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Incorporation of CuZn centers in ZnSe far from equilibrium
SOO22-3697(%)0010&0
Pergamon
INCORPORATION
OF CuZn CENTERS EQUILIBRIUM
J Phys. Chem Solids Vol 58, No. I. pp. 79-84. 1997 Copyright c: 1996 Elsevicr Science Ltd Printed in Great Britain. All nghts reserved OOZZ-3697/97 $17.00 + 0.M)
IN ZnSe FAR FROM
E. D. WHEELER*, JACK L. BOONE*, J. W. FARMER? and H. R. CHANDRASEKHARS *Department of Electrical Engineering, University of Missouri, Rolla, MO 65409, U.S.A. ?University of Missouri Research Reactor and Department of Physics, University of Missouri, Columbia, MO 6521 I, U.S.A. fDepartment of Physics, University of Missouri, Columbia, MO 65211, U.S.A. (Received 16 April 1996; accepted 23 April 1996)
Abstract-An investigation employing nuclear transmutation to probe the effects of copper doping in ZnSe is presented. Three experimental techniques are developed in the investigation. With the first, as-grown ZnSe is irradiated with thermal neutrons which results, after thermal annealing, in the incorporation of Cuzn centers. Observations are consistent with isolated Cuz. being involved in the copper red and copper green emissions in ZnSe but not in the 1; excitonic emission. With the second, it is shown that zinc annealing can be used effectively in investigations involving the irradiation of as-grown ZnSe since the zinc annealing treatment significantly reduces background PL emissions in bulk &Se. The third technique employs homoepitaxial ZnSe layer growth from previously irradiated elemental sources. The epitaxial layers display no dominant 1: excitonic emission and a very low level of deep emissions. Because, with the techniques described here, the copper atoms are introduced at zinc sites after crystal growth processes are complete, the copper atoms are not able to interact with other dopants or lattice defects during growth as they can when incorporated by other means. The absence of interactions during crystal growth permits the unambiguous incorporation, far from equilibrium, of isolated Cuzn centers in ZnSe. Copyright Q 1996Elsevier Science Ltd Keywords: A. optical materials, A. semiconductors, B. epitaxial growth, D. optical properties.
1. INTRODUCTION
ease. Emissions believed to be due to copper are often present in both bulk crystals and in epitaxial layers. Since copper is a common impurity in ZnSe and since it strongly affects luminescence in ZnSe, understanding its behavior in ZnSe is doubly important. One goal in investigations of copper doping has been to develop a microscopic model of the luminescent centers; one obstacle to this goal is the variety of centers which can be present in copper doped II-VI semiconductors. This variety of possible centers, coupled with coppers high diffusion rate, frustrates attempts to clearly identify the nature of copper as an impurity in these compounds since it is possible for CuZn, Cui, or complexes involving copper to be present. In low temperature photoluminescence (PL) measurements, the copper red (Cu-R) emission at 1.97eV [7-91 and the copper green (Cu-G) emission at 2.35eV [7-91 are known to involve copper. Other experimental work employing optically detected magnetic resonance gives evidence the Cu-G center results from a complex involving a copper atom at a zinc site in association with another ion at an interstitial site [lo]. At present, there is some question as to whether the 1: excitonic emission at 2.7830eV involves only zinc vacancies [I I, 121or involves both zinc vacancies and copper related centers [l3, 141.
Copper is an important impurity in wide bandgap zinc chalcogenides [l]. In ZnSe, the incorporation of copper results in a variety of characteristic visible bands [2], and investigations seeking a microscopic model of copper in the zinc chalcogenides have continued for many years. The long standing uncertainty of the role of copper in these materials attests to the need for developing new experimental techniques which may provide additional information on the behavior of this important impurity. It is necessary to gain a clear understanding on the role of copper in the zinc chalcogenides for two reasons. First, copper is the most efficient known activator of luminescence in these compounds [3] which makes understanding its effects important in any effort to control or modify its luminescent properties. Second, copper is a major residual impurity and can be incorporated in zinc chalcogenides via two primary avenues. One is that copper is a major contaminant of zinc [4], so that zinc chalcogenides produced by any process can contain copper; the second avenue is copper’s high diffusion rate in the II-VI compound semiconductors [5,6]. Because of this high diffusion rate, copper can enter the material from growth and processing environments with relative 79
E. D. WHEELER et al.
80
The experimental techniques reported herein allow the effects of isolated Cuz,, centers to be observed in ZnSe and will allow the effects due to Cuz,, centers to be characterized in a straightforward manner.
