Strain Balanced AlGaN/GaN/AlGaN nanomembrane HEMTs

Single crystal semiconductor nanomembranes (NM) are important in various applications such as heterogeneous integration and flexible devices. This paper reports the fabrication of AlGaN/GaN NMs and NM high electron mobility transistors (HEMT). Electrochemical etching is used to slice off single-crystalline AlGaN/GaN layers while preserving their microstructural quality. A double heterostructure design with a symmetric strain profile is employed to ensure minimal residual strain in freestanding NMs after release. The mobility of the two-dimensional electron gas (2DEG), formed by the AlGaN/GaN heterostructure, is noticeably superior to previously reported values of many other NMs. AlGaN/GaN nanomembrane HEMTs are fabricated on SiO2 and flexible polymeric substrates. Excellent electrical characteristics, including a high ON/OFF ratio and transconductance, suggest that III-Nitrides nanomembranes are capable of supporting high performance applications.

The mobility of the 2DEG at the top AlGaN/GaN interface is about 800 cm 2 /V s. HEMT devices have been prepared on SiO 2 /silicon substrates and flexible PET films, with low leakage current and a ON/OFF ratio of 10 7 .

Results
The process flow to fabricate the III-Nitride NM is illustrated in Fig. 1(a)-(f) sequentially. The preparation of NM starts with the epitaxial growth of III-Nitride on sapphire substrate by metal-organic chemical vapor deposition. The NM structure to lift-off is grown on top of a 2 µm GaN buffer layer with a heavily doped n++ GaN layer serving as a sacrificial layer. The lift-off of NM is based on the conductivity-selective electrochemical (EC) etching of the n++ GaN sacrificial layer laterally. ( Figure S1) The EC etching behavior of GaN depends on its doping level as well as the bias voltage 32 . Voltage is typically held constant throughout the etching. The etching rate ranges from 1 to 50 μm/min, varying with electrolytes and bias voltage. To achieve large-area (~cm 2 ) NM within a reasonable time, the epitaxial wafer is lithographically patterned with a 2-D array of via holes spaced at 50 to 100 μms to expose n++ GaN as shown in Fig. 1(b), using Cl-based dry etching. In Fig. 1(c), the n++ GaN is etched from the edge of via windows in an isotropic way while NM and other layers stay intact. Etching proceeds with the advancing and eventually coalescing of the etching fronts; then NMs will separate from the substrate and become suspended in electrolyte solutions as shown in Fig. 1(d). We note that dry-etching masks, either photoresist or SiO 2 , can not only prevent vertical etching through the NM surface but also provide mechanical support during and after electrochemical etching. Once the EC etching is finished with the release of NMs, the floating NMs are gently rinsed in DI water and solvent to remove chemical residuals, and transferred onto host substrates.
There are two challenges in the fabrication of AlGaN/GaN NMs. First, the desirable goal of creating 2DEG at the AlGaN/GaN interface introduces additional electrical current-flowing pathways in the sample during EC etching. Given that EC etching, employed to laterally undercut n++ GaN and release the nanomembranes, is conductivity selective, the presence of the highly conductive 2DEG leads to unintentional and parasitic etching at the AlGaN/GaN interface in addition to etching of the sacrificial layer. (Figure S2(a)) A Fe-doped highly resistive GaN interlayer is therefore employed between the sacrificial layer and the 2DEG as a current-blocking layer 33 .
Considering the memory effect of Fe during growth and its influences on 2DEG, the thickness and doping level of the Fe-GaN layer need to be optimized. A Fe-GaN (25 nm, ~1 × 10 18 cm −3 ) layer is found to prevent vertical current flow into the top AlGaN/GaN interface and curb the undesirable parasitic etching effectively without degrading the 2DEG much.
The second issue in fabricating AlGaN/GaN NMs is the presence of residual heteroepitaxial strain in the NMs after release. Unlike the well-known AlAs/GaAs system where the entire ternary alloy is essentially lattice-matched, the AlN-GaN binary end compounds have a lattice mismatch of 2.4%. Management of residual strain in the composite NM system becomes very critical. By balancing the force and bending moment in a  Figure 2(a)-(c). The thickness of the sandwiched GaN layer is adjusted to keep the strain in AlGaN layers thus the 2DEG density. With a total thickness of 300 nm, the NMs are conformal to and adhere to almost any surfaces by van der Waal's force. The area of NM in Figure 2(a) is 0.5 × 0.5 cm 2 . Larger NM of 1 × 1 cm 2 can also be fabricated. The front surface is protected during the EC etching, thus a smooth surface is expected, and confirmed by AFM measurement in Figure 2(d).
Micro-Raman is used to measure the state of strain in a freestanding DH NM using 532 nm laser as excitation source. The spectra are shown in Fig. 3(a), with exciting laser on the NM and on the edge of NM, respectively. In both cases, majority of Raman scattering signal comes from the GaN layer due to its larger thickness. It is noticed that on the NM, there are only E 2 (High) and A 1 (LO) peaks of GaN visible, which is signature of single crystal wurtzite GaN 35,36 . In contrast, A 1 (TO) and E 1 (TO) peaks are prominent on the edge of the NM, due to coupling of laser into the plano-waveguide like AlGaN/GaN/AlGaN NM, and subsequent scattering. The Raman spectra within NM is distinctive from single crystal nanoporous GaN film where all the above peaks appear together 37 . Figure 3(b) and (c) show SEM image of an N-polar NM and the spatial intensity-ratio mapping the A 1 (TO) to E 2 (high) peak on the NM. The A 1 (TO) is only observable surrounding the edges of the vias. The absence of A 1 (TO) peak within NM region indicates that there is no obvious damage such as vertical etching or cracks within the NM that is well preserved during the electrochemical etching and transfer process.
