Transparent conducting thin films of GalnO,. Julia M. Phillips, J. Kwo, G. A. Thomas, S. A. Carter, R. J. Cava, S. Y. Hou,. J. J. Krajewski, J. H. Marshall, W. F. Peck, ...
Transparent
conducting
thin films of GalnO,
Julia M. Phillips, J. Kwo, G. A. Thomas, S. A. Carter, R. J. Cava, S. Y. Hou, J. J. Krajewski, J. H. Marshall, W. F. Peck, D. H. Rapkine, and R. 6. van Dover AT&T Bell Laboratories, Murray Hill, New Jersey 07974
(Received26 January 1994; acceptedfor publication 2 May 1994) GaInOs is recently identified transparentconducting material which is structurally and chemically distinct from indium tin oxide [R. J. Cava, J. M. Phillips, J. Kwo, G. A. Thomas, R. B. van Dover, S. A. Carter, J. J. Krajewski, W. F. Peck, Jr., J. H. Marshall, and D. H. Rapkine,Appl. Phys. Lett. 64, 2071 (1994)]. We have used both dc reactive sputtering in the on- and off-axis geometriesand pulsed laser deposition to grow films of this material. Layers of pure GaInO, as well as those partially substitutedwith Ge for Ga or Sn for In have been studied. Both growth techniquesare capable of producing films with conductivity -400 (Q cm)-’ and transmission as high as 90% throughout the visible spectrum for -l-,um-thick films. The growth techniques differ in the morphology of the films produced as well as in the degree of dopant incorporation that can be achieved.A post-growth annealin H2 can help producean optimized oxygen content and a reduction of resistivity. Hall measurementsindicate a carrier concentrationup to 4X10m cm-3 for all films and a Hall mobility up to 10 cm’/(V s). Doping appearsto be due both to oxygen vacanciesand aliovalent ion substitution.
A number of transparentconducting oxides (TCO) such as indium tin oxide (In,O,:Sn-ITO), ZnO:Al, and SnOz:F have been investigated extensively.‘-5 Indium tin oxide (In>) has become the TCO of choice for a wide variety of applicationsfrom solar cells to flat panel displays for at least 20 years.lw3Typical IT0 films have a sheet resistanceof 10 a/U [i.e., a conductivity of about 2.5-5X103 (a cm)-l]. Future designswill require a sheet resistanceapproaching1 InlO. This cannot be accomplishedsimply by increasingthe thickness of the ITO, since the thicker film would have unacceptabletransmission characteristics.Even given a sheet resistancein the neighborhoodof 10 aJO, there is a needfor a conductor with better transparencyacrossthe visible spectrum, especially in the green-blueregion. These challenging goals require a major improvement in the materials used as transparentconductors; GabtO is a recently identified transparentconductive material.6 It is structurally distinct from ITO, having a P-Ga,03 structurewith the Ga site tetrahedrallycoordinated and the In site octahedrallycoordinated.Aliovalent elements such as Ge or Sn and/or oxygen vacanciesdope these materials to achieve conductivities of over 300 (a cm)-1 as discussedbelow. In bulk polycrystalline form, the resistivity is comparableto conventional wide-band-gaptransparentconductors such as IT0 while exhibiting superior light transmission, particularly in the blue wavelengthregion of the visible spectrum, and enhancedindex matching with typical glass substratesused in the fabrication of flat panel displays. While the bulk material exhibits highly promising qualities, the primary use of transparentconductive materialsis as coatings on transparentsubstratessuch as glass, fused silica, plastic, and semiconductors. In this letter we report the growth and properties of GaInO, films deposited on glass and quartz substratesby both sputtering and pulsed laser deposition (PLD). The targetsemployedfor thin film growth were prepared by standardceramic processingmethods. Starting materials
were powderedGaz03, Inz03, GeO, and SnO, mixed in the appropriate molar proportions to yield Ga1-xGe,In03 or GaInt-,Sn,O,, for O
