Patent Publication Number: US-2022223687-A1

Title: Nanoscale Thin Film Structure and Implementing Method Thereof

Description:
This invention was made with government support under the Materials Innovation Project No. 2020M3H4A308173112 funded by National Research Foundation of Korea. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Korean Patent Application No. 10-2021-0005539, filed on Jan. 14, 2021, all the benefits accruing therefrom under 35 U.S.C.§ 119, the disclosure of which is incorporated herein in its entirety by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a nanoscale thin film structure and an implementing method thereof, more specifically to a nanoscale thin film structure of which target structure is designed with quantized thickness and a method to implement the nanoscale thin film structure, by which the performance of the manufactured nanodevice can be implemented the same as the designed performance. 
     BACKGROUND 
     Recently, semiconductor technology is beyond the nano-level technology and is approaching the level where atoms should be controlled. Semiconductor devices are getting smaller and thinner, which is comparable to the lattice constant. Especially, Nanoscale devices including quantum wells, such as Quantum Cascade Lasers (QCLs), High Electron Mobility Transistors (HEMTs), Vertical-cavity Surface-emitting Lasers (VCSELs), Light Emitting Diodes (LEDs), Photodetectors, Solar Cells, Field Effect Transistors (FETs), Memories, and Complementary Metal Oxide Semiconductors (CMOS), are very thin with thin film thickness almost equal to the lattice constant. 
     Currently, physical or chemical methods are used for depositing a thin film, and the methods are divided into a Physical Vapor Deposition (PVD) and a Chemical Vapor Deposition (CVD). These methods can be used to implement a nanoscale thin film. Typical PVD is the Sputtering method, and typical CVDs include Metal Organic Chemical Vapor Deposition (MOCVD) or Atomic Layer Deposition (ALD). 
     The MOCVD is a method in which a reactant having atomic species, that constitute a thin film to be deposited in a chamber, is supplied as a source gas, the supplied reactant reaches a substrate and is heated via a wafer carrier or a susceptor, and crystals are formed through thermal decomposition and chemical reaction and are deposited on the substrate. Atomic layer deposition is a deposition method using a phenomenon in which a monoatomic layer is chemically attached, and a thin film can be deposited on a substrate at the level of atomic layers. 
     However, when the thickness of a thin film to be deposited is very thin close to the lattice constant, the performance of the manufactured device may be implemented differently from the designed performance under normal process conditions and structural design methods. Therefore, it is necessary to design such process conditions and structures that can implement the designed performance and the performance of the manufactured device to be the same. 
     Prior art is disclosed in the TEM micrograph of the active region of a lattice-matched AlInAs/GaInAs QCL grown by MBE Cho et al., J. Cryst. Growth 227-228, 1 (2001). 
     SUMMARY 
     An embodiment of the present disclosure provides a nanoscale thin film structure of which target structure is designed with quantized thickness and a method to implement the nanoscale thin film structure by which the performance of the manufactured nanodevice can be implemented the same as the designed performance. 
     In an embodiment, the nanoscale thin film structure comprising a substrate; and a thin film layer deposited on the substrate to a quantized thickness. Said thin film layer is quantized to a minimum unit thickness concerning a growth direction and has a thickness corresponding to an integer multiple of the minimum unit thickness. 
     The thin film layer is formed of a single element, and the minimum unit thickness corresponds to the distance between a reference growth plane and the atom closest to the reference growth plane in the growth direction among atoms forming a crystal structure of the element included in the thin film layer. 
     A reference growth plane is a plane that is formed by atoms corresponding to an origin of the growth direction and is perpendicular to the growth direction. 
     The thin film layer is formed of two or more kinds of elements, and the minimum unit thickness corresponds to the distance between the reference growth plane and a unit growth plane which is formed by a second atom closest to a first atom closest to a reference growth plane in the growth direction among atoms forming a crystal structure of the element included in the thin film layer. 
