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Passivation of a-Si:H-based structuresin KCN and HCN solutions and itsapplication on p-i-n solar cellEmil Pinčík a , Hikaru Kobayashi b , Masao Takahashi b , Róbert

Brunner a & Stanislav Jurečka c

a Institute of Physics SAS , Dúbravská cesta 9, Bratislava , 845 11 ,Slovak Republicb Institute of Scientific and Industrial Research of Osaka Universityand CREST, Japan Science and Technology Organization , 8-1Mihogaoka, Ibaraki, Osaka , Japanc Institute of Aurel Stodola of University of Žilina , Nálepku 1390,031 01, Liptovský Mikuláš , Slovak RepublicPublished online: 17 Jun 2013.

To cite this article: Emil Pinčík , Hikaru Kobayashi , Masao Takahashi , Róbert Brunner & StanislavJurečka (2013) Passivation of a-Si:H-based structures in KCN and HCN solutions and its applicationon p-i-n solar cell, Journal of the Chinese Advanced Materials Society, 1:2, 151-165, DOI:10.1080/22243682.2013.808063

To link to this article: http://dx.doi.org/10.1080/22243682.2013.808063

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Passivation of a-Si:H-based structures in KCN and HCN solutions and

its application on p-i-n solar cell

Emil Pin�c�ıka�, Hikaru Kobayashib, Masao Takahashib, R�obert Brunnera andStanislav Jure�ckac

aInstitute of Physics SAS, D�ubravsk�a cesta 9, 845 11 Bratislava, Slovak Republic; bInstitute ofScientific and Industrial Research of Osaka University and CREST, Japan Science and TechnologyOrganization, 8-1 Mihogaoka, Ibaraki, Osaka, Japan; cInstitute of Aurel Stodola of University of�Zilina, N�alepku 1390, 031 01 Liptovsk�y Mikul�a�s, Slovak Republic

(Received 25 March 2013; revised 9 May 2013; accepted 21 May 2013)

Dominant aim of the contribution is focused on investigation of very thin SiO2/a-Si:Hinterface properties. After PECVD deposition, surface of a-Si:H is not covered byreproducibly defined insulating layer. Therefore, three basic types of oxygen plasmasources were used for treatments of a-Si:H surfaces. Electrically well defined SiO2

very thin film layers are formed by (i) inductively coupled plasma in connection withits applying at plasma anodic oxidation, (ii) RF plasma as the source of positiveoxygen ions for plasma immersion ion implantation process, and (iii) dielectric barrierdischarge ignited at high pressures. In addition, chemical oxidation of a-Si:H surfacewas performed in boiling azeotropic HNO3 solutions. Electrical properties of SiO2/semiconductor interface of both a-Si:H- and Si-based structures were improved bytreatments in KCN and/or HCN solutions. Positive passivation influence of last men-tioned solutions is presented on p-i-n a-Si-H solar cell parameters.

Keywords: plasma–surface interaction; chemical very thin oxides; interface; a-Si:H;NAOS

1. Introduction

Oxygen plasmas are still frequently used for oxidation of semiconductor surfaces and for-

mation of thin oxide layers. Kinetics of thin film oxide growth is strongly influenced by the

oxidized substrate itself and by oxidizing conditions. The silicon dioxide is still frequently

used in MOS FET technology and its thin film growth technology, thin SiO2 and its inter-

face properties with substrates (usually crystalline Si) for long period prevail above many

another types of thin film dielectrics/substrate structures. There are many devices based on

the MOS technology. The MOS structures are created on more types of semiconductors,

but they are still used mainly in the crystalline Si technology. There exist different possibil-

ities how to create silicon dioxide layers. It can be done by thermal heating the sample in

the oxygen atmosphere,[1,2] by implantation of oxygen atoms by plasma immersion ion

implantation (PIII),[3,4] or by etching the surface of the sample by Arþ ions and thus

activated surface exposing to the oxygen atmosphere,[4] etc. All of the above-mentioned

techniques belong to gas phase high- or low-pressure techniques. Some of them can be

used for formation of thin oxide layer on the amorphous hydrogenated silicon (a-Si:H).

*Corresponding author. Email: emil.pincik@savba.sk

� 2013 Chinese Advanced Materials Society

Journal of the Chinese Advanced Materials Society, 2013

Vol. 1, No. 2, 151–165, http://dx.doi.org/10.1080/22243682.2013.808063

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Additional important possibility to create the oxide layer on both crystalline and

amorphous substrates is utilization of chemical solutions for sample surface oxidation.

