- Email: [email protected]
Secondary electron emission spectroscopy and total electron yield measurements for the assessment of near-surface damage in diamond
440
Diamond and Related Materials, 1 (1992) 440 444 Elsevier Science Publishers B.V., Amsterdam
Secondary electron emission spectroscopy and total electron yield measurements for the assessment of near-surface damage in diamond A. Hoffman Applications of Nuclear Physics, Australian Nuclear Science and Technology Organisation, Private Mailbag 1, Menai, New South Wales 2234 (Australia)
S. Prawer School of Physics, The University of Melbourne, Parkville, Vic, 3052, (Australia)
R. Kalish Department of Physics and Solid State, Institute Technion, Haifa 32000 (Israel)
Abstract In this work the sensitivity of secondary electron emission (SEE) spectroscopyin the 0-50 eV range and total electron yield (TEY) for characterising the near-surface region of diamond with varyinglevels of damage is examined. This is accomplishedby SEE and TEY measurements of diamond surfaces subjected to ever increasing doses of I keV Ar ion irradiation. The irradiation gradually damages, amorphises and eventually graphitises the diamond surface thus providing a continuous spectrum of different forms of carbon ranging from crystalline sp3 to amorphous spz bonded carbon. A strong and abrupt charging effect is observed as a function of irradiation dose. This effect was correlated with changes in TEY and sample conductivity. It is found that TEY and SEE are very sensitive measures of the crystalline perfection of the near-surface region of diamond. It is suggested that SEE and TEY be used as sensitive tools for the characterisation of the near-surface region of diamond thin films.
1. Introduction The energy distribution of electrons emitted from solid surfaces under electron bombardment consists of elastically backscattered electrons (used for electron diffraction), inelastically backscattered electrons (used for electron energy loss) and true secondaries (Auger electrons, low energy secondaries, etc.), whose energy distribution consists of a broad band peaking at a few eV. Superimposed on the lower energy range of slowly varying secondary electrons is a fine structure which has been shown to reflect features in the empty density of states above the vacuum level [1]. In particular, the fine structure in the secondary electron emission (SEE) spectrum of diamond (type 2a) has been measured in the 0-50 eV range by the authors [-2] and has been shown to reflect high energy conduction band states; in agreement with other spectroscopic techniques sensitive to the empty density of states and with band structure calculation [-3-5]. Important information may also be obtained from total electron yield (TEY) measurements. TEY is defined as the ratio between the total emitted electron flux and the primary (incident) electron flux. By measuring the current through the sample under electron bombardment, I~, and the primary electron current, lp, then for
0925-9635/92/$5.00 © Elsevier Science Publishers B.V. All rights reserved
a particular primary electron energy, TEY is given by: TEY = (Ip - Is)/Ip = (It . . . . . . . . darics + Ibackscanered)/(lprimary)
(1) In this work the sensitivity of SEE and TEY to crystal damage of diamond was studied. Controlled levels of damage were introduced into the diamond lattice via 1 keV Ar ion irradiation at ever increasing doses. For such irradiations the damage peak is localised within 10-20 ~ from the surface (as calculated by T R I M 88). The SEE and TEY measurements using i-keV primary electrons were performed as function of ion dose in the 1013-1017 ions/cm / range. In addition, the functional dependence of TEY on the primary electron beam energy, Ep, was also measured in the range 5 0 0 < E p < 6 0 0 0 e V , for diamond, graphite and amorphous carbon. These dependences were compared to those obtained from an ion beam irradiated d i a m o n d surface using primary electron energies in the 500 6000 eV range.
2. Experimental The experiments were performed on type 2a natural diamond supplied by Drukkers of Amsterdam. Prior to
A. Hoffman et al. ,' Secondary electron emission spectroscopy and total electron yield measurements
insertion in the UHV system, the diamond window (3 x 3 x 0.25) was ultrasonically cleaned in acetone and rinsed in deionised water. The graphite (HOPG) sample was cleaved by the standard scotch type peeling technique in air. The amorphous carbon was prepared as a thin film by e beam evaporation of a carbon target onto a Si substrate and prior to insertion into the UHV system it was ultrasonically cleaned in acetone and rinsed in deionised water. Auger measurements of the as-prepared samples showed that the C(KLL) line shapes were characteristic of the different carbon allotropes suggesting well-defined impurity-free (to about the 1% level) surfaces. The experiments were all performed in an UHV chamber equipped with a cylindrical minor analyser (CMA). The primary electron energy used in the SEE measurements was 1 keV and a primary current of 0.1 I.tA confined to a spot size of 100 l.tm was used. The spectra were recorded in the d ( E * N ( E ) ) / d E vs. E mode using a 2-V peak to peak modulation. All measurements were done at room temperature. The Ar + irradiation was performed in situ using a VARIAN ion gun. The ion and electron current densities were measured by placing a Faraday cup in the sample position.
