Wide Bandgap III-Nitride Nanomembranes for Optoelectronic

Jul 2, 2014 - Single crystalline nanomembranes (NMs) represent a new embodiment of semiconductors having a two-dimensional flexural character with com...
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Wide Bandgap III-Nitride Nanomembranes for Optoelectronic Applications Sung Hyun Park,† Ge Yuan,† Danti Chen,† Kanglin Xiong,† Jie Song,† Benjamin Leung,† and Jung Han*,† †

Department of Electrical Engineering, Yale University, New Haven, Connecticut 06520, United States S Supporting Information *

ABSTRACT: Single crystalline nanomembranes (NMs) represent a new embodiment of semiconductors having a two-dimensional flexural character with comparable crystalline perfection and optoelectronic efficacy. In this Letter, we demonstrate the preparation of GaN NMs with a freestanding thickness between 90 to 300 nm. Large-area (>5 × 5 mm2) GaN NMs can be routinely obtained using a procedure of conductivity-selective electrochemical etching. GaN NM is atomically flat and possesses an optical quality similar to that from bulk GaN. A light-emitting optical heterostructure NM consisting of p-GaN/InGaN quantum wells/GaN is prepared by epitaxy, undercutting etching, and layer transfer. Bright blue light emission from this heterostructure validates the concept of NM-based optoelectronics and points to potentials in flexible applications and heterogeneous integration. KEYWORDS: Nanomembrane, metalorganic chemical vapor deposition, Gallium nitride, electrochemical etching, heterostructure, epitaxy

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tion devices without the traditional limit in heteroepitaxy or the complexity in wafer bonding.6 So far single crystalline NMs with sizes larger than 1 mm2 have only been produced from Si, Ge, SiGe, GaAs, InGaAs, and InP.7−12 Notable device demonstrations include radio frequency operation of Si-NM transistors, InGaAsP vertical cavity laser, and a hybrid pentacene−Si heterojunction.13−15 GaN is one of the most important semiconductors of our time. Its properties of a near-ultraviolet bandgap and a high breakdown field are enabling phenomenal changes in energyefficient lighting and power distribution industry. Furthermore, creating ultrathin GaN NMs could open up new application fields in flexible electronics and piezoelectronics.16 In spite of many works in making free-standing GaN layers and devices, there has not been any report in producing large-area GaN NMs.17,18 One major reason is the chemical inertness of IIInitride compound semiconductors, making it difficult for the undercut etching of free-standing NMs. In this paper, we demonstrate large-area GaN NMs with a thickness as thin as 90 nm. These NMs are produced based on our recent discovery of conductivity-selective electrochemical (EC) etching of GaN.19−22 Large-area (>5 mm × 5 mm) can be routinely obtained with a dislocation density of ∼108 cm−3, only limited by the quality of the starting epitaxial structures. To complete the proof-of-concept demonstration of new optoelectronic applications using NM, InGaN based NM light-emitting diodes (LEDs) are realized and transferred onto

emiconductor nanomembrane (NM), with a typical thickness less than a few hundred nanometers, represents a new embodiment of the well-known single-crystalline semiconductors. The creation of these inorganic NMs has been enabled by advances in microelectronic fabrication techniques.1 For instance, silicon NMs can be prepared from commercially available silicon-on-insulator (SOI) wafers by selectively etching away the oxide layer, and GaAs NMs by selectively etching of AlAs sacrificial layers from GaAs/AlAs epitaxial structures.2,3 Although the crystalline perfection remains unaltered, an important property in membrane-like structure is the drastic reduction in flexural rigidity (or bending rigidity). The motivation of exploring membrane-based devices comes from the very strong dependence of flexural rigidity on thickness of membrane structure.3 On the macroscopic scale, the phenomenon of “softening” of inorganic semiconductors opens up new possibilities in flexible, stretchable, and bendable devices that used to belong exclusively to organic materials.4 On the microscopic scale, and more intriguingly, semiconductor with reduced flexural rigidity can be used to manipulate the strain of crystalline layer, that is, to change the interatomic bond length, to a degree not typically attainable in conventional epitaxial grown samples without inducing catastrophic failure through fracturing or buckling. NMs thus offer a pathway to alter the physical properties of well-known semiconductors.5 Additionally, semiconductor NMs have the tendency and ability to adhere to a target surface with atomic proximity and conformity. If this process can form an electronically useful interface, NMs can become the building blocks toward the designed, versatile, and layered formation of multifunctional, vertically stacked semiconductor heterojunc© 2014 American Chemical Society

