Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate
© Lee et al.; licensee Springer. 2014
Received: 30 June 2014
Accepted: 5 September 2014
Published: 16 September 2014
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 cm−2 to 2.6 × 107 cm−2. 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.
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 (ηex) 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 ηex was improved up to 35 % by using patterned sapphire substrate (PSS) . 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 UV-LEDs 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.  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 (600°C) 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.
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 μm, 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.
Results and discussion
where L is the EL luminescence, I is the driving current, and P is a constant indicating the contribution of non-radiative recombination to overall recombination balance. A higher value of P constant indicates a higher concentration of non-radiative recombination centers. At the linear region of the power law fitting curve of PD-EL, the obtained P constants for LED I and LED II are 1.82 and 2.21, respectively; non-radiative recombination centers are greatly decreased in LED I. With the driving current increasing, the increasing rate of PD-EL curves of two devices decrease and gradually merge with each other as one. It is dominated by non-radiative Auger recombination, since the driving current in this region is high. The two LEDs are quenched at high driving current, so that the two curves are gradually becoming identical. LED I and LED II have similar geometric structures, except for the nucleation layer which causes different GaN growth crystal quality, that influence performance of the devices.
In this study, a RPD AlN nucleation layer has been utilized to improve the crystal quality of GaN on PSS. The better crystallinity of GaN with RPD AlN nucleation layer compared to that with LT-GaN nucleation layer is confirmed by the XRD spectra. The TEM images show that the RPD AlN nucleation layer possesses good coverage uniformity and effectively suppresses the formation of threading dislocations by eliminating GaN islands on PSS. The room temperature EL spectra of the LED with RPD AlN nucleation layer show stronger luminescence intensity compared to that of conventional LEDs. The LOP of the LED with the RPD AlN nucleation layer is enhanced by 32.6 % compared to that of the LED with the conventional LT-GaN nucleation layer at 350 mA. Both the observations of L-I-V curves and current leakage at reverse bias indicate the improvement of crystal quality brought by AlN nucleation layer. PD-EL measurement has also been conducted on the two LEDs for further confirmation, indicating that less non-radiative recombination centers are performed in the LED with AlN nucleation layer. All observations and analysis have consistently shown that the AlN nucleation layer can significantly improve the performance of a LED by increasing the crystal quality of GaN.
The authors are grateful to the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 100-2218-E-032-001-ET and NSC 101-2623-E-032-002-ET, and we deeply appreciate the support from Optorun Co., Ltd., Japan.
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