Ohmic Graphite-Metal Contacts on Oxygen-Terminated Lightly Boron-Doped CVD Monocrystalline Diamond T ⁎ M

Diamond & Related Materials 92 (2019) 18–24

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Diamond & Related Materials

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Ohmic graphite-metal contacts on oxygen-terminated lightly boron-doped CVD monocrystalline diamond T ⁎ M. De Feudisa,b, , V. Millea, A. Valentina, O. Brinzaa, A. Tallairea, A. Tardieua, R. Issaouia, ⁎⁎ J. Acharda, a Laboratoire des Sciences des Procédés et des Matériaux, CNRS, Université Paris 13, Sorbonne Paris Cité, 93430 Villetaneuse, France b Dipartimento di Matematica e Fisica, Università del Salento, 73100 Lecce, Italy

ARTICLE INFO ABSTRACT

Keywords: A process to obtain ohmic contacts on oxygen-terminated lightly boron-doped CVD monocrystalline diamond Ohmic graphite-metal contacts films was developed. Samples were contacted by Ti/Au metallic pads in the transmission line model (TLM) Graphitization configuration. The electric contacts were placed onto a mesa structure produced on the CVD boron-doped layer. Ion implantation One of the samples was additionally implanted with helium ions at 10 keV in order to induce the formation of a Oxygen terminations graphitic layer underneath the diamond surface before contacting so as to improve electrical conduction. The Lightly boron-doped CVD diamond film electrical performance of both devices was characterized by the TLM method and compared. As a result, the sample with metallic electrodes exhibited a small and non-linear electrical conduction, while the graphitic/ metallic contacts showed an ohmic behaviour with an estimated specific contact resistance as low as − − 3.3 × 10 4 Ω·cm2 for a doping level of a few 1017 cm 3. This approach opens the way to more efficient ohmic contacts on intrinsic or low-doped diamond that are crucial for the development of electronic devices and detectors.

1. Introduction electrodes on diamond films, such as the metallic material used and the diamond surface termination. A typical metallic stacking for diamond Nowadays, synthetic CVD diamond films are widely investigated as ohmic contact is composed of titanium and gold layers [2,10,14]. The power devices [1,2], particle detectors [3,4] and dosimeters [5,6] titanium layer ensures titanium carbide (TiC) formation after thermal thanks to their appreciable properties, such as wide band gap, excep- annealing at about 500 °C [2,14] increasing the contact mechanical tional thermal conductivity, radiation hardness, high breakdown vol- adhesion on the device and the gold layer avoids titanium oxidation. A tage, carrier mobility [1,7]. One of the crucial points for the develop- crucial role in terms of electrical conduction can also be played by the ment of these diamond devices is the manufacturing of good electric diamond surface termination. For example, Volpe et al. [8,14] showed contacts. In the last decades several studies about the metallic electrode that Ti/Au ohmic contacts were obtained on the H-terminated lightly manufacturing on diamond films, for example with ohmic or Schottky boron-doped diamond surface, while by adding an oxidation treatment electrical behaviour, have been carried out [8–12]. However, up to (by vacuum ultra-violet (VUV) ozone treatment [15]) the electrodes now, diamond contacting is still not a simple task due to the inertness proved to have a Schottky behaviour. But one of the drawback of hy- and the wide bang gap of the material. Indeed, diamond contacts drogenated surface is its thermal instability and, even if improvement should satisfy different requirements to be considered as good elec- in terms of specific contact resistance can be obtained on such surface, trodes, such as a good mechanical adhesion, chemical, thermal stability it is highly desirable to develop a deposition process of ohmic contact over time and the possibility of bonding connections. Ohmic contact on lightly boron-doped layer starting from an oxygenated diamond fabrication, which is the main subject of the present paper, is still an surface which is well known to be very stable [16]. opened field for intrinsic and lightly doped diamonds [13], while For low boron-doped oxygen-terminated CVD diamond layers a contacting heavily-doped diamonds is facilitated by the low material non-ohmic electrical behaviour is expected, thus a graphitic layer in- resistivity [12]. Several aspects can affect the performance of metallic duction underneath the diamond surface before the metallic deposition

