Silicon germanium photo-blocking layers for a-IGZO based industrial display

Amorphous indium- gallium-zinc oxide (a-IGZO) has been intensively studied for the application to active matrix flat-panel display because of its superior electrical and optical properties. However, the characteristics of a-IGZO were found to be very sensitive to external circumstance such as light illumination, which dramatically degrades the device performance and stability practically required for display applications. Here, we suggest the use for silicon-germanium (Si-Ge) films grown plasma-enhanced chemical vapour deposition (PECVD) as photo-blocking layers in the a-IGZO thin film transistors (TFTs). The charge mobility and threshold voltage (Vth) of the TFTs depend on the thickness of the Si-Ge films and dielectric buffer layers (SiNX), which were carefully optimized to be ~200 nm and ~300 nm, respectively. As a result, even after 1,000 s illumination time, the Vth and electron mobility of the TFTs remain unchanged, which was enabled by the photo-blocking effect of the Si-Ge layers for a-IGZO films. Considering the simple fabrication process by PECVD with outstanding scalability, we expect that this method can be widely applied to TFT devices that are sensitive to light illumination.

more Ge ratio than Si ratio 21 . However, the difference of UV absorbance depending on components ratio is not enough to satisfy the absorbance condition for a photo blocking layer. In order to find better results, we controlled the thickness of the Si-Ge films which have fixed component ratio (Si:Ge = 1:1). It was found that TFTs with Si-Ge photo blocking layers preserved the initial value including mobility and threshold voltage (V th ) without any oxidation or chemical damages under the light illumination.

Results
The Si-Ge films were synthesized on glass substrates coated by silicon oxide by plasma-enhanced chemical vapor deposition (PECVD). Without the silicon oxide, it is difficult to directly grow the Si-Ge films on the glass substrate. Fig. 1a shows a schematic illustration of the structure of a-IGZO TFTs passivated by the Si-Ge photo-blocking layers with the top gate and electrode contact. Silicon Nitride buffer layers were deposited on the Si-Ge layers for dielectric materials. The synthesis method is more explained in following experimental synthesis parts. Fig. 1b is the side image of the structure of a-IGZO TFTs measured by scanning electron microscopy (SEM). We confirmed that uniform Si-Ge films were placed among the two oxide based dielectric layers without any externally damage to overall structure. The crystallinity of the Si-Ge films synthesized by PECVD was investigated by X-ray diffraction measurements (XRD). As shown in Fig. 2a, the XRD patterns of the Si-Ge films exhibit conventional cubic phase similar to that of bulk silicon (JCPDS No. 27-1402). Reflections of the Ge phase are imperceptible in the XRD patterns, suggesting that Ge exists as a solid solution 22 . The formation of the Si-Ge bonding is more clearly explained by Raman spectrum as shown in Fig. 2b. The Raman peaks at 510, 405 and 295 cm −1 from Si-Si, Si-Ge and Ge-Ge bonding, respectively, are observed. In particular, the additional Raman peaks between 420 and 470 cm −1 , which are the vibrational modes of Si super lattice, are measured. These vibrational modes which are composed of localized Si-Si optical modes whose frequencies are lowered because of the larger mass of adjacent Ge atoms 23 . X-ray photoelectron spectroscopy (XPS) also demonstrates chemical composition and surface oxidation of the Si-Ge films. The Si 2p and the Ge 3d peaks obtained from XPS could be ascribed to elemental Si and Ge, as shown in Fig. 2c,d. Owing to slight surface oxidation, weak peaks related oxygen are also detected around 101.5 eV at Fig. 2c and 33 eV at Fig. 3d, respectively. These characterizations summarize that the elemental phases are originated from Si and Ge elements and their bonding. Figure 3a shows the ultra violet -visible (UV-Vis) spectrum according to the thickness of the Si-Ge compounds. The UV absorbance peak was gradually shifted in which direction increasing the thickness of Si-Ge layers according above the equation. In particular, we confirmed the UV absorbance at 450 nm more detail, which the a-IGZO react with actively ( Fig. 3b) 24 . Photo blocking effect is also proportional to the thickness like as other wavelength. Therefore, it is clear that the higher thickness of Si-Ge compounds has good blocking effects, thereby offering a possibility for superior photo blocking layers in a-IGZO based displays.
Although the higher thickness of the Si-Ge films has outstanding UV absorbance, the electrical superiority is inversely proportional to the thickness. Figure 4a shows the threshold voltage (V th ) is negatively shifted depending on the thickness of the Si-Ge films at dielectric buffer layers 300 nm. (Supplementary Fig. S1) The more charge carrier density, so called free electrons, are accumulated in the Si-Ge films as the thickness of the Si-Ge films is increased. The accumulated electrons make the Si-Ge films more conductive, thus making it a kind of capacitance with a-IGZO. Eventually this accumulated charge carriers cause the a-IGZO channel to shut off while generating tunnelling effects to the Si-Ge layers. Therefore, we optimize the thickness of Si-Ge compounds without any tunnelling effects. In addition, the thickness of the silicon nitride buffer layers could also give rise to the performance of the a-IGZO TFTs, as shown in Fig. 4. (Supplementary Fig. S2) The more deposition of dielectric materials results in the improved efficiency of TFTs due to the decrease in capacitance 25,26 . However, too high thickness of buffer layers lead to surface instability, which makes post processes to fabricate the TFTs very difficult. Hence, we selected the heights of the Si-Ge films and the silicon nitride buffer layers to 300 nm and 200 nm, respectively for   a-IGZO TFTs to light illumination, the V th is negatively shifted (>10 V). In contrast, the a-IGZO TFTs passivated by the Si-Ge photo blocking layers show no change in electrical properties during the light illumination, as shown in Fig. 4d. This result suggests a superior possibility of the Si-Ge films as a photo blocking layers for the a-IGZO based industrial display field under light illumination.
In order to obtain more information of electrical properties on the a-IGZO TFTs passivated by the Si-Ge photo-blocking layers, the effect of light stress time on the change of V th and electron mobility was investigated in detail. The V th changes without and with the Si-Ge passivation layer to the light illumination condition are compared, as shown in Fig. 5a. By applying 10 V gate voltage, V th of pristine a-IGZO is largely negatively shifted (~15 V at 1000 sec) in the direction with increasing the light stress time, while that of the Si-Ge passivated a-IGZO persists similar value compared with initial value. Furthermore, the electron mobility of the a-IGZO TFTs passivated by the Si-Ge layers shows no change under the light stress time, as shown in Fig. 5b. Therefore, the V th shift and electron mobility are greatly dependent on the existence of the Si-Ge photo-blocking layers.

