Two-photon optical-beam-induced current imaging of indium gallium nitride blue light-emitting diodes

An insight into optical beam induced current microscopy: Concepts and applications

Laser scanning optical beam induced current (OBIC) microscopy has become a powerful and nondestructive alternative to other complicated methods like electron beam induced current (EBIC) microscopy, for high resolution defect analysis of electronic devices. OBIC is based on the generation of electron-hole pairs in the sample due to the raster scanning of a focused laser beam with energy equal or greater than the band gap energy and synchronized detection of resultant current profile with respect to the beam positions. OBIC is particularly suitable to localize defect sites caused by metal-semiconductor interdiffusion or electrostatic discharge (ESD). OBIC signals, thus, are capable of revealing the parameters/factors directly related to the reliability and efficiency of the electronic device under test (DUT). In this review, the basic principles of OBIC microscopy strategies and their notable applications in semiconductor device characterization are elucidated. An overview on the developments of OBIC microscopy is also presented. Specifically, the recent progresses on the following three OBIC measurement strategies have been reviewed, which include continuous laser based single photon OBIC, pulsed laser based single photon OBIC, and multiphoton OBIC microscopy for three-dimensional mapping of photocurrent response of electronic devices at high spatiotemporal resolution. Challenges and future prospects of OBIC in characterizing complex electronic devices are also discussed.

Methanol immersion reduces spherical aberration of water dipping lenses at long wavelengths used in multi-photon laser scanning microscopy

Dipping objectives were tested for multi-photon laser scanning microscopy, since their large working distances are advantageous for thick specimens and the absence of a coverslip facilitates examination of living material. Images of fluorescent bead specimens, particularly at wavelengths greater than 850 nm showed defects consistent with spherical aberration. Substituting methanol for water as the immersion medium surrounding the beads corrected these defects and produced an increase in fluorescence signal intensity. The same immersion method was applied to two representative biological samples of fixed tissue: mouse brain labeled with FITC for tubulin and mouse gut in which the Peyer's patches were labeled with Texas Red bilosomes. Tissue morphology was well preserved by methanol immersion of both tissues; the two-photon-excited fluorescence signal was six times higher than in water and the depth of penetration of useful imaging was doubled. No modification of the microscope was needed except the provision of a ring to retain a sufficient depth of methanol for imaging.

Autoconfocal transmission microscopy based on two-photon-induced photocurrent of Si photodiodes

We describe a simple, self-aligned confocal transmission microscopy technique based on two-photon-induced photocurrents of silicon photodiodes. Silicon detectors produce photocurrents in quadratic dependence on incident intensity under the pulsed illumination of light with wavelengths longer than 1.2 microm. We exploit this nonlinear process to reject out-of-focus background and perform depth-sectioning microscopic imaging. We demonstrate a comparable background rejection capability of the technique to linear confocal detection and present three-dimensional imaging in biological specimens.

Tracking the emergence of defect in light emitting semiconductor diodes with two-photon excitation microscopy and spectral microthermography

We demonstrate a multifunctional optical technique for tracking the evolution of defects in live 605 nm LEDs. Photocurrent images, electroluminescence, and spectral reflectance maps are simultaneously acquired and utilized to evaluate LED performance at different injection currents. Free-carrier density profiles in the active region are constructed from photocurrent images that are generated via two-photon excitation (2PE) at 800 nm. A device defect is induced by electrical stress and ripples are observed in the density distribution by 2PE microscopy. The microscopic stress patterns are not revealed with linear excitation. We investigate the local thermal activity in the active region by measuring the spectral reflectance change with injection current. Spectral unmixing separates the electroluminescence and reflectance signals and high-resolution background-free thermal maps are derived to determine the device operational limits and possible connections between structural defect and thermal activity.

The use of optical parametric oscillator for harmonic generation and two-photon UV fluorescence microscopy

Ultrafast lasers have found increasing use in scanning optical microscopy due to their very high peak power in generating multiphoton excitations. A mode-locked Ti:sapphire laser is often employed for such purposes. Together with a synchronously pumped optical parametric oscillator (OPO), the spectral range available can be extended to 1,050-1,300 nm. This broader range available greatly facilitates the excitation of second harmonic generation (SHG) and third harmonic generation (THG) due to better satisfaction of phase matching condition that is achieved with a longer excitation wavelength. Dental sections are then investigated with the contrasts from harmonic generation. In addition, through intra-cavity doubling wavelengths from 525-650 nm are made available for effective two-photon (2-p) excitation with the equivalent photon energy in the UVB range (290-320 nm) and beyond. This new capacity allows UV (auto-) fluorescence excitation and imaging, for example, from some amino acids, such as tyrosine, tryptophan, and glycine.

High-contrast images of semiconductor sites via one-photon optical beam-induced current imaging and confocal reflectance microscopy

We demonstrate a computationally efficient procedure for determining only the semiconductor sites in a confocal reflectance image of an integrated circuit. It utilizes a one-photon optical beam-induced current (1P-OBIC) and confocal reflectance images that are generated from the same focused excitation beam. A 1P-OBIC image is a two-dimensional map of the currents induced by the beam as it is scanned across the circuit surface. A 1P-OBIC is produced by an illuminated semiconductor material if the excitation photon energy exceeds the bandgap. The 1P-OBIC image has no vertical resolution because the 1P-OBIC is linear with the excitation beam intensity. The exclusive high-contrast image of semiconductor sites is generated by the product of the 1P-OBIC image and the confocal image. High-contrast images of the metal sites are also obtained by the product of the complementary OBIC image and the same confocal image.

Three-dimensional observation of internal defects in semiconductor crystals by use of two-photon excitation

We present a method of three-dimensional observation of internal defects in semiconductor crystals for blue lasers by use of two-photon excitation. We excite photoluminescence by using a two-photon process. Since semiconductor materials have intrinsically high absorption in the short-wavelength region, the excitation light of photoluminescence is largely absorbed by the crystals. It is difficult to observe defects in deep regions. Two-photon excitation can overcome this limitation because near-infrared light can be used instead of blue light for excitation of photoluminescence, and the near-infrared light is absorbed at only the focused point. The excitation light can penetrate into the deep regions of the crystal. We succeeded in observing defects in a ZnSe crystal ~200 microm below the crystal surface. This is, as far as we know, the first demonstration of the observation of three-dimensional structures of defects in semiconductor crystals by the use of two-photon excitation.

Multi-photon fluorescence microscopy--the response of plant cells to high intensity illumination

Multi-photon fluorescence microscopy has been cited for its advantage in increased depth penetration due to low linear absorption and scattering coefficient of biological specimen in the near infrared (NIR) range. Because of the need of high peak power for efficiently exciting two-photon fluorescence, the relationship between cell damage and peak power has become an interesting and much debated topic in the applications of multi-photon fluorescence microscopy. It is conceivable that at high illumination intensity, non-linear photochemical processes have impacts on cell physiology and viability in ways much different from low illumination in the linear domain. In this article, we discuss some of the issues in two-photon fluorescence microscopy, including the degree of transparency of the specimen, a comparison of single- and two-photon excited fluorescence spectra, and the cell damage under high intensity illumination, using plant cells as a model.