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The luminescence of copper in zinc oxide
Paper presented at ICDS-12 Amsterdam, August 31 - September 3, 1982
Physica 116B (1983) 492-499 North-Holland Pubfishing Company
THE LUMINESCENCE OF COPPER IN ZINC OXIDE
C. WEST, D.J. ROBBINS, P.J. DEAN and W. HAYES*
Royal Signals and Radar Establishmen t St. Andrews Road, Great Malvern, Worcestershire WR14 3PS, U.K. * Clarendon Laboratory, Parks Road, Oxford, U.K. Magneto-optical data on the low-temperature infrared luminescence characteristic of Cu-doped ZnO are discussed• The salient features of these Zeeman data can be explained by a copper-donor associate in which the associated donor has unpaired spin and the axis of the defect is aligned with the c axis of wurtzite ZnO. The observed relative strengths of the Zeeman components are justified using arguments which involve magnetic mixing and a second-order process which requires a spin flip within the weakly coupled donor. A very small zero-field splitting of ~ 1.3 cm-I deduced from the Zeeman data is attributed to exchange splitting between a photo-excited electron and the Cu2+(3d 9) core.
I.
INTRODUCTION
Zinc oxide is a II-Vl semiconductor which grows with a wurtzite lattice structure and has a direct energy gap at the centre of the Brillouin zone of magnitude ~ 27710 cm -I (3.44 eV) at 4°K. The conduction band is non-degenerate, symmetry r6, while the degeneracy of the valence band is lifted by a negative spln-orbit splitting and hy crystal-field splitting, resulting in a F~ uppermost band with two other bands lying withln an energy range of only ~ 113 cm -I (~ 14 meV). Co~per forms the neutral substitutional centre Cu~ +, which exhibits a number of well-known optlcal spectra I , and is known to compensate n-type conductivity in ZnO due to interstitial Zn or H. Conductivity studies place the ground state of Cu$~(3d 9) only 1530 cm-I (0.19 eV) below the c~duction band 2, requiring a very large energy for the reaction:
Cug<3d)+ CUz (3dlO)+
(1)
This reaction produces the aceeptor state Cu~(3dlO), which is singly negatively charged with respect to the lattice. A set of moderately weakly bound excited hole st~t~s have been identified with this centre in ZnO~,~p the lowest of which relaxes radia~ively to the F4 (C3v) ground state of Cu2+(3d N) giving characteristic green luminescence. Attention has also recently been drawn to an additional broadened luminescence component with similar optical phonon replication to that of the main series in the green luminescence 5. Although no interpretation was offered by Kuhnert and HelbSg 5, it seems most probable that the ~ 225 om-L splitting between the leading members of each series represents the spin-orbit splitting in the ground state of Cu2+(3d9). Thus, the new series originates from radiative hole relaxation into the spin-orbit split excited state, symmetry F~ like the ground state of Cu 2+ in wurtzite ZnO. ~
Absorption 4 or photoluminescence excitation 4'6 studies (PLE) reveal the full set of relatively weakly bound hole states, which also have F 4 symmetry. These bound hole states have been recently related to the three upper valence bands in the pure wurtzite lattice which produce levels of r~ symmetry in the CaV point group of the Cu. impurity 7. Optical a~sorption reveals some o~nthe structure within Cu~(Bd9). This state is split by the cubic crystal field (Td) into 2T 2 and 2E separated by 5800 cm -I. The triple orbital degeneracy of the 2T 2 ground state is lifted by the combined effects of spinorbit coupling and the small trigonal field characteristic of the wurtzite lattice, while the 2E excited hole state is split into two subcomponents by the crystal field. Transitions from the F4 ground state component derived from 2T 2 to both subcomponents of 2E are clearly observed at 5772 cm -I and 5812 cm-I in optical absorption I and fix the F5, F6 + F4 splitting of the 2E substates at 39.5 cm -I. The detailed magnetic properties of these optical absorption components I are consistent with the electron spin resonance properties of the Cu~ + =round • n~ state and with expectatlon of a hole tlghtly bound to copper giving Cu~+(3d9). Optical absorption at elevated temperatures I led to the suggestion of a F4 ÷ F4 splitting in the ground state of 116 cm -I. However, it has been argued elsewhere 6 that this hot absorption component involves a phonon replica and that the smallest ground state splitting must be larger than 116 cm -I, perhaps 225 cm -I as already described. No luminescence has been observed from the 2E subcomponents, in sharp contrast to the behaviour of Cu Z in ZnS 8. There is no clear explanation for thls strlklng difference. It is particularly unexpected since phonon coupling is not very strong in either host, so it seems unlikely that the luminescence should be quenched by multiphonon
C West et al. / The luminescence ofCu-doped ZnO
relaxation from the 2E states in ZnO. However, highly structured infrared luminescence has been observed in ZnO:Cu~at 2°K and correlates strongly with the introduction of Cu both during6c~ystal growth and in subsequent diffusion ' . The infrared luminescence has a strong sharp no-phonon component at 6884 om-i (0.8533 eV), II00 cm -I (136 meV) above the centroid of the absorption lines. Evidently, the 6884 cm -I luminescence cannot origiHate from the energy states attributed to Cu~(Bd9)."± The present paper is concerned with the identification of the Cu-related infrared luminescence, mainly from Zeeman analysis of the 6884 am-I no-phonon lines. We shall see that there is a small unresolved zero-field splitting of 1.3 cm -I arising from the excited state of the luminescence transition. We also show that all of the observable experimental data can be accounted for with an acceptor-donor associate model involving Cu Z , with the sym• . n metry axls of the assoclate selected so as not to reduce the point group symmetry in wurtzite ZnO.
6O00 I
2.
EXPERIMENTAL
The crystals studied in this work were prepared and in some c~ses indlffused by Cu as described by Dean et al v. The general technique of luminescence spectroscopy was also described previously 6. Zeeman measurements were made in the Clarendon Laboratory, Oxford, with a spllt-coll super-conducting magnet producing magnetic fields of up to 8.5T at 2K.
6S00 !
70O0 I
6500
7000
~nO=Cu OF40
6000
c m -1
Figure 1 : The low temperature luminescence spectrum (I.6K) of a centre associated with Cu in ZnO.
3. We had initially considered an alternative model which involved magnetic triplets in the ground and excited states, with the triplet degeneracy lifted by a weak trigonal-field splitting. Such a transition could be attributed to luminescence between the lowest crystal field components of Cu3+(d 8) which would have 3T I (ground) and 3T 2 (excited) levels in a zinc blende lattice. Many aspects of this model proved to be very promising; a sign difference between the trigonal-field splitting for the excited and ground states was predicted and the energy of the transition was not unreasonable. However, the observed nearly isotropic nature of the g values and the closeness of their magnitudes to that of a free electron did not correspond to values predicted by the model. This disagreement and the improbability of a Cu 3+ state in ZnO based on general experience of Cu in binary semlnonductors, favours the alternative donor-acceptor associate model which is discussed in detail in section 3.3, We find that this latter model can also account for many features observed in the PLE spectra of the 6884 cm -I ~O.8533 eV) luminescence reported by Dean et al u.
