Negative photoconductance in heavily doped Si nanowire field-effect

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Negative photoconductance in heavily doped Si nanowire field-effect transistors Eunhye Baek, Taiuk Rim, Julian Schuett, Larysa Baraban, and Gianaurelio Cuniberti Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02788 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on October 1, 2017

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Negative photoconductance in heavily doped Si

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nanowire field-effect transistors

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Eunhye Baek,1 Taiuk Rim,2 Julian Schütt,1 Larysa Baraban,1,3* Gianaurelio Cuniberti1,3

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Dresden, Germany

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Pohang, Korea

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Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062

Department of Creative IT Engineering, Pohang University of Science and Technology, 37673

Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany

Corresponding author: [email protected]

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KEYWORDS

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negative photoconductance, hot electron trapping, interfacial trapping, Si nanowire, indirect band

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gap semiconductor

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ABSTRACT

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We report the first observation of negative photoconductance (NPC) in n- and p-doped Si

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nanowire field-effect transistors (FETs) and demonstrate the strong influence of doping

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concentrations on the nonconventional optical switching of the devices. Furthermore, we show

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that the NPC of Si nanowire FETs is dependent on the wavelength of visible light due to the

1 Environment ACS Paragon Plus

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phonon-assisted excitation to multiple conduction bands with different band gap energies that

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would be a distinct optoelectronic property of indirect band gap semiconductor. We attribute the

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main driving force of NPC in Si nanowire FETs to the photo-generated hot electrons trapping by

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dopants ions and interfacial states. Finally, comparing back- and top-gate modulation, we derive

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the mechanisms of the transition between negative and positive photoconductance regimes in

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nanowire devices. The transition is decided by the competition between the light-induced

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interfacial trapping and the recombination of mobile carriers, which is dependent on the light

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intensity and the doping concentration.

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Negative photoconductance (NPC) is a rare effect since the photoexcitation of charge carriers

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normally enhances the channel conductivity.1 In order to reach the situation, when the channel

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conductivity is decreased (NPC), additional electronic states are required that can compensate a

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generation of photoelectrons. Some of the low dimensional materials (e.g., nanoparticles,

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nanowires and thin film) reveal a negative photoconductance, due to the surface effects

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originating from the high surface-to-volume ratio.2-3 Thus, the large surface area of

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nanostructured materials can potentially generate high density of localized energy states acting as

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traps for charge carriers, sufficient to reverse the type of the channel conductivity. For instance,

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arrays of metal nanoparticles, which are capable of surface plasmon excitations upon light

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illumination, can reveal the NPC due to the presence of interfacial charges.2 On the other hand,

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the NPC in semiconductors is of different nature and is linked to the energy band gap structure.

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In many cases, the NPC has been observed in large band gap semiconductors such as AlN,4 p-

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ZnSe5 or Ga2O3,6 with sub-band gap excitation where photoexcited electrons can be captured by

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extrinsic (e.g., surface oxygen) and intrinsic (e.g., defects) trap states in the middle of the band

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gap. Moreover, since photoexcited electrons are generated via the super-band gap excitation,

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NPC requires additional phenomena like scattering at recombination centers in InN7.

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Photoconductivity studies of Si have a long history8,9 as well as numerous industrial

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realizations10 because of the well-known electronic properties and performance e.g., high speed

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and efficient signal processing and compatibility with various electrical platforms by mature

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integration. The NPC of bulk Si was observed for the first time in cobalt doped Si under the

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infrared light illumination8,11. The localized energy states of dopants in the band gap of Si act as

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a powerful recombination center, which is typical sub-band gap NPC phenomena. During the last

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decade, Si and Si nanostructures, i.e., especially nanowires, have been studied for various optical


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applications, such as photodetectors,12,13 photovoltaics14,15 and solar cells16-17, using advantages

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from 1-dimensinal structure and relying mostly on the phonon-assisted photoexcitation, due to its

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indirect bandgap, and generating a conventional “positive” photocurrent.

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However, despite the well-developed Si photodetectors offered on the market and the

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enormous research and industrial demands of Si nanowires for various optical applications, the

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NPC in Si nanowire devices has not yet been reported. In particular, modern Si nanowire field

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effect transistors (FETs) need proper doping in the conduction channel for effective gate

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modulation and should include insulating layers in contact with the channel area for field effect

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or surface functionalization for bio-18-19 or optical20,21 sensor application. In this situation, the

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devices are working in more complex electrical systems including the charge transfer via defects

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and the interfaces.

