Low-Temperature Electrical Characteristics of Si-Based Device
with New Tetrakis NiPc-SNS Active Layer
ARZU BU¨ YU¨KYAG˘CI YAVUZ,1BUKET BEZG_IN CARBAS,2 SAVAS¸ SO¨ NMEZOG˘LU,3and MURAT SOYLU4,5
1.—Department of Mining Analysis and Technologies, General Directorate of Mineral Research and Exploration, Ankara, Turkey. 2.—Department of Energy System Engineering, Karamanoglu Mehmetbey University, Karaman, Turkey. 3.—Department of Materials Science and Engineering, Karamanoglu Mehmetbey University, Karaman, Turkey. 4.—Department of Physics, Faculty of Arts and Sciences, Bingol University, Bingol, Turkey. 5.—e-mail: soylum74@yahoo.com
A new tetrakis 4-(2,5-di-2-thiophen-2-yl-pyrrol-1-yl)-substituted nickel phthalocyanine (NiPc-SNS) has been synthesized. This synthesized NiPc-SNS thin film was deposited on p-type Si substrate using the spin coating method (SCM) to fabricate a NiPc-SNS/p-Si heterojunction diode. The temperature-dependent electrical characteristics of the NiPc-SNS/p-Si heterojunction with good rectifying behavior were investigated by current–voltage (I–V) mea-surements between 50 K and 300 K. The results indicate that the ideality factor decreases while the barrier height increases with increasing tempera-ture. The barrier inhomogeneity across the NiPc-SNS/p-Si heterojunction re-veals a Gaussian distribution at low temperatures. These results provide further evidence of the more complicated mechanisms occurring in this heterojunction. Based on these findings, NiPc-SNS/p-Si junction diodes are feasible for use in low-temperature applications.
Key words: Phthalocyanine, surface treatments, electrical properties, thermal analysis
INTRODUCTION
Phthalocyanine (Pc) compounds are known as p-type semiconductors with low carrier concentration and low mobility.1 They could form the basis for molecular optoelectronic applications. The physical properties of phthalocyanines enable molecular engineering to modify the molecular structure, as well as chemical stability. Thin films of copper phthalocyanine (CuPc) can be used as charge-in-jection layers for organic-based electronic de-vices.2–9 Nickel phthalocyanine (NiPc) offers the features required to act as an active donor layer in planar heterojunction solar cells. A relatively high mobility value of 1 9 105m2V1cm16 was found for NiPc, while the mobility values for ZnPc,
CoPc, and CuPc were reported to be
7.6 9 109m2V1cm1,7 3 9 107m2V1cm1,8
and 1 9 108m2V1cm1,9 respectively. Films consisting of NiPc and 3,4,9,10-perylene tetracar-boxylic dianhydride (PTCDA) mixture can be useful materials in organic photovoltaic devices.10 The Schottky barrier (SB) is a potential energy barrier between an electrode and metal Pc materials that allows the formation of a junction. Indium tin oxide (ITO)/CuPc/Al diodes show temperature- and bias-dependent regimes, exhibiting a space-charge-lim-ited conduction (SCLC) regime at high bias and low temperature and a Poole–Frenkel (PF)-type effect at low bias and high temperature.11 The dominant conduction mechanism in Au/InPcCl/p-Si/Al heterojunctions was extensively reported in a recent paper.12 Based on the temperature dependence of I–V characteristics, the thermionic emission (TE) conduction mechanism was observed at low forward bias (<0.5 V) whereas space-charge-limited current controlled by a single-trap energy level dominates at voltages higher than 0.5 V. Researchers have em-barked on studies to develop a new device for (Received May 19, 2015; accepted September 30, 2015;
published online October 20, 2015)
Ó2015 The Minerals, Metals & Materials Society
measuring the temperature dependence of electrical resistance and capacitance. In this study, nickel(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (NiTSPc) aqueous solution was employed as a potential material for a temperature sensor in the specific temperature range of 290 K to 350 K.13 Azim-Araghi and Sahebi14 investigated the direct-current (DC) electrical properties of bromoindium phthalocyanine/aluminum interfaces using Al elec-trodes in the temperature range of 298 K to 373 K; the results showed an increase in the SB height and decrease in the width of the Schottky region with increasing temperature up to 333 K, followed by Poole–Frenkel domination at temperatures higher than 333 K. Development of Pc-based junctions to operate at low temperatures is still required.
