# Patent

## Title
Structure for creating ohmic contact in semiconductor devices and methods for manufacture

## Abstract

A semiconductor-to-metal interface with ohmic contact is provided. The interface includes a semiconductor material, a metal layer, and a silicon carbide layer disposed between the semiconductor material and the metal layer. The silicon carbide layer causes the formation of a semiconductor-to-metal interface with ohmic contact. Applications include forming a photovoltaic device with ohmic contact by disposing a layer of silicon carbide over the photovoltaic material before depositing a bottom electrode layer of metal to complete the bottom of a photovoltaic cell.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No. 61/722,693, entitled “Photovoltaic Cell and Methods of Manufacture,” filed on Nov. 5, 2012, U.S. Provisional Application No. 61/655,449, entitled “Structure For Creating Ohmic Contact In Semiconductor Devices And Methods for Manufacture,” filed on Jun. 4, 2012, the entireties of which are incorporated by reference as if fully set forth herein.
This application is related to copending U.S. application Ser. No. 13/844,428, “Photovoltaic Cell and Methods of Manufacture,” filed on even date herewith, the entirety of which is incorporated by reference as if fully set forth herein.


FIELD OF THE INVENTION
The present invention relates to creating ohmic contact in a metal-to-semiconductor interface, and in particular, structure and method of manufacture for achieving such ohmic contact.
BACKGROUND OF THE INVENTION
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.
Certain semiconductor devices, such as photovoltaic cells, are composed of a metal component, such as one or more electrodes, which is in contact with a semiconductor component. Typically a metal-to-semiconductor interface will form a high resistance contact interface such as Schottky barrier, which is an undesirable interface for applications such as a photovoltaic cell or other semiconductor devices.
One approach for reducing contact resistivity is to increase impurities at the contact interface by performing doping processes, such as ion implantation, heat treatment, or other diffusion methods, or by performing chemical etching. These processes are costly and normally require the use of toxic materials and expensive equipment.
BRIEF SUMMARY OF PREFERRED EMBODIMENTS OF THE INVENTION
It is an object of the preferred embodiments of the invention to provide a novel structure for creating an ohmic contact at a metal-to-semiconductor interface, and to provide techniques for manufacturing the structure that does not require uses of toxic additives and doping processes.



BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
FIG. 1 is a block diagram that illustrates a structure for creating an ohmic contact at a metal-to-semiconductor interface, according to embodiments of the invention.
FIG. 2 is a flow diagram that illustrates an example process for manufacturing a structure for creating an ohmic contact at a metal-to-semiconductor interface, according to embodiments of the invention.
FIG. 3 is a block diagram that illustrates the structure configured as a component of a photovoltaic cell, according to embodiments of the invention.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
In the following description numerous specific details such as specific parameters for performing physical vapor deposition methods have been set forth to provide a more thorough understanding of embodiments of the present invention. It will be appreciated however, by one skilled in the art, that embodiments of the invention may be practiced without such specific details or with different implementations for such details. Additionally some well known structures have not been shown in detail to avoid unnecessarily obscuring the present invention.
In accordance with preferred embodiments of the invention, FIG. 1 is a block diagram that illustrates a structure for creating an ohmic contact at a metal-to-semiconductor interface, according to one embodiment of the invention.
Structure 100 comprises semiconductor substrate 106, metal layer 102, and silicon carbide (SiC) buffer 104 between semiconductor substrate 106 and metal layer 102. Ohmic contact 108 is formed in structure 100 at the metal-to-semiconductor interface of semiconductor substrate 106 and metal layer 102 due to the presence of silicon carbide buffer 104.
In some embodiments, metal layer 102 is any metal whose work function is equal to or below that of aluminum (Al), including but not limited to aluminum (Al), gold (Au), copper (Cu), silver (Ag), indium (In), cadmium (Cd), thallium (Tl), tin (Sn), tungsten (W), platinum (Pt), gallium (Ga), zinc (Zn), titanium (Ti), and nickel (Ni), and metallic alloys.
In accordance with preferred embodiments of the invention, FIG. 2 is a flow diagram that illustrates process 200 for manufacturing structure 100. At step 202, semiconductor substrate 106 is cleaned. In some embodiments, semiconductor substrate 106 is a silicon wafer with photovoltaic layer included. Other examples of semiconductor substrate 106 include, but are not limited to silicon in other forms, such as germanium and other semiconductor of elements in group IV. Semiconductor substrate 106 is not limited to an element semiconductor as silicon, but include compound semiconductors as III-V compounds or II-VI compound semiconductor materials.
Semiconductor substrate 106 is cleaned to remove any natural oxide film formed on the surface. An example of a process of cleaning semiconductor substrate 106 includes, but is not limited to, dipping semiconductor substrate 106 in a solution of hydrofluoric acid, rinsing semiconductor substrate 106 in water, and air-drying semiconductor substrate 106. It is appreciated by one of skill in the art that step 202 includes other approaches for removing undesired substances at the surface of semiconductor substrate 106.
At step 204, in some embodiments, an amorphous silicon carbide (a-SiC) layer is deposited on semiconductor substrate 106 by physical vapor deposition (PVD), such as sputtering, to form silicon carbide buffer 104. In some embodiments, a target for performing PVD is beta silicon carbide (β-SiC). An example of a sputtering method used at step 204 includes RF sputtering. In a preferred embodiment of the invention, power of approximately RF 2.2 Watt/cm2, in Ar atmosphere, is used in RF sputtering at step 204, in which the semiconductor substrate 106 is heated to a temperature of 500 K or above. Other examples of sputtering methods used at step 204 include any sputtering method capable of depositing silicon carbide on semiconductor substrate 106.
At step 206, in some embodiments, metal layer 102 is deposited onto silicon carbide buffer 104 formed at step 204 by physical vapor deposition methods, such as sputtering, screen painting, or ink jet printing.
In some embodiments, steps 204 and 206 are dry sputtering processes, performed without using toxic materials, and without the need for pre- and/or post-process steps as would be necessary in normal wet process steps. Accordingly, process 200 provides the benefit of being low-cost and environmentally friendly.
Fabrication conditions for steps 204 and 206, according to some embodiments of the invention, are presented in Table 1.







