Fermi-Level Unpinning Technique with Excellent Thermal Stability

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Fermi-Level Unpinning Technique with Excellent Thermal Stability for n-type Germanium Gwang-Sik Kim, Seung-Hwan Kim, Tae In Lee, Byung Jin Cho, Changhwan Choi, Changhwan Shin, Joon Hyung Shim, Jiyoung Kim, and Hyun-Yong Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10346 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on October 1, 2017

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ACS Applied Materials & Interfaces

Fermi-Level Unpinning Technique with Excellent Thermal Stability for n-type Germanium Gwang-Sik Kim,1 Seung-Hwan Kim,1 Tae In Lee,2 Byung Jin Cho,2 Changhwan Choi,3 Changhwan Shin,4 Joon Hyung Shim,5 Jiyoung Kim,6 and Hyun-Yong Yu1,* 1

School of Electrical Engineering, Korea University, Seoul 02841, Korea

2

School of Electrical Engineering, Korea Advanced Institute of Science and Technology,

Daejeon 34141, Korea 3

Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Korea

4

School of Electrical and Computer Engineering, University of Seoul, Seoul 02504, Korea

5

School of Mechanical Engineering, Korea University, Seoul 02841, Korea

6

Department of Materials Science and Engineering, The University of Texas at Dallas,

Richardson, TX 75080, USA *

corresponding author: [email protected]

KEYWORDS: germanium, metal–interlayer–semiconductor structure, thermal stability, Schottky barrier height, tantalum nitride, aluminum-doped zinc oxide

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ABSTRACT

A metal–interlayer–semiconductor (M–I–S) structure with excellent thermal stability and electrical performance for a non-alloyed contact scheme is developed and considerations for designing thermally stable M–I–S structure are demonstrated based on n-type germanium (Ge). A thermal annealing process makes M–I–S structures lose its Fermi-level unpinning and electron Schottky barrier height reduction effect in two mechanisms: (1) oxygen (O) diffusion from the interlayer to the contact metal due to high reactivity of a pure metal contact with O and (2) interdiffusion between the contact metal and semiconductor through grain boundaries of the interlayer. A pure metal contact such as titanium (Ti) provides very poor thermal stability owing to its high reactivity with O. A structure with a tantalum nitride (TaN) metal contact and a titanium dioxide (TiO2) interlayer exhibits moderate thermal stability up to 400 °C because TaN has much lower reactivity with O than Ti. However, the TiO2 interlayer cannot prevent the interdiffusion process because it is easily crystallized during thermal annealing and its grain boundaries act as diffusion path. A zinc oxide (ZnO) interlayer doped with group-III elements, such as an aluminum-doped ZnO (AZO) interlayer, acts as a good diffusion barrier due to its high crystallization temperature. A TaN/AZO/n-Ge structure provides excellent thermal stability above 500 °C as it can prevent both O diffusion and inter-diffusion processes; hence, it exhibits Ohmic contact properties for all thermal annealing temperatures. This work shows that in order to fabricate a thermally stable and low resistive M–I–S contact structure, the metal contact should have low reactivity with O and a low work-function, and the interlayer should have a high crystallization temperature and a low conduction band offset to Ge. Furthermore, new insights are provided for designing thermally stable M–I–S contact schemes for any semiconductor material that suffers from the Fermi-level pinning problem.


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1. INTRODUCTION Germanium (Ge) has emerged as a sound substitute for silicon (Si) in the next-generation complementary metal-oxide-semiconductor (CMOS) technology, owing to its higher carrier mobility. An ultra-low source/drain (S/D) contact resistance should be achieved to utilize the advantages of Ge in nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs).1 However, it is difficult to obtain an ultra-low contact resistance in n-type Ge (n-Ge) due to the low solubility and high diffusivity of n-type dopants2 and the large electron Schottky barrier height (SBH) induced by severe Fermi-level pinning (FLP) at the metal/Ge interface. When the metal is in contact with Ge, the Fermi level on the metal side is strongly pinned near the edge of the valence band of Ge, resulting in an SBH of approximately 0.55 eV regardless of the value of the metal work-function, as shown in Figure 1a.3,4 Two approaches can be employed to achieve a low-resistance contact in n-Ge: an alloyed germanide technique and a non-alloyed SBH reduction technique. However, the germanide technique can cause a non-uniform contact resistance problem in nanoscale fin field-effect transistors (FinFETs) because multiple fins form a single transistor in recent CMOS technologies and the germanide process is a diffusion-based process performed by thermal annealing. Therefore, a non-alloyed contact structure should be developed to reduce the electron SBH of the metal/n-Ge contact by alleviating the FLP. A metal–interlayer–semiconductor (M–I–S) structure has been developed as a non-alloyed contact structure in n-Ge. The FLP can be alleviated and the electron SBH can be significantly reduced by inserting ultra-thin interlayers between the metal and the n-Ge. The interlayers reduce metal-induced gap states (MIGS), which are the main causes of FLP in Ge, by preventing the penetration of electron wave functions from the metal side into Ge. The Fermi level on the metal side is unpinned when the M–I–S structure is formed, and a metal with a low work function


