Effect of the Doping and the Al Content on the Microstructure and Morphology of Thin Al x Ga 1t-x N Layers Grown by MOCV
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J.H. Mazur1,2, M. Benamara1, Z. Liliental-Weber1, W. Swider1, J. Washburn1, C.J. Eiting3,4, R. D. Dupuis3.
1
E.O. Lawrence Berkeley National Laboratory, MS 62-203, Berkeley, CA 94720
2
New address: Philips Semiconductors, 9651Westoverhills Blvd., San Antonio, TX78251
3
Microelectronics Research Center, The University of Texas at Austin, TX 78712
4
New address: Air Force Research Laboratory, Materials and Manufacturing
Directorate, WPAFB, Dayton, OH 45433-7707
ABSTRACT
AlxGa1-xN {x=30% (doped and undoped), 45% (doped)} thin films were grown by MOCVD on ~2 µm thick GaN layer using Al2O3 substrate. These films were designed to be the active parts of HFETs with nsµ product of about 1016(Vs)-1. The layers were then studied by means of transmission electron microscopy (TEM) techniques. In this paper, it is shown that the AlxGa1-xN layer thickness was non-uniform due to the presence of Vshaped defects within the AlxGa1-xN films. The nucleation of these V-shaped defects has taken place about 20 nm above the AlxGa1-xN/GaN interface. Many of these Vshaped defects were associated with the presence of the threading dislocations propagating from the GaN/Al2O3 interface. We show that the density of these V-shaped defects increases with the doping level and also with the Al mole fraction in the films. The formation mechanism of the V-shaped defects seems to be related to the concentration of dopants or other impurities at the ledges of the growing film. This suggestion is supported by high resolution TEM analysis. The growth front between the V-shaped defects in the lower Al concentration thin films was planar as compared with
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the three-dimensional growth in the doped, higher Al concentration film. This interpretation of the origin of the V-shaped defects is consistent with the observed lowering of the Schottky barrier height in n-doped AlGaN/Ni Schottky diodes.
INTRODUCTION In addition to applications in optoelectronics[1], GaN and related alloys are of interest for applications in high temperature, high frequency and high power electronic devices[2]. In many of these devices, the AlGaN layer is an active part of the device structure and the structural properties of this layer can affect the basic device characteristics. This has been demonstrated and inferred for example in the work of Shiojima, et al [3] who observed lowering of the Schottky barrier height and assigned it to the presence of the structural defects in the n-doped layer. In this work, several AlGaN/GaN HFET structures are analyzed using transmission electron microscopy (TEM).
EXPERIMENTAL The GaN and the AlGaN layers were grown by low pressure MOCVD in a modified EMCORE D125 reactor system. Trimethylgalium (TMGa) and trimethylaluminum (TMAl) were used as column III precursors, while hydrogen-diluted silane (SiH4) was used as the n-type dopant. High purity ammonia (NH3) was used as the N source. The Al2O3 substrates were cleaned for 10 minutes in 10:1:1 H2SO4:H2O2:DI H2O and then for 10 minutes in 2:9 HF:HNO3. The cleaning was followed by
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