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Abstract
We have designed a metal–semiconductor–metal (MSM) solar-blind ultraviolet (UV) photodetector (PD) by utilizing Al0.55Ga0.45N/Al0.4Ga0.6N/Al0.65Ga0.35N heterostructures. The interdigital Ni/Au metal stack is deposited on the Al0.55Ga0.45N layer to form Schottky contacts. The AlGaN hetero-epilayers with varying Al content contribute to the formation of a two-dimensional electron gas (2DEG) conduction channel and the enhancement of the built-in electric field in the Al0.4Ga0.6N absorption layer. This strong electric field facilitates the efficient separation of photogenerated electron-hole pairs. Consequently, the fabricated PD exhibits an ultra-low dark current of 1.6 × 10−11 A and a broad spectral response ranging from 220 to 280 nm, with a peak responsivity of 14.08 A/W at −20 V. Besides, the PD demonstrates an ultrahigh detectivity of 2.28 × 1013 Jones at −5 V. Furthermore, to investigate the underlying physical mechanism of the designed solar-blind UV PD, we have conducted comprehensive two-dimensional device simulations.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The potential applications of optoelectronic detectors operating in the ultraviolet (UV) part of the spectrum are wide-spread [1–5], for example, chemical leak detection, missile warning, fire detection, solar UV monitoring and ozone detection. As a type of ternary compound possessing a wide direct band gap [6], the AlxGa1−xN material system has become one of the most competitive materials owing to its high chemical stability, excellent thermal conductivity, high electron mobility, and radiation hardness [7–9]. Furthermore, by varying Al content, the energy band gap of AlxGa1−xN is tunable from 3.4 eV (GaN) to 6.2 eV (AlN), directly contributing to the corresponding cutoff wavelength to vary from 365 nm to 200 nm [10]. AlGaN becomes intrinsically solar-blind, particularly when the specified percentage of Al element is larger than 38% [11]. The superior material properties of the AlxGa1−xN material system are utilized in the manufacture of UV detectors. Until now, based on AlxGa1−xN material, diverse photodetector (PD) structures have been physically implemented based on PN/PIN junction diodes [12,13], photoconductors [14], Schottky type diode [15–17], and metal–semiconductor–metal (MSM) detectors [18]. In particular, MSM PDs consist of two metal electrodes forming Schottky contacts, one is in positive bias and the other is in reverse bias. This unique configuration renders outstanding characteristics to MSM AlxGa1−xN-based PDs, such as direct compatibility, less complicated manufacture technique, no doping or ohmic contact, and high detectability [19]. However, not only will the crystal quality of the material deteriorate when the Al content increases [20], but also numerous electron trap states are formed at the metal–semiconductor interface [21]. Inhomogeneous Schottky barriers are related to the high dislocation density and defects in AlxGa1−xN films containing high Al element, enabling carrier to tunnel transport under reverse bias (in the dark) that ultimately results in augmented leakage current, i.e., the dark current [22]. Hence, it is necessary to enlarge the discrepancy between the photocurrent and the dark current, which can afford superior responsivity. An available approach to increase the responsivity is the adsorption of organic molecules for the modification of Schottky barrier heights at metal–semiconductor interfaces. As a chemical self-assembly technology [23], peculiar structural units, including polarizability, charge, and dipoles, are utilized to satisfy required functions. Furthermore, regarded as one typical asymmetric structure [24,25], the different barrier heights are formed by using different metals as electrodes at the two ends; this effectively contributes to a higher charge transport and responsivity. In addition, heterostructure has been applied in MSM PDs along with AlxGa1−xN materials that can enhance the polarization-induced electric field in the absorption layer and the ability to collect more carrier [26–28]. Heterojunction interfaces typically lead to accumulation of charges at the interface of one of the materials, forming two-dimensional electron gas (2DEG) [29,30]. Recently, Hsueh et al. reported a p–n ultraviolet photodetector (PD) based on an Al-doped MgZnO/Al0.08Ga0.92N heterojunction which exhibits a dark current of 1.56 µA at –3 V [31]. Yoshikawa et al. developed a solar-blind Al0.6Ga0.4N/Al0.5Ga0.5N MSM type PD that exhibited both high rejection ratios and high photosensitivity owing to the 2DEG layer at the Al0.6Ga0.4N/Al0.5Ga0.5N hetero-interface [32]. However, these works surfer from some drawbacks such as high dark current and poor response speed. In addition, the investigations of AlxGa1−xN-based solar-blind PD with double heterostructures are rarely be reported. It is expected to further improve the performances of AlGaN-based MSM PDs by adopting double heterostructures. Therefore, studying the physical mechanisms of double heterostructures is essential.
