Effects of Electron Irradiation on Deep Centers in High-Purity Semi-Insulating 6H-SiC

Many point-defect–related centers have been investigated in electron-irradiated 6H-SiC by deep-level transient spectroscopy (DLTS). Most of them are believed to be related to vacancies. Our DLTS studies on deep centers produced by electron-irradiation (EI) in conductive epi-6H-SiC are in agreement with the literature data. However, for semi-insulating SiC, DLTS cannot be used for trap studies so we have applied thermally stimulated current (TSC) spectroscopy. At least nine TSC traps have been observed in high-purity/semi-insulating (HPSI) 6H-SiC. To understand the nature of these centers, 1-MeV EI and postirradiation annealing at 600°C were applied to the sample. The TSC spectroscopy and 4.2 K photoluminescence (PL) have been used to study the effects of EI and annealing on the centers in HPSI 6H-SiC. It was found that (1) some of the major EI-induced DLTS centers in conductive 6H-SiC, such as ED1, E₁/E₂, E_i, Z₁/Z₂, and L₉, have TSC counterparts even in as-grown HPSI 6H-SiC; (2) EI-induced TSC centers in HPSI 6H-SiC are due to point defects, which have been confirmed by typical PL lines (such as S, L, and V lines); and (3) the concentration of the 1.1-eV center, which controls material conductivity, can be increased by 1-MeV EI and decreased by 600°C annealing.     

High-resolution DLTS studies of vacancy-related defects in irradiated and in ion-implanted n-type silicon

Using high-resolution Laplace deep-level transient spectroscopy (DLTS), we have compared the electron emission characteristics of vacancy-related defects in silicon. The samples include material irradiated with high-energy protons, material implanted with a heavy ion and silicon irradiated with 2 MeV electrons. We show that in the proton- and electron-irradiated material the DLTS peak in the region of the (- -/-) state of the divacancy at E_c=0.23 eV contains only one feature. The DLTS peak at 250 K which contains the signal derived from the (-/0) state of the divacancy is much larger in ion-implanted silicon than in electron-irradiated silicon. The Laplace DLTS is able to resolve clearly the (-/0) divacancy state and the V–P defect, whereas conventional DLTS shows only a broad peak in that region.     

Effect of Rapid Thermal Annealing on Sputtered CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> Thin Films

Calcium copper titanium oxide (CaCu₃Ti₄O₁₂, abbreviated to CCTO) films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature (RT) by radiofrequency magnetron sputtering. As-deposited CCTO films were treated by rapid thermal annealing (RTA) at various temperatures and in various atmospheres. X-ray diffraction patterns and scanning electron microscope (SEM) images demonstrated that the crystalline structures and surface morphologies of CCTO thin films were sensitive to the annealing temperature and ambient atmosphere. Polycrystalline CCTO films could be obtained when the annealing temperature was 700°C in air, and the grain size increased signifi- cantly with annealing in O₂. The 0.8-μm CCTO thin film that was deposited at RT for 2 h and then annealed at 700°C in O₂ exhibited a high dielectric constant (ε′) of 410, a dielectric loss (tan δ) of 0.17 (at 10 kHz), and a leakage current density (J) of 1.28 × 10⁻⁵ A/cm² (at 25 kV/cm).     

Effects of Annealing on Structural and Electrical Properties of CuInSe<Subscript>2</Subscript> Thin Films Prepared by Hybrid Sputtering/Evaporation Processes

CuInSe₂ (CIS)-based absorber layers have been fabricated on soda-lime glass substrates by co-sputtering Cu/In and evaporating Se. The effects of annealing in the Se environment on the structural and electrical properties of the CIS layers were studied. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used to investigate the surface morphology and crystal structures of the CIS films, respectively. XRD patterns showed that the CIS films had a single chalcopyrite structure with preferential (112) orientation. The asymmetry of the Se 3d x-ray photoelectron spectroscopy (XPS) peak for the as-grown films and a separated peak for the annealed films are indications of the existence of two different valence states in the two kinds of CIS films. The electrical properties of the CIS films were studied by the four-probe method and Hall measurements. The results showed that the as-grown films had lower carrier mobility than the annealed films. However, the as-grown films had higher carrier concentration.     

