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Preparation, characterization and properties of pure and lithium-doped zinc oxide thin films
Thin Solid Films, 32 (1976) 87-91 © Elsevier Sequoia S.A., Lausanne-Printed in Switzerland
87
PREPARATION, CHARACTERIZATION AND PROPERTIES OF PURE AND LITHIUM-DOPED ZINC OXIDE THIN FILMS* G. WEISE, E. M. FECHNER, G. OWSIAN AND D. KRAUT Zentralinstitut fiir FestkiJrperphysik und Werkstofforschung der A kademie der Wissenschaften der DDR, 8027 Dresden, Helmholtzstr. 20 (G.D.R.)
(Received August 25, 1975)
1. INTRODUCTION Thin films of zinc oxide have been prepared previously by both physical and chemical procedures. The application of the sputtering method has been investigated in detail 1-s. Deposition by the thermal evaporation of zinc oxide is unsuccessful because of the high sublimation temperature and the thermal dissociation of zinc oxide vapour into zinc and oxygen. It is feasible to bypass this difficulty by depositing zinc and then oxidizing it, but the adhesive strength of the fdms is considerably reduced 6. Chemical methods used have involved transport reactions 7 and the spraying of organic solutions 8 . This paper reports on the preparation of zinc oxide thin films by a vapour reaction in vacuum involving zinc deposition in a partial oxygen atmosphere and by the oxidation of zinc solutions in an oxyhydrogen flame. The structural, electrical and luminescence properties of the films are also described. 2. THIN FILM PREPARATION 2.1. R e a c t i v e z i n c d e p o s i t i o n
The thin films were prepared in a high vacuum apparatus in which zinc was evaporated from a quartz glass supply vessel by using a capillary. This approach prevents oxidation of the zinc melt in a partial oxygen atmosphere, thus ensuring a constant deposition rate. The zinc vapour beam impinged directly on the substrate, the maximum distance between evaporator and substrate being 5 cm. An oxygen jet was likewise directed onto the substrate. With substrate temperatures between 400 o and 700 °C, partial oxygen pressures ranging from 0.1 to 10 Torr and deposition rates up to 500 A s-1 it was possible to obtain single-phase zinc oxide thin films of good adhesive strength. Table I contains data on the preparation conditions and properties of zinc oxide films deposited by this method. 2.2. O x i d a t i o n o f z i n c s o l u t i o n s in an o x y h y d r o g e n f l a m e
Zinc chloride dissolved in methanol (concentration 0.1-1 M) was sprayed through a cannula into an oxyhydrogen flame. Doping is easily accomplished over wide concentration ranges and was carried out using lithium chloride up to 104 ppm. The throughput of the * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 1-20.
G. WEISE etal.
88 TABLE l
PREPARATION CONDITIONS, STRUCTURE AND PROPERTIES OF ZnO THIN FILMS, DEPOSITED ON GLASS BY REACTIVE EVAPORATION
Sample
P02 (Tort)
TA (°C)
Rate (A s"1 )
d 0zm)
Structure
Resistivity (~2 cm)
Luminescence (nm) Rel. intens.
1 2 3 4 5 6 7a
0.1 1 5 5 10 10 10
500 500 500 500 500 500 500
15 20 15 280 120 5 130
2.3 1.3 1.0 2.5 2.1 1.2 5.8
++ ++ ++ ++ +
0.2 x 101 101 2 x 102 4 x 101 5 x 102 9 x 10 z 5 x 103
495 488 490 522 523 490 540
62 87 50 68 12 90 55
++ Oriented; + moderately oriented. a Li-doped. TABLE II PREPARATION CONDITIONS, STRUCTURE AND PROPERTIES OF ZnO THIN FILMS, DEPOSITED ON QUARTZ GLASS BY OXIDATION OF ZINC CHLORIDE SOLUTIONS
Sample
ZnCI 2 LiCI (mol) (ppm)
TA (°C)
H2/02 (1 h -1)
Rate (A s -1)
d ~m)
Structure
Resistivity (I2 cm)
Luminescence
(nm) Rel. intens.
