Enhancement in optical characteristics of c-axis-oriented radio frequency–sputtered ZnO thin films through growth ambient and annealing temperature optimization

High-quality radio frequency–sputtered ZnO were grown on Si substrates at 400 °C at various partial gas pressures (Ar/Ar+O2). Subsequently, to remove as-grown defects, high temperature annealing from 700 to 900 °C on as-grown samples in constant oxygen flow for 10 s was performed. X-ray diffraction study confirmed the formation of highly crystalline films with a dominant peak at (002). The sample grown in 50% Ar and 50% O₂ ambient exhibited the lowest linewidth (2θ=~0.2728°) and highest stoichiometry. Grain size of the as grown samples decreased with increase in the partial pressure of oxygen till a certain ratio (1:1), and photoluminescence (PL) improved with increase in annealing temperature. Low-temperature (18 K) PL measurements showed a near-band-edge emission peak at 3.37 eV, and the highest peak intensity (more than six orders compared to others with narrow linewidth of ~0.01272 eV) was exhibited by the sample annealed at 900 °C and was six orders higher than that of the as-grown sample. All as-grown samples exhibited dominant visible-range peaks due to emission from defect states.     

Photoluminescence of phosphorous doped Ge on Si (100)

Photoluminescence (PL) of selectively grown phosphorus (P) doped germanium (Ge) is investigated. 350–600 nm thick P-doped Ge is grown on 100 nm thick P-doped Ge buffer layer, which is annealed at 800 °C before the main part of Ge deposition. In the case of Ge deposited at 325 °C, approximately two times higher PL intensity is observed by P doping of ~3.2×10¹⁹ cm⁻³. Further increase of PL intensity by a factor of 1.5 is observed by increasing the growth temperature from 325 °C to 400 °C due to improved crystal quality. Varying PH₃ partial pressure at 400 °C, red shift of the PL occurred with increasing P concentration due to higher bandgap narrowing. With increasing P concentration up to ~1.4×10¹⁹ cm⁻³ at 400 °C the PL peak intensity increases by filling electrons into the L valley and decreases due to enhanced point defect concentration and degraded crystallinity. By post-annealing at 500–800 °C, the PL intensity is further increased by a factor of 2.5 because of increased active P concentration and improved crystal quality. Reduced direct bandgap energy by introducing tensile strain is also observed.     

Effects of defect,carrier concentration and annealing process on the photoluminescence of silicon pn diodes

Silicon pn diodes were fabricated by ion implantation of B and P ions with different doses and subsequent annealing processes. Room temperature photoluminescence (PL) were investigated and the factors affecting the PL intensity were analyzed. Results show that both kinds of pn diodes have PL peak centered at about 1140 nm. Dislocation loops resulted from ion implantation and annealing process may enhance the light emission of silicon pn diode due to its band quantum confinement effect to carriers. The luminescence intensity depends on the carrier concentrations in the implantation region. It should be controlled at the range of 1–6×10¹⁶ cm⁻³. Moreover, the PL intensities of pn diodes with furnace annealing (FA) are higher than those with rapid thermal annealing, and the annealing temperature range for FA is 900–1100 °C.     

The use of hydrogen passivation to fabricate Schottky diodes for DLTS measurements of heavily-doped p+ Si

Schottky diodes were successfully fabricated on p⁺ Si for deep-level transient spectroscopy (DLTS) measurements by the use of hydrogen passivation of boron. Atomic hydrogen was introduced into the near-surface region of boron-doped (1 0 0) CZ Si crystals, which had a resistivity of about 0.01 Ω cm, at temperatures between room temperature and 300°C by exposure to a hydrogen plasma. Rectifying characteristics were obtained for fabricated Schottky contacts on hydrogenated samples. This was due to the carrier concentration decrease in the near-surface region by hydrogen passivation of boron. As the hydrogenation temperatures were increased, the decrease in carrier concentration was significant. Some results of DLTS measurements were given for fabricated diodes.     

