Journal of ELECTRONIC MATERIALS, Vol. 34, No. 5, 2005

Regular Issue Paper

Study of Threading Dislocations in the Plan-View Sample of SiGe/Si(001) Superlattices by Transmission Electron Microscopy YUSUF ATICI Faculty of Arts and Sciences, Department of Physics, Firat University, 23119 Elazig, Turkey. E-mail: [email protected]

Interfacial defects due to a mismatch of 1.378% between substrate and epilayer were examined in a Si0.67Ge0.33/Si(001) superlattice by transmission electron microscopy (TEM). Plan-view specimens from the superlattice were prepared to investigate the defects in the structure. It was observed that 60°-type misfit dislocations associate with point contrast on and at their ends. This point contrast was found to represent threading dislocations by using tilt experiments in the microscope. Consequently, stereo electron microscopy was used to examine the threading dislocations. It was discovered that the threading dislocations are not on the {111} slip planes but can be almost parallel to the [001] zone axis. Key words: Threading dislocations, SiGe/Si(001) superlattices, transmission electron microscopy (TEM)

INTRODUCTION SiGe/Si strained layer structures have attracted much interest to produce new high-performance devices for integrated circuit technology. Therefore, many attempts have been made to improve the structural, electronic, and optical properties of SiGe/Si heterostructures, changing the growth conditions.1–6 Above the critical thickness of SiGe epilayers, structural relaxations are seen in the films introducing misfit and threading dislocations. The presence of dislocations at the interfaces of strained layer structures degrades the device features. Dislocation densities in such structures can be reduced by several methods, for example, the growth of buffer layers on Si substrates, the insertion of graded layers, the growth of superlattices, and a modified lattice mismatch. Epitaxial growth defects in SiGe/Si films grown by molecular-beam epitaxy (MBE) can be produced at the interfaces relaxing the strain in the films.7–14 Transmission electron microscopy (TEM) has been used to examine misfit and threading dislocations, precipitates, pagodas, carbon and oxygen-rich interfacial particles, surface steps, and crystal distortions in (Received April 6, 2004; accepted October 25, 2004) 612

the films. Generation and propagation of the defects have been explained at the interfaces considering the Matthews and Blakeslee mechanism. A number of annealing experiments have shown two orthogonal sets of misfit dislocations generated in a SiGe/Si strainedlayer superlattice at (SLS) 760°C and 940°C.14 The mobility and carrier concentration in SiGe/Si heterostructures can be changed due to annealing processes.15 The dominant scattering mechanism can be found to change to scattering due to uniformally doped impurities from the remote impurity scattering after annealing in samples with wider channel thickness. Thermal diffusion can cause an increase of impurity scattering and a reduction of channel width. The channel reduction causes the sample to be more sensitive to interface roughness scattering and gives rise to carrier localization in the extreme case. In order to improve electrical and optical properties of SiGe/Si, intermixing between Si and Ge should be controlled.5 Consequently a smooth abrupt interface can be produced to increase electron mobilities and light-emission intensities. A Si0.67Ge0.33/Si(001) superlattice was subjected to this study to examine the threading dislocations in the plan-view TEM samples by using the diffraction contrast and stereo electron microscopy techniques.

Study of Threading Dislocations in the Plan-View Sample of SiGe/Si(001) Superlattices by Transmission Electron Microscopy

Fig. 1. A view of misfit dislocations associating with a point contrast on them and at their ends.

EXPERIMENTAL PROCEDURE To improve structural, electrical, and optical features of SiGe/Si heterostructures, a Si0.67Ge0.33/ Si(001) superlattice was grown by MBE at 550°C. The superlattice has 21 repeated SiGe strained layers with a thickness of 200 Å. The thickness of the Si layer in the periodic structure is 450 Å. Plan-view TEM samples were prepared to image the interfacial defects, which are generated due to a mismatch between epilayer and substrate in the superlattice.12–14 A Philips EM430 electron microscope at 250 kV (H. H. Wills Physics Laboratory, Bristol, U.K.) was operated to display the defects and to achieve the stereo electron microscopy technique. RESULTS AND DISCUSSION Strain relaxation can occur by the presence of misfit dislocations at the interfaces of SLS’s. These dislocations in SiGe/Si(001) run along the interfaces

