Patent Publication Number: US-2004041508-A1

Title: Electrode and device using the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Technical Field  
       [0002] The present invention relates to an electrode to inject and emit carrier effectively and a device using the same.  
       [0003] 2. Background Art  
       [0004] A cold cathode can be applied to a field emission display, electron beam exposure, microwave traveling wave tube, image pickup device and so on. It can be also used as an electrode source of a material evaluation device such as an Auger electron spectroscopy using electron beam. Further, it can be used as a light-emitting element for an illumination device or an indicator lamp and other varied applications.  
       [0005] With regard to a cold cathode, an electron-emitting device called spint-type forming a spire using a metal or silicon has been researched and developed. However, low voltage operation, high current operation and reliability of a device have been required in the applications shown above. Under such circumstances, improvement of characteristics of a spint-type cold cathode and new materials for cold cathodes have been researched and developed. Diamonds, aluminum nitride, boron nitride have become a focus of attention as one of materials having negative electron affinity. Recently, syntheses of materials such as carbon nanotubes or carbon nanofibers that can enlarge the electric field concentration factors have been improved significantly and electron emissions at lower voltage have been observed. Applications for a field emission display are expected. However, there were problems about spatial stability in electron emission characteristics of these carbon nanotubes or carbon nanofiber. Further, low voltage operation and high current operation are required.  
       [0006] Moreover, even in an organic light-emitting element which has been developed recently, a problem about carrier injection still remains. Therefore, further improvement in performance is required.  
       OBJECT AND GENERAL DESCRIPTION OF THE INVENTION  
       [0007] Under such circumstances, with regard to improvement of characteristics of spint-type cold cathode, coatings of varied materials on surfaces have been discussed. Moreover, coating technologies have become a focus of attention in order to improve spatial stability in electron emissions from carbon nanotubes or carbon nanofibers. Although, several trials have been carried out so far, it is required to realize more excellent electron emission characteristics, it is required to improve the carrier injection efficiency in development of organic light-emitting element.  
       [0008] An object of the present invention is to provide an electrode to realize more effective emission and injection of carriers than before, regarding to the circumstances mentioned above.  
       [0009] In order to achieve the object mentioned above, an electrode of the present invention has a film on a conductive material to supply carriers and the film includes space charge. When emitting or injecting electrons with an electrode of the present invention, positive space charges are used in the film. When electron holes are injected, negative space charges are used in the film. Higher density of space charge is more preferable. Density of 1×10 17  cm −3  or more is effective.  
       [0010] In addition, metals, semiconductors and graphite can be used for the conductive material.  
       [0011] Further, a surface of the conductive material is characterized by having irregularities or spires. This type of surface can enhance the electric field strength on the surface and efficiency to inject carrier into a film including space charge.  
       [0012] Moreover, a surface of a conductive material is characterized by having indeterminate form or fibrous metals, and by using semiconductors or graphite. Metal flakes, fibers, and carbon nanotubes can be used to enhance the electric field strength on a surface. As mentioned above, it is possible to enhance efficiency to inject carriers into film including space charges.  
       [0013] The film is characterized by including any one of amorphous, crystal grain boundary or impurity atoms.  
       [0014] The film is also characterized by having a thickness of 50 nm or less. A thickness of 10 nm or less shows significant effect and the thinner a film becomes, the higher the effect is. However, 5 to 8 nm is preferable when considering manufacturing process.  
       [0015] An electron-emitting device according to the present invention comprises the electrode as a cathode.  
       [0016] If an electron-emitting device according to the present invention is used for a field emission display, low voltage operation and clear images can be realized.  
       [0017] If an electron-emitting device according to the present invention is used for an electron beam exposure, an electron beam exposure with high resolution and enhanced throughput can be realized.  
       [0018] If an electron-emitting device according to the present invention is used for a microwave traveling-wave tube, high-power microwave output can be obtained.  
       [0019] If an electron-emitting device according to the present invention is used for an image pickup device, clear images can be realized.  
       [0020] If an electron-emitting device according to the present invention is used for an electron beam source of a material evaluation device, it is possible to expect enhanced evaluation accuracy.  
       [0021] In addition, it is characterized in that an electrode according to the present invention can be used for an electrode of a light-emitting element. If an electrode according to the present invention is used for a light-emitting element, vivid emission with high-luminance can be obtained as well as superior illumination and display can be realized.  