441
obtained for the different carbon allotropes. The various peaks were found to be associated with high energy conduction band states as reported previously [2, 6]. In Fig. 2(a) and (b) SEE measurements for diamond as a function of a 1 keV Ar + irradiation dose are shown. As observed from Fig. 2(a), doses as low as 7 x 1012 ions/ cm 2 are sufficient to cause a noticeable change in the spectra, mainly reflected in a change in the relative intensity of the two main peaks in the spectrum. The unirradiated signature is only observed for very high quality material. Above doses of about I x 1014 ions/cm 2 the fine structure is gradually washed out and for a dose of about 2x1015 ions/cm 2 an abrupt charging effect occurs followed by a gradual discharging. At doses exceeding 1×1016 ions/cm 2 the line shape is typical of amorphous carbon (electron beam evaporated), evidencing the transformation of the surface layer of the diamond into a highly disordered sp 2 region. In Fig. 3(a) the energy position of the C(KLL) Auger line is plotted as a function of ion dose (the shift in the C(KLLI peak being similar to the lower energy SEE peak as they are both true secondaries). 3.2. Total electron yield measurement
3. Results 3.1. S E E measurements
In Fig. 1, results of SEE measurements for diamond, graphite and amorphous carbon are shown. As can be seen from this figure, different SEE signatures are
AMORPHOUS C A ~ ~
~,~,
' "q'/ /r" / /
CRAP HiTE
I'I1, /,[~,
i;
]
0
cD
~
r.aa i
/ D AMOND
j
r.~,I
,j' ~
I
o. . . . . . . . . . . . .
..................
ENERGY (eV) Fig. 1. Secondar~ electron emission spectrum (SEE) of daimond (type 2a) graphite {HOPG) and amorphous carbon (e beam evaporated).
The total electron yield, TEY, was measured as a function of the ion irradiation dose of diamond as shown in Fig. 3(b). In Fig. 4, TEY measurements for the weltdefined diamond, graphite, amorphous carbon and the diamond surface after an irradiation dose of l xlO 17 ions:cm 2 are shown as a function of primary electron energy.
4. Discussion As observed from Figs 2(a) and (b), the SEE spectrum of diamond is very sensitive to the crystalline perfection of its near-surface region. A well-known difficulty in applying electron spectroscopies to insulating materials in general and diamond in particular, is line shape distortion due to charging. By defocussing the electron beam and working with low primary currents we found that it is possible to minimise charging effects for the unirradiated diamond surface. The degree of charging was estimated by the shift in the energy position of the C(KLL) Auger line from the graphite value at 272 eV. For the unirradiated diamond surface, the C(KLL) line energy position was measured to be 274 eV. However, for the irradiated diamond surface a strong and abrupt charging effect takes place at a dose of about 2 x 1015 ions/cm 2. Therefore an assessment of the sensitivity of the SEE spectrum to crystal damage is restricted to the level of damage induced by doses up to - 5 x 1014 ions/cm 2, i.e. before strong charging occurs. For this dose level, it is expected that ion irradiation
A. Hoffman et al. / Secondary electron emission spectroscopy and total electron yield measurements
442
7.0x10 TM
(b)
(a)
f
not irradiated
o
t ""
t
'
/
1/
(E-77eV)
(E-35eV)
7.9x1015 b
1.8x101~
4xl 016 6"2xl 017 Z
Z v
"/cm 2) o . . . . . . . . ~'6 . . . . . . . 4'6 . . . . . . . g'6 . . . . . . . 8 o ENERGY (eV)
DOSE (Ar+/cm a) 0
40
20
60
80
ENERGY (eV)
Fig. 2. Secondary electron emission spectra of type 2a diamond irradiated with 1 keV Ar ions under UHV conditions for ion doses between 7 x 1012 and 6 x l0 w ions/cm 2.
