T. W. HALL et al.: Photoelectric Properties of Films of Sb

phys. stat. sol. (a) 2, 327 (1970)

Subject classification: 1.4 and 17; 1.2; 2; 21.7

Department of Pure and Applied Physics, The Queen’s University of Belfast

327

Photoelectric Properties of Amorphous and Crystalline Films of Antimony

BY T. W. HALL, R. M. EASTMENT, and C. H. B. MEE

Thin films of antimony occur in amorphous and crystalline forms. For the thinnest films the amorphous phase is stable; a t a critical thickness (corresponding to an optical transmission of about 30%) the film crystallizes spontaneously. The phase change has been detected by studying the variation of photoelectric emission with thickness for anti- mony films evaporated a t pressures less than 5 x Torr. At the transition there is an abrupt increase of about an order of magnitude in the photoelectric yield. A sudden decrease in the electrical resistance of the film, previously taken as an indication of the phase transition, occurs at a rather greater film thickness. It is suggested that the thinnest films are made up of islands of amorphous antimony; a t the critical thickness crystallization occurs but the island structure is still retained. The decrease in resistance a t greater thick- nesses is probably due to coalescence of the islands. The photoelectric threshold of crystal- line films of antimony thicker than about 15 nm is constant a t (4.52 * 0.01) eV. In the crystalline region the results are a good fit to the Bowler analysis. In the amorphous region the yield Y may be related to the photon energy h v by the empirical relation Y N - (h v - h which has been found to apply to a number of semiconductors. Amorphous films have a threshold in the region of 4.7 eV.

Diinne Antimonschichten existieren in amorpher und l~istalliner Form. Fur die dunn- sten Schichten ist die amorphe Phase stabil; bei einer kritischen Dicke (der optischen Durch- lassigkeit von etwa 30% entsprechend) kristallisiert die Schicht spontan. Die Phasen- anderung wurde bei der Untersuchung der Bnderungen der lichtelektrischen Emission mit der Dicke von Antimonschichten gefunden, die bei Driicken kleiner als 5 x 10-8 Torr auf- gedampft wurden. Am tfbergang tritt ein abrupter Anstieg der photoelektrischen Ausbeute um eine GroBenordnung auf. Ein plotzlicher Abfall des elektrischen Widerstands der Schicht, der friiher als Anzeige des Phasenubergangs angesehen wurde, tritt erst bei groBe- ren Schichtdicken auf. Es wird angenommen, daB die diinnsten Schichten aus Inseln amorphen Antimons bestehen; bei einer kritischen Dicke tritt Kristallisation auf, jedoch bleibt die Inselstruktur noch erhalten. Der Abfall des Widerstands bei groBeren Schicht- dicken wird wahrscheinlich durch die Koaleszenz der Inseln hervorgerufen. Die photo- elektrische Schwelle der kristallinen Antimonschichten, die dicker als 15 nm sind, ist kon- stant bei (4,52 f 0,01) eV. Im kristallinen Bereich lassen sich die Resultate gut an die Bowler-Analyse anpassen. Im amorphen Bereich kann die Ausbeute Y mit der Photonen- energie h Y durch die empirische Beziehung Y - (h v - h yo)* verkniipft werden, die auf eine Anzahl von Halbleitern angewendet werden kann. Amorphe Schichten besitzen eine Schwelle im Bereich von 4,7 eV.

1. Introduction It has been known for many years that thin evaporated films of antimony

can exist in amorphous and crystalline forms [l]. A very thin (of the order of 5nm) film deposited on a substrate a t room temperature gives an electron diffraction pattern of diffuse rings characteristic of an amorphous sample [2, 31.

328 T. W. HALL, R. M. EASTMENT, and C. H. B. MEE

The amorphous antimony film may crystallize spontaneously a t room temper- ature, or the crystallization process may be induced by an increase in the tem- perature or the thickness of the film. The crystallization process has been followed using optical microscopy [4 to 61: for films evaporated under rather poor vacuum conditions (pressures greater than about Torr) the crystalline phase appears to grow in the form of circular areas around nuclei. During crystal- lization the electrical resistance of the film falls, and when the boundaries of the crystalline areas approach one another there is an abrupt decrease in resistance, perhaps over several orders of magnitude, which has been taken as marking the phase transition. The rate a t which Crystallization procceds has been found to be a function of the cleanliness of thc substrate and the pres- sure at which the antimony is evaporated - in short, the process is typically one in which impurities act as nucleation centres for crystallization. Rome 171 and Condas IS] have shown that when the substrate is carefully cleaned and the evaporation pressure is less than about 5 x loT5 Torr the phase transition is characterized by a critical thickness of the film, the boundary between phases following the contour of critical thickness rather than spreading outwards spon- taneously from crystallization nuclei. The transition appears as a discontinuity in the gradient of the graph of the optical transmission of the film versus its thickness. Most resistivity determinations, and the transmission results of Rome [7], indicate a transition thickness of between 14 and 25 nm, but Condas IS], who took particular precautions to obtain films of uniform thickness by evaporation from ring sources [9], suggests that the critical value may be as low as between 9 and 12 nm. The transition occurs a t a transmission to white light of about 30%. Thick films produced by slow evaporation under only moderate vacuum conditions may be heterogeneous, containing islands of the amorphous phase separated from crystalline regions by an oxide film [lo]. As might be expected, evaporation under ultra-high vacuum conditions produces homo- geneous final films, even for very slow evaporation conditions.

