Figures 8.1 (a) and (b) show the HRXRD spectra for the (004) reflection of InAs/GaSb SLS’s, containing 20 periods and grown at 510 °C with V/III ratios of 5.0/1.75 on (001) and 2° off (001) GaSb substrates, respectively. The (001) and misoriented substrates were placed on the Rand L positions of the susceptor, respectively. These SLS’s were grown with different gas switching sequences (details shown in Table 5.1) at the interface (therefore
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forming series A to E). Also included in the figures are spectra of sample M4493 which was not part of this series, but was grown with the same parameters (V/III ratios, periodicity and nominal layer thicknesses) as the layers in the series, and the same gas switching sequence as sample E. Even though sample M4493 differs from sample E in terms of quality, the existence of InSb-like interfacial layers (as shown by the zero’th order satellite peak that appears at a smaller angle than the substrate peak) is common in these two samples. The inclusion of sample M4493 is primarily for comparisons of its structural properties to that of layers forming the series. It is evident that the structural quality of M4493 is better than that of all the layers in the series, as indicated by the number of satellite peaks observed in the XRD spectrum (Bennett et al., 1993, Sankowska et al., 2012)).
Figure 8.1 HRXRD spectra for the (004) reflection of InAs/GaSb SLS’s (containing20 periods)
grown at 510 °C with V/III ratios of 5.0/1.75 on (a) 2° off (001) and (b) (001) GaSb substrate, to illustrate the dependence of the structural properties on the gas
The SLS’s grown on (001) GaSb substrates exhibited lower structural quality compared to their counterparts grown on 2° off (001) GaSb substrates. This is indicated by a lower intensity and a smaller number of distinguishable satellite peaks observed in the XRD spectra
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of SLS’s grown on the former type of substrate (Bennett et al., 1993, Sankowska et al., 2012). When the two spectra for sample A, grown on the two different substrate orientations, are compared, it is evident that the spacing between the satellite peaks of the SLS grown on a (001) substrate is larger than for the SLS grown on a 2° off (001) substrate. This is an indication of a shorter period (Bowen and Tanner, 1998). Also, the separation between the substrate peak and the zero’th order satellite peak is greater for the former type of substrate. This indicates that the perpendicular lattice mismatch between the substrate and the SLS is greater for the (001) substrate (Bragg’s law relates the position of the zeroth order satellite peak to the SLS average lattice constant (Bennett et al., 1993)). The difference in the lattice mismatch observed for the two SLS’s is ascribed to either unequal thicknesses of the GaAs- like interfacial layer (large interfacial layer thicknesses yield a high lattice mismatch (Jasik et
al., 2011)) or unequal thicknesses of the constituent layers, GaSb and InAs (as indicated by
the difference in satellite peak spacing) in the respective SLS’s. When the thickness of the constituent layers is small, interfacial layers form a bigger fraction of the SLS (Haugan et al., 2004). This means that an interfacial layer of the same thickness can lead to a larger lattice mismatch in a SLS with thinner constituent layers than in a SLS with thicker layers.
Due to the lower structural quality exhibited by layers grown on (001) substrates, the influence of the gas switching sequences on the interface type and/or quality is illustrated by considering only the SLS’s grown on misoriented substrates. As a reminder, the gas switching sequences included no growth interruption at either interface (Sample A), and a one second flow of TMIn followed by a four second flow of TMSb over the GaSb surface and a five second flow of H2 over the InAs surface (Sample B). Among the sequences was
also a flow of TBAs and TMSb over the GaSb surface (5 s) and H2 (5 s) over the InAs
surface (Sample C), a 5 s flow of H2 over both the GaSb and InAs surfaces (sample D) and a
flow of H2 (5 s) over the GaSb surface and TMSb (5 s) over the InAs surface (sample E).
Samples A to D have a GaAs-like interfacial layer, as indicated by the appearance of the zero’th order satellite peak at a larger angle than the substrate peak (implying a compression along the substrate normal), while sample E have an InSb-like interfacial layer. The appearance of the zero’th order satellite peak at larger angles than the substrate peak can be explained in terms of GaAs having a smaller lattice parameter as compared to both InAs and GaSb. As a result, the GaAs-like interfacial layer undergoes in-plane tensile strain, while undergoing compressive strain perpendicular to the substrate, which means a shift towards higher 2θ angles in the position of the zero’th order satellite peak (Jasik et al., 2011,
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Sankowska et al., 2012). The existence of an InSb-like interfacial layer leads to the opposite effect (Sankowska et al., 2012).
The presence of GaAs-like interfaces in all samples, except sample E, has been attributed to In segregation on the InAs to GaSb interface (Steinshnider et al., 2000). At the GaSb to InAs interface, this effect has been attributed to an As/Sb exchange (Kaspi et al., 1999, Magri and Zunger, 2001). According to Zhang et al., (Zhang et al., 2006), this could be due to the segregation of Sb atoms that is driven by the much smaller bonding strength of Sb (as compared to that of As) in the grown structure. Magri and Zunger (Magri and Zunger, 2002) found Sb and In to be the segregating species, with the latter having a much higher segregation energy. In sample B, this type of interface is attributed to the carry-over effect at the InAs to GaSb interface. In sample E, flowing TMSb for five seconds over the InAs led to the formation of an InSb-like interface. According to Kaspi et al. (Kaspi et al., 2001), the above mentioned phenomena can also promote the formation of InSb like interfaces at the InAs to GaSb interface, due to the high segregation energy of indium. The sample with the worst structural properties (sample C, for which satellite peaks are hardly distinguishable) was grown with a flow of TBAs and TMSb over the GaSb surface before the growth of the InAs layer. There is a possibility that As substitutes Sb on the GaSb surface, which leads to the segregation of Sb arriving at the surface, thus forming a second phase which promotes rough interfaces.
Among all the samples in the series, sample A (grown without growth interruption at the GaSb and InAs surfaces) exhibits the best structural properties (with up to 3 satellite peaks observed). The spacing between the satellite peaks of this sample is also uniform. The uniformity of the separation between satellite peaks in its spectrum is an indication of the same period throughout the SLS (Waterman et al., 1993, Bowen and Tanner, 1998). The other SLS that exhibited good structural properties is sample B. Up to 3 satellite peaks can be seen also in this spectrum. However, the separation between the zero’th order and first order satellite peaks was not the same as the one between the first and second order satellite peaks. The non-uniform separation of the satellite peaks might indicate a non-uniform periodicity in the SLS. The compressive strain observed in all the samples except sample E is an indication that the soaking of the surface terminated with the less volatile Sb atoms is not as effective as the soaking of the InAs surface. According to Kaspi et al. (Kaspi et al., 1999), the appearance of the zeroth order peak of the SLS below the angular position of the substrate peak means the average lattice parameter of the SLS is larger than that of the substrate.
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