Chunxiao Gao

Experimental analysis and theoretical model for anomalously high ideality factors in ZnO/diamond p-n junction diode

High-quality heterojunctions between p-type diamond single-crystalline films and highly oriented n-type ZnO films were fabricated by depositing the p-type diamond single-crystal films on the Io-type diamond single crystal using a hot filament chemical vapor deposition, and later growing a highly oriented n-type ZnO film on the p-type diamond single-crystal film by magnetron sputtering. Interestingly, anomalously high ideality factors (n≫2.0) in the prepared ZnO/diamond p–n junction diode in the interim bias voltage range were measured. For this, detailed electronic characterizations of the fabricated p–n junction were conducted, and a theoretical model was proposed to clarify the much higher ideality factors of the special heterojunction diode.

Predicting hardness of dense C3N4 polymorphs

We report the calculations of the Vickers hardness of five predicted C3N4 polymorphs by using the microscopic model of hardness. The hardest phase, cubic C3N4, has the hardness of 92.0GPa, softer than diamond, although its modulus is higher than that of diamond. The densest phase, cubic spinel C3N4, has the lowest hardness of 62.3GPa in the five polymorphs. Our analysis suggests that the hardness of simple-structured covalent materials might not exceed that of diamond.

Integrated microcircuit on a diamond anvil for high-pressure electrical resistivity measurement

A multilayer microcircuit on a diamond surface has been developed for high-pressure resistivity measurement in a diamond anvil cell (DAC). Using a film deposition technique, a layer of Mo film was deposited on a diamond anvil as a conductor, topped with a layer of alumina film for insulation. A microelectric circuit was fabricated with a photolithographic shaping method after film encapsulation. With precise control and measurements of all the dimensions of the sample for resistance measurement, including the width of the metallic film and the diameter and thickness of the gasket hole, resistivity of a sample can be accurately determined. This microcircuit can be flexibly fabricated and easily cleaned. It also provides a promising prospect to measure resistivity under in situ high pressure and high temperature. We measured the resistivity of ZnS using this method, and proved the pressure induced phase transition at 13.9–17.9GPa to be a semiconductor to semiconductor transformation.A multilayer microcircuit on a diamond surface has been developed for high-pressure resistivity measurement in a diamond anvil cell (DAC). Using a film deposition technique, a layer of Mo film was deposited on a diamond anvil as a conductor, topped with a layer of alumina film for insulation. A microelectric circuit was fabricated with a photolithographic shaping method after film encapsulation. With precise control and measurements of all the dimensions of the sample for resistance measurement, including the width of the metallic film and the diameter and thickness of the gasket hole, resistivity of a sample can be accurately determined. This microcircuit can be flexibly fabricated and easily cleaned. It also provides a promising prospect to measure resistivity under in situ high pressure and high temperature. We measured the resistivity of ZnS using this method, and proved the pressure induced phase transition at 13.9–17.9GPa to be a semiconductor to semiconductor transformation.

Fabrication of transparent p-n hetero-junction diodes by p-diamond film and n-ZnO film

Abstract ZnO/diamond hetero-junction diodes have been fabricated for the first time. The structure of the diode was n-type ZnO film/p-type diamond film on the {111} surface of a crystalline diamond. The contact between the n- and p-type semiconductors was found to be improved. The ratio of forward current to the reverse current exceeded 120 within the range of applied voltages of −4 to +4 V. The diode possessed an optical transmission of 50–70% in 500–700 nm wavelength regions.

Accurate measurements of high pressure resistivity in a diamond anvil cell

A new technique incorporating a diamond anvil cell with photolithographic and film deposition techniques has been developed for electrical resistivity measurement under high pressure. Molybdenum was sputtered onto a diamond anvil facet and patterned to the desired microcircuit. A sputtered Al2O3 (alumina) layer was then fabricated onto the Mo-coated layer to insulate the thin-film electrodes from the metallic gasket and to protect the electrodes against plastic deformation under high pressure conditions. For better insulation, Al2O3 was also sputtered onto the metallic gasket. The regular shape of the microcircuit makes it convenient to perform an electric current field analysis, hence, accurate resistivity data can be obtained from the measurement. We performed the measurement of nanocrystalline ZnS to 36 GPa and determined its reversibility and phase transition hysteresis.