With 0.6% lattice mismatch between Al 0.25 Ga 0.75 N and GaN, there is noticeable strain distribution in the sandwich structure. Namely, GaN is compressively strained while AlGaN is stretched in lateral direction. Assuming no plastic relaxation of mismatched strain and adopting the same Young's modulus for AlGaN and GaN, the in-plane strain of GaN can be estimated by the strain partition rule to be 0.1% 34 . The strain of the GaN layer can also be measured from the shift of E 2 peak of 569.4 cm −1 compare to freestanding GaN of 567.4 cm −1 30 . The value is 0.11% according to relative Raman shift of 2.0 cm −1 (Δω E2 = Δσ a × k a , ϵ a = Δσ a /M, where Δω E2 , Δσ a is Raman shift, in-plane stress, respectively; the value for coefficient k a , biaxial modulus M is 4.2 cm −1 /GPa, 449.6 GPa) 38,39 . The agreement between calculated and measured results suggests the AlGaN and GaN in the NM share the coherent lattice.
Further investigation of the crystalline quality of the DH NM is done by high resolution X-ray diffraction (XRD) after the NM is transferred (N-polar face up) onto a SiO 2 /Si handle wafer. Reciprocal space mapping (RSM) using the (105) diffraction is shown in Fig. 4(a). The vertical alignment of AlGaN and GaN diffraction points   In conventional AlGaN/GaN heterostructures on epitaxial wafers, the formation of 2DEG comes from both spontaneous and piezoelectric polarizations in the AlGaN layer. In the AlGaN/GaN/AlGaN nanomembrane configuration, 2DEG is also expected at the top hetero-interface, because the AlGaN layer is tensilely strained (=0.5%) according to micro-Raman and XRD measurement. Hall measurement on a 0.5 × 0.5 cm 2 NM shows that, the 2DEG density in NM is 5.4 × 10 12 cm −2 , and the electron mobility is 790 cm 2 /V s. (Figure S5) Considering the reduced strain of the AlGaN layer after NM lift-off and the simplified structure without AlN spacing layer, the values are in agreement with previous results from AlGaN/GaN on sapphire 42 . This mobility is among the highest values reported for single-crystalline semiconductor NMs, and it surpasses most single crystal NMs, for example, Si and GaN 9, 31 . Still, It is likely that the 2DEG is affected by memory effect of Fe doped GaN. Further improvement of 2DEG can be made by replacing Fe-GaN with a carbon doped high resistive GaN layer and adding an AlN spacer. (Supplementary Information).
To complete the proof-of-concept of heterointegration, NM HEMT is fabricated on SiO 2 . The device processing starts with lifting off the AlGaN/GaN/AlGaN NM and transferring it onto SiO 2 /Si substrate with the Ga-polar surface facing up. A shallow ICP etching was conducted (depth = 50 nm) to create mesas with isolated 2DEG layers. Second, Ti/Al/Ni/Au was deposited as source and drain contact metal by evaporation and lift-off, and subsequently annealing at 800 °C for 1 min in N 2 ambient. Later, 50 nm silicon nitride is deposited by PECVD to passivate and anchor the entire NM. Via holes for gate metal were opened by RIE etching, and Ni/Au is deposited as Schottky gate metal. Both via and gate were patterned by E-beam lithography. At last, Ni/Au metal pad is deposited.
The finished device is shown in Fig. 5(a). The channel is 25 µm × 2 in width and 4 µm in length. The gate length is 250 nm. For all the measurements in Fig. 5(b)-(d), only one side of the channel is used. I-V of Schottky gate diode is shown in Fig. 5(b). The reverse current at −4 V is 2.8 × 10 −7 mA and forward current at 4 V is 4.2 mA, with rectification ratio of over 10 7 . The ideality factor is 1.65. The results suggest that the AlGaN/GaN NM is of high crystalline quality with negligible leakage paths. The current-voltage characteristics are shown in Fig. 5(c) and (d). The current density and transconductance is 200 mA/mm and is 90 mS/mm, respectively. We note that this transconductance is a factor of 3 to 4 lower than the conventional AlGaN transistors on sapphire, mainly due to the low-Al content and low 2DEG concentration. Strategy in increasing 2DEG in the AlGaN NM is currently under way.
The fabrication of NM HEMT on flexible, polymeric templates requires further procedural refinement due to a much lower thermal budget. Conventional ohmic contacts to source and drain need a high annealing temperature (usually >750 °C) which is far above the melting point of PET. An additional constraint arises since during the alloying process on thick epilayer structures, metals could diffuse into GaN for over a hundred nanometers 43 , thus altering the electrochemical etching process. The alloyed contacts are often corroded during EC etching, making it not feasible to make ohmic contacts prior to electrochemical etching. As a consequence, the NM HEMTs are initially fabricated on SiO 2 /Si template. After the HEMT is fabricated, the NM HEMT/SiO 2 /Si structure is then flip-chip bonded onto a PET film using SU-8 as an adhesive. Then, the Si substrate is selectively etched away by gas phase XeF 2 . During the substrate etching, the NM device are protected by SiO 2 due to a very high etching selectivity of Si over SiO 2 (likely >10000:1). Finally, the SiO 2 is fully removed to expose the metal pads. (Figure S6 Fig. 6(e) and (f). Comparing to NM HEMTs on SiO 2 /Si, the threshold shifts to a positive direction by around 0.3 V, probably due to thermal strain induced by polymers. Despite of thermal dissipation problem of PET, the ON/OFF ratio of the device is 10 7 , the sub-threshold swing of transconductance is 150 mV/decade. The parameters compared very favorably with other NM transistors 1,9,31 . Finally, there is apparent current drop for HEMTs on PET films with increasing V D after saturation ( Fig. 6(e)), likely due to heating effect. We speculate that the channel temperature has increased during operation as a heating power of over 40 mW is generated within 25 µm × 4 µm channel region. ( Figure S7) At an elevated temperature, resistance of the NM rises from a reduced mobility, reducing effectively V G and g m , respectively 44 . Considering the glass transition of PET film begins at 80 °C and its large thermal expansion coefficient 45 , both heat and strain contribute to affect device performance. While the potential mechanisms are identified, the thermal dissipation can further be enhanced by employing a heat dissipation layer such as UNCD or Graphene 46, 47 .

