- Email: [email protected]
Effects of the ion-implantation on the thermoluminescence spectra of strontium titanate
Nuclear Instruments and Methods in Physics Research B 226 (2004) 549–555 www.elsevier.com/locate/nimb
Effects of the ion-implantation on the thermoluminescence spectra of strontium titanate B. Yang
a,b,*
, P.D. Townsend b, Y. Fan a, R. Fromknecht
c
a
Department of Physics, Beijing Normal University, Beijing 100875, China Science and Technology, University of Sussex, Brighton BN1 9QH, UK Forschungszentrum Karlsruhe, Institut fuer Festkoerperphysik, D-76021 Karlsruhe, Germany b
c
Received 14 April 2004; received in revised form 27 July 2004
Abstract Thermoluminescence of strontium titanate at low temperature is characterised by long wavelength emission centred near 800 nm which contains at least three overlapping luminescence bands. The thermoluminescence is excited by X-ray irradiation at 25 K of an entire sample. Room temperature ion-implantation into the surface layer changes the low temperature TL signals both in terms of their relative intensities and peak temperatures, as well as modifying the emission spectra. Such an intense perturbation of the bulk signals resulting from surface ion beam implantation is extremely unusual. However, it is well documented that for strontium titanate there are a variety of low temperature relaxations of the structure, and even phase transitions, which are highly sensitive to the presence of intrinsic defects, impurities and stresses. The ion-implantation damage in the surface is thus thought to act as a stress nucleation zone from which such relaxations can propagate throughout the entire crystal. There are consequent changes in the thermoluminescence in terms of defect stability and glow peak temperature. Details of such changes and modifications of emission spectra are reported. 2004 Published by Elsevier B.V. PACS: 78.60.K; 6180; 6172.Ww Keywords: Strontium titanate; Thermoluminescence; Phase changes and relaxations
1. Introduction
* Corresponding author. Address: Department of Physics, Beijing Normal University, Beijing 100875, China. Tel.: +86 10 62208417; fax: +86 10 62200141. E-mail address: [email protected] (B. Yang).
0168-583X/$ - see front matter 2004 Published by Elsevier B.V. doi:10.1016/j.nimb.2004.08.002
The wavelength, intensity and temperature dependence of luminescence signals are widely used to provide highly sensitive monitors of imperfections in insulating crystals since variations in local lattice site configurations are reflected by
550
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
variations of the luminescence. The detailed data offer information on the generation, decay and conversion of the various types of defect, especially those associated with the impurities or impurity-intrinsic defect complexes. Less studied is how they change as a function of modifications of the crystalline phases and other structural parameters. Defect studies and recognition of low temperature phase changes, and/or relaxations, have been reported in numerous works for strontium titanate (STO) using many experimental methods [1–7]. There are conflicting reports as to the magnitude, or even presence, of the changes, and it is assumed that the problems occur because the data may be sample or technique dependent. This assumption might imply that such sensitivity in the variations of the lattice structure occurs because the energies involved are small, and thus influenced by defects or stress etc. Consequently the phase transitions or relaxations do not always occur as a particular phase may be stabilised as the result of impurities, stress or other defects. In other words, defects are playing a key role in the phase stability of crystalline STO samples. Since ion beam implantation of the crystal surface is associated with large stress changes extending beyond the implant zone, introduction of very high concentrations of intrinsic defects, and injection of impurities, implantation is likely to be particularly crucial in altering the stability of closely related phase situations. Indeed, recent work on the radioluminescence (RL) of ion-implanted and unimplanted STO has been reported [8] which shows remarkable features indicating that not only are there surface changes, where the implant occurs, but also that this surface treatment can initiate changes throughout the bulk of the crystal. The anomalies of the RL signals were observed in the heavily implanted samples (5 · 1016 to 1 · 1017 ions/cm2 for Au ions, 1 · 1015 to 5 · 1016 ions/cm2 for Sb ions). In those samples, the wavelength and intensity of the RL emissions below 65 K were totally different from those in the unimplanted sample, even though RL is a monitor of bulk properties. The interpretation of that data was that the ion-implantation not only introduced a range of defects and lattice disorder, but the consequent
stress fields penetrate deeper into the lattice. Overall this metastable situation is ideal for the relaxation and nucleation of crystalline phase variants as the sample temperature is changed. Note in particular that in strontium titanate many of the anomalies and discontinuities in properties are thought to originate from discrete temperature related relaxations rather than pure phase changes. Hence, stress and defects may be particularly effective in these cases. The original report on the ion-implanted material is here complemented by thermoluminescence (TL) spectral data from ion-implanted strontium titanate. As appeared in the RL case, good transparent single crystalline samples were used, and so it is possible to contrast TL from bulk and surface modified material. The data presented here confirm that surface implants can influence signals from the entire specimen.
