Band Offsets, Optical Conduction, Photoelectric and Dielectric Dispersion in InSe/Sb2Te3 Heterojunctions

InSe based heterojunction devices gain importance in optoelectronic applications in NIR range as multipurpose sensors. For this reason, InSe/Sb2Te3 heterojunctions are constructed as NIR sensors by the thermal evaporation technique. The structural, optical, dielectric and photoelectric properties of InSe/Sb2Te3 heterojunctions are explored by X-ray diffraction and ultraviolet-visible light spectrophotometry techniques. The structural analyses revealed the preferred growth of polycrystalline hexagonal Sb2Te3 onto amorphous InSe as a major phase. Optically, the coating of Sb2Te3 onto InSe enhanced the light absorbability of InSe by more than 18 times, redshifts the energy band gap, increased the dielectric constant by ~5 times and increased the optical conductivity by 35 times in the NIR range of light. A conduction and valance band offsets of 0.40 and 0.68 eV are determined for the InSe/Sb2Te3 heterojunction devices. In addition, the Drude-Lorentz fittings of the optical conductivity indicated a remarkable increase in the plasmon frequency values upon depositing of Sb2Te3 onto InSe. The illumination intensity and time dependent photocurrent measurements resulted in an enhancement in the photocurrent values by one order of magnitude. The response time of the devices is sufficiently short to nominate the InSe/Sb2Te3 heterojunction devices as fast responding NIR sensors suitable for optoelectronic applications.


Introduction
One of the famous research sectors which are popular in the current century is the design of new classes of semiconducting materials that suits many optoelectronic applications. These semiconductor groups include organic/ inorganic 1 , metal-(double) oxides 2-4 layers and mixed metaloxide nanoparticles 5 . Organic Cr-Co mixed crystals is mentioned suitable for fabrication of micro-nano devices 5 . In addition, Cr-Ni OSC complexes are observed to exhibit energy band gaps of ~1.87 eV which nominates it for use in visible light communications as signal receivers 3 . Moreover, carboxymethyl cellulose zinc thin films which were fabricated using the sol-gel technique and widely tested are found to be ideal materials for use in producing photosensors and solar cells 6,7 . As nanoparticles, Co-Al oxides are reported promising alternative materials for improving the power conversion efficiency of solar cells and for obtaining clean and renewable energy 4 .
Another interesting material is antimony telluride in thin films forms which attracted the attention of research society owing to their wide range of applications. They can exhibit different characteristics that depend on the substrates they grow onto. As for examples, Sb 2 Te 3 films coated onto Bi 2 Te 3 are observed to behave as a planer thermoelectric generator 8 . n-Bi 2 Te 3 /p-Sb 2 Te 3 heterojunctions which are prepared onto Kapton substrates generated open circuit voltage of 0.21 V and output power of 3.3mW/cm 2 . As another example, Sb 2 Te 3 films are employed as templates to enable fast memory switching of phase change materials like GeSb 2 Te 4 9 . Sb 2 Te 3 coated onto Ge-Sb-Te substrates is nominated for as promising heterojunctions for enhancing the speed of response of phase-memory change devices 9 . In addition, Sb 2 Te 3 thin films which are deposited onto CdTe based solar cells, improved the solar cell efficiency up to 8.01% 10 . Moreover, the vertically stacked layers of Sb 2 Te 3 and MoS 2 formed heterojunctions that are suitable for high performance optoelectronic applications 11 . The current rectification ratio of this device reached 10 6 . It also displayed excellent photovoltaic properties. Furthermore, antimony telluride films coated onto Si substrates displayed photodetector characteristics presented by a self-powered fast and broad band photodetection. The response time of this device was less than 40 ms 12 .
