Nano electronics and science unit I introduction, survey of modern electronics

2. 
   Explain about Modern cells?
   

Today, standard solid state QCA cell design considers the distance between quantum dots to be about 20 nm, and a distance between cells of about 60 nm. Just like any CA, Quantum (-dot) Cellular Automata are based on the simple interaction rules between cells placed on a grid. A QCA cell is constructed from four quantum dots arranged in a square pattern. These quantum dots are sites electrons can occupy by tunneling to them.

Figure 2 - A simplified diagram of a four-dot QCA cell.

Figure 3 - The two possible states of a four-dot QCA cell.

2. 
   Describe about Grid arrangements?
   
• 
  Grid arrangements
  
• 
  
  
• 
  Figure 4 - A wire of quantum-dot cells.
  
• 
  Grid arrangements of quantum-dot cells behave in a ways that allow for computation. The simplest practical cell arrangement is given by placing quantum-dot cells in series, to the side of each other. Figure 4 shows such an arrangement of four quantum-dot cells. The bounding boxes in the figure do not represent physical implementation, but are shown as means to identify individual cells.
  
• 
  If the polarization of any of the cells in the arrangement shown in figure 4 were to be controllable (driver cell), the rest of the cells would immediately synchronize to its polarization due to Coulombic interactions between them; much like an instantaneous chain reaction. In this way, a wire of quantum-dot cells is realizable. Although the ability to realize conductive wires does not alone provide the means to perform computation, a complete set of universal logic gates can be constructed using the same principle.
  

1. 
   Explain about logic gates?
   
• 
  Logic gates
  
• 
  Majority gate
  
• 
  
  
• 
  Figure 5 - QCA Majority Gate
  
• 
  The fundamental logic gate in QCA is the majority gate. Figure 5 shows a majority gate with three inputs and one output. Assuming inputs A and B exist in a “binary 0” state and input C exists in a “binary 1” state, the output will exist in a “binary 0” state as the conjunct electrical field effect of inputs A and B is greater than the one of input C. In other words, the majority gate drives the output cell’s state to be equal to that of the majority of the inputs. Now, if the polarization of input C were to be fixed to say, binary 0, the only way the output’s state becomes binary 1, is if input A and B are also 1. Otherwise, the output cell will exhibit a binary 0 state.
  
• 
  Other gates
  
• 
  This conditional behavior is exactly the same as that of an AND gate. Similarly, an OR gate can be constructed using a majority gate with fixed polarization equivalent to binary 1 at one of its inputs. In this way, if any or both of the remaining inputs exist in the binary 1 state, the output will be also in a binary 1 state. Although not certainly based on a majority gate structure, a NOT gate is just as easily realizable. The key principle behind its functionality lies on the fact that placing a cell at 45 degrees with respect of a pair of cells of same polarity, the polarization of the cell will become opposite to that of its driving pair. Figure 6 shows a standard implementation of a NOT logic gate.
  
• 
  
  
• 
  Figure 6 - Standard Implementation of a NOT gate.
  
• 
  
  
• 
  State transition
  

2. 
   Explain about four stages?
   
• 
  Four stages
  
• 
  A QCA clock induces four stages in the tunneling barriers of the cells above it. In the first stage, the tunneling barriers start to rise. The second stage is reached when the tunneling barriers are high enough to prevent electrons from tunneling. The third stage occurs when the high barrier starts to lower. And finally, in the fourth stage, the tunneling barriers allow electrons to freely tunnel again. In simple words, when the clock signal is high, electrons are free to tunnel. When the clock signal is low, the cell becomes latched.
  
• 
  Figure 7 shows a clock signal with its four stages and the effects on a cell at each clock stage. A typical QCA design requires four clocks, each of which is cyclically 90 degrees out of phase with the prior clock. If a horizontal wire consisted of say, 8 cells and each consecutive pair, starting from the left were to be connected to each consecutive clock, data would naturally flow from left to right. The first pair of cells will stay latched until the second pair of cells gets latched and so forth. In this way, data flow direction is controllable through clock zones.
  
