S. J. Pearton, Jiancheng Yang, Patrick H. Cary, F. Ren ...

1 A review of Ga2O3 materials, processing, and devicesS. J. Pearton, Jiancheng Yang, Patrick H. Cary, F. Ren, Jihyun Kim, Marko J. Tadjer, and Michael A. MastroCitation: Applied physics Reviews 5, 011301 (2018);View online: Table of Contents: by the american institute of PhysicsAPPLIED physics REVIEWSA review of Ga2O3materials, processing, and devicesS. ,1,a)JianchengYang,2 Patrick IV, ,2 JihyunKim,3,a)Marko ,4and Michael of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA2 Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA3 Department of Chemical and Biological Engineering, Korea University, Seoul 02841, South Korea4US Naval Research Laboratory, Washington, DC 20375, USA(Received 28 September 2017; accepted 30 October 2017.

2 Published online 11 January 2018)Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics,solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existingtechnologies due to its large bandgap. It is usually reported that there are five different polymorphsof Ga2O3, namely, the monoclinic (b-Ga2O3), rhombohedral (a), defective spinel (c), cubic (d), ororthorhombic (e) structures. Of these, theb-polymorph is the stable form under normal conditionsand has been the most widely studied and utilized. Since melt growth techniques can be used togrow bulk crystals ofb-GaO3, the cost of producing larger area, uniform substrates is potentiallylower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and performance of technologically important high voltage rectifiers and enhancement-modeMetal-Oxide Field Effect Transistors benefit from the larger critical electric field ofb-Ga2O3rela-tive to either SiC or GaN.

3 However, the absence of clear demonstrations of p-type doping inGa2O3, which may be a fundamental issue resulting from the band structure, makes it very difficultto simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of thisreview is to summarize recent advances in the growth, processing, and device performance of themost widely studied polymorph,b-Ga2O3. The role of defects and impurities on the transport andoptical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping,and the development of processing techniques like etching, contact formation, dielectrics for gateformation, and passivation are discussed.

4 Areas where continued development is needed to fullyexploit the properties of Ga2O3are by AIP OF 1 BASIC PROPERTIES AND APPLICATIONS 2 BULK GROWTH TECHNOLOGY OF 6 PROGRESS IN METAL ORGANIC CHEMICALVAPOR DEPOSITION (MOCVD) GROWTH 7 HALIDE VAPOR PHASE EPITAXY (HVPE) ..9 PULSED LASER DEPOSITION (PLD).. 10 MBE OF 10 ATOMIC LAYER EPITAXY (ALE) ANDATOMIC LAYER DEPOSITION (ALD) 12 PROPERTIES AND PROCESSING 13 DOPING AND DEFECTS IN 16 OXIDE p-n JUNCTION HETEROSTRUCTURES.. 19 THEORY OF DEFECTS IN 19 LOW AND HIGH FIELD TRANSPORT IN STRUCTURE .. 21 EPR OF 22 HYDROGEN IN 23 OHMIC CONTACTS TO 24 SCHOTTKY CONTACTS TO 26 WET ETCHING OF 28 DRY ETCHING OF 30 BAND ALIGNMENTS OF DIELECTRICS ON( 201) 32 RADIATION DAMAGE IN 34 THIN FILM SOLAR-BLIND UV DETECTORS.

5 35 POWER RECTIFIERS .. 37 POWER MOSFETs AND MESFETs .. 40Ga2O3-BASED GAS 42 SUMMARY AND 44a)Authors to whom correspondence should be addressed: by AIP , 011301-1 APPLIED physics REVIEWS5, 011301 (2018)INTRODUCTIONG allium oxide (Ga2O3) has a long history, and the phaseequilibria of the Al2O3-Ga2O3-H2O system was first reportedin 1952,1in which the polymorphs ( , different forms orcrystal structures) of Ga2O3and their regions of stabilitywere also identified. There are five commonly identified pol-ymorphs of Ga2O3, labeled asa,b,c,d, 10 These areknown as corundum (a), monoclinic (b), defective spinel (c),and orthorhombic (e), with thedphase commonly accepted asbeing a form of the orthorhombic ,2,7,11 Among thesedifferent phases of Ga2O3, the orthorhombicb-gallia structure(b-phase orb-Ga2O3) is the most stable crystal structure andhas attracted most of the recent attention.

