简介概要

Characterization and performance of graphene-PbSe thin film heterojunction

来源期刊：Rare Metals2021年第1期

论文作者：Bo He Yi-Xuan Ren Tian-Jun Dai Shuang Hou Xing-Zhao Liu

文章页码：219 - 224

摘要：An infrared detector with high responsivity based on graphene-PbSe thin film heterojunction was reported.High-quality PbSe thin film and graphene were prepared by molecular beam epitaxy and chemical vapor deposition,respectively.The physical characteristics of PbSe thin film and graphene were performed using X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS） and Raman measurement.The photo transistor using PbSe thin film as a sensitizer and graphene as a channel to transport excitons exhibits peak responsivity and detectivity up to～420 A·W^-1 and 5.9×10¹¹ Jones(radiation intensity:0.75 mW·cm^-2) at room temperature in the near-infrared（NIR） region,respectively.The high optical response is attributed to the photo-excited holes transferring from PbSe film to graphene under irradiation.Moreover,it is revealed that the responsivity of graphene-PbSe photo transistor is gate-tunable which is important in photodetectors.

详情信息展示

稀有金属(英文版) 2021,40(01),219-224

Characterization and performance of graphene-PbSe thin film heterojunction

Bo He Yi-Xuan Ren Tian-Jun Dai Shuang Hou Xing-Zhao Liu

School of Electronics Science and Engineering,University of Electronic Science and Technology of China

State Key Laboratory of Electronic Thin Films and Integrated Devices

作者简介：Xing-Zhao Liu e-mail:xzliu@uestc.edu.cn;

收稿日期：2 April 2018

基金：financially supported by the National Natural Science Foundation of China (No.51572043);

Characterization and performance of graphene-PbSe thin film heterojunction

Bo He Yi-Xuan Ren Tian-Jun Dai Shuang Hou Xing-Zhao Liu

School of Electronics Science and Engineering,University of Electronic Science and Technology of China

State Key Laboratory of Electronic Thin Films and Integrated Devices

Abstract：

An infrared detector with high responsivity based on graphene-PbSe thin film heterojunction was reported.High-quality PbSe thin film and graphene were prepared by molecular beam epitaxy and chemical vapor deposition,respectively.The physical characteristics of PbSe thin film and graphene were performed using X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS) and Raman measurement.The photo transistor using PbSe thin film as a sensitizer and graphene as a channel to transport excitons exhibits peak responsivity and detectivity up to～420 A·W^-1 and 5.9×10¹¹ Jones(radiation intensity:0.75 mW·cm^-2) at room temperature in the near-infrared(NIR) region,respectively.The high optical response is attributed to the photo-excited holes transferring from PbSe film to graphene under irradiation.Moreover,it is revealed that the responsivity of graphene-PbSe photo transistor is gate-tunable which is important in photodetectors.

Keyword：

PbSe; Graphene; Heterojunction; Infrared photodetectors;

Received： 2 April 2018

1 Introduction

Lead selenide (PbSe),a typicalⅣ-Ⅵcompound semiconductor with a narrow band gap (Eg) of 0.27 eV at room temperature for thin film [ 1, 2, 3] ,is regarded as one of the promising photonic infrared (IR) sensors for room temperature operation.Thermal evaporation in vacuum and chemical deposition are two main methods for preparing PbSe film.In order to obtain good performance in detectivity,sensitization process,in which the as-deposited film is thermally treated in specified atmospheres (such as oxygen),must be carried out [ 4, 5] .Much work has been devoted to this subject,and it is found that oxygen plays an important role in the sensitization process because of introducing deep trap and/or forming p-n junction in PbSe crystallites [ 1, 2, 4, 6, 7] .However,the responsivity (R) of such detectors is still very low (about several A·W^-1) in IR.Recently,graphene has been used as IR photodetectors due to its extremely high carrier mobility and light absorption in a broad wavelength rang [ 8, 9] .Yet the responsivity of such IR detectors is very low(≤6.1 mA·W^-1) because of graphene's low light absorbance (?2%for single-layer graphene) [ 10] .It is reported that phototransistors based on two-dimensional semiconductor materials (such as graphene) and PbSe (or PbS)quantum dots (QDs) had achieved ultrahigh photoresponsivity (～1×10⁶ A·W^-1) in IR region due to high-mobility conductors in two-dimensional (2D) materials,the spotlight merit of QDs,and photo-induced charge transfer between the interface of the heterojunction [ 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] .Nevertheless,such a method increases fabrication complexity because QDs are not compatible with current thin film micromanufacturing techniques and are difficultly fabricated into complex structures.Graphene-PbSe heterojunction photodetectors based on PbSe thin film have rarely been investigated,while PbSe thin film is an important semiconductor widely used for applications in IR region.Accordingly,PbSe thin film and graphene are chose to fabricate a hybrid photodetector that is expected to have high performance.