2. Experimental comments The 5 · 5 · 1 mm strontium titanate (STO) samples were either unimplanted or had been implanted with gold ions at 240 keV or antimony at 260 keV at RT. Ion doses for Au were 5 · 1013, 5 · 1014, 5 · 1016 and 1 · 1017 ions/cm2 and for Sb the doses were 1 · 1015 and 5 · 1016 ions/cm2. None of the samples discussed here had been subject to high temperature annealing. In order to perform thermoluminescence measurements the samples were cooled to 25 K and irradiated with X-rays with a dose of 200 Gy. The thermoluminescence signals were recorded with a high-sensitivity wavelength multiplexed system [8,9] at a heating rate of 6 K/min. Although the irradiations were near 25 K there were non-reproducible variations in the initial TL signals below 35 K and hence the only data presented here are for those above 40 K, where consistent results were obtained. The TL spectrometer has two detectors covering the wavelength ranges 200–410 and 410–830 nm, with a wavelength resolution of 4 nm. All spectra were corrected for the wavelength dependent response of the system. For the implanted samples, the implanted surface was the upper surface facing the spectrometer, i.e. the unimplanted surface was in
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
contact with the heater stage. This arrangement was chosen to try to optimise any features related to the very thin implant layers, however it is likely to influence the emission intensity from the bulk if there is absorption within the implant layer. Such absorption might reduce the intensity of bulk signals but is unlikely to influence their TL peak temperatures.
3. Results The emission spectra are unusual compared with those recorded from most oxides (e.g. quartz, silica, alumina) in that the emission is almost totally confined to intense signals at the red end of the spectrum with negligible signals near 400 nm as are seen in many other oxide materials. Fig. 1 contrasts the signals in isometric plots between
551
unimplanted and implanted samples, from which one notes the dominant glow peak is recorded at about 125–155 K, together with some other weaker TL peaks. No obvious TL signals were observed above 240 K. The emission signals in all samples were only significant in the 750–830 nm in the temperature range of this work. In Fig. 1 the isometric TL spectra are for unimplanted STO and for data taken after the 5 · 1013/cm2 Au implant. On comparing unimplanted and implanted samples, no major additional thermal peaks or emission bands were added by the implantation but in detail there are definite changes in the peak temperatures, the relative intensities of the component TL peaks and some minor spectral broadening (discussed later). The observation of major changes caused by ion-implantation of the surface is remarkable since the thermoluminescence is stimulated by X-ray
(a)
(b) Fig. 1. The X-ray excited thermoluminescence emission spectra of (a) a sample of unimplanted strontium titanate and (b) a sample which had received a 5 · 1013/cm2 Au implant at 300 K.
552
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
irradiation of the sample which includes both the bulk substrate and the very thin surface layer modified by the ion beam implantation. The fact that the bulk volume greatly exceeds the implanted zone by almost 104 times suggests that any additional signal, specifically from the implant zone, would need to be very intense per unit volume in order to contribute measurably to the total signal. If the surface signals are in addition to those from the bulk then both signals should be apparent. The fact that the data differ in terms of TL peak temperature implies that the surface changes must have strongly modified the bulk properties. Further, neither the details of the TL curves nor the spectra are identical between the different samples
as a result of different ion species and implant doses. It is reasonable to consider that the signals originate primarily from the greater volume of bulk material and that most changes must have occurred because of perturbations driven by the surface implantations. Details of the emission spectra are included in Fig. 2 at selected temperatures for pure unimplanted material (Fig. 2(a)) and implanted samples (Fig. 2(b)). The 60, 100, 125 and 185 K temperature slices extracted from the data for Fig. 1(a) are presented in Fig. 2(a). The intensities are normalised at 795 nm. An overall broad band is recorded which can be analysed in terms of at least three emission components at about 780, 795 and
STO 1.2
(a)
185K 125K 100K 60K
1.0 0.8 0.6
Intensity (normalized)
0.4 0.2 0.0
1.2
STO STO:Au(5x10 13 ) STO:Au(1x10 17 ) STO:Sb(5x10 16 )
(b)
1.0 0.8 0.6 0.4 0.2 0.0 750
760
770
780
790
800
810
820
830
Wavelength (nm) Fig. 2. Details of the emission spectra obtained from: (a) the unimplanted material and (b) implanted crystals. The intensities are normalised at 795 nm.
