The future of semiconductor materials

FMI040

Friday the 13th of May, 2005

Shadi Eibpoosh 810330

Lars Gunnar Klason 800829

Elisabeth Lee 801031

Marie Sonestedt 810218

André Wachau 810516

Introduction

The semiconductor revolution

After the Second World War, most industrial sectors as well as information flow mechanisms have gone through revolutionary transformations. Both these distinct trends originate in the development and spreading of the transistors throughout the market. Together with its dropping price, the costs of several already existing techniques such as the telephone and the radio were reduced dramatically, in the same time as the situation created a perfect breeding ground for new communication techniques.

Better communications brought about a whole new type of worldwide competition, higher transparency and better distributional possibilities in all industrial sectors. The conditions imposed by the so called “semiconductor revolution” have during its five decades or so lead to a constant creation of new technologies and solutions, that all shortly after their invention have experienced a rapid world wide spreading and have also been imitated to a large extent. The transistor based technology has thus diffused into many other technology branches, irrevocably changing them, their prerequisites, and the total market.

During the last decades of the 20th century, the electronics industry was very much characterized by rapid changes, and new inventions came about that would have been hard to even imagine only some years before. The most central feature of the semiconductor industry has been - and still is – the constantly dropping prices accompanied by continuously increasing performance abilities.

Moore’s Law

40 years ago, in 1965, Gordon E. Moore, the co-founder of Intel, presented his famous observation that has come to be called Moore’s law. This empirical observation states that at today’s rate of technological development, the complexity of an integrated circuit with respect to minimum component cost will double in about 24 months. Actually, Douglas Engelbart, a co-inventor of the mechanical computer mouse, had made a similar statement already in 1960. Somehow, the law has become famous for it’s predicition of a doubling every 18 months, but this was demented by Moore himself, that in 1975 claimed that he had projected a doubling every 24 months, i.e every two years.Below, in the graph, both a 18-months-line and one with a 24 months cycle is plotted. The fact that only four years had passed since the invention of the first planar integrated circuit, when Moore presented his paper, shows what a brave hypothesis this was for the future.

2

Figure 1 18 and 24 month cycles[18] Figure 2 Intel’s observation of Moore’s law[10]

With the definition that the complexity of a chip is proportional to the amount of transistors, Moore’s law has suprisingly held, and is predicted to be valid for at least 20 more years. The mission of the technology development team at Intel has been to continue to break down barriers to Moore's Law. And still today, Intel belongs to the 20 top semiconductor companies in the world. In the third column in table 1, it can be seen the degree to which semiconductors are the leading product of the companies.

Table 1 Semiconductor Industry — Market Shares & Dominant Chip Manufacturers

In 1977 for example, the personal computer (PC) market was opened for a lot of new actors, leading to a rapid technical development and structural changes in the field. Intel,

3

Motorola, Texas and IBM are some of the earliest actors in the semiconductor field. Except for the personal computers, the computer game industry has clearly shown implications of the exponential technology development that Moore’s law predicts. Maybe the continuous development of new games is one of the most visual proofs of the innovation process in the semiconductor industry. The increased amount of computer power has lead to more and more advanced games, and the enhanced compactness of pixels has caused a graphic that itself has catalyzed the development of many other technical fields, among them the screen technology industry.

The internet could be viewed as an infrastructural solution that supplies communication ways between a potentially infinite amount of data systems. Through this, it can dispose information about any product or service, and besides saving huge amounts of time, it dramatically reduces the customer- and handling-costs of almost all companies around the world. Information has become much more accessible and cheap. This has dramatically changed our society.

The semiconductor market

Today, roughly one third of the worldwide semiconductor production is localized to Japan, while the lion's share of the semiconductor market belongs to the USA with almost 40 %. Europe has about one fourth of the market, with a focus on Grenoble, France. Countries like South Korea and Taiwan are gaining in importance and in amount of share of the total market. Roughly 85% of the semiconductor market of today belongs to Si, where 10-20% of this share is dedicated to SiGe. The II-VI-compounds have only a few percent of the total market, and the rest of the market of course belongs to the III-V-compounds. In terms of annual turnovers, the semiconductor segment is among the most powerful and dominant industrial sectors today, coarsely comparable to the automotive industry.

The semiconductor market reached a total revenue of about 220.0 billion US-Dollar in 2004. Compared to the revenue in 2003 of 178 billion US-Dollar this is an increase of 23.9 % and the market is still expected to grow during the coming years but at a slower pace. According to GARTNER the market will only grow by a few percent in 2005 and 2006 but a pronounced increase is expected for the ensuing years (Fig 3).

