Yung C. Liang

Bulk Photovoltaic Effect at Visible Wavelength in Epitaxial Ferroelectric BiFeO3 Thin Films

Adv. Mater. 2010, 22, 1763–1766 2010 WILEY-VCH Verlag G T IO N While silicon-based diodes have been the dominant solar cell type, novel photovoltaic mechanisms are being explored in pursuit of lower cost or improved efficiency. In a semiconductor photodiode, such as a Si solar cell, photons with energy higher than the band gap are absorbed to produce electron-hole pairs, which are separated by the internal field in the p–n junction and collected with the electrodes. However, a p–n junction is not a prerequisite for the photovoltaic effect. For exitonic solar cells, photon absorption creates excitons, which dissociate at a heterojunction. In materials without a center of symmetry, such as ferroelectric materials, steady-state photocurrent can exist in a homogeneous medium under uniform illumination, a phenomenon called bulk photovoltaic effect (BPVE). BPVE is a fascinating mechanism with many unique features such as extremely large photovoltage, a photocurrent proportional to the polarization magnitude, and charge-carrier separation in homogeneous media. Observed in bulk ferroelectrics in as early as 1950s, BPVE has seen a resurgent interest recently, especially in ferroelectric thin films. It has been proposed that remarkably higher photovoltaic efficiency can be achieved in thin films. On the other hand, open-circuit voltage much larger than the band gap has also been achieved with ferroelectric thin films with in-plane interdigital electrodes, which has led to the development of UV sensors and dosimeters. The ferroelectric thin-film materials under the previous study, such as BaTiO3 and Pb(ZrTi)O3, have wide band gaps (typically larger than 3.3 eV) corresponding to the UV region. BPVE in visible wavelength could lead to the development of new photovoltaic cells or other novel optoelectronic devices. BiFeO3 (BFO), a multiferroic material at room temperature with a band gap near 2.74 eV and a very large remnant ferroelectric polarization, offers a unique opportunity for such an investigation. Appreciable photoconductivity in visible light has been reported in BFO. Optical studies by absorption spectroscopy and spectroscopic ellipsometry have shown that BFO has a direct band gap with high absorption coefficient. Recently, a switchable-diode effect and a visible-light photovoltaic effect has been observed in BFO bulk crystals. However, no value of photovoltage has been reported for BFO single crystals and significant bulk photovoltaic response has not been demonstrated in BFO thin film. It is also unclear if the photovoltaic response in BFO is due to the diode effect. Here, we studied the photovoltaic effect in epitaxial BFO thin films and obtained an open-circuit voltage Voc of 0.3 V. We further demonstrated that photocurrent direction can be switched by the polarization direction of the BFO film and that the ferroelectric polarization is the main driving force of the observed photovoltaic effect. Moreover, the as-deposited BFO films were self-polarized and they could readily function as a photovoltaic cell without any poling. Epitaxial BFO thin films of 170 nm were grown by radiofrequency (RF) magnetron sputter deposition on a (001)c SrTiO3 (STO) substrate, with a 60-nm layer of SrRuO3 (SRO) as the bottom electrode. The resulting films show good epitaxy as determined by high-resolution X-ray diffraction (HRXRD; Supporting Information, Fig. S1). The polarization–electric field (P–E) hysteresis measurement shows a remnant polarization (Pr) of more than 65mCcm 2 with a Au top electrode (Fig. S2). Devices with an indium tin oxide (ITO) top electrode have a slightly smaller Pr. Figure 1a shows the spectral response of the short-circuit current (Jsc) of the BFO film. Highest current density is detected at 460 nm, closely corresponding to the measured BFO band gap of 2.72 eV (Fig. S3). Incident light at 435 nm, slightly above band gap, was used for the current-density–voltage (J–V) measurement (Fig. 1b). The as-deposited samples were electrically poled before measurement. The poling direction is termed positive if a positive bias voltage is applied to the top electrode with the bottom electrode grounded. In the J–Vmeasurement, the applied voltage is positive if a positive bias voltage is applied to the bottom electrode. Fig. 1b shows that for the positively poled samples the photocurrent is positive (i.e., it flows out of the top electrode). In contrast, after the negative poling, the photocurrent direction is reversed. The magnitudes of both the photocurrent and photovoltage are smaller in positively poled samples than in negatively poled ones. Jsc is observed to increase almost linearly with the illumination intensity (Fig. 1c), whileVoc saturates at high illumination intensity (Fig. 1d). At the highest illumination intensitymeasured,Voc in the negatively poled film of 170-nm thickness is 0.286V. The substantialVoc obtainedhere is probably a result of the low conductivity of our samples, which is on the order of 10V 1 cm , six orders of magnitude smaller than that reported by Basu et al. and also much smaller than that reported by Choi. The photovoltaic response for the as-deposited films without any poling was also measured. The results are surprisingly

