optical and thermal depth of shallow traps in zns

G . BAUR et al.: Optical and Thermal Depth of Shallow Traps in ZnS 337

phys. stat. sol. (a) 18, 337 (1973)

Subject classification: 13.4; 6; 13.1; 20.1; 20.3; 22.4.1

Physik-Department der Technischen Universitat Miinchen

Optical and Thermal Depth of Shallow Traps in ZnS BY

G. B A U R ~ ) , R. WENGERT, and V. WITTWER

Thermoluminescence, IR-absorptions and IR-stimulated spectra of very shallow (0.01 to 0.2 eV) traps in variously doped ZnS single crystals were measured in the temperature range of 5 to 100 K. A special monochromator cooled by liquid helium was developed in order to protect the sample from stray I R radiation. The depth of 'the traps investigated here is approximately the energy of optical and of the most energetic acoustical phonons in ZnS. The electrons released from these traps by I R absorption either make luminescent recombination with excited activator terms or may be retrapped with phonon emission. The retrapping probability is high if the energy to be transferred to the lattice is equal to the energy of a combination of phonons which can be readily created at the particular temperature. This retrapping probability can be obtained by comparing absorption and IR-stimulation measurements. Detailed analysis provides values of optical and thermal trap depths as well as the effective mass and the polarisation energy.

An flachen Haftstellen (0,01 bis 0,2 eV) verschieden dotierter ZnS-Einkristalle wurden Thermolumineszenz-, IR-Absorptions- und IR-Stimulationsmessungen durchgefuhrt. Die Tiefen der untersuchten Haftstellen entsprechen etwa der Energie der optischen und der energiereichsten akustischen Phononen in ZnS. Die durch IR-Absorption aus diesen Haft- stellen befreiten Elektronen rekombinieren entweder mit angeregten Aktivatoren unter Emission von sichtbarem Licht oder fallen unter Emission von Phononen in die Haftstellen zuruck. Der Vergleich von Absorptions- und Stimulationsmessungen zeigt dann eine hohe Retrappingwahrscheinlichkeit der befreiten Elektronen, wenn die ans Gitter abzugebende Energie einer Phononenkombination entspricht, die bei dieser Temperatur mit hoher Wahr- scheinlichkeit erzeugt werden kann. Aus der Temperatur- und Wellenliingenabhangigkeit der I R - Absorptions- und IR-Stimulationskurven wurden optische und thermische Trap- tiefen sowie effektive Masse und Polarisationsenergien berechnet.

1. Introduction The existence of shallow traps in IS-VI compounds has been demonstrated

by Riehl et al. [l to 31. These traps can be thermally depopulated in the temperature range from 4.2 to 100K. From the temperatures of electron release the trap depth is estimated to be between 0.01 and 0.2 eV. Such small ionization energies permit optical ionization with I R radiation up to a wavelength of 50 pm. In this wavelength range it is necessary to shield the crystal from thermal IR irradiation from uncooled portions of the experimental equipment.

From thermoluminescence measurements as well as from the temperature dependence of SR-absorption and IR-stimulation spectra the temperature of carrier release from trapping levels may be determined. From these temperatures (of carrier release) thermal trap depths Eth can be computed quantitatively if direct trap-activator transitions can be neglected or the ratio of band-acti-

l) Present address: Institut fur Angewandte Festkorperphysik der Fraunhofer-Gesell- schaft, D-78 Freiburg, EckerstraBe 4, FRG.

22 physica (a) l 8 j l

338 G. BAUR, R. WENGERT, and V. WITTWER

vator to direct trap-activator recombinations is known over the temperature range under consideration. Optical trap depths cannot be evaluated from such experiments. However, from the IR-absorption spectra of these shallow traps thermal and optical trap depth may be evaluated. These quantities may be evaluat,ed more precisely from the wavelength dependence of 1R stimulation if the retrapping probability is independent of the irradiated IR wavelength. If this assumption is not valid, the optical trap depth Eopt can be computed from the longest effective IR wavelength with high precision, but in this case Eti, cannot be evaluated from the shape of the stimulation curve.

2. Theory The traps are assumed due to positive excess charges in the lattice. Thus,

the occupied traps can be described as donor states in the gap. These donor levels are treated in the effective mass approximation. The electrons with effective mass m* are assumed to move in a hydrogen-like potential shielded by the effective dielectric constant E * . The thermal ionization energy is then given by

The optical energy necessary to free the trapped electrons is larger because of the polarisation energy stored in the lattice. In the case of a thermal ionization electron and lattice are in an energetic equilibrium.

