Study of Cobalt-doped Zinc oxide thin film deposited by pulsed Laser deposition and RF sputter

• Main Authors: Chin-WeiHsu, 許峻維
• Other Authors: Jung-Chun Huang
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/91042186446195533708

碩士 === 國立成功大學 === 物理學系 === 102 === ABSTRACT SUMMARY Based on previous study, we have already improve the luminous efficiency of light-emitting diodes (LEDs) by depositing zinc oxide related materials thin film between electrode layer and n-GaN layer. Although we get similar results by depositing cobalt doping zinc oxide on pulse laser deposition(PLD), the precise mechanisms that improve luminous efficiency have not confirm. In this research, we assume zinc oxide thin film will reduce the Efficiency droop effect. In order to solve PLD method can't deposit large size chip which is hard to apply by LEDs industry, we use sputter to reconfirm our result. In this research we focus on cobalt doping zinc oxide structure deposited by sputter and compare with PLD thin film. We use X-ray absorption spectrum including (XAS,XANES,EXAFS) and X-ray diffraction method to analyze our thin film. By changing different depositing parameters(Hydrogen, Temperature, Pressure, Percentage), we can observe the difference between each structure. In this research, we find out that Hydrogen can induce Hydrogen interstitial or oxygen vacancy. When the temperature raising, there are few cobalt interstitial will be observed and the amount of oxygen vacancy will also change. Pressure can also affect thin film structure. We use two target to modulate the percentage of cobalt doping in zinc oxide. At the end, we find out that under depositing parameters(temperature:400℃,pressure: 3.75E-3 mbar, Hydrogen mixed, power: 70w)can get the similar structure to previous result. Key word: Sputter, ZnO , XAS, Efficiency droop INTRODUCTION Light emitting diodes are one of the most important components in optoelectronic industry. Because of low power consumption, high lift time and fast reaction, LEDs research attracts increasing attention in the energy-price-up surging era. Some materials such as GaP, GaAs have been commonly used in yellow-blue LEDs. Facing strong global competition in full color LED industry, developing of high efficiency blue LEDs for illumination are ranged from 130-150 lm/W. Ideally, increasing the LED driving current enables the same lumen output to be achieved with less LED chips or using smaller LEDs. Unfortunately, this straightforward route to cost-per-lumen reduction is not readily available in actual GaN LEDs, One of the most significant challenges facing high-power and large size GaN-based LEDs is the efficiency droop effect which means the decrease in external quantum efficiency (EQE)of an LED with increasing injection current in quantum well. Peak internal quantum efficiency (IQE) occurs at relatively low-current densities , then rolls off as current density increases. Typical GaN-based LEDs have a peak in efficiency, typically at current densities less than 10 A/cm2, above which the efficiency gradually decreases. Despite having been the subject of extensive research efforts for a decade, the physical origin of droop has not been clarified. Several mechanism have been proposed to explain this LED droop effect, including electron leakage, poor hole injection, delocalization carriers, Auger recombination. In this research, we deposit cobalt doping zinc oxide thin film on n-GaN try to modulate mobility of n-GaN which can improve poor hole injection. Based on several reference, n-GaN mobility and carrier concentration will influence efficiency droop effect. Zinc oxide (ZnO), a wide bandgap (3.4 eV) II-VI compound semiconductor, has a stable wurtzite structure with lattice spacing a = 0.325 nm and c = 0.521 nm. It has attracted intensive research effort for its unique properties and versatile applications in transparent electronics. X-ray Absorption Spectroscopy (XAS) includes both Extended X-Ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES). X-ray absorption spectra also contain information about the valence state of elements in materials. If the element is present as a cation, the absorption edge is shifted to lower (higher) photon energy because of the lower (higher) ionization potential The “X-ray absorption near edge structure” (XANES) is due to transitions into unoccupied bound states below the edge of the continuum. The “Extended X-ray absorption fine structure” (EXAFS) above the edge is due to backscattering of the photoelectron to the emitting atom. In our research, we use XRD method to check that lattice