Ga Profile Research Articles

Quantitative analysis of effect of dopant interaction on microstructural, physical, and electrical properties in laser-annealed SiGe:B:Ga film

Lowering the contact resistivity at the metal/semiconductor interface is a necessary requirement in the development of next-generation p-type metal-oxide-semiconductor logic devices. The contact resistivity can be lowered by B and Ga co-ion implantation followed by nanosecond laser annealing (NLA), a state-of-the-art annealing method, which can effectively incorporate dopants into the Si1-xGex lattice, surpassing the solid solubility limit. When only the Ga-doped Si1-xGex (SiGe:Ga) layer is used as a source/drain material, the electrical characteristics deteriorate in terms of bulk resistivity and device reliability owing to the graded Ga profile. However, it is possible to implement a uniform B profile in the B and Ga co-doped Si1-xGex (SiGe:B:Ga) layer during NLA. In this study, we quantitatively investigated the effects of the B and Ga interactions on the microstructure, surface roughness, and lattice parameters of the laser-annealed SiGe:B:Ga films. We observed decreased defects on the surface and improved surface morphology after B co-doping; after analysis through reciprocal space mapping, we deduced that Ga had a higher contribution to the lattice parameter than B at the same B and Ga ion-implantation dose. Furthermore, by measuring the sheet resistance according to the type of dopant (B and Ga), we compared the electrical properties of the laser-annealed III-element-doped Si1-xGex films with the B and Ga profiles after NLA. Our comprehensive study provides information on the behavior and interaction of B and Ga during NLA, thus presenting a path to understanding the physical and electrical properties of SiGe:B:Ga films.

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

A novel method to determine Ga profile in CIGS thin film compound via focused ion beam modification

Ga profile is a critical design in CIGS thin film solar cells to enhance device performance. Many techniques have been exploited to determine Ga profiles including secondary ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), Raman depth profiling and Energy-dispersive X-ray spectrometry in a scanning electron microscope (SEM-EDX) etc. These techniques, when considering deployment in a manufacturing environment, are either time-consuming or expensive. Herein, a novel method is developed in this work to obtain Ga profiles by analyzing the Ga nanodots formed on CIGS cross-section under Focused Ion Beam (FIB) modification. A model of the formation of the Ga nanodots has been established and a mathematical method has been developed to process their depth distribution. Good agreement has been reached between the developed method and AES measurement. This method provides the fast-growing CIGS industry a fast, precise and cost-effective way to determine Ga profiles to realize process monitoring.

Tuning Ga Grading in Selenized Cu(In,Ga)Se2 Solar Cells by Formation of Ordered Vacancy Compound

Severe Ga accumulation at the back contact is regarded as one of the major limiting factors for high efficiency Cu(In,Ga)Se2 (CIGSe) solar cells fabricated by the sequential process. The Ga deficiency near the front surface of absorber leads to the reduction of open‐circuit voltage; however, controlling the Ga grading is quite challenging during the selenization process. Herein, the tuning of Ga profile is demonstrated by forming the ordered vacancy compound (OVC) phase at the beginning of selenization process under high Se partial pressure (PSe) with the precursor composed of a relatively high In content. The OVC phase formed with a columnar structure promotes Ga diffusion through Cu vacancies, resulting in Ga increase at the front side of CIGSe. However, high PSe also increases the thickness of MoSe2, significantly raising the series resistance. Thereby, a 5 nm TiN layer is deposited on top of Mo to suppress the formation of MoSe2. In addition, the extra Na‐contained precursor is added to increase the carrier concentration because the TiN layer also blocks the Na outdiffusion from the substrate. By controlling PSe, interlayer TiN thickness, and carrier concentration, a 16.04% record high efficiency of CIGSe solar cells (sulfur‐free) via elemental Se source is achieved.

Engineering a Ga-Gradient by One-Step Sputtering to Achieve Over 15% Efficiency of Cu(In,Ga)Se2 Flexible Solar Cells without Post-selenization.

