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Defects production and interaction in ion-implanted diamond
Physica 116B (1983) 187-194 North-HollandPublishingCompany
Paper presented at ICDS-12 Amsterdam, August31 - September3, 1982
DEFECTS PRODUCTION AND INTERACTION IN ION-IMPLANTED DIAMOND
A.A. GIPPIUS, V ~S. V A V I L O V , A.M. Z A I T S E V a n d B.S. Z H A K U P B E K O V
P.N. Lebedev Physical Insl~tute of the USSR Academy of Sciences Leninsky prospect 53, Moscow 117924, USSR Point defects production and interaction are studied by luminescence technique under various conditions of implantation and annealing. New data are presented on the structure and transformation of nitrogen defect complexes. The luminescence due to centres containing implanted atoms of hydrogen, helium, neon and some metals is observed for the first time. The importance of interstitital configurations in ionimplanted diamond is emphasized. 1.
INTRODUCTION
Ion implantation as doping technique is particularly important for diamond since it can not be doped b~ diffusion while doping during synthesis has so far presented serious difficulties. Practical applications of ion implantation in diamond [I] developed along with studies of defects produced by implantation. Various techniques were emploxed to monitmr the formation of amorphous la2ers [2,3], the recovery of lattice after annealing and the location of implanted impurities [4]. However data on point defects are scarce [5] largely due to predominance of macroscopic techniques used. In the present work luminescence is employed to stud~ point defects in ion implanted diamonds. Both intrinsic defects and complexes of defects with various impurities are studied in the conditions of high defect density produced by ion implantation. 2. EXPERIIdENTAL Samples of type IIa and Ia natural diamonds with nitrogen content less than 1018cm -3 and >/ I019cm -3 respectively were implanted at room temperature b~ various ions with energy up to 350 keV and isochronally (I h) annealed in vacuum 10-5 Tort over the range 500 to 1400°C. Cathodoluminescence (2-10 keV, 5 mkA)was studied in the range 250-900 nm st 80 K. In order to exclude the change of lifetime or carriers (governed by nonradietive transitions) we took the ratio of the intensit~ I of a given line to that of band A (which is quenched b2 irradiation). Since the amount of centres corresponding to this band did not change after the implantation (up to a definite dose, see § 3.3), the ratio I/A is proportional to the concentration of given centres. In uniaxial stress
0378-4363/83/0000-0000/$03.00 © 1983 North-Holland
studies, besides the usual data on the symmetr~ of centres we used a parameter s (introduced by the formula:
=(~hvm,,x/h~.'°') "1012 Pa-1, ~h~ma x is the maximum shift
where of a split component at a stress o" from the line's position at zero stress),named "softness" of a centre, which is relative change pf transition energ2 for a unit stress [6] . This change depends upon relative deformation of centre's orbitals (at least for orientation degeneracy), which in turn depends upon the local change of elastic constants produced by the centre. Vacancies are known to loose or to soften the lattice [7] while interstitial atoms or substitutional atoms with radii higher than that of a host atom make it harder (M~ssbauer effect studies give evidence of a ver~ high, 1500 kbar, internal pressure at iron atoms implanted in diamond [8] ). It means that defect centres containing vacancies must be softer than interstitial defects. Indeed, for the known centres 741 nm (GR-I, isolated vacancy) and 637.7 nm (vacancy plus substitutional nitrogen atom) the values of "softness"I are 4.5 and 4.1 Pa-I respectively [9] , while for the centre 736 nm (comprising two atoms of Si [10] and regardless of their position in the lattice making it harder because of 50% difference between the radii of Si and C) the value of ~¢ is less than 0.1 Pa "I. 3. DEFECTS NOT RELATED TO THE IDENTITY OF THE IONS 3.1. Ion mass effects Cathodoluminescence spectra of unimplanted samples consist of a band A and a number of weak narrow lines [9], Fig. la. The implantation quenches band A and introduces new lines,some of them
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
188
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(I
F fO °
;, tOc
',,,
~L(c)
kA
I~ ÷ -
F
H+
6oo °C
c
~
C+I
~o
moo T,'C
Figure 4 : Annealing of 5RL and GR-I
Figure 2 : Annealing of TR-12(a) and GR-I (b) centres
