Electrochemical Capacitance-voltage Profiling Research Articles

Multicrystalline, Highly Oriented Thick‐Film Silicon from Reduction of Soda‐Lime Glass

AbstractThe study describes synthesis and characterization of > 10 µm thick multicrystalline (mc), highly oriented, p‐doped silicon layers by aluminothermic reduction of low‐cost soda‐lime glass. X‐ray diffraction shows a highly preferred (111)‐orientation and excellent crystallinity. Low compressive stress and very good crystallinity are confirmed by the peak position and width of the Raman LO‐phonon line, approaching the one of bulk single‐crystalline wafer material. Due to strong bonding to the glass substrate layer, spalling is not observed. A conductive aluminum‐rich oxide layer is formed underneath the silicon, serving as an electrical back‐contact for electronic devices. Using secondary ion mass spectrometry very low concentrations of 1014–1015 cm−3 of impurities are found originating from the soda‐lime glass with an iron content below the detection limit. Furthermore, a plateau‐like, very homogenous Al concentration of ≈4 × 1018 cm−3 over a thickness of ≈10 µm is found, which corresponds to the solubility of Al in Si at the process temperature. Complete electronic activation within the plateau region is confirmed by carrier concentration measurements using electrochemical capacitance–voltage profiling and Raman spectroscopy. Hole concentrations in the range of few 1018 cm−3 are beneficial for the p‐type base material of full‐emitter cell mc‐silicon photovoltaic devices.

Effects of Different Particle-Sized Al Powders on Sintering Properties of Aluminum Paste in Crystalline Silicon Solar Cell

In this paper, the effects of particle size difference of aluminum (Al) powder on physical properties of Al powder and sintering properties of Al paste were investigated. For this purpose, respectively, Al powders with four mean particle sizes were used, which were obtained by two classifications using the same nitrogen atomization process. Thermal analysis results showed that the smaller the Al particle size, the lower the oxidation temperature and the higher the reaction enthalpy, indicating higher reactivity of the Al powder. Four Al powders were prepared into Al paste by the same process, and the resistances of the paste increased with the decrease of the particle size of the aluminum powder. Electrochemical capacitance-voltage profiler (ECV) and scanning electron microscope (SEM) analysis showed that the smaller the Al particle size, the thicker the back surface field (BSF) and the higher the doping concentration of BSF. It was found that the smaller size of Al powder limited the migration of silicon, which resulted in higher concentration of silicon (Si) in the Al-Si liquid phase. This leads to reaching a balance at a higher temperature between the recrystallization of silicon from the alloy liquid and the dissolution of silicon in the liquid alloy and a higher doping concentration and a higher effective BSF doping thickness. Results of the study will provide reference for further study and application of Al paste.

Diagnostics of semiconductor structures by electrochemical capacitance-voltage profiling technique

The properties of interfaces in the heterostructures which frequently govern their operation are of particular importance for the devices containing heterostructures as active elements. Any further improving of the characteristics of semiconductor devices is impossible without a detail analysis of the processes occurring at the interfaces of heterojunctions. At the same time, the results largely depend on the purity of the starting materials and the technology of layer manufacturing. Moreover, the requirements to the composition and distribution of the impurity steadily get stringent. Therefore, the requirements regarding the methods of the impurity control and carrier distribution also become tougher both in the stage of laboratory development of the structure and in various stages of manufacturing of semiconductor devices. Electrochemical capacitance-voltage profiling is distinguished among the methods of electrical diagnostics of semiconductors by the absence of special preparation of the structures and deposition of the contacts to perform measurements, thus providing for gaining information not only about the impurity distribution but also about the distribution of free carriers. The goal of this work is to perform precise measurements of the profiles of free carrier distribution in semiconductor structures of different types, and demonstrate the measuring capabilities of a modern technique for concentration distribution diagnostics, i.e., electrochemical capacitance-voltage profiling. The method allows verification of the layer thickness in semiconductor heterostructures and provide a useful and informative feedback to technologists. To increase the resolution of the method and broad up the range of available test frequencies, a standard electrochemical profiler has been modified. Mapping data for GaAs substrate structure, the profiles of the concentration distribution of the majority charge carriers in SiC structures, GaAs structure with a p – n junction, pHEMT heterostructure, GaN heterostructure with multiple quantum wells, and in a silicon-based solar cell heterostructure are presented. The obtained results can be used to analyze the physical properties and phenomena in semiconductor devices with quantum-sized layers, as well as to improve and refine the parameters of existing electronic devices.

