- Email: [email protected]
Nitrogenated amorphous carbon as a semiconductor
ELSEVIER
Diamond and Related Materials 5 (1996) 401-404
Nitrogenated
R[AMOND RELATED MATERIALS
amorphous carbon as a semiconductor
S.R.P. Silva ay1, B. Rafferty b, G.A.J. Amaratunga a, J. Schwan ‘, D.F. Franceschini d, L.M. Brown b ” Cambridge University, Engineering Department, Cambridge CB2 1 PZ, UK b Cambridge University, Cavendish Laboratory, Cambridge CB3 OHE, UK ’ University of Kaiserslautern, Fachbereich Physik, 67663 Kaiserslautern, Germany d Pontijicia Universidade Catolica, Departamento de Fisica, Rio de Janeiro, Brazil
Abstract
At present, hydrogenated amorphous carbon (a-CH) is a poor electronic material primarily due to the excessive density of defect states in the band gap which act as trapping centres. The ability of nitrogen to improve the semiconducting properties of a-C:H is examined. A reduction in the activation energy for electronic conduction in nitrogenated a-C:H (a-C:H:N) films and the approximately constant optical band gap with increasing N content suggest that N influences the bulk electronic properties of a-CH. Electron spin resonance shows a reduction of the density of gap states in a-C:H:N with increasing N content. Electron energy loss spectroscopy shows the films to be predominantly sp2 bonded with band edge properties which change significantly as a function of the N content. The C:H:N contents of the films were determined by elastic recoil detection analysis and Rutherford backscattering. Keywords: Doping; Optoelectronic
properties; Nitrogen; Electronic defects; a-C:H; a-C:H:N
1. Introduction Use of amorphous carbon (a-C) or diamond-like (DLC) thin films as a mechanical protective coating material is well known [ 1,2]. The hard mechanical properties arise due to the sp3 hybridized tetrahedral bonds between the C-C atoms. Unlike other Group IV amorphous materials, which only form tetrahedrally bonded structures (a-Si, a-Ge), a-C can form tetrahedral sp3, trigonal sp2 and linear sp’ hybridized bonds. The properties of a-C films are therefore closely related to the structure, which in turn is controlled by the deposition process. The possibility of forming sp2 and sp’ bonds should, in theory, allow highly sp3 bonded a-C films to reduce the number of coordination defects (unsaturated dangling bonds) present in the films [ 31. Spin densities measured for hydrogenated a-C (a-C:H) films to date have been in the range 101g-1021 cmm3, indicating the presence of a high number of coordination defects. In the case of a-58, the number of defects measured decreases dramatically below 1016cme3 when hydrogenated, but for a-C:H films the reduction seen in the spin density is minimal. carbon
1Present address: University of Surrey, Department Engineering, Guildford, Surrey, GU2 5XH, England; e-mail: [email protected] Elsevier Science %A. SSDI
0925-9635(95)00446-7
of Electronic
In order to utilize a-C thin films as a semiconductor, they need to have a low defect density of states in the gap. A low defect density is also essential for an unpinned Fermi level to allow electronic doping. Attempts to dope a-C films have led to a reduction in the optical band gap as the dopant concentration increases [4-71. The decrease in the band gap suggests that structural modifications which result from an increased incorporation of dopant atoms lead to increased rc bonding within the a-C structure. This also leads to an increase in the conductivity of the films. Although limited success in the electronic doping of a-C has been reported [8], most attempts have not been successful due to graphitization of the films [4-71. Encouraging results which show an invariance of the optical band gap with increasing nitrogen doping concentration, and a concomitant reduction in the resistivity, have been obtained recently [9-111. In this paper, we study the properties of nitrogenated a-C:H films (a-C:H:N) with the aim of using them as a semiconducting material.
