[Show abstract][Hide abstract] ABSTRACT: Direct current (DC) transport and far infrared photoresponse were studied an InAs/GaSb double quantum well with an inverted band structure. The DC transport depends systematically upon the DC bias configuration and operating temperature. Surprisingly, it reveals robust edge conduction despite prevalent bulk transport in our device of macroscopic size. Under 180 GHz far infrared illumination at oblique incidence, we measured a strong photovoltaic response. We conclude that quantum spin Hall edge transport produces the observed transverse photovoltages. Overall, our experimental results support a hypothesis that the photoresponse arises from direct coupling of the incident radiation field to edge states.
Preview · Article · Jan 2016 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: The Auger lifetime is a critical intrinsic parameter for infrared photodetectors as it determines the longest potential minority carrier lifetime and consequently the fundamental limitations to their performance. Here, Auger recombination is characterized in a long-wave infrared InAs/InAsSb type-II superlattice. Auger coefficients as small as 7.1×10−26 cm6/s are experimentally measured using carrier lifetime data at temperatures in the range of 20 K–80 K. The data are compared to Auger-1 coefficients predicted using a 14-band K·p electronic structure model and to coefficients calculated for HgCdTe of the same bandgap. The experimental superlattice Auger coefficients are found to be an order-of-magnitude smaller than HgCdTe.
Preview · Article · Dec 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Carrier lifetime and dark current measurements are reported for a mid-wavelength infrared InAs0.91Sb0.09 alloy nBn photodetector. Minority carrier lifetimes are measured using a non-contact time-resolved microwave technique on unprocessed portions of the nBn wafer and the Auger recombination Bloch function parameter is determined to be |F1F2|=0.292. The measured lifetimes are also used to calculate the expected diffusion dark current of the nBn devices and are compared with the experimental dark current measured in processed photodetector pixels from the same wafer. Excellent agreement is found between the two, highlighting the important relationship between lifetimes and diffusion currents in nBn photodetectors.
No preview · Article · Nov 2015 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Temperature-dependent
measurements of carrier recombination rates using a time-resolved optical pump-probe technique are reported for mid-wave infrared InAs/InAs1−
type-2 superlattices (T2SLs). By engineering the layer widths and alloy compositions, a 16 K band-gap of ∼235 ± 10 meV was achieved for five unintentionally and four intentionally doped T2SLs. Carrier lifetimes were determined by fitting lifetime models based on Shockley-Read-Hall (SRH), radiative, and Auger recombination processes to the temperature and excess carrier density dependent data. The minority carrier (MC), radiative, and Auger lifetimes were observed to generally increase with increasing antimony content and decreasing layer thickness for the unintentionally doped T2SLs. The MC lifetime is limited by SRH processes at temperatures below 200 K in the unintentionally doped T2SLs. The extracted SRH defect energy levels were found to be near mid-bandgap. Also, it is observed that the MC lifetime is limited by Auger recombination in the intentionally doped T2SLs with doping levels greater than n ∼ 1016 cm−3.
[Show abstract][Hide abstract] ABSTRACT: Strongly coupling metallic nanoresonators with specially designed intersubbandtransitions in quantum-wells results in efficient, saturation-limited second-harmonic (SH) generation. This method also grants full control over the polarization and phase-front of the emitted SH radiation.
[Show abstract][Hide abstract] ABSTRACT: We use epsilon-near-zero modes in semiconductor nanolayers to design a system whose spectral properties are controlled by their interaction with multi-dipole resonances. This design flexibility renders our platform attractive for efficient nonlinear composite materials.
[Show abstract][Hide abstract] ABSTRACT: Time-resolved measurements of carrier recombination are reported for a midwave infrared InAs/InAs0.66Sb0.34 type-II superlattice (T2SL) as a function of pump intensity and sample temperature. By including the T2SL doping level in the analysis, the Shockley-Read-Hall (SRH), radiative, and Auger recombination components of the carrier lifetime are uniquely distinguished at each temperature. SRH is the limiting recombination mechanism for excess carrier densities less than the doping level (the low-injection regime) and temperatures less than 175 K. A SRH defect energy of 95 meV, either below the T2SL conduction-band edge or above the T2SL valence-band edge, is identified. Auger recombination limits the carrier lifetimes for excess carrier densities greater than the doping level (the high-injection regime) for all temperatures tested. Additionally, at temperatures greater than 225 K, Auger recombination also limits the low-injection carrier lifetime due to the onset of the intrinsic temperature range and large intrinsic carrier densities. Radiative recombination is found to not have a significant contribution to the total lifetime for all temperatures and injection regimes, with the data implying a photon recycling factor of 15. Using the measured lifetime data, diffusion currents are calculated and compared to calculated Hg1−xCdxTe dark current, indicating that the T2SL can have a lower dark current with mitigation of the SRH defect states. These results illustrate the potential for InAs/InAs1−xSbx T2SLs as absorbers in infrared photodetectors.
