$\hbox{Ni}(\hbox{Ge}_{1 - x}\hbox{Sn}_{x})$ Ohmic Contact Formation on N-Type $\hbox{Ge}_{1 - x}\hbox{Sn}_{x}$ Using Selenium or Sulfur Implant and Segregation

$Page 1: $\hbox{Ni}(\hbox{Ge}_{1 - x}\hbox{Sn}_{x})$ Ohmic Contact Formation on N-Type $\hbox{Ge}_{1 - x}\hbox{Sn}_{x}$ Using Selenium or Sulfur Implant and Segregation$
746 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 2, FEBRUARY 2013

Ni(Ge1−xSnx) Ohmic Contact Formation on N-TypeGe1−xSnx Using Selenium or Sulfur Implant

and SegregationYi Tong, Genquan Han, Bin Liu, Yue Yang, Lanxiang Wang, Wei Wang, and Yee-Chia Yeo, Member, IEEE

Abstract—The physics of ohmic contact formation for nickelstanogermanide [Ni(Ge1−xSnx)] on n-type germanium–tin(n-Ge1−xSnx) was investigated. Low-resistivity Ni(Ge1−xSnx)was formed on Ge1−xSnx using a 350 ◦C 30-s anneal. Ionimplantation of selenium (Se) or sulfur (S) into n-Ge1−xSnx

followed by nickel stanogermanidation led to the segregationof Se or S at the Ni(Ge1−xSnx)/n-Ge1−xSnx interface. Loweffective electron Schottky barrier height (Φn

B) of 0.12 and0.11 eV was achieved for Ni(Ge1−xSnx)/n-Ge1−xSnx contactswith Se and S segregation, respectively. A simulation study wasalso performed to explain the experimental observations. Seand S atoms could be modeled as donor-like traps near theNi(Ge1−xSnx)/n-Ge1−xSnx interface, modifying the potentialprofile near the contact and giving rise to trap-assisted tunnelingto increase the reverse leakage current for ohmic contactformation.

Index Terms—Nickel stanogermanide, Schottky barrier height(SBH), segregation, selenium (Se), sulfur (S).

I. INTRODUCTION

G ERMANIUM–TIN (Ge1−xSnx) could have high electronmobility and shows promise as an alternative channel

material for n-MOSFETs [1]. Ge1−xSnx channel n-MOSFETswere recently reported [2], [3], and nonself-aligned metallicNi [2] and Ti/Al [3] contacts on n+ Ge1−xSnx source anddrain (S/D) were used. Good self-aligned ohmic contacts withlow Schottky barrier height (SBH) on n+ Ge1−xSnx S/D areneeded. However, there are no reports of self-aligned ohmiccontact formation on n-type Ge1−xSnx (n-Ge1−xSnx). In addi-tion, a challenge for forming metallic contacts on n-type Ge orGe1−xSnx is the pinning of the Fermi level of metallic materialstoward the valence band, leading to a large SBH for electronsand, therefore, a high contact resistance.

One potential approach to form self-aligned ohmic contactis to introduce specific atomic species by implant prior to self-aligned silicidation or germanidation [4]–[7]. Implanted specieswill be pushed to the metal–semiconductor interface, resultingin a low effective SBH (ΦB) and, thus, a low contact resistance

Manuscript received October 2, 2012; revised November 27, 2012; acceptedDecember 4, 2012. Date of publication January 3, 2013; date of currentversion January 18, 2013. This work was supported by the National ResearchFoundation of Singapore under Grant NRF-RF2008-09. The review of thispaper was arranged by Editor H. Jaouen.

The authors are with the Department of Electrical and Computer En-gineering, National University of Singapore, Singapore 119260 (e-mail:[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2012.2233204

Fig. 1. TEM image of the epitaxially grown Ge1−xSnx layer on top of ann-type Ge (100) substrate. The interface between Ge1−xSnx and Ge is clearlyobserved. The quality of the epitaxial Ge1−xSnx is good.

