HfAlOx Films Research Articles

Atomic Layer Deposition of HfAlOx for High-k Gate Dielectrics

Nowadays, the downscaling of CMOS devices requires continued effort to propose high performance components with extremely compact size. Surely, this trend is not achievable without extremely enhanced High-k gate stack materials, inter alia. For years, HfO2-based dielectrics deposited using ALD (Atomic Layer Deposition) technique are perfect dielectrics of choice for various devices and applications (Memristor, resistive random access memory, ferroelectric random access memory, antiferrolectric capacitances, ferroelectric FET...).As such, HfAlOx films present a good compromise between HfO2 and Al2O3 in term of coupling properties and insulating capabilities in comparison with both single HfO2 and Al2O3 films. The purpose of this work is to evaluate binary high-k HfAlOx dielectrics deposited by ALD technique. ALD is well suited for deposition of ultrathin films with exceptional capabilities enabling precise thickness, stoichiometry control and conformal growth at atomic scale. In this work, we focused on Al2O3/HfO2 films fabricated using two approaches: (i) repeatedly alternating the deposition of two different and separate extremely thin oxides (Al2O3 and HfO2; particular focus on HfO2/Al2O3/HfO2/Al2O3/HfO2 multilayer stack with equivalent Al fraction around 30%); (ii) combination of ALD super cycles allowing the formation HfAlOx layers. We analyze the general performance and the behavior of the binary high-k HfAlOx dielectrics (10 nm thin layers deposited at 300°C using TiCl4-TMA-H2O precursors) using physico-chemical (X-ray photoelectron spectroscopy XPS, X-ray reflectometry XRR, wavelength dispersive X-ray fluorescence WDXRF) and electrical measurements (Hg probe measurements, MIM capacitance).The incorporation of Al2O3 in the composition of the HfO2 gate dielectrics allows a perfect monitoring of physico-chemical properties varying from HfO2 layers to Al2O3 layers by controlling the number of Al2O3 and HfO2 ALD cycles. In this respect, we were able to monitor the Al fraction in the range of 5% to ~86% in the mixed binary dielectrics (Figure 1: XRR density of mixed binary layers vs Al/[Al+Hf] ratio, Figure 2: Al and Hf fractions in the mixed layer determined by WDXRF and XPS analysis).The electrical characteristics of the HfAlOx dielectrics confirm the influence of the Al fraction in the HfO2 layer. The leakage currents of the mixed layers decreases with increasing the Al fraction (Figure 3: J/E characteristics of mixed and stacked HfAlOx layers). The breakdown voltages for all mixed HfAlOx layers are greater than that of reference HfO2 layer. On the other hand, the shown multilayer presents an intermediate leakage currents profiles between the two mixed HfAlOx (25%Al and 40%Al). Thus, the incorporation of Al in the construction of HfAlOx system is likely to decrease the number of defects. Furthermore, the fully integrated MIM structures exhibit areal capacitance in the range 10-15 nF/mm2 of ALD-based high-k HfAlOx (10 to 12 nm) depending on the Al fraction. Thus, ALD-HfAlOx with various composition highlight the potential tremendous for high-κ capacitances stack for CMOS semiconductor devices. Figure 1

