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A New Method to Enhance the Transparent Conductivity of UV LEDs

What is the relationship between ITO and LED?
ITO is a transparent electrode material with high electrical conductivity, high visible light transmittance, high mechanical hardness and good chemical stability. Currently ITO film is mainly to improve the LED's light efficiency.
What is the Burstein-Moss effect?
 
Burstein-Moss effect: When the semiconductor is heavily doped, the Fermi level enters the conduction band and the intrinsic light absorption shifts toward the higher energy direction.
In normally doped semiconductors, the Fermi level lies between the conduction band and the valence band. As the n-type doping concentration increases, the Fermi level is slowly pushed into the conduction band due to the accumulation of electrons in the conduction band (which can be simply understood as the water (ice) with increased ice (Fermi level) Electronic) pushed to high).
What is the precursor?
Forebody refers to a specially treated compound used to synthesize and prepare other substances.
Recently, researchers at Sun Yat-sen University invented a process for the preparation of indium tin oxide (ITO) films in LED structures by metal-organic vapor deposition (MOCVD). This method can effectively enhance the transparent conductive properties of UV LEDs.
Usually UV LED according to the wavelength is divided into UVA UVB UVC three types. Currently mainly used for water purification, biological sterilization, medical treatment, UV treatment and other fields.
research process
Although ITO is a transparent conductive layer material in the visible spectral region, the transparency of ITO decreases gradually for the UV region.
As a result, the Sun Yat-sen team managed to broaden the optical band gap to 4.7 eV using MOCVD technology. The bandgap excites photons with wavelengths in the UV region (364 nm).
Usually UV LED according to the wavelength is divided into UVA UVB UVC three types. Currently mainly used for water purification, biological sterilization, medical treatment, UV treatment and other fields.
Figure 1 90nm MOCVD ITO film photoelectric properties
 
(A) Effect of Sn flow rate on electron density and mobility
 
(B) UV Visible Light Transmission in MOCVD Process ITO Films with Different Tin Flow Rates.
 
(C) Comparison of ITO optical band gaps under different processes
Sun Yat-sen University team first used the MOCVD technology on the sapphire surface (growth temperature of about 500 ° C) 90nm ITO film growth, the precursor is trimethyl indium (trimethyl indium), tetra-tin (tetrakis-dimethylamino tin), and oxygen Argon gas mixture. The resultant surface of the material is provided with pyramid-like (100) and triangular (111) particles.
 
After many research experiments, the researchers found that the rate of precursor addition was controlled at 350 cubic centimeter per minute to reach the highest free electron density (2.15 × 1021 / cm3). At the same time, the optical band gap will reach 4.70eV. Normally indium oxide (In2O3 without precursor) has an electron density of only 1.47 × 10 19 / cm 3 and a forbidden band width of 3.72 eV.
The difference in the band gap mainly comes from the Burstein-Moss effect, in which part of the free electrons are concentrated in the conduction band and therefore more photon energy is needed to excite the electrons from the valence band come out. The researchers said using this method broadened the forbidden band width by 0.98eV, a gain near 1eV that is uncommon.
At the same time, the researchers also believe that the MOCVD process can improve the Lattice distortion, which is one reason for the narrow bandgap of ITO.
Figure 2 LED epitaxial structure
 
In general, a magnetron sputtering process can also produce a 120 nm light-transmitting conductive layer compared to the MOCVD process. This process uses a mixture of tin oxide (SnO2) and indium oxide (In2O3) with a composition ratio of 1: 9. The magnetron sputtered material needs to be annealed at 550 ° C and placed in a nitrogen atmosphere for 5 minutes.
By analyzing the spectrum, the peak wavelength of the UV LED was 368 nm (Figure 3a). At this wavelength, the magnetron sputtering process ITO film transmittance of 86%, MOCVD process ITO film transmittance of 95%. However, the magnetron sputtering ITO film resistivity is less than the use of MOCVD ITO film, magnetron sputtering process greater contact resistance.
Figure 3 120nm MOCVD ITO film and magnetron sputtering ITO film photoelectric properties
 
(A) Conductivity of 120 nm MOCVD ITO film and magnetron sputtered ITO film on sapphire substrate, and LED emission spectrum using MOCVD ITO film
 
(B) LED current-voltage characteristics of ITO film using two processes
 
(C) output power and current curve
in conclusion
The ITO film of the MOCVD process can increase the output power by 11.4% and 14.8% respectively at 350mA and 600mA (Fig. 3c). After a number of sample tests, the average operating voltage is 3.45V at 350mA of operating current. The ITO current and voltage curves of ITO films using the above two processes are almost the same (Figure 3b).
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