Obtaining p-type wide-bandgap semiconductors with a bandgap >3.5 eV is still challenging. Here, p–n junction devices based on wide-bandgap (≥4 eV) p-type MnO quantum dots (QDs) and n-type Si-doped GaN are fabricated. The p-MnO QDs are synthesized by cost-effective femtosecond laser ablation in liquid. A simple spray-coating method is used for fabricating the p-MnO/n-GaN-based solar-blind deep UV (DUV) photodetector. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy reveal the MnO QD crystal structure. X-ray photoelectron microscopy analysis reveals good band alignment between p-MnO QDs and n-GaN, demonstrating the (type-II) staggered band alignment p–n heterojunction-based device. Electrical and photocurrent measurements show a high photocurrent response with a low dark current, while superior photo-responsivity (∼2530 mA/W) is achieved, along with self-powered and visible-blind characteristics (265 nm cutoff), demonstrating a high-performance DUV device with high detection limit for low light level applications. This study provides insights into the potential of p-type MnO QDs for III-nitride p–n junction DUV devices.

Deep ultraviolet (DUV) optoelectronic devices have become one of the most essential technologies with applications in a wide range of domains, such as medical treatment, astronomical investigations, sterilization, biomaterial analysis, missile detection, space communications, security systems, and imaging.1–4 However, the currently available commercial DUV optoelectronic devices (mainly photodetectors) still suffer from several issues, such as low selectivity and stability.5–7 For example, Si is an indirect-bandgap material and suffers from low sensitivity in the UV and DUV wavelength range but yields high response in the visible spectral domain, while requiring backup tools for cooling5,8,9 as well as many layers for sufficient performance, which necessitates the complex fabrication process.10 

Wide-bandgap semiconductors (WBGSs) with the bandgap energy ≥3.5 eV can be potential candidates for DUV optoelectronics, including photodetectors,1,9 due to their high breakdown voltage and capacity for operating in the UV and DUV spectral regions without the need for cooling.5,6,11 Particularly, GaN and its alloy with AlN are the most important direct WBGSs for DUV optoelectronic and electronic devices as well as high-frequency and high-power applications that can be used in harsh environments due to their excellent unique properties, such as high electron velocity and good chemical and thermal stabilities.12–20 As a result of their wide bandgap, they are characterized by high breakdown voltage.12,13,21 However, there are no p-type WBGSs with energy ≥4 eV, such as III-nitrides and other materials (e.g., MgZnO, AlGaN, and Ga2O3). Thus, DUV optoelectronic devices based on these materials (operating in the UV-B and UV-C spectral regions) suffer from low performance as the p–n junction structure in these devices is challenging to obtain, which is a prerequisite for DUV device enhancement.22–24 The poor p-type characteristics in these semiconductors are due to high resistivity or lack of p-type stability.24–26 

Such p–n junction structure in DUV photodetector devices retains superior stability and photo-responsivity, while it reduces dark current, compared to devices based on Schottky structure.27 In particular, the staggered band (type-II) p–n heterojunction structure is preferred, which depends on band alignment between the semiconductor layers included in the device structure, further promoting fast spatial separation of electrons and holes.28 

Due to these issues, highly complex processing methods are needed to fabricate such WBGS-based devices,8,26,29 which motivated us in our recent study to explore highly stable p-type WBGS quantum dots (QDs) based on solution-processed manganese oxide (MnO) semiconductors, synthesized by a cost-effective femtosecond laser ablation in liquid (FLAL) technique. These QDs are characterized by a wide bandgap (4–5 eV) suitable for UV-C applications.30 As QDs possess a large surface-to-volume ratio31,32 and can eliminate the interface defects (e.g., dislocations) that are common in heterojunction devices, a high photo-responsivity of solution-processed QDs-based devices was obtained.27,33 These p-MnO QDs comprise of the MnO phase (81.5%) mainly, with some minority phases from (MnOOH) (12%) originating from QD surface states passivated by OH molecules and minor inclusion of Mn2O3 (6.2%) as confirmed by x-ray photoelectron spectroscopy (XPS).30 We also demonstrated the p-type electrical properties of a wide-bandgap semiconductor with the energy gap ≥4 eV based on MnO QDs using field-effect transistor (FET) and Kelvin probe measurements.30 The DUV solar-blind performance of Schottky photodetectors based on these MnO QDs was demonstrated,30 while we reported on a broadband photodetector based on 2D n-MoS2 functionalized by p-MnO QDs to enhance the UV response and reduce the dark current as a result of the straddling p–n junction structure.27 Owing to their excellent optical, environmental, magnetic electrical, and nontoxic properties, these QDs can also be potential candidates for DUV applications.34–36 However, the suitability of the p-type MnO QDs for DUV p–n junction devices based on GaN and other WBGS has never been explored.