Sixty-eight percent aqueous solution of HNO3 at temperature of 121�C during 30 min

was used to create silicon dioxide layer in [5]. The H2O2 [6] or H2SO4/H2O2 solutions [7]

can be also applied for the same purposes.

The paper is focused on investigation of interaction of plasmas with Si-based semi-

conductors. Plasmatic low-energy treatment modifies also composition of “native” defect

structure of materials. The understanding of the consequences of low-energy (from a few

eV to hundreds eV) particle bombardment of semiconductor surfaces is of interest in

many device production technologies as well as for surface-analysis techniques. The

inbuilding of the projectiles, damage induced in the target, and the relocation of a target

atoms or clusters (ion-induced mixing) are of particular importance. Low-energy self-

bombardment of crystalline Si was investigated by Kitabatake.[8–10] The effect of par-

ticles of thermal energies which are of interest for MBE is shown, e.g., in [11,12]. The

effect of argon bombardment is simulated in [13]. Formation of disordered zones after

impacts of keV Si atoms is presented in [14]. It is not clear if overlapping of such ion-

formed zones directly leads to target amorphization or a more complex mechanism should

be supposed. Very interesting paper has been presented by Hensel et al. [15] The size of

the near-surface disordered zones created by the ion impact was studied and compared to

experimental data for the critical fluence of Si amorphization.

a-Si:H contains predominantly metastable defects. The mechanism of metastable

light-induced degradation of hydrogenated amorphous silicon (a-Si:H) remains a key sci-

entific and technological problem over 25 years after the discovery of degradation by

Staebler and Wronski in 1977. Numerous metastability models were proposed to explain

this effect within the first decade, the most widely accepted of these was the 1985 Stutz-

mann, Jackson, and Tsai (SJT) model of weak Si-Si bond breaking by single photocarrier

recombination event. Bond breaking is accompanied by a H hop which stabilizes two

light-induced dangling bonds. During last years, we have presented experimental facts

that three groups of gap states can be observed in the band gap of a-Si:H. They are in

good agreement with the predicted existence of Dh, Dz, and De states of the improved

defect-pool model of Winer [16] and Powell and Dean.[17,18] In accordance with this

model, the distribution of gap states was found to be dependent on the pre-equilibrium

preparation of the sample by bias annealing. According to the defect-pool model, the

energy distribution of gap states (EDOS) is determined by the width of the valence band

tail and by the energy of the equilibrium Fermi-level in the band gap of a-Si:H semicon-

ductor. Depending on the energy of the quasi-Fermi level and dangling bond density and

their spatial distribution, these states act as deep traps or recombination levels. The posi-

tion of the Fermi level in the band gap is responsible for the shape of EDOS in the gap.

The position of the Fermi level can be shifted by bias annealing procedure above the

equilibration temperature. In the “theoretically good intrinsic” a-Si:H the EDOS can

be predefined by bias annealing at temperature about 500 K. Our measurements on

“PECVD prepared real a-Si:H thin films” confirmed that the produced a-Si:H thin films

are slightly of n-type (not intrinsic) and temperatures exceeding 450 K only can be used

successfully for redistribution of any pre-defined EDOS and/or for “definition” of another

EDOS type. The following three distinct distributions usually can be prepared:

� intrinsic EDOS distribution with all three parts Dh, Dz, and De;� n-type distribution by annealing at positive bias leading to formation of dominantly

De groups of states in the EDOS;

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� p-type distribution by annealing at negative bias leading to formation of dominantly

Dh groups of states.

In our last papers we have shown, by our measurements based on charge version of

DLTS (Q-DLTS) on undoped a-Si:H, that gap states with energies 0.63, 0.82, and

1.25 eV (as Dh, Dz, De, respectively) can be observed in volume of the a-Si:H layer

deposited on c-Si. Usually, we have used large voltage steps of 6 V. It is needed to con-

firm that shape of the Q-DLTS spectra approximately corresponds to the EDOS in the

gap of the investigated semiconductor and quantitatively the EDOS can be determined.

