Ga4Se3S interface

Journal of Alloys and Compounds 583 (2014) 180–185

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Optical dynamics of MgO/Ga4Se3S interface A.F. Qasrawi a,b,⇑, Mariam M. Abd-Alrazq a, N.M. Gasanly c a

Department of Physics, Arab-American University, Jenin, West Bank, Palestine Group of Physics, Faculty of Engineering, Atilim University, 06836 Ankara, Turkey c Department of Physics, Middle East Technical University, 06531 Ankara, Turkey b

a r t i c l e

i n f o

Article history: Received 18 May 2013 Received in revised form 28 July 2013 Accepted 3 August 2013 Available online 31 August 2013 Keywords: p–n Junction Solar cell Optical High absorption

a b s t r a c t A new p–n interface made of p-type MgO as an optical window to the n-type Ga4Se3S crystals is investigated by means of optical reflectance, transmittance and absorbance in the incident light wavelength (k) range of 200–1100 nm. The reflectivity spectral analysis as a function of angle of incidence for MgO, Ga4Se3S and the Ga4Se3S/MgO layers revealed Brewster angles of 75°, 80° and 70° with the corresponding dielectric constants of 13.93, 32.16 and eMgO ¼ 7:55eGa4 Se3 S , respectively. To remove Brewster condition of reflection and obtain maximum absorption, the light must be incident from the MgO side. A novel light absorbability is observed. Namely, for all k < 600 nm, the absorbance is dominated by the Ga4Se3S layer. For larger k values, while the crystal absorbance decreases significantly, the bilayer absorbance increased by four times in the visible range and three times in the IR range of spectrum. In the MgO layer, two distinct sets of band tails of the localized states with the widths of 2.30 and 1.26 eV are determined from the absorption spectral analysis. These band tails shift up to 2.32 and 1.44 eV when the interface is constructed. In addition, an indirect energy band gaps (Eg) which are located at 3.10, 2.13 and 1.90 eV for the MgO, Ga4Se3S and the Ga4Se3S/MgO layers, respectively, are determined. The Eg value of the crystal shifts by a 0.23 eV upon bilayer construction. The reflection properties, the band tails, the energy gaps and related shifts make the Ga4Se3S/MgO interface attractive for fabrication of solar cells, narrow barrier resonant tunneling diodes or quantum dots, and as an optical detector for tunable types of lasers. Ó 2013 Published by Elsevier B.V.

1. Introduction The development of a specific-wavelength p–n junction device is essentially important for the realization of transparent electronics as a candidate of next-generation optoelectronics. As for example, a p–n junction of NiO thin film was observed to exhibit good rectifying properties with efficient UV photodiode characteristics. Such device is claimed to provide a suitable solution for low-cost visible blind UV photodetector applications [1]. Similarly, poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) films coated on GaN were studied by photoemission spectroscopy and reflected a type-II energy alignment with band offsets being suitable for efficient photocurrent generation. Such structure is suggested to find application as hybrid optoelectronic devices [2]. Furthermore, a transient photocurrent response of a vertically stacked triple p–n junction structure that is able to detect three different simultaneous colors is designed with the help of a 0.6 lm BiCMOS technology using a pp+ epitaxial wafer without any process modification. This device

⇑ Corresponding author at: Department of Physics, Arab-American University, Jenin, West Bank, Palestine. Tel.: +970 599379412; fax: +970 42510817. E-mail address: [email protected] (A.F. Qasrawi). 0925-8388/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jallcom.2013.08.026