2. Experimental details The films were deposited using a magnetically confined r.f. plasma-enhanced chemical vapour deposition
402
S. R. P. Silva et al. IDiamond and Related Materials 5 (1996) 401-404
(PECVD) system which has been described elsewhere [lo-121. The magnetic confinement increases the fraction of ionized species in the plasma which results in a decrease in the d.c. self-bias voltage when compared with standard r.f. deposition [ 121. An enhanced plasma electron temperature and growth rate are obtained with magnetic confinement. CH4, He and N2 were used as source gases at 100 mTorr. Si( 100) and quartz substrates were placed on the bottom r.f. driven electrode which was cooled to 20 “C. A d.c. self-bias of -60 V was maintained for all depositions carried out in this work. While the CH, and He gas flows were maintained at 5 and 45 seem respectively, the Nz gas flow was varied in the range O-10 seem. Activation energy studies were conducted on gap cell arrangements as well as metalamorphous semiconductor-metal structures. Electron energy loss spectroscopy was carried out on a Vacuum Generator HB501 scanning transmission electron microscope (STEM) at a collection angle of 10 mrad. Samples were prepared by lifting off the a-C:H:N films deposited on Si by dissolving the Si substrate in an HF-HNO,-deionized water mixture and then floating them onto Cu TEM grids. A Brukker EPR spectrometer was used for electron spin resonance measurements, and the optical constants were obtained by analysing both the transmitted and reflected spectra from a-C:H:N films deposited on quartz substrates. The atomic compositions of the films were determined by electron recoil detection analysis (ERDA) coupled with Rutherford backscattering (RBS) and nuclear reaction analysis (NRA). Beams of a particles and deuterons were obtained with an HVEC 4 MeV Van de Graaff accelerator to perform the carbon atid hydrogen depth profiles. The 14N(d,p,)“N reaction (Ed=610 keV) was used to determine the nitrogen contents in the films.
3. Results and discussion The variation of the atomic nitrogen and atomic hydrogen contents in the films as a function of the Nz flow into the chamber is shown in Fig. 1. It can be seen that, after an initial rapid increase in the atomic nitrogen content up to a flow of about 2.5 seem, a more gradual increase in the nitrogen uptake into the a-C:H:N films takes place. Accompanying this increase in the atomic N content is a very small decrease in the H content up to an N, flow of 5 seem. For N, flow rates above 5 seem, the decrease in H content is much more pronounced. The decrease in hydrogen content is also observed as a decrease in the bonded C-H groups and an increase in the bonded C-N and N-H groups in IR absorption spectroscopy [ 111. The IR spectra also confirm that the bonded N content increases as the detected N in the films increases [ 111. The K edge electron energy loss spectra show an
22 14 20
12 s? J cd ’ !I z s *z
10
18
8
YI 4 $ 2
16
6 4
14
2 0 0
2
4
6
8
10
’ F s
12
Nitrogen flow / seem Fig. 1. Atomic nitrogen and atomic hydrogen content as a function of the N, flow into the chamber.
averaged local density of states around the empty conduction band. The C and N K edges are expected at around 285 and 400 eV respectively. The K edge spectra contain a peak corresponding to the 1s to ?I* transitions, followed by a step (approximately 5 eV later) which arises due to the 1s to TV*transitions. The sp2 fraction in a-C:H films has been calculated using the normalized area under the n* peak and comparing this with a 100% sp2 bonded graphite standard [ 131. Using this calibration, the a-C:H:N films show a constant C-C sp2 content of approximately 65% for N2 flows in the range O-7.5 seem, indicating &hat the local carbon environment remains unchanged with increasing atomic nitrogen content in the films [ 121. However, a similar analysis on the N K edge shows that N bonded in the sp2 hybridization state increases from 10% at an N2 flow of 0.5 seem to 60% for an N2 flow of 7.5 seem [12]. The optical Taut gap as a function of the N2 flow is shown in Fig. 2(a). After an initial increase from 1.65 eV to 2.1 eV when the atomic N content reaches 7%, the Taut gap remains constant at around 2.0 eV. This result is unlike any of the previous reports on the electronic doping of a-C:H films using a glow discharge method in that the optical gap does not decrease with increasing N content [ 4-71. The initial increase in the optical band gap with increasing N content is indirect evidence of nitrogen incorporation in the a-C:H films, leading to a reduction in the band tail states due to a more relaxed bonding structure with less strain and distortion. This is seen more clearly in the joint density of states (JDOS), discussed later. Electron spin resonance allows the measurement of the unpaired spins, and hence unbonded electrons, to be determined. The variation of the spin density as a function of the N2 flow is shown in Fig. 2(b). The initial (O-2.5 seem) rapid decrease in the spin density with increasing N content in the films suggests that nitrogen significantly reduces the number of coordination defects by forming an a-C:H:N alloy. The spin density of lower
S. R P. Silva et aLlDiamond and Related Materials 5 (1996) 401-404
403
2.2
a
2.1 % \
2.0
4 M 3
1.9
0" '3
1.8
2 \
0.9
-
$ ti $
0.8 0.7 -
8 ‘8 :
0.6 -
P Y
0.5 0.4 ' ' 0
b
I
I
I
I
I
I
I
2
4
6
8
10
12
14
Nitrogen flow / seem Fig. 3. Activation energy as a function of the N, flow into the chamber.
