Electrically active defects induced by hydrogen and helium implantations in Ge

Results of a study of electrically active defects induced in Sb-doped Ge crystals by implantations of hydrogen and helium ions (protons and alpha particles) with energies in the range from 500 keV to 1 MeV and doses in the range 1×10¹⁰–1×10¹⁴ cm⁻² are presented in this work. Transformations of the defects upon post-implantation isochronal anneals in the temperature range 50–350 °C have also been studied. The results have been obtained by means of capacitance–voltage (C–V) measurements and deep-level transient spectroscopy (DLTS).It was found from an analysis of DLTS spectra that low doses (<5×10¹⁰ cm⁻²) of H and He ion implantations resulted in the introduction of damage similar to that observed after MeV electron irradiation. The Sb–vacancy complex was the dominant deep-level defect in the lightly implanted samples. After implantations with doses higher than 5×10¹⁰ cm⁻² peaks due to more complex defects were observed in the DLTS spectra. Implantations with heavy (5×10¹³ cm⁻²) doses of both H and He ions caused the formation of a sub-surface layer with a high (up to 1×10¹⁷ cm⁻³) concentration of donors. These donors were eliminated by anneals at temperatures in the range 100–200 °C. Heat treatments of the heavy proton-implanted Ge samples in the temperature range 250–300 °C resulted in the formation of shallow hydrogen-related donors, the concentration of which was the highest in a region close to the projected depth of implanted protons. The maximum peak concentration of the H-related donors was higher than 1×10¹⁵ cm⁻³ for a proton implantation dose of 1×10¹⁴ cm⁻².     

Influence of rapid thermal annealing on morphological and electrical properties of RF sputtered AlN films

Aluminum nitride films were deposited, at 200 °C, on silicon substrates by RF sputtering. Effects of rapid thermal annealing on these films, at temperatures ranging from 400 to 1000 °C, have been studied. Fourier transform infrared spectroscopy (FTIR) revealed that the characteristic absorption band of Al–N, around 684 cm⁻¹, became prominent with increased annealing temperature. X-ray diffraction (XRD) patterns exhibited a better, c-axis, (0 0 2) oriented AlN films at 800 °C. Significant rise in surface roughness, from 2.1 to 3.68 nm, was observed as annealing temperatures increased. Apart from these observations, micro-cracks were observed at 1000 °C. Insulator charge density increased from 2×10¹¹ to 7.7×10¹¹ cm⁻² at higher temperatures, whereas, the interface charge density was found minimum, 3.2×10¹¹ eV⁻¹cm⁻², at 600 °C.     

High temperature characterization of high-κ dielectrics on SiC

We have fabricated thin catalytic metal–insulator–silicon carbide based structure with palladium (Pd) gates using TiO₂ as the dielectric. The temperature stability of the capacitor is of critical importance for use in the fabrication of electronics for deployment in extreme environments. We have evaluated the response to temperatures in excess of 450 °C in air and observed that the characteristics are stable. Results of high temperature characterization are presented here with extraction of interface state density up to 650 °C. The results show that at temperatures below 400 °C the capacitors are stable, with a density of interface traps of approximately 6×10¹¹ cm² eV⁻¹. Above this temperature the C–V and G–V characteristics show the influence of a second set of traps, with a density around 1×10¹³ cm² eV⁻¹, which is close to that observed for slow states near the conduction band edge. The study of breakdown field as a function of temperature shows two distinct regions, below 300 °C where the breakdown voltage has a strong temperature dependence and above 300, where it is weaker. We hypothesize that the oxide layer dominates the breakdown voltage at low temperature and the TiO₂ layer above 300 °C. These results at high temperatures confirms the suitability of the Pd/TiO₂/SiO₂/SiC capacitor structure for stable operation in high temperature environments.     