In one of our recent works [13][14][15] , we have also shown that a thin layer (20 nm) of Sb 2 Te 3 can significantly improve the performance of CdS and force it exhibiting negative capacitance effect in the frequency range of 260-1800 MHz. The CdS/Sb 2 Te 3 heterojunction devices were usable as a wide range low pass filters displaying ideal voltage standing wave ratios. In the light of the above mentioned inventions, *e-mail: atef.qasrawi@aaup.edu here in this work, we are motivated to construct a new class of heterojunction devices that get benefit from Sb 2 Te 3 . Namely, Sb 2 Te 3 films of thicknesses of 300 nm are coated onto InSe substrates of thicknesses of 1.0 m µ . The formed InSe/Sb 2 Te 3 heterojunctions are structurally, optically and photo-electrically characterized. The characterizations include the crystallization and growth nature, the light transmittance, reflectance and absorbance, identification of energy band offsets and energy band gaps as well. The dielectric dispersion and optical conductivity dynamics at the InSe/Sb 2 Te 3 interfaces are also included. In addition, the light intensity and time dependencies of the photocurrent are also considered.

Experimental Details
Indium selenide-antimony telluride heterojunction devices are prepared by the thermal evaporation technique using NORM VCM -600 thermal evaporator. The vacuum pressure was kept at 10 -5 mbar. The InSe films were coated onto chemically and ultrasonically cleaned glass substrates. The grown indium selenide thin films were of thicknesses of 1.0 µ m. The grown glass/InSe films were masked and covered with Sb 2 Te 3 thin films. The thickness of Sb 2 Te 3 thin films was 300 nm. This thickness was selected to allow minimum light transmittance and increase the reflectance. The high reflectance values leads to higher values of dielectric constants. The resulting InSe/Sb 2 Te 3 films were coated with Ag pads of areas of 3.14

Results and Discussion
The geometrical design and X-ray diffraction patterns for the InSe/Sb 2 Te 3 thin films which are prepared by a vacuum evaporation technique are shown in Figure 1. In accordance with the figure, no sharp diffraction patterns are observed for InSe films. The broaden XRD patterns of InSe reveal an amorphous nature of structure. On the other hand, Sb 2 Te 3 thin films which are coated onto glass substrates displayed intensive peaks at various diffraction angles. The observed sharp peaks were compared to the standard PDF cards of ) are calculated for the peaks oriented along the (01-1) direction. While the crystallite sizes decreased from 28 to 24 nm, ∈ , and b δ increase from 6.28 10 lines cm × respectively when Sb 2 Te 3 was coated onto InSe substrates. The decrease in the grain size may be assigned to the increased surface roughness upon coating of Sb 2 Te 3 onto InSe substrates 18,19 . As previously mentioned, the increased/decreased in average crystallite size was confirmed by reduced/enhanced dislocation density and micro strain and is assigned to the large difference between the surface energy of the differently oriented planes 20 .
In general, Sb 2 Te 3 crystals are similar in structure to Bi 2 Se 3

16,17
. In these materials, the hexagonal unit cells are formed from quintuple layers oriented along the (0001) direction of the hexagonal unit cell. Each layer is composed of five covalently bonded Te-Sb-Te-Sb-Te atomic sheets. While the interactions between two adjacent quintuple layers are predominated by the weak van der Waals forces, interactions between the atomic sheets which exists within a quintuple layer is strong. Hence, the cleaving planes are centered between the quintuple layers and should always terminate with Te atomic layers. In Sb 4 Te 3 , similar to the structure of (Bi 2 Se 3 ) m (Bi 2 ) two extra Sb layers are added to the sequence of stacking order owing to richness of Sb that results from breaking the weaker bonds in a neighboring quintuple  17 ) and In-Se (296 pm 21 ), interaction between In and Te to form indium telluride is probable. However, since InSe is of amorphous nature, its surface is expected to have large amount of In-Se broken bonds. The capturing of indium ion to a weakly bonded tellurium ion (bond length of In-Te is 272 pm 22 ) leave one Sb ion not bonded. Such variations in the bonding mechanisms could account for the changes in the covalently bonded layers resulting in the appearance of Sb 4 Te 3 or appearance of In 2 Te 3 as well. Figure 2a and b illustrate the transmittance (T ) and reflectance (R) spectra for InSe films before and after coating with Sb 2 Te 3 . It is clear from the figure that the transmittance remarkably decreased by more than fourteen times upon coating of InSe with Sb 2 Te 3 . The local maxima which are observed at 656 nm in the T spectra of InSe disappeared. On the other hand, the reflection spectra of InSe which are displayed in Figure 2b exhibit one absolute and one local minima at an incident light wavelengths (λ ) of 540 and 780 nm, respectively. Coating of InSe with antimony telluride increased the reflectance of InSe, changed the shape of variations of the R λ − dependencies and forced cancellation of the reflectance peaks. The total effects of the Sb 2 Te 3 coatings on the optical properties of InSe are evident from the absorption coefficient (α ) spectra. The absorption coefficient for stacked layers of total thickness d is calculated from the relation 23 when used for glass/InSe films. α spectra for InSe and InSe/Sb 2 Te 3 films are shown in Figure 2c.