I
0
2
4
6
8
10
Nitrogen flow / seem Fig. 2. Optical Taut gap (a) and electron spin density (b) as a function of the N, flow into the chamber.
than 101s cme3 recorded for the most highly nitrogenated a-C:H:N films (N, flow of 15 seem) is one of the lowest reported for any form of a-C/a-C:H film. Although still about two to three orders of magnitude above that of the best a-Si:H films, the defect level is much better than that obtained when using only hydrogen as passivating element. It should be noted that a reduction in spin density may be caused by an increase in the graphitic content in the films and by the defects becoming diamagnetic. Carrier lifetime experiments based on frequency-resolved photoconductive measurements are currently being examined. The electrical conduction mechanism for semiconductors is controlled by the .position of the Fermi level, EF, in the material. Therefore it is important to measure the position of the Fermi level in order to check whether a variation in the conductivity of the films arises due to movement of the Fermi level. If a plot of the conductivity vs. (l/temperature) follows a straight line of nature I = I, exp (- E,/kT), then E’, = ECE - EF and ECE is the energy at the effective edge of the conduction band, E, can be obtained from the slope of the plot. Such Arrhenius plots in the range 40-220 “C were used to obtain the activation energy E, as a function of the N flow (Fig. 3). The slope of the curve at 100 “C was used to obtain the activation energy [ 111. The activation energy shows an initial increase from 0.45 eV up to approximately 0.9 eV at 2.5 seem N, flow, and then decreases gradually to 0.5 eV. A value of
0.45 eV is observed for undoped a-C:H films. Frauenheim et al. [ 141 showed, by molecular dynamic simulations, that undoped a-C:H films have structural defects within the amorphous structure which make the films intrinsically p type. Assuming this to be the case, what we see is the gradual movement of the Fermi level towards the centre of the optical band gap in a-C:H:N with increasing N content up to 2.5 seem. Up to an atomic concentration of 7% (nitrogen flow of 2.5 seem), the a-C:H:N films can be viewed as p type, and the activation energy is measured from the valence band edge if a gradual change in the electronic structure from that of a-C:H to a-C:H:N is assumed. From such a model, when the nitrogen content reaches above 7 at.%, the nitrogen donor level within the density of states in a-C:H:N films can be viewed as having fully compensated the p-type defect states leading to an n-type material. With a further increase in N, a decrease in the activation energy from approximately 0.9 eV to 0.5 eV therefore results, while the optical gap remains. For these highly nitrogenated a-C:H:N films, the activation energy is measured from the conduction band edge to the donor state, which are due to the newly formed N (or mixed C-N) states dominating the conduction process. The JDOS obtained for a-C:H:N films deposited with N, flows of 0, 2.5 and 10 seem is shown in Fig. 4. A full description of the method used to obtain the JDOS is discussed elsewhere [ 121. Briefly, the low loss electron energy loss spectrum is used to obtain the imaginary part of the reciprocal dielectric loss function using the dielectric formulation [ 151. The real part of the dielectric function can then be obtained using the Kramers-Kronig transformation. Then both the real and imaginary parts of the dielectric function can be used to obtain the JDOS via [ 161
404
S. R P. Silva et allDiamond and Related Materials 5 (1996) 401-404
The joint density of states obtained is not quantitative, but is a good measure of the electronic transitions possible around the Fermi level, for instance near the band gap, and it can be used as a qualitative tool to interpret the electronic structure of the a-C:H:N films deposited. The variation seen in the JDOS in Fig. 4 clearly shows the modifications which occur in the electronic structure of the film as the nitrogen content is varied. A sharpening of the pre-edge associated with the rc* bonds around 4 eV in the a-C:H films is clearly seen for the a-C:H:N film deposited with an Nz flow of 2.5 seem. This is confirmed by the increase in the optical band gap. Activation energy and electron spin resonance measurements also confirm that the Fermi level in the material is moving away from the valence band towards the midgap due to a reduction in defects. When the nitrogen content in the films is further increased, a new level close to the conduction band but below that of the rc* carbon level is seen. This may be tentatively assigned to a nitrogen defect band, because it is close to that theoretically predicted by Robertson and Davis [ 171 using tight binding molecular orbital calculations. In the classic case of diamond, nitrogen can sit substitutionally in the C matrix and thus contribute its fifth electron to a singly occupied donor state. Although a similar situation may arise in tetrahedral amorphous carbon (ta-CN) [9], the high sp* content present in a-C:H:N films makes this unlikely. The conduction process that gives rise to electronic doping in these films may originate from a C-N sub-band that arises due to trigonal N3- sites. Three of the electrons in N would then form (r bonds, one electron would pair up with a C 7t state and the fifth electron would move up into the N it* state contributing to the doping effect.
