Characterization of Nanoscale Quantum Dots-in-a-Well Infrared Sensors

Introduction: Infrared detectors have many military, medical, and industrial applications due to their ability to detect radiation in the form of heat. Research groups are striving towards third-generation infrared detectors that have large-area focal plane arrays, improved functionality, and increased operating temperatures [1]. In order to operate at higher temperatures, the dark current (thermally generated electrons) needs to be lowered; then these devices can become more cost-effective by reducing their reliance on expensive cooling equipment. The innovative quantum dots-in-a-well (DWELL) infrared detector has indium arsenide (InAs) dots, three-dimensionally quantum confined, “embedded in” an In0.15Ga0. 85As well [1]. Electrons in InAs dots can become photoexcited and make intersubband transitions (dot to dot, to well, and to continuum) in the conduction band (Figure 1). DWELL detectors decrease dark current and offer greater “control over the operating wavelength,” where the width of the quantum well can be altered to change energy transitions [1]. The DWELL structure was developed from a single In0.15Ga0. 85As well to a dots-in-a-double well (DDWELL) structure with In0.15Ga0. 85As/GaAs wells [1] as shown in Figure 1. Less use of indium for the In0.15Ga0. 85As well reduces the strain that was caused by the lattice mismatch between In0.15Ga0. 85As and GaAs, allowing the growth of more stacks and thus more photocurrent absorption. The Al0.10Ga0. 90As barriers located between the asymmetric wells lowered the dark current by blocking the passage of some thermally generated electrons and prohibited electron coupling between active regions. The focus of this project was to optimize the Al0.10Ga0. 90As barriers by characterizing and comparing devices that have 30, 50, and 65 nm Al0.10Ga0. 90As barriers. Experimental Procedure: The characterization parameters investigated were: currentvoltage characteristics, spectral response, responsivity, and specific detectivity. Each device —single well with 50 nm barrier, double well with 30, 50, or 65 nm barrier —was individually placed into a cryostat, in which the air was vacuumed from the cavity to avoid condensation and then cooled to a low temperature (i.e. 30, 77K) to reduce the signal from thermally generated electrons. First, a semiconductor parameter analyzer was used to obtain current-voltage characteristics for the current from both Figure 1: Schematic DWELL energy (at 0V bias) and DDWELL physical representation. photoexcited and thermally generated electrons. For dark current measurements, a cold shield was used to block any incident light from reaching the device. The temperature of each device was raised from 30°K up to 100°K (150°K for the double well with 65 nm barrier, Sample D) to determine which barrier width was optimal at reducing dark current. Second, the devices’ spectral response at various bias voltages was obtained using a Fourier transform infrared spectrometer (FTIR). Third, responsivity, the signal output per IR power input [2], was used to measure the amount of light absorbed by the detector. For the setup, a blackbody set to a specific temperature (i.e. 527oC) cast an infrared beam which was modulated by a chopper. This frequency was used to trigger