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Switching from Negative to Positive Photoconductivity towards Intrinsic Photoelectric Response in InAs Nanowire Yuxiang Han, Mengqi Fu, Zhiqiang Tang, Xiao Zheng, Xianghai Ji, Xiaoye Wang, Weijian Lin, Tao Yang, and Qing Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13775 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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ACS Applied Materials & Interfaces

Switching from Negative to Positive Photoconductivity towards Intrinsic Photoelectric Response in InAs Nanowire Yuxiang Han†, Mengqi Fu†, Zhiqiang Tang†, Xiao Zheng†, Xianghai Ji‡, Xiaoye §

,*

,*

Wang‡, Weijian Lin§, Tao Yang‡, , Qing Chen†,

†

Key laboratory for the Physics and Chemistry of Nanodevices and Department of

Electronics, Peking University, Beijing 100871, China ‡

Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors,

Chinese Academy of Sciences, Beijing 100083, China, and College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China §

Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing

100190, China

Abstract: Negative photoconductivity (NPC) and positive photoconductivity (PPC) are observed in the same individual InAs nanowires grown by metal-organic chemical vapor deposition. NPC displays under weak light illumination due to photo-excitation scattering centers charged with hot carrier in the native oxide layer. PPC is observed under high light intensity. Through removing the native oxide layer and passivating the nanowire with HfO2, we eliminate the NPC effect and realize intrinsic photoelectric response in InAs nanowire.

Keywords: InAs nanowire, PPC, NPC, carrier scattering, intrinsic photoelectric response

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Introduction III–V and Ge semiconductors are promising as building blocks for the next generation high performance electronic and optoelectronic devices, mainly due to their higher mobilities than Si. Among III–V nanowires (NWs), InAs NWs, due to their narrow bandgap, high electron mobility and easiness to form Ohmic contact with metals, have great potential in high-frequency RF transistor,1 tunnel FET,2 infrared photodetectors and photovoltaic cells.3,4,5 The electrical properties of InAs NWs have been intensively studied and found to be influent by the crystal phase, crystal orientation and diameter of the NWs.6,7,8 Moreover, ultrafast photocurrents and THz emission have been found in InAs NW.9,10 Recently, negative photoconductivity (NPC) of InAs NW has been reported by several groups, but has been explained by different mechanisms. NPC observed in chemical vapor deposition (CVD)-grown InAs NW has been explained by a defective photogating layer (PGL) which can trap photogenerated electrons, so that the residual unpaired holes recombine with carriers in the channel to cause current reduction.11 While, the origin of NPC in molecular beam epitaxy (MBE)-grown InAs NW has been attributed to the depletion of conduction channels by light assisted hot electron trapping.12 We have reported NPC in single crystalline MBE-grown InAs NW without a surface defective layer.13 We found that there are two mechanisms working together, one is related to the photodesorption of water molecules and photo-assisted chemisorption of O2 molecules, and the other one can be attributed to the photogating effect introduced by the native oxide layer outside the NWs. Although NPC in other nanomaterials have also been attributed to the surface oxide layer, there still lack direct evidence to prove it. In this work, we study the NPC effect in the InAs NWs covered by a native oxide layer and grown by metal-organic chemical vapor deposition (MOCVD). The NPC effect is attributed to enhanced carrier scattering by trapped photoelectron in the InOx layer, which is proved by the decreased carrier mobility. Furthermore, the intrinsic positive photoconductivity (PPC) is realized through removing the native oxide layer with NH4Sx solution followed by passivating the NW by HfO2. The wavelength


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dependent NPC and PPC response is also observed and investigated.