results in a highly damaged structure albeit maintaining the long range diamond structure [6]. Within this dose range gradual deterioration of the SEE spectrum occurs and (as observed in Fig. 2(a)) a preferential deterioration of the higher energy peaks seems to take place for the lower doses. The preferential deterioration of the high energy SEE peak as a function of the level of crystal damage is tentatively explained as follows. The higher the energy of a band, the larger its delocalisation in direct space. Therefore it is expected that the sensitivity of a particular electronic state to crystal perfection will increase with its energy, just as observed in our SEE measurements which exhibit the highest sensitivity for the irradiation damage for the highest peak in the SEE spectrum. Considering the very low level of damage required to disrupt the SEE signature of unirradiated diamond, it is clear that the presence of the double structure in the SEE spectrum (as in Figs 1 and 2) can be used as a test for the presence of high quality crystalline diamond. Although more work needs to be done to quantify the technique, the fact that most Auger systems are fitted with a CMA analyser suitable for these measurements should recommend it as a useful adjunct to Auger
spectroscopy in the analysis of diamond and diamond films. Notwithstanding the above, a clear limitation of SEE and other electron spectroscopies is line shape distortion due to charging. As observed from Fig. 3(a) and (b) for doses exceeding about 2x1015 Ar+/cm 2 an extremely strong and abrupt charging effect is observed. This charging effect is closely correlated with a decrease in TEY. As the level of damage increases it decreases from a value of 0.9_+0.1 for the unirradiated diamond until it obtains a value similar to that measured for amorphous carbon (0.65_+0.1). TEY is an exact measure of the balance between the incoming and outgoing electron fluxes; a value equal to 1 will result in the absence of charging effects as the net impinging electron flux is exactly balanced by the total electron current, even for insulating samples. However, a TEY value of less than unity will result in negative charging of the surface and therefore an increase in the observed kinetic energy of the secondary electrons. For TEY values differing from unity the degree of charging depends on the balance between the primary current and the rate of discharge, with the latter primarily dependent on the sample conductivity. Hence it is clear that charging is a result of
A. ffqO'man et al.
0 . 9
Secondary electron emission spectroscopy and u)tal electron yield measurements
443
m
~1.0 ~0.8 Z
0.8
0
Z ~ 0.7 [.-,
~D
,0.6 0.4
~0.6 ,< E~ 0
24
o ~0.2
0.5
,
,
,,,,,
470
0.0
> o
i
i
i
,
i w ~ l
I
,
i
r ~ - J r r ' = r - T ~ - V - r ~ r
~
,
i
I
i
,
i
,
2ooo 400,0 6000 ELECTRON ENERGY (eV)
Fig. 4. Total electron yield as a function of primary electron energy for: (a) ) , diamond; (b) -L'L . graphite (HOPG); (c) I , amorphous carbon; and (d) • . ion beam irradiated diamond (dose= 1 x l 0 iv Ar*,cm2).
"--'420 Z 0
275 .~im ~
~570 0
.<
>
~320
270
I
10 i P
s' i'P
i
4P i
270
ION DOSE ( i o n s / e r a 2) Fig. 3. (al The position of the CIKLL) Auger line as a function of ion dose for irradiation with I keV Ar ion (as for Fig. 2). (b) The total electron yield, TEY, as function of ion dose. For unirradiated diamond TEY=0.9_+0.1 whilst for H O P G and a m o r p h o u s carbon TEY 0.65_+0.1.
For the irradiated diamond surface the dependence of TEY on primary electron current is similar to that measured for graphite or amorphous carbon, i.e. decreases as the primary energy increases. For 3 keV primary electrons low energy secondaries are produced many nanometers below the surface. Considering that the range of damage caused by the I keV ion beam is less than 20 A, the similar functional dependence of TEY to amorphous carbon (or to graphite) suggests that this dependence must be dominated by the state of the nearsurface region rather than any bulk property.
5. Conclusions the deviation of TEY from unity, leading to an inbalance between the magnitude of the impinging and secondary electron fluxes. The observed decrease in TEY is most likely due to the transformation of diamond to amorphous carbon for which it has a value of ~0.65 for a primary electron energy of 1 keV. The decrease in TEY is likely to reflect the transformation between the two carbon phases induced by ion bombardment. For diamond a constant value of 0.9 was obtained for primary energies between 500 and 3000 eV. This dependence of TEY on electron kinetic energy is characteristic for insulators and is due to a self compensating effect [7]. For primary energies larger than 3000 eV, TEY measurements of diamond were not reproducible. This is most likely due to electrical breakdown [8]. For graphite and amorphous carbon TEY decreases as a function of electron energy (in the range measured), in agreement with other reported measurements [9].