• 
  Wire-crossing
  
• 
  Figure 8 - Basic Wire-Crossing Technique.
  
• 
  Wire-crossing in QCA cells is done by using a "plus-sign" pattern, as shown in figure 8. The distances between a plus-sign pattern and a square pattern are exactly the same, allowing for the same Coulombic interactions between electrons in a cell. Thus, when a wire of square cells crosses a wire of plus-sign cells, they do not interact, thus the signals on each wire are preserved.
  
• 
  Fabrication problem
  
• 
  Although this technique is rather simple, it represents an enormous fabrication problem. A new kind of cell pattern potentially introduces as much as twice the amount of fabrication cost and infrastructure; the number of possible quantum dot locations on an interstitial grid is doubled and an overall increase in geometric design complexity is inevitable. Yet another problem this technique presents is that the additional space between cells of the same orientation decreases the energy barriers between a cells ground state and a cell’s first excited state. This degrades the performance of the device in terms of maximum operating temperature, resistance to entropy and switching speed.
  
• 
  Crossbar Network
  
• 
  A different wire-crossing technique, which makes fabrication of QCA devices more practical, was presented by Christopher Graunke, David Wheeler, Douglas Tougaw, and Jeffrey D. Will, in their paper “Implementation of a crossbar network using quantum-dot cellular automata”. The paper not only presents a new method of implementing wire-crossings, but it also gives a new perspective on QCA clocking.
  
• 
  Their wire-crossing technique introduces the concept of implementing QCA devices capable of performing computation as a function of synchronization. This implies the ability to modify the device’s function through the clocking system without making any physical changes to the device. Thus, the fabrication problem stated earlier is fully addressed by: a) using only one type of quantum-dot pattern and, b) by the ability to make a universal QCA building block of adequate complexity, which function is determined only by its timing mechanism (i.e. its clocks).
  
• 
  Quasi-adiabatic switching, however, requires that the tunneling barriers of a cell be switched relatively slowly compared to the intrinsic switching speed of a QCA. This prevents ringing and metastable states observed when cells are switched abruptly. Therefore, the switching speed of a QCA is limited not by the time it takes for a cell to change polarization, but by the appropriate quasi-adiabatic switching time of the clocks being used.
  

4. 
   Explain about parallel to serial?
   

When designing a device capable of computing, it is often necessary to convert parallel data lines into a serial data stream. This conversion allows different pieces of data to be reduced to a time-dependent series of values on a single wire. Figure 9 shows such a parallel-to-serial conversion QCA device. The numbers on the shaded areas represent different clocking zones at consecutive 90-degree phases. Notice how all the inputs are on the same clocking zone. If parallel data were to be driven at the inputs A, B, C and D, and then driven no more for at least the remaining 15 serial transmission phases, the output X would present the values of D, C, B and A –in that order, at phases three, seven, eleven and fifteen. If a new clocking region were to be added at the output, it could be clocked to latch a value corresponding to any of the inputs by correctly selecting an appropriate state-locking period.

Figure 9 - Parallel to serial conversion.

Adding a second serial line to the device, and adding another latching region would allow for the latching of two input values at the two different outputs. To perform computation, a gate that takes as inputs both serial lines at their respective outputs is added. The gate is placed over a new latching region configured to process data only when both latching regions at the end of the serial lines hold the values of interest at the same instant. Figure 10 shows such an arrangement. If correctly configured, latching regions 5 and 6 will each hold input values of interest to latching region 7. At this instant, latching region 7 will let the values latched on regions 5 and 6 through the AND gate, thus the output could be configured to be the AND result of any two inputs (i.e. R and Q) by merely configuring the latching regions 5, 6 and 7.