In this work,a phototransistor based on graphene-PbSe thin film heterojunction with a back-gate geometry was investigated (Fig.1).In the phototransistor,PbSe thin film acted as sensitizer to absorb light and graphene as a channel for excitons (holes) transport.It is reasonable that the irradiation introduced excitons (holes) in PbSe film would transfer to graphene due to the work function mismatch.As a consequence,these holes could circulate many times under the driving of external circuit before they recombined with the photo-excited electrons,which were trapped in PbSe thin film.Our phototransistor exhibits high responsivity and detectivity up to～420 A·W^-1 and5.9×10¹¹ Jones (at gate voltage of V_Gs=35 V and bias voltage of V_Ds=1 V),respectively,which is much larger than that of inpidual graphene or PbSe thin film photodetectors in the IR region.Moreover,the tunable responsivity of the hybrid phototransistor is realized by the back-gate modulation effect.

2 Experimental

2.1 Transfer process of graphene films

The graphene films grown on Cu foil were prepared via chemical vapor deposition (CVD) method [ 20, 21] .First,the graphene films were transferred onto a SiO₂/Si substrate by means of polymethyl methacrylate (PMMA)supported method [ 22, 23] .In brief,PMMA was spincoated on the surface of graphene/Cu foil at 3000 r·min^-1for 1 min and heated at 140℃for 2 min.Then the Cu foil was etched with FeCl3 solution (1 mol·L^-1).Afterward,the PMMA/graphene films were washed by deionized water (DI water) several times to remove the residual metal particles and transferred onto target substrates and then baked at 140℃for 10 min.Finally,the PMMA was removed successively by acetone and ethanol more than four times.

Fig.1 Schematic diagram of graphene-PbSe thin film phototransistor

2.2 Fabrication of graphene field-effect transistors(FETs)

Graphene FETs were fabricated via standard electron beam lithography and lift-off techniques after transferring the graphene film onto the SiO₂/Si substrate.Graphene was patterned to form FETs'channel (50μm in width and100μm in length) via photolithography and oxygen plasma.Then,60-nm Ni film was deposited by electron beam evaporation to define source and drain electrodes.The n⁺silicon layers and the coated 300-nm SiO₂ acted as the back-gate electrode and gate dielectric,respectively.

2.3 Deposition of PbSe thin film

PbSe film was grown by means of molecular beam epitaxy(MBE) using PbSe powder (99.999%) as the source material.The patterns of PbSe thin film (50μm×50μm)on graphene channels were defined by an aligned process with photoresist via photolithography.During the deposition,PbSe powder was heated to 600℃and the base pressure in the MBE chamber was approximately1.8 x 10^-4 Pa and the prepared substrates were unheated.The PbSe thin film deposited at 300 K was p-type [ 24] ,and the thickness of the film was about 70 nm.The photoresist was removed successively by acetone and ethanol more than two times after depositing.

2.4 Characterization or measurements

Rapid annealing (300℃for 3 min,nitrogen atmosphere)was carried out to improve the interface between graphene and PbSe thin film.Electrical measurements were implemented using a semiconductor parameter analyzer (Agilent4155B) in air at room temperature.The wavelength of the incident light used was 1050 nm.

