Carrier Transport and Energy Harvesting in ZnO Nanowires

Joao Pedro Martins Godinhojoao.m.godinho@ist.utl.pt

Instituto Superior Tecnico, Lisboa, Portugal

September 2015

Abstract

ZnO nanowires have been used to enhance carrier collection in dye sensitized solar cells due tolonger optical path with simultaneous ease of carrier collection. Zinc oxide is also interesting forits piezoelectric property, and thus its capability for energy harvesting. We have deposited 100 nmwide and 5 µm long ZnO nanowires by a wet chemical process, and we compared different transportcharacteristics of such nanowires with a homogeneous ZnO thin film, focusing on the surface trappingeffects. Strong variations of the power law exponent as a function of applied electric field in log-logrepresentations of current-voltage characteristics, is interpreted as a manifestation of transport in theTrap-Filled Limit regime, which lead to estimate the density of deep trap states as pt,0 = 4.4 × 1012

cm−3 and the concentration of carriers to be n0 = 1.5 × 109 cm−3. Transient photocurrent measure-ments showed a temperature dependence of the photo-response of the nanowires. When deconvolutedusing Laplace Transform, the signal yielded two peaks in the density-of-states distribution ascribed tosurface trapping at 0.85 eV and 0.95 eV. Admittance spectroscopy showed a peak related to an intrinsicdefect at 0.48 ± 0.02 eV and at 0.44 ± 0.02 eV for the nanowire and thin film sample, respectively.Keywords: Zinc Oxide, Nanowires, Deep-level Defects, Trap-Filled-Limit, Surface Trapping.

1. Introduction

Zinc oxide has been attracting intensive researchdue to its physical and chemical properties, thatallow its usage in a wide range of applications.

Research on ZnO for electronic devices started in1930, and reached its peak in 1980. However, theinterest began to fade away owing to the difficultyof doping ZnO in a p-type semiconductor, which isessential to optoelectronic purposes. The reawak-ening in the researching activity on this compoundhas begun in the nineties, following the growth ofa variety of nanostructures, for instance nanowires,quantum wells and quantum dots, with the aim toobtain the following applications: transparent elec-tronics, ultraviolet (UV) light emitters, piezoelec-tric devices, chemical sensors, spin electronics [1]and an enduring material under a radiation envi-ronment. Doping to p-type has already made someprogress, although it is still a major challenge toovercome, in order to have stable and reproduciblep-type ZnO [2].

One of the most important properties of ZnO iscertainly its piezoelectricity, that arises from thenon-central symmetric structure. Due to it, it hasbeen used in force sensing, acoustics wave resonator,acousto-optic modulator applications, among oth-ers. The search for sustainable nano-power sourcesfor mobile devices is a promising field of research-

ing, offering solutions to nanodevices. A possibleway to obtain such a nano-power source is throughthe piezoelectric effect. With the aim of convert-ing the kinetic energy of environmental vibrationsinto electricity, in 2006 the first piezoelectric gen-erator was demonstrated using ZnO nanowires in[3]. Since then, the usage of nanowires in these ap-plications has been thoroughly studied due to theiradvantages of having high piezoelectric coefficients,high elasticity and high resistance to fatigue [4].

Their electrical properties have also been studiedand the results indicate that nanowire-based devicescan be operated faster than their thin film coun-terpart. Furthermore, concerning optical proper-ties, it has been reported that arrays of nanowireshave been used as light emitting diodes[4], and ithas been shown that quantum size confinement in-creases the exciton binding energy, which as statedbefore, increase the efficiency of the emission. Inaddition, its near-cylindrical geometry and high re-fractive index make ZnO nanowires a candidate foroptical waveguides. Finally, upon adsorption ofmolecules on the surfaces, the conductivity of thematerial changes. In n-type materials it often leadsto a decrease in conductivity. This is the main prin-ciple behind gas sensors. Due to its larger surfaceto volume ratio, nanowires enhance the gas sensingperformance [1].

