WO2016111612A1 - Method of fabricating zinc oxide as transparent conductive oxide layer - Google Patents

Abstract

A method of preparing nanocolumns of zinc oxide (ZnO) on silicon substrates by radio frequency (RF) magnetron sputtering is disclosed. The process is performed using ZnO5 targets of 99.999% purity, in the presence of argon and oxygen gases, RF power of 100 to 300 Watt, at substrate temperature of 300 to 600°C and sputtering pressure of 1 to 10 mTorr. The optimum oxygen percentage over total mixture of argon and oxygen found to deposit ZnO layers of uniform thickness, with dense and fine nanocolumns, is 5 to 10% O2. The nanostructured TCO synthesized can be deposited on top of an LED structure to impose10 rough surface properties, which are useful in improving LEE and reducing light loss. In addition, the average optical transmittance of the ZnO film is 78% in the visible range of 470 nm. The present invention demonstrates advantages such as catalyst-free sputter deposition, formation of dense and uniform ZnO nanocolumn arrays, improved LEE and high transmission in visible spectral range, leading to suitable application in blue InGaN-based15 LEDs.

Description

METHOD OF FABRICATING ZINC OXIDE AS TRANSPARENT CONDUCTIVE OXIDE LAYER

The present invention relates to the preparation of a dense, uniform nanocolumnar transparent conductive oxide (TCO) layer comprising of zinc oxide, wherein its fabrication is performed without the use of catalyst, to be further applied in optoelectronics, with a particular application as transparent conductive oxide film for blue indium gallium nitride (InGaN)-based light emitting diodes (LEDs).

Background of Invention

Transparent conducting films are optically transparent and electrically conductive in thin layers. Transparent conductive oxides (TCO) are doped metal oxides used in optoelectronic devices such as flat panel displays, liquid crystal displays, and photovoltaics (including inorganic devices, organic devices, and dye-sensitized solar cells). Transparent conductive oxides are capable of transporting electrical charge and transmitting visible photons. The unique properties that transparent conductive oxides possess are such as low resistivity, high conductivity, high optical transparency, and high optical band gap. I n order to exhibit high conductivity and transmittance, a carrier concentration on the order of 10 cm^"3 or higher, and a band gap energy above 3 eV are required.

TCO is a wide band gap semiconductor that has a relatively high concentration of free electrons in its conduction band. These arise from three fundamental sources: interstitial metal ion impurities, oxygen vacancies (defects), and doping ions. The high electron-carrier concentration causes absorption of electromagnetic radiation in both the visible and infrared portions of the spectrum.

A light-emitting diode (LED) consists of a chip of semiconducting material doped with impurities to create a p-n junction. It is an electroluminescent device with a broad selection of emission wavelengths. In a typical LED structure, the region capable of emitting light is called the active region, composed of multiple quantum wells. Charge carriers, i.e. electrons and holes, flow into the junction from electrodes with different voltages . When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. Hence, in the electroluminescent effect, the colour of light displayed by the LED (corresponding to the energy of the photon) will be determined by the band gap energy of the semiconductor.

Most blue LEDs are made from gallium nitride (GaN) and indium gallium nitride (InGaN); with wide band gaps. Many studies are directed to the finding of I nGaN-blue LED with optimal characteristics, including producing high light extraction efficiency (LEE) via surface roughening techniques. The light extraction efficiency (LEE) of an LED is defined as the number of photons emitted into free space per second / the number of photons emitted from active region per second. Poor light extraction efficiency is mainly caused by the total internal reflection (TIR) effect. Roughening of the surface can enhance the scattering effect. Examples of methods used to promote a rougher LED surface are such as electron beam and nanoimprint lithography (Lee and Tu, 2011). LED chip manufacturers are looking into patterned sapphire substrates (PSS) manufacturing techniques to maximize light extraction - the technique is known to increase the light emission of active quantum well layers as the result of reduced epitaxial defect density, and PSS can reduce light loss due to the TIR phenomena by enabling a photon scattering effect. Another method is to deposit nanostructured transparent conductive oxide (TCO) layer on top of the LED structure to produce a rough surface.