Results and discussion All the InAs NWs studied in this work were grown by MOCVD.14 The scanning electron microscopy (SEM) image of the sample is shown in Figure 1(a). The MOCVD-grown InAs NWs are characterized to have wurtzite (WZ) phase with large density of stacking faults and covered with 2 nm thick native oxide layer. All the NWs were grown preferentially along WZ direction, as shown in Figure 1(b). InAs NW field effect transistors (FETs) were fabricated on a Si substrate covered with 300 nm SiO2 as gate dielectric. Figure 1(c) is the transfer curve and the device structure of a typical transistor we investigate. The diameter of the InAs NW is 34.4 nm and the channel length is 2.58 µm. The On-Off ratio of the FET is about 104, which is among the highest of MOCVD-grown InAs NW FETs reported in the literature. Figure 1(d) is the output curve of the transistor, the linear relationship between Ids and Vds indicates the contact is Ohmic contact. Unlike the MBE-grown single crystalline InAs NW, the MOCVD-grown NWs are carbon-doped unintentionally by the organic source15 and have higher original electron density and current density. The On-state current of the MOCVD-grown InAs NW in this work is about one order of magnitude larger than that of the MBE-grown InAs NW we reported previously.13 To investigate the photoconductance of the MOCVD-grown InAs NW, the NW FETs were measured in ambient air at room temperature, which is 1.013x105 Pa (78% N2, 21% O2), 20℃ and relative humidity of 50% on a LabRAM HR800 evolution system. First, the Ids-Vgs curves of a back-gate InAs NW FET were measured in dark and under 633 nm light illumination with different intensities. As shown in Figure 2(a), the On-state current decreases obviously from 4.99 µA in dark to 4.12 µA under light illumination with 9.71×10-1 mW/cm2 intensity. With the increase of the light intensity, the On-state Ids decreases first and then increases a little bit. However, the On-state current cannot restore to the dark current level even under intensive light



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illumination with 9.71 × 102 mW/cm2 intensity. So the photocurrent Iphoto (Iphoto=Ilight-Idark, where Ilight and Idark represent Ids under light illumination and in dark) remains negative, this effect is known as NPC. Besides in InAs,11-13 NPC has also been reported in other III–V materials, such as GaAs16 and InN17.

Figure.1 (a) SEM image of the InAs NWs grown on Si substrate by MOCVD. (b) High resolution TEM image and the corresponding Fourier transformation pattern (the low-right inset) of an InAs NW. (c) Ids-Vgs curve and (d) Ids-Vds curves of an InAs NW back-gate FET with its structure schematically shown by the diagram in the inset of (c).

Shown by the Ids-Vgs curves in logarithmic form depicted in the inset of Figure 2(a) and Figure S1 in supporting information, at the Off-state, Ids decreases a little under weak light illumination, which is about 0.73 nA in dark and 0.72 nA at light intensity of 9.71×10-1 mW/cm2. However, current decrease is not very clear at the


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Off-state due to the heat induced current increase shortly after light illumination, this part of current increase compensates NPC to some extent (supported by Figure 5(a) shown later). Our Ids-Vgs curves were taken in 10 seconds upon light illumination, so the NPC effect weakened. While for some nanowires having unsmooth transfer curves, probably due to poor surface conditions, NPC is more obvious at the Off-state (as shown in Figure S2 in supporting information). As the light intensity increases by tens and hundreds of times, Ids increases from 3.06 nA to 67.28 nA and eventually reached 0.12 µA at the light intensity of 9.71×102 mW/cm2 at Vgs= -20 V, indicating a positive photoconductance (PPC) response. The Ids-Vds curves measured at the transistor’s On and Off-states in Figure 2(b) and 2(c) also support the photocurrent variation described above.



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Figure.2 (a) Ids-Vgs curves of a back-gate InAs NW in the dark and under the illumination of the light with its wavelength being 633 nm. The up-left inset is the logarithmic coordinate Ids-Vgs curves in the dark and under the light with intensity of 9.71×10-1 mW/cm2. (b) Ids-Vds curves at Vgs=20 V and (c) Vgs=-20V in the dark and under illumination of light with different intensities, the light intensity of each curve is shown in the legends of (a). (d) Photo induced threshold voltage


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and Vgs swing changes as a function of the light intensity. (e) Photo induced changes of carrier mobility at the maximum transconductance and carrier concentration at Vgs=0V as a function of light intensity.