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should be used to obtain a low electron SBH. For the interlayer, materials with a low conduction band offset (CBO) to Ge and high film conductivity should be inserted to minimize tunneling and series resistance through the contact. Among many interlayer candidates, TiO25–12 and ZnO13–15 are considered the most promising materials in Ge CMOS technology due to their low CBO to Ge. Figure 1b describes the energy band diagram of an M–I–S structure with a reduced electron SBH resulting from the adoption of a contact metal with low metal work function and an interlayer with a low CBO to Ge. In order to successfully transfer an M–I–S structure to realistic uses in Ge electronics, the M–I–S contact structure should be thermally stable to endure the thermal budget from post thermal processes such as the back-end of line (BEOL) process.16 The thermal stability of M–I–S structures has not been widely studied and only a few works have proposed thermally stable M– I–S contact schemes.17 Therefore, an investigation on the thermal stability of M–I–S structures in Ge and development of thermally stable M–I–S structures with excellent electrical performance is needed for the successful transfer of M–I–S structures into practical processes of advanced semiconductor devices as well as Ge-based electronics. In this work, an M–I–S structure with improved thermal stability is developed by incorporating a tantalum nitride (TaN) metal contact and a zinc oxide (ZnO)-based interlayer that can endure thermal processes significantly better, and exhibit even better electrical properties than a titanium (Ti) metal contact and a titanium dioxide (TiO2) interlayer. Furthermore, the loss of the SBH lowering effect of the M–I–S structure during thermal annealing is demonstrated through electrical and morphological analyses of different M–I–S structures, and the important factors that should be considered for designing a thermally stable M–I–S structure are also revealed.


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2. EXPERIMENTAL DETAILS 2.1. Fabrication of M–I–S Structures A moderately arsenic (As)-doped n-type (100) Ge substrate (1×1017 cm–3) was prepared via cyclic cleaning using diluted a 1:25 HF solution and deionized water. Two kinds of interlayers— TiO2 and ZnO—were deposited onto the cleaned Ge substrate by an atomic layer deposition (ALD) process under process temperatures of 250 °C and 150 °C, respectively. For ZnO ALD, a trimethylaluminum (TMA) precursor was added in addition to a main precursor, diethylzinc (DEZ), at a process cycle ratio of 1:6; thus, a low-resistance 2.6 wt.% Al-doped ZnO (AZO) layer was formed. The AZO layer has a high electrical conductivity and was adopted as an interlayer in the Ge M–I–S structure to minimize tunneling and series resistance.15 Both TiO2 and ZnO films are amorphous despite the ALD process, because their thicknesses are very small, i.e., a few nm. A 10 nm thick Ti metal contact and a 50 nm thick Al electrode were evaporated after standard photolithography onto the TiO2/n-Ge and the AZO/n-Ge substrates, and a lift-off process was carried out to fabricate the Ti/TiO2/n-Ge and the Ti/AZO/n-Ge structures. A 5 nm thick TaN metal contact and a 50 nm thick Al electrode were also deposited onto both substrates using DC sputtering to fabricate the TaN/TiO2/n-Ge and the TaN/AZO/n-Ge structures (more details are in the Supporting Information). The Al electrode was etched by an aluminum wet etchant and the TaN contact was etched using inductively-coupled-plasma reactive ion etching (ICP-RIE) with BCl3/Cl2-based plasma after the standard photolithography. 2.2. Evaluation of Thermal Stability Three kinds of M–I–S structures—Ti/TiO2/n-Ge, TaN/TiO2/n-Ge, and TaN/AZO/n-Ge—with various interlayer thicknesses, and two kinds of metal–semiconductor (M–S) structures—Ti/n-


