A Deep-Ultraviolet Photodetector of a Hybrid Organic-Inorganic p-CoPc/n-Ga₂O₃ Heterostructure Highlighting Ultra-Sensitive

Abstract

A practical method for organic–inorganic hybrid heterojunction photodetector by spin-coating the cobalt phthalocyanine (CoPc) solution onto the β-gallium oxide (β-Ga₂O₃) film is available to realize the sensitive detection of ultraviolet light signals. We have carefully measured and calculated the optoelectronic performance of the prepared device. The device demonstrates excellent rectification characteristics under different light intensities, and the rectification ratio reaches 144.96 under 900 μW/cm² at ±5 V. Benefiting from the construction of CoPc/Ga₂O₃ heterojunction, the device shows an extremely low dark current of 5.73 fA, a high detectivity of 1.92 × 10¹⁷ Jones, a responsivity of 18.4 mA/W, and a high light-to-dark current ratio of 3.76 × 10⁶. In addition, the intrinsic physical mechanism of the device is investigated through the energy band diagram under different conditions. The device is equipped with the possibility to work under self-powered mode and has good stability in the air environment.

1. Introduction

Ultra-wide bandgap semiconductor materials, generally with bandgaps of approximately 3.0 to 6.0 eV, are opening up a fascinating and challenging new area in the field of semiconductor materials, quantum information, and functional applications. Beccause some characteristics of wide bandgap semiconductor devices are non-linearly related to the bandgap, they are of higher research value than conventional semiconductor devices [1]. From the view of device structure, low-dimensional devices have fewer defects and higher crystal quality compared with bulk structure devices, thus offering better performance [2]. The ultra-sensitive and ultra-fast nature of two-dimensional (2D) materials in optoelectronic systems led it quickly extending to ultraviolet detection [3]. Most of the ultraviolet radiation from the Sun are absorbed by the stratospheric ozone layer, so the 200 to 280 nm is known as the solar-blind region. Ultraviolet detection has profound impacts on the military and civilian areas, including missile flame sensing, climate monitoring, chemical analysis, and space communication [4]. Therefore, the fabrication of ultraviolet photodetectors based on 2D wide bandgap semiconductor materials is imperative.

In recent years, a series of wide bandgap semiconductors have been applied to ultraviolet photodetectors, such as GaN, SiC, ZnO, Ga₂O₃, ZnMgO, etc. [5]. As a kind of direct bandgap semiconductor, Ga₂O₃ is emerging as the most promising candidate because of its excellent properties [6]. Ga₂O₃ has higher electron mobility, higher breakdown field strength, and higher Baliga’s figure of merit (more than 3000) [7]. Ga₂O₃ has an ideal bandgap of about 4.7 to 4.9 eV, which is well-matched with the solar-blind region, and thus it can better realize ultraviolet detection [8]. Among the five different polymorphs (α, β, γ, δ, and ε) of Ga₂O₃, the β-phase has the best physical and chemical stability and is therefore the most widely studied phase. β-Ga₂O₃ has a monoclinic structure with space group C2/m and it has a melting point of up to 1720 °C [9,10]. In many specific design types of the device, photovoltaic devices are more popular than others due to their superior responsivity and the mature preparation process. Because Ga₂O₃ is an intrinsic n-type semiconductor, the heterojunction photodetector based on Ga₂O₃ is considered to be a practical solution [11]. Bae et al. reported a Cu₂O/Ga₂O₃ p-n heterostructure ultraviolet photodetector prepared by mechanical transfer methods [12]. Ding et al. fabricated an ultraviolet sensor based on GaN/Ga₂O₃ nanowire heterojunctions by chemical vapor deposition, which has good spectral selectivity [13]. Li et al. present a hybrid inorganic-organic Ga₂O₃/PEDOT:PSS 2D heterojunction ultraviolet photodetector with a fast response speed [14].

