Abstract:
The invention is a method of producing an array, or multiple arrays of quantum dots. Single dots, as well as two or three-dimensional groupings may be created. The invention involves the transfer of quantum dots from a receptor site on a substrate where they are originally created to a separate substrate or layer, with a repetition of the process and a variation in the original pattern to create different structures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/522,030 filed on Aug. 4, 2004. 
    
    
     FIELD OF INVENTION 
     This patent specification relates to methods of forming small-scale functional structures. More specifically, it relates to methods of arranging nanoscale building blocks made of atoms and/or molecules into multi-dimensional periodic arrays. Also, it also relates to transferring of the nano-array having uniform size distribution and density for novel optical and electronics devices. 
     BACKGROUND ART 
     As the miniaturization of synthesized functional structures that possess electrical, optical and mechanical functionalities continues to progress rapidly, fabrication techniques based on conventional multi-steps of photolithography and subsequent etching processes appear to be approaching to their practical limits quickly. In the quest for discovering alternative techniques to such “top-down” approaches in which bulk materials are engraved into small-scale functional structures, the concept based on “bottom-up” approaches in which small-scale functional structures are formed by spatially arranging nanoscale building blocks, e.g. atoms and/or molecules, on given foreign substrates have been gaining significant attentions. 
     One of the bottom-up approaches that have been explored extensively in the past ten years is spontaneous self-assembled quantum dot, in particular, coherent, i.e. free from structural defects, small semiconductor inclusions, with a linear order of several tenths of nanometers, in a semiconductor matrix. However, inherent challenges associated with various formation techniques of spontaneous self-assembled quantum dots (SAQDs) have been hindering them from being prosperous approaches to synthesize small-scale functional structures. 
     As the term “spontaneous” indicates, the lack of control on specifically positioning SAQDs into densely-packed multi-dimensional periodic arrays has been a serious issue that needs to be aggressively addressed to ensure flexible tuning of physical properties of the small-scale functional structures consisting of SAQDs. One of diverse approaches that result in arranging SAQDs into periodic arrays, to some extent, as in prior art [T. I. Kamins and R. S. Williams, Appl. Phys. Lett. 71, 1201 (1997)] schematically shown in  FIGS. 1A-1C , is to use engineered strain field generated by pre-formed three-dimensional structures. In  FIG. 1A , a starting substrate  1  is a standard substrate having flat surface. Then, as in  FIG. 1B , the starting substrate  1  is pre-patterned to create a mesa structure  2 . Appropriately designed three-dimensional geometry of the mesa structure  2  provide narrow regions where the formation of SAQDs 3 is energetically favorable, thus SAQDs 3 form along the mesa top as in  FIG. 1C . Although this approach can result in positioning SAQDs 3 in a particular geometrical arrangement, this does not seem to be a feasible way to obtain a densely-packed array because the formation of SAQDs 3 is spatially limited within small regions around the generated strain field. In another prior art (not shown here), an energetic beam consisting of charged particles such as ions can be used to create regions where the formation of SAQDs is energetically enhanced, however, this process would be time-consuming and/or very expensive. 
     Another intrinsic limitation in the formation of SAQDs relates to single crystal substrates on which SAQDs are, in most of cases, formed. Since semiconductor SAQDs are formed under the influence of mechanical strain generated by physical mismatches between a SAQDs material and a substrate material, the substrate necessarily need to be single crystal, putting substantial limitations in terms of choosing substrate materials. 
     On the other hand, organic nanoscale templates for the formation of arrays of nanoscale building blocks are being developed using both artificial and natural materials such as block copolymers, DNA, bacteria, virus, phage and proteins, all of which, unlike semiconductor SAQDs, have a built-in capability of arranging their organic nanoscale building blocks into two-dimensional arrays on a wide range of substrates. These organic nanoscale templates can apparently be used to arrange foreign inorganic nanoscale building blocks, e.g. semiconductor QDs, into two-dimensional arrays characterized by the original organic nanoscale templates. However, as in a prior art [R. A. Mcmillan, et al, “ Ordered nanoparticle arrays formed on engineered chaperonin protein templates”,  Nature Materials 1, 247 (2002).], general incompatibilities in physical properties of such organic nanoscale templates when incorporated as a part of functional device consisting of arrayed inorganic nanoscale building blocks clearly indicate that organic nanoscale templates eventually need to be removed. 
