Lachlan E. Black

On effective surface recombination parameters

This paper examines two effective surface recombination parameters: the effective surface recombination velocity Seff and the surface saturation current density J0s. The dependence of Seff and J0s on surface charge Q, surface dopant concentration Ns, and interface parameters is derived. It is shown that for crystalline silicon at 300 K in low-injection, Seff is independent of Ns only when Q2/Ns   1.5 × 107 cm for accumulation and Q1.85/Ns > 1.5 × 106 cm for inversion. These conditions are commonly satisfied in undiffused wafers but rarely in diffused wafers. We conclude that for undiffused silicon, J0s is superior to the conventional Seff as a metric for quantifying the surface passivation, whereas for dif...

Surface passivation of c-Si by atmospheric pressure chemical vapor deposition of Al2O3

Atmospheric pressure chemical vapor deposition of Al2O3 is shown to provide excellent passivation of crystalline silicon surfaces. Surface passivation, permittivity, and refractive index are investigated before and after annealing for deposition temperatures between 330 and 520 °C. Deposition temperatures >440 °C result in the best passivation, due to both a large negative fixed charge density (∼2 × 1012 cm−2) and a relatively low interface defect density (∼1 × 1011 eV−1 cm−2), with or without an anneal. The influence of deposition temperature on film properties is found to persist after subsequent heat treatment. Correlations between surface passivation properties and the permittivity are discussed.

Effect of boron concentration on recombination at the p-Si–Al2O3 interface

We examine the surface passivation properties of Al2O3 deposited on boron-doped planar ⟨100⟩ crystalline silicon surfaces as a function of the boron concentration. Both uniformly doped and diffused surfaces are studied, with surface boron concentrations ranging from 9.2 × 1015 to 5.2 × 1019 cm−3. Atmospheric pressure chemical vapor deposition and thermal atomic layer deposition are used to deposit the Al2O3 films. The surface recombination rate of each sample is determined from photoconductance measurements together with the measured dopant profiles via numerical simulation, using the latest physical models. These values are compared with calculations based on the interface properties determined from capacitance–voltage and conductance measurements. It is found that the fundamental surface recombination velocity of electrons, Sn0, which describes the chemical passivation of the interface, is independent of the surface boron concentration Ns for Ns ≤ 3 × 1019 cm−3, and in excellent agreement with values ca...

Passivating Contacts for Crystalline Silicon Solar Cells: From Concepts and Materials to Prospects

To further increase the conversion efficiency of crystalline silicon (c-Si) solar cells, it is vital to reduce the recombination losses associated with the contacts. Therefore, a contact structure that simultaneously passivates the c-Si surface while selectively extracting only one type of charge carrier (i.e., either electrons or holes) is desired. Realizing such passivating contacts in c-Si solar cells has become an important research objective, and an overview and classification of work to date on this topic is presented here. Using this overview, we discuss the design guidelines for passivating contacts and outline their prospects.

On the solid phase crystallization of In2O3:H transparent conductive oxide films prepared by atomic layer deposition

Hydrogen-doped indium oxide (In2O3:H) has emerged as a highly transparent and conductive oxide, finding its application in a multitude of optoelectronic devices. Recently, we have reported on an atomic layer deposition (ALD) process to prepare high quality In2O3:H. This process consists of ALD of In2O3:H films at 100 °C, followed by a solid phase crystallization step at 150–200 °C. In this work, we report on a detailed electron microscopy study of this crystallization process which reveals new insights into the crucial aspects for achieving the large grain size and associated excellent properties of the material. The key finding is that the best optoelectronic properties are obtained by preparing the films at the lowest possible temperature prior to post-deposition annealing. Electron microscopy imaging shows that such films are mostly amorphous, but feature a very low density of embedded crystallites. Upon post-deposition annealing, crystallization proceeds merely from isotropic crystal grain growth of t...

Effective surface passivation of InP nanowires by atomic-layer-deposited Al2O3 with POx interlayer

III/V semiconductor nanostructures have significant potential in device applications, but effective surface passivation is critical due to their large surface-to-volume ratio. For InP such passivation has proven particularly difficult, with substantial depassivation generally observed following dielectric deposition on InP surfaces. We present a novel approach based on passivation with a phosphorus-rich interfacial oxide deposited using a low-temperature process, which is critical to avoid P-desorption. For this purpose we have chosen a POx layer deposited in a plasma-assisted atomic layer deposition (ALD) system at room temperature. Since POx is known to be hygroscopic and therefore unstable in atmosphere, we encapsulate this layer with a thin ALD Al2O3 capping layer to form a POx/Al2O3 stack. This passivation scheme is capable of improving the photoluminescence (PL) efficiency of our state-of-the-art wurtzite (WZ) InP nanowires by a factor of ∼20 at low excitation. If we apply the rate equation analysis advocated by some authors, we derive a PL internal quantum efficiency (IQE) of 75% for our passivated wires at high excitation. Our results indicate that it is more reliable to calculate the IQE as the ratio of the integrated PL intensity at room temperature to that at 10 K. By this means we derive an IQE of 27% for the passivated wires at high excitation (>10 kW cm–2), which constitutes an unprecedented level of performance for undoped InP nanowires. This conclusion is supported by time-resolved PL decay lifetimes, which are also shown to be significantly higher than previously reported for similar wires. The passivation scheme displays excellent long-term stability (>7 months) and is additionally shown to substantially improve the thermal stability of InP surfaces (>300 °C), significantly expanding the temperature window for device processing. Such effective surface passivation is a key enabling technology for InP nanowire devices such as nanolasers and solar cells.