Received: March 14, 2014 Revised: June 19, 2014 Published: July 2, 2014 4293

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Figure 1. Process flow of preparing III−N nanomembranes (NMs). (a) Lithography and etching to expose sidewalls of sacrificial layers, (b) electrochemical selective lateral etching of the heavily doped GaN sacrificial layer to undercut the NM, (c) separation of NMs with photoresist (PR) in hydrofluoric acid based solution, (d) extraction and anchoring of NM (with PR) to the target wafer, and (e) removal of PR by wet or dry cleaning after large-area NM has adhered to the host.

Figure 2. (a) Top view Nomarski microscope images of the patterned surface where the vias (10 × 10 μm2) appear as the squares and the formation of membranes (shown as the light-color circles) during electrochemical etching. (b) SIMS profile of undoped NM and heavily doped sacrificial layer. (c) Cross-sectional SEM image of NM after EC etching with highlighted color enhancement for clarity. The membrane (140 nm thick) is protected by PR during etching. The PR also provides mechanical support to the membrane layer during processing. (d) SEM image showing a PR protected rolled up GaN NM resting on GaN grown on sapphire substrate.

new host substrates based on NM fabrication and transfer process. We confirm that the preparation of III-nitride NMs could facilitate heterogeneous integration and new optoelectronic applications. We note that during the preparation of this manuscript, a work on GaN NM was published.23 The epitaxial structures used in the fabrication of III−N NMs are grown by metalorganic chemical vapor deposition (MOCVD). EC etching is used for the fabrication of GaN NM. A distinct advantage of the EC method is that the etching selectivity is caused by a disparity in conductivity rather than chemical constitution; sacrificial layers can therefore be prepared by designed doping profiles within an essentially homoepitaxial structure. A typical layer design is shown in

Figure 1a where all the layers underneath the NM layer are GaN. The MOCVD growth on sapphire begin with an undoped GaN followed by moderately n-doped layer (Si doping concentration of mid 1018 cm−3, 500 nm in thickness) to enhance lateral current flow. This n-type layer is protected by another undoped GaN (500 nm in thickness) above which the sacrificial layer is grown with a Si doping concentration of 1.2 × 1019 cm−3 (the red layer in Figure 1a). A III−N NM layer (consisting of either GaN or GaN/InGaN heterostructures) with a thickness between 90 to 300 nm is grown. Photolithography followed by chlorine-based reactive ion etching (RIE) is used to open vias with a depth that reaches the sacrificial layer to expose the sidewalls of heavily doped n-type GaN. EC etching is performed to etch selectively the n+-GaN 4294