Interpretation of the 1/C2 Curvature and Discontinuity in Electrochemical Capacitance Voltage Profiling of Heavily Ga Implanted SiGe Followed by Melt Laser Annealing

Electrochemical Capacitance Voltage Profiling (ECVP) is one of the most widely used characterization methods in semiconductor industry to measure the activation of dopants in doped semiconductor materials owing to its low-cost and easy-to-use features. Today, there are some specific industrial needs, for instance reduction of the contact resistance in advanced transistors, for which heavily-doped semiconductor materials must be implemented. A clear challenge addressed to ECVP here is that the interpretation of measured data becomes much more complex in such materials because of the appearance of curvatures and discontinuity on the curve given by the inverse of the square of the capacitance within the space charge zone as a function of the applied polarisation voltage. In this paper, we present a case of silicon-germanium doped by heavy gallium ion-implantation and annealed by melt laser annealing, where a metastable dopant activation can be expected. We develop a fitting model with different deep levels, highlighting their possibly different time constants to provide a more reliable interpretation on the measurements.

Integration of electrochemical capacitance–voltage characteristics: a new procedure for obtaining free charge carrier depth distribution profiles with high resolution

A novel electrochemical profiling technique consisting in integration capacitance–voltage characteristics was proposed, developed and proven. The aim is to overcome the insufficient resolution in charge carrier concentration measurements during commonly used electrochemical capacitance–voltage profiling. This is very actual task for modern low-dimensional optoelectronics. According to the introduced “profiling window”, after each etching step the coordinate interval is selected, the concentration distribution in which is to be matched itself and by the first derivative at the edges with the concentration in adjacent intervals. To achieve these conditions the minimum dissipation factor should be maintained for each C–V characteristic at the corresponding etching depth. The method allows both to obtain general free charge carrier concentration depth distribution over unlimited structure depth and to achieve necessary resolution in key regions, even if they are lying at a high depth. Particularly, in GaAs-based heterostructure concentration profile of deep-lying quantum well and a delta layer (ultra-thin (2 nm) profile of dopant with extremely high concentration) were confidently distinguished with magnitude difference of 20% in the dip region in between. Besides, we have shown how quantum-confined objects of different origins give different concentration responses when electrochemical capacitance–voltage profiling with sequential etching. An explanation for this dissimilarity is given.

Back-side-illuminated CCDs for EBCCDs: “dead-layer” compensation

The near-surface “dead-layer” nature and its negative impact on back-side-illuminated charge-coupled devices (CCDs) have been investigated in detail. Special attention is devoted to the analysis of methods for the “dead-layer” eliminating. The study of this phenomenon was carried out using the technique of electrochemical capacitance–voltage profiling by measuring, simulating, and analyzing free charge carrier concentration profiles of back-side-illuminated CCDs, taken at various technological stages. Different techniques of annealing for ion implanted CCDs were used. By analyzing and comparison of free charge carriers concentration profiles, the recommendations for optimization of technology for back-thinned CCDs were proposed, aiming to increase of pulling field and decrease the impact of surface potential on charge carrier transport.

A Scanning Spreading Resistance Microscopy Study on a Laser-Doped Selective Phosphorous Emitter

In this work, we present scanning spreading resistance microscopy (SSRM) measurements of a phosphorous emitter formed via tube furnace diffusion and a laser-doped selective emitter (LDSE) processed by additionally doping the tube furnace diffused emitter. The nonlaser-doped emitter is found to be widely homogeneous along the alkaline textured silicon surface. In contrast, the LDSE features large fluctuations in doping depth, almost constant doping concentration along the (laser-modified) surface within a depth of <0.2 μm, and a steep drop of the doping concentration at the p-n-junction. In some parts of the LDSE, the local doping profile is found to decrease stepwise. These findings are new to the photovoltaic community since the named phenomena are not resolved by common characterization methods such as electrochemical capacitance-voltage (ECV) profiling. The limits of ECV profiling are demonstrated by simulating an ECV profile from an SSRM measurement. In this context, we introduce a new method to correct ECV measurements performed on rough surfaces.

