Silicon Nitride Matrix Research Articles

Dimension- and shape-dependent thermal transport in nano-patterned thin films investigated by scanning thermal microscopy

Scanning thermal microscopy (SThM) is a technique which is often used for the measurement of the thermal conductivity of materials at the nanometre scale. The impact of nano-scale feature size and shape on apparent thermal conductivity, as measured using SThM, has been investigated. To achieve this, our recently developed topography-free samples with 200 and 400 nm wide gold wires (50 nm thick) of length of 400–2500 nm were fabricated and their thermal resistance measured and analysed. This data was used in the development and validation of a rigorous but simple heat transfer model that describes a nanoscopic contact to an object with finite shape and size. This model, in combination with a recently proposed thermal resistance network, was then used to calculate the SThM probe signal obtained by measuring these features. These calculated values closely matched the experimental results obtained from the topography-free sample. By using the model to analyse the dimensional dependence of thermal resistance, we demonstrate that feature size and shape has a significant impact on measured thermal properties that can result in a misinterpretation of material thermal conductivity. In the case of a gold nanowire embedded within a silicon nitride matrix it is found that the apparent thermal conductivity of the wire appears to be depressed by a factor of twenty from the true value. These results clearly demonstrate the importance of knowing both probe-sample thermal interactions and feature dimensions as well as shape when using SThM to quantify material thermal properties. Finally, the new model is used to identify the heat flux sensitivity, as well as the effective contact size of the conventional SThM system used in this study.

Correlation of TEM data with confined phonons to determine strain and size of Ge nanocrystals embedded in SixNy matrix

Models that use phonon confinement fail to provide consistent results for nanocrystal sizes in differing dielectric matrices due to varying stress experienced by nanocrystals in different dielectric environments. In cases where direct measurement of stress is difficult, the possibility of stress saturation as a function of size opens up a window for the use of phonon confinement to determine size. We report on a test of this possibility in Ge: SixNy system. Ge nanocrystals (NCs) embedded in silicon nitride matrix have been fabricated using plasma enhanced chemical vapor deposition (PECVD) followed by post annealing in Ar ambient. Nanocrystal size dependence of Raman spectra was studied taking into account associated stress and an improved phonon confinement approach. Our analysis show same stress for NCs which have sizes below 7.0 nm allowing the use of phonon confinement to determine the nanocrystal size. The results are compared with TEM data and good agreement is observed.

Ge nanocrystals embedded in ultrathin Si3N4 multilayers with SiO2 barriers

Multilayers of germanium nanocrystals (NCs) embedded in thin films of silicon nitride matrix separated with SiO2 barriers have been fabricated using plasma enhanced chemical vapor deposition (PECVD). SiGeN/SiO2 alternating bilayers have been grown on quartz and Si substrates followed by post annealing in Ar ambient from 600 to 900 °C. High resolution transmission electron microscopy (HRTEM) as well as Raman spectroscopy show good crystallinity of Ge confined to SiGeN layers in samples annealed at 900 °C. Strong compressive stress for SiGeN/SiO2 structures were observed through Raman spectroscopy. Size, as well as NC-NC distance were controlled along the growth direction for multilayer samples by varying the thickness of bilayers. Visible photoluminescence (PL) at 2.3 and 3.1 eV with NC size dependent intensity is observed and possible origin of PL is discussed.

Photoluminescence of amorphous and crystalline silicon nanoclusters in silicon nitride and oxide superlattices

The photoluminescence properties of silicon nitride and oxide superlattices fabricated by plasmaenhanced chemical vapor deposition are studied. In the structures annealed at a temperature of 1150°C, photoluminescence peaks at about 1.45 eV are recorded. The peaks are defined by exciton recombination in silicon nanocrystals formed upon annealing. Along with the 1.45-eV peaks, a number of peaks defined by recombination at defects at the interface between the nanocrystals and silicon-nitride matrix are detected. The structures annealed at 900°C exhibit a number of photoluminescence peaks in the range 1.3–2.0 eV. These peaks are defined by both the recombination at defects and exciton recombination in amorphous silicon nanoclusters formed at an annealing temperature of 900°C. The observed features of all of the photoluminescence spectra are confirmed by the nature of the photoluminescence kinetics.

