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Effects of Free Carriers on Piezoelectric Nanogenerators and Piezotronic Devices Made of GaN Nanowire Arrays Chao-Hung Wang, Wei-Shun Liao, Nai-Jen Ku, Yi-Chang Li, Yen-Chih Chen, Li-Wei Tu, and Chuan-Pu Liu*
This study investigates the role of carrier concentration in semiconducting piezoelectric single-nanowire nanogenerators (SNWNGs) and piezotronic devices. Unintentionally doped and Si-doped GaN nanowire arrays with various carrier concentrations, ranging from 1017 (unintentionally doped) to 1019 cm−3 (heavily doped), are synthesized. For SNWNGs, the output current of individual nanowires starts from a negligible level and rises to the maximum of ≈50 nA at a doping concentration of 5.63 × 1018 cm−3 and then falls off with further increase in carrier concentration, due to the competition between the reduction of inner resistance and the screening effect on piezoelectric potential. For piezotronic applications, the force sensitivity based on the change of the Schottky barrier height works best for unintentionally doped nanowires, reaching 26.20 ± 1.82 meV nN−1 and then decreasing with carrier concentration. Although both types of devices share the same Schottky diode, they involve different characteristics in that the slope of the current– voltage characteristics governs SNWNG devices, while the turn-on voltage determines piezotronic devices. It is demonstrated that free carriers in piezotronic materials can influence the slope and turn-on voltage of the diode characteristics concurrently when subjected to strain. This work offers a design guideline for the optimum doping concentration in semiconductors for obtaining the best performance in piezotronic devices and SNWNGs.
1. Introduction C.-H. Wang, W.-S. Liao, Dr. N.-J. Ku, Y.-C. Li, Dr. Y.-C. Chen Department of Materials Science and Engineering National Cheng Kung University Tainan 70101, Taiwan Prof. L.-W. Tu Department of Physics National Sun Yat-Sen University Kaohsiung 80424, Taiwan Prof. C.-P. Liu Department of Materials Science and Engineering Center for Micro/Nano Science and Technology Research Center for Energy Technology and Strategy Advanced Optoelectronic Technology Center National Cheng Kung University Tainan 70101, Taiwan E-mail: [email protected] DOI: 10.1002/smll.201400768
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With the depletion of fossil fuels and safety problems of nuclear power, safe and stable renewable energy sources are desirable. Scavenging waste energy, such as that from vibrations, heat, and wind, has thus received attention[1] because these can be harnessed to provide additional energy sources that are able to be stored and used to mitigate the high demand on traditional energy sources. Waste mechanical energy can be harvested with piezoelectric nanogenerators (NGs)[2] that utilize ZnO nanowires (NWs) to generate electricity in both direct-current (DC) and alternatingcurrent (AC) modes.[3,4] Various semiconductor materials have been applied for enhancing the performance of NGs, such as GaN,[5,6] InN,[7–9] CdS,[10] and CdSe.[11] Group IIInitride semiconductor NWs, which are widely employed in electronic, opto-electronic, and photovoltaic devices,[12–15] © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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are potential candidates for high-outputpower NG applications.[8] The working principle of piezoelectric NGs is based on the piezoelectric effect, where piezoelectric potential (piezopotential) is generated by external strain exerted on non-central symmetric structures. When this effect is applied to electronics, the carriers that transport across the interfaces of the piezoelectric material and another material can be modulated by producing piezoelectric charges at the interfaces. Correspondingly, new types of devices emerge, called piezotronics[16] and piezophototronics[17–19] when coupling with photons. Piezopotential affects the performance of NGs and piezotronic devices. Therefore, the piezoelectric coefficient of materials and the type of deformation are often addressed in the design of such devices,[8,9,20] with much less attention given to carrier concentration, an inherent parameter of semiconductors. Although high carrier concentrations enhance the conductivity of semiconductors,[8] they have been reported to greatly degrade the output voltage of NGs due to the screening effect.[21,22] The effect of carrier concentration on the Schottky bar- Figure 1. a) SEM and b) HR-TEM images of vertically aligned unintentionally doped GaN nanowires. c) Corresponding SAED pattern taken from a single unintentionally doped GaN rier height (SBH) is unknown. Without nanowire in (b). d) Experimental and simulated CBED patterns of a single unintentionally consideration of the carrier concentration, doped GaN nanowire. optimum device performance cannot be achieved. high as 7.95 × 1010 cm−2 due to the low diffusion length of Ga The present study investigates the influence of carrier adatoms, as shown in Figure 1a. The size and density of GaN concentration on the performance of piezoelectric single- NWs can be further tuned by the substrate growth temperananowire NGs (SNWNGs) and piezotronic devices using ture or AlN buffer layer.[23] Owing to the ultrahigh-vacuum arrays of GaN NWs with various doping concentrations as a growth environment, GaN NWs (unintentionally doped) with model system. The output current of piezoelectric SNWNGs high crystal quality and a diameter of around 50 nm were was studied with atomic force microscopy (AFM) in scanning readily obtained. Neither linear nor planar defects, such as mode. The change of the SBH of the piezotronic devices was dislocations or stacking faults, can be seen in the high-resodetermined by examining the current transport behaviors of lution transmission electron microscope (HR-TEM) image GaN NWs under various normal forces. Results show that shown in Figure 1b. The selected-area electron diffraction the carrier concentration affects SNWNGs and piezotronic (SAED) pattern along the ⎡⎣ 2110 ⎤⎦ zone axis in Figure 1c devices in different ways. This work demonstrates that tuning shows that the GaN NW (unintentionally doped) is a singlethe carrier concentration in the piezoelectric material to the crystalline wurtzite structure, growing along the ≤0002≥ right range is critical to the operation efficiency of SNWNGs direction. With this high crystal quality, the dependence of free and piezotronic devices. carriers, crucial characteristics of typical semiconductors, on