is tested as an optical sensor without optical filter and the total data rate of this structure is reported to reach 100 Mbit/s [3]. Generally p–n heterojunctions plays an important role in the development of photovoltaic solar cells. These types of solar cells are reported to have internal crystalline interfaces, which reflect part of the incident light that in turn enhance the performance of heterostructure-based solar cells [4]. Zhu et al. reported that the most commercial common heterostructure solar cells have internal reflection less than 2%, while some potential heterojunction solar cells such as ITO/GaAs, ITO/InP, Si/Ge, polymer/semiconductors and oxide semiconductors may have internal reflection as high as 20%. As a result, Zhu et al. did not support the idea of having a window layer with a lower refractive index than the absorption layer for a solar cell. Further, as the strong internal reflection reduces the conversion efficiency [4] it must be carefully taken into consideration. Recently, we have designed a magnesium oxide based tunneling heterojunction diode produced on Al and InSe films as rectifying substrates [5]. We have observed that when Al thin films are used, the device exhibits tunneling diode behavior of sharp valley at 0.15 V and peak to valley current ratio (PVCR) of 11.4. In addition, the capacitance spectra of the Al/MgO/C device reflected a resonance peak of negative capacitance (NC) values at 44.7 MHz. The

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3. Results and discussion In our previous works we have reported the physical properties of Ga4Se3S single crystals [6–9]. Structurally, Ga4Se3S crystallizes in a hexagonal unit cell. It is novel in the property that it is composed of very thin layers that can be easily cleaved with a layer thickness down to 100 nm. Optically, the energy band gap of the Ga4Se3S single crystal is highly sensitive to the layer thickness. In addition, the indirect energy band gap of the crystal is 2.08 eV at room temperature [6,8]. This energy band gap relate to an absorption edge that coincide with a wavelength of 600 nm (orange in color). Electrically, the crystals are observed to exhibit n-type conductivity with acceptor–donor concentration ratio of 0.97 [8,9]. The room temperature dark electrical resistivity, carrier concentration and Hall mobility of this crystal are found to be 7.7  106 X cm, 8.3  109 cm3 and 98 cm2 V1 s1, respectively. Based on the idea of getting advantage from the reflectivity of the internal crystalline interfaces [4] we have used the Ga4Se3S single crystals as a substrate to the MgO layer. As the latter is transparent near 2.08 eV and exhibits p-type conductivity with carrier concentration of 1015 cm3 [5], the resulting interface of MgO–Ga4Se3S will present a p–n junction that are to be used in the design of optoelectronic devices like solar cells, optical communication detector and light controlled signal amplifier. Working with photonics necessarily needs basic understanding of the nature of light and its properties. Namely, the generation, transport, manipulation, detection, reflection, transmission and absorption of light is regarded as at the heart of photonics. For example, the transmittance and reflectance are used to determine

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Single crystals of Ga4Se3S were grown by the Bridgman method from the stoichiometric melt of the starting materials sealed in evacuated (105 Torr) silica tubes with a tip at the bottom. The ampoule was moved in a vertical furnace through a thermal gradient of 30 °C/cm, between the temperatures 1000 and 650 °C at a rate of 0.5 mm/h. The analysis of X-ray diffraction data showed that Ga4Se3S crystallized in a hexagonal unit cell, with lattice parameters: a = 0.3708 and c = 1.5915 nm. The resulting ingots (orange in color) showed good optical quality and were easily cleaved along the planes, which are perpendicular to the c-axis of the crystal. Typical dimensions of the crystals suitable for electrical measurements were 10  5  0.1 mm3. Magnesium oxide paste (Alfa Aesar-43135) was carefully and homogeneously painted on the surface of the films and left to dry for 24 h. The optical transmittance and reflectance was measured in the incident light wavelength range of 200–1100 nm using Evolution 300 spectrophotometer with VeeMax II variable angle reflectometer attached to the spectrophotometer. The incident light beam is unpolarized and the uncertainty in the reflectivity measurements for this system extends from 0.2% of the measured value at 1100 nm to 1% at 250 nm.

