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Abstract
A metal electrode modification process for AlGaN-based metal-semiconductor-metal (MSM) photodetectors have been introduced to enhance the response of solar-blind ultraviolet (UV) light detection. The hexadecanethiol organic molecules are chemically adsorbed on the electrodes of high-Al-content Al0.6Ga0.4N MSM solar-blind UV photodetectors, which can reduce the work function of the metal electrode and change the height of the Schottky barrier. This modification process significantly increases the photocurrent and responsivity of the device compared with the referential photodetector without modification. Additionally, the adverse effects caused by the surface state and polarization of the AlGaN materials are effectively reduced, which can be beneficial for improving the electrical performances of III-nitride-based UV photodetectors.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultraviolet (UV) optoelectronic devices have a broad application prospect, including missile early warning, environmental monitoring, flame detection, UV astronomy, biological and chemical process sensing and leak detection [1–6], which are becoming a new generation of optoelectronic equipment technology. III-nitride and III- oxides semiconductors are one of the highly anticipated candidate materials for UV photodetectors (PDs) due to their excellent optical and electrical properties [7,8], such as high electron saturation velocity, high breakdown field [9], high heat resistance, and high chemical stability [10]. Moreover, by increasing the proportion of Al content in AlGaN compounds, the band gap of AlxGa1-xN varies from 3.4 eV (GaN) to 6.2 eV (AlN), which will directly cause the cutoff wavelength of the device to vary from 365 nm to 200 nm [11]. Notably, AlGaN-based PDs with an Al-composition ratio greater than 40% are the appropriate candidates for solar-blind UV light detection, because their cut-off wavelength is less than 280 nm, which can effectively avoid the interference of solar background radiation.
So far, various AlGaN-based PD structures have been reported, such as p-n junction diodes, p-i-n diodes, metal-semiconductor-metal (MSM) and Schottky barrier diode structures [12–15]. Among them, MSM AlGaN PDs exhibit great advantages: simple structure, easy to manufacture and high yield, low parasitic capacitance, no ohmic contact or unintentional doping, that is conducive to the manufacture of AlGaN solar-blind UV PDs [16]. However, there existing a large amount of electron trap states at the metal-semiconductor (MS) interface, and the crystal quality of the material will decrease as the Al composition increases [17]. The high dislocation density and point defects in AlGaN films with high Al-composition lead to non-uniform Schottky barriers [18,19], allowing carrier tunneling under reverse bias conditions (in the dark) [18–21]. and would capture or recombine photogenerated carriers [22], resulting in a high reverse bias dark current. In addition, the gap state of the interface would capture or recombine photogenerated carriers [22], so the photocurrent will not be greatly enhanced under UV irradiation, which deteriorates the detection efficiency of AlGaN MSM UV PDs. In order to alleviate the adverse effects, different oxide layers such as SiO2, ZrO2, and HfO2 were inserted at the metal/semiconductor interfaces [23–26]. Besides, it is fascinating that the adsorption of organic molecules can modify the surface state of III-nitride semiconductors. These organic molecules are different from inorganic semiconductors, which exhibits ideal occupied and unoccupied molecular orbital levels [27]. Theoretical and experimental researches have indicated that this method not only can reconstruct the surface, but also change the surface electrical properties without electron transfer between the molecule and the semiconductor. The band bending of Si and GaAs semiconductors were successfully adjusted by organic molecule modification [28,29]. Moreover, the Schottky barrier heights at metal/semiconductor interfaces were demonstrated to be modified by adsorption of organic molecules and reduce the reverse leakage current significantly [30]. Chemical self-assembly technology can conveniently and effectively utilize the characteristics of specific structural units (such as surface characteristics, charge, polarizability, dipoles, etc.) to modify or add required functions [31,32]. It is believed that the chemical modification technology can enhance the performance of semiconductor device.
Thereby, in order to improve the electrical characteristics of high-Al-composition Al0.6Ga0.4N solar-blind UV MSM PDs, we employed organic hexadecanethiol to modify the metal electrodes. It was demonstrated that hexadecanethiol was chemically adsorbed on the surface of the gold electrode to adjust its work function, thereby changing the Schottky barrier height of the solar-blind MSM PD. The enhanced photocurrent gain and responsivity were achieved by using hexadecanethiol modification, and the effect of organic molecular on enhanced performance for AlGaN MSM PDs was explored by energy band diagram.
