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Amorphous-crystalline transition in Ge films by photoemission
Solid State Communications,
Vol. 12, pp.1007—lOll, 1973.
Pergamon Press.
Printed in Great Britain
AMORPHOUS-CRYSTALLINE TRANSITION IN Ge FiLMS BY PHOTOEMISSION L.D. Laude, R.F. Willis and B. Fitton Surface Physics Division, European Space Research Organisation, Noordwijk, Holland (Received 5 January 1973; in revised form 16 February 1973 by M. Balkanski)
Photoelectron second-derivative, energy distribution spectra reveal subtle changes in the density-of-states of vapour-deposited, amorphous Ge films, arising from microstructural ordering processes. Evidence is presented for ‘ideal’ amorphous Ge films, prepared at a deposition temperature T8 = 120 ±20°C,which undergo an abrupt amorphous—crystalline transition upon annealing to TA 230 ±10°C.These films are thought to approach closely the random-network Polk-model. High densities of localized states in the pseudogap are observed in imperfect films. 1 ,2 have emphasized RECENT experimental studies that the disordered—microcrystalline phase transition in Ge and Si films is extremely sensitive to sample preparation technique. Secondary electron emission spectroscopy of vapor-deposited, ‘amorphous’ Ge films has been shown to relate changes in the energy distribution and density of conduction electron states to thermally-activated, microstructural-ordering processes.3 The question remains, however, as to what density-of-states features can be associated with an ideal, ropologically-disordered network, such as that suggested by Polk,4 and how close such a structure can be approached experimentally? Photoelectron energy distribution second-derivative spectra reveal intensity variations associated with density-of-states features, which change as a consequence of thermally-activated, short-range ordering, In order to try to clarify the nature of the amorphous state and the amorphous—crystalline transition in Ge films, we have studied the changes in intensity of photoemission spectral features as a function of the substrate deposition temperature, 1, cooling-rate to 20°C,R, and annealing temperature, TA. These parameters are particularly important for defining the degree of short-range order and homogeneity in vapordeposited Ge films.’~3~5 It is shown that films with a structure, which we believe to closely approach the ‘ideal’ random-network,4 may be prepared by 1007
deposition on vitreous carbon substrates at temperatures in the range, 100< 7 <140°C,followed by controlled slow-cooling (R 10-2 °C/sec)to 20°C. Upon subsequent annealing, these films exhibit an abrupt transition to a microcrystalline structure at a temperature, TA 230 ±10 C. Photoemission from localised stares located in the pseudogap was not observed in these ‘ideal’ films.6 A complex distribution of states tailing into the pseudogap was observed, however, during annealing of high density films deposited at higher substrate temperatures, 140 < 7 200°C,which had been previously fastcooled (R 0.3 C/sec) to 20 C. The results emphasize that the detailed microstructure of high-density, vapor-deposited Ge films is extremely sensitive to preparation technique, and photoelectron energy distribution second-derivative spectra provide a means for monitoring the degree of microstructural-ordering present in these ‘ideal’ films. Films of thickness l000A were deposited from a collimated Ge-crystal source (p-type, 50&2-cm), at rates of~20A/sec onto polished conducting vitreous carbon surfaces. The substrate-to-source distance was 20 cm and the vacuum better than 2 X iO”~Torr. Under such conditions, films with densities close to that of the crystal,1 free of microvoids7 and possessing sharp optical-absorption edges1 are obtained, providing the preparation parameters, 1, R, and TA are
1008
AMORPHOUS—CRYSTALLINE TRANSITION IN Ge FILMS •
I
•
[‘\EDC /
~
A
A valence “,b~d
~Il
B?
EirJ.,\
b
‘ii
/
band dge
~j
..~-
1p~!~9”’\~j.