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the refractive index and absorption coefficient which are key parameters to design optoelectronic devices. In the case of solar cells, the effective absorption can be increased by decreasing the reflection of incident light, increasing the optical path length in the material and increasing the optical intensity of the material. In many cases, the same physical structure of the device produces combination of these effects [10]. For these reasons, here below, we will consider the light reflection, transmission and absorption effects on the MgO–Ga4Se3S heterojunction to give guide to its behavior when used in optoelectronic applications. Fig. 1(a) displays the reflection (R) spectra of a 100.0 lm thin layer of Ga4Se3S single crystal being registered in the incident wavelength range of 200–1100 nm. The angle of incidence of light (hi) was altered in 5° steps from 30° to 80°. The representative data shown in Fig. 1(a) was registered at hi = 45°. The hi = 45° was selected for presentation because at this angle, the incident light intensity exhibit equal parallel and perpendicular components. As it is easily readable from the figure, the crystal reflectivity steadily increases with increasing incident light wavelength up to 544 nm where it then sharply increases reaching a local maximum at 580 nm (inset of Fig. 1(a)). In the wavelength range of 581– 840 nm, the crystal reflectivity decreases with increasing wavelength. In the remaining incident light wavelength range, R linearly increases with increasing k. On the other hand, Fig. 1(b) illustrates the R  k variation for the MgO layer (300 lm) being registered at the same angle of incidence (45°). For all incident k (at MgO surface), the R values are much lower than those of Ga4Se3S single crystal. As for example, at 450, 850 and 1000 nm, the reflectivity ratio of Ga4Se3S to that of MgO is 24.8, 41.9 and 43.6, respectively. The magnesium oxide reflectivity exhibited local maximum and minimum at incident k value of 542 and 836 nm, respectively. These values differ from that observed as 580 and 840 nm for the Ga4Se3S crystals. In general, the RðkÞ values at all incident k for the crystal (Fig. 1(a)) is at least one order of magnitude than those of MgO (Fig. 1 (b)). The reflectivity’s of the interface (diagram is shown in Fig. 1(a)) when light was incident from the Ga4Se3S crystal side (named as Ga4Se3S/MgO) and when the light was incident from MgO side (named as MgO–Ga4Se3S) are also shown in Fig. 1(a) and (b), respectively. As it is clear from Fig. 1(a), the reflectivity of the crystal and the interface (Ga4Se3S/MgO) is the same for all incident k less than 572 nm which is the local maximum of the interface reflectivity. The first readable attenuation via interface construction is the shift in the maximum reflectivity from 580 before the junction is made to 572 nm (enlarged in inset of Fig. 1(a)) after

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capacitance and resistance – voltage characteristics handled at an Ac signal frequency of 100 MHz reflected a build in voltage (Vbi) of 1.29 V and a negative resistance (NR) effect above 2.05 V. The device quality factor (Q)-voltage response was found to be 104. When the Al substrate was replaced by InSe thin film, the tunneling diode valley appeared at 1.1 V. Furthermore, the PVCR, the NR range, the NC resonance peak position, the Q and Vbi exhibited values of 135, 0.94–2.24 V, 39.0 MHz, 105 and 1.34 V, respectively. These devices were found to be suitable for applications as frequency mixers, amplifiers, and monostable–bistable circuit elements (MOBILE). The novelty of these devices attracted our attention to the design of new type of devices being applicable in optical communications as wave mixer and/or amplifiers. Thus, here in this work, we will report the design of new type of devices made of MgO (p)–Ga4Se3S (n) interface that exhibit sharp absorption edge and photonic energy gap suitable for particular optical beam conversion. The role of the angle of incident of light on the device reflectivity and on the attenuation of the energy band gap associated with the interface will be studied in details.

