High-quality Ohmic Contacts Research Articles

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

MgZnO(MZO) interlayers are introduced to form high-quality Ag-based ohmic contacts to p-type GaN ( N a = 4 × 10 17 cm −3) for GaN-based flip-chip light-emitting diodes (LEDs). The MZO (2.5 nm)/Ag (250 nm) contacts become ohmic with specific contact resistance of 3.83 × 10 −4 Ω cm 2 and reflectance of ∼83% at a wavelength of 405 nm when annealed at 530 °C for 1 min in air. It is shown that LEDs made with the MZO interlayers yield reverse leakage currents three orders of magnitude lower than that of LEDs without the interlayers. In addition, the LEDs fabricated with the MZO interlayers show higher output power as compared with the LEDs without the interlayers.

Contacts to ZnO

A review is given of Ohmic and Schottky contacts to n- and p-type ZnO. It is relatively straightforward to form high-quality Ohmic contacts to n-type ZnO, with specific contacts resistivity in the range 10 −6 Ω cm 2 even for unnannealed contacts on strongly n-type layers, while recent work has also shown good results (10 −5–10 −6 Ω cm 2) for Au or Ni/Au annealed at 300–600 °C. Schottky contacts to both n- and p-type ZnO are much lower than expected from the metal work function and the electron affinity of ZnO, suggesting that surface states are important in determining the effective barrier height. The Schottky contacts also show poor thermal stability. For transparent transistors based on ZnO, this suggests that oxide gates are more suitable than metal gates.

Experimental investigation on nonlinearities of PIN photodiodes

In this report, we experimentally investigate nonlinearity of PIN photodiode as a function of incident light power with different bias. It is found that the nonlinearity can be related with effective resistance of this device itself. According to ambipolar diffusion model, the resistance is divided into two parts, i.e. intrinsic region resistance and series resistance originating from non-Ohmic contact. Forward Current (If)–Voltage(Vf) plots indicate that fabrication of high-quality Ohmic contact is necessary to improve linear performance for PIN photodiodes.

Improvement of the luminous intensity of light-emitting diodes by using highly transparent Ag-indium tin oxide p-type ohmic contacts

We have investigated Ag-indium tin oxide (ITO) scheme for obtaining high-quality p-type ohmic contacts for GaN-based light-emitting diodes (LEDs). The Ag(1 nm)-ITO(200 nm) contacts exhibit greatly improved electrical characteristics when annealed at temperatures in the range 400/spl deg/C-600/spl deg/C for 1 min in air, yielding specific contact resistances of /spl sim/10/sup -4/ /spl Omega//spl middot/cm/sup 2/. In addition, the contacts give transmittance of about 96% at 460 nm, which is far better than that of the conventionally used oxidized Ni-Au contacts. It is shown that the luminous intensity of blue LEDs fabricated with the Ag-ITO contacts is about three times higher than that of LEDs with oxidized Ni-Au contacts. This result strongly indicates that the Ag-ITO scheme can serve as a highly promising p-type ohmic contact for the realization of high brightness near ultraviolet LEDs.

Cu -doped indium oxide∕Ag ohmic contacts for high-power flip-chip light-emitting diodes

We have investigated Cu-doped In2O3(CIO)(3nm)∕Ag(250nm) schemes for forming high-quality ohmic contacts to p-type GaN for high-power flip-chip light-emitting diodes (FCLEDs). It is shown that the CIO∕Ag contacts produce specific contact resistance of 1.28×10−5Ωcm2 and reflectance of about 90% at a wavelength of 460nm when annealed at 530°C for 1min in air ambient. It is also shown that unlike single Ag contacts, the CIO∕Ag contacts are fairly stable without surface and interface degradation although annealed at 530°C for 1min in air ambient. In addition, blue multiquantum-well InGaN∕GaN LEDs fabricated with the annealed CIO∕Ag contact layers give forward-bias voltages of around 3.0V at an injection current of 20mA. The results strongly indicate that the CIO∕Ag scheme can be a highly promising p-type contact for high-power GaN-based FCLEDs for solid-state lighting application.