Early photoelectric measurements on antimony by Schulze [11] and Middel 1121 were carried out in vacuum conditions which make the statc of cleanliness of their films very doubtful, and little reliance can be placed on the values of 5.1 and 4.0eV obtained for the threshold photon energy. Apker, Taft, and Dickey [13] investigated the energy distributions of photoelectrons from films the thickness of which was not determined, but which were established as crystallinc by electron diffraction. The evaporation pressure was not stated, but the pressure in the tube after seal-off was less than 5 x Torr. It was found that the photoelectric yields were a good fit to Fowler’s function [14], giving a threshold of 4.56 eV. The depth of the Fermi level below vacuum potential was deduced from the retarding-potential characteristic from a knowledge of the cut-off voltage and the incident photon energy. The value obtained was 4.60 eV, in agreement with the photoelectric threshold. In a later investigation, Taft and Apker [ 151 measured the photoelectron energy distri- butions from amorphous antimony on nickel and germanium substrates. I n this case the depth of the Fermi level below vacuum potential was found to be 4.49 eV, although the photoelectric threshold appeared to be rather higher than that measured for the crystalline phase. Taft and Apker conclude that the amorphous form of antimony behaves as a semiconductor whose Fermi level is about 4.5 eV below vacuum potential, the highest occupied electron energy state being ahout 0.1 eV below the Fermi level.

Photoelectric Properties of Amorphous and Crystalline Films of Sb 329

We report here a detailed investigation of photoelectric emission from anti- mony films in an attempt to detect the effect of the phase transition.

2. Experimental Details Films of antimony were evaporated from an outgassed tungsten boat on to

quartz substrates which were cleaned in acetone, distilled water, and ethanol. The substrates bore painted platinum electrodes a t each side so that the resis- tance of the antimony film could be monitored. Experiments were carried out in a mercury-pumped glass system and in an ion-pumped stainless steel vacuum system. The pressure during antimony evaporation was always less than 5 x x Tom. In each system the antimony film was illuminated through a quartz window ; monochromatic illumination was provided using a mercury arc and quartz prism monochromator. Readings of the photocurrent were taken a t a series of wavelengths as the antimony evaporation proceeded in stages. The transmission of the antimony film was determined for each eva- poration to indicate the thickness of the film.

3. Results Fig. 1 shows the results of an experiment in which the photoelectric emission

from, and the resistance of, an antimony film were determined at various stages of evaporation. The photocurrent remained approximately constant until the transmission of the film had dropped to about 30%. The next stage of evaporation, which produced a reduction in transmission of only 6%, caused a remarkable change in the photoelectric emission, the current increasing by

Fig. 1. The variation of the photoelectric ~ n - i a c i r m fram a n r l tho olrrtrirrl vaciat-nno

0 0 0

B 248nin

~.y.IIIIuy ..., 1.1- "-I" ~IyIy~I"yI.yyII".".ly" I I / I /

40 20 IU 6 of, an antimony film with its thickness L (as measured by its transmission of light 8@ 60

I I ' I

of wavelength 546 nm) --Tro/isinission fo greeii light (%)

330 T. W. HALL, R. M. EASTMENT, and C. H. B, MEE

Fig. 2. Fowler plots for photoelectric emis- sion from antimony films of transmission (a) 33%, (b) 11%. The values of the photo- electric threshold obtained from the plots are: (a) (4.75 f 0.03) e V , (h) (4.518 0,010)

eV

I I

780 190 200 210 hu/ki -

more than an order of magnitude for illumination by light of wavelength 248 nm. This discontinuity in the curve of photocurrent versus transmission is almost certainly associated with the amorphous-crystalline transition, which accord- ing to both Rome [7] and Condas [8] occurs a t a transmission of about 30% to white light. This sudden change in the photoelectric properties of the film is evidently one which involves a reduction in the threshold photon energy. Further evaporation causes a steady increase in the photocurrent. It is inter- esting to note that the resistance between the electrodes on the substrate remained approximately constant a t about 8 x 10l1 the surface resistance of the clean substrate, until a transmission of about 20% was reached. A very abrupt fall in resistance, through six orders of magnitude, occurred a t a significantly smaller transmission than the discontinuity in photoemission. The alternative graduation of the horizontal axis of Fig. 1 in terms of surface density is based on the results of Condas [8]. If thin antimony films have the same density as t,hat of bulk samples (6.7 g ~ r n - ~ ) , the transition occurs a t a film thickness of about 12 nm.