both piezotronic devices and SNWNGs through piezoelectric properties can be exploited. Prior to discussing the influence 2. Results and Discussion of doping concentration, the crystal polarity, which dominates Owing to the same crystal structure, GaN NWs exhibit the properties of SNWNGs and piezotronic devices, needs piezoelectric and semiconducting properties just like ZnO to be claried by fully exploring the mechanisms of power NWs, and both properties can be influenced by crystal generation through GaN NWs on stressing. Accordingly, the quality, required to be examined first. The as-grown GaN n-GaN NWs grown under Si cell temperature at 1400 °C NWs (unintentionally doped) are characterized and the (TSi = 1400 °C) were subjected to a lateral force applied by results are shown in Figure 1. The growth conditions favor the conductive-AFM (C-AFM) Pt-coated tip. The lateral a higher growth rate and GaN NWs grow along the c axis, force bends the n-GaN NWs (TSi = 1400 °C) during tip scanleading to the formation of NW arrays with a density of as ning, with piezoelectric potential produced when the n-GaN small 2014, 10, No. 22, 4718–4725
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reverse-biased, and thus no current flows out.[2] After fully analyzing the crystal polarity, the free carrier effect on both power generating and piezotronic properties can now be discussed. Before investigating the output performance of GaN SNWNGs, the carrier concentrations in various doped GaN NW samples were determined by examining the change in the frequency and line shape of the phonon–plasmon coupled mode of a GaN Raman peak, called the L+ peak.[24] For low doping concentrations, the L+ mode is phonon-like. However, when the carrier concentration is increased, the L+ mode starts to change its character from phonon-like to plasmon-like. With this behavior, the doping concentration can be evaluated using the following empirical equation:[25] n = 1.1 × 10 17 (ω L+ − ω LO )
0.764
( valid for n < 1 × 10 19 cm −3 )
Figure 2. a) Electrical current output of n-GaN nanowires (TSi = 1400 °C) obtained using tip scanning with C-AFM. b) Typical AFM topography image. c) Electrical current signal and topographical line profile superimposed from the area along the scanning direction indicated in (b). d) Schematic of electrical current generation mechanism.
NWs are deformed. Figure 2a depicts the output current characteristics. The maximum output current is about 35 nA under a constant scanning speed and force. To explore the current-generating mechanisms, the surface topography and current-mapping profile of an area with the corresponding scanning direction shown in Figure 2b are superimposed in Figure 2c. This figure clearly demonstrates that a sharp current peak is generated at the initial contact side (i.e., stretched side) of the NWs. Figure 2d depicts a schematic of the piezoelectric-charge-induced potential distribution when a n-GaN NW (TSi = 1400 °C) is subjected to a bending geometry based on its c axis pointing away from the C-AFM tip, as analyzed by matching the simulated and experimental results from the convergent-beam electron diffraction (CBED) patterns shown in Figure 1d. Under this condition, the bending force should result in a negative piezoelectric potential on the stretched side of the NW and a positive piezoelectric potential on the compressed side. During the course of the tip scanning a NW, the tip makes contact with the stretched side first and then the compressed side. The negative piezoelectric potential produced at the stretched side makes the Schottky diode forward-biased, turning on the diode. Current consequently flows through the metal-semiconductor junction and to the external circuit. For the compressed side, the positive piezoelectric potential makes the Schottky diode
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cm −3 (1)
where n is the carrier concentration, ωL+ is the frequency of the L+ mode, and ωLO is the frequency of the uncoupled LO mode (ωLO = 735 cm−1). Figure 3 shows the Raman spectra of a series of GaN NWs grown at various Si dopant temperatures, where the L+ mode is clearly indicated along with the E2high mode
Figure 3. Raman spectra of unintentionally doped and doped GaN nanowires obtained at various Si cell temperatures.
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Table 1. Relationship between the Si cell temperature and free carrier concentration of GaN nanowires extracted from phonon-plasmon coupled mode position of Raman spectra. Si cell temperature [°C]
L+ [cm−1]
Free carrier concentration [cm−3]
0
736.00
1.10 × 1017
1200
747.50
7.58 × 1017
1300
844.23
3.97 × 1018
1350
907.78
5.63 × 1018
1400
1373.88
1.53 × 1019*
Is =
* Overestimated.
at 566.9 cm−1. Based on Equation (1), estimates of the n-type doping concentrations of the GaN NWs are listed in Table 1. The doping concentration varies by two orders of magnitude, starting from 1017 cm−3 for the unintentionally doped sample to about 1019 cm−3 for the most heavily doped sample. Note that the highest doping concentration is beyond the valid range of Equation 1, and likely has a large uncertainty. In the context of the working mechanisms, both SNWNGs and piezotronic devices rely on the same current– voltage curve of a Schottky diode under strain but stress on different operating region. For SNWNGs, the established inner-crystal piezoelectric potential as a current-flow driving force causes the Schottky diode turn on, and then the inner resistance of the material comes to dominate the current output. As for piezotronic devices, the piezoelectric charges mainly influence the Schottky barrier height and govern the diode current near the turn-on voltage. By collecting the change of the turn-on diode current, a force sensor made of a piezotronic device can be achieved. The free carrier concentration can govern inner resistance in accordance with semiconductor physics and Schottky barrier height based on screening effect simultaneously, and should be investigated step by step. Starting from SNWNGs, the doping concentration should affect the conductivity of GaN NWs and thus the output current characteristics of an SNWNG. Figure 4 shows the current outputs of GaN NWs with various doping concentrations obtained using AFM tip scanning. For the unintentionally doped case (carrier concentration = 1.10 × 1017 cm−3), the current output is close to the current detection limit and is regarded as no current generated. This also applies to the lightly doped case (carrier concentration = 7.58 × 1017 cm−3). As the carrier concentration is raised beyond this value, the current output starts to increase, reaching its maximum at a carrier concentration of 5.63 × 1018 cm−3. Of note, the current output then decays with further increasing doping concentration, here with the highest carrier concentration of 1.53 × 1019 cm−3. The results thus demonstrate that the output current of an SNWNG is sensitive to doping concentration. Figure 4f summarizes the statistical data on the current output over a few thousand NWs being scanned for each sample. The maximum current reaches up to 50 nA at a carrier concentration of 5.63 × 1018 cm−3, with variation of the current output due to different deformations during tip small 2014, 10, No. 22, 4718–4725
