Long-wavelength infrared (IR) III-V based devices have long been of interest for potential applications such as chemical sensing and large format IR imaging. Within the III-V family, only the InAs 1-x Sb x bulk alloy in composition range 0.45 ≤ x ≤ 0.8, offers the required bandgap energy ( E g ) from 100 to 125 meV at an operating temperature of 80K and below [1]. Over this composition range however, the InAs 1-x Sb x lattice constant varies from 6.24 to 6.39 Å, where a lack of conventional substrates has restricted progress on the growth and study of this material system. To bridge this lattice-constant gap while maintaining relatively low defect densities, we employed a metamorphic step-graded InAs 1-x Sb x buffer on GaSb, enabling the study of low- E g InAs 1-x Sb x as a function of growth conditions. Using this method, we investigated the effect of substrate temperature ( T sub ) and group V to group III flux ratio (beam equivalent pressure, V/III) on Sb incorporation of the lowest- E g cap layer [2]. We also used x-ray reciprocal space mapping (RSM) to examine the effect of growth conditions on strain and dislocation dynamics. Following these growth studies, we employed the metamorphic InAs 1-x Sb x in an InAs/InAsSb superlattice designed with a cutoff wavelength of 9 µm which leads to improved absorption compared with the lattice-matched counterpart. We first grew, via molecular beam epitaxy, several InAs 1-x Sb x step-graded structures in which the Sb/(As+Sb) flux ratio was varied from 0.05 to 0.50 in 0.05 increments (see figure), under various T sub and V/III. Nomarski imaging revealed smoother surfaces under a V/III=10, the highest ratio we attempted. At this higher V/III, we observed the cross-hatch morphology expected for metamorphic materials and found that the cross-hatch spacing changes, implying a change in dislocation dynamics, with T sub . We then used photoluminescence (PL) to measure the Sb-content in the cap layer as well as compare intensities between samples. We found the highest Sb-incorporation to occur when T sub =415 C and V/III=10, while the most intense samples used T sub =415-430 C and V/III=10 [2]. Using RSM along [110] with (004) and (115) reflections, we identified the Sb composition in each layer. This allowed comparison of Sb-content as a function of Sb/(As+Sb) for various T sub and V/III. The results suggest that V/III has little effect on Sb incorporation, in direct conflict with our previous PL results [2]. To understand the discrepancy between PL and RSM, we measured (004) RSM of the same three samples with the x-ray beam incident along [1-10], revealing extremely different strain relaxation compared to the [110] case (see figure). Asymmetric strain relaxation has been observed in other III-V graded buffer systems and has been explained by different dislocation formation energies and glide velocities along each direction resulting from the core structure of the dislocation being terminated with either a group-III or a group-V element [3]. Transmission electron microscopy is ongoing to further understand the dislocation dynamics in these samples. Taking this all together allowed us to investigate the effect of substrate lattice-constant on strain-balanced InAs/InAsSb superlattices designed for 9 µm cutoff wavelength [4]. Theoretically, by using a larger substrate lattice-constant the superlattice design results in larger electron-hole wavefunction overlap, ultimately increasing photon absorption. Our experimental results confirm this theory, even in the presence of increased threading dislocations inherent to the required lattice-mismatch. [1] I. Vurgaftman et al. JAP 89 , 5815-5875 (2001). [2] Tomasulo et al. J. Vac. Sci. and Technol. B 36 , 02D108 (2018). [3] France et al. J. Appl. Phys. 107 , 103530 (2010); Gelczuk et al., J. Cryst. Growth 310 , 3014 (2008). [4] Affouda, Tomasulo et al., Appl. Phys. Lett. 110 , 181107 (2017). Figure 1

Trace amounts of Mg deposited on a diamond (100) substrate surface facilitate the growth of cubic boron nitride (c-BN) by ion beam-assisted molecular beam epitaxy. Fourier transform infrared spectroscopy indicates that films grown with Mg are cubic, while those without Mg are either hexagonal BN or lacking measurable cubic or hexagonal signatures. Initiating the growth with 0.005 monolayer equivalent of Mg is sufficient to yield epitaxial films with >99% c-BN. Reflection high energy electron diffraction, electron energy loss spectroscopy, and X-Ray photoelectron spectroscopy indicate the surface of the film to be sp(2)-bonded BN, consistent with the results of other groups. High-resolution scanning transmission electron microscopy reveals c-BN with a high density of stacking faults and twinning. A model is proposed by which Mg locally diminishes the energy barrier to dissociation of the as-deposited sp(2)-bonded BN, facilitating the nucleation of c-BN.