i
-oo'o
t0~ t
104~ DOSE,
|04s c m =z
Figure 5 : Dose dependence of band A after annealing at 1400°C 400
~
CO0
?00
.X,.m
Figure I : Cathodoluminescence spectra of unimplanted (a), implanted (b-d) and annealed (e~f) diamonds
260
2BO
320 .~,nm
Figure 3 : Spectra of 5RL-s~stem after annealing at 500°C
due to the known point defects. We have found a clear distinction between the sets of defects created b y ~ + and heavier ions. The implantation of H~ produced e lot of lines (389, 415, 441, 468, 470, 491, 503, 555, 575, 741 nm), while the spectra of samples implanted by heavier ions (except nitrogen, see § 4.1) were dominated by lines 741 and 470 um (Pig. Ib,c). Note the increase of the intensity of lines of intrinsic defects: 1 nm (GRI, i.e. vacancy) and 470 nm TR-12, an interstitial [9] or vacancyinterstitial [11] complex) and the decrease of lines of complexes incorporating impurit~ (nitrogen) atoms: 4 1 5 nm (N3), 503 nm (H3) [9], 575 nm [6,9]. Considerable fraction of defects c~ested by ion implantation is concentrated within disordered regions produced b~ disolace-
~4
380
ment cascades which consist (for comparatively light ions) of a small (if any) amorphous nucleus, a highl3 disordered but cr3stalline boundar~ la~er and a crystalline matrix coutaining a more dilute solution of point defects ~2, 13] . Due to the d~namics of the cascade formation the inner part of the disordered region is vacancy-rich while a large fraction of interstitials is created predominantly at the peripher~ of the cascade and owing to their high mobilit~ quickly migrate into the surrounding lattice where the~ become trap. ped or agglomerate [14] . The p~edominance of luminescence due to GR-I and TR-12 centzes is due to vet2 high concentration of intrinsic defects when the formation of Impurity-defect complexes should saturate° The reverse
189
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
annealing stage (up to 600°C) for TR-12 (Fig.2a) is more pronounced for heavier ions (for Si implantation there is two order of magnitude increase). Similarly, the decrease of GR-I is stronger for heavier ions (Fig.2b). The observed annealing behaviour suggests that the point defects involved belong rather to a disordered region itself than to adjacent p a r t of the crystal. This behaviour is also in agreement with spatial separation (stronger for heavier ions) of vacancies and interstitials in disordered regions, especially if vacancy-interstitial model of TR-12 ~I] (though rather speculative) is accepted. Note also that the reverse annealing of TR-12 occurs in the same temperature range as release of trapped interstitials monitored by spin resonance ~5]. 3.2. 5RL-system This system in UV region consisting of several zero-phonon lines reproduced sharply at interval 236 meV (Fig.3) is thought to be due to an intrinsic defect, p e r h a p s a n a g g r e g a t e [ 9 ] . I t was p r o d u c ed by implantation of any ions, except nitrogen. Reverse annealing of 5RL occurs at the temperatures when vacancies become mobile and GR-I anneals completely (Pig.4). This is in agreement with the vacancy-complex structure of the centre which is further supported b~ its very high "softness" ( ; = 15 Ps -l as compared to 4.5 Pa "J for a single vacancy). The centre is very sensitive to nitrogen content and lattice disorder and is not observed in crystals irradiated and annealed prior to implantation. The intensity of 5RL increases2about linearly with dose up to 1014 cm(for 20 keV He + ) whereupon it drops rapidly to zero. All this imply a rather loose centre which seems to be in contradiction with a ver~ energetic local phonon (236 meV) suggesting a large force constant. The intensity of H-lines of 5RL is markedly increased in He-implanted samples (Pig. 3a,b). The "5RL-H" system anneals at 100oc less temperature than 5RL-C and is even more sensitive to the presence of nitrogen and to the preliminary irradiation. Since there is no isotopical shift of the H-line, but onl~ a changeq + in intensity for the implantation of ~He, it seems that the line is due to some specificity of lattice disorder rather than to the presence of He in the centre. The relative intensity of H-lines is ~ncreased by a factor 3 if energy of He + ions is decreased from 150 to 10 keV. This implies that the centre exists close to the surface. One may speculate about the existence in diamond of a surface damage peak similar to one produced
in silicon by light ions
~6].