scanning. To the best of our knowledge, this is the highest reported output current generated from GaN NWs.[26] The carrier concentration has a huge impact on current output. Although the relationship between output voltage, inner resistance, and the resulting output current of an SNWNG has been previously determined with only a certain carrier concentration,[27] a series of detailed experimental approaches considering the carrier concentration have not been achieved yet. For the SNWNG considered here, the current output is governed by:[27] V0
(2)
( r0 + rc )
where V0 is the piezoelectric potential generated by the strain, Is is the output current, r0 is the inner resistance of the NW including the surface depletion effect,[28] and rc is the contact resistance of the metal-semiconductor interface. The carrier concentration in semiconductor NWs not only strongly modifies the inner resistance but also the carrier transport behavior. In addition, when the electron concentration is increased, the piezoelectric potential is degraded by the screening effect induced by free carriers.[21,22] As a result, the doping concentration affects three terms (V0, rc, and r0) non-linearly, with competition between the electron-concentration-induced inner resistance reductions including the surface depletion change with carrier concentration, contact resistance reductions as exponential dependence on barrier height, which is a function of carrier concentration,[29] and the decreasing piezoelectric potential induced by the screening effect. Therefore, the maximum output current is reached at a doping level of 5.63 × 1018 cm−3, beyond which the piezoelectric potential decreases and thus the output current drops. After investigation of output performances of SNWNGs, the carrier concentration influences carrier transport behavior through interfaces for piezotronic devices as well. The force-dependent current-to-voltage (IV) characteristics and the extracted SBH changes are shown in Figure 5. When a normal compressive force is applied, the SBH decreases since a positive piezoelectric potential is established at the top of the GaN NW in proximity to the metal-semiconductor (GaN-Pt) Schottky interface due to the polarity of the GaN NW. For simplicity, an ideal Schottky diode is assumed by neglecting shunt and series resistance. The thermionic current-voltage relationship is thus given by:[11]
(
I f = AA** T 2 exp −
)
⎛ qVf ⎞ ΦB exp ⎜ − 1⎟ k BT ⎝ nideal k BT ⎠
(3)
where A is the area of the Schottky diode, A** is the effective Richardson constant, T is the temperature, ΦB is the SBH, kB is the Boltzmann constant, q is the electron charge, Vf is the voltage drop on the forward-biased Schottky diode, and nideal is the ideality factor. To verify whether the transport mechanism obeys the thermionic emission and diffusion (TED) model, the curves of logarithmic current versus voltage are shown in the inset of Figure 5b. They confirm that all IV characteristics are dominated by TED. Under the assumption that a change of A** due to the normal force is much smaller than
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Figure 4. Electrical current collected by C-AFM tip scanning of GaN nanowires with electron concentrations of a) 1.10 × 1017, b) 7.58 × 1017, c) 3.97 × 1018, d) 5.63 × 1018, and e) 1.53 × 1019 cm−3. f) Statistics of the current output from (a) to (e).
the strain-induced change in the SBH, the change of the SBH can be derived using Equation 4.[11] The force-dependent SBH change with respect to carrier concentration is shown in Figure 5d. ⎛ I f ( F1 ) ⎞ ΔΦ s ln ⎜ ≈− k BT ⎝ I f ( F2 ) ⎟⎠
(4)
where If(F1) and If(F2) are the forward currents at the normal force of F1 and F2, respectively, and ΔΦs is the change of the SBH. From Figure 5d, the change of the SBH increases linearly with force up to 3 nN for all samples, and further discussion is provided in Supporting Information. More importantly, for a given force, the change of the SBH gradually decreases with increasing carrier concentration. This is mainly caused by the screening effect since the strain state should be identical
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under a given normal force for all samples, where they are almost identical in diameter and length with evidences provided in Supporting Information, and hence should induce equivalent numbers of piezoelectric charges. The higher the free electron concentration, the more the piezoelectric potential is screened, leading to smaller changes in the SBH, as shown in Figure 5c. The force sensitivity values calculated with respect to the SBH change for carrier concentrations from 1017 to 1019 cm−3 are from 26.20 ± 1.82 to 14.70 ± 2.21 meV nN−1, which are equivalent to 1.34 ± 0.58 and 0.90 ± 0.13 ln(A) nN−1, respectively. Notably, the force sensitivity can be decreased by almost two times by increasing the carrier concentration. The maximum force sensitivity in this work is the highest one among all previously reported results as listed in Table 2.[9,11,30–32] Of which, one of the essential strategies for boosting the force sensitivity is to design the NWs geometrically to change its deformation type from
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Figure 5. a) Electrical circuit diagram of force-dependent IV measurement for determining piezotronic effect along with the corresponding energy band diagram. b) IV characteristics of GaN nanowires with carrier concentration of 3.97 × 1018 cm−3 under normal forces from ≈0 nN to –3 nN. Inset is the logarithmic current versus voltage curve accompanied with linear fit curve upon normal force from ≈0 nN to –3 nN. c) Energy band diagram of Schottky diode to compare changes in SBH for various carrier concentrations under a given normal force. d) Changes of SBH with force extracted from force-dependent IV characteristics for three carrier concentrations.
