Kate Black

Permittivity enhancement of hafnium dioxide high-κ films by cerium doping

The effect of cerium doping on the dielectric properties of hafnium dioxide is reported. Thin films of cerium-doped hafnium oxide Cex–Hf1−xO2 (x=0.10,0.17,0.34) have been grown by liquid injection atomic layer deposition. After annealing at 900 °C, all films were transformed from an amorphous state into a stabilized cubic or tetragonal phase. As-deposited films of Ce0.1–Hf0.9O2 showed low hysteresis voltages and negligible flat band voltage shifts. After annealing to form the crystalline cubic or tetragonal phase, the relative permittivity (κ) increased from 25 to 32 at 100 kHz with leakage current densities at ±1 MV cm−1 of ∼1.58×10−5 A cm−2.The effect of cerium doping on the dielectric properties of hafnium dioxide is reported. Thin films of cerium-doped hafnium oxide Cex–Hf1−xO2 (x=0.10,0.17,0.34) have been grown by liquid injection atomic layer deposition. After annealing at 900 °C, all films were transformed from an amorphous state into a stabilized cubic or tetragonal phase. As-deposited films of Ce0.1–Hf0.9O2 showed low hysteresis voltages and negligible flat band voltage shifts. After annealing to form the crystalline cubic or tetragonal phase, the relative permittivity (κ) increased from 25 to 32 at 100 kHz with leakage current densities at ±1 MV cm−1 of ∼1.58×10−5 A cm−2.

Ce(IV) complexes with donor-functionalized alkoxide ligands: improved precursors for chemical vapor deposition of CeO2.

Thin films of ceria (CeO(2)) have many applications, and their synthesis by liquid-injection MOCVD (metal-organic chemical vapor deposition) or ALD (atomic layer deposition) requires volatile precursor compounds. Here we report the synthesis of a series of homoleptic and heteroleptic Ce(IV) complexes with donor-functionalized alkoxide ligands mmp (1-methoxy-2-methylpropan-2-olate), dmap (1-(dimethylamino)propan-2-olate), and dmop (2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)propan-2-olate) and their potential as precursors for MOCVD and ALD of CeO(2). New complexes were synthesized by alcohol exchange reactions with [Ce(OBu(t))(4)]. [Ce(mmp)(4)] and [Ce(dmap)(4)] were both found to be excellent precursors for liquid-injection MOCVD of CeO(2), depositing high purity thin films with very low carbon contamination, and both have a large temperature window for diffusion controlled growth (350-600 °C for [Ce(mmp)(4)]; 300-600 °C for [Ce(dmap)(4)]). [Ce(mmp)(4)] is also an excellent precursor for liquid-injection ALD of CeO(2) using H(2)O as oxygen source and demonstrates self-limiting growth from 150 to 350 °C. [Ce(dmap)(4)] has lower thermal stability than [Ce(mmp)(4)] and does not show self-limiting growth in ALD. Heteroleptic complexes show a tendency to undergo ligand redistribution reactions to form mixtures in solution and are unsuitable as precursors for liquid-injection CVD.

Deposition of ZrO2 and HfO2 thin films by liquid injection MOCVD and ALD using ansa-metallocene zirconium and hafnium precursors

Thin films of ZrO2 and HfO2 have been deposited by liquid injection metalorganic chemical vapour deposition (MOCVD) and atomic layer deposition (ALD) using a range of ansa-metallocene precursors [(Cp2CMe2)ZrMe2] (1), [(Cp2CMe2)ZrMe(OMe)] (2) [(Cp2CMe2)HfMe2] (3), and [(Cp2CMe2)HfMe(OMe)] (4) with O2 (MOCVD) or ozone (ALD) as oxygen source. The crystal structures of the new complexes 2 and 3 have been determined and they are shown to be mononuclear and isostructural to complex 1, containing a chelating [Cp2CMe2] ligand leading to a pseudo-four-coordinate distorted tetrahedral geometry around the central Zr or Hf atom, in agreement with structures from Density Functional Theory (DFT). The ZrO2 and HfO2 films were deposited by MOCVD over the temperature range 400–650 °C and by ALD over the temperature range 175–350 °C. X-Ray diffraction analysis showed that the HfO2 films deposited by MOCVD were amorphous, whereas the ZrO2 films deposited by MOCVD were in the tetragonal phase. Auger electron spectroscopy showed that residual carbon was present in all the films and that the films grown by MOCVD contained more carbon (2.4–17.0 at.%) than the films grown by ALD (1.8–2.8 at.%). The dielectric properties of ZrO2 and HfO2 films deposited by ALD were evaluated using [Al/MO2/n-Si] metal oxide semiconductor capacitor (MOSC) structures which showed that the films had low current leakage densities of less than 6 × 10−7 A cm−2 at ±2 MV cm−1.