In this work, we address the aforementioned issue by developing a p–n junction between MnO QDs and WBGSs (here GaN). Advanced optical and structural analyses demonstrate the material quality as well as a good band alignment between n-type GaN and p-type MnO. The DUV visible-blind p–n photodetector based on p-MnO QDs/n-GaN was also fabricated as a part of this work without the need for complex processing.

Figure 1(a) shows the high-resolution transmission electron microscope (HR-TEM) image of the manganese oxide QDs (hereafter, denoted simply as MnO QDs), indicating that these QDs are crystalline and their size is <7 nm. The FLAL method of MnO QD synthesis is shown in Sec. S1 in the supplementary material (under the Experimental Method section). This result is in line with our previous findings obtained using HR-TEM measurements and electron energy-loss spectroscopy (EELS) compositional maps of HR-TEM images (which confirmed that Mn and O are distributed homogeneously within the QDs).27,30

FIG. 1.

(a) HR-TEM image of MnO QDs. (b) XRD pattern. Raman spectra of MnO QD films on glass substrate obtained using (c) 633 nm and (d) 473 nm excitation lines.

FIG. 1.

(a) HR-TEM image of MnO QDs. (b) XRD pattern. Raman spectra of MnO QD films on glass substrate obtained using (c) 633 nm and (d) 473 nm excitation lines.

Close modal

To study the manganese oxide phases in these QDs, x-ray diffraction (XRD) and Raman measurements were carried out. Figure 1(b) shows XRD 2θ scans, revealing different manganese oxide phases. Using the international center for diffraction XRD (ICDD) data, the peaks located at 35.4° and 40.6° were attributed to (111) and (200) planes of the cubic MnO phase, whereas peaks at 32.5° and 43.7° were attributed to (222) and (420) planes of the Mn2O3 phase, and those located at 29° and 59.5° were assigned to (112) and (224) Mn3O4 planes, respectively. A slight peak shift occurs to higher angles for all peaks except those located at 32.5° and 59.5°, which shift to lower angles compared to bulk material. This observation can be attributed to the quantum size effect of QDs or the presence of impurities, as OH molecules are incorporated in the surface states of the QD.30 These phases concur with our recently published XPS findings, indicating that the solution-processed MnO QDs comprise three phases—MnO, Mn2O3, and MnOOH (passivated with OH)27,30—which is generated as a result of transformations into other phases during the synthesis or due to high-energy laser effects.37 

Raman measurements of the MnO QDs were conducted to further confirm the QD composition. Figures 1(c) and 1(d) show that different Raman spectra were obtained using the 633 and 473 nm excitation laser lines, respectively, to investigate the Raman peaks related to the different manganese oxide phases as different vibration modes are induced by different excitation lines.38 Moreover, these excitation lines were chosen as they yield higher resolution compared to UV lines (not shown) and are more sensitive to oxygen vibration.39 The intense peak at 654.6 cm−1 in the spectrum shown in Fig. 1(c) obtained using 633 nm excitation wavelength and weak peaks at 315 and 365.5 cm−1 are attributed to Mn3O4,39 whereas the shoulder located at 566 cm−1 is attributed to MnO2.

FIG. 2.