The influence of carbon on the structure and photoluminescence of amorphous silicon carbonitride thin films

Silicon carbonitride (SiCN), an intermediate structure between silicon carbide and silicon nitride, has emerged as a material of interest to increase the performance of silicon-based visible luminescence. In this work, thin films of SiCN were deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition using silane, nitrogen, and methane as the gas precursors. The microstructure, composition, and optical properties of the SiCN layers were determined by elastic recoil detection, Rutherford backscattering spectrometry, ultraviolet–visible spectroscopy, variable angle spectroscopic ellipsometry, photoluminescence, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, and X-ray diffraction spectroscopy techniques. The optical and electrical characteristics were linked with the structural properties to investigate the effect of carbon incorporation on the nature of SiCN thin films. Successive increases in the carbon content from 0 to 41.6at.% resulted in an increase in refractive index and growth rate of the films, while band gap narrowing occurred. Structural studies showed that carbon preferred to form bonds with silicon rather than nitrogen in SiCN thin layers. The produced films contained mainly amorphous silicon nitride and silicon carbide structures and small amounts of an amorphous SiCN network. Photoluminescence emission showed two peaks centred at 475 and 550nm. The observed link between the photoluminescence behavior and the composition and structure of SiCN layers revealed the details of the most luminescent SiCN thin film. Silicon nitride (carbon-free) thin films exhibited lower emission intensities indicating that the presence of carbon in the silicon nitride matrix enhanced the visible light emission.

Influence of hydrogen passivation on the luminescence of Si quantum dots embedded in Si3Nx

Silicon quantum dots (Si-QDs) of various diameters were formed by annealing of Si-rich silicon nitride (Si3Nx) films synthesized using plasma-enhanced chemical vapor deposition. To investigate the effect of annealing temperature on the luminescence of this system, samples were annealed at temperatures from 400 to 1000 °C. Photoluminescence (PL), x-ray absorption near edge spectroscopy, elastic recoil detection, and Fourier-transform infrared spectroscopy measurements were used for characterization. The authors found that luminescence originated from both quantum confinement effects (QCE) and defects, and that hydrogen passivation affects the PL intensity. For lower annealing temperatures, radiative recombination due to the QCE of the Si-QDs films was observed. For higher annealing temperatures (above 600 °C), desorption of hydrogen from the sample caused the PL intensity to decrease significantly. Si3Nx films with a lower Si content were less sensitive to this reduction in PL intensity after annealing at high temperatures (above 600 °C). Our results emphasize the importance of hydrogenation of the silicon nitride matrix if Si QDs are to be used in optoelectronic devices.

Synthesis of polymer-derived graphene/silicon nitride-based nanocomposites with tunable dielectric properties

Within the present work, reduced-graphene oxide (rGO)/silicon nitride (Si3N4) nanocomposites were prepared upon pyrolysis of a graphene oxide (GO)-filled polysilazane. The novel preparative approach consists in the synthesis of the polysilazane in the presence of different concentrations of GO, yielding a homogeneous GO/polysilazane composite which was subsequently thermally converted in Ar atmosphere into rGO/Si3N4 nanocomposites. Hot-pressing of the obtained nanocomposite powders delivered monolithic rGO/Si3N4. All prepared samples exhibited the presence of homogeneously dispersed rGO phase within an amorphous or crystalline silicon nitride matrix, as for the as-prepared and hot-pressed samples, respectively. An increasing amount of rGO in the nanocomposites was found to gradually suppress the crystallization of the silicon nitride matrix into α-Si3N4. Moreover, depending on the volume fraction of the graphene phase in the ceramic nanocomposites, different dielectric properties were observed, indicating a facile preparative method to produce materials with tunable electromagnetic waves (EMW) behavior.

