Cadmium telluride (CdTe) thin films grown on indium antimonide (InSb) were examined by low temperature photoluminescence (PL), using an unfocused laser beam with variable excitation power over 300 times, to resolve the long standing issue of the huge variation of the resulting spectra in the deep region (1.40-1.52eV), where the expected phonon replicas alone were hardly observed. The phonon coupling strength, characterized by the Huang-Rhys parameter, or S-factor, as well as the peak width, were both found to increase with the excitation power. The puzzling coexistence of sharp peaks on top of a broad maximum was finally resolved by the awareness of beam intensity variation. Multiple phonon coupling processes can be present simultaneously, resulting in the superposition of narrow peaks on top of the broad maximum. Through the data fitting, three sets of donor-acceptor pair (DAP) recombination emissions with multiple values of Huang-Rhys parameter S can be identified.
© 2017 Optical Society of America
As an important optoelectronic material, CdTe has broad applications in infrared, X-ray and γ-ray detectors, as well as visible photovoltaic devices [1–3]. It possesses unique properties, such as high electrical resistance , high surface recombination velocity , and a direct band gap of about 1.5 eV at room temperature (RT)  and slightly higher than 1.6 eV at low temperature (LT) below 20K [4–6], suitable for photovoltaic applications with low cost and high efficiency (ca. 20%) [2,7]. They have brought up long term interests since 1980s of growing high quality single-crystalline CdTe bulk [5,6,8–10], thin films [11–19], alloys , hetero-structures [1,21,22], superlattices/quantum wells [21–23], quantum dots  and solar cells [2,5,13,25]. Many different substrates have been used for the epitaxial growth of CdTe, including GaAs [13,16,17], InSb [1,6], CdTe [11,12], CdZnTe [21,22], sapphire , alumina , and glass [2,3,25]. Due to a close match of the lattice constants between CdTe (6.4829 Å) and InSb (6.4798 Å) at room temperature (RT), InSb is attractive for the growth of CdTe by molecular beam epitaxy (MBE) [1,11].
In the meantime, low temperature (LT) photoluminescence (PL) has been considered as a primary tool of studying the II-VI semiconductors in general, and CdTe in particular, including, e.g., single-crystalline CdTe [4–13,16,17], poly-crystalline CdTe [14,15,19], and other CdTe-based structures and devices [1,7,21,22,24]. The PL spectra of CdTe were found to exhibit featured emissions in three main sections, viz., (i) excitonic (1.58-1.6 eV), (ii) intermediate (1.52-1.58 eV) and (iii) deep (1.40-1.52 eV) regions [5–7,11,16]. The intermediate spectral region consists of mainly free-to-bond and donor-acceptor-pair (DAP) transitions, while the deep spectral region is generally related to defects or/and complex, as well as DAP transitions [4–8,10,14,16,19].
However, due to the variety of instrumentation and variable measurement conditions, such as temperature and excitation power, the resulting PL spectra of CdTe vary tremendously. At LT, they sometimes show a single broad band , other times a broad band superposed with sharp lines related to the multiple longitudinal optical (LO) phonon replicas [6,7,10,14,15,17,19]. The 1.4 eV deep emissions in CdTe are recognized as defects-related, especially Cd-vacancy and impurity complex, including the so-called A-center [4,5,10,19] and Y-center [4,6,7,10,14,19] defects. Sometimes, well-spaced sharp lines, with the energy separation of 21 meV, due to the CdTe LO phonon coupling with the lattice, appeared. Moreover, the intensity of the mth phonon replica at LT could be described by the classical expression of 
In order to resolve these long standing issues, we employed here LT-PL measurements of CdTe, under variable excitation levels over 365 times, to uncover the causes of the spectrum variation. The coupling strength, the so called S-factor, and the peak breadth (w) were both found to increase with the excitation level. Also, it was revealed that there can be more than one phonon coupling parameters co-existing, resulting in the superposition of narrow peaks on top of the broad maximum. These findings offer a clearer understanding of the physics with respect to the data variation in the literature, making possible the quantitative analysis of the systems in general, and CdTe samples in particular.