3.3. High dose effects Starting from a "critical" dose the lines of point defects (GRI, TR12, 3H) disappear a n d o n l ~ a weak band A r e maine in the spectra (Fig. ld)o Appoximately at the same dose the band A intensity, quenched by the implantation, is not restored by subsequent annealing (Fig.5). Dose dependences (after annealing at 1400°C) of lines due to implanted impurities (N, Si) saturate and even become nonmonotonic. The lines of soft nitrogen containing centres (§ 4.1) broaden (up to ~ 30 meV). All these effects which are due to the interaction between disordered regions (i.e. due to their overlapping) are observed at the implantation doses at least an order of magnitude lower than those at which the saturation of the change of macroscopic parameters was found and attributed to the formation of some kind of amorphous layer [3]. It may indicate that the latter forms (for the light ions used) only after multiple overlapping of disordered regions. This is confirmed by the fact that the estimate of the lateral spread Rp~ = 2 ~ given in [17] for the saturation data of [3] is about two orders of magnitude.less than the calculated value 167 A. We can see that luminescence enables one to detect earlier stages of the process of mounting the disorder which leads eventuslly to the amorphisstion. 4. DEFECTS RELATED TO THE IDENTITY OF THE IONS 4.1. Nitrogen The spectra of N + implanted type IIa crystals (Fig.6a) are dominated, before annealing, by lines 389 and 441.5 nm (GR-1 and TR-12 are relatively weak which is characteristic of N T implantation). Annealing at T > 500°C or electron irradiation produces the 575 nm system (Fig.6b). Then at T > I000°C the centres 503 nm (H3) and 415 nm (N3) appear while the intensity of 389 and 441.5 nm (end to less extent 575 nm) begins to decrease (Fig.6c). Since these centres ore observed in nitrogenlean crystals onl~ after N + implantation it means that all of them incorporate nitrogen atoms. In our samples in agreement with [5] no spin resonance signal due to substitutional N atoms was found. It suggests that the centres 389 and 441.5 nm are related to interstitial forms o~ N. The centres are hard ( ~ < 0 . 1 P a " ) which confirms
190
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
'ISN+IP,IpL.
EIo
>.
~
}..,
I
600 *c v~
I0 =
t..-
10~ tu
W,'c
,,,oo'cl hA. c_.l I 400
5'00
600
700 ,~,nm
Figure 6 : Spectra type IIa crystals implanted by N+
of
~.0 ""
600
fODO
f~tOO
T,-c
Figure 8 : Annealing of 3B9 nm centre~_in uuimplanted and "9E+ implanted samples
Figure 7 : Annealing of the centres created b~ N + implantation
their interstitial nature but does not permit the determination of their s~mmetr~. It is tempting to speculate about various interstitial configurations of which <100> split interstitial site seems to be highly prefgrable for trivalent nitrogen [18, 19]. The 575 nm system is known to be observed in all diamonds, but is particularly strong in type Ib [20] . The latter suggests its relation to a single isolated N atom. This is borne out by linear dependence of 575 nm intensity on I~+ implantation dose. Uniaxial stress data imply the model of 575 nm centre as a N atom in <1007 split interstitial site associated with the nearest (in direction) vacanc~ [6] (the structure is analogous to Acentre i.e. V-O, in Si). The 575 nm centre originates from a configuration different from those of 389 or 441.5 nm centres since it is produced in conditions when no change of 389 and 441.5 nm lines intensity is observed. Dependence of the intensity of 503 nm (H3) and 415 nm (N3) on N+ implantation dose (the second and third power respectively) confirms the models for these centres as complexes comprising two and three N atoms [9] . The H3 centre is soft (I = 4.9 Pa -I) in accord with its being composed of s pair of N atoms (A-aggregate) and one [21] or two ~22] vacancies. The softoess of N3 centre (3.5 Pa -1) (3.5 Pa "]) favours rather the model providing for a vacancy and three N atoms in the nearest neighbour positions ~ than the model with three N atoms bonded to a common carbon atom [23].
Summing up the behaviour of implanted N atoms we see that, immediately after the implantation or after annealing at T < I000°C, they form interstitial and single-atom centres. Then, at T > I000°C they tend to aggregate producing complexes containing 2 or 3 atomB (Fig.7). These data are to be compared to the transformation of isolated substitutioual N into A-aggretate observed at high pressure and T=1600-2000°C [24 , to the dissociation of A-aggregate at T > IBO0°C [25] and to the formation of A-a~gregates and H3 centres at T ~ 1500uC in irradiated diamonds [20] . The irradiation enhancement of aggregation, noted in[20] is even more pronounced in our experiments where this aggregation is observed alread~ at I000°C due to very high degree of disorder produced by ion implantation. The role of lattice disorder and the defect environment is manifest also in the difference between the annealing behavior of the centres 389 nm produced by implantation and those existing in unimplanted samples (Fig.8). 4.2. Metal impurities Specific luminescence lines were observed [26] in type IIa diamonds implanted (with subsequent annealing at 14000C) b~ metal impurities: Cr (741 rim), Zn (518 nm), Ag (398, 401, 405 rim), T1 (a group at 600-700 nm, the strongest 613 nm), Ni (a number of lines, ~ig.9). It was found that for some centres the dependences I(d) of the line intensity
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
191
2 ~.~
:
.~
t"~ --+ i I u
,,
5~/S
5~5 ~ , n m
Figure 1 2 : Isotopical shift of "546 nm" line R~HOYEO LRYER THICICNE$S ~ ,~
Figure 10 : Intensity versus removed layer thickness d, for the lines indicated
l.f +
,T35"
/I
] v]
;Ii V i l /'..l
Lu i
400
i
600
i
i
e
Ne*
i;
I..
