Formation and properties of p-i-n diodes based on hydrogenated amorphous silicon with embedded CrSi2, Mg2Si and Ca2Si nanocrystallites for energy conversion applications

1Institute of Automation and Control Processes of Far Eastern Branch of RAS, Vladivostok, 690041, Radio, 5, Russia 2Institute of Chemical Process Fundamentals of the ASCR, v. v. i., Rozvojová 135, 165 02 Praha 6, Czech Republic 3 Institute of Physics of the ASCR, v. v. i., Cukrovarnická 10/112, 162 00 Praha 6, Czech Republic 4Czech Technical University in Prague, Faculty of Biomedical Engineering, Sitna 3105, 27201 Kladno, Czech Republic


Introduction
Our motivation is to improve efficiency of the energy conversion in diodes based on thin film silicon.It is well know that solar cells based on the hydrogenated amorphous silicon (a-Si:H) thin film have a low absorbance coefficient in the near infrared region compared with the monocrystalline silicon.Therefore we study a possibility to shift the optical absorption edge below the typical band gap energy of a-Si:H (1.7 eV) to increase the near infrared optical absorption.The complementary problem is the generation of near infrared electroluminescence in the a-Si:H diode with embedded nanoparticles where suitable nanoparticles serve as radiative recombination centres.
We consider the semiconducting silicide nanoparticles to be perspective materials for embedding in thin silicon films from the technology point of view as well as because of their suitable electronic properties.CrSi2, Mg2Si, and Ca2Si are narrow band semiconductors with band gaps of 0.35-0.77eV possessing high absorption coefficient in the near infrared region [1][2][3].Moreover, they can be easily grown in the form of nanoparticles or nanolayers on hydrogenated silicon surfaces at a relatively low temperature below 200 °C by several different methods.For example, in our previous work 15-30 Mg2Si/a-Si interlaced multistructures were prepared on a-Si:H surface in the ultra-high vacuum (UHV) by the reactive deposition epitaxy (RDE) at substrate temperature of about 90 o C [4][5].Increased infrared absorption of the a-Si:H layers with embedded Mg2Si has been observed below the a-Si:H optical absorption edge and related to the formation of strong direct transitions in the Mg2Si nanoparticles (NPs) inside amorphous silicon.
In this work the hydrogenated amorphous silicon (a-Si:H) based diode structures Al/a-Si:H(p + )/a-Si:H(i)/(silicides NPs/a-Si)x/a-Si:H(i)/a-Si:H(n + )/ITO/glass with multiple layers (x = 8,…,15) of the embedded narrow band gap semiconducting nanoparticle silicide (CrSi2, Mg2Si and Ca2Si) multistructures have been grown by combining the plasma enhanced chemical vapour deposition (PECVD) and the RDE and studied by in situ and ex situ methods.