Limitations of Control of Electrophysical Characteristics of Quantum-Size Structures by Electrochemical Capacitance–Voltage Profiling

A method is proposed to control the distribution of carrier concentration over the profile of multilayer heteroepitaxial structures (HESs) with quantum-size active region based on the AlGaAs/GaAs heteropair grown with the aid of molecular beam epitaxy using electrochemical capacitance–voltage profiling (ECV). A calculation model for the region of the space charge at the electrolyte–semiconductor interface is developed. The ECV profiles of the HES grown on the GaAs〈100〉 substrates are analyzed. The limiting depths of the space-charge regions are calculated for different concentrations of carriers in the layers under study. Limitations of the method to control the distribution of carrier concentration in the quantum-size region of the HES are outlined.

Active and buffer layers of GaN HEMT: ECV profiling and 2DEG calculation

ABSTRACT Gallium nitride transistors with high electron mobility (HEMT) have been studied by means of electrochemical capacitance–voltage (ECV) technique. The capacitance–voltage characteristics at various frequencies were measured; the free charge carrier depth distributions as well as the intensity of quantum well filling by charge carriers were obtained. The features of 2D electron gas (2DEG) appearance in GaN HEMT, related to spontaneous and piezoelectric polarisation, as well as factors affecting the density of free charge carriers in channel and the performance of GaN HEMTs were described in detail. The efficiency of ECV technique for quality control of both active and buffer layers of GaN HEMT heterostructures is shown. The calculation of 2DEG density for various technological parameters of the interface, such as mutual orientation of AlGaN/GaN layers and composition of Al x Ga1-x N, is carried out taking into account piezoelectric effects.

Through Concentration Profiling of Heterojunction Solar Cells

Electrochemical capacitance–voltage profiling has been used to examine heterojunction solar cells based on single-crystal silicon. Specific features of the electrochemical capacitance–voltage profiling of modern multilayer heterojunction solar cells have been analyzed. The distribution profiles of majority carriers across the whole thickness of the samples were obtained, including, for the first time, those in layers of conducting indium tin oxide.

Adherent Light-Induced Al Plating on Si for Substitution of Ag in Si Solar Cells

Silver (Ag) is one of the most expensive materials used in the manufacture of silicon (Si) solar cells today as the front electrode. The current Si cell manufacturing cost from Si wafers is ~$0.10 per cell and Ag accounts for about half of that cost. Another key issue is the limited reserve of Ag on our planet, with a US Geological Survey estimated amount of 530,000 metric tons [1]. Even if the entire Ag reserve was used for cell manufacturing, we could only supply ~5% of the 2100 global energy demand [2]. Based on material cost, abundance, and resistivity, only two suitable replacements for Ag exist, Cu and Al. Al bears the advantage over Cu since it eliminates the need for a barrier layer such as Ni. Al is also less prone to oxidization often experienced by Cu. We reported light-induced Al electroplating on Si solar cells using a Lewis acidic chloroaluminate ionic liquid [3], demonstrating direct Al plating on Si without a seed layer. This talk focuses on surface preparation and Al plating processes to promote strong adhesion and low contact resistance between plated Al and cell emitter. Sheet resistance and surface morphology of the emitter were monitored as a function of laser power for SiNx removal. Electrochemical capacitance-voltage profiling determined the laser power at which the emitter surface dopant profiles were preserved. A process to remove laser damage and clean the Si surface after laser patterning was developed by etching Si in 1 wt% NaOH. It was found by atomic force microscopy that ~23 s was the minimum time to completely remove the original Si surface which was damaged and contaminated. Al deposits on substrates prepared by this surface cleaning process showed strong adherence in the Scotch tape test. Al plating was performed at different temperatures. The resulting film thickness and morphology were investigated with scanning electron microscopy. The lowest film resistivity was less than 8 µΩ-cm which was obtained at a plating temperature of 50°C and showed a grain size of 2–5 µm. [1] U.S. Geological Survey, Mineral Commodity Summaries (2017). [2] M. Tao, Terawatt Solar Photovoltaics: Roadblocks and Opportunities (Springer, London, 2014). [3] L. Wang, W.-H. Huang, W. J. Shin, M. Tao, B. Deng, and D. Wang, J. Electrochem. Soc., 165, D381 (2018).

Technique for the Electrochemical Capacitance–Voltage Profiling of Heavily Doped Structures with a Sharp Doping Profile

The specific features of applying electrochemical capacitance–voltage profiling to investigate heavily doped structures with a sharp doping profile are considered. Criteria are presented, and recommendations are given for selection of the optimal measurement parameters, and the necessity of increasing the frequency, at which the capacitance is measured during profiling, is substantiated. The described procedure is considered by the example of profiling p-type silicon structures with ion implantation as well as n-GaAs epitaxial and substrate structures for pHEMT devices.

