Recombination dynamics of free and bound excitons in bulk GaN

We report new data on the transient photoluminescence behaviour of free and donor bound excitons in high quality bulk GaN material grown by HVPE. With 266 nm photoexcitation the no-phonon free exciton has a short decay time, about 100 ps at 2 K, assigned to nonradiative surface recombination. The LO replicas of the free exciton have a much longer decay at 2 K, about 1.4 ns, believed to be a lower bound for the bulk radiative lifetimes of the free excitons at 2 K. The donor bound exciton no-phonon lines exhibit a rather short (about 300 ps) nonexponential decay at 2 K, which appears to be dominated by a scattering process. The corresponding LO replicas and the two-electron transitions have a much longer decay. From the latter, the lower bound of the radiative lifetime of the O- and Si-bound excitons are 1800 ps and 1100 ps, respectively.     

Photoluminescence and excitation spectroscopy of the 1.5 μm Er-related band in MBE-grown GaN layers

The infrared photoluminescence at 1.5 m due to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions has been investigated for GaN:Er³⁺ layers grown by MBE. Low temperature high resolution measurements performed under continuous illumination at the wavelength  nm, resonant to one of the intra-4f-shell transitions, revealed that the 1.5 μm band consists of up to eight individual spectral components. In excitation spectroscopy, a temperature dependence splitting of resonant bands was observed. On the basis of these experimental results, a possible multiplicity of optically active centers formed by Er doping in GaN layers is discussed.     

MBE growth of nitride-based photovoltaic intersubband detectors

In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector (QWIP) structures, including the Si-doped GaN/AlN short-period superlattice of the active region, conductive AlGaN claddings and integration of the final device. The growth of Si-doped GaN/AlN multiple quantum well (QW) structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33–1.91 μm wavelength range are measured on samples with QW thickness varying from 1 to 2.5 nm. The absorption exhibits Lorentzian shape with a line width around 100 meV in QWs doped 5×10¹⁹ cm⁻³. To prevent partial depletion of the QWs owing to the internal electric field, we have developed highly-conductive Si-doped AlGaN cladding layers using In as a surfactant during growth. Complete ISB photodetectors with 40 periods of 1 nm-thick Si-doped GaN QWs with 2 nm-thick AlN barriers have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Temperature-dependent photovoltage measurements reveal a narrow (90 meV) detection peak at 1.39 μm.     

GaN/AlN quantum dot photodetectors at 1.3–1.5 μm

We have demonstrated GaN/AlN quantum dots (QD) photodetectors, relying on intraband absorption and in-plane carrier transport in the wetting layer. The devices operate at room temperature in the wavelength range 1.3–1.5 μm. Samples with 20 periods of Si-doped GaN QD layers, separated by 3 nm-thick AlN barriers, have been grown by plasma-assisted molecular-beam epitaxy on an AlN buffer on a c-sapphire substrate. Self-organized dots are formed by the deposition of 5 monolayers of GaN under nitrogen-rich conditions. The dot height is 1.2±0.6 to 1.3±0.6 nm and the dot density is in the range 10¹¹–10¹² cm⁻². Two ohmic contacts were deposited on the sample surface and annealed in order to contact the buried QD layers. The dots exhibit TM polarized absorption linked to the s–p_z transition. The photocurrent at 300 K is slightly blue-shifted with respect to the s–p_z intraband absorption. The responsivity increases exponentially with temperature and reaches a record value of 10 mA/W at 300 K for detectors with interdigitated contacts.     

Photoreflectance investigations of AlGaN/GaN heterostructures with a two dimensional electron gas

Undoped AlGaN/GaN heterostructures with different content and thickness of AlGaN layer are investigated by photoreflectance (PR) spectroscopy. We have observed PR resonances related to an absorption in both GaN and AlGaN layers. The character of these resonances has been analyzed, and PR lines associated with excitonic and band-to-band absorption in the GaN layer and band-to-band absorption in the AlGaN layer have been identified. The transition related to band-to-band absorption possesses characteristic Franz–Keldysh oscillations (FKOs) associated with a built-in electric field. The electric field in the AlGaN layer obtained on the basis of the analysis of FKOs has been found to be in the range of 244–341 kV/cm. The value of the field has been found to decrease with the increase in AlGaN thickness and to increase with the increase in Al concentration. The surface potential for AlGaN layers has been found to increase with the increase in Al mole fraction and has been estimated to be in the range of 1.0–1.7 eV.     

