A Proposal to Research the Future Prospects of Graphene for Electronics Applications

Transcription

1 A Proposal to Research the Future Prospects of Graphene for Electronics Applications Alex Pook EPD 397 Submitted to: Christine Nicometo October 20, 2011

2 Introduction Modern electronic technology is based predominantly on semiconductor materials. The electrical properties of these materials can be controlled by the selective addition of impurities. Electronic devices are built by manipulating those properties. The fundamental building block of most devices is the transistor. Transistors can be used to amplify electrical signals, or as a solid state on/off switch. Integrated circuits, or computer chips, contain millions of transistors built into a thin wafer of semiconducting material. Today, silicon is the material of choice for integrated circuits, but it may not remain so forever. Silicon based devices are an integral part of society, and the market for them is projected to be worth $338 billion in 2012 [1]. The widespread adoption of silicon electronics has largely been enabled by miniaturization, allowing more transistors to fit into the same space and to be used more efficiently. Unfortunately, transistors cannot shrink indefinitely. Current high performance integrated circuits contain transistors with dimensions on the order of tens of nanometers. This scale is only about 100 times larger than that of an atom. It would be impossible to build a transistor smaller than an atom, and quantum effects are likely to derail miniaturization even before it reaches that scale. There is thus a fundamental limit to the trend that has driven electronics technology for the past 40 years. New technology will eventually be needed. Graphene has been touted as a promising candidate for enhancing the performance of electronics [2]. It consists of a sheet of carbon atoms bonded in a honeycomb pattern. Graphene exhibits electronic mobility around 10 times greater than silicon [2]. Electrical signals can thus propagate through graphene much more quickly than through conventional materials. Electronic devices built from graphene could exploit that property, making them faster than silicon devices of comparable size. In addition, the properties of graphene, like those of silicon, can be controlled, making it possible to build electronic devices with it [3]. Recently, graphene has attracted significant attention, even garnering a Nobel Prize for the pioneers in its study. The graphene field has developed quickly. Although it was first isolated only 7 years ago, operational graphene based electronic devices have been demonstrated. Problem Statement While its properties may make it promising, graphene has a long way to go before practical devices are ready. Graphene research has progressed rapidly, but it is far from ready for use as a replacement for silicon in electronics [2]. Several general problems must be solved before it can be truly viable. First, a method for mass producing large high quality graphene sheets must be developed [2]. Next, transistors must be developed with performance comparable to their silicon

3 counterparts [2]. Finally, the transistors must be integrated on a large scale comparable to silicon devices. I propose to research the barriers to the realization of graphene based electronics technology. I will compare graphene and silicon based devices to determine what specifically needs to be improved before graphene can compete, and I will assess potential solutions to these problems. I will also compare graphene manufacturing processes and the devices built with each in order to assess which are the most promising. Objectives I will attempt to achieve the following goals in my research: Analyze the shortcomings of current graphene based technology. Investigate recent progress toward overcoming those shortcomings. Evaluate manufacturing techniques to determine which may be most useful. Compare the performance of graphene based transistors to their silicon counterparts. Assess the near-term potential of graphene based integrated circuits. I intend to characterize the problems facing graphene in terms of manufacturing, transistor performance, and integration. I will investigate progress presented in recent literature, and evaluate its impact on the field. Preliminary Research To assess the feasibility of my plans, I have performed some preliminary research into graphene based devices and their manufacturing processes. I found two review articles which provide broad background information as well as overviews of manufacturing processes and the important properties of graphene. The first [2], is from It covers some epitaxial growth processes (where graphene is grown on top of a substrate such that it fits into the substrate s crystal structure) and assesses some of the applications of graphene in electronics. The second review article [3], which is from 2007, details the fundamental research into graphene s electronic properties. These sources serve primarily to direct my research, pointing toward challenges facing graphene. For example, electrons in graphene to not obey the same quantum mechanical rules that they do in conventional materials, potentially causing problems for graphene based transistors [2,3]. Devices built on graphene must circumvent such issues. I have found several articles describing graphene based transistors. These papers generally detail the manufacturing process as well as performance characteristics, so they are useful for

4 comparisons in both of those areas. The first of these articles [4] involves an epitaxially grown field effect transistor. This is the same type of transistor commonly found in silicon based digital electronics, so it is an important development. The article provides some comparisons with silicon transistors as well, so it is useful for comparative purposes. The next article [5] again involves epitaxial graphene, but proposes a modification to the manufacturing process to increase performance. The new process uses hydrogen to chemically separate the graphene layer from the substrate, and improves some aspects of transistor performance. The last similar source [6] describes transistors built on graphene grown with chemical vapor deposition (CVD). In the CVD process, a substrate is exposed to vapors which react on its surface, leaving behind a graphene film. This is a particularly interesting article because CVD is already used in large scale electronics manufacturing, so the growth process is compatible with existing infrastructure. My final preliminary sources involve the applications of graphene transistors. The first article [7] demonstrates an AC voltage amplifier. It is the first such circuit to show a voltage gain. The second article [8] shows the first graphene based integrated circuit. Both of these articles were published during the summer of 2011, so they are excellent sources for characterizing the current state of integrated graphene devices. The sources I have seen so far provide me with a basis for comparing graphene transistors and manufacturing technologies. I now need to research comparable silicon devices for comparison. I also need to continue researching the general problems graphene faces, such as the unusual behavior of electrons. Much of the information I have on those issues is not entirely current, and since progress has been rapid in this field, there is likely more to be found. Qualifications I was attracted to this topic by media reports of graphene s potential. I have been hearing about it for several years and have often wondered whether the hype was justified. I have a long standing interest in electronics, so I was particularly curious about applications in that area. My undergraduate studies have focused on nanotechnology, and as a result, I am equipped to examine this topic at a deeper level than the popular media. I have taken several classes that are directly relevant to this topic, including quantum mechanics, solid state physics, and design of microelectromechanical systems, which covered semiconductor manufacturing technology. I have also been involved with a major unrelated research project for the past year, so I am familiar with the research process.

