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In this article, a new data treatment based on time-resolved photoluminescence is presented. It works as a streak camera for infrared. A time-resolved photoluminescence spectrum for the HgCd0.33Te0.67 sample at 120 K was performed and analyzed. Typical time-resolved photoluminescence measurements, to compare our results with literature, were conducted. An interpretation of the behavior for three different time constants found in the signal is proposed.

Funding Information: V.K. acknowledges the support of Aalto University Doctoral School, Walter Ahlstro?m Foundation, Elektroniikkainsino?o?rien Sa?a?tio?, Sa?hko?insino?o?riliiton Sa?a?tio?, Nokia Foundation, Finnish Foundation for Technology Promotion (Tekniikan Edista?missa?a?tio?), Waldemar von Frenckell?s Foundation, and Kansallis-Osake-Pankki fund. T.K. acknowledges the support of Aalto University Doctoral School and Walter Ahlstro?m Foundation. T.H. acknowledges the support of the Finnish Cultural Foundation and Walter Ahlstro?m Foundation. The Academy of Finland Photonics Flagship PREIN is acknowledged. The authors thank Lassi Ha?llstro?m for the help with the solar simulator measurements. The authors acknowledge the provision of facilities and technical support by Aalto University at Micronova Nanofabrication Centre and Nanomicroscopy Center. Funding Information: V.K. acknowledges the support of Aalto University Doctoral School, Walter Ahlström Foundation, Elektroniikkainsinöörien Säätiö, Sähköinsinööriliiton Säätiö, Nokia Foundation, Finnish Foundation for Technology Promotion (Tekniikan Edistämissäätiö), Waldemar von Frenckell’s Foundation, and Kansallis-Osake-Pankki fund. T.K. acknowledges the support of Aalto University Doctoral School and Walter Ahlström Foundation. T.H. acknowledges the support of the Finnish Cultural Foundation and Walter Ahlström Foundation. The Academy of Finland Photonics Flagship PREIN is acknowledged. The authors thank Lassi Hällström for the help with the solar simulator measurements. The authors acknowledge the provision of facilities and technical support by Aalto University at Micronova Nanofabrication Centre and Nanomicroscopy Center. Publisher Copyright: © 2022 The Authors. Published by American Chemical Society We report direct growth of InSb nanowires (NWs) and monolithic device fabrication on flexible plastic substrates. The nanowires were grown using metal–organic vapor-phase epitaxy (MOVPE) in self-catalyzed mode. The InSb NWs are shown to form in the zinc-blende crystal structure and to exhibit strong photoluminescence at room temperature. The NW array light-trapping properties are evidenced by reflectance that is significantly reduced compared to bulk material. Finally, the InSb NWs are used to demonstrate a metal–semiconductor–metal photoresistor directly on the flexible plastic substrate. The results are believed to advance the integration of III–V nanowires to flexible devices, and infrared photodetectors in particular.

Semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high temperature requirements and substrate preparation. At the same time, plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance, and biocompatibility. We explore the direct growth of InSb nanowires on flexible plastic substrates by metal-organic vapor phase epitaxy (MOVPE). We synthesize InSb nanowires on polyimide and show that the fabricated NWs are optically active with strong light emission even at RT. Overall, we demonstrate that InSb nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowire-based flexible electronic devices.

The type-2 InAs/InAs1−xSbx superlattices on GaAs substrate with GaSb buffer layer were investigated by comparison of theoretical simulations and experimental data. The algorithm for selection of input parameters (binary and ternary materials) for simulations is presented. We proposed the method of the bandgap energy extraction of the absorption curve. The correct choice of the bulk materials and bowing parameters for the ternary alloys allows to reach good agreement of the experimental data and theoretical approach. One of the key achievements of this work was an electron affinity assessment for the device’s theoretical simulation. The detectivity of the long-/very long-wave InAs/InAs1−xSbx superlattice photoconductors at the level of ~ 8 × 109 cm Hz1/2/W (cutoff wavelength 12 µm) and ~ 9 × 108 cm Hz1/2/W (cutoff wavelength 18 µm) at a temperature 230 K confirmed the good quality of these materials.

