Patent GB-1070621-A (GB)
Title: Improvements in or relating to photo-sensitive devices and methods of manufacturing such devices
IPC: H01L31/102
Filed: 1964-03-10
Abstract: 1,070,621. Photo-cells; television pick-up tubes. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. March 10, 1964 [March 12, 1963], No. 10052/64. Headings HID and H1K. A photo-conductive device has a semiconductor body which consists basically of a metal oxide and which contains an active region of intrinsic or substantially compensated extrinsic conductivity in contact with a thin region of distinctly P-type conductivity adjacent the area of negative current supply to the body. Lead oxide is the metal oxide used in the examples. Lead monoxide layers may be produced by vapour deposition in a low-pressure oxygen atmosphere but it is difficult to control departures from stoichiometry which give rise to the formation of P-type or N-type material. But if the oxide is deposited in an atmosphere containing both oxygen (acceptor) and water vapour (donor) and the relative pressures of these are correctly adjusted a layer of compensated material will be formed. By decreasing the partial pressure of water vapour as deposition continues it is possible to obtain an intrinsic layer bearing the desired P-type surface layer. If a P-type lead monoxide layer formed by deposition in an oxygen or oxygen/rare gas atmosphere is subsequently exposed to a suitable mixture of oxygen and water vapour, if necessary at an elevated temperature, a compensated layer may again be produced. A P- type surface layer may be produced on this by subjecting-it to a bombardment by oxygen ions generated by an electrical discharge in lowpressure oxygen. If the material to be bombarded is of N-type conductivity (perhaps due to the use of excess water vapour in a previous stage) an intense oxygen bombardment will add enough oxygen to produce compensation. Other donor impurities are hydrogen sulphide, hydrogen selenide, and hydrogen telluride. These may replace some or all of the water vapour in the above evaporation or aftertreatment processes. When one of these gases is used it is thought that it reacts with the metal oxide to form water (donor) and to also incorporate the group VI element which enhances the long-wave sensitivity of the layer. Since these three gases are more reactive than water vapour (reactivity increases in the order shown) treatment times and pressures may be reduced and lower temperatures used to obtain similar results. The temperature at which deposition is carried out affects the structure of a lead monoxide layer. If the substrate temperature is less than 60‹ C. a glassy layer is obtained which is fairly transparent and thus has relatively low sensitivity to visible radiation. In the range 60-190‹ C. a polycrystalline layer is formed, and above 190‹ C. a layer is formed containing large crystals which render the layer unsuitable for use in some applications (see below). The application with which the Specification is mainly concerned is the production of vidicontype camera tubes but the manufacture of photoconductive cells is also described. In a vidicon tube the lead monoxide is deposited on to a transparent signal electrode to which the positive supply voltage is connected in operation. The electrode may consist of tin oxide or perhaps gold. Lead, bismuth, and antimony are also suitable and will form N-type contact with the monoxide layer as will the tin oxide. The tube may be used with visible light or with X-rays. The intrinsic layer may be formed by any of various combinations of the treatments outlined above. The substrate temperature during deposition should be kept below 190‹ C. to avoid the formation of large crystals which give rise to visible structure and tendency to white spotting in the transmitted picture. If during the formation of the intrinsic region the various treatments are repeated alternately it is possible that the layer may contain very thin intermediate P and N layers within the intrinsic region but the layer is still considered to be substantially compensated. To reduce the dark current of the tube the surface of the intrinsic layer facing the electron gun is given a thin P-type layer either by a method outlined above or by adding acceptor impurities to the lead monoxide to be evaporated in the last stage of the deposition process. The use of thallous oxide is particularly described but silver, copper, silicon dioxide, or lead fluoride may be used instead. To prevent loss of oxygen from the target in operation as a result of bombardment by stray ions and consequent reduction or loss of P-type conductivity of the surface layer and increase in dark current, a denser non- porous protective layer may be provided on the surface of the target. This may consist of a glassy layer of the metal oxide itself and may be formed by depositing the last part of the layer at low substrate temperature-this is already the case when thallous oxide doping is used to form a P-type surface layer. A very thin layer of insulating material, such as silicon monoxide, may instead form a protective layer as may a non-porous layer of a different semi-conductor, for example vapour deposited layers of antimony trisulphide or selenium. If desired the surface of a protected or unprotected target may be provided with a thin vapour deposited silver layer having no appreciable transverse conductivity. The embodiments described with reference to Figs. 6 and 7 (not shown) are photoconductive cells. Each has a glass substrate bearing alternately placed electrodes. Those which are to be connected together to the positive side of the supply in operation consist of tin oxide or vapour deposited silver. The others are of vapour deposited nickel or platinum. In one embodiment the metal oxide is deposited in separate strips over each pair of adjacent electrodes, that part of each strip over the negative electrode being given distinctly P-type conductivity. In a further embodiment a continuous oxide layer is deposited over all the electrodes, the negative electrodes having first been provided with an oxide coating of P-type conductivity. The methods of depositing and treating the oxide layers are similar to the methods used in forming the vidicon targets. In depositing the oxide layers for photo-cells masks are used and one process is described in which a multiply apertured mask is moved continuously during deposition. To complete a photo-cell, a glass, ceramic, or metal (for example aluminium) cover is sealed to the substrate. The space between cover and substrate is evacuated or contains oxygen at very low pressure. The Specification describes in considerable detail the apparatus and methods used for producing all the devices considered.