Correlation between infrared transmission spectra and the interface trap density of SiO2 films

This work is an attempt to estimate the electrical properties of SiO₂ thin films by recording and analyzing their infrared transmission spectra. In order to study a big variety of films having different infrared and electrical properties, we studied SiO₂ films prepared by low pressure chemical vapor deposition (LPCVD) from SiH₄ + O₂ mixtures at 425 °C and annealed at 750 °C and 950 °C for 30 min. In addition thermally grown gate quality SiO₂ films of similar thickness were studied in order to compare their infrared and electrical properties with the LPCVD oxides. It was found that all studied SiO₂ films have two groups of Si–O–Si bridges. The first group corresponds to bridges located in the bulk of the film and far away from the interfaces, the grain boundaries and defects and the second group corresponds to all other bridges located near the interfaces, the grain boundaries and defects. The relative population of the bulk over the boundary bridges was found equal to 0.60 for the LPCVD film after deposition and increased to 4.0 for the LPCVD films after annealing at 950 °C. Thermally grown SiO₂ films at 950 °C were found to have a relative population of Si–O–Si bridges equal to 3.9. The interface trap density of the LPCVD film after deposition was found equal to 5.47 × 10¹² eV⁻¹ cm⁻² and decreases to 6.50 × 10¹⁰ eV⁻¹ cm⁻² after annealing at 950 °C for 30 min. The interface trap density of the thermally grown film was found equal to 1.27 × 10¹¹ eV⁻¹ cm⁻² showing that films with similar Si–O–Si bridge populations calculated from the FTIR analysis have similar interface trap densities.     

Investigation of Au/Ti/Al ohmic contact to N-type 4H–SiC

In this study, investigation on Au/Ti/Al ohmic contact to n-type 4H–SiC and its thermal stability are reported. Specific contact resistances (SCRs) in the range of 10⁻⁴–10⁻⁶ Ω cm², and the best SCR as low as 2.8 × 10⁻⁶ Ω cm² has been generally achieved after rapid thermal annealing in Ar for 5 min at 800 °C and above. About 1–2 order(s) of magnitude improvement in SCR as compared to those Al/Ti series ohmic systems in n-SiC reported in literature is obtained. XRD analysis shows that the low resistance contact would be attributed to the formation of titanium silicides (TiSi₂ and TiSi) and Ti₃SiC₂ at the metal/n-SiC interface after thermal annealing. The Au/Ti/Al ohmic contact is thermally stable during thermal aging treatment in Ar at temperature in the 100–500 °C range for 20 h.     

Optical and electrical properties of epitaxial (Mg,Cd)xZn1−xO, ZnO, and ZnO:(Ga,Al) thin films on c-plane sapphire grown by pulsed laser deposition

A consistent set of epitaxial, n-type conducting ZnO thin films, nominally undoped, doped with Ga or Al, or alloyed with Mg or Cd, was grown by pulsed laser deposition (PLD) on single-crystalline c-plane sapphire (0 0 0 1) substrates, and characterized by Hall measurement, and UV/VIS optical transmission spectroscopy.The optical band gap of undoped ZnO films at nearly 3.28 eV was shifted by alloying with Mg up to 4.5 eV and by alloying with Cd down to 3.18 eV, dependent on the alloy composition. In addition, highly doped ZnO:Al films show a blue-shifted optical absorption edge due to filling of electronic states in the conduction band.The Hall transport data of the PLD (Mg,Zn,Cd)O:(Ga,Al) thin films span a carrier concentration range of six orders of magnitude from 3 × 10¹⁴ to 3 × 10²⁰ cm⁻³, which corresponds to a resistivity from 5 × 10⁻⁴ to 3 × 10³ Ω cm. Structurally optimized, nominally undoped ZnO films grown with ZnO nucleation and top layer reached an electron mobility of 155 cm²/V s (300 K), which is among the largest values reported for heteroepitaxial ZnO thin films so far.Finally, we succeeded in combining the low resistivity of ZnO:Ga and the band gap shift of MgZnO in MgZnO:Ga thin films. This results demonstrate the unique tunability of the optical and electrical properties of the ZnO-based wide-band gap material for future electronic devices.     

