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Currently, two main ELO technologies exist: the simpler one involves a single growth step after stripe opening. In this one-step-ELO (1S-ELO), growth in the opening remains in registry with the GaN template underneath (coherent part), whereas GaN over the mask extends laterally (wings). This leads to two grades, namely highly dislocated GaN above the openings, and low dislocation density GaN over the masks. With this technique, devices have to be fabricated on the wings. Therefore, conversely, in the two-step-ELO (2S-EL0) process, the growth conditions of the first step are monitored to obtain triangular stripes. Inside these stripes, the threading dislocations arising from the templates are bent by 90° when they encounter the inclined lateral facet. In the second step, the growth conditions are modified to achieve full coalescence. In this two-step-ELO, only the coalescence boundaries are defective. In depth characterisation of these ELO GaN layers reveals that the intermediate stages of the process induce an inhomogeneous impurity incorporation and stress distribution. However, the ELO technology produces high quality GaN, with TDs densities in the mid 106cn-2, linewidths of the low temperature photoluminescence (PL) near band gap recombination peaks below 1 meV, and deep electron traps concentration below 1014cm-3 (compared to mid 1015cm-3 in standard GaN). To further reduce the TDs density, multiple step ELO have also been implented. For applications such as read/write laser light sources of digital versatile disks, higher power and longer operation lifetimes are required, thus necessitating the production of better quality material. Several options are also currently available to pave the way towards self supported high quality GaN. These technologies involve growing thick GaN layers (possibly on MOVPE ELO GaN) and then separating the layers from the substrate. HVPE has proven to be a reliable method to grow GaN with growth rates ranging from 30 to 100 pm/hour. In thick layers (several hundred μm), the mecanisms used for the reduction of dislocations become more efficient. Separation from the starting substrate is currently achieved by either laser lift off, chemically or by strain induced. |
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CITATIONS
Cited by 4 scholarly publications.
Gallium nitride
Epitaxial lateral overgrowth
Dielectrics
Epitaxy
Fabrication
Heteroepitaxy
Laser applications
