Patent Application
20090283865
The present disclosure relates to a process for fabricating doped crystalline semiconductors. In particular, the present disclosure relates to a process that uses a separate solution to introduce dopants intermittently during the electrodeposition of semiconductors. The present disclosure makes it possible to obtain high quality doped crystalline semiconductors.
High quality crystalline compound semiconductors are traditionally deposited by high vacuum processes with gas precursors, such as by molecular beam epitaxy or chemical vapor deposition. Dopant is subsequently introduced into the semiconductors by ion implantation to form electron-rich (n-type) or electron-deficient (p-type) regions in transistors. During ion implantation, dopant ions are accelerated to tens or hundreds of kiloelectron volts before reaching the semiconductor surface. Both gas phase semiconductor deposition process and ion implantation are high vacuum, high cost processes.
With the intensified needs for sustainable energy around the world, photovoltaic solar cells that are based on semiconductor materials has become a promising path. Both single crystalline and polycrystalline semiconductors are applicable in solar cell structures. Therefore there are increasing interests and needs for low cost doped semiconductors. Low cost is critical for the solar cell production in order to be competitive with conventional fossil-based energy resources.
Electrochemical atomic layer deposition (EC-ALD) is a technique for electrodepositing nanofilms, and has been used most extensively to form compound semiconductors as mentioned in U.S. Pat. No. 5,320,736, “Method to electrochemically deposit compound semiconductors”, disclosure of which is incorporated herein by reference. U.S. Pat. No. 5,320,736 relates to a method for depositing epitaxial single crystalline compound semiconductors on top of crystalline metallic substrates. EC-ALD is the electrochemical analog of ALE and ALD, all methods based on the use of surface limited reactions to form deposits with atomic layer control. The advantages of these methodologies are that they can be used to control deposition at the atomic level. The method breaks the deposition process into a sequence of individually controllable steps, thus greatly improving the ability to optimize a process.
Crystalline films can be electrodeposited by using electrochemical atomic layer deposition (EC-ALD). However, electrodeposition has never been used to co-deposit dopants, such as P, B, Al, Sb, Zn, Cd, C, Si, Ge and As during the deposition of semiconductors, such as Si, Ge, III-V, and II-VI compounds. The main reason is because it is difficult when employing conventional electrodeposition techniques to reliably control the dopant concentration at the levels that are used in most integrated circuits.
This disclosure provides for a low power and low cost method to electrodeposit semiconductors and incorporate dopant into the semiconductors during the electrodeposition. It has been found according to the present disclosure that dopants can be co-deposited by absorption or electrodeposition and the dopant concentration can be reliably controlled.
In particular, the present disclosure relates to a process for fabricating doped crystalline semiconductors. One aspect of the present disclosure relates to a process which comprises sequentially electrodepositing by atomic layer electrodeposition at least one atomic monolayer or submonolayer of a first element of a first solution and optionally at least one atomic monolayer or submonolayer of a second element of a second solution on a substrate; and repeating the sequential electrodepositing by atomic layer electrodeposition until at least one film of a semiconductor material is formed on the substrate to provide a semiconductor of a desired thickness. Next, a dopant chemical is introduced by absorption or electrodeposition after a desired number of sequences of making a semiconductor film.
The deposition by atomic layer electrodeposition is repeated until a desired thickness of semiconductor film and introduction of dopant intermittently until the desired final thickness is achieved. The deposited film is then annealed to permit diffusion of the dopant atoms throughout the film to obtain a uniform dopant concentration.
Another aspect of the present disclosure relates to a process which comprises electrodepositing by bulk electrodeposition a desired thickness of a first semiconducting element, compound, or alloy of a first solution on the substrate. Next, a dopant chemical is introduced by using the flow deposition system after a desired thickness of making a semiconductor film.
The deposition by electrodeposition is repeated until a desired thickness of semiconductor film and introduction of dopant intermittently until the desired final thickness is achieved. The deposited film is then annealed to permit diffusion of the dopant atoms throughout the film to obtain a uniform dopant concentration.
