Optoelectronic Device Structures Research Articles

Hybrid n-GaN and polymer interfaces: Model systems for tunable photodiodes

Hybrid optoelectronic device structures offer promising options for a wide range of properties. The functioning of hybrid-bilayer devices is largely determined by their interface. Hybrid-bilayer devices based on wide band gap gallium nitride (GaN) and low band-gap donor/acceptor polymers offer a unique combination and model interface systems for application in photodetectors. A systematic study of the optoelectronic properties, specifically the photocurrent as a function of voltage bias and incident wavelength, has been carried out for n-Gan/polymer bilayer structures. A clear evidence of interface polarization originating from the GaN surface manifests in the current–voltage characteristics and photocurrent response of the hybrid structure.

Solubility limits and phase structures in epitaxial ZnOS alloy films grown by pulsed laser deposition

High-quality ZnO1−xSx thin films were grown epitaxially on c-plane sapphire substrates by pulsed laser deposition using a ZnS ceramic target with varying O2 partial pressures. Single-phase ZnO1−xSx alloys with a wurtzite structure were achieved in composition ranges of 0⩽x⩽0.23 and 0.94⩽x⩽1. Phase separation, i.e., coexistence of ZnO and ZnS in ZnOS ternary alloys was observed for compositions of 0.23<x<0.94. The extended S solubility (up to 0.23) implies that ablating the ZnS target under oxygen atmosphere is an efficient way to incorporate S into ZnO towards ZnOS alloys formation. The work provides additional information on the O solubility (0.06) in the S-rich ZnOS films grown by pulsed laser deposition. These results are of importance when considering ZnO1−xSx for making ZnO based quantum structures for advanced optoelectronic devices.

Semipolar GaN grown on foreign substrates: a review

Non- and semipolar GaN-based optoelectronic device structures have attracted much attention in recent years. Best results have been obtained on small bulk substrates cut from thick c-plane epi-wafers. However, owing to the limited size of such substrates, it is very attractive to study hetero-epitaxial approaches on foreign substrates. In this paper, we review the current state of such studies which eventually lead to large area non- or semipolar nitride structures. The simplest approach is to use planar sapphire or SiC wafers of non-c-plane orientations on which potentially less polar GaN can be grown. However, typically huge dislocation and in particular stacking fault densities evolve. More sophisticated approaches make use of the good GaN growth performance in the c-direction, eventually leading anyway to large area non- or semipolar structures. Several such approaches are discussed in this paper.

2D Photonic Structures for Optoelectronic Devices Prepared by Interference Lithography

Abstract Using the interference lithography based on the two-beam interference method the two-dimensional (2D) photonic structures with different symmetries were prepared in photoresist and III-V compounds surface layers. The 2D square and hexagonal patterns with periods in the range from 270 nm to 2 μm were realized by the interference lithography. Homogeneous 2D pattern of different symmetry and shape was revealed by the atomic force microscope. The interference lithography was applied to the surface patterning of a infrared light emitting diode. Such 2D photonic diode structure shows enhanced emission from the nearfield measurements.

Formation of High Aspect Ratio GaAs Nanostructures with Metal-Assisted Chemical Etching

Periodic high aspect ratio GaAs nanopillars with widths in the range of 500-1000 nm are produced by metal-assisted chemical etching (MacEtch) using n-type (100) GaAs substrates and Au catalyst films patterned with soft lithography. Depending on the etchant concentration and etching temperature, GaAs nanowires with either vertical or undulating sidewalls are formed with an etch rate of 1-2 μm/min. The realization of high aspect ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of a variety of optoelectronic device structures including distributed Bragg reflector (DBR) and distributed feedback (DFB) semiconductor lasers, where the surface grating is currently fabricated by dry etching.

Ge/SiGe multiple quantum well photodiode with 30 GHz bandwidth

We demonstrate high speed Ge/SiGe multiple quantum wells photodiodes using a surface-illuminated vertical p-i-n structure. The device, with a mesa diameter of 12 μm, exhibits a low dark current of 231 nA at −1 V bias. An optical bandwidth of 10 and 26 GHz is measured at −1 and −4 V reverse bias, respectively, and reaches over 30 GHz at a reverse bias of −7 V. These results prove the suitability of Ge/SiGe multiple quantum well structures for high performance optoelectronic devices required for optical interconnection applications.

Light extraction from surface plasmons and waveguide modes in an organic light-emitting layer by nanoimprinted gratings

Organic light-emitting diodes (OLEDs) usually exhibit a low light outcoupling efficiency because a large fraction of power is lost to surface plasmons (SPs) and waveguide modes. In this paper it is demonstrated that periodic grating structures with almost µm-scale can be used to extract SPs as well as waveguide modes and therefore enhance the outcoupling efficiency in light-emitting thin film structures. The gratings are fabricated by nanoimprint lithography using a commercially available diffraction grating as a mold which is pressed into a polymer resist. The outcoupling of SPs and waveguide modes is detected in fluorescent organic films adjacent to a thin metal layer in angular dependent photoluminescence measurements. Scattering up to 5th-order is observed and the extracted modes are identified by comparison to the SP and waveguide dispersion obtained from optical simulations. In order to demonstrate the low-cost, high quality and large area applicability of grating structures in optoelectronic devices, we also present SP extraction using a grating structure fabricated by a common DVD stamp.