Formation of High-Quality Ag-Based Ohmic Contact to p-Type GaN for UV LEDs Using a Tin-Zinc Oxide Interlayer

Formation of High-Quality Ag-Based Ohmic Contact to p-Type GaN for UV LEDs Using a Tin-Zinc Oxide Interlayer

Formation of High-Quality Ohmic Contacts to p-GaN for Flip-Chip LEDs Using Ag∕TiN[sub x]∕Al

We have investigated Ag s2.5 nmd/TiNx s50 nmd/Al s200 nmd contacts for use in GaN-based flip-chip light emitting diodes ~LEDs!. The Ag/TiNx/Al contact becomes ohmic with specific contact resistance of 4.4 3 10 −3 V cm2 when annealed at 430°C for 1 min in nitrogen ambient. It is shown that the continuous Ag interlayer is broken into Ag nanodots when annealed. It is also shown that the TiNx barrier layer effectively hampers the indiffusion of Al toward GaN. Blue LEDs are fabricated using the annealed Ag/TiNx/Al contacts and are compared with those made with the annealed Ni/Au/Al contacts. © 2005 The Electrochemical Society. @DOI: 10.1149/1.1911999# All rights reserved.

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

We report on the formation of high-quality ohmic contacts to p-type GaN (4 × 10 1 7 cm - 3 ) using Ni (5 nm)/Rh (120 nm) and Ni (5 nm)/Au (5 nm)/Rh (120 nm) schemes for use in flip-chip light-emitting diodes (LEDs). Both the Ni/Rh and Ni/Au/Rh contacts become ohmic with specific contact resistances of 10 - 5 to 10 - 6 Ω 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.

Low Resistance and Reflective Mg-Doped Indium Oxide–Ag Ohmic Contacts for Flip-Chip Light-Emitting Diodes

We have investigated an Mg-doped In/sub x/O/sub y/(MIO)-Ag scheme for the formation of high-quality ohmic contacts to p-type GaN for flip-chip light-emitting diodes (LEDs). The as-deposited sample shows nonlinear current-voltage (I--V) characteristics. However, annealing the contacts at temperatures of 330/spl deg/C-530/spl deg/C for 1 min in air ambient results in linear I--V behaviors, producing specific contact resistances of 10/sup -4/--10/sup -5/ /spl Omega//spl middot/cm/sup 2/. In addition, blue LEDs fabricated with the MIO-Ag contact layers give forward-bias voltages of 3.13-3.15 V at an injection current of 20 mA. It is further shown that LEDs made with the MIO-Ag contact layers give higher output power compared with that with the Ag contact layer. This result strongly indicates that the MIO-Ag can be a promising scheme for the realization of high brightness LEDs for solid-state lighting application.

Interfacial reaction effect on the ohmic properties of a Pt/Pd/Au contact on p-type GaN

We have investigated the effect of interfacial reactions between a Pt(7 nm)/Pd(7 nm)/Au(15 nm) layer and p-GaN on its ohmic contact properties by using Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and glancing angle x-ray diffraction. The annealed Pt/Pd/Au contact exhibits linear current-voltage characteristics, which indicates that a high-quality ohmic contact is formed. The Pt/Pd/Au contact shows a specific contact resistivity of 3.1×10−5 Ω cm2 when annealed at 600 °C for 2 min in flowing N2 atmosphere. Both AES and XPS results show that the diffusion of Pt and Pd into the GaN surface region results in the formation of gallide and plays an important role in forming a low resistance ohmic contact by the creation of a highly doped p+-GaN surface region.

