TECHNOLOGY MICROSCOPY
conduction band offset and high polarization charge density at the AlN/GaN interface. AlN can also fulfill other roles: Inserting it between an AlGaN barrier and GaN channel layer improves the electron mobility by reducing alloy scattering.
In both classes of GaN HEMTs, the 2DEG is confined at the AlN/GaN interface with a net positive fixed polarization charge. The transport properties of this electron gas, which impact HEMT performance, are directly related to the roughness and abruptness of the interface, as well as the chemical purity of the AlN. Meanwhile, at the opposite AlN/GaN interface – which has a net negative
polarization charge – deep donor trap states form that are uncovered by deep level transient spectroscopy.
Extensive electrical characterization has been used to try and uncover the physical nature of this trap, but it remains a mystery.
If a characterisation tool is to help to uncover these physical properties of AlN/ GaN heterostructures, it will have to reveal compositional and interfacial profiles of polar AlN/ GaN interfaces, and expose the incorporation of unintentional impurities into the HEMT’s mobility- enhancing AlN interlayers.
crystals. This means that polarization-induced electric fields in N-polar heterostructures are opposite to those in the Ga-polar counterpart, inducing a 2DEG above the wide-bandgap barrier layer, instead of below it (see Figure 1). Thanks to this, N-polar GaN HEMTs with an inverted structure – a GaN channel, an Al(Ga)N barrier and a GaN-buffer – possess an inherent back-barrier that confines electrons and diminishes short-channel effects. The new architecture allows contact to the 2DEG through the channel layer, which has a narrower bandgap and lower surface barrier to electrons, compared with the wide-bandgap Al(Ga)N barrier. Ultimately, this means that N-polar devices have much lower ohmic contact resistances than conventional, Ga-polar HEMTs.
For both Ga-polar and N-polar HEMTs, the idea of employing AlN as the charge-inducing barrier is attracting much attention, because of the large
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www.compoundsemiconductor.net 53
Probing atomic structures Unfortunately, the widely used techniques for scrutinizing semiconductor structures are of limited assistance. Although high-resolution microscopes – namely Transmission Electron Microscopy (TEM) and Scanning TEM (STEM) – provide high lateral spatial resolution imaging and detailed structural information, they are limited to two-dimensional atomic resolution, and they fail to address the critical and fundamental capability of generating three-dimensional atomic resolution. This restriction to just two dimensions also hampers techniques for extracting chemical
Figure 1. Equilibrium band diagrams of (a) generic Ga-polar (0001) and (b) N-polar (0001) heterostructures
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