The photocurrent results were converted to values of photoelectrons per incident photon through a knowledge of the intensity of the light (obtained using a radiation thermopile). The yield results Y were then plotted as lg ( Y / T 2 ) vs. h v / k T . Fig. 2 shows the curves for films of (a) 33% and (b) 11% trans- mission. The solid lines are Fowler plots which were fitted to the experimental points by a least-squares analysis [16]. It is obvious that the points for the 33% transmission film are but a poor fit t o the Fowler plot, and very little significance can be attached t.0 the threshold value obtained ((4.75 &- 0.03) eV). On the other hand, the points for the 11% transmission film are a good fit to the t(heoretica1 curve and yield a photoelectric threshold of (4.518 f 0.010) eV.

Fig. 3 shows the variation of the threshold and the Fowler constant B for films of transmission less than 40%. Values of the threshold for transmissions greater than 40% were above 4.7 eV and had a large standard deviation (about 0.03 eV). A t the phase transition there is an abrupt reduction in the threshold and increase in the Fowler constant; the threshold then decreases to a steady value of about, 4.52 eV for films of transmission less than 15% (corresponding

Photoelectric Properties of Amorphous and Crystalline Films of Sb 331

I I horphou~- - Cryss/o//ine -____

Fig. 3. Variation of the photoelectric threshold and Fowler constant with thickness of an antimony film in the neighbourhood of the amorphous-crystalline

transition

to thicknesses greater than about 17 nm) . Similar behaviour was observ - ed for all the films studied: the aver- age value of the threshold for 33 films more than 15 nm thick was (4.52 &

0.01) eV. The thresholds for amor- phous antimony films showed a much greater scatter ; the average value was (4.7 f 0.1) eV.

4. Discussion Antimony lies in Group VB of the

periodic table, and with arsenic and bis- muth is classed as a semimetal; the valence electrons may be considered to occupy two slightly overlapping ener- gy bands [17]. I n simple terms, in such

d i

I

1 I I

40 N 20 I0 -Transmission fo green fight (%)

&-material there will be fewer conduction electrons than in an ideal metal, but the overlapping bands ensure that there will be some electrons available for conduction, regardless of the temperature. The amorphous form of antimony, like that of arsenic, is regarded as a semiconductor, mainly on the evidence of the negative temperature coefficient of resistance of thin films. However, it may be worth pointing out that this characteristic may merely be due to the isIand structure of the thinnest films; thin films of many well-behaved metals show a resistance which decreases with increasing temperature, the conduction process having an activation energy of the order of 0.1 eV or less [18]. (The activation energies for conduction in this form of antimony are 0.07 eV a t low temperatures and 0.11 to 0.13 eV a t higher temperatures [19, 201.)

Fig. 2 shows that the photoelectric yield results from the amorphous phase of antimony are but a poor fit to the theoretical Fowler plot. If this phase is a conventional semiconductor then, indeed, agreement with a model derived using the simple free-electron theory of metals would not be expected. Kane [21] has summarized the relations between the photoelectric yield Y near the threshold hv, for a number of possible excitation and scattering processes in semiconductors. In general the relationship can be expressed as

Y - (hv - hv,)", (1)

where the exponent n can take the values 1, 3/2,2, or 512. An attempt was made to fit the results of Fig. 2(a) to this relation. It was found that n = 512 gives the best fit of the possible values of n; the threshold deduced using this relation is (4.72 -+ 0.03) eV, a value not significantly different from the (4.75 & 0.03) eV obtained by the Fowler analysis. The 512 exponent may correspond to an

T. W. HALL, R. M. EASTMENT, and C. H. B. MEE 332

3 A

2

7

Fig. 4. Graphs of Y" vs. hv for an antimony film of transmission 3'3%. The threshold value for m =

= 2/5 is (4.72 + 0.03) eV; for T/L = 1/3 i t is (4 67 i- 0.01) eV

indirect optical excitation in the vo- lume or, more likely in such thin, im- perfect. films, to a process in which the roughness of the surface acts as an ab- sorber of momentum 1211. Gobeli and Allen [22] have found that equation (1) with n = 3 gives a good representatoin of photoemission from many scmi- conductors, although the use of this exponent is not supported by the simple theory. Fig. 4 shows that the use of the cubic power law indicates a slightly lower threshold than that obtained from the five-halves law, and with a smaller uncertainty

((4.67 5 0.01) eV). In the amorphous phase photocurrents are so small, and the uncertainty in individual readings so relatively large, that it is not possible to make a clear distinction as to the best method of analysis of the results; it seems that any of the three provides a convenient method for labelling the photoelectric threshold, although the empirical cubic power law gives the smallest uncertainty.

