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Yb luminescence in ion-implanted GaAs
~ ) Pergamon
Solid State Communications, Vol. 96, No. I 1, pp. 839-842, 1995 Elsevier Science Ltd Printed in Great Britain 0038-1098/95 $9.50+.00 O038-1098(95)O0582-X Yb LUMINESCENCE IN ION-IMPLANTED GaAs
JIIOMHHECIIEHIII, Lq Yb B H O H H O - H M I U I A H T H P O B A H H O M
CraAs
V.M. Konnov, T.V. Larikova, N.N. Loyko, V.A. Dravin V.V. Usliakov, A.A. Gippius P.N. Lebedev Physical Institute of the Academy of Sciences of Russia Leninsky prospect 53, Moscow 117924, Russia
(Received 25 July 1995 by L. V. Keldysh) Optical activation of Yb in GaAs was achieved by co-implantation of Yb and group VI elements (O,S,Se,Te). Efficient luminescence complexes "Yb+O+S/Se/Te" showed systematic increase of transition energy with the increase of chalcogen atom size and decrease of participating phonon energy with the increase of the atom mass, Keywords: A. semiconductors, C. impurities in semiconductors, D. electronic states (localized), E. luminescence
1. Introduction Rare-earth (RE) doped Si and III-V semiconductors have received increased attention for their possible use in optoelectronic devices which are expected to combine sharp, temperature stable, atomic-like emission due to internal electronic transitions within 4f-shell of RE impurities with the efficient electrical pumping of this emission by the energy transfer from the system of free electrons and holes, t The efficiency of energy transfer from electrons and holes to RE luminescence ccntres and probability of optical transitions within 4f shell of RE centres, depend upon the structure of luminescence centres, that is on the lattice position of RE impurity and on its possible association with other impurities and/or defects. Due to high chemical activity of RE elements, their associations with other chemical dopants are quite common. It is known that co-doping of RE in II-VI compounds is a prerequisite for their optical activity2 and photoluminescence efficiency of RE in ionic hosts is usually highest for RE complexes. For Er-doped silicon it has been reported that the addition of oxygen and fluorine enhanced the Er 3÷ emissions3. In III-V compounds the intensity of luminescence of Er3÷ in AlxGal_~As was found to he dramatically increased by oxygen co-doping On the other hand, oxygen co-implantation into GaAs:Er generally resulted in weaker Er 3÷ emission than those for the samples implanted with Er alone4.
In-P the luminescent properties of Yb are independent of the growth and doping techniques and that Yb atoms occupy only one type of lattice site (the most probable - substitutional location Ybh) which is made possible by size matching of the Yb3+ and In3÷ ions and favoured also by the admixture of ligand (<111> directional) wave function (covalency effects)S. It was suggested that optical activity of Yb in III-V compounds must be related to its substitutional location largely due to covalency effects which relax the parity selection rule and make parity forbidden dipole transition in the 4f-shell possible6. It was found that optical activity of Yb implanted in III-V binary and ternary compounds correlated with the substitutional fraction of Yb as determined by Rutherford back scattering (RBS)/channeling technique. In GaAs the substitutional fraction of Yb was b r o w the sensitivity of the method S, it was thought to be the reason for very weak (if any) Yb luminescence generally reported for GaAs doped with Yb either by ion implantation or during ¢pitaxial growth. In the present work we demonstrate that Yb can be rendered optically active in GaAs if it is associated with other impurities (in our case oxygen and chalcogens S, Se, Te) which enhance the optical transitions probabilities and/or the efficiency of energy transfer from electrons and holes to Yb luminescence centres.
