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Structural and electrical characterization of epitaxial 4H–SiC layers for power electronic device applications
Materials Science and Engineering B102 (2003) 298 /303 www.elsevier.com/locate/mseb
Structural and electrical characterization of epitaxial 4H SiC layers for power electronic device applications /
L. Scaltrito a, S. Porro a, M. Cocuzza a, F. Giorgis a, C.F. Pirri a, P. Mandracci a, C. Ricciardi a,*, S. Ferrero a, C. Sgorlon b, G. Richieri b, L. Merlin b, A. Castaldini c, A. Cavallini c, L. Polenta c a
INFM and Dipartimento di Fisica, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Turin, Italy b IRCI-International Rectifier Corporation Italia, Via Liguria 29, 10071 Borgaro T.se, Turin, Italy c INFM and Dipartimento di Fisica, Universita` di Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy
Abstract In spite of the high potentiality of silicon carbide (SiC), its technology shows at the moment some limitations, due to the defects present in the crystalline structure. We have focused our analysis on commercial 4H /SiC epitaxial layers. A preliminary investigation has been performed by Optical and Scanning Electron microscopies with the aim to evidence the defect morphology on a large scale. An insight on the defect structure has been obtained by Atomic Force Microscopy, profilometer technique, MicroRaman and Micro-Photoluminescence spectroscopies. Different types of defects such as comets, super dislocations, etch pits and so on, have been characterized finding interesting peculiarities such as different polytypes inclusions. Moreover, the influence of such defects on the SiC electrical performance has been deeply analyzed through the realization of Schottky barriers onto SiC regions including specific kinds of defects, then performing electrical characterization such as current /voltage (I /V ) analysis. Deep Level Transient Spectroscopy (DLTS) yielded the energy position in the SiC gap, the concentration and the capture cross section of two center of recombination. # 2003 Elsevier B.V. All rights reserved. Keywords: Silicon carbide; Micro-Raman spectroscopy; I /V analysis; DLTS analysis PACS numbers: 61.72-p; 78.30-j; 78.40.Fy
1. Introduction The unique combination of a wide band gap, high thermal conductivity, high breakdown field and high saturated electron drift velocity in SiC, determines it as one of the modern microelectronics materials suitable for high voltage, high power and high temperature applications, where Si and GaAs devices cannot be used, as well as for high power microwave devices in the range of 1 /10 GHz [1]. Moreover SiC is appropriate for devices operating in aggressive environments because of its stability to radiation and chemical attacks due to the high energy Si/C bond.
During the last years many efforts have been done by researchers in order to exploit the high potential of silicon carbide. The material quality of the massive SiC and epitaxial layers plays an important role in electrical characteristics. Till now, SiC material quality is not comparable to that of classical semiconductor such as silicon and requires further studies. Aim of this work is the morphological, structural and electrical characterization of 4H /SiC wafers and 4H / SiC epitaxial layers in order to correlate the electrical behavior of electronic power devices, such as Schottky diodes, to the presence of particular defects.