     Therefore, it would be desirable to have a capability of transferring an array of nanoscale building blocks from an original substrate on which the array is preferably formed using a nanoscale template to another substrate on which only the array of nanoscale building blocks resides eventually. It would be further desirable to have a capability of arranging many arrays of nanoscale building blocks into three-dimensional structures. These are necessary to fabricate the novel devices (optical and electrical) having significantly high performances as compared with the bulk-based or non-uniform quantum dot based devices. 
     BRIEF SUMMARY 
     Accordingly, it is an object of the invention to provide the technique to transfer the uniformly distributed quantum dots to the separate substrate by using of which novel devices can be fabricated. 
     According to this invention, it is an object to provide the techniques or methods to create the multi-dimensional quantum dots on single or plurality of layers by transferring from another substrate. 
     It is an object of this invention to provide the manufacturing process of the uniformly distributed quantum dots having pre selected size-distribution and density on the flexible substrate for high performance novel devices. 
     Methods of forming a multi-dimensional array consisting of nanoscale building blocks are described. A two-dimensional periodic array of nanoscale receptors is used as a template by which nanoscale building blocks are weakly captured. A two-dimensional periodicity of the template consisting of nanoscale receptors characterizes a two-dimensional periodicity of nanoscale building blocks captured by the array of nanoscale receptors. The nanoscale building blocks weakly captured by the nanoscale template are subsequently physically transferred onto a foreign substrate by forming strong bindings between the nanoscale building blocks and the foreign substrate, being detached from the nanoscale receptors and resulting in an array of nanoscale building blocks, having a two-dimensional periodicity characterized by the original array of nanoscale template, on the foreign substrate. This transfer technique can be repeated to form three-dimensional array of nanoscale building blocks. 
     According to this invention it is an object to provide the creation of the variable pre-selected sizes quantum dots on the semiconductor or other substrate appropriate for device fabrication. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The invention will be explained in more detail in conjunction with the appended drawings wherein, 
         FIG. 1  is the schematics showing the prior art of the formation of nano-scaled building blocks (e.g. quantum dots). 
         FIGS. 2A to 2E  are the schematics showing the formation of a single nano-scaled building block (e.g. single quantum dot). 
         FIGS. 3A to 3E  are the schematics showing the formation of a two-dimensional array of nanoscaled building blocks (e.g. quantum dots). 
         FIGS. 4A to 4E  are the schematics showing the alternative way of the formation of a two-dimensional array of nanoscaled building blocks (e.g. quantum dots). 
         FIGS. 5A to 5G  are the schematics showing the formation of a three-dimensional array of nanoscaled building blocks (e.g. quantum dots). 
         FIGS. 6A to 6F  are the schematics showing the method of formation of arrays of different sizes nano-scaled building blocks (e.g. quantum dots). 
         FIGS. 7A to 7E  are the schematics showing the method of formation of arrays of different sized nanoscaled building blocks (e.g. quantum dots). 
