ARTICLE pubs.acs.org/JPCC
Enhanced Surface Potential Variation on Nanoprotrusions of GaN Microbelt As a Probe for Humidity Sensing Prasana Sahoo,† Douglas Soares Oliveira,‡ M^onica Alonso Cotta,*,‡ Sandip Dhara,† S. Dash,† A. K. Tyagi,† and Baldev Raj† † ‡
Surface and Nanoscience Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India Instituto de Física “Gleb Wataghin”, UNICAMP, 13083-859 Campinas SP, Brazil
bS Supporting Information ABSTRACT: Surface potential (SP) using Kelvin probe microscopy is employed, as a measure to sense humidity, exploiting localized nanoprotrusions of freestanding GaN microbelts. These belts with distinct nanofeatures are grown using chemical vapor deposition technique in the vapor-solid process. The variation of SP value is associated with the surface charge accumulation. Pronounced enhancement of the SP variation is found to arise from the localized inhomogeneity on the nanoprotrusions of microbelt. Role of oxygen-related native defect complexes is also discussed for the observed SP variation with humidity. Furthermore, the rough surface of belts favors a high level of defects on the surface and appears more sensitive to moisture level in atmosphere. Hence attention is essential for any sensing application of chemical species using GaN. A dissociative path way for the reaction mechanism of water molecules on GaN surfaces has been predicted.
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long with other applications, GaN and its alloys with other group III metal are of great interest as sensors because of their surface sensitivity, high mobility, low drift current, optical transparency, and high thermal breakdown temperature. Its implementation in high-performance electronic and photonic devices, gas, biochemical sensors, and other surface sensitive devices requires a thorough understanding of the surface and interfacial properties for their optimal performances.1-3 In the past decade, chemical sensing based on a variety of onedimensional nitride nanostructures has attracted great attention. Generally, nanowire (NW) sensors are based on a percolated network of nanowires.2,3 However, the poor conduction between different NWs inside the percolated network may limit the system performance, particularly when the electrode distance exceeds the average NW length. This issue may be circumvented by using an individual nanostructure with large surfaceto-volume ratio and hence offering high degree of chemical sensitivity. To date, mats of GaN NWs and AlGaN/GaN high electron mobility transistor structure are particularly used for hydrogen and other gas sensing.3,4 In most of the cases sensing studies have been conducted with dry nitrogen. A few experiments were performed in dry air conditions. However, in real applications for detecting gas, humidity may play a significant role because water vapor in air sometimes perturbs the response of the other sensing components. Therefore, intensive investigation on the effect of humidity of GaN-based nanostructure is required. r 2011 American Chemical Society
In this study, we report the response of nanoprotruded GaN microbelts for the detection of humidity using Kelvin probe force microscopy (KPFM). In KPFM mode, the contact potential between sample and tip is compensated by applying external bias voltages and thus facilitating the simultaneous acquisition of topography and surface potential (SP) images, SP = Δφ/e. Here, Δφ is the difference between tip and sample work functions and e is the electron charge. The observed SP is strongly dependent on the humidity, suggesting that attention to control moisture is essential for sensing applications of chemical species other than water vapor. The growth of GaN belts with nanoprotrusions was performed using conventional chemical vapor deposition (CVD) techniques in a quartz tube furnace at atmospheric pressure in the flow of reacting NH3 (10 sccm) with molten Ga as source material. Growth was carried out at 950 °C for 5 h. A large quantity of free-standing beltlike microstructures of GaN was found on high pure alumina (99.95%) boat without catalyst in the vapor-solid (VS) process. The VS process can be better understood from the presence of adsorbed oxygen in the alumina boat and the formation of gallium oxynitride (GaOxNy) phase during growth. The details of growth processes were reported elsewhere.5 The total belt thickness was found to be 10-15 μm with 1 μm GaN rich layer covering the top surface. The cross Received: December 3, 2010 Revised: January 27, 2011 Published: March 11, 2011 5863
dx.doi.org/10.1021/jp111505m | J. Phys. Chem. C 2011, 115, 5863–5867
The Journal of Physical Chemistry C
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Figure 1. Cross sectional image of belts with the presence of oxygen in GaN microbelt as indicated in EDS analysis from the encircled area and corresponding HRTEM image on the surface nanoprotrusion.
