# Patent

## Title
Carbon nanotube devices with unzipped low-resistance contacts

## Abstract

A method of creating a semiconductor device is disclosed. An end of a carbon nanotube is unzipped to provide a substantially flat surface. A contact of the semiconductor device is formed. The substantially flat surface of the carbon nanotube is coupled to the contact to create the semiconductor device. An energy gap in the unzipped end of the carbon nanotube may be less than an energy gap in a region of the carbon nanotube outside of the unzipped end region.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. Non-Provisional application Ser. No. 13/664,008, entitled “CARBON NANOTUBE DEVICES WITH UNZIPPED LOW RESISTANCE CONTACTS,” filed on Oct. 30, 2012, which is incorporated herein by reference in its entirety.


BACKGROUND
The present invention relates to carbon nanotube field effect transistors (FETs), and more specifically, to improving electrical coupling between contact metals and carbon nanotubes in carbon nanotube field effect transistors.
Carbon nanotube field-effect transistors generally include a carbon nanotube that spans a gap between a source contact and a drain contact and serves as the channel of the transistor, the conductance of which is modulated by a gate separated from the nanotube channel by a dielectric material. There exists a contact resistance at the interfaces between the carbon nanotube and the source/drain contacts due, in part, to difficulties in coupling the cylindrical surface of the carbon nanotube to the contacts. High contact resistance at the interface between the carbon nanotube and either of the contacts will reduce current injection into the nanotube channel, thereby decreasing the performance of the transistor. This problem of contact resistance is exacerbated at technologically relevant nanotube diameters that are less than about 2 nanometers (nm) where a Schottky barrier presents itself at the contact metal/nanotube interface. The band gap of the carbon nanotube increases inversely with respect to diameter and a sufficiently large bandgap ( ̃0.6 eV) is necessary to attain a suitable on-/off-current ratio for digital applications. However, decreasing the diameter to achieve this bandgap leads to larger Schottky barriers and weaker coupling between the contact metal and the carbon nanotube thus increasing the contact resistance.
SUMMARY
According to one embodiment of the present invention, a method creating a semiconductor device includes: unzipping an end of a carbon nanotube to provide a substantially flat surface; forming a contact of the semiconductor device; and coupling the planar surface of the carbon nanotube to the contact to create the semiconductor device.
According to another embodiment of the present invention, a method of creating a carbon nanotube field-effect transistor includes: altering a physical structure of a segment of the carbon nanotube to reduce an energy gap in the altered segment; and coupling the altered segment to a contact to create the carbon nanotube field-effect transistor.
According to another embodiment of the present invention, a method of reducing a contact resistance in a carbon nanotube transistor includes: unzipping a segment of a carbon nanotube; forming a planar surface from the unzipped segment of the carbon nanotube; and coupling the planar surface to a contact of the carbon nanotube transistor.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
FIGS. 1-6 illustrate an exemplary manufacturing process for creating an exemplary carbon nanotube field effect transistor in one embodiment of the present invention, in which:
FIG. 1 illustrates a starting substrate structure including a co-planar local bottom gate formed therein, a dielectric layer formed over the bottom gate, and a carbon nanotube deposited on the dielectric layer and aligned to the local bottom gate;
FIG. 2 illustrates a barrier material and a photoresist layer formed over the structure of FIG. 1;
FIG. 3 illustrates windows formed in the barrier material and a photoresist layer of FIG. 2 that expose ends of the carbon nanotube.
FIG. 4 illustrates unzipped ends of the carbon nanotube, which are formed by chemical treatment of the nanotube in the exposed window areas of FIG. 3;
FIG. 5 illustrates resist layer having windows for formation of source/drain contacts on the unzipped ends of FIG. 4;
FIG. 6 illustrates contact metal deposited onto the unrolled ends of the carbon nanotube;
FIG. 7 shows an exemplary contact formed using the exemplary manufacturing processes disclosed herein; and
FIG. 8 shows an exemplary band gap diagram of a carbon nanotube that has two ends that have been unzipped and to lie flat as disclosed herein.