Experimental
The capacitively coupled radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition (PECVD) with two electrodes configuration was used at the Institute of Physics of the ASCR, v. v. i., in Prague for the deposition of the intrinsic as well as doped a-Si:H thin films on Corning barium borosilicate 7059 glass substrates coated by the transparent and conductive thin film of indium tin oxide (ITO) prepared by magnetron sputtering.At two electrodes configuration the silane diluted by hydrogen is decomposed and a−Si:H thin film of standard quality for solar cells devices is deposited on the surface of electrode heated at about 200°C.The n-type doping is achieved via adding PH3.
The glass substrates with ITO, doped n + -type and intrinsic a-Si:H thin layers were shifted for the ex-situ formation of silicide nanoparticles in the ultra-high vacuum (UHV) reactor installed at the Institute of Automation and Control Processes of Far Eastern Branch of RAS in Vladivostok.Prior loading to UHV, the substrates were cleaned in ethanol.The reactive deposition epitaxy (RDE) of Ca2Si, Mg2Si or CrSi2 multistructures were carried out in the UHV OMICRON chamber with base pressure of 210 -8 Pa equipped with LEED and AES-EELS spectrometers, sublimation sources of Si, Ca, Cr and Mg and quartz sensor of film thickness.The first step of NPs deposition was about 14-21 nm thick a-Si layer deposited at temperatures 100 o C for Mg2Si, 175 o C for CrSi2 and 150 o C for Ca2Si.Five samples were prepared for the purpose of this study.The first sample (50-1) has 9 interlaced layers of CrSi2 (1.15 nm thickness) and Si (6.6 nm) formed by RDE at 175 o C and then covered by amorphous silicon at 175 o C (8.8 nm).The second sample (50-2) has 11 interlaced layers of Mg2Si (1.3 nm)/a-Si(6.6 nm) formed at 100 o C with top a-Si layer (26.4 nm) formed also at 100 o C. The third sample (50-3) has 8 layers of Mg2Si and Ca2Si NPs formed at 150 o C and interlaced by the a-Si.The fourth (50-4) and fifth (50-5) samples contain only Ca2Si layers with two times differ thickness: 2.6 nm and 1.3 nm in layer) and Si (9.9 nm) interlayers and Si top layer (42 nm -only for fifth sample).The electronic structure of silicide multistructures was studied in-situ in the UHV chamber by the Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS).
Prior finishing the diode structure, the morphology and optical properties of silicide nanoparticles deposited on a-Si:H surface were investigated by the atomic force microscopy (AFM) in tapping mode, the optical spectroscopy and the Raman spectroscopy.The optical reflectance spectra of the grown samples were registered using spectrophotometer Hitachi U-3010.The vacuum FT-IR spectrometer VERTEX 80v (Bruker) in the energy range of 0.5-6.2eV (with integrated sphere at 1.5 -6.2 eV) was used for collecting the reflectance (R) spectra.The Raman spectra excited at 473 nm were measured using Olympus BX 51 microscope and Nicolet Almega XR dispersive spectrometer equipped with Olympus BX 51 microscope.
To finish the p-i-n diodes, the samples were transferred from Vladivostok back to Prague.The samples were cleaned for 1 min by hydrogen plasma and then the silicide NPs multilayer structure was covered by the intrinsic hydrogenated silicon and p + layer by PECVD through decomposition of SiH4 with 1% B2H6 diluted in H2.Top Al electrodes were deposited on a-Si:H(p + ) for the formation of an Ohmic contact.Finally, the diode structure had two electrodes: bottom ITO layer and top metallic (Al) electrode.The I-V characteristic's measurements were carried out with use of KEITHLEY 237 High Voltage Source Measure Unit.
The electroluminescence spectra of the p-i-n diodes were measured in the setup, which contain: the Keithley 3340 arbitrary waveform generator as a power supply for diode providing the square pulsed forward voltage bias.The light was collected by the objective with the relative aperture f/1, the monochromator Horiba H20IR (600gr/mm) operating in the spectral range 400-1600 nm with the interchangeable Si and InGaAs photodiodes.Only Si photodiode was used for the purpose of this study providing the spectral range 400-1100 nm with the fixed spectral resolution 4 nm.The signal from the photodiode was read by the lock-in amplifier referenced to the TTL pulses provided by the waveform generator.The electroluminescence spectra were corrected on the spectral efficiency of the monochromator and the detector.

