Interface pn junction arrays with high yielded grown p-Si microneedles by vapor–liquid–solid method at low temperature

Solid-State Electronics 103 (2015) 90–97

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Interface pn junction arrays with high yielded grown p-Si microneedles by vapor–liquid–solid method at low temperature Md. Shofiqul Islam a,⇑, Makoto Ishida b,c a

Department of Electrical and Computer Engineering, Faculty of Engineering, King Abdulaziz University, PO Box 80204, Jeddah 21589, Saudi Arabia Department of Electrical and Electronic Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi 441-8580, Japan c Electronics-Inspired Interdisciplinary Research Institute (EIIRIS), Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi 441-8580, Japan b

a r t i c l e

i n f o

Article history: Received 6 June 2014 Received in revised form 12 September 2014 Accepted 18 October 2014

The review of this paper was arranged by Prof. S. Cristoloveanu Keywords: pn junction arrays p-Si microneedles Vapor liquid solid (VLS) growth in-situ doping Low temperature Vertical active devices

a b s t r a c t In this work we report the fabrication and investigation of the properties of interface pn junction arrays formed at the interface of vertically aligned p-Si microneedles and n-Si substrate. Arrays of boron doped p-Si microneedles were grown on n-Si substrate with the maximum yield of 100% by Au-catalysed vapor– liquid–solid (VLS) growth using in-situ doping with the mixed gas of Si2H6 and B2H6 at temperature less than 700 °C, which is low as compared to the temperature (1100 °C) required by diffusion process to dope Si microneedles after VLS growth. The physical dimension (diameter, length) and position of these p-Si microneedles can be controlled. The variation of growth rate, diameter, conductivity, impurity concentration and hole mobility of these p-Si microneeedles were investigated with the variation of boron doping. The pn junctions, formed with p-Si microneedles having different diameters, were found to exhibit standard diode characteristics. These pn junction embedded Si microneedle arrays might be potential candidate in sensor area applications. Again, low temperature processing would be compatible to integrate these junction arrays with other circuitry on a chip. This work provides one step forward to realize more sophisticated vertical active devices (BJT, MOSFET, etc) with Si microneedles. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Since long years, small dimensional Si microstructures are attracting the attention because of the feasibility of using them in various applications such as micro-electro-mechanical-systems (MEMS), sensors, and device-fabrication. Among them needle-like Si microstructures have drawn the special attention because of its special shape suitable for certain applications e.g. as inserting electrodes into living tissue to collect neural signal, fabricating vertical active devices. Most of the Si microneedles, reported earlier, were fabricated by either etching technique or dicing technology or combination of both [1,2]; however, these techniques suffer from the problem of obtaining high aspect ratio microneedles due to the physical limitations of etching and dicing technology. But from the literature [3] we learnt that needle-like Si single crystal can be grown perpendicular to the Si surface by a special type growth method, the name of which is vapor–liquid–solid (VLS) growth; this method does not need any etching or dicing; it overcomes the problem of obtaining high aspect ratio microneedles. VLS method starts with the formation of dots of catalyst (normally ⇑ Corresponding author. Tel.: +966 569545276. E-mail addresses: [email protected], shofi[email protected] (M.S. Islam). http://dx.doi.org/10.1016/j.sse.2014.10.009 0038-1101/Ó 2014 Elsevier Ltd. All rights reserved.

gold) on Si substrate. When the sample with gold (Au) dots is heated in a vacuum chamber, Au particles combine with Si atoms from the substrate and thus each Au dot forms an Au–Si alloy liquid droplet on the Si surface. Now if a gas source of Si is inserted into the vacuum chamber, the liquid Au–Si alloy droplet absorbs Si atoms from the gas source. After a certain period the liquid alloy droplet becomes supersaturated with Si and then Si atoms start to precipitate at the interface of the liquid alloy droplet and Si surface and the precipitated Si atoms are solidified. As long as the supply of gas source of Si remains available, the precipitation of Si atoms continues and thus needle-like Si crystal grows perpendicular to the Si surface, this way of growing crystal is called VLS growth. In this work, we adopted VLS growth technique to fabricate Si microneedles for our interface pn junction. VLS growth mechanism was introduced in 1964 [3], however, the researchers did not work much with VLS growth system for long time. Recently, for few years, we find that the researchers have been working intensively with VLS grown Si microneedles and applying them in various applications. For example, Kawano et al. investigated the feasibility of using Si microneedles for recording neural signal [4]; some researchers [5] applied VLS-grown Si microneedles as the pins for high-resolution probecard for integrated circuit (IC) testing; some others [6] are using

