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Hybrid nano-scale Au with ITO structure for a

high-performance near-infrared silicon-based

photodetector with ultralow dark current

XINXIN LI,

1,2,3

ZHEN DENG,

1,3,4,

*JUN LI,

1,3

YANGFENG LI,

1,3

LINBAO GUO,

1,2,3

YANG JIANG,

1,3

ZIGUANG MA,

1,3

LU WANG,

1,3

CHUNHUA DU,

1,3,4

YING WANG,

QINGBO MENG,

1,3

HAIQIANG JIA,

1,3,6

WENXIN WANG,

1,3,6

WUMING LIU,

AND HONG CHEN

1,3,6,7

Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed

Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

University of Chinese Academy of Sciences, Beijing 100049, China

Center of Materials and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

The Yangtze River Delta Physics Research Center, Liyang 213000, China

Department of Physics, School of Science, Beijing Jiaotong University, Beijing 100044, China

Songshan Lake Materials Laboratory, Dongguan 523808, China

e-mail: hchen@iphy.ac.cn

*Corresponding author: [email protected]

Received 25 May 2020; revised 4 August 2020; accepted 19 August 2020; posted 20 August 2020 (Doc. ID 398450); published 9 October 2020

An internal photoemission-based silicon photodetector detects light below the silicon bandgap at room temper-

ature and can exhibit spectrally broad behavior, making it potentially suited to meet the need for a near-infrared

pure Si photodetector. In this work, the implementation of a thin Au insertion layer into an ITO/n-Si Schottky

photodetector can profoundly affect the barrier height and significantly improve the device performance. By

fabricating a nanoscale thin Au layer and an ITO electrode on a silicon substrate, we achieve a well-behaved

ITO/Au/n-Si Schottky diode with a record dark current density of 3.7 × 10

−7

A∕cm

at −1Vand a high recti-

fication ratio of 1.5 × 10

at 1V. Furthermore, the responsivity has been obviously improved without sacri-

ficing the dark current performance of the device by decreasing the Au thickness. Such a silicon-based

photodetector with an enhanced performance could be a promising strategy for the realization of a monolithic

integrated pure silicon photodetector in optical communication.

https://doi.org/10.1364/PRJ.398450

1. INTRODUCTION

Optical communication has become one of the most important

technologies in modern society due to its excellent performance

of speed transmission and information capacity, and is widely

operated at wavelengths of 850, 1310, and 1550 nm [1]. Since

there is little light absorption in silicon for wavelengths longer

than 1100 nm, almost all researches have been aimed at

850 nm photodetectors (PDs) in the complementary metal-ox-

ide semiconductor (CMOS) technologies. For 1310 nm and

1550 nm optical receivers, the PDs with excellent performance

are mostly based on compound semiconductors (III–V) and

Ge–Si materials [2–4], both of which have a higher material s

cost. In addition, the compound semiconductors suffer from

not being compatible with the standard Si-based CMOS plat-

form [5–7]. Therefore, the development of Si PDs based on

mature CMOS technology without additional material or pro-

cess steps is an essenti al step for monolithic integrated optical

receivers. To overcome such intrinsic drawbacks and extend the

response wavelength of Si-based PDs to telecom wavelengths,

many new attempts have been proposed, including employing

Si-based quantum dots (QDs) based on quantum confinement

effect (QCE) [8], combining Ge (0.67 eV) with Si [9,10], two-

photon absorption (TPA) [11–13], plasmonic hot carriers [14],

and defect mediated band-to-band photogeneration via mid-

bandgap localized states [15–17]. However, some of these ap-

proaches require complicated designs that limit their overall

application. Recently, the Schottky diode has attracted much

attention due to its unique internal photoemission mechanism

[18–22]. In this configuration, photoexcited carriers from the

metal are emitted to the semiconductor over a potential Φ

called a Schottky barrier (SB), to generate the photocurrent

[23]. Typically, an SB is lower than the intrinsic bandgap E

of a semiconductor, thus allowing the photodetection of pho-

tons with energy hν <E

, which is the basis for silicon-based

Schottky PDs (SPDs) to realize the detection above 1.1 μm

[20,24,25]. For example, PtSi/p-Si SPDs are widely used for

1662

Vol. 8, No. 11 / November 2020 / Photonics Research

Research Article

infrared imaging in the 3–5 μm wavelength range with a

very low SB height (SBH) (0.2 eV) [26,27], while for shorter

wavelength application , Ti, Co, and Ni can be adopted because

their contacts on n-Si have the SBH of 0.5–0.7 eV [28].

However, it is difficult for traditional metal-Si SPDs to achieve

considerable performance in responsivity. Fortunately, trans-

parent conductive glass (ITO) with good conductivity and light

transmission is a promising alternative to metal for allowing

sufficient light to enter the junction [29,30]. One of the chal-

lenges for the ITO/Si detector is that the low SBH leads to high

dark current, which will severely drag down the sensitivity of

the detector. To increase the height of the ITO/Si barrier,

inserting other materials including an insulator and metal be-

tween the ITO and silicon has been performed [18 ,25,31].

