Adrienne D. Stiff-Roberts

High-temperature operation of InAs-GaAs quantum-dot infrared photodetectors with large responsivity and detectivity

We have optimized the growth of multiple (40-70) layers of self-organized InAs quantum dots separated by GaAs barrier layers in order to enhance the absorption of quantum-dot infrared photodetectors (QDIPs). In devices with 70 quantum-dot layers, at relatively large operating biases (/spl les/-1.0 V), the dark current density is as low as 10/sup -5/ A/cm/sup 2/ and the peak responsivity ranges from /spl sim/0.1 to 0.3 A/W for temperatures T=150 K-175 K. The peak detectivity corresponding to these low dark currents and high responsivities varies in the range 6/spl times/10/sup 9//spl les/D/sup */(cm/spl middot/Hz/sup 1/2//W)/spl les/10/sup 11/ for temperatures 100/spl les/T(K)/spl les/200. These performance characteristics represent the state-of-the-art for QDIPs and indicate that this device heterostructure is appropriate for incorporation into focal plane arrays.

Absorption, carrier lifetime, and gain in InAs-GaAs quantum-dot infrared photodetectors

Quantum-dot infrared photodetectors (QDIPs) are being studied extensively for mid-wavelength and long-wavelength infrared detection because they offer normal-incidence, high-temperature, multispectral operation. Intersubband absorption, carrier lifetime, and gain are parameters that need to be better characterized, understood, and controlled in order to realize high-performance QDIPs. An eight-band k/spl middot/p model is used to calculate polarization-dependent intersubband absorption. The calculated trend in absorption has been compared with measured data. In addition, a Monte-Carlo simulation is used to calculate the effective carrier lifetime in detectors, allowing the calculation of gain in QDIPs as a function of bias. The calculated gain values can be fitted well with experimental data, revealing that the gain in these devices consists of two mechanisms: photoconductive gain and avalanche gain, where the latter is less dominant at normal operating biases.

High-performance mid-infrared quantum dot infrared photodetectors

Quantum dot infrared photodetectors (QDIPs) have emerged as attractive devices for sensing long wavelength radiation. Their principle of operation is based on intersublevel transitions in quantum dots (QDs). Three-dimensional quantum confinement offers the advantages of normal incidence operation, low dark currents and high-temperature operation. The performance characteristics of mid-infrared devices with three kinds of novel heterostructures in the active region are described here. These are a device with upto 70 QD layers, a device with a superlattice in the active region, and a tunnel QDIP. Low dark currents (1.59 A cm−2 at 300 K), large responsivity (2.5 A W−1 at 78 K) and large specific detectivity (1011 cm Hz1/2 W−1 at 100 K) are measured in these devices. It is evident that QDIPs will find application in the design of high-temperature focal plane arrays. Imaging with small QD detector arrays using the raster scanning technique is also demonstrated.

Contribution of field-assisted tunneling emission to dark current in InAs-GaAs quantum dot infrared photodetectors

A model for the dark current in quantum dot infrared photodetectors, including thermionic emission and field-assisted tunneling, is developed. The calculated dark currents are in excellent agreement with measured values for a wide range of temperatures (78 K-295 K) and applied bias (0-3 V).

Quantum-dot infrared photodetectors: a review

Quantum-dot infrared photodetectors (QDIPs) are positioned to become an important technology in the field of infrared (IR) detection, particularly for high-temperature, low-cost, high-yield detector arrays required for military applications. High-operating temperature (150 K) photodetectors reduce the cost of IR imaging systems by enabling cryogenic dewars and Stirling cooling systems to be replaced by thermo-electric coolers. QDIPs are well-suited for detecting mid-IR light at elevated temperatures, an application that could prove to be the next commercial market for quantum dots. While quantum dot epitaxial growth and intraband absorption of IR radiation are well established, quantum dot non-uniformity remains as a significant challenge. Nonetheless, state-of-the-art mid-IR detection at 150 K has been demonstrated using 70-layer InAs/GaAs QDIPs, and QDIP focal plane arrays are approaching performance comparable to HgCdTe at 77 K. By addressing critical challenges inherent to epitaxial QD material systems (e.g., controlling dopant incorporation), exploring alternative QD systems (e.g., colloidal QDs), and using bandgap engineering to reduce dark current and enhance multi-spectral detection (e.g. resonant tunneling QDIPs), the performance and applicability of QDIPs will continue to improve.

