氮化铝单晶的湿法化学蚀刻

1 Introduction 

GaN has attracted vast interest due to its unique properties and potential applications in optoelectronic and microelectronic devices. However, the dislocation density in GaN heteroepitaxial layers as high as 108 cm-2 [1] shortens the lifetime of GaN based devices. The chemical compatibility and lattice/thermal expansion match between AlN and GaN make bulk AlN single crystals potentially suitable for GaN epitaxial growth. In addition, the high thermal conductivity (340W/m·K) and high electrical resistivity make AlN ideal for high power devices [2]. Recently, several groups [3] [4] [5] have reported growth of relatively large AlN single crystals, typically at millimeter square scale, produced by the sublimation process, originating from Slack and McNelly's work in 1970s [6]. Understanding the sublimation growth process and the quality of the AlN crystals it produces is key to producing large, low defect density substrates suitable for commercial device fabrication.

Studying the wet etching of AlN epitaxial fifilms on sapphire substrates in various acids and bases, Pearton et al. [10] concluded that only KOH etches AlN appreciably at room temperature, and the etch rate of AlN fifilms is a strong function of crystal quality. They inferred that annealing improves the quality of heteroepitaxial AlN fifilms, as their etch rate was lower than that of as-deposited fifilms.

2 Experimental 

We examined several crystals grown in different furnaces and crucible materials. Sample A was prismshaped needle grown in a graphite heating element furnace using a NbC coated graphite crucible; sample B was a hexagonal platelet grown in the same furnace with a plain graphite crucible; sample C was grown in a microwave-heated furnace; and sample D was grown in a tungsten heating element furnace with a tungsten crucible. Sample A, B, and C employed a self-seeding mechanism, while sample D was a thick AlN fifilm grown directly on a 6H-SiC (Si-face) substrate. Before etching, all samples were cleaned by hydrochloric acid for ten minutes to remove any impurities on the surface. To estimate the appropriate etching time for single crystals, we calculated etch rate of a polycrystalline AlN sample under stirred condition as function of time by measuring the mass and dimension changes due to etching. From this measurement, a standard etching condition for single crystals was set of 10 minutes at 60 °C in 45wt% KOH solution. After etching, all samples were rinsed in 38wt% HCl solution for 5 minutes to neutralize the KOH residues.

3 Results and discussion 

The etch rate of polycrystalline AlN as function of time is shown in Figure 1. Within an hour, the etch rate decreased over 70% from 0.55 µm/min to 0.15 µm/min. Since the solution was well stirred, we suspect that the decrease of etch rate was due to the depletion of the easiest etched crystal planes, instead of etchant depletion at the sample surface.

Fig1

4 Conclusions 

For self-seeded AlN single crystals, the nitrogen polarity (0001) basal plane initially etched rapidly, while the aluminum polarity basal plane, and prismatic (1101) planes were not etched. The etch rate of the nitrogen polarity basal plane eventually decreased to zero, as the surface became completely covered with hexagonal hillocks which were bounded by {1101} planes. The hillock density for the self-seeded AlN crystals studied was typically in the range of 5×107 cm-2 to 109 cm-2. From our analysis of etched AlN crystals, we infer that freely nucleated crystals predominately have the nitrogen to aluminum direction pointing out from the nucleation surface, that is the ends of the AlN crystals facing the source are aluminum polarity