Ted Schuler-Sandy

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.

Carrier lifetime studies in midwave infrared type-II InAs/GaSb strained layer superlattice

The authors report on an investigation of the dependence of the minority carrier lifetime in midwave infrared InAs/GaSb strained layer superlattices on a number of varied parameters: layer placement of two dopants (either Be or Te), and interface treatment between InAs and GaSb layers. In samples where the dopant and doping location was varied, it was found that the nonintentionally doped control sample exhibited the longest lifetimes (∼49 ns at 77 K under low injection), followed by the Be-doped and the Te-doped samples. Regardless of the type of doping, samples with dopants in only the InAs layer appeared to have longer lifetimes [low injection: 15 ns (Be), <3 ns (Te); high injection: 38 ns (Be), 16.2 ns (Te) at 77 K] compared to samples with dopants in the GaSb layer or all layers. However, because trap saturation behavior was observed in the transient photoluminescence (PL) decay, the intensity-dependent PL lifetime is a function of both the minority and majority carrier lifetimes, complicating the interpretation of the data. In samples where the treatment of the InAs/GaSb interface was varied, the sample that demonstrated the longest lifetime had a one-period growth sequence of InAs, an Sb soak, GaSb, and an InSb strain compensation layer. Of the three interface samples investigated, the sample (with a growth sequence of InAs, an Sb soak, GaSb, and a growth interrupt) that demonstrated the shortest lifetime also exhibited a fast initial decay for all injection levels, at only 110 and 150 K. This fast initial decay has been attributed to the appearance of another Shockley–Read–Hall trap level, contributing to a shorter carrier lifetime.

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.

Comparison of superlattice based dual color nBn and pBp infrared detectors

Long-wave infrared (LWIR) detector technologies with the ability to operate at or near room temperature are very important for many civil and military applications including chemical identification, surveillance, defense and medical diagnostics. Eliminating the need for cryogenics in a detector system can reduce cost, weight and power consumption; simplify the detection system design and allow for widespread usage. In recent years, infrared (IR) detectors based on uni-polar barrier designs have gained interest for their ability to lower dark current and increase a detectors operating temperature. Our group is currently investigating nBn and pBp detectors with InAs/GaSb strain layer superlattice (SLS) absorbers (n) and contacts (n), and AlGaSb and InAs/AlSb superlattice electron and hole barriers (B) respectively. For the case of the nBn structure, the wide-band-gap barrier material (AlGaSb) exhibits a large conduction band offset and a small valence band offset with the narrow-band-gap absorber material. For the pBp structure (InAs/AlSb superlattice barrier), the converse is true with a large valence band offset between the barrier and absorber and a small or zero conduction band offset. Like the built-in barrier in a p-n junction, the heterojunction barrier blocks the majority carriers allowing free movement of photogenerated minority carriers. However, the barrier in an nBn or pBp detector, in contrast with a p-n junction depletion layer, does not contribute to generation-recombination (G-R) current. In this report we aim to investigate and contrast the performance characteristics of an SLS nBn detector with that of and SLS pBp detector.

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.

Antimonide-based membranes synthesis integration and strain engineering

Significance In this work we present a versatile method to fabricate antimonide-based heterostructures in membrane form, and we demonstrate the potential of these materials to enable hybrid integration and elastic strain engineering. The relevance of our work is threefold. First, integration of Sb-based compound membranes with Si substrates will potentially solve a number of technological challenges in the fabrication of IR optoelectronic devices based on type II superlattices. Second, transfer of Sb compounds to insulating materials will enable a thorough investigation of electrical transport in the heterostructure via Hall and Van der Pauw measurements. Third, membrane technology applied to Sb-based structures will enable one to engineer strain distributions, which are not obtainable within the limitations of epitaxial growth processes. Antimonide compounds are fabricated in membrane form to enable materials combinations that cannot be obtained by direct growth and to support strain fields that are not possible in the bulk. InAs/(InAs,Ga)Sb type II superlattices (T2SLs) with different in-plane geometries are transferred from a GaSb substrate to a variety of hosts, including Si, polydimethylsiloxane, and metal-coated substrates. Electron microscopy shows structural integrity of transferred membranes with thickness of 100 nm to 2.5 μm and lateral sizes from 24×24μm2 to 1×1 cm2. Electron microscopy reveals the excellent quality of the membrane interface with the new host. The crystalline structure of the T2SL is not altered by the fabrication process, and a minimal elastic relaxation occurs during the release step, as demonstrated by X-ray diffraction and mechanical modeling. A method to locally strain-engineer antimonide-based membranes is theoretically illustrated. Continuum elasticity theory shows that up to ∼3.5% compressive strain can be induced in an InSb quantum well through external bending. Photoluminescence spectroscopy and characterization of an IR photodetector based on InAs/GaSb bonded to Si demonstrate the functionality of transferred membranes in the IR range.

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.

High quantum efficiency mid-wavelength infrared superlattice photodetector

We report high quantum efficiency (QE) MWIR barrier photodetectors based on the InAs/GaSb/AlSb type II superlattice (T2SL) material system. The nBp design consists of a single unipolar barrier (InAs/AlSb SL) placed between a 4 μm thick p-doped absorber (InAs/GaSb SL) and an n-type contact layer (InAs/GaSb SL). At 80K, the device exhibited a 50% cut-off wavelength of 5 μm, was fully turned-ON at zero bias and the measured QE was 62% (front side illumination with no AR coating) at 4.5 μm with a dark current density of 8.5×10-9 A/cm2 . At 150 K and Vb=50 mV, the 50% cut-off wavelength increased to 5.3 μm and the quantum efficiency (QE) was measured to be 64% at 4.5 μm with a dark current of 1.07×10-4 A/cm2 . The measurements were verified at multiple AFRL laboratories. The results from this device along with the analysis will be presented in this paper.

Polarization selective interband transitions in type-II InAs/GaSb superlattices

Polarization dependent photocurrent has been measured and analyzed in type-II InAs/GaSb strained layer superlattices (SLS). Theoretical studies have also been carried out to understand the origin of polarization selective transitions in SLS system.

Colloidal and Epitaxial Quantum Dot Infrared Photodetectors: Growth, Performance, and Comparison

In this review article, the current status of quantum dot infrared photodetectors (QDIPs) compared to competitive infrared (IR) technologies will be evaluated. Carrier generation and some of the other important physical properties of quantum dots (QDs) will be discussed. Recent design improvements of epitaxial and colloidal QDIPs are presented, followed by a thorough discussion on the growth techniques for both types of QDs. Important figures of merit for QDIPs, including dark current, responsivity, detectivity, and noise equivalent differential temperature (NEDT), will be explained, and a comparison will be made between QDIPs performance and other IR technologies in terms of their figures of merit. Keywords: quantum dot; colloidal quantum dot; epitaxial quantum dot; self-assembled quantum dot; infrared photodetector; quantum dot infrared photodetector; QDIP ; responsivity; detectivity; dark current