Extremely low nonalloyed and alloyed contact resistance using an InAs cap layer on InGaAs by molecular-beam epitaxy

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Abstract
Extremely low alloyed and nonalloyed ohmic contact resistances have been formed on n‐type InAs/In0.53Ga0.47As/In0.52Al0.48As structures grown on InP(Fe) by molecular‐beam epitaxy. To insure the accuracy of the small contact resistances measured, an extended transmission line model was used to extrapolate contact resistances from test patterns with multiple gap spacings varying from 1 to 20 μm. For a 150‐Å‐thick InAs layer doped to 2×1018 cm−3 and a 0.1‐μm‐thick InGaAs layer doped to 1×1018 cm−3, a specific contact resistance of 2.6×10−8 Ω* cm2 was measured for the nonalloyed contact, while a resistance less than 1.7×10−8 Ω* cm2 is reported for the alloyed contact. Conventional Au‐Ge/Ni/Au was used for the ohmic metal contact and alloying was performed at 500 °C for 50 s in flowing H2. Using a thermionic field emission model, the barrier height at the InAs/InGaAs interface was calculated to be 20 meV.