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The band edge and higher-lying interband transitions of CuAl(Se0.5S0.5)2 compound have been characterized using thermoreflectance (TR) measurements at room temperature. Single crystals of CuAl(Se0.5S0.5)2 were grown by chemical vapor transport method using ICl3 as the transport agent. The TR measurement results showed three interband transitions denoted as E1, E2, and E3 detected near the band edge of CuAl(Se0.5S0.5)2 chalcopyrite compound. The lowest-energy transition is the main band-edge transition positioned at E1 = 2.96 eV, the second is E2 = 3.05 eV, and the other higher-energy transition is E3 = 3.202 eV, respectively. Polarized-thermoreflectance (PTR) measurements showed that the E1 and E2 transition features are observed only with the linearly polarized light along the ε|| (needle axis) direction while the E3 transition largely appears with the electrical field of ε perpendicular (⊥) to the needle axis. The E1 and E2 transitions may originate from the valence-band top while the E3 transition is closely related to the valence-band splitting in the crystal. To characterize the anisotropic properties of electronic structure of CuAl(Se0.5S0.5)2, PTR measurements were carried out over a wide energy range of 2.5-6 eV. The interband transitions belonging to E|| or E⊥ polarization are respectively identified. On the basis of experimental analysis, the electronic structure near the fundamental band edge of CuAl(Se0.5S0.5)2 has thus been determined.
©2013 Optical Society of America
1. Introduction
The I-III-VI2 chalcopyrite semiconductors (I = Cu, Ag, III = In, Ga, Al, and VI = S, Se) have recently received considerable attention for serving as a potential candidate applied in thin-film solar cells [1,2], optical emitters and optical absorbers [3–5], photocatalysts [6,7], and humidity sensors [8]. The main character of I-III-VI2 compound is its direct band edge with very high absorption coefficient under sunlight [9–11]. Especially, the CuAlX2 (X = S, Se) alloys are the group members with larger band gap in the I-III-VI2 family, which serve as a potential candidate for application as a blue-to-ultraviolet (UV) absorber and emitter [12,13]. The wide-band-gap nature of CuAlX2 (X = S, Se) renders them possessing the possibility for fabrication of optoelectronic devices operated at high-temperature range above room temperature.
CuAlX2 (X = S, Se) compounds usually possess a chalcopyrite phase with tetragonal lattice. The values of direct band gap previously reported were ~2.5 eV for CuAlSe2 [13,14] and ~3.5 eV for CuAlS2 [15,16]. The band gap of CuAlSe2 dominates in the green to blue color region while the absorption range of CuAlS2 works in the UV portion. It may be inferred that the intermediate-composition compound CuAl(Se0.5S0.5)2 may have a direct band gap lying in between that of CuAlSe2 and CuAlS2, which is close to the blue to violet range. However, there is little experimental data on the optical properties of CuAl(Se0.5S0.5)2. It is necessary to have an overall understanding of the optical band-edge property of intermediate compounds in the CuAlX2 series.
In this paper, CuAlSe2, CuAl(Se0.5S0.5)2, and CuAlS2 chalcopyrite crystals have been grown by chemical vapor transport method using ICl3 as the transport agent. X-ray diffraction measurement and energy-dispersive X-ray spectroscopy (EDX) were carried out to verify stoichiometric content of the chalcopyrites. Optical properties of quaternary CuAl(Se0.5S0.5)2 have been characterized using polarized-thermoreflectance (PTR) measurements over a wide energy range of 2.5-6 eV at 300 K. The band-edge and high-lying interband transitions with ε|| (i.e. the needle axis) and ε⊥ polarizations are respectively characterized and identified via the selection rule applied in the PTR spectra. The energy values of interband transitions of CuAl(Se0.5S0.5)2 have been determined accurately from the line-shape analyses of PTR spectra. Transition origins for the PTR features of CuAl(Se0.5S0.5)2 have been assigned. Based on the experimental analyses, the in-plane anisotropy and electronic structure of CuAl(Se0.5S0.5)2 have been characterized.
