# Integration of the Ni/InP system on a 300 mm platform for III-V/Si hybrid lasers

Flore Boyer, Patrice Gergaud, Karen Dabertrand, Denis Mariolle, Christophe

Jany, Fabrice Nemouchi, Magali Grégoire, Quentin Rafhay, Philippe

Rodriguez

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Flore Boyer, Patrice Gergaud, Karen Dabertrand, Denis Mariolle, Christophe Jany, et al.. Integration of the Ni/InP system on a 300 mm platform for III-V/Si hybrid lasers. Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics, 2020, 38 (1), pp.012209. 10.1116/1.5128554 hal-04743063

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# Integration of the Ni/InP system on a 300 mm platform for III-V/Si hybrid lasers ✅

Flore Boyer ⚙️: Patrice Gergaud; Karen Dabertrand; Denis Mariolle; Christophe Jany; Fabrice Nemouchi; Magali Grégoire; Quentin Rafhay; Philippe Rodriguez

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J. Vac. Sci. Technol. B 38, 012209 (2020)

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# Integration of the Ni/InP system on a 300 mm platform for III-V/Si hybrid lasers

Cite as: J. Vac. Sci. Technol. B 38, 012209 (2020); doi: 10.1116/1.5128554

Submitted: 29 November 2019 · Accepted: 10 December 2019 ·

Published Online: 10 January 2020

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Flore Boyer,<fnref n="1" />,<fnref n="2" />,<fnref n="3" />,ᵃ⁾ **Patrice Gergaud,**<fnref n="2" /> **Karen Dabertrand,**<fnref n="1" /> **Denis Mariolle,**<fnref n="2" /> **Christophe Jany,**<fnref n="2" /> **Fabrice Nemouchi,**<fnref n="2" /> **Magali Grégoire,**<fnref n="1" /> **Quentin Rafhay,**<fnref n="3" /> and **Philippe Rodriguez**<fnref n="2" />

## AFFILIATIONS

## ABSTRACT

The integration of III-V/Si hybrid lasers on a 300 mm platform for photonic applications requires the development of dedicated CMOS-compatible contacts, for which nickel-based ones are very good candidates. In this scope, this work presents and compares the impact of in situ preclean based on argon (Ar) or helium (He) plasma on the surface integrity of InP prior to the nickel (Ni) contact deposition. The resulting surface morphology, element distribution, phase formation sequence of the Ni/InP system, and electrical behavior of Ni/n-InP contacts are detailed using morphological, structural, and electrical characterizations. The results show that Ar preclean significantly damages the InP surface by generating high roughness and creating indium (In) dots on the top surface, while He preclean seems to induce lighter damages and no In dots. Although the phase sequence of the Ni/InP system is overall the same for each preclean, the electrical behavior differs depending on the nature of the preclean. On one hand, Ni/n-InP Ar-precleaned contacts exhibit nonohmic behavior for each investigated thermal budget. On the other hand, He-precleaned contacts features ohmic behavior for the as-deposited state and thermal anneals up to 350 °C for 60 s. They, however, become nonohmic after anneals of 400 and 450 °C for 60 s. These results, hence, suggest that the difference of electrical behavior obtained between Ar and He-precleaned Ni/n-InP contacts is due to differences in the state surface and morphology.

Published under license by AVS. https://doi.org/10.1116/1.5128554

## I. INTRODUCTION

The staggering increase of data transfer presently generates a strong demand on networks and a need for faster connectivity over greater distances. In this framework, silicon photonics applications are expected to help support this data transfer increase, by bringing optical data closer to the central processing units (CPUs). This technology relies on the cointegration of (i) a very large scale integration (VLSI) silicon circuit, in order to benefit from its high density, volume production, and low cost, and (ii) an optical circuit, granting high performance communication, higher data transfer rates, improved signal integrity, and low power consumption.<fnref n="1" />⁻<fnref n="3" /> Such cointegration would enable, on the same chip, the conversion of electrical processed data into outgoing light and its transmission through optical fiber, as well as the conversion of incoming light signals into electrical ones.

From a technological point of view, this technology requires the cointegration of silicon based electrical functions and III-V based optical functions on the same optoelectronic die, in a complementary metal oxide semiconductor (CMOS) compatible clean room, either using direct epitaxy of III-V on silicon⁴,⁵ or the bonding of III-V wafers on silicon.⁶,⁷

In the optoelectronic circuit, the light source and amplification is ensured by a III-V/Si hybrid laser. Its active region is composed of multiple quantum wells (MQWs) made from III-V materials. The elaboration of CMOS-compatible contacts on III-V materials is, hence, key for the III-V/Si hybrid laser operation, which features n-InP and p-In₀.₅₃Ga₀.₄₇As accesses. They consist of a metal or an alloy on top of the III-V layer in question.

