Here we report on the experimental results and advanced self-consistent real device simulations revealing a fundamental insight into the non-linear optical response of n+-i-n+ InP nanowire array photoconductors to selective 980 nm excitation of 20 axially embedded InAsP quantum discs in each nanowire. The optical characteristics are interpreted in terms of a photogating mechanism that results from an electrostatic feedback from trapped charge on the electronic band structure of the nanowires, similar to the gate action in a field-effect transistor. From detailed analyses of the complex charge carrier dynamics in dark and under illumination was concluded that electrons are trapped in two acceptor states, located at 140 and 190 meV below the conduction band edge, at the interface between the nanowires and a radial insulating SiOx cap layer. The non-linear optical response was investigated at length by photocurrent measurements recorded over a wide power range. From these measurements were extracted responsivities of 250 A W-1 (gain 320)@20 nW and 0.20 A W-1 (gain 0.2)@20 mW with a detector bias of 3.5 V, in excellent agreement with the proposed two-trap model. Finally, a small signal optical AC analysis was made both experimentally and theoretically to investigate the influence of the interface traps on the detector bandwidth. While the traps limit the cut-off frequency to around 10 kHz, the maximum operating frequency of the detectors stretches into the MHz region. © The Royal Society of Chemistry

High-performance photodetectors operating in the near-infrared (0.75−1.4 μm) and short-wave infrared (1.4−3.0 μm) portion ofthe electromagnetic spectrum are key components in many optical systems.Here, we report on a combined experimental and theoretical study of squaremillimeter array infrared photodetectors comprising 3 million n⁺−i−n⁺ In Pnanowires grown by MOVPE from periodically ordered Au seed particles. Thenominal i-segment, comprising 20 InAs_0.40P_0.60 quantum discs, was grown byuse of an optimized Zn doping to compensate the nonintentional n-doping.The photodetectors exhibit bias- and power-dependent responsivities reachingrecord-high values of 250 A/W at 980 nm/20 nW and 990 A/W at 532 nm/60nW, both at 3.5 V bias. Moreover, due to the embedded quantum discs, thephotoresponse covers a broad spectral range from about 0.70 to 2.5 eV, ineffect outperforming conventional single InGaAs detectors and dual Si/Gedetectors. The high responsivity, and related gain, results from a novel proposed photogating mechanism, induced by the complex charge carrier dynamics involving optical excitation and recombination in the quantum discs and interface traps, which reduces the electron transport barrier between the highly doped n⁺ contact and the i-segment. The experimental results obtained are in perfect agreement with the proposed theoretical model and represent a significant step forward toward understanding gain in nanoscale photodetectors and realization of commercially viable broadband photon detectors with ultrahigh gain. © 2019 American Chemical Society.

Nanowires offer remarkable opportunities for realizing new optoelectronic devices because of their unique fundamental properties. The ability to engineer nanowire heterostructures with large bandgap variations is particularly interesting for technologically important broadband photodetector applications. Here we report on infrared photodetectors based on arrays of InP nanowires with embedded InAsP quantum discs. We demonstrate a strongly reduced dark current in the detector elements by compensating the unintentional n-doping in the nominal intrinsic region of the InP nanowires by in-situ doping with Zn, a crucial step towards realizing high-performance devices. The optimized array detectors show a broad spectral sensitivity at normal incidence for wavelengths from visible to far-infrared up to 20 μm, promoted by both interband and intersubband transitions. Optical simulations show that the unexpected normal incidence response at long wavelengths is due to non-zero longitudinal modes hosted by the nanowires. © 2018 Elsevier B.V.

Semiconductor nanowires have great potential for realizing broadband photodetectors monolithically integrated with silicon. However, the spectral range of such detectors has so far been limited to selected regions in the ultraviolet, visible and near-infrared. Here, we report on the first intersubband nanowire heterostructure array photodetectors exhibiting a spectrally resolved photoresponse from the visible to long-wavelength infrared. In particular, the infrared response from 3-20 mm is enabled by intersubband transitions in low-bandgap InAsP quantum discs synthesized axially within InP nanowires. The intriguing optical characteristics, including unexpected sensitivity to normal incident radiation, are explained by excitation of the longitudinal component of optical modes in the photonic crystal formed by the nanostructured portion of the detectors. Our results provide a generalizable insight into how broadband nanowire photodetectors may be designed, and how engineered nanowire heterostructures open up new fascinating opportunities for optoelectronics.

Semiconductor nanowire (NW) technology has emerged as a key facilitator of novel optoelectronics e.g. solar cells, photodetectors and LEDs. The functional wavelength range of current NW-based photodetectors is typically limited to the visible/ near-infrared region. In this work, we present the first ever reported electrical and optical characteristics of longwavelength IR photodetectors based on large square millimeter ensembles of vertically grown and processed InAsP/InP heterostructure NWs grown on InP substrates1 . More specifically, the MOVPE-grown NWs comprise single or multiple InAsP quantum discs (QDiscs) axially embedded in an n+-i-n+ geometry. The NWs are contacted together in a vertical geometry by uniformly depositing a thin insulating SiO2 layer, selective etching of the oxide from the tip of the NWs followed by sputtering of ITO as a common top contact to all NWs. Using Fourier transform photocurrent spectroscopy, we demonstrate a photoresponse extending from the visible to far infrared1,2. In particular, the infrared response from 3-20 μm is enabled by intersubband transitions in the lowbandgap InAsP quantum discs synthesized axially within the InP NWs. The detector elements exhibit an unexpected sensitivity to normal incident radiation, apparently in contradiction to well-known selection rules for intersubband transitions in quantum wells. From in-depth 2D and 3D optical simulations we attribute this result to an excitation of the longitudinal component of optical modes in the photonic crystal formed by the nanostructured portion of the detectors. Key advantages with the proposed design include a large degree of freedom in choice of material compositions, enhanced optical resonance effects due to periodically ordered NW arrays and the compatibility with silicon substrates. We believe that our novel detector design offers a route towards monolithic integration of compact and sensitive broadband III-V NW detectors with main-stream silicon technology which could seriously challenge existing commercially available photodetectors.

