Issue
Eur. Phys. J. Appl. Phys.
Volume 88, Number 3, December 2019
Disordered Semiconductors: Physics and Applications
Article Number 30302
Number of page(s) 6
Section Thin Films
DOI https://doi.org/10.1051/epjap/2020190253
Published online 03 March 2020

© EDP Sciences, 2020

1 Introduction

Thin film hydrogenated amorphous silicon (a-Si:H) deposited by plasma enhanced chemical vapour deposition (PECVD) has been used for decades in low cost, large area diodes and transistors for applications such as solar cells, TV screens and scanners [1]. The p-i-n structure is the most common type of thin film diode as the intrinsic i-layer can be tailored to optimize the depletion region [2]. Embedding nanoparticles (NPs) into the i-layer is studied for energy conversion applications as well as integrated silicon photonics [3]. However, finding technologically compatible materials with a well-balanced photocurrent is challenging [4]. A crucial issue is the compatibility of the embedded NPs with the host matrix. Further development requires basic research of interface properties between thin layers, nanoparticles and transparent electrodes, as well as a deep understanding of physical processes related to the energy and charge transfer. Previously we reported on PECVD preparation of a-Si:H layers with in-situ embedded Si nanocrystals showing strong photoluminescence [5]. However, the main problem was a relatively low deposition temperature, which was too low for crystallization of most NPs. We partially solved this problem, via cooperation with our Russian partners, by formation of silicon nanocrystals in a-Si:H via nanosecond laser annealing [6] and by formation of two-phase hydrogenated silicon and a germanium layers [7]. We have observed previously that metallic NPs incorporated in thin a-Si:H films influence the effective work function, introduce non-radiative recombination of charge carriers and increase optical absorption in IR region [8]. Embedded metallic NPs increase the intrinsic conductivity of a-Si:H layers by orders of magnitude and induce a low activation energy. Thus, we have shown that the integration of NPs into thin films via in-situ combined PECVD, evaporation and annealing enables a novel approach for designing thin film optoelectronic devices with enhanced properties.

The recent interest in hydrogenated amorphous substoichiometric silicon carbon alloy (a-SiC:H), with a low carbon content, deposited by PECVD has been accelerated by the availability of transparent conductive diamond electrodes, which can withstand hydrogen plasma and elevated temperature [9]. Vibration spectra of a-SiC:H thin layers indicate that CH4 prevents Si crystallization at elevated deposition temperatures and confirms an increasing carbon content up to x = 0.1 for samples grown with SiH4/CH4 flows up to 1:3.

Germanium (Ge) is an indirect band gap semiconductor, however the difference between its direct and indirect band gap is only 140 meV [10]. When Ge is subjected to tensile strain and heavy doping, room-temperature photoluminescence (PL) can be greatly enhanced and therefore making Ge suitable for light-emitting devices [11]. In this paper we show that the integration of Ge NPs via combined PECVD, evaporation and annealing is capable of modifying the electronic structure of a-SiC:H/Boron doped nanocrystalline diamond (B-NCD) diodes to provide measurable infrared electroluminescence.

2 Experimental

2.1 Preparation of B-NCD/Ti electrodes

Conductive B-NCD films were synthesized at approximately 750 °C on fused silica substrates using a CH4/H2/B(CH3)3 (trimethylboron) gas admixture by microwave plasma enhanced chemical vapour deposition (MW PECVD) using a commercial SEKI Technotron AX5010 system. Prior to B-NCD growth, Ti grids were prepared on the substrates by means of deposition by DC magnetron sputtering and micro patterning using a MicroWriter ML™ Laser Lithography system and wet chemical etching, in order to obtain transparent and conductive B-NCD/Ti electrodes, for more detailed information see reference [12].

2.2 Preparation of a-SiC:H diodes

All deposition processes were performed after 12 h heating, pumping and degassing of a stainless steel vacuum chamber and temperature annealing of substrates. The chamber is constructed for in-situ PECVD with a two electrode (capacitive) configuration and in-situ evaporation of material from a tungsten boat. After achieving a residual pressure lower than 10−5 Pa, the chamber was cleaned for 3 minutes in a pure Ar plasma discharge power 9 W [13,14]. Amorphous sub-stoichiometric silicon carbide (a-SiC:H) layers were deposited from silane SiH4 (purity 6.0), methane CH4 (purity 5.0), hydrogen H2 (purity 7.0), diborane B2H6 (diluted 1% in H2 purity 5.0) and phosphine PH3 (diluted 1% in SiH4 purity 5.0) using 13.56 MHz radio frequency PECVD. The substrate temperature during deposition was 300 °C, pressure 30 Pa and power 18 W. H2, SiH4 and CH4 flow rates were 50, 2.365 and 2.365 sccm for non-doped a-SiC:H thin films. 70 nm thick intrinsic a-SiC:H layers were deposited for 12 min. The 25 nm thick p-type boron doped a-Si:H layer and 25 nm thick n-type phosphorus doped layer were prepared using 4% diborane and 1% phosphine addition into SiH4. Finally, an array of 12 metallic electrodes (0.8 × 0.8 mm) was deposited ex situ by vacuum evaporation at room temperature.

