Au Ohmic Contacts Research Articles

Optimization of Ohmic Contact Metallization Process for AlGaN/GaN High Electron Mobility Transistor

In this paper, a manufacturing process was developed for fabricating high-quality AlGaN/GaN high electron mobility transistors (HEMTs) on silicon carbide (SiC) substrates. Various conditions and processing methods regarding the ohmic contact and pre-metal-deposition etching processes were evaluated in terms of the device performance. In order to obtain a good ohmic contact performance, we tested a Ti/Al/Ta/Au ohmic contact metallization scheme under different rapid thermal annealing (RTA) temperature and time. A -based reactive-ion etching (RIE) method was performed before the ohmic metallization, since this approach was shown to produce a better ohmic contact compared to the as-fabricated HEMTs. A HEMT with a 0.5 gate length was fabricated using this novel manufacturing process, which exhibits a maximum drain current density of 720 mA/mm and a peak transconductance of 235 mS/mm. The X-band output power density was 6.4 W/mm with a 53% power added efficiency (PAE).

Ti/Ni/Ti/Au Ohmic contact and Schottky transformation to Al-doped ZnO thin films

A multi-layered Ti (50nm)/Ni (50nm)/Ti (50nm)/Au (50nm) metallic contact to Al-doped ZnO thin films (ND=3.64×1019cm−3) was reported in this paper. The as-deposited Ti/Ni/Ti/Au scheme showed a specific contact resistivity of 2.07×10−4Ωcm2. And a lowest specific contact resistivity of 6.69×10−5Ωcm2 was obtained after annealing at 500°C. The specific contact resistivity showed a decreasing tendency with the annealing temperature up to 500°C. However, for the sample annealed at above 600°C, the I–V curves revealed gradual Schottky transformation. Interface diffusions and reactions induced by annealing treatment were found to be responsible for the decrease of Ohmic contact resistivity and the Schottky transformation.

A novel method for the fabrication of AlGaN/GaN HEMTs on Si (111) substrates

In this paper, the development of a novel manufacturing process is presented for fabricating high-quality AlGaN/GaN high-electron-mobility transistors (HEMTs) on Si (111) substrates. Various material and processing approaches regarding surface passivation, gate oxide, ohmic contact metal, and post-gate annealing are evaluated in terms of device performance. In order to achieve better immunity to current collapse effects, we conducted experiments that investigate the relationship between the AlGaN/GaN HEMTs’ electrical characteristics and different passivation films by plasma-enhanced chemical vapour deposition. In order to obtain a better ohmic contact performance, we tested a Ti/Al/Ta/Au ohmic contact metallisation scheme using different annealing temperatures and annealing times to achieve a lower contact resistance, a more proper line edge definition, and a better surface morphology. A post-gate N2 rapid thermal annealing method done after the gate metallisation process has shown better DC current–voltage output, transfer characteristics, and gate–drain breakdown voltage results compared to the as-fabricated HEMTs. A HEMT with a 0.5-μm gate length, exhibiting a maximum drain current density of 750 mA/mm, a peak transconductance of 220 mS/mm, a unity-gain cut-off frequency of 24.6 GHz, and a maximum frequency of oscillation of 45.4 GHz, was fabricated using this novel manufacturing process; the X-band power performances demonstrate a 5.8-W/mm output power density and a 51 % power-added efficiency.

Ultra-low resistance ohmic contacts to GaN with high Si doping concentrations grown by molecular beam epitaxy

Ti/Al/Ni/Au ohmic contacts were formed on heavily doped n+ metal-polar GaN samples with various Si doping concentrations grown by molecular beam epitaxy. The contact resistivity (RC) and sheet resistance (Rsh) as a function of corresponding GaN free carrier concentration (n) were measured. Very low RC values (<0.09 Ω mm) were obtained, with a minimum RC of 0.035 Ω mm on a sample with a room temperature carrier concentration of ∼5 × 1019 cm−3. Based on the systematic study, the role of RC and Rsh is discussed in the context of regrown n+ GaN ohmic contacts for GaN based high electron mobility transistors.