In our previous work, new types of polymer con-taining the 4-(2,5-di-2-thiophen-2-yl-pyrrol-1-yl) (SNS) substituent were synthesized.15,16 Their oxi-dized states exhibit different colors, making them potential candidates for use in the development of cation-sensing devices. The optical bandgap of polymeric phthalocyanine P(NiPc-SNS) was deter-mined to be 2.27 eV from electronic absorption spectra. Also, these samples show electrochromic and fluorescence properties. Polymeric electrochromic materials are important for applications such as electrochromic displays, electrochromic mirrors, and smart windows. Furthermore, solution-processed light-emitting diodes (LEDs) in series with an inorganic semiconductor using polymeric phthalo-cyanines containing the 4-(2,5-di-2-thiophen-2-yl-pyrrol-1-yl) (SNS) substituent as emitter will attract great interest. Thus, we considered it interesting to focus on the low-temperature current transport of the phthalocyanine complex NiPc-SNS/single-crys-tal p-type silicon junction.
Herein, we report a detailed analysis of the low-temperature behavior of an Al/NiPc-SNS/p-Si heterojunction diode with NiPc-SNS donor active layer. The current–voltage (I–V) characteristics of the Al/NiPc-SNS/p–Si heterojunction were mea-sured in the temperature range from 50 K to 300 K. The observed abnormal behaviors in the tempera-ture-dependent diode parameters have been inves-tigated. The results include valuable information about the temperature-dependent behavior of a metal phthalocyanine (NiPc-SNS).
EXPERIMENTAL PROCEDURES Synthesis of NiPc-SNS
NiPc-SNS was synthesized according to our pre-vious work15 using the following procedure: SNS-PN (0.28 mmol) and nickel(II) chloride (9.1 mg) were dissolved in n-hexanol (3 mL) under nitrogen in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (0.05 mL) and heated with stirring at 140°C for 2 days. The mixture was then cooled to room temperature, and the product was precipitated
by adding the reaction mixture dropwise into etha-nol. The synthetic route is shown in Fig.1.
Fabrication of NiPc-SNS-Based Heterojunc-tion Structures
p-Type silicon wafer with (100) orientation, 1 X to 10 X resistivity, and 400 lm thickness was used as substrate. For the fabrication process, Si wafer was degreased. The cleaning procedure for Si has been given in our previous study.17 High-purity (99.9%) aluminum (Al) was thermally evaporated from a tungsten filament onto the whole back surface of the p-Si wafer under pressure of 107Torr. The Si/Al structure was annealed at 580°C for 3 min in N2 atmosphere with the aim of obtaining a low-resis-tivity ohmic contact. NiPc-SNS solution was spin-coated onto the cleaned p-type silicon crystal in air at 3000 rpm for 30 s. Following the spinning pro-cess, to obtain a rectifying contact on the front surface of the p-Si coated with the NiPc-SNS layer, high-purity (99.9%) aluminum (Al) was thermally evaporated from a tungsten filament using a sha-dow mask onto the surface of the NiPc-SNS/p-Si structure under pressure of 107Torr. The contact area was 0.0314 cm2. Thus, the Al/NiPc-SNS/p-Si heterojunction structure shown in Fig.2 was ob-tained. Current–voltage (I–V) measurements were performed in the temperature range from 50 K to 300 K using a temperature-controlled Janis CCS-350S cryostat and Keithley 4200-SCS semiconduc-tor characterization system connected to a Signa-tone semi-automatic probe station under dark conditions, sweeping the voltage from1 V to 1 V.
