Epitaxial Growth of GaN Films Produced by ECR-Assisted MBE
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Mat. Res. Soc. Symp. Proc. Vol. 395 e 1996 Materials Research Society
must be negligible, and their formation may be instigated by many factors such as strain, impurities and various growth parameters which affect the kinetics of growth. The transport, optical and recombination properties must also be influenced significantly by the coexistence of the two phases in the same film. This is because the wurtzite and zincblende structures of GaN are direct bandgap semiconductors with room temperature energy gaps of 3.4 eV and 3.2 eV respectively [ 1]. In this paper we review the epitaxial growth of GaN in the wutzite and zincblende structures and discuss evidence that certain experimetal results may be related to the coexistence of both phases in the same film. EXPERIMENTAL METHODS A. Thin Film Growth GaN films were grown by the Electron Cyclotron Resonance microwave-plasma assisted Molecular Beam Epitaxy method (ECR-MBE). The deposition system is schematically illustrated in Fig. 1. It consists of a Varian Genii MBE unit with an ASTeX compact ECR source mounted in one of the effusion cell ports. The base pressure of the overall system is 10-" Torr. A reflection high energy electron diffraction (RHEED) setup is an integral part of the apparatus. Conventional Knudsen effusion cells are used for the evaporation of the group III elements as well as the dopants (Si and Mg). Active nitrogen is produced by passing molecular nitrogen through the ECR source at a flow rate, which produces a downstream pressure of 10-5 to 10. 4 Torr.
\ \Quadrupole Mass Spectrometer Rotatable Substrate Heater
Fig. 1 Schematic of the ECR-MBE System
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The wurtzite GaN films were grown on (0001) sapphire substrates while the zincblende GaN films were grown on (001) Si substrates. The sapphire substrates were degreased and etched in H3 PO4 :HzSO4 (1:3) for the removal of surface contaminants and mechanical damage due to polishing and finally were rinsed in de-ionized water. The Sisubstrates were degreased and etched in buffered HF and outgased to 800 °C in the MBE preparation chamber. RHIEED studies of such Si wafer at 400 °C indicate that the (001!) Si surface is unreconstructed (ie. l xi). We found that such an unreconstructed surface is required fo the growth of cubic GaN. B. Design and Characterizlation of the ECR source The ASTeX ECR source is designed with an axial solenoid to generate the magnetic field required for ECR operation. The design of this source promotes efficient electron/gas collisions and results in high density plasma with gas ionization efficiencies as high as 10%. This efficient resonant coupling allows the source to be stabily operated with growth chamber pressure as low as 10- Torr. Due to the large mean free path of the gaseous species at these low background pressures (-1 m at l0.4 Torr), the growth is carried out in the molecular flow regime , where the transport of atoms or molecules in both thermal beams from the effusion cells as well as the beam of activated nitrogen from the ECR source occurs in a collis
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