Sains Malaysiana 37(3)(2008): 285–288
Nanoporous GaN Film Generated by Electro Chemical Etching
(Filem GaN Berliang Nano yang Disediakan Melalui Punaran Elektro Kimia) F.K. YAM, Z. HASSAN & K. M. OMAR
ABSTRACT
This article reports on the studies of structural and optical properties of nanoporous GaN prepared by Pt assisted electro chemical etching. The porous GaN samples were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical transmission (OT). SEM images indicated that the density of the pores increased with etching duration, however, the etching duration has no significant effect on the size and shape of the pores. AFM
measurements exhibited that the surface roughness was increased with etching durations, however, for long etching duration, the increase of the surface roughness became insignificant. OT measurements revealed that the increase of pore density would lead to the reduction of light transmission. The studies showed that the porosity could influence the structural and optical properties of the GaN.
Keywords: Porous GaN; structural properties; optical properties
ABSTRAK
Artikel ini melaporkan kajian sifat-sifat struktur dan optik bagi GaN nanoliang yang disediakan dengan punaran elektro- kimia bantuan Pt. Sampel-sampel GaN berliang dikaji dengan mikroskopi elektron imbasan (SEM), mikroskopi daya atom (AFM) and pemancar optik (OT). Imej-imej SEM menunjukkan bahawa ketumpatan liang bertambah dengan masa punaran, tetapi, masa punaran tiada kesan ketara ke atas saiz dan bentuk liang. Ukuran AFM mendedahkan bahawa kekasaran permukaan bertambah dengan masa punaran, bagaimanapun, bagi masa punaran yang lama, penambahan kekasaran permukaan tidak lagi bermakna. Ukuran OT menunjukkan bahawa penambahan ketumpatan liang boleh mengurangkan pemancaran cahaya. Kajian ini menunjukkan bahawa liang boleh mempengaruhi ciri-ciri struktur dan optik bagi GaN.
Kata kunci: GaN berliang; sifat struktur; sifat optik
INTRODUCTION
The fabrication of porous semiconductors has stimulated much research interest recently. The unique physical properties of the porous layer such as high surface area, shift of bandgap and efficient luminescence are among the special features which show promise in some of the sensing applications (Sohn et al. 2000; Lin et al. 1997). Among porous semiconductors, porous silicon receives enormous attention and has been most intensively studied; however the instability of physical properties has prevented it from large scale application (Fauchet et al. 1995). This leads to the development of other porous semiconductors, for instances, the conventional III-V compounds such as GaAs, GaP and InP; and the wide bandgap materials such as GaN and SiC.
The research in porous GaN is strongly driven by the superior physical properties such as the excellent thermal, mechanical and chemical stability, as well as the potential shift of the bandgap (Li et al. 2002), moreover, it has been reported that porous GaN can be used as intermediate layer for the reduction of substrate induced strain (Inoki et al.
2002; Inoki et al. 2003). Since bulk GaN in wafer size is not available, GaN thin film usually is grown on poor lattice
and thermal mismatch foreign substrates which will result in high residual stress and eventually lead to high density of structural defects. Porous GaN shows promise as a growth template for epitaxial re-growth; this could reduce the density of structural defects significantly and allows the growth of residual-free epitaxial GaN layers. Since the discovery of light emitting porous silicon by Canham in 1990, significant progresses have been made on the studies of the structural, optical as well as mechanical and electrical properties of the porous Si. Comparatively, the study of porous GaN is still in the early stage, many fundamental properties are not well-established.
Porous GaN was prepared by Pt assisted electroless chemical etching. The effect of different etching duration on the morphological, structural and optical characteristics of the porous GaN was investigated.
EXPERIMENTAL METHOD
The unintentionally doped n-type GaN film grown on sapphire substrate was used. The thickness of GaN film is about 3.0 mm with carrier concentration of ~ 4.38 × 1017 cm-3 as determined by Hall Effect measurement. The wafer
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was then cleaved into few pieces. Prior to the metallization, the native oxide of the sample was removed in the 1:20 =
NH4OH:H2O solution, followed by 1:50 HF:H2O.
Subsequently boiling aqua regia (3:1 = HCl:HNO3 ) was used to etch and clean the sample. Porous GaN was generated by Pt assisted electroless etching. Two narrow stripes of Pt with thickness of about 150 nm were deposited on the GaN sample by using sputtering system. The samples were then etched in a solution of 4:1:1
HF:CH3OH:H2O2 under illumination of an UV lamp with 500 W power for 15, 30, 60 and 90 minutes, respectively. After chemical treatment, the samples were removed from the solution and rinsed with distilled water; followed by the removal of the residual Pt by ultrasonic cleaning. The morphological, structural and optical properties of porous GaN samples were characterized by SEM, AFM, and OT.
