Improvement of interfacial and electrical properties of Al2O 3 / n-Ga0.47 In0.53 As for III-V impact ionization MOSFETs

Y. Lechaux, A. Fadjie, S. Bollaert, V. Talbo, J. Mateos, T González, V

Vasallo, Nicolas Wichmann

► To cite this version:

Y. Lechaux, A. Fadjie, S. Bollaert, V. Talbo, J. Mateos, et al., Improvement of interfacial and electrical properties of Al2O 3 / n-Ga0.47 In0.53 As for III-V impact ionization MOSFETs. Journal of Physics: Conference Series, 2015, 647, pp.012062. 10.1088/1742-6596/647/1/012062. http://dx.doi.org/10.1088/1742-6596/647/1/012062

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# Improvement of interfacial and electrical properties of Al₂O₃/n-Ga₀.₄₇In₀.₅₃As for III-V impact ionization MOSFETs

To cite this article: Y Lechaux et al 2015 *J. Phys.: Conf. Ser.* **647** 012062

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# Improvement of interfacial and electrical properties of Al₂O₃/n-Ga₀.₄₇In₀.₅₃As for III-V impact ionization MOSFETs

Y Lechaux¹, A Fadjie¹, S Bollaert¹, V Talbo², J Mateos², T González²,
B G Vasallo² and N Wichmann¹

¹Institute of Electronics, Microelectronics and Nanotechnology, University of Lille 1,
UMR 8520, 59652 Villeneuve d'Ascq, France

²Applied Physics Department, Salamanca University, Plaza de la Merced, s/n
Salamanca 37008, Spain

E-mail: yoann.lechaux@ed.univ-lille1.fr

**Abstract.** In this work, Metal – Oxide – Semiconductor Capacitors (MOSCaps) based on Al₂O₃/n-Ga₀.₄₇In₀.₅₃As interface have been studied. In order to have high MOSFETs performance, it is necessary to improve the semiconductor – oxide interface quality. It is observed that the (NH₄)₂S passivation shows lower interface trap density in the order of 6×10¹¹cm⁻².eV⁻¹. Also, it is observed that O₂ plasma densification after a passivation in a NH₄OH solution improves the electrical behaviour of the charge control. Low interface trap density in the order of 1×10¹²cm⁻².eV⁻¹ was obtained for different treatments presented in this work.

## 1. Introduction

During the past few years, the size reduction of the Metal – Oxide – Semiconductor Field Effect Transistor (MOSFET) was the key for the development of performant Integrated Circuits (ICs). One of the main issues is the reduction of power consumption related to the lowering of supply voltage V<sub>DD</sub>. Today the decrease of V<sub>DD</sub> is limited by the subthreshold swing SS. New device architectures are being investigated in order to have SS less than the 60mV/dec limit. I-MOSFET (Impact ionization MOSFET) is one candidate to obtain SS of few mV/dec [1]. Silicon I-MOSFET showed steep subthreshold swing in the order of 5mV/dec [2] with I<sub>ON</sub> current in the range of 400mA/mm [2] however it requires large V<sub>DD</sub>. III-V materials and engineering of bandgap energy is envisaged to reduce V<sub>DD</sub>. In III-V I-MOSFET, gate oxide with high quality is an issue. Large majority of works used Al₂O₃ oxide deposited by ALD (Atomic Layer Deposition) to have low interface trap density D<sub>it</sub> in the order of 10¹²cm⁻².eV⁻¹ [3] and very few report less D<sub>it</sub> [4]. Recently, paper [5] reports an O₂ plasma treatment of the Al₂O₃ oxide to decrease the formation of oxide during the post-deposition annealing (PDA). In this paper, we present improvement of interfacial and electrical properties of Al₂O₃/n-Ga₀.₄₇In₀.₅₃As using O₂ plasma treatment during the formation of the oxide by ALD.