(Rc). For nickel silicide (NiSi) contacts on silicon (Si), theeffective SBH can be reduced using implantation and segrega-tion of sulfur (S) [8]–[15], selenium (Se) [15]–[17], aluminum(Al) [18]–[23], tellurium (Te) [25], [26], and indium (In) [27].For nickel germanide (NiGe) contacts on n-type germanium(n-Ge), the effective electron SBH was reduced using implanta-tion and segregation of sulfur (S) [28]–[32] and selenium (Se)[32]. For p-type germanium (p-Ge), an ohmic contact can beeasily formed due to strong pinning of the metal Fermi level to-ward the valence band of Ge [28]–[30]. For n-Ge1−xSnx, it willbe very interesting to investigate the feasibility of self-alignedohmic contact formation using implantation and segregation ofselected atomic species.

In this paper, we report the first demonstration of self-alignedNi(Ge1−xSnx) ohmic contacts on n-Ge1−xSnx, featuring ionimplantation and segregation of selenium (Se) or sulfur (S)at the Ni(Ge1−xSnx)/n-Ge1−xSnx interface. Se or S atomswere implanted into n-Ge1−xSnx and were pushed to theinterface between Ni(Ge1−xSnx) and n-Ge1−xSnx duringstanogermanidation, resulting in low effective electron SBHs(Φn

B) of 0.12 and 0.11 eV, respectively. Ohmic behavior wassuccessfully achieved; Se and S segregation peaks were clearlyobserved using secondary ion mass spectrometry (SIMS)measurement.

II. DEVICE FABRICATION

The starting substrate is 150-nm-thick GeSn on an n-type Ge(100) substrate. The substitutional Sn composition is 4.2%, asdetermined by high-resolution X-ray diffraction (XRD). Fig. 1shows the transmission electron microscopy (TEM) images

0018-9383/$31.00 © 2013 IEEE

TONG et al.: Ni(Ge1−xSnx) OHMIC CONTACT FORMATION ON N-TYPE Ge1−xSnx 747

Fig. 2. Schematic of a Ni(Ge1−xSnx)/n-Ge1−xSnx contact withprestanogermanide Se or S implant and segregation. S or Se implant wasperformed prior to the deposition and reaction of Ni with Ge1−xSnx to formNi(Ge1−xSnx) or NiGeSn. Electrical characterization was done by applyingvoltage V on Ni(Ge1−xSnx), and the Al contact is grounded.

of the epitaxially grown GeSn layer on the n-type Ge (100)substrate. The GeSn films were doped using phosphorus (P) atenergies of 50, 130, and 250 keV. For each implant energy, theimplant dose is 1 × 1013 cm−2. A 400 ◦C 5-min rapid thermalanneal (RTA) step was used for P activation.

A 200-nm plasma-enhanced chemical vapor deposition SiO2

was deposited and patterned to define active regions. Here, ex-perimental splits were introduced. The samples either receivedSe or S implant. Se or S implant was performed at energies of8 and 5 keV, respectively, at the same dose of 1 × 1015 cm−2.The target implant range for Se or S is 6.8 and 6.6 nm belowthe GeSn surface, respectively. With the implant conditionsused in this work, the topmost GeSn layer is amorphized,and there should be some crystal damage or disorder beneaththe amorphous–crystalline boundary. From TEM analysis ofS- and Se-implanted Ge samples in a separate experiment, theS-implanted one seems to have less crystal disorder or damagein the Ge lattice as compared with the Se-implanted one.

A 10-nm-thick Ni was deposited after native oxide removalin the active region. This was followed by RTA at 350 ◦C for30 s in N2 ambient for the stanogermanidation. The stanoger-manidation consumed the amorphous GeSn layer. Unreacted Niwas then removed using concentrated sulfuric acid (H2SO4).This completed the formation of self-aligned Ni(Ge1−xSnx)contacts.