Low resistance Ni–Zn solid solution/Pd ohmic contacts to p-type GaN

We have investigated Ni–Zn alloy(5 nm)/Pd(5 nm) metallization schemes for use in forming high-quality ohmic contacts to p-GaN (5 × 1017 cm−3). The as-deposited Ni–Zn alloy/Pd contact exhibits nonlinear current–voltage (I–V) characteristics. However, annealing the contacts at 530 °C for 1 min in either air or N2 ambient improves I–V behaviour drastically. It is shown that the contacts produce specific contact resistance of 6.95 × 10−5 and 4.8 × 10−5 Ω cm2 when annealed in N2 and air ambient, respectively. It is further shown that multi-quantum-well light-emitting diodes fabricated with the annealed Ni–Zn alloy/Pd contact layers give a forward-bias voltage of 3.17 and 3.28 V at an injection current of 20 mA when annealed in air and N2 ambient, respectively.

Low-resistance Pt/Pd/Au ohmic contacts to p-type AlGaN

We report on a Pt/Pd/Au metallization scheme for producing low-resistance ohmic contacts to moderately doped p-AlGaN:Mg (1.5×1017 cm−3). Annealed Pt/Pd/Au contact exhibits linear current–voltage characteristics, showing that a high-quality ohmic contact is formed. The Pt/Pd/Au contact exhibits a specific contact resistivity of 3.1×10−4 Ω cm2 when annealed at 600 °C for 1 min in a flowing N2 atmosphere. Using Auger electron spectroscopy and x-ray photoelectron spectroscopy, a preliminary explanation for ohmic contact formation is described.

High-quality schottky and ohmic contacts in planar 4H-SiC metal semiconductor field-effect transistors and device performance

We have fabricated planar 4H-SiC, metal-semiconductor field-effect transistors (MESFETs) with high-quality metal/SiC contacts. To eliminate potential damage to the gate region caused by etching and simplify the device fabrication process, gate Schottky contacts were formed without any recess gate etching, and an ideality factor of 1.03 was obtained for these gate contacts. The interface state density between the contact metal and SiC was 5.7×1012 cm−2eV−1, which was found from the relationship between the barrier height and the metal work function. These results indicate that the interface was well controlled. Thus, a transconductance of 30 mS/mm was achieved with a 3-µm gate length as the performance figure of these MESFETs with high-quality metal/SiC contacts. Also, a low ohmic contact resistance of 1.2×10−6 Θcm2 was obtained for the source and drain ohmic contacts by using ion implantation.

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

We have investigated high-quality Sb-doped ohmic contacts to p-GaN for use in flip-chip light emitting diodes (LEDs). The Sb-doped contacts produce specific contact resistances of Ω cm2 upon annealing at 430 and 530°C for 1 min in air. It is shown that InGaN blue LEDs fabricated with the Sb-doped 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 contact layers show higher light output power compared with the LEDs with the Ag contacts. © 2004 The Electrochemical Society. All rights reserved.

Low Resistance Ni-Mg Solid Solution/Pt Ohmic Contacts to p-Type GaN

We investigate Ni-Mg solid solution (4 nm)/Pt (6 nm) metallization schemes for use in forming high-quality ohmic contacts to p-GaN:Mg The as-deposited Ni-Mg solid solution/Pt contact reveals nonlinear current-voltage characteristics. Annealing the contacts at 450 and 550°C for 1 min in air ambient, however, improves electrical behaviors significantly. For example, specific contact resistance as low as is obtained, when the contacts are annealed at 450°C. Based on the I-V measurement, Auger electron spectroscopy, and X-ray photoemission spectroscopy results, possible ohmic formation mechanisms for the Ni-Mg solid solution/Pt contacts are discussed. © 2004 The Electrochemical Society. All rights reserved.