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sacrificial layer laterally around the via windows (Figure 1b). As the regions of the undercut etching expand and eventually coalesce, the NM layer is released inside the solution (Figure 1c) and can be transferred through either dry or wet procedures. (Figure 1d). The distance between via windows on the sample can be controlled; depending on the fill factor, the separation distance between windows can be adjusted to anywhere between 25 to 100 μm. Further details on epitaxial structure and hydrofluoric acid (HF) based EC etching can be found in Supporting Information. The NM layer can include GaN-InGaN-AlGaN heterostructures. The use of strained layers in NM24,25 and the possible existence of polarizationinduced charges should create many opportunities in the future. We note that photoresist (PR) used in lithography can serve as both a protection layer during EC etching and an elastic mechanical support to ensure large-area transfer can be achieved. The PR layer can be removed at the end of the transfer for subsequent device fabrication or characterization (Figure 1e). The progress of EC lateral undercut etching of n+-GaN was recorded by Nomarski optical microscope and shown in Figure 2a, where the advancing fronts are clearly seen at different stages. The bright circles observed (in micrographs at 5 and 25 min) are due to the formation of overhang regions with the sacrificial layer removed. The circles eventually coalesce (at 25 min) and the etching tends to slow down as the electrical pathways for current flow become restricted. Quite frequently one observes etching residues (Figure 2a, 40 min, dark spots labeled by red arrows) due to current crowding effect which can be alleviated by optimizing the doping profile and etching condition. The lateral etch rate, though not constant due to the inherent variation of sample impedance during the EC etching, is initially in the range of 1 to 5 μm/min such that large-area NMs can be prepared in less than an hour. The doping profile around the sacrificial layer for a GaN NM is measured by secondary ion mass spectrometry (SIMS) and is shown in Figure 2b. A thin layer of highly doped n-type GaN (Si doping concentration of 4.8 × 1019 cm−3, thickness is 10 nm) was inserted at both the top and bottom of sacrificial layer to enhance the contrast in conductivity. In Figure 2c, the selective etching process is examined by cross-sectional SEM where the n+-GaN underneath the GaN NM layer is selectively etched. Given the excellent selectivity (with more than 5 orders of magnitude difference in etching rate), the thickness of III−N NMs could be precisely controlled by the doping profile to well below 90 nm. At the end of electrochemical etching, NM structures remain very loosely attached to the GaN/sapphire substrate by etching residues or van der Waals bonding. Largescale peel-off from sample edge is sometimes observed, as shown in Figure 2d, for a NM where the center part is still attached to the original GaN/sapphire substrate. Dipping samples into solutions of high surface energy such deionized (DI) water immediately causes separation of the PR/NM layers due to capillary force. The released NMs can subsequently be transferred onto other substrates or surfaces. Figure 3a shows an example of GaN NM interacting with DI water. The asetched GaN NM (with PR) in air has a radius of curvature of only 30 μm (Figure 2d). Upon contacting DI water, in this case with the N-polar side, GaN NM is stretched and unrolled to a radius of curvature of ∼5 mm. Alternatively, if immersed in solutions with a low surface tension, the EC etched NMs will instead remain attached to the original substrate, and requires a dry transfer, using material such as polydimethylsiloxane

Figure 3. (a) Separation and unrolling of a large-area GaN NM on Si substrate with DI water. Inset shows demonstration of 300 nm-thick InGaN based NM structure transferred onto PET film. (b) Plan-view SEM image of the GaN NM after transferred onto Si substrate. (c) Plan-view TEM image showing dislocations with a density comparable to the as-grown GaN layers. Inset shows selective area electron diffraction along [0001] axis showing the single crystallinity of the GaN NM. (d) AFM image of GaN NM after transferred onto SiO2/Si substrate.

(PDMS).26 Inset of Figure 3a shows a InGaN-based NM structure transferred onto a polyethylene terephthalate (PET) template. The NM exhibits a slight greenish tint that is likely due to interference effect. Figure 3b shows the NM transferred onto Si substrate, which demonstrates the conformal surface of NM after EC etching and transfer process. The microstructural property of the separated GaN NM is examined by transmission electron microscopy (TEM). Sample is prepared by transferring a GaN NM onto a copper TEM grid. Figure 3c shows the plan-view TEM image taken under the two beam condition using g = 112̅0.27 Both screw and edge type threading dislocations (TDs) are revealed with a total density of TDs in the upper 108 cm−2 range. This number is nearly identical to the density of dislocations of as-grown GaN NM structure prior to EC etching. The crystallinity is further confirmed by the selected area diffraction (SAD) pattern taken along the [0001] axis (inset of Figure 3c). The clear 6-fold symmetry affirms its single crystallinity. The surface of a transferred GaN NM (onto a SiO2/Si substrate) exhibits atomically flat surface morphology under atomic force microscopy (AFM) (Figure 3d), with a root-mean-square roughness of less than 1.7 nm determined from the area of 10 × 10 μm2, nearly identical to the value from as-grown sample. We conclude that the crystallinity of NM is comparable in quality to as-grown GaN layers and is not compromised in any way during the fabrication process by EC etching or transfer. We note furthermore that conductivity-based etching is completely compatible with the use of bulk GaN substrates28 such that membranes with nearly dislocation-free quality can be produced. After demonstrating the fabrication of GaN NM and crystalline nature of the GaN NMs, we further characterize their optical properties using microphotoluminescence (μ-PL) 4295