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VLS-grown Si microneedles for the fabrication of SiO2 microtubes for drug delivery or extracellular recording in medical applications. From the literature on VLS grown Si microneedles, we find many reports on intrinsic type Si microneedles fabricated by VLS using the gas source of Si only [3–5]. But these intrinsic Si microneedles exhibit low conductivity (104 X1 cm1) that sets barrier to use these needles for some applications; for example, Si-microneedle electrode arrays developed by Kawano et al. [4] require highly conductive microneedles in order to record the small signal from the neurons. Therefore, it demands the doping of Si microneedles to increase the conductivity. Again in the applications mentioned earlier, VLS grown Si microneedles were used as passive element. In order to utilize these microneedles for active device fabrication it also requires the doping of these microneedles. That is, doping of VLS grown Si needles was demanded. In literature we find several reports [7,8] on VLS grown doped Si nanowires, having diameter in nanometer range, and their devices. However, considering our target applications (e.g. using as inserting electrodes into living cell to collect neural signal), we are working with VLS grown Si microneedles, having diameter in micrometer range, to ensure compatible mechanical strength. There are few reports on VLS grown doped Si microneedles. Previous report [9] shows that VLS grown intrinsic Si microneedles could be doped by phosphorous (P) diffusion at 1100 °C after VLS growth. But if we want to integrate these microneedles or device embedded microneedles with other circuitry on a chip, this high-temperature diffusion would be detrimental to on-chip devices. On the other hand, lower-temperature diffusion cannot dope the probes sufficiently; at 900 °C the depth of doping was found to be 0.5 lm from the surface of the probe sidewall [10]. Thus temperature issue puts limitation on realizing doped Si microneedles with on-chip circuitry by conventional way of doping by thermal diffusion. Therefore, at first we were motivated to obtain the doped Si microneedles at low temperature; we introduced an alternate approach of doping by incorporating in-situ doping into VLS growth system and thus we could obtain doped Si microneedles at low temperature (less then 700 °C). In our work with in-situ doping VLS growth, at first, we had approached to fabricate and investigate the properties of doped Si microneedles, which was reported in our previous work [11]. In literature we find that there are very few reports on devices with VLS grown Si microneedles. Therefore, we had approached to fabricate devices with VLS grown Si microneedles. In this paper, we are reporting the development and properties of interface pn junction arrays with p-Si microneedles; here, p-Si microneedles are grown on n-Si substrate by in situ doping VLS growth and thus form pn junction at the interface of microneedle and substrate. In our previous work, the yield (success) of growth of Si microneedles was not so impressive. But in this work, we have come up with very successful growth of p-Si microneedles and realize pn junction arrays with these microneedles; the yield is very impressive (maximum yield is 100%). This work is important for some reasons: due to the special shape microneedles would be suitable for some special applications; pn junction properties can be exploited to use them in sensor area applications; high yielded growth might help to develop very good microneedle array sensor; low-temperature process (in situ doping VLS) would be suitable to integrate this junction array with other on-chip circuitry without damaging on-chip devices.

2. Experimental details 2.1. Sample preparation Since we know from the literature [3] that needle-like Si crystal can be grown in the direction of h1 1 1i by VLS growth mechanism,

n-Si (1 1 1) substrate has been used for our experiment to grow p-Si microneedles vertically standing on Si surface. Fig. 1 illustrates the sample preparation process showing the schematic of the cross section of the sample at probe site. At first, n-Si(1 1 1) was cleaned as per RCA standard, then the substrate was oxidized by wet oxidation at the temperature of 1000 °C for 4 h followed by 10 min dry oxidation and 10 min N2 annealing to form a layer of SiO2 with a thickness of about 820 nm as shown in Fig. 1(a). Then photolithography was carried out to create arrays of circular holes (windows) through the resist as follows: the sample was pre-baked, coated with photoresist, post-baked, UV exposed through mask with circular patterns, developed by NMD3. Then ashing, using the mixer of O2 and CF4, was done to ensure the complete removal of the remaining resist (if any) from the bottom of circular holes. After that SiO2 etching was carried out using buffered hydrofluoric acid (BHF) to create circular holes (windows) through SiO2 layer; a slight side etch is allowed to ensure the complete removal of SiO2 from the bottom of the circular holes to unveil Si surface through the windows. Thus we get the circular holes through photoresist and SiO2 layer with Si surface at the bottom as shown in figure Fig. 1(b). Then Au evaporation was carried out to form an Au film (thickness of 150–220 nm) over this patterned structure as shown in Fig. 1(c). Then by using ultra-sound lift-off process, Au from the surface of resist was removed, however, circular Au dots remained sticking with Si surface at the bottom of the SiO2 windows as shown in Fig. 1(d). Thus we get the sample with Au dots at predetermined microneedle sites where the Si surface was unveiled through SiO2 windows.

