Haller, E.E.

Jeffries, C.D.

Pakulis, E.J.

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Lawrence Berkeley National LaboratoryRecent WorkTitleEPR OF THE LITHIUM ION-DANGLING BOND COMPLEX IN GERMANIUM CONTAINING DISLOCATIONSPermalinkhttps://escholarship.org/uc/item/7vs5983vAuthorsPakulis, E.J.Haller, E.E.Jeffries, C.D.Publication Date1982-08-01eScholarship.org Powered by the California Digital LibraryUniversity of California{j(2-tj LBL-14836 <".d-Preprint Lawrence Berkeley Laboratory UNIV'ERSITY OF CALIFORNIA Materials & Molecular Research Divisi.on Submitted to Journal of Magnetic Resonance EPR OF THE LITHIUM ION-DANGLING BOND COMPLEX IN GERMANIUM CONTAINING DISLOCATIONS E.J. Pakulis, E.E. Haller, and C.D. Jeffries August 1982 .~·t'.\': ....... TWO-WEEK LOAN COpy This is a Library Circulating ~opy _ .' which may be borrowed for two 'weeks .. For a personal retention copy~ call Tech. Info. Division~ Ext. 6782. Prepared for the U.S. Department of Energy under Contract DE-AC03:76SF00098 ...... . l" .- , ~~.. .t. DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. I .\i:f. August 1982 EPR OF THE LITHIUM ION-DANGLING BOND COMPLEX IN GERMANIUM CONTAINING DISLOCATIONS* E. J. Paku1is,t E. E. Haller, and C. D. Jeffries University of California, and Lawrence Berkeley Laboratory, Berkeley, CA 94720 LBL-14836 *This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Office of Health and Environmental Research and Material Sciences Division of the U.S. Department of Energy under Contract Number DE-AC03-76SF00098. tpresent address: IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598. -i-r !.......- -,,,, /' ,=-.,:...... D EPR OF THE LITHIUM ION-DANGLING BOND COMPLEX IN GERMANIUM CONTAINING DISLOCATIONS E. J. Paku1is,* E. E. Haller, and C. D. Jeffries University of California, and Lawrence Berkeley Laboratory, Berkeley, CA 94720 (Received ) We have observed a new EPR spectrum in a lithium-diffused germanium crystal ND-N ~1013 cm-3) containing dislocations. The principal g values are 1.91~ ± 0.002, 1.896 ± 0.002, and 0.855 ± 0.010 along the axes (110), (112), and (111), plus equivalent sets. As this new spectrum is seen in lieu of the dislocation dangling bond spectrum, we attribute it to a lithium ion-dislocation dangling bond complex. 't~ ;, • The behavior of lithium in Ge is of both technological and fundamental interest and has been studied in some detail. 1 - 5 Lithium is a shallow donor, and as an interstitial impurity it diffuses rapidly. It can also be trapped at defects to form shallow donor complexes (for example, LiO). Reiss ~ al. 1 postulate ion pairing between lithium ions and acceptor sites in dislocations. We report here a new EPR spec-trum in lithium diffused dislocated Ge, at liquid helium temperatures, which we attribute to such a complex, where the acceptor sites are the dislocation dangling bonds (DDB). The apparatus used was a helium cooled microwave (22-26 GHz) superheterodyne spectro-meter with an optical port and helium cooled shutter. Magnetic field modulation and lock-in detection were used to record the derivative of the absorption signal. Two samples, one dislo-cation free and one containing dislocations (Zl03 cm-2 ), were cut in the shape of right circular cylinders, 12.5 rom in diameter and 8-10 rom in height (axis along (110»). The samples were saturated with lithium by diffusion from the surface at 400°C. The lithium was then out-diffused for several days at 200°C until a net donor concentration of %1013 cm-3 was achieved. After the out-diffusion virtually all the free lithium has left the crystal and one is left with lithium bound at defects. Each sample in turn was etched, mounted in the spectrometer cavity, and cooled to liquid helium temperature. At T ~ 4K the Ge sample serves as its own micro-wave resonant cavity with a high quality factor Q % 105. S,6 The crystals had oxygen concentrations %1014 cm-3 , so some of the lithium was present in the form of the LiO complex. This complex gives rise to an EPR spectrumS with a g-tensor axially symmetric about the four (111) axes. The principal g values are gil = 0.85 and gl = 1.91. This four-line spectrum is the only lithium related spectrum observed in the lithium diffused dislocation free sample. The dislocated sample had additional lines (~14 g peak-to-peak derivative width), due to lithium at