Discussion
In summary, AlGaN/GaN high mobility transistors in the form of a nanomembrane (~300 nm) have been fabricated. The NM invokes a double-heterostructure design to achieve strain balancing. The crystalline quality of freestanding AlGaN/GaN NM, with dislocation about 5 × 10 8 cm −2 , is well preserved throughout the fabrication process. Transport measurement shows that 2DEG density is 5.4 × 10 12 cm −2 with a mobility of 790 cm 2 /Vs. AlGaN/GaN NM HEMTs are fabricated on both rigid and flexible polymeric substrates. The ON/OFF ratio of the device is greater than 10 7 , with a sub-threshold swing of 150 mV/dec. The study shows that III-Nitrides NM is a promising candidate for high performance electronics.

Methods
Material growth and lift-off. The samples were grown in a horizontal metal-organic chemical vapor deposition (MOCVD) reactor (Aixtron 200/4 RF-S) on 2″ c-plane sapphire substrates. Trimethyl gallium (TMGa), trimethyl aluminum (TMAl), and ammonia (NH 3 ) were used as gallium (Ga), indium (Al), and nitrogen (N) source material, respectively. Silane (SiH 4 ) was used for n-type doping source. After growth, the samples were lithographically patterned with photoresist Shipley 1827; and via holes were etched by inductively coupled plasma (ICP) reactive-ion etching in an Oxford 100 chamber to expose the sidewalls of the highly-doped layer. Subsequently, the AlGaN/GaN/AlGaN double heterostructure nanomembrane (NM) was lifted off from the substrate by electrochemically etching the n++ GaN sacrificial layer using a Keithley 2400 as the voltage source. After lift-off, the NM was transferred onto Si for characterization. Micro Raman mapping was performed by Raman Microscope Thermo Scientific DXRxi using 532 nm laser source and 50× object lens. The crystal characterization was done by PANalytical X'Pert PRO X-ray diffractometer.
Fabrication of HEMT devices. For HEMT on SiO 2 , first, device isolating mesa was created with a height of 50 nm by ICP in a PlasmaTherm 790 etching system. Next, Ti/Al/Ni/Au was deposited as source and drain contacts by CHA 600 E-beam Evaporator, and annealed at 800 °C for 1 min in N 2 ambient. 50 nm silicon nitride was then deposited by plasma-enhanced chemical vapor deposition in PlasmaTherm 70 at 250 °C. The window for gate contact was opened by RIE etching in Unaxis 790, and Ni/Au is deposited as the gate metal. Both the window and gate metal were patterned by E-beam lithography. Lastly, Ni/Au was used as contact pads. For HEMT on PET film, the device on SiO2/Si was bonded with PET film using SU-8 as the agent. Then the Si substrate was removed from the SPTS Xactix e1 Xenon Difluoride (XeF 2 ) Etcher. After fabrication, DC characteristics of the devices were measured with an Agilent 4155 semiconductor parameter analyzer.