Low-resistance and highly-reflective Zn–Ni solid solution/Ag ohmic contacts for flip-chip light-emitting diodes

We have investigated a Zn–Ni solid solution/Ag scheme for use in producing high-quality ohmic contacts for flip-chip light-emitting diodes (LEDs). The as-deposited contact shows nonlinear I–V characteristics. However, oxidizing the contacts at temperatures of 350–550 °C for 1 min in air ambient results in linear I–V behaviors, yielding specific contact resistances of 10−4–10−5 Ω cm2. In addition, LEDs are fabricated with the oxidized Zn–Ni solid solution/Ag contacts and Ag single contacts. The typical I–V characteristics of the LEDs with the annealed Zn–Ni solid solution (2.5 nm)/Ag (200 nm) p-type contact layers reveal a forward-bias voltage of 3.25 V at an injection current of 20 mA, which is much better than that of the LEDs with the Ag (200 nm) contact layers.

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

We report on a promising Rh-based scheme for high-quality ohmic contacts to surface-treated p-GaN:Mg (4×1017 cm−3). It is shown that the two-step surface-treated Rh contacts (10 nm) produce a specific contact resistance of 1.7×10−5 Ω cm2. It is also shown that the two-step treated Rh/Ni (5/5 nm) and Rh/Au (5/5 nm) contacts yield 6.0×10−5 and 9.3×10−6 Ω cm2, respectively. Based on the current–voltage measurement, x-ray photoemission spectroscopy, and Auger electron spectroscopy results, the mechanisms for the formation of the nonalloyed Rh-based ohmic contacts is described and discussed.

Schottky diode back contacts for high frequency capacitance studies on semiconductors

A technique using large-area Schottky diode back contacts has been developed to enable high frequency capacitance studies to be carried out on semiconductors without the need to fabricate high quality ohmic back contacts. This technique will find application for very high resistivity materials or for the characterization of novel semiconductors when a method of producing good ohmic contacts has not been established. In this method a back contact much larger in area than the front contact diode under test is used. It is then found that accurate capacitance–voltage measurements can be made of the ionized doping density and, provided the back contact has sufficient leakage, the built-in potential can also be measured. Such specimens may also be used for characterization using the deep level transient spectroscopy (DLTS) technique and this is demonstrated by obtaining DLTS spectra from very high resistivity silicon specimens containing oxygen precipitates and comparing these to similar spectra obtained from more highly doped material.

Electrical contact behavior of Ni/C60/4H–SiC structures

A C60 interfacial layer between a Ni film and SiC improves the Ohmic contact properties significantly. The C60 film is deposited by the Langmuir–Blodgett method prior to the Ni film deposition on SiC using dc sputtering. High quality Ohmic contacts of Ni/C60/4H–SiC are formed after annealing at 800 °C in Ar for 2 h with a specific contact resistivity of 1.17×10−6 Ω cm2 on SiC with a doping concentration of 1.8×1019 cm−3. Raman spectra reveal that Ohmic contacts are formed after C60 is decomposed and converted to graphitic states. The formation of Ohmic contacts and the graphitic structures in the annealed film of Ni/C60/SiC are similar to that of Ni/C/SiC, for which the Ni catalytic graphitization effects play the determining role in forming Ohmic contacts on SiC. Scanning electron microscopy images show a direct relationship between the graphitized morphological features on the films and Ohmic contact behavior.

Optimization of current spreading metal layer for GaN/InGaN-based light emitting diodes

We have studied the optical and electrical properties of Ni/Au bilayer thin films deposited on GaN/InGaN-based light emitting diodes (LEDs) followed by annealing in air at 550 °C. Oxidation of Ni and intermixing of the resultant NiO with Au lead to a dramatic increase of transparency and resistivity. Metal layers with small Au:Ni ratios (<1) exhibit high resistivity which remarkably affects LED electrical characteristics, but produce high quality ohmic contacts with specific contact resistance in the low 10 −3 Ω cm 2 range. Mismatch between sheet resistances of the NiO/Au layer and n-GaN layer causes current and emission non-uniformity in the LEDs. However, significant efficiency loss was only observed in large area diodes at high operation currents, where the variation of current densities across the whole device was higher than 30%. Finally, based on these findings, the optimum Ni/Au contact is proposed as the p-type current spreading layer for commercial III-nitrides LEDs.