dislocations, superimposed on the four LiO lines. The angular dependence of the combined spectrum, as the magnetic field is rotated in a plane close to (110), appears in Figure 1. The figure also shows computer simu-lated spectra for LiO (dashed) and for Li at dislocations (solid). The simulations were done ,0 . ,0. I ' I" for perfect crystal alignment and were based on the effective spin Hamiltonian ~ ~ ~ ~ 'J{ = I3H • [gLiO + gd-Li] • S, ~ where P is the Bohr magneton, H the magnetic ~ field, and S the spin (the same effective spin, ~ S = 1/2, was used for both centers). gLiO is the spectroscopic splitting tensor for the LiO com-plex and has the principal g values quoted above. gd-Li is the spectroscopic splitting tensor for the Li-dislocation complex and was selected in ~ such a way as to best reproduce the data. gd-Li has principal g values gl = 1.917, g2 = 1.~96, and g3 = 0.855, and principal axes gl= (110), g2 = (112), and g3 = (111) (plus equivalent sets). Note that gd-Li deviates by only ~l% from being axially symmetric about the (111) axes. For each of the four (111) axes (i.e. g3) there are three (110) directions lying in the correspond-ing (111) plane, and gl can be along any of these three (110) directions. g2 also lies in the plane and is perpendicular to gl. Consequently, the spectrum resulting from gd-Lo consists of four branches of three lines eacfi, or 12 lines. ~ For H restricted to the (110) plane, degenera-cies reduce this number to seven lines. In practice, a slight misorientation of the sample can break some of the degeneracies and result in more than seven lines. These are, of course, in addition to the four LiO lines. Recalling that dislocation lines in the tetrahedral structure are oriented along (110) directions,7 we see that the departure of the gd-L ° tensor from axial symmetry is correlate.d to tfie directions of dislocation lines. If we consider in particular the 60 0 dislocation of the shuffle set, Figure 2, then gl is parallel to the dislocation line, g3 is parallel to the DDB's, and g2 is perpendicular to gl and g3· A previous paper6 reported the observation of the spin resonance of the Ge DDB electrons. The DDB g-tensor is axially symmetric about the (111) axes tilted by 1.20 along the (110) direc-tions and has principal g values gil = 0.73 and g~ = 1.89. We did not observe the DDB spectrum in the lithium diffused Ge sample containing dis-locations. In order to explain the absence of the DDB spectrum and the appearance of the d-Li spectrum, we propose a model in which lithium ions form a complex with the DDB's. The complex-ing of the Li to the DDB could also account for the suppression of the 1.20 tilt of the DDB, reSUlting instead in the 1% nonaxiallity of the g-tensor. One might have expected an atomic Li-DDB complex in which the Li saturates the dangling bond. In such a case, however, one would have an S = 0 configuration and no EPR signal. An interesting feature arises when one con-siders the neutral configuration of the Li ion-DDB complex, in which an electron is shallowly bound in some roughly hydro genic orbit. Rather than being distributed throughout the bulk of the crystal, these donors are lined up along the dislocations, where even at concentrations below 10 12 cm- 3 considerable overlap of the donor electron wave functions occurs, resulting in a conducting path along the dislocations. (As in the case of dislocated Si, where the microwave conductivity was found to be seven orders of magnitude larger than the D.C. conductivity,9 we expect this conductivity to be most pronounced at high frequencies, due to kinks and/or other breaks in the translational symmetry along the dislocations.) Supporting this model is our observation of an electron cyclotron resonance signal for the optically shielded dislocated sample comparable to that for the optically pumped dislocation-free sample. The shallow donor electrons are evidently being accelerated along dislocations by the microwave electric field until ionization into the conduction band takes place. The presence of mobile electrons at the dislocations would open up the possibility for spin dependent scattering of those electrons by the Li ion-DDB complex, similar to the previous-ly reported 6 spin dependent scattering of photo-excited electrons by dangling bonds in dislo-cated Ge. In the latter