Phase separation in Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy

We report on the phase separation of Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy. Based on X-ray diffraction, low-temperature (10 K) photoluminescence, and reflection high-energy electron diffraction observations, it is possible to classify the phase of Ga-doped MgZnO layers into three regions depending on the incorporated Ga concentration ([Ga]). Single-phase Mg_0.1Zn_0.9O layers are grown when [Ga] is less than 1×10¹⁸ cm⁻³. For [Ga] between 1×10¹⁸ cm⁻³ and 1×10²⁰ cm⁻³, ZnO and Mg_0.2Zn_0.8O coexist, where electron transport is considered to be via two-channel conduction. When [Ga] exceeds 1×10²⁰ cm⁻³, the Ga-doped MgZnO layers become polycrystalline, where carrier compensation takes place presumably due to grain boundaries.     

Low temperature processed hafnium oxide: Structural and electrical properties

In this work hafnium oxide (HfO₂) was deposited by r.f. magnetron sputtering at room temperature and then annealed at 200 °C in forming gas (N₂+H₂) and oxygen atmospheres, respectively for 2, 5 and 10 h. After 2 h annealing in forming gas an improvement in the interface properties occurs with the associated flat band voltage changing from −2.23 to −1.28 V. This means a reduction in the oxide charge density from 1.33×10¹² to 7.62×10¹¹ cm⁻². After 5 h annealing only the dielectric constant improves due to densification of the film. Finally, after 10 h annealing we notice a degradation of the electrical film's properties, with the flat band voltage and fixed charge density being −2.96 V and 1.64×10¹² cm⁻², respectively. Besides that, the leakage current also increases due to crystallization. On the other hand, by depositing the films at 200 °C or annealing it in an oxidizing atmosphere no improvements are observed when comparing these data to the ones obtained by annealing the films in forming gas. Here the flat band voltage is more negative and the hysteresis on the C–V plot is larger than the one recorded on films annealed in forming gas, meaning a degradation of the interfacial properties.     

Vibrational spectroscopy characterization of low-dielectric constant SiOC:H films prepared by PECVD technique

Low-dielectric constant SiOC:H films were prepared by plasma enhanced chemical vapour deposition (PECVD) from trimethyl-silane (H–Si–(CH₃)₃) and ozone (O₃) gas mixture. The samples were preliminarily annealed at 400 °C in N₂ atmosphere and then in N₂+He plasma. Afterwards, they were treated in vacuum at some fixed temperatures in the range between 400 and 900 °C. Structural investigations of the annealed films were carried out by means of vibrational spectroscopy techniques. FT-IR spectrum of a preliminarily treated sample shows absorption bands due to stretching modes of structural groups like Si–CH₃ at 1270 cm⁻¹, Si–O–Si at 1034 cm⁻¹ and C–H_x in the region between 2800 and 3000 cm⁻¹. No significant spectral change was observed in the absorption spectra of samples annealed up to 600 °C, indicating that the preliminarily treated film retains a substantial structural stability up to this temperature. Above 600 °C, absorption spectra show a strong quenching of H-related peaks while the band due to Si–O–Si anti-symmetric stretching mode shifts towards higher energy, approaching the value observed for thermally grown SiO₂. Raman spectra of samples treated at temperatures T500 °C exhibit both D and G bands typical of sp²-hybridised carbon, due to the formation of C–C bonds within the film which is accompanying the release of hydrogen. The intensity of D and G bands becomes more pronounced in samples annealed at higher temperatures, thus suggesting a progressive precipitation of carbon within the oxide matrix.     