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Ge and TaN/n-Ge structures—were annealed in low-vacuum by using a tube-type furnace with various annealing temperatures in the range of 400 °C to 600 °C for 30 min. A back-side metal contact was provided to minimize the back-side contact resistance of all structures after thermal annealing. 2.3. Electrical and Structural Characterizations The current density-voltage (J–V) characteristics of all the fabricated structures with different thermal annealing conditions were measured to investigate the change of electrical characteristics of M–I–S structures with annealing temperature. The change of the reverse current density (JR) was mainly analyzed for the electrical characterization. The structural modification of M–S and M–I–S structures for various thermal annealing conditions was observed using transmission electron microscopy (TEM). The binding characteristics at interfaces between films and crystallization characteristics of the interlayer for various thermal annealing conditions were also analyzed using X-ray photoelectron spectroscopy (XPS) and Xray diffraction (XRD) measurements, respectively.


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3. RESULTS AND DISCUSSION The Ti/TiO2/n-Ge structure is considered one of the most suitable M–I–S contact structures for Ge-based electronic devices. The Ti has been adopted as the contact metal in the M–I–S structure for n-type semiconductors due to its low work function (approximately 4.33 eV).5–8,11–18 The TiO2 interlayer has been adopted as the interlayer due to its low CBO to Ge.5–12,16–18 In this work, however, the Ti/TiO2/n-Ge structure is demonstrated to have very poor thermal stability. The reasons for this poor thermal stability and two important factors that affect the thermal stability of M–I–S structures are analyzed by first examining the Ti/TiO2/n-Ge structure. 3-1. Selection of the Contact Metal Figure 2a shows the box plots indicating (1) the JR values and its variation and (2) the average JR values for the Ti/TiO2/n-Ge structure at a bias voltage of −0.5 V for different TiO2 thicknesses and after thermal annealing at 400 °C, 500 °C, and 600 °C for 30 min along with data obtained without annealing (i.e., at room temperature (RT)). The figure demonstrates that the value of JR of the M–I–S structure is considerably increased compared to that for the M–S structure, which is consistent with previous findings.16–18 As shown in Figure 2b, insertion of the TiO2 interlayer considerably increases JR by alleviating the FLP and reducing the electron SBH at room temperature. The JR values for each sample without thermal annealing show little variation, indicating that the M–I–S structures using a TiO2 interlayer exhibit uniform and reliable electrical characteristics. However, significant changes appear when both M–S (Ti/n-Ge) and M– I–S (Ti/TiO2/n-Ge) structures are annealed and these structures show opposite behaviors. For example, when both structures are annealed at 400 °C for 30 min, the value of JR increases by a factor of ~40 for the M–S structure whereas it decreases by approximately two orders of


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magnitude for the M–I–S structure. Moreover, the standard deviation (s) of log(JR) values, which indicates the variation of JR, increases remarkably, resulting in unstable and unreliable contact properties for both the M–S and M–I–S structures (more details are in the Supporting Information). The increase of JR and s(log(JR)) for the M–S structure results from the formation of TixGey alloys during the annealing process19 and the As+ dopant segregation effect20,21 while the M–I–S structure returns to the form of the M–S structure. The TixGey alloy reduces the factor of series resistance through the contact because it provides small contact resistance at both metal/TixGey and TixGey/Ge interfaces than the direct Ti contact does. Moreover, the As+ segregation effect can induce a significant effect on the increase of JR even for the moderate doping level of the n-Ge substrate because the substrate doping concentration is close to a branch point that the conduction mechanism is changed from thermionic emission to thermionic field emission. The electrical degradation of the M–I–S structure is due to O diffusion from TiO2 to Ti. Titanium is so reactive with O that it easily gathers O atoms from TiO2.22,23 Because of O diffusion during the annealing process, the TiO2 layer turns into metallic Ti(O) and the metal/interlayer interface becomes difficult to discriminate. Therefore, the M–I–S structure reverts to the M–S structure, which exhibits the Schottky property with a large electron SBH as shown in Figure 2b. This is a mechanism wherein the M–I–S structure loses its SBH reduction effect even at a low annealing temperature below 400 °C when the reactivity of its contact metal with O is very high.22 For the annealing condition of 500 °C, all the structures show significantly larger variations of JR compared to those for 400 °C, i.e., almost a factor of two increases of s(log(JR)) are observed (more details are in the Supporting Information). It is difficult to distinguish Ti/TiO2(1 nm)/n-Ge from Ti/n-Ge by their electrical characteristics. This indicates that most O atoms in the


