Effect of oxygen implantation and adsorption on the emission characteristics of single-crystalline molybdenum

Autor: V. P. Obrezumov, V. A. Koryukin, S. S. Gerashchenko, M. I. Guseva
Rok vydání: 1993
Předmět:
Zdroj: Atomic Energy. 75:885-888
ISSN: 1573-8205
1063-4258
Popis: Ion implantation can alter thermionic emission from metals [1]. Thus, the vacuum work function of thermionicemission transducers whose emitter consists of single-crystalline molybdenum, tungsten, and niobium [2, 3] can be additionally increased by oxygen implantation [4]. We investigated the emission and adsorption characteristics of hot cathodes with implanted additives with the help of a thermionic-emission electron microscope (TEM) [5]. Oxygen ions were implanted using a beam accelerator with accelerating voltages of 40, 50, and 80 kV and doses of 1016-10 TM cm -2 [6]. High-Temperature Emission Properties of Mo (111) and Mo (110) with Implanted Oyxgen. An Mo (III) single crystal was chosen because oxygen strongly influences its work function [7, 8]. The experimental sample consisted of an 8 mm in diameter and 2 mm high disk. Two openings were made in the lateral surface of the disk in order to determine the temperature with an 1~OP-66 pyrometer and a thermocouple. The elemental distribution in the Mo (111) surface layer after implantation of oxygen ions (Fig. 1) was determined by means of Auger analysis and indicates that the implanted oxygen lies at depths greater than 100 nm. Investigation of the emission characteristics of a sample with implanted oxygen before heating at 1500°C showed that the work function does not exceed 4 eV, i.e., it is lower than for Mo (111). After the sample is heated at 1500°C the work function increases rapidly up to 4.7-4.8 eV and remains at this level in a wide temperature range 1300-1700°C. In order to estimate the additional effect of adsorbed oxygen on the work function oxygen was introduced up to a partial pressure of Po2 = 6.7.10 -4 Pa into the working chamber of the TEM. This resulted in an additional increase of the work function by approximately 0.3 eV (Fig. 2). After oxygen was pumped out of the working chamber, the work function returned to its initial value. In order to determine the stability of systems with implanted oxygen, the samples were subjected to forced heat treatment up to 1800°C and the work function was measured at the same time. During forced heat treatments an appreciable growth of peaks corresponding to masses of 16, 17, and 18 and insignificant groWth of the peak for mass 32 occurred in the residual-gas spectrum. The work function gradually decreased as a result of heat treatment (see Fig. 2), the rate of decrease being all the higher (for the same dose of implanted oxygen), the lower the accelerating voltage and the higher the sample temperature at implantation. The spectral emissivity of the surface of the samples was determined in the experiments with the TEM (X = 0.65 g.m). It was established that implantation of oxygen ions in Mo (111) did not change the spectral emissivity. This indicates an inadequate amount of oxygen at the surface. A completely different picture was observed with adsorption of oxygen from a gas sphere with partial pressure 10 -3 Pa. The spectral emissivity increased from 0.38 to 0.41. Thus adsorbed oxygen at a pressure of 1 0 -3 Pa influences the change in the work function and the spectral emissivity much more strongly than does implanted oxygen (with a dose up to 10 TM cm-2). In the course of the investigations it was established that as the dose of implanted oxygen increases from 1.1016 up to 1.1017 cm -2 the maximum work function of Mo (111) increases from 4.5 to 4.7 eV, and does not further increase as the dose increases above this level. Figure 3 shows how the increase in the work f~anction is affected by the implanted
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