8/7/2019 2011 - Mechanisms of Schottky Barrier Control on n-Type Germanium Using Ge3N4 Interlayers - R.R. Lieten - JEC 158

http://slidepdf.com/reader/full/2011-mechanisms-of-schottky-barrier-control-on-n-type-germanium-using-ge3n4 1/5

Mechanisms of Schottky Barrier Control on n-Type GermaniumUsing Ge3N4 Interlayers

R. R. Lieten,a,b,c,z V. V. Afanas’ev,c N. H. Thoan,c S. Degroote,b W. Walukiewicz,a

and G. Borghsb,c

aLawrence Berkeley National Laboratory, Berkeley, California 94720, USAb

IMEC, 3001 Leuven, BelgiumcDepartment of Physics and Astronomy, K. U. Leuven, 3001 Leuven, Belgium

The influence of a few monolayers of crystalline and amorphous Ge3N4 on the Schottky barrier height of n-type Ge has been inves-tigated. Low temperature capacitance–voltage measurements are used to accurately determine the barrier height. Both amorphousand epitaxial Ge3N4 effectively eliminate pinning of the Fermi level at the metal/Ge interface. Metal/Ge3N4/n-Ge contacts there-fore show a linear dependence of the Schottky barrier height with metal work function. Our results indicate that the Fermi levelunpinning is achieved mainly due to the passivation of interface states related to defects at the metal/Ge interface. Aluminum onamorphous and epitaxial Ge3N4 delivers barrier heights of 0.096 0.05 and 0.060.1 eV, respectively, resulting in Ohmic behavior.The formation of epitaxial Ge3N4 requires temperatures above 600C, whereas amorphous layers can be formed at much lower temperatures. Amorphous Ge3N4 can therefore be used to form Ohmic contacts at a low thermal budget.VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3545703] All rights reserved.

Manuscript submitted August 18, 2010; revised manuscript received October 27, 2010. Published February 15, 2011.

In SiGe based high-frequency devices, including n-channel SiGemetal-oxide-semiconductor field-effect transistors (MOSFETs), lowsource and drain resistances are of crucial importance to obtain thedesirably high channel current. To obtain low resistivity contacts, athin layer of very heavily doped semiconductor is most often used.This leads to a thin depletion region, known as a tunnel junction,through which field emission of electrons can take place. However,it is getting increasingly difficult to obtain Ohmic regime withincreasing Ge concentration. Dopants in Ge show a low solid solu-bility, an incomplete activation, and an enhanced diffusion,1 whichresult in a poor quality source–drain (S–D) contacts. Another approach to improve S–D contacts would be a reduction of theSchottky barrier height. The current trough a metal contact, whether from thermionic emission or tunneling, increases exponentially withdecreasing barrier height.2 However, in the ultimate case of n-type

Ge, the Schottky (rectifying) contact is formed independently of themetal work function, as illustrated later in this work. Obtaining lowSchottky barrier heights on n-type germanium seems therefore diffi-cult. It is suggested that intrinsic Ge interface states cause Fermilevel pinning, thus making the metal/Ge barrier height insensitive tothe metal work function.3 The lack of low resistance source anddrain contacts for Ge n-channel MOSFET devices limits the maxi-mum channel current. Schottky barrier S–D MOSFET deviceswould also benefit from low, preferably zero barriers to achieve lowon-state resistance. Formation of Ohmic contacts on n-type Ge rep-resents therefore an important step toward realization of MOSFETsand Schottky barrier S–D transistors in Ge.

Ti germanide contacts and sulfur-passivated Ni germanide con-tacts on n-Ge show barrier height values as low as 0.34 eV (Ref. 4)and 0.15 eV,5 respectively, but current–voltage (I -V ) of these

contacts are still clearly rectifying.5

It has been shown for Si/metalcontacts that imposing a thin insulator between low-work functionmetals and silicon can substantially reduce the Schottky barrier of the junction and decrease the junction resistance.6,7 With thisapproach, a Schottky barrier height for Mg on n-type Si was reducedfrom 0.45 to 0.2 eV, a reduction of the effective barrier height byapproximately 0.25 eV.7

