Denis Mencaraglia

Electronic properties of silicon thin films prepared by hot-wire chemical vapour deposition

Abstract A transition from amorphous to microcrystalline silicon occurs in hot-wire chemical vapour deposition silicon films with increasing dilution of silane with hydrogen. This transition is detected for a dilution ratio R = [SiH 4 ]/[H 2 ] between 10% and 9%, where [SiH 4 ] and [H 2 ] are the silane and hydrogen flow rates, by Raman and optical absorption spectra, and by dark conductivities which are several orders of magnitude larger in microcrystalline as compared to amorphous films. In the microcrystalline films we observe a simultaneous increase of both majority and minority carrier mobility-lifetime products with increasing hydrogen dilution, which is consistent with the measured decrease in sub-gap absorption and defect density deduced from transient photocurrent measurements. This simultaneous increase is in contrast with the general trend observed in amorphous films, where these two quantities vary in opposite ways, and are associated with an improvement of the transport properties of the material. The microcrystalline samples did not show light-induced degradation after prolonged illumination.

Studies of buried interfaces Cu(In,Ga)Se2/CdS XPS and electrical investigations

The formation of interface Cu(In,Ga)Se2/CdS in solar cells is not yet well understood but seems to be the key to further improvements of their performance. This interface depends on many parameters such as the initial chemical state of the CIGS surface or the chemical bath deposition conditions used to grow the CdS layer. In this paper, we focus our attention on the CIGS/CdS hetero-interface at different stages of its formation using mainly XPS studies of buried interfaces which were studied after gradual sputtering. We have investigated interfaces on CIGS submitted to various surface treatments, analogue to those involved in fabrication steps (NH3 dipping…). Kelvin probe and admittance spectroscopy measurements have been also performed on several interfaces prepared in the same conditions to correlate the chemical composition with the electrical response of the buried interfaces.

High current density GaAs/Si rectifying heterojunction by defect free Epitaxial Lateral overgrowth on Tunnel Oxide from nano-seed

Interest in the heteroepitaxy of GaAs on Si has never failed in the last years due to the potential for monolithic integration of GaAs-based devices with Si integrated circuits. But in spite of this effort, devices fabricated from them still use homo-epitaxy only. Here we present an epitaxial technique based on the epitaxial lateral overgrowth of micrometer scale GaAs crystals on a thin SiO2 layer from nanoscale Si seeds. This method permits the integration of high quality and defect-free crystalline GaAs on Si substrate and provides active GaAs/Si heterojunctions with efficient carrier transport through the thin SiO2 layer. The nucleation from small width openings avoids the emission of misfit dislocations and the formation of antiphase domains. With this method, we have experimentally demonstrated for the first time a monolithically integrated GaAs/Si diode with high current densities of 10 kA.cm−2 for a forward bias of 3.7 V. This epitaxial technique paves the way to hybrid III–V/Si devices that are free from lattice-matching restrictions, and where silicon not only behaves as a substrate but also as an active medium.

XPS and electrical studies of buried interfaces in Cu(In,Ga)Se2 solar cells

The active interface of Cu(In,Ga)Se2 solar cells is the key to further improvements of their performance. The formation of this interface is not yet well understood. It depends on the initial state of the CIGS layer, then on the evolution of the interfacial chemistry during the CdS deposition. We present a contribution to its understanding using XPS studies at different steps of the interface formation. Attention has been brought to the surface preparation and to the buried interface CIGS/CdS. Buried interfaces were studied after gradual sputtering. Well-resolved spectra have been obtained. We aimed at a clarification of the role of the various segregated/intermixed phases at the interface. To achieve this, admittance spectroscopy and Kelvin probe measurements have been performed on the same devices to correlate the chemical composition to the electrical responses associated with the buried interface.

