Chemical vapor deposition of ZrxTi1−xO2 and HfxTi1−xO2 thin films using the composite anhydrous nitrate precursors

Zr-Ti and Hf-Ti composite nitrates were successfully developed as single-source precursors for the chemical vapor deposition (CVD) of Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ thin films. The Zr-Ti nitrate can be assumed as a solid solution of the individual Zr and Ti nitrates, and the Zr/Ti molar ratio in the deposited Zr_xTi_1−xO₂ films is consistent with that in the precursor. The Hf-Ti nitrate appears to be a mixture of the Hf and Ti nitrates and the composition of the deposited Hf_xTi_1−xO₂ films depends remarkably on the heating time of precursor. Both Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ films exhibit trade-off properties between band gap and dielectric constant. The obtained results suggest that Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ films are promising candidates for gate dielectric application to improve the scalability and reduce the leakage current of the future complementary metal-oxide-semiconductor (CMOS) devices.     

Microstructure and dielectric properties of La2O3 doped amorphous SiO2 films as gate dielectric material

As a potential gate dielectric material, the La₂O₃ doped SiO₂ (LSO, the mole ratio is about 1:5) films were fabricated on n-Si (0 0 1) substrates by using pulsed laser deposition technique. By virtue of several measurements, the microstructure and electrical properties of the LSO films were characterized. The LSO films keep the amorphous state up to a high annealing temperature of 800 °C. From HRTEM and XPS results, these La atoms of the LSO films do not react with silicon substrate to form any La-compound at interfacial layer. However, these O atoms of the LSO films diffuse from the film toward the silicon substrate so as to form a SiO₂ interfacial layer. The thickness of SiO₂ layer is only about two atomic layers. A possible explanation for interfacial reaction has been proposed. The scanning electron microscope image shows the surface of the amorphous LSO film very flat. The LSO film shows a dielectric constant of 12.8 at 1 MHz. For the LSO film with thickness of 3 nm, a small equivalent oxide thickness of 1.2 nm is obtained. The leakage current density of the LSO film is 1.54 × 10⁻⁴ A/cm² at a gate bias voltage of 1 V.     

Study of HfSiO film prepared by electron beam evaporation for high-k gate dielectric applications

The purpose of this paper is to report some experimental results with HfSiO films formed on silicon substrates by electron beam evaporation (EB-PVD) and annealed at different temperatures. The images of atomic force microscope (AFM) indicated that HfSiO film annealed at 900 °C was still amorphous, with a surface roughness of 0.173 nm. X-ray photoelectron spectroscopy (XPS) analysis revealed that the chemical composition of the film was (HfO₂)₃(SiO₂) and Hf-Si-O bonds existed in the annealed film. Electrical measurements showed that the equivalent oxide thickness (EOT) was 4 nm, the dielectric constant was around 6, the breakdown voltage was 10 MV/cm, the fixed charge density was −1.2 × 10¹² cm⁻², and the leakage current was 0.4 μA/cm² at the gate bias of 2 V for 6 nm HfSiO film. The annealing after deposition effectively reduced trapping density and the leakage current, and eliminated hysteresis in the C-V curves. Annealing also induced SiO₂ growth at the interface.     

Influence of initial surface reconstruction on the interface structure of HfO2/GaAs

We show that the bonding structures and electrical properties of the HfO₂/GaAs interface can be controlled by a choice of the reconstruction on the initial GaAs surface. Electron-beam evaporation of HfO₂ onto the c(4 × 4) surface yielded As-O bonds at the interface, while Ga-O bonds were dominant at the interfaces formed on the (2 × 4) and (4 × 6) surfaces. Influences of the initial surface reconstruction on the interface structure persisted even after annealing at 673 K. Electrical characterization of Ir/HfO₂/GaAs capacitors indicated that the interfacial As-O bonds cause weak Fermi level pinning. It was also suggested that the interfaces dominated by the Ga-O bonds have trapping states in the upper half of the GaAs bandgap.     