Internal Resistive Barriers Related to Zinc Diffusion During the Growth of Inverted Metamorphic Multijunction Solar Cells

Majority carrier barriers at heterointerfaces are a common source of non-linear resistance that hinders concentrator solar cell performance. The source of a particular barrier is often unclear in a multijunction device with numerous heterointerfaces. We demonstrate Zn-dopant diffusion during inverted metamorphic multijunction (IMM) device growth to be one key cause of internal barrier formation. Using an inverted GaAs/GaAs tandem solar cell with a high temperature annealing layer grown in between each subcell, we simulate the annealing conditions of a multijunction growth in a simplified structure. Through analysis of the device by secondary ion mass spectrometry (SIMS) and electrochemical capacitance–voltage profiling, we show that annealing causes Zn to diffuse out of the top cell Ga0.5In0.5P back surface field (BSF) and accumulate in the GaAs base. Through equilibrium band modeling, we show that the resultant doping profile forms an energetic barrier to hole flow in the valence band, which correlates with fill factor losses in the current–voltage curves measured under concentration. When we, instead, employ a C-doped Al0.2Ga0.8As BSF layer in the top cell, we do not observe evidence of a heterojunction barrier. We attribute this difference to the reduced diffusivity of carbon, confirmed by SIMS, as well as more favorable valence band offsets between GaAs and Al0.2Ga0.8As. Finally, we compare 5-junction IMM cells with Al0.2Ga0.8As:C and Ga0.5In0.5P:Zn BSF layers in the GaAs third junction, respectively, and show a significantly improved device performance under concentration when Al0.2Ga0.8As:C is employed. We demonstrate the importance of designing annealing tolerance into multijunction structures that are subjected to extended annealing during growth.

Техника электрохимического вольт-фарадного профилирования сильно легированных структур с резким профилем распределения примеси

AbstractThe specific features of applying electrochemical capacitance–voltage profiling to investigate heavily doped structures with a sharp doping profile are considered. Criteria are presented, and recommendations are given for selection of the optimal measurement parameters, and the necessity of increasing the frequency, at which the capacitance is measured during profiling, is substantiated. The described procedure is considered by the example of profiling p -type silicon structures with ion implantation as well as n -GaAs epitaxial and substrate structures for p HEMT devices.

Сквозное концентрационное профилирование гетероструктурных солнечных элементов

Heterojunction solar cells based on single-crystal silicon were studied by means of electrochemical capacitance-voltage profiling. The features of electrochemical capacitance-voltage profiling of modern multilayer heterojunction solar cells are analyzed. The free charge carriers depth distribution profiles over the entire thickness of the samples, including, for the first time, in the layers of conducting indium tin oxide, are obtained.

EMISSION PROCESSES OF QUANTUM WELL INTERACTION WITH DELTA-LAYER IN pHEMT-HETEROSTRUCTURES

The paper provides experimental and theoretical study of pHEMT heterostructures with quantum well (QW) AlGaAs/InGaAs/GaAs and delta-doped layer used as active layers for fabrication of 4-18 GHz transistors. As the experimental techniques, the electrochemical capacitance-voltage (ECV) profiling and other methods of admittance spectroscopy are applied. Modernization of commercial ECV-profiling setup allows observing for the first time the concentration peak from a near-surface delta-layer of pHEMT heterostructures together with the enrichment peak from the quantum well. In order to optimize the etching speed the crater bottom control is performed by means of AFM. The electrolyte-semiconductor contact capacitance is measured with Agilent RLC-meter. The main theoretical technique used in the research is numerical modeling of nanoheterostructure key electronic features by self-consistent solution of Schrödinger and Poisson equations. The potential line-up for the conduction band bottom is obtained, and the quantized energy levels are calculated. The complex analysis of series of samples is carried out in order to understand the influence of delta-layer position on the level depth and at the carrier concentration. The optimum distance between QW and delta-layer providing the most efficient charge carrier delivery to quantum well is found. The performed research is aimed at improvement of microwave electronic devices allowing increase of the gain coefficient and the transfer characteristic of SHF-transistors.