For an optimal laser operation, these contacts must exhibit (i) an ohmic behavior and (ii) a contact resistivity ($ρ_c$) typically below or equal to $5 \times 10^{-5} \Omega \ cm^2$. This value has been calculated

<footnote n="1">¹STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles, France</footnote>

<footnote n="2">²Univ. Grenoble Alpes, CEA, LETI, F-38000 Grenoble, France</footnote>

<footnote n="3">³Univ. Grenoble Alpes, CNRS, Grenoble INP, IMEP-LAHC, 3 Parvis Louis Neel, F-38000 Grenoble, France</footnote>

<footnote>ᵃ⁾Electronic addresses: flore.boyer@st.com and flore.boyer@cea.fr</footnote>
knowing the maximum contact resistance contribution allowed over the total series resistance of the laser and the contact areas. Due to its particularly fragile and sensitive surface state, InP contacts are the focus of this paper.

Many efficient metallizations used on the InP surface contain noble metals such as gold (Au), palladium (Pd), and platinum (Pt).<sup>8,9</sup> They form unreacted stacks on top of InP and present the advantage of low contact resistivities (between 10<sup>-6</sup> and 10<sup>-7</sup> Ω cm<sup>2</sup>), but they are fabricated using lift-off related processes (nonplanar), which are not compatible with CMOS processes. In addition, Au is known to diffuse into silicon and induce traps as well as contamination issues. For these reasons, noble-metal based metallizations are not suitable for III-V/Si hybrid laser contact integration. An alternative to form ohmic contacts with low contact resistivity is to use CMOS-compatible metals such as titanium (Ti), nickel (Ni), and their alloys. Contrary to inert noble metals, the aim with these CMOS-compatible metals is to generate a fully controlled solid-state reaction between the metal and the InP layer. Several studies using 200 nm CMOS platform have already demonstrated the possibility to obtain a contact resistivity below 5 × 10<sup>-5</sup> Ω cm<sup>2</sup>, especially with the Ni/InP system in which ρ<sub>c</sub> as low as 2.3 × 10<sup>-5</sup> Ω cm<sup>2</sup> has already been obtained.<sup>10</sup>

The topic of this paper is the integration of the Ni/InP contact module on a 300 mm platform, which would, compared to 200 mm platform, allow better quality deposition and uniformity, in addition to reduced production costs. Several contributions could be found in the literature on this system, but no consensus has been reached yet on the phase sequence of the Ni/InP system with increasing annealing temperature. For instance, Yamaguchi et al.<sup>11</sup> have found unreacted Ni until 250 °C, Ni<sub>2</sub>P and Ni<sub>2</sub>InP from 300 and up to 500 °C, and In from 400 and up to 500 °C. On the contrary, Ghegin et al.<sup>12</sup> have identified that the Ni<sub>2</sub>InP phase disappears before 450 °C. A different phase sequence has, however, been observed by Ivey et al.,<sup>13</sup> who have identified the Ni<sub>2</sub>P phase as soon as 250 °C with an increased proportion at 300 °C. Still according to Ivey et al., the Ni<sub>2</sub>P phase is then transformed into the NiP phase before being completely consumed after 400 °C, while NiP and NiP<sub>2</sub> phases are still found at this temperature. This, however, strongly differs from the results of Sands et al.,<sup>14</sup> who observed that after the complete Ni consumption at 200 °C, the system is composed of crystallized Ni<sub>x</sub>InP (2 ≤ x ≤ 3) and composed of Ni<sub>2</sub>InP at 360 °C.

The state interface between the metal and the InP layer has a significant impact on the yield of functioning contacts with suitable electrical performance.<sup>12</sup> In this regard, native oxides on III-V surfaces such as InP usually impede the solid-state reactions between the metal and the semiconductor, hence jeopardizing the electrical characteristics.<sup>10</sup> InP native oxides and organic particles are usually removed using wet-cleaning such as HCl (Ref. 15) or HF (Ref. 16) for instance. Both of these wet treatments are effective to remove the native oxides from the InP surface. Nevertheless, the latter tend to quickly regrow on the InP surface between wet-cleaning and metal deposition, which is why studies on *in situ* dry-preclean have been initiated.<sup>17,18</sup> In this scope, it has been demonstrated using a 200 mm platform<sup>12</sup> that Ar preclean on InP prior to the metal deposition induces a modification of the substrate surface by altering the In-P bonds, leading to an In-rich and P-depleted InP top surface.

Challenges are, hence, great to elaborate the Ni/InP contact using a 300 nm CMOS-compatible platform for future silicon photonic applications. This work is, therefore, dedicated to the analysis of the impact of preclean (Ar vs He) on the Ni/InP system, in terms of the surface roughness, morphology, element distribution, and solid-state reaction. This work also investigates the electrical behavior of Ni/n-InP contacts at different thermal budgets, with either Ar or He preclean plasma.

