Ohmic Contact Performance Research Articles

Mg doping of 3D semipolar InGaN/GaN-based light emitting diodes

The effects of different Mg doping concentrations in the main p-GaN layer and the p-GaN capping layer on the electroluminescence (EL) properties of three-dimensional semipolar InGaN/GaN light emitting diode structures grown on GaN stripes with triangular cross-section were investigated. Secondary ion mass spectrometry analysis revealed the Mg concentration of the 3D semipolar p-GaN, indicating a higher Mg incorporation efficiency on the facet as compared to the facet. The EL output power is low with a too low Mg concentration of 3 × 1019 cm−3, probably due to the inferior hole injection efficiency and stays almost constant with the Mg concentration ranging from 4 × 1019 cm−3 until 1.3 × 1020 cm−3 for the 3D LEDs with the facet. Heavy Mg doping in the p-GaN capping layer is required to achieve good ohmic contact performance.

Effects of rapid thermal annealing on ohmic contact of AlGaN/GaN HEMTs

Ohmic contacts with Ti/Al/Ti/Au source and drain electrodes on AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated and subjected to rapid thermal annealing (RTA) in flowing N2. The wafer was divided into 5 parts and three of them were annealed for 30 s at 700, 750, and 800 °C, respectively, the others were annealed at 750 °C for 25 and 40 s. Due to the RTA, a change from Schottky contact to Ohmic contact has been obtained between the electrode layer and the AlGaN/GaN heterojunction layer. We have achieved a low specific contact resistance of 7.41 × 10−6 Ω·cm2 and contact resistance of 0.54 Ω·mm measured by transmission line mode (TLM), and good surface morphology and edge acuity are also desirable by annealing at 750 °C for 30 s. The experiments also indicate that the performance of ohmic contact is first improved, then it reaches a peak, finally degrading with annealing temperature or annealing time rising.

High power density AlGaAs/InGaAs/GaAs PHEMTs using an optimised manufacturing process for Ka-band applications

In this study, a novel manufacturing process for a 0.1μm T-gate is investigated for producing a high output power performance for the Ka-band frequencies using high-quality AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistors (PHEMTs) on semi-insulated (SI) GaAs substrates. The gate manufacturing process is the most important process due to its intimate relationship with the DC and RF performance of the device. To improve the gate performances of PHEMT devices, we investigated various materials and processing approaches involving a wide gate recess, double exposure by e-beam lithography, and low-damage double-gate passivation methods based on plasma-enhanced chemical vapour deposition (PECVD). To reduce the sensitivity to current collapse effects, we investigate the relationship between the electrical characteristics of the PHEMTs and top and bottom gate-supported passivation films. To improve the ohmic contact performance, we test an AuGe/Ni/Au (200/30/120nm) ohmic contact metallisation scheme using the rapid thermal annealing (RTA) process at temperatures ranging from 450°C 30s. A PHEMT with a gate length of 0.1μm, exhibiting a maximum drain current density of 680mA/mm, a peak transconductance of 500mS/mm, a unity-gain cut-off frequency (fT) of 56GHz, and a maximum frequency of oscillation (fMAX) of 84GHz, is demonstrated using this novel manufacturing process; the Ka-band power performance includes an output power density of 2.4W/mm and a power-added efficiency (PAE) of 44.6%.

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).

Effect of annealing on contact performance and electrical properties of p-type high purity germanium single crystal

Van de Pauw Hall measurement is an effective method to characterize the properties of semiconductors, such as bulk concentration, mobility, and resistivity, all of which are used to describe the purity level in the semiconductors. However, the performance of the ohmic contacts has a direct impact on the reliability and accuracy of the results obtained from the Van de Pauw Hall measurement. In the present work, the influences of different annealing techniques on the performance of the InSn ohmic contacts have been investigated using a High Purity Germanium (HPGe) crystal sample. The results show that the preferred annealing condition is at 400 °C for 1 hour, which has provided a significant improvement of the InSn contact quality and microscopic homogenization of the impurities in the HPGe crystal. The carrier concentration, charge mobility, and resistivity of the sample annealed at 400 °C for 1 hour are 5.772×1010/cm3, 1.883×104× cm2/Vs, and 5.795×103×Ω cm at 77 K, respectively.

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.

Ohmic Contact Properties and Annealing Effect for Au/Ni on p-Type P-Doped ZnO

Au/Ni ohmic contact to p-type P-doped ZnO (P:ZnO) is succeeded and investigated. The specific contact resistance decreases with increasing annealing temperature up to , but increases beyond that. After annealing at , is , a decrease by about 15 times from of the as-deposited contact. The sheet resistance of p-type P:ZnO monotonously increases with the annealing temperature. The mechanisms causing the and variation with the annealing temperature are studied. During thermal annealing in vacuum , competitions exist between Zn out-diffusion and Ni and Au diffusion into ZnO. Zn out-diffusion can improve the ohmic contact performance by producing more acceptors or surface states in the interfacial layer, while Ni and Au diffusion into ZnO can kill the acceptors or surface states in the interfacial layer.

