Tunability in the Optical and Electronic Properties of ZnSe Microspheres via Ag and Mn Doping.

ZnSe microspheres with various Ag and Mn doping levels were prepared by the hydrothermal method using Zn(NO3)2·6H2O and Na2SeO3 as precursors and N2H4·H2O as the reducing agent. The effects of Ag and Mn doping on the phase composition, morphology, and optical and electrical properties of the final products were systematically investigated. A remarkable change in morphology from microspheres with a cubic sphalerite structure to rodlike structure was observed by Ag doping, while the pristine structure was nearly unchanged via Mn doping. Moreover, the band gap of ZnSe microspheres could be tunable in a broad range via controlling the Ag and Mn doping concentration, and ZnSe with high electrical properties could be obtained by doping with an appropriate concentration. The first-principle plane-wave method was carried out to explain the above mentioned experimental results.

Introduction

ZnSe (Eg = 2.7 eV) is an important II–VI wideband gap semiconductor. It is widely used in light-emitting devices, solar cells, and photodetectors because of its high excitation energy and excellent photoelectric performance.[1−4] It is well-known that the microstructure of the ZnSe governs the photoelectric characteristics of devices. The optical and electrical properties of semiconductor materials are the main factors that determine the performance of devices. At present, it is the most widely used way to regulate the photoelectric characteristics of semiconductors through morphology regulation or doping treatment. On the one hand, researchers have conducted extensive investigations on the controllable synthesis of ZnSe with different structures, such as nanowires,[5−7] hollow spheres,[8−11] and microspheres.[12−15] On the other hand, the doping technique by introducing donor impurities or acceptor impurities into the host lattice is one of the most effective ways to achieve regulatory effects, and it can accurately control the transport properties of the semiconductor by adjusting the type of doping element and the doping concentration. However, the self-compensation effect of II–VI semiconductors often leads to unipolar conductive behavior, such as only n-type doping or p-type doping, which is also the main factor limiting the application of optoelectronic devices. Effective n-type and p-type doping are required in practical device applications, so controllable doping of semiconductor materials is particularly important. Up to now, most studies have mainly doped ZnSe by different types of elements. For example, ZnCl2 powder was used as the doping source to realize n-type doping of ZnSe nanowires by means of thermal evaporation.[16] In addition, p-type ZnSe nanowires were successfully prepared using zinc arsenide as the doping source.[17] However, because of the self-compensation effect of ZnSe, it produces unipolar conductive behavior, there are still some problems such as low doping concentration, poor controllability for ZnSe doping, and the complicated experimental operation process, which become the main factors limiting the application of optoelectronic devices.[18] In addition, the mechanism of the change of optical and conductive properties of ZnSe caused by doping has not been studied deeply in theory. Therefore, it is of great importance to reach a facile and controllable doping of ZnSe and understand its underlying doping mechanism. Therefore, we used a simple two-step hydrothermal method to achieve the doping of ZnSe by Ag and Mn. At the same time, the effects of different doping elements on the optical and electrical properties of ZnSe were discussed, and the first principle was used to compare the photoelectric properties of ZnSe before and after doping, and gave a certain mechanism explanation.

In this paper, ZnSe microspheres were synthesized by the hydrothermal method using Zn(NO3)2·6H2O and Na2SeO3 as the precursor and N2H4·H2O as reducing agent.[19] The effects of different doping amounts (Ag, Mn) on the phase composition, morphology, optical and electrical properties of the final product were investigated. When 10 mL of N2H4·H2O was added, the hydrothermal reaction was carried out at 180 °C for 4 h, the final products were mainly ZnSe microspheres of the cubic sphalerite structure; several rodlike structures occurred in the Ag-doped ZnSe sample, while Mn-doped ZnSe retained the microsphere structure. Compared with the pure ZnSe sample, the band gap of Ag:ZnSe showed a red shift as the Ag doping concentration increases, while the band gap of Mn:ZnSe showed a blue shift with the increase of Mn doping concentration. Doping with appropriate concentration can improve the electrical properties of ZnSe. Moreover, the first-principles plane-wave method was adopted to analyze the energy band structures of pure ZnSe, Ag:ZnSe, and Mn:ZnSe,[20] which provided an explanation for the intrinsic mechanisms governing the evolution of the optical and electrical properties of ZnSe via Ag or Mn doping.

