High performance of Ga-doped ZnO transparent conductive layers using MOCVD for GaN LED applications

High performance of Ga-doped ZnO (GZO) prepared using metalorganic chemical vapor deposition (MOCVD) was employed in GaN blue light-emitting diodes (LEDs) as transparent conductive layers (TCL). By the post-annealing process, the annealed 800°C GZO films exhibited a high transparency above 97% at wavelength of 450 nm. The contact resistance of GZO decreased with the annealing temperature increasing. It was attributed to the improvement of the GZO crystal quality, leading to an increase in electron concentration. It was also found that some Zn atom caused from the decomposition process diffused into the p-GaN surface of LED, which generated a stronger tunneling effect at the GZO/p-GaN interface and promoted the formation of ohmic contact. Moreover, contrast to the ITO-LED, a high light extraction efficiency of 77% was achieved in the GZO-LED at injection current of 20 mA. At 350 mA injection current, the output power of 256.51 mW of GZO-LEDs, corresponding to a 21.5% enhancement as compared to ITO-LEDs was obtained; results are promising for the development of GZO using the MOCVD technique for GaN LED applications. ©2013 Optical Society of America OCIS codes: (250.0250) Optoelectronics; (230.3670) Light-emitting diodes References and links 1. S. Nakamura and G. Fasol, The Blue Laser Diode (Springer, Berlin, 1997), pp. 201–260. 2. C. C. Pan, C. M. Lee, J. W. Liu, G. T. Chen, and J. I. Chyi, “Luminescence efficiency of InGaN multiple-quantum-well ultraviolet light-emitting diodes,” Appl. Phys. Lett. 84(25), 5249–5251 (2004). 3. K. C. Shen, W. Y. Lin, D. S. Wuu, S. Y. Huang, K. S. Wen, S. F. Pai, L. W. Wu, and R. H. Horng, “An 83% enhancement in the external quantum efficiency of ultraviolet flip-chip light-emitting diodes with the incorporation of a self-textured oxide mask,” IEEE Electron Device Lett. 34(2), 274–276 (2013). 4. C. T. Lee, U. Z. Yang, C. 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Introduction
The tunable optical properties of GaN-based materials to emit light from the ultraviolet to the visible spectrum, especially for light-emitting diodes (LEDs), has renewed interest in them [1][2][3][4].To obtain more LED output power to satisfy solid-state lighting applications such as indoor lighting, the fabrication of special structures in the LED surface or sapphire has been tried with the aim of increasing the probability of whether photons emitting from the active region escape the LEDs.These attempts to improve light extraction efficiency (LEE) include the use of a photonic crystal, surface texturing, and patterned sapphire substrate [5][6][7][8][9].However, the texturing processes usually require the use of either e-beam lithography or high temperature annealing processes; both processes are unsuitable for mass production and commonly cause metal migration into the semiconductor.It is more favorable to employ high optical transparency and low resistivity transparent conductive oxide layers (TCLs) on LED surface as current spreading layer and hence enhances the LED LEE.Indium tin oxide (ITO) has emerged as one of most promising materials for the TCL in GaN-based LEDs because of its low resistivity and high transparency in the range of visible wavelength.However, indium is a rare metal with high price, and the low thermal stability of ITO leads to a poor device reliability [10].In contrast to ITO, ZnO-based materials have increasingly studied as the alternative TCL in high efficiency LEDs due to their electrical and optical properties similar to ITO, better thermal stability, and smaller lattice mismatch to GaN [11][12][13].Particularly, the Ga-doped ZnO (GZO) was found to be more appropriate to develop the GaN TCLs as compared to that with Al-doped (AZO) since the lower oxidation reactivity of Ga and the smaller difference between the bond lengths of Ga-O and Zn-O [14].Recently, the performance of GaN LEDs with a GZO TCL using atomic layer deposition (ALD) technique has been demonstrated [15].While the potential to improve the output intensity of LEDs has been addressed, details regarding the contribution of enhancing LED output power in LEE using GZO TCL along with the thermal properties of GZO-LEDs are rarely provided.Additionally, due to the refractive index of 2.0 for GZO and 1.7 for ITO [16], it can be expected the critical angle of total internal reflection for GZO/LED is larger than that for ITO/LED.In other word, the area of photons extracted from GZO will expand when the thickness of GZO increases.However, it is not easy to increase the GZO thickness using the ALD technique since the deposition rate of ALD is too slow.Therefore, in this study, we use a metalorganic chemical vapor deposition (MOCVD) technique to fabricate a GZO TCL on GaN-LED with varying the post-annealing time and temperature.The MOCVD technique is a mature method to achieve a high crystal quality film at higher growth rate.Moreover, to completely investigate the effect of GZO in electrical and optical properties, the GZO thickness was fixed the same as the thickness of traditional ITO TCL in LEDs.

