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Transient current measurements after applying the electron-beam pulse on boron-doped homoepitaxial diamond films1
Diamond and Related Materials 7 (1998) 1167–1171
Transient current measurements after applying the electron-beam pulse on boron-doped homoepitaxial diamond films1 Hiroshi Sato a, Hajime Tomokage a,*, Hideo Kiyota b, Toshihiro Ando c a Department of Electronics Engineering, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-80, Japan b Department of Electronics and Information Technology, Kyushu Tokai University, 9-1-1 Toroku, Kumamoto 862, Japan c National Institute for Research in Inorganic Materials, Core Research for Evolutional Science and Technology of Japan, Science and Technology Corporation, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan Received 4 August 1997; accepted 2 March 1998
Abstract The electron-beam induced current (EBIC ) and the transient current after applying an electron beam pulse are measured on the boron-doped diamond films grown homoepitaxially by microwave plasma-assisted chemical vapor deposition. The thermal emission rate of holes at the boron acceptor level of 0.31 eV above the valence band edge are obtained from the transient current measurements. The EBIC image corresponding to the surface roughness on the diamond substrate is observed in the epitaxial film. This suggests that the electrically active defects are introduced in the film during the growth. © 1998 Elsevier Science S.A. Keywords: Diamond; Boron level; Electron beam; Transient current
1. Introduction The electrical characterization of homoepitaxial diamond is important to fabricate high-temperature, highpower, high-frequency electronic devices with diamond thin films. Yasu et al. [1] have measured the Hall effects in boron-doped homoepitaxial layer grown by a microwave plasma-assisted chemical vapor deposition method. They reported the Hall mobilities and hole densities for various CH and B H concentrations. 4 2 6 Kiyota et al. [2], on the other hand, have made C–V and I–V measurements on Schottky barriers. Their results show that the space charge region exists uniformly after the oxidation of the surface layer. Although the electron beam induced current ( EBIC ) technique is one of the powerful tools in investigating the electrically active defects, there are few reports on EBIC measurements on the homoepitaxial diamond films [3]. Mainly EBIC measurements on polycrystalline, natural and high pressure synthetic diamonds have been performed so far [4,5]. On the other hand, the deep levels in a diamond must be controlled in fabricating electronic devices. * Corresponding author. Tel: 0081 928716631; Fax: 0081 928656651; e-mail: [email protected] 1This paper was presented at the Diamond ’97 Conference, Edinburgh, Scotland, August 3–8, 1997. 0925-9635/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 09 2 5 -9 6 3 5 ( 9 8 ) 0 0 17 0 - 8
Glesener [6 ] has measured the boron acceptor level of 0.29 eV above the valence band edge by photoinduced transient current spectroscopy. However, there has been no report on the application of a transient method to the deep levels in the epitaxial diamond films. In this paper, we show the results of scanning deep level transient spectroscopy (SDLTS ) measurements and EBIC measurements on boron-doped homoepitaxial diamond films with Schottky contacts. The transient currents due to the hole emission at the boron acceptor level is calculated and compared with the experimental results. The SDLTS image corresponding to the boron distribution is obtained in the homoepitaxial film. The EBIC image corresponding to the mechanical roughness on the surface of diamond substrate is observed in the epitaxial film. This suggests that the electrically active defects are introduced in the film during the growth along the surface roughness.
