Substrate Effects on Transport and Dispersion in Delta- Doped β-ga 2 O 3 Field Effect Transistors

Transcription

1 1 EMC Substrate Effects on Transport and Dispersion in Delta- Doped β-ga 2 O 3 Field Effect Transistors Chandan Joishi, Zhanbo Xia, Joe McGlone, Yuewei Zhang, Aaron R. Arehart, Steven A. Ringel, Siddharth Rajan Electrical and Computer Engineering, Materials Science and Engineering, The Ohio State University, Columbus, OH, USA Chandan Joishi, Saurabh Lodha EE Department, IIT Bombay, Mumbai, India Funding: DTRA, IMR IITB-OSU Alliance, ONR EXEDE MURI

2 2 Outline EMC Introduction and motivation Semi-insulating substrates Si delta-doped β-ga 2 O 3 MESFETs Results Conclusions

3 3 Outline EMC Introduction and motivation Semi-insulating substrates Si delta-doped β-ga 2 O 3 MESFETs Results Conclusions

4 Wide bandgap semiconductors: applications 4 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu Wide bandgap semiconductors Railways Automotive Power grid Radar Satellite Comm. Military Inverters Power electronics Power supplies Base stations RF electronics Wireless broadband Turbines Ships and vessels Audio amplifiers 5G sub-6ghz 5G mmwave Cell phones Lower loss Smaller system size Harsh environments

5 β-ga 2 O 3 bulk substrates 5 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu β- phase: Monoclinic crystal structure Other polymorphs : α, γ, δ and ε Gallium Oxygen Melt-based growth techniques, availability of high quality native substrates High device yield due to low defect density Controlled n-type doping (Si, Sn, Ge)/ insulating S. J. Pearton et al. APL, 5, (2018) Band gap = 4.6 ev Electron effective mass = 0.28 m 0 (Fe) films first wide bandgap semiconductor ρ = Ω.cm n = cm -3

6 β-ga 2 O 3 : where it stands 6 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu Properties 4H-SiC GaN Ga 2 O 3 Bandgap (ev) Breakdown Field (MV/cm) Relative dielectric constant (ε r ) Electron mobility (cm 2 /Vs) (2DEG) (Bulk) Saturation Velocity (x10 7 cm/s) ~2 Thermal Conductivity (Wcm -1 K -1 ) [010]; 0.11[001] PP oooooo = VV BBBBII MMMMMM 8 S. J. Pearton et al. APL, 5, (2018) II MMMMMMFF BBBB vv ssssss ff ττ High frequency applications: power density not limited by low thermal conductivity

7 β-ga 2 O 3 : where it stands 7 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu Properties 4H-SiC GaN Ga 2 O 3 Bandgap (ev) Breakdown Field (MV/cm) Relative dielectric constant (ε r ) Electron mobility (cm 2 /Vs) (2DEG) (Bulk) Saturation Velocity (x10 7 cm/s) ~2 BFOM/BFOM (Silicon) (εμe C3 ) JFOM/JFOM (Silicon) ( (v sat E c ) 2 ) Substrate cost High High Low Heterojunction No Yes Yes Thermal Conductivity (Wcm -1 K -1 ) [010]; 0.11[001] S. J. Pearton et al. APL, 5, (2018) PP oooooo = VV BBBBII MMMMMM 8 II MMMMMMFF BBBB vv ssssss ff ττ High frequency applications: power density not limited by low thermal conductivity

8 β-ga 2 O 3 : where it stands 8 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu Properties 4H-SiC GaN Ga 2 O 3 Bandgap (ev) Breakdown Field (MV/cm) Relative dielectric constant (ε r ) Electron mobility (cm 2 /Vs) (2DEG) (Bulk) Saturation Velocity (x10 7 cm/s) ~2 BFOM/BFOM (Silicon) (εμe C3 ) JFOM/JFOM (Silicon) ( (v sat E c ) 2 ) Substrate cost High High Low Heterojunction No Yes Yes Thermal Conductivity (Wcm -1 K -1 ) [010]; 0.11[001] S. J. Pearton et al. APL, 5, (2018) PP oooooo = VV BBBBII MMMMMM 8 II MMMMMMFF BBBB vv ssssss ff ττ Low cost substrate High reliability High frequency applications: power density not limited by low thermal conductivity