Finally, 200-nm-thick Al was deposited on the backside ofthe samples. I–V characteristics of the contact devices withan area of 100 × 100 μm2 were measured. Blanket sampleswere also prepared using the same implant and stanogermanida-tion conditions for physical analyses such as XRD and time-of-flight SIMS (TOF-SIMS). Fig. 2 shows a schematic of aNi(Ge1−xSnx)/n-Ge1−xSnx Schottky diode with prestanoger-manide Se or S implant and segregation.

III. RESULTS AND DISCUSSION

A. Material Characterization of Blanket Samples of NickelStanogermanide Films With Selenium or Sulfur Implant

In Fig. 3, XRD characterization was performed to examinethe phase of nickel stanogermanide films. Similar profiles areobserved for the samples that received Se or S implant. It wasreported that the low-resistivity nickel monostanogermanidephase would form after a 350 ◦C 30-s anneal [34]. In this ex-

Fig. 3. XRD characterization of Ni(Ge1−xSnx) films for the samples withSe and S implant. It is found that the phase of Ni(Ge1−xSnx) is nickelmonostanogermanide after a 350 ◦C 30-s anneal. Se and S do not affect theformation of low-resistivity nickel monostanogermanide.

Fig. 4. Room-temperature current–voltage characteristics of Ni(Ge1−xSnx)/n-Ge1−xSnx contact devices formed with prestanogermanide Se or S implant.The contact has an area of 100 × 100 μm2. Φn

B was extracted using anactivation energy method. The extracted Φn

B of the samples with Se and Simplants are 0.12 and 0.11 eV, respectively. The rectifying behavior for thecontrol sample indicates strong Fermi level pinning near the valence band edgeof n-Ge1−xSnx.

periment, the nickel monostanogermanide phase was found forboth Se and S samples, indicating that neither Se nor S implantaffects the phase formation for nickel monostanogermanide.

B. Electrical Characterization of Diodes With Selenium orSulfur Implant

Fig. 4 shows the room temperature I–V characteristics ofNi(Ge1−xSnx)/n-Ge1−xSnx contacts with different implan-tation conditions. The control sample without any implantshows rectifying I−V behavior, giving a large Φn

B . OhmicI–V behavior is clearly observed for samples with Se or Simplant. The increased reverse currents indicate reduction inthe effective SBH for electrons. Fig. 5 shows the cumulativeprobability plot of the reverse current measured at −1 V forNi(Ge1−xSnx)/n-Ge1−xSnx contacts with Se or S implant. Thelower crystal damage for the S-implanted sample as comparedwith the Se-implanted sample may explain the smaller spreadin the reverse current density, as current density may also becaused by defect density due to crystal damage and its device-to-device variation.

Figs. 6 and 7 show the Arrhenius plots used for extraction ofΦn

B of the samples with Se or S implant. The inset shows the


Fig. 5. Cumulative probability plot of the reverse current measured at −1 Vfor Ni(Ge1−xSnx)/n-Ge1−xSnx contacts with Se and S implant.

Fig. 6. Arrhenius plots of Ni(Ge1−xSnx)/n-Ge1−xSnx contacts formedwith prestanogermanide Se implant (8 keV, 1 × 1015 cm−2). The Se-implantedsample with the median value for current density was used in low-temperatureI–V measurement. The inset shows the low-temperature current–voltage char-acteristics used to extract Φn

B . In order to avoid the influence of the voltage dropacross the substrate series resistance, the currents under reverse bias (−0.1 V)at temperatures ranging from 230 to 255 K were used to extract the effectiveSBH. The extracted Φn

B is 0.12 eV.