Preparation of fluorocarbon thin film deposited by soft X-ray ablation and its electrical characteristics and thermal stability

Fluorocarbon films were deposited by soft X-ray ablation of polytetrafluoroethylene (PTFE) and characterized as low-dielectric-constant interlayer dielectrics. Very rapid deposition of such films at approximately 1500 nm/min could be achieved at room temperature. Fourier-transform infrared spectroscopy (FT-IR) measurement results suggest that the films deposited are primarily formed as one-dimensional chains of (-CF₂-)_n which are partially cross-linked. The cross-link density increases with increasing deposition temperature, which improves the thermal stability. However, the dielectric constant of the films increased abruptly above 300 °C. The dielectric constant and leakage current at 1.0 MV/cm of the film deposited at room temperature were approximately 2.1 and 2.0×10⁻⁹ A/cm², respectively.     

Characteristics of sandwich-structured Al2O3/HfO2/Al2O3 gate dielectric films on ultra-thin silicon-on-insulator substrates

Sandwich-structure Al₂O₃/HfO₂/Al₂O₃ gate dielectric films were grown on ultra-thin silicon-on-insulator (SOI) substrates by vacuum electron beam evaporation (EB-PVD) method. AFM and TEM observations showed that the films remained amorphous even after post-annealing treatment at 950 °C with smooth surface and clean silicon interface. EDX- and XPS-analysis results revealed no silicate or silicide at the silicon interface. The equivalent oxide thickness was 3 nm and the dielectric constant was around 7.2, as determined by electrical measurements. A fixed charge density of 3 × 10¹⁰ cm⁻² and a leakage current of 5 × 10⁻⁷A/cm² at 2 V gate bias were achieved for Au/gate stack /Si/SiO₂/Si/Au MIS capacitors. Post-annealing treatment was found to effectively reduce trap density, but increase in annealing temperature did not made any significant difference in the electrical performance.     

Gadolinium oxide high-k gate dielectrics prepared by anodic oxidation

The growth and properties of gadolinium oxide (Gd₂O₃) films prepared by anodic oxidation were investigated. Uniform Gd₂O₃ thin film with good oxide quality was obtained. The X-ray diffraction (XRD) pattern of the Gd₂O₃ films showed that they had a poly-crystalline structure. The dielectric constants of Gd₂O₃ films oxidized at 30 and 60 V are 9.4 and 12.2, respectively. The equivalent oxide thickness (EOT) of the Gd₂O₃ stacked oxide is in the range of 5.8-9.4 nm. The MOS capacitor with Gd₂O₃ exhibits interesting electrical properties. Longer oxidation time reduced the leakage current density for 30 V anodic oxidation but increased the leakage current density for 60 V anodic oxidation. This work reveals that Gd₂O₃ could also be an alternative dielectric for Si substrate and therefore, might pave the way to fabricate CMOS devices in the future.     

Effect of Ti doping on Ta2O5 stacks with Ru and Al gates

The Ti-doped Ta₂O₅ thin films (<10 nm) obtained by rf sputtering are studied with respect to their composition, dielectric and electrical properties. The incorporation of Ti is performed by two methods - a surface doping, where a thin Ti layer is deposited on the top of Ta₂O₅ and a bulk doping where the Ti layer is sandwiched between two layers of Ta₂O₅. The effect of the process parameters (the method and level of doping) on the elemental distribution in-depth of the films is investigated by the time of flight secondary ion mass spectroscopy (ToF-SIMS). The Ti and Ta₂O₅ are intermixed throughout the whole thickness but the layers are very inhomogeneous. Two sub-layers exist in all the samples — a near interfacial region which is a mixture of Ta-, Ti-, Si-oxides as well as TaSiO, and an upper Ti-doped Ta₂O₅ sub-layer. For both methods of doping, Ti tends to pile-up at the Si interface. The electrical characterisation is performed on capacitors with Al- and Ru-gate electrodes. The two types of MIS structures exhibit distinctly different electrical behavior: the Ru gate provides higher dielectric permittivity while the stacks with Al electrode are better in terms of leakage currents. The specific metal-dielectric reactions and metal-induced electrically active defects for each metal electrode/high-k dielectric stack define its particular electrical behavior. It is demonstrated that the Ti doping of Ta₂O₅ is a way of remarkable improvement of leakage characteristics (the current reduction with more than four orders of magnitude as compared with undoped Ta₂O₅) of Ru-gated capacitors which originates from Ti induced suppression of the oxygen vacancy related defects.     