Oxide-bypassed VDMOS (OBVDMOS): an alternative to superjunction high voltage MOS power devices

The superjunction concept has been proposed to overcome the ideal silicon MOSFET limit, but its fabrication was handicapped by the precise charge balance requirement and inter-diffusion problem. We report a novel device structure termed oxide-bypassed VDMOS (OBVDMOS) that requires the well-established oxide thickness control instead of the difficult doping control in translating the limit to a higher blocking voltage. This is done by using metal-thick-oxide (MTO) at the sidewalls of drift region. One can choose to have a higher blocking voltage or increase the background doping. A PiN structure, essentially identical to MOSFET during off state, was fabricated to demonstrate the proposed concept. Its measured BV/sub dss/ of 170 V is 2.5 times higher than measured conventional device BV/sub dss/ of 67 V on the same silicon wafer.

Photovoltaic mechanisms in ferroelectric thin films with the effects of the electrodes and interfaces

It was discovered that the dielectric constant of the electrodes substantially determines the photovoltaic output in (Pb0.97La0.03)(Zr0.52Ti0.48)O3 ferroelectric thin films. With the screening charges distributed extensively away from the electrode interfaces, the use of the electrodes with a high dielectric constant gives rise to dramatically enhanced magnitude of photocurrent in photovoltaic thin films, and extremely high photovoltaic efficiency is theoretically predicted to be possible in ferroelectric ultrathin films or nanostructures.

Epitaxial ferroelectric BiFeO3 thin films for unassisted photocatalytic water splitting

Considering energy band alignment and polarization effect, ferroelectric BiFeO3 thin films are proposed as the photoanode in a monolithic cell to achieve unassisted photocatalytic water splitting. Significant anodic photocurrent was observed in our epitaxial ferroelectric BiFeO3 films prepared from sputter deposition. Both negative polarization charges and thinner films were found to promote the anodic photocatalytic reaction. Ultraviolet photoelectron spectroscopy proved that the conduction and valence band edges of BiFeO3 straddle the water redox levels. Theoretical analyses show that the large switchable polarization can modify the surface properties to promote the hydrogen and oxygen evolutions on the surfaces with positive and negative polarization charges, respectively.

Thickness effects on photoinduced current in ferroelectric (Pb0.97La0.03)(Zr0.52Ti0.48)O3 thin films

Ferroelectric (Pb0.97,La0.03)(Zr0.52,Ti0.48)O3 thin films in different thicknesses were fabricated on Pt∕Ti∕SiO2∕Si substrates through a sol-gel process. Film thickness dependence of photoinduced current was investigated under the illumination of ultraviolet light. A theoretical model was developed to describe the thickness-dependent photocurrent. Both the theoretical model and experimental results showed that the photocurrent increases exponentially with the decrease in film thickness. However, photocurrent may drop in the films with the very small thickness of tens of nanometers and below in which the small size effect of ferroelectricity is considered. This model also reveals the relationship between the photocurrent and the internal electric field or remnant polarization and predicts a small diffusion current in the opposite direction at very low field or polarization region. These results provide useful guides for the design of ferroelectric film photovoltaic devices.