-
Vol. 12, No. 10
are compared with the spectrum from a vacuum cleaved Ge (111) crystal at a photon energy, hi.’ = 7.72 eV. For 7> 20°C,the films were maintained at the deposition-substrate temperature for one hour before slow cooling (10-2 °C/sec)to 20°C.The abscissa scale refers to the energy of the emitted electrons relative to the valence-band edge. The photoelectric threshold was measured to be 4.8 and 4.7 eV With increasing 7 changes are seen to occur in the spectra of the deposited films, which gradually approach in the crystal and amorphous films, respectively. the crystalline spectrum at T~Ge200°C.For films deposited at i 20°C,the spectrum is dominated ,
\
~
\vi • 5
• 6
• 7
• 8
\/
“
r, ~
I
I
5
6
v I
7
8
FIG. 1. (a) Second derivative spectra of photoelectron energy distribution curves (EDC’s) obtained at hi.’ = 7.72 eV from ‘strain-free’ amorphous Ge films, prepared by deposition at substrate deposition temperatures, 20°C i 200°C,and slowly cooled to 20°C at a rate of 10-2 °C/sec.Such a spectrum and its corresponding EDC (dotted line), obtained at the same photon energy from a vacuum cleaved Ge (Ill) crystal, are shown for comparison. The abscissa scale refers to the to valence-band the energy edge. of the(b) emitted Effectelectrons of annealing relative temperature, 150 350°C,on slow-cooled films deposited at 7 = 120 C. The sudden appearance of the crystalline feature (peak A) with subsequent annealing at 240°Cis indicative of a sharp disorderedcrystalline phase transition. The spectra are displaced vertically for clarity, carefully controlled.3 There is evidence5 which suggests that the degree of short-range order present in these films is sensitive to the presence of internal strains, the magnitude of which is dependent on the coolingrate, R, the nature of the substrate and the film thickness. The temperature was carefully regulated to ±10°Cby d.c. resistive heating of the substrate, and the slow cooling-rate, R 10—2 °C/sec(slow-cooling), was adjusted by a step-by-step reduction of the heating current. Photoelectron energy distribution secondderivative spectra were recorded using a conventional retarding-field analyser and voltage modulation.8 In Fig. 1(a), second derivative photoemission spectra from films deposited at 20°C 2 ~ 200°C
by a large peak, centered about & = 7.1 eV, which varies linearly with photon energy, indicative of a valence-band density-of-states maximum located approximately 0.6—0.7 eV below the valence-band edge. A similar feature (peak B) is observed in the crystalline spectrum, the energy of which, &, remains relatively insensitive to the disordered-crystalline transition at this photon energy. Electron scattering within the films is responsible for the weak shoulder at 5.2—5.3 eV, near the photoelectric threshold. For films deposited at 140°C< T~~ 200°C,the crystalline feature at ~ = 5.7 eV (peak A) gradually manifests itself with increasing i, until at i 180°C, 9 Together the spectrum of the films begins to resemble with that peak of thethe A, crystal weakatfeature this photon centered energy. about & = 6.0—6.2 eV,1° whihc is most evident in films deposited at T~ 20°C, Fig. 1(a), remains constant with increasing photon energy, indicating emission from conduction-band density-of-states features located at 5.7 eV (peak A) and 6.1 ±0.2 eV above the valence-band edge. This corresponds to similar structure previously reported in the secondary-electron-emission spectrum, obtained from fast-cooled films.3 The crystalline features observed in the spectra of the slow-cooled films, Fig. 1(a), are indicative of some degree of microcrystalline ordering, which is absent in the spectra of films prepared at 7 = 120°C. Annealing the latter up to TA 220°C,the spectra, Fig. I (b), remain featureless with no evidence of the ‘crystalline’ feature at ?~= 5.7 eV (peak A) until the annealing temperature exceeds 220°C(TA > 220°C). Contrary to the slow evolution of this peak observed during the annealing of fast-cooled films deposited at temperatures in the range 140< 7 200°C,3a transition is observed at TA 220—240°C,indicative of an abrupt disordered-crystalline phase transition.
