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Porous GaN on Si(1 1 1) and its application to hydrogen gas sensor
Sensors and Actuators B 155 (2011) 699–708
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Porous GaN on Si(1 1 1) and its application to hydrogen gas sensor Asmiet Ramizy ∗ , Z. Hassan, Khalid Omar Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 2 November 2010 Received in revised form 11 January 2011 Accepted 20 January 2011 Available online 31 January 2011 Keywords: Porous GaN Gas sensor Heterostructure Electrochemical etching
a b s t r a c t The growth of heterostructure of n-type GaN/AlN/Si(1 1 1) is carried out using the molecular beam epitaxy (MBE) Veeco model Gen II system. The surface morphology of the as-grown GaN sample showed pits on the GaN surface in a ratio small than those found by other research groups. Porous GaN samples were synthesized by an electrochemical etching technique combined with increasing the current density to 75 mA/cm2 . The formation of pore structures are of different sizes, the etched surface became hexagonal, and pore structures are confined to a smaller size. The PL results showed greater blue shift luminescence in comparison to results found by other research groups. The reduction in crystallite size is confirmed by an increase in the broadening of XRD spectra. Raman spectra also displayed a strong band at 522 cm−1 from the Si(1 1 1) substrate, and a small band at 301 cm−1 . These are due to the acoustic phonons of Si. Two Raman active optical phonons are assigned to h-GaN at 139 cm−1 and 568 cm−1 , due to E2 (low) and E2 (high) respectively. The sensitivity of the gas sensor is increased as a function of the hydrogen flow rate and they became much higher compared to the as-grown sample. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Porous semiconductors are considered promising materials for optoelectronic applications in comparison to the bulk materials due to their unique optical and electronic properties. The tunable properties of the porous materials lead to the development of new sensing devices. Moreover, a porous layer can also be used as a buffer or intermediate layer to manage strain during epitaxial regrowth process. Using this growth method may reduce the density of the defects in the epitaxial layer, leading to a high-quality and stress-free layer on the porous template [1,2]. Porous silicon (PS) is a potential candidate for silicon-based optoelectronics applications such as light emitting and light sensing devices [3,4]. Even though PS has attracted more attention, its thermal, chemical, and mechanical instability hinders its large scale applications [5]. These problems lead to the development of other porous semiconductors, such as the conventional III–V compounds including GaAs, GaP and InP as well as the wide band gap materials such as GaN and SiC [6]. GaN is generally grown on sapphire (13.6% lattice mismatch), SiC (4% lattice mismatch), and Si (16%). However, the large lattice mismatches degrade the layer quality [7]. To overcome this, porosity has emerged as an effective tool to control the electronic and optical properties of GaN quantum structures. The characteristics of the surface strongly affect the properties of the semiconductor. Quantum confinement effects control the mechanism of the
∗ Corresponding author. Tel.: +60 174316405; fax: +60 46579150. E-mail addresses: asmat [email protected], [email protected] (A. Ramizy). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.034
luminescence in nanocrystallites [8,9]. The reduction in size to a few nanometers is required for efficient light emission since it produces a modification in the electronic, optical and vibrational properties [10]. A few studies have been reported on porous GaN using electroless wet chemical and photoelectrochemical etching [11–13]. In this work, porous GaN has been synthesized by an electrochemical etching technique. By varying the etching anodization conditions, this technique is more suitable and cheaper for producing highdensity nanostructures with controlled pore size and shapes than the other techniques. There are various applications involving the use of hydrogen as in industrial fabrication processes and hydrogen fuel cells that are used in transportation vehicles. However, because hydrogen gas is flammable and explosive when its concentration in air is more than 4% at room temperature, its detection has become one of the most important safety issues, Also because of impending needs for robust gas sensors operating in harsh environments, nitride groups semiconductor-based hydrogen gas sensors have thus attracted much attention [14,15]. Recently, a hydrogen sensor diode was fabricated with the reactive Ga2 O3 oxide layer directly grown on the GaN layer using a photoelectrochemical oxidation (PEC) method [14]. Wright et al. [16] used Pt-coatings sputtered onto multiple GaN nanowires to enhance their sensitivity for hydrogen at hundreds of ppm level at 25 ◦ C. Pt-coated multiple GaN nanowires showed non-linear relative responses of ∼1.7% at 200 ppm up to ∼1.9% at 2000 ppm H2 in N2 after a 10-min exposure. Today, the optimum electrochemical etching parameters in the nitride groups are still unclear. The objective of this work is to inves-
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Fig. 1. SEM image of the fabricated gas sensor with Pt contact.
tigate the selected etching parameters to control the size, shape, surface chemistry, and emission properties of the nanostructures. This technique could open a new and promising field in the binary nitride groups if it transforms the material’s properties by making them similar to the ternary. To the best of our knowledge, this has never been reported before. 2. Experiment III-nitrides heterostructure of n-type GaN with AlN buffer layer were grown on the n-type Si(1 1 1) substrate with dimension of 3in. using plasma assisted molecular beam epitaxy (PAMBE) Veeco model Gen II system. Source materials of high purity, such as gallium and aluminum, were used in the Knudsen cells. Highly purified nitrogen was channeled to the radio frequency (RF) source to generate reactive nitrogen species. The plasma was operated at a typical nitrogen pressure of 2.5 × 10−5 Torr under a discharge power of 300 W. The growth of III-nitrides on the 3-in. Si(1 1 1) substrate was begun after the standard cleaning procedure using the Radio Corporation of America RCA method .The substrate was then mounted on the wafer holder and loaded into the MBE system. The Si sub-
strate was outgassed in the load-lock and buffer chambers. After outgassing, the Si was transferred to the growth chamber. Prior to the growth of the epilayers, surface treatment of the Si substrate was carried out to remove the SiO2 . The Si substrate was heated to 750 ◦ C, and a few monolayers of Ga were deposited on the substrate for removing the SiO2 by formation of Ga3 O2 . Before the growth of nitride epilayers, a few monolayers of Al were also deposited (1–3 monolayers) at 850 ◦ C on the silicon surface. This was done prior to the introduction of N2 until the 7 × 7 surface reconstruction disappeared to avoid the formation of Six Ny , which is deleterious for the growth of the subsequent epilayers. The buffer or wetting layer, AlN was first grown on the Si substrate. To grow the AlN buffer layer, the substrate temperature was heated up to 860 ◦ C, both of the Al and N shutters were opened simultaneously for 15 min. Subsequently, GaN epilayer was grown on top of the buffer layer for 60 min with substrate temperature set at 845 ◦ C. Porous GaN samples were synthesized by electrochemical etching technique [17]. A GaN wafer with (0 0 0 2) orientation, carrier concentration of 2.1 × 1019 cm−3 , thickness of 0.47 m with radius of 0.25 cm in order to match with the size of the opened-circular of the electrochemical cell, was placed in an electrolyte solution of ethanol 99.999%:HF40%, at varying current densities (25, 50, 75 mA/cm2 ) and constant etching times of 12 min. The Si wafer was immersed in HF acid to remove the native oxide. The electrochemical cell was made of Teflon and has a circular aperture on its bottom under which the silicon wafer was sealed. The cell was a twoelectrode system with a silicon wafer as an anode and platinum as a cathode. The synthesis was carried out at room temperature. After etching processing all samples were rinsed with ethanol and dried in nitrogen shower. Surface morphology and structural properties of nanostructures were analyzed using scanning electron microscopy (SEM), and X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX). Photoluminescence (PL) measurement was also performed at room temperature using an He–Cd laser ( = 325 nm), and Raman scattering has been investigated using an Ar+ Laser ( = 514 nm). To fabricate the gas sensor, Pt Schottky contact with thickness of 200 nm was deposited onto the GaN film using an RF sputtering system. A metal mask consisting of an array of holes with a diameter of about 0.9 mm was used for this purpose, as shown in Fig. 1. In a
Fig. 2. Gas sensor experimental setup.
