Depth-resolved Cathodoluminescence Research Articles

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 ROP modulation of native defects related to Zn or O vacancies.

Boosting Green GaInN/GaN Light-Emitting Diode Performance by a GaInN Underlying Layer

The light output of 530 nm green GalnN/GaN light-emitting diodes on sapphire has been nearly doubled by the insertion of a 130-nm GalnN underlayer (UL) between the n-GaN electron injection layer and the quantum-well (QW) active region. Under variation of the alloy composition, best results were obtained for an x = 6.3% Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> N UL. By low-temperature depth-resolved cathodoluminescence spectroscopy, an interplay of the impurity-related donor-acceptor pair recombination, the UL, and the QW emission has been observed. We propose that the resonance and level alignments between the defect and UL levels reroute excitation toward radiative recombination in the QWs.

Vacancy defect and defect cluster energetics in ion-implanted ZnO

We have used depth-resolved cathodoluminescence, positron annihilation, and surface photovoltage spectroscopies to determine the energy levels of Zn vacancies and vacancy clusters in bulk ZnO crystals. Doppler broadening-measured transformation of Zn vacancies to vacancy clusters with annealing shifts defect energies significantly lower in the ZnO band gap. Zn and corresponding O vacancy-related depth distributions provide a consistent explanation of depth-dependent resistivity and carrier-concentration changes induced by ion implantation.

Depth-resolved cathodoluminescence study of zinc oxide nanorods catalytically grown on p-type 4H-SiC

Optical properties of ZnO nanorods (NRs) grown by vapour–liquid–solid (VLS) technique on 4H-p-SiC substrates were probed by cathodoluminescence (CL) measurements at room temperature and at 5 K complemented with electroluminescence. At room temperature the CL spectra for defect related emission intensity was enhanced with the electron beam penetration depth. We observed a variation in defect related green emission along the nanorod axis. This indicates a relatively poor structural quality near the interface between ZnO NRs and p-SiC substrate. We associate the green emission with oxygen vacancies. Analysis of the low-temperature (5 K) emission spectra in the UV region suggests that the synthesized nanorods contain shallow donors and acceptors.

Diffusion synthesis and electronic properties of Fe-doped ZnO

Fe-doped ZnO was successfully fabricated by thermal in-diffusion of Fe into ZnO crystals. X-ray absorption near edge structure (XANES), photoemission and cathodoluminescence (CL) spectroscopy have been combined to examine the Fe diffusion and its effects on the electronic and optical properties of the crystal. Depth-resolved CL demonstrates that Fe in-diffusion occurs to at least 4 μm depth and results in intense green luminescence, whereas the undoped crystal exhibits only the ZnO near-band-edge emission. XANES and valence-band photoemission show that Fe is incorporated as Fe 2+/3+ ions on substitutional Zn sites. The results suggest that the variation in the CL properties is due to a change in the oxygen vacancy charge state as a result of electron transfer from Fe.

Depth profile of donor–acceptor pair transition revealing its effect on the efficiency of green LEDs

GaInN/GaN light emitting diodes are the primary choice for efficient green light emitters, yet their performance limiting factors have yet to be identified. Here we perform a low-temperature luminescence study in depth-resolved cathodoluminescence. A series of LEDs with a combination of blue and green light emitting quantum wells exhibiting low and high electroluminescence output power were compared. In all samples, a band of donor-acceptor pair recombination was identified at 77 K. In the samples with lowest performance, such transitions located within the active region of the quantum wells had a particular strong contribution to the spectrum. In LEDs of higher performance, such luminescence was substantially suppressed within those layers. We argue that the higher performance of LEDs without such donor acceptor transition bands may be associated with the absence of corresponding dopant impurities and indicate a higher epitaxial perfection within the active quantum well region.

Massive point defect redistribution near semiconductor surfaces and interfaces and its impact on Schottky barrier formation

Abstract Nanoscale depth-resolved cathodoluminescence spectroscopy calibrated with deep level transient spectroscopy of native point defects and capacitance–voltage measurements of free carrier densities, all at the same metal–semiconductor interface, demonstrate that native point defects can (i) increase by order-of-magnitude in densities with tens of nanometers of the semiconductor surface, (ii) alter free carrier concentrations and band profiles with the surface space charge regions, and (iii) dominate the Schottky barrier formation for metal contacts to ZnO and many other single crystal compound semiconductors. The spatial redistribution of electrically active defects within the surface space charge can be understood in terms of temperature-dependent atomic diffusion enabled by low formation energies and driven by strain and electric fields as well as metal-specific chemical reactions near room temperature, consistent with first-principles calculations of interfacial segregation and migration barriers. These results underscore the importance of native point defects in charge transport and barrier formation at semiconductor interfaces.

Surface, bulk, and interface electronic states of epitaxial BiFeO3 films

The authors report on the depth-resolved cathodoluminescence spectroscopy studies of the surface, bulk, and interface-localized electronic states in the band gap of epitaxial BiFeO3 thin films. The BiFeO3 films show a near band edge emission at 2.7 eV and defect emissions at energies varying from 2.0 to 2.5 eV. The overall results clearly suggest that the electronic structure, especially the defect states and their spatial distributions, of BiFeO3 films are strongly dependent on the growth conditions and method, stoichiometry, and strain, so that understanding and controlling them are crucial to optimize BiFeO3 film properties.

