Junhao Chu

Ultrasensitive and Broadband MoS2 Photodetector Driven by Ferroelectrics

A few-layer MoS2 photodetector driven by poly(vinylidene fluoride-trifluoroethylene) ferroelectrics is achieved. The detectivity and responsitivity are up to 2.2 × 10(12) Jones and 2570 A W(-1), respectively, at 635 nm with ZERO gate bias. E(g) of MoS2 is tuned by the ultrahigh electrostatic field from the ferroelectric polarization. The photoresponse wavelengths of the photodetector are extended into the near-infrared (0.85-1.55 μm).

Modulated photoluminescence spectroscopy with a step-scan Fourier transform infrared spectrometer

Fourier transform infrared (FTIR) spectrometer is a powerful tool for studying the photoluminescence (PL) properties of semiconductors, due to its well-known multiplexing and throughput advantages. However, it suffers from internal He–Ne laser disturbance in near-infrared and∕or environmental background thermal emission in mid- and far-infrared spectral regions. In this work, a modulated PL technique is developed based on step-scan (SS)-FTIR spectrometer. Theoretical analysis is conducted, and applications of the technique are given as examples in the PL study of mid-infrared HgCdTe thin films and near-infrared GaInP∕AlGaInP multiple quantum wells, respectively. The results indicate that the He–Ne laser and∕or thermal emission disturbance can be reduced at least 1∕1000 and∕or even 1∕10000, respectively, by the modulated SS-FTIR PL technique, and hence a rather smooth PL spectrum can be obtained even under room temperature for HgCdTe thin films. A brief comparison is given of this technique with previously...

Ferroelectricity in sol-gel derived Ba0.8Sr0.2TiO3 thin films using a highly diluted precursor solution

Barium strontium titanate films with good ferroelectricity have been obtained by a developed sol-gel processing, using a highly dilute spin-on solution. X-ray diffraction and scanning electron microscopy investigations show that large grains with the size of 100–200 nm in the films are formed from highly dilute spin-on solutions with layer-by-layer homoepitaxy. Polarization-electric field and dielectric constant-temperature (er−T) measurements reveal that the ferroelectricity becomes more evident as the grain size increases. The measurements for quality Ba0.8Sr0.2TiO3 ferroelectric films derived from a 0.05 M solution have shown a remnant polarization of 3.5 μC/cm2, a coercive field of 86 kV/cm, and two distinctive phase transitions.

Pyroelectric properties in sol–gel derived barium strontium titanate thin films using a highly diluted precursor solution

The pyroelectric coefficient and pyroelectric infrared response in the sol–gel derived Ba0.8Sr0.2TiO3 thin films have been studied for possible infrared detector applications. The measured pyroelectric coefficient is larger than 3.1×10−4 C/m2 K at temperatures ranging from 10 to 26 °C and reaches the maximum value of 4.1×10−4 C/m2 K at 16 °C. The infrared detectivity of 4.6×106 cm Hz1/2 W−1 has been obtained at 19 °C and 10 Hz in the Ba0.8Sr0.2TiO3 films deposited on thick (500 μm) platinum coated silicon substrates. The better infrared response can be expected by the improvement in the thermal isolation of pyroelectric element and the electrode materials.

Silicon nanowire-array-textured solar cells for photovoltaic application

In this paper, a vertical-aligned silicon nanowires (Si NWs) array has been synthesized and implemented to the Si NW-array-textured solar cells for photovoltaic application. The optical properties of a Si NWs array on both the plane and pyramid-array-textured substrates were examined in terms of optical reflection property. Less than 2% reflection ratio at 800 nm wavelength was achieved. Using leftover monocrystalline Si (c-Si) wafer (125×125 mm2), a 16.5% energy conversion efficiency, with 35.4% enhancement compared to the pyramid-array-textured c-Si solar cells, was made by the Si NW-array-textured solar cells due to their enhanced optical absorption characteristics. However, without SiNx passivation, the short circuit current reduced due to the increased surface recombination when using Si NWs array as surface texturing, indicating that an optimum surface passivation was prerequisite in high-efficiency Si NW-array-textured solar cells.