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ZnO has also attracted intensive research due to,for example, its application as window layers in so-lar cells, either as transparent conductive layers, oras active UV-sensitive front diode in heterostruc-ture solar cells. In particular, in Dye SensitizedSolar Cells (DSSC), nanowires add the advantageof allowing the dye to be attached directly ontothe surface, and thus offering to electrons a directpath to the anode, reducing recombination lossesand therefore increasing the collection efficiency ofthe cell.

As a material with interesting properties and apromising role in future technologies, this work fo-cuses on characterization of ZnO nanowires pre-pared by wet chemical process, by means of study-ing their carrier transport properties, as well astheir capability for energy harvesting. However,most of the work is focused on comparing sur-face related effects in homogeneous thin films ofZnO with those observed in nanowire structures.Pulsed laser deposition (PLD) and a wet chem-ical process are used for deposition, respectively.Both types of samples have been characterized byscanning electron microscopy (SEM), photocurrentspectroscopy (PCS), transient photocurrent anal-ysis (TPC), electrical measurements including IVcurves and admittance spectroscopy. The resultsare tested against a model that considers the ad-sorption of oxygen molecules as the main cause forthe samples behaviour. We point out the similari-ties and differences encountered when applying themodel to occupation of deep defects located at theZnO nanowire surfaces.

2. Background2.1. Trap-Filled-Limit Regime

In semiconductor IV characteristics, neglecting thepossibility of contact barriers, that is assuming anohmic contact, there are in general three to fourregimes. In a phenomenological way, considering asingle set of traps, the following behaviour of thecurrent-voltage curve is expected. For low voltages,the injected carrier concentration is negligible com-pared to the thermally generated ones, so we havean Ohm’s law behaviour. As the voltage increasesthe injected carriers are captured by the trap level,and once the traps are all filled, the current rises,converging afterwards to the Space Charge Lim-ited Current (SCLC) law or trap free square law.And it can be shown that this rise in current af-ter a discrete set of traps is filled is nearly vertical,which is one of the most remarkable results of thistheory, since experimentally such high slope in thecurrent-voltage curve could be misinterpreted as anelectrical breakdown in the material. An importantquantity is the threshold voltage where the currentrises abruptly, which is called the Trap Filled Limitvoltage (VTFL).

For a single set of traps there are two differ-ent situations, the set of traps lies below or abovethe Fermi level, while no voltage is applied. Thefirst one is simpler, since as carriers are injectedthe Fermi level moves up, never crossing the set oftraps, which is not true in latter case. As a resultthere is a fourth regime located between the Ohm’slaw and the TFL regime, denoted Shallow-Trap(ST) regime, which shows a quadratic dependenceof the voltage, like the SCLC. Once, the Fermi levelcrosses the level and the shallow trap is fully oc-cupied, a Trap-Filled Limit regime will be present,followed by the SCLC. In short, the general curvesof these two cases may be represented as in the fig-ure 1, where a deep-trap shows the sequence of Ohm- TFL - SCLC, whereas a shallow-trap shows Ohm- ST - TFL - SCLC.

6log (I)

- log (V)

Ohm

SCLC

TFL

ST

VTFL

Figure 1: Schematic of a Current-Voltage curvewith the four regimes, Ohmic, Space Charged Lim-ited Current (SCLC), Trap-Filled Limit (TFL), andShallow-Trap (ST) regimes.

Lampert and Mark [5] have developed the modelof a single level trap in a semiconductor. Accordingto their model, the regime begins at the designatedTrap-Filled Limit voltage VTFL, which is given as:

VTFL =ept,0L

2

2ε, (1)

where pt,0 is the concentration of traps not occu-pied, e the electron charge, L the space betweencontacts and ε the electric permittivity of the ma-terial. Other important relations from this modelare the following ratios:

J2J1

= 2pt,0n0

, (2)

V2V1

=8

3, (3)

2

where the subscript 2 denotes the end and 1 the be-ginning of the TFL regime. Hence, once identifiedthe Trap-Filled Limit voltage VTFL it is possible toobtain an estimation of pt,0 and n0.

