Lishu Liu

Probing Defects in Nitrogen-Doped Cu2O

Nitrogen doping is a promising method of engineering the electronic structure of a metal oxide to modify its optical and electrical properties; however, the doping effect strongly depends on the types of defects introduced. Herein, we report a comparative study of nitrogen-doping-induced defects in Cu2O. Even in the lightly doped samples, a considerable number of nitrogen interstitials (Ni) formed, accompanied by nitrogen substitutions (NO) and oxygen vacancies (VO). In the course of high-temperature annealing, these Ni atoms interacted with VO, resulting in an increase in NO and decreases in Ni and VO. The properties of the annealed sample were significantly modified as a result. Our results suggest that Ni is a significant defect type in nitrogen-doped Cu2O.

Oxygen vacancies: The origin ofn-type conductivity in ZnO

Oxygen vacancy (VO) is a common native point defects that plays crucial roles in determining the physical and chemical properties of metal oxides such as ZnO. However, fundamental understanding of VO is still very sparse. Specifically, whether VO is mainly responsible for the n-type conductivity in ZnO has been still unsettled in the past fifty years. Here we report on a study of oxygen self-diffusion by conceiving and growing oxygen-isotope ZnO heterostructures with delicately-controlled chemical potential and Fermi level. The diffusion process is found to be predominantly mediated by VO. We further demonstrate that, in contrast to the general belief of their neutral attribute, the oxygen vacancies in ZnO are actually +2 charged and thus responsible for the unintentional n-type conductivity as well as the non-stoichiometry of ZnO. The methodology can be extended to study oxygen-related point defects and their energetics in other technologically important oxide materials.

Engineering of optically defect free Cu2O enabling exciton luminescence at room temperature

Cu2O is an interesting semiconductor with extraordinary high exciton binding energy, however exhibiting weak room temperature excitonic luminescence. The issue was addressed in literature emphasizing a detrimental role of native point defects responsible for optical quenching. Resolving the problem, we propose a method to manipulate the Cu and O vacancies contents opening a gateway for optoelectronic applications of Cu2O. Specifically, applying oxygen lean conditions, we observe a remarkable suppression of VCu enabling strong room temperature exciton luminescence, while manipulating with VO reveals no impact on the signal. As a result, the excitonic signature was interpreted in terms of phonon assisted transitions.

Fluorine doping: a feasible solution to enhancing the conductivity of high-resistance wide bandgap Mg0.51Zn0.49O active components.

N-type doping of high-resistance wide bandgap semiconductors, wurtzite high-Mg-content MgxZn1–xO for instance, has always been a fundamental application-motivated research issue. Herein, we report a solution to enhancing the conductivity of high-resistance Mg0.51Zn0.49O active components, which has been reliably achieved by fluorine doping via radio-frequency plasma assisted molecular beam epitaxial growth. Fluorine dopants were demonstrated to be effective donors in Mg0.51Zn0.49O single crystal film having a solar-blind 4.43 eV bandgap, with an average concentration of 1.0 × 1019 F/cm3.The dramatically increased carrier concentration (2.85 × 1017 cm−3 vs ~1014 cm−3) and decreased resistivity (129 Ω · cm vs ~106 Ω cm) indicate that the electrical properties of semi-insulating Mg0.51Zn0.49O film can be delicately regulated by F doping. Interestingly, two donor levels (17 meV and 74 meV) associated with F were revealed by temperature-dependent Hall measurements. A Schottky type metal-semiconductor-metal ultraviolet photodetector manifests a remarkably enhanced photocurrent, two orders of magnitude higher than that of the undoped counterpart. The responsivity is greatly enhanced from 0.34 mA/W to 52 mA/W under 10 V bias. The detectivity increases from 1.89 × 109 cm Hz1/2/W to 3.58 × 1010 cm Hz1/2/W under 10 V bias at room temperature.These results exhibit F doping serves as a promising pathway for improving the performance of high-Mg-content MgxZn1-xO-based devices.

A three-terminal ultraviolet photodetector constructed on a barrier-modulated triple-layer architecture

We report a novel three-terminal device fabricated on MgZnO/ZnO/MgZnO triple-layer architecture. Because of the combined barrier modulation effect by both gate and drain biases, the device shows an unconventional I–V characteristics compared to a common field effect transistor. The photoresponse behavior of this unique device was also investigated and applied in constructing a new type ultraviolet (UV) photodetector, which may be potentially used as an active element in a UV imaging array. More significantly, the proper gate bias-control offers a new pathway to overcome the common persistent photoconductivity (PPC) effect problem. Additionally, the MgZnO:F as a channel layer was chosen to optimize the photoresponse properties, and the spectrum indicated a gate bias-dependent wavelength-selectable feature for different response peaks, which suggests the possibility to build a unique dual-band UV photodetector with this new architecture.

Self-compensation induced high-resistivity in MgZnO

The degradation of conductivity with increased Mg content for MgxZn1−xO wide bandgap materials has always been a fundamental application-motivated research issue. Herein, the study of self-compensating defects in MgxZn1−xO:F (0 ⩽ x ⩽ 0.29) thin films was performed to reveal their influence on increased resistivity. Our observations solidly evidence that the degradation of conductivity is mainly owing to the increased concentration of Zn vacancy (VZn)-related compensating defects in MgxZn1−xO alloys. The formation enthalpy of intrinsic VZn defects decreases as Mg content (x) increases. Thus, the compensation ratio increases from 0.23 at x = 0 to 0.47 at x = 0.29, resulting in deteriorated conductivity in MgxZn1−xO alloys. Cathodoluminescence (CL) spectra further confirm higher VZn concentrations with increased Mg content. The electron transport is demonstrated to be dominated by an ionized scattering mechanism. Formation of + FO– − V Zn 2 complexes could reduce the concentration of ionized scattering centers and thus increase mobility. These results clarify the reason of increasingly high resistivity in MgxZn1−xO, which is a long-sought-after physics problem in this area, and provide crucial information on controlling the conductivity of MgxZn1−xO alloys.

Ga Zn -V Zn acceptor complex defect in Ga-doped ZnO

Gallium (Ga)-doped ZnO is regarded as a promising plasmonic material with a wide range of applications in plasmonics. In this study, zinc self-diffusion experiments are adopted to disclose the nature of the dominant compensating defect in Ga-doped ZnO isotopic heterostructures. The (GaZn-VZn)− complex defect, instead of the isolated VZn2−, is identified as the predominant compensating acceptor center responsible for the low donor doping efficiency. The comparative diffusion experiments operated by the secondary ion mass spectrometry reveal a ~0.78 eV binding energy of this complex defect, which well matches the electrical activation energy derived from the temperature-dependent Hall effect measurements (~(0.82±0.02) eV). These findings contribute to an essential understanding of the (GaZn-VZn)− complex defect and the potential engineering routes of heavily Ga-doped ZnO.