Organic thin film semiconductors are gaining importance due to their tremendous potential for organic electronics and optoelectronic devices. The performance of organic photovoltaic devices is highly dependent on the quality of the organic film semiconductor, highly uniform and continuous films would contribute to the transport of electrons [15]. The introduction of organic semiconductor layers in heterojunctions can improve unmatched deposition compatibility and device performance [16]. Metal phthalocyanines (MPc) are macrocyclic organic semiconductors with an 18π electron conjugated arrangement, and were first discovered to be semi-conductive in 1948 [17]. The derivatives of phthalocyanines are usually non-toxic, and also have good photoconductivity, physical stability, and chemical stability [18]. The cobalt phthalocyanine (CoPc), as one of the metal phthalocyanines, exhibits a p-type conductivity and high hole mobility [19]. CoPc has been studied widely in theory and practice for its many intriguing characteristics. The thermal stability of CoPc enables it can be used in temperature sensors, and the hydrophobicity of CoPc makes it very suitable for humidity sensors [20]. Meanwhile, CoPc has been used in heterojunctions for ultraviolet detection. Xiao et al. fabricated a self-powered ultraviolet sensor based on n-type porous-GaN/p-type CoPc heterojunctions with high switching ratios [21]. However, a CoPc/Ga₂O₃ heterojunction ultraviolet photodetector has not been reported so far.

In this work, we fabricated a planar heterojunction device based on an n-type Ga₂O₃ film and a p-type CoPc film with a combination of spin-coating and MOCVD methods. The device is highly sensitive to deep ultraviolet light, benefiting from the absorption selectivity of Ga₂O₃ and the construction of heterojunction. The built-in electric field at the heterojunction interface offers the potential for the self-powered operation of the device. X-ray diffraction (XRD) and ultraviolet-visible (UV-vis) were used to examine the quality of Ga₂O₃ and CoPc films. The I-V and I-t curves of the heterojunction devices were tested at various optical powers and voltages. The experiments indicate that the device has excellent rectification characteristics and stable periodic light response. In addition, the device has a high photo-to-dark current ratio and low dark current.

2. Experimental

2.1. Syntheses of CoPc Precursor Solution

Cobalt phthalocyanine (CoPc, purity ≥ 92%) and N,N-Dimethylformamide (DMF, 99%) was purchased from Anhui Cool Biological Engineering Co., Ltd. (Hefei, China) and Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Ten mg of CoPC and one mL of DMF were added into the glass tube and the clear purple precursor solution was obtained after magnetic stirring for 30 min, as shown in Figure 1a. Figure 1b shows the molecular structures of CoPc.

2.2. Syntheses of β-Ga₂O₃

Al₂O₃ substrates were purchased from Hefei Kejing Materials Technology Co., Ltd. (Hefei, China). The Ga₂O₃ film was deposited on Al₂O₃ (0001) substrates by metal-organic chemical vapor deposition (MOCVD). The oxygen (O₂), the triethylgallium (TeGa), and the argon (Ar) were selected as the O source, the Ga source, and the carrier gas, respectively. The flow rate of argon was 65 sccm. The reaction chamber temperature was 790 °C.

2.3. Fabrication of β-Ga2O3/CoPc Heterojunction Device

The preparation process of the heterojunction device is depicted in Figure 1c. The Ga₂O₃ film was ultrasonically cleaned with acetone, alcohol, and deionized water successively to remove contaminants and improve wettability. The Ga₂O₃ film was cut into small pieces (10 mm²) by laser, and half of the small piece was covered with tape. Twenty μL of CoPc precursor solution was taken with a pipette and dropped onto the Ga₂O₃ surface. The spin-coating speed was first 600 rpm for 5 s and then 1500 rpm for 25 s. The two different speeds would result in a more uniform surface. After annealing at 155 °C for 45 min on a heating table, the tape was removed. Au/Ti electrodes were deposited on the surface of Ga₂O₃ and CoPc films by direct-current magnetron sputtering. The area between electrodes is 0.02 mm². The schematic diagram of the cross section of the device is shown in Figure 1d.