(a) Steps involved in the fabrication of the p-MnO/n-GaN device. (b) SEM image of the p-MnO film spray-coated on the n-GaN film (the inset: SEM cross-sectional image showing the thickness of the 9 μm MnO QD film and the 3 μm GaN layer). The XPS spectra of (c) Mn 2p core-level and valence band spectrum for MnO film and (d) Ga 2p core-level and valence band spectrum for GaN. (e) Mn 2p and Ga 2p core-levels for p-MnO QDs/n-GaN heterojunction. (f) Normalized absorption spectra of MnO QDs/GaN and p-GaN samples. (g) Schematic representation of the band alignment at the p-MnO QDs/n-GaN hetero-interface. (h) The energy band diagram of the materials included in our photodetector.

FIG. 2.

(a) Steps involved in the fabrication of the p-MnO/n-GaN device. (b) SEM image of the p-MnO film spray-coated on the n-GaN film (the inset: SEM cross-sectional image showing the thickness of the 9 μm MnO QD film and the 3 μm GaN layer). The XPS spectra of (c) Mn 2p core-level and valence band spectrum for MnO film and (d) Ga 2p core-level and valence band spectrum for GaN. (e) Mn 2p and Ga 2p core-levels for p-MnO QDs/n-GaN heterojunction. (f) Normalized absorption spectra of MnO QDs/GaN and p-GaN samples. (g) Schematic representation of the band alignment at the p-MnO QDs/n-GaN hetero-interface. (h) The energy band diagram of the materials included in our photodetector.

Close modal

The Raman spectrum shown in Fig. 1(d) (using the 473 nm excitation) reveals the presence of a dominant peak at 646 cm−1 and a minor peak at 350 cm−1, which are attributed to the Mn3O4 phase formed during laser ablation.39,40 The 654.6/646 cm−1 lines are due to symmetric stretching of the Mn–O bond of MnOx groups,41 whereas the minor peaks located at 365.5/350 cm−1 correlate with the asymmetric stretching of bridge oxygen species Mn–O–Mn.41 A slight shift is observed in all peaks compared to bulk material, which can be attributed to phonons related to the quantum confinement effect or the strain applied to nanostructure sizes.40 The crystallite size of the QDs is found to exert the most important influence on the intensity and the shift of Raman peaks, while also causing the broadening of asymmetric peaks due to the phonon confinement effect.40–43Figure 1(d) also reveals incorporation of impurity-related defects, as indicated by a D-band at 1348 cm−1 (related to disordered carbons) and a G-band at 1584.5 cm−1 (pertaining to graphite), suggesting that these MnO QDs are doped unintentionally by carbon impurities.44,45 Carbon dopants in the material are expected, as the solution used in this work is ethanol. The weak D-band suggests low defect density, whereas the high intensity of G-band indicates enhanced electronic conductivity.44,45 The Raman and XRD results confirmed the manganese oxide phases in the QDs. It is also noteworthy that when the stability of the film was examined more than 12 months later, no changes in the bandgap values and other properties were noted.

Figure 2(a) shows the device fabrication steps of the p-type MnO QD film spray-coated on n-type GaN, while Fig. 2(b) illustrates the scanning electron microscope (SEM) image of the p-MnO QD film on n-GaN (the cross-sectional image is included in the inset). High-resolution XPS measurements were employed to investigate the band alignment of p-MnO QDs/n-GaN heterojunction and determine the valence band offset (VBO) at their interface, as shown in Fig. 2(c–e). To evaluate VBO at the p-MnO QDs/n-GaN hetero-interface, we calculated the energy difference between the Ga 2p3/2 and Mn 2p3/2 core levels from the p-MnO QDs/n-GaN heterojunction and the energy of Ga 2p3/2 and Mn 2p3/2 core levels relative to the valence band maximum of the GaN and MnO QD films, respectively, using the method proposed by Kraut et al.,46 which expressed as

ΔEV=EMn2p3/2MnOQDfilmEVBMMnOQDfilmEGa2p3/2GaNEVBMGaN+EGa2p3/2GaN/MnOQDEMn2p3/2GaN/MnOQD,
(1)
ΔEC=EgMnOQDfilmEgGaN+ΔEV.
(2)