Vol. 12, No. 10
AMORPHOUS—CRYSTALLINE TRANSITION IN Ge FILMS
Peak A increases in intensity with increasing annealing temperature, the spectrum approaching that of the crystal at TA 300°C.Identical behaviour has been observed during the annealing of films deposited at electron second-derivative spectra have also been re~ 100°Cand slow-cooled to 20°C.Similar photoported for amorphous Si films prepared by vapordeposition.1’
1009
A
~ ~
3OCfC
Similar spectra from films deposited at 140°C< Tturning-off the substrate heating current immediately 8 200°Cand fast-cooled (0.3°C/sec)by simply after deposition, show time-dependent relaxation of 5 While the detailed nature thethis above spectralbehaviour features. is not known, studies on of relaxation films deposited on both silica’0 and vitreous carbon substrates strongly indicate that it is a consequence of the increased stress induced in thin films by the mismatch between the film and substrate atomic networks during the initial stages of deposition and growth. Such stress can reach i09 dyn/cm2 in crystalline film nucleation and growth.’3 Strain effects initiated at the substrate—film interface affect the intrinsic microstructural ordering processes which are observed to occur in films deposited in the range, 140 < 7 <200°C. Fast cooling rates after deposition increase the stress in these films. The resulting internal strains in fastcooled films will have the effect of superimposing additional quantitative disorder14 (due to the distortion of bond lengths and angles from the ideal values of perfect tetrahedral coordination), on essentially topologically disordered15 strain-free films, prepared by slow cooling. This tends to ‘smear-out’ the weak crystalline density-of-states features, which were seen in slow-cooled films deposited at 2~>140°C. Subsequent annealing above 200°C(TA 200°C) of these fast-cooled films deposited at 140°< ~ 200°Cis shown with reference to the spectra from a film deposited at 1 = 180°C,Fig. 2. After relaxation, a spectrum devoid of the crystalline structure at & = 5.7 eV (peak A) is obtained, which upon annealing (TA > 200°C)progressively increases in intensity to resemble that of the crystalline spectrum at TA > 300°C.Simultaneously, additional spectral structure appears at & = 7.8—8.4 eV, which is associated with emission from density-of-states features located in the ‘pseudogap’. The distribution of these localised states varies with increasing annealing temperature in a complex manner for TA >250°C,as shown in Fig. 2. For TA > 450°C,emission from these states is so weak
T
5
6
7 E~(eV)
8
FIG. 2. Effect of annealing temperature, 200< TA < 450°C on strained, fast-cooled films deposited at 180°C.With increasing annealing temperature TA 200°C,the crystalline feature (peak A) at ~ = 5.7 eV increases in intensity and, simultaneously, structure appears at ~ = 7.8—8.4 eV, corresponding to emission from localised states located above the valence-band edge, in the region of the crystalline forbidden band-gap.
that they are no longer resolved, the spectrum closely resembling that of the crystal, Fig. 1(a). These states have also been observed as a weak tailing in the energy distribution curves.16 The evolution of these states in the ‘pseudogap’ of the fast-cooled films appears to be associated with the effect of high stress levels during the growth of local microcrystalline regions3 since similar effects are not observed during the annealing of slow-cooled films, Fig. 1(b). It is probable, though by no means certain, that such states arise due to the presence of broken or ‘dangling’ bonds and other defects, which arise due to bond distortion and subsequent reordering in the strained network. A similar distribution of localised states has been deduced from field effect measurements on amorphous Si films.’7 Furthermore, the present measurements indicate that the Fermi level in fast-cooled films is ill-defined and subject to subsequent annealing treatment. The activation energies for the microstructural reordering processes have been determined to be of the order of 0.02—0.2 eV, in close agreement with the values of contributions to the heat of recrystallization of amorphous Ge films determined from specific heat
1010
AMORPHOUS—CRYSTALLINE TRANSITION IN Ge FILMS
studies18 and theoretical estimates.19 Both electroand vapor-deposited amorphous Ge films have been reported’8 to undergo a slow exothermic reaction during annealing at TA = 200°C,although no crystallization was apparent from X-ray or electron diffraction examination until the annealing temperature exceeded 250°C 18 or thewould substrate deposition 350°C.~ This suggest that thetemperature observed microstructural ordering occurs over a range of less than 15—20 A. These observations, together with those of pre3 underline vious secondary-electron-emission studies the sensitivity of emission from electron states to the degree of short-range microstructural ordering in amorphous Ge films. Weak second derivative spectral features, characteristic of the crystal, first appear in films deposited at substrate temperature, 7 > 140°C, indicative of some degree of short-range ordering extending over second- and third-nearest neighbour distances only. This ordering develops in magnitude with increasing substrate deposition temperature and subsequent annealing, TA > 200°C.3High stress, induced in the deposited thin films by fast cooling,
1.