2. EXPERIMENTAL When zinc is exposed to a flux of thermal neutrons, the Zn@ isotope can capture a thermal neutron by radiative capture, Zn64+n1
+Zn65+y
(1)
The radiative capture process is followed by nuclear decay, via electron capture, to a stable copper isotope
WI, Zn65 + e- + C#
+ v
(2)
where n’ is a thermal neutron, y is a gamma ray, e- is an orbital electron and v is a neutrino. This nuclear decay process has a half-life of 244.1 days. Actually, only 98.54% of the nuclear decays proceed via (2), while 1.46% proceed via positron emission [15]. The decay via positron emission will be neglected here since it results in the same end state as approximately 48% of the decays by electron capture and furthermore, as indicated above, only a small fraction of the nuclear decay events are involved. Three experimental techniques were investigated in the work reported herein. The first involves irradiating as-grown ZnSe wafers, then thermally annealing them to repair neutron damage suffered during irradiation. The second involves irradiating as-grown ZnSe wafers, then annealing them in high purity molten zinc metal. The third involves irradiating elemental sources and then forming homoepitaxial ZnSe layers from the irradiated sources by physical vapor transport. In all three, the long half-life of Zn65 is key to allowing neutron transmutation to be employed in experimental investigations of copper in ZnSe. The 244.1 day half-life provides ample opportunity to anneal irradiated as-grown ZnSe or to grown ZnSe from irradiated elemental sources before significant nuclear decay of the Zn65 nuclei has occurred. With these techniques, Cu 65 dopants are introduced after growth processes are complete, far from thermal equilibrium. Since copper dopants are not present at the time that the post-irradiation processing takes place, they can not form complexes with other dopants or lattice defects during crystal growth. 2.1. Irradiation of as-grown ZnSe with subsequent thermal annealing Wafers cut from boules of ZnSe, grown by seeded physical vapor transport (SPVT) at the Eagle Picher
2.8 2.6 2.4 2.2 2.0 1.6
energy (eV)
Fig. 1. PL spectra of bulk ZnSe (a) unirradiated, annealed control sample, (b) irradiated, annealed sample one month after irradiation, (c) same irradiated, annealed sample four months after irradiation.
Research Laboratories in Miami, Oklahoma, were irradiated with thermal neutrons at the Missouri University Research Reactor (MURR) in Columbia, Missouri, The (100) oriented wafers were irradiated for an ultimate copper concentration of 10” cme3. Lattice damage due to both fast neutrons colliding with the lattice and due to the radiative capture of thermal neutrons must be removed by post-irradiation thermal annealing for the effects of neutron transmutation doping (NTD) to be observable. Because no complete thermal annealing studies of SPVT grown ZnSe have been reported in the literature, an annealing study was conducted during this work. The irradiated crystals, weighing about 1.3 mg and measuring 1 mm x 1 mm x 0.25 mm, were placed in quartz vials which had been previously cleaned, lightly etched in HF, and baked overnight at 1100°C. The small irradiated crystals were then surrounded by a larger quantity of unirradiated ZnSe before the vials were purged several times with high purity argon and then sealed under vacuum. Anneals were conducted at temperatures of 600, 700, 800 or 900°C for times of 1 and 3 h. These temperatures were chosen because similar temperatures have previously been employed in annealing ion-implanted ZnSe [ 161. The PL spectra for the ZnSe samples annealed at 600°C for 3 h are shown in Fig, 1. The spectrum for an unirradiated, annealed control sample is shown in Fig. l(a). The spectrum for an irradiated, annealed sample one month after irradiation is shown in Fig. l(b), and the spectrum for the same irradiated, annealed sample four months after irradiation is shown in Fig. l(c). The three emissions of interest here are the If excitonic emission at 2.7830 eV, the Cu-G emission at 2.35 eV, and the Cu-R emission at 1.97 eV. The If excitonic emission is typical of bulk ZnSe and, as previously discussed, may result from Vz”, or from both Vz,, and Cuz,, centers. The 1: emission has several longitudinal
Incorporation of Cuz” centers in ZnSe
optical phonon replicas spaced 0.03 1 eV apart which, along with the donor bound exciton (DBE) peak at 2.7973 eV, dominate the PL spectrum above approximately 2.6eV in the SPVT grown ZnSe used in this study. The PL spectrum at four months after neutron irradiation displays enhanced Cu-G and Cu-R emissions when compared to the spectrum at one month and may result from the nuclear decay of Zn65 to Cu6’. No significant change occurred in the intensity of the 1: emission. Another change in the PL spectrum at four months compared to the spectrum at one month is an enhanced DBE peak. It is difficult to speculate on the cause of this change since the region between the DBE and If has a high level of background emission, but it may be due to arsenic [17]. Other samples, annealed at higher temperatures, also display an enhanced DBE peak at four months but do not display enhanced Cu-R or Cu-G emissions. It should be noted here that previous work on annealing at temperatures significantly higher than 550°C show that deep levels are generated at these higher annealing temperatures [ 181. 2.2. Irradiation of as-grown ZnSe with subsequent zinc annealing An obstacle to employing bulk as-grown ZnSe in NTD studies of copper doping is the I;’ emission peak consistently present in the PL spectrum. This peak is located very near a possible copper related emission. Moreover, its presence, along with its phonon replicas, results in a high background emission level in the SPVT grown material. The location of 1: and the resulting high background emission level make its presence undesirable in studies investigating the effects of copper in ZnSe. Subsequent to the NTD studies on irradiated SPVT grown ZnSe described above, zinc annealing experiments were conducted in
DAP
DAP-LO I ’ DAP-2L0
81
an attempt to remove or diminish the I;’ emission in bulk SPVT grown ZnSe. These studies were conducted so that, in future NTD studies involving SPVT grown ZnSe, background emission levels might be reduced which would then allow more sensitive observations. Figure 2 shows the PL spectrum displayed by a (100) ZnSe wafer which was annealed in molten zinc at 900°C for 24 h. The remarkable reduction in intensity of the 1; emission, compared to the samples without zinc annealing, is consistent with earlier annealing studies [19, 201 and will be very useful in future NTD studies involving bulk ZnSe. In contrast to these earlier studies, however, Fig. 2 does not display a second dominant excitonic emission and its phonon replicas. In these earlier studies, this second emission was attributed to free excitons and became significant after zinc annealing.
