Ge Plasma Research Articles

Analysis of the extreme ultraviolet broadband spectral characteristics and spatiotemporal evolution of laser-produced germanium plasmas

This paper presents comprehensive spectral measurements and a theoretical analysis of laser-produced germanium (Ge) plasma within the 7.5–17 nm wavelength range, with the measurements being performed using spatiotemporally-resolved emission spectroscopy. We obtained and analyzed the extreme ultraviolet (EUV) spectra and their variations meticulously at various positions adjacent to the target surface. The 3p and 3d excitations of Ge ions spanning from Ge4+ to Ge13+ were calculated using the Cowan configuration interaction codes, with particular emphasis being placed on analysis of the spectral line distributions in the 3d-np, 3d-nf, and 3p-3d, 4s, 4d transition arrays. This analysis indicated the contributions made by these transition arrays to the broadband spectral features that were observed experimentally. By using a steady-state collisional radiation model along with a normalized Boltzmann distribution assumption, we clarified the predominant contributions of the 3d-4p and 3p-3d transitions from the Ge7+ to Ge9+ ions in close proximity to the target surface, along with those from the Ge5+ and Ge6+ ions at greater distances. Furthermore, by combining the results above with a plasma dynamics evolution analysis, we gained insights into how the plasma's transient and nonuniform properties influence the analysis of complex EUV band spectral profiles and examine their implications for spectral diagnostics. These results will be useful for plasma spectral diagnosis and for application to development of pulsed short-wavelength light sources.

Nonsequential two-photon absorption in solid Ge irradiated by an intense x-ray free-electron-laser pulse

We theoretically investigate the formation of highly charged ions in a germanium (Ge) solid driven by intense, ultrashort x-ray pulses and its effect on the cross sections for nonsequential two-photon absorption from the $K$ shell. Our investigation is related to an experiment conducted at the Linac Coherent Light Source, in which $K\ensuremath{\alpha}$ fluorescence was measured to identify nonsequential two-photon ionization. When a solid Ge target is irradiated by an intense x-ray free-electron-laser (XFEL) pulse, it undergoes severe ionization and turns into a plasma state. We employ a Monte Carlo molecular-dynamics approach to simulate the time evolution of Ge plasma formation, and the time-dependent configuration-interaction-singles method for cross-section calculations, taking into account various experimental x-ray beam parameters and Ge charge states created during the plasma formation dynamics. We find that under the given experimental condition at a photon energy of 7200 eV, charged ions are formed quickly (the average charge is $\ensuremath{\approx}+6$ at the peak of the pulse and $\ensuremath{\approx}+10$ at the end of the pulse). The cross sections of Ge for nonsequential two-photon absorption, however, turn out to be insensitive to different charge states, and the average value over all computed data is $(2.61\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}59}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{4}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. Our paper proposes a theoretical framework of photoabsorption cross-section calculations under the influence of plasma formation, when a solid target is employed in XFEL experiments.

The Irradiance-Based Growth of Surface Structures Induced by Nanosecond Laser Pulses on Si and Ge and Their Correlation With Plasma Ion Kinetic Energies and Densities

