Surface Elevation and Strain in Ion Implanted GaN
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Surface Elevation and Strain in Ion Implanted GaN B. Molnar, S. B. Qadri, S. Schiestel1, R. M. Stroud and C. A. Carosella Code 6370, Naval Research Laboratory Washington, DC 20375 1 George Washington University Washington, DC 20052 ABSTRACT We report on the investigation of X-ray strains and surface elevation of ion-implanted GaN as a function of ion fluence. Si, Mg and Ar ion implantation are examined. A continuous increase in lattice constants, a and c occur for doses up to near 1 x 1015/cm2. At higher doses there emerges a volume expansion of the GaN perpendicular to the surface which is an order of magnitude larger than any observed out-of-plane change in strain. The rapid expansion correlates with the onset of GaN amorphization. Transmission electron microscopy indicates that cavities form after 5 x 1016/cm2, 100 keV Ar+ implantation. For low temperature implantation, the cavities lie near the peak of the ion range. For room temperature implantation, the cavities coalesce at the surface and push out the GaN perpendicular to the surface. AFM measurements confirm the presence of large bumps on the surface. The very high dose ion implantation have important consequences to etching rates of GaN and to the ability to produce n+, but not p+ doping with ion implantation. INTRODUCTION GaN and the related nitride compounds are currently the subject of intense study because of the possibility of a wide range of optoelectronic and electronic applications. GaN thin films are presently grown on substrates that have different lattice constants and thermal expansion coefficients than GaN. As the grown film cools down stresses are created which then relax by various processes, such as bending, changing the lattice parameters and cracking the GaN layer. The final state of stress is known to impact the film’s mechanical and optical properties. These properties, in turn, can influence device performance. The incorporation of dopants and defects during ion implantation is also known to lead to large lattice constant variations. These processes create strain near the surface layer. The residual strain that influences the films’ properties is the composite of all the strains. At high dose levels, the ion implantation-induced strain can dominate and influence the mechanical properties of the GaN films. The strain associated with the implantation in GaN has been addressed [1]. One study reported an increase in the out-of-plane lattice parameter for doses below amorphization. Another one reported large swelling for amorphization doses [2]. We have been investigating the n+-and p+ -type ion implantation doping of GaN. In this paper we report both lattice constant changes at low doses, as well as swelling and morphology changes near the amorphization level [3]. Some significant technological implications of very high dose implantation, such as etching, and n+-layer but not p+-region formation, are also reported. G11.53.1
EXPERIMENTS Heteroepitaxial, wurzite GaN layers of 2-3 micron thickness were used in this study. These layers were g
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