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Fig. 3. SEM of porous GaN prepared by electrochemical etching with time of 12 min and different current densities (a) as-grown, (b) 25 mA/cm2 , (c) 50 mA/cm2 , (d) 75 mA/cm2 .
home-made gas sensing chamber, a 2% H2 in 98% N2 gas was used. The sensor is fixed in the gas chamber as shown in Fig. 2(I–V). A measurement using the current–voltage sensor is taken at room temperature with different H2 gas flow rates, at a fixed time of 2 min. 3. Results and discussion Fig. 3(b)–(d) shows the top view SEM image of the porous GaN surface prepared using electrochemical etching with varying current densities, while keeping the etching time at 12 min. Fig. 3(a) shows the surface morphology of the as-grown GaN sample. Pits were found on the GaN surface but they were in a small ratio to that observed by another researcher group which observed that if the carrier concentration value of Si-doped GaN films grown with AlN buffer layers is approximately 1 × 1018 cm−3 , then many grooves, V-shapes, and cracks were noticeable on that surface [18]. Although our sample had a high carrier concentration value of 2.1 × 1019 cm−3 it showed fine and non-cracked formation. The effect of varying current density on the morphology of porous GaN layer is observed. The chemical reaction was initiated at a current density of 25 mA/cm2 , and due to the insufficient structure interaction the etched area was irregular. In addition, the existence of the edges of the remaining non-etched layer was observed as shown in Fig. 3(b), which may be attributed to the shortage etching time in relation to thickness of the porous layer. This means that the electron–hole pairs that were generated were inadequate [19]. When current density was increased to 50 mA/cm2 , well-defined layers of pores with sizes ranging from 5 to 15 nm over the whole surface area were observed, as shown in Fig. 3(c). This means the etch rate in the crystal centers is slow enough so that the grain boundaries are etched at a sufficient accuracy. On the other hand, by increasing the current density to 75 mA/cm2 , the formation of pore structures with different sizes and shapes could be seen in Fig. 3(d); the etched surface became hexagonal, and the pore structures are smaller in size. In addition, the pore walls were very thin with some
Fig. 4. Cross section images of the GaN (a) before (b) and after etching process.
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Fig. 5. AFM images of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities: (a) as-grown, (b) 25 mA/cm2 , (c) 50 mA/cm2 , (d) 75 mA/cm2 .
Fig. 6. EDX of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities (a) as-grown, (b) 25 mA/cm2 , (c) 50 mA/cm2 , (d) 75 mA/cm2 .
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Fig. 7. Etching current density as a function of doped concentration and mobility of carriers.
short thin tips at the top. This indicated that when the etch rate is too fast, the grain boundaries are etched significantly slower than the center of the crystals. This leads to a hexagonal and rough morphology [20]. Fig. 4 presents the cross section of the n-GaN thin films before and after the etching process. The thickness of the AlN buffer layer and n-GaN epilayer are 0.069 m and 0.47 m respectively. We estimated the GaN growth rate to be about 0.5 m/h and the AlN growth rate is 0.27 m/h as shown in Fig. 4(a). The depth of porosity for the selective sample presented was approximately 0.22 m as shown in Fig. 4(b). The thickness of the porous layer increased with respect to increasing current density. When the current density is increased, anodic bias is increased, and the dissolution of silicon extends to the bulk of the wafer. Increased anodic bias results in holes with higher energy, which overcome the barrier at the pore ends and wall. Fig. 5(a) describes the three-dimensional topographic image of the as-grown sample. The etched surface with the pyramidal shape was distributed over the entire surface. The pyramidal shape indicated that the increase in the surface roughness is attributed to the etching parameters affecting the surface characterization. Because the porous surface texture has a high degree of roughness, this sug-
gests a possible use for the porous layer as an antireflective coating since it reduces the light reflection. Because roughness is a function of layer thickness, the layer thickness of an antireflective coating increase as roughness increases; this leads to changes in the optical, electronic and vibrational transitions [17]. The attenuation of the reflectivity is due to scattering and transmission at the porous and bulk interfaces [21]. The etched surfaces range in heights nanoparticles from 50 to 200 nm relative to the etch current density, as shown in Fig. 5(b)–(d). Fig. 6 shows the EDX spectra and the atomic composition of the film elements for unintentionally doped n-type GaN/AlN/Si(1 1 1) films and the porous GaN samples. The peak intensity refers to the concentration of the element in the sample and the atomic composition by percentage. The results indicated that the unintentionally doped n-type GaN/AlN/Si(1 1 1) films are of good quality, without the presence of contaminating elements as shown in Fig. 6(a), in contrast to other research group [22], Fig. 6(b)–(d) reveals the EDX data after the etching process. These indicate the presence of the aluminum (Al) element with a ratio between 1% and 9%. This means that the Al started to appear on the surface and increased gradually in proportion to the etching parameters. This result could open
Fig. 8. PL spectra of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities.
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Fig. 9. Raman spectra of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities.
a new and promising field in the binary III-nitride materials group through the conversion of the properties of the materials to become similar to that in the ternary nitride materials group (AlGaN). This result is also confirmed by the analysis of the PL results later. Fig. 7 presents the Van der Hall measurements using an Ohmic contact. The results showed that the doped concentration decreased as a function of the porosity of the etched films, and the mobility of carriers was increased. This may be due to the formation of nanostructures on the whole surface of the etched film, which leads to increases in the probability of the carrier’s transfer through the surface.
Fig. 8 shows the PL spectra at room temperature for the porous GaN. Higher blue shift luminescence was observed in comparison to the as-grown, and the energy gap of porous GaN is increased as a function of etching current density, as shown in Table 1. The broadening of the band gap energy occurs with the decrease in the crystallite size. These results are different in comparison to results from other research groups which found that porous GaN exhibited a red shift photoluminescence [23,24]. Our results showed greater blue shift luminescence in comparison to other research groups [25,26]. This is attributed to charge carrier quantum confinement. This means that the particles are confined in the lower dimension and the probability of the recombination of the electrons and holes
Fig. 10. XRD spectra of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities.