How can the same FET displays both NPC and PPC just at different gate voltage? To understand the mechanism, we extract light intensity-dependent threshold voltage (Vth), subthreshold gate voltage swing (Vgs Swing), field-effect mobility ( ) and carrier concentration (ne) from Figure 2(a). As shown in Figure 2(d), the threshold voltage shifts continuously from 1.60 V in the dark to -17.57 V under light illumination with the intensity of 9.71×102 mW/cm2, indicating an increase of carrier concentration in the channel. This Vth shift is the opposite to that reported previously in MBE-grown InAs NW13 which is caused by photogate depleting electrons in the channel. The negative shift of Vth can be attributed to photo-generated carriers in the channel like in most photoelectric processes. As some of our transfer curves do not show Ids change of more than 4 orders of magnitude, instead of subthreshold swing (SS), we calculate subthreshold gate voltage swing (Vgs Swing) of the transistors through the linear extension from the maximum slope in the logarithmic coordinate transfer curve. Vgs Swing is observed to increase from 4.23 V/dec to 15.64 V/dec as the light intensity increases, which denotes the decrease of gate control owing to the light illumination. The carrier mobility can be calculated using = / , where is the back-gate capacitance of the NW, is the maximum transconductance at Vds=0.1V and Lg is the channel length. is calculated by the

metallic cylinder on an infinite metal plate model,18 = 2 ( ), where

denotes the vacuum permittivity, , t and r are the relative dielectric constant (3.9 for SiO2), the thickness of the gate dielectric (300 nm) and the radius of the InAs NW (17.2 nm). Normally, field-effect mobility is affected by other factors like the contact and the gate voltage. The present extracted mobility is the field-effect mobility at peak transconductance and roughly reflects the mobility of the material. The calculation shows the field-effect mobility is 2782 cm2/V ∙ s in the dark, which is comparable



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with previous results. However, the carrier mobility decreases with light intensity, as shown in Figure 2(e). The carrier mobility decrease can be induced by the photo-excitation scattering centers formed through charging the defect states by photo-generated electrons.19,20 The increased Vgs Swing also supports an increase of charged defects under light illumination. The values of carrier concentration are extracted through ne=σ/e and σ = /π" #, where σ is the conductivity and R is the NW resistance at Vgs=0 V obtained from the linear region of the Ids-Vgs curves. The extracted carrier concentration is about 1.81×1018/cm-3 in the dark, but it increases with the light intensity, as shown in Figure 2(e). Carrier concentration increase is caused by the photo-generated carrier. Whether PPC or NPC is displayed in the InAs NW FETs is decided by the combined effects of ne and . At the On-state, the carrier concentration in the channel is already high in the dark, the photo-generated carrier is not so important, but the mobility decrease is obvious, so that the total current decreases and NPC is observed. At the Off-state, the carrier concentration in the channel is very low in the dark, so that the photo-generated carrier is dramatic compared with the background carrier concentration. Although the mobility also decreases, the total current increases when the light intensity is high enough at the Off-state. We find that the NPC and PPC widely exist in the present MOCVD-grown InAs NWs. However, the threshold light intensity at which NPC turns to PPC may be affected by other factors, such as the diameter or surface structure of the NW. Figure S2 shows the transfer curves of another InAs FET, in which PPC is observed at the Off-state when the light intensity is above 9.71×102 mW/cm2. Besides the light intensity, the wavelength of the light also influences the photoresponse of the InAs NW FETs. The wavelength-dependent photocurrent response is measured with different wavelength from 650 nm to 1000 nm under the same light intensity of 7.64 mW/cm2 from the InAs NW FET whose transfer curves are displayed in Figure S2. As shown in Figure 3(a), the untreated InAs NW (which is covered by a native oxide layer) shows NPC under light wavelength less than 950 nm,


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and turns to PPC at wavelength of 1000 nm. Meanwhile, the shorter wavelength of the light, the larger NPC would be and the negative photocurrent tends to saturate at short wavelength (e.g. 650 nm). Previous works have suggested that the NPC is caused by light assisted hot electron trapping in oxide layer or PGL,11-12 our previous work on single crystalline MBE InAs NW without defective layer has also indicated the native oxide layer can trap photo-generated electron and cause photogating effect.13 Also due to the existence of the rich surface states, the PPC of InAs NW photodetector is influenced by gas atmosphere.5 However, no one has removed the native oxide layer in channel and realized intrinsic photoconductivity of InAs NW so far.

