Published: April 01, 2011

r 2011 American Chemical Society 2028 dx.doi.org/10.1021/nl2004219 |Nano Lett. 2011, 11, 2028–2031

LETTER

pubs.acs.org/NanoLett

Axial InP Nanowire Tandem Junction Grown on a Silicon SubstrateMagnus Heurlin,*,†,‡,§ Peter Wickert,‡ Stefan F€alt,‡Magnus T. Borgstr€om,† Knut Deppert,† Lars Samuelson,†

and Martin H. Magnusson‡

†Division of Solid State Physics, Lund University, Box 118, 22100 Lund, Sweden‡Sol Voltaics AB, Ideon Science Park, Scheelev€agen 17, 22370 Lund, Sweden

ABSTRACT: Tandem InP nanowire pn-junctions have been grownon a Si substrate using metal�organic vapor phase epitaxy. In situ HCletching allowed the different subcomponents to be stacked on top ofeach other in the axial extension of the nanowires without detrimentalradial growth. Electro-optical measurements on a single nanowiretandem pn-junction device show an open-circuit voltage of 1.15 Vunder illumination close to 1 sun, which is an increase of 67%compared to a single pn-junction device.

KEYWORDS: Nanowires, photovoltaics, doping, III�V, MOVPE

During the past 20 years, interest in nanowires (NWs) andtheir applications have grown tremendously. In particular

III�V semiconductor NWs have shown great potential in severalapplications, including electronics,1,2 light-emitting diodes,3,4

and photovoltaics (PV).5,6 The small lateral dimension of NWsenables high performance materials, for instance, III�V semi-conductors, to be integrated on a Si platform7 while minimizingcrystal defects, such as dislocations and antiphase boundaries thatwould otherwise degrade device performance. Photovoltaic en-ergy conversion based on III�V NW pn-junctions has beenrealized in both axial8 and radial6 NW geometries. The axialgeometry is promising for integrating several pn-junctions in amultijunction solar cell, since it offers a large degree of materialdesign freedom due to efficient strain relaxation for latticemismatched materials.9 It has also been shown that the tunneldiode that connects the different subcells (a key component ofthe multijunction solar cell) can be realized using an axial nþpþ

InP-GaAs heterostructure NW.10 This tunnel diode showedpromising performance numbers on par with and sometimesbetter than those used in thin film multijunction solar cellstoday.11 Tandem junction NW solar cells where all subcompo-nents have been incorporated in the same NW have so far onlybeen realized in the Si material system.12 In order to fully utilizethe potential of NW PV it is important, however, to use materialswith higher absorption coefficients than Si that can reach evenhigher photovoltaic conversion efficiencies, such as direct band-gap III-Vs, while still integrating the NWs on a low cost and largearea platform, preferably Si.

We have previously reported on the implementation of axialsingle junction InPNWs on both native and Si substrates.8 In thisletter, we report on further developments in growth of InP NWson a Si substrate by the vapor�liquid�solid mechanism.13 TheInP NWs were grown using metal�organic vapor phase epitaxy(MOVPE), and were doped in situ to form axial pn-junctions. Toextend the concept of III�V NW PV on Si, we show that twopn-junctions can be connected in series by a tunnel junction

in a single InP NW, resulting in a large increase in open circuitvoltage (Voc).

Silicon n-type (111) wafers with 60 nm Au particles at adensity of 2 μm�2 deposited by an aerosol technique14 were usedas substrates for the NW growth. The InP NWs were grown in alow pressure (100 mbar) commercial MOVPE system (Aixtron200/4) with a total flow of 13 slm, using purified hydrogen as thecarrier gas. The NWs were grown at a temperature of 420 �Cusing trimethylindium (TMIn) and phosphine (PH3) as pre-cursors. Tetraethyltin (TESn)15 and diethylzinc (DEZn)8 wereused for n- and p-type doping, respectively. The doping levelswere modulated by varying the dopant precursor flows. For thepresented NWs, we set out to maximize the p- and n-type dopinglevels by increasing the dopant precursor flow and simulta-neously decreasing the TMIn flow during growth of the tunneljunction. Subsequent investigation, not included in this report,has shown, however, that this effort is not necessary to achievesufficiently high doping levels required for the tunnel junction.Several growth runs were also performed with slightly variedswitching sequence in the tunnel junction, yielding similarresults, that is, substantially higher Voc compared to a singlejunction, demonstrating a reproducible and stable process.Unwanted radial growth on the NW sidewalls was avoided byadding hydrogenchloride (HCl) to the flow during growth,16

thereby eliminating the risk of short circuiting the first (lower)pn-junction. After growth, the NWs were inspected using a field-emission scanning electron microscope (SEM) working at 10 kVand a JEOL 3000F high-resolution transmission electron micro-scope (HRTEM) working at 300 kV. In order to contact theNWs, they were broken off and transferred to an oxidized Sisubstrate. Lateral contacts were defined by electron beamlithography, metal evaporation, and lift off as previouslydescribed.15 Electrical characterization of the contacted NWs

