GOX 2022 Session EP1-WeM: Process & Devices III

Wednesday, August 10, 2022 9:15 AM in Room Jefferson 2-3
Wednesday Morning

Session Abstract Book
(279KB, Oct 9, 2022)
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9:15 AM EP1-WeM-4 Remarkable Improvement of Conductivity in Β-Ga2O3 by High-Temperature Si Ion Implantation
Arka Sardar, Tamara Isaacs-Smith, Sarit Dhar (Auburn University); Jacob Lawson, Neil Merrett (Air Force Research Laboratory, USA)
Monoclinic Beta Gallium Oxide (β-Ga2O3) is emerging as a promising wide bandgap semiconductor for high voltage electronics. Ion implantation is a key process for device fabrication as it provides a unique way to carry out selective area doping with excellent control. It has been demonstrated that Si implantation into (010) β-Ga2O3 at room temperature followed by annealing at ~1000°C, results in an activation efficiency (η)of 63% for Si concentrations up to ~5e19 cm-3. However, for higher concentrations, a severe drop of the η to 6% occurs [1]. In this work, we demonstrate that high-temperature implantation can be used to significantly improve this for heavily implanted β-Ga2O3. In the case of SiC, implantation at > 500°C results in superior conductivity due to lower defect densities and better recrystallization after annealing [2]. Based on this, we performed room temperature (RT, 25°C) and high temperature (HT, 600oC) Si implants into MBE grown 300 nm (010) β-Ga2O3 films with energies of 275 keV and 425 keV through ~110 nm Mo and ~30 nm Al2O3 layers; with a total of fluence of 2.4e15 cm-2 or 4.8e15 cm-2. This was followed by annealing in flowing nitrogen at 970oC for 30 minutes to activate the dopants. SIMS shows the Si profile is ~400 nm deep with an average concentration of ~6.0e19 cm-3 for the lower fluence samples, and expected to be ~1.2e20cm-3 for the higher fluence (SIMS ongoing). No significant difference in surface roughnesses were detected by AFM throughout the process. HRXRD shows structural defects after the implantation and partial crystallization recovery upon annealing, where the advantage was in favor of HT implantation. The ratio of the free electron concentration from Hall measurements and the total amount of Si in β-Ga2O3 was used to determine the activation efficiencies. For the lower fluence, the HT sample shows only a ~6% improvement of η over the RT sample. Remarkably, for the higher fluence, while the RT sample was too resistive for measurement, the HT sample had η close to 70%, with a high sheet electron concentration of 3.3e15 cm-2 and excellent mobility of 92.8 cm²/V·s at room temperature. These results are highly encouraging for achieving ultra-low resistance heavily doped β-Ga2O3 layers using ion implantation, which will be discussed further in this presentation.
[1]K. Sasaki et.al,, Appl. Phys. Express 6, 086502 (2013).
[2]F. Roccaforte, et. al, , Micro 2, 23 (2022).
We acknowledge the support of the Department of Physics, Auburn University.
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9:30 AM Invited EP1-WeM-5 Towards Lateral and Vertical Ga2O3 Transistors for High Voltage Power Switching
Kornelius Tetzner, Joachim Würfl, Eldad Bahat-Treidel, Oliver Hilt (Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH)); Zbigniew Galazka, Saud Bin Anooz, Andreas Popp (Leibniz-Institut für Kristallzüchtung (IKZ))

Gallium Oxide (Ga2O3) power switching devices are expected to boost efficiency of power converters predominately operating at comparatively high bias voltage levels in the kV range. Thanks to the extraordinarily high energy band gap of 4.9 eV a high device breakdown strength of about 8 MV/cm is expected. Thus it is possible to efficiently utilize these properties for very compact power devices with aggressively minimized gate to drain separation. This enables low resistive on-state and low leakage off-state properties. Most Ga2O3 devices introduced so far rely on volume electron transport properties; only a few 2DEG devices have been demonstrated. In any case the values of electron mobility and saturation velocity in Ga2O3 crystals may depend on crystal orientation and did not yet reach properties being comparable to more developed wide band gap semiconductor families such as GaN and SiC. – Nevertheless the benefit of Ga2O3 devices relates to the combination of high breakdown field and electron transport properties and the resulting compact device design strategies are already getting competitive to existing power switching technologies.

The presentation will give an overview on the current status of lateral and vertical Ga2O3 devices with a special emphasis on results obtained at FBH and IKZ [1]. For both cases concepts for epitaxial layer structures and device designs suitable for reaching the targeted performance will be discussed especially in terms of breakdown voltage and channel current density. Critical points for device optimization such as type of gate recess in lateral transistors and concepts of critical electric field reduction in vertical transistors will be addressed.

[1] K. Tetzner, IEEE Electron Device letters, vol. 40, No. 9, (2019), pp. 1503 - 1506.

View Supplemental Document (pdf)
10:00 AM EP1-WeM-7 Comparison of β-Ga2O3 Mosfets With TiW and NiAu Metal Gates for High-Temperature Operation
Nicholas Sepelak (KBR, Wright State University); Daniel Dryden (KBR); Rachel Kahler (University of Texas at Dallas); Jeremiah William (Air Force Research Lab, Sensors Directorate); Thaddeus Asel (Air Force Research Laboratory, Materials and Manufacturing Directorate); Hanwool Lee (University of Illinois at Urbana-Champaign); Katie Gann (Cornell University); Andreas Popp (Leibniz-Institut für Kristallzüchtung); Kyle Liddy (Air Force Research Lab, Sensors Directorate); Kevin Leedy (Air Force Research Laboratory, Sensors Directorate); Weisong Wang (Wright State University); Wejuan Zhu (University of Illinois at Urbana-Champaign); Micheal Thompson (Cornell University); Shin Mou (Air Force Research Laboratory, Materials and Manufacturing Directorate, USA); Kelson Chabak, Andy Green (Air Force Research Laboratory, Sensors Directorate); Ahmad Islam (Air Force Research Laboratory, Sensors Directory)

β-Ga2O3 offers a robust platform for operation of electronic devices at a high temperature because of its large band gap and low intrinsic carrier concentration. We have recently characterized the high temperature performance β-Ga2O3 field effect transistors using different gate metals in vacuum and air ambient at temperatures up to 500 °C.

