Thin Solid Films 519 (2011) 1950–1954

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Photovoltaic and interface state density properties of the Au/n-GaAs Schottky barriersolar cell

Murat Soylu a, Fahrettin Yakuphanoglu b,c,⁎a Department of Physics, Faculty of Sciences, Bingöl University, Bingöl, Turkeyb Department of Metallurgical and Materials Science Engineering, Fırat University, 23119, Elazığ, Turkeyc Department of Physics, College of Science, King Saud University, Riyadh, Saudi Arabia

⁎ Corresponding author. Department of MetallurEngineering, Fırat University, 23119, Elazığ, Turkey. Tfax: +90 424 2330062.

E-mail addresses: fyhanoglu@firat.edu.tr, fyakuphan(F. Yakuphanoglu).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.tsf.2010.10.030

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 January 2010Received in revised form 1 October 2010Accepted 7 October 2010Available online 21 October 2010

Keywords:Gallium arsenideSchottky diodeSolar cellsMetal-semiconductor contacts

The electrical and photovoltaic properties of the Au/n-GaAs Schottky barrier diode have been investigated.From the current–voltage characteristics, the electrical parameters such as, ideality factor and barrier heightof the Au/n-GaAs diode were obtained to be 1.95 and 0.86 eV, respectively. The interface state distributionprofile of the diode as a function of the bias voltage was extracted from the capacitance–voltagemeasurements. The interface state density Dit of the diode was found to vary from 3.0×1011 eV−1 cm−2 at0 V to 4.26×1010 eV−1cm−2 at 0.5 V. The diode shows a non-ideal current–voltage behavior with the idealityfactor higher than unity due to the interfacial insulator layer and interface states. The diode under lightillumination exhibits a good photovoltaic behavior. This behavior was explained in terms of minority carrierinjection phenomenon. The photovoltaic parameters, such as open circuit voltage and short circuit currentdensity were obtained to be 362 mV and Jsc=28.3 μA/cm2 under AM1, respectively.

gical and Materials Scienceel.: +90 424 2370000 6378;

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Metal-semiconductor structures are of important applications inthe electronics industry. These applications consist of microwave fieldeffect transistors, radio-frequency detectors, phototransistors, hetero-junction bipolar transistors, quantum confinement devices and spacesolar cell [1]. The performance and reliability of a Schottky contact arehighly influenced by the interface quality between the depositedmetal and the semiconductor surface. In order to understand theconduction mechanism of the Schottky barrier diodes, many attemptshave been made. Analysis of the current–voltage characteristics ofSchottky barriers on the basis of the thermionic emission diffusiontheory reveals an abnormal decrease in the barrier height and theincrease in the ideality factor with decreasing temperature [2–4].Explanation of the possible origin of such anomalies has beenproposed by taking into account the interface state density distribu-tion [5], quantum-mechanical tunnelling [1,6,7], image-force lower-ing [1] and lateral distribution of BH inhomogeneities [2,8,9]. Inaddition, a Gaussian distribution of the Schottky barrier over thecontact area has been assumed to describe the inhomogeneities asanotherway too [2,10]. The Schottky barrier junctions can operate as a

solar cell and the main advantage of Schottky barrier solar cells iseasiness of production, as they do not require high temperatureprocessing, and thus, the processing cost is remarkably reduced[11,12].

On the other hand, owing to the recent developments on GaAselectronic devices, much information is available about metal contactson gallium arsenide. The stability and reproducibility of contactproperties and formation of a high-quality Schottky barrier height(SBH) are essential prerequisites for device developments [13–17].Gallium arsenide-based photovoltaic devices are preferable not onlyhaving reciprocal properties with the microelectronic technology, butalso having long lifetime and high efficiency. Meanwhile, thephotovoltaic electricity can be used for hydrogen generation, whichis a clean renewable fuel through water electrolysis [18–21]. Thegallium arsenide Schottky barriers remain one of the most promisingstructures for development of semiconductor devices used in con-temporary microelectronics and optoelectronics [20,21].

In the present study, the electrical and photovoltaic properties ofthe Au/n-GaAs Schottky diode have been investigated to determinethe possibility of use in photovoltaic applications as a solar cell. Theaim of this study is to fabricate a solar cell based on the Au/n-GaAsSchottky diode. The parameters influencing the efficiency of a solarcell are the diffusion length of minority charge carriers, recombinationvelocity on the back and front surfaces, solar cell thickness and opticalconfinement [21]. We have evaluated that the Au/n-GaAs Schottkydiode can be prepared in the form of the Schottky solar cell, which ispromoted as a good candidate for a solar cell. Thus, we have at-tempted to resolve this issue.

