Electronic states in a-Si:H/c-Si heterostructures

We have investigated PECVD-deposited ultrathin intrinsic a-Si:H layers on c-Si substrates using UV-excited photoemission spectroscopy (hν = 4–8 eV) and surface photovoltage measurements. For samples deposited at 230 °C, the Urbach energy is minimal, the Fermi level closest to midgap and the interface recombination velocity has a minimum. The a-Si:H/c-Si interface density of states is comparable to that of thermally oxidized silicon interfaces. However, the measured a-Si:H dangling bond densities are generally higher than in thick films and not correlated with the Urbach energy. This is ascribed to additional disorder induced by the proximity of the a-Si:H/c-Si interface and H-rich growth in the film/substrate interface region.     

Influence of the amorphous/crystalline silicon heterostructure properties on planar conductance measurements

We report a quasi-analytical calculation describing the heterojunction between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) at equilibrium. It has been developed and used to determine the carrier sheet density in the strongly inverted layer at the a-Si:H/ c-Si interface. The model assumes an exponential band tail for the defect distribution in a-Si:H. The effects of the different parameters involved in the calculation are investigated in detail, such as the Fermi level position in a-Si:H, the density of states and the band offsets. The calculation was used to interpret temperature dependent planar conductance measurements carried out on (n) a-Si:H/ (p) c-Si and (p) a-Si:H/(n) c-Si structures, which allowed us to confirm a previous evaluation of the conduction band offset, ?E_C = 0.18 ± 0.05 eV, and to evaluate the valence band offset: ?E_V = 0.36 ± 0.05 eV at the a-Si:H/ c-Si heterojunction. The results are placed in the frame of recent publications.     

Interface properties of amorphous-crystalline silicon heterojunctions prepared using DC saddle-field PECVD

The planar conductance technique has been previously used in the study of amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunctions, grown using conventional Radio Frequency Plasma Enhanced Chemical Vapor Deposition (RF-PECVD), to examine the existence of a strong inversion layer at the c-Si surface. In the present study such measurements were undertaken on a series of heterojunctions in order to provide insight into the nature of the electrical interface between the hydrogenated amorphous silicon, deposited using the DC Saddle Field PECVD system, and the underlying crystalline silicon wafer. The films showed good passivation and a strong inversion layer, indicating their amenability for device applications.     

Influence of textured c-Si surface morphology on the interfacial properties of heterojunction silicon solar cells

For the HIT solar cells, the properties of interface between intrinsic thin film and c-Si are critical for the resulting device. The interfacial properties mainly depend on the surface passivation quality of c-Si, which is found to be affected by the morphology of textured surfaces. In this study, four kinds of textured c-Si substrates are fabricated: large pyramids without chemical polished (CP), large pyramids with CP, small pyramids without CP and small pyramids with CP. We investigated the effects of textured-surface morphology on the passivation of c-Si, the thin layer coverage and the interfacial properties of heterojunction prepared by HWCVD. Minority carrier lifetime measurements show that the wafer with small pyramids leads to better surface passivation than the one with large pyramids. The good coverage and contact between the thin film and the substrate can be achieved and no epitaxial growth occurs on the wafer with small pyramids through the study of TEM. Dark I-V measurements reveal that the heterojunction on wafer with small pyramids and CP has low recombination at the a-Si:H/c-Si interface. Our results indicate that the surface with small pyramids and low surface roughness is beneficial to the performance of HIT solar cells.     

Correlation between micro-roughness,surface chemistry,and performance of crystalline Si/amorphous Si:H:Cl hetero-junction solar cells

The correlation between micro-roughness, surface chemistry, and performance of crystalline Si/amorphous Si:H:Cl hetero-junction solar cells is discussed through a deposition study of amorphous Si:H:Cl (a-Si:H:Cl) films by rf plasma-enhanced chemical vapor deposition using a SiH₂Cl₂–H₂ mixture. The degree of H- and Cl-termination on the growing surface determined the degree of micro-roughness at the p-type a-Si:H:Cl/intrinsic a-Si:H:Cl interface and solar cell performance. A higher degree of Cl-termination compared to H-termination was effective to suppress the micro-roughness at the growing surface and oxygen incorporation into the film, as well as chemical reduction of the intrinsic a-Si:H:Cl layer during the underneath p-layer formation. The study showed that a-Si:H:Cl deposited from SiH₂Cl₂ is a potential material for c-Si hetero-junction solar cells with an intrinsic a-Si:H:Cl thin layer.     