     
    
    
     DETAILED DESCRIPTION 
     According to a preferred embodiment illustrated in  FIG. 2A˜2E , a nanoscale receptor  101  is formed on a starting substrate  100 . The size of the nanoscale receptor  101  can be in the range of the effective size of an atom (˜0.1 nm) up to the size of giant organic or inorganic molecules (˜several thousands of nm). The starting substrate  100  on which the nanoscale receptor  101  is formed can be chosen from any preferred organic or inorganic materials that are compatible to the synthesizing processes of the nanoscale receptor  101  and other fabrication processes described in detail infra. Subsequently, as shown in  FIG. 2B , the nanoscale receptor  101  captures a nanoscale building block  102 . The nanoscale receptor may be made from either an organic or inorganic material. The nanoscale building block can be made from the opposite type of material, or of the same material. Inorganic material can be semiconductors, ceramic, metal, or they are alloy. Organic material can be polymers, biomolecules, or proteins. The nanoscale building blocks may be quantum dots, atoms, nanowires, or molecules. The binding between the nanoscale receptor  101  and the nanoscale building block  102  is strong enough to capture and hold the nanoscale building block  102  at the location of the nanoscale receptor  101 , yet it is weak enough to be broken by a competing binding formed in the next process step, in addition, the binding between the nanoscale receptor  101  and the starting substrate  100  is designed to be much stronger than the binding between the nanoscale receptor  101  and the nanoscale building block  102 . Then, as illustrated in  FIG. 2C , a foreign substrate  105  is brought to close proximity of the nanoscale building block  102  so that the surface of the foreign substrate  105  and the nanoscale building block  102  starts interacting physically, forming a stronger binding than that between the nanoscale receptor  101  and the nanoscale building block  102 . The nanoscale building block  102  forming the weak binding with the nanoscale receptor  101  is, then transferred, as in  FIG. 2D , to the foreign substrate  105  when the foreign substrate  105  is brought away from the original substrate  100  on which the nanoscale receptor  101  is formed. Finally, as in  FIG. 2E , the building block  102  on the foreign substrate  105  is obtained. 
     In an alternative preferred embodiment shown in  FIGS. 3A˜3E , a two-dimensional array of nanoscale receptors  201  is formed on a starting substrate  200 . The size of each nanoscale receptor  201  can be in the range as for the nanoscale receptor  101  in  FIG. 2 , supra. The starting substrates  200  on which the array of nanoscale receptors  201  is formed can be any semiconductor and all kind of polymers, ceramics (mentioned later), or any material compatible with this process as for the starting substrate  100  in  FIG. 2 , supra. Subsequently, as shown in  FIG. 3B , the array of nanoscale receptors  201  captures nanoscale building blocks  202 . The nanoscale receptor may be made from either an organic or inorganic material. The nanoscale building block can be made from the opposite type of material, or of the same material. Inorganic material can be semiconductors, ceramic, metal, or they are alloy. Organic material can be polymers, biomolecules, or proteins. The nanoscale building blocks may be quantum dots, atoms, nanowires, or molecules. A specific two-dimensional arrangement of the resulting array comprising nanoscale building blocks  202  represents the specific two-dimensional order of the array of nanoscale receptors  201  formed on the starting substrate  200 , i.e. the information on the specific ordered pattern of the two-dimensional array of nanoscale receptors  201  is transferred to the resulting array of nanoscale building blocks  202 . Then, as illustrated in  FIG. 3C , a foreign substrate  203  is brought to close proximity of the array of nanoscale building blocks  202  so that the surface of the foreign substrate  203  and the surface of nanoscale building blocks  202  on the array of nanoscale receptors  201  starts interacting physically, forming a much stronger binding than that between the array of nanoscale receptors  201  and the array of nanoscale building blocks  202 . The array of nanoscale building blocks  202  forming a weak binding with the array of nanoscale receptors  201  is, then transferred, as in  FIG. 3D , to the foreign substrate  203  when the foreign substrate  203  is brought away from the original substrate  200  on which the array of nanoscale receptors  201  is formed. As implied here, the strength of the binding between the array of nanoscale receptors  201  and the array of nanoscale building blocks  202  needs to be the weakest among all bindings, i.e. the binding between the array of nanoscale receptors  201  and the starting substrate  200  and the binding between the array of nanoscale building blocks  202  and the foreign substrate  203 , involved for this transfer process. As in  FIG. 3E , finally the array of nanoscale building blocks  202  transferred onto the foreign substrate  203  is obtained. 