1951M. Soylu, F. Yakuphanoglu / Thin Solid Films 519 (2011) 1950–1954

2. Experimental details

A Schottky barrier diode was fabricated on an n-type GaAs(Si-doped) substrate with a (100) orientation and a doping concentra-tion of 2.6×1016 cm−3. The substrate was sequentially cleaned withtrichloroethylene, acetone, and methanol and then rinsed in deionisedwater. The native oxide on the surface of GaAs was etched in sequencewith acid solutions (H2SO4:H2O2:H2O=3:1:1) for 60 s, and (HCl:H2O=1:1) for another 60 s. After the substrate is again rinsed indeionised water, it was dried by nitrogen. Low resistance ohmic contact(50 nm) on the back side of the n-GaAs wafer was formed byevaporating of indium at a pressure of 3×10−3 Pa and followed bythermal annealing at 375 oC for 5 min in a nitrogen atmosphere. Then,the above procedureswere also used to clean the front surface. Finally, acircular dot with a thickness of 50 nm and a diameter of approximately2 mm of Au was formed through a molybdenum mask at a pressure of3×10−3 Pa to form the Schottky barrier and the diode contact area wasfound be 3.14×10−2 cm2. The illuminated area for the prepared solarcell was found to be 0.1 cm2. The schematic diagram of the Au/n-GaAsSchottky barrier diode was given in Fig. 1a. Dark and photo current–voltage (I–V) and capacitance–voltage (C–V) measurements of the Au/n-GaAs diode were performed using a KEITHLEY 4200 SCS semicon-ductor characterization system. The photovoltaic measurements weredone using a Small-Area Class-BBA Solar Simulator.

3. Results and discussion

3.1. Current–voltage characteristics of the Au/n-GaAs Schottky diode

Fig. 1b shows the current–voltage characteristics of the Au/n-GaAsSchottky diode at room temperature. As seen in Fig. 1b, the diode

Fig. 1. Schematic diagram of Au/n-GaAs Schottky barrier diode and semi-logarithmicreverse and forward bias current–voltage characteristics at room temperature.

shows a good rectifying behavior. The current–voltage characteristicsof the diode can be analyzed by the following relation [22,23]

I = Io expq V−IRsð Þ

nkT

� �ð1Þ

where I0 is the reverse saturation current given by

I0 = AA*T2 exp − qΦb

kT

� �: ð2Þ

where q is the electronic charge, V is the bias voltage, A* is theRichardson constant, k is the Boltzmann constant, T is the absolutetemperature, Φb is the barrier height and n is the ideality factor and Ais the diode contact area (3.14×10−2 cm2). The values of Φb and nwere determined from the intercept and slope of the forward bias of In(I) vs. V plot, respectively. The Φb and n values were obtained to be0.86 eV and 1.95, respectively. The obtained n value higher than theunity suggests that the diode has a metal-insulator-semiconductorstructure (MIS). The I–V characteristics of the diode deviate fromlinearity due to the series resistance and interfacial layer. Thus, theseries resistance is an effective parameter for the I–V characteristics ofthe diode and it cannot be ignored. Therefore, we used the Nordemethod to calculate the Φb and n values of the diode. In the Nordemethod [24], the following functions are used to calculate diodeparameters,

F Vð Þ = V0

γ− kT

qln

I Vð ÞA*AT2

� �ð3Þ

where γ is an integer greater than n. I(V) is the current obtained fromthe I–V characteristics. The plot of F(V) vs. V for the diode is shown inFig. 2. The F(V) plot gives a minimum point and thus, the barrierheight is calculated by the relation,

Φb = F V0ð Þ + V0

γ− kT

qð4Þ

where F(Vo) is the minimum point of the F(V) function. The barrierheight of the diode was found to be 0.86 eV. The obtained barrierheight for the Au/n-GaAs/In Schottky diode is in agreement with thebarrier height of Ni/n-GaAs/In and Au/n-GaAs Schottky diodes [25,26].Also, in the Norde method, the series resistance is determined by thefollowing relation

Rs =kT γ−nð Þ

qIo: ð5Þ

Fig. 2. Plot of F(V) vs. V of an Au/n-GaAs Schottky barrier diode.

1952 M. Soylu, F. Yakuphanoglu / Thin Solid Films 519 (2011) 1950–1954

The Rs value for the diode was determined using Eq. (5) and wasfound to be 18.5 kΩ. This is considerably higher due to the oxide layerand resistance of the GaAs semiconductor and contact properties. Thiscauses a non-linear behavior for the diode. The Rs value of the studieddiode is higher than that of the Au/n-GaAs Schottky diode [27]. Thisconfirms the higher ideality factor of the studied diode.

