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1. Introduction??
GaSe layered crystals are related to A3B6 binary compounds. Due to the sharp anisotropy of their chemical bonds, strong ion-covalent bonds within layers and weak van der Waals forces between layers, GaSe crystals can be easily intercalated both with atoms and molecules, which makes them promising materials for hydrogen energetics [1, 2] and accumulators of electric energy [3]; moreover, heterostructures with high photosensitivity based on these materials can be applied in solar cells [4-6].
At the same time, it seems interesting to investigate the influence of γ-irradiation on the optical properties of these crystals with the aim of using them as
2. Experimental Methods
GaSe single crystals, both pure and intentionally doped with Cd, Zn and Sn impurities, were grown using the Bridgman method. The concentration of impurities did not exceed 0.01 wt.%.
To study the irradiation influence on optical properties, we chipped plates with a thickness of 3 to 5 mm from GaSe ingots and then cut samples to the dimensions of 5 × 5 mm2 from these plates.
Semiconductor samples prepared in this way were irradiated with γ-quanta in an electron accelerator providing electron energy of 35 MeV and an average beam current close to 250 mA. The electron beam was converted into bremsstrahlung on a tantalum target. The samples were placed at an angle of 90° relative to
3. Results and Discussion
Fig. 1a shows an electron microscopic image of the GaSe crystal surface when the crystal did not contain any intentionally introduced impurities. These crystals were characterised by the presence of spherical formations with a size of 80 to 500 nm. As shown in Ref. [2], these formations are inclusions of the amorphous phase inherent to red monoclinic β-Se precipitating in the course of growth into the interlayer space of GaSe. These inclusions can be easily eliminated by annealing crystals placed in an evacuated ampoule for 2 to 3 h at T = 400 °C. Under these conditions, residual selenium escapes from the crystal and is deposited onto the ampoule walls, and the inclusions observed in the electron microscope disappear.
EDS investigations performed in this work using the INCA ENERGY 450 detector showed that Zn, Cd and Sn impurities were inhomogeneously distributed in the GaSe crystals even at low concentrations (0.01 wt.%). They had mainly accumulated in the vicinity of dislocations (Fig. 1b) generated by impurities in the growing process. When their concentration exceeded 0.1 wt.% (Fig. 1c), inhomogeneous distribution could be observed even using the Primo Star 5 optical microscope.
studied crystal surfaces.
The presence of the fine structure inherent to free excitons in GaSe is, as a rule [8-10], related to stacking fault defects in the crystal layers and the availability of bound excitons with various point defects inside these layers.
As can be seen in Fig. 2, γ-irradiation of non-doped
transition, one can observe P1-P4 bands, the peak positions of which are summarised in Table 2.
As seen in Table 2, the peaks of the P1-P4 emission and absorption bands were shifted relative each other.
It should be noted that similar changes related to the appearance of additional wide bands after ?-irradiation were observed (Fig. 2h) in GaSe:Zn crystals at T = 4.5 K as well. In this case, an additional wide structural P0 arose with two peaks at 608 and 614 nm in the PL spectrum. As seen in Fig. 2h, the halfwidth of this band was close to 80 meV, which allows the assumption (see Table 2) that these peaks are caused by transitions between the DCB and the ICB as well as the acceptor level а1 with participation of one TO and one LO phonons, respectively. In this case, the level а1 is located at 70 meV above the VB, which corresponds to the value obtained in Ref. [18]. The energy position of the а1 level and the scheme of transitions are illustrated in a diagram in Fig. 4e.
Finally, we considered the acceptor level a0 located at 25 ? 2 meV above the top of the VB. The results of our investigations allowed us to assume that transitions from this level to the DCB should take place with the participation of LO or TO phonons. Then, this transition should be accompanied by the appearance of a wide band in the emission spectrum, and at Т = 4.5K this band should possess a peak at the energy 2.075 ? 0.01 meV (? = 597.5 ? 2.5 nm), which should be observed within the range of B4-B6 emission lines related to bound excitons. It is possible that this band(Table 1) was observed in the PL spectra at Т = 4.5 K in GaSe crystals doped with the impurities Zn (? = 597.6 nm), Cd (? = 596.0 nm) and Sn (? = 595.5 nm) with increasing doses of γ-irradiation.
4. Conclusions
We performed electron microscopic, energy dispersion and low-temperature optical and spectroscopic investigations of GaSe crystals, both non-doped and doped with Zn, Sn and Cd impurities at a concentration 0.01 wt.% as well as irradiated with
(165 meV) and acceptor levels (25, 70, 150, 310 and 460 meV) in the forbidden gap of GaSe crystals, the experimentally observed carrier transitions between the levels located in this gap as well as direct and indirect conduction bands with the participation of lattice vibrations.
Acknowledgments
This work was funded by the joint Scientific and Technology Centre of Ukraine and National Academy of Sciences of Ukraine project #22-5228/10.