493
DISCUSSION
3.1 Experimental Results The near-infrared luminescence associated with copper in zinc oxide is shown in Figure i. This spectrum has been previously reported by Dean et al 6 and is dominated by a strong zerophonon (ZP) llne at 6884 cm -I. In addition, Dean et al 6 reported an extra weak ZP llne occurring 3.95 cm -I to higher energy which shows effects of thermalisation with respect to the first component. This line is not visible in the present experiments due to the very low temperature (2K). The phonon side band is highly structured, with main replicas lying near 6365 cm -I and 6290 cm -I. When a magnetic field is applied parallel to the c-axis of the crystal the ZP line at 1.45 microns splits into two components, each component moving linearly with increasing field (Figures 2a, 3a). It is noteworthy that the two components do not extrapolate back to the same origin, indicating a residual zero-fleld splitting. With the magnetic field perpendicular to the c-axis five components are seen (Figures 2b, 3b). Similar data have been reported previously, but lacked the full complement of lines observed in the present spectra, thereby inhibiting a complete discussion (R Heinzeg). Of these five lines, only three survive at high fields, due to thermalisation. Unfortunately, polarisation data appeared to be unreliable and inconclusive in both of these sets of spectra. The crystal was rotated from B [1 c to B i c when the crystal was subject to a magnetic field of 8.5 Tesla. These data are given in Figure 4. It appears that there is considerable switching of intensity between the components. This will prove to be important when considering the symmetries of the energylevels.
C. West e t al.
494
6892
cm -1 BSg~
I
B//c
/
The l u m i n e s c e n c e o f Cu-doped Z n O
~e~2F
I I"
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I 8888 6886
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687~
686B
6868 MAGNETIC
FIELD
(Tesla)
0.
2.
i
I
1 MAGNETIC FIELD
(TesIa)
Figure 2 : The effects of a magnetic field, applied parallel (a), and perpendicular (b) to the c axis of ZnO, on the 6884 cm -] zero phonon luminescence line due to copper at ].6K.
3.2 Determination of the Emitting and Ground States of the Infrared Luminescence Transition If we first consider the experimental data with the magnetic field applied along the c-axis of the crystal (Figure 2a) we see that the two components extrapolate back to different zero-field positions. The zero-field splitting is approximately 1.3 cm -I. The Zeeman data for the magnetic field applied perpendicular to the c-axls (Figure 2b) are also consistent with two levels at zero field. The central line of the experimental data is the only line which is seen to come from the lower-energy zero-field component. The highest-energy line in the B I c Zeeman spectra (Figure 3b) thermalises at high field, indicating an excited state splitting in the magnetic field. However~ the corresponding low-energy line, which should have a large Boltzmann population factor, remains very weak even at high field. These large differences in oscillator strength for the various Zeeman components suggest that very different selection rules apply to the two zero-field-split states in the transition. Further we note that the general magnitudes of the splitting for both directions of magnetic field suggest g values which are near to those for free spin systems. Consider a model where the excited state consists of a spin doublet close to a higher spin
quartet, and where the ground state is also a spin doublet. With suitable choices of g values for these states the experimental data points agree closely with the calculated values over the range 3-9 Tesla, as shown in Figure 2. With the magnetic field applied perpendicular to the c-axis the system would be complicated by mixing of the two excited states, but the high field data should be asymptotic to the lines in the absence of these interactions. The positions of the lines drawn in Figures 2a and 2b are calculated using the arguments of section 3.3 and the parameter values
6exch
= 1.3 cm -I '
gll
(ground state)
gll (excited state:
-
F 6) = 0.12
½ (gll (ground state) + gll (excited state:
F8)) = 2.00
gl (ground state) = 2.00, g± (excited state:
F8) = 1.81
It is seen that the data have a splitting pattern consistent with a quartet - doublet excited level scheme radiating to a doublet ground state. We now discuss the origin of these energy states in terms of the CUZn-donor associate model.
C West et aL / The luminescence ofCu-doped ZnO
495
3.3 The Acceptor-Donor Associate Model DII¢
[nO; ¢u
S~¢
IF 4 0
!
n
1:5
j
~
,
'
,
.:_,&
,
--,
Figure 3 : The experimental data showing how the spectrum due to the donor-acceptor associate described in the text progresses when a magnetic field is applied parallel (a) and perpendicular (b) to the c axis of ZnO (T = 1.6K).
6Be2 . . . . . . . . . . . . . +e~
B=8. 5
eeee
t
Tes]o
ZnO;Cu Dr40
P//c
'
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~
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+o:,oi
¢
l+
-~-,--,~ .~i-
'+'++'J, 4 ~ i ~ ~ i ++ i t~AGNETIC FIELD ORIENT/iT,[ON (de 9)
Figure 4 : The effect of rotating the magnetic field (8.5 Tesla) from a position where the field is parallel to the c axis of ZnO to the position where the field is perpendicular to the e axis on the 6884 om-I zero phonon line due to the axial associate at 1.6K.
The experimental data can be understood in terms of a substitutional • CUz_LL acceptor compensated by , • a double donor specles, the axls of the resulting associate being aligned with the c-axis of the ZnO lattice to preserve the trigonal point group symmetry. The two electrons in the neutral double donor are assumed to be spin-paired in an S-like (a t sy~netry) orbital. In the ground state of the associate one electron is transferred from the donor to the Cu~n+(d9) centre, filling the Cu 3d-shell and leaving an unpaired electron on the donor; this configuration is indicated at the top of Figure 5. The compensated [Cul+(dlO)]" centre carries an effective negative lattice charge, and the ground state of the associate is stabilised by the Coulomb interaction with the ionised donor. For an interionic separation of ~ 2 ~ in the associate, the Coulomb stabilisation energy will be of order 0.9 eV in ZnO. The d I0 configuration of Cuzn is a filled shell and the only magnetic behaviour of the associate in the ground state therefore stems from the spin of the ionised D+(s I) donor. For an S-like donor orbital this level will exhibit all the features of a pure S = ~ state; in particular the g-value will be approximately equal to 2 and will be reasonably isotropic as required by the experimental data. The excited configuration of the associate is assumed to involve promotion of an electron from the d-shell of the [Cuzn(dl0)] - centre into a more spatially-extended orbital of S-like syn~netry but remaining bound to the associate. The excited state can then be presented as [Cu~(d9)]o - e~ - D+(sl), as indicated in the lower part of Figure 5. In this case there are three contributary terms to the angular momentum of the electronic states. First,^$n the point group C3V the ground state for C u ~ ( d 9) has r 4 sy~netry, and can behave in the strong trigonal field of the associate as a pure spin state with no orbital momentum provided that the splitting between the r I and r3 orbital components derived from F5(T d) is sufficiently large, with the F I orbital higher in energy. Secondly, the bound electron eb, which also has r& symmetry, couples to the Cuzn(d9 ) centre, resulting in levels of syrfonetry F I + (r2 + rq). F I represents a singlet spin state, while the pair(F 2 + r R) corresponds to a triplet spin state 3F I. -The zero-field splitting observed experimentally is attributed to a small singlet - t~iplet exchange 2+ 9 )] o - e b splitting of this coupled [Cuzn(d system. The third angular momentum component in the excited state arises from the spin of the donor D+(sl), which iSgaSSumed only weakly coupled to the [Cuzn(d )]o _ eb system. In deriving relative strengths for the Zeeman transitions of the associate it will be assumed that dipolar transitions are allowed only between the {i£1~Cuzn(d~O)])} - D + ( s I) ground state and the {irl~ Cuz~(dg)] - eb)} - D+(s I) excited state of the assoclate, and, further, that the spin of the donor is unchanged in the optical transition.
