Thermal Stability Of Ohmic Contacts Research Articles

Temperature stability features of ohmic contacts resistance to GaAs and GaN based nanoheterostructures

The temperature stability of Ge/Au/Ni/Au ohmic contacts to GaAs nanoheterostructures and Ti/Al/Ni/Au ohmic contacts to GaN nanoheterostructures on silicon substrate was investigated. It has been established that optimization of the RTA process made it possible to obtain ohmic contacts with field emission current flow mechanism. The thermal stability of ohmic contacts for transistors and mesa resistors demonstrated the threshold behavior of the heat treatment temperature. The optimum process parameters for temperature stability and minimum contact resistance were defined. Keywords: ohmic contact, gallium arsenide, gallium nitride.

Особенности температурной стабильности сопротивления омических контактов к наногетероструктурам на основе GaAs и GaN

The temperature stability of Ge/Au/Ni/Au ohmic contacts to GaAs nanoheterostructures and Ti/Al/Ni/Au ohmic contacts to GaN nanoheterostructures on silicon substrate was investigated. It has been established that optimization of the RTA process made it possible to obtain ohmic contacts with field emission current flow mechanism. The thermal stability of ohmic contacts for transistors and mesa resistors demonstrated the threshold behavior of the heat treatment temperature. The optimum process parameters for temperature stability and minimum contact resistance were defined.

In situ XPS spectroscopic study of thermal stability of W/Ni bilayer Ohmic contact to n-type 4H-SiC

The thermal stability of Ohmic contacts to n-type 4H-SiC using Au/Pt/Ni and Au/Pt/W/Ni layers has been systematically investigated after long-time aging in air at 300 °C and 400 °C. The specific contact resistance (ρc) is calculated from current–voltage (I–V) curves. The content of carbon in the contact layer is measured by Raman spectroscopy. A transmission electron microscope and a energy dispersive spectrometer are used to characterize the interface morphology and component distribution. The phase composition of the contact layer at different depths is analyzed in situ by x-ray photoelectron spectroscopy. It is found that Ni2 − xWxSi decomposition plays an important role in improving the thermal stability of the Ohmic contact. The decomposition products (Ni, W, and Si atoms) react with oxygen, and the diffusion of oxygen to the Ni2Si/SiC interface can be reduced to some extent. Compared to Au/Pt/Si/SiC, the Au/Pt/W/Ni/SiC Ohmic contact exhibits better thermal stability. It is expected that the W/Ni/SiC Ohmic contact will be very suitable for high temperature applications.

Understanding the thermal stability of Ohmic contacts

Using X-ray photoelectron spectroscopy, researchers uncover one way that electronics can degrade.

Remote plasma-enhanced atomic layer deposition of metallic TiN films with low work function and high uniformity

Thermal stability of metal/n-GaN contact is critical for its applications in microelectronic and optoelectronic devices. Metal Ti is generally used to make Ohmic contact on n-GaN after high temperature annealing, and the key factor is to form TiN at the interface. To reduce the processing temperature and improve the reliability, metallic titanium nitride (TiN) thin film has been proposed to substitute traditional metals (such as Ti) in the contact structures, due to its low work function and high blocking effect. For this novel approach, the first step is to fabricate high quality TiN films. Here, the authors adopted remote plasma-enhanced atomic layer deposition method to deposit TiN films under well-controlled conditions. Stoichiometric TiN films (Ti:N ∼ 1:1) with low oxygen contamination (<5%) have been deposited uniformly on 2 in. substrates in a large temperature range of 250–400 °C. The work function of TiN films is quite low (∼3.7 ± 0.1 eV) compared to metal Ti (∼4.33 eV), and almost independent to the growth temperature and substrates. Strong Fermi edge and high conductivity indicate excellent metallic property of the TiN films. This study of TiN film growth paves the way to establish a low temperature process and improve the thermal stability of Ohmic contacts for wide band gap semiconductor-based devices.

Role of W in W/Ni Bilayer Ohmic Contact to n-Type 4H-SiC From the Perspective of Device Applications

Ohmic contacts to n-type 4H-SiC using Ni layer and W/Ni bilayer were investigated and compared. The phase composition, electronic states, and carbon structural evolution of the contact layer were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The surface roughness and cross-sectional morphology were characterized by atomic force microscopy and high-resolution transmission electron microscopy. The Keithley 4200-SCS semiconductor parameter analyzer was used to measure the current–voltage curves of the contacts. The specific contact resistance $\rho _{c}$ was calculated based on the circular transmission line model. $\rho _{c}$ is $\textsf {3.2}\times \textsf {10}^{-\textsf {5}}\,\,\Omega \cdot $ cm2 for W/Ni/SiC and $\textsf {4.2} \times \textsf {10}^{-\textsf {4}} \,\,\Omega \cdot $ cm2 for Ni/SiC. Ni2Si with minor W substitution and tungsten carbide with minor Ni substitution were identified as the dominant phases for W/Ni/SiC after annealed. The contact surface morphology is improved, and the content of free carbon and the quantity of voids at the interface are reduced when W is introduced into Ni/SiC. The wire bonding is easy to carry out and the thermal stability of ohmic contact is greatly enhanced. From the perspective of device applications, W/Ni bilayer ohmic contact to n-type 4H-SiC is very competitive.