InAsSb ternary alloys can be considered as an alternative to HgCdTe for mid-wavelength, as well as long-wavelength infrared applications. Its energy bandgap exhibits nonlinearity versus Sb composition, and hence the correct determination of its value is necessary for the effective design and simulation of optoelectronic devices. The commonly used expression on the InAsSb energy bandgap provided by Wieder and Clawson overestimated energy bandgap values at high temperatures. In this paper, we present two modified empirical relationships which are based on the experimental data obtained in our laboratory and published literature. Both are correct for a wide range of Sb molar composition and temperatures.The nonlinearity of the energy gap is described with the adequate bowing parameter. The data collected in our experiment, for samples grown on lattice mismatched GaAs substrates, fits well with a bowing factor of C = 0.72 eV. The minimum bandgap energy that can be reached at room temperature is about 73 meV for a Sb mole fraction, x = 0.63. In comparison, the bandgap at low temperatures collected in the literature for samples grown on lattice-matched GaSb substrates is smaller than the measured values in our experiment and fits well with a parabola with C = 0.9 eV. Keywords: InAsSb, Bandgap energy, Bowing, III-V ternary alloys, Infrared detectors

The InAs/InAs1−xSbx type-II superlattices (T2SLs) grown on GaSb buffer layer and GaAs substrates have recently been applied for detectors for long wavelength infrared (LWIR) range and high operating temperature (HOT) conditions. The detailed modeling of T2SLs minibands structure required for detector’s design optimization process relies on accurate knowledge of the InAs1−xSbx bandgap and band edge position. The k∙p (8 × 8 method) was used to analyze the valence band offset (VBO) between InAs and InAs1−xSbx, and hence the InAs1−xSbx band edge position at required xSb composition. An increase of the VBO leads to the structure transformation from type-IIb to type-II superlattices. The required energy bandgap can be reached by changing the conduction band offset (CBO) and the energy bandgap bowing parameter. The temperature dependence of the experimental results of InAs/InAs1−xSbx T2SLs energy bandgap was found to be comparable with theoretical one when energy bandgap bowing parameter is dependent on temperature. The proper fitting of theoretically calculated and experimentally measured spectral response characteristics was shown. Keywords: InAs/InAs1−xSbx type-II superlattice, T2SLs, Energy-band offset, Infrared detectors, LWIR

We report on of high operating temperature (HOT) long-wavelength infrared radiation (LWIR) type-II superlattices (T2SLs) InAs/InAsSb photoconductor grown on buffered semi-insulating GaAs substrate. The absorber of the device consists of a 300 periods of T2SLs InAs/InAsSb. The high resolution X-ray diffraction (HRXRD), photoluminescence (PL), Hall, Optical Parametric Oscillator (OPO) measurements are presented and analyzed. The detector reached a 50% cut-off wavelength of 9.8 μm and 10.4 μm at 210 K and 230 K, respectively. The time constant of 24 ns is observed at 210 K under 0.5 V bias. The detectivity (D∗) of ∼ 2.0 × 1010 cm Hz1/2/W at 210 K is reached.

This study reports on photoelectrical performance of the long-wave infrared HgCdTe high operating temperature (200–230 K) detector for the fast response time operating under unbiased conditions. The detailed analysis of the response time as a function of device architecture was conducted pointing out optimal working conditions and structure. The response time of the long-wave (x Cd = 0.19) HgCdTe detector for temperatures reached by thermoelectrical cooling (200–230 K) was estimated at the level of τ s

We report on the molecular beam epitaxial growth of 13ML InAs/5ML AlSb type-II superlattice on highly lattice mismatched GaAs (0 0 1) substrate. The high lattice mismatch of 7.35% between the superlattice and GaAs substrate is reduced by the growth of GaSb buffer layer based on interfacial misfit array technique. This latter consists on the instantaneous relieving of the strain at the GaAs/GaSb interface by the formation of a periodic array of 90° misfit dislocations. On the other hand, the strain in the InAs/AlSb superlattice has been compensated by a special shutters sequence that leads to the formation of both AlAs-like and InSb-like interfaces. In addition, the grown superlattice has been deposited at relatively low growth temperature compared to that reported in the literature. Under optimized growth conditions, the InAs/AlSb superlattice exhibit good structural, and optical properties.

The paper reports on a long-wave infrared (cut-off wavelength ~ 9 μm) HgCdTe detector operating under nbiased condition and room temperature (300 K) for both short response time and high detectivity operation. The ptimal structure in terms of the response time and detectivity versus device architecture was shown. The response time of the long-wave (active layer Cd composition, xCd= 0.19) HgCdTe detector for 300 K was calculated at a level of τs~ 1 ns for zero bias condition, while the detectivity − at a level of D* ~ 109cmHz1/2/W assuming immersion. It was presented that parameters of the active layer and P+barrier layer play a critical role in order to reach τs≤ 1 ns. An extra series resistance related to the processing (RS+in a range 5−10 Ω) increased the response time more than two times (τs ~ 2.3 ns).

