the optical properties of cds crystal grown by the sublimation method

Journal of Crystal Growth 218 (2000) 19}26

The optical properties of CdS crystal grown bythe sublimation method

K.J. Hong!,*, T.S. Jeong", C.J. Yoon#, Y.J. Shin#

! Department of Physics, Chosun University, Kwangju 501-759, South Korea"Department of Physics, Suncheon National University, Suncheon 540-742, South Korea

#Department of Physics and Semiconductor Physics Research Center (SPRC), Jeonbuk National University, Jeonbuk 560-756, South Korea

Received 12 April 2000; accepted 17 April 2000Communicated by M. Schieber

Abstract

A cadmium sul"de (CdS) single crystal was grown by the sublimation method without a seed crystal in a two-stagevertical electric furnace. The carrier concentration and mobility obtained from Hall measurements at room temperaturewere 2.90]1016 cm~3 and 316 cm2/V s, respectively. The photoluminescence and the photocurrent measurement of theCdS single crystal have been performed in the temperature ranging from 20 to 293K. From the photoluminescencemeasurement, the energy of the free exciton Ex(A) and Ex(B) has been obtained to be 2.5511 and 2.5707 eV, respectively.The variance of the peak position, intensity, and linewidth of the free excitons as a function of the temperature have beeninvestigated by means of the conventional empirical relations and Toyozawa's theory. The crystal "eld of the CdS and itssplitting energy, *c

3, have been found to be 19.6meV. In the photocurrent measurement, only the Ex(A) exciton peak has

been observed. The energy band gap of the CdS at room temperature was determined to be 2.4749 eV by thephotoluminescence and photocurrent measurement. Also, the temperature dependence of the energy band gap of theCdS, E

'(¹), has been examined. ( 2000 Elsevier Science B.V. All rights reserved.

PACS: 78.20.Wc; 78.55.Et; 81.10.Aj

Keywords: CdS; Sublimation method; Photoluminescence; Photocurrent; Energy band gap

1. Introduction

The energy band gap of CdS having a hexagonalstructure at room temperature has been known tobe about 2.42 eV [1]. Therefore, CdS is one of theinteresting materials used as optoelectronic devicesapplicable to the visible region because the energy

*Corresponding author.E-mail address: [email protected] (K.J. Hong).

of its band gap is in the visible region. Many studiesof the techniques to grow a high-quality crystal andthe fundamental characterization of the grown ma-terial have been carried out. The melting point ofCdS is very high. Thus, the CdS crystal grown atthis high temperatures usually has a high concen-tration of unintentional defects, resulting in thedeviation of stoichiometry. However, in the growthmethod of a CdS single crystal, a sublimationmethod has been known to reduce the growth de-fects because of the advantage of low-temperature

0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 4 9 1 - 7

Fig. 1. Experimental arrangement of the two-stage vertical elec-tric furnace.

growth. Also, the sublimation method is classi"edinto two methods distinguished as attaching or notattaching a tail tube to the bottom of the growthtube. This tail tube contributes to improving thestoichiometry of the CdS crystal in the closed innertube during growing. Consequently, due to the dif-"culty in growing a high-quality CdS single crystal,the room temperature band edge of the CdS hasnot been carried out by using the photolumines-cence (PL) and photocurrent (PC) measurement,although the optical properties of the bulk [2}11]and the "lm of CdS [12,13] have been intensivelyinvestigated. Also, the free exciton transition hasbeen observed only at low temperature; therefore,the room temperature band gap energy was de-termined by extrapolating the low-temperaturemeasurement data. Generally, the absorption ex-periment is adopted to measure the energy bandgap of CdS, this method is not accurate due to thedi$culty in de"ning the position of absorptionedge. Also, a few studies on the temperature de-pendence of the energy band gap of the CdS havebeen carried out by using a photoluminescence anda photocurrent experiment.

In this work, a very large CdS single crystalwas grown in the growth tube with a tail tubeby the sublimation method without a seedcrystal. The PL and the PC measurement of thegrown CdS crystal have been carried out atthe temperature ranging from 20 to 293K. Theobtained PL spectra have been analyzed withrespect to the PL intensity, the linewidth, andthe emission peak position of the free excitons.The temperature dependence of the energy bandgap of the CdS and the free exciton has also beenexamined.