To clarify the influence of pt,0 on the TFL regimeand the general curve of this regime, we solved theanalytic equation that describes an insulator witha deep trap with various values of n0 and pt,0, andplotted the results in figure 2. It was varied thevalues of n0 and the ratio between the two concen-tration A =

pt,0n0

. Changing the ratio, while keepingpt,0 constant, it influences the ohmic regime, as ex-pected since there are more carriers in the material.Besides that it also influences the slope of the TFLregime, which is steeper for higher values of A. Onthe other hand, if the ratio A is kept constant andthe value of pt,0 is changed (fourth curve), it shiftsthe curve to higher voltages, thus the voltage atwhich the TFL begins depends on pt,0.

Figure 2: Analytical solution of the single deep trapinsulator, using dimensionless variables, for differ-ent values of n0 and pt,0 = n0A. Note: These aredimensionless variables.

2.2. Piezoelectric Properties

The piezoelectric effect is closely related with theoccurrence of electric dipole moments in solids.These dipoles can be induced either due to ions ina crystal lattice that exhibits asymmetries in thecharge distribution or by certain molecular groupswith electrical properties. Neighbour dipoles tendto have the same direction forming regions calledWeiss domains. With a mechanical stress applied,the crystal lattice is displaced and the macroscopicpolarization changes. Thus, in these materials thereis a coupling between the electric behaviour and themechanical stress. The opposite effect, applying anelectrical field and obtaining a mechanical deforma-tion is known as converse piezoelectric effect.

If no coupling is considered the electric displace-ment ~D is:

[D] = [ε][E] (4)

and the strain tensor Sij is given by Hooke’s law

Sij = sijklTkl (5)

where ε is the dielectric permittivity tensor, [E] isthe electric field vector, sijkl the compliance (orelastic modulus) tensor and Tkl the stress tensor.The elastic modulus is a fourth rank tensor with81 components, although due to symmetry with re-spect to suffixes i,j and k,l it reduces the free com-ponents to 36. Using Voigt’s notation to contractthe index pairs i,j and k,l , the Hooke’s law may bewritten in a two rank matrix. To take into accountthe coupling between the two effects, the electricdisplacement ~D and the strain tensor S are rewrit-ten as:

[D] = [d][T ] + [εT ][E] (6)

[S] = [sE ][T ] + [d]t[E] . (7)

The [d] is the piezoelectric strain constant matrix,the superscripts T and E indicate that these quanti-ties are held constant and the t superscript indicatesa transpose matrix. The first equation is known asthe direct piezoelectric effect and the second as con-verse piezoelectric effect. Alternatively, the directpiezoelectric equation may be written as function ofthe strain:

[D] = [e][S] + [εT ][E] , (8)

where [e] is the piezoelectric stress constant matrix.Matrices [d] and [e] are connected by the compliancematrix as [d] = [s][e] [6].

A nanowire is a natural cantilever, and so it canbe modelled as a clamped-free beam with circularcross-section, which is statically compressed or bentby a force applied at its free end. Hence, the energyharvesting ability of nanowires can be studied usingthis simple model.

The main goal is then to calculate the electricpotential ϕ that appears in the nanowire, when itis compressed or bent. In order to do so, it wasused the following set of equations: the mechanicalequilibrium equation:

~∇.T = 0 , (9)

the piezoelectric effect equations (eq. 6 and 7), andthe Gauss equation:

~∇. ~D = ρ , (10)

where ρ the charge density.