High-quality heterojunction between p-type diamond single-crystal film and n-type cubic boron nitride bulk single crystal

We presented the results on the fabrication and characterization of high-quality heterojunction between p-type diamond single-crystalline film and n-type cubic boron nitride (c-BN) bulk single crystal. By employing a simple surface diffusion, we prepared the n-type c-BN bulk single crystals with relatively low resistivity (1.0×10−1 Ω cm). Combining p-type diamond films grown by chemical vapor deposition with n-type c-BN, we fabricated a high-quality heterojunction bipolar p–n diode, which the turn-on voltage of the heterojunction was 0.85 V, and the current density reached to 170 A/m2 when the forward bias was applied to 3 V.

Spray reaction prepared FA1−xCsxPbI3 solid solution as a light harvester for perovskite solar cells with improved humidity stability

FA1−xCsxPbI3 solid solution films are prepared as light harvesters for perovskite solar cells by a spray reaction. Cs+ ions can improve the quality of the light harvester film, and the average PCE of devices increases from 11.3% for FAPbI3 to 14.2% for FA0.9Cs0.1PbI3. Moreover, FA0.9Cs0.1PbI3-based devices exhibit remarkable improvement on humidity stability.

An investigation on the pressure-induced phase transition of nanocrystalline ZnS

An in situ energy dispersive x-ray diffraction study on nanocrystalline ZnS was carried out under high pressure up to 30.8 GPa by using a diamond anvil cell. The phase transition from the wurtzite to the zinc-blende structure occurred at 11.5 GPa, and another obvious transition to a new phase with rock-salt structure also appeared at 16.0 GPa-which was higher than the value for the bulk material. The bulk modulus and the pressure derivative of nanocrystalline ZnS were derived by fitting the Birch-Murnaghan equation. The resulting modulus was higher than that of the corresponding bulk material, indicating that the nanomaterial has higher hardness than the bulk material.

Electronic topological transition and semiconductor-to-metal conversion of Bi2Te3 under high pressure

Accurate high pressure in situ Hall-effect and temperature dependent electrical resistivity measurements have been carried out on Bi2Te3, a topological insulator. The pressure dependent electrical resistivity, Hall coefficient, carrier concentration, and mobility show the abnormal inflection points at 8, 12, and 17.8 GPa, indicating that the pressure-induced structural phase transitions of Bi2Te3 can result in a series of changes in the carrier transport behavior. In addition, the Hall coefficient shows a significant discontinuous change at 4 GPa, which is caused by the electronic topological transition. A sign inversion of Hall coefficient from positive to negative is found around 8 GPa. Furthermore, the temperature dependent electrical resistivity shows that the sample undergoes a semiconductor-to-metal conversion around 9.2 GPa, indicating that the insulating gap of Bi2Te3 becomes closed at this pressure. As the metallization occurs in the sample, the topological property of Bi2Te3 disappears.

In situ electrical conductivity measurement of high-pressure molten (Mg0.875,Fe0.125)2SiO4

In situ resistance measurement of mantle mineral under high temperature and pressure has been considered an important method for studying the electrical properties and thermal states of Earth’s interior. Here the authors report the results of the electrical conductivity of molten olivine [(Mg0.875,Fe0.125)2SiO4] on diamond anvil cell with pressure at 13.2GPa and temperature at 3720K. The results indicate that the activation enthalpy of molten olivine is much less than that of solid, and its conductivity is relatively insensitive to temperature. Moreover, at the given temperature range the conductivity of molten olivine exhibits Arrhenius behavior perfectly. Compared to the results of Hawaiian tholeiite provided by Tyburczy and Waff [J. Geophys. Res. 88, 1413 (1983)] at lower pressure and temperature, the pressure effect on molten olivine conductivity is slightly weaker. This method for electrical conductivity measurement on laser-heated diamond anvil cell allows the environment simulation study of unresea...