(Invited) Study of HfO2-Based High-k Gate Insulators for GaN Power Device

Gallium nitride (GaN) high-electron-mobility transistor and vertical GaN devices with metal-oxide-semiconductor (MOS) structure have been widely investigated to suppress leakage currents for applications associated with high power and high frequency [1-3]. Metal oxide (High-k) with a high dielectric constant (k)-value have been employed instead of SiO2 from the viewpoint of increasing the physical thickness [4, 5]. HfO2-based materials including HfO2, HfSiOx and HfAlOx are the candidate materials because of its relatively large bandgap (~6.5 eV) and a high k value (> 13) [6-9]. These films for transistor applications are typically fabricated using atomic layer deposition (ALD) because ALD is compatible with semiconductor fabrication process and has a big advantage of conformal film fabrication on three-dimensional structure. In this study, we report comparison of characteristics among HfO2, HfSiOx and HfAlOx gate insulators of GaN MOS capacitors.Pt-gated n-GaN MOS capacitors with HfO2, HfSiOx and HfAlOx gate insulators were prepared as follows. A single HfO2 layer, (HfO2)m/(SiO2)n and (HfO2)m/(Al2O3)n nanolaminate layers were deposited on n+-GaN/n-GaN epilayer by plasma-enhanced ALD at 300°C. The m/n ratio was kept at 2/1. Next, PDA was carried out at 800°C in N2. HfSiOx and HfAlOx films were formed from (HfO2)m/(SiO2)n and (HfO2)m/(Al2O3)n nanolaminate after PDA, respectively. The thicknesses of these films were ~24 nm. Finally, Pt gate electrode was deposited on gate insulator and Pt/Ti ohmic contact was deposited on the n-GaN substrate.The HfSiOx and HfAlOx films had an amorhous structure while HfO2 film had a polycrystalline structure after PDA at 800°C using transmission electron microscopy and electron diffraction pattern analysis. The HfO2 capacitor exhibited a small breakdown electric field (EBD) value of 2.6 MV/cm, defined at a current density of 1.0 × 101 A/cm2, because leakage currents easily flow along grain boundaries of the polycrystalline structure. On the other hand, the EBD values of the HfSiOx and HfAlOx capacitors increased up to 8.5 MV/cm. From capacitance-voltage measurements, the estimated k values of the HfO2, HfSiOx and HfAlOx films were 17.7, 17.2 and 13.5, respectively, as expected. The flatband voltage hysteresis (620 mV) of the HfO2 capacitor was much larger than those (~10 mV) of the HfSiOx and HfAlOx capacitors, indicating that the number of charge trapping/detrapping sites and frequency responsive defects was significantly decreased in the HfSiOx and HfAlOx capacitors. No frequency dispersion of the HfSiOx and HfAlOx capacitors were observed while the HfO2 capacitor showed a large frequency dispersion. The interface state density (Dit) values at Ec - E = 0.25 eV which estimated using the conductance method of the HfSiOx (~1 × 1011cm-2V-1) and HfAlOx (4.4 × 1011cm-2V-1) capacitors were one magnitude lower than that of the HfO2 capacitor (3.1 × 1012cm-2V-1), indicating that the electrical defects at n-GaN/HfSiOx and n-GaN/HfAlOx interfaces were much less than that of the n-GaN/HfO2 interface.We conclude that the HfSiOx and HfAlOx were the attractive materials as gate insulator because of amorphous structure, higher k, relatively low Dit, and high Ebd.This work was supported by JSPS KAKENHI (Grant No. JP20H02189). A part of this work was also supported by the MEXT “Program for research and development of next-generation semiconductor to realize energy-saving society.” Program Grant Number JPJ005357. The authors wish to thank Dr. A. Ohi, Dr. N. Ikeda, and the members of the nanofabrication group of the National Institute for Materials Science for their support during this study.[1] M. Kodama et al., Appl. Phys. Express 1, 021104 (2008).[2] Z. Yatabe et al., J. Phys. D: Appl. Phys. 49, 393001 (2016).[3] T. Oka et al., Appl. Phys. Express 8, 054101 (2015).[4] T. Hashizume et al., Appl. Phys. Express 11, 124102 (2018).[5] T. Kikuta et al, J. Vac. Sci. Technol. A 35, 01B122 (2017).[6] T. Nabatame et al., Appl. Phys. Express 12, 011009 (2019).[7] E. Maeda et al., Microelectron. Eng. 216, 111036 (2019).[8] R. Ochi et al., AIP Advances 10, 065215 (2020).[9] S. Miyazaki et al., ECS Trans. 92, 11(2019).

Memory characteristics and tunneling mechanism of Pt nano-crystals embedded in HfAlOx films for nonvolatile flash memory devices

A non-volatile flash memory device based on metal oxide semiconductor (MOS) capacitor structure has been fabricated using platinum nano-crystals(Pt–NCs) as storage units embedded in HfAlOx high-k tunneling layers. Its memory characteristics and tunneling mechanism are characterized by capacitance–voltage(C–V) and flat-band voltage-time(ΔVFB-T) measurements. A 6.5V flat-band voltage (memory window) corresponding to the stored charge density of 2.29 × 1013cm−2 and about 88% stored electron reserved after apply ±8V program or erase voltage for 105s at high frequency of 1 MHz was demonstrated. Investigation of leakage current–voltage(J–V) indicated that defects-enhanced Pool-Frenkel tunneling plays an important role in the tunneling mechanism for the storage charges. Hence, the Pt–NCs and HfAlOx based MOS structure has a promising application in non-volatile flash memory devices.