normal to bending force in oblique InN nanorods (NRs).[9,31] Nevertheless, in our findings, the carrier concentration is also critical and more versatile in tuning the force sensitivity, which can be extensively altered from 14.70 ± 2.21 to 26.20 ± 1.82 meV nN−1, implying that carrier concentration plays an crucial role in piezotronic devices as well. More detailed discussions on the effect of carrier concentration on force sensitivity is provided in Supporting Information. Figure 5d not only demonstrates the changes in the SBH, but also gives deep insights for piezoelectric SNWNG and piezotronic applications. In piezotronic devices, such as a force sensor, a high SBH change is required in response to an applied force since the sensitivity is determined by it.[30] Accordingly, a low carrier concentration is preferred. The Table 2. Comparison of force sensitivity with the literature. Materials
Force sensitivity [meV nN−1]
Forcing method by AFM tip
≈0.275
Normal force
30
≈0.962
Bending force
Oblique InN NRs 9,31
≈25
Bending force
ZnO NW 32
0.017 to 0.12 (depending on length and size)
Normal force
GaN NW (This work)
14.70 ± 2.21 to 26.20 ± 1.82 (depending on carrier concentration)
Normal force
CdSe NW 11 GaN NW
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force sensitivity reached as high as 26.20 ± 1.82 meV nN−1 in this study. On the other hand, for SNWNGs, the inner and contact resistances critically affect the overall output efficiency. As a consequence, to obtain high output power from SNWNGs, the inner and contact resistance should be lowered by increasing the carrier concentration to an optimum level. In this study, the maximum output current of a GaN SNWNG was up to 50 nA at a carrier concentration of 5.63 × 1018 cm−3. A relatively high carrier concentration is more favorable for SNWNG applications whereas a low carrier concentration is more favorable for piezotronic applications. Notably, the piezoresistance effect is not the dominant driver responsible for both SNWNGs and piezotronic devices because the asymmetrical current changes involved in forcedependent IV curves provide the strong evidence for piezoelectric effect. The close relation of both mechanisms to carrier concentration is discussed and concluded as follows. For SNWNGs application, the current is generated based on the slope of IV characteristics of a turn-on Schottky diode when the C-AFM tip contacts with the negative piezopotential side of a GaN NW during a tip scanning process. Consequently, the inner and contact resistances governed by carrier concentration are critical for electricity generation because they dominate the slope of IV curve. As for piezotronic devices, the SBH change is extracted from the IV characteristics
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of a turn-on Schottky diode of the same type as SNWNGs. However, the carrier concentration here acts as an interfacial barrier modification layer, which controls the electron transport, the contact resistance, and mainly the turn-on voltage of the Schottky diode that provides the versatility of tuning its force sensitivity. Both devices are established on the turn-on characteristics of a Schottky diode but with different working mechanisms. This implies that changing the slope of the IV curve is related to the bulk property and shifting the turnon voltage is referred to the interfacial property, and the carrier concentration can influence both simultaneously. These results give us a guideline toward designing a SNWNG with a giant output power and a piezotronic device possessing of a ultra-high force sensitivity.
by scanning electron microscopy (SEM, Hitachi SU-8000) and the crystal structure was studied using an aberration-corrected transmission electron microscope (TEM, JEOL JEM-2100F) equipped with a probe Cs-corrector. Conductive AFM (C-AFM, SII SPA-400) with Pt-Ir-coated tips (spring constant = 0.2 N m−1) was used to characterize the output current and force-dependent current-to-voltage (IV) characteristics. All testing conditions including tip scanning speed (0.7 Hz), tip down force (–1 nN), and so on, were kept the same over a scan area of 2 µm × 2 µm for each sample in order to map out the current output distribution and the results are able to be compared fairly. Micro-Raman spectroscopy (HORIBA Jobin Yvon/Labram HR) was utilized to determine the doping concentration of the Si-doped GaN NWs.
3. Conclusion Supporting Information GaN NWs with various electron concentrations were synthesized by PA-MBE to investigate the impact of carrier concentration on SNWNGs and piezotronic devices. The carrier concentration of each sample was determined from the phonon–plasmon coupled mode of Raman peaks. From C-AFM experiments, the maximum output current, obtained at a carrier concentration of 5.63 × 1018 cm−3, was ≈50 nA, which is the highest current ever reported. The optimal output current is explained by the competition between the inner and contact resistance reductions, and the carrier screening effect on the piezoelectric potential. For piezotronic devices, the highest change in the SBH occurs at the lowest carrier concentration of 1.1 × 1017 cm−3, reaching 26.20 ± 1.82 meV nN−1, equivalent to 1.34 ± 0.58 ln(A) nN−1, which is the best value ever reported for GaN NW-based force sensors without optimization. The results show that the doping concentration in semiconductors plays a crucial role in the performance of SNWNGs and piezotronic devices via changes in the inner and contact resistance, and screening effect on the piezopotential. Overall, a relatively high but optimized carrier concentration is favorable for SNWNG applications owing to smaller inner resistance, whereas piezotronic applications require low carrier concentration due to the necessity of a huge change in the SBH.
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This research was supported by the National Science Council under Grant NSC-101–2221-E-006–131-MY3, NSC-101–2622-E-006– 015-CC1, and NSC-101–2622-E-006–005-CC1. The authors also wish to thank the Center for Micro/Nano Science and Technology, National Cheng Kung University, and the NSC Core Facilities Laboratory for Nano-Science and Nano-Technology in Kaohsiung-Pingtung area for providing equipment and technical support.