TABLE 1







Ohmic contact fabrication conditions











Preferred


Parameter
Value Range
embodiment





SiC target
β-SiC target
β-SiC target


a-SiC layer thickness (nm)
  5 to 500
200


SiC sputtering atmosphere and pressure
Ar
Ar


(Pa)
0.1 to 103
1


Sputter RF power for a-SiC (W)
1~4
2.2


Optional semiconductor substrate
over 500K
600K


heating during sputtering


Metal for contact
Al, Au, Cu, Ag,
Al



In, Tl, Sn, W, Ga


Metal layer thickness (nm)
 50 to 1000
500


Metal sputtering atmosphere and
Ar
Ar


pressure (Pa)
0.1 to 103
1


RF power for metal deposition (W/cm2)
1~4
2.2









At step 208, an annealing process may optionally take place. In other embodiments, depending upon the choice of the metal, annealing process 208 is applied between steps 204 and 206.
In accordance with preferred embodiments of the invention, FIG. 3 is a block diagram that illustrates the structure configured as a component of photovoltaic cell 300. Photovoltaic cell 300 comprises photovoltaic layer 302, and semiconductor substrate 106. Photovoltaic layer 302 and semiconductor substrate 106 are disposed between top electrode 304 and bottom electrode 306. Bottom electrode 306 is an example of metal layer 102. Silicon carbide buffer 104 is disposed in between semiconductor substrate 106 and bottom electrode 306. In some embodiments, silicon carbide buffer 104 is deposited onto semiconductor substrate 106 in accordance with the method described with reference to FIG. 2. The metal-to-semiconductor interface between bottom electrode 306 and semiconductor substrate 106 results in ohmic contact 108 due to the presence of silicon carbide buffer 104 in between semiconductor substrate 106 and bottom electrode 306.
Other features, aspects and objects of the invention can be obtained from a review of the figures and the claims. It is to be understood that other embodiments of the invention can be developed and fall within the spirit and scope of the invention and claims.
The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Various additions, deletions and modifications are contemplated as being within its scope. The scope of the invention is, therefore, indicated by the appended claims rather than the foregoing description. Further, all changes which may fall within the meaning and range of equivalency of the claims and elements and features thereof are to be embraced within their scope.

## Claims

1. A semiconductor device with ohmic contact, comprising:
a semiconductor material:
metal layer; and
a silicon carbide layer disposed between the semiconductor material and the metal layer, which causes the metal-to-semiconductor interface located between the metal layer and the semiconductor material to be an ohmic contact.

2. The semiconductor device with ohmic contact of claim 1, wherein the semiconductor material is a photovoltaic material.

3. The semiconductor device with ohmic contact of claim 1, wherein the metal layer is a bottom electrode, and the semiconductor material is a photovoltaic material.

4. The semiconductor device with ohmic contact of claim 1, herein the silicon carbide layer is amorphous silicon carbide.

5. The semiconductor device with ohmic contact of claim 1, wherein the metal layer comprises Al.

6. The semiconductor device with ohmic contact of claim 1, wherein the metal layer comprises any one of Au, Cu, Ag, In, Tl, Sn, W and Ga.

7. The semiconductor device with ohmic contact of claim 1, wherein the silicon carbide layer is deposited onto the semiconductor material by a sputtering process.

8. A method of manufacturing a semiconductor device with ohmic contact, comprising the steps of:
providing a semiconductor material;
forming a silicon carbide layer over the semiconductor material;
forming a metal layer over the silicon carbide layer,
wherein the silicon carbide layer causes the metal-to-semiconductor interface located between the metal layer and the semiconductor material to be an ohmic contact.

9. The method of claim 8, wherein the semiconductor material is a photovoltaic material.

10. The method of claim 8, wherein the metal layer is a bottom electrode, and the semiconductor material is a photovoltaic material.

11. The method of claim 8, wherein the silicon carbide layer is amorphous silicon carbide.

12. The method of claim 8, wherein the metal layer comprises Al.

13. The method of claim 8, wherein the metal layer comprises any one of Au, Cu, Ag, In, Tl, Sn, W and Ga.

14. The method of claim 8, wherein the silicon carbide layer is deposited onto the semiconductor material by a sputtering process.