## II. SAMPLE DESCRIPTION

Two sets of samples have been fabricated for the study of the Ni/InP system. One set is dedicated to the material study, which has undergone physical characterizations, and the other is dedicated to electrical measurements. Although some variations in the process exist between the two sets (mentioned below), the preclean and Ni deposition steps are rigorously identical. The material study has been performed on nonintentionally doped InP epiready wafers. Substrates have been cleaned with 10% HF wet-etching for 1 min in order to remove native oxides, followed by de-ionized water (DIW) rinse. Ar or He plasma treatments have then been performed at room temperature to remove the regrown oxides using a 300 nm Endura® platform and dedicated holders. Reference samples with no preclean were also used. The pressure, gas flux, and RF power ranges of the Ar and He preclean plasma are displayed in Table I.

In Table I, RF1 refers to the control of the ion concentration inside the plasma, while RF2 controls the acceleration of the elements striking the InP surface. A 10 nm thick Ni deposition process via RF-PVD (magnetron sputtering) has then been performed followed by a 7 nm thick TiN capping film deposition at room temperature without air-break, which prevents samples from air contamination. Samples have been annealed by rapid thermal annealing (RTA) at different temperatures: 200, 250, 300, 350, 400, 450, and 550 °C for 60 s under N<sub>2</sub> atmosphere. Thanks to atomic force microscopy (AFM), transmission electron microscopy (TEM), energy dispersive x ray (EDX), and integrated in-plane x-ray diffraction reciprocal space mapping (IPRSM) (which minimizes the crystallographic texture effect and therefore facilitates phase identification), the surface roughness, element distribution, and phase sequence have been analyzed. For TEM-EDX characterizations, a platinum (Pt) technical capping layer has been deposited on the top of the samples for measurement purposes.

Contrary to the samples used for the material study, the electrical samples have been fabricated on 300 nm doped epilayers (n-doped InP with Si, N<sub>d</sub> = 5 × 10<sup>18</sup> at.cm<sup>3</sup>) on semi-insulating InP substrates. Transfer length measurement (TLM) structures have been fabricated according to the module described in Fig. 1.

<table><caption>TABLE I. Recipe parameters values of helium and argon preclean.</caption><thead><tr><th>Parameters</th><th>Argon</th><th>Helium</th></tr></thead><tbody><tr><td>Pressure (mTorr)</td><td>0.1–3</td><td>2–45</td></tr><tr><td>Gas fluxes (SCCM)</td><td>5–150</td><td>5–150</td></tr><tr><td>RF1 (W)</td><td>10–100</td><td>10–100</td></tr><tr><td>RF2 (W)</td><td>100–750</td><td>100–750</td></tr></tbody></table>
<figure><img src="image_12.png" /><figcaption>FIG. 1. TLM process steps: (a) n-InP ($N_d = 5 \times 10^{18}$ at.cm$^{-3}$) doped epi-layer on the InP substrate, (b) 150 nm SiN deposition, (c) lithography and SiN etching, (d) Ni (20 nm) metal deposition, followed by TiN (7 nm) capping layer deposition using 300 nm tools, (e) Ti/Pt/Au (15/25/150 nm) filling, (f) lithography and ion beam etching, and (g) lithography and MESA etching.</figcaption></figure>

Before Ni deposition, the electrical samples have been cleaned with a 10% HCl solution for 30 s in order to remove any native oxides, followed by a DIW rinse. Then, using the same 300 nm Endura® platform and dedicated holders, He or Ar plasma treatments have been performed to remove the regrown oxides, followed by 20 nm thick Ni deposition process via RF-PVD. Without air-break, samples have then been capped by a 7 nm thick TiN film deposited at room temperature, preventing samples from oxidation. The contacts cavities have then been filled with a stack of Ti/Pt/Au (respectively, 15/25/150 nm) for the sake of electrical measurement purposes. Then, samples have undergone RTA at different temperatures, ranging between 250 and 450 °C for 60 s under the $N_2$

atmosphere. The electrical results have been obtained on the TLM modules, using $200 \times 200 \mu m^2$ squared patterns, described in Fig. 2.

### III. MATERIAL RESULTS

#### A. Study of morphology and element distribution for post-preclean and as-deposited states

The surface morphology of the material samples is presented in Fig. 3. In these AFM characterizations, it appears that the morphology of the InP reference sample (i.e., without preclean) [Fig. 3(a)], and InP with He preclean [Fig. 3(b)] are similar: surfaces

<figure><img src="image_13.png" /><figcaption>FIG. 2. Representation of the squared-TLM patterns used for the extraction of the electrical results: (a) top-view and (b) cross section.</figcaption></figure>
are smooth and few particles are observed. Root mean squared (RMS) roughness are in the same range value, respectively, 0.3 and 0.2 nm. Typical RMS roughness for these state surfaces is ranging between 0.2 and 1 nm, mostly free of dots. This hence suggests that He preclean has a moderated impact on the morphology of the InP surface. On the contrary, the RMS roughness of the Ar-precleaned sample [Fig. 3(c)] is more important and reaches 3.5 nm (usually between 3 and 6 nm). In addition, the InP surface precleaned with Ar contains numerous systematic dots and exhibit a morphology strongly different from the InP reference.

