Yonghao Han

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.

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.

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.

High-pressure electrical transport properties of KNbO3: Experimental and theoretical approaches

The electrical transport behavior of SnS under high pressure has been investigated by the temperature dependence of electrical resistivity measurement, the in situ Hall-effect measurement, and the first-principle calculation. The experimental results show that SnS undergoes a semiconductor to semimetal transition at ∼10.3 GPa, and this transition is further substantiated by the band-structure calculation. The total and partial density of states predict that the semimetal character of SnS is attributed to the enhanced coupling of Sn-5s, Sn-5p, and S-3p states with application of pressure. In addition, dramatic changes in electrical transport parameters such as the electrical resistivity, the carrier concentration, and the carrier mobility are observed at 12.6 GPa, which are correlated to the pressure-induced Pnma-Cmcm structural phase transition.

In situ impedance measurements in diamond anvil cell under high pressure

Two-electrode configuration was developed for in situ electrical impedance detecting on diamond anvil cell under high pressure. The metal gasket was used as one electrode and the risk coming from electrical short between sample and interside wall of the gasket was eliminated. The configuration was evaluated and proved to be effective by measuring the electric impedance of nanocrystalline ZnS under high pressure.

Phase transformation and resistivity of dumbbell-like ZnO microcrystals under high pressure

High-pressure Raman spectra and in situ electrical resistivity measurement of the dumbbell-like ZnO microcrystals have been investigated by using the diamond-anvil-cell technique at room temperature. The dumbbell-like ZnO microcrystals were synthesized via a facile solution method under mild conditions. In terms of the Raman results, the dumbbell-like ZnO microcrystals underwent a transition from wurtzite to rock-salt structure with increasing pressure and the phase-transition pressure was about 11.13 GPa. In situ electrical resistivity measurement of the dumbbell-like ZnO microcrystals was performed on a designed diamond anvil cell. The change in electrical resistivity related to the phase structure for the ZnO microcrystals was observed with the applied pressure of up to 34.86 GPa. Moreover, the pressure dependence of the electrical resistivity for the dumbbell-like ZnO microcrystals annealed at different conditions was also investigated.

Finite element analysis of resistivity measurement with van der Pauw method in a diamond anvil cell

Using finite element analysis, the authors studied the steady current field distribution under the configuration of van der Pauw method [L. J. van der Pauw, Philips Tech. Rev. 20, 220 (1958)] for resistivity measurement in a diamond anvil cell. Based on the theoretical analysis, the authors obtained the theoretical accuracy curve of the van der Pauw method. This method provides accurate determination of sample resistivity when the ratio of sample thickness to its diameter is less than 0.45. They found that the contact area between electrode and sample is a key factor in the resistivity measurement accuracy and its size is dependent on the sample diameter for a given measurement accuracy.