Properties of Bi-doped PbTe epitaxial layers and pn junction diodes grown by TDM-CVP

Effects of Bi-doping in PbTe liquid-phase epitaxial layers grown by the TDM-CVP have been investigated. For Bi concentration in the solution, x_Bi, lower than 0.2 at.%, Hall mobility is low. In contrast, for x_Bi>0.2 at.%, Hall mobility is high, while carrier concentration is in the range 10¹⁷ cm⁻³. However, ICP emission analysis shows that, for x_Bi=1.0 at.%, Bi concentration in epitaxial layer is N_Bi=2.3–2.7×10¹⁹ cm⁻³.These results indicate that Bi behaves not only as a donor but also as an acceptor; the nearest neighbor or very near DA pairs are formed. Carrier concentration for Bi-doped layers takes a minimum value at a Te vapor pressure of 2.2×10⁻⁵ Torr for growth temperature 470°C, which is coincident with that of the undoped PbTe. And broad contact pn junctions with highly Bi-doped layers easily cause laser emission compared to undoped junctions. The result suggests that the nearest lattice site Bi–Bi DA pairs behave as strong radiative centers in PbTe.     

Silicon dioxide deposited by ECR-PECVD for low-temperature Si devices processing

Silicon dioxide films have been deposited at temperatures less than 270 °C in an electron cyclotron resonance (ECR) plasma reactor from a gas phase combination of O₂, SiH₄ and He. The physical characterization of the material was carried out through pinhole density analysis as a function of substrate temperature for different μ-wave power (E_w). Higher E_w at room deposition temperature (RT) shows low defects densities (<7 pinhole/mm²) ensuring low-temperatures process integration on large area. From FTIR analysis and Thermal Desorption Spectroscopy we also evaluated very low hydrogen content if compared to conventional rf-PECVD SiO₂ deposited at 350 °C. Electrical properties have been measured in MOS devices, depositing SiO₂ at RT. No significant charge injection up to fields 6–7 MV/cm and average breakdown electric field >10 MV/cm are observed from ramps I–V. Moreover, from high frequency and quasi-static C–V characteristics we studied interface quality as function of annealing time and annealing temperature in N₂. We found that even for low annealing temperature (200 °C) is possible to reduce considerably the interface state density down to 5 × 10¹¹ cm⁻² eV⁻¹. These results show that a complete low-temperatures process can be achieved for the integration of SiO₂ as gate insulator in polysilicon TFTs on plastic substrates.     

ICP dry etching of ZnO and effects of hydrogen

Two different plasma chemistries for etching ZnO were examined. Both Cl₂/Ar and CH₄/H₂/Ar produced etch rates which increased linearly with rf power, reaching values of 1200 Å/min for Cl₂/Ar and 3000 Å/min for CH₄/H₂/Ar. The evolution of surface morphology, surface composition, and PL intensity as a function of energy during etching were monitored. The effect of H in ZnO was studied using direct implantation at doses of 10¹⁵–10¹⁶ cm⁻², followed by annealing at 500–700 °C. The hydrogen shows significant outdiffusion at 500 °C and is below the detection limits of SIMS after 700 °C anneals. SEM of the etched features showed anisotropic sidewalls, indicative of an ion-driven etch mechanism.     

ICP-induced defects in GaN characterized by capacitance analysis

The defects induced by inductively coupled plasma reactive ion etching (ICP-RIE) on a Si-doped gallium nitride (GaN:Si) surface have been analyzed. According to the capacitance analysis, the interfacial states density after the ICP-etching process may be higher than 5.4 × 10¹² eV⁻¹ cm⁻², compared to around 1.5 × 10¹¹ eV⁻¹ cm⁻² of non-ICP-treated samples. After the ICP-etching process, three kinds of interfacial states density are observed and characterized at different annealing parameters. After the annealing process, the ICP-induced defects could be reduced more than one order of magnitude in both N₂ and H₂ ambient. The H₂ ambient shows a better behavior in removing ICP-induced defects at a temperature around 500 °C, and the interfacial states density around 2.2 × 10¹¹ eV⁻¹ cm⁻²can be achieved. At a temperature higher than 600 °C, the N₂ ambient provides a much more stable interfacial states behavior than the H₂ ambient.     

