- Email: [email protected]
ZnO growth by chemically assisted sublimation
ELSEVIER
Journal
of Crystal
Growth
184/185 (1998) 1026-1030
ZnO growth by chemically assisted sublimation J.-M. Ntep, M. Barb&, G. Cohen-Solal, Laboratoire
F. Bailly, A. Lusson,
R. Triboulet”
de Physique des Solides de Bellevue, CNRS, I, Place Aristide Briand, F-921 95 Meudon Cedex, France
Abstract Residual water present in gases or gaseous mixtures such as Hz, Ar or H2 + Hz0 is shown to act as sublimation activator of the vapour phase transport of ZnO. The thermodynamic constants of the water activated sublimation are determined through close spacing vapour transport experiments using a theoretical model. The variation of the growth rate as a function of temperature is found to be not linear. Finally, some optical and structural properties of the layers are studied and compared to those of bulk crystals. No photoluminescence emission that could be attributed to HO or O2 are detected. 0 1998 Elsevier Science B.V. All rights reserved. PACS: 81.05.D~; 81.10.Bk; 81.15.Gh; 78.55.Et; 61.10.N~ Keywords:
ZnO growth; Reactive sublimation;
ZnO characterisation
1. Introduction ZnO is an attractive material as alternative substrate for the growth of GaN epilayers. It shows the same crystallographic structure as GaN with a lattice mismatch less than 1.8%. Because of its large band gap of 3.2 eV, ZnO could also act as a candidate for blue-emitting devices. Its shear modulus, which has been identified to be a key structural signature of the materials Cl], has been calculated using the semi-empirical law proposed in Ref. [2]: in spite of the very high ionicity of its chemical bond, ZnO presents the highest shear modulus of
*Corresponding author. [email protected].
Fax:
+ 33 01 45 07 58 41; e-mail
0022-0248/98/$19.00 c 1998 Elsevier Science B.V. All rights reserved PII SOO22-0248(97)00763-X
the II-VI family, and then the most stable lattice, because of its very small interatomic distance. In order to grow ZnO crystals by the technique of sublimation and chemical vapour transport (SCVT), using different gaseous species and mixtures as sublimation activators, the thermodynamic constants of the transport have been determined using the technique of close spacing vapour transport (CSVT), allowing fast experiments and easy determination of the growth rate, from the theoretical model of Bailly et al. [3]. The structural and electronic properties of the layers grown by CSVT have been assessed by X-ray diffraction, scanning electron microscopy and photoluminescence and compared to the properties of bulk crystals obtained from preliminary growth experiments.
J.-M. Ntep et al. 1 Journal
2. Experimental
of Crystal Growth 1841185 (1998) 1026-1030
procedure
Several papers deal with the bulk growth of ZnO crystals in closed vessels [4-S]. Given the very high ZnO melting point ( - 2OOo”C), growth experiments by sublimation have been implemented. 6N ZnO powder charges have been loaded in silica ampoules sealed off under vacuum, according to a Piper and Polich configuration. Such ampoules have been positioned in a furnace with the ZnO charge at - 1000°C and the top of the ampoule kept at 950°C. No transport has been observed in such conditions. Such gases and gaseous mixtures as Hz, H2 + Hz0 or Ar have been introduced in the growth ampoules. Fast transport to the cold point of the ampoules has been observed under such conditions, demonstrating that these gases and gaseous mixtures were acting as sublimation activators, providing considerable increase of the sublimation rate. The chemical reactions occurring during the vapour growth of ZnO with such gases as hydrogen and water vapours are not well known. A knowledge of thermodynamic constants such as formation enthalpy and activation energy can help to determine suitable conditions of crystal growth. Such constants have been determined using the so-called close spacing vapour transport technique (CSVT) measuring the variation of the growth rate as a function of temperature and using the theoretical model developed by Bailly et al. [3]. The CSVT reactor used is similar to the one described by Robinson et al. [9]. The graphite crucibles have been modified to obtain a fast temperature rise by Joule effect [lo]. The polycrystalline ZnO pieces used as source in the CSVT experiments have been grown by sublimation enhanced by gaseous activators as described above. 1 mm thick and 10 mm diameter sapphire wafers were used as substrates. ZnO ingots have been cut into 1.5 cm diameter and 1 mm thick slices. The face used for the deposition was mechanically polished up to a grain size of 1 urn. The other face was only ground to get parallel surfaces. After cleaning and drying with argon source, substrate, substrate holder and substretch, the whole set-up was positioned carefully in the reactor which was then evacuated to 1O-3 Torr during 20 min. Hy-
1027
drogen was then introduced in the reactor which was again evacuated and finally filled with hydrogen the flow rate of which was fixed at one litre per minute. The substrate was backed out at 750°C in 3 min. while protecting the source with the substretch-holder. After cooling down to lOO”C, substrate, substretch and source were respectively put in contact and submitted to a thermal treatment at the same temperature (600 or 550°C) in 2 min. Substrate and substretch were immediately moved away after switching off heating. When the heating controller indicated lOO”C, a distance of 1 mm was adjusted between the substrate and source. Substrate and source temperatures were then adjusted and the growth occurred during different times. Temperature and growth time are the changeable parameters from one experiment to the other.
