GaN-based Structures Research Articles

Cathodoluminescence study of GaN-based film structures

GaN films grown on sapphire substrate with an emphasis on epitaxial lateral overgrown (ELOG) layers with an array of rhombic shaped mask area as well as InGaN/GaN MQW laser diode layer structures were investigated by cathodoluminescence (CL) spectroscopy and CL imaging at room and low temperatures. The microscopic imaging with a high-spatial resolution clearly reveals the distribution of threading dislocations and point defects in ELOG GaN films. The secondary electron and CL data measured on cleaved faces of laser diodes are analyzed in consideration with luminescence mechanisms in semiconductor heterostructures and around the p - n junction, providing important information on the defects and carrier dynamics in laser diode devices.

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.

Cathodoluminescence study in Al xGa 1 − xN structures

In this work, we consider a 2D model for calculation of cathodoluminescence in GaN-based structures. This model is developed using an extended generation profile and taking into account the influence of the carrier diffusion process, internal absorption and some radiative recombination processes. First, we have investigated the effect of hole diffusion length and the surface recombination velocity on the CL spectra of GaN sample grown at 800 °C by MOVPE method. Then, we have calculated the dependence of CL intensity from AlGaN alloys as a function of Al content and the electron beam energy. Results show a red shift of the CL peaks when the beam energy is varied from 2 to 10 keV at room temperature. The band-edge emission of Al x Ga 1 − x N shifts about 0.49 eV when the Al composition is increased from x = 0.18 to 0.38. Comparison of the experimental spectra with simulations shows a good agreement.

Bose glass and superfluid phase transitions of exciton–polaritons in GaN microcavities

Microcavity exciton–polaritons within GaN-based structures are the object of the present work. The impact of the structural imperfections on the properties of the two-dimensional polariton gas is investigated through the calculation of its phase diagram. We demonstrate that the presence of disorder first induces a quasi-phase transition of the polariton system towards a Bose-glass phase before it becomes superfluid as its density increases. Calculations of the density of states as well as the condensate wavefunction and the related spectrum of elementary excitations in the framework of the Gross–Pitaevskii theory provide further insight into the properties of exciton–polaritons in GaN-based microcavities.

Defects with bright contrast in the induced-current mode in GaN-based light-emitting structures

InGaN/GaN-based light-emitting structures with multiple quantum wells have been studied by the electron beam induced current method. It is shown that the investigation of the dependence of the collected current on the beam energy allows us to evaluate the fraction of excess charge carriers recombining in quantum wells. Two types of defects with bright contrast have been found. It is shown that bright contrast is associated with the formation of enhanced conductivity channels across the structure with quantum wells. The dependence of the contrast on the beam current has been investigated.

Progress in the optical studies of single InGaN/GaN quantum dots

The great success that GaN-based structures have enjoyed in implementing efficient optoelectronic devices has fostered a rapidly expanding interest amongst researchers aimed at understanding their underlying physics. There has been an active debate on the mechanisms that give rise to efficient luminescence in these materials. In this paper we approach these questions through optical studies of single InGaN/GaN quantum dots in the context of the available experimental and theoretical understanding of InGaN structures in general, and of three-dimensional localization in this material in particular. We will also show how it is possible to exploit the various unique properties that nitride-based materials offer, such as the strong inbuilt electric field, in a controlled manner. Such control may in the future prove essential for the implementation of single quantum dot devices in applications such as quantum information processing. We also show how nonlinear spectroscopy provides an invaluable tool in suppressing background luminescence effects inherent in this material.

Micro-Raman probing of residual stress in freestanding GaN-based micromechanical structures fabricated by a dry release technique

In this study, the authors have employed micro-Raman scattering to characterize the residual stress in freestanding GaN-based micromechanical structures on (111)-oriented silicon substrates. Arrays of freestanding cantilevers and microbridges have been fabricated using a combination of dry etching techniques. The Si material beneath the GaN microstructures is removed by a nonplasma XeF2-based dry release technique. Two distinct sets of GaN-based layers on Si(111) with a different amount of growth-induced tensile stress are selected for the fabrication of freestanding cantilevers. The residual stress in these micromechanical structures is determined from the peak shift of the E2-high phonon mode of GaN. Such GaN mechanical structures on Si platforms may be useful for the fabrication of GaN-based microelectromechanical systems and sensors.