Comprehensive study of Ohmic electrical characteristics and optimization of Ti∕Al∕Mo∕Au multilayer Ohmics on undoped AlGaN∕GaN heterostructure

The performance of Ohmic contacts is highly dependent on the metal stack contents and layer thicknesses evaporated, annealing temperature, and annealing time. A four-layer-based metal stack was used to form Ohmic contacts on an undoped AlGaN∕GaN material structure, grown by molecular-beam epitaxy technique on a sapphire substrate. The thicknesses of the overlayers Ti∕Au, or Mo∕Au, investigated were fixed. The rapid thermal annealing time and temperature in a N2 ambient are optimized for Ti∕Al∕Mo∕Au (from bottom to top) metal stack system, with bottom Ti∕Al metal at thickness ratios of 1:5 and 1:6. The lowest contact transfer resistance rt of 0.4Ωmm was obtained for Ti∕Al metal at thickness ratio of 1:5. The sheet resistance Rsh associated with it was 600Ω∕sq. The edge acuity of Ti∕Al∕Mo∕Au Ohmic contacts after annealing has been discussed and examined under scanning electron microscopy for the optimized Ohmic contact transfer resistance associated with the bottom Ti∕Al metal at thickness ratios of 1:5 and 1:6. The straightness of the edge of the metal contact has also been compared with the one using Ti∕Al∕Ti∕Au as the Ohmic metal stack under the condition of same annealing environment and metal thickness.

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).

Ohmic contact formation on inductively coupled plasma etched 4H-silicon carbide

We report on the investigation of ohmic contact formation using sputtered titanium-tungsten contacts on an inductively coupled plasma (ICP) etch-damaged 4H-SiC surface. Transfer length method (TLM) measurements were performed to characterize how ICP-etch damage affects the performance of ohmic contacts to silicon carbide. In order to recover etch damage, high-temperature oxidation (1250°C for 1 h) was evaluated for one of the samples. Some of the etch damage was recovered, but it did not fully recover the etch damage for the sample etched with medium platen power (60 W). From our TLM measurements, the specific contact resistance (ρc of sputtered titanium tungsten on highly doped n+-type 4H-SiC epilayers with a doping of 1.1 × 1019 cm-3 for the unetched reference sample, 30-W etched, and 60-W etched with and without sacrificial oxidation was as low as 3.8 × 10-5 Ωcm2, 3.3 × 10-5 Ωcm2, 2.3 × 10-4 Ωcm2, and 1.3 × 10-3 Ωcm2, respectively. We found that the low-power (30 W) ICP-etching process did not affect the formation of ohmic contacts, and we did not observe any difference between the unetched and the 30-W etched sample from our TLM measurements, having the same value of the ρc. However, medium-platen-power (60 W) ICP etching showed significant influence on the ohmic contact formation. We found that the specific contact resistance is highly related to the surface roughness and quality of the metals, and the lower, specific contact resistance is due to the smoother and denser ohmic contacts.

Microstructure, electrical properties, and thermal stability of Ti-based ohmic contacts to n-GaN

Single Ti layers, single TiN layers, and thin Ti films overlayered with Au were investigated as ohmic contacts to n-type (n 4.5 × 1017 to 7.4 × 1018 cm−3) single-crystal GaN (0001) films. Transmission line measurements (TLM) revealed the as-deposited TiN and Au/Ti contacts on n = 1.2 − 1018 cm−3 to be ohmic with room-temperature specific contact resistivities of 650 and 2.5 × 107minus;5 Ω cm2, respectively. Single Ti layer contacts had high resistance and were weakly rectifying in the as-deposited condition. The three contact/GaN systems exhibited a substantial decrease in resistivity after annealing; the value of ρc was also a function of the carrier concentration in the GaN. The Au/Ti contacts exhibited the lowest resistivity values yet observed in these contact studies, particularly for the more lightly doped n-GaN. The ρc for n = 1.2 × 1018 cm−3 reached 1.2 × 1026 Ω cm2; for n = 4.5 × 1017 cm−3, ρc = 7.5 × 1025 Ω cm2 after annealing both samples through 900 °C. X-ray photoelectron spectroscopy (XPS) and high-resolution cross-sectional transmission electron microscopy (X-TEM) analysis revealed the formation of TiN at the interface of annealed Ti layers in contact with GaN, which is believed to be beneficial for ohmic contact performance on n-GaN.

Electrical and structural properties of Pt/Ti/p+–InAs ohmic contacts

The thermally stable Pt/Ti metallization scheme was applied to Zn doped InAs layers in order to evaluate the influence of the outstanding electrical properties of the latter, such as the narrow band gap and the fact that the surface Fermi level pins in the conduction band, on the ohmic contact performance. These contacts applied to 1×1018, 5×1018, and 1×1019 cm−3 Zn doped InAs layers, grown directly on InP(Fe) substrates, were already ohmic as-deposited with a specific resistance values of 3.0×10−5, 9.8×10−6, and 7.5×10−6 Ω cm−2, respectively. Rapid thermal processing for 30 s at elevated temperatures yielded even a further reduction of the specific contact resistances, with a minimum value of less than 6×10−7 Ω cm2, for a 450 °C anneal. This heat treatment enhanced limited reactions in both the Pt/Ti and Ti/InAs interfaces, but, however, did not lead to the degradation of the stable microstructure.