Nanopipe formation as a result of boron impurity segregation in gallium nitride grown by halogen-free vapor phase epitaxy

Abstract

The work reported herein demonstrated that nanopipes can be formed via a surfactant effect, in which boron impurities preferentially migrate to semipolar and nonpolar facets. Approximately 3 μm-thick GaN layers were grown using halogen-free vapor phase epitaxy. All layers grown in pyrolytic boron nitride (pBN) crucibles were found to contain a high density of nanopipes in the range of 1010 to 1011 cm−2. The structural properties of these nanopipes were analyzed by X-ray rocking curve measurements, transmission electron microscopy, and three-dimensional atom probe (3DAP) tomography. The resulting 3DAP maps showed nanopipe-sized regions of boron segregation, and these nanopipes were not associated with the presence of dislocations. A mechanism for nanopipe formation was developed based on the role of boron as a surfactant and considering energy minima. A drastic reduction in the nanopipe density was achieved upon replacing the pBN crucibles with tantalum carbide-coated carbon crucibles. Consequently, we have confirmed that nanopipes can be formed solely due to surface energy changes induced by boron impurity surface segregation. For this reason, these results also indicate that nanopipes should be formed by other surfactant impurities such as Mg and Si.

Device characteristics of enhancement mode double heterostructure DH‐HEMT with boron‐doped GaN gate cap layer for full‐bridge inverter circuit

“…However, as shown Figures and , the utilization of the lattice‐matched B‐doped GaN cap/AlGaN surface permits low leakage and the high I ON /I OFF current ratio without creating a strain or surface‐related defects, while higher V T in DH‐HEMTs is obtained. The high resistive and improved surface qualities of B in GaN cap layer prevent the leakage of electrons from the gate and offer higher V T and lower I gs in comparison to reported GaN gate HEMTs …”

“…Thus, the B-doped GaN cap layer is effective in restricting the gate tunneling current by raising the effective barrier height with increased gate to channel distance, and by capping the GaN/AlGaN surface with low conductive defects. However, as shown Figures 6 and 7, the utilization of the lattice-matched B-doped GaN cap/AlGaN surface permits low leakage and the high I ON /I OFF current ratio without 17,19 prevent the leakage of electrons from the gate and offer higher V T and lower I gs in comparison to reported GaN gate HEMTs. 9,12 Figure 8 shows the variation of transconductance g m for the B-doped GaN cap DH-HEMT devices with x = 0%, 1%, 3%, 5%, and 7% at V ds = 15 V. The peak g m is 0.182 S/mm, 0.179 S/mm, 0.176 S/mm, 0.15 S/mm, and 0.127 S/mm for the GaN, Al 0.01 Ga 0.99 N, Al 0.03 Ga 0.97 N, Al 0.05 Ga 0.95 N, and Al 0.07 Ga 0.93 N back-barrier/buffers, respectively.…”

“…The introduction of low bandgap B‐doping at GaN cap/AlGaN layer modifies the band structure as shown in Figure . The presence of B‐doping in GaN on AlGaN layer causes a large conduction band offset shows improved Schottky barrier height with low surface‐related defects would cause V T change toward positive (E‐mode type) V T = 1.92 V for Al 0.23 Ga 0.77 N/GaN/Al 0.07 Ga 0.93 N DH‐HEMT devices through the application of high quality B‐doped GaN cap layer on top of Al 0.23 Ga 0.77 N barrier with a GaN channel. It is to be noted that the obtained V T values are better than the reported GaN cap gate HEMT structures .…”

“…Recently, only a limited number of B-doped GaN, especially on metal-organic chemical vapor deposition (MOCVD) technique, has been reported. [16][17][18] The B dopant induces dislocation inclination to reduce dislocation density in the GaN film. 19 Because the atomic radius of B is smaller than both Al and Ga and larger than N, B doping in a GaN layer induces a local strain field.…”

“…In this effort, B is found to be a highly favorable dopant with possible high doping concentration in GaN. Recently, only a limited number of B‐doped GaN, especially on metal‐organic chemical vapor deposition (MOCVD) technique, has been reported . The B dopant induces dislocation inclination to reduce dislocation density in the GaN film .…”