Local structure and effective chemical valency of Mn impurity atoms incorporated in wide-band-gap (Ga,Mn)N epilayers have been investigated by using x-ray absorption fine structure techniques. The x-ray results provide direct evidence for the substitution of majority Mn atoms for the Ga sites in GaN, with an effective valency close to Mn(II), up to a rather high Mn concentration about 2 at. %. A small fraction of the impurity atoms could also form Mn clusters.
The preparation of hexagonal GaN:Mn and GaN:Fe epilayers has been studied by RF-plasma-assisted molecular beam epitaxy. GaN:Fe epilayers exhibit superparamagnetic behavior, presumably due to ferromagnetic inclusions. GaN:Mn epilayers can be expressed in the form of Ga 1−x Mn x N with x up to 0.02, indicating the successful preparation of the GaN-based magnetic alloy semiconductor for the first time. The epilayers are primarily paramagnetic and highly resistive. For epilayers with very high Mn concentration (∼ 10 21 cm −3 ), analysis of the paramagnetic component has revealed the effective spin number S ≈ 2.5 together with the positive paramagnetic Curie temperature. This suggests the presence of ferromagnetic spin exchange between Mn ions.
Recently we have observed the structure images of silicon in the (110), (111) and (100) projection respectively, and then examined the optimum defocus and thickness ranges for the formation of such images on the basis of calculations of image contrasts using the n-slice theory. The present paper reports the effects of a chromatic aberration and a slight misorientation on the images, and also presents some applications of structure images of Si, Ge and MoS2 to the radiation damage studies.(1) Effect of a chromatic aberration and slight misorientation: There is an inevitable fluctuation in the amount of defocus due to a chromatic aberration originating from the fluctuations both in the energies of electrons and in the magnetic lens current. The actual image is a results of superposition of those fluctuated images during the exposure time. Assuming the Gaussian distribution for defocus, Δf around the optimum defocus value Δf0, the intensity distribution, I(x,y) in the image formed by this fluctuation is given by
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