Howard L. Mosbacker

Role of Near-Surface States in Ohmic-Schottky Conversion of Au Contacts to ZnO

A conversion from ohmic to rectifying behavior is observed for Au contacts on atomically ordered polar ZnO surfaces following remote, room-temperature oxygen plasma treatment. This transition is accompanied by reduction of the “green” deep level cathodoluminescence emission, suppression of the hydrogen donor-bound exciton photoluminescence and a ∼0.75eV increase in n-type band bending observed via x-ray photoemission. These results demonstrate that the contact type conversion involves more than one mechanism, specifically, removal of the adsorbate-induced accumulation layer plus lowered tunneling due to reduction of near-surface donor density and defect-assisted hopping transport.

Dominant Effect of Near-Interface Native Point Defects on ZnO Schottky Barriers

The authors used depth-resolved cathodoluminescence spectroscopy and current-voltage measurements to probe metal-ZnO diodes as a function of native defect concentration, oxygen plasma processing, and metallization. The results show that resident native defects in ZnO single crystals and native defects created by the metallization process dominate metal-ZnO Schottky barrier heights and ideality factors. Results for ZnO(0001¯) faces processed with room temperature remote oxygen plasmas to remove surface adsorbates and reduce subsurface native defects demonstrate the pivotal importance of crystal growth quality and metal-ZnO reactivity in forming near-interface states that control Schottky barrier properties.

Remote hydrogen plasma doping of single crystal ZnO

We demonstrate that remote plasma hydrogenation can increase electron concentrations in ZnO single crystals by more than an order of magnitude. We investigated the effects of this treatment on Hall concentration and mobility as well as on the bound exciton emission peak I4 for a variety of ZnO single crystals–bulk air annealed, Li doped, and epitaxially grown on sapphire. Hydrogen increases I4 intensity in conducting samples annealed at 500 and 600 °C and partially restores emission in the I4 range for Li-diffused ZnO. Hydrogenation increases carrier concentration significantly for the semi-insulating Li doped and epitaxial thin film samples. These results indicate a strong link between the incorporation of hydrogen, increased donor-bound exciton PL emission, and increased n-type conductivity.

Thermally driven defect formation and blocking layers at metal-ZnO interfaces

The authors used depth-resolved cathodoluminescence spectroscopy and current-voltage measurements to probe the temperature-dependent formation of native point defects and reaction layers at metal-ZnO interfaces and their effect on transport properties. These results identify characteristic defect emissions corresponding to metal-Zn alloy versus oxide formation. Au alloys with Zn above its eutectic temperature, while Ta forms oxide blocking layers that reduce current by orders of magnitude at intermediate temperatures. Defects generated at higher temperatures and/or with higher initial defect densities for all interfaces produce Ohmic contacts. These reactions and defect formation with annealing reveal a thermodynamic control of blocking versus Ohmic contacts.

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 ...

Impact of near-surface defects and morphology on ZnO luminescence

We have used depth-resolved cathodoluminescence spectroscopy (DRCLS) to measure the distribution of deep level defects at and below the surface of ZnO crystals grown by vapor phase transport, hydrothermal, and melt-growth methods. DRCLS reveals large variations in defect distributions with depth on a nanometer scale that correlate with maps of potential and surface morphology measured by Kelvin probe force and atomic force (AFM) microscopies, respectively. A strong correlation between the optical emission efficiency of the nanoscale subsurface region and the AFM surface roughness reveals a figure of merit for substrate polishing and etching.

Impact of near-surface native point defects, chemical reactions, and surface morphology on ZnO interfaces

The authors used a complement of depth-resolved cathodoluminescence spectroscopy (DRCLS), atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM) to correlate the formation of native point defects with interface chemical reactions as well as surface morphology. A wide array of ZnO crystals grown by both melt and hydrothermal growth methods display orders-of-magnitude variation in 2.1, 2.5, and 3.0eV native point defect optical transitions at their free surface and as a function of depth on a nanometer scale. AFM surface morphology scans taken simultaneously with KPFM potential maps reveal large variations in surface morphology related to the growth method and subsequent processing. Notably, when DRCLS defect emissions are low, the surface roughness is low and the morphology matches its respective KPFM potential map. When DRCLS emissions vary with depth, the morphology and potential maps do not correlate. Indeed, the latter can vary by hundreds of meV across micron square areas. These subsu...

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.