C West et al. / The luminescence o f Cu-doped ZnO
496
AXIAL Cuzn- D ASSOCIATE IN ZnO GROUND STATE
o
0 Cu
..® D+
EXCITED STATE i
® Cu °
D +
Figure 5 : Schematic representation of the associate ground and excited configurations.
Transitions in which the donor spin is flipped during the electronic excitation or relaxation may become allowed by virtue of the small spin coupling assumed for the donor electron, but such transitions will be much weaker than those involving no change in donor spin. Essential features of the model are therefore that the exchange interaction between [Cuzn(d9)]° and e b is only ~ 1.3 cm -I, and that the exchange interaction with the donor electron D+(s I) is even weaker; this second exchange splitting is assumed unresolved in the experimental data. The small magnitude of the resolved exchange interaction~ compared with the ~ 50 cm -I interaction attributed to an analogous excited state of ZnSe:Co I0, may result from the lack ~f Coulomb interaction between the [Cuzn(d=)] ° core and eb in the present case, and from the added perturbation introduced by the ionised donor. One of the two native donor species in ZnO, interstitial Zn (Zn I) or the oxygen vacancy (Vo) , is the most probable candidate for the double donor proposed for the associate. Zn I is known to be a shallow donor, the ionisation energy of the first electron (Zn~ + Zn + + e being ~ 50 meV, comparable to H and Liill,l~. At temperatures > 750°C the solubility of Zn I exceeds 2 x 1017 em -3, and interstitial diffusion is fast II. Rapid quenching is required to retain significant Zn I concentration at room temperature~ although formation of an associate of the kind discussed here would be expected to stabilise Zn I through the ground state Coulomb interaction. Rather less is known about the oxygen vacancy, which is believed to be a [~e~" ~;e~d~l~. The EPR signal of the n + centre) has been detected following irradiation, (gl[ = 1.9948), and shows hyperfine interaction wlth Zn 67 isotopes in the lattice 14.
The essential properties required of the double donor by our model are that it he mobile at the temperature of growth or diffusion in order to for the associate, and that it be relatively shallow so that exchange interactions with the residual donor core spin are weak. In that case the lowest energy excitation involves promotion of an electron from the filled Cu 3d-shell to a spatially-extended orbital centred on the associate, rather than transfer to a localised, tightly-bound donor orbital. In addition the axis of the associate should be closely aligned with the c-axis of the crystal, with a negligible concentration of associates lying offaxis. The first two of these requirements favour Zn I as the associate donor. The axial nature of the defect is also more easily understood in terms of Znl, which may generate a quasi split-interstitial (Cuz_-ZnT) configuration which would strongly favour ~n orientation parallel to the c-axis. In the case of the oxygen vacancy the difference between cationanion bond lengths along the c-axls and those inclined to this axis suggests a Coulomb energy difference of < 0,I eV favouring axial [CUZn(dlO)]- - V~ associates aligned parallel to e; this energy difference appears insufficient to explain the strong axial preference evident in the Zeeman data unless vacancy reorientation is very facile at low temperature, or very strong lattice relaxation also occurs, and the latter is not detected in the optical spectra. For these reasons we shall therefore assign the donor D + as the interstitial Zn~ (4s I) in the following discussions. However, the basic symmetry requirements of the model do not exclude V o as a donor, and further magnetic resonance measurements may be helpful in resolving this question. In our model, therefore, the energy of the electronic transition is determined largely by the difference in inter-ionic Coulomb interaction between the ground and excited configurations of the associate (see Figure 5), and the selection rules are determined mainly by the one-electron transitions associated with the ~] - e b sub-unit. The core spin of the Zn I [CU(4s ~ donor is very weakly coupled to the system. The dominant transitions in the absence of external perturbation will be between the spin singlet levels of the [Cuzn] - e b sub-unit, with no change in the spin orientation of Zn~ (4sl). These transitions are denoted by linetype (a) in Figure 6. When a magnetic field is applied to the system parallel to the c-axis the magnetic field mixing will transfer oscillator strength to the (~ = I, M s = O) + (~ = O, M s = O) transition. These transitions are denoted by linetype (5) in the Figure 6. Further, a field applied perpendicular to the c-axis will transfer oscillator strength to the (S = I, M s = ± i) + (S = O, M s = O) transition (li~etype c in Figure 6). Additional observable components
C. West et aL / The luminescence o f Cu-doped ZnO
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(M. (Cu. e), M,, (Zn))
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Allowed 6 S1ngle£/TrIple~ mlx1,~j bX B//o SLngle£/TrIple~ ~:xtn~ 6X B.i~ A. b I ~ wlbh Z. opln--Fllp As o hub wi£h Zn e p / . " f ' l i p
Figure 6 : The energy level scheme for the donor-aceeptor model described in the text. Transitions which are marked by lines which are not continuous are only allowed by magnetic field mixing and may additionally require a spin fllp of the associate donor electron. It should be noted that the [Cu~+(d9)]°.e b levels are derived from the ground r 4 state of C u ~ ( d 9) coupled L to the spin of the ~ound electron eb, resulting in levels which have FI orbital symmetry and have slnglet and triplet spin symmetries (see text). result from a second-order effect where there is an accompanying spln-fllp within the Zn+(4s I) donor (l~netypes d and e in Figure 6). As a result of the second order nature of these latter transitions, they are much weaker than the other transitions. All of the transitions shown in F~gure 6 are included in the calculated splitting diagrams (Figure 2), with the g-values for the ground and excited states as defined in the previous section. It is seen that there is good qual~tatlve agreement between the strength of the observed and predicted lines. In particular, the strong intensity redistribution in Figure 4b is consistent w~th the transition intensities d~scussed above. Finally, in Section 3.1 we mentioned the presence of an extra weak ZP. line 3.95 cm -I to higher energy than the 6884 cm lines. Thzs energy would be too low for a level which is derived from either the C u9~)(-d or Zn~(4s I) centre in the associate. Therefore we suggest that this line is due to a transition involving an excited state of the electron e5 which is moving in the complex potential which results from short range interactlon w~th the C u ~ ( d 9 ) and long range interactions with the Zn~(4s~).