2.2. VLS growth The sample, with circular Au dots, was then inserted into a highvacuum gas source molecular beam epitaxy (GS-MBE) chamber as shown in Fig. 2(a) for growing needle by VLS mechanism. The chamber was equipped with 100% Si2H6 as the gas source of Si and 1% B2H6 (diluted in 99% H2) as the source of boron (p-type dopant). The system includes a control gate valve (CGV) to control the opening of the exhaust path of the growth chamber, thereby controlling the growth condition, and mass flow meters for controlling the flow rates of B2H6 and Si2H6 to vary the boron-to-silicon ratio in the inlet

Photoresist SiO2 layer

Circular hole

n-Si (111)

(a)

(b)

Au film Au dot (circular)

(c)

(d)

Fig. 1. Cross section of the sample at probe site, illustrating the sample preparation. (a) Formation of SiO2 layer on n-Si (1 1 1) substrate by wet oxidation; (b) circular holes (windows) through photoresist and SiO2 layers created by photolithography and BHF etching; (c) Au film deposited by evaporation; (d) after Au lift-off from resist surface, Au dot remained at the bottom of SiO2 window, this sample is used for VLS growth.

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MBE chamber

Au dot Sample

Mixed gas line Heater Variable leak valve

Holder

n-Si Heating Au-Si dro plet n-Si

Control gate valve TMP

B2H6

Si2H6

Mass flow meter

RP

(a)

Si2H 6 +B2H 6 p-Si needle pn junction n-Si

(b)

Fig. 2. (a) Growth chamber set up for growing p-Si microneedle by VLS mechanism. (b) Au–Si alloy liquid droplet is formed when the sample is heated in growth chamber; when the mixed gas of Si2H6 and B2H6 is inserted into growth chamber, pSi microneedle grows by VLS mechanism on n-Si substrate and thus form pn junction at the interface.

gas system. Then the sample was heated at a temperature around 700 °C to form Au–Si alloy liquid droplet inside the SiO2 window as shown Fig. 2(b). Then, the mixed gas of B2H6 and Si2H6 with the desired ratio was supplied to the growth chamber and hence p-type Si microneedles were grown on n-Si substrate by VLS mechanism as shown in Fig. 2(b), and thus pn junction is formed at the interface of p-Si microneedle and n-Si substrate as shown in Fig. 2(b). 2.3. Measurement The physical properties of the VLS grown Si microneedles were estimated from scanning electron microscopy (SEM) images whereas the electrical characterization was carried out by using a micromanipulator system with tungsten (W) microneedles mounted on it for contacting at the tip of Si microneedle.

Fig. 3. Microscopic view of Au dots with different diameters on the prepared samples. Au dot diameters are: (a) 4 lm, (b) 6 lm, (c) 8 lm and (d) 10 lm.

physical and electrical properties of these p-Si microneedles. The success (yield) of growth of Si microneedles by VLS method depends on various factors such as growth temperature, pressure, condition of Au dots, etc. For the doped needles, dopant species (boron here) is included as an additional factor. In this work, we have grown p-Si microneedles very successfully; the yield is very impressive (highest yield is 100%). Arrays of p-Si microneedles could be obtained successfully with a wide range of boron doping (i.e. B2H6 flow from 0.12 sccm to 1.57 sccm) while Si2H6 flow was kept constant at 1.70 sccm; the highest yield is around 100% for all investigated doping levels as shown in Fig. 5(a), where growth temperature is 680°–690 °C and growth pressure is 0.005– 0.0095 Pa. It is also observed that the growth success with smaller sized Au dots (e.g. diameter 4 lm) is higher than that with larger

3. Results, analysis and discussions The samples (n-Si substrates) were prepared with Au dots with different diameters; Fig. 3 shows the microscopic view of such typical arrays of Au dots having the diameters of 4 lm, 6 lm, 8 lm and 10 lm, with the spacing of 25 lm between two adjacent Au dots. Using these samples having Au dots, we fabricated p-type Si microneedles by in-situ doping VLS method on n-type Si substrate and thus formed pn junctions at the interface. We mixed B2H6 with Si2H6 and then inserted this mixed gas of B2H6 & Si2H6 into GS-MBE chamber to grow p-Si microneedles by VLS mechanism. Thus, p-Si microneedles with different diameters could be successfully grown by VLS method using the sample with Au dots having different diameters. Fig. 4 shows SEM images of such typical arrays of p-Si microneedles with diameters of (a) 1.88 lm, (b) 2.47 lm, (c) 2.94 lm, (d) 3.29 lm, which were grown by VLS using Au dots having diameters of 4, 6, 8 and 10 lm respectively and Au film thickness of 150 nm; growth was carried out at the temperature of 690 °C, at the pressure of 0.007 Pa with Si2H6 flow of 1.70 sccm and B2H6 flow of 1.17 sccm for 90 min; the lengths of the microneedles were around 50 lm. Before going to discuss the properties of interface pn junctions fabricated with VLS grown p-Si microneedles, let us look at

