1.1 A brief review of ultraviolet photodetectors materials
Ultraviolet (UV) photodetectors are used in a wide range of applications, like flame monitoring, pollution analyzers, UV astronomy, medical instruments, or even military affairs. Recently, it attached more interests than before on being integrated with InGaN- or other-materials-based light-emitting diodes (LED) or lasers to form a base constituent for UV-blue optical storage systems. Until now, III-V semiconductors are still the main flow of the materials employed for UV detector applications, which gives a barrier for the integration with integrated circuits.
Fig.1.1 Bandgap versus the lattice constant of materials used in UV detection.
Current UV technology utilizes wide-bandgap materials, as shown in Fig. 1.1 [1].
These include metal zinc oxide and magnesium zinc oxide, III–V materials as well as Schottky-type TiO2 UV photodiodes or GaN-AlGaN compounds [2,3]. The distinctive aspect of III–nitrides is that their bandgap are tuneable within the ultraviolet range.
Other novel approaches include organics and phosphors. Silicon-based detection
utilizes silicon carbide material [4] and amorphous silicon alloys [5,6]. The many UV detectors that revolve around nitride-based heterostructures are grown on incompatible substances. Si nanoparticle-based devices may offer an opportunity for integration on Si.
1.2 Review works
Photovoltaic devices are essential components in photonics [7] and solar-cells [8], as well as even bio-chips [9]. Bulk silicon, which exhibited impressed success in electronics, with indirect narrow band-gap of 1.1 e V, fails to complete with direct wide band-gap materials on applications in ultraviolet (UV)-blue light detection, which recently face an intense demand in optical storage systems [10]. Nanostructures, unlike bulk materials, exhibited size-dependent electronic states [11]. Recent research has shown that reducing the size of a Si crystal to a few tens of atoms (1 nm) effectively creates a “direct” wide-bandgap material [12,13]. UV light couples to the particles efficiently to produce electron-hole pairs. Contrary to bulk Si or large particles, the electron hole pairs do not appreciably recombine via nonradiative processes, allowing charge separation and collection.
Specific techniques such as electrochemical etching [14,15] or porous silicon [16,17] are proposed to prepare ultrasmall Si nano-particles or nano-channels with direct wide-bandgap characteristics for Si based UV detector fabrication. With such a material, they demonstrated the Si nano-structured ultraviolet photodetector by processes which are complex, hard to be controlled, and not fully compatible with the conventional IC processing lines. Table I list the characteristics of comparison with other researches about UV photodetectors.
~1012
Table I Comparison with other researches about UV photodetectors.
1.3 Motivation
Since the discovery of highly efficient photoluminescence (PL) in the visible region from nanometer-sized silicon crystallites (Si nanodot) [22], extensive study on the Si nanodots has been stimulated by their possibility as a light emitting material. At the same time, the Si nanodots are expected to be a photoconductive material for the following reasons: (1) The spectral response characteristics are controlled by changing the band gap with the crystal size of the Si nanodot. (2) The impact ionization rate of the single crystal Si nanodot must be higher than that of amorphous Se and amorphous Si. (3) When a semi-ballistic carrier transport [23] in the Si-nanodot/SiO2 system occurs, the impact ionization rate is expected to be higher than that of the single crystal Si.
In order to form Si-nanodot photoconductive films with carrier multiplication at a low electric field and photoresponse at various wavelengths, the Si nanodots with a uniform dot size distribution are necessary, because the dot size fluctuation brings about a variation in the band gap of the dots. To obtain high photosensitivity for stacked layer structures of Si nanodots, controlling the dot density and the oxide
thickness are of great importance, because the electron tunneling between neighboring Si nanodots limits the transport of the photoexcited carriers. In addition, because defects such as Si dangling bonds act as non-radiative recombination centers of the photoexcited carriers, the low defect density is also a crucial factor.
Recently, we reported that self-assembled mesoporous silica (MS) with extremely large internal surface area. The dielectric constant and reliability of MS films as interlayers strongly depend on their porosity and the amount of moisture taken up [24]. In considering the number of radiative recombination centers within the MS matrix, the porosity (or pore nanostructures) determines the total area of the pore-surfaces (and, thus, the abundances of the emission centers [25-29]) and the nanoscaled surroundings, both of which influence the luminescence efficiency and spectra. The advantages of MS film are high porosity (30~75%), controllable pore diameter (2~10nm), ordering pore channel array, and providing quantum surrounding for doping nanomaterials.
At the nanometer scale, the ratio of the numbers of atoms on the surface and in the bulk of a material increases rapidly. Interfacial properties of a nanostructures material could, therefore, enable new functional devices. In this regard, self-assembled mesoporous silica (MS) is attractive for its extremely large internal surface area and controllable nanoporous structure [30]. Recently, we showed that enhanced blue photoluminescence (PL) in three-dimensional Si nanocrystals embedded in MS had been reported [31].
In this thesis, we demonstrate a high response diode-like UV-visible detector with a capping layer of nc-Si-embedded MS. This new technology is easy and fully compatible with the IC industry on synthesizing dense, uniform, size-tunable Si NCs within mesoporous silica film as efficient far short-wavelength sensing layers. Gain in this photodiodes was found and attributed to an enhancement of reverse bias of
positive voltage by the formation of positively charged capped layer due to photoionization of electrons.
1.4 Reference
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