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Abstract
High responsivity is important for solar blind detection because solar blind signals are generally weak. In this work, we achieved a high responsivity of 48 A/W at 262 nm via Au/MgO/MgZnO/MgO/Au structured photodetectors at a 10 V bias. Wurtzite Mg0.55Zn0.45O films with a band gap of 4.57 eV were grown on c-plane sapphire by pulse laser deposition and MgO films were deposited on MgZnO films as barrier layers to induce ionization impact for avalanche gain. The photodetectors achieved a rejection ratio (R262-nm/R350-nm) of over 103 and a cut-off wavelength of 273 nm. The time-resolved response measurement showed a response time of 453 μs. The devices performed good stability without degeneration of responsivity under repeating illumination. The high performance of the photodetector implied a wide potential for solar blind application.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Solar blind photodetectors (SBPD) sensitive to photons with wavelengths shorter than 280 nm have wide applications in flame detection, missile warning, environmental monitoring and ultraviolet astronomy [1–4]. Available photomultipliers and silicon photodetectors are limited by bulky size, large operating voltage, high cost, or low intrinsic quantum efficiency and complicated optical filters [5]. Therefore, SBPD based on wide band gap semiconductors like SiC or AlGaN have attracted significant attention in recent years. However, these photodetectors suffer from either low sensitivity, color selectivity, or large dislocation densities [6–8]. ZnO, alloying with MgO, is considered to be a promising candidate for SBPD due to its adjustable band gap from 3.3 to 7.7 eV, high electron saturation velocity and good radiation tolerance. Excellent researches have been conducted on MgZnO based photodetectors with different structures like schottky [9], photoconductive [10], metal-semiconductor-metal photodetectors [11, 12], etc. As is known, solar blind signals are so weak that high responsivity is required for photodetection. Avalanche photodetectors have the advantage of large photocurrent and high responsivity because of carrier multiplication induced by ionization impact. In ZnO and MgZnO based ultraviolet photodetectors, avalanche photodetectors have been achieved by applying reverse bias on p-n junctions or introducing MgO as barrier layers [13–15]. Due to the lack of p-type MgZnO, avalanche SBPD has only been realized by growing cubic MgZnO films on p-type silicon [16]. However, extra response induced by silicon is inevitable in such devices. In ref. 14, J. Yu et al reported MgO/MgZnO typed photodetector with a maximum response of 315 nm, which is non-solar-blind. So far there is no report on MgO/MgZnO typed SBPD due to the difficulty in optimizing the MgZnO crystal quality as well as the interface. In this paper, we fabricated avalanche SBPD on wurtzite MgZnO films by introducing MgO as a barrier layer. Wurtzite Mg0.55Zn0.45O films with a band gap of 4.57 eV were grown on c-plane sapphire via pulse laser deposition. Au/MgO/MgZnO/MgO/Au structured photodetectors were fabricated. The SBPD achieved a maximum responsivity of 48 A/W at the wavelength of 262 nm under 10 V bias. Cut-off wavelength is calculated to be 273 nm by decay of e−1. The rejection ratio (R262-nm/R350-nm) was over 103. The SBPD performed a response time of 453 μs and good stability of photoresponse without degeneration under repeated illumination.
2. Experimental
MgZnO films were grown on c-plane sapphire via pulse laser deposition. Laser pulses with a wavelength of 248 nm, an average energy density of 6.2 J/cm2 and a repetition rate of 4 Hz from a KrF excimer laser were impinged on a Mg0.3Zn0.7O ceramic target (99.99% purity) for MgZnO deposition. The distance between the Mg0.3Zn0.7O target and sapphire substrate was fixed at 80 mm. Sapphire substrates were heated to 923 K in an oxygen ambient of 1.0 Pa. The thickness of the MgZnO film is estimated to be 400 nm by a deposition rate of 1.5 nm/min. To introduce a barrier layer, a MgO film of about 20 nm was grown in situ on the MgZnO/sapphire substrate at 573 K in an oxygen ambient of 10 Pa with a deposition rate of 0.5 nm/min. Surface morphology of the sample was observed by scanning electron microscope (SEM) and atomic force microscope (AFM). The crystal structure of the sample was studied using a Bruker D8 Discover x-ray diffractometer (XRD). Absorbance and transmittance measurement were performed by a Shimadzu UV-2550 spectrophotometer to determine the band gap of the films. 100-nm-thick Au interdigital electrodes were deposited on the MgO/MgZnO films by standard photolithography and lift-off route. Spectral response and repetition stability of the photodetector were measured under a Xenon lamp with a monochromator. Time-resolved response was measured under 266-nm laser pulses. Current-voltage (I-V) characteristics were measured by a Keithley 6514 high resistance electrometer.
