A Study And Optimization Of N-polar GaN Materials

In recent years, much attention has been paid for N-polar GaN, because it has some superior characteristics, such as a good 2DEG confinement, the presence of natural back barrier, etc. However, N-polar GaN epi-layers still exhibit rough surface morphology and poor crystal quality. Of course, many growth parameters must be optimized. In the paper, the growth properity and its optimization will be discussed.All the samples used in this paper was grown with MOCVD and measured by PL, HRXRD, AFM etc.al.for the study of their basic character.With the compareation of AlN nucleation layer(NL) in Ga-polar and N-polar GaN materials, it is found that a much higher NH3 flow rate was used during the growth of N-polar NL to maintain the N-polar. Under this circumastance, NL exhibits a morphology which goes against the formation of uniform nucleation island of GaN epitaxial layer. According to this, a two-step growth method which combines high and low NH3 flow rate is present to optimize the material quality of GaN. With this kind of NL, the surface of GaN epi-layer becomes smoother and Oxigen(O) impurity concertration decreases because of AlN growth with low NH3 flow rate could trap more O atoms and less O atoms diffusion from the sapphire substrate to GaN epi-layer.The variation of N-polar GaN epi-layer growth property with different thickness is discusses in the second part of this paper. Thicker of the GaN epi-layer, less dislocation, stress contained in it.And, the variation of C impurity concentration is also studied.Finally, interlayers are used in our N-polar samples to obtain higher crystal quality.With low temperature GaN interlayers, three dimision island growth mode and stacking fault is induced into the epi-layer growth, which will be benifite to the dislocation annihilation.Our experiment indicate that, when low temperature interlayer is insertd between AlN NL and GaN epi-layer, the surface character get obviously promoted, at the same time, edge dislocation and screw dislocation are both reduced as predicted before.AlN interlayer is the other kind of interlayers used in this paper. This AlN interlayer is extremly thin with a thickness of 3nm to keep strain, and could induce stree in GaN epi-layer which enable the bending of dislocationalline especially edge dislocation. Our exeriment has confirm this effect through HRXRD and TEM measurement. The GaN morphology of sample with a AlN interlayer is smoother than sample without it.