A one-step sputtering process using a quaternary target has been demonstrated to be a simple route to form Cu(In,Ga)Se2 (CIGSe) absorber without post-selenization; however, the lack of a Ga-grading structure in the CIGSe absorber confines its efficiency. Here, we demonstrate a one-step cosputtering process to control the Ga profile in the CIGSe absorber on flexible stainless steel substrates. Special attention was paid to the formation of second phases and their effects on the cell performance. Although the normal Ga-grading and efficiency enhancement could be achieved by cosputtering of CIGSe and Ga2Se3 targets, high-energy ion bombardment during the sputtering process might cause the decomposition of the Ga2Se3 target, leading to the formation of Ga2O3 in the CIGSe absorber, which gradually degraded the device performance. We replaced the Ga2Se3 target with a stoichiometric CuGaSe2 target for cosputtering, which can further enhance the cell efficiency due to the elimination of Ga2O3. However, when the Ga content at the back side of CIGSe is further increased by raising the deposition power of the CuGaSe2 target, the phase separation of CuGaSe2 may take place, resulting in the formation of Cu2-XSe and CuGaSe2 at the back side of the CIGSe absorber; therefore, the recombination at the back side is increased. By cosputtering a CIGSe target with a Cu-deficient CuGaSe2 target, we can suppress the formation of second phases and achieve designable normal grading, leading to the highest efficiency of 15.63% without post-selenization on flexible substrates.

Alkali treatment for single-stage co-evaporated thin CuIn0.7Ga0.3Se2 solar cells

KF and NaF treatments were done for single-stage co-evaporated CuIn0.7Ga0.3Se2. The absorber layers were grown on a substrate with an alkali barrier layer and NaF was either added before or after absorber layer growth. No differences were found on the device performance amongst the procedures to add Na. This is expected if the single-stage process does not have a copper rich stage or a Ga gradient, which is likely since there was no change of the elemental fluxes during absorber layer growth and no Ga profile was measured. KF was added by post-deposition only. Current-voltage characteristics were measured and net doping concentrations were determined from capacitance-voltage measurements (CV). We see an improvement of the open-circuit voltage (Voc) with increasing KF amount, and a marginal increase of the fill factor. CV measurements showed increasing net acceptor concentration with increasing KF amount. Time resolved photoluminescence (PL) showed an increased decay time for KF treated cells and the PL peak shape changed. Without KF treatment the PL peak is symmetric, after KF treatment a further peak appears at higher energy in the PL spectrum. This higher energy peak increases in intensity with increasing KF concentration. The same effects were seen in a sample without Na, but here the Voc was limited due to large tailing. Hence both Na and K are required for good cell efficiencies.

Interplay between potassium doping and bandgap profiling in selenized Cu(In,Ga)Se2 solar cells: A functional CuGa:KF surface precursor layer

The progress of selenized Cu(In,Ga)Se2 (CIGSe) solar cells is limited by low open-circuit voltage (Voc), which results from the Ga-deficient surface and undesirable bandgap profile after selenization. Controlling the Ga grading, especially on the CIGSe surface, is challenging but critical for further efficiency improvement. Here, the simple sputtering route with K incorporation is presented to engineer single- or double-graded bandgap. The K incorporation through sputtered precursors can considerably affect the Ga profile in CIGSe during selenization, essentially distinct from reported KF post-deposition treatment, in which the Ga profile keeps unchanged. Using synchrotron-based X-ray absorption spectroscopy and first-principle calculations, we verify that Ga diffusion via Cu vacancies is restrained due to the presence of KCu defects. Therefore, by introducing a CuGa:KF surface layer on the bi-layer precursors, the surface Ga content of CIGSe is increased effectively, achieving a notch-like Ga distribution after selenization. This unique CuGa:KF layer significantly boosts the Voc and yields over 15% efficiency, even with a low reactive Se vapor for selenization. Our approach, completely compatible with the existing fabrication process, offers a new direction for engineering band structure without sulfurization.

Rear Contact Passivation for High Bandgap Cu(In, Ga)Se<formula> <tex>$_{2}$</tex> </formula> Solar Cells With a Flat Ga profile

In this study, Cu(In, Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> solar cells with a high bandgap (1.31 eV) and a flat Ga profile ([Ga]/([Ga]+[In]) ≈ 0.60) were examined. For absorber layer thicknesses varying from 0.60 to 1.45 μm, the Mo rear contact of one set of samples was passivated with an ultrathin (27 nm) Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer with point contact openings, and compared with reference samples where the rear contact remained unpassivated. For the passivated samples, mainly large gains in the short-circuit current led to an up to 21% (relative) higher power conversion efficiency compared with unpassivated cells. The differences in temperature-dependent current voltage behavior between the passivated and the unpassivated samples and the thin and the thick samples can be explained by an oppositely poled secondary photodiode at the rear contact.

