Patent Publication Number: US-2023163260-A1

Title: Micro led and display module having same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation bypass of International Patent Application No. PCT/KR2021/012472, filed on Sep. 14, 2021, with the World Intellectual Property Organization, which is based on and claims priority to Korean Patent Application No. 10-2020-0127489, filed on Sep. 29, 2020, in the Korean Intellectual Property Office, the disclosures of each of which are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     Field 
     The disclosure relates to a micro LED and a display module including the same, and more particularly, to a micro LED including a wavelength conversion layer, and a display module including a plurality of micro LEDs arranged in pixel units on a thin film transistor (TFT) substrate. 
     Description of Related Art 
     Light emitting diodes (LEDs) are being applied in various industrial fields because of their relatively high light emitting efficiency and long lifespan. In particular, LEDs are being commercialized in the field of displays as well as in the field of general lightings. 
     In recently developed LED displays (e.g., a full-color micro LED display), micro LEDs of at least three colors of R/G/B are integrated in one pixel. However, with the trend of high performance and high efficiency of a display, there is a continuing demand for improvement of light emitting efficiency of an LED device. In particular, in the field of development of nano-size LEDs such as micro LEDs, improvement of light emitting efficiency of an LED is emerging as an important task to be resolved together with improvement of procedural efficiency. 
     In the case of a micro LED, non-radiative recombination appears around a side surface formed by mesa etching during a manufacturing process. The non-radiative recombination not only reduces quantum efficiency of a micro LED, but it also becomes a factor for thermal droop. When the temperature of thermal droop increases, light output of a micro LED decreases. 
     Meanwhile, in the conventional wavelength conversion technology applied to a display, fluorescent materials on which quantum dots or rare earth elements are doped are being used widely. Quantum dots are based on semiconductor nano particles such as InP and CdSe. 
     Quantum dots are unstable thermally and chemically. Accordingly, a quantum dot layer provided in an LED package is arranged to be distanced from a micro LED, which is a heat source and a light source, as much as possible for avoiding degradation. For this, an LED package includes a cover layer between a quantum dot layer and a blue micro LED. However, the thickness of a quantum dot layer is being manufactured to exceed 5 µm in general, due to a low absorption coefficient of about 10 4  cm -1  or smaller. Due to this, there is a problem that the thickness of the LED package increases and the structure becomes complex. 
     Also, a phosphor on which a rare earth element is doped is a commonly used material in wavelength conversion. Until now, various types of materials such as yttrium aluminum garnet doped with Ce, CaAlSiN doped with Eu, and SiAlON doped with Eu have been used in the field of lightings and liquid crystal displays. However, due to low absorption coefficients (10 3  cm -   1  or smaller) of these materials in blue and UV wavelength areas, the required thickness of a phosphor on which a rare earth element is doped is scores of µm. Accordingly, a supporting structure is generally required to avoid structural weakness. Also, if there is partial permeation of blue and UV excite lights through a wavelength conversion layer, the permeated excite lights reduce color reproducibility of a display, and thus a color filter that can block excite lights is needed. Due to this, a complex structure and a complex process are required in use of a conventional wavelength conversion material applied to an LED package, and thus there is a problem that the cost increases, and productivity decreases. 
     SUMMARY 
     For resolving the aforementioned problem, the disclosure provides a micro LED of a simple structure including a thin wavelength conversion layer that does not require a color filter or a cover layer, and a display module including the same. 
     In accordance with certain embodiments of the present disclosure, a micro LED is provided. The micro LED includes a first semiconductor layer which has a light emission surface and to which a first electrode is electronically connected, a second semiconductor layer to which a second electrode is electronically connected, an active layer arranged between the first and the second semiconductor layers, and a wavelength conversion layer which is stacked on the light emission surface of the first semiconductor layer. The wavelength conversion layer includes a semiconductor in a chalcopyrite-structure. 
     In accordance with other embodiments of the present disclosure, a display module is provided. The display module includes a TFT substrate including a glass substrate and a thin film transistor (TFT) layer formed on one surface of the glass substrate, and a plurality of micro light emitting diodes (LEDs) which are electronically connected to a plurality of TFT electrodes formed on the TFT layer, the plurality of micro LEDs being arranged in pixel units. Each of the plurality of micro LEDs respectively include a first semiconductor layer which has a light emission surface and to which a first electrode is electronically connected, a second semiconductor layer to which a second electrode is electronically connected, an active layer arranged between the first and the second semiconductor layers, and a wavelength conversion layer which is stacked on the light emission surface of the first semiconductor layer. Each wavelength conversion layer includes a semiconductor in a chalcopyrite-structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and aspects of certain exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like reference numerals denote like elements, and wherein: 
         FIG.  1    is a plan view illustrating a display module including a plurality of pixels having R/G/B sub pixels according to an embodiment of the disclosure; 
         FIG.  2    is a side view schematically illustrating a pixel of a display module according to an embodiment of the disclosure; 
         FIG.  3 A  is a schematic diagram illustrating a micro LED having a red wavelength conversion layer according to an embodiment of the disclosure; 
         FIG.  3 B  is a schematic diagram illustrating a micro LED having passivation layers respectively on both surfaces of a red wavelength conversion layer according to another embodiment of the disclosure; 
         FIG.  4    is a schematic diagram illustrating a micro LED having a green wavelength conversion layer according to still another embodiment of the disclosure; 
         FIG.  5    is a schematic diagram illustrating a micro LED having a blue wavelength conversion layer according to still another embodiment of the disclosure; 
         FIG.  6    is a graph illustrating a relationship between a lattice constant and a corresponding wavelength calculated from band gap energy of a wavelength conversion layer in a chalcopyrite-structure; 
         FIG.  7    is a flow chart illustrating a manufacturing process of a micro LED according to an embodiment of the disclosure; 
         FIG.  8 A  to  FIG.  8 G  are process diagrams sequentially illustrating a manufacturing process of a micro LED according to an embodiment of the disclosure; 
         FIG.  9    is a side view schematically illustrating a pixel of a display module according to an embodiment of the disclosure; 
         FIG.  10    is a side view schematically illustrating another example of a pixel of a display module according to another embodiment of the disclosure; 
         FIG.  11    is a side view schematically illustrating a pixel in a micro LED display according to still another embodiment of the disclosure; and 
         FIG.  12    is a graph comparing an efficiency of a micro LED according to an embodiment of the disclosure with an efficiency of a conventional micro LED. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, various embodiments will be described in more detail with reference to the accompanying drawings. The embodiments described in the disclosure may be modified in various forms. Also, specific embodiments may be illustrated in the drawings, and explained in detail in the detailed description. However, the specific embodiments described in the accompanying drawings are just for making the various embodiments understood easily. Therefore, the technical idea of the disclosure is not limited by the specific embodiments described in the accompanying drawings, but they should be interpreted to include all equivalents or alternatives included in the ideas and the technical scopes of the disclosure. 