3.4 The PLE Spectrum and the Cu-Donor Associate Model The excitation spectra of the 'Cu-green' and 'Cuinfrared' photoluminescence have been contrasted by Dean et al 6, who relied on Broser et al 4 for the general form of the PLE spectrum of the 'Cugreen '~ luminescence. The forms of these spectra are strikingly d~fferent in the energy region common to both, above ~ 2.86 eV, with a smooth decline in optical response for the 'Cu-infrared' luminescence (Figure 7 upper). Figure 7 also shows two broad excitation bands peaking near 1.6 eV and 2.1 eV and a rise near 1.3 eV which is probably instrumental, caused by leakage of the exciting light into the Ge detector. We can account for the main PLE structure shown in Figure 7 using a Cu acceptor-donor associate model. We have already no~ed in Section i that the ground state of the C u ~ ( 3 d 9) acceptor is only about 0.2 eV below Ec, so that the acceptor binding energy EA appropriate to the reaction of Eqn (1) is ~ 3.25 eV. The interstitial donor Zn I pairs with substitutional Cuzn to form a neutral associate according to the reaction described in Section 3.3:
C West et al. / The luminescence o f Cu-doped ZnO
498
CUzn (3d i0)] - + Zn;(4s I) -+ (2)
I [CUzn (3dlO)] Znl (4sl) 1
where the C^v syrmnetry of Cuzn is preserved by alignment o~ the interstitial donor along the c-axis of the ZnO lattice. Any interstitial donor would satisfy the electrostatic aspects of the binding, but the arguments of Section 3.3 implicate a double donor to provide the additional spin necessary in the ground state ~igure 6). Zn$ is the most probable interstitial donor for thls type of associate iN ZnO.
~w
ZnO:Cu 4.2°K PLE
DETECT 'Cu - I N F R A - R E D '
We may first assume that Zn I has a similar radius to the host anion. An upper limit to the interion separation within the associate may then be 3a/8 or ~ 1.95 ~. The well-known expression for the transition energy hv L of a donor-acceptor pair can also be used for associate centres with surprising accuracy when one of the members of the associate has a very large binding energy 15, here the Cuzn acceptor: hv L = E G - (ED + EA) - e2/eR
where E G = 3.4 eV in ZnO, e is the electronic charge and e the static dielectric constant, 8 33 in ZnO For the very large value for E. for Cuzn , m~ - (ED + EA) ~ O, so hv L ~ e /eR. For R = 1.95 ~, Eqn (3) yields hv~ ~ 0.88 eV, (>~7100 cm-l), as a lower limit, in fair agreem e ~ with the observed transition energy of 6884 cm considering the very simplified model we have used. The effective value of EA is reduced by this same energy, which becomes larger if R is less than 3a/8, as is probable for the interstitial donor; translation towards a quasi splitinterstitial configuration would also favour a smaller intra-associate separation. The electron photo-ionisation reaction
I[0u ,.'~s
~:o
2.'~5 ~:5
~.'o 315 2ol
z4 o _ Zn I
has a threshold
1.514
~W-1
(3)
+ eCB
e n e r g y hu T g i v e n by
"-~ h~°LC~
hv T = E G - E A + e2/eR
(5)
CUTOFF
I.'25
1.75 I:S ' PHOTON ENERGY eV
2 .'0
2 .'2 5
Figure 7 : The 4.2K PLE spectrum of the infrared Cu-assoclated luminescence of Figure i showing no significant structure or excitation efficiency near the band gap EG or the exciton energy gaps F~X. The infrared luminescence spectra were usually recorded under excitation on the high-energy wing of the broad band peaking near 2.12 eV. W-I represents leakage of the exciting tungsten iodine light through the filter used in front of the photomultiplier detector. W22 in the lower half of the figure is a Wratten filter between the spectrometer and sample to separate orders of the grating used to analyse the exciting light.
Equation (5) predicts h~ T % i.I eV, or significantly larger if R < 1.95 X. We suggest that the 1.45 eV threshold in Figure 6 m a y b e due to the reaction of Eqn (4). The observed infrared luminescence results from subsequent electron recapture through the excited state [Cu2+(3d9)]geh -Zn~ introduced in Section 3.3. The suggested assignment places this excited state about 0.6 eV below E c if the observed values of hv T and hv L are1~s~, much deeper than a donor in ZnO, ED ~ 0.2 eV ~ ' ~ . This large value contrasts with th& binding energy ~ O.18 eV deduced for the qualitatively similar point-defect bound state [Co3+]eb recently studied in ZnSe 13 . The weak structure indicated near the top of the 1.6 eV PIE component is not well established in these experiments. The ~ 2.0 eV threshold is the approximate energy complement of the 1.45 eV thresholds, implying the reaction Cu2+(3d9 ) o - Zn + +
C West et al. / The luminescence ofCu-doped ZnO
Equation (6) describes the photo-ionisation of the deep Cu acceptor modified by the presence of the interstitial donor. The transition in equation (6) prepares the correct ground state for the infrared luminescence. This luminescence may then result from absorption of a second photon at this centre to produce an electron high in the conduction band according to equation (5), with luminescence again resulting as the final stage of the electron recapture process. 4.
5.
REFERENCES (I)
Dietz, R.E., Kamimnra, H., Sturge, M.D. and Yarlv, A., Phys. Rev. 132 1559 (1963).
(2)
Mollwo, E., Muller G. and Wagner, P., Solid State Commun. 13 1283 (1973).
(3)
Dingle, R., Phys. Rev. Lett., 2 3 5 7 9 (1969).
(4)
Broser, l.J., Germer, R.K., Schulz, H.J.E. and Wiznewski, P., Solid State Electron 21 1597 (1978).
(5)
Kuhnert, R. and Helbig, R., J. Luminesc,26 203 (1981).
(6)
Dean, P.J., Robbins, D.J., Bishop, S.G., Savage, J.A. and Porteous, P., J. Phys. C: Solid State Phys. 1 4 2 8 4 7 (1981).
(7)
Robbins, D.J., Herbert, D.C. and Dean, P.J., J. Phys. C: Solid State Phys. 1 4 2 8 5 9 (1981)
(8)
Broser, I.J., Maier, H. and Schulz, H.$.E., Phys. Rev. 140 A2135 (1965).
(9)
Heinze, R., Dok. der Natur. Dissertion, Tech. Univ. Berlin (1975, unpublished).
CONCLUSION
In this paper we have demonstrated that infrared luminescence transitions known to be intimately connected with the presence of Cu in ZnO have a residual degeneracy which may be lifted by a magnetic field. The levels involved in these transitions can be explained in terms of an associate involving substitutional C u ~ ( d 9) and a double donor, which must be on the crystal c-axls to preserve the trigonal point group symmetry of the Cu. site. The luminescence tranSltlon energy, t~e anomalous Zeeman intensities and the photoluminescence excitation spectra have been explained qualitatively in terms of this model, involving excitation of an electron from a d-orbital into an extended orbital of S-like symmetry bound to the associate. The donor is probably a native defect, Zn I or Vo, the former being favoured by various energy arguments. This type of associate is one of a class often invoked to account for the wellknown difficulties of type conversion in II-VI compound semiconductors. In the present instance the Zn I native donor compensates the aeceptor properties of CUZn. The associate suggested here is similar to those reported by Ennen et a116 to account for optical spectra in GaP:Ni and GaAs:Ni co-doped with donors. However, in those cases, the donors were substitutional, and the transitions observed are simply perturbed intra-d-shell excitations whose energies are slightly shifted in the axial field of the nearby donor.
499
(I0) Robbins, D.J., Dean, P.J., West, C.L. and Hayes, W., Phil. Trans. Roy. Soe. London A304 499 (1982). (II)
Thomas, D.G. and Lander, J.J., J. Chem. Phys. 25 1136 (1956). Thomas]'-D.G., J. Phys. Chem. Solids 3 229 (1957).
(12)
Hutson, A.R., Phys. Rev. 108 222 (1957).
(13)
Soriamo, V. and Galland, D., Phys. Star. Solidi.(B) 7 7 7 3 9 (1976).
