MBE growth of high conductivity single and multiple AlN/GaN heterojunctions

Record-low sheet-resistance of $ 128 O=sq have been obtained in two-dimensional electron gases at ultrathin single AlN/GaN heterojunctions by optimizing the metal fluxes used in molecular beam epitaxy growth. Multiple 2DEGs have been found in AlN/GaN superlattices, with the net electron density measured 4 1 Â 10 14 cm À2 at room temperature. This very high electron density also leads to a further lowering of sheet resistance to values far below what is achievable in single nitride heterojunctions. These low resistance channels are induced entirely by polarization in nominally undoped heterostructures, and are attractive for various device applications such as high speed transistors and intersubband detectors. It has been shown that at single AlN/GaN heterojunctions, two-dimensional electron gases (2DEGs) with sheet densities up to $ 6 Â 10 13 =cm 2 are achievable, with the sheet resistance as low as 150 O=sq [1,2]. High electron mobility transistors (HEMTs) fabricated with such structures have shown high current densities more than 2.3 A/mm and transconductances up to 480 mS/mm [3]. Buffer leakage problems in such HEMTs have also been solved using ultrathin AlN back-barriers [4]. These results have shown that AlN/GaN heterojunctions are promising to push the performance of GaN HEMTs into a new level which the traditional AlGaN/GaN HEMTs are not able to reach. When grown lattice-matched to GaN, the AlN barrier has to be thinner than 5 nm to avoid strain relaxation [5]. An improved AlN/GaN interface has been proven to be important to achieve high 2DEG mobility and hence lower the sheet resistance [1]. These requirements sensitively depend on the MBE growth conditions. We find that the metal fluxes used in MBE growths showed important effects on the 2DEG transport properties in both single and multiple AlN/GaN heterojunctions. By optimizing the metal fluxes, a record-low sheet resistance of $ 128 O=sq has been achieved in single as-grown AlN/GaN heterojunctions. Using the optimal fluxes found for single AlN/GaN heterojunctions, high-mobility 2DEGs are found in AlN/GaN multiple quantum wells (MQWs). The details of these findings are discussed in this paper. 2. Single AlN/GaN heterojunctions The AlN/GaN heterojunctions studied here were grown by MBE in a Veeco Gen 930 system on Fe-doped semi-insulating 1 Â 1 cm 2 GaN templates prepared by metalorganic chemical vapor deposition (MOCVD) on sapphire. Active N 2 was supplied through a Veeco RF plasma source with the plasma power of 275 W, which yielded a growth rate …