P-type semiconductor zinc oxide films process for preparation thereof, and pulsed laser deposition method using transparent substrates

Abstract

A p-type semiconductor zinc oxide (ZnO) film and a process for preparing the film are disclosed. The film is co-doped with phosphorous (P) and lithium (Li). A pulsed laser deposition scheme is described for use in growing the film. Further described is a process of pulsed laser deposition using transparent substrates which includes a pulsed laser source, a substrate that is transparent at the wavelength of the pulsed laser, and a multi-target system. The optical path of the pulsed laser is arranged in such a way that the pulsed laser is incident from the back of the substrate, passes through the substrate, and then focuses on the target. By translating the substrate towards the target, this geometric arrangement enables deposition of small features utilizing the root of the ablation plume, which can exist in a one-dimensional transition stage along the target surface normal, before the angular width of the plume is broadened by three-dimensional adiabatic expansion. This can provide small deposition feature sizes, which can be similar in size to the laser focal spot, and provides a novel method for direct deposition of patterned materials.

1 . An apparatus for depositing transparent thin films and direct deposition of patterned structures on a substrate, comprising;
 a pulsed laser source generating picosecond or femtosecond pulses;   a beam delivery system for guiding said laser pulses through the substrate and for focusing said laser pulses on a target, said beam delivery system comprising a high numerical aperture lens and beam expansion optics, said high numerical aperture lens focusing said pulses into focused spots on the target having a spot size in the range from less than  1  micron to a few microns to generate an ultrafast laser ablation plume of target material by ultra-fast laser ablation comprising picosecond or femtosecond laser ablation;   the material ablated from said target being deposited on said substrate, said substrate being transparent at a wavelength of said picosecond or femtosecond pulses; and   apparatus for translating said substrate with respect to said target, configured such that when said substrate is positioned a first distance from said target, said ablated target material forms a large area thin film on said substrate, and when said substrate is positioned a second distance which is smaller than the first distance from said target, said ablated material forms a patterned structure comprised of micron-scale features on said substrate, utilizing a characteristic narrow angular distribution of the ultra-fast laser ablation plume and   wherein said substrate, said target, and an optical path to propagate and focus a beam from said pulsed laser source to said target are arranged such that a distance between said substrate and said target is adjustable based on said narrow angular distribution and a desired feature size to be deposited.   
     
     
         2 . An apparatus as claimed in  claim 1 , wherein said substrate is positioned at a variable distance from said target, said variable distance ranging from 10 μm to 30 cm. 
     
     
         3 . The apparatus of  claim 1 , wherein said patterned structure has surface features with a feature size as small as approximately said spot size. 
     
     
         4 . The apparatus as claimed in  claim 1 , wherein said large area thin film is grown while laterally translating the substrate with respect to the target. 
     
     
         5 . The apparatus as claimed in  claim 1 , wherein said thin films form a multilayer structure by alternating the growth processes at long and short target-substrate distances and using different target materials. 
     
     
         6 . A method of depositing transparent thin films and direct deposition of patterned structures, comprising;
 providing an ultrafastpulsed laser source, and focusing pulses from said laser source on a target through a transparent substrate so as to ablate or evaporate portions of said target by ultra-fast laser ablation, comprising picosecond or femtosecond laser ablation;   translating said substrate with respect to said target, such that when said substrate is positioned a first distance from said target, said ablated or evaporated target material forms a large area thin film on said substrate, and when said substrate is positioned a second distance which is smaller than the first distance from said target, said ablated or evaporated material forms a patterned structure on said substrate, wherein the pattern comprises micron-scale features which are deposited utilizing a characteristic narrow angular distribution of an ultra-fast laser ablation plume.   
     
     
         7 . The method as claimed in  claim 6 , further comprising a substrate heating system, comprising a CW infrared heating laser. 
     
     
         8 . The method as claimed in  claim 6 , wherein the substrate is positioned near the target for depositing small size features and then moved away from the target to deposit intermediate layers so as to deposit multilayer structures. 
     
     
         9 . The method as claimed in  claim 6 , wherein said translation system positions said substrate at a variable distance from said target, said variable distance ranging from 10 μm to 30 cm. 
     
     
         10 . The method as claimed in  claim 6 , wherein said patterned structures have surface features with a feature size as small as approximately a beam diameter of said laser. 
     
     
         11 . The method as claimed in  claim 6 , wherein said large area thin film is grown while laterally translating the substrate relative to said target. 
     
     
         12 . The method as claimed in  claim 6 , further comprising growing a multilayer film on said substrate by alternatively using different target materials. 
     
     
         13 . The method as claimed in  claim 6 , further comprising forming two-dimensionally patterned structures on said substrate by positioning the substrate at the second distance from the target and laterally translating the substrate relative to said target. 
     