       [0022] If a light-emitting element using an electrode according to the present invention is used for a backlight of a liquid crystal display, a liquid crystal display with high-luminance and less power consumption can be realized.  
       [0023] In addition, a plasma display according to the present invention is characterized by using the electrode as an electrode of a discharge cell in.  
       [0024] An organic light-emitting device according to the present invention comprises the electrode. If an electrode according to the present invention is used in an organic light-emitting device, vivid emission with high luminance and a high quality display device can be realized. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0025]FIG. 1 is a cross-sectional view of a first embodiment of an electron-emitting device according to the present invention;  
     [0026]FIG. 2 is a cross-sectional view of a second embodiment of an electron-emitting device according to the present invention;  
     [0027]FIG. 3 is a cross-sectional view of a third embodiment of an electron-emitting device according to the present invention;  
     [0028]FIG. 4 is a cross-sectional view of a fourth embodiment of an electron-emitting device according to the present invention;  
     [0029]FIG. 5 is a cross-sectional view of a fifth embodiment of a light-emitting element according to the present invention;  
     [0030]FIG. 6 is a cross-sectional view of a sixth embodiment of an organic light-emitting element according to the present invention. 
    
    
     EXPLANATION OF REFERENCES  
     [0031] 21 ,  31 ,  41 ,  51  Substrate  
     [0032] 2 ,  22 ,  32 ,  42 ,  52  Film  
     [0033] 23 ,  33 ,  43  SiOx Film  
     [0034] 24 ,  34 ,  44 ,  54  Extraction electrode  
     [0035] 5 ,  25 ,  35 ,  45 ,  55  Anode electrode  
     [0036] 6 ,  26 ,  36 ,  7 ,  27 ,  37 ,  46 ,  47  Power source  
     [0037] 8 ,  28 ,  38 ,  58  Cathode electrode  
     [0038] 29  Spire part  
     [0039] 30  Gallium nitride layer  
     [0040] 40 ,  50  carbon nanotube or carbon nanofiber  
     [0041] 510  Fluorescent material  
     [0042] 511  Glass tube  
     [0043] 61  Glass substrate  
     [0044] 62  Anode  
     [0045] 63  Hole transporting layer  
     [0046] 64  Emitting layer  
     [0047] 65  Cathode  
     [0048] 66  Boron nitride thin film  
     [0049] 67  Metal  
     DETAILED DESCRIPTION OF BEST MODE FOR THE INVENTION  
     [0050] Embodiments of the present invention will be explained now. The electrode according to the present invention is composed of a film, having a thickness of 50 nm or less, which has space charge corresponding to the present invention on a conductive material surface, and the surface having a carbon nanotube or a carbon nanofiber formed on a conductive material. The use of an electrode of the present invention as a cathode gives an effect on improvement of the characteristic and the reliability of the conventional electron-emitting device. Moreover, it becomes possible to provide a material evaluation device of materials with a field emission display, an electron beam exposure, a microwave traveling-wave tube, an image pickup device and an electron beam by using the electron-emitting device of the present invention. Further, providing a light-emitting device and a high-efficiency organic light-emitting device becomes possible by the use of the electrode according to the present invention.  
     [0051] Embodiments  
     [0052] Specific embodiments of the electron-emitting device, the light-emitting device, and the organic light-emitting device according to the present invention will be explained now.  
     [0053] (Embodiment 1)  
     [0054]FIG. 1 is a schematic cross-sectional view of a first embodiment of an electron-emitting device according to the present invention. An electron-emitting device of embodiment 1 is composed of a substrate  1 , a boron nitride thin film  2 , a SiOx film  3 , an extraction electrode  4 , an anode electrode  5 , a power source  6 ,  7 , a cathode electrode  8 .  
