Letter pubs.acs.org/NanoLett
Correlation of Polarity and Crystal Structure with Optoelectronic and Transport Properties of GaN/AlN/GaN Nanowire Sensors M. I. den Hertog,*,† F. González-Posada,‡ R. Songmuang,§ J. L. Rouviere,⊥ T. Fournier,† B. Fernandez,† and E. Monroy‡ †
Institut Néel CNRS/UJF UPR2940, BP 166, 25 rue des Martyrs, 38042 Grenoble cedex 9, France CEA-CNRS Group “Nanophysique et Semi-conducteurs”, INAC-SP2M, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France § CEA-CNRS Group “Nanophysique et Semi-conducteurs”, Institut Néel-CNRS, BP 166, 25 rue des Martyrs, 38042 Grenoble Cedex 9, France ⊥ CEA-INAC/UJF-Grenoble1 UMR-E, SP2M, LEMMA, PFNC-Minatec Grenoble F-38054, France ‡
ABSTRACT: GaN nanowires (NWs) with an AlN insertion were studied by correlated optoelectronic and aberration-corrected scanning transmission electron microscopy (STEM) characterization on the same single NW. Using aberration-corrected annular bright field and high angle annular dark field STEM, we identify the NW growth axis to be the N-polar [000−1] direction. The electrical transport characteristics of the NWs are explained by the polarization-induced asymmetric potential profile and by the presence of an AlN/GaN shell around the GaN base of the wire. The AlN insertion blocks the electron flow through the GaN core, confining the current to the radial GaN outer shell, close to the NW sidewalls, which increases the sensitivity of the photocurrent to the environment and in particular to the presence of oxygen. The desorption of oxygen adatoms in vacuum leads to a reduction of the nonradiative surface trap density, increasing both dark current and photocurrent. KEYWORDS: Nanowire, GaN, polarity, photocurrent, sensor, scanning transmission electron microscopy
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environment, two major differences are identified: (i) The average steady-state photocurrent measured in vacuum is higher than the one in air/oxygen. (ii) The transient photocurrent shows that the photoresponse in air or oxygen is faster than in vacuum. These observations are assigned to adsorbate-induced variations of the surface band bending and carrier lifetime. Regarding GaN NWs grown along the polar caxis, which is the dominant growth direction for wires synthesized by plasma-assisted molecular-beam epitaxy (PAMBE) and metalorganic chemical vapor deposition, several publications report the quenching of the photoluminescence (PL) in the air, compared to experiments in vacuum.18−20 In
emiconductor nanowires (NWs) are promising candidates for many device applications ranging from electronics1−3 and optoelectronics4−6 to energy conversion7−9 and spintronics.10−13 Their large surface-to-volume ratio can be used advantageously in sensor applications10−14 via direct probing of variations in their optical and/or electrical properties (e.g., photoluminescence, conductance/resistance, impedance) when exposed to various analytes. Especially the chemical inertness and robustness of GaN are highly desirable sensor characteristics, enabling device operation in extreme environments such as high temperature, radiation, and extreme pH levels.15,16 To date, several research groups have investigated the behavior of single GaN NWs as opto-/electrochemical transducers. Chen et al.17 have reported a dependence of the conductivity, photocurrent, and photocurrent decay time of single [10−10]-oriented GaN NWs synthesized by chemical vapor deposition on the measurement environment, notably the presence of oxygen. As a function of the measuring © 2012 American Chemical Society
Received: August 3, 2012 Revised: September 24, 2012 Published: October 3, 2012 5691
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contrast, the photocurrent from c-axis GaN NWs was found to be insensitive to oxygen/air exposure.21,22 An idea to enhance the sensing capabilities of NWs is to promote the current transport through the NW surface while suppressing the core conduction, for example employing NW axial heterostructures to create a potential barrier in the middle of the NW. For GaN NWs an AlN barrier is the most suitable material to inhibit the core-current transport because of the large conduction band offset (around 2 eV) between GaN and AlN, so that thermionic emission is negligible at room temperature. However, heterostructuring III-nitrides requires accounting for the presence of spontaneous and piezoelectric polarization fields which drastically modify the potential landscape and strongly influence the electron transport characteristics.23−27 For this reason it is necessary to identify the crystallographic orientation and the polarity of GaN-based NW heterostructures for a correct understanding of the device performance and proper sensor design. Currently, the