Jason C. Verley

Roughening transition in nanoporous hydrogenated amorphous germanium: Roughness correlation to film stress

Hydrogenated amorphous germanium (a-Ge:H) is a material of interest for optoelectronic applications such as solar cells and radiation detectors because of the material’s potential to extend the wavelength sensitivity of hydrogenated amorphous silicon (a-Si:H). An increase in porosity is observed in amorphous germanium compared to a-Si:H, and this increase in porosity has been correlated with a degradation of the electrical performance. Improved understanding of the mechanisms of porous formation in a-Ge:H films is therefore desirable in order to better control it. In this paper we describe a correlation between film stress and surface roughness, which evolves with increasing thickness of a-Ge:H. A roughening transition from planar two-dimensional growth to three-dimensional growth at a critical thickness less than 800A results in a network of needlelike nanotrench cavities which stretch from the transition thickness to the surface in films up to 4000A thick. Surface roughness measurements by atomic force ...

Fabrication and characterization of large-area 3D photonic crystals

Full bandgap (3D) photonic crystal materials offer a means to precisely engineer the electromagnetic reflection, transmission, and emission properties of surfaces over wide angular and spectral ranges. However, very few 3D photonic crystals have been successfully demonstrated with areas larger than 1 cm2. Large sheets of photonic bandgap (PBG) structures would be useful, for example, as hot or cold mirrors for passively controlling the temperature of satellites. For example, an omni-directional 3D PBG structure emitting only at wavelengths shorter than 8 microns radiates only 7% of what a black body would at 200degK while radiating more than 40% at 400degK. 3D PBG materials may also find application in thermophotovoltaic energy generation and scavenging, as well as in wide field of view spectral filtering. Sandia National Laboratory is investigating a variety of methods for the design, fabrication, and characterization of PBG materials, and three methods are being pursued to fabricate large areas of PBG material. These methods typically fabricate a mold and then fill it with metal to provide a high refractive index contrast, enabling a full 3D bandgap to be formed. The most mature scheme uses silicon MEMS lithographic fabrication means to create a mold which if filled by a novel tungsten deposition method. A second method uses LIGA to create a mold in PMMA, which is filled by electro-deposition of gold, copper, or other materials. A third approach uses nano-imprinting to define the mold, which is filled using evaporative deposition or atomic layer deposition of metals or other materials. Details of the design and fabrication processes and experimental measurements of the structures are presented at the conference

Variable-angle directional emissometer for moderate-temperature emissivity measurements.

We have developed a system to measure the directional thermal emission from a surface, and in turn, calculate its emissivity. This approach avoids inaccuracies sometimes encountered with the traditional method for calculating emissivity, which relies upon subtracting the measured total reflectivity and total transmissivity from unity. Typical total reflectivity measurements suffer from an inability to detect backscattered light, and may not be accurate for high angles of incidence. Our design allows us to vary the measurement angle (θ) from near-normal to ~80°, and can accommodate samples as small as 7 mm on a side by controlling the sample interrogation area. The sample mount is open-backed to eliminate shine-through, can be heated up to 200 °C, and is kept under vacuum to avoid oxidizing the sample. A cold shield reduces the background noise and stray signals reflected off the sample. We describe the strengths, weaknesses, trade-offs, and limitations of our system design, data analysis methods, the measurement process, and present the results of our validation of this Variable-Angle Directional Emissometer.

Argon–germane in situ plasma clean for reduced temperature Ge on Si epitaxy by high density plasma chemical vapor deposition

Demand for integration of near infrared optoelectronic functionality with silicon complementary metal oxide semiconductor (CMOS) technology has for many years motivated the investigation of low temperature germanium on silicon deposition processes. This work describes the development of a high density plasma chemical vapor deposition process that uses a low temperature (<460 °C) in situ germane/argon plasma surface preparation step for epitaxial growth of germanium on silicon. It is shown that the germane/argon plasma treatment sufficiently removes SiOx and carbon at the surface to enable germanium epitaxy. The use of this surface preparation step demonstrates an alternative way to produce germanium epitaxy at reduced temperatures, a key enabler for increased flexibility of integration with CMOS back-end-of-line fabrication.

A new time-dependent analytic model for radiation-induced photocurrent in finite 1D epitaxial diodes

Photocurrent generated by ionizing radiation represents a threat to microelectronics in radiation environments. Circuit simulation tools such as SPICE [1] can be used to analyze these threats, and typically rely on compact models for individual electrical components such as transistors and diodes. Compact models consist of a handful of differential and/or algebraic equations, and are derived by making simplifying assumptions to any of the many semiconductor transport equations. Historically, many photocurrent compact models have suffered from accuracy issues due to the use of qualitative approximation, rather than mathematically correct solutions to the ambipolar diffusion equation. A practical consequence of this inaccuracy is that a given model calibration is trustworthy over only a narrow range of operating conditions. This report describes work to produce improved compact models for photocurrent. Specifically, an analytic model is developed for epitaxial diode structures that have a highly doped subcollector. The analytic model is compared with both numerical TCAD calculations, as well as the compact model described in reference [2]. The new analytic model compares well against TCAD over a wide range of operating conditions, and is shown to be superior to the compact model from reference [2].