High thermal annealing effect on structural and optical properties of ZnO–SiO2 nanocomposite

The effect of annealing temperature on photoluminescence (PL) of ZnO–SiO₂ nanocomposite was investigated. The ZnO–SiO₂ nanocomposite was annealed at different temperatures from 600 °C to 1000 °C with a step of 100 °C. High Resolution Transmission Electron Microscope (HR-TEM) pictures showed ZnO nanoparticles of 5 nm are capped with amorphous SiO₂ matrix. Field Emission Scanning Electron Microscope (FE-SEM) pictures showed that samples exhibit spherical morphology up to 800 °C and dumbbell morphology above 800 °C. The absorption spectrum of ZnO–SiO₂ nanocomposite suffers a blue-shift from 369 nm to 365 nm with increase of temperature from 800 °C to 1000 °C. The PL spectrum of ZnO–SiO₂ nanocomposite exhibited an UV emission positioned at 396 nm. The UV emission intensity increased as the temperature increased from 600 °C to 700 °C and then decreased for samples annealed at and above 800^°C. The XRD results showed that formation of willemite phase starts at 800 °C and pure willemite phase formed at 1000 °C. The decrease of the intensity of 396 nm emission peak at 900 °C and 1000 °C is due to the collapse of the ZnO hexagonal structure. This is due to the dominant diffusion of Zn into SiO₂ at these temperatures. At 1000 °C, an emission peak at 388 nm is observed in addition to UV emission of ZnO at 396 nm and is believed to be originated from the willemite.     

BCB-to-oxide bonding technology for 3D integration

Process optimization of BCB polymer to silicon oxide bonding was investigated. The suitable bonding temperature is about 300 °C, while bond failure of BCB-to-oxide bonding is observed starting from 400 °C. Bonding interface morphologies and bond strengths of BCB-to-oxide bonding were investigated as well. PECVD oxide to BCB bonding has better bonding quality than that of thermal oxide to BCB bonding. Si–O–Si bonds may be the reason of a strong BCB to oxide bonding. Water molecules link BCB and oxide surfaces during the initial contact, while Si–O–Si bonds are formed during bonding. This proposed mechanism of BCB-to-oxide bonding provides a guideline for polymer to oxide hybrid bonding technology in 3D integration.     

Effects of substrate temperature on the degradation of RF sputtered NiO properties

Nickel oxide (NiO) film was grown on Si (100) substrate through RF sputtering of NiO target in Ar plasma at various temperatures ranging from room temperature (RT) to 300 °C. The structural study revealed (200) oriented NiO diffraction peak at RT and at 100 °C, however, by increasing the substrate temperature to 200 °C, intensity of (200) NiO diffraction peak was decreased. At higher temperature (300 °C), crystalline quality of NiO was significantly degraded and the film was decomposed into Ni. The EDS results confirmed an increase of Ni atomic percentage with increase of the substrate temperature. The surface morphology of NiO film at RT and at 100 °C displayed cubical like grains that were changed into elongated grains with further increase of the substrate temperature. The UV–vis reflectance measurements of NiO revealed a small decrease in its band gap by increasing the substrate temperature to 200 °C.     

Low-temperature formation of self-assembled Ge quantum dots on Si(100) under high carbon mediation via solid-phase epitaxy

CMOS-compatible low-temperature formation of self-assembled Ge quantum dots (QDs) by carbon (C) mediation via a solid-phase epitaxy (SPE) has been demonstrated. The samples were prepared by a solid-source molecular beam epitaxy (MBE) system. C and Ge were successively deposited on Si(100) at 200 °C and Ge/C/Si heterostructure was annealed in the MBE chamber. Sparse Volmer-Weber mode Ge dots without a wetting layer were formed for C coverage (θ_C) of 0.25 and 0.5 ML by lowering SPE temperature (T_S) to 450 °C, but small and dense Stranski-Krastanov (SK)-mode Ge QDs with the wetting layer were obtained with increasing C coverage of 0.75 ML even at 450 °C. From the investigation of SPE temperature effect on Ge QD formation for θ_C of 0.75 ML, SK-mode Ge QDs of about 10 nm in diameter and of about 4.5×10¹¹ cm⁻² in density were formed at T_S≥400 °C. The wetting layer of SK-mode QDs was almost constant 0.2-nm thick at T_S≥450 °C. Measurements of chemical binding states of C in Ge QDs and at Ge/Si interface revealed that a large amount of C–Ge bonds were formed in the wetting layer for high C coverage, and the formation of C–Ge bonds, together with the formation of C–Si bonds, enabled the low-temperature formation of small and dense Ge QDs. These results suggest that the C-mediated solid-phase epitaxy is effective to form small and dense SK-mode QDs at low temperature.     

