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Improved free-standing GaN Schottky diode characteristics using chemical mechanical polishing
Applied Surface Science 255 (2008) 3085–3089
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Improved free-standing GaN Schottky diode characteristics using chemical mechanical polishing Arul Chakkaravarthi Arjunan a, Deepika Singh a, H.T. Wang b, F. Ren b, Purushottam Kumar c, R.K. Singh c, S.J. Pearton c,* a b c
Sinmat Inc., Gainesville, FL 32641, United States Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, United States Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 June 2008 Received in revised form 12 August 2008 Accepted 28 August 2008 Available online 6 September 2008
Chemical mechanical polishing of free-standing GaN substrates is found to reduce the reverse leakage current of Ni/Au Schottky diodes fabricated on the Ga-face. The barrier height extracted from current– voltage measurements is found to increase from 0.4 eV on un-treated substrates to 0.75 eV on polished wafers, along with a reduction in diode ideality factor from 2 to 1.5. The electrical measurements are consistent with removal of a defective near-surface region that promotes generation-recombination current. More acidic polishing solutions were found to produce the best Schottky diode characteristics and there was an optimum loading force during CMP of the GaN surface. ß 2008 Elsevier B.V. All rights reserved.
Keywords: GaN Al2O3
1. Introduction GaN Schottky diodes can be used for a variety of applications, including hydrogen gas sensing and power control systems. Schottky rectifiers are a key element of inverter modules because of their high switching speeds and low switching losses, which are important for improving the efficiency of inductive motor controllers and power supplies. In particular, GaN power diodes needed for inverter modules exhibit on-state resistances several orders of magnitude lower than comparable Si devices along with much larger electric field breakdown strengths [1–16]. One of the remaining critical requirements for commercialization is the need for improved Schottky contacts on n-type GaN and AlGaN. The strength of interfacial reactions between the metal and semiconductor plays a key role in determining the quality of the resultant Schottky barriers, while the crystal quality and purity of the near-surface region around the rectifying contact also play a role in determining the performance. The most common Schottky metallizations for AlGaN/GaN HEMTs are based on Pt/Au or Ni/Au. There have been a number of past investigations on the role of the surface or of treatments to the surface to produce optimized Schottky barriers on GaN [17–38]. These have focused on wet chemical cleaning of the surface prior to deposition of the rectifying contacts, the role of dislocations and other extended defects that
* Corresponding author. Tel.: +1 3528461086; fax: +1 3528461182. E-mail address: [email protected]fl.edu (S.J. Pearton). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.08.096
arise from the crystal growth process and the thermal stability of Schottky diodes formed on GaN. To date, there have been no reports of the effect of chemical mechanical planarization of GaN surfaces on the electrical properties of diodes formed on these surfaces. GaN belongs to a class of high hardness materials that are also chemically inert. Presently the state of art techniques for polishing such surfaces employs either hard particles (such as diamond/alumina) or chemically incompatible slurries. Such polishing techniques have either resulted in mechanical damage (e.g. scratches, defects) or non-uniform defective surfaces. The present techniques are unsuitable for developing high quality GaN substrates. In this paper we use Schottky diodes as sensitive indicators of the efficiency of CMP processing of free-standing GaN templates. 2. Experimental The n-GaN structures were obtained from Lumilog, Inc. (France) and consisted of 250 mm free standing (0 0 0 1) oriented GaN templates grown on c-plane Al2O3 substrates by HVPE and removed by differential laser heating that separate the film from the sapphire substrate. The electron concentration obtained from Hall measurements was 5 1017 cm3. A full-area backside Ohmic contact was fabricated with Ti(200 A˚)/Al(800 A˚)/Pt(400 A˚)/ Au(800 A˚) annealed at 800 8C for 30 s under a flowing N2 ambient. The free-standing substrates were polished using a process based on the use of soft colloidal silica nanoparticles combined with specific surfactant additives in unique chemical environment to gently polish the surface of GaN/SiC films. Such a polishing