2.3. ZnSe sources
grown from
irradiated elemental
Homoepitaxial layers of ZnSe are grown, with elemental zinc and selenium as sources, by physical vapor transport in a closed quartz tube as shown in Fig. 3. This technique allows control over vapor stoichiometry and is capable of producing layers with no dominant I;’ peak and a very low level of deep level emissions in the PL spectrum as shown in Fig. 4. This low level of background emissions is crucial if PL is to be employed in observations of coppers effects on the luminescent properties in ZnSe. Layers grown with this technique display a X-ray diffraction rocking curve with a full width at half maximum of less than 25 arc seconds. Details of this growth technique appear elsewhere [21]. By employing this technique, homoepitaxial layers can be grown from elemental selenium and previously irradiated zinc. Post-irradiation growth eliminates neutron irradiation lattice damage and the subsequent required post-irradiation annealing. With this technique, CuZn can be introduced in homoepitaxial layers of ZnSe after the crystal has been grown. As noted previously, the long half life of the Zn65 isotope is crucial in that it provides sufficient time to grow the homoepitaxial layers before significant nuclear decay to CUDShas occurred. PL measurements can then be made as the Zn65 nuclei decay and will permit the observation of copper in ZnSe where the copper is unambiguously known to be in an isolated zinc site.
3. DISCUSSION 2.8
2.6
2.4
2.2
2.0
1.8
energy W)
Fig. 2. PL spectrum of bulk SPVT grown ZnSe annealed in molten zinc at 900°C for 24 h.
When irradiating as-grown ZnSe, Zn6’ nuclei capture thermal neutrons by radiative capture while they are in the ZnSe lattice. Radiative capture by Zn@
E. D. WHEELER et al.
82 vacuum seals
i
n
:
znse substrate
II
I
-___._’
-
.-
I
- IO mtorr
1060°C
1000°C 900°C
~. ----t-z*30
k-
0
50
distance (cm)
Fig. 3. ZnSe homoepitaxial layer growth experimental setup.
2.793
2.779
I
2.6
2.7
2.6 energy
2.6 (av)
2.4
2.3
Fig. 4. PL spectrum of ZnSe homoepitaxial layer. nuclei result in the emission of r-rays with energies of several MeV22 which can impart a recoil in excess of 100 eV to the zinc nuclei. The recoil energy is given by,
ER =
E2
&%
E2 1.3 x li5MeV
(3)
where ER is the recoil energy, E_, is the energy of the emitted y-ray, m, is the nuclei mass, and c is the speed of light. The threshold energy for lattice displacement in ZnSe is not precisely known, but the threshold energies for many other materials is less than the value given above [23]. If the 25eV value given in earlier work for copper [24] is taken as a conservative estimate for the threshold energy for nuclear displacement in ZnSe, it is apparent many Zn6’ nuclei may be displaced from their copper sites in the ZnSe lattice during neutron irradiation. The neutron lattice damage must be repaired in order for the effects of NTD to be observable with PL. The important result obtained from the zinc annealing experiments is that bulk ZnSe can be obtained with a very low level of background emissions at the energies of 2.783, 2.35 and 1.97eV. Since these are precisely the regions of the 17, Cu-G, and Cu-R
emissions, one may well be able to effectively use high quality SPVT grown bulk material in future detailed studies of copper doping in ZnSe. In these future experiments, as-grown ZnSe would first be neutron irradiated, then immediately annealed in molten zinc; the zinc anneal would serve both to repair irradiation damage and to significantly reduce the intensity of the 1: excitonic emission. This postirradiation processing could easily be completed before significant amounts of copper are introduced by nuclear decay. This technique, employing zinc annealing, presents a relatively low level of expcrimental hazard while allowing effective PL monitoring of the energy regions of interest in investigations probing the effects of copper doping in ZnSe. In the experimental technique employing post-irradiation crystal growth, ZnSe homoepitaxial layers are formed from previously irradiated elemental sources with a growth technique described fully elsewhere [21]. Post-irradiation crystal growth permits copper to be introduced at an isolated zinc site after the crystal is grown and ensures the formation of isolated Cuz,, centers. The lattice damage due to neutron irradiation is eliminated since the crystal is grown after neutron irradiation but before nuclear decay occurs. This, in turn, eliminates the need for thermal annealing to repair lattice damage suffered during neutron irradiation. The recoil energies in 50.7% of the nuclear decays have energies of 0.43 eV followed by recoils of energy 10.2 eV while 47.8% of the nuclear decays result in nuclear recoils of 14.1 eV. Since these recoil energies are smaller than the nuclear displacement threshold energies for many other materials [23], the copper dopants will likely remain at a zinc site in the ZnSe during the nuclear decay. That is, the nuclear recoil associated with decay Zn6’ to Cu6’ is likely too small to result in the displacement of the copper dopant from its zinc site. Whether the copper dopants are displaced during nuclear decay is crucial if isolated Cuz,, centers are to be successfully introduced in a manner that is independent of crystal growth processes. If the nuclear decay itself causes displacement of the copper, then some type of post-decay annealing would be necessary in any doping experiment which employs NTD in ZnSe. The high temperatures associated with many anneals would, at least, call into question whether dopant introduction is truly decoupled from crystal growth processes since the high temperatures would permit rearrangements of the atoms in the crystal lattice. The final arrangement of atoms and defects within the crystal lattice may well be different than would be the case in the absence of the copper dopants. If, on the other hand, the decay processes do not produce nuclear displacements, then