2. Experiment
Single crystals of CuAl(Se0.5S0.5)2, CuAlSe2, and CuAlS2 solid solutions were grown by chemical vapor transport (CVT) method using ICl3 as a transport agent [17]. The stoichiometric starting materials were prepared from the elements (Cu: 99.995% pure, Al: 99.9995%, Se: 99.999%, and S: 99.999%). Totally about 10 grams of the starting mixture materials together with an appropriate amount of transport agent (ICl3 about 10 mg/cm3) were introduced into a quartz ampoule (outer diameter 22 mm, inner diameter 17 mm, and length 20 cm), which was then cooled with liquid nitrogen, evacuated to 10−6 Torr and sealed. The mixture in the quartz tube was slowly heated to 850 °C. This slow heating is necessary to avoid any explosions due to the strongly exothermic reaction between the elements. The use of Al ingot is better than Al powder to react with sulfur and selenium, which may avoid any possible explosion during the temperature change. For the CVT growth, the growth temperature was set as 850 °C → 750 °C with a gradient of −5 °C/cm to facilitate the chemical transport. The reaction was maintained for 300 hrs for producing large single crystals. After the growth, the synthetic crystals of CuAl(Se0.5S0.5)2, CuAlSe2, and CuAlS2 contained a lot of needle-like crystals that possess mirror-like as-grown surfaces. For X-ray measurements, several small crystals of as-grown CuAl(Se0.5S0.5)2, CuAlSe2, and CuAlS2 were finely ground and the X-ray powder patterns were taken and recorded by using Cu Kα radiation as the probing light source. EDX measurements were carried out to estimate constituent composition of the compounds. The stoichiometry of the CuAl(Se0.5S0.5)2, CuAlSe2, and CuAlS2 crystals showed approximate agreement with the original content of the starting materials within a reasonable standard error.
TR experiments were carried out over a wide energy range from 2.5 to 6 eV. A 150 W xenon-arc lamp acted as the light source. The reflected light of the sample was detected by a Hamamatsu H3177-51 photomultiplier tube (PMT) and the signal was recorded via an EG&G model 7265 dual phase lock-in amplifier. For thermal perturbation of the samples, a quartz plate acted as the heat sink. The quartz plate coated with a winding path of golden tracks acted as the heating element. The shape of golden path was formed by copper mask using magnetron sputtering [18]. The heating path consisted of two wide tracks at the end sections and one narrow track lying in between them. The narrow track in the middle section is designed to act as a heat generation source when electrical current passes through the heater. The function of the wide tracks at the end sections is to speed up the heat dissipation when electrical power is off. The thin sample of CuAl(Se0.5S0.5)2 crystal with as-grown {112} face was closely attached on the narrow track of the Au path by silicone grease (thermal conductivity of ~2 W/m-K). Thermal modulations of the samples were achieved by indirect heating by supplying current pulses to the Au heating element (resistance about 20 Ω) periodically. Heating pulses of low frequency and long duty cycle seem to be more efficient in the enhancement of spectral amplitude of transition features. Heat generation and dissipation from all the Au tracks and quartz substrate must be well balanced to avoid any long-term increase of temperature in sample (i.e. weak heating disturbance, a cooling fan also used).A 4 Hz square wave with duty cycle 50% and amplitude 0.5 V (i.e. the supplied current is about 0.5 A) is employed in the TR experiments. For PTR measurements, a pair of Glan-Taylor-prism polarizers (210 - 2300 nm) was employed. The experiments were done on the as-grown {112} surface of the CuAl(Se0.5S0.5)2. The angle-dependent PTR measurements were carried out with the linearly polarized light set at ε|| (the needle axis) and ε⊥ polarizations, respectively.