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process results in atomically smooth surface finish with no subsurface damage or scratches. The slurries were formulated so that a thin passivating layer was formed which can be removed by relatively soft silica particles or hybrid particle systems. The formulation consists of addition of chemicals (such as pH changer, surfactants) and particles. The chemicals include (i) pH control chemicals, (ii) oxidizers/mild etchants and (iii) surfactants. The pH was varied from basic to acidic conditions and the pad force varied from 12 to 62 N s. In this work we define basic conditions as those where the pH is below 7 (typically 6 in our case), acidic as when the pH is 8 and more acidic when the pH is 9. It is known that (OH) ions can react with the GaN surface and passivate surface states [6] and therefore the pH may affect the surface electrical properties as well as influence the CMP removal rate. The pH was measured with an Accumet Ap61 meter. In each case, a fixed depth of 0.5 mm of the GaN surface was removed by the CMP process using a proprietary Sinmat pad. The typical removal rates were in the range 0.15– 0.5 mm/h depending on pH and pad pressure. The polishing duration was 1–3 h. We used Sinmat’s nano/nanosponge particle technology that has allowed development of a flexible, defect-free process to polish GaN. The nanosponge particles are significantly smaller and softer than slurry particles currently being used in the Si industry for polishing Cu and create fewer defects in the wafers [39]. The formulations were prepared by mixing these particles under high shear with appropriate chemical additives. The surface roughnesses before and after CMP were quantified by contact mode atomic force microscopy (AFM) measurements. A front-side Schottky metallization scheme of Ni(200 A˚)/ Au(1000 A˚) was used in all experiments. The Ni/Au contacts ranged in diameter from 200 to 800 mm and were patterned by e-beam evaporation through a shadow mask. The current–voltage (I–V) characteristics of the resulting diodes were measured on an Agilent 4145B parameter analyzer. The barrier height for the n-type samples, fb, and diode ideality factor, n, were extracted from the relation for the thermionic emission over a barrier [14] ef eV J F ¼ A T 2 exp b exp nkT kT
Fig. 1. AFM image and line scan of as-received GaN surface.
the positively charged slurry particles and the negatively charged silicate formed on the surface of the substrate being polished lowers the activation energy and aids in the removal of the silicate reaction product [40].
where JF is the current density, A* is the Richardson’s constant for n-GaN, T the absolute temperature, e the electronic charge, k Boltzmann’s constant and V the applied voltage. 3. Results and discussion Fig. 1 shows an AFM image and line scan from the as-received substrate prior to any polishing. The root-mean-square (RMS) roughness is 4.1 nm over a 5 mm 5 mm area. The optimized CMP process was able to significantly smooth the surface, as shown in Fig. 2, where the RMS roughness is now <1 A˚. The periodic structures correspond to atomic ledges formed due to miscut from exact orientation. Fig. 3 shows, under optimized CMP condition (37 N, acidic), the polished surfaces exhibits low roughness over small area (25 mm2) as well as over large area (10,000 mm2). The high pressure, basic slurry polishing and high acidic slurry polishing increases roughness over large area. However, polished samples show complete elimination of scratch marks and atomic terracing as shown in Fig. 1. Previous work in polishing SiC, another very hard semiconductor material, suggests that a large fraction of the reactive chemistry absorbs to the particle surface and that the maximum removal rate is obtained when the surface of the particles is saturated with the reactive molecules [40]. In analogy, in our case, the particles act to deliver the chemistry to the GaN surface via transport on the nanoparticle surface and combined with the kinetic energy imparted by the particles momentum, the GaN surface is removed. In the case of SiC, the attraction between
Fig. 2. AFM image and line scan of GaN surface obtained by CMP. The surface roughness is less than 1 A˚.
A.C. Arjunan et al. / Applied Surface Science 255 (2008) 3085–3089
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Fig. 3. Roughness as a function of polishing conditions obtained from AFM scans over 25, 625, 10,000 mm2 area.
