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Atomic-layer-deposition process enabled carrier mobility boosting in field-effect transistors through a nanoscale ZnO/IGO heterojunction.

Low-temperature (≤ 400 °C), stackable oxide semiconductors are promising as an upper transistor ingredient for monolithic three-dimensional integration. The atomic layer deposition (ALD) route provides a low-defect, high-quality semiconducting oxide channel layer and enables accurate controllability of the chemical composition and physical thickness, as well as excellent step coverage on nano-scale trench structures. Here, we report the realization of a high-mobility electron transistor (HEMT) in a ternary IGZO system using the ALD technique. The HEMT consists of a 10-nm-thick IGO layer as an effective transporting layer and a 3-nm-thick, wide band gap ZnO layer. The formation of a two-dimensional electron gas was enabled by controlling the band gap of the IGO quantum well through In/Ga ratio tailoring and reducing the physical thickness of the ZnO film. A field-effect transistor (FET) with a ZnO/In0.83Ga0.17O1.5 heterojunction channel exhibited the highest field-effect mobility of 63.2 ± 0.26 cm2/Vs, a low subthreshold gate swing of 0.26 ± 0.03 V/decade, a threshold voltage of -0.84 ± 0.85 V, and an ION/OFF ratio of 9 × 108. This surpasses the performance (carrier mobility of ~41.7 ± 1.43 cm2/Vs) of an FET with a single In0.83Ga0.17O1.5 channel. Furthermore, the gate bias stressing test results indicate that FETs with a ZnO/In1-xGaxO1.5 (x = 0.25 and 0.17) heterojunction channel are much more stable than those with a single In1-xGaxO1.5 (x = 0.35, 0.25, and 0.17) channel. Relevant discussion is given in detail on the basis of chemical characterization and TCAD simulation.