Low-defect GaN Research Articles

Improving Optical and Electrical Characteristics of GaN Films via 3D Island to 2D Growth Mode Transition Using Molecular Beam Epitaxy

Molecular beam epitaxy (MBE) is demonstrated as an excellent growth technique for growing a low-defect GaN channel layer, which is crucial for controlling vertical leakage current and improving breakdown voltage (BV) in GaN-based high-electron mobility transistors (HEMTs). The 3D islands to 2D growth mode transition approach was induced by modulating substrate growth temperature (Tsub), displaying an overall improvement in film quality. A comprehensive investigation was conducted into the effects of Tsub on surface morphologies, crystal quality, and the optical and electrical properties of GaN films. Optimal results were achieved with a strain-relaxed GaN film grown at 690 °C, exhibiting significantly improved surface characteristics (root-mean-square roughness, Rq = 0.3 nm) and impressively reduced edge dislocations. However, the film with the smoothest surface roughness, attributed to the effect of the Ga-rich condition, possessed a high surface pit density, negatively affecting optical and electrical properties. A reduction in defect-related yellow emission further confirmed the enhanced crystalline quality of MBE GaN films. The optimized GaN film demonstrated outstanding electrical properties with a BV of ~1450 V, surpassing that of MOCVD GaN (~1180 V). This research significantly contributes to the advancement of MBE GaN-based high electron mobility transistor (HEMT) applications by ensuring outstanding reliability.

Broadband optical nonlinearity and all-optical switching features in low-defect GaN.

GaN is a one of promising materials for nonlinear optical applications. In this work, the broadband nonlinear optical response and potential applications for all-optical switching (AOS) are evaluated in low-defect GaN. In the pump-probe experiments, the ultrafast optical switching times are consistent with pulse widths accompanied with relative weak free-carrier absorption response, and the modulation contrast can reach ∼60% by varying the polarization orientations between the pump and probe lights. In the visible region, the broadband two-photon absorption effect exhibits excellent values for the imaginary part of figure of merit (FOM), providing the possibility of AOS based on nonlinear absorption (magnitude). While in the near-infrared region and under the presence of three-photon absorption, not only the real part of FOM based on Kerr effect is evaluated, but also the maximum light intensity for the usage of AOS based on nonlinear refraction (phase) is determined. The broadband nonlinear optical and AOS features in low-defect GaN will be highly favorable for the applications in the field of integrated nonlinear photonics and photonic circuits.

Effective Leakage Current Reduction in GaN Ultraviolet Avalanche Photodiodes With an Ion-Implantation Isolation Method

We report high-performance homojunction GaN avalanche photodiodes (APDs) grown on a low-defect GaN substrate and fabricated with an ion-implantation isolation method. High-quality p-i-n GaN layers were grown using metalorganic chemical vapor deposition (MOCVD), and an effective device isolation method using optimized nitrogen ion implantation was developed to provide significant leakage current suppression on the etched mesa sidewalls. Fabricated GaN APDs showed an ultralow dark current density <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> up to 50% of the avalanching breakdown voltage region and achieved a photocurrent gain of >106 at a reverse bias of 71.5 V under deep-ultraviolet (DUV) illumination at λ = 280 nm. A further temperature-dependent study of the dark current indicated that a trap-assisted tunneling process predominates the leakage current component that also contributes to the carrier multiplication process near the avalanching breakdown.

