GaN Membrane Research Articles

Optimized photoresponse performances in vertical and horizontal photodetectors based on freestanding GaN membranes

GaN has become an important material in the optoelectronics due to its suitable band gap and excellent physical and chemical properties, however, the grown substrates limit device geometry owing to their poor electrical and thermal conductivity. Herein, horizonal and vertical GaN ultraviolet photodetectors (UV PDs) were fabricated using the freestanding GaN membranes obtained by an epitaxial lift-off technique. The photoresponse behaviors in the fabricated PDs were then optimized by the piezo-phototronic effect. Owing to the synergistic effect of spontaneous polarization and asymmetric electrodes, the vertical UV PD exhibits unique self-powered characteristics with the light on/off of 3.3 × 103 and the responsivity (R) of 114 mA/W at the bias of 0 V. Optimized by the piezo-phototronic effect, the R increases to 168 mA/W at a tension strain of 0.25 %. In comparison, the R of horizontal UV PD decreases, regardless of the compression or tension. Physical mechanisms of the performances optimization in the vertical and horizontal UV PDs are proposed and analyzed. This study opens a pathway to fabricate GaN UV PDs with different device configurations and provides fundamental support for their performance modulation by the piezo-phototronic effect.

Fabrication and transfer printing based integration of free-standing GaN membrane micro-lenses onto semiconductor chips

We demonstrate the back-end integration of optically broadband, high-NA GaN micro-lenses by micro-assembly onto non-native semiconductor substrates. We developed a highly parallel process flow to fabricate and suspend micron scale plano-convex lens platelets from 6" Si growth wafers and show their subsequent transfer-printing integration. A growth process targeted at producing unbowed epitaxial wafers was combined with optimisation of the etching volume in order to produce flat devices for printing. Lens structures were fabricated with 6 − 11 µm diameter, 2 µm height and root-mean-squared surface roughness below 2 nm. The lenses were printed in a vertically coupled geometry on a single crystalline diamond substrate and with µm-precise placement on a horizontally coupled photonic integrated circuit waveguide facet. Optical performance analysis shows that these lenses could be used to couple to diamond nitrogen vacancy centres at micron scale depths and demonstrates their potential for visible to infrared light-coupling applications.

MOVPE Growth of GaN via Graphene Layers on GaN/Sapphire Templates.

The remote epitaxy of GaN epilayers on GaN/sapphire templates was studied by using different graphene interlayer types. Monolayer, bilayer, double-stack of monolayer, and triple-stack of monolayer graphenes were transferred onto GaN/sapphire templates using a wet transfer technique. The quality of the graphene interlayers was examined by Raman spectroscopy. The impact of the interlayer type on GaN nucleation was analyzed by scanning electron microscopy. The graphene interface and structural quality of GaN epilayers were studied by transmission electron microscopy and X-ray diffraction, respectively. The influence of the graphene interlayer type is discussed in terms of the differences between remote epitaxy and van der Waals epitaxy. The successful exfoliation of GaN membrane is demonstrated.

Large-Sized Nanocrystalline Ultrathin β-Ga2O3 Membranes Fabricated by Surface Charge Lithography.

Large-sized 2D semiconductor materials have gained significant attention for their fascinating properties in various applications. In this work, we demonstrate the fabrication of nanoperforated ultrathin β-Ga2O3 membranes of a nanoscale thickness. The technological route includes the fabrication of GaN membranes using the Surface Charge Lithography (SCL) approach and subsequent thermal treatment in air at 900 °C in order to obtain β-Ga2O3 membranes. The as-grown GaN membranes were discovered to be completely transformed into β-Ga2O3, with the morphology evolving from a smooth topography to a nanoperforated surface consisting of nanograin structures. The oxidation mechanism of the membrane was investigated under different annealing conditions followed by XPS, AFM, Raman and TEM analyses.