2.4. Characterization

The crystal quality was examined by XRD (X’Pert PRO, PANalytical) with a Cu K_α (λ~1.5405 Å) radiation, the scanning step is 0.02°. The absorption spectrum was measured by UV-vis (UV-1900, Macy), which can measure in the range of 190–1100 nm. The 254 nm beam was from the mercury lamp (Philips, Amsterdam, The Netherlands, TUV 8W), and the light intensity is changed by adjusting the lamp position. The dark condition was achieved by a closed black metal experiment box. Semiconductor analyzer (Keithley 4200) was employed to measure I-V and I-t curves under dark and light conditions.

3. Results and Discussion

The crystalline structure of the Ga₂O₃ and Al₂O₃ phases is characterized by XRD patterns. In Figure 2a, the diffraction peaks located at 18°, 38°, and 59° are attributed to the ($\overline{2}$01), ($\overline{4}$02), and ($\overline{6}$03) crystal planes of β-Ga₂O₃(JCPDS #43-1012). The results show that the prepared Ga₂O₃ is in β-phase. The sharpest peak at 18° indicates that the growth direction of the β-Ga₂O₃ phase is along the ($\overline{2}$01) direction. All the peak locations and corresponding intensities are well matched to the β-Ga₂O₃ [10]. In Figure 2b, the linear I-V curves of Au/Ti-Ga₂O₃ and Au/Ti-CoPc under dark conditions demonstrate that good Ohmic contacts would not affect the output performance of the heterojunction. In Figure 2c,d, the optical absorption spectrum of the prepared Ga₂O₃ film and CoPc film were measured on sapphire substrates to investigate their optical features. The prepared Ga₂O₃ has a sharp absorption edge around 255 nm, and the bandgap is estimated to be 4.85 eV. The absorption curve of the CoPc film has two main bands, one in the ultraviolet region, called Q-band, the other in the visible region, called B-band, which has a shoulder peak at about 700 nm. According to the absorption curve, we calculated the bandgap values of 1.55 eV and 1.80 eV for the B-band and 3.34 eV for the Q-band. All the results of the optical absorption tests and bandgap calculations are in high agreement with the previous work reported [22,23,24]. Figure 2e show the semi-log scale I-V curves of Ga₂O₃ single device under dark and 300 μW/cm² illumination. Figure 2f show the semi-log scale I-V curves of CoPc/Ga₂O₃ heterojunction device under the same conditions. In comparison with the single device, it is easy to see that the light response of the heterojunction device is obviously improved and has a significant rectification characteristic, which is attributed to the construction of the heterojunction and the formation of built-in electric field [25]. It is worth mentioning that the heterojunction device has an extremely low dark current, measured to be 5.73 fA, 1.57 fA, and 2.59 fA at 5 V, 0 V, and −5 V, respectively.

We have carefully measured the I-V curves of the heterojunction device at various light intensities. The I-V curves for the light intensity ranges of 0.1–0.9 μW/cm², 1–9 μW/cm², 10–90 μW/cm² and 100–900 μW/cm² are displayed in Figure 3a–d, respectively. The test results indicate that the device is very sensitive to changes in ultraviolet light intensity and has a wide sensing test range. The device is capable of detecting weak UV signals even at 0.1 μW/cm², and still has good discrimination for light intensities less than 1 μW/cm². With increasing light intensity, the photocurrent of the heterojunction gradually increases because of the absorption of more photons in the photosensitive region and thus the excitation of more photogenerated carriers [26]. No matter how the light intensity changes, the device shows outstanding rectification characteristics. Under the light intensity of 900 μW/cm², the rectification ratio of the device reaches 144.96 at ±5 V. The photocurrents in different light intensity ranges at 5 V are shown in Figure 3e,f, respectively. The relationship between photocurrent and light power can be fitted by the following formula: I_ph ∝ P^θ, where I_ph, P, and θ are photocurrent, light power, and empirical coefficient. For different light intensity ranges, the values of θ were fitted to 0.97, 0.93, 0.91, and 0.88, respectively. The linearity of the photocurrent decreases with increasing light intensity because large light intensity increases the scattering of carriers and the chance of recombination of electron-hole pairs. The unavoidable defect states in the semiconductor film preparation process would also become the recombination centers of carriers [14].

