Structural properties of alloyed Ti/Al/Ti/Au and Ti/Al/Mo/Au ohmic contacts to AlGaN/GaN

Two alloyed ohmic contact structures for AlGaN/GaN–Ti/Al/Ti/Au and Ti/Al/Mo/Au were studied. Both structures were optimized for minimum ohmic contact resistance. Structures grown on sapphire and SiC substrates were used to investigate structural properties of ohmic contacts to AlGaN/GaN. Ohmic contacts to AlGaN/GaN on SiC showed higher contact resistance values compared to contacts to AlGaN/GaN on sapphire. Ohmic contact metals were etched on samples after annealing. The alloyed interface was studied with backside illumination under an optical microscope. Alloyed inclusions associated with threading dislocations were observed on the surface. For the AlGaN/GaN on SiC sample the inclusion density was an order of magnitude lower than for the sample on sapphire. Conductive atomic force microscopy with carbon nanotube tip was used to investigate topography and conductivity profile of the surface after ohmic contact metal removal by etching.     

Mo/Al/Mo/Au-based ohmic contacts to AlGaN/GaN heterostructures

Mo/Al/Mo/Au metallization scheme as an ohmic contact to undoped AlGaN/GaN heterostructures was investigated. The optimal thicknesses of the metal layers were determined: Mo (10 nm)/Al (60 nm)/ Mo (50 nm)/Au (50 nm). The specific contact resistance of the fabricated ohmic contact is 4.7 × 10^–7 Ohm cm² (0.14 Ohm mm). The microstructure of the contact after annealing was investigated using scanning and transmission electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy. It is shown that a noticeable alloying of metallization into semiconductor upon annealing does not occur, but strong mixing of metals takes place. X-ray diffraction analysis demonstrated the presence of interfacial compounds, namely, Al₂Au, Al_{3 + x} Mo_1–x , AlMo₃, Al₁₂Mo, GaMo₃ and GaAu₂. Investigations of the phase composition of films depending on the thickness ratio of the metallization layers have shown that the formation of Al₂Au phase has a negative effect on the contact surface morphology, and the formation of GaMo₃, Al _x Mo_y phases likely plays the most important role in the ohmic contact formation, which was also confirmed by the method of energy-dispersive analysis.     

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.     

Nanocrack formation in AlGaN/GaN high electron mobility transistors utilizing Ti/Al/Ni/Au ohmic contacts

AlGaN/GaN HEMTs are poised to become the technology of choice in RF and power electronics applications where high operating frequencies and high breakdown voltages are required. The alloyed contacting scheme utilized in the formation of the source and drain contacts of these devices affects the conduction of electrons through the 2DEG from the moment of ohmic contact formation onward to operation in the field. Analysis of the ohmic contacts of as-fabricated and electrically stressed AlGaN/GaN HEMTs, via chemical deprocessing and Scanning Electron Microscopy, indicates the presence of cracks oriented along the [11-20] directions, which nucleate at metal inclusions present under the alloyed ohmic source/drain contact metal. Cracks which form at the edges of these contact regions can extend into the channel region. It appears that electrical biasing induces additional growth in the longest cracks present within the channel regions of these devices.     

Long-term thermal stability of Ti/Al/Mo/Au ohmic contacts on n-GaN

Improved performance of the ohmic contacts on n-GaN has been demonstrated with the use of MoAu as the capping layer on TiAl metallization. Contact resistance as low as 0.13 Θ-mm was achieved in these ohmic contacts when annealed at 850°C for 30 sec. We have studied the long-term thermal stability of these contacts at 500°C, 600°C, 750°C, and 850°C, respectively. The Ti/Al/Mo/Au metallization forms low contact-resistance ohmic contacts on n-GaN that are stable at 500°C and 600°C after 25 h of thermal treatment. The ohmic-contact performance degrades after 10 h of thermal treatment at 750°C, while the contacts exhibit nonlinear current-voltage (I-V) characteristics after 1 h of thermal treatment at 850°C with the formation of oxide on the surface of the contacts accompanied by surface discoloration. The intermetallic reactions taking place in the contacts during the long-term thermal treatments were studied using Auger electron spectroscopy (AES), and the surface morphology was characterized using atomic force microscopy (AFM).     