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 barrier for Au compared with O-face. A comprehensive model accounts for the metal-and polarity-dependent transport properties.

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.

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.

Cathodoluminescence Properties of ZnO Tower-Like Structures Prepared by Thermal Oxidation

ZnO tower-like structures with hexagonal cross section were fabricated by a thermal oxidation method. Spatially resolved cathodoluminescence (CL) intensity images were employed to correlate the luminescent properties with the investigated structure. Local CL monochromatic images revealed that the bodies of ZnO tower-like structures were the major source for a strong green emission. The depth-resolved UV and green CL emissions were examined by changing the accelerating voltage. A part of reabsorbed UV emission is converted into green emission and partly contributes the enhancement of green emission. [DOI: 10.1380/ejssnt.2009.358]

Nanoscale depth-resolved cathodoluminescence spectroscopy of ZnO surfaces and metal interfaces

The electronic properties of ZnO surfaces and interfaces has until recently been relatively unexplored. We have used a complement of ultrahigh vacuum scanning electron microscope (SEM)-based, depth-resolved cathodoluminescence spectroscopy (DRCLS), temperature-dependent charge transport, trap spectroscopy, and surface science techniques to probe the electronic and chemical properties of clean surfaces and interfaces on a nanometer scale. DRCLS reveals remarkable nanoscale correlations of native point defect distributions with surface and sub-surface defects calibrated with capacitance trap spectroscopies, atomic force microscopy, and Kelvin probe force microscopy. The measurement of these near-surface states associated with native point defects in the ZnO bulk and those induced by interface chemical bonding is a powerful extension of cathodoluminescence spectroscopy that provides a guide to understanding and controlling ZnO electronic contacts.

Cathodoluminescence microanalysis of diamond nanocrystals in fused silicon dioxide

MeV carbon ion implantation followed by thermal annealing in a hydrogen-containing atmosphere produces a layer of diamond nanocrystals within fused quartz (SiO2). Cathodoluminescence (CL) microanalysis in a scanning electron microscope has revealed at least three previously unreported low intensity CL emissions from carbon implanted and thermally annealed fused SiO2. The CL emissions are observed at 2.78 eV [full width at half maximum (FWHM) of 0.08 eV], ∼3 eV (FWHM of 0.4 eV), and 3.18 eV (FWHM of 0.11 eV). The peak widths and energies of these emissions are incompatible with any known defects associated with the silicon dioxide host lattice. Nondestructive depth resolved CL microanalysis investigations confirm that these CL emissions originate from the near-surface region, consistent with their association with the layer of diamond nanocrystals.

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 cathodoluminescence spectroscopy on the same samples, which reveals an increase in the intensity of this band within ∼100 nm of the Pd/ZnO interface.

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 study of defects in SrTiO3

The authors report a depth-resolved cathodoluminescence spectroscopy study of defects and their distributions in SrTiO3 single crystals and epilayers. In SrTiO3 single crystals, the dominant defects are oxygen vacancies that locate mainly near the free surface, while Ti interstitials locate further into the bulk. Vacuum annealing increases the density of oxygen vacancies at SrTiO3 surfaces but reduces the density of Ti interstitials located deeper. In epilayers, the density and distribution of the defects depend on the film thickness and stoichiometry. The results reveal a strong dependence of SrTiO3 native point defects and their depth distributions on epitaxial growth and process conditions.

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 subsurface electrical changes are consistent with DRCLS features and emphasize the contribution of surface morphology to electrically active interface defects. The relative strength of near band edge to deep level defect emissions exhibit a threshold dependence on surface roughness.

Theoretical and experimental depth-resolved cathodoluminescence microanalysis of excitonic emission from ZnO epilayers

In this letter, the excitonic emissions from ZnO epilayers were studied by depth-resolved cathodoluminescence (CL) microanalysis. An excellent agreement between the redshift of the experimental depth-resolved CL emission and theoretically simulated values was observed, which clearly identified its origin to be the strong internal absorption in ZnO epilayer. Moreover, the rigorous Monte Carlo simulated CL generation profiles with the correction of self-absorption exhibit the reasonable correspondence with the measured CL intensities. The intensity discrepancies for low and high excitation cases have been interpreted by the occurrence of the excess carrier nonradiative recombination on the surface space charge layer and the exciton nonradiative quenching processing by defects or structural disorders near the interface of ZnO and sapphire.

Surface and near-surface passivation, chemical reaction, and Schottky barrier formation at ZnO surfaces and interfaces

Using a combination of depth-resolved cathodoluminescence spectroscopy, electronic transport, and surface science techniques, we have demonstrated the primary role of native defects within ZnO single crystals as well as native defects created by metallization on metal–ZnO Schottky barrier heights and their ideality factors. Native defects and impurities resident within the ZnO depletion region as well as defects extending into the bulk from the intimate metal–ZnO interface contribute to barrier thinning of, carrier hopping across, and tunneling through these Schottky barriers. Chemical reactions at clean ZnO–metal interfaces lead to metal-specific eutectic or oxide formation with pronounced transport effects. These results highlight the importance of bulk crystal quality, surface cleaning, metal interaction, and post-metallization annealing for controlling Schottky barriers.