2. Experimental details * Corresponding author. Tel.: /39-011-564-7381; fax: /39-011564-7399. E-mail address: [email protected] (C. Ricciardi). 0921-5107/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(02)00726-2
The investigation was performed on commercial 4H / SiC wafers with 2 in. diameter supplied by Cree
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
Research Inc. The substrate was n-type (ND /2 /1019 cm 3), with 88 off-axis silicon faces. The micropipe density declared was 16 /30 cm 2. The epitaxial layer was also n-type (ND /4 /1015cm 3) with a thickness of 7 mm. First of all we have performed a morphological analysis of the wafers with the aim to evidence the defect morphology and density. This investigation has been performed by optical bright-field microscopy (Nikon microscope with magnification up to 2000 /) by which we mapped the defect distribution; each defect was further analyzed by Surface Profiler (Tencor P-10), Scanning Electron Microscopy (Cambridge Stereoscan 90) and Atomic Force Microscopy (DME DS 95-200). Among the structural characterizations Raman spectroscopy is a powerful tool because it is non destructive and requires no special preparation of samples, so it was used in this work to obtain information about 4H /SiC wafers composition. The parameters of the Raman signal such as intensity, width, peak frequency of the bands, provides fruitful information on the crystal quality. The system used in this work was a MicroRaman Renishaw system equipped by an Argon laser with excitation line at 514.5 nm and a cooled CCD camera as a detector, with spectral resolution 3 cm 1, spatial resolution 1 mm, field of depth 2 mm in confocal configuration. In order to obtain information about the influence of defects on the electrical performances, Schottky diodes were fabricated on 4H /SiC n-type wafers with an epitaxial layer and a metal-oxide overlap for electric field termination. Devices realized on a mapped wafer were characterized by I /V analysis, performed by a SMU237 Keithley Source Measure Unit (high voltage source up to 1100 V) and a SMU238 Keithley Source Measure Unit (high current source up to 1 A). A deeper electrical investigation was performed by Deep Level Transient Spectroscopy (DLTS) equipped by a cryogenic system and a hot cell for electrical monitoring from 10 up to 800 K.
3. Results and discussion The preliminary investigation by optical microscope has permitted to obtain a map of the defect present on the surface of the wafer. Fig. 1 reports a typical example of such a mapping, where different type of defects are shown. First of all micropipes are the well known defects in silicon carbide. Their diameter can be up to some tenth of micrometers and a Scanning Electron Microscope image reported in Fig. 2 shows one of these particular defects. Besides the micropipes, etch pits are microscopic tubular cavities with diameter between tenths of micrometer to some micrometers. These cavities do not cross
299
Fig. 1. 4H /SiC wafer */example of defects map obtained after wafer inspection by optical and Scanning Electron Microscopy.
Fig. 2. Morphological micropipe image on the surface of 4H /SiC wafer obtained by Scanning Electron Microscope.
over the entire epitaxial layer, but their thickness is of the order of hundred of nanometers. Fig. 3 reports optical and Scanning Electron Microscope image of comets; their origin can probably be due to the union of micropipes during the epitaxial layer growth. Fig. 4 reports a morphological reconstruction of a comet obtained by Atomic Force Microscopy, as it is shown, the typical dimension of the comets ranges within some micrometers [2,3]. The other defects detected show polygonal shape and were present only on 4H /SiC bulk wafers. Other kind of defects are dislocations, that seem to be ascribed to different plane of growth. It is worth to underline that the surface profiler shows a difference in height up to hundred of nanometers between wafer region separated by a dislocation. Over the highlighted defects, a structural characterization was performed by means of Micro-Raman and Micro-Photoluminesce spectroscopy. Micro probe techniques have the advantage to investigate a short portion of the sample under examination, thus allowing to avoid the contribute of the substrate in the case of evaluation of epitaxial layers quality. In particular Micro-Raman analysis gave information on the presence of different polytypes on the 4H/SiC
300
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
Fig. 3. Investigation of 4H /SiC epilayer. Comets images obtained by Scanning Electron Microscope (left) and Optical Microscope (right).
epitaxial layers [4,5]. Fig. 5 shows in the left the image of a dislocation on one of the samples examined; the Raman peaks shown in the graph evidence the presence of 6H /SiC inclusions in 4H /SiC epitaxial layers on the top of such a dislocation. In the same figure Raman spectrum of epilayers free defects zone is compared with the spectrum yielded by the dislocation. Different polytypes have been found also around the micropipes; for example we have found the presence of 3C /SiC inclusions reported in Fig. 6 and the presence of a strong photoluminescence signal (Fig. 7) centered around 2 eV, which can be due to the presence of extended defects distributed into the energy gap. After the identification on the main defects present of 4H /SiC wafers one of the mapped wafers was processed in order to obtain Schottky diodes and to have
information about defects and their influence in electrical performances of devices [6]. The defect position was carefully identified in a x /y coordinate system and Schottky diodes were realized with a metal-oxide overlap for electric field termination; the layout of the device is described in Fig. 8. The Schottky barrier was formed by thermal evaporation of Titanium. The silicon dioxide was grown by Plasma Enhanced Chemical Vapor Deposition (PECVD), the metal-oxide overlap was fabricated by optical lithography by obtaining an active area of the devices was 1 mm2. Finally the Ohmic contact was grown by thermal evaporation of titanium. Characteristic parameters such as Schottky barrier height (fB), ideality factor (n ) and breakdown voltage (the value of reverse voltage at 1 mA of reverse current)
Fig. 4. Morphological reconstruction obtained by Atomic Force Microscope of a comet.