Figure 2(c) shows the Mn 2p core-level and valence band maximum (VBM) spectra of the bulk MnO QD film. As the binding energy of Mn 2p3/2 is equal to 642.00 eV and VBM is equal to 0.51 eV, the separation between the core-level energy of Mn 2p3/2 and the valence band maximum ΔEMn2p3/2VBMMnOfilm=EMn2p3/2MnOQDfilmEVBMMnOQDfilm for the MnO QD sample was determined to be 641.49 eV. Figure 2(d) shows the Ga 2p core-level and valence band spectra of the bulk GaN sample. The binding energy of Ga 2p3/2 is equal to 1117.62 eV, and VBM is equal to 2.62 eV. Thus, the separation between the core-level energy of Ga 2p3/2 and VBM ΔEGa2p3/2VBMGaN=EGa2p3/2GaNEVBMGaN for GaN is 1115.00 eV. Figure 2(e) shows the Mn 2p and Ga 2p core-level spectra of thin MnO film spray-coated on the GaN film. The binding energies of Mn 2p3/2 and Ga 2p3/2 are found to be 641.00 eV and 1117.09 eV, respectively. The energy difference ΔEGa2p3/2Mn2p3/2GaN/EGa2p3/2GaN/MnOQDfilm=EGa2p3/2GaN/MnOQDEMn2p3/2GaN/MnOQD between Ga 2p3/2 and Mn 2p3/2 core levels is observed to be 476.09 eV. After substitution to Eq. (1), the VBO value was estimated to be ΔEV = 2.58 eV.

To determine the bandgap energy, we carried out absorption measurements. Figure 2(f) depicts the absorption spectra of the p-MnO QDs/n-GaN film and pristine GaN film, confirming the wide bandgap energy of the MnO QD film (EgMnQDfilm = ∼4 eV), while the bandgap of the p-GaN film is estimated to be EgGaN = ∼3.43 eV. However, absorption measurements of MnO QDs in ethanol are provided by Fig. S1 in the supplementary material, indicating that the bandgap value of as-synthesized p-MnO QDs is ∼4.52 eV. Evolving surface state slowly during the spray-coating method [as shown in Fig. 2(a)] can cause such a shift in the absorption edge of QDs due to air exposure.47,48 Thereby, substitution of VBO (ΔEV) obtained from XPS analysis and the Eg values of the MnO QD and GaN films in Eq. (2) allows us to determine the conduction band offset (CBO), or ΔEC, for the p-MnO QDs/n-GaN heterojunction, resulting in ΔEC = 3.15 eV. The offset parameters obtained in this study are shown in the band alignment diagram presented in Fig. 2(g), confirming that this band alignment pertains to staggered (type-II) heterojunction at the interface. As reported in our recently published work, the p-type property of the MnO QDs has been confirmed by studying the FET characteristics and Kelvin probe force microscope.30 Therefore, these analyses confirm a sufficient charge transfer due to the work function difference, as shown in Fig. 2(h), depicting relevant energy levels of the materials used in our photodetector.

As a proof of concept, the performance of a DUV photodetector device based on the p-MnO QDs/n-GaN heterostructure has been demonstrated, as illustrated in Fig. 3, using 244 nm illumination at room temperature (under ambient conditions). The I–V characteristics shown in Fig. 3(a) indicate a typical asymmetrical rectifying behavior of photodetectors based on the p–n junction.49 The I–V log scale curve under illumination [the inset of Fig. 3(a)] further confirms the asymmetrical behaviors of the device. The photodetector was operated using a reverse bias, whereas no response in the forward bias voltage was registered, which is typical for a photodiode based on a p–n heterojunction. For p–n junction devices, forward bias is opposite to potential barrier that causes a significant decrease in the depletion region, speeding up the electron–hole recombination process. Thus, as the recombination time is much shorter than the required transient time, no photoresponse is observed.50 On the other hand, reverse bias applied to the p–n staggered heterojunction under illumination increases the depletion region, which leads to a sufficient built-in electric field that improves electron–hole separation and shortens the transient time.28,49,51 To further confirm that the photoresponse has occurred through the p–n junction, we examined the device using different configurations. For the p-MnO/p-MnO configuration, a weak and noisy response was obtained, whereas no response was induced in the n-GaN/n-GaN configuration.