     Also, terms including ordinal numbers such as ‘the first’ and ‘the second’ may be used to describe various components, but these components are not limited by the aforementioned terms. The aforementioned terms are used only for the purpose of distinguishing one component from another component. 
     In addition, in the disclosure, terms such as “include” or “have” should be construed as designating that there are such characteristics, numbers, steps, operations, elements, components, or a combination thereof described in the disclosure, but not as excluding in advance the existence or possibility of adding one or more of other characteristics, numbers, steps, operations, elements, components, or a combination thereof. 
     Other than the above, in describing the disclosure, where it is determined that detailed explanation of related known functions or features may unnecessarily confuse the gist of the disclosure, the detailed explanation will be abridged or omitted. 
     Herein, on the front surface of a glass substrate, a thin film transistor (TFT) layer for forming a TFT circuit may be arranged, and on the rear surface, a driving circuit for driving the TFT circuit of the TFT layer may be arranged. The glass substrate may be formed as a quadrangle type. Specifically, the glass substrate may be formed as a rectangle or a square. 
     Herein, a structure in which a TFT layer (or a backplane) is stacked on a glass substrate may be referred to as a TFT substrate. The TFT substrate is not limited to a specific structure or type. For example, the TFT substrate may be implemented as an oxide TFT and an Si TFT (poly silicon, a-silicon), an organic TFT, a graphene TFT, or a low temperature polycrystalline silicon (LTPS) TFT. Also, a P-type (or an N-type) metal oxide semiconductor field effect transistor (MOSFET) may be made in an Si wafer complementary metal oxide semiconductor (CMOS) process, and applied. 
     In an embodiment, the front surface of the glass substrate on which a TFT layer is arranged may be divided into an active area and an inactive area. The active area may fall under an area occupied by the TFT layer on one surface of the glass substrate, and the inactive area may fall under an edge area on one surface of the glass substrate. The edge area may include a side surface of the glass substrate. Also, the edge area may be the remaining area excluding the area wherein the TFT circuit is arranged on the front surface of the glass substrate and the area wherein the driving circuit is arranged on the rear surface. Further, the edge area may include a side surface of the glass substrate, and a portion of the front surface of the glass substrate and a portion of the rear surface of the glass substrate adjacent to the side surface. 
     In an embodiment, on the glass substrate, a plurality of front surface connection pads electronically connected with the TFT circuit through a wiring may be formed in the edge area of the front surface, and a plurality of rear surface connection pads electronically connected with the driving circuit through a wiring may be formed in the edge area of the rear surface. The plurality of front surface and rear surface connection pads may be arranged to be respectively inserted at a specific distance from the side surface of the glass substrate to the inner side of the glass substrate. The connection pads respectively formed on the front surface and the rear surface of the glass substrate may be electronically connected by a side surface wiring formed in the edge area of the glass substrate. 
     In an embodiment, on the TFT layer of the glass substrate, a plurality of pixels may be arranged. Each pixel may include a plurality of sub pixels, and one sub pixel may correspond to one micro LED. The TFT layer may include a TFT circuit for driving each pixel. A micro LED may be a semiconductor chip that includes an inorganic light emitting material, and that can emit light by itself when power is supplied. Also, a micro LED may have a flip chip structure wherein anode and cathode electrodes are formed on the same surface, and a light emitting surface is formed on the opposite side of the electrodes. 
     In an embodiment, the number of pixels mounted on the TFT substrate (including the glass substrate and the TFT layer stacked on the glass substrate) may be determined by the resolution of the display. Also, micro LEDs may be arranged on the TFT substrate in pixel units. Herein, the feature of “being arranged in pixel units” means that a plurality of pixels are grouped into “pixel units” having the same pattern and each arranged in respective “pixel areas”. 
     In an embodiment, the TFT layer formed in a stack on the glass substrate is electronically connected to a micro LED. Specifically, the electrode pad of the micro LED may be electronically connected to the electrode pad on the TFT layer, and the electrode of the micro LED and the TFT electrode may have a joint structure in a state of metal binding. 
     In an embodiment, a display module including micro LEDs may be a flat display panel. The micro LEDs may be inorganic light emitting diodes (LEDs) having a size of 100 µm or smaller. The display module including micro LEDs can provide better contrast, faster response time, and higher energy efficiency than a liquid crystal display (LCD) panel which needs a backlight. Both of an organic light emitting diode (OLED) which is an organic light emitting device and a micro LED which is an inorganic light emitting device have good energy efficiency, but a micro LED has better brightness and light emitting efficiency, and a longer lifespan than an OLED. 
