Single Crystal Si Research Articles

Method for Low-Temperature Vacuum-Thermal Cleaning of Surface Single Crystals Si and GaAs

The paper reports on a method of low-temperature vacuum-thermal cleaning of the surface of Si and GaAs single crystals developed by the authors, which consists in implanting Ba+ ions (or alkaline elements) into Si and GaAs crystals preliminarily cleaned by ultra-high vacuum by ion etching and subsequent annealing in two stage at 800 K - 15 minutes and at 1000 K for 30 minutes. The effect of effective cleaning is achieved due to the fact that the introduced ions of Ba+ and alkaline elements, being active, form compounds with impurity atoms (O, C, S, N, etc.) at the first stage and are removed together with impurities after the second stage of heating.

An adjustable Ar ion-beam activation strategy to achieve hydrophilic bonding of Si and diamond by deposited AlN interlayer

As a III-nitride, AlN exhibits high thermal conductivity, excellent insulation, and low thermal expansion. Therefore, AlN is an alternative to the insulating layer in conventional Si-on-insulator (SOI) systems to solve the problem of severe self-heating effects caused by the poor thermal conductivity of buried oxide (BOX, SiO2) layers. This is particularly true for high-power integrated circuit and micro-electro-mechanical system fabrication. However, heat transfer also depends on the bottom Si substrate. With the highest thermal conductivity (≥2000 W/mK) among bulk materials, diamond is a highly attractive option to replace conventional Si substrates. Herein, we describe a low-temperature vacuum-free hydrophilic bonding method based on a thin AlN interlayer to prepare a single-crystal Si layer on a polycrystalline diamond. Through suitable Ar ion-beam activation, the surface nanomorphology and chemical state can be simultaneously modified. Increasing the real contact area and removing the surface heavy oxygen layer are critical factors for achieving complete bonding. Moreover, the relatively mild deposition and bonding conditions of AlN make it compatible with the complex CMOS transistor process and could enable the more flexible integration of diamond into 3D chip packages in the future.

Magnetization steps at the ferromagnetic transition of (Mn,Fe)2(P,Si) single crystals

Single crystals are highly desirable to study intrinsic properties of phase transitions, unfortunately due to an embrittlement at the transformation, (Mn,Fe)2(P,Si) studies were so far limited to polycrystalline samples. Here, we report the growth of (Mn,Fe)2(P,Si) single crystals which can be cycled across their first-order ferromagnetic transition. In single crystals with optimized composition, detailed magnetization measurements reveal a ferromagnetic transition whose shape is significantly different from polycrystalline samples, with unusual discontinuous magnetization steps induced by temperature or magnetic field leading to a stair-like transition. In contrast to the “virgin effect” of (Mn,Fe)2(P,Si) samples, this behavior is reproducible. This unique magnetic response of some (Mn,Fe)2(P,Si) crystals can be correlated to a microstructure very different from that of the bulk and preserving the clamping/release role played by strains and stresses at the ferro-paramagnetic transformation. On top of offering more details on how the first-order magnetic transition develops in time, space and as a function of the driving parameter, which is of importance for designing functional magnetocaloric materials, the growth of cyclable crystals opens new possibilities for future experiments.

Deconvolution of heat sources for application in thermoelectric micro four-point probe measurements

Joule heating is a primary phenomenon responsible for increasing temperature in electronic devices, and consequently, decreasing the lifetime and performance of electronic devices. However, this unwanted Joule heating can itself be used as a local source of heat to map the temperature-dependent material properties. Recently, micro four-point probe (M4PP) showed a promising potential for characterizing the temperature coefficient of resistance (TCR) and the Seebeck coefficient using Joule heating resulting from the measurement current. Here, we use M4PP to estimate the micrometer scale, relative temperature profile resulting from a single heat source. We introduce a triplet of four-point voltages measured at the second harmonic frequency, to deconvolute the thermoelectric voltage from the three individual heat sources involved. This paper tests and documents the validity of the proposed scheme in the 1–40 μm range on highly doped single crystal Si at 300 K, supporting predominantly Fourier heat transport at these scales. The method of deconvolution of heat sources reduces the complexity in evaluation of length- and time-dependent measurements, specifically used in the characterization of thermoelectric properties. The proposed method may also facilitate a more profound understanding of heat transport on the mesoscopic scale.