AlGaN/GaN HEMT器件欧姆接触多步退火工艺的分析与优化

利用Ti/Al/Ni/Au的多步退火工艺能够在 AlGaN/GaN HEMT上制作出低接触电阻,表面形貌平整的欧姆接触。本文通过实验对多步退火工艺的机制进行了详细地分析并对多步退火工艺进行了优化。实验结果显示出调整不同的退火的时间和温度在接触电阻和表面形貌的优化中起到了至关重要的作用。取得高质量的欧姆接触的关键是通过调整退火的时间和温度以平衡金属与金属之间,金属与半导体之间的各种反应的速率。我们利用优化后多步退火工艺在未掺杂AlGaN/GaN结构上制作出了低接触电阻的欧姆接触,其比接触电阻率为3.22?10-7Ω.cm2。     

Effect of annealing temperature on Ti/Al/Ni/Au ohmic contacts on undoped AlN films

The Ti/Al/Ni/Au metals were deposited on undoped AlN films by electron beam evaporation. The influence of annealing temperature on the properties of contacts was investigated. When the annealing temperatures were between 800 and 950℃, the AlN-Ti/Al/Ni/Au contacts became ohmic contacts and the resistance decreased with the increase of annealing temperature. A lowest specific contacts resistance of 0.379 Ω·cm² was obtained for the sample annealed at 950℃. In this work, we confirmed that the formation mechanism of ohmic contacts on AlN was due to the formation of Al-Au, Au-Ti and Al-Ni alloys, and reduction of the specific contacts resistance could originate from the formation of Au₂Ti and AlAu₂ alloys. This result provided a possibility for the preparation of AlN-based high-frequency, high-power devices and deep ultraviolet devices.     

Ta/Au ohmic contacts to n-type ZnO

Ta/Au ohmic contacts are fabricated on n-type ZnO (∼1 × 10¹⁷ cm⁻³) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). After growth and metallization, the samples are annealed at 300°C and 500°C for 30 sec in nitrogen ambient. The specific contact resistance is measured to be 3.2×10⁻⁴ Ωcm² for the as-deposited samples. It reduces to 5.4×10⁻⁶ Ωcm² after annealing at 300°C for 30 sec without significant surface morphology degradation. When the sample is annealed at 500°C for 30 sec, the specific contact resistance increases to 3.3 × 10⁻⁵ Ωcm². The layer structures no longer exist due to strong Au and Ta in-diffusion and O out-diffusion. The contact surface becomes rough and textured.     

Electrical and structural properties of low-resistance Ti/Al/Re/Au ohmic contacts to n-type GaN

Titanium (15 nm)/aluminum (60 nm)/rhenium (20 nm)/gold (50 nm) ohmic contacts to moderately doped n-type GaN (4.07×10¹⁸ cm⁻³) have been investigated as a function of annealing temperature. It is shown that the current-voltage (I–V) characteristics of the contacts are improved upon annealing at temperatures in the range of 550–750°C. Specific contact resistance as low as 1.3 × 10⁻⁶ Ωcm² is obtained after annealing at 750°C for 1 min in a nitrogen ambient. X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core level for the sample annealed at 750°C shifts toward the high binding side by 0.71 eV compared with that of the as-deposited one. It is also shown that the contact does not seriously suffer from thermal degradation even when annealed at 750°C for 30 min. Based on Auger electron spectroscopy (AES), glancing angle x-ray diffraction (GXRD), and XPS results, possible explanations for the annealing-induced improvement of the ohmic behavior are described and discussed.     