In this study, we designed a solar-blind UV PD based on AlxGa1−xN, incorporating an Al0.4Ga0.6N absorption layer between the Al0.55Ga0.45N barrier layer and the Al0.65Ga0.35N buffer layer to form double heterostructures. The heterostructures enhanced the polarization-induced electric field in the absorption layer, facilitating the separation of photogenerated electron-hole pairs and the formation of a 2DEG conduction channel, which results in low dark current. Experimental and simulation methods were employed to explore the characteristics and underlying physical mechanisms of the proposed solar-blind UV PD based on AlxGa1−xN heterostructures.
2. Methods
The AlGaN layers with varying Al content were epitaxially grown on an AlN/sapphire template using metal organic chemical vapor deposition. Initially, a 1.5-µm-thick unintentionally doped (UID) Al0.65Ga0.35N buffer layer was grown on the AlN/sapphire template. This was followed by the deposition of a 300-nm-thick UID Al0.4Ga0.6N absorption layer. Subsequently, a 50-nm-thick UID Al0.55Ga0.45N barrier layer was deposited. 50/100 nm Ni/Au interdigital electrodes were deposited on top of the barrier layer to form Schottky contact by lift-off using e-beam evaporation. Figure 1(a) provides a clear schematic structure of the designed MSM solar-blind UV PD, while Fig. 1(b) presents an optical microscopy image of the fabricated device. The electrodes were patterned to have a length, width, and spacing of 500, 20, and 20 µm, respectively. The effective photosensitive area corresponds to the region between the interdigital electrodes, measuring approximately 500 × 500 µm2.
Fig. 1. (a) Schematic of the MSM solar-blind UV PD based on AlxGa1−xN heterostructures. (b) Optical microscope image of the interdigital electrodes and contacts.
To evaluate the surface roughness of the epitaxial layers, atomic force microscopy (AFM) was employed. The Al mole fractions and crystal quality of the epitaxial layer were estimated using a high-resolution X-ray diffractometer. The optical properties of the device were analyzed using a photoluminescence (PL) spectrometer. The current-voltage (I–V) curves and spectral responses of the fabricated PD were measured using a Keithley 4200 semiconductor parameter analyzer with a 150 W xenon lamp and monochromator. The response spectra were calibrated using a standard Si detector. The noise power density spectra and transient response spectra were collected using an FS-Pro multifunctional semiconductor parameter tester.
To investigate the physical mechanisms and understand the impact of Al0.55Ga0.45N/Al0.4Ga0.6N/Al0.65Ga0.35N heterostructures on device performance, the general-purpose TCAD ATLAS device simulator was utilized. This involved analyzing electric fields, energy bands, and electron and hole concentrations. The polarization effect in the heterostructures was also taken into account. Several physical models were considered in the simulations, including the Fermi-Dirac statistical model, Shockley-Read-Hall recombination model, concentration-dependent mobility model, field-dependent mobility model, and drift-diffusion transport model. The absorption coefficients for AlGaN materials were obtained from Refs. [33,34]. The work function of Ni was set as 5.15 eV to define the Schottky contact [35]. Essential parameters of the AlxGa1−xN materials, such as bandgap, electron affinity, and permittivity, were incorporated in the simulation. The bandgap energy (Eg) in eV was calculated using Eq. (1) mentioned in Ref. [36].
As specified in [38], Eq. (3) can be used to calculate the permittivity E of AlxGa1−xN with different Al proportion.
