Appl. Phys. A 85, 255–263 (2006)
Applied Physics A
DOI: 10.1007/s00339-006-3709-7
Materials Science & Processing
Structural and electrical properties of core–shell structured GaP nanowires with outer Ga2O3 oxide layers
b.-k. kim1 h. oh1 e.-k. jeon1 s.-r. kim1 j.-r. kim1 j.-j. kim1,u j.-o. lee2 c.j. lee3
1
Department of Physics, Chonbuk National University, Jeonju 561-756, Korea Advanced Material Division, Korea Research Institute of Chemical Engineering, Daejon 305-600, Korea 3 Department of Electrical Engineering, Korea University, Seoul 136-713, Korea 2
Received: 30 April 2006/Accepted: 3 July 2006 Published online: 20 September 2006 • © Springer-Verlag 2006 ABSTRACT This paper presents a review of our current experimental research on GaP nanowires grown by a vapor deposition method. Their structural, electrical, opto-electric transport, and gas-adsorption properties are reviewed. Our structural studies showed that a GaP nanowire consisted of a core–shell structure with a single-crystalline GaP core and an outer Ga2 O3 layer. The individual GaP nanowires exhibited n-type field effects. Their electron mobilities were in the range of about 6 to 22 cm2 /V s at room temperature. When the nanowires were illuminated with an ultraviolet light source, an abrupt increase of conductance occurred resulting from carrier generation in the nanowire and de-adsorption of adsorbed OH− or O− 2 ions on the Ga2 O3 surface shell. Using an intrinsic Ga2 O3 shell layer as a gate dielectric, top-gated GaP nanowire field-effect transistors were fabricated and characterized. Like other metal oxide nanowires, the carrier concentration and mobility of GaP nanowires were significantly affected by the surface molecular adsorption of OH or O2 . The GaP nanowire devices were fabricated as sensors for NO2 , NH3 , and H2 gases by using a simple metal decoration technique. PACS 73.63.-b;
1
72.80.Ey; 85.35.-p
Introduction
Gallium phosphide (GaP) is a wide band gap semiconductor with an indirect energy gap the size of 2.24 eV at room temperature. GaP has an important application as a light-emission device in the visible range [1]. Bulk GaP has a relatively good electron mobility of 160 cm2 /V s at room temperature. It has various kinds of impurities such as Mg, Se, and Sn that can also be doped to tailor the electrical properties [2]. Like other semiconducting nanowires, a GaP nanowire is expected to provide many opportunities for us to use in building nanoscale optical, electronic, and various sensor device applications. Based on these physical properties and potential applications, researchers have focused a large effort to synthesize GaP-based nanowires, tubes, and coaxial nanowires using various methods [3–10]. However, although there have been many reports on electronic devices based u Fax: +82-63-270-3320, E-mail:
[email protected]
on semiconducting nanowires [11–13], only a few electronic devices made of GaP nanowires have been reported despite their good and flexible electrical properties for potential use in nano-electronic device applications [14–16]. Here we present the basic structural and electrical properties of single-crystalline GaP nanowires grown by a vapor deposition method. 2
Synthesis of core–shell structured GaP nanowires with a Ga2 O3 outer layer
High-quality single-crystalline GaP nanowires were grown by a simple vapor deposition method. The GaP nanowires were synthesized on an Al2 O3 substrate (10 mm × 5 mm in size) where NiO or CoO catalyst nanoparticles were evenly distributed. First, a nickel (or cobalt) nitrate/ethanol solution (concentration 0.01 M) was dropped onto the surface of the Al2 O3 substrate. After drying the substrate at 400 ◦ C in ambient air, the nanosized NiO or CoO catalyst particles were formed on the Al2 O3 substrate. Then, the catalyzed Al2 O3 substrate was put on the top of a quartz boat filled with a mixture of Ga/GaP powder (Ga: 99.999%, GaP: 99.99%, Sigma-Aldrich, Ga:GaP = 1 : 1 (volume ratio)). The