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Influence of sample geometry on the magnetic-field induced metal-insulator transition in nHgCdTe
0038-1098/87 $3.00 + .OO Perzamon Journals Ltd.
Solid State Communications,Vo1.63,No.7, pp.573-574, 1987. Printed in Great Britain.
ON THE
INFLUENCE OF SAMI'LE CEOMETRY MAGNETIC-FIEI,D INDUCPD METALINSUL~TOR
‘?RANSTTION
TN
n-HgCdTc
J. Gcbh_iit and G. Nimtz II.Physikalisches In&it& IJnivttit%t zu K&,
Z;$z)icher Str.77, Germany
(Received for publication 16 January by B. Miihlschlegel) We reprt oil measurementsof the Hall-resistivi~ as a fun&on of sample geometry liln-HgO8CdO 2Te. The resultsdemonsba+zsthat the data measure3 witiian n-HjCdTe' sample by Shayegan et al.' c@e recently;Irea mere consec,uenceof the sample geome+zy and do not representthe Hall resistivity.The ;nagneto-lmnsrortpropertiesof puce n-IIgCdTe are not iden+&& ?liththcee of n-InFb h ting magnetic fieldsand at low tem~perahrr-es.
Si-?cethe ~discove~~ of a :;lag~e~transport anoma!y in FLice,high qu&ty n-Hgg.8CdO 2Te there has been .strongint~erest in its understan%ng2-10. The ano:ralo~rs beiiatiour is manifestedby the fact that the measured longitudinalmagneto-resistivityis activatetl accordingto pzz = P oexp(a(B)/T),(where a is an activationternperabrre, dependtig on mai;netic wield! I:,) whms at the same time the Hall coefficient RH = P x$B remains essentially constant irrcle:+endent of both 1 and T or even decreasesas followsIYom Fig. 1 curve (a)2-10.Such a behaviour is oppositeto that otsezveo in pure n-InSb at the magnetic-field induced metal-insulatortransition (XT). In InSb both RH and P sz stronglyincrease
with B indicatinq the freeze-out of carders as more and more or them become localizedon donor&es'. The anomalous behaviour of n-HgCdTe was inde endently conWmed by several groups WC&%wideY-lo. Without prejudice to the differentinmtatins, the experimental data of all these independent s&lies incontestably show a co&t or decreasing RH(B) functionatlow temperatures. Quite recently, however, the situationchanged when Shayegan etaL1 pubhshed data, which seem to be identicalto these known &om pure n-InSb as seen in Fig. 2. Since internationallyother 1
/
If /
T= 1.4K /
2
4
3
B(T)5
P
6
rnsrl,
P
XY
30
//
(Rem)
/I (b) ,I 20
1c
4
L
5 B(T)
(1
0
Fig 2
FigI
Hall-&tie vs ma etic fLeld.(a) Sam?nand(b)sample4x2 ple12x2x0.5mm x 0.5 mm3. 573
I
I
I
1
2
3
B(T)
In
4”
Hall resistivityvs magneti &lii for nHgCdTe and n-InSbl. The ratlo of length E width of the HgCdTe sample was s 2 .