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705
Fig. 11. I–V measurement of porous GaN gas sensor with different H2 gas flow rate.
0,80 Air:porous
Schottky barrier height (eV)
0,75 Air: as-grown
0,70
H2 (1sccm) H2 (2sccm)
0,65
0,60
H2 (3sccm)
0,55
H2 (4sccm)
0,50 0
1
2
3
4
Flow rate(sccm) Fig. 12. The effective Schottky barrier height as function of the hydrogen flow rate.
Table 1 The energy gap, FWHM, peak shift, intensity, and the radius of nanocrystillites of different samples obtained from the PL spectra at room temperature. Sample
Peak position (nm)
FWHM (meV)
Peak shift) (nm)
intensity (a.u)
Energy gap (eV)
Nanocrystillites radius (nm)
As-grown 25 mA/cm2 50 mA/cm2 75 mA/cm2
362.0 346.5 345.5 335.0
67 86 90 100
– 15.5 16.5 27.0
4.5 × 105 8.09 × 106 8.2 × 106 1.0 × 10 7
3.42 3.57 3.58 3.70
– 9.5 9.0 7.1
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Fig. 13. Room temperature on–off responses of hydrogen gas sensors measured at constant voltage of 1 V.
is higher in the low dimensional structures. This leads to high emission efficiency from high porosity structures arising from quantum confinement effects. Reducing the dimensions to nanometers drastically changes the physical properties of GaN film, as previously mentioned in the energy dispersive X-ray analysis. An estimate of the size of the GaN nanocrystallite was identified using the effective mass theory. Assuming that there are infinite potential barriers, the of the 3D-confined GaN were obtained as: [27] En (eV) = Eg +
h2 8d2
1 1 + ∗ m∗e mh
Table 2 Peaks position, peak shift, and intensity of different samples obtained from Raman spectra at room temperature. Sample
Peak position (cm−1 )
Intensity (a. u)
Peak shift
As-grown 25 mA/cm2 50 mA/cm2 75 mA/cm2
568.1 566.8 567.4 566.5
498.5 587.6 606.9 786.5
– 1.3 0.7 1.7
(1)
where Eg is the as-grown GaN energy gap, d is the diameter of the spherical particle, and the m∗e , m∗h the electron and hole effective mass, respectively (at 300 K, the m∗e = 0.20m0 , m∗h = 1.0m0 and Eg = 3.42 eV) [28]. From Eq. (1) the radius of nanocrystallites En are summarized in Table 1. The room temperature photoluminescence shows that the increase in intensity is more than 22 times that of the grown sample, with etching current density as shown in Table 1. This strength of photoluminescence output becomes stronger due to increasing porosity. This means the intensity of photoluminescence is proportional to the number of emitted photons on the porous surface. This surface experiences a shrinking of its size caused by deeper etching, which in turn leads to an increase in the number of transitions inside the structure. Fig. 9 shows Raman’s spectra of as-grown GaN and porous GaN, respectively. There is a strong band at 522 cm−1 from the Si(1 1 1) substrate, and a small band at 301 cm−1 , due to the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 cm−1 and 568 cm−1 due to E2 (low) and E2 (high) respectively. Porous GaN spectra displayed a decrease in the Raman intensity relative to as-grown GaN, as shown in Table 2. We believe that, due to the particles confined into the lower dimension, this may be leads to increasing the probability of the particles vibrating in relation to the laser incident beam, which enhances the number of scattered photons per unit incident angle for a given laser power density. Porous GaN spectra were shifted and broadened relative
to the as-grown GaN, which is attributed to the quantum confinement of optical phonons which is distributed upon the etched surface [29]. This means that the nanostructures shape and size are obtained with a high surface area to a small volume ratio that is most sensitive in response to any external effects. Fig. 10 shows a small broadening peak for porous GaN in comparison to as-grown GaN. The data in Table 3 showed that the peak broadening occurred when the current density was gradually increased.In addition, the reduction in crystallite size is confirmed by increases in the broadening of XRD spectra. On the other hand, the lattice constant and peak positions also changed due to chemical treatment. The distorted shape of the porous GaN crystallite led to a change in the phase diffraction. The capability of gas sensors to operate in harsh environmental conditions, such as high-temperature and highly corrosive chemical conditions is one of the important specifications of high quality gas sensors. The large band gap of GaN, chemical stability and Table 3 Different lattice constants and diffraction peak position of different samples derived from XRD analysis. Sample
Peak (◦ )
FWHM (◦ 2Th.)
d-spacing (Å)
c (Å)
As-grown 25 mA/cm2 50 mA/cm2 75 mA/cm2
17.261 17.2983 17.3003 17.3040
0.19 0.29 0.19 0.29
2.59 2.59 2.59 2.59
5.189 5.18 5.18 5.19
A. Ramizy et al. / Sensors and Actuators B 155 (2011) 699–708 Table 4 The sensitivity% of the gas sensors operating at room temperature with different H2 gas flow rate. Sample
Sensitivity (%)
Air:porous H2 (1 sccm) H2 (2 sccm) H2 (3 sccm) H2 (4 sccm)
0.83 7.60 7.77 9.0 13.80
By assuming the flow current mechanism is governed by the thermionic emission conditions, is given by [28] KT ln B = q
mechanical robustness leads to the use of this semiconductor in many harsh applications, including gas sensing operations during chemical reactor processing, fire alarms onboard aircraft and space vehicles, as well as in devices that detect fuel leaks in automobiles and aircraft. A unique advantage of a GaN gas sensor is that it can be integrated with GaN-based solar-blind UV photodetectors or high-power, high-temperature electronics devices on the same chip [30,31]. Recently, there are many reports of GaN gas sensors utilizing Schottky contacts [32–34]. Such types of devices have typical detection abilities in the range of 4.7% H2 in the air [35]. The unique physical properties of the porous GaN establish a special feature, which is considered a promising material for use in sensing applications. Fig. 11 shows measurement of porous GaN gas sensor fabricated using electrochemical parameter etching with different H2 gas flow rate. The result showed that the sensor was able to detect hydrogen in the range of 1–4 sccm with sign of saturation. The sample of GaN and porous GaN showed a good Schottky behavior when operated in the air at room temperature, which indicated the Pt contact is of good quality. On the other hand, both samples changed to Ohmic contact behavior when 1–4 sccm H2 gas was released. This means that the porous surface of Pt/GaN contact has a larger surface area, and this allows hydrogen molecules to dissociate and form atomic hydrogen more efficiently. Moreover, the unique surface morphology offers a higher accumulation of hydrogen at the Pt/porous GaN interface. The performance of the gas sensor was estimated through the measurement of the hydrogen detection sensitivity, which is defined as: [36] IH2 − IAir Iair
(2)
at constant voltage of 3 V.Table 4 shows that the sensitivity of the gas sensor increases as a function of the hydrogen flow rate and becomes much higher compared to the as-grown sample. The increase in the sensitivity can be explained, according to Johansson [37], by an electrical polarization that is given by: V =
Ni i ε
AA∗ T 2 I0
(4)
where I0 is the saturation current density, q is the electron charge, K is Boltzmann’s constant, T is the absolute temperature, A* is the effective Richardson coefficient, A is the contact area, and the theoretical value of A* can calculated using A∗ =
S=
707
(3)
where is the effective dipole moment, Ni is the number of sites per area at the interface, is the coverage of hydrogen atoms at the interface, and ε is the dielectric constant. It should be noted that the role of porous Pt/porous GaN interface is not only limited in providing a larger surface for dissociating the hydrogen molecules more efficiently but also in generating higher. Eq. (2) suggests that V increases with the Ni . Since Pt/porous GaN interface is able to supply higher Ni , this unique structure subsequently produces larger electrical polarization and leads to a drop in the effective Schottky barrier height (SBHs) [38]. The effective Schottky barrier height, as function of the hydrogen flow rate, can be determined from I–V measurement.
