A photoconductor, consisting of a semiconductor with two ohmic contacts, is essentially a radiation-sensitive resistor [13]. The schematic structure and operation of the photoconductor are shown in Figure 1. Usually, the load resistance is much smaller than the device resistance. When a photon with the energy larger than the band-gap energy of semiconductor is absorbed, an electron-hole pair would be produced, thereby changing the electrical conductivity of the semiconductor.

Photoconductors present high internal gain at room temperature, so they have large photoresponsivity and can work without any amplifying equipment. However, this gain is associated with a sublinear behaviour with incident power, poor UV/visible contrast, and persistent photoconductivity effects. Therefore, for a practical application, we must think about internal gain and other parameters according to our need.

ZnO photoconductors have been investigated in detail by a number of groups. Different materials (Au, Al, Pt, Al/Au, Ni/Au, ITO, etc.) have been selected as electrodes [14–26]. Liu et al. demonstrated MSM UV sensitive photoconductors based on high-quality N-doped ZnO epitaxial films grown on R-plane sapphire by metal organic chemical vapor deposition (MOCVD) [14]. The low frequency photoresponsivity with the value of 400 A/W at 5 V bias was obtained. The devices show high response speed. The rise time and fall time were about 1 μs and 1.5 μs, respectively. Xu et al. have fabricated photoconductive UV detector with planar interdigital Al electrodes on c-axis preferred oriented ZnO thin film prepared by RF sputtering on quartz substrate [15]. The detector exhibits fast photoresponse with a rise time of 100 ns and fall time of 1.5 μs. Meanwhile, it shows a big dark current of 38 μA at 5 V bias. We also reported the ZnO UV photoconductor with planar interdigital Au electrodes by RF sputtering [16]. With the applied bias below 3 V, the dark current was below 250 nA. In addition, the transient response measurement revealed fast photoresponse with a rise time of 20 ns as shown in Figure 2. Recently, a back-illuminated vertical-structure ZnO UV detector was fabricated using an indium-tin oxide (ITO) electrode and an Al electrode [17]. At 5 V bias, the dark current was 640 μA and the photocurrent was 16.8 mA under UV illumination (365 nm, 10 μW), indicating the responsivity as high as 1,616 A/W. The response time measurements showed a rise time of 71.2 ns and a decay time of 377 μs.

In order to obtain high performance ZnO photoconductors, different methods such as surface treatment and covering by other materials have been used by several groups [18–20]. The oxygen plasma treatment is found to dramatically enhance the UV detection properties of ZnO, reducing the decay time constant (to below 50 μs) and increasing the on/off ratio of photocurrent (to over 1,000) with high UV responsivity (1–10 A/W) [18]. The reason for this result may be that oxygen plasma treatment can effectively suppress the chemisorption effect and the oxygen vacancy in ZnO films. Additionally, surface HCl treatment [19] and SiO₂ covering [20] on the surface of devices can increase the photoresponsivity, but they also can increase the dark current due to the damage on ZnO films. More recently, Sun and coworkers reported a photoconductive detector based on intentionally Ga-doped ZnO film on quartz by RF sputtering [27]. The transient response measurement revealed photoresponse with a rise time of 10 ns and a fall time of 960 ns, respectively. The results are much faster than those reported in photoconductive detectors based on unintentionally doped n-type ZnO films. And the authors think that the relatively faster response in ZnO:Ga photoconductor may be attributed to the enhancement of tunnel recombination across the potential barriers generated by surface and defects in ZnO:Ga sample, as the similar behavior observed in Si-doped Al_xGa_1−xN photoconductive detectors [28]. Table 1 summarizes the data on responsivity, darkcurrent and response time of ZnO film photoconductors.

In the past few years, MSM photodiodes have become increasingly popular in the research field due to their fundamental advantages [29,30]:

Simple structure.