9 Previous device reports (selected) 9 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu Field-plated MOSFET V BR = 755 V (MBE) NICT (Japan) Wong et.al. IEEE EDL 37, 2 (2016) Schottky Barrier Diode V BR > 1000 V (HVPE) NICT (Japan) Konishi et.al. DRC 2016 MOSFET with f T = 3GHz (MBE) AFRL AJ Green et.al. IEEE EDL 38,6 (2017)

10 Why delta doped FETs? 10 EMC High Mobility in high charge regime Enables scaling of Gate to Channel distance High concentration of 2DEG High gate breakdown voltage (Lowest Elec. Field between gate and channel) High trans-conductance Higher mobility than uniformly doped film (for similar doping) Enables exploration of Ga 2 O 3 - based high frequency devices

11 11 Outline EMC Introduction and motivation Semi-insulating substrates Si delta-doped β-ga 2 O 3 MESFETs Results Conclusions

12 Semi insulating substrates 12 EMC S G D Epilayer Buffer Substrate Substrate requirements: RF and power Low conduction and switching loss Minimization of parasitic capacitance

13 Semi insulating substrates 13 EMC S G D Epilayer Buffer Substrate Substrate requirements: RF and power Low conduction and switching loss Minimization of parasitic capacitance Semi-insulating substrates Fermi level pinning near mid-gap Compensation of residual donors and acceptors

14 Semi insulating substrates 14 EMC S G D Epilayer Buffer Substrate T. Nishioka et al., JAP, 51, 5789 (1980) Substrate requirements: RF and power Low conduction and switching loss Minimization of parasitic capacitance Semi-insulating substrates Fermi level pinning near mid-gap Compensation of residual donors and acceptors Fe as a deep level acceptor The most common transition metal to realize semi-insulating substrates

15 Fe doped semi-insulating substrates 15 EMC InP, GaAs, SiC Efficient SRH centers Fe diffusion during epi growth GaN Current collapse in HEMTs Non radiative center GaN literature

16 16 EMC Fe doped substrates in β-ga 2 O 3 Native n-type substrates from melt growth techniques E c ev a deep level acceptor Diffusion during growth and implantation anneal. Tamura (010) substrates (>10 10 Ω.cm) Wong et al. APL106, (2015) DFT and DLTS- Fe doped Ga 2 O 3 No report of effect of Fe diffusion on β-ga 2 O 3 based device performance

17 17 Outline EMC Introduction and motivation Semi-insulating substrates Si delta-doped β-ga 2 O 3 MESFETs Results Conclusions

18 MBE growth of β-ga 2 O 3 Substrate: Bulk (010) β- Ga 2 O 3 Substrate Temperature: 700 o C O 2 plasma power : 300 W Growth Rate: nm/ hour Ga flux: 4x10-8 Torr 8x10-8 Torr (O-rich conditions) Growth phase diagram Okumura et. al. (Speck Group- UCSB) Growth Rate (nm/hr) Riber M7, O-plasma MBE x x10-7 Ga Flux (Torr) Growth rate along the (010) orientation faster than (-201) Excess Ga on Ga 2 O 3 results in the formation of volatile Ga 2 O (resulting in etching of Ga 2 O 3 ) (010) : 240 nm/hr (-201): 50 nm/hr 18 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu

19 β- Ga 2 O 3 δ- Doped MESFET 19 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu 5 nm S G D n + Ga 2 O 3 UID Ga 2 O 3 (cap) Si δ- doping UID β-ga 2 O 3 (buffer) Fe- doped (010) β-ga 2 O 3 substrate 100 nm buffer 1 μm 0 nm 3 nm As grown surface t RMS ~ 0.6 nm for 100 nm buffer t RMS ~ 0.7 nm for 600 nm buffer S/D contact using regrowth process flow -3.5 nm 600 nm buffer 1 μm

20 Contact regrowth process flow 20 EMC Photoresist SiO 2 Growth Mask 2DEG δ doped MESFET δ doped MESFET δ doped MESFET 1. SiO 2 Deposition Lithography with thick PR 2. ICP/RIE SiO 2 Etch 3. SiO 2 wet etching for the remaining SiO 2 δ doped MESFET δ doped MESFET δ doped MESFET 4. Ohmic recess 5. SiO 2 wet etching for regrowth overlap 6. n++ Ga 2 O 3 regrowth

