Free-standing single crystalline semiconductor membranes have gained intensive attention over the last few years due to their versatile usage in many applications. This material platform possesses a high level of material quality similar to their bulk counterparts because single crystallinity is maintained. Si, Ge, and III–V based membranes have been widely studied for flexible electronic and optoelectronic devices such as thin-film transistors and photodetectors. However, the current status of research and development on free-standing single crystalline wide band-gap membranes is at a relatively early stage compared to IV and III–V based membranes. This review highlights recent advances in free-standing wide band-gap membranes, including GaN, SiC, ZnO, β-Ga2O3, and diamond and their applications. Fabrication techniques of each membrane are presented with material characterization. Some prospects for new research opportunities and challenges are also discussed.

Recent advances in free-standing single crystalline wide band-gap semiconductors and their applications: GaN, SiC, ZnO, β-Ga2O3, and diamond

Progress in metal-organic chemical vapor deposition of high quality N-polar (Al, Ga, In)N films on sapphire, silicon carbide and silicon substrates is reviewed with focus on key process components such as utilization of vicinal substrates, conditions ensuring a high surface mobility of species participating in the growth process, and low impurity incorporation. The high quality of the fabricated films enabled the demonstration of N-polar (Al, Ga, In)N based devices with excellent performance for transistor applications. Challenges related to the growth of high quality N-polar InGaN films are also presented.

Recent progress in metal-organic chemical vapor deposition of $\left( 000\bar{1} \right)$ N-polar group-III nitrides

We report a facile roll-printing method, geometrically confined lateral crystal growth, for the fabrication of large-scale, single-crystal CH3NH3PbI3 perovskite thin films. Geometrically confined lateral crystal growth is based on transfer of a perovskite ink solution via a patterned rolling mould to a heated substrate, where the solution crystallizes instantly with the immediate evaporation of the solvent. The striking feature of this method is that the instant crystallization of the feeding solution under geometrical confinement leads to the unidirectional lateral growth of single-crystal perovskites. Here, we fabricated single-crystal perovskites in the form of a patterned thin film (3 × 3 inch) with a high carrier mobility of 45.64 cm2 V−1 s−1. We also used these single-crystal perovskite thin films to construct solar cells with a lateral configuration. Their active-area power conversion efficiency shows a highest value of 4.83%, which exceeds the literature efficiency values of lateral perovskite solar cells. Wafer-scale deposition of uniform metal halide perovskite single-crystals is a step towards commercialisation. Using geometrically-confined lateral crystal growth, Leeet al., report patterned thin films of highly-aligned single-crystals and achieve lateral solar cells with efficiencies up to 4.83%.

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Wafer-scale single-crystal perovskite patterned thin films based on geometrically-confined lateral crystal growth.

A porous medium is a special type of material where voids are created in a solid medium. The introduction of pores into a bulk solid can profoundly affect its physical properties and enable interesting mechanisms. In this paper, we report the use of mesoporous GaN to address a long-standing challenge in GaN devices: tuning the optical index in epitaxial structures without compromising the structural and electrical properties. By controlling the doping and electrochemical etching bias, we are able to control the pore morphology from macro- to meso- and microporous. The meso- and microporous GaN can be considered a new form of GaN with unprecedented optical index tunability. We examine the scattering loss in a porous medium quantitatively using numerical, semiempirical, and experimental methods. It is established that the optical loss due to scattering is well within the acceptable range. While being perfectly lattice-matched to GaN, the porous GaN layers are found to be electrically highly conductive. As a...

Mesoporous GaN for Photonic Engineering-Highly Reflective GaN Mirrors as an Example

A type of single-crystal gallium nitride mesoporous membrane is fabricated and its supercapacitor properties are demonstrated for the first time. The supercapacitors exhibit high-rate capability, stable cycling life at high rates, and ultrahigh power density. This study may expand the range of crystals as high-performance electrode materials in the field of energy storage.

Gallium Nitride Crystals: Novel Supercapacitor Electrode Materials.

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.

