A. D. BROTHERS Rnd D. M. WIELICZKA: The Exciton Peaks in BIT, and SbT, 201

phys. stat. sol. (b) 80, 201 (1977)

Subject classification: 13.5.1 ; 13.1; 22.5

Benedictine College, Atchison')

Temperature and Hydrostatic Pressure Measurements of the Exciton Peaks in BiIs and SbI,

BY A. D. BROTHERS and D. ill. WIELICZKA

Exciton peak positions are measured as a function of temperature in the range 90 to 270 K and as a function of hydrostatic pressure to 3500 atm in thin film samples of BiI, and SbI,. Values for (aE/aP), are -8.6 x eV/atm and -32 x eV/atm for BiI, and SbI,, respectively. (aE/aT)p for BiI, varies from -0.49 x eV/K near 200 K, and for SbT, the value is found to be +7.3 x eV/K. Estimates of the electron- phonon coefficient, (i3E/aT)V, are made. Comments are made concerning the nature of the ex- citons and the tdnd structure in these materials.

Es werden die Lagen der Exzitonenmaxinia in Abhiingigkeit von der Temperatur zwischen 90 und 270 K als Funktion des hydrostatischen Druckes bis zu 3500 atm in dunnen BiJ,- und SbJ,- Schichten gemessen. Die Werte fur (aE/aP), betragen -8,6 x 10-4eV/atm und -32 x x 10-4 eV/atm fiir BiJ, bzw. SbJ,. (aE/aT)p fur BiJ, variiert von -0,49 x lo4 eV/K bei 100 K bis +0,56 x lo-' eV/K bei 200 K, und fur SbJ, wird ein Wert von +7,3 x lo-' eV/K gefunden. Es werden Berechnungen des Elektron-Phonon-Koeffizienten (aE/BT) durchgefuhrt und die Natur der Exzitonen sowie die Bandstruktur dieser Materialien diskutiert.

eV/K near 100 K to $0.58 x

1. Introduction

Bismuth and antimony triiodides are layered compounds similar to the heavy metal dihalides, which have been more extensively studied. They form in layers with the metal ions partially filling a layer between two layers of iodide ions, with this arrange- ment repeating to form the sample. Both materials are predominantly ionic and crys- tallize with their optic axis normal to the layers [ 11. Because of their structural similar- ity to each other and to the metal dihalides, especially the extensively studied metal diiodides, comparison of the optical properties of BiI, and SbI, to these materials is informative.

2. Experimental

Thin film samples were prepared by evaporation from molybdenum boats onto fused quartz substrates a t room temperature. Sample material was obtained from CERAC/ PURE, Inc. The BiI, material was typically 99.9q/b pure, with less than 0.01% iron and zinc and less than O . O O l ~ o of other metal impurities. The SbI, was typically 99.5% pure with O.Olyo arsenic, 0.008% copper, lead, and iron, and traces of other metal impurities. Typical evaporation pressures were 1 x Torr or less. Sample thickness varied from 750 A to 6600 A with most films studied being about 2000 A. Thicknesses were determined by an interference technique using a inasked film held adjacent to the samples upon evaporation which was then overcoated with silver in a second evaporation. Values of film thickness to the nearest 100 A are used, as this uncertainty is a reasonable estimate for the accuracy of the technique and equipment

l) Atchison, Kansas 66002, USA.

202 A. D. BROTHERS and D. M. WIELICZKA

used for the thickness measurements. The films of SbI, were quite susceptible to niois- ture damage and were handled in a dry nitrogen atmosphere until they were mounted for study. All films were prepared immediately prior to use, and were handled under subdued light, installing them in the sample chambers and cooling the cryostats as rapidly as possible.

Temperature runs were made in a conventional cryostat equipped with a thernio- couple attached near the sample and a heating coil placed between the sample and the liquid nitrogen reservoir. Data for temperature shifts were taken generally from 110 K to near 370 K. Pressure measurements were made a t liquid nitrogen temperature in a cell previously described [2]. Pressures in the range from 1000 to 3500 atm were achieved with this system. An equal number of data points at ascending and descend- ing pressures assured that the effect on the spectra of any further cooling occurring during the pressure runs would be averaged out and also made manifest' any permanent pressure-induced effects.