Low-thermal-budget Au-free ohmic contact to an ultrathin barrier AlGaN/GaN heterostructure utilizing a micro-patterned ohmic recess

A pre-ohmic micro-patterned recess process,is utilized to fabricate Ti/Al/Ti/TiN ohmic contact to an ultrathin-barrier(UTB) AlGaN/GaN heterostructure,featuring a significantly reduced ohmic contact resistivity of 0.56 Ω.mm at an alloy temperat-ure of 550 ℃.The sheet resistances increase with the temperature following a power law with the index of +2.58,while the spe-cific contact resistivity decreases with the temperature.The contact mechanism can be well described by thermionic field emis-sion (TFE).The extracted Schottky barrier height and electron concentration are 0.31 eV and 5.52 × 1018 cm-3,which suggests an intimate contact between ohmic metal and the UTB-AlGaN as well as GaN buffer.A good correlation between ohmic trans-fer length and the micro-pattern size is revealed,though in-depth investigation is needed.A preliminary CMOS-process-compat-ible metal-insulator-semiconductor high-mobility transistor (MIS-HEMT) was fabricated with the proposed Au-free ohmic con-tact technique.     

Microstructural properties of thermally stable Ti/W/Au ohmic contacts on n-type GaN

We have investigated the microstructural and electrical characteristics of Ti/W/Au ohmic contacts on n-type GaN (4.0 × 10¹⁸ cm⁻³) using Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) after annealing at 900 °C. It is shown that the electrical properties are improved upon annealing at 900 °C for 1 min in nitrogen ambient. The 900 °C annealed contact produced a specific contact resistance of 8.4 × 10⁻⁶ Ω cm². It is further shown that the contact exhibits thermal stability during annealing at 900 °C. Based on the Auger electron microscopy and transmission electron microscopy studies, the formation of TiN layer results in an excess of N vacancies near the surface of the GaN layer, which could be the reason for the low-resistance of the Ti/W/Au contact.     

Alloyed Si/Al-based ohmic contacts to AlGaN/GaN nitride heterostructures

For the first time in Russia, the Si/Al/Ti/Au alloyed contact composition is investigated for the formation of ohmic contacts to AlGaN/GaN heterostructures using thermal annealing. The obtained results are compared with those for conventional Ti/Al/Ni/Au ohmic contacts. Use of the composition under investigation makes it possible to decrease the annealing temperature to 675–700°C, which results in improvement in the morphology of alloyed ohmic contacts in comparison with conventional contacts. The value of the contact resistance using the Si/Al-based composition to the AlGaN/GaN heterostructure is obtained in relation to the temperature and annealing duration. It is shown that no qualitative change in the resistance occurs at an annealing duration of several minutes in the temperature range of 700–750°C. In the temperature range of 675–700°C, there is an asymptotic decrease in the contact resistance with increasing annealing duration. The smallest value of the contact resistance amounts to 0.41 Ω mm.     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.     

Au/Ge/Pd ohmic contacts to n-GaAs with the Mo/Ti diffusion barrier

The influences of the As-outdiffusion and Au-indiffusion on the performances of the Au/Ge/Pd/n-GaAs ohmic metallization systems are clarified by investigating three different types of barrier metal structures Au/Ge/Pd/GaAs, Au/Ti/Ge/Pd/ GaAs, and Au/Mo/Ti/Ge/Pd/GaAs. The results indicate that As-outdiffusion leads to higher specific contact resistivity, whereas Au-indiffusion contributes to the turnaround of the contact resistivity at even higher annealing temperature. For Au/Mo/Ti/Ge/Pd/n-GaAs samples, they exhibit the smoothest surface and the lowest specific contact resistivity with the widest available annealing temperature range. Moreover, Auger electron spectroscopy depth profiles show that the existing Ti oxide for the Mo/Ti bilayer can very effectively retard Au-indiffusion, reflecting the onset of the turnaround point at much higher annealing temperature.     