After Ni/TiN deposition, on the He-precleaned [Fig. 3(d)], one can observe a few dots on the surface. The RMS roughness has increased from 0.2 to 0.6 nm after metal deposition, even though the sample exhibits a smoother surface compared to the TiN (7 nm)/Ni (10 nm)/InP sample precleaned with Ar [shown in Fig. 3(e)]. Indeed, in that case, the morphology of the as-deposited TiN (7 nm)/Ni (10 nm)/InP sample precleaned with Ar [Fig. 3(e)] still features the dots observed prior to Ni deposition [Fig. 3(c)]. However, the RMS roughness is reduced from 3.5 to 1.7 nm after metal deposition, which indicates that the Ni and TiN depositions have smoothed the surface.

To separate the impact of the Ni and TiN deposition from the impact of the precleans on the InP surface roughness, AFM observations have been performed on Ni (10 nm)/InP and TiN (7 nm)/InP samples that have not been precleaned: these are the reference samples (Fig. 4). The results confirm that neither Ni nor TiN deposition alters the RMS roughness or surface morphology of the Ni/InP

stacks. In dots alone on the Ar-precleaned surface are hence responsible for the elevated RMS roughness.

To confirm the morphological process induced variations observed by AFM measurements, combined TEM and EDX mapping analyses have been carried out. TEM images of as-deposited samples precleaned with Ar and with He are, respectively, shown in Figs. 5(a) and 5(b). EDX mapping of these same samples is shown in Figs. 5(c)-5(f).

First and foremost, in the case of the Ar-precleaned sample [Figs. 5(a), 5(c), and 5(e)], one can observe the previously discussed high roughness of the sample. A waved surface is, in addition, observed between the unreacted Ni and the InP substrate. One can observe in Figs. 5(c) and 5(e) that this waved interface is due to In accumulation, which corresponds to the dots observed by AFM characterizations in Fig. 3. In addition, we previously demonstrated, thanks to AFM characterizations (Fig. 4), that Ni and TiN depositions follow the shape of the substrate without strong alteration. It therefore confirms through TEM images and EDX mapping that the Ar-precleaned surface is rough due to the presence of In dots.

In the case of the He-precleaned sample [Figs. 5(b)-5(f)], the top InP surface is smoother. In addition, a lighter layer under the unreacted Ni can be observed in Figs. 5(d) and 5(f), composed of Ni, In, and P. The presence of this intermixing layer indicates that Ni diffusion occurs right after deposition and is consistent with the work of Ghegin et al.,¹² in which an amorphous Ni-In-P layer had been identified. The presence of this intermixing layer in the He-precleaned sample also suggests that in the case of Ar

<figure><img src="image_14.png" /><figcaption>FIG. 3. 1 × 1 µm² AFM images of various InP samples. (a) Reference, (b) after He preclean, (c) after Ar preclean, (d) after He preclean and Ni (10 nm)/TiN (7 nm) deposition, and (e) after Ar preclean and Ni (10 nm)/TiN (7 nm).</figcaption></figure>
preclean, no equivalent In bump is observed. In addition, on this same sample, a difference is noticeable between 59.2 and 64.3 nm in the slope of the Ni curve, which contributes to the proof of an intermixing layer composed of In, Ni, and P. Its width is approximately 5 nm thick. This intermixing layer corresponds to the one discussed in Figs. 5(d) and 5(f). Here again, this result confirms that the presence of In dots on the InP top surface most likely hinders the formation of the Ni-In-P intermixing layer.

The origin of the In dots has been discussed in a previous study led by Rodriguez *et al.*,¹⁷ which showed that these dots can be identified as indium (In) and that they are formed by the preclean itself. During preclean, the elements striking the surface deplete the InP top surface from its most volatile element, i.e., phosphorus in this particular case.¹⁹

In this section, it was demonstrated that Ar preclean on the InP surface generates damage on the InP surface by inducing the presence of a high concentration of In dots and a high RMS roughness ($3 \text{ nm} \le \text{RMS} \le 6 \text{ nm}$). It has also been observed that

He preclean generates a significantly lighter RMS roughness (RMS ≤ 1 nm). In addition, according to AFM and TEM-EDX characterizations, in the case of Ar preclean sample, a systematic accumulation of In dots is observed on the top surface of the InP substrate, as well as a high RMS roughness. In the case of He preclean, In accumulation does not systematically occur and the RMS roughness is less important. An intermixing layer composed of In, P, and Ni is observed on He-precleaned surfaces right after Ni deposition, which reveals that In accumulation observed on Ar-precleaned samples hinders the formation of an intermixing layer. In Sec. III B, the morphology of annealed samples and the solid-state reaction of the Ni/InP system are discussed.

## B. Study of annealed samples and solid-state reaction

The physical characteristics of the annealed samples are presented in this section.