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Nowadays optoelectronic integrated systems require active and passive photonic devices, like

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lasers or photodiodes, for data transmission and conventional CMOS devices for computing.22

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Those systems are fast and power-efficient compared to pure electronic circuits, but the optics

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are mainly dedicated to signal transmission, which needs to be converted back to the electronic

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signal for actual data processing in the CMOS devices. In order to actively involve optics into

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processes, hybrid phototransistors or memories, modulated by light, have been developed by

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combining quantum dots or organic films on FETs20,21,23,24. However, several drawbacks still

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exist in the hybrid devices, such as slow switching speed, induced from amorphous organic

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layers and unidirectional current changes upon illumination. In this regard, NPC studies of Si

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nanowire devices would be a critical issue, not only for a deeper understanding of the

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optoelectronic properties of 1-dimensional Si systems, but also for realizing Si-based optical


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processors which have bilateral switching functionality, preserving the speed of Si in the CMOS

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technology.

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In this study, we report the observation of NPC in the Si nanowire FETs with different doping

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concentrations under visible light illumination. The photoconductivity of the devices was

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investigated depending on the doping type (both p- and n-type) and concentration as well as the

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intensity and the wavelength of light. Interestingly, the reduction of the current is observed only

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in the special gate bias condition, which is able to overcome depletion. We also demonstrate that

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the main sources of NPC in FET devices is not only the change of the mobile carrier density in

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the conduction band of a SiNW, but also the strong threshold voltage shift induced by the

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modified depletion, and by interfacial charge from the electron capturing in dopant ions and

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interface states; in other words, the electric field gating effect. Therefore, dopant type and

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concentration play a significant role in the emergence of the NPC effect in SiNW-based FETs.

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Table 1 shows previous studies that claim single dominant phenomenon leading to NPC, for

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instance, sub-band gap trapping in direct band-gap semiconductors or carrier trapping by surface

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states in metal nanoparticles. Unlike those studies, we show that the NPC of heavily doped Si

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nanowire FETs is driven by (i) photoexcited electron trapping by dopant ions in the nanowires

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and (ii) competitive electron transient between interfacial trapping and fast recombination, which

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are dependent on the doping concentration. Therefore, NPC effect is particularly maximized in

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subthreshold region, which is the electrically most sensitive area in the FET operation, that

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agrees the observations of transistor-based sensor applications.25 NPC change of Si has a special

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dependence on the wavelength of visible light area due to the strong correlation with the carrier

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generation rate by phonon-associated excitation in indirect band gap. Finally, we believe that our


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observation of NPC can be considered, as a universal paradigm, extended to any kind of

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nanostructure, having defect and interfacial states.

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Results

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Si nanowire FETs with a honeycomb nanowire network were fabricated on an 8-inch SOI

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wafer using conventional CMOS fabrication technique (see Figure 1(a)-(e)). The lithography

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steps are shown in Figure S1 panels i-viii in Supporting Information. The bare SOI wafer

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consists of a top-Si layer doped with 1016 cm-3 of boron on a 400 nm-thick oxide layer and p-type

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Si substrate doped with 1015 cm-3 of boron. The Si channel area was heavily doped with

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phosphorus to modify the channel conduction properties from the normal inversion mode n-type

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FET to the accumulation mode n-type FET. Therefore, the device is normally in an on-state at

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the gate bias Vg = 0 V, which is advantageous for low power sensor applications. As a result, the

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variations of doping concentration were 1018 cm-3 and 1019 cm-3 of phosphorus and 1016 cm-3 of

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boron respectively. (See details in Methods) Hereafter, we will use the terms ‘n+’, ‘n++’ and ‘p’-

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doped device to designate the abovementioned doping concentrations. As a nanowire channel

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region, a honeycomb structure was designed using electron beam lithography in order to obtain

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higher signal to noise ratio and higher current stability at the subthreshold voltage regime26,27

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(Figure 1(c)-(d)). Figure 1(e) shows a cross section of Si nanowire with 50 nm width and 30 nm

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height, which are covered with a 5 nm layer of thermally grown oxide. The devices have Ag

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contact pads with a Ti adhesion layer on the source and drain area, which were heavily doped to

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form ohmic contacts (See Figure S2 in Supporting Information) with metal transmission lines

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(Figure 1(b)). For illumination, 4 light-emitting diodes (LEDs) emitting in the visible range


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(wavelengths, λ = 405 nm, 470 nm, 530 nm and 625 nm) were used. The transmission line

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method (TLM) measurement was used to verify that there is no plasmonic effect by Ag contact.