1 2 3 4 5 6 7 8 9
0.1 0.1 1 1 1 1 1 1 1
102 103 103 104
600 1000 600 600 600 600 600 600 600
1000/520 1000/520 1000/520 1000/640 1000[800 1000/520 1000/520 1000/800 1000/520
8 6 20 22 25 20 24 i1 11
0.7 1.0 1.0 1.2 0.9 1.2 1.0 1.1 0.7
+ ++ ++ ++ ++ + + + -
5 x 5x 1x 4 x 5x 3x 8x 2x 1x
101 103 104 104 104 104 104 105 106
518 525 520 543 554 608 534 600
5 60 67 30 31 30 60 18
++ Oriented; + moderately oriented. s o l u t i o n c o u l d be c o n t r o l l e d b y t h e a p p l i c a t i o n o f an e x c e s s p r e s s u r e o f an i n e r t gas. T h e s u b s t r a t e u s e d w a s q u a r t z glass. T h e d e p o s i t i o n t e m p e r a t u r e s r a n g e d f r o m 4 0 0 ° to 1 0 0 0 °C, a n d c o u l d be a d j u s t e d b y altering t h e h e a t e r - s u b s t r a t e d i s t a n c e . T h e degree o f o x i d a t i o n o f t h e zinc o x i d e films w a s d e t e r m i n e d b y t h e h y d r o g e n : o x y g e n ratio. H y d r o g e n f l o w rates o f 1 0 0 0 1 h - l a n d o x y g e n f l o w rates o f 5 0 0 - 1 0 0 0 1 h -1 were used. T h e zinc o x i d e d e p o s i t i o n rates u s e d c o v e r e d a range f r o m 5 to 50 A s-1 . T h e p r e p a r a t i o n c o n d i t i o n s a n d p r o p e r t i e s o f t h e z i n c o x i d e films o b t a i n e d are given in Table II. 3. THIN FILM PROPERTIES
3.1. F i l m t e x t u r e T h e z i n c o x i d e t h i n films o n glass p r e p a r e d in a c c o r d a n c e w i t h p r o c e d u r e 2.1 are p o l y c r y s t a l l i n e , as s h o w n in t h e e l e c t r o n m i c r o g r a p h s a n d e l e c t r o n d i f f r a c t i o n p a t t e r n s
PURE AND Li-DOPED ZnO FILMS
89
Fig. 1. Electron micrographs of the surface and electron diffraction patterns for ZnO tiara trims deposited onto glass by reactive evaporation. (a) Sample 1: PO: = 0.1 Torr; r = 15 A s-1, d = 2.3 ~m. (b) Sample 2:PO2 = 1 Torr; r = 20 A s-I ;d = 1.3 tzm. (c) Sample 5:PO2 = 10 Torr; r = 120 A s-l; d = 2.1 ~m. (Fig. 1). The diffraction patterns were obtained by employing RHEED according to the Lebedeff method, focusing either upon the object (e.g. Fig. l(a)) or upon the photographic plate. Patterns l(a) and l ( b ) reveal a pronounced orientation of the crystallites with the c-axis vertical to the substrate plane. At all the partial pressures of oxygen investigated, from 0.1 to 10 Torr, it was possible to obtain oriented zinc oxide thin films at deposition temperatures from 500 o to 600 °C. However the degree of orientation decreased with increase in deposition rate and deposition rates exceeding 100 A s -1 led to a more or less random distribution of orientations of the crystallites, as shown in pattern l(c). The electron micrographs (Figs. l(a) and l(b)) show the dominant morphology of the (0001)crystallites bounded by the six pyramid faces. As the texture patterns in Fig. 2 show, the zinc oxide thin films on quartz glass prepared in the oxyhydrogen flame according to procedure 2.2 are polycrystalline with a preferred orientation parallel to the c-axis. Textured thin films of zinc oxide were obtained under all the test conditions: deposition temperatures from 400 ° to 1000 °C, hydrogen-oxygen ratios of 2:1 and 1 : 1, zinc chloride concentrations of 1-0.1 M and Li dopings up to 104 ppm. The degree of orientation increased with an increase in the deposition temperature. This was most obvious with the 0.1 M zinc chloride solutions which led to more highly oriented films at a deposition temperature of 1000 °C (sample 2) than at a deposition temperature of 600 °C (sample 1). Furthermore, the degree of orientation was reduced with an increase in the doping concentration of lithium (samples 8 and 9).
