Abstract
In the research field of infrared transparent conductive films, alleviating the contradiction between high transmittance and low resistivity is a major scientific challenge. The molecular beam epitaxy (MBE) technique is used to study the effect of growth temperature on the electrical properties of indium-doped cadmium selenide (CdSe:In) films, and a transparent conductive film with a high mobility of 204 cm2·V-1·s-1 and a low resistivity of 6.95×10-5 Ω·m is prepared. The film exhibits a high transmittance of more than 87% in the 1－4 μm waveband. The results of the temperature-dependent characteristics study show that there are three temperature ranges for the conductivity of the film, which are distinguished by the transition of the carrier transportation mechanism. Compared with other reported mid-infrared transparent conductive films, it can be seen that the CdSe:In film shows obvious advantages in carrier mobility and transmittance. This study lays an experimental and theoretical foundation for the application of CdSe:In films in mid-infrared detectors.
0 Introduction
Infrared transparent conductive films have excellent light transmittance and conductivity in the infrared wavelength range and have important application value in infrared detectors. First, as a conductive electrode material, the infrared transparent conductive film is directly integrated with the infrared focal plane device, which requires it to have high conductivity to improve the device's detection capability [1]. Secondly, as a detector window material, it needs to have high infrared transmittance in order to receive infrared radiation and good stability [2]. Therefore, the development of high-performance broadband infrared transparent conductive films is of great significance to the development of future infrared imaging systems.
Currently, the most common infrared transparent conductive film is transparent conductive oxide (TCO) , which can be divided into N-type and P-type according to the doping type. Among them, N-type films mainly include tin-doped indium oxide, fluorine-doped tin oxide and other materials [3-4], which have high transmittance in the visible and near-infrared bands; however, when the wavelength is greater than 2 μm, its transmittance drops sharply, limiting the application of TCO in the mid-wave and long-wave infrared bands [5]. P-type films mainly include CuAlO2, LaCuOS and other materials [6-7], which have high transmittance in the mid-and long-wave infrared bands. However, the mobility of most P-type films (less than 3 cm2 ·V-1 · s-1) and conductivity are not ideal, and in most cases they require high temperature annealing and post-processing and are not suitable for infrared detection systems [8]. In addition, P-type films are usually multi-component compounds, which makes their chemical properties unstable and the preparation conditions are harsh.
The existing material systems are difficult to achieve wide spectrum, high transmittance and high conductivity is the plasmon resonance effect:
(1)
In the formula, ωp is the plasmon resonance frequency; ne is the carrier concentration; m* is the effective mass; e is the electron charge; c0 is the speed of light in vacuum; ε0 is the vacuum dielectric constant; λp is the plasmon resonance wavelength [9-12]. It can be seen that λp decreases with the increase of ne, resulting in a blue shift in the cutoff peak.
Therefore, in order to obtain a thin film material with a wide spectrum and high light transmittance, the carrier concentration cannot be too high. At the same time, according to the relationship between conductivity σ and carrier concentration n and mobility μ, σ=nqμ, it can be seen that high conductivity requires consideration of both carrier concentration and mobility. In summary, the development of new material systems requires both lower carrier concentration and higher mobility. Only in this way can a wide infrared spectrum, high light transmittance and high conductivity be taken into account.
In this paper, a CdSe:In mid-infrared transparent conductive film was prepared using MBE technology, and the photoelectric properties of the CdSe:In infrared transparent conductive film were further improved by studying the physical relationship between the growth temperature and the photoelectric properties . At the same time, by testing the performance parameters of the film at different temperatures, we also studied the effect of the film's electrical properties at different operating temperatures (25－297 Compared with other reported mid-infrared transparent conductive films, this CdSe:In film shows obvious advantages in carrier mobility and transmittance, and has excellent conductivity and mid-infrared transmittance.
1 Experimental procedures
1.1 Material growth
In this paper, MBE technology is used to deposit CdSe:In thin films on infrared transparent substrates such as quartz glass. Before the film is grown, the substrate needs to be ultrasonically cleaned with isopropanol, anhydrous ethanol, and deionized water, and then dried with high-purity nitrogen. The source materials used in the experiment are high-purity cadmium selenide, indium, and selenium, all of which have a purity higher than 99.999%. The doping concentration is changed by controlling the beam ratio of indium to cadmium selenide, and the growth temperature is controlled at 25－350℃. Due to the low Se vapor pressure during the growth process, an additional Se source was used to ensure the film quality.
1.2 Material characterization
X-ray photoelectron Spectrometer (XPS) was used to test the elemental composition and valence state of the samples, scanning electron microscope (SEM) was used to test the micromorphology of the samples, X-ray diffractometer (XRD) was used to test the crystal structure of the films, Fourier transform infrared (FTIR) spectrometer was used to test the infrared transmission spectra of the samples, and Hall effect test system was used to test the electrical parameters of the samples at different operating temperatures, such as carrier concentration, carrier mobility and conductivity.
