J. H. Warner

Graded band gap for dark-current suppression in long- wave infrared W-structured type-II superlattice photodiodes

A new W-structured type-II superlattice photodiode design, with graded band gap in the depletion region, is shown to strongly suppress dark currents due to tunneling and generation-recombination processes. The long-wave infrared (LWIR) devices display 19%–29% quantum efficiency and substantially reduced dark currents. The median dynamic impedance-area product of 216Ωcm2 for 33 devices with 10.5μm cutoff at 78K is comparable to that for state-of-the-art HgCdTe-based photodiodes. The sidewall resistivity of ≈70kΩcm for untreated mesas is also considerably higher than previous reports for passivated or unpassivated type-II LWIR photodiodes, apparently indicating self-passivation by the graded band gap.

W-structured type-II superlattice long-wave infrared photodiodes with high quantum efficiency

Results are presented for an enhanced type-II W-structured superlattice (WSL) photodiode with an 11.3μm cutoff and 34% external quantum efficiency (at 8.6μm) operating at 80K. The new WSL design employs quaternary Al0.4Ga0.49In0.11Sb barrier layers to improve collection efficiency by increasing minority-carrier mobility. By fitting the quantum efficiencies of a series of p-i-n WSL photodiodes with background-doped i-region thicknesses varying from 1to4μm, the authors determine that the minority-carrier electron diffusion length is 3.5μm. The structures were grown on semitransparent n-GaSb substrates that contributed a 35%–55% gain in quantum efficiency from multiple internal reflections.

Dual band LWIR/VLWIR type-II superlattice photodiodes

Multiband detection capability is a critical attribute of practical infrared (IR) sensing systems for use in missile defense detect-and-track applications. This capability, already demonstrated in mercury-cadmium telluride (MCT) photodiodes and quantum well infrared photodetectors (QWIPs), has not previously been explored in type II-superlattices (T2SLs), a newer system which is under consideration to meet next-generation sensor needs. Like QWIPs, T2SLs are composed of layers of III-V compound semiconductors grown by molecular beam epitaxy (MBE), and have an infrared gap that is determined primarily by the layer thicknesses. With the exceptional control of MBE over layer thicknesses and the ability to grow multiple bandgap structures under compatible growth conditions, T2SL-based multiband IR focal plane arrays (FPAs) are expected to have advantages in spectral control and pixel-to-pixel uniformity over MCT. Additionally, T2SLs have intrinsically higher quantum efficiency than QWIPs, in which the optical selection rules for intersubband transitions forbid the absorption of normally incident light. Here we describe the first results for a T2SL dual band detector with independent long-wave and very-long-wave infrared responsivity bands, with cutoffs of 11.4 and 17 μm respectively. The p-n-p device contains W-structured T2SL (WSL) active regions for enhanced band selectivity, owing to the quasi-two-dimensional density of states for WSLs. Photodetector results are demonstrated using a maskset designed to fabricate single-band diodes, 3-terminal dual band devices, and 2-terminal band selectable devices to comply with different dual band FPA read-out architectures.

Recent progress in W- structured type-II superlattice photodiodes

Recently we have achieved significant improvements in the performance of LWIR type-II superlattice photodiodes, with discrete devices beginning to demonstrate dynamic impedance-area product (R0A) levels approaching the MCT trend line and quantum efficiency exceeding 30% in devices without anti-reflection coatings. We discuss the key innovations that have led to these improvements, including modified W-structures, band-gap grading, and hybrid superlattices.

Passivation of W-structured type-II superlattice long-wave infrared photodiodes

A critical step in developing type-II superlattice (T2SL) based LWIR focal plane array (FPA) technology is to achieve high performance levels in FPA pixel-sized devices having 20-40 μm pitch. At this scale, device performance tends to be limited by surface effects along mesa sidewalls which are etched to provide pixel isolation. While control of surface leakage has been achieved for MWIR T2SLs, as evidenced by the availability of commercially produced FPAs, the same cannot be said for LWIR T2SLs. Several groups have approached this problem as strictly a matter of surface treatment, including cleaning, chemical treatment, and dielectric coating or epitaxial overgrowth, but with limited success. Here we describe an approach based on shallow-etch mesa isolation (SEMI), which takes advantage of bandgap grading to isolate devices without exposing narrow-gap LWIR regions on diode mesas sidewalls. The SEMI process consists of defining mesa diodes with a shallow etch that passes only 20-100 nm past the junction of a graded-gap W-structured type-II superlattice p-i-n structure, where the bandgap remains large (>200 meV). A second, deeper etch is then used to define a trench along the chip border for access to the p-contact. As a result, SEMI diodes have only MWIR layers exposed along sidewalls, while the LWIR regions remain buried and unexposed. We also discuss an investigation of surface passivation of GaSb with sulfur using thioacetamide.

High quantum efficiency long-wave infrared photodiodes using W- structured type-II superlattices

Recent improvements in material quality and design have led to large improvements in the quantum efficiency (QE) of long-wave infrared (LWIR) photodiodes based on W-structured type-II superlattices (WSL), which now have achieved external QE of up to 35% on an 11.3 μm cutoff photodiode operating at 80K. While single band and dual band WSLs have been demonstrated with cutoff wavelengths out to 17 μm, the initial devices also showed significant losses of photo-excited carriers resulting in QE levels of ≤ 10%. Here we describe recent results in which these losses have been dramatically reduced by modifying the WSL barrier layers to increase the mini-band width and improve the material properties. An additional 35-55% increase in QE also resulted from the use of semitransparent Te doped n-GaSb substrates that allowed for IR reflections off the backside from the Au plated chip carrier. A series of PIN photodiodes using the improved WSL, with intrinsic regions from 1 to 4 μm thick, were used to study minority carrier transport characteristics in the new structure. As a result of the improved design and material properties, the electron diffusion length in the undoped i-region, as determined from a theoretical fit to the thickness-dependent data, was 3.5 μm, allowing for much higher collection efficiency in PIN photodiodes with intrinsic regions up to 4 μm thick.

Recent progress by mid-IR antimonide type-II W interband cascade lasers and LWIR detectors

Significant recent advances in the high-temperature, high-power performance of type-II antimonide interband cascade lasers (ICLs) operating in the mid-infrared are reported. A 5-stage ICL with a 12μm ridge width and Au electroplating for improved epitaxial-side-up heat sinking operates cw to a maximum temperature of 257 K, where the emission wavelength is 3.7 μm. A similar device with a ridge width of 22 μm emits > 260 mW per facet for cw operation at 80 K (λ = 3.4 μm) and 100 mW at 200 K (λ = 3.6 μm). Beam qualities for the narrowest ridges approach the diffraction limit. The recent development of type-II W photodiodes for the long-wave infrared is also reviewed. A W photodiode with an 11.3 μm cutoff displayed a 34% external quantum efficiency (at 8.6 μm) operating at 80 K. A graded-gap design of the depletion region is shown to strongly suppress dark currents due to tunneling and generation-recombination processes. The median dynamic impedance-area product of 216 Ω-cm2 for 33 devices with 10.5 μm cutoff at 78 K is comparable to that for state-of-the-art HgCdTe-based photodiodes. The sidewall resistivity of ≈70 kΩ-cm for untreated mesas is also considerably higher than previous reports for passivated or unpassivated type-II LWIR photodiodes, apparently indicating self-passivation by the graded bandgap.