Phonon-Assisted Tunneling from <Emphasis Type="Italic">Z</Emphasis><Subscript>1</Subscript>/<Emphasis Type="Italic">Z</Emphasis><Subscript>2</Subscript> in 4H-SiC

n-Type 4H-SiC bulk samples with a net doping concentration of 2.5 × 10¹⁷ cm⁻³ were irradiated at room temperature with 1-MeV electrons. The high doping concentration plus a reverse bias of up to −13 V ensures high electric field in the depletion region. The dependence of the emission rate on the electric field in the depletion region was measured using deep-level transient spectroscopy (DLTS) and double-correlation deep-level transient spectroscopy (DDLTS). The experimental data are adequately described by the phonon-assisted tunneling model proposed by Karpus and Pere.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

Microstructure and Characteristics of Thin-Film La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>Co<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>O<Subscript>3</Subscript>/ZnO:Al Heterocontact CO Sensors Prepared by RF Magnetron Sputtering

Highly sensitive CO gas sensors based on heterocontacts of ZnO:Al on La_0.8Sr_0.2Co_0.5Ni_0.5O₃ (LSCNO) were fabricated successfully. La_0.8Sr_0.2Co_0.5Ni_0.5O₃ thin films were coated on (100) silicon wafers by a sol-gel method including the Pechini process followed by a spin-coating procedure. Then, ZnO:Al films prepared by radiofrequency (RF) magnetron sputtering at various oxygen partial pressures and deposited on as-deposited La_0.8Sr_0.2Co_0.5Ni_0.5O₃ films were investigated. The results revealed that the CO sensing ability of the film prepared with the ratio of O₂/Ar = 5/5 (ratio of volume flow rate) was the worst, owing to the highest (002) plane orientation in the ZnO:Al film. In contrast, the ZnO:Al film prepared with O₂/Ar = 3/7 exhibited better CO sensitivity. Furthermore, all two-layer samples showed higher CO sensitivities than single-layer samples. The CO sensitivity of ZnO:Al/La_0.8Sr_0.2Co_0.5Ni_0.5O₃ thin film was 45% for 500 ppm CO at a sensing temperature of 200°C.     

Memory Effect of Metal-Insulator-Silicon Capacitor with HfO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Multilayer and Hafnium Nitride Gate

Metal-insulator-silicon capacitors have been fabricated using novel insulators of SiO₂/HfO₂-Al₂O₃-HfO₂ (HAH)/Al₂O₃ and metallic HfN gate, exhibiting a program-erasable characteristic. The memory capacitor presents a large memory window of 2.4 V under +12 V program/–14 V erase for 10 ms, no erase saturation, and sufficient electron- and hole-trapping efficiencies such as an electron density of ∼7 × 10¹² cm^–2 under 13 V program for 0.5 ms and a hole density of ∼4 × 10¹² cm^–2 under –12 V erase for 0.5 ms. The observed properties are attributed to the introduction of high permittivity atomic-layer-deposited HAH/Al₂O₃ as well as high work function HfN gate. The related mechanism is addressed accordingly.     

Response of thermoelectric parameters of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> films to secondary recrystallization

Flash evaporation is used to grow Bi_0.5Sb_1.5Te₃ films 1200 nm thick on mica substrates. The average lateral crystallite sizes in the as-grown films are ~800 nm. The (0001) plane in the crystallites is preferentially parallel to the substrate plane. After heat treatment in an argon atmosphere, the effective lateral size of crystallites in which the third-order axis is perpendicular to the substrate plane increased by a factor of 3–5. The crystallites were preferentially oriented in the substrate plane as well. The thermoelectric-power parameter of Bi_0.5Sb_1.5Te₃ films after their heat treatment in an inert environment increased approximately twofold to values close to that of the corresponding single crystals.     