Fig. 4. SEM images of arrays of p-Si microneedles having diameters of (a) 1.88 lm, (b) 2.47 lm, (c) 2.94 lm, (d) 3.29 lm grown by VLS using Au dots having diameters of 4, 6, 8 and 10 lm respectively and Au film thickness of 150 nm.

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Highest Yield (%)

100

(a)

50

0

0.5

1.0

5

Si needle diameter (μm)

sized Au dots (e.g. diameter 10 lm). This might be due to the fact that larger sized Au dot forms larger volume Au–Si alloy liquid droplet, for a larger volume liquid droplet it is difficult to remain as one droplet, sometimes it splits up into pieces, in that case the desired single microneedle does not grow from that droplet, instead, a bunch of nanowires grow at that site. Whereas, for smaller sized Au dot, smaller Au-Si alloy liquid droplet remains as one droplet and hence the desired microneedle grows more successfully. However, for some levels of doping, p-Si microneedles could be grown with the highest yield of 100% with all sized Au dots (diameter 4–10 lm) investigated as shown in Fig. 5(b), which shows the yield results of VLS growth with B2H6 flow rate 1.17 sccm (temperature 690 °C, pressure 0.007 Pa, Si2H6 flow rate 1.70 sccm). The diameters of Si microneedles were measured by SEM image. Several Si microneedles, grown from Au dots having a certain diameter (e.g. 4 lm), were measured and the diameter values were found almost same, no remarkable difference was observed, the average was taken. Similarly, diameters of other microneedles, grown from Au dots with diameter of 6, 8 and 10 lm, were measured. It is found that for a certain Au thickness, Si microneedle diameter increases with the increase of Au dot diameter maintaining a little bit bent curve as shown in Fig. 6, which shows the results for two sets of Au thickness (150 nm, 220 nm). On the other hand, for a certain Au dot diameter, needle-diameter is higher with higher Au thickness (Fig. 6) as expected. By using lower Au thickness and smaller diameter Au dot pattern, Si needles with the diameter in submicron or nanometer scale could be grown. Thus by forming Au dots with desired diameter and thickness, needlediameters were controlled in the range of 1–5 lm. These Si microneedles could be selectively grown at the desired sites by forming SiO2 windows and hence forming Au dots at those sites. For the length and growth rate, several microneedles were measured and the average has been reported. The length of Si microneedles can be controlled by selecting the suitable growth conditions and time. Si microneedles, by VLS method, grow at faster rate than epitaxial Si grown by vapor–solid (VS) epitaxy; in our

6

8

Au thickness = 150 nm

2 5

6

7

8

9

10

Au dot diameter (μm) Fig. 6. Variation of Si microneedle diameter with Au dot diameter. Here, the microneedles were grown by VLS with Au layer thicknesses of 150 nm and 220 nm.

work, p-Si microneedles grow at the rate of 0.4–0.6 lm/min by VLS at the temperature of 680–690 °C, whereas, the literature [12] shows that the epitaxial Si crystal grows by VS epitaxy at a rate of less than 0.05 lm/min at the temperature of around 700 °C. This is because, VLS growth requires smaller activation energy (0.81 eV) [11], whereas, VS growth requires higher activation energy (1.50 eV) [12]. The investigation, regarding the effect of boron doping on the growth rate of p-Si microneedles grown by in-situ doing VLS, shows that the growth rate increases with the increase of the gas ratio of B2H6 to Si2H6 (i.e. increase of boron doping) upto a certain level; after that growth rate decreases as shown in Fig. 7. Here, VLS growths were carried out at the temperature 680–690 °C, pressure 0.005–0.0095 Pa and B2H6/Si2H6 ratio was varied in the order from 102 to 104 ppm. Similar trend of variation of Si growth rate, due to the effect of boron incorporation, was also observed by other researchers; e.g. Herner and Clark [13] reported such growth rate variation in case of poly-Si growth. Boron atom has only three valence electrons, when a boron atom replaces a Si atom of Si crystal structure, boron atom forms three covalent bonds with three surrounding Si atoms using its three valence electrons, but due to the lack of one valence electron it cannot form bond with 4th Si atom in the surrounding, it hunts for accepting electron and hence the potential for bond formation is stronger. Therefore, boron atoms on the growing surface enhance the absorption of Si atoms from Si2H6 gas source and thus results in an accelerated growth until a maxima is reached. For the further incorporation

0.7

10

Au dot diameter (μm) Fig. 5. (a) Highest yield of growth of p-Si microneedles versus B2H6 flow rate. Here VLS growths were carried out at temperature 680°–690 °C, pressure 0.005– 0.0095 Pa, Si2H6 flow rate 1.70 sccm (fixed). (b) Highest yield versus Au dot diameter for VLS growth with B2H6 flow rate 1.17 sccm, Si2H6 flow rate 1.70 sccm, temperature 690 °C and pressure 0.007 Pa.