The data were recorded on a Cary 14 spectrophotometer. Wavelength reproducibil- ity was typically 1 A as determined by the consistency of data from multiple runs a t the same temperature or pressure. Peak positions were determined by bisecting the intercepts of the peak just below the maximum with lines on the strip chart paper. Three peaks a t each temperature or pressure were generally run, and these data aver- aged to produce one data point for determinations of the temperature and pressure coefficients. To study changes in the position of the absorption edge, positions inter- mediate between the first exciton peak and the edge onset were chosen.

Fig. l a displays typical data for the temperature shift and pressure shift of the spec- trum for BiI,. The temperature data illustrate well the difficulty of characterizing the shift of the peak by the shift near the onset of the edge; the shift of the peak

t $ a -0" y 3

.

b

incident photon energy IeV) incident phofon energy lev) -

Fig. 1. Typical absorption spectra for a) BiI, films and b) SbI,. On the left-hand side: 90 K (solid line) and 270 K (dashed line) spectra taken at 1 atm pressure. On the right-hand side: high pressure (solid line) and low pressure (dashed

incident photon energy(eV)- line) spectra taken at 90 K

Temperature and Pressure Measurements of the Exciton Peaks in BiI, and SbI, 203

BiI, BiI, SbI,

here being considerably smaller than the shift of the edge near its onset. The pres- sure data indicate that the edge and the exciton peak shift rigidly.

Fig. 1 b similarly displays typical data for SbI, films. Due to their affinity for water, these films were of lower quality than the BiT, films, and the exciton peaks were cor- respondingly weaker, often blurring into a shoulder a t temperatures only slightly above our lowest attainable ones. I n general our SbI, films were thicker so as to obtain a usu- able peak over a moderate range of temperatures. The pressure spectrum again indi- cates a rigid shifting of the edge and exciton peak with pressure. We also noticed in many samples a pressure-induced enhancement of the peak. A small hysteresis effect in the shifting of the peak position with pressure was also noted, and it was determined that this effect, which caused about a 5% overestimate of our pressure coefficient values, could be eliminated by cycling the sample to the highest pressure used first, then running the experiment. The second pressurization then caused no discrepancies between ascending and descending pressure peak position values. The values of the pressure coefficient reported later are not adjusted for this effect, as it was noted only after some data were obtained.

Table 1 presents our results for the pressure and temperature shifts of the excitons in BiI, and SbI,. The temperature coefficient for BiI, listed in Table 1 is an averaged value for several films of thicknesses ranging from 700 to 2900 A, with data taken be- tween about 110 and 150 K in each case. Czaja e t al. [3], report a temperature coeffi- cient +1.7 x eV/K between 5 and 35 K , above which the coefficient becomes negative.

In most films on quartz substrates the coefficient became positive above 170 K and was constant to near 270K. An averaged value of this positive coefficient was (5.6 f 3.1) x eV/K. The broadening and shape changes in the spectrum a t higher temperatures give rise to t>he large uncertainty reported. The value of the temperature shift, it was further noted, depended on the film thickness; thinner films having a smal- ler shift than thicker ones. A best-fit straight line of the temperature shift versus thick- ness data for Sir, on quartz substrates between 90 and 150 K indicated that a 1000 A film would have a coefficient of almost -0.2 x 10-4 eV/K while a 2000 A film would have a value near -0.6 x eV/K. In two instances a negative coefficient even at the lowest temperatures was not seen, and the shift was positive from 90 to 270 K , and these values of the temperature coefficient were within the range of positive values reported above for all samples above 170 K. The valueof the pressure coefficient for BiI, showed no significant trend as a function of thickness. Both the lower temperature negative value of (BE/aT), and the higher temperature positive value for BiI, are used in Table 1 to calculate the lattice-phonon term. The high temperature positive

(aE/aT)P (wwT 1 (B/%) (wwT 1 (wwv

+7.3 f 3.2 I -32 i 0.6 -6 & 0.1 +l & 3.3

eV/K) (10-6 eV/atm) ( eV/K) ( eV/K)

+0.56 -j= 0.31*) -8.6 i 0.5 -1.6 & 0.1**) -1 & 0.8 -0.49 & 0.12***) , -8.6 0.5 -1.6 f 0.1 -2 & 0.6

Table 1

204 A. D. BROTHERS and D. M. WIELICZKA

coefficient is thought to be the more reliable quantity for the temperature coefficient, since the value of (aE/aT) , has a significant temperature dependence below the Debye temperature.

