In example I, from sample (c) it can be known that thermal cleaning in N2 ambience is also working to improve controllability somehow but not good enough. Thermal desorption in nitrogen might be helpful to remove adsorbed H atoms from the surface of Si and then the adsorption of gallium atoms. However, on the other hand, annealing in nitrogen may cause Si nitridation and deteriorate the growth on III-nitrides on silicon. This argument is reasonable because the edge area of the surface of sample (c) in example I was rough. It is hopeful that the combination of AlN coating and thermal cleaning in nitrogen might be more effective to improve growth controllability by reducing more adsorbed H atoms, yet to be tested.
Some other efforts also have been tried to improve reproducibility and avoid AlN coating.
Although AlN is working to enhance reproducibility, it is time consuming and costly. Adding thermal cleaning in mixture atmosphere of hydrogen and ammonia at high temperature in the end of a growth prior to cooling was tested, as shown in Fig. 2.20. Effect of this measure is not stable and more or less works. Like thermal desorption of Si wafer in nitrogen, it also might be harmful to the growth. Annealing in hydrogen at high temperature may lead to GaN decomposition and destroy the surface of a sample, as discussed in part 2.2.2. So this method was not applied anymore.
Applying AlN coating, experiments of V/III ratio test for AlN buffer layer like that in example II of uncontrollability test were repeated. The result of curvature was shown in Fig.
2.21. It is clear that the performance of GaN is strongly dependent on the growth conditions of AlN buffer layer, such as V/III ratio which shown here. Based on all the experiments above, AlN dummy coating is a practical and effective method to achieve high controllability for the growth of GaN on Si, although it consumes much time and is not recommendable to industrial production.
laboratory, a standard procedure to ensure successful growth of GaN on Si is proposed and established for the first time here. This procedure is shown in Fig. 2.22.
First of all, the purity of gases (ammonia and carrier gases of H2 and N2) and metal-organic sources must be high enough. Gas purity is critical for MOVPE, especially for growing AlN, because it is more sensitive to the gas purity than other composition like GaN.
In this work, almost all possible factors which might lead to Si melt-back had been checked and the problem couldn’t be solved until the gas purifiers for H2 and N2 was exchanged with new ones, although the species of impurity were not clear. After applying new purifiers, the dew point of both H2 and N2 decreased to around -90 ℃ and then water in the carrier gas could be removed.
Secondly, in order to achieve controllable growth, AlN dummy growth is compulsory prior to every real designed growth of GaN on Si. GaN and Ga deposition inside the reactor can contaminate the Si surface during thermal desorption and deteriorate the quality of GaN on Si. AlN dummy coating can cover the GaN and Ga deposition with AlN and keep Si surface from being contaminated by Ga. Without AlN dummy coating, GaN growth on Si would be uncontrollable and insensitive to the variation of growth conditions.
The next critical step is Si cleaning by wet chemical method. The particles, organic pollutants and Si oxide should be removed completely; otherwise GaN quality on it cannot be good. The detail of cleaning was shown in the third step in Fig. 2.22. It is followed by thermal cleaning in the reactor to desorb the possible residual pollutants on Si wafer surface.
Following it, TMAl pre-flowing must be performed to keep Si surface being nitridized.
However, the optimal TMAl pre-flowing time is depending on the type of reactor and TMAl flow rate. Then the following steps are the growth of AlN buffer layer, GaN layer, AlN interlayers, and top GaN layer and so on.
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3 Experimental observation of stress control by conventional AlN
As discussed in chapter 1, the critical problem for the growth of GaN on Si is stress control during and after growth to eliminate cracking. This chapter is about the experimental observation of stress behavior during the growth. The simplest structure of applying AlN buffer layer and AlN interlayers (ILs) to induce compressive stress in GaN layers is adopted in this work to understand the basic effects of AlN layers and elementary mechanisms of stress control. This chapter starts with section 3.1, an introduction, including interpretation of a curvature transition, possible strategies to realize zero curvature (bowing) at room temperature and a basic model to explicate how compressive stress was induced and relaxed in GaN layers. The effects of AlN buffer layer and conventional ILs grown under various conditions were discussed carefully in section 3.2 and 3.3 respectively. The adjustment of curvature by GaN layers was observed in section 3.4. The concept of conventional AlN mentions that AlN is grown under fixed conditions continuously in one step without any special techniques such as patterning, multi-step growth or pulse injection, which would be introduced in chapter 6.