Abstract The laser irradiance-based surface structural growth on Si and Ge has been correlated first time with plasma parameters. The better control over plasma parameters makes manufacturing of various sized and shaped surface structures on the semiconducting materials. The effect of laser irradiances on surface morphology of Si and Ge has been explored. For this purpose, Nd: YAG laser (532 nm, 6 ns, 10 Hz) has been employed as an irradiation source at the various laser irradiances ranging from 4 to 7.1 GW/cm2 under the vacuum condition. Surface modifications of laser-ablated Si and Ge were analyzed by performing scanning electron microscope (SEM) analysis. It has been revealed that laser irradiance plays a significant role in the growth of the micro- and nanostructures on the laser-irradiated target surfaces. The surface morphology of laser-ablated Si and Ge exhibited the formation of various structures such as laser-induced periodic surface structures (LIPSS), cracks, spikes, ridges, and cones. Density and size of these structures have been found to be strongly dependent upon the laser irradiances. SEM analysis exhibits the cones formation at central ablated region of both Si and Ge. These cones become more distinct and pronounced with increasing the laser irradiance due to more energy deposition with Gaussian profile distribution at the central region. Microspikes were observed at boundaries of laser-ablated Si. Whereas, in case of Ge-ablated boundaries, wave-like ridges have been observed, which are then converted into globules at higher laser irradiances up to 7 GWcm−2. LIPSSs were seen at outer boundaries of laser-ablated Ge, whose periodicity varies with the laser irradiances. Faraday cup has been employed in order to probe the kinetic energy and density of laser-induced Si and Ge plasma ions at the similar values of laser irradiances. A correlation at similar values of laser irradiances has been established between the evaluated plasma ion parameters (kinetic energy and density of plasma ions) and observed structures for both materials. This correlation reveals the dependence of kinetic energy and density of plasma ions on the corresponding surface modification of both laser-ablated Si and Ge, as well as enables us for the better understanding of the laser-induced plasma to be used as ion source in various fields ion implantation, surface structuring, and material modification. The results of ion energies are explained by the generation of ambipolar field or self-generated electric field (SGEF) in the expanding plasma due to the charge separation and double-layer structure. The values of SGEF have also been evaluated at different laser irradiances.

A hybrid method for investigating the detailed emission spectra of mid-Z plasmas in non-local thermodynamic equilibrium: Ge as an example

The detailed radiative properties of plasmas in non-local thermodynamic equilibrium (NLTE) are important for determining experimental plasma states. However, a complete detailed-level-accounting approach calculation is impractical for mid- and high-Z elements. Herein, we propose a hybrid method for obtaining the detailed radiative properties of mid-Z NLTE plasmas. First, a large-scale rate equation within the framework of a detailed-configuration-accounting method is established using atomic data in a configuration–configuration formalism. Second, we assume that the population distributions in fine-structure levels belonging to a particular configuration are in equilibrium. Thus, the populations at fine-structure levels are obtained through the populations in the corresponding configurations. Finally, detailed radiative properties are calculated using the populations in fine-structure levels and radiative data in level–level formalism. Such a method can balance computation costs and accuracy. Examples utilizing Ge plasmas demonstrate that the proposed method can better predict detailed structures in emission spectra than the detailed-configuration-accounting method.

Investigation of Energy and Density of Laser-Ablated Si and Ge Plasma Ions Along With Surface Modifications

The energy and density of laser-generated Si and Ge plasma ions have been investigated by using Faraday cup (FC). Nd: YAG laser (532 nm, 6 ns) at different irradiances ranging from 4 to 7.1 GWcm−2 has been employed as an irradiation source. FC measurements reveal two time-of-flight (TOF) ion signal peaks. From the observed TOF signals of ions, the kinetic energy and density of laser induced Si and Ge plasma ions have been evaluated. With increasing the laser irradiance from 4 to 7.1 GWcm−2, the Si ions density varies from $4.9\times 10^{20}$ cm−3 to $5.62\times 10^{20}$ cm−3, whereas for Ge ions, it varies from $11\times 10^{20}$ cm−3 to $12\times 10^{20}$ cm−3. Similarly, the evaluated values of Si ion energy range from 156 to 166 eV, whereas for Ge ion, energy varies from 108 to 124 eV. It was also revealed that both energy and density of plasma ions are strongly dependent on FC distance from the target surface as well as the angle of FC with respect to the target surface normal. By employing an electric probe, self-generated electric field (SGEF) of laser-induced plasmas of both Si and Ge has also been measured and it varies from 94 to 122 and 101 to 158 V/m, respectively, with increasing the laser irradiances from 4 to 6 GWcm−2. The observation of SGEF confirms the existence of double layer potential in laser-induced Si and Ge plasmas. Scanning electron microscope (SEM) analysis has been performed to establish a correlation between number density of the Si spikes and the periodicity of laser-induced periodic surface structure of Ge grown after ablation with evaluated corresponding ion density of Si and Ge plasmas. Surface structures grown on Si and Ge after laser ablation enhance their optical, electrical, and field emission properties to make them useful in solar cells, LED displays, electronic device, as well as for the fabrication of nanograting.