2. Theory It is reported from the photoconductivity measurements that boron atoms introduce an acceptor level 0.37 eV above the valence band edge [7,8]. The C–V measurements indicate that most boron acceptors are
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H. Sato et al. / Diamond and Related Materials 7 (1998) 1167–1171
compensated at room temperature by the donors introduced during the growth. Therefore, the hole concentration p is much lower than the boron concentration. In order to calculate the transient current through the diamond Schottky barrier, we first solve the Poisson’s equation numerically. The ionized boron concentration N− in the equilibrium state is obtained by T p (1) N− = 1 N , T T p+p 1 where N is the total boron concentration and p is the T 1 hole concentration when the Fermi level is located at the boron level. Poisson’s equation to be solved is given by d2V dx2
=−
q e
( p+N −N ), D T
(2)
where q is an electronic charge, e the dielectric constant and N the compensating donor concentration. We D assume that the hole concentration p in the depletion region is given by p=p exp 0
A B −qV kT
,
(3)
where p is the hole concentration in the bulk region, k 0 the Boltzmann constant and T the absolute temperature. Introduction of the symbol n for exp(−qV/kT ) gives [9]
A B dn
dx
=
2q2n2 kTe
G
p (n−1)+N ln(n) 0 D
A
BH
p n+p n 1 . (4) −N ln 0 T p +p 0 1 Eq. (4) is solved numerically in order to obtain the hole concentration p before applying the electron beam e pulse. The hole concentration p during the electron c beam irradiation is assumed to be p +ap where a e 0 indicates the injection level due to an electron beam pulse. If p and p are assumed to be constant with time, e c the electron occupancy function f at the boron level after the electron-beam irradiation is given by
A
B
e e p p − exp[−(e +c p )t] p p e e +c p e +c p p p c p p e R e R p + = − e +c p R+p: R+p: p p e c e p: R , (5) ×exp − 1+ e t: + R R+p: e where p: and p: denote p and p normalized by p , e c c e 0 respectively, e the thermal emission rate of holes, c p p the hole capture coefficient and t: the time normalized by 1/e . In Eq. (5), we neglected the influence of p f=
A
CA
B
BD
Fig. 1. Calculated transient currents after applying an electron-beam pulse for various values of R.
electrons. Since e depends on the boron level E and p p T depends on the Fermi level E , we introduce here the F factor R as R=e /c p . We can calculate f using R p p 0 without knowing the position of E and E . If the free F T carrier concentration in the depletion region is assumed to be low, the transient current Di is given by
P A BA C A BD
Di=qAL N D T
w: R
0
p: 1+ e R
p: ×exp − 1+ e t: dx: , R
R
R+p: c
−
R R+p: e
BA
1−
x: w: R
B
(6)
where A is the cross-section of the electron beam, L D the Debye length given by L =(ekT/q2p )1/2, x: the D 0 distance x normalized by L and w: the normalized D R depletion width. Fig. 1 shows the transient current after applying the electron beam pulse calculated for various values of R with a=0.1, built-in potential 1.0 V, N =1018 cm−3 and N =1016 cm−3. It is shown in Fig. 1 T D that the nonexponential decay increases with decreasing R. The tail of hole concentration near the edge of depletion region causes the nonexponential behavior [10]. Fig. 2 is the transient current calculated by varying a from 10−1 to 10−4 with R=10−3. It is found from Fig. 2 that the nonexponential behavior increases with the injection level a.
3. Experimental The homoepitaxial growth was performed by the microwave plasma-assisted chemical vapor deposition method at 810 °C for 6 h on high-pressure synthetic diamond substrates polished along the (100) plane. The
H. Sato et al. / Diamond and Related Materials 7 (1998) 1167–1171
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Fig. 2. Calculated transient currents after applying an electron-beam pulse. The injection level a is varied from 10−1 to 10−4. Fig. 4. Arrhenius plot of the thermal emission rate. The dotted line shows the result of boron-doped natural diamond obtained by Glesener [6 ].
178 K under zero bias condition. The electron beam was chopped at a period of 100 ms with the width of 2 ms under the acceleration voltage 20 kV. We obtained the emission rate e from the slope of the linear part of p log(Di)−t in Fig. 1. Fig. 4 is the Arrhenius plot of the emission rate e measured between 178 and 233 K. p Assuming that the capture cross-section is independent of temperature, the emission rate is given by the following equation:
Fig. 3. Semilogarithmic plots of transient current versus time after applying an electron-beam purse of 2 ms width under zero bias condition at 178 K.
