Nanoporous GaN Research Articles

Ultrahigh Sensitivity Graphene/Nanoporous GaN Ultraviolet Photodetectors.

Integration of graphene with three-dimensional semiconductors can introduce unique optical and electrical properties that overcome the intrinsic limitation of the materials. Here, we report on the high sensitivity ultraviolet (UV) photodetectors based on monolayer graphene/nanoporous GaN heterojunctions. By investigating the reflectivity, photoluminescence, and Raman spectral characteristics of nanoporous GaN, we find that the increase in the porosity can help to improve its optical properties. The device based on the highest-porosity nanoporous GaN demonstrates rapid and linear response to UV photons, with an ultrahigh detectivity of 1.0 × 1017 Jones and a UV-visible rejection ratio of 4.8 × 107 at V = -1.5 V. We attribute such high sensitivity to the combination of the significantly enhanced light harvesting of high-porosity nanoporous GaN and the unique UV absorption, high mobility, and finite density of states of the monolayer graphene. The high performance together with a simple and low-cost fabrication process endow these graphene/nanoporous GaN heterojunctions with great potential for future selective detection of weak UV optical signals.

Enhanced optical output in InGaN/GaN light-emitting diodes by tailored refractive index of nanoporous GaN

The light to be trapped inside light-emitting diodes (LEDs) greatly affects the luminous efficiency and device lifetime. Abrupt difference in refractive index between the sapphire substrate and GaN-based LEDs causes light trapping by total internal reflection, however, its optical loss has been taken for granted. In this study, we demonstrate that nanoporous GaN can be used as a refractive-index-matching layer to enhance the light transmittance at the sapphire–GaN interface in InGaN/GaN flip-chip light-emitting diodes (FCLEDs). The porosity and the refractive index of the nanoporous GaN layer are controlled by electrochemical etching of n-type GaN layer. The optical output power of FCLEDs with the nanoporous GaN layer grown on flat and patterned sapphire substrates is increased by 355% and 65% at an injection current of 20 mA, respectively, compared with that of an FCLED without the nanoporous GaN layer. The remarkable enhancement of optical output is mostly attributed to the nanoporous GaN layer which drastically increases the light extraction efficiency by decreasing the reflection of light at the sapphire–GaN interface.

Fabrication and improved properties of InGaN-based LED with multilayer GaN/nanocavity structure

Herein we first use the multilayer nanoporous GaN (MNP-GaN) as a substrate for the regrowth of light emitting diode (LED) with InGaN/GaN multiple quantum wells. The large-scale MNP-GaN structure, prepared by a simplified electrochemical etching method in an oxalic acid solution, shows the striking difference in porosity between n-GaN and n+-GaN layer, which is due to the significant difference in the carrier concentration. After the regrowth process, the nanopores in the MNP-GaN layer can be transformed into nanocavities ascribed to mass-transport from high-curvature to low-curvature areas. The photoluminescence lifetime and luminescence intensity of the LED with multilayer GaN/nanocavity structure show a sixfold enhancement compared to those of the reference sample. The luminescence enhancement can be ascribed to the high crystalline quality of multiple quantum wells due to the stress relaxation and the low density of dislocations, as well as the light reflection and scattering effect of the bottom multilayer GaN/nanocavity structures.

A nanoporous GaN photoelectrode on patterned sapphire substrates for high-efficiency photoelectrochemical water splitting

The photoelectrode of highly ordered nanoporous GaN on patterned sapphire substrate (PSS) is exploited to address issues of optical absorption and photocarrier separation efficiency in solar water splitting. The introduced PSS reduces the threading dislocation (TD) density and defects of the GaN epilayer, resulting in the improved crystalline quality, which suppresses recombination of electron-hole pairs. A single-step top-down etching approach is developed to fabricate the nanoporous GaN using an anodic aluminum oxide (AAO) mask conveniently and economically. The highly ordered nanoporous morphologies of AAO membrane are well transferred into the GaN surface grown on PSS via inductively coupled plasma (ICP) dry etching. Surface nanostructuring significantly increases surface-volume ratio of GaN, more incident light is trapped and absorbed by nanoporous structure, and the unabsorbed light scattered by PSS upwards is re-absorbed in nanoporous GaN. The ultraviolet light absorptance and reflectance of nanoporous GaN grown on PSS were improved significantly, close to 89% and 9% respectively. The resulting improved absorption in the nanoporous GaN further enhances the generation of photocarriers. The increasing surface-volume ratio also contributes to increasing of photoelectrochemical (PEC) reaction area and photocarrier separation efficiency, decrease of photocarrier migration distance towards the GaN-electrolyte interface, more holes participate in the PEC reaction, leading to an improved PEC efficiency and photocurrent density by 470% times with respect to planar counterpart. This work will pave the way towards low-cost and mass production of nanoporous GaN photoelectrode for efficient solar water splitting.