DETAILED DESCRIPTION
FIGS. 1-6 illustrate an exemplary manufacturing process for creating an exemplary carbon nanotube field effect transistor in one embodiment of the present invention. FIG. 1 illustrates a first stage of the manufacturing process. A substrate 102 is provided that has a co-planar local bottom gate 104 formed therein, filled with a bottom gate material, such as tungsten, for example. The substrate 102 may include silicon dioxide (SiO2) in various embodiments. Top surfaces of the substrate 102 and of the local bottom gate 104 are substantially aligned to form a substantially planar surface using, for example, chemical mechanical polishing. A high-k dielectric layer 106 is formed on the planar surface of the substrate 102 and local bottom gate 104. The high-k dielectric layer 106 may be deposited using, for example, atomic layer deposition or other suitable deposition methods. Exemplary high-k dielectric material may include HfO2 and Al2O3, among others. The high-k dielectric layer 106 may provide electrical isolation of the underlying gate 104 from transistor source and drain metals formed in later manufacturing processes. A carbon nanotube 108 is deposited on a top layer of the high-k dielectric material 106. The carbon nanotube 108 is generally aligned with the local bottom gate 102 so as to span the local bottom gate 104, such that the ends of the carbon nanotube 108 extend beyond the local bottom gate 104 and over the silicon substrate 102.
FIG. 2 illustrates a second stage of the manufacturing process. An etch barrier material 110 is deposited over the top surfaces of the high-k dielectric layers 106 and the carbon nanotube 108. In various embodiments, the barrier material 110 may be Si3N4, Al2O3 or other suitable barrier material. The etch barrier material serves as both a protective layer in later manufacturing steps to prevent chemical unzipping of the nanotube in a channel region of the carbon nanotube (discussed below) and a means to self-align the contact metals to the unzipped portion of the nanotube in the contact areas of the carbon nanotube (discussed below). A resist layer 112 is then formed on top of the barrier material 110.
FIG. 3 illustrates a third stage of the manufacturing process. In this stage, windows 114a and 114b are formed in the resist layer 112 and transferred to the barrier material 110 by means of chemical etching. Windows 114a and 114b may be formed by standard lithographic processing of the resist layer 112 followed by wet etching of the barrier material 108a. In this way, the pattern defined in the resist layer 112 by windows 114a and 114b is transferred through exposed portions of the barrier material 110 so as to expose ends 108a and 108b of the carbon nanotube 108, while also still covering a middle segment of the carbon nanotube 108 that forms the channel region 108c of the carbon nanotube 108.
FIG. 4 illustrates a fourth stage of the manufacturing process. The resist layer 112 is removed and the ends of the carbon nanotube 108 in the exposed window areas 114a and 114b are unzipped and unrolled to form a planar surface of contact regions 108a and 108b, as disclosed herein. The planar surface is then disposed to lie substantially flat against the high-k dielectric layer 106. In one embodiment, unzipping an end of the carbon nanotube 108 includes forming a cut in the end of the carbon nanotube. In an exemplary embodiment, the cut is along a longitudinal direction of the carbon nanotube 108. Alternately, the cut may be along a spiral direction of the carbon nanotube 108. The cut may be formed by subjecting the exposed carbon nanotube contact regions 108a and 108b of the carbon nanotube 108 to a potassium permanganate (KMnO4) mixture, for example. The potassium permanganate breaks bonds in the surface of the carbon nanotube 108, thereby creating two free edges of the carbon nanotube 108 in the contact regions 108a and 108b. The free edges include dangling bonds that enable the establishment of covalent bonds between contact metal (shown in FIG. 6) and the nanotube i.e. edge contacted devices. The barrier layer 110 may include a wetting layer to reduce air gaps between the barrier layer 110 and the carbon nanotube 108 to prevent access of the potassium permanganate or other chemical treatment to the carbon nanotube channel region 108c. As shown below with respect to FIG. 8, preserving the channel region 108c in its cylindrical form preserves the bandgap in this region.
FIG. 5 illustrates a fifth stage of the manufacturing process. Resist 116 is applied to the surface of the transistor and developed to reopen windows 114a and 114b in the contact regions of the carbon nanotube 108, i.e., at the unrolled ends 108a and 108b of the carbon nanotube 108.
FIG. 6 illustrates a sixth stage of the manufacturing process. Contact metal 120 is deposited in the window areas 114a and 114b onto the contact regions 108a and 108b of the carbon nanotube 108. The contact metal 120 such as palladium for p-type or erbium for n-type transistors may be deposited by electron-beam deposition or other suitable method. The contact metal 120 thus couples to planar surfaces of the contact regions 108a and 108b. Additionally, the contact metal may couple to the free edges of the contact regions 108a and 108b, thereby forming covalent bonds with the dangling bonds of the free edges. The resist 116 is then lifted off of exposed surfaces to complete the carbon nanotube field effect transistor. A subsequent anneal at temperatures in the range from about 200° C. to about 400° C. in one of N2, forming gas, or vacuum may provide further improvements in contact resistance.