reactant gas was CH diluted with H and the boron 4 2 doping was carried out by mixing B H into the reactant 2 6 gas under the pressure of 41 Torr. The ratio of the boron concentration to the carbon one (B/C ) in the reactant gas was 100 ppm. The thickness of the homoepitaxial layer was about 2 mm. After the growth, the low resistance layer near the surface of the film was removed by dipping the sample in a solution of H SO and 2 4 HNO (3:1) at 200 °C. In order to form the Schottky 3 contact, Au was deposited on the diamond film using vacuum evaporation through a mask with a hole of 0.2 mm in diameter. Fig. 3 shows the transient currents Di observed at
e =1.62×106T2 exp(−0.31/kT ) (s−1). (7) p In Fig. 4, the dotted line indicates the result of borondoped natural diamond reported by Glesener [6 ]. It is shown that our result agrees well with that of natural diamond. Fig. 5 shows the scanning electron microscope (SEM ) and electron-beam induced current ( EBIC ) images near the edge of Schottky contact at 290 K under the acceleration voltage 20 kV. The stripe patterns under Au contact was observed in EBIC image, although from SEM image the surface was flat. Since the contrast of EBIC images corresponds to the magnitude of the induced currents by electron-beam, the dark part of EBIC image indicates the place where many electrically active recombination centers exist or the depletion width is small due to the increase in net inpurity concentration. The transient current was measured at the rate window of 2 and 8 ms by scanning the electron beam pulse on the sample. Fig. 6 shows SDLTS images near the edge of Schottky contact at 160 K under the acceleration voltage 20 kV. The stripe patterns as in EBIC images were not observed.
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Fig. 6. SDLTS image near the edge of Schottky contact at 160 K under the acceleration voltage 20 kV.
Fig. 5. SEM (a) and EBIC (b) images near the edge of Schottky contact at 290 K under the acceleration voltage 20 kV.
4. Discussion From C−V measurements the ionized boron concentration is 7×1014 cm−3 in the present sample, while the total boron concentration is estimated as 1018 cm−3. Therefore, the ionization rate is about 10−3, which means that R#10−3. From Hall measurement, Kiyota et al. [11] reported that N #1016 cm−3 for the sample D of 100 ppm B/C ratio. In the numerical analysis, the transient current does not show any prominent change with N if N %N . The nonexponential behavior of D D T transient current agreed well with the theoretical one for R#10−3. Therefore, it is suggested that the free-
carrier tail in the depletion edge region have an influence on the hole emission process. The hole emission rate agreed well with that of a natural diamond. It is expected, therefore, that the epitaxial films have the same behavior of capture and emission as the natural diamond crystal. Kiyota et al. reported that the hopping conduction occurs in the sample with high boron concentration of more than 200 ppm B/C ratio [11]. Since the B/C ratio of the present sample is 100 ppm, the hopping conduction should not be dominant. It is reported that the ionization energy of the boron acceptor is 0.37 eV, but the present result from the thermal emission rate is 0.31 eV. Glesener [12] proposed a cascade mechanism for the carrier capture in natural diamonds in order to explain the activation energy of 0.29 eV obtained from photoinduced transient currents. Our results also support the cascade model for the hole capture. The projected range of electron beam in the diamond film is 2.1 mm at 20 kV by considering the thickness of Au, while the thickness of homoepitaxial layer is 2.0 mm. By changing the acceleration voltage of electron-beam from 3 to 25 kV, we observed almost the same EBIC images. Therefore, the contrast of the EBIC image in Fig. 5b gives the information of homoepitaxial layer. From optical microscopy, we observed the surface roughness on the diamond substrate formed by the mechanical polishing, which exactly correspond to the EBIC image. The electrically active defects corresponding to the surface roughness on the substrate would be induced during the homoepitaxial growth. Since the SDLTS image did not agree with the EBIC one, the electrically active defect observed by EBIC measurements is not the boron acceptor level. The SDLTS image
H. Sato et al. / Diamond and Related Materials 7 (1998) 1167–1171
showed that the boron atoms distribute uniformly in the epitaxial films.