2. Experimeniai GaAs samples were implanted at "High Voltage
As far as ytterbium is concerned it was found that in 839
840
Yb LUMINESCENCE IN ION-IMPLANTED OaAs
Engineering Europe" implanter at room temperature by ions of Yb and co-activators (O, S, Se, Te) using a set of energies and doses to produce a flat profile of dopants with concentrations in the range (10~7+10~9) cm-3 up to a depth 150 nm. We used samples with various concentration of background impurities controlled by secondary ions mass spectroscopy (SIMS) technique. After the implantation the samples were covered with a Si3N4 protective layer and then annealed up to 800°C. The photoluminescence (mostly at 77K) was recorded by the standard lock-in technique using liquid-nitrogencooled photomultiplier. 3. R e s u l t s
In this section we present results referring to impurities and defects contributing to optical activation of Yb in GaAs. We have performed systematic studies of oxygen co-doping of Yb-implanted GaAs basing upon our previous results on Yb-implanted InP and GaAs where it was found that of the three co-dopants used (O, F, Li) oxygen produced the largest enhancement of Yb luminescence7''. Implantation of Yb alone (up to concentrations of I0 ~9cm-3) into CraAs samples with background impurities concentration of about I0 ~ cm-3 produced no luminescence characteristic of f-f Yb3÷ transitions, in agreement with the data reported in the literature. Combined implantation (Yb+O) into the same samples
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produced a number of luminescence lines (to be discussed further) in the spectral range of Yb3+ (2Fsa-2FTa) transitions. For the series of samples with background impurities (Se, O, Si, C etc) concentration of about 10 ms cm-3 the implantation of Yb alone produced appreciable Yb 3÷ luminescence. Its intensity varied by more than an order of magnitude within this series of identically implanted samples. Combined implantation (Yb+O) produced much more intense Yb 3+ luminescence with much lower dispersion of the intensity within the series. In all cases the Yb3÷ luminescence was observed within the range of annealing temperatures (550+700)°C. It can be concluded from these results that oxygen (implanted or background) plays a crucial role in the optical activation of Yb atoms implanted in GaAs. Let us consider the spectra of Yb 3+ related luminescence. The characteristic Yb 3÷ lines are observed in the range (9800+:10200) A, i.e. (1.264+1.215)eV, where in some samples more than one hundred lines can be found, and in the range (10300+11000)~ i.e. (1.203+1.126) eV, with much less intensity. Basing upon the analysis of the spectra for various samples and for various annealing conditions it was possible to separate and nominate for convenience several most important groups of lines referring to various centres (Fig. 1): - X1 and X2 systems with emission lines at 10060 A (1.2321 eV) and 10054,1, (1.2329 eV) respectively; - Yl and Y2 systems differing by the ratio of intensities of lines at 9886 A (1.2533 eV), 9898 A (1.2518 eV), 9905 A (1.2509 eV) and 9908 ~,
(1.2505 eV)
40
(Yb+O)-1019 cm "3 Tan = 650"C 10060-- X1
9
10054 30
.ha
X2
20
_=
A 10
0 98
99
100 A,x102, k
101
Fig. 1. Luminescence spectra ofYb-related X and Y centres for various (A and B) samples.
The lines of Xt/X2 and YI/Y2 systems were found to correlate with much weaker groups of lines from the range (10300+11000)~.. The energy shifts between the correlated lines are 70.6 meV and 69.5 meV for X~ and X2 systems respectively and ~67 meV for Y1/Y2 systems. These weaker lines can be attributed to phonon assisted transitions. Occurrence of various combinations of (Yb+O)related lines in various samples subjected to identical implantation and annealing procedures suggested that the Yb luminescence centres might be complexes incorporating (besides oxygen) some other components. It was natural to look for unidentified components of Ybrelated complexes among the background impurities detected in part of our samples by SIMS technique (Se, O, Si, C etc). To establish the role of at least some of these impurities we performed the co-doping of Ybimplanted GaAs samples (containing relatively low concentration of background impurities) with chalcogens S, Se and Te. In the first experiment we implanted Yb in combination with either S or Se or Te. No Yb-related
Vol. 96, No. II
Yb LUMINESCENCE IN ION-IMPLANTED GaAs
luminescence was found. In the second experiment three species were implanted in the same samples: Yb, O and either S or Se or Te. In this case the Yb-related luminescence was observed (which confirmed the important role played by oxygen in the optical activation of Yb in GaAs) with lines specific for each of the chalcogenide impurities. The following are the data on these luminescence structures referring to (threecomponents) Yb-related centres (Fig. 2). Sulfur (S, centre): implantation of (Yb+O+S) produced a line at 10115 A (1.2254 eV) which at sulfur concentration lO~9cm-3 dominated in the spectrum and was an order of magnitude more intense than X and Y systems at the reference (implanted by Yb+O) parts of the samples. Tellurium (Tet centre): in (Yb+O+Te) implanted samples we found a line at 9984 A (1.2415 eV), which dominated in the spectrum at tellurium concentration 1019cm-3 being about an order of magnitude lower in intensity than the S~ line. Selenium (Sex centre): in the case of (Yb+O+Se) implantation it was difficult to separate the Se-related line at implants concentration (1017÷1018) cm-~. It was only at Se concentration 10~gcm-3 that the line at 10060A (1.2321 eV), previously referred to as X~, became
[. (Yb+S+O)-1019 cm "3 1 0 [ Tan= 660o C 10118
.