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
301
Fig. 5. Dislocation detected on 4H /SiC epilayer. Structural investigation performed by Micro-Raman analysis shows the presence of 6H /SiC inclusions on the top of such a dislocation. The Raman signal is compared with that obtained on the epilayer defect free zone.
were determined by I /V analysis, taking into account that the relationship between current and voltage is given by
J JS exp
qV
nkT
1
qfB JS AT exp kT 2
(1)
where A * is the Richardson constant. For 4H /SiC a value of 146 A cm 2 K 2 is taken [7]. A typical example of current /voltage behavior has been reported in previous work [8]. The analysis was performed on 30 devices: ten without detected defects, ten with the presence of etch pits and ten with the presence of comets. The reported results in Table 1 are the average values. The main influence of defects is on the reverse voltage breakdown (VB). Diodes without defects have shown average VB
values of 460 V (some devices had VB /600 V and the real not reversible breakdown was found at 850 V). The etch pit presence does not modify the barrier height and the ideality factor, but it causes a decrease of 30% in breakdown voltage. It is interesting to note that diodes on comets have reverse voltage lower by one order of magnitude with respect to the defect free diodes. In order to obtain information on deep-level impurities, DLTS was performed on devices without defects in the temperature range 80/600 K. The unintentional incorporation of deep level impurities in the semiconductor materials creates several problems. Impurities could be unintentionally included during crystal growth and device processing. Today’s silicon is grown very pure with unintentional metallic impurity levels less than 1010 cm3. Processing tends to increase impurity densities but many of the impurities are gettered during
Fig. 6. Micro-Raman analysis performed on the edge of the micropipe represented in the optical image on the left. Raman spectra evidence 3C /SiC inclusions.
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
302
Fig. 7. Photoluminescence spectra obtained at room temperature with 514.5 nm excitation wavelength on the top of a dislocation.
conduction edge. The estimated cross sections were 1.1 /1015 cm 2 for S0 center and 5.7 /1012 cm 2 for S1 center and their densities were 8.8 /1012 and 1.14 /1013 cm 3, respectively. The graph reported in Fig. 9 shows the Arrhenius plot and the centers of recombination S0 and S1 detected.
4. Conclusions Fig. 8. 4H /SiC Schottky diode with field plate fabricated on investigated wafers.
An accurate investigation of 4H /SiC bulk and epitaxial layers has been performed. The optical analysis has led to identify different type of defects such as micropipes, etch pits, comets and so on, which were
Table 1 Measured height barrier, ideality factor and voltage reverse breakdown from J /V measurements Defect
fB (eV)
n
VB
None Etch pit Comet
1.11 1.08 1.09
1.16 1.15 1.23
460 347 41
The analysis has been performed on ten devices without defects, ten with the presence of etch pits and ten with the presence of comets.
subsequent processing and, with the help of high temperature processes, impurity levels are kept in the 1010 /1012 cm3 range. Obviously the presence of deeplevel impurities even at very small concentration could be detrimental to the performance of electronic devices [9]. DLTS analysis has shown the presence of two main centers of recombination in 4H /SiC Schottky structures (named S0 and S1) located at energy levels of 0.17 and 1.55 eV, respectively, with respect to the bottom of the
Fig. 9. DLTS Arrhenius plot of a 4H /SiC Schottky diode fabricated on investigated wafers. The analysis shows the presence of two main recombination centers (S0 and S1) located at energy levels of 0.17 and 1.55 eV, respectively, with respect to the bottom of the conduction edge.
