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aPhys. Status Solidi C 8, No. 7–8, 2331–2333 (2011) / DOI 10.1002/pssc.201001163

Fabrication of periodically poled AlN with sub-micron periods Jonathan Wright *, Craig Moe, Anand V. Sampath, Gregory A. Garrett, and Michael Wraback

Sensors and Electron Devices Directorate, US Army Research Laboratory, Adelphi, MD 20783, USA

Received 15 October 2010, revised 10 November 2010, accepted 11 November 2010 Published online 17 June 2011

Keywords AlN, periodic poling, polarity inversion, UV * Corresponding author: e-mail jonathan.wright17@us.army.mil, Phone: 301 394 2117, Fax: 301 394 0310

We report on the fabrication of periodically poled AlN structures using inductive coupled plasma (ICP) etching and plasma-assisted molecular beam epitaxy (PAMBE). Periodically poled AlN structures are fabricated by first depositing Al polar material on c-plane sapphire sub-strates and then inverting the polarity through Mg over-doping. Subsequently, a grating with periods between 250 and 750 nm is defined by e-beam lithography and etched into the film by inductive coupled plasma etching to expose stripes of Al-polar material. ICP etching was optimized to produce vertical sidewalls and smooth Al-

polar regions. Finally, the patterned substrate is regrown by PAMBE to realize a periodically poled AlN structure. The sharpness of the interface between the Al and N po-lar regions is found to be dependent upon ICP etch condi-tions and III-V flux during regrowth. Under III-rich con-ditions, the alignment of the grating with crystallographic orientation has an important role in the final surface mor-phology. While lower III-V flux ratios prevent faceting, regrowth of Al-polar material is inhibited near the side-walls of trenches with high aspect ratios.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction There is growing interest in the fabri-cation of deep ultraviolet (DUV) Nitride-based laser diodes (LDs) operating at wavelengths below 250 nm for water quality monitoring and bio-detection applications. The re-alization of (Al,Ga)N DUV LDs is complicated, however, by doping issues, extended/point defects from growth, and strong polarization fields in these materials. An alternative approach is to employ second harmonic generation (SHG) via quasi-phase matching (QPM) using a visible laser source in conjunction with periodically poled AlN (pp-AlN). Periodic poling of nonlinear materials offers key advantages over traditional nonlinear crystals such as β-barium borate (BBO) by allowing increased interaction length and potentially higher conversion efficiency. With QPM, rapid SHG growth from an incoming pump wave is obtained by changing the polarity periodically throughout the nonlinear crystal [1]. Efficient frequency doubling by QPM requires a constant phase relationship over the course of the nonlinear crystal that is governed by a grating perio-dicity Λ [2]:

( )ωω

ωλnn −

=Λ22

(1)

where λω is the fundamental wavelength at ω, and nω and n2ω are the refractive indices at the fundamental and second harmonic wavelengths, respectively. For SHG at wave-lengths shorter than 250 nm, the period of the pp-AlN structure needs to be less than 1 μm.

Previously, Chowdhury and co-workers have demon-strated the feasibility of SHG using an infrared laser and pp-GaN structures that were fabricated by growing GaN by plasma-assisted molecular beam epitaxy (PAMBE) on al-ternating stripes of bare sapphire and GaN capped AlN buffer layer to realize N-polar and Ga polar regions, re-spectively [2]. To achieve SHG in the deep UV, however, a material with a bandgap wider than GaN must be used. In this study N-polar AlN is grown by inverting III-polar AlN through Mg overdoping, similar to what has been demonstrated for GaN [3-6].This N-polar AlN is used in the fabrication of pp-AlN structures having submicron pe-riods that may be suitable for SHG of deep ultraviolet light by employing inductively coupled plasma reactive ion etching (ICP-RIE) and regrowth by PAMBE.

2 Experiment Submicron period pp-AlN structures are fabricated by initially growing an Al polar AlN film by PAMBE at a substrate temperature of 900 C and then in-verting the polarity of the film by Mg overdoping. Polarity

2332 J. Wright et al.: Fabrication of periodically poled AlN with sub-micron periods

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inversion was confirmed in-situ by observing a 3x surface reconstruction by reflection high-energy electron diffrac-tion (RHEED) after deposition of the Mg overdoped layer. Further details on the growth of N-polar AlN by Mg over-doping will be published elsewhere. Next, e-beam lithog-raphy was used to define a Ni hard etch mask consisting of stripes having spacings between 0.15 and 1 μm. Side-wall angle of the periodic mesa structures, surface roughness of the etched III-face, and overall etch rate were characterized as a function of ICP etch parameters, including pressure, RF bias, ICP power and gas composition. Crystallographic orientation of the growth surface is known to have an ef-fect on surface morphology and faceting [3]. As a result, stripes were aligned along either the m- (1010) or a- (1120) directions. Surface morphology and etch depth were in-vestigated by SEM and AFM. Average initial RMS roughness of the AlN film was 2.25 nm. Finally, the pat-terned film was regrown by PAMBE at different III-V flux ratios to produce a pp-AlN structure.

3 Results and discussion Under all growth conditions employed, the Mg-doped

layer required for polarity inversion is much thicker than that for GaN inverted under similar conditions. In GaN, it is believed the formation of N-Mg-N bonds acting in Ga vacancy sites leads to the creation of inversion domain boundaries, which then grow outward until the entire film is inverted. The minimum thickness of the AlN:Mg layer for full inversion achieved here was 120 nm, possibly be-cause inversion domain boundaries in AlN are smaller than in GaN. Since the AlN periodic structure must be etched deeper to reach the original, III-polar material than for thinner Mg-doped GaN films, the regrowth of the III-face surface will be restricted by the aspect ratio of the trench width to mesa height.