Large-area, liftoff nanoporous GaN distributed Bragg reflectors: Fabrication and application

Abstract A facile one-step electrochemical etching based on NaNO3 solution was developed for use in the chemical lift-off nanoporous- (NP-) GaN distributed Bragg reflectors (DBRs) over a macroscopic area (>1 cm2). The reflectivity of the lift-off DBR mirror is ~85%, which is far lower than that (~97%) of the NP-GaN DBR on the sapphire substrate. The decrease should be attributed to light scattering due to higher roughness in the lift-off mirror. To study its possible applications, tris(8-hydroxyquinoline) aluminum (III) (Alq3) thin film was grown on transferred NP-GaN DBR mirror onto silicon substrate via a thermal evaporation deposition. Compared to the reference Alq3 thin film, the photoluminescence of the Alq3 thin film on the transferred mirror presents significant enhancement and slight blue-shift, which should be attributable to reflectivity enhancement by the DBR and inner channel surface in the DBR mirror, respectively.

Light-output enhancement of InGaN light emitting diodes regrown on nanoporous distributed Bragg reflector substrates

Utilising our novel wafer-scale electrochemical porosification approach which proceeds through the top surface by means of nanoscale vertical etching pathways, we have prepared full 2 inch wafers containing alternating solid GaN and nanoporous GaN (NP-GaN) layers that form distributed Bragg reflectors (DBRs), and have regrown InGaN-based light emitting diode (LED) heterostructures on these wafers. The NP-GaN DBR wafer is epi-ready and exhibits a peak reflectance of 95% at 420 nm prior to growth of the LED heterostructure. We observe a 1.8× enhancement in peak intensity of LED electroluminescence from processed devices, and delayed onset of efficiency droop with increased injection current.

Thermal transport of nanoporous gallium nitride for photonic applications

Recently, nanoporous (NP) GaN has emerged as a promising photonic material in the III-N family. Due to its attractive properties, such as its large refractive index contrast and perfect lattice matching with GaN, as well as its good electrical conductivity, photonic components and devices involving NP GaN have been successfully demonstrated. However, further development of high-performance NP GaN based electrically injected devices, such as vertical-cavity surface-emitting lasers (VCSELs) and edge emitting lasers, requires efficient heat dissipation. Therefore, in this paper, we study thermal conductivity (TC) of NP GaN, especially when incorporated into a practical distributed Bragg reflector (DBR) in a VCSEL device. Through an effective medium model, we study the theoretical effect of NP GaN morphological properties over its TC. We then experimentally measure the TC of NP GaN, with different porosities and pore wall thicknesses, which shows a high agreement with the theoretical model. We also fabricate actual NP GaN DBRs and study the large tunability and interdependence among their TC (1–24 W/m K), refractive index (0.1–1.0), and electrical conductivity (100–2000 S/m) compared to other conventional DBRs. Finally, we perform a finite-element simulation of the heat dissipation within NP GaN-VCSELs, revealing their superior thermal dissipation compared to dielectric DBR based VCSELs. In this regard, this study lays the foundation for nanoscale thermal engineering of NP GaN optoelectronic and photonic devices and paves the way for their successful commercialization.