The element distribution profiles of the Ar-precleaned samples annealed at 350°C for 60 s [Fig. 7(a)] and at 450°C for 60 s [Fig. 7(b)] suggest that there is a reaction between Ni and InP. When the temperature of the RTA increases from 350 to 450°C, the width of the reacted layer increases as well: from 25 to 38 nm, while 10 nm of Ni was originally deposited. In addition, In accumulation between the reacted layer and the TiN capping layer can be observed, which is most likely due to the high concentration of preclean-induced In dots observed in the as-deposited sample. The In peak, therefore, acts as a marker for the reaction. The position of the In accumulation remaining at the same position whatever the annealing temperature indicates that Ni is the diffusing specie.

Regarding the He-precleaned sample annealed at 350°C for 60 s [Fig. 8(a)] and 450°C for 60 s [Fig. 8(b)], In accumulation is not clearly evidenced, which is consistent with previously discussed characterizations. In this case, there is also a reaction between Ni and InP: when the RTA temperature increases from 350 to 450°C, the width of the reacted layer increases as well from 25 to 40 nm. One should note that for a given thermal budget applied to the sample (either 350°C for 60 s or 450°C for 60 s), the width of the reacted layer is overall the same whatever the nature of the preclean, which means that at these temperatures, In accumulation evidenced in the case of Ar-precleaned samples does not hinder anymore the Ni diffusion toward the InP substrate.

The surface roughness of annealed samples has also been investigated through AFM characterizations. The RMS roughness of each studied thermal budget is presented in Fig. 9.

In the case of He preclean (respectively, Ar preclean), from the as-deposited state to 300°C for 60 s, the RMS roughness remains constant and below 1 nm (respectively, below 2 nm). Therefore, for each preclean, the state surface remains the same over this range of temperature. The RMS values of Ar-precleaned samples are, however, twice larger than the RMS values of He-precleaned samples, which is attributed to the high concentration of In dots on Ar-precleaned surfaces. The difference of the surface roughness induced by the difference of preclean also has an impact at higher thermal budgets. In the case of He preclean (respectively, Ar preclean) after 450°C for 60 s, the RMS roughness increases significantly and reaches 3.3 nm (respectively, 11.5 nm). This increase is due to the reacted layer becoming agglomerated at this temperature

<figure><img src="image_15.png" /><figcaption>FIG. 6. EDX profiles of the TiN (7 nm)/Ni (10 nm)/InP system as-deposited with (a) Ar preclean and (b) He preclean.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>FIG. 7. EDX profiles of the TiN (7 nm)/Ni (10 nm)/InP system precleaned with Ar annealed at (a) 350 °C for 60 s and (b) 450 °C for 60 s.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>FIG. 8. EDX profiles of the TiN (7 nm)/Ni (10 nm)/InP system precleaned with He annealed at (a) 350 °C for 60 s and (b) 450 °C for 60 s.</figcaption></figure>

[see Figs. 10(a) and 10(b)]. The higher increase of RMS roughness in the case of Ar-preclean may come from the In dots agglomerating, which could explain the highly degraded state surface at this temperature (not clearly evidenced in Fig. 10).

In addition to the element distribution and morphology variations induced by the annealings, the phase sequence has been investigated using IPRSM analysis. The RTA temperatures have been set to 200, 250, 300, 350, 450, and 550 °C for 60 s. The integrated and indexed IPRSM patterns are presented in Fig. 11.

The diffraction peaks associated with the InP substrate and the TiN capping layer are observed for all annealing temperatures. Unreacted Ni is observed at the earliest stage of the reaction and disappears after 250 °C. This is in agreement with several studies based on Ar preclean.¹¹,¹²,¹⁴ As shown on EDX profiles and maps, an intermixing layer appears in the case of the He-precleaned sample right after Ni deposition and at the lowest applied thermal budget (200 °C for 60 s) in the case of the Ar-precleaned sample. It is most likely that the intermixing layer disappears when crystallized phases appear (300 °C). The stoichiometry of this phase can be

calculated from the semiquantitative EDX profiles, which lead to NiₓInP: 2.2 ≤ x ≤ 2.4. The stoichiometry of this amorphous layer is consistent with previous studies,¹¹,¹⁴ in which the stoichiometry of the intermixing layer has been estimated: NiₓInP: x ≈ 2.7.

It has been found in this paper that for He and Ar-precleaned samples, In phase is always observed. This observation is, however, in disagreement with Yamaguchi *et al.*¹¹ and Ghegin *et al.*,¹² in which In phase does not appear before, respectively, 400 and 300 °C. It is possible that the use of integrated IPRSM analyses in the present study, which are more sensitive to the top surface of the thin film than classical θ/2θ measurements, is accountable for divergences regarding the identification of In at the lowest temperatures.

