Photo-electrochemical etching of free-standing GaN wafer surfaces grown by hydride vapor phase epitaxy

Electrochimica Acta 171 (2015) 89–95

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Photo-electrochemical etching of free-standing GaN wafer surfaces grown by hydride vapor phase epitaxy Junji Murata a, * , Shun Sadakuni b a b

Department of Mechanical Engineering, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

A R T I C L E I N F O

A B S T R A C T

Article history: Received 28 November 2014 Received in revised form 2 March 2015 Accepted 28 April 2015 Available online 30 April 2015

An investigation into the photo-electrochemical (PEC) etching of free-standing GaN wafers produced by hydride vapor phase epitaxial growth (HVPE) has found that etching is only possible with UV illumination in an acidic or basic electrolyte. Through photo-current measurement and X-ray analysis it was determined that lack of etching in a neutral electrolyte can be attributed to the formation of an oxide film on the GaN surface. Surface damage was also found to be a significant factor, with the etching rate and photo current density of surfaces treated by grinding and mechanical polishing being markedly less compared to a finely polished surface. Subsequent investigation of the luminescence and the etching characteristics of the intentionally-introduced scratches indicated that subsurface damage is difficult to remove from GaN by PEC etching due to the trapping of photo-excited carriers. A peculiar surface feature of concentric ring structures made up of alternating small and large pores was observed on the GaN surface along with small island regions, which is attributed to variations in the electronic properties of the GaN crystal that is created during HVPE growth. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Gallium nitride Photo-electrochemical etching Free-standing wafer Hydride vapor phase epitaxy (HVPE) Subsurface damage

1. Introduction Gallium nitride (GaN) and other Group III–V materials have attracted significant attention due to their suitability for use in short-wavelength emitters or detectors, as well as in high-power and high-frequency electronic devices. Achieving the full potential of such devices, however, requires a suitable method for removing selected portions of the GaN surface. Although wet chemical etching techniques are extensively used with conventional semiconductor technology for removing subsurface damage, surface cleaning and nano-structure fabrication, such methods are far less effective in etching GaN due to its inherently high thermal and chemical stability [1]. Indeed, only high temperature treatments involving molten KOH [2] and hot phosphoric acid [3,4] have proven to be capable of etching GaN surfaces, but this creates a high cost and energy consumption. Extensive investigation into the photo-electrochemical (PEC) etching of semiconductors has led to the development of a suitable method for use at room temperature. In this, the surface of a semiconductor immersed in an electrolyte is irradiated by

* Corresponding author. Tel.: +81-6-4307-3474. E-mail address: [email protected] (J. Murata). http://dx.doi.org/10.1016/j.electacta.2015.04.166 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

ultraviolet (UV) light with an optical energy greater than the band gap of the materials. This generates electron-hole pairs, which then initiate a redox reaction at the semiconductor/ electrolyte interface. This method has been used for the chemical lift off of GaN layers [5] and in light-emitting diode (LED) fabrications [6] with a practical etching efficiency, but has so far only been used with epitaxial GaN thin films [7–10] applied to a substrate by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). However, free-standing GaN bulk wafers have been produced quite recently by hydride vapor phase epitaxy (HVPE) [11–14], and the demand for these has significantly increased due to the fact that they represent high-quality GaN crystals with a low dislocation density. This paper therefore looks at the PEC etching of a free-standing HVPE GaN surface. The effect of the electrolyte composition and presence of subsurface damage on the etching rate is evaluated along with the morphology of the etched GaN surface. 2. Experimental The samples used in this study were commercially available free-standing GaN wafers (n-type doped, 300 mm thick) that were produced by hydride vapor phase epitaxial (HVPE) growth, and which had a carrier concentration of (1-3)  1018 cm 3. Unless

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Fig. 3. Optical micrograph of the surface of GaN PEC etched in H3PO4 (pH = 1.0).

