Low-temperature PL spectra generated from as-grown and annealed CdS/CdTe core-shell NWs are presented in this Section. PL spectra of CdS/CdTe structures are complex, but often have three distinct regions: a) near band-edge CdS luminescence (at 2.4 – 2.6 eV); b) CdTe luminescence (at 1.2 -1.6 eV); and c) an intermediate region (at 1.6 – 2.4 eV), in which luminescence can be assigned to deep-levels in CdS or to CdSxTe1-x inter-diffused layers. As
inter-diffusion of S and Te is highly influential on the performance of CdS/CdTe solar cells57, particular attention is paid to the intermediate region of the spectra upon discussing the results.
138
6.4.1 Experimental procedure
Au-catalysed CdTe NWs were grown on CdTe/glass substrates, (see Section 5.4) onto which CdS shell layers were sputter-grown at 200°C, with the shells being ~ 200 nm thick. Both as-grown and annealed (at 400°C, under 100 Torr N2 for 30 mins) core-shell NWs were
analysed by PL spectroscopy, with the NWs being mechanically removed onto Si substrates for measurements. Spectra were excited using the 457 nm line of a linearly polarised argon ion laser (excitation intensities of 30 – 1500 mW/cm2), with the beam incident at 20° to the normal and the samples held at 4K in a closed-cycle He cryostat.
6.4.2 Results
A comparison of the PL spectra of as-grown and annealed CdS/CdTe core-shell NWs is shown in Fig 6.16a (spectra having been corrected for unidentified background signal). Both spectra have a number of sharp features in the region 2.4 – 2.6 eV, which are typical of the CdS near-band edge region (CdS band-gap being 2.578 eV at 4K). A broad structured band also exists in both spectra from 1.4 – 2.2 eV, which consists of overlapping peaks and shoulders and will be referred to as the ‘intermediate band’. In the spectra of the as-grown NWs, some sharp features, overlapping with this broad band, are observed at 1.5 – 1.6 eV and 1.4 – 1.5 eV.
Fig 6.16: a) Comparison of PL spectra excited from as-grown (black line) and annealed (red line) isolated CdS/CdTe core-shell NWs. Annealing was carried out at 400C under 100 Torr N2. b) Deconvolution of the intermediate PL band of the annealed sample – black line shows
experimental data, dashed lines are the deconvoluted peaks and the red line is the fit generated from the deconvolution.
139 Gaussian deconvolutions (carried out using the Fityk software) of these spectra are shown in Fig 6.16b, 6.17 and 6.18. Such deconvolutions must be treated with caution however given that good fits are obtainable simply by the inclusion of more Gaussian peaks. Nevertheless, Fig 6.16b shows a deconvolution of the intermediate band from the spectra of the annealed NWs and Fig 6.17 and Fig 6.18 show deconvolutions of the entire spectra of the as-grown and annealed NWs respectively. The spectra are now described in more detail with reference to these.
In the CdS near-band edge region of the as-grown core-shell NWs (Fig 6.17), sharp peaks are observed at 2.543, 2.501, 2.479, 2.453 and 2.401 eV, and shoulders at 2.555 and 2.519 eV. From I - Lex measurements, exponent values, k, for the 2.555 and 2.543 eV features were
determined to be 1.04 and 1.10 respectively. Loss of spectral resolution made this analysis difficult for the other features. The intermediate region has a broad background peak centred at 1.72 eV, which overlaps with a series of peaks of similar intensity at 2.11, 1.98, 1.87, 1.77, 1.67 and 1.55 eV. The blue peaks are coloured so because they exist at energies previously assigned to transitions in bare CdTe NWs (see Section 6.3); sharp near-band edge CdTe peaks in the 1.53 – 1.59 eV region, and a DAP peak at 1.47 eV.
Fig 6.17: Deconvolution of PL spectra excited from as-grown CdS/CdTe core-shell NWs - black line shows experimental data, dashed lines are the deconvoluted peaks, blue lines are deconvoluted peaks assigned to CdTe emission and the red line is the fit generated from the deconvolution.
140 In the spectra of the annealed core-shell NWs (Fig 6.18), the CdS near-band edge features are still observed but are lower in intensity. A good fit to the intermediate band of the
annealed NWs (Fig 6.16b) could be obtained by using peaks at similar energies as for the as- grown core-shell NWs, i.e. at 2.11, 1.98, 1.87, 1.76, 1.66 and 1.55 eV overlapped with a broad 1.71 eV peak. The broad peak is notably more intense than for the as-grown core-shell NWs.
Fig 6.18:Deconvolution of PL spectra excited from CdS/CdTe core-shell NWs annealed at 400°C - black line shows experimental data, dashed lines are the deconvoluted peaks, and the red line is the fit generated from the deconvolution.
Moreover, upon annealing the features assigned to CdTe are no longer observed, but for a broad-band in the DAP region, and a new CdS feature is observed at 2.24 eV.