New approaches for growth control of GaN-based HEMT structures

This paper reports on new approaches for growth control of GaN-based heterostructures for high frequency and high power application. First in situ methods are presented and their further development discussed [1]. The development leads to a greatly improved observation of growth parameter influences in the MOVPE of GaN. A new growth process is introduced which enhances growth reproducibility [2]. This new growth process is then optimized with respect to the envisaged application. To this end process modeling will be employed. The application envisaged is the AlxGa1-xN/GaN high electron mobility transistor (HEMT). At last device results will be presented. All in all it will be shown how fundamental research can drive technology and how basic knowledge can be employed for process development with respect to device applications.

Fabrication of deeply undercut GaN-based microdisk structures on silicon platforms

The authors demonstrate the use of a dry releasing technique to achieve deeply undercut GaN-based microdisk structures supported by silicon platforms. Varying dimensions of microdisk structures on silicon posts with large air gaps are fabricated by a XeF2-based dry etching of the underlying silicon material. The residual stress variation in these microdisks is studied by high spectral resolution micro-Raman mapping. Such a fabrication technique may effectively improve the light extraction efficiency from GaN-based microdisk light emitting diodes on silicon substrates.

Crystal orientation effects on electronic and optical properties of wurtzite ZnO/MgZnO quantum well lasers

Crystal orientation effects on electronic and optical properties of ZnO/MgZnO QW structures are investigated by taking into account the non-Markovian gain model with many-body effects. These results are compared with those for GaN-based QW structures. In a range of small crystal angles, ZnO/MgZnO QW structures have a lower internal field than GaN/AlGaN and InGaN/GaN QW structures. However, ZnO/MgZnO QW structures show a larger internal field than GaN-based QW structures at crystal angles near $${\theta =50^{\circ}}$$ . The WZ ZnO/MgZnO QW structures are shown to have much larger optical gain than the GaN-based QW structures for small crystal angles. This is because WZ ZnO/MgZnO QW structures have larger matrix element and smaller effective masses than InGaN/GaN QW structures near the (0001) crystal orientation. On the other hand, in the case of the (10 $${\bar{1}}$$ 0) crystal orientation, the optical gain of ZnO/MgZnO QW structures becomes smaller than that of InGaN/GaN QW structures due to the increase of the effective mass. In addition, the ZnO/MgZnO QW structures have a maximum in the optical gain near $${\theta =50^{\circ}}$$ , which can be explained by the fact that the average hole effective mass increases although the matrix element at high carrier density is improved with increasing crystal angle.

Application of ZnO to Passivate the GaN-based Device Structures

AbstractThe fabrication of high-resistivity ZnO-based thin films lattice-matched to AlGaN/GaN structures has been developed. It relies on low-temperature reactive sputter deposition of ZnO:Sb from ZnSb target. Taking into account the hygroscopic nature of ZnO surface, an additional coating by Si3N4 films is applied to ensure the humidity protecition. The developped passivation suppresses leakage currents in Schottky diods, and substantially improves output characteristics of AlGaN/GaN HEMT.

Work function analysis of GaN-based lateral polarity structures by Auger electron energy measurements

Lateral polarity heterostructure (LPH) was grown with adjacent Ga- and N-face domains in order to invert the polarity of the crystal within a periodicity of a few microns. In this study we focus on the analysis of these LPH by Auger electron spectroscopy (AES). Because of the relationship between the Auger electron energy and the Fermi level, AES is a suitable method to identify the domains of a lateral polarity heterostructure. In addition, we discuss the possibility of determining the work function difference of Ga- and N-face GaN. This difference in the work function between Ga-face and N-face GaN is found to be 0.25eV. This difference is caused by a surface band bending.