5. Conclusion We performed the transient current measurements after applying the electron-beam pulse and EBIC measurements on homoepitaxial growth films. The thermal emission rate of holes at a boron level of 0.31 eV above the valence band edge was obtained from the transient current after an electron beam pulse. The nonexponential behavior of the initial decay was explained by the tail of free carrier in the depletion region. The EBIC image corresponding to the mechanical roughness on the surface of diamond substrate was obtained in the epitaxial film. This suggests that the electrically active defects were induced in film along the roughness on the surface of diamond substrate during the growth.
1171
References [1] E. Yasu, N. Ohashi, T. Ando, J. Tanaka, M. Kazo, Y. Sato, H. Kiyota, Diamond Relat. Mater. 4 (1994) 59. [2] H. Kiyota, E. Matsushima, K. Sato, H. Okushi, M. Hda, Appl. Phys. Lett. 67 (1995) 3596. [3] T. Ariki, S. Shikaina, S. Suzuki, Y. Otsuka, T. Maki, T. Kobayashi, Jap. J. Appl. Phys. 33 (1994) L888. [4] R. Heiderhoff, P. Koschinski, M. Maywald, L.J. Balk, P.K. Bachmann, Diamond Relat. Mater. 4 (1995) 645. [5] K. Okano, H. Kiyota, T. Iwasaki, T. Kurosu, M. Hda, T. Nakayama, Appl. Phys. Lett. 58 (1991) 840. [6 ] J.W. Glesener, Appl. Phys. Lett. 63 (1993) 767. [7] M.I. Eremets, Semicond. Sci. Technol. 6 (1991) 439. [8] A.T. Collins, E.C. Lightowlers, Phys. Rev. 171 (1968) 843. [9] E. Schibli, A.G. Milnes, Solid-State Electron. 11 (1968) 323. [10] H. Tomokage, T. Ueta, H. Furuta, T. Miyamoto, J. Appl. Phys. 77 (1995) 2041. [11] H. Kiyota, H. Okushi, T. Ando, Y. Sato, Adv. Mater. (1997) 167 [12] J.W. Glesener, Appl. Phys. Lett. 64 (1994) 217.
Transient current measurements after applying the electron-beam pulse on boron-doped homoepitaxial diamond films1 Hiroshi Sato a, Hajime Tomokage a,*, Hideo Kiyota b, Toshihiro Ando c a Department of Electronics Engineering, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-80, Japan b Department of Electronics and Information Technology, Kyushu Tokai University, 9-1-1 Toroku, Kumamoto 862, Japan c National Institute for Research in Inorganic Materials, Core Research for Evolutional Science and Technology of Japan, Science and Technology Corporation, 1-1 Namiki, Tsukuba, Ibaraki 305, Japan Received 4 August 1997; accepted 2 March 1998
Abstract The electron-beam induced current (EBIC ) and the transient current after applying an electron beam pulse are measured on the boron-doped diamond films grown homoepitaxially by microwave plasma-assisted chemical vapor deposition. The thermal emission rate of holes at the boron acceptor level of 0.31 eV above the valence band edge are obtained from the transient current measurements. The EBIC image corresponding to the surface roughness on the diamond substrate is observed in the epitaxial film. This suggests that the electrically active defects are introduced in the film during the growth. © 1998 Elsevier Science S.A. Keywords: Diamond; Boron level; Electron beam; Transient current
1. Introduction The electrical characterization of homoepitaxial diamond is important to fabricate high-temperature, highpower, high-frequency electronic devices with diamond thin films. Yasu et al. [1] have measured the Hall effects in boron-doped homoepitaxial layer grown by a microwave plasma-assisted chemical vapor deposition method. They reported the Hall mobilities and hole densities for various CH and B H concentrations. 