RU2236033C2 - Method of preparing light-sensitive lead sulfide layers - Google Patents

Abstract

FIELD: light-sensitive materials.

SUBSTANCE: preparation of light-sensitive lead sulfide layers appropriate for use in semiconductor devises sensitive to infrared emission involves chemical precipitation light-sensitive layers on insulation substrate with specified crystallization sites during 60 to 150 min from mixed solution, which is prepared by consecutively adding 8-16% sodium sulfite solution, 6-10% sodium dithionite solution, and 20-25% lead salt solution to 20-25% alkali solution at specified proportion of components followed by ageing for 15 to 60 min.

EFFECT: enabled preparation of quick-responsible lead sulfite layers with high level of integral sensitivity and created controlled process of preparing light-sensitive lead sulfide layers with optimal parameters.

3 cl, 2 tbl, 8 ex

Description

The invention relates to a technology for the manufacture of semiconductor devices sensitive to infrared radiation (IR), and can be used in the manufacture of infrared radiation receivers based on chemical layers of lead sulfide.

In one of the first methods for producing photosensitive layers of lead sulfide by chemical deposition from a solution on an insulating substrate, a solution containing lead acetic acid, thiourea, and hydrazine hydrate was used - a substance that is active from the point of view of the redox process and at the same time creates an alkaline environment (see, e.g. Phys. Rev. Rep, 11,273, 1956).

The layers deposited from such a solution had a small thickness of 0.3-0.4 μm and a low integrated sensitivity in the wavelength region of more than 2.5 μm. In addition, these layers had insufficient speed, which is characterized by a photoconductivity time constant τ; τ at 20 ° С was 500 μs, and upon cooling, tens of ms. At the same time, the high toxicity of hydrazine hydrate hampered the improvement of this method and its application for the manufacture of infrared radiation detectors.

A known method for producing photosensitive layers of lead sulfide, based on the chemical deposition of the photosensitive layer on an insulating substrate from a solution containing lead salt, thiourea, alkali, ethyl alcohol, sodium sulfite and manganese nitrate.

Two baths were used to prepare the precipitated solution.

In the first bath, a solution was prepared containing 70 cm ^{3 of} ethyl alcohol, 100 cm ^{3 of a} 10% aqueous solution of lead salt Pb (NO ₃ ) ₂ and 70 cm ^{3 of a} 14% aqueous solution of alkali NaOH.

In the second bath, a solution was prepared containing 10 g of thiourea and 1 g of sodium sulfite per 100 ml of water, to which 0.5 ml of a 0.1 molar solution of manganese nitrate was added. Then, the resulting solutions were poured together into one bath, from which the photosensitive layer was deposited for 5-30 minutes (see US Pat. No. 3,595,690, cl. 117-211, publ. 1971).

This method, as the closest to the proposed essentially constituent operations, adopted as a prototype.

The main disadvantage of this method is the large constant photoconductivity τ of the resulting layers. At the same time, the value of the integrated sensitivity in the infrared region of the spectrum does not always satisfy the needs of modern infrared equipment.

Meanwhile, the current requirements for infrared devices dictate the need to create photodetectors based on lead sulfide, which have both a sufficiently high integrated sensitivity in a given region of the IR spectrum and a low constant photoconductivity, while operating at room temperature , and during cooling up to 77K.

Known to the authors of the methods, including the method adopted for the prototype, do not allow to obtain photosensitive layers based on lead sulfide that meet such requirements. The present invention solves the problem of producing photosensitive layers based on lead sulfide with an optimal level of parameters for given operating conditions that meets modern requirements for speed and integrated sensitivity.

To solve this problem, in the known method for producing photosensitive layers of lead sulfide by chemical deposition of photosensitive layers on an insulating substrate from a solution containing lead salt, thiourea, alkali, sodium sulfite, the deposition is carried out on a substrate with predetermined crystallization centers for 60-150 min from previously aged within 15-60 minutes after preparation of a mixture of aqueous solutions, which is obtained by sequentially adding to a 20-25% alkali solution an 8-16% solution of sodium sulfite, 6-10% p a solution of sodium dithionite, an 8-15% solution of thiourea and the addition of a 20-25% solution of lead salt at a weight ratio of the components of the mixture: solution of lead salt: solution of thiourea: alkali solution: sodium sulfite solution: sodium dithionite solution, respectively, 1: ( 0.3-0.6) :( 0.3-0.6); (0.3-0.6): (0.1-0.3).

To increase the integral sensitivity and, accordingly, the specific detectivity, the deposition is carried out by successively immersing the substrate in at least two baths containing a mixture of aqueous solutions of the same composition.

To stabilize the bath and, accordingly, increase the percentage of yield of suitable layers, the mixture of solutions additionally contains a 2-5% solution of sodium thiosulfate with a weight ratio of 1: (0.3-0.6): (0.3-0.6): ( 0.3-0.6): (0.1-0.3): (0.05-0.1).

