Observations from the moderate resolution imaging spectroradiometer (MODIS) were used in combination with a large data set of field measurements to map woody above-ground biomass (AGB) across tropical Africa. We generated a best-quality cloud-free mosaic of MODIS satellite reflectance observations for the period 2000-2003 and used a regression tree model to predict AGB at 1 km resolution. Results based on a cross-validation approach show that the model explained 82% of the variance in AGB, with a root mean square error of 50.5 Mg ha −1 for a range of biomass between 0 and 454 Mg ha −1 . Analysis of lidar metrics from the Geoscience Laser Altimetry System (GLAS), which are sensitive to vegetation structure, indicate that the model successfully captured the regional distribution of AGB. The results showed a strong positive correlation (R 2 = 0.90) between the GLAS height metrics and predicted AGB.

In recent years, Ga2O3 solar-blind photodetectors (SBPDs) have received great attention for their potential applications in solar-blind imaging, deep space exploration, confidential space communication, etc. In this work, we demonstrated an ultra-high-performance ε-Ga2O3 metal–semiconductor–metal (MSM) SBPD. The fabricated photodetectors exhibited a record-high responsivity and fast decay time of 230 A/W and 24 ms, respectively, compared with MSM-structured Ga2O3 photodetectors reported to date. Additionally, the ε-Ga2O3 MSM SBPD presents an ultrahigh detectivity of 1.2 × 1015 Jones with a low dark current of 23.5 pA under an operation voltage of 6 V, suggesting its strong capability of detecting an ultraweak signal. The high sensitivity and wavelength selectivity of the photodetector were further confirmed by the record-high responsivity rejection ratio (R 250 nm/R 400 nm) of 1.2 × 105. From the temperature-dependent electrical characteristics in the dark, the thermionic field emission and Poole–Frenkel emission were found to be responsible for the current transport in the low and high electric field regimes, respectively. In addition, the gain mechanism was revealed by the Schottky barrier lowering effect due to the defect states at the interface of the metal contact and Ga2O3 or in the bulk of Ga2O3 based on current transport mechanism and density functional theory calculations. These results facilitate a better understanding of ε-Ga2O3 photoelectronic devices and provide possible guidance for promoting their performance in future solar-blind detection applications.

The Pt/β-Ga2O3 Schottky barrier diode and its temperature-dependent current-voltage characteristics were investigated for power device application. The edge-defined film-fed growth (EFG) technique was utilized to grow the (100)-oriented β-Ga2O3 single crystal substrate that shows good crystal quality characterized by X-ray diffraction and high resolution transmission electron microscope. Ohmic and Schottky electrodes were fabricated by depositing Ti and Pt metals on the two surfaces, respectively. Through the current-voltage (I-V) measurement under different temperature and the thermionic emission modeling, the fabricated Pt/β-Ga2O3 Schottky diode was found to show good performances at room temperature, including rectification ratio of 1010, ideality factor (n) of 1.1, Schottky barrier height (ΦB) of 1.39 eV, threshold voltage (Vbi) of 1.07 V, ON-resistance (RON) of 12.5 mΩ·cm2, forward current density at 2 V (J@2V) of 56 A/cm2, and saturation current density (J0) of 2 × 10−16 A/cm2. The effective donor concentration Nd − Na was calculated to be about 2.3 × 1014 cm3. Good temperature dependent performance was also found in the device. The Schottky barrier height was estimated to be about 1.3 eV–1.39 eV at temperatures ranging from room temperature to 150 °C. With increasing temperature, parameters such as RON and J@2V become better, proving that the diode can work well at high temperature. The EFG grown β-Ga2O3 single crystal is a promising material to be used in the power devices.

No abstract

Ga2O3‐based solar‐blind photodetectors (PDs) are now attracting more and more attention for their potential application in optical imaging, spatial communication, etc. However, the performance of ever‐reported Ga2O3‐based PDs is still not good enough, strongly affected by either the Ga2O3 crystalline quality or the device structure, which severely limits their capability to detect extremely weak signals and achieve future applications. In this work, solar‐blind field‐effect phototransistor based on radio‐frequency‐magnetron sputtered amorphous gallium oxide thin film with ultrahigh photodetection performance are demonstrated. The key feature of the device is gate‐tunable photodetection enabling ultrahigh responsivity of 4.1 × 103 A W−1, external quantum efficiency of 2 × 106%, and detectivity of 2.5 × 1013 Jones under a 70 µW cm−2 weak signal of 254 nm light due to a high internal gain and field effect control of the phototransistor. Furthermore, high‐resolution imaging is achieved for the imaging target by integrating the as‐fabricated photodetectors into the imaging system, which is the first report on solar‐blind imaging of amorphous gallium oxide photodetectors. Such field‐effect phototransistors with ultrahigh performance and excellent imaging capability address a significant step toward the feasibility and practicability of gallium oxide photodetectors in solar‐blind imaging system.

No abstract

Ultraviolet (UV) photodetectors (PDs) have drawn great attention in recent years due to their potential application in civil and military fields. Because of its ultrawide bandgap, low cost, strong radiation hardness, and high thermal and chemical stability with high visible-light transparency, Ga 2 O 3 is regarded as the most promising candidate for UV detection. Furthermore, the bandgap of Ga 2 O 3 is as high as 4.7-4.9 eV, directly corresponding to the solar-blind UV detection band with wavelength less than 280 nm. There is no need of doping in Ga 2 O 3 to tune its bandgap, compared to AlGaN, MgZnO, etc, thereby avoiding alloy composition fluctuations and phase separation. At present, solar-blind Ga 2 O 3 photodetectors based on single crystal or amorphous Ga 2 O 3 are mainly focused on metal-semiconductor-metal and Schottky photodiodes. In this work, the recent achievements of Ga 2 O 3 photodetectors are systematically reviewed. The characteristics and performances of different photodetector structures based on single crystal Ga 2 O 3 and amorphous Ga 2 O 3 thin film are analyzed and compared. Finally, the prospects of Ga 2 O 3 UV photodetectors are forecast.

Different metalloporphyrin model compounds have been synthesized to study the mechanisms of cytochrome P450s with various terminal oxidants, and numerous intermediates have been reported. However, the detailed mechanism of the oxygen atom transfer from iodosylarene to the substrates remains unclear. Here we report the direct ultraviolet-visible spectroscopic observation of the soluble iodosylarene-manganese porphyrin adduct following catalytic oxidation using 2,4,6-tri-tert-butylphenol as the reductant. When the reductant is changed to cisstilbene, the rate-determining step also changes. Both the iodosylarene-manganese porphyrin adduct and [(porphyrin)Mn(V) ¼ O] species may be simultaneously observed. In the absence of reductant, the adduct of iodosylarene with sterically hindered [Mn(meso-tetrakis(2,6-dichlorophenyl)porphinato)Cl] is immediately formed, and smoothly converted into a highvalent [(porpyrinato)Mn ¼ O]. Electrospray ionization mass spectrometry analysis of the reaction further confirms the transformation between these species. This study provides an insight into the mechanism of oxygen transfer within the haem-containing enzymatic systems.