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Electron beam induced thermoluminescence from Li+ implanted Al2O3
353
Nuclear Instruments and Methods in Physics Research B23 (1987) 353-358
North-Holland, Amsterdam
ELECTRON BEAM INDUCED THERMOLUMINESCENCE C.N. WHANG,
FROM Li + IMPLANTED AI ,O,
T.K. KIM, S.T. KANG, Y.K. KOH, and CC. LEE
Depu~t~en~ of Ph.~s~~s, Yonsei b’niversity, Seoul. 120, Kareu
H.S. CHOE Depuriment
of Physics Education, Kyungsang
University, Ji;?iu 620, Korea
Received 7 September 1986 and in revised form 1.5 November 1986
Electron beam induced thermoluminescence (TL) from Li+ implanted Al,Os has been investigated to determine the effect of Li+ in Al,O, on the TL. Electron beam induced TL curves from an unimplanted sample show TL peaks at 375 K, 440 K, and 460 K. In the TL curves from a Li+ implanted sample, the 375 K and 460 K TL peaks disappear, while the 440 K TL peak is enhanced. The 375 K and 460 K peaks are assumed to be associated with F centers and the 440 K TL peak associated with Ft centers, thus the Li+ ions convert F centers to F’ centers through a recombination process with electrons trapped in the F centers. The 440 K TL peak from Li+ implanted Also, is sensitively dependent on the excitation energy of the electrons, which allows the depth profile of Li+ in Al,O, to be determined.
1. In~~on
2. Ex~~ment~
The thermoluminescence (TL) properties of pure and impurity doped A1203 have been of considerable interest over the past several years [1,2]. The electronic trap structure of single crystal Al *OS is of interest from both Eund~ent~ and practical points of view because this material is used as a substrate in the fabrication of silicon on sapphire microelectronic devices [3]. More
A schematic diagram of the experimental equipment is given in fig. 1. The cryostat was specially designed and constructed in order to measure TL and thermally stimulated electron emission (TSEE) following ion implantation and electron irradiation without destroying the good vacuum condition. The cryostat was evacuated to 1 X 1O-6 Torr by an oil diffusion pump with a LN2 cold trap. Aluminium oxide crystals (ESPI K-227, sapphire) are heated up to 600 K about 2 hours before the TL meas~ement in order to eliminate unwanted trapped carriers already existing inside the samples. Li+ ions passing through a magnetic mass analyser are implanted into the Al,O, using a 50 keV accelerator [6] attached to the TL measuring apparatus as shown in fig. 1. The Li’ ion source is of the thermionic emission type, and P-eucryptite is used as the source material. When Li+ ions are implanted, the typical Li’ ion beam current is about 0.5 PA/cm’. The diameter of the Li+ ion beam is controlled by varying the voltage on the electrostatic quadrupole lens and slit so as to implant ions onto the whole surface of the sample. The electron gun was also designed and constructed for this study. Electron irradiation is carried out in the range up to 3 keV. The diameter of the electron beam can be controlled by varying the voltage on the Einzel lens. The flux and incident energy of the electrons
recently, impurity doped A1203 has aroused increased interest in connection with practical applications, such
as radiation dosimeters [4]. The implanted ions can produce electron traps, hole traps or interstitial ions. When irradiated by some kinds of radiation (X-ray, y-ray, particle beams, UV light, etc.), these traps can be filled with electrons or holes. Then if the sample is heated at a constant heating rate, the trapped carriers are thermally released giving rise to TL. In the case where electrons are used as the exciting particles, the T’L intensity is very sensitive to the penetrating range of the electrons and the concentration of the traps [5]. In the present paper, electron beam induced TL from Li+ implanted Al,O, has been studied. Prom the electron beam induced TL curves of the unimplanted and Li+ implanted samples the effect of Li’ on the TL properties of Al,O, and the possibility of determining the depth profiles for Li+ ions in Al,O, using TL are studied. 0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (Nosh-Holland Physics Pub~s~ng Division)
Procedures
C.N. Whung et ~1. / Induced TL
354
TEMPERATURE CONTR0LLE.R
Fig. 1. Schematic
DIGITAL
diagram
MULTIMETER
of the experimental
apparatus
1
ENERGY
for measuring
electron
1.2
kev
1.4
kev
-.-.-.-
1.6 kev
350
the electron beam Prior to Li+ ion implantation, excited TL properties of M203 were studied. Fig. 2 shows the typical electron beam induced TL glow curves as a function of the electron energy. TL intensity in-
400 TEMPERATURE
which
3. Results and discussion
:
___---__
Fig. 2. TL glow curves versus electron
beam excited TL glow curves.
