AVS2013 Session EM+TF-MoM: High-k Gate Oxides for High Mobility Semiconductors I

One of the solutions to the constantly growing demands of the microelectronic industry is to replace Si, the channel material in metal oxide field effect transistors (MOSFETs), with a higher mobility semiconductor. Being a leading candidate, Ge has recently been the center of numerous research works. Ge surface passivation was a major challenge which was almost exclusively addressed in Ge related research. Once a few options for good passivation of Ge were found, the current challenge in Ge MOSFET research is to find a gate dielectric with the desired properties. This dielectric should have a large band gap, large band offsets with respect to Ge, chemical and thermal stability on top of Ge or on top of the passivation layer, and a high dielectric constant (high-k).

In this work the dielectric stack HfMOx/Al₂O₃/GeO₂ with the nominal thicknesses 4/2/0.7 nm was studied on top of p-Ge. GeO₂ served as the passivation layer, while Al₂O₃ was chosen for its wide band gap and band offsets with respect to Ge, and its excellent chemical stability on GeO₂. Since the dielectric constant of Al₂O₃, is not sufficiently high, an additional top high-k dielectric is needed. We have studied Hf-based oxides (HfO₂, HfAlOx, HfGdOx, and HfZrOx) as the top high-k dielectric.

In order to examine the suitability of the dielectric materials, current-voltage (I-V) measurements were conducted. The results show a significant difference between HfAlOx and the other high-k studied. While HfO₂, HfGdOx and HfZrOx display similar I-V curves, with the same trend and the same level of leakage current, the leakage current of the HfAlOx containing sample is one order of magnitude lower.

In order to understand the root cause of this phenomenon, the band alignments of the samples were studied using X-Ray Photoemission Spectroscopy (XPS). Here, again, the HfAlOx sample stands out: only in this high-k dielectric the valence band offset (VBO) with respect to Ge is larger than the VBO of Al₂O₃ (3.6 and 3.1 eV respectively), while for the other dielectrics it is equal to the VBO of Al₂O₃ (3.0-3.1 eV). Examination of the conduction band offsets (CBO) shows no difference between the samples. The CBO of the Hf-based high-k dielectrics with respect to Ge are in the range of 1.5-1.7 eV, while the CBO of Al₂O₃ with respect to Ge is 3.1eV.

2-D nano-materials, like graphene and Van Der Waals layered materials, have attracted lots of attentions as new generation electronic materials for device applications because of their prominent properties such as high-mobility, flexibility and optical transparency compared to conventional semiconductor materials like silicon. Therefore, it is desperately required to develop techniques to integrate robust gate dielectrics with high-quality interfaces for these materials in order to attain maximum performance. To date, a variety of methods including physical vapor deposition, atomic layer deposition, physical assembly among others have been employed in order to integrate dielectrics for graphene based field-effect transistors. Owing to the difficulty in wetting pristine surfaces of graphene, most of the atomic layer deposition methods require a seeding technique involving non-covalent functionalization of their surfaces in order to nucleate dielectric growth while maintaining their intrinsic properties. A review regarding the various dielectric integration schemes will be provided. Effects of ALD process on transport characteristics of 2D channel layers including graphene and TMD (transition metal dichalcogenide) will be also discussed.

This work was financially supported by the SWAN program funded through NRI-SRC.

AlGaN/GaN high electron mobility transistors (HEMTs) are of significant interest for high power, high frequency and high temperature devices due to their large band gaps, high mobility and chemical stability compared to Si and other high mobility III-V materials. However, a large leakage current and frequency dependent current collapse degrades the performance of AlGaN/GaN HEMTs. The surface passivation process consisting of a suitable surface treatment and growth of a high-k oxide layer between the semiconductor and the gate metal is one of the proposed methods to address these issues.

In this study, we investigate atomic layer deposition (ALD) of Al₂O₃ on the native oxide, N₂ plasma and forming gas plasma exposed AlGaN surface, respectively. X-ray photoelectron spectroscopy (XPS), l­o­w ene­r­gy electron diffraction (LEED) is used to determine the interface after N₂ or forming gas plasma exposure. Initial results indicate that N₂ or forming gas plasma is able to remove carbon and decrease the concentration of oxygen. And the contamination level is lower than various alternative wet chemical treatments. The interaction between the Al₂O₃ and the AlGaN surfaces during deposition is also studied by XPS. The XPS and LEED are carried out in-situ by transferring the samples between the plasma chamber, the ALD reactor and the analysis chamber under UHV conditions (< 2 x 10^-10 mbar) to prevent contamination due to atmospheric exposure.