Using a 2D metallic grating, we demonstrate the resonant enhancement of the external quantum efficiency of an nBn structure with a 0.5 mu m-thick-absorber. A total of two structures were studied with different barrier materials: ternary AlInSb and an InAs/AlInSb superlattice (SL). The device using an SL barrier had a diffusion-limited dark current of 6.7 mu A/cm(2) at 150 K, which was four times lower than the ternary-barrier device. The surface plasmon polariton (SPP) resonance wavelength for devices with six different grating periods varied as predicted by simulations. The quantum efficiency (QE) was enhanced by up to 56% by coupling to the SPP mode. A peak external QE of 39% was achieved at 4 mu m with a 1100 nm grating period.

A resonant-cavity detector with peak sensitivity at 4.0 μm reaches 34% external quantum efficiency at room temperature, despite having only five absorbing quantum wells. Multiple passes enhance the peak absorption by nearly x30. © 2019 The Author(s)

Betavoltaic power sources based on the conversion of radioisotope energy to electrical power are considered an appealing option for remote applications due to extended period of operation and high energy densities. However, to be competitive with other power sources, their efficiency must be increased. This can be done through optimization of the beta source and selection of the semiconductor absorber. This paper evaluates available on the market and developing wideband gap semiconductors as prospective absorbers with 3 H and 63 Ni sources. Simulation results indicate that among wide band gap materials 4H-SiC and diamond are two optimal semiconductors due to the combination of good coupling efficiencies with isotope sources and good electronic transport properties. Additionally, having good coupling efficiency, an ultra-wide bandgap, and the capability for both n- and p-type doping, c-BN is a promising material for betavoltaic applications.

We report on an investigation of dark current contributions from common microscale crystallographic defects in type-II InAs/GaInSb superlattice infrared PIN type photodiode structures grown on (100) GaSb substrates and have identified three general classifications. Defects on several wafers of varying design were examined from multiple perspectives to correlate electrical activity with structural properties, to develop the identification and classification scheme. Active defects were first identified by current density vs voltage (J-V) measurements and electron beam induced current (EBIC) scans of individual diodes with micrometer resolution. The EBIC scans were then correlated with plan-view optical and atomic force microscopy images, both before and after anisotropic etch-pit analysis using a newly developed etchant. The atomic scale structure of active and inactive defects was then compared using cross-sectional transmission electron microscopy (TEM) on vertical slices of defects extracted using focused ion beam milling. Analysis of the TEM images yielded important clues as to the structure and root causes of benign and active defects, in which only significant disruptions at the epi-substrate interface appear to play a key role in producing microscale defects that efficiently promote dark current.

We report resonant-cavity infrared detectors with absorbers that consist of only five quantum wells, but exhibiting 34% external quantum efficiency at room temperature at the resonant wavelength of 4.0 μm. The FWHM linewidth is 46 nm, and the peak absorption is enhanced by nearly a factor of 30 over that for a single pass through the absorber. Although the Shockley-Read lifetime in the current material is much shorter than the state of the art, the dark current density is at the level of HgCdTe detectors as quantified by “Rule 07”. The Johnson-noise limited detectivity (D*) at 21°C is 7 × 109 cm Hz½/W. We expect that future improvements in the device design and material quality will lead to higher quantum efficiency, as well as a significant reduction of the dark current density consistent with the very thin absorber.

We report resonant-cavity infrared detectors with 34% external quantum efficiency at room temperature at the resonant wavelength of 4.0 μm, even though the absorber consists of only five quantum wells with a total thickness of 50 nm. The full width at half maximum (FWHM) linewidth is 46 nm, and the peak absorption is enhanced by nearly a factor of 30 over that for a single pass through the absorber. In spite of an unfavorable Shockley-Read lifetime in the current material, the dark current density is at the level of state-of-the-art HgCdTe detectors as quantified by "Rule 07." The Johnson-noise limited detectivity (D*) at 21°C is 7 × 109 cm Hz½/W. We expect that future improvements in the device design and material quality will lead to higher quantum efficiency, as well as a significant reduction of the dark current density consistent with the very thin absorber.