Fig. 4 shows an optical micrograph of the fabricated Schottky diodes on the as-received substrates. We measured I–V characteristics from all diameters of the diodes in order to see the uniformity of the current density across areas a few mm2 to ensure that any changes induced by the CMP process were larger than the diode-to-diode variations. Forward and reverse I–V characteristics from diodes on the asreceived substrates are shown in Fig. 5 (top). Within experimental error, all diode sizes showed the same current density. Similarly, after any particular CMP treatment, we observed that the magnitude of the currents had changed, but the current density within a set of diodes was consistent. An example is shown at the bottom of Fig. 5 for diodes polished under acidic conditions at a pad force of 15 N s. In the forward voltage direction, the current at
Fig. 4. Optical micrograph image of Ni/Au Schottky contacts of different diameters on as-received free-standing GaN substrate.
Fig. 5. I–V characteristics from Ni/Au Schottky diodes on as-received GaN substrate (top) and after CMP under acidic conditions at a pad force of 15 N (bottom).
higher biases is reduced for the larger diodes due to the greater impact of series resistance. The CMP process produced a very significant improvement in diode characteristics, as shown in Fig. 6 (top) for substrates that were polished under acidic conditions as a function of different pad force. This is a result of an optimized removal rate of GaN relative to the introduction of mechanical damage from the nanoparticle abrasion of the GaN surface. There may also be a contribution from surface passivation by the hydroxide ions whose concentration is high under these conditions. Thus, there is a complicated interplay between the leakage current and surface roughness, since passivation of surface states may also play a role. Note the very large decrease in current for all CMP processed diodes relative to the unpolished devices. This is a result of the increase in barrier height on the polished diodes, shown at the bottom of Fig. 6. The unpolished diodes show low barrier heights of 0.40 eV, which is likely to result from the presence of a high density of defects in the near-surface region. By sharp contrast, the polished diodes show more normal barrier heights for Ni of up to 0.75 eV. This is the typical value obtained for Ni Schottky barriers on high quality epi films grown by Metal Organic Chemical Vapor Deposition. In addition, the barrier height is a function of the polishing conditions, with an optimum pad force of 37 N for this particular set of slurry conditions. This is not unexpected, since the pad force will be a factor in determining the relative amount of chemical versus physical removal of GaN during the CMP process. It is also concomitant with AFM observation (Fig. 3). At the optimum pad force of 37 N, the pH of the polishing slurry was also a factor in determining the effective Schottky
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Fig. 6. I–V characteristics from Ni/Au Schottky diodes after CMP under acidic conditions at different pad forces (top) and barrier height as a function of pad force during CMP (bottom).
barrier height. Fig. 7 (top) shows the I–V characteristics from diodes polished at a fixed pad force but different pH. Note that more acidic conditions lead to the maximum barrier height, although any CMP condition leads to significantly improved rectifying properties. The current transport in metal-GaN contacts is mainly due to majority carriers and the two major processes under forward bias are (1) transport of electrons from the semiconductor over the potential barrier into the metal; (2) quantum-mechanical tunneling of electrons through the barrier. In addition, we may have recombination current in the space-charge region and leakage current at the contact periphery. The transport of electrons over the potential barrier is the dominant process for Schottky diodes on moderately doped semiconductors as is the case here. The I–V characteristics for our diodes were adequately described by thermionic emission theory. The increase in barrier height was accompanied by a decrease in diode ideality factor, as shown in Fig. 8. The value of 2 observed in unpolished diodes indicates that generation-recombination is the dominant current transport mechanism in these diodes, whereas the reduction to 1.5 after optimized polishing suggests that the contribution from defects is reduced and true thermionic emission over the barrier is equally important. All of this data suggests that the performance of bulk GaN rectifiers can be significantly improved by CMP. In combination with careful surface cleaning, passivation with low interface state density dielectrics and use of edge termination methods, CMP appears to be a powerful tool for optimizing the performance of GaN diode rectifiers.
Fig. 7. I–V characteristics from Ni/Au Schottky diodes after CMP under acidic conditions at a fixed pad force of 37 N (top) and barrier height as a function of acidic condition during CMP (bottom).
Fig. 8. Diode ideality factor as a function of acidic condition during CMP at a fixed pad force of 37 N.