Results and Discussion

Morphology and X-ray Diffraction

Figure shows the scanning electron microscopy (SEM) images of pure ZnSe, Ag:ZnSe, and Mn:ZnSe samples. When Zn(NO3)2·6H2O, Na2SeO3, and N2H4·H2O are used as the Zn source, Se source, and reducing agent, respectively. ZnSe synthesized in NaOH medium by hydrothermal reaction at 180 °C for 4 h exhibits a microsphere-like morphology, as shown in Figure a,b. However, the morphology of the sample changes after doping with Ag. When doped with 3.125% Ag, the Ag:ZnSe sample shows a rodlike structure in addition to a spherical structure (Figure c), which may arise from the hydrazine hydrate with strong reducibility and silver nitrate with strong oxidation. The reaction of hydrazine hydrate and silver nitrate in the solution act as an effective source of the Ag element, N2H4·H2O and AgNO3 first react and to form the Ag element, resulting in a decrease in the content and concentration of free Ag+ and hydrazine hydrate in the solution. The appearance of the rodlike structure is attributed to the change in the content and concentration of reducing agent.[21−23] For the Mn:ZnSe sample, as shown in Figure d, ZnSe doped with Mn also shows a spherical shape. Compared with the morphology of pure ZnSe, the only difference is that the surface of Mn-doped ZnSe microspheres is much rougher.

Figure 1

Low- and high-magnitude SEM images of (a,b) ZnSe microspheres, (c) 3.125% Ag-doped ZnSe, and (d) 3.125% Mn-doped ZnSe.

As shown in Figure , pure ZnSe, Ag:ZnSe, and Mn:ZnSe samples have the cubic structure and peaks from(111), (220), (311), (400), and (331) planes are in agreement with the peaks referred to in the JCPDS card no. 37-1463. Both Ag:ZnSe and Mn:ZnSe samples show all major peaks of pure ZnSe. Moreover, the additional peaks represented by the black diamond suit correspond to Ag2Se, which are well matched to JCPDS card no. 24-1041, some of which correspond to reflection planes (020), (112), (121), (122), and (220). The diffraction peaks of Ag:ZnSe doped with different proportions of Ag+ show a small angular shift, which may be caused by the lattice expansion caused by Ag+ substitution of Zn2+.[24−26]

Figure 2

X-ray diffraction (XRD) of synthesized pure ZnSe, Ag- and Mn-doped ZnSe samples confirming that all structures have a cubic structure.

For the Mn:ZnSe sample, in addition to the cubic phase of ZnSe, the peaks expressed by black club suit indicate the presence of MnSe, which are well consistent with JCPDS card no. 27-0311. The diffraction peak of the corresponding ZnSe in the Mn:ZnSe sample also shows a small angular shift (Supporting Information Figure S1b), which may be caused by Mn2+ replacing part of Zn2+ into the lattice of the material resulting in lattice expansion.[27]

X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition and the effect of doping on ZnSe. As can be seen from Figure a, all elements of Ag, Zn, and Se can be detected. The binding energies of Zn 2p1/2 and Zn 2p3/2 are at 1021.3 and 1044.5 eV (Figure b), which correspond to Zn2+.[22] The Se element in Figure c presents an asymmetric broad peak that can be fitted to two peaks with binding energies at 53.8 and 54.6 eV, which are assigned to Se 3d5/2 and Se 3d3/2, respectively.[23] In Figure d, the binding energy peaks of the Ag element at 367.9 and 374 eV are attributed to Ag 3d5/2 and Ag 3d3/2, respectively.[24] The radius of Ag+(1.26 Å) is slightly larger than that of Zn2+(0.74 Å), indicating that Ag+ has the possibility of replacing Zn2+ into the lattice of the material. In addition, the diffraction peak in XRD shifted to a small angle because of the lattice change, which show that part of Ag+ substituted for Zn2+ is incorporated into the lattice of ZnSe.

Figure 3

(a) XPS spectrum of ZnSe and Ag:ZnSe (3.125%); (b–d) high-resolution spectrum of Zn 2p, Se 3d, and Ag 3d, respectively.

