BJTs and FETs. In this work heterojunction diodes have been manufactured and characterized. The structure was a GaN or AlGaN n-type region on top of a 6H-SiC p-type substrate. Two different approaches of growing the n-type region were tested. The GaN was grown with the MBE technique using a polycrystalline GaN buffer, whereas the AlGaN was grown with CVD and an AlN buffer. The AlGaN had an aluminum mole fraction of around 0.1. Mesa structures were formed using Cl2 RIE of GaN/AlGaN, which showed good selectivity on 6H-SiC (about 1:6). A Ti metallization with subsequent RTA was used as contact to GaN and AlGaN, and the contact to 6H-SiC was liquid InGa. Both I –V and C–V measurements were performed on the heterojunction diode. The ideality factor of the diodes, doping concentration of the SiC, and the band alignment of the heterojunction were extracted. © 1999 Elsevier Science S.A. All rights reserved.
1. Introduction SiC has several properties that makes it a promising candidate for future power devices. One interesting possibility with the SiC crystal structure is that it can be used as a substrate for GaN or AlGaN epitaxial growth. This makes it possible to create heterojunctions in SiC technology, which is a very powerful tool when designing devices. Several reports have covered this subject recently [1 – 4]. The devices manufactured are usually heterojunction bipolar transistors (HBTs) and heterostructure field effect transistors (HFETs). The HBTs are often made with a SiC base and collector, with the wider bandgap nitride as the emitter. In the HFETs SiC is only used as an inactive substrate to grow the structure on, and various compositions of Alx Ga1 − x N is used to create the heterostructures. GaN is unintentionally n-type doped during growth, which is believed to originate from the N-vacancy [5] or be due to silicon or oxygen impurities [6,7]. Since it is
* Corresponding author.
difficult to reduce the background doping it is preferable to use GaN or AlGaN as the n-type region in SiC heterojunction design. The carrier concentration of unintentionally doped AlGaN decreases with aluminum content [8], but silicon can be used to intentionally dope the AlGaN to the desired level. It is difficult to grow GaN and AlGaN on SiC due to the 2–3% lattice mismatch, so it is common to use a buffer layer to relax the interface and nucleate the growth. The lattice match improves with aluminum content, and for AlN the lattice mismatch is only 1%. In this work two approaches have been tested for the buffer layer. The first, tested with GaN, was a polycrystalline GaN layer, and the second approach, tested with AlGaN, was an AlN buffer layer [9,10].
2. Experimental The starting material for the heterodiodes was p-type SiC substrates with a 4 mm thick p-type epitaxial layer with doping around 1016 cm − 3. The first step was to grow the GaN and AlGaN epitaxially on the substrates
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 5 2 6 - 1
E. Danielsson et al. / Materials Science and Engineering B61–62 (1999) 320–324
using molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) respectively. The GaN layer was undoped, with an unintentional doping causing a carrier concentration in the 1018 cm − 3 region. The epi-layer was approximately 0.5 mm thick. Between the GaN layer and the SiC there was 20 nm thick polycrystalline GaN buffer grown at low temperature to relax the interface. The AlGaN was Si doped slightly above 1018 cm − 3, and the aluminum mole fraction was around 0.1. The epi-layer was approximately 0.5 mm thick, with a 12 nm AlN buffer layer between the AlGaN and the SiC. The AlN only has a 1% lattice mismatch on SiC, but the wide bandgap of AlN (: 6.2 eV) [11] will produce a rather high barrier for the carriers. The AlGaN sample was relaxed and had a fine pattern of cracks in the epi-layer. Mesa structures were formed using a reactive ion etch (RIE) with Cl2 as active gas. The etch rate was around 150 nm min − 1 for GaN and AlGaN. On SiC the etch rate was only around 25 nm min − 1, which corresponds to a selectivity of 1:6 for this etch. The chamber pressure was 100 mTorr and the RF-power was set to 200 W, and photoresist was used as etch mask. Sputtering of titanium was used to deposit a TiN/Ti structure as metallization on the GaN and AlGaN. Between the first and second titanium layer the samples were annealed by rapid thermal annealing (RTA) at 950°C for 30 s in a N2 ambient to form the TiN. The first layer was made thin (around 50 nm) to limit the reaction with GaN. Recent reports of titanium as contact to GaN has shown low contact resistivity after RTA [12,13], and TLM measurements on both the GaN and the AlGaN samples showed ohmic behavior with low contact resistance (rC :5·10 − 6 Vcm2). The low contact resistance is probably due to the formation of TiN at the Ti/GaN interface which creates N-vacancies, and the highly doped region of the GaN near the interface will produce a very thin tunneling barrier for carriers [12,13]. To check how deep the reaction is under the metal contact the metal was etched off with a mixture of HNO3 and HF (1:1) and the structure was raster scanned with a stylus profilometer. This measurement did not show any large consumption of the GaN under the contact. The backside contact to the p-type 6H-SiC was liquid InGa, and since the doping of the SiC substrate was rather low ( :1018 cm − 3) the contact characteristics and the series resistance of the substrate results in an early current saturation of the heterodiodes. Both C–V and I – V measurements were performed on the heterodiodes at elevated temperatures, ranging from room temperature up to 300°C.