Experimental
In this manuscript, a 220 nm-thick GZO film was grown on a sapphire and a blue LED sample with a wavelength of 450 nm using a modified Emcore-D180 MOCVD system, respectively.Diethylzinc (DEZn), Triethylgallium (TEGa), and oxygen (99.999%) were used as the GZO precursors.Ar (99.999%) was used as the carrier gas, passing through the TEGa bubbler to deliver the DEZn and TEGa vapors to the reactor.The growth condition of GZO was controlled at a lower pressure (15 Torr) and low temperature (350°C) for 15 min.The samples were annealed at 600°C and 800°C for 2 min in N 2 ambient, and the transmittance spectra were determined from the N&K analyzer (model: 1280, N and K Tech.).Contact resistances between GZO and p-GaN layer were evaluated by transmission-line model (TLM) method and the thermal properties of GZO TCL were further investigated through atomic force microscopy (AFM) and double-crystal X-ray diffraction (XRD) measurements.An ITO film with the same thickness also was deposited on the sapphire and the blue LED by E-gun evaporation as contrasted sample.The both ITO-LEDs and GZO-LEDs were fabricated using standard photolithography and etching processes.The current-voltage (I-V) characteristics of TCL-LEDs were determined using a semiconductor parameter analyzer (Keithley, 2400 sourcemeter), and the output power of LEDs was measured with a calibrated integrating sphere.

