Improving the electron mobility of polycrystalline InN grown on glass substrates using AlN crystalline orientation layers

Thin-film transistors (TFTs) are key components in large-area electronic systems such as liquid crystal displays and micro light-emitting diode displays.^1–4 The most popular materials for the channel semiconductors of TFTs are amorphous silicon and polysilicon. However, amorphous silicon suffers from low electron mobility, and polysilicon has limited scalability due to the complexity of its fabrication process. The other group of channel materials is metal oxides such as InGaZnO, but it also exhibits low electron mobility. Hence, new materials that simultaneously offer high electron mobility and production scalability are highly sought after.

InN has the highest electron mobility and peak electron velocity among the group III nitrides,^5,6 making it one of the most promising materials for TFTs. In addition, it is known that the optimum growth temperature for (In-polar) InN films falls within 400–500 °C,⁷ at which glass substrates for TFTs can survive. Although growth of high-quality InN epitaxial films has been achieved by the use of plasma-assisted molecular-beam epitaxy (MBE)^5,7–11 on single-crystalline substrates including GaN, AlN, and Si, there are few reports on the detailed characteristics of InN films deposited on glass substrates.¹² We have recently reported that c-axis-oriented InN films can be grown on glass substrates covered with amorphous HfO₂ orientation layers.¹³ However, as HfO₂ layers were prepared by atomic layer deposition (ALD) using a metalorganic gas, they contain carbon impurities and organic contaminations.¹⁴ These contaminants are possibly desorbed from HfO₂ and incorporated into InN films during the growth process. In this study, we investigate the use of sputtered AlN as an orientation layer for improving the crystallinity and electron mobility of InN films on glass substrates. It is known that sputtered species with high kinetic energy result in the formation of densely packed c-plane AlN.¹⁵ Moreover, we fabricate InN-TFTs with AlN orientation layers to demonstrate a practical application of the developed InN films.

AlN with a thickness of 2–30 nm and InN with a thickness of 60–195 nm were grown on synthetic SiO₂ glass substrates (strain point of ∼1000 °C) in the same pulsed sputtering deposition (PSD) chamber. The growth temperature of InN was fixed at 460 °C, whereas AlN was grown at temperatures ranging from room temperature to 700 °C. The deposition rates of AlN and InN were approximately 175 nm h⁻¹, which were controlled by tuning sputtering power of Al and In targets, respectively. InN was grown under the In-rich condition so that In adlayers and droplets form on the growth front. After the growth, the residual In droplets were removed by HCl solution (∼10M) to ensure precise electrical characterization. The crystallinity, surface flatness, and electron mobility of the InN layer were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Hall-effect measurements. The InN-TFTs with AlN orientation layers were fabricated by conventional photolithography followed by a lift-off process. The structure of the TFT will be described in detail later.

Figure 1 shows the XRD patterns of 185-nm-thick InN films grown at 460 °C on glass substrates with and without an AlN orientation layer. The InN film grown without the AlN orientation layer exhibited XRD peaks assigned to various diffractions, including $101¯0$⁠, 0002, $101¯1$⁠, $112¯0$⁠, and $112¯2$⁠, indicating that the InN directly grown on glass has a polycrystalline structure without any preferred orientation. This finding is consistent with that in our previous study.¹³ However, InN grown on a 10-nm-thick AlN layer only exhibited XRD peaks assigned to 0002 and 0004 diffractions, indicating that the insertion of AlN causes the InN to grow with a c-axis orientation. In addition, the XRD of the InN films recorded with a high-resolution X-ray diffractometer (Bruker D8 DISCOVER) revealed that the full width at half maximum of a 0002 X-ray rocking curve for the InN grown on AlN/glass was 2.1°, whereas that for the InN directly grown on glass could not be calculated due to its weakness. SEM images of InN films grown with and without the AlN orientation layer (shown in Fig. 2) showed that the surface flatness of the InN was improved by the presence of the AlN layer. The microstructure of the InN grown on the AlN layers was further investigated with bright-field transmission electron microscopy (TEM) as shown in Fig. 3. The TEM image shows that the 143-nm-thick InN film grown at 460 °C atop an 11-nm-thick AlN layer on a glass substrate contained fine crystallite columns with widths in the range of 20–35 nm. Furthermore, selected area diffraction revealed that each grain consisted of c-axis-oriented single-crystal InN, but the in-plane crystallographic orientation of the InN grains was randomly distributed.

Next, the effects of the growth temperature and thickness of the AlN orientation layer on the crystallinity of the resulting InN were investigated. Figure 4(a) shows the XRD patterns for 185-nm-thick InN films grown on AlN orientation layers with various thicknesses ranging from 0 to 30 nm with constant AlN and InN growth temperatures of 460 °C. Figure 4(b) shows the XRD patterns for 185-nm-thick InN films grown at 460 °C on 10-nm-thick AlN. The deposition temperature of AlN was ranged from room temperature to 700 °C. All of the InN films that were grown on AlN orientation layers in Fig. 4 exhibited XRD peaks corresponding to 0002 and 0004 diffractions regardless of the AlN thickness and growth temperature.