In a hydrogen-like approximation Eopt and Eth are given by [4]

I with

F,, and E~ are the static and high frequency dielectric constants. If the donor states can be represented as a series of the wave functions of the first conduction band, perturbation theory applied to the effective mass theory yields for the absorption coefficient [ 5 ]

with 32 1/2 ?I: e2 h6 N

a. = F;(mx)712(u*)5'

( E energy of the light quanta, N concentration of donors, a* reduced Bohr r'a- dius, n refractive index.) The optical delivery starts with a minimum I R energy larger than Eopt. The shape of the absorption curveisdependent on

This theory is valid only for monoenergetic traps, i.e. interaction cif the E o p + and Eth.

1R with the lattice has been neglected.

Optical and Thermal Depth of Shallow Traps in ZnS 339

Pig. 1. Sketch of the cryostat. 1 Room temperature part; 2 liquid nitrogen cooled part; 3 liquid helium cooled part; 4 sample holder (variable temperature); 5 I R soiircr; G chopper (77 K); 7 IR monochromator (10 K); 8a optical observation; 8b IR detector

3. Experimental

Since the energy of shallow traps in ZnS corresponds to the energy of the emission maximum of a black-body radiator a t room temperature, a new apparatus was designed and constructed to minimize extraneous I R radiation in spectral glow, IR-absorption, and IR-stimulation measurements. The primary constituent is an I R monochromator cooled by liquid helium. Such cooling is necessary since equipment a t 300 K (approximated by a black-body radiator) produces a quantum flux density of 1016 photons/s em2 at the sample. This IR radiation would ionize the occ-upied traps instantaneously. At 10 K, the black-body emission capable of emptying the traps is reduced by a factor 1014. Therefore a cryostat (Fig. 1) was developed which cools the surrounding of the sample to liquid helium temperature independent of the temperature of the sample. The helium cooled parts were insulated by a liquid nitrogen shield. The filters a t the windows for UV excitation and optical observation were cooled with liquid helium and liquid nitrogen which reduced stray IR by a factor of about 10lo. The sample was irradiated through an I R inonochromator which was cooled to liquid helium temperature. I ts disperging elements were two cir- cular variable interference filters (OCLI) transmitting in the wavelength region from 2.5 to 5 pm and from 4.2 to 16 pm respectively. Broad band filters transmitting from 17.5 to 24.5 pm and from 26.5 to 35 pm allowed measurements a t longer wavelengths. The IR of a gauged tungsten lamp source (Osram) could be blocked or modulated by a cooled shutter or a cooled chopper respectively. 22 *

340 G. BAUR, R.. WENGERT, and V. WITTWEX

temp. I contr..

recorder

Fig. 2. Diagram of the experimental apparatus. A, B, and C optical entrances; D chopper (77 K); E I R monochromator (10 I<); S shutter (10 K); F monochromator drive;

L UV source; P sample holder (variable temperature)

For IR detection a Cu doped germanium detector (Type RPY 40 Mullard) was used a t 10 K. The reduction of background noise and the use of a PAR lock-in amplifier allowed very precise absorption and stimulation measurements.

A diagram of the experimental equipment is given in Fig. 2 . The temperature of the sample could be fixed or scanned between 5 and 100K by electronic control. The samples were excited by different mercury lamps in the wavelength range of 360 to 430 nm. After a decay time of 5 min spectral glow curves were measured with a heating rate of 3 K/min.

IR-absorption and stimulation measurements were performed both on the unexcited as well as on the excited crystal t o get the absorption and stimulated emission due to the emptying of the shallow traps. The measurements were taken by an automatic control circuit. For each wavelength and temperature the crystals were excited to saturation. After a fixed decay time the cooled shutter was opened and the IR-absorption and stimulation signals were recorded.

4. Crystals For the measurements five differently doped ZnS single crystals were used.

They are listed in Table 1. By an appropriate selection of different doping

Optical and Thermal Depth of Shallow Traps in ZnS 34 1

ZnS (Cu, Al) ZnS (Cu, Al, I) ZnS (Cu, I) ZnS (Cu, C1) ZnS (Al, I)

combinations, characteristic properties of activators, traps, and the lattice were separated. (The structure of all crystals was a mixed cubic hexagonal one.)

2 x lo1* A1 535 and 460 (& 5) 101s ~ i , 1017 I 535 and 465 (& 5) 2 x 1018 Cu, IOl7 I 535 and 460 (* 5) 101s CU, 1019 c i 530 and 460 ( * 5) 1018 AI, 1017 I 505 (* 5)

T a b l e 1 Doping and emission maxima of the investigated crystals

crystal I doping [cm-~] I emission maxima (nm)

The recombination centres (activators) in crystals 1 to 4 were copper centres. The typical green and blue emission was observed. Crystal 5 showed an emission a t 505 nm due to recombination to A-centres (self-activated centres). All crystals contained a high concentration of AP+, C1- or I- which cause a positive excess charge to the lattice. These coactivators are assumed to act as shallow electron traps [6] which can to a good approximation, be described by the shallow donor model.

