Berlin 2005 – scientific programme
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DS: Dünne Schichten
DS 22: Optische Spektroskopie dünner Schichten II
DS 22.9: Talk
Tuesday, March 8, 2005, 16:30–16:45, TU HS110
Characterization of GaN/AlxGa1−xN quantum well structures by spectroscopic ellipsometry and photoluminescence — •U. Rossow, D. Fuhrmann, T. Retzlaff, N. Riedel, and A. Hangleiter — TU Braunschweig, Inst. f. Techn. Physik, 38106 Braunschweig; b.postels@tu-bs.de
In this contribution we combine the optical methods spectroscopic ellipsometry and photoluminescence (PL) to characterize AlxGa1−xN-layers, GaN/AlxGa1−xN single and multi quantum-well (SQW, MQW) and GaN-AlN MQW structures. The QW structures are promising for various devices such as LEDs or lasers in the UV range. Challenges are on one hand the crystalline quality of AlxGa1−xN as well as the high strain due to lattice and thermal mismatch in the heterostructures. On the other hand it turned out that thickness (quantum wells) and composition xAl need to be carefully controlled in order to achieve reproducible emission wavelengths. To determine both parameters ellipsometry is usually thought to be the best choice. However, for the GaN SQW structures which are typically 1-3nm grown with AlxGa1−xN barriers of typically xAl∼ 0.23 we observe no contribution directly related to QW transitions. This is in contrast to GaN MQW structures where features appear in the spectra. All investigated samples show strong PL signals indicating that material quality is not the cause. It is likely that for such thin SQW structures the signal is too weak to be detected at room temperature (typical signal-to-noise ratio). The problem is even worse due to the fact that strong piezoelectric fields exist in the quantum wells and therefore spectral features tend to be weaker.