Nanowire array ensembles contacted in a vertical geometry are extensively studied and considered strong candidates for next generations of industrial scale optoelectronics. Key challenges in this development deal with optimization of the doping profile of the nanowires and the interface between nanowires and transparent top contact. Here we report on photodetection characteristics associated with doping profile variations in InP nanowire array photodetectors. Bias-dependent tuning of the spectral shape of the responsivity is observed which is attributed to a Schottky-like contact at the nanowire-ITO interface. Angular dependent responsivity measurements, compared with simulated absorption spectra, support this conclusion. Furthermore, electrical simulations unravel the role of possible self-gating effects in the nanowires induced by the ITO/SiO_x wrap-gate geometry. Finally, we discuss possible reasons for the observed low saturation current at large forward biases.  

Radial GaInP/AlGaInP nanowire array light-emitting diodes (LEDs) are promising candidates for novel high-efficiency solid state lighting due to their potentially large strain-free active emission volumes compared to planar LEDs. Moreover, by proper tuning of the diameter of the nanowires, the fraction of emitted light extracted can be significantly enhanced compared to that of planar LEDs. Reports so far on radial growth of nanowire LED structures, however, still point to significant challenges related to obtaining defect-free radial heterostructures. In this work, we present evidence of optically active growth-induced defects in a fairly broad energy range in vertically processed radial GaInP/AlGaInP quantum well nanowire array LEDs using a variety of complementary experimental techniques. In particular, we demonstrate strong infrared electroluminescence in a spectral range centred around 1 eV (1.2 μm) in addition to the expected red light emission from the quantum well. Spatially resolved cathodoluminescence studies reveal a patchy red light emission with clear spectral features along the NWs, most likely induced by variations in QW thickness, composition and barriers. Dark areas are attributed to infrared emission generated by competing defect-assisted radiative transitions, or to trapping mechanisms involving non-radiative recombination processes. Possible origins of the defects are discussed. © 2017 IOP Publishing Ltd

Nanowire semiconductors are promising candidates for optoelectronic applications such as solar cells, photodetectors and lasers due to their quasi-1D geometry and large surface to volume ratio. The functional wavelength range of NW-based detectors is typically limited to the visible/near-infrared region. In this work, we present electrical and optical properties of novel IR photodetectors based on large square millimeter ensembles (>1million) of vertically processed semiconductor heterostructure nanowires (NWs) grown on InP substrates which operates in longer wavelengths. InP NWs comprising single or multiple (20) InAs/InAsP QDics axially embedded in an n-i-n geometry, have been grown on InP substrates using MOVPE. The NWs are contacted in vertical direction by ALD deposition of 50 nm SiO₂ as an insulating layer followed by sputtering of ITO and evaporation of Ti and Au as top contact layer. In order to extend the sensitivity range to the mid-wavelength and long-wavelength regions, the intersubband transition within conduction band of InAsP QDisc is suggested. We present first experimental indications of intersubband photocurrent in NW geometry and discuss important design parameters for realization of intersubband detectors. Key advantages with the proposed design include large degree of freedom in choice of materials compositions, possible enhanced optical resonance effects due to periodically ordered NW arrays and the compatibility with silicon substrates. We believe that our novel detector design offers the route towards monolithic integration of compact and sensitive III-V NW long wavelength detectors with Si technology.

Avalanche photodetectors (APDs) are key components in optical communication systems due to their increased photocurrent gain and short response time as compared to conventional photodetectors. A detector design where the multiplication region is implemented in a large band gap material is desired to avoid detrimental Zener tunneling leakage currents, a concern otherwise in smaller band gap materials required for absorption at 1.3/1.55 μm. Self-assembled III-V semiconductor nanowires offer key advantages such as enhanced absorption due to optical resonance effects, strain-relaxed heterostructures, and compatibility with mainstream silicon technology. Here, we present electrical and optical characteristics of single InP and InP/InAsP nanowire APD structures. Temperature-dependent breakdown characteristics of p⁺-n-n⁺ InP nanowire devices were investigated first. A clear trap-induced shift in breakdown voltage was inferred from I-V measurements. An improved contact formation to the p⁺-InP segment was observed upon annealing, and its effect on breakdown characteristics was investigated. The band gap in the absorption region was subsequently varied from pure InP to InAsP to realize spatially separate absorption and multiplication APDs in heterostructure nanowires. In contrast to the homojunction APDs, no trap-induced shifts were observed for the heterostructure APDs. A gain of 12 was demonstrated for selective optical excitation of the InAsP segment. Additional electron-beam-induced current measurements were carried out to investigate the effect of local excitation along the nanowire on the I-V characteristics. Simulated band profiles and electric field distributions support our interpretation of the experiments. Our results provide important insight for optimization of avalanche photodetector devices based on III-V nanowires. © 2017 American Chemical Society