Abstract
In this work, an AlGaN/GaN-HEMT heterostructure is exemplarily studied by a strict place-to-place correlational approach in order to help clarify some open questions in the wide field of reliability topics. Especially, vertical leakage current, its relation to dislocations in general, and specific types in particular are investigated on a highly defective material. With the aid of atomic force microscopy (AFM) in tapping mode, cathodoluminescence imaging, defect selective etching, and energy dispersive X-ray, the material’s defect content around the device relevant two dimensional electron gas is analyzed. The total dislocation density, as well as the density of threading screw, edge, and mixed type dislocations, is systematically investigated directly. The obtained result is statistically much more significant than is possible by conventional transmission electron microscopy studies and more precise than the results obtained by the indirect method of rocking curve analysis. The method of conductive AFM allowed mapping of variations in the vertical leakage current, which could be correlated with variations in barrier leakage or gate leakage. Spots of locally high leakage current could be observed and directly assigned to dislocations with a screw component, but with significant differences even within the same group of dislocation types. The electrical activity of dislocations is discussed in general, and a fundamental model for a potential dislocation driven vertical leakage is proposed.
Highlights
Due to its outstanding properties, GaN is one of the most promising materials for power electronics and high frequency applications and has been a subject of intense research over the last few years.[1,2,3,4] Most commonly used devices are AlGaN/GaN high electron mobility transistors (HEMTs), making use of the piezoelectrical nature of GaN and the formation of an electrically highly conductive two dimensional electron gas (2DEG) located right below the heterointerface.[5]
The result from topography measurements by Tapping-atomic force microscopy (AFM) is depicted in Fig. 1 and is typical for GaN-capped AlGaN/ GaN-HEMT heterostructures: flat terraces separated by atomic steps are indicative for step flow growth
Small depressions are expected to form at locations of threading edge dislocations (TEDs), whereas medium ones are related to threading screw dislocations (TSDs) and large ones to mixed types
Summary
Due to its outstanding properties, GaN is one of the most promising materials for power electronics and high frequency applications and has been a subject of intense research over the last few years.[1,2,3,4] Most commonly used devices are AlGaN/GaN high electron mobility transistors (HEMTs), making use of the piezoelectrical nature of GaN and the formation of an electrically highly conductive two dimensional electron gas (2DEG) located right below the heterointerface.[5]. A huge effort has been spent on reducing the dislocation density on the one hand and on providing a vertically good electrically insulating behavior of the whole layer stack on the other hand Today, this is achieved by growing thick multi-layered buffer structures to adapt the mismatched crystal lattices and thermal expansion coefficients of Si and GaN and by growing C-doped GaN-layers for compensating the intrinsic n-type behavior of GaN to a semi-insulating one.[20–22]. The dislocation density of AlGaN/GaN-HEMT structures grown on Si is typically in the order of 109 cm−2, i.e., many dislocations are covered by a conventional device.[25] This makes it generally difficult to perform such sophisticated investigations. Direct evidence is given as to what dislocation types are most critical with respect to vertical leakage in our investigated AlGaN/GaN-HEMT heterostructure grown on Si, and the distribution of those dislocations in the device relevant region around the 2DEG is analyzed. The presented work-flow and results are important for upcoming studies involving test structures and for directly drawing conclusions from the underlying material about the device characteristics
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