Ohmic Contact Formation Mechanism Research Articles

Ohmic Contact Properties Depending on AlGaN Layer Thickness for AlGaN/GaN High Electron Mobility Transistor Structures

Mo/Al/Ti or TiN/TiSi2ohmic contacts were fabricated on AlGaN/GaN HEMT structures having various thicknesses of AlGaN layers. Optimum thicknesses depending on annealing temperature were found, which should be correlated with the contact formation mechanism. AlGaN/GaN HEMTs (high electron mobility transistors) have drawn lots of attention for high frequency and power applications owing to high mobility of 2DEG. One of the important issues of the devices is formation of ohmic contacts with low contact resistance. Since contact metal layers are usually placed on the AlGaN layer under which two-dimensional electron gas (2DEG) is induced, current passes through the AlGaN layer, a barrier layer for electrons, should be formed. Various models of the contact formation mechanism, such as the intrusion of metal into 2DEG [1] and formation of n-type region due to N vacancies [2], have been discussed. However, comprehensive understanding for the mechanism is still insufficient so far. We expect that optimum AlGaN thickness is present to obtain low contact resistance because of a trade-off between increase in the resistance by increasing the thickness due to thicker barrier and that by decreasing the thickness due to depletion of 2DEG. In this work, we revealed the prospect experimentally, and discussed mechanism based on the phenomenon. The wafers used in this study consist of 30-nm-thick undoped AlGaN with GaN layer epitaxially grown on Si (111) substrates. The top AlGaN layers were thinned by the cyclic etching in which oxidation in O3 ambient and removal by hydrofluoric acid were carried out, so that samples with various AlGaN thicknesses were prepared. Contact electrodes, Mo/Al/Ti (35/60/15nm) or TiN/TiSi2 (45/20nm) [3], were formed by sputtering followed by subsequent annealing (400-1100 oC for 1min. in N2) on the samples, as shown in Fig. 1. Contact resistances were evaluated by the TLM method. A typical characteristic of the contact resistance vs. AlGaN thickness for the case of relatively low annealing temperature (650oC) is shown in Fig.2. The optimum AlGaN thickness to get the lowest resistance, 11.6 nm in this case, was clearly observed. On the other hand, in the case of higher annealing temperature (950oC or 1100oC), the monotonous decrease in contact resistance with the thickness was observed as shown in Fig.3. The increase in contact resistance in the region thinner than 11.6 nm is speculated to be due to depletion of 2DEG, which is common to the both low and high temperature cases. While, the increase in contact resistance for thicker layers at low temperature annealing, as shown in Fig.2, is probably attributed to high resistivity in the AlGaN layer. However, in the case of high temperature annealing, as shown in Fig.3, some mechanism of the metal intrusion and/or the conversion to n-type to overcome the thick barrier layer should work effectively. In conclusion, dependences of contact resistance on thickness of the AlGaN layer were observed, and they indicated existence of the optimum thickness. Discussion of these properties correlated with mechanism of ohmic contact formation will be useful for further understanding of the contact technology for AlGaN/GaN HEMTs.

Role of Al and Ti for ohmic contact formation in AlGaN/GaN heterostructures

A mechanism for ohmic contact formation using Ti/Al based metals on AlGaN/GaN heterostructures has been investigated by measuring temperature dependence of sheet electron density (ns) and mobility (μ). It was found that both ns and μ at room temperature for Ti/Al deposited sample were increased by annealing in vacuum, while not for Al/Ti deposited one. The results, especially increase in μ, cannot be understood by the conventional ohmic formation model, including Ti-N (nitrogen) complex formation or N vacancy formation. As the most probable mechanism for the increase in ns and μ, we have proposed a model, in which tensile strain is induced by the reaction of Ti/Al and AlGaN after annealing.