3.2. ElectHcal resistance and luminescence properties The thin films of zinc oxide were supplied with silver electrodes by thermal evaporation and the electrical resistance was measured by means o f the four-point probe method.
90
G. WEISE etal
Fig. 2. Electron micrographs of the surface and electron diffraction patterns for ZnO thin films on quartz glass, prepared by the oxidation of zinc chloride solutions. (a) Sample 3: TA = 600 °C; H2/O 2 = 1000/5201 h -1 ; ZnCI2 = 1 M. (b) Sample 8: TA = 600 °C; H2/O 2 = 1000/800 1h-1 ; ZnC12 = 1 M; LiC1 = 103 ppm. (c) Sample 2: TA = 1000 °C; H2/O 2 = 1000/5201 h -1 ; ZnC12 = 0.1M. The zinc oxide films deposited according to procedure 2.1 showed the following dependences of the specific resistance on the preparation conditions (Table I). For a deposition temperature of 500 °C and partial oxygen pressures from 0.1 to 10 Torr the specific resistance rose from 1 to 103 ~ cm. With higher deposition rates at a constant oxygen pressure the specific resistance was less, as evidenced by samples 3 and 4 (oxygen pressure of 5 Torr) and samples 6 and 7 (10 Torr). Zinc oxide is a non-stoichiometric compound since it is deficient in oxygen, but increased oxygen pressures and reduced deposition rates raised the proportion of oxygen in the zinc oxide lattice, thus causing the specific resistance to increase. Li doping at an optimum oxygen pressure of 10 Torr resulted in a further increase in the specific resistance to 104 f2 cm, in conformity with the action of monovalent cations as acceptors in zinc oxide. In the thin films of zinc oxide prepared by procedure 2.2 the specific resistances covered a range from 101 to 106 ~2 cm (Table II). For 0.1 M zinc chloride solutions it was possible to obtain specific resistances of less than 104 ~ cm (samples 1 and 2). At a deposition temperature of 600 °C optimum specific resistances of 104 ~2 cm could be obtained for 1 M solutions. As the amount of oxygen in the gas mixture was raised, the resistance rose to 5 x 104 ~2 cm (samples 3-5). Solutions doped with Li up to 104 ppm resulted in the resistance increasing to 106 ~2 cm. By using a high pressure mercury vapour lamp and an SPM2 monochromator it was possible to investigate the luminescence of zinc oxide thin films due to monochromatic ultraviolet excitation. The location of the emitted luminescence band and its relative intensity (Tables I and II) were determined. In pure zinc oxide thin films the greencoloured luminescence band was displaced towards shorter wavelengths when the degree of orientation increased. Doping with lithium displaced the luminescence towards longer wavelengths of orange colour. This displacement was discernible to a limited
PURE AND Li-DOPED ZnO FILMS
91
extent even if the degree of orientation in the thin films decreased. The extremely low relative intensities (samples 1 and 2, Table II) are evidently brought about by the small crystallite sizes. ACKNOWLEDGMENTS Our thanks are due to Dr. Edelmann, ZFW Dresden, for the structure investigations, and to Dr. Rauch, Carl-Zeiss-Jena, for the luminescence measurements. REFERENCES 1 2 3 4 5 6 7 8
G. A. Rozgonyi and W. J. Polito, J. Vac. Sci. Technol., 6 (1969) 115. T. Hada, K. Wasa and S. Hayakawa, Thin Solid Films, 7(1971) 135. D. L. Raimondi and E. Kay,./. Vac. ScL Technol., 7 (1970)96. R. Inaba, T. Ishiguro and N. Mikoshiba, Jpn. J. Appl. Phy~, 10 (1971) 1493. K. W. Schalimova and K. M. Botnew, Kristallografiya, 13 (1968) 679. G. Heiland, E. Mollwo and F. St6ckmann, Solid State Phys., 8 (1958) 191. D. Galli and J. E. Coker, Appl. Phy~ Lett., 16 (1970)439. J. McK. Nobbs and F. C. GiUespie,J. Phys. Chem. Solids, 31 (1970) 2553.