case, the spin resonance of the DDB electrons was electrically detected, Le., the magnetic dipole transition triggered a change in conductivity which resulted in an observable change (both increases and decreases were seen, depending on the sample) in absorp-tion of energy from the microwave electric field. We cannot rule out the possibility that the d-Li spectrum has also been electrically detected (although only increases in absorption were observed), particularly in view of the small number of spins involved which, consider-ing the number of lines and their width, would be difficult to detect using conventional EPR magnetic dipole transitions. In summary, we have found that when lithium is diffused into a Ge crystal containing /\ dislocations, a new EPR spectrum emerges. This new spectrum is not observed in a lithium-diffused, dislocation-free crustal. The princi-pal axes of the g-tensor correspond to the orientation of dislocation lines and their dangling bonds. The DDB EPR spectrum seen in ultrapure dislocated Ge is not observed in the lithium-diffused sample. The simple model of a lithium ion-DDB complex is consistent with all of our observations. Electrons are apparently so shallowly bound to these ions that they are easily promoted by the microwaves into the con-duction band, resulting in a strong electron cyclotron resonance signal in the absence of optical excitation. Acknowledgement - This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Office of Health and Environmental Research and Material Sciences Division of the U.S. Department of Energy under Contract Number DE-AC03-76SF00098. * Present address: IBM Thomas J. Watson Research Center, Yorktown Heights, New York. 1. H. Reiss, C. S. Fuller, and F. J. Morin, Bell Syst. Tech. J. 35, 535 (1956). 2. R. L. Aggarwal, P. Fisher, V. Mourzine, and A. K. Ramdas, Phys. Rev. 138. A882 (1965). 3. R. J. Fox, IEEE Trans. Nuc1. Sci. 13, 367 (1966). --4. E. M. Bykova, L. A. Goncharov, T. M. Lifshits, V. I. Sidorov, and R. N. Hall, Fiz. Tekh. Po1uprov. 9, 1966 (1975) [Sov. Phys. Semicond. 1, 1228 (1976)]. 5. E. E. Haller and L. M. Fa1icov, Phys. Rev. Lett. 41, 1192 (1978). 6. E. ~ Paku1is and C. D. Jeffries, Phys. Rev. Lett. 47, 1859 (1981). 7. See, e.g., H. Alexander and P. Haasen, in Solid State Physics: Advances in Research and Applications, Vol. 22, edited by Frederick Seitz. David Thornbu11, and Henry Ehrenreich (Academic Press, New York, 1968), p. 27. 8. J. Hornstra, J. Phys. Chem. Solids 5, 129 (1958). -9. V. A. Grazhu1is, V. V. Kveder, V. Yu. Mukhina, and Yu. A. Osip'yan, JETPLett. 24, 142 (1976). [Pis'ma Zh. Eksp. Teor. Fiz. 1i,-r64 (1976)]. Figure 1 - Angular dependence of the EPR lines observed for the lithium-diffused Ge sample con-taining dislocations as the magnetic field is rotated in the approximate ClIO) plane. Insert shows the continuation of the lines for low values of g near (110). ND-NA% 10 13 cm- 3 • Dis-location density ~103 cm-2 T = 2K. f = 23.49 GHz. Points are measured values. Lines are computer simulations based on perfect crystal alignment and g values quoted in the text. Solid lines: lithium at dislocations. Dashed lines: LiO complex. The size of the deviation from calculations is consistent with a misori-entation ~lo. Note that lines degenerate for perfect alignment can become resolved for other orientations. Figure 2 - Germanium crystal structure including one 60 o-dislocation line, d, with its row of .dislocation dangling bonds, and Burgers vector, ~ b. See Reference 8. 1.3 1.4 1.7 9 1.8 ! 15" ~ ! ! , , ! 1.5 1.7 1.8 1.9 (100) XBL 826-10393 Fig. 1 p,., , ""V (\ \~\ II ( I] ~ I a 819-6501 Fig. 2 " This report was done with support from the Department of Energy. Any conclusions or opinions expressed in this report-represent solely those of the author(s) and not necessarily those of The Regents of the University of California, the Lawrence Berkeley Laboratory or the Department of Energy. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Department of Energy to the exclusion of others that may be suitable. -'"'''':, ...c;!!",,~, TECHNICAL INFORMATION DEPARTMENT ( , LA WRENCE BERKELEY LAB ORA TOR Y UNIVERSITY OF CALIFORNIA " BERKELEY, CALIFORNIA 94720 " o ~;~ "t"'~'<t"' o 

EPR OF THE LITHIUM ION-DANGLING BOND COMPLEX IN GERMANIUM CONTAINING DISLOCATIONS

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