4. Summary and conclusions CMP of the surface of free-standing GaN substrates is found to produce significant improvements in the Schottky barrier properties of Ni/Au contacts evaporated on the polished surface. This indicates that there is a defective near-surface region in the asgrown samples that promotes generation-recombination current and that this region can be effectively removed by CMP under conditions that do not introduce polishing damage into the GaN.
A.C. Arjunan et al. / Applied Surface Science 255 (2008) 3085–3089
Acknowledgments The work at UF is supported by NSF (CTS-0301178), ONR Grant N000140710982 monitored by Igor Vodyanoy, and State of Florida, Center of Excellence for Nano-Bio Application. The authors would like to thank National Science Council of Taiwan for partial financial support (Contract No. NSC 96-2112-M-008-002). The work at Sinmat Inc. is supported by NSF SBIR: Grant # IIP 0646586. References [1] A.P. Zhang, G.T. Dang, F. Ren, H. Cho, K.P. Lee, S.J. Pearton, J.-I. Chi, T.E. Nee, C.M. Lee, C.C. Chuo, IEEE Trans. Electron Dev. 48 (2001) 407. [2] R.T. Kemerley, H.B. Wallace, M.N. Yoder, Proc. IEEE 90 (2002) 1059. [3] T. Ericsen, Proc. IEEE 90 (2002) 1077. [4] G.T. Heydt, B.J. Skromme, Mater. Res. Soc. Symp. Proc. 483 (1998) 3. [5] M. Trivedi, K. Shenai, J. Appl. Phys. 85 (1999) 6889. [6] S.J. Pearton, F. Ren, A.P. Zhang, K.P. Lee, Mater. Sci. Eng. R30 (2000) 55. [7] Z.Z. Bandic, D.M. Bridger, E.C. Piquette, T.C. McGill, R.P. Vaudo, V.M. Phanse, J.M. Redwing, Appl. Phys. Lett. 74 (1999) 1266. [8] A.P. Zhang, J.W. Johnson, F. Ren, J. Han, A.J. Polyakov, N.B. Smirnov, A.V. Govorkov, J.M. Redwing, K.P. Lee, S.J. Pearton, Appl. Phys. Lett. 78 (2001) 823. [9] A.P. Zhang, G. Dang, F. Ren, J. Han, A.Y. Polyakov, N.B. Smirnov, A.V. Govorkov, J.M. Redwing, X.A. Cao, S.J. Pearton, Appl. Phys. Lett. 76 (2000) 1767. [10] J.W. Johnson, J.R. LaRoche, F. Ren, B.P. Gila, M.E. Overberg, C.R. Abernathy, J.I. Chyi, C.C. Chuo, T.E. Nee, C.M. Lee, K.P. Lee, S.S. Park, J.I. Park, S.J. Pearton, Solid-State Electrochem. 45 (2001) 405. [11] J.W. Johnson, A.P. Zhang, W.B. Luo, F. Ren, S.J. Pearton, S.S. Park, Y.J. Park, J.-I. Chyi, IEEE Trans. Electron Dev. 49 (2002) 32. [12] J.W. Johnson, B. Luo, F. Ren, D. Palmer, S.J. Pearton, S.S. Park, Y.J. Park, Solid-State Electron. 46 (2002) 911. [13] K.H. Baik, Y. Irokawa, F. Ren, S.J. Pearton, S.S. Park, Y.J. Park, J. Vac. Sci. Technol. B 20 (2002) 2169. [14] A.P. Zhang, G.T. Dang, F. Ren, H. Cho, K.P. Lee, S.J. Pearton, J.-I. Chti, T.E. Nee, C.C. Chuo, Solid-State Electron. 44 (1999) 619. [15] T.G. Zhu, D.J. Lambert, B.S. Shelton, M.M. Wong, V. Chowdhurg, R.D. Dupis, Appl. Phys. Lett. 77 (2000) 2918.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Improved free-standing GaN Schottky diode characteristics using chemical mechanical polishing Arul Chakkaravarthi Arjunan a, Deepika Singh a, H.T. Wang b, F. Ren b, Purushottam Kumar c, R.K. Singh c, S.J. Pearton c,* a b c
Sinmat Inc., Gainesville, FL 32641, United States Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, United States Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 June 2008 Received in revised form 12 August 2008 Accepted 28 August 2008 Available online 6 September 2008
Chemical mechanical polishing of free-standing GaN substrates is found to reduce the reverse leakage current of Ni/Au Schottky diodes fabricated on the Ga-face. The barrier height extracted from current– voltage measurements is found to increase from 0.4 eV on un-treated substrates to 0.75 eV on polished wafers, along with a reduction in diode ideality factor from 2 to 1.5. The electrical measurements are consistent with removal of a defective near-surface region that promotes generation-recombination current. More acidic polishing solutions were found to produce the best Schottky diode characteristics and there was an optimum loading force during CMP of the GaN surface. ß 2008 Elsevier B.V. All rights reserved.
