Prism coupling as a non destructive tool for optical characterization of (Al, Ga) nitride compounds
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ABSTRACT An optical characterization technique is proposed for GaN based compounds deposited on sapphire. In AlGaN films grown by MOCVD, the film optical behavior and the substrate to layer interface are qualified from the measured optical data. The experimental and theoretical approach used for this purpose is described in detail. The results clearly show bending effects at the interface which may be related to structural defects; a good agreement with transmission electronic microscopy analysis is obtained.
INTRODUCTION There is currently an increasing interest in the growth of high quality nitride related materials for fabrication of short-wavelength devices for displays [1-2]. This has been initiated by use low temperature buffer layers (AlN or GaN) [1]. Until now, most of the studies for the group III nitrides have dealt with the improvement of films quality, the characterization of physical properties has become an integral part of research for the understanding of material behavior and performance under operating conditions. Numerous accurate tools as transmission electronic microscopy (TEM) [3], scanning probe microscopy (SCM) [4] techniques are largely used for the qualification of the layers. Furthermore, the increasing need of feedback informations has spurred the development of original characterization methods. In the following, the high sensitivity of prism coupling for the characterization of the interface properties is demonstrated.
EXPERIMENTAL
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The AlGaN films were grown on (0001) sapphire substrates at a low pressure using a metalorganic vapor phase epitaxy (MOVPE) apparatus. A low-temperature AlN nucleation layer of thickness 10nm was first deposited on sapphire at 800°C. Growth temperature of the active layers was carried out at 1170°C. Details of the growth procedure were published elsewhere [5]. The optical properties of AlGaN / AlN heterostructures were measured by using a guided-wave technique based of the prism coupling. This technique was described in details by Tien et al [6]. In short, a He-Ne laser beam emitting at a wavelength of 632.8nm is coupled into the AlGaN layer using a rutile prism (TiO2) through the evanescent field in the air-film gap. By measuring the reflected intensity versus the angle of incidence α, we draw the guided-mode spectrum of the samples (figure 1). With the optical axis normal to the surface, the ordinary and extraordinary modes are respectively excited using transverse electric (TE) and transverse magnetic (TM) polarized light. The same layers were examined by transmission electron microscopy using cross section samples prepared in the conventional way of mechanical polishing followed by ion milling. Photodiode
Beam Splitter Rutile Prism
Laser He - Ne Sample Lens Pressure
λ
θ
GONIOMETER
Figure 1 Experimental set up of the prism coupling method
RESULTS In the TE mode spectrum of the AlGaN films, six sharp reflectivity dips are observed at certain angles α which correspond to
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