Influence of thermal treatment of low dielectric constant SiOC(H) films using MTES/O2 deposited by PECVD

Low dielectric constant SiOC(H) films are deposited on p-type Si(100) substrates by plasma enhanced chemical vapor deposition (PECVD) using methyltriethoxysilane (MTES, C₇H₁₈O₃Si) and oxygen gas as precursors. The SiOC(H) films are deposited at room temperature, 100, 200, 300 and 400 °C and then annealed at 100, 200, 300 and 400 °C temperatures for 30 min in vacuum. The influence of deposition temperature and annealing on SiOC(H) films are investigated. Film thickness and refractive index are measured by field emission scanning electron microscopy and ellipsometry, respectively. Chemical bonding characteristics of as-deposited and annealed films are investigated by Fourier transform infrared (FTIR) spectroscopy in the absorbance mode. As more carbon atoms are incorporated into the SiOC(H) films, both film density and refractive index are decreased due to nano pore structure of the film. In the SiOC(H) film, CH₃ group as an end group is introduced into OSiO network, thereby reducing the density to decrease the dielectric constant thereof. The dielectric constant of SiOC(H) film is evaluated by C-V measurements using metal-insulator-semiconductor (MIS), Al/SiOC(H)/p-Si structure and it is found to be as low as 2.2 for annealed samples deposited at 400 °C.     

Secondary ion species containing nitrogen atoms from plasma-enhanced chemical vapor deposited silicon oxide films on silicon

Secondary ion species from plasma-enhanced chemical vapor deposited (PECVD) SiO₂ films have been investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Comparative studies of PECVD SiO₂ films prepared using a mixture of SiH₄/N₂O reaction gas at 400 °C with thermally oxidized SiO₂ films grown at 900 °C were carried out in the mid-range mass spectra from 95 to 165 amu. Small amounts of ion species containing nitrogen atoms, including Si₂O₂N⁺, Si₃O₂N⁺and Si₃O₃N⁺, were detected in the SiO₂ bulk from the PECVD SiO₂ films. Furthermore, large amounts of Si₃O₂N⁺ and Si₂O₃N⁻ were found at the interface between silicon and the SiO₂ films. Depth analysis showed that the intensity peak shapes of these ion species containing nitrogen atoms at the interface were closely coincident with those of Si₃O₃⁺ corrected by subtracting the influence of the SiO₂ matrix. The variation in the spectra of these ion species clearly indicates that two types of structures of oxynitride exist for the PECVD SiO₂ films in the SiO₂ bulk films and at the interface. These are likely produced by the reaction of reactive gas with SiO₂ and silicon surfaces where dangling bonds of silicon may exist in the different form.     

Electrical characteristics of polycrystalline Si layers embedded into high-k Al2O3 gate layers

The electrical characteristics of polycrystalline Si (poly Si) layers embedded into high-k Al₂O₃ (alumina) gate layers are investigated in this work. The capacitance versus voltage (C-V) curves obtained from the metal-alumina-polysilicon-alumina-silicon (MASAS) capacitors exhibit significant threshold voltage shifts, and the width of their hysteresis window is dependent on the range of the voltage sweep. The counterclockwise hysteresis observed in the C-V curves indicates that electrons originating from the p-type Si substrate in the inversion condition are trapped in the floating gate layer consisting of the poly Si layer present between the top and bottom Al₂O₃ layers in the MASAS capacitor. Also, current versus voltage (I-V) measurements are performed to examine the electrical characteristics of the fabricated capacitors. The I-V measurements reveal that our MASAS capacitors show a very low leakage current density, compared to the previously reported results.     