FIG. 3.

Electrical properties of the p-MnO QDs/n-GaN device under DUV illumination (244 nm and 28.6 mW/cm2) under reverse bias. (a) I–V measurements under illumination and dark conditions (the log scale of I–V curve under illumination is shown in the inset). (b) Transient current–time characteristics under −1 V. (c) Voltage dependence of photo-responsivity and detectivity. (d) Photo-responsivity as a function of power density (the inset shows the log –log scale photocurrent as a function of power density). (e) Self-powered transient characteristics at 0 V. (f) Relative photo-responsivity vs wavelength (solar-blind characteristic).

FIG. 3.

Electrical properties of the p-MnO QDs/n-GaN device under DUV illumination (244 nm and 28.6 mW/cm2) under reverse bias. (a) I–V measurements under illumination and dark conditions (the log scale of I–V curve under illumination is shown in the inset). (b) Transient current–time characteristics under −1 V. (c) Voltage dependence of photo-responsivity and detectivity. (d) Photo-responsivity as a function of power density (the inset shows the log –log scale photocurrent as a function of power density). (e) Self-powered transient characteristics at 0 V. (f) Relative photo-responsivity vs wavelength (solar-blind characteristic).

Close modal

Transient measurements (with several on/off cycles) were performed under different reverse bias voltages (ranging from −0.5 to −2.5 V), as shown in the supplementary material (Fig. S2). The photoresponse signals obtained under all bias settings possess good signal-to-noise characteristics and high stability, indicating efficient photocurrent generation in this range. Figure 3(b) shows the transient photoresponse obtained using −1 V bias. The photocurrent increases by two orders of magnitude compared to dark current, from ∼0.56 nA under dark conditions to ∼93.2 nA under illumination (Iph/Id = ∼1.8 × 102). A considerable low dark current is observed, which is suitable for capturing weak light signals (high detection limit).

To confirm the excellent functionality of our device, photo-responsivity and detectivity were investigated, as these are the most important device figures of merit. Figure 3(c) shows that the photo-responsivity under 28.6 mW/cm2 of 244 nm laser illumination increases as the reverse bias increases. To further investigate the device functionality, the photo-responsivity of the p-MnO QDs/n-GaN device under different power densities was examined. As shown in Fig. 3(d), the photo-responsivity decreases as the illumination power density increases. This is to be expected, as increasing the light power density results in a greater number of charge carriers (electrons and holes). Consequently, the likelihood of electron–hole pairs recombining is greater than that of holes drifting to the anode and electrons to the cathode, which increases the forward photocurrent and decreases the depletion layer width.52 Therefore, lower power density can create sufficiently strong electric field to increase the depletion layer width, with low noise and enough carrier density transported due to low recombination probability. With 244 nm illumination, superior photo-responsivity (∼2.53 × 103 mA/W) was obtained at low light power 0.08 mW/cm2 compared to the values reported in literature for solar-blind DUV photodetectors53–58 (Table S1 in the supplementary material), while the photo-responsivity of our p–n device is one order of magnitude higher than that obtained for pristine MnO QD Schottky photodetector,30 confirming significant enhancement due to the p–n heterojunction structure and good band alignment between p-MnO QDs and n-GaN. These results demonstrate a high light detection limit and high sensitivity.

