Ultrathin AlN‐Based HEMTs Grown on Silicon Substrate by NH3‐MBE

AlN‐based HEMTs grown on silicon by ammonia‐assisted molecular beam epitaxy (NH3‐MBE) are demonstrated and studied. As shown by photoluminescence, the very thin GaN channel (35–55 nm‐thick) is compressively strained on the 250 nm‐thick relaxed AlN buffer layer grown on silicon substrate. The structure is then completed by an 8 nm‐thick AlN barrier and 2 nm‐thick GaN cap. Despite an ultrathin total epilayer (only ≈300 nm‐thick), a high 2DEG density (≈2.7 × 1013 cm−2) is measured by Hall effect. Room temperature mobility values, as high as 636 cm2 V−1 s−1, are measured. They are higher than those reported on similar structures grown on sapphire, SiC and bulk AlN substrates, showing the interest of growing such structures on silicon substrate. Also, low contact resistance values are obtained, as low as 0.18 Ω mm, by using a basic fabrication process.