3. Results The thickness variation as a function of time e(t) is displayed in Fig. 1 for a source temperature of 550°C. The substrate temperature has been varied from 400 to 500°C. The source temperature was fixed at 550°C. The experimental results are well fitted by the equation e(t) = $ t3’4 (solid line). The curve of the instantaneous rate as a function of time is then not linear, as shown in Fig. 2. The growth rate decreases first sharply and then slowly as a function of time. This leads to consider growth rates at identical growth times for a given source
1
10
100
log(t)) Fig. 1. Thickness as a function of time; the solid line is the theoretical fit, while squares are experimental points.
1028
J.-M. Ntep et al. /Journal
of Crystal Growth 1841185 (1998) 1026-1030
temperature. Only the instantaneous rate is taken into account. The variation of the ZnO growth rate as a function of the inverse substrate temperature is given by Eq. (1) according to Ref. [3], and shown in Fig. 3. The experimental points are values recorded at both source temperatures of 550 and 600°C. The growth time was 4 min for each experiment.
with CI= Q/R and /? = AH/R, Q is the activation energy, AH the reaction enthalpy, T the source temperature, 0 the substrate temperature and R the gas constant. 2 = @,r& where QSis a combination of the pressures associated with the reacting gas, y and 5 are the pre-exponential constants of the Arrhenius relations. The solid and dashed lines correspond to the fit according to Eq. (1) for source temperatures of 550 and 600°C respectively. Experimental points are presented as circles. T, is the source temperature. Using Eq. (l), the activation energy Q = 0.63 eV and the formation enthalpy AH = 0.42 eV, have been determined. The good fit between theoretical curves and experimental points, as shown in Fig. 3, is obtained for 1 = 2.995 x 10” AImin, CI= 7.245 x lo3 K and B = 4.35 x lo3 K. This fit remains valid for different source temperatures to within about a hundredth of electron-volt showing that the growth follows satisfactorily the model of Bailly et al. [3]. The equation giving the energy of sublimation is generally expressed [3] as
l/0(@, T, A,a, P) i
e-a/(T+273)
l+e
e
a/(T+273)
-a(l/(T+273)-
_
e-fi/(@+273)
)
l/(0+273))
(-
(1)
al gj
0.5
0
10
20
30
Qsublimation = AH + Qcondensatiom
40
where AH = 0.41 eV (9.7 kcal/mole), Qcondensation = 0.63 eV (14.5 kcal/mole) and Qsublimation = 1.04 eV (24.2 kcal/mole).
t (min) Fig. 2. Instantaneous
growth
rate as a function
I’s = 600 ‘%!
of time.
I
I
I
I
I
1.4
1.5
1.6
1.7
Ts = 550 “C
1.1
1.2
I .3
1.8
100010 (K) Fig. 3. ZnO growth rate as a function of the inverse substrate experimental points; solid lines: theoretical fit.
temperature
for two source temperatures
(T,): (550 and 600°C). Circles:
J.-M. Ntep et al./JournalofCrystal Growth 184J18.5(1998) 1026-1030
The following table gives the slopes which can be approximated as activation energies of the variation of the growth rates (In scale) as a function of the inverse source temperature l/T,. They show an uncertainty within half an eV. They are not the same for any AT.
