Yufeng Dong

Zn- and O-face polarity effects at ZnO surfaces and metal interfaces

Depth-resolved cathodoluminescence spectroscopy, current-voltage, capacitance-voltage, and deep level transient spectroscopy of ZnO (0001) Zn- and (0001¯) O-polar surfaces and metal interfaces show systematically higher Zn-face near band edge emission and lower near-surface defect emission. Even with remote plasma decreases of the 2.5 eV near-surface defect emission, (0001)-Zn face emission quality still exceeds that of (0001¯)-O face. Ultrahigh vacuum-deposited Au and Pd diodes on as-received and O2/He plasma-cleaned surfaces display a strong polarity dependence that correlates with defect emissions, traps, and interface chemistry. A comprehensive model accounts for the polarity-dependent transport properties and their correlations with carrier concentration profiles.Depth-resolved cathodoluminescence spectroscopy, current-voltage, capacitance-voltage, and deep level transient spectroscopy of ZnO (0001) Zn- and (0001¯) O-polar surfaces and metal interfaces show systematically higher Zn-face near band edge emission and lower near-surface defect emission. Even with remote plasma decreases of the 2.5 eV near-surface defect emission, (0001)-Zn face emission quality still exceeds that of (0001¯)-O face. Ultrahigh vacuum-deposited Au and Pd diodes on as-received and O2/He plasma-cleaned surfaces display a strong polarity dependence that correlates with defect emissions, traps, and interface chemistry. A comprehensive model accounts for the polarity-dependent transport properties and their correlations with carrier concentration profiles.

Defects at oxygen plasma cleaned ZnO polar surfaces

Depth-resolved cathodoluminescence spectroscopy (DRCLS) reveals the evolution of surface and near surface defects at polar surfaces with remote oxygen plasma (ROP) treatment. Furthermore, this evolution exhibits significant differences that depend on surface polarity. ROP decreased the predominant 2.5 eV defect emission related to oxygen vacancies on the O face, while creating a new 2.1 eV defect emission on the Zn face that increases with ROP time. The surface-located 2.1 eV emission correlates with carrier profiles from capacitance-voltage measurements and a shift of the E3 trap to higher binding energy from deep level transient spectroscopy (DLTS). This result suggests that ROP generates Zn vacancies on the Zn face which act as compensating acceptors at the surface and in the near surface region. Secondary ion mass spectrometry (SIMS) shows no polarity dependence due to impurities. We conclude that the near-surface deep level optical emissions and free carrier densities of ZnO depend strongly on the RO...

Surface traps in vapor-phase-grown bulk ZnO studied by deep level transient spectroscopy

Deep level transient spectroscopy, current-voltage, and capacitance-voltage measurements are used to study interface traps in metal-on-bulk-ZnO Schottky barrier diodes (SBDs). c-axis-oriented ZnO samples were cut from two different vapor-phase-grown crystals, and Au- and Pd-SBDs were formed on their (0001) surfaces after remote oxygen-plasma treatment. As compared to Au-SBDs, the Pd-SBDs demonstrated higher reverse-bias leakage current and forward-bias current evidently due to higher carrier concentrations, which might have been caused by hydrogen in-diffusion through the thin Pd metal. The dominant traps included the well-known bulk traps E3 (0.27 eV) and E4 (0.49 eV). In addition, a surface-related trap, Es (0.49 eV), is observed but only in the Pd-SBDs, not in the Au-SBDs. Trap Es is located at depths less than about 95 nm and shows an electron capture behavior indicative of extended defects. A possible correspondence between trap Es and the well-known 2.45 eV green band is suggested by depth-resolved ...

Interplay of native point defects with ZnO Schottky barriers and doping

A combination of depth-resolved electronic and structural techniques reveals that native point defects can play a major role in ZnO Schottky barrier formation and charged carrier doping. Previous work ignored these lattice defects at metal–ZnO interfaces due to relatively low point defect densities in the bulk. At higher densities, however, they may account for the wide range of Schottky barrier results in the literature. Similarly, efforts to control doping type and density usually treat native defects as passive, compensating donors or acceptors. Recent advances provide a deeper understanding of the interplay between native point defects and electronic properties at ZnO surfaces, interfaces, and epitaxial films. Key to ZnO Schottky barrier formation is a massive redistribution of native point defects near its surfaces and interfaces. It is now possible to measure the energies, densities, and in many cases the type of point defects below the semiconductor-free surface and its metal interface with nanosca...