2. Experimental procedure

A very large CdS single crystal was grown in thelaboratory by using a sublimation method froma vapor-phase technique. The two-stage verticalelectric furnace is shown in Fig. 1. The growth tubewithout a seed crystal is prepared with the tail tubeas a reservoir for stoichiometry. The CdS powder(ESPI, K982, 6N) was degassed at 2003C for 2 hunder &10~6Torr. 12 g of the degassed powder

and 60mg sulfur were placed in the growth tubeand the reservoir.

The sample was grown via three steps. First, inorder to stabilize the vapor pressure of the growthtube the temperature of the source and growthparts was increased gradually to 1025 and 11653C,respectively. This temperature was then maintainedfor 24 h. The growth tube was pulled up to 9 cm for14 h using a puller with a speed-control motor. The"nal temperature of the source part, the growthpart, and the tail tube was 1165, 1150, and 1103C,respectively.

The second step is a growing stage. The pull-upspeed of the growth tube was 0.38mm/h and thusthe distance moved in 105h was 4 cm. The temper-ature was kept within $13C during growth. The"nal step was carried out in order to obtain defect-free samples. We pulled up the growth tube furtherand lowered the temperature of the tube at a rate of203C/h for 21 h in order to avoid samples withcracks. The growth tube was then removed fromthe electric furnace. The growth temperature wasset to be 11653C for the source part, 11503C for thegrowth part, and 1103C for the tail tube, during

20 K.J. Hong et al. / Journal of Crystal Growth 218 (2000) 19}26

Fig. 2. Photograph of the grown CdS single crystal.

Fig. 3. Photoluminesence spectra of the CdS single crystal mea-sured at di!erent temperatures.

the growth of the CdS crystal. The pull-up speed ofthe growth tube was controlled to be 0.38 mm/h.Fig. 2 shows a crystal grown via these steps.

In the measurement of the PL and the PC, theexciton emitted from the band edge has been re-ported to be strongly dependent on the surfacecondition of the crystal. Thus, only those samplescleaved o! the bulk CdS along (1 0 11 0) plane wereused in this experiment, since the crystal surfaceground and polished has a possibility of a!ectingthe PL spectra [14]. The photoluminescence of thesample was performed at temperature ranging from20 to 293K in the low temperature cryostat equip-ment (AP Inc. CSA 202B, DE 202S). The surface ofthe CdS sample was illuminated by 325nm ultra-violet light emitted from a He}Cd laser (Kimon,10mW) in which the light was polarized perpen-dicular to the c-axis of the (1 0 11 0) plane, and thelight coming from the sample was dispersed witha monochromator (Jarrel Ash, 82-020, F"0.5m).The dispersed light was detected with a photomul-tiplier tube (RCA C3-1034) and then converted intoa current. This current was recorded on an X}Yrecorder (MFE, 815M) with ampli"cation bya lock-in ampli"er (EG&G 5208).

To measure the photocurrent spectra, the elec-trode was placed on both ends of the sample andconnected to a wire. After this, the sample wasmounted on the holder in the low-temperaturecryostat equipment. The photocurrent spectrummeasurement was done while the monochromaticlight emitted from a halogen lamp passed through

a chopper which is illustrated on the sample, as thetemperature varied from 20 to 293K.

3. Results and discussion

3.1. Structural and electrical properties

The crystal structure of CdS is hexagonal asseen from the typical di!raction peaks from planessuch as (1 0 11 0), (0 0 0 2) and (1 0 11 1) using the X-raypowder method. From the measured patterns, thelattice constants a

0and c

0obtained by the extra-

polation method are 4.132 and 6.712As , respective-ly. This value agrees well with the values obtainedby Kittel [15]. The plane perpendicular to thegrowth direction of the crystal was the M0 0 0 1Nplane as determined by the back-re#ection Lauemethod. From the measured Laue patterns weknow that the c-axis of the grown crystal is alongthe growth direction, as shown in Fig. 2.