4 m∗ K h3
(5)
where h is Plank’s constant. For n-type GaN, m* = 0.20 m0 is the effective electron mass for GaN. Fig. 12 showed reduced effective Schottky barrier height as function of the hydrogen flow rate. Fig. 13 shows the on–off responses of the sensor measured at room temperature with a constant voltage of 1 V. The response and recovery time measured for the sensor are about 3 and 2.5 min, respectively. The sensor showed a good sensitivity as a function of time with the minimum flow rate 1 sccm of the test gas (H2 ) which means it exhibited a good performance. The maximum sensitivity is attained in more than 10 and 20 min (shut off-time) after cutting off the hydrogen gas and this may accrue due to the trapping of hydrogen atoms from the defects at the Pt/PGaN interface and these atoms would then diffuse through the Pt thin film and moving towards the Pt/PGaN contact interface which leads to polarizing, thus forming a dipole layer near the interface. Once the dipole layer is formed at the interface culminate, returning to the initial value is prevented [39,40]. 4. Conclusions The surface morphology of the as-grown GaN showed fine and non-cracked formation. The effect of varying the current density on the morphology of porous GaN layer is observed. By increasing the current density to 75 mA/cm2 , the formations of pore structures are different in size and shape. The etched surface becomes hexagonal, and pores structures are confined to smaller sizes. EDX data revealed that, after the etching process, the presence of the aluminum (Al) element with a ratio between 1% and 9% can be detected, which means that the Al element started to appear in the surface and increased gradually in proportion to the etching parameters. This result could open a new and promising field in the binary III-nitride materials group through the conversion of the properties of the materials to become similar to that in the ternary nitride materials group (AlGaN). This result is also confirmed by the analysis of the PL measurement, which showed greater blue luminescence, the energy band gap became 3.70 eV after the etching process. This value of the energy band gap is similar to that in the ternary nitride group (AlGaN). The lattice constant and peak positions also changed due to chemical treatment. The sensitivity of the gas sensor is increased to the highest level, as a function of the hydrogen flow rate and becomes much higher in comparison to the as-grown sample. Acknowledgement Support from FRGS grant and Universiti Sains Malaysia are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2011.01.034.
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[30] B.P. Luther, S.D. Wolter, S.E. Mohnley, High temperature Pt Schottky diode gas sensors on n-type GaN, Sensors Actuators B 56 (1999) 164. [31] A. Lloyd, A. Baranzahi, P. Tobias, I. Lundstrom, High temperature sensors based on metal–insulator–silicon carbide devices, Phys. Status Solidi A162 (1997) 493. [32] J. Schalwig, G. Müller, U. Karrer, M. Eickhoff, O. Ambacher, M. Stutzmann, L. Görgens, G. Dollinger, Hydrogen response mechanism of Pt–GaN Schottky diodes, Appl. Phys. Lett. 80 (2002) 1222. [33] O. Weidemann, M. Hermann, G. Steinhoff, H. Wingbrant, A. Lloyd Spetz, M. Stutzmann, M. Eickhoff, Influence of surface oxides on hydrogen-sensitive Pd:GaN Schottky diodes, Appl. Phys. Lett. 83 (2003) 773. [34] Kim. Jihyan, F. Ren, B.P. Gila, C.R. Abernathy, S.J. Pearton, Reversible barrier height changes in hydrogen-sensitive Pd/GaN and Pt/GaN diode, Appl. Phys. Lett. 82 (2003) 739. [35] Yun Feng, Chevtchenko Serguei, Moon Yong-Tae, Morkoc¸ Hadis, J. Timothy, John Fawcett, T. Wolan, GaN resistive hydrogen gas sensors, Appl. Phys. Lett. 87 (2005) 73507. [36] H.-I. Chen, Y.-I Chou, C.-Yi Chu, A novel high-sensitive Pd/InP hydrogen sensor fabricated by electroless plating, Sensors Actuators B 85 (2002) 10. [37] M. Johansson, I. Lundström, L.G. Ekedahl, Bridging the pressure gap for palladium metal–insulator–semiconductor hydrogen sensors in oxygen containing environments, J. Appl. Phys. 84 (1998) 44. [38] F.K. Yam, Z. Hassan, A.Y. Hudeish, The study of Pt Schottky contact on porous GaN for hydrogen sensing, Thin Solid Films (2007) 515. [39] S.N. Das, A.K. Pal, Hydrogen sensor based on thin film nanocrystalline n-GaN/Pd Schottky diode, J. Phys. D: Appl. Phys. 40 (2007) 7291. [40] F. Razia, A. Iraji zada, B.F. Rahimia, Investigation of hydrogen sensing properties and aging effects of Schottky like Pd/porous Si, Sensors Actuators B 146 (2010) 53.