AT(K)
50
75
100
125
E, (eV)
1.48
0.89
0.88
1.44
Eq. (2) allows to determine activation energies. The pseudoactivation energies calculated from the values given above vary from 0.97 to 1.14 eV and are different from those determined above. The uncertainty of experimental values makes you predict such a difference. - AH/kT F(~GAK~T,@)=
-+e_AH,;T_e_AH,kB
e-a((l/T)k(l/o))
+a
1
+
e-a((l/T)-W@H’
(2)
Although the equilibrium reactions taking place during the growth remain unknown, we have attributed the role of catalyst to the residual water present in the ampoules, either intentionally introduced or remaining as trace in the gas used. The following reactions between ZnO and Hz0 or H2 in the temperature range used show a positive free enthalpy and cannot consequently occur (using the HSC chemistry software of Outokumpu Research): ZnO(s)
+ H,(g) * Zn(g) + H,O(g),
ZnO(s)
+ H,O(g) * Zn(g) + H,(g) + O,(g),
ZnO(s) + 2H,O(g) ZnO(s)
+ H,O(g)
0 Zn(OH),
+ H,O(g),
o Zn(OH),.
The role of residual water has already several times been reported [3,6,11]. We have performed successful experiments showing that argon introduced in the ampoules as residual gas led also to vapour transport. These experiments aimed at demonstrating that hydrogen was not the only chemical agent responsible for the oxide transport.
1029
Water plays the role of activator. Another proof is that transport does not occur under vacuum.
4. ZnO characterisation Photoluminescence spectra (N 1.6 K) of layers grown by CSVT under hydrogen (Fig. 4) are similar to those reported by Solbrig et al. [12]. The presence of the well-resolved band edge excitonic emissions, with the free exciton peak X at 3.41 eV, demonstrates the good quality of the material. Spectra of layers present the same features as those of bulk crystals, but additional lines below 3.3 eV. Zinc shows a tendency to occupy interstitial sites, creating coloured centres at the origin of the orange colour frequently observed on the oxide [S]. It is worth noting that no emission that could be attributed to HO or O2 are detected in both crystals and layers. SEM observations show that layers grown under argon present facets indicating that argon with a residual water pressure should be more suitable to the growth of good quality crystals than hydrogen. This could be due to the reducing role of hydrogen on the oxide. X-ray diffraction patterns measured on layers are in good agreement with the ASTM card of ZnO, on both intensity and line positions. Laue patterns do not show any preferential orientation. Electronic microprobe observations show a green luminescence that could be explained by the presence of zinc excess in interstitial positions [6].
5. Conclusions Residual water present in gases or gaseous mixtures such as hydrogen, argon, Hz + Hz0 has been found to activate the sublimation of ZnO, according to a mechanism that has not yet been identified. The determination of the thermodynamic constants of the transport with water acting as a sublimation activator has been achieved through close spacing vapour transport experiments, allowing fast and easy determination of the growth rate, using a theoretical model previously reported [3]. The variation of the growth rate as a function of temperature
1030
J-M.
Ntep et al. /Journal
of Crystal Growth 1841185 (1998) 1026-1030
layer
bulk -
b
I
I
I
I
I
I
2.4
2.6
2.8
3.0
3.2
3.4
Energy Fig 4. Photoluminescence
spectra
of ZnO layers grown
has been found to be not linear. The energies of activation, sublimation and condensation and the enthalpy of formation have been calculated. The same values allow a good fit of the variation of the growth rates as a function of temperature for two different source temperatures, indicating the general character of the model used. The observation of the photoluminescence spectra of the well resolved band edge excitonic emissions shows the good quality of the material. No emission that could be attributed to HO or O2 are detected: water used as sublimation activator does not seem to act as a dopant in ZnO. This fundamental approach opens now the wayto the controlled ZnO growth by vapour transport activated with water.
4
WI
by CSVT under hydrogen
atmosphere
compared
to bulk ZnO.
References [l] C. V&it, Mater. Sci. Eng. B 43 (1997) 60. [2] C. Virii, J. Crystal Growth 184/185 (1998) 1061. [3] F. Bailly, G. Cohen-Solal, J. Mimila-Arroyo, J. Electrochem. Sot. 126 (1979) 1604. [4] M. Shiloh, J. Gutman, J. Crystal Growth 11 (1971) 105. [S] W. Piekarczyk, S. Gazda, T. Niemyski, J. Crystal Growth 12 (1972) 272. [6] K. Matsumoto, K. Konemura, G. Shimaoko, J. Crystal Growth 71 (1985) 99. [7] K. Matsumoto, G. Shimaoko, J. Crystal Growth 86 (1988) 410. [S] K. Matsumoto, J. Crystal Growth 102 (1990) 137. [9] P.H. Robinson, RCA Rev. 24 (1968) 574. [lo] J. Mimila-Arroyo, 3rd cycle thesis, Paris, 1978. [11] J. Mimila, R. Triboulet, Mat. Lett. 24 (1995) 221. [12] C. Solbrig, E. Mollwo, Solid State Commun. 5 (1967) 625.