800 .~.nrn i
Figure 9 : Spectra of: (s~ Ni + implanted diamonds, deep laser; (b) Ni + implsnted, surface layer; (c) the same as (b) minus Aand B-bands; (d) synthetic diamonds
~oo
~00
$00
600 &,nm
BOO
~ooo T,'C
Figure 13: Annealing of "546 nm" (H,D) and "535 nm" (He) centres
F i g u r e 11 : S p e c t r a of He+, Ne + and C+ i m p l a n t e d diamonds
I versus thickness of e removed layer d was very close to that for band B (which is related to the distribution of gross disorder produced by implantation) while for the others I(d) were much broader (Fig.10). The former are soft and their lines are broadened (especisll~ in the damaged layer near the surface) while the latter ere hard and no broadening was detected. It suggests that metal impurities can produce optical centres both of vscsnc~ and interstitial type. Since the lines 4B4, 451 , 885 nm are observed both in Ni + implanted natural end (unimpleted) synthetic diamonds (Pig.9) it means that the latter contain optically active Ni atoms introduced during the growth (Ni is used as a
catalyst). The similarity between the spectra of synthetic and I~i-implanted natural diamonds exists onl3 for the surface layer of the latter where soft Ni centres dominate. Synthetic diamonds produce very week if an~ luminescence of hard (interstitial?) Ni centres observed in deeper lasers of implsted crystals. Similarly, in natural (unimplented)diemonds the luminescence of the soft vacancy-nitrogen centres (N3,H3, 575 um) is always much stronger than that of hard, probsbl3 interstitial nitrogen centres (389, 441.5 rim) while the latter dominate (up to T ~ 600°C)in the spectra of N implsnted crystals. Thus interstitital impurity centres seem to be characteristic of ion implantstion° The absence of such ceutres
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
192
in natural and synthetic diamonds (doped in equilibrium conditions) is probably due to lower stability of interstitial configurations. 4.3 H~drogen
5 6
and neutral gases
We have observed for the first tSme iD diamonds implanted by H +, D *, He ~, Ne ~ lines which can be attributed to centres containing these impurities (Fig. t1,12). Some of the lines are common to several impurities: "513 nm" with the wavelength changing slightly from 512.76 nm (H) to 513.66 nm (He) and 513.96 nm (Ne); "535 um" ( 534.9 (D), 535.4 (He), 535.8 (Ne) ] ; "560 nm" { 559.96 (He), 559.80 (Ne)} . This behsviour implies "mass effect" in the centres whose electronic structure is perturbed by the trapped impurities. The lines 522 nm and "546 nm" are observed only in He and H(D) implanted samples respectively which indicates that these impurities play more specific role in the formation of the centres. Phonon coupling is weak for all the centres, the 522 nm (He) centre was found to be ver~ hard. The nonmonotonic annealing curves (Fig. 13) suggest complicated kinetics of formation and destruction of the centres. The curves for He and D are rather similar, while for H an additional peak is observed suggesting a process which overcompensates the distruetion of the centres. This process is believed to be the release of b4drogen from the sources, probably ma~ma dropless, where it is "stored" [27~ These resultsdemonstratethat hydrogen,which is now recognized as one of the major impurities in diamond [27] , is capable of forming optically active centres which provides an effective tool of controlling the content of this impurity. The authors are indebted to A.V.Spitsin and V.A.Dravin for their assistence in the present work.
7 B 9 10
11 12 13 14 15 16
17 18 19 20 21
22 23 REFERENCES : I 2 3 4
Kozlov, S.F., Belcarz, E., Hage-Ali, M.,Stuck, R., Siffert, P., Nucl. Instr. Meth. !17 (1974) 277-288 Brosius, P.R., Corbett, J.W. Bourgoin, J.C., Phys. Stat. Sol. (a) 21 (1974) 677-684. Morhange, J.F., Beserman, R., Bourgoin J.C. Japanese Jrnl. Appl. Phys. 14 N 4 (1975) 544-548. Braunstein, G., Kalish, R., Appl. Phys. Lett. 38 (b) (19B1) 416-418.