Electronic structure and morphology of the silicide nanoparticle multistructures
The sample 50-1 has the transition metal (Cr) silicide with composition closed to CrSi2 forming CrSi2 islands on Si surface.This is confirmed by appearance of Cr AES peak at 36 eV in AES spectra (Fig. 1 a) after Cr deposition on a Si amorphous layer at 175 o C and by shift a bulk plasmon in the EELS spectrum in the energy position of 19.8 eV (Fig. 1 b), which is closed to well-known CrSi2 bulk plasmon position (20.6 eV [6]).The difference in plasmon positions could be explained by coexistence of Si and CrSi2 islands, which do not cover the entire substrate surface.After Cr-Si multilayer growth procedure co-deposition of eight Cr and Si thin layers on the first layer led to  some intermixing of CrSi2 and Si and conservation of Cr AES peak (Fig. 1 a) that correspond to island growth mode and partial covering by silicon layer of CrSi2 nanoparticles (NPs).By EELS data (Fig. 1 b) the bulk plasmon of silicon is shifted in high energy region due to small contribution from CrSi2.So, we can speculate about CrSi2 NPs formation in each Cr-Si layer.After the growth of top Si layer an amorphous and continuous layers were characterized by AES and EELS techniques (Fig. 1).Therefore, a multilayer structure with embedded CrSi2 NPs has been proved.
Multilayer sample with Mg and Si deposition at 100 o C was grown by the same procedure as the first sample with CrSi2 NPs.The AES data have shown the formation of Mg peak at 43 eV and Si peak at 91 eV without contributions of C (272 eV) and O2 (510 eV) peaks on the stages of first Mg deposition (Fig. 2 a, line "Mg") and after growth of Mg-Si multilayers (Fig. 2 a, line "Si-Mg").Changes in the fine structure of Si AES peak clearly correspond to Mg silicide formation.After the first Mg deposition on the Si:H layer two plasmons were formed in EELS spectrum: surface plasmon at 9.3 eV and bulk one at 15.2 eV with large width (Fig. 2 b. line "Mg").Their positions do not exactly correspond to Mg2Si (9.4 -9.7 eV and 14.1 -14.2 eV [7]) that testifies about formation of Mg2Si thin film consisting of small and polycrystalline grains with partial contribution of silicon substrate between silicide grains.After growth of Mg-Si multilayers the main contribution is observed from silicon bulk plasmon (Fig. 2 b, line "Si-Mg").Since by the AES data a real Mg peak is simultaneously seen due to larger escape length of electrons as compared with EELS data, one can conclude that embedded of Mg2Si NPs were formed under thin Si layer.After the growth of Si top layer only silicon is visible by AES and EELS data (Fig. 2 a,b).
Nanoparticles of Ca2Si were formed during Ca deposition on the Si-H layer at 150 o C that confirmed by AES data (Fig. 3 a, line "Ca") due to changes in the fine structure of Si AES peak and appearance of Ca peak at 44 eV.In EELS spectrum (Fig. 3 b, line "Ca") new peaks have also appeared: 5.2 eV (interband transition), 10.5 eV (surface plasmon) and 14.0 eV (bulk plasmon) without any contribution of bulk Si plasmon for amorphous Si (17.8 eV).It is known that Ca2Si has surface plasmon of 9.6 -10.4 eV and bulk plasmon of 13.4-14.0eV [8].So, we can speak about formation of near continuous Ca2Si film on the first stage of Ca deposition at 150 o C.After deposition of Ca-Si multilayer structure changes in AES (Fig. 3 a, line "Ca-Si") and EELS (Fig. 3 b, line "Ca-Si") spectra were observed.Co-existence of Ca2Si and Si features were observed in the fine structure of AES spectrum (Fig. 3 b, line "Ca-Si") and in positions and width of surface and bulk plasmons of EELS spectrum (Fig. 3 b, line "Ca-Si").The position of bulk plasmon (16.4 eV) and big width correspond to main contribution of Si and small one of Ca in EELS spectrum.Si top layer did not deposit in the experiment for this sample.But for the fifth sample with grown Ca2Si layer of smaller Ca thickness it was done and shown by AES and EELS data that Ca2Si NPs or films was fully covered.Morphology data obtained by AFM and SEM has shown that all samples have near the same roughness and consist from silicon grain (from Si top layer) with sizes of about 70-120 nm and roughness of about 6.1-9.5 nm.Two images are presented on Fig. 4 for samples with embedded Mg2Si NPs (a) and Ca2Si NPs (b).They have minimal root mean square roughness 7.27 nm and 6.07 nm, correspondingly.Grains have not a cut, so we can testify that they are in amorphous state.

