TECHNOLOGY INTEGRATION
of templates with a smooth ELOG InP layer that has an overall width of around 15 μm.
We have used these templates as a foundation for the MOCVD growth of InGaAsP-based multi-quantum well structures emitting at 1.55 μm. Forming such structures has not been easy, due to loading effects – selective growth can result in thicknesses and composition of the wells and barriers being far different from those on a planar substrate. However, after conducting many experiments, we have refined the dielectric mask pattern so that the loading effect is reduced during MOCVD growth.
The HVPE reactor at KTH has been used to form InP layers on silicon substrates with SiO2
stripes.
of InP these defects can climb, but what is interesting – and critically important – is that they follow a specific angle associated with a crystal plane. What this means is that if the ratio of the thickness of the opening to the height of this trench is chosen carefully, defects can annihilate at the walls, and therefore block defects not only beneath the mask, but also within the opening.
By carrying out a series of experiments, we have optimised the growth of coalesced ELOG InP in ten openings that are 300 nm across and spaced 1 μm apart. This has led to the formation
Figure 2. Atomic force microscopy reveals the high degree of flatness of coalesced ELOG InP on silicon.
Growth of a polar material onto a non-polar layer – such as the growth of InP on silicon – can create anti-phase domains, which are formed when bonds between identical atoms occur along particular directions. These domains can be eradicated by turning to growth on off-orientated substrates, but even this step may not prevent new defects arising. That’s because imperfections can also appear at the coalescent junction between two parallel growth fronts, and form due to the interaction of an epitaxial layer with the dielectric thin film. So, in order to achieve a defect-free epitaxial layer all over the dielectric mask, it is essential to avoid coalescence of parallel growth fronts and obtain a dielectric film with a smooth surface and sidewalls.
Figure 1. Epitaxial layer overgrowth holds the key to forming defect-free InP on silicon.
Thanks to these insights, we have formulated our experiments in such a way that we have been able to grow almost-defect- free, isolated large areas of ELOG InP on silicon (see Figure 4). These un-coalesced stripes of InP were formed via growth in 1 μm-wide openings separated by 20 μm. The openings were defined by conventional optical lithography −
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www.compoundsemiconductor.net Issue VI 2014 Copyright Compound Semiconductor
Atomic force microscopy reveals the smooth epitaxial surface after multi-quantum-well growth (see Figure 2). However, this active region is by no means perfect, and there is inhomogeneity in the thicknesses of the multi-quantum wells, according to images provided by transmission electron microscopy (see Figure 3). It is possible that this is caused by the tremendous loading effect during selective growth of multi-quantum wells on a very small area. However, it might also result from unevenness caused by the coalescence of parallel growth fronts during ELOG.
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