Formation Of Ge Nanocrystals Research Articles

Crystallization of Germanium-SiO2 composite films via nanosecond laser pulse irradiation

Germanium (Ge)-SiO2 composite films fabricated by using radio frequency magnetron co-sputtering were subjected to nanosecond (ns) laser pulse irradiation. The intense laser irradiation effects on as deposited composite films lead to the modification and the formation of Ge nanocrystals (NCs) embedded in SiO2. The as-deposited and laser treated samples have been characterized with X-ray diffraction (XRD) and micro-Raman spectroscopy to understand the formation of Ge NCs due to laser irradiation. The respective Ge NCs related peaks in XRD pattern as well as the Ge-Ge related optical phonon vibrational peaks in micro-Raman spectra confirm the formation of Ge NCs. Transmission electron microscopy images indicate the formation of Ge NCs after the laser irradiation. The effects of laser pulse energy and the irradiation time play a vital role in the crystallization of the as-deposited films. The changes in the Ge NCs size related shifts in the peak position of Ge optical phonons from the Raman spectra of the respective samples are explained with Ge-Ge optical phonon vibrational model. The observed photoluminescence emission in the green and red region of the visible spectrum from the laser treated samples are explained using the phenomenon of creation of Ge NCs in the Ge+SiO2 composite films and the oxygen defects created in SiO2 during the ns laser treatment.

Formation of crystalline Si1-xGex top layers by ion implantation in crystalline silicon

Semiconductor nanocrystals incorporated in a dielectric film are widely studied as potential candidates to exceed the Shockley-Queisser theoretical conversion limit of photovoltaic cells. In this context, Ge nanocrystals embedded in SiO2 films seem to be among the best candidates. However, the charges generated in the dielectric film are hard to collect. For this reason, it would be better if the charges were generated in a semiconductor matrix such as silicon, which has better conductivity. However, implanted Ge atoms have poor mobility in a silicon matrix and thus Ge nanocrystals formation is not likely. But even if the formation of Ge nanocrystals seems difficult, it would still be interesting to form crystalline Si1-xGex alloys. This work investigates the formation of the Si1-xGex films by ion implantation and their crystallinity. 36 keV Ge ions were implanted in crystalline Si substrates, with fluences ranging from 5 × 1015 to 1.5 × 1017 Ge/cm2 at temperatures up to 600 °C. Rutherford Backscattering Spectrometry (RBS), Raman Spectroscopy, and Transmission Electron Microscopy (TEM) were used to investigate the microstructure of the Si1-xGex alloys. It is shown that germanium is mostly incorporated in the crystal network in substitutional sites. XRD, Raman spectroscopy, and TEM confirm that the Si1-xGex layer on top of the c-Si substrate is monocrystalline. TEM also indicates the possible presence of nanostructures, extended defects or both. Implantation was also carried out at temperatures up to 600 °C, with the objective of preserving the crystallinity and promoting Ge diffusion into nanoclusters. RBS shows that the Ge profile is more extended in depth for the sample implanted at 600 °C, compared to a room temperature implantation. As the energy of the ions is the same in both samples, this indicates that Ge is able to diffuse in depth during the implantation at 600 °C compared to implantation at ambient temperature. However, RBS/C shows that the minimal yield is higher for the implantation at 600 °C, indicating a high concentration of interstitials or that crystallinity is deteriorated, as confirmed by TEM.

Segregation-induced formation of Ge nanocrystals in silicon oxide

The investigation of initial stage of Si1 – xGex alloy deposition and clarification of Ge nanocrystal formation mechanism has been carried out. It was found that at the initial stages of growing layers of Si1 – xGex alloys, the density of island nuclei Si1 – xGex increases by a factor of 2.5–3.4 compared to the density of polycrystalline silicon islands (from 1.07 ⋅ 1011 to 1.90 ⋅ 1011 cm–2 and from 3.1 ⋅ 1010 to 4.3 ⋅ 1010 cm–2 respectively). A decrease in the thickness of the layer corresponding to the end of the induction period and the formation of a continuous Si1 – xGex layer to 8–10 nm (for polycrystalline silicon, the thickness of a similar layer is approximately 22 nm) has been established. It is shown that the Ge nanocrystal formation is occurred by segregationist pushback of Ge atoms by the SiO2 /Si1 – xGex oxidation front and oxidation through grain boundaries during oxidation of Si1 – xGex thin layers, produced by chemical vapor deposition. The MOS structure with array of Ge nanocrystal, which has the hysteresis capacitance characteristics of 1.7–1.8 V and leakage current density from 1.5 ⋅ 10–16 to 2.2 ⋅ 10–16 A/µm2 was obtained.