Experimental demonstration of using nano-photonic crystal sensor systems for submicron damage detection, quantification, and diagnoses

Photonic crystals (PC) are artificially fabricated crystals with a periodicity in the dielectric function. The spectral signature of such crystals is intricately tied to their underlying crystal lattice structural parameters. A result of this is that significant spectral changes can occur if damage is induced in the photonic crystal. In this work we present preliminary experimental results that demonstrate the possible use of photonic crystals as sensors for the detection, quantification and diagnosis of sub-micron damage. The experimentally observed variation in the reflection spectra of the photonic crystals is related to the damage induced in the material. A novel damage metric, based on principles of fuzzy pattern recognition, is introduced and is used to identify and quantify micro-damage in the photonic crystal. The corresponding damage metric is also presented and discussed. The detailed fabrication steps, as well as the advantages and limitations of this new approach are also addressed. It is concluded that photonic crystals can be successfully used for micro-damage quantification.

An integrated numerical approach for microdamage detection using nanophotonic sensors

Photonic bandgap materials (PBM) are synthetic materials that artificially manufactured at the nano-scale to control light propagation. These crystals have the ability to control light propagation in three dimensions by opening a frequency gap in which light is forbidden to propagate. When light is reflected by a nano photonic (NP) crystal a spectral signature that is directly related to its crystalline structure periodicity can be observed. It is suggested here that microscale damage in a substrate attached to the NP sensor might result in a significant change in the spectral signature of the NP sensor, hence allowing for micro-scale damage detection and quantification. To demonstrate the use of sensors for microdamage detection in structural materials an integrated numerical modelling approach was used. The approach augments two numerical methods to simulate the effect of microdamage in the material substrate on the spectrum signature of NPC sensors. First, the finite element method (FEM) was used to simulate structural response of the NP sensor under strain induced in the substrate with and without substrate damage. Second, the results of the finite element analysis were used as inputs to simulate the optical response of the NP sensors using the finite difference time domain method (FDTD). The integrated numerical approach was applied to a wood pile NP sensor attached to a silicon substrate. The numerical analysis showed promising results. Changes in the NP spectral signatures due microdamage in the silicon substrate were successfully identified.

Precision Laser Annealing of Focal Plane Arrays

We present results from laser annealing experiments in Si using a passively Q-switched Nd:YAG microlaser. Exposure with laser at fluence values above the damage threshold of commercially available photodiodes results in electrical damage (as measured by an increase in photodiode dark current). We show that increasing the laser fluence to values in excess of the damage threshold can result in annealing of a damage site and a reduction in detector dark current by as much as 100x in some cases. A still further increase in fluence results in irreparable damage. Thus we demonstrate the presence of a laser annealing window over which performance of damaged detectors can be at least partially reconstituted. Moreover dark current reduction is observed over the entire operating range of the diode indicating that device performance has been improved for all values of reverse bias voltage. Additionally, we will present results of laser annealing in Si waveguides. By exposing a small (<10 um) length of a Si waveguide to an annealing laser pulse, the longitudinal phase of light acquired in propagating through the waveguide can be modified with high precision, <15 milliradian per laser pulse. Phase tuning by 180 degrees is exhibited with multiple exposures to one arm of a Mach-Zehnder interferometer at fluence values below the morphological damage threshold of an etched Si waveguide. No reduction in optical transmission at 1550 nm was found after 220 annealing laser shots. Modeling results for laser annealing in Si are also presented.

Precision laser annealing of silicon devices for enhanced electro-optic performance

We present results from laser annealing experiments in Si using a passively Q-switched Nd:YAG microlaser. Exposure with laser at fluence values above the damage threshold of commercially available photodiodes results in electrical damage (as measured by an increase in photodiode dark current). We show that increasing the laser fluence to values in excess of the damage threshold can result in annealing of a damage site and a reduction in detector dark current by as much as 100x in some cases. A still further increase in fluence results in irreparable damage. Thus we demonstrate the presence of a laser annealing window over which performance of damaged detectors can be at least partially reconstituted. Moreover dark current reduction is observed over the entire operating range of the diode indicating that device performance has been improved for all values of reverse bias voltage. Additionally, we will present results of laser annealing in Si waveguides. By exposing a small (<10 um) length of a Si waveguide to an annealing laser pulse, the longitudinal phase of light acquired in propagating through the waveguide can be modified with high precision, <15 milliradian per laser pulse. Phase tuning by 180 degrees is exhibited with multiple exposures to one arm of a Mach-Zehnder interferometer at fluence values below the morphological damage threshold of an etched Si waveguide. No reduction in optical transmission at 1550 nm was found after 220 annealing laser shots.

A new time-dependent analytic compact model for radiation-induced photocurrent in epitaxial structures

Photocurrent generated by ionizing radiation represents a threat to microelectronics in radiation environments. Circuit simulation tools that employ compact models for individual electrical components (SPICE, e.g.) are often used to analyze these threats. Historically, many photocurrent compact models have suffered from accuracy issues due to the use of empirical assumptions, or physical approximations with limited validity. In this paper, an analytic model is developed for epitaxial diode structures that have a heavily-doped sub-collector. The analytic model is compared with both numerical TCAD calculations and the compact model described in reference [1]. The new analytic model compares well against TCAD over a wide range of operating conditions, and is shown to be superior to the older compact model [1]. The methods put forth in this paper could also be applied to model devices with similar physics, such as photonic and power devices.