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 (35-55nm-thick) is compressively strained on the 250nm-thick relaxed AlN buffer layer grown on silicon substrate. The structure is then completed by an 8nm-thick AlN barrier and 2nm-thick GaN cap. Despite an ultrathin total epilayer (only approximate to 300nm-thick), a high 2DEG density (approximate to 2.7x10(13)cm(-2)) is measured by Hall effect. Room temperature mobility values, as high as 636cm(2)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 0.18mm, by using a basic fabrication process.