Keywords: GaN Al2O3
1. Introduction GaN Schottky diodes can be used for a variety of applications, including hydrogen gas sensing and power control systems. Schottky rectifiers are a key element of inverter modules because of their high switching speeds and low switching losses, which are important for improving the efficiency of inductive motor controllers and power supplies. In particular, GaN power diodes needed for inverter modules exhibit on-state resistances several orders of magnitude lower than comparable Si devices along with much larger electric field breakdown strengths [1–16]. One of the remaining critical requirements for commercialization is the need for improved Schottky contacts on n-type GaN and AlGaN. The strength of interfacial reactions between the metal and semiconductor plays a key role in determining the quality of the resultant Schottky barriers, while the crystal quality and purity of the near-surface region around the rectifying contact also play a role in determining the performance. The most common Schottky metallizations for AlGaN/GaN HEMTs are based on Pt/Au or Ni/Au. There have been a number of past investigations on the role of the surface or of treatments to the surface to produce optimized Schottky barriers on GaN [17–38]. These have focused on wet chemical cleaning of the surface prior to deposition of the rectifying contacts, the role of dislocations and other extended defects that
* Corresponding author. Tel.: +1 3528461086; fax: +1 3528461182. E-mail address: [email protected]fl.edu (S.J. Pearton). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.08.096
arise from the crystal growth process and the thermal stability of Schottky diodes formed on GaN. To date, there have been no reports of the effect of chemical mechanical planarization of GaN surfaces on the electrical properties of diodes formed on these surfaces. GaN belongs to a class of high hardness materials that are also chemically inert. Presently the state of art techniques for polishing such surfaces employs either hard particles (such as diamond/alumina) or chemically incompatible slurries. Such polishing techniques have either resulted in mechanical damage (e.g. scratches, defects) or non-uniform defective surfaces. The present techniques are unsuitable for developing high quality GaN substrates. In this paper we use Schottky diodes as sensitive indicators of the efficiency of CMP processing of free-standing GaN templates. 2. Experimental The n-GaN structures were obtained from Lumilog, Inc. (France) and consisted of 250 mm free standing (0 0 0 1) oriented GaN templates grown on c-plane Al2O3 substrates by HVPE and removed by differential laser heating that separate the film from the sapphire substrate. The electron concentration obtained from Hall measurements was 5 1017 cm3. A full-area backside Ohmic contact was fabricated with Ti(200 A˚)/Al(800 A˚)/Pt(400 A˚)/ Au(800 A˚) annealed at 800 8C for 30 s under a flowing N2 ambient. The free-standing substrates were polished using a process based on the use of soft colloidal silica nanoparticles combined with specific surfactant additives in unique chemical environment to gently polish the surface of GaN/SiC films. Such a polishing
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A.C. Arjunan et al. / Applied Surface Science 255 (2008) 3085–3089
process results in atomically smooth surface finish with no subsurface damage or scratches. The slurries were formulated so that a thin passivating layer was formed which can be removed by relatively soft silica particles or hybrid particle systems. The formulation consists of addition of chemicals (such as pH changer, surfactants) and particles. The chemicals include (i) pH control chemicals, (ii) oxidizers/mild etchants and (iii) surfactants. The pH was varied from basic to acidic conditions and the pad force varied from 12 to 62 N s. In this work we define basic conditions as those where the pH is below 7 (typically 6 in our case), acidic as when the pH is 8 and more acidic when the pH is 9. It is known that (OH) ions can react with the GaN surface and passivate surface states [6] and therefore the pH may affect the surface electrical properties as well as influence the CMP removal rate. The pH was measured with an Accumet Ap61 meter. In each case, a fixed depth of 0.5 mm of the GaN surface was removed by the CMP process