Study of a Single Coaxial Silicon Nanowire forOn-Chip Integrated Photovoltaic Application

Oka Kurniawan and Er Ping LiComputational Electronics and PhotonicsInstitute of High Performance Computing

Singaporekurniawano@ihpc.a-star.edu.sg

Abstract—Nanowires have been identified as a promisingalternative for future electronics. A single coaxial nanowirephotovoltaic device has been fabricated to power up nanoelec-tronic sensors and logic gates. The current work calculates andanalyses the performance of the coaxial silicon nanowire usinga semi-classical method. The current-voltage characteristics arecalculated for both under dark and AM1.5G illumination. Someinteresting phenomena is observed from the two-dimensionalplot of the potential. Furthermore, the effect of the surfacerecombination, which is significant in nanoscale devices, is alsostudied.

Index Terms—Photovoltaic cells, Nanotechnology, Quantumwires.

I. INTRODUCTION

Nanoelectronics provide a promising alternative to enhancethe performance of electronic devices as well as to continue thescaling of Moore’s law. Logic circuits have successfuly beenfabricated using nanowires [1]. Not only that, researchers fromHarvard have shown that self-powered nanosystems are notfar beyond our reach [2]. These self-powered nanosystems usesolar energy and convert them into electrical power throughphotovoltaic effect. In Ref. [2], a silicon nanowire pH sensorwas powered by a single coaxial silicon nanowire photovoltaicdevice. The same paper also shows the operation of ANDlogic gates fabricated from nanowires and powered by twocoaxial silicon nanowires photovoltaic devices. Our currentwork studies the performance of this coaxial silicon nanowirephotovoltaic device using a semi-classical simulation.

In the earlier experiments, nanowires were mostly usedsimply as a conducting channel for enhancing the electrontransport [3], [4]. It was then shown that silicon nanowireshave the potential to give an improved optical absorption [5].This means that silicon nanowires can be used as photovoltaicdevices where photogeneration occurs. However, for a pho-tovoltaic device to work properly, a built-in electric field isnecessary for electron-hole charge separation. In other words,either a p-n junction or a Schottky barrier must be present inthe device.

There are two ways to realize a photovoltaic device fromsilicon nanowires. The first is to fabricate a p-n junction siliconnanowire [2], [6]–[8], and the second is to create a Schottkycontact in one of the nanowire’s electrode [9]. In this paper,we will investigate the coaxial p-n junction silicon nanowireconfiguration (Fig. 1). The advantage of this configuration is

(a)

(b)

Fig. 1: Simulation scheme of a coaxial nanowire. The nanowirehas a diameter of 360 nm and a length of 1µm. Thicknessof the i-layer and n-type layer is set to 80 nm and 100 nmrespectively.

that the charge separation can occur efficiently throughoutthe nanowire lengths along the radial direction. Since theseparation occurs in the radial direction, which is smaller thanthe minority carrier diffusion length, the bulk recombinationdecreases significantly, and hence, improves the efficiency.

The core p-type nanowire is usually synthesized by meansof a vapour-liquid-solid (VLS) method. Afterwards, siliconshells can be deposited at a higher temperature and lowerpressure to allow for the axial growth of the silicon nanowirecore. During this deposition, phosphine is used as the n-typedopant in the outer shell. The cross-section of such a coaxialnanowire is shown in Fig. 1 (b).

We recognize that very few studies were done using sim-ulation tools. Most of the works in the literature consists ofeither experiments or analytical analysis with approximations[11]. In this work we pursue the analysis using a numericalsimulation to give a better picture and understanding of theworking of the on-chip nanowire photovoltaic device.

ISIC 2009314

We study the current-voltage characteristic of the coaxialnanowire and compared them to an experimental data obtainedin [2]. The transport properties were calculated using semi-classical method and were found to agree with experiments.We also plot the current-voltage characteristics under both darkand AM1.5 Global solar illumination. It was found that ourshort circuit current is about the same as the experiment whileour open circuit voltage is slightly larger. The potential plotshows that most of the potential drop occurs near the anode.Moreover, the built-in electric fields are not uniform along theaxis of the nanowire, but rather decrease as it moves awayfrom the anode. The surface recombination was also studiedand was shown to affect the performance of the device.

