TECHNOLOGY MICROSCOPY “ At the critical sub-100-nm length scale, three-dimensional resolution
achievable by atom probe tomography offers unique insights into the structural and chemical properties of active device layers and interfaces for growth optimization
massive difference between the two samples (see Figure 5). A well-defined AlN interlayer exists in the sample grown by MBE, but for both MOCVD- grown samples there is no distinct AlN interlayer, but rather an unintentionally graded AlGaN alloy.
Armed with this knowledge, it is far easier to interpret the Hall data for these samples, which were gathered at room temperature and 77 K and obtained via the van der Pauw Hall technique (see table 1). Our measurements, which show a deviation of below 5 percent between the sheet resistances measured across each sample, reveal that the lower the gallium content in the AlN interlayer, the higher the sheet concentration. For the sample that has a 2 nm-thick interlayer and was grown by plasma-assisted MBE, we recorded higher mobilities due to reduced alloy
disorder and interface roughness scattering. Meanwhile, the MOCVD-grown equivalent with an unintentional AlGaN interlayer is plagued by the combination of increased alloy disorder and interface roughness scattering, which both drag down 2DEG mobilities. Interestingly, despite high gallium concentrations in the interlayer, a very high 2DEG mobility was measured in the MOCVD- grown heterostructure that nominally contained a 0.7 nm-thick AlN layer. Note that we cannot infer that unintentional gallium incorporation into AlN will occur in all MOCVD reactors. It is in fact quite probable that our particular gas flow sequence played a role in the unintentional incorporation of residual gallium in the boundary layer.
”
Values for electron mobility have been put in perspective by performing a measurement at 300K on an MOCVD-grown AlGaN/GaN heterostructure without an AlN interlayer. The mobility in this structure was 1600 cm2
/Vs, about 500 cm2 /Vs
lower than both the sample grown by plasma- assisted MBE and featuring a 2 nm-thick AlN layer, and the MOCVD-grown structure that was designed to have an AlN layer that is 0.7 nm-thick. Our work showcases the capability and value of site-specific atom probe analysis on electronic devices. At the critical sub-100-nm length scale, the three-dimensional resolution achievable by atom probe tomography offers unique insights into the structural and chemical properties of active device layers and interfaces for growth optimization. Atom probe tomography can thus be a very powerful tool in correlating the electrical properties of a device to material properties. These capabilities, along with the means to provide a wealth of quality information, will strongly impact the processing and analysis of devices and components.
Figure 5. Atom probe microscopy reveals that different growth techniques lead to variations in the composition of a thin interlayer that is nominally made of AlN. Measuremnents were made of the three-dimensional distribution of gallium atoms from AlGaN/AlN/GaN heterostructures grown by plasma-assisted MBE with 2 nm interlayer (a) and MOCVD with 2 nm and 0.7 nm interlayer ((c) and (e), respectively). Group III-site chemical composition profiles (b) (d) and (f) were taken from (a), (c) and (e), respectively
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www.compoundsemiconductor.net June 2013
£ This work was supported by the ONR DEFINE and DRIFT MURIs (D. Green, H. Dietrich, P. Maki) and made use of the central facilities supported by the NSF MRSEC at UCSB. A portion of this work was done in the UCSB Nanofabrication Facility, part of the NSF-funded National Nanotechnology Infrastructure Network.
© 2013 Angel Business Communications. Permission required.
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