GROWTH OF SiC SUBSTRATES

In recent years SiC has metamorphisized from an R&D based materials system to emerge as a key substrate technology for a significant fraction of the world production of green, blue and ultraviolet LEDs. Emerging markets for SiC homoepitaxy include high-power switching devices and microwave devices. Applications for heteroepitaxial GaN-based structures on SiC substrates include lasers and microwave devices. In this paper we review the properties of SiC, assess the current status of substrate and epitaxial growth, and outline our expectations for SiC in the future.

Fabrication of GaN-based nanoscale device structures utilizing focused ion beam induced Pt deposition

In this work we have demonstrated nanoscale GaN device structures made from individual GaN nanowires and electrical contacts utilizing focused ion beam (FIB) induced Pt deposition. These GaN nanowires were grown by direct reaction of Ga vapor with NH3 and had diameters ranging from 100nmto250nm and lengths up to 200μm. As-grown nanowires were dispersed on SiO2 coated p++ Si substrate. A 30keV Ga+ ion beam was used to dissociate (trimethyl)methylcyclopentadienyl-platinum precursor for depositing Pt contacts to GaN nanowires. FIB-deposited Pt contacts to GaN nanowires showed nonlinear I-V characteristics, which turned linear after annealing at 500°C for 30s in argon. Resistivity of a GaN nanowire measured using a four terminal contact geometry fabricated by depositing Pt with a FIB was in the range of 5×10−3Ωcm. Temperature dependent resistivity measurement of the GaN nanowire revealed semiconducting behavior with a weak temperature dependence of the resistivity. In this study both Ohmic and Schottky contacts to GaN nanowires have been realized with FIB-deposited Pt contacts. Barrier height and ideality factor have been extracted for the metal-GaN nanowire Schottky junctions, which had low reverse breakdown voltage and large ideality factor of 18. Modulation of the current through the nanowire was achieved by applying a bias to the Si substrate acting as a backgate. n-type depletion mode behavior was observed in the GaN nanotransistor, which was consistent with the impurity related background concentrations expected in this type of growth method. The heat generation during FIB deposition and the ion damage appeared to cause noticeable swelling of the nanowires under the Pt contacts. Electron beam induced Pt deposition was also used to fabricate electrical contacts to the nanowires.

The study of lateral carrier transport in structures with InGaN quantum dots in the active region

GaN-based structures with InGaN quantum dots in the active region, which emit in the blue and green spectral ranges, are studied. The structures grown by both the conventional method and with the use of special procedures of the growth of the active region were compared. The use of these special growth modes stimulates the activated phase decomposition, which leads to the formation of quantum dots with substantially larger localization depth of electrons. It is shown that the formation of such deep quantum dots, like the formation of larger inhomogeneities of the active region, substantially suppresses the lateral carrier transport. This effect improves the characteristics of light-emitting diode structures at a low injection level and increases the temperature stability of quantum efficiency.

Influence of Overheating Effect on Transport Properties of AlGaN/GaN Heterostructures