     A reference growth plane is a plane that is formed by atoms corresponding to an origin of the growth direction and is perpendicular to the growth direction, and a unit growth plane is a plane that is formed by the second atom and is perpendicular to the growth direction. 
     The minimum unit thickness is calculated using a trigonometric function when the crystal structures of the element included in the thin film layer is a crystal structure of which axis angle is not defined to a specific value. 
     The substrate includes a predetermined pattern, and the thin film layer has a thickness corresponding to an integer multiple of the minimum unit thickness in 0° to 180° direction concerning the top surface of the substrate. 
     The maximum thickness of the thin film layer is about 100 nm, and the thickness of the thin film layer includes an error range of about 49% or less of the minimum unit thickness. 
     The thin film layer is deposited using a physical vapor deposition method or a chemical vapor deposition method. The total process time for depositing the thin film layer is the sum of no-growth time and an integer multiple of the unit process time corresponding to the quantized thickness. Said no-growth time is about 0.01 to 10 seconds. 
     The thin film layer is deposited in a plurality of growth times and the no-growth time is added to each growth process. No-growth time is set up to a different time depending on the type of element included in the thin film layer. 
     The total process time includes an error time of about 49% or less of the unit process time. The thin film layer includes any one of Group II, III, IV, V, and VI elements and their compounds. 
     The thin film layer is applied to any one of Quantum Cascade Laser (QCL) devices, HEMTs, Vertical Cavity Surface Light Emission Lasers (VCSEL), LEDs, Photodetectors, Solar cells, Field Effect Transistors (FETs), Memories, and CMOS devices. 
     In an embodiment, an implementing method of nanoscale thin film structure comprising the steps of preparing a substrate; and depositing a thin film layer on the substrate to a quantized thickness. The thin film layer is quantized to a minimum unit thickness concerning a growth direction and has a thickness corresponding to an integer multiple of the minimum unit thickness. 
     The thin film layer is formed of a single element, and the minimum unit thickness corresponds to the distance between a reference growth plane and the atom closest to the reference growth plane in the growth direction among atoms forming a crystal structure of the element included in the thin film layer. A reference growth plane is a plane that is formed by atoms corresponding to an origin of the growth direction and is perpendicular to the growth direction. 
     The thin film layer is formed of two or more kinds of elements, and the minimum unit thickness corresponds to the distance between the reference growth plane and a unit growth plane which is formed by a second atom closest to a first atom closest to a reference growth plane in the growth direction among atoms forming a crystal structure of the element included in the thin film layer. 
     A reference growth plane is a plane that is formed by atoms corresponding to an origin of the growth direction and is perpendicular to the growth direction, and a unit growth plane is a plane that is formed by the second atom and is perpendicular to the growth direction. 
     The minimum unit thickness is calculated using a trigonometric function when the crystal structures of the element included in the thin film layer is a crystal structure of which axis angle is not defined to a specific value. 
     The substrate includes a predetermined pattern, and the thin film layer has a thickness corresponding to an integer multiple of the minimum unit thickness in 0° to 180° direction concerning the top surface of the substrate. 
     The maximum thickness of the thin film layer is about 100 nm. The thickness of the thin film layer includes an error range of 49% or less of the minimum unit thickness. 
     The step of depositing the thin film layer comprises performing a process of either a physical vapor deposition method or a chemical vapor deposition method. The step of depositing the thin film layer comprises deciding the total process time as the sum of no-growth time and an integer multiple of the unit process time corresponding to the quantized thickness. 
     Said no-growth time is about 0.01 to 10 seconds. The step of depositing the thin film layer is performed in a plurality of growth times, and no-growth time is added to each growth process. 
     Said no-growth time is set up to a different time depending on the type of element included in the thin film layer. The total process time includes an error time of about 49% or less of the unit process time. 
     The thin film layer includes any one of Group II, III, IV, V, and VI elements and their compounds. The thin film layer is applied to any one of Quantum Cascade Laser (QCL) devices, HEMTs, Vertical Cavity Surface Light Emission Lasers (VCSEL), LEDs, Photodetectors, Solar cells, Field Effect Transistors (FETs), Memories, and CMOS devices. 