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1 nm thick TiO2 layer diffuse to the Ti layer and are consumed, and the metallization process forming TixGey occurs in Ti/TiO2(1 nm)/n-Ge at a similar level to that in Ti/n-Ge. Interestingly, the results of thermal annealing at 600 °C are contrary to those observed at 500 °C. The values of JR of Ti/n-Ge and Ti/TiO2(1 nm)/n-Ge decrease and those of Ti/TiO2(2 nm)/n-Ge and Ti/TiO2(3 nm)/n-Ge increase compared to those at 500 °C. As shown in Figure 2b, the reason for the decrease of the average value of JR in Ti/n-Ge and Ti/TiO2(1 nm)/n-Ge structures after thermal annealing at 600 °C is the appearance of an abnormal exponential J–V shape at the reversebiased region, which differs from Ohmic (linear) and Schottky (logarithmic) curves (more details are in the Supporting Information). This abnormal J–V shape begins to appear when the interlayer is absent or very thin; thus, it is a result of considerable metallization of the Ge surface by the metal. The relationship between the interlayer thickness and the appearance of the abnormal J–V shape indicates that O diffusion occurs first, and is subsequently followed by metallization of the Ge surface. Oxygen diffusion and metallization occur in this specific order because metallization of the Ge surface is impeded by the interlayer (more details are in the Supporting Information). The structural deformation was verified using TEM analyses of the Ti/TiO2/n-Ge structure with and without thermal annealing at 600 °C as shown in Figure 3. Clear interfaces are visible in Ti/TiO2/n-Ge before the thermal annealing as shown in Figure 3a. However, after thermal annealing at 600 °C, the TiO2 interlayer disappears and most of the Ti atoms and even Al atoms diffuse into Ge, thus forming a metal-Ge alloy, which results in very rough and unreliable interface characteristics as shown in Figure 3b. From the electrical and morphological analysis, it is concluded that the degradation of the M–I–S structure during thermal annealing occurs in two sequential processes: (1) disappearance of the interlayer through O diffusion to the contact metal and (2) metallization of Ge by the


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contact metal or even the electrode. Therefore, the first consideration for a thermally stable M–I– S structure should be the prevention of O diffusion from the interlayer to the contact metal and, accordingly, a metal with low reactivity with O atoms must be adopted as the contact metal of the M–I–S structure. Thus, an improvement of thermal stability of the M–I–S structure should be achieved by initially blocking O diffusion from the interlayer to the contact metal. Due to its low reactivity with O atoms, a metal nitride is a good candidate for replacing a pure metal contact such as Ti. Metal nitrides have been widely used as a diffusion barrier for Cu interconnections or a gate metal in conventional CMOS processes due to its excellent thermal stability.24,25 The incorporation of nitrogen (N) in pure metals such as Ti and Ta can suppress the dissolution of O atoms and enable the film to maintain its resistivity by reducing the temperature coefficient of resistivity.26 Among metal nitrides, TaN is considered the most appropriate contact metal for the M–I–S structure in n-Ge because it has a very low work-function in the range of 3.4 – 4.1 eV.27 The N content of the TaN film was determined by balancing its reactivity with O and the film resistivity (more details are in the Supporting Information). Figure 4a shows the electrical characteristics of the TaN/TiO2/n-Ge structure, which replaces Ti with TaN in the Ti/TiO2/n-Ge structure. The TaN/n-Ge structure shows three orders of magnitude larger JR values compared to those for Ti/n-Ge, and it will be discussed later. The TaN/TiO2/n-Ge structures exhibit JR values comparable to those for Ti/TiO2/n-Ge structures without thermal annealing. This indicates that the work function of the fabricated TaN film is comparable to that of Ti; thus, TaN is also suitable as the contact metal for the M–I–S structure in n-Ge. The change of electrical characteristics of the TaN/TiO2/n-Ge structure with thermal annealing temperature is quite different from that of the Ti/TiO2/n-Ge structure. In contrast to the