A Comparative Optimization of Electrical Properties and Surface Morphology of Ti/Al/Ta/Au Ohmic Contacts in AlGaN/GaN HEMTs on Si(111), Sapphire, 4H-SiC Substrates

In the process of characterizing AlGaN/GaN HEMTs on Si (111), Sapphire, 4H-SiC substrates, various Rapid Thermal Annealing (RTA) conditions for the Ti/Al/Ta/Au ohmic contact process and the resulting surface analysis have been investigated. In order to achieve a low ohmic contact resistance (RC) and a high quality surface morphology, we tested seven steps (800 °C to 920 °C) annealing temperatures and two steps (15, 30 sec) annealing times. According to these annealing temperatures and times, the optimal ohmic resistance of 3.62 × 10-6 Ohm • cm2 on Si(111) substrate, 9.44 × 10-6 Ohm • cm2 on Sapphire substrate and 1.24 × 10-6 Ohm • cm2 on 4H-SiC substrate are obtained at an annealing temperature of 850 °C and an annealing time of 30 sec, 800 °C and an annealing time of 30 sec and 900 °C and an annealing time of 30 sec, respectively. The surface morphologies of the ohmic contact metallization at different annealing temperatures are measured using an Atomic Force Microscope (AFM). AFM morphology Root Mean Square (RMS) level determines the relationship of the annealing temperature and the annealing time for all of the samples. According to these annealing temperatures and times, the optimal ohmic surface RMS roughness of 13.4 nm on Si(111) substrate, 3.8 nm on Sapphire substrate and 2.9 nm on 4H-SiC substrate are obtained at an annealing temperature of 850 °C and an annealing time of 30 sec, 800 °C and an annealing time of 30 sec and 900 °C and an annealing time of 30 sec, respectively.

Role of Ti/Al relative thickness in the formation mechanism of Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures

In this work, Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures with different Ti/Al relative thicknesses were fabricated and characterized. A contact with Ti/Al relative thickness of 20/180 nm presented non-linear current–voltage response and bumpy surface morphology, while a contact with Ti/Al relative thickness of 20/120 nm demonstrated low contact resistance and flat surface morphology. We verify the existence of two electron transport mechanisms, namely carrier tunnelling and direct link through a TiN conductive pathway, by scanning transmission electron microscopy and corresponding electron dispersive x-ray spectra. Ti/Al relative thickness exerts a decisive influence on the alloy reaction processing. Superfluous Al consumes Ti, thus impeding the formation of low-resistance TiN and N vacancies. As a result, electron transport is restricted by the low-efficiency tunnelling mechanism. At the same time, the consumption of Ni causes damage to its blocking effect and arouses the free diffusion of metal alloys, which should be responsible for the rough surface.

Fabrication of CdMnTe semiconductor as radiation detector

In this work, devices have been fabricated as to be nuclear detector, from p-CdMnTe. Three devices were prepared: First planar detector with (Au) ohmic contact on both sides; Second as schottky type device of In-CdMnTe with (Au) ohmic contact on the back face; and thirdly metal oxide semiconductor (MOS) device for which the samples annealed under Oxygen flow for (2 h.) at 700 °C, this device (MOS) when connected to conventional nuclear spectroscopy gave reasonable but not high quality signals due to irradiating with 137Cs, 241Am and (137Cs + 241Am). Matlab- software based on numerical methods is used to analyze the data. Resolution through the Full Width at Half Maximum and the area under the peak and the error for this detector are presented.