A focused pulsed X-ray beam is used to determine how the redistribution of the electric field by the gate-connected field plate affects single-event transient (SET) susceptibility of an AlGaN/GaN Schottky-gate HEMT on SiC. SETs generated by scanning the X-ray beam across the HEMT depend strongly on the presence of the field plate, radiation strike location, bias conditions, and X-ray photon energy. For the particular HEMT we tested, the gate-connected field plate reduces the electric-field strength near the edge of the gate by a factor of approximately 2, which results in faster decaying transients and less collected charge.

For diffusion limited nBn detectors, using an absorption layer much thinner than the optical attenuation length and minority carrier diffusion length can improve the dark current. As the absorber thickness decreases, the lower dark current increases the signal-to-noise ratio to provide greater sensitivity or higher temperature operation. However, if the quantum efficiency (QE) also decreases with absorber thickness, the advantage of reduced dark current is eliminated. Here we discuss the use of a metallic grating to couple the incident light into laterally-propagating surface plasmon polariton (SPP) modes, so as to increase the effective absorption length. We fabricate the gratings using a deposited Ge layer, which provides a uniform grating profile without increasing the dark current. Using this process in conjunction with a 0.5 μm-thick InAsSb absorber lattice-matched to GaSb, we demonstrate an external QE of 34% for T = 78–240 K. An nBn structure with an InAs0.8Sb0.2 absorber that is grown metamorphically on GaSb using a step-graded InGaSb buffer has a peak external QE of 39% at 100 K, which decreases to 32% by 240 K. Finally, we demonstrate that a grating with SPP resonance near the bandgap extends the absorption band, and can potentially reduce the dark current by a factor of 3–8 in addition to the 5× reduction due to the thinner absorber.

Betavoltaic power sources based on the conversion of radioisotope energy to electrical power are considered an appealing option for remote applications due to extended period of operation and high energy densities. However, to be competitive with other power sources, their efficiency must be increased. This can be done through optimization of the beta source and selection of the semiconductor absorber. This paper evaluates available on the market and developing wideband gap semiconductors as prospective absorbers with 3H and 63Ni sources. Simulation results indicate that among wide band gap materials 4H-SiC and diamond are two optimal semiconductors due to the combination of good coupling efficiencies with isotope sources and good electronic transport properties. Additionally, having good coupling efficiency, an ultra-wide bandgap, and the capability for both n- and p-type doping, c-BN is a promising material for betavoltaic applications.

Betavoltaic converters are attractive power sources due to their long duration, continuous discharge and highenergy densities in comparison with other modes of energy harvesting. The main obstacle to widen the application of betavoltaics is their low power output. 2D Monte Carlo numerical simulations coupled with drift-diffusionmodeling were conducted to evaluate commercially available and emerging wideband gap semiconductors as potential absorbers with widely available tritium (H-3) isotope. Results show that power enhancement can be achieved by using emerging ultra wide bandgap material such as diamond. Using those materials, >40% increase in the power density in comparison with current 4H-SiC technology can be achieved.

InAs1-xSbx is a unique semiconductor as it possesses the lowest bandgap (Eg) of the conventional III–V materials, yielding emission out to nearly 15 μm at room temperature. As such, it is well-suited as the absorber material in long-wavelength infrared (IR) applications such as chemical sensing and large format IR imaging. However, the compositions at the longest wavelengths are significantly lattice-mismatched to conventional substrates. Overcoming this lattice-mismatch requires using thick graded buffers, and to date, little work has focused on the effect of growth conditions on the Sb-incorporation or optical quality of the longest-wavelength InAs1-xSbx materials. Here, the authors investigate the molecular beam epitaxy (MBE) growth of this potential absorber material by using solid source MBE to grow InAs1-xSbx on step-graded buffers on GaSb substrates. The authors have achieved cap-layer compositions as high as x ≈ 0.55 under various substrate temperatures (Tsub) and V/III beam equivalent pressure ratios. By using InAs1-xSbx as the grading material as well as the cap material, the authors can measure Sb-content as a function of growth conditions over many compositions. The author's results show good agreement between x and Eg compared with previous reports, obtaining T = 8 K photoluminescence (PL) emission approaching 11 μm. However, the authors also found that Sb-incorporation falls off for x > 0.2 and worsens with increasing Tsub, suggesting that Sb-incorporation rates increase with reduced adatom mobility. Additionally, the PL of the samples show improved intensity when either Tsub or V/III were increased, while cross-sectional transmission electron microscopy revealed a significant decrease in threading dislocation density with increased V/III. Ultimately, the authors found that under the proper growth conditions, optically active InAs1-xSbx with x over 0.55 can be attained, and with an optimized grading structure, could be a low-cost alternative to HgCdTe for long-wavelength infrared optoelectronics.