Electrical Characteristics of Ti/Al Ohmic Contacts to Molecular Beam Epitaxy-Grown N-polar n-type GaN for Vertical-Structure Light-Emitting Diodes

We investigated the electrical properties of Ti(30 nm)/Al(200 nm) contacts to molecular beam epitaxy-grown N-polar n-GaN with different carrier concentrations. Samples with carrier concentration of 1.2 × 1018 cm−3 showed nonohmic behaviors when annealed at 300°C, but ohmic at 500°C and 700°C. All samples with carrier concentration of 2.0 × 1019 cm−3 exhibited ohmic behavior. x-Ray photoemission spectroscopy (XPS) results showed that, for samples with carrier concentration of 1.2 × 1018 cm−3, the Ga 2p core levels shift to lower or higher binding energy upon annealing at 300°C or above 500°C, respectively. Scanning transmission electron microscopy (STEM) results showed that, for samples with carrier concentration of 1.2 × 1018 cm−3, a wurtzite AlN layer (∼2 nm thick) formed at the metal/GaN interface when the samples were annealed at 500°C. An interfacial wurtzite AlN layer also formed upon annealing at 700°C, but its thickness was ∼4 nm. Based on the XPS and STEM results, the ohmic contact formation and degradation mechanisms are described and discussed.

The Formation Mechanism of Ni-Based Ohmic Contacts to 4H-n-SiC

In this work the electrical properties of Ni and Ni2Si contacts on n-type 4H-SiC were correlated to the strong structural changes at the contact/SiC interface upon annealing. We can conclude that only δ-Ni2Si grains play a main role in determining electrical transport properties of the Ni-based ohmic contacts to n-SiC. It is presumed that a recrystallization and texturization of δ-Ni2Si phase on (0001)SiC-surface during high temperature annealing (> 900°C) contributes to the change of barrier heights, as well as specific contact resistance of contacts.

Low-resistivity Ni/Pt Ohmic contacts to p-type N-doped ZnO

Low-resistivity Ni/Pt Ohmic contacts to p-type N-doped ZnO were fabricated in this paper. Ni/Pt Ohmic contacts showed a lowest specific contact resistivity of 3.81 × 10 −6 Ω cm 2 after 450 °C annealing, which is lower than any other reported Ohmic contacts to p-type ZnO. The interface reactions during annealing and the Ohmic contact formation mechanism were investigated by secondary ion mass spectrometry measurements. The results showed that Ni out-diffused to react with Pt and Zn out-diffused a little, too. These interface diffusions as well as activation of N acceptors during annealing induced such a low specific contact resistivity.

Formation mechanism of Ohmic contacts on AlGaN∕GaN heterostructure: Electrical and microstructural characterizations

The electrical characteristics and interfacial reactions of Ti∕Al∕Mo∕Au metallization on AlGaN∕GaN heterostructures at various annealing temperatures ranging from 400to950°C have been investigated in an effort to elucidate the Ohmic contact formation mechanism. A transition from Schottky to Ohmic electrical behavior occurred at ∼500°C. This transition was engendered by the formation of a thin epitaxial TiN layer on the AlGaN. Binary and ternary intermetallic compound formations were observed to have formed as a result of intermixing and reactions among the metals. The Mo layer remained continuous until 700°C, where it disintegrated into particles embedded in the metallic matrix. An optimal contact performance was obtained for Ti∕Al∕Mo∕Au on AlGaN∕GaN at 850°C; this was correlated to TiN island formation along dislocations penetrating into the heterostructure beyond the plane of the two-dimensional electron gas. Overannealing degradation mechanism at 950°C is also explained based on structural characterization.

A Cu-based alloyed Ohmic contact system on n-type GaAs

An alloyed Pd∕Ge∕Cu Ohmic contact to n-type GaAs is reported for the first time. The Pd∕Ge∕Cu Ohmic contact exhibited a very low specific contact resistance of 5.73×10−7Ωcm2 at a low annealing temperature of 250°C. This result is comparable to the reported Pd∕Ge and Au∕Ge∕Ni Ohmic contact systems to n-type GaAs with doping concentrations about 1×1018cm−3. The Ohmic contact formation mechanisms and microstructure evolution were investigated using secondary ion mass spectrometry, x-ray diffraction, transmission electron microscopy, and energy dispersive spectrometer. The Ohmic contact behavior was related to the formation of Cu3Ge and PdGaxAsy compounds after annealing.

