We analyse by phonon spectroscopy low lying phonon scattering states from defects that are introduced by the diffusion of Zn into thick Si wafers

Bracht, Hartmut

Groß, Peter

Lassmann, Kurt

Staiger, Joachim

Stolwijk, Nicolaas A.

OPUS Online Publikationen der Universität Stuttgart

Materials Science Forum Vols. 143-147 (1994) pp 675-680© (1994) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.143-147.675All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of thepublisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 129.69.141.12-12/10/09,13:36:26)Materials SCIence l'orum VOIS. 14;)-14 1 (lYY4J pp. O/J-O(JV © Trans Tech Publications. Switzerland PHONON SPECTROSCOPY OF DEFECTS CORRELATED WITH THE DIFFUSION OF ZN INTO SI J. STAIGER!, P. GROSSI, K. LASSMANN i , H. BRACHT2, AND N.A. STOLWIJK2 1 Universitat Stuttgart, 1. Physikalisches Insti t ut , Pfaffenwaldring 57, D-70550 Stuttgart 2Universitat Munster , Institut fur Metallforschung, Wilhelm-Klemm-StraBe 10, D-48149 MUIlster Key words: Si:Zn, double acceptor, phonon spectroscopy, diffusion ABSTRACT We analyse by phonon spectroscopy low lying phonon scattering states from defects that are introduced by the diffusion of Zn into thick Si wafers. INTRODUCTION The two f s-holes bound to a substitutional double acceptor in a diamantine semiconductor at low temperatures would form a sixfold degenerate ground state were it not for the hole-hole interaction that within Tel splits it into at most 3 states: f) , f 3 , and f5 [1] . The ordering of these states may be according to Hund's rule with f) (.1=0) highest or it may be inverted (i.e. the S-like f) lowest ) by a strong attractive central potential [2J . The (near) degeneracy of this ground state multiplet may add the complication of a .1ahn-Teller effect [3] which may increase the number of possible scattering resonances. The level separations of the ground state multiplets of double acceptors in Ge were found by opti cal methods to li e in the range of meV where phonon spectroscopy with superconduct-ing tunnel junctions is a direct spectral method with high sensitivity and resolution . Such experiments applied to Ge:Zn and Ge:Be [4] have essentially confirmed the optical results: A non-degenerate ground state and a rather broad (~ 1.4 me V) excited state at 2.4 me V for Ge:Zn and for Ge:Be a triplet with zero stress separations below .3 meV. Including also the optical data for the Hg and Mg double acceptors in Ge a simple relationship between binding energy and ground state splitting is not obvious. The reason may be a delicate interplay of the concurrent mechanisms enumerated above. In Si t he situation is even more complicated by the tendency of the corresponding elements to associate with other defects or to reside on interstitial sites [5 ,6, 7J . Recent Hall and DLTS-measurements [8,5] have confirmed older work in that Zn may prevail as a substitutional double acceptor in Si under appropriate conditions. From IR-absorption due to transitions between ground and excited states [9,1OJ it was concluded in addition that the double acceptor ground state is a multiplet of at least three states with separations in the meV range and additional splittings under uni axial stress. However, position and total number of these states are not unequivocal from these optical data. Therefore, measurements involving only transitions within the ground state multiplet are of interest. 676 Defects in Semiconductors 17EXPERIMENTAL In this paper we analyse by phonon spectroscopy low lying phonon scattering states that are introduced by the diffusion of Zn into thick Si wafers [11]. Zn was diffused from an elemen-tal source into originally n-type Si ([P] = 5.1012 cm -3 and, for partial compensation, [P] = 1.1.1015 cm-3 ) by annealing at about 1000°C. The predominant configuration of Zn in Si is the substitutional double acceptor Zns. Partial compensation by a deep donor (possibly Zni) is found right after the diffusion and eliminated after an additional heat treatment at 600°C for 15 minutes. Concentration-depth profiles of Zns were determined after this 600°C