In this paper, we present experimental results of photoluminescence for series of InAs:Si heavily doped samples, with doping level varying from 1.6 × 1016 cm-3 to 2.93 × 1018 cm-3. All samples were grown using MBE system equipped with a valved arsenic cracker. The measurements were performed in the temperature range of 20 K to 100 K. Although the Mott transition in InAs appears for electron concentrations above 1014 cm-3, Burstein-Moss broadening of photoluminescence spectra presented in this article was observed only for samples with concentration higher than 2 × 1017 cm-3. For the samples with lower concentrations two peaks were observed, arising from the band gap and defect states. The intensity of the defect peak was found to be decreasing with increasing temperature as well as increasing concentration, up to the point of disappearance when the Burstein-Moss broadening was visible.

In this work we report simulation and experimental results for an MWIR HgCdTe photodetector designed by computer simulation and fabricated in a joint laboratory run by VIGO Sytems S.A. and Military University of Technology. The device is based on a modified N+pP+heterostructure grown on 2”., epiready, semi-insulating (100) GaAs substrates in a horizontal MOCVD AIX 200 reactor.The devices were examined by measurements of spectral and time responses as a function of a bias voltage and operating temperatures. The time response was measured with anOptical Parametric Oscillator(OPO) as the source of ~25 ps pulses of infrared radiation, tuneable in a 1.55–16 μm spectral range. Two-stage Peltier cooled devices (230 K) with a 4.1 μm cut-off wavelength were characterized by 1.6 × 1012cm Hz1/2/W peak detectivity and < 1 ns time constant forV> 500 mV.

Fast response is an important property of infrared detectors for many applications. Currently, high-temperature long-wavelength infrared HgCdTe heterostructure photodiodes exhibit subnanosecond time constants while operating under reverse bias. However, nonequilibrium devices exhibit excessive low-frequency 1/f noise that extends up to MHz range, representing a severe obstacle to their widespread application. Present efforts are focused on zero-bias operation of photodiodes. Unfortunately, the time constant of unbiased photodiodes is still at the level of several nanoseconds. We present herein a theoretical investigation of device design for improved response time and detectivity of long-wavelength infrared HgCdTe photodiodes operating at 230 K in zero-bias mode. The calculation results show that highly doped p-type HgCdTe is the absorber material of choice for fast photodiodes due to its high electron diffusion coefficient. The detectivity of such a device can also be optimized by using absorber doping of N A = 1 × 1017 cm−3. Reduction of the thickness is yet another approach to improve the device response. Time constant below 1 ns is achieved for an unbiased photodiode with absorber thickness below 4 μm. A tradeoff between the contradictory requirements of achieving high detectivity and fast response time is expected in an optically immersed photodiode with very small active area.

This paper reports on N+- $\pi $ interface influence on the performance: dark current and response time of multilayer long-wave infrared mercury cadmium telluride (MCT - HgCdTe) N+ $\pi$ P+p+n+ detector under high operating temperature conditions, $T = 230$ K. A detailed analysis of the dark current and response time as a function of device architecture, i.e., N+- $\pi $ interface and applied bias was performed to optimize the working conditions. Dark current of the long-wave HgCdTe detector with 50% cutoff wavelength of $\lambda _{\mathbf {\textit {co}}} \approx 10.6~\mu \text{m}$ at $T = 230$ K was reduced from $\approx ~62$ to 12 A/cm $^{\mathrm {\mathbf {2}}}$ , while response time could be reduced to $\approx ~52$ ps for low-voltage operation, $V = 200$ mV.

InAsSb epilayers grown on GaAs substrates by molecular beam epitaxy have been studied using X-ray diffraction and Raman scattering. X-ray diffraction was used to determine the mole fraction of presented samples. In Raman spectrum, we analyzed for each sample not only the position of LO InAs and InSb phonons but also intensities of those. We found correlation between intensities ratio of LO phonons and the mole fraction of measured samples and we proposed a method how to calculate InAsSb mole fraction only using Raman spectroscopy.