4

The market share of the important technologies for 2004 is shown in Fig 4.

During the past few years the growth in Asia especially China has been the highest. South-East Asia is with a fraction of 42% on the world market the region with the highest semiconductor consumption. Until 2000 the USA was the biggest market with a share of 33%. Today it is the smallest market with 18%. Europe and Japan have fractions of 19% and 21% respectively and remained more or less stable.

5

Figure 3 World wide semiconductor forecast [8]

Figure 4 Market percentage by device types[8]

The rapid growth of the Chinese market is expected to continue with about twice the pace of the entire semiconductor industry over the next five years (Fig 5).

Modern semiconductors

Present Si is the far most important semiconductor in the commercial market. In fact it dominates the market; as approximately 85 % of the fabrication of electronic devices being produced includes this material. Si is used in discrete devices and integrated circuits (IC), the central processing unit in microcomputers and ignition module in modern automobiles. The reason why it is so commonly used is because of its relative simplicity and low cost.

GaAs on the other hand is being used in a significant amount of applications ranging from laser diodes to high speed ICs, i.e. optical devices and applications that handle high frequency. The reason for this is that it has superior electron transport properties and special optical properties. Along with GaAs, InP based materials (III-V compounds) are also being used for certain optical devices. The reason for using these materials in optical devices is because of the size of the direct band gap.

HgCdTe and InSb are other semiconductors that have a low band gap (Eg<0.2 eV). These are used within the field of infrared detectors (IR), to detect thermal radiation or simply for night vision.

Besides these semiconductors, there are a lot of semiconductors which aren’t that commercially big on the market, e.g. ZnO. So far these are just utilized in certain niche applications. However ZnO is starting to being more and more used within the diode market, for e.g. it recently won an environmental award at Chalmers. Which semiconductor that is used for a specific device depends on its frequency and

6

Figure 5 Revenue consumption forecast for the Chinese semiconductor market, 2004-2008[8]

wavelength. Electronic devices care more about a certain frequency; while optical ones on the other hand have wavelength as a parameter.

Material problems in microelectronics

First let’s divide microelectronics into two parts, i.e. as sensors and integrated circuits. By doing so there are some material problems that are related to the semiconductor. A relevant material problem for silicon is “the replacement of the oxide layer with a high-dielectric-constant-material to avoid the gate current leakage in thin oxides and the finding of suitable low-dielectric-constant-materials to reduce capacitive coupling in nm-scale devices” (Andersson, 2005).

When it comes to processing, management and flow of information the problems are found in photonics. There are problems with the semiconductors´ band gap (i.e. material limitations) as well as with finding semiconductors with a wide band gap used for short wavelength light sources. An example of the latter one could be blue and green light emitting diodes and lasers.

We also have problems in the interfacing of information; that is between the man and machine. A typical problem for the semiconductor device can e.g. be speech recognition.

Material fabrication and processing

The fabrication process has three defined steps.

Material growth Processing Device assessment and analysis

Material growth is, as the name implies, growth of single crystals. This can be Si (which will be explained in more details further down) or GaAs. But there is also a possibility for an epitaxial growth, where a single crystal is grown on another substrate which is cut from a crystal.

During the processing two stages will take place. First the pattern will “be transferred by light from a mask into a layer of a polymer resist on the wafer surface” (Andersson, 2005). Then a wet chemical or dry etching will take place. A result of this will be a pattern generation on the surface of the crystal, or an integrated circuit with contacts on a piece of material.

The analysis is the final step, and is basically a quality control of the devices.

7

Crystal growth

As Si is the dominant semiconductor on the market, this chapter will focus on how ultra pure Si is obtained.

Obtaining ultra pure Si

A problem with this semiconductor is that silicon never occurs alone in the nature as an element. Single crystal Si used in device production is in fact a man-made material. The different steps, which involve separation and purification, needed to obtain this ultra pure silicon can bee seen from Fig 6, and will now be further explained.

Figure 6 Summary of the process employed to produce ultra pure silicon

The whole process starts by heating Silica with carbon in an electric furnace. Then very impure silicon is produced. In this process the carbon atoms remove the oxygen from the silica atoms, and thus leave behind impure Si. The next step in the process is to chlorinate the ferrosilicon to yield SiCl4 or SiHCl3. Both of these are liquids at room temperature. The reason why this is favorable is because liquids are much easier to purify, compared to solids. Thus, a number of purifying procedures will now take place, resulting in ultra pure SiCl4. The final step is now to reduce this in a hydrogen atmosphere, yielding the ultra pure Si.