3 Results and discussion

3.1 Characterization of graphene and PbSe film

The Raman spectrum of pristine graphene is shown in Fig.2a.The measurements were performed at room temperature with a Renishaw spectrometer at 532 nm.The G band and 2D band are located at 1585 cm^-l and2671 cm^-1,respectively.It is found that the G to 2D peak ratio (IG/I2D) is 0.45 and the full width at half maximum of the 2D peak is 40 cm^-1,indicating that the graphene is monolayer [ 25] .The absence of D peak proves the defectsfree characteristic of our single-layer graphene.From the Raman characterization results,it can be drawn a conclusion that the graphene transferred onto SiO₂/Si substrate is monolayer and in good quality.

Fig.2 Characterization of graphene and PbSe thin film:a Raman spectrum of pristine graphene and b XRD pattern of PbSe thin film

Figure 2b shows X-ray diffraction (XRD) pattern of the deposited PbSe thin film.According to Fig.2b,the peaks of XRD pattern are well matched with the cubic PbSe phase (JCPDS file No.78-1902).The diffraction peaks correspond to reflections from (111),(200),(220),(311),(222),and (400) planes,except the one from the Si substrate (located at 69°).Moreover,the obvious predominant peak suggests a preferred orientation along the (200) plane.For XRD,the diffraction intensity of a certain orientation is the accumulation effect of all equivalent crystal planes,called multiplicity factor [ 26] .In addition,the XRD pattern of the sample corresponds to PbSe film with some traces of PbO and the weak diffraction signals are labeled by star(*).The formation of PbO can be due to the effect of air exposure on the sample during the test.

The high-resolution Se 3d and Pb 4f X-ray photoelectron spectroscopy (XPS) spectra of the deposited PbSe thin film are shown in Fig.3a,b,respectively.The spectrum (from 48 to 64 eV) in Fig.3a is dominated by Se^2-3d core level.As can be seen,the spectrum is broadened and decomposed into two components located at 52.8 and 53.9 eV (corresponding to 3d_5/2 and 3d_3/2levels) [ 27] .The two components correspond to Se^2-of PbSe.The spectrum is dominated by two features (located at～137 and～143 eV) due to the 4f_5/2 and 4f_7/2 spinorbit coupled doublet of Pb in Fig.3b.The feature in each band of the doublet (137.5 and 142.8 eV) is attributed to the Pb²⁺of PbSe [ 28] .In addition,the atomic ratio of Pb/Se calculated on the basis of Pb(4f) and Se(3d)spectrum is approximately 7:4,which shows that the product is rich in lead.

3.2 Photoelectric properties of graphene-PbSe heterojunction

Figure 4a presents the drain-source current (I_Ds) of the graphene-PbSe photo transistor as a function of drainsource voltage (V_DS) under dark and illumination at gate voltage of V_GS=0 V.It is observed that the I_DS shows a linear dependence on the V_DS,which indicates a good ohmic contact between graphene and metal.The inset is the enlarge of the curve from V_DS=0.50-0.70 V.Responsivity (R) and detectivity (D*) of our device are determined by the equation below:

where Iill and Idark are the channel current values under illumination and dark.P,E_e,e,and S are power density,irradiance,the charge of an electron,and the effective area under illumination,respectively [ 1, 29] .

Accordingly,the R of the phototransistor is～54 A·W^-1 (P=0.75 mW·cm-2 at V_DS=1 V,V_GS=0 V),which is much higher than that of inpidual graphene(≤6.1 mA·W^-1) [ 30] or PbSe thin film photodetectors.Moreover,the D*of 6.1×10¹⁰ Jones is achieved.Clearly,PbSe thin film does play a key role in the photocurrent generation in our device.

