High temperature operation of quantum dots-in-a-well infrared photodetectors
Introduction
Quantum dot infrared photodetectors (QDIP) technology has been emerging as a competitive technology for high performance 3rd generation infrared imaging system. The key requirements of 3rd generation imaging systems are high operating temperature, large format arrays and multicolor operation. QDIPs, due to the zero dimensional carrier confinement, mature GaAs fabrication technology and the richness of energy levels in the quantum dots, can fulfill all these requirements. Most of these advantages result from three dimensional carrier confinement inside the quantum dots, which leads to much higher carrier lifetime as compared to quantum well infrared detectors (QWIPs). There have been several demonstrations of QDIPs at higher operating temperatures [1], [2], [3], [4], [5] especially in the midwave infrared regime. It has been predicted [6], [7] that QDIPs could replace quantum well infrared photodetectors as the dominant technology in the field, owing to the ability of high temperature operation.
The self assembled growth of quantum dots in Stranski–Krastanow (SK) growth mode makes it hard to precisely control the shape and size of the quantum dots, which makes controlling the peak wavelength difficult. The dots-in-a-well (DWELL) design [8], [9], [10], [11], [12], [13], in which InAs quantum dots are placed in an InGaAs–GaAs or an InGaAs–GaAs–AlGaAs quantum well, effectively solves this problem and allows precise control of the peak wavelength by simply changing the width of the quantum well. The infrared absorption is a result of intersubband transitions from quantum dot ground state to quantum well bound state or the continuum [9], [14]. DWELL offers additional advantages such as superior optical quality of the quantum dots due to strain relaxation [15], [16] in the InGaAs QW and optimum growth temperature for capping layer materials.
The ability to independently tune the ground state energy, which is dominated by the quantum dot properties, and the excited energy state, which is a function of quantum well and barrier properties offers unique advantage to DWELL detectors over both QWIP and QDIP type of devices. Systematic studies of different transitions in QWIPs have been previously reported [18], however no such study has been reported on QDIPs as it is hard to control the energy levels in the quantum dots. In this study, we have tuned the excited state in the quantum well of the DWELL structure with respect to the barrier to obtain different types of transitions [17], such as bound to bound (B–B), bound to quasi-bound (B–Q) (two devices) and bound to bound (B–B). The full width half maximum of the spectral response decreases progressively from B–C to B–B transitions. The measured responsivity shows that at lower biases, the responsivity decreases with decreasing FWHM (going from B–C to B–B), while at higher biases the exact reversal of the trend is observed. B–C transitions have unity escape probability for photoexcited electrons hence show high responsivity at lower bias. On the other hand, B–B transitions have best absorption coefficient due to better wave function overlap between the two states, but poor escape probability at lower biases. These designs allow optimizing the detector performance for the required spectral width, wavelength, photoconductive gain and absorption quantum efficiency. In this study, very high detectivities of ∼4 × 1011 cm Hz1/2 W−1 at 77 K and ∼4 × 108 cm Hz1/2 W−1 at 220 K for f/2 optics have been obtained. The measured background limited performance (BLIP) temperature for B–Q device is ∼100 K for 300 K background with 2π field of view (FOV). High performance FPAs have been fabricated using these devices, which have average NEDT of 44 mK at 80 K operating temperature, for f/2.3 optics.
Section snippets
Device designs
Four DWELL devices with varying quantum well thicknesses were fabricated and characterized. The samples were grown in solid source molecular beam epitaxy machine. The B–C, B–Q2 and B–B samples consist of 20 stacks of InAs quantum dots placed in In0.15Ga0.85As quantum wells, each separated with 50 nm of Al0.08Ga0.92As barriers. B–Q1 device has only 5 stacks in the active region. Devices B–Q2 and B–B have an additional 50 nm of Al0.08Ga0.92As barriers near the top and bottom contact, which serve as
Results
The radiometric measurements were performed to calculate the responsivity and detectivity of the structure. The responsivity, plotted in Fig. 2a for all the samples, shows that at low electric fields, the B–C sample has largest responsivity, which decreases with decreasing FWHM. This trend reverses at higher electric field, owing to higher absorption in B–B structures. At lower electric fields, the extraction probability of electrons below the barrier is very small, which results in lower
Conclusion
In conclusion, we have reported a systematic study of different transitions for DWELL detectors working at high operating temperatures. The systematic study allows for greater tunability in the device designs, with the ability to control the peak wavelength, spectral width, absorption coefficient, photoconductive gain, activation energies for a fixed barrier composition. Ability to independently control the ground state and excited state energy in the DWELL architecture offers unique advantage
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