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
observed in details by profilometry and AFM techniques; on such highlighted defects structural analysis performed by Micro-Raman spectroscopy has shown the evidence of inclusion of different polytypes such as 3C /SiC and 6H /SiC on 4H /SiC wafers. The electrical measurements performed on Schottky diodes fabricated on the investigated wafers show a strong influence on reverse voltage breakdown of etch pits and comets. DLTS analysis carried out on free defect diodes evidences the presence of two centers of recombination.
References [1] G.L. Harris (Ed.), Properties of Silicon Carbide, vol. 13, IEE Emis Datareviews, 1995 (and references therein).
303
[2] M. Sasaki, Y. Nishino, S. Nakashima, H. Harima, Materials Science Forum 264 /268 (1998) 41 /44. [3] W.L. Zhou, P. Pirouz, J.A. Powell, Materials Science Forum 264 / 268 (1998) 417 /420. [4] S. Nakashima, H. Harima, Physica Status Solidi (a) 162 (1997) 39 / 64. [5] J.C. Bourton, L. Sun, M. Pophristic, S.J. Luckacs, F.H. Long, Z.C. Feng, I.T. Ferguson, Journal of Applied Physics 84 (1998) 6268 / 6273. [6] Q. Wabah, A. Ellison, A. Henry, E. Janze´n, C. Hallin, J. Di Persio, R. Martinez, Applied Physics Letters 76 (2000) 2725 / 2727. [7] A. Itoh, Proceedings of International Symposium on Power and Semiconductor Devices (1995) p. 101. [8] L. Scaltrito, S. Ferrero, S. Porro, F. Giorgis, P. Mandracci, C.F. Pirri, C. Ricciardi, G. Richieri, C. Sgorlon, L. Merlin, A. Cavallini, A. Castaldini, Materials Science Forum (2002), in press. [9] G. Verzellesi, G. Meneghesso, A. Cavallini, E. Zanoni, EE Electron Device Letters 22 (12) (2001) 579 /581.
Structural and electrical characterization of epitaxial 4H SiC layers for power electronic device applications /
L. Scaltrito a, S. Porro a, M. Cocuzza a, F. Giorgis a, C.F. Pirri a, P. Mandracci a, C. Ricciardi a,*, S. Ferrero a, C. Sgorlon b, G. Richieri b, L. Merlin b, A. Castaldini c, A. Cavallini c, L. Polenta c a
INFM and Dipartimento di Fisica, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Turin, Italy b IRCI-International Rectifier Corporation Italia, Via Liguria 29, 10071 Borgaro T.se, Turin, Italy c INFM and Dipartimento di Fisica, Universita` di Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy
Abstract In spite of the high potentiality of silicon carbide (SiC), its technology shows at the moment some limitations, due to the defects present in the crystalline structure. We have focused our analysis on commercial 4H /SiC epitaxial layers. A preliminary investigation has been performed by Optical and Scanning Electron microscopies with the aim to evidence the defect morphology on a large scale. An insight on the defect structure has been obtained by Atomic Force Microscopy, profilometer technique, MicroRaman and Micro-Photoluminescence spectroscopies. Different types of defects such as comets, super dislocations, etch pits and so on, have been characterized finding interesting peculiarities such as different polytypes inclusions. Moreover, the influence of such defects on the SiC electrical performance has been deeply analyzed through the realization of Schottky barriers onto SiC regions including specific kinds of defects, then performing electrical characterization such as current /voltage (I /V ) analysis. Deep Level Transient Spectroscopy (DLTS) yielded the energy position in the SiC gap, the concentration and the capture cross section of two center of recombination. # 2003 Elsevier B.V. All rights reserved. Keywords: Silicon carbide; Micro-Raman spectroscopy; I /V analysis; DLTS analysis PACS numbers: 61.72-p; 78.30-j; 78.40.Fy
1. Introduction The unique combination of a wide band gap, high thermal conductivity, high breakdown field and high saturated electron drift velocity in SiC, determines it as one of the modern microelectronics materials suitable for high voltage, high power and high temperature applications, where Si and GaAs devices cannot be used, as well as for high power microwave devices in the range of 1 /10 GHz [1]. Moreover SiC is appropriate for devices operating in aggressive environments because of its stability to radiation and chemical attacks due to the high energy Si/C bond.