Results and discussion
The transmission spectra with the wavelength range from 200 to 1000 nm of the ITO and the GZOs with annealing at 600°C and 800°C were measured and shown in Fig. 1.One can see that the both GZO films showed a higher transparency than 96% at the wavelength range of 500-1000 nm.Contrast, the transmittance of ITO was only about 90-92%.Moreover, the transmittance of the annealed 800°C GZO film at 450 nm was equal to 97%, which is higher than the transmittance of 92 and 95% of the annealed 600°C GZO and the ITO film, respectively.Furthermore, the inset of Fig. 1 shows the XRD patterns of GZO films after annealing 600°C and 800°C for 2 min.Three XRD peaks representing (100), (002), and (101) crystal planes were observed at 2θ = 32.38°,34.56°, and 36.82°,respectively, on the both annealed GZO films.Except for the (101) ZnO plane, the intensity of (002) and (100) peak of the annealed 800°C GZO film was stronger and sharper than that of the annealed 600°C sample.The results of the annealed 800°C GZO film were attributed to the reconstruction of GZO during high temperature annealing process, which improved the GZO crystal quality and increased its transmittance.
The I-V characteristics and electrical properties of the as-deposited, annealed 600°C and 800°C GZO on the p-GaN were measured by TLM method and Hall measurement, as shown in Fig. 2    annealing temperature increases to 800°C.However, the trend in electron concentration raised from 3.16 × 10 20 to 1.47 × 10 21 cm −3 , then decreased to 5.43 × 10 20 cm −3 .Generally, for as-deposited GZO film, high defect densities were formed in the GZO film due to the low temperature deposition, which resulted in a lower mobility.When the GZO sample was annealed at 600°C, the structural defects in GZO film started to be decomposed and reconstructed, and hence increased the mobility and electron concentration [17,18].For the 800°C annealing sample, the change of electron concentration can be ascribed to the reduction of Zn sites owning to the occurrence of Zn diffusion, which is further discussed in Fig. 3.In Fig. 2, it still exhibited a nonlinear I-V characteristic in the annealed 600°C GZO/p-GaN sample, although the characteristic was better than that of as-deposited one.The ohmic property was clearly observed in the annealed 800°C GZO/p-GaN sample, and the specific contact resistance was estimated to be 8.7 × 10 −3 Ω-cm 2 .While the specific contact resistance of the anneal 800°C sample is slightly higher than that of ITO (6.2 × 10 −3 Ω-cm 2 ), the advantage of applying MOCVD to the growth of the GZO film will be more significant as the GZO thickness increases because of the increment in carrier concentration [19].XPS results were employed to better understand the ohmic contact behavior of GZO/p-GaN, as illustrated in Fig. 3.In Fig. 3(a), the distribution of Ga, O, Zn and N in the as-deposited sample are seen to be stable.However, after annealing at 800°C, some differences in Zn concentration were found (see Fig. 3(b)).The Zn concentration was significantly decreased in the GZO region and exhibited diffusion into the p-GaN layer.Due to the low deposition temperature of GZO, some ZnO were decomposed during the annealing process to create extra Zn atom and inter-diffused into the p-GaN region.Moreover, the XPS spectra of Ga2p at the surface of p-GaN at the as-deposited and annealed 800°C states was examined.It was found that the Ga XPS core level peak at the p-GaN surface after annealing 800°C was shifted from 1116.6 to 1116.0 eV.The peak-shift implied that an increase of hole concentration occurred at the surface of p-GaN.A similar phenomena in XPS peak-shift of Ga and increased hole concentration at p-GaN surface were presented in Ref. 12 and was attributed to the inter-diffused Zn atom.The behavior of Zn diffused in the p-GaN is similar to the role of Mg-doping in GaN, which allows the increase of the hole concentration in p-GaN surface to form a p + -GaN layer [12,20].As a bias of reverse voltage is applied across this GZO/p + -GaN junction, the electron tunneling will dominate the carrier transfer mechanism [15,21].This can explain that the ohmic contact characteristic was observed in the annealed 800°C sample (Fig. 2).In addition, the influence of thermal annealing in surface morphology was presented in Fig. 4. It clearly indicated that the root-mean-squared (RMS) roughness rose appreciably from 2.87 to 7.90 nm as the annealing temperature was extended from the as-deposited state to the annealing at 800°C.
Figure 5 shows the voltage and output power versus the injection current characteristics of the ITO-LEDs and GZO-LEDs.The forward voltages at 20 mA for both LEDs were approximately 2.67 V.At this point, the output power was approximately 18.41 and 21.22 mW for the ITO-LEDs and the GZO-LEDs, respectively.The external quantum efficiency (EQE) of the GZO-LEDs was calculated to be 38.5%, which is an increase of 15.2% compared to that of the ITO-LEDs (~33.4%).The 15.2% enhancement in the EQE originated from the enhanced LEE of LED.The LEE of 67% and 77% were evaluated for ITO-LEDs and GZO-LEDs based on the 50% internal quantum efficiency of the blue LEDs.When the LED chips was driven with a 350mA injection current, the output power of the LEDs with ITO and GZO TCL were measured to be 211.21 and 256.51mW, respectively.The output power of the GZO-LEDs was enhanced by a factor of 21.5% compared to that of the ITO-LEDs.The increased light output power of GaN LEDs with GZO was attributed to four main factors: (1) the higher transmittance and (2) the lower resistivity of the annealed GZO film, (3) the low specific contact between the interface of GZO and p-GaN resulted from the tunneling effect, and (4) the roughened GZO surface.When the current was driven in the GZO-LEDs, a uniform current distribution was generated due to the low-resistivity GZO TCL.By the low specific resistance at the GZO/p-GaN interface, the current was more effectively injected into the LEDs, increasing the radiative recombination efficiency and obtained higher output power.In addition, the high transmittance property and roughened surface of GZO TCL also contributed in enhancing the light scattering via the change of the refractive index difference.

Conclusion
In this study, we successfully fabricated the GZO TCL on GaN-based blue LED structure using MOCVD.The GZO film shows high transmittance of 97% at wavelength of 450 nm and low contact resistance of 8.7 × 10 −3 Ω-cm 2 to p-GaN, which results in a high LEE of 77% in GZO-LED.Moreover, compared to the ITO-LEDs, it was found that the light output power of the LEDs with GZO was increased to be 256.51mW.It clearly indicates the GZO films prepared using MOCVD can be potentially useful in GaN LED applications.

Fig. 1 .
Fig. 1.Transmittance spectra of ITO and GZO films with annealing 600°C and 800°C.The inset shows XRD in 2θ scan mode of the annealed 600°C and 800°C GZO films on sapphire substrate.

Fig. 2 .Fig. 3 .
Fig. 2. I-V curves of the ITO and the GZO on the p-GaN surface.

Fig. 5 .
Fig. 5. Output power as a function of injection current for GZO-LEDs and ITO-LEDs.