Indium bound exciton luminescence in silicon

Solid State Communications, Vol. 27, pp. 705—708. © Pergamon Press Ltd. 1978. Printed in Great Britain.

0038.4098/78/0815—0705 $02.00/0

INDIUM BOUND EXCITON LUMINESCENCE IN SILICON R. Sauer, W. Schmid, and J. Weber Physikalisches Institut der Universitàt Stuttgart, 0-7000 Stuttgart 80 Federal Republic of Germany

(Received June 23

1978 by E. Moliwo)

A high—resolution photoluminescence study is presented of the bound ex— citon line and its 4 meV low energy satellite in Si:In. The excitation and temperature dependence and the decay times suggest that the satellite line originates from the decay of an exciton bound to the indium acceptor. We tentatively correlate this line with excitations of the final state indium acceptors into a vibrational excited level.

The luminescence from excitons bound to shallow donors and acceptors in silicon is associated with low energy satellites meV belowexcitons the principal 1. Fora few donor bound the BE emission lines satellites form sets of up to seven lines with re— markable spectral, kinetic, and symmetry dependent properties. The physical nature of the underlying excitonic states is under current discussion. Expla— nations in terms of specifically structured multiple bound exciton (MBE) models were recently advanced by several authors2.3’4 ; this interpretation, how— ever, is not generally accepted5. For acceptor bound excitons, the spectral features of the satellites are heterogeneous. While the boron induced lumines— cence strongly resembles the donor associated exci— ton series1, the three satellites of Al and Ga bound exciton lines which were recently reported differ in that in both cases the first satellite is a nonthermalizing doublet and is relatively less widely spaced from the BE line6’7. Following’a preceding prediction2, the doublet satellite was ascribed to

decay after pulsec laser excitation. We used three Czochralski grown silicon samples (Hoboken and Wack Chemitronic) with indium in con centrations ofhomogeneously Sxl&5cm3, doped 1x1O16~m3 , a~d5x1017c:

The samples were excited by cw Ar — or Kr — ion i~ts lines (514 nm, ~ W; 647 nm, 800 mW) or by light pulses from an Ar - ion laser. Since the lifetime o indium bound excitons is very short, t = 2.7 nsec11 this laser system was run in mode locking operation producing pulses of 500 psec puls length at a repetition rate of 10 nsec. In order to observe the decay of the luminescence signal over more than one order of magnitude it was necessary to select single pulses from the mode lock pulse chain with the help of a cavity dumper. Carefully ad— justed the dumper reduced at a resulting rate of about 1 MHz the intensity of the pulses following the excitation puls by a factor of > 100. The peak power of the excitation puls amounted to 20 W. Edge luminescence spectra of Si:In are shown in Fig. 1. The BE emission line, 1n 1,is observed as the recombination of an electron and a hole in a NP, TA—, and TO—phonon assisted replicas. For the bound two-exciton complex leaving behind the bound lower doping levels, the NP emission has1’°lines, a linecited which areother generated the low coupling are = considerably broadened i~E excitonstates in one or the of theby two lying of ex- however, width of I~E 0.35 meV. The BETA and yielding BE the two 3 = 3/2—holes in the A°Xcomplex8. As a re— 0.9 meV or ~E = 0.7 meV, respectively. These finement of the experimental results, Lyon et al.9 linewidths differ from the familiar situation for very recently demonstrated that for Si:Ga the first silicon bound excitons where normally the BETO satellite line is a non—thermalizing triplet which emission is more strongly broadened by the phonon exactly mirrors the thermalizing triplet structure coupling than the BETA emission’2. The satellite of the BE emission line. Assuming ground—state to line labelled 1°l,Aappears 4. 1±0.1 meV below the ground—state transitions the new observation gives In 1 principal BE emission in the NP spectrum. It further evidence for the existence of acceptor bound can also be observed in the TO spectrum at largely 9 also reported a satellite line for relative to the In We estimate that its intensity two—exciton complexes. reduced resolution. Lyon et al. 1 bound exciton line is comparabl. Si:In spaced 4.0 meT below the principal BE line corn— to the case of the NP spectrum. The NP satellite is ponent which by analogy with Al and Ga doped Si might much broader (tIE 1.4 meV) than the associated 17 two—exciton complex. This linefrom was the alsodecay seenofbya crn3, theFor In the higher doping level, [In] = 5x10 be considered as originating BE line. 8 in their investigation of flounced low 1energy resulting in a broadened boundtail exciton line develops a pro— vouk and bound acceptor Lightowlers exciton cathodolurninescence as a NP— BE lineshape and impeding a definite assessment and TO—phonon—assisted replica labelled U 1 or U4 , of the satellit&s intensity and linewidth. respectively. A similar satellite in the TA spectrum of Fig. In this paper, we present high—resolution photo— spaced 4.3±0.1meV below the In1 principal bound and line Lightowlers8 deserves comment. luminescence measurements of the In bound exciton 1° Vouk exciton and labelled U2 in special accordance with The The and temperature dependence these same spectral position with respect to the ~ line line excitation and the satellite 4 meV below the BE of emission lines was investigated as well as the luminescence suggests to consider the U 2 satellite as the TA 705