Determination of activation energy of technological thermodonors in uniaxially deformed [formula omitted

A study of the tensoresistance and tenso-Hall effect for the neutron-doped silicon single crystals after high-temperature annealing was conducted. It was established that the features of tensoeffects for these n−Si single crystals are caused by the change under the uniaxial deformation of the activation energy of the deep levels of thermodonors, which belong to different of their charge states. Dependencies of the activation energy of thermodonors on the uniaxial pressure based on the solutions of the electroneutrality equations were obtained. The obtained tensosensitivity for the neutron-doped n−Si single crystals due to the ionization of the deep level of the thermodonor E=EC−257meV at the uniaxial pressures P>0.7GPa can find its practical use in the development of high uniaxial pressure sensors based on such silicon single crystals.

Athermal annealing of pre-existing defects in crystalline silicon

Systematic investigations of electronic energy loss (Se) effects on pre-existing defects in crystalline silicon (Si) are crucial to provide reliance on the use of ionizing irradiation to anneal pre-existing defects, leading to successful implementation of this technology in the fabrication of Si-based devices. In this regard, the Se effects on nonequilibrium defect evolution in pre-damaged Si single crystals at 300 K has been investigated using intermediate-energy ions (12 MeV O and Si ions) that interact with the pre-damaged surface layers of Si mainly by ionization, except at the end of their range where the nuclear energy loss (Sn) is no longer negligible. Under these irradiation conditions, experimental results and molecular dynamics simulations have revealed that pre-existing disorder in Si can be almost fully annealed by subsequent irradiation with intermediate-energy incident ions with Se values as low as 1.5–3.0 keV/nm. Selective annealing of pre-existing defect levels in Si at room temperature can be considered as an effective strategy to mediate the transient enhanced diffusion of dopants in Si. This approach is more desirable than the regular thermal annealing, which is not compatible with the processing requirements that fall below the typical thermal budget.

X-Ray Diffraction and Raman Spectroscopy Analyses of GaSb-Enriched Si Surface Formed by Applying Diffusion Doping Technique

The paper studies the properties of surface and near-surface region of a single crystalline silicon sample doped with atoms of Ga (AIII) and Sb (BV). n-type single-crystal Si wafers were chosen as substrates, and samples were size of 8×10×0.5 mm3. For diffusion into silicon, Ga and Sb impurities were used with a purity of 99.999 and 99.998, respectively. The authors propose that a new heterostructure might form in the near-surface region of silicon that could be engineered by applying a relatively cheap diffusion method. The experimental and analysis results show that the composition and absorption spectrum of silicon start manifest certain changes, and can be used in the future as a functional material for solar cells. The result showed that randomly located islands with an average diameter of 1–15 µm are formed on the substrate surface. X-ray diffraction analysis was carried out using a Rigaku diffractometer to study the crystallographic parameters of islands formed with the participation of Ga and Sb atoms on the silicon surface. The energy spectrum was studied on Nanofinder High End Raman spectrometer (LOTIS TII) in order to determine the presence of complexes of Ga and Sb atoms within islands formed as a result of diffusion. The optical emission spectra in the new structure were studied using a Lambda 950 spectrophotometer. The measurements were carried out at room temperature, i.e., at 300°K. Having studied the results of X-ray analysis, Raman spectroscopy, and optical spectroscopy, the authors have revealed that Ga and Sb atoms form new Si0.44(GaSb)0.56 and Si0.75(GaSb)0.25-type binary compounds on Si surface.