CIGS absorber layer with double grading Ga profile for highly efficient solar cells

It is well-known that the band gap grading in CIGS solar cells is crucial for achieving highly efficient solar cells. We stimulate a CIGS solar cell and investigate the effects of the band gap grading on performance of the CIGS solar cell, where Ga/(Ga + In) ratio (GGI) at back (Cb) and front (Cf) of the absorber layer are considered constant. Our simulations show that by increasing the GGI at middle of CIGS absorber layer (Cm), the JSC decreases and VOC increases independent of the distance of the Cm from the back contact (Xm). For Cm lower than Cf, JSC increases and VOC decreases when the Xm shifts to the front of the CIGS layer. The behavior of JSC and VOC became reverse for the case of Cm greater than Cf. Almost in all of the structures, efficiency and FF have same behaviors. Our simulations show that the highest efficiency is obtained at Cm = 0.8 and Xm = 200 nm.

Controlled back slope of Ga/(In+Ga) profile in Cu(In,Ga)Se2 absorber fabricated by multi layer precursor method for improvement of its photovoltaic performance

Cu(In,Ga)Se2 (CIGS) absorbers with several Ga/III, Ga/(In+Ga) ratio, profiles were deposited by the so-called “multi layer precursor method” consisting of Ga-Se/In-Se/Cu-Se stacks. The effects of average Ga/III ratios and back slopes of the Ga/III profiles on the photovoltaic performance were investigated. The optimum average Ga/III ratio of approximately 0.36 leads to the appropriate average band-gap energy as well as sufficiently long carrier lifetime in CIGS absorber, thus enhancing open-circuit voltage. In addition, the back slope of 0.50μm−1, defined as the ratio of the change in Ga/III ratio to the change in the depth range from 1 to 2μm from CIGS surface, induces back surface field of 0.34V/μm, thereby increasing short-circuit current density. Ultimately, the CIGS absorber on flexible stainless steel substrate with the double graded Ga/III profile (the average Ga/III of 0.36 and the back slope of 0.50μm−1) was fabricated by the multi layer precursor method. This results in the 14.36%-efficient CIGS solar cell on the flexible substrate without an anti-reflective layer.

High-fluence Ga-implanted silicon—The effect of annealing and cover layers

The influence of SiO2 and SiNx cover layers on the dopant distribution as well as microstructure of high fluence Ga implanted Si after thermal processing is investigated. The annealing temperature determines the layer microstructure and the cover layers influence the obtained Ga profile. Rapid thermal annealing at temperatures up to 750 °C leads to a polycrystalline layer structure containing amorphous Ga-rich precipitates. Already after a short 20 ms flash lamp annealing, a Ga-rich interface layer is observed for implantation through the cover layers. This effect can partly be suppressed by annealing temperatures of at least 900 °C. However, in this case, Ga accumulates in larger, cone-like precipitates without disturbing the surrounding Si lattice parameters. Such a Ga-rich crystalline Si phase does not exist in the equilibrium phase diagram according to which the Ga solubility in Si is less than 0.1 at. %. The Ga-rich areas are capped with SiOx grown during annealing which only can be avoided by the usage of SiNx cover layers.

Na effect on flexible Cu(In,Ga)Se2 photovoltaic cell depending on diffusion barriers (SiOx, i-ZnO) on stainless steel

Cu(In,Ga)Se2 (CIGS) based-photovoltaic (PV) cells with different diffusion barriers of SiOx and i-ZnO were fabricated on stainless steel (STS) substrate and their electrical characteristics were investigated by measuring J–V curves under illuminated and dark conditions. The physical properties of the CIGS film depending on type of diffusion barrier were also analyzed using X-ray diffraction and secondary ion mass spectroscopy. The efficiency of the CIGS-PV cell with i-ZnO barrier was approximately 2% higher than that with the SiOx barrier. Through the analysis of dark J–V curves, we discovered that distinctive defects were formed in the band gap of CIGS based on which diffusion barrier contacted the STS. The diffraction pattern showed a slightly different tendency of the peak intensity ratio of (220/204)/(112) in the PV cell with the i-ZnO barrier, which was slightly higher than that in the PV cell with SiOx barrier. In elemental depth profile, a deficient Ga profile was observed near the surface of the CIGS film with the SiOx barrier, and an abundant Na profile within the CIGS film with the i-ZnO barrier was detected. This is attributed to a difference in thermal conduction through the diffusion barriers during CIGS film growth, originating from the larger thermal conductivity of ZnO compared with SiOx.