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In the following, we demonstrate the influence of the light illumination on the conductivity of

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Si nanowire FET devices. Figure 1(f) and (g) show the transfer and output characteristics of the

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n+-doped (Nd = 1018 cm-3) device under illumination at λ = 625 nm with various light power

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intensities. Interestingly, the threshold voltage (Vth) of the device increases under illumination,

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which causes a distinct decrease of the photocurrent as the light intensity increases (see inset,

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Figure 1(f)). The output characteristics of the device also support the clear NPC behavior of Si

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nanowire FETs (Figure 1(g)). Figure 1(h) shows reverse switching of the channel current (Id)

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under light illumination. The magnitude of the light-induced reversed current switching increases

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(see stages i-v in Figure 1(h)) as the light intensity increases in the time domain.. Contrary to

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expectations that the current should increase due to increasing number of channel carriers by

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photoexcitation,28 the Id is decreasing when the gate bias and light intensity satisfy certain

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conditions.

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Next, we demonstrate the condition of gate potential that leads to the NPC in Si nanowire

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FETs. Figure 2(a) shows that the subthreshold slope of devices increases as the light intensity

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increases. Therefore, the NPC is not able to be observed at low Vg. In order to determine the

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dependence between the NPC and the gate bias, the current change ratio (∆Id (%))is extracted

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from the transfer curve (see Figure 2(b)). When exposed to the light of very low power intensity

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(< 400 µW/cm2), the overall subthreshold area shows strong reduction of current. After the

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nanowire channel is fully formed (Vg>>Vth), the NPC effect is still present although is rather

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weak. As the light power intensity increases (> 400 µW/cm2), the off current of the transfer curve

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also increases because of photoexcitation. This leads to the increase of the ∆Id (%) at low Vg and


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NPC is switched to conventional positive photoconductance (PPC). Therefore, the best area to

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observe the NPC in FET devices covering wide range of light intensities is the subthreshold area

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near Vg = Vth,dark.

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Since the visible light photons have much higher energy (1.9 eV < Eph < 3.06 eV) than the

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band gap of Si (Eg = 1.1 eV), photoexcited electrons behave as hot electrons in Si nanowires. In

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general, hot electrons can create trap states in an oxide layer29 and these additional trap states

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increase the subthreshold slope (SS) accordingly to the following equation,

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(1)

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where Cox is oxide capacitance, q is the elementary charge, K is Boltzmann constant, T is the

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temperature ∆Nit is density change of interface state by light illumination and ∆SS is the change

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of the subthreshold slope.30 The degradation of the subthreshold slope in FET devices is

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observed by the interface trap creation under light illumination30,31. Hence, the increased SS by

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photo-induced hot electrons raises the current at low Vg. For this reason, the NPC is only

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observed at high e Vg to cause the channel depletion.

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From now on, we will discuss how various factors, such as doping concentration and light

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intensity, influence the NPC of Si nanowire devices (Figure 3). Since strong NPC was observed

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near Vth, we extracted the light-induced threshold voltage shift (

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the Id-Vg curve for various doping concentration of nanowires (Figure 3(a)). Using the constant

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current method, Vg at Id = 200 nA was defined as Vth. Since Vth was changed under the light

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illumination, the photoconductance in FET devices are mainly due to the shift of Vth. Under

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illumination with light of low intensity light (< 200 µW/cm2), all devices show positive ∆Vth

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leading to the observation of the NPC effect regardless of the dopant type. However the variation

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of ∆Vth with the change of the light intensity is different with the dopant type and concentration. 8 Environment ACS Paragon Plus

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The heavily n-doped devices reveal much stronger NPC effect than the lightly p-doped devices

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do. For the n+-doped devices, Vth increases depending on the light intensity, which is a standard

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NPC behavior. Contrarily, although the n++-doped device shows similar amplitude of the NPC

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with the n+-doped device at the beginning at lower light intensities illuminating devices, the

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amplitude of ∆Vth decreases with higher light intensities. Finally, weakly p-doped devices reveal

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relatively weak NPC effect at lower intensities of illumination. As light intensity increases,

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however, positive photoconductance (PPC) behavior, such as the fast decrease of Vth, was

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observed.