Size- and position-controlled Ge nanocrystals separated by high-k dielectrics

Germanium nanocrystals embedded in high-k dielectric matrices are of main interest for infrared sensing application, as a role model for Ge-based nanoelectronics passivation or for nonvolatile memory devices. The capability of the size control of those nanocrystals via rapid thermal processing of superlattice structures is shown for the [Ge–TaZrOx/TaZrOx]n, [Ge–TaZrOx/SiO2/TaZrOx]6, and [TaZrOx/Ge–SiO2]n superlattice systems. All superlattices were deposited by radiofrequency magnetron sputtering. Transmission electron microscopy (TEM) imaging confirms the formation of spherically shaped nanocrystals. Raman scattering proved the crystallization of Ge above 700°C. The TaZrOx crystallizes above 770°C, associated with a phase separation of Ta2O5 and ZrO2 as confirmed by x-ray diffraction. For the composite layers having 3 nm and 6 nm thickness, the size of the Ge nanocrystals correlates with the deposited layer thickness. Thicker composite layers (above 9 nm) form two fractions of nanocrystals with different sizes. An additional SiO2 layer in the [Ge–TaZrOx/SiO2/TaZrOx]6 superlattice stacks facilitates the formation of larger and better separated Ge nanocrystals. The deposition of Ge-SiO2 composite layers separated by pure TaZrOx illustrates the barrier effect of TaZrOx against Ge diffusion. All three material systems allow the controlled formation of Ge nanocrystals in amorphous matrices at temperatures above 700 and below 770°C.Graphical abstract

Control of germanium diffusion using low quantities of co-implanted silicon isotopes

The thermal diffusion of Ge implanted into SiO2 films growth on a Si substrate has been studied by nuclear analyses and μ-Raman spectroscopy with and without the presence of co-implanted 30Si and 29Si barriers, each located from both sides of the Ge implanted distribution. Combination of Rutherford backscattering spectroscopy and Resonant nuclear reaction analysis shows that, under thermal activation at 1100°C, implanted Ge diffuses differently toward the sample surface and the SiO2/Si interface due to the occurrence of Ge outgassing effects, as well as the non-homogenous distributions of the implanted ion species and the defects they have generated inside SiO2. A maximum local atom concentration of co-implanted silicon as low as ∼1.6 at. % is found to completely block the germanium diffusion in both directions, leading to the formation of Ge nanocrystals and Si/Ge aggregates evidenced by μ-Raman spectroscopy. In addition to highlighting the role of Si excess on the Ge trapping mechanism, such a result makes the nominal silicon oxide stoichiometry and composition two crucial parameters to stabilize Ge during high temperature annealing, which explains the strong discrepancies reported for the Ge thermal diffusion coefficient in the literature.

Thermally Stimulated Evolution of Optical and Structural Properties of Germanium-Doped Alumina Films

Ge-doped Al2O3 films were deposited on long, non-rotated quarts substrate by magnetron co-sputtering of Ge and Al2O3 space-apart targets. This approach allowed to vary Ge content along the film length in the range 5-85 at.%. Simultaneous annealing treatment of such long samples permitted to investigate the evolution of optical and structural properties of the films with different Ge content. It was observed that all as-deposited films and those annealed at TA=550 °C during 50 min in nitrogen atmosphere were amorphous whatever Ge content. The formation of Ge nanocrystals was observed after treatment at TA=600 °C during 1-15 min. The higher Ge content, the shorter annealing time was required to form Ge nanocrystals. This annealing stimulated also an appearance of photoluminescence in visible-near infrared spectral range. Three specific bands centered at about 590 nm, 620 nm and 700-730 nm were detected. Their relative contribution was depended on Ge content determining the shape and peak position of luminescence spectrum. Two former components were ascribed to the carrier recombination via the defects in Ge oxide shell covered Ge nanocrystals, whereas the latter one was assumed to originate from carrier recombination in amorphous Ge clusters or via defects at Ge-cluster/Al2O3 interface. This statement was supported by the heat treatment of the films at TA=700-800 °C caused significant decrease of Ge content due to Ge out-diffusion from film volume via formation of volatile GeO.