Memory characteristics and tunneling mechanism of Ag nanocrystal embedded HfAlOx films on Si83Ge17/Si substrate

A nano-floating gate memory capacitor consisting of a stack of 3nm-thick HfAlOx tunneling layer, self-organized Ag nanocrystals (NCs), and a 6nm-thick HfAlOx control layer, has been fabricated on compressively strained p-type Si83Ge17/Si(100) substrates by radio-frequency magnetron sputtering. The Ag-NCs with a size of 5–8nm and a density of 5.7×1012/cm2 are well dispersed in the amorphous HfAlOx matrix. Counterclockwise hysteresis capacitance–voltage curve with a memory window of ~2V, corresponding to a charge storage density of about 1.3×1013electrons/cm2, is observed in this memory capacitor. The accumulation capacitance of this memory capacitor has no obvious decrease during electrical stressing process within a period of 104s, but the memory window gradually becomes narrower, and only 54% stored charges are retained in the Ag-NCs after 105s stressing. Defect-enhanced Poole–Frenkel tunneling is found to be responsible for the degradation of memory properties.

Influences of different oxidants on the characteristics of HfAlOx films deposited by atomic layer deposition

A comparative study of two kinds of oxidants (H2O and O3) with the combinations of two metal precursors [trimethylaluminum (TMA) and tetrakis(ethylmethylamino) hafnium (TEMAH)] for atomic layer deposition (ALD) hafnium aluminum oxide (HfAlOx) films is carried out. The effects of different oxidants on the physical properties and electrical characteristics of HfAlOx films are studied. The preliminary testing results indicate that the impurity level of HfAlOx films grown with both H2O and O3 used as oxidants can be well controlled, which has significant effects on the dielectric constant, valence band, electrical properties, and stability of HfAlOx film. Additional thermal annealing effects on the properties of HfAlOx films grown with different oxidants are also investigated.

Starting layer dependence of the atomic-layer-deposited HfAlOx films on GaAs

This study reports the effects of the starting layer on the interface and electrical properties of HfAlOx films on GaAs, when the atomic layer deposition (ALD) was performed with tetrakis(ethylmethylamino)hafnium and trimethylaluminum precursors as well as an H2O oxidant. By using the Al2O3-first process, more decrease in the number of Ga–O bonds near the interface region was achieved than by using the HfO2-first process, which resulted in a reduction of the interface state density and the frequency dispersion. Furthermore, the Al2O3-first process decreased the border trap density, as well as the voltage stress-induced electron trapping, as estimated from the hysteresis characteristics of the capacitance–voltage and the leakage current measurement under a ramped-voltage stress condition. Based on these results, we experimentally demonstrate the importance of the cycling sequence during the ALD-HfAlOx process on GaAs.

Amorphous Oxide Semiconductor Memory Using High-k Charge Trap Layer

Amorphous oxide semiconductor memory devices with HfInZnOx as the channel layer and high k dielectric stacks as the charge storage medium were fabricated. HfO2 and Al2O3 and HfAlOx films were examined as the charge trap layers. The drain current - gate voltage transfer curves of the fabricated charge trap memories show a large hysteresis due to the electron trapping and de-trapping at the interfaces between the high-k charge storage layer and the SiO2. The device structure and operational scheme for the amorphous oxide semiconductor charge trap memories were suggested based on these properties.

High Density MIM Capacitors Using HfAlOx

Amorphous HfAlOx films deposited by RF magnetron co-sputtering of HfO2 and Al2O3 targets at a substrate temperature of 50{degree sign}C have been used for the fabrication of Metal-insulator-metal capacitor structures. Effects of different metal electrodes (Au and Al) are investigated and the electrical properties are compared. The electrical properties of the MIM capacitors are investigated by using current-voltage (I-V) and capacitance-voltage (C-V) characteristics in the 1 kHz - 1 MHz frequency range. Using high work function (5.1 eV) top electrode Au, low leakage current and low energy dissipation is obtained in high density (~5fF/μm2) HfAlOx MIM capacitors with improved voltage coefficients of capacitance values. The leakage current density is approximately one order of magnitude higher than devices with Al electrodes. In addition, effects of constant voltage stressing and temperature on the leakage current and voltage linearity are also investigated. Stress voltage frequency dependence of quadratic voltage coefficient of capacitance is discussed.