Recently, we have shown that a similar approach can be used for n-type Ge.8 A few monolayers of single-crystal Ge3N4 effectivelyreduce the barrier height of Al contacts on n-type Ge from 0.6 tobelow 0.3 eV. We have also shown that the Fermi level pinning of metal contacts on n-type Ge could be reduced significantly, allowingone to control the barrier height by varying the work function of themetal. Since then, other groups have demonstrated a similar effect

for germanium using AlOx,9,10 GeOx,10 SiN,11 and MgO.12 Sput-tered AlOx and GeOx did not unpin the Fermi level but shifted theFermi level pinning point.10 AlOx deposited by Al evaporation andsubsequent oxidation showed a partial unpinning of the Fermi level,however , without linear relation between barrier height and workfunction.9 Sputtered SiN showed Fermi level pinning for thicknessesbelow 2.0 nm and an unpinned Fermi level for the thickness of 2.0 nm.11 Epitaxial MgO showed a barrier height of 0.35 eV for Fecontacts.12

The mechanisms of Fermi level unpinning for metal contacts bythe introduction of an insulating layer are still under debate. Ini-tially, we explained our results by the effective passivation of sur-face dangling bonds.8 Several groups explain their results bysuppressing the penetration of metal wave functions into the Gebandgap.9– 11 Recently, Zhou et al. confirmed our explanation by

showing that, in the case of epitaxial MgO, interface states relatedto native defects on Ge surfaces play a more important role in Fermilevel pinning than the penetration of metal electron wavefunctions.12

In the present work, we provide additional evidence for the latter mechanism. We have investigated the influence of a few monolayersof crystalline and amorphous Ge3N4 on the Schottky barrier heightof metal contacts on n-type Ge. Low temperature capacitance–volt-age (C-V ) measurements are used to accurately determine the barrier height. We will show that barrier heights of 0.09 eV or lower can beachieved on n-type Ge for Al contacts. We also demonstrate that theFermi level becomes unpinned through observation of a linear rela-tionship between barrier height and metal work function.

Experimental

Standard n-type Ge(111) substrates with a carrier concentrationof around 1 Â 1016 cmÀ3 and resistivity of  $0.15 X cm were used.These substrates were chemically cleaned to remove metallic con-tamination, particles, and native oxide from the surface, just beforeloading into an ultrahigh vacuum (UHV) system. Subsequently,annealing in vacuum ($1 Â 10À9 Torr) at 500C was performed todegas the samples. The cleanliness of the surface was confirmed byreflection high energy electron diffraction (RHEED), which showeda reconstructed surface. On this clean Ge surface, either crystalline(epitaxial) or amorphous Ge3N4 was formed by exposure to nitrogenplasma. Recently, we have experimentally shown that exposure of Ge(111) to a nitrogen plasma below 550C leads to the formation of an amorphous Ge3N4 layer while a thin crystalline Ge3N4 layer isformed at temperatures above 600C.13 The Ge

3N4

formed at hightemperature is most likely b-Ge3N4 with a hexagonalz E-mail: ruben.lieten@gmail.com

Journal of The Electrochemical Society, 158 (4) H358-H362 (2011)0013-4651/2011/158(4)/H358/5/$28.00 VC The Electrochemical Society

H358

Downloaded 16 Feb 2011 to 146.103.254.11. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

8/7/2019 2011 - Mechanisms of Schottky Barrier Control on n-Type Germanium Using Ge3N4 Interlayers - R.R. Lieten - JEC 158

http://slidepdf.com/reader/full/2011-mechanisms-of-schottky-barrier-control-on-n-type-germanium-using-ge3n4 2/5

crystallographic structure that can only form an epitaxial layer onthreefold symmetric Ge(111). The bandgap of  b-Ge3N4 has beencalculated to be 3.1 eV.14 In this work, crystalline Ge3N4 was grownat 700C, whereas amorphous Ge3N4 was formed at 200C. After the growth, the samples were taken out of the UHV system and im-mediately loaded into a metal deposition system to suppress oxideformation. Metal contacts with a diameter of 500 lm were deposited

using metal evaporation through a shadow mask. Complete cover-age of the sample by metal was used as the backside contact.Because of the large surface area, Ohmic behavior could beobserved for this contact. We have repeated this experiment for dif-ferent metals: Al, Cr, Co, Au, and Pt. In order to prevent the metalfrom reacting with ambient air, a Au capping layer was deposited ontop of Al, Cr, and Co contacts. X-ray photoelectron spectroscopymeasurements revealed a Ge3N4 thickness of 1.2 nm for both thecrystalline and amorphous Ge3N4. No postdeposition anneal wasperformed. C-V  measurements were performed between the frontand backside contacts at 77 K to extract the Schottky barrier height.15 As comparison, metal contacts were prepared on n-typeGe(111) and Ge(001) substrates without Ge3N4 interlayers. Thenative surface oxide was removed prior to contact deposition byexposure to dilute HF solution.