3. Results and discussion
Figure 1(a) shows the powder X-ray diffraction pattern of the chalcopyrites of CuAlS2, CuAl(Se0.5S0.5)2, and CuAlSe2 crystals in the angular range of 25-65°. Several peak features that, respectively, indexed to the tetragonal chalcopyrite phase for CuAlX2 are observed in Fig. 1(a). The strongest intensity of the (112) plane shows the preferred orientation of the CuAlX2 chalcopyrite crystals. As shown in Fig. 1(a), the diffraction peaks of (112), (103), (200), (211), (220), (204), (312), and (116) are simultaneously detected in the X-ray diffraction patterns while the angle position of each peak shifts to higher diffraction angle with respect to the increase of S composition from CuAlSe2 to CuAlS2. For CuAl(Se0.5S0.5)2, the angle value of each diffraction peak just lies in between the corresponding values of CuAlSe2 and CuAlS2. This situation indicates that the three compounds are crystallized in the same chalcopyrite phase with lower lattice constant in CuAlS2 but higher lattice constant in CuAlSe2. Lattice constants of a and c for the chalcopyrites present in Fig. 1(a) can be calculated using a tetragonal equation and their values are determined to be a = 5.33Å, c = 10.45 Å for CuAlS2, a = 5.46Å, c = 10.67 Å for CuAl(Se0.5S0.5)2, and a = 5.60Å, c = 10.95 Å for CuAlSe2, respectively. The values of a and c of CuAl(Se0.5S0.5)2 are close to the medium values between CuAlS2 and CuAlSe2 in matching with a linear relation of Vegard’s law. To further verify the stochiometry of the CuAl(Se0.5S0.5)2 compound, EDX measurements of the three as-grown chalcopyrites were implemented. Figure 1(b) displays the EDX results of the CuAlS2, CuAl(Se0.5S0.5)2, and CuAlSe2 crystals. The stoichiometric contents for the three compounds show that the composition of each chalcopyrite approximately agrees well with the original content of the starting material before crystal growth within a standard error less than 0.6%. The homogeneity of the samples is also good because they are single crystals.
Fig. 1 (a) Powder X-ray diffraction patterns of CuAlS2, CuAl(Se0.5S0.5)2 and CuAlSe2 crystals. (b) EDX spectra and content analysis of CuAlS2, CuAl(Se0.5S0.5)2 and CuAlSe2.
The upper part in Fig. 2 shows the unpolarized TR spectra of CuAlS2, CuAl(Se0.5S0.5)2, and CuAlSe2 single crystals measured on the as-grown {112} face of the chalcopyrites near the band edge. The dashed lines are the experimental data and solid lines are least-square fits to a Lorentzian line-shape function appropriate for the interband transitions expressed as [19]:
where i is the respective transition, and and are amplitude and phase of the line shape, and and are the energy and broadening parameter for the interband transitions of the chalcopyrite crystals. Where the value of n in Eq. (1) is assigned as n = 2, responsible for excitonic transitions [20]. The assignment of n value for band-edge exciton is based on previous temperature-dependent TR measurements of CuAlS2 from 30 to 340 K [5], where a higher-order excitonic feature (i.e. the E2 feature of CuAlS2 in Fig. 2) showed thermal ionization at T ≥ 320K. The E1-E3 transitions of CuAl(Se0.5S0.5)2 and CuAlSe2 near band edge (see Fig. 2) are hence temporarily assigned as excitonic-like features and taking as the n value of n = 2 for line-shape analysis herein. According to the line shape analysis by using Eq. (1), the obtained transition energies of E1, E2, and E3 of the chalcopyrite crystals are listed in Table 1 and the values are also depicted