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p-type conductivity and green photoluminescence of CuGaS2 grown by iodine transport
Journal of Luminescence 6 (1973) 140-142. North-Holland Publishing Company
LETTERS TO THE EDITOR p-TYPE CONDUCTIVITY
AND GREEN
PHOTOLUMINESCENCE
OF
CuGaS 2 G R O W N B Y I O D I N E T R A N S P O R T J.L. SHAY, P.M. BRIDENBAUGH, B. TELL and H.M. KASPER Bell Telephone Laboratories, Incorporated, Holmdel and Murray Hill, New Jersey, U.S.A.
Received 6 October 1972 Crystals of CuGaS2 grown from the vapor phase by iodine transport can be made usefully p-type (resistivity of 1 ~ cm at room temperature) by post annealing in saturated sulfur vapor, At 2°K these crystals exhibit efficient green photoluminescence.
It has recently been reported [ 1] that the chalcopyrite compound CuGaS 2 grown from the melt has a direct bandgap (2.4 eV at 300°K), and can be prepared usefully p-type. It is the purpose of this note to report that CuGaS 2 crystals prepared by iodine transport can also be made usefully conducting p-type, and that such crystal manifest a characteristic and efficient green photoluminescence at low temperatures. Our interest in iodine transport arises from tile possibility of using this growth technique to epitaxially deposit CuGaS 2 onto the n-type I I - V I compounds such as CdS and ZnS. This represents another potential method for fabricating heterostructures between the p-type Cu I - I I I - V I 2 compounds and the n-type I I - V I sulfides and selenides. The crystals were grown by iodine transport in a manner similar to that described by Honeyman [2]. Stoichiometric quantities of elemental Cu, Ca, and S 2 with a total weight of ~ 2 grams, were sealed in an evacuated quartz ampoule of 20 cc volume together with 5 mg cc -1 of iodine, The ampoule was placed in an oven with temperature gradient of 800°C to 700°C and maintained at this temperature for 72 h. Such a growth yielded many platelets of CuGaS 2 having a slightly greenish color. The largest platelets had dimensions of 1 X 5 X 0.05 ram. The crystal habit of the platelets suggested the chalcopyrite structure with prominent (112) facets. D e b y e Scherrer photographs confirmed that the crystals had the chalcopyrite structure and the lattice constants of bulk CuGaS 2 (a = 5.328 A, c = 10.462 A). In fig. 1 we show the photoluminescence spectra of a CuGaS 2 crystal grown by iodine transport. The emission was excited with a 50 mw H e - C d laser at 4416 A (Spectra Physics), dispersed by a Jarrell Ash 3/4 in double spectrometer, and detected with an EMI 9558 photomultiplier. Virtually every crystal showed the same line 140
J.L. Shay et al., Conductivity and photoluminescence
141
PHOTON ENERGY (eV) 2.50
2.40
2..30
PHOTOLUMINESCENCE CuGaS2:I 2OK
>I.-Z ILl
l*Z
~sulphur ed
Z 0 if) if) hl hl > i---J hl rr
5000
5200 WAVELENGTH
5400
5600
(~,)
Fig. 1. Photoluminescence spectra of CuGaS2: I at 2°K.
spectrum with approximately the same intensity. Apparently the energies and relative amplitudes of the lines are characteristic of a recombination mechanism induced in CuGaS 2 by the presence of iodine. The spectrum consists of a series of equally spaced lines separated by 4.6 meV with the highest energy and dominant line at 2.402 eV (5163 )k). Although additional experiments will be required to positively identify the physical mechanism producing this emission, the regular spacing of the lines and the general shape of the spectrum suggests that it results from local mode sidebands of a bound exciton recombination. It has previously been shown [ 1] that annealing CuGaS 2 at ~ 700°C in an environment of excess sulfur leads to useful p-type conductivity, while samples intentionally doped with donors such as C1 and Cd become semi-insulating ( p > 106~ cm). Since it is known that large quantities of the halogens are incorporated during vapor transport growth of II-VI sulfides and selenides, it might have been expected that such CuGaS 2 crystals would be insulating and remain high resistivity in spite of a sulfur anneal. Indeed, as grown crystals were found to be insulating (p > 106 ~2-cm). Several iodine transported CuGaS 2 crystals were sealed in an evacuated quartz ampoule together with sufficient sulfuI to produce a saturation sulfur pressure at 700°C, and annealed at that temperature for 30 hours. After quenching to room temperature, the excess sulfur was removed with warm CS 2. A four point probe (collinear) revealed that these crystals had a resistivity of typically 1 g2 cm, and a thermal probe indicated p-type conductivity. Hence, the extent of iodine
142
J.L. Shay et aL, Conductivity and photoluminescence
incorporation does not diminish the potential for useful p-type conductivity. The photoluminescence spectrum of an annealed crystal is also shown in fig. 1 for the same experimental conditions as for the unannealed crystal. Tile line spectrum has given way to a much broader spectrum of about the same peak intensity which is consequently much brighter. By measuring the emission in a small solid angle normal to the emitting surface with a silicon photodiode, we estimate that the 2°K photoluminescence efficiency of annealed samples CuGaS 2 is ~ 10%.
References [ 1] B. Tell, J,L. Shay and H.M. Kasper, Phys. Rev. B4 (1971) 2463; J. Appl. Phys. 43 (1972) 2469 [2] W.N. Honeyman and K.H. Wilkinson, J. Phys. D: Appl. Phys. 4 (1971) 1182. [3] J.D. Cuthbert and D.G. Thomas, J. Appl. Phys. 39 (1968) 1573. [4] D.M. Roessler, J. Appl. Phys. 41 (1970) 4589.