Low energy secondary electron emission spectroscopy is very sensitive to the surface perfection of diamond crystals. Damage levels induced by doses as low as 7 x 10~2 Ar+/cm z are sufficient to modify the spectrum, diminishing first the magnitude of the peaks corresponding to the highest energy states. The usefulness of SEE (and other electron spectroscopies) is often limited by problems associated with charging. Although charging is not a problem for unirradiated diamond, diamonds with moderate levels of damage display strong charging which is associated with a deviation from unity of the total electron yield. This deviation is due to the gradual transformation to amorphous carbon under the influence of ion bombardment. The dependence of TEY on the primary electron energy, E v, is also modified by the ion beam; unirradiated diamond displays a value of TEY = 0.9+0.1 independent of Ep, in the 500 3000 eV range
444
A. Hoffman et al. / Secondary electron emission spectroscopy and total electron yield measurements
whilst irradiated diamond, amorphous carbon and graphite all display a monotonic decrease of TEY as a function of Ep.
References 1 R. F. Willis, B. Filton and G. S. Painter, Phys. Rev. B, 9 (1974) 1926. 2 A. Hoffman, M. Folman and S. Prawer, Phys. Rev. B, 1991, 44 (199l) 4680.
3 F.J. Himpsel, J. F. v o n d e r Veen and D.E. Eastman, Phys. Rev., B22 (1980) 1967. 4 J. F. Mowz, F. J. Himpsel, G. Hollinger, G. Hughes and J. L. Jordan, Phys. Rev. Lett., 54 (1985) 1960. 5 G.S. Painter, D. E. Ellis and A. R. Lubinsky, Phys. Rev., B4 (1971) 3610. 6 R. Kalish, T. Bernstein, B. Shopiro and A. Talmi, Radiat. Eft., I2 (1980) 153. 7 J. Cazoux, J. Appl. Phys., 59(5) (1986) 1418. 8 H. W. Werner and N. Warmoltz, J. Vac. Sci. Technol. A, 2 (1984) 726. 9 K. Ohya, K. Nishimura and I. Mori, Jap. J. Appl. Phys., 30(5) (1991) 1093, and references therein.
Diamond and Related Materials, 1 (1992) 440 444 Elsevier Science Publishers B.V., Amsterdam
Secondary electron emission spectroscopy and total electron yield measurements for the assessment of near-surface damage in diamond A. Hoffman Applications of Nuclear Physics, Australian Nuclear Science and Technology Organisation, Private Mailbag 1, Menai, New South Wales 2234 (Australia)
S. Prawer School of Physics, The University of Melbourne, Parkville, Vic, 3052, (Australia)
R. Kalish Department of Physics and Solid State, Institute Technion, Haifa 32000 (Israel)
Abstract In this work the sensitivity of secondary electron emission (SEE) spectroscopyin the 0-50 eV range and total electron yield (TEY) for characterising the near-surface region of diamond with varyinglevels of damage is examined. This is accomplishedby SEE and TEY measurements of diamond surfaces subjected to ever increasing doses of I keV Ar ion irradiation. The irradiation gradually damages, amorphises and eventually graphitises the diamond surface thus providing a continuous spectrum of different forms of carbon ranging from crystalline sp3 to amorphous spz bonded carbon. A strong and abrupt charging effect is observed as a function of irradiation dose. This effect was correlated with changes in TEY and sample conductivity. It is found that TEY and SEE are very sensitive measures of the crystalline perfection of the near-surface region of diamond. It is suggested that SEE and TEY be used as sensitive tools for the characterisation of the near-surface region of diamond thin films.