Opto-Structural Properties of Silicon Nitride Thin Films Deposited by ECR-PECVD

Amorphous hydrogenated silicon nitride thin films a-SiNx:H (abbreviated later by SiNx) were deposited by Electron Cyclotron Resonance plasma enhanced chemical vapor deposition method (ECR-PECVD). By changing ratio of gas flow (R = NH3/SiH4) in the reactor chamber different stoichiometric layers x = [N]/[Si] ([N] and [Si] atomic concentrations) are successfully deposited. Part of the obtained films has subsequently undergone rapid thermal annealing RTA (800°C/1 s) using halogen lamps. Optical and structural characterizations are then achieved by spectroscopic ellipsometry (SE), ion beam analysis and infrared absorption techniques. The SE measurements show that the tuning character of their refractive index n(λ) with stoichiometry x and their non-absorption properties in the range of 250 - 850 nm expect for Si-rich SiNx films in the ultraviolet UV range. The stoichiometry x and its depth profile are determined by Rutherford backscattering spectrometry (RBS) while the hydrogen profile (atomic concentration) is determined by Elastic Recoil Detection Analysis (ERDA). Vibrational characteristics of the Si-N, Si-H and N-H chemical bonds in the silicon nitride matrix are investigated by infrared absorption. An atomic hydrogen fraction ranging from 12% to 22% uniformly distributed as evaluated by ERDA is depending inversely on the stoichiometry x ranging from 0.34 to 1.46 as evaluated by RBS for the studied SiNx films. The hydrogen loss after RTA process and its out-diffusion depend strongly on the chemical structure of the films and less on the initial hydrogen concentration. A large hydrogen loss was noted for non-thermally stable Si-rich SiNx films. Rich nitrogen films are less sensitive to rapid thermal process.

Si quantum dots in silicon nitride: Quantum confinement and defects

Luminescence of amorphous Si quantum dots (Si QDs) in a hydrogenated silicon nitride (SiNx:H) matrix was examined over a broad range of stoichiometries from Si3N2.08 to Si3N4.14, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiNx films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH4 and NH3 gases. The compositional and optical properties were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, and energy-filtered transmission electron microscopy. Ultraviolet-laser-excited PL spectra show multiple emission bands from 400 nm (3.1 eV) to 850 nm (1.45 eV) for different Si3Nx compositions. There is a red-shift of the measured peaks from ∼2.3 eV to ∼1.45 eV as Si content increases, which provides evidence for quantum confinement. Higher N content samples show additional peaks in their PL spectra at higher energies, which we attribute to defects. We observed three different ranges of composition where Tauc band gaps, PL, and PL lifetimes change systematically. There is an interesting interplay of defect luminescence and, possibly, small Si QD luminescence observed in the intermediate range of compositions (∼Si3N3.15) in which the maximum of light emission is observed.

Raman study of light-emitting SiNx films grown on Si by low-pressure chemical vapor deposition

Si-rich silicon nitride (SRSN) films were deposited on Si wafers by low pressure chemical vapor deposition (LPCVD) technique and, subsequently, annealed at (800–1200) °C to form Si precipitates. The composition of SiNx films was measured by Rutherford backscattering spectrometry (RBS). Two sets of samples differed by the amount of excessive Si (Siexc) in silicon nitride were studied. Evolution of Si nanoclusters from amorphous to crystalline ones during high temperature treatment was examined by Raman scattering (RS) spectroscopy. The amorphous Si clusters were already revealed in as-deposited SiNx while the annealing results in their crystallization. The crystalline nanoprecipitates are only registered in nitride films after annealing at 1200°C. A dependence of Raman scattering intensity from the Si wafer on the temperature of annealing of SiNx/Si structures was revealed. This information was used to explain the phase transformations in SRSNs during high temperature treatments. The peculiarities of photoluminescence (PL) spectra for two sets of Si-rich SiNx films are explained taking into account the contribution from the quantum confinement effect of Si nanocrystals and from the native defects in silicon nitride matrix, such as N- and K-centers.

Photoluminescence from silicon nanocrystals embedded in silicon nitride fabricated by low-pressure chemical vapor deposition followed by high-temperature annealing

The photoluminescence (PL) from silicon nanocrystals (Si-ncs) embedded in an amorphous silicon nitride matrix was examined both experimentally and through theoretical simulations. The film was prepared using low-pressure chemical vapor deposition with subsequent high-temperature annealing. The experimental parameters required for the PL modeling were determined using Raman spectroscopy. A novel method to estimate the nitrogen content, which allowed the determination of both the Urbach energy and the Tauc gap, was reported. The luminescence could be attributed to different origins, namely, Si-ncs, amorphous silicon nanodots, nitrogen and silicon defects, and amorphous matrix. A comparison between the experimental results and the modeling indicated that the existing models are unable to satisfactorily explain the observed PL.