3.2. Capacitance–voltage characteristics and interface state densityproperties of the Au/n-GaAs Schottky diode

In a capacitance–voltage (C–V) measurement carried out at asufficiently high frequency, the charges at the interface cannot followthe alternating current signal. This will occur when the time constantis too long to permit the charge to move in and out of the states inresponse to an applied signal. Thus, for a Schottky barrier diodefabricated on an n-type semiconductor, the depletion layer capaci-tance can be expressed as [28]

1C2 =

2 Vbi + Vr−kT = qð ÞqεsεoA

2Ndð6Þ

where, Vbi is the built-in voltage, Vr is the reverse voltage, A is the areaof the diode (3.14×10−2 cm2), εs is the dielectric constant for n-GaAs ,which is 13.1 [13–15], εo=8.85×10−14 F/cm and Nd is the donorconcentration and it can be obtained by the following expression

Nd =2

qεsεoA2

1d C−2� �

= dV

" #: ð7Þ

The barrier height for the diode is obtained by the following relation

ΦC−Vb = VbiC2 +

kTq

lnNc

Ndð8Þ

where Vbi is built-in potential, C2=1/n is a constant, here n is anideality factor and Nc(4.3×1017cm−3) is the effective density ofstates in the conduction band of n-GaAs at 300 K [13–15]. Fig. 3shows the C−2−V plot for the Au/n-GaAs Schottky diode under aspot frequency of 1 MHz. The reverse bias C−2−Vplot of the diodeshows a non-linear curve. The non-linear behavior of the curve canresult from the interface states. The Vbi and Nd values are obtainedfrom the intercept and slope of the extrapolated C−2-V lines with the

Fig. 3. The C−2–V characteristic of the Au/n-GaAs Schottky barrier diode at roomtemperature.

Vr axis, respectively. Then, the value ofΦbC-V was calculated by means

of Eq. (8) and was found to be 0.90 eV. The obtained ΦbC-V value is

higher than that of the ΦbI−V value at room temperature. For the

difference in barrier height values obtained from I–V and C–Vmeasurements, some general reasons have been mentioned in theliterature, such as surface contamination at the interface, deepimpurity levels, an intervening insulating layer, quantum-mechan-ical tunnelling, image-force lowering and edge leakage currents[5,7,22]. There have also been some reports, showing that thediscrepancy between barrier heights measured by different techni-ques might be associated with the instrumentation problems;namely, the way to determine a true space-charge capacitancefrom capacitance–voltage data or a large series resistance, whichcould affect the value determined from current–voltage data [5,29].In the case of the interface layer, the interface states and fixed surfacecharge affect the reverse bias and forward bias characteristics ofSchottky devices. Therefore, it can be said that the performance andreliability of Schottky devices depend on the interface layer and fixedsurface charge. The interface states play an important role in thedetermination of metal/semiconductor structures. The density of theinterface states can be calculated from the capacitance–voltagemeasurements. The capacitance of the interfacial layer is effectivelyin series with the capacitance of the depletion region. For the metal-insulator-semiconductor diode having an interface, the idealityfactor is higher than the unity. We can determine the interfacestate density using a combined high–low frequency capacitancemethod [30]. In this method, the low frequency capacitance for thediode is defined as,

1CLF

=1Cox

+1

Cs + Citð9Þ

where Cox is the capacitance measured in strong accumulation, Cs isthe space-charge capacitance, and Cit is the interface layer capacitanceand solving of Eq. (9) yields,

Ci =1CLF

− 1Cox

� �−1−Cs: ð10Þ

At higher frequencies, the capacitance is expressed as,

CHF =Cs + Cox

CsCox: ð11Þ

Combining of Eq. (10) and Eq. (11) yields

Cit =1CLF

− 1Cox

� �−1− 1

CHF− 1

Cox

� �−1ð12Þ

and the interface state density can be determined by the followingrelation,

Dit =1qA

1CLF

− 1Cox

� �−1− 1

CHF− 1

Cox

� �−1ð13Þ

where A is the diode contact area (3.14×10−2 cm2), CLF is the lowestvalue of the low frequency (10 kHz) capacitance, and CHF is the highfrequency (1 MHz) capacitance [31]. It is well known that theinterface charges respond to the alternating current signal especiallyat a low frequency (b10 kHz) and yield an excess capacitance, whichdepends on the frequency, whereas at high frequencies (≥500 kHz),(capture or emission time τ is much larger than 1/ω), the charges atthe interface states cannot follow an ac signal [32–38]. This makes acontribution of the interface state capacitance to the total capacitancenegligibly small. From themeasured low frequency capacitance under10 kHz and high frequency capacitance under 1 MHz curves, the Dit

values were determined and the plot of Dit vs. V is shown in Fig. 4a.

Fig. 5. Current–voltage characteristics of the Au/n-GaAs Schottky barrier diode underdark and illumination conditions.