(Invited) (Photo-)Electrochemical Reactions on Semiconductor Surfaces:Si and III-V Surfaces – Atomic and Electronic Structure

Semiconductor/electrolyte interfaces are used in many application areas where strong non linear variations in the electric performance depending on applied potential or due to illumination play a decisive role. Often a simplified explanation on charge redistribution across the semiconductor/electrolyte junctions is applied considering only the charge redistribution involving the bulk and the electrolyte following an idealized semiconductor /metal Schottky barrier model. But as will be addressed in our contribution strong deviations are usually observed in real devices which are due to Fermi level pinning effects and adsorbate related double layer potential shifts of the band edges. This is especially true for photoelectrochemical cells used in water splitting devices. For this reason we will present detailed surface science studies on the properties of Si and III-V surfaces espcially by applying photoelectron spectroscopy.In so-called model experiments the effect of surface states found on differently doped Si(111) and (100) surfaces is investigated in relation to the resulting surface electronic structure after appying different surface preparation and processing procedures. Additionally the reactivity and the changes in electronic structure following a „frozen electrolyte“ junction formation will be presented after water exposure at low sample temperature. The results are compared to photoelectrochemical cells based on Si single crystals as well as thin film multiabsorber cells used as photocathodes.For III-V semiconductors at first the surface properties of etched InP, GaP and GaInP (100) surfaces will be compared and further modified by subsequent vacuum based processing steps as heating and sputtering. The best surfaces are in their surface composition very close to samples prepared by MOCVD. Also for the III-V materials the interaction with H2O was investigated. In all cases Fermi level pinning close to the valence band maximum was observed and compared to the resulting band diagrams of photoelectrochemical devices used for HER.In combining photoelectrochemical studies with surface science investigations on the electrode properties using directly contacted but also buried junction photoelectrodes it can be shown that defect levels related to remaining dangling bond states play a dominant role for the performance of the devices. The concentration of the remaining defect states influence the obtained performance in dependence of the interfacial contact composition preferring the formation of buried junctions. In conclusion we suggest that surface science studies of photoelectrodes before and after electrochemical studies are essential and need to be further developed for in operando experiments with special emphasis also in the valence band regime to obtain improved insights into semiconductor/electrolyte junction properties.

Determination of Seebeck coefficient originating from phonon-drag effect using Si single crystals at different carrier densities

The phonon-drag effect is useful for improving the thermoelectric performance, especially the Seebeck coefficient. Therefore, the phonon and electron transport properties of Si single crystals at different carrier densities were investigated, and the relationship between these properties and the phonon-drag effect was clarified. Phonon transport properties were determined using nanoindentation and spot-periodic heating radiation thermometry. The electron transport properties were determined based on the electrical conductivity of Si. The diffusive Seebeck coefficient derived from the electron transport properties was in good agreement with previous reports. However, the value of the phonon-drag Seebeck coefficient derived from the phonon transport properties is very low. This phenomenon suggests that phonons with a normal mean free path (MFP) do not contribute to the increase in the Seebeck coefficient; however, phonons with a long MFP and low frequency increase the Seebeck coefficient via the phonon-drag effect. Moreover, the phonon-drag effect was sufficiently pronounced even at 300 K and in the heavily doped region. These features are key in designing thermoelectric materials with enhanced performance derived from the phonon-drag effect.

Elimination of Domain Boundaries Accelerates Diffusion in MOFs by an Order of Magnitude: Monolithic Metal‐Organic Framework Thin Films Epitaxially Grown on Si(111) Substrates

AbstractMany properties of the emerging class of metal‐organic frameworks (MOFs) depend crucially on defect concentrations, as in case of other solids. In order to provide reference systems with nearly perfect structure and low defect density, a procedure to grow MOFs epitaxially on cm‐sized Si(111) single crystals is developed. The crystalline metal‐organic thin films are in high registry with the substrate's crystal lattice, as demonstrated by synchrotron‐based grazing incidence X‐ray diffraction (GI‐XRD) experiments. The corresponding reduction of MOF defect density is shown to have striking effects on the properties of these porous frameworks. The most pronounced difference concerns mass transport. An increase in the diffusion coefficient of guest molecules by one order of magnitude relative to the same MOF materials with normal defect densities is observed.