Distributed Bragg Reflectors for GaN-Based Vertical-Cavity Surface-Emitting Lasers

A distributed Bragg reflector (DBR) is a key building block in the formation of semiconductor microcavities and vertical cavity surface emitting lasers (VCSELs). The success in epitaxial GaAs DBR mirrors paved the way for the ubiquitous deployment of III-V VCSELs in communication and mobile applications. However, a similar development of GaN-based blue VCSELs has been hindered by challenges in preparing DBRs that are mass producible. In this article, we provide a review of the history and current status of forming DBRs for GaN VCSELs. In general, the preparation of DBRs requires an optimization of epitaxy/fabrication processes, together with trading off parameters in optical, electrical, and thermal properties. The effort of epitaxial DBRs commenced in the 1990s and has evolved from using AlGaN, AlN, to using lattice-matched AlInN with GaN for DBRs. In parallel, dielectric DBRs have been studied since 2000 and have gone through a few design variations including epitaxial lateral overgrowth (ELO) and vertical external cavity surface emitting lasers (VECSEL). A recent trend is the use of selective etching to incorporate airgap or nanoporous GaN as low-index media in an epitaxial GaN DBR structure. The nanoporous GaN DBR represents an offshoot from the traditional epitaxial approach and may provide the needed flexibility in forming manufacturable GaN VCSELs. The trade-offs and limitations of each approach are also presented.

Fabrication and properties of wafer-scale nanoporous GaN distributed Bragg reflectors with strong phase-separated InGaN/GaN layers

High photoluminescence (PL) efficiency InGaN/GaN multiple quantum well (MQW) active layers have been fabricated by metalorganic chemical vapor deposition (MOCVD) and followed by electrochemical (EC) etching. The MQW active layers were embedded nanoporous GaN (NP-GaN) distributed Bragg reflectors (DBRs). The PL intensity of MQW structure on the NP GaN mirror was several times higher than for as-grown sample, which should be attributable to improved light extraction efficiency (LEE) due to the increased light-extracting surface area and buried DBR mirror, and improved internal quantum efficiency (IQE) due to the relaxation of strain in the MQWs embedded in the NP-GaN DBRs.

Lift-Off Mechanism of GaN Thin Films with Buried Nanocavities Investigated by SEM and TEM

A process to slice and separate GaN layers with buried nanocavities was presented via an annealing process of nanoporous GaN (0002) thin films at 1050°C in an NH3 ambient. We were able to separate and lift off GaN layers over a macroscopic area ( $$ \ge $$ cm2). The several growth stages of buried nanocavities were examined by scanning/transmission emission microscopy techniques. During the early stage of annealing, the annealing leads to variations in morphology from nanopores with rough sidewalls to columnar or hexagonal nanopores with smooth sidewalls. At the same time, amorphous gallium oxide and single crystal GaN with high dislocation density, which are formed by electrochemical etching, are converted into GaN with a perfect crystal lattice. Subsequently, the columnar or hexagonal nanopores are sealed, cut, and then spheroidized. Above a sufficiently high porosity (> 50%), the nanocavities will overlap one another and coalesce, allowing a stand-alone GaN thin film to be lifted off the original substrate.

Encapsulation of methylammonium lead bromide perovskite in nanoporous GaN

Halide perovskites hold exceptional promise as cheap, low temperature solution-processed optoelectronic materials. Yet they are hindered by poor structural and chemical stability, rapidly degrading when exposed to moisture or air. We demonstrate a solution-phase method for infiltrating methylammonium lead bromide perovskite (CH3NH3PbBr3, or MAPbBr3) into nanoporous GaN which preserved the green photoluminescence of the perovskite after up to 1 year of storage under ambient conditions. Besides a protective effect, confinement within the porous GaN matrix also resulted in a blueshift of the perovskite emission with decreasing pore size, suggesting an additional templating effect of the pores on the size of the perovskite crystals within. We anticipate that our method may be generalised to related perovskite materials, offering a route to producing composites of interest for use in optoelectronic devices for various applications.

Fabrication and properties of high quality InGaN-based LEDs with highly reflective nanoporous GaN mirrors

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices. In this paper, we first report the use of the nanoporous GaN (NP-GaN) DBR as a template for regrowth of InGaN-based light-emitting diodes (LEDs). The wafer-scale NP-GaN DBR, which is fabricated by electrochemical etching in a neutral solution, has a smooth surface, high reflectivity (>99.5%), and wide spectral stop band width (>70 nm). The chemical composition of the regrown LED thin film is similar to that of the reference LED, but the photoluminescence (PL) lifetime, PL intensity, and electroluminescence intensity of the LED with the DBR are enhanced several times compared to those of the reference LED. The intensity enhancement is attributed to the light reflection effect of the NP-GaN DBR and improved crystalline quality as a result of the etching scheme, whereas the enhancement of PL lifetime is attributable to the latter.

InGaN/GaN microdisks enabled by nanoporous GaN cladding.