Luminescence and microstructure of Er/O co-doped Si structures grown by MBE using Er and SiO evaporation

Er and O co-doped Si structures have been prepared using molecular-beam epitaxy (MBE) with fluxes of Er and O obtained from Er and silicon monoxide (SiO) evaporation in high-temperature cells. The incorporation of Er and O has been studied for concentrations of up to 2×10²⁰ and 1×10²¹ cm⁻³, respectively. Surface segregation of Er can take place, but with O co-doping the segregation is suppressed and Er-doped layers without any indication of surface segregation can be prepared. Si_1−xGe_x and Si_1−yC_y layers doped with Er/O during growth at different substrate temperatures show more defects than corresponding Si layers. Strong emission at 1.54 μm associated with the intra-4f transition of Er³⁺ ions is observed in electroluminescence (EL) at room temperature in reverse-biased p–i–n-junctions. To optimize the EL intensity we have varied the Er/O ratio and the temperature during growth of the Er/O-doped layer. Using an Er-concentration of around 1×10²⁰ cm⁻³ we find that Er/O ratios of 1 : 2 or 1 : 4 give higher intensity than 1 : 1 while the stability with respect to breakdown is reduced for the highest used O concentrations. For increasing growth temperatures in the range 400–575°C there is an increase in the EL intensity. A positive effect of post-annealing on the photoluminescence intensity has also been observed.     

Optimization of electric properties of high-k zirconium dioxide by varying deposition and annealing conditions

ZrO₂ thin films with a smooth surface were synthesized on silicon by atomic vapor deposition™ using Zr[OC(CH₃)₃]₄ as precursor. The maximum growth rate (7 nm min⁻¹) and strongest crystalline phase were obtained at 400 °C. The increase of the deposition temperature reduced the deposition rate to 0.5 nm min⁻¹ and changed the crystalline ZrO₂ phase from cubic/tetragonal to monoclinic. These films showed no enhancement of the dominating monoclinic phase by annealing. The values of the dielectric constant (up to 32) and leakage current density (down to 1.2×10⁻⁶ A cm⁻² at 1×10⁶ V cm⁻¹) varied depending on the deposition temperature and film thickness. The midgap density of interface states was N_it=5×10¹¹ eV⁻¹ cm⁻². The leakage current and the density of interface states were lowered by the annealing to 10⁻⁷ A cm⁻² at 1×10⁶ V cm⁻¹ and to 10¹⁰ eV⁻¹ cm⁻², respectively. However, this also led to a decrease of the dielectric constant.     

Optical and electrical characterization of the electron beam gun evaporated TiO2 film

We report measured evolutions of the optical band gap, refractive index and relative dielectric constant of TiO₂ films obtained by electron beam gun evaporation and annealed in an oxygen environment. A negative shift of the flat band voltage with increasing annealing temperatures, for any film thickness, is observed. A dramatic reduction of the leakage current by about four orders of magnitude to 5×10⁻⁶ A cm⁻² (at 1 MV cm⁻¹) after 700°C and 60 min annealing is found for films thinner than 15 nm. The basic carrier transport mechanisms at different ranges of applied voltage such as hopping, space charge limited current and Fowler–Nordheim is established. An equivalent SiO₂ thickness in order of 3.5 nm is demonstrated.     