321
Fig. 1. Results from I– V measurements for heterodiodes on the MBE-sample plus one diode on the CVD sample at different temperatures. Note that the measured voltage is the cathode voltage, so the forward voltage is negative.
3. Results
3.1. I–V measurements The I–V measurements were performed with a HP4156 parameter analyzer on a probe station, and the temperature was controlled using a hot chuck. The measurements were made on circular diodes with diameters of 50, 100 and 200 mm, and at temperatures of 25 (RT), 100, 150, 200 and 300°C. Representative results from the measurements are shown in Fig. 1 and Fig. 2. From the measurements the ideality factor in the linear region is extracted and compared for the different temperatures. The ‘turn-on’ voltage for all of the heterodiodes were lower than expected for wide bandgap junctions, only :1.2 V when it should be around 2.5 V. This could be due to a defect level in the p-type SiC 1.25 eV above the valence band. The defect level acts as a recombination center when electrons
Fig. 2. Results from I – V measurements for different diameters of heterodiodes on the MBE-sample.
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determine the band alignment Eq. (1) is used, where VD is determined from the C–V measurement as the intercept with the x-axis.
2 1 2 = + (VD − V) 2 qoGaNNGaN qoSiCNSiC CD
(1)
When VD is known the band offset at the conductance band is calculated using Eq. (2). The doping is extracted from the slope, but it can only be determined if the doping on one side is known or much larger than the other side [15]. In the calculations the doping of the GaN layer was assumed to be 5×1018 cm − 3, but since the dependence is logarithmic this value is not so crucial. Fig. 3. Results from C–V measurements of heterodiodes from the CVD-sample. This shows better linear dependence than the MBEsample. The band lineup in the inset is exaggerated.
from the GaN conductance band to tunnel to this level at forward bias [14]. The MBE sample with the polyGaN buffer has a lower forward voltage drop at high currents than the CVD sample with the AlN buffer, so the I– V measurement analysis is mainly performed on the MBE sample. In Fig. 1 results from the MBE sample at different temperatures is presented, and one result from the CVD sample is also included as comparison. The CVD sample has : 50 times lower current density at 5 V compared to the MBE sample, which is probably due to the extra barrier for carriers that is introduced with the AlN buffer, see inset of Fig. 3. The influence from the cracks in the AlGaN epi-layer is uncertain. The MBE sample has a linear region around −1.7 V where the current scales with area. This is the region where the curves overlap in Fig. 2, since the y-axis is current density. The smaller area diodes has a longer linear region, which probably is due to smaller leakage through the poly-GaN since the grain boundary area is smaller and the probability of defects is lower. The temperature measurement shows the expected overall increase in current at elevated temperatures, see Fig. 1. The ideality factors for different temperatures are summarized in Table 1, which have mainly been extracted from the 50 mm diodes, since the linear regions were often too short for the other diameters. The same problem occurs at 300°C, since the leakage current at low voltages shortens the linear region.
4. C–V measurements C–V measurements on heterojunctions can be used to extract band alignment and doping of the lower doped side (in this case the p-type SiC region). To
DEC = qVD − EG (SiC)+dSiC + dGaN dSiC = kT ln
NSiC NGaN , dGaN = kT ln NV (SiC) NC (GaN)
(2)
The C–V measurements were made using a HP4280 capacitance meter. The measurements were performed at elevated temperatures, but only on 200 mm diodes since the resolution of the capacitance meter was limited. At RT the C–V measurements had resolution problems and these values are unreliable for analysis. The aim was to estimate the band alignment, but on the MBE sample with the poly-GaN buffer this is a very uncertain measurement since the band alignment measurement is sensitive to interface states and fixed charge at the heterojunction. The heterojunction on the CVD sample should have less interface states and fixed charge than the MBE-sample, since this sample has an AlN buffer. However, the cracks in the epi-layer may introduce some interface states. The AlN buffer can also interfere with the band alignment measurement, because the band alignment is calculated from the built in potential, which is dependent on the heterojunction dipole charge. However, the AlN buffer layer influences two heterojunction charges, one between the SiC and AlN and the other between AlN and AlGaN. The dipole charge at these heterojunctions should cancel out, so that the extracted potential is the difference in conduction band between AlGaN and 6H-SiC. With Table 1 Ideality factors from the I–V measurements at different temperatures Sample
Diameter (mm)
Temperature (°C)
Ideality factor, h
MBE MBE MBE MBE MBE MBE MBE
50 50 100 50 100 50 100
25 100 100 150 150 200 200
1.28 1.24 1.22 1.23 1.20 1.20 1.19
E. Danielsson et al. / Materials Science and Engineering B61–62 (1999) 320–324
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Table 2 C–V measurements result for the CVD-samplea Temperature (°C)
NA (6H-SiC) (cm−3)
VD (V)
dVD (50%) (V)
DEC (eV)
Diode id.
