Improved electrical characteristics and reliability of amorphous InGaZnO metal-insulator-semiconductor capacitor with high κ HfOxNy gate dielectric

High κ HfO_xN_y film was deposited on amorphous InGaZnO (a-IGZO) by radio-frequency reactive sputtering using an HfO₂ target in nitrogen plus argon ambience, the electrical characteristics and reliability of a-IGZO metal-insulator-semiconductor (MIS) capacitors were investigated. Experimental results indicate that the nitrogen incorporation into HfO₂ can produce a strong nitride interfacial barrier layer, thus lead to reducing the interface state density, suppressing the hysteresis voltage, and decreasing the gate-leakage current. Improved performance has been achieved for HfO_xN_y gate dielectric a-IGZO MIS capacitors, with a interface state density of 5.1 × 10¹¹ eV⁻¹ cm⁻², a gate-leakage current density of 3.9 × 10⁻⁵ A/cm² at V_fb + 1 V, an equivalent permittivity of 24, and a hysteresis voltage of 105 mV. Moreover, the enhanced reliability of Al/HfO_xN_y/a-IGZO MIS capacitor is observed with a small degradation of electrical characteristics after a high field stressing at 10 MV/cm for 3600 s.     

High-mobility pentacene thin-film transistor by using LaxTa(1−x)Oy as gate dielectric

Pentacene organic thin-film transistors (OTFTs) using La_xTa_(1−x)O_y as gate dielectric with different La contents (x = 0.227, 0.562, 0.764, 0.883) have been fabricated and compared with those using Ta oxide or La oxide. The OTFT with La_0.764Ta_0.236O_y can achieve a carrier mobility of 1.21 cm² V⁻¹s⁻¹s, which is about 40 times and two times higher than those of the devices using Ta oxide and La oxide, respectively. As supported by XPS, AFM and noise measurement, the reasons lie in that La incorporation can suppress the formation of oxygen vacancies in Ta oxide, and Ta content can alleviate the hygroscopicity of La oxide, resulting in more passivated and smoother dielectric surface and thus larger pentacene grains, which lead to higher carrier mobility.     

Atomic layer deposition of high capacitance density Ta2O5-ZrO2 based dielectrics for metal-insulator-metal structures

We have investigated electrical properties of laminated atomic layer deposited films: ZrO₂-Ta₂O₅, ZrO₂-Nb₂O₅-Ta₂O₅, ZrO₂-Ta_xNb_1−xO₅ and Ta₂O₅-Zr_xNb_yO_z. Even though the capacitances of laminates were often higher compared to films of constituent materials with similar thickness, considerably higher charge storage factors, Q, were achieved only when tetragonal ZrO₂ was stabilized in ZrO₂-Ta₂O₅ laminate and when the laminate thickness exceeded 50 nm. The decreased Q values in the case of most laminates were the result of increased leakage currents. In the case of thinner films only Ta₂O₅-Zr_xNb_yO_z stack possessed capacitance density and Q value higher than reference HfO₂. Concerning the conduction mechanisms, in the case of thinner films, the Ta₂O₅ or Ta_xNb_1−xO₅ apparently controlled the leakage either by Richardson-Schottky emission or Poole-Frenkel effect.     

Defect-engineering in SiC by ion implantation and electron irradiation

Three examples are given, which show that ion implantation and electron irradiation can drastically modify the electrical properties of SiC and SiC-based MOS capacitors. (1) It is demonstrated that sulphur ions (S⁺) implanted into 6H-SiC act as double donors with ground states ranging from 310 to 635 meV below the conduction bandedge. (2) Co-implantation of nitrogen (N⁺) - and silicon (Si⁺) - ions into 4H-SiC leads to a strong deactivation of N donors. Additional experiments with electron (e⁻)-irradiated 4H-SiC samples (E(e⁻) = 200 keV) support the idea that this deactivation is due to the formation of an electrically neutral (N_x-V_C, y)-complex. (3) Implantation of a surface-near Gaussian profile into n-type 4H-SiC followed by a standard oxidation process leads to a strong reduction of the density of interface traps D_it close to the conduction bandedge in n-type 4H-SiC/SiO₂ MOS capacitors.     