     In an embodiment, in the display module, black matrices may be formed among the plurality of micro LEDs arranged on the TFT layer. The black matrices can improve the contrast ratio by blocking leakage of light in the ambient part of micro LEDs adjacent to one another. 
     In an embodiment, the display module may further include a touch screen panel arranged on the side wherein the plurality of micro LEDs emit light, and also, the display module may include a touch screen driver for driving the touch screen panel. 
     In an embodiment, the display module may further include a rear substrate that is arranged on the rear surface of the glass substrate, and is electronically connected through a flexible printed circuit (FPC), etc., and also, the display module may further include a communication device that can receive data. 
     In an embodiment, the glass substrate on which micro LEDs are mounted and a side surface wiring is formed may be referred to as a display module. Such a display module may be installed and applied in a single unit on wearable devices, portable devices, handheld devices, and various kinds of electronic products or electronic components which need displays. Also, the display module may be applied as a matrix type to display devices such as monitors for personal computers (PCs), high resolution TVs and signage (or, digital signage), and electronic displays through a plurality of assembly arrangements. 
     Hereinafter, the display module according to an embodiment of the disclosure will be described in detail with reference to the drawings. 
       FIG.  1    is a plan view illustrating a display module including a plurality of pixels having R/G/B sub pixels according to an embodiment of the disclosure, and  FIG.  2    is a side view schematically illustrating a pixel of a display module according to an embodiment of the disclosure. 
     Referring to  FIG.  1   , the display module  10  may include a TFT substrate  30 , a panel driver (not shown), and a processor (not shown). 
     Referring to  FIG.  2   , the TFT substrate  30  may include a glass substrate  31  and a TFT layer  33  formed on the front surface of the glass substrate  31 . The TFT layer  33  may include a plurality of pixel areas  40 . Each pixel area  40  may include a plurality of sub pixels and a plurality of pixel circuits for driving each sub pixel. The plurality of sub pixels in the pixel area  40 , and their arrangement in the pixel area  40 , may be the “pixel unit” described earlier herein. The plurality of sub pixels and the plurality of pixel circuits arranged in each pixel area  40  will be described in detail below. 
     The TFT substrate  30  is formed such that gate lines and data lines intersect with one another, and pixel circuits may be formed in the area provided by the intersection. 
     The panel driver drives the plurality of pixel circuits of the TFT substrate  30  according to control by the processor. The panel driver may include a timing controller, a data driver, and a gate driver. 
     The timing controller may receive input of an input signal, a horizontal synchronization signal, a vertical synchronization signal, and a main clock signal, etc. from the outside and generate an image data signal, a scan control signal, a data control signal, a light emission control signal, etc., and may provide the signals to the TFT substrate  30 , the data driver, the gate driver, etc. 
     In particular, the timing controller may apply a control signal MUX Sel R, G, B for selecting one sub pixel among R, G, B sub pixels to the driving circuit. 
     The data driver (or a source driver) is a means for generating a data signal, and receives image data of R/G/B components, etc. from the processor and generates a data voltage (e.g., a pulse width modulation (PWM) data voltage, a pulse amplitude modulation (PAM) data voltage). Also, the data driver may apply the generated data signal to the TFT substrate  30 . 
     The gate driver is a means for generating various kinds of control signals such as a control signal SPWM(n), a control signal SPAM, etc., and transmits the generated various kinds of control signals to a specific row (or, a specific horizontal line) of the TFT substrate  30 , or transmits the signals to the entire lines. Also, the gate driver may apply a driving voltage VDD to the driving voltage terminal of the driving circuit, depending on embodiments. 
     All or a part of the data driver and the gate driver may be implemented on the TFT layer  33 , or implemented as a separate semiconductor IC, and arranged on the rear surface of the glass substrate  31 . 
     The processor controls the overall operations of the display module  10 . In particular, the processor may drive the TFT substrate  30  by controlling the panel driver, and thereby make the plurality of pixel circuits perform the aforementioned operations. 
     For this, the processor may be implemented as one or more of a central processing unit (CPU), a micro-controller, an application processor (AP) or a communication processor (CP), and an ARM processor. According to an embodiment of the disclosure, the processor may control the panel driver to set the pulse width of a driving current according to a PWM data voltage, and set the amplitude of a driving current according to a PAM data voltage. Here, the TFT substrate  30  is arranged in n rows and m columns, and the processor may control the panel driver such that a PWM data voltage is applied in row units (or horizontal line units). Also, the processor may control the panel driver such that a PAM data voltage is applied generally to the entire sub pixels of the TFT layer  33 . Afterwards, the processor may control the panel driver such that a driving voltage VDD is applied at once to the plurality of pixel circuits included in the TFT layer  33 , and a linearly changing voltage (a swipe voltage) is applied to the PWM driving circuits of each of the plurality of pixel circuits, and thereby display an image. 
     Herein, the processor and the timing controller are described as separate components, but depending on embodiments, the timing controller may perform the function of the processor, without the processor. 
     Referring to  FIG.  2   , in each pixel area  40  formed on the TFT layer  33 , a plurality of sub pixels  50  (such as sub pixels  51 ,  52 ,  53 ), and a plurality of pixel circuits  61 ,  62 ,  63  for driving each sub pixel may be arranged. 
     The TFT layer  33  is formed on the front surface of the glass substrate  31 , and it may include a multi-layer structure. As an example, the TFT layer  33  may include a buffer layer stacked on the front surface of the glass substrate  31 , a gate insulation layer stacked on the buffer layer, an inter-layer insulation layer stacked on the gate insulation layer, and a plurality of passivation layers sequentially stacked on the inter-layer insulation layer. Also, the TFT layer  33  may include a wiring electronically connected to a voltage (VDD, VSS) terminal. 