AlGaN/GaN HEMTs grown on silicon (001) substrates by molecular beam epitaxy

We report the realization of an AlGaN/GaN HEMT on silicon (001) substrate with noticeably better transport and electrical characteristics than previously reported. The heterostructure has been grown by molecular beam epitaxy. The 2D electron gas formed at the AlGaN/GaN interface exhibits a sheet carrier density of 8×10¹² cm⁻² and a Hall mobility of 1800 cm²/V s at room temperature. High electron mobility transistors with a gate length of 4 μm have been processed and DC characteristics have been achieved. A maximum drain current of more than 500 mA/mm and a transconductance g_m of 120 mS/mm have been obtained. These results are promising and open the way for making efficient AlGaN/GaN HEMT devices on Si(001).     

DLTS study of n-type GaN grown by MOCVD on GaN substrates

Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm² of substrate A and on an area of 1×1 cm² of substrate B. Two traps labeled B1 and B2 are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.     

Gain mechanisms in field‐free InGaN layers grown on sapphire and bulk GaN substrate

The gain mechanisms and recombination dynamics of InGaN layers strongly depend on the structural properties of the substrate material. The 4.5 nm and 9.5 nm thick layers were grown by metal organic chemical vapor deposition on two different substrates (sapphire and GaN) with different dislocation densities. Time‐resolved photoluminescence spectroscopy at high excitation densities identifies the saturation of nonradiative recombination centers through excited carriers as a major gain mechanism. The prime argument is an unusual nonexponential luminescence decay. This was confirmed by a lower threshold of the optical gain for the structures grown on GaN with lower dislocation densities. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micro-photoluminescence of GaN quantum dots embedded in 100 nm wide cylindrical AlN pillars

Individual pillars were etched from a sample embedding a single plane of GaN/AlN quantum dots, deposited by molecular beam epitaxy on a sapphire substrate. Pillars with diameters ranging from 0.1 to 5 μm were fabricated by electron-beam lithography and SiCl₄ reactive ion etching. The PL from a single pillar could be measured by using a confocal microscope, with a spatial resolution of 600 nm. We report an intense PL signal from pillar diameters as small as 0.1 μm at room temperature. By increasing the power of the excitation laser from 0.05 to 200 μW, we induced a blue-shift of the PL energy peak from 2.38 to 2.86 eV, accompanied by a substantial broadening of the PL line. This is explained by the photo-induced screening of the internal electric field, which is close to 10 MV/cm in GaN/AlN heterostructures. Finally we report and tentatively explain a photodarkening effect, i.e., the progressive decrease of the PL intensity over two orders of magnitude, after one hour of continuous laser excitation. However, this effect does not seem to be correlated to the etching process.     

Microstructural assessment of InN-on-GaN films grown by plasma-assisted MBE

The structural properties of InN thin films, grown by rf plasma-assisted molecular beam epitaxy on Ga-face GaN/Al₂O₃(0001) substrates, were investigated by means of conventional and high resolution electron microscopy. Our observations showed that a uniform InN film of total thickness up to 1 μm could be readily grown on GaN without any indication of columnar growth. A clear epitaxial orientation relationship of , was determined. The quality of the InN film was rather good, having threading dislocations as the dominant structural defect with a density in the range of 10⁹–10¹⁰ cm⁻². The crystal lattice parameters of wurtzite InN were estimated by electron diffraction analysis to be a=0.354 nm and c=0.569 nm, using Al₂O₃ as the reference crystal. Heteroepitaxial growth of InN on GaN was accomplished by the introduction of a network of three regularly spaced misfit dislocation arrays at the atomically flat interface plane. The experimentally measured distance of misfit dislocations was 2.72 nm. This is in good agreement with the theoretical value derived from the in-plane lattice mismatch of InN and GaN, which indicated that nearly full relaxation of the interfacial strain between the two crystal lattices was achieved.     

Optical properties of InGaN quantum dots

We present time-resolved and spatially-resolved photoluminescence (PL) measurements of InGaN inclusions in a GaN matrix. The structures were grown by metal-organic chemical vapor deposition on sapphire and Si(111) substrates. Nonresonant pulsed excitation yields a broad PL peak, while resonant excitation into the nonresonant PL intensity maximum results in an evolution of a sharp resonant PL peak, having a spectral shape defined by the excitation laser pulse and a radiative decay time close to that revealed for PL under nonresonant excitation. Observation of a resonantly excited narrow PL line gives clear proof of the quantum dot (QD) nature of luminescence in InGaN–GaN samples. Cathodoluminescence (CL) and micro-PL measurements demonstrate sharp emission lines from single QD states. The recombination dynamics of single QD’s and the whole QD ensemble were investigated. Monoexponential decay was observed for the PL of single QD’s. For similar transition energies different time constants were obtained. Therefore the nonexponential decay observed for the whole ensemble is attributed to the coexistence of QD’s having similar ground-state transition energies, but significantly different electron–hole overlap.     