Zinc oxide (ZnO), an inorganic compound, is an n-type semiconductor owing to intrinsic defects such as interstitial zinc atoms (ZnJ and oxygen vacancies (V_Q) in its structure; there has also been attempts to develop p-type ZnO (Damiani and Mansano, 2012). Zinc oxide is a potentially suitable material for use as TCO layer in optoelectronic devices due to its wide band gap of 3.37 eV and high exciton binding energy of 60 meV, which results in bright room temperature emission from ZnO. ZnO films with a hexagonal wurtzite structure have wide optical energy band gap of ~3.3 eV. An additional key quality of ZnO is that it has an isoelectric point (IEP) of 9.5. Compared to silicon dioxide (S1O2), whose IEP ranges from 1.7 to 3.5, ZnO's superior IEP allows for the absorption of proteins by electrostatic interaction.

Materials that are actively researched in the field are doped Zn thin films such as aluminium-doped zinc oxide (AZO) and gallium-doped zinc oxide (GZO). It is found that AZO films are stable in a reducing ambient condition, more readily available compared to indium tin oxide (abundant), and less expensive. Zinc oxide is currently viewed as a promising substitute for indium tin oxide as TCO films; as ZnO is low cost, nontoxic, and chemically stable under exposure to hydrogen plasma, in comparison to indium tin oxide. In recent years, a large focus in ZnO has shifted toward biosensing and optoelectronic applications utilizing nanostructures. Zinc oxide nanostructures have a wide range of application in areas such as solar cells, field emission devices, chemical and biological sensors, photocatalysts, light emitting devices including light emitting diodes (LEDs) and laser diodes (LDs). However, a disadvantage of ZnO may be that the electrical properties of undoped ZnO films are subjected to stoichiometric deviations resulting from oxygen vacancies and interstitial zinc atoms.

Generally, the deposition methods of ZnO films and nanostructures available in the prior art are described by Gomez and Tigli (2013), which include: vapour-liquid-solid (VLS), general physical vapour deposition (Furnace), electron beam physical vapour deposition (EBPVD), cathode arc physical vapour deposition (Arc-PVD), pulse laser deposition (PLD), ion beam sputtering (IBS), atmospheric pressure chemical vapour deposition (APCVD), low- pressure chemical vapour deposition (LPCVD), plasma-enhanced chemical vapour deposition (PECVD), metal-organic chemical vapour deposition (MOCVD), hydrothermal-based chemical approach. Magnetron sputtering is a physical vapour deposition method that creates plasma in the chamber to sputter the target material. The process is initiated in a glow discharge in the vacuum chamber, under controlled gas flow. In sputter deposition, atoms are vaporized from the source material by bombarding them with energized particles. The energized particles kinetically knock atoms in the surface from their equilibrium states. The atoms then begin to travel to the negatively-charged target and undergo further collisions. The process continues, causing atoms to be ejected or sputtered from the target surface. Collecting the sputtered particles from the target on a substrate then enables the deposition of a thin layer of coating. Advantages of RF magnetron sputtering are such as: the use of a closed magnetic field to trap electrons, capable of producing films of uniform thickness, high-speed growth, good reproducibility, and large area deposition.

According to Gao et al (2003), catalysts such as Au, Co, and NiO have been introduced in the synthesis processes of aligned one-dimensional ZnO nanostructures, to guide their oriented growth. In their work, dispersed metallic Sn droplets function as catalysts to guide the oriented growth of ZnO, which in fact is a homoepitaxial process with the bulk ZnO microrods. Au is also used as catalyst in a study by Rosli et al (2014), whereby aluminum-doped zinc oxide (AZO) nanorods thin films are grown by RF magnetron sputtering on Au catalyst layers. The Au catalyst layer is said to facilitate the nucleation sites for AZO nanorods growth on substrate, resulting in AZO nanorods thin films having crystalline hexagonal wurtzite structure with individual nanorods on the film surface.

The properties of ZnO films are strongly dependent on the preparation method and deposition parameters employed. Growth conditions such as doping, substrate type, temperature, and ambient atmosphere, play important roles in determining the structural, electrical, and optical properties of thin films. Therefore, in the prior art, different deposition methods have been explored to synthesize nanostructures for particular industrial application.