using a proprietary Sinmat pad. The typical removal rates were in the range 0.15– 0.5 mm/h depending on pH and pad pressure. The polishing duration was 1–3 h. We used Sinmat’s nano/nanosponge particle technology that has allowed development of a flexible, defect-free process to polish GaN. The nanosponge particles are significantly smaller and softer than slurry particles currently being used in the Si industry for polishing Cu and create fewer defects in the wafers [39]. The formulations were prepared by mixing these particles under high shear with appropriate chemical additives. The surface roughnesses before and after CMP were quantified by contact mode atomic force microscopy (AFM) measurements. A front-side Schottky metallization scheme of Ni(200 A˚)/ Au(1000 A˚) was used in all experiments. The Ni/Au contacts ranged in diameter from 200 to 800 mm and were patterned by e-beam evaporation through a shadow mask. The current–voltage (I–V) characteristics of the resulting diodes were measured on an Agilent 4145B parameter analyzer. The barrier height for the n-type samples, fb, and diode ideality factor, n, were extracted from the relation for the thermionic emission over a barrier [14] ef eV J F ¼ A T 2 exp b exp nkT kT
Fig. 1. AFM image and line scan of as-received GaN surface.
the positively charged slurry particles and the negatively charged silicate formed on the surface of the substrate being polished lowers the activation energy and aids in the removal of the silicate reaction product [40].
where JF is the current density, A* is the Richardson’s constant for n-GaN, T the absolute temperature, e the electronic charge, k Boltzmann’s constant and V the applied voltage. 3. Results and discussion Fig. 1 shows an AFM image and line scan from the as-received substrate prior to any polishing. The root-mean-square (RMS) roughness is 4.1 nm over a 5 mm 5 mm area. The optimized CMP process was able to significantly smooth the surface, as shown in Fig. 2, where the RMS roughness is now <1 A˚. The periodic structures correspond to atomic ledges formed due to miscut from exact orientation. Fig. 3 shows, under optimized CMP condition (37 N, acidic), the polished surfaces exhibits low roughness over small area (25 mm2) as well as over large area (10,000 mm2). The high pressure, basic slurry polishing and high acidic slurry polishing increases roughness over large area. However, polished samples show complete elimination of scratch marks and atomic terracing as shown in Fig. 1. Previous work in polishing SiC, another very hard semiconductor material, suggests that a large fraction of the reactive chemistry absorbs to the particle surface and that the maximum removal rate is obtained when the surface of the particles is saturated with the reactive molecules [40]. In analogy, in our case, the particles act to deliver the chemistry to the GaN surface via transport on the nanoparticle surface and combined with the kinetic energy imparted by the particles momentum, the GaN surface is removed. In the case of SiC, the attraction between
Fig. 2. AFM image and line scan of GaN surface obtained by CMP. The surface roughness is less than 1 A˚.
A.C. Arjunan et al. / Applied Surface Science 255 (2008) 3085–3089
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Fig. 3. Roughness as a function of polishing conditions obtained from AFM scans over 25, 625, 10,000 mm2 area.
Fig. 4 shows an optical micrograph of the fabricated Schottky diodes on the as-received substrates. We measured I–V characteristics from all diameters of the diodes in order to see the uniformity of the current density across areas a few mm2 to ensure that any changes induced by the CMP process were larger than the diode-to-diode variations. Forward and reverse I–V characteristics from diodes on the asreceived substrates are shown in Fig. 5 (top). Within experimental error, all diode sizes showed the same current density. Similarly, after any particular CMP treatment, we observed that the magnitude of the currents had changed, but the current density within a set of diodes was consistent. An example is shown at the bottom of Fig. 5 for diodes polished under acidic conditions at a pad force of 15 N s. In the forward voltage direction, the current at
Fig. 4. Optical micrograph image of Ni/Au Schottky contacts of different diameters on as-received free-standing GaN substrate.
