Epitaxial Ir Research Articles

Analysis of Surface Microstructures Formed on Ir Substrate under Different Bias Conditions by Microwave Plasma Chemical Vapor Deposition

To understand the process window of bias‐enhanced nucleation of diamond, it is important to study surface modifications of Ir substrates that occur under different conditions. Herein, various microstructures formed on Ir substrates are reported. The first microstructure contains rectangular particles with a size of several hundred nanometers obtained at a relatively high pressure of 12.5 kPa and a high temperature of 880 °C; these particles consist of epitaxially grown Ir3Si species. The second microstructure includes particles oriented along the [110] direction; these particles, however, consist of etched Ir pillars with a height of 400 nm when the substrate temperature increases to 1110 °C. Both microstructures form because of Ir sputtering due to the bombardment of energetic charged ions and the transport of Ir surface atoms. This study demonstrates two types of surface modifications under different bias‐enhanced nucleation conditions and provides a method for preparing an epitaxial Ir–Si compound that can be applied in various silicon‐based electronic devices.

Engineering the spin conversion in graphene monolayer epitaxial structures

Spin Hall and Rashba–Edelstein effects, which are spin-to-charge conversion phenomena due to spin–orbit coupling (SOC), are attracting increasing interest as pathways to manage rapidly and at low consumption cost the storage and processing of a large amount of data in spintronic devices as well as more efficient energy harvesting by spin-caloritronics devices. Materials with large SOC, such as heavy metals (HMs), are traditionally employed to get large spin-to-charge conversion. More recently, the use of graphene (gr) in proximity with large SOC layers has been proposed as an efficient and tunable spin transport channel. Here, we explore the role of a graphene monolayer between Co and a HM and its interfacial spin transport properties by means of thermo-spin measurements. The gr/HM (Pt and Ta) stacks have been prepared on epitaxial Ir(111)/Co(111) structures grown on sapphire crystals, in which the spin detector (i.e., top HM) and the spin injector (i.e., Co) are all grown in situ under controlled conditions and present clean and sharp interfaces. We find that a gr monolayer retains the spin current injected into the HM from the bottom Co layer. This has been observed by detecting a net reduction in the sum of the spin Seebeck and interfacial contributions due to the presence of gr and independent from the spin Hall angle sign of the HM used.

Epitaxial growth of atomic-scale smooth Ir electrode films on MgO buffered Si(1 0 0) substrates by PLD

Atomic-scale smooth Ir films have been deposited on MgO buffered Si(1 0 0) by the pulsed laser deposition (PLD) technique. The whole growth process of the bilayer films was in situ monitored by using reflection high-energy electron diffraction (RHEED) apparatus. The Ir/MgO/Si(1 0 0) heterostructures were also characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The RHEED observations show that the growth mode of the heterostructures is 2D layer-by-layer growth. The crystalline quality of epitaxial Ir films is comparable to that of single crystals. The achievement of the single-crystal-like epitaxial Ir films with atomic-scale smooth surfaces (Ra=0.47 nm) is ascribed to the improved crystalline quality of the MgO buffer layer.

Heteroepitaxial diamond film growth: the a-plane sapphire–iridium system

A substrate system with suitable physical and chemical properties is a necessary component for achieving large-scale heteroepitaxial diamond growth. We have developed a two-stage process to produce (001) diamond films using a-plane (112̄0) α-Al 2O 3 (sapphire) as a substrate. Epitaxial (001) iridium films are first grown on terraced, vicinal a-plane sapphire by ultra-high vacuum electron-beam evaporation at 950 °C. The epitaxial relationship, Ir(100)‖Al 2O 3(112̄0) with Ir[011]‖Al 2O 3 [11̄00], was determined by analysis of X-ray and electron backscattering diffraction. For a 300-nm thickness of Ir, a (200) rocking curve yielded a linewidth of 0.2°. After transfer to a CVD system, a diamond film was grown on the Ir layer by low-pressure chemical vapor deposition using methane and hydrogen. A key step in the process is the initial exposure of the epitaxial Ir to a d.c.-biased plasma that leads to uniform coverage of the surface by a dense array of diamond crystallites prior to growth. A subsequent growth step for approximately 60 min yields a continuous and highly homogenous (001) diamond film over areas more than 50 mm 2. The epitaxial relationship between diamond and the Ir substrate is diamond(001)‖Ir(001) with diamond[100]‖Ir[100]. The films have also been characterized by AFM, SEM, TEM, and Raman. As a result of the superior thermomechanical properties of sapphire, this heteroepitaxial system may enable wafer-size heteroepitaxial diamond growth in the near future.