In addition, the electrical characteristics of the InN films prepared with and without AlN orientation layers on glass substrates were evaluated. Figure 5(a) shows the relationship between the thickness and the electron mobility of the polycrystalline InN film. The highest electron mobility, 427 cm² V⁻¹ s⁻¹, was observed in the 195-nm-thick InN film grown on an AlN orientation layer. The results show that the electron mobilities of InN grown with AlN orientation layers were much higher than those of InN grown directly on the glass substrate without the orientation layer. This result indicates that controlling the crystalline orientation of the polycrystalline InN is important to achieve high electron mobility. Although the results clearly indicated that the electron mobility increases with the thickness of the film, we focus on the thinner InN films in the following experiments because they are more suitable for TFT applications, in which the whole InN layer must be depleted by applying a bias. Electron mobilities of the polycrystalline InN grown on glass substrates are lower than epitaxial InN films grown on AlN/sapphire templates by MBE,¹⁶ which indicates that electron is surely scattered by the grain boundaries of InN. Figure 5(b) shows the correlation between the electron mobility and the electron concentration in InN films prepared with and without AlN orientation layers on glass substrates. For comparison, data for InN prepared on glass with HfO₂ orientation layers¹³ are also shown. The electron concentrations of the InN films grown with AlN are lower than those grown without it. In contrast, the InN grown on HfO₂ exhibited a high electron concentration. Since the HfO₂ orientation layers were grown by ALD, the interface between HfO₂ and InN can be expected to be severely contaminated by organic compounds during the growth. These contaminants¹⁷ may also be incorporated in the InN films as impurities, resulting in the observed higher electron concentration. On the other hand, AlN and InN were grown in the same PSD chamber continuously without exposure to the air. Hence, the formation of a chemically clean interface between AlN and InN can be expected, leading to the observed lower electron concentration of InN grown on AlN compared with that of InN grown on HfO₂. In addition, the InN grown on AlN exhibited the highest electron mobility due to its higher crystallinity. Based on these results, it is hypothesized that InN grown on AlN orientation layers can be used to improve the performances of TFT devices.

Furthermore, the temperature dependences of the electron mobility and electron concentration of the 195-nm-thick c-axis-oriented InN film grown on AlN/glass were investigated. The results are summarized in Fig. 6. As the temperature decreased, the electron mobility of the InN increased while its electron concentration remained almost constant, which indicates that the polycrystalline InN behaves like a degenerate semiconductor. This temperature dependence is qualitatively similar to that of a thin InN epilayer grown on single-crystalline substrates,¹⁸ which is possibly explained by the fact that electron mobility of the degenerate InN is less affected by dislocations than InN with low electron concentration.¹⁹ 

Finally, InN-TFTs on AlN orientation layers were fabricated by PSD on SiO₂/Si substrates. The InN channel layer and the AlN orientation layer were both 5 nm thick, and the thermally grown SiO₂ on Si was 50 nm thick. The TFT consists of a polycrystalline InN channel etched in a mesa, source/drain electrodes of Ti/Al/Ti/Au, a gate electrode of Au, and a 20-nm-thick AlN dielectric layer deposited at room temperature by PSD. An optical microscopy image and the schematic structure of the TFT are shown in Figs. 7(a) and 7(b), respectively. The gate length, channel length, and channel width, were 5, 15, and 60 μm, respectively. The output characteristics of the TFT, shown in Fig. 7(c), indicate that the TFT operates in depletion mode: the InN sandwiched between the gate AlN insulator and the AlN orientation layers serves as a conduction path for electrons, even at a gate bias of 0 V. The maximum drain current density was as high as 16 mA/mm. The obtained current density is lower than that of single-crystal InN/AlN-FET fabricated on Si(111) substrates,²⁰ which indicates that the crystallinity of InN affects the conductivity and FET characteristics. Further improvement of crystallinity of InN on glass substrates will lead to an increase in the current density of FETs. Then, a negative gate voltage was applied to the TFT, the drain current notably decreased. The on-to-off current ratio was approximately 10², and the field-effect mobility was calculated to be 60 cm² V⁻¹ s⁻¹.

In conclusion, a highly c-axis-oriented InN film with high electron mobility was grown on a glass substrate by first coating the substrate with an AlN crystalline orientation layer. The c-axis-oriented InN grown on the orientation layer exhibited higher electron mobility with lower electron concentration than the InN grown directly on glass substrates. The observed maximum electron mobility was 427 cm² V⁻¹ s⁻¹. In addition, as a practical application, a polycrystalline InN-TFT with an AlN orientation layer and an AlN gate insulator was successfully fabricated and operated with a field-effect mobility of 60 cm² V⁻¹ s⁻¹.

This work was supported by JST ACCEL and JSPS KAKENHI (Grant Nos. JPMJAC1405, JP16H06414, and JP18K04955).