Optimization of Metamorphic Materials on GaAs Grown by MBE

Licentiatavhandling, 2010

Advanced epitaxial technologies such as molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE) have enabled the idea of semiconductor heterostructures, which built up the foundation of the fast developing information and communication technology nowadays. Lattice mismatch has been a problem limiting designs of semiconductor heterostructures. Restricted by the availability of large and high-quality commercial substrates, only a small range of materials with a lattice constant close to certain substrate, such as GaAs and InP, could be chosen. Metamorphic growth is one of the solutions by which a virtual substrate with a desired lattice constant can be obtained after growing a mismatched but nearly relaxed buffer layer on a conventional substrate. The main challenges of this method are the rough interface and a high threading dislocation (TD) density in the active region of the devices. The TD problem is more severe for optoelectronic devices, such as lasers, which have a large device area, and therefore can easily contain TDs in the active region. Although there have been notable progresses for metamorphic optoelectronic devices in recent years, further reduction of the TD density is still required to improve the performance and make them competitive with existing products. In this work, we investigate and optimize growth schemes of metamorphic buffer layers grown on GaAs substrate by MBE. Effects of both n- and p-type doping on material quality in alloy graded InGaAs buffers with different parameters, such as grading profiles, grading slopes, In compositions and thicknesses are studied systemically. Moreover, further TD reduction by nitrogen incorporation in metamorphic buffers is demonstrated. The physical origin is found to be due to both the strain and the lattice hardening effect. These results show that by proper designs of the metamorphic buffers there are great potentials to further improve the quality of metamorphic heterostructures and enhance the performance of metamorphic optoelectronic devices.