574
MAGNETIC-FIELD INDUCED METAL-INSULATORTRANSITION IN n-HgCdTe
groups have never ohserved such a behaviour with similarcrystals,the new resultapublishedby Shayegan et aL1 sound sensational and provocative.At the same time similar results were publishedin other journals by this groupll. The data were oblzrined from only a few sample, which moreover, did not have the proper geometry to accurately measure the Hall resistivitycomponent. Namely, the samples investigated had sizes where the ratio of length to width was only 2 11. Such a sample geometry is not appropriatefor an accurate Hall measurement, as quoted by Putley13 in a now classical book on Hall effect.The sample geometry even dominates eventually the measured pseudoHall resistivityin materials with strongly nonisotropic &v&y. Tri qered by the new and very interesting datalrFT we have studied the infLuence of the sample size on the mm v& of i+y. We have measured the results presented in Fig. 1 with a sample having a ratio of length to width of 6 (a) and another one with a ratio of only 2 (b) as & in 1,II" Obviouslyshorteninga sample does indeed generate a pseudo-~ xy whichincreases with B in
Vol. 63, No. 7
a way measured by Shayegan et al.llllas displayed in Fig. 2. The non-linearincreaseof %y with B is due to contributions from pzz and pxx giving the mi&aadingimprez&on that n-HgCdTe behaves as we are used to &om InSb. We would like to mention another important point in me asuting the bulk redstivitycomponents. The saturation of the nasistiviity components Pzz and P xx or of the pseudo-P,y in Fig. 2 is a consequence of an n+-layer on HgCdTe crystals, as was lirstshown by Stadler et aL14-". This low resistivity film evenmy dominates all the resisIivity components with increasing magnetic field and decreasingtempexaizre. Summarizing, there is a magnetittansportan* maly in n-HgCdTe: The Hall coef6cient and Pxy near the MIT are not identicalwith those of other pure narrow-gap se~ondu&xs as e. g. InSb (Fig 2)5,10,and the conductivitv is activated4r5r10. The ‘P . data published by -Shayegan et aLI," are notx?-&xesentative due to an improper sample geometry and therefore have to be discarded tim further discussions on the magnet&xanspxt propertiesof HgCdTe.
References M. Shayegan, V.J. Goldman, H. D. Drew, N. A. Q Fortune, J. S. Brooks, Solid State Comm. 817 (1986) G. Nimtz,
B. Schlicht,E. Tyssen, R. Domhaus, L. D. Haas, Solid State Comm. 2 669 (1979) B. Schlicht,G. Nimtz, Lecture Notes in Physics 1_5& 383 (1982) J. Gebhardt,G. Nimtz, B. Schlicht, J. P. Stadler, Phys. Rev. B32, 5449 (1985) G. Nimtz and J. GebhardL Proc. 18. Int Conf. Phys. Semicond. Strxckholm,August 11-15, 1986, page 1197; Ed. 0. En-m, World Scientific A. Raymond, J. L. Robert, R. L. AuJombard, C. Bousquet, D. Valassiades, M. Royer, Lechire Notes in Physics,152, 387 (1982) A. B. Aleinikov,P. I. Baranskii., A. V. Zhidkov, Solid State Comm. s 75 (1983) Yu. G. Arapov, A. B. Davydov, M. L. Zvereva, Sov. Phys. SeV. I. Stiifeev, M. Tsidil'kovskii, micond. 11, 885 (1984) G. de Vos, F. Herlach,H. W. Myron, J. Phys. C s 2509 (1986) 10 B. A. Aronzon, A. V. Kopylov and E. Z. Meilik-
hov, Sov. Phys. Semicond. a 515 (1986) 11 M. Shayegan, H. D. Drew, D. A. N&son, P. M. Tedrow, Phys. Rev. B 3l_,6123 (1985) M. Shayegan, H. D. Drew, D. A. Nelson, P. M. Tedrow, Phys. Rev. B g 6952 (1985) V. J. Goldman, M. Shayegan, H. D. Drew, Phys. Rev. Lettersz 1056 (1986) V. J. Goldman, H. D. Drew, M. Shayegan, Proc. 18. Int. Conf. Phys. Semicond, Strzkholm August II-15 (1986) 12 Prof. D. Drew, private communication,September 3rd 1986 13 E. H. Put&y, The Hall Effectand Related Phenomena, ButterworthLondon (1960),p. 42-62 I4 J. P. Stadler,G. Nimtz, B. Schlicht,G. Remenyi, Solid State Comm. g 67 (1984) 15 J. P. Stadler,G. Nimtz, H. Maier,J. Ziegler, J. Phys. D B 2277 (1985) 16 G. Nimiz, J. Gebhardt,B. Schlicht, J. P. Stadler, Phys. Rev._.Letterss 443 (1985) I7 J. P. Stadler,G. Nimtz and G. Remenyi, SoBci State Comm. s 459 (1986)
Solid State Communications,Vo1.63,No.7, pp.573-574, 1987. Printed in Great Britain.