measured by a copper-constantan thermocouple is attached to the upper surface of the sample.
irradiated into the unimplanted Al,O, or Lif ion implanted samples were varied. Following evacuation down to 1 x 10m6 Torr, the Li+ ion implantation and electron irradiation are performed consecutively at room temperature. TL is detected by a PM tube mounted at the observation window. A variable heating rate is used over the entire temperature range, 300-500 K. The anode current from the PM tube is measured with an electrometer and displayed on the Y axis of an X-Y recorder. The temperature is
ELECTRON
fromLI + implunted AlJO_,
450
500
c K )
energy with fixed electron
dose of 8.0
x
lOI cm
‘.
C. N. Whang et al. / Induced TL
creases with increasing electron energy, which means that the TL intensity is affected sensitively by the depth of penetration of the electrons. In this study, we assume a uniformly irradiated cross-sectional area for all energies and a uniform distribution of traps. The irradiated volume is then proportional to the penetrating depth of the electrons and the TL intensity is also proportional to the number of traps in the irradiated volume. The results shown in fig. 2 strongly support this assumption and are similar to Lasky and Moran’s study [7] of the TL response of LiF to 0.1-S keV electrons in which they measured the penetration range of electrons in LiF using electron induced TL. Fig. 3 shows the TL glow curves for 1.6 keV electrons as a function of dose. A prominent glow peak occurs at 440 K and two shoulders at 375 and 460 K. The TL intensity at 440 K increases with increasing electron dose. TL intensity was measured with a constant heating rate of 6 K/min. The 440 K TL peak is well known [8] and is used in routine UV dosimetry. Kawamura and Royce [9] obtained this peak in TSC from X-ray irradiated Also,. They suggested that this peak was associated with electrons rather than holes. Fullerton and Moran [lo] also suggested that the defect center associated with this peak had a large capture cross-section for electrons similar to the Ff center. Overwhelming evidence has shown that F, Ff, and V centers exist in AlzOs [ll] and that the F and F+ centers are responsible for the electron traps [12]. The
fromLi + implanted AI,O,
355
optical absorption bands at 6.1 eV and 4.8 eV have been associated with F and F+ center, respectively [13]. Prior to TL experiments, optical absorption data was available [14] which showed, only the 6.1 eV absorption band, which means that we have primarily F centers in our samples. We assume that the F centers convert to F+ centers due to ionization during electron irradiation. If an electron is thermally released during TL measurement, it could recombine with a F+ center, converting it again to an excited F center whose deexcitation would produce the 440 K TL peak, while residual F centers would produce the 375 and 460 K TL peaks. Therefore as dosage or energy of electrons is increased, the concentration of residual F centers is reduced because of increasing ionization probability. In order to determine the effect of Li+ ion implantation on the electron beam excited TL, TL measurements were performed after Li+ implantation. The results are shown in fig. 4. Li+ ions were implanted at an energy of 10 keV and a dose of 8 X 1014 cm- 2. The electrons were irradiated with an energy of 1 keV and the same flux as the Li+ ions. The TL peak at 440 K is enhanced by a factor of 2 over the TL intensity of the unimplanted sample, while the two shoulders at 375 and 460 K disappear in the TL curves after Li+ ion implantation. Lee and Crawford [15] studied the F centers in Al,O, and found that F centers can be converted to F+ centers; that is, the thermally released holes recombine
2.7
ELECTRON ELECTRON
ENERGY DOSE
:
1.6
keV
:
A
c
c 3
.a.++-.
3.4X1d4cmZ
_.-._
6.8X10’4cni2
-----
2.0x1d6cmZ
1.8 d : ”
: 3 2 f 0.9
-
_I I-
300
350
400 TEMPERATURE
Fig. 3. TL glow curves versus electron
dose irradiated
450
500
CK)
to the sample with electron
energy of 1.6 keV.