     In an alternative preferred embodiment illustrated in  FIGS. 4A˜4E , unlike the process described with  FIGS. 3A˜3E  supra, instead of transferring an entire array of nanoscale building blocks  202  on an array of nanoscale receptors  201  in  FIG. 3D , it would be beneficial that a part of an array of nanoscale building blocks on a starting substrate is transferred on to a foreign substrate. A two-dimensional array of nanoscale receptors  301  in  FIG. 4A  is formed on a starting substrate  300 . The size of each nanoscale receptor  301  can be in the range as for the nanoscale receptor  101  in  FIG. 2 , supra. The starting substrate  300  on which the array of nanoscale receptors  301  is formed can be any semiconductor and all kind of polymers, ceramics (mentioned later), or any material compatible with this process. Subsequently, as shown in FIG.  4 B, the array of nanoscale receptors  301  captures nanoscale building blocks  302  that can be made of a variety of organic or inorganic materials. A specific two-dimensional arrangement of the resulting array comprising nanoscale building blocks  302  represents the specific two-dimensional order of the array of nanoscale receptors  301  formed on the starting substrate  300 , i.e. the information on the specific two-dimensional pattern of the two-dimensional array of nanoscale receptors  301  is transferred to the resulting array of nanoscale building blocks  302 . Parts of the array of nanoscale building blocks  302  are, then, selectively covered with masks  303  to prevent the specific parts of the array of nanoscale building blocks  302  from being transferred. Then, as illustrated in  FIG. 4C , a foreign substrate  304  is brought to close proximity of the array of nanoscale building blocks  302  so that the surface of the foreign substrate  304  and the surface of nanoscale building blocks  302  not covered by the mask  303  on the array of nanoscale receptors  301  starts interacting physically, forming much stronger bindings than those between the array of nanoscale receptors  301  and the array of nanoscale building blocks  302 . The array of nanoscale functional building blocks  302  forming a weak binding with the array of nanoscale receptors  301  is, then transferred, as in  FIG. 4D , to the foreign substrate  304  when the foreign substrate  304  is brought away from the original substrate  300  on which the array of nanoscale receptors  301  is formed. As implied here, the strength of the binding between the array of nanoscale receptors  301  and the array of nanoscale building blocks  302  is designed to be the weakest among all bindings involved for this transfer process. As in  FIG. 4E , finally a part of the array of nanoscale building blocks  302  selectively transferred onto the foreign substrate  304  is obtained. 
     In another preferred embodiment shown in  FIGS. 5A-5G , the transfer methods described in  FIGS. 2 ,  3 , and  4  supra are repeated as many times as necessary to construct three-dimensional arrayed structures comprising nanoscale building blocks.  FIG. 5A  illustrates an array of nanoscale building blocks  401  transferred, as in  FIG. 3  and  FIG. 4 , onto a foreign substrate  400 , then, in  FIG. 5B , the array of nanoscale building blocks  401  is planarized  403  to provide flat surface to the next array of nanoscale building blocks to be transferred. Meantime, as in  FIG. 5C , an array of nanoscale receptors  405  is formed on a starting substrate  404 . An array of nanoscale building blocks  406  is captured by the array of nanoscale receptors  405 . Then, the array of nanoscale building blocks  406  prepared in  FIG. 5D  is transferred by bringing the planarized array of nanoscale building blocks  403  prepared in  FIG. 5B  to the close proximity to the array of nanoscale building blocks  406 , subsequently, the second array of nanoscale building blocks  406  is transferred on to the planarized array of nanoscale building blocks  403 , resulting in multi-level of the array of nanoscale building blocks as shown in  FIG. 5G . The transfer process can be repeated as many times as necessary. 