MSM photodiodes are comprised of two back-to-back Schottky diodes by using an interdigitated electrode configuration on top of an active light collection region. A schematic image of the MSM photodiodes structure is shown in Figure 3. This photodetector cannot operate at a zero bias. MSM photodiodes are inherently fast due to their low capacitance per unit area and are usually transit time limited, not RC time constant limited. With electron beam lithography, the electrode width and spacing can be made with submicron dimension which greatly improves the speed. The biggest drawback of MSM photodetectors is their intrinsic low responsivity. MSM detectors exhibit low photoresponsivity mainly because the metallization for the electrodes shadows the active light collecting region.

ZnO MSM photodiodes have been fabricated by different methods, such as MOCVD [31,32], laser assisted molecular beam deposition (LAMBD) [33], radio frequency (RF) magnetron sputtering [34–36], atomic-layer deposition (ALD) [37] and molecular beam epitaxy (MBE) [38,39]. In 2001, Liang et al. [32] fabricated MSM photodiodes by using Ag as Schottky contact metal. A low frequency photoresponsivity of 1.5 A/W at a bias of 5 V was obtained. The dark current of the device at 5 V bias was in the order of 1 nA (see Figure 4). The photoresponse of the detector showed a fast component with a rise time of 12 ns, and a fall time of 50 ns.

After that, MSM photodetectors based on laser-annealed ZnO films with Au and Cr metals were demonstrated by LAMBD [33]. Figure 5 shows the dark and photocurrent characteristics of laser-annealed ZnO MSM photodetectors with Au and Cr metals. The dark current for the Au film was found to be three orders lower in magnitude compared to the Cr film. However, its responsivity was also reduced from 1.05 mA/W to 11.3 μA/W since Au has a higher work function, hence higher barrier height than Cr. Meanwhile, Shan et al. have fabricated the ZnO MSM photodiodes by ALD method [37]. The photodetector shows an obvious broad response to the UV spectrum shorter than 400 nm, and the cutoff response wavelength is located at around 390 nm (see Figure 6). By increasing the bias applied, the maximum responsivity of the photodetector increases almost linearly in the range from 3 to 15 V.

According to the results in reference [33], in order to achieve high performance MSM UV photodetectors, it is important to achieve large Schottky barrier height at metal–semiconductor interface. A large barrier height leads to small leakage current and high breakdown voltage, which could result in improved photocurrent to dark current contrast ratio [39]. To achieve a large Schottky barrier height on ZnO, one can choose metals with high work functions [40]. Different metals have been selected as electrode materials, such as Au, Ag, Pt, Ni, Pd, Cr, Al, Ru, etc. [31–39]. The Schottky barrier height at the Ru/ZnO, Ag/ZnO, Pd/ZnO and Ni/ZnO was evaluated to be 0.76, 0.736, 0.701 and 0.613 eV, respectively [38,39]. Higher Schottky barrier height can realize lower darkcurrent. However, the responsivity and the quantum efficiency decrease with the increase of the Schottky barrier height.

More recently, Ji et al. have demonstrated UV photodetectors based on the ZnO epitaxial films grown on poly(ethylene terephthalate) (PET) flexible substrates by RF sputtering [36]. The device with a stack structure of ZnO/Ag/ZnO/PET served as a Schottky-type MSM photodetector with a ZnO cap layer. The photodetector with a ZnO cap layer shows a much higher UV-to-visible rejection ratio of 1.56 × 10³ than that without. This can be attributed to the photocurrents that are not only significantly increased in the UV region but also slightly suppressed in the visible region for such a novel structure. With an incident wavelength of 370 nm and an applied bias of 3 V, the responsivities of both photodetectors with and without a ZnO cap layer are 3.80 × 10³ and 2.36 × 10³ A/W, which correspond to quantum efficiencies of 1.13 and 0.07%, respectively. However, the photodetector with a ZnO cap layer has larger leakage currents than that without.

