PDF Electrical thermal transport properties ofTIGaSez single crystals

Electrical

and

thermal transport properties ofTIGaSez single crystals

A. T. Nagat', S. E. Al Garnil, . S. Bahabril, G. Attia³, S. R. Al harbi ^l,

And _-\. A. AI Ghamdi2.

1- physics Department, Girls colleg2~ ofEdllcation, king Abdulaziz wtiversitv-KSA.

2- physics Department, Faculty ofSc:'21Ices, king Abdll/aziz lIlliversity-KSA . 3- physics Department, Fayolllll 1I11i'.2rsity, Egypt.

ABSTRACT. The electrical conducti·.~:y (u) and Hall coefticient (R_H) of single crystals prepared by a special modified Bri·:~an technique have been investigated over the temperature range 245-495 K. Our ::-o-.estigation showed that our samples are p-type conducting. The dependence of t::~ Hall mobility on temperature was presented graphically. The forbidden energy g2? was calculated and found to be 2.1 eV whereas the ionization energy of the impuricy level was 0.36 eV. The values of the electrical conductivity, Hall coefficient and c::=.rrier concentration at room temperature were 1.87"10-⁶

n-

^I cm", 3.98" 10⁹ cm' C' :a.,d 1.57x 10⁹ cm·^l respectively. The Hall mobility at room temperature (IlW was found to be 7.46x _{I 0' cm}² V·ls·'. Also, the thermoelectric power (TEP) was investigated in the t=perature range 271-493 K. The combination of the electrical and thermal measuremc:::r:s in the present investigation makes it possible to find various physical parameters SUCJJ as mobilities, effective mass, relaxation times, diffusion coefficients and diffusion le:::gths both for majority and for minority carriers, Also figure of merit was determined. These parameters reveal the general behavior of this semiconductor.

I-Introduction

The ternary semiconducting cha1coge:1ide with the fonnula ABX2(A, B represent metal atoms, X represents cha1cogen atoms) hay;: been studied intensively in recent years. Recently ternary thallium chalcogenides received a great deal of attention due to their opti.:al and electrical properties, in view of possible optoelectronic device application[I,2J. The Ternary compound TIGaSe2 belong to the class of I1I-III-Vh type semiconductors and have potential application[3,41. Due to this applicability :here is a need for studying its detailed physical properties. Much interest has recently OIl ternary chalcogenide TIGaSe2 compound_ which possesses both ferroelectric and semiconductor properties[5,6]. TIGaSe2 crystalEzes in monoclinic system and belongs to a space group of

c:

h at room temperature. The interest of these materials is simulated not only by their fundamental properties but also by possible practical application[7J. The photoconducti..-ity of TIGaSe2 single crystal were investigated in the temperature range 78-300 K[Sl. The photoluminescence (PL) spectra of III-III-VI::! single crystals (e.g. TIGaSe2, TIGaS2, TlInS2) were studied[9-11j. Also the spectral dependence of photoconductivity and the near band edge absorption was reported[12J. The thennal expansion properties have been studied[131. Moreoyer, gamma irradiation effect on the electrical properties[l-l] and the behavior of TIGaSez crystals near phase transition in static elect:ic field have also been reported[151. The low temperature photoluminescence (PL) and infrared (IR) spectra of TIGaSe2 crystals was published[II,16J. Ramman spectra of TIGaSe2 cr::.stal at different temperatures are discussed[I7J. T.le optical properties of layered single crYSTals of TIGaSe2 have been studied[lsJ. The dielecTric characteristic of TIGaSez was reportd^{fl9J •} In spite of all the above reported studies, E:erature still lacks of the infonnation abo'..lt Hall properties, the carrier effective masses, tte impurity le\'el, mobility of charge carrier:; 35 well as relaxation time, diffusion coefficien:,. diffusion length, and the dominat sc.:!ttering mechanisms in TlGaSe2 crystals. Thus tce aim of this \\"ork is to report these properties

\ l

2-Experimental details \

The TIGaSe2 compounds v;ere synthesized by fusing initial components consisting of\

extra pure elements (purity 99.999%). The crystal were synthesized and grvwn in eVJcuated\

quartz ampoules. The silica z::::poule was e\'acuated to 10-⁶ Torr and sealed under vacuum. '.

The ampoule, with its charge. was phced in a three-zone tube furnace which was designed and constructed in our laborc.:ories. More details about the apparatus, the electrical system and the mechanical system we:-e previously published[20J• The silica tube '.'."as kept for 10 h in the first zone, where the tempe:-ature was higher than the crystallization ter:1?erature. The melt was shaken during heating several times. Then the ampoule was drawn with a rate of 2 mmfh and allowed to enter the second zone in which the temperature was corresponding to the crystallization temperature[211. In the final stage as the ampoule began to enter the third zone, the solidification process was achieved, since the temperature inside this zone was below the crystallization temperature. T'1e duration for obtaining TlGaSe2 in single crystal form was about fourteen days. X-ray an?lysis and DTA investigation confirmed that TIGaSez is a single crystal. The XRD patterns show that these crystals have monoclinic structure with the lattice parameters of a=IO.756°A, b=1O.1730oA, c=15.596°A and P=99.92°A specimens for measurements were prepared for electrical conductivity and Hall effect in rectangular shape with mean dimensions 7x2.2x 1 mm³ . Ohmic contacts were formed on the specimen surfaces by means of silver paste and the ohmic nature of the contact was checked by recording the current voltage characteristic. The dc compensation method was adopted for measuring voltage without drawing appreciable current by using a Tensely UJ33E potentiometer As to the sensitivity of our potentiometer the error limits are not exceeding I%, All measurements were carried out under vacuum with 0.5 tesla magnetic field strength. The magnetic field was oriented parallel to the cleange plane i.e H 1. C ( where the C-axis is perpendicular to the cleavage plane)[3J. Details of the experimental arrangements and cryostat was described previollslyl22J• For measuring the thennoelectric power (TEP), the sample was prepared in a cylindrical shape. The length of sample shoud be as short as possible, bur the cross sectional area should be as large as possible. A t\VO parts holder was used for making the temperature difference along the crystal, i:1 a direction perpendicular to the natural cleavage plane, for investigation the thermoelectr.;:: power. The sample (5.6 mm length) was supported between the two holders. A temperatL:re gradient of about 5-10K was maintain~d by two electric heaters. One of them stands at one end of the sample and the other one surrounding the whole sample body. The accuracy of the measurement was enough because the potential difference and the temperature were measured simultaneously. Also these measurements were done under vacuum for preventing oxidation of the sample or water vapor effect. The temperature was measured with the aid of a calibrated thennocouple. Details of the apparatus, working chamber and method of measu;-ements have been published[23.241.

3-Results and discussion

Typical data presented in i"igure 1 show the conductivity as a function of temperature in the range 245-495 K. The CL:;-ve is quit similar to the simple semiconci:..:ctor behavior. It should be noted that the CUITe in figure I, three regions can be distinguished. Beginning from the low temperature, the elee-rical conductidty (0) increased slowly wit:: temperature, and this is due to the fact that the c 2rrier concentration in this region is determ ::1ed by the number of ionized acceptors liberated :-:om the impurity level. From this region the ionization energy was calculated, indicating th",' ~he acceptor le\"el lies 0.36 eV above the :Jp of the \alance band. The (i-T CUITe passes t::rough an intermediate region, 309-441. This is the transition from impurity to intrinsic con'::lcti'lity which depends on the carrier concentration and their mobilities (this will be clear if one observes both figures 3 and 4 in that te;nperature range I.

At temperature above 441 K, :..:"e conductivity increases rapidly because of the carrier being

2

excited from the extended state of the valance C 3.nd into the conduction band. So the width of the forbidden energy gap can be calculated. It

:5

found to be 2.1 eY. From figure I the value of G, at room temperature equals 1.87x 10-⁶ (fL:my'. Since Hall measurements.are important, figr..:re 2 is constructed in the temperature range 248-484 K. Figure 2 shows tfie dependence of RH against the temperature. The positive sip of Rll indicates that the majority carriers a~=

ho!::,s. It can be seen from figure 2 that the Hall coefficient decreases with the rise i:-r tem;>erature, but above 400 K it decreases very :ccpidly. Determination of the energy gap fro:-:l Ha:1 data is possible from this relation. The ba::-_j width of the energy gap calculated from slope of the curve in the intrinsic region was found to be 2.1 eY. The values of the Hail coefficient and carrier concentration at room te:-:lperature are 3.98x 10⁹ cm3('I and 1.57x 10'*

cm-³ respectively. The temperature dependence of the Hall mobility for TIGaSez is shown in figue 3. It was found that the exponent n iG the relation PH a Tn(below 330 K) is 1.5 indicating that the scattering of the carriers is i;dluenced by the impurities i.e the impurities play an important role in this range of tempera[,J.re. We wil! able to say the mobility increase with temperature. As temperature decreases, !he mobility due to impurity ion scattering decreases too. In the high temperature range (T >330 K), the carrier scattering mechanism is the scattering on thennal lattice vibration, which causes the mobility to decrease with the temperature increase. The mobility decreases a.::cording to the law fJ.H

a. r

7.2 This leads to the assumption that phonon scattering is dominate. At room temperature the Hall mobility is JlH =7.46 x 10³ cm y-Is·'. Figure 2 4 illustrates the variation of the current carrier density against the temperature. Figure 4 is helpful for understanding figure 1. Also from the basis of the relation Pi = (N N )1I2 _{c v} e-llEgl2KT =ce-~gI2KT . Thus the energy gap (Mg) can be calculated from the slope of the curve in the intrinsic region which is good agreement with the valt:e obtained from the conductivity and Hall effect \vork and also agrees with value in(3.251.The result for the temperature dependen;::e of TEP of single crystal of TIGaSe2 in the tem;>erature range 271-493 K are presented in figure 5. At the beginning of the curve TEP increases as the temperature rises, reaching a maximum value at a equal to 563 ~lYK-1 <:1

T=307. A sharp drop ofTEP is observed until it reaches 24 ~l\'K-1 at 343 K. The decrease of a vakes, at temperature higher than 307 K, is regarded as a result of the compensatio:1 processes. Above 343 K the TEP increases with increasing temperature. The obscm::d positive thermoelectric power value in the ent£:-e temperature range investigated result fro'-:1 the fact that the hole concentration is greater tbn that of electrons, that is, the m:1tcrial is P­

type_ This Agrees with result obtained from H311 effect data and -Bublished data{J·²⁵ 1. In the

inrrmsic region the

^TEP

:.0 ~etr;::d(:~::f:ll:Fi~~·ti::i-) 1

where K is Boltzmann's constant, b the ratio of mobilities, Mg the width of the forbidden oap 3.nd ^1/1' and m' are the effective masses of electrons and holes respectively. This fonnub.

o n p

precicts that a plot ora as a function of the reci;-rocal of the temperature in the intrinsic range sho·...:ld be a straight line. The plot of the thermoelectric power (a) as a function of the recqrocal of the temperature is shown in figure :1. Since equals 2.1 eV from Hall data ard we are assuming that

m:/m;

dose not \3ry with temperature, it was found th3:

.

746 10' ?V· ^{j -}

b ^:.i"1

I

f.1 _p ::: 1.32 .Thus, usmg f.1_{p :::} .' x . :m- s ^I at room temperature means th::.:

2 1

J.L 9.85xl0³ cm y. s· ^I .The ratio of electron lnd hole effective ~asses

m'/m'

^evaluated

is known that the themlOelectric pc·.\cr of a semiconductor, when one type of carriers dominates, is given by the following relation{27]

- K

r.,

^{I ( ph 3}

]J

a - -;;

l-- nl

² (2;:-II!:KT)3!2

plotting the above relation between a c:1d In T predicts that TEP increases with temperature in the temperature range corresponding :J the impurity region as shown in figure 7. From the intercept of the line (in the impurity region) with the a axis we got 111::::: 2.647 / 10.^3: ;:g.

Combining this value with the above r::::io 1II:/IlI~ allows us to determine the effective mass of the electrons. This value equals 2.247/' 10-^3" kg. The calculated values of the effecti \'c masses both for minority and for majority ca:ners can be used for determination of the relaxa!ion time for both current carners. This \'2.:ue for holes turns out to be 1.23 x 10·12s, \vhereas for electrons is equals 1.38x 10·15S. The diffusion coefficients for holes and electrons can be

2 1 2 1

deduced to be Dp=193.02 cm s· and D,,=254.79 cm s· respectively. The diffusion consta"t is inversely proportional to the effective mass of hole and electrons. So this result is quite logical. Another important parameter can be ~stimated, that is the diffusion length. The values of Lp and

Ln

were found to be L54xlO"cm and 5.93 xlO·⁷cm for hole and electrons respectively. Figure 8 depicts the dependence of the TEP on the natural logarithm of the charge carrier concentration. The main conclusion from this curve is that a decreases sharply and linearly as the concentration increase.

Figure 9 shows the dependence of the thermoelectric power coefficient on the natural logarithm of the electrical conductivit':,. The following relation can be applied

K [ ^r 2(2Jrm;KT)3/2 e

;l) 1

a = - A+In ^{3 -} Ina

e , (2;rh)

This behavior which govers the relati,:)n between the electrical conducti vity and tho: TEl' is similar to that of a versus p.

By using the results of measurements of the electrical conductivity 0, sc-:bcck coe nicknt ⁽¹ and published val uel2⁸] of thermal conductivity K, the figure of merit (Z>

ror

^{Tl< jaS::~ at r(lum}

temperature is calculated to be 9.62x I 0.¹¹ K! . This indicated that Ollr he,;t sample TI( ;:!:)C2

can be use as high efficiency thermoelectric element.

4-Conclusion

In the present paper, the electrical conductivity, Hall effect and themlOelcctric power of TIGaSe2 were reported. This work re".-ealed that crystalline thallium-gallium·diselenide h:ts a semiconducting nature with a P-type conductivity. It has an energy gap of 2.1 eV and activation energy of acceptors of 0.36 eV. Combination of the electrical conductivity, Hall effect and thermoelectric power data a!1O\vs us to deduce many important physical parameters such as the mobilities, effective mass-es, diffusion coefficient, diffusion length, as well as relaxation time for the majority and t;:e minority carriers. The scattering mechanism of the charge carners was discllssed in the ;:.-resent study. The efficiency of thennoelectr.c power conversion also investiaated throuah c::terrnination of figure of merit Z. _{, 0} ~

4

I: A A

A

"

'A~A~ \:0.

...^~~

~" A

Fig. (1) The temperature dependence of electrical conductivity (0) for TIGaSe2 single crystal.

...,.:

::..

' - ' ,-,

~;...

. -

- A

Fig. (2) The dependence of RHT^3/2 cgainst the temperature for T1GaSe2 single ;::rystal.

-

-

-

- -

A A

• A

Fig (3) Variation of In p with In T for a TIGaSe2 single crystal.

10

9

"

6 ­

5+---....--...-,---,

5A 56 5.8 60 6.

In T ( K)

Fig (4) Hall mobility 2.5 a function of temperature for TIGaSe: single crystal.

6

( , "

600

Fig (5) Temperature dependence ofTEP for TlGaSe2 single crystal.

600 ­

-.

400 ­

. . . ^.. .

^--

^.. ­

,-..

..

~ ...

:i

>

d ^'-'

200 ­

.

:

. .

.."" ^~

. .

-.

'.

'.'

'.

0

2.0 24 2.8 3.2 3.6 4.0

10]/ T ( K-¹⁾

Fig (6) Plot of (a) against 10 ³fT for TlGaSe: single crystal.

600

..

Fig (7) The relation behveen (a) and In T for TIGaSe2 single crystal.

(8) The dependence of the TEP on the natural logarithm of the charge carrier concentration for TIGaSe2 single crystal.

8

Fig (9) The dependence of the thennoelectric power coefficient ·:'TI the natural logaritl,.l' of t::e electrical conductivity for TIGaSe2 singk :rystaL

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