JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.1, FEBRUARY, 2016 ISSN(Print) 1598-1657 http://dx.doi.org/10.5573/JSTS.2016.16.1.039 ISSN(Online) 2233-4866

Manuscript received May. 21, 2015; accepted Jan. 9, 2016 1 Daegu Center, Defense Agency for Technology and Quality, 52-8 Dongwon-ro 28-gil Suseong-gu, Daegu 706-803, Republic of Korea 2 Department of Analysis Assessment, Defense Agency for Technology and Quality, 402, Dongjinro, jinju-si, Gyeongsangnam-do, 660-031, Republic of Korea E-mail : sbh85@dtaq.re.kr

Enhanced Efficiency of Transmit and Receive Module with Ga Doped MgZnO Semiconductor Device by

Growth Thickness

Bo-Hyun Shim1,*, Hee-Jin Jo2, Dong-Jin Kim1, and Jong-Mok Chae1

Abstract—The structural, electrical properties of Ga doped MgZnO transparent conductive oxide (TCO) films by ratio-frequency(RF) magnetron sputtering were investigated. Ga doped MgZnO TCO films were deposited on the sapphire substrates at 200 varying growth thickness 200 to 600 nm. The optical properties of Ga doped MgZnO TCO films were showed above 85% transmittance from 300 to 1000 nm region. In addition, the current density (JSC) of Cu(In,Ga)Se2 (CIGS) solar cells was improved by using the MgZnO:Ga films of 500 nm thickness because of outstanding electrical properties. The Cu(In,Ga)Se2 solar cells with MgZnO:Ga transparent conducing layer yielded an efficiency of 9.8% with current density (31.8 mA/cm2), open circuit voltage (540.2 V) and fill factor (62.2) under AM 1.5 illumination. Index Terms—Solar cells; sputtering; electrical properties; optical properties

I. INTRODUCTION

Cu(In,Ga)Se2 (CIGS), which is the chalcopyrite compound semiconductors with a direct band gap and excellent optical absorption coefficient, solar cells have

highest efficiency for photovoltaic application [1]. The CIGS has emerged as low-cost, high efficiency solar cells that dramatically improve the economics of solar photovoltaic to replace expensive crystalline silicon solar cells [2]. Currently, the Al- or Ga-doped ZnO window layer has been widely used for the CIGS solar cells due to its good opto-electronic properties such as high transparency (>80%) and electrical conductivity(>103 Ω-cm-1) [3-5]. However, transparent conductive oxide (TCO) materials with high transmittance and electronic properties have been still required to achieve the high efficiency solar cells [6-8]. As an alternative, MgZnO:Ga TCO has been researched for available band-gap engineering through Mg concentration [9, 10]. By alloying MgO into the ZnO layer, the optical transmission can increase in the near ultraviolet and visible region.

The papers related to MgZnO TCO layer indicated that it is possible to provide large energy band gap of about 3.9 eV without phase change from hexagonal to cubic structure. Increasing energy band gap with Mg alloying is expected to improve current density that is a key component of the solar cells. Also, MgZnO TCO layer has obtained relatively outstanding electrical properties as minimizing resistance by using dopant materials of Ga atoms whose atomic radius is similar with Zn atoms.

In this paper, we investigated the dependence of structural, electrical, and optical properties of MgZnO:Ga thin films on growth thickness during RF sputtering process and confirmed that the MgZnO:Ga films are applicable CIGS solar cells as a function of TCO layer.

40 BO-HYUN SHIM et al : ENHANCED EFFICIENCY OF TRANSMIT AND RECEIVE MODULE WITH GA DOPED MGZNO …

2. EXPERIMENTAL PROCEDURE

MgZnO:Ga films were deposited on the glass substrates by RF magnetron sputtering with different growth thickness from 200 to 600 nm. The Ga concentration of the ZnO:Ga targets was fixed by 3 wt. High-purity Ar gas was used as the plasma source, and the gas flow rate was regulated at 50 sccm. (working pressure)

The CIGS device structure for the use of J-V characteristic was composed with soda-lime glass (3 mm), molybdenum (1 μm) by sputtering method, CIGS (2.5 μm) by co-evaporation, CdS (50 nm) using chemical bath deposition method, un-doped ZnO (50 nm), MgZnO:Ga (200 to 600 nm) layer by sputtering method and silver-grid by e-beam evaporation.

The electrical, optical and structural properties of MgZnO:Ga were investigated by using Hall effect measurement in Var-der-Pauw, ultraviolet-visible (UV-VIS) spectroscopy, x-ray diffraction (XRD). In addition, the current density and quantum efficiency of CIGS solar cells was investigated by a K3000 solar cell measurement system and K3100 spectral IPCE measurement system.