     [0055] In this embodiment, silicon was used for the substrate  1 . On the substrate, 10 nm of the boron nitride thin film  2  was deposited by the plasma chemical vapor deposition (CVD) method using boron trichloride and nitrogen gas. Next, sulfur atoms were added to the boron nitride thin film  2  by concentration of 1×10 18  cm −3 . Further, 800 nm of the SiOx thin film  3  and Ti (20 nm)/Au (500 nm) as a metal for the extraction electrode  4  were formed on the boron nitride thin film  2  by the electron-beam evaporation method. Still further, AL (500 nm) as the cathode electrode  8  was electron-beam evaporated on the backside of the silicon substrate  1 . After that, a metal for the extraction electrode  4  and the SiOx thin film  3  were removed by etching in the photolithography process to form a window with a diameter of 5 μm. After a surface of the boron nitride thin film  2  exposed in the window was processed with hydrogen plasma, in a vacuum chamber, a metal plate to be the anode electrode  5  was made to oppose to the boron nitride thin film  2  with a distance of 125 μm. The extraction electrode  4  was grounded, the cathode electrode  8  and the anode electrode  5  were applied with bias respectively, and an emission current was measured at a vacuum degree of 8×10 −7  Torr or less. The anode voltage was stabilized at 500V and the cathode voltage was changed. Electron emission started by impressing 10V to the cathode electrode  8 . High emission current of 0.1 mA was obtained by impressing 30V.  
     [0056] Electron emission characteristics were researched, and further, roughness of a film surface was evaluated by depositing the boron nitride thin film with a thickness of 10 nm on a flat silicon substrate by above mentioned method, without preparing the extraction electrode  4 , and making the distance between the boron nitride thin film and the anode electrode  5  stabilized at 125 μm. The flat silicon substrate surface was evaluated to have a surface roughness of 0.3 to 0.7 nm while the boron nitride film with a thickness of 10 nm was evaluated to have a surface roughness of 0.6 to 1.2 nm. Assume an electric field concentration factor being 1 on the flat silicon substrate, and the electron affinity of silicon (4.05 eV) being comparable to a surface potential. Comparing to the assumption, when the boron nitride with a thickness of 10 nm is used, the electric field concentration factor is evaluated as 10—actually, it is overestimated—the height of the potential barrier is estimated about 0.6 eV. Thus, a significant and practical reduce of the height of the potential barrier becomes possible based on the present invention, and the reduction of an electron emission threshold electric field can be expected.  
     [0057] Introduction of a film according to the present invention other than a boron nitride film can reduce an effective potential barrier height and improve electron emission characteristics. In this embodiment, a boron nitride film was used, however, it is possible to use all materials according to the present invention other than boron nitride. In the embodiment, a boron nitride film was synthesized by the plasma assist CVD method. However, varied preparation method such as metal organic chemical vapor deposition (MOCVD) method, molecular beam epitaxial (MBE) method, sputtering method may be used.  
     [0058] In the present embodiment, the boron nitride thin film  2  added with sulfur impurities was used, however, a boron nitride thin film  3  added with atoms such as lithium, oxygen, silicon to be donor impurities may be also used. The same impurities may be used for compounds composed of group 3 and nitride atoms other than above mentioned boron nitride.  
     [0059] In this embodiment, silicon was used as material of a substrate. However, a substrate may be made by using varied types of conductors and semiconductors such as other metals, gallium arsenide, indium phosphorus, silicon carbide, gallium nitride. In this embodiment, Ti/Au was used for a metal for the extraction electrode  4 . However, Cr instead of Ti, or various metals instead of Au may be also used. If a semiconductor substrate is used, any metals that can form an Ohmic electrode may be used as a metal for the cathode electrode  8 . If a conductor substrate is used, a substrate itself may be used as a cathode electrode.  
     [0060] (Embodiment 2)  
     [0061]FIG. 2 is a schematic cross-sectional view of a second embodiment of an electron-emitting device according to the present invention. The electron-emitting device formed a spint-type spire shape on the silicon substrate  1  provided with the boron nitride carbon film of the present invention is composed of a substrate  21 , a boron nitride carbon thin film  22 , a SiOx film  23 , an extraction electrode  24 , an anode electrode  25 , a power source  26 ,  27 , a cathode electrode  28  and a spire shape  29 .  