0 L (Yb+Se+O) ~ 10-1
.
.
.
,
,10060 Sel
[ ]
,
o [ 660 C
~0[ (Yb+Te+O) 5 I ~ ~L.,~.I
ii
, r.
19984
i ....
o t v I,I (Yb+O) 5I
~
7
Or,,.,i
98
....
99
7°°°c I ....
i
0 C 0 X. I ,.,
• I
....
100 101 Axl02,~,
|
102
Fig. 2. Spectra of Yb-related centres for various combinations of co-dopants.
dominant in the spectrum, so it was evident that X~ centre was in fact (Yb+O+Se) complex. The lines St, Set and Tet, similar to X and Y systems, were found to correlate with much weaker lines (supposedly, phonon-assisted transitions) shined from the (zero-phonon) lines St, Sel and Tel by 90.7, 70.6 and 56.7 meV respectively. Basing upon these results it can be suggested that considerable fraction (if not all) of luminescence lines observed in (Yb+O) implanted samples might be due to other background impurities, still to be identified. The role of radiation defects in the formation of Yb-related centres is not clear. There is no direct evidence that the centres under discussion incorporate radiation defects. On the other hand, we have found other Yb-related centres which seem to incorporate some components of radiation damage. The experimental conditions were the following. GaAs samples implanted with (Yb+O) and annealed to optimize Yb-related luminescence (in particular, X and Y systems) were irradiated with 1 MeV protons with the fluence of 10 m4cm-2, which quenched completely both the edge emission and Yb-related luminescence. Annealing at relatively low temperatures (-~400°C) produced new luminescence structures ZI (9964 A, 1.2440 eV) and Z2 (9972 A, 1.2430eV) which dominated in the spectra within the annealing temperature range (400-500)°C while X and Y systems were very weak. After annealing at ~550°C Z-system disappeared and at 600°C X and Y systems appeared again. Z-centres can be either the initial (prior to proton irradiation) X or Y centres modified by the capture of some component of proton induced radiation damage (released at higher temperatures) or some new (Yb+O)-based complexes incorporating radiation damage. 4. Discussion
In the present work we have shown that, contrary to the general belief, ytterbium in GaAs can be optically active. Among optically active Yb-related centres we have identified complexes comprising Yb, O and chalcogen impurities S, Se, Te. Formation and destruction of Yb related complexes imply complicated reactions between their components, at least some of them being mobile at the corresponding annealing temperatures. These complex luminescence centres are rather efficient, the intensity of luminescence of, say, (Yb+O+S) complex is comparable to the intensity of Yb related luminescence in GaP and even in In_P, where high optical activity of Yb was attributed to isolated substitutional Yb 3+ ions. It should be born in mind that the correlation between optical activity of Yb and Er and their lattice position is not definitely established. First Of all, it is not evident that
841
842
'To LUMINESCENCE IN ION-IMPLANTED GaAs
the substitutional RE atoms detected by RBS/channeling measurements are the same RE atoms that are seen in luminescence experiments. Moreover there are some data which show that Er atoms loose their optical activity when located substitutionally 9. It is not excluded that Yb atoms can move from interstitial sites to substitutional sites when they form optically active complexes with O and S (or Se, Te), as it was observed for (Er+O) doping of GaAs ~°. Alternatively, it can be suggested that lattice position of RE atoms might not be the only (and even not the most important) factor of their optical activity if they are incorporated in some complex aggregated centres. Basing upon our data we can draw some preliminary conclusions concerning the correlation of the atomic structure of Yb-related complexes and their energy