# Diamond delta doped structures exhibiting ultra-sharp interfaces

C. Mer-Calfati, Nicolas Tranchant, P. Volpe, Jean-Charles Arnault, P.

Bergonzo, F. Jomard

► To cite this version:

C. Mer-Calfati, Nicolas Tranchant, P. Volpe, Jean-Charles Arnault, P. Bergonzo, et al., Diamond delta doped structures exhibiting ultra-sharp interfaces. EuMIC 2014 - 9th European Microwave Integrated Circuits Conference, Oct 2014, Rome, Italy. pp.144-145, 10.1109/EuMIC.2014.6997812. cea-01901744

HAL Id: cea-01901744

https://cea.hal.science/cea-01901744v1

Submitted on 6 Nov 2025

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# Diamond Delta Doped Structures
Exhibiting Ultra-Sharp Interfaces

C. Mer-Calfati, N. Tranchant, P.N. Volpe,
J-C. Arnault, P. Bergonzo

CEA, LIST, Diamond Sensors Laboratory,
F-91191 Gif-sur-Yvette, France

F. Jomard

GEMaC-CNRS/UVSQ,
1 place Aristide Briand, 92195 Meudon, France

**Abstract**— Boron delta-doping of diamond has appeared as a promising viable approach for the fabrication of high performance RF power transistors taking advantage of diamond properties. Here structures based on p-/p+/p- multilayers were synthesised on (100) HPHT Ib substrates using MPCVD. An original gas injector system was developed enabling to significantly improve the sharpness of both interfaces between p+ and p- layers with a good reproducibility. SIMS profiles recorded on the doping transients still demonstrate an asymmetry of the interface sharpness from 7nm/decade to 2nm/dec on the p-/p+ and the p+/p- interfaces, respectively. The observed differences are here explained and confirmed experimentally, and result from the combination of ion mixing with the effect of the surface roughness, thus limiting the SIMS resolution in depth. The MRI (Mixing Roughness Information) model then allows to evaluate the real value of the delta thickness achievable using this technique to 7 nm and the negative and positive gradients to identical values, namely of 1.4 nm/dec.

**Keywords**— Diamond Delta Structures

Diamond extreme properties make it a very attractive material for RF electronic applications (wide band gap, high electrical breakdown voltage, high carrier velocity and high thermal conductivity) [1][2]. Single crystal diamond is an excellent candidate for high-voltage and high-power applications [3]. High frequency RF switches are based on "delta" MESFET architecture [4] that consists in a very thin p+ layer containing highly boron content (ideally 2 nm layer with [B] > 3.1020 at/cm3) sandwiched between two "low boron doped" p- layers (500 nm for the bottom layer and 20 nm for the top layer with [B] ≈ 1016 at/cm3) [5][6]. For such thin p+ layer, the conduction occurs in the p- doped layers due to holes delocalization from the narrow delta layers. The main experimental challenge is to control both the p+ thickness and the sharpness of the doping interfaces to allow carrier delocalization within the surrounding p- layer that must exhibit high mobility.

We previously demonstrated the feasibility of ultra sharp p+/p- and p-/p+ interfaces with doping gradient values above four decades using an original injection system [7]. This innovative configuration enables to inject the pre-cursor gases at the very close vicinity of the holder via a re-tractable quartz tube (Fig 1). Consequently, gradients of boron concentrations were significantly reduced, down to 7 nm/decade and 2 nm/decade for the p+/p- and p-/p+ inter-face gradients, respectively.

In the present paper, we report on the growth of reproducible delta-doped layers (p-/p+/p-) with nanometric p+ layer. Secondary Ion Mass Spectrometry (SIMS) phenomena inherent to the technique such as mixing and roughness effects were taken into account to extract the real p+ thickness from SIMS profiles using the MRI model (Mixing Roughness Information model) [8].

<figure><img src="image_2.png" /><figcaption>Figure 1: Configuration of the injection gas system using a retractable quartz tube.</figcaption></figure>

Boron doped diamond layers were grown using Microwave-Plasma-enhanced Chemical Vapour Deposition (MPCVD) on Ib (100) HPHT diamond substrates purchased from Sumitomo. The dopant gas used is trimethylboron (TMB). Oxygen was added during growth (with an optimum for a [O₂]/[H₂] = 0.25 %) to achieve p⁻ layers with a boron content below the residual one in the reactor (< 5x10¹⁶ at.cm⁻³)[9][10]. Such p⁻ layers exhibit high carrier mobility values (1250 cm²/V.s) according to RT Hall effect measurements [11].

