Comparing Magnetron Sputtered ScN Films Grown on Sapphire (1 0 -1 0) and (1 -1 0 2) Substrates Tobin Muratore1,2, Said Elhamri1,2, Amber Reed2, John Cetnar3, David C Look4, Kurt Eyink2, Hadley Smith1,2, Zachary Biegler2,5 1. Department of Physics, University of Dayton, Dayton, OH 45469 2. Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright Patterson Air Force Base 45433 3. Sensors Directorate, Air Force Research Laboratory, Wright Patterson Air Force Base 45433 4. RESEARCH CENTER, WRIGHT STATE UNIVERSITY, DAYTON, 45435; 5. Department of Electro-optics, University of Dayton, Dayton, OH 45469 Goal: To understand how growth parameters impact film properties (crystallinity, resistivity, electron density and mobility) Introduction Experimental Setup Previous Work: ScN (111) on Sapphire (001) and ScN (001) on MgO (001) Ultra high-vacuum, controllably- Sputtering on the Single Ion Level ScN (111) on Sapphire (0001) • (ScN) is closely lattice matched to GaN (A unbalanced reactive DC-powered Magnetron Power Optimization magnetron sputtering system • Note the improvement in crystal quality semiconductor of interest for high power devices) and 25 W Magnetron 40 W between the two samples (see below), this 50 W Electromagnetic 75 W serves as a good growth template 100 W Coil 200 W change in crystal quality was reflected in almost Load Lock doubled carrier concentration and increased • ScN is a thermally and mechanically robust semiconductor Tantalum mobility. (samples grown at nominally same Foil

(direct bandgap 2.1-2.4 eV) with high electron mobility and Intensity Deposition conditions) low resistivity Chamber • This result indicates an unintentional change in Substrate Substrate growth parameters, most likely caused by Location Sample Base Pressure: • Epitaxial ScN has been grown on sapphire [(0001), (10-10), Holders variation in heater resistance ~10-8 Torr (1-102)] using MBE, HVPE, and other methods 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Growth Pressure: 100 W (1st Grow) 2q-w(°) 100000 100 W (2nd Grow) ~20 mTorr 10000 • Our group has successfully grown epitaxial ScN [(111),(001)] Plasma Target to Substrate • Room temp. mobility 1000 on Sapphire (0001) and MgO (001), respectively, using Distance: for ScN (111) ranged [1] 2 100 from 8-14 cm /Vs over (a.u) Intensity magnetron sputtering Controlled Growth Parameters ~5 cm (a.u.) Intensity 10 • Substrate Temperature (500-900°C) the tested magnetron 1 • • Magnetron Power (50 - 200 W) power settings The goal of this study is to grow high crystal quality ScN 0.1 32 34 36 38 40 42 44 33 35 38 40 43 45 [(002), (022)] films with high electron mobility and low • Coil Current (0 - 6 A) 2q-w(°) 2q-w(°) resistivity using a magnetron sputtering system on sapphire • N2 Fraction Al2O3 Substrate Orientation c = 1.291 nm ScN (001) on MgO (001) [(1-102), (10-10)] substrates, respectively Hall Mobility Carrier Concentration Magnetron Power Optimization

• To that end, we investigated the impact of various Al2O3 Crystal Structure (Wurtzite) magnetron sputtering growth conditions on the quality of these ScN films. These conditions include substrate a = .4785 nm temperature, magnetron power, fraction and ion flux

• To evaluate the quality of these films, various Highlights of Previous Studies characterization methods were used, including x-ray Scandium Nitride Properties: diffraction, atomic force microscopy, and Hall effect • Rock salt structure •Oxygen and fluorine impurities play a significant role in • Semiconducting: determining the electrical properties of the ScN films[1] • Results from these various characterization methods on the Direct band gap = 2.1-2.4 eV •For the ScN (002) grown on MgO (001), room temperature ScN films are presented here • High carrier concentration: mobility was in the range of 66 - 97 cm2/Vs with carrier 1019- 1021 cm-3 concentration on the order of 1020 – 1021 cm-3 [1] Results ScN (002), Grown on Sapphire (1 -1 0 2) *All Samples have nominal thickness of 50 nm* ScN (022), Grown on Sapphire (1 0 -1 0) For the Substrate T series: Relaxed d-spacing = .2220 nm Conditions for the coil current series: Magnetron Power = 100 W Relaxed d-spacing = .2029 nm ScN (002) Substrate T Mobility and Resistivity Substrate T: 809°C 50 3.0 Coil Current = 4 A Magnetron Power: 100 W

Mobility ) Resistivity Sapphire (10-10) Magnetron Power

Sapphire (1-102) Substrate T XRD 2.5 Nitrogen Fraction = 75% (15.0 mTorr) Nitrogen Fraction = 75% (15.0 mTorr) cm 40 50 W 846 °C Sapphire (10-10) Substrate T XRD 75 W Sapphire (1 0 -1 0) Coil Current (Ion Flux) Series 883 °C 2.0 W- Al O (2 0 -2 0) 883 °C Regrow 100 W 2 3

