STRESS RELAXATION IN SILICON AT LOW TEMPERATURES. A. Kohno, Z. Lu, Y. Soejima, A. Okazaki, Department of Physics, Kyushu University, Fukuoka 812-81, Japan

The effect of uniaxial external stresses on the lattice spacing of silicon (444) has been examined in a temperature range 25-305 K by means of high-angle double-crystal X-ray diffractometry (HADOX); the relative precision of the lattice spacing [[Delta]] d/d is +/-3x10^-6. It is found that the effect is not at all consistent with the theory of elasticity. The stress brings about a change in d that is 10³ times as large as that expected from the values of elastic compliance. The effect is anisotropic, and most remarkable when the stress is applied along [110]; the stress of 5.4 kPa, only 1/20 of atmospheric pressure, results in [[Delta]]d/d~4xl0^-5. The effect is just significant, that is, [[Delta]]d/d~lxl0^-5, when the stress is along [111]; it is intermediate for the stress along [112]. Previous X-ray diffraction experiments by HADOX and the Bond method showed that d of (444) is not a one-to-one function of temperature; a variety of d vs temperature curves depending on stress, thermal history etc. were observed. It was shown that the temperature where d shows a minimum, or the minimum value of d can be the index of the d vs T relation. This is more clearly observed in the present d vs T results with the stress as a parameter. This means that the anomaly so far observed in d vs T relation in silicon is directly related to the negative thermal expansion. It is also found that the response to the external stress is very slow; the relaxation time is of the order of one hour. The behaviour is similar to that characteristic to viscoelastics. This explains the anomalies observed in the ultrasonic attenuation.