Electrical characterization and nanoscale surface morphology of optimized Ti/Al/Ta/Au ohmic contact for AlGaN/GaN HEMT

Good ohmic contacts with low contact resistance, smooth surface morphology, and a well-defined edge profile are essential to ensure optimal device performances for the AlGaN/GaN high electron mobility transistors [HEMTs]. A tantalum [Ta] metal layer and an SiNx thin film were used for the first time as an effective diffusion barrier and encapsulation layer in the standard Ti/Al/metal/Au ohmic metallization scheme in order to obtain high quality ohmic contacts with a focus on the thickness of Ta and SiNx. It is found that the Ta thickness is the dominant factor affecting the contact resistance, while the SiNx thickness affects the surface morphology significantly. An optimized Ti/Al/Ta/Au ohmic contact including a 40-nm thick Ta barrier layer and a 50-nm thick SiNx encapsulation layer is preferred when compared with the other conventional ohmic contact stacks as it produces a low contact resistance of around 7.27 × 10-7 Ω·cm2 and an ultra-low nanoscale surface morphology with a root mean square deviation of around 10 nm. Results from the proposed study play an important role in obtaining excellent ohmic contact formation in the fabrication of AlGaN/GaN HEMTs.

Micro and nano analysis of 0.2 Ω mm Ti/Al/Ni/Au ohmic contact to AlGaN/GaN

As GaN technology continues to gain popularity, it is necessary to control the ohmic contact properties and to improve device consistency across the whole wafer. In this paper, we use a range of submicron characterization tools to understand the conduction mechanisms through the AlGaN/GaN ohmic contact. Our results suggest that there is a direct path for electron flow between the two dimensional electron gas and the contact pad. The estimated area of these highly conductive pillars is around 5% of the total contact area.

GaN detector development for particle and X-ray detection

We report on preliminary alpha particle and X-ray measurements on a number of prototype GaN PIN diodes. The aim of the study was to investigate the potential use of GAN based radiation detectors for radiation hard, high temperature, solar blind space applications. The devices have a planar structure consisting of a 2μm epitaxial GaN layer grown on a highly doped n-type AlxGa1−xN nucleation layer, which in turn is deposited on a p-type 4H–SiC substrate. Au ohmic contacts were applied to the top of the GaN layer and the bottom of the substrate. A number of different sized devices were tested with contact diameters ranging from 0.4mm to 0.7mm. All devices showed good diode behaviour with reverse leakage currents in the tens to hundreds of micro-amp range. C–V measurements showed that the GaN layers were fully depleted for biases >20V. When exposed to a 5.5MeV alpha particle source, the devices showed a spectroscopic response with energy resolutions of ∼25% FWHM at room temperature (RT) and 10V bias and 20% FWHM at −50°C. These values are consistent with the previous measurements. No response to 60keV photons could be measured.

High temperature induced failure in Ti/Al/Ni/Au Ohmic contacts on AlGaN/GaN heterostructure

Ti/Al/Ni/Au (200/1200/500/2000 Å) Ohmic contact on AlGaN/GaN was prepared and it was subjected to thermal aging experiments. Thermal processing at 400 and 500 °C did not change the contact resistance significantly, while high temperature storage at 600 °C resulted in a surge in the contact resistance. The Al–Au alloy in the contact metal is believed to re-melt because its lowest melting temperature is 525 °C. The liquid of Al–Au alloy is observed to diffuse to the AlGaN surface and consume some AlGaN layer. In addition, voids are found to be produced during thermal process, which can reduce the effective contact area and thus lead to higher contact resistance. The TEM and EDX results of Ohmic contact’s cross sectional images provide evidence for this proposed mechanism.

Low-Resistive Ohmic Contacts for AlGaN Channel High-Electron-Mobility Transistors Using Zr/Al/Mo/Au Metal Stack

An AlxGa1-xN/AlyGa1-yN high-electron-mobility transistor (HEMT) with AlGaN as a channel layer has been fabricated on a sapphire substrate for high-output-power and high-frequency electronic applications. One of the key process steps for the AlGaN-channel HEMT is to ensure low resistivity for source/drain ohmic contacts. In this work, the electrical characteristics of Zr/Al/Mo/Au ohmic contacts for AlGaN-channel HEMTs were investigated at annealing temperatures from 850 to 1000 °C. An AlGaN-channel HEMT was fabricated with Al contents of 0.3 and 0.55 for the channel and barrier layer, respectively. A minimum ohmic contact resistivity of 2.6×10-4 Ω cm2 was achieved for the Al0.55Ga0.45N/Al0.3Ga0.7N heterostructure after annealing at 950 °C.