A method of chemical spray pyrolysis for the deposition of ZnO on various substrates is disclosed in US Patent No. 8808801 B2, wherein the method described the preparation of highly structured ZnO layers comprising ZnO nanorods or nanoneedles, at moderate deposition temperatures of the substrate, from about 400 to 600°C. The substrate can be glass, silicon, quartz, or metal oxide covered glass. The nanocolumnar ZnO layers are consisting of well-developed hexagonal nanorods of single crystal ZnO, with high purity, and with lengths from 50 nm up to 7 μιη, the diameter of rods could be varied from some tens of nanometers up to 1 micron. An aqueous or aqueous alcoholic solution comprising zinc chloride or zinc acetate as precursors is prepared and sprayed onto the preheated substrate so that the precursor reacts to form ZnO layer on the substrate. Different methods can be used for heating the substrate. Fine droplets of said solution are produced by atomizing the solution using ultrasonic or pneumatic spray techniques. The solvent evaporates when the droplets reach the substrate and the precursor reacts to form a pl urality of zinc oxide nanorods or nanoneedles.

Reference may also be made to the article by Wei et al (2013), which disclosed a method for deposition of ZnO thin films on GaN buffer layer-decorated Si (111) substrates by pulsed laser deposition (PLD). GaN is the preferred buffer layer because it has similar crystal structure as ZnO (i.e. wurtzite), and the lattice mismatch is 1.8% on the c-plane. GaN thin films are grown on Si (111) substrate by PLD at growth temperature of 800°C using a GaN ceramic target. Following the deposition, samples are annealed in a high-temperature tube quartz furnace. Subsequently, ZnO thin films are fabricated on GaN (111) template by PLD at growth temperature of 400°C in 0₂ ambience, with a pressure of 1.3 Pa using a ZnO ceramic target. Results have indicated that the ZnO/GaN/Si film is two-dimensionally grown with flower-like structure, while the ZnO/Si film is in the (002) orientation grown with an incline columnar structure. Hence, the quality of ZnO thin film is improved due to epitaxial growth of crystalline grain by GaN epitaxial layer. UV emission of ZnO thin film fabricated on GaN/Si substrate is higher than that fabricated on Si substrate only. Therefore, the authors suggested suitable application of nanopillar array ZnO/GaN heterostructure in deep UV emission LED devices.

According to the related literature and prior arts described, the TCO layer comprising pure ZnO have limited light extraction efficiency (LEE). An improved fabrication method is required to fabricate ZnO with increased LEE.

Summary of Invention The object of the present invention is to provide a method for the fabrication of pure zinc oxide as transparent conductive oxide layer by RF magnetron sputtering, for further application in optoelectronics, specifically for the improvement of LEE in blue InGaN-based LEDs. To incorporate the beneficial characteristics of ZnO onto substrate such as silicon (111), RF magnetron sputtering with particular deposition parameters is applied in the present technique, in which the method eliminates the use of a cata lyst.

The method for the preparation of nanocolumnar ZnO layer on a substrate according to the present invention briefly comprises the steps of pretreatment of substrates, evacuating the sputtering chamber to base pressure of 1 x 10^"6 Torr, pre-sputtering of target to remove contaminants, and depositing ceramic ZnO target of 99.999% purity onto Si (111) substrate by RF magnetron sputtering. Deposition parameters include the following: target-to-substrate distance of 10 to 15 cm, rotation of sample holder at 1 to 10 rpm, deposition time of 60 min, RF power of 200 Watt, deposition temperature and pressure of 300 to 600°C and 1 to 10 mTorr respectively, in discharge gas and oxygen gas mixture, wherein the oxygen percentage of the gas composition is 5 to 10%. The morphology of the ZnO nanocolumns is therefore controlled by the deposition parameters in RF magnetron sputtering.

Zinc oxide grains synthesized are non-homogeneous, polycrystalline, and are manifesting dense and fine nanocolumnar structures for a ll RF-sputtered samples. The films are also distributed with uniform thickness. The average thickness of the ZnO layer is between 311 to 326 nm, the average grain size is between 30 to 100 nm, and the lateral size of the nanocolumns is between 30 to 60 nm.

The nanostructured TCO synthesized can be deposited on top of an LED structure to impose rough surface properties, which are useful in improving LEE and reducing light loss. In addition, the average optical transmittance of the ZnO film is 78% in the visible range of 470 nm. In summary, the present invention demonstrates advantages such as catalyst-free sputter deposition, formation of dense and uniform ZnO nanocolumn arrays, improved LEE and high transmission in visible spectral range, suitable for application in blue InGaN-based LEDs.