Comparing with photoconductor and MSM photodiode, a Schottky photodiode has many advantages in the aspects of high quantum efficiency, high response speed, low dark current, high UV/visible contrast, and possible zero-bias operation [13,41]. The schematic structure and operation of the photoconductor are shown in Figure 7(a,b), respectively. Schottky diodes in their simplest form consist of a metal layer that contacts a semiconductor. The metal/semiconductor junctions exhibit rectifying behavior. The rectifying property of the metal-semiconductor contact arises from the presence of an electrostatic barrier between the metal and the semiconductor which is due to the difference in work functions Φ_m and Φ_s of the metal and semiconductor, respectively. For example, for a metal contact with an n-type semiconductor, Φ_m should be greater. For the detail work mechanism of Schottky photodiodes, Razeghi et al. have described clearly in Razeghi, et al. [13].

In 1986, Fabricius and coworkers first fabricated a ZnO Schottky photodiode using sputtering system [10]. The diode structure consisted of a glass substrate with a Mn electrode as the bottom contact material to the ZnO-Au diode. The diodes exhibited conventional I-V characteristics. Due to recombination in the polycrystalline ZnO layers and the contact layers the quantum efficiency was of the order of 1%. Moreover, ZnO Schottky barrier diodes exhibited a rise time around 20 μs and a decay time of 30 μs in the time response measurements. After that, several groups have prepared ZnO Schottky photodiodes by different methods [42–44]. Oh et al. have demonstrated ZnO Schottky barrier diodes on (0001) GaN/Al₂O₃ substrates by plasma-assisted molecular-beam epitaxy [43]. These ZnO Schottky barrier diodes show a reverse saturation current of ∼10⁻⁸ A in the dark, and they present a large current buildup of ∼10³ A under ultraviolet light illumination, with maintaining stable diode characteristics. The ZnO Schottky barrier diodes have a large bandwidth of 195 nm, where the short-wavelength cutoff and the long-wavelength cutoff are 195 and 390 nm, respectively. Additionally, the devices have a time constant of 0.36 ms.

In order to investigate the effect of polarization on the response properties, two types of Schottky photodiodes based on Zn-polar and O-polar ZnO films have been fabricated by the hydrothermal growth method [44]. Figure 8(a) shows the typical current-voltage characteristics of the Schottky photodiodes with Pt electrodes. The current increases at 0.8 V with increase in the voltage when the voltage is applied to the Zn-polar sample. On the other hand, in the case of the O-polar sample, the forward biased current increases at 0.4 V, which is smaller than that of the Zn-polar sample. Furthermore, the dark current density of the O-polar device is larger by five orders of magnitude than that of the Zn-polar device. The spectral responses of both Zn-polar and O-polar devices are shown in Figure 8(b). The responsivity and external quantum efficiency for the Zn-polar sample are 0.185 A/W and 62.8%, respectively, at a wavelength of 365 nm, and those for the O-polar sample are 0.09 A/W and 31.0%, respectively. From those measurements, it was found that the responsivity for the Schottky barrier of the Zn surface is two times higher than that for the O surface. The polarity dependence in the Schottky photodiode was attributed to the difference in surface reactivity and/or the defect density in the ZnO substrate surface. These results indicate that it is possible to fabricate high-performance ZnO Schottky photodiodes based on Zn-polar ZnO films.

After that, the ZnO Schottky photodiodes have been demonstrated using transparent polymer as Schottky electrodes [41]. Poly (3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) was selected as electrode material due to the large internal transmittance of nearly 100% in a wide wavelength range from 250 to 800 nm, in addition to a resistivity of as low as 10⁻³ Ω cm and a large work function of 5.0 eV [45]. The quantum efficiency as high as unity in ultraviolet region and a visible rejection ratio of about 10³ were achieved in the spectral response of the photodiode under zero-bias condition. The normalized detectivity of the photodiode was evaluated to be 3.6 × 10¹⁴ cm Hz^1/2 /W at 370 nm. It is expected that transparent polymers can be used as Schottky electrodes instead of metals for ZnO photodiodes.