As described above ultra pure silicon is derived from the separation and purification process. However silicon is not a single crystal, but instead polycrystalline, which means that additional processing is needed to form large single crystals used in device fabrication. The most famous process used to create these large crystals is called the Czochralski method.

SilicaVery impure Si

SiCl4

LiquidUltra pure SiCl4

Ultra pure polycrystalline Si

Reduced in presence of C

Chlorinated Distilledetc.

Reduced in hydrogen atmosphere

8

What are the promises for GaN in comparison to ZnO, SiC etc?

With the development of technology going towards power devices with tougher requirements (e.g. higher breakdown voltage, switching frequencies, efficiency, and reliability) that Si-based devices no longer can handle, scientists are confronted with the need of finding new semiconductor materials. An answer to these prayers could and is believed to be wide band gap materials, which with larger band gaps than both silicon and gallium arsenide have the following advantages:

A larger operating temperature range The ability to function as base for visible-range, light-emitting devices High critical breakdown fields, Ecr

High radiation resistance

In the following section, attempts will be made to compare gallium nitride (GaN), which is a wide band gap semiconductor, to other wide band gap semiconductor materials such as silicon carbide (SiC) and zinc oxide (ZnO). As can be seen below in table 2 and 3, diamond has the widest band gap amongst the listed semiconductors.

Table 2 Physical characteristics of Si and main wide band gap semiconductors[14]

9

Table 3 Properties of wurtzite ZnO[4]

As for GaN, SiC and ZnO, although the band gaps are quite similar for all three materials, GaN holds a slightly higher band gap energy. Due to this wider band gap, GaN is also operable at slightly higher temperatures than both SiC and ZnO, since semiconductors with wider band gaps can operate at higher temperatures. However, this maximum operating temperature, shown in table 4 for GaN and two polytypes of SiC1, should in this case of wide band gap semiconductors also be related to the Debye temperature TD, which determines the thermal stability of a semiconductor. If TD > Toperable, the maximum working temperature will decrease. With respect to this, GaN is as seen in Fig 7 clearly inferior to the SiC polytypes. Moreover, the electric breakdown field can be related to the breakdown voltage of a diode as follows:

VB = εrEc/(2qND )

where q is the charge of an electron and ND is the doping density.

1 Unfortunately, the same normalized figures of merit and parameters couldn’t be found for ZnO.

10

Figure 7 Debye temperature TD and the maximum operating temperature Toperable versus the band gap for certain semiconductors[3]

The breakdown voltages calculated for the same doping concentration in different semiconductor materials are plotted normalized to Si in Fig 8. As can be seen in this figure, diamond has the most superior theoretical breakdown voltages of the listed wide band gap semiconductors with a value that is 514 times more than that of Si, while 6H-SiC, 4H-SiC and GaN hold the corresponding values of 56, 46 and 34 respectively.

Figure 8 Maximum Breakdown Voltage of a power device at the same doping density normalized to Si[14]

11

Also seen in table 2 are the thermal conductivity values of the different materials. It turns out that GaN has the worst thermal conductivity, which indirectly indicates the capability of a device of a given material to operate at higher temperatures. Table 4 The basic parameters of some semiconductors2[14]

Table 4 shows some commonly known figures of merits of the different semiconductors. The values in this table have been normalized to the ones of Si, and the higher the value the better performance is shown by the material in the corresponding category. As can be seen, diamond is once again far more superior to Si as well as GaAs in comparison to SiC and GaN which both show similar performances.

A difference between GaN and SiC as well as diamond is that GaN is a direct band gap material (as is ZnO), which makes GaN more suited for optoelectronic devices and radio frequency uses due to its high frequency performance. It should be noted though that ZnO has a larger exciton binding energy than GaN (~60meV in comparison to ~25meV), which should imply that one would obtain brighter light emission from ZnO photonics than from corresponding ones of GaN. Furthermore, studies show that Schottky diodes of GaN have a slightly faster switching speed and lower losses than corresponding SiC Schottky diodes. On the other hand, the forward voltage drop of a GaN Schottky diode is much higher than that of a SiC Schottky diode due to the wider band gap of GaN.

