Fast semiconductor thermoelectric devices

Fast semiconductor thermoelectric devices

Sensors and Actuators, FAST SEMICONDUCTOR U BIRIUIOLZ, 179 12 (1987) 179 - 184 R FETTIG THERMOELECTRIC DEVICES* and J ROSENZWEIG Xnstdut fur ...

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Sensors and Actuators,

FAST SEMICONDUCTOR U

BIRIUIOLZ,

179

12 (1987) 179 - 184

R FETTIG

THERMOELECTRIC

DEVICES*

and J ROSENZWEIG

Xnstdut fur angeumndte Phystk der Unavemtat Karlsruhe, Kaaserstrasse 12, Karlsruhe (F R G )

O-7500

(Recewed

May 21, 1986, m rewsed form October

3,1986,

accepted December

17,1986)

Abstract

The parameters of a thermoelectnc radlatlon detector on a BlzTeJ basis were calculated Maxunum speclflc detectlvlty requires a special geometic relation between the receiver area and the thermocouple leg dunenslons. To obtam a response tune m the masecond range, thermocouple legs approxunately 100 pm long have to be produced. A flash evaporation equipment for the preparation of small thermocouples was developed, allowmg the deposltlon of films up to 200 pm thick Senuconductor alloys of the system B12_ $3b,Tes+,, were used for p- and ntype legs A mlcrothermocouple with film thickness of 100 pm exhlblted a response time of 10 ms when used as a Peltler element

1. Introduction According to Fig 1, thermocouples can be operated either as heat pumps (Peltrer elements) or as heat engmes (Seebeck elements) Accordmg to the [email protected] theory, the efficiency of the elements mcreases with the dnnenslonlessnumber M=

&J

-T K

where (Y 1s the thermoelectnc power, u 1s the specific electical conductivity and K 1s the specific thermal conductwlty of the thermocouple legs Usmg semiconductor alloys based on B12Te3,a value M = 1 can be obtamed at room temperature [l] For fast response of the thermocouples, a small heat capacity, I e , small geometnc dlmenslons of the thermocouple legs, IS required *Paper presented Technical Conference), 0250-6874/87/$3

50

at SensorsBad Nauhelm,

Technology and Apphcatlons F R G , March 17 - 19,1986 0 Elsewer Sequola/Pnnted

(3rd

International

m The Netherlands

180 Pettier

element

Seebeck

load

Fig

1 Senuconductor

element

reststance

thermocouple

used as heat pump or heat engme respectively

2. Prmclples for optunum dunenslonal deslgro of thermoelectnc radmtlon detectors The problems related to optunum desw of fast thermoelectic devices ml1 now be discussed for the example of a Seebeck micro-element to be used as an mfrared radlatlon detector [2,3] Accordmg to Fig 2, a black area A receives ra&atlon mth heat current den&y JH This current generates a temperature difference AT = Th - T, along the thermocouple legs with length 2 and cross section A*/2. The relevant parameters of the thermoelectic radlatlon detector are summmed m Table 1 idrived

luckrecewwureo

tat

Fig

2 Thermoelectric

radlatlon

A

electrode

detector

The spectilc detectlvlty D* can be calculated as a function of the quantlty A*/Z, which characterizes the geometic dlmenslons of the thermocouples For this purpose we use a formula for the thermal res&ance RH=

1 4asAT3 + K:

%s follows from the balance equation of the total heat current absorbed by the receiver area of the thermocouple JHA = 4osAT3AT

+ K$

AT

where os IS the Stefan-Boltzmann constant The electrical resistance of the thermocouple is

181 TABLE

1

Parameters of a radlatlon detector according to Fig 2 (k, Boltzmann’s constant, T, absolute temperature, RH, thermal resistance, R,, electrlcal resistance, Af bandwldth of the ampllfier,Ca heat capacity of the thermocouple, C,, speclflc heat capacity per umt volume) The thermoelectric power of the detector (S-Q IS assumed to be 2cr, wrth arp= -G = Q, where ap and QL,are the Seebeck coefflclents of the p- and n-type semxonductor legs Parameter

Defmltlon

Formula

Unit

Signal voltage (d c) Noise voltage

v,

v, = 2aAT = 2CK&&A

V

v, = (4kTR,&f)“*

V

2adT r= -

VW-’

Responslvlty

=2&H

JHA NEp

Noise eqmvalent power

(4kTRdf

r

D=

1 NEP

D=

Specific detectlvlty

D* = D(AAf)“2

D*=

T =

R&w

)l’*

W

2mH

Detectlvlty

Response time

=

2-H

(4kTR&f)l’* Pc~.R~A”~ (4kTR,)1’2

-j-z

CV

-

l2

w-1

cm Hz”~

W-l

s

2K

R,=

41 OA*

Insertmg these relations obtain

2 4a,AT3

+ K:

into the formula for the specific detectlvlty,

we

(kT)1’2

The speclflc detectlvlty D* exhibits a maxunum If the dunenslons of the detector are chosen to sat&y the maxlmlzmg relation

Accordmg to ths condltlon the heat current through the thermocouple legs equals the heat current re-emitted by radlatlon from the receiver area In this case the maxunum value of the detectlvlty 1s

182

This result may be compared with the general lnnlt of the detectlvlty for thermal detectors associated mth the fluctuations of the background radlatlon [2, 33

D*Iim

1 =

4(askT5)1’2

Hence the specific detectlvvlty of the optmuzed detector can be written as M”/2

D*max =-D

2

llm

thermoelectmc

radiation

*

As mentioned above, thermoelectmc fore at 300 K a speclflc detectlvlty

material with M = 1 1s avmlable There-

D*max = 0 5Dllm* = 1 X lOlo cm Hz~‘~ W-l

may be realized For a good infrared detector not only a high detectlvlty D* but also a small response time T 1s required This tnne 1s a function of the length I of the legs According to Table 1, the response time of the detector IS proportional to 12 Hence the legs of the thermocouple should be as short as possible The influence of the contact resistance, however, requires a finite length of the legs A reahstlc value IS I = 100 pm, correspondmg to T = 5 ms m the case of B12Te3 alloys For this value of I, the maximum detectlvlty can be achieved if the ratlo of thermocouple cross section to the recewer area 1s A*/A = lop3 m order to satisfy the maxnnlzmg relation deduced above Thermocouple legs with such dlmenslons can hardly be prepared by mechanical treatment of the semlconductor matenal Therefore mlcrotechmques such as vacuum deposltlon, etching etc have to be consldered 3. Preparation of mlcrothermocouples

with low response tune

We prepared thermocouples of the required dlmenslons usmg flash evaporation techniques This method allows the deposltlon of blsmuththe components of which exhlblt different vapour antunony-tellundes, pressures We developed the high-vacuum flash evaporation equipment shown m Fig 3 Usmg this apparatus, we could achieve deposltlon layers vvlth a maximum thickness of 200 E.crn Ground material with a composltlon according to Table 2 falls from a contamer through a movmg aperture on a slide and finally through the substrate vertically mto a hot crucible The crucible temperature IS mamtamed at 700 “C Tungsten chips inside the crucible enlarge the hot surface and prevent the grams from being expelled dvectly The composl-

183

\ ?F

st.tbsfmre’

wrfh hwfef WUfW

coo/Jfg

crucrble

[email protected]

sptral

tungsfen

chrps



Wg 3 Flash evaporation equipment for the preparation of thick layers

TABLE 2 Thermoelectric properties of the deposited layers Starting material

%T%

7

Blo dQSb136Te3 7

-140 160

640 610

1 3 x 10” 1 3 x 10”

03 04

tlon of the vapour produced by the flash evaporation of the grams 1s similar to that of the startmg mater& The thermoelectnc power and the electzxal reastance of the layers were measured durmg the depoatlon process at a substrate temperature of 180 “C The results are hsted m Table 2 The achieved data for M correspond to the flash deposltlon results reported m refs. 4 and 5 The construction of a three-dlmenslonal thermocouple usmg the pand n-type evaporated legs requires very good electrxal contacts Sliver IS the best mater& for the bridge electrode To prevent the sfiver from dlffusmg mto the semiconductor, we deposited a barner of nickel An addltlonal layer of bismuth reduces the specrflc contact resistance to lo-’ Q cm* Figure 4 shows the construction and dlmenslons of a thermocouple prepared

184

10

Ftg 4 ConstructIon of a fast thermocouple produced by evaporation Fig 5 Temperature difference AT across the Peltler mxro-element U, under pulsed current I

and element voltage

m this way. A thermocouple mth these geometrrcal dunenslons ISespectiy suxtable for apphcatlon as a Peltler micro-element If a pulsed current of rectangular waveform flows through the thermocouple, the bmdge temperature changes penodrcally (Fig 5(a)) From the voltage across the element (Fig 5(b)), which contams an ohmic drop and a thermoelectnc voltage, the response tune can be determined. In I&B case we obti r = IO ms The thermal conductmltles were calculated from a numerical fit (Table 2) For apphcatlon as mfrared radlatlon detectors, the dlmenslons of the thermocouples have to be changed accordmg to the condltlons estabhshed m Section 2

References U Blrkholz, Thermoelektnsche Bauelemente, m W Heywang (ed ), Amorphe und polykrastallme Halblezter, Spxmger, Berlm, 1984, p 77 R C Jones, The ultrmate sensltlvlty of radlatlon detectors, m R D Hudson and J W Hudson(eds ), Infrared Detector& Dowden, Hutchmson and Ross, Stroudsburg, PA, USA,1975,p 283 E H Putley, Thermal detectors, m R J Keyes (ed ), Optrcal and Infrored Detectors, Sprmger, Berlin, 1977, p 71 J W C Harpster, Proc 2nd Inf Conf Themoelectracal Energy Comers, Arlmgton, TX, 1978, IEEE, New York, 1978, p 43 B Chmagha, F Vallana and G Boslo, Energy Comers, 16 (1976) 55