Abstract
We report on our multi–pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3μm)/ AlN(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X–ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson–Schrödinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2–DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance.
Highlights
We report on our multi–pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3μm)/ as well in the interlayer (AlN)(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE)
The total V–defect density estimated from atomic force microscope (AFM), scanning electron microscope (SEM) and plan view transmission electron microscope (TEM) images are 1.1±0.2×109 cm−2, 2.4±1×109 cm−2 and 4.5±2×109 cm−2 respectively
We surmise that this reasonably high mobility is due to the smooth interfaces between the Al(Ga)N/GaN and InAl(Ga)N as revealed by scanning transmission electron microscope (STEM)
Summary
In the nitride ternary alloys the optimum growth temperatures of the end compounds are quite different, especially using metal organic vapor phase epitaxy (MOVPE), i.e. AlN (> 1200◦C), GaN (≈ 1000◦C), and InN (< 550◦) This is the case for their covalent bond lengths;[6] the growth conditions need to be very well controlled such phenomena as the predicted miscibility gap in these alloys 7 may lead to phase separation,[8,9] ordering,[10,11,12] composition fluctuations[13,14] and even growth disruption.[15] Poor growth conditions can give rise to layers containing high densities of crystallographic defects[16,17,18,19] and even cracks.[20] Recently unintentional Ga incorporation in the InAlN layers has been reported which adds to the list of growth challenges for InAlN thin films.[21,22,23,24] There are two possible explanations given for the unintentional Ga incorporation in the InAlN layers. Possible routes to minimize unintentional Ga incorporation and the role of unintentional Ga incorporation on the HEMT properties are discussed
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