Received: February 4, 2011Revised: March 20, 2011

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was carried out using a Cascade 11000B probe station with anintegrated temperature control. A 100 W (Osram) halogen lampalong with fiber optics to focus the light was used as a light source.For calibration of the light source, a GaAs solar cell with knowncharacteristics when exposed to a standard AM1.5 spectrum wasused, and the corresponding illumination level was estimated byshort circuit current scaling. For the temperature dependentmeasurements, measurements were performed at 298 K bothbefore and after the series in order to estimate the errors relatedto the measurement setup.

Each of the two photocurrent producing junctions consists of ann-type Sn doped section, a nominally intrinsic (i) section, and ap-type Zn doped section. After the i section, the DEZn flow waslinearly increased, while flowing through the reactor cell, during 30 sto the set growth value for the p-type section. Figure 1a,b shows SEMimages of InP NWs where the doping has been modulated toproduce a nip-pþnþ-nip-pþ structure. Figure 1c is a schematic of the

structure. After growth, the NWs were 3.1 μm tall and had no visibletapering (i.e., no unwanted radial growth) due to the HCl in situetching. Importantly, HCl also eliminated parasitic growth on the Sisubstrate, which may proceed via the samemechanism that removesparasitic growth on the NW sidewalls, that is, HCl reacting withInP to form volatile species. Investigations of the influence of HClwhile growing Si NWs have shown that the Si surface may also bepassivated by a chlorination process, inhibiting parasitic growth.17

The elimination of parasitic growth results in a well-defined currentpath from the NW pn-junction to the substrate when the NWs arecontacted monolithically. The wide base of the NWs, visible in theSEM micrograph, originates from the nucleation, which has beenoptimized for vertical InP NW growth on the nonpolar Si(111)surface. All inspected NWs show a change in morphology about2μmabove the substrate surface (see Figure 1b). From theHRTEMimage of this section, depicted in Figure 2a, we observe a change incrystal structure, from a mixture of zinc blende and wurtzite to zincblende with periodic twin planes (see Figure 2b). This is consistent

Figure 1. (a) Overview SEM image of InP NW tandem junctions growing perpendicular to the Si substrate. (b) Higher magnification SEM image witharrow indicating a change in morphology. The inset shows a higher magnification of the area indicated by the arrow. The etched NWs show no visibletapering and no substrate growth can be observed on the Si substrate. The lateral scale bar is 2 μm in (a) and 200 nm in (b). Images were acquired with a30� tilt with respect to theNWgrowth direction. (c) Schematic over the grown structure indicating the doping type and level throughout the wire. Greenrepresents n-type doping, blue represents p-type doping, and white represents nominally intrinsic sections. A darker color represents higher doping.Dimensions in (c) are not to scale.

Figure 2. HRTEM of a tandem junction InP NW viewed in the Æ110æzone axis. (a) Overview image of the tunnel junction. In the interfacebetween pþ and nþ doping the crystal structure changes from zincblende (pþ) to a mixture of wurtzite and zinc blende (nþ). The arrowindicates the NW growth direction and position of the Au particle.(b) HRTEM image of the pþ section of the tunnel junction showing azinc blende crystal structure with periodic twin planes. Scale bar is 40 nmin (a) and 20 nm in (b).

Figure 3. (a) I�V characteristics of a single (red) and tandem (black)junction NW device. The dashed lines correspond to measurementswithout illumination and solid lines correspond to measurements withillumination. Under illumination the single junction shows a Voc = 0.69V, Isc = 4 pA and FF = 58%, while the tandem junction shows aVoc = 1.15V, Isc = 3 pA and FF = 48%. (b) Temperature dependent measurementof Voc for the tandem junction device. The black line indicates a linear fitof the data points corresponding to Voc(T) = 2.49 V � T � 4.8 mV/K.The illumination power corresponds to 0.9 suns in both figures, asmeasured by a reference GaAs solar cell.

