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CP Liu

Publications and source records attributed to CP Liu.

5 recordsLinked to original sources
PubMed2000

Strain evolution in coherent Ge/Si islands

Strain evolution of coherent Ge islands on Si(001) is measured using a newly developed transmission electron microscopy technique based on two-beam dark-field strain imaging. The strain measurements show that a metastable Ge island shape is involved in the shape transition between pyramids and domes; this shape is more readily observed for growth at 550 than 600 degrees C because of the slower rate at which islands cross the kinetic barrier between shapes. The strain relaxation changes discontinuously between pyramids and domes, indicating that the underlying shape transition is first order.

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PubMed2000

TEM measurement of strain in coherent quantum heterostructures

We report on a transmission electron microscopy (TEM) technique that can be used to measure strain due to individual nanometer-scale coherent heterostructures such as quantum dots or inclusions. The measurement relies on two-beam imaging and on an approximation that employs a universal model for lattice plane bending. We demonstrate that analysis is simple and accurate. Using this method, we measured the average strain in dome-shaped Ge islands grown on Si (0 0 1). We found that the method of specimen preparation can significantly affect the observed strain in these islands.

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PubMed1999

Quantitative analysis of ultrathin doping layers in semiconductors using high-angle annular dark field images.

It is well known that the high-angle annular dark field (HAADF) technique in scanning transmission electron microscopy is an incoherent imaging process in the lateral (xy) plane. However, as a consequence of the existence of partial coherence in the z direction, accurate quantitative interpretation of image intensity is difficult. The effects of coherence in the z direction can be reduced by increasing the inner collector angle of the annular detector so that the scattering from atoms in the z direction is essentially incoherent. We thus show that it is feasible to quantify the total As concentration of ultrathin InAsxP1-x layers in InP in a simple but accurate way using a thickness integrated Bloch wave calculation including phonon scattering with a large inner collector angle of the annular detector of around 150 mrad. We compare the As composition derived from this approach with that from the Fresnel method and high resolution imaging. We also show that the non-linear variation of the HAADF intensity with thickness is consistent with our simpler simulations for such conditions. Therefore, this approach enables us easily and quickly to quantify compositions using HAADF images. The tetragonal distortion due to lattice mismatch is also shown to influence the contrast and has been included in the calculations.

Journal Article↗
PubMed1999

Energy-filtered transmission electron microscopy of multilayers in semiconductors.

Quantitative analysis of novel semiconductors with wide or ultrathin multilayers of atomic dimensions is very important in order to control electronic and optical properties, but rather difficult due to the limited resolution in most techniques. In this paper we attempt to assess how effectively the total As dopant concentration in ultrathin As doped layers in InP and the Ti atomic fraction in a TixAl1-xN multilayer can be analysed quantitatively using energy-filtered imaging. These two materials have characteristic edges located at widely different energy losses, with the L edge of As being above 1000 eV, while that of Ti is around 450 eV. We have quantified the As concentration using the three-window technique and theoretical cross-sections and we find that the resolution limit is dominated by the signal-to-noise ratio in this delta-doped specimen. However, the accuracy of the Ti atomic fraction in TixAl1-xN can be as good as 10 at% for specimens of uniform thickness made by focused ion beam milling. We will compare our results with measurements of the composition made using Fresnel contrast, high resolution imaging and high angle annular dark field techniques.

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