During the last years many efforts have been done by researchers in order to exploit the high potential of silicon carbide. The material quality of the massive SiC and epitaxial layers plays an important role in electrical characteristics. Till now, SiC material quality is not comparable to that of classical semiconductor such as silicon and requires further studies. Aim of this work is the morphological, structural and electrical characterization of 4H /SiC wafers and 4H / SiC epitaxial layers in order to correlate the electrical behavior of electronic power devices, such as Schottky diodes, to the presence of particular defects.
2. Experimental details * Corresponding author. Tel.: /39-011-564-7381; fax: /39-011564-7399. E-mail address: [email protected] (C. Ricciardi). 0921-5107/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(02)00726-2
The investigation was performed on commercial 4H / SiC wafers with 2 in. diameter supplied by Cree
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
Research Inc. The substrate was n-type (ND /2 /1019 cm 3), with 88 off-axis silicon faces. The micropipe density declared was 16 /30 cm 2. The epitaxial layer was also n-type (ND /4 /1015cm 3) with a thickness of 7 mm. First of all we have performed a morphological analysis of the wafers with the aim to evidence the defect morphology and density. This investigation has been performed by optical bright-field microscopy (Nikon microscope with magnification up to 2000 /) by which we mapped the defect distribution; each defect was further analyzed by Surface Profiler (Tencor P-10), Scanning Electron Microscopy (Cambridge Stereoscan 90) and Atomic Force Microscopy (DME DS 95-200). Among the structural characterizations Raman spectroscopy is a powerful tool because it is non destructive and requires no special preparation of samples, so it was used in this work to obtain information about 4H /SiC wafers composition. The parameters of the Raman signal such as intensity, width, peak frequency of the bands, provides fruitful information on the crystal quality. The system used in this work was a MicroRaman Renishaw system equipped by an Argon laser with excitation line at 514.5 nm and a cooled CCD camera as a detector, with spectral resolution 3 cm 1, spatial resolution 1 mm, field of depth 2 mm in confocal configuration. In order to obtain information about the influence of defects on the electrical performances, Schottky diodes were fabricated on 4H /SiC n-type wafers with an epitaxial layer and a metal-oxide overlap for electric field termination. Devices realized on a mapped wafer were characterized by I /V analysis, performed by a SMU237 Keithley Source Measure Unit (high voltage source up to 1100 V) and a SMU238 Keithley Source Measure Unit (high current source up to 1 A). A deeper electrical investigation was performed by Deep Level Transient Spectroscopy (DLTS) equipped by a cryogenic system and a hot cell for electrical monitoring from 10 up to 800 K.
3. Results and discussion The preliminary investigation by optical microscope has permitted to obtain a map of the defect present on the surface of the wafer. Fig. 1 reports a typical example of such a mapping, where different type of defects are shown. First of all micropipes are the well known defects in silicon carbide. Their diameter can be up to some tenth of micrometers and a Scanning Electron Microscope image reported in Fig. 2 shows one of these particular defects. Besides the micropipes, etch pits are microscopic tubular cavities with diameter between tenths of micrometer to some micrometers. These cavities do not cross
299
Fig. 1. 4H /SiC wafer */example of defects map obtained after wafer inspection by optical and Scanning Electron Microscopy.
