The Investigation of Hybrid PEDOT:PSS/β-Ga2O3 Deep Ultraviolet Schottky Barrier Photodetectors

In this paper, the hybrid β-Ga2O3 Schottky diodes were fabricated with PEDOT:PSS as the anode. The electrical characteristics were investigated when the temperature changes from 298 K to 423 K. The barrier height ϕb increases, and the ideality factor n decreases as the temperature increases, indicating the presence of barrier height inhomogeneity between the polymer and β-Ga2O3 interface. The mean barrier height and the standard deviation are 1.57 eV and 0.212 eV, respectively, after taking the Gaussian barrier height distribution model into account. Moreover, a relatively fast response speed of less than 320 ms, high reponsivity of 0.6 A/W, and rejection ratio of R254 nm/R400 nm up to 1.26 × 103 are obtained, suggesting that the hybrid PEDOT:PSS/β-Ga2O3 Schottky barrier diodes can be used as deep ultraviolet (DUV) optical switches or photodetectors.

In this work, the hybrid Schottky diode was fabricated with PEDOT:PSS polymer and the mechanically exfoliated β-Ga 2 O 3 flakes from the high quality β-Ga 2 O 3 substrate. The electrical characteristics of the diodes were investigated in the temperature region between 298 K and 423 K. Furthermore, the I-V measurements under the UV illumination were carried out, the responsivity was measured, and the transient behavior of the photocurrent was also analyzed.

Experimental Methods
The β-Ga 2 O 3 flakes with the thicknesses of 15-25 μm were mechanically exfoliated from the (100) β-Ga 2 O 3 substrate with the electron concentration of 7 × 10 16 cm −3 . For the electron density is 2-3 orders of magnitude higher than that in the unintentionally doped Ga 2 O 3 epilayer deposited on sapphire substrate in [30] and the highly conductive PEDOT:PSS films was used in this paper, so the pn heterojunction was formed in [30] while Schottky junction was formed in this paper [30]. Figure 1a shows the schematic diagram of the hybrid PEDOT:PSS/β-Ga 2 O 3 Schottky diode. The β-Ga 2 O 3 flakes were cleaned in acetone, ethanol, and deionized water with ultrasonic agitation and then immersed into the HF: H 2 O (1:10) solution to remove surface oxides. Then, the deposition of Ti/Au(20 nm/100 nm) metal stack was carried out on the whole back side, and the rapid thermal processing at 470°C under N 2 atmosphere was conducted for 60 s to decrease the ohmic contact resistance. After spin coated onto the surface of β-Ga 2 O 3 flake for three times, PEDOT:PSS was baked on an electric hotplate at 150°C, and the baking duration was 15 min. Subsequently, isolated devices with the area of 1 mm × 2 mm were obtained. From the HRTEM image of Fig. 1b, we can observe that the atoms are regularly arranged and few atomic column misalignments are present, indicating a high crystal quality of the β-Ga 2 O 3 flake. As shown in Fig. 1c, d, the FWHM of HRXRD is about 35.3 arcsec, and the root mean square (RMS) is estimated to be 0.19 nm, illustrating the superior crystal quality and smooth surface.

