Indium Tin Oxide Ohmic Contact Research Articles

All‐Oxide Transparent Vertical Indium Tin Oxide and Aluminum‐Doped Zinc Oxide/β‐Ga2O3 Schottky Diodes

Herein, the fabrication and characterization of low‐resistance all‐oxide transparent vertical β‐Ga2O3 diodes using indium tin oxide (ITO) and aluminum‐doped zinc oxide (AZO) Schottky contacts are reported. It is shown that an ITO ohmic contact to n+‐β‐Ga2O3 substrate is formed after annealing in N2 at 800 °C. Both AZO‐ and ITO‐based Schottky diodes show well‐behaved current–voltage characteristics. Average Schottky barrier heights and ideality factors are 0.99 and 1.05 eV and 0.95 and 1.03 eV for AZO and ITO Schottky contacts, respectively. The on‐off current ratio is about 21010 and 11010 for AZO and ITO Schottky contact, respectively. Moreover, the on‐state resistance is about 6–7 and 4–5 mΩ cm2 for AZO and ITO Schottky contact, respectively, and is 20–35 times lower than for previously reported transparent β‐Ga2O3 Schottky diodes.

Formation of indium–tin oxide ohmic contacts for β-Ga2O3

Sputter-deposited indium–tin oxide (ITO) electrodes became ohmic contacts for unintentionally doped β-Ga2O3(010) substrates with a carrier concentration of 2 × 1017 cm−3 after rapid thermal annealing in a wide range of annealing temperatures of 900–1150 °C. The formation of an ohmic contact is attributed to interdiffusion between ITO and β-Ga2O3, as evidenced by the results of transmission electron microscopy and energy-dispersive X-ray spectroscopy. The interdiffusion decreases the band gap and increases the donor concentration of β-Ga2O3 at the interface, and forms an intermediate semiconductor layer desirable for carrier transport. The ITO ohmic contact is particularly useful for future β-Ga2O3 devices operated at high temperatures.

Performance of GaN-Based LEDs with Nanopatterned Indium Tin Oxide Electrode

The indium tin oxide (ITO) has been widely applied in light emitting diodes (LEDs) as the transparent current spreading layer. In this work, the performance of GaN-based blue light LEDs with nanopatterned ITO electrode is investigated. Periodic nanopillar ITO arrays are fabricated by inductive coupled plasma etching with the mask of polystyrene nanosphere. The light extraction efficiency (LEE) of LEDs can be improved by nanopatterned ITO ohmic contacts. The light output intensity of the fabricated LEDs with nanopatterned ITO electrode is 17% higher than that of the conventional LEDs at an injection current of 100 mA. Three-dimensional finite difference time domain simulation matches well with the experimental result. This method may serve as a practical approach to improving the LEE of the LEDs.

Temperature-dependent contact resistivity of radio frequency superimposed direct current sputtered indium tin oxide ohmic contact to p-type gallium nitride

The temperature-dependent contact resistivity of radio frequency (RF) superimposed direct current (DC) sputtered indium tin oxide (ITO) contacts to p-type gallium nitride (p-GaN) films as well as the temperature-dependent sheet resistivity of p-GaN films were investigated to understand the carrier transport mechanism of the sputtered ITO ohmic contacts to p-GaN. As the measurement temperature was decreased from 400 to 200K, the contact resistivity increased by three orders of magnitude. Furthermore, the sheet resistivity of the p-GaN increased linearly with exp (1/Temperature(T))1/4 from 200 to 340K, indicating variable-range hopping (VRH) conduction via the Mg-related deep level defect (DLD) band. Based on the VRH conduction model, the effective barrier height between the sputtered ITO and the DLD band were calculated to be 0.12eV, which is sufficiently low to explain the formation of the low contact resistivity of the RF superimposed DC sputtered ITO contacts to p-GaN (~10−2Ω-cm2).

Hydrogen gas generation using n-GaN photoelectrodes with immersed Indium Tin Oxide ohmic contacts

An n-GaN photoelectrochemical (PEC) cell with immersed finger-type indium tin oxide (ITO) ohmic contacts was demonstrated in the present study to enhance the hydrogen generation rate. The finger-type ITO ohmic contacts were covered with SiO₂ layers to prevent the PEC cell from generating leakage current. Using a 1M NaCl electrolyte and external biases, the typical photocurrent density and gas generation rate of the n-GaN working electrodes with ITO finger contacts were found to be higher than those with Cr/Au finger contacts. The enhancement in photocurrent density or gas generation rate can be attributed to the transparent ITO contacts which allowed the introduction of relatively more photons into the GaN layer. No significant corrosion was observed in the ITO layer after the PEC process compared with the Cr/Au finger contacts which were significantly peeled from the GaN layer. These results indicate that the use of n-GaN working electrodes with finger-type ITO ohmic contacts is a promising approach for PEC cells.