4. Conclusions The electronic and optical properties of nitrogenated a-C:H films have been studied. The results obtained suggest that bonded atomic nitrogen reduces coordination defects and also leads to a reduction in network strain and distortion. Electronically, increasing the N content in a-C:H:N tends to move the Fermi level across the mid-gap towards the conduction band edge producing a compensated semiconductor at a nitrogen content of about 7%. The electron energy loss spectra-derived JDOS confirms that N incorporation leads to a reduction in the tail state density. The JDOS also shows quite clearly that, when the N content is increased above
4,000 ,
I
I
I
,
I
20
25
3,500 3,000 2,500 2,000 1,500 1,000 500 w
.c 0
10 her;;/
7
30
eV
Fig. 4. Joint density of states obtained for a-C:H:N films deposited with N, flows into the deposition chamber ok (a) 0 seem; (b) 2.5 seem; (c) 10 scorn.
7 at.%, a distinct sub-band appears below the carbon conduction band states. The n-type behaviour of a-C:H:N is postulated as originating from the activation of N donor electrons into the sub-band states. The results taken together suggest that a-C:H:N may be a viable semiconductor material for electronic applications. Carrier lifetime measurements are currently being pursued in order to examine the defect states within the a-C:H:N films in more detail.
References [l] [2] [ 31 [4] [S] [6] [7] [S]
[9] [lo] [ 1l] [ 121 [13] [14] [ 151 [ 161 [17]
H. Tsai and D.B. Bogy, J. Vat. Sci. Technol. A, 5 (1987) 3287. J. Robertson, Adv. Phys., 35 (1986) 317. G.A.J. Amaratunga et al., Diamond Relat. Mater., 4 (1994) 637. D.I. Jones and A.D. Stewart, Philos. Mag. B, 46 (1982) 423. J. Schwan, W. Dworschak, K. Jung and H. Ehrhardt, Diamond Relat. Mater., 3 (1994) 1034. 0. Amir and R. Kalish, J. Appl. Phys., 70 (1991) 4958. 0. Stenzel et al., Phys. Status Solidi, 140 (1993) 179. B. Meyerson and F.W. Smith, Solid State Commun., 41 (1982) 23. VS. Veerasamy et al., Phys. Rev. B, 48 (1993) 17{16954. S.R.P. Silva, K.J. Clay, S.P. Speakman and G.A.J. Amaratunga, Diamond Relat. Mater., 4 (1995) 977. S.R.P. Silva and G.A.J. Amaratunga, Thin Solid Films, in press. S.R.P. Silva et al., in preparation. S.D. Berger, D.R. McKenzie and P.J. Martin, Philos. Mag. Lett., 57 (1988) 285. Th. Frauenheim, U. Stephan, P. Blaudeck and G. Jungnickel, Diamond Relat. Mater., 3 (1994) 462. R.F. Egerton, Electron Energy I&s Spectroscopy in the Electron Microscope, Plenum, New York, 1986. N.W. Ashcroft and N.D. Mermin, Solid State Physics, Saunders College Publication, New York, 1976, Appendix K. J. Robertson and C.A. Davis, Diamond Relat. Mater., 4 (1995) 441.