Vol. 12, No. 10
produces strain effects which tend to smear-out any weak density-of-states features due to such small-scale ordering. The presence of such strains can also give rise to a high density of localised states in the pseudogap upon subsequent annealing. Films deposited at too low a substrate temperature, T 8 < 100°C,are known to 7 The present contain microvoids and other defects. observations suggest that high-density amorphous Gefilms, which closely approach the ideal random network-model,4 may be prepared only by slow-cooling films deposited at 100°< T 8 < 140°C.This is mdicated by the behaviour of a filmannealed, deposited at 1 = 120°C,Fig. 1(a), andslow-cooled subsequently 150< TA <350°C,Fig. 1(b), which undergoes an abrupt disordered-microcrystalline phase transition during annealing at TA 230°C.Photoemission from such films indicate a featureless conduction-band density-of-states in agreement with previous optical work.1 Acknowledgements The authors thank Prof. H. Fritzsche for his valuable comment regarding reference 6 and M.R. Barnes for his enthusiastic and very competent technical assistance. —
REFERENCES DONOVAN T.M., SPICER W.E., BENNETT J.M. and ASHLEY E.J.,Phys. Rev. B2, 397 (1970); SPICER W.E. and DONOVAN T.M., J. Non-Cryst. Solids 2, 66 (1970).
2.
BRODSKY M.H., TITLE R.S., WEISER K. and PETTIT G.D.,Phys. Rev. Bi, 2632 (1970).
3. 4.
WILUS R.F., LAUDE L.D. and FITTON B.,Phys. Rev. Len’. 29, 220(1972). POLK D.E.,J. Non-Cryst. Solids 5, 365 (1971).
5.
LAUDE L.D., WILLIS R.F. and FITTON B., to appear in Proc. XI tnt. Conf on the Physics ofSemiconductors, Warsaw, (1972).
6.
This does not necessarily imply that there is an absence of states tailing into the gap. Based on the assumption (a) the excitation probability and (b) the escape probability are the same for both extended and localised states. Pierce and Spicer” arrive at a figure of 3 X 3 as a (possible) lower limit on the observation of localised states, which can be resolved 10i9 by states vacuum eV~ photoemission. cm MOSS S.C. and GRACZYK J.F.,Phys. Rev. Lett. 23, 1167 (1969);GALEENER E.J.,Phys. Rev. Lett.27, 421 (1971) and 27, 1716 (1971); DONOVAN T.M. and I-IEINEMANN K.,Phys. Rev. Lett. 27, 1794 (1971). BARNES M.R. and LAUDE L.D.,Rev. Sci. Instr. 42, 1191 (1971). In the crystal, the evolution of peak A (Fig. I) with varying photon energy is characteristics of direct transitions about the L symmetry direction. (KRAMER B., Phys. Status Solidi (b) 47, 501(1971); and LAUDE L.D., to be published). Films deposited at 20°Con fused silica substrates show a more intense structure at & = 6.0—6.2eV, indicative of a higher degree of micro-crystalline ordering imposed on these films by the substrate compared with those obtained with vitreous carbon substrates. This underlines the importance of the choice of the substrate in limiting any orienting effects on subsequent film growth.
7. 8. 9.
10.
Vol. 12, No. 10
AMORPHOUS—CRYSTALUNE TRANSITION IN Ge FILMS
1011
11.
PIERCE D.T. and SPICER W.E., Phys. Rev. B5, 3017 (1972).
12.
CHOPRA K.L., Thin Film Phenomena, p. 277, McGraw-Hill, New York (1969).
13.
PALATNIK L.S., SAVITSKII B.A., USENKO N.YU. and FEDOVENKO A.I., Soviet Phys. Solid State Phys. 13, 3033 (1972); BEHRNDT K.H.,J. appi. Phys. 37, 3841 (1966).
14.
ECONOMOU E.N. and COHEN M.H.,Phys. Rev. Lett. 25, 1445 (1970).
15.
WEAIRE D. and THORPE M.F., Phys. Rev. B4, 2508 (1971).
16. 17.
LAUDE L.D.,WILLIS R.F. and FITTON B., to be published. SPEAR W.E. and LE COMBER P.G.,J. Non-Cryst. Solids 8—10,727(1970).
18.
CHEN H.S. and TURNBULLD.,J. app!. Phys. 40,4214(1969).
19.
GRIGOROVICI R. and MANAILA R.,Nature 226, 143 (1970).
20.
THEYE M.L., Opt. Commun. 2, 329 (1970).
Les spectres deuxiéme-dérivée de distributions énergétiques de photo-diectrons font apparaitre des variations dans Ia densité-d’état de films de Ge amorphes, qui sont associées a l’ordre dans ces films. Les films amorphes obtenus a une temperature de deposition 7’~= 120 ±20°Ccristallisent abruptement aprés recuit a TA 230 ± 10°C.Ces films pourraient avoir une structure trés proche de Ia configuration aléatoire du modéle de Polk. Des densitCs elevées d’états localisCs dans le pseudogap sont observCes dans les films ‘imparfaits’.