II. PHOTOVOLTAIC SIMULATIONS

We simulate the device shown in Fig. 1 using ATLAS,a 2D/3D semiconductor device simulator [10]. Due to itssymmetry, we simulate the device only in two dimensions anduse the cylindrical coordinate system. The radius of the corep-type nanowire is 100 nm. The thicknesses of the i-layer andthe n-type layer are 80 nm and 100 nm respectively. This givesus a nanowire with a diameter size of 360 nm. This diametersize is about the same as size of the nanowires in [2]. Thelength of our nanowire, on the other hand, is set to be onemicron which is smaller than the one in [2]. The reason forusing a smaller nanowire length is to save computational time.The electrodes are placed so as to immitate the configurationshown in Fig. 1 (a).

The transport properties are calculated using the drift-diffusion model. The validity of using such a semi-classicalapproach for the nanowire size considered in this work hasbeen discussed in [11], [12]. In the calculation, we enabledthe Shockley-Read-Hall recombination and concentration de-pendent mobility. We first solved the Poisson-Schrodingerequations self-consistently to see whether any quantum effectcan be observed. But the results suggest that the quantum con-finement effect is pretty small. Using an infinite potential wellapproximation, the first bound state is only about 0.01 meVabove the conduction band, which supports the conclusion ofthe self-consistent calculation.

The dark current-voltage characteristic was first obtained.The potential and the band energy of the coaxial nanowireswere calculated self-consistently. After the dark propertieswere obtained, the nanowire was illuminated with AM 1.5Global solar irradiance. The spectrum of this solar illuminationis shown in Fig. 2. To perform this illumination we enabledthe multi-spectral photogeneration in the simulator. The powerof the solar spectrum was scaled by 1.5103 to give a shortcircuit current in the pico Ampere range.

The generation rate is calculated from

G η0

»P pλqLλ

hcα exppαxqdλ (1)

where the integration is over the wavelength λ, P pλq is thepower spectral density of the light source, L is the factor repre-senting the cummulative loss due to reflections, transmissions,

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Fig. 2: Solar spectrum air mass 1.5 global (AM 1.5 G)irradiance.

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Fig. 3: Dark current-voltage characteristic in comparison withthe data obtained from [2]. The doping concentration wasestimated to give a turn on voltage of about 0.7 V, and thecontact resistance is added to give the correct slope.

and absorption, α is the absorption coefficient, and x is theradial distance from the surface. The absorption coefficient isin turn calculated from

α 4πλk (2)

where k is the imaginary part of the optical refraction index.

III. RESULTS AND DISCUSSION

Fig. 3 shows the current-voltage characteristic of the simula-tion result under the dark condition. We also compare the resultwith that obtained from Fig. 2 of [2]. The results agree with theexperimental data. The turn on voltage for our simulation isabout 0.7 V and the slope for high bias is about 6 106

A/V or 167 kΩ. Note that in this simulation we take intoaccount the contact resistance by adding a series resistanceto the contact. The work in [2] did not report the dopingconcentration of their devices. And so in our simulation, thedoping was estimated to give about 0.7 V turn on voltage. Thevalue of the concentration is about 7.41016 cm3, which isabout the same as the one reported in [9].

Now, it is interesting to plot the two dimensional potentialprofile of the device under dark condition. Fig. 4 shows usthis plot. The first plot is the potential profile under zero bias,while the second one is under 1.0 V bias. Notice that even

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(a)

(b)

Fig. 4: (a) Potential profile under zero bias. (b) Potential Profilewith the anode is biased by 1.0V. The anode is located at thetop left while the cathode is at the right bottom. The dark blacklines distinguishes the three regions of the nanowire, left: p-type, center: i-layer, right: n-type.

under zero bias, we can see some potential gradient due tothe p-n junction. This potential difference implies that thereis a built-in electric field inside the material. We can obtainsome interesting observations by taking a cut of the potentialprofile at y 500 nm. There we can see the usual p-njunction potential profile but with a smaller built-in electricfield of slightly above 0.17 eV. Now at y 1µm, the built-in electric field is smaller and roughly can be taken to be0.17 eV. However, near the anode on the top, we observe asignificant potential drop and a larger built-in electric field. Abuilt in electric field of about 0.7 eV can be observed whentaking a cut line at y 0. Hence, most of the potential dropoccurs at the p-type core nanowires near the anode. This alsoimplies a drop in the electron concentration or an increase inthe hole concentration near the anode. We can conclude thenthat the built-in electric field decreases along the length of thenanowires from about 0.7 eV near the anode to about 0.17 eVnear the cathode.