Recent Progress in Ohmic Contacts to Silicon Carbide for High-Temperature Applications

During the past few decades, silicon carbide (SiC) has emerged as the most promising wide-bandgap semiconductor for high-temperature, high-frequency, and high-power applications. All its attractive properties depend critically on and are often limited by the formation of Ohmic contacts to SiC. Although impressive progress has been made, improvements to further reduce the specific contact resistance without lowering the stability of Ohmic contacts during long-term aging are still indispensable. In this regard, we present a review of recent progress in Ohmic contacts to n- and p-type SiC reported in literature. The mechanism of Ohmic contact to SiC, surface preparation methods, and performance of Ohmic contacts are discussed. Emphasis is placed on the thermal stability of Ohmic contacts to SiC. To date, extremely low specific contact resistance values (10−7 Ω cm2 to 10−6 Ω cm2) have been obtained after high-temperature (>800°C) annealing. Moreover, great efforts have been made to achieve good Ohmic contact performance (10−6 Ω cm2 to 10−5 Ω cm2) under low-temperature annealing (<600°C) or even without annealing. During long-term aging at high temperature (300°C to 1000°C), surface morphology degradation, formation of unwanted Kirkendall voids, interdiffusion between metallization stacks and/or SiC substrates, and especially severe oxidation in oxygen-containing atmospheres can be responsible for electrical degradation of Ohmic contacts. These critical issues are discussed along with future perspectives.

Silicon on insulator pressure sensor based on a thermostable electrode for high temperature applications

A high temperature silicon on insulator pressure sensor utilising a Ti/TiN/Pt/Au electrode is presented for improving the thermal stability of ohmic contacts, which can work stably at high temperatures of up to 500°C. To analyse the characteristics of the electrode at high temperatures, a special test structure is measured using the linear transmission line method and Auger electron spectroscopy. To solve the measurement problem, a novel calibration setup is designed to calibrate the absolute pressure sensor at extremely high temperatures. The measurement results have shown that the pressure sensor has a nonlinearity error of 0.17%FS and a sensitivity of 0.24 mV/kPa with a measurement range of 30–150 kPa at 500°C, indicating the good thermal stability of the ohmic contacts.

Thermal stability of Ohmic contacts to InN

Ti ∕ Al ∕ Ni ∕ Au and Ti∕Al∕TiB2∕Ti∕Au Ohmic contacts on n-type InN were investigated as a function of annealing temperature. A minimum specific contact resistance of 1.6×10−6Ωcm−2 was obtained for the TiB2-based Ohmic contacts on n-type GaN with n∼1018cm−3 after annealing at 900°C. The minimum value with the Ni-based metallization was higher (6×10−6Ωcm2). The measurement temperature dependence of contact resistance was similar for both Ti∕Al∕TiB2∕Ti∕Au and Ti∕Al∕Ni∕Au, suggesting that the same transport mechanism was present in both types of contacts. The TiB2-based Ohmic contacts displayed superior thermal stability, suggesting that it is a superior diffusion barrier at these temperatures than Ni.

Thermal stability of ohmic contacts between Ti and Se-passivated n-type Si(001)

The thermal stability of the interface between Ti and Se-passivated n-type Si(001) is investigated. As-deposited Ti contacts show ohmic characteristics. After annealing at temperatures from 200 °C to 480 °C in air, the Se-passivated samples are much more resistant to Schottky transition as compared to the bare samples. The bare samples turn to Schottky below 300 °C, whereas the passivated samples are ohmic all the way to 400 °C. A difference of over 100 °C is observed between the two types of samples. Schottky barrier heights are determined using capacitance–voltage and activation–energy methods. The bare samples reach a maximum barrier height of 0.39 eV as determined by activation–energy measurements, while the passivated samples maximize at 0.19 eV. We deduce that a monolayer of Se suppresses silicidation between Ti and Si by eliminating dangling bonds on Si(001).

Ohmic Contact to p-GaN Using a Strained InGaN Contact Layer and Its Thermal Stability

We have formed ohmic contacts to p-GaN using a strained p-InGaN contact layer, and achieved the lowest contact resistance of 1.1×10-6 Ω-cm2 at room temperature by optimizing the contact layer thickness and its In mole fraction. We have also evaluated thermal stability of ohmic contacts to p-GaN using the strained p-InGaN contact layer. The contact resistance decreased to 2×10-7 Ω-cm2 at 100°C, and increased with elevating temperature above 100°C. In the temperature range up to 400°C, the contact resistances of the samples with the p-InGaN contact layer were smaller than those of the samples without the contact layer. Furthermore, the ohmic characteristics of the strained p-InGaN contact layer were less degraded even after the thermal process, compared with those of the sample without a contact layer. These results indicate that the strained p-InGaN contact layer is favorable for practical application.

