Electrochemical Capacitance Voltage Measurements Research Articles

Low-temperature aluminum doped and induced polysilicon and its application as partial rear contacts on p-type silicon solar cells

Low-temperature contact techniques offer benefits for various solar cells, including tandem solar cells, which risk degradation from high-temperature contact fabrication for top cell materials, such as in III-V/Si multijunction cells. This paper presents aluminum (Al) doped and induced poly-Si (formed through annealing at 190 °C for 5 min) as localized contacts on p-type silicon absorbers. This approach avoids the high-temperature processes required for poly-Si formation and the toxic gases involved in in-situ doping through chemical vapor deposition. Raman spectroscopy confirms poly-Si formation due to Al-induced recrystallization of a-Si:H(i), with electrochemical capacitance-voltage (ECV) measurements validating Al doping in the newly-formed poly-Si. The contact stack of p-Si/a-Si:H(i)/Al exhibits ohmic behavior after annealing at 190 °C for 5 min, achieving a contact resistivity of ∼13.2 mΩ⋅cm2. Finally, a cell featuring a localized Al-doped poly-Si contact structure is fabricated, realizing a 3% absolute efficiency improvement compared to a full-area Al back contact cell.

An In-Depth Study of the Boron and Phosphorous Doping of GeSn

We have investigated the in situ doping of GeSn with Ge2H6, SnCl4, B2H6 (p-type) or PH3 (n-type) at 349 °C, 100 Torr on Ge Strain-relaxed Buffers, themselves on Si(001) substrates. Our aim was to replace the boron and phosphorous doped Ge layers used in previous pin structures with GeSn:B and GeSn:P. The Sn content from Wavelength Dispersive X-ray Fluorescence (WDXRF) was constant around 6.5% for low F(B2H6)/F(Ge2H6) and F(PH3)/2*F(Ge2H6) Mass-Flow Ratios (MFRs). It fell down to 4.9% (GeSn:B) and 6.0% (GeSn:P) for higher MFRs due to the dopant precursors that catalyzed the Ge Growth Rate component. The B atomic concentrations (from Secondary Ion Mass Spectrometry (SIMS)) increased almost linearly with the MFR, reaching values of at most 6.3 × 1019 cm−3. B atoms were almost fully electrically active based on Electrochemical Capacitance Voltage (ECV) measurements. WDXRF and SIMS also gave us access to the atomic P concentrations, with values as high as 3.9 × 1020 cm−3. Maybe because of the formation of SnmPnV nanoclusters, the P electrical activation decreased for the highest flows. Only the GeSn:B layers grown at the three highest MFRs exhibited some B and/or Sn surface segregation. Otherwise, a smooth surface with a cross hatch along 〈110〉 was present.

Evaluation of secondary electron intensities for dopant profiling in ion implanted semiconductors: a correlative study combining SE, SIMS and ECV methods

This study evaluates the secondary electron (SE) dopant contrast in scanning electron microscopy (SEM) and helium ion microscopy (HIM) on boron implanted silicon sample. Complementary techniques like secondary ion mass spectrometry and electrochemical capacitance voltage (ECV) measurements are used to understand the dopant profile and active dopant distribution before and after a thermal firing, a step carried out to remove implantation damage and to electrically activate the implanted boron. Thermal firing resulted in an activation efficiency of 33%. HIM showed higher contrast than SEM having more defined peak with a lower background contribution. Variations in dopant concentration near the peak maximum were observed in ECV measurements, which was not observed in the intensity profiles from both SEM and HIM. This study demonstrates the effectiveness of SE dopant profiling as a quick tool to map the electrically active dopant concentrations even in far-from-equilibrium materials such as ion implanted samples.

