Investigation of Si/SiGe/Si heterostructure implanted by H ion and annealed in vacuum and dry O2 ambient

The 20-nm-thick Si cap layer/74-nm-thick Si_0.72Ge_0.28 epilayer/Si heterostructures implanted by 25 keV H⁺ ion to a dose of 1×10¹⁶ cm⁻² were annealed in ultra-high vacuum ambient and dry O₂ ambient at the temperature of 800 °C for 30 min, respectively. Rutherford backscattering/ion channeling (RBS/C), Raman spectra, high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) were used to characterize the structural characteristics of the Si_0.72Ge_0.28 layer. Investigations by RBS/C demonstrated that the crystal quality of the Si/Si_0.72Ge_0.28/Si heterostructure sample implanted by 25 keV H⁺ in conjunction with subsequent annealing in dry O₂ ambient is superior to that of identical sample annealing in ultra-high vacuum ambient. The less strain relaxation of SiGe layer of the Si/Si_0.72Ge_0.28/Si heterostructures implanted by H ion and annealed in dry O₂ ambient at the temperature of 800 °C for 30 min could be doublechecked by Raman spectra as well as HRXRD, which was compared with that in an identical sample annealed in ultra-high vacuum ambient for identical thermal budget. In addition, the SiGe layer of the H-implanted Si/SiGe/Si heterostructural sample annealed in dry O₂ ambient accompanied by better crystal quality and less strain relaxation made its surface morphology superior to that of the sample annealed in ultra-high vacuum ambient at the temperature of 800 °C for 30 min, which was also verified by AFM images.     

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.     

Role of a Si0.95Ge0.05 epilayer cap on boron diffusion in silicon under inert and dry oxidizing ambient annealing

Silicon (Si) and Si with a 60 nm Si_0.95Ge_0.05 epilayer cap (Si_0.95Ge_0.05/Si) were implanted with 60 keV, 1×10¹³ cm⁻² boron (B) followed by annealing in nitrogen (N₂) or dry oxygen (O₂) in two different anneal conditions. B⁺implantation energy and dose were set such that the B peak is placed inside Si in Si_0.95Ge_0.05/Si samples and concentration independent B diffusion is achieved upon annealing. For samples annealed above 1075 °C, Ge diffusing from the Si_0.95Ge_0.05 epilayer cap in Si_0.95Ge_0.05/Si samples reached the B layer inside Si and resulted in retarded B diffusion compared to the Si samples. For annealing done at lower temperatures, diffusion of Ge from Si_0.95Ge_0.05 epilayer cap does not reach the B layer inside Si. Thus B diffusion profiles in the Si and Si_0.95Ge_0.05/Si samples appear to be similar. B diffusion in dry oxidizing ambient annealing of Si_0.95Ge_0.05/Si samples further depends on the nature of Si_0.95Ge_0.05 oxidation which is set by the duration and the thermal budget of the oxidizing anneal.     

Characterization of reduced pressure chemical vapor deposited Si0.8Ge0.2/Si multi-layers

We have investigated the Si_0.8Ge_0.2/Si multi-layer grown directly onto the Si (001) substrates using reduced pressure chemical vapor deposition. The thicknesses of the Si_0.8Ge_0.2/Si multi-layer were determined using transmission electron microscopy. From the results of energy-dispersive X-ray spectroscopy and X-ray diffraction analyses on the Si_0.8Ge_0.2/Si multi-layer, Ge composition in the Si_1?xGe_x layers was determined as ～20% and the value of residual strain ε of the Si_0.8Ge_0.2 layer is calculated to be 0.012. Three peaks are observed in Raman spectrum, which are located at approximately 514, 404, and 303 cm^?1, corresponding to the vibration of Si–Si, Si–Ge, and Ge–Ge phonons, respectively. The photoluminescence spectrum originates from the radiative recombinations both from the Si substrate and the Si_0.8Ge_0.2/Si multi-layer. For the Si_0.8Ge_0.2/Si multi-layer, the transition peaks related to the quantum well region observed in the photocurrent spectrum were preliminarily assigned to e–hh and e–lh fundamental excitonic transitions.     