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behavior exhibited by the structure with the Ti contact metal, both M–S and M–I–S structures with the TaN contact metal at the thermal annealing condition of 400 °C retain their SBH reduction effects to some degree, and still show Ohmic or quasi-Ohmic characteristics with almost unchanged JR variation, as shown in Figure 4b (more details are in the Supporting Information). A slight degradation of JR of the M–I–S structures originates from the resistivity degradation of the Al electrode after thermal annealing: the film resistivity of the Al(50 nm)/TaN(5 nm) stack increases from 50 µΩ·cm to 111 µΩ·cm (more details are in the Supporting Information). Figure 4 shows that the thermal stability of the M–I–S structure with the TiO2 interlayer is considerably improved by introducing TaN instead of Ti because the TaN metal contact prevents O diffusion from the interlayer to the contact metal due to its low reactivity with O. Very few O atoms move to the TaN metal contact; thus, the interlayer does not collapse and the effect of the M–I–S structure is maintained after thermal annealing at 400 °C. The TaN/TiO2/n-Ge structure, however, exhibits very low values of JR that are more than two orders of magnitude lower than those without thermal annealing, corresponding to the Schottky J–V property, shown in Figure 4b, after thermal annealing at 500 °C. The presence of the TiO2 interlayer is not evident after thermal annealing at 500 °C; thus, the structure shows the Schottky M–S property. The values of JR of all structures increase with larger variations after thermal annealing at 600 °C, as shown in Figure 4a and Figure 4b. This is due to the metallization of Ge with Ta and Al, which results from the diffusion of Ta from TaN and Al from the electrode to Ge, as shown in Figure 5a and Figure 5b (more details are in the Supporting Information). The metallization process corresponding to the diffusion of atoms in the contact metal to Ge results from two factors. First, the TaN metal contact of the M–I–S structure must have lower thermal stability than the typical TaN film used as a diffusion barrier in the BEOL process because the N


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content in this work is intentionally lowered to obtain a low film resistivity24 (more details are in the Supporting Information). Second, the TiO2 interlayer is easily crystallized at a moderate annealing temperature of approximately 400 °C or less and it cannot act as a diffusion barrier between the contact metal and Ge because the inter-diffusion process can easily occur along grain boundaries of a poly-crystalline TiO2 film.17,28 Figure 5c shows XRD patterns of TiO2 layer for the thermal annealing temperatures, which show crystallization characteristics of amorphous TiO2 layer. From the XRD analysis, it is found that the amorphous TiO2 layer is easily crystallized at 400 °C and more crystallized at higher temperature. The first problem cannot be overcome because high film resistivity is not desired for the contact scheme of semiconductor devices. Therefore, the only method to enhance the thermal stability of the M–I–S structure with the TaN metal contact is to adopt an interlayer with a higher crystallization temperature than that of TiO2 as well as a low CBO to Ge. 3-2. Selection of the Interlayer The thermal stability of M–I–S structures with TiO2 in n-Ge can be significantly improved by adopting a thermally stable TaN film as the contact metal. However, the failure of the M–I–S structure is observed when the TaN/TiO2/n-Ge structure is annealed at temperatures over 500 °C because the TiO2 interlayer becomes crystallized during thermal annealing and cannot act as a diffusion barrier between the contact metal and Ge. Thus, the interlayer selection must be considered for improving the thermal stability of the structure. A ZnO-based interlayer can be the best option in terms of thermal stability and electrical properties. Many studies have shown that a ZnO-based layer is a good diffusion barrier between metals and semiconductors, or between metals themselves29,30 because ZnO has a higher


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crystallization temperature of more than 500 °C. Therefore, ZnO can maintain its amorphous properties after thermal annealing, which is desirable for better diffusion blocking.28 In terms of electrical properties, a ZnO-based interlayer has a nearly zero CBO to Ge; thus, it has a very small tunneling resistance between the metal and Ge, similar to that for the TiO2 interlayer.13–15 Furthermore, a ZnO-based interlayer has an additional advantage over the TiO2 interlayer in the doping process of the interlayer. An increase in the conductivity of the interlayer is important because it can considerably improve the current flow through the M–I–S structure by reducing the tunneling resistance and series resistance of the interlayer.11,13,14 In contrast to doping of TiO2, which is relatively difficult,6 ZnO can be easily doped by incorporating some amount of n-type dopant such as aluminum (Al) or gallium (Ga) to reduce its tunneling resistance and series resistance further15. Moreover, those dopants in ZnO can help suppress its crystallization process when the film undergoes thermal annealing by increasing its crystallization temperature.31 Zinc oxide has a nearly zero CBO to Ge and a higher crystallization temperature than TiO2. Moreover, the group-III n-type dopants can reduce its tunneling and series resistance and increase its crystallization temperature, which improves the electrical performance and thermal stability of the M–I–S structure. Therefore, a ZnO interlayer doped with group-III elements is considered as the best interlayer material for improving both the electrical and thermal characteristics. Accordingly, an AZO film wherein ZnO is doped with Al was selected as the interlayer in this work. Figure 6a shows the electrical characteristics of the TaN/n-Ge and TaN/AZO/n-Ge structures with different AZO thicknesses and thermal annealing conditions. As shown in Figure 6b, the TaN/n-Ge structure has JR values comparable to those of the TaN/AZO/n-Ge structure, because the Ge-N bonds that facilitate reduction of the electron SBH are formed at the TaN/n-Ge


