Jill A. Nolde

Analysis and performance of type-II superlattice infrared detectors

We discuss the current performance of long-wavelength infrared photodetectors based on type-II superlattices, and the projected characteristics for diffusion-limited operation. For optimized architectures such as graded-gap and abrupt-heterojunction designs, the dark currents are strongly dominated by Shockley-Read (SR) rather than Auger processes. A factor of 10 improvement over the demonstrated SR lifetimes would lead to a factor of 4 lower dark current than state-of-the-art HgCdTe devices.

Interband cascade laser operating cw to 257K at λ=3.7μm

A five-stage interband cascade laser with 12μm ridge width and Au electroplating for improved epitaxial-side-up heat sinking operates cw to a maximum temperature of 257K, where the emission wavelength is 3.7μm. The device emits 100mW/facet for cw operation at 80K, 54mW at 200K, and 10mW at 250K. The beam quality is within twice the diffraction limit for injection currents up to 14 times the lasing threshold at 120K.

Broad-stripe, single-mode, mid-IR interband cascade laser with photonic-crystal distributed-feedback grating

We have demonstrated an electrically pumped photonic-crystal distributed-feedback laser with an interband cascade active region emitting at 3.3μm. At 78K, the stripe of width 400μm emits up to 67mW of cw power into a single spectral mode with side-mode suppression ratio ≈27dB. The full width at half maximum of the far-field divergence angle is ≈0.5°, which combined with the near-field profile yields an effective M2 of 1.7–2.0.

High-temperature interband cascade lasers emitting at λ=3.6–4.3μm

The authors report the operating characteristics of ten-stage interband cascade lasers from two wafers with room-temperature wavelengths of 4.1 and 4.3μm. For 150-μm-wide stripes, the threshold current densities are as low as 4.8A∕cm2 at 78K (cw) and 1.15kA∕cm2 at room temperature (pulsed). At 78K, the cw wall-plug efficiency for an 11-μm-wide ridge with 0.5-mm-long cavity and coated facets is 27%, while a 3-mm-long cavity emits a maximum cw power of 200mW. Devices from the two wafers have maximum cw operating temperatures of 261K (λ≈4.0μm) and 243K (λ≈4.2μm).

LWIR high performance focal plane arrays Based on type-II strained layer superlattice (SLS) materials

Type-II strained layer superlattices (SLS) are a rapidly maturing technology for infrared imaging applications, with performance approaching that of HgCdTe1,2,3,4. Teledyne Imaging Sensors (TIS), in partnership with the Naval Research Laboratory (NRL), has recently demonstrated state-of-the-art, LWIR, SLS 256 × 256 focal plane arrays (FPAs) with cutoff wavelengths ranging from 9.4 to 11.5 μm. The dark current performance of these arrays is within a factor of 10-20 of (state-of-the-art) HgCdTe. Dark current characteristics of unpassivated and passivated devices exhibit bulk-limited behavior, essential for FPA applications. TIS has also demonstrated rapid substrate thinning processes for increased infrared transmission through the GaSb substrate. In addition to this work, this presentation will discuss the recent developments of 1K x 1K LWIR SLS FPAs.

Recent developments in type-II superlattice-based infrared detectors

Much has been accomplished in the last few years in advancing the performance of type-II superlattice (T2SL) based infrared photodiodes, largely by focusing on device and heterostructure design. Quantum efficiency (QE) has increased to 50% and higher by using thicker absorbing layers and making use of internal reflections, and dark currents have been reduced by over a factor of ten by using bandstructure engineering to suppress tunneling and generation-recombination (G-R) currents associated with the junction. With performance levels of LWIR T2SL photodiodes now within an order of magnitude of that of HgCdTe (MCT) based technology, however, there is renewed interest in understanding fundamental materials issues. This is needed both to move performance toward the theoretical Auger limit, and to facilitate the task of transitioning T2SL growth from laboratories to commercial institutions. Here we discuss recent continuing efforts at NRL to develop new device structures for enhanced detector performance, and to further our understanding of this material system using advanced structural and electronic probes. Results from electron beam induced current (EBIC) imaging and analysis of point defects in T2SL photodiodes will be presented, showing differentiated behavior of bulk defect structures. We will also describe a study comparing intended vs. as-grown T2SL photodiode structures by crosssectional scanning microscopy (XSTM). Using parameters extracted from the XSTM images, we obtain detailed knowledge of the composition and layer structures through simulation of the x-ray diffraction spectra.