87
PREPARATION, CHARACTERIZATION AND PROPERTIES OF PURE AND LITHIUM-DOPED ZINC OXIDE THIN FILMS* G. WEISE, E. M. FECHNER, G. OWSIAN AND D. KRAUT Zentralinstitut fiir FestkiJrperphysik und Werkstofforschung der A kademie der Wissenschaften der DDR, 8027 Dresden, Helmholtzstr. 20 (G.D.R.)
(Received August 25, 1975)
1. INTRODUCTION Thin films of zinc oxide have been prepared previously by both physical and chemical procedures. The application of the sputtering method has been investigated in detail 1-s. Deposition by the thermal evaporation of zinc oxide is unsuccessful because of the high sublimation temperature and the thermal dissociation of zinc oxide vapour into zinc and oxygen. It is feasible to bypass this difficulty by depositing zinc and then oxidizing it, but the adhesive strength of the fdms is considerably reduced 6. Chemical methods used have involved transport reactions 7 and the spraying of organic solutions 8 . This paper reports on the preparation of zinc oxide thin films by a vapour reaction in vacuum involving zinc deposition in a partial oxygen atmosphere and by the oxidation of zinc solutions in an oxyhydrogen flame. The structural, electrical and luminescence properties of the films are also described. 2. THIN FILM PREPARATION 2.1. R e a c t i v e z i n c d e p o s i t i o n
The thin films were prepared in a high vacuum apparatus in which zinc was evaporated from a quartz glass supply vessel by using a capillary. This approach prevents oxidation of the zinc melt in a partial oxygen atmosphere, thus ensuring a constant deposition rate. The zinc vapour beam impinged directly on the substrate, the maximum distance between evaporator and substrate being 5 cm. An oxygen jet was likewise directed onto the substrate. With substrate temperatures between 400 o and 700 °C, partial oxygen pressures ranging from 0.1 to 10 Torr and deposition rates up to 500 A s-1 it was possible to obtain single-phase zinc oxide thin films of good adhesive strength. Table I contains data on the preparation conditions and properties of zinc oxide films deposited by this method. 2.2. O x i d a t i o n o f z i n c s o l u t i o n s in an o x y h y d r o g e n f l a m e
Zinc chloride dissolved in methanol (concentration 0.1-1 M) was sprayed through a cannula into an oxyhydrogen flame. Doping is easily accomplished over wide concentration ranges and was carried out using lithium chloride up to 104 ppm. The throughput of the * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 1-20.
G. WEISE etal.
88 TABLE l
PREPARATION CONDITIONS, STRUCTURE AND PROPERTIES OF ZnO THIN FILMS, DEPOSITED ON GLASS BY REACTIVE EVAPORATION
Sample
P02 (Tort)
TA (°C)
Rate (A s"1 )
d 0zm)
Structure
Resistivity (~2 cm)
Luminescence (nm) Rel. intens.
1 2 3 4 5 6 7a
0.1 1 5 5 10 10 10
500 500 500 500 500 500 500
15 20 15 280 120 5 130
2.3 1.3 1.0 2.5 2.1 1.2 5.8
++ ++ ++ ++ +
0.2 x 101 101 2 x 102 4 x 101 5 x 102 9 x 10 z 5 x 103
495 488 490 522 523 490 540
62 87 50 68 12 90 55
++ Oriented; + moderately oriented. a Li-doped. TABLE II PREPARATION CONDITIONS, STRUCTURE AND PROPERTIES OF ZnO THIN FILMS, DEPOSITED ON QUARTZ GLASS BY OXIDATION OF ZINC CHLORIDE SOLUTIONS
Sample
ZnCI 2 LiCI (mol) (ppm)
TA (°C)
H2/02 (1 h -1)
Rate (A s -1)
d ~m)
Structure
Resistivity (I2 cm)
Luminescence
(nm) Rel. intens.
