Low-Resistance and Reflective Ni/Rh and Ni/Au/Rh Contacts to p-GaN for Flip-Chip LEDs

Abstract

We report on the formation of high-quality ohmic contacts to p-type GaN (4 ϫ 10 17 cm Ϫ3 ) using Ni ͑5 nm͒/Rh ͑120 nm͒ and Ni ͑5 nm͒/Au ͑5 nm͒/Rh ͑120 nm͒ schemes for use in flip-chip light-emitting diodes ͑LEDs͒. Both the Ni/Rh and Ni/Au/Rh contacts become ohmic with specific contact resistances of 10 Ϫ5 to 10 Ϫ6 ⍀ cm 2 , when annealed at 350°C for 2 min in air ambient. LEDs are fabricated using the Ni/Au/Rh and Ni/Au contact layers, which give a forward-bias voltage of 3.32 and 3.45 V at injection current of 20 mA, respectively. This indicates that the Rh-based contacts could be suitable for high power flip-chip LEDs.GaN-based compound semiconductors are of great technological importance for their applications in optoelectronic devices, such as blue light-emitting diodes ͑LEDs͒ and blue laser diodes ͑LDs͒. In particular, for solid-state lighting application, the fabrication of high brightness LEDs is essential. To realize such LEDs, the achievement of high extraction efficiency in LED structures is important. It was shown that high extraction efficiency in GaN-based LEDs could be obtained by the use of the flip-chip configuration where emitted light is extracted through the sapphire substrate. 1,2 For flip-chip LEDs ͑FCLEDs͒, the fabrication of highly reflective and low resistance contacts to p-GaN is crucial. So far, efforts have been mainly made to develop low resistivity ohmic contacts for p-GaN using a number of different metallization schemes. 3-13 For the flip-chip configuration, however, the reflectivity of ohmic contacts is a key factor. Al, Ag, and Rh are known to be the best metallic reflectors. For example, Hibbard et al., 1 investigating Al-and Ag-based flip-chip configuration contacts, showed that oxidized Ni/Au bilayers/Al ͑or Ag͒/ Ni/Au multiplayer contacts produce much better electrical and optical performance compared with conventional Ni/Au contacts. However, Al contacts produce poor ohmic, and Ag contacts suffer from poor adhesion problem and agglomeration during postdeposition annealing. 11,12 Song et al., 14 investigated another reflective Rh contacts and showed that the nonalloyed Rh ͑10 nm thick͒ contacts yield a low specific contact resistance of ϳ10 Ϫ5 ⍀ cm 2 , when the samples were two-step surface-treated using a buffered oxide etch solution prior to metal deposition. It was suggested that the nonalloyed Rh-based contacts could be a promising scheme for the fabrication of high-performance optical devices. However, single Rh contacts suffered from thermal degradation when annealed at temperatures in excess of 300°C. 15 Thus, in this work, Ni and Ni/Au interlayers are introduced to improve the thermal stability of the single Rh contacts. We have investigated Ni ͑5 nm͒/Rh ͑120 nm͒ and Ni ͑5 nm͒/Au ͑5 nm͒/Rh ͑120 nm͒ ohmic contacts for FCLEDs as a function of annealing temperature. It is shown that the Ni/Rh and Ni/Au/Rh contacts produce specific contact resistances as low as 5.13 ϫ 10 Ϫ5 and 9.51 ϫ 10 Ϫ6 ⍀ cm 2 , respectively, when annealed at 350°C for 2 min in air ambient. Multi-quantu...

Ag and zinc oxide-doped indium oxide ohmic contacts to p-type GaN for flip-chip LED applications

“…Accordingly, two kinds of p-type ohmic contact with high reflectivity and low contact resistance have been investigated to develop low-resistance ohmic contacts to p-type GaN with light reflectance to achieve high light-extraction efficiency FCLEDs. The first one is based on the multi-metal contact, such as Ir/Ag [9], Pd/Ni/Al/Ti/Au [10], Ni/Ag/Au [11] and Ni/Au/Rh [12]; the other is based on transparent conductive oxide (TCO)/Ag contact, such as MIO/Ag [13] and ICO/Ag [14]. In the Ir/Ag contacts [9], the specific contact resistance and the reflectivity are about 3.26 × 10 −4 cm 2 and 70-80%, respectively.…”