     The disclosed technology can have the following effects. However, this does not mean that a specific embodiment could include all the following effects or only the following effects, so the scope of the disclosed technology should not be construed as being limited thereby. 
     According to the nanoscale thin film structure and implementing the method of the present disclosure, a target can be designed in a predetermined rule with a quantized thickness, and the device can be implemented having the same performance with a designed performance. 
     In addition, the nanoscale thin film structure and its implementation method according to an embodiment of the present invention can control the growth rate corresponding to the thickness by adjusting the total process time to include no-growth time, and implement a thin film with a flat surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain preferred embodiments of the present disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view showing a nanoscale thin film according to an embodiment of the present disclosure. 
         FIG. 2  is a view showing types of crystal structures. 
         FIGS. 3A to 3G  are diagrams for explaining a minimum unit thickness for a basic cubic structure and a basic hexagonal structure. 
         FIG. 4  is a diagram illustrating a processing time of a nanoscale device according to an embodiment of the present disclosure. 
         FIGS. 5A and 5B  are exemplary diagrams illustrating a nanoscale thin film structure according to an embodiment of the present disclosure. 
         FIGS. 6A and 6B  are exemplary diagrams illustrating a quantum cascade laser structure according to a comparative example of the present disclosure. 
         FIGS. 7A to 7C  are views showing a thin film layer implemented in a nanoscale thin film structure according to an embodiment of the present disclosure. 
         FIG. 8  is a graph showing the growth rate according to the thickness of the thin film layer shown in  FIGS. 7A to 7C . 
         FIG. 9  is a graph showing the thickness of the thin film layer according to the processing time. 
         FIGS. 10A and 10B  show the measurement result of an Atomic Force Microscope (AFM) of a thin film layer according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described in detail concerning the accompanying drawings so that a person who has ordinary skill in the art can easily implement the present disclosure.  FIG. 1  is a view showing a nanoscale thin film according to an embodiment of the present disclosure.  FIG. 2  is a view showing types of crystal structures.  FIGS. 3A to 3G  are diagrams for explaining a minimum unit thickness for a basic cubic structure and a basic hexagonal structure.  FIG. 4  is a diagram illustrating a processing time of a nanoscale device according to an embodiment of the present disclosure. 
     Referring  FIGS. 1 , a nanoscale thin film structure  100  according to an embodiment of the present disclosure can be applied to Quantum Cascade Laser (QCL) devices, HEMTs, Vertical Cavity Surface Light Emission Lasers (VCSEL), LEDs, Photodetectors, Solar cells, Field Effect Transistors (FETs), Memories, and CMOS devices. the nanoscale thin film structure  100  may comprise a substrate  110  and a thin film layer  120 . 
     The substrate  110  is a substrate capable of supporting and growing the thin film layer  120 , and the type of material is not limited and can be a thin film layer different from the thin film layer  120 . The substrate  110  may include a flat surface or a predetermined pattern. For example, the substrate  110  may include at least one protruding pattern, and the pattern may have an inclined surface or may be formed in various shapes such as polygons and circles. 
     The thin film layer  120  is deposited on the substrate  110  to a quantized thickness. Specifically, the thin film layer  120  may be quantized to a minimum unit thickness concerning the growth direction and has a thickness corresponding to an integer multiple of the minimum unit thickness. Here, the integer may include a range of 1 to 600. Namely, in one embodiment of the present disclosure, the thin film layer  120  is deposited with discontinuous thickness rather than a continuous thickness by setting the minimum unit thickness as one deposition unit. 
     The thin film layer  120  may include any one of Group II, III, IV, V, and VI elements and compounds thereof. The maximum thickness of the thin film layer  120  may be about 100 nm. The thickness of the thin film layer  120  may include an error range, and the error range can be about 49% or less of the minimum unit thickness. 