3. Materials and MethodsThroughout this work four samples of ZnOnanowires were studied and one additional sam-ple of ZnO thin film in order to compare between

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Table 1: Samples studied in this work.Sample Structure Contact Distance Growth Method

PS1, PS2 and PS3 Nanowire 10 µm Chemical bathNWQ Nanowire 300 µm Chemical bath

MEFT4 Thin film 300 µm PLD

these two nanostructures. As listed in table 1 sam-ples PS1, PS2 and PS3 are very similar, consist-ing in nanowires deposited on Corning glass sub-strates using a wet chemical process. A schematicof these samples is shown on figure 3, illustratingthe nanowire layer on top of the gold inter-digitalcontacts. Notice that the voltage was applied onthe contacts C1 and C2. The NWQ sample has alonger contact distance of 300 µm and instead of be-ing deposited on Corning glass it was prepared ona quartz glass substrate. The reference ZnO film,MEFT4, was deposited on quartz glass substrateby pulsed laser deposition (PLD). All samples havegold metal contcts.

Figure 3: Schematic of the nanowire samples. Atyellow we have the gold contacts (C1 and C2) andthe cylinders represent the nanowires.

To investigate the optical properties of the struc-tures, particularly the band gap Eg and the Ur-bach energy Eu, photocurrent spectroscopy wasperformed. To make sure the sample reached aquasi steady state, after selecting a new wavelengthand the acquiring the current there was an intervalof 12 minutes. The voltage was supplied by a lock-inamplifier, which connected to the acquisition com-puter by GPIB, and the current reading was doneusing a Keithley 6485 picoammeter also connectedto the computer by GPIB. It was used an opticalsystem to focus the light on the sample, which con-sisted in optical lenses and a monochromator thatwas controlled by the acquisition routine written inLabview. The light source used was a 75 W Xelamp.

Considering the model developed by Lampertand Mark, the IV characteristics were measured inorder to investigate the defects states in the sam-ples. Studying the slopes present in the IV curvesand determining at which voltage the TFL regimestarts, it is possible to infer about the depth of thetrap (shallow or deep trap) and estimate the con-

centration of unoccupied trap states pt,0 and thecarrier concentration n0.

To further investigate the defects in the nanowirestructure, transient photocurrent (TPC) and ad-mittance spectroscopy (AS) were performed at dif-ferent temperatures. Both techniques give us in-sight about the defects and traps levels within thematerial. Furthermore, by performing TPC for along interval of time and AS for high frequencies,these two techniques become complementary. Sincethe TPC will be sensible to traps with long emis-sion times and AS to the shorter ones. Thus, atthe end one obtains a wide range in emission times,where each peak or distribution can be associatedto a given trap level.

It was used the same setup as in PCS, but themonochromator was removed, increasing this waythe intensity of light on the sample. Once the lightsource was turned off, the current flowing throughthe sample was monitored for a full hour, while thevoltage across the sample was kept constant at 0.1V. A Peltier cell and a thermocouple were used toset and read the sample temperature.

In order to measure the capacitance of the sam-ples it was used a Solartron 1260 impedance anal-yser, with which the real and imaginary parts ofthe impedance were measured. Then consideringthe sample as a resistance in parallel with a capac-itor, the resistance and capacitance were deduced.The measurements were obtained using a zero biasvoltage, a 0.1 V modulation voltage and 15 s of ac-quisition time. In order to study the effect of surfacetrapping, the sample was placed inside the vacuumchamber, and the measurements were firstly madeat atmospheric pressure and afterwards in vacuumconditions at 10−3 mbar.

4. Results and DiscussionIn this chapter we present the main results and willpropose a model to explain those findings.

This work started with the five samples afore-mentioned. However, not all measurements weremade with each sample. Due to the fact that, PS1and PS2 got damaged during a transient photocur-rent experiment, where it was used a Nd:YAG lasersystem. In an attempt to have a nanowire sam-ple with the same contact distance as the referencesample, the ZnO thin film (MEFT4), the sampleNWQ was made. However, it showed a higher re-sistance when compared with the other nanowiresamples. While at room temperature PS1, PS2 andPS3 show values around 300 Ω, NWQ showed 5 MΩ.Furthermore, with usage its value increased. Thismight mean that the deposited metal contact wasnot thick enough, so that maybe it was not uni-form and likely over the various measurements thecontacts got damaged. Therefore, special focus willgiven to the results from PS3 and MEFT4, and in

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this extended abstract of the thesis only results ofthese two samples will be shown.