In0.75Ga0.25As channel layers with record mobility exceeding 12,000 cm2/Vs for use in high-κ dielectric NMOSFETs

In_0.75Ga_0.25As channel layers with a record mobility exceeding 12,000 cm²/Vs for use in high-κ dielectric NMOSFETs have been fabricated. The device structures which have been grown by molecular beam epitaxy on 3″ semi-insulating InP substrate comprise a 10 nm strained In_0.75Ga_0.25As channel layer and a high-κ oxide based dielectric layer (κ ≅ 20). Electron mobilities of 12,033 and 7,042 cm²/Vs have been measured for sheet carrier concentrations n_s of 2.5 × 10¹² and 6 × 10¹² cm⁻², respectively.     

Interaction of ion-implantation-induced interstitials in B-doped SiGe

B-doped Si_0.77Ge_0.23 of various surface-doping levels was used to investigate the evolution of implant damage and the corresponding transient enhanced diffusion of boron as a function of boron concentration. These layers were implanted with a non-amorphizing 60 keV, 1×10¹⁴ cm⁻² Si, and annealed at 750 °C. Plan-view transmission electron microscopy (PTEM) confirmed the formation and dissolution of dislocation loops. Transient enhanced diffusion (TED) is evident in the surface doped SiGe, but the low diffusivity of interstitials in Si_0.77Ge_0.23 and the presence of interstitial traps inhibited TED at the deeper B marker layer.     

Improvement in negative differential conductance characteristics of hole resonant-tunneling diodes with high Ge fraction Si/strained Si1−xGex/Si(1 0 0) heterostructure

Hole resonant-tunneling diodes (RTD) with Si/strained Si_1?xGe_x heterostructures epitaxially grown on Si(1 0 0) have been fabricated and improvement in negative differential conductance (NDC) characteristics for high Ge fraction such as x = 0.5 was investigated. It is clearly shown that SiH₄ exposure at low temperatures of 400–450 °C just after Si_1?xGe_x epitaxial growth is effective to suppress surface roughness in atomic order. In the case of the RTD with x = 0.48, NDC characteristics for 1.4-nm thick Si barriers were observed at higher temperatures around 270 K than that for 2.4-nm thick Si barriers. By increasing the Ge fraction to x = 0.58, NDC characteristics were also observed at higher temperatures around 290 K than that with x = 0.48.     

Effects of post-annealing on the passivation interface characteristics of AlGaN/GaN high electron mobility transistors

We examined the effects of post-annealing in forming-gas ambient on the spin-on-dielectric (SOD)-buffered passivation as well as the conventional plasma-enhanced chemical vapor deposition (PECVD) Si₃N₄ passivation structure in association with the quantitative analysis of defects at the passivation interfaces of AlGaN/GaN high electron mobility transistors (HEMTs). Before the annealing, the interface state densities (D_it) of the PECVD Si₃N₄ are one-order higher (10¹²–10¹³ cm⁻² eV⁻¹) than those of the SOD SiO_x (10¹¹–10¹² cm⁻² eV⁻¹) as derived from C–V characterization. Clear reduction in D_it from the PECVD Si₃N₄ is extracted to a level of 10¹¹–10¹² cm⁻² eV⁻¹ with a stronger absorption from Si–N peak in Fourier transform infrared spectroscopy spectra after the post-annealing. On the other hand, negligible difference in D_it value is obtained from the SOD SiO_x. In this paper we propose that much lower measurement levels (~156 mA/mm) before the annealing and substantial recovery (~13% increase) after the annealing in maximum drain current density of the AlGaN/GaN HEMTs with Si₃N₄ passivations are due to the original higher density before the annealing and greater reduction in D_it of the PECVD Si₃N₄ after the annealing. Significant reduction after the annealing in gate–drain leakage current (from ~10⁻³ to ~10⁻⁵ A, 100-μm gate width) of the HEMTs with the Si₃N₄ passivation is also supposed to be attributed to the reduction of D_it.     

The effect of temperature on the growth and properties of green light-emitting In0.5Ga0.5N films prepared by reactive sputtering with single cermet target

We have grown In_0.5Ga_0.5N films on SiO₂/Si (100) substrate at 100–400 °C for 90 min by rf reactive sputtering with single cermet target. The target was made by hot pressing the mixture of metallic indium, gallium and ceramic gallium nitride powder. X-ray diffraction (XRD) measurements indicated that In_0.5Ga_0.5N films had wurtzite structure and showed the preferential (1 0 -1 0) diffraction. Both SEM and AFM showed that In_0.5Ga_0.5N films were smooth and had small roughness of 0.6 nm. Optical properties were measured by photoluminescence (PL) spectra from room temperature to low temperature of 20 K. The 2.28 eV green emission was achieved at room temperature for all our InGaN films. The electrical properties of In_0.5Ga_0.5N films on a SiO₂/Si (100) substrate were measured by the Hall measurement at room temperature. InGaN films showed the electron concentration of 1.51×10²⁰–1.90×10²⁰ cm⁻³ and mobility of 5.94–10.5 cm² V⁻¹ s⁻¹. Alloying of InN and GaN was confirmed for the sputtered InGaN.     

