Ja Soon Jang

Low-resistance Pt/Ni/Au ohmic contacts to p-type GaN

We report on a Pt (20 nm) Ni (30 nm)/Au (80 nm) metallization scheme for low-resistance ohmic contacts to the moderately doped p-type GaN:Mg (3×1017u200acm−3). Both as-deposited and annealed Pt/Ni/Au contacts on p-GaN exhibit linear current–voltage characteristics, showing that a high-quality ohmic contact is formed. The Pt/Ni/Au scheme shows the specific contact resistance of 5.1×10−4u200aΩu200acm2 when annealed at 350u200a°C for 1 min in a flowing N2 atmosphere.

Formation of low resistance Pt ohmic contacts to p-type GaN using two-step surface treatment

Two-step surface treatment is introduced to obtain low resistance Pt contacts to p-type GaN. The first step is performed after the mesa etching process using buffered oxide etch (BOE) and ammonium sulfide [(NH4)2Sx]. This is followed by the second step using BOE. The Pt contact, that was simply BOE treated, yields 2.1(±0.9)×10−2 Ωu200acm2. However, the contact which was treated sequentially using ultrasonically boiled BOE (10 min) and boiled (NH4)2Sx (10 min), produces a specific contact resistance of 2.0(±3.5)×10−5 Ωu200acm2. To the best of our knowledge, this is the lowest contact resistance reported hitherto for the contacts on p-GaN. The effective Schottky barrier heights (SBHs) of the differently surface-treated contacts were determined using the Norde and current–voltage methods. It is shown that the SBHs are dependent upon the surface treatment conditions.

Mechanisms for the reduction of the Schottky barrier heights of high-quality nonalloyed Pt contacts on surface-treated p-GaN

X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical bonding and electronic properties of the interfaces between Pt and p-GaN layers that were two-step surface treated using a buffered-oxide etch solution, and hence, to understand the surface-treatment time dependence of the Schottky barrier height (SBH). Current–voltage (I–V) measurements show that the effective SBH decreases with increasing surface-treatment time. The XPS results show that as the treatment time increases, the Ga 2p and Pt 4f core levels for the 20-min-treated samples shift toward the lower-binding-energy side by 0.6 and 1.5 eV, respectively, compared to the 0.5-min-treated one. It is further shown that the intensity of the oxygen core-level peak decreases with increasing treatment time. Based on the I–V and XPS results, the observed reduction of the effective SBHs is attributed to the combined effects of the effective removal of the native oxide and the shift of the surface Fermi level toward the valence-band ...

Electronic transport mechanisms of nonalloyed Pt Ohmic contacts to p-GaN

We report on the electronic transport mechanisms for nonalloyed Pt Ohmic contacts to p-GaN which were surface treated using a buffered oxide etch solution and (NH4)2Sx. Measurements show that the value of the effective Richardson constant (A**) is 12 Au200acm−2u200aK−2, which is considerably smaller than the theoretical value of 103.8 Au200acm−2u200aK−2. Based on Hall-effect results, the two-step surface-treated contact is modeled to consist of a Pt/p+-/p-GaN structure, and the conventionally treated contact consists of a Pt/p-GaN structure. The theoretical results obtained using these models are compared with the experimental data. It is shown that for the conventionally treated contact thermionic emission dominates the current flow, whereas for the two-step surface-treated contact, field emission is dominant.

Schottky barrier characteristics of Pt contacts to n-type InGaN

Schottky barrier behaviors of Pt contacts to n-InGaN have been investigated by means of current-voltage (I-V) and capacitance-voltage (C-V) methods. It is found that the Schottky barrier heights (SBHs) determined by thermionic emission (TE) and thermionic field emission (TFE) modes using the I-V data are quite different from each other. However, the SBHs obtained by the TFE mode are fairly similar to theoretically calculated values, which are in good agreement with the results obtained by the C-V method. It is also shown that the SBHs and the ideality factors calculated by the TE and TFE modes decrease with increasing annealing temperature. The different SBHs obtained by the TE and TFE modes, the annealing temperature dependence of the SBHs, and the ideality factors are described and discussed in terms of the presence of different types of native point defects near the InGaN surface.