4.1. MorphologyThe nanostructures grown with this method haveapproximate lengths of 5 µm and widths of 100nm. SEM images show a wide variety of flakes,nanowires and crystallites. In some images thereare some bunches of nanowires on top of a first layerof nanowires. These bunches may have grown in thesolution and then got deposited on top of the other,or due to some impurities or defects they started togrow on top of them. Figure 4 shows two SEM im-ages of PS3 with different magnifications.

Figure 4: SEM images of PS3.

4.2. Energy HarvestingFrom the set of equations described before (eq. 6to 10) it was studied two possible configurations toharvest energy from nanowires, axial compressionand bending. For the first case an analytical ex-pression was obtained:

V = −e33ε33

F

EAl , (11)

where V is the electric potential generated, e thepiezoelectric coefficient matrix, ε the electric per-mittivity, F the applied force, E the Young modulusand A the cross section of the nanowire. The otherconfiguration lead to a Poisson equation ∇2ϕ =− ρε11

that was solved numerically, where the chargedensity is given by:

ρ =F

EIy [e33 − 2νe31 − e15(1 + ν)] , (12)

with I = π4 r

4, r the radius of the nanowire and νthe Poisson coefficient.

Considering nanowires with 5 µm length and 100nm width and an applied force of 80 nN, the com-pression configuration yielded 5.5 V while the bend-ing the nanowire results in 0.05 V. Figure 5 showsthe solution to the Poisson equation obtained withthese parameters. The value of each constant usedwas: E = 142 GPa, ν = 0.323, ε11 = ε22 = 7.77,ε33 = 8.91, e31 = −0.543 Cm−2, e33 = 1.203 Cm−2

and e15 = −0.444 Cm−2.

Figure 5: Electric potential within a nanowire whenit is bent. The axis X and Y are indices of the gridused to solve the Poisson equation, and the circledefines the nanowire.

It is possible to conclude that regarding the en-ergy harvesting, the nano-generators based in ax-ial compression are more efficient, as well as thosethat whose nanowires are thinner and taller. Fur-thermore, the axial compression shows a linear re-sponse, which makes this nanostructure useful forforce sensing, owing to this sought property in anysensor.

4.3. Photocurrent SpectroscopyThe photocurrent spectrum obtained for PS3 is infigure 6. Thus, assuming that the photocurrent,that is, the difference between total and dark cur-rent, is proportional to the absorption coefficient, aTauc plot of the form (Iphhν)2 was performed. Ityielded a band gap energy of 3.2±0.3 eV, which is in

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agreement with the literature. From this spectrumthe Urbach Tail was also estimated to be aroundthe 33 ± 1 meV, which indicate a good crystallinestructure.

Figure 6: Photocurrent spectrum and Tauc plot ofPS3.

4.4. Current-Voltage CharacteristicsThe results for the PS3 are presented in the figure 7,where various Current-Voltage (or IV) curves wereobtained at different temperatures. The first thingto note is the clear change in behaviour after 0.5V, especially for lower temperatures. Firstly, in thefirst regime, for all temperatures analysed (temper-atures lower than 28oC were excluded as their sig-nals for low voltages are within the noise), the slopein the log-log representation is compatible with ornear 1. Thus, this is the ohmic regime, meaningthat there is a negligible injection barrier.

Figure 7: Current-Voltage curves of PS3 at varioustemperatures.

