C. Z. Gu

Visible-blind ultraviolet photodetector based on double heterojunction of n-ZnO/insulator-MgO/p-Si

Exploiting a double heterojunction of n-ZnO/insulator-MgO∕p-Si grown by molecular beam epitaxy, a visible-blind ultraviolet (UV) photodetector has been fabricated. The photodetector shows a rectification ratio of ∼104 at ±2V and a dark current of 0.5nA at a reverse bias of −2V.The photoresponse spectrum indicates a visible-blind UV detectivity of our devices with a sharp cut off at the wavelength of 378nm and a high UV/visible rejection ratio. The key role of the middle insulating MgO layer, as a barrier layer for minority carrier transport, has been demonstrated.

Dual-band MgZnO ultraviolet photodetector integrated with Si

We have constructed a dual-band ultraviolet photodetector by growing high quality MgxZn1−xO layers on Si substrate with molecular beam epitaxy. The device performance was studied by current-voltage, capacitance-voltage, spectra photoresponse, and time-resolved photoresponse characterizations. It demonstrates a high UV/visible light rejection ratio of more than 2 orders of magnitude and a fast response speed of less than 100 ms. The cutoff wavelength can be at solar-blind (280 nm)/visible-blind (301 nm) region by applying 1 V forward/2 V reverse bias. The working principle of the dual-band photodetector was finally investigated by interpretation of the specific carrier transport behavior with the energy band diagram.

Comparative study of n-MgZnO/p-Si ultraviolet-B photodetector performance with different device structures

A comparative study of n-MgZnO/p-Si UV-B photodetector performance was carried out with different device structures. The experimental results demonstrate superior photoresponse characteristics of the p-n heterojunction detector against the Schottky type metal-semiconductor-metal counterpart, including a sharper cutoff wavelength at 300 nm, a larger peak photoresponsivity of 1 A/W, and a faster response speed. The role of built-in field and low interface scattering in p-n heterojunction is explored, and the energy band diagram of n-MgZnO/p-Si is employed to interpret the efficient suppression of visible light photoresponse from Si substrate, revealing the applicability of this heterostructure in fabrication of deep ultraviolet detectors.

Interface engineering of high-Mg-content MgZnO/BeO/Si for p-n heterojunction solar-blind ultraviolet photodetectors

High-quality wurtzite MgZnO film was deposited on Si(111) substrate via a delicate interface engineering using BeO, by which solar-blind ultraviolet photodetectors were fabricated on the n-MgZnO(0001)/p-Si(111) heterojunction. A thin Be layer was deposited on clean Si surface with subsequent in situ oxidation processes, which provides an excellent template for high-Mg-content MgZnO growth. The interface controlling significantly improves the device performance, as the photodetector demonstrates a sharp cutoff wavelength at 280 nm, consistent with the optical band gap of the epilayer. Our experimental results promise potential applications of this technique in integration of solar-blind ultraviolet optoelectronic device with Si microelectronic technologies.

Field electron emission from individual diamond cone formed by plasma etching

Field electron emission properties of individual diamond cone were investigated using a customized double-probe scanning electron microscope system. The diamond cone was formed by maskless ion sputtering process in bias-assisted hot filament chemical vapor deposition system. The as-formed sharp diamond cone coated with high-sp2-content amorphous carbon exhibited high emission current of about 80μA at an applied voltage of 100V. The field emission was stable and well in consistent with the conventional Fowler-Nordheim emission mechanism, due to a stabilization process in surface work function. It has demonstrated the possibility of using individual diamond cone as a point electron emission source, because of its high field electron emission ability and stable surface state after the process of work function stabilization.

Field emission from high aspect ratio tubular carbon cones grown on gold wire

Tubular carbon cones (TCCs) with nanometer-sized tips and micrometer-sized roots, having a herringbone hollow interior surrounded by helical sheets of graphite coiling around, were grown on Au wires by hot filament chemical vapor deposition (HFCVD). These TCCs exhibit excellent field emission properties with a very low threshold field of 0.27V∕μm and a corresponding current density of about 1μA∕cm2; and a stable emitting current density of 1.9mA∕cm2 can be obtained at only 0.6V∕μm. Their low effective work function of ∼0.0056eV and their conical bases—which effectively reduce the screening effect due to sufficient distance between adjacent tubular cones—are both favorable to field emission enhancement.

Effectively enhanced oxygen sensitivity of individual ZnO tetrapod sensor by water preadsorption

In this letter, individual ZnO tetrapod was designed as a multiterminal sensor. The gas-sensing properties of water and oxygen of the device have been investigated. We found that if water was preadsorbed on the device, the sensitivity of oxygen can be effectively enhanced; meanwhile, the response time can also be remarkably shortened. The mechanism of these phenomena, we propose, is the adsorption competition of the two kinds of molecules. Our results show that the preadsorption of appropriate gas can enhance the performance of the sensors, which is helpful for sensor designing.

Effect of grain size and pores on the dielectric constant of nanocrystalline diamond films

The nanocrystalline diamond films with different morphologies and roughness were synthesized by a bias-assisted hot filament chemical vapor deposition method. It was found that the nanocrystalline diamond film exhibited low-k dielectric properties with the increase of CH4 concentration during diamond deposition. The low-k nanocrystalline diamond film with grain size of around 40nm and dielectric constant of 2.4 was obtained at the CH4 concentration of 16% and the bias of −140V. The low dielectric constant can be mainly attributed to the decrease of diamond grain sizes and the formation of more nanopores in as-grown nanocrystalline diamond film, both of which were discussed in details based on the grain size determined band gap expansion effect and the two-phase dielectric mixing model, respectively.

Enhanced photoluminescence from porous silicon by hydrogen-plasma etching

Porous silicon (PS) was etched by hydrogen plasma. On the surface a large number of silicon nanocone arrays and nanocrystallites were formed. It is found that the photoluminescence of the H-etched porous silicon is highly enhanced. Correspondingly, three emission centers including red, green, and blue emissions are shown to contribute to the enhanced photoluminescence of the H-etched PS, which originate from the recombination of trapped electrons with free holes due to SiO bonding at the surface of the silicon nanocrystallites, the quantum size confinement effect, and oxygen vacancy in the surface SiO2 layer, respectively. In particular, the increase of SiOx(x<2) formed on the surface of the H-etched porous silicon plays a very important role in enhancing the photoluminescence properties.

Time-resolved broadband analysis of split ring resonators in terahertz region

Split ring resonators (SRRs) in micrometer scale were fabricated by photolithography and lift-off technologies. The transmission and dispersion characteristics of SRR were measured by terahertz spectroscopy in broadband time domain. Significant resonances at about 0.88 and 2.01THz have been observed, where two stop-band gaps are formed in the dispersion spectrum of SRR. It is suggested that these two minimum dips in the transmission spectrum are induced by dipole and magnetodipole resonances. The results show good agreement with the effective medium theory and the numerical calculation of electromagnetic propagation using finite-difference time-domain method.