I-V Characteristics and Barrier Height
As presented in Fig. 2a, the I-V characteristics of the hybrid PEDOT:PSS/β-Ga 2 O 3 Schottky barrier diodes were investigated when the temperature changes from 298 K to 423 K. The current increases monotonously with the temperature and the semi-log I-V curves show the linear behavior as the forward voltage bias less than 1.5 V. As the forward bias voltage further increases, the slope of the semi-log I-V curves gradually reduces, and the forward current approaches 6~8 × 10 −4 A, indicating that the series resistance causes the I-V curve deviating from the linearity. In addition, the reverse leakage current is less than 10 −9 A at -3 V, and the I on /I off ratio is up to 10 6 at room temperature, illustrating a rectifying behavior as good as inorganic β-Ga 2 O 3 Schottky diodes [11][12][13][14][15].
According to the equation where V is the bias voltage, T and k are the absolute temperature and the Boltzmann constant, respectively. The ideality factor n and the reverse saturation current I s can be extracted from the y-axis intercepts and the slopes of the linear extrapolation of the semi-log I-V curves at different temperatures. Although the ideality factor n of the ideal Schottky diode is equal to 1, it is always larger than 1 to some extent in actual device. The deviation of the thermal emission (TE) model becomes much greater as n increases. According to the expression ϕ b ¼ kT q ln ½ AA Ã T 2 I s , we can obtain the Schottky barrier height ϕ b at different temperatures, as shown in Fig. 2b. The increase in temperature causes ϕ b to increase from 0.71 eV to 0.84, 0.87, 0.90, 0.93, and 0.96 eV while n to decrease from 4.27 to 3.42, 3.35, 3.29, 3.06, and 2.86. For n much larger than 1, suggesting other conducting mechanisms, such as field effect or thermal field effect, contributing to the current transport and resulting in the difference between pure TE model and the I-V characteristics, which has been illustrated in the wide bandgap SBDs, including GaN and SiC [31][32][33][34].
For ϕ b and n are temperature-dependent, the inhomogeneity of barrier height should be considered at PEDOT:PSS and β-Ga 2 O 3 interface. Considering the Gaussian distribution of the barrier height, the inhomogeneous barrier height may be described as 2kT and the variation of n with T is given by 2kT , where ϕ b0 and σ s are the mean barrier height and the standard deviation, respectively, ρ 2 and ρ 3 are the temperature-dependent voltage coefficients, and the voltage deformation of the Schottky barrier height (SBH) distribution was quantified by them (Fig. 3a). ϕ b0 and σ s can be calculated from the intercept and the slope of the ϕ b versus q/2kT curve, about 1.57 eV and 0.212 eV, respectively. At the same time, ρ 2 and ρ 3 are evaluated to be 0.4 eV and 0.02 eV from the intercept and slope of the (1/n − 1) versus q/2kT plot. Compared with ϕ b0 , σ s is not small, illustrating the existence of barrier inhomogeneity at PEDOT:PSS/β-Ga 2 O 3 interface [35].
By considering the barrier height inhomogeneity, the relationship between the reverse saturation current I s and the mean barrier height ϕ b0 can be modified as Inð I s kT . It can be discerned from Fig. 3b that the plot of the ln ðI s =T 2 Þ − ðq 2 σ 2 s =2k 2 T 2 Þ versus 1/kT is a straight line, from which we can extract the effective Richardson constant A * of 3.8 A cm −2 K −2 , one order magnitude smaller than the theoretical Richardson constant of 40.8 A cm −2 K −2 with the β-Ga 2 O 3 effective mass of m* = 0.34 m 0 [36,37]. Thus, the temperature-dependent ϕ b and n, in other words, the Gaussian distribution of the barriers over SBHs can be used to explain the barrier inhomogeneity at the PEDOT:PSS/β-Ga 2 O 3 interface.

