1. Introduction
Semiconductor-based ultraviolet (UV) photodiodes have been continuously developed that can be widely used in various commercial, civilian areas, and military applications, such as optical communications, missile launching detection, flame detection, UV radiation calibration and monitoring, chemical and biological analysis, optical communications, and astronomical studies, etc. [1-2]. All these applications require very sensitive devices with high responsivity, fast response time, and good signal-to-noise ratio is common desirable characteristics. Currently, light detection in the UV spectral range still uses Si-based optical photodiodes. Due to the Si-based photodiodes are sensitive to visible and infrared radiation, the responsivity in the UV region is still low [3-5]. To avoid these disadvantages, wide-bandgap materials (such as diamond, SiC, III-nitrides and wide-bandgap II–VI materials) are under intensive studies to improve the responsivity and stability of UV photodiodes, because of their intrinsic visible-blindness [6].
Among them, zinc oxide (ZnO) is another wide direct bandgap material due to its sensitive and UV photoresponse in the UV region [7-9]. ZnO has attracted attention as a promising material for optical devices, owing to its large direct band gap energy of 3.37 eV and a large exciton binding energy of 60 meV at room temperature compared to other II-VI semiconductors [10-12]. Therefore, ZnO is promising for use in light-emitting diodes (LEDs), laser diodes (LDs), ultraviolet (UV) detection devices [12-15]. Several deposition methods have been employed for the growth of ZnO layers, including metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), sol-gel and spray pyrolysis [16-20]. The synthesis of
Since the quality of ZnO materials plays a key role in determining the performance of UV photodiodes. This chapter reviews the recent progress in Si-based heterostructure (UV) photodiodes, including
2. ZnO/Si UV photodiodes
Fabrication of a
Figure 2(a) shows the plots of the
Responsivity
where
Kim et al. [28] were demonstrated utilizing radial heterojunction nanowire diodes (RNDs) array consisting of
Figure 4(a) shows the photoresponsivity spectra under a forward bias of 0.5 V. It is clear the UV responsivities of RND2 and RND6 are higher than that of the planar thin film diode (PD) under a forward bias. Such as compared to a PD, a RND2 (6 μm) resulted in a ~2.7 times enhancement of the UV responsivity at λ=365 nm in the forward bias. In addition, the enhanced UV photoconductive response in ZnO NWs may be attributed to the presence of oxygenrelated hole-trap states at the NW surface [29]. As a result, RNDs can improve the UV photodetection sensitivity due to the high surface area to volume ratio. In this case, the UV responsivities at λ=365 nm were detected to be 0.23, 0.42, and 0.63 A/W for PD, RND2, and RND6, respectively. Owing to the short penetration depth, the carrier generation normally occurs near the surface. It indicates surface scattering and recombination decrease the carrier lifetime. Figure 4(b) shows the photoresponsivity spectra of RNDs compared to the PD under a reverse bias. The values of the visible/UV responsivity at λ=700 nm and 365 nm were 17.2 A/W for RND6 and 0.86 A/W for PD. It appears that the ZnO surface can be depleted by the surface oxygen absorption according to the hole-trapping mechanism [29]. Therefore, both the UV and visible photoresponsivities of the RNDs were better than that of a planar PD, owing to the enlarged surface area to volume ratio, efficient carrier collection, and improved light absorption.
3. ZnO/SiO2/Si UV photodiodes
3.1. Ultrathin SiO2 films
Many the various types of photodiodes which include homojunction, heterojunction and metal-semiconductor-metal (MSM) photodiodes much attention has been paid in recent years to metal-oxide-semiconductor (MOS) structures [30-33]. An ultrathin silicon dioxide (SiO2) films has been the most commonly used material for diffusion barriers and insulating layers for various applications in MOS devices due to its properties such as low defect density, high thermal stability, high resistivity, high electric insulating performance, high reliability, and reasonable dielectric constant [34,35]. In general, an ultrathin SiO2 films (≤ 1 nm) was formed on the silicon substrate that the silicon/SiO2 interface becomes crucial for good transistor behavior. Several fabrication methods have been employed for the formed of ultrathin SiO2 films, such as rapid thermal oxidation (RTO) [36], oxidation with excited molecules and ions [37,38], plasma oxidation [39,40], photo-oxidation [34,41], ozone oxidation [43], metal-promoted oxidation [44], anodic oxidation [45,46] and nitric acid (HNO3) vapor oxidation [47,48] etc. When a reverse bias is applied to a MOS photodiode, the energy bands in the semiconductor bend and a potential well is formed between the oxide and the semiconductor. Electron-hole pairs generated near the junction by incident light will be stored in the potential well, and current transport occurs through the oxide layer via tunneling.
Recently, Chen et al. [49-51] reported the
3.2. ZnO/SiO2/Si UV photodiodes
In 2003, Jeong et al. [52] presents
Additionally, we found that an intermediate SiO2 ultrathin film can improve the quantum efficiency and the responsivity by decreasing the surface state density and increase the tunneling photocurrent [49-51]. Figure 6 (a) shows a schematic cross-section of the complete structure. The inset in this figure shows a schematic cross-sectional TEM image of nanostructure
Figures 7(a) and 7(b) present a schematic band diagram to elucidate the current components. Based on Figure 7(a), the dark current can be described as [30,53]
where
where
Figure 8(a) plots the responsivity as a function of (
Figure 8(b) plots the
as a function of wavelength for both a
4. ZnO/SiO2/Si UV photodiodes in a strong magnetic field
In recent years, diluted magnetic semiconductors (DMSs) are attracted much great scientific interest because of their unique spintronics properties with potential technological applications. Consequently, the high Curie temperature ferromagnetism of ZnO and related materials, doped with transition metal (TM) ions, is also expected to have applications in spintronics, including in information storage and data-processing devices [56]. The electronic, optical and magnetic properties of TM-doped ZnO and related materials have been studied extensively [57-64]. However, the behavior and characteristics of ZnO optoelectronic devices in a magnetic field have seldom been investigated. Photodiodes with a
Figure 10 (a) plots the
The total current can be described as
where
Figure 10(b) a plots the total current at a reverse bias of 1 V as a function of the magnetic field. In the case of non-illumination, applying a magnetic field only slightly changed the total current because of the absence of photo-ionization. However, under illumination,
Figure 11(a) and 11(b) show the
Figure 12(a) plots the responsivity as a function of wavelength for a photodiode with the
where the gain factor,
Figure 13(a) plots the photocurrent as a function of wavelength in the range of 350-410 nm for a
Hence, according to the discussion above, a carrier transport model can be used to descript the magneto-induced current. Figure 13(b) shows that the dark current and photocurrent can be respectively described as [30,49,53]
where
where the subscript
5. Conclusions
In summary, both
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