Description of Embodiments

Hereinafter, an embodiment of present invention is described in detail.

The catalyst approach for ZnO deposition usually involves Au (Gomez and Tigli, 2013). However, while using catalysts allow for highly controlled growth, the current invention implements a physical vapour deposition process without the use of a catalyst to eliminate any possible contamination or impurities.

The deposited ZnO layer by the catalyst-free RF magnetron sputtering method retains rough surface properties that can improve the light extraction efficiency of blue InGaN-based LEDs; which is important as total internal reflection causes light to be trapped and degenerate into heat energy, resulting in inefficient LEDs.

Photons are emitted from the active quantum well layers by electron-hole recombination, and they escape the light emitting diode into free space. In an ideal situation, all of the photons emitted by the active layers would be extracted as light output. However, in reality, a large number of the emitted photons do not escape the LED for various reasons. One key obstacle to such ideal light extraction is the TIR effect, caused by the high refractive index of gallium nitride (GaN) versus the refractive index of free space (about 2.5 to 1.0). A large number of photons generated from the active region bounce back into the LED and are trapped inside, eventually dissipating as heat.

In the RF magnetron sputtering equipment set up, a cathode and an anode are placed in opposite of one another, in a vacuum chamber. The negatively charged electrode is the ceramic ZnO target (4 in.), containing Zn and 0₂ elements, with a purity of 99.999% or 5N purity. The target and substrate are assembled at a distance within the range of 10 to 15 cm; preferably, the distance is kept at 14 cm. The sample holder is designed to support 8 in. silicon wafer. Si (111), and constant rotations of 1 to 10 rpm are maintained during the deposition process. Optimum rotation for deposition is 8 rpm. In the present invention, the ZnO film is grown on silicon (111), as the lattice matches the hexagonal wurtzite structure, simulating the growth of ZnO on the wurtzite structure of GaN.

In the pre-treatment step, the Si(lll) substrates are ultrasonically cleaned in acetone, propanol and deionized water for 5 min and blown dry in nitrogen gas before loading into sputtering chamber. Prior to the sputtering deposition, the chamber is evacuated down to 1 x 10^"6 Torr by rotary and turbomolecular pumps, to set the base pressure. The target is pre-sputtered for 10 minutes prior to each deposition, to avoid contamination.

RF magnetron sputtering of zinc oxide film is operated at constant RF power of 100 to 300 Watt, say 200 Watt at substrate temperature of 300 to 600 °C, say 500 °C. Deposition time maintained for all the samples is 60 min. Sputtering pressure is 1 to 10 mTorr, say 5 mTorr, and the sputtering gases are argon (discharge gas) and oxygen (reactive gas) respectively. The supply of mixture of argon and oxygen is performed with oxygen percentage over mixture of argon and oxygen mixture is 5 to 10%. The optimum percentage is 7%. This method for RF magnetron sputtering of ZnO is catalyst-free.

The structure and morphological properties of the ZnO layer synthesized, according to experimental conditions, are characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), and UV-Vis spectroscopy.

The ZnO film deposited exhibits polycrystallinity. High atom mobility at high temperatures may allow the atoms to occupy more suitable positions in the lattice, thus enhancing crystallinity. The average film thickness is within the range of 311 to 326 nm. The average grain size of ZnO films sputtered deposited by RF magnetron method is within the range of 30 to 100 nm. The lateral size of the nanocolumns is between 30 to 60 nm. The ZnO grown at temperature of 500°C is almost stress free and exhibits more pronounced ZnO nanocolumnar structure. In addition, there is a reduction in compressive stress due to the atomic peening effect, attributed to the bombardment of atoms in the ZnO film. Therefore, it is proposed that the formation of defects in the film is primarily caused by the re-evaporation process.

Grains grown at 500°C are uniform and sharp-faceted, and hence the surface is non- homogeneous. There is also a high grain boundary density, whereby the defects and adsorbed species are usually present at the grain boundaries.

At higher sputtering temperatures, there appear to be a larger number of grains, larger distribution of grains of high vertical size, increased peak-to-valley ratio, and increased RMS roughness value. The resistivity and conductivity of the ZnO film deposited according to this manner is not disclosed, however, the inventors addressed that high conductivity has been reported for ZnO grown at the lowest oxygen percentage due to the formation of intrinsic donors, such as zinc interstitials and oxygen vacancies in the film. The dense and uniform nanocolumnar array of ZnO exhibits high optical transmittance of 78% in visible region at 470 nm; the film efficiently transmits blue light with optical transmittance of 78% in this region.

