Substrate effects on the strain relaxation in GaN/AlN short-period superlattices

ABSTRACT: We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.     

Metal organic vapour-phase epitaxy growth of GaN wires on Si (111) for light-emitting diode applications

GaN wires are grown on a Si (111) substrate by metal organic vapour-phase epitaxy on a thin deposited AlN blanket and through a thin SiN_x layer formed spontaneously at the AlN/Si interface. N-doped wires are used as templates for the growth of core-shell InGaN/GaN multiple quantum wells coated by a p-doped shell. Standing single-wire heterostructures are connected using a metallic tip and a Si substrate backside contact, and the electroluminescence at room temperature and forward bias is demonstrated at 420 nm. This result points out the feasibility of lower cost nitride-based wires for light-emitting diode applications.     

Improving the emission efficiency of MBE-grown GaN/AlN QDs by strain control

ABSTRACT: The quantum-confined stark effect induced by polarization has significant effects on the optical properties of nitride heterostructures. In order to improve the emission efficiency of GaN/AlN quantum dots [QDs], a novel epitaxial structure is proposed: a partially relaxed GaN layer followed by an AlN spacer layer is inserted before the growth of GaN QDs. GaN/AlN QD samples with the proposed structure are grown by molecular beam epitaxy. The results show that by choosing a proper AlN spacer thickness to control the strain in GaN QDs, the internal quantum efficiencies have been improved from 30.7% to 66.5% and from 5.8% to 13.5% for QDs emitting violet and green lights, respectively.     

Structural characteristics of single crystalline GaN films grown on (111) diamond with AlN buffer

Hexagonal GaN films with the [0001] direction parallel to the surface normal were grown on (111) oriented single crystalline diamond substrates by plasma-assisted molecular beam epitaxy. Pre-treatments of the diamond surface with the nitrogen plasma beam, prior the nucleation of a thin AlN layer, eliminated the inversion domains and reduced the density of threading dislocations in the GaN epilayers. The films have an in-plane epitaxial relationship [1010]GaN//[110]diamond. Thus GaN (0001) thin films of single epitaxial relationship and of single polarity were realised on diamond with AlN buffer.     

Intersubband absorption properties of high Al content AlxGa1?xN/GaN multiple quantum wells grown with different interlayers by metal organic chemical vapor deposition

High Al content Al_xGa_1−xN/GaN multiple quantum well (MQW) films with different interlayers were grown by metal organic chemical vapor deposition. These MQWs were designed to achieve intersubband (ISB) absorption in the mid-infrared spectral range. We have considered two growth conditions, with AlGaN interlayer and GaN/AlN superlattice (SL) interlayer, both deposited on GaN-on-sapphire templates. Atomic force microscopy images show a relatively rough surface with atomic-step terraces and surface depression, mainly dominated by dislocations. High-resolution X-ray diffraction and transmission electron microscopy analyses indicate that good crystalline quality of the AlGaN/GaN MQW layer could be achieved when the AlGaN interlayer is inserted. The ISB absorption with a peak at 3.7 μm was demonstrated in MQW films with AlGaN interlayer. However, we have not observed the infrared absorption in MQW films with GaN/AlN SL interlayer. It is believed that the high dislocation density and weaker polarization that resulted from the rough interface are determinant factors of vanished ISB absorption for MQW films with the GaN/AlN SL interlayer.     