Differences in the reaction kinetics and contact formation mechanisms of annealed Ti∕Al∕Mo∕Au Ohmic contacts on n-GaN and AlGaN∕GaN epilayers

Ti ∕ Al -based multilayer metallizations are usually used interchangeably for n-GaN and AlGaN∕GaN epilayers. Our investigations show that, although excellent Ohmic performances were obtained on both cases, the reaction pathways/kinetics and Ohmic contact formation mechanisms of Ti∕Al∕Mo∕Au metallization on n-GaN and AlGaN∕GaN are different. In the case of n-GaN, the reaction proceeds by the formation of continuous TiN layer at the metal/GaN interface. Low-barrier Schottky contact or tunneling contact is the possible contact formation mechanisms. On AlGaN∕GaN heterostructures, the existence of the AlGaN layer significantly modifies the reaction pathway, where discrete TiN protrusions form nonuniformly along threading dislocations. The TiN protrusions penetrate through the AlGaN layer and form direct electrical link between the two-dimensional electron gas and the metal contacts which would otherwise be separated by the AlGaN barrier layer. A contact formation mechanism, similar to the “spike mechanism,” which dominates the Ohmic contact formation in GaAs- and InP-based devices, is proposed for the carrier transport in the case of Ti∕Al∕Mo∕Au contacts on AlGaN∕GaN. Kinetically, the formation rate of TiN is lower on AlGaN layer than on n-GaN, a result that is rationalized by the formation enthalpies of different nitrides.

Mechanism for Ohmic contact formation on Si3N4 passivated AlGaN∕GaN high-electron-mobility transistors

Recent experiments have shown that in situ passivation by Si3N4 of AlGaN∕GaN high-electron-mobility transistors results in improved electrical characteristics. Transmission electron microscopy techniques have been applied to study the metal contact formation on top of passivated AlGaN∕GaN structures. Contrary to unpassivated AlGaN∕GaN, the AlGaN top layer is not consumed by a typical Au∕Mo∕Al∕Ti metal stack. Instead, a thin AlN interface layer is formed, being the key factor in the Ohmic contact formation. The formation of this AlN is believed to be due to extraction of N atoms out of the AlGaN. The resulting N vacancies, electrical donors, create a conducting channel through the AlGaN.

Ni Graphite Intercalated Compounds in Ohmic Contact Formation on SiC

Ohmic contacts on SiC have been investigated extensively in the past decade. However, the mechanism for ohmic contact formation has been a troublesome issue. The interfacial structures at the atomic scale responsible for forming ohmic contacts have not been revealed. Our previous results have shown that carbon can form ohmic contacts on SiC after thermal annealing, and that an interfacial carbon layer between Ni and the SiC improves the contacts significantly. In this study, we have investigated the interactions between Ni and carbon, and ohmic contact formation on SiC using x-ray diffraction (XRD) and Raman spectroscopy. After annealing, ohmic behavior was observed and Ni graphite intercalated compounds (GICs) were found on Ni/C/SiC structures. Unlike conventional graphite intercalated compounds, the Ni atoms substitute for carbon atoms in the graphitic networks in these Ni-GICs. XRD peaks at 21.6° due to the Ni graphitic intercalation compound (Ni-GIC) and at 26.3° due to graphite have been observed. The distance between graphitic sheets is 0.403nm in the Ni graphite intercalated compounds, whereas it is ~20% larger in the graphite. The thickness of the interfacial carbon layer does not affect the formation of Ni-GIC.

An Approach to Improving the Morphology and Reliability of n-SiC Ohmic Contacts to SiC Using Second-Metal Contacts

While nickel ohmic contacts to n-type silicon carbide have good electrical properties, the physical contact, and therefore the reliability, can be poor. An approach is described for using the good electrical properties of Ni ohmic contacts while using another metal for its desired mechanical, thermal and/or chemical properties. In the present work, once the Ni contacts have been annealed forming nickel silicides and achieving low contact resistance, they are etched off. Removing the primary Ni contacts also eliminates the poor morphology, voids, and at least some of the excess carbon produced by the Ni/SiC reaction. The Ni contacts are then replaced by a second contact metal. This second metal displays low contact resistance as-deposited, indicating that the critical feature responsible for the ohmic contact has not been removed by the primary contact etch. Not only does this approach provide more flexibility for optimizing the contact for a given application, it also provides some insight into the ohmic contact formation mechanism.