Growth rate (μm/min)

Highest Yield (%)

(b)

4

3

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Au thickness = 220 nm

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B2H6 flow rate (sccm)

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4

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0.5 Experimental data Fitted curve

0.4

0.3

0

2000

4000

6000

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B2H6/Si2H6 ratio (ppm) Fig. 7. The effect of boron doping on the growth rate of p-Si microneedles fabricated by in-situ doping VLS growth at 680°–690 °C and 0.005–0.0095 Pa. The growths were carried out varying B2H6/Si2H6 ratio in the order from 102 to 104 ppm.

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of boron, the growth rate decreases. Makino and Nakamura [14] also observed that grain growth is suppressed for high boron doped polycrystalline silicon. They attributed this suppression of grain growth to the ‘‘excess’’ precipitation of boron at the grain boundary. Similarly, in our experiment, ‘‘excess’’ boron on the growing surface might be one reason for the decrease of growth rate at higher boron concentration. In addition, for VLS growth, there is Au–Si alloy liquid droplet at the tip of Si microneedle. From literature we see that Pan et al. [8] reported the effect of B2H6 on the microstructure of boron doped silicon nanowires grown by VLS method. They found that in case of higher doping level, the wires were much shorter than expected and there were lack of Au tips at the ends. That is, the addition of B2H6 was responsible for the continuous loss of Au along the growing nanowire; it is predicted that the loss of Au occurs through the solid defects caused due to the boron doping. Other researchers also reported such Au diffusion. With the loss of Au, the size of the Au–Si alloy liquid droplet decreases; as a result the vapor–liquid interface surface area decreases which causes less absorption of Si atoms and hence growth rate decreases. Therefore, Au loss might be another reason for decreasing growth rate of VLS grown p-Si microneedles at higher boron doping. The effect of boron doping on the diameters of p-Si microneedles, grown by VLS with certain Au dot pattern and certain Au thickness, is shown in Fig. 8; the trend of diameter variation (at first increases then decreases) is similar to that of microneedle length variation (Fig. 7), however, diameter variation is smaller. The diameter variation can be attributed to two reasons: Si growth on the side-wall of the needle by vapor–solid (VS) method and the change of Au–Si alloy droplet size because of Au loss due to doping. At low doping, Au loss is not much; diameter variation is mainly due to side-growth by VS method; the strong tendency of boron atom for bond formation increases side-growth and hence the diameter increases with doping. At high level doping, the ‘‘excess’’ boron on the growing surface decreases side-growth and hence decreases the diameter; another reason, at high B doping Au-loss increases that decreases the Au–Si alloy droplet size that decreases liquid–needle interface area and hence the diameter decreases. Since Si growth rate by VS method is much smaller than that by VLS method, diameter variation is smaller than length variation. The conductivity and impurity concentration of VLS grown p-Si microneedles were determined by the average from the measurements of several microneedles. The conductivity was found to change in the range from 104 to 2.2 X1 cm1 and the impurity concentration was found to change in the order from 1012 to 5  1016 cm3 with the variation of B2H6/Si2H6 ratio in the order from 102 to 104 ppm, as shown in Fig. 9. These results of our VLS