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In order to demonstrate the driving source for the transformation of the NPC into the PPC with the Vth shift, we use a standard equation describing the Vth in FET devices,

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(2)

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where

, Qi, Qd and Cox are the required potential compensating the working function

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difference and the channel formation, oxide and interface charge, channel charge in the depletion

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region and the oxide capacitance, respectively. Eq. 2 can be modified to include electrical

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changes induced by the light illumination:

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(3)

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where ∆Vsub is the change of the substrate potential by the illumination, Qit is the light-induced

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interfacial trapped charge and Qdt is the light-induced dopant-trapped charge. Since the substrate

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is p-type Si, photogenerated mobile charges can change the interfacial potential. Therefore, we

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measured the change of the substrate potential (Figure 3(a)). The detailed data and method is

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shown in the Figure S3 in Supporting information. Since the range of ∆Vsub is comparable with


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∆Vth, the initial shift of Vth in the devices can be driven by ∆Vsub when the light intensity is low.

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However, extra ∆Vth that is exceeding ∆Vsub in the heavily n-doped devices could be induced by

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Qit and Qdt.

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We consider very low light intensity (< 500 µW/cm2) where ∆Vth exceeds ∆Vsub. In order to

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induce the positive change of Vth, the sum (Qit + Qdt) should be negative. Firstly, Qdt can

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compensate or strengthen the depletion charge (c.f., Qd < 0 in the n-type devices) depending on

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the dopants type. In n-doped devices, positive donor ions capture electrons (Qdt < 0) to increase

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Vth. On the other hand, p-doped devices have negative acceptor ions, which capture holes (Qdt >

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0) to decrease Vth. Therefore, n-doped devices show an extra increase in Vth. Also, Qit would be

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a negative value since the photoexcited electrons are filled in the defect states, thereby raising the

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Vth. As the light intensity gradually increases, Qit and Qdt are strongly affected by the

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recombination rate (discussed later). Therefore, we can conclude that the photoexcited electrons

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captured by interfacial states and donor ions are the main driving force of NPC in heavily n-

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doped nanowires.

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Figure 3(b) shows the light intensity dependence of the light-induced current change with fixed

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Id,OFF ≈ 100 nA, which is chosen to be near to the Vth of devices based on the result of Figure

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2(b). The photocurrent switching with respect to the doping concentration and extraction of ∆Id

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are shown in Figure S4 in Supporting information. It is notable that the heavily n-doped

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devices show an opposite change of the photocurrent, originating mainly from the Vth change as

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discussed above. ∆Id gradually increases with the increase of the light intensity because of the

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photoexcitation of electrons in the nanowires. With the equal change of light intensity, however,

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the lightly p-doped devices show strong PPC behavior. This figure clearly shows the main

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differences between dopant types. The PPC is strongly disturbed in the heavily n-doped devices.


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The inset in Figure 3(b) shows a detail variation of the current with the light intensity on a

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logarithmic scale. In the heavily n-doped devices, ∆Id decreases exponentially with the light

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intensity and, ∆Id increases, even though ∆Id remains negative. It implies that the net ∆Id is a sum

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of the exponential decay and growth of the current with the change of the light intensity.

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In order to verify the NPC in the nanowires without a substrate effect, we fabricated a top gate

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electrode on the nanowires, shown in Figure 4(a) and (b). A thin platinum electrode was formed

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on the middle of the honeycomb nanowire array covered with an oxide gate dielectric. Pt is

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chosen to protect the nanowire from any additional optical effects like surface plasmonic effects

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in the visible range. The back gate was floating in order to electrically isolate the nanowire from

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the substrate. The fabrication steps are shown in Method.

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Figure 4(c) shows the change of the photo-induced current with the various intensities of the

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light, extracted using the same method as for Figure 3(b). Even when the substrate effect is

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eliminated, the photocurrent is reduced when the sample is illuminated with a low light intensity.

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The current change ratio in the top-gate device is much smaller than that in the back gate devices

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due to the absence of the strong potential drop. Similar to the data shown in the inset in Figure

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3(b), ∆Id of the top-gate device also decreases when illuminated with light of low intensity and

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increases as the light intensity increases. The ∆Id is attributed to the combination of the two

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major phenomena such as carrier trapping and photogeneration that are inducing NPC and PPC,

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respectively.

