Ding-Yuan Chen

ZnO thin-film transistors with polycrystalline (Ba,Sr)TiO3 gate insulators

The electrical characteristics of ZnO thin-film transistors with high-k (Ba,Sr)TiO3 gate dielectrics are presented. The ZnO and (Ba,Sr)TiO3 thin films were deposited on Pt, exhibiting polycrystalline characteristics. The thin-film devices demonstrated transistor behavior over the range of 0–10V with a stable threshold voltage of approximately 1.2V. The field effect mobility, subthreshold slope, and on/off ratio were measured to be 2.3cm2V−1s−1, 0.25V∕decade, and 1.5×108, respectively. The measured transistor performance characteristics suggest that ZnO∕(Ba,Sr)TiO3 structures are well suited for both polycrystalline thin-film transistors for display applications and future higher performance transistors based on single crystal ZnO.

Analysis and design optimization of electrooptic interferometric modulators for microphotonics applications

Scaling the electrode layout (electrode gap, electrode length) down to microscale dimensions extends the application of electrooptic (EO) modulators to microphotonics. In this paper, design criteria are set up to minimize the switching voltage of microscale EO Mach-Zehnder interferometric modulators. Mach-Zehnder interferometric modulators under different directions of electric field with respect to the optic axis are analyzed. Three expressions of the intensity output characteristics are presented for various crystal classes and compared in terms of conditions of validity and design applications. The analysis in this paper suggests that the switching voltage is strongly related with the direction of the electric field relative to the optic axis. For the 4-mm BaTiO/sub 3/ (r/sub 33/=28 pm/V,r/sub 51/=820 pm/V) Mach-Zehnder modulators, r/sub 51/ is utilized when the electric field is applied normal to the optic axis. In this configuration, the thermal stability and polarization insensitivity improve but the extinction ratio becomes a function of the electrode length. The phase-retardation expression is useful to find a suitable modulator length and maximize the extinction ratio. Some of the discussions also apply to Fabry-Pe/spl acute/rot interferometric modulators.

Electronic properties of ZnO epilayers grown on c-plane sapphire by plasma-assisted molecular beam epitaxy

Plasma-assisted molecular beam epitaxy was used to grow thin films of ZnO on c-plane sapphire substrates. The crystalline properties of the layers as measured by x-ray diffraction were found to improve with lower growth temperatures, where the full width at half maximum (FWHM) of the (0002) peak x-ray rocking curves was shown to be in the range of 100to1100arcsec. The surface roughness and crystalline quality were shown to be dependent on the Zn∕O flux ratio. The electronic properties were found to be improved for higher growth temperatures with carrier concentrations in the range of 1×1017–5×1018cm−3 and electron mobilities ranging from 80to36cm2∕Vs. P-type doping of the ZnO films was accomplished by introducing nitrogen into the oxygen plasma with resulting carrier concentration and hole mobility of 2.8×1018cm−3 and 9cm2∕Vs, respectively.

Optical waveguiding in BaTiO3∕MgO∕AlxOy∕GaAs heterostructures

Thin films of BaTiO3 with MgO buffer layers were deposited on patterned GaAs substrates incorporating AlxOy for optical confinement. The inclusion of AlxOy layers are used to provide a means for obtaining thick optical confinement layers as a substitute for MgO cladding layers which have large thermal expansion mismatch with respect to GaAs and BaTiO3 that typically result in thin film cracking. Deposition on patterned features was found to reduce thin film cracking and is attributed to a reduction in thin film stress resulting from thermal expansion mismatch. A maximum ridge width of 10–20 μm is estimated for 1-μm-thick BaTiO3 thin films. Optical waveguiding was observed in BaTiO3∕MgO∕GaAs∕AlxOy∕GaAs ridges suggesting the potential application of these structures for integrated optoelectronics.