Dominant aim of the contribution is focused on investigation of very thin SiO2/a-Si:H

interface properties. Because after PECVD deposition surface of a-Si:H is not covered by

reproducibly defined insulating layer, three basic types of oxygen plasma sources were

used for the first time in our laboratories for treatments of a-Si:H surfaces and their recov-

ering by electrically well defined SiO2 very thin film layer: (i) inductively coupled plasma

in connection with its application in plasma anodic oxidation, (ii) RF plasma as the source

of positive oxygen ions for PIII process, and (iii) dielectric barrier discharge ignited at

high pressures. In addition, wet chemical oxidation of a-Si:H samples was performed in

HNO3 solutions. This process is very cheap and ultra-thin and very thin SiO2 layers with

increased atomic densities (e.g., in comparison with deposited layers) can be prepared.

Electrical properties of SiO2/semiconductor interface of both a-Si:H- and Si-based struc-

tures were improved by treatment in KCN and/or HCN solutions. We demonstrated posi-

tive influence on parameters of passivated p-i-n a-Si-H solar cell structures.

In addition, thicker insulating overlayer enable to realize bias annealing experiments

leading to the redistribution of defect states in the volume of a-Si:H. This redistribution

can be optimized from view of parameters of thin film p-i-n solar cells.

The properties of formed plasmatic SiO2/a-Si:H structures are investigated mainly by

charge version of deep level transient spectroscopy (Q-DLTS) on corresponding metal-

oxide-semiconductor (MOS) structures.

2. Experiment

2.1. Preparation of samples

N-type of crystalline Si (100) supporting wafers of 1-20 V cm conductivity after Radio

Corporation of America (RCA) cleaning were used for preparation of a-Si:H/Si structures.

Preparation of intrinsic a-Si:H: device-quality intrinsic a-Si:H layers of the thickness

of �0.9 mm (i) or 0.5 mm (ii) were deposited on the n-type Si(100) oriented crystals

(phosphorus doped of 0.001–10 V cm conductivity) in a 13.56 MHz rf excited parallel

plate plasma enhanced chemical vapor deposition system

(1) in laboratories of Princeton University: from pure SiH4 and its mixture with H2

at the plasma power of �40 W at the sample temperatures of 250�C;(2) in laboratories of Osaka University: deposition conditions for KCN (HCN)

treated samples: SiH4 – 10 sccm, H2 – 90 sccm, 6 W (0.03 Wcm�2), 120 Pa, at

sample temperature of 160�C.

Aluminum MOS electrodes of hexagonal shape were evaporated on top of oxide

layers from tungsten boat. The back metal contact of MOS structures was made also of

aluminum. The back-side of the sample was etched before the Al deposition.

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2.2. Description of technological apparatuses, techniques, and technological conditions

The set of corresponding technological parameters, thicknesses of formed insulating SiO2

layers, and brief evaluation of oxide/a-Si:H interface defect state densities is presented in

Table 1. The SiO2 thicknesses and interface defect state densities were evaluated from

C-V and DLTS measurements.

2.2.1. Plasma anodic oxidation (scheme of the equipment is illustrated in Figure 1)

It was performed in the standard HV equipment with RF inductive coupled oxygen

plasma at frequency 380 kHz, pressure 30 Pa in the constant current regime at current

density of 1 mAcm�2, and temperature of the sample 200�C.

2.2.2. Low-energy Ar ion beam assisted oxidation – LEIBAO (see Figure 2)

Ion beam oxidation treatment was made in two steps at the sample temperature of 80�C.First, the treatment by 500 eV argon positively charged particles with the dose of �1016

per cm2. Second, the covering of the exposed surface by an insulator was performed by a

low-energy ion beam composed of oxygen and hydrogen only, with the dose of �2�1015per cm2. For the low-energy ion beam exposure of the surface, we have used commercial

ion beam etching system of LPAI Comp. with Kaufman plasma source.

2.2.3. Plasma immersion ion implantation – PIII (see Figure 3)

This technique was performed under following conditions: pressure O2 of 50 Pa;

UAccelerating � –1000 V; at 25�C when process started; 30 min. duration, resulting oxide

thickness: 6–8 nm.

2.2.4. Oxidation in coplanar dielectric barrier discharge device (see Figures 4 and 5)

The coplanar DBD was used to create dominant set of MOS structures. Electrodes of

dielectric barrier discharge were embedded in the dielectric. The discharge was ignited in

pure oxygen at pressure 133 kPa in flowing regime in the gap between the sample and

dielectric surfaces. Distance of the treated sample to the surface of dielectric was 0.25 �10�3 m. Generated plasma was in the direct contact with the sample surface. Additional

process parameters: frequency 14 kHz; voltage 7 kV; distance of electrodes 0.5 mm;

width of electrodes 1.65 mm; dielectric was Al2O3 based; exposure time 30 s (sample

bd1) and 60 s (bd2); room temperature conditions.