356
C. N. Whang et al. / Induced TL from Li + implanted AlJO_, 6
-.-.-
Li+
ION
IMPLANTED
UNIMPLANTED A
300
350
450
400 TEMPERATURE
500
CK)
Fig. 4. Electron beam excited TL glow curves of Li+ ion implanted sample and unimplanted sample. The dotted line is TL glow curve from Li+ Implanted sample, and solid line is the TL glow curve from unimplanted sample. The electrons were irradiated with an energy of 1 keV. with F centers and produce Ft centers. We suggest that Li+ ions will produce singly charged interstitial cations and then recombine with F centers like holes to produce F+ centers. Therefore the 375 K and 460 K TL peaks associated with F centers would disappear, while the 440 K TL peak associated with F+ centers would be enhanced as shown in fig. 4. Optical absorption measurements confirmed [14] the presence of Ft centers (4.8 eV absorption band) in the sample implanted with a 1 X 10’” cm-* dose of 10 keV Li+ ions. If this assumption is correct, the TL intensity of the 440 K peak is directly proportional to the concentration range of electrons of Li+ ions within the penetration [16]. The electrons and ions irradiated into solid materials show Gaussian concentration distributions. Therefore, if the peak position of electron concentration coincides with the peak position of Li+ ion concentration, the TL intensity would be maximized. While the penetration range of electrons could be shorter or longer than the penetration range of Li+ ions, the TL intensity would be smaller than maximum intensity when the penetrating range of electrons coincides with that of the Li+ ions. Therefore the depth profile of the Li+ ion distribution could be determined by varying the bombarding energy of the electrons. In order to determine the depth profile of the Li+ ions in Al,O, using the TL technique based on this assumption, electrons were irradiated into Li+ im-
planted samples with energy varying from 0.4 keV to 2 keV; then TL measurements were performed. The results are shown in fig. 5, where the Li+ ions were implanted with an energy of 10 keV and a dose of 8 x lOi cm 2. The dose of electrons is the same as that of the Lit ions. The incident energy of the Li’ ions corresponds to a projected range of about 320 A by the well-known LSS theory [17]. The projOected range of 1 keV electrons in Al,O, is about 290 A [18], which nearly coincides with the projected range of 10 keV Li’ ions. In fig. 5, the TL intensity for the 1 keV electron irradiated Al,Os is larger than that for the 0.6 keV and 1.6 keV electron irradiated ones by a factor of 100. Therefore these results strongly support our assumption. Recently, the electron beam induced TSEE technique which is based on the same assumption as ours has been of interest as a nondestructive method for investigating depth profiles or damage profiles [19,20]. Arnold and Vook [21] presented the first measurements on the depth distribution of energy deposited in electronic processes by H+, He+ using radiophotoluminescence. and Oi implantation Malik et al. [22] recognized that the TL technique can be used as a means of determining the quality of high purity quartz and that it holds high promise for providing comparable information for the Al-M+ center of quartz with M+ either Li+ or Nat. There are, however, few studies using the TL technique for damage profile
C.N. Whung et al. / Induced TL from Li + imphnted
ELECTRON
ENERGY
_______
0.6
kev
-.-.-.-
1 .O
kev
1.6
kev
300
350
ion implanted
400
450
51
0
c K >
sample. The 10 keV energy of Li+ electron corresponds to 290 .&
impurity concentration profile measurements. Our results show that the TL technique should be recognized as having the potential to nondestructively measure impurity concentrations from a simple experiment, if a TL glow peak corresponds to the presence of a particular type of defect, and the intensity of the peak is proportional to the concentration of this type of defect. Fig. 6 shows the TL intensity from the Li+ implanted sample as a function of electron energy or penetrating range of electrons. The experimental point at given electron energies is the TL intensity of the Li+ implanted sample at 440 K less the TL intensity of the unimplanted sample at 440 K. The projected range of electrons was calculated by Young’s empirical formula [18]. Even though the experimental projected range (R r) of Li+ ions is slightly different from the theoretical one, the standard deviation of R,(A RJ is similar to the theoretical one; the value of AR, an fig. 6 is about 55 A, while the theoretical value is 48 A [23]. In thermal processes such as TL, TSC and TSEE, the most important parameters are activation energy, frequency factor and kinetic order [24]. These parameters for the Li+ ion implanted sample are evaluated in this study. In fig. 4, the fwhm of the TL curve for the Li+ implanted sample is AT = AT_ + AT+ = 63 K, and the half-width on the high temperature side of T, is 25 K (AT+ = 25 K). Therefore AT+/AT is 0.4, which means that the 440 K TL peak is due to a recombinaor
357
:
TEMPERATURE
Fig. 5. TL glow curves of Li’
AlJO_,
ion corresponds
to 320 A and 1 keV energy
of
tion dominated process or 1st order kinetics [24]. The TL intensity for the 1st order kinetic process the initial stage of the heating process is described
at as
lo22
Fig. 6. TL intensity of 440 K peak for Li+ implanted sample as a function of electron range. Electron range is calculated by Young’s formula, R = 0.0115 E1.35, where R is expressed in mg/cm* and E in keV.
