Using selective-area growth and selective-area etching on (−102) β-Ga2O3 substrates to fabricate plasma-damage-free vertical fins and trenches

Abstract

We have demonstrated selective-area growth and selective-area etching on SiO2-masked (−102) β-Ga2O3 substrates using a HCl-based halide-vapor-phase epitaxy system that is capable of performing both growth and gas etching without plasma excitation. Since the surface of the (−102) substrate is perpendicular to the (100) plane, which has the lowest surface energy, we were able to use both methods to fabricate plasma-damage-free fins and trenches with (100)-faceted vertical sidewalls on windows striped along the [010] direction with high processing accuracy. Furthermore, since the [010] window direction is aligned parallel to the majority of dislocations and line-shaped voids in the substrate—which extend along the [010] direction and could potentially act as leakage paths—such crystal defects are unlikely to appear on the surfaces of the resulting fins and trenches. We believe that these selective-area growth/etching techniques can greatly accelerate research on, and the development of, β-Ga2O3-based vertical/lateral devices with fins or trenches.

Plasma-free anisotropic selective-area etching of β-Ga 2 O 3 using forming gas under atmospheric pressure

“…However, the bottom surface of the etched areas showed relative roughness, which increased with the process temperature ( Figures 5(a4),5(a5), 5(b4) , and 5(b5)). Such surface roughness is consistent with observations from HCl gas etching on ( 02) substrates [ 25 ], suggesting that roughening is a typical result of gas etching on the ( 02) plane. The observed roughening of the ( 02) bottom surfaces relative to the (100) sidewall surfaces at elevated temperatures can be attributed to the difference in the roughening transition temperatures associated with the free energies of steps on the respective planes.…”

“…Figure 4 (a1) shows a top-view SEM image of an etched hole beneath a circular window, where the contour of the etched shape forms a hexagon elongated along the [010] direction. This shape points to the emergence of (100)- and {310}-faceted sidewalls, similar to findings from our previous HCl gas etching study on ( 02) substrates [ 25 ]. The (100) plane has the lowest surface energy density [ 26 ], while {310} planes, being oxygen-close-packing planes, are expected to have relatively low surface energy densities [ 28 ].…”

“…The aspect ratio of vertical anisotropy, defined as the height of the (100) sidewall divided by the side-etched length, varied from 7.0 to 8.4 at 750°C and from 13 to 30 at 950°C. These values surpass the aspect ratios of 2.7 to 6.7 reported for HCl gas etching on ( 02) substrates at 1038°C [ 25 ]. The capability of achieving vertical etching with smooth sidewall profiles highlights the potential of forming-gas etching as a viable plasma-free anisotropic dry etching technique.…”

“…This orientation coincides with the primary dislocation and void/nanopipe directions within the crystal [ 36–38 ], making it particularly suitable for vertical anisotropic etching. This avoids undesired crystal defects on the surface, favoring vertical trench and fin device applications [ 25 ]. For detailed methodologies and findings from our previous experiments on ( 02) substrates, including HCl gas etching, readers are referred to our published studies [ 25 , 39 , 40 ].…”

“…This avoids undesired crystal defects on the surface, favoring vertical trench and fin device applications [ 25 ]. For detailed methodologies and findings from our previous experiments on ( 02) substrates, including HCl gas etching, readers are referred to our published studies [ 25 , 39 , 40 ].…”