characteristics. First of all, one can see the systematic change of the optical transition energy within a group of similar centres: Yb+O+S/Se/Te. Transition energy for S~, Se~ and Te, centres (1.2254, 1.2321 and 1.2415 eV) increases and the nepheioxetic shift, i.e. the difference between the free Yb3+ ion transition (1.2705 eV) and the transition energy of a given centre (45.1, 38.4 and 29.0 meV) decreases with the increase of the coactivator size. If we accept the assignment of weak lines in the region (10300+.'11000) A to phonon-assisted transitions then another trend is seen: the phonon energy for S~, Set and Te~ centres (90.7, 70.6 and 56.7 meV) decreases with the increase of the S/Se/Te
Vol. 96, No. I 1
coactivator mass. Basing upon the present data it is not possible to separate the roles of the two coactivators as far as the energy structure of the luminescence centres is concerned: the definite correlation between the transition energies and properties of one of the coactivators (S/Se/Te) does not exclude the contribution of another (oxygen). Oxygen seems to be an ever-present component of Yb-related luminescence centres in GaAs (though no centres with oxygen co-activator alone have so far been found), so it is possible that its main role is in the energy transfer process. 5. Conclusion
It has been shown that Yb in GaAs can be optically active provided it is associated with other impurities (oxygen and chalcogenides S, Se, Te). In view of these data it can be suggested that lattice position of RE atoms might not be the only (and even not the most important) factor of their optical activity. Oxygen has been shown to play a crucial role in optical activation of Yb3+ ions, possibly due to its participation in the energy transfer process.
Acknowledgements - - This work was supported by the Russian Foundation for Fundamental Research (project N~. 93-02-16122), National Program "Physics of Solid State Nanostructures" (project N~. 1-008) and by the International Science Foundation (grants N~.N~. 9U000, 9U300).
References
1. Rare Earth Dooed Semiconductors. ed. G.S.Pomrenke, P.B.Klein, D.W.Langer, Material Research Society Symposium Proceedings, v301, 1993, Material Research Society, Pittsburgh, Pennsylvania. 2. R.Boyn, "4f-4fLuminescenc¢ of Rare-Earth Centres in II-VI Compounds", phys. star. sol. (b), v.48, pp.11-47, 1988. 3. J.Michei, J.L.Benton, R.F.Ferrante, DC.Jacobson, D.J.Eaglesham, E.A.Fitzgerald, Y.-HXie, J.M.Poate and L.C Kimerling, "Impurity Enhancement of the 1.54-gm Er3÷Luminescence in Silicon", J. Appl. Phys., v.70, No.5, pp.2672-2677, 1991. 4. J.E.Colon, DWEIsaesser, Y.K.Y¢o, R.L.Hengehold and G.S.Pomrenke,"Luminescence Study of the Intra-4f Emission from GaAs:(Er+O) and AlxGat_xAs : (Er+O)", in [1], ppA69-174. 5. A.Kozanecki and R.Greetzschel, "Lattice Location and Optical Activity of Yb in III-V Senticonducting Compounds", J. Appl. Phys., v.68, No.2, pp.517-522, 1990.
6. A.Kozanecki, "Correlation of the Location in Crystal Lattice and Optical Activity of the Yb Impurity in III-V Compounds", in [ 1], pp.219-224. 7. V.M.Konnov and NN.Loiko, "Photoluminescence of In_P Subjected to Joint Implantation of Yb and F, O, or Li", Bulletin of the Lebedev Physics Institute, No.12, pp.20--23, 1993, Allerton Press, Inc./New York. 8. V.MKonnov, N.N.Loiko and V.ADravin, "The Effect of O, F and Li on Luminescence of GaAs Implanted with Yb", Bulletin of the Lebedev Physics Institute, No.3, pp.20-23, 1994, Allerton Press, Inc./New York. 9. AKozanecki, M.Chan, C.Jeynes, B.Sealy and K.Homewood, "Lattice Location of Erbium Implanted into GaAs", Sol. St. Comm., v.78, No,8, pp.763-766, 1991. 10. K.Takahai, A.Taguchi and YHorikoshi, "Atomic Configuration of the Er-O Luminescence Center in Er-doped GaAs", J. Appl. Phys., v.76, No.7, pp.4332-4339, 1994.