Au-Doped Indium Tin Oxide Ohmic Contacts to p-Type GaN

Indium tin oxide (ITO) thin films doped with Au, Ni, or Pt (3.5 at.% to 10.5 at.%) were deposited on p-GaN epilayers (Mg ~4 × 1019 cm−3) using direct-current (DC) sputter codeposition. It was found that undoped ITO con- tacts to p-GaN exhibited leaky Schottky behavior, whereas the incorporation of a small amount of Au (3.5 at.% to 10.5 at.%) significantly improved their ohmic characteristics. Compared with standard Ni/ITO contacts, the Au-doped ITO contacts had a similar specific contact resistance in the low 10−2 Ω cm−2 range, but were more stable above 600°C and more transparent at blue wavelengths. These results provide support for the use of Au-doped ITO ohmic contact to p-type GaN in high-brightness blue light-emitting diodes.

InGaN/AlGaN Ultraviolet Light-Emitting Diode with a Ti3O5/Al2O3 Distributed Bragg Reflector

Here, we report InGaN/AlGaN ultraviolet (UV) light-emitting diodes (LEDs) using distributed Bragg reflectors (DBRs). The DBRs consist of 11 layers of alternating quarter-wave thickness Ti3O5 and Al2O3 deposited onto the indium–tin-oxide ohmic contact layer of a 385 nm UV LED by ion-assisted electron beam evaporation. Numerical calculations showed that the angle averaged reflectivity of the GaN/DBR structure at 385 nm was 12% higher than that of a conventional GaN/Ag structure. The measured light output–current–voltage of the DBR UV LEDs at 20 mA current injection showed an increase of 15% in output power in comparison to a conventional Ag reflector with no degradation in electrical properties. We attributed this light output enhancement to the increased light extraction from the higher DBR reflectivity.

Formation of low-resistance and transparent indium tin oxide ohmic contact for high-brightness GaN-based light-emitting diodes using a Sn–Ag interlayer

We report on the formation of low-resistance and highly transparent indium tin oxide (ITO) ohmic contacts to p-GaN using a Sn–Ag alloy interlayer. Although the as-deposited Sn–Ag(6 nm)/ITO(200 nm) contacts show non-ohmic behaviors, the scheme becomes ohmic with specific contact resistance of 4.72×10 −4 Ω cm 2 and produce transmittance of ∼91% at wavelength of 460 nm when annealed at 530 °C. Blue light-emitting diodes (LEDs) fabricated with the Sn–Ag/ITO contacts give forward-bias voltage of 3.31 V at injection current of 20 mA. LEDs with the Sn–Ag/ITO contacts show the improvement of the output power by 62% (at 20 mA) compared with LEDs with Ni/Au contacts.

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.

Investigation of indium–tin-oxide ohmic contact to p-GaN and its application to high-brightness GaN-based light-emitting diodes

Indium–tin-oxide (ITO) contacts to p-GaN exhibit ohmic characteristics by inserting a 10-nm-thick p-In 0.1Ga 0.9N layer as an intermediate. The specific contact resistivity of 4.5 × 10 −2 Ω cm 2 was obtained while annealing ITO/p-GaN contacts at 500 °C. Based on the variation of contact resistivity with respect to temperature, the dominant transport mechanism of ITO/p-GaN structure tended from thermionic field emission to thermionic emission when the post-annealing temperature was raised from 400 °C to 600 °C. From the XPS, XRD and SIMS results, the outdiffusion of gallium atoms and the formation of Ga–O bonds could introduce the gallium vacancies and increase the net concentration of holes, which would benefit the carrier tunneling through the interface. The GaN-based light-emitting diodes (LEDs) with 500 °C-annealed ITO contacts exhibited the forward voltage of 3.43 V and output power of 4.30 mW at a 20-mA-current injection. Although the forward voltage showed a little higher than the conventional LEDs by 0.2 V, the external quantum efficiency and power efficiency were enhanced by about 46% and 36%, respectively. As for the life test, LEDs with 500 °C-annealed ITO contacts presented a similar reliability as the conventional LEDs. Therefore, ITO contacts can make GaN-based LEDs highly bright and reliable in practice.

Gallium nitride photoconductive ultraviolet sensor with a sputtered transparent indium–tin–oxide ohmic contact

A gallium nitride (GaN) photoconductive ultraviolet sensor with a sputtered transparent indium–tin–oxide (ITO) contact is presented, in which a maximum photo-responsivity of 327 A/W at a bias of 5 V and a wavelength of 366 nm is achieved and attributed to good ohmic contact between ITO and n-type GaN layer. It is shown that as-deposited and annealed ITO films deposited onto n-type GaN using a radio frequency sputtering produce linear current–voltage curves that are believed to be the origin of the high photo-responsivity. In addition, annealing is shown to improve transmittance through the ITO films.

Indium tin oxide ohmic contact to highly doped n-GaN

The electrical characteristics of the indium tin oxide (ITO) contacts on n-GaN with various doping concentrations have been studied. Ohmic behavior was observed for ITO films on highly doped n-GaN ( n=1×10 19 cm −3) without thermal annealing and the measured specific contact resistance was 5.1×10 −4 Ω cm 2. This result could be attributed to the formation of a tunneling junction on the heavily n-type GaN surface. However, as the thermal annealing was performed to the ITO/n-GaN ( n=1×10 19 cm −3) Ohmic contact, it exhibited Schottky characteristics. This result might be due to the microscopic interfacial reaction among In, Sn, O and GaN and their alloys which extend into GaN films, thereby influencing the electrical properties of ITO/n-GaN contacts.