Super-transition-array calculations for synthetic spectra and opacity of high-density, high-temperature germanium plasmas

The synthetic emission spectra and opacity of high-density, high-temperature germanium (Z=32) plasma from super-transition-array (STA) calculations are presented. The viability of the STA model, which is based on a statistical superconfigurations accounting approach for calculating the atomic and radiative properties, is examined by comparing and contrasting its results against the available experimental data and other theoretical calculations. First, we focus on the emission data. To model the data, the Eulerian radiation-hydrodynamics code FastRad3D is used in conjunction with STA to obtain the STA-required inputs, namely, the time-dependent temperature and density profiles of the Ge plasmas. Consequently, we find that STA results fit the experimental spectrum reasonably well, reproducing the main spectral features of 2p−3d,2s−3p, and 2p−4d transitions from the laser-heated germanium layer buried in plastic [High Energy Density Phys., 6 (2010) 105]. However, careful comparison between experimental and theoretical results in the photon-energy regions of ~ 1.7 keV shows some degrees of disparity between the two. This may be due to the non-LTE effects and the presence of spatial gradients in the sample. Limitations of STA to model the experimental spectrum precisely is expected and underscoring the difficulty of the present attempts as the model assumed local thermodynamics equilibrium population dynamics. Second, we examine the STA calculated multi-frequency opacities for a broad range of Ge plasma conditions covering the L- and M-shell spectral range. Comparing with a hybrid LTE opacity code which combines the statistical super-transition-array and fine-structure methods [High Energy Density Phys., 7 (2011) 234], impressively good agreement is found between the two calculations. In addition, the sensitivity of the opacity results in various plasma temperatures and mass densities is discussed. The ionized population fraction and average ionization of the Ge plasma are also described. Comparisons of STA results in the observed spectrum and opacity are considerably close while offering the advantage of computational speed and its capability of treating hot and dense high-Z plasmas.

Influence of feeding thermally peroxidized soybean oil on growth performance, digestibility, and gut integrity in finishing pigs.

Consumption of peroxidized lipids has been shown to reduce pig performance and energy and lipid digestibility. Objectives of the current study were to evaluate the effect of feeding soybean oil (SO) with different levels of peroxidation on growth performance, lipid, N, and GE digestibility, plasma Trp, and gut integrity in finishing pigs. Fifty-six barrows (46.7 ± 5.1 kg initial BW) were randomly assigned to one of four diets in each of two dietary phases, containing either 10% fresh SO (22.5 °C) or thermally processed SO (45 °C for 288 h, 90 °C for 72 h, or 180 °C for 6 h), each infused with of 15 L/min of air. Peroxide values were 2.0, 17.4, 123.6, and 19.4 mEq/kg; 2,4-decadienal values were 2.07, 1.90, 912.15, and 915.49 mg/kg; and 4-hydroxynonenal concentrations were 0.66, 1.49, 170.48, and 82.80 mg/kg, for the 22.5, 45, 90, and 180 °C processed SO, respectively. Pigs were individually housed and fed ad libitum for 81 d to measure growth performance, including a metabolism period to collect urine and feces for determination of GE, lipid, N digestibility, and N retention. Following the last day of fecal and urine collection when pigs were in the metabolism crates, lactulose and mannitol were fed and subsequently measured in the urine to evaluate gut permeability, while markers of oxidative stress were evaluated in plasma, urine, and liver. There were no differences observed in ADFI (P = 0.91), but average daily gain (ADG) and gain:feed G:F were decreased in pigs fed 90 °C SO diet (P ≤ 0.07) compared to pigs fed the other SO diets. Pigs fed the 90 and 180 °C SO had the lowest (P = 0.05) DE as a % of GE compared to pigs fed the 22.5 °C SO, with pigs fed the 45 °C SO being intermediate. Lipid digestibility was similarly affected (P = 0.01) as energy digestibility, but ME as a % of DE was not affected by dietary treatment (P = 0.16). There were no effects of lipid peroxidation on N digested, N retained, or the urinary lactulose:mannitol ratio (P ≥ 0.25). Pigs fed the SO processed at 90 and 180 °1C had lower concentrations (P < 0.01) of plasma Trp compared to pigs fed the 22.5 and 45 °C SO treatments. Pigs fed 90 °C SO had the greatest (P < 0.01) concentrations of F2-isoprostane in plasma and urine thiobarbituric acid reactive substances compared to the other SO treatments. These results indicate that the change in FA composition and/or the presence of lipid peroxidation products in peroxidized SO may reduce ADG, G:F, and digestibility of GE and ether extract, but has little impact on N digestibility and balance or on gut permeability.

