Over the past decade, one-dimensional nanostructures, including nanowires(NW), nanorods, nanotubes and nanobelts, with various compositions and morphology have been fabricated by many approaches^[1-5]. Among a large variety of semiconductors, metal oxides have attracted much research interest because of their unique electronic and optoelectronic properties^[6-8]. In particular, zinc oxide(ZnO) has a direct and wide band gap of 3.37 eV, a large excition binding energy of 60 meV, and its ability to form a varity of nanostructured configurations^[9], which is emerging as a promising candidate for designing nanostructures in the field of short wavelength optoelectronic devices^[10-11]. Such as light-emitting diodes^[12], sensors^[13], solar cells^[14] and field-effect transistors^[15]. However, the questions of ZnO should be given out. In order to optimize the electrical and optical properties of ZnO, doping the ZnO nanostructures with various elements has been widely used^[16]. Kim et al. reported the Al-doped ZnO nanostructures^[17], which indicated the optical transparency tend to be degraded due to the enhanced scattering caused by the dopping. Yayapao et al. reported the Dy-doped ZnO nanostructures^[18], which showed weak near-band-edge-emission in the UV region and a strong broad band deep-level-emission, and the intensity of green emission decreased with the percent of Dy. Shi et al. reported the Co-doped ZnO and the result showed a pronounced red shift of UV emission with the increases of Co doping concentration for ZnO nanorods^[19]. Among these elements, Mg-doping in ZnO is preferred because it can modulate the band gap within a certain range from 3.37 to 7.7 eV as MgO has a larger band gap(7.7 eV) than that of ZnO. The ionic radius of Mg²⁺ (0.057 nm) is very close to Zn²⁺ (0.006 nm), therefore, the replacement of Zn by Mg does not give rise to significant changement in lattice constants^[20-25]. At present, Mg doped ZnO nanostructures have been deposition(PLD)^[26], sol-gel deposition^[27], hydrothermal synthesis^[28]and chemical vapor deposition(CVD) etc. Among these methods, CVD is one of the most important approaches for growing high quality nanostructures, which has several advantages mainly including high crystallinity, controlled size and dimensionality^[29-30].

In this paper, MgZnO NWs have been successfully fabricated via CVD using the mixture of the zinc oxide powder and commerical graphite poweder and Mg powder as the precursor material without any catalysts. The advantages of this method include facile and catalyst-free growth of Mg_xZn_1-xO NWs on SiO₂/Si substrate and the subsequent transfer-free fabrication of electronic or optoelectronic devices.

The SiO₂/Si wafer was cleaned with acetone, ethanol and deionized water in turn under the ultrasound, and then the wafer was dried with a compressed nitrogen. The substrate is heated for 2 min at 100 ℃ being coated with AZ3100 photoresist in a spin coater at 3500 r/min. Then, the periodical square pillar microstructure with a top area size of 10 mm × 10 mm and a height of around 800 nm was fabricated on substrate surfaces by RIE(Magnetic Enhanced Reactive Ion Etching, SF₆∶CHF₃ = 10∶40) after a UV lithography process.

The growth of Mg_xZn_1-xO nanowires was performed in a horizontal tube furnace. This system contained a quartz tube vacuum chamber 100 cm long and 10 cm in diameter. A smaller one-ended quartz tube, 50 cm long, and 2 cm in diameter that contained precursor materials(ZnO and Mg) and substrate was placed into the vacuum chamber. A mixture of the zinc oxide powder(99.99%) and commercial graphite powder(weight ratio 1∶1) was used as the precursor material of fabricating ZnO, and Mg powder(99.99%) was also used as the doped material. The precursor material of ZnO was placed at the quartz coat, and the SiO₂/Si substrate was placed over the quartz coat, while the Mg powder was placed at the closed end of the smaller quartz tube. Then the smaller quartz tube was inserted into the the center of the low-temperature region of the quartz tube vacuum chamber, and the quartz coat was inserted into at the center of the high-temperature region of the quartz tube vacuum chamber.