Studies of heteroepitaxial growth of diamond

Large-scale heteroepitaxial growth of diamond depends critically on the development of a suitable lattice-matched substrate system. Oxide substrates, notably MgO and SrTiO 3, on which thin epitaxial films of iridium serve as a nucleation layer for diamond have already shown considerable promise. We describe here improvements in the growth of single crystal diamond by low-pressure microwave plasma-enhanced CVD. Oxide substrates with flat, low-index surfaces form the initial basis for the process. Iridium was deposited on heated substrates in a UHV electron-beam evaporation system resulting in epitaxial films, typically 150–300 nm thick, with Ir (1 0 0) parallel to the surface of all substrates as confirmed by X-ray and electron backscattering diffraction. Following Ir deposition, the samples were transferred to a CVD reactor where a bias-enhanced nucleation step induced a dense condensate that completely covered the Ir surface. Uniform nucleation densities of order 10 12 cm −2 were observed. Interrupted growth studies, carried out at intervals from seconds to minutes subsequent to terminating the nucleation step, revealed a rapid coalescence of grains. One hour of growth resulted in a smooth, nearly featureless, (0 0 1) diamond film. For extended growth runs, slabs of diamond were grown with thickness as great as 38 μm and lateral dimensions near 4 mm. The crystals were transparent in visible light and cleaved on (1 1 1) planes along 〈1 1 0〉 directions, similar to natural diamond. Of particular significance is the successful use of sapphire as an underlying substrate. Its high crystalline perfection results in epitaxial Ir films with X-ray linewidths comparable to those grown on SrTiO 3. However, Al 2O 3 possesses superior interfacial stability at high temperatures in vacuum or in a hydrogen plasma with a better thermal expansivity match to diamond. Since sapphire is available as relatively inexpensive large diameter substrates, these results suggest that wafer-scale growth of heteroepitaxial diamond should be feasible in the near future.

Epitaxial Growth of a (101) Pb(ZrxTi1-x)O3 Film on an Epitaxial (110) Ir/(100) ZrN/(100) Si Substrate Structure

We deposited an Ir film on the epitaxial (100) ZrN/(100) Si substrate and deposited a Pb(ZrxTi1-x)O3 (PZT) film serially, using the sputter deposition method. The (110)-oriented Ir and (101)-oriented PZT regions in the deposited films were increased by increasing the substrate temperature TS for the Ir film from 600 to 800°C while the (100)-oriented Ir and (001)-oriented PZT regions decreased. At TS=800°C, we can obtain a predominant (110)-oriented epitaxial Ir film on the epitaxial (100) ZrN film. On this epitaxial Ir film, a predominant (101)-oriented epitaxial PZT film can also be obtained around TS=600°C. We estimated the ferroelectric property of the (101) PZT film, compared with the (101)+(001) PZT film. It was found that the remanent polarization and the coercive field of the (101) PZT film were smaller than those of the (101)+(001) PZT film.

High deposition rate of epitaxial (1 0 0) Iridium film on (1 0 0)YSZ/(1 0 0)Si substrate by RF sputtering deposition

We investigated the crystalline quality and resistivity of an epitaxial Ir film deposited on an epitaxial (1 0 0)YSZ/(1 0 0)Si substrate by RF magnetron sputtering method. It has been reported that the orientation of Ir film deposited on YSZ film depends on the deposition rate. In our case, the surface of the YSZ/Si structure was cleaned by diluted HF solution, just before loading the sample to the sputtering chamber. It was found that even when the sputtering gas pressure was increased to 15 Pa and the deposition rate from 0.5 to 60 nm/min, respectively, the Ir film was still epitaxially grown with a high (1 0 0) orientation. This is because the diluted HF solution treatment cleaned off the YSZ/Si surface. When the deposition rate was increased, the crystalline quality and resistivity of the film were decreased and increased, respectively.