Temperature dependence of emission intensity in femtosecond laser-induced Ge plasma

The Ge(i) 422.66 nm line in femtosecond laser-induced germanium plasma at different initial sample temperatures is investigated.

Magnetic field effect on laser-induced breakdown spectroscopy and surface modifications of germanium at various fluences

AbstractThe effect of the transverse magnetic field on laser-induced breakdown spectroscopy and surface modifications of germanium (Ge) has been investigated at various fluences. Ge targets were exposed to Nd: YAG laser pulses (1064 nm, 10 ns, 1 Hz) at different fluences ranging from 3 to 25.6 J/cm2 to generate Ge plasma under argon environment at a pressure of 50 Torr. The magnetic field of strength 0.45 Tesla perpendicular to the direction of plasma expansion was employed by using two permanent magnets. The emission spectra of laser-induced Ge plasma was detected by the laser-induced breakdown spectroscopy system. The electron temperature and number density of Ge plasma are evaluated by using the Boltzmann plot and stark broadening methods, respectively. The variations in emission intensity, electron temperature (Te), and number density (ne) of Germanium plasma are explored at various fluences, with and without employment of the magnetic field. It is observed that the magnetic field is responsible for significant enhancement of both excitation temperature and number density at all fluences. It is revealed that an excitation temperature increases from Te,max,without B = 16,190 to Te,max,with B = 20,123 K. Similarly, the two times enhancement in the electron density is observed from ne,max,without B = 2 × 1018 to ne,max,with B = 4 × 1018 cm−3. The overall enhancement in Ge plasma parameters in the presence of the magnetic field is attributed to the Joule heating effect and adiabatic compression. With increasing fluence both plasma parameters increase and achieve their maxima at a fluence of 12.8 J/cm2 and then decrease. In order to correlate the plasma parameters with surface modification, scanning electron microscope analysis of irradiated Ge was performed. Droplets and cones are formed for both cases. However, the growth of ridges and distinctness of features is more pronounced in case of the absence of the magnetic field; whereas surface structures become more diffusive in the presence of the magnetic field.

Nanosecond pulsed laser ablation of Ge investigated by employing photoacoustic deflection technique and SEM analysis

Nanosecond pulsed laser ablation phenomena of single crystal Ge (100) has been investigated by employing photoacoustic deflection as well as SEM analysis techniques. Nd: YAG laser (1064nm, 10ns, 1–10Hz) at various laser fluences ranging from 0.2 to 11Jcm−2 is employed as pump beam to ablate Ge targets. In order to evaluate in-situe ablation threshold fluence of Ge by photoacoustic deflection technique, Continuous Wave (CW) He–Ne laser (632nm, power 10mW) is employed as a probe beam. It travels parallel to the target surface at a distance of 3mm and after passing through Ge plasma it causes deflection due to density gradient of acoustic waves. The deflected signal is detected by photodiode and is recorded by oscilloscope. The threshold fluence of Ge, the velocity of ablated species and the amplitude of the deflected signal are evaluated. The threshold fluence of Ge comes out to be 0.5Jcm−2 and is comparable with the analytical value. In order to compare the estimated value of threshold with ex-situe measurements, the quantitative analysis of laser irradiated Ge is performed by using SEM analysis. For this purpose Ge is exposed to single and multiple shots of 5, 10, 50 and 100 at various laser fluences ranging from 0.2 to 11Jcm−2. The threshold fluence for single and multiple shots as well as incubation coefficients are evaluated. It is observed that the value of incubation co-efficient decreases with increasing number of pulses and is therefore responsible for lowering the threshold fluence of Ge. SEM analysis also reveals the growth of various features such as porous structures, non-uniform ripples and blisters on the laser irradiated Ge. It is observed that both the fluence as well as number of laser shots plays a significant role for the growth of these structures.