Effect of substrates on lasing properties of GaN transferable membranes

Transferable membranes were separated from the GaN epitaxial sheet with lift-off (LLO) technology. After the LLO process, the GaN membrane was removed onto various substrates, and then we can create different interfacial conditions, such as GaN attached with Si and GaN attached to Au. The substrate dependent photoluminescence (PL) properties of GaN membranes are studied by PL and time-resolution photoluminescence (TRPL) measurements in situ. Lasing oscillations are observed in the GaN membranes. The lasing features – which include the lasing spectra, Q factor, laser threshold, mode number, carrier dynamics and resonance mechanism – are systematically analyzed. We obtain, for GaN on a Si substrate, two series of ultraviolet lasing emissions with Fabry-Pérot (F–P) mode at the same excitation position. Furthermore, lasing of the GaN membranes can be altered by Au film at the back end of it via the phenomenon of lasing mode merger and red shift. Comparing with the Si substrate, Au film on the back can balance the lasing oscillations through decreasing the threshold value, unifying the resonant modes, and increasing the carrier combination rate. This research provides a feasible method for the realization of flexible membrane lasers.

Design of GaN-Based PCSEL With Temperature-Insensitive Lasing Wavelength

Considering the compensation of thermo-optic coefficients (TOC) between GaN-based cavity and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> photonic crystal (PhC) layer, we have theoretically designed a photonic crystal surface emitting laser (PCSEL) with temperature-insensitive wavelength. The structure includes a freestanding GaN membrane with embedded multiple InGaN/GaN quantum well (QW) layer and the top TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PhC layer. The high refractive index (RI) of PhC and the freestanding membrane cavity structure cooperate to enable a stronger field-coupling in PhC, thus fulfill the athermal coupling condition. Interestingly, the wavelength temperature dependence is only 0.0015 nm/°C for such PCSEL with a 82% PhC confinement factor, and its athermal property is proportional to the field confinement factor in TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PhC layer. To illustrate, a general formula has been developed after the investigations of thermo-optic effect and thermal expansion effect.

(Invited) Flexible and Stretchable Microwave Electronics

he development of microwave electronics has greatly advanced applications including communication, sensing, detection, and so on. The recently emerging markets like wearable devices, biomedical sensors, and foldable large-area microwave systems impose new requirements on microwave electronics, including flexibility, low weight, small size, and even stretchability. The unique mechanical properties of flexible and/or stretchable electronics renders them perfect candidates for applications like biomedical sensors and wearable devices, where the device needs to be attached or wrapped on nonplanar surfaces. Microwave electronics in flexible and/or stretchable form can substantially expand the application of these devices by eliminating lengthy connection wires and implementing modern wireless techniques. Therefore, flexible/stretchable electronics including fundamental components, functional circuits, and complete systems have attracted great interest from both academia and industry. The development of flexible microwave electronics has focused on fundamental flexible components like transistors and passive components in the past decades. As the heart of modern electronics, conventional semiconductor-based transistors have transformed into the flexible form through various mechanisms including substrate removal and exfoliation of the active layer on the surface. Silicon (Si), the most widely studied semiconductor material, has been extensively investigated for flexible microwave transistors. By employing technologies like strained channel and nanoimprinting, flexible Si thin-film transistors (TFTs) with excellent high-frequency performance were obtained. In addition, to extend the application of flexible transistors toward higher frequency and higher power, compound semiconductor-based flexible microwave transistors attracted increasing attention due to their superior material properties like high electron mobility, wide bandgap, and so on. The flexible GaInP/GaAs heterojunction bipolar transistor (HBT) and flexible AlGaN/GaN high electron mobility transistor (HEMT) have shown higher operating frequency than flexible Si TFTs. Besides superior high-frequency performance, flexible GaN HEMTs also show great potential for high-power flexible microwave electronics due to their wide bandgap and excellent thermal stability. The demonstrated flexible GaN HEMT can sustain a dissipation power of 0.5 W while achieving maximum oscillation frequency of 115 GHz. Besides the flexible transistors, flexible passive components operating at the GHz range, i.e. necessary parts of microwave circuits, are demonstrated as well. The operating frequency of inductors and capacitors have been pushed to beyond 10 GHz with optimization in device design. As the flexible microwave fundamental components continue to improve, flexible integrated circuits (ICs) consisting of fundamental components are being developed as well. Simple flexible circuits like switches and filters have been demonstrated by integrating Si or Ge diodes and passive components like inductors and capacitors. Flexible rectifiers, the critical component of wireless power transferring, are demonstrated using membrane GaAs Schottky diodes and flexible capacitors, which can harvest electromagnetic energy at WLAN band. Recently, a flexible power amplifier operating at 5.5 GHz with output power around 10 dBm has been reported. A reliable fabrication process flow has been developed to integrate small membrane GaN HEMT and flexible inductors and capacitors on polyimide thin film. Due to the excellent thermal reliability of GaN HEMT and heat spreader based on large coplanar copper ground plane, the amplifier can retain reliable operation with GaN membrane size as small as 0.5 mm by 0.5 mm. Compared to the rapid advancement of flexible microwave electronics, the development of stretchable microwave components is quite restricted due to the increased fabrication complexity and degraded device performance. The existing stretchable microwave components focus on passive components, especially the interconnect transmission lines. Due to the rigidness and fragility of semiconductor materials, stretchable microwave devices are typically made by connecting compact high-performance rigid/flexible semiconductor devices with stretchable interconnects. We have demonstrated a stretchable transmission line using a twisted structure, which can operate up to 50 GHz with minimized electromagnetic leakage. Recently, we successfully fabricated stretchable inductors operating at the GHz range. By connecting polyimide encapsulated compact spiral inductors with serpentine interconnects, the stretchable inductors inherit the high performance of a spiral inductor as well as the stretchability of serpentine lines. Although several key issues, including effective thermal management and enhanced reliability of the devices, require further research, flexible and stretchable microwave electronics have shown great promise in reshaping the existing electronics.