Three different types of samples of InP nanowires, i.e. undoped, doped with Si and doped with Te, were grown and measured using SEM and Raman spectroscopy. Scanning Electron Microscope (SEM) images showed differences in the length, homogeneity and curvature of the nanowires. The most homogenous wires, grown most perpendicular to the surface, were those Si doped. They were also the shortest. Raman spectroscopy showed that the nanowires doped with Si had the lowest Full Width at Half Maximum (FWHM) TO band, which suggests the highest crystal quality of these wires. For the wires doped with Te, which were the most inhomogeneous, a low energy acoustic band was also observed, which suggests the lowest crystal quality of these structures.

The highly Be-doped InAs layer has been grown on semi-insulating GaAs (100) substrate by Molecular Beam Epitaxy. Very good quality of the layer has been attested by high resolution scanning electron microscope (HR-SEM), X-ray diffraction (XRD) and the Raman spectra. The parallel and perpendicular residual strain are determined to be – 1.17 × 10-3 , and 1.12 × 10-3 , respectively. Moreover, the absorbance (ABS) and photoluminescence (PL) spectra were collected in order to estimate the bandgap narrowing. The 10 meV bandgap shrinking for 1.7×1018 cm-3 acceptor concentration suggests necessity of reexamining the Jain et al model [Jain, S. C., et al. - JAP 68(7): 3747-3749] in the context of actual values of InAs valence-bands effective-masses.

In this work we present photoluminescence measurements of InAs 0.916Sb0.084 bulk material grown in MBE VIGO/MUT laboratory [1–2]. Photoluminescence spectra showed the occurrence of two peaks in temperature range 20K-40K, one of them comes from the band-to-band transition while the another is the result of exciton transition [3].

The initial stage of the growth of graphene on SiC with the underlying mechanism of carbon layer early stage formation on the single crystal silicon carbide surface was studied using silicon sublimation technique. The obtained buffer layer is organized in a form of carbon regions with 10% of sp 3 defects separated 10–15 A. Raman spectroscopy was used to assess the degree of the buffer layer’s disorder. The intensity of I ( D ) and I ( G B ) buffer peaks was found to be proportional to the number of defects. Although the layer is not fully saturated with carbon atoms, it remains impenetrable. However, sublimation from the steps side walls which are not covered by the buffer layer is possible. It was observed that in the vicinity of the macro-step edges the sublimation is more effective, which leads to the production of additional free C atoms, filling the buffer layer structure, subsequently decreasing sp 3 hybridization, to about 1–2%. This healing process which also continues during the graphene layer growth is reflected in a decrease in D line intensity and finally in formation of the well-organized buffer layer.

In this work we compare two InAs/GaSb superlattice samples grown in MBE VIGO/MUT laboratory on 2 inch (001) GaAs substrate, using MBE technique. Both samples have the same architecture, however their growth processes were conducted at different temperatures. For sample A the growth temperature was equal 668 K (395°C), for sample B 588 K (315°C). Photoluminescence measurements were performed at 30 K. For sample A there is no photoluminescence signal, while spectrum for sample B consists of two peaks: bandgap peak at 0.5 eV and deep state peak at 0.25 eV. X-ray diffraction (XRD) measurements indicate that sample A has better crystallographic quality than sample B. Raman spectra consists of low energy peaks (20-100 cm-1) which confirm the existence of superlattice for both samples [4]. Additionally, for sample A there are peaks related to Sb precipitates. It suggests that except the InAs/GaSb superlattice there is an additional Sb layer which may disturb band structure of superlattice and cause the disappearance of photoluminescence for sample A.

In this work we compare two superlattices: InAs/GaSb (sample A) and InAs/InAsSb (sample B). Both samples were grown in MBE VIGO/ MUT laboratory on 2 inch (001) GaAs substrate using MBE technique. We characterized quality and thickness of the samples using three methods: photoluminescence, X-ray diffraction (XRD) and Raman scattering. Period of superlattice layers was obtained using Raman scattering and XRD measurements. For sample A it was equal 5.3 nm and 4.76 nm for InAs and GaSb layers respectively, for sample B 8.3 nm and 9.4 nm. Photoluminescence spectrum for sample A exhibits two peaks: band gap peak at 0.5 eV and deep state peak at 0.25 eV. Spectrum for sample B consists of one band gap peak at 0.17 eV.

