GOX 2022 Session EP1-WeM: Process & Devices III

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.

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 (I_DS-V_GS) characteristics of a device were measured at various temperatures in vacuum and air. Extracted I_MAX/I_MIN 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 I_D-V_D characteristics showed no current degradation up to 450 ^oC. At 500 ^oC, the device exhibited a drop in I_D; 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 I_MAX/I_MIN < 10² at 450 ^oC because of contact degradation. The barrier height of ϕ_B~ 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 ϕ_B 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 ϕ_B 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.

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 (μ_FE) measurements and FET current characteristics. Few samples were processed for regrown ohmic contacts to minimize contact resistance. R_C 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. R_sh values from TLM measurements matched closely with Hall measurements. RT μ_FE measured on FatFET structures (L_G ~110um, L_GS/L_GD ~ 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 L_GS/L_G/L_GD = 1/2.5/5 um show max ON currents of ~200 mA/mm and ~130 mA/mm respectively. MESFETs show very high I_ON/I_OFF ~ 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.