Structural and light emitting properties of silicon-rich silicon nitride films grown by plasma enhanced-chemical vapor deposition

Si-rich Silicon nitride films fabricated by plasma enhanced chemical vapor deposition (PECVD) on p-type silicon substrates were investigated by means of photoluminescence (PL) and X-ray diffraction (XRD) methods. The film stoichiometry was controlled via varying the NH3/SiH4 ratio (R) in the range R=0.56–1.0. Thermal annealing at 1100°C for 30min in the nitrogen flow was applied to form the Si nanocrystals (NCs) embedded in the films. XRD experiment has revealed also the formation of hexagonal silicon nitride NCs with the sizes of 26–30nm in the films grown with R=1. When R decreasing, the sizes of silicon nitride NCs decreases down to 2.8–3.0nm (R=0.63). In the films grown with R=0.56 the amorphous silicon nitride, amorphous Si and crystallized Si phases have been detected. PL study showed the non-monotonic behavior of PL intensity with the R decreasing. The highest PL intensity was detected for the films grown with R=0.63. PL spectra of all films were found to be complex containing a set of PL bands peaked at about 2.8–3.0, 2.5–2.7, 2.1–2.2 and 1.8–2.0eV. Temperature behavior of PL spectra was investigated in the range 20–300K that allowed analyzing the nature of each PL component. It was shown that the peak position of all PL bands varies with the SiNx stoichiometry and Si NC sizes, demonstrating a “red” shift with R decreasing. It was revealed that the former three PL bands did not change their peak positions versus temperature of measurement that permits to assign them to the carrier recombination via radiative defects in the silicon nitride matrix. The PL band at 1.8–2.0eV has demonstrated a “blue” spectral shift with cooling similar to the Si band gap shrinkage. Based on this behavior, the 1.8–2.0eV PL band has been attributed to exciton emission inside of Si NCs. The role of silicon nitride NCs in photoluminescence and its excitation is discussed.

Simulation of localized surface plasmon resonance of hexagonal Ag nanoarrays and amorphous oxidized silicon nitride

Simulation on the properties of localized surface plasmon resonance (LSPR) of different sized hexagonal Ag nanoarrays embedded in the amorphous oxidized silicon nitride(a-SiNx:O) matrix has been carried out by using COMSOL Multiphysics and FDTD Solution simulation software. Through the calculation of the scattering and absorption cross section of Ag array with different radius, we find that the position of extinction peaks red-shift from 460 to 630 nm when the radius of nanoparticles of hexagonal Ag arrays increases from 25 to 100 nm with the distance between particles 100 nm. The enhanced scattering cross section of the localized surface plasmon (LSP) and blue-shift of the extinction peak can be obtained by tunning the distance between Ag nanoparticles from 100 to 50 nm with the radius of Ag nanoparticles fixed at 50 and 75 nm, respectively. However the mismatch between the extinction peak of hexagonal Ag nanoarrays and the blue light emission of 460 nm from a-SiNx:O films still exists. The novel overlap between the scattering cross section of LSP from hexagonal Ag arrays with a radius of 25 nm and the blue light emission of a-SiNx:O films at 460 nm further confirms that the hexagnoal Ag arrays with a radius of 25 nm is the optimal option to enhance the blue light emission from a-SiNx:O films. Therefore, strong coupling between LSP and blue light emission at 460 nm from a-SiNx:O films with a thickness of 70 nm can be realized when the radius of Ag nanoparticle is 25 nm. We also investigate the enhancement of near field radiative intensity of LSP from hexagnoal Ag arrays with a radius of 25 nm. When the excitation wavelength is 460 nm, the maximum enhancement of near field intensity of LSP from hexagnoal Ag arrays with a radius of 25 nm reaches 1.46104 V/m. The calculated polarization intensity and charge distribution of hexagonal Ag nanoparticle with a radius of 25 nm embedded in a-SiNx:O films reveal that the enhancement of electromagnetic field-intensity is through the dipolar plasmon coupling with the excitons in a-SiNx:O films in bright field mode under the excitation of 460 nm. Further calculation of perpendicular radiative intensity for LSP from the hexagonal Ag array with a radius of 25 nm embedded in a-SiNx:O films indicates that the maximum radiative intensity can be realized in a-SiNx:O matrix with an optimum thickness of 30 nm for a-SiNx:O films. Our theoretical calculations and analysis can provide valuable reference for the design of Si-base blue LED with light emission around 460 nm.