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construction. For all k > 572 nm, the reflectivity follows the same trend like that of the crystal but with less R% values. On the other hand, when light was incident from MgO side to the interface (Fig. 1(b)), the reflectivity of the heterojunction significantly reduces to very low values that exhibit a local maximum shift to 588 nm. This peaks shifts from 542 nm before the junction was made to 588 nm when the interface was established. It is also of importance to notice that the R values of MgO layer and MgO–Ga4Se3S is less than 1%. The low value of R indicates either high transmittance or high absorbance of this layer. The reflectivity of a thinner layer of MgO is not easily readable. As the effective absorption of materials is increased by decreasing the reflection of incident light on the material’s surface [10], the study of effect of the angle of incidence of light on the material’s reflectivity is necessary to enhance the absorbability. Fig. 2(a)–(d) displays the reflectivity – angle of incidence (hi) dependencies for the Ga4Se3S crystal, MgO layer, Ga4Se3S/MgO interface when light was incident from Ga4Se3S side and for MgO–Ga4Se3S interface when light was incident from MgO side, respectively. The curves represent R  hi dependencies at various wavelengths that extent from UV to near IR spectrum. For all incident wavelengths, the Ga4Se3S exhibit maximum reflectivity at an angle of 50°. For each incident k, the R  hi curve shifts up for all k < 600 nm. Beyond which the R  hi curves start shifting down. This behavior is assigned to the absorption edge that appears near 600 nm [6]. It is also notable that for the reflectivity of Ga4Se3S crystal which is presented in Fig. 2(a), the R values sharply decreases with increasing angle of incidence for all hi > 50°. It reaches near zero value at hi of 75°. For the MgO layer presented in Fig. 2(b), local and absolute maximum reflection amplitudes are observed at hi of 35 and 70°, respectively. The R value exhibits minima that tend to zero at 80°. When the light was incident from the Ga4Se3S side to the interface (p–n junction in Fig. 2(c)), two local maximum points are observed at 45° and 60°. The R  0 value appears at hi of 70°. On the other hand, when light is incident from the MgO side (Fig. 2(d)), the local and absolute maximum reflections are observed at 50° and75°, respectively. The R  0 value is not observed for this side of interface.

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It is worth noting that the maximum value of the reflectivity as 12% which was observed for the substrate still represents an acceptable value compared to commercial solar cells. However, because the reflectivity of the substrate highly influences the solar cell efficiency, minimization of the reflectivity enhances the absorber’s internal quantum efficiency. Thus, the above mentioned study helps revealing the maximum internal quantum efficiency associated with angle of incidence of light. Recalling that the minimum reflectivity (R  0) relate to the condition that at the relative hi which is called Brewster angle (hB) in this case, the light strike the surface so that there is a 90° angle between the reflected and refracted rays. As a result, the reflected light will be linearly polarized with a direction of polarization being parallel to the plane of the interface. In other words, for any two materials that exhibit two unequal dielectric constants (e1 and e2), the condition for the Brewster angle is given as pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 1 hB ¼ sin ð e2 =ðe1 þ e2 Þ. At hB the reflected wave consists of the perpendicularly polarized component of the wave alone. Thus, for any general wave (polarized or unpolarized), the reflected wave at the Brewster angle of incidence is linearly polarized perpendicular to the plane of incidence [11]. Even though the parallel component of the reflectivity (R//) of incident light is zero at Brewster, this does not imply that the total reflectivity at that condition is zero, because the perpendicular part of reflectivity (R\) is naturally not vanishing at Brewster angle. The zero value of total reflectivity obtained in our measurement indicates that both of the parallel and perpendicular components are zero at this angle. This strange behavior may be ascribed to the crystalline nature of the crystal. Ga4Se3S crystal is a layer crystal which is composed very thin piles of crystal plates. As some of which may causes surface inhomogeneity. They eliminate the perpendicular reflectivity component of light [12]. Another worth noting reason is the high absorption and transmission abilities of the layers. According to Fresnel’s equations R\ increases with the increase in the refractive index of the medium, and the Brewster angle shifts towards increasing incidence angles. Increasing the absorption level increases R// and decreases the R\/R// ratio. Thus to get large R\/R// ratio, the refractive index must be large and