determination of the polarity of PAMBE-grown GaN NWs remains under debate.28−32 In this Letter, we present a new NW sensor design, where the insertion of an AlN barrier is proposed as a material engineering solution to inhibit the electron transport through the NW core, confining the current close to the NW sidewalls. This surface conduction results in an enhancement of the photocurrent sensitivity to the environment, due to the amplification of the effect of adsorbates on the surface band bending and carrier lifetime. We report the direct correlation of optoelectronic and transport properties with structural properties measured by aberration-corrected scanning transmission electron microscopy (STEM) on the same single GaN-AlNGaN NW device, and we describe the effect of the polarity and heterostructure design on the optoelectronic transport characteristics of the sensor. Methods. Growth of n-i-n GaN NWs was carried out by PAMBE on Si (111) under N-rich conditions at a substrate temperature ∼790 °C.33 The NWs are defect-free and have a length of 1.2−1.5 μm with a diameter of 30−80 nm. For n-type doping of the NW extremities to ensure electrically reliable configurations using ohmic contacts, the Si cell temperature was set to the value that yields a Hall electron concentration of ∼2 × 1019 cm−3 in two-dimensional (2D) GaN layers. To perform the characterization of single nano-objects, the NWs were dispersed in ethanol solution by sonication on an array of homemade 50 nm thick Si3N4 membranes with a window size of 100 μm. Arrays of nitride membranes were fabricated starting from a 300 μm thick silicon (100) wafer with a layer of stoichiometric Si3N4 of 50 nm thick on each side deposited by low-pressure chemical vapor deposition (LioniX BV, The Netherlands). Then, windows and lines were opened on one side in the Si3N4 layer by UV lithography and a sequential reactive ion etching step to locally remove the Si3N4 layer. Subsequently, silicon was etched through the backside in a KOH bath at 80 °C for several hours, until the other Si3N4 layer was reached. The so-defined membrane arrays were cleaned in 65% HNO3 at 80 °C for 1 h. Single NWs deposited on the Si3N4 membranes were contacted using e-beam lithography and e-beam metallization. An Ar plasma was used directly prior to the metallization step to ensure a clean GaN− metal interface. The electrodes consisted of Ti/Al (10/120 nm). In a previous work26 we have connected homogeneous ndoped GaN NWs and n-i-n NWs using the above-described procedure and found linear I−V behavior and resistances in the
order of a few 100 kΩ, which can hence be considered as an upper limit for the contact resistance. In total, six single GaN NWs were properly contacted; three of them had the AlN barrier centered between the contacts whereas the other three had the AlN barrier very close to or underneath one of the contacts. In the literature, connected NW devices on commercial nitride membranes are described with Ge,34 bismuth,35 tungsten,36 and In2Se337 NWs. I−V characteristics were recorded using an Agilent 4155C semiconductor parameter analyzer. For photocurrent measurements the NWs were biased and connected in series with a load resistance (RL = 200 kΩ). The photocurrent signal was recorded using a TDS2022C oscilloscope, and the spectral response was characterized using a 450 W Xe-arc lamp coupled to a Gemini 180 monochromator. At the exit, the light was directed with an optic fiber and focused onto the NW device. The photocurrent as a function of the optical power was analyzed using an Ar+ laser (wavelength λ = 244 and 488 nm) as excitation source. All the measurements presented in this work were performed at room temperature. High angle annular dark field (HAADF) STEM was performed on a probe Cs corrected FEI Titan using a semiconvergence angle ∼17 mrad and inner and outer detector angles of ∼50−280 mrad, at acceleration voltages of 200 or 300 kV. Annular bright field (ABF) and HAADF STEM were performed on a probe corrected Titan Ultimate using a semiconvergence angle of ∼23 mrad and inner and outer detector angles of 15−24 mrad on the ABF detector, 31−103 mrad on the ADF detector (averaged image of Figure 1f), and 125−670 mrad on the HAADF detector at an acceleration voltage of 200 kV. Systematically, electronic transport and optoelectronic properties were characterized before STEM observations to avoid the modification of the NWs by the highenergy electron irradiation.38 We have simulated the potential profile in the NWs using the Nextnano3 8-band-k.p Schrödinger-Poisson equation solver.39 2D simulations were performed with the [000−1] axis along the z direction and the [10−10] axis along the x direction. The strain-minimization option was applied to the freestanding NW structure, so