otherwise noted, all samples were treated prior to testing by fine polishing [15,16] to obtain a damage-free surface with a typical root-mean-square surface roughness of just 0.1 nm. As shown in Fig. 1, GaN samples immersed in an electrolyte were illuminated to a controlled intensity using a Hg-Xe lamp (LIGHTNING CURE L9588, Hamamatsu photonics). In order to determine the etching depth, part of the sample surface was masked with a polyimide film and the step formed was measured using a stylus profiler. The pH of the electrolyte was controlled to 1.0, 6.86 or 13.0 by adding H3PO4, phosphate buffered solution (PBS) or KOH, respectively. To provide an electrical contact, a Pt/Cr bilayer was applied to the back of the GaN wafer by electron-beam evaporation. The photo-induced current flow between the sample and a Pt counter electrode was measured using a digital multimeter, with no bias being applied between the two electrodes. All etching experiments were conducted at room temperature. The surface morphology of the etched samples was observed using confocal laser scanning microscopy (LEXT OLS3100, Olympus), optical interferometric microscopy (NewView 200CHR, Zygo), and scanning electron microscopy (SEM; S-4800, Hitachi). A chemical analysis of the sample surface was also conducted using X-ray photoelectron spectroscopy (XPS; Quantum 2000, ULVAC PHI) with an Al Ka X-ray source. 3. Results and discussion

ring structures with some islands structure that is believed to originate from the non-uniformity of the electronic properties of GaN crystals produced by HVPE, which is discussed in more detail further on. In contrast, there was no step evident between the illuminated and masked region in the case of the neutral (pH = 6.8) solution, suggesting that this failed to produce any significant etching of the GaN surface. As shown in Fig. 4, the current density flowing from the Pt electrode to the GaN samples varies depending on the pH of the electrolyte. That is, in an acidic or basic electrolyte the current density remains almost constant during UV illumination, while in a neutral electrolyte the current density decreases rapidly with time. As shown in Fig. 5, PEC etching is initiated by the generation of

UV on H3PO4 (pH=1.0) 4 Current density / mA cm-2

Fig. 1. Schematic view of the experimental setup used.

Figs. 2 (a)-(c) show cross-sectional profiles of GaN surfaces subjected to UV illumination for 20 min in various electrolytes, which reveals that the etching depths in acidic (pH = 1) and basic (pH = 13) electrolytes are both approximately 1.5–2.0 mm. As shown in Fig. 3, these two electrolytes also produced concentric

Height / μm

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0 0

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360 540 Time / s

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Fig. 4. Photo-current density between a GaN wafer and Pt cathode as a function of time. UV illumination started from 1 min under all electrolyte conditions.

(b)

1

PBS (pH=6.8)

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3.1. Effect of electrolytes

2

KOH (pH=13.0)

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Illuminated region

0 -1 -2

0

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2 Distance / mm

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1

2 Distance / mm

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4 0

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2 Distance / mm

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Fig. 2. Cross-sectional profiles of GaN surfaces treated in (a) H3PO4 (pH = 1.0), (b) phosphate buffered solution (pH = 6.8) or (c) KOH (pH =13.0) electrolytes under a UV intensity of 5.2 mW/cm2 for 20 min.

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Fig. 5. Schematic view of the PEC etching process. (a) Electron-hole pairs are generated by UV energy. (b) Carriers are separated along the potential gradient in the depleted layer. (c) Holes react with the oxidant at the semiconductor/electrolyte interface. (d) An oxide film is formed on the semiconductor surface, and the surface is etched if this oxide is soluble in the electrolyte.

electron-hole pairs through excitation by optical energy, with subsequent oxidation of the semiconductor due to reaction between holes in the valence band and oxidants in the electrolyte. Thus, only if the resulting oxide is soluble in the electrolyte is the semiconductor surface etched. According to the pH/potential equilibrium diagram (Pourbaix diagram) [17], Ga oxide (Ga2O3) is stable under neutral conditions (a pH range of 4–12), but dissolves under acidic or basic conditions to form Ga3+ or Ga(OH)4 ions, respectively. The XPS spectra shown in Fig. 6 demonstrates how the Ga 3d spectrum obtained from the surface of GaN treated in the

(a)

