【World Express】Liquid atomization epitaxy method for β-Ga₂O₃ devices

The β-Ga₂O₃ MESFET produced using mist epitaxy validated the promise of this low-cost growth technique

      In an ideal world, compound semiconductor devices are produced on a homogeneous substrate with relatively simple equipment, offering opportunities for low-cost manufacturing, high-performance chips.

      Unfortunately, for most categories of compound semiconductor devices, it is not possible to meet both conditions. But power transistors made with β-Ga₂O₃, a collaboration between Kyoto and Nagoya universities in Japan and Panasonic, are said to meet these requirements.

      They claim that β-Ga₂O₃, produced on a homogeneous substrate using the fog chemical vapor phase deposition (mist CVD) method, offers a new route, a low-cost growth technique that is much simpler than MBE and MOCVD, and has already been used by Japanese company Flosfia to manufacture α-Ga₂O₃ power devices based on sapphire substrates.

      Shizou Fujita of Kyoto University, a spokesperson for the collaboration, told Compound Semiconductors that the team was able to grow transparent conductive oxides (such as ZnO), drawing on previous experience using atomized vapor phase deposition.

      "One of the challenges is to apply this simple and cost-effective technique to the growth of semiconductor crystals, which requires extremely low impurity incorporation and smooth surfaces", Fujita commented.

      One benefit, he adds, is that Oxygen is no longer used as an impurity, as is the case with GaAs, InP, SiC and GaN. However, there are still some obstacles that need to be resolved, such as impurity doping and the design of atomized vapor phase deposition reactors to smoothly introduce fog particles into the reaction tube without condensation.

      Fujita and colleagues produced MESFETs by loading semi-insulated β-Ga₂O₃ (010) substrates into a homemade hot-wall type atomized vapor phase deposition system and depositing N-type Ga₂O₃ layers. The precursors of Gallium and Silicon that provide N-type dopants are (acetylacetone) Gallium and Chloride - (3-cyanopropyl) -dimethylsilane. Using an oxygen carrier and a diluent gas, the team supplied the precursor mist to a horizontal reactor heated to 700°C to 800°C.

      The high temperatures reduced the growth rate. Therefore, the team chose a growth temperature of 750°C with a growth rate of only 750 nm/hr, which is much lower than the 3.2 μm/hr value reported by another group, which used a precursor with a higher concentration of gallium to prepare Ga₂O₃ films by atomized vapor phase deposition.

      β-Ga₂O₃ MOSFET offers competitive performance in terms of transconductance and drain current, but there are problems related to pinch-off and breakdown voltages.

      β-Ga₂O₃ MOSFET offer competitive performance in transconductance and drain current, but there are issues related to pinch-off and breakdown voltages.

      After growing the 200 nm thick β-Ga₂O₃ layer, Fujita and colleagues made the mesa structure through conventional lithography and inductively coupled plasma-reactive ion etching, then used silicon injection to form the source and drain regions. After adding Ti/Au metal layers to these two regions, the team deposited a 60 nm thick SiN passive-layer, which was subsequently etched by inductively coupled plasma-reactive ion etching in the gate region to allow for the addition of Ti/Pt/Au contact layers. Finally, this form of etching removed the SiN passivation layer above the gate and drain regions.

      Electrical measurements using the Van der Pauw method determined a mobility of 80 cm² V^-1 s^-1, which is said to be comparable to or only slightly lower than β-Ga₂O₃ films grown by MBE and MOCVD. The maximum transconductance reached 46 mS mm^-1 and the peak drain current reached 240 mA mm^-1, basically reflecting the characteristics of Ga₂O₃.

      However, some devices did not exhibit pinching, indicating insufficient suppression of the leakage path at the interface between n-Ga₂O₃ and substrate. Other disadvantages are that the breakdown voltage is only 195 V and the on-resistance is 30Ωmm, which can be solved by optimizing the device structure and growth conditions.

      Another problem is that it is difficult to obtain a carrier concentration below 10¹⁷ cm^-3, which is necessary for normally off transistors and devices that provide high breakdown voltages.

"In atomized vapor phase deposition, unintentional doping in source chemicals or quartz tubes is the next stage to address," Fujita said.