Low-bias, high-temperature performance of a normal-incidence InAs/GaAs vertical quantum-dot infrared photodetector with a current-blocking barrier

The growth, fabrication, and characterization of a low-bias, high-temperature, InAs/GaAs vertical quantum dot infrared photodetector with a single Al0.3Ga0.7As current-blocking barrier are described and discussed. A specific detectivity ≈3×109 cm Hz1/2/W is measured at normal incidence for a detector temperature of 100 K at a bias of 0.2 V, and detector characteristics are measured for temperatures as high as 150 K. The equivalence of the activation energy and photoionization energy for thermionic emission in quantum dots is also verified. The superior low bias performance of the photodetector ensures its compatibility with commercially available silicon read-out circuits necessary for the fabrication of a focal plane array.

Raster-scan imaging with normal-incidence, midinfrared InAs/GaAs quantum dot infrared photodetectors

We demonstrate normal incidence infrared imaging with quantum dot infrared photodetectors using a raster-scan technique. The device heterostructure, containing multiple layers of InAs/GaAs self-organized quantum dots, were grown by molecular-beam epitaxy. Individual devices have been operated at temperatures as high as 150 K and, at 100 K, are characterized by λpeak=3.72 μm, Jdark=6×10−10 A/cm2 for a bias of 0.1 V, and D*=2.94×109 cm Hz1/2/W at a bias of 0.2 V. Raster-scan images of heated objects and infrared light sources were obtained with a small (13×13) interconnected array of detectors (to increase the photocurrent) at 80 K.

Influence of rapid thermal annealing on a 30 stack InAs/GaAs quantum dot infrared photodetector

The research at the Australian National University was supported by the Australian Research Council and the work at the University of Michigan was supported by DARPA under Grant No. DAAD19-00-1-0394.

RIR-MAPLE deposition of multifunctional films combining biocidal and fouling release properties

Multifunctional films with both antimicrobial activity and fouling-release ability based on a biocidal quaternary ammonium salt (QAS) and thermo-responsive poly(N-isopropylacrylamide) (PNIPAAm) were deposited on substrates using resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE). The surface properties of these films were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), and water contact angle measurements. The biocidal and release properties of the films were tested against Escherichia coli K12 and Staphylococcus epidermidis. At 37 °C, the deposited film facilitated bacterial attachment and killed a majority of attached bacteria. Decrease of the temperature to 25 °C promoted the hydration and at least partial dissolution of PNIPAAm, leading to bacterial detachment from the film. To enhance the retention of PNIPAAm on the substrate, a small amount of (3-aminopropyl)triethoxysilane (APTES) was incorporated as a stabilizer, resulting in a ternary film with biocidal activity and bacterial-release ability after several attach-kill-release cycles. The simplicity and universality of RIR-MAPLE to form films on a wide range of substrata make it a promising technique to deposit multifunctional films to actively mitigate bacterial biofouling.

Antimicrobial and bacteria-releasing multifunctional surfaces: Oligo (p-phenylene-ethynylene)/poly (N-isopropylacrylamide) films deposited by RIR-MAPLE

Antimicrobial oligo (p-phenylene-ethynylene) (OPE) films have previously been demonstrated to show effective ultraviolet A (UVA) light-induced biocidal activity; however, a serious problem arises from the accumulation of dead bacteria and debris on the films that limits their effectiveness and application. In this work, we address this challenge by incorporating thermally-responsive poly (N-isopropylacrylamide) (PNIPAAm), which provides on-demand bacteria-releasing functionality. Multifunctional surfaces comprising blended films of OPE and PNIPAAm were deposited on substrates by resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) using a sequential co-deposition mode. In this way, RIR-MAPLE enabled the deposition of multifunctional films with surface properties and film functionality that can be tailored, precisely and systematically, by controlling the chemical composition of the deposited film. The surface properties of these films were characterized by UV-visible (UV-vis) absorbance spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and water contact angle measurements. The interactions between bacteria and the deposited films were tested using two model bacteria: Escherichia coli K12 (Gram-negative) and Staphylococcus epidermidis (Gram-positive). The antimicrobial and bacteria-release properties of the blended films were controlled by varying the OPE/PNIPAAm ratio in the RIR-MAPLE emulsion target, providing an easy way to optimize the multifunctional surface. The OPE/PNIPAAm blended films with optimized composition killed a majority of attached E. coli bacteria at 37 °C and under UVA exposure, and the dead bacteria were then removed from the films simply by rinsing with water at 25 °C.