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Researchers show low temperature deposition of GaN for TFTs

Study demonstrates possibility of using GaN layers to make flexible transparent substrates

Researchers at the Bilkent University in Turkey have grown GaN thin film transistors (TFT) with a thermal budget below 250 degC. The study, they say, demonstrates the possibility of using low-temperature atomic layer deposition (ALD)-grown GaN layers to make stable flexible/transparent TFT devices.

TFTs, usually based on amorphous Si (a-Si), are the driving elements of liquid crystal display technology. However, due to low carrier mobility in a-Si, high fabrication thermal budget, and strong absorption of visible light, a-Si is not suitable for flexible and transparent electronics applications. Transparent metal oxides, in particular ZnO, have been proposed instead but stability remains an important problem.

As a transparent semiconducting material with a band-gap of 3.4eV, GaN is another option for the active layer of TFTs, however the two main deposition techniques for the use of epitaxial GaN films require high deposition temperatures.

To use GaN in settings with limited thermal budget, such as back end of line (BEOL) and flexible substrates, low-temperature deposition of GaN is being pursued using various methods such as sputtering, pulsed laser deposition (PLD), and atomic layer deposition (ALD).

ALD offers the most uniform and conformal deposition even at sub-nanometer thickness levels, say the researchers. In their paper in Applied Physics Letters, the team at Bilkent describe the development of a hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD)-grown GaN based TFT with the lowest reported thermal budget so far, keeping the entire layer growth and device fabrication steps below 250degC.

A 3D picture of the proposed TFT is shown above (a) , and a scanning electron microscope (SEM) image of the top view of the device is shown below (b).

The researchers report that the deposited GaN thin film has a polycrystalline wurtzite structure with a crystallite size of 9.3nm using GIXRD and LPA, respectively. Elemental analysis of the films revealed the low amount of oxygen in HCPA-ALD based GaN thin films. Output characteristics of the TFTs are obtained which show that the fabricated devices exhibit n-type enhancement mode field effect transistor behavior with clear pinch-off and saturation characteristics. Transfer characteristics of the devices show that the fabricated transistors have on-to-off ratios of 2 x 10 3. Finally, the effect of the positive gate bias stress on threshold voltage of the devices is studied, and reasonable threshold voltage shifts for a device with a considerably thick gate insulator are obtained.

Reference: Low temperature thin film transistors with hollow cathode plasma-assisted atomic layer deposition based GaN channels by S. Bolat et al, Appl. Phys. Lett. 104, 243505 (2014); http://dx.doi.org/10.1063/1.4884061

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