Vertical-Injection AlGaInP LEDs with n-AlGaInP Nanopillars Fabricated by Self-Assembled ITO-Based Nanodots
© Ryu et al. 2015
Received: 17 August 2015
Accepted: 30 August 2015
Published: 15 September 2015
The light output power of AlGaInP-based vertical-injection light-emitting diodes (VI-LEDs) can be enhanced significantly using n-AlGaInP nanopillars. n-AlGaInP nanopillars, ~200 nm in diameter, were produced using SiO2 nanopillars as an etching mask, which were fabricated from self-assembled tin-doped indium oxide (ITO)-based nanodots formed by the wet etching of as-deposited ITO films. The AlGaInP-based VI-LEDs with the n-AlGaInP nanopillars provided 25 % light output power enhancement compared to VI-LEDs with a surface-roughened n-AlGaInP because of the reduced total internal reflection by the nanopillars at the n-AlGaInP/air interface with a large refractive index difference of 1.9.
Given the recent strong interest in the epitaxial quality of AlGaInP-based materials, the application of AlGaInP-based light-emitting diodes (LEDs) has been extended to automotive lighting, full-color displays, and visible light communications, which require high-brightness and high-power operation [1–4]. A vertical-injection geometry through wafer-bonded Si conductive substrates with a reflective electrode has been suggested as a solution to increase the light output power of AlGaInP-based LEDs because vertical-injection LEDs (VI-LEDs) yield better current spreading, good heat dissipation, and simple packaging [5, 6]. On the other hand, the light extraction efficiency (LEE) of AlGaInP VI-LEDs is limited by the total internal reflection because of the large difference in refractive index between the n-AlGaInP (n ∼ 2.9) and air (n = 1.0) .
Several beneficial methods that can enhance the LEE of AlGaInP LEDs have been reported. AlGaInP LEDs with a roughened surface, textured surface, and truncated pyramid geometry were reported to enhance the LEE by increasing the critical angle and the probability of escape of emitted light from an air/semiconductor interface [8–10]. Omni-directional reflectors as a p-type electrode and an air-hybrid distributed Bragg reflector structure were also reported to improve the LEE of AlGaInP LEDs significantly [11, 12]. Recently, Wenjing et al. suggested that the fabrication of AlGaInP-based nanorod LEDs using self-assembly metal layer nanomasks can enhance the probability of emitted light escaping from nanorod LEDs . Although the nanorods fabricated using Au metal clusters as nanomasks improved the LEE of AlInGaP LEDs greatly by enhancing the probability of escape of emitted light, this approach cannot be implemented in high-power AlGaInP-based VI-LEDs with wafer-bonded Si conductive substrates, because the formation of Au metal clusters requires high temperature annealing (>400 °C), which is significantly higher than that of wafer bonding between the Si conductive wafer and AlGaInP LED wafer.
The present study focused on improving the LEE of high-power AlGaInP-based VI-LEDs with wafer-bonded Si conductive substrates. For this purpose, n-AlGaInP nanopillars with a diameter of ~200 nm were fabricated using self-assembled ITO-based nanodots as etching nanomasks. This approach is a promising method for producing high-power AlGaInP-based VI-LEDs with wafer-bonded Si conductive substrates because ITO-based nanodots can be produced by the wet etching of as-deposited ITO films without an annealing process. The results show that the light output power of AlGaInP-based VI-LEDs with n-AlGaInP nanopillars can be improved considerably compared to that of the VI-LEDs with surface-roughened n-AlGaInP, which is a widely implemented method for increasing the LEE of AlGaInP-based VI-LEDs.
AlGaInP-based LEDs emitting a 610-nm wavelength were grown on 2-in. (100) GaAs substrates by metal-organic vapor phase epitaxy (MOVPE). The AlGaInP-based LED structure consisted of a GaInP etching stop layer and an n-GaAs contact layer grown on an n-GaAs buffer layer, a 2-μm-thick Si-doped n-AlGaInP cladding layer, an undoped active layer with 20 period AlGaInP/GaInP multiple quantum wells (MQWs), a Mg-doped p-AlGaInP layer, and a thick p+-GaP window layer.
To fabricate the AlGaInP-based VI-LEDs with a chip size of 1 mm2, Ni/Ag/Ni reflectors were deposited onto the p+-GaP contact layer and annealed at 350 °C for 1 min to ensure ohmic contact. Adhesive/barrier/bonding layers consisting of Ni/Cr/Ni/Au/Sn/Au were then deposited onto the Ni/Ag/Ni reflectors, and the AlGaInP-based LEDs on the GaAs substrates were bonded to the p-Si conductive substrates at 350 °C for 1 min in a nitrogen environment. After wafer bonding, the GaAs substrate and GaInP etching stop layer were removed using a NH4OH-based chemical etching solution. The Ni/Ge/Au contacts were then deposited on the n-GaAs contact layer, and Ti/Au boding pad metals were deposited on n-AlGaInP and the back side of the p-Si conducting substrate. Finally, n-AlGaInP nanopillars, ~200 nm in diameter, were produced using SiO2 nanopillars as an etching mask, which were fabricated by self-assembled ITO-based nanodots formed by wet etching of the as-deposited ITO films. For comparison, surface-roughened n-AlGaInP was also fabricated by wet etching of the n-AlGaInP layer using a H3PO4-based solution.
After fabricating the AlGaInP-based VI-LEDs with n-AlGaInP nanopillars or surface-roughened n-AlGaInP, the wafers were diced and the AlGaInP-based VI-LED chips were mounted onto the TO-18 headers with no epoxy encapsulation. All subsequent measurements were carried out using a conventional integration sphere. The n-AlGaInP nanopillars and ITO-based nanodots were examined by scanning electron microscopy (SEM).
Results and Discussion
This paper reports an improvement of the LEE of the AlGaInP-based VI-LEDs with wafer-bonded Si conductive substrates using the n-AlGaInP nanopillars. Nanopillars with a diameter of ~200 nm were produced using SiO2 nanopillars as an etching mask, which were fabricated by self-assembled ITO-based nanodots formed by the wet etching of as-deposited ITO films without an annealing process. The height of the n-AlGaInP nanopillars were varied from 350 to 900 nm, and n-AlGaInP nanopillars with a height of 350 nm were sufficient to increase in the probability of emitted light escaping from the air/semiconductor interface. The AlGaInP-based VI-LEDs with the n-AlGaInP nanopillars provided 25 % light output power enhancement compared to the VI-LEDs with the surface-roughened n-AlGaInP because the height of the n-AlGaInP nanopillars was four times higher than that of the triangle-like-roughened n-AlGaInP.
This study was supported financially by the Basic Science Research Program through the NRF of Korea funded by the Ministry of Education (NRF-2014R1A6A1030419 and NRF-2015R1D1A3A01019050).
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