     In an embodiment, three sub pixels  51 ,  52 ,  53  form one pixel. Each sub pixel may be a light emitting device, for example, a micro LED. Herein, the terms ‘a sub pixel’ and ‘a micro LED’ are used interchangeably. 
     As depicted, an example wherein the plurality of sub pixels includes three micro LEDs corresponding to red/green/blue (R/G/B) colors is described, but the disclosure is not necessarily limited thereto. That is, the plurality of sub pixels may alternatively include two micro LEDs of red/blue (R/B), red/green (R/G), or green/blue (G/B), or three micro LEDs of red/blue/white (R/B/W), or four micro LEDs of red/green/blue/white (R/G/B/W), red/green/green/white (R/G/G/W), or red/green/blue/yellow (R/G/B/Y), or five micro LEDs of red/green/blue/yellow/cyan (R/G/B/Y/C). In each case, the number of the pixel circuits may correspond to the number of the sub pixels. 
     The plurality of sub pixels  51 ,  52 ,  53  respectively include first electrodes (anode electrodes)  51   a ,  52   a ,  53   a  and second electrodes (cathode electrodes)  51   b ,  52   b ,  53   b . 
     The first electrodes  51   a ,  52   a ,  53   a  of the respective sub pixels are respectively soldered to first TFT electrodes  35   a ,  36   a ,  37   a , and are connected to a driving voltage (VDD) terminal through the first TFT electrodes  35   a ,  36   a ,  37   a . Also, the second electrodes  51   b ,  52   b ,  53   b  of the respective sub pixels are respectively soldered to second TFT electrodes  35   b ,  36   b ,  37   b , and are connected to a ground voltage (VSS) terminal through the second TFT electrodes  35   b ,  36   b ,  37   b . 
     The plurality of pixel circuits  61 ,  62 ,  63  are electronically connected with the respective sub pixels  51 ,  52 ,  53 , and control the respective corresponding sub pixels  51 ,  52 ,  53  to be driven to be turned on or turned off. 
     The R/G/B sub pixels  51 ,  52 ,  53  illustrated in  FIG.  2    are an example, and they may respectively be formed as below. The red sub pixel  51  may include a blue micro LED having a red wavelength conversion layer stacked on the light emitting surface of the blue micro LED. The green sub pixel  52  may include a blue micro LED having a green wavelength conversion layer stacked on the light emitting surface of the blue micro LED. The blue sub pixel  53  may include a blue micro LED without a wavelength conversion layer. 
     Hereinafter, the structure of a micro LED according to an embodiment of the disclosure will be described. While an exemplary embodiment of a flip chip horizontal micro LED is described and illustrated herein, the disclosure is not limited thereto, and the structure may be a vertical micro LED that is not illustrated in the drawings. 
       FIG.  3 A  is a schematic diagram illustrating a micro LED having a red wavelength conversion layer according to an embodiment of the disclosure. 
     Referring to  FIG.  3 A , the maximum length L of the upper surface of the first micro LED  51  emitting a red light may be, for example, 60 µm or smaller. 
     The first micro LED  51  may include a first semiconductor layer  511 , a second semiconductor layer  513 , an active layer  515  formed between the first and second semiconductor layers, a first LED electrode  516   a  formed on the first semiconductor layer, a second LED electrode  516   b  formed on the second semiconductor layer, and a wavelength conversion layer  518  formed in a stack on the light emission surface  511   a  of the second semiconductor layer. 
     The first semiconductor layer  511  is a conductivity type semiconductor layer, and it may be an n-type (or a p-type) semiconductor layer. The second semiconductor layer  513  is a conductivity type semiconductor layer, and it may be a p-type (or an n-type) semiconductor layer. 
     For example, if the first semiconductor layer  511  is an n-type semiconductor layer, the second semiconductor layer  513  may be a p-type semiconductor layer, and in contrast, if the first semiconductor layer  511  is a p-type semiconductor layer, the second semiconductor layer  513  may be an n-type semiconductor layer. 
     The active layer  515  formed between the first and second semiconductor layers  511 ,  513  may include a so-called multiple-quantum-well (MQW) or a single-quantum-well (SQW). 
     The first electrode  516   a  is electronically connected to the first semiconductor layer  511 , and it may be formed of any one of Al, Ti, Cr, Ni, Pd, Ag, Ge, and Au, or an alloy thereof. For ohmic contacts between the first electrode  516   a  and the first semiconductor layer  511 , an electronic conductive oxide such as indium tin oxide (ITO) and ZnO may be used. 
     The second electrode  516   b  is electronically connected to the second semiconductor layer  513 , and it may be formed of any one of Al, Ti, Cr, Ni, Pd, Ag, Ge, and Au, or an alloy thereof. For ohmic contacts between the second electrode  516   b  and the second semiconductor layer  513 , an electronic conductive oxide such as indium tin oxide (ITO) and ZnO may be used. 
     The micro LED may be a blue micro LED having a light emission wavelength of 450-490 nm or a UV micro LED having a light emission wavelength of 360-410 nm including the first semiconductor layer  511 , the second semiconductor layer  513 , the active layer  515 , the first LED electrode  516   a , and the second LED electrode  516   b  described above. In this case, the blue and UV micro LEDs are an example, and they may be a semiconductor based on AlInGaN. 
     The wavelength conversion layer  518  may include poly-crystalline films of chalcopyrite-structure semiconductors. Here, “a chalcopyrite-structure” means a crystalline structure. A chalcopyrite-structure semiconductor may be described as a I-III-VI 2  compound in general, and here, I, III, and VI indicate element groups of the periodic table. 