This represents the flexibility to implement 16 functions, leaving the physical design untouched. Additional serial lines and parallel inputs would obviously increase the number of realizable functions. However, a significant drawback of such devices is that, as the number of realizable functions increases, an increasing number of clocking regions is required. As a consequence, a device exploiting this method of function implementation may perform significantly slower than its traditional counterpart.
Unit – III
Section – A

1. 
   What is Fabrication?
   

Generally speaking, there are four different classes of QCA implementations: Metal-Island, Semiconductor, Molecular, and Magnetic.

2. 
   What is metal island?
   

Ans: The Metal-Island implementation was the first fabrication technology created to demonstrate the concept of QCA.

3. 
   What is molecular?
   

Ans: A proposed but not yet implemented method is method consists of building QCA devices out of single molecules. The main advantages of such implementations include: highly symmetric QCA.

1. 
   Explain about the metal islands?
   

Generally speaking, there are four different classes of QCA implementations: Metal-Island, Semiconductor, Molecular, and Magnetic.

The Metal-Island implementation was the first fabrication technology created to demonstrate the concept of QCA. It was not originally intended to compete with current technology in the sense of speed and practicality, as its structural properties are not suitable for scalable designs. The method consists of building quantum dots using aluminum islands. Earlier experiments were implemented with metal islands as big as 1 micrometer in dimension. Because of the relatively large-sized islands, Metal-Island devices had to be kept at extremely low temperatures for quantum effects (electron switching) to be observable.

A proposed but not yet implemented method is method consists of building QCA devices out of single molecules. The main advantages of such implementations include: highly symmetric QCA cell structure, very high switching speeds, extremely high device density, operation at room temperature, and even the possibility of mass-producing devices by means of self-assembly. A number of technical challenges, including choice of molecules, the design of proper interfacing mechanisms, and clocking technology remain to be solved before this method can be implemented.

Magnetic QCA –commonly referred to as MQCA (or QCA: M), is based on the interaction between magnetic nanoparticles. The magnetization vector of these nanoparticles is analogous to the polarization vector in all other implementations. In MQCA, the term “Quantum” refers to the quantum-mechanical nature of magnetic exchange interactions and not to the electron-tunneling effects. Devices constructed this way could operate at room temperature.

Although QCA solves most of the limitations of CMOS technology, it also brings its own. Research suggests that intrinsic switching time of a QCA cell is at best in the order of terahertz. However, the actual speed may be much lower, in the order of megahertz for solid state QCA and gigahertz for molecular QCA, due to the proper quasi-adiabatic clock switching frequency setting. Additionally, solid-state QCA devices cannot operate at room temperature. The only alternative to this temperature limitation is the recently proposed “Molecular QCA” which theoretically has an inter-dot distance of 2 nm and an inter-cell distance of 6 nm. Molecular QCA is also considered to be the only feasible implementation method for mass production of QCA devices.

2. 
   Give brief description about the steps for producing quantum dots embedded matrix?
   

• 
  Depositing a precursor on the substrate, the precursor including at least one first metal or a metal compound
  

• 
  Contacting the deposited precursor and uncovered areas of the substrate with a gas-phase reagent including at least one second metal and/or a chalcogen
  

• 
  Initiating a chemical reaction between the precursor and the reagent by raising a temperature thereof simultaneously with or subsequent to the contacting so that the matrix consists exclusively of elements of the reagent.
  

The present invention relates to a method for producing quantum dots embedded in a matrix on a substrate, and to quantum dots embedded in a matrix, produced using the method.

In objects having a size of only a few nanometers, which are known as quantum dots, nanodots, or nanoislands, the freedom of motion of the electrons is restricted in all three spatial directions (“zero-dimensional system”). Thus, the linear dimension in all three directions is less than the de Broglie wavelength of the charge carriers. Such quantum dots have a greatly modified electronic structure from the corresponding bulk semiconductor material and, in particular, the density of states becomes more like that for molecules. Quantum dots have a discrete energy spectrum and, in some aspects, behave similarly to atoms, which is due to the quantum nature of the electronic structure. However, unlike with atoms, it is possible to influence the size and electronic structure. Due to the small electrical capacitance of the quantum dots, the addition of a further electron to the electrons already present in the quantum dot (“single-electron tunneling”) requires a certain amount of energy, ranging from several tens of meV to several hundreds of meV (“Coulomb blockade”). This effect allows for controlled quantization of the current flow through the quantum dot. The size and shape of the quantum dots are dependent on the production method and the elements used. At present, quantum dots are mainly used in nanooptics and nanoelectronics, for example, in photo detectors and semiconductor lasers, and also in solar cells. In particular, the formation of binary, ternary, or multinary compound semiconductor quantum structures in a semiconductor matrix is becoming increasingly important in the manufacture of efficient solar cells.