In this work, the presence of In in the case of Ar-precleaned samples seems consistent with previous observations made in this paper, which showed the presence of In dots on top of the InP surface. For instance, EDX profiles of Ar-precleaned samples exhibit the In accumulation phenomenon in the as-deposited state in Fig. 6(a), after 350 °C for 60 s and 450 °C for 60 s in Figs. 7(a) and 7(b). In the case of the He-precleaned surface, an In-rich layer
<figure><img src="image_18.png" /><figcaption>FIG. 9. RMS roughness of TiN (7 nm)/Ni (10 nm)/InP for different thermal budgets in the case of Ar and He preclean.</figcaption></figure>

is nonetheless highly possible. Indeed, it can be explained by the fact that even though this preclean is less damaging for the InP surface, it is still strong enough to deplete the InP surface of phosphorus, which is volatile, therefore generating an In-rich surface. In addition, the volume of material analyzed during TEM-EDX characterization is small and no In accumulation has been observed in He-precleaned samples, while the volume analyzed during IPRSM measurements is more important and has confirmed the presence of In-rich He-precleaned samples. Therefore, the difference of analyzed volume could also explain the differences regarding In identification.

<figure><img src="image_19.png" /><figcaption>FIG. 10. TEM images of TiN (7 nm)/Ni (10 nm)/InP annealed at 450 °C for 60 s with (a) Ar preclean and (b) He preclean.</figcaption></figure>

At 300 °C, three phases appear simultaneously: Ni₂P, Ni₂InP, and Ni₃P. These three phases remain stable until at least 550 °C. Two phases out of these three (Ni₂P and Ni₂InP) have also been observed by Yamaguchi *et al.*¹¹ from 300 to 500 °C. This is also overall consistent with Ghegin *et al.*, nevertheless, Ni₃P and Ni₂InP were not identified at 450 °C,¹² and Yamaguchi *et al.* have not identified the Ni₃P phase.¹¹ The divergences between the observed phases can be attributed here also to difference in employed characterization techniques. Indeed, intermetallics, in general, are submitted to textural effects,²⁰,²¹ and conventional θ/2θ techniques do not allow for a complete identification of each texture. IPRSM is, however, less subjected to the textural effect; therefore, the analysis is expected to be more accurate. It was previously established that there was no difference between the Ni/InP phase sequence

<figure><img src="image_20.png" /><figcaption>FIG. 11. Integrated IPRSM patterns of the TiN (7 nm)/Ni (10 nm)/InP system precleaned with (a) argon and (b) helium plasma annealed at various RTA.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>FIG. 12. TiN (7 nm)/Ni (10 nm)/InP system precleaned with Ar or He and its behavior with growing temperatures.</figcaption></figure>

obtained with Ar or He preclean; nevertheless, the analysis of IPRSM patterns exhibits a few differences regarding the orientations of several phases obtained with either Ar or He preclean. For instance, in the Ar-precleaned samples in Fig. 11(a), the (222) plane of the Ni₃P (50.6°) phase is identified, while it is not the case in the He-precleaned samples [Fig. 11(b)]. It can also be noticed that in the He-precleaned system, the (021) plane of the Ni₂P (44.7°) phase is identified, while it is not in the Ar-precleaned system. Other differences of orientation might be highlighted. Nevertheless, the overall phase sequence remains the same for both systems.

So far, the morphological and structural aspects of the Ni/InP contacts elaborated using a 300 mm platform have been discussed. More specifically, the impact of the nature of the preclean (Ar vs He) prior to Ni deposition over the surface roughness, surface morphology, element distribution, and phase sequence has been investigated. The RMS roughness values of Ar-precleaned samples has been found to be higher than He-precleaned samples. It is the case for as-deposited states as well as for annealed ones. This is due to the presence of In dots generated by the Ar-preclean. EDX profiles of Ar-precleaned samples annealed at 350 and 450 °C for 60 s indicated the presence of a high In accumulation between the TiN capping layer and the reacted layer. In this section, it has been demonstrated that the phase sequence of the Ni/InP system is overall the same with either Ar or He preclean.

### C. TiN (7 nm)/Ni (10 nm)/InP model

In this section, the mechanisms involved in the TiN (7 nm)/Ni (10 nm)/InP system based on the characterization results detailed in Subsections III A and III B are discussed. Figure 12 presents schemes of these mechanisms for each studied preclean.

First, in the case of Ar preclean, a large amount of In dots is generated on the InP surface, which is not the case with He preclean. Then, after Ni and TiN deposition, in the case of He preclean Ni diffuses toward InP and forms an intermixing layer, while in the case of Ar preclean, the presence of In dots hinders the Ni diffusion, and no intermixing layer is formed.

After an annealing of 250 °C for 60 s, in both cases, Ni is entirely consumed. Therefore, the thermal energy is high enough for Ni to diffuse despite In accumulation. After 350 °C for 60 s, three phases are present: Ni₂P, Ni₃P, and Ni₂InP. After 450 °C for 60 s, as will be discussed in the following, the phases are still present, but agglomeration occurs and the layer becomes discontinued.

