Growth of CuGaSe2 film by molecular beam epitaxy

Microelectronics Journal 27 (1996) 53-58 Copyright © 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0026-2692/96/$15.00

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Growth of CuGaSe2 film by molecular beam epitaxy Akimasa Yamada, Yunosuke Makita, Shigeru Niki, Akira Obara, Paul Fons and Hajirne Shibata ElectrotechnJcalLaboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305,Japan Tel: +81 (298)58-5607, Fax: +81 (298)55-9664

Cu-Ga-Se films of different composition prepared by MBE technique were examined by reflection high energy electron diffraction, electron ]probe for micro analysis, X-ray diffraction and photolurniaaescence.The valence stoichiometry of the films is fulfilledat unity molecularity, showing that CuGaSe2 is possible to be grown by MBE. The valence stoichiometry is almost conserved in a wide range of molecularity of the films. It is proven that the films have chalcopyrite structure over a remarkably wide range of Curich composition and that the CuGaSez films are epitaxiaUy grown with the c-axis perpendicular to the (001) plane of GaAs substrate. The low temperature photoluminescence on epitaxiallygrown films of nearly stoichiometric and Curich composition show sharp emission peaks at 1.71 eV attributed to exciton recombination, indicating that the film quality is rather high.

1. Introduction C

uGaSe2 is a promising material for optical nonlinear devices and for solar cell active layers as similar as CulnSe2. T h e conversion effciency of CuGaSe2 solar cells studied up to now, however, has been limited about 7% [1-7]. Although close investigation on the material is requested, it is quite difficult to grow a good quality large crystals of CuGaSe2 because of the peritectic melting o f the stoichiometric

CuGaSe2 at 1030°C [8]. In this work, molecular beam epitaxial (MBE) growth o f CuGaSe2 films on (001) oriented GaAs substrate was examined to get high quality material and to investigate the influence of growth parameters. CuxGarSe z films of different composition were prepared by changing molecular beam flux by using a conventional type o f MBE machine. T h e molecularity ([Cu]/[Ga]) and the valence stoichiometry (2[Se]/([Cu]+3[Ga])) are discussed in relation to the Cu/Ga ratio o f the molecular beam flux, where [M] stands for the mole fraction o f element " M " of the grown films measured by electron probe for micro analysis (EPMA). T h e crystallographic structure o f the films was characterized by reflection high energy electron diffraction (R_HEED) during growth and by X-ray diffr c d o n (XR,D) after growth. T h e electro-optical properties were studied by low temperature photoluminescence (PL). X R D patterns for the polycrystalline films grown on glass substrates indicated tetragonal structure in rather wide range o f [Cu]/[Ga] ratio especially for Cu-rich composition. CuGaSe2 epitaxial films were obtained on (001) GaAs substrate with the c-axis perpendicular to the

53

A. Yamada et al./Growth of CuGaSe2 film by MBE

(001) plane of the substrate. The material quality is highly estimated through PL characteristics showing a sharp emission peak at 1.71 eV that is attributed to exciton recombination.

2. Experimental procedure The elemental sources, Cu(7N), Ga(7N) and Se(6N), were evaporated using Knudsen's effusion cells in a conventional type of MBE machine. (001) oriented GaAs substrates were degreased with organic solvents then etched with the chemical mixture of NH4OH, H 2 0 2 and H 2 0 in a ratio of 5:2:10. The substrates were h d d at 480°C during growth. The temperatures of Cu- and Se-cells were fixed at 1070°C and about 180°C, respectively. The flux of Se molecular beam should be sufficiently larger than that of the others. Ga-cell temperature was varied between 840°C and 940°C to change [Cu]/[Ga] ratio of the films. R_HEED patterns were observed to look the initial state of the substrate surfaces and to monitor the progress of film growth. Polycrystalline Cu=GaySez films were also fabricated on glass (Coming 7059) substrates under the same molecular beam conditions in order to identify crystallographic phases by X R D measurements and to measure compositions by EPMA. X R D patterns were also used to characterize epitaxial growth of CuxGaySe= films on GaAs substrates.

through films on glass substrates. The refractive index of the films was estimated at 3.0. The thickness of the films grown for 1 hour ranged from 0.7 #m to 1.5 #m depending on the rate of Ga molecular beam flux or on molecularity of the films. EPMA measurement was carried out to the films grown simultaneously on glass substrates in order to avoid spurious signals from Ga in GaAs substrate coming through the grown films. The disagreement of [Cu]/[Se] ratios was not more than + 5 % between the films grown on GaAs and those on glass substrates. Since [Cu]/[Se] ratio is free from the principal constituents of the two types of substrates, the composition of films on GaAs substrates can be estimated through the value for films on glass substrates with the accuracy mentioned above. The molecularity and the valence stoichiometry of the films are shown in Fig. 1 as a function of Cu/Ga ratio of the molecular beam flux. The molecularity of the films is nearly proportional to the Cu/Ga flux ratio, the factor being about 0.8. The valence stoichiomerry is nearly conserved but deviates a little to Se-poor side for Cu-rich molecularity, and vice -&