More recently, a method to effectively suppress the unwanted increase of the leakage current of ZnO-based Schottky diodes in vacuum was presented by means of a dielectric passivation [42]. Additionally, post-metal deposition annealing has been selected to improve the performance of ZnO based Schottky photodetectors [46]. Ali and coworker found that the performance of the device improves with increasing post-metal deposition annealing temperature up to 250 °C approximately. For annealing temperature beyond 250 °C the performance of the device degrades drastically. The variation in the electrical and photoresponse properties of ZnO based Schottky photodetectors can be attributed to combined effects of interfacial reaction and phase transition during the annealing process. These results suggest that both annealing and dielectric passivation are the viable approach to enhancing the performance of ZnO based Schottky photodetectors.

A p-n junction photodiode is just a p-n junction diode that has been specifically fabricated and encapsulated to permit light penetration into the vicinity of the metallurgical junction. p-n and p-i-n photodiodes have the advantages of fast responding speed, low dark current, and working without applied bias. Therefore, p-n and p-i-n photodiodes are the most suitable choice for future space application. The schematic structure and I-V characteristic of p-n photodiode are shown in Figure 9. The total current can be expressed as following equation: (1) I ( V ) = I s   [ exp ( eV / nkT ) − 1 ] − eGwhere I_s is the saturation current, V is the applied voltage, n is the ideality factor, k is Boltzmann’s constant, T is absolute temperature and G is the generation rate. I_s [exp(eV/nkT) − 1] and −eG correspond to dark current and photocurrent, respectively.

In 2001, Yang et al. reported the fabrication of ZnMgO photoconductor by PLD and investigated the photoconductive properties [90]. Mg_0.34Zn_0.66O thin films with a band gap of 4.05 eV were epitaxially grown on c-plane sapphire substrates. Based on the Mg_0.34Zn_0.66O films, planar geometry photoconductive type metal–semiconductor–metal photodetectors were fabricated. The interdigital metal electrodes, which were defined on ∼1500 Å Cr/Au bilayer by conventional photolithography and ion milling, are 250 μm long, 5 μm wide, and have an interelectrode spacing of 5 μm [see Figure 16(a)]. Figure 16b shows the linear I–V curves both in dark and under 308 nm light illumination. Under 5 V bias, the measured average dark current is ∼40 nA, which is close to the calculated dark current based on the resistivity of Mg_0.34Zn_0.66O. Upon UV illumination (308 nm, 0.1 μW), the photocurrent jumped to 124 μA at 5 V bias, indicating a responsivity of ∼1,200 A/W. This responsivity value is comparable to that of ZnO (400 A/W at 5 V bias, 2–16 μm interelectrode spacing) and GaN (2,000 A/W at 5 V bias, 10 μm interelectrode spacing) photoconductive detectors [14,91]. The spectral response of a Mg_0.34Zn_0.66O UV detector under front illumination is plotted in Figure 16(c). The peak response is found at 308 nm. The cutoff wavelength is ∼317 nm, and the visible rejection (R308 nm/R400 nm) is more than four orders of magnitude, indicating a high degree of visible blindness. Figure 16d shows the temporal response of a Mg_0.34Zn_0.66O UV detector with 3 V bias and 50 V load. The 10%–90% rise and fall time are 8 ns and ∼1.4 μs, respectively. The excess lifetime of trapped carriers, especially the trapped holes in n-type semiconductors should be responsible for the slow decay process [90]. After that, Ghosh et al. have fabricated Mg_xZn_1−xO (x = 0 − 0.08) thin films on glass substrate by sol-gel technique and photoconductivity of the thin films have been investigated in vacuum, hydrogen, oxygen and air [92]. The authors found that the I_ph/I_d for the Mg_xZn_1−xO films increases in vacuum, hydrogen with increase in x, the increase being maximum in vacuum for the film with x = 0.05. The I_ph/I_d does not change much for oxygen and it remains almost constant for air. The photoresponse is much slower in the vacuum, hydrogen and oxygen ambient compared to that in air.