(Invited) Process Monitoring of 100 GaN-on-Diamond Wafers

The introduction of chemical vapor deposition (CVD) diamond as a viable substrate for GaN HEMTs [1], [2] makes GaN-on-diamond an attractive means to run higher power densities through the GaN HEMT compared to traditional GaN on host substrates. While power densities of 20 and 40W/mm have been demonstrated for pulsed devices on silicon carbide (SiC) [3], continuous powers exceeding 10W/mm will reduce the lifetime of the GaN HEMTs due to self-heating. GaN-on-diamond holds a promise of reducing the problems associated with self-heating and allows for increases in the power density of HEMT devices. To date, GaN-on-diamond has been shown to allow for higher areal power densities than GaN-on-SiC [4], [5]. The process of making GaN-on-diamond was first demonstrated more than 10 years ago [1], [2] has evolved and improved over the years from broken samples 1cm on a side with a 25 micron (800 W/mK thermal conductivity) diamond substrate to a 4” process with a 100 micron (1500 W/mK) diamond substrate. In this paper we will look at wafer level electrical comparisons of GaN-on-diamond to GaN on host substrates and at process monitoring for the 100 GaN-on-diamond wafers. The process we use to make GaN-on-diamond is to transfer an epitaxial layer of GaN from a silicon substrate to diamond. In this process we flip the GaN grown on silicon onto a sacrificial substrate and remove the growth silicon. At this point the AlGaN transition layers (TLs) which are necessary to grow low defect density GaN on silicon are etched away. The TLs while critical to the growth of low defect GaN are themselves highly defective and have poor thermal properties [6]. It is therefore advantageous both electrically and thermally to remove these layers. Once the TLs are removed we protect the GaN back side with a nucleation/adhesion layer and we grow the CVD diamond. The standard metric for GaN-on-diamond is the thermal boundary resistance (TBR) which measures how efficiently the heat can be transferred from the GaN to the diamond. With collaborators we have been able to demonstrate a drop from 135 m2K/GW to as low as 10 m2K/GW by removing the TLs and maintaining a consistent adhesion layer thickness of silicon nitride between the GaN and the diamond [7]. Process monitoring shows that we can consistently maintain a 35 +/- 5nm SiN adhesion layer between the GaN and the diamond. In this paper we will present process monitoring data of this and other parameters for the last 100 GaN wafers. Less well established are the electrical metrics for comparing GaN-on-diamond to GaN on host substrates. One important metric of electrical performance for GaN-HEMTs is the flat capacitance at high reverse bias. It allows us to characterize the presence of traps and/or doping at the GaN to host interface [8]. In our case the GaN to diamond interface. We measure this flat capacitance by taking the capacitance voltage (C-V) profile. Results show that we can take GaN-on-silicon which has a flat band capacitance of 25pF and turn it into GaN-on-diamond with a flat band capacitance of ~ 2pF Figure[1a]. This low capacitance is indicative of negligible GaN buffer and epi interface charge/doping. The capacitance of the GaN-on-diamond is now comparable to GaN-on-silicon carbide Figure [1b]. Figure 1

Epitaxial Growth of High Quality GaN Films on Lattice Matched Metallic Layers

In recent years there have been great advances in wide bandgap electronic and optoelectronic devices based on GaN materials. However, problems arising from growth on non-native substrates, such as sapphire or Si, continue to present major hurdles for development of higher performance and lower cost devices. For example, complex layer structures and processes have been developed to successfully mitigate some of the problems arising from mismatch in both lattice constant (LC) and coefficient of thermal expansion (CTE) between GaN and foreign substrates. However, the added complexity and costs reduce the utility of these techniques for many applications. In the past, GaN growth on metallic substrates, buffer layers and interlayers have been studied by few groups for a number of applications including thermal management in high power devices, back contact for vertical-transport devices, backside mirror for light emitters and detector, and as threading dislocation filters. The major obstacles for the growth of high quality GaN films in these studies have been 1) deposition of single crystal metal film with proper orientation on foreign substrates, 2) avoiding strong chemical reactions and interface mixing during the nucleation of GaN on the metal surface, 3) minimizing mismatch in LC and CTE to avoid high dislocation density, stress cracks or sever wafer bowing, and 4) achieving adequate thermal and chemical stability of the GaN/metal/substrate structure for the demanding growth conditions encountered in deposition of III-nitride films for example by hydride vapor phase epitaxy. In this work we have studied high quality growth of GaN by RF-plasma assisted molecular beam epitaxy (PA-MBE) and metal organic chemical vapor deposition (MOCVD) on high-quality, single crystal refractory metals deposited by plasma sputtering on c-plane sapphire and Si(111) substrates. We will first discuss the deposition and optimization of fully relaxed high quality metal layer structures that are properly oriented and lattice matched for the growth of low defect GaN films on c-plane sapphire and Si(111). We will then compare the nucleation and growth of GaN by PA-MBE and MOCVD on these metal layers. Some initial results such as x-ray diffraction scans for GaN grown on these lattice-matched metal alloys are quite comparable to high quality GaN grown on c-plane sapphire. Finally, we present some examples of improved wide bandgap devices based on GaN films grown on metal layers. Figure 1

Defect-Related Luminescence in Undoped GaN Grown by HVPE

Hydride vapor phase epitaxy (HVPE) is used for the growth of low-defect GaN. We have grown undoped films on sapphire and investigated them using steady-state and time-resolved photoluminescence (PL). One of the dominant PL bands in high-quality GaN grown by HVPE is the green luminescence (GL) band with a maximum at 2.4 eV. This PL band can be easily recognized in time-resolved PL measurements due to its exponential decay even at low temperatures (<50 K), with a characteristic lifetime of 1–2 μs. As the temperature increases from 70 K to 280 K, the PL lifetime for the GL band increases by an order of magnitude. This unusual phenomenon can be explained on the assumption that the electron-capture coefficient for the GL-related defect decreases with temperature as T −2.6.