Thermal stability of compressively strained Si/relaxed Si1-xCx heterostructures formed on Ar ion implanted Si (100) substrates

Thermal stability of compressively strained Si/relaxed Si_1-xC_x heterostructure formed with the defect control by Ar ion implantation was investigated. It was found that compressive strain is sustained up to 900 °C without prominent change in surface roughness. From the X-ray diffraction reciprocal space mapping, it was found that relaxed Si_1-xC_x layer is stable up to at least 800 °C, and compressively strained Si_1-xC_x with relatively large thickness is formed by annealing at temperatures higher than 900 °C owing to redistribution of C atoms. These results indicate that the compressively strained Si/relaxed Si_1-xC_x heterostructure formed by Ar ion implantation technique is available up to at least 800 °C and has a potential to be used at more than 900 °C.     

Effects of surface passivation in porous silicon as H2 gas sensor

This paper investigates the effects of surface passivation in porous silicon (PS) as a hydrogen gas sensor. Two types of sample have been prepared, one with typical HF anodizing solution and the other with the presence of peroxide (H₂O₂) in the solution. The Fourier transform infrared (FT-IR) measurements on the PS layer on the Si substrate showed that the typical PS surface is characterized by chemical species like Si–H and Si–O. Samples anodized with peroxide based (H₂O₂) solution showed a PS structure with higher porosity (～80%) and better surface passivation (higher concentration of Si–O and Si–H species) compared to those not treated with peroxide. Peroxide based PS sample fabricated as an H₂ gas sensor showed better electrical (I–V) sensitivity compared to those without peroxide, which has been associated with good surface passivation. Surface passivation in peroxide based PS is also maintained at higher temperatures (100 °C).     

Structural and electrical studies of Cu-doped CdO prepared by solid state reaction

This paper reports synthesis, crystal structure and electrical properties of Cu-doped CdO (CdO:Cu) powders. X-ray diffraction shows that majority of the samples are monophase and has the cubic structure. The limit solubility of Cu ions in CdO lattice is found to be 2 mol% (after heating at 900 °C), whereby the impurity phase was determined to be the monoclinic-CuO. For monophase CdO:Cu samples synthesized at 900 °C, the lattice parameter decreased with increasing Cu concentration. Electrical conductivity of undoped CdO and 2 mol% Cu-doped CdO (after heating at 900 °C) were found to be 79 and 191 Ω^?1 cm^?1, respectively, at 100 °C and 912 and 1549 Ω^?1 cm^?1, respectively, at 900 °C. Thus, it appears that electrical conductivity slightly increases with Cu doping. Finally, the activation energy of monophase CdO:Cu (after heating at 900 °C) is shown to decrease with Cu concentration.     

Purification of metallurgical-grade silicon powder via chemical attack by hydrofluoric and nitric acids followed by thermal treatment

We investigated a novel process for purifying metallurgical-grade silicon (MG-Si). MG-Si powder was first treated to form a thin porous silicon layer. This was heated at 900 °C under oxygen to weaken impurity–Si bonds. Samples were then chemically etched with dilute aqueous hydrofluoric acid. To understand the mechanisms in this purification process, structural, chemical composition and optical properties of MG-Si powder before and after treatment were characterized using Fourier-transform infrared (FTIR), inductively coupled plasma-atomic emission (ICP-AES), and photoluminescence (PL) spectroscopy techniques. FTIR studies of treated MG-Si powder revealed the formation of a thin porous silicon layer on the top surface, as evidenced by SiH_x vibration peaks. PL spectra show that 30-min HF etching of MG-Si led to an increase in red emission, indicating the formation of porous silicon and suggesting a decrease in impurities. ICP-AES revealed that the process led to significant decreases in the concentrations of 15 different elemental impurities.     