that the unit cells of the strained material are allowed to deform along all two spatial directions (x and z) in order to minimize the elastic strain energy. An air layer of 10 nm is defined around the structure to allow surface relaxation of strain. At the air/NW interface, a negative charge density of 2 × 1012 cm−2 is introduced to account for the charge at surface states (low estimation values for a chemically clean GaN msurface40), which are assumed to be located at the valence band edge.40,41 The GaN and AlN parameters used in the calculations are summarized in ref 42. NW Polarity Determination. Figure 1a shows a HAADF STEM low-magnification image of a contacted NW device on an electron-transparent silicon nitride membrane. The NWs under study are n-i-n structures consisting of two Si-doped ntype edges and an undoped middle section with a nominal length of 400 nm. The NWs grow tapered outward with the top diameter larger than the base diameter, as previously reported in the case of Si-doped NWs.43 A single 3−5 nm thick AlN barrier was inserted in the center of the undoped section. Highresolution images around the AlN barrier (the square region in Figure 1a) are displayed in Figure 1b,c. In Figure 1b, the NW is viewed along the [2−1−10] direction, whereas in Figure 1c the NW is tilted by 60° around its ⟨0001⟩ growth direction and viewed along the [11−20] zone axe (ZA). These two directions 5692
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AlN shell is visible as a darker line on the sidewall of the NW base. This AlN shell is covered by a GaN shell whose thickness is slightly different for the four observed NW {10−10} facets: 5.3 and 2.2 nm for the top and bottom (0−110) facets of Figure 1b, respectively, and 5 nm for the top and bottom (1− 100) facets of Figure 1c. These measurements are made in close proximity to the 3.6 nm thick AlN barrier. These shells are due to the AlN and GaN radial deposition during growth. Figure 1e presents a zoom of a high-resolution ABF STEM image of the NW, compared with the schematic description of the GaN crystal structure (Figure 1d). In spite of the small distance between the Ga and N columns (0.1119 nm) and noise in the STEM image, the Ga−N dumbbell can be observed in the ABF image, with darker contrast on the heavier Ga atom than on the lighter N atom. An averaged HAADF zoom is presented in Figure 1f together with an intensity profile in Figure 1g. The light N atom can be directly observed in the ABF image (Figure 1e), whereas in the HAADF image, as shown in ref 44, an intensity profile taken along the [0001] direction around a Ga column can be used to position the N columns: on one side of the Ga column, there is a small tunnel with a N column situated at 0.3229 nm while on the other side, there is the Ga−N bond and a N atom situated at 0.1956 nm. The side of the tunnel is characterized by a dip (indicated by green solid arrows) in the intensity profile (Figure 1g). Both dark and bright field atomic resolution images (Figure 1e−g) unambiguously demonstrate that these NWs grow along the [000−1] direction, in agreement with recent results by de la Mata et al.32 We did not observe any polarity inversion or any other crystallographic defect. Influence of the NW Polarity and Heterostructure on Transport Properties. Figure 2 summarizes the HAADF STEM structural and transport behavior characterization of three GaN NW devices with 3.6 nm thick AlN axial barriers in the center of the device. The average thickness of the GaN shell around the AlN sidewall barrier increases from 4 nm (device in Figures 2a,b), to 8 nm (device in Figures 2c,d), and to 10 nm (device in Figures 2e,f), as summarized in Table 1. (We refer to the contact geometry of these devices as “aligned” as the AlN barrier is well centered between the contacts.) Figure 2g compares the current−voltage (I−V) characteristics of these three devices. The thicker part of the GaN NW, the NW top, is systematically connected to ground (left side in Figure 2). To understand the asymmetry of the transport, we have simulated the potential profile in the NWs (Figure 3). Figure 3a illustrates the conduction band at zero bias. As the NWs exhibit
Figure 1. (a) HAADF STEM image of a contacted NW; the NW top is on the left. The inset schematically shows the two observation directions of the NW. (b, c) Zoom of the area around the AlN barrier indicated by the square shown in (a) viewed along [2−1−10] (b) and [11−20] (c). (d) Schematic representation of the GaN crystal structure viewed along [11−20]. (e) Convoluted atomic resolution ABF and (f) averaged HAADF STEM image obtained on the same NW viewed along the [11−20] axis with a superposition of the GaN atomic structure. (g) Intensity profile along the superimposed line in image (f).