γ-Al2O3 nanoparticle production by arc-discharge in water: in situ discharge characterization and nanoparticle investigation

γ-Al2O3 crystalline nanoparticles (NPs) are for the first time produced by the arc-discharge in water method. The discharge is studied in situ during the NP formation by simultaneously recording the voltage and current waveforms, emitted light intensity and time-resolved optical emission spectra and snapshots of the discharge. The microstructure of the NPs is investigated by high-resolution transmission electron microscopy, the chemical purity is confirmed by x-ray photoelectron spectroscopy and the crystalline structure by x-ray diffraction. The results experimentally demonstrate the successive phases that lead to the supersaturation condition governing this gas-phase bottom–up process for dielectric NP production.

Silver Ink Formulations for Sinter-free Printing of Conductive Films

Inkjet printing offers an attractive method for the deposition of metal interconnects in electronic systems and enables a low-cost, environmentally friendly route to manufacture. However, virtually all current metal inkjet processes require post-deposition sintering treatments to achieve the optimum electrical conductivity, because the growth mechanism involves coalescence of discrete nanoparticles. A manufacturing process that reduces the number of steps by directly printing silver, removing the need to sinter the printed metal, would be highly advantageous. Here we describe a, sinter-free process that results in the direct printing of crystalline silver. This process exploits the chemistries developed for Atomic Layer Deposition (ALD), to form the basis of a new ink formulation, which we term; Reactive Organometallic inks (ROM). These ROM ink formulations are capable of depositing low temperature, high conductivity metal films, without the need for subsequent sintering treatments. To reduce the temperature for direct formation of metallic Ag, we have added an alcohol as a catalytic reducing agent to dissociate the organometallic component. Silver films printed from our novel ROM ink, on a glass substrate at 120 °C, are electrically conductive with a typical resistivity as low as 39.2% that of bulk silver, without the need for sintering.

The optical properties of vertically aligned ZnO nanowires deposited using a dimethylzinc adduct

The optical properties of zinc oxide nanowires are critically influenced by the growth process. Herein, we describe a metal-organic chemical vapour deposition (MOCVD) process for the growth of ZnO nanowires with improved optical properties. A tetrahydrofuran adduct is used to control the reactivity of dimethylzinc to enable this. Vertically aligned zinc oxide nanowires have been grown on Si(111) substrates by liquid injection MOCVD, using a solution of [Me(2)Zn(tetrahydrofuran)] in the presence of oxygen. The ZnO morphology becomes nanowire-like in a narrow temperature range centred about 500 degrees C. Above and below this temperature range, the ZnO is deposited in the form of polycrystalline films. The ZnO nanowires grow from a polycrystalline nucleation layer, with the (0002) c-axis parallel to the Si[111] substrate orientation. High-resolution electron microscopy reveals a highly crystalline nanowire microstructure. Resonance enhanced ultraviolet Raman spectroscopy shows that the ratio of first- and second-order longitudinal optic modes is commensurate with electron-phonon coupling effects observed previously in ZnO nanostructures. Photoluminescence exhibits intense near band-edge emission with a full width at half-maximum of 110 meV at room temperature and shows negligible defect-related visible emission.

Liquid injection ALD and MOCVD of lanthanum aluminate using a bimetallic alkoxide precursor

Lanthanum aluminate has been deposited for the first time by liquid injection atomic layer deposition (ALD) using a bimetallic alkoxide precursor, [LaAl(OiPr)6(iPrOH)]2 with H2O as oxygen source. The ALD growth rate was constant in the temperature range 160–300 °C, but ALD growth was not fully self-limiting. Comparisons are made with film growth by liquid injection MOCVD. Auger electron spectroscopy (AES) showed that the films contained no detectable C contamination (detection limit ∼0.5%) and XRD showed that films remained amorphous at temperatures below 850 °C. Films grown by ALD were all La deficient with La : Al ratios of 0.50–0.61; films grown by MOCVD at 300 and 450 °C have La : Al ratios of 1.24 and 0.84, respectively. C–V and leakage current data for films grown by ALD and MOCVD are presented and a LaAlOx film (La : Al = 0.54) film had a permittivity (k) of ∼13.