3C-SiC epitaxial layers were grown on on-axis (100)- and (110)-oriented silicon substrates in a RF heated horizontal hot-wall CVD reactor. We focused on the relationship between the pressure used during carbonization growth and the crystalline quality of obtained layers. A comprehensive analysis of the surface morphology of the 3C-SiC carbonization layer was performed, after which two pressure values guaranteeing top crystalline quality were determined. The carbonization layer with the lowest surface roughness was obtained at around 20 mbar, while for 30 mbar, the surface roughness was slightly worse but more homogenous. To sum up, we successfully grew 1.5 µm thick 3C-SiC layers on silicon substrates with both orientations using the said pressure conditions.

The InAs1−xSbx ternary alloy band gap nonlinearly depends on the composition, which provides the opportunity for use of this material in devices operating in a wide range of infrared radiation. We present experimental results for InAs1−xSbx samples for Sb composition from 0.1 to 0.8. The most common way to determine it is by using a high resolution X-ray diffractometer. In a previous article, we showed that energies of folded longitudinal acoustic and folded transverse acoustic Raman peaks are linearly correlated with mole fraction. In this work, we will illustrate how to determine mole fraction using peak energy and calculate the bowing parameter for InAs1−xSbx at 300 K.

The InAs1–xSbx ternary alloy band gap nonlinearly depends on the composition, which gives the opportunity to use this material in devices operating in a wide range of infrared radiation. We present experimental results for InAs1–xSbx samples for Sb composition from 0.1 to 0.8. Raman spectrum of InAs and InSb in the range of 20 - 250 cm-1 exhibits two main peaks: LO peak and 2TA peak. On the other hand in the same range for InAsSb in Raman spectra two additional peaks (round 44 cm-1 and round 130 cm-1) appear, while they are not existing neither in pure InAs nor in InSb. Energies of 44 cm-1 and 130 cm-1 are close to phonon energy for momentum equals K and L for InAs. This type of peaks are usually called DALA and DATA and it is commonly accepted that they come from disorder in crystal field. We propose other explanation-existence of zone folding in phonon dispersion curves as a result of CuPt unit cell of InAsSb being twice larger than typical ZB unit cell.

X-ray diffraction, Raman spectroscopy and Optical absorption estimates of the thickness of graphene multi layer stacks (number of graphene layers) are presented for three different growth techniques. The objective of this work was focused on comparison and reconciliation of the two already widely used methods for thickness estimates (Raman and Absorption) with the calibration of the X-ray method as far as Scherer constant K is concerned and X-ray based Wagner-Aqua extrapolation method.

Features associated with short and prolonged growth time in the chemical vapor deposition process of growth of graphene stacks on SiC (0001) substrate are reported. In particular general bimodal (as far as d002 interlayer spacing is concerned) distribution of graphene species across the surface of the samples is observed. It consists of thin few layer graphene coverage of most of the sample surface accompanied by thick graphite-like island distribution. It points to the two independent channels of graphene stacks growth with two characteristic growth rates.

The results of optical investigation of hydrogenated epitaxial bilayer graphene are presented. A softening and an increase of the intensity of the in-plane anti-symmetric phonon mode are observed at 0.2eV. It is suggested that they both originate from coupling of the optically active phonon mode to virtual electronic transitions, which is related to the band structure of bilayer graphene and leads to the “charged phonon” effect. In addition, it is noted that optically active phonon peaks have pronounced Fano shapelines. It is argued that the Fano shapeline is attributed to the interaction of the phonon mode with a continuum of electronic transitions in valence bands of hydrogenated bilayer graphene.

It is fully confirmed that the development of the new HgCdTe long-wave (8−12 μm) infrared radiation (LWIR) detector has been driven by applications requiring high frequency response ( τ s) and operation at higher temperature (HOT - hot operating temperature, T > 200 K) [1]. Not only time response but also detectivity directly linked with τ s of the HOT HgCdTe detector should be optimized. The HOT HgCdTe’s performance is limited by Auger processes and to circumvent that issue the N+/π/P+n+ device’s designs has been proposed to suppress generation - recombination (GR) processes, i.e. combination of exclusion and extraction heterojunctions (π is a p-type doping region) [2−5]. The nominally sharp interfaces in N+/π/P+n+ (especially N+π) layered HgCdTe heterostructures are affected by interdiffusion during technological process leading to significant composition and doping grading occurring during HgCdTe growth by MOCVD [6,7]. Mentioned composition and doping grading should be controlled to optimize frequency performance of the devices. In this paper we present short analysis of the time response depending on type and doping grading of N+/π/P+n+ HOT LWIR HgCdTe structure. Time response of the long-wave HgCdTe detector with 50% cut-off wavelength ( λ c), ≈ 10.6 μm at T = 230 K, V = 200 mV was reduced form ~ 160 ps to ~ 52 ps by optimization of the N+π interface even for low voltage operation ~ 200 mV.