5. Results

5.1 Tempepature-depenqent experiments

The IR-absorption coefficients a( T) due to the absorption of the occupied shallow traps can be calculated from the temperature dependence of the trans- mission intensities. Fig. 3 shows a( 1') together with the corresponding glow curve for ZnS (Cu, I). The turning point of the a ( T ) curve and the first maximum of the glow curve mark the temperature a t which the shallow traps are emptied. The agreement between these two methods is satisfactory within experimental error. The temperatures where the traps empty are listed in the following Table 2.

Fig. 3. Glow emission (Iglolxr), IR-stimulated emission (Ist,im) and optical density for IR absorption at occupied shallow traps (a) in dependence on temperature for a ZnS (Cu, I) crystal. IR used 12pm;

emission a t 535 nm

I

I /

11 za 40 60 80 I( I temperuture (h'-

342

Zn8 (Ca, Al) 605 f 30 519 5 30 ZnS (Cu, Al, I) 491 f 30 ZnS (Cn, I) 1 :;t i: 1 470 f 30

G . BAUR, R. WENGERT, and V. WITTWER

1.16 * 0.12 1.19 * 0.12 1.23 f 0.12

T a b l e 2 Temperatures of trap emptying from glow

and IR-absorption measurements

crystal I temperature of trap emptying [K]

ZnS (Cu, Al) ZnS (Cu, Al, I) ZnS (Cu, I) ZnS (Cu, C1) ZnS (Al, I)

47 47 50 53 41 and 56

Since the ratio of direct trap-activator transitions to band-activator transitions is unknown one cannot compute precise values for the thermal trap depth from the emptying temperatures listed above. The intensity of IR-stimulated emission decreases rapidly with increasing temperature. We suppose that this effect is due to a temperature dependent retrapping probability for the released electrons.

5.2 Absorption experiments at 5 U

At 5 K the absorption coefficients 01 due to shallow traps were measured as a function of I R wavelength. The absorption spectra for ZnS (Cu, Al) and ZnS (Cu, I) are shown in Fig. 4. The longest effective I R wavelength corresponds to the optical trap depth Eopt. Applying the shallow donor model, the shape of the absorption curve is dependent on Eopt and Eth. The values of Eopt and Et,, calculated with a leas; square fit are listed in Table 3. The absorption coefficients were measured with an error of 2% in energy and 5% in intensity. The good agreement of theory and experiment justifies the application of the shallow donor model to the occupied shallow traps in ZnS.

T a b l e 3 Optical trap depth Eopt and thermal trap depth E,, calculated with

a least square f i t from the absorption coefficients

I

5.3 IR stimulation at 5 K

IR-stimulation experiments are a more sensitive method to determine the optical trap depth Eopt from the longest effective IR wavelength. E+,, may be calculated from the shape of the stimulation curve, assuming that each electron released by IR causes a luminescent recombination or that the retrapping probability is independent on the irradiated I R wavelength. The absorption probability function calculated in the shallow donor model can then be fitted to the stimulation intensities corrected for lattice absorption.

Before discussing the results, the 1R-absorption and emission intensities used in the experiments will be given. At 12 pm the I R flux a t the sample


v [crn-7 - Fig. 5 . IR-stimulated emission a t 535 nm for a ZnS (Cu, Al) (upper part) and a ZnS (Cu, I) crystal

(lower part) a t 5 K 4 2 ]I,, , , ,\", , ! I

0 500 700 900 1100 1300 7500 7700 7900

G icm-'i - Fig. 4. Optical density for IR absorption a t occupied shallow traps for a ZnS (Cu. Al) (upper part) and a ZnS (Cu, I) crystal (lower part) at 5 I<. The solid line is calculated with the

shallow donor model. The circles are experimental values

was 7 x W. About 10% (7 x 1013 quanta/cm2 s) was absorbed by the occupied shallow traps. The emission intensity, stimulated by IR absorbed at shallow traps, was found to be 7 x 1012 quanta/ em2 s. Therefore approximately 10% of the electrons released by IR radiation of 12 pm make luminescent recombinations with activator terms. The analogous quantum efficiency for radiation of 15 pm was found to be only 5%. A comparison of the absorption coefficients and the fluorescence intensities shows a de- creasing quantum efficiency with increasing IR wavelength. In addition t o this effect, sharp minima for the stimulated emission were observed for discrete irradiated IR energies. Stimulation curves for ZnS (Cu, Al) and ZnS (Cu, I) are shown in Fig. 5 . The minima in the stimulated emission appear a t energies corresponding to multiples of the energies of LO phonotis of the ZnS lattice, listed in Table 4.