     In other preferred embodiments, several sections  501 ˜ 503  that are spatially separated each other are pre-formed on a starting substrate  500  as in  FIG. 6A . A wide variety of geometrical arrangements, shapes and the number of sections of the pre-formed sections on starting substrates, which are not shown in  FIG. 6A , would be apparent to a person skilled in the art in view of the preset disclosure. Each section, then, is filled with nanoscale receptors  504 ˜ 506 , each of which is specifically designed to capture nanoscale building blocks having a specific size as illustrated in  FIG. 6B , therefore, the multiple sections  501 ˜ 503  accommodate the nanoscale receptors  504 ˜ 506  that can capture a variety sizes of nanoscale building blocks  507 ˜ 509  as in  FIG. 6C . Geometrical arrangement, size and the number of the pre-formed sections and the nanoscale receptors illustrated in  FIGS. 6A and 6B  are obviously just one example among a wide variety of choices and not limitations. As described in  FIGS. 4A-4E  supra, in  FIG. 6D , a foreign substrate  510  is brought to close proximity of the arrays of nanoscale building blocks  507 ˜ 509  so that the surface of the foreign substrate  510  and the surface of nanoscale building blocks  507 ˜ 509  starts interacting physically, forming much stronger binding than that between the array of nanoscale receptors  501 ˜ 503  and the arrays of nanoscale building blocks  504 ˜ 506 . The arrays of nanoscale building blocks  507 ˜ 509  forming the weak binding with the arrays of nanoscale receptors  504 ˜ 506  are, then transferred, as in  FIG. 6E , to the foreign substrate  510  when the foreign substrate  510  is brought away from the original substrate  500 , leaving the arrays of nanoscale receptors  504 ˜ 506  formed on the pre-formed sections  501 ˜ 503  on the original substrate  500 . Finally, as in  FIG. 6F , the arrays of different pre-designed quantum dots with selected location can be formed. 
     In an alternative preferred embodiment illustrated in  FIGS. 7A-7E , nanoscale receptors having a variety of sizes  601 ˜ 603  can be varied in a starting substrate  600  in such a way that the surface of the nanoscale receptors  601 ˜ 603  can be at a variety of levels with respect to that of the surface of the starting substrate  600 . By way of an example and not way of limitation, the surface of nanoscale receptors  601 ˜ 603  can be at the same level the surface of the original substrate  600  is at as shown in  FIG. 7A . In  FIG. 7A , the nanoscale receptors  601 ˜ 603  can be simply geometrical indentations formed on the starting substrate  600  as well as nanoscale receptors  601 ˜ 603  consisting of dissimilar materials to the starting substrate  600 . A wide variety of geometrical arrangements, shapes and the number of nanoscale receptors  601 ˜ 603  buried in the starting substrate  600 , which are not shown in  FIG. 7A , would be apparent to a person skilled in the art in view of the present disclosure. As in  FIG. 7B , nanoscale building blocks  604 ˜ 606  are, then, formed on the nanoscale receptors  601 ˜ 603 . As in  FIG. 7C , a foreign substrate  607  is, then, brought to close proximity of the surface of the nanoscale functional building blocks  604 ˜ 606  so that the surface of the foreign substrate  607  and the surface of nanoscale building blocks  604 ˜ 606  starts interacting physically, forming much stronger bindings than those between the array of nanoscale receptors  601 ˜ 603  and the nanoscale building blocks  604 ˜ 606 . The nanoscale building blocks  604 ˜ 606  forming weak bindings with the nanoscale receptors  601 ˜ 603  are, then transferred, as in  FIG. 7E , to the foreign substrate  607  when the foreign substrate  607  is brought away from the original substrate  600 , leaving the nanoscale receptors  601 ˜ 603  formed on the original substrate  600 . 
     According to this invention, the nano-scaled blocks can be quantum dots, atoms, or molecules on the substrate or the layer of the materials. For example, CdSe quantum dots can be formed using the organic receptor like protein template and can be transferred to the foreign substrate (e.g. ZnS) or to the layer of material (e.g. ZnS layer) to form the quantum dot based optical devices. Arrays of single layered quantum dots or three-dimensional quantum dots can be formed to enhance the device performance. 
     According to this invention, the nano-scaled blocks can be transferred to the foreign substrate or to the layer of foreign material. The foreign substrate or the layer of material can be any semiconductor such as Si, Ge, InP, GaAs, ZnS, CdTe, ZnCdTe etc. The substrate can cover also all kinds of polymers or ceramics such as AlN, Silicon-oxide etc. 
     Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, reference to the details of the preferred embodiments is not intended to limit their scope. 
     Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching here is set forth. 
     The present invention is expected to be found practically use in the novel device fabrication using the substrate whereon the quantum-dots formation is not possible using the conventional techniques as mentioned in the prior art. The proposed invention can be used for fabricating wide display, imaging devices, low threshold laser, quantum confinement devices (optical and electronics) etc.