Cross-contamination in single-chamber processes for thin-film silicon solar cells

Boron (B) and phosphorus (P) cross-contamination for single-chamber deposited a-Si:H, μc-Si:H, and a-Si:H/μc-Si:H tandem solar cells has been investigated by studying their impact on the different layers of solar cells. To reduce the B and P cross-contamination into the i-layer and p-layer, respectively, to a tolerable level, for a-Si:H and μc-Si:H cells a 15' evacuation cycle prior to the i-layer deposition is applied. The effect of P cross-contamination into the i-layer is strongly reduced by the p-layer deposition and a 15’ evacuation cycle prior to the i-layer deposition. The p-layer is assumed to cover up or to fix (in form of P-B complexes) most of the P at the chamber walls. This leads to high quality μc-Si:H cells and a-Si:H cells with only slightly reduced performance. Here, a soft-start of the a-Si:H i-layer led to high quality cells, presumably due to reduced P recycling. Further, there is no need to clean the process chamber with, e.g. NF₃, after each p-layer, as applied in many industrial processes. Instead, many cells are deposited without cleaning the process chamber. We established a single-chamber tandem cell process with 15' evacuation cycles prior to the μc-Si:H p-layer and to each i-layer with a cell efficiency of ~ 11.1%.     

H NMR in μc-Si:H

NMR, IR, ESR, Raman scattering and X-ray diffraction measurements were performed in μc-Si:H prepared by various methods. Results of H NMR in some films are qualitatively similar to those i n a-Si:H, but the NMR lines exhibit a motional narrowing. Other films which exhibit sharp IR peaks exhibit H NMR signal shape different from that in a-Si:H.     

Variation of the defect density in a-Si:H and μc-Si:H based solar cells with 2 MeV electron bombardment

Improvement of the performance of solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon requires understanding of the role of the deep defects – dangling bonds – in the bulk of the intrinsic a-Si:H or μc-Si:H absorber layers. A straightforward way to understand how these defects may affect the performance of the cells is to investigate changes in the device performance upon variation in the defect density.In the present work solar cells with a-Si:H and μc-Si:H absorber layers were exposed to 2 MeV electron bombardment. The performance of the cells after various bombardment doses and annealing steps was evaluated in view of the changes in the defect density of intrinsic layers, measured with ESR on nominally identical absorber layers irradiated in parallel with the cells.The defect density was varied over a range of 2 orders of magnitude. In the solar cells a strong degradation of performance is observed upon irradiation with the biggest effect on the short circuit current density J_SC for both types of absorber layers. In most cases both V_OC and J_SC recover after the final annealing step (at 160 °C) for both types of cells.     

Determination of the defect density in thin film amorphous and microcrystalline silicon from ESR measurements: The influence of the sample preparation procedure

Accurate evaluation of the defect density (N_D) is of high relevance for the optimization of thin film silicon. The spin density (N_S) measured in ESR experiments is often used as a measure for the density of deep defects in the material, assuming that all defects are in a paramagnetic charge state. However, exposure to air, water, or acid during ESR sample preparation can potentially change the N_S in a sample and lead to misinterpretation of N_D. We have investigated how the preparation procedures of a Si thin film ESR sample may affect the properties of its ESR spectrum. Samples of different structural composition from highly crystalline μc-Si:H to a-Si:H deposited by PECVD on Mo-foil, Al-foil and ZnO:Al were studied for different states of exposure to ambient conditions and annealing. N_S measured directly after sample preparation and after air exposure was found to be higher than N_S measured in the annealed state. Particularly in highly crystalline material this discrepancy may reach one order of magnitude. On the other hand in a-Si:H and medium crystalline μc-Si:H relevant for applications, the difference in N_S between air-exposed and annealed conditions is smaller. ESR measurements performed at 40 K suggest that atmospheric exposure leads to charging of the defect states, which in turn influences the evaluated spin density.     

Investigation of gap states in phosphorous-doped ultra-thin a-Si:H by near-UV photoelectron spectroscopy

A variant of photoelectron spectroscopy with near-UV light excitation was established and applied to an n-type doping series of ultra-thin a-Si:H layers (layer thickness ～10 nm). Using this technique, the position of the surface Fermi level E_Fs is obtained and the density of recombination active defect states in the a-Si:H band gap down to ～10¹⁵ states/cm³ can be detected. Defect densities are generally about one order of magnitude higher than in the bulk of thicker (several 100 nm) layers, and the minimum achievable distance of E_Fs from the conduction band is ～360 mV for doping with 10⁴ ppm PH₃. The optimum doping for the fabrication of solar cells is almost one order of magnitude lower. This discrepancy may be explained by enhanced recombination at the a-Si:H/c-Si interface at high doping levels, and in addition by an efficient recombination pathway where charge carriers tunnel from c-Si via a-Si:H band tail states into the a-Si:H and subsequently recombine at dangling bond states.     

The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries

Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder.However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses.     

Electron spin resonance studies of microcrystalline and amorphous silicon irradiated with high energy electrons

Paramagnetic defects in μc-Si:H and a-Si:H with various structure compositions were investigated by electron spin resonance (ESR). The defect density was varied by high energy electron bombardment and subsequent annealing. The spin density increases by up to 3 orders of magnitude. In most cases the initial spin density can be restored upon annealing at 160 °C.     