Section IV describes the impact of these different identified physical characteristics on their electrical behavior.

### IV. ELECTRICAL RESULTS

The electrical behavior of the Ni/n-InP contacts are presented as follow: first, I-V curves are discussed (Fig. 13), from which the nature of the contact, either ohmic or nonohmic, is determined. Then, in the case of ohmic contacts, ρ_c values (in Ω cm²) presented in Fig. 14 are discussed. Each ρ_c value discussed in this paper is the average value of measurements performed on 9 unit dies. Each error bar, therefore, represents the variability over 9 unit dies.

The I-V characteristics (extracted from TLM measurements) of Ni (20 nm)/n-InP (Si-doped, N<sub>d</sub> = 5 × 10<sup>18</sup> at.cm<sup>-3</sup>) precleaned with Ar exhibit a nonohmic behavior from the as-deposited state to 450 °C [see Figs. 13(a)–13(c)]. Indeed, for each thermal budget investigated and distances between the pads (from 20 to 120 µm), the I-V curves never exhibit a straight line behavior. This could be attributed to the Ar preclean impact on the InP surface (high roughness and poor morphology).

He-precleaned contacts are, however, ohmic from the as-deposited state to 350 °C for 60 s [see Figs. 13(d) and 13(e)]. Indeed, straight I-V curves are obtained in this range of temperatures for each distance between pads. The corresponding contact resistivity values are presented in Fig. 14.

It should be noted that between the as-deposited state and 350 °C, the contact resistivity values range from 1.88 × 10<sup>-5</sup> to 2.86 × 10<sup>-5</sup> Ω cm<sup>2</sup>, which are roughly equivalent values; nevertheless, the phase sequence indicates different phases at these temperatures (see Fig. 15): indeed, in the as-deposited state, unreacted Ni and In are present, while at 350 °C, unreacted Ni is absent, and the
<figure><img src="image_22.png" /><figcaption>FIG. 13. I-V curves of TiN (7 nm)/Ni (20 nm)/n-InP (N<sub>d</sub> = 5 × 10<sup>18</sup> at.cm<sup>-3</sup>) annealed at (a) 250 °C for 60 s, (b) 350 °C for 60 s, and (c) 450 °C for 60 s using Ar preclean and (d) 250 °C for 60 s, (e) 350 °C for 60 s, and (f) 450 °C for 60 s using He preclean.</figcaption></figure>

three phases Ni₃P, Ni₂P, and Ni₂InP are identified. Furthermore, it has been established in previous works<sup>11,12,22</sup> that the Ni₂P phase is the most favorable for low contact resistivity on the Ni/InP system, which would have suggested that the contact resistivity at 350 °C should have been reduced compared to contact resistivities at lower temperatures. Therefore, in our study, the correlation between the electrical behavior and the phase sequence is not clearly evidenced. Regarding previous studies, Fatemi *et al.*<sup>23</sup> have found that Ni₃P is the cause of low contact resistivity values as they have obtained ρ<sub>c</sub> values in the low 10<sup>-7</sup> Ω cm<sup>2</sup> range after an annealing of 400 °C for 40 min with the following stack: In/Ni₂P/Ni₃P/n-doped InP (Si, 1.7 × 10<sup>18</sup> at.cm<sup>-3</sup>). The authors have also demonstrated

that extending the annealing time leads to the formation of the following stack: In/Ni₂P/n - InP, and an increase of contact resistivity reaching the 10⁻⁴ Ω cm² range. In this case, the phase sequence has a clear correlation with the electrical behavior of the contact. Nevertheless, this result is in disagreement with the results of Appelbaum *et al.*<sup>24</sup> who have obtained on Ni₂P/n-doped InP (Sn, 2 × 10<sup>18</sup> at.cm⁻³) after 400 °C a contact resistivity of 3 × 10<sup>-6</sup> Ω cm². A summary of the different electrical results from the literature is presented in Table II.

He-precleaned contacts presented in this work become non-
ohmic after 400 °C [see Fig. 13(f)]. The shift in the electrical
behavior of the TiN (7 nm)/Ni (20 nm)/n-InP samples is most
<figure><img src="image_23.png" /><figcaption>**FIG. 14.** Contact resistivity of Ni (20 nm)/n-InP (N<sub>d</sub> = 5 × 10<sup>18</sup> at.cm<sup>-3</sup>) with He PC. The horizontal line symbolizes the upper limit not to be exceeded for the sake of contact resistance contribution.</figcaption></figure>

likely morphology-related. Indeed, after 450 °C for 60 s, the contact layer is discontinued [see Fig. 10(b)], most likely resulting from the agglomeration of Ni<sub>2</sub>P, Ni<sub>3</sub>P, and Ni<sub>2</sub>InP. This layer morphology is, therefore, not electrically favorable, which explains the non-ohmic behavior of the contact. In our study, the overall difference of electrical behavior between Ar and He-precleaned contacts surfaces is most likely related to the previously discussed surface morphology differences. These results highlight the importance of the impact of the surface morphology and roughness over the electrical quality of the contact.