ON THE
INFLUENCE OF SAMI'LE CEOMETRY MAGNETIC-FIEI,D INDUCPD METALINSUL~TOR
‘?RANSTTION
TN
n-HgCdTc
J. Gcbh_iit and G. Nimtz II.Physikalisches In&it& IJnivttit%t zu K&,
Z;$z)icher Str.77, Germany
(Received for publication 16 January by B. Miihlschlegel) We reprt oil measurementsof the Hall-resistivi~ as a fun&on of sample geometry liln-HgO8CdO 2Te. The resultsdemonsba+zsthat the data measure3 witiian n-HjCdTe' sample by Shayegan et al.' c@e recently;Irea mere consec,uenceof the sample geome+zy and do not representthe Hall resistivity.The ;nagneto-lmnsrortpropertiesof puce n-IIgCdTe are not iden+&& ?liththcee of n-InFb h ting magnetic fieldsand at low tem~perahrr-es.
Si-?cethe ~discove~~ of a :;lag~e~transport anoma!y in FLice,high qu&ty n-Hgg.8CdO 2Te there has been .strongint~erest in its understan%ng2-10. The ano:ralo~rs beiiatiour is manifestedby the fact that the measured longitudinalmagneto-resistivityis activatetl accordingto pzz = P oexp(a(B)/T),(where a is an activationternperabrre, dependtig on mai;netic wield! I:,) whms at the same time the Hall coefficient RH = P x$B remains essentially constant irrcle:+endent of both 1 and T or even decreasesas followsIYom Fig. 1 curve (a)2-10.Such a behaviour is oppositeto that otsezveo in pure n-InSb at the magnetic-field induced metal-insulatortransition (XT). In InSb both RH and P sz stronglyincrease
with B indicatinq the freeze-out of carders as more and more or them become localizedon donor&es'. The anomalous behaviour of n-HgCdTe was inde endently conWmed by several groups WC&%wideY-lo. Without prejudice to the differentinmtatins, the experimental data of all these independent s&lies incontestably show a co&t or decreasing RH(B) functionatlow temperatures. Quite recently, however, the situationchanged when Shayegan etaL1 pubhshed data, which seem to be identicalto these known &om pure n-InSb as seen in Fig. 2. Since internationallyother 1
/
If /
T= 1.4K /
2
4
3
B(T)5
P
6
rnsrl,
P
XY
30
//
(Rem)
/I (b) ,I 20
1c
4
L
5 B(T)
(1
0
Fig 2
FigI
Hall-&tie vs ma etic fLeld.(a) Sam?nand(b)sample4x2 ple12x2x0.5mm x 0.5 mm3. 573
I
I
I
1
2
3
B(T)
In
4”
Hall resistivityvs magneti &lii for nHgCdTe and n-InSbl. The ratlo of length E width of the HgCdTe sample was s 2 .