Nitrogen-doped Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films for nonvolatile memory

Nitrogen-doped Ge₂Sb₂Te₅ (GST) films for nonvolatile memories were prepared by reactive sputtering with a GST alloy target. Doped nitrogen content was determined by using x-ray photoelectron spectroscopy (XPS). The crystallization behavior of the films was investigated by analyzing x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that nitrogen doping increases crystallization temperature, crystallization-activation energy, and phase transformation temperature from fcc to hexagonal (hex) structure. Doped nitrogen probably exists in the grain vacancies or grain boundaries and suppresses grain growth. The electrical properties of the films were studied by analyzing the optical band gap and the dependence of the resistivity on the annealing temperature. The optical band gap of the nitrogen-doped GST film is slightly larger than that of the pure GST film. Energy band theory is used to analyze the effect of doped nitrogen on electrical properties of GST films. Studies reveal that nitrogen doping increases resistivity and produces three relatively stable resistivity states in the plot of resistivity versus annealing temperature, which makes GST-based multilevel storage possible. Current-voltage (I-V) characteristics of the devices show that nitrogen doping increases the memory’s dynamic resistance, which reduces writing current from milliampere to microampere.     

Low Fatigue in Epitaxial Pb(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> on Si Substrates with LaNiO<Subscript>3</Subscript> Electrodes by RF Sputtering

Epitaxial PZT (001) thin films with a LaNiO₃ bottom electrode were deposited by radio-frequency (RF) sputtering onto Si(001) single-crystal substrates with SrTiO₃/TiN buffer layers. Pb(Zr_0.2Ti_0.8)O₃ (PZT) samples were shown to consist of a single perovskite phase and to have an (001) orientation. The orientation relationship was determined to be PZT(001)[110]∥LaNiO₃(001)[110]∥SrTiO₃ (001)[110]∥TiN(001)[110]∥Si(001)[110]. Atomic force microscope (AFM) measurements showed the PZT films to have smooth surfaces with a roughness of 1.15 nm. The microstructure of the multilayer was studied using transmission electron microscopy (TEM). Electrical measurements were conducted using both Pt and LaNiO₃ as top electrodes. The measured remanent polarization P _r and coercive field E _c of the PZT thin film with Pt top electrodes were 23 μC/cm² and 75 kV/cm, and were 25 μC/cm² and 60 kV/cm for the PZT film with LaNiO₃ top electrodes. No obvious fatigue after 10¹⁰ switching cycles indicated good electrical endurance of the PZT films using LaNiO₃ electrodes, compared with the PZT film with Pt top electrodes showing a significant polarization loss after 10⁸ cycles. These PZT films with LaNiO₃ electrodes could be potential recording media for probe-based high-density data storage.     

Investigation of Ga oxide films directly grown on n-type GaN by photoelectrochemical oxidation using He-Cd laser

By using a He-Cd laser in a chemical solution of H₃PO₄ with a pH value of 3.5, Ga oxide films were directly grown on n-type GaN. From the energy-dispersive spectrometer (EDS) measurement and x-ray diffraction (XRD) measurement, the grown Ga oxide film was identified as (104) α-Ga₂O₃ structure. A small amount of phosphors existed and bonded with oxygen on the grown films. The as-grown films were amorphous. From the XRD analysis, it is evident that annealing of the α-Ga₂O₃ films led to a change in the microstructure from an amorphous to a polycrystalline phase. In addition, the as-grown low-density films gradually became dense films during the annealing process. Furthermore, the surface roughness of the annealed films also gradually decreased. Hexagonal pinholes on the grown films were observed. The density of the hexagonal pinholes was similar to the defect density of the n-type GaN. From the cross-sectional transmission electron microscopy (TEM) micrographs, it is evident that the hexagonal pinholes originated from defects in the n-type GaN.     