Except Auger 7, p-type HgCdTe is mostly limited by technology dependent Shockley-Read-Hall generation-recombination processes. One of the ways to reduce the trap density is a growth of the (100) HgCdTe on GaAs substrates. That orientation allows reaching lower carrier concentration ∼ 5×1014 cm−3 in comparison to the commonly used (111) orientation ∼ 5×1015 cm−3 in mid-wave infrared range. In addition Shockley-Read-Hall traps density could be reduced to the level of ∼ 4.4×108 cm−3. The theoretical simulations related to the utmost performance of the (100) HgCdTe Auger suppressed structures are presented. Dark current is reported to be reduced by more than one order of magnitude within the range ∼ 6×10−2−3×10−3 A/cm2. Detectivity increases within range ∼ 3−12×1011 cmHz1/2/W (wavelength ∼ 5 μm) at temperature 200 K and voltage 200 mV.

Erbium-doped fiber laser passively mode-locked by bilayer graphene is presented. The graphene layers were grown by chemical vapor deposition (CVD) on Cu substrate and transferred onto a fused silica window, forming a saturable absorber (SA). Low non-saturable losses and modulation depth as high as 55% allowed to achieve soliton pulses with over 11 nm bandwidth and nearly-transform limited 315 fs duration at 1564 nm center wavelength. The paper describes the design of the laser construction, as well as the graphene-SA preparation process. Our study demonstrates, that CVD-Cu graphene transferred on glass substrate may be used for efficient mode-locking of fiber lasers.

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ∼1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.

Polytype instability of SiC epitaxial films was the main focus of attention in the experiment performed since this factor has a decisive influence on graphene growth, which was the second stage of the experiment. Layers deposited in various initial C/Si ratios were analyzed. Our observations indicate that the initial C/Si ratio in epitaxial growth is a crucial parameter determining which polytype will be grown, in particular for cubic (3C) or hexagonal (4H) polytypes. If the initial C/Si ratio was close to its final value, the dominant polytype was 4H. On the other hand, when the initial C/Si ratio was close to zero, 3C became the major polytype in spite of a non favourable growth temperature. The results for graphene growth on an epi-SiC layer and a bulk substrate, in which case the dominant polytype was 4H, are also presented. These results indicate that layers on epitaxial 4H-SiC are thicker, more relaxed and have better quality in comparison with samples on 4H-SiC substrates. Morphology and defects in SiC epilayers were analyzed using Nomarsky optical microscopy, scanning electron microscopy (SEM) and high resolution X-ray diffraction (XRD). Graphene quality was characterized by Raman spectroscopy.

The paper provides a deeper understanding of key-parameters of epitaxial graphene growth techniques on SiC. At 16000C, the graphene layer is continuous and covers a large area of the substrate. Significant differences in the growth rate could be observed for different reactor pressures and the polarity of SiC substrates as well as for the substrate miscut and surface quality. In addition, graphene thickness uniformity and mechanism of ridges creation was examined.

The resistive switching processes are investigated at the nano-scale in graphene oxide. The modification of the material resistivity is driven by the electrical stimulation with the tip of atomic force microscope. The presence of water in the atmosphere surrounding graphene oxide is found to be a necessary condition for the occurrence of the switching effect. In consequence, the switching is related to an electrochemical reduction. Presented results suggest that by changing the humidity level the in-plane resolution of data storage process can be controlled. These findings are essential when discussing the concept of graphene based resistive random access memories. (C) 2015 AIP Publishing LLC.

Graphene grown by a sublimation technique was studied by Scanning Electron Microscopy (SEM) and micro-Raman spectroscopy. The measurement area of a sample was marked and investigated using both systems, as a result of which SEM images were directly compared with Raman maps. In this work we show that a correlative analysis of Energy Selective Backscattered electrons detector (EsB), In-Lens figures and Raman maps of shape and intensity of the 2D band is adequate to determine graphene layer thickness with the precision of SEM and reliability of Raman spectroscopy.