Ga-O

Ga-N

neutral solution can be decomposed into three distinct components at 19.6 eV (Ga-N), 20.5 eV (Ga-O) and 23 eV (O 2s). Although the Ga-O component was also observed with the GaN treated in acidic or basic solution, the peak intensity of this and the O 2s spectra were markedly lower. This indicates that a Ga oxide layer was formed in the neutral solution by UV illumination, but as shown in Fig. 2, this oxide was not etched. Instead, this oxide inhibits charge transfer at the semiconductor/electrolyte interface, thus resulting in the decrease in photocurrent evident in Fig. 4. In previous reports [18,19], the surface morphology was found to vary

Ga3d

O 2s

(b)

O1s

Counts / a. u.

Counts / a. u.

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N 2s

(c)

PBS (pH6.8) H3PO4 (pH1.0) KOH (pH13.0)

26

24 22 20 18 16 Binding energy / eV

14

540

535 530 Binding energy / eV

525

Fig. 6. XPS Ga 3d spectra obtained from the surface of GaN treated in (a) PBS (pH=6.8), (b) H3PO4 (pH=1.0), or (c) KOH (pH=13.0) electrolytes. (d) O1s spectra.

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with the electrolyte used due to the difference in redox potential and the solubility of the oxide in acids. Consequently, there is a need for further investigation of the photocurrent and chemical state of the etched surface in different acids.

6 Etching rate 5

Current density

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4

60

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40

2

20

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grinding

mechanical polishing

fine polishing

Current density / mA cm-2

Etching rate / nm min-1

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Fig. 7. Comparison of the etching rate and photo-current density in KOH solution for different GaN surface finishes.

3.2. Effect of subsurface damage Fig. 7 shows a comparison of the PEC etching rate and photoinduced current density in a KOH solution (pH = 13.0) with different surface finishes. It is evident from this that mechanical polishing produces an etching rate and current density that is markedly lower than what is achieved with a sample prepared by fine polishing. Furthermore, no apparent etching or photo-induced current were observed with a ground surface, which is confirmed by the lack of change in surface morphology in the laser confocal scanning microscopy images taken before and after etching (Fig. 8 (a)). In contrast, there is clear evidence of etching on the surface of the mechanically-polished sample; however, the scratches induced by mechanical polishing were not removed and still remain on the etched surface, as indicated by the arrows in Fig. 8 (b). The resistance of these scratches to etching is also evident in the magnified SEM image in Fig. 8 (c), which suggests that subsurface damage caused by mechanical machining is difficult to remove by

Fig. 8. Laser confocal scanning microscopy images of the surface of GaN after and (inset) before etching. The initial surfaces were machined by (a) grinding and (b) mechanical polishing. (c) SEM image showing an etched GaN surface that has retained scratches that were introduced by mechanical polishing.

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PEC treatment. The fact that the ground surface was entirely covered with such subsurface damage therefore explains its extremely low etching rate. To evaluate the effect of mechanical abrasion on the PEC etching of GaN, a scratch was intentionally introduced into a fine polished GaN surface using a scratch tester with a single point diamond tool. As can be seen in the interferometric microscopy image in Fig. 9, this produced a scratch with a depth of approximately 60 nm. Evaluation of the GaN surface surrounding this scratch by scanning micro-photoluminescence (m-PL) at room temperature excited using a focused He-Cd laser (l = 365 nm), which as shown in Fig. 10, found that the PL intensity of this region is approximately two orders of magnitude lower than that of the unscratched region. This suggests that photo-excited carriers are trapped by recombination centers introduced by mechanical abrasion; and indeed, a similar result has been previously reported by Aida et al. [20,21] with regards to the cathode luminescence of mechanicallypolished GaN surfaces. The scratched sample was subsequently treated by PEC etching in KOH solution (pH = 13.0), with Fig. 11 showing an optical interferometric image obtained from its surface. It is apparent from this image that a step height of approximately 1 mm was formed in the illuminated area of the GaN surface along the intentional scratch, thus confirming that it was not removed by etching. This clearly differs from the conventional wet etching of GaN by hot phosphoric acid or molten KOH, where crystal defects are preferentially removed and etching pits are formed on the GaN surface [2–4]. Instead, such crystal defects form a recombination center in the band gap that traps photo-excited carriers. This prevents charge transfer at the semiconductor and Pt cathode/ electrolyte interfaces, and as a result the surface around such defects is not etched. A similar phenomenon was observed in the PEC etching of a GaN film with a high dislocation density [7], in which dislocations also acted as a carrier trap and therefore proved