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
810 nm. The component bands can be found in every spectrum, but their relative intensities differ with temperature. In addition, the changes may subsume small variations caused by movements of the component peaks with temperature. The shorter wavelength components are relatively weaker at lower temperatures. The emission spectra of the main TL peak in the unimplanted, the 5 · 1013, 1 · 1017 Au-implanted and the 5 · 1016 Sb-implanted samples are contrasted in Fig. 2(b). Once again the first estimate suggests that three bands are required to model the overall shape. Within the precision of the data the same three components might be adequate to describe the observations. Only minor differences of the band positions and the relative intensities between the three bands were detected in the 1 · 1017 Au- and 5 · 1016 Sb-implanted samples. The relative intensities between the emission bands in the 5 · 1013 Au-implanted example are obviously different from those in other samples. It is difficult to tell precisely the exact band positions in that sample, as some of the overlapping bands having almost equal intensities. The glow curves integrated over the 790–800 nm spectral range in the heavily Sb ion-implanted (1 · 1015 and 5 · 1016/cm2), the heavily Au ion-implanted (5 · 1016 and 1 · 1017/cm2) and the lightly Au-implanted (5 · 1013 and 1 · 1014/cm2) samples studied in this work are all shown in Figs. 3(a)– (c) respectively. For reference purposes the TL of an unimplanted sample is provided. The common features found in all the three figures are that firstly, the peak temperature of the main TL peak in the implanted samples is higher than that in the unimplanted sample, and secondly the intensity of that peak in the implanted samples is lower than that in the unimplanted one. For the gold implants the maximum peak temperature displacement occurs for the ion dose of 1014 ions/cm2 with an upward shift in temperature of 25. Normal reproducibility of these measurements is always much better than 5, hence the peak movements displayed here are very significant. Similar peak movement effects are noted for the antimony implants. In the range between 40 and 90 K other minor TL peaks differ between the various sample conditions.
553
It is also apparent that there are variations in the peak positions and intensities of the minor TL signals. There was not a clear pattern for these changes. Two weak glow peaks are seen below 90 K and one at about 185 K, as shown in Figs. 3(a) and (b). These were detected in the unimplanted and heavily implanted samples. The peak temperatures of the two peaks below 90 K, as well as that of the main peak, move to higher temperature in the implanted samples. The low temperature peak shifts increase with the larger doses whereas the 185 K peak remains at almost the same temperature in most of the glow curves. By contrast, in the samples implanted with low doses, as shown in Figs. 1(a) and 3(c), small TL peaks were observed below 90 K and near 215 K, and the shift of the temperature of the main TL peak is even larger than that in the heavily implanted samples. As mentioned earlier the selected experimental geometry was to place the implanted faces nearest to the spectrometers. This introduces some absorption of signals from the bulk but might have aided detection of any signals directly from the implant layer. Indeed the signals are less than for the unimplanted sample, but on comparing the glow curves of the 5 · 1013, 1 · 1014 and 5 · 1016/cm2 Au ionimplanted samples, it is found that the intensity of the main TL peak in the former two samples is even lower than that for the higher dose case. A simple model of absorption increasing with implant dose is thus compromised.