distance change and secondary phase form when growth parameters change. But XRD can't precise observe whether cobalt substitute zinc place or not, and this method can just determine cobalt interstitials and vacancy formed roughly by lattice distance changed. So we use XAS to analyze thin film structure more precisely. MATERIALS AND METHODS We deposit cobalt doping zinc oxide by sputter. First, we compare two conditions, one is pure Ar gas, the other is 5％ H2 gas mixed. Second, we modulate different parameters, for example, temperature:(400-750℃),pressure(1E-2,3.75E-3,1E-3mbar), power:70w.Third,we use two targets(ZnO,CZO) with different power to modulate cobalt percentage in zinc oxide. Last, we compare with PLD thin film, which substrate temperature is 300℃,frequency is 2HZ, energy is 40mJ. X-ray Absorption(XAS,XANES, EXAFS) is measured in NSRRC 20A,07 beamline in Taiwan. Structural characterization was carried out by X-ray diffraction (XRD). Surface characterization was carried out by SEM, thickness of thin film was carried out by α-step, all were measuring in NCKU. RESULTS AND DISCUSSION First, we compare with the pure argon gas condition and five percentage hydrogen gas mixed condition. In XRD result, we can rule out secondary phase formed and we can observe a slightly move at 34 degree toward small angle when hydrogen gas mixed. It implies lattice distance increase that may cause by hydrogen interstitial. In XAS spectrum we can observe oxygen vacancy formed. In cobalt L-edge spectrum can find out that unoccupied states of cobalt are increasing when hydrogen mixed with argon and oxygen unoccupied states are decreasing. Because of oxygen vacancy formed, the amount of oxygen is decrease which hybridization with cobalt. This cause the peak and area in oxygen K-edge spectrum decrease. In XANES and EXAFS spectrum, we can rule out cobalt is metallic or cobalt oxide form in crystal, because of the shape of our spectrum is totally different to reference. So that we can confirm that cobalt substitute zinc. Second, we change different substrate temperature from 400-750℃. In XRD spectrum, we observe the lattice distance decrease when temperature raised. So we assume that hydrogen will escape from substrate when temperature raised. So the ability of forming oxygen vacancy decrease. In XAS spectrum, we can find out that the amount of oxygen vacancy decrease and unoccupied states increase in oxygen K-edge spectrum when temperature raised. At 750℃,the unoccupied states of oxygen are decrease. Based on reference, it may cause by cobalt interstitial. In XANES and EXAFS spectrum, we can confirm that cobalt substitute zinc whether temperature change or not. Because the cobalt interstitials amount are small, so it can't influence XRD and EXAFS spectrum. Third, we modulate pressure parameter (1E-2,3.75E-3,1E-3mbar),and we find out 3.75E-3 mbar is the better parameter of our sputter system. Obviously, the 3.75E-3 mbar condition FWHM of ZnO(002) peak is more narrow then the other. And photon energy of1E-2,1E-3mbar condition are shifting toward small angle that means the cobalt valence are more approaching to zero. Fourth, we can successfully use ZnO and CZO targets to deposit different cobalt percentage thin film by modulate different power of target, but sacrifice thin film quality. In XAS spectrum, different power of ZnO target can lead different cobalt behavior hybrid with oxygen. Last, we compare PLD with sputter result, we can observe at (temperature:400℃,pressure: 3.75E-3 mbar, Hydrogen mixed, power: 70w) this condition the structure that sputter is similar to PLD result whether in XRD or XAS spectrum. Although, the peak shape are slightly different, we approach our initial goal. CONCLUSION In Hydrogen condition, argon mix Hydrogen can produce oxygen vacancy but also formed some interstitial site. In Temperature condition, when the substrate temperature raise up, we can observe that may decrease hydrogen interstitial and little cobalt interstitials formed at 750.In Pressure condition,3.75E-3mbar is a better growth pressure in our system. In Co-sputter condition, we can modulate Co concentration by using CZO and ZNO target Co-sputter. But quality of thin film we be destroy slightly. we find out the growth condition at(temperature:400℃,pressure: 3.75E-3 mbar, Hydrogen mixed, power: 70w)can approach our previous PLD result. Because of the poor electrical properties of CZO film by sputter, we have to co-doping other materials( Al, Ga…) to improve its electrical properties. Then we can deposit on GaN substrate to check our result and apply on LED.