2.3 Preparation of embedded Ge NPs

A 1 nm thick Ge layer was deposited on the 17 nm thick i-layer a-SiC:H surface by vacuum evaporation at a substrate temperature of 300 °C and a residual pressure 10−5 Pa. The evaporation speed of Ge was 0.1 nm/s and thickness of evaporated Ge was monitored by a crystal quartz sensor. For each vaporization cycle, 3 nm of Ge was first evaporated onto the shutter and then 1 nm of Ge was evaporated onto the sample. Then the Ge layer was coated by a 17 nm thick a-SiC:H i-layer using PECVD. The process was repeated 3 times. Diode without NPs was deposited for comparison.

2.4 Setup for optoelectrical characterization of light emitting diodes

To study the energy and charge transfer through nanostructured interfaces a highly sensitive optoelectronic system for measuring I–V characteristics of small area photodiodes integrated with electroluminescence (EL) spectroscopy is required. I–V characteristics were measured in dark and under white light illumination in a pulsed mode using an arbitrary waveform generator, a picoammeter and a white LED 100 mW/cm2 synchronized by reference frequency [15]. Trapezoid waveform pulses and a protective resistor R 0= 100 Ω were used to eliminate commutation voltage spikes induced by an inductive load. Samples were illuminated through glass substrates masked by an aperture in order to illuminate only a selected diode.

A new setup for EL spectroscopy used in this work has evolved from our previous setup designed for steady-state PL spectroscopy in the near infrared and visible spectral range 0.75−3 eV [16]. The new setup shares with the previous one an optical detection unit composed of f/1 objective, Horiba H20IR monochromator with 600 g/mm holographic grating, Si and InGaAs photodetectors with fW sensitivity and a lock-in amplifier. Collected emitted light is split to photodetectors by a dielectric mirror and a signal from the photodetectors is read via a multiplexer, which switches between them at an energy of 1.3 eV. The spectral response was calibrated by a calibrated halogen lamp in wavelength scale followed by an additional correction [17].

2.5 A setup for a transmission electron microscopy

Transmission electron microscopy (TEM) analyses were performed with a FEI Tecnai F20 S/TEM, which is equipped with an X-TWIN lens, a high brightness field emission electron gun operated at 200 kV, a 4 Mpixel CCD camera for imaging in convectional mode, a high angle annular dark field detector working in scanning mode and an EDAX energy-dispersive X-ray spectrometer (EDX) system working over the range of 0–40 keV with a 20 eV per channel dispersion for elemental composition analysis. Micrographs were processed by Digital Micrograph 3.20.1314.0 software (Gatan, USA). Diffraction patterns were interpreted using Process Diffraction V_8.7.1 Q software [18] and the ICDD PDF-2 database [19].

3 Theoretical

The diode current I is given by the implicit function in equation (1) (1)

Here U is an applied voltage, kB  = 1.381×10−23 J/K the Bolzmann constant, q = 1.602×10−19C the elementary charge, T = 300 K room temperature, n an ideality factor (n = 1 for diffusion currents, n = 2 for generation-recombination currents [20]), R P a parallel resistance, R S a serial resistance, R 0 a protective resistance, I 0 a saturation current and I ph a photocurrent. The parallel resistance was given by a differential resistivity at U ≈ 0, whereas the total serial resistance was found by fitting and extrapolating the function R ≈ R 0 + R S  + e abU with auxiliary parameters a, b at U > 1V. A non-linear fit of the equation (1) was carried out for U < 0.5V via Wolfram Mathematica software using only two constrained fitting parameters: I 0 and n.

4 Results and discussion

Bright field TEM image in Figure 1a shows dark inclusions with sizes in the range of 10s of nm embedded in light background of the amorphous phase. The diffraction pattern in Figure 1b obtained from the area depicted in Figure 1a shows two types of circles. First, broad diffusive circles, typical for amorphous materials, and a second system consisting of sharp circles formed from bright diffraction spots typical for small nanocrystals. The sharp diffraction spots are fully assignable to diamond nanocrystals (PDF 65-6329 from ICDD PDF-2 database [19]), which are also shown in the dark field image in Figure 1c. Dark field image in Figure 1d depicted by beams from the diffuse ring show Ge crystal nuclei (PDF 89-3833) with a diameter of about 1 nm embedded in the amorphous phase. Due to its mainly amorphous character, it was not possible to distinguish crystal nuclei of Ge in amorphous Ge phase from crystal nuclei present in amorphous silicon carbide phase. The presence of Ge was confirmed by EDX analyses.