Robbins and Thomas [23] and Bahadur and Chaudhary [24] have obtained electron micrographs of antimony films. In both cases the evaporation pressure was higher than in the present investigation, but the mode of growth of the films may provide an explanation of the photoelectric behaviour described here. A film about 10 nm thick shows a completely discontinuous island structure, the island dimensions being about 50 nm [24]; electron diffraction patterns confirm the amorphous nature of the antimony within the islands. The transi- tion to the crystalline form may be induced by electron bombardment; the island structure is maintained and there is no preferred orientation. Thicker, continuous films may show individual crystallites of more than 1 pm in linear dimension, the crystallites being oriented with their (1 1 l} planes parallel to the substrate [23]. It is suggested that the results of Fig. 1 may be explained by such a process. For films of transmission down to about 30%, the antimony film is amorphous with a discontinuous island structure. Because of this struc- ture the electrical resistance is very high, and photoelectric emission is limited by the film resistance. For films whose transmission is between 30 and 20%, crystallization has taken place, but the island structure is maintained. The resistance thus remains high, but the photocurrent is increased by an order of magnitude as the work function decreases and the Fowler constant increases at the phase change. The very rapid fall of resistance at a transmission of just below 20% represents the coalescence of the islands and the formation of an oriented polycrystalline film. For further increases in thickness the photo- electric threshold remains constant, the increase in yield being accounted for by the greater number of photons which are absorbed in thicker films. This is reflected by the increase in the Fowler constant p.

Photoelectric Properties of Amorphous and Crystalline Films of Sb 333

The value of the photoelectric threshold for crystalline antimony films more than about 15 nm thick ((4.52 f 0.01) eV) is in reasonable agreement with the value quoted by Apker, Taft, and Dickey [13] (4.56 eV). However, there exists an unexplained discrepancy between the photoelectric results and the depth of the Fermi level below vacuum potential as measured by contact potential difference methods. Taft and Apker [15] found that the photoelectric threshold and the surface potential of crystalline antimony were virtually indistinguishable (4.56 and 4.60 eV); contact potential difference measurements [25 to 271 give a surface potential of between 4.1 and 4.2 eV, a few tenths of 1 eV less than the values of the threshold.

5. Conclusions The phase transition between amorphous and crystalline films of antimony

may be detected by an abrupt increase in the photoelectric yield which occurs a t an optical transmission of about 30% t o green light. The sudden decrease in the electrical resistance of the film, which is often taken as an indication of the phase transition, occurs a t a significantly greater film thickness. The photo- electric threshold of crystalline antimony films thicker than about 15 nm is (4.52 f 0.01) eV; amorphous films have a threshold in the region of 4.7 eV.

Acknowledgements

Thanks are due to Prof. D. J. Bradley for the provision of laboratory facilities. Two of us (T.W.H. and R.M.E.) wish to thank the Northern Ireland Ministry of Education for the award of Advanced Course Studentships. R. Greer gave skilled assistance in the construction of the glass vacuum system and experi- mental tubes.

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[lo] E. CREMER and E. RUEDL. Z. Phys. 161, 487 (1958). [Ill R. SCHULZE, Z. Phys. 912, 212 (1934). [12] V. MIDDEL, Z. Phys. 106, 358 (1937). [13] L. APKER, E. TAFT, and J. Diclisu, Phys. Rev. 76, 270 (1949). [14] R. H. FOWLER, Phys. Rev. 38, 45 (1931). [15] E. TAFT and L. APRER, Phys. Rev. 96. 1496 (1954). [lci] C. LE.4, B. H. BLOTT, and C. H. B. MEE, Appl. Optics 8, 203 (1969). [17] J. ZIMAN, Priiiciples of the Theory of Solids, Cambridge University Press, Cambridge

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1964 (p. 103).

334 T. W. HALL et al. : Photoelectric Properties of Films of Sb

[23] K. G. ROBBINS and J. M. THOMAS, Nature (London) 217, 1261 (1968). 1241 K. BAHADUR and K. L. CHAUDHARY, Appl. Phys. Letters 15, 277 (1969). [25] 0. KLEIN and E. LANCE, Z. Elektrochem. angew. phys. Chem. 44, 542 (1938). [26] B. J. HoPxrNs, A. J. MARDEN, and D. PARKER, quoted by B. J. HOPKXNS and J. C-

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(Received March 31, 1970)