J. Zeller

AlGaN/InGaN/GaN blue light emitting diode degradation under pulsed current stress

This study focused on the performance of commercial AlGaN/InGaN/GaN blue light emitting diodes (LEDs) under high current pulse conditions. The results of deep level transient spectroscopy (DLTS), thermally stimulated capacitance, and admittance spectroscopy measurements performed on stressed devices, showed no evidence of any deep‐level defects that may have developed as a result of high current pulses. Physical analysis of stressed LEDs indicated a strong connection between the high intrinsic defect density in these devices and the resulting mode of degradation.

Degradation of blue AlGaN/InGaN/GaN LEDs subjected to high current pulses

Short-wavelength, visible-light emitting optoelectronic devices are needed for a wide range of commercial applications, including high-density optical data storage, full-color displays, and underwater communications. In 1994, high-brightness blue LEDs based on gallium nitride and related compounds (InGaN/AlGaN) were introduced by Nichia Chemical Industries. The Nichia diodes are 100 times brighter than the previously available SiC blue LEDs. Group-III nitrides combine a wide, direct bandgap with refractory properties and high physical strength. So far, no studies of degradation of GaN based LEDs have been reported. Our study, reported in this paper, focuses on the performance of GaN LEDs under high electrical stress conditions. Our observations indicate that, in spite of a high defect density, which normally would have be fatal to other III-V devices, defects in group-III nitrides are not mobile even under high electrical stress. Defect tubes, however, can offer a preferential path for contact metals to electromigrate towards the p-n junction, eventually resulting in a short. The proposed mechanism of GaN diode degradation raises concern for prospects of reliable lasers in the group-III nitrides grown on sapphire.

Ultrafast Processes in Highly Excited Wide-Gap Dielectric Thin Films

The response of wide-gap dielectric oxide materials to high-intensity femtosecond laser pulses is studied. The scaling of the threshold for dielectric breakdown with pulse duration and bandgap in the sub-picosecond regime was investigated using thin oxide films. A phenomenological model can explain the observations as the interplay of multiphoton ionization, impact ionization, and carrier relaxation. Transient reflection and transmission measurements were performed to monitor the temporal evolution of the dielectric constant that reflect processes in the excited electron gas and the lattice dynamics at excitation densities close to breakdown. The time-dependent dielectric constant is retrieved from the reflection and transmission data using an algorithm that takes into account standing waves formed by pump and probe in the film. A theoretical description based on the solution of the Boltzmann equation for the electron and phonon system after excitation including the formation of self-trapped excitons is presented. The contribution of individual scattering processes to the material response is studied and compared to the experimental data.

Spectro-temporal characterization of a femtosecond white-light continuum by transient-grating diffraction

An optically induced transient grating in a wide-bandgap dielectric is used to characterize the dispersion of a femtosecond white-light continuum over a 700 nm spectral range and with a time resolution of approximately 20 fs. With this technique, the dependence of the continuum dispersion on the focusing into the nonlinear material is investigated. Implications for laser spectroscopy experiments with continuum pulses are discussed.

Femtosecond pulse damage behavior of oxide dielectric thin films

Pulse duration and band-gap scaling of the laser breakdown threshold fluence of oxide dielectrics were measured using various (TiO2, Ta2O5, HfO2, Al2O3, and SiO2) single layer thin films. The observed scaling with pulse duration was explained by an empirical model including multi-photon and avalanche ionization, and conduction band electron decay. The results suggest the formation of self-trapped excitons on a sub-ps time-scale, which can cause significant energy transfer to the lattice. At constant pulse duration, the band-gap scaling was found to be approximately linear. This linear scaling can be explained by the Keldysh photo-ionization theory and avalanche ionization in the flux-doubling approximation.

Anomalous nonlinear photoresponse in a InGaN/GaN heterostructure

The nonlinear (third to fourth order) as well as linear photoconductivity in a Gallium nitride/Indium-Gallium nitride (GaN/InGaN) heterostructure is investigated using femtosecond pulses in the infrared (IR) and near ultraviolet (UV). An anomalous IR photoresponse is explained by a four level model for the GaN region including defect density fluctuations and nonlinear carrier transport phenomena. The same model also explains the observed subpicosecond noninstantaneous IR response of the photodetector. The linear UV photoresponse originates in the InGaN region. Design guidelines for GaN-based nonlinear photodetectors used in autocorrelation measurements are suggested.

Femtosecond Nonlinear Microscopy of Photodetectors

Diffraction and bandwidth limited pulses from 10 to 100 femtoseconds in the focal plane of microscope objectives are used to perform multiphoton current and luminescence microscopy of nonlinear photodetectors. Information on material parameters and the electric field distribution is obtained.

Electrical Properties of Nichia AlGaN/InGaN/GaN Blue LEDs in a Wide Current/Temperature Range

Studies of electrical characteristics of nitride-based light emitting diodes (LEDs) are of interest as they can shed light on carrier transport across the p-n heterojunction. In addition, they provide a convenient way of investigating degradation processes associated with high electrical stress. We present electrical characteristics of Nichia NLPB500 blue LEDs based on AlGaN/InGaN/GaN material system in the temperature range of 9-340 K. Two components of current are identified. High-density current stress leads to diode degradation by shunt formation caused by metal electromigration. At 8 K, blue emission was observed at currents as small as 20 nA.

Femtosecond breakdown and pre-breakdown behavior in thin dielectric films

Dielectric oxide and fluoride films used for optical coatings are studied with femtosecond laser pulses with respect to their breakdown and pre-breakdown behavior. A phenomenological model with only three figures of merit is used to explain the measured breakdown thresholds for pulse durations from 25 fs to 1 ps. The temporal evolution of the dielectric constant in the pre-breakdown regime is obtained from transient reflection and transmission measurements after taking into account standing wave effects of pump and probe. In addition to electron-electron and electron-phonon scattering processes, the creation of a new sample state after a few hundred fs is observed. The experimental data are explained with a computer simulation based on the Boltzmann equation.

Femtosecond dynamics of highly excited dielectric thin films

Highly excited wide-gap dielectrics are well suited to study the evolution of non-equilibrium conduction band electrons in solids. Compared to semiconductors, the measurable quantities in femtosecond pump-probe experiments – transient reflection (R) and transmission (T) – are less affected by processes such as gap shrinkage and band filling, simplifying the data interpretation. The measurements were performed on three oxides (TiO2, Ta2O5 and HfO2) thin films (~ 500 nm) with different band gaps (3.3 eV, 3.8 eV, 5.1 eV). Pulses at 800 nm and 25 fs duration excited the samples to ~60% of the threshold for dielectric breakdown. Besides their technical importance as optical coatings, we chose thin films to avoid self-focusing. The difficulty when working with thin films is the presence of standing waves (Fabry-Perot effects) of pump and probe, strongly influencing the transmission and reflection behavior after excitation. An algorithm to retrieve the change of the complex dielectric constant, ∆e(t)= ∆eR(t) − i∆e(t), from measured ∆R(t) and ∆T(t) data was developed [1]. This algorithm calculates the optical response of the film due to the periodic excitation by the pump through multi-photon absorption and impact ionization. Detailed studies were performed to characterize and maximize the stability of the algorithm with respect to experimental uncertainties. Figure 1 shows the so obtained time dependent dielectric function for two samples.