6 The different poly-morphs can be either insulators or conductors, depending onthe growth resulting crystals are layeredmaterial, similar to the behavior of GaSe and GaTe. TableIsummarizes the lattice parameters, crystal structure, and someof the bulk properties of the polymorphs of Ga2O3. The origi-nal studies indicated that the other polymorphs of Ga2O3con-vert to the stableb-form with heat this review, we will primarily focus on the monoclinicstructuredb-Ga2O3polymorph, which is attracting interest forpower electronic devices, as well as solar-blind UV photodetec-tors, photocatalysts, gas sensors, solar cells, phosphors, and trans-parent conducting films for electrodes on a variety ofoptoelectronic ,3,12 20Ga2O3is optically transparent to 250 nm and is electrically conducting, making it useful as awindow on some types of optical devices.

7 In terms of deviceapplications, thin films of polycrystallineb-Ga2O3containing Ovacancies have long beenknown as sensors for a variety of gasesincluding H2,CH4,CO,andO2which change the electrical con-ductivity upon 25In addition, the conduction elec-tron spins in this material produces a magnetic memory effectthat is stable to above room temperature (RT).26,27 The review will cover basic properties of Ga2O3, the roleof defects and impurities, the status of doping studies, as wellas developments in device fabrication processes such as etch-ing and contacting, gate dielectrics, and passivation films. Thecurrent status of power electronic devices, solar-blind UVphotodetectors, and gas sensors will also be PROPERTIES AND APPLICATIONS OF Ga2O3 The number of publications on Ga2O3has accelerated inrecent years, as is evident from , due to the interest inelectronic and photonic devices with capabilities beyond exist-ing technologies.

8 There has been a healthy balance of experi-mental and theoretical investigations of the properties of ,6,8 11,28,29 The monoclinic phase ofb-Ga2O3isthe stable one under normal conditions of temperature and pres-sure, and can be converted into other phases at higher pressuresor ,29 For example, it undergoes a transition tothe hexagonala-Ga2O3phase at a pressure of GPa at1000 high pressure phase can remain as a metastableTABLE I. Summary of the properties of (A )Refractiveindex, nOpticaldielectricconstantVolumeexpansio nat 1200 KBulk modulus(300 K, GPa)CommentReferencesaa, b ,c 185 Corundum, rhombohedralstructure, space group R 3c,bandgap larger than all otherpolymorphs ( eV)Yoshiokaet al.

9 ,4 Stepanovet al.,2and Heet ,b ,c 150 Monoclinic structure, spacegroup C2/mKohnet al.,10 Stepanovet al.,2and Heet spinel, cubic struc-ture, space group Fd 3mStepanovet bixbyite. Suggestedto be a nanocrystalline formofe-Ga2O3 Royet , Playfordet ,b ,c structure,space group Pna21 Yoshiokaet Krollet 1. Number of publications on gallium oxide since 1952. A total of2492 papers which had either Ga2O3 or Gallium Oxide in the title havebeen published in the last 65 years (Data: Thomson Reuters).011301-2 Phys. , 011301 (2018)phase if quenched to room ,6 Thebphase is theonly stable phase up to 1800 C, while the rhombohedralcorundumaphase is metastable but can exist under ambientconditions.

10 Theb-phase can be transformed into thea-phaseunder hydrostatic pressure at higher to the relative difficulty in isolating the different phasesin pure crystalline form,31 34much of what we know about theproperties has come from theory, especially density functionaltheory (DFT). For example, Yoshiokaet the lat-tice parameters, space groups, and volume expansivity normal-ized to room temperature for the different polymorphs. Theresults of these calculations are shown in (a).Theb-phasehas the lowest volume expansion and this increases in the orderb,e,a,andd. The expansivity of thea-phase is in good agree-ment with the reported experimental calculated bulkmoduli for the different phases are shown in (b)4and theseincrease in the orderb,e,d,andabelow 400 the structural parameters, band struc-ture, and Debye temperature ofb-Ga2O3, also calculatedfrom density functional theory.