27 (1985) 3696-3699. (in Russian)
[13] P. Schmid, J.P. Voitchovsky, A. Mercier, Impurity effects on low temperature photoluminescence of GaSe, Phys. Status Solidi A 21 (1974) 443-449.
[14] Y.I. Zhirko, Investigation of the light absorption mechanisms near exciton resonance in layered crystals: Part 2. N = 1 state exciton absorption in GaSe, Phys. Status Solidi B 219 (2000) 47-61.
[15] Y.P. Gnatenko, Z.D. Kovalyuk, P.A. Skubenko, Band gap edge luminescence of GaSe crystals doped by iron group impurities, Ukr. Fiz. Zhurn. 27 (1982) 838-842. (in Russian)
[16] S. Shigetomi, T. Ikari, H. Nishimura, Photoluminescence spectra of p-GaSe doped with Cd, J. Appl. Phys. 69 (1991) 7936-7938.
[17] H. Karagac, M. Parlac, O.Karabulut, U. Serincan, Structural, electrical and optical properties of Ge implanted GaSe single crystals grown by Bridgman technique, Cryst. Res. and Technology 41 (2006) 1159-1166.
[18] J.F. Sanchez-Roy, D. Errandonea, A. Segura, L. Roa, A. Chevy, Tin-related double acceptors in gallium selenide single crystals, J. Appl. Phys. 83 (1998) 4750-4755.
[19] K. Zeeger, Semiconductor Physics (Springer Series in Solid State Science Vol. 40, Springer, Berlin, 1982, pp. 35-45.
[20] Y.I. Zhirko, Excitons in layered p-GaSe crystals with a two-dimension hole gas, Photoelectronic 13 (2004) 41-46.
[21] T.Y. Park, S.C. So, M.S. Jin, Photoluminescence spectra of undoped and Cu-doped GaSe single сrystals, J. Korean Phys. Soc. 38 (2001) 754-757.
[22] C.H. Chung, S.R. Hahn, H.L. Park, H.T. Kim, S.I. Lee, Photoluminescence of Mn-, Cr-doped and undoped?-GaSe, J. Luminescence 41 & 42 (1988) 4005-4006.
[23] C.S. Yoon, W.T. Kim, Hole traps in GaSe:Co Single crystals, Sol. St. Comm. 62 (1987) 583-586.
[24] V. Augelli, C. Manfredotti, R. Murri, L. Vasanelli, Hall-mobility anisotropy in GaSe, Phys. Rev. B 17 (1978) 3221-3226.
GaSe layered crystals are related to A3B6 binary compounds. Due to the sharp anisotropy of their chemical bonds, strong ion-covalent bonds within layers and weak van der Waals forces between layers, GaSe crystals can be easily intercalated both with atoms and molecules, which makes them promising materials for hydrogen energetics [1, 2] and accumulators of electric energy [3]; moreover, heterostructures with high photosensitivity based on these materials can be applied in solar cells [4-6].
At the same time, it seems interesting to investigate the influence of γ-irradiation on the optical properties of these crystals with the aim of using them as
2. Experimental Methods
GaSe single crystals, both pure and intentionally doped with Cd, Zn and Sn impurities, were grown using the Bridgman method. The concentration of impurities did not exceed 0.01 wt.%.
To study the irradiation influence on optical properties, we chipped plates with a thickness of 3 to 5 mm from GaSe ingots and then cut samples to the dimensions of 5 × 5 mm2 from these plates.
Semiconductor samples prepared in this way were irradiated with γ-quanta in an electron accelerator providing electron energy of 35 MeV and an average beam current close to 250 mA. The electron beam was converted into bremsstrahlung on a tantalum target. The samples were placed at an angle of 90° relative to
3. Results and Discussion
Fig. 1a shows an electron microscopic image of the GaSe crystal surface when the crystal did not contain any intentionally introduced impurities. These crystals were characterised by the presence of spherical formations with a size of 80 to 500 nm. As shown in Ref. [2], these formations are inclusions of the amorphous phase inherent to red monoclinic β-Se precipitating in the course of growth into the interlayer space of GaSe. These inclusions can be easily eliminated by annealing crystals placed in an evacuated ampoule for 2 to 3 h at T = 400 °C. Under these conditions, residual selenium escapes from the crystal and is deposited onto the ampoule walls, and the inclusions observed in the electron microscope disappear.
EDS investigations performed in this work using the INCA ENERGY 450 detector showed that Zn, Cd and Sn impurities were inhomogeneously distributed in the GaSe crystals even at low concentrations (0.01 wt.%). They had mainly accumulated in the vicinity of dislocations (Fig. 1b) generated by impurities in the growing process. When their concentration exceeded 0.1 wt.% (Fig. 1c), inhomogeneous distribution could be observed even using the Primo Star 5 optical microscope.
studied crystal surfaces.
The presence of the fine structure inherent to free excitons in GaSe is, as a rule [8-10], related to stacking fault defects in the crystal layers and the availability of bound excitons with various point defects inside these layers.