4 2 6 Kiyota et al. [2], on the other hand, have made C–V and I–V measurements on Schottky barriers. Their results show that the space charge region exists uniformly after the oxidation of the surface layer. Although the electron beam induced current ( EBIC ) technique is one of the powerful tools in investigating the electrically active defects, there are few reports on EBIC measurements on the homoepitaxial diamond films [3]. Mainly EBIC measurements on polycrystalline, natural and high pressure synthetic diamonds have been performed so far [4,5]. On the other hand, the deep levels in a diamond must be controlled in fabricating electronic devices. * Corresponding author. Tel: 0081 928716631; Fax: 0081 928656651; e-mail: [email protected] 1This paper was presented at the Diamond ’97 Conference, Edinburgh, Scotland, August 3–8, 1997. 0925-9635/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 09 2 5 -9 6 3 5 ( 9 8 ) 0 0 17 0 - 8
Glesener [6 ] has measured the boron acceptor level of 0.29 eV above the valence band edge by photoinduced transient current spectroscopy. However, there has been no report on the application of a transient method to the deep levels in the epitaxial diamond films. In this paper, we show the results of scanning deep level transient spectroscopy (SDLTS ) measurements and EBIC measurements on boron-doped homoepitaxial diamond films with Schottky contacts. The transient currents due to the hole emission at the boron acceptor level is calculated and compared with the experimental results. The SDLTS image corresponding to the boron distribution is obtained in the homoepitaxial film. The EBIC image corresponding to the mechanical roughness on the surface of diamond substrate is observed in the epitaxial film. This suggests that the electrically active defects are introduced in the film during the growth along the surface roughness.
2. Theory It is reported from the photoconductivity measurements that boron atoms introduce an acceptor level 0.37 eV above the valence band edge [7,8]. The C–V measurements indicate that most boron acceptors are
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H. Sato et al. / Diamond and Related Materials 7 (1998) 1167–1171
compensated at room temperature by the donors introduced during the growth. Therefore, the hole concentration p is much lower than the boron concentration. In order to calculate the transient current through the diamond Schottky barrier, we first solve the Poisson’s equation numerically. The ionized boron concentration N− in the equilibrium state is obtained by T p (1) N− = 1 N , T T p+p 1 where N is the total boron concentration and p is the T 1 hole concentration when the Fermi level is located at the boron level. Poisson’s equation to be solved is given by d2V dx2
=−
q e
( p+N −N ), D T
(2)
where q is an electronic charge, e the dielectric constant and N the compensating donor concentration. We D assume that the hole concentration p in the depletion region is given by p=p exp 0
A B −qV kT
,
(3)
where p is the hole concentration in the bulk region, k 0 the Boltzmann constant and T the absolute temperature. Introduction of the symbol n for exp(−qV/kT ) gives [9]
A B dn
dx
=
2q2n2 kTe
G
p (n−1)+N ln(n) 0 D
A
BH
p n+p n 1 . (4) −N ln 0 T p +p 0 1 Eq. (4) is solved numerically in order to obtain the hole concentration p before applying the electron beam e pulse. The hole concentration p during the electron c beam irradiation is assumed to be p +ap where a e 0 indicates the injection level due to an electron beam pulse. If p and p are assumed to be constant with time, e c the electron occupancy function f at the boron level after the electron-beam irradiation is given by
A
B
e e p p − exp[−(e +c p )t] p p e e +c p e +c p p p c p p e R e R p + = − e +c p R+p: R+p: p p e c e p: R , (5) ×exp − 1+ e t: + R R+p: e where p: and p: denote p and p normalized by p , e c c e 0 respectively, e the thermal emission rate of holes, c p p the hole capture coefficient and t: the time normalized by 1/e . In Eq. (5), we neglected the influence of p f=
A
CA
B
BD
Fig. 1. Calculated transient currents after applying an electron-beam pulse for various values of R.