The present disclosure is also concerned with a doped crystalline semiconductor obtained by the above disclosed processes.
Still other objects and advantages of the present disclosure will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described preferred embodiments, simply by way of illustration of the best mode contemplated. As will be realized the disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the disclosure. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.
In order to facilitate an understanding of the present disclosure, reference will be made to the figures where like numerals in different views refer to the same component.
More particularly,
A represents a component of or the material of the semiconductor and n+ being its valence. Examples of materials for A are Si, Ge, Group III and Group II elements.
When the semiconductor is a compound material such as a III-V or II-VI semiconductor or SiGe, the second component 54 is electrodeposited. In particular, a second element 54 represent by Bn+ of a second solution onto the layer of the first element 50. B being a component of the semiconductor and m+ being its valence. Examples of materials for B are Group V and Group VI elements, and Si and Ge.
The electodeposition for 50 and 54 in this exemplary method provides monolayers or submonolayers by EC-ALD method or under-potential method. The term “submonolayer” refers to a layer that does not extend over all of the underlying substrate.
The above steps are repeated (see
After the desired thickness of the semiconductor film is formed; a dopant chemical 56 represented as Dx+ is deposited by EC-ALD method or under-potential deposition. See
Typical concentrations of the dopant is about 0.1% to about 10% and more typically about 0.5% to about 5%.
The above steps are followed by repeating the deposition of a certain thickness of semiconductor film and introduction of dopant intermittently until the final desired thickness is achieved. Typical final layer thicknesses are about 10 nanometers to about 10 microns and more typically about 100 nanometers to about 1 micron.
Next the deposited film is annealed to improve the crystallinity of the semiconductor film and cause the dopant atoms to diffuse throughout the film to obtain uniform dopant concentration. See
Next the same semiconductor solutions are introduced as in the first step, and this sequence is repeated until the target thickness is built.
After this, the structure is annealed as discussed herein above.
The following non-limiting examples are provided to further illustrate the present disclosure.
The EC-ALD cycle used to deposit InAs involved sequential depositions of In and As. The cycle used for depositing InAs is as follows: the As solution was flushed into the cell for 2 s (40 mL/min), and held quiescent for 8 s, all at the potential chosen for As deposition. Blank solution was then flushed through the cell for 3 s, followed by filling the cell with the In solution for 2 s, and holding quiescent for 15 s for deposition. The cycle was completed by flushing with blank for 3 s. This cycle ideally results in the deposition of one monolayer of the compound. The deposition potentials used for making InAs on Au substrate include the deposition of As at −0.82 V and In at −0.78V (vs Ag/AgCl reference electrode). The XPS analysis, as shown in
Incorporation of pure P on a semiconductor or metallic surface is difficult because once a monolayer of P is deposited, the surface becomes self-limiting because of its insulating property. For low dopant concentration cases, a monolayer of P after a certain thickness is likely to be sufficient. If it is required to have a substantial amount of P, more than a couple of monolayer, a carrier metal is codeposited with P, such as Nip, and other conductive P alloys or compounds.
The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “having” or “including” and not in the exclusive sense of “consisting only of.” The terms “a”, “an” and “the” as used herein are understood to encompass the plural as well as the singular.
All publications, patents and patent applications cited in this specification are herein incorporated by reference, and for any and all purpose, as if each individual publication, patent or patent application were specifically and individually indicated to be incorporated by reference. In the case of inconsistencies, the present disclosure will prevail.
The foregoing description of the disclosure illustrates and describes the present disclosure. Additionally, the disclosure shows and describes only the preferred embodiments but, as mentioned above, it is to be understood that the disclosure is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art.
The embodiments described hereinabove are further intended to explain best modes known of practicing it and to enable others skilled in the art to utilize the disclosure in such, or other, embodiments and with the various modifications required by the particular applications or uses. Accordingly, the description is not intended to limit it to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.