Ga gradient behavior of CIGS thin films prepared through selenization of CuGa/In stacked elemental layers

In this study, Cu (In, Ga)Se2 (CIGS) film was formed using a two-stage process. Bilayer CuGa/In metallic precursors were sequentially deposited using both CuGa alloy and In targets in magnetron sputtering. Then, the Se layer was coated on the sputter-deposited CuGa/In precursors through evaporation. The diffusion behaviors of each element during the post-selenization annealing process were determined based on FESEM, XRD, and SIMS. The higher annealing temperature was proposed to have improved Ga diffusion toward the film surface, and thus, achieved the growth of more uniform, more densely packed, and larger grains. Moreover, Na and O contents were found to be correlated due to the strong affinity between Na and O. The distribution of Na seems to be similar to the Ga profile, which may be ascribed to the diffusion of Ga, which was retarded by Na and O. The CIGS films fabricated by the two-step method consisted of a single chalcopyrite phase, and the efficiency of the cell fabricated by the films was 2.56%.

The effect of high growth temperature on Cu(In,Ga)Se2 thin film solar cells

The morphological, elemental distribution and electrical performance effects of increasing the Cu(In,Ga)Se2 (CIGS) growth substrate temperature are studied. While the increased substrate growth temperature with no other modifications led to increased CIGS grain size, it also resulted in depth profile flattening of the [Ga]/([Ga]+[In]) ratio. Tuning the Ga profile in the high temperature process led to a more desirable [Ga]/([Ga]+[In]) depth profile and allowed a comparison between high and standard temperature. Devices prepared at higher temperature showed an improved grain size and the electrical performance is very similar to that of the reference sample prepared at a standard temperature.

Effect of Substrate Temperature on Preferred Orientation and Ga Composition Profile of Co-Evaporated Cu(In,Ga)Se2

Cu(In,Ga)Se2 (CIGS) films were deposited by the three-stage co-evaporation process and the effect of the second stage substrate temperature on device performance were evaluated. With increasing the substrate temperature, the preferred orientation of CIGS was found to be changed from (112) to (220), however, the Ga double grading became weaker and nearly flat for the (220) textured CIGS grown at 570°C. Despite of (220) preferred orientation, the conversion efficiency decreased mainly due to the poor fill factor and open circuit voltage. It is suggested that increasing the growth temperature in order to get (220) preferred CIGS may destroy the advantage of double-graded Ga profile.

A study on composition, structure and optical properties of copper-poor CIGS thin film deposited by sequential sputtering of CuGa/In and In/(CuGa+In) precursors

Copper-poor CIGS thin films were fabricated by using two precursor of CuGa/In and In/(CuGa+In) onto the Mo coated soda-lime glass (SLG) by the sequential sputtering of CuGa and In targets. The CIG precursors were converted into CIGS absorption thin film by selenization process. The X-ray diffraction (XRD) patterns of CIGS absorber from CuGa/In precursor exhibits strong (112) preferred orientation that is more stable than (220)/(204) as compared to In/(CuGa+In) precursor. The elemental composition uniformity onto the surface and along depth were extensively analyzed with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). For as-fabricated CIGS thin films from In/(CuGa+In) and CuGa/In precursors, the atomic (at%) composition values of [Cu]//[In+Ga]/=1.14, 1.08 and [Ga]/[In+Ga]=0.33, 0.20 were observed, respectively that is well matched for highest efficient CIGS-based solar cell so far. SIMS confirmed that the Ga profile is not through the depth of CIGS thin film attributed a consistent band gap of 1.04 and 1.08eV. Further, PL spectrum of CIGS absorber formed by CuGa/In precursor exhibits relatively narrow emission peak as compared to In/(CuGa+In) precursor. It is attributed to decrease defect density with uniform composition in the CIGS absorber. The carrier concentration (Np) found to increase from 1020 to 1021cm−3 orders with the increase of Cu/(In+Ga) at.% from 0.93 to 1.02, which is related to the increasing carrier concentration for stoichiometric CIGS films.