24 25
26 27
Lee, Y.H., Brosius, P.R., Corbett, J.W., Phys. Stat.. Sol. (a) 50 (1970) 237-243 Zaltsev, A.H., Gippius, A.A., Vavilov, V.S., Piz. Techn. Poluprov. 16(3) (1882) 397-403. Larkins, F.P., Stoneham, A.L;., Jrnl. Phys. C: Solid St. Ph2s. 4 (1971) 143-163. Sawicka, B.D., Sawicki, J.A., de Waard H., Phys. Lett. B5A N 5 (1981~ 303-307Walker, J., Rep. Progr. Phys. 42 (1979) 1605-1659. Zaitsev, A.M., Vavilov, V.S., Gippius, A.A., Kratkie Soobshchen. Fiz. P.1~.Lebedev Phys. Inst. 10 (1981) 20-23. Gippius, A.A., Zaitsev, A.}{., Vavilov, V.S., Fiz. Techn. Poluprov. 16 (3) (1982) 404-411. Kimerling, L.C., Paste. J.M., Inst. Phys. Conf. Set. N 23 (1975) 12614B. Physical Processes in Irradiated Semiconductors, ed. Smirnov~ L.S. ("Nauka", Novosibirsk, 1977). Nelson, R.S., Inst. Phys. Conf. Set. N 16 (1973) 140-158. Clark, C.D., Duncan, I., Lomer, J.K., ~:q~ippe2, P.W.. Proc. Brit. Ceram. Soc. I (1964J 1-B. Gashtoldt, V.N., Gerasimenko, i~.N., Dvurechenskii, A.V., Smirnov, L.S., Fiz. Techn. Poluprov. 9 (5) (1975) 835-839. Bourgoin, J.C., Diamond Research (1975) 24.28. Mainwood, A., Larkins, F.P., Stoneham, A.M., Sol. St. Electron. 21 (1978) 1431-1435. Evans, S., Proc. Phys. Soc. Lond. A370 (1980) 107-129. Collins, A.T., Inst. Pl~s. Conf. Set. N 46 (1979) 327-333. Davies, G., Nazare, ~4.H., Inst. Phys. Conf. Set. N 31 (1977) 332338. Sobolev, E.V., Abstracts of the Second All-Union Confer. on Wide Gap Semicond. /Leningrad, 1979) 20. Davies, G., Welbourn, C.M., Loubset, J.H.N., Diamond Research (1972) 21-32. Chrenko, R.M., Tuft, R.E., Strong, H.M., Nature 270 (1977) 141-144. Brozel, IV!.R., Evans, T., Stephenson, R.F., Proc. R.Soc. A361 (1978) 109-127. Vavilov, V.S., Gippius, A.A., Zaitsev, A.M., Zhakupbekov, B.S., Fiz. Techn. Poluprovod. (to be published). Sellschop, J.P.P., Madiba, C.C.P., Annegarn, H.J., Nucl. Instr. Meth. 168 (1980) 529-534.
Paper presented at ICDS-12 Amsterdam, August31 - September3, 1982
DEFECTS PRODUCTION AND INTERACTION IN ION-IMPLANTED DIAMOND
A.A. GIPPIUS, V ~S. V A V I L O V , A.M. Z A I T S E V a n d B.S. Z H A K U P B E K O V
P.N. Lebedev Physical Insl~tute of the USSR Academy of Sciences Leninsky prospect 53, Moscow 117924, USSR Point defects production and interaction are studied by luminescence technique under various conditions of implantation and annealing. New data are presented on the structure and transformation of nitrogen defect complexes. The luminescence due to centres containing implanted atoms of hydrogen, helium, neon and some metals is observed for the first time. The importance of interstitital configurations in ionimplanted diamond is emphasized. 1.
INTRODUCTION
Ion implantation as doping technique is particularly important for diamond since it can not be doped b~ diffusion while doping during synthesis has so far presented serious difficulties. Practical applications of ion implantation in diamond [I] developed along with studies of defects produced by implantation. Various techniques were emploxed to monitmr the formation of amorphous la2ers [2,3], the recovery of lattice after annealing and the location of implanted impurities [4]. However data on point defects are scarce [5] largely due to predominance of macroscopic techniques used. In the present work luminescence is employed to stud~ point defects in ion implanted diamonds. Both intrinsic defects and complexes of defects with various impurities are studied in the conditions of high defect density produced by ion implantation. 2. EXPERIIdENTAL Samples of type IIa and Ia natural diamonds with nitrogen content less than 1018cm -3 and >/ I019cm -3 respectively were implanted at room temperature b~ various ions with energy up to 350 keV and isochronally (I h) annealed in vacuum 10-5 Tort over the range 500 to 1400°C. Cathodoluminescence (2-10 keV, 5 mkA)was studied in the range 250-900 nm st 80 K. In order to exclude the change of lifetime or carriers (governed by nonradietive transitions) we took the ratio of the intensit~ I of a given line to that of band A (which is quenched b2 irradiation). Since the amount of centres corresponding to this band did not change after the implantation (up to a definite dose, see § 3.3), the ratio I/A is proportional to the concentration of given centres. In uniaxial stress
0378-4363/83/0000-0000/$03.00 © 1983 North-Holland
studies, besides the usual data on the symmetr~ of centres we used a parameter s (introduced by the formula:
=(~hvm,,x/h~.'°') "1012 Pa-1, ~h~ma x is the maximum shift
where of a split component at a stress o" from the line's position at zero stress),named "softness" of a centre, which is relative change pf transition energ2 for a unit stress [6] . This change depends upon relative deformation of centre's orbitals (at least for orientation degeneracy), which in turn depends upon the local change of elastic constants produced by the centre. Vacancies are known to loose or to soften the lattice [7] while interstitial atoms or substitutional atoms with radii higher than that of a host atom make it harder (M~ssbauer effect studies give evidence of a ver~ high, 1500 kbar, internal pressure at iron atoms implanted in diamond [8] ). It means that defect centres containing vacancies must be softer than interstitial defects. Indeed, for the known centres 741 nm (GR-I, isolated vacancy) and 637.7 nm (vacancy plus substitutional nitrogen atom) the values of "softness"I are 4.5 and 4.1 Pa-I respectively [9] , while for the centre 736 nm (comprising two atoms of Si [10] and regardless of their position in the lattice making it harder because of 50% difference between the radii of Si and C) the value of ~¢ is less than 0.1 Pa "I. 3. DEFECTS NOT RELATED TO THE IDENTITY OF THE IONS 3.1. Ion mass effects Cathodoluminescence spectra of unimplanted samples consist of a band A and a number of weak narrow lines [9], Fig. la. The implantation quenches band A and introduces new lines,some of them
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
188
Tf( 1~ >.
