Research Article The Insulator to Superconductor Transition ...downloads.hindawi.com/journals/acmp/2015/963768.pdfResearch Article The Insulator to Superconductor Transition in Ga-Doped

Research ArticleThe Insulator to Superconductor Transition inGa-Doped Semiconductor Ge Single Crystal Induced bythe Annealing Temperature

Y B Sun12 Z F Di1 T Hu1 and X M Xie1

1State Key Laboratory of Functional Materials for Informatics Shanghai Institute of Microsystem and Information TechnologyChinese Academy of Sciences Shanghai 200050 China2University of Chinese Academy of Sciences Beijing 100049 China

Correspondence should be addressed to T Hu thumailsimaccn

Received 26 December 2014 Revised 8 March 2015 Accepted 19 March 2015

Academic Editor Viorel Sandu

Copyright copy 2015 Y B Sun et alThis is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

We have fabricated the heavily Ga-doped layer in Ge single crystal by the implantation and rapid thermal annealing method Thesamples show a crossover from the insulating to the superconducting behavior as the annealing temperature increases Transportmeasurements suggest that the superconductivity is from the heavily Ga-doped layer in Ge

1 Introduction

Since the superconductivity was observed in metal in 1911intensive efforts have been performed to find new super-conductor such as high 119879

119888cuprate [1] and pnictide [2] The

application of superconductor on the traditional industrynow motivates scientists to find the superconductivity insemiconductor materials since it can solve the problemof power consumption of integrated circuit and improvethe efficiency of the device As the superconductivity isinduced in the diamond doped with boron [3] and the silicondoped with boron [4] the Ga-implanted germanium [5]with a total dose of 2 times 1016 cmminus2 Ga was recently foundto show superconductivity at the low temperature with thesuperconducting transition temperature lower than the ger-manium heavily doped with 4 times 1016 cmminus2 Ga [6] The originof superconductivity in the heavily doped semiconductorshowever is an open question yet even after the intensivestudy [6ndash8] In this work we fabricated the heavily Ga-dopedGe with a 30 nm SiO

2cover layer by implanting Ga into the

Ge single crystals and performed the rapid thermal annealing(RTA) on these samples After performing the scanningelectron microscope (SEM) and transport measurement wefind that the annealing temperature can induce an insulator

to superconductor transition in Ga-doped Ge samples Ourstudy of superconductivity in the Ga-implanted Ge combinesthe superconductor and semiconductorwhichwill contributeto application of superconductor on the traditional semicon-ductor industry

2 Experimental Details

We chose the (1 0 0) oriented and P doped germanium asthe substrate in this experiment On the top of the germa-nium we grew a 30 nm SiO

2cover layer through plasma-

enhanced chemical vapor deposition (PECVD) After thatwe implanted 4 times 1016 Ga cmminus2 at an ion energy of 100 keVinto it Here the SiO

2capping layer can prevent the surface

degradation [6] except the implanted region during implan-tation as shown in Figure 1

We annealed the Ga-doped Ge samples by the rapidthermal annealing (RTA) for 60 s in flowing Ar atmosphereat various temperatures The RTA was reported to play animportant role in recrystallizing the amorphous implantedlayer which leads to the formation of the highly Ga-dopedlayer at the SiO

2Ge interface [6 8] We therefore investigate

the surface structure of the samples by the scanning electronmicroscope (SEM) The surface degradation does not occur

Hindawi Publishing CorporationAdvances in Condensed Matter PhysicsVolume 2015 Article ID 963768 4 pageshttpdxdoiorg1011552015963768

2 Advances in Condensed Matter Physics

(a) (b)

Figure 1 Surface structure investigated with SEM in the (100) oriented Ge single crystal which exposed at 4 times 1016 cmminus2 Ga dose at an ionenergy of 100 keV (a) As-implanted sample (b) Sample annealed at 800∘C RTA in the flowing 119860119903 atmosphere

10

1

01

R(kΩ)

Unannealed

300∘C

500∘C

600∘C

700∘C

780∘C

800∘C 830

∘C850

∘C

0 100 200 300

T (K)(a)

3 4 5 6 7 8(b)

100

098

096

R(T)R(Tc)

Tc

T998400c

600∘ C 700∘ C

780∘ C

800∘ C

830∘ C850

∘C

(c)