grown p-Si microneedles are found reasonably consistent with other reported results on p-Si growth by other methods. For example, S. Ghosh and his group [15] reported boron doped microcrystalline p-Si grown by photochemical vapor deposition method, for the purpose of solar cell application, with the dark conductivity varying within the range of 106–100 X1 cm1 by varying B2H6/SiH4 ratio in the order of 104–102 (equivalent to 102–104 ppm); Li [16] reported that in boron doped bulk p-Si dopant densities are found to change from 4.5  1014 to 3.2  1018 cm3 with the resistivity of 102–102 X-cm (equivalent conductivity 102–102 X1 cm1); another report by Tsao and Sah [17] shows that the resistivity of boron doped bulk p-Si changes from 100 to 0.4 X-cm (equivalent conductivity 102–2.5 X1 cm1) with the impurity concentrations of 2  1014–5  1016 cm3. Hole mobility (lp) in these VLS grown p-Si microneedles was estimated using the theoretical relation, lp = r/qp (where r is conductivity; q is electron charge; and p is impurity concentration). Hole mobility was found to decrease with the increase of B2H6/Si2H6 ratio (i.e. boron doping) as shown in Fig. 10; mobility decreases due to increased scattering with increased number of dopant atoms. Hole mobility varies in the range of 470– 380 cm2 V1 s1; which are found consistent with reported hole mobility in p-Si grown by other methods [16], for the doping density levels investigated in our work. It is to note that in this work the impact of surface scattering effect and crystal orientation on mobility were not studied. Now, let us look at the characteristics of interface pn junction, which was fabricated by growing p-Si microneedle by in-situ doping VLS method on n-Si (1 1 1) substrate. Current–voltage (I–V) characteristic of interface pn junction was measured by using one contact with tungsten microneedle at the tip of Si microneedle and another contact at the backside to the substrate as shown in Fig. 11(a); a micromanipulator system was used for placing the tungsten microneedle at the tip of Si-microneedle for the measurement. We measured I–V of multiple pn junctions made with same dimension Si microneedles to verify the perfection of the measurement; they were found very close to each other; then a representative I–V has been accepted. Fig. 11(b) shows the I–V characteristic of such a typical interface pn junction formed at the interface of n-Si substrate and p-Si microneedle with diameter of 4.53 lm and length of 25.63 lm, which was grown by VLS at 675 °C with B2H6/Si2H6 ratio of 8941 ppm. The impurity concentration of n-Si substrate was estimated as Nd = 2.5  1017 cm3 and that of p-Si microneedle of this junction was Na = 3  1016 cm3. The current level in I–V characteristic of our pn junction (Fig. 11) is low, because p-Si microneedle (impurity concentration in the

Conductivity (Ω-1cm-1)

2.5 2.0 1.5

Au thickness = 150 nm Au dot diameter = 4 Au dot diameter = 8

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B2H6/Si2H6 ratio (ppm)

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B2H6/Si2H6 ratio (ppm) Fig. 8. Variation of the diameters of p-Si microneedles, fabricated by in-situ doping VLS growth, with the variation of boron-doping (i.e. B2H6/Si2H6 ratio).

Fig. 9. Conductivity and impurity concentration of VLS grown p-Si microneedles at various levels of boron doping (i.e. B2H6/Si2H6 ratio). Conductivity changes in the range from 104 to 2.2 X1 cm1 and impurity concentration changes in the order from 1012 to 5  1016 cm3 for B2H6/Si2H6 ratio change in the order from 102 to 104 ppm.

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d xpo ¼ NaNþN w ¼ 0:174 lm; the values are consistent with the

500

d

Mobility (cm2V-1s-1)

480 460 440 420 400 380 360 0

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B2H6/Si 2H6 ratio (ppm)

113 log

Fig. 10. Variation of hole mobility in VLS grown p-Si microneedles with B2H6/Si2H6 ratio (i.e. boron doping); hole mobility is found to decrease with the increase of doping.

(a)

Tungsten needle pn junction

V

GND 60

20 0 Current ( μA)

Current (μA)

(b)

-40 -60 -80 -20

-10

impurity concentration in low doped region; for our pn junction (N = 3  1016 cm3) we obtain calculated value ecr  4.75  105 V/cm; again from the reported results [18] on ecr versus N for one sided pn junction, we find the similar value (4.75  105 V/ cm) of ecr for N = 3  1016 cm3. After getting the value of ecr, the breakdown voltage VBR of one sided pn junction can be calcue2

p-needle

-20

N 1016

cr ; for our pn junclated by theoretical equation [18] as: V BR ¼ 2qN

n-Sub

40

dimensions of our pn junction. From Fig. 11 we also see that the reverse bias breakdown of interface pn junction occurs at around 15 V. In this pn junction, neither p-region (Na = 3  1016 cm3) nor n-region (Nd = 2.5  1017 cm3) is heavily doped, therefore, breakdown occurs here due to avalanche mechanism because for tunnelling (Zener) breakdown to occur the impurity concentration in both regions should be greater than 1018 cm3. Avalanche breakdown occurs when reverse bias voltage is so large that the maximum electric field in depletion region exceeds a critical value ecr. The critical field ecr for one sided pn junction can be calculated by an empir410  5  V/cm, where N is the ical expression [18] as: ecr ¼

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Voltage (V)