⁎ Correspondence to: M. De Feudis, Laboratoire des Sciences des Procédés et des Matériaux, CNRS, Université Paris 13, Sorbonne Paris Cité, 93430 Villetaneuse, France. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (M. De Feudis), [email protected] (J. Achard). https://doi.org/10.1016/j.diamond.2018.12.009 Received 3 October 2018; Received in revised form 10 December 2018; Accepted 11 December 2018 Available online 12 December 2018 0925-9635/ © 2018 Elsevier B.V. All rights reserved. M. De Feudis et al. Diamond & Related Materials 92 (2019) 18–24 has been considered. Indeed, diamond graphitization just under the 30 s, respectively. The implantation dose was estimated to be about − surface can create electrically active defects and consequently reduces 1.4 × 1017 ions·cm 2. This treatment was performed at room tem- − the depletion region width and/or decrease the potential barrier height perature and in secondary vacuum condition (pressure of 10 5 mbar). at the diamond/metal interface [10]. Diamond graphitization has al- Following implantation, a thermal annealing was carried out at 1000 °C − ready been demonstrated by laser or high energy ion implantation on for 60 min in secondary vacuum (10 6 mbar) [36,37]. Metallic contacts the diamond surface or into the bulk [7,17,18]. Currently both tech- composed of titanium and gold layers in the TLM configuration were niques have been successfully used to produce ohmic graphitic contacts deposited on a CVD diamond mesa structure by several micro-fabrica- [19–25]. A previous study to combine diamond metallization and gra- tion steps widely discussed in the following Section 3.2. Several char- phitization techniques was carried out by Chen et al. [26] by using acterization techniques were used to investigate the samples during all argon ion implantation at 40 keV. They produced ohmic graphite/metal the processing steps. Structurally investigations were performed by a contacts but with a specific contact resistance which remained rela- Raman spectrometer (Jobin-Yvon HR800) using the 632.8 excitation − tively high (≈ a few Ω.cm 2). In order to improve this result, the main line, morphological analyses were carried out by a SEM (Leica S440) idea of our work was to produce a graphitic layer just underneath the and a profilometer, while diamond surface electrical measurements lightly boron-doped diamond surface using a low ion implantation were executed by four-point probes method (using a Keithley 4200-SCS energy (10 keV) and a low mass ion source (helium). Subsequently we model). Finally, the metal contacts were electrically studied by the TLM manufactured metallic pads in a Transmission Line Model configuration method using a Yokogawa 7651 programmable DC source and a Femto (TLM) [27] on a mesa structure to confine the current lines in the CVD Amp DLCPA-200 amplifier controlled by a LabVIEW software. boron-doped active layer. The use of a low boron-doped layer allows ascertaining the charge carrier availability required for the contact electrical characterization [28]. 3. Results and discussion For completeness, it is worth reporting that in literature, a suc- cessful work about diamond contacting on oxygen-terminated lightly 3.1. Ion implantation-induced diamond graphitization − doped diamond films (2.5 × 1017 cm 3) has already been reported. Chen's et al. [29] succeed in ohmic metal contact fabrication using a Ti/ The contacting process required to overcome the diamond graphi- Pt/Au deposited on a CVD mesa structure obtaining a very low specific tization threshold which is estimated to be about − − contact resistance (1.3 × 10 5 Ω·cm2). Nevertheless nowadays, thir- 1022–1023 vacancies·cm 3 [38,39]. Based on TRIM (Transport of Ions in teen years after this paper, the fabrication of good ohmic metallic Matter [40]) simulations we calculated that under our implantation − contacts on this kind of diamond film (oxygen-terminated and lightly conditions about 1023 vacancies·cm 3 were produced with a maximum doped) is still not a simple task and remains an open field [8,13,14]. penetration depth of about 50 nm in diamond. For example, the profile Therefore, the idea of this work is also to show that, even if the metallic of vacancies generated by an incident helium ion into a diamond film is diamond contacting gives rise to non-ohmic electrical behaviour or reported in Fig. 1. The implanted layer seems to spread under the ohmic one but with a high specific contact resistance, it can be possible diamond surface at about a depth of 50 nm following a warped Gaus- to improve the electrical conduction at the diamond/metal interface by sian shape distribution. inducing a graphitic layer underneath the diamond surface. From an experimental point of view, the ion implantation treatment was first tested onto an intrinsic CVD diamond film in order to more 2. Materials and instrumentations easily evaluate any electrical and structural change. In Fig. 2, a sample exposed to the implantation treatment using a ring-like shadow mask to Intrinsic and lightly boron-doped homoepitaxial diamond films highlight the optical contrast between the irradiated and unirradiated were grown by Microwave Plasma Assisted Chemical Vapour zones is shown. The graphitic layer, spatially localized by the mask, is Deposition (MPACVD) technique [30–33]. The reactors were two clearly visible since the irradiated zone appears black, in contrast with BJS150 model provided by Plassys company. One of them is exclusively the unirradiated one which remained yellow due to nitrogen impurities dedicated to the growth of intrinsic diamond and has never been con- in the HPHT substrate. taminated with boron. In this work intrinsic diamond films were used to To study the structural evolution of the diamond sample exposed to study the graphitization process while boron doped ones to perform the ion bombardment, micro-Raman analyses were carried out before electrical characterizations. The substrates were (100) oriented type-Ib and after the treatment in particular to estimate, step by step, the HPHT diamonds having dimensions of 3 × 3 × 1.5 mm3 provided by diamond-graphite phase change. Micro-Raman spectra registered on Sumitomo company. The substrate temperature was adjusted to 900 °C, the gas pressure to 140 mbar, the microwave power to 2.4 kW and the 0.06 ) total gas flux to 200 sccm (standard cubic centimeters per min). The -1 Å methane and oxygen concentrations were set at 5% and 0.25%, re-