4. Discussion As mentioned initially, the earlier radioluminescence data [8] indicated that surface ion beam implantation strongly modified the radioluminescence signals which arise from the bulk of the material, and the suggestion was that the distorted surface layer was effective in nucleating relaxations and/or phase transitions of the bulk material during heating and cooling of the crystals. In RL data the effects included a sharp intensity discontinuity near 65 K in the near infra-red emission bands and strong new visible/ emission bands at lower temperatures. The detailed origin
554
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
STO:Sb 15000
(a)
unimplanted 1x1015 /cm2 5x1016 /cm2
12000 9000 6000 3000 0 5000
(b)
STO:Au unimplanted(x30%) 5x1016 /cm 2 1x1017 /cm2
Intensity (arb. unit)
4000 3000 2000 1000 0 5000
STO:Au
(c)
unimplanted(x30%) 5x1013 /cm2 1x1014 /cm2
4000 3000 2000 1000 0 40
90
140
190
240
Temperature (K) Fig. 3. The TL glow curves obtained from the unimplanted, antimony and gold implanted samples of STO. The data are the integrations of the red wavelength emission.
of sites which could initiate the changes was unclear but lattice distortions, imperfections, nanoparticle precipitates and/or stress fields are all possible candidates. Indeed, for a sensitive metastable crystal structure the relaxations might be induced by any of these factors. The sensitisation effect depends on the degree of damage, or amorphisation of the implant layer and the altered radioluminescence features were most clearly observed in the heavily implanted samples. By contrast, for the thermoluminescence data the spectral changes are modest, except for the low
dose gold implants. Further, the only intense signals were observed at the longer wavelengths (i.e. there were no visible emission bands at low temperatures as for the RL). Nevertheless, the temperatures of the glow peaks are significantly shifted as the result of ion beam implantation. Note that in TL only effects during a heating cycle can be observed. There are obvious differences between the radioluminescence and thermoluminescence in terms of dose dependence. For the TL from the gold implanted samples the TL peak shifts do not follow a smooth displacement with
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
dosage. This is not in conflict with implantation induced stress since in many cases (glass, silica, lithium niobate, LiF) stresses initially increase and then undergo relaxations caused by plastic flow, defect aggregation or precipitation of nanostructures within the implanted region. Hence the net stress will differ between the heavily and lightly implanted samples and at high doses there might be stress relief. Hence in the radioluminescence case, the high dose implantation plays a key role which may not be mirrored exactly in the thermoluminescence, since the two techniques probe rather different defect characteristics. In spectral terms, for variations of a specific defect type in relaxed or modified lattice environments, electron–hole recombination occurs which is sensitive to the local structure, so small wavelength shifts can occur. However, for the broad bands seen here the spectral changes will appear to be relatively minor as the result of phase changes or relaxations. More significant is that the lattice modifications will define the binding energies of charges at defect sites and so will introduce temperature shifts in thermoluminescence as a result of structural variations. For a material such as STO, where there are known to be small structural relaxations, then TL will be a sensitive probe of such variations. This is precisely the feature reported here where as a result of surface implants the TL peaks move in temperature. Additionally, the changes can modify the efficiency of radiative recombination processes. This behaviour is expected from a homogeneous source but totally unexpected for samples where only the surface has been modified. As mentioned earlier, the bulk volume greatly exceeds the implanted zone by almost 104 times, hence the only way in which the TL peaks can be displaced on the temperature axis (as seen for the Sb implants, and the lower dose Au implants), is because the stresses generated in the implant zone, significantly modify the bulk lattice and hence the major bulk signal TL. New TL signals from the implant zone may exist, but they are not readily identifiable in the presence of the
555
intense bulk emission signals. For the very high gold dose implants there is some suggestion that the implant layers have restructured, and consequently have a different influence on the bulk material. The main, and surprising, conclusion unequivocally remains that ion beam implantation of the surface of strontium titanate results in bulk relaxations which can be monitored via the thermoluminescence, as well as via radioluminescence.
5. Conclusion The changes in the details of the TL emission spectra indicate that ion implantation of the surface of strontium titanate induces relaxations throughout the bulk of the samples. This once again underlines the nature of structural sensitivity in this material to the presence of intrinsic defects, impurities and stresses.
Acknowledgements We are grateful for funding from EPSRC, the Royal Society and the NSFC (10244007).