Vol. 12, pp.1007—lOll, 1973.
Pergamon Press.
Printed in Great Britain
AMORPHOUS-CRYSTALLINE TRANSITION IN Ge FiLMS BY PHOTOEMISSION L.D. Laude, R.F. Willis and B. Fitton Surface Physics Division, European Space Research Organisation, Noordwijk, Holland (Received 5 January 1973; in revised form 16 February 1973 by M. Balkanski)
Photoelectron second-derivative, energy distribution spectra reveal subtle changes in the density-of-states of vapour-deposited, amorphous Ge films, arising from microstructural ordering processes. Evidence is presented for ‘ideal’ amorphous Ge films, prepared at a deposition temperature T8 = 120 ±20°C,which undergo an abrupt amorphous—crystalline transition upon annealing to TA 230 ±10°C.These films are thought to approach closely the random-network Polk-model. High densities of localized states in the pseudogap are observed in imperfect films. 1 ,2 have emphasized RECENT experimental studies that the disordered—microcrystalline phase transition in Ge and Si films is extremely sensitive to sample preparation technique. Secondary electron emission spectroscopy of vapor-deposited, ‘amorphous’ Ge films has been shown to relate changes in the energy distribution and density of conduction electron states to thermally-activated, microstructural-ordering processes.3 The question remains, however, as to what density-of-states features can be associated with an ideal, ropologically-disordered network, such as that suggested by Polk,4 and how close such a structure can be approached experimentally? Photoelectron energy distribution second-derivative spectra reveal intensity variations associated with density-of-states features, which change as a consequence of thermally-activated, short-range ordering, In order to try to clarify the nature of the amorphous state and the amorphous—crystalline transition in Ge films, we have studied the changes in intensity of photoemission spectral features as a function of the substrate deposition temperature, 1, cooling-rate to 20°C,R, and annealing temperature, TA. These parameters are particularly important for defining the degree of short-range order and homogeneity in vapordeposited Ge films.’~3~5 It is shown that films with a structure, which we believe to closely approach the ‘ideal’ random-network,4 may be prepared by 1007
deposition on vitreous carbon substrates at temperatures in the range, 100< 7 <140°C,followed by controlled slow-cooling (R 10-2 °C/sec)to 20°C. Upon subsequent annealing, these films exhibit an abrupt transition to a microcrystalline structure at a temperature, TA 230 ±10 C. Photoemission from localised stares located in the pseudogap was not observed in these ‘ideal’ films.6 A complex distribution of states tailing into the pseudogap was observed, however, during annealing of high density films deposited at higher substrate temperatures, 140 < 7 200°C,which had been previously fastcooled (R 0.3 C/sec) to 20 C. The results emphasize that the detailed microstructure of high-density, vapor-deposited Ge films is extremely sensitive to preparation technique, and photoelectron energy distribution second-derivative spectra provide a means for monitoring the degree of microstructural-ordering present in these ‘ideal’ films. Films of thickness l000A were deposited from a collimated Ge-crystal source (p-type, 50&2-cm), at rates of~20A/sec onto polished conducting vitreous carbon surfaces. The substrate-to-source distance was 20 cm and the vacuum better than 2 X iO”~Torr. Under such conditions, films with densities close to that of the crystal,1 free of microvoids7 and possessing sharp optical-absorption edges1 are obtained, providing the preparation parameters, 1, R, and TA are
1008
AMORPHOUS—CRYSTALLINE TRANSITION IN Ge FILMS •
I
•
[‘\EDC /
~
A
A valence “,b~d
~Il
B?
EirJ.,\
b
‘ii
/
band dge
~j
..~-
1p~!~9”’\~j.
