Solid State Communications 143 (2007) 228–231www.elsevier.com/locate/ssc

Deposition of n-type nanocrystalline SiC films and current transportmechanisms in nanocrystalline SiC/crystalline Si heterojunctions

Wei Yu, Chun-Sheng Wang∗, Wan-Bing Lu, Shuang-Kui Cui

College of Physical Science and Technology, Hebei University, Baoding 071002, People’s Republic of China

Received 12 May 2007; accepted 17 May 2007 by A.H. MacDonaldAvailable online 24 May 2007

Abstract

Phosphorus doped n-type nanocrystalline silicon carbide (nc-SiC) films were deposited on crystalline Si (c-Si) substrates at a low substratetemperature using helicon wave plasma chemical vapor deposition techniques. The current transport behaviors of nc-SiC/c-Si heterojunctionswere measured in the temperature range of 100–290 K. It has been shown that the deposited SiC films reveal a high crystalline degree in 6Hpolytype and the fabricated nc-SiC/c-Si diode shows a typical abrupt heterojunction with good rectifying performance. The transport currentsatisfies a recombination-tunneling mechanism at forward bias, in which the recombination process determines the current in a small bias voltagevalue range, while the tunneling process becomes dominant when the voltage is higher than 2.5 V. Meanwhile an inversion behavior exists at lowtemperature regions in the current–voltage plot due to series resistance. At reverse bias, the current behavior is mainly controlled by the thermalemission of minority carries and their subsequent multi-step tunneling through defect states at the interface.c© 2007 Elsevier Ltd. All rights reserved.

PACS: 73.40.Lq; 73.40.-c

Keywords: A. Heterojunctions; A. Surfaces and interfaces; D. Electronic transport

SiC/Si heterostructures are of considerable interest inrealizing wide-band-gap emitters or windows in solar cells,photodetectors and electroluminescent devices [1–3]. In virtueof the superior properties of SiC such as wide band gap, highthermal conductivity, and high chemical stability, the opticaland electronic devices of this material can reach a wide rangeof spectral emission or response with a high efficiency.

The performance of devices greatly depends on the qualityof the films being used. Polycrystalline SiC films on siliconsubstrates are usually deposited by chemical vapor depositionor high temperature annealing techniques [4]. CrystallineSiC/p-Si heterojunctions with high breakdown voltage and highreverse current have also been implemented [5]. However, thequality of devices is hard to control since dopants redistributeand interface defects increase due to the high processingtemperature [6]. Higher quality SiC/Si heterojunction diodescan be fabricated using nc-SiC films because the nc-SiCfilms can be deposited at relatively low temperature, which

∗ Corresponding author. Tel.: +86 0312 6133516; fax: +86 0312 5011174.E-mail address: wangchunsheng@mail.hbu.edu.cn (C.-S. Wang).

0038-1098/$ - see front matter c© 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2007.05.015

provides better compatibility of fabrication of silicon carbidewith the current silicon technology. Furthermore, the SiCnanocrystallites have the characteristics of an enlarged band gapand the enhancement of electron–hole radiative recombinationdue to quantum confinement effects, which make nc-SiC/Siheterojunctions most promising as short-wavelength lightemitters [7,8]. However, there are few reports on the depositionand doping of nc-SiC to date, and much less on the physicalproperties of the nc-SiC/Si heterojunction.

In this letter, phosphorus doped n-type nc-SiC layers aredeposited on c-Si at a low substrate temperature of 500 ◦Cby helicon wave plasma chemical vapor deposition (HWP-CVD) using PH3 as doping source. Furthermore, n–p nc-SiC/c-Si heterojunction diodes are fabricated by using n-type nc-SiC films and p-type c-Si substrates. The current transportmechanisms of this nc-SiC/c-Si heterojunction are explored bymeasuring its I –V properties under different temperatures.

The nc-SiC layers were deposited by HWP-CVD techniquesin a vacuum system using a radio frequency source [9]. High H2diluted SiH4+CH4 were used as the reactant gases and PH3 wasused as the donor doping gas, with doping ratio (Ph3/SiH4) of

W. Yu et al. / Solid State Communications 143 (2007) 228–231 229

Fig. 1. FTIR transmittance spectra of the nc-SiC samples.