(14)
Locher, D.R., and Meese, J.M., IEEE Trans Nuc. Sci. NS-19 237 (1972).
(15)
Dean, P.J., Progress in Solid State Chemistry edited by J. O. McCaldin and G. Somorjai, Pergamon, Oxford (1973)pl.
(16)
Ennen, H., Kaufmann, U. and Schneider, J., Appl. Phys. Lett. 38 355 (1981).
ACKNOWLEDGEMENTS
Two of us (DJR and PJD) wish to acknowledge earlier discussions with Professor F Williams in which the possibility of assigning these transitions to a general donor-acceptor associate was considered.
Copyright
~
Controller, HMSO, London 1982.
Physica 116B (1983) 492-499 North-Holland Pubfishing Company
THE LUMINESCENCE OF COPPER IN ZINC OXIDE
C. WEST, D.J. ROBBINS, P.J. DEAN and W. HAYES*
Royal Signals and Radar Establishmen t St. Andrews Road, Great Malvern, Worcestershire WR14 3PS, U.K. * Clarendon Laboratory, Parks Road, Oxford, U.K. Magneto-optical data on the low-temperature infrared luminescence characteristic of Cu-doped ZnO are discussed• The salient features of these Zeeman data can be explained by a copper-donor associate in which the associated donor has unpaired spin and the axis of the defect is aligned with the c axis of wurtzite ZnO. The observed relative strengths of the Zeeman components are justified using arguments which involve magnetic mixing and a second-order process which requires a spin flip within the weakly coupled donor. A very small zero-field splitting of ~ 1.3 cm-I deduced from the Zeeman data is attributed to exchange splitting between a photo-excited electron and the Cu2+(3d 9) core.
I.
INTRODUCTION
Zinc oxide is a II-Vl semiconductor which grows with a wurtzite lattice structure and has a direct energy gap at the centre of the Brillouin zone of magnitude ~ 27710 cm -I (3.44 eV) at 4°K. The conduction band is non-degenerate, symmetry r6, while the degeneracy of the valence band is lifted by a negative spln-orbit splitting and hy crystal-field splitting, resulting in a F~ uppermost band with two other bands lying withln an energy range of only ~ 113 cm -I (~ 14 meV). Co~per forms the neutral substitutional centre Cu~ +, which exhibits a number of well-known optlcal spectra I , and is known to compensate n-type conductivity in ZnO due to interstitial Zn or H. Conductivity studies place the ground state of Cu$~(3d 9) only 1530 cm-I (0.19 eV) below the c~duction band 2, requiring a very large energy for the reaction:
Cug<3d)+ CUz (3dlO)+
(1)
This reaction produces the aceeptor state Cu~(3dlO), which is singly negatively charged with respect to the lattice. A set of moderately weakly bound excited hole st~t~s have been identified with this centre in ZnO~,~p the lowest of which relaxes radia~ively to the F4 (C3v) ground state of Cu2+(3d N) giving characteristic green luminescence. Attention has also recently been drawn to an additional broadened luminescence component with similar optical phonon replication to that of the main series in the green luminescence 5. Although no interpretation was offered by Kuhnert and HelbSg 5, it seems most probable that the ~ 225 om-L splitting between the leading members of each series represents the spin-orbit splitting in the ground state of Cu2+(3d9). Thus, the new series originates from radiative hole relaxation into the spin-orbit split excited state, symmetry F~ like the ground state of Cu 2+ in wurtzite ZnO. ~
Absorption 4 or photoluminescence excitation 4'6 studies (PLE) reveal the full set of relatively weakly bound hole states, which also have F 4 symmetry. These bound hole states have been recently related to the three upper valence bands in the pure wurtzite lattice which produce levels of r~ symmetry in the CaV point group of the Cu. impurity 7. Optical a~sorption reveals some o~nthe structure within Cu~(Bd9). This state is split by the cubic crystal field (Td) into 2T 2 and 2E separated by 5800 cm -I. The triple orbital degeneracy of the 2T 2 ground state is lifted by the combined effects of spinorbit coupling and the small trigonal field characteristic of the wurtzite lattice, while the 2E excited hole state is split into two subcomponents by the crystal field. Transitions from the F4 ground state component derived from 2T 2 to both subcomponents of 2E are clearly observed at 5772 cm -I and 5812 cm-I in optical absorption I and fix the F5, F6 + F4 splitting of the 2E substates at 39.5 cm -I. The detailed magnetic properties of these optical absorption components I are consistent with the electron spin resonance properties of the Cu~ + =round • n~ state and with expectatlon of a hole tlghtly bound to copper giving Cu~+(3d9). Optical absorption at elevated temperatures I led to the suggestion of a F4 ÷ F4 splitting in the ground state of 116 cm -I. However, it has been argued elsewhere 6 that this hot absorption component involves a phonon replica and that the smallest ground state splitting must be larger than 116 cm -I, perhaps 225 cm -I as already described. No luminescence has been observed from the 2E subcomponents, in sharp contrast to the behaviour of Cu Z in ZnS 8. There is no clear explanation for thls strlklng difference. It is particularly unexpected since phonon coupling is not very strong in either host, so it seems unlikely that the luminescence should be quenched by multiphonon
C West et al. / The luminescence ofCu-doped ZnO
relaxation from the 2E states in ZnO. However, highly structured infrared luminescence has been observed in ZnO:Cu~at 2°K and correlates strongly with the introduction of Cu both during6c~ystal growth and in subsequent diffusion ' . The infrared luminescence has a strong sharp no-phonon component at 6884 om-i (0.8533 eV), II00 cm -I (136 meV) above the centroid of the absorption lines. Evidently, the 6884 cm -I luminescence cannot origiHate from the energy states attributed to Cu~(Bd9)."± The present paper is concerned with the identification of the Cu-related infrared luminescence, mainly from Zeeman analysis of the 6884 am-I no-phonon lines. We shall see that there is a small unresolved zero-field splitting of 1.3 cm -I arising from the excited state of the luminescence transition. We also show that all of the observable experimental data can be accounted for with an acceptor-donor associate model involving Cu Z , with the sym• . n metry axls of the assoclate selected so as not to reduce the point group symmetry in wurtzite ZnO.
6O00 I
2.
EXPERIMENTAL
The crystals studied in this work were prepared and in some c~ses indlffused by Cu as described by Dean et al v. The general technique of luminescence spectroscopy was also described previously 6. Zeeman measurements were made in the Clarendon Laboratory, Oxford, with a spllt-coll super-conducting magnet producing magnetic fields of up to 8.5T at 2K.
6S00 !
70O0 I
6500
7000
~nO=Cu OF40
6000
c m -1
Figure 1 : The low temperature luminescence spectrum (I.6K) of a centre associated with Cu in ZnO.
3. We had initially considered an alternative model which involved magnetic triplets in the ground and excited states, with the triplet degeneracy lifted by a weak trigonal-field splitting. Such a transition could be attributed to luminescence between the lowest crystal field components of Cu3+(d 8) which would have 3T I (ground) and 3T 2 (excited) levels in a zinc blende lattice. Many aspects of this model proved to be very promising; a sign difference between the trigonal-field splitting for the excited and ground states was predicted and the energy of the transition was not unreasonable. However, the observed nearly isotropic nature of the g values and the closeness of their magnitudes to that of a free electron did not correspond to values predicted by the model. This disagreement and the improbability of a Cu 3+ state in ZnO based on general experience of Cu in binary semlnonductors, favours the alternative donor-acceptor associate model which is discussed in detail in section 3.3, We find that this latter model can also account for many features observed in the PLE spectra of the 6884 cm -I ~O.8533 eV) luminescence reported by Dean et al u.