3. 
   Give description about background art?
   

The most frequently used method for producing quantum dots is Stranski-Krastanov epitaxial growth, which is based on a strained crystal lattice of the semiconductor growing on the substrate. As a result of this lattice strain, the growing layer does not grow uniformly. Instead, small nanometer-sized islands are formed, which constitute the quantum dots. Using this method, the size and density of the quantum dots can be controlled to a certain degree, while control of the arrangement and position is possible only to a very limited extent. Other methods for producing quantum dots use the methodology of scanning probe microscopy. These methods allow excellent control over the size and position of the quantum dots. However, they are sequential methods, in which each quantum dot must be produced individually. Therefore, such methods can be used only to a limited extent for devices having a large number of quantum dots.

The in-situ creation of quantum dots in a matrix is known, for example, from U.S. 2004/0092125 A1. There, a dielectric precursor is coated onto a thin metal layer on a substrate and gradually heated, whereby the metal layer and the precursor are gradually stacked on each other, so that quantum dots are formed from the precursor in the metal layer. U.S. Pat. No. 6,242,326 B1 discloses a method for producing quantum dots, in which GaAs quantum dots are formed from Ga droplets and coated with a passivation layer which is formed of a buffer layer and a barrier layer. A similar method is described in KR 1020010054538 A. Japanese document JP 2006080293 A discloses a method of self-organized formation of InAs quantum dots on a GaAs layer, the quantum dots being embedded in a GaAs matrix. Further, it is known from U.S. Pat. No. 5,229,320 to deposit quantum dots through a porous GaAs membrane on an AlGaAs substrate, and to subsequently grow a matrix of AIGaAs for embedding purposes. A method for manufacturing a polymer containing dispersed nanoparticles is known from DE 601 08 913 T2. In that approach, first a polymer precursor is deposited, on which nanoparticles are subsequently distributed as quantum dots. The polymer is cross-linked by application of heat, thereby embedding the quantum dots into the matrix.

The closest prior art to the present invention is represented by DE 694 11 945 T2, which discloses a method in which, first, a soluble precursor of a metal or a metal compound is dissolved in a vaporizable solvent. Then, the dissolved precursor is sprayed onto a substrate as finely distributed, nanometer-sized droplets. Thus, in this known method, the structure and distribution of the quantum dots are no longer dependent on the material and the substrate. The relatively severe limitations of the epitaxial growth method do not occur. The deposited nanostructured precursor is then brought into contact with a chalcogen-containing reagent, so that a chemical reaction occurs at room temperature to form quantum dots of a desired material composition comprised of the precursor and the reagent. The solvent may be vaporized before, during or after the chemical reaction. A polymer is additionally added to the solvent, and serves primarily to coat the dissolved precursor in the solvent and to prevent the nanoparticles from agglomerating during spray deposition. In addition, the polymer is deposited on the substrate, forming a matrix in which the quantum dots are embedded. A polymer matrix of this kind which is made of a transparent plastic has a certain refractive index for optical applications and may be stacked with other polymer layers of different refractive indices. The polymer is not subjected to a chemical reaction; it does not interact with the reagent. Materials other than a polymer cannot be used in the known method to form the matrix, because there matrix formation is merely a secondary effect, the matrix being formed on the substrate as a simple precipitate. The main purpose of the polymer used is to prevent the dissolved precursor particles from agglomerating, and therefore, must have corresponding materials and properties.