1 2 3 4 5 6 7 8 9
0.1 0.1 1 1 1 1 1 1 1
102 103 103 104
600 1000 600 600 600 600 600 600 600
1000/520 1000/520 1000/520 1000/640 1000[800 1000/520 1000/520 1000/800 1000/520
8 6 20 22 25 20 24 i1 11
0.7 1.0 1.0 1.2 0.9 1.2 1.0 1.1 0.7
+ ++ ++ ++ ++ + + + -
5 x 5x 1x 4 x 5x 3x 8x 2x 1x
101 103 104 104 104 104 104 105 106
518 525 520 543 554 608 534 600
5 60 67 30 31 30 60 18
++ Oriented; + moderately oriented. s o l u t i o n c o u l d be c o n t r o l l e d b y t h e a p p l i c a t i o n o f an e x c e s s p r e s s u r e o f an i n e r t gas. T h e s u b s t r a t e u s e d w a s q u a r t z glass. T h e d e p o s i t i o n t e m p e r a t u r e s r a n g e d f r o m 4 0 0 ° to 1 0 0 0 °C, a n d c o u l d be a d j u s t e d b y altering t h e h e a t e r - s u b s t r a t e d i s t a n c e . T h e degree o f o x i d a t i o n o f t h e zinc o x i d e films w a s d e t e r m i n e d b y t h e h y d r o g e n : o x y g e n ratio. H y d r o g e n f l o w rates o f 1 0 0 0 1 h - l a n d o x y g e n f l o w rates o f 5 0 0 - 1 0 0 0 1 h -1 were used. T h e zinc o x i d e d e p o s i t i o n rates u s e d c o v e r e d a range f r o m 5 to 50 A s-1 . T h e p r e p a r a t i o n c o n d i t i o n s a n d p r o p e r t i e s o f t h e z i n c o x i d e films o b t a i n e d are given in Table II. 3. THIN FILM PROPERTIES
3.1. F i l m t e x t u r e T h e z i n c o x i d e t h i n films o n glass p r e p a r e d in a c c o r d a n c e w i t h p r o c e d u r e 2.1 are p o l y c r y s t a l l i n e , as s h o w n in t h e e l e c t r o n m i c r o g r a p h s a n d e l e c t r o n d i f f r a c t i o n p a t t e r n s
PURE AND Li-DOPED ZnO FILMS
89
Fig. 1. Electron micrographs of the surface and electron diffraction patterns for ZnO tiara trims deposited onto glass by reactive evaporation. (a) Sample 1: PO: = 0.1 Torr; r = 15 A s-1, d = 2.3 ~m. (b) Sample 2:PO2 = 1 Torr; r = 20 A s-I ;d = 1.3 tzm. (c) Sample 5:PO2 = 10 Torr; r = 120 A s-l; d = 2.1 ~m. (Fig. 1). The diffraction patterns were obtained by employing RHEED according to the Lebedeff method, focusing either upon the object (e.g. Fig. l(a)) or upon the photographic plate. Patterns l(a) and l ( b ) reveal a pronounced orientation of the crystallites with the c-axis vertical to the substrate plane. At all the partial pressures of oxygen investigated, from 0.1 to 10 Torr, it was possible to obtain oriented zinc oxide thin films at deposition temperatures from 500 o to 600 °C. However the degree of orientation decreased with increase in deposition rate and deposition rates exceeding 100 A s -1 led to a more or less random distribution of orientations of the crystallites, as shown in pattern l(c). The electron micrographs (Figs. l(a) and l(b)) show the dominant morphology of the (0001)crystallites bounded by the six pyramid faces. As the texture patterns in Fig. 2 show, the zinc oxide thin films on quartz glass prepared in the oxyhydrogen flame according to procedure 2.2 are polycrystalline with a preferred orientation parallel to the c-axis. Textured thin films of zinc oxide were obtained under all the test conditions: deposition temperatures from 400 ° to 1000 °C, hydrogen-oxygen ratios of 2:1 and 1 : 1, zinc chloride concentrations of 1-0.1 M and Li dopings up to 104 ppm. The degree of orientation increased with an increase in the deposition temperature. This was most obvious with the 0.1 M zinc chloride solutions which led to more highly oriented films at a deposition temperature of 1000 °C (sample 2) than at a deposition temperature of 600 °C (sample 1). Furthermore, the degree of orientation was reduced with an increase in the doping concentration of lithium (samples 8 and 9).