In order to measure the Hall e!ect by the Vander Pauw method at room temperature, thecrystal was cut along the (1 0 11 0) plane. The mea-sured carrier concentration and mobility were2.90]1016 cm~3 and 316 cm2/V s, respectively.

3.2. Photoluminescence spectra

Fig. 3 shows a typical PL spectra obtained inthe temperature range from 20 to 293K. Several

K.J. Hong et al. / Journal of Crystal Growth 218 (2000) 19}26 21

Fig. 4. Exciton linewidth as a function of temperature between20 and 293K.

emission peaks in the 20KPL spectrum wereobserved at 2.5707 eV (482.3 nm), 2.5511 eV(486.0 nm), 2.5449 eV (487.2 nm), 2.5149 eV(493.0 nm), and 2.4772 eV (500.5 nm). The peak at2.5511 eV exhibits a very sharp and strong inten-sity, which is believed to correspond to the A-freeexciton, Ex(A). This has been reported to be ob-served at 2.553 eV by Langer et al. [16], which wastaken from the re#ectance measurements, and2.5537 eV by Thomas et al. [4], which was obtainedfrom the absorption experiments. Our result is alsoin close agreement with the value measured at 10 Kby Lovergine et al. [13] from a CdS epilayer grownon CdTe substrate. The emission peak at 2.5707 eVon the shoulder is shifted to the shorter wavelengthregions and is believed to correspond to the B-freeexciton, Ex(B). This peak was only observed in thetemperature range between 20 and 100 K. Also, theEx(B) exciton peak has been reported to appearat 2.570 eV in the low-temperature re#ectancemeasurements by Thomas et al. [2]. The emissionpeak at 2.5449 eV is thought to be associated withthe neutral donor bound exciton, (D3,X). This ex-citon is known to originate from the Cd excess, anda recombination from bound exciton to neutraldonor. This (D3,X) peak has been observed to be at2.5464 [17] or 2.5443 eV [13], respectively. Twopeaks having a weak intensity at 2.5149 and2.4772 eV are considered to be associated with thelongitudinal optical (LO) phonon replicas of theEx(A) exciton emission, because the energy di!er-ence between these peaks is equal to a multiple ofthe LO phonon energy [1, p.64], such as 0.0362 eV(1LO) and 0.0739 eV (2LO). In the PL measure-ment, the observation of the free exciton and itsassociated LO phonon replicas suggests that thecrystal grown in this laboratory is of high quality,because the emission peak of the exciton can onlybe observed when the interaction of the long rangecoulomb coupling between the electron and thehole exists. And the broad peak of Ex(A) was ob-served only in the PL spectrum measured at 293 Kwhereas other peaks did not appear in the longerwavelength regions. The broad peak of 2.0944 eV(592 nm) observed in the longer wavelength regionsis considered to be attributable to defects whichoriginated from the S-vacancy (or Cd-interstitial)atom.

3.3. The temperature dependence of the free exciton

As shown in Fig. 3, the PL spectra show that theposition of the emission peaks tend to shift toshorter wavelength regions with decreasing temper-ature, and the emission peaks continue to quenchwith increasing temperature.

As has been described above, the broad peak at2.4455 eV in the PL spectrum at 293K correspondsto the Ex(A) exciton emission. Generally, exciton isthermally dissociated when the thermal energy ex-ceeded the binding energy of exciton (k¹'Ex").Therefore, exciton is only observed when themeasurement temperature is satis"ed with the rela-tion ¹*Ex"/k. Thus, the lifetime of exciton reduc-es with increasing temperature and the linewidth ofits energy broadens according to the uncertaintyprinciple of *E"+/*q, where *q is the excitonlifetime. Conversely, the linewidth of exciton peaktends to be narrower due to the reduction of theimpurity scattering in the crystal with decreasingtemperature. Thereby, the exciton linewidth can bedetermined by measuring the exciton lifetimebroadening.

Fig. 4 illustrates the values of the excitonlinewidth in the temperature range between 20 and293K. This "gure contains the solid line expressed


Fig. 5. PL intensity quenching of the free exciton as a functionof temperature.