C.N. Whang et al. / Induced TL from Li + implanted AlJO
358
follows [25]: I(T)
=.rns
exp(-E/kT),
(1)
where s is the frequency factor, E is the thermal activation energy, n,, is the initial concentration of free carriers trapped in trap centers, k is Boltzmann’s constant. Eq. (1) suggests that one should plot In Z(T) as a function of l/T. This method, which is the well-known initial rise method, was used to determined the thermal activation energy of the 440 K TL peak. This method yields an activation energy of 1.12 eV, which is in good agreement with the result of Kawamura [9] of 1.12 f 0.1 eV, and also the result of Cooke et al. [8]. The frequency factor s is calculated from the following expression [26], s = (b/k)(E/Ti)
exp(E/kT,),
(2)
where T, is the TL peak temperature and b is the heating rate (K/s). The frequency factor was found to be 4.5 x 1O’O s-l. The frequency factor obtained by Cooke [8] is 7.0 X 10” s-l and Kawamura’s result [9] is 1.0 x 10” s-r. Depth profiling experiments using the TL technique for Na+ and K+ implanted into Al,O, and SiO, are in progress in our laboratory. And optical absorption measurements for ion implanted Al,O, and SiO, are also in progress.
4. Conclusions Electron beam induced TL characteristics of Li+ implanted and unimplanted Al,O, have been investigated over the temperature range 300-500 K. Three TL peaks are obtained at 375, 440 and 460 K from the unimplanted sample. After Li+ implantation, the 440 K TL peak is enhanced, while the 375 and 460 K TL peaks disappear. The 440 K TL peak is assumed to be associated with the F+ center which has a large electron capture cross-section, and it is suggested that the other peaks are associated with the F center. Also, Li+ ions are assumed to form singly charged interstitial cations during or after implantation and then to recombine with electrons trapped in F centers to produce the F+ centers. From the electron beam induced TL curves of Li+ implanted Al,O, and unimplanted samples, we confirmed the possibility of depth profile determination using the TL technique; the TL measurement is shown
to be useful for nondestructive determination profiles like TSEE in this study.
of depth
References HI P.W. Levy, Phys. Rev. 123 (1961) 1226. PI W. Runciman, Solid State Commun. 6 (1968) 537. ]31 K.F. Galloway and S. Mayo, IEEE Trans. Electron Devices ED-25 (1978) 549. and S. Rudin, Health Phys. 19 (1970) 141 R.S. McDougall 281. ]51 V.K. Jain, S.P. Kathusia, and A.K. Ganguly, J. Phys. C7 (1974) 3810. 161 C.C. Lee, Y.K. Koh, and C.N. Whang, J. Korean Phys. sot. 12 (1979) 77. ]71 J.B. Lasky and P.R. Moran, J. Appl. Phys. 50 (1979) 4951. J. Appl. 181 D.W. Cooke, H.E. Roberts and J.C. Alexander, Phys. 49 (1978) 3451. and B.S.H. Royce, Phys. Status Solidi A50 ]91 S. Kawamura (1978) 66a. HOI G.D. Fullerton and P.R. Moran, Med. Phys. 1 (1974) 161. ]lll K.H. Lee and J.H. Crawford Jr., Phys. Rev. B15 (1977) 4065. Phys. Status Solidi A57 ]l21 A. Rehavi and N. Kristianpoller, (1980) 221. ]l31 B. Jeffries and G.P. Summers, J. Appl. Phys. 51 (1980) 3984. ]l41 C.N. Whang, C.C. Lee, T.K. Kim, and H.S. Choe, to be published. ]l51 K.H. Lee and J.H. Crawford Jr., Phys. Rev. B19 (1979) 3217. 1161 E.J. Kobetich and R. Katz, Phys. Rev. 170 (1968) 391. ]l71 H.E. Schiott, Radiat Eff. 6 (1970) 107. 1181 J.R. Young, Phys. Rev. 103 (1956) 292. T. Kakiage, Y. Kido, K. Oda, and T. 1191 M. Kawanishi, Yamamoto, Jpn. J. Appl. Phys. Suppl. 24 (4) (1985) 238. ]201 H. Glaefeke and H.J. Fitting, Prod. 6th Int. Symp. on Exoelectron Emission and Application (Ahrenshoop, GOR, 1979) p. 178. 1211 G.W. Arnold and F.L. Vook, Radiat. Eff. 14 (1972) 157. ]221 D.M. Malik, E.E. Kohnke, and W.A. Shibley, J. Appl. Phys. 52 (1981) 3600. ]231 J.W. Mayer, L. Eriksson, and J.A. Davies, Ion Implantation in Semiconductors (Academic Press, London, 1970) p. 32. ~241 R. Chen and Y. Kirsh, Analysis of Thermally Stimulated Processes (Pergamon, New York, 1981) p. 160. ~251 G.F.J. Garlick and K. Gibson, Proc. Phys. Sot. (London) 60 (1948) 574. ]261 Ref. [24], p. 9.
Nuclear Instruments and Methods in Physics Research B23 (1987) 353-358
North-Holland, Amsterdam
ELECTRON BEAM INDUCED THERMOLUMINESCENCE C.N. WHANG,
