Characterization of Al x Ga 1-x N/ Al y Ga 1-y N Distributed Bragg Reflectors Grown by Plasma Assisted Molecular Beam Ep
- PDF / 525,044 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 72 Downloads / 264 Views
Characterization of AlxGa1-xN/ AlyGa1-yN Distributed Bragg Reflectors Grown by Plasma Assisted Molecular Beam Epitaxy H. Klausing, F. Fedler, T. Rotter, D. Mistele, O. Semchinova, J. Stemmer, J. Aderhold and J. Graul Laboratorium für Informationstechnologie, Universität Hannover, GERMANY
ABSTRACT AlxGa1-xN/ AlyGa1-yN Distributed Bragg Reflectors (DBRs) with up to 45 periods have been grown on (0001) sapphire substrates by r.f. plasma-assisted molecular beam epitaxy (PAMBE) with the aid of two Al effusion cells. Several samples were grown with an Al mole fraction varying between 0.38 ≤ x ≤ 1 (0 ≤ y ≤ 0.4) at temperatures of up to 890 °C. In all samples, an AlxGa1-xN buffer layer was used to prevent cracking of the quarter wave stack and improving surface morphology by choosing the Al content so that strain energy in the DBR structure would be compensated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations were performed to determine the thickness of the quarter wave layer periods and the Al mole fraction of corresponding AlxGa1-xN single layers. Room-temperature calibrated reflection and transmission (R&T) measurements were performed. Thus stray and self-absorption of the DBRs were extracted from reflectance and transmittance. The thickness of the quarter wave layers was designed such that the measured peak reflectances appeared between 346 nm to 421 nm. The dispersion data, including refractive indices and absorption coefficients, used in the calculation were extracted from R&T measurements done on the above mentioned AlxGa1-xN single layers. INTRODUCTION Distributed Bragg Reflectors (DBRs) based on GaN/AlxGa1-xN have been fabricated and investigated by several groups [1-9] in order to push the development of vertical cavity surface emitting lasers (VCSELs) and resonant-cavity light emitting diodes for the near UV and visible spectral range. Nevertheless there have been only few attempts [10-11] to realize AlxGa1-xN/ AlyGa1-yN based DBRs which are suited to be monolithically integrated in VCSEL structures containing GaN/AlxGa1-xN quantum wells. Redwing et al. [10] have observed stimulated emission at 363.5 nm by optically pumping a 10 µm GaN active layer from the side, sandwiched between 30 period Al0.12Ga0.88N/ Al0.40Ga0.60N DBRs. Ambacher et al. [12] have investigated AlxGa1-xN single layers by different spectroscopic methods in order to determine absorption coefficients and refractive indices. Therewith the design and reflectivity of AlxGa1-xN/ AlyGa1-yN based Bragg mirrors could be calculated using the transfer matrix method. The influence of stress induced defects however, especially at the interfaces of a multilayer stack, which can cause subbandgap absorption by non-radiative transitions, was not taken into consideration. Thus it is difficult to evaluate the self-absorption of an AlGaN based DBR only with the dispersion data of corresponding AlxGa1-xN single layers. Our work focuses on how the composition and contrast of high and low index of refraction material and the number of p
Data Loading...