1%, so the conduction type of the deposited nc-SiC films is ntype. During the nc-SiC growth, the working pressure, substratetemperature, plasma power and magnetic field were fixed at0.7 Pa, 500 ◦C, 500 W and 200 G, respectively. Both p type(8–10 � cm) c-Si(100) and quartz were used as the substrates.The films deposited on quartz substrate were used for opticaltransmittance and electrical conductivity measurements, thefilms deposited on c-Si were used for Fourier transforminfrared (FTIR) spectroscopy and x-ray diffraction (XRD)measurements. The conductivity of the deposited nc-SiC is0.02 S cm−1. The nc-SiC/c-Si heterojunction was preparedas follows. After the usual cleaning, the native oxide wasremoved from the surface of the c-Si substrate by immersion in10% hydrofluoric acid just before introduction to the vacuumchamber. After a 400-nm-thick nc-SiC layer was depositedon the substrates, a 100-nm-thick Ni electrode with an areaof 0.2 mm2 was sputtered on both sides of the substrates toform ohmic contacts. The schematic plot of the nc-SiC/c-Siheterojunction diodes is shown in the inset of Fig. 3. Its I –Vcharacteristics were measured by HP-100 meters and the lowtemperature was supplied by a liquid nitrogen cryostat.

Fig. 1 shows the IR absorption spectra of the deposited nc-SiC films, the main band at around 800 cm−1 correspondsto the Si–C stretching vibration mode. In addition, two weakabsorptions located at around 2100 and 2900 cm−1 can beattributed to Si–H and C–H stretching vibration modes [10].These results indicate the bonding structure of the film ismainly in the form of Si–C bonds. To deduce the crystallinefraction of the nc-SiC films, we have proceeded to deconvolutethe predominant 800 cm−1 band as shown in the inset ofFig. 1 [11]. The Gaussian peaks at 788 and 870 cm−1

are related to amorphous Si–C bond stretching and Si–CH2wagging vibration modes, respectively. The Lorentzian peak at802 cm−1 is ascribed to the stretching mode of Si–C bond in thecrystalline phase. The crystalline fraction of the film obtainedfrom the ratio of Lorentzian component to the total area is morethan 60%.

Fig. 2 shows the XRD spectra of the nc-SiC films, whichwas carried out in the glancing mode. Incidence of the x-raybeam was chosen close to the sample surface (1◦) in order toavoid substrate effects. As can be seen, there are three broadpeaks at 2θ ≈ 33.8◦ and 35.7◦. The relatively large width

Fig. 2. XRD spectra for the deposited nc-SiC films.

Fig. 3. The room temperature linear scale measurements of the I –Vcharacteristics and the schematic cross section of the diode.

Fig. 4. Energy band diagram of the n–p nc-SiC/c-Si heterojunction.

of the diffraction lines is indicative of small value of the SiCcrystallites. Comparison of the diffraction position and relativeintensity with those of ASTM card suggests the predominantpolytype of the film is 6H-SiC phase. The observed peakscan be attributed to the (101), (102) and (110) diffractions,respectively. The average crystallite size, as estimated by thehalf width at full maximum of the (101), (102) diffractions,using the Scherrer equation, is 6 nm (±10%) [12].

The room temperature current density–voltage (J–V )

characteristic of the n–p nc-SiC/c-Si heterojunction is plottedin Fig. 3. The n–p heterojunction shows a good rectificationratio of about 5 × 102 at ±5 V and the reverse current densityis equal to 2 × 10−6 A/cm2. To explore the current transportmechanisms, the energy band diagram of this n–p nc-SiC/c-Si heterojunction is tentatively shown in Fig. 4. An abruptheterojunction is assumed to form at the interface between nc-SiC and c-Si. Since the difference of electron affinity betweenc-Si and SiC is small, the conduction band has a smallerdiscontinuity than the valence band (1Ec < 1Ev) [13]. Thedepletion region is formed near the junction and its electric fieldpoints to the nc-SiC side. The interface depletion region width

230 W. Yu et al. / Solid State Communications 143 (2007) 228–231

Fig. 5. (a) J–V characteristics of the heterojunction at different temperatures.(b) Temperature dependence of reverse current density at different bias.

and the position of Fermi level can be controlled by the biasvoltage applied on this dioxide.