The potential plot under 1.0 V bias also reveal some inter-esting observation. The potential drop now is more uniformalong the length of the nanowire as compared the zero biaspotential profile. We can observe as well that there are regionswith very little barrier for electrons to move from the n-type

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Fig. 5: Current-voltage characteristic under AM1.5G illumina-tion. The short circuit current is about 0.490 nA and the opencircuit voltage is about 0.470 V. This results in a power ofabout 230 pW.

shell to the p-type core. This is a similar effect normally foundin the forward bias case of a planar p-n junction.

Under zero bias, when the light shines on the nanowire, theelectron-hole pairs are generated, and the built-in electric fieldseperates the charges which then contribute as electric current.This is measured as a negative current on the anode. Hence,the solar illumination causes the current-voltage characteristicto shift downwards.

The current-voltage characteristic under AM1.5G illumina-tion is shown in Fig. 5. It can be seen that the open circuitvoltage Voc is about 0.470 V and the short circuit current Isc

is about 0.490 nA. We note that our open circuit voltage ishigher than [2], while our short circuit current is about thesame as the value stated in [2], which is about 0.503 nA.

Note that the calculated available photocurrent is about0.780 nA, whereas the short circuit photocurrent is about 0.490nA. This means that about 62% of the carriers contribute as acurrent in the external circuit. The power calculated from theopen circuit voltage and the short circuit current is about 230pW. This is larger than the one reported in [2] which is about72 pW. This is because our device has a larger open circuitvoltage. Nevertheless, the values are still reasonable sincesilicon nanowire photovoltaic elements can produce from about50 pW to about 200 pW per nanowire at 1-sun illumination.To work as a nanoscale power supplies, the output power mustbe higher than a few nanoWatt. In order to increase the power,the light intensity should be larger, or several nanowires shouldbe used together.

Next, we plot the photogeneration rate along the radialdirection. This is shown in Fig. 6. We expect that the curveis higher than the exponential behaviour of intensity decayin a bulk material. The reason is that in this simulationwe illuminate the nanowire all around. And hence, there isillumination from other sides of the nanowire, which resultsin a higher absorption by the nanowire. In reality, however, theillumination usually only occurs from only one direction ratherthan all around. In this case, a three dimension simulationmust be performed to study the photogeneration rate insidethe nanowire.

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Fig. 6: Photogeneration rate along the radial direction. Thedistance is measured from the axis of the nanowire (x 0nm) up to the surface of the nanowire (x 180 nm).

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Fig. 7: Effect of surface recombination on the current-voltagecharacteristic. Surface recombination velocity of 1 104 cm/sreduces the short circuit current to about 0.342 nA.

Another factor that we study is the surface recombination atthe outer most nanowire shell. To take into account the surfacerecombination, we added another thin layer of oxide and varythe surface recombination velocity. Fig. 7 shows the effect ofadding a surface recombination value of 1104 cm/s under thesame illumination condition which we previously considered.It can be seen that the effect on the short circuit current ismuch larger than the effect on the open circuit voltage.

Fig. 8 shows the variation of the short circuit current in thepresence of the surface recombination velocity. It can be seenthat the effect of the surface recombination is to reduce thephotocurrent. The short circuit current reduces to about 0.1 nAfor a very high surface recombination velocity. The drop seemsto saturate at the lower and higher recombination velocityvalues. It is important, therefore, to study further the surfacerecombination mechanism at nanowire and how it affects theother performance parameters of the coaxial silicon nanowirephotovoltaic device.

IV. CONCLUSION

We have simulated and modeled the coaxial siliconnanowire used for photovoltaic devices. It was shown byother researchers that such a device is able to power upnanoelectronics devices that require power in the range ofnanoWatt. We have studied the mechanism of the nanowire

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Fig. 8: Plot of short circuit current versus surface recombina-tion velocity.

solar cell by investigating the potential profile, the current-voltage characteristic, the photogeneration rate, and the surfacerecombination velocity. But, more studies are still needed,especially on the surface recombination since this is a majorrecombination phenomena in nanoscale devices. Future workwill focus on how to improve the performance and efficiencyof such device by designing the geometry and its materialproperties.

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