Effect of sample temperature on laser-induced semiconductor plasma spectroscopy

We investigate the temperature dependence of the emission spectrum of a laser-induced semiconductor (Ge and Si) plasma. The change in spectral intensity with the sample temperature indicates the change of the laser ablation mass. The reflectivity of the target surface is reduced as the sample is heated, which leads to an increase in the laser energy coupled to the surface of the sample and eventually produces a higher spectral intensity. The spectral intensities are enhanced by a few times at high temperatures compared with the cases at low temperatures. The spectral intensity of Ge is enhanced by 1.5 times at 422.66 nm, and 3 times at 589.33 nm when the sample temperature increases from 50°C to 300°C. We can obtain the same emission intensity by a more powerful laser or by less pulse energy with a higher sample temperature. Based on experimental observations we conclude that the preheated sample can improve the emission intensity of laser-induced semiconductor plasma spectroscopy.

Comprehensive study and design of scaled metal/high-k/Ge gate stacks with ultrathin aluminum oxide interlayers

Advanced metal/high-k/Ge gate stacks with a sub-nm equivalent oxide thickness (EOT) and improved interface properties were demonstrated by controlling interface reactions using ultrathin aluminum oxide (AlOx) interlayers. A step-by-step in situ procedure by deposition of AlOx and hafnium oxide (HfOx) layers on Ge and subsequent plasma oxidation was conducted to fabricate Pt/HfO2/AlOx/GeOx/Ge stacked structures. Comprehensive study by means of physical and electrical characterizations revealed distinct impacts of AlOx interlayers, plasma oxidation, and metal electrodes serving as capping layers on EOT scaling, improved interface quality, and thermal stability of the stacks. Aggressive EOT scaling down to 0.56 nm and very low interface state density of 2.4 × 1011 cm−2eV−1 with a sub-nm EOT and sufficient thermal stability were achieved by systematic process optimization.

Single-pulse picking at kHz repetition rates using a Ge plasma switch at the free-electron laser FELBE.

We demonstrate a system for picking of mid-infrared and terahertz (THz) radiation pulses from the free-electron laser (FEL) FELBE operating at a repetition rate of 13 MHz. Single pulses are reflected by a dense electron-hole plasma in a Ge slab that is photoexcited by amplified near-infrared (NIR) laser systems operating at repetition rates of 1 kHz and 100 kHz, respectively. The peak intensity of picked pulses is up to 400 times larger than the peak intensity of residual pulses. The required NIR fluence for picking pulses at wavelengths in the range from 5 μm to 30 μm is discussed. In addition, we show that the reflectivity of the plasma decays on a time scale from 100 ps to 1 ns dependent on the wavelengths of the FEL and the NIR laser. The plasma switch enables experiments with the FEL that require high peak power but lower average power. Furthermore, the system is well suited to investigate processes with decay times in the μs to ms regime, i.e., much longer than the 77 ns long pulse repetition period of FELBE.

High-order harmonic generation during propagation of femtosecond pulses through the laser-produced plasmas of semiconductors

The generation of high-order harmonics of the ultrashort pulses of Ti:sapphire laser in the plasma plumes produced during laser ablation of various semiconductors (Te, Se, Si, As, Sb, and Ge) is reported. Application of two-color pump allowed the generation of enhanced odd and even harmonics. The resonance-induced enhancement of single harmonics was observed in the Ge, Se, Sb, As, and Te plasmas. The enhanced 35th harmonic obtained from selenium plasma was twelve times stronger than the neighboring harmonics. We also demonstrate the quasi-phase-matching of the harmonics generated in the As plasma.

Microplasma generated at solid–solid interface and its application to nanostructure formation

The dynamics of the microplasmas created at a transparent solid–solid interface were investigated extensively. Microplasmas were generated at an interface between a carbon (C) or a germanium (Ge) target and a SiO2 substrate by irradiating a KrF excimer laser beam onto the target, and the dynamics of the plasmas were investigated with the aid of optical emission spectroscopy. Although the emission spectra that contained the characteristic emission lines and the absorption bands originated from C and Ge species were observed from the C and Ge plasmas without the SiO2 substrate, identical spectra were obtained from both the plasmas created at the C–SiO2 and Ge–SiO2 interfaces. Furthermore, the target and the Si substrate surfaces were examined with a scanning electron microscope. The SiO2 substrate was smoothly etched and a nanostructure of a chain-like morphology was also observed in the deposits on the SiO2 substrate.