Biographies Asmiet Ramziy received the B.Sc. degree and the Masters degree in Physics from Mustansiriya University- collage of science, Iraq in 1996 and 1999, respectively, currently Ph.D. student at Universiti sains Malaysia, Lecturer at Al-Anbar University, Iraq, since 2001. Participated in many national and international conferences and workshops and published 15 papers in ISI journals and 6 papers in non-ISI journals. Research interested in nanostructured materials and their applications, Laser-Induced Etching and porous solar cell. Zainuriah Hassan received the B.Sc. (Magna Cum Laude) degree and the Masters degree in Physics from Western Michigan University, USA in 1983 and 1985, respectively, and Ph.D. degree in Experimental Condensed Matter Physics from Ohio University, USA in 1998. She was a Research Associate at Ohio University from 1997 to 1998, and a Visiting Research Scholar under the Fulbright Program at Department of Electrical and Computer Engineering, University of Minnesota, USA in 2004/2005. At present, she is the Dean, and formerly the Deputy Dean (Academic and Student Development) and Chair of the Engineering Physics Program at School of Physics, Universiti Sains Malaysia. She was promoted to Professor in 2009 and is attached to the Condensed Matter, Applied and Engineering Physics group. She has received several awards, scholarships and recognitions which includes Outstanding Scholarship Award and Deans List from Western Michigan University, Graduate Scholarship from Ohio University, Excellent Service Awards, Fulbright Research Scholar Award, Sanggar Sanjung (Hall of Fame) Awards, and Merit Rewards. Her main focus of research is on wide band gap semiconductor materials, in particular III-Nitrides (GaN and related alloys) growth/deposition and characterization, as well as fabrication, characterization, and simulation/design of optoelectronic and electronic devices based on III-nitrides and other semiconductor materials. She has published more than 280 papers in international and national journals and proceedings. She is currently the Editor of Journal of Physical Science and a member of the Materials Research Society, Optical Society of America, IEEE, Fulbright Association, Malaysian Solid State Science and Technology Society, and Malaysian Institute of Physics. Dr. Khalid Omar lecturer and researcher at the School of Physics, Universiti Sains Malaysia (USM). Received Ph.D. in Physics/ Laser Technology from Jamia Millia Islamia (central university) New Delhi, India in September 2003. Continued an illustrious teaching career since 1987. Participated in many national and international conferences and workshops and published sixty seven papers in ISI journals and non-ISI journals as well as in proceedings of conferences since 2007. Interested in Laser-Induced Etching, Nanostructure materials, porous solar cell and laser-medical application.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Porous GaN on Si(1 1 1) and its application to hydrogen gas sensor Asmiet Ramizy ∗ , Z. Hassan, Khalid Omar Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 2 November 2010 Received in revised form 11 January 2011 Accepted 20 January 2011 Available online 31 January 2011 Keywords: Porous GaN Gas sensor Heterostructure Electrochemical etching
a b s t r a c t The growth of heterostructure of n-type GaN/AlN/Si(1 1 1) is carried out using the molecular beam epitaxy (MBE) Veeco model Gen II system. The surface morphology of the as-grown GaN sample showed pits on the GaN surface in a ratio small than those found by other research groups. Porous GaN samples were synthesized by an electrochemical etching technique combined with increasing the current density to 75 mA/cm2 . The formation of pore structures are of different sizes, the etched surface became hexagonal, and pore structures are confined to a smaller size. The PL results showed greater blue shift luminescence in comparison to results found by other research groups. The reduction in crystallite size is confirmed by an increase in the broadening of XRD spectra. Raman spectra also displayed a strong band at 522 cm−1 from the Si(1 1 1) substrate, and a small band at 301 cm−1 . These are due to the acoustic phonons of Si. Two Raman active optical phonons are assigned to h-GaN at 139 cm−1 and 568 cm−1 , due to E2 (low) and E2 (high) respectively. The sensitivity of the gas sensor is increased as a function of the hydrogen flow rate and they became much higher compared to the as-grown sample. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Porous semiconductors are considered promising materials for optoelectronic applications in comparison to the bulk materials due to their unique optical and electronic properties. The tunable properties of the porous materials lead to the development of new sensing devices. Moreover, a porous layer can also be used as a buffer or intermediate layer to manage strain during epitaxial regrowth process. Using this growth method may reduce the density of the defects in the epitaxial layer, leading to a high-quality and stress-free layer on the porous template [1,2]. Porous silicon (PS) is a potential candidate for silicon-based optoelectronics applications such as light emitting and light sensing devices [3,4]. Even though PS has attracted more attention, its thermal, chemical, and mechanical instability hinders its large scale applications [5]. These problems lead to the development of other porous semiconductors, such as the conventional III–V compounds including GaAs, GaP and InP as well as the wide band gap materials such as GaN and SiC [6]. GaN is generally grown on sapphire (13.6% lattice mismatch), SiC (4% lattice mismatch), and Si (16%). However, the large lattice mismatches degrade the layer quality [7]. To overcome this, porosity has emerged as an effective tool to control the electronic and optical properties of GaN quantum structures. The characteristics of the surface strongly affect the properties of the semiconductor. Quantum confinement effects control the mechanism of the
∗ Corresponding author. Tel.: +60 174316405; fax: +60 46579150. E-mail addresses: asmat [email protected], [email protected] (A. Ramizy). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.034
luminescence in nanocrystallites [8,9]. The reduction in size to a few nanometers is required for efficient light emission since it produces a modification in the electronic, optical and vibrational properties [10]. A few studies have been reported on porous GaN using electroless wet chemical and photoelectrochemical etching [11–13]. In this work, porous GaN has been synthesized by an electrochemical etching technique. By varying the etching anodization conditions, this technique is more suitable and cheaper for producing highdensity nanostructures with controlled pore size and shapes than the other techniques. There are various applications involving the use of hydrogen as in industrial fabrication processes and hydrogen fuel cells that are used in transportation vehicles. However, because hydrogen gas is flammable and explosive when its concentration in air is more than 4% at room temperature, its detection has become one of the most important safety issues, Also because of impending needs for robust gas sensors operating in harsh environments, nitride groups semiconductor-based hydrogen gas sensors have thus attracted much attention [14,15]. Recently, a hydrogen sensor diode was fabricated with the reactive Ga2 O3 oxide layer directly grown on the GaN layer using a photoelectrochemical oxidation (PEC) method [14]. Wright et al. [16] used Pt-coatings sputtered onto multiple GaN nanowires to enhance their sensitivity for hydrogen at hundreds of ppm level at 25 ◦ C. Pt-coated multiple GaN nanowires showed non-linear relative responses of ∼1.7% at 200 ppm up to ∼1.9% at 2000 ppm H2 in N2 after a 10-min exposure. Today, the optimum electrochemical etching parameters in the nitride groups are still unclear. The objective of this work is to inves-
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Fig. 1. SEM image of the fabricated gas sensor with Pt contact.