25 25 100 100
1.9×1016 1.8×1016 2.2×1016 1.9×1016
2.82 2.26 2.24 2.33
0.3 0.3 0.09 0.1
0.11 −0.46 −0.37 −0.28
1 2 1 2
a
The variable dVD is the deviation from the fitted line where 50% of the measured data is located.
Table 3 C–V measurements result for the MBE-samples, the diameter is 200 mma Temperature (°C)
NA (6H-SiC) (cm−3)
VD (V)
dVD (50%) (V)
DEC (eV)
Diode id.
25 25 100 100 150 150 200 200 300 300
8.9×1015 8.3×1015 9.6×1015 9.8×1015 1.0×1016 1.1×1016 1.1×1016 1.0×1016 1.0×1016 1.0×1016
5.57 4.94 4.40 3.90 4.17 4.92 4.44 5.32 3.66 4.20
1.5 0.7 0.4 0.3 0.4 0.5 0.4 0.5 0.3 0.3
2.88 2.24 1.81 1.31 1.65 2.40 1.99 2.88 1.37 1.92
Unknown Unknown 1 2 1 2 1 2 1 2
a
The variable dVD is the deviation from the fitted line where 50% of the measured data is located.
this in mind the results for the CVD sample is summarized in Table 2, and the results for the MBE-sample are summarized in Table 3. The extracted values for DEC for the MBE samples are clearly unrealistic. Representative C–V measurements on the MBE-sample and CVD-sample at 100°C are shown in Fig. 3 and Fig. 4 respectively. The results from the MBE-sample clearly show that there is a large spread in the built in potential, VD, and this most likely due to the poly-GaN interface. The CVD-sample shows a much smaller spread in VD, and this is probably due to a much better interface than the MBE-sample regarding interface states and fixed charge. It is also interesting to note that the calculated DEC is positive for the MBE-samples and negative for the CVD-samples. In the calculation a positive sign of DEC means that the SiC conduction band has lower electron energy than the GaN or AlGaN conduction band, see the insets in Fig. 3 and Fig. 4 where DEC is shown with positive sign. No temperature dependence can be seen from these measurements regarding VD.
cially the poly-GaN buffer layer. The low ‘turn-on’ voltage of the measured diodes [14] has not been successfully implemented in the simulations. The simulations have been used to check the qualitative effect on I–V measurements of different heights of the AlN barrier in the conduction band, i.e. altering the electron affinity for the AlN, see Fig. 5. Simulations of C–V measurements on the AlGaN/SiC heterodiodes resulted in a DEC of − 0.34 eV, which in good agreement with the corresponding measurement of the CVD sample. The electron affinities used in the simulations were 4.12
5. Simulations Simulations were performed with the MEDICI program, with models developed for GaN and AlGaN. The SiC model is based on [16], and the GaN and AlGaN models are presented in [17]. The buffer layers of the heterodiodes are very difficult to simulate, espe-
Fig. 4. Results from C – V measurements of heterodiodes from the MBE-sample. The distortion of the curve is probably from the poly-GaN buffer layer. The band lineup in the inset is exaggerated.
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It was impossible to determine any band alignment information from C–V measurements of the MBEsample due to the poly-GaN buffer, since the measured built in potential showed a large spread between different diodes. The CVD-sample was more successful and an estimate of the conduction band offset from the 100°C data is −0.32 eV. The cracks in the CVD epi-layer could introduce some uncertainty to this measurement, but the spread in measured built in potential is small so the influence is believed to be low.
References
Fig. 5. Results from I–V simulations for a 1D heterodiode with an AlN buffer layer and different AlN affinities.
V for 6H-SiC and 4.32 V for GaN (assumes the same for AlGaN) [17,18]. All the simulations have been simulated at elevated temperature (600 K), since the simulation of wide bandgap materials and heterojunctions introduces numerical problems with low leakage currents and discontinuities in the energy bands respectively. The simulations show that a high barrier in the conduction band from the AlN buffer results in a high forward voltage drop for the heterodiode, just as seen when comparing measurements on diodes from the CVD-sample (with the AlN buffer) and the MBE-sample (with poly-GaN buffer).
6. Conclusions Heterodiodes have successfully been manufactured on 6H-SiC. MBE grown GaN with a poly-GaN buffer layer showed better forward characteristics than the CVD grown AlGaN with an AlN buffer. This is probably due to a high barrier in the conduction band from the AlN buffer. All the diodes showed a low ‘turn on’ voltage, which is believed to be caused by a trap level in 6H-SiC. The ideality factor was between 1.20 and 1.30 in the linear region of the I– V characteristics.
.
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