TECHNOLOGY MICROSCOPY
interface between AlN and GaN, while the bottom interface between this pair of materials is diffuse. Meanwhile, the opposite situation occurs in our Ga-polar structure: The top interface is chemically diffuse, and the bottom one is abrupt.
What about artefacts? Artefacts can plague atom probe measurements. We took this into account when interpreting our results, investigating the possibility that the tip shape changed during the measurement, due to differences in the evaporation field of AlN and GaN. This evaporation field is higher for the former material, ruling out any possibility of an artefact causing our results, which would require an opposite state of affairs for differences in evaporation fields.
Our findings are strengthened by atom probe analyses on N-rich GaN/AlN/GaN heterostructures grown by both MBE-based techniques. Measurements confirmed observed asymmetry did not originate from the metal-rich, plasma- assisted MBE process. What we have concluded is that all GaN/AlN/GaN interfaces are asymmetric, and that inversion of crystal polarity reverses the characteristics observed in these regions, regardless of growth conditions (Figure 4a).
To provide an independent validation of our atom probe results, we employed STEM, high-angle annular dark-field imaging to scrutinise our N-polar samples grown by MBE with ammonia as the nitrogen source. A cross-sectional micrograph obtained with this technique reconfirms the asymmetry of the GaN/AlN/GaN interfaces, with the top interface appearing abrupt, while the bottom one looks diffuse. A line intensity profile across the GaN/AlN/GaN interfaces clearly shows this asymmetry. In agreement with the results from atom probe tomography, TEM images and line intensity profiles indicate a diffuse interface of about one unit cell.
The upshot of this study is that we have revealed how the polarity at GaN/AlN interfaces impacts their structural and compositional characteristics. If they are positively polarized, they are abrupt; but if they are negatively polarized, they are diffuse (this conclusion holds whether the heterostructure is Ga-polar or N-polar). It is possible to relate these findings to the well-known electrical properties of GaN/AlN/GaN heterostructures. The highly polar interfaces between GaN and AlN seek to minimize their electrostatic potential energy, through screening of polarization charges by free carriers. At a positively polarized interface, neutralization occurs via the accumulation of a 2DEG – electrons come from intrinsic donors, and for Ga-polar structures these come from the surface; but for N-polar, they come from traps. In contrast, it is not possible to neutralise a negatively polarized interface with a two- dimensional hole gas, because very few holes
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exist in unintentionally doped III-N materials. This observation tallies with the results of calculations by scientists at Rockwell International in the late 1970s. According to their work, interfaces with a large charge imbalance reconstruct and become chemically diffuse.
Adding AlN interlayers We have also studied three different AlGaN/AlN/ GaN HEMT structures that were grown by either plasma-assisted MBE or MOCVD. They feature an AlN interlayer that reduces the penetration of the electron wavefuction into the AlGaN barrier, and ultimately minimises the impact that alloy scattering can have on 2DEG mobility.
For MOCVD growth, the optimal thickness of the AlN layer is 0.7 nm, while for plasma-assisted MBE it is 2 nm. In addition, we grew a structure with a 2 nm-thick AlN layer by MOCVD: Although we knew that the thicker MOCVD interlayer would degrade material quality, we felt that it would provide a useful comparison with the MBE-grown sample. All samples were scrutinized with atom probe microscopy, and the findings compared with results of Hall electrical data. Atom probe microscopy proved very insightful, revealing a
Figure 4. (a) Chemical profiles of aluminium and gallium of the pure AlN layer grown by ammonia MBE (b) High angle annular dark-field STEM image of the AlN layer shown in (a). The step-terrace features at the interfaces were due to a 4º miscut. The enclosed region in (b) is enlarged in (c) along with the corresponding line intensity profile in (d)
The power of atom probe tomography is associated with its capability to generate three-dimensional maps of material with atomic resolution
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