Fig. 5. I–V characteristics from Ni/Au Schottky diodes on as-received GaN substrate (top) and after CMP under acidic conditions at a pad force of 15 N (bottom).
higher biases is reduced for the larger diodes due to the greater impact of series resistance. The CMP process produced a very significant improvement in diode characteristics, as shown in Fig. 6 (top) for substrates that were polished under acidic conditions as a function of different pad force. This is a result of an optimized removal rate of GaN relative to the introduction of mechanical damage from the nanoparticle abrasion of the GaN surface. There may also be a contribution from surface passivation by the hydroxide ions whose concentration is high under these conditions. Thus, there is a complicated interplay between the leakage current and surface roughness, since passivation of surface states may also play a role. Note the very large decrease in current for all CMP processed diodes relative to the unpolished devices. This is a result of the increase in barrier height on the polished diodes, shown at the bottom of Fig. 6. The unpolished diodes show low barrier heights of 0.40 eV, which is likely to result from the presence of a high density of defects in the near-surface region. By sharp contrast, the polished diodes show more normal barrier heights for Ni of up to 0.75 eV. This is the typical value obtained for Ni Schottky barriers on high quality epi films grown by Metal Organic Chemical Vapor Deposition. In addition, the barrier height is a function of the polishing conditions, with an optimum pad force of 37 N for this particular set of slurry conditions. This is not unexpected, since the pad force will be a factor in determining the relative amount of chemical versus physical removal of GaN during the CMP process. It is also concomitant with AFM observation (Fig. 3). At the optimum pad force of 37 N, the pH of the polishing slurry was also a factor in determining the effective Schottky
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Fig. 6. I–V characteristics from Ni/Au Schottky diodes after CMP under acidic conditions at different pad forces (top) and barrier height as a function of pad force during CMP (bottom).
barrier height. Fig. 7 (top) shows the I–V characteristics from diodes polished at a fixed pad force but different pH. Note that more acidic conditions lead to the maximum barrier height, although any CMP condition leads to significantly improved rectifying properties. The current transport in metal-GaN contacts is mainly due to majority carriers and the two major processes under forward bias are (1) transport of electrons from the semiconductor over the potential barrier into the metal; (2) quantum-mechanical tunneling of electrons through the barrier. In addition, we may have recombination current in the space-charge region and leakage current at the contact periphery. The transport of electrons over the potential barrier is the dominant process for Schottky diodes on moderately doped semiconductors as is the case here. The I–V characteristics for our diodes were adequately described by thermionic emission theory. The increase in barrier height was accompanied by a decrease in diode ideality factor, as shown in Fig. 8. The value of 2 observed in unpolished diodes indicates that generation-recombination is the dominant current transport mechanism in these diodes, whereas the reduction to 1.5 after optimized polishing suggests that the contribution from defects is reduced and true thermionic emission over the barrier is equally important. All of this data suggests that the performance of bulk GaN rectifiers can be significantly improved by CMP. In combination with careful surface cleaning, passivation with low interface state density dielectrics and use of edge termination methods, CMP appears to be a powerful tool for optimizing the performance of GaN diode rectifiers.
Fig. 7. I–V characteristics from Ni/Au Schottky diodes after CMP under acidic conditions at a fixed pad force of 37 N (top) and barrier height as a function of acidic condition during CMP (bottom).
Fig. 8. Diode ideality factor as a function of acidic condition during CMP at a fixed pad force of 37 N.
4. Summary and conclusions CMP of the surface of free-standing GaN substrates is found to produce significant improvements in the Schottky barrier properties of Ni/Au contacts evaporated on the polished surface. This indicates that there is a defective near-surface region in the asgrown samples that promotes generation-recombination current and that this region can be effectively removed by CMP under conditions that do not introduce polishing damage into the GaN.
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