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spectroscopy. In Figure 4a, we plot the normalized μ-PL emission measured from as-grown GaN NM structure on

relaxation of the compressive strain typically present for GaN epilayers grown on sapphire.31 Using a published value of kPL a = 27 meV GPa−1 for the in-plane deformation potential (or PL constant),32 the magnitude of the reduction of in-plane stress is estimated to be 537 MPa (ΔE = kPL, a × Δσ, σ is the in-plane biaxial stress). The state (or the change of the state) of stress is also investigated with Raman scattering focusing on the E2 Raman mode, which is known to be sensitive only to stress.33 As shown in Figure 4b, for the as-grown GaN, the spectrum shows distinct E2 (high) near 569.8 cm−1, in accordance with the Raman selection rules for wurtzite GaN on sapphire substrate.34 The E2 (high) peak is shifted to 567.4 cm−1 for the freestanding NM sample. This value of 567.4 cm−1 closely agrees with the reported E2 (high) peak from bulk GaN samples. The difference of 2.4 cm−1 indicates a compressive stress of 571 MPa is present in the as-grown GaN structure (ΔωE2= kRaman,a × Δσ, kRaman,a = 4.2 cm−1 GPa−1 is the Raman calibration constant32 and σ is the in-plane biaxial stress), which is subsequently relaxed in the membrane. We note the two values of stress relaxation fall within earlier experimental measurements of the thermal stress,which has a range of 360−650 MPa.31,32,35,36 To complete the proof-of-concept demonstration of the utility of large-area III-nitride NMs, we fabricated InGaN/GaN NM LEDs with a vertical current injection scheme. The NM LED structure consisted of a 125 nm-thick unintentionally doped GaN on the n-side, five pairs of In0.2Ga0.8N (3 nm)/GaN (8 nm) quantum wells, and a 125 nm-thick Mg-doped GaN (Mg concentration of 2 × 1019 cm−3). After the epitaxial growth, a metallization process for Ni/Au was first performed, followed by the opening of via windows and the EC undercut etching. The 300 nm-thick LED structure together with Ni/Au and PR were then separated and transferred onto an Al-coated n-Si(100) substrate. Figure 5a shows an optical micrograph (100 × ) of a transferred LED NM on the Al/Si substrate. The top p-electrode of each LED assume a “tic-tac-toe” grid pattern around 9 (3 × 3) via windows with a lateral dimension of 100 × 100 μm2 (red grid in Figure 5a). In this preliminary investigation, the n-type contact is formed with the LED NM pressed against the Al surface, followed by annealing at 200 °C for 5 min. The GaN/InGaN NM LED turns on at around 4 V, clear blue emission was observed at less than 5 V (Figure 5b and the inset of Figure 5c). The relatively high resistance and turn-on voltage are likely due to the nonuniform electrical contact at the NM-substrate interface, as suggested by the intensity gradient in EL image (Figure 5b), especially near the probe tip. New processing schemes that will improve the conformal contacting are being investigated. Electroluminescence (EL) is collected from 0.2 to 3.5 mA, corresponding to current density of at least from 2 to 35 A/cm2 as shown in Figure 5c. The observed blue shift in EL emission wavelength by about 1.5 nm with injection current from 0.2 to 3.5 mA is attributed to the quantum-confinement Stark effect and band filling of the localized states by excitons.37 EL collected at 1 mA is also compared with separate PL measurement of the same membrane at roughly the same position (Figure 5d). A slight broadening and red shift in emission peak suggest that local heating for the NM LED might play a role. Further improvements in the LED growth and fabrication process, including contact annealing, doping optimization, and improved carrier transport across the interface between NM LED and new host substrate, could greatly improve the LED