catalyzed Al2 O3 substrate was placed near the Ga/GaP source with about 3 – 5 mm distance. After loading the quartz boat into a quartz tube of a furnace system, the furnace tube was evacuated by a rotary pump. The temperature was increased, at a rate of 35 ◦ C/min, to preset a reaction temperature. The GaP nanowires were synthesized in the temperature range of 850 – 1000 ◦ C for 60 min under a constant flow of Ar (500 sccm). After the reaction, white-colored materials appeared on the surface of the alumina substrate. The wirelike products observed on the substrate were randomly oriented nanowires. The nanowires had an average diameter of about 50 nm and lengths up to hundreds of micrometers. The nanowires had uniform diameters and showed very clean surfaces without any amorphous material deposits as shown in scanning electron microscope (SEM) images. Energydispersive X-ray spectroscopy (EDX) was employed to analyze the nanowires. The results indicated that the nanowires were composed of Ga, P, and O elements [6]. Transmission electron microscopy (TEM) analysis is more useful in obtaining detailed information on the structure of nanowires. Figure 1a shows a low-magnification TEM
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Applied Physics A – Materials Science & Processing FIGURE 1 (a) A low-magnification TEM image of one of the GaP nanowires. (b) A medium-magnified TEM image of a typical GaP nanowire consisting of a GaP core and an outer oxide layer. (c) A HRTEM image of the core (GaP) and the outer shell layer (gallium oxide). (d) A HRTEM image of the microstructure of the core (GaP) and the outer shell layer (GaPO4 and Ga2 O3 ). The inset shows the SAED pattern
image of one of the fabricated GaP nanowires. Figure 1b and c show a medium-magnified TEM image of these GaP nanowires. As shown in Fig. 1b and c, a GaP nanowire has a coaxial structure of GaP and gallium oxide. We surmised that the outer gallium oxide layer might have grown during the reaction process in the quartz tube as a result of oxygen elements introduced from supplying a carrier gas. We used a normal grade Ar carrier gas (99.9%). We considered that the oxygen elements originated from residual oxygen in the apparatus at a high growth temperature. Figure 1d shows a high-resolution TEM (HRTEM) image which indicates the microstructure of the core and the outer oxide layer. The HRTEM and selected-area electron diffraction (SAED) clearly demonstrated that the core was zinc blende-structured GaP and the outer oxide layer consisted of a double layer of orthorhombic structured GaPO4 and amorphous Ga2 O3 layers. The thickness of the outer oxide layer was about 4 – 6 nm. We presumed that the GaPO4 and Ga2 O3 layers were formed on the surface of the GaP nanowire during the reaction process in the quartz tube. 3
urements. Figure 2 shows a typical SEM image of a GaP nanowire with two Ti/Au electrodes. The typical diameters of the GaP nanowire, except the outer oxide layer, were in the range of 30 to 50 nm. Figure 3a and b show the source–drain current change as a function of gate voltage (Vg ) in the range of ±10 V at 280 K and 80 K, respectively. The insets in Fig. 3 show the logarithmic plots of the I –Vg curves and the I –V curves as a function of Vg at each temperature. An application of a positive gate voltage increased the conductance. This behavior is a signature of an n-type field-effect transistor. Such an ntype electrical property in an undoped GaP nanowire results from natural defects such as the P vacancies and Ga interstitials. Similar intrinsic n-type gate responses resulting from vacancies have also been observed in GaN, In2 O3 , and ZnO nanowires [11–13, 17]. When Vg was smaller than − 7.5 V at the source–drain bias voltage of V = + 1.5 V, the device was fully depleted at 280 K. The device showed good turn-on characteristics with a large on/off ratio greater than 8 × 104 (= 800 nA in the on state and 10 pA in the off state) at 280 K.