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Thickness of the films was observed from interference fringes of infrared light transmitted

54

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3. Results and discussion

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The electro-optical properties were studied by PL measurements. Samples were cooled to 2 K and excited by 514.5 nm line of Ar+-ion laser. The power was 10mW. PL emissions were dispersed by a l m single monochromator, detected by a Peltier-dooled GaAs-photocathode photomultiplier, amplified by lock-in technique, and collected by computerized system.

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CuxGaySez/glass

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-1.5 -1.0 -0.5 0.0 0.5 Cu/Ga ratio of molecular beam flux (log)

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Fig. 1. Molecularity and valence stoichiometry of CuxGar_ Sex films as a function of Cu/Ga ratio in molecular beam flux provided.

Microelectronics Journal, Vol. 27, No. 1

versa. Almost no deviation of the valence stoichiometry is shown where molecularity = 1, that is, stoichiometric CuGaSe2 can be grown by MBE. The ternary composition diagram of the films grown by MBE is shown in Fig. 2. No evidence of self-selectivity or self-tuning of the composition to the stoichiometric CuGaSe2 can be seen, but any intermediate composition can be realized in the Cu2Se-Ga2Se3 pseudobinary system. X R D patterns are shown in Fig. 3 for polycrystalline films of Cu-rich, nearly stoichiometric and Ga-rich compositions. In curves for Cu-rich and nearly stoichiometric compositions, (220) and (204) peaks are clearly separated, showing that the films have the chalcopyrite type crystallographic structure in these compositions. The d-values calculated from the X R D peaks rightly fit to those of" CuGaSe2 in JCPDS data. The lattice constants for films with near stoichiometric CuGaSe2 composition are calculated to be a = 5 . 6 2 A and[ c--11.03A, which agree well with those in a literature [9]. The crystallographic [112] direction of crystal grains in the films is preferentially oriented to the normal of substrates. The full width half maximum Se

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Fig. 2. Ternary composition diagram of CuxGarSe= films grown by molecularbeam deposition.

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Diffraction angle 2e (degree)

Fig. 3. X-ray diffraction patterns for CuxGarSez polycrystallinefilmsgrown on glasssubstrates. (FWHM) of these diffraction peaks is sufficiently small, indicating that the crystallinity is fairly good for stoichiometric and Cu-rich composition. Secondary phases can be formed in the composition far from unity molecularity. The dominant secondary phase in further Cu-rich region appears to be Cu2Se. Since the X R D pattern for the Ga-rich composition resembles that of CuGaSe2, but lacks the features of chalcopyrite structure, the films of Ga-rich composition are considered to have sphalerite crystalline structure. F W H M of (112) diffraction for Ga-rich films is larger than that for the Curich one. This means that the grain size of the former is smaller comparing to the latter. Only (00g) diffraction peaks appear in X R D patterns in the case of films grown on (001) GaAs substrates (Fig. 4). This indicates that epitaxial growth is performed. The value of the lattice distance, d, calculated from the diffi'action peaks agrees with 1/8th of the c-axis lattice constant reported for CuGaSe2 [9]. No trace of (h00) diffraction can be seen at the expected angle,

55

A. Yamada et al./Growth of CuGaSe2 film by MBE

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XRD (Cu-K~) CuxGaySez/GaAs

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Fig. 4. X-ray diffr cfion patterns for Cu=Ga~Se=epitaxial filmsgrownon (001) GaAssubstrates. indicating that unique growth of epitaxial films is performed. The lattice constant of the a-axis for CuGaSe2 is very close to that of GaAs, with lattice mismatch of 0.71%. Moreover, the tetragonal distortion of CuGaSe2 is rather large; c/ 2a=0.982. Therefore, the a-axis of CuGaSe2 might preferentially follow the <100> orientation in the (001) plane of the GaAs surface, and consequently CuGaSe2 grows epitaxially with the c-axis perpendicular to the GaAs(001) plane of the substrate surface. The estimated value c = 10.96 A is a little smaller compared with the value in the literature [9], and also with that of the polycrystalline films. The shrinkage of the film thickness is expected from the fact that thermal expansion of GaAs is larger than that of CuGaSe2. Since epitaxial films are possibly relaxed at growth temperature from the strain caused by lattice misfit, they might be expanded in the surface plane after cooling down, then consequently compressed perpendicular to the plane. It is interesting that the main second phase (possibly CuSe) for very Cu-rich epitaxial films is different to that (Cu2Se) of the polycrystalline films.

