Nicole DiLello

Photonic ADC: overcoming the bottleneck of electronic jitter

Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated for many years as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits using a photonic ADC built from discrete components. This accuracy corresponds to a timing jitter of 15 fs - a 4-5 times improvement over the performance of the best electronic ADCs which exist today. On the way towards an integrated photonic ADC, a silicon photonic chip with core photonic components was fabricated and used to digitize a 10 GHz signal with 3.5 effective bits. In these experiments, two wavelength channels were implemented, providing the overall sampling rate of 2.1 GSa/s. To show that photonic ADCs with larger channel counts are possible, a dual 20-channel silicon filter bank has been demonstrated.

Photonic Analog-to-Digital Conversion with Electronic-Photonic Integrated Circuits

Photonic Analog-to-Digital Conversion (ADC) has a long history. The premise is that the superior noise performance of femtosecond lasers working at optical frequencies enables us to overcome the bottleneck set by jitter and bandwidth of electronic systems and components. We discuss and demonstrate strategies and devices that enable the implementation of photonic ADC systems with emerging electronic-photonic integrated circuits based on silicon photonics. Devices include 2-GHz repetition rate low noise femtosecond fiber lasers, Si-Modulators with up to 20 GHz modulation speed, 20 channel SiN-filter banks, and Ge-photodetectors. Results towards a 40GSa/sec sampling system with 8bits resolution are presented.

Thin film a-Si/c-Si 1−x Ge x /c-Si heterojunction solar cells with Ge content up to 56%

Thin film a-Si(n⁺)/c-Si_1-xGe_x(p)/c-Si(p⁺) heterojunction solar cells are fabricated with Ge content up to 56 atomic percent. Solar cells with junction layers consisting of Si, Si_0.75Ge_0.25, Si_0.59Ge_0.41, and Si_0.44Ge_0.56 are compared to study the effect of increasing Ge concentration. The measured short-circuit current (J_sc) increases from ~14 mA/cm² for Si cells to 21 mA/cm² for the Si_0.44Ge_0.56 cells, for one light pass and a 2 μm-thick SiGe layer. The results show an open-circuit voltage (V_oc) of 0.61 V for Si cells, dropping to 0.32 V for Si_0.44Ge_0.56, consistent with the reduction in band-gap. Quantum efficiency measurements highlight the improved spectral response for higher Ge percentages. Physics based TCAD simulations combined with the experimental results are used to extract lifetime and interface velocity.

Effect of germanium fraction on the effective minority carrier lifetime in thin film amorphous-Si/ crystalline-Si1xGex/crystalline-Si heterojunction solar cells

The effect of germanium fraction on the effective minority carrier lifetime (τeff) for epitaxial Si1-xGex layers is extracted using measurements on amorphous(a) Si(n+)/crystalline(c)-Si1-xGex(p)/crystalline(c)-Si(p+) heterojunction solar cells with x = 0.25, 0.41 and 0.56. The τeff extracted for Si0.75Ge0.25 is ∼1 μs, decreasing to ∼ 40 ns for Si0.44Ge0.56. In addition, the band-gap voltage offset (Woc) increases from 0.5 eV for Si to 0.65 eV for 56% Ge indicating an increase in non-radiative recombination consistent with the reduction in effective lifetime.

Development of low dark current SiGe-detector arrays for visible-NIR imaging sensor

SiGe based Focal Plane Arrays offer a low cost alternative for developing visible- NIR focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based IRFPAs will take advantage of Silicon based technology, that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance comparison for the SiGe based VIS-NIR Sensor with performance characteristics of InGaAs, InSb, and HgCdTe based IRFPAs. Various approaches including device designs are discussed for reducing the dark current in SiGe detector arrays; these include Superlattice, Quantum dot and Buried junction designs that have the potential of reducing the dark current by several orders of magnitude. The paper also discusses approaches to reduce the leakage current for small detector size and fabrication techniques. In addition several innovative approaches that have the potential of increasing the spectral response to 1.8 microns and beyond.

Characterization of SiGe-detector arrays for visible-NIR imaging sensor applications

SiGe based focal plane arrays offer a low cost alternative for developing visible- near-infrared focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based foal plane arrays take advantage of silicon based technology that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance characteristics for the SiGe based VIS-NIR Sensors for a variety of defense and commercial applications using small unit cell size and compare performance with InGaAs, InSb, and HgCdTe IRFPAs. We present results on the approach and device design for reducing the dark current in SiGe detector arrays. The electrical and optical properties of SiGe arrays at room temperature are discussed. We also discuss future integration path for SiGe devices with Si-MEMS Bolometers.

Impact of post-metallization annealing on Ge-on-Si photodiodes passivated with silicon dioxide

Ge-on-Si photodiodes were fabricated from epitaxial germanium films grown by low-pressure chemical vapor deposition. These vertical p-i-n diodes were passivated with SiO2 deposited by chemical vapor deposition. It is found that the incorporation of a post-metallization anneal reduces the dark current by 1000X for small-area devices, with 10 × 10 μm diodes exhibiting a dark current of 8 nA at −1 V. Metal-oxide-semiconductor capacitors were also fabricated using the same materials and annealing conditions. Capacitance-voltage measurements indicate that the anneal changes the surface condition of the germanium from depletion to accumulation, lowering the photodiode perimeter leakage current.

Development of SiGe arrays for visible-near IR applications

SiGe based focal plane arrays offer a low cost alternative for developing visible- near-infrared focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based foal plane arrays take advantage of silicon based technology that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper will discuss performance characteristics for the SiGe based VIS-NIR Sensors for a variety of defense and commercial applications using small unit cell size and compare performance with InGaAs, InSb, and HgCdTe IRFPAs. We will present results on the approach and device design for reducing the dark current in SiGe detector arrays. We will discuss electrical and optical properties of SiGe arrays at room temperature and as a function of temperature. We will also discuss future integration path for SiGe devices with other Silicon-based technology for defense and Commercial Applications.

Design Considerations for SiGe-based Near-Infrared Imaging Sensor

Low cost IR Sensors are needed for a variety of Military and Commercial Applications. SiGe based IR Focal Plane Arrays offer a low cost alternative for developing near IR sensors that will not require cooling and can operate in the visible and NIR bands. The attractive features of SiGe based IRFPAs will take advantage of Silicon based technology, that promises small feature size and compatibility with the low power silicon CMOS circuits for signal processing. A feasibility study of an infrared sensor based on SiGe material system and its performance characteristics are presented. Simulations comparing the sensitivity of the SiGe detector with spectral cutoff wavelength of 1.6 micron to other IR Focal Plane arrays are discussed. Measured electrical and optical characteristics of Ge-on-Si photodetectors are also presented.

Characterization and Performance Analysis of LPCVD Germanium-on-Silicon C-Band Photodiodes

Spatially-resolved photoresponse and bias-dependent modulation measurements of large area vertically-illuminated germanium photodiodes are presented. These measurements are compared with finite-element device simulations to theorize possible sources of performance limitations revealed by different measurement conditions.