Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate

Chia-Yu Lee An-Jye Tzou Bing-Cheng Lin Yu-Pin Lan Ching-Hsueh Chiu Gou-Chung Chi Chi-Hsiang Chen Hao-Chung Kuo Ray-Ming Lin 0 Chun-Yen Chang 0 Department of Electronic Engineering, Chang-Gung University , Taoyuan 333, Taiwan The flip chip ultraviolet light-emitting diodes (FC UV-LEDs) with a wavelength of 365 nm are developed with the ex situ reactive plasma deposited (RPD) AlN nucleation layer on patterned sapphire substrate (PSS) by an atmospheric pressure metal-organic chemical vapor deposition (AP MOCVD). The ex situ RPD AlN nucleation layer can significantly reduce dislocation density and thus improve the crystal quality of the GaN epitaxial layers. Utilizing high-resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the crystalline quality of the epitaxial layer with the (RPD) AlN nucleation layer is better than that with the low-temperature GaN (LT-GaN) nucleation layer. The threading dislocation density (TDD) is estimated by transmission electron microscopy (TEM), which shows the reduction from 6.8 107 cm2 to 2.6 107 cm2. Furthermore, the light output power (LOP) of the LEDs with the RPD AlN nucleation layer has been improved up to 30 % at a forward current of 350 mA compared to that of the LEDs grown on PSS with conventional LT-GaN nucleation layer. - Background The emission wavelength of GaN-based semiconductor, a directly transitional wide bandgap material, is theoretically capable of covering the whole visible spectrum from UV to IR, and GaN-based semiconductors attract considerable attention due to their continuously expanding applications for optoelectronic devices, such as light emitting diodes (LEDs) and laser diodes (LDs) [1,2]. Recently, the applications of UV-LEDs with emission wavelengths of about 365 nm are widely expanding, such as in sterilization, medicine, biochemistry, water purification system, light sources for optical recording, fluorescence analyzer, biological sensor, and air purification systems. However, the external quantum efficiency (ex) of UV-LEDs is still much lower than blue LEDs, including the differences between LED structural design, chip area, or other package design. Yamada et al. reported that ex was improved up to 35 % by using patterned sapphire substrate (PSS) [3]. The enhanced light extraction efficiency by scattering the emission light in the epi-layers has been considered, and also related reports demonstrate that the crystal quality can be enhanced by using PSS [4-6]. Despite this, the performance of UVLEDs is sensitive to defects in epitaxial layer because of the lack of localized states in the multiple-quantum-well (MQW) active regions [7,8]. Therefore, improvement of GaN crystal quality for UV-LED is a crucial issue in order to promote related applications. A nucleation layer of GaN hetero-epitaxially grown on PSS is the most important factor for suppressing the formation of threading dislocation densities (TDDs). Lai et al. [9] have recently reported that the ex situ sputtered AlN nucleation layer prepared by radio-frequency (RF) sputtering could reduce the TDDs of GaN and enhance the LED performances due to improvement on crystal quality. The surface of PSS could be damaged by recoil argon ions, though, owing to higher bias voltage (200 ~ 400 V) of RF sputtering system and a short distance from the target to the sample. Thus, it is necessary to deposit AlN nucleation layer on PSS but not cause PSS surface damages. In this study, we demonstrated an UV-LEDs with an ex situ reactive plasma deposited (RPD) AlN nucleation layer on PSS. Comparing the RF sputtering system, the RPD system utilizes a lower bias voltage (15 ~ 20 V), and the distance between the target and the sample is longer. It is practical for avoiding the substrate from being damaged. Moreover, the deposition temperature of RPD AlN nucleation layer was kept at high temperature (600C) that could lead to the preferred orientation growth. Systematic experiments and investigations have been described in detail, which showed an up to 30 % output performance increase by using RPD AlN nucleation layer on PSS. Methods All samples were grown on 2-in. PSS by an AP-MOCVD system. The PSS was prepared using a cone pattern on the (0001) sapphire, which was fabricated by inductively coupled plasma reactive ion etching in order to etch (0001) the sapphire-coated cone-shaped photoresistant layer. The bottom diameter, the center-to-center spacing, and the height of the PSS were 2.5, 3, and 1.5 m, respectively. After preparing the patterned substrates, a 25-nm-thick RPD AlN nucleation layer was deposited onto the PSS by Optorun RPD system (Optorun Co., Ltd., Saitama, Japan). During an epitaxial process, trimethylgallium (TMGa), trimethylaluminum (TMAl), trimethylindium (TMIn), and ammonia (NH3) were employed as the reactant source materials for Ga, Al, In, and N, respectively. Hydrogen and nitrogen were used as carrier gases, and silane and bis-cyclopentadienyl magnesium (Cp2Mg) were used as sources for n-type and p-type dopants, respectively. Two samples were prepared: sample 1 was a device with a 3-m-thick unintentionally doped GaN (u-GaN) layer which was grown on PSS using RPD AlN nucleation layer at 1,150C where ELOG method was applied for fully coalesced GaN layer and the RPD AlN nucleation layer without thermal annealing treatment. By contrast, sample 2 has a 25-nm-thick low-temperature GaN (LT-GaN) nucleation layer, grown on PSS at a temperature of 520C with thermal annealing treatment before the u-GaN epitaxial layer at 1,150C. Following, the GaN-based LED structures were grown on both samples identically; the LED structures consisted of a 2.5-m-thick n-type Al0.02Ga0.98N layer (n-doping is 5 1018 cm3) with a temperature of 1,150C, ten pairs of InGaN/InAlGaN MQWs with a 2.5-nm-thick un-doped well and a 12.5nm-thick Si-doped barrier as active layers grown at 830C, a 15-nm-thick Mg-doped Al0.3Ga0.7N and a 10-nm-thick Mg-doped Al0.1Ga0.9N electron blocking layers (EBL) grown at 1,050C (p-doping = 1 1017 cm3), a 50-nmthick Mg-doped GaN cap layer (p-doping = 5 1017 cm3) grown at 1,030C, and a 4-nm-thick p-type InGaN contact layer. The crystalline qualities of these LED samples with RPD AlN nucleation layer (i.e., LED I) and LT-GaN nucleation layer (i.e., LED II) were then investigated by performing high-resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). Subsequently, the LED mesa with a pattern of 45 45 mil2 was defined and fabricated by photolithography and dry etching. A transparent conduction indium tin oxide (ITO) layer was employed to be a p-type ohmic contact layer. Finally, a Ni/Ag/Pt and Ti/Pt/Au metallization was deposited as p-type and n-type electrodes, respectively. After conventional LED processes, flip chip technology was applied for better light extraction and heat dissipation [10]. The LED chips with patterned sapphire substrate were flip chip bonded onto silicon submount (...truncated)