As shown in the Figure , the peak of the Mn element occurs in the XPS spectrum of Mn:ZnSe. The binding energies of Zn 2p1/2 and Zn 2p3/2 are at 1021.4 and 1044.5 eV (Figure b), which correspond to Zn2+. The Se element in Figure c is fitted to two peaks with binding energies at 53.8 and 54.6 eV, respectively, which are attributed to Se 3d5/2 and Se 3d3/2, respectively. The peak at 641.8 eV is attributed to Mn 2p3/2 (Figure d), and the valence state of the corresponding Mn element is +2.[28,29] The radius of Mn2+ is 0.80 Å, and the radius of Zn2+ is 0.74 Å, the results show that Mn2+substituted Zn2+ doping into the ZnSe lattice would cause lattice malformation, and confirm to the result of the diffraction peak of Mn:ZnSe shifts to a small angle in the XRD diagram, which also demonstrate that Mn2+ partially replaces Zn2+ into ZnSe lattice.

Figure 4

(a) XPS spectrum of ZnSe and Mn:ZnSe (3.125%); (b–d) high-resolution spectrum of Zn 2p, Se 3d, and Mn 2p, respectively.

Optical Analysis

As shown in Figure a, the absorption edge of pure ZnSe is at 502 nm, and the corresponding band gap is 2.47 eV, which is shifted by 0.23 eV from bulk ZnSe (Eg = 2.7 eV), which is due to the agglomeration of microspheres as seen in SEM images. Compared with the absorption edge of pure ZnSe, the absorption edge of the Ag:ZnSe sample decreases with the increase of the molar concentration of Ag doping (Figure a). While the absorption edge of the Mn:ZnSe sample becomes larger as the molar concentration of Mn doping increases (Figure a).

Figure 5

(a) UV–visible spectra and (b) Tauc plots of ZnSe and Ag-doped ZnSe with varying Ag concentration.

Figure 6

(a) UV–visible spectra and (b) Tauc plots of ZnSe and Mn-doped ZnSe with varying Mn concentration.

The Tauc relation was used to obtain the band gap of samples. Compared with the band gap of pure ZnSe (2.47 eV), with the increase of Ag doping molar concentration, the band gap of Ag:ZnSe can be adjusted to 2.56 eV when the Ag doping concentration reaches 9.375% (Figure b). While the band gap of the Mn-doped ZnSe sample decreased, and the band gap of Mn:ZnSe can be modulated to 1.9 eV when the Mn concentration is 9.375% (Figure b) (Supporting Information Table S1).

First-Principles Calculation

In order to facilitate the analysis of the influence of doping on the electronic structure of ZnSe, the electronic band structure of the pure ZnSe crystal was first calculated, as shown in Figure a, where the Fermi level shifts at zero point. As can be seen from Figure a, ZnSe is a direct-band gap semiconductor material, and the valence band top and the conduction band bottom are located at the Γ point of the Brillouin area. Figure b shows the energy band structure of ZnSe after 3.125% Ag doping. Compared with the energy band diagram of pure ZnSe, it can be found that the Fermi level of crystal shift downward into the valence band after Ag doping, together with new energy levels near the Fermi level, which are associated with the localized Ag states hybridized with ZnSe bulk states. Therefore, Ag-doped ZnSe produces a p-type semiconductor.

Figure 7

(a) Band structure of pure ZnSe and (b) 3.125% Ag-doped ZnSe. Red dashed line is Fermi energy.

We also evaluated the evolution of band structures with increasing the doping concentration of Ag. As shown in Figure , where three different concentrations of 3.125, 6.25, and 9.375% of Ag are plotted, the Fermi level shifts downward steadily indicating an increasing hole concentration. With heavily doped Ag, there exists clear band splitting and new levels in the valence band, whereas the conduction bands keep almost the same for the three doped cases.

Figure 8

(a) Band structure of 3.125% Ag-doped, (b) 6.25% Ag-doped and (c) 9.375% Ag-doped ZnSe. Red dashed line is Fermi energy.

Concerning the effect of Mn doping, as can be seen from Figure , the introduction of Mn causes significant states splitting while the Fermi level maintains within the mid of the band gap. However, the Mn doping forms new flat levels in the conduction band around 1.5 eV. Such new empty states can host the light excited carriers which can explain the observed increase in light absorption.

Figure 9

(a) Band structure of pure ZnSe and (b) 3.125% Mn-doped ZnSe. The black (red) lines correspond to spin up (down) states.

