Distribution of fixed oxide charge in MOS structures with ALD grown Al2O3 studied by capacitance measurements

We analyze the fixed oxide charge in Al
₂
O
₃
grown by thermal and plasma enhanced ALD at 100 and 200 °C using capacitance-voltage measurements on MOS structures with different Al
₂
O
₃
thickness. For both ALD techniques and deposition temperatures, Al
₂
O
₃
shows negative fixed oxide charge with density in the range of 2-7×10
¹²
cm
^-2
, most likely located at the Al
₂
O
₃
/SiO
₂
interface as inferred from the linear dependence of flat-band voltage on Al
₂
O
₃
thickness. The break-down field ranges from 4 to 8 MV/cm illustrating a high quality of the Al
₂
O
₃
layers. Post-deposition annealing was found to stabilize the Al
₂
O
₃
dielectric constant determined to be similar to 9. Control of the negative Al
₂
O
₃
fixed charge represents a promising way to e.g. enhance the threshold voltage shift of GaN based MOS heterostructure FETs towards normally-off operation.