Low Resistance Nonalloyed Ohmic Contacts Research Articles

Non-alloyed ohmic contacts to (010) β -Ga2O3 with low contact resistance

Low resistance non-alloyed ohmic contacts are realized by a metal-first process on homoepitaxial, heavily n+ doped (010) β-Ga2O3. The resulting contacts have a contact resistance (Rc) as low as 0.23 Ω-mm on an as-grown sample and exhibit nearly linear ohmic behavior even without a post-metallization anneal. The metal-first process was applied to form non-alloyed contacts on n+ (010) β-Ga2O3 grown by metal-organic chemical vapor deposition (MOCVD) as well as suboxide molecular beam epitaxy. Identical contacts fabricated on similar MOCVD samples by conventional liftoff processing exhibit highly rectifying Schottky behavior. Re-processing using the metal-first process after removal of the poor contacts by conventional methods does not improve the contacts; however, addition of a Ga-flux polishing step followed by re-processing using a metal-first process again results in low resistance, nearly linear ohmic contacts. The liftoff process, therefore, does not reliably render nearly linear ohmic behavior in non-alloyed contacts. Furthermore, no interface contamination was detected by x-ray photoelectron spectroscopy. This suggests that during the initial liftoff processing, a detrimental layer may form at the interface, likely modification of the Ga2O3 surface, that is not removable during the contact removal process but that can be removed by Ga-flux polishing.

Tunneling current through non-alloyed metal/heavily-doped SiC interfaces

In this paper, tunneling phenomena at metal/heavily-doped SiC Schottky (non-sintered) interfaces are reviewed, and a design guideline for low-resistance non-alloyed SiC ohmic contacts is proposed. Carrier transport at Schottky contacts on heavily-doped SiC epitaxial layers is quantitatively described by direct tunneling (DT) including both of the thermionic field emission (TFE) and field emission (FE) when the doping density (Nd) is higher than mid-1017cm−3. As for p-type SiC, tunneling of holes in the split-off band is the dominant conduction mechanism due to its light effective mass (0.21m0). Phosphorus ion (P+) implantation makes the interface tunneling current larger by several orders of magnitude compared with that at contacts on epitaxial layers with almost the same Nd, which is plausibly explained by trap-assisted tunneling (TAT) through implantation-induced defect levels. The contact resistivity (ρc) at non-alloyed ohmic contacts formed on heavily P+-implanted SiC (Nd>4×1018cm−3) is significantly reduced thanks to the contribution of TAT especially when Nd is lower than about 1020cm−3, while ρc decreases with increasing Nd according to the DT theory in a higher Nd range (>1020cm−3). At a very high Nd of 2×1020cm−3, an extremely low ρc of 1–2×10−7Ωcm2 is demonstrated for non-alloyed Mg and Ti contacts. Through the experiment and calculation of tunneling current, a model to predict ρc at non-alloyed ohmic contacts taking account of the contributions of TAT and DT is proposed, and a design guideline for low-resistance non-alloyed ohmic contacts is presented as follows; a very low ρc (<10−6Ωcm2) is achievable without sintering by lowering the interface barrier height (≤1eV) and utilizing high-dose ion implantation (≥3×1019cm−3).

Reactive sputtering of III-N materials for applications in electronic devices

ABSTRACTGallium Nitride (GaN) and other III-N semiconductors are rapidly gaining importance in high power and high frequency electronic applications. III-N material based devices are fabricated on heterostructures that are usually grown by high vacuum techniques such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). However, in many applications, it is necessary to regrow thin cap layers of III-N materials during device fabrication. One such application is regrowth of ohmic contacts to III-N devices. Heavily doped n+ GaN, or InGaN grown by MBE or MOCVD is used to obtain low resistance non-alloyed ohmic contacts to GaN based devices. However, from a commercial point of view, this becomes difficult because of the high cost and lack of availability of ultra high vacuum (∼1x10-10 Torr) techniques in most clean room facilities. Reactive sputtering provides a cheaper and more ubiquitous alternative for the growth of thin cap layers on parent MOCVD III-N heterostructures during device fabrication. In this work, we explore the possibility of using reactive sputtering as a method to grow III-N materials as ohmic contacts to GaN based devices.