Evaluation of thermally activated defects behaviors in nitrogen-doped Czochralski silicon single crystals using deep level transient spectroscopy

Thermally activated defect behaviors in nitrogen (N)-doped Czochralski silicon (Cz-Si) single crystals were investigated using deep level transient spectroscopy and quasi-steady-state photoconductance to confirm the crystals’ applicability in insulated gate bipolar transistors (IGBTs). The thermally activated defects, which were probably N-vacancy complexes and degraded the minority carrier lifetime, were detected with extremely low densities in N-doped Cz-Si compared with N-rich floating zone Si single crystals after heat treatments at 500 °C, resulting in a high remaining value of minority carrier lifetime. The difference was assumed to come from whether vacancies were released in the Si matrix during heat treatment. For the Cz-Si, vacancies were assumed to be strongly bound with oxygen atoms with concentrations of 1017 atoms cm−3. Therefore, vacancies were not released during heat treatment, resulting in low remaining N-vacancy complex densities. N-doped Cz-Si are potential materials for IGBTs because of their low densities from thermally activated defects.

MICROSTRUCTURE OF A CrSi2 TRANSITION LAYER PRODUCED BY HOT PRESSING OF Cr AND Si

Hot pressing of a Si single crystal in the bulk of electrolytic Cr powder at 1213 K, with subsequent annealing in air, leads to the formation of an intermediate polycrystalline silicide layer at the interface between the initial components. The phase composition and microstructure of the transition layer and its vicinity were investigated by scanning electron microscopy, X-ray energy-dispersive microanalysis, and electron backscatter diffraction. The transition layer has a crystal structure of the hexagonal phase of chromium disilicide (sp. gr. P6222). An additional annealing up to 120 h leads to insignificant recrystallization of small grains into larger ones.

Synthesis and characterization of half-Heusler ScPtBi films via three-source magnetron co-sputtering on Nb superconductor buffer layer

The half-Heusler compounds exhibit simultaneous electrical, optical, and magnetic properties hence extensive research efforts have recently been dedicated to their development in diverse applications. Herein, we detail the fabrication and magnetic characterization of thin film ScPtBi was grown at 300 °C on Nb-buffered Si (100) single crystals (Nb/Si (100)) by using magnetron co-sputtering. X-ray diffraction examinations have unveiled the polycrystalline growth nature of the films. It is seen that the obtained ScPtBi thin film is strained by 1.4% compared to the single crystal of ScPtBi. The morphological observations, conducted by scanning electron microscopy (SEM), revealed that the Nb buffer layer has a smooth and crack-free surface. Moreover, it has been observed that the Bi islands are formed on the surface of ScPtBi film, which is confirmed by the high-resolution scanning transmission electron microscopy (HR-STEM) analyses. The presence of the weak antilocalization (WAL) effect has been observed in the samples. The Hikami-Larkin-Nagaoka (HLN) equation was used to analyze experimental data, aiming to calculate the phase coherence length (lφ) and the coefficient α. As the temperature increased, the phase coherence length decreased from 39 nm to 24 nm. The coefficient α was determined to be in the range of 500 to 1100. As a result, it has been concluded that the presence of the WAL effect might be connected to the strong spin-orbit coupling (SOC) nature of the films instead of from topologically protected surface states.