References [1] J.F. Scott, H. Ledbetter, Z. Physik B 104 (1997) 635. [2] E.K.H. Salje, M.C. Gallardo, J. Jime´nez, F.J. Romero, J. del Cerro, J. Phys. Condens. Matter 10 (1998) 5535. [3] N. Kulagin, J. Dojcilovic, Physica B 269 (1999) 49. [4] J.F. Scott, J. Phys. Condens. Matter 11 (1999) 8149. [5] C. Ang, J.F. Scott, Zhi Yu, H. Ledbetter, J.L. Baptista, Phys. Rev. B 59 (1999) 6661. [6] C. Ang, Z. Yu, J. Hemberger, P. Lunkenheimer, A. Loidl, Phys. Rev. B 59 (1999) 6665. [7] W. Schranz, P. Sondergeld, D. Havlik, E.K.H. Salje, A.V. Kityk, J.F. Scott, Phys. Rev. B 61 (2000) 946. [8] B. Yang, P.D. Townsend, R. Fromknecht, Nucl. Instr. and Meth. B 217 (2004) 60. [9] P.D. Townsend, M. Maghrabi, B. Yang, Nucl. Instr. and Meth. B 191 (2002) 767.
Effects of the ion-implantation on the thermoluminescence spectra of strontium titanate B. Yang
a,b,*
, P.D. Townsend b, Y. Fan a, R. Fromknecht
c
a
Department of Physics, Beijing Normal University, Beijing 100875, China Science and Technology, University of Sussex, Brighton BN1 9QH, UK Forschungszentrum Karlsruhe, Institut fuer Festkoerperphysik, D-76021 Karlsruhe, Germany b
c
Received 14 April 2004; received in revised form 27 July 2004
Abstract Thermoluminescence of strontium titanate at low temperature is characterised by long wavelength emission centred near 800 nm which contains at least three overlapping luminescence bands. The thermoluminescence is excited by X-ray irradiation at 25 K of an entire sample. Room temperature ion-implantation into the surface layer changes the low temperature TL signals both in terms of their relative intensities and peak temperatures, as well as modifying the emission spectra. Such an intense perturbation of the bulk signals resulting from surface ion beam implantation is extremely unusual. However, it is well documented that for strontium titanate there are a variety of low temperature relaxations of the structure, and even phase transitions, which are highly sensitive to the presence of intrinsic defects, impurities and stresses. The ion-implantation damage in the surface is thus thought to act as a stress nucleation zone from which such relaxations can propagate throughout the entire crystal. There are consequent changes in the thermoluminescence in terms of defect stability and glow peak temperature. Details of such changes and modifications of emission spectra are reported. 2004 Published by Elsevier B.V. PACS: 78.60.K; 6180; 6172.Ww Keywords: Strontium titanate; Thermoluminescence; Phase changes and relaxations
1. Introduction
* Corresponding author. Address: Department of Physics, Beijing Normal University, Beijing 100875, China. Tel.: +86 10 62208417; fax: +86 10 62200141. E-mail address: [email protected] (B. Yang).
0168-583X/$ - see front matter 2004 Published by Elsevier B.V. doi:10.1016/j.nimb.2004.08.002
The wavelength, intensity and temperature dependence of luminescence signals are widely used to provide highly sensitive monitors of imperfections in insulating crystals since variations in local lattice site configurations are reflected by
550
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
variations of the luminescence. The detailed data offer information on the generation, decay and conversion of the various types of defect, especially those associated with the impurities or impurity-intrinsic defect complexes. Less studied is how they change as a function of modifications of the crystalline phases and other structural parameters. Defect studies and recognition of low temperature phase changes, and/or relaxations, have been reported in numerous works for strontium titanate (STO) using many experimental methods [1–7]. There are conflicting reports as to the magnitude, or even presence, of the changes, and it is assumed that the problems occur because the data may be sample or technique dependent. This assumption might imply that such sensitivity in the variations of the lattice structure occurs because the energies involved are small, and thus influenced by defects or stress etc. Consequently the phase transitions or relaxations do not always occur as a particular phase may be stabilised as the result of impurities, stress or other defects. In other words, defects are playing a key role in the phase stability of crystalline STO samples. Since ion beam implantation of the crystal surface is associated with large stress changes extending beyond the implant zone, introduction of very high concentrations of intrinsic defects, and injection of impurities, implantation is likely to be particularly crucial in altering the stability of closely related phase situations. Indeed, recent work on the radioluminescence (RL) of ion-implanted and unimplanted STO has been reported [8] which shows remarkable features indicating that not only are there surface changes, where the implant occurs, but also that this surface treatment can initiate changes throughout the bulk of the crystal. The anomalies of the RL signals were observed in the heavily implanted samples (5 · 1016 to 1 · 1017 ions/cm2 for Au ions, 1 · 1015 to 5 · 1016 ions/cm2 for Sb ions). In those samples, the wavelength and intensity of the RL emissions below 65 K were totally different from those in the unimplanted sample, even though RL is a monitor of bulk properties. The interpretation of that data was that the ion-implantation not only introduced a range of defects and lattice disorder, but the consequent
stress fields penetrate deeper into the lattice. Overall this metastable situation is ideal for the relaxation and nucleation of crystalline phase variants as the sample temperature is changed. Note in particular that in strontium titanate many of the anomalies and discontinuities in properties are thought to originate from discrete temperature related relaxations rather than pure phase changes. Hence, stress and defects may be particularly effective in these cases. The original report on the ion-implanted material is here complemented by thermoluminescence (TL) spectral data from ion-implanted strontium titanate. As appeared in the RL case, good transparent single crystalline samples were used, and so it is possible to contrast TL from bulk and surface modified material. The data presented here confirm that surface implants can influence signals from the entire specimen.
