Z.-B. Tian

Barrier Engineered Infrared Photodetectors Based on Type-II InAs/GaSb Strained Layer Superlattices

We present the design, growth, fabrication, and characterization of unipolar barrier photodiodes, pBiBn, based on type-II InAs/GaSb superlattice for midwave and longwave infrared detection. Design optimization of barriers using bandgap and band-offset tailorability of InAs/GaSb/AlSb superlattice system, their advantages and evolution of heterostructure designs are discussed for both the regimes. Dark current densities of 1.6 × 10^-7 and 1.42 × 10^-5 A/cm² are measured at 77 K for midwave and longwave detectors with cutoff wavelengths of 5 and 10 μm, respectively. Responsivities of 1.3 (QE = 38%) and 1.66 A/W (QE = 23.5%) are measured at 4.2 and 8.7 μm for the midwave and longwave, respectively, at 77 K. Shot noise limited peak detectivity of 8.9 × 10¹² and 7.7×10¹¹ cm-Hz^1/2-W^-1 are observed at -10 and -40 mV for midwave infrared and longwave infrared detectors, respectively, at 77 K.

InAs/GaAs p-type quantum dot infrared photodetector with higher efficiency

An InAs/GaAs quantum dot infrared photodetector (QDIP) based on p-type valence-band intersublevel hole transitions as opposed to conventional electron transitions is reported. Two response bands observed at 1.5–3 and 3–10 μm are due to transitions from the heavy-hole to spin-orbit split-off QD level and from the heavy-hole to heavy-hole level, respectively. Without employing optimized structures (e.g., the dark current blocking layer), the demonstrated QDIP displays promising characteristics, including a specific detectivity of 1.8×109 cm·Hz1/2/W and a quantum efficiency of 17%, which is about 5% higher than that of present n-type QDIPs. This study shows the promise of utilizing hole transitions for developing QDIPs.

Gallium free type II InAs/InAsxSb1-x superlattice photodetectors

We report on a mid-wave infrared (λ50% cut-off ∼5.4 μm at 77 K) Ga-free Type II InAs/InAsxSb1-x (x = 0.65) superlattice detector Radiometric measurements reveal a quantum efficiency of 20% (λ = 4 μm, 77 K) with a dark current density of 2.1 × 10−4 A/cm2 (−10 mV) with spectral response observable up to 210 K. Although the Shockley-Read-Hall lifetime is expected to be longer for Ga-free superlattices, the dark current density is larger than that of conventional InAs/GaSb superlattice detectors. This is attributed to increased probability of carrier tunneling due to reduced valence and conduction band-offsets in InAs/InAsSb SL system.

High operating temperature interband cascade focal plane arrays

In this paper, we report the initial demonstration of mid-infrared interband cascade (IC) photodetector focal plane arrays with multiple-stage/junction design. The merits of IC photodetectors include low noise and efficient photocarrier extraction, even for zero-bias operation. By adopting enhanced electron barrier design and a total absorber thickness of 0.7 μm, the 5-stage IC detectors show very low dark current (1.10 × 10−7 A/cm2 at −5 mV and 150 K). Even with un-optimized fabrication and standard commercial (mis-matched) read-out circuit technology, infrared images are obtained by the 320 × 256 IC focal plane array up to 180 K with f/2.3 optics. The minimum noise equivalent temperature difference of 28 mK is obtained at 120 K. These initial results indicate great potential of IC photodetectors, particularly for high operating temperature applications.

Electron barrier study of mid-wave infrared interband cascade photodetectors

In this paper, we report our experimental investigation on the influence of electron barrier (eB) in mid-infrared interband cascade photodetectors. Even though earlier theoretical projection indicates that an eB with 2-pairs GaSb/AlSb quantum wells (QWs) is sufficient to block electrons direct tunneling between stages, our experimental results show that a thicker (with 6-pairs of GaSb/AlSb QWs) electron barrier could significantly reduce the device dark current, with little influence on the optical performance. The 5-stage devices have demonstrated a dark current density of 1.10 × 10−7 A/cm2 (at −5 mV) and a Johnson-limited D* of 1.81 × 1011 cmHz1/2/W (at 3.8 μm) at 150 K, respectively.

Study of valence-band intersublevel transitions in InAs/GaAs quantum dots-in-well infrared photodetectors

The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a well-preserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The bias-independent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias.

Mid-Infrared Interband Cascade Photodetectors With Different Absorber Designs

We report an experimental investigation on the influence of absorber thicknesses in mid-infrared interband cascade (IC) photodetectors. The electrical and optical properties of these five-stage IC detectors are characterized in detail over a wide operating temperature range. The IC detectors are operational above 400 K under zero bias, with a 50% cutoff of 4.56 μm and external quantum efficiency (single pass, no antireflection coating) up to 10.1% at room temperature. The dark current in the IC detectors is at 10 μA/cm2 at -10 mV, with a Johnson-limited D* of 1.10 × 1011 Jones at 200 K. Our experimental results show that both the optical response and the noise performance in the quantum-engineered IC detectors improve with increased discrete absorber thicknesses. It is also suggested that the dark current in the IC detectors is determined by tunneling components at lower temperatures, and becomes diffusion-limited at higher temperatures.

Mid-infrared metamorphic interband cascade photodetectors on GaAs substrates

Antimony-based mid-infrared interband cascade (IC) photodetectors fabricated on (001) GaAs substrates are reported. By using a “buffer-free” interfacial misfit array growth method, an overall good crystalline quality is obtained on the largely lattice-mismatched GaAs substrate. The GaAs-based IC detectors show comparable optical performance, with similar electrical performance at temperatures higher than 140 K, as compared to the reference devices grown on GaSb substrate. The GaAs-based IC detectors demonstrate dark current density of 2.63 × 10−6 A/cm2 at 180 K, which is about twice as compared to that grown on GaSb substrate, with Johnson-limited D* of 1.06 × 1011 Jones at 180 K and 4.0 μm. The results indicate that IC detector design is robust and relatively insensitive to the material quality, and metamorphic IC detector is viable for large-format infrared focal plane array applications.

High temperature terahertz response in a p-type quantum dot-in-well photodetector

Terahertz (THz) response observed in a p-type InAs/In0.15Ga0.85As/GaAs quantum dots-in-a-well (DWELL) photodetector is reported. This detector displays expected mid-infrared response (from ∼3 to ∼10 μm) at temperatures below ∼100 K, while strong THz responses up to ∼4.28 THz is observed at higher temperatures (∼100–130 K). Responsivity and specific detectivity at 9.2 THz (32.6 μm) under applied bias of −0.4 V at 130 K are ∼0.3 mA/W and ∼1.4 × 106 Jones, respectively. Our results demonstrate the potential use of p-type DWELL in developing high operating temperature THz devices.

Noise, gain, and capture probability of p-type InAs-GaAs quantum-dot and quantum dot-in-well infrared photodetectors

We report experimental results showing how the noise in a Quantum-Dot Infrared photodetector (QDIP) and Quantum Dot-in-a-well (DWELL) varies with the electric field and temperature. At lower temperatures (below ∼100 K), the noise current of both types of detectors is dominated by generation-recombination (G-R) noise which is consistent with a mechanism of fluctuations driven by the electric field and thermal noise. The noise gain, capture probability, and carrier life time for bound-to-continuum or quasi-bound transitions in DWELL and QDIP structures are discussed. The capture probability of DWELL is found to be more than two times higher than the corresponding QDIP. Based on the analysis, structural parameters such as the numbers of active layers, the surface density of QDs, and the carrier capture or relaxation rate, type of material, and electric field are some of the optimization parameters identified to improve the gain of devices.