Journal
of Crystal
Growth
184/185 (1998) 1026-1030
ZnO growth by chemically assisted sublimation J.-M. Ntep, M. Barb&, G. Cohen-Solal, Laboratoire
F. Bailly, A. Lusson,
R. Triboulet”
de Physique des Solides de Bellevue, CNRS, I, Place Aristide Briand, F-921 95 Meudon Cedex, France
Abstract Residual water present in gases or gaseous mixtures such as Hz, Ar or H2 + Hz0 is shown to act as sublimation activator of the vapour phase transport of ZnO. The thermodynamic constants of the water activated sublimation are determined through close spacing vapour transport experiments using a theoretical model. The variation of the growth rate as a function of temperature is found to be not linear. Finally, some optical and structural properties of the layers are studied and compared to those of bulk crystals. No photoluminescence emission that could be attributed to HO or O2 are detected. 0 1998 Elsevier Science B.V. All rights reserved. PACS: 81.05.D~; 81.10.Bk; 81.15.Gh; 78.55.Et; 61.10.N~ Keywords:
ZnO growth; Reactive sublimation;
ZnO characterisation
1. Introduction ZnO is an attractive material as alternative substrate for the growth of GaN epilayers. It shows the same crystallographic structure as GaN with a lattice mismatch less than 1.8%. Because of its large band gap of 3.2 eV, ZnO could also act as a candidate for blue-emitting devices. Its shear modulus, which has been identified to be a key structural signature of the materials Cl], has been calculated using the semi-empirical law proposed in Ref. [2]: in spite of the very high ionicity of its chemical bond, ZnO presents the highest shear modulus of
*Corresponding author. [email protected].
Fax:
+ 33 01 45 07 58 41; e-mail
0022-0248/98/$19.00 c 1998 Elsevier Science B.V. All rights reserved PII SOO22-0248(97)00763-X
the II-VI family, and then the most stable lattice, because of its very small interatomic distance. In order to grow ZnO crystals by the technique of sublimation and chemical vapour transport (SCVT), using different gaseous species and mixtures as sublimation activators, the thermodynamic constants of the transport have been determined using the technique of close spacing vapour transport (CSVT), allowing fast experiments and easy determination of the growth rate, from the theoretical model of Bailly et al. [3]. The structural and electronic properties of the layers grown by CSVT have been assessed by X-ray diffraction, scanning electron microscopy and photoluminescence and compared to the properties of bulk crystals obtained from preliminary growth experiments.
J.-M. Ntep et al. 1 Journal
2. Experimental
of Crystal Growth 1841185 (1998) 1026-1030
procedure
Several papers deal with the bulk growth of ZnO crystals in closed vessels [4-S]. Given the very high ZnO melting point ( - 2OOo”C), growth experiments by sublimation have been implemented. 6N ZnO powder charges have been loaded in silica ampoules sealed off under vacuum, according to a Piper and Polich configuration. Such ampoules have been positioned in a furnace with the ZnO charge at - 1000°C and the top of the ampoule kept at 950°C. No transport has been observed in such conditions. Such gases and gaseous mixtures as Hz, H2 + Hz0 or Ar have been introduced in the growth ampoules. Fast transport to the cold point of the ampoules has been observed under such conditions, demonstrating that these gases and gaseous mixtures were acting as sublimation activators, providing considerable increase of the sublimation rate. The chemical reactions occurring during the vapour growth of ZnO with such gases as hydrogen and water vapours are not well known. A knowledge of thermodynamic constants such as formation enthalpy and activation energy can help to determine suitable conditions of crystal growth. Such constants have been determined using the so-called close spacing vapour transport technique (CSVT) measuring the variation of the growth rate as a function of temperature and using the theoretical model developed by Bailly et al. [3]. The CSVT reactor used is similar to the one described by Robinson et al. [9]. The graphite crucibles have been modified to obtain a fast temperature rise by Joule effect [lo]. The polycrystalline ZnO pieces used as source in the CSVT experiments have been grown by sublimation enhanced by gaseous activators as described above. 1 mm thick and 10 mm diameter sapphire wafers were used as substrates. ZnO ingots have been cut into 1.5 cm diameter and 1 mm thick slices. The face used for the deposition was mechanically polished up to a grain size of 1 urn. The other face was only ground to get parallel surfaces. After cleaning and drying with argon source, substrate, substrate holder and substretch, the whole set-up was positioned carefully in the reactor which was then evacuated to 1O-3 Torr during 20 min. Hy-
1027
drogen was then introduced in the reactor which was again evacuated and finally filled with hydrogen the flow rate of which was fixed at one litre per minute. The substrate was backed out at 750°C in 3 min. while protecting the source with the substretch-holder. After cooling down to lOO”C, substrate, substretch and source were respectively put in contact and submitted to a thermal treatment at the same temperature (600 or 550°C) in 2 min. Substrate and substretch were immediately moved away after switching off heating. When the heating controller indicated lOO”C, a distance of 1 mm was adjusted between the substrate and source. Substrate and source temperatures were then adjusted and the growth occurred during different times. Temperature and growth time are the changeable parameters from one experiment to the other.