1. Introduction The energy distribution of electrons emitted from solid surfaces under electron bombardment consists of elastically backscattered electrons (used for electron diffraction), inelastically backscattered electrons (used for electron energy loss) and true secondaries (Auger electrons, low energy secondaries, etc.), whose energy distribution consists of a broad band peaking at a few eV. Superimposed on the lower energy range of slowly varying secondary electrons is a fine structure which has been shown to reflect features in the empty density of states above the vacuum level [1]. In particular, the fine structure in the secondary electron emission (SEE) spectrum of diamond (type 2a) has been measured in the 0-50 eV range by the authors [-2] and has been shown to reflect high energy conduction band states; in agreement with other spectroscopic techniques sensitive to the empty density of states and with band structure calculation [-3-5]. Important information may also be obtained from total electron yield (TEY) measurements. TEY is defined as the ratio between the total emitted electron flux and the primary (incident) electron flux. By measuring the current through the sample under electron bombardment, I~, and the primary electron current, lp, then for
0925-9635/92/$5.00 © Elsevier Science Publishers B.V. All rights reserved
a particular primary electron energy, TEY is given by: TEY = (Ip - Is)/Ip = (It . . . . . . . . darics + Ibackscanered)/(lprimary)
(1) In this work the sensitivity of SEE and TEY to crystal damage of diamond was studied. Controlled levels of damage were introduced into the diamond lattice via 1 keV Ar ion irradiation at ever increasing doses. For such irradiations the damage peak is localised within 10-20 ~ from the surface (as calculated by T R I M 88). The SEE and TEY measurements using i-keV primary electrons were performed as function of ion dose in the 1013-1017 ions/cm / range. In addition, the functional dependence of TEY on the primary electron beam energy, Ep, was also measured in the range 5 0 0 < E p < 6 0 0 0 e V , for diamond, graphite and amorphous carbon. These dependences were compared to those obtained from an ion beam irradiated d i a m o n d surface using primary electron energies in the 500 6000 eV range.
2. Experimental The experiments were performed on type 2a natural diamond supplied by Drukkers of Amsterdam. Prior to
A. Hoffman et al. ,' Secondary electron emission spectroscopy and total electron yield measurements
insertion in the UHV system, the diamond window (3 x 3 x 0.25) was ultrasonically cleaned in acetone and rinsed in deionised water. The graphite (HOPG) sample was cleaved by the standard scotch type peeling technique in air. The amorphous carbon was prepared as a thin film by e beam evaporation of a carbon target onto a Si substrate and prior to insertion into the UHV system it was ultrasonically cleaned in acetone and rinsed in deionised water. Auger measurements of the as-prepared samples showed that the C(KLL) line shapes were characteristic of the different carbon allotropes suggesting well-defined impurity-free (to about the 1% level) surfaces. The experiments were all performed in an UHV chamber equipped with a cylindrical minor analyser (CMA). The primary electron energy used in the SEE measurements was 1 keV and a primary current of 0.1 I.tA confined to a spot size of 100 l.tm was used. The spectra were recorded in the d ( E * N ( E ) ) / d E vs. E mode using a 2-V peak to peak modulation. All measurements were done at room temperature. The Ar + irradiation was performed in situ using a VARIAN ion gun. The ion and electron current densities were measured by placing a Faraday cup in the sample position.
441
obtained for the different carbon allotropes. The various peaks were found to be associated with high energy conduction band states as reported previously [2, 6]. In Fig. 2(a) and (b) SEE measurements for diamond as a function of a 1 keV Ar + irradiation dose are shown. As observed from Fig. 2(a), doses as low as 7 x 1012 ions/ cm 2 are sufficient to cause a noticeable change in the spectra, mainly reflected in a change in the relative intensity of the two main peaks in the spectrum. The unirradiated signature is only observed for very high quality material. Above doses of about I x 1014 ions/cm 2 the fine structure is gradually washed out and for a dose of about 2x1015 ions/cm 2 an abrupt charging effect occurs followed by a gradual discharging. At doses exceeding 1×1016 ions/cm 2 the line shape is typical of amorphous carbon (electron beam evaporated), evidencing the transformation of the surface layer of the diamond into a highly disordered sp 2 region. In Fig. 3(a) the energy position of the C(KLL) Auger line is plotted as a function of ion dose (the shift in the C(KLLI peak being similar to the lower energy SEE peak as they are both true secondaries). 3.2. Total electron yield measurement
3. Results 3.1. S E E measurements
In Fig. 1, results of SEE measurements for diamond, graphite and amorphous carbon are shown. As can be seen from this figure, different SEE signatures are
AMORPHOUS C A ~ ~
~,~,
' "q'/ /r" / /
CRAP HiTE
I'I1, /,[~,
i;
]
0
cD
~
r.aa i
/ D AMOND
j
r.~,I
,j' ~
I
o. . . . . . . . . . . . .
..................
ENERGY (eV) Fig. 1. Secondar~ electron emission spectrum (SEE) of daimond (type 2a) graphite {HOPG) and amorphous carbon (e beam evaporated).