2. Experimental comments The 5 · 5 · 1 mm strontium titanate (STO) samples were either unimplanted or had been implanted with gold ions at 240 keV or antimony at 260 keV at RT. Ion doses for Au were 5 · 1013, 5 · 1014, 5 · 1016 and 1 · 1017 ions/cm2 and for Sb the doses were 1 · 1015 and 5 · 1016 ions/cm2. None of the samples discussed here had been subject to high temperature annealing. In order to perform thermoluminescence measurements the samples were cooled to 25 K and irradiated with X-rays with a dose of 200 Gy. The thermoluminescence signals were recorded with a high-sensitivity wavelength multiplexed system [8,9] at a heating rate of 6 K/min. Although the irradiations were near 25 K there were non-reproducible variations in the initial TL signals below 35 K and hence the only data presented here are for those above 40 K, where consistent results were obtained. The TL spectrometer has two detectors covering the wavelength ranges 200–410 and 410–830 nm, with a wavelength resolution of 4 nm. All spectra were corrected for the wavelength dependent response of the system. For the implanted samples, the implanted surface was the upper surface facing the spectrometer, i.e. the unimplanted surface was in
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
contact with the heater stage. This arrangement was chosen to try to optimise any features related to the very thin implant layers, however it is likely to influence the emission intensity from the bulk if there is absorption within the implant layer. Such absorption might reduce the intensity of bulk signals but is unlikely to influence their TL peak temperatures.
3. Results The emission spectra are unusual compared with those recorded from most oxides (e.g. quartz, silica, alumina) in that the emission is almost totally confined to intense signals at the red end of the spectrum with negligible signals near 400 nm as are seen in many other oxide materials. Fig. 1 contrasts the signals in isometric plots between
551
unimplanted and implanted samples, from which one notes the dominant glow peak is recorded at about 125–155 K, together with some other weaker TL peaks. No obvious TL signals were observed above 240 K. The emission signals in all samples were only significant in the 750–830 nm in the temperature range of this work. In Fig. 1 the isometric TL spectra are for unimplanted STO and for data taken after the 5 · 1013/cm2 Au implant. On comparing unimplanted and implanted samples, no major additional thermal peaks or emission bands were added by the implantation but in detail there are definite changes in the peak temperatures, the relative intensities of the component TL peaks and some minor spectral broadening (discussed later). The observation of major changes caused by ion-implantation of the surface is remarkable since the thermoluminescence is stimulated by X-ray
(a)
(b) Fig. 1. The X-ray excited thermoluminescence emission spectra of (a) a sample of unimplanted strontium titanate and (b) a sample which had received a 5 · 1013/cm2 Au implant at 300 K.