At high temperatures, the slope approaches thevalue of 2, which is the expected value for a SCLCregime. This supports the hypothesis of low trapconcentration at high temperatures, as an ideallytrap-less semiconductor would go from ohmic toSCLC, without presenting a TFL behaviour. Ei-

ther that or a given deep trap is only present atlow temperatures, and at high temperatures it van-ishes, leaving behind a dominant shallow trap level.In order to know which hypothesis is correct, a pos-sible solution would be to scan over a wider rangein voltages, and look whether the slope keeps ap-proximating the value 2 or if it is observed a TFLbehaviour. The first would mean the sample is ina SCLC regime and the second one would mean aShallow Trap regime.

The estimation of the concentration pt,0 comesfrom the crossover voltage, that is, the voltage atwhich the sample changes from ohmic to TFL. Re-garding this estimation, the results were not so con-clusive due to the limitation in measuring at lowtemperatures, where the TFL regime is more sig-nificant. The value obtained for the crossover didnot change significantly with the temperature (onlyfor ones higher than 28oC), being around 0.50±0.02V. If in fact the second regime observed at high tem-peratures is either SCLC or ST and not TFL, then itwould explain why the crossover did not change sig-nificantly, and more importantly it would mean thatfor higher temperatures the crossover voltage is notthe VTFL, but rather the voltage at which the SCLCor the Shallow Trap Regime starts. Although, as-suming that the VTFL would be around 0.5 V, pt,0and n0 would take the values (4.4±0.2)×1012 cm−3

and (1.5± 0.1)× 109 cm−3, respectively.

Figure 8: Current-Voltage curves of MEFT4 at var-ious temperatures.

Regarding the thin film, MEFT4, the IV curvesobtained are in figure 8, where the sample behavedalways as ohmic. The contact distance betweenthese two samples varies by a factor of 30, and so theelectric field inside the samples varies by the sameamount. Therefore, if the TFL regime in MEFT4were to happen at the same magnitude of the elec-tric field as in PS3, it means that the range in volt-age had to go up to 15 V - 20 V in order to observe

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this high-slope regime.

4.5. Transient Photocurrent

The current decays for different temperatures areshown in figure 9 for PS3 and MEFT4. While thephotocurrent decay curves for the MEFT4 are quitesimilar, the curves of PS3 at 10oC and 50oC differby almost an order of magnitude. The photo re-sponse of the nanowires changed significantly withtemperature, where at 10oC the sensitivity to lightwas lower. Note that for every temperature the timeof exposure to light was kept the same. A possiblereason to explain this is to say that low tempera-tures favour the adsorption of molecules on the sur-faces. Since the high photo response reported on theZnO nanowires has been attributed to the photo-desorption of oxygen molecules O2 [7, 8, 9]. In thedark, oxygen is adsorbed on the ZnO surface defectscapturing electrons, which is illustrated in contin-uous lines in figure 10. As the adsorbed moleculesare charged a depletion region is created, bendingthe conduction and valence band. Once light withenergy above the band gap energy is shone on thesample, the photogenerated holes discharge the ad-sorbed oxygen molecules (dashed line in fig. 10)and hence leading to the desorption of oxygen andthe trapping of the hole. As a result, the recombi-nation rate of the photogenerated electrons reduces,their lifetime increases and consequently the currentincreases as well, owing to the unbalanced concen-tration of both carriers. However, if the adsorp-tion rate is favoured, in other words if it increases,the lifetime of the photogenerated electrons will beshortened. Therefore this might be the reason forthe low photo response observed at 10oC in PS3.This effect, due to the larger surface to volume ra-tio is more relevant to the nanowires than to thethin film.

Figure 9: Transient photocurrent of PS3 andMEFT4.

When deconvoluted the transient curve yielded

Figure 10: Illustration of the adsorption (contin-uous line) and desorption (dashed line) of oxygenmolecule.

