High Temperature Hardness of Bulk Single Crystal GaN
- PDF / 71,271 Bytes
- 5 Pages / 595 x 842 pts (A4) Page_size
- 86 Downloads / 242 Views
INTRODUCTION GaN (gallium nitride) and its alloys are promising as a wide band-gap, high temperature semiconducting material for application as blue- and ultraviolet-lightemitting devices, and high power/high frequency devices [1]. GaN materials are grown on various foreign substrates with large lattice mismatch and significant differences of thermal expansion coefficient. Such heteroepitaxial structures are inevitably accompanied by the introduction of various kinds of extended defects such as dislocations, prismatic stacking faults, etc. [2]. It is well recognized that the reduction in density of threading defects does lead to improvement of optical and electrical properties of GaN [3]. Thus, efforts are being made in controlling film stress to reduce the occurrence of detrimental defects during GaN hetero-epitaxial growth to obtain high performance devices. Understanding of dislocation behavior in this material is also indispensable. The obstacle to conducting such an investigation has been the difficulty in growing bulk GaN crystal of a suitable size for deformation test. A few investigations are available on dislocation motion or hardness of GaN at room temperature. Drory et al. [4] measured the hardness and fracture toughness of GaN by a conventional hardness test. Nowak et al. [5] recently evaluated the yield strength of GaN by nanoindentation. Maeda et al. [6] observed the viscous motion of dislocations under electron irradiation using a transmission electron microscope operating at 200kV. Yonenaga [7] presumed empirically that the activation energy for dislocation motion in GaN is presumed to be 2 eV. This paper reports the hardness of GaN at elevated temperature in comparison with those of other typical materials such as Si, GaAs, etc. As far as we know, this is the first information on the mechanical strength of GaN at elevated temperature and is important as a basis for the control of dislocation generation during crystal growth.
F99W3.90
EXPERIMENT GaN single crystals were prepared from a high quality GaN thick film grown on a 2-inch diameter (0001) sapphire substrate by using hydride vapour phase epitaxy (HVPE) together with the selective growth through SiO2 windows by means of the facet initiated epitaxial lateral overgrowth (FIELO) technique. The details on the growth procedure are described elsewhere [8]. Finally, the thick grown layer was removed from the substrate. As a result, crack-free GaN single crystals of 0.5 mm thickness with mirror-like surfaces were successfully obtained and were sufficiently thick to be regarded as being bulk material. The density of grown-in dislocations was as low as 107 cm-2 [9]. Hardness measurements on the crystals were carried out by the Vickers indentation method using a pyramidal diamond indentor. The applied indentation load P was 0.5 - 5 N, and the dwell time 30 seconds, for every temperature tested in the range from room temperature to 1200°C in a high purity Ar gas atmosphere. Four indents were formed at every temperature for the Ga ( 0001 ) and N ( 000 1
Data Loading...