RESULTS AND DISCUSSION Temperature-Dependent Electrical Charac-teristics of the Al/NiPc-SNS/p-Si heterostruc-ture
The forward- and reverse-bias current–voltage (I–V) characteristics of the Al/NiPc-SNS/p-Si heterojunction were measured in the temperature range from 50 K to 300 K (Fig.3). As can be seen from this figure, the I–V characteristics changed with temperature and showed excellent rectifying behavior with rectification ratio of 1.31 9 104 (±1 V) at 300 K. This value is higher than that of the p-NiPc/n-Si heterojunction reported by El-Nahass.18 A Au/InPcCl/p-Si/Al heterojunction device exhibited rectifying behavior with rectifica-tion ratio of 102 at ±1 V as determined at room temperature.12 As the temperature decreases, the reverse saturation current does too. On the con-trary, the forward-bias current shifts towards higher voltage values with decreasing temperature, indicating the validity of the TE theory over a cer-tain voltage region. Furthermore, the nonideal behavior in the curves may be caused by instru-mental problems and poor measurement equipment calibration, as well as interface effects between the
two electrically contacted semiconductors or para-sitic inductance.
The main diode parameters as a function of the forward J–V characteristics were determined on the basis of TE theory. The current–voltage relation is given as follows19: J¼ J0exp qðV JRsAÞ nkT 1 exp qðV JRsAÞ kT : (1)
The saturation current density J0is given by J0¼ AT2exp
qUb0
kT
; (2)
where k, n, T, Ub0, A, A*, q, J0, and V have common meanings well known from our previous work.20 Fig. 1. Synthesis route of 4-(2,5-di-2-thienyl-1H-pyrrol-1-yl)phthalonitrile and of NiPc-SNS complex.
Fig. 2. Schematic cross-section of the Al/NiPc-SNS/p-Si hetero-junction diode.
Fig. 3. Semilog current–voltage characteristics of Al/NiPc-SNS/p-Si heterojunction diode at various temperatures.
A*is the Richardson constant and has the theoret-ical value of 32 A cm2K2 for holes in p-type sili-con.19 The term V JRsA is specified with respect to the voltage drop in the diode. The ideality factor and apparent barrier height (BH) of a diode can be determined experimentally from the slope and intercept, respectively, of the linear region of a plot of In(I) versus voltage. The required equations are as follows: n¼ q kT dV d lnðIÞ ; (3) Ub0¼ kT q ln AAT2 I0 : (4)
The BH (Ub0) and ideality factor (n) values were
determined in the temperature range from 50 K to 300 K from the y-axis intercept and the slope of ln[J/{1 exp(q(V JRsA)/kT)}] versus V. The val-ues of n and Ub0are shown as functions of
temper-ature in Fig. 4. The ideality factor increases while Ub0 decreases as the temperature decreases. This
behavior confirms that the current transport is dominated by not only TE. Barrier tunneling, gen-eration–recombination in the depletion region, im-age force lowering, and the charge distribution near the interface are some of the different mechanisms. The main issue with barrier inhomogeneity is its explanation based on the nonuniformity of interfa-cial charges, defects, interface roughness, and grain boundaries.21–23These results suggest that a patchy model for the inhomogeneities will be more suc-cessful, considering the presence of higher or lower barrier patches. The pinch-off effect can be observed for patch sizes less than the depletion region width.24–26 Transitions throughout a potential-en-ergy surface are evaluated as nonlinear saddle-point problems. The variation with temperature of the BH for the Al/NiPc-SNS/p-Si heterojunction is inversely related to that of the ideality factor. The correlation between the experimental n and Ub0 is
shown in Fig.5. The straight line yields a homoge-neous BH of approximately 0.818 eV, assessing in a linear format. The abnormal behavior is associated with the current flowing through the low barrier regions. To characterize the low barrier patches, the following parameters are introduced:
c3¼27R 2 0D 4 ¼ 27C 3gV2 bb; (5)
where g = ese0=qNd. D is the BH reduction at the
interface of the patch (when the homogeneous value is considered), R0 is the radius of a circular patch, and Vbbis the interface band bending of the uniform barrier outside the patches. The resulting current through the heterojunction, taking into account TE theory, is as follows27–29: Ipatch¼ AT2 4pcg2=3 qbVbb2=3 ! exp bUb0þ bcVbb1=3 g1=3 ! exp bVð 1Þ ½ : (6) A different current mechanism mode should operate with decreasing temperature. It is confirmed that the assumption of an inhomogeneous barrier dis-tribution is valid in this structure.