     The minimum unit thickness according to an embodiment of the present disclosure may be set up according to the number and the crystal structure of the elements constituting the thin film layer  120 . The types of crystal structures that the thin film layer  120  may include are classified into fourteen (14) types as shown in  FIG. 2 . 
     A French physicist Auguste Bravais, known for his work in crystallography, studied the apparent structure of solid crystals and the arrangement of particles and discovered that the possible structure of a unit cell in three-dimensional space is classified into 7 structures, more specifically into 14 structures. The 7 structures are called the Crystal system, and the 14 structures are called the Bravais Lattice. 
     Here, the unit cell may be distinguished by distances of the length, width, and height cell edges (a, b, c) and the angles formed between them (α, β, γ). For example, the cubic system has a shape of a=b=c and α=β=γ=90° and classified into primitive, body-centered, and face-centered structures. The hexagonal system has a basic structure of a=b≠c, α=β=90°, and γ=120°. 
     When the thin film layer  120  according to an embodiment of the present disclosure is formed of a single element, the minimum unit thickness corresponds to the distance between the reference growth plane and the atom closest to the reference growth plane in the growth direction. Here, the reference growth plane corresponds to a plane that is formed by atoms corresponding to an origin of the growth direction and perpendicular to the growth direction. 
     For example, when the thin film layer  120  is composed of a single element and the crystal structure of the element is a cubic structure, as shown in  FIG. 3A , if the growth direction is the [001] direction, then the minimum unit thickness corresponds to the distance L between the reference growth plane  10  and the atoms  20  closest to the reference growth plane  10  in the growth direction, i.e., the height c. 
     When the thin film layer  120  is composed of a single element and the crystal structure of the element is a hexagonal structure, as shown in  FIG. 3C , if the growth direction is the [001] direction, then the minimum unit thickness corresponds to c and, the growth direction is the direction, the minimum unit thickness corresponds to a/2. And, if the growth direction is the direction, as shown in (c) of  FIG. 3D , then the minimum unit thickness corresponds to (√{square root over (3)}α)/4. 
     On the other hand, when the thin film layer  120  is composed of two or more kinds of elements, the minimum unit thickness corresponds to the distance between the reference growth plane and a unit growth plane. The unit growth plane corresponds to a plane that is formed by the second atom closest to the first atom closest to the reference growth plane and perpendicular to the growth direction. 
     For example, when the thin film layer  120  is composed of two types of elements and the crystal structure of the element is a cubic structure, as shown in  FIG. 3E , the minimum unit thickness corresponds to the distance L between the reference growth plane  10   a  and the unit growth surface  10   b  in the growth direction (indicated as an arrow). For example, if the growth direction is the [001] direction, then L can be c/2. 
     Here, the reference growth plane  10   a  is a plane which is formed by atoms A 1  corresponding to an origin of the growth direction and perpendicular to the growth direction, and the unit growth plane  10   b  corresponds to a plane which is formed by the third atoms A 3  closest to the second atoms A 2  closest to the reference growth plane  10   a  and perpendicular to the growth direction. 
     Similarly, when the thin film layer  120  is composed of two types of elements and the crystal structure of the element is a hexagonal structure, as shown in  FIG. 3G , the minimum unit thickness corresponds to the distance L between the reference growth plane  10   a  and the unit growth surface  10   b . The distance L can be produced differently according to the growth direction (indicated as an arrow). For example, if the growth direction is the [001] direction, then L can be c/2. 
     On the other hand, when the crystal structures of the element included in the thin film layer  120  is a crystal structure of which axis angle is not defined to a specific value, that is, a trigonal system, a monoclinic system, and a triclinic system, the minimum unit thickness is calculated using a trigonometric function between the distance on the axis and the angle. For example, in the case of a trigonal system, if the growth direction is [ 001 ], the distance L between the atom closest to the basic growth plane and the basic growth plane is calculated using the relation of c*sin β=L. 