We demonstrate a strategy for increasing the operating temperatures of nBn midwave infrared (MWIR) focal plane arrays, based on the use of two-dimensional plasmonic gratings to enhance the quantum efficiency (QE) of structures with very thin absorbers. Reducing the absorber volume correspondingly reduces the dark current in a diffusion-limited photodiode, while light trapping mediated by the plasmonic grating increases the net absorbance to maintain high QE. The plasmonically enhanced nBn MWIR sensors with absorber thicknesses of only 0.5 μm exhibit peak internal QEs as high as 57%, which enables a 5-fold reduction in dark current. Numerical simulations indicate the potential for further improvement.

The electron beam induced current (EBIC) mode of a scanning electron microscope (SEM) is a widely used technique for the quantitative assessment of minority carrier diffusion length and surface recombination. Point source (one-dimensional) and extended source (two-dimensional) analytical models are two widely used approaches to assess this information in geometry where the electron beam (e-beam) is parallel to the p-n junction. In this article, a two-dimensional (2D) analytical model is evaluated and compared with 2D finite element numerical simulations, where the electron beam-solid interaction is modeled using a Monte Carlo simulation coupled with a drift-diffusion solver. The simulations are computed for both low and high level injection conditions. The effect of an e-beam injection level on the shape of EBIC profiles is analyzed to evaluate limitations of the analytical models.

The narrow band gap and staggered band alignment of InAsSb alloys make it possible to engineer type-II superlattices (T2SLs) for mid-wave and long-wave (LW) infrared sensors operating in the 3–12 μm range. However, InAs/InAsSb T2SLs that are strain balanced to the underlying GaSb substrate have much lower absorption coefficients for LWIR operation because of the larger superlattice (SL) period, leading to reduced electron-hole overlap. The absorption coefficient of T2SLs can be greatly improved by growing on metamorphic buffers (MBs) with reduced lattice mismatch to the InAsSb layers, which allows the SL period to be reduced. For this study, MBs were capped with InAs/InAsSb T2SLs to assess the suitability of the materials for detector applications by X-ray diffraction and photoluminescence lifetime measurements. We show that the absorption of T2SLs can be significantly increased with no apparent degradation in the minority-carrier lifetime.

Tunnel junctions with low differential resistance and high transparency have been grown by molecular beam epitaxy on GaSb substrates. The resulting devices have been characterized and analyzed using a combination of electrical measurements and modeling. These devices have importance to multi-junction solar cells grown on GaSb substrates. The structures contain an n-type InAs quantum well embedded in a GaSb p/n junction, exploiting the high tunnel probability at the broken gap interface between p-type GaSb and n-type InAs, whilst having a minimal impact on the transparency of the device.

Longwave infrared detectors using p-type absorbers composed of InAs-rich type-II superlattices (T2SLs) nearly always suffer from high surface currents due to carrier inversion on the etched sidewalls. Here, we demonstrate reticulated shallow etch mesa isolation (RSEMI): a structural method of reducing surface currents in longwave single-band and midwave/longwave dual-band detectors with p-type T2SL absorbers. By introducing a lateral shoulder to increase the separation between the n+ cathode and the inverted absorber surface, a substantial barrier to surface electron flow is formed. We demonstrate experimentally that the RSEMI process results in lower surface current, lower net dark current, much weaker dependence of the current on bias, and higher uniformity compared to mesas processed with a single deep etch. For the structure used, a shoulder width of 2 μm is sufficient to block surface currents.

A tunnel junction has been developed with an application to multijunction solar cells grown on GaSb and analyzed using a combination of electrical device measurements and modeling. The device employs an InAs quantum well embedded in a GaSb p/n junction, exploiting the high tunnel probability at the broken-gap interface between p-type GaSb and n-type InAs and having a minimal impact on the transparency of the device. The concept is extended to wider bandgap heterointerfaces using Al(0.)2Ga(0.8)Sb, achieving a differential resistance of 4.08 x 10(-4) Omega cm(2). (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)