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– in the [110] and [110] directions on a (001) plane forming an orthogonal set.14 If they end in the interfacial plane, they thread up to the wafer surface, forming threading dislocations, or annihilate with other dislocations of the same Burgers vector.8,9 A – micrograph taken in g  220 in Fig. 1 demonstrates misfit dislocations that are generated due to a mismatch of 1.3785% between epilayer and substrate, at the interfaces of SiGe/Si SLS. It can be seen that the misfit dislocations associate with a point contrast on them and at their ends (some marked). Stereo electron microscopy exhibited that these point contrast features are threading dislocations and generate new dislocations at high temperatures.14 Two stacking faults, generated due to a compression in SiGe layers, are also seen in the micrograph in Fig. 1. Diffraction contrast experiments showed that these stacking faults are extrinsic.16 Figure 2 shows a pair of micrographs taken with the stereo electron microscopy technique. This has been achieved in the electron microscope with tilting stages by monitoring Kikuchi patterns and tilting – the specimen to maintain a (220) reflection. The visibility criterion has been considered when the pictures were taken under the two-beam condition on both sides of [001] zone axis. The first picture in – Fig. 2 has been taken in g  220 with an angle 5° away from the [001] zone axis, and the second one has been taken from the other side of the zone axis using the same angle and reflection. Thus, the tilt angle between the two micrographs is 10°. If we have a look at the misfit dislocations in these micrographs, we can see the differences of misfit dislocations in the two micrographs. For instance, the dislocation (end indicated) in the figure changes length in one of the pictures after tilting the specimen under the two-beam condition. Thus, the technique used reveals that these dots on/at the end of misfit dislocations are dislocations that thread up through the epilayer from one interface to another forming threading dislocations in Si0.67Ge0.33/Si(001)

Fig. 2. Stereomicrographs of a threading dislocation in the SiGe/Si superlattice.

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superlattice. The misfit dislocation in the figure runs – along the [110] direction and, at the point indicated in the picture, threads up through the epilayer to another interface. At that interface, it also runs along – – the [110] direction. The same a/2[011] type Burgers vectors have been obtained for these dislocations using the conventional grain boundary technique. The threading segment in Fig. 2 has a line direction u. In order to characterize this threading segment, it must be considered in three dimensions. Figure 3a shows a vector in three dimensions that is attributed to a dislocation line. The line direction of the vector, u, is represented by

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– – – u  a/√2[110]  b/√2[110]  c[001] This equation can be applied to the threading segment in Fig. 2. The stereographic projection in Fig. 3b and the sketch of the threading segment in Fig. 3c indicate in which directions a and b are to be taken. If the equation above is applied to this segment, the line direction of the segment is given as – –– –– u  a/√2[1 10]  b/√2[110]  c[001] Accordingly, a and b can be measured from the picture, then c calculated. The values of a and b were measured as 281 Å and 238 Å (2 Å), respectively,

Fig. 3. Schematic drawing of the threading dislocation in Fig. 2 showing how to take the measurements to find the line direction of threading segment.

Study of Threading Dislocations in the Plan-View Sample of SiGe/Si(001) Superlattices by Transmission Electron Microscopy

and c was found to be 2720 Å, as shown in the sketch in Fig. 3d. Thus, the threading dislocation line direc– tion is u  [108] putting a, b, and c values in the equation above. This line direction is close to the growth direction. The line directions for the threading segments indicated with arrows in Fig. 4 have been obtained following the same method above. The tilt angle between the pairs was 12°. The micrographs have – been taken in g  220. The line directions of thread– ing segments shown in the pictures are u  [2,0,15], [3,0,15], [105], and [107], respectively. These line directions are close to the [001] zone axis too. More experimental studies have been carried out on the threading dislocations in plan-view SiGe/Si(001) samples using the stereo technique. The results are summarized in Table I and Fig. 5, which is a stereographic projection. The angles between these line directions and the zone axis are also tabulated in the table.

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Table I. Line Directions and Angles between u and [001] Obtained u 107 2,0,13 2,0,15 3,1,14 105 3,1,13 108 109 – 2,0,19

Zone Axis

0 (°)

[001] [001] [001] [001] [001] [001] [001] [001] [001]

8.13 8.75 7.59 12.72 11.31 13.67 7.13 6.34 6.0

Consequently, the line directions of the threading dislocations in the plan-view TEM specimens are nearly parallel to [001]. Thus, the threading dislocations detected in SiGe/Si SLS may not lie on a slip plane. Therefore, the strain relaxation in the structure could occur by climb. The climb mechanism is

– Fig. 4. Stereomicrographs of threading dislocations in different plan-view samples, taken in g  220 with a tilt angle 6° away from [001].

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ACKNOWLEDGEMENTS The author thanks Professor David Cherns (H.H. Wills Physics Laboratory, Bristol University, Bristol, UK) for his help throughout the project. The TEM work was carried out at H.H. Wills Physics Laboratory. REFERENCES

Fig. 5. A stereographic projection showing the line directions of threading dislocations.

attributed to either diffusion of vacancies or the formation of interstitial atoms. Nevertheless, noticeable dislocation climb is improbable at the low growth temperatures. To prevent the relaxation of strain by diffusion or dislocation formation, SiGe/Si films can be grown by MBE at 500°C.17 For such a metastable layer in a device structure, several processing steps have to be employed, e.g., selective etching, implantation, thermal activation, and so on. The typical temperature for these steps is between 600°C and 900°C. Diffusion may take place depending on the concentration of Ge at these temperatures.17,18 Thus, a climb mechanism of threading dislocations could be expected.

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