We have developed a technique so that both transmission electron microscopy and microphotoluminescence can be performed on the same semiconductor nanowire over a large range of optical power, thus allowing us to directly correlate structural and optical properties of rotationally twinned zinc blende InP nanowires. We have constructed the energy band diagram of the resulting multiquantum well heterostructure and have performed detailed quantum mechanical calculations of the electron and hole wave functions. The excitation power dependent blue-shift of the photoluminescence can be explained in terms of the predicted staggered band alignment of the rotationally twinned zinc blende/wurzite InP heterostructure and of the concomitant diagonal transitions between localized electron and hole states responsible for radiative recombination. The ability of rotational twinning to introduce a heterostructure in a chemically homogeneous nanowire material and alter in a major way its optical properties opens new possibilities for band-structure engineering.

/pdf/optical-properties-of-rotationally-twinned-inp-nanowire-3eobeug0tr.pdf

Optical properties of rotationally twinned InP nanowire heterostructures.

Methylammonium lead iodide perovskite has attracted considerable recent interest for solution processable solar cells and other optoelectronic applications. The orthorhombic-to-tetragonal phase transition in perovskite can significantly alter its optical, electrical properties and impact the corresponding applications. Here, we report a systematic investigation of the size-dependent orthorhombic-to-tetragonal phase transition using a combined temperature-dependent optical, electrical transport and transmission electron microscopy study. Our studies of individual perovskite microplates with variable thicknesses demonstrate that the phase transition temperature decreases with reducing microplate thickness. The sudden decrease of mobility around phase transition temperature and the presence of hysteresis loops in the temperature-dependent mobility confirm that the orthorhombic-to-tetragonal phase transition is a first-order phase transition. Our findings offer significant fundamental insight on the temperature- and size-dependent structural, optical and charge transport properties of perovskite materials, and can greatly impact future exploration of novel electronic and optoelectronic devices from these materials.

/pdf/size-dependent-phase-transition-in-methylammonium-lead-32l8ydhb4v.pdf

Size-dependent phase transition in methylammonium lead iodide perovskite microplate crystals

High‐Efficiency III‐V Multijunction Solar Cells

The band-gap (Egp) and valence band offset (ΔEv) energies of pseudomorphic GaAsSb layers on GaAs substrate are determined from temperature-dependent photoluminescence measurements on GaAsSb/GaAs and GaAsSb/GaAlAs quantum wells grown by molecular beam epitaxy. A clear evidence of staggered type-II band alignment of GaAsSb relative to GaAs and a value of 1.05 for the valence band offset ratio (Qv) are proposed. Finally, through a detailed comparison of these values with those published previously, we have shown that the scatter in Qv found in the literature (<1 to 2.1) is closely dependent on the exact determination of Egp. Particularly, we have shown that the strain dependence of the deformation potential is important in the calculation of the strain energy contribution to Egp.

Temperature-dependent valence band offset and band-gap energies of pseudomorphic GaAsSb on GaAs

Low-temperature photoluminescence spectra of an 1r1~,~sG~,s~P alloy and a p-type GaAs/ Ino,b.sGauszP multiple quantum well, both grown by molecular beam epilaxy, have been obtained under hydrostatic pressures from 0 to 6 GPa. The zero-pressure extrapolation of the InGaP to GaAs( I?) transitions yields a 0.40*0.02 valence-band offset, and hence only a small, 0.06 =t 0.02 eY, conduction-band offset. These offset values are in agreement with measured values of the confinement energy versus well width.

https://www.engr.colostate.edu/ece/faculty/robinson/pdf/journals/1991_spain.pdf

Band offset of GaAs/ln 0,48Ga0,52P measured under Ihydrostatic pressure

Interfacial characteristics of Ga0.51In0.49P/GaAs heterostructures grown by metal‐organic vapor‐phase epitaxy in the temperature range from 600 °C to 730 °C were studied. Photoluminescence (PL) measurements have been used for this purpose. A PL peak with an energy of about 1.425 eV (870 nm) was continuously observed in samples containing the GaInP‐to‐GaAs interface. Excitation power dependent PL measurements show that this peak belongs to an excitonic recombination. Furthermore, a strong blue‐shift of this PL‐peak energy was observed as the excitation power increased. We attribute the 870 nm peak to the radiative recombination of spatially separated electron‐hole pairs and suggest the type‐II band alignment at the ordered GaInP to GaAs heterointerface under growth conditions reported here. Further investigations using x‐ray diffraction measurements and simulations with dynamical theory show that the lower and upper interfaces are not equivalent. This explains the absence of type‐II transition in most GaAs‐to‐GaInP lower interfaces.

