Taming transport in InN

Taming transport in InN Joel W. Ager III and Nate R. Miller Materials Sciences Division, Lawrence Berkeley National Lab, Berkeley, CA 94720 USA Emcore, Albuquerque, NM 87123 USA Keywords Indium nitride, thermopower, doping, conductivity The large electron affinity of InN, close to 6 eV and the largest of any III-V semiconductor, creates a strong driving force for native donor formation, both in the bulk and at surfaces and interfaces. Moreover, all InN surfaces, regardless of crystal orientation or doping, have been observed to have a surface accumulation layer of electrons, which interferes with standard electrical measurements. For these reasons, until recently, it was uncertain whether or not compensation by donor defects would prevent “real” p-type activity (i.e. existence of sufficiently shallow acceptors and mobile holes). A coordinated experimental approach using a combination of electrical (Hall effect) and electrothermal (Seebeck coefficient) measurements will be described that allows definitive evaluation of carrier transport in InN. In Mg-doped InN films, the sensitivity of thermopower to bulk hole conduction, combined with modeling of the parallel conducting layers (surface/bulk/interface), enables quantitative measurement of the free hole concentration and mobility. In undoped (n-type) material, combined Hall and thermopower measurements, along with a considering of the scattering mechanisms, leads to a quantitative understanding of the crucial role of charged line defects in limiting electron transport. 1 Introduction Depending on the band offset values used, the electron affinity of InN is in the range of 5.5-5.8 eV [1-3]. This is the largest electron affinity of all group III-V semiconductors and has a number of important consequences for the study of electron and hole transport in this material. Based on considerations of both the charge neutrality level (also known as the branch point energy) [4,5] and of the amphoteric native defect (Fermi level stabilization) model [6,7], InN would be expected to have its surface Fermi level pinned high above the conduction band minimum. Indeed, surface pinning corresponding to a Fermi level position in the range of 0.7-0.9 eV above the CBM has been observed experimentally by a number of techniques including electron energy loss spectroscopy, x- ray photoelectron spectroscopy, angle-resolved photoemission, and electrolyte contacted capacitance voltage (CV) measurements [8-12]. The large electron affinity of InN also affects bulk conduction properties. Native point defects have a strong driving force to be donors. Energetic particle irradiation experiments have shown that their average energy is 0.9 eV above the CBM, similar to the position of the surface pinning energy [13,14]. Both hydrogen and also certain types of dislocations are also predicted to be donors in InN [15,16]. As a result, undoped InN thin films grown to date by molecular beam epitaxy have been degenerate n-type, with the lowest reported electron concentration in the low 10 17 cm -3 range [17,18]. The presence of electron-rich surface layers and the strong propensity for the formation of native donors created challenges for producing p-type InN. While CV techniques were able to show that Mg forms acceptors, as in GaN [11], direct measurement of the transport properties had been prevented by the n-type surface inversion layer. Recently, thermopower [19-22] and Hall effect/conductivity studies [23-24] have shown that Mg doping produces mobile holes in InN, but there are only a few reports concerning the hole concentration and mobility. In this work, we are “taming” the transport of InN in two ways. (1) We use the ability of an electrolyte to form an insulating surface layer to deplete InN’s surface accumulation/inversion layers to reveal the bulk transport properties of InN epitaxial films [25]. (2) We take advantage of the sensitivity of thermopower measurements to bulk hole conduction to obtain the free hole concentration and mobility in p-type InN:Mg films grown by MBE. The additional use of thermopower yields a more precise assessment of hole conduction properties than with Hall/conductivity analysis alone. 2 Experimental Methods for the MBE growth of the films used in this study are described in the literature by our collaborators [17,26,27]. Hall effect measurements were measured using a 3000 Gauss magnet with contacts placed in the van der Pauw configuration. Thermopower measurements were performed using a system Corresponding author: JWAger@lbl.gov