Elaine D. Haberer

Vertically oriented GaN-based air-gap distributed Bragg reflector structure fabricated using band-gap-selective photoelectrochemical etching

A three-period vertically oriented GaN-based air-gap distributed Bragg reflector structure was fabricated using band-gap-selective photoelectrochemical (PEC) etching. The epitaxial structure consisted of an Al0.08Ga0.92N∕(In0.04Ga0.96N∕In0.07Ga0.93N) superlattice structure, wherein the InGaN layers served as sacrificial layers during PEC etching. Microreflectance measurements yielded an average enhancement in the reflected signal of ∼12-fold over the wavelength range of 550–650 nm, when compared with the signal from a dry-etched GaN surface.

GaN blue photonic crystal membrane nanocavities

GaN-based photonic-crystal membrane nanocavities with Q factors up to 800 have been realized at the wavelength of ∼480nm. The tuning behavior agrees well with numerical calculations using the finite-difference time-domain method. Theoretically, the lowest energy mode of a cavity that consists of seven missing holes in the Γ-K direction promises a Q factor as high as 4×104 with a mode volume of about 1.3×(λ∕n)3.GaN-based photonic-crystal membrane nanocavities with Q factors up to 800 have been realized at the wavelength of ∼480nm. The tuning behavior agrees well with numerical calculations using the finite-difference time-domain method. Theoretically, the lowest energy mode of a cavity that consists of seven missing holes in the Γ-K direction promises a Q factor as high as 4×104 with a mode volume of about 1.3×(λ∕n)3.

Cl2 reactive ion etching for gate recessing of AlGaN/GaN field-effect transistors

An effective gate recess etch process has been applied to AlGaN/GaN modulation-doped field-effect transistors (MODFETs), utilizing low power Cl2 reactive ion etching. In comparison to GaAs-based materials, GaN shows a greater robustness to ion damage under ion bombardment at very low ion energies (<70 V). It suggests that a viable gate recess etch process is possible. Recessed gate AlGaN/GaN MODFETs with gate to drain breakdown higher than −80 V have been demonstrated with an optimized lower power Cl2 reactive ion etching.

Free-standing, optically pumped, GaN∕InGaN microdisk lasers fabricated by photoelectrochemical etching

GaN-based, mushroom-shaped microdisk lasers were fabricated using band-gap selective photoelectrochemical etching. The optically pumped microdisks had well-defined, distinct modes at excitation powers ranging from about 8to16W∕cm2. Modal linewidths of 0.09nm were reported, which was near the resolution of the measurement equipment. Quality factors for the microdisks were >4600. The observed lasing threshold was 12.1W∕cm2. At higher excitation powers, heating effects and degradation were observed in the optical response of the microdisks.

Visible resonant modes in GaN-based photonic crystal membrane cavities

Photonic crystal membrane cavities play a key role as building blocks in the realization of several applications, including quantum information and photonic circuits. Thus far, there has been no work on defect cavities with active layers emitting in the UV to green range of the spectrum based on the (Al,In,Ga)N material system. While this material system has great potential for a new generation of optoelectronic devices, there are several obstacles for the fabrication of GaN-based membrane cavities, including the absence of a conventional selective chemical wet etch. Here, we demonstrate the first fabrication of fully undercut GaN photonic crystal membranes containing an InGaN multiquantum well layer, fabricated using band-gap-selective photoelectrochemical etching. A postfabrication coating of Ta2O5 is used to tune the cavity modes into resonance with the quantum well emission, and the fabricated membranes exhibit resonant modes with Q=300.

Removal of thick (>100nm) InGaN layers for optical devices using band-gap-selective photoelectrochemical etching

We report on band-gap-selective photoelectrochemical (PEC) etching of thick InGaN layers for use in optical devices, such as GaN microdisks, distributed Bragg reflectors, and two-dimensional photonic crystal membranes. Three InGaN sacrificial layer structures are studied: a 300nm InGaN layer, an InGaN∕GaN superlattice, and an InGaN∕InGaN superlattice. Calculated equilibrium band diagrams of the epitaxial structures are used to explain the observed etching behavior. The strong piezoelectric-induced fields within the InGaN sacrificial layers are found to greatly affect carrier confinement and etching behavior. As a demonstration of the etching technique, a free-standing GaN microdisk on an InGaN post is fabricated.

Highly sensitive hydrogen sulfide (H2 S) gas sensors from viral-templated nanocrystalline gold nanowires

A facile, site-specific viral-templated assembly method was used to fabricate sensitive hydrogen sulfide (H2S) gas sensors at room temperature. A gold-binding M13 bacteriophage served to organize gold nanoparticles into linear arrays which were used as seeds for subsequent nanowire formation through electroless deposition. Nanowire widths and densities within the sensors were modified by electroless deposition time and phage concentration, respectively, to tune device resistance. Chemiresistive H2S gas sensors with superior room temperature sensing performance were produced with sensitivity of 654%/ppm(v), theoretical lowest detection limit of 2 ppb(v), and 70% recovery within 9 min for 0.025 ppm(v). The role of the viral template and associated gold-binding peptide was elucidated by removing organics using a short O₂ plasma treatment followed by an ethanol dip. The template and gold-binding peptide were crucial to electrical and sensor performance. Without surface organics, the resistance fell by several orders of magnitude, the sensitivity dropped by more than a factor of 100 to 6%/ppm(v), the lower limit of detection increased, and no recovery was detected with dry air flow. Viral templates provide a novel, alternative fabrication route for highly sensitive, nanostructured H2S gas sensors.

Channeling as a mechanism for dry etch damage in GaN

Etch damage of GaN was investigated using a quantum-well probe structure. A clear decrease in photoluminescence (PL) intensity was observed and was aggravated with increasing ion-beam voltage. The magnitude of decrease in PL intensity was much larger than expected, even greater than for GaAs subjected to similar etch conditions. Angle-dependent bombardment studies were carried out to investigate channeling as a damage mechanism in GaN. The large decrease in PL intensity observed near normal incidence or along the [0001] direction suggests that channeling is a damage mechanism for low-energy bombardment in GaN.

Enhanced diffusion as a mechanism for ion-induced damage propagation in GaN

Although GaN is a chemically inert, thermally stable material, it has demonstrated sensitivity to ion damage generated by dry etch processes such as reacting ion etching and inductively coupled plasma etching. Recombination-enhanced diffusion is an important mechanism which has been observed in other III–V semiconductor systems. In this study we examine the possibility of enhanced diffusion in GaN using quantum well (QW) probe structures. The deeper QWs (750 and 1000 A deep) showed a steady decrease in relative photoluminescence (PL) intensity with time, providing evidence of the cooperative effects of channeling and defect diffusion in deep etch damage propagation in GaN. In contrast, shallow QWs (150 and 250 A from the surface) showed a slight decrease followed by a gradual increase in relative PL intensity with time which was explained by defect annihilation. Exposure to above band gap illumination, used to simulate and enhance carrier generation during etch, appears to speed defect annihilation in hig...

Enhanced photogenerated carrier collection in hybrid films of bio-templated gold nanowires and nanocrystalline CdSe.

Hybrid films of bio-templated gold nanowires and chemical bath deposited nanocrystalline CdSe were fabricated. The conductivity of the gold nanowires within the hybrid material was controlled by gold electroless deposition. Photocurrent measurements were taken on gold nanowire films, CdSe chemical bath deposited films, and hybrid films. The incorporation of gold nanowires within the hybrid material clearly increased the extraction of photogenerated carriers within the CdSe. Photocurrent showed a direct correlation with gold nanowire conductivity.