Contributors:

• Roberto Fornari (Head of the research - Physics Dept. – Parma Italy)

• A. Baraldi, C. Borelli, V. Montedoro, A. Parisini, M. Pavesi (Physics Dept.)

• D. Delmonte, F. Fabbri, C. Ferrari, E. Gombia (IMEM-CNR)

• G. Calestani, F. Mezzadri (Chemistry Dept.)

• I. Cora, B. Pecz (Inst. Technical Phys. Budapest)

• D. Klimm (IKZ, Berlin)

• H. J. von Bardeleben, J. L. Cantin (Sorbonne Université, Paris, France)

• S. Leone (Fraunhofer IAF, Friburgo)

Ga2O3 research in IMEM:

Status and perspectives

Matteo BosiIMEM-CNR Institute, Parma (Italy)

matteo.bosi@imem.cnr.it

Novel oxide semiconductors

Ga2O3 key properties:

➢ Bandgap: 4.7 – 4.9 eV

➢ Breakdown field: 8 MV cm-1

➢ Bulk crystals and substrates are easily grown

➢ n-type doping is possible

Applications:

➢ Superior characteristics for high-power application with respect to SiC and GaN

➢ Potentialities for performances beyond GaN and SiC for very high power applications

➢ Expected to be cheaper compared to SiC and GaN for certain classes of devices

➢ Solar blind UV-detectors

Our research:

IMEM: Ga2O3 growthPhysics dept. UniPR: characterization, processingNational and international network of collaborators: theory, characterization

team

• Low power (10 W – 1 kW) / low voltage (<400V) power supplies for consumer electronics

• Mid power (1 kW – 100 kW) / mid voltage (1.2 kV) electric motor control, PV inverters, electric vehicles, UPS

• High power (MW – GW) / high voltage (> 2 kV)rail transport, ship, wind mills, large PV farms, smart power grid

Energy distribution & conversion

• Low power (10 W – 1 kW) / low voltage (<400V) power supplies for consumer electronics

• Mid power (1 kW – 100 kW) / mid voltage (1.2 kV) electric motor control, PV inverters, electric vehicles, UPS

• High power (MW – GW) / high voltage (> 2 kV)rail transport, ship, wind mills, large PV farms, smart power grid

GaN, SiCMature technologyStill very expensive

Main issue: substrate cost!

Which technology for very high V?

Energy distribution & conversion

Ga2O3:•Emerging material•Potentially cheaper•Increased performance

GaN, SiCMature technologyStill very expensive

• Low power (10 W – 1 kW) / low voltage (<400V) power supplies for consumer electronics

• Mid power (1 kW – 100 kW) / mid voltage (1.2 kV) electric motor control, PV inverters, electric vehicles, UPS

• High power (MW – GW) / high voltage (> 2 kV)rail transport, ship, wind mills, large PV farms, smart power grid

Energy distribution & conversion

→ Replace silicon with wide-bandgap / high breakdown field semiconductors:lower switching losshigher efficiencyreduced weight and sizehigher operating temperatures, less cooling requirements

More efficient energy conversion = energy saving

Energy distribution & conversion

Interest in Ga2O3 is increasing rapidly…

Novel oxide semiconductors

Pub. Date Citations now Citations Mar ‘19Hetero-epitaxy of ε-Ga2O3 layers by MOCVD and ALD giu-16 42 35

Crystal structure and ferroelectric properties of ϵ-Ga2O3 films grown on (0001)-sapphire nov-16 42 29The real structure of ε-Ga2O3 and its relation to κ-phase feb-17 32 20

ε-Ga2O3 epilayers as a material for solar-blind UV photodetectors feb-18 18 9Thermal stability of ε-Ga2O3 polymorph nov-17 12 7

Dedicated conference, workshop, focused sessions

Main drivers: Japan, USA (US-Army), Germany

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Scopus: Ga2O3 OR “gallium oxide” in ABS-TITLE-KEY

Aims➢ Basic material science to realize, characterize and understand a “new” compound➢ Build test devices (electrical characterization, optical sensors)

What we do➢ Ga2O3 epitaxy with home made MOVPE on sapphire substrates and GaN templates➢ Doping: SiH4 (in situ), Sn (diffusion, ex situ)➢ Electrical contacts and electrical characterization➢ Structural characterization: XRD, TEM (Budapest)

Critical issues➢ Ga2O3 has several polymorphs:

most interesting: b- Ga2O3 (stable, grown at 800-900 °C, device development)our phase: e- Ga2O3 - metastable but still technologically relevant (low growth T)

➢ Limits of our reactor: low temperature, homogeneity

➢ Upgrade of the growth system?Higher temperature Deposition homogeneity over 2’’

Novel oxide semiconductors

4.41 Ev3.92 Ev 6.18 Ev

Detection of weak UV radiation on vis-IR backgrounds:

• There is no terrestrial background at less than 280 nm (UVC)• Heat sources (flames, jet engines, or missile plumes) emit UVC

Easy detection of emitters at wavelengths less than 280 nm:→ “solar blind” UV detector

Applications:

➢ detect missile plume, airplane engines (security)➢ flame monitoring in boiler control and fire-safety equipment ➢ UV exposure control in photolithographic and binder curing processes➢ imaging of UV objects in astronomy➢ UV sensing elements in advanced medical and biological instruments

e-Ga2O3 is suitable for detection of deep-UV radiation • Very basic test photoresistor already proven

Solar blind UV photodetectors

Bandgap of e-Ga2O3 ~ 4.6 eV (270 nm)

Doping and electrical characterization

Doping of epilayer (n-type)

• SiH4 addition to the gas phase during the growth• Sn by diffusion ex-situ (sputtering + thermal annealing)

Electrical contactstest: Ti/Au, ITO/Au

Contact resisitivity is difficult to measure: 1 (doped) – 30 (undeoped) Ωcm2

Best undoped e - Ga2O3 films have RT resistivity in the range of 107 -108 Ωcm

Doped layers:RT resistivity in the range 1-10 Ωcmmobility is very low (2-3 cm2 / V s)Hall density (difficult to measure) ~ 1017 cm-3

Si acts as shallow dopant (EPR spectroscopy)

Perspectives

Integration of Ga2O3 and GaN (Fraunhofer IAF, Freiburg)

• Expitaxial deposition studies: Ga2O3 / GaN and GaN/Ga2O3

• Thanks to the high spontaneous and piezoelectric polarization of e- Ga2O3, a 2DEG seems to be formed atthe interface between e-Ga2O3 on GaN, as evidenced by preliminary CV measurements and as supportedby theoretical calculations.

• Novel HEMT?

Reactor upgrade?Funding request by Physics Dept. to University of Parma (~ 200.000€ for a new deposition chamber)

switch to b polytype -> Very high voltage devices?improved homogeneity on 2’’

Technological improvements• Epitaxy (deposition parameters, doping process, role of the carrier gas)• Improvement of metal ohmic contacts• Deep UV photodetector

Articoli pubblicati:

http://dx.doi.org/10.1016/j.jcrysgro.2016.03.013 https://dx.doi.org/10.1021/acs.inorgchem.6b02244 http://dx.doi.org/10.1039/C7CE00123Ahttps://dx.doi.org/10.1016/j.actamat.2017.08.062https://doi.org/10.1016/j.matchemphys.2017.11.023https://doi.org/10.1063/1.5054395https://doi.org/10.1063/1.5050982