Photo Sensitivity Enhanced By the Modulation of Oxide Thickness in MIS(p) Structure
Authors: Chen Hung-Yu , Hwu Jenn-Gwo 
Published: February 2020

It was found that the ratio of light to dark current can be enhanced by coupling effect with simple MIS(p) on concentric MIS structure [1]. Gate with bias can largely decrease dark current, and thereby improve the ratio. In this work, to research deeper about it, the enhancement of ratio with gate voltage is concentrated on the modulation of oxide thickness.
The topside and cross section of the studied MIS(p) structure with the concentric pattern is shown in Fig. 1(a). The radius of inner circle r1 is 85um. The space between inner and outer gate, i.e., s, is 20um. The outer radius of the ring is 300um. Here, the inner circle is defined as diode and the outer ring is defined as gate. Also, according to the previous work [1], a specific gate voltage of VX is defined as when it causes the lowest diode dark current in ID-VG curve (the inset in Fig. 1(b)) and therefore exhibits the largest enhanced ratio. Fig. 1(b) a shows I-V curve of light current and dark current with VG=VX in two oxide thickness of 2.5nm and 3nm. The ratio is defined by the light current to dark current at 2.5V which is located at the diode current saturation region. Comparing the ratio of light to dark current in two thickness, it is obvious to show that the ratio is dependent on oxide thickness and MIS structure with a thicker oxide might have a higher ratio.
To further study the relation of ratio and thickness, samples with different thickness were explored to examine their light to dark current ratios as shown in Fig. 2(a). From Fig. 2(a), it obviously shows that the ratio is highly increased for thicker oxide. Also, in order to see the effect of illumination, three strength of illumination with 131lux, 306lux, and 410lux were studied. Fig. 2(b) shows that thicker oxide indeed has a higher ratio than thinner counterpart no matter what the strength of illumination is.
To analyze the dependency of the saturation current on oxide thickness, the dark current, light current and the ratio of light to dark current are separately presented for various thicknesses as shown in Fig. 3(a). It’s clear to indicate that the dark current reduces with the oxide thickness and light current has no obvious relation with thickness. Hence, the discussion is divided into two parts. To discuss about dark current, Fig. 3(b) demonstrates of the carrier and two currents part of dark diode current from the gate region under VG=VX in dark environment. Because gate voltage is biased in the minimum diode current which is near VFB, minority carrier gathered under gate region is very few. Then, the diffusion current from gate to diode is very small due to the subtle divergence of carrier concentration. Therefore, the diffusion current by coupling effect is largely reduced and contributed less to the barrier height modulation of diode. As a result, diode current is mainly dominated by the injection current part from the gate region. In Fig. 3(c), because the gate current which induces the injection current is strongly dependent on oxide thickness, it implies that thicker sample which has lower injection current has lower saturation current. It confirms that dark current is clearly decreased with increasing thickness under VG=VX in Fig. 3(a). For the light current part, because the light current is also contributed from light-induced electron-hole pairs, the light current is not solely dependent on oxide thickness. Hence, light current does not show similar dependence of thickness. The ratio of light to dark current is largely enhanced by the largely decrease of dark current due to the increase of oxide thickness. Furthermore, the ratio of light to dark current increases with the stronger strength of illumination in the thicker MIS sensor as shown in Fig. 2(b). The result indicates that there is a larger light current in the stronger illumination because of more electron-hole pairs. Therefore, the ratio of light to dark current is increased with the strength of illumination. In conclusion, the modulation of oxide thickness is of importance to the photo sensitivity of MIS(p) tunnel diode.
This work was supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. MOST 105-2221-E-002-180-MY3 and MOST 106-2221-E-002-196-MY3.
[1] W.T. Hou and J.G.Hwu*, 2017, “Photo Response Enhancement in MIS(p) Tunnel Diode via Coupling Effect by Controlling Neighboring Device Inversion Level”. Electronic and Photonic Devices and Systems, ECS Journal of Solid State Science and Technology, 6(10), pp. Q143-147, 2017

Figure 1