AVS 62nd International Symposium & Exhibition
    Surface Science Thursday Sessions
       Session SS+AS+EM+EN-ThA

Paper SS+AS+EM+EN-ThA6
Probing 2-DEG at InN Surface by Electrolyte-Gated Raman Spectroscopy

Thursday, October 22, 2015, 4:00 pm, Room 113

Session: Atomistic Modeling of Surface Phenomena & Semiconductor Surfaces and Interfaces - II
Presenter: Tommaso Brazzini, Lawrence Berkeley Lab, University of California, Berkeley
Authors: E. Alarcon Llado, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
T. Brazzini, Lawrence Berkeley Lab, University of California, Berkeley
J.W. Ager, Lawrence Berkeley National Laboratory (LBNL)
Correspondent: Click to Email
Indium nitride has attracted much attention as its narrow bandgap (~0.67eV) expands the range of the direct gaps of the group III-N alloys into the visible and near-IR and thus offers an outstanding potential for solar energy conversion and optoelectronic applications. However, experimental demonstration of high efficiency In-rich III-V pn rectification junctions has been hampered by the existence of an intrinsic interface electron accumulation layer, which seems to persist regardless of surface treatment. The large capacitance of the Helmholtz double layer that forms on a surface of an object in contact with an electrolyte allows the 2-DEG at the surface of InN to be tuned and even depleted. Using this effect, we demonstrated the first pn rectification behavior in InN.1

In addition, the 2-DEG accumulation layer affects not only the electrical properties, but also has brought many controversies in the interpretation of optical experiments. Raman spectroscopy probes not only the lattice dynamics in a crystal, but also the electronic structure and free carriers. In particular, the interaction between the free electrons at the surface and the longitudinal optical (LO) phonon in InN has been addressed by several studies. However some questions still remain.

In this work, we present an in-situ micro-Raman study that confirms the presence of a surface related Raman mode in InN and shows its interaction with accumulated electrons at the surface. Electrolyte gated Raman spectroscopy (EGRS) on InN layers was performed in order to modulate and in-situ probe the surface electron accumulation region in InN. A reversible shift of the LO phonon with the applied gate potential is found (see figure 1). The peak position and shift depends on the probing light energy, however it is independent of bulk doping. We explain these findings by Martin’s double scattering mechanism and bandgap narrowing at the surface tuned by the gate voltage. InN nanocolumns were also investigated by EGRS. The LO mode lies at higher frequencies in all nanocolumn samples. This fact corroborates the nature of the scattering mechanism, which is strongly dependent on the surface orientation. In summary, our results clearly demonstrate the surface origin of this feature and allow the fundamental study and understanding of the electronic structure of InN.

1.Alarcón-Lladó, E. et al. PN junction rectification in electrolyte gated Mg-doped InN. Appl. Phys. Lett. 99, 102106 (2011).