552
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
irradiation of the sample which includes both the bulk substrate and the very thin surface layer modified by the ion beam implantation. The fact that the bulk volume greatly exceeds the implanted zone by almost 104 times suggests that any additional signal, specifically from the implant zone, would need to be very intense per unit volume in order to contribute measurably to the total signal. If the surface signals are in addition to those from the bulk then both signals should be apparent. The fact that the data differ in terms of TL peak temperature implies that the surface changes must have strongly modified the bulk properties. Further, neither the details of the TL curves nor the spectra are identical between the different samples
as a result of different ion species and implant doses. It is reasonable to consider that the signals originate primarily from the greater volume of bulk material and that most changes must have occurred because of perturbations driven by the surface implantations. Details of the emission spectra are included in Fig. 2 at selected temperatures for pure unimplanted material (Fig. 2(a)) and implanted samples (Fig. 2(b)). The 60, 100, 125 and 185 K temperature slices extracted from the data for Fig. 1(a) are presented in Fig. 2(a). The intensities are normalised at 795 nm. An overall broad band is recorded which can be analysed in terms of at least three emission components at about 780, 795 and
STO 1.2
(a)
185K 125K 100K 60K
1.0 0.8 0.6
Intensity (normalized)
0.4 0.2 0.0
1.2
STO STO:Au(5x10 13 ) STO:Au(1x10 17 ) STO:Sb(5x10 16 )
(b)
1.0 0.8 0.6 0.4 0.2 0.0 750
760
770
780
790
800
810
820
830
Wavelength (nm) Fig. 2. Details of the emission spectra obtained from: (a) the unimplanted material and (b) implanted crystals. The intensities are normalised at 795 nm.
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
810 nm. The component bands can be found in every spectrum, but their relative intensities differ with temperature. In addition, the changes may subsume small variations caused by movements of the component peaks with temperature. The shorter wavelength components are relatively weaker at lower temperatures. The emission spectra of the main TL peak in the unimplanted, the 5 · 1013, 1 · 1017 Au-implanted and the 5 · 1016 Sb-implanted samples are contrasted in Fig. 2(b). Once again the first estimate suggests that three bands are required to model the overall shape. Within the precision of the data the same three components might be adequate to describe the observations. Only minor differences of the band positions and the relative intensities between the three bands were detected in the 1 · 1017 Au- and 5 · 1016 Sb-implanted samples. The relative intensities between the emission bands in the 5 · 1013 Au-implanted example are obviously different from those in other samples. It is difficult to tell precisely the exact band positions in that sample, as some of the overlapping bands having almost equal intensities. The glow curves integrated over the 790–800 nm spectral range in the heavily Sb ion-implanted (1 · 1015 and 5 · 1016/cm2), the heavily Au ion-implanted (5 · 1016 and 1 · 1017/cm2) and the lightly Au-implanted (5 · 1013 and 1 · 1014/cm2) samples studied in this work are all shown in Figs. 3(a)– (c) respectively. For reference purposes the TL of an unimplanted sample is provided. The common features found in all the three figures are that firstly, the peak temperature of the main TL peak in the implanted samples is higher than that in the unimplanted sample, and secondly the intensity of that peak in the implanted samples is lower than that in the unimplanted one. For the gold implants the maximum peak temperature displacement occurs for the ion dose of 1014 ions/cm2 with an upward shift in temperature of 25. Normal reproducibility of these measurements is always much better than 5, hence the peak movements displayed here are very significant. Similar peak movement effects are noted for the antimony implants. In the range between 40 and 90 K other minor TL peaks differ between the various sample conditions.
553
It is also apparent that there are variations in the peak positions and intensities of the minor TL signals. There was not a clear pattern for these changes. Two weak glow peaks are seen below 90 K and one at about 185 K, as shown in Figs. 3(a) and (b). These were detected in the unimplanted and heavily implanted samples. The peak temperatures of the two peaks below 90 K, as well as that of the main peak, move to higher temperature in the implanted samples. The low temperature peak shifts increase with the larger doses whereas the 185 K peak remains at almost the same temperature in most of the glow curves. By contrast, in the samples implanted with low doses, as shown in Figs. 1(a) and 3(c), small TL peaks were observed below 90 K and near 215 K, and the shift of the temperature of the main TL peak is even larger than that in the heavily implanted samples. As mentioned earlier the selected experimental geometry was to place the implanted faces nearest to the spectrometers. This introduces some absorption of signals from the bulk but might have aided detection of any signals directly from the implant layer. Indeed the signals are less than for the unimplanted sample, but on comparing the glow curves of the 5 · 1013, 1 · 1014 and 5 · 1016/cm2 Au ionimplanted samples, it is found that the intensity of the main TL peak in the former two samples is even lower than that for the higher dose case. A simple model of absorption increasing with implant dose is thus compromised.