sectional and high-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100F) images of the tip region are shown in Figure 1. The cross sectional TEM sample was prepared using standard focused ion beam (FIB) lift-off technique. The elemental analysis using energy-dispersed X-ray spectroscopy (EDS) shows the presence of oxygen in the microbelt and suggests the possible formation of GaOxNy phases. For the SP measurement, the GaN belts were transferred to clean Cu substrates, which were electrically grounded. Commercially available metallorganic chemical vapor-deposited GaN films (6-7 μm thick) on sapphire were electrically contacted through the top layer to avoid capacitance effects. Photoluminescence (PL) studies were performed using microPL setup with 325 nm excitation. Topography and SP images were acquired simultaneously using an Agilent 5500 with a three-lock-in amplifier in the amplitude modulation KPFM.6,7 For the measurements, VAC bias at frequencies in the range 10-15 kHz plus a DC bias, VDC, were applied between tip and sample. The detector signal amplitude at the VAC frequency should be proportional to (VDC - Δφ)VAC ∂C/∂z, where C and z are the capacitance and distance between tip and sample, respectively.8 The SP images thus show the values of the DC bias that minimize this amplitude at each measured point. These images show only SP variations along the image, since lock-in adjustments are made so that the initial SP value is close to zero. The KPFM measurements were carried out in an environmental chamber where humidity levels in the atmosphere could be controlled by injecting pure N2 flow. Dry atmosphere refers to humidity levels below 15%. Topography and KPFM potential fluctuation maps of the nanoprotruded microbelts are shown in Figures 2a,b. The rms roughness on these surfaces is in the range 30-40 nm. A clear evidence for the presence of nanoprotrusion along the surface of the microbelts is shown (inset Figure 2a) using field-emission scanning electron microscopy (FESEM; Zeiss SUPRA 40). Moreover, large steps (between 0.1 and 0.4 μm in height) can also be observed (marked as dotted lines in Figure 2a and
Figure 2. Topography (left) and SP (right) images for the (a,b) GaN microbelt with nanoprotrusions and (c,d) GaN film, both under dry atmosphere. Inset in (a) shows the presence of nanoprotrusion along the surface of the microbelts using FESEM. The regular variations of height in (c) are steps on the surface of the GaN film. The inset in (d) shows the atomic configuration of the VGa in the 3- and 2- charge states following Figure 1 of ref 14. Dotted lines and circles are guide to surface features mentioned in the text. Solid circle and box represent surface features related to extended defects discussed in the text.
corresponding KPFM image Figure 2b) along the belt surface with enhanced roughness due to larger crystallites exposed at the edges. Topographic and the corresponding SP value on a isolated surface feature is marked with dotted circle. The electrical images acquired simultaneously on dry atmospheres show a more uniform background with smaller SP values associated to the larger grains 5864
dx.doi.org/10.1021/jp111505m |J. Phys. Chem. C 2011, 115, 5863–5867
The Journal of Physical Chemistry C
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Figure 3. (a) Topography, (b) phase, and (c) SP images attributed to the oxide and oxynitride phases of the nanoprotruded region on GaN microbelts.
and to the step edges present in the image. The decrease in SP values at these regions reaches over 1 V. In this context, it is worth mentioning that the SP images show minimal artifacts or signal crosstalk due to the large roughness variation in the images,9 as indicated by the presence of steps in Figure 2a,b. We compare our result with commercially available GaN films. Figure 2c,d shows the topography and SP images of the GaN film under dry atmosphere. The film is completely different from the belts, showing very smooth morphologies where mono or bilayer step edges (along the dotted line drawn in Figure 2c,d) are observed. The SP image (Figure 2d) shows that the potential values are lower (∼50 mV) in some regions closer to step edges. A lower SP value means it is lower in electron potential, that is, excess negative fixed charges as indicated by the dark spot in the squared region (Figure 2d). The likely origin of these extended defects is clustering of Ga vacancies (VGa; inset in Figure 2d showing a possible configuration for a single vacancy), which is discussed later in details. Some small depressions, or pits, which can be associated to threading dislocations (marked with a circle in Figure 2c), are also observed in the GaN film. The total dislocation density in the film is estimated as 2 108 cm-2. The SP value increases at the pit (circle in Figure 2d), indicating a carrier depleted region; a higher SP signal corresponds to regions with higher value in an electron energy diagram, that is, presence of positive charges or depletion of electrons.10 Our result is in agreement with earlier observations of Hansen et al.11 of similar surface structures, where a smaller capacitance, which is consistent with a larger depletion depth, was observed near the dislocations relative to the regions between dislocations. The variation in SP values in nanoprotruded microbelts, however, is at least 1 order of magnitude larger than those from the GaN film. This indicates a variation of Fermi level energy, which generally speaking may arise from different crystallographic phases, dopant levels, or defect densities present in these regions toward the mid gap of the semiconductor. High-magnification AFM images of topography, phase, and SP (Figure 3a,b,c) provide significant input in order to understand this material issue. Large variations of phase (Figure 3b) are observed at the nanoprotusions, indicating a drastic change in the viscoelastic properties of the surface. The corresponding electrical image (Figure 3c) shows that the larger SP variation is localized at these same regions and not necessarily correlated to the height variation along the surface. When a large number of such regions are present in the images, the SP profile around them is also minimized with regard to the surrounding surface. These features altogether indicate the presence of a different material within the microbelt. The differences in SP value larger than 500 mV can be attributed to the presence of gallium oxide phases on the surface.12 However, both the resulting strain at the interface with GaN as well as the compositional variation
Figure 4. Micro-PL spectra of GaN belt at different locations as stated in the text. The near band edge emission was clearly resolved into three PL peaks at 3.47 and 3.1-3.25 eV corresponding to the direct band-toband transition and DAP recombination, respectively. The peak ∼2.2 eV is assigned to YL band. The insets show a single belt with spots 1, 2, and 3 marked, the typical YL band corresponding to spot 2 in the low-energy region of the spectra and schematic configuration of VGa-ON complex following Figure 1 of ref 14.