tigate the selected etching parameters to control the size, shape, surface chemistry, and emission properties of the nanostructures. This technique could open a new and promising field in the binary nitride groups if it transforms the material’s properties by making them similar to the ternary. To the best of our knowledge, this has never been reported before. 2. Experiment III-nitrides heterostructure of n-type GaN with AlN buffer layer were grown on the n-type Si(1 1 1) substrate with dimension of 3in. using plasma assisted molecular beam epitaxy (PAMBE) Veeco model Gen II system. Source materials of high purity, such as gallium and aluminum, were used in the Knudsen cells. Highly purified nitrogen was channeled to the radio frequency (RF) source to generate reactive nitrogen species. The plasma was operated at a typical nitrogen pressure of 2.5 × 10−5 Torr under a discharge power of 300 W. The growth of III-nitrides on the 3-in. Si(1 1 1) substrate was begun after the standard cleaning procedure using the Radio Corporation of America RCA method .The substrate was then mounted on the wafer holder and loaded into the MBE system. The Si sub-
strate was outgassed in the load-lock and buffer chambers. After outgassing, the Si was transferred to the growth chamber. Prior to the growth of the epilayers, surface treatment of the Si substrate was carried out to remove the SiO2 . The Si substrate was heated to 750 ◦ C, and a few monolayers of Ga were deposited on the substrate for removing the SiO2 by formation of Ga3 O2 . Before the growth of nitride epilayers, a few monolayers of Al were also deposited (1–3 monolayers) at 850 ◦ C on the silicon surface. This was done prior to the introduction of N2 until the 7 × 7 surface reconstruction disappeared to avoid the formation of Six Ny , which is deleterious for the growth of the subsequent epilayers. The buffer or wetting layer, AlN was first grown on the Si substrate. To grow the AlN buffer layer, the substrate temperature was heated up to 860 ◦ C, both of the Al and N shutters were opened simultaneously for 15 min. Subsequently, GaN epilayer was grown on top of the buffer layer for 60 min with substrate temperature set at 845 ◦ C. Porous GaN samples were synthesized by electrochemical etching technique [17]. A GaN wafer with (0 0 0 2) orientation, carrier concentration of 2.1 × 1019 cm−3 , thickness of 0.47 m with radius of 0.25 cm in order to match with the size of the opened-circular of the electrochemical cell, was placed in an electrolyte solution of ethanol 99.999%:HF40%, at varying current densities (25, 50, 75 mA/cm2 ) and constant etching times of 12 min. The Si wafer was immersed in HF acid to remove the native oxide. The electrochemical cell was made of Teflon and has a circular aperture on its bottom under which the silicon wafer was sealed. The cell was a twoelectrode system with a silicon wafer as an anode and platinum as a cathode. The synthesis was carried out at room temperature. After etching processing all samples were rinsed with ethanol and dried in nitrogen shower. Surface morphology and structural properties of nanostructures were analyzed using scanning electron microscopy (SEM), and X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX). Photoluminescence (PL) measurement was also performed at room temperature using an He–Cd laser ( = 325 nm), and Raman scattering has been investigated using an Ar+ Laser ( = 514 nm). To fabricate the gas sensor, Pt Schottky contact with thickness of 200 nm was deposited onto the GaN film using an RF sputtering system. A metal mask consisting of an array of holes with a diameter of about 0.9 mm was used for this purpose, as shown in Fig. 1. In a
Fig. 2. Gas sensor experimental setup.
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Fig. 3. SEM of porous GaN prepared by electrochemical etching with time of 12 min and different current densities (a) as-grown, (b) 25 mA/cm2 , (c) 50 mA/cm2 , (d) 75 mA/cm2 .
home-made gas sensing chamber, a 2% H2 in 98% N2 gas was used. The sensor is fixed in the gas chamber as shown in Fig. 2(I–V). A measurement using the current–voltage sensor is taken at room temperature with different H2 gas flow rates, at a fixed time of 2 min. 3. Results and discussion Fig. 3(b)–(d) shows the top view SEM image of the porous GaN surface prepared using electrochemical etching with varying current densities, while keeping the etching time at 12 min. Fig. 3(a) shows the surface morphology of the as-grown GaN sample. Pits were found on the GaN surface but they were in a small ratio to that observed by another researcher group which observed that if the carrier concentration value of Si-doped GaN films grown with AlN buffer layers is approximately 1 × 1018 cm−3 , then many grooves, V-shapes, and cracks were noticeable on that surface [18]. Although our sample had a high carrier concentration value of 2.1 × 1019 cm−3 it showed fine and non-cracked formation. The effect of varying current density on the morphology of porous GaN layer is observed. The chemical reaction was initiated at a current density of 25 mA/cm2 , and due to the insufficient structure interaction the etched area was irregular. In addition, the existence of the edges of the remaining non-etched layer was observed as shown in Fig. 3(b), which may be attributed to the shortage etching time in relation to thickness of the porous layer. This means that the electron–hole pairs that were generated were inadequate [19]. When current density was increased to 50 mA/cm2 , well-defined layers of pores with sizes ranging from 5 to 15 nm over the whole surface area were observed, as shown in Fig. 3(c). This means the etch rate in the crystal centers is slow enough so that the grain boundaries are etched at a sufficient accuracy. On the other hand, by increasing the current density to 75 mA/cm2 , the formation of pore structures with different sizes and shapes could be seen in Fig. 3(d); the etched surface became hexagonal, and the pore structures are smaller in size. In addition, the pore walls were very thin with some
Fig. 4. Cross section images of the GaN (a) before (b) and after etching process.
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Fig. 5. AFM images of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities: (a) as-grown, (b) 25 mA/cm2 , (c) 50 mA/cm2 , (d) 75 mA/cm2 .
Fig. 6. EDX of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities (a) as-grown, (b) 25 mA/cm2 , (c) 50 mA/cm2 , (d) 75 mA/cm2 .
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Fig. 7. Etching current density as a function of doped concentration and mobility of carriers.
short thin tips at the top. This indicated that when the etch rate is too fast, the grain boundaries are etched significantly slower than the center of the crystals. This leads to a hexagonal and rough morphology [20]. Fig. 4 presents the cross section of the n-GaN thin films before and after the etching process. The thickness of the AlN buffer layer and n-GaN epilayer are 0.069 m and 0.47 m respectively. We estimated the GaN growth rate to be about 0.5 m/h and the AlN growth rate is 0.27 m/h as shown in Fig. 4(a). The depth of porosity for the selective sample presented was approximately 0.22 m as shown in Fig. 4(b). The thickness of the porous layer increased with respect to increasing current density. When the current density is increased, anodic bias is increased, and the dissolution of silicon extends to the bulk of the wafer. Increased anodic bias results in holes with higher energy, which overcome the barrier at the pore ends and wall. Fig. 5(a) describes the three-dimensional topographic image of the as-grown sample. The etched surface with the pyramidal shape was distributed over the entire surface. The pyramidal shape indicated that the increase in the surface roughness is attributed to the etching parameters affecting the surface characterization. Because the porous surface texture has a high degree of roughness, this sug-
gests a possible use for the porous layer as an antireflective coating since it reduces the light reflection. Because roughness is a function of layer thickness, the layer thickness of an antireflective coating increase as roughness increases; this leads to changes in the optical, electronic and vibrational transitions [17]. The attenuation of the reflectivity is due to scattering and transmission at the porous and bulk interfaces [21]. The etched surfaces range in heights nanoparticles from 50 to 200 nm relative to the etch current density, as shown in Fig. 5(b)–(d). Fig. 6 shows the EDX spectra and the atomic composition of the film elements for unintentionally doped n-type GaN/AlN/Si(1 1 1) films and the porous GaN samples. The peak intensity refers to the concentration of the element in the sample and the atomic composition by percentage. The results indicated that the unintentionally doped n-type GaN/AlN/Si(1 1 1) films are of good quality, without the presence of contaminating elements as shown in Fig. 6(a), in contrast to other research group [22], Fig. 6(b)–(d) reveals the EDX data after the etching process. These indicate the presence of the aluminum (Al) element with a ratio between 1% and 9%. This means that the Al started to appear on the surface and increased gradually in proportion to the etching parameters. This result could open
Fig. 8. PL spectra of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities.