ECV Doping Profile Measurements in Silicon Using Conventional Potentiostat

Formation of p–n junctions by phosphorus diffusion from liquid dopant in Si was investigated using a custom-built procedure for performing electrochemical capacitance–voltage (ECV) measurements. The feasibility of using a potentiostat equipped with an impedance module was investigated using an algorithm and experimental setup developed for the ECV technique. Using impedance spectroscopy and amperometry methods in a controlled manner, the dopant concentration was measured in the depth range of 600 nm to 900 nm. Sequential etching in NH4F (0.1 M) electrolyte using current–time (I–t) testing under ultraviolet (UV) illumination was applied to etch the surface of n-type silicon and estimate the diffusion depth. Initial current–voltage (I–V) tests were used to determine the bias voltage. The results of the proposed method were compared with those obtained using a commercially available ECV profiler. A conventional parallel equivalent circuit model of the Schottky junction was used to describe the electrolyte–silicon barrier, leading to good agreement between the proposed method and commercial ECV analysis results.

Electrochemical capacitance-voltage profiling of nonuniformly doped GaAs heterostructures with SQWs and MQWs for LED applications

Light-emitting heterostructures with single and multiple GaAs/InGaAs quantum wells have been investigated by means of electrochemical capacitance-voltage (ECV) profiling. Capacitance-voltage characteristics were measured; concentration profiles of free charge carriers over the heterostructure depth as well as the intensity of quantum well filling by charge carriers were obtained. In heterostructures with a single quantum well (QW), we considered limitations of capacitance techniques for undoped QW profiling, which are situated near the metallurgic border of the p-n junction. We made a detailed consideration of phenomena related to Debye smearing and we developed and analyzed the dependence of the space charge region width on the doping. Special attention was paid to investigation of "the blind" area. This was inspired by the practical problem from capacitance spectroscopy of semiconductors, when the researcher poses the task of obtaining a free charge carrier depth distribution profile as deep as possible in the space charge region, i.e. where the intensity of the electric field is maximum. Generally, the active QW of a LED heterostructure is placed deep in the space charge region, so reaching these regions is extremely important for practical problems. We present an evolution of capacitance-voltage characteristics during ECV profiling of nonuniformly doped p-n-heterostructures. For a heterostructure with multiple quantum wells, we registered a response from 6 QWs.

Specific Features of the Electrochemical Capacitance–Voltage Profiling of GaAs LED and pHEMT Structures with Quantum-Confined Regions

GaAs light-emitting (LED) and HEMT structures with δ-doped regions, InGaAs/GaAs quantum wells, and surface layers of InAs/GaAs quantum dots were studied by means of the electrochemical capacitance- voltage profiling technique. The concentration depth profiles of free charge carriers were obtained. Charges accumulated in quantum wells and quantum dots, as well as the doping levels of the emitter and δ layers were determined. The band structure and free carrier density distribution over the depth of the samples with different quantum well geometry were simulated. The specific features of electrochemical capacitance- voltage profiling in different heterostructure types are analyzed. A method of integration of capacitance- voltage curves at each etching stage was suggested. This method provides the efficient separation of responses from closely located layers, particularly the quantum well and δ layer.

Germanium diffusion with vapor-phase GeAs and oxygen co-incorporation in GaAs

Vapor-phase germanium diffusion has been demonstrated in Zn-doped and semi-insulating GaAs in sealed ampoules with GeAs powders and excess arsenic. Secondary-ion-mass spectroscopy (SIMS) profiles indicate the presence of unintentional co-incorporation of oxygen in high densities (&amp;gt;1017/cm3) along with diffused germanium donors whose concentration (&amp;gt;&amp;gt;1018/cm3) determined by electro-chemical capacitance-voltage (ECV) profiler shows significant compensation near the surface. The source of oxygen mainly originates from the GeAs powder which contains Ge-O surface oxides. Variable-temperature photoluminescence (PL) shows that in GeAs-diffused samples, a broad peak ranging from 0.86-1.38 eV with the peak position around 1.1 eV predominates at low temperatures while the near band-edge luminescence quenches. The broad band is attributed to the GeGa-VGa self-activated (SA) centers possibly associated with nearby oxygen-related defect complex, and its luminescence persists up to 400 K. The configurational-coordinate modeling finds that the SA defect complex has a thermal activation energy of 150-180 meV and a vibrational energy 26.8 meV. The presence of oxygen does not much affect the SA emission intensity but may have influenced the peak position, vibration frequency and activation energy as compared to other common donor-VGa defects in GaAs.