Metal-induced lateral crystallization of germanium thin films

Recent advances in polycrystalline Ge layers formed through solid-phase crystallization have demonstrated carrier mobilities superior to those of single-crystal Si. In this study, we thoroughly examined the phenomenon of metal-induced lateral crystallization in Ge, a topic that had garnered significant attention in Si research and had shown promise in improving the performance of thin-film transistors. Of the 24 metals tested (Au, In, Bi, Pb, Ga, Ag, Al, Sn, Zn, Sb, Fe, Nb, Mg, Mn, Co, Cr, Mo, Zr, Cu, Ni, Pd, Ta, Ti, and W), 20 were found to promote the low-temperature (<400 °C) lateral growth of Ge layers on glass. Based on the reaction type (eutectic or compound), reaction temperatures, diffusion coefficients, and domain matching analyzed using a materials informatics method, the growth morphology was categorized into three types: alloy, dendrite, and uniform. The knowledge of metal-induced growth obtained in this study has broad implications for a variety of materials and holds potential for developing the next generation thin-film devices with common and inexpensive substrates.

A Wide Frequency Range C-V and G-V Characteristics Study in Schottky Contacts with a BODIPY-Pyridine Organic Interface

In this study, it was aimed to produce an organic interface layered Schottky diode structure and frequency effect on capacitance-conductance-voltage measurements. In this context, phosphor doped n-type Si single crystal has been used as a semiconductor substrate with a 1-20 Ω.cm resistivity, (100) surface oriention, 2 inches in diameter and 350 μm thickness. The (E)-5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-2-(2-(pyridin-2-yl)vinyl)-5H-54,64-dipyrrolo [1,2-c:2',1'-f] [1,3,2] diazaborinine (BODIPY-Pyridine) thin film was coated on n-Si using the spin coating technique.&#x0D; Ohmic and rectifier contacts were coated by evaporation of indium (In) and gold (Au) using a thermal evaporation system and Au/ BODIPY-Pyridine/n-Si/In Schottky diode was fabricated. Capacitance-voltage (C-V) and conductance-voltage (G-V) measurements of this structure were gained at different frequencies in the dark. Contingent on the frequency, the series resistance (Rs) and the interface state density (Nss) values were identified by using the conductance and Hill-Coleman method, respectively.

Evaluation of numerical simulation of constitutional supercooling during heavily Boron-Doped silicon crystal growth using Cz method

Using the Czochralski (Cz) method, it has been established that heavily doped silicon (Si) single crystals with impurities such as boron (B) can negatively impact the quality of grown crystals due to morphological instabilities such as cellular growth caused by constitutional supercooling for doping higher than 1 × 1019 cm−3. To produce high-quality heavily doped Si single crystals, constitutional supercooling must be prevented during growth. In this study, we used a three-dimensional numerical simulation to predict constitutional supercooling during the growth of heavily B-doped Cz-Si single crystals, considering the transport of B in the melt and the segregation effect. We compared the numerically predicted constitutional supercooling region with experimental data to assess its accuracy and found that the cellular growth region observed in the experiment was in good agreement with the region of constitutional supercooling predicted by the simulation. Moreover, we considered the effects of different pulling rates and crystal lengths during the growth process on constitutional supercooling, and the limitations of numerical modelling.

Strategy of preparing SmCo based films with high coercivity and remanence ratio achieved by temperature and chemical optimization