2. Experimental details
Figure 1(a) shows the schematic structure of AlGaN solar-blind UV MSM PD with hexadecanethiol modification. The epitaxial structure was composed of an AIN buffer layer and an unintentionally doped AlGaN active layer on the sapphire substrate. The pressure in the chamber for AlGaN growth is maintained at 50 mbar, with the growth temperature of 1200°C, and V/III ratio of about 300. Ni (30 nm)/Au (100 nm) interdigital electrode structures were fabricated on AlGaN surface by using standard photolithography, thermal evaporation and lift-off processes. The interdigital electrode was composed of 21 electrode fingers with an effective area of 300×300 µm2, and the width and spacing of the interdigital electrodes were both 7µm, as shown in Fig. 1(b).
Fig. 1. (a) The schematic structure of the AlGaN MSM PD with hexadecanethiol modification. (b) Top-view photograph of the fabricated MSM PD. (c) Schematic illustration of hexadecanethiol organic molecules bonded to the metal electrode. (d) Chemical schematic of hexadecanethiol (C16H33SH).
Afterwards, the surface of the MSM PD’s interdigital electrodes was modified by hexadecanethiol, the chemical structure of which is shown in Fig. 1(d). The modification process was as following: before modification, the sample was thoroughly rinsed with acetone and isopropanol, then dried with deionized N2. The self-assembled molecule hexadecanethiol (CH3 (CH2) 15-SH) (Aldrich) was dissolved in ethanol with a concentration of (1∼3) × 10−3 M. The PD samples with gold electrodes were immersed in the solution for at least 48 hours to form a uniform and densely packed monolayer, as schematic illustrated in Fig. 1(c). After the self-assembly adsorption was completed, the samples were rinsed with toluene, acetone and ethanol, finally dried with a flow of deionized N2.
The AlGaN epitaxial film was characterized by high-resolution X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The I-V characteristics of AlGaN solar-blind MSM PDs with and without electrode modification were measured by probe station with Keithley Semiconductor parameter analyzer (SCS-4200). The spectral responsivities of the two PDs were measured by using a monochromator fitted with a 500 W xenon lamp as the excitation source.
3. Result and discussion
Figure 2(a) shows Omega-2theta XRD pattern of the AlGaN (002) plane. Two diffraction peaks correspond to the AlN buffer layer and AlGaN epi-layer are observed. In addition, the crystal quality of the AlGaN epitaxial film can be characterized by the full width at half maximum (FWHM) of the XRD rocking curve, which is attributed to the fact that FWHM reflects the dislocation density of the material. The FWHM of the AlGaN (102) and (002) planes are 1106 and 837 arcsec, respectively, as shown in Fig. 2(b). A 10×10 µm2 AFM image of the AlGaN surface shown in Fig. 2(c) indicates that typical step-flow structures dominate the surface morphology, and the surface root-mean-square (RMS) roughness is 2.698 nm. Figures 2(d) and 2(f) show the dark field TEM images of AlGaN/AlN interfaces, which clearly exhibits the trend and position distribution of the dislocation line. By calculating the number of dislocation lines, it is estimated that the screw and edge dislocation densities of the AlGaN active layer are approximately 3.4×108 cm-2 and 1.19 × 109 cm-2, respectively.
Fig. 2. (a) Omega-2theta XRD pattern of the AlGaN (002) plane. (b) XRD rocking curves of AlGaN (002) and (102) planes. (c) 10×10 µm2 AFM image of AlGaN epitaxial layer. (d, e) TEM images of the AlGaN/AlN interface.