Method for inferring the mechanical strain of GaN-on-Si epitaxial layers using optical profilometry and finite element analysis

GaN-on-Si has become a useful fabrication route for many GaN devices and applications, but the mechanical stress incorporated throughout the material stack can impact the viability of this approach. The transfer printing of GaN membrane devices, a promising emerging technology, is most effective with flat membranes, but in practice many GaN structures released from their Si substrate are highly bowed due to the strain in the epitaxial nitride stack. Our approach uses the optical profiles of epitaxial wafers and membranes as inputs for inferring the mechanical strain state of the material by multi-variable numerical model fitting using COMSOL Multiphysics. This versatile, adaptable and scalable method was tested on samples from two GaN-on-Si wafers, revealing the relationship between built-in strain and material bow in principal-component fashion, returning 3–4×10−4 strain estimates for the AlGaN (compressive) and GaN (tensile) layers, and suggesting the occurrence of plastic deformation during transfer printing.

Absorption in ultrathin GaN-based membranes: The role of standing wave effects

A methodology is described to extract the absorption coefficient spectrum and exciton oscillator strength of GaN layers and GaN/AlGaN quantum wells by analyzing microtransmittance experiments in high-quality, free-standing membranes with thicknesses in the 160–230 nm range. The absorbance of a subwavelength GaN membrane is found to be an oscillating function of its thickness, in keeping with the standing wave effect. We analyze our results using two alternative models including interference effects and extract identical absorption coefficient values. The room-temperature absorption coefficient of bulk GaN membranes at the main exciton peak is found to be 9 × 104 cm−1. In the case of GaN/AlGaN quantum wells, the enhancement and blue shift of the excitonic absorption are observed, as a result of quantum confinement.

Design of micro-nano grooves incorporated into suspended GaN membrane for active integrated optics

Micro-nano grooves incorporated into a suspended GaN sheet is proposed for active and passive monolithic integration of silicon based InGaN/GaN blue LEDs. Rigorous finite element method (FEM) simulation is performed to investigate the efficiency of coupling enhancement and regulation effect. The imported efficiency from active source to passive waveguide is significantly improved, especially for small angle incidences and small membrane thickness, due to the active grating coupling effect. The overall imported efficiency increased by 240% in the ±30° incident angle range, and 140% in the ±70° range, with 300 nm membrane thickness and 260 nm grating period. Waveguide gratings exhibit filter and extractor properties respectively at different parameters, realizing direct monolithic modulation to light source. This study demonstrates new possibilities for integrated optics and innovative blue LED integrated applications.

Learning mechanisms in memristor networks based on GaN nanomembranes

We demonstrate experimentally that single crystalline GaN nanomembranes arranged in simple networks exhibit learning mechanisms such as habituation and dishabituation followed by storage of the response to a certain electrical stimulus. These artificial learning mechanisms are analogous to non-associative learning processes which are identical in simple animals and human beings. We found that the learning time depends on the number of GaN membranes in parallel, and this parameter decreases by 30% when three memristors are connected in parallel compared to the learning time of a single memristor. Moreover, an increased number of parallel memristors reduces the eventual asymmetry in the temporal response of the circuit at positive and negative step voltages.