2 Results and Discussion
2.1 Effect of growth temperature on film properties
2.1.1 Effect of growth temperature on film crystal structure and morphology
Figure1 (a) shows the XRD patterns of CdSe:In films grown at different temperatures. At 200℃, the obtained film has no obvious characteristic peaks, indicating that the film is amorphous or has poor crystallinity at this temperature. At 350℃, the sample began to show multiple diffraction peaks . The peak positions were25.3°, 35.1°, 42.1°, 45.9° and 49.8°, corresponding to the (002) , (102) , (110) , (103) and (112) crystal planes of the CdSe wurtzite structure, respectively [13], indicating that these films have polycrystalline properties and grow in an orientation toward the (110) and (103) planes. As the temperature continues to rise to 350℃, the diffraction peak intensity increases, indicating that the film crystallinity is improved. In addition, no characteristic peaks of impurities such as In, InSe or In2Se3 were found, indicating that the In element was successfully doped into the CdSe lattice, rather than impurities such as In2Se3 crystals. Figure1 (b) is 200 The cross-sectional SEM images of the film at 200℃. It can be seen that the film thickness is about 800 nm, the thickness is uniform and the surface is flat. Figure1 (c) and Figure1 (d) show the cross-sectional SEM images of the film at 200℃ Surface SEM image of the film at 370℃. It can be seen that the film is composed of crystal particles of a certain size, the particles are uniform and have no obvious cracks or obvious defects, which is conducive to the application of the film in the fields of infrared detectors and passivation layers.
Fig.1(a) XRD images of CdSe:In films grown at different temperatures; (b) cross-sectional SEM images of CdSe:In films grown at 200℃; (c) － (d) surface SEM images of CdSe:In films grown at 200℃ (different magnifications)
2.1.2 Chemical element analysis of CdSe:In
The composition and valence state of the elements in the CdSe:In film were analyzed by XPS, as shown in Figure2. The binding energy of all spectra was calibrated with C 1s. Figures 2 (b) , 2 (c) and 2 (d) show the Gaussian distribution fitting spectra of Cd 3d, Se3d and In 3d of the CdSe:In film, respectively. Two different characteristic peaks of 404.8 eV (Cd 3d5/2) and 411.6 eV (Cd 3d3/2) were observed from the Cd 3d spectrum in Figure2 (b) . According to previous studies, Cd exists in the form of Cd 2+ [14]. Figure2 (c) shows the Gaussian fitting of the Se3d spectrum. It can be seen that the two adjacent strong characteristic peaks are53.6 eV and 54.4 eV, corresponding to the Se3d5/2 peak and the Se3d3/2 peak, respectively. Compared with the NIST XPS database, the valence state of selenium can be determined to be-2 [15]. In addition, through fitting analysis, there is also a small characteristic peak at the position of 55.5 eV. According to the XPS database, it is found that it represents the energy level associated with the surface selenium atoms [16-17]. This is because an additional selenium source is used during the film growth process, resulting in a trace amount of selenium residue on the surface of the CdSe:In film. At the same time, the In element was also measured from the film. As shown in Figure2 (d) , there are two different characteristic peaks at 444.6 eV and 452.1 eV, representing the In 3d5/2 peak and the In 3d3/2 peak, respectively. This shows that the In element mainly exists in the form of In 3+ [18], confirming that the In ions are successfully doped into the CdSe lattice, rather than simply attached to the inside and surface of the CdSe film, which is consistent with the previous XRD analysis .
Fig.2XPS spectra of CdSe:In thin films grown at 200℃: (a) XPS full spectrum; (b) Cd 3d spectrum; (c) Se 3d spectrum; (d) In 3d spectrum
2.1.3 Effect of growth temperature on thin film electrical properties
Figure3 shows the changes in the electrical properties of the films at different growth temperatures. The results show that all the films are n-type doped semiconductors. It can be found that when the film growth temperature is 25℃, the conductivity and resistivity of the film are 5.6×103 S/m and 1.78×10-4 Ω·m. As shown in the XRD pattern in Figure1, the higher resistance is due to the poorer crystallinity at lower growth temperature.
As the growth temperature increases, the carrier concentration and carrier mobility of the film first increase and then decrease. This is because when the growth temperature increases from 25℃ rise to 200℃, at higher temperatures, In atoms are successfully incorporated into the CdSe film to provide more free electrons. At the same time, higher temperatures help the material recrystallize, thereby improving the crystal quality of the film, reducing film defects, and reducing the grain boundary scattering of charge carriers. The above combined effects lead to an increase in carrier concentration and carrier mobility, ultimately leading to the conductivity and resistivity reaching their optimal values at this moment, which are 1.438 × 104 S/m and 6.95 × 10-5 Ω·m.
As the growth temperature continues to rise, the carrier concentration and carrier mobility of the film begin to drop sharply, and the conductivity also decreases. This is due to the excessive loss of selenium at too high a temperature, and is also related to the inter-grain scattering caused by the agglomeration of crystals at too high a temperature.
Fig.3Effect of growth temperature on the electrical properties of CdSe:In thin films.