As seen from the figure, the absorption coefficient values for InSe follow a sharp trend of variation in two easily distinguishable high (3.30-1.90 eV) and low (1.70-1.30 eV) energy regions. α -spectra of InSe/Sb 2 Te 3 displays only one sharp trend of variation in the range of 2.73-1.31 eV. It is also clear from Figure 2c that the coating of Sb 2 Te 3 onto InSe films redshifts the absorption coefficient spectra and It means that coating of Sb 2 Te 3 onto InSe highly enhanced the light absorbability of InSe. The smaller the energy value is, the larger the light absorbability. The highest recordable R λ values is 18.5 at 1.27 eV. The enhanced light absorbability makes the InSe more suitable for optoelectronic applications. The increase in the absorption coefficient and light absorbability can be assigned to the increase in the volume of the absorbing material upon InSe/Sb 2 Te 3 interfacing 25 . It could also be assigned to the higher efficiency of energy transfers that is reached by suppressing the unbonded or dangling ions at the InSe surface 26 . The enhanced absorption characteristics may have resulted from the formation of the clusters of Sb 4 Te 3 and In 2 Te 3 at the ultrathin interface. These two minor phases were observed in the XRD patterns ( Figure 1).
To reveal clearer idea about the changes in the optical transitions as a result of stacking of Sb 2 Te 3 with InSe, Tauc's equation, was employed. Theoretically n is equal to 2, 1/2, 3 or 3/2 for the indirect allowed ( IA g E ), direct allowed ( dA g E ), indirect forbidden ( IF g E ) and direct forbidden ( dF g E ) transitions, respectively 3,27 .The decision about the favorable transition type is taken in accordance with the equation that linearly  28 and laser sublimation and chemical deposition techniques 29 . The value of the energy band gap for InSe/Sb 2 Te 3 bilayers being 0.90 eV mostly arises from interactions between indium and tellurium at the ultrathin interface. As we suggested through the XRD analyses the bonding between indium and tellurium forced formation of Sb 4 Te 3 or In 2 Te 3 as minor phase in the structure of Sb 2 Te 3 . Interactions between In and Te forms In 2 Te 3 . The energy band gap of In 2 Te 3 is reported to exhibit values in the range of 0.80-0.99 eV 30,31 . The bonding energy of In-Te being 218.0 kJ/mol is much larger than that of In-Sb (151.9 kJ/mol 32 ), of Sb-Se (183.89 kJ/mol 33 ) and of In-Se (48.2 kJ/mol 34 ). The larger the bonding energy is, the stronger the interaction with the surrounding atoms, the more difficult the vacancy formation 35 . Other studies which take into account the linear optical properties and nonlinear absorption behaviors of Sb 2 Se 3 and Sb 2 Te 3 have shown that the metallic character of Sb 2 Te 3 is much stronger than that of Sb 2 Se 3

36
. The attenuation in the optical parameters in these materials is assigned to the pronounced metal character 36,37 . Replacement of Se by Te in Sb 2 Se 3 redshifted the energy band gap from 1.20 eV to 0.30 eV 38 .
