(Invited) Nature of Point Defects at High-Mobility Semiconductor/Interfaces Probed by Electron Spin Resonance: Thermal GaAs/GaAs-Oxide Structures

The introduction of III-V semiconductors, such as GaAs, as high-mobility channels in field effect transistors has been hindered by the presence of a large density of electron states (defects) at the GaAs/oxide interface¹, leading to strong Fermi-level (E_f) pinning, the origin of which is not fully atomically identified and quantified. This lack of adequate interface (surface) passivation has been a main obstacle on the route to development of high-performance III-V MOS technology.

            As to the atomic origin of the interface states causing Fermi level pinning, on theoretical grounds, various types of native point defects such as interfacial As-As dimers (like-atom bonds), As_Ga antisites, Ga dangling bonds (DBs), and elemental As, have been projected with levels in the GaAs band gap². X-ray photoelectron spectroscopy has concluded that elimination of interfacial As-oxides is a key requisite for E_f unpinning³.

            Aiming atomic identification of electrically detrimental interface traps, we here present results of a first experimental extensive low-temperature multi-frequency (K-and Q-band) study by conventional ESR -the tool of choice when it comes to atomically identify paramagnetic point defects- of thermally grown c-GaAs/GaAs-oxide entities. In an exploring approach, for ESR sensitivity reasons, two groups of samples were prepared: (1) GaAs powders, thermally oxidized (O₂; 1.1 atm) in the temperature range T_ox=350 - 615 °C for various times (22-390 min). (2) Oxidized (430–555 °C) slices (2 ´ 10 mm²), cut with their long edge along a á1-10ñ direction.

            As a major GaAs-related result, the study reveals the generation of As_Ga⁺ antisites upon thermal oxidation in registry with the GaAs substrate. Importantly, no such signal could be observed on a stack of slices of the as-received (100)GaAs substrates. Evaluated from T_ox~350 °C onward, the generation is observed in densities increasing with T_ox, reaching alarmingly high levels (~1×10¹³ cm^-2), thus providing solid independent evidence of substantial interfacial As enrichment, appearing as endemic for oxidation of GaAs. Comparison with literature data, based on salient ESR parameters, would indicate the observed AsGa⁺ antisite to concern the electrically well identified EL2 deep double donor center. It thus provides an answer on atomic scale of how a major part of excess As gets incorporated at the interface.

            Besides As_Ga⁺, four more ESR signals have been observed. An eye catching one concerns an anisotropic behaving signal, which is only observed on oxidized c-GaAs slices after additional vacuum-UV irradiation (10.02 eV). The spectrum is composed of a quartet centered at g~2.27 with average hyperfine splitting constant A= 87 G for the applied magnetic field along the (100)GaAs sample normal; it might stem from a V_Ga center. Other single-component isotropic behaving spectra are observed at g~2.06 and g~1.937. Also observed is the substitutional Fe dopant (quintet ESR spectrum) in the parent GaAs substrate, centered at g~ 2.07. The atomic nature of the originating point defects is addressed starting from a comparative analysis of the data obtained on oxidized c-GaAs powders and (100)GaAs slices.

            In summary, the current ESR study on thermally oxidized c-GaAs has revealed several types of point defects. This includes the As_Ga⁺ (EL2) antisite as a predominant defect, the work thus providing convincing first atomic identification of an inherent detrimental interface defect. As to realization of device-grade semiconductor/insulator interfaces, it is clear that oxidation of the GaAs substrate interface (surface) is to be strictly avoided.

¹ C. W. Wilmsen, in Physics and Chemistry of III-V Compound Semiconductor Interfaces (Plenum, New York, 1985).

² J. Robertson, Microelectron. Eng. 86, 1558 (2009; L. Lin and J. Robertson, Appl. Phys. Lett. 98, 082903 (2011).

³ W. Wang, C. L. Hinkle, E. M. Vogel, K. Cho, and R. M. Wallace, Microelectron. Eng. 88, 1061 (2011).