Structural and Electrical Properties of Ta ax La(1−a)x O y Thin Films

Effect of annealing temperature on the characteristics of sol–gel-driven Ta _ax La_(1?a)x O _y thin film spin-coated on Si substrate as a high-k gate dielectric was studied. Ta _ax La_(1?a)x O _y thin films with different amounts of a were prepared (as-prepared samples). X-ray diffraction measurements of the as-prepared samples indicated that Ta_0.3x La_0.7x O_y film had an amorphous structure. Therefore, Ta_0.3x La_0.7x O _y film was chosen to continue the present studies. The morphology of Ta_0.3x La_0.7x O _y films was studied using scanning electron microscopy and atomic force microscopy techniques. The obtained results showed that the size of grain boundaries on Ta_0.3x La_0.7x O _y film surfaces was increased with increasing annealing temperature. Electrical and optical characterizations of the as-prepared and annealed films were investigated as a function of annealing temperature using capacitance–voltage (C–V) and current density–voltage (J–V) measurements and the Tauc method. The obtained results demonstrated that Ta_0.3x La_0.7x O _y films had high dielectric constant (≈27), wide band gap (≈4.5 eV), and low leakage current density (≈10^?6 A/cm² at 1 V).     

High-k TixSi1−xO2 thin films prepared by co-sputtering method

We report on high-k Ti_xSi_1−xO₂ thin films prepared by RF magnetron co-sputtering using TiO₂ and SiO₂ targets at room temperature. The Ti_xSi_1−xO₂ thin films exhibited an amorphous structure with nanocrystalline grains of 3-30 nm having no interfacial layers. The XPS analyses indicate that stoichiometric TiO₂ phases in the Ti_xSi_1−xO₂ films increased due to stronger Ti-O bond with increasing TiO₂ RF powers. In addition, the electrical properties of the Ti_xSi_1−xO₂ films became better with increasing TiO₂ RF powers, from which the maximum value of the dielectric constant was estimated to be ∼30 for the samples with TiO₂ RF powers of 200 and 250 W. The transmittance of the Ti_xSi_1−xO₂ films was above 95% with optical bandgap energies of 4.1-4.2 eV. These results demonstrate a potential that the Ti_xSi_1−xO₂ thin films were applied to a high-k gate dielectric in transparent thin film transistors as well as metal-oxide-semiconductor field-effect transistors.     

Dependence of electrical properties on interfacial layer of Ta2O5 films

The change in the thickness and chemical states of the interfacial layer and the related electrical properties in Ta₂O₅ films with different annealing temperatures were investigated. The high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy analyses revealed that the 700 °C-annealed Ta₂O₅ film remained to be amorphous and had the thinnest interfacial layer which was caused by Ta-silicate decomposition to Ta₂O₅ and SiO₂. In addition, the electrical properties were improved after annealing treatments. Our results suggest that an annealing treatment at 700 °C results in the highest capacitance and the lowest leakage current in Ta₂O₅ films due to the thinnest interfacial layer and non-crystallization.     

Investigation of Ta2O5/SiO2/4H-SiC MIS capacitors

Tantalum pentoxide (Ta₂O₅) deposited by pulsed DC magnetron sputtering technique as the gate dielectric for 4H-SiC based metal-insulator-semiconductor (MIS) structure has been investigated. A rectifying current-voltage characteristic was observed, with the injection of current occurred when a positive DC bias was applied to the gate electrode with respect to the n type 4H-SiC substrate. This undesirable behavior is attributed to the relatively small band gap of Ta₂O₅ of around 4.3 eV, resulting in a small band offset between the 4H-SiC and Ta₂O₅. To overcome this problem, a thin thermal silicon oxide layer was introduced between Ta₂O₅ and 4H-SiC. This has substantially reduced the leakage current through the MIS structure. Further improvement was obtained by annealing the Ta₂O₅ at 900 °C in oxygen. The annealing has also reduced the effective charge in the dielectric film, as deduced from high frequency C-V measurements of the Ta₂O₅/SiO₂/4H-SiC capacitors.     

Influence of the metal electrode on the characteristics of thermal Ta2O5 capacitors