Among these materials, the insulator materials have a high

series resistance, while an Ag layer at least 8 nm thick is needed

to form a high barrier with Si due to the poor film forming

ability of Ag, resulting in poor light transmittance of the elec-

trode and thus a reduced response. In recent studies, the Au/Si

SPDs with different kinds of plasmonic designs have had better

responsivity, but the dark current for these structures is still

high [32–34].

In this work, we demonstrated the ITO/Au/n-Si PDs with

the best record of dark current density on the order of

1 × 10

−7

A∕cm

(−0.1Vto −2V) at present. The mechanism

for such low dark current density is a high SBH, resulting from

a high work function of the Au insertion layer into the ITO/Si.

Furthermore, for the new structure, the transparency of the

electrode has been obviously improved and higher photocur-

rent is obtained by thinning the Au film. As a silicon-based

SPD with a record low dark current density, one can envision

its role for operation from visible light to a 1550 nm wave-

length without damaging the light responsivity.

2. METHODS

A. Preparation of the ITO/Au/n-Si PDs

Figure 1 illustrates the process flow of making ITO/Au/n-Si

SPDs. The PDs were fabricated with a commercial epi-ready

n-type (0.1–1 Ω·cm, 400  10 μm) silicon substrate. First,

the PD areas were defined by ultraviolet lithography using a

negative resist (AR-U 4030) in leaf patterns with an area of

0.0007 cm

. Before being sent to the electron beam evapora-

tion (Ohmiker -50B) chamber for Au deposition at room tem-

perature, the patterned substrates were dipped in a dilute HF

solution (H

O:HF  10:1 for 1 min) for oxide removing.

During the evaporation process, the deposition pressure was

maintained below 2 × 10

−6

Pa and the deposition rate was

0.17 Å/s (1 Å = 0.1 nm) controlled by the current with a volt-

age at 8 kV. In addition, the plate where the substrates were

placed rotated at a rate of 8 r/min to ensure the uni formity

of the film. After that, 100 nm ITO was immediately deposited

on the Au film by a doub le chamber mag netron sputtering sys-

tem (Shenyang Defeng Technology) at room temperature

under Ar∕O

atmosphere. During the sputtering process,

the thickness was controlled by the sputtering time with a dep-

osition rate of 0.93 Å/s. Finally, the ohmic contact on the back-

side was realized by a 300 nm Al film using Ohmiker-50B with

a deposition rate of 1.7 Å/s at room temperature after removing

the oxide by a dilute HF solution (H

O:HF  10:1 for 1 min).

To protect the ITO/Au electrode from the HF solution, the

positive resist (AZ 6130) was used as the protective layer during

the removing process. The source of Au and Al in the Ohmiker-

50B was the molten metal that came from the particles (purity,

99%; particles diameter, 0.5 cm) by heating an electron beam

in the crucible before evaporating; the source of the ITO was a

bulk target (purit y, 99.99%; In

:SnO

= 9:1, mass ratio).

B. Characterization and Measurement

The surface morphology of the ITO/Au/n-Si PDs was analyzed

using a field-emission scanning electron microscope (SEM)

(SUS5500) monitored with accelerating voltage (30 kV) and

an atomic force microscope (AFM) (Bruker, Multimode8) in

a ScanAsyst mode. While the interfaces between the ITO, Au,

and n-Si substrate were analyzed by the SEM (SUS5500) and

high resolution transmission electron microscope (HRTEM)

(JEM-2200FS). The transmission spectra were acquired by a

UV-VIS-NIR light spectrophotometer (Shimadzu, UV 3600

Plus). Since the Si substrate is opaque in nature, the ITO layer

(100 nm) and the ITO (100 nm)/Au (2 nm, 3 nm, 4 nm,

6 nm) multilayers, processed in the same manner as their Si

counterparts, were deposited on glass substrates to obtain

the transmission of the electrodes. In addition, the transmission

of the glass had been removed during the transmission measure-

ment. At the same time, the resistivity of the ITO/Au/glass was

analyzed by a Hall effect testing instrument.

The I–V measurements were performed by the use of a

Keithley 4200 in the forward and reverse regions at 2V

by 10 mV per step, while the temperature dependent I–V char-

acteristics were acquired by a standard electrical probe station

(Lakeshore Cryotronics) and a Keithley 4200 semiconductor

parameter analyzer from −1Vto 1 V by a step of 10 mV.

In addition, the photocurrent at the certain wavelength was

obtained with a Keithley 4200 from −2Vto 2 V when the

detector was normally incident by 1064 nm (2 mW) or

Fig. 1. Schematic process flow for the formation of ITO/Au/n-Si

SPDs. (a) The PD areas were defined on an n-type Si substrate by

ultraviolet lithography. (b) The patterned Si was sent to the electron

beam evaporation chamber to grow Au film at room temperature.

ble chamber magnetron sputtering system at room temperature under

Ar∕O

atmosphere. (d) The ohmic contact on the backside was real-

ized by a 300 nm Al film using electron beam evaporation.

Research Article

Vol. 8, No. 11 / November 2020 / Photonics Research 1663

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