Ferroelectric properties of epitaxial Bi 4Ti 3O 12 films deposited on epitaxial (1 0 0) Ir and (1 0 0) Pt films on Si by sputtering

A heteroepitaxial ferroelectric Bi 4Ti 3O 12 (BIT) film was prepared on the bottom electrode of the epitaxial Ir or Pt film by reactive sputtering with Ar+O 2 gas. The Ir or the Pt film was deposited on an epitaxial (1 0 0) YSZ film/(1 0 0) Si substrate structure by sputtering with Ar gas. X-ray diffraction patterns showed that (0 0 1) BIT films were heteroepitaxially grown on epitaxial (1 0 0) Ir and Pt films with a 45°-rotated planar orientation. However, Rutherford backscattering spectroscopy measurements observed the existence of a number of voids in the BIT film on Ir/YSZ/Si structure in addition to interdiffusion of chemical components of its structure. On the other hand, the BIT film deposited on Pt film exhibited normal density with little interdiffusion. A polarization–voltage hysteresis loop was observed for the BIT film on Pt while it was hardly observed for the BIT film on Ir. Thus, the existence of voids in BIT films deposited on Ir films and the layer interdiffusion are possible causes behind the loss of the ferroelectric property of the BIT film.

Crystalline and ferroelectrical properties of heteroepitaxial (100) and (111) Pb(Zr xTi 1− x)O 3 films on Ir/(100)(ZrO 2) 1− x(Y 2O 3) x/(100)Si structures

PZT films were deposited on the epitaxial Ir/(100)(ZrO 2) 1− x (Y 2O 3) x /(100)Si structures by sputtering. On the epitaxial (100) and the (100)+(111) Ir films, the tetragonal (001) PZT films were heteroepitaxially grown with a cube-on-cube relationship. Also, on the epitaxial (111) Ir film, the tetragonal (111) epitaxial PZT film was grown with quadruple domain. The crystalline quality of the epitaxial (001) PZT film was much better than that of the epitaxial (111) PZT film. The leakage and P– E characteristics of the epitaxial (001) PZT film on the (100) Ir film was superior to the (001) PZT film on the (100)+(111) Ir film and to the (111) PZT film.

Improvement of Surface Crystalline Quality of an Epitaxial (100)ZrN Film as a Bottom Electrode Diffusion Barrier for Ferroelectric Capacitors

AbstractWe fabricated the double epitaxial layers of Ir on barrier metal ZrN for Pb(ZrxTi1−x)O3 (PZT), using reactive sputtering. In order to remove the surface oxide layer of the ZrN film prior to depositing the Ir film, we used a new treatment method, in which the ZrN film is dipped into a 0.5% buffered HF solution for less than 30 seconds and then immediately dipped into a hydrazine (N2H4) monohydrate solution for 60 seconds. Also, in order to suppress the generation of twin boundaries and to improve crystalline quality, the ZrN film is deposited using a two-step method, which consists of an initial high temperature deposition step at 850s°C followed by a lower temperature step at 600°C. On the ZrN film prepared by these methods, we can obtain a mainly (110)-oriented epitaxial Ir film.

Electrical properties of Ir-silicide formation on p-Si(100) in ultra-high vacuum

An epitaxial Ir-silicide film was grown on top of a p-Si(100) substrate a temperature of 450°C in an ultra-high vacuum. The epitaxial Ir-silicide film was identified to be Ir 3Si 4 with four types of epitaxial modes. The average Schottky barrier height of the epitaxial Ir 3Si 4/p-Si(100) diode at 60–100 K was determined to be 0.177 eV with an ideality factor of 1.12. In contrast, a polycrystalline IrSi/p-Si(100) diode was formed by conventional room-temperature deposition and annealing at high temperatures, and its average Schottky barrier height was 0.157 eV with an ideality factor of 1.08. The difference in Schottky barrier height was attributed to the merged effect of phase composition and microstructure difference between Ir 3Si 4 and IrSi silicides. The epitaxial Ir 3Si 4/p-Si(100) diode has higher n values than polycrystalline IrSi/p-Si(100) due to more interfacial states of oxygen remaining at the Ir 3Si 4/p-Si(100) interface.

