Contents lists available at ScienceDirect

Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 17 (2014) 134–137

1369-80http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/mssp

Metallic conduction behavior in selenium-hyperdoped silicon

Shaoxu Hu n, Peide Han, Peng Liang, Yupeng Xing, Shishu LouState Key Lab on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083,People's Republic of China

a r t i c l e i n f o

Available online 8 October 2013

Keywords:SiliconIon implantationSelenium supersaturationMetallic conduction

01/$ - see front matter & 2013 Elsevier Ltd.x.doi.org/10.1016/j.mssp.2013.09.001

esponding author. Tel: þ86 10 82304817.ail address: hushaoxu@semi.ac.cn (S. Hu).

a b s t r a c t

Hall measurements at 80–300 K are performed on crystalline silicon doped with seleniumexceeding the equilibrium solid solubility limit using ion implantation combined withfurnace annealing. The temperature dependence of free carrier density and sheet conductivityof the Se doping layer changes with implantation dose. Metallic conduction behavior is wellobserved in the sample doped with selenium to be 7.4�1020/cm3. The overlapping betweenSe impurity states below Si conduction band might give a microscopic explanation.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Hyperdoping as a promising technique is being used tocreate novel semiconductors with unique properties. Sili-con hyperdoped by chalcogenide impurities has shownsignificant sub-band-gap absorption [1–10], activating itspotential applications in infrared detectors [11–15], andimpurity band photovoltaics [16–18]. This material wasinitially produced by pulsed laser irradiation of siliconwafers in SF6 atmosphere [1–4,6]. Recently, an alternativemethod of ion implantation followed by pulsed laser melting[5,7,9] has been developed to fabricate chalcogen-rich sili-con, and insulator-to-metal transition (IMT) was observed inthis class of materials [19–21]. IMT has been extensivelyinvestigated and characterized in boron or phosphorus-doped silicon [22–26], but reports on IMT arising fromdeep states such as chalcogens are scarce. These deepstates can absorb sub-band-gap photons, but generally actas recombination centers that are not favorable for thecollection of photogenerated carriers. However, the non-radiative recombination effects can be suppressed by delo-calization of electrons of the impurity states via Mott transi-tion [26], which will extremely improve the performance ofSi-based optoelectronic devices. Therefore, it is of profound

All rights reserved.

significance to carry out comprehensive research on metallicphase transition related to chalcogen deep states.

In this paper, metallic conduction in Se-hyperdopedsilicon prepared using ion implantation followed by fur-nace annealing rather than pulsed laser melting is inves-tigated and analyzed.

2. Experimental details

Si (100) wafers (boron doping, 5–10 Ω cm, single-polished) were implanted with 100 keV Se ions to a doseof 1�1014, 1�1015, or 6�1015/cm2 on the polished side.Prior to ion implantation, silicon wafers were convention-ally RCA (Radio Corporation of America) cleaned and thenative oxide was removed by dipping in a HF:H2O (1:10)solution. The as-implanted wafers were then cleaned in aH2SO4:H2O2 (5:1) solution, ethanol, and deionized water,followed by furnace annealing at 700 1C for 30 min in anitrogen flow of 2.5 L/min. The annealed samples wereselected for optoelectronic measurements. A calculationmodule of TRIM (the Transport of Ions in Matter) [27] wasused to simulate the distribution profiles of Se ions in thedoping layer. Secondary ion mass spectrometry (SIMS) wasused to measure Se atom distribution profiles in siliconsurface layer. Hall measurements were carried out at 80–300 K with an even step of 20 K. A cryogenic cycle systemequipped with a liquid nitrogen cryostat assists in this

S. Hu et al. / Materials Science in Semiconductor Processing 17 (2014) 134–137 135

measurement. The absorptance A (A¼1�R�T) over therange 400–2500 nm was determined from the directlymeasured reflectance (R) and transmittance (T) using anAvaspec-2048�14/NIR256-2.5 fiber spectrophotometerequipped with an integrating sphere detector.

3. Results and discussion

Fig. 1 shows Se atom concentration versus depth distribu-tions of the samples implanted at different doses. Thesimulated Se atom distribution profile for the sampleimplanted with selenium ions at a dose of 6�1015/cm2 isalso presented by using a TRIM calculation module. It is notedthat the implanted Se atoms are primarily distributed in adepth of 200 nm below silicon surface. This value is takenas the effective implantation depth. For the three samplesimplanted with 1�1014, 1�1015, and 6�1015 Se/cm2,the peak atom concentration located at 90 nm is correspond-ingly 1.5�1019/cm3,1.4�1020/cm3, 7.4�1020/cm3, far beyondselenium equilibrium solubility limit of 1017 /cm3 in silicon[28]. In addition, the measured Se atoms distribution liesdeeper away the silicon surface than the simulated Se atomsdistribution, but the Se peak concentrations are almostthe same.