The total electron yield, TEY, was measured as a function of the ion irradiation dose of diamond as shown in Fig. 3(b). In Fig. 4, TEY measurements for the weltdefined diamond, graphite, amorphous carbon and the diamond surface after an irradiation dose of l xlO 17 ions:cm 2 are shown as a function of primary electron energy.
4. Discussion As observed from Figs 2(a) and (b), the SEE spectrum of diamond is very sensitive to the crystalline perfection of its near-surface region. A well-known difficulty in applying electron spectroscopies to insulating materials in general and diamond in particular, is line shape distortion due to charging. By defocussing the electron beam and working with low primary currents we found that it is possible to minimise charging effects for the unirradiated diamond surface. The degree of charging was estimated by the shift in the energy position of the C(KLL) Auger line from the graphite value at 272 eV. For the unirradiated diamond surface, the C(KLL) line energy position was measured to be 274 eV. However, for the irradiated diamond surface a strong and abrupt charging effect takes place at a dose of about 2 x 1015 ions/cm 2. Therefore an assessment of the sensitivity of the SEE spectrum to crystal damage is restricted to the level of damage induced by doses up to - 5 x 1014 ions/cm 2, i.e. before strong charging occurs. For this dose level, it is expected that ion irradiation
A. Hoffman et al. / Secondary electron emission spectroscopy and total electron yield measurements
442
7.0x10 TM
(b)
(a)
f
not irradiated
o
t ""
t
'
/
1/
(E-77eV)
(E-35eV)
7.9x1015 b
1.8x101~
4xl 016 6"2xl 017 Z
Z v
"/cm 2) o . . . . . . . . ~'6 . . . . . . . 4'6 . . . . . . . g'6 . . . . . . . 8 o ENERGY (eV)
DOSE (Ar+/cm a) 0
40
20
60
80
ENERGY (eV)
Fig. 2. Secondary electron emission spectra of type 2a diamond irradiated with 1 keV Ar ions under UHV conditions for ion doses between 7 x 1012 and 6 x l0 w ions/cm 2.
results in a highly damaged structure albeit maintaining the long range diamond structure [6]. Within this dose range gradual deterioration of the SEE spectrum occurs and (as observed in Fig. 2(a)) a preferential deterioration of the higher energy peaks seems to take place for the lower doses. The preferential deterioration of the high energy SEE peak as a function of the level of crystal damage is tentatively explained as follows. The higher the energy of a band, the larger its delocalisation in direct space. Therefore it is expected that the sensitivity of a particular electronic state to crystal perfection will increase with its energy, just as observed in our SEE measurements which exhibit the highest sensitivity for the irradiation damage for the highest peak in the SEE spectrum. Considering the very low level of damage required to disrupt the SEE signature of unirradiated diamond, it is clear that the presence of the double structure in the SEE spectrum (as in Figs 1 and 2) can be used as a test for the presence of high quality crystalline diamond. Although more work needs to be done to quantify the technique, the fact that most Auger systems are fitted with a CMA analyser suitable for these measurements should recommend it as a useful adjunct to Auger
spectroscopy in the analysis of diamond and diamond films. Notwithstanding the above, a clear limitation of SEE and other electron spectroscopies is line shape distortion due to charging. As observed from Fig. 3(a) and (b) for doses exceeding about 2x1015 Ar+/cm 2 an extremely strong and abrupt charging effect is observed. This charging effect is closely correlated with a decrease in TEY. As the level of damage increases it decreases from a value of 0.9_+0.1 for the unirradiated diamond until it obtains a value similar to that measured for amorphous carbon (0.65_+0.1). TEY is an exact measure of the balance between the incoming and outgoing electron fluxes; a value equal to 1 will result in the absence of charging effects as the net impinging electron flux is exactly balanced by the total electron current, even for insulating samples. However, a TEY value of less than unity will result in negative charging of the surface and therefore an increase in the observed kinetic energy of the secondary electrons. For TEY values differing from unity the degree of charging depends on the balance between the primary current and the rate of discharge, with the latter primarily dependent on the sample conductivity. Hence it is clear that charging is a result of
A. ffqO'man et al.
0 . 9
Secondary electron emission spectroscopy and u)tal electron yield measurements
443
m
~1.0 ~0.8 Z
0.8
0
Z ~ 0.7 [.-,
~D
,0.6 0.4
~0.6 ,< E~ 0
24
o ~0.2
0.5
,
,
,,,,,
470
0.0
> o
i
i
i
,
i w ~ l
I
,
i
r ~ - J r r ' = r - T ~ - V - r ~ r
~
,
i
I
i
,
i
,
2ooo 400,0 6000 ELECTRON ENERGY (eV)
Fig. 4. Total electron yield as a function of primary electron energy for: (a) ) , diamond; (b) -L'L . graphite (HOPG); (c) I , amorphous carbon; and (d) • . ion beam irradiated diamond (dose= 1 x l 0 iv Ar*,cm2).
