Effects of Nitrogen Doping on Nanocrystalline Diamond/p-type Si toward Solar Cell Applications

Chii-Ruey Lin1,2, a*, Da-Hua Wei1,2, b, Minh-Khoa BenDao2, c 1Department of Mechanical Engineering and Institute of Manufacturing Technology, National Taipei

University of Technology, Taipei, 106, Taiwan

2Graduate Institute of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei, 106, Taiwan

acrlin@ntut.edu.tw, bdhwei@ntut.edu.tw, cbenkhoavn@gmail.com

Keywords: solar cell, Raman spectroscopy, nanocrystalline diamond

Abstract.

In this work, a solar cell structure of nitrogen-doped nanocrystalline diamond (NCD:N)/p-type silicon

was fabricated using microwave plasma jet chemical vapour deposition technique. The effects of

nitrogen doping level on the structure, optical, and electrical of the as-grown NCD:N was discussed.

The results showed that the micro structure, surface roughness, electrical properties, and optical

properties were affected by the nitrogen doping. Additionally, the agglomeration of the film was

increased with the higher concentration of CN species when the ratio of doped nitrogen increased.

The roughness of the film was Rms:16.5 nm ~ 20.4 nm and the wettability was increased (contact

angle 94.4o ~ 64.6

o). The optical transmittance was decreased (87% ~ 72%) with the higher nitrogen.

The results of Hall measurements showed that the carrier concentration increased 2 order (1016 cm-3

to 1018 cm-3

) through nitrogen doping. The solar cell was made by NCD: N compound with p-type

silicon. The photoelectric conversion efficiency was 2.8%. The open-circuit voltage was 0.52 V. The

short-circuit current was 3 mA and the fill factor was 0.38.

Introduction

Over the past few decades, solar energy have attain much attention all over the world due to the

increasing of the global demand in energy consumption as well as the limitation of fossil fuels. Also,

with various owned advantages, solar cells have been developed for many important applications

such as defense and space technologies due to stability, compact design, and relative high energy

conversion in operations [1-3]. However, since the opto-electric conversion efficiency in solar cell

degrades as result of “solar wind” and UV irradiation [4,5], it is all of importance to improve the

radiation resistance, optical as well as thermal properties of the cell.

Nanocrystalline diamond (NCD) has been investigated recently [6-9] due to its unique combination

of outstanding mechanical, electrical, optical, and chemical inertness properties, leading to

developments of this metarials for operation at high temperature and in harsh environment which

aimed at replacing Si and other metal elements. It is well known that the electrical characteristics of

NCD can be controlled by doping with elements in III, IV groups during growth process or through

incorporation of metals in the films. This indeed open up potential applications of NCD films in

photovoltaic fields such as diamond-based solar cell. However, it should be noticed here that a p-n

junction and the doping level of diamond films are considered as the key factors for highly

performance of these devices and should be controlled. In this paper, we present here the deposition

of nitrogen-doped nanocrystalline diamond films on p-type Si substrate for solar cell application. The

effect of nitrogen doping level on the characteristics of NCD films as well as photovoltaic behavior of

the whole structure were studied.

Advanced Materials Research Vol. 918 (2014) pp 59-63Online available since 2014/Apr/17 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.918.59

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Experimental details

The polished p-type Si substrate (having size of 15 × 15 mm and 1 mm thick), initially were cleaned

with methanol and acetone solution, following by a diamond nucleation enhancement using a

suspension of nanodiamond particles. NCD films (0.3-2.5 m) were then growth onto the pre -treated

Si substrate using microwave plasma-jet chemical vapour deposition (MPJCVD) technique with

H2/CH4 gas mixture as precursors, which was described elsewhere [10].

The doping was carried out during the deposition process by adding nitrogen into the precursor gas

mixture with various doping level of 0÷20%. Gold and aluminum electrodes were fabricated by

magnetron sputtering on the surface of NCD films and Si substrate, respectively. In order to obtained

Ohmic contact of Al/Si/NCD/Au multilayer structure, annealing procedures were carried out at 450 oC in Ar ambient. Surface characteristics of as-deposited NCD:N films were examined by scanning

electron microscope (SEM) and atomic force microscopy (AFM) using Veeco Multimode operating

in the tapping mode. The nanocrystalline diamond structure grown with different nitrogen doping

level was investigated by Raman scattering spectroscopy with He-Ne laser in backscattering

geometry. Electrical properties of NCD:N films on p-type Si substrate were measured using

Van-der-Pauw method in the room temperature.

Results and Discussion

Fig. 1 shows SEM images of the diamond films deposited under 35 Torr of working pressure, 4% of

CH4 concentration, and with different nitrogen incorporation level of 0%, 1%, 3%, 10%, and 20% for

3 hours.

Fig. 1. SEM images for the NCD films grown at various nitrogen concentrations: (a) 0%, (b) 1%, (c)

3%, (d) 5%, (e) 10% and (f) 20%.

