Lead-Doped Silicon With Enhanced Semiconductor Properties

1. Field of the Invention

This invention relates to optoelectronic devices, especially solar cells, and is more particularly directed to such devices containing a new and improved silicon component having semiconductor properties, and methods for making the silicon.

2. Description of the Prior Art

When producing optoelectronic components, in particular solar cells consisting of silicon, one endeavours to use silicon which is as inexpensive as possible. It is known that the efficiency factor of the cells, the ratio of the overall irradiation energy per surface unit to the solar energy which is usable in photo-voltaic fashion per surface unit then becomes accordingly lower.

At present, an efficiency factor of 15-18% is expected in monocrystalline silicon a factor of 8-10% in poly-crystalline silicon (in extreme cases involving large mono-crystalline zones even up to 12%), and a factor of 1-3% in amorphous silicon.

An object of the present invention is to provide a method and means to substantially improve the efficiency factor, in particular in poly-crystalline silicon. The invention can also be advantageously used in amorphous and even mono-crystalline silicon and is not limited to poly-crystalline silicon.

With the foregoing and other objects in view, there is provided in accordance with the invention a process for the production of silicon having semiconductor properties with improved optoelectronic properties, which comprises adding an agent to the silicon which increases the carrier life time.

In accordance with the foregoing, there is provided an optoelectronic device containing a silicon component having semiconductor properties, said silicon having incorporated an agent which increases the carrier life time.

There is provided in accordance with the invention a method for increasing the efficiency of an optoelectronic device containing a silicon component having semiconductor properties, which comprises incorporating in silicon to be used as said silicon component, an agent which increases the carrier life time.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a method for manufacturing silicon with semiconductor properties, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The invention, however, together with additional objects and advantages thereof will be best understood from the following description.

The invention is based on the realization that the efficiency factor of optoelectronic semiconductor components runs parallel with the carrier life time of the material and that the carrier life time is reduced by powers of ten as a result of high recombination rates at interstitial positions of the silicon, at the surfaces and at the pn-junctions of the semiconductor body, and in poly-crystalline silicon even at the grain boundaries of the silicon.

The objective on which the invention is based therefore leads to the problem which has not been solved as yet, namely to reduce the recombination rate.

The process for the production of silicon having semiconductor properties therefore provides in accordance with the invention that an agent which increases the carrier life time is added to the extremely pure silicon. Preferably, this agent is added to the poly-crystalline silicon during its production. Expediently, the poly-crystalline silicon is obtained by means of thermal decomposition of a gaseous silicon compound, preferably silico-chloroform (SiHCl.sub.3) or silicon tetrachloride (SiCl.sub.4), which is mixed with carrier gas, such as hydrogen. The silicon is deposited onto a carrier body, preferably consisting of silicon, which is heated by direct passage of current.

Lead is used as the agent which increases the carrier life time. Therefore, during the deposition of the poly-silicon from the gas phase lead is added in order to increase the carrier life time. The lead is preferably added to the reaction gas in the form of lead iodide (PbI.sub.2), lead chloride (PbCl.sub.2), lead tetraethyl (Pb(C.sub.2 H.sub.5).sub.4) or lead tetramethyl (Pb(CH.sub.3).sub.4). The lead compound can be added in a separate gas stream. However, it is likewise possible to charge the reaction gas mixture with the lead compound used and to adjust the quantity of the lead to be added in a simple fashion by selecting the temperature of the lead compound, i.e., increasing or decreasing the temperature of the lead compound.

The atomic hydrogen which reduces the carrier life time can be increasingly found at interstitial positions, grain boundaries, surfaces and pn-junctions, and it is necessary to concentrate these hydrogen radicals to form innocuous molecular hydrogen. Lead causes the atomic hydrogen to react in the cooling phase of the silicon and thus to precipitate, preferably diffusing into the atmosphere.

Although the inserted lead remains in the silicon it has no detrimental effects. Lead itself has no effect which influences the p- and n-conductivity; consequently it possesses no dopant properties.

The quantity of added lead is 10.sup.-6 to 10.sup.-9, preferably 10.sup.-7 to 10.sup.-8 parts by weight of the silicon. However, limits with respect to excessive lead additions are to be expected where lamellae consisting of metallic lead, coherent layers of lead or small lead metal points form. In particular, it is necessary to avoid them on the inner surfaces of the poly-silicon.

Among the lead compounds which are suitable for common decomposition with the silicon compound, a substance, such as lead tetraethyl (Pb(C.sub.2 H.sub.5).sub.4) which is liquid at room temperature, has the advantage of a high vapor pressure. A high vapor pressure implies a simple insertion into silicon and thus a simpler outlay in terms of equipment.

Intake into the reaction gas mixture is possible without heating in the case of tetraethyl lead because it is a liquid. Tetramethyl lead (Pb(CH.sub.3).sub.4) can likewise be used without additional heating.

Solid bodies, such as lead iodide (PbI.sub.2) having a melting point at 412.degree. C. and lead chloride (PbCl.sub.2) having a melting point at 498.degree. C. are also suitable as a lead addition.

The lead can be supplied to the reaction gas in a subsidiary stream, for example through a separate gas stream. In terms of apparatus it is easier to accommodate lead in the main gas stream. The hydrogen as carrier gas is mixed with a silicon compound, for example silicon chloroform or silicon tetrachloride, and the mixture subsequently charged with lead iodide or another lead compound.

Lead iodide and lead chloride may be accommodated in containers in order to let the reaction gas circulate around them. Even the extent of the incorporation of lead in silicon can be controlled with the aid of the temperature to which the lead compound is brought. More lead can be incorporated when the temperatures are increased.

A reduction in the recombination rate in the silicon is also achieved in that an agent which increases the carrier life, preferably lead, is diffused into the extremely pure silicon. The lead can either be diffused in metallic form or from one of the compounds which are specified in detail in the above. However, lead halides are less suitable as lead compounds because in addition they display depletion phenomena.

In the diffusion process in accordance with the invention, lead can be diffused into a preferably poly-crystalline silicon rod as a whole in a sealed container. Likewise, a mono-crystalline or amorphous silicon rod can be divided into disks and lead diffused into the disk stack in an ampoule which is completely sealed.

To the silicon rod may be gaseous lead, which is either vaporized or originates from a gaseous compound, during the noncrucible zone melting. However, the evaporation rate of lead has to be considered here. In any event, lead incorporation of a magnitude of 10.sup.-8 parts by weight can be achieved during the zone melting.

The reaction container is heated to approximately 1250.degree. C. for the diffusion and the lead is diffused in the course of several hours, for example in the course of three days.

A dopant, preferably phosphorus, which determines the conductivity of the silicon can be diffused together with the lead diffusion. A lead addition within the limits of 10.sup.-6 to 10.sup.-9, preferably 10.sup.-7 to 10.sup.-8, parts by weight of silicon can be achieved by means of a process in accordance with the present invention.

In the so-called ampulla using lead, 0.5 mg. of metallic lead was used for 500 g of silicon disks. The hermetically sealed and evacuated ampoule was then heated to 1250.degree. C. and the lead was diffused for 24 hours. However, equally good results could also be obtained with the aid of 1 mg of lead acetate for 500 g of silicon disks, with the ampoule likewise heated to 1250.degree. C. for 24 hours.