Fig. 5 shows typical J–V characteristics of the n–p nc-SiC/c-Si heterojnction measured at various temperatures of120–290 K. It exhibits a good rectifying behavior even at lowtemperature. In the low forward bias region (0 < V < 2.5 V),the current density J increases exponentially with the appliedvoltage V [14]

J = J0[exp(qV/nkT ) − 1] (1)

where J0 is the saturation current density determined bytemperature and q is the electron charge, k is Boltzmann’sconstant. The ideality factor, n, is estimated to be 1.89 at roomtemperature and decreases with temperature. Such an increasebehavior suggests that the diode forward current is dominatedby recombination processes through defects at the SiC/Siinterface [15]. There are two facts that would be responsible forthese defects. The first is the large lattice mismatch and thermalmisfit between Si and nc-SiC, and the second is the H or Sidangling bonds and interface defects between the amorphousmatrix and nc-SiC grains. The defect states act as the effectiverecombination centers, through which the electrons in the nc-SiC layers can recombine with the holes in the c-Si underlow forward bias. Because under low bias voltage, the energyof most electrons is so small that it can not pass throughthe sharp potential barrier directly, the recombinant process

through defect states is predominant. The ideality factor of thisheterojunction shows a slow increase with temperature whichagrees with the fact that the density of these defect states at theinterface is temperature dependent.

The current departure from exponential increase occurs asthe applied voltage is higher than 2.5 V, as shown in Fig. 5.The slope of the log J–V curves reveals a noticeable decreasewith temperature which is in agreement with the electrontunneling mechanism [16]. When the applied voltage increases,the electrons have enough energy to reach the bottom of thesharp potential barrier as shown in the Fig. 4, then they canpass the potential barrier though a tunneling process. As 1Ecis smaller than 1Ev , the barrier height for electrons is smaller.The electron current is dominant, it is the electrons from thenc-SiC side tunneling through the interface barrier and thenrecombining with the holes in the c-Si substrate.

In addition, at low current densities, the voltage drop acrossthe diode decreases with increasing temperature owing to thereduction of the built-in potential. By contrast, at high currentdensities, the voltage drop at a given J increases with increasingtemperature. This behavior is called inversion. An “inversion”point can be seen at 5 V for this nc-SiC/c-Si dioxide. Suchan “inversion” is also observed in Si homojunction diodes andSiC diodes [17]. At higher temperature, 290 K, it is can befound that the “inversion” in the J–V curve disappears and thecurrent density is much higher than that at low temperature.These phenomena relate to the temperature dependence of theresistivity of Si [18].

The reverse current is in a low value range when thetemperature is low, as seen in Fig. 5(a), whereas it increasesrapidly with increasing temperature as compared with theforward current. Its temperature dependence under differentbiases is shown in Fig. 5(b). The nearly straight line in thesemilogical J–T plot shows that the reverse current increasesexponentially with the temperature, which can be described by

J = J0 exp (T/T0) (2)

where J0 and T0 are constant. This behavior clearly illustratesthat the reverse conduction is dominated by the current dueto a multi-step tunneling process of electrons generated in thedepletion region of c-Si [19]. According to this mechanism,minority carriers for the narrow-bandgap semiconductorsubsequently undergo trapping and thermal emission via thestates in the interface space charge region, and an electron fromthe c-Si passes through the multi-step path in the interfacespace charge region until it recombines with a hole in thenc-SiC. It can be considered that the microstructure andconductivity should be quite different in different kinds of nc-SiC and c-Si. These effects, together with different fabricationparameters, would influence the transport mechanisms in theheterojunctions. Further work is needed to study these effectson the carrier transport behaviors for improving the rectifyingcharacteristics of nc-SiC/c-Si heterojunctions.

In conclusion, high quality phosphorus doped n-type 6Hnc-SiC layers have been successfully deposited on p-typesilicon substrates at low substrate temperature. Furthermore,heterojunction diodes have also been fabricated by using

W. Yu et al. / Solid State Communications 143 (2007) 228–231 231

nc-SiC films and c-Si substrates. The conduction mechanismsof the heterojunction are investigated by current–voltagecharacteristic measurements. At low forward bias, the currentincreases exponentially which is governed by recombinationprocesses, while, at higher forward bias, the tunnelingof electrons from nc-SiC to c-Si becomes dominant. Thethermal emission of minority carries and subsequent multi-steptunneling through defect states at the interface are likely theorigin of reverse currents.

Acknowledgements

This work is supported by the Natural Foundation of Hebeiprovince, People’s Republic of China under Grant No 503129and No E2006000999.

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