Improved performance on multi-crystalline silicon solar cells by optimizing aluminum back surface field process

Traditional aluminum back surface field (Al-BSF) multi-crystalline silicon (mc-Si) solar cells have been favored by the market for a long time due to their low cost. However, the Al-BSF formed after Al screen printing and firing restrict the doping profile and the passivation effect. Here we report a scheme to produce the Al-BSF in advance (before phosphorus diffusion) by using the additional preparation processes consisting of three successive steps, namely Al screen printing, annealing and Al removing. The Al removing process helps optimize the depth of Al-BSF, while the annealing process and the subsequent diffusion process helps improve the doping profiles. As a result, the passivation performance of such improved Al-BSF is greatly improved in terms of electrochemical capacitance voltage measurements (ECV) and minority carrier lifetime characterizations. Also the average conversion efficiency for such mc-Si cells with improved Al-BSF reaches 18.73%, which is 0.27% higher than that of the same batch of traditional Al-BSF solar cells. This approach represents an efficient method for further improving mc-Si solar cells.

Delta-doped β-(AlxGa1−x)2O3/Ga2O3 heterostructure field-effect transistors by ozone molecular beam epitaxy

Heterojunction field-effect transistors based on the β-(AlxGa1−x)2O3/Ga2O3 heterostructure grown by ozone-assisted molecular beam epitaxy were demonstrated for the first time. Al composition ratios in the 14%–23% range were validated using x-ray diffraction on the three samples grown for this study. Electrochemical capacitance-voltage (ECV) measurements showed the presence of a charge sheet in the delta-doped (AlxGa1−x)2O3 barrier layer. Secondary ion mass spectroscopy and ECV measurements also revealed an unintentional Si peak at the (AlxGa1−x)2O3/Ga2O3 interface. Direct current (IDS-VGS) and transconductance (Gm-VGS) measurements demonstrated depletion-mode transistor operation as well as the presence of a parallel conduction channel. A one-dimensional Poisson model suggested that dopant redistribution in the delta-doped region could cause a secondary channel to form in the barrier in addition to the primary channel near the (AlxGa1−x)2O3/Ga2O3 interface under certain conditions met in these samples. Fabricated devices on sample A did not exhibit breakdown up to the measurement limit of 1100 V, with stability after ten cycles. A maximum output drain current density of 22 mA/mm was measured on sample B. Room temperature Hall measurements yielded a sheet carrier density of 1.12 × 1013 cm−2 with corresponding Hall mobility of 95 cm2/V s in sample C.

Effect of germanium auto-diffusion on the bond lengths of Ga and P atoms in GaP/Ge(111) investigated by using X-ray absorption spectroscopy.

The germanium auto-diffusion effects on the inter-atomic distance between the nearest neighbors of the Ga atom in GaP epilayers are investigated using high-resolution X-ray diffraction (HRXRD) and X-ray absorption spectroscopy. The GaP layers grown on Ge (111) are structurally coherent and relaxed but they show the presence of residual strain which is attributed to the auto-diffusion of Ge from the results of secondary ion mass spectrometry and electrochemical capacitance voltage measurements. Subsequently, the inter-atomic distances between the nearest neighbors of Ga atom in GaP are determined from X-ray absorption fine-structure spectra performed at the Ga K-edge. The estimated local bond lengths of Ga with its first and second nearest neighbors show asymmetric variation for the in-plane and out-of-plane direction of GaP/Ge(111). The magnitude and direction of in-plane and out-of-plane microscopic residual strain present in the GaP/Ge are calculated from the difference in bond lengths which explains the presence of macroscopic residual tensile strain estimated from HRXRD. Modified nearest neighbor configurations of Ga in the auto-diffused GaP epilayer are proposed for new possibilities within the GaP/Ge hetero-structure, such as the conversion from indirect to direct band structures and engineering the tensile strain quantum dot structures on (111) surfaces.

Dissolution of donor-vacancy clusters in heavily doped n-type germanium

The n-type doping of Ge is a self-limiting process due to the formation of vacancy-donor complexes (D n V with n ⩽ 4) that deactivate the donors. This work unambiguously demonstrates that the dissolution of the dominating P4V clusters in heavily phosphorus-doped Ge epilayers can be achieved by millisecond-flash lamp annealing at about 1050 K. The P4V cluster dissolution increases the carrier concentration by more than three-fold together with a suppression of phosphorus diffusion. Electrochemical capacitance–voltage measurements in conjunction with secondary ion mass spectrometry, positron annihilation lifetime spectroscopy and theoretical calculations enabled us to address and understand a fundamental problem that has hindered so far the full integration of Ge with complementary-metal-oxide-semiconductor technology.