As can be seen in Fig. 2, γ-irradiation of non-doped
transition, one can observe P1-P4 bands, the peak positions of which are summarised in Table 2.
As seen in Table 2, the peaks of the P1-P4 emission and absorption bands were shifted relative each other.
It should be noted that similar changes related to the appearance of additional wide bands after ?-irradiation were observed (Fig. 2h) in GaSe:Zn crystals at T = 4.5 K as well. In this case, an additional wide structural P0 arose with two peaks at 608 and 614 nm in the PL spectrum. As seen in Fig. 2h, the halfwidth of this band was close to 80 meV, which allows the assumption (see Table 2) that these peaks are caused by transitions between the DCB and the ICB as well as the acceptor level а1 with participation of one TO and one LO phonons, respectively. In this case, the level а1 is located at 70 meV above the VB, which corresponds to the value obtained in Ref. [18]. The energy position of the а1 level and the scheme of transitions are illustrated in a diagram in Fig. 4e.
Finally, we considered the acceptor level a0 located at 25 ? 2 meV above the top of the VB. The results of our investigations allowed us to assume that transitions from this level to the DCB should take place with the participation of LO or TO phonons. Then, this transition should be accompanied by the appearance of a wide band in the emission spectrum, and at Т = 4.5K this band should possess a peak at the energy 2.075 ? 0.01 meV (? = 597.5 ? 2.5 nm), which should be observed within the range of B4-B6 emission lines related to bound excitons. It is possible that this band(Table 1) was observed in the PL spectra at Т = 4.5 K in GaSe crystals doped with the impurities Zn (? = 597.6 nm), Cd (? = 596.0 nm) and Sn (? = 595.5 nm) with increasing doses of γ-irradiation.
4. Conclusions
We performed electron microscopic, energy dispersion and low-temperature optical and spectroscopic investigations of GaSe crystals, both non-doped and doped with Zn, Sn and Cd impurities at a concentration 0.01 wt.% as well as irradiated with
(165 meV) and acceptor levels (25, 70, 150, 310 and 460 meV) in the forbidden gap of GaSe crystals, the experimentally observed carrier transitions between the levels located in this gap as well as direct and indirect conduction bands with the participation of lattice vibrations.
Acknowledgments
This work was funded by the joint Scientific and Technology Centre of Ukraine and National Academy of Sciences of Ukraine project #22-5228/10.
27 (1985) 3696-3699. (in Russian)
[13] P. Schmid, J.P. Voitchovsky, A. Mercier, Impurity effects on low temperature photoluminescence of GaSe, Phys. Status Solidi A 21 (1974) 443-449.
[14] Y.I. Zhirko, Investigation of the light absorption mechanisms near exciton resonance in layered crystals: Part 2. N = 1 state exciton absorption in GaSe, Phys. Status Solidi B 219 (2000) 47-61.
[15] Y.P. Gnatenko, Z.D. Kovalyuk, P.A. Skubenko, Band gap edge luminescence of GaSe crystals doped by iron group impurities, Ukr. Fiz. Zhurn. 27 (1982) 838-842. (in Russian)
[16] S. Shigetomi, T. Ikari, H. Nishimura, Photoluminescence spectra of p-GaSe doped with Cd, J. Appl. Phys. 69 (1991) 7936-7938.
[17] H. Karagac, M. Parlac, O.Karabulut, U. Serincan, Structural, electrical and optical properties of Ge implanted GaSe single crystals grown by Bridgman technique, Cryst. Res. and Technology 41 (2006) 1159-1166.
[18] J.F. Sanchez-Roy, D. Errandonea, A. Segura, L. Roa, A. Chevy, Tin-related double acceptors in gallium selenide single crystals, J. Appl. Phys. 83 (1998) 4750-4755.
[19] K. Zeeger, Semiconductor Physics (Springer Series in Solid State Science Vol. 40, Springer, Berlin, 1982, pp. 35-45.
[20] Y.I. Zhirko, Excitons in layered p-GaSe crystals with a two-dimension hole gas, Photoelectronic 13 (2004) 41-46.
[21] T.Y. Park, S.C. So, M.S. Jin, Photoluminescence spectra of undoped and Cu-doped GaSe single сrystals, J. Korean Phys. Soc. 38 (2001) 754-757.
[22] C.H. Chung, S.R. Hahn, H.L. Park, H.T. Kim, S.I. Lee, Photoluminescence of Mn-, Cr-doped and undoped?-GaSe, J. Luminescence 41 & 42 (1988) 4005-4006.
[23] C.S. Yoon, W.T. Kim, Hole traps in GaSe:Co Single crystals, Sol. St. Comm. 62 (1987) 583-586.
[24] V. Augelli, C. Manfredotti, R. Murri, L. Vasanelli, Hall-mobility anisotropy in GaSe, Phys. Rev. B 17 (1978) 3221-3226.