-
Vol. 12, No. 10
are compared with the spectrum from a vacuum cleaved Ge (111) crystal at a photon energy, hi.’ = 7.72 eV. For 7> 20°C,the films were maintained at the deposition-substrate temperature for one hour before slow cooling (10-2 °C/sec)to 20°C.The abscissa scale refers to the energy of the emitted electrons relative to the valence-band edge. The photoelectric threshold was measured to be 4.8 and 4.7 eV With increasing 7 changes are seen to occur in the spectra of the deposited films, which gradually approach in the crystal and amorphous films, respectively. the crystalline spectrum at T~Ge200°C.For films deposited at i 20°C,the spectrum is dominated ,
\
~
\vi • 5
• 6
• 7
• 8
\/
“
r, ~
I
I
5
6
v I
7
8
FIG. 1. (a) Second derivative spectra of photoelectron energy distribution curves (EDC’s) obtained at hi.’ = 7.72 eV from ‘strain-free’ amorphous Ge films, prepared by deposition at substrate deposition temperatures, 20°C i 200°C,and slowly cooled to 20°C at a rate of 10-2 °C/sec.Such a spectrum and its corresponding EDC (dotted line), obtained at the same photon energy from a vacuum cleaved Ge (Ill) crystal, are shown for comparison. The abscissa scale refers to the to valence-band the energy edge. of the(b) emitted Effectelectrons of annealing relative temperature, 150 350°C,on slow-cooled films deposited at 7 = 120 C. The sudden appearance of the crystalline feature (peak A) with subsequent annealing at 240°Cis indicative of a sharp disorderedcrystalline phase transition. The spectra are displaced vertically for clarity, carefully controlled.3 There is evidence5 which suggests that the degree of short-range order present in these films is sensitive to the presence of internal strains, the magnitude of which is dependent on the coolingrate, R, the nature of the substrate and the film thickness. The temperature was carefully regulated to ±10°Cby d.c. resistive heating of the substrate, and the slow cooling-rate, R 10—2 °C/sec(slow-cooling), was adjusted by a step-by-step reduction of the heating current. Photoelectron energy distribution secondderivative spectra were recorded using a conventional retarding-field analyser and voltage modulation.8 In Fig. 1(a), second derivative photoemission spectra from films deposited at 20°C 2 ~ 200°C
by a large peak, centered about & = 7.1 eV, which varies linearly with photon energy, indicative of a valence-band density-of-states maximum located approximately 0.6—0.7 eV below the valence-band edge. A similar feature (peak B) is observed in the crystalline spectrum, the energy of which, &, remains relatively insensitive to the disordered-crystalline transition at this photon energy. Electron scattering within the films is responsible for the weak shoulder at 5.2—5.3 eV, near the photoelectric threshold. For films deposited at 140°C< T~~ 200°C,the crystalline feature at ~ = 5.7 eV (peak A) gradually manifests itself with increasing i, until at i 180°C, 9 Together the spectrum of the films begins to resemble with that peak of thethe A, crystal weakatfeature this photon centered energy. about & = 6.0—6.2 eV,1° whihc is most evident in films deposited at T~ 20°C, Fig. 1(a), remains constant with increasing photon energy, indicating emission from conduction-band density-of-states features located at 5.7 eV (peak A) and 6.1 ±0.2 eV above the valence-band edge. This corresponds to similar structure previously reported in the secondary-electron-emission spectrum, obtained from fast-cooled films.3 The crystalline features observed in the spectra of the slow-cooled films, Fig. 1(a), are indicative of some degree of microcrystalline ordering, which is absent in the spectra of films prepared at 7 = 120°C. Annealing the latter up to TA 220°C,the spectra, Fig. I (b), remain featureless with no evidence of the ‘crystalline’ feature at ?~= 5.7 eV (peak A) until the annealing temperature exceeds 220°C(TA > 220°C). Contrary to the slow evolution of this peak observed during the annealing of fast-cooled films deposited at temperatures in the range 140< 7 200°C,3a transition is observed at TA 220—240°C,indicative of an abrupt disordered-crystalline phase transition.