     The chemical formula of the wavelength conversion layer  518  may be basically described as XYZ 2 . Here, X includes group I elements such as Ag and Cu, Y includes group III elements such as In, Ga, and Al, and Z includes group VI elements such as Se and S. 
     The wavelength conversion layer  518  is excited by the blue micro LED or the UV micro LED, and emits a red color. The wavelength conversion layer  518  may have a thickness necessary for absorbing an excitation light, for example, a thin thickness of about 2 µm at the maximum. Accordingly, the thickness of the wavelength conversion layer  518  may be thinner than the total sum of the thicknesses of the first and second semiconductor layers  511 ,  513  and the active layer  515 . 
     For example, if the total sum of the thicknesses of the first and second semiconductor layers  511 ,  513  and the active layer  515  of the blue micro LED or the UV micro LED is 4 µm, the thickness of the red wavelength conversion layer  518  may be 2 µm at the maximum. 
     Also, in the first micro LED  51  according to an embodiment of the disclosure, an excitation light from the active layer  515  of the blue micro LED or the UV micro LED may be totally absorbed at the wavelength conversion layer  518 , and thus a separate color filter for blocking the remaining excitation light is not needed. 
     The operation of the first micro LED  51  emitting a red light according to an embodiment of the disclosure is as follows. 
     When a current is applied to the blue micro LED from the TFT circuit through the first and second electrodes  516   a ,  516   b , radiative recombination of the carrier is generated at the active layer  515 . Accordingly, a light corresponding to the band gap energy is emitted from the active layer  515 . 
     The light from the blue micro LED excites the carrier on the wavelength conversion layer  518 . Then, the radiative recombination of the excited carrier is continued to radiation from the wavelength conversion layer  518 . For example, the wavelength conversion layer  518  may emit a red light by emission of a blue light emitted from the blue micro LED located in the lower part of the wavelength conversion layer  518 . 
       FIG.  3 B  is a schematic diagram illustrating a micro LED having passivation layers respectively on both surfaces of a red wavelength conversion layer according to another embodiment of the disclosure. 
     Referring to  FIG.  3 B , in the first micro LED  51 ′, for improving the light emitting efficiency of the red wavelength conversion layer  518 , first and second passivation layers  519   a ,  519   b  may be formed in a stack respectively on the upper surface and the lower surface of the wavelength conversion layer  518 . 
     The first and second passivation layers  519   a ,  519   b  may be formed of any one material among ZnS, (Mg)ZnO, Al 2 O 3 , SiNx, SiON, and SiO 2 . The first and second passivation layers  519   a ,  519   b  may repress surface recombination of the carrier excited on the red wavelength conversion layer  518 . The thickness of each of the first and second passivation layers  519   a ,  519   b  may be in a range of from 10 nm to 1000 nm. 
       FIG.  4    is a schematic diagram illustrating a micro LED having a green wavelength conversion layer according to an embodiment of the disclosure. 
     Referring to  FIG.  4   , the second micro LED  52  emitting a green light may be formed of the same structure as the aforementioned first micro LED  51  emitting a red light. However, the second micro LED  52  is different from the first micro LED  51  in that it includes a wavelength conversion layer  528  emitting a green light. 
     The second micro LED  52  may include a first semiconductor layer  521 , a second semiconductor layer  523 , an active layer  525  formed between the first and second semiconductor layers, a first LED electrode  526   a  formed on the first semiconductor layer, a second LED electrode  526   b  formed on the second semiconductor layer, and a wavelength conversion layer  528  formed in a stack on the light emission surface  521   a  of the second semiconductor layer. 
     The operation of the second micro LED  52  emitting a green light according to an embodiment of the disclosure is as follows. 
     When a current is applied to the blue micro LED from the TFT circuit through the first and second electrodes  526   a ,  526   b , radiative recombination of the carrier is generated at the active layer  525 . Accordingly, a light corresponding to the band gap energy is emitted from the active layer  525 . 
     The light from the blue micro LED excites the carrier on the wavelength conversion layer  528 . Then, the radiative recombination of the excited carrier is continued to radiation from the wavelength conversion layer  528 . For example, the wavelength conversion layer  528  may emit a green light by emission of a blue light emitted from the blue micro LED located in the lower part of the wavelength conversion layer  528 . 
       FIG.  5    is a schematic diagram illustrating a micro LED having a blue wavelength conversion layer according to an embodiment of the disclosure. 
     Referring to  FIG.  5   , the third micro LED  53  emitting a blue light may be formed of the same structure as the aforementioned first micro LED  51  emitting a red light. However, the third micro LED  53  is different from the first micro LED  51  in that it includes a wavelength conversion layer  538  emitting a blue light, and uses a UV micro LED instead of a blue micro LED. 
     The third micro LED  53  may include a first semiconductor layer  531 , a second semiconductor layer  533 , an active layer  535  formed between the first and second semiconductor layers, a first LED electrode  536   a  formed on the first semiconductor layer, a second LED electrode  536   b  formed on the second semiconductor layer, and a wavelength conversion layer  538  formed in a stack on the light emission surface  531   a  of the second semiconductor layer. 
     The operation of the third micro LED  53  emitting a blue light according to an embodiment of the disclosure is as follows. 
     When a current is applied to the blue micro LED from the TFT circuit through the first and second electrodes  536   a ,  536   b , radiative recombination of the carrier is generated at the active layer  535 . Accordingly, a light corresponding to the band gap energy is emitted from the active layer  535 . 