Typically, implantation energy loss distribution in siliconsurface implanted with 100 keV Se ions is similar to that in100 keV As ions implanted sample, because they have roughlythe same mean projected range and range deviation.As reported by Liu et al. [29], when annealing temperatureexceeds 600 1C, the damage in the 100 keV, 1�1015 As ions/cm2 implanted sample is almost repaired and the sheetresistance nearly reaches the minimum value. Raman spectrahave approved that the disordered Se doping layers are almostrestored to single crystalline after furnace annealing at 700 1Cfor 30min [30]. As annealing temperature increases from 500to 900 1C, the structural phase gradually changes from theamorphous to the crystalline. The deformed silicon latticesstart to rebuild at 600 1C when the implantation-introduceddefects are partially removed. The pure crystalline phase at700 1C suggests that the effects of the implantation defects onthe optoelectronic behavior of Se doped samples can beneglected. The annealed samples were subsequently mountedonto a cold finger with a chip carrier in a liquid nitrogen

Fig. 1. Depth-dependent Se atom concentration distributions in p-typesilicon implanted with 100 keV Se ions to a dose of 1�1014, 1�1015, and6�1015/cm2. The simulated Se atom distribution profile for the sampleimplanted with selenium at a dose of 6�1015/cm2 is also presented.

cryostat for Hall measurements. The negative Hall coefficientsindicate n-type conductivity, and nþp junctions are formedbetween the Se-doped layers and the silicon substrates.

Fig. 2a and b shows the temperature dependence offree carrier density and sheet conductivity of the Se dopinglayers of the annealed samples. In Hall measurements, thethickness of the Se doping layer is assumed to be 200 nmaccording to the measured Se SIMS distribution profileshown in Fig. 1. As shown in Fig. 2a, carrier density in Sedoping layer increases reasonably with implantation doseincreasing. Selenium is a double electron donor in silicon.Generally, as implantation dose increases, Se doping con-centration increases. For the sample implanted with a highdose, after thermal annealing, more selenium atomsshould be activated and located at substitutional sites,resulting in a high density of free carriers shown in Fig. 2a.For implantation doser1�1015 Se/cm2, carrier densityshows a direct increase with the testing temperature.Electron ionization of selenium dopant in silicon can bedescribed as a function of temperature. With an increase intemperature, more electrons will be ionized and activatedto conduction band becoming free carriers. However,as implantation dose reaches 6�1015 Se/cm2, a distinctinverse correlation between carrier density and temperatureis well observed. This exotic behavior incontrovertiblycorrelates with the carrier transport mechanism in theSe supersaturated layer. Selenium at dilute density is knownto introduce a variety of electronic energy states that reside

Fig. 2. Carrier density (a), sheet conductivity (b) versus temperature forp-type silicon implanted with 100 keV Se ions to a dose of 1�1014, 1�1015,and 6�1015/cm2, followed by furnace annealing at 700 1Cor 30 min.

Fig. 3. Wavelength-dependent absorptance for p-type silicon implantedwith 100 keV Se ions to a dose of 1�1014, 1�1015, and 6�1015/cm2,followed by furnace annealing at 700 1C for 30 min. The absorptance ofthe bare silicon is shown for reference, and its remained infraredabsorption beyond 1200 nm is caused by the integrating sphere.

S. Hu et al. / Materials Science in Semiconductor Processing 17 (2014) 134–137136

0.1–0.6 eV below Si conduction band edge [31]. Se deeplevels at Ec�0.30 eV and Ec�0.53 eV have been reportedby us previously in the literature [32]. As supersaturatedselenium atoms (E1021/cm3) are implanted into a thinsilicon layer, the delocalization of electron wave functionsmight occur through electron–electron interactions thatscreen the Coulomb field between a parent donor and itselectron [26]. As a result, the discrete Se energy levels wouldshift and broaden until merge with the conduction band,causing a transition to metallic conduction. This is fullyconsistent with the temperature-dependent sheet conduc-tivity shown in Fig. 2b. At a low density of Se doping, thesheet conductivity takes on a nearly exponential increasewith the temperature. This indicates that the conductivity sis an activated process and can be expressed by

s¼ s0 exp �Ea=kT� � ð1Þ

Here, s0 is an initial constant, k is the Boltzmann constant, T isthe Kelvin temperature, and Ea represents the activationenergy value calculated from the slope of log (s) versus1000/T curve. By calculation, Ea is 8.5 and 7.0 meV forthe samples implanted with 1�1014 and 1�1015 Se/cm2,respectively. These low activation energy values imply that Sediscrete levels begin to interact on each other, and donor-electron delocalization is coming into being. However, it ispossible to have a negative activation energy value whensheet conductivity decreases with an increase in temperaturein the case of the sample implanted with 6�1015 Se/cm2. Thismeans that electrons can transport freely between Se impuritybands and the conduction band without external activation.In terms of metallic conduction, the thermal vibration of Seatoms in silicon lattices becomes intense as temperatureincreases, thus prevents the directional drift of free electronsin silicon and ultimately reduces the sheet conductivity.