Vol. 12, No. 10
AMORPHOUS—CRYSTALLINE TRANSITION IN Ge FILMS
Peak A increases in intensity with increasing annealing temperature, the spectrum approaching that of the crystal at TA 300°C.Identical behaviour has been observed during the annealing of films deposited at electron second-derivative spectra have also been re~ 100°Cand slow-cooled to 20°C.Similar photoported for amorphous Si films prepared by vapordeposition.1’
1009
A
~ ~
3OCfC
Similar spectra from films deposited at 140°C< Tturning-off the substrate heating current immediately 8 200°Cand fast-cooled (0.3°C/sec)by simply after deposition, show time-dependent relaxation of 5 While the detailed nature thethis above spectralbehaviour features. is not known, studies on of relaxation films deposited on both silica’0 and vitreous carbon substrates strongly indicate that it is a consequence of the increased stress induced in thin films by the mismatch between the film and substrate atomic networks during the initial stages of deposition and growth. Such stress can reach i09 dyn/cm2 in crystalline film nucleation and growth.’3 Strain effects initiated at the substrate—film interface affect the intrinsic microstructural ordering processes which are observed to occur in films deposited in the range, 140 < 7 <200°C. Fast cooling rates after deposition increase the stress in these films. The resulting internal strains in fastcooled films will have the effect of superimposing additional quantitative disorder14 (due to the distortion of bond lengths and angles from the ideal values of perfect tetrahedral coordination), on essentially topologically disordered15 strain-free films, prepared by slow cooling. This tends to ‘smear-out’ the weak crystalline density-of-states features, which were seen in slow-cooled films deposited at 2~>140°C. Subsequent annealing above 200°C(TA 200°C) of these fast-cooled films deposited at 140°< ~ 200°Cis shown with reference to the spectra from a film deposited at 1 = 180°C,Fig. 2. After relaxation, a spectrum devoid of the crystalline structure at & = 5.7 eV (peak A) is obtained, which upon annealing (TA > 200°C)progressively increases in intensity to resemble that of the crystalline spectrum at TA > 300°C.Simultaneously, additional spectral structure appears at & = 7.8—8.4 eV, which is associated with emission from density-of-states features located in the ‘pseudogap’. The distribution of these localised states varies with increasing annealing temperature in a complex manner for TA >250°C,as shown in Fig. 2. For TA > 450°C,emission from these states is so weak
T
5
6
7 E~(eV)
8
FIG. 2. Effect of annealing temperature, 200< TA < 450°C on strained, fast-cooled films deposited at 180°C.With increasing annealing temperature TA 200°C,the crystalline feature (peak A) at ~ = 5.7 eV increases in intensity and, simultaneously, structure appears at ~ = 7.8—8.4 eV, corresponding to emission from localised states located above the valence-band edge, in the region of the crystalline forbidden band-gap.
that they are no longer resolved, the spectrum closely resembling that of the crystal, Fig. 1(a). These states have also been observed as a weak tailing in the energy distribution curves.16 The evolution of these states in the ‘pseudogap’ of the fast-cooled films appears to be associated with the effect of high stress levels during the growth of local microcrystalline regions3 since similar effects are not observed during the annealing of slow-cooled films, Fig. 1(b). It is probable, though by no means certain, that such states arise due to the presence of broken or ‘dangling’ bonds and other defects, which arise due to bond distortion and subsequent reordering in the strained network. A similar distribution of localised states has been deduced from field effect measurements on amorphous Si films.’7 Furthermore, the present measurements indicate that the Fermi level in fast-cooled films is ill-defined and subject to subsequent annealing treatment. The activation energies for the microstructural reordering processes have been determined to be of the order of 0.02—0.2 eV, in close agreement with the values of contributions to the heat of recrystallization of amorphous Ge films determined from specific heat
1010
AMORPHOUS—CRYSTALLINE TRANSITION IN Ge FILMS
studies18 and theoretical estimates.19 Both electroand vapor-deposited amorphous Ge films have been reported’8 to undergo a slow exothermic reaction during annealing at TA = 200°C,although no crystallization was apparent from X-ray or electron diffraction examination until the annealing temperature exceeded 250°C 18 or thewould substrate deposition 350°C.~ This suggest that thetemperature observed microstructural ordering occurs over a range of less than 15—20 A. These observations, together with those of pre3 underline vious secondary-electron-emission studies the sensitivity of emission from electron states to the degree of short-range microstructural ordering in amorphous Ge films. Weak second derivative spectral features, characteristic of the crystal, first appear in films deposited at substrate temperature, 7 > 140°C, indicative of some degree of short-range ordering extending over second- and third-nearest neighbour distances only. This ordering develops in magnitude with increasing substrate deposition temperature and subsequent annealing, TA > 200°C.3High stress, induced in the deposited thin films by fast cooling,
1.
Vol. 12, No. 10
produces strain effects which tend to smear-out any weak density-of-states features due to such small-scale ordering. The presence of such strains can also give rise to a high density of localised states in the pseudogap upon subsequent annealing. Films deposited at too low a substrate temperature, T 8 < 100°C,are known to 7 The present contain microvoids and other defects. observations suggest that high-density amorphous Gefilms, which closely approach the ideal random network-model,4 may be prepared only by slow-cooling films deposited at 100°< T 8 < 140°C.This is mdicated by the behaviour of a filmannealed, deposited at 1 = 120°C,Fig. 1(a), andslow-cooled subsequently 150< TA <350°C,Fig. 1(b), which undergoes an abrupt disordered-microcrystalline phase transition during annealing at TA 230°C.Photoemission from such films indicate a featureless conduction-band density-of-states in agreement with previous optical work.1 Acknowledgements The authors thank Prof. H. Fritzsche for his valuable comment regarding reference 6 and M.R. Barnes for his enthusiastic and very competent technical assistance. —
REFERENCES DONOVAN T.M., SPICER W.E., BENNETT J.M. and ASHLEY E.J.,Phys. Rev. B2, 397 (1970); SPICER W.E. and DONOVAN T.M., J. Non-Cryst. Solids 2, 66 (1970).