of observation are compatible with a NW that has a hexagonal section with six {10−10} facets (inset Figure 1a), so that in each direction two separate sets of facets are visible (parallel to the electron beam). In these HAADF STEM images the darker areas correspond to the AlN barriers, as Al is a lighter element than Ga and therefore scatters fewer electrons on the HAADF detector. In both zoomed images (Figures 1b,c) a 0.5 nm thick
Figure 2. (a) HAADF STEM images (a, c, e) and corresponding zoom of the area around the AlN barrier (b, d, f) of NW devices I, II, and III, respectively. Devices I and III are viewed along [2−1−10] while device II is viewed along [10−10]. The diameter d at the AlN barrier and the respective average GaN shell thickness (“shell”) are indicated in (b, d, f). (g) Smoothed I−V characteristics measured in the dark. The top of the NW (left side in images a, c, and e) was connected to ground. 5693
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current under reverse bias is generally related to the surface leakage46 that involves dangling bonds, consistent with the fact that the lowest current level is observed for device II with the smallest diameter and corresponding surface area. By comparing aligned devices, as the device in Figure 4a,b with the AlN barrier centered between the metal contacts, with misaligned devices where the AlN barrier is close or underneath a metal contact and the other contact is located at the thicker top of the NW (Figure 4c,d), we can identify the influence of the AlN barrier on the current level. I−V measurements of these two kinds of devices, performed in the dark and under ultraviolet (UV) illumination, are represented in Figure 4e. In the dark, the current level is 3 orders of magnitude higher in misaligned devices than in aligned devices (see also Table 1), which confirms the blocking effect of the AlN barrier. Under illumination, misaligned and aligned NW devices present photocurrents on the order of 1 and 0.1 μA, respectively, at 1 V bias. This implies that the insertion of the AlN barrier results in an enhancement of the photodetector ON/OFF current ratio but is associated with a slight degradation of the photocurrent level, whose origin will be discussed in the next section. The insertion of the AlN barrier within the n-i-n GaN NW does not degrade the spectral selectivity of the structures: n-i-n GaN NWs incorporating an AlN barrier show a typical spectral response with a cutoff at around 360 nm wavelength and a visible rejection of more than 3 orders of magnitude (not shown), similar to the spectral selectivity obtained in n-i-n GaN NWs without an insertion.18 A particularity of n-i-n NWs with an AlN barrier centered between the contacts is that they present a photoresponse at zero bias, unlike the n-i-n NWs. A photovoltage of ∼0.5 μV is measured in device I, when illuminating with 100 μW/cm2 at λ = 350 nm wavelength, whereas no photovoltaic response is observed when exciting with visible light. This photovoltage is associated with the polarization-induced internal electric field, which separates the photogenerated electron−hole pairs in the vicinity of the AlN barrier. Sensitivity to the Environment: Prospects for Chemical Sensors. The insertion of an AlN barrier is a material
Table 1. Summary of Shell Thickness, Diameter, and Current Level at 1 V Bias Measured in Aligned Devices, Where the AlN Barrier Is Well Centered between the Contacts, and Misaligned NW Devices, Where the AlN Barrier Is Close to or Underneath a Contact device I II III a
contact geometry aligned aligned aligned misaligned devices
GaN shell (nm) 4 8 10
NW diam (nm) at the barrier position
current at 1 V (nA)
40 38 42 37−45
0.10 0.24 2.0 1100−10000
a
Range of diameters and current values derived from three misaligned devices.
N-polarity, the difference in spontaneous and piezoelectric polarization at the barrier results in a downward band bending with accumulation of electrons at the top GaN/AlN interface and an upward band bending at the bottom AlN/GaN interface (see Figure 3b). The band distortion extends into the GaN shell inducing an asymmetric potential barrier as shown in Figure 3c. Surface states result in a complete electron depletion of the undoped NW sections away from the barrier and an upward band bending at the NW sidewalls (see Figure 3d). For this reason the GaN shell becomes a conduction channel for holes, which is expected to be the dominant transport path. Indeed, for the presented AlN barrier thickness (3.6 nm) and knowing the conduction band offset of 2 eV between GaN and AlN, we can rule out both field-emission and thermionic transport through the barrier. The direction of forward conduction observed in Figure 2g is consistent with the band structure in the GaN shell (Figure 3c). Looking back at the experimental results in Figure 2g, a higher forward current is observed for increasing shell thickness (see also comparison in Table 1), which supports the hypothesis that the conduction takes place via the surface pathway created by the GaN shell. Conduction through a GaN shell has also been reported by Rigutti et al.45 in the case of GaN NWs containing GaN/AlN superlattices. The leakage
Figure 3. (a) Nextnano3 two-dimensional simulation of the NW conduction band profile. The simulated structure was based on the NW device I, taking into account the AlN and GaN shell dimensions and the AlN barrier thickness extracted from STEM images. (b) Conduction band edge profiles along z in the [0001] direction at the center of the NW at x = 32 nm, (c) along z in the [0001] direction in the center of the GaN shell at x = 12.5 nm, and (d) along x in the [10−10] direction at z = 49.6 nm. The N-terminated NW top is on the left, oriented in the same direction as the STEM images shown in Figure 2. 5694
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Figure 5. I−V characteristic from device I measured in the air and in vacuum, in the dark and under UV illumination.