Investigation of strontium tantalate thin films for high-k gate dielectric applications

Strontium tantalate (STO) films were grown by liquid-delivery (LD) metalorganic chemical vapor deposition (MOCVD) using Sr[Ta(OEt)₅(OC₂H₄OMe)]₂ as precursor. The deposition of the films was investigated in dependence on process conditions, such as substrate temperature, pressure, and concentration of the precursor. The growth rate varied from 4 to 300 nm/h and the highest rates were observed at the higher process temperature, pressure, and concentration of the precursor. The films were annealed at temperatures ranging from 600 to 1000 °C. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and ellipsometry indicated that the as-deposited and the annealed films were uniform and amorphous and a thin (>2 nm) SiO₂ interlayer was found. Crystallization took place at temperatures of about 1000 °C. Annealing at moderate temperatures was found to improve the electrical characteristics despite different film thickness (effective dielectric constant up to 40, the leakage current up to 6×10⁻⁸ A/cm², and lowest midgap density value of 8×10¹⁰ eV⁻¹ cm⁻²) and did not change the uniformity of the STO films, while annealing at higher temperatures (1000 °C) created voids in the film and enhanced the SiO₂ interlayer thickness, which made the electrical properties worse. Thus, annealing temperatures of about 800 °C resulted in an optimum of the electrical properties of the STO films for gate dielectric applications.     

Low-temperature conductance measurements of surface states in HfO2–Si structures with different gate materials

Metal-oxide-semiconductor capacitors based on HfO₂ gate stack with different metal and metal compound gates (Al, TiN, NiSi and NiAlN) are compared to study the effect of the gate electrode material on the trap density at the insulator–semiconductor interface.C–V and G–ω measurements were made in the frequency range from 1 kHz to 1 MHz in the temperature range 180–300 K. From the maximum of the plot G/ω vs. ln(ω) the density of interface states was calculated, and from its position on the frequency axis the trap cross-section was found. Reducing temperature makes it possible to decrease leakage current through the dielectric and to investigate the states located closer to the band edge.The structures under study were shown to contain significant interface trap densities located near the valence band edge (around 2×10¹¹ cm⁻²eV⁻¹ for Al and up to (3.5–5.5)×10¹² cm⁻² eV⁻¹ for other gate materials). The peak in the surface state distribution is situated at 0.18 eV above the valence band edge for Al electrode. The capture cross-section is 5.8×10⁻¹⁷ cm² at 200 K for Al–HfO₂–Si structure.     

Effect of annealing on the properties of low-k nanoporous SiO2 films prepared by sol–gel method with catalyst HF

Using hydrofluoric acid (HF) as catalyst, nanoporous SiO₂ thin film was synthesized by sol–gel method. By scanning electron microscopy, Fourier transform infrared spectra, thermo gravimetric and differential thermal analysis, ellipsometry, capacitance–voltage and current–voltage measurements, the effects of annealing on film properties were discussed in detail. The introduction of HF results in the less polarizability, the preferable microstructures and the improved thermal stability of the nanoporous silica films. After thermal annealing at 450 °C, the crack-free films with strong hydrophobicity, ultra-low dielectric constant of 1.65, porosity of 78%, and leakage current density of 1.3 × 10⁻⁸ A cm⁻² were obtained.     

Examination and evaluation of La2O3 as gate dielectric for sub-100 nm CMOS and DRAM technology

High-k gate dielectric La₂O₃ thin films have been deposited on Si(1 0 0) substrates by molecular beam epitaxy (MBE). Al/La₂O₃/Si metal-oxide–semiconductor capacitor structures were fabricated and measured. A leakage current of 3 × 10⁻⁹ A/cm² and dielectric constant between 20 and 25 has been measured for samples having an equivalent oxide thickness (EOT) 2.2 nm. The estimated interface state density D_it is around 1 × 10¹¹ eV⁻¹ cm⁻². EOT and flat-band voltage were calculated using the NCSU CVC program. The chemical composition of the La₂O₃ films was measured using X-ray photoelectron spectrometry and Rutherford backscattering. Current density vs. voltage curves show that the La₂O₃ films have a leakage current several orders of magnitude lower than SiO₂ at the same EOT. Thin La₂O₃ layers survive anneals of up to 900 °C for 30 s with no degradation in electrical properties.