On the Formation of Amorphous Ge Nanoclusters and Ge Nanocrystals in GeSixOy Films on Quartz Substrates by Furnace and Pulsed Laser Annealing

Nonstoichiometric GeO0.5[SiO2]0.5 and GeO0.5[SiO]0.5 germanosilicate glassy films are produced by the high-vacuum coevaporation of GeO2 and either SiO or SiO2 powders with deposition onto a cold fused silica substrate. Then the films are subjected to furnace or laser annealing (a XeCl laser, λ = 308 nm, pulse duration of 15 ns). The properties of the samples are studied by transmittance and reflectance spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. As shown by analysis of the Raman spectra, the GeO[SiO] film deposited at a substrate temperature of 100°C contains amorphous Ge clusters, whereas no signal from Ge–Ge bond vibrations is observed in the Raman spectra of the GeO[SiO2] film deposited at the same temperature. The optical absorption edge of the as-deposited GeO[SiO2] film corresponds to ~400 nm; at the same time, in the GeO[SiO] film, absorption is observed right up to the near-infrared region, which is apparently due to absorption in Ge clusters. Annealing induces a shift of the absorption edge to longer wavelengths. After annealing of the GeO[SiO2] film at 450°C, amorphous germanium clusters are detected in the film, and after annealing at 550°C as well as after pulsed laser annealing, germanium nanocrystals are detected. The crystallization of amorphous Ge nanoclusters in the GeO[SiO] film requires annealing at a temperature of 680°C. In this case, the size of Ge nanoclusters in this film are smaller than that in the GeO[SiO2] film. It is not possible to crystallize Ge clusters in the GeO[SiO] film. It seems obvious that the smaller the semiconductor nanoclusters in an insulating matrix, the more difficult it is to crystallize them. In the low-temperature photoluminescence spectra of the annealed films, signals caused by either defects or Ge clusters are detected.

In-situ transmission electron microscopy studies of the crystallization of N-doped Ge-rich GeSbTe materials

Abstract

Thermally Induced Evolution of “Ge(OH)2”: Controlling the Formation of Oxide-Embedded Ge Nanocrystals

Germanium nanocrystals (GeNCs) hold great promise as active materials in many applications including solar cells, biological imaging, Bragg reflectors, light-emitting diodes, and nonvolatile memory devices. However, because of the size-, shape-, and composition-dependent nature of their properties, it is essential that methods affording high-purity, well-defined GeNCs be developed. Herein, we report a systematic investigation of a series of “Ge(OH)2” precursors using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and solid-state 73Ge NMR and correlate their properties with the GeNCs obtained from their thermal processing. This study provides important insights into the role of the Ge(OH)2 internal structure and will allow for future rational design and practical application of GeNCs.

Dense Ge nanocrystals embedded in TiO2 with exponentially increased photoconduction by field effect

Si and Ge nanocrystals in oxides are of a large interest for photo-effect applications due to the fine-tuning of the optical bandgap by quantum confinement in nanocrystals. In this work, dense Ge nanocrystals suitable for enhanced photoconduction were fabricated from 60% Ge in TiO2 amorphous layers by low temperature rapid thermal annealing at 550 °C. An exponential increase of the photocurrent with the applied voltage was observed in coplanar structure of Ge nanocrystals composite films deposited on oxidized Si wafers. The behaviour was explained by field effect control of the Fermi level at the Ge nanocrystals-TiO2 layer/substrate interfaces. The blue-shift of the absorption gap from bulk Ge value to 1.14 eV was evidenced in both photocurrent spectra and optical reflection-transmission experiments, in good agreement with quantum confinement induced bandgap broadening in Ge nanocrystal with sizes of about 5 nm as found from HRTEM and XRD investigations. A nonmonotonic spectral dependence of the refractive index is associated to the Ge nanocrystals formation. The nanocrystal morphology is also in good agreement with the Coulomb gap hopping mechanism of T–1/2 -type explaining the temperature dependence of the dark conduction.