enhanced effect of adsorbed oxygen on the surface-current transport in devices containing the AlN insertion. In the literature, there are several reports of an enhancement of both photocurrent and PL in vacuum, particularly in nonintentionally doped GaN NWs and in NWs with large (>100 nm) diameter, which is explained by taking into account a modification of the surface band bending at the NW sidewall facets.21,50,51 At ambient conditions, oxygen adsorbs on the NW surface and depletes a larger NW volume. Furthermore, oxygen reduces the nonradiative recombination time, resulting in a faster quenching of the NW PL emission in the air in comparison to experiments in vacuum.18,19 In the case of the NWs in this study, with diameters around 30−40 nm, their undoped sections should be entirely depleted, as predicted by our simulations (Figure 3) or just by applying the abrupt depletion approximation to a cylindrical structure.52−55 Therefore, the effect on the surface band bending is limited and the dominant mechanism modifying the photocurrent is likely to be the activation of a nonradiative recombination path associated with adsorbed oxygen. In vacuum, oxygen is desorbed from the surface, and hence increased photocurrent and longer photocurrent decay times are observed. Conclusions. In conclusion, we have presented a correlated study of the polarity and structural characteristics of single GaN n-i-n NWs with a single AlN insertion, their transport properties (in the dark and under illumination), and their sensitivity to the measurement atmosphere. High-resolution STEM images reveal the presence of an AlN/GaN shell structure around the GaN base of the NW. The nitrogen polarity of the NWs is unambiguously identified in aberrationcorrected high-resolution images obtained in ABF and HAADF STEM. The detailed structural analyses allow simulation of the NW potential profile and correlation with the physical properties of the device. The AlN insertion is shown to suppress the electron flow through the GaN core and confine the current in the radial GaN shell, close to the NW sidewalls, which increases the sensitivity of the photocurrent to the environment.
Figure 4. (a) HAADF STEM images (a, c) and corresponding zoom on the area around the AlN barrier (b, d) of an aligned NW device on the [11−20] ZA with the AlN barrier centered between the contacts (a, b) and a misaligned NW device respectively (c, d) showing the AlN barrier underneath the Ti−Al contact, indicated by an arrow. (e) I−V measurements (in the air) of the aligned and misaligned NW devices in the dark and under UV illumination (wavelength λ = 350 nm using 0.2 mW/cm2 laser excitation power). The bright particles of around 2 nm in diameter visible in (b, d) are not intrinsic to the NW growth or lithography process but due to an involuntary gold contamination of the plasma cleaner used to clean the sample prior to STEM observation.
engineering solution to inhibit the electron transport through the NW core and force the current to flow through the GaN shell. This surface conduction path should result in an enhancement of the sensitivity of the NW to the environment, due to an amplification of the effect of adsorbates on the surface band bending and carrier lifetime. To assess this issue, we have studied the conductivity behavior of an n-i-n NW with an AlN barrier (device I) in the dark and under UV illumination, as a function of the environment (in the air and in vacuum), as illustrated in Figure 5. Both the dark current and the photocurrent increase under vacuum. The enhancement of the dark current in vacuum was already reported for both undoped and n-i-n GaN NWs.47−49 However, the photocurrent measured in similar n-i-n structures without the AlN insertion was found insensitive to the environment in similar measurements performed in vacuum and in the air.22,49 We can therefore conclude that the enhancement of the photocurrent in vacuum reported here is associated with the presence of the AlN insertion and hence to the surface nature of the conduction. Indeed, the photocurrent under vacuum in the GaN/AlN/GaN heterostructured NWs reaches the level observed in misaligned devices measured in the air, suggesting that the difference reflected in Figure 4e is related to the
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 5695
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ACKNOWLEDGMENTS Partial financial support from the French FMN-SMINGUE 2011, the French CNRS and CEA METSA network, the ANR2011-NANO-027 “UVLamp” project, the EU ERC-StG “TERAGAN” (#278428) project, and the Société Française Des Microscopies (SFμ) for a travel grant is acknowledged. We thank J. Dussaud, Y. Curé, and Y. Genuist for their technical support.
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dx.doi.org/10.1021/nl302890f | Nano Lett. 2012, 12, 5691−5696