in the lower part of Fig. 2 for comparison. The energy values of E1-E3 show an increase with respect to the increase of S content in the CuAl(Se1-xSx)2 (0≤x≤1) series due to the reduction in lattice constants. The obtained values of fitting parameter of broadening parameter Γi from Eq. (1) are also included in Table 1 for comparison. The broadening parameter Γ1 of main band-edge transition E1 gradually increases with increasing Se content from CuAlS2, CuAl(Se0.5S0.5)2 to CuAlSe2. It indicates that the CuAlS2 possesses higher crystal quality among the three compounds. The inference can also be verified from the observation that the pure sulfide compound has the highest hardness and bond strength observed (i.e. by mechanical test) in the three as-grown chalcopyrite crystals. As shown in Fig. 2, the transition energies of CuAl(Se0.5S0.5)2 near band edge are E1 = 2.96 ± 0.01 eV, E2 = 3.050 ± 0.008 eV, and E3 = 3.202 ± 0.008 eV, respectively. The values of E1, E2 and E3 are lying in between the corresponding energy values obtained for CuAlSe2 and CuAlS2 as listed in Table 1 and shown in the lower part of Fig. 2. The E1 transition in the three compounds is the main band-edge exciton of the wide-band-gap chalcopyrites. The band-edge structure of CuAlX2 (X = S, Se) is mainly determined by X p→p, which belongs to the “intra-atomic” transition [21]. The p→p “intra-atomic” transition is a special character of I-III-VI2 chalcopyrites and it is significant for very high absorption coefficient to make promising as a solar cell material. For the electronic structure of CuAlX2, the upper valence band (EV) of CuAlX2 is dominated by Cu-d and X-p hybrid interactions and the conduction band (EC) includes the contributions from X-s/p, Cu-s/p, and Al-s/p evident by density-of-states calculations [22]. The E1 is assigned to be a fundamental band-edge exciton from EV top to EC bottom at the Γ point. For the EV top of the chalcopyrite, the degenerate Γ15 band splits into Γ4V and Γ5V due to uniaxial crystal field [23,24]. The EV top of CuAlS2 is S p-like consisting of a higher Γ4V and a lower split Γ5V. The E1 transition of CuAlX2 is hence assigned as the transition of Γ4V to Γ1C. For the origin of the E2 transition, previously temperature-dependent measurements of CuAlS2 revealed that it might arise from the higher exciton level above the E1 transition [5]. The polarization-dependent optical measurements also verified that E2 of CuAlS2 has the same polarization dependence as that of E1 [25]. The E2 transition near the band edge of CuAl(Se0.5S0.5)2 may also have the same origin as E1 at the Γ point in the valence band. The origin of the E3 feature for CuAlX2 is inferred to come from the split valence band Γ5V and it will be verified by PTR measurements later.Fig. 2 Experimental TR spectra of CuAlS2, CuAl(Se0.5S0.5)2 and CuAlSe2 single crystals measured on the as-grown {112} crystalline plane at 300 K. The lower part depicts the energy values of E1, E2, and E3 for the three chalcopyrite compounds.
Table 1. Values of the Fitting Results of Transition Energies and Broadening Parameters in CuAlSe2, CuAl(Se0.5S0.5)2, and CuAlS2 Single Crystals Obtained by TR Measurements at 300 K