Polarity-related asymetry at ZnO surfaces and metal interfaces

Clean ZnO (0001) Zn- and (0001¯) O-polar surfaces and metal interfaces have been systematically studied by depth-resolved cathodoluminescence spectroscopy, photoluminescence, current-voltage and capacitance-voltage measurements, and deep level transient spectroscopy. Zn-face shows higher near band edge emission and lower near surface defect emission. Even with remote plasma decreases of the 2.5eV near surface defect emission, (0001)-Zn face emission quality still exceeds that of (0001¯)-O face. The two polar surfaces and corresponding metal interfaces also present very different luminescence evolution under low-energy electron beam irradiation. Ultrahigh vacuum-deposited Au and Pd diodes on as-received and O2∕He plasma-cleaned surfaces display not only a significant metal sensitivity but also a strong polarity dependence that correlates with defect emissions, traps, and interface chemistry. Pd diode is always more leaky than Au diode due to the diffusion of H, while Zn-face is better to form Schottky barr...

Metal Contacts on Bulk ZnO Crystal Treated with Remote Oxygen Plasma

To study the quality of thin metal/ZnO Schottky contacts (SCs), temperature-dependent current-voltage (I-V), capacitance-voltage, deep level transient spectroscopy, and photoluminescence measurements were performed using bulk, vapor-phase ZnO, treated by remote oxygen plasma (ROP). Au∕ZnO and Pd∕ZnO contacts on both O and Zn faces are compared as a function of the ROP processing sequence and duration. We find that (i) as the duration of ROP treatment increases from 2to4h, Au∕ZnO contacts on the Zn face, deposited before ROP treatment, become rectifying, while those on the O face remain Ohmic; (ii) with long-term ROP treatments prior to metallization, both Au∕ZnO and Pd∕ZnO show high-quality SCs; however, their I-V characteristics can be significantly degraded by electric field and high temperatures; (iii) ROP treatment can cause more H removal on the Zn face than on the O face, resulting in a decrease in the near-surface carrier concentration for the Zn face only; (iv) in addition to the dominant bulk-trap E3, surface traps, E6/E7 and E8, and Es, can be observed in Au∕ZnO and Pd∕ZnO SCs, respectively, on the Zn face, with shorter ROP treatment; and (v) with long-term ROP treatment, E3 (or L2) significantly increases and shifts in Au∕ZnO SCs on the Zn face.To study the quality of thin metal/ZnO Schottky contacts (SCs), temperature-dependent current-voltage (I-V), capacitance-voltage, deep level transient spectroscopy, and photoluminescence measurements were performed using bulk, vapor-phase ZnO, treated by remote oxygen plasma (ROP). Au∕ZnO and Pd∕ZnO contacts on both O and Zn faces are compared as a function of the ROP processing sequence and duration. We find that (i) as the duration of ROP treatment increases from 2to4h, Au∕ZnO contacts on the Zn face, deposited before ROP treatment, become rectifying, while those on the O face remain Ohmic; (ii) with long-term ROP treatments prior to metallization, both Au∕ZnO and Pd∕ZnO show high-quality SCs; however, their I-V characteristics can be significantly degraded by electric field and high temperatures; (iii) ROP treatment can cause more H removal on the Zn face than on the O face, resulting in a decrease in the near-surface carrier concentration for the Zn face only; (iv) in addition to the dominant bulk-tra...

Depth-resolved cathodoluminescence spectroscopy as a probe of defect structure in oxides

Depth-resolved cathodoluminescence spectroscopy (DRCLS) is a powerful technique for probing the nature of defects in oxides, both electronically and spatially on a nanometer scales. The information derived from this technique provides a tool to guide the growth and processing of state-of-the-art semiconductors and dielectrics for micro- and opto-electronics. DRCLS is particularly effective in probing electronic and chemical structure within ultrathin films, beyond the capabilities of conventional techniques. This talk highlights the capabilities of DRCLS with recent results from conventional oxides such as ZnO, to complex oxides such as the perovskite titanates, and the high-K dielectric HfO2. These studies establish the physical nature of native point defects in these materials as well as their spatial distribution on a nanometer scale. Deep level transient and optical spectroscopies (DLTS and DLOS), capacitance-voltage, as well as atomic force microscopy (AFM) combined with Kelvin Force Probe Microscopy (KPFM) provide methods to calibrate the observed luminescence features in terms of defect densities and carrier concentrations in these materials. DRCLS combined with these calibration techniques reveal dramatic increases in defect densities within tens of nanometers of surfaces and interfaces. In turn, such defect segregation has major effects on metal-semiconductor Schottky barrier formation, dielectric loss in capacitance structures at RF frequencies, and interface trapping in metal-oxide-semiconductor structures. For all these electronically-active oxides, DRCLS provides a rapid, non-destructive and highly sensitive method to evaluate localized electronic states and guide the growth and processing of these materials to achieve state-of-the-art device structures.