Nanomaterials and low dimensional structures have been used for a wide variety of device applications.  To achieve higher performance of such devices, a characterization technique in nano-scale is required.  In this paper, we present strain and crystal structure evaluation with high spatial resolution using a nano-beam electron diffraction (NBD) [1] for GaAsP/GaAs strained superlattice layers.  

  Following the growth of a 2 mm thick GaAs_0.65P_0.35 layer and a 100 nm thick GaAs layer on the (001) GaP substrate, 6 pairs of the GaAs_0.65P_0.35(4 nm)/GaAs(4 nm) superlattice layers were grown.  The NBD measurement was carried out using a TEM at an acceleration voltage of 120 kV.  The electron beam size was about 15 nm in diameter.

TEM observation showed that the 100 nm GaAs layer consisted of (001) planes and slopes with an inclination angle of about 15° from the (001) plane.  The superlattice layers on the (001) plane showed uniform thickness while those on the slope showed variable thickness.  The NBD measurement indicated that the constant strain was introduced in the superlattice on the (001) plane while the non-uniform strain was introduced in the superlattice on the slope.  From the NBD measurement, it was also found that the crystal orientation of the GaAsP superlattice layers on the slope was inclined by 1° from the [001] direction. 

Using the strain values and thicknesses of the layers, the average mismatches of each pair of the superlattice were calculated.  In the superlattice on the slope, the average mismatches decreased obviously with growth.  It is believed that the change of the strain and the thickness contributed to decrease the strain effect.  On the other hand, the average mismatches in the superlattice on the (001) plane showed much smaller values than that on the slope.  The smaller values were the reason for the growth of uniform superlattice layers. 

The formation mechanism of the different superlattice structures on the (001) plane and on the slope will be discussed. 

Reference

[1] K. Saitoh, H. Nakahara, N. Tanaka, Microscopy, accepted (2013).

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