Transparent and conductive zinc oxide film with low growth temperature

Abstract

The present invention relates to a novel zinc oxide thin film having hydrogen (H) and gallium (Ga) dopants. Advantageously, the activation temperature is low. The co-doped zinc oxide is highly conductive, transparent, chemically stable, easily deposited on a variety of substrates, including flexible or plastic substrates, and is well suited for electrical or optical applications. By co-doping with two impurities, both sides of the zinc oxide lattice contribute to the film conductivity resulting in high electron concentration and high mobility. The co-doped zinc oxide thin film has an increased Fermi level and a reduced work function that is less than 3 eV. The co-doped zinc oxide is crystal clear and transparent even when grown at relatively low processing temperatures. In another preferred embodiment of the present invention, a novel low-temperature activation indium tin oxide (ITO) thin film comprising tin oxide co-doped with indium (In) and hydrogen is disclosed.

I claim:  
     
         1 . A transparent, chemically stable, conductive wide band gap semiconductor material comprising an oxide having a first dopant and a second dopant, said oxide exhibiting free electrons in the conduction band.  
     
     
         2 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 1  wherein said oxide comprises zinc oxide, said first dopant comprises gallium and said second dopant comprises hydrogen.  
     
     
         3 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 1  wherein said first dopant comprises gallium.  
     
     
         4 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 3  wherein said second dopant comprises hydrogen.  
     
     
         5 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 4  wherein said oxide comprises zinc oxide.  
     
     
         6 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 4  wherein the concentration of said first dopant is between about 0.5 atomic wt % and about 2.0 atomic wt %.  
     
     
         7 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 6  wherein the concentration of said second dopant is about four times the concentration of said first dopant in said co-doped zinc oxide thin film.  
     
     
         8 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 4  wherein the concentration of said second dopant is between about one to about four times the concentration of said first dopant in said co-doped zinc oxide thin film.  
     
     
         9 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 1  wherein said oxide comprises indium oxide, said first dopant comprises tin and said second dopant comprises hydrogen.  
     
     
         10 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 9  wherein the concentration of said tin is between 0.5 atomic wt % and about 2.0 atomic wt % and the concentration of said hydrogen is about four times the concentration of said tin.  
     
     
         11 . The transparent, chemically stable, conductive wide band gap semiconductor material of  claim 9  wherein the concentration of said tin is between 0.5 atomic wt % and about 2.0 atomic wt % and the concentration of said hydrogen is between about one to four times the concentration of said tin.  
     
     
         12 . A transparent conductive wide band gap semiconductor material comprising a co-doped oxide having a first dopant and a second dopant, said co-doped oxide exhibiting free electrons in the conduction band and optical transparency sufficient to transmit at least 75% of incident light.  
     
     
         13 . The transparent conductive wide band gap semiconductor material of  claim 12  wherein said oxide has an electron concentration of at least 5×10 20  cm −3 .  
     
     
         14 . The transparent conductive wide band gap semiconductor material of  claim 12  wherein said oxide has a low work function.  
     
     
         15 . The transparent conductive wide band gap semiconductor material of  claim 12  wherein said co-doped oxide cathode has an optical transmission spectra absorption threshold below 350 nanometers whereby light having a wavelength below said absorption threshold is not transmitted through said transparent cathode.  
     
     
         16 . The transparent conductive wide band gap semiconductor material of  claim 12  wherein said co-doped oxide has an optical transmission spectra absorption threshold below 375 nanometers whereby light having a wavelength below said absorption threshold is not transmitted through said transparent cathode when said co-doped zinc oxide has been grown at a temperature below 50° C.  
     
     
         17 . The transparent conductive wide band gap semiconductor material of  claim 12  wherein said co-doped oxide comprises a zinc oxide having a gallium dopant and a hydrogen dopant.  
     
     
         18 . An improved deposition process for growing co-doped zinc oxide thin film having a low growth temperature for dopant activation comprising the steps of: 
 providing a zinc oxide target having at least one impurity;    maintaining a substrate at a selected temperature;    providing a mixture of oxygen and gaseous hydrocarbons between said target and said substrate; and    depositing, by deposition means, a thin film of co-doped zinc oxide on said substrate.    
     
     
         19 . The process of  claim 18  wherein said step of providing at least one impurity in said zinc oxide target comprises the step of providing about 2 atomic wt % of gallium.  
     
     
         20 . The process of  claim 18  wherein said at least one impurity in said zinc oxide target comprises about 1.5 atomic wt % of gallium.  
     
     
         21 . The process of  claim 18  wherein said at least one impurity in said zinc oxide target comprises between 0.2 atomic wt % and 2 atomic wt % of gallium.  
     
     
         22 . The process of  claim 18  wherein said gaseous hydrocarbon is selected from the following: ethane, propane or methane.  
     
     
         23 . The process of  claim 18  wherein said mixture comprises about four parts oxygen and one part gaseous hydrocarbon.  
     
     
         24 . The process of  claim 18  wherein said deposition means comprises a pulsed laser deposition chamber.  
     
     
         25 . The process of  claim 18  wherein said zinc oxide thin film comprises gallium and hydrogen dopants.  
     
     
         26 . The process of  claim 18  wherein said gallium dopant comprises about 0.5 atomic wt %.  
     
     
         27 . The process of  claim 18  wherein each gallium atom stabilizes at least one hydrogen atom.  
     
     
         28 . The process of  claim 22  wherein said depositing step comprises pulsed laser deposition to decompose said gaseous hydrocarbon and to controllably vaporize said target.  
     
     
         29 . The process of  claim 18  further comprising the step of maintaining said substrate temperature at less than 200° C.  
     
     
         30 . The process of  claim 18  further comprising the step of maintaining said substrate temperature in the temperature range of between 35° C. and 200° C. during deposition of said thin film.  
     
     
         31 . The process of  claim 30  wherein said substrate temperature is maintained between about 170° C. and 175° C. during deposition of said thin film.  
     
     
         32 . The process of  claim 30  wherein said substrate temperature is maintained below 50° C. during deposition of said thin film.  
     
     
         33 . The process of  claim 21  wherein said gaseous hydrocarbons is selected from methane, propane or ethane.  
     
     
         34 . The process of  claim 21  wherein said mixture comprises about four parts oxygen and one part methane.  
     
     
         35 . The process of  claim 18  wherein said at least one impurity in said zinc oxide target comprises about 1.5 atomic wt % of gallium and said mixture comprises about four parts oxygen and one part methane.  
     
     
         36 . The process of  claim 18  wherein said at least one impurity in said zinc oxide target comprises between 0.2 atomic wt % and 2 atomic wt % of gallium and said mixture comprises about four parts oxygen and one part methane.  
     
     
         37 . The process of  claim 29  wherein said at least one impurity in said zinc oxide target comprises between 0.2 atomic wt % and 2 atomic wt % of gallium and said mixture comprises about four parts oxygen and one part methane.  
     
     
         38 . The process of  claim 35  wherein said co-doped zinc oxide has a transmittance at least 75% of visible light.  
     
     
         39 . The process of  claim 36  wherein said co-doped zinc oxide has a resistivity of less than 2.5×10 −4  ohms per square centimeter and an electron concentration of about 5.2×10 20 .