Thin-film heteroepitaxy by the formation of the dilatation dipole ensemble

It is shown that in substantially anisotropic media, such as a crystal with cubic lattice symmetry, the analogous dilatation centers can strongly attract each other considerably decreasing the total elastic energy. Such attractive centers form stable objects of a new type, the elastic dilatation dipoles. By the example of heteroepitaxy of the film of silicon carbide SiC on a silicon substrate, the calculations of elastic energy of the system are performed. It is shown that the elastic energy can relax completely only due to the ensemble of dilatation dipoles. A new growth method of SiC on Si is suggested and implemented experimentally. In this method, the elastic energy relaxes due to the ensemble of elastic dilatation dipoles. It is shown experimentally that the heteroepitaxial SiC films grown do not contain cracks and lattice-misfit dislocations despite the tremendous difference in lattice parameters. Thick low-defect GaN and AlN layers are grown on the SiC/Si templates obtained. The laboratory model of the operating light-emitting diode is originally obtained based on these structures.

Near-Infrared Absorption in Lattice-Matched AlInN/GaN and Strained AlGaN/GaN Heterostructures Grown by MBE on Low-Defect GaN Substrates

We have investigated near-infrared absorption and photocurrent in lattice-matched AlInN/GaN and strained AlGaN/GaN heterostructures grown by molecular-beam epitaxy (MBE) on low-defect GaN substrates for infrared device applications. The AlGaN/GaN heterostructures were grown under Ga-rich conditions at 745°C. Material characterization via atomic force microscopy and high-resolution x-ray diffraction indicates that the AlGaN/GaN heterostructures have smooth and well-defined interfaces. A minimum full-width at half-maximum of 92 meV was obtained for the width of the intersubband absorption peak at 675 meV of a 13.7 A GaN/27.5 A Al0.47Ga0.53N superlattice. The variation of the intersubband absorption energy across a 1 cm × 1 cm wafer was ±1%. An AlGaN/GaN-based electromodulated absorption device and a quantum well infrared detector were also fabricated. Using electromodulated absorption spectroscopy, the full-width at half-maximum of the absorption peak was reduced by 33% compared with the direct absorption measurement. This demonstrates the suitability of the electromodulated absorption technique for determining the intrinsic width of intersubband transitions. The detector displayed a peak responsivity of 195 μA/W at 614 meV (2.02 μm) without bias. Optimal MBE growth conditions for lattice-matched AlInN on low-defect GaN substrates were also studied as a function of total metal flux and growth temperature. A maximum growth rate of 3.8 nm/min was achieved while maintaining a high level of material quality. Intersubband absorption in AlInN/GaN superlattices was observed at 430 meV with full-width at half-maximum of 142 meV. Theoretical calculations of the intersubband absorption energies were found to be in agreement with the experimental results for both AlGaN/GaN and AlInN/GaN heterostructures.

Injection-level-dependent internal quantum efficiency and lasing in low-defect GaN nanowires

Measurements of temperature-dependent and time-resolved photoluminescence (PL) on individual GaN nanowires revealed PL lifetimes and values of internal quantum efficiency (IQE) that increased with excitation fluence. With sufficient injection levels, radiative recombination dominated within the nanowire temperature range of 75 K to 175 K, as indicated by the T3/2 temperature dependence of the free-exciton PL lifetimes for this bulk material. The IQE was close to unity here. Free-carrier recombination became more significant as temperatures increased toward room temperature, but excitonic recombination remained important with ultrashort excitation pulse fluences as high as 190 μJ/cm2. The IQE at room temperature fell to a value between 3% and 30% depending on the nature of the recombination, and, considering both excitonic and free-carrier recombination, the effective IQE was roughly 15%. Temperature-dependent measurements of lasing thresholds in optically pumped nanowires showed lower thresholds at temperatures where excitonic radiative recombination was strong, indicating a possible persistence of excitoniclike behavior with high injected carrier densities at temperatures below T = 170 K.

XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C–SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar + ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2–4 keV energy and fluence ranging from 3 × 10 13 ions/cm 2 to 1 × 10 18 ions/cm 2. Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N 2 + ions and 7.2 × 10 17 ions/cm 2 are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar + ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.