The effect of various electrodes (Al, W, TiN) deposited by evaporation (Al) and sputtering (W, TiN) on the electrical characteristics of thermal thin film (15-35 nm) Ta₂O₅ capacitors has been investigated. The absolute level of leakage currents, breakdown fields, mechanism of conductivity, dielectric constant values are discussed in the terms of possible reactions between Ta₂O₅ and electrode material as well as electrode deposition process-induced defects acting as electrically active centers. The dielectric constant values are in the range 12-26 in dependence on both Ta₂O₅ thickness and gate material. The results show that during deposition of TiN and Al a reaction that worsens the properties of Ta₂O₅ occurs while there is not an indication for detectable reduction of Ta₂O₅ when top electrode is W, and the leakage current is 5-7 orders of magnitude lower as compared to Al and TiN-electroded capacitors. The high level of leakage current for TiN and Al gate capacitors are related to the radiation defects generated in Ta₂O₅ during sputtering of TiN, and damaged interface at the electrode due to a reaction between Al and Ta₂O₅, respectively. It is demonstrated that the quality of the top electrode affects the electrical characteristics of the capacitors and the sputtered W is found to be the best. The sputtered W gate provides Ta₂O₅ capacitors with a good quality: the current density <7 × 10⁻¹⁰ A/cm² at 1 V (0.7 MV/cm, 15 nm thick Ta₂O₅). W deposition is not accompanied by an introduction of a detectable damage leading to a change of the properties of the initial as-grown Ta₂O₅ as in the case of TiN electrode. Damage introduced during TiN sputtering is responsible for current deterioration (high leakage current) and poor breakdown characteristics. It is concluded that the sputtered W top electrode is a good candidate as a top electrode of storage capacitors in dynamic random access memories giving a stable contact with Ta₂O₅, but sputtering technique is less suitable (favorable) for deposition of TiN as a metal electrode due to the introduction of radiation defects causing both deterioration of leakage current and poor breakdown characteristics.     

Annealing effect on physical and electrical characteristics of thin HfO2, HfSixOy and HfOyNz films on Si

We report the effect of annealing on electrical and physical characteristics of HfO₂, HfSi_xO_y and HfO_yN_z gate oxide films on Si. Having the largest thickness change of 0.3 nm after post deposition annealing (PDA), HfO_yN_z shows the lowest leakage current. It was found for both as-grown and annealed structures that Poole-Frenkel conduction is dominant at low field while Fowler-Nordheim tunneling in high field. Spectroscopic ellipsometry measurement revealed that the PDA process decreases the bandgap of the dielectric layers. We found that a decreasing of peak intensity in the middle HfO_yN_z layer as measured by Tof-SIMS may suggest the movement of N toward the interface region between the HfO_yN_z layer and the Si substrate during the annealing process.     

Morphology and mechanical properties of intermetallic compounds in SnAgCu solder joints

This paper presents the studies to determine hardness and elastic modulus of intermetallic compound (IMC) layers in lead-free solder joints using nanoindentation technique. Two types of surface finishes, i.e., organic solderability preservative (OSP) and electrolytic Ni/Au on Cu pad, with Sn3.5Ag0.5Cu solder balls of 330 μm in diameter are studied, and the intermetallic layers are identified to be Cu₆Sn5, Cu₃Sn and (Ni_y,Cu_1−y)₃Sn₄. The thicknesses of these IMC layers are only few microns at reflowed conditions (less than 2.3 μm). Therefore, probing mechanical properties of thinner IMCs using nanoindentation techniques poses immense difficulties and challenges. In this study, taper-mounted samples are used rather than standard cross-sectional mounted for solder joints. This taper sample gives a larger area for nanoindentation measurements. The elastic modulus and hardness of IMC layers are determined based on the parameter P/S² (load/stiffness²) as a function of the indentation depth to minimise the effects of underlying UBM or solder materials. The modulus of Cu₆Sn₅, Cu₃Sn, (Cu_x,Ni_1−x)₆Sn₅ and (Ni_y,Cu_1−y)₃Sn₄ layer are found to be 112.0 ± 5.1 GPa, 135.5 ± 4.3 GPa, 165.0 ± 11.3 GPa and 136.8 ± 5.8 GPa; whereas the hardness values are found to be 6.8 ± 0.4 GPa, 6.6 ± 0.5 GPa, 7.2 ± 0.9 GPa and 8.2 ± 1.0 GPa, respectively. Thus, the IMC layers in the order of increasing hardness and modulus are found to be Cu₆Sn₅, Cu₃Sn, (Cu_x,Ni_1−x)₆Sn₅ and (Ni_y,Cu_1−y)₃Sn₄.     

Flexible metal-insulator-metal capacitor using plasma enhanced binary hafnium-zirconium-oxide as gate dielectric layer

We have used a sol-gel spin-coating process to fabricate a new metal-insulator-metal capacitor comprising 10-nm thick binary hafnium-zirconium-oxide (Hf_xZr_1−xO₂) film on a flexible polyimide (PI) substrate. The surface morphology of this Hf_xZr_1−xO₂ film was investigated using atomic force microscopy and scanning electron microscopy, which confirmed that continuous and crack-free film growth had occurred on the PI. After oxygen plasma pre-treatment and subsequent annealing at 250 °C, the film on the PI substrate exhibited a low leakage current density of 3.22 × 10⁻⁸ A/cm² at −10 V and maximum capacitance densities of 10.36 fF/μm² at 10 kHz and 9.42 fF/μm² at 1 MHz. The as-deposited sol-gel film was oxidized when employing oxygen plasma at a relatively low temperature (∼250 °C), thereby enhancing the electrical performance.     