Interface annealing characterization of Ti/Al/Au ohmic contacts to p-type 4H-SiC

Ti/Al/Au ohmic contacts to p-type 4H-SiC in terms of a different annealing time and Ti composition are reported. At 1050℃, proper increase in annealing time plays a critical role in the Schottky to ohmic contact conversion. With the optimized annealing time, the contact with a high Ti content yields a lower specific contact resistivity(ρ_c) of 6.4×10^-5 Ω·cm² compared with the low-Ti contact. The annealed surface morphology and phase resultants were examined by scanning electron microscopy(SEM) and X-ray diffraction(XRD), respectively. For the better ohmic contact, element distribution and chemical states were qualitatively identified by X-ray photoelectron spectroscopy(XPS) depth analysis. In particular, the presence of C and a Si-related phase was discussed and associated with the change in the surface status of the as-grown epilayer of 4H-SiC during annealing. The results reveal that the out-diffused C and Si atoms, with an approximate atomic ratio of 1:1 in the contact layer, can combine to form an amorphous Si-C state. The polycrystalline graphite instead of an unreacted C cluster in the whole alloyed structure and an extra nanosize graphite flake on the outermost surface of the annealed contact were confirmed by Raman spectroscopy.     

Au/Pt/Ti/Ni ohmic contacts to p-ZnTe

Ohmic contacts of Au/Pd/Ti/Ni to p-ZnTe show a minimum specific contact resistance of 10^-6 Ωcm² for a p-type doping level of 3×10¹⁹ cm^-3 and at an annealing temperature of 300°C. The Ni and Ti layers are very effective in improving the electrical properties of these contact     

Morphological and electrical comparison of Ti and Ta based ohmic contacts for AlGaN/GaN-on-SiC HFETs

The morphology and impact on leakage currents of two different ohmic metal stacks for GaN based transistor devices is investigated in this work. The results have implications for the performance and reliability of a GaN transistor device. A low temperature Ta based and a higher temperature anneal Ti based metallization are compared. The low temperature process shows a smoother metal semiconductor interface together with several orders of magnitude lower vertical and lateral leakage compared to the conventional higher temperature process. In addition to the leakage tests, back bias ramping experiments are performed unveiling potential advantages of the conventional approach in mitigating current collapse. However the low leakage will enable higher voltage operation making the low temperature process the preferable choice for high power RF applications, if simultaneously current collapse can be controlled.     

Enhancement-mode AlGaN/GaN HEMTs on silicon substrate

High-performance enhancement-mode AlGaN/GaN HEMTs (E-HEMTs) were demonstrated with samples grown on a low-cost silicon substrate for the first time. The fabrication process is based on a fluoride-based plasma treatment of the gate region and postgate annealing at 450 /spl deg/C. The fabricated E-HEMTs have nearly the same peak transconductance (G/sub m/) and cutoff frequencies as the conventional depletion-mode HEMTs fabricated on the same wafer, suggesting little mobility degradation caused by the plasma treatment.     

Nonalloyed ohmic contacts for high-electron-mobility transistors based on AlGaN/GaN heterostructures

A microwave field-effect transistor with nonalloyed ohmic contacts is fabricated using the technique of regrowing a heavily doped region under the contact metallization by molecular beam epitaxy through a preliminarily formed dielectric mask. The fabricated field-effect transistor with a gate length of 0.18 µm and a total width of 100 µm has a current–amplification cutoff frequency of 66 GHz and ohmic contact resistivity of 0.15-0.18 Ω mm.     

Thermally stable Ge/Cu/Ti ohmic contacts to n-type GaN

The performance of a novel Ge/Cu/Ti metallization scheme on n-type GaN has been investigated for obtaining thermally and electrically stable low-resistance ohmic contacts. Isochronal (2 min.) anneals in the 600–740°C temperature range and isothermal (690°C) anneals for 2–10 min. duration were performed in inert atmosphere. For the 690°C isothermal schedule, ohmic behavior was observed after annealing for 3 min. or longer with a lowest contact resistivity of 9.1 × 10⁻⁵ Ωcm² after the 10 min. anneal for a net donor doping concentration of 9.2 × 10¹⁷ cm^−Ω3. Mean roughness (R_a) for anneals at 690°C was almost constant at around 5 nm, up to an annealing duration of 10 min., which indicates a good thermal stability of the contact scheme.