3. RESULTS AND DISCUSSION

The crystal structure of MgZnO:Ga films was investigated by XRD for different growth thickness as shown in Fig. 1. The peak confirmed at 34.45° was attributed to (002) diffraction of the hexagonal wurtzite structure with highly c-axis plane. The peak intensity of (002) plane was higher with increasing growth temperature. This behavior implied that the MgZnO:Ga films was stabilized due to the removal of defects. Also, Other ZnO peaks at 35.56° and 72.60° were indicated at growth thickness of 600 nm. This characteristics depend for (001) and (004) diffraction.

The surface morphology with different growth thickness of MgZnO:Ga films was obtained by SEM analysis as shown in Fig. 2. The MgZnO:Ga films has changed a uniform and dense structure when the growth thickness of films decreases. Also, the grain size of the MgZnO:Ga films was improved with increasing growth thickness. This feature was attributed to the activation energy of the MgZnO:Ga flims. The activation energy

was decreased when the growth thickness of films increases. The activation energy(Ea = Ec – Ef) can be determined as the difference between the Fermi energy level to conduction band. When the activation energy was low, it can be explained by the higher energy level. In addition, the Fermi level closed to conduction band and it was electrically stable through external force. The electrical conductivity of the material according to temperature was affected by concentration higher than carrier relaxation time (σ = σ0 exp(-Ea/KBT) → Inσ = In σ0 -Ea/KBT). Therefore, grain boundary in the surface morphology was removed and electrical properties of MgZnO:Ga films was enhanced.

Fig. 3 showed the surface roughness of MgZnO:Ga films with different growth thickness by AFM method. The surface roughness was increased due to the growth thickness from 200 nm to 600 nm. However the result which be measured about the surface roughness was not affect to the optical properties of MgZnO:Ga films due to the incomplete variation of the surface roughness.

The variation of electrical properties for MgZnO:Ga films was measured as shown in Table 1. The mobility and electrical resistivity of MgZnO:Ga films at the 200 nm thickness were 8.38 cm2/V-s and 1.41×10-3 Ω-cm, respectively, whereas the values of films at the 500 nm growth thickness indicated 14.8 cm2/V-s and 1.15×10-3 Ω-cm, respectively. The outstanding electrical properties were attributed to the high mobility. These characteristics resulted from the enhancement of surface morphology. The electrical mobility of MgZnO:Ga films was decreased by the grain boundary which acted as free

Fig. 1. XRD patterns of the MgZnO:Ga films with different Temperature. The intensity is increased due to improvement of crystallinity by annealing.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.1, FEBRUARY, 2016 41

carrier trap. The uniform and crack-free grains lead to increasing the mobility of films. Thus, electrical properties was confirmed to the MgZnO:Ga films grown at the 500 nm thickness.

The optical transmittance ranging from 300 to 1000 nm of MgZnO:Ga films were organized in Fig. 4. Most MgZnO:Ga films showed high transparency of over 90% in the visible region. The regulated values offered that MgZnO:Ga films were very useful for applications demanding outstanding transmittance. In the visible region, the difference of the transmittance about 600 nm growth thickness was clearly revealed.

In comparison to transmittance of MgZnO:Ga films that was deposited about 600 nm thickness, transmittance of 500 nm growth thickness MgZnO:Ga films was superb from 380 nm to 780 nm region.

It implied that high transmittance of TCO films as minimizing the loss of light in the visible region played an important role to improve current density of solar cells.

Fig. 5 showed current density-voltage(J-V) characteristics including fitted curves of CIGS devices with the MgZnO:Ga films. Performance parameters and fitted results obtained from the J-V characteristics are summarized in Table 2. The device with MgZnO:Ga

Table 1. Electrical properties of MgZnO:Ga TCO with growth thickness

200 nm 300 nm 400 nm 500 nm 600 nm Resistivity (Ω-cm-3) 1.41 1.31 1.28 1.15 1.09 Mobility (cm2/v-s) 8.36 8.94 11.4 14.8 15.9

Concentration (1020/cm3) 5.35 5.94 7.85 7.15 6.20

300 400 500 600 700 800 900 10000

20

40

60

80

100

Tr

ansm

ittan

ce (%

)Wavelength (nm)

MgZnO:Ga(200nm) MgZnO:Ga(300nm) MgZnO:Ga(400nm) MgZnO:Ga(500nm) MgZnO:Ga(600nm)

Fig. 4. UV-visible transmittance spectra in wavelength region from 300 to 1000 nm MgZnO:Ga films deposited at different thickness.

Fig. 2. SEM morphologies of the MgZnO:Ga films at various thickness. The grain boundary is removed by increasing growth thickness.