493
DISCUSSION
3.1 Experimental Results The near-infrared luminescence associated with copper in zinc oxide is shown in Figure i. This spectrum has been previously reported by Dean et al 6 and is dominated by a strong zerophonon (ZP) llne at 6884 cm -I. In addition, Dean et al 6 reported an extra weak ZP llne occurring 3.95 cm -I to higher energy which shows effects of thermalisation with respect to the first component. This line is not visible in the present experiments due to the very low temperature (2K). The phonon side band is highly structured, with main replicas lying near 6365 cm -I and 6290 cm -I. When a magnetic field is applied parallel to the c-axis of the crystal the ZP line at 1.45 microns splits into two components, each component moving linearly with increasing field (Figures 2a, 3a). It is noteworthy that the two components do not extrapolate back to the same origin, indicating a residual zero-fleld splitting. With the magnetic field perpendicular to the c-axis five components are seen (Figures 2b, 3b). Similar data have been reported previously, but lacked the full complement of lines observed in the present spectra, thereby inhibiting a complete discussion (R Heinzeg). Of these five lines, only three survive at high fields, due to thermalisation. Unfortunately, polarisation data appeared to be unreliable and inconclusive in both of these sets of spectra. The crystal was rotated from B [1 c to B i c when the crystal was subject to a magnetic field of 8.5 Tesla. These data are given in Figure 4. It appears that there is considerable switching of intensity between the components. This will prove to be important when considering the symmetries of the energylevels.
C. West e t al.
494
6892
cm -1 BSg~
I
B//c
/
The l u m i n e s c e n c e o f Cu-doped Z n O
~e~2F
I I"
• •
j'"
cm-% I
I 8888 6886
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8888[
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,4:;
68861 $884
6884 6882
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888~ ~878
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8882~
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6872
687~
687~
686B
6868 MAGNETIC
FIELD
(Tesla)
0.
2.
i
I
1 MAGNETIC FIELD
(TesIa)
Figure 2 : The effects of a magnetic field, applied parallel (a), and perpendicular (b) to the c axis of ZnO, on the 6884 cm -] zero phonon luminescence line due to copper at ].6K.
3.2 Determination of the Emitting and Ground States of the Infrared Luminescence Transition If we first consider the experimental data with the magnetic field applied along the c-axis of the crystal (Figure 2a) we see that the two components extrapolate back to different zero-field positions. The zero-field splitting is approximately 1.3 cm -I. The Zeeman data for the magnetic field applied perpendicular to the c-axls (Figure 2b) are also consistent with two levels at zero field. The central line of the experimental data is the only line which is seen to come from the lower-energy zero-field component. The highest-energy line in the B I c Zeeman spectra (Figure 3b) thermalises at high field, indicating an excited state splitting in the magnetic field. However~ the corresponding low-energy line, which should have a large Boltzmann population factor, remains very weak even at high field. These large differences in oscillator strength for the various Zeeman components suggest that very different selection rules apply to the two zero-field-split states in the transition. Further we note that the general magnitudes of the splitting for both directions of magnetic field suggest g values which are near to those for free spin systems. Consider a model where the excited state consists of a spin doublet close to a higher spin
quartet, and where the ground state is also a spin doublet. With suitable choices of g values for these states the experimental data points agree closely with the calculated values over the range 3-9 Tesla, as shown in Figure 2. With the magnetic field applied perpendicular to the c-axis the system would be complicated by mixing of the two excited states, but the high field data should be asymptotic to the lines in the absence of these interactions. The positions of the lines drawn in Figures 2a and 2b are calculated using the arguments of section 3.3 and the parameter values
6exch
= 1.3 cm -I '
gll
(ground state)
gll (excited state:
-
F 6) = 0.12
½ (gll (ground state) + gll (excited state:
F8)) = 2.00
gl (ground state) = 2.00, g± (excited state:
F8) = 1.81
It is seen that the data have a splitting pattern consistent with a quartet - doublet excited level scheme radiating to a doublet ground state. We now discuss the origin of these energy states in terms of the CUZn-donor associate model.
C West et aL / The luminescence ofCu-doped ZnO
495
3.3 The Acceptor-Donor Associate Model DII¢
[nO; ¢u
S~¢
IF 4 0
!
n
1:5
j
~
,
'
,
.:_,&
,
--,
Figure 3 : The experimental data showing how the spectrum due to the donor-acceptor associate described in the text progresses when a magnetic field is applied parallel (a) and perpendicular (b) to the c axis of ZnO (T = 1.6K).
6Be2 . . . . . . . . . . . . . +e~
B=8. 5
eeee
t
Tes]o
ZnO;Cu Dr40
P//c
'
L
t-I l*SI~
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ea86
6as4
O
O
O
O
O
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~
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0
¢
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0
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see2 6eae /
E+aTsI eats [
~
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+o:,oi
¢
l+
-~-,--,~ .~i-
'+'++'J, 4 ~ i ~ ~ i ++ i t~AGNETIC FIELD ORIENT/iT,[ON (de 9)
Figure 4 : The effect of rotating the magnetic field (8.5 Tesla) from a position where the field is parallel to the c axis of ZnO to the position where the field is perpendicular to the e axis on the 6884 om-I zero phonon line due to the axial associate at 1.6K.
The experimental data can be understood in terms of a substitutional • CUz_LL acceptor compensated by , • a double donor specles, the axls of the resulting associate being aligned with the c-axis of the ZnO lattice to preserve the trigonal point group symmetry. The two electrons in the neutral double donor are assumed to be spin-paired in an S-like (a t sy~netry) orbital. In the ground state of the associate one electron is transferred from the donor to the Cu~n+(d9) centre, filling the Cu 3d-shell and leaving an unpaired electron on the donor; this configuration is indicated at the top of Figure 5. The compensated [Cul+(dlO)]" centre carries an effective negative lattice charge, and the ground state of the associate is stabilised by the Coulomb interaction with the ionised donor. For an interionic separation of ~ 2 ~ in the associate, the Coulomb stabilisation energy will be of order 0.9 eV in ZnO. The d I0 configuration of Cuzn is a filled shell and the only magnetic behaviour of the associate in the ground state therefore stems from the spin of the ionised D+(s I) donor. For an S-like donor orbital this level will exhibit all the features of a pure S = ~ state; in particular the g-value will be approximately equal to 2 and will be reasonably isotropic as required by the experimental data. The excited configuration of the associate is assumed to involve promotion of an electron from the d-shell of the [Cuzn(dl0)] - centre into a more spatially-extended orbital of S-like syn~netry but remaining bound to the associate. The excited state can then be presented as [Cu~(d9)]o - e~ - D+(sl), as indicated in the lower part of Figure 5. In this case there are three contributary terms to the angular momentum of the electronic states. First,^$n the point group C3V the ground state for C u ~ ( d 9) has r 4 sy~netry, and can behave in the strong trigonal field of the associate as a pure spin state with no orbital momentum provided that the splitting between the r I and r3 orbital components derived from F5(T d) is sufficiently large, with the F I orbital higher in energy. Secondly, the bound electron eb, which also has r& symmetry, couples to the Cuzn(d9 ) centre, resulting in levels of syrfonetry F I + (r2 + rq). F I represents a singlet spin state, while the pair(F 2 + r R) corresponds to a triplet spin state 3F I. -The zero-field splitting observed experimentally is attributed to a small singlet - t~iplet exchange 2+ 9 )] o - e b splitting of this coupled [Cuzn(d system. The third angular momentum component in the excited state arises from the spin of the donor D+(sl), which iSgaSSumed only weakly coupled to the [Cuzn(d )]o _ eb system. In deriving relative strengths for the Zeeman transitions of the associate it will be assumed that dipolar transitions are allowed only between the {i£1~Cuzn(d~O)])} - D + ( s I) ground state and the {irl~ Cuz~(dg)] - eb)} - D+(s I) excited state of the assoclate, and, further, that the spin of the donor is unchanged in the optical transition.