Atomic layer deposition of HfO2, Al2O3, and HfAlOx using O3 and metal(diethylamino) precursors

Tetrakis-diethylamino hafnium (TDEAH), tris-diethylamino aluminum (TDEAA), and ozone were used for the atomic layer deposition (ALD) of HfO2, Al2O3, and HfAlOx films. The ALD rates were measured to be 1.1 Å/cycle for HfO2 and 1.3 Å/cycle for Al2O3. The ALD temperature windows were found to be between 200 and 325 °C for TDEAA, and between 200 and 275 °C for TDEAH. The overlap of these ALD windows between 200 and 275 °C is critical for ALD of the composite film, HfAlOx. In addition to the overlapping ALD temperature windows, the two metal precursors have similar thermal characteristics, as shown by TGA and differential scanning calorimetry. As-deposited films and films postannealed at 600 and 800 °C films were analyzed using Fourier transformed infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction (XRD) techniques. FTIR spectra revealed interfacial oxide growth during deposition of both HfO2 and Al2O3 whose thickness increased with annealing temperature. The FTIR data also indicated hydroxyl and nitrate groups in the films; these species were removed after annealing in Ar at a temperature of ⩾600 °C. Both FTIR and XRD results indicated the crystallization of pure HfO2 after annealing at temperatures as low as 600 °C. On the other hand, pure Al2O3 remained amorphous after annealing at temperatures up to 800 °C. XRD data of the composite HfAlOx film show that films deposited by alternating five cycles of HfO2 and one cycle of Al2O3 remained amorphous after annealing at 600 °C. Rutherford backscattering analysis of HfAlOx deposited with a varied number of alternating HfO2 and Al2O3 cycles demonstrated a strong correlation between the cyclic dosage of TDEAA and TDEAH and the film composition.

Temperature-dependent structural stability and optical properties of ultrathin Hf–Al–O films grown by facing-target reactive sputtering

The structural stability and optical properties of ultrathin HfAlOx films grown by facing-target reactive sputtering, depending on the postannealing temperature, have been determined via x-ray photoelectron spectroscopy and spectroscopic ellipsometry (SE). By virtue of the chemical shifts of Hf4f, Al2p, and Si2p core-level spectra, it has been found that the structural stability of HfAlOx∕Si system sustains up to 800°C. However, the breaking of the Hf–Al–O bond and the phase separation take place drastically at the annealing temperature of 900°C. In particular, the information of an interfacial Si–O–Si bond as the dominant reaction during the postannealing treatment has been observed, confirmed by Fourier transform infrared spectroscopy. Analysis by SE, based on the Tauc-Lorentz model, has indicated that increase in the refractive index and reduction in thickness has been observed as a function of annealing temperature, originating from the annealing-induced higher packing density. The change of the complex dielectric functions and bandgap Eg correlated with the annealing temperature are discussed in detail.

Room-temperature weak ferromagnetism of amorphous HfAlOx thin films deposited by pulsed laser deposition

The room-temperature weak ferromagnetism of amorphous HfAlOx thin films deposited by pulsed laser deposition on various substrates in oxygen-defective ambient is demonstrated. The magnetization is independent of film thickness, but depends on substrates and deposition temperatures. A magnetic moment of ∼0.26μB per HfAlOx f.u. is recorded for HfAlOx films deposited under optimized conditions [deposited at 600°C on (001) sapphire in high vacuum]. It is argued that interfacial defects are one of the possible sources of the weak ferromagnetism.

HfAlO and HfSiO Based Dielectrics for Future DRAM Applications

The temperature stability of HfSiOx and HfAlOx films was investigated in the annealing temperature range of 700-1000 {degree sign}C. The main focus of this work is to compare mixed and laminate films in terms of crystallographic phase, capacitance enhancement and leakage current degradation. Single HfO2 layers in the laminate film fully crystallize into monoclinic phase. Mixed films remain nanocrystalline up to a temperature of 800- 900{degree sign}C and fully crystallize in to the cubic phase for higher temperatures. The different crystallization temperatures and crystallographic phases are reflected in the annealing temperature dependence of capacitance and leakage current. Mixed films show a significant increase of capacitance and leakage current after crystallization. For laminates the capacitance remains constant for annealing temperature up to 1000{degree sign}C and the leakage current shows a slight degradation only. Using temperature dependent leakage current measurements the formation of trap states at average levels of 0.2-0.3 eV below conduction band edge of the dielectrics is shown.