ABSTRACTAlGaN/GaN heterostructures can operate at very high power in dc and rf regimes due to the superior properties of III-nitride materials. At the same time, the higher level of dissipated power causes higher overheating in the structures. Considering the fact that the temperature is a driving force for aging processes in devices, it is clear that the investigations of thermal effects in 2DEG of GaN-based structures are of great importance. Additionally, such studies result in a better understanding of the transport features which affect device performance, and allow us to determine the conditions under which hot-electron effects play the dominant role in electron transport. In this report we focus on thermal effects in transmission line model (TLM) patterns. The experimental results and numerical simulations of self-heating effects in TLM patterns processed on heterostructures with different layer design (different Al mole fraction of barrier layer, different substrates) are analyzed. The GaN-based heterostructures were grown by MOCVD. One of the investigated samples consists of a nucleation layer of AlGaN (16% Al), an undoped GaN buffer layer and an AlGaN (33%) undoped barrier layer. This type of structure was formed on sapphire and SiC substrates. Additionally, another two types of structures were grown on sapphire substrate: with wide bandgap barrier layer (with Al mole fraction of 75%), and with a thin AlN spacer layer (5nm) in between the AlGaN barrier layer (23 nm) and the GaN buffer layer (1300 nm). All samples were processed with TLM patterns with standard Ti/Al/Ti/Au contact metallization and annealed for 40s at 800° C. The conducting channel of the devices has a width W = 100 μm with intercontact lengths L of 1, 5, 10, 15, 20, 25, 30, 35 μm. The average overheating temperature over the active channel area for each structure type is estimated. The strong influence of the Al mole fraction of the barrier layer and also the substrate type of the structure on noise and transport properties is revealed and analyzed. It was shown that the implementation of wide bandgap barriers and high thermal conductance substrates allows us to improve the overall performance of the structures significantly. Optimal conditions for the observation of hot-electron effects are determined.

Use of AlInN layers in optical monitoring of growth of GaN-based structures on free-standing GaN substrates

When lattice matched to GaN, the AlInN ternary alloy has a refractive index ∼7% lower than that of GaN. This characteristic can be exploited to perform in situ reflectometry during epitaxial growth of GaN-based multilayer structures on free-standing GaN substrates, by insertion of a suitable Al0.82In0.18N layer. The real-time information on growth rates and cumulative layer thicknesses thus obtainable is particularly valuable in the growth of optical resonant cavity structures. We illustrate this capability with reference to the growth of InGaN∕GaN multiple quantum-well structures, including a doubly periodic structure with relatively thick GaN spacer layers between groups of wells. Al0.82In0.18N insertion layers can also assist in the fabrication of resonant cavity structures in postgrowth processing, for example, acting as sacrificial layers in a lift-off process exploiting etch selectivity between Al0.82In0.18N and GaN.

Light Emission Properties of GaN-Based Laser Diode Structures

Cathodoluminescence is applied for evaluation of in-depth and in-planevariations of light emission from two types of GaN-based laser diode struc-tures. We evaluate in-depth properties of the laser diode emission anddemonstrate that potential °uctuations still afiect emission of laser diodesfor e-beam currents above thresholds for a stimulated emission.PACS numbers: 85.30.{z, 78.60.Hk, 68.37.Hk

In Ga N ∕ Ga N multi-quantum-well structures on (111)-oriented bonded silicon-on-insulator substrates

We report growth of InGaN∕GaN multiple quantum wells (MQWs) on (111)-oriented bonded silicon-on-insulator (SOI) substrates by metalorganic chemical vapor deposition (MOCVD). Prior to MOCVD growth of MQWs, about a 1.2μm thick GaN layer was deposited on SOI substrate with a high-temperature transitional buffer layer. The growth conditions were tuned to realize blue-green emission peaks centered around 420–495nm from such MQWs on SOI. X-ray diffraction, atomic force microscopy, scanning electronic microscopy, and photoluminescence techniques were used to characterize these MQWs. Such an approach to realize multicolor light-emitting layers on SOI substrates is suitable for the integration of InGaN∕GaN-based optoelectronic structures on SOI-based micro-optoelectromechanical systems and sensors.

Z-contrast imaging of AlN exclusion layers in GaN field-effect transistors

The structural characteristics of AlN exclusion layers used to enhance the electron mobility in GaN-based field-effect transistor structures are investigated using scanning transmission electron microscopy. It is shown that a peak in electron mobility is achieved for an AlN exclusion layer with a nominal thickness of 2.3 nm, although significant compositional grading at the interfaces appears to be present. For longer growth times (i.e., 30 s), a transition to three-dimensional growth occurs, roughening the GaN/AlN/AlGaN interfaces. This roughening is likely to be associated with scattering of carriers in the two-dimensional electron gas (2DEG) consistent with an observed increase in 2DEG sheet resistivity.