Assignments of each of these features to types of transitions are summarised in Table 6.5, having been made with reference to literature data (the reference being noted in the final column), these being discussed below.
Chapter 6: Structural, optical and electrical characterisation of CdTe and CdS/CdTe nanowires
141 Table 6.5: Photoluminescence bands excited from as-grown CdS/CdTe nanowires in this work and their assignments. See Figs 6.16 – 6.18 and text in Section 6.4.3. The references used for each assignment are included in the final column.
142
6.4.3 Discussion
First, the near-band edge CdS luminescence is discussed. Imada et al.58 state that, due to spin-orbit coupling, the valence band-edge in wurtzite CdS splits into three separate sub- bands, which allows three different free-exciton transitions, namely FE(A), FE(B) and FE(C). Imada calculated that the FE(A) transition is at 2.555 eV, and it has been estimated
experimentally at 2.551 and 2.553 eV by PL59, 60, and at 2.554 eV by absorption61
measurements. The small shoulder observed here at 2.555 eV on the high energy side of the 2.543 peak is therefore assigned to the FE(A). The exponent values of k = 1.04 for this feature is consistent with it being excitonic in nature – the observation of luminescence attributable to a free exciton only being common for single crystal CdS. The sharp intense peak at 2.543 eV is assigned to a donor-bound exciton, D0X, according to literature values from Jeong (2.545 eV)59, Yu (2.546 eV)62 and Lovergine63 (2.544 eV), its exact position depending on which donor impurity is involved. In Yu’s work, the exciton was bound to a F or Cl impurity,
although whilst CdS has been sputtered in the presence of CHF3 in the equipment used here in
previous work, no such F-containing gases were used in these growth experiments. The peak also has a k-value (1.10) consistent with it being excitonic.
The assignment of the features at 2.519, 2.501 and 2.479 eV are not clear: they may be phonon replicas of the FE(A) and D0X peaks – the LO phonon energy in CdS is 37 meV, and the 2.519 peak is separated by 36 meV from the FE(A). Ekimov et al.64 attribute luminescence in this region to transitions involving a free hole and the donor (hD0). Peaks observed at 2.453 and 2.414 eV have previously been attributed to the Y-band65 – excitons localised at
dislocations – and to DAP luminescence66 involving VS-2 respectively.
Secondly, the intermediate band has been deconvoluted into a series of bands at 2.11, 1.98, 1.87, 1.77, 1.67 and 1.55, and a broad band at 1.72 eV (shown in Fig 6.17 and 6.18, but more clearly in Fig 6.16b). Although deconvolution of spectra of low-resolution must be treated with caution, this model is supported by previous work: Abken et al.67 labelled this region as the ‘infra-red/red band’, and observed each of these peaks, (each within 10 meV of the peaks observed here, and each of comparable FWHM) in CBD grown CdS, assigning them (not including the peak at 2.12 eV) to the radiative decay of electrons trapped in surface states to the valence band – this may be expected to be significant for NWs that have a high surface-to- volume ratio. The broad feature similar to the one at 1.72 eV has been observed by Cuthbert68 (1.72 eV), Vuylsteke69 (1.73 eV) and Gemain70 (1.65 eV), with the former assigning it to DAP luminescence involving Te doping of CdS – an increase in its intensity was observed with
143 increased Te doping - and the others assigning it to DAP luminescence involving VS-2. The
FWHM observed here (0.47 eV) is highly comparable to that observed by Cuthbert (0.44 eV) for this peak. The peak at 2.12 eV has previously been associated with DAP transitions involving Cdi-VCd complexes formed during S re-evaporation67, 71.
In the range 1.53 – 1.59 eV, three peaks were observed, which are likely to be attributable to CdTe luminescence, as discussed in Section 6.3, as is the broad DAP band at 1.47 eV – their spectral positions being consistent with those listed in Table 6.1.
A number of differences are observed between the PL spectra of as-grown core-shell NWs and those annealed at 400°C. Upon annealing: a) the intensity of the near band edge CdS luminescence was reduced; b) the CdTe luminescence was much less intense and could not be resolved upon deconvolution; c) the intermediate band increased in intensity, particularly the 1.72 eV peak which is related to Te doping of S and/or S vacancies; and d) a new peak at 2.24 eV was observed, similar ‘green-yellow’ luminescence being associated with Cd interstitials and/or S loss67. The reduction of CdS near-band edge luminescence and the enhancement of features related to S vacancies implies the loss of S upon annealing, either to the atmosphere via re-evaporation, or by diffusion into the CdTe core. Moreover, the quenching of CdTe emission and enhancement of CdS:Te related emission would imply the diffusion of Te into CdS. It is therefore postulated that annealing promotes inter-diffusion of S and Te at the CdS/CdTe interface in these core-shell NWs, despite the lower density of grain boundaries present – inter-diffusion is typically most prominent at grain boundaries72.