Solid State Communications, Vol. 96, No. I 1, pp. 839-842, 1995 Elsevier Science Ltd Printed in Great Britain 0038-1098/95 $9.50+.00 O038-1098(95)O0582-X Yb LUMINESCENCE IN ION-IMPLANTED GaAs
JIIOMHHECIIEHIII, Lq Yb B H O H H O - H M I U I A H T H P O B A H H O M
CraAs
V.M. Konnov, T.V. Larikova, N.N. Loyko, V.A. Dravin V.V. Usliakov, A.A. Gippius P.N. Lebedev Physical Institute of the Academy of Sciences of Russia Leninsky prospect 53, Moscow 117924, Russia
(Received 25 July 1995 by L. V. Keldysh) Optical activation of Yb in GaAs was achieved by co-implantation of Yb and group VI elements (O,S,Se,Te). Efficient luminescence complexes "Yb+O+S/Se/Te" showed systematic increase of transition energy with the increase of chalcogen atom size and decrease of participating phonon energy with the increase of the atom mass, Keywords: A. semiconductors, C. impurities in semiconductors, D. electronic states (localized), E. luminescence
1. Introduction Rare-earth (RE) doped Si and III-V semiconductors have received increased attention for their possible use in optoelectronic devices which are expected to combine sharp, temperature stable, atomic-like emission due to internal electronic transitions within 4f-shell of RE impurities with the efficient electrical pumping of this emission by the energy transfer from the system of free electrons and holes, t The efficiency of energy transfer from electrons and holes to RE luminescence ccntres and probability of optical transitions within 4f shell of RE centres, depend upon the structure of luminescence centres, that is on the lattice position of RE impurity and on its possible association with other impurities and/or defects. Due to high chemical activity of RE elements, their associations with other chemical dopants are quite common. It is known that co-doping of RE in II-VI compounds is a prerequisite for their optical activity2 and photoluminescence efficiency of RE in ionic hosts is usually highest for RE complexes. For Er-doped silicon it has been reported that the addition of oxygen and fluorine enhanced the Er 3÷ emissions3. In III-V compounds the intensity of luminescence of Er3÷ in AlxGal_~As was found to he dramatically increased by oxygen co-doping On the other hand, oxygen co-implantation into GaAs:Er generally resulted in weaker Er 3÷ emission than those for the samples implanted with Er alone4.
In-P the luminescent properties of Yb are independent of the growth and doping techniques and that Yb atoms occupy only one type of lattice site (the most probable - substitutional location Ybh) which is made possible by size matching of the Yb3+ and In3÷ ions and favoured also by the admixture of ligand (<111> directional) wave function (covalency effects)S. It was suggested that optical activity of Yb in III-V compounds must be related to its substitutional location largely due to covalency effects which relax the parity selection rule and make parity forbidden dipole transition in the 4f-shell possible6. It was found that optical activity of Yb implanted in III-V binary and ternary compounds correlated with the substitutional fraction of Yb as determined by Rutherford back scattering (RBS)/channeling technique. In GaAs the substitutional fraction of Yb was b r o w the sensitivity of the method S, it was thought to be the reason for very weak (if any) Yb luminescence generally reported for GaAs doped with Yb either by ion implantation or during ¢pitaxial growth. In the present work we demonstrate that Yb can be rendered optically active in GaAs if it is associated with other impurities (in our case oxygen and chalcogens S, Se, Te) which enhance the optical transitions probabilities and/or the efficiency of energy transfer from electrons and holes to Yb luminescence centres.