2.2.5. Oxidation in HNO3 solution

The samples were oxidized in azeotropic solution of HNO3.

2.2.6. Passivation procedure by cyanide treatment

For presentation of very important process of passivation of defect states at the oxide/

semiconductor interface and/or in semiconductor thin film volumes the aqueous or

MeOH solutions of KCN or HCN were used – see for more details, e.g., in [19]. The cya-

nide method can passivate Si surface states, Si/SiO2 interface states, and defects in amor-

phous Si – see, e.g. [20–23].

Passivation procedure in KCN and/or HCN solutions is usually applied before forma-

tion of ultrathin or very thin SiO2 layers to exclude difficulties with diffusion of CN

radicals of the solution to the a-Si:H sub-surface volume. The native oxide surface layer

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Table

1.

Listofexperim

entaltechniques

usedat

form

ationofverythin

SiO

2/a-Si:H

structures.Itcontainsalso

correspondingthicknessofSiO

2layersandbrief

evaluationofSiO

2/a-Si:Hinterfacedefectstatedensities.

Conditions

Results

Oxidation

method

Oxygen

pressure

[Pa]

Accelerating

voltage[V

]Exposure

time[s]

Sam

ple

temperature

[�C]

SiO

2thickness

[nm]

SiO

2/a-Si:H

interface

Native

105:aircondition

–fewmonths

room

temp.

�2wellpassivated

byhydrogen

Anodic

30

20–60

600

200

6–8

standardproperties

Arbeam

assisted

I10�1:Ar,O2,H2

�500

30

room

temp.

�8high-interface

statedensity

Arbeam

assisted

II10�1:Ar,O2,H2

�700

420

room

temp.

�6standardproperties

Plasm

aim

mersion

ionim

plantation

50

�1000

1800

start:25

6–8

standardproperties

Coplanar

dielectric

barrier

discharge

105

variable

(14kHz,

7kV)

60

room

temp.

6–8

defectstates

well

passivated

by

hydrogen

Liquid

chem

ical

oxidation

––

14,400

room

temp.

4–6

standardproperties

Liquid

chem

ical

oxidationþ

HCN

passivation

––

14,400

room

temp.

4–6

defectstates

wellpassivated

byCNgroup

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of a-Si:H is before passivation removed by suitable chemical etching. The “open” surface

and porosity of the a-Si:H layer enable to reach CN radicals depth of approx. 10 nm. We

have to admit that there is problem with time stability of formed Si-CN bonds. This must

be more carefully investigated simultaneously with process of aging of passivated solar

cells.

3. Experimental methods

For investigation of electrical properties of prepared structures (in the form of metal-

oxide-semiconductor (MOS) ones) following methods were used: charge version of deep

level transient spectroscopy (Q-DLTS), measurements of capacitance vs. voltage in time

domain (C-V), and X-ray diffraction at grazing incidence.

Figure 1. Scheme of the plasma anodic oxidation system.

Figure 2. Sketch of the oxidation apparatus with low-energy ion beam source.

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3.1. Charge version of deep level transient spectroscopy (Q-DLTS)

The modified charge-based correlation DLTS method was used to study the metal-

oxide-amorphous semiconductor (MOS) structures. This method is based on the correla-

tion approach, i.e., the transient response of the MOS structure under periodically applied

voltage steps to the gate electrode is measured, and subsequently a weighted combination

of the charge signals obtained at various sampling events is achieved. The sampling

events, as well as the weighting coefficients used in our work, are chosen with the aim to

Figure 3. Sketch of equipment for PIII.

Figure 4. Scheme of the oxidation system in which dielectric barrier discharge (DBD) was used.

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improve the selectivity in the measured spectrum. The resulting correlated charge DLTS

signal is described by a weighted summation of the contributions from three channels.

The parasitic output charge due to the presence of a leakage current is eliminated by the

used filtering scheme. Figure 6 illustrates principles of the DLTS.

The following formula is valid for the measured Q-DLTS signal coming from a deep

trap:

DQ Cox

Cox þ Cs

� �q�w�NT �expð�entÞ; ð1Þ

Figure 5. Photo of the DBD ignited on the semiconductor surface.