2.1.4 Optical properties of CdSe:In films under optimal conditions
The CdSe:In film with the best electrical performance was tested by FTIR spectrometer in the range of 1.2－5.0 μm. The transmission spectrum is shown in Figure4. It can be seen that the average transmittance of the film in the near-infrared to mid-infrared wavelength range (1 to 4 μm) exceeds 87%. The obvious characteristic absorption peak at 4－4.5 μm wavelength is determined by the optical properties of the quartz substrate and has nothing to do with the characteristics of the CdSe:In film. This result shows that the CdSe:In film not only has excellent electrical conductivity, but also has good transmittance in the mid-infrared band.
Fig.4Transmission spectrum of CdSe:In thin film sample under optimal conditions.
2.2 Temperature-dependent properties of CdSe:In thin films under optimal conditions
Figure5 (a) shows the curve of the conductivity of the CdSe:In film grown under optimal conditions as a function of ambient temperature. The change in conductivity with ambient temperature is distinguished by the change in the carrier transport mechanism, which is essentially the transition of carriers from the subband of the valence band to the conduction band. From the figure, it can be seen that the curve is mainly divided into three parts: when the temperature is higher than 150 K, its slope is approximately linear; at 90－150 K, the slope begins to become an obvious nonlinear change; when the temperature is lower than 90 K, the range of conductivity change with temperature is extremely small. This is basically consistent with the experimental phenomenon of Pathinetam PD et al . [19]. The change of conductivity with temperature at higher temperatures can be expressed by equation (2) [20]:
(2)
In the formula, σ1 and σ2 represent the pre-exponential factors; ΔE1 and ΔE2 represent the energy required for excitation; k represents the Boltzmann constant; T represents temperature (K) .
When the temperature is higher than 150 K, carriers are excited from the valence band to the conduction band, causing a change in conductivity (regarded as the intrinsic range) . Therefore, in this temperature range, carrier drift and thermal excitation are the main conduction pathways.
When the temperature is 90－150 K, the change in conductivity is mainly due to the existence of localized states around the Fermi level EF caused by indium doping, which causes charge carrier hopping conduction (regarded as a non-intrinsic interval) .
At temperatures below 90 K, the change in conductivity is very small, which is due to the freezing effect of carriers caused by low temperature. From formula (2) , it can be found that when the temperature is lower than 90 At 2.5 K, the extremely low conductivity means that the energy required for excitation is extremely low. The extremely low excitation energy may be caused by variable-range hopping (VRH) conduction of carriers [19, 21-23] .
Fig.5(a) The relationship between lnσ and 1000/T in the temperature range of 25－298 K; (b) The relationship between ln (σT1/2) and T-1/4 in the temperature range of 25－80 K
The VRH conduction mechanism of carriers can be explained as the hopping of carriers between fixed states of adjacent EF in a low temperature environment. The VRH conduction mechanism of carriers can be given by equation (3) [19] :
(3)
Where σ0 represents the pre-exponential factor. The value of the exponent S is determined by the nature of the jump process: optimizing the jump probability and assuming a slowly varying density of states around EF. If the density of states of EF is constant, then S=1/4. T0 represents the characteristic temperature coefficient, which is determined by the density of states and EF [19] :
(4)
Where a represents a measure of the spatial extension of the wave function exp (-ax) associated with the local state. Figure5 (b) shows that the relationship curve between conductivity ln (σT1/2) and T-1/4 in the temperature range of 25－80 K can more clearly reveal the VRH conduction mechanism of carriers. As can be seen from the figure, its slope changes linearly. This is highly consistent with the Mott-VRH model, confirming the existence of the VRH conduction mechanism of carriers in the sample. The slope of the straight line in the figure is the value of the characteristic temperature coefficient.
Finally, in order to evaluate the performance advantages of the obtained CdSe:In infrared transparent conductive film, we compared it with several typical infrared transparent conductive films reported in the literature, as shown in Table1. It can be seen that the CdSe:In infrared transparent conductive film studied in this paper has obvious advantages in carrier mobility and carrier concentration, while the transmittance and resistivity can also maintain the same level as other films. This comparison proves that the CdSe:In infrared transparent conductive film obtained in this paper has excellent electrical properties in the mid-infrared band, which is conducive to its practical application in infrared detectors and other fields.
Table1Performance comparison of several mid-infrared transparent conductive films
3 Conclusion
CdSe:In infrared transparent conductive films were grown using MBE technology, and the effect of growth temperature on the electrical properties of the films was studied. The results show that when the growth temperature is 200℃, the film has the best comprehensive performance, and the resistivity can reach 6.95×10-5 Ω·m, between 1 and 4 The CdSe:In film has a high transmittance of more than 87% in the near-infrared to mid-wave infrared band. The temperature variation of the electrical properties of the CdSe:In film shows that there are three different temperature ranges, which are distinguished by the transformation of the carrier transport mechanism. Finally, by comparing with other reported infrared transparent conductive films, it is found that the CdSe:In infrared transparent conductive film has obvious advantages in carrier mobility and carrier concentration while maintaining high transmittance and low resistivity.
CdSe:In infrared transparent conductive film obtained in this paper and the study of its temperature variation law have laid a solid theoretical foundation for practical application in infrared detectors. In the future, the CdSe:In transparent conductive film will be combined with PbSe mid-infrared detection materials to study its practical application in mid-infrared detectors.