600∘C

700∘C

780∘C

800∘C

830∘C

850∘C

T (K)100 150 200 250 300

R(kΩ)

24

22

20

18

Figure 2 (a) 119879 dependence of the sheet resistance 119877◻for samples with the different annealing temperatures (b)The enlarged plot of (a) near

the superconducting transition temperature 119879119888 (c) The thermal hysteresis curves for the samples annealed at temperature window (600∘Cndash

850∘C) The arrows show the direction of the thermal cycle

on the surface of the sample because of the protection of theSiO2capping as discussed before But there are many discrete

holes on the surface as a result of high dose implantationFor the as-implanted samples the holes are discrete islandscontaining gallium but for the samples annealed at 800∘Cthe gallium islands inside the holes percolate which can beregarded as superconducting grains of gallium percolating inthe normal state germanium matrix

To clarify the properties of the different annealed sampleswe performed the electronic transport measurements onthose annealed samples We contacted the electrodes bythe bonding lines and carried out the measurements inthe usual four-terminal geometry at the temperature from25 K to 300K in a physical property measurement system(PPMS) The resistance was measured by ETO option whoseexcitation current is 100 120583A The temperature dependence of

resistance in the as-implanted state and annealed at varioustemperatures under Ar flowing atmosphere is presented inFigure 2

Figure 2(a) is the 119879 dependence of the sheet resistance 119877◻

for the unannealed and annealed samples The 119877◻s decreases

as the annealing temperature increasesThe logarithmic scaleof 119877◻here shows how dramatic the electronic transport

can be influenced by the annealing temperature For dif-ferent annealing temperature it shows a transition fromthe insulating to superconducting behavior Upon coolingthe as-implanted samples show an increasing resistancewhich could reflect intrinsic semiconductor property of Gewhile upon increasing the annealing temperature the sheetresistance decreases which could be due to the change ofthe discrete Ga islands to percolating ones as discussedabove

Advances in Condensed Matter Physics 3

10

08

06

04

02

R(H)R(0)minus1

T = 25K300

∘C

600∘C

700∘C

780∘C

800∘C

830∘C

850∘C

minus12 minus8 minus4 0 4 8 12

H (kOe)

(a)R(Ω)

6

5

4

T (K)

1T

H = 0T

60

55

50

45

40

35

Tc(K

)

0 2 4 6 8

H (kOe)

2 3 4 5 6

(b)

Figure 3 (a) The 119867 dependence of normalized magnetic resistance (119877(119867)119877(0) minus 1) measured at 119879 = 25K for the samples annealed attemperature ranging from 300∘C to 850∘C (b) 119879 dependence of the sheet resistance 119877

◻at various magnetic fields 119867 applied perpendicular

to the surface for the samples annealed at 800∘CThe inset shows the field-temperature (119867 minus 119879) phase diagram

At the annealing temperatures ranging from 600∘C to850∘C the samples show a superconducting transition below7K as shown in Figure 2(b) There is a gradual drop ofsheet resistance but zero resistance is not possible becauseof the discontinued superconducting path in the sampleWhen focusing on the low temperature transport carefullywe observed a detailed dependence of the superconductingproperties on the annealing conditions There is an optimalannealing temperature window (600∘Cndash800∘C) Out of thattemperature window the superconductivity is weak Thereare two critical temperatures in the samples with annealingtemperature at 830∘C and 850∘C

It was shown in Figure 2(c) that the samples with super-conducting behavior display a thermal hysteresis loop duringthe thermal cycle and the arrows mark the direction of thosethermal cycles The thermal hysteresis observed here showsthe presence of the first order phase transition which couldbe attributed to the phase transition of the Ga grains fromendothermic and exothermic process [9] The gallium withexothermic process actually shows a similar superconductingtransition temperature [10 11] as the 119879

119888in Figure 2(b) The

superconducting gallium grains actually play an importantrole in the process Based on [9] the 120573-Ga 120574-Ga and 120575-Ga show the similar hysteresis loop in the single-energyX-ray absorption measurements and the reason is theirendothermic and exothermic process during thermal cycleSo we attribute the hysteresis loop to the gallium and relatethe superconductivity to the hysteresis loop Particularly the119879119888of 120573-Ga is 6 K It therefore clarifies the reason that the

superconducting behavior is related to thermal hysteresis asobserved in this experiment