574
MAGNETIC-FIELD INDUCED METAL-INSULATORTRANSITION IN n-HgCdTe
groups have never ohserved such a behaviour with similarcrystals,the new resultapublishedby Shayegan et aL1 sound sensational and provocative.At the same time similar results were publishedin other journals by this groupll. The data were oblzrined from only a few sample, which moreover, did not have the proper geometry to accurately measure the Hall resistivitycomponent. Namely, the samples investigated had sizes where the ratio of length to width was only 2 11. Such a sample geometry is not appropriatefor an accurate Hall measurement, as quoted by Putley13 in a now classical book on Hall effect.The sample geometry even dominates eventually the measured pseudoHall resistivityin materials with strongly nonisotropic &v&y. Tri qered by the new and very interesting datalrFT we have studied the infLuence of the sample size on the mm v& of i+y. We have measured the results presented in Fig. 1 with a sample having a ratio of length to width of 6 (a) and another one with a ratio of only 2 (b) as & in 1,II" Obviouslyshorteninga sample does indeed generate a pseudo-~ xy whichincreases with B in
Vol. 63, No. 7
a way measured by Shayegan et al.llllas displayed in Fig. 2. The non-linearincreaseof %y with B is due to contributions from pzz and pxx giving the mi&aadingimprez&on that n-HgCdTe behaves as we are used to &om InSb. We would like to mention another important point in me asuting the bulk redstivitycomponents. The saturation of the nasistiviity components Pzz and P xx or of the pseudo-P,y in Fig. 2 is a consequence of an n+-layer on HgCdTe crystals, as was lirstshown by Stadler et aL14-". This low resistivity film evenmy dominates all the resisIivity components with increasing magnetic field and decreasingtempexaizre. Summarizing, there is a magnetittansportan* maly in n-HgCdTe: The Hall coef6cient and Pxy near the MIT are not identicalwith those of other pure narrow-gap se~ondu&xs as e. g. InSb (Fig 2)5,10,and the conductivitv is activated4r5r10. The ‘P . data published by -Shayegan et aLI," are notx?-&xesentative due to an improper sample geometry and therefore have to be discarded tim further discussions on the magnet&xanspxt propertiesof HgCdTe.
References M. Shayegan, V.J. Goldman, H. D. Drew, N. A. Q Fortune, J. S. Brooks, Solid State Comm. 817 (1986) G. Nimtz,
B. Schlicht,E. Tyssen, R. Domhaus, L. D. Haas, Solid State Comm. 2 669 (1979) B. Schlicht,G. Nimtz, Lecture Notes in Physics 1_5& 383 (1982) J. Gebhardt,G. Nimtz, B. Schlicht, J. P. Stadler, Phys. Rev. B32, 5449 (1985) G. Nimtz and J. GebhardL Proc. 18. Int Conf. Phys. Semicond. Strxckholm,August 11-15, 1986, page 1197; Ed. 0. En-m, World Scientific A. Raymond, J. L. Robert, R. L. AuJombard, C. Bousquet, D. Valassiades, M. Royer, Lechire Notes in Physics,152, 387 (1982) A. B. Aleinikov,P. I. Baranskii., A. V. Zhidkov, Solid State Comm. s 75 (1983) Yu. G. Arapov, A. B. Davydov, M. L. Zvereva, Sov. Phys. SeV. I. Stiifeev, M. Tsidil'kovskii, micond. 11, 885 (1984) G. de Vos, F. Herlach,H. W. Myron, J. Phys. C s 2509 (1986) 10 B. A. Aronzon, A. V. Kopylov and E. Z. Meilik-
hov, Sov. Phys. Semicond. a 515 (1986) 11 M. Shayegan, H. D. Drew, D. A. N&son, P. M. Tedrow, Phys. Rev. B 3l_,6123 (1985) M. Shayegan, H. D. Drew, D. A. Nelson, P. M. Tedrow, Phys. Rev. B g 6952 (1985) V. J. Goldman, M. Shayegan, H. D. Drew, Phys. Rev. Lettersz 1056 (1986) V. J. Goldman, H. D. Drew, M. Shayegan, Proc. 18. Int. Conf. Phys. Semicond, Strzkholm August II-15 (1986) 12 Prof. D. Drew, private communication,September 3rd 1986 13 E. H. Put&y, The Hall Effectand Related Phenomena, ButterworthLondon (1960),p. 42-62 I4 J. P. Stadler,G. Nimtz, B. Schlicht,G. Remenyi, Solid State Comm. g 67 (1984) 15 J. P. Stadler,G. Nimtz, H. Maier,J. Ziegler, J. Phys. D B 2277 (1985) 16 G. Nimiz, J. Gebhardt,B. Schlicht, J. P. Stadler, Phys. Rev._.Letterss 443 (1985) I7 J. P. Stadler,G. Nimtz and G. Remenyi, SoBci State Comm. s 459 (1986)