The work is supported by the AOARD under AFOSR Grant No. FA2386-11-14077

Here we present ambient pressure XPS studies of the atomic layer deposition of HfO₂ on InAs: By performing subsequent half-cycle steps of the ALD process within the reaction cell of an ambient pressure XPS system, we were able to monitor the slowed down ALD reaction by XPS and thus obtain fully in-situ and real-time XPS measurements of the high-k deposition on III-V semiconductors for the first time. The experiments were performed at the HP-XPS endstation of synchrotron beamline I511 at the MAX IV Laboratory [3]. Tetrakis(dimethylamino)hafnium (TDMA-Hf) and water were used as precursors, deposited at pressures between 10^-3 and 10^-2 mbar.

After an activation time of up to several minutes, a complete reduction of the As-oxides within about 20 s could be seen in the real-time As 3d spectra during exposure to TDMA-Hf. Hf 4f spectra show a significant amount of hafnium to be present on the surface only when the As-oxide reduction sets in, indicating that the reaction occurs immediately as soon as hafnium precursors are adsorbed on the surface. During the second half-cycle, surface hydroxylation occurs within about a minute as seen in O 1s spectra during water exposure. N 1s and C 1s spectra taken under vacuum conditions between subsequent ALD half-cycles change reversibly between different N- and C-based components. If the substrate temperature is reduced from 220°C to 160°C, an incomplete As-oxide reduction together with significant adsorption of non-reacting hafnium precursor material on the surface is observed. We will discuss the dependence of the chemical reactions on further ALD conditions and the relevance of such ambient pressure XPS studies for an improved understanding and control of the semiconductor-oxide interface.

[1] Hinkle et al., Curr. Opin. Solid St. M. 15, 188 (2011)

[2] Timm et al., Appl. Phys. Lett. 97, 132904 (2010)

[3] Schnadt et al., J. Synchrotron Radiat. 19, 701 (2012)

Growing suitable high-k dielectrics for high-performance III-V metal-oxide semiconductor field effect transistor (MOSFET) devices remains a challenge because native oxides on III-V semiconductors contain a high number of interfacial defects. In particular, atomic layer deposition (ALD) of Al₂O₃ has been thoroughly studied, featuring a “self-cleaning” phenomenon or reduction process of the initial native oxide by trimethyl aluminum. Despite notable progress in improving the interface quality, the number of interfacial defects still prevents the manufacturing of quality MOSFET devices. A fundamental understanding of the chemical composition of the interface and its evolution during processing is important to make further progress. In this study, in-situ infrared (IR) spectroscopy is used to examine the growth at 300°C of aluminum silicate (AlSiO_x) using trimethylaluminum (TMA), silicon tetrachloride (SiCl₄) and heavy water (D₂O) on degreased native oxides and chemically-treated (5% vol. HF and 10% vol. (NH₄)₂S) InP(100) surfaces. After an initial TMA exposure, the formation of Al-O-P species is observed, with a loss of In(PO₃)₃ and gain of x-(PO₄) (x= In and Al) (at 1007 and 1145 cm^-1, respectively) along with the formation of Al-O-Al bonds (absorption band at 800 cm^-1). This observation is consistent with the “self-cleaning” effect whereby the native oxides are reduced by formation of aluminum oxide and different chemical species. Upon the subsequent D₂O exposure, the loss of the 1217 cm^-1 and 2942 cm^-1 bands indicates the removal of the aluminum-bound methyl groups through ligand exchange. Interestingly, a subsequent pulse of SiCl₄ gives rise to a vibrational mode at 1060 cm^-1, assigned to Si-O-P. This indicates that SiCl₄molecules primarily react surface P-O(D) groups to form a silicon phosphate structure on the native oxides. After the subsequent D₂O exposure, further growth of the mode at 1060 cm^-1 suggests the continued formation of surface Si-O-P. The presence of AlSiO_x is observed on the degreased native oxide InP(100) substrates after eight supercycles. In conclusion, a complex consisting of primarily of Al-O-P develops initially, associated with a self-cleaning mechanism. As further growth develops, a complex of Al-O-Si becomes apparent, as indicated by the appearance and growth of the mode at 1151 cm^-1. The information derived from this study makes it possible to optimize the growth conditions for tailored aluminum silicate layers on InP surfaces. This work is made possible by National Science Foundation as a part of the U.S. –Ireland R&D Partnership (Grant no. NSF-ECCS-0925844) and Science Foundation Ireland [Grant No.09/IN.1/I2633.]