Discussion
In conclusion, we synthesized the Si-Ge films by PECVD and investigated the effects of the photo blocking Si-Ge layers on the electrical properties of a-IGZO semiconductors. Although the thicker Si-Ge films shows stronger the light absorbance, the electrical performance of a-IGZO TFTs is inversely proportional to the thickness of the Si-Ge films because of accumulated free electrons. The thickness of the dielectric buffer layers changes the charge capacitance and surface instability of the TFTs, which also considerably affects the device performance. Therefore, we optimize the thicknesses of the Si-Ge films (~200 nm) and the buffer layers (~300 nm). After up to 1,000 s exposure to light, the V th and electron mobility of the a-IGZO TFTs passivated by the Si-Ge photo-blocking layers were unchanged. Considering the simple fabrication process by PECVD with outstanding scalability, we expect that this method can be widely applied to various metal-oxide TFT devices that are sensitive to light illumination.

Synthesis.
Using plasma enhanced chemical vapour deposition (PECVD), the 100 nm SiO 2 layers were deposited on a glass substrate for the efficient synthesis of the Si-Ge films. The SiO 2 /glass substrate was placed on a quartz flat inside of 4-inch quartz tube. The Si-Ge films were synthesized on SiO 2 /glass substrates through the PECVD method, using silane (10 sccm) and germane (10 sccm) with vacuum pumping at 370 °C at initial low vacuum (~25 mTorr). After 5~10 min of direct exposure to the plasma (100 W) at that temperature, a large-area Si-Ge films formed on the SiO 2 /glass substrate. Then, silicon nitride buffer layers also deposited on the substrate using PECVD with same method above.
Characterization. The entire structure was investigated by field-emission scanning electron microscopy (FESEM, AURIGA Carl Zeiss). The crystal phases were collected using an X-ray diffractometer (D8-Advance, Bruker Miller Co.) with Cu Ka1 irradiation. The Raman spectra were obtained by a Raman spectrometer (RM 1000-Invia, Renishaw, 514 nm). Structure bonding in the structure is analysed by XPS carried out with a KRATOS AXIS-His model in Research Institute of Advanced materials. The electrical properties were measured by Agilent 2602.