3. Results The thickness variation as a function of time e(t) is displayed in Fig. 1 for a source temperature of 550°C. The substrate temperature has been varied from 400 to 500°C. The source temperature was fixed at 550°C. The experimental results are well fitted by the equation e(t) = $ t3’4 (solid line). The curve of the instantaneous rate as a function of time is then not linear, as shown in Fig. 2. The growth rate decreases first sharply and then slowly as a function of time. This leads to consider growth rates at identical growth times for a given source
1
10
100
log(t)) Fig. 1. Thickness as a function of time; the solid line is the theoretical fit, while squares are experimental points.
1028
J.-M. Ntep et al. /Journal
of Crystal Growth 1841185 (1998) 1026-1030
temperature. Only the instantaneous rate is taken into account. The variation of the ZnO growth rate as a function of the inverse substrate temperature is given by Eq. (1) according to Ref. [3], and shown in Fig. 3. The experimental points are values recorded at both source temperatures of 550 and 600°C. The growth time was 4 min for each experiment.
with CI= Q/R and /? = AH/R, Q is the activation energy, AH the reaction enthalpy, T the source temperature, 0 the substrate temperature and R the gas constant. 2 = @,r& where QSis a combination of the pressures associated with the reacting gas, y and 5 are the pre-exponential constants of the Arrhenius relations. The solid and dashed lines correspond to the fit according to Eq. (1) for source temperatures of 550 and 600°C respectively. Experimental points are presented as circles. T, is the source temperature. Using Eq. (l), the activation energy Q = 0.63 eV and the formation enthalpy AH = 0.42 eV, have been determined. The good fit between theoretical curves and experimental points, as shown in Fig. 3, is obtained for 1 = 2.995 x 10” AImin, CI= 7.245 x lo3 K and B = 4.35 x lo3 K. This fit remains valid for different source temperatures to within about a hundredth of electron-volt showing that the growth follows satisfactorily the model of Bailly et al. [3]. The equation giving the energy of sublimation is generally expressed [3] as
l/0(@, T, A,a, P) i
e-a/(T+273)
l+e
e
a/(T+273)
-a(l/(T+273)-
_
e-fi/(@+273)
)
l/(0+273))
(-
(1)
al gj
0.5
0
10
20
30
Qsublimation = AH + Qcondensatiom
40
where AH = 0.41 eV (9.7 kcal/mole), Qcondensation = 0.63 eV (14.5 kcal/mole) and Qsublimation = 1.04 eV (24.2 kcal/mole).
t (min) Fig. 2. Instantaneous
growth
rate as a function
I’s = 600 ‘%!
of time.
I
I
I
I
I
1.4
1.5
1.6
1.7
Ts = 550 “C
1.1
1.2
I .3
1.8
100010 (K) Fig. 3. ZnO growth rate as a function of the inverse substrate experimental points; solid lines: theoretical fit.
temperature
for two source temperatures
(T,): (550 and 600°C). Circles:
J.-M. Ntep et al./JournalofCrystal Growth 184J18.5(1998) 1026-1030
The following table gives the slopes which can be approximated as activation energies of the variation of the growth rates (In scale) as a function of the inverse source temperature l/T,. They show an uncertainty within half an eV. They are not the same for any AT.
AT(K)
50
75
100
125
E, (eV)
1.48
0.89
0.88
1.44
Eq. (2) allows to determine activation energies. The pseudoactivation energies calculated from the values given above vary from 0.97 to 1.14 eV and are different from those determined above. The uncertainty of experimental values makes you predict such a difference. - AH/kT F(~GAK~T,@)=
-+e_AH,;T_e_AH,kB
e-a((l/T)k(l/o))
+a
1
+
e-a((l/T)-W@H’
(2)
Although the equilibrium reactions taking place during the growth remain unknown, we have attributed the role of catalyst to the residual water present in the ampoules, either intentionally introduced or remaining as trace in the gas used. The following reactions between ZnO and Hz0 or H2 in the temperature range used show a positive free enthalpy and cannot consequently occur (using the HSC chemistry software of Outokumpu Research): ZnO(s)
+ H,(g) * Zn(g) + H,O(g),
ZnO(s)
+ H,O(g) * Zn(g) + H,(g) + O,(g),
ZnO(s) + 2H,O(g) ZnO(s)
+ H,O(g)
0 Zn(OH),
+ H,O(g),
o Zn(OH),.
