Two of the key challenges in the realisation of focal plane arrays based on type-II InAs/GaSb superlattices (T2SL) are
the difficulty in achieving a good sidewall profile and the increased dominance of surface leakage current as the device
dimensions shrink. We report the electrical and morphological results of test pixels for mid-wave infrared T2SL
photodiodes etched using a Cl2/Ar based inductively coupled plasma reactive ion etching (ICP-RIE) process and
passivated using SU-8 epoxy photoresist. The etch rate and sidewall surface morphology of GaSb, InAs, and InAs/GaSb
T2SL materials are compared after dry etching under the same conditions, leading to the determination of an optimal
etch rate. The effect of surface treatment using selected wet chemical etchants before passivation on the surface leakage
current is presented. Limitations of the dry etching recipe and further improvement of the sidewall verticality and
smoothness are also discussed. Good sidewall profiles and
bulk-limited dark currents are demonstrated for T2SL
photodiodes etched to depths between 1.5 and 3.5 μm with a pitch size down to 12 μm.
InGaAsN is a promising material system to enable low-cost GaAs-based detectors to operate in the telecommunication
spectrum, despite the problems posed by the low growth temperature required for nitrogen incorporation. We
demonstrate that InGaAsN p+-i-n+ structures with nominal In and N fraction of 10% and 3.8%, grown by molecular
beam epitaxy (MBE) under non-optimal growth conditions, can be optimized by post growth thermal annealing to match
the performance of optimally grown structures. We report the findings of an annealing study by comparing the
photoluminescence spectra, dark current and background concentration of the as-grown and annealed samples. The dark
current of the optimally annealed sample is approximately 2 μA/cm2 at an electric field of 100 kV/cm, and is the lowest
reported to date for InGaAsN photodetectors with a cut-off wavelength of 1.3 μm. Evidence of lower unintentional
background concentration after annealing at a sufficiently high temperature, is also presented.
In this work, we present the study on In0.53Ga0.47As/GaAs0.51Sb0.49 type-II heterojunction PIN diodes and
Separate Absorption, Charge and Multiplication (SACM) APDs utilising In0.52Al0.48As as the multiplication layer and
In0.53Ga0.47As/GaAs0.51Sb0.49 type-II heterostructures as the absorption layer. In0.52Al0.48As lattice matched to InP has been
shown to have superior excess noise characteristics and multiplication with relatively low temperature dependence
compared to InP. Furthermore, the type-II staggered band line-up leads to a narrower effective bandgap of approximately
0.49 eV corresponding to the APD cut off wavelength of 2.4 μm. The device exhibited low dark current densities near
breakdown. The device also exhibited multiplication in excess of 100 at 200 K.
The InAs/GaSb Type II strained layer superlattice (SLS) is promising III-V material system for infrared (IR) devices due to the ability to engineer its bandgap between 3-30 μm and potentially have many advantages over current technologies such as high uniformity smaller leakage current due to reduced Auger recombination which are crucial for large IR focal plane arrays. However, an issue with this material system is that it relies on growth on GaSb substrates. These substrates are significantly more expensive than silicon, used for HgCdTe detectors, lower quality and are only available commercially as 3" diameters. Moreover it has to go through thinning down before it could be hybridized to readout integrated circuits. GaAs substrate is a possible alternative. We report on growth and characterisation of Type-II InAs/GaSb SLS photodiodes grown on GaAs substrates for mid-wave infrared with peak responses of 3.5 μm at 77K and 4.1 μm at 295K. Comparisons with similar structure grown on GaSb substrates show similar structural, optical and electrical characteristics. Broadening of X-ray rocking curves were observed on the structure grown on GaAs substrate. A full width half maximum (FWMH) of 25.2 arc sec. for the superlattice was observed near ~30.4 degree for the structure on GaSb substrate compared to near ~30.4 degree for structure grown on GaAs. However peak responsivity values of ~ 1.9 A/W and ~ 0.7 A/W were measured at 77K and 295K for devices grown on GaAs substrate. Room temperature responsivity suggests that these photodiodes are promising as high temperature IR detectors.