NMR study of μc-Si:H

The behavior of hydrogen in glow-discharge (GD) μc-Si:H has been characterized by ¹H NMR. The ¹H spectra consist of two components with different linewidths. The linewidth (FWHM) of the narrow component is about 0.5 kHz at ν₀ = 90 MHz, being much narrower than has been observed in GD a-Si:H deposited under a conventional low RF-power condition. It has been demonstrated that the 0.5-kHz FWHM component originates from the hydrogens in motional narrowing state, and such fast-moving hydrogens are incorporated both in μc-Si:H and high-power deposited a-Si:H.     

Structure of the ESR spectra of thin film silicon after electron bombardment

Defect creation by MeV electron bombardment of a-Si:H and μc-Si:H thin films is used to explore hidden features of the electron spin resonance spectra. Different dynamics of creation and annealing for different paramagnetic states is expected and found. In a-Si:H the g-value of the db resonance does not change after irradiation, but a pair of satellites is observed on its wings. In the spectra of μc-Si:H three additional lines can be extracted after irradiation, overlapping with the central resonance. Careful analysis of the spectra shows also modification of the dangling bond resonance in μc-Si:H that is compatible with variations of two components of the spectra and supports the model of two dominant defect states in μc-Si:H.     

An X-ray scattering study of hydrogenated nanocrystalline silicon with varying crystalline fraction

Using X-ray diffraction (XRD) and small angle X-ray scattering (SAXS), we probed the nanostructural features of several PECVD grown nc-Si:H thin films with varying crystalline volume fraction. XRD results of a mixed phase film, 70% a-Si:H and 30% c-Si:H, show these crystallites have a preferred [220] orientation in the growth direction. Another film with approximately 90% c-Si also shows elongated grains, but with a preferred [111] orientation. The SAXS results also show an increase in scattering intensity when compared to the mixed phase material. In the mixed phase material, models show that the electron density fluctuations between the amorphous and crystalline phases are not enough to explain the measured SAXS scattering. Hydrogen clustered at the crystallite boundaries and in void regions of the a-Si phase must be included as well.     

Origin of the Voc enhancement with a p-doped nc-SiOx:H window layer in n-i-p solar cells

The introduction of a nc-SiOx:H material as window layer in single junction a-Si:H n-i-p solar cell leads to a V_oc enhancement of 80 mV compared to a μc-Si:H p-layer. According to numerical modeling of the V_oc, both the higher work function p-layer and the conduction band offset (CBO) at the i/p interface match well with the experimental V_oc increase with the oxygen content. Using the differential temperature method, the built-in voltage (V_bi) of the cells with the two different p-layers is measured to be similar, agreeing well with the CBO model. Thus we attribute the improvement of the V_oc to the reduction of recombination at the i/p interface, as a consequence of the CBO in this region.     

X-ray emission spectra and electronic structure of amorphous silicon

X-ray K and L emission bands of c-Si and a-Si are reported and compared with available XPS and UPS measurements. The experimental results for a-Si are found to be consistent and in excellent agreement. From comparison of the experimental results with available electron density-of-states calculations based on different structure models of the amorphous state, we conclude that only ST-12 structure and the Polk-Boudreaux model provide results that are compatible with experiment.Furthermore we have studied the high energy L satellite band of c-Si and a-Si.     

Similarities and differences in the field-induced doping effect of n-type and of intrinsic a-Si : H

Field-induced doping was studied in doped and undoped (intrinsic) a-Si:H samples. It was found that field-induced doping occurs in n-type and intrinsic materials. Capacitance and current measurements on doped and undoped Schottky-diode samples show changes in the capacitance and conductivity after heat treatments at 400 K while keeping the specimen under high electric field. These changes depend on the field direction and can be reversed by a new heat treatment with a zero bias. Field-induced doping effects are attributed mainly to changes in the density of shallow states in n-type materials but in the intrinsic material it is mainly due to increases in the number of deep states.     

Distribution of hydrogen in low temperature passivated amorphous silicon (a-Si:H) films from neutron reflectivity

Hydrogen plays a critical role in the passivation of dangling bonds in hydrogenated amorphous silicon (a-Si:H) to enable acceptable semiconducting characteristics during operation in devices. Low temperature processing enables fabrication of high performance transistors on flexible substrates such as plastic or stainless steel foils, but also leads to a decrease in the stability of the electronic performance. Generation of defects at the a-Si:H/insulator (hydrogenated silicon nitride, SiN:H) during electrical use due to localized heating will lead to decreased performance unless the dangling bonds are passivated in-situ by residual hydrogen. For this reason, the distribution of hydrogen within a-Si:H may be critical to understanding their aging phenomena. Here the distribution of hydrogen within both a-Si:H and SiN:H layers is probed with sub-nanometer resolution using neutron reflectivity. The hydrogen concentration within the bulk of the a-Si:H (11 ± 2 at.%) and SiN:H (18 ± 3 at.%) agree well with previous reports, but the increased resolution of the neutron measurement is able to identify an approximate three fold increase in the concentration within 2 nm of the semiconductor-insulator interface. This enhanced hydrogen content may act in the short-term as a sink to passivate any dangling bonds formed during operation.