5 Management Plan Below is an outline for my plan to complete the MTR project. The first three weeks of October will consist mostly of proposal preparation. After the proposal is completed, I will shift focus to the MTR itself. I will first research the problems facing graphene technology and then the solutions. I will draft sections of the MTR as I research the topics relevant to them. After the first week of November, I will focus on editing the paper and beginning preparing the presentation. I will try to complete as much as possible before Thanksgiving week, as I will likely be less productive than usual during that time. Task 10/3 10/10 10/17 10/24 10/31 11/7 11/14 11/21 11/28 12/6 MTR Proposal Research Write Draft Edit Draft Prepare Presentation MTR Research Manufacturing Techniques Transistor Performance Integration Current Progress Write Draft Edit Draft Prepare Presentation Conclusion Electronic technology has made great strides over the past 40 years. It has now become so widespread that our society is dependent on it. It is essential for our continued technological growth that we investigate new materials and new techniques to enhance the performance of our devices. Graphene is a promising potential avenue for that enhancement. Its properties make it an excellent candidate for electronic applications, and the great attention it has received makes it likely these applications will be explored sooner rather than later. Although it is not ready in any practical sense, graphene technology is progressing rapidly. Eventually, it may even replace silicon as the predominant material in electronics. Where this technology takes us will depend on how we solve the problems it currently faces, so it is imperative that we evaluate solutions with an eye towards the future.

6 References [1] WSTS Semiconductor Market Forecast Spring Internet: June 7, 2011 [Oct ]. This is the website for an organization called World Semiconductor Trade Statistics. It is composed of 64 semiconductor companies representing a large portion of the semiconductor market. I intend to use this as a source for statistics that emphasize the importance of semiconductor technology. [2] A. K. Geim, Graphene: status and prospects, Science, vol. 324, no. 5934, pp , June This article reviews the state of graphene technology as of It discusses the recent history of graphene research and summarizes its important properties. Furthermore, it describes contemporary fabrication processes and devices. It speculates of future research as well. Since research into graphene has progressed rapidly in recent years, this source is not entirely up to date, but it provides a wealth of information on what to look for to assess the technology as well as some background information that will be useful for understanding other sources. [3] A. Geim, and K. Nevoselov, The Rise of Graphene, Nature Materials, vol. 6, no. 3, pp , Mar This is another review article which gives background information on graphene. It includes detailed explanations of graphene s electronic properties and why they would advantageous in electronics. [4] Y. Lin, et al., 100-GHz transistors from wafer-scale epitaxial graphene, Science, vol. 327, no. 5966, pp. 662, Feb This article describes the fabrication and characterization of some graphene based field effect transistors. The transistors were fabricated with an epitaxial growth process, which warrants further investigation. The description of the transistors serves as a basis for comparison with silicon technology and between fabrication technologies. [5] J. Robinson, et al., Epitaxial Graphene Transistors: Enhancing Performance via Hydrogen Intercalation, Nano Letters, vol. 11, no. 9, pp , Aug This article demonstrates a method for improving the performance of epitaxially grown graphene. The process involves the use of hydrogen to partially separate the graphene from its substrate. Transistors built on this process were demonstrated and their performance compared to other graphene transistors. This method will be part of the comparison between manufacturing techniques and transistors.

7 [6] Z. Juang, et al., Direct formation of wafer scale graphene thin layers on insulating substrates using chemical vapor deposition, Nano Letters, vol. 11, no. 9, pp , Aug This source reports on a process for fabricating high quality graphene using chemical vapor deposition. That process is used commonly in the production of large scale electronic. Since it is already well developed and infrastructure is in place to utilize it, CVD is a promising technology. The article also gives a characterization of devices produced with the process. It contains many excellent points for comparison between fabrication technologies, and is especially useful because it is recent. [7] S. Han, K. Jenkins, A. Garcia, A. Franklin, A. Bol, and W. Haensch, High-frequency graphene voltage amplifier, Nano Letters, vol. 11, no. 9, pp , Aug The voltage amplifier presented in this paper is an example of a potentially useful device based on graphene. It provides performance characteristics for the device that may be useful for comparison to other technologies. The device is also an example of an application that may come to fruition within the next few years. [8] Y. Lin, et al., Wafer-scale graphene integrated circuit, Science, vol. 332, no. 6035, pp , June This source demonstrates one of the first graphene based integrated circuits. It provides a template for the future development of such devices. It was made with an epitaxial growth process similar to that shown in source [3], and shows the potential of that process. This is perhaps the best source I have found for gauging the current state of the technology, and should provide some valuable insight into future developments in the field.