Figure 6. Electrical principle of charge version of deep level transient spectroscopy (Q-DLTS).

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where w stands for the excited part of the depletion region, NT for the trap density, Cox and

Cs for the capacitance of the oxide and that of the space charge layer, respectively. The

emission rate from the trap is written as:

en ¼ n�sn�Nc�exp �DE

kT

� �; ð2Þ

where n is the mean thermal velocity of electrons, sn is the capture cross section,

and Nc is the effective density of states in the conduction band. The DLTS response

of the above-mentioned structure was measured at sampling events chosen to fit the

following scheme: t1, 2t1, and 4t1. Sometimes, two sampling times t1 and t2 are used,

only, at which t2 ¼ 2�t1. The resulting correlated charge DLTS signal Sc can be

described as

Sc ¼ Qðt1Þ � 1:5Qð2t1Þ þ 0:5Qð4t1Þ: ð3Þ

3.2. Capacitance–voltage measurements (C-V)

Capacitance of the MOS diodes as a function of the gate bias was measured by the

improved feedback charge measurements in the time domain. The feedback charge C-V

method was described for the first time by Mego [24] and commercial Keithley stand-

alone instrument is available. Originally, the latter has been intended for measurements

of the steady-state (dc) time-domain capacitance of semiconductor devices. Bias step Du0is applied to the unknown capacitance C of the investigated structure and consequently

the change (time dependent) of output voltage Du of the integrator is registered. The

capacitance C is given by the relation C ¼ CF(Du/Du0), where CF ¼ 3.3 nF and Du0 ¼60 mV are used in our experiments. The sampling time was set to 10 ms. CQ-V measure-

ments and Q-DLTS were carried out using the home-made spectrometer developed at the

Institute of Physics of SAS Bratislava. For the Q-DLTS, we used following parameters:

period ¼ 63 ms, pulse ¼ 48 ms.

It is needed to stress that:

(1) before the start of Q-DLTS measurements on any MOS structure after its prepa-

ration it has been (before measurements)

annealed at 500 K under rough vacuum conditions given by used Q-DLTS

cryostat (at pressure approx. 100 Pa) for 30 min;

(2) each Q-DLTS behavior has been performed from liquid nitrogen temperature to

500 K. Construction materials of the MOS sample support inside of LN cryostat

enabled sample heating up to 550 K.

4. Results and discussion

4.1. Interface: anodic oxide/a-Si:H

The following Q-DLTS record – see Figure 7 – illustrates “the standard” EDOS in which

all three components of the defect states can be observed. The sample was prepared at

low positive bias of the oxidized sample and, therefore, group De and high-temperature

tail is well developed. The positive bias of the sample caused during the oxide growth at

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200�C bias self-annealing of the samples and led to the formation of corresponding shape

of the DLTS spectra and/or formation of De group of defect states. Well H passivated out-

ermost a-Si:H layer was consumed or transformed to another one during the oxide

growth.

Bias annealing of the sample at negative voltages inside of the measurement cryostat

has led to the redistribution of the defect states to the “square from”. Missing of the high-

temperature tail confirmed existence of all three groups of defect states in the interface

region (at least in the interface region, because large voltage step was used).

4.2. Interface: DBD oxide/a-Si:H

The data measured by Q-DLTS and C-V on the MOS structures with oxides prepared

by the DBD method are presented in Figure 8 (60s exposure) respectively. Q-DLTS spec-

trum does not contain expected electron emission from deep defect states. The sample

reached the accumulation capacitance. The hysteresis of the sample can be explained by

the presence of the free negative charge in the oxide volume and/or at the oxide/a-Si:H

interface. We suppose that this mobile charge can be related with hydrogen atomic or

molecular ions produced during the oxide growth. The missing emission of defect states

we relate with interesting effect – we suppose that hydrogen evolving of the a-Si:H layer

during the oxide growth could diffuse also deeper into the SiO2/a-Si:H interface and it

causes additional passivation of interface defect states by hydrogen. It can be supposed

that higher atmospheric pressure of the surrounding plasma and/or gas during the DBD

process contributes to this process and/or increases its effectiveness.

4.3. Interface: PIII oxide/a-Si:H

This oxide was prepared at high negative bias and the result is shown in Figure 9. The

DLTS spectrum of samples does not contain high-temperature tail and the shape corre-

sponds to bias annealed sample at negative biases. Duration of plasma implantation pro-

cedure really enabled to take place to such process at higher temperatures – we estimate

that the temperature of the surface region could increase to the value of 200�C during the

oxide formation. Unfortunately, the temperature of the surface was not measured directly.