Suspended GaN beams and membranes on Si as a platform for waveguide-based THz applications

We demonstrate the suitability of employing suspended GaN beams on a Π-shaped Si frame for waveguide-based cryogenic THz components and systems. This concept addresses major challenges and provides eased device handling, cryogenic operation, micron-alignment possibilities, high integratability and allows the electrical contacting by using bonding wires. In particular, a balanced hot electron bolometer (HEB) mixer was implemented for frequencies at 1.3 THz with state-of-the-art IF performance, which combines micro-machined all-metal waveguide components in conjunction with a suspended GaN beam. In addition, in order to accomplish a proper design of active or passive components, the accurate knowledge of the effective dielectric constant at THz frequencies is crucial when such membranes are employed. Thus, a direct measurement method based on a resonance structure and an S-parameter measurement between 1 THz and 1.5 THz is also presented.

GaN Membrane With Nano-Grooves for Single-Band Coupling in the Entire Visible Wavelength Range

Nano-groove arrays incorporated into a freestanding GaN membrane are proposed for coupling with visible light. The coupling of transverse electric mode light over the entire visible wavelength region into and out of the planar membrane is thoroughly investigated herein by performing a finite element method analysis. Coupling can be regulated by device parameters such as groove depth, membrane thickness, grating period, and duty cycle. Single-band coupling featuring tunable coupling bands can be realized, with the peak dual coupling efficiency reaching 0.55. This study paves the way for realizing planar photonics devices at specific single wavelengths. The proposed device is a suitable candidate for use in optical filters, de-multiplexers, planar photonic sensors, and other devices.

Etch tapered angle on the influence of GaN membrane grating reflectance spectra

The technique of backside thinning is used to obtain thinner freestanding GaN membrane. This technique is helpful for producing stronger reflectance peaks and to reduce the reflectance spectra quantity and reflectance interference in the visible light range. Ion beam etching (IBE) was used to fabricate a GaN membrane grating, and the size of the tapered angle of which has an important affect on the reflectance spectra modes. Therefore, we presented a GaN membrane grating model with tapered angle which is built based on a GaN-on-silicon wafer structure. The influence of the varied tapered angle on the reflectance spectra of grating was investigated. Meanwhile, affects of different parameters like the grating thickness, membrane thickness, grating period, filling factor, incident beam based on a certain tapered angle were also analyzed on reflectance spectra of the GaN grating. Rigorous coupled wave analysis (RCWA) was used as theoretical principle for numerical simulations on the GaN grating model in this study. The results of this report are very useful for fabricating GaN reflectance photonic devices, such as refractive index sensor, couplers and so on.

Ultracompact Multilayer Fabry–Perot Filter Deposited in a Micropit

We fabricate a multilayer Fabry–Perot filter (FPF) on a GaN-on-silicon platform. Being significantly different from conventional thin film FPFs formed on smooth flat surfaces, this FPF is deposited in a micropit, and takes a suspended GaN membrane as its carrier, which makes the whole structure ultracompact. The reflection spectra of the FPF are characterized by using an angle-resolved microreflection measurement system. The experimental results show that its spectra generally have a considerable blue shift in comparison with that of its conventional counterpart formed on the smooth flat surface; moreover, its thickness is not uniform but varies along its lateral dimension, suggesting that its spectral properties are highly localized. Resonable explanations are provided to illuminate its particular spectral phenomena. Finally, the localized thicknesses of the thin films composing the FPF are obtained by fitting the measured reflection curves with the simulated ones. This kind of FPF may be integrated monolithically into a batch of suspended membrane devices to endow them with the desired wavelength selectivity.

GaN Membrane Supported SAW Pressure Sensors With Embedded Temperature Sensing Capability

