Composition, Optical Resonances, and Doping of InP/InGaP Nanowires for Tandem Solar Cells: a Micro-Raman Analysis

We present a micro-Raman study of InP/InGaP tandem junction photovoltaic nanowires. These nanowires render possible InGaP compositions that cannot be made in thin films due to strain. The micro-Raman spectra acquired along the nanowires reveal the existence of compositional changes in the InGaP alloy associated with the doping sequence. The heavily Zn-doped InxGa1–xP (x is the In molar fraction) side of the tunnel diode is Ga rich, x = 0.25, with respect to the n-type and intrinsic segments of the top cell, which are close to the nominal composition of the NWs (x = 0.35). The p-type end segment is still Ga-rich. Electromagnetic resonances are observed in the tunnel diode. The Raman signal arising from the InGaP side of the tunnel diode is significantly enhanced. This enhancement permits the observation of a Raman mode that can be associated with an LO phonon plasmon coupled mode (LOPCM). This mode has not been previously reported in the literature of InGaP, and it permits the Raman characterization of the tunnel diode. The analysis of this mode and its relation to the LO phonon modes of the alloy, InP-like and GaP-like, allows to establish an apparent one-mode behavior for the phonon plasmon coupling. It indicates that hole plasma couples to the GaP-like LO mode. The LOPCMs are modeled using the Lindhard Mermin formalism for the dielectric function.


S2. High-resolution transmission electron microscopy
High-resolution transmission electron microscopy (HRTEM) measurements were carried out in a JEOL JEM 3000F.The nanowires were deposited on a carbon grid for TEM observation.

S3. LOPCM modeling
The Raman spectra are analysed using a dielectric model based on the Lindhard-Mermin susceptibility taking into account only the heavy holes (HH) contribution.The main parameters used in our calculations are listed in Table SI The differential Raman cross section for the LOPCM's of a doped two-mode ternary alloy A x B 1−x C can be expressed as: Here is the average high-frequency dielectric function, and the dielectric function of the alloy is given by: where ε ∞ is the high frequency dielectric constant, χ h (ω) is the electric susceptibility of the free-charge plasma, and χ i (ω, x) is the ionic susceptibility contributions from each sublattice given by: T O,i and ω 0 LO,i , with i = A, B, are the TO and LO phonon frequencies of the pure endmember compounds, ω T O,i is the TO phonon frequency of the alloy i sublattice, and Γ i is the phonon damping parameter.
However, as the results suggest that the phonon plasmon coupling can be treated as one mode behavior, we only have taken into account the GaP-sublattice in the calculations, so the cross term in Eq.( 1) has been eliminated.Thus, we have used the one mode Lindhard-Mermin formalism.
The constants A i introduced in Eq.( 1) are defined as: with C 0 i the Faust-Henry coefficient for the pure end-member compound.
We used the Lindhard-Mermin model to calculate the free hole contributions to the susceptibility (χ h ): where the heavy-hole intraband contributions to the Lindhard susceptibility was given by: where f h (E h F , T, k) is the Fermi distribution function for a hole plasma with Fermi energy E F at temperature T , and E(k) is the energy dispersion of the alloy conduction band.

S4. Energy-dispersive X-ray spectroscopy (EDX)
The chemical composition analysis was carried out using a Scanning Electron Microscope (FEI-QUANTA 200FEG, Hillsboro, OR, USA) with Energy Dispersive Spectrometry.The system has a Schottky's Filament Field Emission Cannon, and the results were achieved at 10 kV.An EDAX Genesis micro-probe (Mahwah, NJ, USA) was used for elemental microanalysis.

Fig
Fig. SI.1 illustrates the Raman spectra obtained from nanowires of different lengths, demonstrating an overall similarity, although with some distinctions attributed to the different growth rate for different NW lengths.

Figure SI. 1 :
Figure SI.1:Selected Raman spectra representative of each sector of the NW for different length NW a) 2.5 µm, b) 6 µm, and c) 14 µm.

Figure
Figure SI.2: a) HRTEM image of the InP bottom cell, b) Fast Fourier Transform (FFT) of the HRTEM a).

Fig
Fig.SI.2a and Fig.SI.2b show a high-resolution TEM image and the corresponding fast Fourier transform (FFT), which simulates the experimental electron diffraction pattern (EDP), of a region of the InP bottom cell of the nanowire.The diffraction spots were indexed as (111) and (220) reflections of the ZB InP phase.

Fig
Fig.SI.3a and Fig.SI.3b show a high-resolution TEM image and the corresponding FFT of the interface of InP/InGaP axially heterostructured NW which corresponds to the InGaP/InP tunnel junction.The FFT shown in Fig.SI.3b reveals the existence of two ZB structures that are aligned along the (111) axis.The spots marked in red correspond to the [011]-InGaP planes, and those marked in blue to [011]-InP.The inverted images for both set of planes are shown in Fig.SI.3c and Fig.SI.3d,where the InGaP tunnel junction region (Fig.SI.3c) and the InP region (Fig.SI.3d) are well defined.

Figure
Figure SI.3: a) HRTEM of the tunnel junction between InP and InGaP subcells, b) FFT of the TEM image shown in a), c) inverted image extracted from the indexed planes marked in red in figure b) that corresponds to [011]-InGaP and, d) inverted image extracted from the indexed planes marked in blue in figure b) that corresponds to [011]-InP.

Figure
Figure SI.4: a) TEM image of a InP/InGaP nanowire, b) electron diffraction patter (EDP) of the complete nanowire where two ZB structures in the [011] zone axis are identified.The planes marked in red belongs to ZB [011]-InGaP and the planes marked in blue to ZB [011]-InP.

Fig
Fig.SI.4a and Fig.SI.4b display a TEM image and the corresponding experimental electron diffraction pattern of a InP/InGaP nanowire.The diffraction spots were indexed showing the existence of two ZB structures.According to the previous HRTEM (Fig.SI.3) it can be identified ZB [011]-InGaP planes in red and ZB [011]-InP planes in blue.This result confirms the ZB structure in both InGaP and InP over the complete nanowire.

Figure SI. 5 :
Figure SI.5:SEM image of the nanowire with the numbered points where the EDX measurements have been carried out, and the concentration of In and Ga in each point measured.
.1.Table SI.1 List of parameters used in the LOPCM model.