Fig. 2. Morphological micropipe image on the surface of 4H /SiC wafer obtained by Scanning Electron Microscope.
over the entire epitaxial layer, but their thickness is of the order of hundred of nanometers. Fig. 3 reports optical and Scanning Electron Microscope image of comets; their origin can probably be due to the union of micropipes during the epitaxial layer growth. Fig. 4 reports a morphological reconstruction of a comet obtained by Atomic Force Microscopy, as it is shown, the typical dimension of the comets ranges within some micrometers [2,3]. The other defects detected show polygonal shape and were present only on 4H /SiC bulk wafers. Other kind of defects are dislocations, that seem to be ascribed to different plane of growth. It is worth to underline that the surface profiler shows a difference in height up to hundred of nanometers between wafer region separated by a dislocation. Over the highlighted defects, a structural characterization was performed by means of Micro-Raman and Micro-Photoluminesce spectroscopy. Micro probe techniques have the advantage to investigate a short portion of the sample under examination, thus allowing to avoid the contribute of the substrate in the case of evaluation of epitaxial layers quality. In particular Micro-Raman analysis gave information on the presence of different polytypes on the 4H/SiC
300
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
Fig. 3. Investigation of 4H /SiC epilayer. Comets images obtained by Scanning Electron Microscope (left) and Optical Microscope (right).
epitaxial layers [4,5]. Fig. 5 shows in the left the image of a dislocation on one of the samples examined; the Raman peaks shown in the graph evidence the presence of 6H /SiC inclusions in 4H /SiC epitaxial layers on the top of such a dislocation. In the same figure Raman spectrum of epilayers free defects zone is compared with the spectrum yielded by the dislocation. Different polytypes have been found also around the micropipes; for example we have found the presence of 3C /SiC inclusions reported in Fig. 6 and the presence of a strong photoluminescence signal (Fig. 7) centered around 2 eV, which can be due to the presence of extended defects distributed into the energy gap. After the identification on the main defects present of 4H /SiC wafers one of the mapped wafers was processed in order to obtain Schottky diodes and to have
information about defects and their influence in electrical performances of devices [6]. The defect position was carefully identified in a x /y coordinate system and Schottky diodes were realized with a metal-oxide overlap for electric field termination; the layout of the device is described in Fig. 8. The Schottky barrier was formed by thermal evaporation of Titanium. The silicon dioxide was grown by Plasma Enhanced Chemical Vapor Deposition (PECVD), the metal-oxide overlap was fabricated by optical lithography by obtaining an active area of the devices was 1 mm2. Finally the Ohmic contact was grown by thermal evaporation of titanium. Characteristic parameters such as Schottky barrier height (fB), ideality factor (n ) and breakdown voltage (the value of reverse voltage at 1 mA of reverse current)
Fig. 4. Morphological reconstruction obtained by Atomic Force Microscope of a comet.
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
301
Fig. 5. Dislocation detected on 4H /SiC epilayer. Structural investigation performed by Micro-Raman analysis shows the presence of 6H /SiC inclusions on the top of such a dislocation. The Raman signal is compared with that obtained on the epilayer defect free zone.
were determined by I /V analysis, taking into account that the relationship between current and voltage is given by
J JS exp
qV
nkT
1
qfB JS AT exp kT 2
(1)
where A * is the Richardson constant. For 4H /SiC a value of 146 A cm 2 K 2 is taken [7]. A typical example of current /voltage behavior has been reported in previous work [8]. The analysis was performed on 30 devices: ten without detected defects, ten with the presence of etch pits and ten with the presence of comets. The reported results in Table 1 are the average values. The main influence of defects is on the reverse voltage breakdown (VB). Diodes without defects have shown average VB
values of 460 V (some devices had VB /600 V and the real not reversible breakdown was found at 850 V). The etch pit presence does not modify the barrier height and the ideality factor, but it causes a decrease of 30% in breakdown voltage. It is interesting to note that diodes on comets have reverse voltage lower by one order of magnitude with respect to the defect free diodes. In order to obtain information on deep-level impurities, DLTS was performed on devices without defects in the temperature range 80/600 K. The unintentional incorporation of deep level impurities in the semiconductor materials creates several problems. Impurities could be unintentionally included during crystal growth and device processing. Today’s silicon is grown very pure with unintentional metallic impurity levels less than 1010 cm3. Processing tends to increase impurity densities but many of the impurities are gettered during
Fig. 6. Micro-Raman analysis performed on the edge of the micropipe represented in the optical image on the left. Raman spectra evidence 3C /SiC inclusions.