C West et al. / The luminescence o f Cu-doped ZnO
496
AXIAL Cuzn- D ASSOCIATE IN ZnO GROUND STATE
o
0 Cu
..® D+
EXCITED STATE i
® Cu °
D +
Figure 5 : Schematic representation of the associate ground and excited configurations.
Transitions in which the donor spin is flipped during the electronic excitation or relaxation may become allowed by virtue of the small spin coupling assumed for the donor electron, but such transitions will be much weaker than those involving no change in donor spin. Essential features of the model are therefore that the exchange interaction between [Cuzn(d9)]° and e b is only ~ 1.3 cm -I, and that the exchange interaction with the donor electron D+(s I) is even weaker; this second exchange splitting is assumed unresolved in the experimental data. The small magnitude of the resolved exchange interaction~ compared with the ~ 50 cm -I interaction attributed to an analogous excited state of ZnSe:Co I0, may result from the lack ~f Coulomb interaction between the [Cuzn(d=)] ° core and eb in the present case, and from the added perturbation introduced by the ionised donor. One of the two native donor species in ZnO, interstitial Zn (Zn I) or the oxygen vacancy (Vo) , is the most probable candidate for the double donor proposed for the associate. Zn I is known to be a shallow donor, the ionisation energy of the first electron (Zn~ + Zn + + e being ~ 50 meV, comparable to H and Liill,l~. At temperatures > 750°C the solubility of Zn I exceeds 2 x 1017 em -3, and interstitial diffusion is fast II. Rapid quenching is required to retain significant Zn I concentration at room temperature~ although formation of an associate of the kind discussed here would be expected to stabilise Zn I through the ground state Coulomb interaction. Rather less is known about the oxygen vacancy, which is believed to be a [~e~" ~;e~d~l~. The EPR signal of the n + centre) has been detected following irradiation, (gl[ = 1.9948), and shows hyperfine interaction wlth Zn 67 isotopes in the lattice 14.
The essential properties required of the double donor by our model are that it he mobile at the temperature of growth or diffusion in order to for the associate, and that it be relatively shallow so that exchange interactions with the residual donor core spin are weak. In that case the lowest energy excitation involves promotion of an electron from the filled Cu 3d-shell to a spatially-extended orbital centred on the associate, rather than transfer to a localised, tightly-bound donor orbital. In addition the axis of the associate should be closely aligned with the c-axis of the crystal, with a negligible concentration of associates lying offaxis. The first two of these requirements favour Zn I as the associate donor. The axial nature of the defect is also more easily understood in terms of Znl, which may generate a quasi split-interstitial (Cuz_-ZnT) configuration which would strongly favour ~n orientation parallel to the c-axis. In the case of the oxygen vacancy the difference between cationanion bond lengths along the c-axls and those inclined to this axis suggests a Coulomb energy difference of < 0,I eV favouring axial [CUZn(dlO)]- - V~ associates aligned parallel to e; this energy difference appears insufficient to explain the strong axial preference evident in the Zeeman data unless vacancy reorientation is very facile at low temperature, or very strong lattice relaxation also occurs, and the latter is not detected in the optical spectra. For these reasons we shall therefore assign the donor D + as the interstitial Zn~ (4s I) in the following discussions. However, the basic symmetry requirements of the model do not exclude V o as a donor, and further magnetic resonance measurements may be helpful in resolving this question. In our model, therefore, the energy of the electronic transition is determined largely by the difference in inter-ionic Coulomb interaction between the ground and excited configurations of the associate (see Figure 5), and the selection rules are determined mainly by the one-electron transitions associated with the ~] - e b sub-unit. The core spin of the Zn I [CU(4s ~ donor is very weakly coupled to the system. The dominant transitions in the absence of external perturbation will be between the spin singlet levels of the [Cuzn] - e b sub-unit, with no change in the spin orientation of Zn~ (4sl). These transitions are denoted by linetype (a) in Figure 6. When a magnetic field is applied to the system parallel to the c-axis the magnetic field mixing will transfer oscillator strength to the (~ = I, M s = O) + (~ = O, M s = O) transition. These transitions are denoted by linetype (5) in the Figure 6. Further, a field applied perpendicular to the c-axis will transfer oscillator strength to the (S = I, M s = ± i) + (S = O, M s = O) transition (li~etype c in Figure 6). Additional observable components
C. West et aL / The luminescence o f Cu-doped ZnO
sq
(M. (Cu. e), M,, (Zn))
B//=
8_!I_o
Mj +3./2
(I, I/m (
__liT,
I/2)
--
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[ Z n + (401 ) ]
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¢
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l
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•
m ,
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f
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.
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,
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-1, +i -I,+I
-1, +1
•
; I i i 'I , i I ~,
,r~ - _~
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---[]
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[Cu ~"( 4 9 ] ~. e,- +
[Cu" (dlm) ] -
497
- I , +I
Allowed 6 S1ngle£/TrIple~ mlx1,~j bX B//o SLngle£/TrIple~ ~:xtn~ 6X B.i~ A. b I ~ wlbh Z. opln--Fllp As o hub wi£h Zn e p / . " f ' l i p
Figure 6 : The energy level scheme for the donor-aceeptor model described in the text. Transitions which are marked by lines which are not continuous are only allowed by magnetic field mixing and may additionally require a spin fllp of the associate donor electron. It should be noted that the [Cu~+(d9)]°.e b levels are derived from the ground r 4 state of C u ~ ( d 9) coupled L to the spin of the ~ound electron eb, resulting in levels which have FI orbital symmetry and have slnglet and triplet spin symmetries (see text). result from a second-order effect where there is an accompanying spln-fllp within the Zn+(4s I) donor (l~netypes d and e in Figure 6). As a result of the second order nature of these latter transitions, they are much weaker than the other transitions. All of the transitions shown in F~gure 6 are included in the calculated splitting diagrams (Figure 2), with the g-values for the ground and excited states as defined in the previous section. It is seen that there is good qual~tatlve agreement between the strength of the observed and predicted lines. In particular, the strong intensity redistribution in Figure 4b is consistent w~th the transition intensities d~scussed above. Finally, in Section 3.1 we mentioned the presence of an extra weak ZP. line 3.95 cm -I to higher energy than the 6884 cm lines. Thzs energy would be too low for a level which is derived from either the C u9~)(-d or Zn~(4s I) centre in the associate. Therefore we suggest that this line is due to a transition involving an excited state of the electron e5 which is moving in the complex potential which results from short range interactlon w~th the C u ~ ( d 9 ) and long range interactions with the Zn~(4s~).