Figure 11: Density of States obtained from Tran-sient Photocurrent of PS3 and MEFT4.

two peaks at 0.85 eV and 0.95 eV, in both sam-ples. To convert lifetime of the observed peaksinto energy, the following expression was used τ =1013 exp

(EkT

). The second peak was also reported

in [7] with ZnO thin film and using the same de-convolution method. Both peaks change in magni-tude with temperature for the PS3, whereas for theMEFT4 they did not change significantly. Thus, itis likely that these peaks are associated with sur-face adsorption, rather than with intrinsic defects,as the thin film is not so sensible to surface effects asthe nanowire structure. Each peak might be asso-ciated to the adsorption of two different molecules,or with two possible lattice sites where the samemolecule can establish a chemical bond. The ex-periment that we carried out does not allow us tomake further conclusions. In order to do so, thesurrounding atmosphere has to be controlled.

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4.6. Admittance Spectroscopy

Except for the nanowire sample PS3 at 10oC and20oC, at lower frequencies the capacitance presenta positive slope, which is not expected by the the-ory. Thus, this might be due to a limitation inthe impedance analyser to read low capacitances atlower frequencies, particularly lower than 100 Hz.

In both cases, at atmospheric pressure or in vac-uum, both samples have a peak around 300 Hzand 400 Hz, and PS3 has a second peak at 6 kHzwhile MEFT4 has one at 40 kHz. However, thefirst one can be an artefact due to the limitationof the analyser at low frequencies, and this ”jump”in capacitance might be the point where the devicestarts reading correctly. At atmospheric pressureand 10oC, instead of a peak near the 350 Hz, PS3showed a broader peak centred at 200 Hz. Onceagain except for the sample PS3 at 10oC and 20oC,there are no meaningful changes in the spectrum ofboth samples between atmospheric pressure and invacuum. Therefore, the defects detected are likelyto be intrinsic ones instead of being due to surfacetrapping. Further, the peaks at 6 kHz for nanowiresand 40 kHz for thin film, may be due to intrinsicdefects favoured by the respective growth methodused. The nanowires were grown by wet chemicalprocess while the thin film by PLD.

Using the same expression as in TPC that relatesthe lifetime of a trap with its distance in energyfrom the nearest band, the energy of these peakswere estimated and then compared with values forintrinsic defects in [10]. The peak observed in PS3at 6 kHz with 0.48±0.02 eV was associated to singlycharged or neutral zinc interstitial (Zn+i or Zni).And the one at 40 kHz or 0.44 ± 0.02 eV observedin MEFT4 was associated to neutral zinc interstitial(Zni).

The figure 16 shows all four trap states identi-fied in this work by transient photocurrent and ad-mittance spectroscopy, in their respective locationinside the band gap.

5. Conclusions

Nanowire samples were grown by wet chemical pro-cess and their characteristics were compared witha thin film grown by PLD. Morphology of thenanowires was revealed by SEM images, which pre-sented a wide variety of flakes, nanowires, and crys-tallites. Photocurrent spectroscopy showed thatnanowire samples have a band gap of 3.2 ± 0.3 eVand a small Urbach tail may indicate a good crys-talline structure.

We observed a strong variation of power law ex-ponent as a function of temperature and appliedelectric field, on the IV curves. It was not pos-sible to estimate the temperature dependence ofVTFL, due to the limitation of measuring at low

Figure 12: Density of States from Admittance Spec-troscopy of PS3 at atmospheric pressure.

Figure 13: Density of States from Admittance Spec-troscopy of PS3 in vacuum.

temperatures, and this way observe any changein the value of the concentration of unoccupiedtraps pt,0. However, we estimated it to be aroundpt,0 = (4.4 ± 0.2) × 1012 cm−3 and the concentra-tion of carriers to be n0 = (1.5 ± 0.1) × 109 cm−3,assuming VTFL = 0.5 V.

