III-nitride materials include gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) and their respective alloys (e.g., AlGaN, InGaN, AlInGaN and AlInN). In particular, gallium nitride materials (GaN and its alloys) have attractive properties including the ability to efficiently emit blue light, the ability to transmit signals at high frequency, and others. Accordingly, gallium nitride materials are being widely investigated in many microelectronic applications such as transistors, field emitters, and optoelectronic devices. Semiconductor structures that include gallium nitride material regions oftentimes include regions of other III-nitride materials (e.g., AlN). Such layers, for example, may function as buffer or intermediate layers positioned between a substrate and an overlying gallium nitride material region.
III-nitride materials have different crystal structures and properties than many common substrates including silicon, silicon carbide and sapphire. Thus, when III-nitride materials are formed on such substrates, these differences may lead to the formation of defects including dislocations. For example, dislocations may result from differences between the lattice constants of the substrate and the III-nitride material; differences between the thermal expansion coefficients of the substrate and the III-nitride material; as well as, substrate quality including misorientation and defects.
Dislocations are linear imperfections in a crystalline array of atoms. Types of dislocations include edge dislocations, screw dislocations and mixed dislocations (which have an edge component and a screw component). The presence of dislocations in III-nitride materials that are in the vicinity of the active region in a device can impair device performance. For example, the dislocations can function as scattering centers which effect electron transport and, thus, impair electrical performance. Also, the dislocations can function as non-radiative recombination centers which reduce performance of opto-electronic devices. Furthermore, dislocations can lead to inhomogeneities in composition and formation of macro-defects which can also negatively effect device performance.
Certain conventional vertical growth processes (i.e., growth that proceeds in a vertical direction from the underlying layer) form III-nitride material regions having screw dislocation densities of greater than about 1012/cm3. Lateral growth processes can produce localized areas within III-nitride material regions having low defect densities, while other areas within the regions have relatively high defect densities. Lateral growth processes are typically more complex than vertical growth processes.