Incorporation
of CuZn centers in ZnSe
a technique allowing post-irradiation, pre-decay crystal growth does allow crystal growth to be decoupled from doping [17] and permits the incorporation of isolated Cuz,, centers in Z&e. The experimental techniques described herein allow copper doping in ZnSe to be probed in new ways. For example, it is known that the behavior of many dopants in wide bandgap materials change as the concentration of dopants is increased [25]. With the techniques developed in this investigation, high concentrations of isolated Cuzn dopants, in excess of lOI crnvm3,can be incorporated in ZnSe in a manner that is independent of crystal growth mechanisms. The question can be addressed as to whether the changes in dopant behavior is a property of the isolated substitutional dopant or, rather, is due to some interaction which is present during crystal growth. The method of dopant incorporation in the present investigation is fundamentally different when compared to other doping techniques. In the techniques reported here, isolated Cuz,, dopants are introduced, in bulk, after crystal growth is complete while, in other techniques, dopants are introduced at a growing crystal surface or at high temperatures. The dopants presence at a surface during growth or its presence at elevated temperature during growth permits greater degree of interaction with other dopants and lattice defects when compared with the doping technique presented herein. The doping techniques described here may indeed represent the furthest limit from equilibrium possible in the incorporation of the Cuzn dopant in ZnSe. Since some emissions due to copper in ZnSe may be due to Cuzn and other emission may be due to complexes involving copper, the experimental techniques described here will be a valuable aid in the development of a microscopic model for the various emissions due to copper in ZnSe. Any observed changes in the PL or infrared (IR) spectra during the decay of the ZN6’ nuclei can be unambiguously assigned to Cuzn centers in ZnSe. Further, through the use of neutron irradiation in ZnSe, additional new experiments can be designed. It becomes possible to treat the temperature and pressure at the time of dopant incorporation as experimental variables. Experiments such as these could yield valuable information on the structure and stability of the Cuzn center in ZnSe.
4. CONCLUSIONS
The investigation employs three techniques using nuclear transmutation doping in investigations of the effects of copper in ZnSe. In the first technique, as-grown bulk ZnSe samples are irradiated with
83
thermal neutrons which likely result, after thermal annealing, in the incorporation of Cuzn in ZnSe. Initial results are consistent with Cuzn being involved in the copper red and copper green emissions in ZnSe but not in the I;’ excitonic emission. Since the presence of the 1: excitonic emission, along with its phonon replicas, results in a high level of background emission and causes difficulties in interpreting PL spectra, zinc annealing experiments were performed which showed that the 1: excitonic emission can be removed in SPVT material. In future NTD studies of copper doping in ZnSe, zinc annealing can be employed in order to obtain a more sensitive experimental technique employing as-grown bulk ZnSe. The third technique employs homoepitaxial layer growth from previously irradiated elemental sources. This technique produces layers with a very low level of background emission and allows &heunambiguous incorporation of isolated Cuzn centers in ZnSe. Since, with the techniques described herein, the copper dopants are introduced after the growth processes are complete, there is no possibility that the copper dopants can form complexes as they can when they are present during crystal growth. The experimental techniques presented here will allow solid progress to be made in determining the role of copper in ZnSe. Acknowledgements-X-ray performed by J. Thomas
diffraction of Pittsburg
measurements were State University in Pit&burg, Kansas. E.D.W. was partially supported by a GAANN research fellowship from the U.S. Department of Education.
REFERENCES I. Curie D., Luminescence in Crystals. Methuen (1963). 2. Curie D. and Prener J. S., in Physics and Chemistry ofllVI Compounds. Eds M. Aven and J. S. Prener, North Holland, Amsterdam (1967). 3. Robbins D. J.. Dean P. J.. Simmonds P. E. and Tews H.. in Deep Centersfor Semiconductors. Ed. S. T. Pantelides, Gorden and Breach, Reading (1986). 4. Mathewson C. H., Zinc, rhr Meral, its Alloy and Compounds. Reinhold (2959). H. H., in Physics and Chemistr_v of Ii-VI 5. Woodbury Compounds. Eds M. Aven and J. S. Prener, North Holland, Amsterdam (1967). 6. Shaw D., in Widegap II- VI Compounds,for Optoelectronic Application. Ed. H. Ruda, Chapman Hall (1992). I. Stringfellow G. B. and Bube R. H., Phj:r. Rev. 171,903 (1968). 8. Grimmeiss H. G.. Ovrin C. and Mach R., J. Appl. Phys. 50,6328 (1979). 9. Godlewski M., Lamb W. E. and Cavenett B. C., Sol. St. Comm. 39, 595 (1981). 10 Pate1 J. L., Davies J. J. and Nichols J. E.. J. Phys. C14, 5545 (1981). 11 Pohl LJ. W., Kudlek G. H., Klimakow A. and Hoffmann A., J. Cr~~stal Growfh 138,385 (1994). 12. Dean P. J., Pitt A. D., Skofnick M. S., Wright P. J. and Cockayne B., J. Crystal Growth 59, 301 (1982). 13. Shi-Min Huang, Yasuo Nozue and Kenzo Igaki, Jpn. J. Appl. Phys. 22, L420 ( 1983).
