Length Change of Electron-Irradiated Germanium

Abstract

The purpose of this Letter is to report length contractions in n-type germanium due to electron bombardment at liquid-helium temperature and to propose the interpretation that these length contractions are produced by an electronic effect which dominates over the expansions caused by the atomic effect. In contrast to n type, the length changes of a p-type sample irradiated under the same conditions are smaller by more than an order of magnitude.Simultaneous length and conductivity changes have been observed in degenerate rc-type germanium single crystals during bombardment by 4.5-MeV electrons at liquid-helium temperature and during subsequent isochronal annealing. The irradiated portion of the sample measured 5 mm long, 4 mm wide, and 0.7 mm thick. The linear-accelerator electron beam passed through the sample in the [001] direction, and the length changes were measured in the perpendicular [110] direction. Changes in length of the sample produced changes in the spacing of a system of parallel plate capacitors, and these capacitors were compared with a standard capacitor by means of an ac ratio bridge. All capacitors were of the three-terminal type and the method of measurement was similar in principle to that used by White. 1 The sample and supporting framework for the capacitors was shaped from one large single crystal of germanium to minimize extraneous effects. The sensitivity and long-term stability of the system were such that changes in length on the order of 0.3 angstrom were detectable. Annealing was of the isochronal type and was accomplished by means of a heater and double exchange gas-vacuum system. The sample was held at each annealing temperature for 10 minutes before recooling to liquidhelium temperature. All measurements were made at liquid-helium temperature.Degenerate w-type material was chosen for our initial experiments, rather than intrinsic material as used by other investigators, since the electrical measurements by Klontz and Mac-Kay 2 and the stored-energy measurements by Singh and MacKay 3 have shown that irradiation effects in w-type material are more than 100 times greater than in/>-type material. Figure 1 shows the length change in a degenerate rc-type sample as a function of the irradiation flux. From this plot &L/L = -6.2 x 10~2 4 per incident electron/cm 2 . Figure 2 shows the results for the same sample during a later run consisting of a relatively light bombardment followed by an isochronal anneal with points taken every 5 degrees. The general characteristics of the length and conductivity curves are quite similar. An annealing peak between 35 and 40°K is evident in both curves. A second sample of degenerate n -type material showed similar behavior.Irradiation of a degenerate />-type sample at liquid-helium temperature resulted in a barely detectable contraction. The length-change effect is at least an order of magnitude smaller in p type than in n type. The fact that length 609

Photostrictive materials

“…The sign of the photostriction is determined by the pressure susceptibility of the energy gap which is positive in germanium leading to lattice dilation. [61][62][63] The spectral response of the photostriction and photoconductivity was measured in Ref. 59 .…”

Thermal Conductivity of Electron-Irradiated Germanium

“…Electrical measurements, 11 ' 12 length change, 13 and stored-energy measurements 14 have given much larger production rates of defects in degenerate ^-type germanium irradiated at liquid-helium temperatures than in degenerate ^-type germanium. For alpha-particle irradiation, however, Gobeli did observe fairly large production rates for both degenerate ^-type and ^-type Ge near 4°K.…”

Recent applications of semiconductor techniques in the study of nuclear radiations

“…T o discuss the detailed processes of charge formation and collection, it is simplest to follow the treatment given by Northrop and Simpson (1962) for a homogeneous conductivity counter. The same principles of charge collection can equally well be applied to junction counters where the field is non-uniform.…”

“…This is particularly true if the charge loss occurs through trapping or recombination of predominantly only one type of carrier, as may arise if the primary particle enters the counter near one electrode. Northrop and Simpson (1962) have considered how the geometrical position of the primary ionization, relative to the electrodes, affects charge loss and have derived simplified expressions for the subsequent line broadening. Their treatment is not, however, accurate in the situation where X is comparable with d. Day et al (1966, see also Dearnaley and Northrop 1966, pp.…”

“…In semiconductors, these regions are more or less depleted of free carriers and are sometimes termed 'voids'. They can be seen by transmission electron microscopy (Parsons et al 1962, Bertolotti et al 1963, Hemment and Gunnersen 1966, and may contribute to appreciable length changes in the irradiated sample (Vook and Baluffi 1959, Vook 1962, North and Buschert 1964, Stevens 1966. Their effect on electrical parameters and the perturbation of collecting field € may extend well beyond (e.g.…”