Self-limiting growth of ZrO<Subscript>2</Subscript> films on a Si(100) substrate using ZrCl<Subscript>4</Subscript> and O<Subscript>2</Subscript> under atmospheric pressure

The ZrO₂ films were deposited onto a Si(100) substrate using an alternate reaction of ZrCl₄ and O₂ under atmospheric pressure. It is found that the growth rate of ZrO₂ film depends on the growth conditions, such as growth temperature, partial pressure of the sources being supplied, and exposure time of the substrate to the gaseous sources. Self-limiting growth of the ZrO₂ was achieved in the range of the growth temperature of 673–923 K. The x-ray diffractogram of the ZrO₂ films showed a typical diffraction pattern assigned to the tetragonal polycrystalline phase. The obtained ZrO₂ films were of smooth and uniform surface. It was found that the [O]/[Zr] ratio of the ZrO₂ films are similar to that of the ZrO₂ bulk.     

Synthesis and Characterization of In<Subscript>2</Subscript>S<Subscript>3</Subscript> Thin Films Deposited by Chemical Bath Deposition on Polyethylene Naphthalate Substrates

Indium sulfide (In₂S₃) thin films were deposited on polyethylene naphthalate (PEN) by chemical bath deposition (CBD). The materials were characterized by ultraviolet (UV)–visible spectroscopy, x-ray photoelectron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), scanning electron microscopy (SEM), and x-ray diffraction (XRD) to investigate the influence of the polymeric substrate on the resulting thin In₂S₃. The films showed polycrystalline (cubic and tetragonal) structure. A reduction of the ordering of the polymeric chains at the surface of the PEN was also observed, demonstrated by the appearance of two infrared bands at 1094 cm⁻¹ and 1266 cm⁻¹. Presence of oxygen during the early stages of In₂S₃ growth was also identified. We propose a reaction mechanism for both the equilibrium and nucleation stages. These results demonstrate that In₂S₃ can be deposited at room temperature on a flexible substrate.     

Growth of Nanocrystalline ZnSe:N Films by Pulsed Laser Deposition

Nanocrystalline zinc-blende-structured ZnSe:N films have been deposited on GaAs(100) substrates by pulsed laser deposition (PLD). The growth of the nanocrystalline ZnSe:N films is found to be greatly affected by the pressure of ambient N₂. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) results show that the morphologies of the as-grown films are sensitive to the ambient pressure at a fixed substrate temperature of 300 °C, and the sizes of the as-grown ZnSe:N nanocrystals increase as the ambient pressure increases from 0.1 Pa to 100 Pa. The average sizes of the as-grown nanocrystals are estimated to be about 19 nm, 29 nm, and 71 nm for 0.1 Pa, 1 Pa, and 100 Pa ambient N₂ pressure, respectively. X-ray photoelectron spectroscopy analyses show that the N-doping concentration in the as-grown film is over 10²¹ cm⁻³. Raman spectra demonstrate the broadening of the longitudinal optical (LO) phonon and transverse optical (TO) phonon modes of the ZnSe nanocrystals. Based on these analyses, the mechanism of the formation of ZnSe:N nanocrystals is discussed.     

Electron traps created by high temperature annealing in MBE n-GaAs

An electron trap with a thermal activation energy of 0.83 eV from the conduction band is common in the deep level transient spectroscopy (DLTS) spectra of vapor phase epitaxial (VPE) n-GaAs, but is not observed in the DLTS spectra of as-grown molecular beam epitaxial (MBE) n-GaAs. We show here that this trap is created during high temperature annealing of MBE samples with a Si₃N₄, encapsulant. The trap concentration is correlated with the annealing temperature and time, suggesting the outdiffusion of a constituent atom resulting in the formation of a vacancy or vacancy-complex. Other electron traps observed in the DLTS spectra of asgrown MBE n-GaAs are annealed out for temperatures at or above 800° C.     

