Transport properties of niobium-silicon superconductor-semiconductor junctions

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Physica B 204 (1995) 183-189

Transport properties of niobium-silicon superconductor-semiconductor junctions Th. Becker a' t, M. Mficka, Ch. Heiden u'* lnstitut fiir Schicht- und lonentechnik, Forschungszentrum Ji~lich GmbH (KFA), Postfach 1913, 52425 Jiilich, Germany bInstitut fi~r Angewandte Physik der Justus Liebig Universiti~t, Heinrich-Buff-Ring 16, 35392 Gieflen, Germany

Abstract We have prepared superconductor-semiconductor-superconductor (SSmS) junctions and-studied their transport properties. The junctions were designed in a planar geometry using niobium as superconductor and silicon as semiconductor. Supercurrents due to the proximity effect in silicon have been observed. The junctions showed the AC-Josephson effect under microwave irradiation. Subgap structures in the I - V characteristics could be explained by multiple Andreev-reflections. The electrical properties are strongly determined by the interface between niobium and silicon. A comparison of the barrier transmissivity deduced from the lcRN-product and the Schottky barrier height is given.

1. Introduction During recent years, a considerable interest in superconductor-semiconductor hybrid devices has emerged. Two terminal devices, like super-Schottky diodes [1-3] have been investigated for use in mixers and detectors. In case of three-terminal devices, various concepts are discussed in the literature like 'hot electron' transistors [4], SUBSITs [5] (superconducting-base semiconductor isolated transistor), and superconducting field effect transistors [6] (SuFET or JoFET). The basic elements for the latter ones are superconductor-semiconductor-superconductor (SSmS) junctions. Because the properties of SuFETs are determined by the properties of the SSmSjunctions, the investigation of these junctions is of considerable interest. Here, remarkable work has been performed on junctions with various geometries and sub-

strates [7-15]. In the semiconductor electronics, silicon seems to be the most promising material for integrated circuits at the time. Therefore, it seems advisable to realise SSmS-junctions on silicon to consider an integration of such junctions (and transistors) with state-of-theart semiconductor electronics. The transport properties of the SSmS-junctions are mostly determined by the interface between superconductor and semiconductor. Depending on the channel length, the coherence length ~sm of the Cooper pairs in the semiconductor and the interface transparency, a proximity-effect induced supercurrent through the SSmS-junction may be observed. Otherwise, the 1-Vcurve usually has the shape of two super-Schottky diodes connected in series.

2. Preparation of SSmS-junctions * Corresponding author. 1Present address: Omikron Vakuumphysik GmbH, Idsteiner Str. 78, 65232 Taunusstein, Germany.

For ease of preparation, our SSmS-junctions have been realised in a planar structure. Low doped

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Th. Becker et al./ Physica B 204 (1995) 183-189

( < 102°m -3) silicon wafers were used as substrate material. Through a window in a thermally grown, 300 nm thick oxide on top of the wafer, the surface is locally highly n++-dope d. The diffusion of phosphorus as dopant has been performed using a spin-on dopant in a rapid thermal processing furnace. After the thermal diffusion, the remaining silicate glass was carefully removed with a 10% HF-etch. From high resolution SIMS-measurements the junction depth and the carrier concentration of the highly doped areas can be deduced to 70nm and 5 x 1026m -3, respectively. Using electron beam lithography, a fine PMMA line, defining the channel length and width, was then structured on top the highly doped areas. For this purpose, the substrate was coated with a single layer of 400nm thick PMMA of 462k molecular weight. The channel structure was directly written by an electron beam in a modified SEM. With this method, channel lengths of less than 80nm could be realised (cf. Fig. 1). The channel width usually is 121am. After developing the PMMA, the samples were dipped in buffered HF to remove any oxide layer at the silicon surface. Prior to the following niobium sputtering, a slight ion milling was performed in situ, to remove residual adsorbats from the silicon surface. In order not to damage the semiconductor in the channel region, the niobium patterning was performed by a lift-off process. The thickness of the niobium film was about 30nm. When using thicker films, an incomplete lift-off often shorted the junctions because of metallic residues. The critical temperature of the niobium films was 7.7 K. Since the exposure field of our SEM used for the electron-beam lithography is only 200 x 200 lam2, the contact pads were structured using optical lithography.