Fig. 1(a) shows the case of without nitrogen doping, a continuous diamond film is observed with

uniform diamond grain having size of few ten nanometers. From Fig. (b)-(f), it is found that the

surface morphologies of the as-grown NCD:N were significantly changed with the nitrogen

incorporation which the surface granular can be observed, indicating defects of the microstructure of

the films. Doping with 10% N2, SEM image of NCD film reveals serious of grain cluster with

granular shape and around 100 nm in size while the hole appeared on the film doped with 20%

nitrogen caused by as-deposited non-continuous diamond film. These can be explained through the

CN species during diamond growth as result of the appearance of nitrogen in the plasma. It is reported

in previous works that the CN species could have etching and bombardment effect on the nucleation

layer in the diamond growth process [11], leading to the surface granular of nitrogen doped NCD

films. This phenomena seems to be similar to the hydrogen etching effect in diamond growth during

CVD process with H2/CH4 surrounding. Moreover, it is not unreasonable to consider that these

60 Micro Nano Devices, Structure and Computing Systems III

existence of energetic CN species may have effects on the low C2 and CH3 which were the main

species for diamond growth. Therefore, corresponding to the increasing of nitrogen doping level,

thefilms thickness reduced from 2.587 µm to 385 nm. The aforementioned effects of nitrogen

incorporation on the diamond film growth can be investigated further using optical electron

spectroscopy [12].

Fig. 2. AFM images of nanocrystalline diamond films grown with different nitrogen concentrations:

(a) 0% (b) 1% (c) 3% (d) 5% (e) 10% and (f) 20%.

Fig. 2 shows AFM data of as-grown diamond films with various nitrogen incorporation level. It is

clear, from Fig. 2 (a), that the as-prepared NCD film possesses relatively smooth surface (Rms~16.5

nm) which ensures well junction for further electrical applications as p-n based devices. However,

the surface roughness of the NCD:N films were found to increase from 18.177 nm to 20.39 nm as a

function of N2 doping (Fig. 2(b)-(d)). This is in good agreement with the SEM results, according to

which, the aggregation of diamond grain on the film surface caused by CN species. Doping with 20%

nitrogen, AFM result of diamond film exhibits significantly reduction in roughness as obtained 9.884

nm of surface roughness. It should be noted here that, this is originated from the balance of the

diamond growth and etching ratio caused by excessive CN species in the plasma. In other words,

diamond grains cannot be enlarged due to a low diamond growth species and the aforementioned

etching effect of CN species on as-nucleated diamond.

Fig. 3. Raman spectra for the NCD films grown in H2-4% CH4 chemistries with addition of N2

concentration ranged from 0% to 20%.

Advanced Materials Research Vol. 918 61

The significant changes in microstructure of NCD:N films with various nitrogen doping level can be

investigated by Raman spectroscopy (Fig. 3). The as-prepared intrinsic diamond film possesses

characteristic peaks of NCD includes 1140 cm-1

of C-H bond (polyacetylene, trans- structure), 1332

cm-1

of diamond phase carbon, 1350 and 1580 cm-1

of D and G band of sp2-structure carbon,

respectively. The peak at 1190 cm-1

is observed in the spectra of NCD:N films and gradually

strengthens as a function of nitrogen doping level, exhibit that the nitrogen incorporation contribute

in the formation of C-N bonds at grain boundaries. Raman spectra of NCD:N film deposited with

20% nitrogen doping shows a weak peak of sp3 bonding at 1332 cm

-1 which seems to be overlapped

by D-band peak (at ~1350 cm-1

), demonstrating that the film possesses small diamond grain size and

relative high ratio of grain boundaries.

The electrical characteristics of the NCD:N films includes carrier concentration, mobility, and

conductivity are shown in Table 1. All the measurements were conducted by Van Der Pauw method.

Carrier concentration was found to remarkably increase with nitrogen doping concentration. NCD

film doped with 10% of nitrogen concentration exhibits the highest carrier concentration, lowest

mobility and lowest conductivity due to high nitrogen incorporation and high grain boundaries.

Table 1. The carrier concentration, mobility and conductivity of the NCD:N films prepared with

various nitrogen concentrations.

NCD:N

(%)

Carrier concentration

(cm-3

)

Mobility

(cm2/Vs)

Conductivity

(Ω-1

cm-1

)

0 -4.72 1016

745 5.64

1 -2.51 1017

70.4 2.83

3 -8.70 1017

51.0 7.12

5 -4.35 1018

2.75 1.92

10 -8.95 1018

1.62 10-4

2.33 10-4

20 -4.90 1013

45 3.53 10-4

Conclusion

In this paper, home-made microwave plasma chemical vapour deposition system was employed to

prepare nitrogen doped nanocrystalline diamond films. It is found that the nitrogen incorporation in

the microstructure of the films have significant effects on their electrical and morphology

characteristics. The results of SEM and AFM showed that the surface granular and surface roughness

were increased with increasing the nitrogen ratio in the plasma. Also, Raman spectra and XPS

showed that the grain boundary was increased with increasing the nitrogen ratio in the plasma. The

carrier concentration of nitrogen-doped nanocrystalline diamond films was increased with two order

from 1016

to 1018

with increasing nitrogen concentration in the plasma. The above results show a

prospects of NCD:N in solar cell applications.

Acknowledgement

This work was financially supported by the mail research projects of the National Science Council of

Republic of China under Grant numbers NSC 101-2221-E-027-009.

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