The fabrication of nanoporous (NP) GaN is proposed as a generic technique to create out-of-plane index guiding for nitride microcavities. Compared to the conventional undercut technique, the proposed technique forms uniformly a low-index NP-GaN layer beneath the entire microcavity. Therefore, it supports all cavity modes (with different cavity geometries), while the undercut technique only supports the modes that reside at the circumference of a circular microcavity. As a proof of concept, GaN microdisk cavities were fabricated with the NP-GaN as the bottom low-index medium. A cold cavity with Q>2,000 was reported under continuous-wave pumping. Lasing was demonstrated with threshold optical pumping power Pth∼60 kW/cm2 for the r=10 μm microdisk and Pth∼7 kW/cm2 for the r=50 μm microdisk. A rate equation analysis was performed to estimate the spontaneous coupling factor β∼1E-3, which was one order of magnitude higher than the previous report of a nitride microdisk laser with an InGaN quantum well active region. Therefore, NP GaN was proven to be a suitable replacement of the undercut technique for future nitride microcavities applications.

Light Emission by Nanoporous GaN Produced by a Top-Down, Nonlithographical Nanopatterning

Temperature-dependent photoluminescence (PL) spectroscopy is carried out to probe radiative recombination and related light emission processes in two-dimensional periodic close-packed nanopore arrays in gallium nitride (np-GaN). The arrays were produced by nonlithographic nanopatterning of wurtzite GaN followed by a dry etching. The results of Raman spectroscopy point to a small relaxation of the compressive stress of ~0.24 GPa in nanoporous vs. bulk GaN. At ~300 K, the PL emission is induced by excitons and not free-carrier interband radiative recombinations. An evolution of the emission spectra with T is confirmed to be mainly a result of a decay of nonexcitonic PL emission and less of spectral shifts of the underlying PL bands. A switching of excitonic PL regime observed experimentally was analyzed within the exciton recombination-generation framework. The study provides new insights into the behaviors and physical mechanisms regulating light emission processes in np-GaN, critical to the development of nano-opto-electronic devices based on mesoscopic GaN.

Fabrication, annealing, and regrowth of wafer-scale nanoporous GaN distributed Bragg reflectors

A wafer-scale distributed Bragg reflector (DBR), which consists of perfectly lattice-matched polar (0001) GaN and nanoporous GaN layers, was fabricated by electrochemical etching method in an oxalic acid solution, presenting high-reflectivity (>90%) and wide spectral stop-band width (~80 nm). Annealing of the DBR at 950 °C leads to a significant enhancement in the peak reflectivity (>95%) due to the mass transport process. According to the above mentioned research, InGaN-based light emitting diodes (LEDs) were regrown on the DBR templates. The photoluminescence intensity of the LED structure on the etched template was four times higher than for standard LED.

Enhanced piezoelectric output of NiO/nanoporous GaN by suppression of internal carrier screening

The efficiency of piezoelectric nanogenerators (PNGs) significantly depends on the free carrier concentration of semiconductors. In the presence of a mechanical stress, piezoelectric charges are generated at both ends of the PNG, which are rapidly screened by the free carriers. The screening effect rapidly decreases the piezoelectric output within fractions of a second. In this study, the piezoelectric outputs of bulk- and nanoporous GaN-based heterojunction PNGs are compared. GaN thin films were epitaxially grown on sapphire substrates using metal organic chemical vapor deposition. Nanoporous GaN was fabricated using electrochemical etching, depleted of free carriers owing to the surface Fermi-level pinning. A highly resistive NiO thin film was deposited on bulk- and nanoporous GaN using radio frequency magnetron sputter. The NiO/nanoporous GaN PNG (NPNG) under a periodic compressive stress of 4 MPa exhibited an output voltage and current of 0.32 V and 1.48 μA cm−2, respectively. The output voltage and current of the NiO/thin film-GaN PNG (TPNG) were three and five times smaller than those of the NPNG, respectively. Therefore, the high-resistivity of NiO and nanoporous GaN depleted by the Fermi-level pinning are advantageous and provide a better piezoelectric performance of the NPNG, compared with that of the TPNG.

