We report on low strain quantum dot infrared photodetectors (QDIP) with 80 dot in a well (DWELL) stacks. These
QDIPs have been grown with lattice matched Al0.1Ga0.9As barriers and GaAs wells allowing a large number of stacks to
be grown leading to an increased absorption volume. The QDIPs show a strong spectral response that varies
significantly with applied bias, with four distinct peak wavelengths ranging from 5.5μm to 10.0μm. The highly tunable
nature of the intrinsic responses makes these QDIPs very attractive as multispectral imagers in the MWIR and LWIR
regions. The spectral diversity of these QDIPs has been exploited using an algorithm to produce a highly versatile
algorithmic spectrometer. The algorithm assigns a specific weighting factor to each of the intrinsic responses and then
sums these weighted responses to achieve any desired spectral shape. Triangular narrowband filters have been
synthesised in this way with full width at half maximums (FWHM) as narrow as 0.2μm. The QDIPs can be used to
image objects in the MWIR and LWIR regions by measuring the photocurrent generated at each specific bias and
summing them using the calculated weighting factors for every wavelength of interest. This technique has been
successfully used to capture the radiated power from a blackbody source through IR filters with different centre
wavelengths and bandwidths as a function of wavelength in the LWIR and MWIR regions.
Quantum dot infrared photodetectors (QDIP) have established themselves as promising devices for detecting infrared (IR) radiation for wavelengths <20μm due to their sensitivity to normal incidence radiation and long excited carrier lifetimes. A limiting factor of QDIPs at present is their relatively small absorption volume, leading to a lower quantum efficiency and detectivity than in quantum well infrared photodetectors and mercury cadmium telluride based detectors. One means of increasing the absorption volume is to incorporate a greater number of quantum dot (QD) stacks, thereby increasing the probability of photon capture. Growth of InAs/InGaAs dot-in-a-well (DWELL) QDIPs with greater than 10 stacks is challenging due to the increased strain between layers, leading to high dark current. It is known that strain can be reduced in QDIPs by reducing the width of the InGaAs well and incorporating a second well consisting of GaAs and barriers consisting of AlGaAs. A number of InAs/InGaAs/GaAs DWELL QDIPs with 30-80 stacks have been grown, fabricated and characterised. Dark current in these layers appears to be constant at given electric field, suggesting strain does not increase significantly if the number of QD stacks is increased. IR spectral measurements show well defined peaks at 5.5μm, 6.5μm and 8.4μm. In this work a comparison between dark current, noise, gain, responsivity and detectivity in these layers is presented and compared to existing data from conventional DWELL QDIPs.
We report measurements on a series of quantum dot infrared photodetectors grown with different combinations of
monolayer thicknesses (2.2. 2.55 and 2.9 ML) and quantum dot layer sheet doping densities (6×10<sup>10</sup> cm<sup>-2</sup> and 12×10<sup>10</sup>
cm<sup>-2</sup>). The dark current and noise current were higher in devices grown with sheet doping density of 12×10<sup>10</sup> cm<sup>-2</sup>. At a
given bias voltage the dark current and the noise current was found to be lowest in devices having 2.55 ML and sheet
doping density of 6×10<sup>10</sup> cm<sup>-2</sup>. This combination gives a sheet doping density to dot density ratio of approximately unity.
Highest gain was achieved in devices with 2.55 ML and sheet doping density of 6×10<sup>10</sup> cm<sup>-2</sup>.