Document U.S. 2003/0129311 A1 describes a method which is similar but in which first a porous template is formed. The pores of the polymer are subsequently filled with a precursor solution from which the quantum dots are then formed.

Secton – C

1. 
   Explain about the object of the invention?
   

Starting from the aforementioned prior art, it is an object of the present invention to provide a method for producing quantum dots embedded in a matrix, which will allow any polymer-free matrices to be produced in a controlled manner without any impairing limitations to the method. The matrix composition should be selectable independently of the quantum dot properties, but should co-determine the material composition of the quantum dots, resulting in concordant compositions of the quantum dots and the matrix. Further, the production of the quantum dots should remain independent of the severe limitations of the epitaxial growth method. In addition, the method should be simple, inexpensive and rugged, and should preferably enable the manufacture of compound semiconductor-based products which may be used, in particular, in solar cell technology.

The approach for achieving these objectives will become apparent from the method claim. Advantageous embodiments of the invention are given in the dependent claims and will be described in more detail below in connection with the invention.

The present invention provides a method for producing quantum dots embedded in a matrix on a substrate. In this method, first, quantum dots are deposited on the substrate from a precursor of at least one first metal or a metal compound. In this process, the highly structured or nanostructured deposition determines the geometry and density of the quantum dots. In this manner and through the selection of the precursor, the electronic properties of the quantum dots are determined independently of the substrate, which allows for the use of a variety of different substrates, such as simple glass, metal-coated glass, monocrystalline wafers, polycrystalline layers, films. There is no coupling, for example, to strained lattice states in a crystalline substrate. The use of a precursor in the present invention eliminates the link between the final structure size and the self-organization of the quantum dots during the process. There are various known methods for depositing quantum dots, which will be mentioned further below.

After deposition of the quantum dots, the quantum dots and the substrate regions which are not covered by the dots are brought into contact with a gas-phase reagent. This reagent is comprised of at least one second metal and/or a chalcogen and contains all elements of the matrix to be formed, while the matrix is composed exclusively of elements of the reagent. The chemical reaction between the precursor and the reagent is brought about by raising the temperature simultaneously with or subsequent to said contact (annealing step). The contact between the reagent and the deposited quantum dots causes the precursor to undergo a chemical reaction leading to the final material composition of the quantum dots.

In regions where the reagent contacts the exposed substrate, a matrix is formed from the elements of the reagent in a corresponding stoichiometric ratio. The matrix also deposits above the converted quantum dots, so that the quantum dots are completely embedded in the in-situ formed matrix after the reaction with the reagent is completed.

Thus, the method of the present invention enables quantum dots to be produced using elemental metals or metal compounds as a precursor in a gas-phase reaction with multinary or elemental chalcogens. In the method of the present invention, the gas-phase reaction step serves simultaneously, i.e., in situ in one and the same method step, to grow the matrix in the preferred form from a binary, ternary or multinary compound semiconductor in which the quantum dots are embedded, also in the form of a compound semiconductor (having one or more metal components more than the matrix). The gas-phase reaction step may be carried out such that the precursor is reacted directly to form the final product, for example, by using increased process temperature, or such that this reaction is performed in a subsequent, separate annealing step. The structural, electronic and optical properties of the final product are determined by the dimensions of the precursor structure, the precursor elements used, and the elements used in the gas-phase reaction. There are various methods available for producing the precursor and for the gas-phase reaction.

The deposition of metallic precursors in the form of islands of desired dimensions, which are then converted preferably into semiconductor structures in the subsequent gas phase-based processing step, can be done using a variety of methods, such as evaporation, sputtering, lithographic processes, Focused Ion Beam, scanning probe microscopy-based methods, electrochemical deposition techniques, and the ILGAR and SILAR wet-chemical methods.