Phase separation and interfacial reaction of high-k HfAlOx films prepared by pulsed-laser deposition in oxygen-deficient ambient

Ultrathin high-k HfAlOx films on silicon wafers are prepared by pulsed-laser deposition in oxygen-defective ambient. The precipitation of HfOx clusters from HfAlOx matrix is revealed by x-ray photoelectron spectroscopy. It is argued that the HfOx clusters react with silicon to form Hf-silicide interfacial layer at a temperature as low as 600°C, which evolves into Hf-silicate in the subsequent postannealing at 700°C for 30s in oxygen ambient. The optimized fabrication conditions developed to avoid the formation of interfacial layer is employed to prepare high-quality HfAlOx films of excellent electrical characteristics, such as a dielectric constant of 17.7, a small equivalent oxide thickness of 0.66nm, a flatband voltage of 0.45V, and a low leakage current density of 53.8mA∕cm2 at 1V gate voltage.

「ゲート絶縁膜／Ｓｉ界面の評価」低速陽電子ビームを用いたｈｉｇｈ‐κ膜の空隙評価

Positron annihilation is an established technique for investigating defects in solids. Using this technique, thin Hf0.3Al0.7Ox films fabricated by an atomic-layer-deposition technique were characterized. It was found that positrons in the HfAlOx films annihilate from the trapped state by open spaces that exist intrinsically in their amorphous structure. The line-shape parameter S and the positron lifetime corresponding to the HfAlOx films decreased with increasing oxygen content in the annealing atmosphere. This fact was attributed to the shrinkage of the open spaces due to the change in the matrix structure of amorphous HfAlOx. A clear correlation between the mean size of the open spaces in HfAlOx and the suppression of transient leak current was observed.

Characterization of Hf0.3Al0.7Ox Fabricated by Atomic-Layer-Deposition Technique Using Monoenergetic Positron Beams

Thin Hf0.3Al0.7Ox films fabricated by an atomic-layer-deposition technique were characterized using monoenergetic positron beams. From the measurements of the Doppler broadening spectra of annihilation radiation and the lifetime spectra of positrons for 3- and 7-nm-thick HfAlOx films deposited on SiON(0.9 nm)/Si, it was found that positrons in the HfAlOx films annihilate from the trapped state by open spaces that exist intrinsically in their amorphous structure. The line-shape parameter S and the positron lifetime corresponding to the HfAlOx films decreased with increasing oxygen content (0.004–1%) in the annealing atmosphere. This fact was attributed to the shrinkage of the open spaces due to the change in the matrix structure of amorphous HfAlOx. The increase in oxygen content was also found to decrease the transient current of n+-polycrystalline-Si/HfAlOx/SiON/Si. The decrease in the mean size of the open spaces and the resultant hardening of the matrix structure are considered to suppress the dielectric polarization of HfAlOx.

Impact of Rapid Thermal O2 Anneal on Dielectric Stack Structures of Hafnium Aluminate and Silicon Dioxide Formed on Si(100)

3–7-nm-thick HfAlOx (Hf/Hf+Al=29%) films were deposited on thermally grown ∼0.9-nm-thick SiO2/Si(100) by atomic layer chemical vapor deposition (ALCVD) at 350°C. After postdeposition anneal (PDA) in 0.2% O2 ambience at 1050°C for 1 s, we evaluated the compositional profile of HfAlOx/SiO2 dielectric stack structures by X-ray photoelectron spectroscopy (XPS) in combination with oxide thinning in a dilute HF solution and observed Si diffusion and incorporation into the HfAlOx network by a few at.% through the pregrown SiO2 layer. The energy band alignment in the dielectric stack structure on Si(100) after the PDA was determined by the energy band gap of the ALCVD HfAlOx films and the valence band offset at the interfaces between HfAlOx and Si(100), which were measured by analyzing the energy loss spectra of O1s photoelectrons and the valence band spectra of the dielectric stack structures, respectively. Also, by the total photoelectron yield spectroscopy (PYS) of the dielectric stack structures, filled defect states as many as 1–3×1017 cm-3 eV-1 were estimated in the HfAlOx layer in the energy region of 0.4 to 0.8 eV from the energy position corresponding to the Si valence band top.

