A team of 海角社区 scientists including Boris Polevoi, Dmitrii Zherebtsov, Dmitrii Zhivulin, Danil Nenarokomov, Aleksandr Vorontsov, jointly with Dmitrii Godovskii from Moscow, have presented a technology of creating photoresist films based on cadmium- and lead sulphides, using the vacuum deposition method.
Photoresistance is a phenomenon when the conductivity of a material, weakly conducting electrical current in darkness, increases when exposed to light. The brighter the light, the lower the resistance. Modern engineers will see it as a potential use for informatics. If you cover, for instance, a silicon substrate with a thin film of a photoresist material, you will get a “chip” that could be controlled by a light ray. Or, a sensor reacting to the environment condition.
Two photoresist materials are known of: lead sulphide and cadmium sulphide. Each of these has its own “specialization”. Lead sulphide is one of the most sensitive materials in the visible and near infrared region of spectrum. Its “partner”, cadmium sulphide, works excellently in the blue-green region of spectrum.
What if we could combine these properties? To achieve that, a “solid solution” of the two sulphides is created. By regulating the shares of lead sulphide and cadmium sulphide in it, we could obtain a film composition “fine-tuned” for the required region of the light spectrum.
Chemical synthesis of such films is not convenient for electronics. That is why the group of the Ural scientists proposed to use vacuum methods: magnetron sputtering and electron-beam evaporation. These methods suit the most when creating modern microcircuits.
In the course of the experiment, the researchers created two types of films.
Using the method of DC magnetron sputtering, sitall (glass-ceramics) and oxidized-silicon substrates were coated with films containing 4% of cadmium. The film thickness varied from 50 to 500 nanometres. And in order to compare them, similar samples were coated with films containing 12% of cadmium using electron-beam evaporation.
Some samples after deposition were annealed at 250?°C during one hour in order to improve their properties. Next, nickel electrical contacts were applied to the films.
The best photoresistic effect was detected in the films containing 4% of cadmium obtained by magnetron sputtering. Applied to silicon substrates, these films demonstrated their response when exposed to light in over just 25 microseconds! Meanwhile, the resistance in darkness and the resistance in light differed five-fold. Variants with glass-ceramics substrate yielded weaker results.
Why did silicon turn out to be better than sitall? A silicon substrate is not just a mechanical support layer. There is a probability that a heterojunction is formed on the boundary between the film and silicon. This is a structure that facilitates the separation of photoelectric charges in itself. This opens up an opportunity of integrating photoresist layers right into silicon chips.
As to the films applied using electron-beam evaporation, they did not show any photoresistic effect. The researchers assume that the high-intensity electron beam overheated the material and destroyed its photosensitive structure. And though this method allowed to apply films of pure cadmium sulphide, their reaction to light was much slower: 125-500 microseconds instead of 25 microseconds.
The 25-microseconds reaction makes these films suitable to be used for such tasks as registering sparks, flame, and laser light.