The core Structure of Pure Edge Threading Dislocations in GaN Layers Grown on [0001] SiC or Sapphire by MBE
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ABSTRACT The best layers of GaN grown on SiC or sapphire contain high densities of threading dislocations, which do not seem to exhibit a critically important electrical activity. It is possible that the electrical activity of these dislocations may change with time. The large majority of the threading dislocations are a type, with 1/3 < 1120 > Burgers vector and their line parallel to the [0001] growth direction. The atomic structure of the a threading dislocations was found to correspond to 5/7 or 8 atoms rings core with rather equal frequency. The 8 atoms ring cores contain dangling bonds and will present favourable sites for atomic impurities which eventuallly can modify the good performances of the devices.
INTRODUCTION Due to their large band gap (1.9 InN - 3.2 GaN - 6.2 eV AIN), wurtzite nitrides have now become subject of research in order to fabricate high performance optoelectronic and power devices which can be operated from visible to deep ultraviolet range of the optical spectrum. Until now these materials have not yet been fabricated in bulk form so the active layers are made by epitaxy over various substrates. Highly efficient devices have now been made using layers which exhibit very high densities of threading dislocations [1]. These are a, a + c and c dislocations with their lines mainly parallel to the c growth axis[2,3]. It has been shown that the large majority of these dislocations are of a type, so they are pure edge[3]. It is suspected that these high densities are responsible for the low lifetimes which are obtained in laser diodes. Until now, the atomic structure of the dislocations has been investigated in cubic materials [4, 5]. In hexagonal materials, only one case of the dissociation of the 1/3 [1120] {10T0} dislocation has been investigated in titanium using high resolution electron microscopy (HREM) observations in comparison with the available models in the litterature. It was shown that its core was slightly extended over more than lnm and three models for the dissociation were taken into account[6]. To our knowledge, no experimental data exist for the atomic structure of the 1/3 [1 120] dislocation in wurtzite materials although some of them have been under investigations for a long time [7, 8]. In the ceramic AIN, much work has been done on the structure of the basal stacking fault, which in some cases have been shown to lead to the formation of inversion domains [9,10] and results are now available on the prismatic stacking faults 1/2{ 1 2 101 in epitaxial GaN and AIN layers grown by MBE [11,12]. The origin of the a dislocations was shown to be the mosaic growth of the GaN which leads to epitaxial layers made of individual islands rotated about the c axis [2,13]. In optimised 10 2 layers, it was shown that the rotation angles could be less than 1V[2,14]. However, when 10 cm dislocations are present, angles as large as 4' for the rotation between mosaic grains can be 459
Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
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