21 Contact regrowth process flow 21 EMC Channel δ doped MESFET Source Drain 7. BOE lift off Regrown Ga 2 O 3 on SiO 2 lift off in BOE n ++ regrown Ga 2 O 3 n ++ regrown Ga 2 O 3 Regrown Ga 2 O 3 completely covers crevice plane Contact resistance ~ 0.35 Ohm-mm SEM image of channel and regrown contact region

22 22 Outline EMC Introduction and motivation Semi-insulating substrates Si delta-doped β-ga 2 O 3 MESFETs Results Conclusions

23 SIMS profiling 23 EMC UID β-ga 2 O 3 Fe- doped (010) β-ga 2 O 3 substrate Fe concentration (atoms/cm 3 ) Detection limit Depth (nm) Fe diffusion in buffer layer nm before detection limit

24 SIMS profiling 24 EMC UID β-ga 2 O 3 Fe- doped (010) β-ga 2 O 3 substrate Fe concentration (atoms/cm 3 ) nm Detection limit 100 nm Depth (nm) Fe diffusion in buffer layer nm before detection limit 100 nm and 600 nm buffer thickness studied

25 Two terminal IVs 25 EMC D Mesa D Current (µa/mm) nm buffer 100 nm buffer Voltage (V) Hall mobility (cm 2 /Vs) Electron mobility Charge density Buffer thickness (nm) Charge density (x10 13 cm -2 ) Buffer leakage < 40 na/mm at 200V Increase in Hall mobility for thick buffer. Increase in charge density with thickness Back-depletion from Fe Energy (ev) E C (600 nm buffer) E C (100 nm buffer) E F Towards substrate Depth (nm)

26 2-DEG profile and I DS -V GS characteristics 26 EMC 2018 joishi.1@osu.edu rajan.21@osu.edu C G (µf/cm 2 ) nm buffer f = 1 MHz V GS (V) Doping concentration ( cm -3 ) nm 100 nm Position (nm) 100 nm buffer I DS (A/mm) nm buffer V DS = 10V V GS (V) I DS V GS g m g m (ms/mm) Similar position of the 2-DEG from charge profile Three terminal transfer characteristics I ON /I OFF ~ 10 5 I OFF ~ 2 μa/mm, limited by Ni/Au/Ni gate V P = -8 V (100 nm), -11 V (600 nm) I DS (A/mm) V DS = 10V I DS V GS g m V GS (V) g m (ms/mm)

27 DC-RF dispersion 27 EMC (V GSQ, V DSQ ) = (-12, 15) (V GSQ, V DSQ ) = (-15, 15) Voltage V GS V DS 5 ms V GSQ I DS (A/mm) V GS = 2V, V GS = -2V Line : DC Symbols : 5 µs I DS (A/mm) V GS = 2 V, V GS = -2 V Line : DC Symbols : 5 µs 5 µs time V DSQ nm buffer V DS (V) V DS (V) 600nm buffer For DC and pulsed IVs, 0.1% duty cycle Pulsed IV Current collapse for 100 nm buffer Reduced knee-walkout dispersion for 600 nm buffer Better characteristics with 600 nm buffer (R ON - R ON, (0, 0) ) /R ON, (0, 0) nm buffer 100 nm buffer V GSQ = -12 V V GSQ = -15 V V DSQ (V)

28 Conclusions 28 EMC δ-doping a promising approach for scaled RF devices based on β-ga 2 O 3 High current density and transconductance Fe diffusion into the buffer exhibits current collapse Hall mobility seen to increase with buffer thickness Buffer thickness greater than 600 nm can enable better transport and dispersion properties Thank you! Fe concentration (atoms/cm 3 ) Detection limit Depth (nm) Hall mobility (cm 2 /Vs) Electron mobility Charge density Buffer thickness (nm) Charge density (x10 13 cm -2 ) I DS (A/mm) V GS = 2 V, V GS = -2 V Line : DC Symbols : 5 µs V DS (V)