RESULTS AND DISCUSSION SCANNING ELECTRON MICROSCOPY
Figure 1 shows the SEM images of the porous GaN samples generated under different durations. For 15 minutes sample, the etching was in the initial stage, it started to form ridges and valleys, surface became relatively rough, however no pore was found. For 30 minutes sample, ridges and valleys were formed with pores observed at the valley area. On the other hand, the samples etched for 60 minutes or longer duration, the surface morphology was found to be similar, suggesting that an etching saturation was reached, at this stage, high density of pores were observed. The etching duration has no significant effect on the pores. The size and shape of the pores remained relatively constant for those samples etched for 30, 60 and 90 minutes. From the
SEM images, the pore sizes were estimated to be around 50 to 80 nm.
(a) As grown (b) 15 minutes
(e) (f)
90 minutes 90 minutes under high magnification
FIGURE 1. SEM images of the samples etched under different duration. (a) As grown, (b) 15 minutes, (c) 30 minutes, (d) 90 minutes and (f) 90 minutes under high magnification
(c) 30 minutes (d) 60 minutes
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ATOMIC FORCE MICROSCOPY
Figure 2 shows the AFM measurements of the porous GaN samples under different etching durations. Table 1 summarizes the surface roughness of the samples analyzed by AFM. From Table 1, the root mean square (RMS) of the porous GaN samples increased with the etching durations.
The increase of RMS was found to be proportional to the etching durations from 15 to 60 minutes, after 60 minutes, i.e. 90 minutes, the further increase of RMS became insignificant. These observations could be further supported by SEM images shown in Figure 1 in which the surface morphology was changed gradually with the etching durations, for 15 minutes etched sample, ridges and valleys formed with no pores, 30 minutes etching would deepen the etched area (valleys) with the formation of pores, 60 minutes would further increase the etching process with the generation of higher density of pores, however, further etching duration i.e. 90 minutes, the etching process seemed to reach a saturated stage, in which there were little change in the density of pores and surface roughness as revealed by both SEM and AFM measurements.
TABLE 1. The surface roughness (root mean square) of the samples measured by AFM
Sample RMS (nm)
As grown 5.65
15 min 7.11
30 min 15.83
60 min 30.79
90min 32.93
OPTICAL TRANSMISSION
Figure 3 shows the optical transmission spectra of the samples, it is interesting to note that for higher wavelengths (λ>370 nm) transmission (%) is gradually reduced with the etching durations of the porous GaN samples, and transmission (%) became constant after 60 minutes.
The gradual reduction of the transmission could be attributed to the change of the pore density of the chemical treated GaN samples, from SEM images, in which for 15 min sample, only ridges and valley was observed and no pore was found, however for 30, 60 and 90 minutes samples, the pore densities were estimated as 0.7 × 1010, 1.1 × 1010 and 1.1 × 1010 cm-2, respectively. Obviously, with the increase of the pore density, the photon transmission became lower.
Sharp decline transmission profiles of as-grown and 15 minutes samples were observed in Figure 3, but these were not found for the high pore density porous samples (60 min and 90 min). The drop of the transmissions started from 373 nm, and approached to zero transmission at about 363.0 to 364.5 nm (or Eg, equivalent to 3.402 to 3.416
(a) 15 minutes (b) 30 minutes
(c) 60 minutes (d) 90 minutes
FIGURE 2. AFM micrographs of the porous GaN samples showing different surface topography
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eV). For the as-grown sample, the intercept was found at lower wavelength among the samples.
CONCLUSION
SEM, AFM and OT have been used to characterize the porous GaN samples fabricated by Pt assisted electroless chemical etching. SEM images showed that the density of the pores increased with the etching duration; however, the etching duration has no significant effect on the size and shape of the pores. AFM measurements exhibited that the surface roughness was increased with etching durations; however, no significant increase of RMS for long etching duration.
OT measurements revealed that the light transmission was gradually reduced with the increase of pore density.
Porosity in GaN films could influence the structural and optical properties of the material.
ACKNOWLEDGEMENT
This work was conducted under 304/PFIZIK/637040 short term grant. The support from Universiti Sains Malaysia is gratefully acknowledged.
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F.K. Yam, Z. Hassan & K.M. Omar School of Physics
Universiti Sains Malaysia 11800 Penang,
Malaysia
Received : 12 June 2007 Accepted : 31 October 2007
FIGURE 3. Optical transmission spectra of the samples, inset shows the enlargement