High resolution X-ray emission spectra from picosecond laser irradiated Ge targets

Investigations of a high resolution X-ray emission spectrum in the range 0.66–0.75 nm obtained by irradiating a Germanium target with high-power p-polarized, 40 picosecond laser radiation at 532 nm wavelength was done. Spectra in the wavelength region of 2l-4l′ and 2l-5l′ L-shell transitions in F-like, Ne-like and Na-like germanium ions were recorded using the FSSR-2D spectrometer equipped with a spherically bent quartz crystal with a spectral resolution λ/Δλ better than 5000. Spectral lines were compared with theoretical values obtained using the LANL plasma kinetic code ATOMIC. Fair agreement between experimental and theoretical spectral lines has been observed, which allowed to measure enough high bulk electron temperature values of 560 eV and electron density of ∼1021 cm−3 in Ge plasma irradiated by rather small commercial high repetition rate Nd:YAG laser system.

A generalized quasi-sum relations for oscillator strengths in transition arrays: theoretical study of the opacity of Ge plasmas

We present a simplified relativistic configuration interaction method (SRCI), by which all the energy levels and oscillator strengths in each transition array can be calculated. There exist generalized quasi-sum relations for the calculated oscillator strengths in the relevant transition arrays by the SRCI and by an unresolved transition array model (UTA). Based on UTA and the detailed configuration accounting (DCA), with a fully relativistic treatment incorporated with the quantum defect theory, the X-ray absorption spectra for any middle- and high- Z plasmas or composite plasmas can be calculated with much less computational efforts. The gross features of calculated spectra by DCA-UTA are in agreement with the relevant experimental measurements, except some detailed structures in some narrow spectral ranges. Such detailed structures can be calculated by SRCI incorporated with DCA-UTA. As an illustrative example, the absorption spectra of Ge plasmas are calculated by DCA-UTA-SRCI and are in agreement with the experimental opacity data. Therefore, the theoretical method (DCA-UTA-SRCI) verified by experimental measurements will not only be a basic tool to provide “precision” opacity data for the inertial confinement fusion research (ICF) and studies in stellar physics but also can be used to analyze the relevant diagnostic measurements for ICF plasmas.

Coherent phonon emission in the supersonic expansion of photoexcited electron-hole plasma in Ge

Coherent subnanosecond acoustic pulses generated by a laser-excited electron-hole plasma in a germanium single crystal are detected by a time-resolved pump-probe photodeflection technique. It is found that the front duration of these hypersound pulses is controlled by the time of plasma diffusion at supersonic velocities. The characteristic velocity of plasma diffusion evaluated from the experiments exceeds the longitudinal sound velocity in germanium by a factor of 1.5. The hypersound pulse shapes provide evidence for supersonic diffusion of the electron-hole plasma in a semiconductor at room temperature on a subnanosecond time scale.

Theoretical Simulation of the Transmission Spectra of Fe and Ge Plasmas

Using the detailed configuration accounting with the term structures treated by the unresolved transition array model, we present a method to calculate the transmission spectra of hot dense plasmas. Due to the fully relativistic treatment, incorporated with the quantum defect theory to calculate the atomic parameters of the huge number of configurations with high principal quantum number, we can study the opacities for any middle- and high-Z plasmas with much less computational efforts. We can also conveniently identify the dominant configurations and the detailed features of transition arrays, which are very helpful for the diagnostic of the plasma conditions. The accuracy of our theoretical model is tested by comparing the calculated transmission spectra of Fe and Ge plasmas with the bench mark experimental data.

Magnetic confinement of electron-hole plasma in Ge

Far-infrared magneto-absorption due to electron-hole plasma is measured in highly photo-excited pure Ge below 15 K. A low field anomaly is observed in thin samples under high excitation conditions. We assigned the anomaly to electron-hole plasma confined by the magnetic field. The drift velocity of the electron-hole plasma was estimated to be 7 × 10 3 cm/ s in zero magnetic field.