Basic electrical transport properties of GaP/Ga2 O3 nanowires
A basic study of the electrical properties of GaP nanowires is crucial for developing their future applications in nanoelectronics. Electrical measurements were performed on individual GaP nanowires with a Ga2 O3 oxide shell (GaP/Ga2 O3 ). The single nanowire was configured as a field-effect transistor (FET) for the basic study of transport properties. Individual GaP nanowires were prepared on a heavily doped Si substrate covered with a 500-nm-thick thermally grown SiO2 layer. The Si substrate was used as a back gate to control the source–drain current. Since the GaP nanowire had an outer oxide layer of thickness 4 to 6 nm, we removed the Ga2 O3 layer by chemical etching before the metal electrode (Ti/Au) deposition for the electrical meas-
FIGURE 2
electrodes
The SEM image of a typical GaP nanowire with two Ti/Au
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I–Vg curves as a function of V at (a) 280 K and (b) 80 K, respectively. The source–drain bias varies from + 1.5 V to − 1.5 V with the 10 voltage steps. The upper insets show the logarithmic plot of the I–Vg curves for the positive source–drain bias voltage. The lower insets show the corresponding I–V curves as a function of Vg = +10 V to −10 V with the 10 voltage steps FIGURE 3
The on-state linear resistances of our devices were typically in the range of 0.5 to 5 MΩ at room temperature. Figure 3b shows the device’s characteristics at 80 K. The threshold voltage at V = + 1.5 V for the complete depletion increased from − 7.5 V at 280 K to 0 V at 80 K. The device exhibited a rapid decrease of the number of charge carriers in the channel. The electron mobility of this device at 280 K was estimated to be about 6.7 cm2 /V s. It decreased to 1.4 cm2 /V s at 4.2 K. As the temperature was lowered, the mobility decreased with a T 3/2 dependence in the high-temperature region (T > 100 K). The mobility deviated from T 3/2 de-
pendence in the lower-temperature region (T < 100 K) as shown in Fig. 4a. A T 3/2 dependence of the carrier mobility is known to originate from impurity scattering [18]. The carrier density of our nanowire was in the range of 1017 – 1018 cm−3 . The presence of such large intrinsic vacancies and defects introduced a significant amount of scatterings even at room temperature. The typical values of the mobilities of the GaP nanowire FETs at room temperature were in the range of about 6 – 22 cm2 /V s. Although this value was slightly higher than that of the B-doped silicon nanowire [19], it was much lower than the electron mobility of bulk GaP (160 cm2 /V s) and other semiconducting nanowires such as
FIGURE 4 (a) Electron mobility as a function of temperature which followed a T 3/2 dependence in the high-temperature region (T > 100 K). (b) Temperature-dependent I–V curves of the GaP nanowire channel. (c) Conductance as a function of the inverse temperature (1/T ). (d) Schematic band diagram at the GaP/metal electrode
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Applied Physics A – Materials Science & Processing
GaN and ZnO [11, 20]. Such a low carrier mobility suggested a diffusive nature of electron transport in the GaP/Ga2 O3 nanowire, possibly because of the surface scatterings between the outer GaO and the inner crystalline GaP core. The temperature-dependent transport properties of GaP nanowires have also been studied in order to explain their detailed transport mechanism. Figure 4b presents the typical temperature-dependent current–voltage ( I –V ) characteristics of a GaP nanowire. As the temperature was lowered, the I – V characteristics became highly non-linear and exhibited gap structures in the range of −27 mV < V < 26 mV at 4.2 K. The temperature-dependent conductance, G(T), was fitted to a thermal activation form, G(T) ∼ exp(−E a /kB T ), with an activation energy of E a = 7.6 meV, in the high-temperature region (80 K < T < 300 K) as shown in Fig. 4c. G(T) had a different slope at low temperatures. The activation energy, E a , obtained from fitting of the high-temperature conductance to the thermal activation form, was related to the energy barrier ( E B ) between the conduction-band edge of the nanowire, E C , and the Fermi level of the metal electrode, E B ≈ E CF = E C − E F . E CF acted as an energy barrier to overcome electrons passing through the nanowire (Fig. 4d). When the thermal energy kB T was larger than the energy barrier, E B , thermal activation was a dominant transport mechanism. However, when kB T became smaller than the energy barrier (in our case T < 80 K), thermally activated transport was suppressed. Other transport mechanisms, such as the quantum tunneling of electrons