Fig. 1 shows the etch rate, sidewall angle, and surface roughness of the etched III-face as a function of ICP etch parameters. Corresponding negative dc bias is also shown at each etch condition. There are a number of points to note regarding the etch character of ICP-RIE on N-polar AlN. Effect of etch rate, side-wall angle and RMS surface roughness with pressure variation is shown in Fig. 1(a). RF and ICP powers were kept at 100 and 500 W, respec-tively, using a Cl2/BCl3 flow rate of 18 and 2 sccm. An in-crease in pressure directly correlates to a decrease in etch rate and slight improvement in side-wall angle and III-face (trench bottom) surface roughness.

Figure 1(b) shows the etch trends examined as a func-tion of RF power. ICP power was held at 500W. Again, a Cl2/BCl3 gas chemistry with a flow rate of 18 and 2 sccm was used. Pressure was adjusted to 5 mTorr. Increasing RF (Fig. 1(b)) and ICP (not shown) powers lead to an in-creased etch rate, a decrease in side-wall angle, and little change to the roughness of the III-face surface.

Gas composition of the plasma had the greatest effect on etch character. Figure 1(c) shows the dependence of etch rate, III-face rms roughness and side-wall angle on the

percentage of Cl2 in the etching plasma. RF and ICP pow-ers were kept at 100 and 500 W, respectively, while using a pressure of 5 mTorr. The total flow rate was kept con-stant at 20 sccm and the Cl2 content varied from 25 to 100%. Higher % Cl2 leads to an increase in etch rate, however this rate is limited above ~50% Cl2. More impor-tantly, increasing the % Cl2 to 100% creates the smallest sidewall angles while maintaining low roughness of the III-face surface.

Figure 1 N-polar AlN etch rates, side-wall angle and rms rough-ness values as a function of ICP processing parameters.

The trends observed in Fig. 1 can be understood as fol-lows. Increasing plasma pressure in an electronegative Cl-atmosphere reduces the positive ion flux leading to a de-crease in etch rate [9]. Additionally, increasing pressure reduces the mean-free path of ions, slowing the etch rate

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by limiting surface desorption occurring by physical sput-tering [7]. Higher ICP power creates a greater number of reactive ions and neutrals in the plasma, while increased RF power improves the efficiency of surface sputtering by reactive ions [8], both of which lead to an increased etch rate. While BCl3 acts to remove non-volatile etch by-products created by the radicals from the Cl2, it works to roughen the surface at the same time [7]. Additionally, in-creasing percent BCl3 potentially decreases etch selectivity between the mask and AlN, so a large reduction in side-wall angle with increasing % Cl2 is observed. Etch trends shown in Fig. 1 follow similar trends for III-face AlN ICP etching. Although etch comparisons between III- and N-polar AlN are not found in the literature, similar contrasts for GaN show minimal differences [9]. From the overall etch character described above, periodicity and appropriate duty cycles are maintained with moderate ICP and RF powers while using high pressure in a Cl2 rich environment. Additionally, optimal, nearly vertical sidewalls are pro-duced with high pressure and under large Cl2 percentage.

Figure 2 AlN regrowth with 750 nm original periodic spacing aligned to A) <1120> direction and B) <1010> direction.

Figure 3 Periodic structured AlN with 750 nm spacing aligned to <1010> direction A) before regrowth and B) after regrowth.

For AlN regrowth, an initial Al beam equivalent pres-sure between 6.71 and 6.24 x 10-8 Torr was used based on previous standard growth of AlN on sapphire. Periodic patterns were aligned to either the a- (1120) or m- (1010) direction. Hexagonal faceting was observed in regrowth samples for patterns in both directions, as shown in Fig. 2. Bars aligned in the a-direction exhibited jagged morphol-ogy along the m-plane resulting in rough, sloped sidewalls. Bars aligned in the m-direction gave smoother, stable a-plane facets, but still showed moderately rough island-type growth.

Lowering the Al flux during regrowth was successful in eliminating the hexagonal faceting. At an Al beam

equivalent pressure of 5.77 x 10-8 Torr, the periodically poled structure reveals smooth surface morphology both at the III- and N-faces and cleaner, more vertical side-walls than for samples regrown with higher III-V ratios, as Fig. 3 shows. However, there are large areas at the side-walls where growth is absent caused by the high aspect ratio be-tween trench width and mesa height. This problem could be addressed by adjusting the growth parameters to com-pensate for uneven growth rates at different polarities or by changing the period spacings to give impinging atoms to the surface better access to side-wall corner areas.

4 Conclusions Sub-micron structures with lateral periodic inversion of the AlN polarity were completed by e-beam lithography, ICP etching, and MBE regrowth. Po-larity inversion of AlN was achieved through Mg over-doping by PAMBE. ICP etch parameters were varied to obtain optimal morphologies for regrowth, with high pres-sure and a high Cl2 chemistry producing the smoothest sur-faces and most vertical side-walls. Regrowth morphology is heavily dependent on III-V flux ratio; III-rich growth conditions cause faceting at the surface while V-rich con-ditions prevent such faceting. Although periodic poling is possible, nanoscale patterning combined with high aspect ratios between polar surfaces prevents growth at the side-walls, leading to nano-trenches between N- and III-polar materials. If this problem can be addressed, periodically poled AlN structures may help for realization of deep UV laser diodes by frequency doubling.

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