The CdTe samples were grown on InSb (001) by MBE. A typical film studied here was grown under 225°C and growth rate of 0.66 μm per hour, with a thickness of 1.32 μm. The sample was characterized by a variety of techniques including the infrared (IR) reflectance, spectroscopic Ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and so on, confirming the formation of film and its good surface and interface properties. It was measured by double-crystal X-ray diffraction (DCRXD) showing a single crystalline narrow XRD peak with a full width at half maxima (FWHM) of 20.3 arcsec, characteristic of high single crystalline perfection, for which an additional evidence was provided from sharp (narrower than 0.7 meV) exciton emission lines of PL spectra. LT-PL measurement system consists of, an Ar+ laser of 514.5 nm as the excitation source, a SPEX-1400 spectrometer, a cooled RCA C31034 photomultiplier (PM), a lock-in amplifier, and computers for the control and data processing. To achieve low excitation power level, necessary for the observation of weak PL features, the laser beam was unfocused on the sample surface, with the spot size of about 1 mm. The sample was immersed into liquid helium inside a Dewar connected to a mechanical pump, through pumping, cooled down to 2K, below the λ-point, leading no bubbles found in liquid helium, which helps to enhance the signal-to-noise ratio of spectra. During the experiments, because of limited amount of liquid helium in the Dewar, we first used two days to perform rough 2K PL scans to find proper excitation powers and measurement parameters such as scanning interval, lock-in constants and scale etc for use, and in the 3rd day, run five 2K PL scans with nice signal-to-noise ratios at the chosen excitation levels. The outputs from the Ar+-laser were set in digital values and we used a laser power meter to measure the excitation level before the sample, which are marked in Fig. 1.
The resulting LT-PL spectra are displayed in Fig. 1, with 770-880 nm (1.605 eV down to 1.417 eV) and five different excitation power levels over 365 times. The strongest Aα was set by a factor of x1 in intensity and all other ranges of spectra were set by corresponding intensity factors, to display all spectra within a graph and to well observe their variations.
It is obvious that, there is no qualitative difference in spectra above 1.51 eV, except for the PL intensities. The left part of Fig. 1 exhibits high-resolution spectra for energies in 1.575-1.605 eV, representing clear free-exciton (FE) features of Xn = 1, XUP and Xn = 2, and sharp bound exciton (BE) lines. The strongest Aα line at 1.5892 eV with a FWHM of 0.6 meV has been attributed to the excitons bound to a neutral acceptor, and the second strong Aγ at 1.5903 eV with a FWHM of 0.5 meV is a BE from another neutral acceptor. The lines of Dμ, Dλ and Dκ were assigned to the recombination of excitons bound to donor impurities or complexes [7,11,14]. The middle part of Fig. 1 shows the DAP transitions R (1.555 eV), P (1.548 eV) and Q (1.542 eV), and their phonon replicas, R1, P1 and Q1, in the intermediate emission region (1.51-1.57 eV).
However, more dramatic changes by the excitation power level are found on the right side of Fig. 1, in the so-called deep emission region (1.41-1.51 eV). At the lowest excitation of 0.2 mW, it shows sharp lines of B, B1, B2 and B3 with a fixed interval of 21 meV, i.e. the CdTe LO phonon energy [9,11]. As the excitation power increases, the 1st phonon replica B1, raises its peak intensity much faster than the zero phonon line (ZPL) B, and so does the 2nd phonon replica B2. More astonishingly, as the excitation power further increases, a broader maximum starts to appear near the B3, and eventually become dominating the whole 1.4 eV spectral region.
It can be observed that B, B1 and B2 have indeed asymmetric line shapes, and at least two transitions are involved. Another C band is located at the right side of the B band. This C band and its phone replicas C1 and C2 possess asymmetric line shapes too. In addition, the broad maximum starts to emerge from B3 rather than B1, indicating its association with a higher S-factor (Eq. (1)), or a stronger phonon coupling.
A more systematic analysis of the spectra becomes possible now, which can obtain detailed information of the S-factors, the phonon coupling strengths. Expanding from the theory of Huang-Rhys , the intensity of the complex PL spectra can be described by the sum of phonon replicas (m) of DAP emission :Eq. (2) becomes a summation of Eq. (1). The summation over the phonon replicas (m) from 0 to 3, as the experimental results in Fig. 1, shows four individual bands, B, B1, B2 and B3.
The simulated results of Eq. (2), summed over 3 different S-factors (3 components), are plotted against the experimental data in Fig. 2, for the deep region of the CdTe PL spectra at 2K. The fitting parameters for each component, viz., peak intensity I0, Huang-Rhys factor S, peak energy E0, and peak width w, were obtained as shown in Fig. 3. It is seen that, the small S components dominate the low excitation curve, showing narrow lines and low order phonon replicas, whereas large S components take over the high excitation, presenting broad maxima and high order phonon replicas, but inevitably accompanied by the small S components. Small amounts of blue shifts of the peak positions can be found for various components, characteristic of DAP transitions [4,5,7,16]. Three DAP series, component-1, 2 and 3, specified by 3 slightly different ZPLs emissions, 1.498, 1.476 and 1.494 eV, respectively, can be obtained from the lowest excitation, Fig. 2(a).