FROM Li + IMPLANTED AI ,O,
T.K. KIM, S.T. KANG, Y.K. KOH, and CC. LEE
Depu~t~en~ of Ph.~s~~s, Yonsei b’niversity, Seoul. 120, Kareu
H.S. CHOE Depuriment
of Physics Education, Kyungsang
University, Ji;?iu 620, Korea
Received 7 September 1986 and in revised form 1.5 November 1986
Electron beam induced thermoluminescence (TL) from Li+ implanted Al,Os has been investigated to determine the effect of Li+ in Al,O, on the TL. Electron beam induced TL curves from an unimplanted sample show TL peaks at 375 K, 440 K, and 460 K. In the TL curves from a Li+ implanted sample, the 375 K and 460 K TL peaks disappear, while the 440 K TL peak is enhanced. The 375 K and 460 K peaks are assumed to be associated with F centers and the 440 K TL peak associated with Ft centers, thus the Li+ ions convert F centers to F’ centers through a recombination process with electrons trapped in the F centers. The 440 K TL peak from Li+ implanted Also, is sensitively dependent on the excitation energy of the electrons, which allows the depth profile of Li+ in Al,O, to be determined.
1. In~~on
2. Ex~~ment~
The thermoluminescence (TL) properties of pure and impurity doped A1203 have been of considerable interest over the past several years [1,2]. The electronic trap structure of single crystal Al *OS is of interest from both Eund~ent~ and practical points of view because this material is used as a substrate in the fabrication of silicon on sapphire microelectronic devices [3]. More
A schematic diagram of the experimental equipment is given in fig. 1. The cryostat was specially designed and constructed in order to measure TL and thermally stimulated electron emission (TSEE) following ion implantation and electron irradiation without destroying the good vacuum condition. The cryostat was evacuated to 1 X 1O-6 Torr by an oil diffusion pump with a LN2 cold trap. Aluminium oxide crystals (ESPI K-227, sapphire) are heated up to 600 K about 2 hours before the TL meas~ement in order to eliminate unwanted trapped carriers already existing inside the samples. Li+ ions passing through a magnetic mass analyser are implanted into the Al,O, using a 50 keV accelerator [6] attached to the TL measuring apparatus as shown in fig. 1. The Li’ ion source is of the thermionic emission type, and P-eucryptite is used as the source material. When Li+ ions are implanted, the typical Li’ ion beam current is about 0.5 PA/cm’. The diameter of the Li+ ion beam is controlled by varying the voltage on the electrostatic quadrupole lens and slit so as to implant ions onto the whole surface of the sample. The electron gun was also designed and constructed for this study. Electron irradiation is carried out in the range up to 3 keV. The diameter of the electron beam can be controlled by varying the voltage on the Einzel lens. The flux and incident energy of the electrons
recently, impurity doped A1203 has aroused increased interest in connection with practical applications, such
as radiation dosimeters [4]. The implanted ions can produce electron traps, hole traps or interstitial ions. When irradiated by some kinds of radiation (X-ray, y-ray, particle beams, UV light, etc.), these traps can be filled with electrons or holes. Then if the sample is heated at a constant heating rate, the trapped carriers are thermally released giving rise to TL. In the case where electrons are used as the exciting particles, the T’L intensity is very sensitive to the penetrating range of the electrons and the concentration of the traps [5]. In the present paper, electron beam induced TL from Li+ implanted Al,O, has been studied. Prom the electron beam induced TL curves of the unimplanted and Li+ implanted samples the effect of Li’ on the TL properties of Al,O, and the possibility of determining the depth profiles for Li+ ions in Al,O, using TL are studied. 0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (Nosh-Holland Physics Pub~s~ng Division)
Procedures
C.N. Whung et ~1. / Induced TL
354
TEMPERATURE CONTR0LLE.R
Fig. 1. Schematic
DIGITAL
diagram
MULTIMETER
of the experimental
apparatus
1
ENERGY
for measuring
electron
1.2
kev
1.4
kev
-.-.-.-
1.6 kev
350