Effects of Various Reductants and Surfactants on the Nanostructure of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Synthesized by a Hydrothermal Process

Various reductants and surfactants were used to investigate their effects on the structure of Bi₂Te₃ nanopowders prepared via hydrothermal synthesis at 150°C for 24 h. X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were applied to analyze the phase distributions, microstructures, and grain sizes of the as-grown Bi₂Te₃. The results showed that the grain sizes and morphologies of the synthesized powders were mainly related to the effects of the reductants on the nucleation rate and the surface-adsorbed energy of the diverse surfactants on special crystal planes. Some surfactants could control the growth of the Bi₂Te₃ crystals and promote one-dimensional growth of nanorods during hydrothermal synthesis.     

Thermoelectric Properties of In<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>N Alloys

We report on the experimental investigation of the potential of InGaN alloys as thermoelectric (TE) materials. We have grown undoped and Si-doped In_0.3Ga_0.7N alloys by metalorganic chemical vapor deposition and measured the Seebeck coefficient and electrical conductivity of the grown films with the aim of maximizing the power factor (P). It was found that P decreases as electron concentration (n) increases. The maximum value for P was found to be 7.3 × 10⁻⁴ W/m K² at 750 K in an undoped sample with corresponding values of Seebeck coefficient and electrical conductivity of 280 μV/K and 93␣(Ω cm)⁻¹, respectively. Further enhancement in P is expected by improving the InGaN material quality and conductivity control by reducing background electron concentration.     

Effects of trimethylindium on the purity of In0.5Al0.5P and In0.5Al0.5as epilayers grown by metalorganic chemical vapor deposition

In_0.5Al_0.5P lattice-matched to GaAs and In_0.5A1_0.5As lattice-matched to InP epilayers were grown by atmospheric pressure metalorganic chemical vapor deposition (AP-MOCVD). The effect of trimethylindium on the purity of the as-grown layers was systematically studied using secondary ion mass spectroscopy (SIMS), deep level transient spectroscopy (DLTS), and capacitance-voltage (C-V) measurements. The SIMS results showed that oxygen is the main impurity in all layers and the oxygen concentration in InAlP was approximately one to four orders of magnitude higher than the oxygen concentration found in InALAs when the same indium source was used, indicating that more oxygen was introduced by the phosphine source than by the arsine source. Two electron traps in the InAlP epilayers and four electron traps in the InALAs epilayers were observed in this study. When a high-purity indium source was used, the best InAlP epilayer showed only one deep electron trap at 0.50 eV while the best InALAs epilayer showed no deep levels measured by DLTS. In addition, we also found that a high concentration of oxygen is related to the high resistivity in both material systems; this suggests that semi-insulating (SI) materials can be achieved by oxygen doping and high quality conducting materials can only be obtained through the reduction of oxygen. The oxygen concentration measured by SIMS in the best InALAs epilayer was as low as 3 × 10¹⁷ cm⁻³.     

Formation and crystallization of silicon nanoclusters in SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:H films using femtosecond pulsed laser annealings

SiN _x :H films of different compositions grown on glass and silicon substrates using plasma-chemical vapor deposition at a temperature of 380°C have been subjected to pulsed laser annealings. The treatments are performed using titanium-sapphire laser radiation with a wavelength of 800 nm and a pulse duration of 30 fs. Structural changes in the films are studied using Raman spectroscopy. Amorphous silicon nanoclusters are detected in as-grown films with molar fractions of excess silicon of ∼1/5 and larger. Conditions required for pulsed crystallization of nanoclusters were determined. According to the Raman data, no silicon clusters were detected in as-grown films with a small amount of excess silicon (x > 1.25). Pulsed treatments resulted in the formation of silicon nanoclusters 1–2 nm in size in these films.