     Referring to  FIG. 1  again, the thin film layer  120  according to an embodiment of the present disclosure may have a thickness corresponding to an integer multiple of the minimum unit thickness in 0° to 180° direction concerning the top surface of the substrate  110 . For example, when substrate  110  includes a predetermined pattern, the thin film layer  120  may be deposited with different minimum unit thicknesses regarding the pattern surface. Namely, the minimum unit thickness may be set up based on the crystal growth plane to the pattern surface. 
     Furthermore, the thin film layer  120  may be formed in a stacked structure of single materials or different materials. When the thin film layer  120  is formed in a stacked structure of different materials, the minimum unit thickness may be set up corresponding to each of the different materials. 
     The thin film layer  120  can be deposited using a physical vapor deposition method or a chemical vapor deposition method. The total process time for depositing the thin film layer  120  may be set up as the sum of no-growth time and the time corresponding to an integer multiple of the unit process time for the minimum unit thickness. Here, no-growth time is a time during which the thin film layer  120  is not grown at the initial stage of the process and can be about 0.01 to 10 seconds. The total process time may further include an error time of about 49% or less of the unit process time. 
     The total process time for forming the thin film layer  120  according to an embodiment of the present disclosure can be set up as the time obtained by adding an integer multiple of the unit process time t 2  to the no-growth time t 1  as shown in  FIG. 4 . In this case, the thin film layer  120  may grow by a minimum unit thickness for each unit process time t 2  after the no-growth time t 1  to maintain a constant growth rate. 
     The embodiment of the present disclosure is not limited to the above, and the thin film layer  120  may be grown in a plurality of dividend growth times to improve surface roughness. Namely, the growth of the thin film layer  120  can be restarted after stopping one or more times. In this case, the processing time of the thin film layer  120  may be set up by adding the no-growth time t 1  according to the number of restarting of the growth process. In addition, according to an embodiment of the present disclosure, the no-growth time t 1  may be set up to a different time depending on the type of element included in the thin film layer  120 . 
       FIGS. 5A and 5B  are exemplary diagrams illustrating a nanoscale thin film structure according to an embodiment of the present disclosure,  FIG. 5A  is a schematic diagram for growth and  FIG. 5B  is a diagram showing crystal structure.  FIGS. 6A and 6B  are exemplary diagrams illustrating a quantum cascade laser structure according to a comparative example of the present disclosure. 
     Referring to  FIGS. 5A and 5B , the nanoscale thin film structure may be formed in a structure in which InGaAs and InAlAs are alternately stacked. InGaAs and InAlAs are group III-V compound semiconductor materials and have a face-centered cubic structure. When InGaAs and InAlAs are grown in lattice matching on the InP substrate, the minimum unit thickness for each of InGaAs and InAlAs may correspond to ½ of the lattice constant (a) of InP. In this embodiment, (a) is 0.58688 nm. Namely, InGaAs and InAlAs are grown at a thickness of a/2. 
       FIGS. 6A and 6B  are for the prior art, TEM micrograph of the active region of a lattice-matched AllnAs/GaInAs QCL grown by MBE Cho et al., J. Cryst. Growth 227-228, 1 (2001), and as can be seen from  FIG. 6A , the Quantum Cascade Laser is designed to have a thin flat layer of which thicknesses are consecutive (analog), such as 1.1, 1.2, 1.3 2.5, or 3.5 nm, etc. Dividing the numbers by the half of lattice constant (a), the ratio of the thickness 2.5 nm to a/2 is 8.52, 3.5 nm to a/2 is 11.93, and 1.1 nm to a/2 is 3.75. Therefore, 2.5, 3.5, 1.1 nm are substantially impossible thicknesses to deposit even if the 49% of a/2 is considered as the error thickness. 
     Namely, despite that the thickness between the minimum unit thicknesses is practically impossible to grow if a design is made with continuous numerical value without considering this, then the thickness may not be implemented as it was designed, and finally, a difference in the device performance may occur between the design and the product. Since the energy level of electrons changes according to the thickness of the well layer in the Quantum Cascade Laser device, when the thickness difference occurs, the oscillation wavelength changes, and the flow of electrons is disturbed, thereby causing output loss. 