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.

Development of Conductive SiCx:H as a New Hydrogenation Technique for Tunnel Oxide Passivating Contacts.

Conductive hydrogenated silicon carbide (SiCx:H) is discovered as a promising hydrogenation material for tunnel oxide passivating contacts (TOPCon) solar cells. The proposed SiCx:H layer enables a good passivation quality and features a good electrical conductivity, which eliminates the need of etching back of SiNx:H and indium tin oxide (ITO)/Ag deposition for metallization and reduces the number of process steps. The SiCx:H is deposited by hot wire chemical vapor deposition (HWCVD) and the filament temperature (Tf) during deposition is systematically investigated. Via tuning the SiCx:H layer, implied open-circuit voltages (iVoc) up to 742 ± 0.5 mV and a contact resistivity (ρc) of 21.1 ± 5.4 mΩ·cm2 is achieved using SiCx:H on top of poly-Si(n)/SiOx/c-Si(n) stack at Tf of 2000 °C. Electrochemical capacitance-voltage (ECV) and secondary ion mass spectrometry (SIMS) measurements were conducted to investigate the passivation mechanism. Results show that the hydrogenation at the SiOx/c-Si(n) interface is responsible for the high passivation quality. To assess its validity, the TOPCon stack was incorporated as rear electron selective-contact in a proof-of-concept n-type solar cells featuring ITO/a-Si:H(p)/a-Si:H(i) as front hole selective-contact, which demonstrates a conversion efficiency up to 21.4%, a noticeable open-circuit voltage (Voc) of 724 mV and a fill factor (FF) of 80%.

Influence of Zn–Si donor–acceptor pair traps in invert InGaP/GaAs dual-junction solar cell by MOCVD

In In(Ga)P materials, Zn atoms diffuse into the adjacent Si doping layer at high temperatures, forming Zn–Si donor–acceptor pairs which function as non-radiation carrier recombination centers. The traps lead to decreased carrier diffusion length and separation efficiency in the space charge region of InGaP subcell, resulting in degradation of photoelectric conversion efficiency. In this paper, by adopting an InGaP p–i–n junction structure, the invert growth thin film cell was able to reach the same external quantum efficiency performance as the upright InGaP/GaAs dual-junction solar cell. The Zn doping peak was not observed at the Si doping interface layer in the electrochemical capacitance–voltage measurement. A 675 nm peak was observed at 5 and 30 K and absent in 77 K, as was resolved by low-temperature PL measurement. The peak showed a strong correlation with the Zn diffusion into adjacent Si doping InGaP layer. We speculate that the 675 nm peak represented the energy of Zn–Si donor–acceptor pair traps.

Characterization of tunnel oxide passivated contact with n-type poly-Si on p-type c-Si wafer substrate

The junction properties of tunnel silicon oxide (SiOx) passivated contact (TOPCon) with n-type poly-Si on p-type c-Si wafer are characterized using current-voltage (J-V) and capacitance-voltage (C-V) measurements. The dark J-V curves show a standard diode characteristic with a turn-on voltage of ∼0.63 V, indicating a p-n junction is formed. While the C-V curve displays an irregular shape with features of 1) a slow C increase with the decrease of the magnitude of reverse bias voltage, being used to estimate the built-in potential (Vbi), 2) a significant increase at a given positive bias voltage, corresponding to the geometric capacitance crossing the ultrathin SiOx, and 3) a sharp decrease to negative values, resulting from the charge tunneling through the SiOx layer. The C of depleting layer deviates from the normal linear curve in the 1/C2-V plot, which is caused by the diffusion of P dopants from the n-type poly-Si into the p-type c-Si wafer as confirmed by the electrochemical capacitance-voltage measurements. However, the 1/C2+γ-V plots with γ > 0 leads to linear curves with a proper γ and the Vbi can still be estimated. We find that the Vbi is the range of 0.75–0.85 V, increases with the increase of the doping ratio during the poly-Si fabrication process, and correlates with the passivation quality as measured by the reverse saturated current and implied open circuit voltage extracted from transient photoconductivity decay.