In 2003, Yang and coworkers reported ZnMgO MSM UV photodiodes [93]. Wide-band-gap cubic-phase MgZnO thin films were grown on Si (100) with a thin SrTiO₃ buffer layer by PLD [93]. Photodetectors fabricated on Mg_0.68Zn_0.32O/SrTiO₃/Si show peak photoresponse at 225 nm, which is in the deep UV region. At the same time, Takeuchi et al. have fabricated Mg_xZn_1−xO epitaxial composition spreads where the composition across the chip is linearly varied from ZnO to MgO [94]. The continuously changing band gap across the spread is used as a basis for compact broadband photodetector arrays with a range of detection wavelengths separately active at different locations on the spread film. The composition-spread photodetector is demonstrated in the wavelength range of 290–380 nm using the ZnO to Mg_0.4Zn_0.6O region of the spread as shown in Figure 17.

After that, several groups have reported the production of ZnMgO MSM photodiodes by different methods, such as RF sputtering [86–88,95], MBE [76,96,97], MOCVD [77–79], Sol-Gel [98,99] and PLD [100]. Zn_0.8Mg_0.2O MSM UV photodiodes were fabricated on quartz by RF sputtering [86]. The photodetectors showed a peak responsivity at 330 nm. The ultraviolet-visible rejection ratio (R330 nm/R400 nm) was more than four orders of magnitude at 3 V bias. The photodetector showed fast photoresponse with a rise time of 10 ns and a fall time of 170 ns (see Figure 18). The thermally limited detectivity was calculated to be 3.1 × 10¹¹ cm Hz^1/2W⁻¹ at 330 nm. In addition, a composite target was selected to prepare ZnMgO films by RF magnetron sputtering and the Mg composition of the samples can be controlled easily, even at a high growth temperature [88]. The schematic diagram of the composite ZnMgO–Zn target was shown in Figure 19a. Figure 19b shows the UV–visible absorption spectra of Zn_1−xMg_xO films with different x values. The absorption edges of pure ZnO, Zn_0.82Mg_0.18O, Zn_0.72Mg_0.28O, Zn_0.6Mg_0.4O, Zn_0.4Mg_0.6O and Zn_0.3Mg_0.7O were at about 385 nm, 370 nm, 330 nm, 295 nm, 240 nm and 225 nm, respectively. The phase separation was evident for compositions whose absorption edge was between 290 and 240 nm. The typical I–V characteristics of the Zn_0.6Mg_0.4O film based MSM detector were shown in Figure 19c. The device exhibited a very low dark current (lower than 200 pA for |V_bias| < 95 V), and it was not broken down even when the applied bias voltage is larger than 100 V. Furthermore, the MSM deep ultraviolet photodetector based on the wurtzite Zn_0.6Mg_0.4O film exhibits a peak responsivity at 270 nm and a very sharp cutoff wavelength at around 295 nm [see Figure 19(d)]. Recently, cubic Mg_0.70Zn_0.30O thin films have been prepared on quartz substrates by RF magnetron sputtering, and an MSM structured photodetector was fabricated based on the film [95]. The peak responsivity of the photodetector was at about 225 nm, with a very sharp cutoff wavelength at about 230 nm. The dark current of the photodetector was only 2 pA at 3 V bias. These results indicated that RF sputtering system can be used to fabricate high performance solar-blind UV photodetectors.

Joike et al. have fabricated single-phase wurtzite Zn_1−xMg_xO alloy films with 0 < x < 0.45 on (111)-oriented Si substrates by MBE [97]. The cutoff wavelength of Zn_1−xMg_xO UV photodetectors with the x values of 0, 0.10, 0.26 and 0.34 is at ∼375, ∼350, ∼315 and ∼300 nm, respectively. Du and coworkers have demonstrated that the interfacial layer plays a key role in suppressing phase segregation in the MgZnO layer [76]. A single-phase wurtzite Mg_0.55Zn_0.45O thin film with a bandgap of 4.55 eV was successfully synthesized on quasi-homo Mg_0.2Zn_0.8O buffers by RF-plasma assisted MBE. The photodetector based on Mg_0.55Zn_0.45O thin film shows a sharp cut off of responsivity at 277 nm and the responsivity peak is at 266 nm, which demonstrates that the device is a real solar-blind photodetector.