The Schottky barrier height of metal–semiconductor contacts isoften determined by I -V  measurements. This measurement requiresthe knowledge of the Richardson constant A* to determine the bar-rier height.2 The Richardson constant often depends on the process-ing conditions of the metal contact. C-V  measurements do not relyon this value and are therefore more accurate.15 To calculate thebarrier height from I -V  measurements, the contact area needs to beknown accurately, which is not the case for C-V measurements. Fur-thermore, C-V  measurements are less prone to defects at the inter -face that act as recombination centers and affect I -V measurements.2

The advantage of low temperature C-V  measurements is that itreduces the leakage currents allowing for accurate C-V  measure-ments to be extended to the voltages close to the flatbandconditions.

Results

The dependence of the capacitance on the applied voltage pro-vides information on the flatband voltage and thus on the Schottkybarrier height. The capacitance–voltage behavior of a Schottkydiode on a n-type semiconductor is given by the equation

C

A¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiqK Se0N D

2 V bi À V À kT =qð Þ;

s 

where A, V , N D, V bi, and K Se0 are the semiconductor contact area,the applied voltage, the donor concentration, the built in potential,and the permittivity of the semiconductor, respectively. PlottingCÀ2 vs V  gives a linear dependence with slope À2/A2qK Se0N D, and

intercept on the V -axis of V bi – kT/q. Figures 1 and 2 show C

À2

plot-ted against voltage for metal contacts on n-type Ge(111) andGe(001), respectively. A linear behavior is observed down to zerovoltage. At positive bias, the barrier to the metal for electrons in thegermanium decreases and the current increases, which makes the ca-pacitance model invalid. Therefore, linear extrapolation of  CÀ2 isneeded to accurately determine the intercept point with the voltageaxis. The Schottky barrier height UB is then calculated by the equa-tion UB ¼ V bi þ EC À EF, where EC and EF are the conduction bandenergy and Fermi level energy, respectively. The Schottky barrier heights versus metal work function are plotted in Figs. 3 and 4. It isclear that the Schottky barrier height for direct metal contacts on n-type Ge is almost independent of the metal work function. The sur-face orientation of the germanium also does not have any measura-ble effect on the barrier height values. The difference in the barrier height of Al, Cr, Co, Au, and Pt contacts for (111) and (001) surfaceis –0.02, 0.05, 0.06, –0.02 and –0.01 eV, respectively, which is close

to the accuracy limit of the measurement. A difference of 0.06 eVhas been previously reported for Ni contacts on Ge(111) andGe(001).16

As seen in Figs. 5, a linear dependence of CÀ2 is observed downto zero voltage for metal contacts on n-type Ge with amorphousGe3N4 interlayers. In contrast, as is shown in Fig. 6 for crystallineGe3N4 interlayers, a linear behavior could only be observed by sub-tracting a series capacitance of 1.7 Â 10À10 F. This series capaci-tance cannot be modeled by the thin Ge3N4 layer, which shouldhave a much larger capacitance when modeled as parallel plate ca-pacitor. Most likely, this capacitance is associated with a depletedlayer near the Ge surface, created reated during the high temperaturedeposition by out diffusion of dopants. The Schottky barrier heights,obtained from the C-V measurements, are plotted as functions of themetal work function in Fig. 7 and 8 and summarized in Table I. Incase of amorphous Ge3N4 the Al contacts show a very low barrier height and corresponding high leakage current. Therefore C-V mea-surement was not possible for these contacts and the barrier heightwas deduced from the current density at 77 K (1.5 A/cm2):0.096 0.05eV. The dependence of the barrier height on the metalwork function clearly indicates that the introduction of a thin amor-phous or crystalline Ge3N4 layer eliminates the Fermi level pinningand, therefore, allows for control of the barrier by choosing a metalwith appropriate work function.

Figure 1. (Color online) CÀ2 against applied voltage for metal contacts onn-type Ge(111), measured at 77 K.