In order to characterize band-edge transitions of CuAl(Se0.5S0.5)2 in detail, PTR experiments were carried out with the electric fields of ε|| needle axis (E||) and ε⊥ needle axis (E⊥) at room temperature. The upper part in Fig. 3 displays the unpolarized and PTR spectra of a needle-like CuAl(Se0.5S0.5)2 crystal. The crystal outline and measurement configuration of PTR for CuAl(Se0.5S0.5)2 are shown in the lower part of Fig. 3. The measurements were done on the as-grown {112} plane of the CuAl(Se0.5S0.5)2 and the linearly-polarized light beams (E|| and E⊥) with a spot size of ~hundred μm2 were incident on the sample surface. The dashed lines in Fig. 3 are the experimental spectra and solid lines are the least-square fits using Eq. (1) which yield transition energies indicated with arrows. The polarization dependence of band-edge transitions of CuAl(Se0.5S0.5)2 clearly indicates that E1|| and E2|| transitions are merely present in the E|| spectrum while the E3 feature is dominant with the E⊥ polarization (E3⊥). A slightly and weakened transition feature with the E|| polarization (E3||) was also detected in Fig. 3. The polarization dependence of the band-edge transitions verifies that E1 and E2 are probably the band-edge excitons at the valence-band top and the E3 transition arises from the valence-band splitting. The polarization-dependent optical results of CuAlS2 also showed that E1 and E2 are band-edge excitons and E3 comes from the valence-band splitting [25]. For CuAlSe2, a previous In-solvent-grown sample by traveling-heating method (containing 4% In) also indicated that the lowest band-edge transition (E~2.450 eV) is mainly along the E|| direction while the transition of valence-band splitting (E~2.704 eV) is predominantly along the E⊥ polarization observed by spectral ellipsometer [14]. The previous results of polarization dependence of CuAlS2 and CuAlSe2 [14,25] facilitated the assignment of transition origins for the E1-E3 transitions in CuAl(Se0.5S0.5)2 in Fig. 3. As shown in Fig. 3, the unpolarized TR spectrum shows a reasonable superposition of the E|| and E⊥ polarized spectra for the chalcopyrite. The obtained transition energies of E1, E2 and E3 from PTR in Fig. 3(a) matched well with those listed in Table 1 obtained by unpolarized TR measurements. Also shown in Fig. 2 and Table 1, the energy separations of E1 and E3 are 0.154 eV for CuAlS2, 0.242 eV for CuAl(Se0.5S0.5)2, and 0.301 eV for CuAlSe2, respectively. With increasing Se content in the CuAl(Se1-xSx)2 (0≤x≤1), valence-band splitting is highly separated owing to the inter-substitution of S and Se atoms with different atomic size in the chalcopyrites. This situation was also observed in the other optical measurements at 77K for CuAlX2 (X = S, Se) [26]. For the main band-edge transition, the energy value of E1 is 2.53 ± 0.01 eV for CuAlSe2 (see Table 1), 2.96 ± 0.01 eV for CuAl(Se0.5S0.5)2, and 3.486 ± 0.005 eV for CuAlS2, respectively. The E1 value of CuAl(Se0.5S0.5)2 lies in between those of CuAlS2 and CuAlSe2 and approaches the medium value of the two end members. The energy of valence-band splitting of ~0.242 eV obtained by TR for CuAl(Se0.5S0.5)2 is also close to the value of ~0.212 eV for a previous CuAl(SxSe1-x)2 crystal with x = 0.53 at 77 K [26]. The valence-band splitting of CuAl(Se0.5S0.5)2 may arise from the influence of uniaxial crystal field and spin-orbital splitting in the tetragonal chalcopyrite crystals [14,27].
Fig. 3 (a) Polarization-dependent TR spectra of CuAl(Se0.5S0.5)2 with E|| and E⊥ polarizations from 2.5 to 6.0 eV. The unpolarized TR spectrum is also included for comparison. The polarization dependences of E1 to E10 transitions for CuAl(Se0.5S0.5)2 are clearly shown. The lower part shows the crystal outline of CuAl(Se0.5S0.5)2 and the measurement configuration of PTR experiment.