Dependences of the S-parameter, peak energy position and band half width of three components on excitation intensity can be seen clearly in Fig. 3. Three ZPL emissions, 1.498, 1.476 and 1.494 eV, i.e. component-1, 2 and 3, respectively, are observed to shift toward higher energy with excitation power but at different rates, which are characteristic of donor-acceptor pair (DAP) emission nature [6,10,11,14,16,19]. The component-1 has the ZPL peak shift little within 1 meV. All values of their full width at half maximum (FWHM) are about or less than 0.6 meV. Such sharp DAP transition in the deeper 1.4 eV region for CdTe were scarcely observed in the literature. Previously, M. Soltani et al.  studied on PL of Bridgman-grown CdTe doped with As and Sb, which showed a 1.499 eV PL band at their Fig. 2, with a FWHM of ~6 meV but without any description. A sharp 1.499 eV PL emission, with a FWHM of ~2.5 meV, from MBE-grown undoped CdTe on CdTe substrate was observed under the excitation below the shallow donor energy level by Z. Yu et al. , which might be related to hydrogenation. C. Onodera et al.  recently reported the LT-PL analysis of CdTe on GaAs for the 1.4 eV region emissions, which consist of a ZPL band (7.8 meV wide) located slightly less but close to 1.50 eV and four phonon replicas superposed on a broad background, with s of 1.1. Our 1.499 eV – series DAP transitions, i.e. component-1, possess low Huang-Rhys parameter s values of 1.05 under 0.2 mW and ~1.25 under all other excitations, indicating a weak phonon coupling strength for this shallow DAP process. This set of DAPs (ZPL and three phonon replicas) possess narrow line width of 0.6 meV and energy position very little changes as the excitation power level changed by 365 times, which makes us to assume that only single DAP, not complex, is involved to the 1.499 eV DAP series transitions.
The component-2 has the ZPL at ~1.47 eV with slight change on the excitation power, in a total shift of ~3 meV only. The FWHMs are slightly increased from 1.67 meV to 3 meV, with very low S-factors of less than 0.3. It could be due to the Y-band presumably related to defects associated to the dislocations with low S-values [4,6,9,10]. S. Hildebrandt et al.  studied the Y- luminescence band at 1.476 eV from un-doped (111) CdTe single crystals, and measured between 2 and 11 K having a rather large linewidth of 8 to 9 mev as well as an anomalously weak LO phonon coupling with a Huang-Rhys factor S = 0.16 to 0.4. This defect-related Y-band was considered to originate from the polar Te(g) glide dislocation segments in CdTe [6,9,10]. J. Procházka et al.  investigated the PL of indium-doped CdTe single crystals grown by the vertical gradient freeze method using above- and below-bandgap excitation at temperatures of 4.5-20 K and found the position of ZPL of “1.4 eV band” located at different photon energies in the samples without In-doping. They recognized that the “1.4 eV band” is more complicated than a simple transition from the recombination in DAPs to the recombination of free electron with hole bound to acceptor, and suggested that the indium A-center at 145-150 meV above the valence band forms the complex consisting of Cd vacancy, VCd, and In-donor occupied Cd position, InCd, which contributed to the 1.4 eV band emissions . By the way, even for CdZnTe (with ~10% Zn) single crystals, G. Yang et al.  performed LT micro-PL and mapping, and showed the defect-related broad D band at 1.5 eV contributed from the A center, [InCd+ - VCd2-]-.
The component-3 exhibits another type of variation with excitation power, quite different from other two components. At three lower excitations, the overall 1.4 eV spectra looked as wave-like shape with the zero-phonon and replicas broad bands connected but with the ZPL E0 at a same energy of 1.494 eV. The ZPLs have the large FWHMs of 5.80-7.13 mW. As the excitation increased to 14.3 and 73 mW, the FWHM increased to larger values of 12.6-16.5 meV. The Huang-Rhys coupling factor S also increased greatly from 1.5 to 6.2. Under two lower excitations with low s of 1.5-2.2, four bands of zero phonon and replicas can be distinguished, while at 4.3 mW with s of 2.4, the four-bands structured feature was deemed weak. As the excitation goes to 14.3 mW with s of 3.1, the four-band structure developed into a smooth and broad band spreading over the entire 1.4 eV region with the maximum peak energy at 1.45 eV. This broad band increased its overall intensity further as the excitation reaches 73 mW with a large s of 6.2, which is expected for much stronger phonon-electron coupling.