Improved reliability of AlGaN/GaN-on-Si high electron mobility transistors (HEMTs) with high density silicon nitride passivation

We have systematically studied the effects of Si_xN_1 − x passivation density on the reliability of AlGaN/GaN high electron mobility transistors. Upon stressing, devices degrade in two stages, fast-mode degradation and followed by slow-mode degradation. Both degradations can be explained as different stages of pit formation at the gate-edge. Fast-mode degradation is caused by pre-existing oxygen at the Si_xN_1 − x/AlGaN interface. It is not significantly affected by the Si_xN_1 − x density. On the other hand, slow-mode degradation is associated with Si_xN_1 − x degradation. Si_xN_1 − x degrades through electric-field induced oxidation in discrete locations along the gate-edges. The size of these degraded locations ranged from 100 to 300 nm from the gate edge. There are about 16 degraded locations per 100 μm gate-width. In each degraded location, low density nano-globes are formed within the Si_xN_1 − x. Because of the low density of the degraded locations, oxygen can diffuse through these areas and oxidize the AlGaN/GaN to form pits. This slow-mode degradation can be minimized by using high density (ρ = 2.48 g/cm³) Si₃₆N₆₄ as the passivation layer. For slow-mode degradation, the median time to failure of devices with high density passivation is found to increase up to 2× as compared to the low density (ρ = 2.25 g/cm³) Si₄₃N₅₇ passivation. A model based on Johnson-Mehl-Avrami theory is proposed to explain the kinetics of pit formation.     

Effects of RTA temperatures on conductivity and micro-structures of boron-doped silicon nanocrystals in Si-rich oxide thin films

In this work, the B-doped Si rich oxide (SRO) thin films were deposited and then annealed using rapid thermal annealing (RTA) to form SiO₂-matrix silicon nanocrystals (Si NCs). The effects of the RTA temperatures on the structural properties, conduction mechanisms and electrical properties of B-doped SRO thin films (BSF) were investigated systematically using Hall measurements, Fourier transform infrared spectroscopy and Raman spectroscopy. Results showed that the crystalline fraction of annealed BSF increased from 41.3% to 62.8%, the conductivity was increased from 4.48×10⁻³ S/cm to 0.16 s/cm, the carrier concentration was increased from 8.74×10¹⁷ cm⁻³ to 4.9×10¹⁸ cm⁻³ and the carrier mobility was increased from 0.032 cm² V⁻¹ s⁻¹ to 0.2 cm² V⁻¹ s⁻¹ when the RTA temperatures increased from 1050 °C to 1150 °C. In addition, the fluctuation induced tunneling (FIT) theory was applicable to the conduction mechanisms of SiO₂-matrix boron-doped Si-NC thin films.     

Role of Schottky barrier height at source/drain contact for electrical improvement in high carrier concentration amorphous InGaZnO thin film transistors

We report the fabrication of bottom-gate thin film transistors (TFTs) at various carrier concentrations of an amorphous InGaZnO (a-IGZO) active layer from ~10¹⁶ to ~10¹⁹ cm⁻³, which exceeds the limit of the concentration range for a conventional active layer in a TFT. Using the Schottky TFTs configuration yielded high TFT performance with saturation mobility (μ_sat), threshold voltage (V_TH), and on off current ratio (I_ON/I_OFF) of 16.1 cm²/V s, −1.22 V, and 1.3×10⁸, respectively, at the highest carrier concentration active layer of 10¹⁹ cm⁻³. Other carrier concentrations (<10¹⁹ cm⁻³) of IGZO resulted in a decrease of its work function and increase in activation energy, which changes the source/drain (S/D) contact with the active layer behavior from Schottky to quasi Ohmic, resulting in achieving conventional TFT. Hence, we successfully manipulate the barrier height between the active layer and the S/D contact by changing the carrier concentration of the active layer. Since the performance of this Schottky type TFT yielded favorable results, it is feasible to explore other high carrier concentration ternary and quaternary materials as active layers.     