The stability of the device’s response to light has become the touchstone for measuring device performance. In Figure 4a, the I-t curves were measured with periodical on/off ultraviolet illumination with a light intensity of 100 to 900 μW/cm² at 5 V. The photocurrent increases steadily as the light intensity increases, the device can instantaneously respond to ultraviolet light and has a stable photoresponse at different light intensities. The I-t curves were measured with periodic on/off ultraviolet illumination with voltages from 1 to 5 V under 1000 μW/cm² light intensity, as shown in Figure 4b. As the voltage increases the photocurrent increases, the high voltage not only reinforces the diffusion rate of photogenerated carriers but also inhibits the recombination of electron-hole pairs [27]. A constant photocurrent can be obtained by controlling the external voltage of the device. In Figure 4c, the I-t curves were measured with periodical on/off ultraviolet illumination of light intensities from 10 to 90 μW/cm² at 0 V. The device can generate photocurrent without external voltage, and the photocurrent will change with light intensity, demonstrating that the device has the possibility to work well under self-powered mode. The typical overshoot feature of photocurrent comes from the increased carrier concentration in a short period of time. Substantial amounts of electrons and holes would not be extracted from the heterojunction timely because the voltage of the built-in electric field is too small [28]. This phenomenon can be alleviated by the applied voltage in Figure 4a,b. In general, the device is sensitive to ultraviolet light and has a stable periodic response at different voltages and different light intensities. For a more detailed comparison of the I-t curves, we further analyzed the response speed of the devices. An enlarged graph of one cycle period with 90 μW/cm² under self-powered mode was shown in Figure 4d. The rise and decay time (τ_r and τ_d) are tested for times when the photocurrent rises from 10 to 90% and falls from 90 to 10%, which are estimated to be 5.09 s and 7.51 s, respectively. The response time in self-powered mode is slow because the driving force of the built-in electric field is not enough to make the carriers flow at a faster rate. Figure 4e show the fitted I-t curves for Ga₂O₃ single device with 300 μW/cm² at 3 V. Figure 4f show the fitted I-t curves for CoPc/Ga₂O₃ heterojunction device under the same conditions. The rise and decay time are fitted according to the second-order exponential equation: $I=I_0+A{e}^{-t/{\tau }_1}+B{e}^{-t/{\tau }_2}$, where I₀ is the steady-state photocurrent, A and B are constants, τ₁ and τ₂ are relaxation constants. The rise and decay time (τ_r1 and τ_d1) of the CoPc/Ga₂O₃ heterojunction are fitted to be 0.18 s and 0.35 s, which are faster than the former, demonstrating the construction of heterojunction could enhance the response speed of the device [29].