Figure 2. (Color online) C

À2

against applied voltage for metal contacts onn-type Ge(001), measured at 77 K.

Journal of The Electrochemical Society, 158 (4) H358-H362 (2011) H359

Downloaded 16 Feb 2011 to 146.103.254.11. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

8/7/2019 2011 - Mechanisms of Schottky Barrier Control on n-Type Germanium Using Ge3N4 Interlayers - R.R. Lieten - JEC 158

http://slidepdf.com/reader/full/2011-mechanisms-of-schottky-barrier-control-on-n-type-germanium-using-ge3n4 3/5

The slope of CÀ2 in function of applied voltage V , in the case of metal contacts on bare Ge(111), shows a standard deviation of 7.0%. This variation can be explained by a difference in carrier con-centration N D or contact surface area A. As the same bulk Ge sub-strates were used, a difference in surface area or contact diameter ismore likely than a difference in carrier concentration. A variation incontact diameter with standard deviation of 1.8% would explain theobserved variation in CÀ2 slope. In case of amorphous Ge3N4, thestandard deviation in CÀ2 slope is 5.7%, which can be explained bya contact diameter standard deviation of 1.4%. These variations incontact diameter are reasonable for contact formation by shadowmasking. The variation in contact surface area or contact diameter has no influence on the calculation of the barrier height from C-V measurements.

The S parameter, given by the slope of the dependence of theSchottky barrier height on the metal work function, is a measure of the Fermi level pinning of metal contacts. Barrier heights of metalcontacts on bare germanium are weakly dependent on the metalwork function and, therefore, are characterized by a low value of theS parameter. In determination of the S parameter for germanium,one should take into account that the maximum achievable barrier height is limited by the bandgap. Therefore, a linear dependence of 

the barrier height on the metal work function is expected only for the limited range of the metal work functions. In the case of Ge3N4

interlayers, we did not observe saturation of the barrier height,except for the Pt contacts. We, therefore, calculate the parameter S without taking into account the values for Pt. The values of  Sparameter increase from 0.02 for bare Ge to 0.37 for amorphous and0.43 for crystalline Ge3N4 interlayer. Aluminum contacts on amor-phous and epitaxial Ge3N4 show barrier heights of 0.096 0.05 and0.06 0.1 eV at 77 K, respectively. These are the lowest barrier heights reported for metal contacts on n-type Ge.

The difference between the metal work function of Al [(4.17 eV(Ref. 17)] and the Ge electron affinity [4.1 eV (Ref. 18)] is 0.07 eV.Hence, a barrier height of 0.07 eV would be expected for an Al/ n-Ge contact in the absence of surface states and interface dipoles.Metals with a work function lower than that of aluminum, e.g., Mg,could be used to reduce the Schottky barrier height further.

Discussion

In the following section, we will attempt to provide a qualitativeexplanation of our results. The Fermi level pinning occurs at semi-conductor surfaces and interfaces because of the presence of 

Figure 3. (Color online) Schottky barrier heights of metal contacts on n-type Ge(111), deduced from C-V  measurements at 77 K. The Schottky bar-rier for metal contacts on n-type Ge is almost independent of the metal workfunction.

Figure 4. (Color online) Schottky barrier heights of metal contacts on n-type Ge(001), deduced from C-V  measurements at 77 K. The Schottky bar-rier for metal contacts on n-type Ge is almost independent of the metal workfunction.

Figure 5. (Color online) CÀ2 against applied voltage for metal contacts onn-type Ge(111) with a few monolayers of amorphous Ge3N4, measured at 77K. No measurement was possible on Al contacts because of very low barrier height and corresponding high leakage currents.

Figure 6. (Color online) CÀ2 against applied voltage for metal contacts onn-type Ge(111) with a few monolayers of single crystalline Ge3N4, measured

at 77 K. The data have been corrected by subtracting a series capacitance toobtain linear behavior.