The Richardson plot of the saturation current helps to determine the effective barrier height. The equation can be rewritten as follows:
Ln J0 T2 ¼ LnAqU j b0 kT : (7)
The Richardson plot of ln(J0T2) versus T1 is shown in Fig.6. The curve shows nonlinear behav-ior due to the spatial distribution of BHs and interface-state-induced potential fluctuations, indi-cating upward bowing at low temperatures but lin-earity at high temperatures. The intercept and slope of the least-squares fit to the linear portion of the Richardson plot give the values of the activation energy and Richardson constant A*, respectively. The values of the activation energy and Richardson constant were found to be 0.40 eV and 8.33 A K2cm2, respectively. The value of 8.33 A cm2K2is not con-sistent with the theoretical value of 32 A cm2K2 for holes in p-type Si. The modified Richardson equation can be reordered as follows:
Ln J0 T2 q 2r2 s0 2k2T2 ¼ LnAqU j b0 kT : (8)
The plot of lnðJ0=T2Þ ðq2r2s0=2k2T2Þ versus 1000/T
is shown in Fig.6. This modified Richardson plot exhibits quite good linearity over the whole tem-perature range. From this, the zero-bias mean BH
Ujb0 and A* were determined to be 0.77 eV and 74 A K2cm2, respectively. There is a mismatch between the Richardson constant values. Thus, even though a thin oxide layer does not exist on the Fig. 4. Temperature dependence of the zero-bias BH (Ujb0) and
silicon substrate, electron tunneling through the NiPc-SNS layer will occur.
The barrier inhomogeneity is described by a Gaussian distribution (GD) function of patch barrier height. The expression for the current through the diode with a normalized GD function can be ex-pressed as30 J¼ AT2 exp qV nkT 1 Zþ1 1 PðUjbÞ exp qU j b kT ! dUb; (9) PðUbÞ ¼ 1 rs ffiffiffiffiffiffi 2p p exp ð U j b U j bÞ 2 2r2 s " # : (10)
The net current density is given as follows:
J¼ AT2exp q kT U j b qr2 s 2kT exp qV kT 1 exp qV kT : (11)
Considering a linear bias dependence of both the mean BH (Ujb) and the square of the standard deviation (r2
s) with coefficients q2 and q3 (i.e.,
UjbðVÞ ¼ Ujb0þ q2V and r2sðVÞ ¼ r2s0þ q3V), the
cur-rent density can be rewritten as follows: J¼ J0exp qV napkT 1 exp qV kT ; (12) J0¼ AT2exp qUap kT ; (13)
where nap is the apparent ideality factor and Uapis
the apparent barrier height. These parameters are given as follows31: Uap¼ U j b0 qr2 s0 2kT; (14) 1=napðTÞ 1 ¼ q1ðTÞ ¼ q2þ qq3 2kT; (15)
where q1, q2, and q3 are voltage coefficients that depict the voltage deformation of the BH distribu-tion, while Ujb0 and rs0 are the mean BH and its
standard deviation at zero bias (V = 0), respectively. Uap varies as a function of the distribution
param-eters such as T, rs0, and Ujb0. Figure7shows a plot
of Uap versus 1/2kT. The values of U j
b0and rs0 were
obtained from the intercept and slope of the linear fit as 0.71 and 0.068 eV, respectively. The rs0value
of 0.068 confirms the validity of the interface inhomogeneity, indicating the relative barrier homo-geneity. These findings suggest that the inhomo-geneous barriers of the Al/NiPc-SNS/p-Si junction diode are related to the low-temperature I–V characteristics. The variation of [(1/n) 1] versus 1/2kT is shown in Fig. 7. The values of q2 and q3 were found to be 0.48 and 3.28 9 103eV, respec-tively, giving an idea of the existence of voltage deformation.
