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Plastic substrate deformation

Plastic deformation of the substrate is an important topic and often a limitation for the maximum thickness of prestrained layers to achieve crack-free GaN-on-silicon substrates [33-38]. At typical AlN and GaN growth temperatures

Nomarski microscopy view on two differently plastically deformed samples o

Fig. 3.4. Nomarski microscopy view on two differently plastically deformed samples on Si(111). The slip lines in Si are well visible and, depending on the strength of deformation and position on the substrate, striations are visible in one, two, or all three directions, including an angle of 120°. (Reprinted from [36] with permission.)

(>1000°C) silicon is a relatively soft material and does not withstand high stresses which occur for prestrained layers in excess of several-micron thickness. If plastic deformation occurs, the slip planes of Si are usually well visible by differential interference contrast (Nomarski) microscopy (Fig. 3.4). The deformation of the substrate, though, leads to a non-reproducible high bow, since once deformed the substrate is permanently bowed, even if the layer is removed. Weak plastic deformation, e.g., just the onset, can be acceptable, especially when only part of the wafer, such as the edges, are affected. However, very often this cannot be well controlled. Another effect that was observed for 100 and 150 mm substrates were single cracks with additional slip planes that appear reproducible only in specific regions of the wafer edge. These can be correlated with the impact of the substrate flat region on the substrate symmetry. Typically a substrate flat and to a smaller portion a substrate notch are breaking the substrate rotational symmetry and, when high stresses are applied, lead to a preferred anisotropy orientation of the bow. Therefore, the combination of the substrate flat and crystalline symmetry leads to slip lines and cracks occurring at the same single position of the wafer, but not being repeated at positions one would expect from the crystal symmetry solely, e.g., after a rotation of 120° and 240° for Si(111) substrates.

There are also differences in the stiffness of silicon substrates. These differences were already investigated in the 1980s [39]. Here it was found that Czochralski and float-zone grown substrates differ strongly in their stiffness, which was attributed to their impurity concentration. It has been observed that indeed substrates with high concentrations of dopants such as oxygen or nitrogen, but also typical dopants such as As or P, do decrease the effect of plastic deformation in GaN epitaxy. At the high growth temperatures of GaN on Si and the high compressive stresses that are required during growth to avoid crack formation after cooling, very high impurity concentrations are advisable which are not standard in the silicon industry, where a high purity is typically targeted by the crystal grower. Therefore, at present one is best advised to order special custom-made substrates to efficiently reduce the effect of plastic deformation and avoid differences for different Si substrate batches. Even a Si substrate with a slightly lower quality can be acceptable, since the impact of a lower quality on the AlN/GaN layers is rather small and the benefit of an efficient slip blocker can be bigger.

Due to their high crystalline perfection and typically high purity, float- zone substrates are very sensitive to plastic deformation, and the maximum layer thickness achievable is significantly lower when compared to Czochralski- grown Si.

Another way to postpone the effect of plastic deformation is by using thicker substrates. This not only delays the onset of plastic deformation but also reduces the overall bow during growth, and by this helps to improve layer homogeneity. But also thicker substrates are limited in thickness, due to the demands of the crystal growth and processing equipment. And even when wafer thinning is applied prior to processing, the higher cost also has to be taken into account.

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