3. Result and discussion
The crystalline quality and alloy compositions of the AlxGa1−xN hetero-epilayers were characterized using AFM and high-resolution X-ray diffraction (HRXRD) techniques. Figure 2(a) presents an AFM image illustrating the surface morphology obtained from a typical 10 × 10 µm2 scan area. The root mean square roughness was determined to be 5.191 nm, indicating a reasonably good morphological quality of the epitaxial layer surface. In Fig. 2(b), the omega-2theta HRXRD pattern of the epitaxial films displays three distinct diffraction peaks at approximately 17.50°, 17.79°, and 18.02°. These peaks correspond to Al mole fractions of 0.40, 0.65, and 1, respectively. On account of the weak diffraction signal, it is challenging to differentiate the 50-nm-thick Al0.55Ga0.45N layer from the peaks of the Al0.65Ga0.35N and Al0.4Ga0.6N layers. Figure 2(c) clearly demonstrates the XRD rocking curve scan of the AlN layer. The full width at half-maximum (FWHM) values for the AlN (102) and (002) planes were determined as 440 and 369 arcsec, respectively, indicating an ideal crystalline quality. Room temperature photoluminescence (PL) spectrum is shown in Fig. 2(d), with a narrow normalized intensity peak observed in the band emission near the cutoff wavelength (280 nm) of Al0.4Ga0.6N. The FWHM of 16.46 nm can also be observed.
Fig. 2. (a) Typical 10 × 10 µm2 AFM image of the Al0.55Ga0.45N layer. (b) Omega-2theta X-ray diffraction (XRD) pattern of the wafers (002) plane for the designed device. (c) X-ray diffraction (XRD) rocking curves of the AlN (102) and (002) plane. (d) Photoluminescence spectra of the Al0.4Ga0.6N absorption layer.
Figure 3(a) displays the measured IV curves of the fabricated MSM PD with AlxGa1−xN hetero-epilayers under dark and illuminated conditions, using an incident light wavelength of 270 nm and an incident optical power density of 2.56 µW/cm2. The dark current is remarkably low at 1.6 × 10−11 A, while the photocurrent reaches 2.1 × 10−7 A at an applied bias of −20 V. This low dark current is attributed to the formation of a high Schottky barrier between the metal Ni and the Al0.55Ga0.45N layer. Consequently, the sensitivity value at −20 V is 1.3124 × 106%, calculated as (Iphoto − Idark)/Idark × 100%. Figure 3(b) demonstrates the responsivity as a function of wavelength, showing broad responses ranging from 220 nm to 280 nm. The MSM PD is capable of detecting solar-blind UV resulting from this wavelength range. The spectral responsivity gradually increases as the applied bias on the Schottky contact increases at the same wavelength. The response peak at an applied bias of −20 V is 14.08 A/W under illumination at 270 nm. The spectral responsivities sharply decrease near the wavelength of 280 nm, which aligns closely with the energy bandgap of the Al0.4Ga0.6N absorption layer. Furthermore, the designed AlxGa1−xN-based MSM PD exhibits a normalized photocurrent-to-dark current ratio (NDRD) of 8.8 × 1011 W−1 at −20 V, which is defined as the ratio of responsivity to dark current.
Fig. 3. (a) I–V curves of the fabricated device in dark and under 270 nm UV illumination. (b) Spectral responsivity in terms of the incident light wavelength at different applied biases.
One crucial characteristic of photodetectors is the detectivity D*, which describes the normalized signal-to-noise performance of a photodetector [39,40]. In this work, the flicker (1/f) noise is taken into account for an accurate evaluation of detectivity. Accordingly, the noise power spectrum measured in the frequency range of 1 Hz to 10 kHz under different applied biases is depicted in Fig. 4(a). The spectral density of the noise power increases with increasing reverse bias.
Fig. 4. (a) Noise power density spectra of the designed PD measured at various applied biases. (b) Time-dependent photo-response characteristics for the designed PD measured at -8 V and under 270 nm UV illumination.