Structural and optical properties of near-UV LEDs grown on V-grooved sapphire substrates fabricated by wet etching

V-grooved sapphire substrates (VGSS) were fabricated by a simple wet etching process with SiO 2 stripe masks along 〈 1 1 ¯ 2 0 〉 orientation of the sapphire substrate and a mixed solution of H 2SO 4 and H 3PO 4. The growth of low-defect GaN template was optimized by two-step growth technique of metalorganic vapor deposition (MOCVD), resulting in the threading dislocation (TD) density of 2–4×10 7 cm −2 in the entire region of the GaN template. The epitaxial structure of near-UV light emitting diode (LED) was grown on the GaN templates on both the VGSS and the flat sapphire substrate (FSS) in order to compare the characteristics of their structural and optical properties. The internal quantum efficiency and the extraction efficiency of the LED structure grown on the VGSS were remarkably increased when compared to the conventional LED structure grown on the FSS. It seems to be attributed to the reduction in the TD density of the GaN template on the VGSS and the decrease in the number of times of total internal reflections of the light flux due to the V-grooved pattern, respectively. The increase in optical output power of the LED grown on the VGSS agreed well with the expected value based on the simulation of the commercial Light Tool program and temperature-dependent photoluminescence (PL) intensities.

Transmission electron microscopy study of defect reduction in two-step lateral epitaxial overgrown nonplanar GaN substrate templates

Transmission electron microscopy (TEM) is carried out to characterize the extended defect reduction in low-defect nonplanar GaN substrate templates grown by lateral epitaxial overgrowth (LEO). The LEO nonplanar GaN substrate template has a trapezoidal cross section with smooth (0 0 0 1) and { 1 1 2 ¯ 2 } facets. We demonstrate here the dislocation distribution and behavior in both ordinary LEO and two-step LEO. Penetration of threading dislocations (TDs) beyond mask windows is observed in ordinary LEO substrates. In two-step LEO substrates, which utilize the tendency for TDs to bend 90° at certain plane interfaces, only a type dislocations with Burgers vector b = 1 3 〈 1 1 2 ¯ 0 〉 are generated in the upper part above the TD bending zone between two mask windows with a density of ∼8×10 7 cm −2, and there are almost no dislocations in the LEO wing region. This approach provides a promising path to produce low-defect GaN substrate templates for high-performance buried heterostructure lasers.

Thick AlN layers grown by HVPE on sapphire substrates

ABSTRACT Thick low defect AlN and AlGaN layers grown on ultra violet (UV) transparent substrates are considered as promising substrate materials for the UV light emitters and detectors. Electrically insulating thick AlN layers may serve as the substrates for AlGaN/GaN-based high power high electron mobility transistors (HEMTs). In this paper we report on crack-free up to 20 µm thick AlN layers grown by stress control HVPE on 2-inch sapphire substrates. As-grown surface had a characteristic pyramidal morphology. Being t hick enough, AlN layers can be polished to improve surface roughness. The minimum full width at half maximum (FWHM) values of AlN ω-scan x-ray (00.2) and (10.2) rocking curves was about 500 and 1000 arcsec, respectively. The XRD analysis was applied for the threading dislocation density evaluation in grown AlN layer. Screw dislocation density was found to be (3-7)x10 8 cm -2 for the layers from 3 to 12 µm thick. INTRODUCTION Hydride vapour phase epitaxy (HVPE) is known to produce low defect GaN substrate materials for electronic and optoelectronic GaN-based devices. High quality HVPE grown GaN layers and free-standing material have been recently demonstrated by several research teams as the substrates for GaN-based power devices [1], high-frequency transistors [2], and blue lasers [3]. However, GaN is not optically transparent in a UV spectral region for wavelengths shorter than 360 nm and wider band gap material are needed for deep UV applications. Due to large band gap (6.2 eV), high thermal conductivity (3.3 W/cm-K) and close thermal and lattice match to AlGaN layers, AlN is promising substrate material for advanced high power UV emitting devices (LEDs and LDs), UV detectors, ultra high power high frequency electronic devices (AlGaN/GaN HEMTs). However, bulk AlN substrates of required size (2-inch diameter and larger) are not available. One way to get over this issue is to fabricate AlN template substrates comprising thick low defect AlN layer grown on a foreign substrate. The main technical challenge here is to minimize stresses in such template substrate and to avoid cracking of the AlN layer. Recently we have reported on stress control HVPE technology to grow thick (>50µm) crack-free AlN layers on SiC substrates [4]. In this paper we report on crack-free AlN layers up to 20 µm thick grown by HVPE on 2-inch sapphire substrates. Results of 3-15 µm thick layers characterization are discussed.

Growth and characterization of low defect GaN by hydride vapor phase epitaxy

Low defect density GaN boules were produced using hydride vapor phase epitaxy. A high growth rate of over 100 μm/h enabled the growth of GaN boules longer than 3 mm in length. GaN wafers generated from the boules were characterized by X-ray diffraction, synchrotron white beam X-ray transmission topography, and microscopy of etched surfaces. It was found that the dislocation density decreased with the thickness of the grown material. GaN samples with dislocation density less than 10 4 cm −2 were produced. The high crystal quality of the GaN samples was further demonstrated with a full-width at half-maximum of 38 arcsec for the GaN(0 0 0 4) double crystal X-ray rocking curve.