As such, properties of this invention such as high transmission, rough surface morphology, and improved LEE, appropriately allow the application of the ZnO film in devices incorporating blue InGaN-based LEDs. Due to properties such as nanosized junction and excellent waveguiding function of the nanorods, ZnO-based heterostructures light- emitting diodes exhibit significantly improved electroluminescence performance. Different morphology of ZnO thin film will be observed with different substrate type, growth temperature, ambient atmosphere (e.g. oxygen partial pressure), and film thickness. At high temperature, films exhibited higher doping efficiency, higher carrier concentration, lower resistivity, and higher Hall mobility (not for all applications). The increasing resistivity at higher pressures is derived from a lower concentration of oxygen vacancy, which reduces the number of free electrons.

The change in film thickness affects the electrical and optical properties of the deposited film. As film thickness increases, electrical conductivity increases while transmittance decreases. The conductivity increases because of the increase in grain size, and decrease in grain boundary scattering. At the same time, more photons are absorbed when the thickness increases, therefore lower transparency. Accordingly, a method to prepare a dense, uniform nanocolumnar transparent conductive oxide (TCO) layer comprising of zinc oxide is described. The fabrication is performed without the use of catalyst, to be further applied in optoelectronics, with a particular application as transparent conductive oxide film for blue indium gallium nitride (InGaN)-based light emitting diodes (LEDs).

List of Non-Patent Citations: Damiani, L. R., & Mansano, R. D. (2012), 'Zinc oxide thin films deposited by magnetron sputtering with various oxygen/argon concentrations', Journal of Physics: Conference Series, Vol. 370, doi:10.1088/1742-6596/370/l/012019.

Gao, P. X., Ding, Y., & Wang, Z. L. (2003), 'Crystallographic orientation-aligned ZnO nanorods grown by a tin catalyst', Nano Letters, Vol. 3, No. 9, pp. 1315-1320.

Gomez, J. L., & Tigli, O. (2013), 'Zinc oxide nanostructures: from growth to application'. Journal of Materials Science, Vol. 48, pp. 612-624. Lee, Y. C, & Tu, S. H. (2011), 'Improving the light-emitting efficiency of GaN LEDs using nanoimprint lithography', in Bo, C. (ed.). Recent Advances in Nanofabrication Techniques and Applications, InTech, Europe, pp. 173-196.

Rosli, A. B., Marbie, M. M., Herman, S. H., & Ani, M. H. (2014), 'Gold-catalyzed growth of aluminium-doped zinc oxide nanorods by sputtering method'. Journal of Nanomaterials, Vol. 2014.

Wei, X. Q., Zhao, R. R., Shao, M. H., Xu, X. J., Huang, J. Z. (2013), 'Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate', Nanoscale Research Letters, Vol. 8, pp. 112-118.

Claims

Claims

1. A method of preparing transparent conductive oxide layer, comprising:

preparing a substrate in a magnetron sputtering chamber;

preparing a zinc oxide target;

setting the chamber at temperature of 300 to 600 °C;

rotating the substrate holder at 1 to 10 rpm;

depositing zinc oxide on substrate using RF power of 100 to 300 W;

depositing zinc oxide on substrate at sputtering pressure of 1 to 10 mTorr; and supplying mixture of discharge gas and oxygen.

2. The method of claim 1, wherein setting the chamber was performed at temperature of 500 °C.

3. The method of claim 1, wherein rotating the substrate is performed at 8 rpm.

4. The method of claim 1, wherein depositing zinc oxide on substrate is performed using RF power of 200W.

5. The method of claim 1, wherein depositing zinc oxide on substrate is performed at sputtering pressure of 5 mTorr.

6. The method of claim 1, wherein depositing zinc oxide is performed at a distance of 10 to 15 cm between the target and the substrate.

7. The method of claim 1, wherein supplying mixture of discharge gas and oxygen is performed using argon and oxygen, said oxygen at 5 to 10% from total percentage of argon and oxygen.

8. The method of claim 1, wherein supplying mixture of discharge gas and oxygen is performed using argon and oxygen, said oxygen at 7% from total percentage of argon and oxygen.