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Fig. 9. Raman spectra of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities.
a new and promising field in the binary III-nitride materials group through the conversion of the properties of the materials to become similar to that in the ternary nitride materials group (AlGaN). This result is also confirmed by the analysis of the PL results later. Fig. 7 presents the Van der Hall measurements using an Ohmic contact. The results showed that the doped concentration decreased as a function of the porosity of the etched films, and the mobility of carriers was increased. This may be due to the formation of nanostructures on the whole surface of the etched film, which leads to increases in the probability of the carrier’s transfer through the surface.
Fig. 8 shows the PL spectra at room temperature for the porous GaN. Higher blue shift luminescence was observed in comparison to the as-grown, and the energy gap of porous GaN is increased as a function of etching current density, as shown in Table 1. The broadening of the band gap energy occurs with the decrease in the crystallite size. These results are different in comparison to results from other research groups which found that porous GaN exhibited a red shift photoluminescence [23,24]. Our results showed greater blue shift luminescence in comparison to other research groups [25,26]. This is attributed to charge carrier quantum confinement. This means that the particles are confined in the lower dimension and the probability of the recombination of the electrons and holes
Fig. 10. XRD spectra of porous GaN prepared by electrochemical etching with time of 12 min and varying current densities.
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Fig. 11. I–V measurement of porous GaN gas sensor with different H2 gas flow rate.
0,80 Air:porous
Schottky barrier height (eV)
0,75 Air: as-grown
0,70
H2 (1sccm) H2 (2sccm)
0,65
0,60
H2 (3sccm)
0,55
H2 (4sccm)
0,50 0
1
2
3
4
Flow rate(sccm) Fig. 12. The effective Schottky barrier height as function of the hydrogen flow rate.
Table 1 The energy gap, FWHM, peak shift, intensity, and the radius of nanocrystillites of different samples obtained from the PL spectra at room temperature. Sample
Peak position (nm)
FWHM (meV)
Peak shift) (nm)
intensity (a.u)
Energy gap (eV)
Nanocrystillites radius (nm)
As-grown 25 mA/cm2 50 mA/cm2 75 mA/cm2
362.0 346.5 345.5 335.0
67 86 90 100
– 15.5 16.5 27.0
4.5 × 105 8.09 × 106 8.2 × 106 1.0 × 10 7
3.42 3.57 3.58 3.70
– 9.5 9.0 7.1
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Fig. 13. Room temperature on–off responses of hydrogen gas sensors measured at constant voltage of 1 V.
is higher in the low dimensional structures. This leads to high emission efficiency from high porosity structures arising from quantum confinement effects. Reducing the dimensions to nanometers drastically changes the physical properties of GaN film, as previously mentioned in the energy dispersive X-ray analysis. An estimate of the size of the GaN nanocrystallite was identified using the effective mass theory. Assuming that there are infinite potential barriers, the of the 3D-confined GaN were obtained as: [27] En (eV) = Eg +
h2 8d2
1 1 + ∗ m∗e mh
Table 2 Peaks position, peak shift, and intensity of different samples obtained from Raman spectra at room temperature. Sample
Peak position (cm−1 )
Intensity (a. u)
Peak shift
As-grown 25 mA/cm2 50 mA/cm2 75 mA/cm2
568.1 566.8 567.4 566.5
498.5 587.6 606.9 786.5
– 1.3 0.7 1.7
(1)
where Eg is the as-grown GaN energy gap, d is the diameter of the spherical particle, and the m∗e , m∗h the electron and hole effective mass, respectively (at 300 K, the m∗e = 0.20m0 , m∗h = 1.0m0 and Eg = 3.42 eV) [28]. From Eq. (1) the radius of nanocrystallites En are summarized in Table 1. The room temperature photoluminescence shows that the increase in intensity is more than 22 times that of the grown sample, with etching current density as shown in Table 1. This strength of photoluminescence output becomes stronger due to increasing porosity. This means the intensity of photoluminescence is proportional to the number of emitted photons on the porous surface. This surface experiences a shrinking of its size caused by deeper etching, which in turn leads to an increase in the number of transitions inside the structure. Fig. 9 shows Raman’s spectra of as-grown GaN and porous GaN, respectively. There is a strong band at 522 cm−1 from the Si(1 1 1) substrate, and a small band at 301 cm−1 , due to the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 cm−1 and 568 cm−1 due to E2 (low) and E2 (high) respectively. Porous GaN spectra displayed a decrease in the Raman intensity relative to as-grown GaN, as shown in Table 2. We believe that, due to the particles confined into the lower dimension, this may be leads to increasing the probability of the particles vibrating in relation to the laser incident beam, which enhances the number of scattered photons per unit incident angle for a given laser power density. Porous GaN spectra were shifted and broadened relative
to the as-grown GaN, which is attributed to the quantum confinement of optical phonons which is distributed upon the etched surface [29]. This means that the nanostructures shape and size are obtained with a high surface area to a small volume ratio that is most sensitive in response to any external effects. Fig. 10 shows a small broadening peak for porous GaN in comparison to as-grown GaN. The data in Table 3 showed that the peak broadening occurred when the current density was gradually increased.In addition, the reduction in crystallite size is confirmed by increases in the broadening of XRD spectra. On the other hand, the lattice constant and peak positions also changed due to chemical treatment. The distorted shape of the porous GaN crystallite led to a change in the phase diffraction. The capability of gas sensors to operate in harsh environmental conditions, such as high-temperature and highly corrosive chemical conditions is one of the important specifications of high quality gas sensors. The large band gap of GaN, chemical stability and Table 3 Different lattice constants and diffraction peak position of different samples derived from XRD analysis. Sample
Peak (◦ )
FWHM (◦ 2Th.)