Wide bandgap III-nitride nanomembranes for optoelectronic applications

Nanoporous (NP) gallium nitride (GaN) as a new class of GaN material has many interesting properties that the conventional GaN material does not have. In this paper, we focus on the mechanical properties of NP GaN, and the detailed physical mechanism of porous GaN in the application of liftoff. A decrease in elastic modulus and hardness was identified in NP GaN compared to the conventional GaN film. The promising application of NP GaN as release layers in the mechanical liftoff of GaN thin films and devices was systematically studied. A phase diagram was generated to correlate the initial NP GaN profiles with the as-overgrown morphologies of the NP structures. The fracture toughness of the NP GaN release layer was studied in terms of the voided-space-ratio. It is shown that the transformed morphologies and fracture toughness of the NP GaN layer after overgrowth strongly depends on the initial porosity of NP GaN templates. The mechanical separation and transfer of a GaN film over a 2 in. wafer was demonstrated, which proves that this technique is useful in practical applications.

Mechanical properties of nanoporous GaN and its application for separation and transfer of GaN thin films.

By using a novel conductivity-based selective electrochemical etching, we have introduced nanometer-sized pores into GaN to significantly decrease its refractive index while maintaining its crystallinity. Such advantages of nanoporous (NP) GaN can thus overcome the epitaxial (strain) and optical (index) limitations of AlGaN, which has been used as the cladding layers in InGaN laser diodes for two decades. Compared to Al0.1Ga0.9GaN cladding, which has a refractive index contrast (Δn) of 0.04 to GaN, the Δn of NP-GaN to GaN can be engineered to be over 0.4 without inducing any tensile strain. The high Δn not only increases the optical confinement factor (Γ) from lower than 3% in the state-of-the-art (SOTA) InGaN laser diodes to 9% but also offers a broad tunability of the Γ and the modal gain as well. We have observed a clear correlation between the Γ and the modal gain by using the variable stripe length method and have demonstrated an over 100% increase of the modal gain by engineering the Γ of the wavegu...

Optical Engineering of Modal Gain in a III-Nitride Laser with Nanoporous GaN

Epitaxial lateral overgrowth (ELO) of nitrogen-polar (0001) GaN (N-polar GaN) by metalorganic vapor deposition has been studied to achieve a high microstructural quality of N-polar GaN. The influence of growth conditions on lateral growth is investigated, and a correlation of growth conditions with the observed inversion of polarity is established. Most of the observed trends for N-polar ELO are contrary to those reported for Ga-polar experiments. Such differences are explained by considering the property of surface reactivity of N-polar GaN with hydrogen species. On the basis of the trends of the occurrence (or absence) of polarity inversion, an atomistic model is proposed to explain the origin of polarity inversion. This model also allows us to control the process and to completely eliminate polarity inversion, resulting in fully coalesced, purely N-polar GaN with improved crystalline quality.

Epitaxial Lateral Overgrowth of Nitrogen-Polar (0001̅) GaN by Metalorganic Chemical Vapor Deposition

Large-area, free-standing and single-crystalline GaN nanomembranes are prepared by electrochemical etching from epitaxial layers. As-prepared nanomembranes are highly resistive but can become electronically active upon optical excitation, with an excellent electron mobility. The interaction of excited carriers with surface states is investigated by intensity-dependent photoconductivity gain and temperature-dependent photocurrent decay. Normally off enhancement-type GaN nanomembrane MOS transistors are demonstrated, suggesting that GaN could be used in flexible electronics for high power and high frequency applications.

Ge Yuan

Papers

Wide bandgap III-nitride nanomembranes for optoelectronic applications

Mechanical properties of nanoporous GaN and its application for separation and transfer of GaN thin films.

Optical Engineering of Modal Gain in a III-Nitride Laser with Nanoporous GaN

Epitaxial Lateral Overgrowth of Nitrogen-Polar (0001̅) GaN by Metalorganic Chemical Vapor Deposition

Single Crystal Gallium Nitride Nanomembrane Photoconductor and Field Effect Transistor