3. Measurements For the measurements, the samples were mounted on an epoxy printed circuit board and electrical connections were made by ultrasonic wire bonding. Most measurements were performed at 4.2 K in a helium storage dewar. For measurements at lower temperatures, we used a Hebath cryostat with a pumping system. State-of-the-art electronics was used to measure the I - V characteristics and their derivatives. A number of samples have been prepared with critical currents in the range of 0.3 to 50 laA. The IcRN-product was usually about 50 laV, one junction exhibited an IcRN = 230 ~V. It was found, that the transmission probability of the interface is of crucial importance for the electrical properties of the SSmS-junctions. With the assumption, that the degenerated semiconductor can be treated like a metal in the model of free electrons, the Cooper pair transport across a superconductor-semiconductor junction can be

Fig. 1. SEM photograph of a SSmS-junctionmade by a lift-off technique. The channel length and width are 80 nm and 13 lam, respectively.

described in the proximity-effect model after deGennes [17, 18]. Here, the order parameter in the semiconductor decays exponentially with the coherence length of the Cooper pairs in the semiconductor ~Sm = (h3 p/6rtrn* ek T)l/2(3~2n ) 1/3 ,

(1)

where kt is the carrier mobility, m* the carrier effective mass, n the carrier concentration, and T the temperature. Hall measurements yield a carrier mobility of /t = 45cm2/Vs at 4.2K for our highly doped layers. Together with n = 5× 1026m -3, the coherence length ~sm is calculated to ~Sm = 22 nm (T = 4.2 K). The temperature dependence of the critical current can be written as Ic oz D ~ x e x p ( - Cx/T ) with C,D constants. The experimental values and a least-squares mean fit is shown in Fig. 2. The measured values correspond well to the theoretical prediction. To verify the Josephson behaviour of the SSmS-junctions, they were irradiated with microwaves. The I V characteristic of a SSmS-junction with and without microwave radiation is shown in Fig. 3. Its critical current is 1.35 p.A, and the frequency of the applied microwave field is Vrf 12 GHz. Therefore, the steps in the I - V characteristics occur at voltages n × 24.8 ~tV. A comparison of the step height with the simulations performed by Russer [19] lead to a parameter F = hvrf/2eR1c .~ 0.5. From this value, the resistance of the junction is calculated to be R = 37 fL which is comparable to the normal resistance deduced from the I V characteristics RN = 33 f~. We only observed a few steps in the I V characteristic. This is consistent with the simulations of Russer [-19], who predicts an increasing number of steps while the normalised amplitude decreases. None of the examined =

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Th. Becket et al./ Physica B 204 (1995) 183 189 22

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temperature[K] Fig. 2. Temperature dependence of the critical current ofa SSmS-junction.The crosses are measured values, the solid line is a last mean square fit (cf. text).

Fig. 3. Current voltage characteristic of a SSmS-junction with (a) and without (b) microwave irradiation (v = 12 GHz). The vertical scale is 0.5 p_A/divand the horizontal scale is 20p_V/div.

samples showed subharmonic steps in the I V characteristics under radiation. This indicates, that the junction has a sinusoidal current-phase relation [20]. Some of the junctions exhibited subgap structures in the I - V characteristics at voltages below the gap voltage. This results in peaks in the d V / d l curve at voltages II, = 2As~he with n an integer. With silicon as semiconductor, subgap structures have been observed in P b - S i - P b step edge junctions [21] and in Nb-Si Nb junctions containing a thin silicon membrane z2 at 1.2 K. Subgap structures in N b - S i - N b junctions with a current through a silicon ridge appeared at voltages less than

0.3mV ( T = 1.96K), which may be explained by an induced gap in the semiconductor [23]. These subgap structures can be explained by multiple Andreev-reflections [24] at the interface between the semiconductor and the superconductor [25-27]. At the boundary, an electron from the semiconductor will be reflected as a hole, and a Cooper pair is injected into the superconductor. The probability of this process is determind by the strength Z of the barrier between the superconductor and the semiconductor. The transmissivity of the barrier is then (1 + Z2) -1 [25]. For Z > 10 (small transmissivity), the I - V characteristic is like that of a tunnel

186

Th. Becker et al./ Physica B 204 (1995) 183-189

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voltage [mVl Fig. 4. Current-voltage characteristic of a SSmS-junction. An extrapolating line to determine the excess current is shown. junction. Dependent on the barrier strength Z, the junctions show either an excess or a deficit current. This current can be determined by extrapolating the linear region of the I - V curve to zero voltage (cf. Fig. 4). The excess current of the junction shown here is 0.55 laA, the critical current 0.7 p,A. The normalised I - V curve and the normalised excess current versus Z are shown in Fig. 5 (after Flensberg et al. [27]). The barrier strength of the junction described above is therefore Z ~ 1. The d V/dl characteristic of the junction shown in Fig. 4 is plotted in Fig. 6. The voltages corresponding to the peaks are specified. All of our junctions showing a supercurrent exhibit an excess current as well. This is similar to - -