3. Results and discussion
Figure 1 shows surface morphology of the bare MgZnO films and MgO/MgZnO films grown on c-plane sapphire. It can be seen from both SEM (Figs. 1(a) and (b)) and AFM (Figs. 1(c) and 1(d)) images that the roughness of the surface increases after MgO deposition. Root mean square roughness of the bare MgZnO films and MgO/MgZnO films were 1.49 nm and 2.46 nm, respectively. Figure 2(a) shows XRD pattern of the MgO/MgZnO films. Two obvious peaks at 34.84° and 41.70° are observed, corresponding to (002) planes of wurtzite MgZnO and (006) planes of sapphire, respectively. No noticeable peak could be found around 36.6° for (111) planes of cubic phase MgZnO, which indicates that no phase segregation occurred. The inset of Fig. 2(a) shows magnification of XRD pattern and a minor peak at 37.52° could be found, which could be attributed to (111) planes for the MgO layer. Full width at half maximum of the MgZnO (002) diffraction peak at 34.84° is calculated to be 0.31° by Gauss fitting profile. For pure ZnO, typical diffraction of (002) planes occurs at 34.36°. In our samples, Mg content of 55% induces a decrease of lattice constant from 5.20 Å to 5.15 Å along c axis, according to the shift of (002) planes from 34.36° to 34.84°. Figure 2(b) shows the composition analysis of the bare MgZnO and MgO/MgZnO films by energy dispersive spectroscopy (EDS). The composition of the bare MgZnO films was estimated to be Mg0.55Zn0.45O. Desorption of Zn species and the condensation of Mg species on the growing surface lead to a higher Mg content (0.55) in the MgZnO films than that in the Mg0.3Zn0.7O ceramic target [17]. A much higher Mg signal can be found in the MgO/MgZnO films due to the MgO layer. It is worth noting that the Zn signals derive from the MgZnO films underneath the MgO, considering the thickness of MgO is only 20 nm. Absorbance and transmittance of the samples are presented in Fig. 2(c). It can be seen that the samples are almost transparent for photons with wavelength longer than 280 nm. As is known, MgO has a band gap of about 7.7 eV, corresponding to the wavelength of 161 nm. Hence, the MgO layer in the MgO/MgZnO structure will not induce optical absorbance within the measuring wavelength range (200 to 800 nm). It is rational to assume that the observed absorbance around 280 nm can be attributed to the MgZnO layer. For direct band gap semiconductor, the optical absorption follows formula as below [18]:
where B is a constant related to some basic physical constants and specific characteristics of a semiconductor, α is the absorption coefficient, h is the Planck’s constant, v is the frequency of the incident photon and Eg is the optical band gap of the semiconductor. In the inset of Fig. 2(c), (αhv)2 is plotted versus hv by scattered circles in the inset. By drawing a tangent (red line), the band gap is determined to be 4.57 eV, which is within the solar blind region.
Fig. 1 SEM images of (a) MgZnO films and (b) MgO/MgZnO films and AFM images of (c) MgZnO films and (d) MgO/MgZnO films on c-plane sapphire.
Fig. 2 (a) XRD pattern of the MgO/MgZnO films on c-plane sapphire. Inset shows the minor peak at 37.52°, which might be attributed to MgO (111). (b) EDS of the MgO/MgZnO and bare MgZnO films. (c) Absorbance and transmittance of the MgO/MgZnO films on sapphire. Inset shows (αhv)2 versus hv, referring to a band gap of 4.57 eV.
Interdigital electrodes of 100-nm-thick Au were deposited on the samples. Optical microscope image and schematic diagram of the prepared Au/MgO/MgZnO/MgO/Au photodetector are shown in Figs. 3(a) and 3(b), respectively. I-V characteristics of the photodetector are presented in Fig. 3(c). Light current was measured under illumination of 262 nm ultraviolet light with a power density of 1.11 mW/cm2 from a Xenon lamp. It can be seen that the dark current begins to increase exponentially at about 9 V, which indicates a breaking down behavior and carrier multiplication process. Light and dark current are shown in logarithm scale in the inset of Fig. 3(c). Light current was 3 orders of magnitude higher than the dark one at 9 V, referring to a signal-to-noise ratio of over 103. Photocurrent (Iph) is calculated as light current minus dark current and approximately equals to light current, considering it is much higher than the dark current. Avalanche gain versus voltages is denoted by a blue line in the inset of Fig. 3(c). The avalanche gain indicates how many times the carriers are multiplied in the avalanche process and could be calculated as the ratio of multiplied photocurrent over unmultiplied photocurrent. Therefore, the breaking down voltage of 9 V could be regarded as the critical voltage at which the photocurrent starts to be multiplied. The avalanche gain is calculated to be 1.6 at 10 V. It is worth noting that the gain increases rapidly versus voltage.
