Surface origin and control of resonance Raman scattering and surface band gap in indium nitride

Esther Alarcón-Lladó,Joel W Ager,Tommaso Brazzini

doi:10.1088/0022-3727/49/25/255102

Abstract

Resonance Raman scattering measurements were performed on indium nitride thin films under conditions where the surface electron concentration was controlled by an electrolyte gate. As the surface condition is tuned from electron depletion to accumulation, the spectral feature at the expected position of the (E1, A1) longitudinal optical (LO) near 590 cm−1 shifts to lower frequency. The shift is reversibly controlled with the applied gate potential, which clearly demonstrates the surface origin of this feature. The result is interpreted within the framework of a Martin double resonance, where the surface functions as a planar defect, allowing the scattering of long wavevector phonons. The allowed wavevector range, and hence the frequency, is modulated by the electron accumulation due to band gap narrowing. A surface band gap reduction of over 500 meV is estimated for the conditions of maximum electron accumulation. Under conditions of electron depletion, the full InN bandgap (Eg = 0.65 eV) is expected at the surface. The drastic change in the surface band gap is expected to influence the transport properties of devices which utilize the surface electron accumulation layer.

Highlights

The origin of the intense peak appearing in the resonance Raman spectrum of wurtzite InN films in the vicinity of the expected (E1,A1) longitudinal optical (LO) frequency has been of considerable interest for the last decade
We find that the intensity and frequency of the forbidden LO Raman feature is reversibly changed by the external potential, showing its surface
By changing the band bending at the surface, both carrier density and band structure are modulated, which in turn changes range of LO phonon wavevectors which contribute to the scattering

Summary

Introduction

The origin of the intense peak appearing in the resonance Raman spectrum of wurtzite InN films in the vicinity of the expected (E1,A1) longitudinal optical (LO) frequency has been of considerable interest for the last decade. In contrast to GaAs and other III-V semiconductors, where the LO phonon couples to free carriers and forms coupled L- and L+ modes [1], the LO-like scattering in InN and in In-rich InGaN has only a small shift in frequency regardless of electron (hole) concentration [2,3]. A number of groups have attributed the LO-like feature observed in the z( , )z scattering geometry from c-axis oriented wurtzite InN films to an unscreened A1(LO) mode with a large wavevector (q) [4,5]. Some authors have attributed the wavevector non-conservation to the presence of bulk impurities.[6] this explanation appears to be inconsistent with Cusco et al who confirmed the conservation of wavevector in the scattering process in bulk InN by the observation of the L- phonon-plasmon coupled mode in a series of doped InN layers [7]

Results

Discussion

Conclusion