Low-resistance nonalloyed ohmic contacts on undoped ZnO films grown by pulsed-laser deposition

We report on the formation of nonalloyed Ti and Ni ohmic contacts to ZnO films grown by pulsed-laser deposition. The experimental results show a lower barrier height of the Ti/ZnO samples than that of the Ni/ZnO samples (due to the lower work function of Ti than Ni), suggesting the Fermi-level unpinning at the interfaces. Based on the thermionic-emission or the thermionic-field-emission model, we found weak barrier-height dependence of the contact resistivity, implying that the presence of hydroxide in ZnO (i.e. the formation of the narrow depletion region at the metal/ZnO interface) resulted in the excess current component related to tunnelling, which led to the formation of the low-resistance nonalloyed metal/ZnO contact. The measurement temperature dependence of the contact resistivity revealed that the dominant current transport mechanism is field emission.

Ti-based nonalloyed Ohmic contacts for Al0.15Ga0.85N∕GaN high electron mobility transistors using regrown n+-GaN by plasma assisted molecular beam epitaxy

A technique for regrowing n+-GaN layers has been developed to realize nonalloyed Ohmic contacts using plasma assisted molecular beam epitaxy. The contact resistance and device performance were measured of a recessed-gate with the regrowth and of recessed-source/drain AlGaN∕GaN high electron mobility transistors (HEMTs). With the regrown n+-GaN layers and recessed drain/source, a low contact resistance of 0.6Ωmm was obtained for Ti∕Au contacts to AlGaN. The peak drain current (IDS,max) and maximum transconductance (gm,max) of the AlGaN∕GaN HEMTs with nonalloyed Ohmic contacts were 573mA∕mm and 60mS∕mm, respectively. These results demonstrate that the regrowth of highly doped GaN layers is crucial in achieving low-resistance nonalloyed Ohmic contacts for the HEMT structures.

Low resistance nonalloyed Al-based ohmic contacts on n-ZnO:Al

Abstract We have investigated the electrical properties of nonalloyed Al, Al/Au, and Al/Pt ohmic contacts on n-type ZnO:Al (2×10 18 cm −3 ). All Al-based nonalloyed ohmic contacts on the n-ZnO:Al reveal linear current–voltage behavior with low specific contact resistivity of 8.5×10 −4 (Al), 8.0×10 −5 (Al/Au) and 1.2×10 −5 Ω cm 2 (Al/Pt), respectively. Using secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spectroscopy (XPS) depth profiles, it was found that the O atoms in the ZnO:Al layer outdiffused to Al metal layer while the Al atoms indiffused to the surface region of ZnO:Al. This interdiffusion between Al and O atoms at room temperature results in an increase of doping concentration in the surface region of the ZnO:Al and reduces a specific contact resistivity of the Al-based ohmic contacts without thermal annealing process.

Formation of low resistance nonalloyed ohmic contacts to p‐type GaN by KrF laser irradiation

AbstractKrF excimer‐laser‐irradiation treatment in a N2 atmosphere is introduced to improve the electrical properties of p ‐type GaN and so to form low resistance nonalloyed Ni/Au ohmic contacts. The as‐grown sample exhibits non‐linear electrical behavior. However, the samples laser‐annealed in N2 ambient become ohmic with specific contact resistance of 8.9(±0.6)×10–5 Ωcm2. Hall measurement and current‐voltage‐temperature results show that the laser irradiation is effective in increasing a hole concentration in the surface region of p ‐GaN, leading to both the enhancement in field emission and the reduction of the effective Schottky barrier heights at the metal‐semiconductor interface. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A 0.1-μm MHEMT millimeter-wave IC technology designed for manufacturability

We describe and discuss our approach to improved manufacturability of millimeter-wave InP-type HEMT IC technology. The main ingredients are the use of (1) GaAs (rather than InP) as substrate, with a buffer that grades the lattice constant to that of the high-mobility Ga 0.47In 0.53As channel; (2) low-resistance non-alloyed ohmic contacts with good reliability and reproducibility; (3) an e-beam process that produces 0.1-μm T-gate fingers with one exposure and (4) two-step selective etching of the gate trough for good uniformity. The process is compatible with standard GaAs FET IC front and back end unit processes, including backside vias. The yield, uniformity and performance of devices and circuits made on this metamorphic (MHEMT) material are consistently as good as those we get using InP substrates. The reliability of our devices, whether made on GaAs or InP substrates, is limited by impact ionization in the channel, which has also been linked to R d-degradation and burnout. At low-noise drain bias ( V d=1 V), where the impact ionization is small, we have demonstrated reliability, i.e. extrapolated MTTF at 125°C channel temperature is >10 6 h. This is as good as FETs made on InP substrates.

