Simona Podda

High brightness GaN LEDs degradation during dc and pulsed stress

This paper reports an analysis of InGaN/GaN LEDs degradation under dc and pulsed current conditions. The analysis was carried out by means of current-voltage and optical power measurements, scanning electron microscopy and EDS maps. Identified degradation modes were leakage and generation/recombination current increase, series resistance increase and output power decrease, related to the high temperatures reached by the devices during stress. Failure analysis revealed degradation of anode contacts and Ti-W reflector, which can be related to the measured series resistance increase. Comparison between dc and pulsed stress carried out with the same average current indicated that pulsed driving does not imply an acceleration in the degradation rate, except for the lowest duty cycles.

Failure modes and mechanisms of DC-aged GaN LEDs

This paper presents failure modes observed in long-term aging of high-brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non-radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.

An Automatic Alignment Procedure for a Four-Source Photometric Stereo Technique Applied to Scanning Electron Microscopy

This paper presents an automatic alignment procedure for a four-source photometric stereo (PS) technique for reconstructing the depth map in the scanning electron microscope (SEM). PS, which is based on the so-called reflectance map, used several images of a surface to estimate the surface depth at each image point, in which the Lambertian reflectivity function is the simplest. In the SEM, the backscattered electron emission, which is one of the most important signals, is nearly Lambertian, and to simplify matters, SEM images are intrinsically grayscale maps. The possibility of having electron-PS at the SEM is assumed, taking advantage of one of the most exciting features of the technique, which returns true numerical 3-D models instead of some depth illusion from ordinary pictures.

Stability and performance evaluation of High Brightness Light Emitting Diodes under DC and pulsed bias conditions

This paper presents an experimental analysis of high brightness light emitting diodes (HBLEDs) performance and stability under dc and pulsed current bias. Three different families of HBLEDs from three leading manufacturers have been considered. The analysis was carried out by means of current-voltage, integrated optical power and electroluminescence measurements, and failure analysis. After an initial characterization of the electrical, optical and thermal behavior of the devices, a set of ageing tests was carried out, both under dc and pulsed bias conditions. Identified degradation modes were efficiency decrease, series resistance increase, leakage current increase, and modifications of the emitted spectrum. Characterization of devices behavior during stress indicated (i) generation of non-radiative components, (ii) degradation of the anode contacts and bonding wires, (iii) degradation of the phosphorous layer conversion efficiency and (iv) of the plastic package as possible responsible of the electrical and optical degradation of the LEDs. Comparison between dc and pulsed stress carried out using the same average current level and different duty cycle values showed that the use of pulsed bias can reduce the degradation rate with respect to dc bias. However, for duty cycles lower than 20%, fast degradation and abrupt ruptures can take place, due to the high peak current levels.

Image alignment for 3D reconstruction in a SEM

In a previous paper [1] a procedure to extract the third dimension from back-scattered electron has been proposed. The 3D recovery process involves the acquisition of four images and the computation of surface gradients; thus, a specific algorithm finds the depth map of the given surface. The image acquisition process may require or specific instrumentation (4-sector, independent channel axial BS detector), or very standard SEM configuration and specimen rotation/translation. In this last case an alignment procedure should be developed, which results troublesome because of the different image shadowing. The paper deals with an alignment method based on the parallel acquisition of the BS images and of their respective uniformly shaded twins obtained by the specimen current detector, or by the standard annular axial BS detector.

XEBIC at the Dual Beam

Abstract After its golden age during the decades of the development of Silicon IC technology, nowadays Electron Beam Induced Current (EBIC) technique comes back to importance mostly on photonic and high speed electron devices based on compound semiconductors. A new approach for recording a fast EBIC embedded in a Dual Beam system is presented. The combined use of multi-slicing FIB, SEM and EBIC enables 3D tomography of P–N junctions and drives TEM preparation in failure analysis studies of leaky junctions.

Improvements in automated Photometric Stereo 3D SEM

In a previous paper [1] an Automatic Alignment Procedure for a 4-Source Photometric Stereo (PS) technique has been proposed to reconstruct the third dimension in the Scanning Electron Microscope (SEM). This method, as opposed to the well known stereoscopy, returns true numerical 3-D models instead of some depth illusion from ordinary pictures. In brief the PSbased 3D recovery method consists of the acquisition of 4 BackScattered (BS) images, taken from the same viewpoint but under different lighting directions, from which the gradient of the surface and its “intrinsic brilliance” are first separately extracted using gray PS [2], and then the depth map is recovered by integration of the gradient vector field using the conjugate gradient method [3]. In standard equipped SEM it is necessary to rotate the specimen under the fixed detector and to sequentially acquire four images of the same specimen area upon imposing three 90° rotation steps on the specimen, and then rotating back the images. To align the images, the availability of isotropic shaded image is essential, so a twin set of images is acquired: one for alignment purposes and the other for 3D reconstruction. In this paper we use results of [1] to improve the performances of the automatic procedure so far limited to the acquisition and alignment of the image. The next natural step is to develop a completely automated tool that enables a standard instrument and an unskilful SEM user to access the 3D domain. The missing piece of the jigsaw is to automate the initial set up of the two detectors: 1) off-axis (BS) electron detector and 2) the available one that can produce isotropic shading, which may either be the Specimen Current detector or the complete circular axial BS detector. In Fig 1a and 1b the two types of images used for the 3D evaluation and the alignment procedure have been respectively shown: the first one shows the reflectance image, whereas the second one has accentuated brightness and contrast and is useful to alignment process. To preserve the depth information for 3D reconstruction, the off-axis BSE detector image needs to not have any level saturation but it must cover the highest histogram values range, so brightness and contrast must be properly set. The modern digital SEM have an oscilloscope (sometimes called videoscope) to visualize the signal excursion in the frame: it’s straightforward that, in order to have a perfect image, the signal must be included between the minimum (black) and the maximum (white) without saturation. On the contrary, the isotropic shadow image should has contrast variations in order to have well defined image patterns for the automated alignment procedure. Some examples are illustrated in Fig. 1 where the 3D reconstruction of a detail of a 1euro coin and of a microfossil have been shown. The automatic brightness and contrast set up of the two detectors leaves to the SEM operator the only duty of choosing the interested area and of setting focus and magnification, daily routine operations for any microscope user.

3D Sculptures From SEM Images

In a general framework for exploiting the automation capabilities in modern SEM, two main branches has been investigated: the web-based remote microscopy [1] and the 3D metrology by Photometric Stereo (PS). Focusing on this second topic, in a previous paper [2] an Automatic Alignment Procedure for a 4-Source Photometric Stereo (PS) technique was presented for metrically reconstructing the third dimension in the Scanning Electron Microscope (SEM). The method, developed on a ESEM Quanta FEI-200 platform [3] equipped by standard ETH and solid state annular BSE detectors, has been completely automated, and made easily implemented on any standard equipped SEM.

SEM Remote Control with a 3D Option

The remote control of a scientific instrument is a topic gaining more and more attention between the instrument users and operators. The project presented in this article starts from the mix of two separated research works and evolves first obtained results. The mixed works are the realization of an application to remote-control Scanning Electron Microscope (SEM), and the implementation of a procedure to reconstruct 3D surfaces.