Effects of Annealing Temperature on Structural, Optical and Electrical Properties of Ge Nanocrystals Embedded in GeOx Matrix

In this paper, the effects of annealing temperature on the growth of Ge nanocrystals embedded in GeOx matrix have been investigated. GeOx thin films deposited on Si and quartz substrates using electron beam evaporation were annealed under nitrogen ambience at 650, 750 and 850 °C. The pristine and annealed samples have been characterized using X‐ray diffraction, Raman spectroscopy, Atomic force microscopy, UV‐Visible spectroscopy and Semiconductor Device Analyzer (SDA). It is evident from XRD that the as‐deposited films are amorphous in nature, whereas the annealed films upto 750°C reveal the coexistence of crystalline Ge and GeO2. However, a further increase in temperature reflects only GeO2 phase and disappearance of Ge peak. The sharp and intense peak at 299 cm−1 for the films annealed up to 750 °C in the Raman spectra indicates the formation of Ge nanocrystals and the peak around 436 cm−1 is attributed to crystalline GeO2. The optical band gap of pristine and annealed films was calculated using UV–Vis spectroscopy. The I–V measurements of annealed films show the ohmic behavior. The results reveal that the annealing temperature significantly affects the crystallinity, size of nanocrystals and optical band gap of films. The impact of annealing temperature on the structural, optical and electrical properties of nc‐Ge embedded in GeOx matrix has been reported in detail.

Specific features of the ion-beam synthesis of Ge nanocrystals in SiO2 thin films

The systematic features of the formation of Ge nanocrystals in SiO2 thin films implanted with Ge+ ions and then subjected to high-temperature annealing (1130°C) are studied in relation to hydrostatic pressure. It is established that annealing at atmospheric pressure is accompanied by the diffusion of Ge atoms from the implantation region to the Si substrate and does not induce the formation of Ge nanocrystals. An increase in pressure during annealing yields a deceleration in the diffusion of germanium into silicon and is accompanied by the formation of twinned lamellae at the Si/SiO2 interface (at pressures of ~103 bar) or by the nucleation and growth of Ge nanocrystals (at pressures of ~104 bar) in the SiO2 film. The results are discussed on the basis of the concept of a change in the activation volume of the formation and migration of point defects under conditions of compression.

Structure and infrared photoluminescence of GeSi nanocrystals formed by high temperature annealing of GeOx/SiO2 multilayers

Germanium and GeSi nanocrystals were synthesized in SiGeO2 glass by high temperature annealings of GeOx(5 nm)/SiO2(5 nm) multilayers. According to electron microscopy data, the size distribution and stoichiometry of the nanocrystals depend on the annealing temperature (700, 800, or 900 °C). Spatial redistribution of Ge with the formation of large faceted nanocrystals located near the Si substrate and GeSi intermixing at the substrate/film interface were observed. In the case of the 900 °C annealed sample, we note that some nanocrystals have a pyramid-like shape. Infrared absorption spectroscopy demonstrates that intermixing takes place between the GeOx and SiO2 layers leading to the formation of SiGeO2 glass. Raman spectroscopy confirms the formation of Ge nanocrystals after annealing at 700 °C and GeSi nanocrystals after annealing at 800 and 900 °C. For all annealed samples, we report the observation of infrared photoluminescence (PL) at low temperatures in the spectral range 1300–2100 nm. The observation of PL at wavelengths close to 2000 nm may be due to defect-induced radiative transitions in the nanocrystals.