(Invited) Rectifiers, Mos Diodes and LEDs Made of Fully Porous GaN Produced by Chemical Vapor Deposition

GaN is an important wide band-gap semiconductor in electronics and optoelectronics. In its porous form is particularly interesting for developing optoelectronic devices with improved efficiency, such as LEDs with enhanced efficiency and sensors with enhanced sensitivity. Through chemical vapour deposition (CVD) [1], we have shown that it is possible to produce nanoporous GaN without any etching or chemical post-growth treatment, with the porosity being present only on the (0001) face of the material. Low resistivity ohmic Pt and Au metallic contacts were demonstrated on porous n-type GaN by the formation of intermetallic seed layers through the vapour-solid-solid (VSS) mechanism [2]. Also, we have been able to develop p-type porous GaN by doping with Mg, with a charge carrier concentration of the order of 1018 cm-3 [3]. By tuning the concentration of Mg, introduced as Mg2N3 in the CVD system, it has been possible to form a polycrystalline high-k oxide between an ohmic metallic alloy interlayer contact and the porous GaN, while maintaining a clean interface, that allowed to fabricate a MOS-type diode on silicon in a single growth regime [4]. Through the careful selection of the substrate it has also been possible to produce porous GaN epitaxial layers [5] that allow for the fabrication of high quality partially and fully porous GaN rectifying p-n junctions, through a two step CVD process, and show their behaviour as diodes with effective uniform conduction under a green technology [6]. Here, we will also present the recent results we obtained in the light emission of these structures. Thus, these porous junctions have potential applications in high brightness unencapsulated LEDs with enhanced light emitting properties and high surface area sensors with improved sensitivity. [1] Carvajal & Rojo, Crystal Growth Des., 9 (2009) 320 [2] Bilousov et al., ACS Appl. Mater. Interfaces 4 (2012) 6927 [3] Bilousov et al., Appl. Phys. Lett. 103 (2013) 112103 [4] Bilousov et al., Chem. Mater. 26 (2014) 1243 [5] Bilousov et al., CrystEngComm 16 (2014) 10255; Bilousov et al., ACS Appl. Mater. Interfaces 6 (2014) 17954 [6] Carvajal et al., ECS Transactions 66 (2015) 163 Figure 1

Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers

In this work, we have succeeded in growing an approximate 2-inch self-separated thick GaN wafer by hydride vapor phase epitaxy with an introduction of a sacrificial layer of nanoporous GaN. Such nanoporous GaN layer is invented by using the HVPE growth of thin GaN layer on the spin-coating silica nanosphere layer followed by a hydrofluoric acid etching to the silica nanosphere layer. It has been found that the nanoporous GaN layer, enabling a reduction of stickiness between thick GaN films and the substrates, plays a significant role in the self-separation of thick GaN films during the cooling process. However, the thickness of the nanoporous GaN layer is another key issue to achieve good quality self-separated GaN thick films. In our study, we suggest that the nanoporous GaN layer with a thickness of approximately 150–240 nm can best serve as the sacrificial layer in self-separation process. Raman spectroscopy also indicates the self-separated thick GaN films by using the proposed approach are virtually strain-free.

Fabrication and properties of self-standing GaN-based film with a strong phase-separated InGaN/GaN layer in neutral electrolyte

We found an electrochemical method which can be used to rapidly fabricate GaN-based thin film with phase-separated InGaN/GaN multiple quantum well (MQW) via an electrochemical etching technique. Compared with the as-grown sample, light extraction efficiency is significantly improved for the etched samples due to the increased light-extracting surface area and improved internal quantum efficiency which is due to the relaxation of strain in the MQWs embedded in the nanoporous n-GaN layers. The several growth stages of nanoporous GaN were examined by microscopic technique, indicating the variation from dislocation defects in the GaN/porous GaN interface to oxidation of GaN, and finally to formation of GaNO3 which is soluble in water.

Porosity-induced weakening of piezoresponse in GaN layers by means of piezoelectric force microscopy

Nanoporous (NP) GaN thin films with low nucleation density were fabricated via UV-assisted electrochemical etching approach. Under different applied biases, two NP samples with different porosities were fabricated. Piezoelectric characteristics of the fabricated samples were investigated using piezoresponse force microscopy. It was found that the nanopores have great influence on its piezoelectric characteristics such as weakening of piezoresponse and change of polarization orientation. During the performance of devices, the weakening of piezoresponse should be propitious to the optoelectronic and electronic stabilities of GaN-based devices.