2. Experimeniai GaAs samples were implanted at "High Voltage
As far as ytterbium is concerned it was found that in 839
840
Yb LUMINESCENCE IN ION-IMPLANTED OaAs
Engineering Europe" implanter at room temperature by ions of Yb and co-activators (O, S, Se, Te) using a set of energies and doses to produce a flat profile of dopants with concentrations in the range (10~7+10~9) cm-3 up to a depth 150 nm. We used samples with various concentration of background impurities controlled by secondary ions mass spectroscopy (SIMS) technique. After the implantation the samples were covered with a Si3N4 protective layer and then annealed up to 800°C. The photoluminescence (mostly at 77K) was recorded by the standard lock-in technique using liquid-nitrogencooled photomultiplier. 3. R e s u l t s
In this section we present results referring to impurities and defects contributing to optical activation of Yb in GaAs. We have performed systematic studies of oxygen co-doping of Yb-implanted GaAs basing upon our previous results on Yb-implanted InP and GaAs where it was found that of the three co-dopants used (O, F, Li) oxygen produced the largest enhancement of Yb luminescence7''. Implantation of Yb alone (up to concentrations of I0 ~9cm-3) into CraAs samples with background impurities concentration of about I0 ~ cm-3 produced no luminescence characteristic of f-f Yb3÷ transitions, in agreement with the data reported in the literature. Combined implantation (Yb+O) into the same samples
Vol. 96, No. 11
produced a number of luminescence lines (to be discussed further) in the spectral range of Yb3+ (2Fsa-2FTa) transitions. For the series of samples with background impurities (Se, O, Si, C etc) concentration of about 10 ms cm-3 the implantation of Yb alone produced appreciable Yb 3÷ luminescence. Its intensity varied by more than an order of magnitude within this series of identically implanted samples. Combined implantation (Yb+O) produced much more intense Yb 3+ luminescence with much lower dispersion of the intensity within the series. In all cases the Yb3÷ luminescence was observed within the range of annealing temperatures (550+700)°C. It can be concluded from these results that oxygen (implanted or background) plays a crucial role in the optical activation of Yb atoms implanted in GaAs. Let us consider the spectra of Yb 3+ related luminescence. The characteristic Yb 3÷ lines are observed in the range (9800+:10200) A, i.e. (1.264+1.215)eV, where in some samples more than one hundred lines can be found, and in the range (10300+11000)~ i.e. (1.203+1.126) eV, with much less intensity. Basing upon the analysis of the spectra for various samples and for various annealing conditions it was possible to separate and nominate for convenience several most important groups of lines referring to various centres (Fig. 1): - X1 and X2 systems with emission lines at 10060 A (1.2321 eV) and 10054,1, (1.2329 eV) respectively; - Yl and Y2 systems differing by the ratio of intensities of lines at 9886 A (1.2533 eV), 9898 A (1.2518 eV), 9905 A (1.2509 eV) and 9908 ~,
(1.2505 eV)
40
(Yb+O)-1019 cm "3 Tan = 650"C 10060-- X1
9
10054 30
.ha
X2
20
_=
A 10
0 98
99
100 A,x102, k
101
Fig. 1. Luminescence spectra ofYb-related X and Y centres for various (A and B) samples.
The lines of Xt/X2 and YI/Y2 systems were found to correlate with much weaker groups of lines from the range (10300+11000)~.. The energy shifts between the correlated lines are 70.6 meV and 69.5 meV for X~ and X2 systems respectively and ~67 meV for Y1/Y2 systems. These weaker lines can be attributed to phonon assisted transitions. Occurrence of various combinations of (Yb+O)related lines in various samples subjected to identical implantation and annealing procedures suggested that the Yb luminescence centres might be complexes incorporating (besides oxygen) some other components. It was natural to look for unidentified components of Ybrelated complexes among the background impurities detected in part of our samples by SIMS technique (Se, O, Si, C etc). To establish the role of at least some of these impurities we performed the co-doping of Ybimplanted GaAs samples (containing relatively low concentration of background impurities) with chalcogens S, Se and Te. In the first experiment we implanted Yb in combination with either S or Se or Te. No Yb-related
Vol. 96, No. II
Yb LUMINESCENCE IN ION-IMPLANTED GaAs
luminescence was found. In the second experiment three species were implanted in the same samples: Yb, O and either S or Se or Te. In this case the Yb-related luminescence was observed (which confirmed the important role played by oxygen in the optical activation of Yb in GaAs) with lines specific for each of the chalcogenide impurities. The following are the data on these luminescence structures referring to (threecomponents) Yb-related centres (Fig. 2). Sulfur (S, centre): implantation of (Yb+O+S) produced a line at 10115 A (1.2254 eV) which at sulfur concentration lO~9cm-3 dominated in the spectrum and was an order of magnitude more intense than X and Y systems at the reference (implanted by Yb+O) parts of the samples. Tellurium (Tet centre): in (Yb+O+Te) implanted samples we found a line at 9984 A (1.2415 eV), which dominated in the spectrum at tellurium concentration 1019cm-3 being about an order of magnitude lower in intensity than the S~ line. Selenium (Sex centre): in the case of (Yb+O+Se) implantation it was difficult to separate the Se-related line at implants concentration (1017÷1018) cm-~. It was only at Se concentration 10~gcm-3 that the line at 10060A (1.2321 eV), previously referred to as X~, became
[. (Yb+S+O)-1019 cm "3 1 0 [ Tan= 660o C 10118
.