3.2. ElectHcal resistance and luminescence properties The thin films of zinc oxide were supplied with silver electrodes by thermal evaporation and the electrical resistance was measured by means o f the four-point probe method.
90
G. WEISE etal
Fig. 2. Electron micrographs of the surface and electron diffraction patterns for ZnO thin films on quartz glass, prepared by the oxidation of zinc chloride solutions. (a) Sample 3: TA = 600 °C; H2/O 2 = 1000/5201 h -1 ; ZnCI2 = 1 M. (b) Sample 8: TA = 600 °C; H2/O 2 = 1000/800 1h-1 ; ZnC12 = 1 M; LiC1 = 103 ppm. (c) Sample 2: TA = 1000 °C; H2/O 2 = 1000/5201 h -1 ; ZnC12 = 0.1M. The zinc oxide films deposited according to procedure 2.1 showed the following dependences of the specific resistance on the preparation conditions (Table I). For a deposition temperature of 500 °C and partial oxygen pressures from 0.1 to 10 Torr the specific resistance rose from 1 to 103 ~ cm. With higher deposition rates at a constant oxygen pressure the specific resistance was less, as evidenced by samples 3 and 4 (oxygen pressure of 5 Torr) and samples 6 and 7 (10 Torr). Zinc oxide is a non-stoichiometric compound since it is deficient in oxygen, but increased oxygen pressures and reduced deposition rates raised the proportion of oxygen in the zinc oxide lattice, thus causing the specific resistance to increase. Li doping at an optimum oxygen pressure of 10 Torr resulted in a further increase in the specific resistance to 104 f2 cm, in conformity with the action of monovalent cations as acceptors in zinc oxide. In the thin films of zinc oxide prepared by procedure 2.2 the specific resistances covered a range from 101 to 106 ~2 cm (Table II). For 0.1 M zinc chloride solutions it was possible to obtain specific resistances of less than 104 ~ cm (samples 1 and 2). At a deposition temperature of 600 °C optimum specific resistances of 104 ~2 cm could be obtained for 1 M solutions. As the amount of oxygen in the gas mixture was raised, the resistance rose to 5 x 104 ~2 cm (samples 3-5). Solutions doped with Li up to 104 ppm resulted in the resistance increasing to 106 ~2 cm. By using a high pressure mercury vapour lamp and an SPM2 monochromator it was possible to investigate the luminescence of zinc oxide thin films due to monochromatic ultraviolet excitation. The location of the emitted luminescence band and its relative intensity (Tables I and II) were determined. In pure zinc oxide thin films the greencoloured luminescence band was displaced towards shorter wavelengths when the degree of orientation increased. Doping with lithium displaced the luminescence towards longer wavelengths of orange colour. This displacement was discernible to a limited
PURE AND Li-DOPED ZnO FILMS
91
extent even if the degree of orientation in the thin films decreased. The extremely low relative intensities (samples 1 and 2, Table II) are evidently brought about by the small crystallite sizes. ACKNOWLEDGMENTS Our thanks are due to Dr. Edelmann, ZFW Dresden, for the structure investigations, and to Dr. Rauch, Carl-Zeiss-Jena, for the luminescence measurements. REFERENCES 1 2 3 4 5 6 7 8
G. A. Rozgonyi and W. J. Polito, J. Vac. Sci. Technol., 6 (1969) 115. T. Hada, K. Wasa and S. Hayakawa, Thin Solid Films, 7(1971) 135. D. L. Raimondi and E. Kay,./. Vac. ScL Technol., 7 (1970)96. R. Inaba, T. Ishiguro and N. Mikoshiba, Jpn. J. Appl. Phy~, 10 (1971) 1493. K. W. Schalimova and K. M. Botnew, Kristallografiya, 13 (1968) 679. G. Heiland, E. Mollwo and F. St6ckmann, Solid State Phys., 8 (1958) 191. D. Galli and J. E. Coker, Appl. Phy~ Lett., 16 (1970)439. J. McK. Nobbs and F. C. GiUespie,J. Phys. Chem. Solids, 31 (1970) 2553.