Characterization of Mn1.56Co0.96Ni0.48O4 films for infrared detection

Mn1.56Co0.96Ni0.48O4 films with spinel structure were prepared on Al2O3 substrate by chemical solution deposition method. The microstructure of the films was studied by atomic force microscope and field-emission scanning electron microscope. The current-voltage characteristics showed Ohmic conductivity in the temperature range of 245–295K. The conduction was described by a variable range hopping model for a parabolic density of states. The advantages of high characteristic temperature, as well as high transition temperature (201K) between ferromagnetic and paramagnetic phases make the Mn1.56Co0.96Ni0.48O4 films very promising for infrared detection, especially for functional devices by integrating magnetic and electronic properties of the materials.

Optical properties of PbZrxTi1−xO3 on platinized silicon by infrared spectroscopic ellipsometry

A method of analyzing infrared spectroscopic ellipsometry (IRSE) measurement data is proposed for lead zirconate titanate PbZrxTi1−xO3 (PZT) thin films grown on Pt/Ti/SiO2/Si substrates with x=0.3 and 0.5. The IRSE data measured at an angle of incidence 75° for x=0.3 and 70° for x=0.5 are fitted by a dielectric function formula. The refractive index and extinction coefficient of PZT with x=0.3 and 0.5 are determined in the spectral range of 2.5–12.5 μm. As the wavelength increases, the refractive index decreases, on the contrary, the extinction coefficient increases. The absorption coefficient for x=0.5 is greater than that for x=0.3 by a factor of 1.5. The effective static ionic charges have also been derived, which are smaller than they would be in a purely ionic material for PZT thin films.

Giant spin-orbit splitting in a HgTe quantum well

We have investigated beating patterns in Shubnikov-de Haas oscillations for HgTe/Hg_{0.3}Cd_{0.7}Te(001) quantum wells with electron densities of 2 to 3 X 10^{12} cm^{-2}. Up to 12 beating nodes have been observed at magnetic fields between 0.9 and 6 T. Zero magnetic field spin-orbit splitting energies up to 30 meV have been directly determined from the node positions as well as from the intersection of self-consistently calculated Landau levels. These values, which exceed the thermal broadening of Landau levels, k_B T, at room temperature, are in good agreement with Rashba spin-orbit splitting energies calculated by means of an 8 X 8 kp Kane model. The experimental Shubnikov-de Haas oscillations are also in good agreement with numerical simulations based on this model.

Domain stabilization effect of interlayer on ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin film

A charge injection material, poly(3,4-ethylene dioxythioohene)-poly(styrene sulfonic) acid, is introduced as a buffer layer between metal electrodes and a ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer ultrathin film. The buffer layer not only prevents the reactions between P(VDF-TrFE) layer and the top electrode during metallization process, which leads to the loss of polarization, but also supplies the amount of charges needed for compensation of the ferroelectric dipoles to stabilize the domain during switching process so that the fatigue property was improved tremendously. The sandwiched structure shows prominent ferroelectric properties in a 50 nm P(VDF/TrFE) film. A coercive electric field of 52 MV/m and a remnant polarization of 88 mC/m2 are recorded. With two additional organic interlayers, 33 nm thick in total, the coercive field decreases. After more than 1×107 cycles of switching, the polarization remains as high as 68 mC/m2. Moreover, the cell still has good ...

Band-to-band optical absorption in narrow-gap Hg1-xCdxTe semiconductors

The infrared absorption spectra have been measured at temperatures from 4.2 to 300 K for thin bulk samples of Hg1−xCdxTe with compositions, x, from 0.170 to 0.443 with thicknesses smaller than 10 μm. The spectral range of the investigations includes the exponential absorption edges and intrinsic absorption spectra determined by electron transitions from the valence band to the conduction band. From the measured absorption spectra, the energy‐band gap was determined by the photon energy at the absorption transition point between the Urbach exponential region and the Kane region. The data were fitted by a calculation of the intrinsic optical transition using Kane’s theory. The dependence of the energy‐band gap on composition and temperature was determined in the alloy composition range from x=0 to 0.443 and for x=1, for temperatures from T=4.2 to 300 K. The absorption coefficient, αg, at the band‐gap energy versus composition and temperature, and the exponential rule satisfied by the absorption edge are als...