W.D. Herzog

Time-resolved photoluminescence studies of free and donor-bound exciton in GaN grown by hydride vapor phase epitaxy

Time-resolved photoluminescence (PL) spectroscopy was used to study the radiative recombination of free and donor-bound excitons in unintentionally doped GaN grown by hydride vapor phase epitaxy. Low temperature (4 K), time-integrated PL spectra identified the free exciton (A), the donor-bound exciton peak ∼6 meV below, and the acceptor-bound exciton ∼20 meV below the free exciton peak. A radiative recombination lifetime of 295 ps for the free exciton and 530 ps for donor-bound exciton were found at 4 K. The decay of the free exciton remained single exponential to room temperature, with an increase in lifetime to 530 ps, consistent with the thermal excitation of exciton states.

Beam divergence and waist measurements of laser diodes by near-field scanning optical microscopy

We demonstrate the use of near-field scanning optical microscopy (NSOM) for the measurement of the beam properties of single quantum well, graded index separate confinement heterojunction ridge laser diodes. Using NSOM, we measure the field intensity in the transverse plane at near field and as a function of distance from the facet. The divergence of the laser beam and the beam waists in vertical and lateral dimensions are directly measured and the astigmatism of the mode is determined. In the near field, we observe a nearly ideal Gaussian shape in the vertical dimension which is consistent with the beam divergence as measured in the far field. In the lateral dimension, the beam shape deviates from the ideal Gaussian since the mesa structure of the laser diode provides an effective step-index waveguide. The non-Gaussian structure of the mode is also observed in the beam divergence properties.

NEAR-FIELD OPTICAL BEAM INDUCED CURRENT MEASUREMENTS ON HETEROSTRUCTURES

We report near‐field optical beam induced current (NOBIC) measurements on semiconductor quantum well (QW) structures. A subwavelength fiber tip is coupled with a tunable laser source and scanned over a sample surface. The induced photocurrent reveals the compositional profile of quantum structures. Semiconductor QW structures were designed and fabricated by molecular beam epitaxy (MBE) to study the wavelength dependence and resolution capability of NOBIC. We demonstrated that the resolution of this technique strongly depends on the aperture size. For aperture sizes that allow for coupling of evanescent fields from the tip into the semiconductor as propagating fields, the resolution strongly depends on the excitation wavelength due to the variation of the optical penetration depth. For smaller apertures, the optical field remains evanescent in the semiconductor and resolution is essentially independent of the wavelength.

Near-field optical studies of semiconductor heterostructures and laser diodes

Near-field optical microscopy and spectroscopy is emerging as a powerful tool for the investigation of semiconductor structures. Tunable excitation combined with sub-wavelength resolution is providing an unprecedented level of detail on the local optical properties of semiconductor structures. Recent near-field optical studies have addressed issues of laser diode mode profiling, minority carrier transport, near-field photocurrent response of quantum-well structures and laser diodes, imaging of local waveguide properties, and location and studies of dislocations in semiconductor thin films. We present results on the intrinsic resolution limitations of near-field photoconductivity in quantum-well heterostructures and demonstrate that the resolution depends strongly on the amount of evanescent and propagating field components in the semiconductor. Spectroscopic mode-profiling of high-power laser diode emission details the spatial dependence of multiple spectral modes. This paper presents an overview of NSOM techniques for semiconductor systems, its limitations, and present status.

Beam steering in narrow-stripe high-power 980-nm laser diodes

We used near-field scanning optical microscopy to measure the optical beam characteristics of weakly-guided narrow-stripe high-power laser diodes. For both facets of the device, we correlate changes in the near-field optical characteristics with beam steering and the kink in the light output versus operating current curve. Our measurements demonstrate that: 1) frequency-locking of lateral modes exists for operating currents below the kink in the L-I curve; 2) beam steering is a result of a shift of in the beat-length pattern inside the laser cavity of the frequency-locked lateral modes; and 3) the kink in the L-I curve results from a sudden increase in the beam waist of the guided-modes.

Photoluminescence microscopy of InGaN quantum wells

Submicron spatial resolution photoluminescence is used to assess radiative efficiency and spatial uniformity of GaN/InGaN heterojunctions. Room temperature photoluminescence of multiple InGaN quantum wells with GaN barriers fabricated by electron-cyclotron resonance assisted molecular beam epitaxy was measured as a function of position on a facet perpendicular to the layer structure. Our high resolution studies reveal that the radiative recombination for the InGaN quantum wells is 50–60 times more efficient than for the underlying GaN film.

Spectroscopy of competing mechanisms generating stimulated emission in gallium nitride

Two competing recombination mechanisms of stimulated emission in the vicinity of 145 K have been directly observed in the temperature dependence of the optical emission spectra for high-quality, unintentionally doped gallium nitride. Our analysis of the spectra indicates that exciton-exciton scattering is responsible for stimulated emission below 145 K, while at higher temperatures an electron-hole plasma becomes the dominant mechanism.

Thermal imaging by infrared near-field microscopy

Heat dissipation is a major concern in integrated circuit (IC) design. Mapping the temperature distribution of ICs plays a very important role in detecting fabrication and material defects and also verifying the temperature distribution predicted by thermal simulations. We describe a non-invasive, affordable and easy to maintain thermal imaging system capable of measuring devices under normal operating conditions. The operating principle of our thermal imaging system is based on the application of near-field scanning optical microscopy (NSOM) to IR thermography.

Characterization of Materials and Devices by Near-Field Scanning Optical Microscopy

Near field scanning optical microscopy (NSOM) is a recent technique where a tapered single-mode optical fiber probe is scanned over a sample surface at a height of a fraction of the wavelength. The tapered fiber provides a tiny aperture ( a , ˜ 70nm) through which light is coupled and can yield resolutions as high as ˜, λ/40. We have used both room and low-temperature NSOM to study the local spectroscopic characteristics of a wide variety of material systems, from quantum dots and wires, to ordered GaInP, to heterojunctions and optoelectronic devices. Low temperature near-field photoluminescence spectroscopy was used to study spectral emission maps of a set of samples of GaInP epilayers with varying degrees of ordering. The samples exhibit two peaks, a low energy (LE) and a high energy (HE) peak. Our data are inconsistent with expectations that the LE peak is due to emission from domain boundaries and alternative models will be discussed. NSOM spectral maps can yield information about the spatial dependence of the local optical matrix elements. NSOM data on the emission mode structure of strained (In, Ga)As quantum well lasers has yielded new information on the source kinks in the light response at high currents, while local photocurrent spectroscopy using the tip as a point source of photons provides analysis of the semiconductor layer composition.

Beam steering in narrow-stripe, high-power 980 nm laser diodes

High-power, 980 nm laser diodes are currently used to optically pump erbium-doped fiber-amplifiers for telecommunication systems. In this capacity, the laser diode must reliably provide high output power in a stable optical beam with efficient power coupling to the waveguide-mode of the fiber amplifier. To this end, narrow-stripe devices capable of single spatial mode operation are utilized. These devices typically employ weak lateral guiding to ensure single mode operation while maximizing the spatial size of the fundamental mode to reduce the peak power density and thus prevent catastrophic optical damage on the output facet. We employed near-field scanning optical microscopy (NSOM) to image the output of a 980 nm, graded-index, separate-confinement, heterostructure (GRINSCH) laser diode that exhibits a kink in the light versus current (L-I) curve and lateral beam steering. Measurements were performed on the output of both the antireflection and high-reflection coated facets. We correlate the NSOM images of the laser near-field with far-field measurements of beam-steering as a function of the laser diode operating current with an emphasis on the behavior at around the L-I kink.