Journal of The Electrochemical Society, 158 (4) H358-H362 (2011)H360

Downloaded 16 Feb 2011 to 146.103.254.11. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

8/7/2019 2011 - Mechanisms of Schottky Barrier Control on n-Type Germanium Using Ge3N4 Interlayers - R.R. Lieten - JEC 158

http://slidepdf.com/reader/full/2011-mechanisms-of-schottky-barrier-control-on-n-type-germanium-using-ge3n4 4/5

rechargeable states, which shield the semiconductor from externallyapplied electric fields. Charged surface states can be attributed toseveral types of defects like surface dangling bonds or vacancydefects in the immediate subsurface region of Ge. In the case of ger-manium, dangling bonds show an amphoteric behavior, i.e., theycan either give or accept an electron. The nature (donor or acceptor)of the defects depends on the location of the bulk Fermi energy withrespect to the Fermi level stabilization energy EFS, located at about4.9 eV (Ref. 19) below the vacuum level, i.e., close to the Ge va-lence band edge. Location of  EFS coincides with the so-calledcharge neutrality level that is defined as the energy separatingdonor-like from acceptor-like states.3 Because the surface Fermienergy is pinned close to the valence band, as is shown in Fig. 9, allmetal contacts on n-type Ge lead to rectifying behavior. Another distinctly different concept of the Fermi energy pinning considers a

situation in which electrons in the metal layer tunnel into the semi-conductor gap, resulting in metal induced gap states that can becharged and reduce the dependence of the Schottky barrier heighton the metal work function.20

We have argued previously that the introduction of a thin Ge 3N4

layer 8 unpins the Fermi level by passivating the interface states. Inthis case, the band diagram of metal contacts on germanium can berepresented by Fig. 10, with the assumption that the Ge3N4 does notintroduce a dipole at the Ge3N4/Ge interface. Others have explained

the unpinning of the Fermi level by the suppr ession of penetrationof metal wave functions into the Ge bandgap.10,11 This conclusionwas drawn from a dependence of the barrier height on the thicknessof the insulator layer. An optimum thickness of 2.0 nm hasbeen reported for sputtered SiN,11 leading to a barrier of 0.1 eVfor Al. A barrier of 0.4 eV was obtained for 1.0 nm SiN. However,sputtered GeOx and AlOx did not unpin the Fermi level but shiftedthe position of Fermi level pinning.10 This indicates that it is impor-tant not only to separate the metal from the semiconductor but alsoto sufficiently passivate the semiconductor surface. Furthermore,our experiments show that 1.2 nm crystalline or amorphous Ge3N4

effectively unpins the Fermi level, where, in case of SiN, a thicknessof 2.0 nm is required.11 If the pinning is only due to the penetrationof metal wave functions, a Ge3N4 thickness beyond 2.0 nm wouldbe required because of the smaller bandgap and band offsets of 

Ge3N4 (Ref. 14) with Ge in respect with SiN. We have alsoobserved a barrier height of 0.1 eV for 0.6 nm crystalline Ge3N4,again indicating that passivation of the Ge surface is required tounpin the Fermi level for Al contacts. Recently, the role of interfacestates related to native defects on Ge surfaces has been shown toplay a more important role in Fermi level pinning than the penetra-tion of metal electron wave functions.12,21 Although we cannotexclude the possibility that metal induced gap states play some rolein the Fermi level pinning, the most likely and consistent explana-tion of our results is that the pinning is caused by the dangling bondsor other defects. The growth of Ge3N4 removes the defects by con-suming the top defective layer, thus forming a high quality interfacebetween Ge and Ge3N4.

13 Amor phous Ge3N4 has been shown to bea good passivation of Ge(001).22 It is also been shown by theoreticalcalculations that crystalline Ge3N4 is a passivation layer for 

Ge(111) surfaces.

14

We suggest that a thin Ge3N4 layer, regardless

Figure 8. (Color online) Schottky barrier height of metal contacts on n-typeGe with a few monolayers of crystalline Ge3N4, determined by low tempera-

ture C-V measurements. Metals with increasing work function were used: Al,Cr, Co, Au, and Pt.

Table I. The barrier height of metal contacts on n-type(1111)with a few monolayers of crystalline and amorphous Ge3N4 meas-ured by low temperature C–V  technique with accuracy of 0.1 and0.03 eV, respectively. The barrier height for Al contacts withamorphous Ge3N4 interlayers was measured by low temperatureI–V : 0.0960.05 eV. Also added are the metal work functions fordifferent metals from Ref. 17 and the barrier height of bare metalcontacts on n–Ge(111) and Ge(001) with accuracy of 

0.03 eV.