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interface during the TaN sputtering process.32 The effect of Ge-N bonds to reduce the electron SBH of the metal/n-Ge contact has been also studied with a TiN/n-Ge structure.1,33 The Ge-N bonds form an interface dipole at the metal nitride/Ge interface and the dipole can reduce the SBH of the contact.32 The presence of Ge-N bonds was verified using an X-ray photoelectron spectroscopy (XPS) analysis on the TaN/n-Ge structure as shown in Figure 7a, where the binding energy of the Ge-N bond is located at 397.6 eV in the N 1s spectra.34 The values of JR for the TaN/AZO/n-Ge structures are almost a factor of three larger than those of the TaN/TiO2/n-Ge structures without thermal annealing; moreover, there is little dependence of JR on the interlayer thickness due to the improved interlayer conductivity by the incorporation of Al dopants.11,15 After thermal annealing at 400 °C, the electrical properties of TaN/AZO/n-Ge are hardly degraded and the values of JR still show little variation. However, the TaN/n-Ge structure exhibits a much larger decrease in JR, of approximately a factor of 10 compared to only a factor of 2 for the M–I–S structures. A slight degradation of JR in the M–I–S structures results from a conductivity decrease of the Al electrode, and the degradation window demonstrates good consistency with that of the Al electrode (more details in the Supporting Information). For thermal annealing at 500 °C, the values of JR for the M–I–S structures are almost maintained whereas those of the M–S structure revert to the Schottky contact shown in Figure 6b. After the thermal annealing at 500 °C, the properties of the TaN/n-Ge structure change into Schottky properties, due to the dissociation of Ge-N bonds at the TaN/n-Ge interface in the process of thermal annealing as shown in Figure 7b. For the M–S structure, most of the Ge-N bonds are dissociated after thermal annealing at 500 °C with the TaN film not deformed as shown in Figure 8a; therefore, the TaN/n-Ge structure becomes close to a metal/n-Ge structure with FLP


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and a large SBH. Figure 9a and Figure 9b show that the TaN/AZO(2.8 nm)/n-Ge structure shows very clear interface and presence of the AZO layer even after the thermal annealing at 500 °C. Thermal annealing at 600 °C does not make the TaN/AZO/n-Ge structures show Schottky characteristics; however, larger variations and a slightly larger JR reduction of approximately a factor of 20 compared to that without thermal annealing are observed. Although the electrical characteristics of M–S and M–I–S structures appear to be similar, their structural characteristics are entirely different. In contrast to the collapse of the TaN contact region for the TaN/n-Ge structure shown in Figure 8b, the TaN contact region in the TaN/AZO/n-Ge structure shown in Figure 9c does not collapse or move to the Ge, and the amorphous AZO interlayer is not crystallized, as shown in Figure 9d, after the thermal annealing at 600 °C. From these results, it is concluded that the interlayer can block the diffusion of metal from the contact to Ge and the AZO interlayer can act as a sound diffusion barrier. The reason for the ability of the AZO film to act as a diffusion barrier between the contact metal and Ge is that the crystallization temperature of the amorphous ZnO film is considerably higher than that of the amorphous TiO2 film and the Al dopants further increases its crystallizing temperature; therefore, the ZnO-based film does not provide grain boundaries that act as diffusion paths for Ta and Ge atoms.31 Figure 10 shows a compilation of the average values of JR for all structures considered in this article. The M–S and M–I–S structures with the Ti contact metal show the most drastic changes with thermal annealing temperatures. The Ti/n-Ge structure shows an increased current density after thermal annealing because of the metallization of Ge. Furthermore, the Ti/TiO2/nGe structures exhibit Schottky properties after thermal annealing at 400 °C, and the metallization process occurs after thermal annealing at temperatures of over 500 °C. After thermal annealing at