Physical and Electrical Properties of HfAlOx Films Prepared by Atomic Layer Deposition Using NH3/Ar Plasma

The effects of introducing NH3/Ar plasma pulses during the atomic layer deposition (ALD) of HfAlOx films and during the in situ nitridation of an interfacial layer (IL) on the properties of HfAlOx-FETs were investigated. The interaction between HfAlOx and IL and the dopant penetration were suppressed by the introduction of a NH3/Ar plasma pulse during ALD, because it increases the nitrogen content at the interface of HfAlOx/IL. Moreover, the nitrogen content in the IL could be controlled by adjusting the number of NH3/Ar plasma pulses used for in situ nitridation. No threshold voltage shift due to the nitridation of the Si substrate was observed, indicating that the nitrogen profile was well controlled when using a NH3/Ar plasma pulse for in-situ nitridation. It was found that in-situ nitridation with 5 cycles of NH3/Ar plasma pulses was optimum for reducing C–V hysteresis and improving carrier mobility.

Application of XPS time-dependent measurement to the analysis of charge trapping phenomena in HfAlOx films

We found that the Si 2p photoelectron peak binding energy of a Si substrate covered with a 3nm thick HfAlOx film increased to a saturation value during X-ray irradiation. This shift towards a higher binding energy indicates an increase in the amount of positive charges in the film, and the density of hole traps is estimated to be significantly higher than that in a SiO2 film. Before X-ray irradiation, the interface Fermi level coincided with the valence-band maximum of the Si substrate, which indicates that the Si surface band bends upward. This means that there were negative charges already in the film before X-ray irradiation. Investigating the effects of the interlayer beneath the HfAlOx film on these trapped charges reveals that the observed hole trapping and initial electron trapping are intrinsic to the HfAlOx film.

Roles of nitrogen incorporation in HfAlOx(N) gate dielectrics for suppression of boron penetration

We have investigated impacts of nitrogen incorporation into HfAlOx films on the gate leakage currents and the flat band voltage VFB shifts. Also, a role of O–Hf–N bonding states in HfAlOx(N) in suppression of boron penetration is discussed. The nitrogen concentration CN in HfAlOx(N) was controlled by changing the NH3 annealing temperature at the step of the layer-by-layer deposition and annealing process as well as the O2 annealing temperature of post-deposition annealing. The CN over 10 at. % in HfAlOx(N) films effectively suppressed boron penetration as revealed by very slow diffusion in Hf3N4 films and the formation of boron-nitrogen remote complex in HfOx(N), preserved the amorphous structure and reduced the VFB shift compared to the case of HfAlOx without nitrogen incorporation, while CN exceeding 13 at. % led to a significant increase of the gate leakage current. This suggests the excess O–Hf–N bond formation enhanced the leakage current.

Effect of Hf Sources, Oxidizing Agents, and NH3/Ar Plasma on the Properties of HfAlOx Films Prepared by Atomic Layer Deposition

The effect of Hf sources, oxidizing agents, and NH3/Ar plasma on the properties of HfO2 and HfAlOx films prepared by atomic layer deposition (ALD) have been investigated. The use of tetrakis(ethylmethylamino)hafnium: Hf(NEtMe)4 (TEMAHf) as a source of hafnium has avoided the problems associated with hafnium tetrachloride (HfCl4), such as poor throughput and the generation of a large number of particles. This source has also been effective in reducing the level of residual chlorine (Cl). However, the level of residual carbon (C) increased. Hydrolysis was found to be more effective than oxidation using O3 in removing ligands from the TEMAHf and/or trimethylaluminum (TMA); the film formed using O3 contained a higher level of C than that using H2O, resulting in the deterioration in field-effect transistor (FET) properties, such as the subthreshold swings and the mobilities. The total levels of residual impurities (C+Cl) were successfully reduced through the combination of TEMAHf and NH3/Ar plasma, leading to improvements in FET properties, particularly in the reduction of the gate leakage current.