-3 773°C 901 °C 125 W 30 809°C 0 A 920 °C Al2O3 (2 0 -2 0) 150 W

1.5 10 846°C (

(1 -1 0 2) 0 -1 (1 920 °C Regrow

(2 -2 0 4) 0 -2 (2 200 W 2 A ScN (0 2 2) 2 (0 ScN

3 3 882°C

975 °C 3

ScN (002) ScN 919°C 4 A O

O 20 2

2 1.0

956°C 6 A 0) -2 0 (2

Al 3 3

Al 992°C

ScN (0 2 2) 2 (0 ScN O

0.5 2

10 Al

ScN (0 2 2) 2 (0 ScN Mobility (cm²/Vs) Mobility

0.0 Resistivity Intensity(a.u.) 0

880 890 900 910 920 930 940 950 960

Intensity(a.u) Intensity(a.u.)

Substrate T (°C) (a.u.) Intensity 920°C: RMS = .162 nm Conditions for the Substrate T series: Magnetron Power = 100 W 55 60 65 70 75 2q-w(°) Coil Current = 4 A 55 60 65 70 75 55 60 65 70 75 ScN (022) Magnetron Power Electrical Properties 20 30 40 50 60 2q-w(°) 0.26 2q-w(°)

Nitrogen Fraction = 75% (15.0 mTorr) ) 42 0.24 ScN (022) Substrate T Electrical Properties Mobility 50

2q-w(°) Resistivity 0.22 cm Mobility 0.15 ) 40

Resistivity 0.20 Conditions for the nitrogen fraction series:

W- cm

48 0.14 38 0.18 Substrate T: 809°C -3 -3 Conditions for the Magnetron Power Series: W- 0.16

Sapphire (1-102) Power Series XRD 0.13 36 Coil Current = 4 A -3 -3

0.14 10 Substrate T: 883°C 46 ( 0.12 Magnetron Power: 100 W 50 W 10 0.12 ( 34 75 W Coil Current = 4 A 0.10 44 32 Sapphire (1 0 -1 0) N2 Fraction Series XRD 100 W Nitrogen Fraction = 75% (15.0 mTorr) 0.11 0.08

(1-102) 0.06

3 3 150 W (2-204) 30

ScN (002) Magnetron Power Series Electrical Properties 0.10 (cm²/Vs) Mobility Al O (2 0 -2 0) 3 3

O 42 0.04 25% 2 3

Mobility (cm²/Vs) Mobility 2

200 W O

2 ) Al 50 50 % ScN(002) Mobility 0.02

0.40 Resistivity

Al 28 0.09

Resistivity Resistivity 0.00 75 % 45 cm 40 750 800 850 900 950 40 60 80 100 120 140 160 180 200 220 0.35 100 %

40 W- Substrate T (°C) Magnetron Power (W) ScN (022)

0.30 -3

35 10

( 956°C:

Intensity(a.u.) Conditions for the Magnetron Power Series:

30 0.25 RMS = 4.583 nm Intensity(a.u) 25 Substrate T: 809°C 0.20 20 Coil Current = 4 A Mobility (cm²/Vs) Mobility 0.15 Nitrogen Fraction = 75% (15.0 mTorr)

15 846 °C: Resistivity Resistivity 10 0.10 RMs = 3.378 nm 20 30 40 50 60 40 60 80 100 120 140 160 180 200 220 55 60 65 70 75 2q-w (°) Magnetron Power (W) 2q-w(°) Summary of Study Future Work • • The crystalline quality of the grown films is not as of high quality as that of ScN Continue N2-fraction study for (1 0 -1 0) orientation and initiate this study for the (1 -1 0 2) • (111). Revisit samples showing atypical crystalline or electrical results • Expand XRD scans to determine crystal structure • Current results indicate (022) ScN is of better quality than (002) ScN based on • Initiate XPS to better understand composition (impurities significantly impact electrical XRD. properties) • Substrate temperature is much more important for the (002) ScN than the (022) • Additional Hall effect measurements of ScN (002) and (022) films. Acknowledgements

• The results of our N2 fraction study indicate that optimal growth conditions for This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-17RYCOR490-RX. TM would like to thank the Southwestern Ohio (002) require a lower N2 fraction than that of (111) ScN. • The mobility of all measured samples, regardless of the orientation, is lower than Council for Higher Education (SOCHE) that of ScN films grown on MgO (001), but mobility is higher than that of the ScN References 1. Electronic transport in degenerate (100) scandium nitride thin films on magnesium oxide (111). substrates ,Appl. Phys. Lett. 113, 192104 (2018); https://doi.org/10.1063/1.5050200 Submitted: 27 July 2018 . Accepted: 25 October 2018 . Published Online: 08 November 2018