The solid lines in this Fig. are the experimental data, and the dashed lines are fitted well by
where Sn(f) is the spectral density of the noise power, and K and α are two fitting parameters. From the measured curves, the α values are found to be nearly unity across the measured frequency range, indicating that the low-frequency noise in the designed device is predominantly dominated by 1/f noise. The total power of the noise current can be estimated by integrating Sn(f) over the frequency range:Therefore, the noise equivalent power (NEP) can be calculated by
where R is the responsivity. The specific detectivity D* of the PD is determined by where A is the active area of the PD, and BW is the measured bandwidth. For a bias voltage of −5 V, and a given bandwidth of 10 kHz, the NEP is calculated as 2.19 × 10−13 W. Consequently, the specific detectivity (D*) for the fabricated device is estimated to be 2.28 × 1013 cmHz1∕2W−1 (Jones). The low NEP value and high detectivity value of the designed PD based on AlxGa1−xN hetero-epilayers are attributed to the effective suppression of leakage current. Figure 4(b) illustrates the time-dependent photo-response of the PD at an applied bias of −8 V, measured under periodic 270 nm irradiation. It can be observed that the PD exhibits on/off switching behavior, indicating the generation and decay of carriers in response to the periodic turning on and off of the xenon lamp. In detail, the fabricated PD presents a rise time (τr) of 0.71s and a decay time (τd) of 0.48 s. Here the rise time is defined as the time required for the response to increase from 10 to 90% of the peak value, whereas the decay time is the opposite. Hence, the solar-blind UV PD demonstrates excellent stability and reproducibility. As summarized in Table 1, compared with various UV PDs reported in other studies, the designed PD in this work shows excellent optoelectronic performances.
Table 1. Comparison of performances of various UV PDs
To highlight the superiority of AlGaN hetero-epilayers with different Al contents, a comparison is made between the designed PD (Device N) and a reference PD (Device R) through theoretical simulations, focusing on the electric field profile and energy band. The inset of Fig. 5(a) displays a cross-sectional view of Device R, while Fig. 5(a) and 5(b) show the electric field distribution and energy band diagram of Device R at a bias of −5 V, respectively. It is evident that the simulated average electric field within 300 nm beneath the surface is only 0.03 MV/cm in the dark, which is still higher than under illumination. Furthermore, the energy band exhibits a slight bending when Device R is in the dark, attributed to the Schottky contact barrier in the simulation. On illumination, the extent of energy band bending decreases due to the influence of the built-in electric field.
Fig. 5. Simulated electric field, energy band diagram in dark and on illumination at –5 V for (a), (b) Device R and (c), (d) Device N. The inserts show the cross-sectional view of the device R and N.
The two-dimensional diagram of Device N is shown in the inset of Fig. 5(c). In the dark state, the built-in electric field in the absorption layer of Device N is calculated to be as high as 0.18–0.25 MV/cm, as illustrated in Fig. 5(c). Under the same conditions, it can be observed that there exists a stronger electric field in the Al0.55Ga0.45N barrier layer and a slightly weaker built-in field in the Al0.65Ga0.35N buffer layer. Thus, the polarized Al0.55Ga0.45N/Al0.4Ga0.6N and Al0.4Ga0.6N/Al0.65Ga0.35N heterostructures lead to a significantly enhanced electric field in both the absorption layer and the barrier layer. When Device N is illuminated, the photogenerated electron-hole pairs in the absorption layer are separated toward different hetero-interfaces under the influence of the built-in field, and the polarization-effect-induced interface charges are partially screened, resulting in a weakened electric field. However, even under 270 nm UV illumination, the electric field in the Al0.4Ga0.6N absorption layer of Device N can still reach values as high as 0.02–0.13 MV/cm, surpassing that of Device R. Moreover, Fig. 5(d) clearly shows that the energy band of Device N exhibits significant bending in the dark due to the polarization-effect-induced charges at the Al0.55Ga0.45N/Al0.4Ga0.6N and Al0.4Ga0.6N/Al0.65Ga0.35N hetero-interfaces. Based on the energy band diagram, it can be inferred that a strong built-in field exists in the absorption layer which directs toward the Al0.4Ga0.6N/Al0.65Ga0.35N interface from the Al0.55Ga0.45N/Al0.4Ga0.6N interface. Similarly, there is a strengthened built-in field pointing toward the surface in the Al0.55Ga0.45N barrier layer and an electric field pointing toward the Al0.4Ga0.6N/Al0.65Ga0.35N interface in the Al0.65Ga0.35N buffer layer. Furthermore, the energy band in the Al0.4Ga0.6N absorption region exhibits minimal bending under illumination compared to in the dark due to the screening effect. Therefore, the designed Device N with AlxGa1−xN heterostructures exhibits significantly enhanced polarization-effect-induced built-in field in the absorption layer compared to Device R, as confirmed by the simulated electric field and energy bands.