Physical properties of nanometer graphene oxide films partially and fully reduced by annealing in ultra-high vacuum

The properties of reduced graphene oxide (GO) are reported from a non-chemical reduction method. Ultra-high vacuum annealing of GO films in the thickness of 1–80 nm was studied by XPS, AFM, UV-Vis-NIR, Raman, and TEM to observe the controlled removal of oxygen. We observed the loss of hydroxyl (C-OH) at low temperatures (<600 °C) followed by the complete loss of carbonyls (C = O) and epoxy (C-O-C) species by 1200 °C. As oxygen was removed, we observed a decrease in the layer spacing between the GO sheets and a concurrent decrease in the film resistance. While the Raman spectroscopy showed no change with reduction, indicating no change in the overall defect density or the general structure of the GO, the transmission spectra showed a shift in the transmission minimum from 245 nm to 260 nm, and a total decrease in transmission above 800 nm occurs as the films visibly darken. TEM indicated that there is turbostratic stacking of the graphene layers as the reduction occurs, leading us to conclude that at a cer...

Resonant quantum efficiency enhancement of midwave infrared nBn photodetectors using one-dimensional plasmonic gratings

We demonstrate up to 39% resonant enhancement of the quantum efficiency (QE) of a low dark current nBn midwave infrared photodetector with a 0.5 μm InAsSb absorber layer. The enhancement was achieved by using a 1D plasmonic grating to couple incident light into plasmon modes propagating in the plane of the device. The plasmonic grating is composed of stripes of deposited amorphous germanium overlaid with gold. Devices with and without gratings were processed side-by-side for comparison of their QEs and dark currents. The peak external QE for a grating device was 29% compared to 22% for a mirror device when the illumination was polarized perpendicularly to the grating lines. Additional experiments determined the grating coupling efficiency by measuring the reflectance of analogous gratings deposited on bare GaSb substrates.

Effect of the oxide-semiconductor interface on the passivation of hybrid type-II superlattice long-wave infrared photodiodes

In order to be commercially viable, the type-II superlattice (T2SL) LWIR focal plane array technology will require the development of effective passivation of exposed surfaces. Here we investigate the relationship between the thickness and composition of the native oxide at the T2SL-SiO2 interface and the diode performance in terms of sidewall resistivity. Device performance is compared between samples with untreated surfaces, those for which the native oxides have been removed at various intervals prior to SiO2 deposition, and samples for which oxide growth was promoted by ozone exposure with and without a prior oxide strip. InAs- and GaSb-capped pieces were processed in an identical manner and studied using X-ray photoelectron spectroscopy (XPS). From these spectra, the compositions and thicknesses of the surface oxides just prior to SiO2 deposition were determined, complementing the electrical characterization of devices. Correlation of the performance and surface composition is presented.

MBE growth of Sb-based type-II strained layer superlattice structures on multiwafer production reactors

Ga(In)Sb/InAs-based strained-layer superlattices (SLS) have received considerable attention recently for their potential in infrared (IR) applications. These heterostructures create a type-II band alignment such that the conduction band of InAs layer is lower than the valence band of Ga(In)Sb layer. By varying the thickness and composition of the constituent materials, the bandgap of these SLS structures can be tailored to cover a wide range of the mid-wave and long-wave infrared (MWIR and LWIR) absorption bands. Suppression of Auger recombination and reduction of tunneling current can also be realized through careful design of the Type-II band structure. The growth of high-quality Ga(In)Sb/InAs-based SLS epiwafers is challenging due to the complexity of growing a large number of alternating thin layers with mixed group V elements. In this paper, the development of a manufacturable growth process by molecular beam epitaxy (MBE) using a multi-wafer production reactor will be discussed. Various techniques were used to analyze the quality of the epitaxial material. Structural properties were evaluated by high-resolution x-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM). Optical properties were assessed by low-temperature photoluminescence measurements (PL). Surface morphology and roughness data as measured by Nomarski optical microscope and atomic force microscope (AFM) will be presented. Device characteristics such as dynamic impedance, responsivity, quantum efficiency, and J-V characteristics of photodiodes fabricated using our SLS epiwafers will be discussed.