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extinction coefficient must be small. On the other hand, if the extinction coefficient of the crystal is very large and the refractive index at the measured IR wavelength (1000 nm) is small, then observation of R\ – 0 values will not be possible [12]. Applying the critical reflectivity condition for the three of the four cases discussed above one can estimate the dielectric constant at the Brewster angles. For the Ga4Se3S crystal the R  0 condition which is satisfied at 75° reveals a dielectric constant of 13.93. For MgO layer, at hi = 80°, e2 is 32.16. For Ga4Se3S/MgO interface, the two materials satisfy the reflection condition at 70°. For this case eMgO ¼ 7:55eGa4 Se3 S . On the other hand, when light is incident from MgO side, the Brewster angle hit was not observed which means that the necessary condition for its existence is not satisfied [11]. It is advantageous to mention that calculation of dielectric constant of the single and bilayer as function of wavelength to reveal the dielectric dispersion is not possible in the current study due to the absence of light polarizability in our instrument. It is also not possible to determine the dielectric constant at each angle of incidence due to the same reasons. Optimization of the dielectric constant for unpolarized light gets use from Fresnel’s equations with mathematical approximations that may lead to inappropriate results. In addition the change in the dielectric constant upon interface construction is assigned to the depletion layer at the p–n junction. The depletion region has an extra polarization associated with the electron and hole separations which in turn alters the dielectric constant. Because the light is always incident from all directions, the condition for maximum and minimum reflectivity plays a vital role in the performance of optoelectronic devices. As for example, if the p–n junction is to be used as solar cell, then in solar cells, the top layer of the interface is used as a window layer (MgO layer) for collection of photo-generated carriers and it has a large band gap, while the absorption layer (Ga4Se3S) with a relatively small band gap is the main layer used to generate the photocarriers. Blockage of light into the absorption layer by the top layer will result in a severe deterioration of the photovoltaic cell performance. This means that the use of multi-antireflection layer structure with inter-medium refractive indices between layers 1 and 2 must be carefully considered to reduce the interface reflection [4]. However, because the total reflectivity of MgO is always less than 1% and because the Brewster condition is not satisfied when light is incident from MgO side, its (Brewster condition) effect on light blockage into the absorption layer is negligible. Thus, because the above analysis reflects the suitability of using MgO as window to the Ga4Se3S absorption layer, in the forthcoming discussions, the light will be considered as incident from MgO window to the absorption layer.

Fig. 3 displays the transmittance (T) spectra being registered at normal incidence for the single crystal, the MgO layer and the combination of the layers. Both layers were designed so that they have the same thickness of 100 lm (reflectivity of MgO is highly reduced). As it is noticeable from the figure, the T of the magnesium oxide layer (enlarged in the inset of Fig. 3) exhibits a sharply growing transmittance spectrum in the range of 315–380 nm. Above 380 nm T of MgO continues increasing with increasing wavelength even for light incident at wavelengths in the IR region. Similarly, the Ga4Se3S crystal exhibits a sharp increase in the incident wavelength range of 554–590 nm. The interface layer of Ga4Se3S/MgO reflected sharp transmittance region of 556–586 nm when light was incident from the Ga4Se3S side. The T values of the interface show no difference when light was incident from the MgO side. In addition, as the inset of Fig. 3 displays, above 556 nm, because the TðkÞ values and trend for Ga4Se3S/MgO is close to those of MgO, the transmittance of the interface appears to be controlled by the MgO layer. The absorbance (A) or optical density – which is known as the logarithmic ratio of the radiation falling upon the crystal, to the radiation transmitted through it-is calculated [13] and displayed in Fig. 4. The figure reflects a novel excellent behavior of the absorbance at the interface. Particularly, the figure show that the window layer (MgO) exhibit the highest absorbance values that can absorb all light energies extending from UV to IR range. The absorbance of the Ga4Se3S crystal exhibit constant value of 1.07 for all incident k values greater than 570 nm. On the other hand, the absorbance of the Ga4Se3S/MgO interface exhibits an interesting absorption spectrum. Namely, the A values are the same as those of the crystal for all incident k < 544 nm. For k > 584 nm all the A values are the same as those of MgO layer. The MgO layer increased the absorbance of the crystal beyond the absorption edge from 1.07 to 4.05 at 670 nm and from 1.03 to 3.18 at 1000 nm (for example). This result is remarkable as it means that the absorption spectral range and level of the crystal has increased four times in the visible range and three times in the IR range. With this interface construction a high level of absorption for wide spectrum of incident light that extends from ultraviolet regions to infrared is possible. The non-vanishing absorbance in the studied structures indicates the existence on interband transitions in these materials. These transitions can be ascribed to the existence of the defects that introduces alternative energy levels between the intrinsic bands and as a result subsequently reduces the band gap [14]. To reveal more detailed information about the optical behavior and optical energy band gap of these layers, the absorption coefficient (a) of the samples was calculated using the relation T = (1  R1)(1  R2)ead with R1 and R2 being the reflectivity’s of