[1] [2] [3] [4] [5] [6] [7]
4. Experimental Section
[8]
Arrays of GaN NWs were grown on a 2-inch highly doped n-type Si(111) substrate using a radio-frequency plasma-assisted molecular beam epitaxy (PA-MBE) system (SVTA 35-Nitride MBE). The NWs were grown at high substrate temperatures via diffusion control under a nitrogen-rich environment without using any catalysts. The nitrogen flow, plasma generator power, Ga cell temperature, and substrate temperature were 2 sccm, 350 W, 910 °C, and 760 °C, respectively. For fair comparisons, all the GaN NW samples were grown under the same conditions except for the dopant flux for 2 h. Silicon was used as the n-type dopant. The silicon cell temperature was varied from 1200 to 1400 °C to control the vapor pressure in order to increase electron concentration. Once the n-GaN NWs were grown, the morphologies were investigated
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[9] [10] [11] [12] [13] [14] [15]
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Received: March 21, 2014 Revised: May 19, 2014 Published online: July 10, 2014
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Effects of Free Carriers on Piezoelectric Nanogenerators and Piezotronic Devices Made of GaN Nanowire Arrays Chao-Hung Wang, Wei-Shun Liao, Nai-Jen Ku, Yi-Chang Li, Yen-Chih Chen, Li-Wei Tu, and Chuan-Pu Liu*
This study investigates the role of carrier concentration in semiconducting piezoelectric single-nanowire nanogenerators (SNWNGs) and piezotronic devices. Unintentionally doped and Si-doped GaN nanowire arrays with various carrier concentrations, ranging from 1017 (unintentionally doped) to 1019 cm−3 (heavily doped), are synthesized. For SNWNGs, the output current of individual nanowires starts from a negligible level and rises to the maximum of ≈50 nA at a doping concentration of 5.63 × 1018 cm−3 and then falls off with further increase in carrier concentration, due to the competition between the reduction of inner resistance and the screening effect on piezoelectric potential. For piezotronic applications, the force sensitivity based on the change of the Schottky barrier height works best for unintentionally doped nanowires, reaching 26.20 ± 1.82 meV nN−1 and then decreasing with carrier concentration. Although both types of devices share the same Schottky diode, they involve different characteristics in that the slope of the current– voltage characteristics governs SNWNG devices, while the turn-on voltage determines piezotronic devices. It is demonstrated that free carriers in piezotronic materials can influence the slope and turn-on voltage of the diode characteristics concurrently when subjected to strain. This work offers a design guideline for the optimum doping concentration in semiconductors for obtaining the best performance in piezotronic devices and SNWNGs.
1. Introduction C.-H. Wang, W.-S. Liao, Dr. N.-J. Ku, Y.-C. Li, Dr. Y.-C. Chen Department of Materials Science and Engineering National Cheng Kung University Tainan 70101, Taiwan Prof. L.-W. Tu Department of Physics National Sun Yat-Sen University Kaohsiung 80424, Taiwan Prof. C.-P. Liu Department of Materials Science and Engineering Center for Micro/Nano Science and Technology Research Center for Energy Technology and Strategy Advanced Optoelectronic Technology Center National Cheng Kung University Tainan 70101, Taiwan E-mail: [email protected] DOI: 10.1002/smll.201400768
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With the depletion of fossil fuels and safety problems of nuclear power, safe and stable renewable energy sources are desirable. Scavenging waste energy, such as that from vibrations, heat, and wind, has thus received attention[1] because these can be harnessed to provide additional energy sources that are able to be stored and used to mitigate the high demand on traditional energy sources. Waste mechanical energy can be harvested with piezoelectric nanogenerators (NGs)[2] that utilize ZnO nanowires (NWs) to generate electricity in both direct-current (DC) and alternatingcurrent (AC) modes.[3,4] Various semiconductor materials have been applied for enhancing the performance of NGs, such as GaN,[5,6] InN,[7–9] CdS,[10] and CdSe.[11] Group IIInitride semiconductor NWs, which are widely employed in electronic, opto-electronic, and photovoltaic devices,[12–15] © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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are potential candidates for high-outputpower NG applications.[8] The working principle of piezoelectric NGs is based on the piezoelectric effect, where piezoelectric potential (piezopotential) is generated by external strain exerted on non-central symmetric structures. When this effect is applied to electronics, the carriers that transport across the interfaces of the piezoelectric material and another material can be modulated by producing piezoelectric charges at the interfaces. Correspondingly, new types of devices emerge, called piezotronics[16] and piezophototronics[17–19] when coupling with photons. Piezopotential affects the performance of NGs and piezotronic devices. Therefore, the piezoelectric coefficient of materials and the type of deformation are often addressed in the design of such devices,[8,9,20] with much less attention given to carrier concentration, an inherent parameter of semiconductors. Although high carrier concentrations enhance the conductivity of semiconductors,[8] they have been reported to greatly degrade the output voltage of NGs due to the screening effect.[21,22] The effect of carrier concentration on the Schottky bar- Figure 1. a) SEM and b) HR-TEM images of vertically aligned unintentionally doped GaN nanowires. c) Corresponding SAED pattern taken from a single unintentionally doped GaN rier height (SBH) is unknown. Without nanowire in (b). d) Experimental and simulated CBED patterns of a single unintentionally consideration of the carrier concentration, doped GaN nanowire. optimum device performance cannot be achieved. high as 7.95 × 1010 cm−2 due to the low diffusion length of Ga The present study investigates the influence of carrier adatoms, as shown in Figure 1a. The size and density of GaN concentration on the performance of piezoelectric single- NWs can be further tuned by the substrate growth temperananowire NGs (SNWNGs) and piezotronic devices using ture or AlN buffer layer.