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality Al xGa 1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al x Ga 1− x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al x Ga 1− x N was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al 0.1Ga 0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al 0.65Ga 0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al x Ga 1− x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.

Free‐standing zinc‐blende (cubic) GaN substrates grown by a molecular beam epitaxy process

AbstractIn this paper bulk, free‐standing, zinc‐blende (cubic) GaN wafers grown by plasma‐assisted molecular beam epitaxy (PA‐MBE) are described. GaN layers of up to 60 microns in thickness have been grown. Characterisation measurements confirm the cubic nature of the GaN crystals and show that the fraction of hexagonal material present is not more than ∼10%. Cubic (001)GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0001) c‐axis wurtzite GaN. Free standing GaN wafers grown by PA‐MBE make ideal lattice‐matched substrates for the growth of cubic GaN‐based structures for optoelectronic devices, and high power and high frequency electronic applications. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth and characterization of free-standing zinc-blende (cubic) GaN layers and substrates

In this paper, we describe bulk, free-standing, zinc-blende (cubic) GaN wafers grown by plasma-assisted molecular beam epitaxy. We have grown GaN layers of up to 60 µm in thickness. We present the data from characterization measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the material that is hexagonal in nature is not more than about 10% in the best thick samples. Cubic (0 0 1) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0 0 0 1) c-axis wurtzite GaN. Therefore, the free-standing GaN wafers we have grown would make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high-power and high-frequency electronic applications.

InGaN: An overview of the growth kinetics, physical properties and emission mechanisms

This article reviews the fundamental properties of InGaN materials. The growth kinetics associated with the growth parameters, such as growth temperatures, V/III ratios, and growth rates which influence the quality of the InGaN epilayers , are briefly described. An overview of the properties of the InGaN alloys, such as the optical, structural and electrical characteristics, is presented. The design and fabrication of novel optoelectronic device structures require an accurate knowledge of the band gap as a function of alloy composition; therefore, attention is paid to Vegard’s law and the bowing parameter; in addition, the major factors leading to the uncertainties of the bowing parameter of InGaN are addressed. Apart from that, the determination of indium composition by X-ray diffraction (XRD) using different assumptions and various equations are summarized. The erroneous measurements of the indium composition by using this technique are also described. Finally, different emission mechanisms of the strained InGaN quantum wells proposed by different groups of researchers are also discussed.

Analysis of reflectance and modulation spectroscopic lineshapes in optoelectronic device structures

We discuss the spectral lineshapes of reflectance and modulated reflectance (MR) measurements on optoelectronic device structures such as epi-layers, quantum wells (QWs), vertical-cavity surface emitting-lasers (VCSELs) and resonant-cavity light-emitting diodes (RCLEDs). We consider the various methods for the extraction of built-in electric fields and band-gap energies from Franz-Keldysh oscillations (FKO), using the example of a tensilely strained InGaAs QW system, whose InGaAsP barriers yield strong FKO. We describe how critical point transition energies can be easily obtained by eye from Kramers-Kronig (KK) transforms of low field or QW modulation spectra, using the example of the modulated transmittance spectra of dilute-nitrogen InGaAsN p-i-n structures. We also discuss how the ordinary reflectivity spectrum, usually acquired at the same time as the MR signal, may also be exploited to extract layer thicknesses and compositions, and information about the active QW absorption spectrum in VCSEL and RCLED structures.

Properties of InN grown by High-Pressure CVD

AbstractGroup III-nitride compound semiconductors (e.g. AlN-GaN-InN) have generated considerable interest for use in advanced optoelectronic device structures. The fabrication of multi-tandem solar cells, high-speed optoelectronics and solid state lasers operating at higher energy wavelengths will be made possible using (Ga1-y-xAlyInx)N heterostructures due to their robustness against radiation and the wide spectral application range. To date, the growth of indium rich (In1-xGax)N films and heterostructures remains a challenge, primarily due to the large thermal decomposition pressures in indium rich group III-nitride alloys at the optimum growth temperatures. In order to control the partial pressures during the growth process of InN and related alloys, a unique high-pressure chemical vapor deposition (HPCVD) system with integrated real-time optical monitoring capabilities has been developed. We report initial results on InN layers grown at temperatures as high as ∼850°C with reactor pressures around 15 bar. Such process conditions are a major step towards the fabrication of indium rich group III-nitride heterostructures that are embedded in wide band gap group III-nitrides. Real-time optical characterization techniques are applied in order to study the gas phase kinetics and surface chemistry processes during the growth process.For an ammonia to TMI precursor flow ratio below 500, multiple phases with sharp XRD features are observed. Structural analysis perform by Raman scattering techniques indicates that the E2 high mode improves as NH3:TMI ratio is decreased to below 500. Optical characterization of these InN layers indicates that the absorption edge shifts from down from 1.85 eV to 0.7 eV. This shift seems to be caused by a series of localized absorption centers that appear as the indium to nitrogen stoichiometry varies. This contribution will correlate the process parameters to results obtained by XRD, Raman spectroscopy and optical spectroscopy, in order to assess the InN film properties.