through the energy barrier, possibly also played an important role in electron transport through the barrier [13]. For further clarification of the electrical transport properties of the nanowires, we measured the conductance of a GaP nanowire as a function of back-gate voltage, Vg , at several temperatures as shown in Fig. 5. As the temperature decreased below 77 K, the aperiodic current oscillations started to appear, along with the background n-type gating effect. As the temperature was lowered, the oscillations became clearer. The observed oscillations were related to the Coulomb oscillations. A single-electron charging effect was expected in this system, as we assumed a weak coupling between the nanowire and the metal electrodes inferred from a contact barrier resistance far greater than the quantum resistance (∼ 25 kΩ) at low temperatures. The contact barriers blocked the electron from tunneling into the nanowire, thus yielding a Coulomb island between the two confined metal electrodes [21]. A rough estimation of the single-electron
charging energy of the nanowire was obtained from the relations E c = e2 /2C = 0.48 meV, where the capacitance of the nanowire was given by C ≈ 2πε0εL/ln(4h/d) = 168 aF, where L was the source–drain length (3 µm); the variable, d , was the diameter of the nanowire (38 nm); and h and ε were the thickness (500 nm) and dielectric constant (4) of the SiO2 . Therefore, the Coulomb oscillations were expected to be observed below T = 5.53 K, which was much lower than our experimental observation (T < 77 K). The charging energies determined from the geometrical factor were less than the experimentally determined value, which could be attributed to the multiple island formation within the nanowire [21]. 4
Top-gated GaP FET with a Ga2 O3 gate dielectric layer
Using the fact that our GaP nanowires had a core– shell structure with the outer Ga2 O3 shell, it was possible to fabricate top-gated GaP FETs using an intrinsic Ga2 O3 shell layer as a gate dielectric. The top-gate electrodes were fabricated by defining electrode patterns between source and drain electrodes with an extra electron beam lithography step and Au lift-off without the oxide etching process. Highly effective channel–gate coupling was expected in the top-gated GaP FET. Figure 6a shows the SEM image of the typical top-gated GaP FET. As is shown in the schematic diagram of Fig. 8b, source and drain electrodes were defined on the GaP layer after the chemical etching process. Top-gate electrodes were patterned directly on the outer Ga2 O3 layer. Figure 7 a and b show the current–voltage ( I –V ) characteristics from the GaP nanowire device in Fig. 6 between electrodes 1 and 4 at different top- and back-gate voltages. In this experiment, top gates were adopted as gate electrodes. With a top gate, the conduction channel in a GaP nanowire FET closed almost completely around − 1.5 V. Here, the same gate-bias voltages as the top gate were used for comparison. It is clear that the top gate showed much better gate coupling as compared with the back gate. The trans-conductance value by a local top gate was about 62 nA/V. If we assumed that the top gate covered a large portion of the nanowire’s surfaces, we were able to approximate the system as a coaxial cylinder. The capacitance of a coaxial cylinder was calculated as C = 2πεε0 L/ln(rb /ra ), where L is the length of the cylinder and ra and rb represent the outer and inner radii of the cylinder, respectively. If we assumed
FIGURE 5 (a) The current between source and drain electrodes as a function of Vg at several temperatures. The bias voltage was 20 mV. (b) I–Vg curves at 4.2 K. The bias voltage varied from 0 to +100 mV with the six voltage steps
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Structural and electrical properties of core–shell structured GaP nanowires with outer Ga2 O3 oxide layers
FIGURE 6 (a) A SEM image of a typical local top-gated GaP nanowire FET. (b) A schematic diagram of a top-gated GaP nanowire FET. While the electrodes 1 and 4 were used as drain and source electrodes, 2 and 3 were used as top gates
that the dielectric constant of our Ga2 O3 shell was 10, the length of the cylinder 3.5 µm, and the inner and outer radii of the core–shell GaP nanowire 38 nm and 50 nm, respectively, the capacitance of a coaxial GaP nanowire was around 7.1 × 10−15 F. This capacitance was an order of magnitude larger than the back-gate capacitance, 2.34 × 10−16 F. The calculated mobility from this device was around 7 cm2 /V s [15]. 5
Hetero-junction between the GaP core and Ga2 O3 outer layers