Metallization scheme for highly low-resistance, transparent, and thermally stable Ohmic contacts to p-GaN

We report on a promising metallization scheme for high-quality Ohmic contacts to surface-treated p-GaN:Mg (2–3×1017u200acm−3). It is shown that the as-deposited Pt/Ru contact produces a specific contact resistance of 7.8(±2.2)×10−4u200aΩu200acm2. However, annealing of the contact at 600u200a°C for 2 min results in a resistance of 2.2(±2.0)×10−6u200aΩu200acm2. It is also shown that the light transmittance of the annealed contact is 87.3% at 470 nm. Furthermore, the surface of the contact annealed at 600u200a°C for 30 min is found to be very smooth with a rms roughness of 0.8 nm. These results strongly indicate that the Pt/Ru can be a suitable scheme for the fabrication of high-performance laser diodes or other devices.

Low-resistance and thermally stable indium tin oxide Ohmic contacts on strained p‐In0.15Ga0.85N∕p‐GaN layer

We report on the formation of low-resistance and thermally stable indium tin oxide (ITO) Ohmic contacts on p‐GaN using a strained p‐InGaN (5nm) layer. Unlike as-deposited ITO contacts on p‐GaN, the as-deposited contacts to a strained p‐InGaN∕p‐GaN layer exhibit Ohmic behavior. The current-voltage characteristics of the ITO∕InGaN∕GaN contacts are further improved upon annealing at 550°C for 1min and the specific contact resistance is 3.2(±0.8)×10−5Ωcm2. In addition, the contact resistivity and surface morphology of the ITO∕InGaN∕GaN contacts annealed for 30min are only slightly increased, confirming the thermal stability of this scheme. Based on the electrical and Auger spectroscopic data, the low contact resistivity and thermal stability of the ITO∕InGaN∕GaN contacts are described in terms of the polarization effect, increased acceptor concentration, and formation of diffusion barrier layer at the interface.

Low Resistance and High Reflectance Pt/Rh Contacts to p-Type GaN for GaN-Based Flip Chip Light-Emitting Diodes

We report on a Pt(8 nm)/Rh(200 nm) metallization scheme for use in low resistance and high reflectance ohmic contacts to p-GaN for use in flip chip light-emitting diodes (FCLED) applications. The thermal annealing of a Pt/Rh contact at 500°C led to linear current-voltage characteristics with a specific contact resistance of 9.0 X 10 - 5 Ω cm 2 while the as-deposited Pt/Rh contact showed nonohmic characteristics. It was also shown that the 500°C annealed contact produces a high reflectance of 62% at a wavelength of 460 nm. Auger electron spectroscopy depth profiles and glancing-angle X-ray diffraction results indicated that the low contact resistance of the annealed sample can be attributed to the formation of Ga-Pt and Ga-Rh intermetallic phases. Furthermore, the electrical performance of an FCLED with the Pt/Rh reflector scheme was superior to that of a FCLED with a Ni/Au/Ag reflector scheme.

Interfacial Reaction of Ni/Pt/Au Contact Schemes to p‐Type GaN

Glancing angle X‐ray diffraction and Auger electron spectroscopy have been used to investigate interfacial reactions between the Ni(20 nm)/Pt(30 nm)/Au(80 nm) contacts and p‐GaN . The metallization schemes were annealed at temperatures ranging from 500 to 700°C for 30 s in a flowing Ar atmosphere. The anneal of the sample at 500°C resulted in ohmic behavior with a specific contact resistance of . However, the anneal at temperatures ≥600°C led to the degradation of the ohmic property. It is shown that Ga‐(Pt,Ni) phases such as and , and a Pt‐Ni solid solution are formed upon annealing at 500°C, while in addition to the , new phases of and GaAu are formed upon annealing at temperatures 600°C. As for the Pt‐Ni solid solution, the lattice parameter decreases significantly with increasing annealing temperatures. A qualitative explanation is given to describe why the characteristic of the contacts changed from ohmic to rectifying behavior with increasing temperature.

Ultrahigh transparency of Ni/Au ohmic contacts to surface-treated p-type GaN

We report on ultrahigh-transparency and low-resistance Ni/Au ohmic contacts to surface-treated p-GaN:Mg (3.6×1017u200acm−3). It is shown that annealing at 500u200a°C for 1 min in a N2 ambient improves ohmic contact properties. Specific contact resistance is measured to be 5.0(±1.0)×10−3 and 2.5(±1.0)×10−3u200aΩu200acm2 for the as-deposited and annealed samples, respectively. It is also shown that the light transmittance is 90.3(±0.6) and 97.3(±0.8) % (at 470 nm) for the as-deposited and annealed contacts, respectively. Furthermore, the surface of the annealed contact is found to be fairly smooth with a root-mean-square roughness of 0.84 nm. These results are compared with those previously reported for the Ni/Au contacts.