Patent GB-910449-A (GB)
Title: Improvements in or relating to semi-conductor devices
IPC: C30B13/00
Filed: 1959-05-08
Abstract: 910,449. Semi-conductor devices. PHILIPS ELECTRICAL INDUSTRIES Ltd. May 8, 1959 [May 13, 1958], No. 15884/59. Drawings to Specification. Class 37. In a method of manufacturing a semi-conductor device having a body of CdTe which is converted, at least in part, into high ohmic CdTe, the body or the relevant part thereof is provided with a predominant concentration of donor impurities up to a content of at least 10<SP>15</SP>/Cm<SP>3</SP> and heated to a temperature between 500‹ C. and the maximum melting temperature of CdTe in a partial Cd pressure which is lower than the Cd pressure of stoichiometric CdTe at the treatment temperature but higher than the Cd pressure of solid CdTe which is in equilibrium with its liquid at such temperature, for a time sufficient to bring the body or the part to a specific resistance of at least 10<SP>5</SP> ohm-cms., preferably higher than 10<SP>6</SP> ohmcms. The explanation is advanced that, due to the predominance of donor impurities, a plurality of deep-lying energy levels are formed around the centre of the prohibited energy zone, about equal in number to the incorporated donor levels, and that, during cooling after the tempering treatment, the CdTe becomes high ohmic due to the high activation energy of these deep-lying levels. Suitable donor impurities mentioned are In, Ga, Cl, I, Br; preferably, non-volatile impurities are introduced into the melt, while volatile impurities are diffused into the solid CdTe. The CdTe body may be poly- or mono-crystalline or in the pulverulent state, and, when partial conversion is required, a body uniformly doped may be tempered for a time so short that only a superficial conversion occurs, or the donors may be provided locally; alternatively, the whole body may be converted, the portions not required to be high ohmic being subsequently given a desired conductivity by conventional techniques. Preferred donor concentrations are 10<SP>16</SP> to 10<SP>18</SP>/Cm<SP>8</SP>, especially 101<SP>7</SP> to 5 X 10<SP>17</SP>/Cm<SP>3</SP>. Temperatures of from 700‹ to 1000‹ C. are preferred. In one example, a number of monocrystalline rods obtained from CdTe uniformly doped with In to concentrations of 1.3 x 10<SP>17</SP>/Cm<SP>3</SP>, 2 Î 10<SP>17</SP>/ Cm<SP>3</SP> and 5 x 10<SP>17</SP>/Cm<SP>3</SP> are converted to specific resistances of 10<SP>7</SP> to 10<SP>8</SP> ohm-cms. by being heated in an atmosphere of Cd and Te, the relative pressures of which are adjustable. A temperature-pressure diagram (not reproduced) shows the partial Cd pressure as a function of the treatment temperature, and a table in the Specification shows the treatment duration; the latter is shortened to produce high ohmic superficial layers. In other examples, Ga replaces In as the donor impurity; intrinsic CdTe is heated in an atmosphere containing a volatile donor (Br) and a partial Cd pressure simultaneously; and an In-doped rod is pulverized, pressed into a pill, and sintered at 1000‹ C., in a flow of N 2  which has been led over heated CdTe. Applications mentioned are crystal diodes, transistors and particularly photo-sensitive devices, e.g. photo-diodes, infrared telescopes, image intensifiers, camera tubes, X-ray dosimeters &c.