FIG. 7 shows an exemplary contact 700 formed using the exemplary process disclosed herein. Carbon nanotube 702 includes a cylindrical channel region 702a and an unzipped contact region 702b. The cylindrical channel region 702a spans the gated section of the transistor. The unzipped contact region 702b is laid substantially flat against the high-k dielectric 704 and contact 706 is formed thereon. The width W of the unzipped contact region 702b is related to the diameter dCNT of the carbon nanotube (W=πdCNT). The unzipped contact region 702b and the contact 706 form a substantially seamless contact. Current enters the unzipped contact region 702b from the contact 706 either at the free edges 710 (as shown by exemplary current 720) or through the surface 712 (as shown by exemplary current 722).
FIG. 8 shows an exemplary band gap diagram 800 of a carbon nanotube that has ends that have been unzipped and flattened as disclosed herein to form a carbon nanotube field-effect transistor. The exemplary band gap diagram 800 includes a first bandgap region 802 that corresponds to the bandgap of the cylindrical channel region 702a of the carbon nanotube of FIG. 7, also referred to herein as the channel region. The exemplary band gap diagram 800 also includes second bandgap regions 804 that correspond to a band gap diagram for the unzipped contact regions 702b of the carbon nanotube of FIG. 7. In the band gap diagram 800, E0 is a vacuum energy level. Energy gap EG1 is an energy gap of the carbon nanotube in the cylindrical channel region 702a and is the energy difference between a valence band energy level EV1 and the conduction band energy level EC1 in the channel region. In various aspects, the energy gap EG1 is inversely proportion to a diameter of the carbon nanotube (EG1 ̃1/dCNT). EG2 is the energy gap in the unzipped region of the carbon nanotube that forms the contacts to the source and/or drain. The EG2 is the energy difference between valence band energy level EV2 and conduction band energy level EC2.
In an exemplary embodiment, energy band gap EG2 in unzipped region 702b is less than the energy band gap EG1 in the cylindrical channel region 702a. For example, for a 1.2 nm carbon nanotube, the band gap values for the carbon nanotube channel region EG1 ranges from about 650 meV to about 700 meV and the width of the unzipped region is approximately 3.8 nm. Experimentally, the band gap of the unzipped contact region EG2 is Eg=α/W, wherein α=0.7-1.5 i.e. the bandgap of a graphene nanoribbon. Thus, the band gap values for the unzipped region EG2 ranges from about 185 meV to about 395 meV, roughly half of the channel bandgap EG1.
EFM is the Fermi level of the metal or a potential energy level for an electron as defined by the Fermi-Dirac distribution, for example. ΦM is a metal work function of the contact metal or the energy required to move an electron from a Fermi energy level to the vacuum level. A Schottky barrier is denoted by ΦB. The Schottky barrier ΦB is defined as the difference between a minimum in the conduction band and the metal Fermi level EFM for an n-type semiconductor and is a difference between the valence band maximum and the metal Fermi level for a p-type semiconductor. The Schottky barrier arises from a mismatch between the semiconductor Fermi level and the contact metal Fermi level at the interface. The Schottky barrier ΦB generally is present when the band gap is within a range that is useful for digital transistors  ̃0.6 eV. The Schottky barrier generally inhibits current injection into a valance band for a p-type device. As shown in the exemplary band gap diagram 800, the Schottky barrier rises to a level above EV2. Thus for a carbon nanotube device without unzipped or flattened ends (i.e., where EV2=EV1 and EG2=EG1), the Schottky barrier inhibits injection of electrons directly into the contact region 702b. Similar arguments can be made for inhibiting injection into the conduction band for an n-type device. The band diagram 800 also shows that for the second bandgap region 804 (where EG2<EG1 and EV2 is close to EFM) the Schottky barrier is much smaller or vanishes altogether in the unzipped contact region 702b. Therefore, reducing the energy gap Eg in the second bandgap region 804 lowers the Schottky barrier. As a result, the contact resistance associated with the Schottky barrier is reduced or eliminated. Thus, an applied bias in the gated region 702b simply modulates EG1 and therefore the current through the channel as in a conventional metal-oxide-semiconductor (MOS) device.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof.
The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated
The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.
While the exemplary embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.