Very low resistance nonalloyed ohmic contacts using low-temperature molecular beam epitaxy of GaAs

Ex situ nonalloyed ohmic contacts were made to n- and p-type GaAs using low-temperature molecular beam epitaxy. For n-type GaAs, Ag, and Ti/Au nonalloyed contacts displayed specific contact resistitivities of mid 10−7 Ω cm2. For p-type GaAs, nonalloyed Ti/Au contacts with specific contact resistivities of about 10−7 Ω cm2 were obtained.

Pt/Ti/n-InP nonalloyed ohmic contacts formed by rapid thermal processing

Low resistance nonalloyed ohmic contacts of e-gun evaporated Pt/Ti to S doped n-InP 5×1017, 1×1018, and 5×1018 cm−3 have been fabricated by rapid thermal processing. The contacts to the lower doped substrates (5×1017 and 1×1018 cm−3) were rectifying as-deposited as well as after heat treatment at temperatures lower than 350 °C. Higher processing temperatures stimulated the Schottky to ohmic contact conversion with minimum specific contact resistance of 1.5×10−5 and 5×10−6 Ω cm2, respectively, as a result of rapid thermal processing at 450 °C for 30 s. Heating at a temperature of 550 °C again yielded a Schottky contact. The contact to the 5×1018 cm−3 InP was ohmic as deposited with a specific contact resistance value of 1.1×10−4 Ω cm2. Supplying heat treatment to the contact caused a decrease of the specific contact resistance to a minimum of 8×10−7 Ω cm2 as a result of rapid thermal processing at 450 °C for 30 s. In all cases, this heat treatment caused a limited interfacial reactions between the Ti and the InP, and resulted in an almost abrupt interface. Heating at temperatures higher than 500 °C resulted in an interfacial intermixing and a mutual migration and reaction of the Ti and the semiconductor elements. The Pt/Ti bilayer structure was highly tensile as deposited (5×109 dyn cm−2) and became stress-free as a result of the interfacial reactions which took place while heating the samples to temperature of 400 °C or higher.

Extremely low-resistance nonalloyed ohmic contacts on molecular beam epitaxially grown p-type Ge

Extremely low-resistance nonalloyed ohmic contacts have been formed on p-type Ge grown on GaAs by molecular beam epitaxy as part of a research effort into GaAs/Ge heterojunction devices. Using evaporated Ti/Al metallisation, specific contact resistances well below 1 × 10−6Ωcm2 were achieved for 0heavily doped Ge (p>1 × 1019cm−3). The investigated Ge layers were grown as the base region of AlGaAs/Ge/GaAs HBTs. The influence of the low base resistance on the high-frequency performance of HBTs was studied by simulations.

GaAs metal-semiconductor field-effect transistor with extremely low resistance nonalloyed ohmic contacts using an InAs/GaAs superlattice

GaAs metal-semiconductor field-effect transistors (MESFETs) with extremely low resistance nonalloyed ohmic contacts have been demonstrated. The contact structure consists of an n+-InAs/GaAs strained-layer superlattice and an InAs cap layer. Contact resistances as low as 0.036 Ω mm have been measured. These results represent the smallest figures reported to date for GaAs field-effect transistors. Nonalloyed MESFETs with 1 μm gate lengths had transconductances of about 210 mS/mm.

Low-resistance nonalloyed ohmic contacts on p-type GaAs using GaSb/GaAs strained-layer superlattices

Employing a GaSb/GaAs strained-layer superlattice and a GaSb cap, specific contact resistivities as low as 3.2×10−7 Ω cm2 have been realized for nonalloyed ohmic contact to p-type GaAs. This contact structure is shown to give low contact resistances irrespective of the contact metals, including AuBe, AuGe/Ni/Au, and Au. Excellent thermal stability for AuBe contacts was obtained when sintered.

Pt/Ti/p-In0.53Ga0.47As low-resistance nonalloyed ohmic contact formed by rapid thermal processing

Very low resistance nonalloyed ohmic contacts of Pt/Ti to 1.5×1019 cm−3 Zn-doped In0.53Ga0.47As have been formed by rapid thermal processing. These contacts were ohmic as deposited with a specific contact resistance value of 3.0×10−4 Ω cm2. Cross-sectional transmission electron microscopy showed a very limited interfacial reacted layer (20 nm thick) between the Ti and the InGaAs as a result of heating at 450 °C for 30 s. The interfacial layer contained mostly InAs and a small portion of other five binary phases. Heating at 500 °C or higher temperatures resulted in an extensive interaction and degradation of the contact. The contact formed at 450 °C, 30 s exhibited tensile stress of 5.6×109 dyne cm−2 at the Ti/Pt bilayer, but the metal adhesion remained strong. Rapid thermal processing at 450 °C for 30 s decreased the specific contact resistance to a minimum with an extremely low value of 3.4×10−8 Ω cm2 (0.08 Ω mm), which is very close to the theoretical prediction.