Permittivity enhancement and dielectric relaxation of doped hafnium and zirconium oxide

The frequency dispersion of La doped zirconia and cerium doped hafnia films is considered. Doping at concentration of approximately 10% stabilizes crystalline phases with higher-k values. The dielectric relaxation of La doped zirconia films is more severe than the cerium doped hafnia.

Spontaneous Hyperinsulinism Due to Islet-cell Adenoma

Spontaneous hyperinsulinism resulting from a tumour of the islets of Langerhans is not a common condition, and the diagnosis is easily overlooked. Failure in diagnosis usually results in the early death of the patient, while diagnosis followed by operative removal of the tumour cures the majority of patients. Nicholls (1902) first described an adenoma of the islet cells of the pancreas, but this was an incidental finding at necropsy and no clinical details of the case were recorded. Following the discovery of insulin, Harris (1924) suggested that symptoms might be caused by excess secretion of the hormone, and the first demonstration of an insulin-secreting tumour was by Wilder et al. (1927); they explored the pancreas of a doctor, aged 40, who had suffered from attacks of spontaneous hypoglycaemia for eighteen months, and found a carcinoma in the tail of the pancreas with metastases in the liver. The cells of the carcinoma bore a striking resemblance to islet cells, and insulin was extracted from one of the liver metastases. The first successful operative removal of an insulinsecreting tumour was by Roscoe Graham in 1929 (Howland et al., 1929), who found a small tumour in the body of the pancreas of a woman of 52 who had a seven-years history of attacks of spontaneous hypoglycaemia. The tumour was composed of islet cells, and as it lacked a capsule it was thought possibly to be a carcinoma, but no metastases were found. This lack of a capsule has been a feature of a number of islet-cell tumours subsequently studied, and is not now regarded as an indication of malignancy. Another successful operation was performed at about the same time, for Harvey Cushing (1930), in his Lister Lectures on neurohypophysial mechanisms, mentioned that a small islet-cell adenoma which had caused recurrent attacks of hypoglycaemia had recently been removed in his clinic. Since then the condition has attracted much attention in the United States, no fewer than 38 cases being reported from the Mayo Clinic in twenty years (Lopez-Kruger and Dockerty, 1947), while reviews have been published on both medical (Crain and Thorn, 1949) and surgical aspects of the problem (Howard et al., 1950). In Great Britain the number of reported cases is comparatively small. Apart from Nichollss early pathological observation, the first report is that of Barnard (1932), who found an islet-cell adenoma at the post-mortem examination of a patient in whom the diagnosis had been suspected clinically but who had declined further investigation or operation; postmortem reports have also been published by Cairns and Tanner (1933), Lawrence et al. (1942), Webster and Blades (1952), and Cunningham et al. (1952). The first successful operative removal of an adenoma in Great Britain seems to be that reported by Fraser et al. (1938), and nine other reports of single cases successfully treated by operation have subsequently been published (Rudd and Walton, 1941-2; Rayner and Rogerson, 1943 ; Haines, 1946; Dickie, 1946; Holmes et al., 1946; Bishton and Malins, 1947; Murray, 1949; Wood, 1951; Smith and Cochran, 1952); and Cohen (1950) has recorded two cases. There has also been a single report of an islet-cell carcinoma with liver metastases (Brearley and Laws, 1950), and one case in which attacks of spontaneous hypoglycaemia were thought to be caused by diffuse hyperplasia of the islet cells without tumour formation (Graham and Oakley, 1950). Richardson and Russell (1952) have reported five cases from the London Hospital; operation was carried out in three of these; in two an islet-cell adenoma was removed with complete relief of symptoms, while in the third patient the tumour was a carcinoma with a metastasis in the liver. We have had under our care in St. Bartholomews Hospital in the course of four years three patients with attacks of spontaneous hyperinsulinism, in each of which an islet-cell adenoma has been removed, with a successful result.

GaN-based radial heterostructure nanowires grown by MBE and ALD

A combination of molecular beam epitaxy (MBE) and atomic layer deposition (ALD) was adopted to fabricate GaN-based core/shell NW structures. ALD was used to deposit a HfO2 shell of onto the MBE grown GaN NWs. Electron transparent samples were prepared by focussed ion beam methods and characterized using state-of-the-art analytical transmission and scanning transmission electron microscopy. The polycrystalline coating was found to be uniform along the whole length of the NWs. Photoluminescence and Raman spectroscopy analysis confirms that the HfO2 ALD coating does not add any structural defect when deposited on the NWs.