Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar (112¯2) oriented In0.12Ga0.88N quantum well

Infrared Photodetectors based on Nanowire Arrays with Embedded Quantum Heterostructures

Optical sensors operating in the infrared range of the electromagnetic spectrum are key components in a variety ofapplications including optical communication, night vision, medical diagnosis, surveillance, and astronomy.Semiconductor nanowires have great potential for realizing broadband infrared photodetectors with excellentresponsivity, low dark current and low noise, and a unique compatibility with commercial silicon-based electronics.In this thesis work, comprising three published articles in Nano Letters, we synthesized, characterized andmodeled disruptive infrared photodetectors based on InP nanowires with axially embedded InAsP quantum discs.In the first article, we made a combined study of design, growth, device processing and optoelectronic propertiesof n+−i−n+ InP detector elements comprising 4 million periodically ordered nanowires in arrays, including either asingle or 20 InAsP quantum discs. Optimized Zn compensation of the residual non-intentional n-dopants in the isegmentsuppressed the dark current at room-temperature to a few pA/NW. The detector elements exhibit astrong broadband photoresponse with contributions from both the InP and InAsP segments with a thresholdwavelength of about 2.0 μm and a bias-tunable responsivity reaching 7 A/W@ 1.38 μm at 2 V bias.In the second article, we performed an in-depth experimental and theoretical investigation of the responsivity ofoptimized photodetectors under different illumination conditions. The photodetectors exhibit strongly bias andpower-dependent responsivities reaching record-high values of 250 A/W at 980 nm/20 nW and 990 A/W at 532nm/60 nW, both at 3.5 V bias. Complementary real device modeling revealed a new photogating mechanism,induced by the complex charge carrier dynamics involving optical excitation and recombination in the quantumdiscs and interface traps, which reduces the electron transport barrier between the n+ segment and the i-segmentunder illumination.Finally, in the last article, we demonstrate the first intersubband photocurrent response in a nanowireheterostructure array photodetector. The infrared response from 3 to 20 μm is enabled by intersubband transitionsin the low-bandgap InAsP quantum discs. The intriguing optical characteristics, including unexpected sensitivity tonormal incident radiation, are partly explained by excitation of the longitudinal component of optical modes in thephotonic crystal formed by the nanostructured portion of the detectors.Our results show that properly designed arrays of axial nanowire heterostructures are promising candidates forrealization of commercially viable broadband photodetectors