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Fig. 4. The absorption spectra for the Ga4Se3S, MgO and Ga4Se3S/MgO. The inset shows the a  E dependencies.

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the first and second surfaces, respectively, (R1 = R2 for single layer) and d is the layer thickness [15]. The calculated data of a  E dependence are displayed in the inset of Fig. 4. One worth of noting observation in the a  E spectra of MgO and Ga4Se3S/MgO layers are the non-vanishing continuously decreasing values of the absorption coefficient beyond the absorption edge. It indicates the existence of tail states in the energy band diagrams of these materials. According to Tauc [16], it is possible to separate three distinct regions in the absorption edge spectrum of semiconductors. The first is the weak absorption tail, which originates from defects and impurities, the second is the exponential edge region, which is strongly related to the structural randomness of the system and the third is the high absorption region that determines the optical energy gap. In the exponent edge where the absorption coefficient is governed by the relation a = a0 exp (E/Ee), where ao is constant and Ee characterizes the slope of the exponential edge region and it is the width of the band tails of the localized states that exists in the layers. The data plotted in the inset of Fig. 5(a) illustrates the semi-logarithmic plot of the a as function of E in the photon energy region 1.14–1.26 eV. The slopes presented by solid lines reveals the width of the band tails of the localized states as 1.26 eV in MgO and 1.44 eV in the Ga4Se3S/MgO layer. Similarly in the energy region of 1.28–1.71 eV which is presented in Fig. 5(a), the Ee is determined as 2.30 and 2.32 eV for MgO and Ga4Se3S/MgO interface, respectively. The latter data indicates that the localized tail states are most probably present due to its existence in MgO layer. As the ln (a)  E data of the Ga4Se3S crystal reveal no systematic variation, it can be stated that no dominant localized states do exist in the energy band gap of the Ga4Se3S single crystals. The broaden band tails in MgO was also observed through the scanning tunneling spectroscopy studies of the electronic structure of 1.5–3 nm (1 0 0) textured MgO layers grown on (0 0 1) Fe [17]. Thick MgO layers is reported to exhibit a bulk like band gap of 5–7 eV and sparse localized defect states with characteristics that was assigned to the oxygen and, in some cases, Mg vacancies. Thin MgO layers exhibited an electronic structure indicative of interacting defect states that forms band tails that can extend to ±0.5 V of the Fermi level. These vacancy defects were attributed to the compressive strain arising from the MgO/Fe lattice mismatch. Following our previously described methods in our related published works [6,7], we have recalculated the energy band gap of the