The main drawback of this procedure is missing group of Dh defect states in the spectrum!

Figure 7. Q-DLTS records obtained on MOS structures prepared by plasma anodic oxidation ofa-Si:H.

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We suppose direct relation of this phenomenon with interaction of accelerated positive

ions with the a-Si:H surface region during the formation of oxide layer.

Figure 10 illustrates structural changes of the Ar beam treated a-Si:H/Si sample (pre-

pared by the LAIBAO) observed by X-ray diffraction at grazing incidence obtained at

angle 1�. The Ar atom energy did not exceed 300 eV. There are important changes indi-

cated as formation of microcrystallites at the a-Si:H/Si interface. They can be strongly

related with changes of interface state density profile observed by DLTS in the a-Si:H

band gap. The similar structural changes can be induced by plasma exposure of a-Si:H/Si

structures by PIII method.

4.4. Interfaces: chemical oxide/a-Si:H

Figure 11 shows QDLTS spectrum of the sample NAOS oxide/a-Si:H before and after

passivation procedure in HCN aqueous solution performed at room temperature. After

Figure 8. Q-DLTS and corresponding C-V characteristics recorded on MOS structure prepared by30s exposure of a-Si:H surface to DBD discharge.

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passivation procedure any interface defect states were detected on the MOS under condi-

tions (temperature, bias, voltage step, etc.) given by used Q-DLTS method. We suppose

almost all dangling Si bonds contributing to the DLTS spectra were passivated by CN.

4.5. a-Si:H-based solar cell

Figure 12 illustrates I-V curves measured before and after passivation procedure on sim-

ple thin film pin solar cell deposited with corresponding electrodes on Corning glass

substrate.

The increase of the solar cell efficiency is attributed predominantly to passivation of

Si dangling bonds by CN atomic group. The similar effect is observed also by DLTS –

see Figure 11. Passivation state of the solar cells should be stable up to 800�C.

Figure 9. Q-DLTS records obtained of MOS structures prepared by PIII.

Figure 10. Records of X-ray diffraction at grazing incidence performed at angle 1� on as-preparedand low-energy Ar ion beam exposed a-Si:H. After the surface exposure to Ar beam very thin SiO2

has been formed.

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5. Conclusions

We have shown by investigation of corresponding MOS structures (mainly using charge

version of DLTS measurements (Q-DLTS) of samples prepared on undoped a-Si:H) that

gap states with energies 0.63, 0.82, and 1.25 eV (as Dh, Dz, De, respectively) can be

observed in volume of the a-Si:H layer deposited on c-Si. The manner of preparation of

very thin SiO2 layer in surface region of a-Si:H strongly influences resulting distribution

of defect states. Chemical way leads to observation (by charge version of DLTS) of

“standard” distribution of defect states consisting of three basic components which can be

partly eliminated by HCN aqueous solutions. The passivation procedure using HCN water

Figure 11. Q-DLTS records of MOS structures with very thin oxide prepared by azeotropic HNO3

solution on a-Si:H before and after passivation in HCN aqueous solution.

Figure 12. I-V curves measured before and after passivation procedure on thin film p-i-n a-Si:Hsolar cell deposited with corresponding electrodes on Corning glass substrate.

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solution was successfully applied on thin film pin a-Si:H solar cell structure deposited on

Corning glass resulting in considerable increase of solar cell efficiency.

Acknowledgements

The authors thank for support to Japan Society for the Promotion of Science (JSPS) and Slovakiangrant agencies: VEGA: project No. 2/0076/12 and APVV project No. APVV-0888-11. The authorswould like to thank also to Assoc. Prof. Miroslav Zahoran of FMPI Comenius University Bratislava(Slovak Republic) for his strong cooperation in the field of the PIII sample treatments. In addition,the authors thank to Prof. Helena Gleskov�a of University of Strathclyde in Glasgow (United King-dom) for the preparation of special types of a-Si:H/Si and a-Si:H/glass structures. The authorswould like to acknowledge to Dr. Matej Jergel and Ing. J. Rusn�ak of Institute of Physics SAS Brati-slava (Slovakia) for their kind cooperation in the fields of X-ray diffraction at grazing incidence andelectrical measurements of the MOS structures.

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