     
         14 . The method as claimed in  claim 6 , further comprising forming a multilayer structure by alternating growth processes at long and short target-substrate distances and using different target materials. 
     
     
         15 . The method as claimed in  claim 6 , further comprising forming two-dimensionally patterned structures, comprising arrays of dots and/or lines, by directly writing onto the substrate by positioning the substrate at the second distance to the target and laterally translating the substrate. 
     
     
         16 . The method as claimed in  claim 6 , wherein three-dimensionally patterned structures, comprising combinations of arrays of dots and/or lines, and thin film layers, are deposited by controlling the distance between substrate and target and laterally translating the substrate. 
     
     
         17 . The method as claimed in  claim 6 , wherein said substrate comprises at least one of sapphire, SiC, quartz, fused silica, glass, and transparent polymer. 
     
     
         18 . The method as claimed in  claim 6 , wherein said target comprises at least one of alumina (Al 2 O 3 ), silica (SiO 2 ), metal oxides (MgO, ZnO, TiO 2 , ZrO 2 , Nb 2 O 5 ), transparent conductive oxides (TCOs), wide gap III-V and II-VI semiconductors and their alloys (GaN, AlN, ZnS, ZnSe, ZnTe), and polymers. 
     
     
         19 . An apparatus for depositing transparent thin films and direct deposition of patterned structures on a substrate, comprising;
 a target;   a pulsed laser source generating picosecond or femtosecond pulses;   a beam delivery system for guiding said laser pulses through the substrate and for focusing said laser pulses to a spot size of around 1 micron at said target, for evaporating material of said target by ultra-fast laser ablation;   a substrate onto which material removed from said target is deposited, said substrate being transparent at a wavelength of said picosecond or femtosecond pulses; and   means for depositing a large area film, and for depositing micron size features onto said substrate by utilizing a characteristic narrow angular distribution of ultrafast laser plume, to enable the deposition of micron sized features when said substrate is positioned near the target, and a large area film when said substrate is positioned more distant from the target.   
     
     
         20 . The apparatus as claimed in  claim 19 , wherein a gaseous Knudsen layer results from an interaction of said picosecond or femtosecond pulses with said target, and said substrate is positioned within said gaseous Knudsen layer for deposition of micron-size features, and at a greater distance for depositing large area films. 
     
     
         21 . The apparatus as claimed in  claim 19 , wherein said narrow angular distribution of ultrafast laser plume is half the maximum value at an angle, θ 1/2 , which is less than approximately 20 degrees. 
     
     
         22 . A method for depositing transparent thin films and direct deposition of patterned structures on a substrate, comprising;
 providing a target;   generating picosecond or femtosecond pulses with a pulsed laser source;   guiding said laser pulses through the substrate with a beam delivery system;   focusing said laser pulses to a spot size of about 1 micron at said target, for evaporating material of said target by ultra-fast laser ablation;   providing a substrate onto which material removed from said target is deposited, said substrate being transparent at a wavelength of said picosecond or femtosecond pulses; and   depositing a large area film and depositing micron size features onto said substrate by utilizing a characteristic narrow angular distribution of ultrafast laser plume, to enable the deposition of micron sized features when said substrate is positioned near the target, and a large area film when said substrate is positioned distanced from the target.   
     
     
         23 . The method as claimed in  claim 22 , wherein a gaseous Knudsen layer results from an interaction of said picosecond or femtosecond pulses with said target, and said substrate is positioned within said gaseous Knudsen layer for deposition of micron-size features, and at a greater distance for depositing large area films. 
     
     
         24 . The method as claimed in  claim 22 , wherein said narrow angular distribution of ultrafast laser plume is half the maximum value at an angle, θ 1/2 , which is less than approximately 20 degrees. 
     
     
         25 . A method of depositing transparent thin films and direct writing of patterned structures, comprising;
 providing a pulsed laser source, and focusing said laser on a target through a substrate that is transparent to the wavelength of said laser, so as to ablate portions of said target using the energy of said laser; and   translating said substrate with respect to said target, such that said ablated material is deposited on said substrate and forms patterned structures,   wherein three-dimensional structures, comprising combinations of arrays of dots, lines or both, and thin film layers, are deposited by varying the distance between said substrate and said target, using different target materials alternately, and laterally translating said substrate.   
     
     
         26 . An apparatus for depositing transparent thin films and direct deposition of patterned structures on a substrate, comprising;
 a pulsed laser source,   a beam delivery system for guiding said laser through the substrate and focusing it on a target;   a substrate onto which material ablated or evaporated from said target is deposited, said substrate being transparent at a wavelength of the pulse laser source; and   an x-y-z translation system for defining a deposition feature size by translating said substrate with respect to said target along a z-direction so as to change a distance between said substrate with respect to said target, and for translating said substrate laterally in x and y directions with respect to said target, so that said ablated material can be deposited on said substrate, thereby forming patterned structures with a defined deposition feature size.