Evidence of type‐II band alignment at the ordered GaInP to GaAs heterointerface

A highly strained In0.5Ga0.5P spacer is introduced into an HFET grown by MOVPE on InP substrate for the first time. Despite the large lattice mismatch and the group V exchange at the channel-spacer interface the transport data are not affected (µH, 300K = 11 300 cm2/Vs, ns = 2 x 1012 cm-2). However the large bandgap energy of InGaP results in improved gate leakage, drain-conductance and drain breakdown. At VDS = 10 V a voltage gain of vu > 100 is maintained. HFETs with Lg = 0.7µm exhibit gm = 360 mS/mm, ID, max(VDS = 6V) = 550 mA/mm and a cutoff frequency fmax of 150 GHz. At VDS = 5.5V an AC output power of 0.6 W/mm is achieved indicating the capability for high power HFET application on InP substrates.

High breakdown voltage InGaAs/InAlAs HFET using In0.5Ga0.5P spacer layer

Ga0.51In0.49P layers grown by metal‐organic vapor‐phase epitaxy in the temperature range from 600 to 730 °C and with various Si‐doping concentrations were studied. X‐ray diffraction was used to observe the ordering in Ga0.51In0.49P layers. The rocking curves of {115} reflection have shown the existence of ordering in Ga0.51In0.49P even in Si‐doped samples with an electron concentration up to 1×1019 cm−3. The samples with ordered structure exhibit an additional {1/2, 1/2, 5/2} reflection. However, no such reflection, which is related to group‐III sublattice ordering, was observed in GaAs, AlGaAs, and GaInP layers grown at 730 °C. Photoluminescence (PL) and transmission electron diffraction (TED) results confirm these observations. TED patterns and especially the intensity of the additional ordering spots of Ga0.51In0.49P samples are almost unchanged in the studied doping range up to 1.7×1018 cm−3. An increase of PL peak energy attributed to the ordered structure randomization was not observed. The PL peak shift for highly doped Ga0.51In0.49P was found to be only due to the Burstein–Moss effect.

Analysis of ordering in GaInP by means of x‐ray diffraction

The role of hydrogen in low-temperature MOVPE growth and carbon doping of In0.53Ga0.47As for InP-based HBT

Photoluminescence analysis of Ga0.51In0.49P/GaAs single‐quantum well structures grown by metal‐organic vapor‐phase epitaxy in the temperature range from 570 to 720 °C have been carried out. Besides the GaAs band‐edge emissions, all SQW samples studied here exhibit a dominant long‐wavelength peak, which is attributed to the spatially indirect transition due to the type‐II band alignment of Ga0.51In0.49P/GaAs heterojunctions. The energy of the type‐II PL emission has been found to depend strongly on the growth temperature indicating the strong influence of the growth temperature on the band alignment. The shifts of the type‐II PL emission have been used to estimate the growth temperature dependent conduction and valence band discontinuity of the Ga0.51In0.49P/GaAs heterojunction. X‐ray diffraction measurements and simulations using the dynamical theory were carried out to study the influence of the growth temperature on the unintended interfacial layers.

A. Lindner

Papers

Evidence of type‐II band alignment at the ordered GaInP to GaAs heterointerface

High breakdown voltage InGaAs/InAlAs HFET using In0.5Ga0.5P spacer layer

Analysis of ordering in GaInP by means of x‐ray diffraction

The role of hydrogen in low-temperature MOVPE growth and carbon doping of In0.53Ga0.47As for InP-based HBT

Growth temperature dependent band alignment at the Ga0.51In0.49P to GaAs heterointerfaces