4. Discussion As mentioned initially, the earlier radioluminescence data [8] indicated that surface ion beam implantation strongly modified the radioluminescence signals which arise from the bulk of the material, and the suggestion was that the distorted surface layer was effective in nucleating relaxations and/or phase transitions of the bulk material during heating and cooling of the crystals. In RL data the effects included a sharp intensity discontinuity near 65 K in the near infra-red emission bands and strong new visible/ emission bands at lower temperatures. The detailed origin
554
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
STO:Sb 15000
(a)
unimplanted 1x1015 /cm2 5x1016 /cm2
12000 9000 6000 3000 0 5000
(b)
STO:Au unimplanted(x30%) 5x1016 /cm 2 1x1017 /cm2
Intensity (arb. unit)
4000 3000 2000 1000 0 5000
STO:Au
(c)
unimplanted(x30%) 5x1013 /cm2 1x1014 /cm2
4000 3000 2000 1000 0 40
90
140
190
240
Temperature (K) Fig. 3. The TL glow curves obtained from the unimplanted, antimony and gold implanted samples of STO. The data are the integrations of the red wavelength emission.
of sites which could initiate the changes was unclear but lattice distortions, imperfections, nanoparticle precipitates and/or stress fields are all possible candidates. Indeed, for a sensitive metastable crystal structure the relaxations might be induced by any of these factors. The sensitisation effect depends on the degree of damage, or amorphisation of the implant layer and the altered radioluminescence features were most clearly observed in the heavily implanted samples. By contrast, for the thermoluminescence data the spectral changes are modest, except for the low
dose gold implants. Further, the only intense signals were observed at the longer wavelengths (i.e. there were no visible emission bands at low temperatures as for the RL). Nevertheless, the temperatures of the glow peaks are significantly shifted as the result of ion beam implantation. Note that in TL only effects during a heating cycle can be observed. There are obvious differences between the radioluminescence and thermoluminescence in terms of dose dependence. For the TL from the gold implanted samples the TL peak shifts do not follow a smooth displacement with
B. Yang et al. / Nucl. Instr. and Meth. in Phys. Res. B 226 (2004) 549–555
dosage. This is not in conflict with implantation induced stress since in many cases (glass, silica, lithium niobate, LiF) stresses initially increase and then undergo relaxations caused by plastic flow, defect aggregation or precipitation of nanostructures within the implanted region. Hence the net stress will differ between the heavily and lightly implanted samples and at high doses there might be stress relief. Hence in the radioluminescence case, the high dose implantation plays a key role which may not be mirrored exactly in the thermoluminescence, since the two techniques probe rather different defect characteristics. In spectral terms, for variations of a specific defect type in relaxed or modified lattice environments, electron–hole recombination occurs which is sensitive to the local structure, so small wavelength shifts can occur. However, for the broad bands seen here the spectral changes will appear to be relatively minor as the result of phase changes or relaxations. More significant is that the lattice modifications will define the binding energies of charges at defect sites and so will introduce temperature shifts in thermoluminescence as a result of structural variations. For a material such as STO, where there are known to be small structural relaxations, then TL will be a sensitive probe of such variations. This is precisely the feature reported here where as a result of surface implants the TL peaks move in temperature. Additionally, the changes can modify the efficiency of radiative recombination processes. This behaviour is expected from a homogeneous source but totally unexpected for samples where only the surface has been modified. As mentioned earlier, the bulk volume greatly exceeds the implanted zone by almost 104 times, hence the only way in which the TL peaks can be displaced on the temperature axis (as seen for the Sb implants, and the lower dose Au implants), is because the stresses generated in the implant zone, significantly modify the bulk lattice and hence the major bulk signal TL. New TL signals from the implant zone may exist, but they are not readily identifiable in the presence of the
555
intense bulk emission signals. For the very high gold dose implants there is some suggestion that the implant layers have restructured, and consequently have a different influence on the bulk material. The main, and surprising, conclusion unequivocally remains that ion beam implantation of the surface of strontium titanate results in bulk relaxations which can be monitored via the thermoluminescence, as well as via radioluminescence.
5. Conclusion The changes in the details of the TL emission spectra indicate that ion implantation of the surface of strontium titanate induces relaxations throughout the bulk of the samples. This once again underlines the nature of structural sensitivity in this material to the presence of intrinsic defects, impurities and stresses.
Acknowledgements We are grateful for funding from EPSRC, the Royal Society and the NSFC (10244007).
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