Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 4142–4146 Part 1, No. 7A, July 1999 c 1999 Publication Board, Japanese Journal of Applied Physics In Situ X-Ray Topographic Study of Sweeping of Impurity Ions in Quartz Crystals M. T. SEBASTIAN 1,2, ∗ , R. A. BECKER 2 and H. KLAPPER 3 1 Regional Research Laboratory, Trivandrum 695019 India 2 Institut fur Kristallographie, RWTH Aachen, Germany 3 Mineralogical Institut der Universitat Bonn, Poplesdorf Schloss, Bonn Germany (Received November 5, 1998; accepted for publication February 4, 1999) X-ray topographic studies were made on alpha quartz crystals with in situ electric field using graphite and silver electrodes at elevated temperatures in vacuum. A strong dark striation contrast appeared on the topographs near the anode side of the crystals when recorded with electric field along the c-axis. The effect is attributed to lattice deformation by space charge collection caused by the removal of the charge compensating impurity ions. The striation contrast disappeared as the temperature was raised above the stability range of the alpha phase. A strong contrast also appeared on the topographs near the cathode side of the crystals when silver electrodes were used. This contrast was due to the accumulation of the silver electrode material. The contrast at the anode end due to sweeping was predominant when the dc electric field was along the c-axis whereas the contrast near the cathode end was present irrespective of the direction of the electric field inside the crystals. Spotty contrast due to the evolution of fluid inclusion with increasing temperature has also been observed in some crystals. KEYWORDS: quartz, X-ray topography, sweeping, solid state electrolysis, electrodiffusion of impurity ions ∗ E-mail: mts@csrrltrd.ren.nic.in 4142 1. Introduction Among the oxides, quartz is a unique material. It is ex- tremely stable and found in nature as large crystals or grown in the laboratory with even higher purity. It’s most important property, piezoelectricity, in combination with excellent me- chanical behaviour allows quartz to be used in a variety of electronic devices. In most of the applications of quartz, im- perfections play an important role and crystals of very high quality are often required. For example the quality factor ( Q) of a quartz resonator directly depends on the point de- fects present in the crystal in addition to surface finish, mass loading, stresses and design overtone. Aluminium invariably gets substituted for silicon in both synthetic and natural quartz crystals. From an ionic point of view aluminium is a + 3 en- tity in a + 4 site. The trivalent aluminium can make only three of the bonds normally made by a silicon and hence a negative charge on the site due to the fourth non-bonding oxygen. This charge is compensated by an alkali ion (Li + , Na + ) or a proton (H + ) or holes trapped at the oxygen ions. The aluminium- hole centres are formed by the escape of an electron from the outer orbit of a non-bonding oxygen adjacent to the alu- minium. Sodium and lithium are important impurity charge compensators in synthetic crystals since Na 2 CO 3 and Li 2 CO 3 are often used as mineralisers for the hydrothermal growth of the crystals. Because of the coulombic attractive force of the interstitial ions with aluminium and because of their high mobility, these charge compensators are usually located adja- cent to the substitutional aluminium ions. The quartz struc- ture contains large channels of about 2.5 A running parallel to the c-axis (see Fig. 1). The impurity charge compensators normally lie in these structural channels adjacent to the sub- stitutional aluminium ions. 1, 2) Temperature variations, elec- tric field or irradiation causes the weakly bound alkali ions to move through the large structural channels. This leads to frequency changes in resonators made from such quartz crystals. 2, 3) The easy migration of impurity ions through the large c-axis tunnels has been exploited by several authors 4–7) to purify quartz crystals. Commercially the process is called Fig. 1. Projection of the quartz structure showing the c-axis structural channel. sweeping. Commercially air sweeping is employed to re- place the weakly bound alkalis present in as-grown crystals with hydrogen which is relatively strongly bound by means of O–H bonding. The sweeping is reported 8–14) to lower the production of etch channels significantly which is very important in the production of devices by photolithographic techniques and of very high frequency bulk wave oscillator crystals. Sweeping prior to device fabrication has been re- ported 2, 3) to improve the performance of resonators and in- creases the radiation hardness of quartz resonators used in precision oscillators. The variations in frequency of unswept crystals are traceable to impurities and defects. In the present paper we report a detailed in situ X-ray topographic investi- gation of vacuum sweeping of quartz crystals to study what happens or what damages are being produced in the crystals during the high temperature solid state electrolysis. 2. Experimental In the present investigation y-cut synthetic quartz crystal plates (Sawyer Research Products) of about 1 mm thickness were used. The crystals were polished, chemically etched to remove the surface defects and then mounted on a high tem- perature, high voltage vacuum camera described schemati- cally in Fig. 2. This camera is quite convenient for in situ