Enhanced germanium precipitation and nanocrystal growth in the Ge+ ion-implanted SiO2 films during high-pressure annealing

Abstract The effect of pressure employed during subsequent annealing of the Ge + -ion implanted SiO 2 layers on the Ge nanocrystal formation was studied. Ge + ions implanted in the thin SiO 2 layers formed Gauss-like profiles with a Ge peak concentration varied from 1 to 12 at%. Subsequent annealing was carried out at temperature 600–1130 °C under pressures 1–1.2×10 4 bar. Strong effect of the pressure on the Ge atom distribution was obtained. High-temperature annealing under pressure within the range of 1–10 3 bar resulted in the out-diffusion of germanium from the SiO 2 layer to the Si substrate. As the pressure reached 1.2×10 4 bar, Ge migration to the Si/SiO 2 interface was prevented. At that, the Ge nanocrystal growth within the ion-implanted region of the SiO 2 film took place. The nanocrystal size was investigated as a function both of the Ge atom concentration and the annealing temperature. The obtained results show a diffusion-controlled nanocrystal growth mechanism. The high-pressure (1.2×10 4 bar) diffusion coefficient of germanium in silicon dioxide was estimated as a function of the temperature and expressed by D= 1.1×10 −10 exp ( −1.43 eV/kT) cm 2 /s.

Optical, structural and electrical characterizations of stacked Hf-based and silicon nitride dielectrics

High-k stacked dielectric structures were fabricated by a combination of RF magnetron sputtering and plasma-enhanced chemical vapor deposition. Their structural properties were studied versus deposition and annealing conditions by means of attenuated total reflection and high-resolution transmission electron microscopy techniques. All samples demonstrated smoothed surface (with a roughness below 1nm) and abrupt interfaces between the different stacked layers. No crystallization of Hf-based layers was observed after annealing at 800°C for 30min, demonstrating their amorphous nature and phase stability upon annealing. Uniform capacitance–voltage characteristics were measured along the wafers for all stacks. Besides, after round-voltage sweep they demonstrate significant flat-band voltage hysteresis due to charging of the stack caused by carrier injection from the substrate. These phenomena were found to be more pronounced for the stacks with pure HfO2 layers. The stacked structures were implemented for the formation of Ge nanocrystals by means of ion implantation followed by the thermal treatment mentioned above. It was found that the spatial distribution of Ge crystallites in stacked dielectrics affects significantly their electrical properties including the trapping of charge.

Size and Shape Controlled Semiconductor Nanocrystals Synthesized by RF‐Sputtering Techniques for Electronic and Optoelectronic Applications

AbstractGroup IV (Ge and Si) nanocrystals embedded in a dielectric matrix were prepared via rf magnetron sputtering by the deposition of superlattices of stoichiometric and substoichiometric oxide layers and subsequent annealing. By this way Ge nanocrystals in a (Ta)ZrOx and Si nanocrystals in an SiO2 matrix were prepared in a size and shape controlled manner. In case of the Ge‐ZrO2 system, elongated Ge nanocrystals in a crystallized ZrO2 matrix were formed for annealing temperatures above 660 °C. Starting from a Si3N4 interface of the Si wafer, a preferential orientation between the Ge and ZrO2 lattice can be stated. Doping the ZrO2 matrix additionally with Ta increases the crystallization temperature of the ZrO2 matrix and leads to the formation of Ge nanocrystals embedded in an amorphous TaZrOx matrix. The Ge nanocrystals were investigated by photoluminescence measurements and implemented into metal‐insulator‐semiconductor capacitor to investigate their potential for charge trapping devices. The prepared nano floating gate devices show good charging characteristics with a maximum memory window of 5 V and a slope of the writing voltage vs. memory window characteristics of 0.8‐1. All luminescence centers seen in the Ge‐(Ta)ZrOx system can be attributed to defects either from the Ge nanocrystal surface, the (Ta)ZrOx matrix or the Si substrate itself. Si nanocrystals were fabricated in the same way in an amorphous SiO2 film after annealing above 900 °C. The formed Si nanocrystals act as superior sensitizer for Er3+ ions which were ion implanted into the films followed by an additional subsequent anneal, varied between 700 and 1000 °C. The Si nanocrystal ‐ Er3+ system was successfully implemented into a slot waveguide structure showing a strong polarization dependent field enhancement in both emission and excitation geometry. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Metastable Ge nanocrystalline in SiGe matrix for photodiode