2.
BRODSKY M.H., TITLE R.S., WEISER K. and PETTIT G.D.,Phys. Rev. Bi, 2632 (1970).
3. 4.
WILUS R.F., LAUDE L.D. and FITTON B.,Phys. Rev. Len’. 29, 220(1972). POLK D.E.,J. Non-Cryst. Solids 5, 365 (1971).
5.
LAUDE L.D., WILLIS R.F. and FITTON B., to appear in Proc. XI tnt. Conf on the Physics ofSemiconductors, Warsaw, (1972).
6.
This does not necessarily imply that there is an absence of states tailing into the gap. Based on the assumption (a) the excitation probability and (b) the escape probability are the same for both extended and localised states. Pierce and Spicer” arrive at a figure of 3 X 3 as a (possible) lower limit on the observation of localised states, which can be resolved 10i9 by states vacuum eV~ photoemission. cm MOSS S.C. and GRACZYK J.F.,Phys. Rev. Lett. 23, 1167 (1969);GALEENER E.J.,Phys. Rev. Lett.27, 421 (1971) and 27, 1716 (1971); DONOVAN T.M. and I-IEINEMANN K.,Phys. Rev. Lett. 27, 1794 (1971). BARNES M.R. and LAUDE L.D.,Rev. Sci. Instr. 42, 1191 (1971). In the crystal, the evolution of peak A (Fig. I) with varying photon energy is characteristics of direct transitions about the L symmetry direction. (KRAMER B., Phys. Status Solidi (b) 47, 501(1971); and LAUDE L.D., to be published). Films deposited at 20°Con fused silica substrates show a more intense structure at & = 6.0—6.2eV, indicative of a higher degree of micro-crystalline ordering imposed on these films by the substrate compared with those obtained with vitreous carbon substrates. This underlines the importance of the choice of the substrate in limiting any orienting effects on subsequent film growth.
7. 8. 9.
10.
Vol. 12, No. 10
AMORPHOUS—CRYSTALUNE TRANSITION IN Ge FILMS
1011
11.
PIERCE D.T. and SPICER W.E., Phys. Rev. B5, 3017 (1972).
12.
CHOPRA K.L., Thin Film Phenomena, p. 277, McGraw-Hill, New York (1969).
13.
PALATNIK L.S., SAVITSKII B.A., USENKO N.YU. and FEDOVENKO A.I., Soviet Phys. Solid State Phys. 13, 3033 (1972); BEHRNDT K.H.,J. appi. Phys. 37, 3841 (1966).
14.
ECONOMOU E.N. and COHEN M.H.,Phys. Rev. Lett. 25, 1445 (1970).
15.
WEAIRE D. and THORPE M.F., Phys. Rev. B4, 2508 (1971).
16. 17.
LAUDE L.D.,WILLIS R.F. and FITTON B., to be published. SPEAR W.E. and LE COMBER P.G.,J. Non-Cryst. Solids 8—10,727(1970).
18.
CHEN H.S. and TURNBULLD.,J. app!. Phys. 40,4214(1969).
19.
GRIGOROVICI R. and MANAILA R.,Nature 226, 143 (1970).
20.
THEYE M.L., Opt. Commun. 2, 329 (1970).
Les spectres deuxiéme-dérivée de distributions énergétiques de photo-diectrons font apparaitre des variations dans Ia densité-d’état de films de Ge amorphes, qui sont associées a l’ordre dans ces films. Les films amorphes obtenus a une temperature de deposition 7’~= 120 ±20°Ccristallisent abruptement aprés recuit a TA 230 ± 10°C.Ces films pourraient avoir une structure trés proche de Ia configuration aléatoire du modéle de Polk. Des densitCs elevées d’états localisCs dans le pseudogap sont observCes dans les films ‘imparfaits’.