[23] Owing to the ultrahigh-vacuum arrays of GaN NWs with various doping concentrations as a growth environment, GaN NWs (unintentionally doped) with model system. The output current of piezoelectric SNWNGs high crystal quality and a diameter of around 50 nm were was studied with atomic force microscopy (AFM) in scanning readily obtained. Neither linear nor planar defects, such as mode. The change of the SBH of the piezotronic devices was dislocations or stacking faults, can be seen in the high-resodetermined by examining the current transport behaviors of lution transmission electron microscope (HR-TEM) image GaN NWs under various normal forces. Results show that shown in Figure 1b. The selected-area electron diffraction the carrier concentration affects SNWNGs and piezotronic (SAED) pattern along the ⎡⎣ 2110 ⎤⎦ zone axis in Figure 1c devices in different ways. This work demonstrates that tuning shows that the GaN NW (unintentionally doped) is a singlethe carrier concentration in the piezoelectric material to the crystalline wurtzite structure, growing along the ≤0002≥ right range is critical to the operation efficiency of SNWNGs direction. With this high crystal quality, the dependence of free and piezotronic devices. carriers, crucial characteristics of typical semiconductors, on both piezotronic devices and SNWNGs through piezoelectric properties can be exploited. Prior to discussing the influence 2. Results and Discussion of doping concentration, the crystal polarity, which dominates Owing to the same crystal structure, GaN NWs exhibit the properties of SNWNGs and piezotronic devices, needs piezoelectric and semiconducting properties just like ZnO to be claried by fully exploring the mechanisms of power NWs, and both properties can be influenced by crystal generation through GaN NWs on stressing. Accordingly, the quality, required to be examined first. The as-grown GaN n-GaN NWs grown under Si cell temperature at 1400 °C NWs (unintentionally doped) are characterized and the (TSi = 1400 °C) were subjected to a lateral force applied by results are shown in Figure 1. The growth conditions favor the conductive-AFM (C-AFM) Pt-coated tip. The lateral a higher growth rate and GaN NWs grow along the c axis, force bends the n-GaN NWs (TSi = 1400 °C) during tip scanleading to the formation of NW arrays with a density of as ning, with piezoelectric potential produced when the n-GaN small 2014, 10, No. 22, 4718–4725
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reverse-biased, and thus no current flows out.[2] After fully analyzing the crystal polarity, the free carrier effect on both power generating and piezotronic properties can now be discussed. Before investigating the output performance of GaN SNWNGs, the carrier concentrations in various doped GaN NW samples were determined by examining the change in the frequency and line shape of the phonon–plasmon coupled mode of a GaN Raman peak, called the L+ peak.[24] For low doping concentrations, the L+ mode is phonon-like. However, when the carrier concentration is increased, the L+ mode starts to change its character from phonon-like to plasmon-like. With this behavior, the doping concentration can be evaluated using the following empirical equation:[25] n = 1.1 × 10 17 (ω L+ − ω LO )
0.764
( valid for n < 1 × 10 19 cm −3 )
Figure 2. a) Electrical current output of n-GaN nanowires (TSi = 1400 °C) obtained using tip scanning with C-AFM. b) Typical AFM topography image. c) Electrical current signal and topographical line profile superimposed from the area along the scanning direction indicated in (b). d) Schematic of electrical current generation mechanism.
NWs are deformed. Figure 2a depicts the output current characteristics. The maximum output current is about 35 nA under a constant scanning speed and force. To explore the current-generating mechanisms, the surface topography and current-mapping profile of an area with the corresponding scanning direction shown in Figure 2b are superimposed in Figure 2c. This figure clearly demonstrates that a sharp current peak is generated at the initial contact side (i.e., stretched side) of the NWs. Figure 2d depicts a schematic of the piezoelectric-charge-induced potential distribution when a n-GaN NW (TSi = 1400 °C) is subjected to a bending geometry based on its c axis pointing away from the C-AFM tip, as analyzed by matching the simulated and experimental results from the convergent-beam electron diffraction (CBED) patterns shown in Figure 1d. Under this condition, the bending force should result in a negative piezoelectric potential on the stretched side of the NW and a positive piezoelectric potential on the compressed side. During the course of the tip scanning a NW, the tip makes contact with the stretched side first and then the compressed side. The negative piezoelectric potential produced at the stretched side makes the Schottky diode forward-biased, turning on the diode. Current consequently flows through the metal-semiconductor junction and to the external circuit. For the compressed side, the positive piezoelectric potential makes the Schottky diode
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cm −3 (1)
where n is the carrier concentration, ωL+ is the frequency of the L+ mode, and ωLO is the frequency of the uncoupled LO mode (ωLO = 735 cm−1). Figure 3 shows the Raman spectra of a series of GaN NWs grown at various Si dopant temperatures, where the L+ mode is clearly indicated along with the E2high mode
Figure 3. Raman spectra of unintentionally doped and doped GaN nanowires obtained at various Si cell temperatures.
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Table 1. Relationship between the Si cell temperature and free carrier concentration of GaN nanowires extracted from phonon-plasmon coupled mode position of Raman spectra. Si cell temperature [°C]
L+ [cm−1]
Free carrier concentration [cm−3]
0
736.00
1.10 × 1017
1200
747.50
7.58 × 1017
1300
844.23
3.97 × 1018
1350
907.78
5.63 × 1018
1400
1373.88
1.53 × 1019*
Is =
* Overestimated.
