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Jul 23rd, 2014
Gallium nitride thin-film transistors produced at less than 250°C
Turkey’s Bilkent University has used hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) to make gallium nitride (GaN) thin-film transistors (TFTs) at temperatures below 250°C [S. Bolat et al, Appl. Phys. Lett., vol104, p243505, 2014]. The researchers claim this as the lowest reported process temperature for GaN transistors.
Thin-film transistors are usually produced using amorphous silicon. While these devices are used as the driving element in liquid crystal displays, low carrier mobility, high fabrication thermal budget, and strong absorption of visible light limits applications and is not suitable for flexible and transparent electronics.
Layers of 77nm aluminium oxide and 11nm GaN were deposited in a window in the SiO2 at 200°C using a modified Fiji 200-LL ALD reactor from Ultratech/Cambridge NanoTech Inc. X-ray analysis showed the GaN film to be polycrystalline with average crystallite size of 9.3nm. The aluminium oxide provided the gate insulator and the GaN provided the channel; the bottom gate was the p++-silicon substrate itself.
Figure 2: (a) Output and (b) transfer characteristics of HCPA-ALD-based GaN TFTs.
Positive bias stress testing was carried out with measurement of the threshold voltage after applying a field of 2.5MV/cm between the gate and grounded source-drain electrodes. A shift in threshold voltage indicates charge trap states at semiconductor/insulator interfaces or within the aluminium oxide. The shift increased with duration of the stress stage of the test. At 200 seconds, the shift was around 5.5V; this increased to 7.3V for 1000 seconds of stressing. The researchers point out that the 1000-second shift in high-performance zinc oxide-based TFT is larger.
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