High-quality nonalloyed rhodium-based ohmic contacts to p-type GaN

Electrochem. Solid-State Lett.

Leem

et al. 2005

Self Cite

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...

supporting

confidence: 91%

mentioning

confidence: 99%

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

Park

Electrochem. Solid-State Lett.

Leem

et al. 2005

Self Cite

“…3(a), there is no significant intermixing between the oxidized Ag-ITO film and metallic Al layer, and each layer is well defined. However, it is clear that even if the thin Ag film outdiffused toward the surface during oxidizing the Ag/ITO layer, a small amount of Ag remains near the GaN surface resulting in forming Ag solid solution including Ga atoms, in accordance with the AES result reported by Song et al [6,7]. For the oxidized Ag-ITO film/Al scheme in annealed at 330 °C in vacuum, Fig.…”

supporting

confidence: 84%

“…In particular, owing to their thermal stability, both Rh and Al contacts have been considered as potentially important reflectors for the fabrication of highpower flip-chip LEDs. However, single Rh contacts suffer from thermal degradation when annealed at temperatures in excess of 300 °C, and Al contacts do not show ohmic characteristics to p-type GaN [3,4]. Thus, in this work, we have investigated Zn/Rh and oxidized Ag-ITO/Al schemes for the formation of low resistance and highly reflective p-ohmic contacts for FCLEDs.…”

mentioning

confidence: 97%

Low resistance and highly reflective ohmic contacts to p ‐type GaN using transparent interlayers for flip‐chip light emitting diodes

Phys. Status Solidi (c)

Kwak

Hong

et al. 2006

PACS 73.40. Cg, 85.60.Jb Zn (3 nm)/Rh (100 nm) and oxidized Ag-ITO (100 nm)/Al (200 nm) schemes for the formation of low resistance and highly reflective ohmic contacts for flip-chip light emitting diodes were investigated. It is shown that annealing the samples at 430 and 530 °C for 1 min in air results in good ohmic behaviors with specific contact resistances of ~ 10 -5 Ωcm 2 LEDs fabricated using the Zn/Rh contact layers give forwardbias voltages of 3.09-3.12 V at the injection current of 20 mA. Furthermore, the oxidized Ag-ITO film/Al contacts also show good ohmic characteristics with contact resistivity of 10 -5 Ωcm 2 when annealed at 330 °C for 5 min in vacuum. LEDs fabricated using the oxidized Ag-ITO film/Al contacts layers give forward-bias voltages of ~ 3.40 V at 20 mA. It is clearly shown that a transparent contact interlayer is necessary to make high-quality ohmic contacts to p-type GaN for thermally stable flip-chip LEDs using Rhand Al-reflectors.

“…Several attempts have been hitherto made to develop high-quality ohmic contacts to p-type GaN using Rh reflectors. 2,3 For example, Song et al, 2 showed that nonalloyed Rh ͑10 nm thick͒ contacts yield a low specific contact resistance of ϳ10 −5 ⍀ cm 2 , when the samples were two-step surface-treated using a buffered oxide etch solution prior to metal deposition. However, single Rh contacts suffer from thermal degradation when annealed at temperatures in excess of 300°C.…”

mentioning

confidence: 99%

Reflective and Low-Resistance Zn∕Rh Contacts to p-Type GaN for Flip-Chip Light-Emitting Diodes

Electrochem. Solid-State Lett.

Hong

et al. 2005

Self Cite

We report on the formation of high-quality ohmic contacts to p-type GaN ͑5 ϫ 10 17 /cm 3 ͒ using a Rh ͑100 nm͒ layer combined with a 3 nm thick Zn interlayer for high-power flip-chip light-emitting diodes ͑LEDs͒. The as-deposited sample produces a nonlinear behavior. However, the samples annealed at 430 and 530°C for 1 min in air become ohmic with a contact resistivity of ϳ10 −5 ⍀ cm 2 . Measurements show that the reflectivity of the samples annealed at 530°C is 73% at 460 nm. LEDs fabricated using the Zn/Rh contact layers give forward-bias voltages of 3.09-3.12 V at an injection current of 20 mA.High-performance flip-chip light-emitting diodes ͑LEDs͒ are of significant technological importance for applications in solid-state lighting. 1 In order to realize such LEDs, the development of p-type ohmic electrodes having high reflectivity as well as low contact resistance is crucial. Ag, Al, and Rh are known to be the best metallic reflectors. In particular, owing to its thermal stability, a Rh contact has been considered as a potentially important electrode for the fabrication of high-power flip-chip LEDs. Several attempts have been hitherto made to develop high-quality ohmic contacts to p-type GaN using Rh reflectors. 2,3 For example, Song et al., 2 showed that nonalloyed Rh ͑10 nm thick͒ contacts yield a low specific contact resistance of ϳ10 −5 ⍀ cm 2 , when the samples were two-step surface-treated using a buffered oxide etch solution prior to metal deposition. However, single Rh contacts suffer from thermal degradation when annealed at temperatures in excess of 300°C. 4 Recently, Lee et al. 3 used an 8 nm thick Pt interlayer to improve the ohmic behavior of Rh contacts and showed that Pt/Rh contacts give a specific contact resistance of 9.0 ϫ 10 −5 ⍀ cm 2 and reflectance of 62% at 460 nm when annealed at 500°C. In this work, to improve the thermal stability of single Rh contacts, a Zn layer is added inbetween a Rh layer and a GaN layer. Use of a Zn layer is advantageous since Zn atoms could act as an acceptor dopant 5 or transform into transparent conducting oxide, such as ZnO, when annealed in air. The results show that the Zn͑3 nm͒/Rh͑100 nm͒ contacts produce specific contact resistances as low as ϳ10 −5 ⍀ cm 2 , when annealed at 430 and 530°C for 1 min in air. GaN-based LEDs fabricated using the Zn/Rh ohmic contact layers give forward-bias voltages of 3.09-3.12 V at an injection current of 20 mA.Metallorganic chemical vapor deposition ͑MOCVD͒ was used to grow 2 m thick unintentionally doped GaN layers on ͑0001͒ sapphire substrates. This was followed by the growth of 1 m thick p-GaN:Mg layer ͑n a = 5 ϫ 10 17 /cm 3 ͒. The GaN layers were ultrasonically degreased with trichloroethylene, acetone, methanol for 5 min in each step, and then rinsed with deionized water. After cleaning the samples using a buffered oxide etch ͑BOE͒ solution for 20 min, 6 the GaN layers were blown dry by nitrogen gas. Circular transfer length model ͑CTLM͒ patterns were defined by the photolithographic technique to measure the specific...