When we applied high bias voltages between the contact electrode and the top gate, current started to flow. Figure 8a shows I –V characteristics measured between electrodes 1 and 2 (source and top gate). Highly non-linear, diodelike I –V behavior were observed, which supported the breakdown of Ga2 O3 as an insulating layer. The current increased
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after the threshold voltage of + 1.2 V for the forward bias condition. Breakdown did not occur over −3 V for the reverse bias condition. To explain such a rectifying behavior, we modeled the system as a hetero-junction of two semiconductors with different gap energies. Figure 8b shows a schematic diagram of the system. When n-doped smaller gap GaP (∼ 2.7 eV) was brought into contact with slightly doped wide gap Ga2 O3 (∼ 3.8 eV), the Fermi level ( E F ) was constant across the junction achieved by transferring electrons from the smaller gap GaP to Ga2 O3 . This made band bending of the conduction band and valence band as illustrated in the schematic diagram. The current increased exponentially in the forward bias region, resulting from the lowering of the barrier for electron transport. The barrier height increased for the reverse bias regions. We also studied the gate dependence of the rectifying junction. Figure 8a shows I –V curves from the GaP/Ga2 O3 hetero-junction with back-gate voltages from −2 V to 2 V. The current from the GaP/Ga2 O3 hetero-junction increased with positive back-gate bias, indicating an n-type gate response. Such an n-type response can be expected in a sense that our GaP nanowire acted as an n-doped semiconducting channel. When we applied negative gate bias to the heterojunction, electron depletion occurred in the GaP layer, thereby suppressing electron transfer from the GaP to the Ga2 O3 layer [15]. 6
The photo-electric effect of GaP/Ga2 O3 nanowires
Since GaP has a direct band gap of 2.78 eV at k = 0 and an indirect energy gap of 2.24 eV, we expected that the electrical transport through our device should be very sensitively affected by exposure to light having a wavelength shorter than the cut-off wavelength of about 554 nm. To study the photo-response of our device, we measured the source– drain current under exposure to ultraviolet (UV) light in air. To completely deplete the carriers in the nanowire, we first applied a sufficiently negative bias voltage, Vg = −15 V, to the gate. As a result, the nanowire then became non-conducting. Next, we measured the I –V characteristics of the GaP FET in the dark state and under UV exposure. When the device was exposed to the UV light source (200–600 nm), the resistance in the nanowire decreased abruptly from a non-conducting state to 0.5–1 MΩ at room temperature. Figure 9 shows the time-dependent photo-response of our device as the UV lamp
(a) I–V curves of the GaP nanowire between electrodes 1 and 4 as a function of the top-gate voltage Vg . Top-gate voltages in the range of − 1.4 V to + 1.4 V were applied with 10 voltage steps. (b) I–V curves of the GaP nanowire as a function of back-gate voltage. We applied a range of − 1.4 V to + 1.4 V backgate bias for comparison with the 10 voltage steps [15] FIGURE 7
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Applied Physics A – Materials Science & Processing (a) I–V curves between contact electrode and top gate with back-gate voltages from −2 V to 2 V with the 10 voltage steps. Inset: current change as a function of backgate voltage. (b) Schematic band diagram for GaP/Ga2 O3 junction [15]
FIGURE 8
was switched on and off. This change showed the reversible switching actions of the GaP/Ga2 O3 nanowire between the high- and low-resistance states. Next, we investigated the effect of UV light upon the gate-modulation curve by using several optical filters. Figure 10a shows the I –Vg curves of the GaP FET upon exposure to UV light with a WG-320 (λ ≥ 400 nm), a WG-320 (λ ≥ 320 nm), and a WG-250 (λ ≥ 250 nm) long wavelength pass filter, respectively. The threshold gate voltage of −10 V for the initial off state shifted to −15 V upon exposure to UV light with λ ≥ 320 nm. By tuning the gate-bias voltage, we were able to select the particular UV range to give channel current through the nanowire. For example, when the gate-bias voltage was applied at −15 V (−12 V), only the λ ≥ 225 nm (320 nm) UV light delivered channel current (Fig. 10b). From the threshold voltage shift in the gate-response curves, it was possible to estimate the change of carrier concentrations resulting from UV exposure with λ ≥ 320 nm by the simple relation ∆n = C∆Vt /L . From the channel length of the device L = 1.34 µm, C = 8.75 × 10−2 fF, and ∆Vt = 5 V, a carrier concentration change coming from UV exposure with λ ≥ 320 nm was estimated as n ∼ 3.34 × 10−1 nm−1.