Diamond and Related Materials 5 (1996) 401-404
Nitrogenated
R[AMOND RELATED MATERIALS
amorphous carbon as a semiconductor
S.R.P. Silva ay1, B. Rafferty b, G.A.J. Amaratunga a, J. Schwan ‘, D.F. Franceschini d, L.M. Brown b ” Cambridge University, Engineering Department, Cambridge CB2 1 PZ, UK b Cambridge University, Cavendish Laboratory, Cambridge CB3 OHE, UK ’ University of Kaiserslautern, Fachbereich Physik, 67663 Kaiserslautern, Germany d Pontijicia Universidade Catolica, Departamento de Fisica, Rio de Janeiro, Brazil
Abstract
At present, hydrogenated amorphous carbon (a-CH) is a poor electronic material primarily due to the excessive density of defect states in the band gap which act as trapping centres. The ability of nitrogen to improve the semiconducting properties of a-C:H is examined. A reduction in the activation energy for electronic conduction in nitrogenated a-C:H (a-C:H:N) films and the approximately constant optical band gap with increasing N content suggest that N influences the bulk electronic properties of a-CH. Electron spin resonance shows a reduction of the density of gap states in a-C:H:N with increasing N content. Electron energy loss spectroscopy shows the films to be predominantly sp2 bonded with band edge properties which change significantly as a function of the N content. The C:H:N contents of the films were determined by elastic recoil detection analysis and Rutherford backscattering. Keywords: Doping; Optoelectronic
properties; Nitrogen; Electronic defects; a-C:H; a-C:H:N
1. Introduction Use of amorphous carbon (a-C) or diamond-like (DLC) thin films as a mechanical protective coating material is well known [ 1,2]. The hard mechanical properties arise due to the sp3 hybridized tetrahedral bonds between the C-C atoms. Unlike other Group IV amorphous materials, which only form tetrahedrally bonded structures (a-Si, a-Ge), a-C can form tetrahedral sp3, trigonal sp2 and linear sp’ hybridized bonds. The properties of a-C films are therefore closely related to the structure, which in turn is controlled by the deposition process. The possibility of forming sp2 and sp’ bonds should, in theory, allow highly sp3 bonded a-C films to reduce the number of coordination defects (unsaturated dangling bonds) present in the films [ 31. Spin densities measured for hydrogenated a-C (a-C:H) films to date have been in the range 101g-1021 cmm3, indicating the presence of a high number of coordination defects. In the case of a-58, the number of defects measured decreases dramatically below 1016cme3 when hydrogenated, but for a-C:H films the reduction seen in the spin density is minimal. carbon
1Present address: University of Surrey, Department Engineering, Guildford, Surrey, GU2 5XH, England; e-mail: [email protected] Elsevier Science %A. SSDI
0925-9635(95)00446-7
of Electronic
In order to utilize a-C thin films as a semiconductor, they need to have a low defect density of states in the gap. A low defect density is also essential for an unpinned Fermi level to allow electronic doping. Attempts to dope a-C films have led to a reduction in the optical band gap as the dopant concentration increases [4-71. The decrease in the band gap suggests that structural modifications which result from an increased incorporation of dopant atoms lead to increased rc bonding within the a-C structure. This also leads to an increase in the conductivity of the films. Although limited success in the electronic doping of a-C has been reported [8], most attempts have not been successful due to graphitization of the films [4-71. Encouraging results which show an invariance of the optical band gap with increasing nitrogen doping concentration, and a concomitant reduction in the resistivity, have been obtained recently [9-111. In this paper, we study the properties of nitrogenated a-C:H films (a-C:H:N) with the aim of using them as a semiconducting material.