     The light from the UV micro LED excites the carrier on the wavelength conversion layer  538 . Then, the radiative recombination of the excited carrier is continued to radiation from the wavelength conversion layer  538 . For example, the wavelength conversion layer  538  may emit a blue light by emission of a UV light emitted from the UV micro LED located in the lower part of the wavelength conversion layer  538 . In this case, the wavelength conversion layer  538  may only be excited by the UV micro LED due to the energy requirement for transition between bands. 
       FIG.  6    is a graph illustrating a relationship between a lattice constant and a corresponding wavelength calculated from band gap energy of a wavelength conversion layer in a chalcopyrite-structure. 
     Referring to  FIG.  6   , the wavelength conversion layer in a chalcopyrite-structure provided in the micro LED according to an embodiment of the disclosure may cover a wide wavelength range reaching UV rays from infrared rays by changing chemical components. 
     As the wavelength conversion layer adjusts the composition of X, Y, and Z to form a display application, the band gap may correspond to necessary light emission wavelength areas, i.e., red (600-750 nm), green (520-560 nm), and blue (450-490 nm). Additional elements such as Zn, Sn, Si, and Te may be doped on the wavelength conversion material for improving the material characteristic. 
     The wavelength conversion layer in a chalcopyrite-structure formed of a I-III-VI 2  compound has the following characteristic. 
     The wavelength conversion layer has a direct transition-type band structure. Such a characteristic indicates that the wavelength conversion layer has high possibility of emitting a strong light. 
     Also, the wavelength conversion layer has a big absorption coefficient (-10 5  cm -1 ). Such a characteristic indicates that the thickness necessary for absorbing an excitation light may have a maximum width of about 2 µm. Accordingly, the film thickness of the wavelength conversion layer including the chalcopyrite-structure semiconductor may be thinner than the total thickness of the first and second semiconductor layers and the active layer. For example, while the total thickness of the first and second semiconductor layers and the active layer of the blue micro LED (or the UV micro LED) is 4 µm, the thickness of the wavelength conversion layer may be about 2 µm. Also, in the micro LED device according to an embodiment of the disclosure, an excitation light from the active layer of the blue micro LED (or the UV micro LED) can be totally absorbed at the wavelength conversion layer, and thus a color filter is not needed for blocking the remaining excitation light. 
     In addition, the wavelength conversion layer has low sensitivity of electronic and optical characteristics regarding crystalline defects. Such a characteristic indicates possibility that the wavelength conversion layer has high performance in a polycrystalline form. Accordingly, the wavelength conversion layer based on the I-III-VI 2  compound may be formed of a thin polycrystalline film, and it can operate as a wavelength conversion layer having high performance without a color filter. 
       FIG.  7    is a flow chart illustrating a manufacturing process of a micro LED according to an embodiment of the disclosure, and  FIG.  8 A  to  FIG.  8 G  are process diagrams sequentially illustrating a manufacturing process of a micro LED according to an embodiment of the disclosure. 
     Referring to  FIG.  8 A , on the wafer  100 , for example, a stacked semiconductor epitaxial structure for forming a blue micro LED through an epitaxial process is formed in operation S 21  according to an embodiment of the disclosure. 
     A blue micro LED is used as an example, but the structure stacked on the wafer is not limited to a structure for forming a blue micro LED, and may instead be, for example, a structure for forming a UV micro LED. 
     The semiconductor epitaxial structure having a stacking order of the first semiconductor layer  110 , the active layer  120 , and the second semiconductor layer  130  is formed on the wafer  100  through epitaxial growing technologies such as metalorganic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), and sputtering. The wafer  100  is generally a monocrystalline bulk wafer of sapphire, Si, SiC, GaAs, GaP, and GaN. 
     The first semiconductor layer  110  is a conductivity type semiconductor layer, and it may be an n-type semiconductor layer, and the second semiconductor layer  130  is a conductivity type semiconductor layer, and it may be a p-type semiconductor layer. In contrast, the first semiconductor layer  110  may be a p-type semiconductor layer, and the second semiconductor layer  130  may be an n-type semiconductor layer. 
     Referring to  FIG.  8 B , a mesa etching processing is performed on the semiconductor epitaxial structure in operation S 22  according to an embodiment of the disclosure. 
     The mesa etching process may be performed, for example, by photolithography and a dry etching process. Accordingly, in the semiconductor epitaxial structure, a plurality of blue micro LEDs may be independently formed respectively at a specific interval. 
     In this case, one surface of the first semiconductor layer  110  (the surface contacting the wafer  100 ) is used as a light emission surface. 
     Also, a first electrode  151  is formed on the first semiconductor layer  110 , and a second electrode  153  is formed on the second semiconductor layer  130  in operation S 23 . 
     The first and second electrodes  151 ,  153  may be formed, for example, by a lift-off process, and here, for ohmic contacts between the first electrode  151  and the first semiconductor layer  110  and between the second electrode  153  and the second semiconductor layer  130 , an electronic conductive oxide such as indium tin oxide (ITO) and ZnO may be used. Such ohmic substances may be deposited on the first and second semiconductor layers  110 ,  130  by various technologies such as sputtering, evaporation, and spin coating. 
     Herein, a structure including the first semiconductor layer  110 , the active layer  120 , the second semiconductor layer  130 , and the first and second electrodes  151 ,  153  is referred to as a blue micro LED  200 . 
     Referring to  FIG.  8 C , a passivation layer  160  is formed on the surface of the blue micro LED  200  in operation S 24  according to an embodiment of the disclosure. 
     In this case, the passivation layer  160  is formed so that it does not cover the first and second electrodes  151 ,  153  when it is formed in a stack on the surface of the blue micro LED  200 . 