the electron beam Prior to Li+ ion implantation, excited TL properties of M203 were studied. Fig. 2 shows the typical electron beam induced TL glow curves as a function of the electron energy. TL intensity in-
400 TEMPERATURE
which
3. Results and discussion
:
___---__
Fig. 2. TL glow curves versus electron
beam excited TL glow curves.
measured by a copper-constantan thermocouple is attached to the upper surface of the sample.
irradiated into the unimplanted Al,O, or Lif ion implanted samples were varied. Following evacuation down to 1 x 10m6 Torr, the Li+ ion implantation and electron irradiation are performed consecutively at room temperature. TL is detected by a PM tube mounted at the observation window. A variable heating rate is used over the entire temperature range, 300-500 K. The anode current from the PM tube is measured with an electrometer and displayed on the Y axis of an X-Y recorder. The temperature is
ELECTRON
fromLI + implunted AlJO_,
450
500
c K )
energy with fixed electron
dose of 8.0
x
lOI cm
‘.
C. N. Whang et al. / Induced TL
creases with increasing electron energy, which means that the TL intensity is affected sensitively by the depth of penetration of the electrons. In this study, we assume a uniformly irradiated cross-sectional area for all energies and a uniform distribution of traps. The irradiated volume is then proportional to the penetrating depth of the electrons and the TL intensity is also proportional to the number of traps in the irradiated volume. The results shown in fig. 2 strongly support this assumption and are similar to Lasky and Moran’s study [7] of the TL response of LiF to 0.1-S keV electrons in which they measured the penetration range of electrons in LiF using electron induced TL. Fig. 3 shows the TL glow curves for 1.6 keV electrons as a function of dose. A prominent glow peak occurs at 440 K and two shoulders at 375 and 460 K. The TL intensity at 440 K increases with increasing electron dose. TL intensity was measured with a constant heating rate of 6 K/min. The 440 K TL peak is well known [8] and is used in routine UV dosimetry. Kawamura and Royce [9] obtained this peak in TSC from X-ray irradiated Also,. They suggested that this peak was associated with electrons rather than holes. Fullerton and Moran [lo] also suggested that the defect center associated with this peak had a large capture cross-section for electrons similar to the Ff center. Overwhelming evidence has shown that F, Ff, and V centers exist in AlzOs [ll] and that the F and F+ centers are responsible for the electron traps [12]. The
fromLi + implanted AI,O,
355
optical absorption bands at 6.1 eV and 4.8 eV have been associated with F and F+ center, respectively [13]. Prior to TL experiments, optical absorption data was available [14] which showed, only the 6.1 eV absorption band, which means that we have primarily F centers in our samples. We assume that the F centers convert to F+ centers due to ionization during electron irradiation. If an electron is thermally released during TL measurement, it could recombine with a F+ center, converting it again to an excited F center whose deexcitation would produce the 440 K TL peak, while residual F centers would produce the 375 and 460 K TL peaks. Therefore as dosage or energy of electrons is increased, the concentration of residual F centers is reduced because of increasing ionization probability. In order to determine the effect of Li+ ion implantation on the electron beam excited TL, TL measurements were performed after Li+ implantation. The results are shown in fig. 4. Li+ ions were implanted at an energy of 10 keV and a dose of 8 X 1014 cm- 2. The electrons were irradiated with an energy of 1 keV and the same flux as the Li+ ions. The TL peak at 440 K is enhanced by a factor of 2 over the TL intensity of the unimplanted sample, while the two shoulders at 375 and 460 K disappear in the TL curves after Li+ ion implantation. Lee and Crawford [15] studied the F centers in Al,O, and found that F centers can be converted to F+ centers; that is, the thermally released holes recombine
2.7
ELECTRON ELECTRON
ENERGY DOSE
:
1.6
keV
:
A
c
c 3
.a.++-.
