introduce

Gallium nitride has attracted widespread interest due to its unique properties and potential applications in optoelectronic and microelectronic devices. However, dislocation densities as high as 108 cm-2 in GaN heteroepitaxial layers can shorten the lifetime of GaN-based devices.  The chemical compatibility and lattice/thermal expansion matching between AlN and GaN make bulk AlN single crystals potentially suitable for GaN epitaxial growth. In addition, high thermal conductivity (340 W/m K) and high electrical resistivity make AlN ideal for high-power devices.  . AlN has a polar wurtzite structure consisting of closely packed hexagonal layers of alternating cationic (Al3+) and anionic (N3−) layers stacked together along the c-axis. Thus, the base plane can be positively or negatively polarized.  The polarity of AlN is very important to control the impurity doping and piezoelectric effect in epitaxial GaN films. This paper reports the etching study of AlN single crystals prepared by different sublimation growth methods in Hualin Kona.

experiment

We studied several crystals grown in different furnace and crucible materials. Sample A is a prismatic needle grown in a graphite heating element furnace using an NbC-coated graphite crucible; Sample B is a hexagonal platelet grown in the same furnace as a normal graphite crucible; Sample C is grown in a microwave oven; Sample D Grown in a tungsten heating element furnace with a tungsten crucible. Samples A, B and C employ a self-seeding mechanism, while sample D is a thick AlN film grown directly on a 6H-SiC (silicon side) substrate. Before etching, all samples were rinsed with hydrochloric acid for ten minutes to remove any impurities from the surface. To estimate the appropriate etching time for single crystals, we calculated the etching rate versus time for polycrystalline AlN samples under stirring conditions by measuring the mass and dimensional changes induced by etching. Based on this measurement, the standard etching conditions for single crystals were set at 60 °C for 10 min in a 45 wt% potassium hydroxide solution. After etching, all samples were rinsed in 38 wt% hydrochloric acid solution for 5 min to neutralize KOH residues.

Results and discussion

The SEM images of sample A (before and after etching) are shown in Figure 2. Obviously, the planes perpendicular to the base plane are not etched. Rapid etching was observed on the (0001) plane of the substrate, resulting in the formation of hexagonal hillocks. By analogy to the results reported for GaN, we conclude that the basal plane has nitrogen polarity. Etching also occurs on crystal planes inclined less than 90° from the basal plane (Fig. 2d). The hillock density of the crystal is about 5×107 cm-2.

Figure 3 shows the etching effect of sample b. Figure 3a shows the AlN crystal before etching; images 3b and 3c are for 10 min of etching, and all other images are for an additional 20 min of etching. Hexagonal hillocks are again observed on the (0001) base as shown in Figures 3c and 3d. Figure 3e gives a zoomed-in overview of these hillocks.  The diameter of the hillock is about 1 m in 3c (10 min etching) and about 2 m in 3d (additional 20 min etching). Considering the small crystal size and the self-convection of the solution at high temperature (60 °C), we believe that the loss of etchant is not significant, that is, the concentration of KOH does not change. We therefore conclude that the etch rate decreases with time as the area of the exposed (0001) plane decreases towards zero.

In summary

For AlN single crystals, initially the nitrogen polar (0001) basal planes are rapidly etched, while the aluminum polar basal planes and prism (1101) planes are not etched. The etch rate of the nitrogen-based plane eventually drops to zero because the surface is completely covered by hexagonal hills surrounded by the plane. The densities of the studied AlN crystals are usually between 5 × −2 and −2 . From our analysis of etched AlN crystals, we deduce that freely nucleated crystals have predominantly nitrogen to Al orientation, pointing from the nucleating surface, ie, the end of the AlN crystal facing the source is the polarity of Al.