**1. Introduction**

Almost three decades have passed since Boeck and Borghs published their paper on heteroepitaxy versus epitaxial lift-off techniques [1], inspired by the seminal papers of Yablonovitch on epitaxial lift-off (ELO) [2,3]. Since then, a large number of scientific publications have been released regarding thin film epitaxy and film transfer to a variety of host substrates [4]. These publications show that ELO and related techniques are an outgrowth of mature epitaxial growth techniques, such as metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE); they inherently need each other for future microelectronics and nanoelectronics progress.

Although many various two-dimensional materials, such as graphene, MoS2, WS2, and WSe<sup>2</sup> [5,6], are intensively studied at present, long well-established thin III-V heterostructures with high-mobility two-dimensional electron gases (2DEG) are exceptionally appropriate for ELO [2,7,8] and for emerging break-through three-dimensional hybrid microelectronic and nanoelectronic technologies [9,10].

In the early years, ELO was mainly used for the separation of relatively thick layers (up to 800 nm and more), and supporting organic layers were very often used in the separation process to increase the sacrificial AlAs etching efficiency and to facilitate the transfer of separated layers [11].

**Citation:** Gregušová, D.; Dobroˇcka, E.; Eliáš, P.; Stoklas, R.; Blaho, M.; Pohorelec, O.; Hašˇcík, Š.; Kuˇcera, M.; Kúdela, R. GaAs Nanomembranes in the High Electron Mobility Transistor Technology. *Materials* **2021**, *14*, 3461. https://doi.org/10.3390/ma14133461

Academic Editor: Fabrizio Roccaforte

Received: 29 May 2021 Accepted: 16 June 2021 Published: 22 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Currently, by contrast, very (ultra) thin, high-quality, single crystalline inorganic semiconductor nanomembranes, with thickness that matches the length scales of important quantum physical processes, are released from their growth substrates and transferred to host substrates. This allows for studies of the basic physics of such nanomembranes [12]. The nanomembranes are released using highly selective etching processes, which utilize sacrificial layers of semiconductor materials, mostly (but not only) AlAs [12,13]. The nanomembranes are attached by van der Waals forces to foreign substrates, such as sapphire, GaN, and plastic flexible substrates [14], and they are used to prepare hybrid equivalents of devices that cannot be produced monolithically [15].

We pursue the hybrid integration of MOCVD-grown III-V- and III-N-based devices, in which GaAs-based heterostructure transistors with excellent dc and high-frequency properties are integrated with devices based on nitride semiconductors on host substrates.

The motivation for our work was to prepare flexible devices based on high electron mobility 2DEG III-V structures. Very thin nanomembranes are extremely flexible, but they can exhibit a low mobility because electrons are increasingly scattered due to interactions at both nanomembrane surfaces. Moreover, the technology of such devices is much more difficult than that of thicker ones. However, 2DEG III-V heterostructures as thin as 100 nm allow for the processing of nanomembranes with mobilities that are comparable with those of the monolithic heterostructures. As such, nanomembranes are very flexible; they can stick to various substrates with van der Waals forces.

#### **2. Materials and Methods**

This paper reports on the growth, ELO, and transfer of an AlGaAs/InGaAs/GaAs high electron mobility transistor (HEMT) heterostructure to sapphire. The as-grown heterostructure and transferred nanomembrane heterostructure were characterized by high-resolution X-ray diffraction (HRXRD), photoluminescence (PL), and scanning electron microscopy (SEM), and they were used to produce HEMTs, whose performance was studied. The heterostructure contained a strained 10 nm 2DEG In0.23Ga0.77As channel, which makes the ELO process difficult. This work is inspired by previous studies on hybrid transistor technologies, such as [16–20].

The 2DEG AlGaAs/InGaAs/GaAs heterostructure used in this experiment was designed with the intention to test the limits of heterostructure membrane lift-off and transfer. Therefore, the heterostructure was very thin. Its thickness after separation was nominally ≈ 100 nm. In addition, it was appropriate for the preparation of good-quality HEMTs, which are used to evaluate the quality of nanomembrane transfer by looking into the device transport properties.

The 2DEG channel of the heterostructure was based on a strained In1-XGaXAs layer, whose thickness and composition were optimized by calculation and experimentation to prevent the generation of misfit dislocations, which is necessary to achieve high electron mobilities and sheet concentrations. Similarly, an appropriate doping level was found to fill the 2DEG channel with electrons and inhibit the formation of a parallel conducting channel with a low electron mobility in the delta doped layer. The strained In1-XGaXAs layer was grown close to the center of the heterostructure to avoid a significant bowing of the heterostructure nanomembrane during and after separation from the growth substrate.

The heterostructure was grown using low-pressure MOVPE in an Aixtron AIX 200 reactor (Aixtron, SE Dornkaulstr. 2, 52134 Herzogenrath, Germeny) at 700 ◦C. Hydrogen was used as the carrier gas. The precursors were trimethylgallium, trimethylindium, trimethylaluminum, arsine, and diluted silane.

The heterostructure consisted of a GaAs (001) substrate, 300 nm AlAs sacrificial layer, GaAs buffer layer, 10 nm 2DEG In0.23Ga0.77As channel, 4 nm Al0.3Ga0.7As spacer, deltadoped Si layer, 30 nm Al0.3Ga0.7As top layer, and 5 nm GaAs cap layer. The delta-doped Si layer was prepared during a 30 s growth interruption under an overpressure of arsine. The growth rates of AlAs, GaAs, In0.23Ga0.77As, and Al0.3Ga0.7As were 0.29, 0.17, 0.2, and 0.23 nm/s, respectively. The heterostructure was designed to have a nominal sheet electron

concentration of 2.10<sup>12</sup> cm−<sup>2</sup> . The as-grown heterostructure exhibited a sheet electron concentration of 3.4 <sup>×</sup> <sup>10</sup><sup>12</sup> cm−<sup>2</sup> and Hall mobility of 4590 cm2<sup>V</sup> −1 s −1 . − − −