     On the other hand, if a Quantum Cascade Laser device is implemented according to an embodiment of the present disclosure, the QLC device can be implemented as the same as the designed thickness by limiting the thickness of InGaAs and InAlAs to integer multiples of the minimum unit thickness that can be grown. Due to this, the performance of the designed device may match the performance of the actual device. 
       FIGS. 7A to 7C  are views showing a thin film layer implemented in a nanoscale thin film structure according to an embodiment of the present disclosure,  FIG. 7A  is a Transmission Electron Microscopy (TEM) photo,  FIGS. 7B and 7C  are diagrams of the results of converting the measurements of the thickness into integers. 
       FIGS. 7A to 7C  show that the result of measurement for the thickness of the thin film layer  120  by TEM which is designed to be an integer multiple of the minimum unit thickness, and InGaAs and InAlAs are alternately deposited by using an organometallic vapor deposition method, in which a group III gas is injected in a sufficient group V gas atmosphere. The values obtained by dividing the thicknesses of InGaAs of 6.784, 3.085, and 0.643 nm measured as above by half of the lattice constant (a) are 23.11886587, 10.25252181, and 2.191248637 nm respectively. When these are converted to integers, the errors between all measured thicknesses and the converted integers are within 25% of the lattice constant (a). Namely, when the thin film layer  120  is designed to be an integer multiple of the minimum unit thickness according to an embodiment of the present disclosure and the device is manufactured, the actual thickness can be implemented within the error range of the design thickness. 
       FIG. 8  is a graph showing the growth rate according to the thickness of the thin film layer shown in  FIGS. 7A to 7C , and  FIG. 9  is a graph showing the thickness of the thin film layer according to the processing time. 
     As can be seen from  FIG. 8 , when the total process time of the thin film layer  120  is an integer multiple of the minimum unit thickness, though the source gas is supplied at the beginning of the process, the growth rate decreases as the thickness decreases due to the no-growth time in which the thin film layer  120  does not grow. Furthermore, the growth rate according to the thickness varies upon the material of the thin film layer  120 . 
     Accordingly, the process may be performed including the no-growth time in the total process time in one embodiment of the present disclosure. In this case, as shown in  FIG. 9 , the unit growth time corresponding to the minimum unit thickness is constantly repeated after the no-growth time of about 0.95 seconds. Due to this, since the thickness of the thin film layer  120  is deposited in proportion to the unit growth time, the growth rate according to the thickness can be maintained at a constant value. Also, according to an embodiment of the present disclosure, the optimized process conditions for the thin film layer  120  can be designed by adjusting the no-growth time differently for each material of the thin film layer  120 . 
       FIGS. 10A and 10B  show the measurement results of an Atomic Force Microscope (AFM) of a thin film layer according to an embodiment of the present disclosure. 
     When the thin film layer  120  is grown without considering the no-growth time, as shown in  FIG. 10A , partially unnecessary nucleation may exist on the surface. However, when the thin film layer  120  is deposited in consideration of the no-growth time according to the embodiment of the present disclosure, as shown in FIG. B, the growth of crystal nuclei may be minimized to have a flat surface. 
     As described above, in a nanoscale thin film structure and an implementing method according to an embodiment of the present disclosure, the design thickness can be implemented by designing the thin film layer by quantizing it to a minimum unit thickness that can be grown. In addition, by adjusting the total process time with the no-growth time of the thin film layer, the growth rate according to the thickness can be constantly controlled, and it implementation of a flat surface with minimized crystal nuclei growth can be possible. Accordingly, the performance of the manufactured device can be implemented to be the same as the designed performance. 
     The above description of the present invention is for illustrative purposes only, and a person having ordinary skill in the art to which the present invention pertains will be able to understand that other specific forms can be easily modified without changing the technical spirit or essential features of the present invention. Therefore, the embodiments described above are illustrative and non-limiting in all respects. The scope of the present invention is outlined in the claims rather than in the detailed description, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be interpreted as being included in the scope of the present invention.