Temperature dependence of the dielectric properties of polymer composite based RF capacitors

The temperature dependence of the dielectric properties of high Q polymer-based composites was discussed in terms of microstructural differences with various filler contents. Liquid coatable high Q polymer-based composites with 3-4× improvement in dielectric constant and precisely controlled temperature coefficient of capacitance (TCC) were developed with Ta₂O₅ as the inorganic filler. The base polymer showed a negative TCC of −250 ppm/°C from room temperature to 125 °C. Measurement of TCC with samples containing Ta₂O₅ greater than 30 vol.% showed TCC of ±50 ppm/°C with moderate capacitance density, whereas no detectable improvement in the TCC was observed in samples having 20 vol.% Ta₂O₅. The electron micrographs of this low filler content sample gave a clear indication of clustering, i.e. particle agglomeration. The homogeneity of the particle distribution was more pronounced in the sample with 40 vol.% Ta₂O₅.     

Constant current stress-induced leakage current in mixed HfO2-Ta2O5 stacks

The electrical characteristics of HfO₂-Ta₂O₅ mixed stacks under constant current stress (CCS) at gate injection with 20 mA/cm² and stressing times of 50 and 200 s have been investigated. A very weak effect of the stress on the global dielectric constant, on fast and slow states in the stack as well as on the dominant conduction mechanism is detected. The most sensitive parameter to the CCS is the leakage current. The stress-induced leakage current (SILC) is voltage and thickness dependent. The pre-existing traps govern the trapping kinetics and are a key parameter to evaluate the stress response. Two processes - positive charge build-up and new bulk traps generation - are suggested to be responsible for SILC: the domination of one of them depends on both the film thickness and the stressing time. The positive charge build-up is localized close to the gate electrode implying gate-induced defects could be precursors for it. It is established that unlike the case of single SiO₂ layer, the bulk traps closer to the gate electrode control SILC in the mixed Ta₂O₅-HfO₂-based capacitors.     

High-power broad-area InGaNAs/GaAs quantum-well lasers in the 1200 nm range

High-power broad-area InGaNAs/GaAs quantum-well (QW) edge-emitting lasers on GaAs substrates in the 1200 nm range are reported. The epitaxial layers of the InGaNAs/GaAs QW laser wafers were grown on n⁺-GaAs substrates by using metal-organic chemical vapor deposition (MOCVD). The thickness of the InGaNAs/GaAs QW layers is 70 Å/1200 Å. The indium content (x) of the In_xGa_1−xN_yAs_1−y QW layers is estimated to be 0.35-0.36, while the nitrogen content (y) is estimated to be 0.006-0.009. More indium content (In) and nitrogen content (N) in the InGaNAs QW layer enables the laser emission up to 1300 nm range. The epitaxial layer quality, however, is limited by the strain in the grown layer. The devices were made with different ridge widths from 5 to 50 μm. A very low threshold current density (J_th) of 80 A/cm² has been obtained for the 50 μm × 500 μm LD. A number of InGaNAs/GaAs epi-wafers were made into broad-area LDs. A maximum output power of 95 mW was measured for the broad-area InGaNAs/GaAs QW LDs. The variations in the output powers of the broad-area LDs are mainly due to strain-induced defects the InGaNAs QW layers.     

Conduction mechanisms in Ta2O5 stack in response to rapid thermal annealing

The influence of the rapid thermal annealing (RTA) in vacuum at 1000 °C on the leakage current characteristics and conduction mechanisms in thermal Ta₂O₅ (7-40 nm) on Si has been studied. It was established that the effect of RTA depends on both the initial parameters of the films (defined by the oxidation temperature and film thickness) and annealing time (15-60 s). The RTA tends to change the distribution and the density of the traps in stack, and this reflects on the dielectric and leakage properties. The thinner the film and the poorer the oxidation, the more susceptible the layer to heating. The short (15 s) annealing is effective in improving the leakage characteristics of poorly oxidized samples. The RTA effect, however, is rather deleterious than beneficial, for the thinner layers with good oxygen stoichiometry. RTA modifies the conduction mechanism of Ta₂O₅ films only in the high-field region. The annealing time has strong impact on the appearance of a certain type of reactions upon annealing resulting to variation of the ratio between donors and traps into Ta₂O₅, causing different degree of compensation, and consequently to domination of one of the two mechanisms at high fields (Schottky emission or Poole-Frenkel effect). Trends associated with simultaneous action of annealing and generation of traps during RTA processing, and respectively the domination of one of them, are discussed.     