The compositional variations in InSe and Sb 2 Te 3 could also play main role in determining the values of the energy band gaps. Mn, Fe and Co doped ZnO films displayed wide variety in the energy band gap value depending on the doping agent and content 39,40 . For InSe variation of atomic composition of In was sufficient to change the energy band gap from 1.88 eV to 1.12 eV as indium content increases 41 .
In accordance with the energy band theory, InSe and Sb 2 Te 3 exhibit electron affinities ( qχ ) 4.55 and 4.15 eV, respectively. These values lead to a conduction band offset ( c E ∆ ) of 0.40 eV. Since the energy band gap of Sb 2 Te 3 is 0.28 eV 12 , the energy band gap difference is 1.42/1.08 eV. As a result, the valence band offset is 1.02 eV/0.68 eV. The most preferable value is 0.68 eV as it should be between the two direct allowed transitions energy band gaps of InSe and Sb 2 Te 3 . The value of v E ∆ is sufficiently large to allow using InSe/Sb 2 Te 3 stacked layers in optoelectronic applications and thin film transistor technology. As mentioned, the larger the valence band offset at the interface, the less band bending is required to achieve good tunneling contacts 42 . As Figure 4a shows, coating of InSe with Sb 2 Te 3 highly enhanced the values of dielectric constant and changes its shape of variation. While the r ε spectra of InSe display two peaks centered at 2.31 and 1.63 eV, the spectra of InSe/Sb 2 Te 3 display one peak at 2.31 eV. In the spectral region of 1.63-1.14 eV, the dielectric constant of InSe  ) at which the dielectric constant of InSe exhibits a maxima is attributed to the direct forbidden transitions from the valence to the conduction band of InSe. The critical energy value being 2.31 eV is close to the previously observed at 2.40 eV. This peak was assigned to the electronic transitions between the top of the valence band which is composed of p x,y orbitals along (110) direction in k-space and the bottom of the conduction band formed from p z -like orbitals along (001) direction in k-space 44 .
It is also clear from Figure 4b  ) is always larger than one. The lower the incident photon energy, the larger the value of .
R σ The enhancement in the optical conductivity reach more than 35 times at 1.19 eV. The enhanced optical conductivity is mentioned to be beneficial in designing photonic and optoelectronic devices such as solar cells and light-emitters 45 . It is also regarded as promising features for use in visible light communications as signal receivers 46,47 .
As a confirmation procedure for the usability of the InSe/Sb 2 Te 3 as optoelectronic devices, we first compute the optical conductivity parameters by using the Drude-Lorentz model for optical conduction 46,47 . In this model the optical conductivity is given by the equation, In the above equation, the electron-plasmon reduced frequency is e w . It represents the reduced resonant frequency that takes into account the effects of the interband transitions on the optical conductivity function.
Here n is the free carrier density, * m is effective mass of charge carriers. From these parameters, the drift mobility can also be estimated using the relation, The computed conductivities that best fit the experimental data are shown by solid black circles in Figure 4b. While computing the optical conductivities the effective masses of electrons in InSe and of holes in Sb 2 Te 3 were taken as .  Table 2. As the table shows, for the first three oscillators ( , , n 1 2 3 = ), while the scattering time at femtosecond level is shortened and the drift mobility is decreased, the free carrier density and plasmon frequency are highly increased upon interfacing of InSe with Sb 2 Te 3 . The plasmon frequency increased from 0.50 GHz to 2.9 GHz at reduced oscillator angular frequency of 2.0  (1.96-2.30 eV). The values of the plasmon frequency are high enough to nominate the constructed heterojunction device as microwave resonators. Since larger plasmon frequencies are reachable, it is possible to suggest that interfacing of InSe with Sb 2 Te 3 is more effective for production microwave cavities than InSe/Ge/InSe 47 or than InSe/CdSe heterojunctions 21 . The shorter scattering time means the electronic friction has increased as a result of heterojunction formation. This is expected because electronic motion finds energy barriers at the depletion region of the n − InSe/ p − Sb 2 Te 3 interfaces. It is also worth reminding that the valence band offsets is large enough to control electron -hole recombination at the interface. In addition, the band bending mechanisms at the interface requires re-alignment of the Fermi level. As the energy of Fermi level increases more higher-order plasmonic levels can be expected 49 .