Figure 3(e) demonstrates the good functionality of this p–n junction photodetector with self-powered characteristics under 0 V and 244 nm illumination. The time-dependent photoresponse under 0 V shows a photocurrent of ∼10.5 nA, with a higher self-powered photo-responsivity (∼14.9 mA/W) compared to Schottky MnO photodetectors.30 This result further confirms that electrons and holes are photo-generated and efficiently separated without external bias due to the sufficient internal bias in the depletion layer between p-MnO and n-GaN. This finding shows that this device can operate with extremely low power consumption.33 

To demonstrate that our DUV device exhibits solar- or visible-blindness characteristics, we performed wavelength-dependent photoresponse measurements. Figure 3(f) shows the relative photo-responsivity of our device as a function of the illumination wavelength. The photo-responsivity decreases rapidly as the wavelength increases, whereby zero photo-responsivity is attained at λ = 400 nm with a sharp cutoff at λ = ∼265 nm in the DUV range, thus confirming that the device is a solar-blind photodetector. In a p–n type-II heterojunction, both layers need to be excited to generate a sufficient number of photo-generated carriers. If the light energy is below the MnO QD bandgap, the photo-generated carriers from GaN are not sufficient to produce photoresponse, while no electron–hole pairs can be photo-generated below the bandgap energies of both GaN and MnO QDs (at λ ≥ 400 nm). The weak photoresponse in the 300–400 nm range (i.e., below ∼4 eV) shown in Fig. 3(f) is similar to that observed in pristine MnO QD Schottky photodetectors,30 suggesting that the photoresponse in this range can be attributed to the surface defects in the MnO QDs. These findings confirm that the photodetector device operates efficiently in the DUV range (UV-B and UV-C) and exhibits visible-blind characteristics, which, in turn, reduces the interface between low- and high-energy signals, making it suitable for space communications.9,30 Therefore, our DUV photodetector can be used in sensors with no interaction with solar radiation.

The photocurrent rise (tr) and decay (td) times were estimated using a double-exponential function as described in the supplementary material (Sec. S3),59–61 which is the best fitting attained, as slow and fast components are observed for both tr and td (see Fig. S3). The rise and decay time were estimated to be tr1 = ∼0.06 s, tr2 = ∼0.87 s, and td1 = ∼0.05 s and td2 = ∼1.22 s, respectively. These values are comparable to those reported for QD-based DUV photodetectors by other authors.9,30,53,62 The relatively long rise/decay time can be due to carriers trapped by defects in the MnO QD layer.27 To elucidate the device characteristics and the kinetics of photo-generation and transportation through the p–n junction, we studied the photocurrent response as a function of the power density (Pd) of illuminated light using the Iph=aPdk correlation (a is the proportionality constant and k is the photosensitivity factor). The k value was estimated to be <1 using the logarithmic-scale fitting, as shown in the inset of Fig. 3(d), suggesting that the inclusion of trap states at the junction (such as the MnO QD-related defects) might have induced different photoelectric effects, or complex processes of electron–hole generation and recombination routes.63 These complex processes can be responsible for the slow and fast rise and decay times.

In conclusion, we have developed a high-performance solar-blind DUV photodetector based on p-MnO QDs/n-GaN. Our findings demonstrated that p-type MnO QDs are suitable for WBGSs and nitride devices based on the p–n junction structure. Structural analyses revealed several manganese oxide phases within the QDs. Solution-processed MnO QDs were spray-coated on n-GaN to fabricate a DUV p–n device, overcoming complex fabrication processing. XPS analyses revealed a type-II heterojunction between GaN and MnO QDs and good band alignment. Electrical characterizations showed superior device photoresponse, solar blindness, and self-powered characteristics. These p-type MnO QDs that are easily processed for device fabrication by spray-coating can be potentially used for the development of large-scale DUV optoelectronic devices.

See the supplementary material for the experimental method as well as detailed experimental data of absorption, electrical characterizations, and a comparison between our device characteristics and those reported in literature.

The authors thank KAUST for financial support. This work was supported by the base fund (No. BAS/1/1319-01-01). D.A. acknowledges the support from the Deanship of Scientific Research of Taif University for Taif University Researchers Supporting Project No. TURSP-2020/261.

The authors have no conflicts to disclose.

H.A. and I.S.R. contributed equally to this work.

The data that support the findings of this study are available within the article and its supplementary material.

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Supplementary Material