Volkmar Dierolf

Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

Optimization of internal quantum efficiency (IQE) for InGaN quantum wells (QWs) light-emitting diodes (LEDs) is investigated. Staggered InGaN QWs with large electron-hole wavefunction overlap and improved radiative recombination rate are investigated for nitride LEDs application. The effect of interface abruptness in staggered InGaN QWs on radiative recombination rate is studied. Studies show that the less interface abruptness between the InGaN sub-layers will not affect the performance of the staggered InGaN QWs detrimentally. The growths of linearly-shaped staggered InGaN QWs by employing graded growth temperature grading are presented. The effect of current injection efficiency on IQE of InGaN QWs LEDs and other approaches to reduce dislocation in InGaN QWs LEDs are also discussed. The optimization of both radiative efficiency and current injection efficiency in InGaN QWs LEDs are required for achieving high IQE devices emitting in the green spectral regime and longer.

Growths of staggered InGaN quantum wells light-emitting diodes emitting at 520–525 nm employing graded growth-temperature profile

Three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520–525 nm were grown by metal-organic chemical vapor deposition by employing graded growth-temperature profile. The use of staggered InGaN QW, with improved electron-hole wave functions overlap design, leads to an enhancement of its radiative recombination rate. Both cathodoluminescence and electroluminescence measurements of three-layer staggered InGaN QW LED exhibited enhancements by 1.8–2.8 and 2.0–3.5 times, respectively, over those of conventional InGaN QW LED.

Enhanced room-temperature luminescence efficiency through carrier localization in AlxGa1−xN alloys

AlGaN samples grown by plasma-assisted molecular-beam epitaxy on sapphire (0001) substrates, with 20%–50% Al content and without the use of indium, show intense room-temperature photoluminescence that is significantly redshifted, 200–400meV, from band edge. This intense emission is characterized by a long room-temperature lifetime (∼375ps) comparable to that seen in low defect density (∼108cm−2) GaN. Room-temperature monochromatic cathodoluminescence images at the redshifted peak reveal spatially nonuniform emission similar to that observed in In(Al)GaN alloys and attributed to compositional inhomogeneity. These observations suggest that spatial localization enhances the luminescence efficiency despite the high defect density (>1010cm−2) of the films by inhibiting movement of carriers to nonradiative sites.

Characterization of new erbium-doped tellurite glasses and fibers

Tellurite glasses are promising candidates for optical fiber laser and amplifier applications because of their excellent optical and chemical properties. The emission spectrum from erbium in tellurite glasses is almost twice as broad as the corresponding spectrum in silica. In this presentation, we report the results of a two-prong investigation of new tellurite glasses: a Raman study that provides detail information on the microscopic structure of these glasses, and a study of the erbium emission in fibers fabricated from these glasses. Specifically, we report on the emission from fibers of different lengths and for different pumping schemes.

Three-dimensional grain boundary spectroscopy in transparent high power ceramic laser materials.

Using confocal Raman and fluorescence spectroscopic imaging in 3-dimensions, we show direct evidence of inhomogeneous Nd(3+) distribution across grain boundaries (GBs) in Nd(3+):YAG laser ceramics. It is clearly shown that Nd(3+) segregation takes place at GBs leading to self-fluorescence quenching which affects a volume fraction as high as 20%. In addition, we show a clear trend of increasing spatial inhomogeneities in Nd(3+) concentration when the doping levels exceeds 3 at%, which is not detected by standard spectrometry techniques. These results could point the way to further improvements in what is already an impressive class of ceramic laser materials.

Directionally controlled 3D ferroelectric single crystal growth in LaBGeO5 glass by femtosecond laser irradiation

Laser-fabrication of complex, highly oriented three-dimensional ferroelectric single crystal architecture with straight lines and bends is demonstrated in lanthanum borogermanate model glass using a high repetition rate femtosecond laser. Scanning micro-Raman microscopy shows that the c-axis of the ferroelectric crystal is aligned with the writing direction even after bending. A gradual rather than an abrupt transition is observed for the changing lattice orientation through bends up to approximately 14 degrees. Thus the single crystal character of the line is preserved along the bend through lattice straining rather than formation of a grain boundary.

Continuous-wave optical parametric terahertz source

Here, we present a continuous-wave optical parametric terahertz light source that does not require cooling. It coherently emits a diffraction-limited terahertz beam that is tunable from 1.3 to 1.7 THz with power levels exceeding 1 microW. Simultaneous phase matching of two nonlinear processes within one periodically-poled lithium niobate crystal, situated in an optical resonator, is employed: The signal wave of a primary parametric process is enhanced in this resonator. Therefore, its power is sufficient for starting a second process, generating a backwards traveling terahertz wave. Such a scheme of cascaded processes increases the output power of a terahertz system by more than one order of magnitude compared with non-resonant difference frequency generation due to high intracavity powers. The existence of linearly polarized terahertz radiation at 1.35 THz is confirmed by analyzing the terahertz light with metal grid polarizers and a Fabry-Pérot interferometer.

Direct-write method for domain inversion patterns in LiNbO3

We have developed a technique to directly write stable ferroelectric domain patterns into lithium niobate crystals using visible light from an argon laser tightly focused to a diffraction limited spot in a confocal luminescence microscope. The patterns produced exhibit feature sizes down to 2 μm and show very little strain. The coercive field was reduced by as much as 30%. From the dynamics of the process, we attribute the reduction in coercive field to space charge fields produced by photoionization of defects. We have ruled out the possibility that the reduction in coercive field is simply a result of increased temperature by making local temperature measurements using the Stokes/anti-Stokes intensity ratio in confocal Raman measurements.

Site-selective spectroscopy of Er in GaN

We investigated different Er3+ defect sites found in Er-doped GaN layers by site-selective combined excitation-emission spectroscopy and studied the role of these sites in different direct and multistep excitation schemes. The layers were grown by molecular beam epitaxy and were 200 nm thick. Two majority sites were found along with several minority sites. The sites strongly differ in excitation and energy transfer efficiencies as well as branching ratios during relaxation. For this reason, relative emission intensities from these sites depend strongly on emission and excitation. The sites were identified for several transitions and a comprehensive list of energy levels has been compiled. One of the minority sites appears strongly under ultraviolet excitation above the GaN band gap suggesting that this site is an excellent trap for excitation energy of electron-hole pairs.

Excitation of Eu3+ in gallium nitride epitaxial layers: Majority versus trap defect center

We report studies of the excitation mechanism of Eu ions in situ doped during organometallic vapor-phase epitaxy (OMVPE) of GaN. We find that the bright red emission under above-band gap excitation originates primarily from an incorporation site that exhibits high excitation efficiency but occurs in low relative abundance ( 97%).