[Show abstract][Hide abstract] ABSTRACT: We use cross-sectional scanning tunneling microscopy (STM) to reconstruct the monolayer-by-monolayer composition profile across a representative subset of MBE-grown InAs / InAsSb superlattice layers and find that antimony segregation frustrates the intended compositional discontinuities across both antimonide-on-arsenide and arsenide-on-antimonide heterojunctions. Graded, rather than abrupt, interfaces are formed in either case. We likewise find that the incorporated antimony per superlattice period varies measurably from beginning to end of the multilayer stack. Although the intended antimony discontinuities predict significant discrepancies with respect to the experimentally observed high-resolution x-ray diffraction spectrum, dynamical simulations based on the STM-derived profiles provide an excellent quantitative match to all important aspects of the x-ray data.
No preview · Article · Feb 2015 · Journal of Crystal Growth
[Show abstract][Hide abstract] ABSTRACT: We experimentally demonstrate efficient second harmonic generation using a metamaterial-coupled III-V heterostructure at mid-infrared wavelengths. Our approach exploits the large second-order nonlinear susceptibilities of intersubband transitions and the near-field enhancement and polarization manipulation flexibility offered by metamaterial resonators.
[Show abstract][Hide abstract] ABSTRACT: We present an electrodynamic model of strongly coupled metamaterial/intersubband-transition systems that can be used to predict and maximize Rabi splittings. This model can also be used to optimize metamaterial structures that enhance second-order nonlinear processes.
[Show abstract][Hide abstract] ABSTRACT: We have fabricated a superconductor (Ta)-InAs/GaSb bilayer-superconductor (Ta)
device that has a long mean free path and can preserve the wavelike properties of particles (electrons and holes) inside the junction. Differential conductance measurements were carried out at low temperatures in this device, and McMillan-Rowell like oscillations (MROs) were observed. Surprisingly, a much larger Fermi velocity, compared to that from Shubnikov-de Haas oscillations, was obtained from the frequency of MROs. Possible mechanisms are discussed for this discrepancy.
Preview · Article · Oct 2014 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Measurements of carrier recombination rates using a time-resolved pump-probe technique are reported for mid-wave infrared InAs/InAs1−xSbx type-2 superlattices (T2SLs). By engineering the layer widths and alloy compositions, a 16 K band-gap of ≃235 ± 10 meV was achieved for all five unintentionally doped T2SLs. Carrier lifetimes were determined by fitting a rate equation model to the density dependent data. Minority carrier lifetimes as long as 10 μs were measured. On the other hand, the Auger rates for all the InAs/InAsSb T2SLs were significantly larger than those previously measured for InAs/GaSb T2SLs. The minority carrier and Auger lifetimes were observed to generally increase with increasing antimony content and decreasing layer thickness.
[Show abstract][Hide abstract] ABSTRACT: The effect of defects on the dark current characteristics of MWIR, III-V nBn detectors has been studied. Two different types of defects are compared, those produced by lattice mismatch and by proton irradiation. It is shown that the introduction of defects always elevates dark currents; however the effect on dark current is different for nBn detectors and conventional photodiodes. The dark currents of nBn detectors are found to be more tolerant of defects compared to pn-junction based devices. Defects more weakly increase dark currents, and cooling reduces the defect-produced dark currents more rapidly in nBn detectors than in conventional photodiodes.
[Show abstract][Hide abstract] ABSTRACT: In recent years, impressive demonstrations related to quantum information processing have been realized. The scalability of quantum interactions between arbitrary qubits within an array remains however a significant hurdle to the practical realization of a quantum computer. Among the proposed ideas to achieve fully scalable quantum processing, the use of photons is appealing because they can mediate long-range quantum interactions and could serve as buses to build quantum networks. Quantum dots or nitrogen-vacancy centres in diamond can be coupled to light, but the former system lacks optical homogeneity while the latter suffers from a low dipole moment, rendering their large-scale interconnection challenging. Here, through the complete quantum control of exciton qubits, we demonstrate that nitrogen isoelectronic centres in GaAs combine both the uniformity and predictability of atomic defects and the dipole moment of semiconductor quantum dots. This establishes isoelectronic centres as a promising platform for quantum information processing.
No preview · Article · May 2014 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: Since InGaP/GaAs heterojunction bipolar transistors (HBTs) are utilized in a wide variety of RF and other applications, a great deal has been learned about their reliability. Nevertheless, this knowledge is limited by the fact that most reliability studies heavily emphasize the stress-related evolution of a single parameter, the DC current gain, beta. We have found that interrupted stressing experiments, with complete characterization of HBTs during the interruption, can give a more complete picture of the degradation that occurs during bias stressing of HBTs. We have previously correlated electrical signatures with degradation in step-stressing of packaged InGaP/GaAs HBTs . In this work we will present some results of step-stress experiments which have been conducted at the wafer level for InGaP/GaAs HBTs. Wafer-level evaluation is advantageous due to the potential for greater automation and flexibility in the testing as well as the ability to identify problems at an earlier stage in the process.