More recently, ZnMgO MSM photodetectors have also been fabricated by MOCVD. Ju et al. have prepared the MgZnO thin films with Mg content from 0.5 to 0.7 by MOCVD [77]. A series of solar blind UV photodetectors with their cutoff wavelength varying from 225 to 287 nm has been realized based on these thin films. The representative solar-blind photodetector shows a UV/visible rejection ratio of about four orders of magnitude and a dark current of 15 pA under 10 V bias.

In 2008, Endo et al. reported the fabrication and characteristics of Pt/Mg_xZn_1−xO Schottky photodiodes on a ZnO Single Crystal [101,102]. The Mg_xZn_1−xO film was deposited on a ZnO single crystal substrate by RF magnetron sputtering method. The optical bandgap of the Mg_0.59Zn_0.41O film obtained from the spectral transmittance and reflectance was 4.6 eV. The fabricated photodiode consisted of an anti-reflection SiO₂ film, semitransparent Schottky Pt electrode, Mg_0.59Zn_0.41O film, n⁺-ZnO single crystal substrate and Pt/Ti ohmic electrode. The ideality factor of the photodiode, obtained from the current–voltage characteristics, was 1.3. The maximum responsivity was 0.015 A/W at the wavelength of 220 nm. After that, Nakano et al. demonstrated the Schottky photodiodes consisting of a Mg_xZn_1−xO (x ≤ 0.43) thin film and a transparent conducting polymer, poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate) by MBE [103]. Spectral response of Mg_xZn_1−xO Schottky photodiodes was characterized under a zero-bias condition at room temperature. The cut-off wavelengths could be varied systematically by changing x, keeping high quantum efficiency near unity and small Urbach’s energy up to sufficiently high Mg content (x ≤ 0.43). More recently, Schottky photodiodes based on Au-ZnMgO/sapphire are demonstrated covering the spectral region from 3.35 to 3.48 eV, with UV/VIS rejection ratios up to ∼10⁵ and responsivity as high as 185 A/W [104]. Both the rejection ratio and the responsivity are shown to be largely enhanced by the presence of an internal gain mechanism. The authors think that during illumination the Schottky photodiodes become highly ohmic due to the large contribution of tunneling, whose primary origin is the photoexcitation of trapped carriers at acceptor-like deep levels. This tunneling current causes a large internal gain under reverse bias, which is a function of the photon flux. At the same time, Zhu et al. have demonstrated a Au/MgO/MgZnO metal-oxide-semiconductor-structured photodetector by MOCVD [105]. The responsivity of the photodetector was about two orders of magnitude larger than that of the Au/MgZnO metal-semiconductorstructured photodetector fabricated under the same procedure except that no MgO layer was introduced. The detectivity of the photodetector can reach 1.26 × 10¹³ cm Hz^1/2/W, almost one order of magnitude larger than that of Si photodetectors that are widely employed for UV detection currently. The authors think that the enhanced responsivity should be attributed to the carrier multiplication occurring in the MgO layer via impact ionization. Their results may provide a facile route to ultraviolet photodetectors with high internal gain.