Novel Wideband Eddy Current Device for the Conductivity Measurement of Semiconductors

We report on the development and application of a brand-new contactless method based on eddy currents with a view to designing a generic apparatus for the characterization of some transport properties of a large range of semiconductors. The eddy current probe, constituted of a coil connected to a transmission line, interacts with the semiconductor wafer under inspection. The innovative approach of this letter consists in measuring the impedance of the coil by reflectometry using a broadband multicarrier test signal, i.e., containing multiple frequencies. An analytical electromagnetic model of the coil-wafer interaction is then used to estimate the conductivity of the wafer. This process results in a new contactless conductivity measurement system that exhibits a very wide conductivity measurement range and allows the characterization of a large variety of semiconductor materials. The device is also very fast potentially allowing the measurement of transport properties of semiconductors in fast transient conditions. As a practical example, the performance of our device is demonstrated by estimating the conductivity of a set of crystalline silicon wafers.

Subgap modulated photocurrent spectroscopy and its application to the study of the solar cell absorber defect distributions

In this paper, a theoretical background of subgap modulated photocurrent experiment is presented. It allows the investigation of the density of states DOS distribution, directly from the active region of a semiconductor heterojunction device. The junction is illuminated with a modulated subgap light excitation i.e., light with photon energy lower than the band gap of the active layer. Under specific considerations for the applied reverse bias voltage and the bias-light level, a simple theoretical relation of the imaginary part of the photocurrent versus the modulation angular frequency allows the determination of the energy profile of the gap states. This technique has been successfully applied to a Ga free CuIn,GaSe2 based solar cell to investigate the DOS distribution in the band gap of the absorber. Two distinct defect distributions have been exhibited in the absorber layer of the studied solar cell.

GaAs microcrystals selectively grown on silicon: Intrinsic carbon doping during chemical beam epitaxy with trimethylgallium

The monolithic integration of III-V semiconductors on silicon and particularly of GaAs has aroused great interest since the 1980s. Potential applications are legion, ranging from photovoltaics to high mobility channel transistors. By using a novel integration method, we have shown that it is possible to achieve heteroepitaxial integration of GaAs crystals (typical size 1 lm) on silicon without any structural defect such as antiphase domains, dislocations, or stress, usually reported for direct GaAs heteroepitaxy on silicon. However, concerning their electronic properties, conventional free carrier characterization methods are impractical due to the micrometric size of GaAs crystals. In order to evaluate the GaAs material quality for optoelectronic applications, a series of indirect analyses such as atom probe tomography, Raman spectroscopy, and micro-photoluminescence as a function of temperature were performed. These revealed a high content of partially electrically active carbon originating from the trimethylgallium used as the Ga precursor. Nevertheless, the very good homogeneity observed by this doping mechanism and the attractive properties of carbon as a dopant once controlled to a sufficient degree are a promising route to device doping.

Structural, Optoelectronic and Electrical Properties of GaAs Microcrystals Grown from (001) Si Nano-areas

