Fabrication and characterization of n-In0.4Ga0.6N/p-Si solar cell

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Letter

Fabrication and characterization of n-In

0.4

Ga

0.6

N/p-Si solar cell

Binh-Tinh Tran

a

, Edward-Yi Chang

a,n

, Hai-Dang Trinh

a

, Ching-Ting Lee

b

, Kartika Chandra Sahoo

a

,

Kung-Liang Lin

a

, Man-Chi Huang

a

, Hung-Wei Yu

a

, Tien-Tung Luong

a

, Chen-Chen Chung

a

,

Chi-Lang Nguyen

a

Department of Materials Science and Engineering, National Chiao Tung University, 1001 University Road, Hsinchu, Taiwan

b_{Department of Electrical Engineering, National Cheng Kung University, Taiwan}

a r t i c l e

i n f o

Article history:

Received 18 December 2011 Received in revised form 21 February 2012 Accepted 26 March 2012 Available online 19 April 2012 Keywords: MOCVD InGaN Si Solar cell ITO

a b s t r a c t

Electro-optic characteristics of a fabricated n-In0.4Ga0.6N/p-Si hetero-structure solar cell on Si substrate

with Al and ITO (or Ti/Al/Ni/Au) materials for p and n-type contacts were investigated in this letter. The solar cell devices with ITO as n-type contacts were also compared to the solar cell using Ti/Al/Ni/Au as n-type contact in this study. High short-circuit current density observed for solar cell with ITO as n-type contacts due to the increased amount of light reaching the solar cell. The device with ITO contact exhibited an open-circuit voltage (Voc) of 1.52 V and a short-circuit current density (Jsc) of 8.68 mA/cm2

with 54% ﬁll factor. The conversion and external quantum efﬁciency (EQE) of the solar cell were 7.12 and 20.8%, respectively. Besides, a relationship between Vocand In content in the InxGa1 xN alloys for

this type of solar cell was also derived.

1. Introduction

The InxGa1 xN alloys attract a lot of attention for

optoelec-tronic applications due to their bandgap range (i.e. continuously from 0.67 eV to 3.4 eV ) and higher theoretical mobility, high saturation velocity, and high absorption rate. This material system is very promising for multijunction solar cell application. The theoretical calculation indicates that InxGa1 xN alloys with

about 40% In content can fulfill the requirements as active material for solar cells with solar energy conversion efficiency greater than 50%[1]. However, to develop this III-nitride material, several major challenges need to be overcome. First, there is a lack of a suitable substrate in terms of lattice and thermal expansion coefficient mismatches for the epitaxial growth of

InxGa1 xN with low dislocation density. Usually, SiC and sapphire

are the most common substrates used to grow III-nitride materi-als, but with high dislocation density in the range of 107 _to

1010_cm2_[2_,₃_{]. Second, the large lattice mismatch between InN}

and GaN usually leads to a miscibility gap, which can cause phase separation[4], V-pit defects, and InN segregation[5,6]. Therefore, it is difﬁcult to grow high indium content InxGa1 xN ﬁlms with

good quality, which have great potential for solar cell applica-tions, on these substrates. There are only a few reports on

InxGa1 xN ﬁlms grown on Si substrate with In content below

40% and no reports so far on fabrication of InxGa1 xN on Si

substrate with high In content (x 440%) for solar cell application. In this letter, we report on the fabrication and characterization of a n-In0.4Ga0.6N/p-Si hetero-structure solar cell. We also compare

the characteristics of the solar cell fabricated using two different n-type contact materials. The detailed development process for the n-In0.4Ga0.6N/p-Si hetero-structure has been previously

reported in[7].

2. Experimental procedures

The growth of In0.4Ga0.6N ﬁlm on GaN/AlN/Si(111) substrate

was carried out using a Metal Organic Chemical Vapor Deposition (MOCVD) in an EMCORE Model D-180 rotating disk vertical reactor and the detailed growth process has been reported as mentioned above. In brief, the growth process consists of a 50 nm AlN buffer grown on Si substrate at 1010 1C followed by a 0.6-

m

m-thick GaN film deposition at 1030 1C. Finally, a 300-nm-m-thick In0.4Ga0.6N film was grown epitaxially on top of the GaN film at

740 1C with the growth pressure of 300 Torr. Then we checked the material quality of the n-In0.4Ga0.6N/p-Si hetero-structure by

doing a photoluminescence measurement (PL) on 3 different positions on the wafer surface before the fabrication of the solar cell. The fabricated solar cells had a cell area of 1 1 cm2_{. The}

radius pattern for n-type electrode on each cell was 0.33 mm and was fabricated by direct deposition of Ti/Al/Ni/Au (200/1200/250/ 1000 ˚A) on the top of the In0.4Ga0.6N ﬁlm by electron beam

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Corresponding author. Tel.: þ886 3 5131536. E-mail address: [email protected] (E.-Y. Chang).

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evaporation (device-A). The p-type contact was formed by eva-poration of 1000- ˚A-thick Al on the backside of p-Si substrate. For device-B a 2000 ˚A thick ITO layer was used as n-type contact while keeping all other structures the same as in device-A. The In0.4Ga0.6N ﬁlm was intrinsically n-type due to the donor-like

nature of the native defects of the material and the Si substrate was boron doped p-type substrate with sheet resistance of 5–10

O

cm. The electron concentration was in the order of 1019_cm3 _{as obtained from the Hall measurements. We}

pre-sented the fabrication process of device-B inFig. 1, and device-A is similar to that of device-B. The current–voltage characteristics of the devices were measured under AM 1.5G illumination (one sun, air mass 1.5 global spectrum).