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600 °C, no clear differences are visible between the electrical and structural characteristics of M– S and M–I–S structures. In contrast, the TaN-based structures can endure the thermal annealing process significantly better; therefore, the TaN/TiO2/n-Ge structure can maintain its electrical characteristics for annealing temperatures up to 400 °C. However, the TaN/TiO2/n-Ge structures start to exhibit Schottky properties after thermal annealing at 500 °C, and the metallization process occurs at 600 °C. In contrast to other structure, the TaN/AZO/n-Ge structure only exhibits a monotonic change as the annealing temperature increases because the M–I–S stack is well maintained since the AZO interlayer can preserve its amorphous property and act as a diffusion barrier during the thermal annealing process, which results in a clear interface even after thermal annealing at 600 °C. Table 1 summarizes the effective electron SBH of all the discussed structures for different annealing conditions. The effective electron SBH is extracted only for the case wherein the contact structure shows the Schottky J–V characteristics. The Ti/n-Ge structure without thermal annealing exhibits an electron SBH of 0.548 eV corresponding to a severe FLP. The effective electron SBH of the Ti/n-Ge structure after thermal annealing is still large despite the metallization of Ge because the JR increase of the structure after thermal annealing does not come from the SBH reduction effect. The Ti/TiO2/n-Ge structure initially shows Ohmic J–V characteristics; however, it exhibits Schottky properties with a large electron SBH after thermal annealing at 400 °C and 500 °C. Two TaN metal contact-based structures—TaN/n-Ge and TaN/TiO2/n-Ge—show improved thermal stability. Both structures maintain their Ohmic J–V characteristics up to an annealing temperature of 400 °C, subsequently adopt Schottky properties at 500 °C, and eventually show Ohmic-like properties at 600 °C. The effective SBH values of the two structures, after the thermal annealing at 500 °C, are similar, i.e., 0.514 eV for TaN/n-Ge and


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0.5 eV for TaN/TiO2/n-Ge. However, they exhibit Schottky properties for different reasons. Schottky properties in the former result from the dissociation of Ge-N bonds whereas, Schottky properties in the latter result from crystallization and subsequent disappearance of the TiO2 interlayer. The TaN/AZO/n-Ge structure is the only structure that maintains its Ohmic J–V characteristics for all thermal annealing conditions in the table. From this result, it is concluded that the proposed M–I–S contact scheme adopting the TaN metal contact and the AZO interlayer has significantly better thermal stability properties than other structures studied in this work.


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4. SUMMARY AND CONCLUSION An M–I–S structure with excellent thermal stability and electrical performance was developed by incorporating a TaN metal contact and an AZO interlayer on n-Ge. The Ti/TiO2/n-Ge structure, which is the most widely studied M–I–S structure in Ge, was shown to lose its reduced SBH after thermal annealing, because it has very poor thermal stability due to the high reactivity of the Ti contact metal and the low crystallization temperature of the TiO2 interlayer. The thermal stability of the M–I–S structure with the TiO2 interlayer was considerably enhanced by replacing Ti with TaN, which has a lower reactivity with O than Ti, as well as the low metal work function; this aids in achieving a low SBH. However, the structure could not maintain its Ohmic characteristics and its SBH increased at annealing temperatures above 500 °C, because the TiO2 interlayer becomes easily crystallized during thermal annealing, and inter-diffusion between the contact metal and Ge occurs through grain boundaries of TiO2. The thermal stability of the M–I– S structure with the TaN metal contact was further improved by adopting an AZO interlayer, which acts as a good diffusion barrier between the contact metal and Ge, because it has a higher crystallization temperature and can maintain its amorphous film properties. Furthermore, the AZO layer provides excellent electrical performance due to its low tunneling and series resistance induced by its low CBO to Ge and high film conductivity. Therefore, the proposed M– I–S structure—TaN/AZO/n-Ge—exhibited significantly improved thermal stability at more than 500 °C with excellent electrical performance corresponding to Ohmic J–V characteristics and a low SBH. The thermally stable M–I–S structure successfully developed in this work is expected to facilitate its introduction into both practical Ge-based devices and any semiconductor devices exhibiting FLP problems.


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ASSOCIATED CONTENT Supporting Information. Trade-off between thermal stability and film resistivity of TaN, Standard deviations of log(JR) showing JR variation for each contact structure, Abnormal J–V characteristics of Ti contact metal case induced by metallization of Ge, Resistivity degradation of Al electrode during thermal annealing, Elemental mapping of TaN/TiO2/n-Ge and TaN/n-Ge after thermal annealing at 600 °C.

ACKNOWLEDGMENTS This work was supported in part by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT, and Future Planning(2017R1A2B4006460), in part by the Technology Innovation Program (10048594, Technology Development of Ge nMOS/pMOS FinFET for 10nm Technology Node) funded by the Ministry of Trade, Industry & Energy (MI Korea), and in part by Nano·Material Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning(2016M3A7B4910426).


