Mg-doping and free-hole properties of hot-wall MOCVD GaN

Abstract

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range ([Formula: see text] cm−3 up to [Formula: see text] cm−3) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance–voltage (C–V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art [Formula: see text]-GaN layers with a low resistivity and a high free-hole density (0.77 [Formula: see text] cm and [Formula: see text] cm[Formula: see text], respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures.

Incorporation of Magnesium into GaN Regulated by Intentionally Large Amounts of Hydrogen during Growth by MOCVD

“…We point out that the atomic concentration of Mg in the reference GaN layer H19 is below the threshold value beyond which the atomic concentrations of Mg and H decouple (Figure 1a) and a saturation in the incorporation of MgH complex becomes relevant upon high levels of Mg doping. [10,11] Next, the reference H19 has been repeated by maintaining the same Cp 2 Mg/TMGa precursor ratio, except for the hydrogen flow being set to ever larger amounts of 21, 23, and 25 slm, and the layers have been accordingly labeled as H21, H23, and H25. The individual Mg depth profiles measured in each of the layers have been plotted in Figure 2a.…”

“…We point out that the atomic concentration of Mg in the reference GaN layer H19 is below the threshold value beyond which the atomic concentrations of Mg and H decouple (Figure 1a) and a saturation in the incorporation of MgH complex becomes relevant upon high levels of Mg doping. [ 10,11 ]…”

Mg segregation at inclined facets of pyramidal inversion domains in GaN:Mg

“…The amount of Mg bound to PIDs ( ) could account for the concentration of electrically inactive Mg ( ) measured in this sample 14 . This would imply that the formation of PIDs is the main limiting factor of p -conductivity when using Mg concentrations in the low-to-mid and above.…”

“…However, it should be noted that the difference between the H-passivated Mg acceptors before annealing of (assuming all the available H is bound to Mg) is larger than the net acceptor concentration after annealing ( ). This suggests the presence of compensating donors, most probably nitrogen vacancies, V , and Mg-V complexes, the density of which depend on the MOCVD growth conditions 14 .…”

“…Post-growth annealing is therefore required for the H dissociation and out-diffusion to achieve p -type conductivity in GaN 12 . However, at high Mg concentrations the total Mg concentration is higher than the ionized acceptor or free hole concentrations, meaning Mg is only partially incorporating into electrically active sites or the Mg acceptors are partially compensated 13 , 14 . The whereabouts of the inactive dopants have been discussed and include the formed structural defects.…”