0

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20

Voltage (V) Fig. 11. (a) Technique of measuring I–V characteristic of interface pn junction. (b) I– V characteristic of a typical pn junction formed at the interface of n-Si (1 1 1) substrate and p-Si microneedle having length of 25.63 lm and diameter of 4.53 lm grown by VLS at 675 °C with B2H6/Si2H6 ratio of 8941 ppm. The close-up shows the magnified view near the threshold point.

order of 1016 cm3) offers very high resistance, for example, the pSi microneedle of the aforementioned junction offers the resistance of about 8  105 X. The contacting tungsten needle also includes some resistance in the path. However, from the magnified view near the cut-in point, we see that the built-in potential of the junction is about 0.7 V, which is similar to that of a standard Si diode. If we calculate the built-in potential (Vbi) theoretically, we obtain ln Nna N2 d ¼ 0:78 V, which is consistent with our experimental V bi ¼ kT q i

result. Although our pn junction is structurally different from standard ones, if we use the theoretical equations of standard pn junctions to see the depletion range, at equilibrium condition we obtain rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi depletion region width, W ¼ 2qV bi NNaa NNd ¼ 0:195lm, the penetrad

a w ¼ 0:021 lm, tion of depletion region into n-Si substrate, xno NaNþN d

the penetration of depletion region into p-Si microneedle,

tion (N = 3  1016 cm3) we obtain calculated value VBR  25 V; again from the reported results [18] on avalanche breakdown voltage VBR versus N for one-sided Si pn junction we find the similar value (25 V) of VBR for N = 3  1016 cm3. However, the magnitude of the breakdown voltage of our pn junction is 15 V. Compared with the theoretically calculated value and/or reported result, the breakdown voltage of our pn junction is found lower. The difference might be due to the following reasons: our pn junction is not perfectly one-sided junction because the level of impurity concentrations of n- and p-regions are very close, not big difference; physical structure is different from that of a standard diode; due to the lack of passivation there might be leakage through the fringing electric field near the edge of the junction (edge-effect). From the investigation of pn junctions with Si microneedles having different diameters it is found that in forward bias, the cut-in of the junctions occurs at around 0.7 V, after the cut-in point the larger forward current flows with junction having larger diameter microneedle as shown in Fig. 12(a); on the other hand, in reverse bias, breakdown occurs at around 15 V, after the breakdown the larger reverse breakdown current flows with larger diameter junction as shown in Fig. 12(b). This is expected as the resistance is reduced with the larger diameter microneedle; the figures show the results of three pn junctions with Si microneedles having length of about 25 lm and diameters of 3.6, 4.5 and 5.1 lm. Now let us discuss some important issues regarding temperature and Au diffusion. In this work we fabricated Si microneedles by VLS at the temperature around 700 °C. One question may arise, whether 700 °C is low enough for integrating Si microneedles with on-chip devices without damaging the devices. In a previous work [10] MOSFETs were fabricated on Si (1 1 1) substrates and Au dots were placed at the drain regions of the MOSFETs in order to grow Si microneedles; then Si microneedles were grown at the drain regions by VLS at 700 °C for 2 h. The electrical characteristics of the MOSFETs were measured before and after the VLS process. After the VLS process, no changes in the MOSFET characteristics were observed and the results confirmed that VLS growth at a temperature of 700 °C allows integration of Si microneedles with onchip devices without deterioration of the devices. Still if any application demands, VLS growth may be carried out at further lower temperature (down to 500 °C) but in that case yield (success) of growth of Si microneedles would be less.

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Current (μA)

96

60

(a)

40

Forward bias p-Si miconeedle diameter 3.6 µm

vertical structure of the device. Unlike to the reported methods, ours does not need the etching of the substrate for making the vertical structure since the Si microneedle grows directly on the substrate by VLS mechanism. 4. Applications

4.5 µm 5.1 µm 20

0

0

5

10

15

Voltage (V) 0

(b)

Reverse bias

Current (μA)

-20 -40 p-Si microneedle diameter -60

3.6 µm 4.5 µm

-80

-25

5.1 µm

-20

-15 -10 Voltage (V)

-5

0

Fig. 12. Forward and reverse characteristics of interface pn junctions with p-Si microneedles having different diameters. Microneedles are of 25 lm long and diameters of 3.6, 4.5 and 5.1 lm.