SmCo based films with excellent intrinsic magnetic properties have promising applications in micro-electro-mechanical system (MEMS). However, due to the complexity of phase composition and uncontrollable crystallization degree of SmCo hard magnetic phase in the film, both the coercivity (Hc) and remanence (Mr) of films are difficult to enhance simultaneously. In this paper, SmCo based films were deposited with a Cr underlayer and capping layer on single crystal Si substrates via magnetron sputtering process. The effects of annealing parameters and Sm/Co atomic ratio on the phase structure and coercivity of films are discussed. By adjusting the Sm/Co atomic ratio from 1:5 to 1:4, Co soft magnetic phase disappears and the single phase SmCo5 is obtained, leading to the increase of coercivity of the films from 30 to 34 kOe. The influence of deposition temperature and Cu doping on magnetic properties of SmCo based films was investigated. When the deposition temperature increases from room temperature to 250 °C, the coercivity will further increase from 34 to 51 kOe. However, a severe kink is observed in the demagnetization curves due to the poor exchanged coupling. An analysis of transmission electron microscopy (TEM) confirms that the average size of non-hard magnetic amorphous phase exceeds the effective exchanged coupling length of SmCo5, which contributes to the decoupling and low remanence ratio. Therefore, doping Cu and applying a post-annealing process can significantly improve the crystallization degree of the films. Both the coercivity and the remanence ratio of the demagnetization curves are greatly enhanced. We propose a plausible strategy to prepare the SmCo based films with high coercivity and remanence ratio by temperature and chemical optimization, which can be utilized in high performed MEMS devices.

Effects of seed layer thickness and post-annealing process on crystalline quality of β-Ga2O3 films prepared on Si (100) substrate by RF magnetron sputtering

Ga2O3 films were deposited on Si (100) substrates via radio frequency magnetron sputtering at room temperature. The large lattice mismatch between Ga2O3 and single crystal Si resulted in poor crystalline quality and cracking of the as-deposited Ga2O3 films. To improve the crystalline quality of the Ga2O3 films and promote the combination of Ga2O3 materials with the current Si-based process, a method combining pre-deposited β-Ga2O3 seed layer and post-annealing process was proposed. Structure, morphology and composition tests showed that when the seed layer was 9 nm and the annealing temperature was 800 °C, the prepared film was high crystalline quality β-Ga2O3 growing along a single (2‾ 01) out-of-plane orientation, and the film thickness was about 160 nm. The seed layer provided nucleation sites for the β-Ga2O3 films, which alleviated the lattice mismatch between the β-Ga2O3 films and Si substrates and prevented the film from cracking. In addition, the appropriate annealing temperature could promote the recrystallization of the film, thereby improving the quality of Ga2O3. This work will promote the development of highly integrated and low-cost Ga2O3-based optoelectronic and power devices on Si substrates. In the meantime, it also contributes to solving the problem of poor heat dissipation of Ga2O3 devices.

Light–induced transition of charged carrier transport in symmetric metal–semiconductor–metal contacts

Metal–semiconductor (M–S) contact is a basic element in semiconductor integrated circuit. Recent studies on photodetectors extensively focus on M–S contact owing to the easy fabrication and superior performance. Here, we report a light–induced transition of charged carrier transport in planar Cu/Si–doped GaAs single crystal (Si:GaAs) symmetric double Schottky contacts. Different from conventional photodetector, illumination not only gains the carrier concentration but also changes the charged carrier transport mechanism at Cu/Si:GaAs contact. The light-induced transition of charged carrier transport is reflected by the change of shape of current–voltage (I–V) curve caused by illumination. By the construction of theoretical model and its fitting to experimental data, we point out that carrier transport mechanism at Cu/Si:GaAs contact changes from tunneling effect to thermionic emission by illumination. Among that, photovoltaic effect is the crucial driving force. This study presents a novel mechanism for converting optical signal to electrical signal by tailoring Schottky contact in photodetector, which provide a design prototype to new–type optoelectronic device.

Band alignment at the strontium germanate interface with silicon

Photocatalytic water splitting is a promising strategy for large-scale clean energy production. However, efficient and low-cost solid-state photocatalysts are still lacking. We present here first-principles calculations for investigating the suitability of an epitaxial layer of strontium germanate on a Si(100) single crystal as a photocathode. Conduction and valence band offsets at the interface between these two semiconductors were determined using state-of-the-art approximations of density functional theory for the accurate prediction of band alignments. The resulting band lineup is also confirmed by inspection of the spatially resolved density of states. It is concluded that the conduction band offset of the investigated heterostructure is favorable for photocathodic functionality.