Figure 2 shows dark I–V characteristics of a-SiC:H/B-NCD diodes with and without Ge NPs. The dark I–V characteristics were modelled for U < 0.5 V by the implicit function I = I(U,I) defined in equation (1). Evaluated parameters are summarized in Table 1. The Reference sample without Ge NPs has an ideality factor of n ≈ 1 indicating that diffusion currents are dominant except for very low currents where leakage currents dominate. The fact that the |I (U) | appears as an even function for |U| < 0.2V indicates that the exponential term is negligible and the diode behaves as an resistor with resistivity Rp and leakage current . Due to the relatively high leakage current, the saturation current I0 cannot be directly observed at reverse voltages. Comparison of evaluated parameters in Table 1 indicates deterioration of diode properties in diode with embedded Ge NPs. Moreover, the discrepancy between measured data and fitted curve for U > 0.5 V, shown in Figure 2, is significantly higher for diode with embedded Ge NPs. The forward current is limited in both samples at high voltages by the total serial resistivity R 0 + R S leading to . For both samples, serial resistivity RS is dominated by the resistivity of the B-NCD electrodes.

Figure 3 shows I–V characteristics of a-SiC:H/B-NCD diodes with and without Ge NPs measured under visible light illumination 100 mW/cm2 with the current recalculated to the contact area. The s-shaped I–V characteristic measured under this illumination indicates deterioration of the diode performance due to charge accumulation [21]. The s-shaped I–V characteristics near the open circuit voltage Uoc is not observed in the sample without Ge NPs, indicating negligible charge accumulation near the a-SiC:H/B-NCD interface. However, very strong s-shaped I–V characteristic is observed in the diode with embedded Ge NPs. Indeed, Figure 3 shows a significantly lower photocurrent density Iph , open circuit voltage Uoc , fill factor FF and the energy conversion efficiency η for the diode with embedded Ge NPs. We relate this effect to charge accumulation near Ge NPs. It should be also noted that the open circuit voltage Uoc measured under light illumination is inversely proportional to the saturated current I0 evaluated from the dark I–V characteristic [20].

Figure 4 compares near infrared EL spectra of a-SiC:H/B-NCD diodes without Ge NPs and diodes with embedded Ge NP measured as a function of current density. The EL intensity of the diode without Ge NPs is very low with no clear maxima in the measured spectral range. Thus, the measured signal could just be related to thermal radiation caused by resistive heating. On the other hand, the diode with embedded Ge NPs clearly shows enhanced EL with a maximum at about 0.85 eV, which then diminishes above 1.2 eV. The total intensity of EL increases linearly with the current density, see Figure 5.

thumbnail Fig. 1

(a) Bright field TEM of a-SiC:H/B-NCD with embedded Ge NPs, (b) diffraction pattern related to area shown in 5. (a) with dark field apertures marked, (c) dark field TEM image showing diamond NPs (aperture (c)) and (d) dark field TEM image showing an even coverage of the sample area by Ge crystal nuclei with a diameter of approximately 1 nm, embedded in an amorphous phase.

thumbnail Fig. 2

Dark I–V characteristics of a-SiC:H/B-NCD diodes without (Ref.) and with (Ge NPs) embedded Ge NPs (points). I–V characteristics were fitted at U < 0.5 V with the implicit function I = I (U, I) described in equation (1) (curves).

Table 1

Parameters of a-SiC:H/B-NCD diodes with (Ge NPs) and without (Ref.) embedded Ge NPs evaluated from I–V characteristics measured in dark and under white light illumination with a power density of 100 mW/cm2, saturation current density I 0, ideality factor n, serial resistivity Rs , parallel resistivity Rp , short photocurrent density I ph at U = 0 V, open circuit voltage U oc, fill factor FF, and light conversion efficiency η. Current values were recalculated for current densities using the contact area.

thumbnail Fig. 3

I–V characteristics of a-SiC:H/B-NCD diodes without (Ref.) and with (Ge NPs) embedded Ge NPs measured under visible light illumination with a power density of 100 mW/cm2. Current values were recalculated for current densities using the contact area. Smooth curves were added for clarity to join the measured points.

thumbnail Fig. 4

Near infrared electroluminescence spectra of a-SiC:H/B-NCD diodes without (A-Reference sample) and with (B) embedded Ge NP measured under the same conditions at different current densities indicated by values expressed in mA/cm2. Smooth curves were added for clarity to interpolate the measured points.

thumbnail Fig. 5

Total electroluminescence of a-SiC:H/B-NCD diode with embedded Ge NP as a function of current density I integrated over a 0.75–2 eV spectral range compared to a diode without embedded NPs (Ref. sample). Lines were calculated by linear interpolation of measured data shown as points.