Interface analysis of Ti/Al/Ti/Au ohmic contacts with regrown n+‐GaN layers using molecular beam epitaxy

The regrowth technique of highly doped n‐type GaN layers is reliable and effective for lowering the ohmic contact resistance. The interface between metal contacts with Ti/Al/Ti/Au and regrown n+‐GaN/GaN layers were analyzed in detail with transmission electron microscopy. During the annealing process, Ti metals and N atoms diffusing from GaN layers formed TiN epitaxial layers between metal alloys and n+‐GaN layers. The orientational relationship between GaN and TiN was [1 0 0]GaN//[−1 1 0]TiN verified by nano‐beam diffraction. Al atoms diffused through the GaN layers and formed thin AlGaN phase. Al content was confirmed as 60% by high‐resolution transmission electron microscopy images. Electron energy loss spectroscopy showed that Si dopants were confined within n+‐GaN layers. These results show that in regrowth technique both TiN layers and Si dopants affect the contact properties because the formation of TiN layers can induce nitrogen vacancies from GaN, while Si‐doped GaN layers can enhance the tunneling effect through the metal contacts resulting in reduced contact resistance. Copyright © 2011 John Wiley & Sons, Ltd.

Structural characterization of V/Al/V/Au Ohmic contacts to n-type Al0.4Ga0.6N

This paper investigates the temperature dependence of the specific resistance in annealed V/Al/V/Au (15 nm/85 nm/20 nm/95 nm) contacts on n-Al0.4Ga0.6N. Contacts annealed at 700 °C and higher temperatures show Ohmic behaviour. Annealing at 800 °C produces the lowest contact resistance. Samples annealed at 800 °C have been analysed by using cross-sectional transmission electron microscopy and an energy dispersive x-ray spectrum. Limited reaction depths are observed between V-based contacts and n-AlGaN. The VN grains are found to form in the contact layer of the annealed samples, which can be considered as the key to the successful formation of Ohmic contact. The contact layer adjacent to AlGaN material consists of V-Al-Au-N, AlN and AlAu alloys.

Annealing temperature dependence of Ohmic contact resistance and morphology on InAlN/GaN high electron mobility transistor structures

Ti/Al/Ni/Au Ohmic contact metallization on InAlN/GaN heterostructures both with and without a thin GaN cap layer was annealed at different temperatures. The minimum transfer resistance for the contacts of 0.65 Ω mm (specific contact resistivity of 2×10−5 Ω cm2) was achieved after 800 °C annealing for structures without the GaN cap, while those with the cap exhibited their lowest resistance at higher temperatures. The contact morphology showed considerable roughening by 750 °C but the carrier mobility was stable until annealing temperatures of 850 °C. Diffuse scattering experiments showed that the morphological roughness of the InAlN/GaN interface increased as a result of annealing at these temperatures and the data were consistent with outdiffusion of Ga into the InAlN. Unpassivated high electron mobility transistors with a gate dimension of 0.7×180 μm2 were fabricated using these contacts and showed a maximum drain current of 1.3 A/mm and an extrinsic transconductance of 366 mS/mm. The presence of the GaN cap increased the effective barrier height of Ni/Au Schottky contacts from 0.91 to 1.01 eV on the heterostructure.