2. Experimental details The films were deposited using a magnetically confined r.f. plasma-enhanced chemical vapour deposition
402
S. R. P. Silva et al. IDiamond and Related Materials 5 (1996) 401-404
(PECVD) system which has been described elsewhere [lo-121. The magnetic confinement increases the fraction of ionized species in the plasma which results in a decrease in the d.c. self-bias voltage when compared with standard r.f. deposition [ 121. An enhanced plasma electron temperature and growth rate are obtained with magnetic confinement. CH4, He and N2 were used as source gases at 100 mTorr. Si( 100) and quartz substrates were placed on the bottom r.f. driven electrode which was cooled to 20 “C. A d.c. self-bias of -60 V was maintained for all depositions carried out in this work. While the CH, and He gas flows were maintained at 5 and 45 seem respectively, the Nz gas flow was varied in the range O-10 seem. Activation energy studies were conducted on gap cell arrangements as well as metalamorphous semiconductor-metal structures. Electron energy loss spectroscopy was carried out on a Vacuum Generator HB501 scanning transmission electron microscope (STEM) at a collection angle of 10 mrad. Samples were prepared by lifting off the a-C:H:N films deposited on Si by dissolving the Si substrate in an HF-HNO,-deionized water mixture and then floating them onto Cu TEM grids. A Brukker EPR spectrometer was used for electron spin resonance measurements, and the optical constants were obtained by analysing both the transmitted and reflected spectra from a-C:H:N films deposited on quartz substrates. The atomic compositions of the films were determined by electron recoil detection analysis (ERDA) coupled with Rutherford backscattering (RBS) and nuclear reaction analysis (NRA). Beams of a particles and deuterons were obtained with an HVEC 4 MeV Van de Graaff accelerator to perform the carbon atid hydrogen depth profiles. The 14N(d,p,)“N reaction (Ed=610 keV) was used to determine the nitrogen contents in the films.
3. Results and discussion The variation of the atomic nitrogen and atomic hydrogen contents in the films as a function of the Nz flow into the chamber is shown in Fig. 1. It can be seen that, after an initial rapid increase in the atomic nitrogen content up to a flow of about 2.5 seem, a more gradual increase in the nitrogen uptake into the a-C:H:N films takes place. Accompanying this increase in the atomic N content is a very small decrease in the H content up to an N, flow of 5 seem. For N, flow rates above 5 seem, the decrease in H content is much more pronounced. The decrease in hydrogen content is also observed as a decrease in the bonded C-H groups and an increase in the bonded C-N and N-H groups in IR absorption spectroscopy [ 111. The IR spectra also confirm that the bonded N content increases as the detected N in the films increases [ 111. The K edge electron energy loss spectra show an
22 14 20
12 s? J cd ’ !I z s *z
10
18
8
YI 4 $ 2
16
6 4
14
2 0 0
2
4
6
8
10
’ F s
12
Nitrogen flow / seem Fig. 1. Atomic nitrogen and atomic hydrogen content as a function of the N, flow into the chamber.
averaged local density of states around the empty conduction band. The C and N K edges are expected at around 285 and 400 eV respectively. The K edge spectra contain a peak corresponding to the 1s to ?I* transitions, followed by a step (approximately 5 eV later) which arises due to the 1s to TV*transitions. The sp2 fraction in a-C:H films has been calculated using the normalized area under the n* peak and comparing this with a 100% sp2 bonded graphite standard [ 131. Using this calibration, the a-C:H:N films show a constant C-C sp2 content of approximately 65% for N2 flows in the range O-7.5 seem, indicating &hat the local carbon environment remains unchanged with increasing atomic nitrogen content in the films [ 121. However, a similar analysis on the N K edge shows that N bonded in the sp2 hybridization state increases from 10% at an N2 flow of 0.5 seem to 60% for an N2 flow of 7.5 seem [12]. The optical Taut gap as a function of the N2 flow is shown in Fig. 2(a). After an initial increase from 1.65 eV to 2.1 eV when the atomic N content reaches 7%, the Taut gap remains constant at around 2.0 eV. This result is unlike any of the previous reports on the electronic doping of a-C:H films using a glow discharge method in that the optical gap does not decrease with increasing N content [ 4-71. The initial increase in the optical band gap with increasing N content is indirect evidence of nitrogen incorporation in the a-C:H films, leading to a reduction in the band tail states due to a more relaxed bonding structure with less strain and distortion. This is seen more clearly in the joint density of states (JDOS), discussed later. Electron spin resonance allows the measurement of the unpaired spins, and hence unbonded electrons, to be determined. The variation of the spin density as a function of the N2 flow is shown in Fig. 2(b). The initial (O-2.5 seem) rapid decrease in the spin density with increasing N content in the films suggests that nitrogen significantly reduces the number of coordination defects by forming an a-C:H:N alloy. The spin density of lower
S. R P. Silva et aLlDiamond and Related Materials 5 (1996) 401-404
403
2.2
a
2.1 % \
2.0
4 M 3
1.9
0" '3
1.8
2 \
0.9
-
$ ti $
0.8 0.7 -
8 ‘8 :
0.6 -
P Y
0.5 0.4 ' ' 0
b
I
I
I
I
I
I
I
2
4
6
8
10
12
14
Nitrogen flow / seem Fig. 3. Activation energy as a function of the N, flow into the chamber.