56

RHEED patterns for the films on GaAs substrates became brighter and more streaky just after starting growth. The patterns showed characteristic spots for the chalcopyrite structure with the c-axis perpendicular to the surface plane [10]. The patterns indicated that films sometimes contain twin crystals. PL spectra of the films grown on GaAs substrates are shown in Fig. 5 in relation to film composition. A sharp emission No. 1 appears with double peaks at 1.713eV and 1.710eV for the films near stoichiometry. Since these energies are just below the gap energy of CuGaSe2, 1.729 eV (at 77K) [11], by 16meV and 19meV, respectively, emission No. 1 is attributed to recombination ofexcitons [12]. The energy difference 16 meV from the band gap agrees with the binding

1.75

700

1.7

720

Photon energy (eV) 1.65 1.6 1.55

740

760 Wavelength

780

800 (nm)

1.5

820

840

Fig. 5. Low temperature photoluminescencespectra of Cu=GacSezepitaxialfilmson GaAssubstratesin relationto film composition.

Microelectronics Journal, Vol. 27, No. 1

energy of free excitons reported in [13]. The estimate here is supported by the fact that the excitation power dependence of the intensity of this emission is more than unity, and also that the dissociation energy is confirmed to agree with the binding energy through the temperature dependence of PL intensity. Details will appear in a forthcoming pubhcation. Excitonic emissions are observed in Cu-rich films far from the stoichiometric composition of CuGaSe2. It is remarkable that the crystallographic structure of the films is still chalcopyrite and that the quality of the films is good fbr even a strongly Cu-rich composition. The most prominent emission, No. 2 at 1.622 eV near stoichiometry, has an energy close to that reported to be attributable to the state of Se vacancy [14]. This interpretation is quite probable because the Cu-rich films are Se-poor. The Se vacancy is considered to act as a donor [15]. Emission No. 2 is considered to be a freeto-bound (FB) transition from observations of PL excitation power dependence. The intensity of emission No. 2 varies hnearly with excitation power and its FWHM increases with increasing excitation power. This feature means that free carriers participate through band filling. Peak No. 3 at 1.588eV is regarded as one phonon rephca of No. 2 (1LO) judging from the excitation power dependence and the energy difference of 34 meV that is close to the value 34.5 meV measured by reflectivity [16]. Peak No. 4 at 1.554 eV is probably 2LO of emission No. 2. All these emissions shift together to the lower energy with increasing Cu content and imply a decrease of band gap energy with increasing Cu content. The emission No. 6 from the GaAs substrates is confirmed by the energy at the peak, and also by the intensity increasing under excitation by longer wavelength laser light, which can penetrate deeper through the films. Ga-rich films show a broad emission band, No. 7, centered at 1.58 eV, similar to that reported as a donor (Se vacancy') to acceptor (Cu vacancy)

pair recombination [14]. Emission No. 7 shows a large blue shift with increasing excitation power, which implies that emission No. 7 can be attributed to donor-to-acceptor recombination. Emission No. 5 at 1.530eV has an energy similar to that observed in iodine-treated CuGaSe2 [12, 14]. Emission No. 5 is so sharp compared with the other that it seems of value to find new states. In contrast to the Cu-rich region, the PL spectra change drastically if the composition is just a little Cu-poor. A detailed discussion on PL properties will be presented elsewhere. 4. Conclusion

The valence stoichiometry of CuxGa, Sez thin films grown by MBE is completed( at unit molecularity ([Cu]/[Ga] ratio). This means it is possible to grow CuGaSe2 by MBE. The valence stoichiometry (2[Se]/([Cu]+3[Ga])) is almost conserved over a wide range of molecularity of the films, though it deviates a little to the Se-poor side for Cu-rich molecularity, and vice versa. ~ patterns for the polycrystalline films grown on glass substrates indicate formation of the chalcopyrite structure in a wide range of Cu-rich compositions. R3qEED and XR_D patterns for the films on the GaAs substrates show that CuGaSe2 films are epitaxially grown with the c-axis perpendicular to the (001) plane of GaAs substrate. The low temperature PL measurements on epitaxially grown films show a notable dependence on molecularity of the film. For nearly stoichiometric and Cu-rich compositions, sharp emission peaks appear at 1.71eV which are attributed to exciton recombination. Several other narrow band emissions which can be attributed to free-to-bound transitions and their phonon replicas are observed. For Garich films, rather broad band emissions appear, which are attributable to donor-to-acceptor transitions.

57

A. Yamada et al./Growth of CuGaSe2 film by MBE

Acknowledgement T h e authors wish to express their thanks to Dr. Y u k o Y o k o y a m a for her advice in the measurement of EPMA.

[8] [9]

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