Breit-Wigner-Fano resonances in the photoconductivity of semiconductors: Experiment

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$ o l i d S t a t e Cmmmnlcations, Vol.47,No.3, pp.167-169, 1983. P r i n t e d in Great B r i t a i n .

0038-1098/83 $3.00 + .00 Pergamon Press Led.

BREIT-W]rGNER.FANO RESONANCES IN THE PHOTOCONDUCTIVITY OF SEMICONDUCTORS: EXPERIMENT R. Baron and M. If. Young Hughes Re~eareh Laboratories Malibu, California 90265 and T. C. McGill California Institute of Technology Pas~lena, California 01125 (Received 23 February 1983 by A. A. Mar,~dudin)

We report the observation of new structure in the photoconduetivity spectra of the single acreptorq (13, AI, Ca, and In) in Si. This s t r , c t u r e occurs at frequencies corresponding to an energy equal to the excitation energy of Ismnd P.~/2 states from the grouml state plus the energy of a near zone center optleal phonon. Igased on this obqervation tlw structure is nttrlhuted to 12relt-Wigner-F:tno reson:mees due to the mixing of the excited bound states of the ~ c e p f o r plus an optical phonon with the continuum of unbound states.

$i and attribute it to a B W F resonance involving the above-noted ground state mixing with the continuum.

We have observed previously unreported structure in the photoconductivity spectra of p-type dopants (B, A], Ga, and In) in Si. Dips are found in the photoconductivity spectra at frequencies corresponding to excitation energies from the ground state to a number of the P3/2 bound states plus a near zone center optical phonon. This structure is attributed to mixing between bound states consisting of an excited acceptor plus optical phonon and the continuum of unbound states resulting in a Breit-Wigner- Fano (BWF) resonance t. Further, for the case of B in Si, a dip in the photoconductivity spectrum is also found at 519 cm - t , the energy of a near zone center optical phonon. For this structure, the state that mixes with the continuum of unbound states is the acceptor ground state plus the optical phonon. This mixing can only occur if the optical phonon energy is greater than the acceptor binding energy, a condition which of the above dopants, only B satisfies. The theory of the photoconductivity and absorption in this ease is presented in the accompanying pape r2. Recently Watkins and Fowler3 have reported a single dip in the absorption spectra of p-type dopants {B, AI, and Ga) in Si. This dip occurs at an energy corresponding to the excitation of a hole from the IS3/2 to the 2P3/2 state plus a tone center optical phonon. They attributed this dip in the absorption to a B W F resonance involving the above-noted excited state mixing with the continuum. The photoconductivity spectra are found to show a substantially larger amount of structure, a difference which is accounted for in the theory 2. Watkins and Fowler also observe the dip at 519 cm - I for B in

The phot~onductivity spectra were measured using a Beckman I]Ro12 spectrometer modified to provide an external dispersed beam, which was focused on the detector sample mounted in a Janis Variotemp dewar. The spectra were corrected for the dewar window absorption. The AI, Ga, and In crystals were float-zone grown with 3.0 × 1016¢m - 3 , 3.0 X 101ecru - s and 1.4 × 10 l r e m - 3 of AI, Ga, and In, respectively. The B spectrum was obtained from a Czochralski grown sample doped with 1.4 X 1014B/c"m3 and 5.7 X 101Sln/cm 3. In the wavelength range where the B spectrum was measured, the In acceptors do not make a measurable contribution to the photoconductivity. All detector samples had an optical thickness of 0.1 cm, giving ,~L ---0.031, 2.6, 1.6, and 1.2 for the B, AI, Ga, and In doped detectors. A typical photoconductivity spectrum for indiumdoped Si is shown in Fig. 1. The fine structure associated with the excited states is shown clearly and i~ identified as "BWF resonance". The spectrum also illustrates the relationship of the excited state BWF resonance structure to the normal P3/2 and P1/2 excitation spectrum 4. The P3/2 excitation spectrum appears as absorptions against a background of photoconductive response from an acceptor level shallower than In (the indium x-level) which is present in the material 5. The P I / 2 excitation spectrum appears as peaks in the photoconductivity. These peaks occur for all three dopants under all measurement conditions of temperature 167