Similarly, we compare the band structures with respect to the increase of the Mn doping concentration from 3.125 % to 9.375% (Figure ). The direct band gap characteristic of ZnSe is maintained after doping Mn element. In the conduction band, the localized levels associated with Mn broadens, leading to significant splitting of the spin-up and spin-down states of host ZnSe.

Figure 10

(a) Band structure of 3.125% Mn-doped, (b) 6.25% Mn-doped and (c) 9.375% Mn-doped ZnSe. Red dashed line is Fermi energy.

Electrical Property

The I–V measurement of pure ZnSe, Ag- and Mn-doped ZnSe was carried out at room temperature using the Keithley Source Meter, the device structure was as shown in Figure S2. It is obvious that a nonlinear rectifying characteristic behavior occurs because of the metal/semiconductor junction (Figures and 12). Compared with that of pure ZnSe, it is observed that a much higher current from films of Ag:ZnSe when the doping molar concentration is above 3.125%, especially when the doping concentration reaches 6.25%, the current of Ag:ZnSe increases dramatically, and reaches to more than 2 orders of magnitude at 5 V when the doping concentration increased to 9.375%.

Figure 11

I–V characteristics of pure ZnSe and Ag:ZnSe samples with different doping ratios at room temperature.

Figure 12

I–V characteristics of pure ZnSe and Mn:ZnSe samples with different doping ratios at room temperature.

Figure shows the current of pure ZnSe and Mn:ZnSe with different doping molar concentrations of Mn. The similar phenomenon can be found and the current of Mn:ZnSe increases with the increase of doping concentration, and the current rises to the highest one when the doping concentration increased to 9.375%. It confirms that the conductivity of the sample can be enhanced by appropriate doping treatment.

Conclusions

ZnSe, Ag- or Mn-doped ZnSe were synthesized by the hydrothermal method. The results showed that different concentrations of doping sources could cause significant changes in the morphology, optical and electrical properties of the composite. The doping of Ag led to the formation of rodlike structure, while Mn:ZnSe maintained the original microsphere morphology of ZnSe. Compared with pure ZnSe, optical properties indicated that the band gap caused by Ag doping increased, while the band gap caused by Mn doping decreased with the increase of doping concentration. In addition, compared with the electrical properties of pure ZnSe, the doping of Ag and Mn caused an increase in the current of ZnSe. When the doping concentration was 9.375%, the current of Mn:ZnSe exhibited the highest. Through the theoretical calculation of the first-principle plane-wave method, the abovementioned experimental results were explained from the perspective of the energy band. These results indicate that controlled doping of ZnSe is expected to expand its application in optoelectronic devices.

Experimental Section and Computational Details

ZnSe microspheres were synthesized using the protocol we previously reported.[16] Typically, Zn(NO3)2·6H2O(0.03M) and sodium selenite (Na2SeO3) (0.03M) were chosen as source materials and 10 mL of hydrazine hydrate(N2H4·H2O) was used as reducing agent. With the alkaline medium of NaOH (0.67 M), the reaction was carried out at 180 °C for 4 h to synthesize ZnSe microspheres. For the synthesis of Ag:ZnSe and Mn:ZnSe samples, the protocols were similar to that of ZnSe microspheres mentioned above. The only difference was that the proper amount of AgNO3 or Mn(NO3)2·4H2O was added as source materials together with the as-synthesized ZnSe to obtain various doped molar concentrations of Ag- or Mn-doped ZnSe (3.125, 6.25, and 9.375%). The structure of ZnSe, Ag- or Mn-doped ZnSe were explored by scanning electron microscope (FE-SEM, 7800F) and XRD. The optical properties and the I–V characteristics of samples were carried out by the UV–vis spectrophotometer and Keithley Source meter 2400, respectively.

Our first-principles calculations were carried out by using Vienna ab initio simulation package (VASP) package.[17] The GGA-PBE functional was adopted to describe the exchange–correlation interaction and a kinetic energy cutoff of 400 eV was used. All the structures were fully relaxed until the forces exerted on each atom are less than 0.005 eV/Å. The relaxed lattice constant of bulk ZnSe is 5.754 Å. To simulate the doped ZnSe, we built a 2 × 2 × 2 supercell together with a 2 × 2 × 2 Monkhorst–Pack grid for k-point sampling. The doping of Ag or Mn atoms was modeled by directly substituting Zn atoms with Ag or Mn atoms, respectively.