Fig. 7. Arrhenius plots of Ni(Ge1−xSnx)/n-Ge1−xSnx contacts formedwith prestanogermanide S implant (5 keV, 1 × 1015 cm−2). The inset showsthe low-temperature current–voltage characteristics used to extract Φn

B . Theextracted Φn

B is 0.11 eV.

low-temperature I–V characteristics of the Ni(Ge1−xSnx)/n-Ge1−xSnx contacts. The details of the method of extractionof Φn

B can be found in [40]–[47]. We used the currents underreverse bias for extraction of the effective electron SBH [35],where the effects of series resistance and junction nonidealityare not significant. Φn

B was extracted using the activationenergy method [29], [41], [42]. The current under low reverse

Fig. 8. (a) Depth profiles of the implanted species in Ni(Ge1−xSnx)/n-Ge1−xSnx contacts with S implant (5 keV, 1 × 1015 cm−2). (b) Depth pro-files of the implanted species in Ni(Ge1−xSnx)/n-Ge1−xSnx contacts withSe implant (8 keV, 1 × 1015 cm−2). Obvious S segregation peak was found atthe Ni(Ge1−xSnx)/n-Ge1−xSnx interface, whereas the Se segregation peakwas located inside the Ni(Ge1−xSnx) layer. It is believed that Se and S atomswere pushed to the interface due to the snowplow effect.

bias was measured at temperatures ranging from 230 to 255 Kin steps of 5 K. According to the thermionic emission (TE)theory [29], [41], [42], temperature dependence of the reversecurrent IR of the Schottky contacts can be expressed as

ln

(IRT 2

)= ln(AA∗∗)− qΦn

B

kT(1)

where A is diode area, A∗∗ is effective Richardson constant,k is Boltzman constant, q is the electronic charge, and Tis temperature. The slope can be obtained from a plot ofIR/T

2 versus 1000/T . ΦnB was calculated from the slope. The

extracted ΦnB of the samples with Se and S implants are 0.12

and 0.11 eV, respectively. Introduction of Se or S lowered ΦnB

of Ni(Ge1−xSnx)/n-Ge1−xSnx contacts. Based on the experi-mental conditions used in this work, S and Se are comparablein the extent of Φn

B reduction. However, it is important to pointout that the extent of Φn

B reduction is expected to depend onprofile of Se and S relative to the Ni(Ge1−xSnx)/n-Ge1−xSnx

interface and, possibly, the stanogermanidation conditions.To further investigate the impact of Se or S implant on Φn

B

reduction, Fig. 8 shows the depth profiles of the implantedspecies in Ni(Ge1−xSnx)/n-Ge1−xSnx as obtained by TOF-SIMS measurement. It is found that implanted Se and S atomsare pushed to the Ni(Ge1−xSnx)/n-Ge1−xSnx interface due tothe snowplow effect during stanogermanidation, and clear seg-regation peaks of Se and S are observed. It is found that Se seg-regation peak is located inside the Ni(Ge1−xSnx) film, whereasS segregation peak is at the interface of Ni(Ge1−xSnx) andn-Ge1−xSnx. It is believed that the Φn

B reduction observed is


Fig. 9. Energy band diagram of a Ni(Ge1−xSnx)/n-Ge1−xSnx contactwithout any implant and segregation. ECEFEC , and EV are the Fermi energylevel, conduction, and valence band edge, respectively. Electrons may surmountthe actual Schottky barrier by TE.

attributed to Se or S segregation in Ni(Ge1−xSnx)/n-Ge1−xSnx

contacts.

C. Mechanisms for Reduction in the Effective SBH inSelenium- or Sulfur-Implanted Nickel Germanium TinContacts

Here, we will discuss a possible mechanism that leadsto the significant increase in the reverse bias current forNi(Ge1−xSnx)/n-Ge1−xSnx contacts. Fig. 9 shows an energyband diagram for a Ni(Ge1−xSnx)/n-Ge1−xSnx contact. Ac-cording to the TE theory [29], [41], [42], carriers with excessenergy higher than the SBH can surmount the barrier, leadingto a TE current, as shown in (1). Electrons with lower energymay surmount the Schottky barrier if the Fermi level of themetal contact is nearer to the conduction band of Ge1−xSnx

or if the SBH is smaller. In this paper, the XRD analysisreveals that the Se or S implant has a negligible impact on thebulk property of nickel stanogermanide film, and a change inthe workfunction of nickel stanogermanide is not expected. Spassivation or segregation has been proposed to cause Fermilevel depinning on Ge, so as to reduce the electron SBH withoutmodifying the metal workfunction [29], [43]. Based on thesimulation results and the physical understanding, we proposeanother possible model involving trap-assisted tunneling (TAT),which could coexist with Fermi level depinning for explainingthe Φn