To obtain the further understanding of the character ofsuperconducting state we measured the magnetic field 119867dependence of the sheet resistance of the samples annealedfrom 300∘C to 850∘C The normalized magnetoresistance(119877(119867)119877(0) minus 1) is plotted in Figure 3(a) (The data isshifted for clarity) Here the 119867 is perpendicular to the ab-plane of samples As shown in Figure 3(a) the samplesshow the largest magnetoresistance variation at the annealingtemperature window between 600∘C and 800∘Cwhichmightsuggest the strongest superconducting character there Wealso performed the 119879 dependence of sheet resistance 119877

◻

measurement for the sample annealed at 800∘C with theapplied magnetic fields 119867 perpendicular to the 119886119887-plane ofthe sample (Figure 3(b)) It is shown in Figure 3(b) that thesuperconducting transition is suppressed by the applied field119867We can define the critical temperature119879

119888as the 2 drop of

normal state resistance for the sample measured at different119867 The inset in Figure 3(b) is the 119867 minus 119879 phase diagram ofthe samplesThe119867

1198882(0 K) at zero temperature was estimated

as the 10 drop of 1198671198882at 2 K which is approximately 05119879

Such a criterion is the same as the early work [6] By using thestandard theory we can calculate the value of the Ginzburg-Landau coherence length 120585GL We deduce the 120585GL = 26 nmvia 1206010= 2120587119867

11988821205852

GL where 1206010 is the magnetic flux quantum

3 Summary

In summary we have fabricated heavily Ga-doped Ge with a30 nm SiO

2cover layerThe ion implantation and subsequent

RTA annealing were employed to form a Ga-doped layerat the SiO

2Ge interface The RTA annealing enables Ga to


redistribute in the sample and realize the superconductingcircuits Ga forms metallic precipitates in the Ge matrixWith increasing annealing temperature the samples showa crossover from the insulator to the superconductor The119879119888is the same with the galliumrsquos transition temperature and

the hysteresis loop is related to gallium So the electronictransport measurements indicate that the Ga-doped layerplays a leading role in the superconductivity

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgment

This work is supported by the ldquoStrategic Priority ResearchProgram (B)rdquo of the Chinese Academy of Sciences Grant noXDB04040300

References

[1] J G Bednorz and K A Muller ldquoPossible high 119879119888supercon-

ductivity in the Ba-La-Cu-O systemrdquo Zeitschrift fur Physik BCondensed Matter vol 64 no 2 pp 189ndash193 1986

[2] Y Kamihara T Watanabe M Hirano and H Hosono ldquoIron-based layered superconductor La[O

1minus119909F119909]FeAs (x= 005-012)

with T119888= 26 Krdquo Journal of the American Chemical Society vol

130 no 11 pp 3296ndash3297 2008[3] E A Ekimov V A Sidorov E D Bauer et al ldquoSuperconductiv-

ity in diamondrdquo Nature vol 428 no 6982 pp 542ndash545 2004[4] E Bustarret C Marcenat P Achatz et al ldquoSuperconductivity

in doped cubic siliconrdquo Nature vol 444 no 7118 pp 465ndash4682006

[5] T Herrmannsdorfer V Heera O Ignatchik et al ldquoSuper-conducting state in a gallium-doped germanium layer at lowtemperaturesrdquo Physical Review Letters vol 102 no 21 ArticleID 217003 2009

[6] J Fiedler V Heera R Skrotzki et al ldquoSuperconducting Ga-overdoped Ge layers capped with SiO

2 structural and transport

investigationsrdquo Physical Review B vol 85 no 13 Article ID134530 2012

[7] J Fiedler V Heera R Skrotzki et al ldquoSuperconducting filmsfabricated by high-fluence Ga implantation in Sirdquo PhysicalReview B vol 83 no 21 Article ID 214504 2011

[8] V Heera A Mucklich M Posselt et al ldquoHeavily Ga-dopedgermanium layers produced by ion implantation and flashlamp annealing structure and electrical activationrdquo Journal ofApplied Physics vol 107 no 5 Article ID 053508 2010

[9] A Di Cicco ldquoPhase transitions in confined gallium dropletsrdquoPhysical Review Letters vol 81 no 14 pp 2942ndash2945 1998

[10] H M Jaeger D B Haviland A M Goldman and B GOrr ldquoThreshold for superconductivity in ultrathin amorphousgallium filmsrdquo Physical Review B vol 34 no 7 pp 4920ndash49231986