The inventive method allows to implement a controlled process for producing chemical photosensitive layers of lead sulfide and optimize their parameters in accordance with operating conditions, i.e., by varying the deposition mode, solution composition and duration of the process, it is possible to obtain layers with increased sensitivity in a certain areas of the infrared spectrum, either with increased speed, or to obtain the optimal combination of the values of integral sensitivity and speed Wii.

High performance of the obtained layers was achieved, firstly, by the composition of the solution from which the deposition was carried out, and secondly, by using a substrate with predetermined crystallization centers. Crystallization centers can be created either mechanically, or by chemical or ion etching, or in any other suitable way.

The condition for the suitability of the substrate is the absence of microcracks and the uniform distribution of crystallization centers over the area of the substrate.

In the process of the studies conducted by the authors, it was found that the layers having the largest coarse crystallites (of the order of 1.2 microns) have the greatest speed. To form such crystallites at crystallization centers, the layers were deposited from a solution with relatively low supersaturation with lead sulfide. However, the integrated sensitivity of such layers was low. This is due to the fact that in the initial period of deposition, layers of relatively pure, pure lead sulfide are formed in connection with its extremely small solubility product (of the order of 10 ^-21 ), which have insignificant photosensitivity. The experiments conducted by the authors showed that high photosensitivity is achieved on layers having an impurity composition. Therefore, in the proposed method to increase photosensitivity, the substrate is immersed in the reaction bath 15-80 minutes after its preparation. Thereby, the pure non-photosensitive phase of lead sulfide is removed from the deposited layer, which is formed at the beginning of precipitation from a freshly prepared bath.

However, with this deposition mode, an undesirable shift of the spectral distribution of sensitivity towards short waves is observed, while the level of photosensitivity in the infrared region is insufficient. In order to obtain a photosensitivity value that meets modern requirements, the layer is built up from several bathtubs of the same composition. As a result, highly sensitive layers were obtained without loss of speed. It was found that the larger the number of baths used and the shorter the deposition time from each bath with the same solution composition, the narrower the spectral sensitivity range of the deposited layers.

Thus, the proposed method allows you to control the distribution of spectral sensitivity and the position of its maximum. The composition of the reaction bath was selected by numerous experiments, during which the percentages of the solutions included in it and their weight ratios that were optimal for the process were identified. Also, through experiments, the permissible duration of the deposition process and the exposure time of solutions in the deposition mode from several baths were determined.

Going beyond the stated ratios in terms of the composition of the reaction bath, as well as the duration of the deposition and the exposure time of the solutions before the deposition begins, leads either to a loss of layer parameters or to a deterioration in the quality and structure of the layer and its adhesion to the substrate.

Implementation examples

By the proposed method, prototypes of photosensitive layers of lead sulfide were made and their parameters studied. The deposition of layers was carried out at a temperature of 20-25 ° C on substrates of quartz KB. A predetermined uniform distribution of crystallization centers was created by mechanically matting the substrate using viscous resins.

The solutions were mixed in the following sequence: alkali solution, sodium sulfite solution, sodium dithionite solution, thiourea solution, after which a lead salt solution was gradually added over 10-12 seconds. This sequence of mixing the components of the bath helps to preserve the redox properties of dithionite and sodium sulfite, because in an alkaline environment, they do not decompose to the same extent as in water and in air. The mixture of solutions was transferred to crystallizers with substrates horizontally fixed in the holders. Layers of lead sulfide were obtained on the lower surface of the substrates.

The composition of the solutions, the mode and duration of deposition are shown in table 1.

The parameters of the obtained layers are presented in table.2.

Thus, the proposed method made it possible for the first time to obtain low inertia layers of lead sulfide with a high level of integral sensitivity. At the same time, for the first time, changing the deposition conditions, it became possible to control the photosensitivity spectrum.

Claims (3)

1. The method of obtaining photosensitive layers of lead sulfide by chemical deposition of photosensitive layers on an insulating substrate from a solution containing lead salt, thiourea, alkali, sodium sulfite, characterized in that the deposition is carried out on a substrate with predetermined crystallization centers for 60-150 min from previously aged within 15-60 minutes after the preparation of a mixture of aqueous solutions, which is obtained by sequentially adding to a 20-25% alkali solution an 8-16% solution of sodium sulfite, a 6-10% solution of dithio sodium nitrate, an 8-15% solution of thiourea and the addition of a 20-25% solution of lead salt at a weight ratio of the mixture components: solution of lead salt: solution of thiourea: alkali solution: sodium sulfite solution: sodium dithionite solution, respectively, 1: (0 , 3-0.6) :( 0.3-0.6) :( 0.3-0.6) :( 0.1-0.3). 2. The method according to claim 1, characterized in that the deposition is carried out by successive immersion of the substrate in at least two baths containing a mixture of aqueous solutions of the same composition. 3. The method according to claim 1, characterized in that the mixture of solutions additionally contains a 2-5% solution of sodium thiosulfate with a weight ratio of the mixture, respectively, 1: (0.3-0.6) :( 0.3-0.6 ) :( 0.3-0.6) :( 0.1-0.3) :( 0.05-0.1).