I
0
2
4
6
8
10
Nitrogen flow / seem Fig. 2. Optical Taut gap (a) and electron spin density (b) as a function of the N, flow into the chamber.
than 101s cme3 recorded for the most highly nitrogenated a-C:H:N films (N, flow of 15 seem) is one of the lowest reported for any form of a-C/a-C:H film. Although still about two to three orders of magnitude above that of the best a-Si:H films, the defect level is much better than that obtained when using only hydrogen as passivating element. It should be noted that a reduction in spin density may be caused by an increase in the graphitic content in the films and by the defects becoming diamagnetic. Carrier lifetime experiments based on frequency-resolved photoconductive measurements are currently being examined. The electrical conduction mechanism for semiconductors is controlled by the .position of the Fermi level, EF, in the material. Therefore it is important to measure the position of the Fermi level in order to check whether a variation in the conductivity of the films arises due to movement of the Fermi level. If a plot of the conductivity vs. (l/temperature) follows a straight line of nature I = I, exp (- E,/kT), then E’, = ECE - EF and ECE is the energy at the effective edge of the conduction band, E, can be obtained from the slope of the plot. Such Arrhenius plots in the range 40-220 “C were used to obtain the activation energy E, as a function of the N flow (Fig. 3). The slope of the curve at 100 “C was used to obtain the activation energy [ 111. The activation energy shows an initial increase from 0.45 eV up to approximately 0.9 eV at 2.5 seem N, flow, and then decreases gradually to 0.5 eV. A value of
0.45 eV is observed for undoped a-C:H films. Frauenheim et al. [ 141 showed, by molecular dynamic simulations, that undoped a-C:H films have structural defects within the amorphous structure which make the films intrinsically p type. Assuming this to be the case, what we see is the gradual movement of the Fermi level towards the centre of the optical band gap in a-C:H:N with increasing N content up to 2.5 seem. Up to an atomic concentration of 7% (nitrogen flow of 2.5 seem), the a-C:H:N films can be viewed as p type, and the activation energy is measured from the valence band edge if a gradual change in the electronic structure from that of a-C:H to a-C:H:N is assumed. From such a model, when the nitrogen content reaches above 7 at.%, the nitrogen donor level within the density of states in a-C:H:N films can be viewed as having fully compensated the p-type defect states leading to an n-type material. With a further increase in N, a decrease in the activation energy from approximately 0.9 eV to 0.5 eV therefore results, while the optical gap remains. For these highly nitrogenated a-C:H:N films, the activation energy is measured from the conduction band edge to the donor state, which are due to the newly formed N (or mixed C-N) states dominating the conduction process. The JDOS obtained for a-C:H:N films deposited with N, flows of 0, 2.5 and 10 seem is shown in Fig. 4. A full description of the method used to obtain the JDOS is discussed elsewhere [ 121. Briefly, the low loss electron energy loss spectrum is used to obtain the imaginary part of the reciprocal dielectric loss function using the dielectric formulation [ 151. The real part of the dielectric function can then be obtained using the Kramers-Kronig transformation. Then both the real and imaginary parts of the dielectric function can be used to obtain the JDOS via [ 161
404
S. R P. Silva et allDiamond and Related Materials 5 (1996) 401-404
The joint density of states obtained is not quantitative, but is a good measure of the electronic transitions possible around the Fermi level, for instance near the band gap, and it can be used as a qualitative tool to interpret the electronic structure of the a-C:H:N films deposited. The variation seen in the JDOS in Fig. 4 clearly shows the modifications which occur in the electronic structure of the film as the nitrogen content is varied. A sharpening of the pre-edge associated with the rc* bonds around 4 eV in the a-C:H films is clearly seen for the a-C:H:N film deposited with an Nz flow of 2.5 seem. This is confirmed by the increase in the optical band gap. Activation energy and electron spin resonance measurements also confirm that the Fermi level in the material is moving away from the valence band towards the midgap due to a reduction in defects. When the nitrogen content in the films is further increased, a new level close to the conduction band but below that of the rc* carbon level is seen. This may be tentatively assigned to a nitrogen defect band, because it is close to that theoretically predicted by Robertson and Davis [ 171 using tight binding molecular orbital calculations. In the classic case of diamond, nitrogen can sit substitutionally in the C matrix and thus contribute its fifth electron to a singly occupied donor state. Although a similar situation may arise in tetrahedral amorphous carbon (ta-CN) [9], the high sp* content present in a-C:H:N films makes this unlikely. The conduction process that gives rise to electronic doping in these films may originate from a C-N sub-band that arises due to trigonal N3- sites. Three of the electrons in N would then form (r bonds, one electron would pair up with a C 7t state and the fifth electron would move up into the N it* state contributing to the doping effect.
