InxAl1-xN/AlN/GaN-based high electron mobility transistor (HEMT) structures are epitaxially grown on 200 mm diameter Si(111) substrates by a metal organic chemical vapor deposition technique. The structural and electrical properties of the nitride layers are addressed by high-resolution X-ray diffraction and Hall effect measurements. The fabricated HEMTs with Au-free contact metallization schemes have resulted in an ON-OFF current ratio ∼105. Ti/Al/Ni/W-based Ohmic source/drain contacts, with 1.5 μm RuOx Schottky gate on In0.15Al0.85N/AlN/GaN HEMTs at VDS = 10 V, show a maximum transconductance (gm) and saturation drain current (IDSAT) of 0.19 ± 0.01 S/mm and 0.82 ± 0.02 A/mm, respectively, with a reasonable uniformity from the center to edge of a 200 mm diameter epiwafer.

Articles you may be interested inTi/Al/Ti/Ni/Au ohmic contacts on AlGaN/GaN high electron mobility transistors with improved surface morphology and low contact resistanceThe authors have studied the electrical characteristics of Hf/Al/Ta ohmic contacts on In 0.18 Al 0.82 N/GaN heterostructure grown on Si (111) substrate. With annealing at 600 C in vacuum (which is $200 C lower than that for traditional Ti/Al/Ni/Au contacts), a minimum ohmic contact resistance of $0.58 XÁmm and specific contact resistivity of $6.75 Â 10 À6 XÁcm 2 are obtained. The minimum contact resistance of Hf/Al/Ta contacts is comparable to that of Ti/Al/Ni/Au contacts. Owing to the lower annealing temperature, Hf/Al/Ta contacts exhibit better surface morphology and edge acuity. More importantly, Hf/Al/Ta contacts show a smooth interface with In 0.18 Al 0.82 N/GaN, whereas spike structures that penetrate the In 0.18 Al 0.82 N layer are observed for Ti/Al/Ni/Au contacts. As a result, the source-carrier-injection induced breakdown mechanism is reduced in the In 0.18 Al 0.82 N/GaN-on-Si high electron mobility transistors (HEMTs) with Hf/Al/Ta ohmic contacts, thereby leading to an improved three-terminal off-state breakdown voltage by about 100 V ($53.5% improvement), in comparison to Ti/Al/Ni/Au based HEMTs.

The electrical characteristics of In0.18Al0.82N/GaN MOSHEMTs-on-Si (111) with Y2O3 gate dielectric of thickness between 5 to 30 nm are reported. A positive shift in threshold voltage, Vth, with respect to Schottky gate HEMTs is observed, which is in contrast to that for ZrO2 gate dielectric MOSHEMTs. An analytical Vth model is proposed to quantify the fixed oxide charges at the Y2O3/In0.18Al0.82N interface (nox,intf) and in the Y2O3 bulk (nox,bulk). The negative polarity of the fixed oxide charges (nox,intf, nox,bulk) is believed to be the reason for the abovementioned positive Vth shift. X-ray photoelectron spectroscopy analysis reveals that a thin interfacial layer (IL) may have formed as a result of spontaneous reaction between dielectric Y2O3 and In0.18Al0.82N/GaN heterostructure, leaving Y2O3 bulk on top for thick dielectric film. We believe nox,intf is located in the IL and that its polarity and magnitude are related to the constituents of the IL, which comprise mainly YAlOxNy and its sub-oxynitrides. It is also observed that Y2O3 bulk has negative fixed charges (nox,bulk) with a magnitude that depends on film densification upon thermal annealing treatment.

The mechanisms of ohmic contact formation and carrier transport of low temperature (600 GaN based high mobility electron transistors (HEMTs) are of interest for applications in high-temperature, high-speed and high-power devices owing to the wide bandgap, high saturation velocity and high critical breakdown field of GaN.1,2 To realize high performance in AlGaN/GaN and InAlN/GaN HEMTs, good ohmic contacts with low resistivity are critical. Although very low contact resistance can be achieved by means of regrowth technique, 3 alloyed ohmic contacts are still most widely used due to their fabrication simplicity and low cost. The typical ohmic metal stacks are Ti/Al based (e.g., Ti/Al/X/Au), where Ti is the bottom layer, followed by Al, a diffusion barrier layer X (where X = Ni, Mo and Ti, etc.) and a Au capping layer. [4][5][6][7][8][9][10][11] In order to realize a low contact resistivity for the aforementioned ohmic contact scheme, the metal stack is usually annealed at high temperature (∼800• C), which leads to the presence of randomly distributed TiN inclusions, so-called contact inclusions or contact spikes, formed at the metal-semiconductor interface. 8,12 The presence of contact inclusions has shown to be vital to achieving good ohmic properties by Kim et al. 13 for InAlN/GaN HEMTs, as contact inclusions provide direct electron transport paths through the InAlN barrier layer to the 2DEG channel. On the other hand, these contact inclusions can also lead to lower breakdown voltage for GaN based HEMTs. 14,15 In our recent study, 16 we have proposed a low thermal budget (≤600• C) Hf/Al/Ta ohmic contact scheme on unintentionally doped In 0.18 Al 0.82 N/GaN grown on Si substrate, which has demonstrated low contact resistivity, smooth surface morphology and metalsemiconductor interface. In addition, the In 0.18 Al 0.82 N/GaN HEMTs fabricated with Hf/Al/Ta ohmic contacts have shown significantly improved breakdown voltage, compared to devices with traditional Ti/Al/Ni/Au ohmic contacts, owing to the source-carrier-injection mechanism suppressed by the smooth metal-semiconductor interface in the former. Our low thermal budget Hf/Al/Ta ohmic contacts show that even with the absence of contact inclusions, good ohmic contact properties can be achieved. This is in contrast to the result of Kim et al., which has shown the necessity of contact inclusions to ohmic contact formation. We noted that our InAlN layer is much thinner than that of Kim et al., hence may not need the presence of contact inclusions for good ohmic contact formation. In addition, it is noted that although analytically studies on the transport mecha- * Electrochemical Society Active Member.z E-mail: a0068100@nus.edu.sg; elecef@nus.edu.sg nism of ohmic contacts with spike structures on AlGaN/GaN 17,18 and InAlN/GaN 13,19 heterostructures have been reported, similar studies for ohmic contacts with a smooth metal-semiconductor interface on InAlN/GaN have not been reported. Hence, the Hf/Al/Ta contacts can be used to understand the carrier transport mechani...

A gate-first CMOS compatible process at low thermal budget to fabricate InAlN/GaN-on-Si MOSHEMTs is reported. This is made possible by a good quality LaAlO3 gate dielectric (k ~ 21.8) and Hf/Al/Ta ohmic contacts that require low annealing temperature. The reverse leakage current of the W/LaAlO3 gate stack remains low (< 3´10-7 mA/mm at -10 V) upon ohmic contact annealing at 600 oC. The fabricated LaAlO3 gate dielectric MOSHEMTs demonstrate a reasonable DC performance (Idsat ~ 510 mA/mm, Gmax ~ 44 mS/mm) for a 1 μm gate length device.