Exciton-polariton propagation in A1GaN/GaN quantum-well waveguides probed by time-resolved photoluminescence T. N. Oder, J. Li, J. Y. Lin, and H. X. Jiang Department ofPhysics, Kansas State University, Manhattan, KS 66506-260 1 ABSTRACT The propagation properties of light in AIGaN/GaN multiple-quantum-well (MQW) waveguides have been studied by time- resolved photoluminescence (PL) spectroscopy. The waveguides were patterned with fixed width of 0.5 j.im but orientations varying from 3oo to 600 relative to the a-axis of GaN by electron-beam lithography and inductively-coupled plasma (ICP) dry etching. The peak position and line-width of the emission peak were found to vary systematically with orientations of the waveguides and followed the six-fold symmetry of the wurtzite structure. This is explained in terms of anisotropy of the exciton/carrier diffusion coefficient along the different crystal orientations of the semiconductor materials. We also observed a remarkable decrease in the PL intensity as well as increase in time delay of the temporal response as the location of the laser excitation spot on the waveguide is varied. These observations can be understood in terms ofexciton-polariton propagation in the waveguides. The speed of generated polaritons with energy corresponding to the well transitions in the waveguides was determined from the time delay ofthe temporal response to be approximately (1.26 0.16) x iO rn/sec. The optical loss in the waveguides was determined to be about 5 - 8 cm' for different excitation intensities. The implications of these results to waveguiding in optical devices based on the 111-nitride semiconductors are discussed. Keywords: AIGaN/GaN quantum wells, waveguides, submicron time-resolved PL, exciton-polariton 1. INTRODUCTION The group 111-nitride semiconductors consisting ofAIN, GaN, InN and their alloys have been intensively studied as they are very promising materials for many optoelectronic device applications such as blue-green and UV light emitting diodes (LEDs), laser diodes (LDs), UV solar blind detectors, and high-temperathreThigh-power electronic devices [1]. However, not so many studies have been done on submicron structures in the HI-nitrides due to the difficulties involved in fabrication and characterization. When structural dimensions are reduced to submicron sizes, significant changes in properties such as carrier and photon dynamics ofthe semiconductor structures will result. There is therefore a great need to study low-dimensional optical and electronic systems such as waveguide structures especially in the Ill-nitride semiconductors in order to elucidate the basic physics governing the optical properties in submicron size stmctures. These may open doors for new applications as well as contribute to the improvement of the design of existing devices where such structures are used. There is also interest in investigating the potential ofthe ifi-nitrides for waveguide materials. Our group has previously fabricated structures of a few pm in dimension using photolithography [2—6]. It is difficult to achieve structures ofdimensions less than 1 im in the ifi-nitride materials using standard photolithography. Also, because the III- nitrides are hard materials, the usual method to achieve pattern-transfer is by high-density plasma etching. Here, we summarize the results from the fabrication and optical study ofsubmicron waveguide patterns based on AIGaN/GaN multiple quantum wells (MQW). Our results reveal that the peak position and line-width of the emission peak vary systematically with the waveguide orientations. We explain these observations in terms of anisotropy of carrier or exciton diffusion coefficient in a quasi one- dimensional (1D) structure. In addition, we observed a systematic decrease ofthe emission intensity as the location ofthe incident laser spot is moved further away from one end of the waveguide. We determined the optical loss in the waveguides to be 5 - 8 cm' for different excitation intensities. From the temporal response, we have determined the propagation speed of light in the waveguide, with energy corresponding to the well transitions in the MQW, to be (1.26 0.16) x 1O' rn/sec. No results for light propagation in waveguides based on these materials have previously been obtained to our knowledge. These results are explained in terms ofthe propagation properties ofexciton-polaritons in the waveguides. Ultrafast Phenomena in Semiconductors VI, Kong-Thon F. Tsen, Jin-Joo Song, Hongxing Jiang, Editors, Proceedings of SPIE Vol. 4643 (2002) © 2002 SPIE · 0277-786X/02/$15.00 258 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/01/2014 Terms of Use: http://spiedl.org/terms