In-plane gate transistors in AlxGa1−xN/GaN heterostructures written by focused ion beams

Focused ion beam implantation of gallium and dysprosium was used to locally insulate the near-surface two-dimensional electron gas of Al_xGa_1−xN/GaN heterostructures. The threshold dose for insulation was determined to be 2×10¹⁰ cm⁻¹ for 90 keV Ga⁺ and 1×10⁹ cm⁻¹ for 200 keV Dy²⁺ at 4.2 K. This offers a tool not only for inter-device insulation but also for direct device fabrication. Making use of “open-T” like insulating line patterns, in-plane gate transistors have been fabricated by focused ion beam implantation. An exemplar with a geometrical channel width of 1.5 μm shows a conductance of 32 μS at 0 V gate voltage and a transconductance of around 4 μS, which is only slightly dependent on the gate voltage.     

Growth and characterisation of GaN with reduced dislocation density

Two GaN MOVPE growth methods to reduce the threading dislocation (TD) density have been explored. The combined effects of (1) in situ SiN_x masking of the sapphire substrate and (2) starting the epitaxial growth at low V-to-III ratio on the GaN film quality were studied by atomic force microscopy, transmission electron microscopy and high-resolution X-ray diffraction. It was found that the annealing condition of the low-temperature nucleation layer after in situ SiN_x masking is critical in order to decrease the density of nucleation sites and hence increase the average grain size to about 5 μm. However, the coalescence of large grains with vertical side facets results in the formation of dense bundles of TDs at the grain boundaries combined with large numbers of basal-plane dislocation loops throughout the film. The formation of these dislocations can be prevented by starting the epilayer growth at low V-to-III ratio, resulting in the formation of grains with inclined side facets. The interaction of the TDs with the inclined side facets causes the dislocations to bend 90 as the grains grow in size and coalesce. GaN films with dislocation densities as low as 1×10⁸ cm⁻², giving full-width at half-maximum values of 180 and 220 arcsec for respectively (002) and (302) omega scans, were achieved by the combination of in situ masking and low V–III ratio epilayer growth. Hall carrier mobility values in excess of 900 cm² V ⁻¹ s⁻¹ were deduced for Si-doped layers.     

Optical properties of InGaN/GaN double quantum wells with varying well thickness

The optical properties and recombination kinetics of the InGaN/GaN double quantum well (DQW) structures with different well thickness (L_w) have been studied by means of photoluminescence (PL), time-resolved PL, and cathodoluminescence (CL) measurements. With increasing quantum well thickness up to 4 nm, the PL emission energy decreases and the blueshift of the PL emission energy increases with increasing excitation density. On the other hand, the PL emission energy of the DQWs with L_w=16 nm is higher than that of the DQWs with L_w=4 nm, and is independent of the excitation density. With increasing L_w from 1 to 4 nm, the PL decay times increase. In contrast, the decay times of 16 nm DQWs are faster than those of 4 nm DQWs. These different results for 16 nm DQWs such as the blueshift of the emission energy, the decrease of the excitation density dependence, and the increase of recombination rate can be ascribed to the relaxation of the piezoelectric field. We also observed the inhomegeneity in the CL spectra of the DQWs with L_w=1 nm on 1 μm scale.     

Magneto-optic phase shift in plasmon propagation along garnet/semiconductor-on-insulator structures

This paper discusses the magneto-optic phase shift in garnet/semiconductor-on-insulator plasmonic waveguides. We consider two structures: (1) Y IG/Si/SiO₂ and (2) Y IG/GaInAsP/AlInAs-oxide. A dispersion relation for the plasmon propagation has been derived and the nonreciprocal phase shift of the fundamental TM mode has been determined at wavelengths of 1.55 μm, 1.5 μm and 1.3 μm.     