In order to further evaluate the photoelectric characteristics of the CoPc/Ga₂O₃ heterojunction device, some parameters of importance are analyzed here, including responsivity (R), detectivity (D*), and photo-to-dark current ratio (PDCR). The responsivity is defined as the photocurrent produced per unit incident photon, representing the response efficiency of the device to the optical signal. The detectivity is defined as the noise figure of the device, reflecting the device’s ability to detect weak optical signals in the noise environment. PDCR is defined as the signal-to-noise ratio of the device, representing the ability to resist noise. R, D*, and PDCR can be expressed by the following formulas: $R=\left(I_{ph}-I_d\right)/P_{\lambda }S$, $D^{*}=R/{\left(2e{I}_d/S\right)}^{1/2}$ and $PDCR=\left(I_{ph}-I_d\right)/I_d$, where I_ph, I_d, P_λ, S, and e are photocurrent, dark current, incident light power, effective area, and electron charge, respectively. Figure 5a shows the variation of R and D* with the applied voltage at 0.1 μW/cm² light intensity. The results show that R and D* have the same trend, which both increase as the forward or negative voltage increases. When the voltage is 5 V, the R-value is 18.4 mA/W, and the D*-value is as high as 1.92 × 10¹⁷ Jones, which shows the excellent detection ability of the device. Figure 5b shows the variation of PDCR with light intensity at different voltages. The results show that PDCR increases with increasing light intensity at the same voltage. With the light intensity of 900 μW/cm², the PDCR-values are 5.04 × 10⁵ and 3.76 × 10⁶ at 1 V and 5 V, which reveal the good noise immunity of the device.

For an integrated understanding of the operating mechanism of the CoPc/Ga₂O₃ heterojunction device, the energy band alignment before contact and after contact are shown in Figure 6a,b, under dark and 254 nm light are shown in Figure 6c,d, and under forward and reverse voltage are shown in Figure 6e,f. Combining the bandgap calculation results in this work with previous reports, the conduction band (Ec) and valence band (Ev) of Ga₂O₃ were determined to be 4.0 eV and 8.85 eV, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of CoPc were determined to be 3.4 eV and 4.95 eV [16,30]. The work functions of Ga₂O₃ and CoPc are 4.06 eV and 4.4 eV, respectively [30,31]. The band bending is caused by the difference in the Fermi energy levels of Ga₂O₃ and CoPc. According to Anderson’s law, the offsets of the conduction band and valence band (ΔEc and ΔEv) were determined to be 0.6 eV and 3.9 eV, respectively [32]. The noise of the heterojunction comes mainly from the dark current. The lower dark current of the device is due to rare intrinsic carriers without illumination. After the contact between Ga₂O₃ and CoPc, electrons flow from Ga₂O₃ to CoPc, and holes flow from CoPc to Ga₂O₃. In order to maintain the equilibrium of the Fermi energy levels of the heterostructure, the energy band of Ga₂O₃ would move lower and bend upward at the interface, and the energy band of CoPc would move higher and bend downward at the interface. With the diffusion movement of carriers, the built-in electric field is formed at the heterojunction interface, and the formation of the space charge region leads to the depletion layer [33]. When the device exposed to ultraviolet light, the electron-hole pairs are excited in Ga₂O₃, which are separated under the effect of the built-in electric field and flow to the corresponding electrodes, resulting in a circuit inside the heterojunction, so the device can operate under self-powered mode. When a forward voltage is applied to the device, the direction of the external voltage is opposite to the built-in electric field, the width of the depletion layer decreases, and carriers can easily transfer through the barriers, resulting in a high current. When the reverse voltage is applied to the device, the direction of the external voltage is the same as the built-in electric field, the width of the depletion layer increases, and the carrier transport is suppressed, resulting in a limited current [34]. The experimental results show that the device has a superior rectification characteristic, which is consistent with the analysis of the energy band diagram.

4. Conclusions

In summary, an organic–inorganic hybrid heterojunction based on Ga₂O₃ and CoPc films was constructed into a planar ultraviolet photodetector. The device shows good wavelength selectivity and sensitivity to ultraviolet light, and it can work stably under different light intensities. The device has a dark current as low as 5.7 fA, which indicates its good immunity to interference. The device has a high D* of 1.92 × 10¹⁷ Jones, an R of 18.4 mA/W, and a PDCR of 3.76 × 10⁶. Therefore, the prepared CoPc/Ga₂O₃ type-II heterojunction device may find great potential in ultraviolet detection, especially in weak light signals and noisy environments.