Barrier height (eV)

Ge(001) Ge(111)

Metal Work function (eV) no Ge3N4 no Ge3N4 c-Ge3N4 a-Ge3N4

Al 4.17 0.70 0.72 0.02 0.09

Cr 4.5 0.65 0.59 0.10 0.41Co 5.0 0.70 0.63 0.21 0.50

Au 5.38 0.71 0.73 0.49 0.67Pt 5.64 0.72 0.72 0.54 0.68

Figure 9. Metal/semiconductor contact with interface states before (leftside) and after (right side) contact. The interface states pin the Fermi level

close to the charge neutrality level U0, implying a Schottky barrier independ-ent of the metal work function.

Figure 7. (Color online) Schottky barrier height of metal contacts on n-typeGe with a few monolayers of amorphous Ge3N4, determined by low tempera-ture C-V measurements. Metals with increasing work function were used: Al,Cr, Co, Au, and Pt.

Journal of The Electrochemical Society, 158 (4) H358-H362 (2011) H361

Downloaded 16 Feb 2011 to 146.103.254.11. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

8/7/2019 2011 - Mechanisms of Schottky Barrier Control on n-Type Germanium Using Ge3N4 Interlayers - R.R. Lieten - JEC 158

http://slidepdf.com/reader/full/2011-mechanisms-of-schottky-barrier-control-on-n-type-germanium-using-ge3n4 5/5

whether it is amorphous or crystalline, will reduce the number of dangling bonds at the Ge surface and, therefore, the interface statedensity. The difference in Pauling electronegativity of nitrogen andgermanium is 1.0, which would make the Ge–N bonds partiallyionic. From this polarized pure covalent bonding, the Fermi levelpinning is expected to be less effective than in case of pure covalentbonding.23 The Ge3N4 also separates the metal from the semicon-ductor and, therefore, suppresses the penetration of metal wavefunctions into the Ge. However, as mentioned previously, webelieve this effect is less important than the passivation of interfacedefects.

Nitridation of germanium is well suited to obtain a very thin (<2nm) and smooth layer of amorphous or epitaxial Ge3N4 that coversthe germanium surface completely. This has been demonstrated byHRTEM imaging for 1.2 nm of Ge3N4 on germanium.13 In general,thin (2 nm) and smooth layers can be obtained by epitaxialgrowth, where adsorbed species (atoms or molecules) diffuse over the surface until a low energy lattice site is reached. Obtaining thin(2 nm) layers that cover the surface completely seems much moredifficult when using sputtering, as the surface diffusion is signifi-cantly reduced at temperatures commonly used for sputtering.Layers of less than 2 nm thickness, obtained by sputtering, are notsmooth enough to completely cover the surface that will still exhibitdangling bonds. Increasing the thickness up to 2 nm or more leadsto better surface coverage and reduction in the number of danglingbonds. This would explain the thickness dependence observed bygroups that use sputter deposition.9– 11

Summary

In summary, we have investigated the influence of a few mono-layers of amorphous and crystalline Ge3N4 on metal contacts on Ge.

Low temperature capacitance–voltage measurements are used toaccurately determine the barrier height. Both amorphous and epitax-ial Ge3N4 effectively unpin the Fermi level, resulting in a linear de-pendence of barrier height on metal work function with pinning fac-tor  S of 0.43 and 0.37 for a-Ge3N4 and c-Ge3N4, respectively.Aluminum contacts on 1.2 nm amorphous and epitaxial Ge3N4

show barrier heights of 0.096 0.05 and 0.06 0.1 eV, respectively.

While the epitaxial growth of Ge3N4 requires temperatures above600C, amorphous layers of Ge3N4 can be formed at muchlower temperatures. As no postdeposition annealing is necessary,amorphous Ge3N4 can be used to form Ohmic contacts on n-type Geat a low thermal budget and can easily be transferred to the SiGealloys of different composition. Our results provide a convincingsupport to the claim that the Fermi level unpinning is achieved due

to the passivation of interface states related to defects at the metal/ Ge interface. The experimental results have been explained in termsof (1) reducing the trap density at the Ge3N4/Ge interface by termi-nating Ge dangling bonds and (2) the ionic character of the covalentGe–N binding, which limits the number of Ge3N4 surface states. AsGe3N4 is a wide bandgap semiconductor, this layer should be keptthin enough to allow electrons to tunnel easily through it. The lowbarrier heights of metal/Ge3N4 contacts on n-type Ge can be used toimprove contact resistances and Schottky barrier S–D transistors.Furthermore, this contact is not stretched into the Ge as for germa-nides and is therefore shallow. In the case of Ge-based fie ld effecttransistors, Ge3N4 might be used for both gate passivation13,22 andcontact improvement.