Silicon Tunnel Junctions Produced by Ion Implantation and Diffusion Processes for Tandem Solar Cells

This work reports on the fabrication of silicon tunnel junctions that can be used in silicon-based bottom cells for tandem solar cells. The tunnel junction was formed on n-type monocrystalline silicon wafers using the following two different processes: 1) a thermal diffusion of boron followed by an arsenic ion implantation; and 2) successive implantation steps of boron and arsenic ions. For both the processes, a rapid thermal annealing step was used to activate the dopants and cure the defects generated by the ion implantation. The activated dopant distribution across the tunnel junction was determined by electrochemical capacitancevoltage (ECV) measurements. The quality of the device was assessed from the peak-to-valley ratio and peak current density of its currentvoltage characteristics, as well as the resistivity. As a significant result, a peak current density value and a peak-to-valley ratio of approximately 50 A/cm2 and 1.7, respectively, were obtained for two consecutive implantations of boron and arsenic, followed by thermal annealing at 925C for 30 min. A very narrow depletion zone of 3.9nm was calculated from the ECV dopant concentrations measurements under full depletion approximation. Finally, solar cells fabricated using this optimized process resulted in a conversion efficiency of 10.1 against 12 obtained from the reference monocrystalline solar cell.

Role of Oxygen for Boron Diffusion From Borosilicate Glass Layers Deposited by Atmospheric Pressure Chemical Vapor Deposition

We investigate the impact of different oxygen concentrations during boron diffusion at 950°C using borosilicate glass (BSG) layers deposited by atmospheric pressure chemical vapor deposition. Therefore, we vary the boron concentration in the BSG layer and the oxygen concentration in the gaseous ambient of the high-temperature step. Characterization includes sheet resistance as well as electrochemical capacitance voltage measurements for the determination of the resulting boron doping profile. In addition, we perform quasi steady-state photoconductance measurements to investigate the influence of the oxygen concentration on the recombination parameters of the boron-diffused surface. Low recombination current densities J 0 of J 0 = 38 fA/cm² at a sheet resistance of 54 Ω/sq are observed. Finally, we show that the oxygen concentration is a crucial parameter to prevent the formation of a boron-rich layer at the silicon surface.

Rapid, deep dopant diffusion in crystalline silicon by laser-induced surface melting

As solar cell back contact schemes have improved in recent years, the non-ideal emitter region has become a dominant contributor to overall losses in solar cell efficiency. Our analysis shows that these losses can be greatly reduced by creating a lightly (< 1018 cm−3), uniformly doped emitter that is about 10 µm deep. However, the constraints imposed by current manufacturing techniques, namely using a tube-furnace for solid state dopant diffusion, limits the emitter to a much more highly doped (~ 1020 cm−3) and shallow (< 500 nm) region. A laser-based process by which dopants are diffused in a liquid silicon state can overcome these limitations. In this work, we provide semiconductor device simulations that compare the effects of these two types of emitters on device performance. Then, we used a heat transport model to demonstrate that a focused laser line beam can create a molten region at the surface of a silicon wafer that would allow dopants to diffuse deeply within a short amount of time. We then performed the laser processing with a scanned laser line beam on preheated silicon samples. The resulting junction depth (> 10 µm) is measured by cross-sectional etching and electron beam-induced current measurements. Electrochemical capacitance-voltage measurements are used to measure both the depth as well as the dopant profile resulting from the laser-driven process. We also analyse dislocation defects induced by the large thermal gradients inherent to the laser-based process which would greatly limit the device performance.

Functionalization of SiO2 Surfaces for Si Monolayer Doping with Minimal Carbon Contamination.

Monolayer doping (MLD) involves the functionalization of semiconductor surfaces followed by an annealing step to diffuse the dopant into the substrate. We report an alternative doping method, oxide-MLD, where ultrathin SiO2 overlayers are functionalized with phosphonic acids for doping Si. Similar peak carrier concentrations were achieved when compared with hydrosilylated surfaces (∼2 × 1020 atoms/cm3). Oxide-MLD offers several advantages over conventional MLD, such as ease of sample processing, superior ambient stability, and minimal carbon contamination. The incorporation of an oxide layer minimizes carbon contamination by facilitating attachment of carbon-free precursors or by impeding carbon diffusion. The oxide-MLD strategy allows selection of many inexpensive precursors and therefore allows application to both p- and n-doping. The phosphonic acid-functionalized SiO2 surfaces were investigated using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy, whereas doping was assessed using electrochemical capacitance voltage and Hall measurements.