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Fig. 10. (a) Monochromatic scanning m-PL image of a GaN surface around an intentionally introduced scratch (PL wavelength = 365 nm). (b) Cross-sectional PL intensity profile along the dashed line in (a).

difficult to remove by PEC etching. This resulted in a columnar structure being formed on the etched GaN surface, but more importantly, this suggests the possibility of using focused energy beam technology with the PEC treatment to produce nanostructures in the surface of GaN surface without the need for a mask. 3.3. Morphology of the etched GaN surface The surface morphology of the etched GaN surfaces was investigated in detail using SEM, and as shown in Fig. 12 (a), this revealed the surface to be almost entirely composed of concentric ring structures with some islands between them. Similar features were reported by K. Motoki et al. [12,13] on GaN wafers grown by HVPE on GaAs substrates using a SiO2 mask, with the intensity of the band-edge luminescence in the concentric structures being markedly higher than in the islands. This difference in morphology

Fig. 9. (a) Optical interferometric microscopy image of a scratch that was intentionally introduced to a finely polished GaN surface, and (b) cross-sectional profile along the dashed line in (a).

Fig. 11. Optical interferometric microscopy image of a PEC etched GaN surface showing that the surface around an intentionally introduced scratch is not removed.

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Fig. 12. (a) Scanning confocal laser microscopy image of an etched GaN surface, and SEM images of the areas framed by (b) solid and (c) dashed rectangles in (a). (d) Magnified SEM image of the framed area in (c).

and luminescence properties was attributed to the difference in growth direction; i.e., crystal growth is lateral in the concentric structure region, but vertical in the island region. Su et al. [22] reported that these islands exhibit a lower carrier concentration than the concentric rings due to differences in the incorporation of impurities during crystal growth. This non-uniformity of the carrier concentration is believed to be dependent on the growth direction (i.e., the growth rate); it being reported by Park. et al. [23] that electrochemical etching in the dark can remove highly-doped n+-GaN, whereas unintentionally-doped n-GaN showed complete resistance to etching without UV illumination. Tseng et al. [19] suggested that etching in the dark involves tunneling between the semiconductor/electrolyte interface, which is inhibited by the larger depletion layer in unintentionally doped n-GaN. In contrast, PEC etching is initiated by electron/hole injection that is induced by UV illumination, and thus the correlation between the etching characteristics and carrier concentration is believed to be less pronounced than in electrochemical etching. However, Lewandowska et al. [24] have identified that the PEC etching characteristics are significantly affected by the carrier concentration of GaN crystal, which would explain the anomalous surface features evident in Fig. 12 (a). The magnified SEM image in Fig. 12 (b) shows that a nano porous structure exists in the etched GaN surface in the form of an alternating concentric double structure of large and small pores (60–80 and 30–40 nm in diameter, respectively) surrounding the center of the ring structures. This alternating nano-structure is also believed to be due to a difference in the carrier density of the GaN film. Although the alternating non-uniformity of the carrier concentration in the horizontally-grown region is not yet fully understood, it is believed to be caused by the rotation of the substrate during crystal growth [25]. Chen et al. [26] have also reported that the diameter of the pores formed on the surface of