Self-assembled nc-Si/a-SiNx: H quantum dots thin films: An alternative solid-state light emitting material

Nanocrystalline silicon quantum dots (QDs) of varying size from ~5.4 to 2.2nm embedded in amorphous silicon-nitride matrix (nc-Si-QDs/a-SiNx:H) were prepared via ICP-CVD (13.56MHz), using a mixture of silane (SiH4), ammonia (NH3) gases in H2-diluted plasma, by changing the low deposition pressure in a narrow range, 10–50mTorr at low substrate temperature (250°C). Strong visible photoluminescence (PL) tunable over a significantly wide range (1.67–3.02eV) has been observed. The origin of the PL is mostly attributed to band-to-band recombination due to the quantum confinement effect (QCE) in the nanocrystalline silicon QDs. A large amount of atomic hydrogen flux that originates due to the high degree dissociation of the gas molecules in high-density inductively coupled plasma (ICP) passivates well the nonradiative dangling bonds and helps in growing plenty of ultra-nanocrystallites that demonstrate intense visible photoluminescence. The red-shift of Raman peak and the corresponding line broadening have been associated to the confinement of optical phonons within the nc-Si QDs. The widening of band gap and tune-ability of visible photoluminescence over a notably wide range along with significantly high electrical conductivity of the material demonstrates enormous promise for its utilization in the fabrication of effective solid-state light emitting devices.

Optical properties of silicon nanocrystals embedded in Si3N4 matrix measured by spectroscopic ellipsometry and UV-Vis-NIR spectroscopy

In this paper, we report a spectroscopic ellipsometry study of the optical properties of silicon nanocrystals (Si-ncs) embedded in silicon nitride matrix. The nanocomposite thin-films were elaborated by radiofrequency plasma enhanced chemical vapor deposition from ammonia and silane precursors, followed by high temperature annealing. Bruggeman effective medium approximation combined with the Tauc–Lorentz dispersion law was found to be an appropriate model in describing the ellipsometric data, and provided a fine determination of the dielectric functions or complex permittivity of Si-ncs embedded in silicon nitride. It is shown that the dielectric functions of Si-ncs undergo a large reduction in amplitude and broadening compared to the dielectric function of the bulk crystalline Si. Consequently to the disappearance of direct transition energy E1 and E2, the imaginary part ϵ2 of the dielectric function of Si-ncs exhibits a single line shape centered between E1 and E2. With decreasing Si-ncs size, we observe a red-shift of ϵ2 which cannot be attributed to bandgap expansion, but is better explained by electron–phonon interactions in the case of a Si3N4 matrix with high Young modulus. According to Tauc–Lorentz dispersion law, the obtained bandgap values of Si-ncs are between 1.58 eV and 1.67 eV for Si-ncs with diameters from 4.6 nm to 3.8 nm, which is in good agreement with measurements from UV-Vis-NIR spectroscopy.

Role of ammonia in depositing silicon nanoparticles by remote plasma enhanced chemical vapor deposition

The optimal development of silicon photonics could improve the current state of microelectronics technology. However due to the multiple parameters involved in the elaboration processes, it is not a simple task to fabricate and reproduce silicon emitting structures. In this paper we discuss the role played by ammonia in depositing silicon nanoparticles (Si-NPs) embedded in silicon nitride matrix by remote plasma enhanced chemical vapor deposition (RPECVD). We found that by varying the ammonia flow, the deposition rate changes. It was observed by high resolution transmission electron microscopy (HRTEM) that higher density and a lower average size of Si-NPs is achieved by increasing the ammonia flow. We recognized a linear behavior between the ammonia flow used to deposit a sample and its maximum photoluminescence (PL) emission peak. Finally we propose a model that allows to predict the maximum PL peak for a given ammonia flow within the range of 403–540nm.