Characteristics of UV Photodetector
As described above, the hybrid β-Ga 2 O 3 Schottky diode exhibits a good rectifying characteristics; the ratio of I on / I off up to 10 6 in dark state at room temperature. The lower dark current I dark of 9.4 nA@V bias = − 4 V can be determined from Fig. 4a, indicating a lower noise characteristic. While under the normal incidence of 254 nm wavelength with the photodensity of 150 μW/cm 2 , the photocurrent I photo reaches 112 nA@V bias = − 4 V. In addition, the photodetector shows a weak photovoltaic effect with a photocurrent of 0.45 nA at 0 V and an open-circuit voltage (V oc ) of 0.15 V, much less than 0.9 V in reference [38], which may be attributed to the carrier density difference and the resulting Fermi level variation. Figure 4b represents the linear I photo versus V bias at various P light . The device shows the dependence of I photo on the P light , and the I photo increases non-linearly with the P light , in other words, at different V bias, the plots of I photo versus P light demonstrate an obvious superlinear behavior, as shown in Fig. 4c. In order to elucidate the mechanism of the superlinear behavior, Fig. 4e presents the energy diagram of the PEDOT:PSS and β-Ga 2 O 3 before contact. The electron affinity and the bandgap of β-Ga 2 O 3 are 4.0 eV and 4.9 eV, respectively. The lowest unoccupied molecular orbital (LUMO) is 3.3 eV, and the highest occupied molecular orbital of PEDOT:PSS is 5.2 eV [39]. As they came to contact, a Schottky barrier was formed. When the device is illuminated and the reverse bias is applied to the electrodes of the Schottky diodes, the photo generated electron-hole pairs are separated rapidly by the electric field and the holes drift to the anode while the electrons to the cathode, as shown in Fig. 4f. For the presence of traps at the PEDOT: PSS/β-Ga 2 O 3 interface, the holes are trapped at the interface states and produce net positive charges, reducing the effective Schottky barrier height, more carriers flowing across the Schottky junction, and improving the I photo . Figure 4d presents the photo to dark current ratio (PDCR) curves under different P light . As the voltage bias shifts from 0V to − 1.2V, the PDCR increases gradually and then decreases with the voltage bias becoming more negative, the higher PDCR above 20 is achieved at a V bias of − 1.2 V and a P light of 150 μW/cm 2 .
The time-dependent photoresponse characteristics of hybrid photodetector are studied by using square wave light with a period of 10 s under the V bias of − 1.2 V and a P light of 150 μW/cm 2 . After several illumination cycles, devices reach the stable on-state I photo at the given P light and V bias , as represented in Fig. 5a. The rise time and decay time are 319 ms and 270 ms [40,41], respectively, much less than those of devices fabricated on epitaxial β-Ga 2 O 3 films or β-Ga 2 O 3 flakes [35,42,43] but longer than the data in [31]. For the existence of double heterojunction in [31], PEDOTT:PSS/Ga 2 O 3 upper junction and Ga 2 O 3 /p-Si lower junction, the photogenerated carriers can be separated more effectively by the double built-in electric fields than the only one PEDOTT:PSS/ Ga 2 O 3 junction in this paper. Therefore, less carriers can be captured by the defects in [31], resulting in the shorter rise time and decay time. Furthermore, the overshooting feature can be observed from the shapes of photoresponse curves with a wedgy head at the lower P light of 150 μW/cm 2 than that occurred at the P light of 600 μW/cm 2 in [30] for the effective collection of photogenerated carriers under the reverse bias of − 1.2 V rather than 0 V. Figure 6 depicts the responsivity characteristics versus the illumination optical λ under the V bias of − 1.2 V. The maximum responsivity R max of 0.62 A/W is achieved at a λ of 244 nm and the corresponding external quantum efficiency(EQE) of 3.16 × 10 2 % calculated by the expression EQE = hcR max /(eλ), much higher than that obtained in [30,38] for the effective collection of photogenerated carriers, where R max is the peak responsivity, and h is the Plank constant. e and λ are the electronic charge and the illumination wavelength, respectively. As the wavelength is longer than 290 nm, the photoresponsivity is lower than 1 × 10 −3 , illustrating a much better spectral selectivity in the hybrid β-Ga 2 O 3 devices. At the same time, the rejection ratio of R 254 nm /R 400 nm is determined to be 1.26 × 10 3 . Compared with the reported inorganic Ga 2 O 3 photodetector [43][44][45][46][47][48][49], the hybrid device possesses a higher photoresponsivity, faster response speed and larger UV/visible rejection ratio, implying a promising solar blind photodetectors with high performance.

Conclusions
We have fabricated PEDOT:PSS/β-Ga 2 O 3 hybrid Schottky barrier diode. The Schottky barrier height ϕ b and ideality factor n are dependent on temperature, indicating that the Schottky barrier height was inhomogeneous at PEDOT:PSS/β-Ga 2 O 3 interface. The mean barrier height and standard deviation can be evaluated to be 1.57 eV and 0.212 eV, respectively, based on the Gaussian barrier height distribution model. Furthermore, the characteristics of PEDOT:PSS/β-Ga 2 O 3 DUV Schottky barrier photodetectors were also investigated. A higher responsivity of 0.6 A/W, rejection ratio of R 254 nm /R 400 nm = 1.26 × 10 3 , EQE of 3.16 × 10 4 % and a faster response speed of less than 320 ms are achieved, suggesting that the hybrid Schottky barrier diodes can be used as DUV optical switches or photodetectors.