5 Conclusions

a-SiC:H/B-NCD photodiodes without and with embedded Ge NPs were prepared from a mixture of H2, SiH4, CH4, PH3 and B2H6 by PECVD combined with in-situ vacuum evaporation and annealing at an elevated temperature of 300 °C. The presence of Ge NPs embedded in amorphous phase has been confirmed by TEM and EDX analyses. No s-shaped photocurrent has been observed near the open circuit voltage Uoc in diode without embedded NPs indicating only negligible charge accumulation near the a-SiC:H/B-NCD interface. The low EL background in the near infrared region makes the diode suitable for studies of radiative recombination related to charge transfer on nanostructured or at the surface of embedded nanoparticles. We have shown that the integration of Ge NPs is capable of modifying the electronic structure of a-SiC:H thin films in such a way that the EL intensity is strongly enhanced and the intensity of EL correlates with the current density. This enables novel approaches for designing thin film optoelectronic devices with enhanced properties. However, photodiodes with embedded Ge NPs displayed deterioration of I–V characteristics, and therefore the further optimization is still required.

Acknowledgments

The work was supported by the CSF project 19-02858J and by the Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 − CZ.02.1.01/0.0/0.0/16_019/0000760). Dr. Michal Kohout has greatly contributed to the preparation of Ti layers for the construction of BNCD/Ti grid electrodes.

Author contribution statement

Zdenek Remes: electroluminescence spectroscopy, modeling I-V characteristics. Jiri Stuchlik: presenting author, preparation of a-SiC:H diodes with embedded Ge NPs. The-ha Stuchlikova: preparation of metallic contacts and I-V characteristics. Jaroslav Kupcik: HRTEM. Vincent Mortet, Andrew Taylor and Petr Ashcheulov: B-NCD electrodes. Vladimir Alekseevich Volodin: consultations and ideas related to NPs embedded in thin semiconducting films.

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Cite this article as: Zdenek Remes, Jiri Stuchlik, The-ha Stuchlikova, Jaroslav Kupcik, Vincent Mortet, Andrew Taylor, Petr Ashcheulov, Vladimir Alekseevich Volodin, Electroluminescence of thin film p-i-n diodes based on a-SiC:H with integrated Ge nanoparticles, Eur. Phys. J. Appl. Phys. 88, 30302 (2019)

All Tables

Table 1

Parameters of a-SiC:H/B-NCD diodes with (Ge NPs) and without (Ref.) embedded Ge NPs evaluated from I–V characteristics measured in dark and under white light illumination with a power density of 100 mW/cm2, saturation current density I 0, ideality factor n, serial resistivity Rs , parallel resistivity Rp , short photocurrent density I ph at U = 0 V, open circuit voltage U oc, fill factor FF, and light conversion efficiency η. Current values were recalculated for current densities using the contact area.

All Figures

thumbnail Fig. 1

(a) Bright field TEM of a-SiC:H/B-NCD with embedded Ge NPs, (b) diffraction pattern related to area shown in 5. (a) with dark field apertures marked, (c) dark field TEM image showing diamond NPs (aperture (c)) and (d) dark field TEM image showing an even coverage of the sample area by Ge crystal nuclei with a diameter of approximately 1 nm, embedded in an amorphous phase.

In the text
thumbnail Fig. 2

Dark I–V characteristics of a-SiC:H/B-NCD diodes without (Ref.) and with (Ge NPs) embedded Ge NPs (points). I–V characteristics were fitted at U < 0.5 V with the implicit function I = I (U, I) described in equation (1) (curves).

In the text
thumbnail Fig. 3

I–V characteristics of a-SiC:H/B-NCD diodes without (Ref.) and with (Ge NPs) embedded Ge NPs measured under visible light illumination with a power density of 100 mW/cm2. Current values were recalculated for current densities using the contact area. Smooth curves were added for clarity to join the measured points.

In the text
thumbnail Fig. 4

Near infrared electroluminescence spectra of a-SiC:H/B-NCD diodes without (A-Reference sample) and with (B) embedded Ge NP measured under the same conditions at different current densities indicated by values expressed in mA/cm2. Smooth curves were added for clarity to interpolate the measured points.

In the text
thumbnail Fig. 5

Total electroluminescence of a-SiC:H/B-NCD diode with embedded Ge NP as a function of current density I integrated over a 0.75–2 eV spectral range compared to a diode without embedded NPs (Ref. sample). Lines were calculated by linear interpolation of measured data shown as points.

In the text

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