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Figure 4(d) shows the energy band diagram of the Si and SiO2 interfaces that describe the

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physical origin of the NPC and PPC. When photons are absorbed by Si, electron-hole pairs are

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created with the optical generation rate g. Due to the high energy of visible photons, hot

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electrons are generated in the conduction band with a density of nhot, leaving hot holes, phot in the


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valence band. Mobile hot electrons can thermally transit to interface states, Eit and the edge of

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the conduction band with the transit time constant τit and τ', respectively. The density change of

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electrons at conduction band edge, ∆n, combines the thermally relaxed hot electrons and the

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detrapped electrons from the interface states with the detrapping time constant, τdit. The electrons

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at the conduction band edge are recombined with the holes at the valence band edge, ∆p. Since

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Si is an indirect band gap semiconductor, the recombination process must involve the defect

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states transition in the band gap with the recombination time constant τr.

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In order to understand the effect of the interfacial trapping on the change of the

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photoconductance of the devices, we consider a model describing the dynamics of the density

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change of the photoexcited carriers:

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(4)

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(5)

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(6)

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(7)

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(8)

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where nit is the density of the interface trapped electrons and Nit is interface state density. The

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photoconductivity change can be expressed with the excess mobile carriers under illumination: 3

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(9)

where µn and µp are the mobility of electrons and holes, respectively.


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In the steady state, we can obtain the solutions of the Eqs (4)-(8) depending on the interface

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trapped electrons.3 If the light intensity is very low, then most of the interface trap states are

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empty (nit Eg), excited electrons could

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be captured by defect states (Ed) in the band gap between the L-band minimum and the X-band

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minimum during the momentum change by phonon absorption or emission. This is similar to the

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sub-bandgap trapping reported previously 4-6. This phenomenon is expected only in indirect band

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gap semiconductors. Thus, quasi sub-bandgap trapping would be highly probable with the red

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light absorption, which could enhance the NPC.

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In conclusion, we have demonstrated a negative photoconductance effect in Si nanowire FETs

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with different doping concentrations as well as under illumination conditions, i.e. light

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wavelengths and intensities. It is the first observation of the NPC, induced by the hot trapped

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carriers in nano-scaled semiconductors with indirect gap. It is an important message for the

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readership audience working with Si-based nanotechnology (and nanowires particularly), which

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remains dominant in semiconductor industry. The main sources of the NPC are the light induced

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Vth shift by photoexcited electrons trapped at the interface (outside of nanowire) and dopants ions

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(inside of nanowire). The interfacial trap state explains the doping concentration dependence, but

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the dopants ion trapping becomes important for the doping types (n- or p-type). The NPC of

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nanowire devices depends on doping concentration, such that heavily n-doped devices show a


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strong NPC behavior due to its longer interfacial trapping time. Also, the NPC and PPC occurs

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by means of light intensity which determines the carrier generation rate competing with the

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carrier recombination life time. NPC effect appears differently at wavelengths in visible light

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region due to the phonon assisted excitation to multi-conduction bands in the indirect band gap

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Si.

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Finally, the analysis of heavily doped Si nanowire based devices and comparison with NPC

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observed in other nanomaterials, leads to a conclusion that the NPC is a universal phenomenon

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for low-dimensional systems, due to the stronger influence of the surface/interface states. The

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controllable bipolar optical current switching may open novel possibilities in Si nanostructure-

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based electronic applications like optical integrated logic circuits, photonic function generators

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or even bio-/chemosensor applications. For the latter, since NPC phenomenon is observed in the

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subthreshold regime of the transfer characteristics, bio-/chemical nanosensors, typically driven

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in this regime, can potentially benefit from this new discovery. In particular, NPC can be

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considered as a measure of biochemical gating of the nanoscaled FET devices.