anneal on simultaneously diffused high-ohmic samples of same thickness using the spreading-resistance technique [11 , 12] . Some of the non-compensated, annealed samples with higher concentration were measured by IR-absorptioll and predominant Zns was found [13] with a ground state multiplet with separations as reported in [18]. At the cryogenic temperatures of our experiments the acceptors compensated by the phospho-rous or the diffusion-induced donors could be neutralized by white light. Uniaxial stress up to 170 MPa was applied in <100>, <110>, or <111> directions along the length of the typically 2 x5 X 1.5 mm3 samples for the investigation of symmetries from stress-induced splittings and shifts. Phonons were generated and detected by 1 mm 2 snperconducting tunnel junctions made from Al or Sn evaporated onto opposite side fa.ces of the samples. The spectral range of phonon energies is given by the properties of the Al and Sn junctions as phonon generators and detectors: For the Sn generator, Al detector configuration (short cut [Sn---+AI]) it extends from ~0.3 meV (AI detector threshold) to 1.2 meV (limit of reabsorption for phonons of energies larger than the Sn generator gap). For the Al generator, Sn detector configuration ([ AI---+Sn]) it extends above 1.2 me V, the Sn detector threshold . Rea.bsorption within the weak coupling superconductor Al is small enough to allow quasimonochromatic phonon emission up to at least 14 meV [14]. RESULTS phonon frequency -[GHz] 0 100 200 300 400 500 600 700 800 900 ~ #8226·27 '2 [Zu) = lxlOls/cmJ = ~ without stress '" Al ~ Fig. 1 Phonon scattering spectra beyond 1.2 meV ([Al---+Sn] con-i O. figuration) for a sample in the = 011 1 as diffused state and then after 'iii '" 600°C annealing to remove a deep .s IJ compensating donor. Concentra-~ ~ tion of Zn~: 1.1015 cm-3 . Oi indi-"0 cates the resonance of interstitial 0 2 4 oxygen present in the sample at phonon energy - [meVl concentrations below 1016 cm-3 . Depending on the sample conditions and parameters at zero stress a number of lines up to about 3.4 meV was observed of which the most distinct are labeled Al through Ag . Some of Materials Science Forum Vols. 143-147 677".UtOi IUic eo 0 these are visible in Fig. 1 beyond the Sll detector threshold at 1.2 meV for a sample in the as diffused state and also after annealing. (Oi is due to a resonance of interstitial oxygen [15].) Since the annealing removes the partially compensating deep donor and thus increases the concentration of Zn~ we attribute A). and A4 to Zn~. At and As vary in depth independently from each other and also from A). and A4 under varying experimental conditions and thus should belong to two other defects, possibly to the complexes Zn(X1) and Zn(X2) of [16J. Only if As is weak and narrow a line A6 can be distinguished as shown in Fig. 2. From the correlation with sample conditions this line seems also to be associated with Zn~. phonon frequency - [GHzl 100 200 300 400 500 600 700 800 900 Si:Zn ~ annealed '§ without stress .Q different [Zn] I. ~ a: 4 .• 8xlOI4/cm3 b: 4 •• 8xlOIJ/cm3 c: 4 •. 8xlOI4/cmJ o 2 phonon energy - [meV] 4 Fig. 2 Phonon scattering spectra for three non-compensated, an-nealed samples with differing con-centrations of Zn~ showing addi-tional lines A6 and A3. A3 seems to be correlated with AI. Line As appears after neutralizing donors and acceptors in an annealed P-compensated sample by bandgap radiation (Fig. :3). This line is also foulld in non-compensated, annealed material without illumination and therefore also attributed to Zn? o #S236a phonon frequency - [GHz] 100 200 300 concentration fCIII3 [Zn] = 8xlO l4 [PI = bl0ls after illumination before illumination S.