Ga- and N-rich condition

The bottom-up method employing either MBE or MOVPE technique provides different possibilities, for growing the epilayer crystal which is decided by what growth condition is used. In this case, let us consider one of the III-V material: GaN crystal growth on our substrate.

To grow GaN, we need Ga and N atoms being supplied from the respective source, where Ga source cell and nitrogen gas or ammonia for MBE or TMGa and ammonia for MOCVD. In here, we can choose either the grown crystal is situated in Ga-rich or N-rich condition.

Depending on the what environmental that GaN crystal is grown, its structure is largely influenced by the majority of atoms dominating the growth of the crystal. It is well-established that Ga-rich condition prefers formation of GaN planar film-2D grow (for example a paper from Ramachandran C, et al from Carnegie Mellon University), while N-rich condition promotes GaN nanowire-3D grow (for example a paper from Callarco R and Marso M from Research Center Julich). There is an important point to be understood in here. What is actually Ga- and N-rich conditions?

I notice there are two important factors in determining how the growth is either Ga-rich or N-rich condition. First is the growth temperature and second is the flux contributing to the growth of GaN epilayer.

Growth temperature can be designed from the very beginning and we can directly assume what is actually the preferred condition is. Ga start desorbing at temperature around 600 C, both for GaAs (source) and GaN (source) system. Then Ga atoms situated at the growth above this range of temperature (higher than 600 C) will start to desorb from the surface of the substrate, and at the same time, only N atoms stay there. As a result, we have more N atoms and therefore this condition is referred as N-rich condition. For the growth conducted at the temperature below 600 C, Ga atoms will likely to stay in the surface of the substrate. This situation is called Ga-rich condition.

Determining flux is rather bit hard. One has to calibrate with the GaN planar film growth, where two condition has to be applied (Ga- and N-rich). To calculate Ga growth rate, the condition has to be in N-rich. Just imagine when the surface is saturated with N, then the only factor that governs the growth is limited Ga atoms due to the desorption. In the other hand, N growth rate can be calculated by using Ga-rich condition. Too much Ga atoms will saturate the substrate surface resulting Ga atoms can’t do much in controlling the structure growth. In fact, the limited presence of N will direct the growth of GaN. I learnt this from Heying et al (2000) and Koblmueller (2003). Next, one has to divide the ratio of Ga/N. If the value is more  than 1, it means it is Ga-rich condition, while value less than 1 is N-rich condition.

Growth of GaN nanowire itself is normally done at high temperature and Ga/N<1, implying the growth being done in N-rich condition. I encountered papers from Calleja’s group from Spain, where there is a possibility of having nanowire at Ga-rich condition (1, 2). Despite grown on high substrate temperature, Ga/N is more than 1. I am still looking the answer for this question. It might be that high growth temperature is more dominant?

This is approximately the same with other III-V semiconductor material, such as GaAs in Ga- or As-rich condition.