## Claims

1. A semiconductor device, comprising:
a carbon nanotube disposed on a substrate; and
an unzipped end of the carbon nanotube that provides a substantially flat surface, wherein the substantially flat surface of the carbon nanotube is configured to couple to a contact of the semiconductor device.

2. The semiconductor device of claim 1, wherein the unzipped end of the carbon nanotube further comprises one of a surface related to a longitudinal cut at the end of the carbon nanotube and a surface related to a spiral cut at the end of the carbon nanotube.

3. The semiconductor device of claim 1, wherein the contact is further coupled to a free edge of the substantially flat surface created by unzipping the carbon nanotube.

4. The semiconductor device of claim 1, wherein the substantially flat surface of the carbon nanotube is sandwiched between the contact and the substrate.

5. The semiconductor device of claim 4, wherein the carbon nanotube is disposed on a surface of a high-k dielectric so that the unzipped end of the carbon nanotube is substantially flat on the surface of the high-k dielectric, and the contact is formed on top of the substantially flat surface of the carbon nanotube.

6. The semiconductor device of claim 1, wherein the coupling between the substantially flat surface is configured to couple to the contact to reduce a contact resistance between the contact and the carbon nanotube.

7. The semiconductor device of claim 1, wherein the contact is one of a source contact and a drain contact and the carbon nanotube is a gated channel of the semiconductor device.

8. A carbon nanotube field-effect transistor, comprising:
a carbon nanotube;
an end segment of the carbon nanotube having a substantially flat surface resulting from unzipping the end segment of the carbon nanotube and unrolling the unzipped end segment; and
a contact configured to couple to the unzipped end segment of the carbon nanotube to create the carbon nanotube field-effect transistor.

9. The transistor of claim 8, wherein an energy gap in the unzipped end segment is less than an energy gap in a segment of the carbon nanotube that is outside of the unzipped end segment.

10. The transistor of claim 8, wherein unzipping the end segment further comprises one of forming a longitudinal cut at the end segment and forming a spiral cut at the end segment.

11. The transistor of claim 8, wherein an edge of the end segment is coupled to the contact.

12. The transistor of claim 8, wherein the end segment is sandwiched between the contact and a substrate of the transistor.

13. The transistor of claim 8, wherein the unzipping the end segment reduces a contact resistance between the contact and the carbon nanotube.

14. The transistor of claim 8, wherein the contact is one of a source contact and a drain contact and the carbon nanotube is a gated channel of the transistor.

15. A carbon nanotube transistor, comprising:
a contact;
a carbon nanotube; and
an unzipped end segment of the carbon nanotube having a substantially flat surface, wherein the unzipped end segment is configured to couple to the contact to reduce a contact resistance between the carbon nanotube and the contact.

16. The carbon nanotube transistor of claim 15, wherein the unzipped end segment of the carbon nanotube further comprises one of a segment formed by a longitudinal cut at an end of the carbon nanotube and a segment formed by a spiral cut at an end of the carbon nanotube.

17. The carbon nanotube transistor of claim 15, wherein the unzipped end segment further comprises at least one free edge having dangling carbon bond, wherein the free edge is coupled to the contact.

18. The carbon nanotube transistor of claim 15, wherein the unzipped end segment of the carbon nanotube is configured to be sandwiched between the contact and a substrate.

19. The carbon nanotube transistor of claim 15, wherein an energy gap in the unzipped end segment is less than an energy gap of a segment of the carbon nanotube that is outside of the unzipped end segment.

20. The carbon nanotube transistor of claim 15, wherein the carbon nanotube is a gated channel and the unzipped end segment of the carbon nanotube is configured to couple to a contact that is one of a source contact and a drain contact.