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INDIUM BOUND EXCITON LUMINESCENCE IN SILICON

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Vol. 27, No. 7

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replica of the Inl,A satellite. This interpretation, however, meets several obstacles: (a) the U2 line is much narrower In~ bound exciton line in contrast to thethan NP the spectrum. The halfwidth, SE < 0.5 meV, may rather indicate a NP exciton transition correlated with a very deep binding

center different In, (b) we find the U 8 also observed the from U 2 about intensity of to large magnitude In1 relative in in different the higher NP to the spectrum, samples, than InTA the luminescence (c) but intensity Vouk at least and comparably ratio Lightowlers one order In115 nescence and same changing crystal; intensities between in particular, 2 satellite, varying different across the samples but U2 obtained a line single cut is from sample lumithe reference in of 16 Vouk times In diffused and as 8. Lightowlers intense They Si. did Bence as not the that we observe BETA support this line this line the in satellite is Fig. conception not 9 oriof due crystal ginally to a growth. correlated different impurity with In impurities introduced but during is rather In of Fig.the2, In1 we plot the NP luminescence intensities and In1 lines as a function of cw excitation in a double’~ogarithmicdiagram. For both emission lines the experimental points can be fitted by straight lines. The exponents obtained from these experimental power laws in the intensity-to-pumping relations are 1.15 for In1 and fall between 1.1 and 1.2 for the In115 satellite. Lyon et al. ~ also mentioned that the observed pump— power dependence of the satellite line is similar to that of the BE emission line, but details were not reported. We have also measured the intensity ratio In1/1n115 in the NP spectrum as a function of tern— perature from P = 2K up to P = 15K. The ratio remains constant in this temperature interval, and

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we find a value of 41±3at a spectral resolution of 1 ineV. In the sams interval, the absolute In? intensitie, were reduced by a factor of 1~B. This intensity ratio is compatible with that found by Vouk and LightowlersB at 11.3K and at an approximate resolution of 0.7 meV as can be estimated

Vol. 27, No. 7

INDIUM BOUND EXCITON LUMINESCENCE IN SILICON

from Fig. 10 of their paper. Fig. 3 shows the decay of luminescence inten—

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initial bound exciton ground state which is also responsible for the In 1 luminescence line. This interpretation demands for a 4.1 ineV ex— cited level of the In acceptor final state. The relative strength of the satellite to the principal BE line on order I to 85 (40) at a resolution of 0.1 may (1 meV) for the relatively low doped with an allowed leaving the silicon samples exciton suggests transition that we are concerned isolated, undisturbed impurity in an excited state of the same even parity I’B syninetry as the ground state. The postulated 4.1 meV excitation

sities after pulsed excitation. The decay is for both lines exponential over one to two orders of magnitude. From a least squares fit we find associated decay times r 2.7 nsec (In1) and r 2.8 nsec (In~,5).The bound exciton In1 decay time and was in discussed connection was already11reported a recent in paper by one with of a localized the authors phononless Auger effect as the life— time governing decay process for donor and accep— tor bound excitons in Si. Within a 5% experimental error, the In~and In~, 5decay times are identical. The constancy of the In~/In~,5 intensity ratio as a function of temperature, the equal and nearly linear intensity—to—pumping relations and the identical decay times of lines In1 and In1,5 are charac— teristically different from all other cases of donor or acceptor bound excitons and their asso— ciated low energy satellites. It was shown for instance that (a) the satellite lines vanish much ture for P and Li’ the andBEfor B8 bound exciton temperacommore rapidly than line at increased plexes, (b) the satellite intensities of the BE luminescence induced by these impurities follow power laws at increased excitation levels with exponents essentially larger than the exponent one for the BE lines1 , and (c) the satellites of the BE emission due to Li, P, As, B, Al, and Ga impurities decay with time constants which are about

energy is very small and exceeds by far the esti— mated energy of the lowest electronic excited r~ state which should be similar to the lowest p—like excited Fj state on order 140 meV~. A number of recent experiments in Si:In, how— ever, actually indicate a 4 — 5 meV excited In level associated with vibronic coupling of the hole and characteristic of deep impurities. This level 6 as from conductivity was independently deduced as thermal well from microwave measurements18 and was directly observed in phonon phonon attenuation’ spectroscopy39. The existence of low lying excited states for deep acceptor systems — e.g. also in GaAs:Mn — may be connected with the modification of the acceptor ground state by the dynamic Jahn Teller effect15. The four degenera~edacceptor ground state components are mixed by coupling to vibrational modes of symmetry E or T 2. The resulting the undisturbed groundofstate. The JTlike pa— vibronic ground acceptor state is again F8 symmetry 15. tential Schad and gives LaBmann’6 rise to investigated excited states the ultrasonic of F8 symmetry attenuation due to neutral In acceptors in Si as a function of temperature. They found a pronounced