at 566.9 cm−1. Based on Equation (1), estimates of the n-type doping concentrations of the GaN NWs are listed in Table 1. The doping concentration varies by two orders of magnitude, starting from 1017 cm−3 for the unintentionally doped sample to about 1019 cm−3 for the most heavily doped sample. Note that the highest doping concentration is beyond the valid range of Equation 1, and likely has a large uncertainty. In the context of the working mechanisms, both SNWNGs and piezotronic devices rely on the same current– voltage curve of a Schottky diode under strain but stress on different operating region. For SNWNGs, the established inner-crystal piezoelectric potential as a current-flow driving force causes the Schottky diode turn on, and then the inner resistance of the material comes to dominate the current output. As for piezotronic devices, the piezoelectric charges mainly influence the Schottky barrier height and govern the diode current near the turn-on voltage. By collecting the change of the turn-on diode current, a force sensor made of a piezotronic device can be achieved. The free carrier concentration can govern inner resistance in accordance with semiconductor physics and Schottky barrier height based on screening effect simultaneously, and should be investigated step by step. Starting from SNWNGs, the doping concentration should affect the conductivity of GaN NWs and thus the output current characteristics of an SNWNG. Figure 4 shows the current outputs of GaN NWs with various doping concentrations obtained using AFM tip scanning. For the unintentionally doped case (carrier concentration = 1.10 × 1017 cm−3), the current output is close to the current detection limit and is regarded as no current generated. This also applies to the lightly doped case (carrier concentration = 7.58 × 1017 cm−3). As the carrier concentration is raised beyond this value, the current output starts to increase, reaching its maximum at a carrier concentration of 5.63 × 1018 cm−3. Of note, the current output then decays with further increasing doping concentration, here with the highest carrier concentration of 1.53 × 1019 cm−3. The results thus demonstrate that the output current of an SNWNG is sensitive to doping concentration. Figure 4f summarizes the statistical data on the current output over a few thousand NWs being scanned for each sample. The maximum current reaches up to 50 nA at a carrier concentration of 5.63 × 1018 cm−3, with variation of the current output due to different deformations during tip small 2014, 10, No. 22, 4718–4725
scanning. To the best of our knowledge, this is the highest reported output current generated from GaN NWs.[26] The carrier concentration has a huge impact on current output. Although the relationship between output voltage, inner resistance, and the resulting output current of an SNWNG has been previously determined with only a certain carrier concentration,[27] a series of detailed experimental approaches considering the carrier concentration have not been achieved yet. For the SNWNG considered here, the current output is governed by:[27] V0
(2)
( r0 + rc )
where V0 is the piezoelectric potential generated by the strain, Is is the output current, r0 is the inner resistance of the NW including the surface depletion effect,[28] and rc is the contact resistance of the metal-semiconductor interface. The carrier concentration in semiconductor NWs not only strongly modifies the inner resistance but also the carrier transport behavior. In addition, when the electron concentration is increased, the piezoelectric potential is degraded by the screening effect induced by free carriers.[21,22] As a result, the doping concentration affects three terms (V0, rc, and r0) non-linearly, with competition between the electron-concentration-induced inner resistance reductions including the surface depletion change with carrier concentration, contact resistance reductions as exponential dependence on barrier height, which is a function of carrier concentration,[29] and the decreasing piezoelectric potential induced by the screening effect. Therefore, the maximum output current is reached at a doping level of 5.63 × 1018 cm−3, beyond which the piezoelectric potential decreases and thus the output current drops. After investigation of output performances of SNWNGs, the carrier concentration influences carrier transport behavior through interfaces for piezotronic devices as well. The force-dependent current-to-voltage (IV) characteristics and the extracted SBH changes are shown in Figure 5. When a normal compressive force is applied, the SBH decreases since a positive piezoelectric potential is established at the top of the GaN NW in proximity to the metal-semiconductor (GaN-Pt) Schottky interface due to the polarity of the GaN NW. For simplicity, an ideal Schottky diode is assumed by neglecting shunt and series resistance. The thermionic current-voltage relationship is thus given by:[11]
(
I f = AA** T 2 exp −
)
⎛ qVf ⎞ ΦB exp ⎜ − 1⎟ k BT ⎝ nideal k BT ⎠
(3)
where A is the area of the Schottky diode, A** is the effective Richardson constant, T is the temperature, ΦB is the SBH, kB is the Boltzmann constant, q is the electron charge, Vf is the voltage drop on the forward-biased Schottky diode, and nideal is the ideality factor. To verify whether the transport mechanism obeys the thermionic emission and diffusion (TED) model, the curves of logarithmic current versus voltage are shown in the inset of Figure 5b. They confirm that all IV characteristics are dominated by TED. Under the assumption that a change of A** due to the normal force is much smaller than
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Figure 4. Electrical current collected by C-AFM tip scanning of GaN nanowires with electron concentrations of a) 1.10 × 1017, b) 7.58 × 1017, c) 3.97 × 1018, d) 5.63 × 1018, and e) 1.53 × 1019 cm−3. f) Statistics of the current output from (a) to (e).
the strain-induced change in the SBH, the change of the SBH can be derived using Equation 4.[11] The force-dependent SBH change with respect to carrier concentration is shown in Figure 5d. ⎛ I f ( F1 ) ⎞ ΔΦ s ln ⎜ ≈− k BT ⎝ I f ( F2 ) ⎟⎠
(4)
where If(F1) and If(F2) are the forward currents at the normal force of F1 and F2, respectively, and ΔΦs is the change of the SBH. From Figure 5d, the change of the SBH increases linearly with force up to 3 nN for all samples, and further discussion is provided in Supporting Information. More importantly, for a given force, the change of the SBH gradually decreases with increasing carrier concentration. This is mainly caused by the screening effect since the strain state should be identical
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under a given normal force for all samples, where they are almost identical in diameter and length with evidences provided in Supporting Information, and hence should induce equivalent numbers of piezoelectric charges. The higher the free electron concentration, the more the piezoelectric potential is screened, leading to smaller changes in the SBH, as shown in Figure 5c. The force sensitivity values calculated with respect to the SBH change for carrier concentrations from 1017 to 1019 cm−3 are from 26.20 ± 1.82 to 14.70 ± 2.21 meV nN−1, which are equivalent to 1.34 ± 0.58 and 0.90 ± 0.13 ln(A) nN−1, respectively. Notably, the force sensitivity can be decreased by almost two times by increasing the carrier concentration. The maximum force sensitivity in this work is the highest one among all previously reported results as listed in Table 2.[9,11,30–32] Of which, one of the essential strategies for boosting the force sensitivity is to design the NWs geometrically to change its deformation type from
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Figure 5. a) Electrical circuit diagram of force-dependent IV measurement for determining piezotronic effect along with the corresponding energy band diagram. b) IV characteristics of GaN nanowires with carrier concentration of 3.97 × 1018 cm−3 under normal forces from ≈0 nN to –3 nN. Inset is the logarithmic current versus voltage curve accompanied with linear fit curve upon normal force from ≈0 nN to –3 nN. c) Energy band diagram of Schottky diode to compare changes in SBH for various carrier concentrations under a given normal force. d) Changes of SBH with force extracted from force-dependent IV characteristics for three carrier concentrations.