The study of Ni (20 nm)/n-InP (Si-doped, N<sub>d</sub> = 3 × 10<sup>18</sup>at.cm<sup>-3</sup>) using Ar preclean has been investigated in a previous study<sup>10</sup> using a 200 mm platform. Thanks to TLM measurements performed on the same mask as used in this study (see patterns in Fig. 2), it has been found that the contacts exhibited ohmic behavior in the as-deposited state, as well as after 340 °C for 60 s and 350 °C for

<figure><img src="image_24.png" /><figcaption>**FIG. 15.** Summary of the TiN (7 nm)/Ni (10 nm)/InP phase sequence.</figcaption></figure>

<table><caption>**TABLE II.** State of the art: Electrical results on Ni/n-InP contact.</caption><thead><tr><th>ρ<sub>c</sub><br>(Ω cm<sup>2</sup>)</th><th>Stack/n-InP</th><th>N<sub>d</sub><br>(at.cm<sup>-3</sup>)</th><th>T-t<br>(°C) (s)</th><th>Reference</th></tr></thead><tbody><tr><td>Low 10<sup>-7</sup></td><td>In/Ni<sub>2</sub>P/Ni<sub>3</sub>P</td><td>1.7 × 10<sup>18</sup></td><td>400 °C – 40 mn</td><td>23</td></tr><tr><td>10<sup>-4</sup></td><td>In/Ni<sub>2</sub>P</td><td>1.7 × 10<sup>18</sup></td><td>400 °C – ≥ 40 mn</td><td>23</td></tr><tr><td>3 × 10<sup>-6</sup></td><td>Ni<sub>2</sub>P</td><td>2 × 10<sup>18</sup></td><td>400 °C – ?</td><td>24</td></tr><tr><td>1.9 × 10<sup>-5</sup></td><td>See Fig. 15</td><td>5 × 10<sup>18</sup></td><td>250 °C for 60 s</td><td>Our work</td></tr></tbody></table>

60 s. The extracted contact resistivities were, respectively, equal to 5.9 × 10<sup>-5</sup>, 2.3 × 10<sup>-5</sup>, and 2.6 × 10<sup>-5</sup> Ω cm<sup>2</sup>. One of the elements that could explain the difference between the Ar-precleaned contacts obtained with 200 and 300 mm lies in equipment settings. Indeed, gas fluxes, RF1 and RF2, are similar from 200 to 300 mm equipment, but one parameter that changes is the volume chamber (V<sub>300</sub> > V<sub>200</sub>). According to basic thermodynamic considerations, when the volume chamber increases, the kinetic energy of a preclean element striking the surface is higher. This could explain why Ar-precleaned contacts obtained with 300 mm platform are more damaged than Ar-precleaned contacts obtained with 200 mm platform.

## V. CONCLUSIONS

In the framework of the integration of III-V/Si hybrid lasers on a 300 mm platform, a CMOS-compatible contact module has to be developed. In this context, the Ni/InP system with either He or Ar plasma preclean prior to Ni deposition has been studied. We demonstrated that the nature of the dry-preclean used to remove the remaining native oxides significantly affects the InP surface. Indeed, Ar preclean leads to a damaged surface with important roughness containing a high concentration of In dots, while He preclean leads to a smoother surface and no or very few In dots. The phase sequence of the Ni/InP system remains the same with either preclean, nevertheless. The extracted electrical characteristics show that Ar-precleaned contacts exhibit a nonohmic behavior for each investigated thermal budget, which is a direct result of a rough and damaged surface morphology, and a high In accumulation on the InP top surface. He-precleaned contacts are, however, ohmic from the as-deposited state to 350 °C for 60 s and become non-ohmic after 400 °C for 60 s. The shift from ohmic to nonohmic behavior is attributed to a discontinued reacted layer and an increased surface roughness.

In this paper, a complete material study of the Ni/InP system was realized. The latter allowed us to understand and explain the electrical results obtained. Using the adequate integration scheme, the Ni/InP system complies with the initial requirements in terms of specific contact resistivity. Integration of the CMOS-compatible contact module for future III-V/Si hybrid lasers using on a 300 mm platform is, therefore, demonstrated.

## ACKNOWLEDGMENTS

The authors would like to thank Nicolas Coudurier, Christophe Lecouvey, Sylvie Favier, Nicolas Chevalier, Jérémy Da Fonseca, Vincent Amalbert, and Cédric Rostaing (CEA Leti) for their contributions. The authors also thank David Carrara and Almae Technologies for providing the doped epitaxial layers. This work was
supported by the French National Research Agency (ANR) under the "Investissements d'avenir" Program Nos. ANR 10-AIRT-0005 (IRT NANO-ELEC) and ANR 10-EQPX-0030 (EQUIPEX FDSOI 11) and by the Nano2022 project.

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