GOLD-DOPED SILICON

was

From trial fits to the resistivity data for n-type, gold-doped specimens, 8A chosen to be. None of these last four values enters significantly into the present calculations. From trial fits to these resistivity data, 8 was chosen to be 8. For each crystal, N was calculated from eq (4) with the assumption that N = 0 and 'd

N. =
Au

a

0; values are listed in table 2.

Although the calculated curves generally fit the experimental data, there are some deviations which appear to be systematic. One possible source of these deviations may be the discrepancy between the computed doping densities for low resistivity crystals (<1 cm) shown in the table and those obtained experimentally [9]. This discrepancy arises in part because of difficulties with the calculation of ionized impurity and mixed mobilities. Two other possible sources of the deviations, compensation by donor impurities unintentionally introduced during the gold diffusion and the previously observed discrepancy between total and electrically active gold [10], are also being considered. Errors in the activation analysis may also contribute to

the discrepancies.

Resistivity vs. Gold Density in n-Type Silicon The measured room temperature resistivity of phosphorus-doped silicon wafers diffused with gold is shown in figure 9. Before the addition of gold the initial resistivity, at room temperature was

0.3, 1.0, 5.3, 75, 380, or 2300 N.cm as indicated on the figure. The gold density ranged from about 1014 cm-3 to about 1017 cm-3. At large gold densities, the conductivity type converts from n to p. Specimens with positive Hall coefficients are plot

ted with solid symbols in the figure. For the lower resistivities a sharp increase in resistivity is observed when the gold density is between one and two times the density of the shallow donor in agreement with earlier observations [10].

Theoretical curves, based on the energy-level model described above, were calculated for each set of data. The theory predicts that the resistivity increases sharply when the gold and shallow donor densities are equal while the experimental results show the increase occurs when the gold density is up to twice the shallow donor density. Elsewhere, however, the theoretical and experimental plots have generally similar form. For the curves plotted in figure 9, E and were taken d as 1.067 eV and 1⁄2, respectively, values appropriate to phosphorus [8]. Other parameters were chosen as described above; values of Nå, calculated as above for each crystal, are listed in table 2. (W. R. Thurber and W. M. Bullis)

gd V

Plans: Hall effect and resistivity measurements as a function of temperature will be resumed to obtain additional data on the energy levels of gold in silicon. As mentioned above, the calculated shallow dopant densities for low resistivity silicon differ somewhat from accepted values due to difficulties with the calculation of ionized impurity and mixed mobilities. Ways of combining the ionized impurity and

GOLD-DOPED SILICON

Boron or Phosphorus Doping Densities of Silicon Crystals
Computed from Measured Room Temperature Resistivity

Table 2

lattice mobilities will be compared and the calculated values will be checked against experimental ones.

3.4. INFRARED METHODS

Objective: To study infrared methods for detecting and counting impurity and defect centers in semiconductors and, in particular, to evaluate the suitability of the infrared response technique for this purpose.

Progress: Infrared response measurements were completed on specimens of five germanium crystals representative of material currently available for use in lithiumdrifted germanium gamma-ray detector fabrication. The diameters of the crystals were approximately 5 cm. For two of the specimens, NBS-817 and NBS-818, IRR spectra of type 2, characteristic of detectors which exhibit hole trapping, were obtained (NBS Tech. Note 733, pp. 17-21). These results are in agreement with the exhibition of hole trapping by the detectors. The IRR spectra of diodes NBS-819 and NBS-820 were also of type 2, and that of NBS-821 was identified as type 5, characteristic of detectors which exhibit electron trapping. The detector characteristics of these latter three diodes have not yet been determined. (H. E. Dyson and A. H. Sher)

The IRR spectrum of NBS-14S, a lithium-drifted silicon detector fabricated at NBS, was obtained both before and after heating in an argon atmosphere to 800°C for

3.1.

approximately 3 h. The effect of heat treatment, shown in figure 10, is similar to that observed in germanium (NBS Tech. Note 598, pp. 14-15). The major features in the IRR spectrum before heat treatment (curve A) are absent in the spectrum after heat treatment (curve B). In the case of germanium, this behavior was in agreement with findings reported in the literature of a continuum of energy levels resulting from thermal damage rather than discrete levels. (Y. M. Liu, H. E. Dyson, and A. H. Sher) Plans: Study of IRR of germanium and silicon diodes will continue with emphasis on the identification of energy levels and the determination of their effects on detector performance and IRR response. Further IRR analysis on silicon devices not compensated with lithium will be resumed.