Fig. 3 (a) Optical microscope image of the Au/MgO/MgZnO/MgO/Au photodetectors. 18 pairs of interdigital electrodes, with a finger width and a gap of 4 μm, a length of 230 μm. The active area: 250 × 300 μm2. (b) Schematic diagram of the photodetectors and the measurement circuit. (c) I-V characteristic under illumination (λ = 262 nm, Xenon lamp) and dark condition. At about 9 V, dark current starts performing significant multiplication. Inset: light and dark current in logarithm scale and the avalanche gain as a function of voltages in a linear scale.
Voltage-dependent responsivity spectra are measured and shown in Fig. 4(a). The photodetector performs a consistent response characteristic at different voltages. Hence, normalized responsivity spectrum of the photodetector measured at 5 V is presented in logarithm scale in Fig. 4(b) for understanding the response characteristic. The photodetector shows a maximum responsivity at 262 nm and a rejection ratio (R262-nm/R350-nm) of over 103 is obtained. The cut-off wavelength is about 273 nm, estimated as the responsivity decays by e−1, which is appropriate for solar blind photodetection. The cut-off wavelength of 273 nm is also consistent with the band gap (4.57 eV) measured in Fig. 2(c). Especially, the responsivity decreases to 6% of the maximum at the wavelength of 280 nm, which indicates good rejection of non-solar-blind signals. High responsivity is important for solar blind detection because solar blind signals are generally weak. Responsivity (Rλ), defined as the photocurrent under illumination power of 1 W, is calculated by Rλ = Iph/PλS, where Iph is the photocurrent, Pλ is power density of the incident light and S is the active area of the photodetector. In this case, Iph approximately equals to the light current in Fig. 3(c) and S is estimated as 250 × 300 μm2. Voltage-dependent responsivity is presented in the inset of Fig. 4(b). The maximum responsivity is found to be 48 A/W at 10 V. To the best of our knowledge, this is the highest responsivity for solar blind detection based on MgZnO materials and it is comparable to that of solar blind avalanche photodetector based on Ga2O3 [19].
Fig. 4 (a) Response spectra of the photodetector at 3, 4, 5 V. (b) Normalized responsivity spectrum of the photodetector, measured at 5 V. Inset shows responsivity as a function of operating voltage. At 10 V, responsivity reached as high as 48 A/W. Rejection ratio (R262-nm/R350-nm) was over 103.
Time-resolved response, generally indicates a response time of the photodetector. Time-resolved response of the photodetector is measured under 266-nm laser pulses (pulse: about 20 ns, laser spot: about 1 mm2). Results are presented in Fig. 5(a). The inset shows time-resolved response in logarithmic scale, illustrating two exponential processes (as denoted by a blue and a black dash line). Decay life-time of the photodetector can be estimated by fitting the scattered experimental data via a 2-order exponential decay formula shown below:
where I is the Time-resolved response current of the photodetector, I0, I1, I2 are fitting constants, t is time. τ1 and τ2 are decay life times. The best fitting of the experiment data yields τ1 = 453 μs and τ2 = 4.69 ms. This result reveals that two response mechanisms exist in the gain of responsivity. The faster life time of 453 μs could be attributed to the avalanche gain, and the slower one of 4.69 ms could be attributed to the photoconductive mechanism. The response time of 453 μs is relatively slower than that of generally reported avalanche photodetectors (10−9∼10−6 s) [3, 13], but still acceptable for those common civil applications, in which ultrafast detection is not required. For avalanche photodetector, stability of response is also important. To evaluate the stability of the photodetector, time-resolved response has been measured under repeated illumination of 250-nm ultraviolet light from a Xenon lamp with a density of about 1 mW/cm2 and results are shown in Fig. 5(b). The photodetector performs a reproducible response without any degeneration, which is indispensable for practical solar blind applications.Fig. 5 (a) Time-resolved response of the photodetector. The red curve was the fitting of the scattered experiment data using a 2-order exponential decay formula. Inset shows data in logarithmic scale. (b) Repeating test of the photodetector, showing good stability of the photodetector without degeneration of responsivity with 250-nm ultraviolet light repeatedly on and off.