Minimising bulk lifetime degradation during the processing of interdigitated back contact silicon solar cells

AbstractIn this work, we develop a fabrication process for an interdigitated back contact solar cell using BBr3 diffusion to form the p+ region and POCl3 diffusion to form the n+ regions. We use the industry standard technology computer‐aided design modelling package, Synopsys Sentaurus, to optimize the geometry of the device using doping profiles derived from electrochemical capacitance voltage measurements. Cells are fabricated using n‐type float‐zone silicon substrates with an emitter fraction of 60%, with localized back surface field and contact holes. Key factors affecting cell performance are identified including the impact of e‐beam evaporation, dry etch damage, and bulk defects in the float zone silicon substrate. It is shown that a preoxidation treatment of the wafer can lead to a 2 ms improvement in bulk minority carrier lifetime at the cell level, resulting in a 4% absolute efficiency boost.

Electrochemical capacitance–voltage measurements and modeling of GaAs nanostructures with delta-doped layers

The paper presents the results of electrochemical capacitance-voltage profiling and simulation of quantum-sized semiconductor structures with quantum wells and delta-doped layers based on gallium arsenide. The experimental ECV data were obtained by superposition of measured capacitance-voltage characteristics during the gradual etching of the nanostructure. As a result of simulation, the concentration distribution and energy lineups for structures with delta-layers and quantum wells in gallium arsenide were calculated. The results of simulation are in qualitative agreement with the experimental results and data found in literature.

Simultaneous measurement of doping concentration and carrier lifetime in silicon using terahertz time-domain transmission

In this work, we present a measurement approach enabling the simultaneous determination of sheet resistance and carrier lifetime in semiconductor samples. It is based on a classic Terahertz (THz) time-domain transmission spectroscopy scheme extended by quasi-steady state optical excitation. The carrier lifetime is determined by contactless THz probing of the increase in sheet conductance associated with quasi-steady-state excitation. Combining a successive etch-back of the surface with repeated THz measurements yields a depth profile of the doping concentration and the carrier lifetime, which is important for the optimization of the emitter of solar cells, for instance. The viability of our approach is demonstrated by investigating a phosphorous doped emitter of a silicon solar cell with the THz approach and comparing the results with electrochemical capacitance voltage measurements.

Oxidation Drive-In to Improve Industrial Emitter Performance by POCl3 Diffusion

In this paper, we investigate the influence of different N2/O2 ambient compositions during POCl3 diffusion on the emitter quality, characterized by secondary ion mass spectrometer and electrochemical capacitance–voltage measurements ( J 0E) and internal quantum efficiency. It is presented that O2 concentration during the drive-in step strongly impacts both emitter electrical properties and cell performances, supported by the loss breakdown analysis. This process can be easily adapted to the industry, improving the emitter quality for screen-printed solar cells.

A Comparison of Zinc and Carbon Doped on the Atomic Interdiffusion of InGaAs/AlGaAs Quantum Wells Laser Structures After Annealing

We have compared a zinc and a carbon doped on the atomic interdiffusion of InGaAs/AlGaAs after annealing using Electrochemical capacitance voltage (EC-V), X-ray diffraction and photoluminescence (PL) measurements. Electrochemical capacitance voltage measurements revealed that the carrier concentration in the Zn-doped p++GaAs contact layers decreased after annealing at 900oC for 60 sec, indicating that some of the Zn acceptors were passivated or outdiffused from the surface. In contrast to the C-doped samples, an increase of carrier concentration was observed after annealing. X-ray rocking curve confirmed this result where the amount of lattice contraction increase after annealing which is attributed to the presence of the substitutional carbon CAs. Photoluminescence results showed that a large energy shift was observed in the Zn-doped samples compare with C- doped samples. Photoluminescence measurements after etching to various depth showed similar luminescence defects in both Zn- and C-doped samples. Photocurrent measurements showed the quality of quantum well was improve after annealing in C-doped samples due to activation of carbon doped.