Figure 4. (a) Room temperature μ−PL peak emission from the GaN NM and as-grown GaN on sapphire. Both of them peak at approximately 3.4 eV, and show an emission at the low energy region. Inset shows 10 K LT-PL spectra of emission from the GaN NM and as-grown GaN on sapphire. (b) Raman shift measured from as-grown GaN on sapphire substrate (black squares) and from the NM (red dots) transferred onto sapphire show strain relaxation of GaN NM.

sapphire (after via formation on the surface) and a transferred GaN NM with thickness of 90 nm on sapphire substrate under an excitation light with a wavelength of 325 nm from He−Cd laser. Sapphire is used as the handle wafer to prevent any artifacts of luminescence from the underlying substrates. Both the transferred NM and as-grown NM structure exhibit μ-PL peak emissions at approximately 3.4 eV and the characteristic yellow luminescence (YL) between 2.0 and 2.5 eV, from the near band-edge (NBE) and the deep-level transitions, respectively.29 Details of NBE luminescence is further investigated with low temperature μ-PL, as shown in the inset of Figure 4a. The main luminescence peak for as-grown sample at 3.4890 eV is designated as free A exciton emission (FXA). The high energy shoulder (3.4936 eV) and the lower energy peak (3.4828 eV) are interpreted to be from the free B exciton (FX B ) and the donor bound exciton (DBE), respectively, which agrees with the conventional interpretation of low-temperature PL.30 The free-standing GaN NM has a similar emission spectrum peaked at 3.4745 eV, most likely from the same FXA emission. The disappearance of the DBE peak from the NM sample is due to the fact that the GaN NM is not intentionally doped; it is likely that the DBE emission from the as-grown sample comes from the underlying sacrificial n+-GaN layer. The fact that FXA and FXB are observed, in a semiconductor membrane with two free surfaces separated by less than 100 nm, again demonstrates the electronic quality and device compatibility. The observed red shift in emission energy (for both FXA and FXB) by about 14.5 meV is due to the 4296

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microphotoluminescence and Raman scattering measurements. This research was supported by National Science Foundation (NSF) under Award No. CMMI-1129964, and facilities use were supported by YINQE and NSF MRSEC DMR 1119826.

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Figure 5. (a) Optical micrograph of NM LED transferred onto Al coated Si substrate with Ni/Au as the p-electrode (highlighted for clarity). (b) Blue emission from the NM LED. (c) EL of the NM LED with driving current from 0.2 mA to 3.5 mA. Inset shows the current− voltage characteristics of the NM LED. (d) PL of the NM LED (in black), after NM was transferred onto Al/Si(100) anchor substrate, and EL of NM LED under 10 V bias (in red).

performance. Nevertheless, the demonstration of our NM is a solid first step to realize III-nitride flexible or piezoelectronic devices. In conclusion, we have demonstrated a procedure to prepare III-nitride NMs with large-area for device applications. We have shown that the microstructural, morphological, and optical properties of the free-standing GaN NMs are essentially identical to those from state-of-the-art thick GaN epilayers. The effectiveness of this procedure is validated by the fabrication of a vertical, blue LED from a 300 nm InGaN/GaN heterostructure NM. With the methodology demonstrated here, III-nitride active structures are no longer confined rigidly in conventional epitaxial matrix but can be interfaced with a wide variety of surroundings. Because the III-nitride NMs are naturally compatible with flexible hosts and can be incorporated into vertical stacks with other two-dimensional or layered materials, preparation of III-nitride NM structures could bring forth new optoelectronic device configurations.

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ASSOCIATED CONTENT

* Supporting Information S

Description of the materials and methods. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Prof. Euijoon Yoon and Daeyoung Moon in Seoul National University for help with the 4297

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