The pressure shift of a point intermediate between the exciton peak position and the onset of the absorption edge of a BiI, film was run at room temperature after the same film was run a t low temperature, yielding a shift that was about 50% larger than the low temperature pressure shifts; the same sized increase as noted by Grant and Yoffe [4].

The temperature coefficient of the exciton peak for SbI, appeared to be somewhat thickness dependent, although the data here were less conclusive, with thick films possibly having a somewhat smaller shift than thinner ones. N o significant alteration in the pressure coefficient over a range of film thicknesses of 2000 to 3500 A was de- tected.

BiI, samples prepared on glass substrates exhibit quite different temperature coeffi- cients. Below about 180 K the shifts were positive, and somewhat smaller in magnitude than the negative shifts seen on quartz substrates in this region. For temperatures between 180 and 230 K the values of the temperature coefficient for samples on glass substrates were negative and comparable to the negative shifts seen at lower tempera- tures on quartz substrate samples. Above 230 K the coefficients were positive on glass substrate samples, but somewhat larger than the positive coefficients seen on quartz substrates in this temperature region. The value of the pressure coefficient for BiI, saiiiple on glass substrate was seen to be -7.2 x eV/atm, only slightly lower than the values obtained on quartz substrates.

Measurement of the temperature coefficient for SbI, films prepared on glass sub- strates yielded various values, all positive, but typically lower than t,he averaged value for films on quartz. A pressure run with one sample on a glass substrate produced the same coefficient as with quartz substrates, but a spectrum of this sample run after the application and release of the pressure revealed a new, prominent sharp peak lying at, about 0.1 eV lower energy from the original exciton peak. The pressure shift of this peak proved to be about 20% smaller than that of the original peak in a second cycling of pressure. This sample also exhibit other unresolved structure near the exciton peak which was not seen in the quartz substrate samples.

Under a 4 0 ~ magnification BiI, films on quartz substrates appeared somewhat granular, with small islands of darker color interspersed uniformly throughout the film. Films on glass substrates appeared grainier and had considerably more imper- fections, perhaps caused by the rougher surface of the glass. SbI, films were not ob- served under a microscope since they deteriorate rapidly in air.

The erratic and inconsistent behavior of samples prepared on glass substrates sug- gests that adhesion to the substrate may affect the data more strongly than for quartz substrates. The appearance of the second peak in the pressure run above may be attri- buted to a phase change in part of the SbI, sample, or, perhaps more likely, to the appearance of a pressure induced second polytype. Only the results from films evap- orated on quartz substrates will subsequently be discussed.

3. Discussion The location of the first exciton peak in BiI, has been variously reported as 2.06 to

2.10 eV [5]. Our values ranged from 2.05 to 2.08 eV over the thirteen samples studied. Czaja et. al. [3], report a band seen in some samples a t 1.93 eV, which is apparently not due to impurities. This band appeared prominently in one of our samples, which also did not exhibit an exciton band in the range listed above. The second, spin-orbit, split, exciton peak has been reported to be a t 2.48 eV. We detected this peak only once.

Temperature and Pressure Measurements of the Exciton Peaks in BiI, and SbI, 205

A second line in the lower energy exciton series was reported a t 2.30 eV [5] but we did not see this peak, or any structure a t 2.30 eV, except in a sample purposely left over- night in a dark room in a dessicator prior to running an absorption spectrum, which suggests the peak may be due to an oxidation product of BiI, [6]. No distinct structure to higher wavelengths from the first exciton peak was seen in our samples as was seen in crystal samples by Gross e t al. [7]. Other structures typically seen in our films a t about 2.70 eV and 3.30 eV has not been previously described.

Our peak position in nine samples of SbI, films ranged from 2.62 to 2.66 eV, with the exception of an extremely thick film (6600 A), whose peak was found a t 2.43 eV. Others report positions of 2.57 eV and approximately 2.60 eV [8, 91. On one sample a shoulder appeared a t 2.54 eV which may be ascribed to an atypical polytype being present in the sample. All peak positions reported above are a t a sample temperature of approximately 90 K and corrected to a pressure of 1 atm.

The consistency of the values from several samples for each coefficient reported in Table 1 indicates that the same polytype was formed in each instance since the values for these coefficients are substantially different for differing polytypes [ 101.