The critical Se doping concentration Nd for metallicconduction can be well estimated using Mott transitionmodel as

N1=3d an

h ¼ C0; ð2Þwhere C0 is a constant, typically of the order 0.26, and an

h isdefined as dopant dependent Bohr radius given by [33]

an

h ¼1

4πε0

q2

2εrEdð3Þ

Here, ε0, εr is the permittivity of free space and silicon,respectively, q is the charge of an electron, and Ed denotesthe first ionization energy of selenium in silicon reportedto be 0.3 eV [32,34]. In this case, Nd is deduced to be2.0�1021/cm3. This value is rough in agreement with theexperimentally determined Se peak density of 7.4�1020/cm3. The difference arises mainly from the refereed Ed.

The occurrence of Se impurity band-crossing can beindirectly reflected by the enhanced infrared absorptancedisplayed in Fig. 3. All the implanted samples wereannealed at 700 1C for 30 min. Compared with the baresilicon, the Se doped samples show an increased sub-band-gap absorption at wavelengths longer than 1200 nm. Thisarises credibly from the selenium dopant rather than fromthe implantation-introduced structural defects. Usually,defects lying in Si midgap can also generate infraredabsorption. However, as shown in the previously reported

literature [30], the Raman peak fixed at 520 cm�1 indicatesthat the disordered Se doping layer is restored to crystallineafter furnace annealing at 700 1C and the implantationdamage is almost eliminated. The near infrared absorptanceis insensitive to the implantation doser1�1015 Se/cm2, butit increases abruptly as implantation dose approaches6�1015 Se/cm2. It predicts that new atomic configurationsarise [35], dopant electrons are no longer bound to theimpurity states. Instead, Se impurity levels begin to overlapand extend into impurity bands. As a result, sub-band-gapphotons can be absorbed in the Se-hyperdoped layers byexciting electronic transitions between Si band edges and Seimpurity bands. The absorption of low energy photons isconsidered as a favorable way to improve the efficiency ofsilicon based solar cells via the impurity photovoltaic effect.

4. Conclusions

The electronic properties of Se-hyperdoped silicon wereinvestigated using temperature dependent Hall measure-ments. Metallic conduction was observed in the sample withSe atom concentration of 7.4�1020/cm3, which is close tothe expected critical Mott transition density of selenium insilicon. The overlapping between Se impurity levels is judgedto be responsible for the metallic conduction behavior. As ahigh density of Se atoms are doped into silicon, electronicinteractions between neighboring atoms are intensified,discrete Se levels overlap and ionized electrons can transportfreely in the metal-like Se hyperdoping layer. In addition, thesuddenly increased sub-band-gap absorption suggests theformation of Se impurity bands in Si band gap. To be honest,more efforts need to be made in the near future forilluminating the exact mechanism under the metallic con-duction behavior in Se-hyperdoped silicon with notableinfrared absorption, facilitating silicon's potential applica-tions in the fabrication of high efficiency solar cells.

Acknowledgments

This work was supported by the National NaturalScience Foundation of China (Nos. 61275040, 60976046,

S. Hu et al. / Materials Science in Semiconductor Processing 17 (2014) 134–137 137

61021003), the Major State Basic Research DevelopmentProgram of China (973 Program) (No.2012CB934200) andby Chinese Academy of Sciences (No.Y072051002).

References

[1] C. Wu, C.H. Crouch, L. Zhao, J.E. Carey, R. Younkin, J.A. Levinson,E. Mazur, R.M. Farrell, P. Gothoskar, A. Karger, Applied Physics Letters78 (2001) 1850.

[2] C.H. Crouch, J.E. Carey, J.M. Warrender, M. J. Aziz, E. Mazur, F.Y. Genin, Applied Physics Letters 84 (2004) 1850.

[3] C. H. Crouch, J. E. Carey, M. Shen, E. Mazur, F. Y. Genin, AppliedPhysics A 79 (2004) 1635.

[4] M.A. Sheehy, L. Winston, J.E. Carey, C.M. Friend, E. Mazur, Chemistryof Materials 17 (2005) 3582.