B reduction caused by Se or S segregation.The presence of Se or S atoms at the metal–semiconductor

interface has been reported to reduce the effective ΦnB , thus

giving a high reverse current [8]–[15], [28]–[32]. In this paper,Se and S segregation should be responsible for the achievementof the ohmic contact formation on n-Ge1−xSnx. Se and S areknown to act as donor impurities in Ge and introduce theshallow donor-like trap levels below the conduction band of Ge[46]. S has a trap level of 0.18 eV below the conduction bandof Ge, whereas Se has double trap levels of 0.14 and 0.28 eVbelow the conduction band of Ge [46]. Se and S may havesimilar behavior in Ge1−xSnx having a low Sn composition.The presence of the ionized Se or S traps might cause bandbending of the semiconductor substrate, thus reducing thebarrier depletion width and increasing the electron tunnelingprobability [25], [32], [43]. A high reverse bias current isconsistent with a low Φn

B or the formation of an ohmic contact.

Fig. 10. (a) Depth profile of sulfur used in numerical simulation is plottedusing a solid line, which fits well to the experimental SIMS sulfur profile (in cir-cles). The profile of ionized sulfur traps was extracted from numerical simula-tion. (b) Simulated energy band diagram of the Ni(Ge1−xSnx)/n-Ge1−xSnxcontacts with and without S implant and segregation. EF , EC , and EV are theFermi energy level, conduction, and valence band edge, respectively.

To verify this postulation, simulation was performed toexamine the energy band diagram of the Ni(Ge1−xSnx)/n-Ge1−xSnx contacts with or without S or Se implant usingSynopsys technology computer-aided design tools [47]. S wassimulated as donor-like traps with a trap level at 0.18 eV belowthe conduction band of GeSn, whereas Se was simulated asdonor-like traps with double trap levels at 0.14 and 0.28 eVbelow the conduction band of GeSn in the simulation [46].The S or Se distribution within the substrate is modeled witha piecewise distribution profile that closely matches the S or Seprofile obtained from the SIMS analysis. Fermi–Dirac carrierstatistics was used to simulate the energy band diagram of theNi(Ge1−xSnx)/n-Ge1−xSnx Schottky contact with the mod-eled S or Se traps at 300 K. The workfunction of Ni(Ge1−xSnx)used in the simulation is set to 4.61 eV. The substrate is dopedwith a uniform n-type doping of 1 × 1018 cm−3. The band gapof GeSn with a Sn composition of 4.2% is 0.59 eV [48], [49].The electron affinity of GeSn with 4.2% Sn is 4.051 eV [50].

Fig. 10(a) shows the actual, the modeled, and the ionizedS traps in the simulation. The model of incomplete ioniza-tion of impurities is used to calculate the ionization rate of


Fig. 11. (a) Depth profile of selenium used in numerical simulation isplotted using a solid line, which fits well to the experimental SIMS se-lenium profile (in circles). The profile of ionized selenium traps was ex-tracted from numerical simulation. (b) Simulated energy band diagram ofthe Ni(Ge1−xSnx)/n-Ge1−xSnx contacts with and without Se implant andsegregation. EF , EC , and EV are the Fermi energy level, conduction, andvalence band edge, respectively.

S or Se traps. The ionized S profile shows near-completeionization of the S traps in the first few nanometers awayfrom the Ni(Ge1−xSnx)/n-Ge1−xSnx interface, and the per-centage ionization becomes lower as one moves away from theNi(Ge1−xSnx)/n-Ge1−xSnx interface. This might be explainedby a partial occupancy of the donor-like trap level with electronswhile is caused by the lowering of the conduction band andthe donor-like trap level toward the Fermi level in the deepern-Ge1−xSnx region [25], [42]. Fig. 10(b) shows the simulatedenergy band diagrams of the sample with the ionized S trapsnear the Ni(Ge1−xSnx)/n-Ge1−xSnx interface. It is clearlyobserved that the addition of the ionized S traps leads to a sharpdownward band bending of Ge1−xSnx. To examine the effectof the ionized Se traps, the similar simulation was performed,as shown in Fig. 11.