(e.g., formation of intermediate GaOxNy phase which is also found in the present study) can originate the gradual increase in SP as one move away from the oxide core.12 To understand the GaN defect states in these nanostructures, room-temperature micro-PL spectra were taken at different location of a single belt using a He-Cd laser (∼325 nm) excitation. The spectra exhibit emission in the ultraviolet range at 3.47 eV (Figure 4), which is attributed to the direct band-toband transition peak of GaN. The second dominant emission peak has been observed around 3.1-3.25 eV from different parts of the microbelt; base region corresponds to spot 1 and spot 2, 3 are referred to middle and tip region (in one of the insets in Figure 4) of the belts, respectively. This emission has been assigned to shallow donor-acceptor pair (DAP) combination, which arises from zero-phonon transitions and LO-phonon replicas associated with distant DAP recombination.13 The strong red shift in the emission (3.1-3.25 eV) band may be attributed to the rough surfaces of belts, having high level of defects on the surface. We have observed a quenching of the direct band-to-band transition peak intensity than that of DAP originated peaks. This issue is discussed later in the light of different configurations of native defects. The undesirable broad emission band, centered on 2.2 eV, is so-called yellow luminescence (YL) (typically shown for spot 2 in one of the insets of Figure 4). The key factor responsible for YL band results from the native defects. The VGa is the most likely candidate from an 5865
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Figure 5. SP surface profiles at (a) dry atmosphere and (b) 20% humidity level. The normalized density of SP points in each image is show in (c); outset shows variation with 5% increase in moisture condition over a small SP range (>100 mV). The inset in (c) shows the schematic of water adsorption on GaN screening the electrical features of the original surface.
energetic point of view.14 The atomic geometry of the VGa (inset in Figure 2d) is characterized by a strong outward relaxation of the surrounding N atoms. The- V-Ga corresponds to a transition of 3- to the 2- charge state, E2 /3 ≈ 1.1 eV above the valence band and thus is indicative to a deep acceptor level.14 A schematic of the VGa formation is shown in Figure 4. However, an alternative defect configuration of VGa and oxygen antisite in Figure 4) corresponding to (ON) complex (one of the insets the similar energy transition E1 /2 ∼ 1.1 eV is also reported as the origin of YL band. Presence of oxygen-related defects in the GaN microbelts can be correlated to the quenching of intensity for band edge transition with respect to that for the DAP originated peaks. Native oxide may lead to nonradiative recombination process in GaN15,16 and may cause the Fermi level to be pinned at a slightly deeper position. Thus, the YL band in the microbelt may be accounted due the formation of VGa-ON complex (Figure 4). The native VGa-ON complex certainly alters the electron shielding containing a negative fixed or interface charge and hence may be consistent with the observed enhanced SP variation along the sample surface.14 However, the scenario changes when SP images are acquired under larger humidity levels. In that case, the average SP variation on the surface drops, and the localized regions attributed to the oxide cores can no longer be clearly observed. Even a small humidity variation (Figure 5a,b) can alter the electrostatic profile of the sample as compared to the dry image (Figure 2). Nevertheless we have acquired SP images under larger humidities which are shown in the Supporting Information (Figure S1); they are increasingly difficult to make as humidity reaches ∼50%. The average SP differences are dependent on the region of the sample which is scanned with an example in the Supporting Information Figure S2 showing a region where SP differences are within noise levels. The normalized density of points in each image (Figure 5c) provides statistical average SP value on exposure of moisture over an extended region. A variation in density of SP value is readily deconvoluted with even 5% increase in moisture condition shown over a small SP range
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>100 mV (outset of Figure 5c). This behavior indicates a screening of the original surface electrical features due to water adsorption. It is also observed for GaN films, but the enhanced SP variation along the nanoprotruded belt surface provides a distinct phenomenological evidence for the screening process. The prominent presence of VGa-ON complexes (inset in Figure 4) in the belt may be assumed to be responsible for the accumulation of negative charges on the surface and the subsequent increase in the band bending leading to an enhanced response of humidity in the microbelt. Moreover, the diminished contrast in SP images with larger moisture levels, shown generally and specially in the encircled region, is also a clear indication of screening by surface charges.10 We have also studied the normalized density of SP points corresponding to the humidity levels of 46% as compared to that