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Methods

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Device fabrication

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Si nanowire FETs were fabricated on three 8-inch SOI wafers that consist of a 40 nm-thick

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top-Si layer (Boron, 1016 cm-3), a 400 nm-thick buried oxide layer and the 725 µm-thick p-type

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Si substrate. To implant phosphorus ions in the top Si layer, the 20 nm SiO2 buffer layer was

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deposited using PECVD at 300 ℃. After that, phosphorus ions are implanted with an energy of

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15 kEV and the concentrations of dopants were 1013 cm-2 for 1018 cm-3 and 1014 cm-2 for 1019 cm-

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samples. Rapid thermal annealing followed, at 1,000 ℃ for 20s in N2 atmosphere to activate

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dopants. Finally, the buffer oxide layer was stripped in 1:100 dHF for 2~3 min. Consequently,

11

one of the wafers has a boron concentration of 1016 cm-3 and the other two wafers have

12

phosphorus concentration of 1018 cm-3 and 1019 cm-3, respectively. An active area including the

13

channel, the source and the drain region was defined for electrical isolation of devices using

14

photolithography and inductively coupled plasma reactive ion etching (ICP-RIE). The source and

15

the drain region were formed using phosphorus ion implantation with a concentration of 5×1020

16

cm-3, and dopant activation followed, using the same recipe above. A honeycomb nanowire was

17

patterned on the channel region using electron beam lithography and etched with ICP-RIE. The

18

pattern width of the nanowire was 50 nm and the length of the nanowire was 8 µm. A 5-nm thick

19

gate oxide layer was grown on the nanowire using a wet oxidation furnace at 850 °C for

20

passivation and post-processing. To form the source and the drain electrodes, a 500 nm Ag layer

21

on 50 nm Ti adhesion layer was deposited using an electron-beam evaporator and liftoff process

22

was followed. Finally, the whole wafer area except the nanowire channel region and metal

23

contact pad was passivated with 2 µm thick SU-8 epoxy-based photoresist to protect the long


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1

transmission line from unwanted contamination. The Figure S1 in Supporting information

2

shows the schematics of all steps for the device fabrication.

3 4

Top gate fabrication

5

Top gate electrodes were fabricated by patterning PMMA 950k using electron beam lithography,

6

followed by a lift-off process with sputtered platinum. The resist was spin-coated on the

7

honeycomb nanowire devices at a speed of 1000 rpm for 60 s, resulting in a 120 nm thick

8

PMMA film. The top electrode pattern was written by electron beam. Then, the samples were

9

immersed into the H2O:IPA(1:3) development solution for 3 min and cleaned in isopropanol.

10

After, a thin chromium adhesion layer (3nm) was thermally evaporated and a 30 nm platinum

11

layer was sputtered on it. The chip was immersed into acetone for 15 min to remove the PMMA

12

layer. Applying this protocol allowed connection of the gate electrode to the honeycomb

13

nanowires with a thin Pt electrode with a width of 650nm which covers approx. 8% of the

14

nanowire area. Finally, the sample was annealed in 200 ℃ to reduce the contact resistance.

15 16

Photocurrent measurement

17

A 4-channel light emitting diode (LED) driver (DC-4100, Thorlabs) which includes 4 visible

18

LEDs (λ = 405 nm, 470 nm, 530 nm and 625 nm) was used as a visible light source. The driver

19

could control the power intensity of light illumination and select the wavelength. The driver was

20

connected to the collimator through liquid waveguide to illuminate the target device area with

21

equivalent light power. The collimator was installed on the hand-made metal dark box with 5cm-

22

height.


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The bias generation and current measurement of FET devices were performed using Keithley

2

2600B at room temperature. The Keithley device was controlled by manual lab-view program.

3


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FIGURES

2 3

Figure 1. Structure and electrical characteristics of honeycomb Si nanowire FETs under

4

illumination (a) Schematic diagram of the Si nanowire FETs under light illumination. (b)

5

Microscopic image of Si nanowire devices with source and drain transmission line. Scanning

6

electron microscopy (SEM) image of (c) Si nanowire channel area and (d) honeycomb structure

7

of nanowires. (d) Transmission electron microscopy (TEM) image of cross-section of nanowire

8

and thermal SiO2 layer. (Scale bar : (b) 100 µm, (c) 10 µm, (d) 1 µm , (e) 20 nm)

9

(f) Transfer (Id-Vg) and (g) output (Id-Vd) characteristics of an n+-doped nanowire device under

10

illumination ((f) Vd = 0.5 V, (g) Vg = 2.3 V) (h) Photocurrent switching characteristics on the

11

time domain with increasing light intensity, (i) 0.022, (ii) 0.089, (iii) 0.442, (iv) 1.8, (v) 8.9

12

mW/cm2 respectively. (Vds = 0.5 V, Vg = 2 V) λ = 625 nm for (f)-(h).