Acknowledgments

R.R.L acknowledges support as Research Fellow of the BelgianAmerican Educational Foundation (BAEF) and as Research Fellowof the Research FoundationFlanders (FWO). Erwin Vandenplas isacknowledged for assistance in metal depositions.

References

1. C. O. Chui, L. Kulig, J. Moran, W. Tsai, and K. C. Saraswat, Appl. Phys. Lett., 87,091909 (2005).

2. Dieter Schroder, Semiconductor Material and Device Characterization, John Wil-ley & Sons, New York (1998).

3. A. Dimoulas, P. Tsipas, A. Sotiropoulos, and E. K. Evangelou, Appl. Phys. Lett.,89, 252110 (2006).

4. D. D. Han, Y. Wang, D. Y. Tian, W. Wang, X. Y. Liu, J. F. Kang, and R. Q. Han,Microelectron. Eng., 82, 93 (2005).

5. K. Ikeda, Y. Yamashita, N. Sugiyama, N. Taoka, and S. Takagi, Appl. Phys. Lett.,88, 152115 (2006).

6. M. A. Sobolewski and C. R. Helms, Appl. Phys. Lett., 54, 638 (1989).7. D. Connelly, C. Faulkner, P. A. Clifton, and D. E. Grupp, Appl. Phys. Lett., 88,

012105 (2006).8. R. R. Lieten, S. Degroote, M. Kuijk, and G. Borghs, Appl. Phys. Lett., 92, 022106

(2008).9. Y. Zhou, M. Ogawa, X. H. Han, and K. L. Wang, Appl. Phys. Lett., 93, 202105

(2008).10. T. Nishimura, K. Kita, and A. Toriumi, Appl. Phys. Express, 1, 051406 (2008).11. M. Kobayashi, A. Kinoshita, K. Saraswat, H. S. P. Wong, and Y. Nishi, J. Appl.

Phys., 105, 023702 (2009).12. Y. Zhou, W. Han, Y. Wang, F. Xiu, J. Zou, R. K. Kawakami, and K. L. Wang,

Appl. Phys. Lett., 96, 102103 (2010).13. R. R. Lieten, S. Degroote, M. Kuijk, and G. Borghs, Appl. Phys. Lett., 91, 222110

(2007).14. M. Yang, S. J. Wang, Y. P. Feng, G. W. Peng, and Y. Y. Sun, J. Appl. Phys., 102,

013507 (2007).15. J. D. van Otterloo and L. J. Gerritsen, J. Appl. Phys., 49, 723 (1978).16. F. G. Allen and A. B. Fowler, J. Phys. Chem. Solids, 3, 107 (1957).

17. CRC Handbook of Chemistry and Physics, 85th ed., pp. 12–130., CRC (2004).18. H. J. Kuhr and W. Ranke, Solid State Commun., 61, 285 (1987).19. W. Walukiewicz, Physica B, 302–303, 123 (2001).20. V. Heine, Phys. Rev. B, 138, 8 (1965).21. K. Yamane, K. Hamaya, Y. Ando, Y. Enomoto, K. Yamamoto, T. Sadoh, and M.

Miyao, Appl. Phys. Lett., 96, 162104 (2010).22. T. Maeda, T. Yasuda, M. Nishizawa, N. R. Miyata, Y. Morita, and S. Takagi,

Appl. Phys. Lett., 85, 3181 (2004).23. S. Kurtin, T. C. McGill, and C. A. Mead, Phys. Rev. Lett., 22, 3 (1969).

Figure 10. Metal/Ge3N4/semiconductor contact without interface statesbefore (left side) and after (right side) contact, assuming the Ge3N4 does notintroduce a dipole at the Ge3N4/Ge interface. The Ge3N4 reduces Fermi levelpinning by passivating germanium dangling bonds. The Schottky barrier height is dependent on the metal work function. The Fermi level is thereforenot pinned.

Journal of The Electrochemical Society, 158 (4) H358-H362 (2011)H362