Growth and Properties of Bulk Single Crystals of GaN
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INTRODUCTION A successful fabrication of the superbright green and blue (InGaAI)N light emitting diodes (LEDs) confirms that Ill-V nitrides play a dominant role with respect to other semiconductor families in the development of short-wavelength optoelectronics [1]. The short life time of IIVI - based devices, due mainly to the low energy of defects formation in these compounds, currently prevents commercialization. The performance of GaN-based LEDs, on the other hand, despite large lattice mismatch and differences in thermal expansion between the sapphire substrate and constituent epitaxial layers, is surprisingly high. This finding suggests that the presence of a large number of extended defects, e.g. dislocations, need not be detrimental to the development of LED-related applications for nitrides. Semiconductor lasers operating in the blue and ultraviolet range of the spectrum represent the next goal to be achieved. It is a common belief that the inadequate structural quality of IIl-V nitride epitaxial films is the main limitation here. Different approaches to the solution of the above problem have been proposed very recently. Low/high temperature buffer layers made of GaN or AIN films lead to a significant improvement in the quality of heteroepitaxially grown layers of various nitrides [2-61. Another idea consists of using new substrates with properties well matched with the nitride compounds. For example, LiGaO 2 , LiAIO 2 [7] or AIMg 2 04 [8] have been found very promising. However, the use of GaN single crystals as substrates for the homoepitaxial growth of GaN represents the most natural way to overcome difficulties caused by a large lattice mismatch and differences in thermal expansion between the substrate and an epitaxial film. Homoepitaxy has become possible owing to the growth of wurtzite GaN crystals in the form of platelets with dimensions of a few millimeters. A high-pressure, high-temperature method has been employed for this purpose [9]. 15 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
The other important aspect of the existence of well-characterized bulk single crystals of GaN consists in the opportunity of using them as a standard and reference material. Strain effects, present in GaN layers grown on lattice mismatched substrates with a different thermal expansion, modify various properties of GaN films. For example, excitonic transitions studied by photoluminescence exhibit not only a broadening of the related bands but also a shift of their energetic positions [10]. The purpose of this review paper is to describe the method used for the growth of GaN bulk, single crystals. Properties of this material will be also discussed. Then, we will present some preliminary results on the homoepitaxial growth of a GaN-film on a GaN-substrate. The second part of this paper is devoted to studies which were undertaken with the purpose of answering some basic questions about the nature of GaN which have implications for devices, in particular: i) an origin of a dominant donor in undope
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