Fig. 3. AFM morphologies of the MgZnO:Ga films at various thickness.

42 BO-HYUN SHIM et al : ENHANCED EFFICIENCY OF TRANSMIT AND RECEIVE MODULE WITH GA DOPED MGZNO …

films of 500 nm thickness exhibits the performance parameters with a current density (Jsc) of 31.8 mA/cm2, a open-circuit voltage (Voc) of 0.540 V, a fill factor (FF) of 62.2%, and an efficiency (Eff.) of 9.8%, as compared with those of the device with MgZnO:Ga films of 600 nm thickness Jsc of 30.9 mA/cm2, Voc of 0.540 V, FF of 60.0%, and Eff. of 9.4%. In order to extract the parameters, the J-V curves were fitted using a general diode equation. The fill factor can be obtained to the ratio of the output of the multiplication with open-circuit voltage and short circuit current. It was maximum rectangle area in the current-voltage curve (FF = Im∙Vm ∕ Isc∙Voc). The measurement of the series resistance was obtained from the linear portion of the direction of easy flow in the Fig. 5, determined series resistance from the voltage drop of the I1 and I2 in the areas which excellent linearity (Rs = V2 – V1 / I2 – I1). The parallel resistance was a problem when the defective junction and leakage current flows much. The measurement of the parallel resistance was calculated from the linear portion of the reverse characteristic.

The efficiency of the device with MgZnO:Ga films of 500 nm thickness is attributed to high transmittance of the MgZnO:Ga films in the visible region, as shown in Fig. 5.

4. CONCLUSIONS

We have prepared high transparent and conductive MgZnO:Ga films via RF magnetron sputtering system. The properties of the MgZnO:Ga films are strongly

dependent on the growth thickness. It was obtained that transmittance was higher than 85% in the near-ultraviolet and visible region. The lowest electrical resistivity of 1.15×10-3 Ω-cm can be confirmed by the MgZnO:Ga films at 500 nm thickness. In spite of its superior electrical properties of films deposited over 600 nm thickness, it was found that 500 nm is the most appropriate process condition for the application of CIGS device. In order to improve properties of MgZnO:Ga films, further investigations are in progress. These ways are regarded as interesting approach for promising candidate that can be used window layer in opto-electronic device.

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0.0 0.1 0.2 0.3 0.4 0.5 0.6-10

0

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30

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Voc (V)

J sc

(mA

/cm

2 ) Device with 200 nm MgZnO:Ga films Device with 300 nm MgZnO:Ga films Device with 400 nm MgZnO:Ga films Device with 500 nm MgZnO:Ga films Device with 600 nm MgZnO:Ga films

Fig. 5. Opto-electronic properties of CIGS solar cells with MgZnO:Ga films.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.16, NO.1, FEBRUARY, 2016 43

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Bo-Hyun Shim was born in Busan, Korea, on 1985. He received the B.S. degree in the Department of Nano Semiconductor Engineering from Korea Maritime and Ocean Univer- sity, Korea, in 2011 and M.S. degree in Applied Physics and Optical

Engineering from Gwangju Institute of Science and Technology (GIST), Korea, in 2013, respectively. He joined at Defense Agency for Technology and Quality (DTaQ), where he has been working in the area of electro optical tracking systems (EOTS). His research interests electronic materials (solar cells, lighting emitting diode) and optics devices.

Hee-Jin Jo received the B.S degrees in the Department of Semiconductor Science and Technology Engineering from Chonbuk National University, Korea, in 2011 and M.S degree in Advanced Materials Engineering from Gwangju Institute of Science

and Technology (GIST), Korea, in 2013, respectively. She joined at Defense Agency for Technology and Quality (DTaQ), where she has been working in the area of analysis about military supplies. She interests electronic materials (solar cells, lighting emitting diode) and optics devices.

Dong-Jin Kim received the B.S and M.S degrees in the Department of Electricity and Electronic Enginee- ring and Electric Wave Engineering from Korea University, Korea, in 2009, 2011, respectively. He joined at Defense Agency for Technology

and Quality (DTaQ), where she has been working in the area of Tactical Information Communication Network (TICN). His research interests high-speed links, memory circuits, and ultra low-power analog.

Jong-Mok Chae received the B.S degrees in the Department of Elec- tricity and Electronic Enginee- ring from Myongji University, Korea, in 1990 and M.S degree in Military Strategy from Korea National Defense University, Korea, in 2009,

respectively. He joined at Defense Agency for Technology and Quality (DTaQ), where she has been working in the area of Tactical Information Communication Network (TICN). His research interests high-speed links, memory circuits, and ultra low-power analog.