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E. D. WHEELER et al.
14. Isshiki M. and Masumoto K., J. Phys. C: Sol. St. Phys. 21,277l (1988). 15. Table of Isotopes 7th Ed. Eds C. Michael Lederer and Virginia S. Shirley, Wiley-Interscience, New York (1978). 16. Wu Z. L., Merz J. L., Werkhoven C. J., Fitzpatrick B. J. and Bhargava R. N., Appl. Phys. Left. 40, 345 (1982). 17. Wheeler E. D., Boone J. L., Farmer J. W. and Chandrasekhar H. R., Phys. Rev. B 53, 15,617 (1996). 18. Yodo T. and Yamashita K., Appl. Phys. Lett. 54, 1778 (1989). 19. Isshiki M., Kyotani T., Masumoto K., Uchida W. and Suto S., Phys. Rev. B36,2568 (1987).
20. Cotal H. L., Markey B. G., McKeever S. W.S., Cantwell G. and Harsch W. C., Physica BItI!!, 103 (1993). 21. Wheeler E. D., Boone J. L. and Cantwell G., to be published. 22. Bartholomew G. A., Doveika A., Eastwood K. M., Monaro S., Groshev L. V., Demidov A. M., Pelekhov V. I. and Sokolovskii L. L., Nuclear Data 3,367 (1967). 23. Greenwood L. R., J. Nuclear Materials 108and 10!3,21 (1982). 24. Eggen D. T. and Laubenstein M. J., Phys. Rev. 91,238 (1953). 25. Neumark G. F., in Widegap II-VI Compoud for Optoelectronic Applications.Ed. H. Ruda, Chapman Hall (1992).
Pergamon
INCORPORATION
OF CuZn CENTERS EQUILIBRIUM
J Phys. Chem Solids Vol 58, No. I. pp. 79-84. 1997 Copyright c: 1996 Elsevicr Science Ltd Printed in Great Britain. All nghts reserved OOZZ-3697/97 $17.00 + 0.M)
IN ZnSe FAR FROM
E. D. WHEELER*, JACK L. BOONE*, J. W. FARMER? and H. R. CHANDRASEKHARS *Department of Electrical Engineering, University of Missouri, Rolla, MO 65409, U.S.A. ?University of Missouri Research Reactor and Department of Physics, University of Missouri, Columbia, MO 6521 I, U.S.A. fDepartment of Physics, University of Missouri, Columbia, MO 65211, U.S.A. (Received 16 April 1996; accepted 23 April 1996)
Abstract-An investigation employing nuclear transmutation to probe the effects of copper doping in ZnSe is presented. Three experimental techniques are developed in the investigation. With the first, as-grown ZnSe is irradiated with thermal neutrons which results, after thermal annealing, in the incorporation of Cuzn centers. Observations are consistent with isolated Cuz. being involved in the copper red and copper green emissions in ZnSe but not in the 1; excitonic emission. With the second, it is shown that zinc annealing can be used effectively in investigations involving the irradiation of as-grown ZnSe since the zinc annealing treatment significantly reduces background PL emissions in bulk &Se. The third technique employs homoepitaxial ZnSe layer growth from previously irradiated elemental sources. The epitaxial layers display no dominant 1: excitonic emission and a very low level of deep emissions. Because, with the techniques described here, the copper atoms are introduced at zinc sites after crystal growth processes are complete, the copper atoms are not able to interact with other dopants or lattice defects during growth as they can when incorporated by other means. The absence of interactions during crystal growth permits the unambiguous incorporation, far from equilibrium, of isolated Cuzn centers in ZnSe. Copyright Q 1996Elsevier Science Ltd Keywords: A. optical materials, A. semiconductors, B. epitaxial growth, D. optical properties.
1. INTRODUCTION
ease. Emissions believed to be due to copper are often present in both bulk crystals and in epitaxial layers. Since copper is a common impurity in ZnSe and since it strongly affects luminescence in ZnSe, understanding its behavior in ZnSe is doubly important. One goal in investigations of copper doping has been to develop a microscopic model of the luminescent centers; one obstacle to this goal is the variety of centers which can be present in copper doped II-VI semiconductors. This variety of possible centers, coupled with coppers high diffusion rate, frustrates attempts to clearly identify the nature of copper as an impurity in these compounds since it is possible for CuZn, Cui, or complexes involving copper to be present. In low temperature photoluminescence (PL) measurements, the copper red (Cu-R) emission at 1.97eV [7-91 and the copper green (Cu-G) emission at 2.35eV [7-91 are known to involve copper. Other experimental work employing optically detected magnetic resonance gives evidence the Cu-G center results from a complex involving a copper atom at a zinc site in association with another ion at an interstitial site [lo]. At present, there is some question as to whether the 1: excitonic emission at 2.7830eV involves only zinc vacancies [I I, 121or involves both zinc vacancies and copper related centers [l3, 141.