PHOTOCONDUCTIVlTY OF SEMICONDUCTORS: EXPERIMENT

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Fig. 1 . A typical photoconductivity spectrum for Si:In. The ordinate is the response per watt of incident flux. The fine structure discussed in this paper is labelled "BWF resonance'. and electric field, and indicate that a significant fraction of the holes excited to the P1/2 excited states make a transition to the Pa/2 valence band before returning to the ground state. For comparison the energies and relative strengths of these absorption spectra 4 are shown in Fig. 1. The photoconductivity spectra in the region of the fine structure for the various acceptors is shown in Fig. 2. The lines marked on the figure show the energy positions of the I S 3 / 2 " ' n P s / 2 excitations4 plus 519 crn - 1 , the energy of a near zone center optical phonon3,0. The dip at 519 c m - ~ associated with the ground state is clearly evident in the Si:B spectrum. In the figure the spectra are aligned so that the 2t>t peak (the transition from the ground state to the 2P1/2 state 4) is in the same position for each acceptor. This procedure will very closely line up the structure from the same excited states for the different acceptors. The similarity between the structure in all the spectra is quite striking. The primary difference between the structure is the distinct broadening of the dip due to the second excited state in Si:Ga. This broadening is due to the near degeneracy of the 3P3/2 state for Ga with the energy of a near zone center optical phonon4, 7. Much more structure is observed in the photocon~ ductivit.v spectra than in the absorption spectra 3. In the absorption spectra only the ISa/z--2P3/2 dip is observed clearly, while in the photoconductivity spectra a much larger number or excited states are observed. The dip at 519 em - 1 is strong in both the absorption and photoconductivity spectra. The temperature dependence of the structure for Skin is shown in Fig. 3. The data show that as the tempera. ture is increased the dips begin to disappear. The dip due to the excited state closest to the valence band disappears first followed at higher temperatures by the dips

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Fig. 2. Photoconductivity spectra or Si doped with B, AI, Ga, and In. The spectra have been shifted so that the peaks due to the transition to the 2P1/2 state are lined up. The spectra above each photoconductivity spectrum are the energies for transitions to various excited states of the acceptor 4 plus the energy of a zone center optical phonon, 519 cm - l 5. The Si:B spectrum also shows the dip at 519 cm - 1 and two strong dips marked L caused by lattice absorption. The peaks at 1100 cm - 1 are an artifact of the spectrometer.

due to deeper states. Similar results have been observed for AI and Ga. The temperature dependence is what is expected for a BWF mechanism, since as the temperature increases the excited states within kT of the valence band edge broaden and become effectively unbound. We would expect that this would lead to a decrease in the magnitude of each individual dip with dips due to more highly excited states showing a more rapid temperature dependence than dips due to less highly excited states. In summary, we have observed a previously unreported structure in the photoconductivity spectra of ptype dopants in Si. We have proposed a BWF resonance model similar to that proposed by Watkins and Fowler to explain their observations of structure in the absorption spectra. The proposed model explains the structure as a resonant interference in the photoconductive process between unbound continuum states and a state consisting of a bound excited states plus a phonon. In the theoretical work of Chang and McGill2, a detailed model of this process is developed. The theoretical line

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shales are in good agreement with the experimental line shapes presented here. Chang and McGill are also able to explain the observation that a large number of excited states are observed in the photoconductivity spectra as compared to only one excited state in the absorption spectra. The authors would like to acknowledge useful discussions with Y. C. Chang. One of us (TCM) gratetully acknowledges the support of the Office of Naval Research under Contract No. N00014-75-C-0423.

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References

1. G. Breit and E. P. Wigner, Phys. Rev. 49, 519 (1936); and U. Fano, Phys. Rev. 124, 1866 (1961). 2. Y.C. Chang and T. C. McGill, Solid State Comm. xxx, xxx (1983); the accompanying paper. 3. G.D. Watkins and W. B. Fowler, Pb,rs. Rev. B16, 4524 (19T7). 4. A. Onton, P. Fisher, and A. K. Ramdas, Phys.Rev. t63, 6 ~ (l~V). 5. R. Baron, M. H. Young, .I.K. Neeland, and O. J. Marsh, Appl. Phys. Lett. 30, 594 (1977).

6.

G. Dolling, in "Inelastic Scattering of Neutrons in Solids and Liquids" (LAEA, Vienna, 1963), Vol. II, p. 37.

7. H. R. Chransekhar, A. K. Ramdas, and S. Rodriquez, Proceedings of 13th International Conference on the Physics of Semiconductors, Rome, edited by F. G. Fumi (Tipografla Marves, Rome, 1976) p. 259.