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GaP for THz

Our evaluation study of GaP/Si0.8Ge0.2/Si(001) heterostructures for potential HBT applications started with theoretical consideration of critical thickness (hcrit) for pseudomorphic GaP growth on pseudomorphic Si0.8Ge0.2/Si(001) substrates. Preliminary theoretical model calculations by Fischer et al. [10] were carried out revealing a critical thickness of GaP on Si(001) of about 64 nm (Fig. 1). Although the calculated value by this model is a thermodynamical value and experimental parameters are usually higher due to kinetic hindrance for defect injection in pseudomorphic layers, the value hcrit of GaP might be too low, as typical HBT designs require a collector thickness in the range of several hundred nanometres [2]. In this respect, it is interesting to point out that nitrogen incorporation in GaP can be used to reduce the misfit and thus to substantially increase hcrit (see Fig. 1) [11].

Fig. 1

 

 

First experimental results of 170 nm thick GaP layers deposited by GSMBE on top of 20 nm Si0.8Ge0.2/Si(001), 4°-off oriented substrates were obtained. Cross section TEM studies (Fig. 2(a)) show a crystalline and closed, but still defective GaP layer. Nevertheless, no clear indications of misfit dislocations were found in our HRTEM images. Most of these observed defects are SF and MT, which are located near the interface between GaP and Si0.8Ge0.2. The AFM study (Fig. 2(b)) shows in addition over a bigger scale (2 x 2 µm²) a surface root mean squared (rms) roughness of 19.6 nm. These results report strong evidence that these defects are mainly growth defects. Such growth defects do not nucleate by plastic relaxation of strained closed 2D thin film structures, but mostly during the coalescence process of a film structure formed by initial 3D island nucleation processes [12-13].

Dark field {002} TEM technique was used to detect the presence of APDs in the GaP layer by the characteristic contrast change [7, 8, 12]. However, the observed APDs (in form of triangular shaped structures located near the defective GaP/Si0.8Ge0.2 interface) disappear by self-annihilation of crossed anti-phase boundaries after about 70 nm GaP thickness. It is mentioned that, besides APD detection, SF´s are found inside and outside of APDs (indicate by arrows in Fig. 2(c)).

Fig. 2

To determine the epitaxy relationship and the pseudomorphic character of the GaP/Si0.8Ge0.2/Si(001) heterostructure on a global scale, different XRD measurements were performed. Figure 3 shows for instance a reciprocal space mapping (RSM) of the asymmetric (-2 -2 4) reflections of Si, GaP and Si0.8Ge0.2 Bragg peaks. The observable sharp Si(-2 -2 4) signal originate from the high quality Si(001) substrate. In comparison, the Si0.8Ge0.2(-2 -2 4) reflection depicts a lower signal intensity and exhibits an ellipsoidal shape. The small full width at half maximum (FWHM) value in Qx direction indicates also a high crystal quality of the SiGe layer. Due to the finite thickness of 20 nm, FWHM parameter in Qz direction is bigger than in case of the Si(-2 -2 4) signal. In consequence of identical Qx positions of the Si0.8Ge0.2(-2 -2 4) and the Si(-2 -2 4) signals, both layers have the same in-plane lattice constant. Otherwise, a relaxation of Si0.8Ge0.2 would be expressed by a Qx peak shift to a position between Si(-2 -2 4) peak position and the (0,0) origin direction of reciprocal space (indicated by arrow in Fig. 3). In summary, no relaxation processes has taken place so that the 20 nm Si0.8Ge0.2 layer remains pseudomorphic on Si(001) after 170 nm GaP deposition. This is an important result for potential HBT collector application of GaP [2].

In contrast, the deposited GaP layer is characterised by a broad GaP(-2 -2 4) reflection with diffusive scattering, suggesting the presence of structural defects [14]. In addition, GaP(-2 -2 4) is neither situated at its Qz bulk position nor at the value estimated with the help of the Poisson ratio for pseudomorphic GaP [5]. Further, the Qx position is also partly shifted (towards arrow in Fig.3), confirming a partial relaxation of the deposited GaP layer. This result indicates that the 170 nm deposited GaP layer is crystalline and (001) oriented, but contains structural defects and grows partially relaxed (≈ 40%) on the Si0.8Ge0.2/Si(001) substrate.

Fig. 3

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