The gramicidin channel ion permeation free-energy profile: Direct and indirect effects of CHARMM force field improvements

A revised CHARMM force field for tryptophan residues is studied as well as a new grid-based correction algorithm, called CMAP, using molecular dynamics simulations of gramicidin A (1JNO) embedded in a lipid bilayer (DMPC) with 1 mol/kg NaCl or KCl saline solution. The conformational stability of the interfacial side chains is studied, which shows good stability on the 10 ns time scale. The revised force field for the tryptophan side chain produces, in the decomposition, a Na(+) PMF(Trp) profile that is consonant with the prediction from the experimental results, analyzed with rate theory by Durrant et al. (2006), but in stark contrast to the prediction of the original CHARMM force field, version 22. However, the effect is diluted in the PMF profile due to indirect effects mediated by other components of the system (polypeptide, lipid molecules, ions, and water molecules). CMAP corrections to the L-amino acids help reduce the excessive translocation barrier. Decomposition demonstrates that this effect is due to effects on the K(+) PMF(H(2)O) profile rather than on the K(+) PMF(gA) profile. The results have been confirmed to be robust using an alternative umbrella-potential method. Further force field balancing efforts (direct and indirect) are required for future studies to evaluate whether these effects give rise to predictions that are consistent with those observables extracted from real experiments.

Cu(InGa)Se2 solar cells on a flexible polymer web

AbstractDeposition of CuInGaSe2 films on 6‐inch wide Mo coated polyimide web substrate in a roll‐to‐roll vapor deposition system from elemental sources is described. Material transport from the sources to the moving web substrate has been modeled by combining an evaporative effusion model and the gas flow kinetics and by experimentally determining the flux intensity profile. The model gives a reasonably good approximation of the Ga profile in the films. Poor adhesion of the selenide film to molybdenum has been resolved by depositing a thin layer of β‐(Ga0.8In0.2)2Se3 precursor layer. Two‐dimensional compositional mapping by energy dispersive spectroscopy of 5‐feet‐long web gave 0.88 ± 1.9% and 0.28 ± 2.5% respectively for Cu/(Ga+In) and Ga/(Ga+In) ratios, indicating highly uniform film composition. This compositional uniformity translated to the uniformity in the devices fabricated on the web. The open‐circuit voltages of the devices from the centerline of the 5‐feet web were measured to be 0.529 ± 0.86%. A two‐dimensional device efficiency survey gave 9.2 ± 1%. The process was able to produce high‐quality material, as defined by the best device efficiency, for a wide range of Cu/(Ga+In) ratios. The best efficiency achieved so far was 12.1% with MgF2 anti‐reflection coating. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Cu(In, Ga)(Se, S)2 Absorbers Formed by Rapid Thermal Processing of Elemental Precursors: Analysis of Thin Film Formation and Implementation of A Large Area Industrial Process

AbstractLarge area Cu(In, Ga)(Se, S)2 thin films (CIGSSe) for solar modules are processed by rapid thermal annealing of stacked elemental layers. A pilot line for 60×90 cm2 absorbers and 30×30cm2 modules is now up and running. Modules with efficiencies at 12% are fabricated with excellent structural and electrical uniformity. For the optimization of electrical performance the selenization and sulfurization process is analyzed by x-ray fluorescence (XRFA), x-ray photo-electron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and current voltage (I-V) measurements. Sequentially processed CIGSSe films show characteristic depth distribution profiles of gallium and sulfur. Based upon our previously published model obtained by In-Situ XRD analysis the Ga profile can be explained by the strongly inhibited formation of binary gallium-selenides. The sulfur incorporation is investigated by comparing XRFA data and SIMS profiles of samples from different sulfurization recipes. A characteristic dependence of S on the Cu/(In+Ga) ratio is observed. Two different mechanisms for sulfur incorporation are proposed. It will be shown that the accumulation of sulfur towards the back is predominantly due to the incorporation into an intermediate molybdenum sulfo-selenide layer. Device characterization shows that the Ga and S profiles lead to a favorable double band gap grading structure.

High hydrogen concentrations produced by segregation into p+ layers in silicon

Gallium-implanted p+ layers in Si were exposed to atomic hydrogen from a plasma. It was found that very large hydrogen concentrations, up to 7.5 times larger than the peak Ga concentration of 7×1019/cm3, segregated into the p+ layer during treatment at 200 °C. The shape of the hydrogen concentration profile was similar to that of the Ga profile. Ion channeling showed that the H atoms did not occupy simple high-symmetry sites in the lattice, and electron microscopy revealed the presence of extended {111} stacking fault defects associated with the layer of high hydrogen concentration. A mechanism to account for these findings is suggested.