Daniel R. Doutt

Depth-resolved subsurface defects in chemically etched SrTiO3

Depth-resolved cathodoluminescence spectroscopy of atomically flat TiO2-terminated SrTiO3 single crystal surfaces reveals dramatic differences in native point defects produced by conventional etching with buffered HF (BHF) and an alternative procedure using HCl–HNO3 acidic solution (HCLNO), which produces three times fewer oxygen vacancies before and nearly an order of magnitude fewer after pure oxygen annealing. BHF-produced defect densities extend hundreds of nanometers below the surface, whereas the lower HCLNO-treated densities extend less than 50 nm. This “Arkansas” HCLNO etch and anneal method avoids HF handling and provides high-quality SrTiO3 surfaces with low native defect density for complex oxide heterostructure growth.

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...

Nanoscale mapping of temperature and defect evolution inside operating AlGaN/GaN high electron mobility transistors

We used depth-resolved microcathodoluminescence spectroscopy (DRCLS) and Kelvin probe force microscopy (KPFM) to measure and map the temperature distribution and defect generation inside state-of-the-art AlGaN/GaN-based high electron mobility transistors (HEMTs) on a scale of tens of nanometers during device operation. DRCLS measurements of near band edge energies across the HEMT’s source-gate-drain regions reveal monotonic temperature increases across the submicron gate-drain channel, peaking under the drain side of the gate. DRCLS defect emissions mapped laterally and localized depthwise near the two-dimensional electron gas interface increase with device operation under the drain-side gate and correlate with higher KPFM surface potential maps.

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.

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...

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...

Field-induced strain degradation of AlGaN/GaN high electron mobility transistors on a nanometer scale

Nanoscale Kelvin probe force microscopy and depth-resolved cathodoluminescence spectroscopy reveal an electronic defect evolution inside operating AlGaN/GaN high electron mobility transistors with degradation under electric-field-induced stress. Off-state electrical stress results in micron-scale areas within the extrinsic drain expanding and decreasing in electric potential, midgap defects increasing by orders-of-magnitude at the AlGaN layer, and local Fermi levels lowering as gate-drain voltages increase above a characteristic stress threshold. The pronounced onset of defect formation, Fermi level movement, and transistor degradation at the threshold gate-drain voltage of J. A. del Alamo and J. Joh [Microelectron. Reliab. 49, 1200 (2009)] is consistent with crystal deformation and supports the inverse piezoelectric model of high electron mobility transistor degradation.

Strain and Temperature Dependence of Defect Formation at AlGaN/GaN High-Electron-Mobility Transistors on a Nanometer Scale

We use depth-resolved cathodoluminescence spectroscopy (DRCLS), Kelvin probe force microscopy (KPFM), and surface photovoltage spectroscopy (SPS) on a nanometer scale to map the temperature, strain, and defects inside GaN high-electron-mobility transistors. DRCLS maps temperature at localized depths, particularly within the 2-D electron gas region during device operation. KPFM maps surface electric potential across the device, revealing lower potential patches that decrease rapidly with increasing off-state stress. CL spectra acquired at these patches exhibit defect emissions that increase with both on- and off-state stresses and that increase with decreasing surface potential. SPS also reveals features of deep level gap states generated after device operation that reduce near-band-edge emission and increase surface band bending. Our nanoscale measurements are consistent with defect generation by inverse piezoelectric field-induced stress at the gate edge on the drain side at high voltage.

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.