Influence of crystallinity of as-deposited Ge film on formation of quantum dot in carbon-mediated solid-phase epitaxy

The influence of crystallinity of as-deposited Ge films on Ge quantum dot (QD) formation via carbon (C)-mediated solid-phase epitaxy (SPE) was investigated. The samples were fabricated by solid-source molecular beam epitaxy (MBE). Ge/C/Si structure was formed by sequential deposition of C and Ge at deposition temperature (T_D) of 150–400 °C, and it was heat-treated in the MBE chamber at 650 °C. In the case of amorphous or a mixture of amorphous and nano-crystalline Ge film grown for T_D ≤250 °C, density of QDs increased with increasing T_D due to the increase of C-Ge bonds in Ge layer. Ge QDs with diameter of 9.2±2.1 nm were formed in the highest density of 8.3×10¹¹ cm⁻² for T_D =250 °C. On the contrary, in the case of polycrystalline Ge film for T_D ≥300 °C, density of QDs decreased slightly. This is because C incorporation into Ge layer during SPE was suppressed due to the as-crystallized columnar grains. These results suggest that as-deposited Ge film in a mixture of amorphous and nano-crystalline state is suitable to form small and dense Ge QDs via C-mediated SPE.     

Structural and electrical characterization of CoNiO monolayer as copper diffusion barrier in integrated circuits

Diffusion barrier properties of CoNiO monolayer, deposited by Langmuir Blodgett (LB) technique, were studied against the diffusion of copper through SiO₂. Cu/CoNiO/SiO₂/Si and Cu/SiO₂/Si test structures were prepared and compared for this purpose. These test structures were annealed at temperatures starting from 100 °C up to 650 °C in vacuum. Samples were characterized using Energy Dispersive X-ray Spectroscopy (EDS), Atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscope (SEM), four probe resistivity measurement, Capacitance-Voltage (C‒V), Current-Voltage (I‒V) characterization techniques. EDS and AFM confirmed the composition and structure of the deposited monolayer. Thermal stability was studied using X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and four probe techniques. Results indicated that structure with barrier was stable up to 600 °C whereas its counterpart could sustain only up to 300 °C. Sheet resistance of Cu/SiO₂/Si structure starts increasing at 300 °C and that of Cu/CoNiO/SiO₂/Si test structure was almost unchanged up to 600 °C in. SEM analysis of samples annealed at different temperatures also confirmed the XRD and four probe results. Biased Thermal Stress (BTS) was applied to the samples and its effect was observed using C‒V analysis. C‒V curves showed that in the presence of CoNiO barrier layer there was no shift in the C‒V curve even after 120 min of BTS while in the absence of barrier there was a significant shift in the C‒V curve even after 30 min of BTS. Leakage current density (j_L) was plotted against the BTS duration under same BTS conditions. It was found that the Cu/CoNiO/SiO₂/Si stack could survive about two times more than the Cu/SiO₂/Si stack.     

Influence of annealing temperature on the structural and optical properties of nanocrystalline zirconium oxide

Nanocrystalline zirconium oxide powder was prepared by sol-gel method using zirconyl chloride octahydrate (ZrOCl₂·8H₂O) and ethylenediaminetetraacitic acid (EDTA) in ammonium hydroxide (NH₄OH) solution. The as-synthesized complex product was annealed at 650 °C, 750 °C and 850 °C for 2 h to get fine ZrO₂ powder. These samples were further analyzed by Scanning electron microscopy (SEM), X- ray diffraction (XRD), Energy-dispersive X- ray spectroscopy (EDX), UV-vis analysis, Fourier transform infrared (FT-IR) spectroscopy, Photoluminescence spectroscopy (PL) and Raman Spectroscopy to study their structural and optical properties. The structural studies revealed that nanocrystalline ZrO₂ powder exhibits monoclinic phase with variation in crystallite size with annealing temperature. The UV–vis absorption band edge of ZrO₂ decreases from 514 nm to 451 nm as annealing temperature rises from 650 °C to 750 °C. It seems that the drastic reduction in band gap energy may be one of the novel unexpected characteristics of ZrO₂. The FTIR analyses further confirmed the formation of nanocrystalline monoclinic ZrO₂. PL analysis revealed the novel emission peaks at 305 and 565 nm. The Raman spectroscopy confirmed the transformation of amorphous zirconium hydroxide to m-ZrO₂ with increase in temperature from 650 °C to 850 °C.     