For CVD growth, the temperature, the pressure and the microwave power were respectively at 850°C, 120 mbar and 600 W. Other CVD parameters are summarized in the Table 1. Boron concentrations were measured by SIMS in a secondary negative ion detection using Cs⁺ ions primary of 14.5 keV and an incidence angle of 27°. The effect of injection time was first investigated to determine the best compromise to achieve the highest boron content and the thinnest full width at half maximum (FWHM) of the p⁺ layer. Two p⁺ layers were deposited on the same substrate (Fig 2a) while a third p⁺ layer was grown on another substrate with an injection time of 105 s.
For the three p+ layers, the maximum boron concentration and the FWHM measured from SIMS profiles are plotted on Fig 2b versus injection time. The best compromise is obtained for an injection time of 90 s leading to a maximum boron content of $2 \times 10^{20}$ at/cm$^3$ and a FWHM of 8 nm. Indeed, for a longer injection time, the FWHM becomes too large to obtain a functional p+ layer for delta MESFET architecture (Fig 2b). The boron concentration is lower than $5 \times 10^{19}$ at.cm$^{-3}$ for the shortest injection time. The reproducibility of the p+ layers is confirmed from the SIMS analysis, as they reproducibly exhibit a residual boron concentration of $3 \times 10^{16}$ at/cm$^3$ (Fig 2a).

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Figure 2: (a) SIMS profiles of two p+ layers grown with injection times of 50 sec and 90 sec; (b) [B]max concentration and FWHM of the p+ layers for injection times ranging from 50 to 105 s.</figcaption></figure>

<table><caption>TABLE I. DIAMOND GROWTH CONDITIONS</caption><thead><tr><th>Layer Type</th><th>CH<sub>4</sub>/<br>H<sub>2</sub><br>(%)</th><th>O<sub>2</sub>/H<sub>2</sub><br>(%)</th><th>(B/C)gas<br>(ppm)</th><th>Injection time<br>(sec)</th><th>Growth rate<br>(mm/h)</th></tr></thead><tbody><tr><td>p-</td><td>1</td><td>0.25</td><td>Residual</td><td>-</td><td>0.55</td></tr><tr><td>p+</td><td>0.6</td><td>-</td><td>21400</td><td>45-90-<br>105</td><td>-</td></tr></tbody></table>

Two p-/p+/p- structures were synthesized with the same injection time (90 s) on two different HPHT Ib (100) substrates to check the reproducibility of the process. SIMS profiles provided on Fig 3 are identical exhibiting the same

positive and negative gradients (2 nm/decade and 7 nm/decade) and a FWHM of 8 nm. The two samples exhibit the same boron concentrations, $3 \times 10^{16}$ at.cm$^{-3}$ and $2 \times 10^{20}$ at.cm$^{-3}$ for the p+ and p+ layers, respectively. The high reproducibility and control of boron concentrations for p-/p+/p- multilayers were demonstrated.

<figure><img src="image_5.png" /><figcaption>Figure 3: SIMS profiles recorded on two p-/p+/p- delta layers grown on different substrates.</figcaption></figure>

When nanometric layers (< 10 nm) are characterized by SIMS, several artifacts may affect the depth resolution, and namely mostly ion mixing and the effect of the surface roughness [12] [13]. These effects can here explain the asymmetry of the observed SIMS profiles and the lower dynamic for the negative gradient (7nm/decade instead of 2nm/decade).

In conclusion, we reported on the controlled growth of p-/p+/p- multilayers with a high reproducibility for boron concentrations. This was achieved using an innovative gas injector. SIMS profiles were fitted with the MRI model to take into account the ion mixing and the surface roughness effects. As these effects are considered, p-/p+ and p+/p- interfaces present the same gradient of 1.4 nm/dec.

## ACKNOWLEDGMENT

The authors would like to thank the ANR for its financial support through the Deltadiam-ANR-08-BLAN-0195 contract.

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