The simulation shows that the presence of the ionized S orSe traps near the Ni(Ge1−xSnx)/n-Ge1−xSnx interface reducesthe barrier depletion width for tunneling and also increases theprobability of TAT of electrons through the barrier region [25],[32], [44]. Fig. 12 shows the simulated I−V characteristics

Fig. 12. Simulated I−V characteristics of the S-implanted Ni(Ge1−xSnx)/n-Ge1−xSnx contacts with and without turning on the TAT model.

of the S-implanted Ni(Ge1−xSnx)/n-Ge1−xSnx contacts, withthe TAT model turned on or turned off. Electrons with lowerenergy than the actual SBH may tunnel from Ni(Ge1−xSnx)to n-Ge1−xSnx by TAT, giving a larger reverse bias current,which is manifested as a smaller effective electron SBH. Thiscould contribute to the ohmic contact formation on n-Ge1−xSnx

in this work. This points toward further tuning of the im-plant conditions of segregation species and also the stanoger-manidation conditions so as to push more S or Se atomsto the Ni(Ge1−xSnx)/n-Ge1−xSnx interface for further Φn

B

reduction. A higher concentration of segregated S and Se nearthe Ni(Ge1−xSnx)/n-Ge1−xSnx interface would give rise to alarger tunneling current, which is good for the ohmic contactformation.

IV. CONCLUSION

Self-aligned Ni(Ge1−xSnx) contacts with Se or S segrega-tion on n-Ge1−xSnx were investigated. Ohmic behavior wasachieved, and low effective SBHs of 0.11 and 0.12 eV wereextracted for the samples with S or Se implant, respectively.It is found that the Se or S implant and segregation does notaffect the phase formation of nickel monostanogermanide. Theobserved Φn

B reduction is caused by Se or S segregation withinNi(Ge1−xSnx)/n-Ge1−xSnx contacts, which may enhance theTAT of electrons.

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Yi Tong received the M.Eng. degree from NationalUniversity of Singapore (NUS), Singapore. He iscurrently working toward the Ph.D. degree at NUS.

Genquan Han received the Ph.D. degree from theInstitute of Semiconductors, Chinese Academy ofSciences, Beijing, China. He is currently with Na-tional University of Singapore, Singapore.

Bin Liu received the B.Eng. degree in electricalengineering from National University of Singapore(NUS), Singapore, in 2008. He is currently workingtoward the Ph.D. degree at NUS.

Yue Yang received the B.Eng. degree in electricalengineering from National University of Singapore(NUS), Singapore, in 2008. She is currently workingtoward the Ph.D. degree at NUS.

Lanxiang Wang received the B.S. degree fromXi’an Jiaotong University, Xi’an, China. He is cur-rently working toward the Ph.D. degree in electri-cal engineering at National University of Singapore,Singapore.

Wei Wang received the Ph.D. degree from theInstitute of Semiconductors, Chinese Academy ofSciences, Beijing, China, in 2011. He is cur-rently with the Department of Electrical and Com-puter Engineering, National University of Singapore,Singapore.

Yee-Chia Yeo (S’98–M’02) received the B.Eng.(first class honors) and M.Eng. degrees from Na-tional University of Singapore (NUS), Singapore,and the M.S. and Ph.D. degrees from the Universityof California, Berkeley. He is currently with NUS.

Documents

$\hbox{Ni}(\hbox{Ge}_{1 - x}\hbox{Sn}_{x})$ Ohmic Contact Formation on N-Type $\hbox{Ge}_{1 - x}\hbox{Sn}_{x}$ Using Selenium or Sulfur Implant and Segregation