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Ga4Se3S crystal, the MgO layer and the Ga4Se3S/MgO assuming an indirect allowed electronic transition energy band gap (Eg) in which (aE)1/2 / (E  Eg). The plots of this equation are presented in Fig. 5(b). The data fits crosses the energy axis at 3.10, 2.13 and 1.90 eV for the MgO, Ga4Se3S and the Ga4Se3S/MgO layers, respectively. The energy band gap is shifted by a 0.23 eV upon bilayer construction. As the error in calculating the energy band gap is 0.007 eV, the slight difference between the previously published (2.08 eV) and currently calculated (2.13 eV) values of the energy band gap of the Ga4Se3S is ascribed to the differences in the thickness of the measured layers. For the previously published data the sample thickness was 200 lm and in the current work it is 100 lm. The energy band gap of the MgO layer is known to exhibit different values that may range from 7.8 eV to 2.5 eV. The energy band gap values of MgO are well known in its wide variety depending on the preparation condition, calculation method and reaction type. Boer and Groot [18] reported the sensitivity of energy band gap of MgO on the oxygen vacancies and Mg and O ionic radiuses. Namely, in the presence of 3S states of oxygen the direct energy band gap ranges from 4.2 to 4.4 eV. In the absence of the 3S-O states the direct Eg values vary from 15.4 to 5.4 eV for Mg/O ionic radius ratio of 2.0–0.8, respectively. On the other hand, the band structure of MgO which was determined in the light of experimental lattice constant values [19,20], by setting the 2S-O and 2p-O and the 3S-Mg states as the valence band states, showed the bottom of the conduction band to be almost due to pure 3S-Mg energy band states. The calculated density of states for the MgO composite with this structure revealed three pronounced peaks: a low laying one from the semicore O-2S states and two from the valence O-2p states. The separation between the p- and S-states is found to be 13.5 eV. Likewise, the separation between the two groups of O-p states was calculated and found to be 2.7 eV. The decrease in the energy band gap of the n-type Ga4Se3S single crystal from 2.13 eV to 1.90 eV when joint to p-MgO layer may be ascribed to the lattice mismatch, defects associated with heterojunction in addition to the free carrier type available for the conduction. Similar decrease in the energy band gap with increasing number of graphene layers on MgO (1 1 1) was also observed by Gaddam et al. [21]. Although the energy band gap shift at the p– n interface is only 0.23 eV, which may not be suitable for field effect transistor technology which need a gap difference of

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Fig. 5. (a) The ln (a)  E and (b) the (aE)1/2  E dependencies for the Ga4Se3S and Ga4Se3S/MgO bilayer. The inset of (a) displays ln (a)  E in the energy range of 1.14–1.26 eV. Inset of (b) shows the (aE)1/2  E for MgO layer.

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0.5–1.0 eV, it may be suitable for the production of other point contact devices like Schottky solar cells and narrow barrier resonant tunneling diodes [5] or quantum dots. In addition, the energy band gap of the new structure with the narrow band off set at the interface make this new type of devices attractive to be used in optical communications as detector for tunable types of lasers. 4. Conclusions In this work the Ga4Se3S thin crystals are used as the substrate for the MgO layer. Both layers were of the same thickness. The optical reflectivities of the single MgO and Ga4Se3S layers and the Ga4Se3S/MgO bilayer were recorded as a function of incident light wavelength in the range of 200–1100 nm at an incident angle in the range of 30–80°. The optical reflectivity spectra have shown that the reflectivity of the interface is mostly controlled by the Ga4Se3S crystal, and the dielectric ratio of the MgO to the Ga4Se3S at the Brewster angle of 70° equals to 7.55. Further, the normal light absorbance reflected a relatively large absorbance values. The absorption ability increased significantly for the bilayer. In addition, the calculated band tails and energy band gap reflected the suitability of the n–p junction layer as a promising candidate for optoelectronic device production. Acknowledgments The authors would like to acknowledge and submit thanks to the rector of the Arab American University (AAUJ) Prof. Dr. Mahmoud Abu Mouis for his own efforts in supporting the research labs at the department of physics. Thanks also go to scientific research committee members for their financial support and

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