Electronic properties of ferroelectric BaTiO3∕MgO capacitors on GaAs

Thin films of MgO and BaTiO3 were deposited on (001) GaAs substrates using molecular beam epitaxy and pulsed laser deposition, respectively. X-ray diffraction scans indicate crystalline MgO and BaTiO3 thin films with preferential c-axis orientation. Capacitors fabricated from the BaTiO3∕MgO∕GaAs structures demonstrate ferroelectric hysteresis behavior with a remenant polarization of 0.4μC∕cm2. Small-signal capacitance measurements indicate an effective dielectric constant for the BaTiO3∕MgO structure of 45.5e0. A hysteresis behavior indicative of ferroelectric switching is observed in the capacitance–voltage characteristics with a memory window of approximately 2 V. The observation of ferroelectric behavior in these materials are promising for future multifunctional devices on GaAs.

Electronic properties of ferroelectric BaTiO{sub 3}/MgO capacitors on GaAs

Thin films of MgO and BaTiO3 were deposited on (001) GaAs substrates using molecular beam epitaxy and pulsed laser deposition, respectively. X-ray diffraction scans indicate crystalline MgO and BaTiO3 thin films with preferential c-axis orientation. Capacitors fabricated from the BaTiO3∕MgO∕GaAs structures demonstrate ferroelectric hysteresis behavior with a remenant polarization of 0.4μC∕cm2. Small-signal capacitance measurements indicate an effective dielectric constant for the BaTiO3∕MgO structure of 45.5e0. A hysteresis behavior indicative of ferroelectric switching is observed in the capacitance–voltage characteristics with a memory window of approximately 2 V. The observation of ferroelectric behavior in these materials are promising for future multifunctional devices on GaAs.

Hysteretic electro-optic response in ferroelectric thin films

A model is developed to explain the hysteretic electric field dependence of the electrooptic coefficient in ferroelectric thin films. The reversible electric polarization and the tunable dielectric susceptibility of the ferroelectric thin film are proposed to explain the hysteretic ρ-E (electrooptic coefficient- applied electric field) loop. An empirical model used in ferroelectric capacitors to predict the high frequency C-V curve is utilized here to find the field dependence of the nonlinear susceptibility. The tunable susceptibility can also explain the peaked characteristics of the ρ-E loop. We also show that the linear electrooptic effect in ferroelectric thin films could produce the pseudo-quadratic electrooptic effect on field-induced birefringence as a result of the switchable spontaneous polarization of ferroelectrics. Thus, a careful interpretation of the field-induced birefringence is required to avoid misleading conclusions. This model provides a fundamental understanding to the tunability of the electrooptic coefficient and is useful for the electrooptic characterization of the ferroelectric thin films.

Integration of BaTiO/sub 3/ferroelectric thin films with GaAs for functional devices

The integration of ferroelectric oxides with GaAs could enable monolithic optoelectronic integrated circuits, advanced metal-insulator-semiconductor structures, and multifunctional optoelectronic devices. BaTiO/sub 3/ is a good candidate for this integration due to the large spontaneous polarization and large electro-optic effects, but presents a challenge due to the large lattice mismatch and difficulty in achieving a crystalline oxide interface on GaAs. In this paper we investigate the deposition of BaTiO/sub 3/ onto GaAs by pulsed laser deposition using varied substrate preparation techniques and MgO buffer layers. Highly oriented [001] BaTiO/sub 3/ thin films are achieved exhibiting spontaneous polarization characteristic of ferroelectric material.

Deposition Of BaTiO 3 Thin Films And MgO Buffer Layers On Patterned GaAs Substrates For Integrated Optics Applications

This work addresses the need for thick layers of ferroelectric thin films on semiconductors for integrated optics applications. The deposition of BaTiO 3 thin films with MgO buffers on patterned GaAs substrates is presented as an approach to achieve crack-free optical waveguiding structures. Cracking and peeling of the thin films are observed on patterns with lateral dimensions exceeding 60 microns and nearly crack-free thin films for patterns with lateral dimensions of a few microns. The cracking and peeling of the thin films is attributed to thermal expansion mismatch during the heating and cooling steps of the deposition process. A thin film stress and fracture model is used to analyze the phenomenon. Reduced cracking and peeling on the patterned features are attributed to strain relief on the patterned features. The inclusion of thick Al x O y buffer layers obtained through wet-oxidation of AlGaAs prior to BaTiO 3 /MgO deposition are presented as a means of obtaining electro-optic waveguide structures on GaAs.