Formation Mechanism of Low Contact Resistance PdZn-based Ohmic Contacts for p-type InP

Recently, Sb(3 nm)/Zn(20 nm)/Pd(20 nm) ohmic contact materials to p-type InP which provides high reliable, low contact resistance after annealing at temperature around 375 ◦ C have been developed in our laboratories (where a slash (/) sign indicates the deposition sequence). The use of these contact materials for photodiodes made it possible to achieve simultaneous preparation of both p and n-type ohmic contacts, which resulted in significant reduction of the fabrication process steps. In the present paper in order to understand the mechanism of ohmic contact formation, the microstructural analysis of the interfaces between the Sb/Zn/Pd ohmic contacts and the Zn-doped p-type InP was carried out by X-ray diffraction, cross-sectional transmission electron microscopy, secondary ion mass spectroscopy, and electrochemical capacitancevoltage measurement. The present experiments proposed a new contact formation mechanism: reduction of the ohmic contact resistances was obtained primarily due to suppression of Zn (doped in the InP) outdiffusing from the InP substrate during annealing by the Pd 2InP layers which contained Zn. Thus the direct contact of the Pd2InP layers to the InP substrate was found to be essential to prevent reduction of the net acceptor concentration near the InP surface and also to improve the thermal stability of ohmic contacts. The roles of Pd and Sb added to the contact materials also contributed to the reduction of contact resistance and their roles will be discussed.

Thermal stability of OHMIC contacts to InN

Ohmic contacts based on the use of InN have been developed which show low contact resistance and good adhesion even after annealing at temperatures up to 600°C. Samples with thin InN layers actually showed improvement in contact resistance with annealing in this range. InN layer thicknesses ≤1000 Å also showed good adhesion over this range, while thicker samples exhibited delamination at temperatures as low as 400°C. Annealing at 700°C produced severe degradation of the contact resistance due to decomposition of the InN and diffusion of the In into the WSi x .

The thermal stability of ohmic contact to n-type InGaAs layer

The thermal stability of ohmic contact to n-type InGaAs layer is investigated. When Ni/Ge/Au is used as the contact metal, the characteristics of the ohmic contact are degraded after thermal treatment. The specific contact resistance of (Ni/Ge/Au)-InGaAs ohmic contact after annealing at 450°C is about 15 times larger than that of as-deposited sample. This is due to the decomposition of InGaAs and the interdiffusion of Ga and Au. A new phase of Au4ln appears after annealing at 300°C. While in the case of Ti/Pt/Au, Au does not penetrate into the InGaAs layer as revealed by secondary ion mass spectroscopy. The specific contact resistance of (Ti/Pt/Au)-InGaAs ohmic contact after annealing at 450°C is eight times larger than that of as-deposited sample. Therefore, the thermal stability of (Ti/Pt/Au)-InGaAs ohmic contact is better than that of (Ni/Ge/Au)InGaAs ohmic contact.

Thermal Stability of Ohmic Contacts to n-InxGa1−xN

ABSTRACTThe microstructural properties and interdiffusion reactions of Au/Ge/Ni, Ti/Pt/Au, WSix and AuBe contacts on GaN and In0.5Ga0.5N have been examined using Scanning Electron Microscopy and Auger Electron Spectroscopy. The WSix contacts possess excellent thermal stability and retained good structural properties at annealing temperatures as high as 800°C on GaN. The electrical characteristics of WSix contacts on In0.5Ga0.5N had a specific contact resistivity of 1.48×10−5Ωcm2 and an excellent surface morphology following annealing at 700°C. The increase in contact resistance observed at higher temperatures was attributed to intermixing of metal and semiconductor. In contrast the Ti/Pt/Au and Au/Ge/Ni contacts were stable only to ≤ 500°C. AuBe contacts had the poorest thermal stability, with substantial reaction with GaN occurring even at 400°C. The WSix contact appears to be an excellent choice for high temperature GaN electronics applications.

Reliability characteristics of ohmic contacts for AlGaGs/GaAs HBTs

The thermal stability of ohmic contacts for AlGaAs/GaAs HBTs is presented. Ti/Pt/Au, Ti/Pt/Au and Cr/Pt/Au, and AuGe/Ni/Ti/Pt/Au were investigated for emitter, base, and collector contacts, respectively. As a result of 200°C storage tests, it was found that these contacts did not limit the reliability of AlGaAs/GaAs HBTs.