Thermal expansion of GaN at low temperatures - a comparison of bulk and homo- and heteroepitaxial layers
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EXPERIMENTAL Different types of samples were investigated: bulk GaN, homoepitaxial GaN and heteroepitaxial GaN layers grown by MOCVD and MBE. The investigated GaN bulk crystals were grown at the High Pressure Research Center, Unipress, Warsaw by the hightemperature high-pressure method at 1800 K and 15 kbar [5]. They have hexagonal platelet shape with lateral sizes of 3 to 4 mm. One of the bulk crystals was Mg-doped, the other was nominally undoped and overgrown by MOCVD with a 3 µm thick undoped GaN layer [6]. For this, a substrate temperature of 1050°C was used similar to that applied for MOCVD of the two heteroepitaxial GaN layers deposited on c-plane sapphire. The growth conditions for these layers provided by S. DenBaars (University of California Santa Barbara) and S. Nakamura (Nichia Chemical Industries) can be found in ref. [7] and [8]. In addition, a 4.5 µm thick GaN layer grown by MBE with an electron cyclotron plasma source on c-plane sapphire at a substrate temperature of 820°C was investigated. In contrast to the MOCVD layers, this layer was deposited directly onto the nitridated substrate without growing a low-temperature buffer before. A high-resolution X-ray diffractometer Philips X´Pert MRD equipped with a Cu sealed anode, a four crystal monochromator and a triple crystal analyzer was used for the measurements. This diffractometer was extended by a continous flow X-ray cryostat of Oxford Instruments enabling temperature dependent measurements from 10 to 630 K. The lattice parameters were calculated from the scattering angles directly measured by triple axis 2θ scans [9]. RESULTS The progress in the growth of GaN bulk crystals over the last few years has resulted in crystal platelets with lateral extensions of up to 10 mm characterized by very narrow Xray diffraction profiles [5] which are a prerequisite for the accurate determination of lattice parameters [9]. However, highly resolved reciprocal space maps (RSMs) for such crystals as shown in figure 1 for an overgrown GaN substrate indicate that even the small GaN crystals obviously can consist of several macroscopic grains. Within the area of about 1 mm2 illuminated by the X-ray beam, we found typically two or three grains contributing to the scattered signal as indicated by corresponding sharp intensity maxima in the (00l) RSMs at different qx values. From the difference in qx the tilt of the crystallographic orientation between different grains can be calculated to be in the range of a few hundred arcseconds. Additionally, the grains are characterized sometimes by slightly different lattice parameters as it is indicated by the small shift of the intensity maxima A and B along qz in figure 1. The changes in lattice parameter c by 8⋅10-4 Å between different grains can be ascribed to different impurity concentrations pointing to a possible preferential accumulation of impurities in some grains.
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a)
b)
intensity [a. u.]
substrate
layer
A B 126.0
126.1
2 [deg]
Figure 1. Triple axis 2θ/ω scan for the (006) reflection (a) and the
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