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Figure 1. Energy band diagrams of (a) M–S structure and (b) M–I–S structure adopting contact metal with low metal work-function and interlayer with low CBO to Ge.


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Figure 2. (a) Box plots of JR values of Ti/n-Ge and Ti/TiO2/n-Ge structures with different TiO2 thicknesses and thermal annealing conditions and (b) J–V characteristics of Ti/n-Ge and Ti/TiO2(1 nm)/n-Ge structures without thermal annealing and Ti/TiO2(1 nm)/n-Ge structure with thermal annealing at 400 °C and 600 °C. (b insets) Energy band diagrams of each structure with their thermal annealing conditions.


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Figure 3. TEM images of Al electrode/Ti/TiO2(1 nm)/n-Ge structure (a) without thermal annealing and (b) with thermal annealing at 600 °C. (a inset) Cross-sectional schematic of Al electrode/Ti/TiO2/n-Ge structure.


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Figure 4. (a) Box plots of JR values of TaN/n-Ge and TaN/TiO2/n-Ge structures with different TiO2 thicknesses and thermal annealing conditions and (b) J–V characteristics of TaN/TiO2(3 nm)/n-Ge structure without thermal annealing and after thermal annealing at 400 °C, 500 °C, and 600 °C. (b insets) Energy band diagrams of each structure with their thermal annealing conditions.


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Figure 5. (a), (b) TEM images of Al electrode/TaN/TiO2(1 nm)/n-Ge structure after thermal annealing at 600 °C, and (c) XRD patterns of 50 nm thick TiO2 layer with different thermal annealing conditions: ‘A’ means an anatase phase of TiO2. (a inset) Cross-sectional schematic of Al electrode/TaN/TiO2/n-Ge structure.


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Figure 6. (a) Box plots of JR values of TaN/n-Ge and TaN/AZO/n-Ge structures with different AZO thicknesses and thermal annealing conditions and (b) J–V characteristics of TaN/n-Ge structure without thermal annealing, TaN/n-Ge structure after thermal annealing at 500 °C, and TaN/AZO(2.8 nm)/n-Ge structure without thermal annealing and after thermal annealing at 400 °C and 500 °C. (b insets) Energy band diagrams of each structure with their thermal annealing conditions.


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Figure 7. N 1s XPS spectra obtained from the TaN/n-Ge structure (a) without thermal annealing and (b) with various thermal annealing conditions.


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Figure 8. TEM images of Al electrode/TaN/n-Ge structure (a) after thermal annealing at 500 °C and (b) 600 °C. (a inset) Cross-sectional schematic of Al electrode/TaN/ n-Ge structure.


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Figure 9. TEM images of (a), (b) Al electrode/TaN/AZO(2.8 nm)/n-Ge structure after thermal annealing at 500 °C, and (c) Al electrode/TaN/AZO(1.4nm)/n-Ge structure and (d) TaN/AZO(5 nm)/n-Ge structure after thermal annealing at 600 °C. (a inset) Cross-sectional schematic of Al electrode/TaN/AZO/n-Ge structure.


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Figure 10. Average JR values of Ti/TiO2/n-Ge, TaN/TiO2/n-Ge, and TaN/AZO/n-Ge structures with various interlayer thicknesses and thermal annealing conditions.


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Table 1. Effective electron SBH of Ti/n-Ge, Ti/TiO2(3 nm)/n-Ge, TaN/n-Ge, TaN/TiO2(3 nm)/nGe, and TaN/AZO(2.8 nm)/n-Ge structures with various thermal annealing conditions. ‘Ohmic’ indicates that the structure shows Ohmic or quasi-Ohmic J–V characteristics and the ‘*’ sign indicates that metallization of the Ge surface occurs. Annealing temperature No annealing 400 °C 500 °C 600 °C

Effective electron SBH [eV] Ti/n-Ge 0.548 (± 0.01) 0.456 (*) (± 0.012) 0.534 (*) (± 0.009) Ohmic (*)

Ti/TiO2/n-Ge

TaN/n-Ge

TaN/TiO2/n-Ge

TaN/AZO/n-Ge

Ohmic

Ohmic

Ohmic

Ohmic

Ohmic

Ohmic

Ohmic

0.514 (± 0.004) Ohmic (*)

0.500 (± 0.007) Ohmic (*)

0.512 (± 0.009) 0.527 (± 0.004) Ohmic (*)

Ohmic Ohmic


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Table of Contents Graphic


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Fermi-Level Unpinning Technique with Excellent Thermal Stability

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