Structural optimization of HfTiSiO high-k gate dielectrics by utilizing in-situ PVD-based fabrication method

We investigated the optimum structure for Ti-containing Hf-based high-k gate dielectrics to achieve EOT scaling below 1 nm. TiO₂/HfSiO/SiO₂ trilayer and HfTiSiO/SiO₂ bilayer structures were fabricated by a newly developed in-situ PVD-based method. We found that thermal diffusion of Ti atoms to SiO₂ underlayers degrades the EOT-J_g characteristics. Our results clearly demonstrated the impact of the trilayered structure with TiO₂ capping for improving EOT-J_g characteristics of the gate stack. We achieved an EOT scaling of 0.78 nm as well as reduced gate leakage of 7.2 × 10⁻² A/cm² for a TiO₂/HfSiO/SiO₂ trilayered high-k dielectric while maintaining the electrical properties at the bottom interface.     

Current conduction mechanism in Al/p-Si Schottky barrier diodes with native insulator layer at low temperatures

The forward bias current-voltage (I-V) characteristics of Al/p-Si (MS) Schottky diodes with native insulator layer were measured in the temperature range of 80-300 K. The obtained zero bias barrier height Φ_B0(I-V), ideality factor (n) and series resistance (R_s) determined by using thermionic emission (TE) mechanism show strong temperature dependence. There is a linear correlation between the Φ_B0(I-V) and n because of the inhomogeneties in the barrier heights (BHs). Calculated values from temperature dependent I-V data reveal an unusual behaviour such that the Φ_B0 decreases, as the n and R_s values are increasing with decreasing absolute temperature, and these changes are more pronounced especially at low temperatures. Such temperature dependence of BH is contradictory with the reported negative temperature coefficient of the barrier height. In order to explain this behaviour we have reported a modification in the expression reverse saturation current I_o including the n and the tunnelling factor (αΧ^1/2δ) estimated to be 15.5. Therefore, corrected effective barrier height Φ_bef.(I-V) versus temperature has a negative temperature coefficients (α = −2.66 × 10⁻⁴ eV/K) and it is in good agreement with negative temperature coefficients (α = −4.73 × 10⁻⁴ eV/K) of Si band gap. In addition, the temperature dependent energy distribution of interface states density N_ss profiles was obtained from the forward bias I-V measurements by taking into account the bias dependence of the Φ_e and n. The forward bias I-V characteristics confirm that the distribution of N_ss, R_s and interfacial insulator layer are important parameters that the current conduction mechanism of MS Schottky diodes.     

Field electron emission from HfNxOy thin films deposited by direct current sputtering

HfN_xO_y thin films were deposited on Si substrates by direct current sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM indicates that the film is composed of nanoparticles. AFM indicates that there are no sharp protrusions on the surface of the film. XRD pattern shows that the films are amorphous. The field electron emission properties of the film were also characterized. The turn-on electric field is about 14 V/μm at the current density of 10 μA/cm², and at the electric field of 24 V/μm, the current density is up to 1 mA/cm². The field electron emission mechanism of the HfN_xO_y thin film is also discussed.     

Morphological and luminescent characteristics of GaN dots deposited on AlN by alternate supply of TMG and NH3

GaN dots were deposited on AlN underlayers by alternate supply of trimethylgallium (TMG) and ammonia (NH₃) in an inductively heated quartz reactor operated at atmospheric pressure. Various growth parameters including deposition temperature, TMG admittance and pulse time between TMG and NH₃ exposures were proposed to investigate the influence of growth parameters on the size distribution of GaN dots. It appears that GaN dots with uniform size distribution can be achieved under certain growth conditions. Based on the study of atomic force microscopy (AFM), high deposition temperature was found to be in favor of forming large GaN dots with small dot density. Decrement of TMG flow rate or reduction in the number of growth cycle tends to enable the formation of GaN dots with small dot sizes. The results of room temperature (RT) cathodoluminescence (CL) measurements of the GaN dots exhibit an emission peak at 3.735 eV. A remarkable blue shift of GaN dot emission was observed by reduced temperature photoluminescence (PL) measurements.     