noirepycnacavforebmu 0.05 spectively, in a purified hydrogen flow. Boron doping has been ensured μ by adding diborane (B2H6) as dopant source [34]. Roughly 5 m-thick 0.04 films were grown. The boron concentration was approximately esti- − mated to 4 × 1017 cm 3 by cathodoluminescence analyses from the 0.03 free to bound exciton ratio in agreement with reference [35]. The CL measurements were carried out by using a Scanning Electron Micro- scope (SEM) ZEISS EVO MA-15 coupled with a Horiba-Jobin-Yvon 0.02 HCLUE cathodoluminescence system at a temperature of 110 K. In order to induce diamond-graphite phase transition underneath the diamond 0.01

surface a home-made helium ion implanter was used. This experimental N( setup created a circular ion beam, with a gun diameter of 3 mm. With 0.00 this implanter, it is possible to locally modify the pressure in the gun 0 20 40 60 80 100 120 even if the ion flux control remains limited as well as the energy dis- Penetration depth (nm) tribution in the irradiated material. For this experiment, the ion ac- celeration energy was set at the low value of 10 keV to limit the pe- Fig. 1. TRIM simulation of the vacancy profile produced by a He ion implanted fi netration depth, while the current and the time were set at 300 μA and into a diamond lm.

19 M. De Feudis et al. Diamond & Related Materials 92 (2019) 18–24

− (G peak) and disordered graphite contribution at 1350 cm 1 (D peak). Then, Fig. 3c shows the structural change of the implanted diamond surface after thermal annealing. The annealing role is to transform the glassy and amorphous material into graphite for the highly damaged zone, while to restore back the broken diamond bonds in the surrounding areas [38]. As expected, the Raman spectrum of the annealed − sample shows a modification of the previous 1300–1600 cm 1 band and the G and D broad peaks sharpen gradually becoming well defined. These peaks confirm the formation of a graphite-like layer [17]. In addition, a sharp diamond peak can again be observed due to the re- storing phenomenon. In order to study the electrical conduction changes induced on intrinsic CVD monocrystalline diamond by helium implantation, four- point probe current-voltage (I(V)) measurements were carried out. Before doing these measurements, the sample was exposed to a VUV- light irradiation treatment (172 nm) for 45 min in an oxygen atmo- sphere at 2 mbar in order to avoid any electrical conduction due to the residual hydrogen termination of as-grown CVD diamond films [15]. The first electrical investigation of the intrinsic sample was done before Fig. 2. Ion implanted CVD monocrystalline intrinsic diamond. A ring-like the ion implantation, however, the insulator nature of diamond did not shadow mask was used to cover part of the diamond surface. allow measuring I(V) curves. I(V) measurements were thus registered on the sample after both ion implantation and annealing (Fig. 4). No 1000 1200 1400 1600 1800 2000 relevant electrical change was found indicating no specific electrical 2 2 After annealing improvement of the sp -bonded graphite as compared with glassy sp - (c) bonded carbon. The plot shows an electrical conduction with an ohmic behaviour, which is typical from graphitic materials [7,19,20]. A resistance value of 1 × 102 Ω was extrapolated from the data fitting and consequently, the sheet resistivity value was estimated equal to 282 Ω/ □ using Eq. (1) [41]:

.u.a(ytisnetn π V ρ = ··F □ ln 2 I (1)

After ion implantation where F is the correction factor which takes into account the sample (b) limited geometry and the probe position on the surface, which was computed equal to 0.6 in a previous work [42] by the Method of Images [43]. This shows that we successfully improved contacts on intrinsic

inama diamond through ion bombardment-induced graphitization of the surface. For completeness, it must be noted that even if theoretical simulations suggests that ion implantation treatment has induced damages

R) underneath the diamond surface, more precisely at a depth of about 50 nm, it cannot rule out changes in terms of implanted layer thickness Intrinsic diamond (a) once the annealing treatment has been carried out. Indeed, in one of our

After ion implantation and annealing Linear fit 1.0x10-3

-4

) 5.0x10 A(

1000 1200 1400 1600 1800 2000 tner 0.0 -1 r

Wavenumber (cm ) uC

Fig. 3. Raman spectroscopic evolution of the diamond and graphite peaks re- -5.0x10-4 gistered on (a) the as-grown, (b) ion implanted and (c) annealed CVD intrinsic diamond film. -1.0x10-3 the initial intrinsic CVD diamond film, after the ion implantation and -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 after thermal annealing are shown in Fig. 3a, b and c respectively. A typical Raman spectrum of an intrinsic CVD diamond, with a well-de- Voltage (V) fi −1 ned diamond peak localized at 1332 cm is obtained (Fig. 3a). On Fig. 4. Current-voltage measurements performed with the four-point probe the contrary, the spectrum of Fig. 3b presents a broad band in the range method and registered on the intrinsic CVD diamond sample after ion im- − 1300–1600 cm 1, which corresponds to the formation of glassy carbon plantation and annealing. The ohmic behaviour is confirmed by the linear fitof − peaks containing both amorphous graphite contribution at 1580 cm 1 the data.

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Fig. 5. Mesa structure and TLM contact microfabrication steps. recent works [42], diamond graphitization process induced by this kind 0.014 Mesa profile of ion implantation treatment has been carefully studied, finding that an uncertainty have to be considered between the ion implantation 0.012 theoretical forecasts and the final experimental evidence. In particular, 0.010 while the initial simulations suggested the most damaged layer for- )mm( mation at 50 nm underneath the diamond surface, the subsequent 0.008 analyses performed by transmission electron microscopy showed the presence of a 110 nm thick graphitic layer at the diamond surface. This thgieH 0.006 difference was ascribed to the intermedium annealing treatment which 0.004 converted definitely the non-diamond phase into the graphitic one, ff causing an expansion of the damaged layer due to the di erent material 0.002 densities. We believe that the same kind of uncertainty has to be taken in consideration for this work. 0.000