4. Conclusions The electronic and optical properties of nitrogenated a-C:H films have been studied. The results obtained suggest that bonded atomic nitrogen reduces coordination defects and also leads to a reduction in network strain and distortion. Electronically, increasing the N content in a-C:H:N tends to move the Fermi level across the mid-gap towards the conduction band edge producing a compensated semiconductor at a nitrogen content of about 7%. The electron energy loss spectra-derived JDOS confirms that N incorporation leads to a reduction in the tail state density. The JDOS also shows quite clearly that, when the N content is increased above
4,000 ,
I
I
I
,
I
20
25
3,500 3,000 2,500 2,000 1,500 1,000 500 w
.c 0
10 her;;/
7
30
eV
Fig. 4. Joint density of states obtained for a-C:H:N films deposited with N, flows into the deposition chamber ok (a) 0 seem; (b) 2.5 seem; (c) 10 scorn.
7 at.%, a distinct sub-band appears below the carbon conduction band states. The n-type behaviour of a-C:H:N is postulated as originating from the activation of N donor electrons into the sub-band states. The results taken together suggest that a-C:H:N may be a viable semiconductor material for electronic applications. Carrier lifetime measurements are currently being pursued in order to examine the defect states within the a-C:H:N films in more detail.
References [l] [2] [ 31 [4] [S] [6] [7] [S]
[9] [lo] [ 1l] [ 121 [13] [14] [ 151 [ 161 [17]
H. Tsai and D.B. Bogy, J. Vat. Sci. Technol. A, 5 (1987) 3287. J. Robertson, Adv. Phys., 35 (1986) 317. G.A.J. Amaratunga et al., Diamond Relat. Mater., 4 (1994) 637. D.I. Jones and A.D. Stewart, Philos. Mag. B, 46 (1982) 423. J. Schwan, W. Dworschak, K. Jung and H. Ehrhardt, Diamond Relat. Mater., 3 (1994) 1034. 0. Amir and R. Kalish, J. Appl. Phys., 70 (1991) 4958. 0. Stenzel et al., Phys. Status Solidi, 140 (1993) 179. B. Meyerson and F.W. Smith, Solid State Commun., 41 (1982) 23. VS. Veerasamy et al., Phys. Rev. B, 48 (1993) 17{16954. S.R.P. Silva, K.J. Clay, S.P. Speakman and G.A.J. Amaratunga, Diamond Relat. Mater., 4 (1995) 977. S.R.P. Silva and G.A.J. Amaratunga, Thin Solid Films, in press. S.R.P. Silva et al., in preparation. S.D. Berger, D.R. McKenzie and P.J. Martin, Philos. Mag. Lett., 57 (1988) 285. Th. Frauenheim, U. Stephan, P. Blaudeck and G. Jungnickel, Diamond Relat. Mater., 3 (1994) 462. R.F. Egerton, Electron Energy I&s Spectroscopy in the Electron Microscope, Plenum, New York, 1986. N.W. Ashcroft and N.D. Mermin, Solid State Physics, Saunders College Publication, New York, 1976, Appendix K. J. Robertson and C.A. Davis, Diamond Relat. Mater., 4 (1995) 441.