Raman piezospectroscopy of high quality 6H-SiC crystals is presented. The crystals used in experiments were grown by the seeded physical vapor transport method. Uniaxial stress up to 0.9 MPa, obtained using a spring apparatus, was applied along [11–20] and [10–10] directions. It was found that the application of uniaxial stress led to different energy shifts of the observed phonon excitations in the investigated 6H-SiC crystals. The obtained pressure coefficients vary in the range 0.98–5.5 cm−1 GPa−1 for different transverse optical phonon modes. For longitudinal optic phonon modes pressure coefficients in the range 1.6–3.6 cm−1 GPa−1 were found. The data obtained could be useful in evaluation of local strain fields in SiC based structures and devices including epitaxial graphene.

The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps similar to 10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be similar to 190 Omega mu m for step height h(S) = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications. (C) 2014 AIP Publishing LLC.

Optical absorption and Raman scattering studies of epitaxial graphene structures obtained by annealing of carbon terminated face of $4H\text{-SiC}(000\text{\ensuremath{-}}1)$ on-axis substrates using standard chemical-vapor deposition reactor are presented. Two series of samples grown at different argon pressures in the reactor and different annealing times were studied. Optical absorption and Raman scattering were used to determine the number of graphene layers formed on the substrate surface. The observed dependence of the number of graphene layers formed on annealing time and argon pressure strongly indicates that the growth kinetics of graphene is limited by Si evaporation and two-dimensional Si diffusion.

Raman Studies of Defects in Graphene Grown on SiC K. Grodecki, R. Bozek, J. Borysiuk, W. Strupinski, A. Wysmolek, R. Stepniewski and J.M. Baranowski Institute of Electronic Material Technology, Wolczynska 133, 01-919 Warsaw, Poland Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Hoza 69, 00-681 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32/46, 02-668 Warsaw, Poland The Raman scattering studies of multi-layer graphene obtained by high temperature annealing of carbon terminated face of 4H-SiC(000-1) substrates are presented. Intensity ratio of the D and G bands was used to estimate the average size of the graphene flakes constituting carbon structures. The obtained estimates were compared with flake sizes from atomic force microscopy data. We found that even the smallest structures observed by atomic force microscopy images are much bigger than the estimates obtained from the Raman scattering data. The obtained results are discussed in terms of different average flake sizes inside and on the surface of the multi-layer graphene structure, as well as different type of defects which would be present in the investigated structures apart from edge defects.

In the present work, we demonstrated a new method of graphene films transferring applying a marker-frame instead of polymer-like films as a support. This method is an alternative that is much faster, cheaper and freely available to all. Our technique guarantees cleaner graphene surface and no polymer residues on it. Consequently, it significantly increases the applicability of graphene. For instance, one can think of using graphene as a component of nanocomposites or coupling it with other nanomaterials. As an example we showed graphene films suspended on GaN nanowires. The characterization of the properties of graphene transferred following the enhanced frame method was performed by Raman spectroscopy, as well as by carrying out SEM imaging and conducting TEM investigations.

In this Letter, we demonstrate a mode-locked Er-doped fiber laser incorporating antimony telluride (Sb2Te3) topological insulator (TI) as a saturable absorber (SA). The laser was capable of generating 270 fs-short soliton pulses at 1560 nm wavelength, which are the shortest solitons generated with a TI-based saturable absorber so far. In order to form a saturable absorber, a bulk piece of Sb2Te3 was deposited on a side-polished single-mode fiber with the presence of a low refractive index polymer. Such saturable absorber exhibits modulation depth at the level of 6% with less than 3 dB of non-saturable losses. Our study shows that TI-based saturable absorbers with evanescent field interaction might compete with SAs based on carbon nanomaterials, like graphene or nanotubes. Additionally, thanks to the interaction with the evanescent field, the material is not exposed to high optical power, which allows to avoid optical or thermal damage.

Graphene grown by chemical vapor deposition on 4H-SiC (0001) was studied using micro-Raman spectroscopy and atomic force microscopy (AFM). AFM revealed that the graphene structure grown on on-axis substrates has a stepped morphology. This is due to step bunching, which results from etching in hydrogen as well as from the process of graphene formation itself. It was shown by micro-Raman spectroscopy that the properties of graphene present on step edges and on terraces are quite different. Graphene on terraces is uniform with a relatively small thickness and strain fluctuations. On the other hand, graphene on step edges has a large thickness and strain variations occur. A careful analysis of micro-Raman spatial maps led us to the conclusion that the carrier concentration on step edge regions is lowered when compared with terrace regions.