GaN treated by electrochemical etching correlates to the carrier density of n-GaN. Taking this into account, the concentric double structures most likely originate from the inhomogeneous carrier concentration, with regions of large pores corresponding to areas of low carrier concentration areas and vice versa. In contrast, the pore diameter in the vertically grown island regions is almost uniform at around 60 nm, as shown in Fig. 12 (c) and 12 (d), which is due to the in-plane uniformity of the carrier concentration inside the vertically grown island region. The surface morphology of the etched GaN surface is therefore considered to be unique to lateral HVPE growth, which would explain why it has not been observed previously in the etched surfaces of heteroepitaxial GaN films. In this work, we focused on the Ga-polar (0001) surface because of its superior chemical inertness to the N-polar (000-1) surface. However, unlike the hexagonal pits formed on the Ga-polar surface shown in Fig. 12, it is predicted that protruding hexagonal pyramids would form on the N-face of the GaN surface through PEC etching; as is indeed the case with GaN etching using an aqueous HCl solution [22]. 4. Conclusion This study has demonstrated that the surface of free-standing GaN wafers grown by HVPE is etched under UV illumination in acidic or basic electrolytes, but not in a neutral solution, which is attributed to the fact that the oxide film formed on the GaN surface is insoluble under neutral conditions. Nevertheless, subsurface damage caused by mechanical finishing proved to be difficult to remove by PEC etching under any conditions due to photo-excited carriers being trapped by the recombination centers introduced by mechanical abrasion. The surface morphology of the etched GaN consisted of islands dispersed amongst concentric rings of alternating small and large pores. This is attributed to the variation

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in electronic properties that is created during HVPE growth, with vertically grown islands corresponding to an area of higher carrier concentration, and horizontally grown rings to a lower carrier concentration. This horizontally grown region is believed to consist of alternating bands of high and low carrier density, which results in a concentric double structure in the etched GaN surface. This would explain why similar features have not been observed with GaN grown by other methods. Acknowledgements This research was partially supported by JSPS KAKENHI Grant Number 24760110. The authors would also like to thank Professor K. Yamauchi and Y. Sano (Osaka University) for fruitful discussion, and Editage (www.editage.jp) for providing English language editing. References [1] D. Zhuang, J.H. Edgar, Wet etching of GaN, AlN, and SiC. a review, Mater. Sci. Eng. R 48 (2005) 1–46. [2] P.J. Wellmann, S.A. Sakwe, F. Oehlschlager, V. Hoffmann, U. Zeimer, A. Knauer, Determination of dislocation density in GaN layers using KOH defect MOVPE grown etching, J. Cryst. Growth 310 (2008) 955–958. [3] A. Shintani, S. Minagawa, Etching of GaN Using Phosphoric Acid, J. Electrochem. Soc. 123 (1976) 706–713. [4] P. Visconti, K.M. Jones, M.A. Reshchikov, R. Cingolani, H. Morkoc, R.J. Molnar, Dislocation density in GaN determined by photoelectrochemical and hot-wet etching, Appl. Phys. Lett. 77 (2000) 3532–3534. [5] L.W. Jang, D.W. Jeon, A.Y. Polyakov, H.S. Cho, J.H. Yun, D.S. Jo, J.W. Ju, J.H. Baek, I. H. Lee, Free-Standing GaN Layer by Combination of Electrochemical and Photo-Electrochemical Etching, Appl Phys Express 6 (2013) 061001 (1)-(4). [6] Y. Zhang, B. Leung, J. Han, A liftoff process of GaN layers and devices through nanoporous transformation, Appl. Phys. Lett. 100 (2012) 181908 (1)-(4). [7] J.A. Bardwell, J.B. Webb, H. Tang, J. Fraser, S. Moisa, Ultraviolet photoenhanced wet etching of GaN in K2S2O8 solution, J. Appl. Phys. 89 (2001) 4142–4149. [8] M.S. Minsky, M. White, E.L. Hu, Room-temperature photoenhanced wet etching of GaN, Appl. Phys. Lett. 68 (1996) 1531–1533. [9] C. Youtsey, I. Adesida, G. Bulman, Highly anisotropic photoenhanced wet etching of n-type GaN, Appl. Phys. Lett. 71 (1997) 2151–2153. [10] C. Youtsey, I. Adesida, L.T. Romano, G. Bulman, Smooth n-type GaN surfaces by photoenhanced wet etching, Appl. Phys. Lett. 72 (1998) 560–562.

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