To investigate the origin of the UV response from our GaP/Ga2 O3 nanowires, we measured the current through the Ga2 O3 layer by having the electrodes at the surface of the Ga2 O3 outer layer directly without the chemical etching process, not the GaP core region. Figure 11a and b show current measured from two different devices with the electrodes on the Ga2 O3 layer. As shown in the figure, a very sharp increase in conductance occurred with UV illumination. When the sample was illuminated with UV with λ ≥ 320 nm, a small step-like increase was observed, while a sharp increase of conductance occurred with λ ≥ 225 nm of UV exposure. Such an abrupt increase of conductance was related to the charge carrier generation in the outer Ga2 O3 shell layer with UV illumination. Recently, there was a report of a photodetector based on Ga2 O3 nanowires which indicated very fast recovery and response times [22]. The physical background for the photo-responses in GaP/Ga2 O3 nanowires can be explained by two major mechanisms. The first mechanism is the electron–hole pair generation by UV illumination with higher energy than the band gap of the GaP nanowire. The other mechanism for conductivity enhancement could be the O2 or OH adsorption that occurred on the nanowire surface. Our GaP nanowires had a Ga2 O3 surface shell that was normally depleted as a result of molecular adsorption in the ambient atmosphere. Upon expo− sure to UV light, the surface adsorbed O− 2 or OH ions, that provided depletion of electrons in the Ga2 O3 shell and then recombined with photo-generated holes to give back the O2 molecules. In this case, the Ga2 O3 shell also contributes in part as a conducting channel in addition to the semiconducting GaP core channel. 7
FIGURE 9
on and off
Current change in the nanowire when the UV source was turned
Adsorption of molecules on the surface of the GaP/Ga2 O3 nanowires
Several kinds of defects or vacancies on the metal oxide nanowires are very active electrically and chemically. The carrier concentration and mobility are significantly affected by the surface molecular adsorption [23–25]. The surface Ga2 O3 outer layer in our GaP nanowire was also exposed to molecular adsorption. With surface adsorption of molecules at the vacancy sites, electrons will be transferred from the surface of the nanowire to the physically
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FIGURE 10 (a) I–Vg curves measured with UV
off and UV illumination with λ ≥ 225 nm, 320 nm, and 400 nm optical filters. (b) Channel current change with changing optical filters, λ ≥ 225 nm, 280 nm, 320 nm, and 400 nm at Vg = −15 V. Only λ ≥ 225 nm delivered the channel current at Vg = −15 V
FIGURE 11 The currents through the GaO outer layer as a function of time resulting from UV illumination from two different devices, (a) and (b) [15]
FIGURE 12 (a) I–Vg curves at different pressure
conditions. (b) I–Vg change during F2 adsorption at every 1 s
adsorbed OH or O2 , etc., leading to the surface depletion layer [26]. First, we explored the effect of ambient air on the electrical transport properties of the GaP nanowires. Electrical measurement was carried out in a vacuum chamber which we evacuated down to 10−4 Torr. After pumping down the chamber to 10−4 Torr from the atmosphere, the threshold voltage (Vt ) shifted in the negative Vg direction by 3 V as shown in Fig. 12a. Such a negative shift in Vt during pumping was also observed in metal oxide nanowires such as ZnO and In2O3 nanowires [17, 27]. After exposing the sample to air for 30 min, Vt recovered to the original Vt in the ambient air. While the mobility of the device that can be estimated from the slope of the I –Vg curve did not show a significant change with pumping, the number of carriers increased greatly. We estimated the increased number of carriers by the simple relation ∆n = C∆Vt /L = 3 × 10−2 nm, with L = 1 µm and C = 10−2 fF. When the nanowire was exposed to a reactive gas molecule such as F2 , the channel current decreased significantly as shown in Fig. 12b. The number and, in particular, the mobility of the carriers decreased rapidly.
The shift of Vt was closely related to adsorption of molecules. In fact, reversible gas adsorptions were reported from bulk semiconductor gas sensors such as Ga2 O3 and SnO2 [28, 29]. This fact suggested that the shift of Vt in our device was associated with chemical doping by the adsorption of ambient air, especially OH or O2 , on the Ga2 O3 outer layer surface. For Ga2 O3 bulk sensors, H2 O and OH groups at the surface tended to attract electrons and form a positive space charge region. Therefore, the depletion layer resulted in increased resistance of the sensors. In our core–shell structured GaP/Ga2 O3 nanowires, the surface adsorption was expected to mainly occur on the surface of the Ga2 O3 outer shell. This surface absorption influenced the electrical transport of the channel. Second, we investigated the effect of oxidizing NO2 gas and reducing NH3 gas on the electrical transport properties of GaP nanowires. Unlike semiconducting single-walled carbon nanotubes that show a sharp increase of conductance with oxidizing NO2 gas and a decrease of conductance with reducing NH3 [30], pristine GaP nanowire devices do not exhibit such extreme sensitivity. However, it is possible to