The role of residual water has already several times been reported [3,6,11]. We have performed successful experiments showing that argon introduced in the ampoules as residual gas led also to vapour transport. These experiments aimed at demonstrating that hydrogen was not the only chemical agent responsible for the oxide transport.
1029
Water plays the role of activator. Another proof is that transport does not occur under vacuum.
4. ZnO characterisation Photoluminescence spectra (N 1.6 K) of layers grown by CSVT under hydrogen (Fig. 4) are similar to those reported by Solbrig et al. [12]. The presence of the well-resolved band edge excitonic emissions, with the free exciton peak X at 3.41 eV, demonstrates the good quality of the material. Spectra of layers present the same features as those of bulk crystals, but additional lines below 3.3 eV. Zinc shows a tendency to occupy interstitial sites, creating coloured centres at the origin of the orange colour frequently observed on the oxide [S]. It is worth noting that no emission that could be attributed to HO or O2 are detected in both crystals and layers. SEM observations show that layers grown under argon present facets indicating that argon with a residual water pressure should be more suitable to the growth of good quality crystals than hydrogen. This could be due to the reducing role of hydrogen on the oxide. X-ray diffraction patterns measured on layers are in good agreement with the ASTM card of ZnO, on both intensity and line positions. Laue patterns do not show any preferential orientation. Electronic microprobe observations show a green luminescence that could be explained by the presence of zinc excess in interstitial positions [6].
5. Conclusions Residual water present in gases or gaseous mixtures such as hydrogen, argon, Hz + Hz0 has been found to activate the sublimation of ZnO, according to a mechanism that has not yet been identified. The determination of the thermodynamic constants of the transport with water acting as a sublimation activator has been achieved through close spacing vapour transport experiments, allowing fast and easy determination of the growth rate, using a theoretical model previously reported [3]. The variation of the growth rate as a function of temperature
1030
J-M.
Ntep et al. /Journal
of Crystal Growth 1841185 (1998) 1026-1030
layer
bulk -
b
I
I
I
I
I
I
2.4
2.6
2.8
3.0
3.2
3.4
Energy Fig 4. Photoluminescence
spectra
of ZnO layers grown
has been found to be not linear. The energies of activation, sublimation and condensation and the enthalpy of formation have been calculated. The same values allow a good fit of the variation of the growth rates as a function of temperature for two different source temperatures, indicating the general character of the model used. The observation of the photoluminescence spectra of the well resolved band edge excitonic emissions shows the good quality of the material. No emission that could be attributed to HO or O2 are detected: water used as sublimation activator does not seem to act as a dopant in ZnO. This fundamental approach opens now the wayto the controlled ZnO growth by vapour transport activated with water.
4
WI
by CSVT under hydrogen
atmosphere
compared
to bulk ZnO.
References [l] C. V&it, Mater. Sci. Eng. B 43 (1997) 60. [2] C. Virii, J. Crystal Growth 184/185 (1998) 1061. [3] F. Bailly, G. Cohen-Solal, J. Mimila-Arroyo, J. Electrochem. Sot. 126 (1979) 1604. [4] M. Shiloh, J. Gutman, J. Crystal Growth 11 (1971) 105. [S] W. Piekarczyk, S. Gazda, T. Niemyski, J. Crystal Growth 12 (1972) 272. [6] K. Matsumoto, K. Konemura, G. Shimaoko, J. Crystal Growth 71 (1985) 99. [7] K. Matsumoto, G. Shimaoko, J. Crystal Growth 86 (1988) 410. [S] K. Matsumoto, J. Crystal Growth 102 (1990) 137. [9] P.H. Robinson, RCA Rev. 24 (1968) 574. [lo] J. Mimila-Arroyo, 3rd cycle thesis, Paris, 1978. [11] J. Mimila, R. Triboulet, Mat. Lett. 24 (1995) 221. [12] C. Solbrig, E. Mollwo, Solid State Commun. 5 (1967) 625.