Strain Balanced AlGaN/GaN/AlGaN nanomembrane HEMTs

Abstract

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.

Figure 1

Fabrication procedure of III-Nitride NM. (a) Epitaxial growth of NM to be lift-off with an underneath n++ GaN layer, (b) photolithograph and dry etching to expose n++ GaN sidewalls with arrays of vias, (c) selective undercut etching of n++ GaN by electrochemical etching, (d) separation of NM from epitaxial wafer after full undercut, (e) removal of photoresist and cleaning the NM, (f) transfer the freestanding NM onto a host substrate.

Figure 2

(a) An AlGaN/GaN/AlGaN NM on SiO₂/Sapphire, (b) Microscopy image of a NM on SiO₂/Si with diagonal spacing of 100 µm, (c) and (d) SEM and AFM images of the Ga-polar surface of a NM, respectively.

Figure 3

(a) The Raman spectrum of DH NM with the exciting laser spot on the NM and on the edge of NM. (b) SEM image of an N-polar NM with diagonal via spacing of 25 µm (c) Spatial mapping of intensity ratio of the A₁ (TO) to E₂ (high) peak. The scanning area is 60 × 60 µm², indicated by red square in (b).

Figure 4

High resolution X-ray diffraction of DH NM on SiO₂/Si. (a) The (105) reciprocal space mapping. (b) (002) ω/2θ scan. (c) (002) rocking curve of GaN. (d) (102) rocking curve of GaN.

Figure 5

NM HEMT on SiO₂/Si. (a) The SEM image of a device with two separate 25 µm × 4 µm channels. Electrodes are colored and labeled. I-V is measured with one channel. (b) I_G-V_G of the Schottky gate in linear (black) and logarithmic (red) scales. (c) I_D-V_D at different gate bias from −3 V to 2 V, with step of 0.5 V. (d) Transfer characteristics (I_D-V_G) with V_D of 4 V (red curve). Measured transconductance g_m is also shown in black.

Figure 6

NM HEMT on polyester substrate. (a) Photo of NM HEMTs on PET with bend radius of about 3 cm. (b) NM HEMT microscopy image with back illumination, showing the transparency of the NM. (c) Front side of active region. (d) Backside of active region. (e) I_D-V_D with gate bias from −3 V to 2 V, with step of 0.5 V. (f) I_D-V_G with V_D of 4 V.