The HBTs are stressed at room temperature at a constant collector voltage. Stressing is initiated at a given base current, held for a fixed time, which is typically 10 s. The applied stress is stepped up to a higher level of base current and the process is iterated until a failure is recorded. Full DC parametric electrical characterization of the HBT gets carried out initially and after certain predefined stress steps. Although the stress is typically carried near room temperature, it is understood that the junction of the HBT is effectively self-heated and can reach extremely high temperatures.
A certain set of degradation processes are responsible for stress-induced failures and these occur singly or in combinations. In addition, many HBTs within a wafer and among duplicate wafers have not only a number of common failure modes, but a tendency towards a consistently realized destructive power limit. The consistency of these results and the utility of electrical signature analysis lead us to conclude that wafer-level step-stressing is an excellent tool for quickly monitoring the HBT quality. We have also found many commonalities in the wafer-level and package level results for the ceramic packages used in a previous study .
The degradation of HBTs by these stressing methods leads eventually to a large drop in current gain. Thus, the interrupted step stressing method arrives at the same end state as continuous, single-parameter monitoring, but with much more clarity about the degradation processes that occur on the way to the end state.
The interrupted step-stress process in combination with electrical signature analysis is very likely to find the weak link in an HBT process. We will discuss the utility of the technique in terms of the opportunities to ruggedize the process. It is also possible that the identified weak link is fundamental to the technology or inconsequential in terms of meeting reliability requirements. In that case, wafer-level step-stressing is still an excellent tool for early identification of quality issues that may affect reliability through monitoring the consistency (or lack thereof) in degradation observed across different wafers over different periods of time.
* A.G. Baca, A.J. Scruggs, A. Gorenz, T.R. Fortune, J.F. Klem, R.D. Briggs, J.B. Clevenger, G.A. Patrizi, and C.T. Sullivan, “A Survey of Electrical Signatures Characteristic of Step-Stressed InGaP/GaAs HBTS,” ECS Transactions vol. 50 (6), pp. 273-282 (2012).
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
[Show abstract][Hide abstract] ABSTRACT: We present the design, realization and characterization of strong coupling between an intersubband transition and a monolithic metamaterial nanocavity in the mid-infrared spectral range. We use a ground plane in conjunction with a planar metamaterial resonator for full three-dimensional confinement of the optical mode. This reduces the mode volume by a factor of 1.9 compared to a conventional metamaterial resonator while maintaining the same Rabi frequency. The conductive ground plane is implemented using a highly doped n<sup>+</sup> layer which allows us to integrate it monolithically into the device and simplify fabrication.
[Show abstract][Hide abstract] ABSTRACT: The interaction between cavity modes and optical transitions leads to new coupled light-matter states in which the energy is periodically exchanged between the matter states and the optical mode. Here we present experimental evidence of optical strong coupling between modes of individual sub-wavelength metamaterial nanocavities and engineered optical transitions in semiconductor heterostructures. We show that this behaviour is generic by extending the results from the mid-infrared (~10 μm) to the near-infrared (~1.5 μm). Using mid-infrared structures, we demonstrate that the light-matter coupling occurs at the single resonator level and with extremely small interaction volumes. We calculate a mode volume of 4.9 × 10(-4) (λ/n)(3) from which we infer that only ~2,400 electrons per resonator participate in this energy exchange process.
Full-text · Article · Nov 2013 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: Minority carrier lifetimes in doped and undoped mid-wave infrared InAs/InAsSb
type-II superlattices (T2SLs) and InAsSb alloys were measured from 77-300 K. The
temperature-dependent lifetimes were analyzed using Shockley-Read-Hall (SRH), ra-
diative, and Auger recombination, allowing the contributions of the various recom-
bination mechanisms to be distinguished and the dominant mechanisms identified.
For the T2SLs, SRH recombination is the dominant mechanism below 200 K and
Auger recombination above 200 K. The alloy lifetimes are limited by radiative and
Auger recombination through the entire temperature range, with SRH not making a
significant contribution. The data allowed the extraction of SRH defect level energies.
[Show abstract][Hide abstract] ABSTRACT: We present ultra-strong light-matter interaction of a metamaterial mode and an intersubband transition for normal incidence radiation in the mid-infrared spectral region. The anti-crossed lines show a splitting of 15% of the central frequency.
[Show abstract][Hide abstract] ABSTRACT: Step-stress experiments on high-voltage Npn InGaP/GaAs HBTs are shown to reveal a number of degradation mechanisms, singly or in combinations: defect buildup in the emitter depletion region, defect buildup in the neutral base region, possible degradation of Ohmic contacts or increase in epitaxial layer resistances. Defect buildup in the emitter depletion region often precedes other types of degradation. Two less commonly reported degradation mechanisms are also suggested: base Ohmic metal punch-through to the collector and deterioration of the die attach material. It was found that the vast majority of devices failed within 5% of a maximum attainable power level, but when stressed just shy of this catastrophic level, electrical characteristics typically degraded gradually.
No preview · Article · Mar 2013 · ECS Transactions