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Applied Physics A – Materials Science & Processing FIGURE 13 An AFM image of a GaP nanowire after Pd decoration. About 2–4 nm sized Pd clusters were deposited on the GaP nanowire device
FIGURE 14 (a) I–Vg curves from the GaP nanowire
device before the Pd decoration at V = 1.2 V, 0.7 V, and 0 V, respectively. The inset shows a SEM image of the device. (b) Electrical transport characteristics of the GaP nanowire device before and after the Pd decoration
improve the sensitivity of a GaP nanowire device by using a simple metal decoration technique. Kolmakov et al. showed that they can improve the sensitivity of SnO2 nanowire gas sensors by decorating them with Pd nanoparticles [31]. Kim et al. showed an enhancement of gas sensitivity in singlewalled carbon nanotube based sensors by Al nanoparticle decorations [32]. The concept used in those previous articles was to tune the sensitivity of devices either by creating local nanoscale Schottky barriers or by using the chemical reactivity of catalytic metal particles. Therefore, we intentionally created local nanoscale Schottky barriers in our GaP nanowire devices by simply decorating them with Pd nanoparticles. Since GaP nanowire devices exhibit n-type transport behavior, it was possible to create local Schottky barriers by using Pd nanoparticle decorations, Pd being a typical high work function metal. To fabricate GaP nanowire devices functionalized with Pd nanoparticles, very thin (< 1 nm) layers of Pd were deposited directly on the GaP nanowire device by using a thermal evaporation method. Figure 13 shows an atomic force microscope (AFM) image of the Pd-decorated GaP nanowire. As shown in the figure, about 2–4 nm sized Pd nanoparti-
cles were evenly distributed on the GaP nanowire. Figure 14a and b shows electrical transport properties measured from a GaP nanowire device before and after the Pd decoration. The GaP device showed typical n-type transport behavior and we observed decreased conductance following the Pd decoration. Such a decreased conductance, following the Pd nanoparticle decorations, can be explained by the formation of nanoscale Schottky barriers. When Pd nanoparticles are adsorbed on a GaP nanowire, electrons are transferred from the GaP nanowire to the Pd nanoparticles. This appears as a decrease of conductance for an n-type GaP nanowire device. Therefore, we ‘shrunk’ the effective channel of the GaP nanowire by the Pd nanoparticle decorations. Figure 15 shows the sensor operations of the Pd-decorated GaP nanowire devices. As shown in Fig. 15a, a decrease of conductance with NO2 and an increase of conductance with NH3 gas have been observed from the Pd-decorated GaP nanowire device. This device did not show any response before the Pd nanoparticle decorations. Also, the H2 -sensitive increase of conductance was observed from the Pd-decorated device that was immune to H2 before the decoration (inset of Fig. 15b).
FIGURE 15 (a) Electrical response from the Pd-decorated GaP nanowire device for NO2 and NH3 exposure. (b) Hydrogen-gas sensitivity of the Pd-decorated GaP nanowire device. Here, the concentration of the H2 gas was 100 ppm
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Summary
This paper reported our research on the structural and electrical properties of core–shell structured GaP/ Ga2 O3 nanowires grown by simple vapor deposition. The individual GaP nanowires exhibited n-type field effects with an on/off ratio as high as 105 . Their electron mobilities were in the range of about 6 to 22 cm2 /V s at room temperature. The current in the nanowire increased abruptly from a non-conducting state under various UV illuminations. This resulted from carrier generation in the nanowires and de-adsorption of adsorbed O− 2 ions on the Ga2 O3 surface shell. The nanowires showed good reversible switching actions between high- and low-resistance states. By tuning the gate-bias voltage, we could choose the particular UV range to give channel current through the nanowire. The GaP nanowire devices having both a field effect and a gate-tunable optical switching effect could be very useful for building a new class of active nanoscale opto-electric devices. Top-gated GaP nanowire field-effect transistors, using an intrinsic Ga2 O3 shell layer as a gate dielectric, have been fabricated. As expected, top-gated GaP nanowire FETs exhibited more effective gate–channel coupling when compared to the conventional back-gated one. Diode-like I –V characteristics were observed between the source and top-gate electrode. This can be explained by the formation of a mid-gap GaP/wide-gap Ga2 O3 hetero-junction. Like other metal oxide nanowires, the carrier concentration and mobility of GaP nanowires were significantly affected by the surface molecular adsorption of OH or O2 . The GaP nanowire devices can be fabricated as sensors for NO2 , NH3 , and H2 gases by using a simple metal decoration technique. ACKNOWLEDGEMENTS This work was supported by the Electron Spin Science Center at POSTECH, and the Core Technology Development Project by ITEP, Korea.
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