Pt/Ti Low Resistance Non-Alloyed Ohmic Contacts to InP-Based Photonic Devices

ABSTRACTPt/ri low resistance non-alloyed ohmic contacts to p-InP-based contact layers in photonic devices, which were formed by rapid thermal processing (RTP), were studied. E-gun evaporated Pt/Ti metallization deposited onto 1.5· 1019 cm−3 Zn doped In0.53Ga0.47 As yielded the best electrical performance. These contacts were ohmic as deposited with a specific contact resistance value of 3.0 · 10−4 Ωcm2. RTP at higher temperatures led to decrease of the specific contact resistance to 3.4 · 10−8 Ωcm2 (0.08Ωmm) as a result of heating at 450°C for 30 sec. This heat treatment caused only a limited interfacial reaction (about 20 nm thick) between the Ti and the InGaAs, resulted in a thermally stable contact and induced tensile stress of 5.6 · 109 dyne · cm−2 at the metal layer but without degrading the adhesion. Heating at temperatures higher than 500°C resulted in an extensive interaction and degradation of the contact.

Extremely low resistance nonalloyed ohmic contacts on GaAs using InAs/InGaAs and InAs/GaAs strained-layer superlattices

Employing a structure consisting of n+-InAs/InGaAs and InAs/GaAs strained-layer superlattices (SLS’s) grown by molecular beam epitaxy on GaAs films, nonalloyed contact resistances less than 8.5×10−8 Ω cm2 have been obtained. Self-consistent simulations show that these extremely small nonalloyed contact resistances are due to the suppression of the depletion depth in the GaAs channel and tunneling through the SLS layer. Similar structures on InGaAs channels have led to nonalloyed specific contact resistances of about 1.5×10−8 Ω cm2. These results represent the smallest figures reported for these important material systems.

Very low resistance nonalloyed ohmic contacts to Sn-doped molecular-beam epitaxial GaAs

This paper presents the results of an investigation conducted to understand the formation of very low resistance nonalloyed ohmic contacts to Sn-doped nGaAs grown by molecular-beam, epitaxy (MBE). The measured specific contact resistance is in good agreement with a new thermionic field-emission model presented in this paper that accounts for a number of physical phenomena that are critical in determining the carrier transport mechanisms across the metal-semiconductor interfaces. The experimental and theoretical results are more than two orders of magnitude smaller than previous contact resistance calculations. This discrepancy predominantly occurs because previous contact theories do not account for metal wave functions and include only conduction band effects when calculating the electron wave vector in the semiconductor energy gap. A specific contact resistance as low as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_{c} \approx 2 \times 10^{-7} \Omega</tex> .cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at T = 300 K is obtained for the nonalloyed contacts fabricated in this work.

Low-resistance nonalloyed ohmic contacts to Si-doped molecular beam epitaxial GaAs

We have found evidence that the surface depletion charge density in molecular beam epitaxial n-GaAs doped heavily with Si approaches the Si concentration. In situ metallization of the as-grown surface of GaAs uniformly doped with Si at 1×1020 cm−3 yields a specific contact resistivity of 1.3 μΩ cm2, indicating a space-charge density about equal to the silicon density despite a measured bulk electron density of 4×1018 cm−3. This contact resistivity is among the lowest for nonalloyed ohmic contacts to n-GaAs. We attribute the large discrepancy between surface space-charge density and bulk electron density to the amphoteric behavior of silicon in GaAs. Surface Fermi-level pinning and arsenic stabilization create a surface depletion region where donor site selection predominates, whereas the extrinsic electron density in the bulk causes self-compensation.

Low contact resistance nonalloyed ohmic contacts to Zn-implanted P+ GaAs

Contact resistance of nonalloyed metal contacts to Zn-implanted shallow (∼1000 A) p+ GaAs layers was investigated. Contact resistance as low as 9×10−7 Ωcm2 was achieved for evaporated Ti/Pt/Au contacts to Zn 40 keV and Zn 100 keV double-implanted layers with a peak concentration of 7×1019/cm3.