In order to investigate the effect of electrode modification on the current-voltage (I-V) characteristic of the AlGaN MSM PDs, Figs. 3(a) and 3(b) exhibit the dark currents and photocurrents of AlGaN MSM PDs with and without electrode modification. It is found that the dark currents (Id) of referential AlGaN MSM PD without modification are 4.95×10−11 A (−20 V) and 2.17×10−10 A (+20 V). While the Id of AlGaN MSM PD with electrode modified by hexadecanethiol are 3.59×10−10A (− 20 V) and 6.68×10−10A (+ 20 V). As for the photocurrents (Ip) of the two PDs illuminated by 225 nm monochromatic UV light, the referential device exhibits a photocurrent of 3.64×10−10A at −20 V and 9.68×10−10 A at +20 V, while the Ip of hexadecanethiol modified PD are 2.9×10−9A at −20 V and 8.18×10−9 A at +20 V, indicating an order of magnitude increasement compared to the referential PD. The increase of dark- and photo-current indicates that the Schottky barrier of the modified device is reduced, which proves that the electrode modification of the device is successful and effective. It is believed that the electron-hole pairs are generated in the depletion zone near the surface under UV illumination. Then the electrons are quickly swept out of the junction, but the holes are trapped and retained in the depletion zone. The trapped holes produce excess positive space-charge in the junction. The positive charge below the semiconductor surface is compensated by the negative surface charge, which further reduces the barrier height, thereby enhances the photocurrent [33]. After electrodes of the AlGaN MSM PD are modified by hexadecanethiol, although the dark current increases slightly, the photocurrent obtaines a higher gain effect compared to the referential PD. The high gain of photocurrent meets the demand for high response to weak UV detection. It is also worth noting that the breakdown voltage of the AlGaN PD increases slightly from 373 V to 377 V after electrode modification, as shown in Fig. 3(c).
Fig. 3. (a) Dark currents and (b) photocurrents of AlGaN MSM PDs with and without electrode modification. (c) Electrical breakdown characteristics of the two PDs.
Figure 4 shows the spectral responsivity of the two devices under different bias voltages. It can be seen that the PD devices with or without modification exhibit a steep cut-off effect at the wavelength of ∼250 nm, which agrees well with the band gap of AlxGa1-xN compound with an Al content of 0.6. Meanwhile, it can be clearly seen that from the overall trend of the response curves of 0-10 V, the order of magnitude of the spectral responsivity continues to rise under the bias voltage from low to high. The highest peak responsivity obtained at 10 V is 0.793A/W and 7.598A/W for PDs without and with hexadecanethiol modification, respectively, indicating approximately one order of magnitude of enhancement after electrode modification due to the enhanced internal gain of the modified device, and the internal gain may be mainly attributed to the trapping of carriers at the semiconductor/metal interface [34–37]. In addition, as the reverse bias voltage continues to increase, the Schottky barrier would be significantly reduced by the image force effect [34]. Moreover, the electric field in the depletion region can be enhanced with the increase of the bias voltage, which reduces the height of the Schottky barrier and induces increased photocurrent.
Fig. 4. Spectral response of the AlGaN solar-blind MSM PD (a)without and (b)with electrode modification under different bias voltages.
Figure 5 shows schematic energy band diagrams of metal-AlGaN contacts without and with hexadecanethiol modification. The local surface state of AlGaN has a continuous energy distribution within the forbidden band and is inconsistent with the Fermi level. Thus a net charge will be generated on the surface, forming a surface potential ${V_s}$. It can be seen that when a metal with a higher work function is in contact with AlGaN, electrons will enter the metal from the semiconductor, forcing their Fermi levels to the same height. Because metal and AlGaN have different work functions, they form a parabolic barrier, which is called the Schottky barrier ${\phi _{Bn}}.$, the energy band diagram is reflected in Fig. 5(a) [38–40]. The Schottky barrier measured relative to the Fermi level is given by:
where ${\phi _{Bn}}$ is the Schottky barrier height (SBH), ${\phi _m}$ is the work function of the metal, ${\chi _s}$ is the electron affinity of the semiconductor, and ${V_s}$ is the surface potential formed by the net charge. The change in the height of the Schottky barrier of the photodetector will significantly affect the photo-dark current, responsivity, and quantum efficiency. Obviously, the Schottky barrier height can be changed by adjusting the work function of the metal. The work function of metal can be regarded as a method for extending the wave function of free electrons on the metal surface into a vacuum. Exposing the metal to the electric field caused by the dipole monolayer causes the wave function of the free electrons to expand to a greater extent into the vacuum [41]. Therefore, electrons are more easily extracted from the metal, resulting in a decrease in the work function of the metal. As demonstrated above, one of the ways to tune the work function of metals is by inserting polar molecules that can self-assemble on the metal and form a highly ordered, thin layer with a dipole in the desired direction, which will cause the charge to redistribute on the metal surface. The effective dipole created by this self-assembled molecular (SAM) tunes the work function of the metal. The application of the dipole on the metal can be accomplished by physical or chemical methods of adsorbing molecules on the electrode surface. Alkane thiols are known to form such SAMs on metals. At the metal/molecular interfaces, the electric dipole associated with the gold-sulfur bond will change the work function of the metal electrode caused by the thiol-based monolayers. In this work, hexadecanethiol (C16H33SH) is chemically adsorbed to the gold electrode via the solution self-assembly phase to form a monolayer with dipoles. According to previous report, by changing the adsorption reaction time or the concentration of the configuration solution, the work function of the gold electrode can be varied from 5.1 eV to 4 eV [41]. The tunable work function of the metal electrode will cause the adjustment of the Schottky barrier, which in turn influences the performance of the AlGaN-based PDs. As displayed in Fig. 5(b), the work function of the metal decreases due to the adsorption of hexadecyl mercaptan, the barrier height generated by the metal-semiconductor contact will also decrease, which will allow more photogenerated carriers to pass through. Therefore, the enhancements of photocurrent and responsivity of the AlGaN solar-blind MSM PD are obtained with electrode modification.Fig. 5. Schematic energy band diagrams of metal-AlGaN contact, and (b) metal-AlGaN with hexadecanethiol modified on the metal surface.
Finally, I-V characteristics in dark as a function of temperature for the solar-blind AlGaN MSM PD with hexadecanethiol modification is shown in Fig. 6. It is found that the dark currents increase with the temperature increased from 300 K to 370 K. Moreover, the dark current shows a significant exponential increase under the bias voltage below 4 V, indicating that the current has a strong dependence on the bias voltage in the low field state. Therefore, thermionic-field emission (TFE) is believed to be dominant at the low bias voltage [42], and the current on the Schottky contact in the MSM PD is limited by the thermally assisted tunneling of electrons from the metal to the semiconductor conduction band at low bias voltages. However, when the applied bias voltage is higher than 8 V, the dark current exhibits a slight linear increase under high voltages, indicating a Poole-Frenkel emission (PFE) for carriers transport from the trap state in the semiconductor body to the continuous conduction state [43]. The results demonstrate that the solar-blind AlGaN MSM PDs with electrode modification are promising for high-temperature application.
Fig. 6. I-V characteristics of Al0.6Ga0.4N MSM PD under dark conditions at different temperatures from 300 to 370 K.
4. Conclusion
In summary, we propose a high-Al-composition Al0.6Ga0.4N solar-blind MSM PD with electrode modification by using hexadecanethiol. The modified AlGaN MSM PD exhibits a solar-blind photo-response with the cut-off wavelength located at ∼250 nm, and a high peak responsivity of 7.598A/W at 10 V. More importantly, the electrical performance of the AlGaN PD with modification is obviously enhanced compared to the referential device without modification, which is due to the reduced Schottky barrier height caused by hexadecanethiol modification. The underlying mechanism has been explored by the energy band diagrams of metal-AlGaN contact with hexadecanethiol modified on the metal surface. In addition, the I-V curves measured from room temperature to 370 K indicate that the solar-blind AlGaN MSM PDs with electrode modification are potential for high-temperature UV detection applications.
Funding
National Natural Science Foundation of China (61974056); Jiangsu Provincial Key Research and Development Program (BE2020756); Natural Science Foundation of Jiangsu Province (BK20190576); Science and Technology Development Foundation of Wuxi (N20191002); State Key Laboratory of Food Science and Technology (JUFSTR20180302); Fundamental Research Funds for the Central Universities (JUSRP22032); Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCY20_1769).
Disclosures
The authors declare no conflicts of interest.
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