     [0062] The boron nitride carbon thin film  22  according to the present invention is formed at the spire shape  29  using an n-type silicon substrate  1  (111) on which the spire shape part  29  having the electrode  24 . A 10 nm of the boron nitride carbon thin film  22  (composition ratio, boron 0.4, carbon 0.2, nitrogen 0.4) was deposited using boron trichloride, methane and nitrogen gas by the plasma assist chemical vapor deposition method. Sulfur atoms were added to the boron nitride carbon thin film  22  to make a concentration of 1×10 18  cm −3 . Al (500 nm) as the cathode electrode  28  was electron-beam evaporated on the backside of the silicon substrate  1 . After processing a surface of the boron nitride carbon thin film  22  with hydrogen plasma, in a vacuum chamber, a metal plate to be the anode electrode  25  was made to oppose to the spire shape part  29  having the boron nitride carbon thin film  22  at a distance of 125 μm. Extraction electrode  24  was grounded, the cathode electrode  28  and the anode electrode  25  were applied with bias respectively, and an emission current was measured at a vacuum degree of 8×10 −7  Torr or less. The anode voltage was stabilized at 500V and the cathode voltage was changed. A high emission current of 0.1 mA was obtained by impressing 20V to the cathode electrode  28 .  
     [0063] In this embodiment, a boron nitride carbon thin film was used, however, other materials mentioned above such as boron nitride may be used.  
     [0064] (Embodiment 3)  
     [0065]FIG. 3 is a schematic cross-sectional view of a third embodiment of an electron-emitting device according to the present invention. An electron-emitting device of embodiment 3 is composed of a substrate  31  onto which an n-type gallium nitride layer  30  is formed, boron nitride carbon thin film  32 , SiOx film  33 , extraction electrode  34 , anode electrode  35 , power source  36 ,  37 , cathode electrode  38 .  
     [0066] A wafer wherein the n-type gallium nitride layer  30  added silicon was grown by 1 μm on the n-type silicon substrate  31  (111) by the metal organic chemical vapor deposition was used as a substrate. Hydrogen plasma is generated by microwave to process a surface of the gallium nitride layer  30 . Processing was performed for five minutes by setting a microwave output to 300W, hydrogen flow to 50 sccm and gas pressure to 40 Torr. A flat surface of the gallium nitride layer  30  changes into a surface with irregularities of several decades nm. A 10 nm of the boron nitride carbon thin film  32  (composition ratio, boron 0.4, carbon 0.2, nitrogen 0.4) was deposited using boron trichloride, methane and nitrogen gas by the plasma assist chemical vapor deposition method thereon. Sulfur atoms were added to the boron nitride carbon thin film  32  to make a concentration of 1×10 18  cm −3 . Then, 800 nm of the SiOx thin film  33  and Ti (20 nm)/Au (50 nm) as a metal for the extraction electrode  34  were formed with the electron-beam evaporation method on the boron nitride carbon thin film  32 . Further, Al (500 nm) as the cathode electrode  38  was electron-beam deposited on the backside of the silicon substrate  31 . After that, a metal for the extraction electrode  34  and the SiOx thin film  33  were removed by etching in the photolithography process to form a window with a diameter of 5 μm. After a surface of the boron nitride thin film  32  exposed in the window was processed with hydrogen plasma, in a vacuum chamber, a metal plate to be the anode electrode  35  was made to oppose to the boron nitride carbon thin film  32  with a distance of 125 μm. The extraction electrode  34  was grounded, bias was applied to the cathode electrode  38  and the anode electrode  35  respectively, and an emission current was measured at a vacuum degree of 8×10 −7  Torr or less. An anode voltage was stabilized at 500V and a cathode voltage was changed. A high emission current of 0.1 mA was obtained by impressing 30V to the cathode electrode  38 .  
     [0067] In this embodiment, irregularities were prepared on a surface by using hydrogen plasma processing. Gases including oxygen, chlorine or fluorine may be also used for gases to generate plasma for forming irregularities on a surface.  
     [0068] (Embodiment 4)  
     [0069]FIG. 4 is a schematic cross-sectional view of a fourth embodiment of an electron-emitting device according to the present invention. This is the electron-emitting device wherein a carbon nanofiber  40  and a boron nitride carbon thin film according to the present invention are formed on a metal substrate  41 , composed of a substrate  41 , a boron nitride carbon thin film  42 , a SiOx film  43 , an extraction electrode  44 , an anode electrode  45 , and a power source  46  and  47 .  