Transfer characteristics (I_DS vs.VGS) of the graphenePbSe transistor under dark and illumination at V_DS=1 V are shown in Fig.4b.Both of the two curves reveal a typical Dirac point (V_Dirac)(the charge neutrality point)where the channel resistance is maximum.Compared with the transfer characteristics under dark and illumination,it is found that the V_Dirac shifts to more positive value under illumination,indicating a photo-induced p-doping effect of graphene [ 30] .In addition,an increase in the I_Ds under illumination at V_GS below V_Dirac (～50 V) is shown in Fig.4b.The photo-induced current varies and reaches the smallest at V_GS=-49 V.And when V_GS is higher than V_Dirac,a decrease in I_DS is observed.

Fig.3 XPS patterns of deposited PbSe thin film:a high-resolution Se 3d spectra and b high-resolution Pb 4f spectra

Fig.4 Optoelectronic characteristic of graphene-PbSe hybrid photodetector:a drain-source current (I_DS) of the graphene-PbSe phototransistor as a function of drain-source voltage (V_DS) under dark and illumination at gate voltage of V_GS=0 V (inset being enlarge of curve from V_DS=0.50-0.70 V),b transfer characteristics (I_DS vs.V_GS) of graphene-PbSe transistor under dark and illumination at V_DS=1 V

3.3 Optical responses mechanism study

To get a better understanding of the origin of optical responses,band structure of PbSe thin film and graphene are first considered.The electron affinity and band gap (E_g)of bulk PbSe at 300 K are—4.21 eV (with respect to the vacuum) and 0.27 eV,respectively [ 1, 2] .In our phototransistor,the PbSe thin film deposited at room temperature is p-type [ 24] .And thus the Fermi energy (E_F) of PbSe is close to its valence band (E_VB).The Fermi level of intrinsic graphene is 4.5 eV at the Dirac point [ 14] .It is reasonable that irradiation introduced excitons (holes) were transferred from PbSe to graphene because of the different work function at the interface.In the transfer characteristics,a photo-induced p-doping effect of graphene under illumination also indicates that holes excited by IR light in PbSe transfers into graphene and leaves the excited electrons trapped in the PbSe film simultaneously.The excited electrons trapped in PbSe film can attract more holes in the graphene so that a higher V_GS is needed to obtain the Dirac point in the graphene FET [ 30] .

Fig.5 Band diagrams explaining photo-induced holes transfer for a gate voltage below V_Dirac (V_GS<VD_irac) and b gate voltage higher than V_Dirac (V_GS>V_Dirac)

As shown in Fig.5a,channel current in the graphene FET is dominated by holes at gate voltage below V_Dirac。The channel current of the phototransistor increases under illumination with NIR light,which is ascribed to the injection of photo-induced holes from PbSe to graphene.Furthermore,hardly any photo-induced current at V_GS=—49 V is observed due to that the Fermi energy of the graphene is located at a position below the energy level of PbSe and thus photo-induced holes transferring is forbidden.At V_GS=～60 V,photo-induced current decrease to zero because the holes transferred from PbSe are just recombined by the electrons which are accumulated by the gate voltage in the graphene channel.The highest responsivity (～420 A·W^-1) and detectivity (5.9×10¹¹ Jones)are obtained at V_GS=35 V,where the Fermi level of graphene is at the best position for the photo-excited holes to transfer from PbSe to graphene.On the other hand,the channel current is dominated by electrons when the gate voltage is higher than V_Dirac,as shown in Fig.5b.The decreased channel current under illumination is attributed to the decrease in electrons which are recombined by photo-excited holes transferring from PbSe.The above results suggest that the phototransistor is gate-tunable which is important in photodetectors.

4 Conclusion

In this work,a high responsivity photodetector based on graphene-PbSe thin film heterojunction was fabricated.The high optical response is attributed to the transfer of photo-excited holes from PbSe film to graphene under irradiation with NIR light.As graphene⁵ s ambipolar properties,the transistors'channel current under illumination is increased due to the injection of photo-induced holes from PbSe to graphene at gate voltage below V_Dirac.When the gate voltage is higher than V_Dirac,the decreased current under illumination is attributed to the decrease in electrons which are recombined by photo-excited holes transferring from PbSe.The responsivity of the device is tuned by regulating the gate voltage,indicating that it is gate-tunable which is important in photodetectors.