Damage and Lattice Strain in Ion-Irradiated Al x Gai- x As
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ABSTRACT Radiation-induced damage and strain in AlxGal-xAs (x=.5 to 1) were investigated by measurements of the lattice parameter using x-ray diffraction. Irradiations employed MeV C, Ar and Au ion beams with a substrate temperature of 80 K. For samples with high Al content, the out-ofplane lattice parameter increased with fluence at low doses, saturated, and then decreased to nearly its original value. The in-plane lattice parameter did not change, throughout. These results were independent of the irradiation particle when scaled by damage energy. For the Al.sGa.5As samples, however, the out-of-plane lattice parameter increased monotonically with dose to large strains until the layer amorphized. Selected samples were examined by high resolution and conventional transmission electron microscopy (TEM). Channeling Rutherford backscattering spectrometry (CRBS) was also employed to monitor the buildup of damage in many samples. Recovery of the lattice parameter during subsequent thermal annealing was also investigated.
INTRODUCTION It has been observed that thin films of AlxGal~xAs subjected to ion irradiation possess a high resistance to damage accumulation [1-4]. In GaAs/AlAs superlattices, for example, the AlAs layers remained crystalline even when subjected to doses of heavy ion irradiation far in excess of those required to amorphize the GaAs layers [2]. At very high doses, the AlAs layers do eventually become amorphous, but the amorphous phase nucleates heterogeneously at one of the amorphous GaAs interfaces and grows across the film [5]. Another example of damage resistance in AlAs is from in-situ TEM studies of thin Al.85Ga.15As/GaAs films bombarded with Xe+ or Kr+. Each ion impact created an amorphous zone in pure GaAs films, but in the Al.85Ga.15As no amorphization or damage was observed [3]. Thus, it is well documented that AlAs has an increased resistance to damage buildup during ion irradiation, but it is not known how much damage exists or how the damage accumulates. Most of the information about damage in AlAs derives from TEM or CRBS experiments, which are rather insensitive to point defects or small clusters of point defects. The
purpose of the current research was to examine the damage in AlxGalxAs more carefully by using high-resolution x-ray measurements of the lattice parameter, and more systematically by irradiating with various ions and at several doses. Finally, the annealing behavior of the damage was investigated between 80 K and 773 K. The samples used in this study consisted of a single layer of AlxGal-xAs (x=.50, .75, .85, and 1.0), 180 nm thick, grown by MOCVD (metalorganic chemical vapor deposition) on a 2 inch diameter (001) GaAs substrate with a thick buffer layer [6]. The films were sealed with a 100 nm thick GaAs cap. Since AlxGal-xAs is slightly oversized compared to GaAs, the AlxGal-xAs layer was initially under compressive stress. Irradiation doses were varied from those not quite sufficient to amorphize the surrounding GaAs to those over three orders of magnitude higher. By irra
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