(I
F fO °
;, tOc
',,,
~L(c)
kA
I~ ÷ -
F
H+
6oo °C
c
~
C+I
~o
moo T,'C
Figure 4 : Annealing of 5RL and GR-I
Figure 2 : Annealing of TR-12(a) and GR-I (b) centres
i
-oo'o
t0~ t
104~ DOSE,
|04s c m =z
Figure 5 : Dose dependence of band A after annealing at 1400°C 400
~
CO0
?00
.X,.m
Figure I : Cathodoluminescence spectra of unimplanted (a), implanted (b-d) and annealed (e~f) diamonds
260
2BO
320 .~,nm
Figure 3 : Spectra of 5RL-s~stem after annealing at 500°C
due to the known point defects. We have found a clear distinction between the sets of defects created b y ~ + and heavier ions. The implantation of H~ produced e lot of lines (389, 415, 441, 468, 470, 491, 503, 555, 575, 741 nm), while the spectra of samples implanted by heavier ions (except nitrogen, see § 4.1) were dominated by lines 741 and 470 um (Pig. Ib,c). Note the increase of the intensity of lines of intrinsic defects: 1 nm (GRI, i.e. vacancy) and 470 nm TR-12, an interstitial [9] or vacancyinterstitial [11] complex) and the decrease of lines of complexes incorporating impurit~ (nitrogen) atoms: 4 1 5 nm (N3), 503 nm (H3) [9], 575 nm [6,9]. Considerable fraction of defects c~ested by ion implantation is concentrated within disordered regions produced b~ disolace-
~4
380
ment cascades which consist (for comparatively light ions) of a small (if any) amorphous nucleus, a highl3 disordered but cr3stalline boundar~ la~er and a crystalline matrix coutaining a more dilute solution of point defects ~2, 13] . Due to the d~namics of the cascade formation the inner part of the disordered region is vacancy-rich while a large fraction of interstitials is created predominantly at the peripher~ of the cascade and owing to their high mobilit~ quickly migrate into the surrounding lattice where the~ become trap. ped or agglomerate [14] . The p~edominance of luminescence due to GR-I and TR-12 centzes is due to vet2 high concentration of intrinsic defects when the formation of Impurity-defect complexes should saturate° The reverse
189
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
annealing stage (up to 600°C) for TR-12 (Fig.2a) is more pronounced for heavier ions (for Si implantation there is two order of magnitude increase). Similarly, the decrease of GR-I is stronger for heavier ions (Fig.2b). The observed annealing behaviour suggests that the point defects involved belong rather to a disordered region itself than to adjacent p a r t of the crystal. This behaviour is also in agreement with spatial separation (stronger for heavier ions) of vacancies and interstitials in disordered regions, especially if vacancy-interstitial model of TR-12 ~I] (though rather speculative) is accepted. Note also that the reverse annealing of TR-12 occurs in the same temperature range as release of trapped interstitials monitored by spin resonance ~5]. 3.2. 5RL-system This system in UV region consisting of several zero-phonon lines reproduced sharply at interval 236 meV (Fig.3) is thought to be due to an intrinsic defect, p e r h a p s a n a g g r e g a t e [ 9 ] . I t was p r o d u c ed by implantation of any ions, except nitrogen. Reverse annealing of 5RL occurs at the temperatures when vacancies become mobile and GR-I anneals completely (Pig.4). This is in agreement with the vacancy-complex structure of the centre which is further supported b~ its very high "softness" ( ; = 15 Ps -l as compared to 4.5 Pa "J for a single vacancy). The centre is very sensitive to nitrogen content and lattice disorder and is not observed in crystals irradiated and annealed prior to implantation. The intensity of 5RL increases2about linearly with dose up to 1014 cm(for 20 keV He + ) whereupon it drops rapidly to zero. All this imply a rather loose centre which seems to be in contradiction with a ver~ energetic local phonon (236 meV) suggesting a large force constant. The intensity of H-lines of 5RL is markedly increased in He-implanted samples (Pig. 3a,b). The "5RL-H" system anneals at 100oc less temperature than 5RL-C and is even more sensitive to the presence of nitrogen and to the preliminary irradiation. Since there is no isotopical shift of the H-line, but onl~ a changeq + in intensity for the implantation of ~He, it seems that the line is due to some specificity of lattice disorder rather than to the presence of He in the centre. The relative intensity of H-lines is ~ncreased by a factor 3 if energy of He + ions is decreased from 150 to 10 keV. This implies that the centre exists close to the surface. One may speculate about the existence in diamond of a surface damage peak similar to one produced
in silicon by light ions
~6].