3.4X1d4cmZ
_.-._
6.8X10’4cni2
-----
2.0x1d6cmZ
1.8 d : ”
: 3 2 f 0.9
-
_I I-
300
350
400 TEMPERATURE
Fig. 3. TL glow curves versus electron
dose irradiated
450
500
CK)
to the sample with electron
energy of 1.6 keV.
356
C. N. Whang et al. / Induced TL from Li + implanted AlJO_, 6
-.-.-
Li+
ION
IMPLANTED
UNIMPLANTED A
300
350
450
400 TEMPERATURE
500
CK)
Fig. 4. Electron beam excited TL glow curves of Li+ ion implanted sample and unimplanted sample. The dotted line is TL glow curve from Li+ Implanted sample, and solid line is the TL glow curve from unimplanted sample. The electrons were irradiated with an energy of 1 keV. with F centers and produce Ft centers. We suggest that Li+ ions will produce singly charged interstitial cations and then recombine with F centers like holes to produce F+ centers. Therefore the 375 K and 460 K TL peaks associated with F centers would disappear, while the 440 K TL peak associated with F+ centers would be enhanced as shown in fig. 4. Optical absorption measurements confirmed [14] the presence of Ft centers (4.8 eV absorption band) in the sample implanted with a 1 X 10’” cm-* dose of 10 keV Li+ ions. If this assumption is correct, the TL intensity of the 440 K peak is directly proportional to the concentration range of electrons of Li+ ions within the penetration [16]. The electrons and ions irradiated into solid materials show Gaussian concentration distributions. Therefore, if the peak position of electron concentration coincides with the peak position of Li+ ion concentration, the TL intensity would be maximized. While the penetration range of electrons could be shorter or longer than the penetration range of Li+ ions, the TL intensity would be smaller than maximum intensity when the penetrating range of electrons coincides with that of the Li+ ions. Therefore the depth profile of the Li+ ion distribution could be determined by varying the bombarding energy of the electrons. In order to determine the depth profile of the Li+ ions in Al,O, using the TL technique based on this assumption, electrons were irradiated into Li+ im-
planted samples with energy varying from 0.4 keV to 2 keV; then TL measurements were performed. The results are shown in fig. 5, where the Li+ ions were implanted with an energy of 10 keV and a dose of 8 x lOi cm 2. The dose of electrons is the same as that of the Lit ions. The incident energy of the Li’ ions corresponds to a projected range of about 320 A by the well-known LSS theory [17]. The projOected range of 1 keV electrons in Al,O, is about 290 A [18], which nearly coincides with the projected range of 10 keV Li’ ions. In fig. 5, the TL intensity for the 1 keV electron irradiated Al,Os is larger than that for the 0.6 keV and 1.6 keV electron irradiated ones by a factor of 100. Therefore these results strongly support our assumption. Recently, the electron beam induced TSEE technique which is based on the same assumption as ours has been of interest as a nondestructive method for investigating depth profiles or damage profiles [19,20]. Arnold and Vook [21] presented the first measurements on the depth distribution of energy deposited in electronic processes by H+, He+ using radiophotoluminescence. and Oi implantation Malik et al. [22] recognized that the TL technique can be used as a means of determining the quality of high purity quartz and that it holds high promise for providing comparable information for the Al-M+ center of quartz with M+ either Li+ or Nat. There are, however, few studies using the TL technique for damage profile
C.N. Whung et al. / Induced TL from Li + imphnted
ELECTRON
ENERGY
_______
0.6
kev
-.-.-.-
1 .O
kev
1.6
kev
300
350
ion implanted
400
450
51
0
c K >
sample. The 10 keV energy of Li+ electron corresponds to 290 .&
