Hetero-Augmentation of Semiconductor Materials

This invention relates to the augmentation or supplementation of semiconductor materials by hetero-atoms, and, more particularly to the doping or alloying of semiconductor materials by hetero-atoms.

In order to provide semiconductor materials with prescribed electrical and physical characteristics, it is common practice to add hetero-atoms to the material. For example, silicon can be used in the production of photovoltaic devices for the conversion of sunlight to electrical energy by the introduction of dopants, such as phosphorus or boron atoms to produce a layered photovoltaic cell. A typical photovoltaic cell of amorphous material includes an intrinsic layer of a semiconductor, such as silicon, which is sandwiched between respective doped layers to provide for the respective generation of holes and electrons in the presence of solar excitation. Thus, the generator of holes can be produced by the boron doping of silicon, while the generator of electrons can be produced by the phosphorus doping of silicon.

The most common technique for doping amorphous materials involves the introduction of the dopant in gaseous form into a reaction chamber with basic feed stock, which is generally a semiconductor hydride. While this procedure can produce satisfactory devices, it is necessary to carefully control the rate of deposition and the concentration of the dopant or hetero-atom.

Accordingly, it is an object of the invention to facilitate the production of semiconductor materials having prescribed characteristics.

A related object is to facilitate the production of doped and alloyed semiconductors. Still another related object is to facilitate the production of doped silicon and germanium as well as their alloys.

A further object of the invention is to achieve enhanced control over the rate at which the deposition of doped and alloyed semiconductors takes place.

A still further object of the invention is to achieve increased control over the concentration of the dopant or alloying atoms that are introduced as hetero-atoms into semiconductor materials.

In accomplishing the foregoing and related objects, the invention provides for the introduction of one or more hetero-atoms into a semiconductor by reacting the mixture of a gaseous host semiconductor precursor and a gaseous compound of the host and the hetero-atom.

The hetero-atom may be used to either dope or alloy the semiconductor depending upon the hetero-atom.

In accordance with one aspect of the invention, the reaction can take place pyrolytically, by electrical discharge, or by photo-chemical decomposition.

In accordance with another aspect of the invention, the reaction temperature is at or below the customary range for the material and the reaction pressure is nominal or below that ordinarily used for the material in question.

In accordance with yet another aspect of the invention, the compound of the host precursor and the hetero-atom is a silyl or germyl dopant or alloyant. Suitable dopants include phosphorus, arsenic, and nitrogen. Suitable alloyants include other semiconductors and nitrogen.

In accordance with another aspect of the invention, the precursor is a semiconductor hydride or a mixture of hydrides including those of silicon and germanium.

Other aspects of the invention will become apparent after considering several illustrative embodiments, taken in conjunction with the drawings in which

FIG. 1 is a flow chart for the practice of the invention; and

FIG. 2 is a schematic diagram of a reaction chamber and accessories for the practice of the invention.

With reference to the drawings, FIG. 1 sets forth a flow chart 100 for the practice of the invention.

The invention commences with a host semiconductor precursor in step 101, that is to be fed to a reaction chamber. Typical precursors include silicon and germanium hydrides, but other precursors may be employed as well.

In addition, for doping, it is necessary to provide, in step 102, a compound of the host precursor and a hetero-atom. In the case of semiconductor precursors, the compound is represented by the general formula (1):

where:

x is 0, 1, or 2 for dopant compounds;

A is a semiconductor such as silicon or germanium; and

B is a dopant such as phosphorus or arsenic.

In the final step 103 of the process the mixture of the precursor and the dopant compound is decomposed in a reaction chamber.

For alloying, the compound is represented in one instance by the general formula (2):

where:

x is 0, 1, or 2

A is a semiconductor; and

B' is a semiconductor or tin.

For alloying with nitrogen the formula for the compound becomes (3):

Where A, x and H are as for equation (1).

FIG. 2 shows an illustrative reaction chamber 200 which includes a substrate 201, for example of glass, ceramic, or metal, upon which the reaction products are formed. The gaseous precursor is applied at an inlet 210 through a valve 211 from a source 212. The doping compound is applied through an inlet 220 through a valve 221 from a source 222. Reaction takes place pyrolytically (by application of heat), electrically (by, for example an electrical discharge) or photo-chemically (by the application of spectral energy) from a suitable source (not shown). The byproducts of the reaction are exhausted from the reaction chamber at an outlet 230 which is maintained at a suitable pressure.

The invention is further illustrated by the following non-limiting examples:

The growth rate of boron-doped amorphous silicon is higher than that for phosphorus-doped silicon produced in conventional fashion. It is speculated that this results from the adverse effect of phosphine molecules on the reaction source. For example, in the case of CVD there is a "catalytic" poisoning of the film by phosphine. Similarly in the case of photo-chemical vapor deposition, the phosphine molecules are believed to absorb light to a greater extent than the predominant host silicon hydride.

The invention overcomes the foregoing difficulties by the employment of a phosphorus source compound where a silicon-phosphorus bond pre-exists. As a result the doping does not involve the decomposition of two chemically distinct compounds and the incorporation of dopant atoms into a host matrix.

Accordingly, the reaction chamber is used for the co-decomposition of a silicon hydride and a silicon hydride derivative which contains the desired dopant. Examples of such silicon hydride derivatives include silyl phosphines beginning with monosilyl phosphine (SiH.sub.3 PH.sub.2) and including disilyl phosphine (SiH.sub.3 PHSiH.sub.3) and trisilyl phosphine (SiH.sub.3 PSiH.sub.3 SiH.sub.3). Suitable control over the silicon-phosphorus ratio is obtained by proportioning the phosphine or phosphines in relation to the feed stock of silicon hydrides, inert gases and/or hydrogen.

The result is a phosphorus doped amorphous silicon hydride in which the reaction time was significantly reduced in comparison with the prior art technique of conventional phosphorus doping.

Example I is repeated with a germyl phosphine substituted for the silyl phosphine and germanium hydride substituted for silicon hydride. The result is a suitable phosphorus-doped germanium.

To prepare a wide bandgap material of amorphous silicon hydride with a nitrogen bond, a silicon hydride feed stock is reacted with a silyl amine as the nitrogen source.

The silyl amines have the formula (SiH.sub.3).sub.1+x NH.sub.2-x where x was 0, 1, or 2.

The result is a wide bandgap material that is produced more quickly than for the conventional reaction of silicon hydrides and ammonia.

In order to achieve arsenic doping of a semiconductor, the conventionally employed arsenic dopant of arsine (AsH).sub.3 is replaced by silyl arsine.

Reaction conditions are the same as for arsine, but the reaction takes place in a shorter time and produces a suitable arsinically doped semiconductor.

The foregoing examples are repeated using a mixed silicon germanium hydride as the feed stock material.

The results are similar to those achieved for single hydrides as feed stock material.

In order to produce a tin alloy with a semiconductor, a silyl tin hydride is reacted with a silicon hydride feed stock under conventional operating conditions.

The result is a silicon tin alloy which is formed in considerably less time than is required for conventional tin alloying.

Other aspects of the invention will be apparent to those of ordinary skill in the art who will appreciate that the foregoing disclosure is illustrative only.