Also shown in Fig. 3, there are about ten interband transitions of E1-E10 detected in the unpolarized TR spectrum of CuAl(Se0.5S0.5)2 from 2.5 to 6 eV. The transitions show distinct polarization dependence of E|| and E⊥ as depicted in the upper part of Fig. 3. All the obtained transition energies of different polarizations by using Lorentzian line-shape fits are listed in Table 2 for comparison. The E1||, E2||, E5||, E6||, E7||, and E8|| transitions are mainly present along the E|| polarization in the PTR spectrum while E3⊥, E4⊥, E9⊥, and E10⊥ features are shown largely along the E⊥ polarized spectrum as shown in Fig. 3. Strong and distinct polarization dependences of E|| and E⊥ for CuAl(Se0.5S0.5)2 are observed in the PTR spectra ofenergy range from 2.5 to 6 eV. It means that the electronic structure in this energy range is not dominant by the highly-symmetry of spherical-like s orbital but may come from the anisotropic angular-oriented p- or d-like electron states located near the valence-band top of CuAl(Se0.5S0.5)2. The valence-band structure of the CuAlX2 (X = S, Se) is mainly influenced by the effect of p-d hybridations (i.e. Cu d- X p) [21]. The incorporation of copper d orbital causes the main difference between the band structures of chalcopyrite and zinc-blende crystals. The p-d hybridation also causes the p-d repulsion in EV, which reduces the band gap of the chalcopyrites relative to their II-VI zinc-blende analogs [21]. The p-d repulsion also lifts up the X p states to form valence-band maximum and conduction-band minimum. For the EC bottom, X p-like Γ1C is the lowest, with increasing energies, the bands are Γ3C and Γ2C, respectively [21]. The band-gap transition is hence determined by X ΓV(p) (bonding state) to X ΓC(p*) (antibonding state). Previous X-ray photoelectron spectroscopy (XPS) study showed that the main valence band of CuAlS2 is constructed by Cu 3d – S 3p hybridations with energies ~-6 to 0 eV below EV [17]. One XPS peak feature closely related to the Cu 3d electrons was also located at ~-2.2 eV below the EV top in the valence band [17]. Because most of the interband transitions of the chalcopyrites are at the Γ point, the possible origins of the E1-E10 features in the PTR spectra of CuAl(Se0.5S0.5)2 (see Fig. 3) can be respectively assigned and are listed in Table 2 for comparison. The E1 and E2 transitions have the same polarization dependence of E|| which are the band-edge excitons caused by chalcogen X Γ4V(p) → X Γ1C(p*) [23,24]. The E3 transition comes from split X Γ5V(p) → X Γ1C(p*) and the E4 transition originates from the split X Γ5V(p) → X Γ3C(p*). The Cu 3d states are very important contribution in the valence band of CuAl(Se0.5S0.5)2 and which may arise from –1.8 eV to –3 eV below the EV top (i.e. 0 eV) [17]. The E5 to E10 transitions are closely correlated with the Cu 3d states, where the E5 and E6 features (E|| polarization) originate from Cu ΓV(d) → X Γ1C(p*). E7 and E8 features (E||) are determined by the interband transitions of Cu ΓV(d) → X Γ3C(p*). The E9 and E10 transitions with E⊥ polarization are coming from the interband transitions of split Cu ΓV(d) → X Γ1C(p*) and split Cu ΓV(d) → X Γ3C(p*), respectively. On the basis of experimental analysis of PTR, the electronic band structure for the CuAl(Se0.5S0.5)2 near band edge is hence being identified.
Table 2. Interband Transition Energies and Polarization Dependences of the PTR Transition Features Obtained for CuAl(Se0.5S0.5)2 Single Crystal on {112} Plane at 300 Ka
4. Conclusions
The band-edge and electronic properties of CuAl(Se0.5S0.5)2 are characterized by polarized thermoreflectance measurements at room temperature. The main band-edge exciton of CuAl(Se0.5S0.5)2 was determined to be 2.96 eV at 300 K. The energy value of the main band-edge transition was lying in between those of CuAlS2 and CuAlSe2, and its value was close to the medium value of the two end members. The maximum valence-band splitting for the CuAl(Se0.5S0.5)2 by PTR was determined to be ~0.242 eV. The value of valence-band splitting is larger than that of 0.154 eV for CuAlS2 but smaller than that of 0.301 eV for CuAlSe2. There are about ten interband transitions detected in the PTR measurements of CuAl(Se0.5S0.5)2 between 2.5 and 6.0 eV. Clear and distinct polarization dependence of the interband transitions was observed for CuAl(Se0.5S0.5)2. This result confirms that the main valence band of the chalcopyrite is composed of the hybridations of Cu d and X p orbitals. The d and p orbitals in EV are anisotropic angular-oriented electronic band states. The transition origins for the interband transitions with E|| and E⊥ polarizations in the CuAl(Se0.5S0.5)2 are respectively identified. The electronic band structure of CuAl(Se0.5S0.5)2 is thus being realized.
Acknowledgments
This work was sponsored by the National Science Council of Taiwan under the grant No. NSC101-2622-E-011-016-CC3.
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