1953M. Soylu, F. Yakuphanoglu / Thin Solid Films 519 (2011) 1950–1954

The schematic band diagram illustrating the potential sources ofsurface and interface states, which can trap charge carriers in the GaAsdevice, is shown in Fig. 4b [34,35]. The interface density Dit rangesfrom 3.0×1011eV−1 cm−2 to 4.26×1010 eV−1cm−2, when the biasvoltage varies from 0 V to 0.5 V. This confirms that the Dit valuechanges with applied voltage and this change confirms the presenceof various positions inside the GaAs gap.

3.3. Photovoltaic properties of the Au/n-GaAs Schottky diode

Fig. 5 shows the I–V plots of the diode under dark and illuminationconditions. As seen in Fig. 5, the reverse current increases stronglywith the illumination. This suggests that the diode exhibits aphotovoltaic behavior. Thus, the solar cell characteristics of the Au/n-GaAs diode are shown in Fig. 6. As seen in Fig. 6, the current–voltagecharacteristics of the diode give a short circuit current Isc, and opencircuit voltage, Voc. The plots of Isc and Voc vs. P are shown in Fig. 7. Theshort circuit current and open circuit voltage values of the deviceincrease illumination intensity. The current–voltage relation for theSchottky barrier solar cell is expressed by the following relation,

I = Ip− Io expq V−IRsð Þ

npkT

!−1

!ð14Þ

Fig. 4. a) Plot of Dit vs. V of the Au/n-GaAs Schottky barrier diode. b) Schematic banddiagram illustrating the potential sources of surface and interfacial trapping to beexpected in an MIS GaAs device.

where Ip is the photocurrent, and np is ideality factor of the solar cell.The I–V characteristics of the solar cell can be analyzed by thefollowing relation,

Voc = npkTq

� �ln

IscIo

� �: ð15Þ

The ideality factor was determined from the logIsc vs. Voc plot bymeans of Fig. 7 and was found to be 1.85. This value is lower than thatof the ideality factor obtained from dark current–voltage measure-ments. This indicates that the performance of the Au/n-GaAs underthe illumination condition is improved. The Au/n-GaAs Schottky solarcell indicates the lower Isc values due to very high series resistance ofthe studied diode.

For the analysis of the photovoltaic mechanism of the Au/n-GaAsSchottky solar cell, the relation between short circuit current densityand illumination intensity is defined as,

Jsc = BPα ð16Þ

Fig. 6. The solar cell characteristics of the Au/n-GaAs diode.

Fig. 7. Plots of Jsc and Voc vs. P of the Au/n-GaAs diode.

1954 M. Soylu, F. Yakuphanoglu / Thin Solid Films 519 (2011) 1950–1954

where B is a constant, α is a exponent and P is the intensity of light. Inorder to determine the α value, the plot of Jsc vs. P was fitted by bothlinear and non-linear fitting functions and the fitting line was shownon the graph in a double logarithmic scale and it was seen that thefigure obey a power law with R2=0.993 value and the α value wasfound to be 0.85. This suggests that the plot of Jsc vs. P obeys a powerlaw. It is well known that the α value lies between 0.5 and 1.0 forcontinuous distribution of trapping centers [39]. Thus, the obtained αvalue confirms the presence of continuous distribution of traps [39].The current at a given voltage for the diode under illumination ishigher than that of under dark. This indicates that the lightillumination increases the production of electron-hole pairs. The Au/n-GaAs solar cell shows a photovoltaic behavior with a maximumopen circuit voltage Voc of 360 mV and short circuit current Jsc of28.3 μA/cm2 under AM1. The fill factor was found to be 0.48. The Jscand Voc values of the studied Schottky type solar cell are lower thanthat of typical commercial solar cells. To improve the efficiency of thestudied solar cell, we have to take into considerations the propertiesof materials constructing a solar cell and to develop new structures. Itis evaluated that this work is useful as a basis for the search of Au/n-GaAs Schottky type solar cell and more competitive Au/n-GaAs-basedsolar cells, despite the fact that Voc and Jsc are lower than thosereported in the literature.

4. Conclusions

The electrical and photovoltaic properties of the Au/n-GaAsSchottky barrier diode have been investigated by means of I–V andC–V measurements at room temperature. The Au/n-GaAs Schottkydiode exhibits a rectifying behavior with obtained electronic para-meters. The photovoltaic measurements indicate that the Au/n-GaAsSchottky structure is a photovoltaic cell. It is evaluated that the Au/n-GaAs Schottky diode could be used as a micropower source that isintegrated into a larger GaAs-based electronic circuit.

Acknowledgements

This work was supported by the Feyzi AKKAYA Scientific ActivatesSupporting Fund (FABED). One of author wishes to thank FABED for ayoung scientist grant.

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