Two-dimensional transverse cross-section nanopotentiometry of actively driven buried-heterostructure multiple-quantum-well lasers

We report results of two-dimensional local potential measurement of the transverse cross-section of operating buried-heterostructure (BH) multiple-quantum-well lasers. The measured two-dimensional image of potential distribution resolved clearly the multiquantum-well active region and the p-n-p-n current-blocking structure of the BH laser, showing close correlation to the scanning spreading resistance microscopy image. Nanopotentiometry measurements were also performed on the p-n-p-n current-blocking structure of a BH laser under different forward bias voltages. The nanopotentiometry results provide direct insight into the behavior of p-n-p-n current-blocking layers intended to minimize current leakage. Our results demonstrate the application of nanopotentiometry to the delineation of complex buried structures in quantum optoelectronic devices.

Electrical behavior of double-sided metal/porous silicon structures for optoelectronic devices

Porous silicon (PS)-based devices consisting of a metal/PS/Si/PS/metal structure were investigated in this study for potential use in photodiodes and solar cells. The PS layers act as front antireflective coating for short wavelength light and back surface reflector/diffuser for high wavelength radiation. The PS layers were formed by the chemical etching of crystalline silicon. Different metals such as Au/Cr, W, Ta, WTi, Cr and Ti were sputter deposited onto both PS surfaces. The electrical behavior of the resulting devices was determined to ascertain the most appropriate metallic contact for the development of these optoelectronic devices.

CONDUCTIVE COPPER SULFIDE THIN FILMS ON POLYIMIDE FOILS FOR OPTICAL AND OPTOELECTRONIC APPLICATIONS

Copper sulfide thin films of 75 nm and 100 nm thickness were coated on Kapton foils (PI) of 25 nm thickness by floating them on a chemical bath. The foils were annealed at 150°C-400°C in N 2 converting the coating from CuS to Cu 1.8 S . The sheet resistance of the annealed coatings (100 nm) is 10-50 ohms/square which is almost unaltered after immersion in dilute HCl for 30-120 min. The infrared reflectance predicted for the coatings is 67%-77% at a wavelength 2.5 μm, which is nearly what is experimentally observed. The coated PI has a transmittance (25-35%) peak located around 550-600 nm. These thermally stable conductive coatings on PI foils might be used as conductive substrates for optoelectronic device structures.

Accuracy and reproducibility of the electrochemical profiler

The electrochemical CV profiler (ECV) plays a key role in the characterisation of compound epitaxial structures. Epi suppliers and their customers need a procedure that ensures that the carrier concentration in any given layer lies within agreed limits. Carrier concentrations in optoelectronic device structures and HBTs are usually specified around ±20%, with tighter limits being applied to MESFET and HEMT structures. By understanding the factors that affect the accuracy and reproducibility of the ECV method, the standard deviation of the measurement, for uniformly doped epilayers, can be reduced to around 2%.

Oxidation Kinetics and Microstructure of Wet-Oxidized MBE-Grown Short-Period AlGaAs Superlattices

AbstractThe kinetics of the wet oxidation process of MBE-grown high-Al-content AlAs/Al0.6Ga0.4As short-period superlattices (SPSLs) was investigated and compared to AlGaAs alloys and pure AlAs. We found that alloys and superlattices (SLs) have different oxidation characteristics. These differences were attributed to traces of the superlattice structure in the oxidized material. The microstructure and chemistry of SPSLs with an equivalent composition of Al0.98Ga0.02As was studied, using transmission electron microscopy, energy-dispersive x-ray spectroscopy, Rutherford backscattering, and nuclear reaction analysis for hydrogen-profiling. We also report on the mechanical stability of oxidized SPSL layers in optoelectronic device structures.

Nondestructive electroluminescence characterization of as-grown semiconductor optoelectronic device structures using indium–tin–oxide coated electrodes

We describe an arrangement for nondestructive electroluminescence measurements to characterize as-grown semiconductor optoelectronic device structures using indium–tin–oxide film coated on glass as a separate transparent electrode. The usefulness of this technique, and its applicability over a range of temperatures, are demonstrated by measurements on pieces of bare as-grown wafers of two different laser structures.

Confined phonons and phonon-mode properties of III–V nitrides with wurtzite crystal structure

Stimulated by the recent interest in the use of nitride-based III–V wurtzite structures for optoelectronic and electronic devices, this paper reports on the application of the Loudon model for uniaxial crystals to derive the Fröhlich interaction Hamiltonian as well as the electron–optical-phonon scattering rate in wurtzite crystals. This paper also presents experimental analyses of the mode behavior of phonons in wurtzite crystals.