3.4 The PLE Spectrum and the Cu-Donor Associate Model The excitation spectra of the 'Cu-green' and 'Cuinfrared' photoluminescence have been contrasted by Dean et al 6, who relied on Broser et al 4 for the general form of the PLE spectrum of the 'Cugreen '~ luminescence. The forms of these spectra are strikingly d~fferent in the energy region common to both, above ~ 2.86 eV, with a smooth decline in optical response for the 'Cu-infrared' luminescence (Figure 7 upper). Figure 7 also shows two broad excitation bands peaking near 1.6 eV and 2.1 eV and a rise near 1.3 eV which is probably instrumental, caused by leakage of the exciting light into the Ge detector. We can account for the main PLE structure shown in Figure 7 using a Cu acceptor-donor associate model. We have already no~ed in Section i that the ground state of the C u ~ ( 3 d 9) acceptor is only about 0.2 eV below Ec, so that the acceptor binding energy EA appropriate to the reaction of Eqn (1) is ~ 3.25 eV. The interstitial donor Zn I pairs with substitutional Cuzn to form a neutral associate according to the reaction described in Section 3.3:
C West et al. / The luminescence o f Cu-doped ZnO
498
CUzn (3d i0)] - + Zn;(4s I) -+ (2)
I [CUzn (3dlO)] Znl (4sl) 1
where the C^v syrmnetry of Cuzn is preserved by alignment o~ the interstitial donor along the c-axis of the ZnO lattice. Any interstitial donor would satisfy the electrostatic aspects of the binding, but the arguments of Section 3.3 implicate a double donor to provide the additional spin necessary in the ground state ~igure 6). Zn$ is the most probable interstitial donor for thls type of associate iN ZnO.
~w
ZnO:Cu 4.2°K PLE
DETECT 'Cu - I N F R A - R E D '
We may first assume that Zn I has a similar radius to the host anion. An upper limit to the interion separation within the associate may then be 3a/8 or ~ 1.95 ~. The well-known expression for the transition energy hv L of a donor-acceptor pair can also be used for associate centres with surprising accuracy when one of the members of the associate has a very large binding energy 15, here the Cuzn acceptor: hv L = E G - (ED + EA) - e2/eR
where E G = 3.4 eV in ZnO, e is the electronic charge and e the static dielectric constant, 8 33 in ZnO For the very large value for E. for Cuzn , m~ - (ED + EA) ~ O, so hv L ~ e /eR. For R = 1.95 ~, Eqn (3) yields hv~ ~ 0.88 eV, (>~7100 cm-l), as a lower limit, in fair agreem e ~ with the observed transition energy of 6884 cm considering the very simplified model we have used. The effective value of EA is reduced by this same energy, which becomes larger if R is less than 3a/8, as is probable for the interstitial donor; translation towards a quasi splitinterstitial configuration would also favour a smaller intra-associate separation. The electron photo-ionisation reaction
I[0u ,.'~s
~:o
2.'~5 ~:5
~.'o 315 2ol
z4 o _ Zn I
has a threshold
1.514
~W-1
(3)
+ eCB
e n e r g y hu T g i v e n by
"-~ h~°LC~
hv T = E G - E A + e2/eR
(5)
CUTOFF
I.'25
1.75 I:S ' PHOTON ENERGY eV
2 .'0
2 .'2 5
Figure 7 : The 4.2K PLE spectrum of the infrared Cu-assoclated luminescence of Figure i showing no significant structure or excitation efficiency near the band gap EG or the exciton energy gaps F~X. The infrared luminescence spectra were usually recorded under excitation on the high-energy wing of the broad band peaking near 2.12 eV. W-I represents leakage of the exciting tungsten iodine light through the filter used in front of the photomultiplier detector. W22 in the lower half of the figure is a Wratten filter between the spectrometer and sample to separate orders of the grating used to analyse the exciting light.
Equation (5) predicts h~ T % i.I eV, or significantly larger if R < 1.95 X. We suggest that the 1.45 eV threshold in Figure 6 m a y b e due to the reaction of Eqn (4). The observed infrared luminescence results from subsequent electron recapture through the excited state [Cu2+(3d9)]geh -Zn~ introduced in Section 3.3. The suggested assignment places this excited state about 0.6 eV below E c if the observed values of hv T and hv L are1~s~, much deeper than a donor in ZnO, ED ~ 0.2 eV ~ ' ~ . This large value contrasts with th& binding energy ~ O.18 eV deduced for the qualitatively similar point-defect bound state [Co3+]eb recently studied in ZnSe 13 . The weak structure indicated near the top of the 1.6 eV PIE component is not well established in these experiments. The ~ 2.0 eV threshold is the approximate energy complement of the 1.45 eV thresholds, implying the reaction Cu2+(3d9 ) o - Zn + +
C West et al. / The luminescence ofCu-doped ZnO
Equation (6) describes the photo-ionisation of the deep Cu acceptor modified by the presence of the interstitial donor. The transition in equation (6) prepares the correct ground state for the infrared luminescence. This luminescence may then result from absorption of a second photon at this centre to produce an electron high in the conduction band according to equation (5), with luminescence again resulting as the final stage of the electron recapture process. 4.
5.
REFERENCES (I)
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CONCLUSION
In this paper we have demonstrated that infrared luminescence transitions known to be intimately connected with the presence of Cu in ZnO have a residual degeneracy which may be lifted by a magnetic field. The levels involved in these transitions can be explained in terms of an associate involving substitutional C u ~ ( d 9) and a double donor, which must be on the crystal c-axls to preserve the trigonal point group symmetry of the Cu. site. The luminescence tranSltlon energy, t~e anomalous Zeeman intensities and the photoluminescence excitation spectra have been explained qualitatively in terms of this model, involving excitation of an electron from a d-orbital into an extended orbital of S-like symmetry bound to the associate. The donor is probably a native defect, Zn I or Vo, the former being favoured by various energy arguments. This type of associate is one of a class often invoked to account for the wellknown difficulties of type conversion in II-VI compound semiconductors. In the present instance the Zn I native donor compensates the aeceptor properties of CUZn. The associate suggested here is similar to those reported by Ennen et a116 to account for optical spectra in GaP:Ni and GaAs:Ni co-doped with donors. However, in those cases, the donors were substitutional, and the transitions observed are simply perturbed intra-d-shell excitations whose energies are slightly shifted in the axial field of the nearby donor.
499
(I0) Robbins, D.J., Dean, P.J., West, C.L. and Hayes, W., Phil. Trans. Roy. Soe. London A304 499 (1982). (II)
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ACKNOWLEDGEMENTS
Two of us (DJR and PJD) wish to acknowledge earlier discussions with Professor F Williams in which the possibility of assigning these transitions to a general donor-acceptor associate was considered.
Copyright
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Controller, HMSO, London 1982.