The devices fabricated using TiW refractory metal gate and Al2O3 gate dielectric exhibited stable operation up to 500 oC in vacuum and up to 450 oC in air [1]. Transfer (IDS-VGS) characteristics of a device were measured at various temperatures in vacuum and air. Extracted IMAX/IMIN for the vacuum test reduced from ~104 to 102 as temperature was increased up to 500 oC. During the vacuum characterization, the contact resistance remained unchanged at all temperatures and, therefore, device characteristics showed no degradation once devices were brought back to RT even after several hours of device operation at 500 °C in vacuum.

The devices, fabricated with Ni/Au gate metal and Al2O3 gate dielectric, exhibited stable operation up to 500 oC in air [2]. The measured ID-VD characteristics showed no current degradation up to 450 oC. At 500 oC, the device exhibited a drop in ID; however, device characteristics recovered once the device is brought back to RT, even after 20 hours of device operation at 500 °C.

For tests in air ambient, both Ni/Au and Ti/W devicesobserved an increase in current with temperature due to activation carriers from dopants/traps in the device, however, both exhibited IMAX/IMIN < 102 at 450 oC because of contact degradation. The barrier height of ϕB~ 1.0 eV and 0.77 eV was calculated for the TiW/Al2O3 and the NiAu/Al2O3 interfaces, respectively using thermionic emission theory. Thought the values of ϕB for the Ti/W contacts was consistent with that expected from the work-function difference between TiW and Al2O3, the devices with Ni/Au yielded lower ϕB presumably due to the diffusion of Ni and the partial crystallization of the Al2O3 dielectric [3]. Our results suggest that with appropriate choice of metals and gate dielectrics, the stable 500 oC operation using β-Ga2O3 is achievable.

[1] Sepelak et al., “High-temperature operation of β-Ga2O3 MOSFET with TiW refractory metal gate,” DRC, 2022.

[2] Sepelak et al., “First Demonstration of 500 °C Operation of β-Ga2O3 MOSFET in Air,” CSW, 2022

[3] Islam et al., "Thermal stability of ALD-grown SiO2 and Al2O3 on (010) β-Ga2O3 substrates," DRC, 2022.

10:15 AM EP1-WeM-8 High Electron Mobility Si-doped β-Ga2O3 MESFETs
Arkka Bhattacharyya (University of Utah); Saurav Roy (University of California at Santa Barbara); Praneeth Ranga (University of Utah); Sriram Krishnamoorthy (University of California at Santa Barbara)

A hybrid low temperature - high temperature (LT-HT) buffer/channel stack growth is demonstrated using MOVPE with superior carrier mobility values. An LT-grown (600°C) undoped Ga2O3 buffer (250-330 nm thick) is grown followed by transition layers to a HT (810°C) Si-doped Ga2O3 channel layers (~220 nm) without growth interruption. The (010) Fe-doped Ga2O3 substrates were cleaned in HF for 30 mins prior to channel growth. From Hall measurements, this stack design is shown to have an effective RT Hall mobility values in the range 162 – 184 cm2/Vs for doped channel electron densities of 1.5-3.5×1017 cm-3 measured on multiple samples/substrates. These mobility values are higher than the state-of-the-art values in Ga2O3 literature. Two types of (010) Fe-doped Ga2O3 bulk substrates were used in this study: 5×5 mm2 diced pieces from 10×15 mm2 EFG-grown substrates from NCT, Japan and 2-inch CZ-grown bulk substrates from NG Synoptics, USA.

The charge and transport properties were also verified using CV, TLM, field-effect mobility (μFE) measurements and FET current characteristics. Few samples were processed for regrown ohmic contacts to minimize contact resistance. RC values of 1-2 Ω.mm were achieved. 3D electron densities were verified by CV measurements. Channel charge profile (from CV) showed the absence of any active parasitic charge below the buffer layer. Rsh values from TLM measurements matched closely with Hall measurements. RT μFE measured on FatFET structures (LG ~110um, LGS/LGD ~ 1um) showed peak values of 158 and 168 cm2/Vs in the doped region for electron densities of 3.5×1017 cm-3 and 2.1×1017 cm-3 respectively, which are also the highest values to be ever reported. MOSFETs and MESFETs with device dimensions LGS/LG/LGD = 1/2.5/5 um show max ON currents of ~200 mA/mm and ~130 mA/mm respectively. MESFETs show very high ION/IOFF ~ 1010 and ultra-low reverse leakage. OFF-state voltage blocking capabilities of these devices will be reported.

These buffer-engineered doped high-mobility Ga2O3 channel layers with superior transport properties show great promise for Ga2O3 power devices with enhanced performance.

Acknowledgement: This material is based upon work supported by the II-VI foundation Block Gift Program 2020-2022. This material is also based upon work supported by the Air Force Office of Scientific Research under award number FA9550-21-0078 (Program Manager: Dr. Ali Sayir). We thank AFRL sensors directorate for discussions. View Supplemental Document (pdf)
10:30 AM BREAK
Session Abstract Book
(279KB, Oct 9, 2022)
Time Period WeM Sessions | Abstract Timeline | Topic EP Sessions | Time Periods | Topics | GOX 2022 Schedule