A near-unity unselective absorption spectrum is desirable for high-performance photovoltaics. Nanowire arrays are promising candidates for efficient solar cells due to nanophotonic absorption resonances in the solar spectrum. The absorption spectra, however, display undesired dips between the resonance peaks. To achieve improved unselective broadband absorption, we propose to enclose distributed Bragg reflectors (DBRs) in the bottom and top parts of indium phosphide (InP) nanowires, respectively. We theoretically show that by enclosing only two periods of In_0.56Ga_0.44As/InPDBRs, an unselective 78% absorption efficiency (72% for nanowires without DBRs)is obtained at normal incidence in the spectral range from 300 nm to 920 nm. Under oblique light incidence, the absorption efficiency is enhanced up to about 85% at an incidence angle of 50º. By increasing the number of DBR periods from two to five, the absorption efficiency is further enhanced up to 95% at normal incidence. In this work we calculated optical spectra for InP nanowires, but the results are expected to be valid for other direct band gap III-V semiconductor materials. We believe that our proposed idea of integrating DBRs in nanowires offers great potential for high-performance photovoltaic applications. ©2018 Håkan Pettersson et al., published by De Gruyter, Berlin/Boston.

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.

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.  

Geometrically designed III-V nanowire arrays are promising candidates for optoelectronics due to their possibility to excite nanophotonic resonances in absorption spectra. Strong absorption resonances can be obtained by proper tailoring of nanowire diameter, length and pitch. Such enhancement of the light absorption is, however, accompanied by undesired resonance dips at specific wavelengths. In this work, we theoretically show that tilting of the nanowires mitigates the absorption dips by exciting strong Mie resonances. In particular, we derive a theoretical optimum inclination angle of about 30 degrees at which the inclined nanowires gain 8% in absorption efficiency compared to vertically standing nanowires in a spectral region matching the intensity distribution of the sun. The enhancement is due to engineering the excited modes inside the nanowires regarding the symmetry properties of the nanowire/light system without increasing the absorbing material. We expect our results to be important for nanowire-based photovoltaic applications. © 2017 SPIE.

Geometrically designed III-V nanowire arrays are promising candidates for disruptive optoelectronics due to the possibility of obtaining a strongly enhanced absorption resulting from nanophotonic resonance effects. With normally incident light on such vertical nanowire arrays, the absorption spectra exhibit peaks that originate from excitation of HE1m waveguide modes in the constituent nanowires. However, the absorption spectra typically show dips between the absorption peaks. Conventionally, such weak absorption has been counteracted by either making the nanowires longer or by decreasing the pitch of the array, both alternatives effectively increasing the volume of absorbing material in the array. Here, we first study two approaches for compensating the absorption dips by exciting additional Mie resonances: 1) oblique light incidence on vertical InP nanowire arrays and 2) normal light incidence on inclined InP nanowire arrays. We then show that branched nanowires offer a novel route to achieve broadband absorption by taking advantage of simultaneous excitations of Mie resonances in the branches and guided HE1m modes in the stem. Finite element method calculations show that the absorption efficiency is enhanced from 0.72 for vertical nanowires to 0.78 for branched nanowires under normal light incidence. Our work provides new insight for the development of novel efficient photovoltaics with high efficiency and reduced active material volume.

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

Core–shell nanowires offer great potential to enhance the efficiency of light-emitting diodes (LEDs) and expand the attainable wavelength range of LEDs over the whole visible spectrum. Additionally, nanowire (NW) LEDs can offer both improved light extraction and emission enhancement if the diameter of the wires is not larger than half the emission wavelength (λ/2). However, AlGaInP nanowire LEDs have so far failed to match the high efficiencies of traditional planar technologies, and the parameters limiting the efficiency remain unidentified. In this work, we show by experimental and theoretical studies that the small nanowire dimensions required for efficient light extraction and emission enhancement facilitate significant loss currents, which result in a low efficiency in radial NW LEDs in particular. To this end, we fabricate AlGaInP core–shell nanowire LEDs where the nanowire diameter is roughly equal to λ/2, and we find that both a large loss current and a large contact resistance are present in the samples. To investigate the significant loss current observed in the experiments in more detail, we carry out device simulations accounting for the full 3D nanowire geometry. According to the simulations, the low efficiency of radial AlGaInP nanowire LEDs can be explained by a substantial hole leakage to the outer barrier layer due to the small layer thicknesses and the close proximity of the shell contact. Using further simulations, we propose modifications to the epitaxial structure to eliminate such leakage currents and to increase the efficiency to near unity without sacrificing the λ/2 upper limit of the nanowire diameter. To gain a better insight of the device physics, we introduce an optical output measurement technique to estimate an ideality factor that is only dependent on the quasi-Fermi level separation in the LED. The results show ideality factors in the range of 1–2 around the maximum LED efficiency even in the presence of a very large voltage loss, indicating that the technique is especially attractive for measuring nanowire LEDs at an early stage of development before electrical contacts have been optimized. The presented results and characterization techniques form a basis of how to simultaneously optimize the electrical and optical efficiency of core–shell nanowire LEDs, paving the way to nanowire light emitters that make true use of larger-than-unity Purcell factors and the consequently enhanced spontaneous emission. © 2017American Chemical Society