This paper presents the fabrication and characterization of a GHz operating surface acoustic wave (SAW)-based pressure sensor on a 1.2- $\mu \text{m}$ -thin GaN membrane. Two types of interdigitated transducers are manufactured using electron beam nanolithography to obtain finger and interdigit spacing widths, one with 170 nm and the other 200-nm-half pitch. Micromachining techniques are used to obtain the 1.2- $\mu \text{m}$ -thin membrane. The resonance frequency shift of the SAW, the pressure sensitivity, $s_{p}$ , as well as the pressure coefficient of frequency (PCF), were experimentally determined and analyzed, both for the Rayleigh as well as for the symmetrical Lamb propagation mode, in the 1 to 7 Bar pressure range. Record values for $s_{p}$ (up to 6 MHz/Bar) and PCF (up to 537 ppm/Bar) have been obtained, especially for the symmetrical Lamb propagation mode also due to the very high frequency operation (5–11.5 GHz). The effect of different orientations of the SAW device (in the $\textrm {[1}\bar {\textrm {1}} \textrm {00]}$ and $\textrm {[11}\bar {\textrm {2}} \textrm {0]}$ directions) on the frequency response and sensitivity is also analyzed. The possibility to determine simultaneously the pressure and the temperature with the same SAW structure operating as a dual sensor has been demonstrated.

Ultra-low threshold polariton lasing at room temperature in a GaN membrane microcavity with a zero-dimensional trap

Polariton lasers are coherent light sources based on the condensation of exciton-polaritons in semiconductor microcavities, which occurs either in the kinetic or thermodynamic (Bose-Einstein) regime. Besides their fundamental interest, polariton lasers have the potential of extremely low operating thresholds. Here, we demonstrate ultra-low threshold polariton lasing at room temperature, using an all-dielectric, GaN membrane-based microcavity, with a spontaneously-formed zero-dimensional trap. The microcavity is fabricated using an innovative method, which involves photo-electrochemical etching of an InGaN sacrificial layer and allows for the incorporation of optimally-grown GaN active quantum wells inside a cavity with atomically-smooth surfaces. The resulting structure presents near-theoretical Q-factors and pronounced strong-coupling effects, with a record-high Rabi splitting of 64 meV at room-temperature. Polariton lasing is observed at threshold carrier densities 2.5 orders of magnitude lower than the exciton saturation density. Above threshold, angle-resolved emission spectra reveal an ordered pattern in k-space, attributed to polariton condensation at discrete levels of a single confinement site. This confinement mechanism along with the high material and optical quality of the microcavity, accounts for the enhanced performance of our polariton laser, and pave the way for further developments in the area of robust room temperature polaritonic devices.

Transparent, Flexible Piezoelectric Nanogenerator Based on GaN Membrane Using Electrochemical Lift-Off.

A transparent and flexible piezoelectric nanogenerator (TF PNG) is demonstrated based on a GaN membrane fabricated by electrochemical lift-off. Under shear stress on the TF PNG by finger force (∼182 mN), the GaN membrane effectively undergoes normal stress and generates piezoelectric polarization along the c-axis, resulting in the generation of piezoelectric output from the TF PNG. Although the GaN layer is 315 times thinner than the flexible polyethylene terephthalate (PET) substrate, the low Young's modulus of PET allows the GaN membranes to absorb ∼41% of the applied strain energy, which leads to their large lattice deformation under extremely low applied stress. Maximum output voltage and current values of 4.2 V and 150 nA are obtained, and the time decay of the output voltage is discussed.

Study of the Effect of the Operating Frequency of a GaN Lamb Wave Device to Viscosity and Protein Sensing

Shear-operating frequency is known to affect the sensitivity of acoustic wave sensor devices to both chemical and biological sensing, e.g., the detection of solutions’ viscosity and protein adsorption. Lamb wave devices have been considered to be limited to low operating frequencies (5–40 MHz) due to their configuration. Here, acoustic devices based on single crystalline GaN membrane were used for liquid-based applications by monitoring the lowest symmetrical mode S0 of a Lamb wave in the range of 142–458 MHz. We evaluated the response of the acoustic devices by monitoring the amplitude and phase of the wave during the application of various concentrations of glycerol on the device surface. In addition, we demonstrated a strong correlation between mass sensitivity and device operating frequency, as measured during the specific binding of biotinylated IgG on the neutravidin modified membrane. Importantly, our results indicate that a decisive parameter in choosing the optimum frequency is the nature of the final application. Lower operating frequencies ( $\sim 300$ MHz) should be selected for biosensing applications.

Memristive GaN ultrathin suspended membrane array

We show that ultrathin GaN membranes, with a thickness of 15 nm and planar dimensions of 12 × 184 μm2, act as memristive devices. The memristive behavior is due to the migration of the negatively-charged deep traps, which form in the volume of the membrane during the fabrication process, towards the unoccupied surface states of the suspended membranes. The time constant of the migration process is of the order of tens of seconds and varies with the current or voltage sweep.