-0.002 0.0 0.1 0.2 0.3 0.4 0.5 3.2. Metal contacts and mesa structure manufacturing Width (mm)

fi fi Titanium‑gold metal contacts in a TLM configuration with a mesa Fig. 6. CVD diamond mesa structure pro le registered by pro lometry. structure were prepared on two lightly boron-doped diamond films. The main micro-fabrication technological steps are schematically presented of the diamond surface was performed using an argon source before any in Fig. 5. metallic deposition steps ([Ar+] = 9 sccm, t = 120 s) [2,44](Fig. 5, He+ ion induced graphitization was performed just before any step #6). Then, the titanium and gold layers deposition (50 and 200 nm technological micro-fabrication steps (Fig. 5, step #1) for one of the 2 thick, respectively) were carried out (Fig. 5, step #7). By a subsequent samples. Then, a silica (SiO2) mask with a thickness of 2 μm and a photoresist coating and an optical lithography process the TLM pad photoresist layer were deposited (Fig. 5, steps #2 and #3). An optical shape was transferred, while a chemical gold etching (by potassium photolithographic step was performed to transfer the mesa shape onto iodide solution heated at 50 °C) and a plasma titanium etching (by a the photoresist (Fig. 5, step #4) and afterwards, three plasma etching mixture of Cl2 and Ar) allowed getting the final contacts on CVD mesa steps using different mixtures of CHF3 and oxygen were performed diamond (Fig. 5, step #8, #9, #10 and #11, respectively). In particular, (Fig. 5, step #5). In particular, the first etching step was carried out on these final Au and Ti etching treatments were very important to avoid

SiO2 layer not protected by the photoresist in order to remove it just the presence of any conductive material among the pads. Before per- around the mesa structure. Then, the uncovered diamond material was forming those on the two diamond samples we optimized the recipes on etched to obtain the mesa structure. In this step we chose for safety to silicon wafers after having deposited similar Ti/Au bilayers. It is worth etch for a long time reaching about 13 μm of depth (which is showed reporting that for these tests the metallic layers were 20% thicker than subsequently) even if a shorter treatment would have been sufficient ones deposited on diamonds (50/200 nm) to ensure the complete considering the CVD layer thickness (5 μm). During this step, the gra- etching. Afterwards, an additional diamond etching step was performed phitic layer around the mesa was removed as the used gas mixture was by ICP to remove a thin layer of about 150 nm containing the graphitic reactive with any carbon phase. Finally, an additional SiO2 etching step material among the pads (Fig. 5, step #12). The same recipe was pre- was performed to remove it from the top of the mesa structure. viously tested on the sample in Fig. 2 confirming the complete removal In Fig. 6 the final CVD diamond mesa profile investigated by a of the graphitic material. Finally, an annealing treatment at 500 °C profilometer is shown. The structure proves to be sharp and 13 μm high. during 60 min in secondary vacuum conditions was performed to pro- As the contact mechanical strength on the diamond surface is an mote titanium carbide formation (Fig. 5, step #13) [14]. The final pad important aspect for both electrical measurement and wire bonding dimensions were 100 × 150 μm2 and the space between each pad operations, in order to strengthen this adhesion, a short pre-treatment

21 M. De Feudis et al. Diamond & Related Materials 92 (2019) 18–24

(a) (b)

1 mm 100 µm

Fig. 7. CVD boron-doped diamond with TLM-like metallic contacts. (a) Lightly tilted top view of the full sample: the central mesa/TLM zone surrounding by different additional patterns is visible. (b) Tilted enlargement of the mesa/TLM zone (in the center of the picture) showing the height of the area with respect to the surrounding surface. increased from 20 to 50 μm by a step of 10 μm. SEM pictures of a full carriers (Fig. 7c). processed sample are shown in Fig. 7. Subsequently, in order to determine the specific contact resistance, the total resistance values were extracted from each I(V) curve and fi 3.3. Contact electrical characterization plotted as a function of the pad distance d (Fig. 9b). The linear t fol- lows the Eq. (2) [27]:

To characterize electrically both samples, TLM measurements were RSH RTC=+2·R d carried out and compared. A typical I(V) curve registered on the non- w (2) graphitized sample with metallic contacts is presented in Fig. 8.As where R is the total resistance, R the contact resistance, R the sheet expected for diamond with a low boron doping level, a non-linear be- T C SH resistivity and w the contact width. From the X axis intercept of this haviour was clearly obtained. This non-linearity is confirmed for each linear fit it was also possible to extrapolate the Transfer Length L pad distance. T which corresponds to the real contact length crossed by the current On the contrary, as illustrated in Fig. 9a, the sample with graphitic- lines. It is worth noting that for contact length L ≥ 1.5 L , which is this metallic contact shows an ohmic electrical behaviour whatever the pad T case, the specific contact resistance ρ can be estimated by the Eq. (3) distance (d). The electrical resistances were calculated to 540, 700 and C [41]: 920 Ω for pad distances of 20, 30 and 40 μm, respectively. The last μ curve (pad distance of 50 m) and the related electrical resistance value ρC = RLwCT·· (3) could not be investigated due to a micro-fabrication problem causing a fi short circuit between the fourth and fifth pads. De nitely, considering that RC and LT values proved to be equal to Ω μ fi The fact that the graphitic layer affects positively the conductivity at 66 and 3.3 m, respectively, the estimated speci c contact resistance ρ −4 Ω 2 the diamond/metal interface can be ascribed to the generation of C was about 3.3 × 10 ·cm , namely one of the lowest values re- electrically active defects in agreement with the interface band diagram ported for oxygen-terminated lightly doped diamonds [13,26]. This ff model presented by Tachibana et al. [10]. By this model it has been di erent electrical behaviour between the two samples can be ascribed demonstrated that the diamond surface treatments producing defects at to the generation of electrically active graphitic defects in the ion im- fl the semiconductor/metal interface can reduce the depletion region planted diamond which improve the charge carrier ow at the semi- width increasing the probability of tunneling (Fig. 7b) and/or decrease conductor/metal interface. Therefore, this diamond contacting method the effective potential barrier height promoting the flow of the charge could open the way to the fabrication of ohmic graphitic-metallic contacts on oxygen-terminated and lightly doped diamonds for different applications such as the n-type doped films which are still an 8.0x10-7 opened field. 6.0x10-7 4. Conclusion -7

) 4.0x10 A(tne -7 We used helium ion implantation treatment (10 keV, 2.0x10 − 1.4 × 1017 ions·cm 2) followed by annealing to generate a graphite rr 0.0 layer underneath the surface of different intrinsic or lightly boron- uC doped diamond single crystal CVD films. Raman spectroscopic analyses -2.0x10-7 confirmed the formation of sp2 bonded carbon and 4-point probe measurements led to a sheet resistivity of 282 Ω/□ for an intrinsic film. -7 -4.0x10 In order to assess the benefit of using such shallow implanted graphitic -7 layers on electrical contacts, lightly boron-doped oxygen-terminated -6.0x10 diamond samples were contacted by Ti/Au contacts in a TLM config- -1.0 -0.5 0.0 0.5 1.0 uration over a mesa structure. Current-voltage characterizations of the Voltage (V) metal contacts have been performed showing that, without the graphitization step, non-linear behaviour was registered. On the contrary, Fig. 8. A typical current-voltage curve for the lightly boron-doped sample with with the additional He+ implantation step, a linear behaviour was μ − metal contacts (pad distance of 20 m). obtained leading to a specific contact resistance of 3.3 × 10 4 Ω·cm2.

22 M. De Feudis et al. Diamond & Related Materials 92 (2019) 18–24

(a) (b) 3 -4 1x10 2.0x10 µ )mhO(ecnatsiserlatoT 3 d = 20 m 1x10 Resistance -4 d = 30 µ m 2 1.5x10 9x10 Linear fit -4 d = 40 µ m 2 )A(tnerruC 1.0x10 8x10 2 5.0x10-5 7x10 2 0.0 6x10 5x102 -5 -5.0x10 4x102 -1.0x10-4 3x102 2 -1.5x10-4 2x10 2 -4 1x10 -2.0x10 0 -0.10 -0.05 0.00 0.05 0.10 0 5 10 15 20 25 30 35 40 45 Voltage (V) Pad distance (µm)

Fig. 9. (a) Current-voltage curves for the lightly boron-doped sample with graphite-metal contacts for diﬀerent pad distances d (blue squares 20 μm, red circles 30 μm and green triangles 40 μm). (b) Total resistance as a function of the pad distance and linear ﬁt.

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