1. Tentative Method of Test for Interstitial Atomic Oxygen Content of Silicon by Infrared Absorption, ASTM Designation F121-70T, Annual Book of ASTM Standards, Part 8. Available as a separate reprint from American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103.

2.

Standard Method of Test for Crystallographic Perfection of Silicon by Preferential Etch Techniques, ASTM Designation F47-70, Annual Book of ASTM Standards, Part 8. Available as a separate reprint from American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103.

REFERENCES

3. Standard Method for Measuring Resistivity of Silicon Slices with a Collinear Four-Probe Array, ASTM Designation F84-72, Annual Book of ASTM Standards, Part Available as a separate reprint from American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103.

4.

5.

6.

7.

8.

9.

10.

3.2.

8.

Tentative Method of Test for Sheet Resistance of Silicon Epitaxial Layers using a Collinear Four-Probe Array, ASTM Designation F374-73T, Annual Book of ASTM Standards, Part 8. Available as a separate reprint from American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103.

Mazur, R. G., and Dickey, D. H., A Spreading Resistance Technique for Resistivity Measurements in Silicon, J. Electrochem. Soc. 113, 255-259 (1966).

Severin, P. J., Measurements of Resistivity of Silicon by the Spreading Resistance Method, Solid-State Electronics 14, 247-255 (1971).

Gupta, D. C., and Chun, J. Y., A Semiautomatic Spreading Resistance Probe, Rev. Sci. Instrum. 41, 176-179 (1970).

Adley, J. M., Poponiak, M. R., Schneider, C. P., Schumann, P. A., and Tong, A. H., The Design of a Probe for the Measurement of the Spreading Resistance of Semiconductors, Semiconductor Silicon, R. R. Haberecht and E. L. Kern, eds., pp. 721-735 (Electrochemical Society, Princeton, New Jersey, 1969).

Buehler, M. G., Peripheral and Diffused Layer Effects on Doping Profiles, IEEE Trans. Electron Devices ED-19, 1171-1178 (1972).

Grove, A. S., Physics and Technology of Semiconductor Devices, p. 312, (John Wiley & Sons, New York, 1967).

Generation-Recombination-Trapping Centers

1. Buehler, M. G., Impurity Centers in pn Junctions Determined from Shifts in the Thermally Stimulated Current and Capacitance Response with Heating Rates, SolidState Electronics 15, 69-79 (1972).

2.

Byczkowski, M., and Madigan, J. R., Minority Carrier Lifetime in p-n Junction Devices, J. Appl. Phys. 28, 878-881 (1957).

3.3. Gold-Doped Silicon

Blakemore, J. S., Semiconductor Statistics, pp. 118-120 (Pergamon Press, New York, 1962).

2. Collins, C. B., Carlson, R. O., and Gallagher, C. J., Properties of Gold-Doped Silicon, Phys. Rev. 105, 1168-1173 (1957).

3.

4.

5.

6.

Macfarlane, G. G., McLean, T. P., Quarrington, J. E., and Roberts, V., Fine Structure in the Absorption-Edge Spectrum of Si, Phys. Rev. 111, 1245-1254 (1958).

Barber, H. D., Effective Mass and Intrinsic Concentration in Silicon, Solid-State Electronics 10, 1039-1051 (1967).

Ludwig, G. W., and Watters, R. L., Drift and Conductivity Mobility in Silicon, Phys. Rev. 101, 1699-1701 (1956).

Brooks, H., Theory of the Electrical Properties of Germanium and Silicon, Advances in Electronics and Electron Physics, Vol. VII, L. Marton, ed., pp. 156160 (Academic Press, New York, 1955).

REFERENCES

7. Brückner, B., Electrical Properties of Gold-Doped Silicon, Phys. Status Solidi A 4, 685-692 (1971).

8. Conwell, E. J., Properties of Silicon and Germanium: II, Proc. IRE 46, 1281-1300 (1958).

9. Irvin, J. C., Resistivity of Bulk Silicon and of Diffused Layers in Silicon, Bell System Tech. J. 41, 387-410 (1962).

10. Bullis, W. M., and Strieter, F. J., Electrical Properties of n-Type Silicon Doped with Gold, J. Appl. Phys. 39, 314-318 (1968).