It is interesting to remind that the refractive index is related to the effective dielectric constant through the relation, 2 eff n ε = . When the refractive index, ( ) n λ , is analyzed using the single-effective-oscillator models 50 using the relation, with d E being the dispersion energy of the oscillator whose energy o E , the plot of ( ) . The oscillator energy being 1.38 eV is very close to the one we determined as 1.31 eV ( Table 2) for InSe in the IR range from Drude-Lorentz approaches 50 . For InSe/Sb 2 Te 3 , the slope of the fitting is positive indicating that using the single oscillator model is not valid for this heterojunction. However, with linearly summed oscillators, the optimization of optical conductivity parameters is possible.
As a second confirmation procedure of the applicability of the proposed InSe/Sb 2 Te 3 heterojunctions as visible light communication receivers, two Ag pads of areas of ~4.0 2 10 − × cm 2 were located at 0.4 cm from each other. As shown in the inset of Figure 1, one of the pads is on the surface of InSe and the other is on the surface of Sb 2 Te 3 . The light was irradiated from a daylight light emitting diode onto the back side of the heterojunction devices to record the time varying signals. It was also back irradiated with  ) between ph I and P. Cases where ~. g 0 5 indicates that the density of charge carries trapped below the Fermi level are less than the density of carriers trapped above the Fermi level. Figure 5b illustrates the time dependent illuminated current response to daylight LED of fixed intensity (2.7 klux). This LED is selected because it is available everywhere and emits light in a wide range of spectra that suits visible light communication tools. Once the LED is turned on, the current increases by more than 29 times. Large symmetry between the "ON" and "OFF" states can be detected from the figure.
The response of the current within one completed irradiation cycle is independent from the previous poling. It is interesting to mention that, with the sufficiently large values of valance band offsets being the 1.02 eV/0.68 eV, acceptable values of drift mobility (~10-3.57 cm 2 /Vs), large plasmon frequency and fast responsivity, our proposed InSe/Sb 2 Te 3 interfaces can be employed to fabricate multifunctional devices. Namely within an area of less than 0.2x0.2 cm 2 , coated onto conducting transparent substrate, comprising the two layers (InSe, Sb 2 Te 3 ) and top contacting distanced areas of ~ 10 -3 cm 2 as metal electrodes, one may use this mini-device as photodetectors, fast switches, microwave band filters, plasmonic resonators and as optical media suitable for nonlinear optical applications.

Conclusions
In this study, we have shown that the formation of InSe/Sb 2 Te 3 bilayers results in remarkable enhancements in optoelectronic properties of the InSe layers. Namely, stacking of InSe with antimony telluride redshifts the energy band gap, increases both of the light absorbability and optical conductivity in the IR region of light. The numerical estimations have shown that the bilayers are more effective for infrared light sensing and related applications. Remarkable improvement in the photocurrent response to infrared light signals is also achieved. The computed optical conductivity parameters indicated an increase in the plasmon frequency values nominating the heterojunction device for optoelectronic applications. Values of plasmon frequency that reaches 9.0 GHz are attractive as they show the ability of using the InSe/Sb 2 Te 3 interfaces as band filters suitable for 4G technology. When employed as field effect transistors, the devices allow propagation of effectively mobile carriers (3.57 cm 2 /Vs).