0 L (Yb+Se+O) ~ 10-1
.
.
.
,
,10060 Sel
[ ]
,
o [ 660 C
~0[ (Yb+Te+O) 5 I ~ ~L.,~.I
ii
, r.
19984
i ....
o t v I,I (Yb+O) 5I
~
7
Or,,.,i
98
....
99
7°°°c I ....
i
0 C 0 X. I ,.,
• I
....
100 101 Axl02,~,
|
102
Fig. 2. Spectra of Yb-related centres for various combinations of co-dopants.
dominant in the spectrum, so it was evident that X~ centre was in fact (Yb+O+Se) complex. The lines St, Set and Tet, similar to X and Y systems, were found to correlate with much weaker lines (supposedly, phonon-assisted transitions) shined from the (zero-phonon) lines St, Sel and Tel by 90.7, 70.6 and 56.7 meV respectively. Basing upon these results it can be suggested that considerable fraction (if not all) of luminescence lines observed in (Yb+O) implanted samples might be due to other background impurities, still to be identified. The role of radiation defects in the formation of Yb-related centres is not clear. There is no direct evidence that the centres under discussion incorporate radiation defects. On the other hand, we have found other Yb-related centres which seem to incorporate some components of radiation damage. The experimental conditions were the following. GaAs samples implanted with (Yb+O) and annealed to optimize Yb-related luminescence (in particular, X and Y systems) were irradiated with 1 MeV protons with the fluence of 10 m4cm-2, which quenched completely both the edge emission and Yb-related luminescence. Annealing at relatively low temperatures (-~400°C) produced new luminescence structures ZI (9964 A, 1.2440 eV) and Z2 (9972 A, 1.2430eV) which dominated in the spectra within the annealing temperature range (400-500)°C while X and Y systems were very weak. After annealing at ~550°C Z-system disappeared and at 600°C X and Y systems appeared again. Z-centres can be either the initial (prior to proton irradiation) X or Y centres modified by the capture of some component of proton induced radiation damage (released at higher temperatures) or some new (Yb+O)-based complexes incorporating radiation damage. 4. Discussion
In the present work we have shown that, contrary to the general belief, ytterbium in GaAs can be optically active. Among optically active Yb-related centres we have identified complexes comprising Yb, O and chalcogen impurities S, Se, Te. Formation and destruction of Yb related complexes imply complicated reactions between their components, at least some of them being mobile at the corresponding annealing temperatures. These complex luminescence centres are rather efficient, the intensity of luminescence of, say, (Yb+O+S) complex is comparable to the intensity of Yb related luminescence in GaP and even in In_P, where high optical activity of Yb was attributed to isolated substitutional Yb 3+ ions. It should be born in mind that the correlation between optical activity of Yb and Er and their lattice position is not definitely established. First Of all, it is not evident that
841
842
'To LUMINESCENCE IN ION-IMPLANTED GaAs
the substitutional RE atoms detected by RBS/channeling measurements are the same RE atoms that are seen in luminescence experiments. Moreover there are some data which show that Er atoms loose their optical activity when located substitutionally 9. It is not excluded that Yb atoms can move from interstitial sites to substitutional sites when they form optically active complexes with O and S (or Se, Te), as it was observed for (Er+O) doping of GaAs ~°. Alternatively, it can be suggested that lattice position of RE atoms might not be the only (and even not the most important) factor of their optical activity if they are incorporated in some complex aggregated centres. Basing upon our data we can draw some preliminary conclusions concerning the correlation of the atomic structure of Yb-related complexes and their energy characteristics. First