electrons. Since e depends on the boron level E and p p T depends on the Fermi level E , we introduce here the F factor R as R=e /c p . We can calculate f using R p p 0 without knowing the position of E and E . If the free F T carrier concentration in the depletion region is assumed to be low, the transient current Di is given by
P A BA C A BD
Di=qAL N D T
w: R
0
p: 1+ e R
p: ×exp − 1+ e t: dx: , R
R
R+p: c
−
R R+p: e
BA
1−
x: w: R
B
(6)
where A is the cross-section of the electron beam, L D the Debye length given by L =(ekT/q2p )1/2, x: the D 0 distance x normalized by L and w: the normalized D R depletion width. Fig. 1 shows the transient current after applying the electron beam pulse calculated for various values of R with a=0.1, built-in potential 1.0 V, N =1018 cm−3 and N =1016 cm−3. It is shown in Fig. 1 T D that the nonexponential decay increases with decreasing R. The tail of hole concentration near the edge of depletion region causes the nonexponential behavior [10]. Fig. 2 is the transient current calculated by varying a from 10−1 to 10−4 with R=10−3. It is found from Fig. 2 that the nonexponential behavior increases with the injection level a.
3. Experimental The homoepitaxial growth was performed by the microwave plasma-assisted chemical vapor deposition method at 810 °C for 6 h on high-pressure synthetic diamond substrates polished along the (100) plane. The
H. Sato et al. / Diamond and Related Materials 7 (1998) 1167–1171
1169
Fig. 2. Calculated transient currents after applying an electron-beam pulse. The injection level a is varied from 10−1 to 10−4. Fig. 4. Arrhenius plot of the thermal emission rate. The dotted line shows the result of boron-doped natural diamond obtained by Glesener [6 ].
178 K under zero bias condition. The electron beam was chopped at a period of 100 ms with the width of 2 ms under the acceleration voltage 20 kV. We obtained the emission rate e from the slope of the linear part of p log(Di)−t in Fig. 1. Fig. 4 is the Arrhenius plot of the emission rate e measured between 178 and 233 K. p Assuming that the capture cross-section is independent of temperature, the emission rate is given by the following equation:
Fig. 3. Semilogarithmic plots of transient current versus time after applying an electron-beam purse of 2 ms width under zero bias condition at 178 K.
reactant gas was CH diluted with H and the boron 4 2 doping was carried out by mixing B H into the reactant 2 6 gas under the pressure of 41 Torr. The ratio of the boron concentration to the carbon one (B/C ) in the reactant gas was 100 ppm. The thickness of the homoepitaxial layer was about 2 mm. After the growth, the low resistance layer near the surface of the film was removed by dipping the sample in a solution of H SO and 2 4 HNO (3:1) at 200 °C. In order to form the Schottky 3 contact, Au was deposited on the diamond film using vacuum evaporation through a mask with a hole of 0.2 mm in diameter. Fig. 3 shows the transient currents Di observed at
e =1.62×106T2 exp(−0.31/kT ) (s−1). (7) p In Fig. 4, the dotted line indicates the result of borondoped natural diamond reported by Glesener [6 ]. It is shown that our result agrees well with that of natural diamond. Fig. 5 shows the scanning electron microscope (SEM ) and electron-beam induced current ( EBIC ) images near the edge of Schottky contact at 290 K under the acceleration voltage 20 kV. The stripe patterns under Au contact was observed in EBIC image, although from SEM image the surface was flat. Since the contrast of EBIC images corresponds to the magnitude of the induced currents by electron-beam, the dark part of EBIC image indicates the place where many electrically active recombination centers exist or the depletion width is small due to the increase in net inpurity concentration. The transient current was measured at the rate window of 2 and 8 ms by scanning the electron beam pulse on the sample. Fig. 6 shows SDLTS images near the edge of Schottky contact at 160 K under the acceleration voltage 20 kV. The stripe patterns as in EBIC images were not observed.
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H. Sato et al. / Diamond and Related Materials 7 (1998) 1167–1171
Fig. 6. SDLTS image near the edge of Schottky contact at 160 K under the acceleration voltage 20 kV.
Fig. 5. SEM (a) and EBIC (b) images near the edge of Schottky contact at 290 K under the acceleration voltage 20 kV.