LETTERS TO THE EDITOR p-TYPE CONDUCTIVITY
AND GREEN
PHOTOLUMINESCENCE
OF
CuGaS 2 G R O W N B Y I O D I N E T R A N S P O R T J.L. SHAY, P.M. BRIDENBAUGH, B. TELL and H.M. KASPER Bell Telephone Laboratories, Incorporated, Holmdel and Murray Hill, New Jersey, U.S.A.
Received 6 October 1972 Crystals of CuGaS2 grown from the vapor phase by iodine transport can be made usefully p-type (resistivity of 1 ~ cm at room temperature) by post annealing in saturated sulfur vapor, At 2°K these crystals exhibit efficient green photoluminescence.
It has recently been reported [ 1] that the chalcopyrite compound CuGaS 2 grown from the melt has a direct bandgap (2.4 eV at 300°K), and can be prepared usefully p-type. It is the purpose of this note to report that CuGaS 2 crystals prepared by iodine transport can also be made usefully conducting p-type, and that such crystal manifest a characteristic and efficient green photoluminescence at low temperatures. Our interest in iodine transport arises from tile possibility of using this growth technique to epitaxially deposit CuGaS 2 onto the n-type I I - V I compounds such as CdS and ZnS. This represents another potential method for fabricating heterostructures between the p-type Cu I - I I I - V I 2 compounds and the n-type I I - V I sulfides and selenides. The crystals were grown by iodine transport in a manner similar to that described by Honeyman [2]. Stoichiometric quantities of elemental Cu, Ca, and S 2 with a total weight of ~ 2 grams, were sealed in an evacuated quartz ampoule of 20 cc volume together with 5 mg cc -1 of iodine, The ampoule was placed in an oven with temperature gradient of 800°C to 700°C and maintained at this temperature for 72 h. Such a growth yielded many platelets of CuGaS 2 having a slightly greenish color. The largest platelets had dimensions of 1 X 5 X 0.05 ram. The crystal habit of the platelets suggested the chalcopyrite structure with prominent (112) facets. D e b y e Scherrer photographs confirmed that the crystals had the chalcopyrite structure and the lattice constants of bulk CuGaS 2 (a = 5.328 A, c = 10.462 A). In fig. 1 we show the photoluminescence spectra of a CuGaS 2 crystal grown by iodine transport. The emission was excited with a 50 mw H e - C d laser at 4416 A (Spectra Physics), dispersed by a Jarrell Ash 3/4 in double spectrometer, and detected with an EMI 9558 photomultiplier. Virtually every crystal showed the same line 140
J.L. Shay et al., Conductivity and photoluminescence
141
PHOTON ENERGY (eV) 2.50
2.40
2..30
PHOTOLUMINESCENCE CuGaS2:I 2OK
>I.-Z ILl
l*Z
~sulphur ed
Z 0 if) if) hl hl > i---J hl rr
5000
5200 WAVELENGTH
5400
5600
(~,)
Fig. 1. Photoluminescence spectra of CuGaS2: I at 2°K.
spectrum with approximately the same intensity. Apparently the energies and relative amplitudes of the lines are characteristic of a recombination mechanism induced in CuGaS 2 by the presence of iodine. The spectrum consists of a series of equally spaced lines separated by 4.6 meV with the highest energy and dominant line at 2.402 eV (5163 )k). Although additional experiments will be required to positively identify the physical mechanism producing this emission, the regular spacing of the lines and the general shape of the spectrum suggests that it results from local mode sidebands of a bound exciton recombination. It has previously been shown [ 1] that annealing CuGaS 2 at ~ 700°C in an environment of excess sulfur leads to useful p-type conductivity, while samples intentionally doped with donors such as C1 and Cd become semi-insulating ( p > 106~ cm). Since it is known that large quantities of the halogens are incorporated during vapor transport growth of II-VI sulfides and selenides, it might have been expected that such CuGaS 2 crystals would be insulating and remain high resistivity in spite of a sulfur anneal. Indeed, as grown crystals were found to be insulating (p > 106 ~2-cm). Several iodine transported CuGaS 2 crystals were sealed in an evacuated quartz ampoule together with sufficient sulfuI to produce a saturation sulfur pressure at 700°C, and annealed at that temperature for 30 hours. After quenching to room temperature, the excess sulfur was removed with warm CS 2. A four point probe (collinear) revealed that these crystals had a resistivity of typically 1 g2 cm, and a thermal probe indicated p-type conductivity. Hence, the extent of iodine
142
J.L. Shay et aL, Conductivity and photoluminescence
incorporation does not diminish the potential for useful p-type conductivity. The photoluminescence spectrum of an annealed crystal is also shown in fig. 1 for the same experimental conditions as for the unannealed crystal. Tile line spectrum has given way to a much broader spectrum of about the same peak intensity which is consequently much brighter. By measuring the emission in a small solid angle normal to the emitting surface with a silicon photodiode, we estimate that the 2°K photoluminescence efficiency of annealed samples CuGaS 2 is ~ 10%.
References [ 1] B. Tell, J,L. Shay and H.M. Kasper, Phys. Rev. B4 (1971) 2463; J. Appl. Phys. 43 (1972) 2469 [2] W.N. Honeyman and K.H. Wilkinson, J. Phys. D: Appl. Phys. 4 (1971) 1182. [3] J.D. Cuthbert and D.G. Thomas, J. Appl. Phys. 39 (1968) 1573. [4] D.M. Roessler, J. Appl. Phys. 41 (1970) 4589.