Figure.3 Photocurrent response of InAs NW FETs to the light with different wavelengthes and

with the intensity of 7.64 mW/cm2. The FETs are at the Off-state and the NW is covered with (a) or without (b) a native oxide layer. The FETs are at the On-state and the NW is covered with (c) or without (d) a native oxide layer.



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To reduce the unfavorable effects caused by the surface states on the electrical and optical properties of InAs NW, several methods have been developed to remove the native oxide and passivate the surface such as Ar plasma dry etch,21 in situ atomic hydrogen cleaning22 and cover a layer of HfO2/Al2O3 by atomic layer deposition (ALD).23 However, Ar plasma tends to cause lattice damage of InAs NW and cannot prevent regrowth of an oxide layer. HfO2/Al2O3 grown by ALD can effectively prevent oxide regrowth and reduce a small amount of As2O3, but cannot clean InOx effectively. In situ hydrogen cleaning is not compatible with the device fabrication process. The method widely adopted to remove the native oxide layer is NH4Sx solution etching.24,25 Here, we treat the channel of InAs NW FET with NH4Sx solution. And then, the transistor is passivated with 10 nm HfO2 by ALD. Figure 3(b) shows the wavelength-dependent photocurrent response of the treated InAs NW to the light illumination under the same condition as Figure 3(a). NPC is vanished at all the wavelength, only PPC can be observed. This NPC to PPC transformation is repeatable on different transistors. The photocurrent is the highest under 650 nm light and decreases as the wavelength increases. This wavelength-dependent photoresponse property is the same as reported previously in MOCVD-grown InAs NW26. The photoelectric response at the transistor’s On-state (Vgs=20V) is also wavelength-dependent. Before removing the native oxide layer, NPC is observed when the light wavelength is short (Figure 3(c)), similar to that at the Off-state. However, the threshold wavelength from NPC to PPC is 900nm (1.38 eV) at the On-state, indicating a higher energy is required for the electrons to get trapped in the oxide layer and induce NPC, possibly due to the necessary to overcome the severe electron-electron scattering at the On-state. PPC at the On-state is also observed at all the wavelength after removing the oxide layer (Figure 3(d)). The value of the photocurrents at the On-state are all much larger than that at the Off-state at different light wavelength except at 650 nm. The photocurrent does not change so significantly with the wavelength as that at the Off-state and larger photocurrent lies at longer wavelength. This is possibly because although the photocurrent at the On-state is large, the dark current and the current under light at the On-state are even larger, being at


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least one order of magnitude higher than the photocurrent, so that the photocurrent extracted through Iphoto=Ilight-Idark is not very accurate. Herein, we confirm that the NPC originates from the native oxide layer, and by removing the native oxide layer, the intrinsic photoelectric response (which is PPC) is achieved in InAs NWs. Based on the results shown above, we propose the mechanism of NPC in MOCVD-grown InAs NW as depicted in Figure 4. As shown in Figure 4 (a), when the material is illuminated with photons with energy much higher than InAs NW’s bandgap, photogenerated electrons can be trapped in the native oxide layer and act as scattering center, corresponding to process Ⅰ and Ⅱ in Figure 4(a). Besides, part of the excited hot electrons can relax to the conduction band (CB) and increase the carrier concentration (process III) or further recombine with holes in the valance band (VB), noted as process Ⅳ. For light with different intensities, the total response will be different as described below in detail. (i) Under weak intensity light illumination, photoexcitation induces additional charge carriers, leading to an increase in carrier concentration in the channel (Figure 4(b)) although a small part of holes can recombine with the electrons in the channel (Figure 4 (c)). On the other hand, the motivated hot electrons get trapped in the native oxide layer and then work as carrier scattering center to decrease the carrier mobility (Figure 4(c)). Specially, at the On-state (Vgs≥Vth), the enhanced electron-electron scattering at higher carrier concentration can further decrease the carrier mobility. The impact from the carrier mobility decrease is superior to the carrier concentration increase and NPC is displayed on the whole. (ii) Under high intensity light illumination at the Off-state (Vgs

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