Raman and IR Studies of InN

Absorption and reflection in the near infrared spectral region have been employed to study the fundamental bandgap, phonons and the free carrier absorption in wurtzite InN films [1-7], Raman spectroscopy has been used to identify the phonon modes [8,9]. 

The first investigation of the phonon modes in binary InN was an extrapolation of the Gai-xInxN (0 x 1) alloy modes in reflection towards the binary compound [1], A typically high free carrier concentration in the mid 1020 cm 3 range controls the absorption (Drude absorption) in the infrared and must also account for the broadened Reststrahlen band in pure InN films (e.g. in [1]). In this case infrared active phonons couple to the plasma of the free electrons forming phonon-plasmon coupled modes [10,11], However, layers of low carrier concentration have been achieved and pure LO phonon energies have been derived in Raman spectroscopy. Resonant Raman spectroscopy at 514 nm has been performed, assigning five of the six Raman allowed zone centre phonon modes [8,9] (TABLE 1). 

Despite resonant excitation conditions (Egmp(InN) 1.9 eV) the Raman spectrum of InN strongly resembles that of GaN although shifted to softer modes. Note, however, that the sequence of E2(LO) and Ai(LO) appears to be inverted compared to GaN. The Ei symmetry assignment of the reflection modes [1] was performed in [7] and by the present authors after a re-evaluation of the data. In addition, the large value of 694 cm 1 indicates an E1(LO)-plasmon coupled mode. It may be assumed that phonon frequencies in heteroepitaxial InN are subject to stress conditions in a similar way to that in heteroepitaxial GaN. 

Raman spectroscopy in InN is expected to become an important tool for the characterisation of doping and stress conditions in heteroepitaxial material and device structures. 

The authors have the pleasure to thank Prof. H. Amano for good collaboration and fruitful discussion. C. Wetzel thanks Prof E.E. Haller and Dr. J.W. Ager for previous collaborations. This work was partly supported by the Ministry of Education, Science, Sports and Culture of Japan (contract nos. 09450133 and 09875083, and High-Tech Research Center Project) and JSPS Research for the Future Program in the Area of Atomic Scale Surface and Interface Dynamics under the project of Dynamic Process and Control of Buffer Layer at the Interface in Highly-Mismatched Systems. 

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