The value of the temperature coefficient for BiI, has been reported in thin crystals to be -2.7 x 10-4 eV/K [ 111 and -2.6 x 10-4 eV/K [ 11, the latter value for crystals ranging from 3500 to 5800 A. Extrapolation of our thickness versus temperature coef- ficient plot for thin film samples yields a value of -2.3 x eV/K for a film thick- ness of 5800 A, in good agreement with the above values. The value of - ( G & 1) x x 10-8eV/atm for the pressure coefficient of BiI, reported by Gulyamov [12] is marginally lower than our values in films, the discrepancy possibly explained by the thicker crystal platelets (lo4 A) used by Gulyamov, although a thickness versus pres- sure coefficient plot of our data showed only a small trend toward higher values at greater film thickness.

A comparison of data for the temperature coefficient of the exciton peak in SbI, with others results is interesting. Tubbs [5] suggests that the shift of the absorption edge onset, given as -9 x eV/K from work by Turyanitsa and Chepur [13], may be used to characterize the peak shift. From Fig. 1 b, however, i t can be seen that the peak and the absorption edge near its onset have quite different temperature coefficients. Evans [la] reports a value for the temperature coefficient in absorption a t the edge onset of approximately -1 x 10-3 eV/K in films evaporated on glass sub- strates. Vashchenko [ l l ] reports a shift of magnitude 2 x eV/K of reflection bands at 2.0 and 3.0 eV in SbI, monocrystals.

Our pressure coefficient for the SbI, exciton peak is nearly double Gulyamov’s value [ 121, however, his data were obtained from photoconductivity measurements in single crystal platelets. Our value for the edge shift in SbI, a t points intermediate be- tween the edge onset and the first exciton peak is (1.5 f. 0.8) x eV/K, somewhat smaller, but also of positive sign, than our value of the exciton peak shift. Gulyamov [12] reports an increase in photocurrent in S b h samples at hydrostatic pressures to 14 x lo3 atm, which may be related to the pressure-induced peak enhancement that we observe in SbI, films. The increase in the pressure coefficient with thickness for excitons in BiI, cannot be unambiguously interpreted, since the observed pressure shift arises from both the shift of the energy gap associated with the exciton and the pressure dependence of the exciton binding energy. As the film thickness increases, the exciton is likely to be less tightly bound, hence the binding energy term pressure dependence will decrease. However, the predominant change likely involves the thickness dependence of the energy gap pressure shift, as i t is much larger in value than the exciton binding energy. This effect, therefore, cannot be used to support a three- dimensional rather than a two-dimensional exciton model.

206 A. D. BROTHERS and D. M. WIELICZKA: The Exciton Peaks in BiT, and SbT,

The size of the pressure shift has been relat,ed to the nature of the valence band by Paul [15]; s-like bands shifting with pressure more than p- or d-like bands. The upper valence band in BiI,, like that of PbI,, is thought to be coniprised of iodide 5P states and metal ion 6s states [a]. Our very large pressure coefficients for both BiI, and SbI, suggest a predominantly s-like valence band for these materials, with SbI, having the larger s-like contribution.

The quite different pressure coefficients for BiI, and SbI, suggest that the cationic exciton model is appropriate for these materials, as an anionic model would predict very similar pressure shifts.

Using the thermodynamic relationship

illustrates that the observed temperature coefficient arises from two effects: changes in the electron-phonon interaction, (aE/aT) ,,; and changes in the lattice parameter, as determined from the pressure coefficient. It is unlikely that the ratio of B / x , the thermal expansion coefficient divided by the isothermal compressibility, for either material would differ widely from this ratio for PbI,, hence the roughly estimated terms listed in the last two colunins of Table 1 suggest that t,he electron-phonon inter- action has the predominant influence on the temperature coefficient in Bi13, but prob- ably not in SbI,. The anomalous positive sign for (i3E/aT)v in SbI, arises most likely from the larger experimental uncertainties in determining the temperature coefficient of that material, and due to this large variation no real difference in the electron- phonon term can be discerned between BiI, and SbT,.

Aehowledge?nente

We wish to thank the Research Corporation for supporting this work, Dr. David W. Lynch, Iowa State University, for his comments on the manuscript, and Mr. Joseph Gress for measurement of the film thicknesses.

References

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(Received November 11, 1976)