Another question may arise, what about the effect of diffusion of Au (used as catalyst for VLS) at 700 °C on device performance? In a previous report [10], two types of NMOSFETs on Si (1 1 1) substrates were prepared, one type without Au dot and one type with Au dots; these were introduced into the GS-MBE vacuum chamber; the Si substrates were heated from the backside to a temperature of 700 °C for 2 h during VLS growth. It is known that diffused Au atoms remove majority carriers from the conduction band in n-type Si substrates or from the valence band in p-type substrates. As a result, the resistance of the substrates increases. Also the threshold-voltage of the MOSFETs may shift due to additional Au charge. The diffusion length and the solid solubility of Au in Si are given as a function of the diffusion temperature and time. For VLS growth at a temperature of 700 °C for 2 h, the Au-diffusion length was estimated to be approximately 200 lm, which was larger than the distance between Au dot and the channel of the investigated MOSFET. Therefore, the Au would have completely diffused into the channel region of the MOSFET. The solid solubility of the Au in the channel region was estimated to be of the order of 1013 cm3. However, the electrical characteristics of NMOSFETs with and without Au dots were found same after VLS growth, i.e. Au diffusion did not alter the MOSFET characteristics after VLS growth; this is because the concentration of diffused Au at 700 °C was lower than the doping concentration in substrate (1016 cm3). To achieve Au concentrations of more than 1016 cm3, temperatures higher than 1000 °C are required. In the current work of interface pn junction with pSi microneedles, the doping concentrations of both substrate and Si microneedles are higher than 1016 cm3, therefore, the effect of Au diffusion (at 700 °C) on device performance may be neglected. Comparing with other vertical diode structures in literature [19] we can see that they required the etching out of the substrate. Special etching-techniques were adopted to form the desired

Si microneedles can be grown by VLS method with the diameter 1–5 lm and length upto 150 lm. As the growth time progresses, base of the microneedle gets side-growth by VS method; also Au–Si alloy size decreases due to Au loss that decreases the microneedle-diameter, so, its shape becomes tapered. Therefore, because of the size and shape, Si microneedles would be suitable to use as inserting electrodes into living cell to collect neural signals and pn junction at the interface can serve as a part of signal processing circuitry. Again, since temperature affects I–V characteristics of a pn junction, these Si microneedles embedded with pn junction can be used to collect temperature information from living cell. Again if a force is applied to a microneedle it will experience a pressure at the bottom (interface) and this will change the properties of pn junction located at that place. Therefore, pn junction embedded Si microneedles can be used as pressure sensors. Since the Si microneedles are vertically aligned on Si substrate, microneedle body surface area will increase the effective sensing area, hence smaller chip area would provide better performance while they will be used for sensing gas, chemical, etc. These pn junction microneedles may also be used as optical devices such as solar cells, photodetectors, photosensitive emitter array devices, etc. Arrayed fashion of Si microneedles has been arranged to cover an area of sensing (e.g. collecting neural signals or temperature information from living cells in an area). The specifications (needle diameter, height, pitch, number of needles) of the arrays have been chosen arbitrarily in this work; however, they can be tailored as required by specific application. 5. Conclusions Interface pn junction arrays have been fabricated by growing p-Si microneedles on n-Si substrate by in-situ doping VLS using the mixed gas of Si2H6 and B2H6 at low temperature (less than 700 °C) using Au dots as catalyst. The success (yield) of growth was very impressive (highest yield was 100%); however, the growth success with smaller sized Au dots was higher. By patterning Au dots with the diameter of 4–10 lm and Au thickness of 150–220 nm, needle-diameter could be controlled in the range of 1–5 lm; the position of these microneedles can be controlled by forming SiO2 windows and hence Au dots at the desired sites. Needle-length can be controlled by varying growth conditions and time. VLS grown p-Si microneedles were found to grow at the rate of 0.4–0.6 lm/min, much faster than Si growth by other methods. Growth rate and diameter of p-Si microneedles were found to increase with boron doping in low doping level and decrease in higher doping level. By varying B2H6/Si2H6 ratio (i.e. boron to Si ratio) in the order of 102–104 ppm in VLS system, we could control the conductivity (of VLS grown p-Si microneedles) in the range of 104–2.2 X1 cm1, impurity concentration 1012– 5  1016 cm3, hole mobility 470–380 cm2 V1 s1. The pn junctions, formed at needle-substrate interface, exhibit the standard diode characteristics with built-in potential around 0.7 V and reverse breakdown voltage around 15 V. This needle-like interface pn junction array may be applied in various sensor applications. VLS growth system does not need etching/dicing to form vertical structure and in-situ doping VLS requires low temperature, therefore, it would be compatible to integrate these pn junction embedded Si microneedle array with other circuitry by standard

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IC process followed by in situ doping VLS growth. In future, it is expected to realize more advanced vertical active devices such as BJT, and MOSFET, with VLS grown Si microneedles. Acknowledgments One of the authors (MSI) acknowledges the support from King Abdulaziz University, Saudi Arabia. The authors would like to acknowledge the support by ‘‘Grant-in-Aid for Scientific Research’’ and ‘‘Global COE Program’’ both from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). References [1] [2] [3] [4]

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