An innovative approach is being investigated to develop III–V compounds on silicon (Si) substrates with the purpose to offer a technological alternative for the development of high efficiency solar cells ( ∼ 30 %). Until now, germanium (Ge) substrate has been the privileged material for the development of III–V multi-junctions (MJ) solar cells mainly dedicated to space applications. Ge offers several advantages, namely the lattice matching to Si and its use as a bottom cell in the MJ. However, the main drawback remains the cost of Ge substrates, which makes it inappropriate for terrestrial photovoltaic (PV) applications. New routes for high efficiency MJ solar cells are expected through the significant improvements of the selective area epitaxy (Li et al., J Appl Phys 103:106102, 2008; Deura et al., J Cryst Growth 310:4768–4771, 2008; Hsu et al., Appl Phys Lett 99:133115, 2011) allowing defect free III–V compounds to be grown on Si substrates patterned with dielectric films. In this work, Si nanoscale areas opened through a SiO2 layer ( < 1 nm) formed on (001) Si have been used to grow GaAs microcrystals by chemical beam epitaxy (CBE) in the temperature range 550–600 ∘C (Renard et al., Appl Phys Lett 102:191915, 2013). Structural, optoelectronic and electrical properties of GaAs microcrystals have been analyzed at room temperature by micro-Raman, photoluminescence and conductive probe atomic force microscopy (CP-AFM). The fine structure of crystals (facet orientations, crystal defects) has also been investigated by transmission electron microscopy (TEM). Linear polarized Raman spectroscopy performed on multiple microcrystals shows exclusively the TO mode which is typically expected for (110) GaAs plane orientations and/or heavily n-type Si-doped GaAs (Zardo et al., Phys Rev B 80:245324, 2009). TEM confirms that all facets are {110}, but unintentionally Si doping cannot be excluded. Indeed, PL measure-ments point out a red shift for the microcrystals for which nucleation seeds were created by silane exposure. CP-AFM imaging of GaAs microcrystals performed at + 1 and − 1 V, respectively, points out a current rectification behavior confirmed by local I–V measure-ments (Fig. 37.1). These results can be interpreted as a sign of the presence of a p-n junction, which agrees well with the p-type doping of Si substrates used in this study (1–5 Ωcm) and the unintentionally n-type doping of GaAs microcrystals suggested by PL measurements (Pavesi and Henini, Microelectron J 28:717–726, 1997).

Chapter 7:III–V Solar Cells

The III–V semiconductor materials provide a range of opto-electronic properties well suited to bandgap engineering and high efficiency solar cells. The design process, III–V growth and fabrication methods are described for homogeneous and heterogeneous structures, and the magnitude of fundamental thermal and radiative losses for important III–V solar cell materials calculated. An analytical model is presented, analysing solar cell performance in detailed terms of processes in the space charge region and charge neutral layers of solar cells. The model formulates the solar cell radiative efficiency as a function of bias, providing a quantitative measure of how close devices come to the ideal efficiency limit. Single junction pin and record efficiency pn GaAs cells are analysed and their radiative efficiency quantified, concluding that radiatively dominated behaviour is reached in the more efficient np design. Tandem and triple junction III–V concepts are reviewed and efficiency limits placed in the context of achievable designs. Experimental data are modelled for both structures and the radiative efficiency quantified. The more radiatively efficient tandem design is found to be closer to its fundamental efficiency limit for a radiatively dominated dual junction structure, as a consequence of lower non-radiative recombination rates. The application of III–V materials to quantum confined structures is finally reviewed with specific regard to the quantum well solar cell and its demonstrated 90% radiative efficiency at high bias as a result of the lower bandgap undoped multiple quantum well region.

Growth route toward III-V multispectral solar cells on silicon

To date, high efficiency multijunction solar cells have been developed on Ge or GaAs substrates for space applications, and terrestrial applications are hampered by high fabrication costs. In order to reduce this cost, we propose a breakthrough technique of III-V compound heteroepitaxy on Si substrates without generation of defects critical to PV applications. With this technique we expect to achieve perfect integration of heterogeneous Ga1-xInxAs micro-crystals on Si substrates. In this paper, we show that this is the case for x=0. GaAs crystals were grown by Epitaxial Lateral Overgrowth on Si (100) wafers covered with a thin SiO2 nanostructured layer. The cristallographic structure of these crystals is analysed by MEB and TEM imaging. Micro-Raman and Micro-Photomuminescence spectra of GaAs crystals grown with different conditions are compared with those of a reference GaAs wafer in order to have more insight on eventual local strains and their cristallinity. This work aims at developping building blocks to further develop a GaAs/Si tandem demonstrator with a potential conversion efficiency of 29.6% under AM1.5G spectrum without concentration, as inferred from our realistic modeling. This paper shows that Epitaxial Lateral Overgrowth has a very interesting potential to develop multijunction solar cells on silicon approaching the today 30.3% world record of a GaInP/GaAs tandem cell under the same illumination conditions, but on a costlier substrate than silicon.