13


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Figure 2. The current change depending on gate bias under light illumination (a) Transfer

3

characteristics (log scale) of the n+-doped Si nanowire FETs upon illumination. (b) The current

4

change (

) in light condition as a function of Vg. (λ = 625nm)

5

6 7

Figure 3. Doping concentration dependence of photoconductivity. (a) Threshold voltage shift

8

(∆Vth) of Si nanowire devices with various doping concentration depending on the light power

9

intensity, which is compared with the substrate potential change by illumination (blue dashed


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1

line). (b) Photocurrent change of the devices with various doping concentration depending on

2

light power intensity. The inset graph is photocurrent change vs. logarithmic scale of light power

3

intensity in logarithmic scale, which shows exponential reduction of current at low light intensity

4

(< 1 mW/cm2). The grey lines are fitted curves. (λ = 625 nm)

5

6 7

Figure 4. Negative photoconductivity of the Si nanowire device without substrate effect. (a)

8

Schematic diagram of Si nanowire FETs with top platinum electrode. (b) SEM images of the top

9

electrode configuration on a honeycomb nanowire device. (c) Photocurrent change of the n+-

10

doped top gate devices depending on light power intensity. The dark current level was 100 nA.

11

(λ = 625 nm) The dashed lines are fitted curves. (d) The schematic energy band diagram of Si


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and SiO2 interface explaining the hot carrier generation by light illumination, interfacial trapping

2

and release of excess electrons and recombination process via defect states.

3

4 5

Figure 5. (a) Wavelength dependence of photocurrent change of the n+-doped back gate device

6

with weak and strong light power intensity. (b) The schematic energy band diagram of Si under

7

visible light illumination. The energy band gap of Γ- and L- valley of Si and the energy of red (λ

8

= 625 nm) and violet (λ = 405 nm) light are shown in the diagram.

9


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1

TABLES material metal

direct semicond.

indirect semicond.

2

Page 24 of 29

dim.

excitation

Au nanoparticle

0D

-

AlN nanowire ZnSe nanowire Ga2O3 nanobelt

1D 1D 1D

subbandgap

hole trapping by surface oxygen

InN thin film

2D

MoS2 monolayer

2D

superbandgap

InNAs nanowire

1D

scattering by recombination centers increasing effective mass by trion hot-carrier trapping by surface states

Au-doped Ge

Bulk

Co-doped Si

Bulk

P-doped Si nanowire

1D

NPC mechanism plasmonic change by charged SAMs

ref.

2009

[2]

2010 2011 2011

[4] [5] [6]

2010

[7]

2014

[34]

2015

[3]

subbandgap

recombination by positive donor ions

1960 1966 1971

[35] [8] [11]

superbandgap

gating effect by hot electron trapping in dopants and interface states

2016

this work

Table 1. Negative photoconductance observed in different systems and its mechanism

3 parameter

life time

ref.

0.01 - 1 s [36] 0.1 - 1 ns 1 ps (at R.T.) Nd = 1019 cm-3 18

4

year

-3

[37]

0.1 µs [38]

Nd = 10 cm

1 µs

Na = 1016 cm-3

100 µs

[39]

Table 2. Experimental values of carrier life time in doped Si

5


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1

ASSOCIATED CONTENT

2

Supporting Information.

3

Additional figures for device fabrication step, substrate potential measurement, optical current

4

switching and wavelength dependent photoconductivity. This material is available free of charge

5

via the Internet at http://pubs.acs.org.

6 7

AUTHOR INFORMATION

8

Corresponding Author

9

*E-mail: [email protected]

10

Notes

11

The authors declare no competing financial interest.

12 13

ACKNOWLEDGMENT

14

This work was supported by the German Excellence Initiative via the Cluster of Excellence

15

EXC1056 “Center for Advancing Electronics Dresden” (CfAED) and the MSIP (Ministry of

16

Science, ICT and Future Planning), Korea, under the “ICT Consilience Creative Program” (IITP-

17

R0346-16-1007) supervised by the IITP (Institute for Information & communications

18

Technology Promotion). We further acknowledge the support from the Initiative and Networking

19

Fund of the Helmholtz Association of German Research Centers through the International

20

Helmholtz Research School for Nanoelectronic Networks (IHRS NANONET) (No.

21

VH‐KO‐606). We would like to give special thanks to Nils Puetz for support in top-electrode

22

patterning and Anh Thuy Phuong Nguyen for reading and discussion.

23 25 Environment ACS Paragon Plus

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Negative photoconductance in heavily doped Si nanowire field-effect

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