we conclude that nature of half of13. theHence BE decay times (for thethefirst satellite) theless In bound exciton satellite line In or completely different from the BE satellites 1,5 is in those materials. Instead, the experimental prop— erties described above all conspire to suggest that the In 1,5 satellite line originates from the same

relaxation maximum which could only be understood

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25

708

INDIUM BOUND EXCITON LUMINESCENCE IN SILICON

in terms of an Orbach process, i.e. the relaxation via an excited vibronic level of the vibronic r

8

ground state distribution which results from local strain splitting. Fits of the experimental data by an Arrhenius—Orbach relaxation formula yielded 17. an excitation energy S = 4.2±0.3meV. Experimental De Combarieu LaBmann18 haveare also deduced an details and aand full discussion given by Schad excited In level from the reduction of the thermal conductivity at low temperatures as compared to pure Si, and obtained S in the energetic range of 4 — 5 meV. Further evidence for the excited In level by direct phonon spectroscopy was very recently given by Schenk et al.19 who observed phonon absorption peaks at till = A = 3.9 may, 4.1 meV, and 4.2 may in various slightly In doped Si samples using different superconducting tunneling junctions with high frequency quasimonochromatic phonons.

Vol. 27, No. 7

The detection of an excited In acceptor level at very similar energies S in three completely different experiments confirms the existence of the level. The energy 5 4.2±0.3maV coincides with the position of the excited state which we Consatellite The temperature, and dude fromline. the properties of the. excitation, In bound exciton time dependent behaviour of the satellite luminescence support the assignment of this line to a bound exciton decay leaving the In acceptor in the excited vibronic state. Acknowledgement — It is a pleasure to thank K. LaB— mann for numerous helpful discussions. We are, grateful to P.C. McGill and E.C. Lightowlers for providing preprints of their papers. The financial support of the Deutsche Forschungsgemeinschaft (SF8 67) is gratefully acknowledged.

References 1. An early review is given by R. SAU11IR, Proc.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19.

12th Internat. Conf. Phys. Semicond., ed. M. Pilkuhn, B.G. Teubner, Stuttgart 1974, p. 42 P.3. DEAN, D.C. HERBERT, D. BINBERG, and W.J. CHOYKE, Phys. Rev. Lett. 37, 1635 (1976) G. KIRCZENOW, Solid State Cairns. 21, 713 (1977); Can. 3. Phys. 55,1787 (1977) M.L.W. TEEWALT, Solid State Comm. 21, 937 (1977); Can. 3. Phys. 55, 1463 (1977) — R. SAUER and 3. WEBER, Phys. Rev. Lett. 36, 48 (1976); ibid. 39, 770 (1977) M.L.W. THEWALT, Phys. Rev. Lett. 38, 521 (1977) E.C. LIGHTOWLERS and M.0. HENRY, 3. Phys. C: Solid State Phys. 10, L 247 (1977) M.A. VOUK and E.C. LIGHTOWLERS, 3. Lum. 15, 357 (1977) S.A. LYON, D.L. SMITH, and T.C. MCGILL, Phys. Rev. B 17, 2620 (1978) The satellite line was by the present authors first observed 1975. Spectra were at this time supplied to lip. Schad and are contained in ref. 17. W. SCHMID, phys. stat. aol. (b) 84, 529 (1977) M.L.W. THEWALT, C. KIRCZENOW, R.R. PARSONS, and R. BABRIE, Can. 3. Phys. 54, 1728 (1976) N. SCHMID, thesis, Stuttgart 1977; partial publication in progress A. ONTON, P. FISHER, and A.K. RAMDAS, Phys. Rev. 163, 686 (1967) T.N. MORGAN, Phys. Rev. Lett. 24, 887 (1970) Hp. SCHAD and K. LASSMANN, Phys. Lett. 56A, 409 (1976); Proc. 2nd Inter— nat. Conf. on Phonon Scattering in Solids, ed. L.J. Wallis, V.W. Rampton, and A.F.G. Wyatt, Plenum Press, New York (1976), p. 337 Hp. SCHAD, thesis, Stuttgart 1976, unpublished A. DE COMBARIEU and K. LASSMANN, Proc. see ref. 16, p. 340 and private communication H. SCHENK, Diplomarbeit, Stuttgart 1977, unpublished; H. SCHENK, W. FORKEL, and W. EISENMENGER, Verhandlungen DPG 1, 328 (1978)