Tunable oxide-bypassed trench gate MOSFET: breaking the ideal superjunction MOSFET performance line at equal column width

The superjunction (SJ) MOSFET power device is recognized for its higher blocking capability and lower on-state resistance that break the conventional unipolar silicon limit. However, SJ devices below 100 V rating incur the constraint of unrealistically narrow column widths , and their performance is greatly handicapped due to difficulties in the formation of perfectly charge-balanced SJ p-n columns by current process technology. Based on an alternative approach of the tunable oxide-bypassed (TOB) SJ MOSFET concept, a TOB-UMOS device of 79 V rating has been successfully fabricated for the first time. Laboratory measurements indicate that the device breaks the ideal SJ MOSFET performance line at equal column width of 3.5 /spl mu/m, and potentially the ideal silicon limit as well.

Stability of photovoltage and trap of light-induced charges in ferroelectric WO3-doped (Pb0.97La0.03)(Zr0.52Ti0.48)O3 thin films

The stability of photovoltage in WO3-doped (Pb0.97La0.03)(Zr0.52Ti0.48)O3 (PLWZT) ferroelectric thin films was investigated. For in-plane polarized configuration, with a greatly enhanced electrode gap, the reduction ratio of photovoltage during multicycle UV illumination was significantly smaller and stability of photovoltage was greatly improved over the sandwich capacitor configuration. The ferroelectric-metal interfacial effects including Schottky barriers and polarization screening due to the trap of photoinduced charges at interfaces were found to determine the magnitude, stability, and even the polarity of the photovoltage, particularly for the electrode-sandwiched PLWZT thin films.

120 V interdigitated-drain LDMOS (IDLDMOS) on SOI substrate breaking power LDMOS limit

A new device structure named IDLDMOS is proposed to overcome the power LDMOS limit (R/sub on, sp/ /spl prop/ BV/sub dss//sup 2.5/). The concept is based on replacing LDMOS lightly doped n-drift region by moderately doped alternating p and n layers of suitable dimension and doping. Off state requirement is achieved by mutual lateral-depletion of the alternating layers. Using small identical lateral width for both p and n layers, a doping concentration of up to two orders of magnitude higher than n-drift concentration in a conventional case can he achieved to reduce the on-resistance R/sub on/. The simulated 120 V IDLDMOS on SOI substrate has shown a R/sub on/ value that is about 38% of the corresponding R/sub on/ value of a conventional n/sup -/ LDD type LDMOS. At a R/sub on, sp/ value of 1.15 m/spl Omega/-cm/sup 2/ with BV/sub dss/ of 124 V, IDLDMOS has exceeded the conventional LDMOS limit. Compared to conventional LDMOS, IDLDMOS is less prone to quasisaturation at high gate and drain voltage due to its higher drain doping. Isothermal simulation has shown that there was no deterioration in both AC and transient performance between the two devices. Nevertheless, the lower V/sub d, sat/ of LDLDMOS is expected to yield a higher g/sub m/ at the same level of current conduction as in the conventional structure.

Design of smart power synchronous rectifier

In low-output-voltage DC/DC power converters, power losses due to the conduction of rectifying devices are significant. Using synchronous rectifiers instead of the conventional fast recovery diodes or Schottky diodes is an effective solution to this problem in most topologies. However, for synchronous rectifiers to perform effectively, this requires an external gate drive with proper sensing and timing control circuits. This can increase the complexity and cost in power converter hardware implementation. For the first time, a smart power synchronous rectifier (SPSR), which is a two-terminal MOS rectifier, is designed to overcome this difficulty. The SPSR integrates a simple control unit with a power MOSFET into a smart module to form a self-controlled synchronous rectifier. It has great advantages over the conventional discrete circuit composition, such as integrated gate control, precise timing switching and fast transient response, which are suitable for applications in high-frequency pulsewidth modulation (PWM) power converter circuits.

A simple technology for superjunction device fabrication: polyflanked VDMOSFET

The charge compensation based novel superjunction (SJ) MOSFET outperforms its conventional counterparts. However, the production of SJ devices is limited by a complicated and costly fabrication process. In this letter, a feasible technology for polyflanked vertical double-diffused MOS SJ structure, as in Gan et al. (2001), is introduced and demonstrated to have greatly reduced fabrication costs, simplified processes, and overcome the interdiffusion problem of SJ columns. This brings forth the new milestone that SJ MOS devices can now be fabricated by standard cleanroom facilities.