[11] H Parr and J Feder ldquoSuperconductivity in beta-PhaseGalliumrdquoPhysical Review B vol 7 no 1 pp 166ndash181 1973

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Computational Methods in Physics

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Soft MatterJournal of

Hindawi Publishing Corporationhttpwwwhindawicom

AerodynamicsJournal of

Volume 2014


PhotonicsJournal of


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ThermodynamicsJournal of


(a) (b)

Figure 1 Surface structure investigated with SEM in the (100) oriented Ge single crystal which exposed at 4 times 1016 cmminus2 Ga dose at an ionenergy of 100 keV (a) As-implanted sample (b) Sample annealed at 800∘C RTA in the flowing 119860119903 atmosphere

10

1

01

R(kΩ)

Unannealed

300∘C

500∘C

600∘C

700∘C

780∘C

800∘C 830

∘C850

∘C

0 100 200 300

T (K)(a)

3 4 5 6 7 8(b)

100

098

096

R(T)R(Tc)

Tc

T998400c

600∘ C 700∘ C

780∘ C

800∘ C

830∘ C850

∘C

(c)

600∘C

700∘C

780∘C

800∘C

830∘C

850∘C

T (K)100 150 200 250 300

R(kΩ)

24

22

20

18

Figure 2 (a) 119879 dependence of the sheet resistance 119877◻for samples with the different annealing temperatures (b)The enlarged plot of (a) near

the superconducting transition temperature 119879119888 (c) The thermal hysteresis curves for the samples annealed at temperature window (600∘Cndash

850∘C) The arrows show the direction of the thermal cycle

on the surface of the sample because of the protection of theSiO2capping as discussed before But there are many discrete

holes on the surface as a result of high dose implantationFor the as-implanted samples the holes are discrete islandscontaining gallium but for the samples annealed at 800∘Cthe gallium islands inside the holes percolate which can beregarded as superconducting grains of gallium percolating inthe normal state germanium matrix

To clarify the properties of the different annealed sampleswe performed the electronic transport measurements onthose annealed samples We contacted the electrodes bythe bonding lines and carried out the measurements inthe usual four-terminal geometry at the temperature from25 K to 300K in a physical property measurement system(PPMS) The resistance was measured by ETO option whoseexcitation current is 100 120583A The temperature dependence of

resistance in the as-implanted state and annealed at varioustemperatures under Ar flowing atmosphere is presented inFigure 2

Figure 2(a) is the 119879 dependence of the sheet resistance 119877◻

for the unannealed and annealed samples The 119877◻s decreases

as the annealing temperature increasesThe logarithmic scaleof 119877◻here shows how dramatic the electronic transport

can be influenced by the annealing temperature For dif-ferent annealing temperature it shows a transition fromthe insulating to superconducting behavior Upon coolingthe as-implanted samples show an increasing resistancewhich could reflect intrinsic semiconductor property of Gewhile upon increasing the annealing temperature the sheetresistance decreases which could be due to the change ofthe discrete Ga islands to percolating ones as discussedabove


10

08

06

04

02

R(H)R(0)minus1

T = 25K300

∘C

600∘C

700∘C

780∘C

800∘C

830∘C

850∘C


H (kOe)

(a)R(Ω)

6

5

4

T (K)

1T

H = 0T

60

55

50

45

40

35

Tc(K

)

0 2 4 6 8

H (kOe)

2 3 4 5 6

(b)










◻







11988821205852


3 Summary










Acknowledgment


References




1minus119909F119909]FeAs (x= 005-012)



















FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




10

08

06

04

02

R(H)R(0)minus1

T = 25K300

∘C

600∘C

700∘C

780∘C

800∘C

830∘C

850∘C


H (kOe)

(a)R(Ω)

6

5

4

T (K)

1T

H = 0T

60

55

50

45

40

35

Tc(K

)

0 2 4 6 8

H (kOe)

2 3 4 5 6

(b)










◻







11988821205852


3 Summary










Acknowledgment


References




1minus119909F119909]FeAs (x= 005-012)



















FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics








Acknowledgment


References




1minus119909F119909]FeAs (x= 005-012)



















FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics








FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics



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Research Article The Insulator to Superconductor Transition ...downloads.hindawi.com/journals/acmp/2015/963768.pdfResearch Article The Insulator to Superconductor Transition in Ga-Doped