Combining transient photocurrent decay and ad-mittance spectroscopy a wide range of time scalesof trap lifetimes were probed, from 1 to 3×103 sec-onds with transient photocurrent and from 10−5 to10−2 seconds with admittance spectroscopy. Twopeaks were identified in transient photocurrent andattributed to surface trapping. These peaks wereestimated with an energy of 0.85 eV and 0.95 eV,and they might be due to two different moleculesadsorption on the surface or to a single moleculespecies, for instance oxygen, chemically bonding intwo different sites in the ZnO lattice. Consideringthe model proposed to explain the photoconductiv-

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Figure 14: Density of States from Admittance Spec-troscopy of MEFT4 at atmospheric pressure.

Figure 15: Density of States from Admittance Spec-troscopy of MEFT4 in vacuum.

ity of the nanowires, it was concluded that the con-centration of unoccupied traps pt,0 increases withtemperature. That is, these traps may be regardedas the adsorbed oxygen molecules that adsorb to thesurface, capturing an electron from the conductionband, and the change in trap concentration may beregarded as how much the adsorption is favouredwith temperature.

The admittance spectroscopy of both samplesshowed a peak at 350 Hz, which might be anartefact. Additionally PS3 had a peak at 6 kHz(0.48±0.02 eV) while MEFT4 at 40 kHz (0.44±0.02eV), which were associated with intrinsic defects,namely to singly charged or neutral zinc interstitial(Zn+i or Zni) and neutral zinc interstitial (Zni),respectively.

In the IV curves and in the TPC experiments,PS3 changed its behaviour around 30oC - 40oC

EV

EC

Ei

0.85 eV 0.95 eV

0.48 eV

Zni or Zn+i

0.44 eV

Zni

Figure 16: Trap states identified with TransientPhotocurrent and Admittace Spectroscopy.

while in the admittance spectroscopy its behaviourchanged from 10oC to 20oC. This can be due to fa-tigue of the sample, as the experiments were carriedby the same order as exposed in this work, henceadmittance spectroscopy was the last experimentperformed. On the other hand, the surrounding at-mosphere was not monitored throughout this work.Thus, this shift in temperature at which the sam-ple changes its behaviour, might be owed to thevariations of the air composition, such as humid-ity. Further, the TPC suggested a higher increasein pt,0 than it was observed in IV curves, rememberthat VTFL is proportional to pt,0. Consequently, itis suggested as a future work to perform these ex-periments in a controlled surrounding atmosphere,as well as in vacuum for the case of IV charac-terization and transient photocurrent. This wouldallow to answer the question just mentioned, andthe one raised the TPC about the origin of the twopeaks. A similar approach was used in [8], wherethe persistent photoconductivity (or PPC effect) ofsingle ZnO nanowire was investigated as a functionof the surrounding atmosphere. The experimentswere performed in vacuum, and in Argon and Ni-trogen atmospheres. This is similar to what wasdescribed in this work as TPC. However, in thispublication only the decay rates were studied andthe density of states related to the decay were not.

Concerning the capability of these samples as en-ergy harvesters, we presented a model and sim-ulation results of the voltage yielded by a singlenanowire, when it is compressed or bent. Using thedeveloped model the compression configuration wasshown to be the most efficient. Experiments, how-ever, will have to be done in a later stage

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AcknowledgementsI would like to acknowledge my supervisor, Prof.Reinhard Schwarz, and co-supervisor, Prof. RachidAyouchi for accepting me as their student andfor their invaluable support, guidance and ideasthroughout all this work.

Besides my supervisors, I would like to expressmy gratitude to Dr. Tania Braz for all the discus-sions we had and who was always willing to help.Also a worthy mention is Prof. Umesh Mardolcarfor his help and contribution, throughout my entiredissertation work, for the admittance spectroscopymeasurements and for the assembly of the vacuumsystem.

My sincere thanks also goes to Prof. Pedro San-guino and Prof. Ricardo Franco for preparing thenanowire samples.

Finally, I would like to thank my family andfriends, who were always supporting me and en-couraging me with theirs best wishes.

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