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
302
Fig. 7. Photoluminescence spectra obtained at room temperature with 514.5 nm excitation wavelength on the top of a dislocation.
conduction edge. The estimated cross sections were 1.1 /1015 cm 2 for S0 center and 5.7 /1012 cm 2 for S1 center and their densities were 8.8 /1012 and 1.14 /1013 cm 3, respectively. The graph reported in Fig. 9 shows the Arrhenius plot and the centers of recombination S0 and S1 detected.
4. Conclusions Fig. 8. 4H /SiC Schottky diode with field plate fabricated on investigated wafers.
An accurate investigation of 4H /SiC bulk and epitaxial layers has been performed. The optical analysis has led to identify different type of defects such as micropipes, etch pits, comets and so on, which were
Table 1 Measured height barrier, ideality factor and voltage reverse breakdown from J /V measurements Defect
fB (eV)
n
VB
None Etch pit Comet
1.11 1.08 1.09
1.16 1.15 1.23
460 347 41
The analysis has been performed on ten devices without defects, ten with the presence of etch pits and ten with the presence of comets.
subsequent processing and, with the help of high temperature processes, impurity levels are kept in the 1010 /1012 cm3 range. Obviously the presence of deeplevel impurities even at very small concentration could be detrimental to the performance of electronic devices [9]. DLTS analysis has shown the presence of two main centers of recombination in 4H /SiC Schottky structures (named S0 and S1) located at energy levels of 0.17 and 1.55 eV, respectively, with respect to the bottom of the
Fig. 9. DLTS Arrhenius plot of a 4H /SiC Schottky diode fabricated on investigated wafers. The analysis shows the presence of two main recombination centers (S0 and S1) located at energy levels of 0.17 and 1.55 eV, respectively, with respect to the bottom of the conduction edge.
L. Scaltrito et al. / Materials Science and Engineering B102 (2003) 298 /303
observed in details by profilometry and AFM techniques; on such highlighted defects structural analysis performed by Micro-Raman spectroscopy has shown the evidence of inclusion of different polytypes such as 3C /SiC and 6H /SiC on 4H /SiC wafers. The electrical measurements performed on Schottky diodes fabricated on the investigated wafers show a strong influence on reverse voltage breakdown of etch pits and comets. DLTS analysis carried out on free defect diodes evidences the presence of two centers of recombination.
References [1] G.L. Harris (Ed.), Properties of Silicon Carbide, vol. 13, IEE Emis Datareviews, 1995 (and references therein).
303
[2] M. Sasaki, Y. Nishino, S. Nakashima, H. Harima, Materials Science Forum 264 /268 (1998) 41 /44. [3] W.L. Zhou, P. Pirouz, J.A. Powell, Materials Science Forum 264 / 268 (1998) 417 /420. [4] S. Nakashima, H. Harima, Physica Status Solidi (a) 162 (1997) 39 / 64. [5] J.C. Bourton, L. Sun, M. Pophristic, S.J. Luckacs, F.H. Long, Z.C. Feng, I.T. Ferguson, Journal of Applied Physics 84 (1998) 6268 / 6273. [6] Q. Wabah, A. Ellison, A. Henry, E. Janze´n, C. Hallin, J. Di Persio, R. Martinez, Applied Physics Letters 76 (2000) 2725 / 2727. [7] A. Itoh, Proceedings of International Symposium on Power and Semiconductor Devices (1995) p. 101. [8] L. Scaltrito, S. Ferrero, S. Porro, F. Giorgis, P. Mandracci, C.F. Pirri, C. Ricciardi, G. Richieri, C. Sgorlon, L. Merlin, A. Cavallini, A. Castaldini, Materials Science Forum (2002), in press. [9] G. Verzellesi, G. Meneghesso, A. Cavallini, E. Zanoni, EE Electron Device Letters 22 (12) (2001) 579 /581.