Non-destructive Raman evaluation of a heavily doped surface layer fabricated by laser doping with B-doped Si nanoparticles

The heavy B-doping of an intrinsic Si(1 0 0) wafer has been performed by irradiating a B-doped Si nanoparticle film on the surface of the Si(1 0 0) substrate with energy densities of 8.0 and 16.0 J/cm² by 532-nm laser light. The thicknesses of the heavily doped surface layers were investigated using Raman spectroscopy. The observed 488.0-nm-excited Raman bands were decomposed into two bands: a Fano-type band due to the heavily doped Si surface layer and a Voigt band due to the lightly doped, intrinsic Si region. The analysis of the Fano-type band indicated that the carrier concentration of the heavily doped region was larger than approximately 10¹⁹ cm⁻³. Based on the two-state model, the thicknesses of the heavily doped surface layers were 480 and 630 nm for the samples prepared with energy densities of 8.0 and 16.0 J/cm², respectively. These values were consistent with those obtained by secondary ion mass spectroscopy (SIMS).     

Enhancing charge transport in copper phthalocyanine thin film by elevating pressure of deposition chamber

Copper phthalocyanine (CuPc)-based thin film transistors were fabricated using CuPc films grown under different deposition pressure (P_dep) (ranging from 1.8 × 10⁻⁴ Pa to 1.0 × 10⁻¹ Pa). The transistor performance highly depended on P_dep. A field-effect mobility of 2.1 × 10⁻² cm²/(V s) was achieved under 1.0 × 10⁻¹ Pa. Detailed investigations revealed that P_dep modulates the molecular packing and orientation of the organic films grown on a SiO₂/Si substrate and influences the charge transport. Furthermore, from a device physics point of view, contact resistance of the fabricated transistors decreased when P_dep increased, which was beneficial in reducing energy consumption.     

A study on phase transformation of SnOx thin films prepared by reactive magnetron sputtering

In the paper, SnO_x thin films were deposited by reactive magnetron sputtering from a tin target in O₂ containing working gas. The evolution from Sn-containing SnO to tetravalent SnO₂ films was investigated. The films could be classified into three groups according to their optical band gaps, which are E_g<2.5 eV, E_g=3.0–3.3 eV and E_g>3.7 eV. The electric measurements show that high conductivity can be obtained much easier in SnO₂ than in SnO films. A high electron mobility of 15.7 cm² V⁻¹ s⁻¹, a carrier concentration of 1.43×10²⁰ cm⁻³ and a resistivity of 2.8×10⁻³ Ω cm have been achieved in amorphous SnO₂ films. Films with the optical band gap of 3.0–3.3 eV remain amorphous though the substrate temperature is as high as 300 °C, which implies that °btaining high mobility in p-type SnO is more challenging in contrast to n-type SnO₂ films.     

Photophysical properties of nano Si/SiOx composites in Al/composite/mono Si structures for green light emitting and photodetector-Schottky diodes

The concept of the self-formation of a nanocrystallite (nc-) Si/SiO_x : Si_zO_yAl nanocomposite at the Al/oxidized porous silicon interface in the result of solid-phase processes between Al and oxidized porous Si (PS) and the influence of its composition on photophysical properties were developed and experimentally confirmed for the Si chip with optical intra-chip interconnect consisting of light emitting and photodetector diodes and alumina waveguide on oxidized PS surface with aluminum electrodes. The peculiarities of nanocomposite photophysical properties (the refractive index, photoluminiscence (PL) peak situation, PL spectrum shape in the green range) have been shown to be due to the quantum confinement effects (revealed by XPS, Raman spectroscopy) and depend on the Al presence in the nanocomposite (obtained by XPS, IR spectroscopy). The experimental confirmation of this concept is (i) the shift of the nc-Si valence band relatively to that of monocrystalline Si (c-Si) on 0.2–0.7 eV for nc-Si size in 2.5–6.5 nm range; (ii) the decrease of Si nanocrystallite size in the Al presence; (iii) the approach of the value of the refractive index of nc-Si : SiO : Si₂O₃ : Si_zO_yAl nanocomposite at λ=236 nm to that of porous Si with 45% porosity and (iv) the stable green PL spectra in the Si_zO_yAl presence in the nanocomposite.     