Optical properties of the silicide nanoparticle multistructures
Investigation of reflectance and transmittance spectra of all grown samples have shown a strong contribution of interference features in both amorphous and hydrogenated silicon and in ITO layer at photon energies smaller 2.5 eV that is specially confirmed for sample with embedded CrSi2 NPs (Fig. 5 a, lines: "ITO T before CrSi2", "CrSi2 R" and "CrSi2 T").In addition, the thicknesses of ITO layer together with a-Si:H layers slightly different for Mg2Si and Ca2Si (Mg) NPs samples that influence on the absorption edge, amplitude and maxima positions in interference features in each grown multilayer sample (Fig. 5) as compared with transmittance of grown layers for CrSi2-sample (Fig. 5 a, "ITO T before CrSi2" and "CrSi T" curves).Interference features in grown layers and limited range of transparency of glass substrate, glass substrate with a-Si:H layer (Fig. 5 c) and especially the ITO layer strongly complicate analysis and calculations of optical properties of embedded silicide nanoparticles for all silicides: CrSi2 (Fig. 5 a), Mg2Si (Fig. 5 b) and Ca2Si(Mg) (Fig. 5 c).It is known that band gap of CrSi2 film is about 0.35 eV [1], for Mg2Si is about 0.76 eV [2], but for Ca2Si the first experimental data offered a band gap value of about 1.02 eV [8] from Hall temperature measurements.Since the quantity of Cr, Mg and Ca silicides is small in grown multilayers (smaller than 30 nm), a real absorbance in silicides can occur only at absorption coefficient higher than 510 5 cm -1 and photon energies higher than 1.0 eV.For example, for CrSi2 multistructures the sample transmittance edge at 0.8 eV is determined mainly by ITO layer transmittance edge (Fig. 5 a), since the CrSi2 absorption coefficient at 0.8 eV is high enough (=710 4 cm -1 , [9]), but it gives a small contribution in the sample transmittance due to the small common Cr disilicide thickness (30 nm).Mg2Si NPs sample has the smallest silicide thickness (19 nm) and maximal a-Si thickness (99 nm) that strongly shifts maxima in the interference features relatively "ITO T before CrSi2" curve (Fig. 5 b) and decreases an absorbance in the energy range of 0.8-1.0eV.For Ca2Si(Mg) sample the transmittance edge at 0.8 eV is not exactly coincided with transmittance edge of "ITO T before CrSi2" curve (Fig. 5 c) due to the optical band gap of Ca2Si could be smaller than the electrical one (1.02eV [8]).We only propose the real strong absorption in Ca2Si layers of grown samples beginning at energies smaller than 0.8 eV.Therefore, the maximal silicide absorption, corresponding to minima of sample's transmittance, was observed for CrSi2, Mg2Si and Ca2Si (Mg) NPs (Fig. 5 a,b,c) in the photon energy range of 1.0 -2.0 eV.
Raman spectra of the grown samples have confirmed the increased contribution of embedded silicide NPs in larger quantities of Mg silicide as compared with first our work [3] and comparable or higher contribution of CrSi2 and Ca2Si NPs (Fig. 6).For all grown samples silicon is present in an amorphous (483 -489 cm -1 and 909 -959 cm -1 ) form.Silicon is hydrogenated for samples with CrSi2 and Mg2Si NPs, as seen from Si-H vibration (2023 -2043 cm -1 ).The maximal loss of hydrogen was observed for samples with Ca2Si NPs (negligible peak at 1980 cm -1 , Fig. 6 c).Small contribution of CrSi2 NP under the Si top layer is confirmed by peak at 342 cm -1 (Fig. 6 a).The peak at 323 cm -1 corresponds to small quantity of Mg2Si NPs.The sample with embedded Ca2Si NPs has a very small Raman peak at 320 cm -1 , which is not the same as for thick Ca2Si film (360 cm -1 [8]).It is possible due to small grain sizes of Ca2Si NPs inside the silicon matrix.