The relatively high responsivity (48 A/W at 10 V) could be attributed to a generated-multiplied mechanism, which can be divided into 3 steps: 1. The photoresponse of the MgZnO layer. In this step, incident solar blind photons are absorbed and photogenerated carriers are produced. 2. The photogenerated carriers drift from the MgZnO to the MgO layer. 3. The drifting carriers are involved in the carrier multiplication in the MgO layer. The avalanche process is shown by a schematic energy band alignment diagram in Fig. 6. Since rare data has been reported about the electron affinity of MgZnO, the energy band alignment of MgO/MgZnO heterojunction remains unknown. However, it can be estimated by calculating the conduction band offset (ΔEc) and valence band offset (ΔEv) between MgO and MgZnO. The estimation are as follows: the band gap of MgO, ZnO are known to be 7.7 eV, 3.37 eV, respectively. ΔEc(MgO/ZnO) and ΔEv(MgO/ZnO) (ΔEc and ΔEv between MgO and ZnO) are calculated to be 3.55 eV and 0.78 eV [20]. ΔEc(MgZnO/ZnO) and ΔEv(MgZnO/ZnO) (ΔEc and ΔEv between MgZnO and ZnO) are calculated to be 1.08 eV and 0.12 eV with a ratio of ΔEc: ΔEv = 9:1 [21] using the MgZnO band gap of 4.57 eV, measured in Fig. 2(c). Therefore, the relative values between the energy bands of ZnO, MgZnO and MgO can be drawn, as shown in Fig. 6(a). ΔEc(MgO/MgZnO) and ΔEv(MgO/MgZnO) (ΔEc and ΔEv between MgO and MgZnO) can be calculated to be 2.47 eV and 0.66 eV as below:
Fig. 6 (a) Relative values between the energy bands of ZnO, MgZnO, and MgO. (b) Schematic energy band alignment of the Au/MgO/MgZnO/MgO/Au photodetector.
By introducing MgO films, two Schottky barriers are connected back-to-back in the Au/MgO/MgZnO/MgO/Au photodetectors. Hence, one of the barrier will be at reverse bias and the other one at forward bias under any polarity of applied voltages. As a result, voltages will be mostly applied on the barrier at reverse bias. The conduction and valence bands of MgO at reverse bias will bend much more significantly than that at forward bias. Furthermore, it is rational to assume that most of the voltage will be applied on the MgO layer due to the dielectric nature [20]. Therefore, the conduction and valence bands of MgO will bend much more significantly than that of MgZnO (Fig. 6(b)). Under applied voltage, the electric field in the barrier layer at reverse bias is enhanced and could be over 106 V/cm at 9 V, considering that the thickness of MgO is about only 20 nm. Under dark condition, carriers in MgZnO films are drifted into the MgO layer and considerably accelerated by the strong electric field. Carrier multiplication occurs via impact ionization process [3, 22] as follows: once the accelerated carriers have gained enough kinetic energy and impact the lattice of the MgO layer, extra electrons and holes will be excited. These extra carriers again will be accelerated and impact the MgO lattice, generating more and more carriers. As a result, carrier multiplication occurs. Under illumination, photo-generated electrons and holes in MgZnO films will be also separated and drifted into the MgO layer, inducing a much stronger carrier multiplication. In this way, avalanche gain has been achieved and high responsivity has been obtained.
Comparison between our device and other reported MgZnO based SBPDs is shown in Table 1, where W is short for wurtzite and C for cubic. Generally, MgZnO photodetectors cannot achieve both high responsivity and fast response at the same time. Our device, however, has reach the highest responsivity and an acceptable rejection ratio and response speed.
Table 1. Comparison between our device and typical MgZnO based solar blind photodetectors.
4. Conclusions
In conclusion, wurtzite Mg0.55Zn0.45O with a band gap of 4.57 eV films have been grown on c-plane sapphire by pulse laser deposition. Thin MgO films are deposited on the MgZnO surface as barrier layers. Based on the samples, Au/MgO/Mg0.55Zn0.45O/MgO/Au photodetectors have been fabricated with a maximum responsivity of 48 A/W (10 V) at 262 nm and a rejection ratio (R262-nm/R350-nm) of over 103. The high responsivity is attributed to avalanche multiplication via impact ionization process in the MgO barrier layer. A cutoff wavelength of about 273 nm is observed. The response time of the photodetector is 453 μs, which is acceptable for practical solar blind detection. The photodetectors have shown stable response to repeating illumination. These results indicate our devices may find many potential applications in solar blind optoelectronics.
Funding
National Natural Science Foundation of China (11204097, U1530120, 51602353).
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