Due to the influence of other two DAP components with weaker phonon-couplings and E0 at 1.499 eV and 1.47 eV, the total-all and experimental 1.4 eV PL emissions with the observed broad band peak down-shifted a little to 1.44 eV, as marked in Fig. 1(e). Hence a clear description on the 1.4 eV spectral variations has been obtained and the underneath physics is revealed in details. The 1.4 eV broad band was originated from the DAP transitions at low excitation with four-band structure of zero-phonon band centered at 1.494 eV and its phonon replicas separated by the CdTe LO phonon energy of 21 meV. As the excitation power increased, the four-band structure gradually developed into a smooth broad band peaked at 1.45 eV. This 3rd set of DAP formed the broad 1.4 eV smooth band and other two sets of DAP constructed the overlapped sharp peaks as appeared in Fig. 1(d) and 1(e).
Quite recently, V. Kosyak et al.  studied the LT-PL spectra of CdTe and Cd1-xZnxTe thick films deposited on the Mo coated glass by the close-spaced vacuum sublimation (CSVS) and observed, at LT-PL from CdTe, a band at 1.495eV which was signed to the DAP type complex [VCd–D], where D is residual donor and VCd is a cadmium vacancy.
As already shown by many authors, the CdTe 1.4 eV deep PL emissions, either at low or high temperature, display mostly a broad band, with or without overlapping sharp lines of zero-phonon and phonon replicas [5,6,10,13–16,19]. Our experiment, on the other hand, provides a full coverage of the possible scenarios, allowing the clarification of the physics behind, and the identification of the multiple phonon coupling processes. Furthermore, the data fitting leads to the separation and attribution of the Huang-Rhys factors, which were found to vary with the excitation power and no longer a constant.
In addition to the deep DAPs in the 1.4-eV region and their coupling with phonons, Fig. 1 has also shown the exciton-phonon couplings and 1.5-eV emission-phonon couplings. The excitons of Aα, Aγ, E and F have their weaker 1st order phonon replicas of Aα1, Aγ1, E1 and F1, indicating smaller S-factors than those from deep 1.4-eV DAP-phonon couplings, discussed above. So do the 1.5-eV recombination-phonon couplings. The exciton-phonon coupling phenomena have arisen more attention and been studied from CdTe-based quantum dots (QDs) [29–31], ZnO-based multiple quantum wells (MQWs) , ZnO nanotwins  and CdSeS nanocrystals . Besides PL, other techniques were employed to study the exciton-phonon couplings, such as, the resonantly excited PL and PL excitation (PLE) at 6 K with the finding that in the regime of weak exciton-LO phonon coupling the strength of this interaction may strongly increase for small QD’s , the time-resolved (TR) PL at 20 K and TD-PL in 20-110 K of the broad QDs emission band variation, and determining the factors related to exciton-phonon coupling and deteriorated the optical performance of CdTe/ZnTe QDs in terms of quantum efficiency , the TD-PL and UV-Vis-NIR absorption of CdTe/CdS QDs to understand the exciton–phonon interaction and radiative and nonradiative relaxation of carriers in these QDs , the TD-PL and absorption from the ZnO-based MQWs and discussion on the localization of excitons, the influence of exciton–phonon interaction and quantum-confined Stark effects , the RT UV-Vis absorption on free exciton and strong exciton phonon coupling from ZnO tein-nanorods , and the temperature and composition dependent excitonic luminescence and exciton-phonon coupling in CdSeS nanocrystals . Nevertheless, the exciton-phonon coupling and the DAP-phonon coupling are worthy to investigate further penetrative for the CdTe-based thin films, quantum and nano-structures.
To conclude, LT-PL spectra of CdTe crystalline thin film on InSb were examined by an unfocused laser beam, allowing for the detection of extremely weak luminescence signals with excellent signal-to-noise ratio. Through the excitation power variation over 365 times, multiple phonon coupling processes were resolved, leading to the separation and attribution of Huang-Rhys phonon coupling factors. It was found that both the coupling strength, the S-factor, and the peak breadth (w) increase with the excitation power. Moreover, it was revealed that there can be more than one phonon coupling parameters co-existing, with the superposition of narrow peaks on top of the broad maximum, often observed but unexplained before. The findings offer hopefully a clearer picture of the physics, behind the tremendous data variation reported in this letter and found in the literature.
National Natural Science Foundation of China (NSFC) (61367004); Natural Science Foundation of Guangxi Province (2013GXNSFFA019001).
We acknowledge Prof. W. J. Choyke for supporting experiments, Dr. R. F. C. Farrow for MBE growth, Prof. Jianyi Lin and Dr. Wenyi Chang on programming, and Mr. Deng Xie for technical help.
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