     [0070] The carbon nanofiber  40  was formed on the metal substrate  41 , on which the boron nitride carbon thin film  42  according to the present invention was formed. A 10 nm of the boron nitride carbon thin film  42  (composition ratio, boron 0.4, carbon 0.2 and nitrogen 0.4) was deposited using boron trichloride, methane and nitrogen gas by the plasma assist chemical vapor deposition method thereon. Sulfur atoms were added to the boron nitride carbon thin film  42  by concentration of 1×10 18  cm −3 . Then, 800 nm of the SiOx thin film  43  and Ti (20 nm)/Au (500 nm) as a metal for the extraction electrode  44  were formed with the electron beam deposition method on the boron nitride carbon thin film  42 . After that, a metal for the extraction electrode  44  and the SiOx thin film  43  were removed by etching in the photolithography process to form a window with a diameter of 5 μm. After a surface of the boron nitride thin film  42  exposed in the window was processed with hydrogen plasma, in a vacuum chamber, a metal plate to be the anode electrode  45  was made to oppose to the boron nitride carbon thin film  42  at a distance of 125 μm. The extraction electrode  44  was grounded, the metal substrate  41  was used as a cathode electrode, bias was applied to the metal substrate  41  and the anode electrode  45  respectively, and an emission current was measured at a vacuum degree of 8×10 −7  Torr or less. An anode voltage was stabilized at 500V and a cathode voltage was changed. A high emission current of 0.1 mA was obtained by impressing 10V to the metal substrate  41 .  
     [0071] In embodiments 2 to 4, as materials of an electron emission part shown in embodiment 1, it is possible to use any one of compounds of group 3 atoms according to the present invention and nitrogen atoms, and oxides including nitrogen-boron-carbon, boron carbide, carbon nitride, boron. In embodiments 1 to 4, two or more electron emission parts may be prepared on a single substrate to realize an array.  
     [0072] (Embodiment 5)  
     [0073]FIG. 5 is a schematic cross-sectional view of a fifth embodiment of a light-emitting element using an electron-emitting device according to the present invention. This is a light-emitting element (lamp) wherein a carbon nanofiber  50  and boron nitride carbon thin film according to the present invention are formed on a metal substrate  51 , composed of a substrate  51 , a boron nitride carbon thin film  52 , an extraction an electrode  54 , an anode electrode  55 , a cathode electrode  58 , a fluorescent material  510 , and a glass tube  511 .  
     [0074] The carbon nanofiber  50  is made on the metal substrate  51 , on which the boron nitride carbon thin film  52  according to the present invention is formed. A 10 nm of the boron nitride carbon thin film  52  (composition ratio, boron 0.4, carbon 0.2 and nitrogen 0.4) was deposited using boron trichloride, methane and nitrogen gas by the plasma assist chemical vapor deposition method. Sulfur atoms were added to the boron nitride carbon thin film  52  by concentration of 1×10 18  cm −3 . The element was put into the glass tube  511  having the mesh extraction electrode  54  and the anode electrode  55  formed on the fluorescent material  510  and vacuum-sealed. By impressing 400V to the extraction electrode  54  against the cathode electrode  58  and 10 kV to the anode electrode  55 , a current of 500 μA was obtained and a light emission was observed.  
     [0075] (Embodiment 6)  
     [0076]FIG. 6 is a schematic cross-sectional view of a sixth embodiment of an organic light-emitting element using an electrode according to the present invention. An anode  62  using an ITO transparent electrode is formed on a glass substrate  61 , on which a hole transporting layer  63 , an emitting layer  64  are formed using an organic thin film. A cathode  65  is composed of a boron nitride thin film  66  and a metal (lithium or magnesium)  67  with a smaller work function. Using a cathode according to the present invention improves injection efficiency of electron and provides an organic light-emitting element with luminescence characteristics improved.  
     [0077] Effect of the Invention  
     [0078] As mentioned above, efficiency of emission and injection of carrier is improved with an electrode having a film including any one of atoms such as oxygen, nitrogen, carbon, silicon, boron that have space charge in a film according to the present invention. An electron-emitting device with an electrode according to the present invention enables operations with lower voltage and higher current. Those effects and reliability are improved by forming irregularities, amorphous forms, and fibrous substances on a surface of a conductive material. By this, a high-efficiency electron-emitting device is provided. It is efficient as a key device in a material evaluation device and light-emitting device using a display device, electron beam photolithography machine, image pickup device. Making an organic light-emitting device by using an electrode according to the present invention improves luminance and allows wide range of practical applications as a display unit.