d-spacing (Å)
c (Å)
As-grown 25 mA/cm2 50 mA/cm2 75 mA/cm2
17.261 17.2983 17.3003 17.3040
0.19 0.29 0.19 0.29
2.59 2.59 2.59 2.59
5.189 5.18 5.18 5.19
A. Ramizy et al. / Sensors and Actuators B 155 (2011) 699–708 Table 4 The sensitivity% of the gas sensors operating at room temperature with different H2 gas flow rate. Sample
Sensitivity (%)
Air:porous H2 (1 sccm) H2 (2 sccm) H2 (3 sccm) H2 (4 sccm)
0.83 7.60 7.77 9.0 13.80
By assuming the flow current mechanism is governed by the thermionic emission conditions, is given by [28] KT ln B = q
mechanical robustness leads to the use of this semiconductor in many harsh applications, including gas sensing operations during chemical reactor processing, fire alarms onboard aircraft and space vehicles, as well as in devices that detect fuel leaks in automobiles and aircraft. A unique advantage of a GaN gas sensor is that it can be integrated with GaN-based solar-blind UV photodetectors or high-power, high-temperature electronics devices on the same chip [30,31]. Recently, there are many reports of GaN gas sensors utilizing Schottky contacts [32–34]. Such types of devices have typical detection abilities in the range of 4.7% H2 in the air [35]. The unique physical properties of the porous GaN establish a special feature, which is considered a promising material for use in sensing applications. Fig. 11 shows measurement of porous GaN gas sensor fabricated using electrochemical parameter etching with different H2 gas flow rate. The result showed that the sensor was able to detect hydrogen in the range of 1–4 sccm with sign of saturation. The sample of GaN and porous GaN showed a good Schottky behavior when operated in the air at room temperature, which indicated the Pt contact is of good quality. On the other hand, both samples changed to Ohmic contact behavior when 1–4 sccm H2 gas was released. This means that the porous surface of Pt/GaN contact has a larger surface area, and this allows hydrogen molecules to dissociate and form atomic hydrogen more efficiently. Moreover, the unique surface morphology offers a higher accumulation of hydrogen at the Pt/porous GaN interface. The performance of the gas sensor was estimated through the measurement of the hydrogen detection sensitivity, which is defined as: [36] IH2 − IAir Iair
(2)
at constant voltage of 3 V.Table 4 shows that the sensitivity of the gas sensor increases as a function of the hydrogen flow rate and becomes much higher compared to the as-grown sample. The increase in the sensitivity can be explained, according to Johansson [37], by an electrical polarization that is given by: V =
Ni i ε
AA∗ T 2 I0
(4)
where I0 is the saturation current density, q is the electron charge, K is Boltzmann’s constant, T is the absolute temperature, A* is the effective Richardson coefficient, A is the contact area, and the theoretical value of A* can calculated using A∗ =
S=
707
(3)
where is the effective dipole moment, Ni is the number of sites per area at the interface, is the coverage of hydrogen atoms at the interface, and ε is the dielectric constant. It should be noted that the role of porous Pt/porous GaN interface is not only limited in providing a larger surface for dissociating the hydrogen molecules more efficiently but also in generating higher. Eq. (2) suggests that V increases with the Ni . Since Pt/porous GaN interface is able to supply higher Ni , this unique structure subsequently produces larger electrical polarization and leads to a drop in the effective Schottky barrier height (SBHs) [38]. The effective Schottky barrier height, as function of the hydrogen flow rate, can be determined from I–V measurement.
4 m∗ K h3
(5)
where h is Plank’s constant. For n-type GaN, m* = 0.20 m0 is the effective electron mass for GaN. Fig. 12 showed reduced effective Schottky barrier height as function of the hydrogen flow rate. Fig. 13 shows the on–off responses of the sensor measured at room temperature with a constant voltage of 1 V. The response and recovery time measured for the sensor are about 3 and 2.5 min, respectively. The sensor showed a good sensitivity as a function of time with the minimum flow rate 1 sccm of the test gas (H2 ) which means it exhibited a good performance. The maximum sensitivity is attained in more than 10 and 20 min (shut off-time) after cutting off the hydrogen gas and this may accrue due to the trapping of hydrogen atoms from the defects at the Pt/PGaN interface and these atoms would then diffuse through the Pt thin film and moving towards the Pt/PGaN contact interface which leads to polarizing, thus forming a dipole layer near the interface. Once the dipole layer is formed at the interface culminate, returning to the initial value is prevented [39,40]. 4. Conclusions The surface morphology of the as-grown GaN showed fine and non-cracked formation. The effect of varying the current density on the morphology of porous GaN layer is observed. By increasing the current density to 75 mA/cm2 , the formations of pore structures are different in size and shape. The etched surface becomes hexagonal, and pores structures are confined to smaller sizes. EDX data revealed that, after the etching process, the presence of the aluminum (Al) element with a ratio between 1% and 9% can be detected, which means that the Al element started to appear in the surface and increased gradually in proportion to the etching parameters. This result could open a new and promising field in the binary III-nitride materials group through the conversion of the properties of the materials to become similar to that in the ternary nitride materials group (AlGaN). This result is also confirmed by the analysis of the PL measurement, which showed greater blue luminescence, the energy band gap became 3.70 eV after the etching process. This value of the energy band gap is similar to that in the ternary nitride group (AlGaN). The lattice constant and peak positions also changed due to chemical treatment. The sensitivity of the gas sensor is increased to the highest level, as a function of the hydrogen flow rate and becomes much higher in comparison to the as-grown sample. Acknowledgement Support from FRGS grant and Universiti Sains Malaysia are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2011.01.034.
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Biographies Asmiet Ramziy received the B.Sc. degree and the Masters degree in Physics from Mustansiriya University- collage of science, Iraq in 1996 and 1999, respectively, currently Ph.D. student at Universiti sains Malaysia, Lecturer at Al-Anbar University, Iraq, since 2001. Participated in many national and international conferences and workshops and published 15 papers in ISI journals and 6 papers in non-ISI journals. Research interested in nanostructured materials and their applications, Laser-Induced Etching and porous solar cell. Zainuriah Hassan received the B.Sc. (Magna Cum Laude) degree and the Masters degree in Physics from Western Michigan University, USA in 1983 and 1985, respectively, and Ph.D. degree in Experimental Condensed Matter Physics from Ohio University, USA in 1998. She was a Research Associate at Ohio University from 1997 to 1998, and a Visiting Research Scholar under the Fulbright Program at Department of Electrical and Computer Engineering, University of Minnesota, USA in 2004/2005. At present, she is the Dean, and formerly the Deputy Dean (Academic and Student Development) and Chair of the Engineering Physics Program at School of Physics, Universiti Sains Malaysia. She was promoted to Professor in 2009 and is attached to the Condensed Matter, Applied and Engineering Physics group. She has received several awards, scholarships and recognitions which includes Outstanding Scholarship Award and Deans List from Western Michigan University, Graduate Scholarship from Ohio University, Excellent Service Awards, Fulbright Research Scholar Award, Sanggar Sanjung (Hall of Fame) Awards, and Merit Rewards. Her main focus of research is on wide band gap semiconductor materials, in particular III-Nitrides (GaN and related alloys) growth/deposition and characterization, as well as fabrication, characterization, and simulation/design of optoelectronic and electronic devices based on III-nitrides and other semiconductor materials. She has published more than 280 papers in international and national journals and proceedings. She is currently the Editor of Journal of Physical Science and a member of the Materials Research Society, Optical Society of America, IEEE, Fulbright Association, Malaysian Solid State Science and Technology Society, and Malaysian Institute of Physics. Dr. Khalid Omar lecturer and researcher at the School of Physics, Universiti Sains Malaysia (USM). Received Ph.D. in Physics/ Laser Technology from Jamia Millia Islamia (central university) New Delhi, India in September 2003. Continued an illustrious teaching career since 1987. Participated in many national and international conferences and workshops and published sixty seven papers in ISI journals and non-ISI journals as well as in proceedings of conferences since 2007. Interested in Laser-Induced Etching, Nanostructure materials, porous solar cell and laser-medical application.