Optical and Microstructural Characterization of the Effects of Rapid Thermal Annealing of CdTe Thin Films Grown on Si (100) Substrates

Journal of Electronic Materials, Jun 2005 by Neretina, S, Sochinskii, N V, Mascher, P

RESULTS AND DISCUSSION

Structural Quality

A Philips scanning electron microscope 515 with a lanthanum hexaboride filament was employed to characterize the film and interfacial microstructure. Figure 1 presents a scanning electron microscopy (SEM) image of a CdTe/Si interface. A cross section of the CdTe/Si structure shows well-defined and flat heterojunction interfaces. The image reveals a good adhesion to the substrate with no evidence of cracks through the bulk.

X-ray diffraction (XRD) techniques were employed to study the structural properties of the CdTe films as a function of annealing temperature. The 2θ scan (Fig. 2) and single-crystal rocking curve (Fig. 3) data were obtained using an automated x-ray diffractometer. The x-ray beam samples large areas of the film; therefore, the results are averaged. The crystallographic orientation of the film was determined to be (111). The lattice constant α is 6.483 [Angstrom].

The XRD patterns of the CdTe epilayers are shown in Fig. 2. One peak at 23.756° corresponding to the (111) plane appears for all films. Furthermore, the (111) peak becomes sharper and stronger with increasing RTA temperature. The XRD results indicate the progressive improvement in the structural quality of the CdTe films with annealing temperature. It can be explained that CdTe was grown on Si as a highly ordered polycrystalline layer with (111) preferential orientation. As the annealing temperature increases, there is a gradual transition toward forming a perfect layer resulting in better crystalline quality of the CdTe film.

The rocking curve linewidth (full-width at half-maximum (FWHM)) is considered a measure of the structural perfection of the interface region. The widths of the x-ray rocking curves for CdTe films show dramatic changes as a function of annealing temperature (Fig. 3). The FWHMs of the rocking curves vary from 9.21° for as-grown films to 4.72° for a film annealed at 600°C. This demonstrates a significant improvement in CdTe/Si crystalline quality at higher RTA temperatures.

The quantitative measurements of CdTe/Si structural quality such as the FWHM and net area values obtained from x-ray 2θ scan and single-crystal rocking curve are shown in Tables I and Table II, respectively. It is seen that increasing annealing temperature up to 550°C significantly improves the epitaxial structure. However, as is also evident from Table I and Table II, the peak intensity clearly decreases with further increasing RTA temperatures. This observation indicates the deterioration in the film quality at temperatures above 550°C.

Surface Morphology

Surface morphology has been investigated by employing atomic force microscopy (AFM). The AFM images were collected in the contact mode using a Digital Instrument Nanoscope Scanning Probe Microscope (Santa Barbara, CA).

Figure 4 shows the surface view of CdTe/Si layers as grown and annealed at 500°C. The images exhibit a uniform fine-grained morphology. Because of the large difference in lattice constant and thermal expansion coefficient, accumulated mechanical strains cause the formation of a high density of microcracks in the CdTe epilayers. A large number of surface defects has been observed in as-grown CdTe/Si, which is typical for surface morphology of a strained layer. Further, we can see that rapid thermal annealing greatly reduces, but does not completely eliminate, the defect density. In addition, at higher annealing temperatures, the CdTe grains crystallize to form larger grains (Fig. 5), which are consistent with the XRD patterns in Fig. 2. However, increasing the temperature above 550°C results in a deterioration of the surface quality.