Novel Pd/Ge/Cu Ohmic Contact to n-Type GaAs

A low contact resistance Pd/Ge/Cu ohmic contact to n-type GaAs is successfully developed. The Pd/Ge/Cu ohmic contact exhibits a very low contact resistivity of 5.73 x 10-7 Ω-cm2 at a low annealing temperature (250 {degree sign}C). This result is comparable to the reported Pd/Ge and Au/Ge/Ni ohmic contact systems to n- type GaAs with doping concentrations about 1x1018 cm-3. The ohmic contact formation mechanisms and microstructure reactions were investigated using x-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM).

Approach to optimizing n-SiC Ohmic contacts by replacing the original contacts with a second metal

Nickel (Ni) contacts to n-type silicon carbide (n-SiC) have good electrical properties, but the physical contact, and therefore the reliability, is poor. An approach is described for using the good electrical properties of Ni Ohmic contacts, while using another metal for its superior topological, mechanical, thermal, or chemical properties for the chosen application. In the present work, we show that once the Ni contact has been annealed, forming nickel silicides, it can be etched off and replaced by a second metal chosen for its desired properties. This second metal displays an as-deposited contact resistance as low as the original annealed Ni contact, indicating that the critical feature responsible for Ohmic contact formation has not been removed when the silicide phases were etched off. Not only does this approach provide more flexibility for optimizing the contact for a given application but it also provides some insight into the Ohmic contact formation mechanism.

Mechanism of ohmic contact formation in AlGaN/GaN high electron mobility transistors

We tested various ohmic contact metallization schemes on AlGaN/GaN HEMTs to achieve low contact resistance, good surface morphology and proper line edge definition. Mo, Ni and Pt intermediate layers in these schemes replaced the intermediate Ti layer interposed between Al and Au in Ti/Al/Ti/Au contacts. We recorded almost similar values of the contact resistance lying in the range of 0.3–0.5 Ω mm on all the metallization stacks, but a significant difference was observed in their surface morphology and line edge definition. The formation of particular intermetallic compounds due to different intermediate layers was found to be responsible for this difference. Ti/Al/Ti/Au/WSiN contacts possessed excellent surface morphology. Mo based contact not only delivered good surface morphology and low contact resistance but also proper line edge definition. We found that the formation of the Al–Mo phase and GaMo3 compound was the key factor responsible for its remarkable performance.

Noninterfacial-nitride formation ohmic contact mechanism in Si-containing Ti∕Al∕Mo∕Au metallizations on AlGaN∕GaN heterostructures

Excellent ohmic contact formation to undoped-AlGaN∕GaN heterostructures is achieved by the incorporation of silicon into Ti∕Al∕Mo∕Au metallization. Contact resistance and specific contact resistivity as low as 0.16Ωmm and 6.77×10−7Ωcm2, respectively, have been obtained for the Ti∕Si∕Al∕Si∕Mo∕Au scheme. Transmission electron microscopy has revealed that the ohmic contact formation mechanism depends on the competing interfacial nitride and silicide formation reactions, the latter giving rise to improved contact performance. The formation and penetration of non-nitride reaction products of Si-containing Al–Au intermetallics to depths beyond the AlGaN∕GaN interface were observed. The superior ohmic performance of these schemes suggests that silicide-based low Schottky barrier contact formation may be preferable to an interfacial nitride contact mechanism in achieving improved ohmic behavior.

Ohmic contact formation mechanism of Ta∕Al∕Mo∕Au and Ti∕Al∕Mo∕Au metallizations on AlGaN∕GaN HEMTs

A comparative study of Ta∕Al∕Mo∕Au and Ti∕Al∕Mo∕Au metallizations for AlGaN∕GaN high electron mobility transistors is presented. By the optimization of surface treatment schemes and annealing temperature, contact resistances of 0.172 and 0.228Ωmm, and specific contact resistivities of 2.96×10−7 and 1.09×10−6Ωcm2 were obtained for Ti∕Al∕Mo∕Au and Ta∕Al∕Mo∕Au, respectively. Auger electron spectroscopy (AES), x-ray diffraction (XRD), and transmission electron microscopy (TEM) were utilized to study microstructural changes occurring in the metallization layers as a result of heat treatment. Results indicate dynamic systems of severe intermixing between atoms from the metallization layers and epilayers resulting in changes in surface morphology, as determined by atomic force microscopy (AFM). The formation of intermetallics and interfacial compounds was observed. The combined effect of surface treatment and annealing-induced reaction is proposed as the mechanisms for low-resistance Ohmic contact formation.