Values for coLo = 350 cm-l and wTO = 272 cm-I ( k = 0) given by Baars and Koschel [7 ] were used. These minima in the stimulated emission are due to an increase of the retrapping probability, assuming that the energy of the released electrons corresponds to the energy of one of the phonon combinations listed above. Therefore, Eth cannot be evaluated by fitting the absorption probability function of the shallow donor model to the IR-stimulated emission intensities. The values of Eopt calculated from the longest effective IR wavelength are in good agreement with the values evaluated from the absorption coefficients.

quanta/cm2 s or approximately

344

~~

phonon combinations . (cm-I )

G. BAUE, R. WENCERT, and V. WITTWER

ZnS

T a b l e 4 Comparison of IR energies a t the minima in the stimulated emisison spectra

with the energies of LO and TO phonons 1 ZnS (Cu, Al) 1 1 1015 1 yi: 1 1720 minima in stimulated j, 25 & 25 & 25

emission ZnS (Cu, I) 1040 1400 1750 & 25 + 25 25 + 25 (ern-')

2LO 5LO 700 1750

5.4 Polarisation energies and effective electroll mass

If traps are emptied optically there is a polarisation energy left in the lattice. This energy can be calculated as the difference of the optical and the thermal trap depth. Some examples are shown in the following table :

T a b l e 5 Polarisation energies calculated from optical

and t.hermal t rap depths

Epol (cm-') I 86 f 15 I 93 15 I 109 $. 15

From Eopt, Eth, and E, the effective mass m* was calculated using (2). The values of m* calculated using Eopt and Eth from Table 3 and F, = 9.6 are listed in Table 6 for severaldifferent crystals. These results agree with the experimeiital

Table 6 Effective mass m* calculated with Eopt and &h

from absorption measurements -

sample I ZnS (cu, ~ 1 ) 1 ZnS (cu, AI, I) 1 ZnS (cu, I)

m+/mo 1 0.33 + 0.03 I 0.31 f 0.03 1 0.28 & 0.03 __ ~~ ~ -~ - __ - - -

values given by Kukimoto et al. [5], and Miklosz and Weehler [S]. Calculations of Cardona [9] and Eckelt e t al. [lo] give values of (0.35 to 0.40) m,.

6. Concluding Remarks

ZnS crystals containing AP+, C1-, and I- in high concentration have shallow traps which can be ionized thermally near 50 K. IR-absorption measurements at these occupied shallow traps show that the shallow donor model can be applied to calculate thermal and optical trap depths with high precision. From comparison of absorption coefficients and stimulated emission i t is concluded that, the retrapping probabilitJy of the electrons, ionized by IR, is dependent on the irradiated IR energy.


Acknowledgemeiits

The authors are grateful to Prof. N. Riehl for his interest in this work and for stimulating discussions. We also wish to thank our colleagues Dr. L. Mader and H. P. Braun for their support, Dr. H. Deuling, TU Berlin, for the least square fitting program, and Prof. S. Shionoya, University of Tokyo, and Dr. A. Rauber, Institut fur Angewandte Festkorperphysik, Freiburg, for supplying us with the crystals. This work was supported by the Praunhofer-Gesellschaft.

References [l] N. RIEHL, Proc. Internat. Conf. Luminescence, Budapest 1966 (p. 974). [2] G. BAUR, N. RIEHL, and P. THOMA, Z. Phys. 206, 229 (1967). [3] G. BAUR, L. MADER, and N. RIEHL, Z. Naturf. 24a, 1296 (1969). [4] J. H. SIMPSON, Proc. Roy. SOC. 231A, 308 (1955). [5] H. KURIMOTO, S. SHIONOYA, T. KODA, and R. HIOKI, J. Phys. Chem. Solids 29, 935

(1968). [6] J. SCHNEIDER, Proc. Internat. Conf. 11-VI Semiconducting Compounds, Providence

1967, W. A. Benjamin, New York, Amsterdam 1967 (p. 40). [7] J. BAARS and W. KOSCHEL, private communication. [8] J. C. MIKLOSZ and R. G. WHEELER, Phys. Rev. 163,913 (1967). [9] M. J. CARDONA, J. Phys. Chem. Solids 24, 1543 (1963).

[lo] P. ECKELT, 0. MADELUNG, and S. TREUSCH, Phys. Rev. Letters 18,656 (1967).

(Received April 24, 1973)

optical and thermal depth of shallow traps in zns

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