Theoretical Analysis of Current Crowding Effect in Metal/AIGaN/GaN Schottky Diodes and Its Reduction by Using Polysilicon in Anode

There exists a current crowding effect in the anode of AIGaN/GaN heterojunction Schottky diodes, causing local overheating when working at high power density, and undermining their performance. The seriousness of this effect is illustrated by theoretical analysis. A method of reducing this effect is proposed by depositing a polysilicon layer on the Schottky barrier metal. The effectiveness of this method is provided through computer simulation. Power consumption of the polysilicon layer is also calculated and compared to that of the Schottky junction to ensure the applicability of this method.     

Ferroelectric polarization-controlled two-dimensional electron gas in ferroelectric/AlGaN/GaN heterostructure

The control effect of the ferroelectric polarization on the two-dimensional electron gas (2DEG) in a ferroelectric/AlGaN/GaN metal–ferroelectric–semiconductor (MFS) structure is theoretically analyzed by a self-consistent approach. With incorporating the hysteresis nature of the ferroelectric into calculation, the nature of the control effect is disclosed, where the 2DEG density is depleted/restored after poling/depoling operation on the MFS structure. The orientation of the ferroelectric polarization is clarified to be parallel to that of the AlGaN barrier, which, based on an electrostatics analysis, is attributed to the pinning effect of the underlying polarization. Reducing the thickness of the AlGaN barrier from 25 nm to 20 nm leads to an improved control modulation of the 2DEG density from 36.7% to 54.1%.     

Microstructure and interfacial properties of HfO2-Al2O3 nanolaminate films

High-k HfO₂-Al₂O₃ composite gate dielectric thin films on Si(1 0 0) have been deposited by means of magnetron sputtering. The microstructure and interfacial characteristics of the HfO₂-Al₂O₃ films have been investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry (SE). Analysis by XRD has confirmed that an amorphous structure of the HfO₂-Al₂O₃ composite films is maintained up to an annealing temperature of 800 °C, which is much higher than that of pure HfO₂ thin films. FTIR characterization indicates that the growth of the interfacial SiO₂ layer is effectively suppressed when the annealing temperature is as low as 800 °C, which is also confirmed by spectroscopy ellipsometry measurement. These results clearly show that the crystallization temperature of the nanolaminate HfO₂-Al₂O₃ composite films has been increased compared to pure HfO₂ films. Al₂O₃ as a passivation barrier for HfO₂ high-k dielectrics prevents oxygen diffusion and the interfacial layer growth effectively.     

Growth and Characterization of A1GaN/A1N/GaN HEMT Structures with a Compositionally Step-Graded A1GaN Barrier Layer

A new A1GaN/A1N/GaN high electron mobility transistor （HEMT） structure using a compositionally step-graded A1GaN barrier layer is grown on sapphire by metalorganic chemical vapour deposition （MOCVD）. The structure demonstrates significant enhancement of two-dimensional electron gas （2DEG） mobility and smooth surface morphology compared with the conventional HEMT structure with high A1 composition A1GaN barrier. The high 2DEG mobility of 1806 cm2/Vs at room temperature and low rms surface roughness of 0.220 nm for a scan area of 5μm×5 μm are attributed to the improvement of interracial and crystal quality by employing the stepgraded barrier to accommodate the large lattice mismatch stress. The 2DEG sheet density is independent of the measurement temperature, showing the excellent 2DEG confinement of the step-graded structure. A low average sheet resistance of 314.5Ω/square, with a good resistance uniformity of 0.68%, is also obtained across the 50 mm epilayer wafer. HEMT devices are successfully fabricated using this material structure, which exhibits a maximum extrinsic transconductance of 218 mS/ram and a maximum drain current density of 800 mA/mm.     

Photoluminescence characteristics from amorphous SiC thin films with various structures deposited at low temperature

Hydrogenated amorphous SiC thin films deposited at low substrate temperature (100 °C) show the different bonding configurations and microstructures which depend on the carbon concentrations in the films controlled by the gas ratio R of methane to silane during the deposition. Photoluminescence characteristics are investigated for these samples with different structures. A strong luminescence in red light region can be observed for samples deposited with low gas ratio R which is significantly reduced its intensity with increasing the carbon concentrations in the films. On the other hand, the luminescence bands located at blue-green light region are detected under UV light excitation for samples deposited with high gas ratio R, which can be associated with the existence of amorphous SiC clusters in the films.