Transparent Conductive Oxide (TCO) Gated Ingaas Mosfets for Front-Side Illuminated Short-Wave Infrared Detection
Authors: Maeda Tatsuro , Oishi Kazuaki , Ishii Hiroyuki , Chang Wen Hsin , Shimizu Tetsuji , Shimizu Tetsuji , Endoh Akira , Fujishiro Hiroki , Koida Takashi 
Published: July 2022

TCO gated InGaAs MOSFETs are demonstrated as a high responsivity and broadband short-wave infrared (SWIR) photodetector under front-side illumination (FSI). InGaAs MOSFET structures have many advantages for monolithic integration with optical communication devices with O, C, and L bands and Si-LSI.
The TCO gated InGaAs MOSFETs were fabricated through the gate-last method as shown in Fig. (a) [1]. A 300-nm-thick p-type In0.53Ga0.47As layer was grown on an InP (001) wafer using MOCVD. After the deposition of a 10-nm-thick Al2O3 protection layer by ALD, the InGaAs active region was defined by chemical wet etching for mesa isolation. For the formation of the source/drain region, Si ions were implanted with 2×1014 cm-2 at 15 keV, followed by RTA at 450 °C for 10 min to achieve dopant activation. After splitting the Al2O3 protection layer, a 10-nm-thick Al2O3 gate oxide layer was deposited again. As TCO gate, Ce and H co-doped In2O3 (In2O3: Ce, H) with an electron mobility of ~160 cm2/V·s and a carrier density of ~ 2×1020 cm-3 were used [2, 3]. The amorphous TCO gate, which were deposited onto the Al2O3 layer using the ion plating method, were lifted off and crystallized by annealing at 250 °C for 30 min. Finally, Ti/Au formed for the S/D and gate contact after post-metallization annealing (PMA) at 250 °C for 30 min.
The thin TCO films exhibited a very low specific contact resistance of 9.01 × 10-8 W·cm2 and a sheet resistance of 18.7W/sq. We confirmed the normal n-channel MOSFET operation with an I
on/I
off ratio of > 1×104. S.S. and D
it values were 134 mV/dec. and 5.2 × 1012 eV-1cm-2, respectively. The peak G
m was observed at V
G= 0.5 V, and the extracted peak field-effect mobility was 620 cm2/V·s. The threshold voltage negatively shifted to -0.38 V, operating in enhancement mode owing to the work function of the TCO (4.2 eV), which is smaller than the Fermi level energy of p-type InGaAs (5.09 eV).
Figure (b) shows the I
D - V
G characteristics of InGaAs MOSFETs at V
G = 1 V in the dark and under illumination at the excitation wavelength of 1550 nm with an incident optical power of 5213 nW. SWIR light was illuminated in the channel area (L/W = 10 μm/10 μm) of TCO gated InGaAs MOSFET with a spot size of 10 μm. Under SWIR illumination, an increase in can be observed in the entire gate bias. Owing to the normally-on operation of the TCO gated InGaAs MOSFET, a photocurrent is detected even if the gate bias is 0 V. The photocurrent increased with the gate bias and reached the maximum in the vicinity of V
G = 0.5 V, which corresponds to that of the maximum transconductance. The primary photo-response when the device is under illumination is known as the photovoltaic effect [4, 5]. The photo-generated holes move to the source region and accumulate at the potential barrier between the source and the channel. This results in the effective reduction of a potential barrier for electrons at the source edge. These phenomena lead to a decline in the threshold voltage of the MOSFET, which leads to the increase of the drain current. The threshold voltage of the TCO gated InGaAs MOSFETs in the dark was -0.35 V, and this value shifted to -0.45 V under illumination. Because is proportional to the mobility at the MOS interface, the improvement of the interface quality of the TCO/Al2O3/InGaAs MOS structures can enhance the sensitivity.
Owing to the SWIR transparent TCO, we observed a responsivity of more than 1 A/W in the range of 1000 nm to 1800 nm, which includes the entire optical communication wavelength bands (1260–1625 nm). Because the maximum sensitivity of InGaAs photodiode was approximately 1 A/W at approximately 1550 nm [6], it is demonstrated that TCO gated InGaAs MOSFETs is a highly sensitive device in the SWIR region owing to the gain mechanism in the photoFETs.
[1] T. Maeda, et al., Appl. Phys. Lett., 119, 192101 (2021).
[2] T. Koida, et al., Status Solidi A, 215, 1700506 (2018).
[3] T. Koida, et al., Status Solidi A, 218, 2000487 (2021).
[5] Y. Takanashi, et al., IEEE Electro. Dev. Lett., 19, 472 (1998).
[6] http://www.thorlabs.com.

Figure 1