     The passivation layer  160  may be formed of substances such as Al 2 O 3 , SiO 2 , and SiN. The passivation layer may be formed by various technologies such as atomic layer deposition, e-beam deposition, sputtering, chemical vapor deposition, and spin coating. 
     Although not illustrated in the drawings, after the passivation layer  160  is formed, a mirror layer such as distributed Bragg reflectors (DBR) may be formed in a stack on an outer surface the passivation layer for improving light extracting efficiency of micro LEDs. 
     Referring to  FIG.  8 D , the carrier substrate  260  is moved to the side of the wafer  100 , and one surface of the carrier substrate  260  is attached to the plurality of blue micro LEDs  200  in operation S 25  according to an embodiment of the disclosure. 
     The carrier substrate  260  is a substrate for transferring the plurality of micro LEDs  210  (refer to  FIG.  8 G ) to be separated from the wafer  100  to the TFT substrate  30 . 
     On one surface of the carrier substrate  260 , an adhesion layer  270  is formed. The plurality of blue micro LEDs  200  may achieve a stable adhered state to the carrier substrate  260  by the adhesion layer  270 . 
     The adhesion layer  270  may also be referred to as a dynamic release layer (DRL), and may be formed of a material that can be easily separated from the carrier substrate  260  (e.g., polyimide) when transferring the micro LEDs  210  (refer to  FIG.  8 G ) to be described below to the TFT layer  33  of the TFT substrate  30  (refer to  FIG.  2   ) by a transfer method using laser. The laser transfer method is a method of, for example, locating the carrier substrate  260  on the upper side of the TFT substrate  30  at a specific interval, and then heating a portion of the adhesion layer  270  to which the micro LEDs  210  are fixed by irradiating a laser beam on the carrier substrate, and thereby separating the micro LEDs  210  from the carrier substrate  260  and transferring them to the TFT substrate  30 . 
     Referring to  FIG.  8 E  and  FIG.  8 F , the plurality of blue micro LEDs  200  attached to the carrier substrate  260  and the wafer  100  are separated by irradiating a laser beam on the rear surface of the wafer  100  in operation S 25  according to an embodiment of the disclosure. 
     The wafer  100  is separated from the plurality of blue micro LEDs  200  by technologies such as pulverizing, wet etching, dry etching, and laser lift off (LLO). The plurality of blue micro LEDs  200  are supported by the carrier substrate  260  having the adhesion layer  270  on the surface during the process of being separated from the wafer  100 . 
     Referring to  FIG.  8 G , in the state of having been separated from the wafer  100 , a wavelength conversion layer  170  is formed in a stack on the light emission surface  130   a  of the second semiconductor layer  130  of each blue micro LED  200  in operation S 26  according to an embodiment of the disclosure. 
     As described above, the micro LED  210  according to an embodiment of the disclosure may include a blue micro LED  200  and a blue micro wavelength conversion layer  170 . 
     The wavelength conversion layer  170  may be formed of a semiconductor polycrystalline thin film in a chalcopyrite-structure. In this case, the wavelength conversion layer  170  may be, for example, deposited as a thin film on the light emission surface  130   a  of the second semiconductor layer  130  of the blue micro LED  200  by technologies such as sputtering and coevaporation. 
     The composition of the wavelength conversion layer  170  may be adjusted to suit the wavelength required by the display application. For example, the composition of the wavelength conversion layer  170  is adjusted by the ratio of supplied element source substances such as Cu, Ag, In, Ga, Al, S, and Se. 
     Referring to  FIG.  2    again, the plurality of micro LEDs  51 ,  52 ,  53  may be connected to the TFT circuit of the TFT substrate  30  and/or a driver circuit such as an Si-based integrated circuit through a transfer process. Here, the transfer process may be, for example, laser-based transfer, stamp-based transfer, roll transfer, etc. 
     The first electrodes  51   a ,  52   a ,  53   a  and the second electrodes  51   b ,  52   b ,  53   b  of the respective micro LEDs are electronically connected to the first TFT electrodes  35   a ,  36   a ,  37   a  and the second TFT electrodes  35   b ,  36   b ,  37   b  of the TFT layer  33 . Accordingly, the respective micro LEDs  51 ,  52 ,  53  become self-luminous sub pixels of the display module  10 , and form one pixel. 
     One pixel may include three sub pixels R/G/B. Hereinafter, various combinations of three sub pixels will be described with reference to  FIG.  9    to  FIG.  11   . 
       FIG.  9    is a side view schematically illustrating a pixel of a display module according to an embodiment of the disclosure. In  FIGS.  9 ,  10   a    is a display module,  300  is a TFT substrate,  310  is a glass substrate, and  330  is a TFT layer. 
     Referring to  FIG.  9   , a pixel formed on the display module  10   a  may include a red sub pixel  351  emitting a red light, a green sub pixel  352  emitting a green light, and a blue sub pixel  353  emitting a blue light. 
     The red sub pixel  351  may include a first blue micro LED  351   a , and a red wavelength conversion layer  351   b  formed as a thin film on the light emission surface of the first blue micro LED  351   a . In the red sub pixel  351 , the red wavelength conversion layer  351   b  is excited by a blue light emitted from the first blue micro LED  351   a , and emits a red light. 
     The green sub pixel  352  may include a second blue micro LED  352   a , and a green wavelength conversion layer  352   b  formed as a thin film on the light emission surface of the second blue micro LED  352   a . In the green sub pixel  352 , the green wavelength conversion layer  352   b  is excited by a blue light emitted from the second blue micro LED  352   a , and emits a green light. 
     The blue sub pixel  353  may include a third blue micro LED  353  emitting a blue light, without a wavelength conversion layer. 
