Abstract
I describe the vacancy-interstitial model ofDX centers, in which the substitutional donor atoms relax toward interstitial sites when they capture electrons. This appears to be the only known model ofDX centers which can account for the structure observed by Mooneyet al. in DLTS spectra of dilute Al x Ga1-x As alloys. By comparing its predictions with the DLTS and Hall data from Al x Ga1-x As alloys of low Al content one can establish values for many of the relevant parameters, such as the enthalpies and entropies (degeneracies) associated with sites surrounded by differing numbers of Al atoms. The data also show that the distribution of donor configurations among the four available neighboring interstitial sites does not attain thermal equilibrium among states of differing energies before being released, although transitions (through tunneling) among states of equal energy may be possible. The model is seen to be consistent with those treated theoretically by Chadi and Chang forDX and by Dabrowski and Scheffler for EL2 centers.
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References
See the recent review by P. M. Mooney, J. Appl. Phys.67, R1 (1990).
P. M. Mooney, T. N. Theis and S. L. Wright, in Defects in Semiconductors 15, ed. G. Ferenczi, Mater. Sc. Forum, Vols. 38–41 (Trans Tech Publications, Switzerland, 1989) p. 1109.
T. Baba, M. Mizuta, T. Fujisawa, J. Yoshino and H. Kukimoto, Jpn. J. Appl. Phys.28, L891 (1989).
T. N. Morgan, in Defects in Semiconductors 15, ed. G. Ferenczi, Mater. Sc. Forum, Vols. 38–41 (Trans Tech Publications, Switzerland, 1989) p. 1079.
After this paper was presented, the author recognized that an alternate model, the Exchanged-Site (X-S) model, in which the substitutional Si atom interchanges with a neighboring As, behaves similarly and is in even closer agreement with experiment. This model, which will be discussed in a sub-sequent paper, leads to the same equations and statistical interpretation presented here.
An upper limit to the energy difference expected between the Ga-Si and Al-Si bonds is given by the difference in bond energies for GaP and AIP crystals. As listed by W. A. Harrison, Electronic Structure, (W. H. Freeman, San Francisco, 1980), Table 7.3, this difference is 250 meV per bond. Second nearest-neighbor interactions are always much smaller, so that they could not account for the observed 100 meV differences.
Although one expects transitions to occur through a range of states near the barrier energy, they will be almost indistinguishable from transitions through a single “barrier state,” as assumed here. (See T. N. Theis and P. M. Mooney, MRS meeting, Boston, 1989.)
J. Dabrowski and M. Scheffler, Phys. Rev. Lett.60, 2183 (1988).
If the barrier state is the Jahn-Teller state, its energy will differ slightly from that needed to surmount the barrier and reach the conduction band.
In this paper I assume that the trap captures one electron to becomeDX°, the ‘positiveU’ case, althoughDX may be a ‘negativeU’ center and capture two electrons to becomeDX. Calculations based on the ‘negativeU’ model have been made and will be published separately. I have found that the fitting parameters—the energies and entropies—are approximately the same for both.
There is no physical justification for this choice. When, as in this case, capture is through a highly-excited electron state, a choice ofv = 0 or 1 could be justified. As the exact value chosen is unimportant, I have usedv = 2 to simplify the comparison with other work.
E. Calleja, A. Gomez, A. Criado and E. Munoz, in Defects in Semiconductors 15, ed. G. Ferenczi, Mater. Sc. Forum, Vols. 38–41 (Trans Tech Publications, Switzerland, 1989) p. 1115.
T. N. Theis, T. N. Morgan, B. D. Parker and S. L. Wright, Ibid, p. 1073.
T. N. Theis, private communication.
T. N. Theis, P. M. Mooney and S. L. Wright, Phys. Rev. Lett.60, 361 (1988).
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Morgan, T.N. Analysis of the vacancy-interstitial model ofDX centers. J. Electron. Mater. 20, 63–70 (1991). https://doi.org/10.1007/BF02651967
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DOI: https://doi.org/10.1007/BF02651967