New Perspectives on Surface Passivation : Understanding the Si-Al2O3 Interface

The book addresses the problem of passivation at the surface of crystalline silicon solar cells. More specifically, it reports on a high-throughput, industrially compatible deposition method for Al2O3, enabling its application to commercial solar cells. One of the main focus is on the analysis of the physics of Al2O3 as a passivating dielectric for silicon surfaces. This is accomplished through a comprehensive study, which moves from the particular, the case of aluminium oxide on silicon, to the general, the physics of surface recombination, and is able to connect theory with practice, highlighting relevant commercial applications.

POx/Al2O3 stacks : Highly effective surface passivation of crystalline silicon with a large positive fixed charge

Thin-film stacks of phosphorus oxide (POx) and aluminium oxide (Al2O3) are shown to provide highly effective passivation of crystalline silicon (c-Si) surfaces. Surface recombination velocities as low as 1.7 cm s−1 and saturation current densities J0s as low as 3.3 fA cm−2 are obtained on n-type (100) c-Si surfaces passivated by 6 nm/14 nm thick POx/Al2O3 stacks deposited in an atomic layer deposition system and annealed at 450 °C. This excellent passivation can be attributed in part to an unusually large positive fixed charge density of up to 4.7 × 1012 cm−2, which makes such stacks especially suitable for passivation of n-type Si surfaces.Thin-film stacks of phosphorus oxide (POx) and aluminium oxide (Al2O3) are shown to provide highly effective passivation of crystalline silicon (c-Si) surfaces. Surface recombination velocities as low as 1.7 cm s−1 and saturation current densities J0s as low as 3.3 fA cm−2 are obtained on n-type (100) c-Si surfaces passivated by 6 nm/14 nm thick POx/Al2O3 stacks deposited in an atomic layer deposition system and annealed at 450 °C. This excellent passivation can be attributed in part to an unusually large positive fixed charge density of up to 4.7 × 1012 cm−2, which makes such stacks especially suitable for passivation of n-type Si surfaces.

Effective passivation of silicon surfaces by ultrathin atomic-layer deposited niobium oxide

This letter reports on effective surface passivation of n-type crystalline silicon by ultrathin niobium oxide (Nb2O5) films prepared by atomic layer deposition (ALD) and subjected to a forming gas anneal at 300 °C. A champion recombination parameter J0 of 20 fA/cm2 and a surface recombination velocity Seff of 4.8 cm/s have been achieved for ultrathin films of 1 nm. The surface pretreatment was found to have a strong impact on the passivation. Good passivation can be achieved on both HF-treated c-Si surfaces and c-Si surfaces with a wet-chemically grown interfacial silicon oxide layer. On HF-treated surfaces, a minimum film thickness of 3 nm is required to achieve a high level of surface passivation, whereas the use of a wet chemically-grown interfacial oxide enables excellent passivation even for Nb2O5 films of only 1 nm. This discrepancy in passivation between both surface types is attributed to differences in the formation and stoichiometry of interfacial silicon oxide, resulting in different levels of chemical passivation. On both surface types, the high level of passivation of ALD Nb2O5 is aided by field-effect passivation originating from a high fixed negative charge density of 1–2 × 1012 cm−3. Furthermore, it is demonstrated that the passivation level provided by 1 nm of Nb2O5 can be further enhanced through light-soaking. Finally, initial explorations show that a low contact resistivity can be obtained using Nb2O5-based contacts. Together, these properties make ALD Nb2O5 a highly interesting building block for high-efficiency c-Si solar cells.This letter reports on effective surface passivation of n-type crystalline silicon by ultrathin niobium oxide (Nb2O5) films prepared by atomic layer deposition (ALD) and subjected to a forming gas anneal at 300 °C. A champion recombination parameter J0 of 20 fA/cm2 and a surface recombination velocity Seff of 4.8 cm/s have been achieved for ultrathin films of 1 nm. The surface pretreatment was found to have a strong impact on the passivation. Good passivation can be achieved on both HF-treated c-Si surfaces and c-Si surfaces with a wet-chemically grown interfacial silicon oxide layer. On HF-treated surfaces, a minimum film thickness of 3 nm is required to achieve a high level of surface passivation, whereas the use of a wet chemically-grown interfacial oxide enables excellent passivation even for Nb2O5 films of only 1 nm. This discrepancy in passivation between both surface types is attributed to differences in the formation and stoichiometry of interfacial silicon oxide, resulting in different levels of ...

Degeneracy and bandgap narrowing in the semiconductor electron-hole product

It is shown that previously proposed expressions for the semiconductor electron-hole product, which purport to separate the influence of carrier degeneracy and bandgap narrowing, fail to properly delineate these effects. A reformulation of the expression in which the two effects are successfully separated is proposed, valid when the majority carrier concentration is independent of bandgap narrowing, as occurs in the common case of low injection and quasi-neutrality. It is shown that under other conditions, the two effects may not be treated in isolation but that only their combined effect or the marginal effect of one or the other on the total electron-hole product may be assessed. Convenient expressions are provided for the latter case. The revised expression provides both conceptual and computational advantages for semiconductor device modelling.