normal to bending force in oblique InN nanorods (NRs).[9,31] Nevertheless, in our findings, the carrier concentration is also critical and more versatile in tuning the force sensitivity, which can be extensively altered from 14.70 ± 2.21 to 26.20 ± 1.82 meV nN−1, implying that carrier concentration plays an crucial role in piezotronic devices as well. More detailed discussions on the effect of carrier concentration on force sensitivity is provided in Supporting Information. Figure 5d not only demonstrates the changes in the SBH, but also gives deep insights for piezoelectric SNWNG and piezotronic applications. In piezotronic devices, such as a force sensor, a high SBH change is required in response to an applied force since the sensitivity is determined by it.[30] Accordingly, a low carrier concentration is preferred. The Table 2. Comparison of force sensitivity with the literature. Materials
Force sensitivity [meV nN−1]
Forcing method by AFM tip
≈0.275
Normal force
30
≈0.962
Bending force
Oblique InN NRs 9,31
≈25
Bending force
ZnO NW 32
0.017 to 0.12 (depending on length and size)
Normal force
GaN NW (This work)
14.70 ± 2.21 to 26.20 ± 1.82 (depending on carrier concentration)
Normal force
CdSe NW 11 GaN NW
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force sensitivity reached as high as 26.20 ± 1.82 meV nN−1 in this study. On the other hand, for SNWNGs, the inner and contact resistances critically affect the overall output efficiency. As a consequence, to obtain high output power from SNWNGs, the inner and contact resistance should be lowered by increasing the carrier concentration to an optimum level. In this study, the maximum output current of a GaN SNWNG was up to 50 nA at a carrier concentration of 5.63 × 1018 cm−3. A relatively high carrier concentration is more favorable for SNWNG applications whereas a low carrier concentration is more favorable for piezotronic applications. Notably, the piezoresistance effect is not the dominant driver responsible for both SNWNGs and piezotronic devices because the asymmetrical current changes involved in forcedependent IV curves provide the strong evidence for piezoelectric effect. The close relation of both mechanisms to carrier concentration is discussed and concluded as follows. For SNWNGs application, the current is generated based on the slope of IV characteristics of a turn-on Schottky diode when the C-AFM tip contacts with the negative piezopotential side of a GaN NW during a tip scanning process. Consequently, the inner and contact resistances governed by carrier concentration are critical for electricity generation because they dominate the slope of IV curve. As for piezotronic devices, the SBH change is extracted from the IV characteristics
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of a turn-on Schottky diode of the same type as SNWNGs. However, the carrier concentration here acts as an interfacial barrier modification layer, which controls the electron transport, the contact resistance, and mainly the turn-on voltage of the Schottky diode that provides the versatility of tuning its force sensitivity. Both devices are established on the turn-on characteristics of a Schottky diode but with different working mechanisms. This implies that changing the slope of the IV curve is related to the bulk property and shifting the turnon voltage is referred to the interfacial property, and the carrier concentration can influence both simultaneously. These results give us a guideline toward designing a SNWNG with a giant output power and a piezotronic device possessing of a ultra-high force sensitivity.
by scanning electron microscopy (SEM, Hitachi SU-8000) and the crystal structure was studied using an aberration-corrected transmission electron microscope (TEM, JEOL JEM-2100F) equipped with a probe Cs-corrector. Conductive AFM (C-AFM, SII SPA-400) with Pt-Ir-coated tips (spring constant = 0.2 N m−1) was used to characterize the output current and force-dependent current-to-voltage (IV) characteristics. All testing conditions including tip scanning speed (0.7 Hz), tip down force (–1 nN), and so on, were kept the same over a scan area of 2 µm × 2 µm for each sample in order to map out the current output distribution and the results are able to be compared fairly. Micro-Raman spectroscopy (HORIBA Jobin Yvon/Labram HR) was utilized to determine the doping concentration of the Si-doped GaN NWs.
3. Conclusion Supporting Information GaN NWs with various electron concentrations were synthesized by PA-MBE to investigate the impact of carrier concentration on SNWNGs and piezotronic devices. The carrier concentration of each sample was determined from the phonon–plasmon coupled mode of Raman peaks. From C-AFM experiments, the maximum output current, obtained at a carrier concentration of 5.63 × 1018 cm−3, was ≈50 nA, which is the highest current ever reported. The optimal output current is explained by the competition between the inner and contact resistance reductions, and the carrier screening effect on the piezoelectric potential. For piezotronic devices, the highest change in the SBH occurs at the lowest carrier concentration of 1.1 × 1017 cm−3, reaching 26.20 ± 1.82 meV nN−1, equivalent to 1.34 ± 0.58 ln(A) nN−1, which is the best value ever reported for GaN NW-based force sensors without optimization. The results show that the doping concentration in semiconductors plays a crucial role in the performance of SNWNGs and piezotronic devices via changes in the inner and contact resistance, and screening effect on the piezopotential. Overall, a relatively high but optimized carrier concentration is favorable for SNWNG applications owing to smaller inner resistance, whereas piezotronic applications require low carrier concentration due to the necessity of a huge change in the SBH.
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This research was supported by the National Science Council under Grant NSC-101–2221-E-006–131-MY3, NSC-101–2622-E-006– 015-CC1, and NSC-101–2622-E-006–005-CC1. The authors also wish to thank the Center for Micro/Nano Science and Technology, National Cheng Kung University, and the NSC Core Facilities Laboratory for Nano-Science and Nano-Technology in Kaohsiung-Pingtung area for providing equipment and technical support.
[1] [2] [3] [4] [5] [6] [7]
4. Experimental Section
[8]
Arrays of GaN NWs were grown on a 2-inch highly doped n-type Si(111) substrate using a radio-frequency plasma-assisted molecular beam epitaxy (PA-MBE) system (SVTA 35-Nitride MBE). The NWs were grown at high substrate temperatures via diffusion control under a nitrogen-rich environment without using any catalysts. The nitrogen flow, plasma generator power, Ga cell temperature, and substrate temperature were 2 sccm, 350 W, 910 °C, and 760 °C, respectively. For fair comparisons, all the GaN NW samples were grown under the same conditions except for the dopant flux for 2 h. Silicon was used as the n-type dopant. The silicon cell temperature was varied from 1200 to 1400 °C to control the vapor pressure in order to increase electron concentration. Once the n-GaN NWs were grown, the morphologies were investigated
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