The study of epitaxial graphene layers grown on SiC by two techniques, namely, the traditional Si sublimation method and the recent chemical vapor deposition (CVD) using temperature induced shift of the Raman 2D line, is presented. The measurements of thermal shift rate of 2D line on 4 H-SiC(0001) allowed us to determine notable differences in interaction of graphene with SiC substrate. The obtained results show that graphene layers grown by Si sublimation of 4 H-SiC(0001) are pinned strongly to the substrate. In contrast, the layers of graphene grown on 4 H-SiC(0001) substrates by CVD showed much weaker pinning. It was found that the film consisting of two or three graphene layers grown by CVD was already unpinned and thus showing Raman shift expected for freestanding graphene. The obtained differences in pinning of epitaxial graphene layers are explained in terms of basic growth mechanism differences between these two methods: graphene growth by Si sublimation is a “bottom-up” process and by CVD—a “top-...

Transmission electron microscopy (TEM) investigations of epitaxial graphene, grown on on-axis and 8° off-axis C-terminated 4H–SiC (0001¯) surfaces are presented. The TEM results provide evidence that the first carbon layer is separated by 3.2 A from the C-terminated SiC surface. It was also found that thick graphene layers grown on on-axis SiC (0001¯) are loosely bound to the SiC substrate. Moreover, the structural observations reveal a certain degree of disorder between the graphene planes, which manifests itself in a rotation of the layers and in an increase in the interplanar spacing between certain carbon layers from 3.35 A, which is characteristic for graphite, up to 3.7 A. Graphene grown on 8° off-axis SiC (0001¯) substrates covers the steps of SiC and as a result disorder seems to be not as pronounced as it is on the on-axis SiC (0001¯) substrate.

Transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and micro-Raman investigations of epitaxial graphene on 4H-SiC on-axis and 4° off-axis are presented. The STM images show that there is superimposed on 1×1 graphene pattern the carbon nanomesh of honeycomb 6×6 structure with the lattice vector of 17.5 A. The TEM results give evidence that the first carbon layer is separated by 2 A from the Si-terminated SiC surface and that subsequent carbon layers are spaced by 3.3 A. It is also found in TEM that the graphene layers cover atomic steps, present on 4° off-axis SiC(0001) surface, indicating a carpetlike growth mode. However, a bending of graphene planes on atomic steps of SiC apparently leads to generation of stress which leads to creation of edge dislocations in the graphene layers.

The temperature sensitivity of the double-barrier separate confinement heterostructure (DBSCH) laser diodes (LDs), intended for high-power, low vertical beam divergence emission, was shown to be somewhat lower than that of the large optical cavity (LOC) devices of similar beam divergences. For the lowest beam divergences, it still remains considerably high, resulting in low characteristic temperatures T0 not exceeding 80 K in the LDs of the vertical beam divergences below 15°. In this work, the decrease in the T0 values of such tensile-strained GaAsP/(AlGa)As DBSCH LDs has been found primarily due to an increased, thermally activated occupation of the heavy-hole subband and, to a minor degree, occupation of the second quantum level, both giving rise to transitions that do not contribute to the optical gain. This is caused by the higher and more temperature-dependent at-threshold band filling for devices of reduced confinement factor (Γ) aimed at achieving low beam divergence. This phenomenon seems to be c...

HgCdTe detectors designed to detect mid-wavelength (3–5 μm) infrared radiation must be cooled to reach the required performance. The cooling requirement makes the sensor system both expensive and bulky and the fundamental goal is to reach higher operating temperature condition preserving near background limited performance with high detectivity and high speed response at the same time. In order to reach higher operating temperature condition the thermal generation rate must to be suppressed under the photon generation rate. Except Auger 7 generation-recombination process, p-type HgCdTe is mostly limited by technology dependent Shockley-Read-Hall generation-recombination mechanism. One of the ways to reduce the trap density is a growth of the (100) HgCdTe on GaAs substrates. That orientation allows reaching lower carrier concentration ~5 × 1014 cm−3 in comparison to the commonly used (111) orientation ~5 × 1015 cm−3 in mid-wavelength infrared range. In addition, it was presented that Shockley-Read-Hall traps density could be reduced to the level of ~4.4 × 108 cm−3. The theoretical simulations related to the utmost performance of the (100) HgCdTe Auger suppressed structures are presented. Dark current is reported to be reduced by more than one order of magnitude within the range ~6 × 10−2–3 × 10−3 A/cm2. Detectivity increases within range ~3–12 × 1011 cm Hz1/2/W (wavelength ~5 μm) at temperature 200 K and voltage 200 mV.