Vacancy assisted diffusion

Vacancy assisted diffusion

The origins of the short carrier trap time and annealing process have been extensively studied [2]–[10]. In the defect-band (AsGa antisite defect) model, the short carrier trap time τCL has been attributed to the high density of AsGa antisite defects. AsGa defects migrate and are eventually captured by As precipitates during the annealing process [4]. The key to shortening the carrier trap time is to control the defect density, as the carrier trap time is inversely proportional to it [4]. Excess arsenic atoms distort the GaAs structure during the crystal growth. Therefore, the arsenic defect density will be high if the arsenic pressure during molecular beam epitaxial (MBE) crystal growth is high. We have recently showed that an LT-GaAs sample grown under a higher arsenic pressure has a shorter carrier trap time for the case of 250◦C growth temperature and as grown and annealed conditions [9], [10]. About 15 years ago, it was reported that semiinsulating GaAs samples have properties similar to LT-GaAs when ions are implanted to GaAs substrate with high doses [11]. Therefore, ion-implanted GaAs substrates can be an alternative of LT-GaAs…………..In spite of these studies, the origin of the short carrier trap time, or the nature of the carrier trap states has not been clarified yet. This also means that the diffusion dynamics of ion-implanted GaAs caused by annealing has not been understood yet. However, for the precise control of the properties of GaAs, an understanding of the nature of the carrier trap states and also the annealing dynamics is important. Here, we discuss the annealing induced diffusion dynamics of As ion-implanted GaAs. The AsGa antisite defects are considered to work as the trap states of carriers [4],[15], [17], and the density of AsGa antisite defects in As ion implanted GaAs is high [14]. Therefore, the discussion of the annealing dynamics of As-ion implanted GaAs becomes simple. If other ions are implanted to GaAs, the discussion may not be so simple because of the possibility of the production of other defects which may trap carriers.

Annealing Induced Diffusion Dynamics in As Ion-Implanted GaAs,
IEICE TRANS. ELECTRON., VOL.E90–C, NO.1 JANUARY 2007







SOME NANO FIGURES

With about 25% of global government investments 1 billion of 4 billion, the US accounts for about 50% of highly cited papers, 60% of patents, and about 70% of startup companies (Nanobusiness, 2004) in nanotechnology world wide. Industry investment in the US has exceeded the NNI in R&D and almost all major companies in traditional and emerging fields have nano groups at least to survey the competitions, Small Times reported 1455 US nanotech companies in March 2005, with roughly half being small businesses, and 23000 new jobs were created in small start-up Nano companies, the NNI SBIR investment was about 80 million dlrs in FY 2005, more than 200 small businesses with a total budget of approx 60 M dlrs have received support from NSF alone, since 2001, many of these are among the 600 pure play nanotechnology companies formed in the US since 2001, in a survey by Small Times.

All Fortune 500 companies in emerging materials, electronics, and pharmaceuticals have had nanotech acivities since 2003.



INNOVATION AND ALL THAT

In 1970 US companies represented 66% of the stock market capitalization of all of the companies in the world. By 1990 even after 20 years of remarkable growth, US companies accounted for only 33% of the world’s total.
Data from the Frank Russell Co, Tacoma, WA

Innovation takes place in a broad pattern in an economic context that is termed as knowledge economy.

Innovation theory is concerned with learning, which is the core process for the creation of knowledge.

Management is another important dimension of innovation for it defines the organisational structures and means trough which innovation and related activities are accomplished.

Innovation is crucial in knowledge economy for predicting the future and providing accurate information in respect of the past. There is a risk of inaccurate prediction which requires systematic revision to compare the prediction with events. Here engineering may be called upon for identifying pattern, which is a formulation for adapting experiences through a sense of order with the knowledge theory to predict the future outcomes. This process will result in rational decision making that ultimately reduces the risks involved.

Michael Polanyi demonstrates the prevalence and importance of tacit knowledge in our daily experience simply by challenging us to express how it is that we can invariably recognize the faces of those we know. The consistency of this ability is reflected in our system of laws, for recognizing someone’s face is considered to be positive identification and is among the most compelling forms of evidence that can be entered in a court of law. Tacit knowledge is part of everything that we do and say, and as it is inherent in our very thinking, it is deeply embedded in the way that we work. When you describe something you miss all the information that is embedded in tacit knowledge of the context.

Fourth Generation, L Morris, W Miller, Wiley, 1999




Silicon and silicon dioxide have very different lattice constants – that is spacing between their atoms. It is therefore difficult to imagine that the interface between them can be electronically perfect. GaAs and AlAs on the other side have almost equal lattice spacing, and two crystals can be perfectly placed on top of each other. The formation of super lattices of such layers of semiconductors has, in fact, been one of the bigger achievements of recent semiconductor technology and was made possible by new techniques of crystal growth (molecular beam epitaxy, metal organic chemical vapour deposition, and the like). Quantum wells, wire and dots have been the subject of extremely interesting research and have enriched quantum physics for example, by the discovery of the Quantum Hall Effect and the Fractional Quantum Hall effect. Use of such layers has also brought significant progress to semiconductor electronics. The concept of modulation doping (selective doping of layers, particularly involving pseudomorphic InGas) has led to modulation doped transistors that hold the current speed records and are used for microwave applications. The removal of the doping to neighbouring layers has permitted the creation of the highest possible electron mobility and velocities. The effect of resonant tunnelling has also been shown to lead to ultrafast devices and applications that reach to infrared frequencies, encompassing in this way both optics and electronics applications. When it comes to large scale integration however the tyranny from the top has favoured silicon technology. Silicon dioxide, as an insulator is superior to all possible III-V compound materials; and its interface with silicon can be made electronically perfect enough, at least when treated with hydrogen or deuterium.


Feynman noticed that nature has already made use of nanostrutures in biological systems with greatest success. Why do we not copy nature? Take for example biological ion channels. These are tiny pores formed by protein structures. T heir opening can be as small as a few one tenths of a nanometer. Ion currents are controlled by these pores that have opening and closing gates much as transistors have.

Brenner D, Handbook of Nanoscience, Engineering, and Technology, Taylor & Francis, 2007