·G. •. ~-'M'-~ AI·Det 0.5 1 phonon energy - [meV] 1.5 Fig. :3 Example for spectroscopy in the [Sn-+AIJ-configuration): Observation of a line As be-yond the AI-detector threshold (at ~O.3 meV) after illumina-tion of a partially P-compensated sample, i.e. after neutralization of P and Zns. In the [Al-+SnJ con-figuration a line AT at 1.86 meV is visible in this P-compensated material only before illumination. It may be correlated with a Zn-P complex. The changes in the spectra induced by bandgap illumination in partially compensated mate-rial are stable after switch off at the working temperature of ~1 K. There is a characteristic difference between compensation by phosphorous and by the diffusion-induced donor: in the 678 Defects in Semiconductors 17first case the changes are annihilated at a tempera.t ure below 77 K as expected for thermal excitation of the 44 meV donor P and in the second case at a temperature above 77 K and below 200 K, indicating a cleeper clonor. Neutralizing illumination cleepens Al i.e. it should belong to a defect with acceptor character. The zero stress positions of the lines we attribute to Zn~ are given below (in meV) and compared with the optical data from [9,18]: This work: .3 (As) [9]: [1 8]: 1.92 (A2 ) 1.85 2.1 2.85 3.16 3.1 3.4 4.0 It is only for A2 that all three data sets coincide; for the rest they are more or less at variance wi t h each other. The energy of As may be too small to be detectable optically. On the other hand , the selection rul es for phonon t ransitions may leave levels undetected by phonon spec-troscopy. E.g., transitions starting from the supposed thermally occupied, final state belonging to As have not been found , in contrast to the case of the first and second excited rotational states of interstitial oxygen in Ge [17]. In any case: it seems clear from the three data sets that there are more than three states belonging to the Zn~ ground st ate, i.e. more than would be expected if only Coulomb interaction between t.he two holes would be effective. In addition: the stress dependence of the level shifts a.nd spl ittings should be distinctly nonlinear with such large level separations [1] . Instead, we find tha.t a ll lines shift or spli t linearly under uniaxial stress in the three main direct ions as shown in Fig. 4 for different condit ions in the < 100> and < 111> directions. Also in previous optical measurements [18] li near shifts and splittings have been found for stress in the < III > and < 100> directions but a nonlinear dependence for stress in < 110> . We do not find a large difference for the deformation potential constants in the <100> and <111> directions: A2 shifts (splits) with 29 meV/GPa in <100>, 23 meV/GPain <111> and in < 110>, whereas Kaufmann et al. [18] find for the ground state splitting 12 meV /GPa in < 111 > and 78 meV /GPa in <100> . 3.5 3.5 <;p<> Si:Zn o A4 3 Si:Zn p II [111] not annealed J:Al D • A2a ;; 2.5 P II [100] 9''' Il AZb a.l • Az 9''' § .. • As 2 2 .. tJ' ,0 t • As t 1.5 'I-~. .. ..0 ,.,; II A6 1.5 '\(> Cl"~ 9,'0 o As .. .. o A8 .. .. .. , A.cf ~., o A9 1 \,0. o.(tJ"b 0.J..... • o A9 'I'l,'" ,r )jt.ll('"" • Au 0.5 0.5 o' .... t'6"'~, ~~ • An 00 00 • A 1500 14 stress - [bar 1 Fig. 4 Stress dependence of various lines in the <100> and <11 1> directions. In this special < 111 >-sample Al was rather weak. For most lines the widths are of the order .1 meV and compare well with those observed for single and double acceptors in Ge and single acceptors in Si and thus should be determined by Materials Science Forum Vols. 143-147 679h'UtOs JUJU COlO: eu lifetime broadening which is also consistent with the ohserved deformation potential constant. The stress coupl ing constant of Al in <100> is only 2.2 meV fGPa, and correspondingly this line is relatively narrow . This suggests that Al may not be due to an electronic transition. Zn is apparently an effective recombination center in Si: illumination of the samples, though producing the above mentioned changes in the depth of the various absorption lines , did not result in significant changes of the resistivity as compared to analogous situations for single acceptors in Si [19]. This may be the reason that we could not observe a phonoconductivity signal corresponding to Zn+ which could have produced by the illumination. We are grateful to A. 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Phonon spectroscopy of defects correlated with the diffusion of Zn into Si

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