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42 eV in solution and in film, respectively, and a HOMO level of 5.09 eV. PDTDP and PC₇₁BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of J_SC and V_OC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μ_h and μ_e, which affects the J_SC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53 cm² V⁻¹ s⁻¹ at 250 °C and 0.36 cm² V⁻¹ s⁻¹ at 310 °C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1 cm² V⁻¹ s⁻¹ at V_d = −5 V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.     

Paramagnetic Ge dangling bond type defects at (1 0 0)Si1−xGex/SiO2 interfaces (Invited Paper)

The work addresses the occurrence of Ge dangling bond type point defects at Ge_xSi_1?x/insulator interfaces as evidenced by conventional electron spin resonance (ESR) spectroscopy. Using multifrequency ESR, we report on the observation and characterization of a first nontrigonal Ge dangling bond (DB)-type interface defect in SiO₂/(1 0 0)Ge_xSi_1?x/SiO₂/(1 0 0)Si heterostructures (0.27 ? x ? 0.93) manufactured by the condensation technique, a selective oxidation method enabling Ge enrichment of a buried epitaxial Si-rich SiGe layer. The center, exhibiting monoclinic-I (C_2v) symmetry is observed in highest densities of ～7 × 10¹² cm^?2 of Ge_xSi_1?x/SiO₂ interface for x ～ 0.7, to disappear for x outside the ]0.45–0.87[ interval, with remarkably no copresence of Si P_b-type centers. Neither are trigonal Ge DB centers observed, enabling unequivocal spectral analysis. Initial study of the defect passivation under annealing in molecular H₂ has been carried out. On the basis of all data the defect is depicted as a Ge P_b1-type center, i.e., distinct from a trigonal basic Ge P_b(0)-type center (Ge₃Ge). The modalities of the defect’s occurrence as unique interface mismatch healing defect is discussed, which may widen our understanding of interfacial DB centers in general.     

Optimization of N2/Ar gas flow ratio for the realization of nitrogen-doped p-type ZnCdO thin films synthesized by RF magnetron sputtering

Nitrogen doped ZnCdO films [ZCO:N] have been grown on quartz substrates by radio frequency (RF) reactive magnetron sputtering technique, and the effect of the ratio of nitrogen to argon gas flow [N₂:Ar] on their electrical, microstructure and optical properties were investigated by Hall effect, energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscope (TEM), optical absorbance and photoluminescence (PL) measurements. The results indicate that all the ZCO:N films are of hexagonal wurtzite structure with highly (002) preferential orientation. As the N₂:Ar increases from 0:1 to 4:1, the absorption edge for the samples exhibits blue shift. Hall effect measurement results indicate that the N₂:Ar exerts an immense influence on the p-type conduction conversion for ZCO:N film. It is found that ZCO:N film deposited at the N₂:Ar of 1:2 shows the optimal p-type behavior, which has a carrier concentration of 1.10×10¹⁷ cm⁻³, a mobility of 3.28 cm²V⁻¹s⁻¹ and a resistivity of 17.3 Ω cm. Compared with the other samples, ZCO:N film fabricated at the relatively lower N₂:Ar possesses the superior crystal quality, luminescent and electrical properties. Additionally, a possible mechanism of p-type conduction for ZCO:N film was discussed in this work.     

DC and microwave characteristics of Lg 50 nm T-gate InAlN/AlN/GaN HEMT for future high power RF applications

The DC and microwave characteristics of L_g = 50 nm T-gate InAlN/AlN/GaN High Electron Mobility Transistor (HEMT) on SiC substrate with heavily doped n+ GaN source and drain regions have demonstrated using Synopsys TCAD tool. The proposed device features an AlN spacer layer, AlGaN back-barrier and SiN surface passivation. The proposed HEMT exhibits a maximum drain current density of 1.8 A/mm, peak transconductance (g_m) of 650 mS/mm and ft/fmax of 118/210 GHz. At room temperature, the measured carrier mobility, sheet charge carrier density (n_s) and breakdown voltage are 1195 cm²/Vs, 1.6 × 10¹³ cm⁻² and 18 V respectively. The superlatives of the proposed HEMTs are bewitching competitor for future monolithic microwave integrated circuits (MMIC) applications particularly in W-band (75–110 GHz) high power RF applications.