3.3. High dose effects Starting from a "critical" dose the lines of point defects (GRI, TR12, 3H) disappear a n d o n l ~ a weak band A r e maine in the spectra (Fig. ld)o Appoximately at the same dose the band A intensity, quenched by the implantation, is not restored by subsequent annealing (Fig.5). Dose dependences (after annealing at 1400°C) of lines due to implanted impurities (N, Si) saturate and even become nonmonotonic. The lines of soft nitrogen containing centres (§ 4.1) broaden (up to ~ 30 meV). All these effects which are due to the interaction between disordered regions (i.e. due to their overlapping) are observed at the implantation doses at least an order of magnitude lower than those at which the saturation of the change of macroscopic parameters was found and attributed to the formation of some kind of amorphous layer [3]. It may indicate that the latter forms (for the light ions used) only after multiple overlapping of disordered regions. This is confirmed by the fact that the estimate of the lateral spread Rp~ = 2 ~ given in [17] for the saturation data of [3] is about two orders of magnitude.less than the calculated value 167 A. We can see that luminescence enables one to detect earlier stages of the process of mounting the disorder which leads eventuslly to the amorphisstion. 4. DEFECTS RELATED TO THE IDENTITY OF THE IONS 4.1. Nitrogen The spectra of N + implanted type IIa crystals (Fig.6a) are dominated, before annealing, by lines 389 and 441.5 nm (GR-1 and TR-12 are relatively weak which is characteristic of N T implantation). Annealing at T > 500°C or electron irradiation produces the 575 nm system (Fig.6b). Then at T > I000°C the centres 503 nm (H3) and 415 nm (N3) appear while the intensity of 389 and 441.5 nm (end to less extent 575 nm) begins to decrease (Fig.6c). Since these centres ore observed in nitrogenlean crystals onl~ after N + implantation it means that all of them incorporate nitrogen atoms. In our samples in agreement with [5] no spin resonance signal due to substitutional N atoms was found. It suggests that the centres 389 and 441.5 nm are related to interstitial forms o~ N. The centres are hard ( ~ < 0 . 1 P a " ) which confirms
190
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
'ISN+IP,IpL.
EIo
>.
~
}..,
I
600 *c v~
I0 =
t..-
10~ tu
W,'c
,,,oo'cl hA. c_.l I 400
5'00
600
700 ,~,nm
Figure 6 : Spectra type IIa crystals implanted by N+
of
~.0 ""
600
fODO
f~tOO
T,-c
Figure 8 : Annealing of 3B9 nm centre~_in uuimplanted and "9E+ implanted samples
Figure 7 : Annealing of the centres created b~ N + implantation
their interstitial nature but does not permit the determination of their s~mmetr~. It is tempting to speculate about various interstitial configurations of which <100> split interstitial site seems to be highly prefgrable for trivalent nitrogen [18, 19]. The 575 nm system is known to be observed in all diamonds, but is particularly strong in type Ib [20] . The latter suggests its relation to a single isolated N atom. This is borne out by linear dependence of 575 nm intensity on I~+ implantation dose. Uniaxial stress data imply the model of 575 nm centre as a N atom in <1007 split interstitial site associated with the nearest (in
Summing up the behaviour of implanted N atoms we see that, immediately after the implantation or after annealing at T < I000°C, they form interstitial and single-atom centres. Then, at T > I000°C they tend to aggregate producing complexes containing 2 or 3 atomB (Fig.7). These data are to be compared to the transformation of isolated substitutioual N into A-aggretate observed at high pressure and T=1600-2000°C [24 , to the dissociation of A-aggregate at T > IBO0°C [25] and to the formation of A-a~gregates and H3 centres at T ~ 1500uC in irradiated diamonds [20] . The irradiation enhancement of aggregation, noted in[20] is even more pronounced in our experiments where this aggregation is observed alread~ at I000°C due to very high degree of disorder produced by ion implantation. The role of lattice disorder and the defect environment is manifest also in the difference between the annealing behavior of the centres 389 nm produced by implantation and those existing in unimplanted samples (Fig.8). 4.2. Metal impurities Specific luminescence lines were observed [26] in type IIa diamonds implanted (with subsequent annealing at 14000C) b~ metal impurities: Cr (741 rim), Zn (518 nm), Ag (398, 401, 405 rim), T1 (a group at 600-700 nm, the strongest 613 nm), Ni (a number of lines, ~ig.9). It was found that for some centres the dependences I(d) of the line intensity
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
191
2 ~.~
:
.~
t"~ --+ i I u
,,
5~/S
5~5 ~ , n m
Figure 1 2 : Isotopical shift of "546 nm" line R~HOYEO LRYER THICICNE$S ~ ,~
Figure 10 : Intensity versus removed layer thickness d, for the lines indicated
l.f +
,T35"
/I
] v]
;Ii V i l /'..l
Lu i
400
i
600
i
i
e
Ne*
i;
I..