Effect of Si doping on GaN/AlN multiple-quantum-well structures for intersubband optoelectronics at telecommunication wavelengths

We present a study of the effect of Si doping localization on the optical and structural properties of GaN/AlN multiple-quantum-well structures for intersubband (ISB) absorption at 1.55 μm. Samples were either undoped or Si doped in different regions (barrier, quantum well (QW), middle of barrier or middle of QW). Structural characterization by atomic force microscopy and X-ray diffraction does not show significant differences in the crystalline quality. All doped samples present room-temperature p-polarized ISB absorption of about 1%–2% per pass, with a line width of 80–90 meV. In contrast, undoped samples present a weaker ISB absorption with a record line width of 40 meV. Both photoluminescence (PL) and ISB absorption display structured shapes whose main peaks correspond to monolayer fluctuations of the well thickness. The emission and absorption line widths depend on the Si doping concentration, but not on the Si location.     

GaN micromachined FBAR structures for microwave applications

This paper presents the manufacturing of GaN membrane supported F-BAR structures. The 2.2 μm thick GaN layer was grown using MOCVD techniques on a high-resistivity 111-oriented silicon substrate. Conventional contact lithography, electron-gun Ti/Au evaporation and lift-off techniques were used to define top-side metallization of the the FBAR structures. Bulk micromachining techniques were used for the release of the GaN membrane. The bottom-side metallization of the micromachined structure was obtained by means of sputtered gold. S-parameter measurements have shown a pronounced resonance around 1.2 GHz. The extracted value of acoustic velocity is in good agreement with those reported by other authors on materials fabricated by other methods. The demonstrated FBAR function in epitaxially grown GaN layers opens new avenues for a low-cost monolithic integration with GaN-based electronics and sensing devices.     

Growth of bulk GaN in a vertical hydride vapour phase epitaxy reactor

Using the hydride vapour phase epitaxy technique, we have grown 2-inch diameter bulk GaN material with a thickness up to 2 mm. The growth was performed in a vertical hot-walled reactor at atmospheric pressure. In this geometry, the process gases are distributed from the bottom upwards through the reactor. We present recent results on growth and characterization of the bulk GaN material. The structural and optical properties of the layers have been studied using decorative etching, optical microscopy, scanning electron microscopy, X-ray diffraction, cathodoluminescence, and low temperature photoluminescence.     

Dependences of structural and electrical properties on thickness of polycrystalline Ga-doped ZnO thin films prepared by reactive plasma deposition

Polycrystalline Ga-doped (Ga content: 4 wt%) ZnO (GZO) thin films were deposited on glass substrates at 200 C by a reactive plasma deposition with DC arc discharge technique. The dependences of structural and electrical properties of GZO films on thickness, ranging from 30 to 560 nm, were investigated. Carrier concentration, n, and Hall mobility, μ, increases with increasing film thickness below 100 nm, and then the n remains nearly constant and the μ gradually increases until the thickness reaches 560 nm. The resistivity obtained of the order of 10⁻⁴ Ω cm for these films decreases with increasing film thickness: The highest resistivity achieved is 4.4×10⁻⁴ Ω cm with n of 7.6×10²⁰ cm⁻³ and μ of 18.5 cm²/V s for GZO films with a thickness of 30 nm and the lowest one is 1.8×10⁻⁴ Ω cm with n of 1.1×10²¹ cm⁻³ and μ of 31.7 cm²/V s for the GZO film with a thickness of 560 nm. X-ray diffraction pattern for all the films shows a hexagonal wurtzite structure with its strongly preferred orientation along the c-axis. Full width at half maximum of the (002) preferred orientation diffraction peak of the films decreases with increasing film thickness below 100 nm.     

Integration of GaN thin films to SiO2–Si(100) substrates by laser lift-off and wafer fusion method

A process methodology has been adopted to bond GaN thin films to Si(100) substrates using the combination of laser lift-off and direct wafer fusion. Using optimum excimer laser conditions, 2–10 μm of GaN is lifted-off from sapphire. The lifted-off thin film is cleared from gallium residual and then suitably treated in a hydrofluoric, nitric and acetic acid mixture to render the surface hydrophilic. This treatment provides van der Waals bonds to immediately contact bond with SiO₂–Si(100) substrate at room temperature. The bonds are further strengthened by a high temperature annealing at 650 C for 2 h. The structural and mechanical characteristics of the bonded structure reveal uniform and high quality bonding. The optical characteristics of the transferred bonded film on SiO₂–Si(100) substrate exhibit similar properties to that of GaN on sapphire. In a similar manner, high-brightness blue LEDs were transferred from sapphire to SiO₂–Si(100) substrate with no deterioration in the electrical and optical performance of the device.