Just as mentioned above, it is still a challenge to fabricate reliable p-type ZnO or ZnMgO. Therefore, very few reports can be found about ZnMgO p-n homojunction photodiodes [85,106]. In 2007, the first Zn_0.76Mg_0.24O p-n homojunction photodiode has been prepared on (0001) Al₂O₃ substrate by MBE [85]. Ni/Au and In metals deposited using vacuum evaporation were used as p-type and n-type contacts, respectively. The schematic structure of the device was shown in Figure 20(a). Current-voltage measurements on the device showed weak rectifying behavior [see Figure 20(b)]. Both p and n electrodes are good ohmic contacts. This result indicates that the rectifying behavior comes from the p-n junction instead of the metal-semiconductor contacts. In Figure 20(c), the response spectra indicated that peak responsivity of the device is at around 325 nm. The ultraviolet-visible rejection ratio (R325 nm/R400 nm) of four orders of magnitude was obtained at 6 V bias. The photodetector showed fast photoresponse with a rise time of 10 ns and fall time of 150 ns as shown in Figure 20(d). In addition, the thermally limited detectivity was calculated as 1.8 × 10¹⁰ cm Hz^1/2/W at 325 nm, which corresponds to a noise equivalent power of 8.4 × 10⁻¹² W/Hz^1/2 at room temperature.

After that, Shukla reported on Zn_1−xMg_xO homojunction photodetectors on (0001) sapphire substrate fabricated by a PLD system [106]. Ti-Au and Ni-Au metals deposited using vacuum evaporation were used as n-type and p-type contacts, respectively. The dark current of the Zn_1−xMg_xO photodetectors are smaller than 20 pA at the bias voltage of 10 V and smaller than 2 nA at a bias less than 40 V. The cutoff wave length of the Zn_1−xMg_xO photodetectors varied from 380 nm to 284 nm and the corresponding rejection decreased from 886 to 842 with the increase of Mg content (x) from 0 to 0.34. The decrease of rejection ratio can be attributed to the degradation in the crystallinity of the Zn_1−xMg_xO films due to the doping to Mg. Meanwhile, Li and coworkers demonstrated p-Mg_0.2Zn_0.8O/n-ZnO heterojunction ultraviolet photodiode on a sapphire substrate by MBE [96]. The current–voltage measurement indicates that the heterojunction has a weak rectifying behaviour with a turn-on voltage of ∼5 V. The spectral response measurement shows that the photodiode has a peak responsivity at around 340 nm, and it has a wide detection range in the ultraviolet region from 400 to 320 nm. The response in the long and short wavelength region is due to the contribution of the n-ZnO and p-MgZnO layers, respectively. The ultraviolet–visible rejection ratio (R340 nm/R500 nm) of two orders of magnitude was obtained at a reverse bias of 8 V. More recently, the spectral response of back- and front-surface-illumination MgZnO/ZnO p-n ultraviolet photodetector have been investigated [107]. The peak responsivity at 330 nm for the device under back-illumination is about four times larger than that of the device under front-illumination under the same reverse bias. Comparing with front-illumination, enhancement in peak responsivity at 330 nm under back-illumination was achieved for the reduction of the surface recombination velocity.

Additionally, the single-phased wurtzite ZnO based oxide system Zn_1−x−yBe_xMg_yO had been successfully prepared with a bandgap continuously modulated from 3.7 to 4.9 eV, and the advantage of crystalline and optical properties had been demonstrated compared with those of ZnMgO and BeZnO [108]. Therefore, it is expected that Zn_1−x−yBe_xMg_yO based UV photodetectors may have high performance. Considering the difficulty of achieving stable and reproducible high-quality p-type ZnO layers, zero-biased solar-blind photodetectors based on the n-Zn_1−x−yBe_xMg_yO/p-Si heterojunction with a cutoff wavelength of 280 nm were fabricated by PLD [109]. The responsivity of the device was 0.003 A/W at zero bias with a UV/visible rejection ratio of more than two orders of magnitude. Furthermore, the improvement in responsivity was achieved by enhancing the carrier collection efficiency using a thin Al-doped ZnO contact layer, and the peak responsivity significantly improved to 0.11 A/W at zero bias, corresponding to a high external quantum efficiency of 53%. The rise time achieved was as fast as 20 ns. Although the fall time was 250 μs, it could be potentially shortened by preparing the contact layer with higher crystalline quality. This work demonstrates the possibility of a wurtzite ZnBeMgO oxide system in realizing high performance zero-biased photodetector with a typical solar-blind photoelectric response characteristic.