Wear damage of Si3N4-graphene nanocomposites at room and elevated temperatures

Abstract Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt.% of multilayered graphene (MLG) platelets were studied and compared to monolithic Si3N4. The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at temperatures 25 °C, 300 °C, 500 °C, and 700 °C in dry sliding. Addition of such amounts of MLG did not lower the coefficient of friction. Graphene platelets were integrated into the matrix very strongly and they did not participate in lubricating processes. The best performance at room temperature offers material with 3 wt.% graphene, which has the highest wear resistance. At medium temperatures (300 °C and 500 °C) coefficient of friction of monolithic Si3N4 and composite with 1%MLG reduced due to oxidation. Wear resistance at high temperatures significantly decreased, at 700 °C differences between the experimental materials disappeared and severe wear regime dominated in all cases.

Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting

Cf–Si3N4 sandwich composites were prepared by gelcasting using α-Si3N4 powder, SiC-coated carbon fibers and sintering additives as starting materials. The microstructure and composition, dielectric properties of Cf–Si3N4 sandwich composites were investigated. SEM and EDS analysis results reveal that the SiC interphase could effectively overcome incompatibility between carbon fiber and silicon nitride matrix under the condition of pressure-less sintering at 1700°C. The investigation of microwave absorbing property reveals that, compared with the Si3N4 ceramics, both the real (ε׳) and imaginary (ε׳׳) permittivity of Cf–Si3N4 sandwich composites show strong frequency dispersion characteristics at X-band. Microwave absorption ability of the Cf–Si3N4 sandwich composites are significantly enhanced compared with pure Si3N4 ceramic, and the reflection loss gradually decreases from −3.5dB to −14.4dB with the increase of frequency, while the pure Si3N4 ceramic keeps at −0.1dB. Particularly, the relationship between permittivity of Cf–Si3N4 sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. Results showed that both ε׳ and ωε׳׳ are inversely proportional to the frequency square ω2, and the predicted results agree quite well with the measured data.

Fabrication and characteristic of non-oxide fiber tow reinforced silicon nitride matrix composites by low temperature CVI process

Non-oxide fiber tow reinforced silicon nitride matrix composite was fabricated by low temperature CVI process with PyC as interphase. The tensile strength of the C and SiC fiber tow composites were 547MPa and 740MPa, respectively. The difference in tensile strength was analyzed based on the length, amount of pull-out fiber and also interface bonding. The infiltration uniformity of CVI silicon nitride (SiN) matrix within SiC fiber tow was comparable with that of CVI SiC matrix. These results suggested that the low temperature CVI process is suitable for the fabrication of fiber reinforced SiN matrix composites with proper interface bonding and high strength.

In-situ fabrication and characterization of ordered Ge QDs in Si3N4 matrix without barrier layers by rf-magnetron sputtering

In this work, in-situ fabrication and characterization of ordered arrays of germanium (Ge) quantum dots (QDs) embedded in silicon nitride matrix were studied. A single layer of Ge and Si3N4 composite was deposited by rf-magnetron sputtering with substrate heating. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy were used to investigate structural properties of the Ge QDs. TEM images showed that uniformly sized Ge QDs were formed in diagonal QD arrays grown from the substrate. The average size estimated from XRD was in agreement with the QD size observed in TEM images. Chemical composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS), confirming the Ge0 state of the Ge–Ge bond in the QDs. Phonon confinement in the QDs was demonstrated by phonon peak broadening in Raman spectra. The confinement and stress effect resulted in phonon peak shifting to 297cm−1. With decreasing QD size, E1 and E2 transitions estimated from spectroscopic ellipsometry were blue-shifted, indicating the quantum confinement effect. This closely spaced Ge QD structure would be potentially advantageous for third generation photovoltaic applications as this structure could improve electrical conductivity while confining the QD growth.