Fig. 6. Photocurrent spectra of the CdS single crystal measuredat di!erent temperatures.

by the values of the exciton linewidth derived fromToyozawa's equation [18] which is given by

>"A/(e+u@kT!1)#C, (1)

where C is the constant linewidth at low temper-ature and +u represents the average energy of thephonon participating in the quenching process. IfC of Eq. (1) is replaced by the exciton linewidthvalue of 7.3meV measured at 20 K, and +u is sub-stituted by the value of 38.4mV obtained from theexperimental data, the values caculated from Eq. (1)were plotted as the solid line in the "gure which "tswell the data of the exciton linewidth obtained fromthis experiment. The 38.4meV of +u is found to becorresponding to the LO phonon energy of CdS,which is known to be 38meV.

Fig. 5 shows the variance of the PL intensity ofthe free exciton participating in the quenching phe-nomenon over the temperature range from 20 to293K. The variance of the PL intensity as a func-tion of temperature is well expressed by [19]

IPL

"I1/[1#C

1¹3@2 exp(!e

1/k¹)]

#I2/[1#C

2¹3@2 exp(!e

2/k¹)], (2)

where C is the "tting parameter and e is the activa-tion energy. Fig. 5 shows that the values calculatedfrom Eq. (2) agree well with the experimental data

derived based on the two-step quenching processmodel. In this model, the activation energy is usedas two values such as 29.6 and 9.1meV. The activa-tion energy of 29.6meV corresponds with the ther-mal dissociation of the free exciton at the groundstate and is also in good agreement with the excitonbinding energy of 29.4meV.

3.4. Photocurrent spectra

Fig. 6 shows the spectra obtained from the PCwith monochromatic light polarized perpendicularto the c-axis of the (1 0 11 0) plane. The peaks at theshort and long wavelength region were observed.The peak at the short wavelength region corres-ponds to the Ex(A) exciton and the peak at the longwavelength region is thought to originate from thenative defects in the crystal. However, the peakscorresponding to the Ex(B) and Ex(C) exciton werenot observed in the PC spectra measured at thetemperature range from 293 to 20 K. The exciton isgenerated by the attraction between a hole and anelectron when the carrier concentration is low; theelectrons are scattered due to the mutual interac-tion between electrons, when the carrier concentra-tion is high [20]. Thus, this result is brought aboutby the fact that the exciton is scattered by the


Fig. 7. Experimental values of the free exciton and energy bandgap as function of temperature between 20 and 293K.

electrons, because the carrier density of the electronis of the order of 1016 cm~3. In the PC measure-ment, an amount of the PC decreased except nearthe Ex(A) exciton peak in the short wavelengthregion. When the light illustrate the sample, most ofthe incident light is absorbed at the surface of thesample and the electrons and holes generated bythe incident light disappear as result of mutualrecombination [21]. Only the excited electrons inthe valence band excited by the absorbed lighttransit from the !

9(A) of the valence band to the

Ex(A) below the conduction band. Immediately,the transitted electrons #ow out of both sides ofthe electrodes. Consequently, the PC peaks corre-sponding to the Ex(A) in the short wavelengthregion are guided to the electrodes.

3.5. The temperature dependence of the energy bandgap of CdS

Fig. 7 displays the energy band gap, the PL andthe PC peak position of the CdS as a function oftemperature. The energy band gap of the CdS mea-sured at 293K was 2.4749 eV. This value was in-

cluded as the exciton binding energy of 29.4meVthat was experimently determined irrespective ofthe temperature dependence. This band gap energyshows slightly larger than the value of 2.45 eV re-ported by Wang [22] at 300K but smaller than thevalues obtained by Pankove [23] at 300K andMadelung [24] at 293K, which are 2.53 and2.485 eV, respectively. Generally, the band gap en-ergy of CdS at room temperature is adopted to be2.42 eV.