3. Results and discussion

The PL measurements of the In0.4Ga0.6N ﬁlm were conducted

at room temperature (RT) with an incident wavelength of 325 nm and the measured spectral peaks at 3 different positions of the wafer are presented in Fig. 2. The main PL peak shows good results across the wafer and the weak peaks beside the main peak in Fig. 2 correspond to Fabry–Perot interferences between the GaN/Si interface and the surface [8]. These weak PL peaks are related to the phase separation of InxGa1 xN due to the weak

bonding between In–N and Ga–N. However, this result is in good agreement with the reports of InxGa1 xN grown on other

sub-strates[9–12].

To investigate the device characteristics, the dark current of the n-In0.4Ga0.6N/p-Si hetero-structure solar cells were measured and

presented inFig. 3. The leakage current of device-A (used Ti/Al/Ni/ Au as n-type contact) was higher than device-B (used ITO as n-type contact) and the values of the leakage current were 3.7 106_and

0.48 107A under a bias of 7 V for device-A and device-B, respectively. This result indicates that the n-In0.4Ga0.6N/p-Si

hetero-structure solar cell using ITO as n-type contact is better than Ti/Al/Ni/Au. Fig. 4 shows the current density versus the voltage characteristics of the devices under AM 1.5G. The mea-sured Voc and Jsc for device-B were 1.52 V and 8.68 mA/cm2,

respectively. It can be observed that the Voc and Jscof device-B

Fig. 1. Solar cell structure with Al and ITO materials for p and n-type contacts (device-B), respectively.

Fig. 2. Photoluminescence spectra of the samples measured at 3 different posi-tions on the wafer conducted at room temperature.

Fig. 3. Dark current–voltage curves for the n-In0.4Ga0.6N/p-Si solar cell.

Fig. 4. Typical current–voltage characteristics of the n-In0.4Ga0.6N/p-Si solar cell

with Ti/Al/Ni/Au and ITO contact materials.

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were larger than those of device-A due to its transparent contact (ITO), which is attributed to more light hitting the device and the improvement in the leakage current. For device-A, we can easily observe that the cell only slightly responds to the white light because it only responds to ultraviolet light [13,14] and this phenomenon needs further investigation. The ﬁll factor and con-version efﬁciency of the n-In0.4Ga0.6N/p-Si hetero-structure solar

cell were calculated using the measured current density and voltage curves fromFig. 4. The calculated ﬁll factor and conversion efﬁciency for device-B were 54 and 7.12%, respectively. However, the Vocof device-B was still smaller as compared to other reports

[14–16]. From this comparison, it can be found that the Vocvalue

decreases with the increase of In content. This decrease in Voccould

be due to either the degradation of electro-optic or material properties or both. This can be explained using the equation written below[16] VOC¼ Eg q þ nkT q lnðJscÞ nkT q lnðJ00Þ ð1Þ

where Egis the energy bandgap, KT/q the thermal voltage, n the

ideality factor, Jscthe short-circuit current density and J00a weak

temperature dependent pre-factor. From this equation, the third term is related to the quality of material, and the second term, in contrast to the ﬁrst term, changes very little when the indium content changes and Vocdecreases linearly with Egbased on the

Eq. (1). However, the experimental results show not fully linear as plotted inFig. 5. The problem can be attributed to the defects in materials. The three-ﬁrst square points 1, 2 and 3 inFig. 5are taken from Refs.[14–16] and the last one is our data measured from n-In0.4Ga0.6N/p-Si hetero-structure solar cell (device-B). From the

data, it is clear that the Voc for hetero-structure device still

decreases with the increase of In content though there is a small deviation. The results with different In contents samples follow the same variation and clearly indicate that In content plays an important role in determining Vocfor the InxGa1 xN solar cell.

Fig. 6shows the EQE for the device-A and device-B. We used Acton monochrometer to disperse Xe lamp for this measurement under AM 1.5G condition at RT, and a calibrated silicon photo-detector was used to detect the incident light power. The EQE was estimated from the ratio between the number of incident photons and the number of measured electrons. This figure shows the quantum efficiency as a function of excitation wavelength. The results demonstrate that device-A and device-B deliver an exter-nal quantum efficiency of about 19.0% and 20.8%, respectively. Device-B shows an improved external quantum efficiency as compared to device-A and both device structures show com-parable performances as compared with complicated structure reported previously[17].

4. Conclusions

In summary, we report the fabrication of n-In0.4Ga0.6N/p-Si

solar cell on Si substrate for the first time. We investigated ITO and Ti/Al/Ni/Au as n-type contact and found that the device with ITO as n-type contact demonstrated an enhanced short-circuit current density due to the increased amount of light that was absorbed by the solar cell as compared to the device using Ti/Al/ Ni/Au as n-type contact. The device with ITO contact showed good photovoltaic performances and clear spectral response with a fill factor of about 54%, an external quantum efficiency of 20.8 and 14% at 375 and 390 nm, respectively, and the overall conver-sion efficiency of 7.12%. This is the highest converconver-sion efficiency reported for an InxGa1 xN based solar cell. Overall, the study

demonstrates the potential of using InxGa1 xN for high efﬁciency

solar cell applications.

Acknowledgment

The authors would like to thank the Taiwan National Science Council (under research grant nos. NSC98-2923-E-009-002-MY3 and NSC99-2221-E-009-170-MY3) for providing the ﬁnancial support. References

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