Copper is an important impurity in wide bandgap zinc chalcogenides [l]. In ZnSe, the incorporation of copper results in a variety of characteristic visible bands [2], and investigations seeking a microscopic model of copper in the zinc chalcogenides have continued for many years. The long standing uncertainty of the role of copper in these materials attests to the need for developing new experimental techniques which may provide additional information on the behavior of this important impurity. It is necessary to gain a clear understanding on the role of copper in the zinc chalcogenides for two reasons. First, copper is the most efficient known activator of luminescence in these compounds [3] which makes understanding its effects important in any effort to control or modify its luminescent properties. Second, copper is a major residual impurity and can be incorporated in zinc chalcogenides via two primary avenues. One is that copper is a major contaminant of zinc [4], so that zinc chalcogenides produced by any process can contain copper; the second avenue is copper’s high diffusion rate in the II-VI compound semiconductors [5,6]. Because of this high diffusion rate, copper can enter the material from growth and processing environments with relative 79
E. D. WHEELER et al.
80
The experimental techniques reported herein allow the effects of isolated Cuz,, centers to be observed in ZnSe and will allow the effects due to Cuz,, centers to be characterized in a straightforward manner.
2. EXPERIMENTAL When zinc is exposed to a flux of thermal neutrons, the Zn@ isotope can capture a thermal neutron by radiative capture, Zn64+n1
+Zn65+y
(1)
The radiative capture process is followed by nuclear decay, via electron capture, to a stable copper isotope
WI, Zn65 + e- + C#
+ v
(2)
where n’ is a thermal neutron, y is a gamma ray, e- is an orbital electron and v is a neutrino. This nuclear decay process has a half-life of 244.1 days. Actually, only 98.54% of the nuclear decays proceed via (2), while 1.46% proceed via positron emission [15]. The decay via positron emission will be neglected here since it results in the same end state as approximately 48% of the decays by electron capture and furthermore, as indicated above, only a small fraction of the nuclear decay events are involved. Three experimental techniques were investigated in the work reported herein. The first involves irradiating as-grown ZnSe wafers, then thermally annealing them to repair neutron damage suffered during irradiation. The second involves irradiating as-grown ZnSe wafers, then annealing them in high purity molten zinc metal. The third involves irradiating elemental sources and then forming homoepitaxial ZnSe layers from the irradiated sources by physical vapor transport. In all three, the long half-life of Zn65 is key to allowing neutron transmutation to be employed in experimental investigations of copper in ZnSe. The 244.1 day half-life provides ample opportunity to anneal irradiated as-grown ZnSe or to grown ZnSe from irradiated elemental sources before significant nuclear decay of the Zn65 nuclei has occurred. With these techniques, Cu 65 dopants are introduced after growth processes are complete, far from thermal equilibrium. Since copper dopants are not present at the time that the post-irradiation processing takes place, they can not form complexes with other dopants or lattice defects during crystal growth. 2.1. Irradiation of as-grown ZnSe with subsequent thermal annealing Wafers cut from boules of ZnSe, grown by seeded physical vapor transport (SPVT) at the Eagle Picher
2.8 2.6 2.4 2.2 2.0 1.6
energy (eV)
Fig. 1. PL spectra of bulk ZnSe (a) unirradiated, annealed control sample, (b) irradiated, annealed sample one month after irradiation, (c) same irradiated, annealed sample four months after irradiation.
Research Laboratories in Miami, Oklahoma, were irradiated with thermal neutrons at the Missouri University Research Reactor (MURR) in Columbia, Missouri, The (100) oriented wafers were irradiated for an ultimate copper concentration of 10” cme3. Lattice damage due to both fast neutrons colliding with the lattice and due to the radiative capture of thermal neutrons must be removed by post-irradiation thermal annealing for the effects of neutron transmutation doping (NTD) to be observable. Because no complete thermal annealing studies of SPVT grown ZnSe have been reported in the literature, an annealing study was conducted during this work. The irradiated crystals, weighing about 1.3 mg and measuring 1 mm x 1 mm x 0.25 mm, were placed in quartz vials which had been previously cleaned, lightly etched in HF, and baked overnight at 1100°C. The small irradiated crystals were then surrounded by a larger quantity of unirradiated ZnSe before the vials were purged several times with high purity argon and then sealed under vacuum. Anneals were conducted at temperatures of 600, 700, 800 or 900°C for times of 1 and 3 h. These temperatures were chosen because similar temperatures have previously been employed in annealing ion-implanted ZnSe [ 161. The PL spectra for the ZnSe samples annealed at 600°C for 3 h are shown in Fig, 1. The spectrum for an unirradiated, annealed control sample is shown in Fig. l(a). The spectrum for an irradiated, annealed sample one month after irradiation is shown in Fig. l(b), and the spectrum for the same irradiated, annealed sample four months after irradiation is shown in Fig. l(c). The three emissions of interest here are the If excitonic emission at 2.7830 eV, the Cu-G emission at 2.35 eV, and the Cu-R emission at 1.97 eV. The If excitonic emission is typical of bulk ZnSe and, as previously discussed, may result from Vz”, or from both Vz,, and Cuz,, centers. The 1: emission has several longitudinal