A mixture of the zinc oxide powder(99.99%) and commercial graphite powder(weight ratio 1∶1) was used as the precursor material of fabricating ZnO, and Mg powder(99.99%) was also used as the doped material. The precursor material of ZnO was placed at the quartz coat, and the SiO₂/Si substrate was placed over the quartz coat, while the Mg powder was placed at the closed end of the smaller quartz tube. Then the smaller quartz tube was inserted into the the center of the low-temperature region of the quartz tube vacuum chamber, and the quartz coat was inserted into at the center of the high-temperature region of the quartz tube vacuum chamber.

The precursor material of ZnO and Mg were heated up to 1000 ℃ and 760 ℃, respectively, and the temperature was maintained about 30 minutes during the growth process of the Mg_xZn_1-xO nanowires, and the high purity Ar gas, as carrier gas, was fed into the furnace at one end at the rate of 100 sccm, while the other end was connected to rotary pump to keep the system to a certain pressure. When the temperature was heated to the designed temperature, the high purity O₂ gas was introduced into the quartz tude at the rate of 2 sccm. After the chamber cooled down to the room temperature, the white materials appeared on the substrates.

SEM was used to characterize the morphology of the products. Figure 1 shows the SEM of the MgZnO NWs. Figure 1(a) and Figure 1(b) showed the top-view SEM images of the MgZnO NWs and EDS, respectively. From the Figure 1(b), it was clear that only the Zn, Mg and O peaks were observed while no other impurities were detected. Figure 1(c) showed the image of sample used for TEM. Figure 1(d) revealed that structure of the MgZnOnanowire was hexagonal structure(PDF#36-1451), the lattice parameter was 2.81 Å(1 Å=0.1 nm), 5.06 Å, respectively, which meaned the Mg ions started to replace the Zn ions. Meanwhile, the corresponding chemical composition of the grown MgZnO NWs was determined through the EDS.

In order to further confirm the formation of MgZnO NWs, the X-ray diffraction(XRD) and Photo-luminescence(PL) of ZnO NWs and MgZnO NWs were checked, showed in Figure 2. XRD patterns of NWs samples were taken to study the crystallographic information on the nanowires. From the diffraction peaks shown in Figure 2(a), it could deduce that the structure of MgZnO NWs was hexagonal, just like ZnO. Meanwhile, the normalized PL spectra of the MgZnO samples are shown in Figure 2(b), the PL spectra showed an obvious peak at the 378 nm. Comparing with the ZnO PL, the MgZnO showed an apparent hypsochromic shift, which was caused by the change of the band-gap with Mg substitution. Owing to Mg doping the ZnO nanowires, the Burstein-Moss effect is deemed to be the reason of the hypsochromic-shift.

Current-voltage(I-V) measurements were carried out at ambient condition to investigate the electrical properties of the fabricated UV photosensor in dark and under UV illumination. Metal electrodes could be directly fabricated through screen printing or shadow-mask assisted deposition method onto the as-obtained MgZnO nanowires.

In our experiment, silver paint was directly applied onto MgZnO nanowire arrys to form the electrods. The photoresponse tests were conducted in a dark environment with UV illumination (254 nm, 0.03 mW/cm²). The "on" and "off" of illumination were controlled by a metal chopper. The photoresponse of UV illumination are shown in Figure 3. Figure 3(b) shows the I-T characteristic curves of MgZnO nanowires photodetector with and without light illumination.

To further analyze the photoconductive properties, the chemical states of NWs surface with and without UV radiation can be clarified based on the previous findings. Under dark conditions, the surface of MgZnO NWs absorbed oxygen from the atmosphere and formed a depletion layer, thereby producing negatively charged ions. As a result, the absorbed oxygen molecules on the surface of the MgZnO NWs trapped some of the free electrons, whereas the mobility of the remaining electrons decreased because of the depletion layers created on the surface.