Epitaxial Ir layers on SrTiO 3 as substrates for diamond nucleation: deposition of the films and modification in the CVD environment

Iridium films on SrTiO3(001) have recently proven to be a superior substrate material for the heteroepitaxy of diamond thin films by chemical vapour deposition in the effort towards the realization of single crystal diamond films. In this paper we report on the growth and structural properties of iridium (Ir) films deposited by electron-beam evaporation on SrTiO3(001) surfaces varying the deposition temperature between 280 and 950°C. The films were studied by scanning electron microscopy, atomic force microscopy and X-ray diffraction. At the highest temperature film growth proceeds via three-dimensional nucleation, coalescence and subsequent layer-by-layer growth. The resulting films show a cube-on-cube orientation relationship with the substrate and a minimum mosaic spread of 0.15°. Towards lower deposition temperatures the orientation spread increases only slightly down to ∼500°C while the surface roughness, after passing through a maximum at ∼860°C, decreases significantly. For the lowest temperatures (below 500°C) the mosaic spread rises accompanied by the occurrence of twins until the epitaxial order is lost. Plasma treatment in the diamond deposition reactor at high temperature (920°C) yields low nucleation densities and modifies the Ir surface. At the same time {111} facets show a significantly higher structural stability as compared with {001} facets. Nucleation at 700°C results in highly aligned diamond grains with low mosaic spread and a vanishing fraction of randomly oriented grains, proving the superior properties of Ir films on SrTiO3 for diamond nucleation as compared with pure silicon substrates.

The effect of Si/Ir codeposition ratio on the Ir silicide/Si(100) interface roughness

Epitaxial Ir suicide films were grown on p-Si(100) substrates at 450 °C by codeposition of Ir and Si with different Si/Ir atomic ratios. Epitaxial Ir 3Si 4 was grown on Si(100) as pure Ir was deposited at a substrate temperature of 450 °C. The Ir 3Si 4/Si(100) interface remained smooth although intermixing occurred during deposition of Ir on Si(100). Ir 3Si 4 was the phase formed on Si(100) even with higher codeposition ratios, but the Ir 3Si 4/Si(100) interface became rougher at higher codeposition ratios. The interface roughness occurring with higher Si/Ir codeposition ratios was attributed to the spatial inhomogeneity of the Si/Ir ratio on a Si(100) surface during the codeposition. The different degrees of intermixing due to the inhomogeneous surface would make the Ir 3Si 4/Si(100) interface rough. To obtain a sharp interface, deposition of pure Ir on Si(100) at high temperature is suggested.

High-temperature stability of heteroepitaxial Ir-silicide/SiGe layers co-deposited on Si(100) under ultra-high vacuum

Heteroepitaxial Ir-silicide/SiGe layers have been grown on top of p-Si(100) at a substrate temperature of 450 °C under ultra-high vacuum. The epitaxial Ir-silicide layer was determined to be Ir 3Si 4 with four types of epitaxial modes. Reflection high-energy electron diffraction was used to examine the film quality. The high-temperature stability of the epitaxial Ir 3Si 4/SiGe layers was investigated by rapid thermal annealing the Ir 3Si 4/SiGe layers at 550, 750 and 950 °C for 20 s. The chemical composition, microstructures and surface morphologies of the layers were characterized by using Auger electron spectroscopy, grazing angle incidence X-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. A critical transition temperature existed between 550 and 750 °C for the Ir 3Si 4/SiGe layers, beyond which the Ir 3Si 4/SiGe layers were unstable during the rapid thermal annealing process. The Ir 3Si 4 layer became a mixture of dual Ir-silicide phases containing Ge atoms as the instability occurred. Because of the thermal instability, the conventional guard-ring fabrication should be performed before deposition of epitaxial films.