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withprevious reports on a change in crystal structure fromwurtzite tozinc blende when introducing DEZn while growing InP NWs.18

Growth rate calibrations of the different NW sections also showedthat this section is the pþ section of the tunnel junction connectingthe two nip junctions. The same change in crystal structure andmorphology can also be observed at the top of the NWs whereanother pþ section was grown to facilitate formation of an Ohmiccontact.

Figure 3a shows I�Vmeasurements of a full tandem junctionstructure together with the I�V characteristics of a singlejunction, fabricated by contacting only the top junction of atandem structure. Without illumination and for small forwardbias the single junction current (I) follows an exponentialdependence on the applied bias (V), I � exp(eV/nkBT), wheree is the electron charge, kB is the Boltzmann constant, T thetemperature, and the ideality factor, n, is 1.8. Under illumination,the single and tandem junction NW devices show a Voc of 0.69and 1.15 V, respectively. The addition of a second junction thusincreases Voc by 67%. Further, the single and tandem junctionsshow short-circuit currents (Isc) of 4.0 and 3.0 pA and fill-factors(FFs) of 58 and 48%, respectively. The measurements wereperformed with an illumination power corresponding to 0.9 suns,as measured by the reference GaAs solar cell. This results in apower conversion efficiency of 3.3% for the single junction deviceand 0.5% for the tandem junction device. When determining thedevice efficiency, the area under which the NW device absorbslight was set to be the entire projected area of the NW. Thisefficiency therefore represents a lower bound for our devices. Adifferent way to estimate the efficiency would be to determine theactive area of the device, that is, the area where photogeneratedcarriers contribute to the current.12,19 Assuming that excitons areefficiently split only in the depletion region, estimated to thelength of the intrinsic sections, results in an efficiency of 5% forthe single junction and 1.8% for the tandem junction. Bothefficiency estimations results in a lower efficiency for the tandemjunction device compared to the single junction device. This ismainly due to an increased length of the segments used toestimate the efficiency of the tandem junction NW deviceresulting in a larger projected area. The output power of bothdevices is the same (1.6 pW) indicating that additional lossmechanisms are present in the tandem junction device out-weighing the benefits of a higher Voc. It should also be noted thatthe two photocurrent producing junctions of the tandem deviceare not current matched, hence the current delivered by thedevice is limited by the junction which produces the lowestphotocurrent.

The temperature dependence of Voc between 215 and 293 Kfor a single junction InP NW (not shown) yields a lineardependence of Voc versus temperature as expected.20 Theextrapolated value of Voc(T = 0 K) = 1.32 V is comparable tothe bandgap of InP in the center of the measurement interval(Eg = 1.36 eV at 254K21). Figure 3b shows a temperature dependentmeasurement ofVoc for the tandem junction device. A linear fit tothe measurement points results in Voc(T) = 2.49 V � T � 4.8mV/K. The extrapolated Voc(T = 0 K) for the tandem junctiondevice is thus 2.49 V, which is somewhat lower than expected forideal voltage addition. We speculate that this discrepancy isrelated to a nonideal tunnel junction.

In conclusion, we have grown tandem junction InP NWs on aSi substrate. By using in situ etching for total control over axialand radial growth we connected two photocurrent producing pn-junctions in series by a tunnel junction. An increase inVoc by 67%

compared to a single junction device was observed. We believethat this represents a first and significant step toward realizinghigh efficiency multijunction solar cells that can be fabricated onlarge area and low cost Si substrates. Further developments of pn-junctions in III�V alloys may in conjunction with our results leadto new high-efficiency solar cells with bandgaps tailored to thesolar spectrum, not easily achieved with planar multijunctiontechnology.

’AUTHOR INFORMATION

Corresponding Author*E-mail: magnus.heurlin@ftf.lth.se.

Present Addresses§Currently at Lund University but performed part of this work atSol Voltaics AB.

’ACKNOWLEDGMENT

The authors wish to acknowledge Jerry M. Olson for helpfuldiscussions and Christian Nilsson for performing the tempera-ture-dependent measurements on a single junction nanowire.This project is performed within the Nanometer StructureConsortium at Lund University (nmC@LU) in cooperationwith Sol Voltaics AB and is supported by the Swedish ResearchCouncil (VR), the Swedish Foundation for Strategic Research(SSF), the Knut and Alice Wallenberg Foundation (KAW), andby the European Commission under Contract number 214814 inthe seventh Framework Programme, as well as by the SwedishVINNOVA Green Nano program. It is based on a project thatwas funded by E.ON AG as part of the E.ON InternationalResearch Initiative. Responsibility for the content of this pub-lication lies with the authors.

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