MOVPE Growth of High Electron Mobility AlGaN/GaN Heterostructures
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ABSTRACT We have fabricated AlxGal_xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm 2 /Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5x10 12 cm- 2 . The undoped AlxGal-xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH 4 . The effect of intentional n-type doping of the AlxGal-xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped AlxGal-xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100A thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped AlxGal-xN. These initial results demonstrate that the electrical properties of AlxGaj_xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.
INTRODUCTION GaN and its alloys with AIN are currently under investigation as candidate materials for electronic devices operating at elevated temperatures. The high saturated drift velocity of electrons in GaN, combined with the large conduction band discontinuities and high mobilities achievable in AlxGal-xN/GaN heterostructures offers the potential of improved performance for GaN-based devices in high power microwave applications [1]. Prior work in this area has confirmed the existence of a twodimensional electron gas (2DEG) at the AlxGal-xN/GaN heterointerface [21, and AlxGal_xN/GaN high electron mobility transistors (HEMTs) with high transconductances and promising dc and microwave operating characteristics have recently been demonstrated [3-5]. Further advances in the performance of GaN-based HEMTs are expected to occur with improvements in material quality. However, to date, there has been relatively little work on characterizing the electrical properties of undoped and intentionally doped AlxGalixN/GaN heterojunctions. Initial AlxGal_xN/GaN heterostructures exhibited low 2DEG mobilities [6]. Possible reasons for this include high background carrier concentrations causing parallel conduction in unintentionally doped n-type GaN and AlxGal_xN layers and poor quality heterointerfaces. Increased 2DEG mobilities, in the range of 5000 cm 2 Ns at 150K, have recently been reported in heterostructures grown by metalorganic vapor phase epitaxy (MOVPE) [7]. The higher mobilities were attributed to improved AlxGal-xN made possible by the use of trimethylamine-alane as the Al precursor rather than the more conventional trimethy- or triethylaluminum sources. 201
Mat. Res. Soc. Symp. Proc. Vol. 395 0 1996 Materials Research Society
We have fabricated AlxGal.xN/GaN heterojunctions on (0001) sapphire substrates with high electron mobilities (5750 cm 2 /Vs at 16K) and sheet carrier densities (8.5x10 12 cm- 2 ) u
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