[5] T.G. Kim, J.M. Warrender, M.J. Aziz, Applied Physics Letters 88 (2006)241902.

[6] V. Zorba, N. Boukos, I. Zergioti, C. Fotakis, Applied Optics 47 (2008)1846.

[7] B.P. Bob, A. Kohno, S. Charnvanichborikarn, J.M. Warrender,I. Umezu, M. Tabbal, J.S. Williams, M.J. Aziz, Journal of AppliedPhysics 107 (2010) 123506.

[8] M. Tabbal, T. Kim, D.N. Woolf, B. Shin, M.J. Aziz, Applied Physics A 98(2010) 589.

[9] S.H. Pan, D. Recht, S. Charnvanichborikarn, J.S. Williams, M.J. Aziz,Applied Physics Letters 98 (2011) 121913.

[10] M.J. Smith, M. Winkler, M.J. Sher, Y.T. Lin, E. Mazur, S. Gradecak,Applied Physics A 105 (2011) 795.

[11] J. E. Carey, C. H. Crouch, M. Shen, E. Mazur, Optics Letters 30 (2005)1773.

[12] Z. Huang, J.E. Carey, M. Liu, X. Guo, E. Mazur, J.C. Campbell, AppliedPhysics Letters 89 (2006) 033506.

[13] R.A. Myers, R. Farrell, A.M. Karger, J.E. Carey, E. Mazur, AppliedOptics 45 (2006) 8825.

[14] A.J. Said, D. Recht, J.T. Sullivan, J.M. Warrender, T. Buonassisi,P.D. Persans, M.J. Aziz, Applied Physics Letters 99 (2011) 073503.

[15] S. Hu, P. Han, S. Wang, X. Mao, X. Li, L. Gao, Semiconductor Scienceand Technology 27 (2012) 102002.

[16] L. Cuadra, A. Marti, A. Luque, IEEE Transactions on Electron Devices51 (2004) 1002.

[17] V.V. Iyengar, B.K. Nayak, M.C. Gupta, Solar Energy Materials andSolar Cells 94 (2010) 2251.

[18] M.J. Sher, M.T. Winkler, E. Mazur, Materials Research Bulletin 36(2011) 439.

[19] M.T. Winkler, D. Recht, M.J. Sher, A.J. Said, E. Mazur, M.J. Aziz,Physical Review Letters 106 (2011) 178701.

[20] X. Li, P. Han, L. Gao, X. Mao, S. Hu, Applied Physics A 105 (2011) 1021.[21] E. Ertekin, M.T. Winkler, D. Recht, A.J. Said, M.J. Aziz, T. Buonassisi,

J.C. Grossman, Physical Review Letters 108 (2012) 026401.[22] C. Yamanouchi, K. Mizuguchi, W. Sasaki, Journal of Physical Society

of Japan 22 (1967) 859.[23] T.F. Rosenbaum, R.F. Milligan, M.A. Paalanen, G.A. Thomas,

R.N. Bhatt, W. Lin, Physical Review B 27 (1983) 7509.[24] M.N. Alexander, D.F. Holcomb, Reviews of Modern Physics 40 (1968)

815.[25] T. G. Castner, N. K. Lee, G. S. Cieloszyk, G. L. Salinger, Physical Review

Letters 34 (1975) 1627.[26] N.F. Mott, Reviews of Modern Physics 40 (1968) 677.[27] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, Nuclear Instruments and

Methods in Physics Research B 268 (2010) 1818.[28] A.A. Taskin, B.A. Zaitsev, V.I. Obodnikov, E.G. Tishkovskii, Semicon-

ductors 34 (2000) 312.[29] S. Liu, W. Lu, C. Ji, T. Chang, A study of the laser and thermal

annealing behavior by Rutherford backscattering, in: The 4th Inter-national Conference on Ion Beam Analysis, June 25–29, 1979.

[30] S. Hu, P. Han, Y. Mi, Y. Xing, P. Liang, Y. Fan, Materials Science inSemiconductor Processing 16 (2013) 987.

[31] E. Janzen, R. Stedman, G. Grossmann, H. G. Grimmeiss, PhysicalReview B 29 (1984) 1907.

[32] L. Gao, P. Han, X. Mao, Y. Fan, S. Hu, C. Zhao, Y. Mi, Chinese PhysicsLetters 28 (2011) 036108.

[33] P.P. Edwards, M.J. Sienko, Physical Review B 17 (1978) 2575.[34] H.R. Vydyanath, J.S. Lorenzo, F.A. Kroger, Journal of Applied Physics

49 (1978) 5928.[35] G.A. Thomas, M. Capizzi, F. Derosa, R.N. Bhatt, T.M. Rice, Physical

Review B 23 (1981) 5472.