Spatially-Resolved Photoluminescence and Raman Study on the GaN/Substrate Interface
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ABSTRACT We performed spatially-resolved photoluminescence and Raman experiments on the substrate interface region of wurtzite GaN layers. We found that the broad "yellow" photoluminescence band is strong only near the interface. Our investigations reveal that both the substrate interface and a region of structural reorientation of the layer near the interface act as source of the photoluminescence. The Raman-scattering experiments show that at least a portion of the GaN layer near the substrate interface is oriented in such a way that the c-axis of the layer is parallel to the substrate interface. At a distance about 30 pm away from the interface the layer reorients by turning the c-axis by 90' into a direction perpendicular to the substrate interface. INTRODUCTION The wide-bandgap semiconductor GaN has attracted considerable attention over the last years because of its application as a basic material for optoelectronic devices working in the blue and UV spectral region, such as blue laser diodes [1]. Apart from the near-bandgap excitonic and donor-acceptor-pair luminescence GaN shows an unwanted broad "yellow" photoluminescence band between 2.0 and 2.5 eV at low temperatures [2]. Recently, intensive work has been done to clarify its origin, and some authors connect this luminescence band with the inherent property of GaN to be automatically n-type conductive [3-5]. However, this issue is still controversial. Similar luminescence bands are also known from several II-VI semiconductors like ZnS [6]. They are often interpreted as a recombination between shallow donors and deep acceptors in which the donors or the acceptors were build by anion or cation vacancies. The creation of such intrinsic vacancies is very probable near surfaces, interfaces or grain boundaries. In order to clarify wether the broad "yellow" photoluminescence band is an intrinsic property of GaN or caused by defects located near the interface to the substrate, we performed spatially-resolved photoluminescence and Raman measurements on hexagonal GaN samples which were grown on sapphire. We found that the luminescence band is strong only in a region near the interface and hence not an intrinsic property of GaN. Our investigations reveal that the photoluminescence originates from both the substrate interface and a region of structural reorientation near the interface where the c-axis of the GaN layer rotates by 90' from a direction parallel into a direction perpendicular to the substrate interface.
577 Mat. Res. Soc. Symp. Proc. Vol. 395 ©1996 Materials Research Society
EXPERIMENT The samples investigated were undoped wurtzite GaN layers grown on [0001] sapphire using hydride vapor phase epitaxy (HVPE) with thicknesses of 220 pm, 230 pim and 400 pim and a free carrier concentration of about 1.1017 cm- 3 . The spatially-resolved photoluminescence and Raman experiments were carried out using a Dilor XY800 triple-grating spectrometer with a charge-coupled device (CCD) detector and confocal optics. The sample was excited either parallel (in-plane) or
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