4. Discussion From C−V measurements the ionized boron concentration is 7×1014 cm−3 in the present sample, while the total boron concentration is estimated as 1018 cm−3. Therefore, the ionization rate is about 10−3, which means that R#10−3. From Hall measurement, Kiyota et al. [11] reported that N #1016 cm−3 for the sample D of 100 ppm B/C ratio. In the numerical analysis, the transient current does not show any prominent change with N if N %N . The nonexponential behavior of D D T transient current agreed well with the theoretical one for R#10−3. Therefore, it is suggested that the free-
carrier tail in the depletion edge region have an influence on the hole emission process. The hole emission rate agreed well with that of a natural diamond. It is expected, therefore, that the epitaxial films have the same behavior of capture and emission as the natural diamond crystal. Kiyota et al. reported that the hopping conduction occurs in the sample with high boron concentration of more than 200 ppm B/C ratio [11]. Since the B/C ratio of the present sample is 100 ppm, the hopping conduction should not be dominant. It is reported that the ionization energy of the boron acceptor is 0.37 eV, but the present result from the thermal emission rate is 0.31 eV. Glesener [12] proposed a cascade mechanism for the carrier capture in natural diamonds in order to explain the activation energy of 0.29 eV obtained from photoinduced transient currents. Our results also support the cascade model for the hole capture. The projected range of electron beam in the diamond film is 2.1 mm at 20 kV by considering the thickness of Au, while the thickness of homoepitaxial layer is 2.0 mm. By changing the acceleration voltage of electron-beam from 3 to 25 kV, we observed almost the same EBIC images. Therefore, the contrast of the EBIC image in Fig. 5b gives the information of homoepitaxial layer. From optical microscopy, we observed the surface roughness on the diamond substrate formed by the mechanical polishing, which exactly correspond to the EBIC image. The electrically active defects corresponding to the surface roughness on the substrate would be induced during the homoepitaxial growth. Since the SDLTS image did not agree with the EBIC one, the electrically active defect observed by EBIC measurements is not the boron acceptor level. The SDLTS image
H. Sato et al. / Diamond and Related Materials 7 (1998) 1167–1171
showed that the boron atoms distribute uniformly in the epitaxial films.
5. Conclusion We performed the transient current measurements after applying the electron-beam pulse and EBIC measurements on homoepitaxial growth films. The thermal emission rate of holes at a boron level of 0.31 eV above the valence band edge was obtained from the transient current after an electron beam pulse. The nonexponential behavior of the initial decay was explained by the tail of free carrier in the depletion region. The EBIC image corresponding to the mechanical roughness on the surface of diamond substrate was obtained in the epitaxial film. This suggests that the electrically active defects were induced in film along the roughness on the surface of diamond substrate during the growth.
1171
References [1] E. Yasu, N. Ohashi, T. Ando, J. Tanaka, M. Kazo, Y. Sato, H. Kiyota, Diamond Relat. Mater. 4 (1994) 59. [2] H. Kiyota, E. Matsushima, K. Sato, H. Okushi, M. Hda, Appl. Phys. Lett. 67 (1995) 3596. [3] T. Ariki, S. Shikaina, S. Suzuki, Y. Otsuka, T. Maki, T. Kobayashi, Jap. J. Appl. Phys. 33 (1994) L888. [4] R. Heiderhoff, P. Koschinski, M. Maywald, L.J. Balk, P.K. Bachmann, Diamond Relat. Mater. 4 (1995) 645. [5] K. Okano, H. Kiyota, T. Iwasaki, T. Kurosu, M. Hda, T. Nakayama, Appl. Phys. Lett. 58 (1991) 840. [6 ] J.W. Glesener, Appl. Phys. Lett. 63 (1993) 767. [7] M.I. Eremets, Semicond. Sci. Technol. 6 (1991) 439. [8] A.T. Collins, E.C. Lightowlers, Phys. Rev. 171 (1968) 843. [9] E. Schibli, A.G. Milnes, Solid-State Electron. 11 (1968) 323. [10] H. Tomokage, T. Ueta, H. Furuta, T. Miyamoto, J. Appl. Phys. 77 (1995) 2041. [11] H. Kiyota, H. Okushi, T. Ando, Y. Sato, Adv. Mater. (1997) 167 [12] J.W. Glesener, Appl. Phys. Lett. 64 (1994) 217.