of all, one can see the systematic change of the optical transition energy within a group of similar centres: Yb+O+S/Se/Te. Transition energy for S~, Se~ and Te, centres (1.2254, 1.2321 and 1.2415 eV) increases and the nepheioxetic shift, i.e. the difference between the free Yb3+ ion transition (1.2705 eV) and the transition energy of a given centre (45.1, 38.4 and 29.0 meV) decreases with the increase of the coactivator size. If we accept the assignment of weak lines in the region (10300+.'11000) A to phonon-assisted transitions then another trend is seen: the phonon energy for S~, Set and Te~ centres (90.7, 70.6 and 56.7 meV) decreases with the increase of the S/Se/Te
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coactivator mass. Basing upon the present data it is not possible to separate the roles of the two coactivators as far as the energy structure of the luminescence centres is concerned: the definite correlation between the transition energies and properties of one of the coactivators (S/Se/Te) does not exclude the contribution of another (oxygen). Oxygen seems to be an ever-present component of Yb-related luminescence centres in GaAs (though no centres with oxygen co-activator alone have so far been found), so it is possible that its main role is in the energy transfer process. 5. Conclusion
It has been shown that Yb in GaAs can be optically active provided it is associated with other impurities (oxygen and chalcogenides S, Se, Te). In view of these data it can be suggested that lattice position of RE atoms might not be the only (and even not the most important) factor of their optical activity. Oxygen has been shown to play a crucial role in optical activation of Yb3+ ions, possibly due to its participation in the energy transfer process.
Acknowledgements - - This work was supported by the Russian Foundation for Fundamental Research (project N~. 93-02-16122), National Program "Physics of Solid State Nanostructures" (project N~. 1-008) and by the International Science Foundation (grants N~.N~. 9U000, 9U300).
References
1. Rare Earth Dooed Semiconductors. ed. G.S.Pomrenke, P.B.Klein, D.W.Langer, Material Research Society Symposium Proceedings, v301, 1993, Material Research Society, Pittsburgh, Pennsylvania. 2. R.Boyn, "4f-4fLuminescenc¢ of Rare-Earth Centres in II-VI Compounds", phys. star. sol. (b), v.48, pp.11-47, 1988. 3. J.Michei, J.L.Benton, R.F.Ferrante, DC.Jacobson, D.J.Eaglesham, E.A.Fitzgerald, Y.-HXie, J.M.Poate and L.C Kimerling, "Impurity Enhancement of the 1.54-gm Er3÷Luminescence in Silicon", J. Appl. Phys., v.70, No.5, pp.2672-2677, 1991. 4. J.E.Colon, DWEIsaesser, Y.K.Y¢o, R.L.Hengehold and G.S.Pomrenke,"Luminescence Study of the Intra-4f Emission from GaAs:(Er+O) and AlxGat_xAs : (Er+O)", in [1], ppA69-174. 5. A.Kozanecki and R.Greetzschel, "Lattice Location and Optical Activity of Yb in III-V Senticonducting Compounds", J. Appl. Phys., v.68, No.2, pp.517-522, 1990.
6. A.Kozanecki, "Correlation of the Location in Crystal Lattice and Optical Activity of the Yb Impurity in III-V Compounds", in [ 1], pp.219-224. 7. V.M.Konnov and NN.Loiko, "Photoluminescence of In_P Subjected to Joint Implantation of Yb and F, O, or Li", Bulletin of the Lebedev Physics Institute, No.12, pp.20--23, 1993, Allerton Press, Inc./New York. 8. V.MKonnov, N.N.Loiko and V.ADravin, "The Effect of O, F and Li on Luminescence of GaAs Implanted with Yb", Bulletin of the Lebedev Physics Institute, No.3, pp.20-23, 1994, Allerton Press, Inc./New York. 9. AKozanecki, M.Chan, C.Jeynes, B.Sealy and K.Homewood, "Lattice Location of Erbium Implanted into GaAs", Sol. St. Comm., v.78, No,8, pp.763-766, 1991. 10. K.Takahai, A.Taguchi and YHorikoshi, "Atomic Configuration of the Er-O Luminescence Center in Er-doped GaAs", J. Appl. Phys., v.76, No.7, pp.4332-4339, 1994.