Amorphous Si1−xGex films were prepared by co-sputtering on an oxidized Si wafer, followed by rapid thermal annealing to form nanocrystal films. The formation of Ge nanocrystals was not at thermodynamic equilibrium formed in the amorphous Si1−xGex matrix. High-resolution transmission electron microscopy was used to characterize the increase in the size of the grains in the Ge nanocrystals as the Ge content increased. The Ge nanocrystals have a greater absorption in the near-infrared region and higher carrier mobility than SiGe crystals, and the variation in their grain sizes can be used to tune the bandgap. This characteristic was exploited herein to fabricate n-Si1−xGex/p-Si1−xGex p–n diodes on insulating substrates, which were then examined by analyzing their current–voltage characteristics. The rectifying property became stronger as the fraction of Ge in the Si1−xGex films increased. The Si1−xGex diodes are utilized as photodetectors that have a large output current under illumination. This paper elucidates the correlations between the structural, optical and electrical properties and the p–n junction performance of the film.

Ge Nanostructures Embedded in ZrO2 Dielectric Films for Nonvolatile Memory Applications

Thin films of pure Ge and ZrO2, composite Ge-ZrO2 layers and [Ge-ZrO2/ZrO2]40 multilayers were fabricated by confocal radio frequency magnetron sputtering. The structural and optical properties of these samples were studied by spectroscopic ellipsometry, Raman scattering and Fourier transform infrared spectroscopy in dependence of the chemical composition and the annealing treatment. It could be shown that rapid thermal annealing stimulates a phase separation process within the Ge-ZrO2 composite films leading to a formation of Ge nanocrystals and tetragonal ZrO2. The starting point of this process depends significantly on the Ge content: the higher the Ge content, the lower is the crystallization temperature. Compared to the monoclinic phase, the tetragonal ZrO2 phase offers an enhanced permittivity.

Photoluminescence of Ultraviolet Initiated Green Emission from Electrochemically Deposited Germanium Films on (100) Silicon

Photoluminescence (PL) spectra of the germanium deposited nanostructures grown on silicon (Si) substrate showed a weak UV peaked at 353 nm (∼3.4 eV), violet-blue at 402 nm (∼3.1 eV), and a broad green band emission in the 470–500 nm (∼2.46 eV) range at room temperature when illuminated by 325 nm line laser beam. Electrochemical growth was monitored and evaluated using scanning electron microscopy (SEM), Raman spectroscopy, and PL. SEM images indicated varied current-density-dependent stoichiometry. The origin of the emissions is attributable to GeO2 defect centers, exciton confined in a quantum well, and to unbridged oxygen hole centers. These results perhaps are an indication that the blue luminescence is correlated with the formation of Ge (or GeO2) nanocrystals. The radiative recombinations of excitons confined in the nanocrystals are assessed with a possibility of contributing to the shift in the peak.

Infrared luminescence of annealed germanosilicate layers

In the light of growing importance of semiconductor nanocrystals for photonics, we report on the growth and characterization of annealed germanosilicate layers used for Ge nanocrystal formation. The films are grown using plasma enhanced chemical vapor deposition (PECVD) and post-annealed in nitrogen at temperatures between 600 and 1200°C for as long as 2h. Transmission electron microscopy (TEM), Raman scattering and photoluminescence spectroscopy (PL) has been used to characterize the samples both structurally and optically. Formation of Ge precipitates in the germanosilicate layers have been observed using Raman spectroscopy for a variety of PECVD growth parameters, annealing temperatures and times. Ge–Ge mode at ~300cm−1 is clearly observed at temperatures as low as 700°C for annealing durations for 45min. Raman results indicate that upon annealing for extended periods of time at temperatures above 900°C; nanocrystals of few tens of nanometers in diameter inside the oxide matrix and precipitation and interdiffusion of Ge, forming SiGe alloy at the silicon and oxide interface take place. Low temperature PL spectroscopy has been used to observe luminescence from these samples in the vicinity of 1550nm, an important wavelength for telecommunications. Observed luminescence quenches at 140K. The photoluminescence data displays three peaks closely interrelated at approximately 1490, 1530 and 1610nm. PL spectra persist even after removing the oxide layer indicating that the origin of the infrared luminescent centers are not related to the Ge nanocrystals in the oxide layer.