For the designed UV PD, the efficient separation of electrons and holes to different regions is prompted by the strong electric field. Consequently, there is a sharp reduction in the overlap between electron and hole profiles, leading to a longer carrier lifetime and improved responsivity. The distributions of electron and hole concentrations in Device N are shown in Fig. 6(a) and 6(b), respectively. It is evident that electrons and holes accumulate separately at the Al0.55Ga0.45N/Al0.4Ga0.6N and Al0.4Ga0.6N/Al0.65Ga0.35N interfaces, whether Device N is in the dark or not, confirming the effect of the enhanced electric field. Additionally, both the electron and hole concentrations reach magnitudes up to 1018 cm−3 at different hetero-interfaces. Furthermore, under illumination, the electrical conducting layer extends deeper, and the total carrier concentration becomes higher compared to in the dark, as intuitively shown in the inset of Fig. 6(a) and 6(b). Under these conditions, it is primarily the electrons gathered at the Al0.55Ga0.45N/Al0.4Ga0.6N interface that form a 2DEG conduction channel, contributing to the current. Despite the ultrahigh electron concentration at the hetero-interface in the dark, the Al0.55Ga0.45N barrier layer hampers the transmission of electrons in the channel, helping to reduce the dark current to some extent. The inherent presence of the built-in field prevents holes from being swept into the conduction channel, which is conducive to noise reduction. The enhancement of the built-in field and the formation of a single type of carrier conduction, namely the 2DEG conduction channel, indicate that AlxGa1−xN hetero-epilayers have positive effects in Device N.
Fig. 6. Simulated (a) electron concentration distributions and (b) hole concentration distributions for the device N in dark and on illumination at –5 V. Inserts: Corresponding two-dimensional chromaticity diagrams.
4. Conclusion
In conclusion, we have successfully fabricated and investigated a MSM solar-blind UV PD based on AlxGa1−xN heterostructures. The designed device exhibits a remarkably low dark current of 1.6 × 10−11 A at an applied bias of −20 V. It demonstrates a high peak responsivity of 14.08 A/W at −20 V, with a sharp cutoff wavelength near 280 nm. Moreover, the device achieves an ultrahigh detectivity of 2.28 × 1013 Jones at –5 V, owing to the effective suppression of leakage current through the inclusion of the Al0.55Ga0.45N barrier layer.
Through theoretical simulations comparing the electric field and energy band of the reference PD, we have confirmed that the use of AlxGa1−xN-based double heterostructures improves the efficiency of separating photogenerated electron-hole pairs thanks to the enhanced polarization-induced built-in field in the absorption layer. The carrier concentration distribution further demonstrates the presence of a single type of carrier conduction and confinement of the hole current, indicating the positive role of the enhanced electric field. These simulation results provide a clear explanation for the observed experimental characteristics. Thus, the adoption of AlxGa1−xN hetero-epilayers proves to be beneficial in achieving the excellent performance of the designed device.
Funding
National Natural Science Foundation of China (61974056, 62374075, 62375028, 62074019, 62104084, 62174016); Fundamental Research Funds for the Central Universities (JUSRP22032); Science and Technology Program of Suzhou (SZS2020313); Jiangsu Provincial Key Research and Development Program (BE2020756).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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