Incorporation of Cuz” centers in ZnSe
optical phonon replicas spaced 0.03 1 eV apart which, along with the donor bound exciton (DBE) peak at 2.7973 eV, dominate the PL spectrum above approximately 2.6eV in the SPVT grown ZnSe used in this study. The PL spectrum at four months after neutron irradiation displays enhanced Cu-G and Cu-R emissions when compared to the spectrum at one month and may result from the nuclear decay of Zn65 to Cu6’. No significant change occurred in the intensity of the 1: emission. Another change in the PL spectrum at four months compared to the spectrum at one month is an enhanced DBE peak. It is difficult to speculate on the cause of this change since the region between the DBE and If has a high level of background emission, but it may be due to arsenic [17]. Other samples, annealed at higher temperatures, also display an enhanced DBE peak at four months but do not display enhanced Cu-R or Cu-G emissions. It should be noted here that previous work on annealing at temperatures significantly higher than 550°C show that deep levels are generated at these higher annealing temperatures [ 181. 2.2. Irradiation of as-grown ZnSe with subsequent zinc annealing An obstacle to employing bulk as-grown ZnSe in NTD studies of copper doping is the I;’ emission peak consistently present in the PL spectrum. This peak is located very near a possible copper related emission. Moreover, its presence, along with its phonon replicas, results in a high background emission level in the SPVT grown material. The location of 1: and the resulting high background emission level make its presence undesirable in studies investigating the effects of copper in ZnSe. Subsequent to the NTD studies on irradiated SPVT grown ZnSe described above, zinc annealing experiments were conducted in
DAP
DAP-LO I ’ DAP-2L0
81
an attempt to remove or diminish the I;’ emission in bulk SPVT grown ZnSe. These studies were conducted so that, in future NTD studies involving SPVT grown ZnSe, background emission levels might be reduced which would then allow more sensitive observations. Figure 2 shows the PL spectrum displayed by a (100) ZnSe wafer which was annealed in molten zinc at 900°C for 24 h. The remarkable reduction in intensity of the 1; emission, compared to the samples without zinc annealing, is consistent with earlier annealing studies [19, 201 and will be very useful in future NTD studies involving bulk ZnSe. In contrast to these earlier studies, however, Fig. 2 does not display a second dominant excitonic emission and its phonon replicas. In these earlier studies, this second emission was attributed to free excitons and became significant after zinc annealing.
2.3. ZnSe sources
grown from
irradiated elemental
Homoepitaxial layers of ZnSe are grown, with elemental zinc and selenium as sources, by physical vapor transport in a closed quartz tube as shown in Fig. 3. This technique allows control over vapor stoichiometry and is capable of producing layers with no dominant I;’ peak and a very low level of deep level emissions in the PL spectrum as shown in Fig. 4. This low level of background emissions is crucial if PL is to be employed in observations of coppers effects on the luminescent properties in ZnSe. Layers grown with this technique display a X-ray diffraction rocking curve with a full width at half maximum of less than 25 arc seconds. Details of this growth technique appear elsewhere [21]. By employing this technique, homoepitaxial layers can be grown from elemental selenium and previously irradiated zinc. Post-irradiation growth eliminates neutron irradiation lattice damage and the subsequent required post-irradiation annealing. With this technique, CuZn can be introduced in homoepitaxial layers of ZnSe after the crystal has been grown. As noted previously, the long half life of the Zn65 isotope is crucial in that it provides sufficient time to grow the homoepitaxial layers before significant nuclear decay to CUDShas occurred. PL measurements can then be made as the Zn65 nuclei decay and will permit the observation of copper in ZnSe where the copper is unambiguously known to be in an isolated zinc site.
3. DISCUSSION 2.8
2.6
2.4
2.2
2.0
1.8
energy W)
Fig. 2. PL spectrum of bulk SPVT grown ZnSe annealed in molten zinc at 900°C for 24 h.
When irradiating as-grown ZnSe, Zn6’ nuclei capture thermal neutrons by radiative capture while they are in the ZnSe lattice. Radiative capture by Zn@
E. D. WHEELER et al.
82 vacuum seals
i
n
:
znse substrate
II
I
-___._’
-
.-
I
- IO mtorr
1060°C
1000°C 900°C
~. ----t-z*30
k-
0
50
distance (cm)
Fig. 3. ZnSe homoepitaxial layer growth experimental setup.