The variance of the energy band gap as a func-tion of temperature is well given by [25]

E'"E

'(0)!a¹2/(b#¹), (3)

where a and b are constant. When a and b are givento be 3.06]10~3 eV/K and 2156, respectively, andthe band gap energy at 0 K, E

'(0), is estimated to be

2.5825 eV, the curve plotted by Eq. (3) is closelymatched with the experimentally measured valuesas shown in Fig. 7. The band gap energy of the E

'(0)

used in this plot is smaller than that of 2.5831 eVobtained by Aven et al. [26], and similar to thevalue of 2.5826 eV reported by Thomas et al. [27].

The CdS single crystal is a hexagonal structureand well known to have an anisotropy along thecrystalline c-axis. A crystal "eld is known to begenerated due to the anisotropy of the crystal.Many studies on the crystal "eld of the II}VI com-pound family has been carried out extensively byusing the following method such as the measure-ment of the spectra of the re#ectivity [28], thephotoconductivity [6,28], and the photocurrentmeasurement [29] at low temperatures. As hasbeen explained by Cho [30], if the valence band ofCdS is p-like and the conduction band is s-like atthe ! point, the conduction band including the spinhas a !

7symmetry, and the valence band splits into

double degenerate states with the symmetries of!9(A), !

7(B) and !

7(C). The uppermost one in the

symmetry of the valence bands is the !9(A), the

middle one is the !7(B), and the lower one is

the !7(C). As a result, its e!ective mass strongly

depends on the direction of k.In this work, the valence band splitting of the

CdS crystal by the crystal "eld has been observedwith the PL measurement. The valence band en-ergy of !

9(A) and !

7(B) are found to correspond to

the Ex(A) exciton energy and the Ex(B) exciton


energy, respectively, the exciton binding energy be-ing 29.4meV. The crystal "eld splitting, *c

3, is the

energy di!erence between the !9(A) and !

7(B) and

the spin orbit splitting, *S >O.

, is the energy di!er-ence between the !

7(B) and !

7(C). Based on the

above relations, *c3

was found to be 0.0196 eVcalculated from the di!erence between the Ex(A)energy of 2.5511 eV and the Ex(B) energy of2.5707 eV, in this experiment. The value of *

C3for

CdS has been found by several groups. Thomaset al. [2,4] obtained 0.016 eV utilizing photore#ec-tance measurement, Park et al. [6] found 0.018 eVusing photoconductivity measurement, and Shinet al. [29] reported 0.026 eV employing photo-current measurement. Also, Qi et al. [10] found0.019 eV and Blackmore [31] reported 0.02 eVusing photoluminescence measurement. Amongthem, our value show a close agreement with Qiet al. and Blackmore.

4. Conclusions

We have grown a single crystal of CdS withouta seed crystal using the sublimation method ina two-stage vertical electric furnace. From theX-ray di!raction pattern we determined that thesingle crystal of CdS exhibits a hexagonal structureand its c-axis is along the symmetry axis of thegrowth tube. The carrier concentration and mobil-ity obtained from Hall measurements at room tem-perature were 2.90]1016 cm~3 and 316 cm2/V s,respectively. The optical properties of the CdSsingle crystal have been investigated as a functionof temperature. In the PL measurement, the Ex(A)exciton, the Ex(B) exciton, and their LO-phononreplicas peak have been observed all together. Thisresult suggests that the crystal grown in this labor-atory has a high quality. Also, the thermal broaden-ing of the free exciton has been well described byToyozawa's theory. The average energy value of +uneeded to calculate Toyozawa's theory was decidedto be 38.4meV and this value was found to be ingood agreement with the LO-phonon energy of theCdS. From the PC measurement, only the Ex(A)exciton was observed because of the scattering pro-cessing between the free excitons and electrons.

The temperature dependence of the free excitonhas been well described by Varshni's relation. The

energy band gap of E'(¹) as a function of tem-

perature was derived to be expressed byE

'(¹)"2.5825 eV!(3.06]10~3)¹2/ (2156#¹).

This result was not found to follow the convec-tional linear relationship suggested by Bube. Andthe energy band gap of the CdS single crystalat room temperature has been determined to be2.4749 eV. Also, the crystal "eld splitting of thevalance band edges was observed and its energy hasbeen determined to be 0.0196 eV.

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the optical properties of cds crystal grown by the sublimation method

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