     As described above, in a combination of R/G/B sub pixels, the red and green sub pixels  351 ,  352  respectively have a red wavelength conversion layer  351   b  and a green wavelength conversion layer  352   b , but the blue sub pixel  353  may be formed without a blue wavelength conversion layer. 
       FIG.  10    is a side view schematically illustrating another example of a pixel of a display module according to another embodiment of the disclosure. In  FIGS.  10 ,  10   b    is a display module,  1300  is a TFT substrate,  1310  is a glass substrate, and  1330  is a TFT layer. 
     Referring to  FIG.  10   , a pixel formed on the display module  10   b  may include a red sub pixel  1351  emitting a red light, a green sub pixel  1352  emitting a green light, and a blue sub pixel  1353  emitting a blue light. 
     The red sub pixel  1351  may include a first blue micro LED  1351   a , and a red wavelength conversion layer  1351   b  formed as a thin film on the light emission surface of the first blue micro LED  1351   a . In the red sub pixel  1351 , the red wavelength conversion layer  1351   b  is excited by a blue light emitted from the first blue micro LED  1351   a , and emits a red light. 
     The green sub pixel  1352  may include a green micro LED  1352  emitting a green light, without a wavelength conversion layer. 
     The blue sub pixel  1353  may include a blue micro LED  1353  emitting a blue light, without a wavelength conversion layer. 
     As described above, in a combination of R/G/B sub pixels, the red sub pixel  1351  has a red wavelength conversion layer  1351   b , but the green and blue sub pixels  1352 ,  1353  may be formed without a blue wavelength conversion layer. 
       FIG.  11    is a side view schematically illustrating a pixel in a micro LED display according to still another embodiment of the disclosure. In  FIGS.  11 ,  10   c    is a display module,  2300  is a TFT substrate,  2310  is a glass substrate, and  2330  is a TFT layer. 
     Referring to  FIG.  11   , a pixel formed on the display module  10   c  may include a red sub pixel  2351  emitting a red light, a green sub pixel  2352  emitting a green light, and a blue sub pixel  2353  emitting a blue light. 
     The red sub pixel  2351  may include a first UV micro LED  2351   a , and a red wavelength conversion layer  2351   b  formed as a thin film on the light emission surface of the first UV micro LED  2351   a . In the red sub pixel  2351 , the red wavelength conversion layer  2351   b  is excited by a UV light emitted from the first UV micro LED  2351   a , and emits a red light. 
     The green sub pixel  2352  may include a second UV micro LED  2352   a , and a green wavelength conversion layer  2352   b  formed as a thin film on the light emission surface of the second UV micro LED  2352   a . In the green sub pixel  2352 , the green wavelength conversion layer  2352   b  is excited by a UV light emitted from the second UV micro LED  2352   a , and emits a green light. 
     The blue sub pixel  2353  may include a third UV micro LED  2353   a , and a blue wavelength conversion layer  2353   b  formed as a thin film on the light emission surface of the third UV micro LED  2353   a . In the blue sub pixel  2353 , the blue wavelength conversion layer  2353   b  is excited by a UV light emitted from the third UV micro LED  2353   a , and emits a blue light. 
     As described above, in a combination of R/G/B sub pixels, the red and green sub pixels  2351 ,  2352  respectively have a red wavelength conversion layer  2351   b  and a green wavelength conversion layer  2352   b , but the blue sub pixel  2353  may be formed without a blue wavelength conversion layer. 
     The red and green wavelength conversion layers  2351   b ,  2352   b  may be excited by the blue micro LED and the UV micro LED. However, the blue wavelength conversion layer  2353   b  may only be excited by the UV micro LED due to the energy requirement for transition between bands. 
     As described above, the micro LED may include a wavelength conversion layer, which may be a semiconductor layer in a chalcopyrite-structure based on the I-III-VI 2  compound. The thickness of the wavelength conversion layer is formed to be thinner than the total thickness of the blue micro LED (the first and second semiconductor layers and the active layer). 
     Accordingly, the micro LED according to an embodiment of the disclosure can have a simple structure and an efficient light emitting capacity. For example, in the micro LED according to an embodiment of the disclosure, an encapsulated structure required in an LED having a phosphor according to the conventional technology is not needed, a separate cover layer that makes a wavelength conversion layer become far from the micro LED does not need to be formed, and a color filter layer making only light from a wavelength conversion layer pass through does not need to be formed. 
     The micro LED according to an embodiment of the disclosure exerts higher efficiency compared to a conventional red micro LED, as shown in the graph of  FIG.  12   . 
     In  FIG.  12   , the dotted line is a curved line of a conventional red micro LED based on AlGaInP, and the solid line is a curved line of a blue micro LED including a red wavelength conversion layer. It can be seen that the efficiency of a low current operation area (0.1 - 10 A/cm 2 ) generally used in a micro LED operation in a display module has been greatly improved compared to a conventional red micro LED. 
     Also, in a structure including a blue micro LED and a red wavelength conversion layer stacked thereon, thermal stability in the strength of a red light can be improved. 
     As described above, a display module to which micro LEDs according an embodiment of to the disclosure are applied uses micro LEDs that are highly efficient and thermally stable as components forming sub pixels, and thus the display module can have high color reproducibility, low power consumption, a high dynamic range, and a high contrast. 
     While preferred embodiments of the disclosure have been shown and described, the disclosure is not limited to the aforementioned specific embodiments, and it is apparent that various modifications may be made by those having ordinary skill in the technical field to which the disclosure belongs, without departing from the gist of the disclosure as claimed by the appended claims. Further, it is intended that such modifications are not to be interpreted independently from the technical idea or prospect of the disclosure. 
     The disclosure relates to a micro LED and a display module including the same.