## 2. Device structure and experimental details

MOSCAPs were realised from a structure of n-doped Ga₀.₄₇In₀.₅₃As lattice matched on InP wafer as shown in figure 1. The structure is made of a high n-doped GaInAs layer (N: 1×10¹⁹cm⁻³) to have a low resistance n-type ohmic contact and a low n-doped GaInAs layer (N: 2×10¹⁶cm⁻³) close to the oxide – semiconductor interface. The first step of fabrication process consist on cleaning, degreasing

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the surface and passivation before oxide growth by ALD. Then the thermal Al₂O₃ is deposited at 250°C from trimethylaluminium (TMA) and water (H₂O) precursors. Three samples were prepared. For the sample I, the native oxide was etched with a HCl solution diluted in deionized water (EDI) with a volume ratio of 1:3 during 2min. Then a 5% dilute (NH₄)₂S solution was used to passivate the surface during 10min [3]-[6]. For the sample II, a 4% dilute NH₄OH solution was used to passivate the surface during 10min. Then 4nm Al₂O₃ was deposited by ALD at 250°C. For a third sample III, an O₂ plasma was used after 2nm oxide deposition on a sample treated with a dilute NH₄OH solution. Then 2nm oxide was deposited to have an oxide thickness of 4nm. On each sample, a PDA at 600°C during 1min under N₂H₂ atmosphere was used to stabilize the oxide. All treatments are resumed in table 1. The gate contact was realised with the evaporation of a sequence of Ni/ Au (100nm/ 250nm). The source n-type ohmic contact was realised with the evaporation of a sequence of Ti/ Pt/ Au on the highly n-doped GaInAs layer. The cross section of MOS gate stack was observed by the means of transmission electron microscopy showed in figure 2. We can observe in the middle the 4nm of non-densified Al₂O₃ surrounded by GaInAs at left and Ni metal at right.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Structure of MOSCaps device at the end of the fabrication process.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** TEM picture of the MOS structure, we can see in the middle of Al₂O₃ with a uniform thickness of 4nm.</figcaption></figure>

<table><caption>**Table 1.** Summary of different treatments and oxide deposition.</caption><thead><tr><th>Sample</th><th>Surface treatments</th><th>Oxide Thickness (nm)</th><th>Densification</th><th>Post Deposition Annealing (°C)</th></tr></thead><tbody><tr><td>I</td><td>HCl + (NH<sub>4</sub>)<sub>2</sub>S</td><td>4</td><td>no</td><td>600</td></tr><tr><td>II</td><td>NH<sub>4</sub>OH</td><td>4</td><td>no</td><td>600</td></tr><tr><td>III</td><td>NH<sub>4</sub>OH</td><td>4</td><td>yes</td><td>600</td></tr></tbody></table>

### 3. Results and discussion

#### 3.1. *Passivation effect*

We compared dilute (NH₄)₂S solution to passivate the surface of the sample I and dilute NH₄OH solution to passivate the surface of the sample II. To that purpose, capacitance – voltage (C – V) measurements were done at ambient temperature in function of gate voltage Vgs for various frequencies of the dynamic signal applied on the gate (figure 3). In figure 3(a), we represent C-V measurements for sample I and C-V measurements for sample II in figure 3(b). We can notice in both samples a high frequency dispersion in accumulation between Vgs=1V and Vgs=2V due to leakage current [7]. The sample I treated with the (NH₄)₂S solution has showed around depletion, between Vgs=-0.75V and Vgs=0.25V, a lower frequency dispersion in comparison with sample II treated with the NH₄OH solution, indicating a lower interface trap density. However the identical stretch-out
between $V_{gs}=-0.75$V and $V_{gs}=0.25$V on both samples has showed no significant modification in charge control. Furthermore, we can notice a “bump” in inversion, between $V_{gs}=-2$V and $V_{gs}=-0.5$V which indicates inversion assisted by interface trap states [8].

<figure><img src="image_6.png" /><figcaption>**Figure 3.** C – V characteristics of the MOS structure Al₂O₃/n-GaInAs for sample I passivated with (NH₄)₂S solution (a), and for sample II passivated with NH₄OH solution (b).</figcaption></figure>