Analysis on the new mechanisms of low resistance stacked Ti/Al Ohmic contact structure on AlGaN/GaN HEMTs

A novel stacked Ti/Al based Ti/Al/Ti/Al/Ti/Al/Ti/Al/Ni/Au Ohmic contact structure is optimized. Compared with the conventional alloyed Ti/Al/Ni/Au Ohmic contact structure, the novel Ohmic contact structure can obtain much lower contact resistance and specific contact resistivity. Through analysis of x-ray diffraction spectra, cross-section transmission electron microscopy images and corresponding electron dispersive x-ray spectroscopy spectra in the novel stacked Ti/Al based Ohmic structure, the reactions between metals and the AlGaN layer were proven to be stable, uniform and continuous, which produced smooth contact interface. In addition, the top Au layer was prevented from diffusing downwards to the metal/AlGaN interface, which degraded the Ohmic performance.

Thermal Storage of AlGaN/GaN High-Electron-Mobility Transistors

The thermal stability and electrical characteristics of GaN high-electron-mobility transistors (HEMTs) were investigated. Storage tests were carried out at 400°C for 48 h to study the ohmic-contact stability by means of the transmission line model. It was found that Ti/Al/Ni/Au ohmic contacts were stable and had superior thermal performance, but the Schottky contact may be more sensitive to the temperature. After thermal storage for 48 h at 400°C, the Schottky barrier height was increased, and the ideality factor decreased. Two types of isolation structures were also investigated under the same condition. DC tests were implemented to study the phenomenon and provide feedback for potential process improvements.

Applications of TEM imaging, analysis and electron holography to III-nitride HEMT devices

This paper reviews some of our recent studies of III-nitride high electron mobility transistor (HEMT) devices using advanced electron microscopy methods. Sample preparation protocols have been developed that can routinely provide thin, electron-transparent specimen regions of uniform thickness extending across entire HEMT devices. The use of focused-ion-beam (FIB) thinning facilitated access to specific device regions, although structural damage and imaging artifacts can result unless suitable precautions are taken during milling to minimize ion-beam damage. The extent of gallium-ion sidewall implantation during FIB milling, and surface damage caused by deposition of Pt protective layers, have been assessed. As-processed device structures have been examined by conventional diffraction contrast imaging as well as high-resolution phase contrast imaging, while nanospectroscopy and nanoscale elemental mapping have been used to measure local variations in chemical composition. Annealing of Ti/Al/Ni/Au ohmic contacts for AlInN/AlN/GaN devices lead to the formation of TiN contact inclusions that are invariably located at mixed-type threading dislocations originating from the underlying GaN layers. Some preliminary observations of device structures after extended periods of operation and after device failure have also been made. The technique of off-axis electron holography has been used to quantify two-dimensional electrostatic fields within cross-sectioned devices with nanometer-scale resolution. Polarization fields of 6.9 MV/cm and a two-dimensional electron gas of ∼2.1 × 10 13/cm 2 have been measured for an AlInN/AlN/GaN heterostructure. Methods suitable for in situ biasing of HEMT samples during electron holography observations have also been explored.

Analysis of surface roughness in Ti/Al/Ni/Au Ohmic contact to AlGaN/GaN high electron mobility transistors

A mechanism of the formation of the bulges on the surface of Ti/Al/Ni/Au Ohmic contact in AlGaN/GaN high electron mobility transistors is proposed. According to the analysis of TEM images and corresponding electron dispersive x-ray spectra, the bulges were found to consist of Ni–Al alloy in the body and Au–Al alloy surrounding. We deduce that the bulges were formed due to Ni–Al alloy aggregation in some local areas during the rapid thermal annealing process, which accounts for the rough surface morphology.

Ohmic contact properties of non-polar a-plane GaN films on r-plane sapphire substrates

The properties of Ti/Al/Ni/Au Ohmic contacts on n-type a-plane GaN epitaxial layers directly grown on r-plane sapphire substrates are reported. The minimum specific contact resistance of ∼10−5 Ω cm2 was achieved after annealing at 650–700 °C. Ohmic contact properties were measured using transmission line method patterns oriented in both the m- and c-axis directions of a-plane GaN. The sheet resistance of a-plane GaN along the c-axis was two times higher than that along the m-axis, which shows significant electric anisotropy in the two orientations.