Dislocation-induced nonuniform interfacial reactions of Ti∕Al∕Mo∕Au ohmic contacts on AlGaN∕GaN heterostructure

Transmission electron microscopy (TEM) is utilized to elucidate the postannealing interfacial microstructure of Ti∕Al∕Mo∕Au metallization with AlGaN∕GaN heterostructures to gain insight into the formation mechanism of low-resistance ohmic contacts. The reaction between the metal and the AlGaN layer does not proceed uniformly. Localized penetration through the AlGaN layer beyond the two-dimensional electron gas (2DEG) is observed while partial consumption of the AlGaN layer is noted in other areas. Analytical TEM analyses confirm that the main reaction product is TiN. A correlation between the appearance of TiN islands and threading dislocations is observed. Threading dislocations serve as short-circuit diffusion channels, and are responsible for the nonuniform reaction. TiN islands have a large total area of intimate contact with the 2DEG, and since no tunneling of electron through the AlGaN is required, a low-resistance ohmic contact is obtained.

Photoemission study on interfacial reaction of Ti/n-type GaN

In order to understand the mechanism of ohmic-contact formation by the annealing of Ti electrodes on n-type GaN, we have investigated the changes in the energy-band structure of Ti/n-type GaN depending on annealing temperature using photoemission spectroscopy. Valence-band spectrum for an as-deposited sample was explained by the simple summation of Ti and GaN spectra. Spectral line shapes significantly changed by annealing at 500 and 700 °C, suggesting the formation of a TiN layer. The peak shifts of Ga 3d and N 1s core levels are interpreted as the energy-band bending and interfacial reaction with the formation of the TiN layer. It is revealed that the ohmic-contact formation by the annealing is attributed to the formation of GaN with nitrogen vacancies, which is consistent with the current–voltage characteristics.

Mechanism for ohmic contact formation of Ni∕Ag contacts on p-type GaN

The mechanism for ohmic contact formation of Ni∕Ag contacts on p-type GaN was investigated using synchrotron photoemission spectroscopy. A low contact resistivity of 6.6×10−5Ωcm2 was obtained from Ni(50Å)∕Ag(1200Å) contact after annealing at 500°C in O2 ambient. Ni out-diffused to form a NiO and Ag in-diffused into the contact interface during the oxidation annealing. Out-diffused Ga atoms from GaN could dissolve in the Ag layer to form Ag–Ga solid solutions, leaving Ga vacancies below the contact. Ga vacancies could increase the net hole concentration and reduce the surface band bending, resulting in the ohmic contact formation.

Electrical properties and microstructure of ternary Ge∕Ti∕Al ohmic contacts to p-type 4H–SiC

The high-power SiC devices require ohmic contact materials, which are prepared by annealing at temperatures lower than 800°C. Recently, we demonstrated in our previous paper [J. Appl. Phys. 95, 2187 (2004)] that an addition of a small amount of Ge to the conventional binary Ti∕Al contacts reduced the ohmic contact formation temperature by about 500°C, and this ternary contacts yielded a specific contact resistance of approximately 1×10−4Ωcm2 after annealing at a temperature as low as 600°C. In this paper, the electrical properties and the microstructures of the Ge∕Ti∕Al contacts (where a slash “/” indicates the deposition sequence) were investigated by current-voltage measurements and transmission electron microscopy observations, respectively, in order to understand the ohmic contact formation mechanism. Ti3SiC2 compound layers (which were previously observed at the metal/SiC interface in the Ti∕Al ohmic contacts after annealing at temperatures higher than 1000°C) were observed to grow epitaxially on the SiC surface after annealing at temperatures as low as 600°C. The Ti3SiC2 layers were believed to act as a p-type intermediate semiconductor layer, which played a key role to reduce the Schottky barrier height at the contacting metal/SiC interface. Further reduction of the contact resistances of the Ge∕Ti∕Al contacts would be achieved by increasing the coverage of the Ti3SiC2 layers on the SiC surface.