## 3.2. Oxide densification effect

We studied the densification effect using O₂ plasma in the sample III passivated with a dilute NH₄OH solution to improve the oxide – semiconductor interface quality. The post deposition O₂ plasma was used on the structure Al₂O₃/Ge by Zhang et al. in order to improve GeOₓ oxide [9]-[10], they succeed to have low value of $D_{it}$ in the range of 10¹¹cm⁻².eV⁻¹. In our case, the treatment consists on a densification of the oxide near interface by the mean of an O₂ plasma across 2nm of oxide. This treatment decreases the quantity of mobile charges in the oxide and fills dangling bonds after oxide deposition. Then 2nm of Al₂O₃ were added to have a total thickness of 4nm. C – V characteristics at ambient temperature in function of $V_{gs}$ are showed in figure 4. We can notice at low frequency the minimum capacitance value is lower for the sample III than for sample II even though both are passivated with NH₄OH solution. Furthermore, the small stretch-out between $V_{gs}=-0.5$V and $V_{gs}=0.5$V for sample III indicates that the O₂ plasma improves the charge control by the gate and might improve the subthreshold swing SS of I-MOSFETs. And as for sample II, the large frequency dispersion in accumulation could be due to the high leakage current [7].

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Capacitance measurements of the MOS structure Al₂O₃/n-GaInAs with an oxide densification treatment (sample III).</figcaption></figure>

## 3.3. Interface trap density

We extracted the interface trap density calculated by High frequency – Low frequency (HF-LF) method for each sample shown in figure 5 in function of gate – source voltage $V_{gs}$. All samples show $D_{it}$ with the same order of magnitude, in the range of 1×10¹²cm⁻².eV⁻¹. However, we can notice a low $D_{it}$ of 6.5×10¹¹cm⁻².eV⁻¹ for the sample I passivated with (NH₄)₂S solution compared to $D_{it}$ of about 9.2×10¹¹cm⁻².eV⁻¹ and 1.5×10¹²cm⁻².eV⁻¹ for samples II and III respectively. But the extraction of $D_{it}$ in samples II and III may not be accurate because of the large frequency dispersion in accumulation.
Moreover, we have extracted flatband voltage V<sub>FB</sub> in order to calculate the charge density Q<sub>f</sub> in the oxide from Q<sub>i</sub>=(V<sub>FB</sub>-Φ<sub>MS</sub>)/C<sub>OX</sub> (Table 2) where C<sub>OX</sub> is oxide capacitance and Φ<sub>MS</sub> ideal metal-semiconductor workfunction difference. We observed a decrease of charge density for sample III with densification and a shift of flatband voltage to its theoretical value (1).

$$ \Phi_{MS} = \Phi_{M} - X - \frac{E_{g}}{2 \cdot q} - \frac{K \cdot T}{q} \cdot \ln\left(\frac{N_{D}}{n_{i}}\right) \quad (1) $$

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Extraction of the interface trap density D<sub>it</sub> in function of the applied voltage V<sub>gs</sub> from HF-LF method.</figcaption></figure>

<table><caption>**Table 2.** Calculated oxide charge density in function of the flatband voltage for each sample.</caption><thead><tr><th></th><th>HCl + (NH<sub>4</sub>)<sub>2</sub>S</th><th>NH<sub>4</sub>OH</th><th>NH<sub>4</sub>OH + densif.</th><th>Theory</th></tr></thead><tbody><tr><td>Qf (x10<sup>11</sup>cm<sup>-2</sup>)</td><td>64</td><td>45.1</td><td>1.14</td><td>-</td></tr><tr><td>V<sub>FB</sub> (V)</td><td>-1,27</td><td>-0,96</td><td>-0,15</td><td>-0.13</td></tr></tbody></table>

## 4. Conclusion

In summary, we studied effect of surface treatment on Al<sub>2</sub>O<sub>3</sub>/ n-Ga<sub>0.47</sub>In<sub>0.53</sub>As interface. We have demonstrated a low D<sub>it</sub> of about 6.5×10<sup>11</sup>cm<sup>-2</sup>.eV<sup>-1</sup> for sample treated with dilute (NH<sub>4</sub>)<sub>2</sub>S solution as surface treatment before oxide deposition. We have reported improvement in charge control by the gate using an O<sub>2</sub> plasma after oxide deposition, even if we obtained large interface trap density. This process needs to be optimized in order to build I-MOSFETs using higher dielectric permittivity oxide like HfO<sub>2</sub>.

## 5. Acknowledgments

This work has been supported by the Agence Nationale de la Recherche through the project ANR-13-JS03-0001.

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