Another disadvantage of GaN compared to SiC is that GaN doesn’t have a native oxide – something that SiC has and which is required for MOS devices. Also, as opposed to SiC as well as ZnO, there aren’t any thick GaN substrates available commercially due to the limitations of present technology of growing pure GaN. In other words, GaN is grown heteroepitaxy using substrates of other materials, e.g. SiC. This leads to a very high dislocation density in the films in general, even though research in the past years has

2 Please note that table 4 is normalized against Si.

12

resulted in GaN growth with relatively lower dislocation density than before. However, yet another disadvantage of GaN, this time to ZnO, is that the latter can be deposited with reasonable quality at lower temperature than GaN, which means that transparent junctions on cheap substrates, such as glass, are possible. This may lead to for example low-cost UV lasers with important applications in high-density data storage systems and solid state lighting.

The future for molecular electronics

First of all, what is molecular electronics? It is a system consisting of nanometre size, atomically precise electronic devices and they are most often made of discrete molecular parts. Secondly, why do we need molecular electronics? Because we are soon reaching the end of making conventional techniques better and smaller and as we do so the costs are growing rapidly. Other downsides to the silicon technology are the massive heat dissipation, the leakage between the devices and there is a physical limit to how small the devices can be made. It is believed that we can replace the technology of today with a few molecules acting as switches, connections, wires and other logic devices. Scientists have already managed to create switches that can be switched on and off hundreds of times as opposed to only once. Improving the switches further will eventually lead to the fabrication of molecular RAM. These switches are made of molecules called catenanes (see Fig 9 ), consisting of two interlinked rings of atoms. To control the switching you need to either remove or replace an electron. They can sense each other and line up efficiently. Researchers are hoping that this will lead to the possibility to make molecular computers that learn and get smarter when used.

Figure 9 Shows some catenanes molecules [13]

13

It is a whole new way of thinking, where instead of the top-down method, where a large piece of silicon is carved into shape, we have the bottom-up method, where the device is (self-) assembled from single atoms and molecules. It will be possible to run massive parallel computing on devices as small as a coin and the power of billions of microprocessors can be contained in one single device. As for storage of data, there will be Atomic Resolution Storage products in which one single atom can hold one bit leading to 100 million times higher information density than on the hard disks of today.

The creation of nanoscale sensors means that it is possible to incorporate them into practically any material. Nanoparticles in textiles allowing the fabrics weave to expand when it is hot and construction materials to change in the presence of fire are both examples of advanced materials. Combine these materials with sensors and they will be able to react to their surroundings. An example is to use it for intelligent food packaging, by using biosensors to detect changes in the food.

References

1. Dr Lennart Ramberg, Docent Jan Jörnmark, Globala förkastningar, Studentlitteratur (2004)

2. Nathan Rosenberg, Den tekniska förändringens ekonomi, SNS Förlag (1997)3. A.A. Lebedev and V.E. Chelnokov, Semiconductors, Wide-gap semiconductors

for high-power electronics, Volume 33, Number 9, September (1999)4. S.J.Pearton, et al., Recent advances in processing of ZnO, J. Vac. Sci. Technol.

B 22(3), May/June (2004)5. Robert F. Pierret, Semiconductor Device Fundamentals, ISBN 0-201-54393-1,

(1996)6. Thorvald Andersson, Compendium in Semiconductors and heterostructure,

Chalmers (2005)7. Professor James R. Shealy at the Electrical and Computer Engineering’s

Department at Cornell University, Ithaca, N.Y., USA 8. Semiconductor Forecast Update, 1Q05, Gartner Consulting (2005),

http://www.gartner.com/teleconferences/attributes/attr_120076_115.pdf9. http://wps2a.semi.org/wps/portal 10. ftp://download.intel.com/research/silicon/moorespaper.pdf 11.http://search.eb.com.proxy.lib.chalmers.se/search?

ct=&query=microelectronics+industrial+segment+economy12.Encyclopedia Britannica online, http://search.eb.com/13.www.wpi.edu/News/Conf/Molecular/ Presentations/mcgimpsey.ppt 14. “Wide Bandgap Semiconductors for Utility Applications”, Leon M. Tolbert, Burak

Ozpineci, S. Kamrul Islam, and Madhu S. Chinthavali, University of Tennessee and Oak Ridge National Laboratory, 2003. http://powerelec.ece.utk.edu/pubs/iasted_2003_wide_bandgap.pdf

15.Unbounding the future: Glossary, http://www.foresight.org/UTF/Unbound_LBW/Glossary.html

14

16.Molecular Electronics and Quantum Dots – Properties http://www.azonano.com/details.asp?ArticleID=564

17.http://www.heise.de/newsticker/result.xhtml?url=/newsticker/meldung/ 56573&words=Halbleitermarkt

18.http://www.wikipedia.com

15