Thermal storage effects on AlGaN/GaN HEMT

The effects of thermal storage on GaN–HEMT devices grown on SiC substrate have been investigated by DC and pulsed electrical measurements, breakdown measurements (by means of a Transmission Line Pulser, TLP), and optical and electron microscopy. After 3000 h of thermal storage testing at 300 °C, only a limited reduction of the DC drain saturation current and of the transconductance peak was observed (20% and 25% decrease, respectively). However, pulsed measurements on aged devices clearly highlight a dramatic current collapse effect that has been attributed to a creation of surface traps in the gate-to-drain and gate-to-source access region. On-state breakdown characterization carried out on aged devices did not highlight any noticeable changes with respect to the untreated devices similarly to the DC characterization. Failure analyses have demonstrated that a loss of adhesion of the passivation layer was responsible for the observed trap formation. An improved passivation deposition process was therefore developed, including a surface cleaning procedure aimed at preventing passivation detaching. The devices fabricated using this new procedure do not show any enhancement of trapping effects up to 500 h of thermal stress at 300 °C.     

Dependence of the optoelectronic properties of selenium-hyperdoped silicon on the annealing temperature

Selenium-hyperdoped silicon was prepared by ion implantation at 100 eV to a dose of 6×10¹⁵ Se/cm², followed by furnace annealing at 500–900 °C for 30 min. A phase transition from amorphous to crystalline was observed for the sample annealed at 600 °C. Carrier density in the Se doping layer gradually increases with the annealing temperature and a high carrier/donor ratio of 7.5% was obtained at 900 °C. The effects of annealing temperature on the rectifying behavior and external quantum efficiency of n⁺p junctions formed on Se-hyperdoped silicon were also investigated. We found that 700 °C was the optimal annealing temperature for improving the crystallinity, below-bandgap absorption, junction rectification and external quantum efficiency of Se-doped samples.     

Synthesis,characterization and optical band gap of La2CuO4 nanoparticles

Pure La₂CuO₄ nanoparticles were synthesized via sol–gel process using stearic acid as complexing reagent. This method consists of the formation of an organic precursor, with metallic cations homogeneously distributed throughout the matrix. The gel was calcined at 700 °C, 800 °C and 900 °C for 4 h. The as-prepared La₂CuO₄ nanoparticles were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy and diffuse reflectance spectroscopy. Results show that the purity of La₂CuO₄ crystallites increases with the increase of heat treatment temperature from 700 °C to 900 °C. Optical properties show that La₂CuO₄ crystallites have broad absorption in the UV–vis region and the corresponding band gap is 1.24 eV.     

A high Schottky barrier between Ni and S-passivated n-type Si(1 0 0) surface

By minimizing surface states with sulfur passivation, a record-high Schottky barrier is achieved with nickel on n-type Si(1 0 0) surface. Capacitance–voltage measurements yield a flat-band barrier height of 0.97 eV. Activation-energy and current–voltage measurements indicate ～0.2-eV lower barriers for the Ni/Si(1 0 0) junction. These results accompany a previously-reported record-high Schottky barrier of 1.1 eV between aluminum and S-passivated p-type Si(1 0 0) surface. The operation of these metal/Si(1 0 0) junctions changes from majority-carrier conduction, i.e., a Schottky junction, to minority-carrier conduction, i.e., a p–n junction, with the increase in barrier height from 0.97 eV to 1.1 eV. Temperature-dependent current–voltage measurements reveal that the Ni/S-passivated n-type Si(1 0 0) junction is stable up to 110 °C.     

Improvement in crystal quality of ZnO film on Si substrate by using a homo-buffer layer

ZnO thin films without and with a homo-buffer layer have been prepared on Si(1 1 1) substrates by pulsed laser deposition (PLD) under various conditions. Photoluminescence (PL) measurement indicates that the optical quality of ZnO thin film is dramatically improved by introducing oxygen into the growth chamber. The sample deposited at 60 Pa possesses the best optical properties among the oxygen pressure range studied. X-ray diffraction (XRD) results show that the films directly deposited on Si are of polycrystalline ZnO structures. A low-temperature (500 °C) deposited ZnO buffer layer was used to enhance the crystal quality of the ZnO film. Compared to the film without the buffer layer, the film with the buffer layer exhibits aligned spotty reflection high-energy electron diffraction (RHEED) pattern and stronger near-band-edge emission (NBE) with a smaller full-width at half-maximum (FWHM) of 98 meV. The structural properties of ZnO buffer layers grown at different temperatures were investigated by RHEED patterns. It is suggested that the present characteristics of the ZnO epilayer may be raised further by elevating the growth temperature of buffer layer to 600 °C.