800 .~.nrn i
Figure 9 : Spectra of: (s~ Ni + implanted diamonds, deep laser; (b) Ni + implsnted, surface layer; (c) the same as (b) minus Aand B-bands; (d) synthetic diamonds
~oo
~00
$00
600 &,nm
BOO
~ooo T,'C
Figure 13: Annealing of "546 nm" (H,D) and "535 nm" (He) centres
F i g u r e 11 : S p e c t r a of He+, Ne + and C+ i m p l a n t e d diamonds
I versus thickness of e removed layer d was very close to that for band B (which is related to the distribution of gross disorder produced by implantation) while for the others I(d) were much broader (Fig.10). The former are soft and their lines are broadened (especisll~ in the damaged layer near the surface) while the latter ere hard and no broadening was detected. It suggests that metal impurities can produce optical centres both of vscsnc~ and interstitial type. Since the lines 4B4, 451 , 885 nm are observed both in Ni + implanted natural end (unimpleted) synthetic diamonds (Pig.9) it means that the latter contain optically active Ni atoms introduced during the growth (Ni is used as a
catalyst). The similarity between the spectra of synthetic and I~i-implanted natural diamonds exists onl3 for the surface layer of the latter where soft Ni centres dominate. Synthetic diamonds produce very week if an~ luminescence of hard (interstitial?) Ni centres observed in deeper lasers of implsted crystals. Similarly, in natural (unimplented)diemonds the luminescence of the soft vacancy-nitrogen centres (N3,H3, 575 um) is always much stronger than that of hard, probsbl3 interstitial nitrogen centres (389, 441.5 rim) while the latter dominate (up to T ~ 600°C)in the spectra of N implsnted crystals. Thus interstitital impurity centres seem to be characteristic of ion implantstion° The absence of such ceutres
A.A. Gippius et al. / Defects production and interaction in ion-implanted diamond
192
in natural and synthetic diamonds (doped in equilibrium conditions) is probably due to lower stability of interstitial configurations. 4.3 H~drogen
5 6
and neutral gases
We have observed for the first tSme iD diamonds implanted by H +, D *, He ~, Ne ~ lines which can be attributed to centres containing these impurities (Fig. t1,12). Some of the lines are common to several impurities: "513 nm" with the wavelength changing slightly from 512.76 nm (H) to 513.66 nm (He) and 513.96 nm (Ne); "535 um" ( 534.9 (D), 535.4 (He), 535.8 (Ne) ] ; "560 nm" { 559.96 (He), 559.80 (Ne)} . This behsviour implies "mass effect" in the centres whose electronic structure is perturbed by the trapped impurities. The lines 522 nm and "546 nm" are observed only in He and H(D) implanted samples respectively which indicates that these impurities play more specific role in the formation of the centres. Phonon coupling is weak for all the centres, the 522 nm (He) centre was found to be ver~ hard. The nonmonotonic annealing curves (Fig. 13) suggest complicated kinetics of formation and destruction of the centres. The curves for He and D are rather similar, while for H an additional peak is observed suggesting a process which overcompensates the distruetion of the centres. This process is believed to be the release of b4drogen from the sources, probably ma~ma dropless, where it is "stored" [27~ These resultsdemonstratethat hydrogen,which is now recognized as one of the major impurities in diamond [27] , is capable of forming optically active centres which provides an effective tool of controlling the content of this impurity. The authors are indebted to A.V.Spitsin and V.A.Dravin for their assistence in the present work.
7 B 9 10
11 12 13 14 15 16
17 18 19 20 21
22 23 REFERENCES : I 2 3 4
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