"--'420 Z 0
275 .~im ~
~570 0
.<
>
~320
270
I
10 i P
s' i'P
i
4P i
270
ION DOSE ( i o n s / e r a 2) Fig. 3. (al The position of the CIKLL) Auger line as a function of ion dose for irradiation with I keV Ar ion (as for Fig. 2). (b) The total electron yield, TEY, as function of ion dose. For unirradiated diamond TEY=0.9_+0.1 whilst for H O P G and a m o r p h o u s carbon TEY 0.65_+0.1.
For the irradiated diamond surface the dependence of TEY on primary electron current is similar to that measured for graphite or amorphous carbon, i.e. decreases as the primary energy increases. For 3 keV primary electrons low energy secondaries are produced many nanometers below the surface. Considering that the range of damage caused by the I keV ion beam is less than 20 A, the similar functional dependence of TEY to amorphous carbon (or to graphite) suggests that this dependence must be dominated by the state of the nearsurface region rather than any bulk property.
5. Conclusions the deviation of TEY from unity, leading to an inbalance between the magnitude of the impinging and secondary electron fluxes. The observed decrease in TEY is most likely due to the transformation of diamond to amorphous carbon for which it has a value of ~0.65 for a primary electron energy of 1 keV. The decrease in TEY is likely to reflect the transformation between the two carbon phases induced by ion bombardment. For diamond a constant value of 0.9 was obtained for primary energies between 500 and 3000 eV. This dependence of TEY on electron kinetic energy is characteristic for insulators and is due to a self compensating effect [7]. For primary energies larger than 3000 eV, TEY measurements of diamond were not reproducible. This is most likely due to electrical breakdown [8]. For graphite and amorphous carbon TEY decreases as a function of electron energy (in the range measured), in agreement with other reported measurements [9].
Low energy secondary electron emission spectroscopy is very sensitive to the surface perfection of diamond crystals. Damage levels induced by doses as low as 7 x 10~2 Ar+/cm z are sufficient to modify the spectrum, diminishing first the magnitude of the peaks corresponding to the highest energy states. The usefulness of SEE (and other electron spectroscopies) is often limited by problems associated with charging. Although charging is not a problem for unirradiated diamond, diamonds with moderate levels of damage display strong charging which is associated with a deviation from unity of the total electron yield. This deviation is due to the gradual transformation to amorphous carbon under the influence of ion bombardment. The dependence of TEY on the primary electron energy, E v, is also modified by the ion beam; unirradiated diamond displays a value of TEY = 0.9+0.1 independent of Ep, in the 500 3000 eV range
444
A. Hoffman et al. / Secondary electron emission spectroscopy and total electron yield measurements
whilst irradiated diamond, amorphous carbon and graphite all display a monotonic decrease of TEY as a function of Ep.
References 1 R. F. Willis, B. Filton and G. S. Painter, Phys. Rev. B, 9 (1974) 1926. 2 A. Hoffman, M. Folman and S. Prawer, Phys. Rev. B, 1991, 44 (199l) 4680.
3 F.J. Himpsel, J. F. v o n d e r Veen and D.E. Eastman, Phys. Rev., B22 (1980) 1967. 4 J. F. Mowz, F. J. Himpsel, G. Hollinger, G. Hughes and J. L. Jordan, Phys. Rev. Lett., 54 (1985) 1960. 5 G.S. Painter, D. E. Ellis and A. R. Lubinsky, Phys. Rev., B4 (1971) 3610. 6 R. Kalish, T. Bernstein, B. Shopiro and A. Talmi, Radiat. Eft., I2 (1980) 153. 7 J. Cazoux, J. Appl. Phys., 59(5) (1986) 1418. 8 H. W. Werner and N. Warmoltz, J. Vac. Sci. Technol. A, 2 (1984) 726. 9 K. Ohya, K. Nishimura and I. Mori, Jap. J. Appl. Phys., 30(5) (1991) 1093, and references therein.