2.793
2.779
I
2.6
2.7
2.6 energy
2.6 (av)
2.4
2.3
Fig. 4. PL spectrum of ZnSe homoepitaxial layer. nuclei result in the emission of r-rays with energies of several MeV22 which can impart a recoil in excess of 100 eV to the zinc nuclei. The recoil energy is given by,
ER =
E2
&%
E2 1.3 x li5MeV
(3)
where ER is the recoil energy, E_, is the energy of the emitted y-ray, m, is the nuclei mass, and c is the speed of light. The threshold energy for lattice displacement in ZnSe is not precisely known, but the threshold energies for many other materials is less than the value given above [23]. If the 25eV value given in earlier work for copper [24] is taken as a conservative estimate for the threshold energy for nuclear displacement in ZnSe, it is apparent many Zn6’ nuclei may be displaced from their copper sites in the ZnSe lattice during neutron irradiation. The neutron lattice damage must be repaired in order for the effects of NTD to be observable with PL. The important result obtained from the zinc annealing experiments is that bulk ZnSe can be obtained with a very low level of background emissions at the energies of 2.783, 2.35 and 1.97eV. Since these are precisely the regions of the 17, Cu-G, and Cu-R
emissions, one may well be able to effectively use high quality SPVT grown bulk material in future detailed studies of copper doping in ZnSe. In these future experiments, as-grown ZnSe would first be neutron irradiated, then immediately annealed in molten zinc; the zinc anneal would serve both to repair irradiation damage and to significantly reduce the intensity of the 1: excitonic emission. This postirradiation processing could easily be completed before significant amounts of copper are introduced by nuclear decay. This technique, employing zinc annealing, presents a relatively low level of expcrimental hazard while allowing effective PL monitoring of the energy regions of interest in investigations probing the effects of copper doping in ZnSe. In the experimental technique employing post-irradiation crystal growth, ZnSe homoepitaxial layers are formed from previously irradiated elemental sources with a growth technique described fully elsewhere [21]. Post-irradiation crystal growth permits copper to be introduced at an isolated zinc site after the crystal is grown and ensures the formation of isolated Cuz,, centers. The lattice damage due to neutron irradiation is eliminated since the crystal is grown after neutron irradiation but before nuclear decay occurs. This, in turn, eliminates the need for thermal annealing to repair lattice damage suffered during neutron irradiation. The recoil energies in 50.7% of the nuclear decays have energies of 0.43 eV followed by recoils of energy 10.2 eV while 47.8% of the nuclear decays result in nuclear recoils of 14.1 eV. Since these recoil energies are smaller than the nuclear displacement threshold energies for many other materials [23], the copper dopants will likely remain at a zinc site in the ZnSe during the nuclear decay. That is, the nuclear recoil associated with decay Zn6’ to Cu6’ is likely too small to result in the displacement of the copper dopant from its zinc site. Whether the copper dopants are displaced during nuclear decay is crucial if isolated Cuz,, centers are to be successfully introduced in a manner that is independent of crystal growth processes. If the nuclear decay itself causes displacement of the copper, then some type of post-decay annealing would be necessary in any doping experiment which employs NTD in ZnSe. The high temperatures associated with many anneals would, at least, call into question whether dopant introduction is truly decoupled from crystal growth processes since the high temperatures would permit rearrangements of the atoms in the crystal lattice. The final arrangement of atoms and defects within the crystal lattice may well be different than would be the case in the absence of the copper dopants. If, on the other hand, the decay processes do not produce nuclear displacements, then
Incorporation
of CuZn centers in ZnSe
a technique allowing post-irradiation, pre-decay crystal growth does allow crystal growth to be decoupled from doping [17] and permits the incorporation of isolated Cuz,, centers in Z&e. The experimental techniques described herein allow copper doping in ZnSe to be probed in new ways. For example, it is known that the behavior of many dopants in wide bandgap materials change as the concentration of dopants is increased [25]. With the techniques developed in this investigation, high concentrations of isolated Cuzn dopants, in excess of lOI crnvm3,can be incorporated in ZnSe in a manner that is independent of crystal growth mechanisms. The question can be addressed as to whether the changes in dopant behavior is a property of the isolated substitutional dopant or, rather, is due to some interaction which is present during crystal growth. The method of dopant incorporation in the present investigation is fundamentally different when compared to other doping techniques. In the techniques reported here, isolated Cuz,, dopants are introduced, in bulk, after crystal growth is complete while, in other techniques, dopants are introduced at a growing crystal surface or at high temperatures. The dopants presence at a surface during growth or its presence at elevated temperature during growth permits greater degree of interaction with other dopants and lattice defects when compared with the doping technique presented herein. The doping techniques described here may indeed represent the furthest limit from equilibrium possible in the incorporation of the Cuzn dopant in ZnSe. Since some emissions due to copper in ZnSe may be due to Cuzn and other emission may be due to complexes involving copper, the experimental techniques described here will be a valuable aid in the development of a microscopic model for the various emissions due to copper in ZnSe. Any observed changes in the PL or infrared (IR) spectra during the decay of the ZN6’ nuclei can be unambiguously assigned to Cuzn centers in ZnSe. Further, through the use of neutron irradiation in ZnSe, additional new experiments can be designed. It becomes possible to treat the temperature and pressure at the time of dopant incorporation as experimental variables. Experiments such as these could yield valuable information on the structure and stability of the Cuzn center in ZnSe.
4. CONCLUSIONS
The investigation employs three techniques using nuclear transmutation doping in investigations of the effects of copper in ZnSe. In the first technique, as-grown bulk ZnSe samples are irradiated with
83
thermal neutrons which likely result, after thermal annealing, in the incorporation of Cuzn in ZnSe. Initial results are consistent with Cuzn being involved in the copper red and copper green emissions in ZnSe but not in the I;’ excitonic emission. Since the presence of the 1: excitonic emission, along with its phonon replicas, results in a high level of background emission and causes difficulties in interpreting PL spectra, zinc annealing experiments were performed which showed that the 1: excitonic emission can be removed in SPVT material. In future NTD studies of copper doping in ZnSe, zinc annealing can be employed in order to obtain a more sensitive experimental technique employing as-grown bulk ZnSe. The third technique employs homoepitaxial layer growth from previously irradiated elemental sources. This technique produces layers with a very low level of background emission and allows &heunambiguous incorporation of isolated Cuzn centers in ZnSe. Since, with the techniques described herein, the copper dopants are introduced after the growth processes are complete, there is no possibility that the copper dopants can form complexes as they can when they are present during crystal growth. The experimental techniques presented here will allow solid progress to be made in determining the role of copper in ZnSe. Acknowledgements-X-ray performed by J. Thomas
diffraction of Pittsburg
measurements were State University in Pit&burg, Kansas. E.D.W. was partially supported by a GAANN research fellowship from the U.S. Department of Education.
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