German document DE 694 11 945 T2, which represents the art closest to the present invention, describes a method for depositing a dissolved precursor. This method can also be used in the present invention, provided that the precursor used is soluble. In a refinement, said method advantageously uses a liquid-phase precursor which is dissolved in a vaporizable solvent. The precursor/solvent mixture is then sprayed onto the substrate in the form of droplets using special nozzles, possibly while applying an electric field. In this process, care must be taken to prevent agglomeration of the precursor. In this invention, it is not possible to add polymers as a separating agent, because they would also be incorporated into the matrix, leading to unwanted effects. The solvent may be vaporized before, during or after the initiation of the chemical reaction between the precursor and the reagent, so that the final product of the quantum dots is obtained by a wet-chemical or a dry-chemical reaction.

In addition to depositing the precursor in dissolved form, a solid-phase precursor may also be used to advantage. This precursor is then deposited on the substrate in highly structured to nanostructured form using special yet simple methods. For example, a solid-phase precursor may be deposited on the substrate in the form of nanoparticles simply by sprinkling them thereon. The nanoparticles may also be selectively deposited using, for example, micromanipulators.

A similar variety of processes are available for the gas-phase reaction of the desired elements in the reagent with the metallic precursors. The method of the present invention preferably uses semiconducting elements. Depending on the desired final product, which generally contains at least one metal and/or a chalcogen, there are different combinations of elemental, binary, ternary or multinary metallic precursors available which can be reacted with corresponding elemental, binary, ternary or multinary chalcogenides in the reagent. Preferably, therefore, binary, ternary or multinary compound semiconductors are formed for the quantum structure and the matrix. Elements from groups I through VI are preferably used for this purpose.

While the structural, electronic and optical properties of the quantum structures, preferably compound semiconductor structures, produced are mainly dependent on the elements used in the precursors and the gas-phase reaction, additional, typical properties of quantum dots are to be

2. 
   Give example of fabrication process?
   

Initially, metallic dots of Cu as the precursor PC having lateraled and vertical dimensions in the nanometer range are deposited on a substrate SU of glass (non-conductive) or of molybdenum-coated glass (conductive). The deposition of precursor PC is done by evaporation using a suitable mask for nanopatterning the metal being deposited. However, the deposition can also be done using physical vapor deposition, molecular beam epitaxy, chemical transport methods (chemical vapor deposition, metal-organic chemical vapor deposition, etc.), or chemical or electrochemical methods (SILAR, ILGAR, electrode position, chemical bath deposition, etc.). Substrate SU, together with metal precursors PC, is then subjected to an annealing step, which allows reaction with gaseous reagent RG which, in this case, contains Ga and Se. Depending on the temperature and other process parameters, such as time and pressure, the gaseous components react with the Cu, forming the ternary compound CuGaSe ₂ in the form of nanometer-sized quantum dots. The process parameters are selected such that these ternary quantum dots are formed in a matrix of a binary compound (Ga ₂ Se ₃ ), which is deposited simultaneously with the reaction that forms the ternary quantum dots (see the figure). In the process, matrix MA initially deposits on substrate SU, and then also on the converted quantum dots QD, so that quantum dots QD are finally embedded in matrix MA. The process kinetics determining the size and shape of the resulting Nano-sized structures can be controlled by the process parameters, which include, inter alia, the process temperature, the saturation conditions in the gas phase at the corresponding substrate temperature, and the duration of the process.

A simple method for the fabrication of metal nanoparticles is introduced. Heating metal–organic crystals in vacuum results in the formation of well-defined metal particles embedded in a carbon matrix. The method is demonstrated for iron phthalocyanine. At 500 °C homogeneously distributed iron nanoparticles with a reasonably narrow size distribution form by nucleation and ripening. After this initial phase the formation kinetics changes drastically. The particles move in the matrix to incorporate material. The 'gluttony' phase shows astonishing similarities with the search for nutrition of living micro-organisms. Particle formation, ripening and gluttony are followed in situ by transmission electron microscopy.

3. 
   Explain about transconductance?
   

From Wikipedia, the free encyclopedia

For small signal alternating current, the definition is simpler: