Low-temperature sensors based on telluride microcrystals

Sensors and Actuarors A, 30 (1992) 55-58

Low-temperature S S Varshava

55

sensors based on telluride microcrystals

and L N Pelekh

Lvov Polytechmcal Instrtute, ul Mra

12 Lvov-13, 290013 (l&ante)

V V Vamberg Imtrlute of Semxonductors,

Academy of Sciences of the Ukrarnran SSR, Prospect Nauki 45, KEV-28 2S2028 (Ukmme)

Abstract Cryogemc thermoreslstlve and magnetoreststlve sensors based on sermconductor mxrocrystals of Te, Te-Se sohd solutions, PbTe and PbSnTe have been developed The galvanomagnetlc propemes of the sensors are studled m a wide range of cryogemc temperatures The sensors have very small outhne dlmenslons and their performances m many cases may be described by suffinently simple formulae

Introduction Their small outline dlmenslons and a vanety of useful electrlcal properties make different whlskerlike mlcrocrystals very attractive for manufacturmg various electromc sensors Recent mvestlgatlons on the technology of whisker-hke crystals of semiconductors such as Te, Te-Se solid solutions, PbTe and PbSnTe [ 1,2] permit us to develop new cryogenic resistive sensors, which may be used as thermometers and for measurmg magnetic field strength In this paper we are concerned wth the temperature dependence of resistance over the range 300 to 1 5 K and the dependence of the magnetoresistance on magnetic field at hqmd hehum temperatures (T < 4 2 K) for the sensors under conslderatlon

Fabrication and experimental procedures The crystals of Te and Te-Se alloys containing up to 2 5 wt % Se, which were whisker- or nbbon-shaped, were grown m sealed quartz tubes by the vapour transport reaction method m the presence of bromme and at the crystalhzatlon zone temperature t, N 300 “C [l] The smallest acceptor dopant density was observed for the pure Te samples, its value was about 2 5 x 1015cmv3 (at 77 K) Using one crucible it was possible to

obtain several hundred crystals suitable for practical applications The boundary dimensions of the sensmg elements were (l-5) mm x (0 05-O 2) mm x (0 050 2) mm Leads attached to the samples for electrical measurements were made by electropulse welding of 30 pm diameter gold nucrowlre Sensors without encapsulation were made, as well as those with dfierent types of cases The PbTe and Pb, _-xSn, Te (with x N 0 2) crystals were grown by the same method at r, B 450 “C In this case we obtained bulk crystals and wafers up to 0 3 mm thick with face dlmenslons of up to 1 mm (rarely 2 mm) [ 21 The malonty earner denslties at room temperature were n x lOI cme3 and p x (3 2-5 6) x lOI cmm3, respectively Measurements of the reslstivlty p for all the senuconductor matenals studied were camed out by the four-probe method The resistance of actual sensors was measured by the two-probe method Furthermore, measurements of the Hall coefficient were carned out over the range 1 5 to 300 K and of the magnetoresistance under magnetic fields from 0 to 4 5 T at 4 2 K, as well as under fields from 0 to 1 5 T over the temperature range 300 to 1 5 K

Experimental results and discussion First, we consider results for the sensors based Elsewer Sequoia

on Te and Te-Se solid solution Typical cahbratlon curves of the sensors under conslderatlon are shown m Fig 1, and their mam parameters are hsted m Table 1 The wful feature of the Te and Te-Se sensors studied 1s the reslstlvlty versus temperature dependence, which can be written for the temperature range 1 7 to 30 K as p=A-BlgT

obtamed by extrapolating p(T) to lg T = 0, and those of B were calculated from the slope of p =f(lg T) By analogy with A from eqn ( 1) for resutlvlty, the values of R, for the real sensors tested by the two-probe method are calculated and presented m Table 1 as well It 1s found that there 1s a good correlation between A and B In general, as the content of Se m the Te-Se solution mcreases, the low-temperature thermal sensltmty at first decreases and then increases again The value of the free-carrier density obtained from Halleffect measurements m the Impurity region by temperature varies correspondmgly from 2 2 x lOI to lOI cm-3 The correspondmg Hall moblhties pLHat liquid hehum temperatures (1 e , T = 3 K) are 500 to 9000 cm* V-’ s-’ Thus one has a good opportunity to control the thermometric parameters by variation of the Se content From the magnetoreslstance studies (Fig 2) we found that at 4 2 K the magnetoreslstance (AR/R) dependence on the magnetic field B rangmg from 0 to 3 T was close to a quadratic form, above about 3 T it might be fitted by the linear function

(1)

where A, B = constants and T = absolute temperature (IL) Slmllar results were reported recently for Te [3] and for some other narrow-band semlconductors, for example InSb [4] This behavlour has not yet been reported for Te-Se solid solutions This temperature dependence IS very attractive for thermometric apphcatlons due to its simple form The values of A m eqn (1) and Table 1 were

AR/R

05

10

15

20

25

i_qT

Rg I Cahbratmn curves for sensors based on Te and Te-Se Curves are numbered as the specimens m Table I T = absolute temperature (K)

TABLE Specimen number

I 2 3

(2)

where a, b = constants, which are listed m Table 1 For the strongest field the magmtude of the magnetoreslstance IS more than 100% In the case of the pure tellurmm samples, the linear part of the R(B) dependence covers the range 0 3 to 35T In the case of PbTe and PbSnTe, the electrical properttes differ from those for Te and Te-Se The reslstlvlty versus temperature dependences m this case consist of two parts at low temperature (1 7 to 40 K) p IS nearly constant, and at T > 40 K p increases with temperature For PbTe the magnetoreslstance as a function of magnetic field 1s fitted by a linear expresslon over the range 0 to 1 6 T It should be noted that this characterlstlc

I 0

=a+bB

1 Parameters of Te and Te-Se sensors Magnetoresishve

parameters

parameters

Se content (wt %)

Thermoreslstwe A (Qcm)

B (a cm)

R, (Q)

(I (%)

b (%/T)

0 07 20

46 05 2 05

2 54 026

1800 260 215

-125 -32 5 -28 0

435 34 30

105

57

16

12

10

08

06

04 02

0

2

I

3

4

B(T)

FIN 2 Transverse magnetoresIstance YS magnetic field curves for Te and Te-Se sensors at 4 2 K (Curves are numbered as III Rg I )

clude that whisker-like crystals of such materials as Te, Te-Se solid solutions, PTe and PbSnTe are useful for apphcations as cryogenic sensors The sensors based on Te and Te-Se have an attractive reslstlvlty versus temperature dependence for cryogenic thermometry, which 1s described as R N lg T over a wide range of cryogenic temperatures These sensors are also useful for measurements of magnetic field strength at fixed temperature due to the simple forms of the magnetoreslstance versus magnetic field characterlstlcs (quadratic and linear) Moreover, these may detect a magnetic field direction, because of the well-known significant difference between longltudmal and transverse magnetoreslstances [51 Finally, it should be noted that the sensors considered above are characterized by very small dlmenslons and their performance 1s described m many cases by simple formulae

AR x

l-POTe(42K) I

020

0

“-P6Te(l6K) a-P6SnTe

02

04

06

(4

08

2K)

References

10

(2

B(T)

Fig 3 Transverse magnetoreslstance vs magnetic field curves for PbTe and PbSnTe sensors at hqmd hehum temperatures

weakly depends on temperature (Fig 3) This slmphfies the practical apphcatlon of such sensors for measurmg magnetic field strength as compared to those based on Te and Te-Se whlskets In the case of PbSnTe samples, the magnetoresistance versus magnetic field dependence has a more complicated form with an inflection at B = 0 S- 1 T and a significance variation of this dependence with temperature at T > 4 2 K and B>lT

Conclusions The results presented above allow us to con-

1 S S Varshava and L N Pelekh, The growth processes and properties of filament crystals of a sohd solution m tellurium, Izv Akad Nauk SSSR Neorg Mater , 21 ( 1985) 183 I- 1834 2 S S Varshava, S N Bekesha, I V Kurllo and L N Pelekh, Morphology and propertles of SnTe and PbTe crystals grown from the gas phase, Izv Akad Nauk SSSR Neorg Mater, 23 (1987) 1286-1290 3 T Ikan, T H Berger and F Levy, Electrical propertIes of vapour grown tellunum smgle crystals, Mater Res Bull, 21 (1986) 99105 4 K Komatsubara and Kodalra-shl, Thermometer for measurmg very low temperatures, US Parent No 3 473 385 ( OCI 21, 1986) 5 P N Gorley, V S Radchenko and V A Shenderovsku, Transport Propertres in Tellurium, Naukova Dumka, Kiev, 1987

Biographies Slavomtr Varshava received a doctor’s degree m 1978 He works at the research laboratory of Lvov Polytechmcal Institute on the physics and technology of whisker semiconductor crystals He also works on their practical apphcatlons as electronic sensors Lyubov Pelech graduated m 1976 from the Physics Department of Lvov State Umverslty Smce 1979 she has worked at the same laboratory

58

on the physics and technology of whisker semlconductor crystals Victor Vamberg received a doctor’s degree m 1984 Since 1976 he has worked at the Institute of Semiconductors, Academy of Sciences of the

Ukramlan SSR, Ukraine, on the technology and low-temperature electron properties of neutron transmutation doped germamum, combmmg this work with the development of low- and ultra-lowtemperature resistance thermometers based on different semiconductors