The low-temperature characteristics of junction diodes can be explained using other theories such as thermionic field emission (TFE) or field emission (FE). The saturation current density is given as follows32,33:
Js¼ A pE00 Ub V þ n h i1=2 =kT coth E00 kT exp n kT Ubþ n E0 ; (16) E0¼ E00coth qE00 kT ; (17)
Fig. 5. Variation of the zero-bias BH (Ujb0) versus the ideality factor (n) in the investigated temperature range.
Fig. 6. Richardson plot of LnðJ0=T2Þ versus 1000/T and modified
Richardson plot of LnðJ0=T2Þ ðq2r2s0=2k2T2Þ versus 1000/T, and
its linear fit for the heterojunction diode according to a Gaussian distribution of barrier heights.
E00¼ qh 2 Na me s 1=2 ; (18)
where n is known as the Fermi energy relative to the top of the valence band and equals kT/[qln(Nv/Na)]. E0 is the tunneling parameter, E00 is the
charac-teristic energy and corresponds to the transmission probability, m* is the tunneling effective mass, es is
the dielectric coefficient (es¼ 11:7e0for silicon34), Na is the free carrier concentration and Nv is the effective density of states in the valence band. De-vices fabricated using degenerate materials typi-cally exhibit tunneling characteristics. The penetrable barrier might be small enough to allow a tunneling current to pass. The values of E00 were
determined to be 1.43 meV and 1.23 meV at 300 K and 50 K, respectively. When E00iikT, E00 kT,
and E00hhkT, FE, TFE, and TE are dominant,
respectively. In addition, the ideality factor (n) is related to the tunneling process as35
ntun¼ qE00 kT coth qE00 kT : (19)
A comparison between the experimentally and the-oretically determined ideality factors is given in Fig.8 as a function of temperature. It seems that there are more complicated mechanisms in the temperature range of 50 K to 300 K.
To evaluate some diode parameters, including the whole forward voltage region, Norde’s function36 is defined as follows: FðVÞ ¼V c kT q ln IðVÞ AAT2 ; (20) Rs¼ kTðc nÞ qI0 : (21)
The Norde function is shown in Fig.9as a function of temperature. From the Norde method, the value of the series resistance (Rs) was found to be 7.65 9 106ohm at 50 K. It is seen that Rsincreases with decreasing temperature. The measured in-crease of Rs with decrease of temperature may be explained especially by the lack of free charge and the decreasing effective charge carrier mobilities in the NiPc-SNS film layer at low temperatures.37 Freeze-out of carriers begins, causing an increase in series resistance, when the temperature is de-creased.38,39
CONCLUSIONS
An Al/NiPc-SNS/p-Si heterojunction diode was prepared by deposition of tetrakis 4-(2,5-di-2-thio-phen-2-yl-pyrrol-1-yl) substituted nickel phthalo-cyanine (NiPc-SNS) film on p-type Si substrate using the spin coating technique. Analysis of the temperature dependence of the I–V characteristics Fig. 7. Zero-bias BH (Ujb0) and [(1/nap) 1] versus 1/(2kT) plots and
their linear fits for the Al/NiPc-SNS/p-Si heterojunction diode according to a Gaussian distribution of barrier height.
Fig. 8. Normalized E00/kT values as a function of temperature.
Fig. 9. Plots of F(V) versus V of Al/NiPc-SNS/p-Si heterojunction diode at various temperatures.
was applied to determine the variation of the diode parameters in the temperature range from 50 K to 300 K. The parameters depend strongly on tem-perature. In particular, at low temperatures, the current transport mechanism can be explained on the basis of low-SB areas and tunneling. The results provide an insight into the Gaussian distribution of barrier height that prevails at low temperatures. The decrease of the series resistance indicates that there is a large freeze-out of carriers with decreas-ing temperature.
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