Heteroepitaxial silicon-carbide nitride films with different carbon sources on silicon substrates prepared by rapid-thermal chemical-vapor deposition

Cubic crystalline silicon-carbon nitride (Si_1−x−yC_xN_y) films have been grown successfully using various carbon sources by rapid-thermal chemical-vapor deposition (RTCVD). The characteristics of the Si_1−x−yC_xN_y films grown with SiH₃CH₃, C₂H₄, and C₃H₈ are examined and compared by x-ray photoelectron spectroscopy (XPS) spectra, scanning electron microscopy (SEM) images, and transmission electron microscopy (TEM) patterns. The XPS spectra show that the differences of chemical composition and chemical-bonding state are co-related to the C bonding type of the different carbon source. The SEM images and TEM analysis indicate that the better Si_1−x−yC_xN_y film can be obtained using C₃H₈ gas as the carbon source. In addition, correlations between the growing stages to the microstructure of the cubic-crystalline Si_1−x−yC_xN_y films have been illustrated in detail.     

Influence of oxidation temperature on the microstructure and electrical properties of Ta2O5 on Si

The effect of the oxidation temperature (673-873 K) on the microstructural and electrical properties of thermal Ta₂O₅ thin films on Si has been studied. Auger electron spectroscopy and X-ray photoelectron spectroscopy results revealed that the films are non-stoichiometric in the depth; an interfacial transition layer between tantalum oxide and Si substrate, containing presumably SiO₂ was detected. It has been found by X-ray diffraction that the amorphous state of Ta₂O₅ depends on both the oxidation temperature and the thickness of the films—the combination of high oxidation temperature (>823 K) and thickness smaller than 50 nm is critical for the appearance of a crystal phase. The Ta₂O₅ layers crystallize to the monoclinic phase and the temperature of the phase transition is between 773 and 823 K for the thinner layers (<50 nm) and very close to 873 K for the thicker ones. The electrical characterization (current/voltage; capacitance/voltage) reveals that the optimal oxidation temperature for achieving the highest dielectric constant (∼32) and the lowest leakage current (10⁻⁸ A/cm² at 1 MV/cm applied field) is 873 K. The results imply that the poor oxidation related defects are rather the dominant factor in the leakage current than the crystallization effects.     

Investigation of graded InxGa1−xP buffer by Raman scattering method

The compositional changes of In_xGa_1−xP graded buffer inserted between GaP substrate and subsequently grown In_0.36Ga_0.64P homojunction LED structure were investigated by Raman spectroscopy. The indium content of In_xGa_1−xP interlayers was increased in eight steps with thickness of 300 nm and constant compositional change Δx_In between the steps. The properties of In_xGa_1−xP graded buffer along the structure cross-section have been studied by Raman back scattering method and the changes in GaP LO and TO phonons were investigated. Raman shift of 13 cm⁻¹ in GaP-like LO₁ phonon was measured on beveled [100]surface for compositional change of In_xGa_1−xP layer in the range of 0<x_In<0.32. The measurements on the cleaved edge of the sample in [011] direction revealed a strong TO phonon at 366 cm⁻¹ and weak LO phonon peak at 405 cm⁻¹ in GaP substrate. By reaching the graded In_xGa_1−xP region the intensity of TO phonon decreases and appearance of considerable TO₁ phonon shift up to 350 cm⁻¹ for In content x_In=0.16 was observed. For upper graded layers with x_In from 0.16 to 0.24 the position of GaP-like TO₁ was constant and can be ascribed to relaxation of lattice mismatched thin In_xGa_1−xP graded upper layers in the structure.     

The role of defects on D-C conduction in Ta2O5 films

Extended Abstract Thin film tantalum oxide capacitors have been used extensively in the electronic and the telecommunication industry.¹ One of the most important parameters that measures the quality of the capacitor is its leakage current. In spite of a large amount of effort to study the conduction mechanism in Ta₂O₅ films, even the basic question of whether conduction is electrode-limited or bulk-limited has not been satisfactorily answered.² In a previous publication,³ it was shown that the conduction current in a metal-Ta₂O₅-metal device is independent of the work function of the metal electrode, for r-f sputtered Ta₂O₅ films.⁴ Furthermore, we have shown that there exists drastic differences in the magnitude of the conduction current through M-I-M devices with similar Ta₂0₅/metal interfaces but progressively different bulk oxide.₅ Therefore, the conduction mechanism cannot be electrode-limited even if interface irregularities are postulated to exist.₆ In this talk, direct evidence that the conduction mechanism is indeed bulk-limited will be presented. Detailed results will be published elsewhere.⁵