impurity concentration profile measurements. Our results show that the TL technique should be recognized as having the potential to nondestructively measure impurity concentrations from a simple experiment, if a TL glow peak corresponds to the presence of a particular type of defect, and the intensity of the peak is proportional to the concentration of this type of defect. Fig. 6 shows the TL intensity from the Li+ implanted sample as a function of electron energy or penetrating range of electrons. The experimental point at given electron energies is the TL intensity of the Li+ implanted sample at 440 K less the TL intensity of the unimplanted sample at 440 K. The projected range of electrons was calculated by Young’s empirical formula [18]. Even though the experimental projected range (R r) of Li+ ions is slightly different from the theoretical one, the standard deviation of R,(A RJ is similar to the theoretical one; the value of AR, an fig. 6 is about 55 A, while the theoretical value is 48 A [23]. In thermal processes such as TL, TSC and TSEE, the most important parameters are activation energy, frequency factor and kinetic order [24]. These parameters for the Li+ ion implanted sample are evaluated in this study. In fig. 4, the fwhm of the TL curve for the Li+ implanted sample is AT = AT_ + AT+ = 63 K, and the half-width on the high temperature side of T, is 25 K (AT+ = 25 K). Therefore AT+/AT is 0.4, which means that the 440 K TL peak is due to a recombinaor
357
:
TEMPERATURE
Fig. 5. TL glow curves of Li’
AlJO_,
ion corresponds
to 320 A and 1 keV energy
of
tion dominated process or 1st order kinetics [24]. The TL intensity for the 1st order kinetic process the initial stage of the heating process is described
at as
lo22
Fig. 6. TL intensity of 440 K peak for Li+ implanted sample as a function of electron range. Electron range is calculated by Young’s formula, R = 0.0115 E1.35, where R is expressed in mg/cm* and E in keV.
C.N. Whang et al. / Induced TL from Li + implanted AlJO
358
follows [25]: I(T)
=.rns
exp(-E/kT),
(1)
where s is the frequency factor, E is the thermal activation energy, n,, is the initial concentration of free carriers trapped in trap centers, k is Boltzmann’s constant. Eq. (1) suggests that one should plot In Z(T) as a function of l/T. This method, which is the well-known initial rise method, was used to determined the thermal activation energy of the 440 K TL peak. This method yields an activation energy of 1.12 eV, which is in good agreement with the result of Kawamura [9] of 1.12 f 0.1 eV, and also the result of Cooke et al. [8]. The frequency factor s is calculated from the following expression [26], s = (b/k)(E/Ti)
exp(E/kT,),
(2)
where T, is the TL peak temperature and b is the heating rate (K/s). The frequency factor was found to be 4.5 x 1O’O s-l. The frequency factor obtained by Cooke [8] is 7.0 X 10” s-l and Kawamura’s result [9] is 1.0 x 10” s-r. Depth profiling experiments using the TL technique for Na+ and K+ implanted into Al,O, and SiO, are in progress in our laboratory. And optical absorption measurements for ion implanted Al,O, and SiO, are also in progress.
4. Conclusions Electron beam induced TL characteristics of Li+ implanted and unimplanted Al,O, have been investigated over the temperature range 300-500 K. Three TL peaks are obtained at 375, 440 and 460 K from the unimplanted sample. After Li+ implantation, the 440 K TL peak is enhanced, while the 375 and 460 K TL peaks disappear. The 440 K TL peak is assumed to be associated with the F+ center which has a large electron capture cross-section, and it is suggested that the other peaks are associated with the F center. Also, Li+ ions are assumed to form singly charged interstitial cations during or after implantation and then to recombine with electrons trapped in F centers to produce the F+ centers. From the electron beam induced TL curves of Li+ implanted Al,O, and unimplanted samples, we confirmed the possibility of depth profile determination using the TL technique; the TL measurement is shown
to be useful for nondestructive determination profiles like TSEE in this study.
of depth
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