Electrical and electro-optical properties of a-Si:H p-i-n diodes with embedded silicide nanoparticle multistructures
After growth of embedded silicide NPs in amorphous silicon prepared samples were transferred in PE CVD system for growth of p + Si layer and following deposition of Al stripped layer.After some dry etching p-i-n diode structures were formed in accordance with scheme (Fig. 7 a) and I-V measurements in dark were done.I-V characteristics were shown on Fig. 7 (b) for all p-i-n diodes.Diodes No 50-1 and No 50-2 with CrSi2 and Mg2Si NPs have the minimal direct current at bias of 2-3 V and maximal reverse current as compared with p-i-n diodes based on Ca2Si NPs embedded in amorphous Si layer.The sample No 50-5 with 15 multilayers of Ca2Si with minimal Ca portion during deposition has the best I-V characteristics.So, we can propose that minimal sizes of NPs resulted to the best relation of direct to reverse current as compared with larger Ca2Si grains in sample No 50-4 with 8 layers and with p-i-n diodes prepared in the first article on the base of embedded of Mg2Si NPs [4].The main problem of formed p-i-n diodes is a large reverse current with compared of p-i-n diodes formed directly on hydrogenated amorphous silicon layers [11].Measurements of photoresponse on the prepared p-i-n diodes are in progress now.
Usually for a-Si:H layers a broad photoluminescence (PL) signal is observed at 1.0 -1.5 eV at 10 K with maximum at 1.25 eV with the ability to record the signal electroluminescence (EL) at room temperature [11].Unlike the immeasurable PL on grown multilayered structures, the EL signal was observed from the p-i-n diodes on their base at room temperature in the near infrared spectral region.For p-i-n diodes EL region can be divided on one sharp EL peak at 1.2-1.3eV and two smaller ones at 1.4 -1.5 eV and 1.5-1.8eV (Fig. 8).The EL signal appeared at forward bias voltage beginning from 2.0-2.5 V (Fig. 8 a,b).With increase of the bias the EL signal amplitude grows nonlinear.The same character of the increase was observed for second and third EL peaks (Fig. 8 a,b) without any shift on photon energy scale that testifies a strong influence of interference features on the EL signal at 1.2-1.8eV as on optical spectra (Fig. 4).The maximal EL signal was observed for p-i-n diode with Ca2Si NPs at 3.5 V.Only the weak EL was observed for p-i-n diode with CrSi2 NPs at photon energy of 1.5-1.7 eV and the voltage bias 3.5 eV (Fig. 8 c).The EL position is determined by real thickness of grown a-Si layers together with embedded silicide layers, ITO and a-Si:H layers and also with density of the radiative states, which is proportional to absorbance of silicide NPs.The maximal width of EL signal from 1.2 to 2.0 eV was observed for Mg2Si based p-i-n diode (Fig. 8a).So, we can propose existence of wider EL signal without   contribution of interference features.Radiative transitions are determined by first direct transitions in Ca2Si and Mg2Si NPs [3,12] at 1.2 -1.3 eV, which have a strong oscillator force.This is correlated with strong absorption in grown structures with Ca2Si and Mg2Si NPs at 1.2-1.3eV (Fig. 4 b,c).The EL signal for CrSi2 p-i-n diode at 1.5-1.7 eV is also determined by strong direct transitions in CrSi2 at these energies [1,9] and radiative recombination in CrSi2 NPs.Since photoluminescence on grown multistructures with laser excitation was not observed at room temperature, the nonradiative recombination is main factor.So, in the p-i-n diode EL is possible due to formation of carrier's avalanche at large direct biases (2.5-3.5 V) and radiative recombination in the CrSi2, Ca2Si and Mg2Si nanoparticles embedded in Si matrix.The small PL signal at 5 K was firstly observed for Ca silicide films grown at 500 o C on Si(111) substrate at 0.9-1.1 eV [12] that proved a possibility of emitting recombination in Ca silicide films.

Conclusion
Hydrogenated amorphous silicon p-i-n diodes with embedded CrSi2, Mg2Si and/or Ca2Si nanoparticle multistructures have been prepared combining two deposition methods − PECVD and RDE.It was shown that the high quantity silicide NPs can be formed by co-deposition of metal and silicon at substrate temperature from 100 to 175 o C as confirmed by AES, EELS, optical transmittance and Raman spectroscopy.The I-V curves of the a-Si:H p-i-n diodes with embedded silicide NP multistructures have shown maximal forward current for Ca2Si nanoparticles with minimal sizes and larger quantities of embedded multilayers.The room temperature electroluminescence spectra have been measured for p-i-n diodes with embedded CrSi2, Ca2Si and Mg2Si NP multistructures.The nature of electroluminescence demands additional studies.Photoresponse measurements on diode structures are in progress now.

Fig. 1 .
Fig. 1.AES (a) and EELS (b) spectra of different steps of the formation of the CrSi2 NP multistructures on a-Si:H surface.

Fig. 2 .
Fig. 2. AES (a) and EELS (b) spectra of different steps of the formation of the formation of the Mg2Si NP multistructures on a-Si:H surface.

Fig. 3 .
Fig. 3. AES (a) and EELS (b) spectra of different steps of the formation of the Ca2Si NP multistructures on a-Si:H surface.

Fig. 8 .
Fig. 8. Room temperature electroluminescence spectra at different direct biases: (a) a-Si:H p-i-n diode with embedded Mg2Si NPs multistructure and (b) a-Si:H p-i-n diode with embedded Ca2Si(Mg) NPs multistructure; and EL spectra for all p-i-n diodes with embedded (Cr, Mg and Ca) silicide NP multistructures at maximal direct bias (3.5 V) (c).