${\rm O_2} + 2{e^ - } \to \rm O_2^ - $ (1)

The water vapor molecules absorbed by the surface of MgZnO NWs also enhanced the depletion layer. The water vapor molecules captured not only free electrons but also free holes, thereby further lowering the conductivity NWs. Consequently, H₂O molecules significantly affected the conductivity more than O₂ molecules in MgZnO NWs.

$ {\rm{H_2}O} + 4{h^ + } \to \frac{1}{2}{\rm O_2} + 2{\rm H^{\rm{ + }}} $ (2)

$ 2{\rm {H_2}O} + 4{e^ - } \to {\rm H_2} + 2{\rm O{H^ - }} $ (3)

Electron-hole pairs were generated ( $hv \to {h^ + } + {e^ - }$) after applying UV light with 254 nm. The photogenerated holes were separated from the electrons by strong local electric fields^[31-32], which reduced the electron-hole recombination process, thereby increasing the carrier lifetime. Consequently, the conductivity increased because of the increase in carrier density. The holes then relocated to the surface and suppressed the depletion region by discharging the adsorbed oxygen ions, thereby forming photo-desorbed oxygen.

The response time is another important indicator of merit of a photodetector. To examine the response time of the MgZnO nanowires UV photodetector, the time-dependent photo-current at 3 V bias with multiple UV on/off cycles was measured, in which both the "on" and "off" times of the UV illumination were 20 s. It was well-known oxygen molecules absorbed at surface of MgZnO acting as electron acceptors to form O₂ by capturing free electrons from the surface of MgZnO in dark and created a low conductive depletion layer near the surface of nanowires.

Upon UV illumination, the photogenerated holes in MgZnO migrated to the surface and neutralized the O₂ ions, while the unpaired electrons significantly enhanced the conductivity of the sample. As shown in Figure 3(b), upon UV illumination, the current would first rapidly ramp from the dark current, followed by a slow increase; and as UV illumination was off, the current would first promptly fall, and then slowly decay to around the original level. These observed time-resolved photocurrent course could be described by a fast photoresponse process followed by a slow one, and the latter one was governed by the low rate of the turnover of oxygen chemisorption/desorption on the NWs surface. The dependence of both rose and decay of photocurrent on time could be well described by second-order decay functions as follows:

$ I = {I_0} + A{{\rm e}^{ - t/{t_1}}} + B{{\rm e}^{ - t/{t_2}}} $ (4)

Where, I₀, A, and B are constants, t₁ and t₂ are time constants.

Figure 3(c) and Figure 3(d) showed a typical rise and decay stage of the time-resolved photocurrent variation curve, respectively. By fitting the photocurrent data with the time, it was estimated the time constants for rise stage are t_r1 = 0.47 s, t_r2 = 3.99 s, with relative weight factors of 64% and 36% respectively; while the time constants for decay stage are t_d1 = 0.60 s, t_d2 = 2.01 s, with relative weight factors of 40% and 60%, respectively.

It was expected that the performance of our MgZnO nanowire photodetectors could be further improved by employing thermal annealing, plasmonic nanoparticles modifica-tion, heterojunction creation, and so on. For the rise stage, the fast process was a result of photocarriers generation excited by UV illumination, however, the slow one was governed by readsorption of oxygen molecules on MgZnO surface; when the UV illumination was off, the fast decay process was related to photocarrier recombination, and the slow one was controlled by the slow physisorption of oxygen molecules.

In summary, it was demonstrated that the MgZnO nanowire arrays could be grown on SiO₂/Si substrate using the CVD without predeposited catalyst or seed-layer. The advantages of this method include facile and safe achievement of the growth of MgZnO nanowires on SiO₂/Si substrate; and the Mg content of MgZnO could be adjusted by changing the ratio of the source materials and the subsequent transfer-free fabrication of electronic or optoelectronic devices. The PL spectra of the MgZnO samples show abvious blue-shift. The MgZnO nanowire UV photodetector was fabricated by a transfer-free process presented high responsibility performance. The strategy shown here would greatly reduce the complexity in nanodevice fabrication processes and significantly prompt the application of MgZnO nanostructures in nano-electronics and optoelectronics.