Low Resistance and Highly Reflective Sb-Doped SnO[sub 2]/Ag Ohmic Contacts to p-Type GaN for Flip-Chip LEDs

Abstract

We have investigated high-quality Sb-doped SnO 2 /Ag ohmic contacts to p-GaN for use in flip-chip light emitting diodes ͑LEDs͒. The Sb-doped SnO 2 /Ag contacts produce specific contact resistances of ϳ10 Ϫ4 ⍀ cm 2 upon annealing at 430 and 530°C for 1 min in air. It is shown that InGaN blue LEDs fabricated with the Sb-doped SnO 2 /Ag contacts give a forward-bias voltage of 3.18 V at 20 mA, while LEDs with Ag contacts show 3.36 V. It is further shown that the LEDs made with the Sb-doped SnO 2 /Ag contact layers show higher light output power compared with the LEDs with the Ag contacts.Group III-nitride semiconductors have been extensively investigated because of their technological importance for the fabrication of light emitting diodes ͑LEDs͒, laser diodes, and photo-detectors. [1][2][3] In particular, GaN-based LEDs are important for practical applications such as solid-state lighting and full-color displays. 4,5 Commercially available GaN-based LEDs, however, have some limits to the application for solid-state lighting. For commercial top-emitting GaN-based LEDs grown on sapphire substrates, light is extracted through a semi-transparent current spreading layer on the top of p-GaN, which limits the light extraction efficiency. 4 To overcome this problem, flip-chip LEDs have recently been introduced. 6,7 In this configuration, LEDs are fabricated with highly reflective metals and light is extracted through the transparent sapphire substrate rather than a partially absorbing current spreading layer. Ag and Al are the most commonly used reflectors for flip-chip LEDs, although Ag was shown to be better than Al due to its higher reflectivity and better ohmic behavior. 4,8 However, Ag suffers from poor adhesion to p-GaN and agglomeration upon annealing, which degrade its electrical and thermal behaviors. 4,8,9 Thus, recently, multilayer contacts consisting of a thin oxidized Ni/Au bilayer overcoated with a thick Al or Ag layer were investigated to minimize the drawbacks of single Ag contacts. 8 However, because Au itself absorbs and scatters the extracted light, and contributes to the lowering of light output, Au-free contact layers have been investigated. For example, Gessmann et al., 10 investigating an ITO/Ag contact to p-GaN, showed that LEDs with the new scheme produces significantly improved output power compared to that with the Ni/Au contact. Therefore, in this work, we have also investigated a transparent conducting oxide ͑Sb-doped SnO 2 )-based contact to p-GaN. The Sb-doped SnO 2 layer was employed as a first layer because of its high transmittance of about 95% at 460 nm. The Sb-doped SnO 2 /Ag contacts produce a good ohmic contact with specific contact resistances of ϳ10 Ϫ4 ⍀ cm 2 when annealed at 430 and 530°C in air ambient. The electrical and optical performances of LEDs fabricated with the annealed Sbdoped SnO 2 /Ag contacts are characterized and compared to those of LEDs with the annealed Ag contact.Metallorganic chemical vapor deposition grown 1.0 m thick p-GaN layers (5 ϫ 10 17 cm Ϫ3 ͒ were ultrason...

Improvement of the reverse leakage behavior of Ag-based ohmic contacts for GaN-based light-emitting diodes using MgZnO interlayer

“…(It should be, however, noted that the electrical measurements were performed on a 200 nm-thick MZO film, but not 2.5 nm thick ones.) Second, it could be associated with the formation of Ag-Ga solid-solution [3][4][5]12], as expected from the GDS result. The formation of the Ag-Ga solid-solution produces deep acceptor-like Ga vacancies near the GaN surface region under the contacts, resulting in an increase in carrier concentration at the surface region and so the reduction of the Schottky barrier height [13][14][15].…”

“…One possible reason why the MZO/Ag reflector produces higher reflectance that that of the annealed Ag contact could be related to the thermal instability of Ag contacts. In other words, the single Ag electrode experiences thermal degradation, such as the formation of interfacial voids upon annealing [3,4]. The formation of the interfacial voids results in the formation of a very rough Ag/GaN interface and so the Ag reflector has a very rough surface.…”

“…Ag is the most commonly used reflectors for FCLEDs, because it has high reflectivity and gives relatively good ohmic behavior [1]. However, a single Ag electrode experiences problems, such as agglomeration and the formation of interfacial voids upon annealing, which degrades the electrical and optical performances of LEDs [1][2][3][4]. To resolve such problems, various interlayers were introduced between Ag and p-GaN.…”

“…To resolve such problems, various interlayers were introduced between Ag and p-GaN. For example, Leem et al [3] showed that LEDs fabricated with an Sb-doped tin oxide layer result in higher reflectivity and output performance as compared with LEDs with single Ag reflectors. Song et al [4,5] also reported that Ag-based contacts combined with Cu-and Mg-doped * Corresponding author.…”

Improved Light Output of GaN-Based Light-Emitting Diodes by Using AgNi Reflective Contacts

“…The introduction of interlayers significantly enhanced the light output of GaN-based LEDs. [8][9][10][11][12] In addition, Ag-based alloy layers, such as AgAl, AgCu, and AgMg, have also been used to solve the problem of Ag-only contacts; for example, Kim et al, 13 investigating the electrical and optical properties of AgAl alloy (3 at.% Al) contacts to p-type GaN, showed that LEDs fabricated with AgAl contacts exhibited much higher light output than did those with Ag-only contacts. Kim and Lee,14 investigating LED performance with Ag-based contacts, such as AgNi, AgCu, and AgAl, showed that, after annealing at 400°C to 450°C, LEDs fabricated with Ag-based contacts yielded higher power efficiency as compared with those with Ag-only contacts.…”