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the results of Serfaty et al. [21], whereas Heslinga et al. [28] reported about a deficit current for their junctions. One of the SSmS-junctions without supercurrent exhibit a I V characteristic with a shape similar that of the quasi-particle characteristics of a leaky tunnel junction (cf. Fig. 7). From the deficit current one can deduce Z ~ 1.3. Compared to the calculated l - V c u r v e in Fig. 5, the agreement is obvious. An irreversable crossover from tunnelling (large Z, deftcit current) to metallic behavior (small Z, excess current) has been observed, when applying a large current to a SSmS-junction. The I - V characteristic and its derivative is shown in Fig. 8. The electrical behaviour of the junction is like two super-Schottky diodes, connected in series. The small maximum in the conductance reflects the superconducting gap at voltages V ~ 3 mV. The junction clearly shows a deficit current. After recording this characteristic, a current of about 30mA was applied for a few seconds to the junction. The I - V characteristic and its derivative, as measured after the current pulse was applied is shown in Fig. 9. The shape now exhibits an excess current, and the onset of a supercurrent can possibly be seen. Together with the change in the shape of the characteristic the resistance of the junction has been lowered by a factor of 10. A similar behaviour can be observed by applying voltage pulses to the junction or by heating it in a high vacuum chamber at temperatures of 600°C for a few seconds. However, the interface could never be changed in a way, that a supercurrent could be observed. A crossover from tunnelling to,metallic behaviour has been reported by Kleinsasser et al. [29] in the system Nb-InGaAs. Depending on the carrier concentration,

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Th. Becker et al./ Physica B 204 (1995) 183-189

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Fig. 6. d V/dl characteristic of the junction shown in Fig. 4. The voltages corresponding to the peaks are specified.

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voltage EV] Fig. 8. I - Vcharacteristic and its derivative o f a SSmS-junction before applying a large current through it. The deficit current can clearly be seen.

188

Th. Becker et al./ Physica B 204 (1995) 183-189 1 0.8 0.(~ 0.4

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Fig. 9. I V characteristic of the junction shown in Fig. 8 after applying 30 mA for a few seconds to the junction. The shape switches to a more metallic behaviour with a smaller resistance. The junction now exhibits an excess current. they obtain a more metallic (high doping) or tunnelling (low doping) behaviour of the junction. In our case the change from tunnelling to metallic behaviour is most likely due to a burn out of the tunnel barrier induced by the applied current pulse, or an inter diffusion of atoms close to the interface due to heating. This may also lower the barrier height. A barrier lowering due to the formation of a silizide is not probable, because the Schottky barrier heights are nearly the same, ~Nb = 0.6eV [30], ~NbSi= 0.59eV [31]. The transport properties of the SSmS-junctions are strongly determined by the properties of the interface between the superconductor and the semiconductor. For example, one can straightforward calculate the expected normal resistance from the channel conductivity. The resistivity of the highly doped regions is PSm = 2.8 X 10-4f~cm. With an estimated homogeneous current distribution over the junction depth and the width of the channel, one obtains a normal resistance of 0.2 f~. In contrast, the measured resistances are 100 to 1000 times higher. The ratio A of the values of the order parameter in the superconducting and semiconducting electrodes can be calculated using an approach by van Duzer et al. [32], using the measured values of critical current, normal resistance and channel length (as examined by a SEM). For most of the junctions, we calculate A = 0.02, only one junction with a larger IcRN has A = 0.04. From this value, also the gap in the semiconductor can be estimated to Asm = 0.5meV. This roughly corresponds with the measured induced gap in silicon by Nishino et al., As,, = 0.32 meV [33]. In the BTK-model [25] for the multiple Andreevreflections [24], the transmissivity TBrKcan be deduced from the parameter Z: Tax~, = (1 + Z2) -~. From the measured excess current, the parameter Z can be determined. For our junctions Z ~ 1 and therefore Ta'rK "~ 0.5.

It is questionable, whether this value can be correlated to the Schottky barrier between silicon and niobium, or not. The intrinsic Schottky barrier of niobium on silicon has been determined by Heslinga et al. [31] with q~Bo = 0.605 eV. Taking into account the barrier lowering at higher dopings, the (theoretical) Schottky barrier should be q~Nb= 0.3 eV. With the WKB-approximation, one can calculate the transmissivity of the barrier to TWKB= 0.8. This is an unexpected high value and cannot be correlated with the measurements. On the other hand, the WKB-approximation fails in the high doping range, leading to an overestimated value. A large amount of surface states [34, 35] also play an important role, so that the Fermilevel can be fixed in the bandgap [36]. This effect results in a higher Schottky barrier and therefore lower transmissivity of the interface. Calculating the Schottky barrier from the deduced transmissivity of TBrK = 0.5, one obtains q~Nb= 0.46eV, which seems to be realistic.

Acknowledgements We gratefully acknowledge stimulating discussions with L. Vescan. Thanks also to R. Apetz for various CV measurements and to T. Sch~ipers for the Hall measurements.

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Th. Becker et al./ Physica B 204 (1995) 183-189

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