Electrical characterization and nanoscale surface morphology of optimized Ti/Al/Ta/Au ohmic contact for AlGaN/GaN HEMT

Good ohmic contacts with low contact resistance, smooth surface morphology, and a well-defined edge profile are essential to ensure optimal device performances for the AlGaN/GaN high electron mobility transistors [HEMTs]. A tantalum [Ta] metal layer and an SiN_x thin film were used for the first time as an effective diffusion barrier and encapsulation layer in the standard Ti/Al/metal/Au ohmic metallization scheme in order to obtain high quality ohmic contacts with a focus on the thickness of Ta and SiN_x. It is found that the Ta thickness is the dominant factor affecting the contact resistance, while the SiN_x thickness affects the surface morphology significantly. An optimized Ti/Al/Ta/Au ohmic contact including a 40-nm thick Ta barrier layer and a 50-nm thick SiN_x encapsulation layer is preferred when compared with the other conventional ohmic contact stacks as it produces a low contact resistance of around 7.27 × 10^-7 Ω·cm² and an ultra-low nanoscale surface morphology with a root mean square deviation of around 10 nm. Results from the proposed study play an important role in obtaining excellent ohmic contact formation in the fabrication of AlGaN/GaN HEMTs.     

Hydride Vapor Phase Epitaxy of GaN Thick Films by the Consecutive Deposition Process Using GaCl3

GaN buffer and main layers were grown by the conventional hydride vapor phase epitaxy technique using GaCl₃ consecutively. The deposited buffer layers were investigated by atomic force microscopy and X-ray analysis. To examine the behavior of the buffer layers at main layer growth temperature, heat treatment was conducted at 900°C. Based on the results of the buffer layer study, GaN thick films were grown at 1050°C. Optimum deposition conditions of buffer layer from the buffer and main layer studies generally coincided. On the φ scanning pattern, the GaN films grown on (0001) Al₂O₃ were single-crystalline. Band-edge emission dominated photoluminescence was observed at room temperature.     

Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate

Zinc oxide thin films have been obtained on bare and GaN buffer layer decorated Si (111) substrates by pulsed laser deposition (PLD), respectively. GaN buffer layer was achieved by a two-step method. The structure, surface morphology, composition, and optical properties of these thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, infrared absorption spectra, and photoluminiscence (PL) spectra, respectively. Scanning electron microscopy images indicate that the flower-like grains were presented on the surface of ZnO thin films grown on GaN/Si (111) substrate, while the ZnO thin films grown on Si (111) substrate show the morphology of inclination column. PL spectrum reveals that the ultraviolet emission efficiency of ZnO thin film on GaN buffer layer is high, and the defect emission of ZnO thin film derived from Zn_i and V_o is low. The results demonstrate that the existence of GaN buffer layer can greatly improve the ZnO thin film on the Si (111) substrate by PLD techniques.     

Comparison of GaN and AlN nucleation layers for the oriented growth of GaN on diamond substrates

In this study, {0001} oriented GaN crystals have been grown on freestanding, polycrystalline diamond substrates using AlN and GaN nucleation layers (NLs). XRD measurements and SEM analysis showed that the application of a thin AlN NL gives the best structural results, because AlN has a thermal expansion coefficient in between GaN and diamond and thus delocalizes the stress to two interfaces. The optical quality of the layers, investigated with Raman microscopy and photoluminescence spectroscopy, is similar. Although no lateral epitaxy is obtained, new insight is gained on the nucleation of GaN on diamond substrates facilitating the growth of GaN epilayers on polycrystalline diamond substrates.     

Domain epitaxy of crystalline BeO films on GaN and ZnO substrates

We demonstrated the growth of wurtzite-crystalline beryllium oxide (BeO) thin films on GaN and ZnO substrates using atomic layer deposition (ALD). Single-crystalline BeO were epitaxially grown on GaN. Despite the inherently large lattice mismatch of BeO and GaN atoms, the 6/5 and 7/6 domain-matched structures dramatically reduced the residual strain in BeO thin films. On the other hand, the lattice mismatch of BeO and ZnO was not effectively accommodated in the mixed domains. X-ray diffraction (XRD) confirmed the in-plane crystallization of BeO-on-substrates in the (002){102}_BeO||(002){102}_Sub orientation and relaxation degrees of 20.8% (GaN), 100% (ZnO). The theoretical critical thicknesses of BeO for strain relaxation were 2.2 μm (GaN) and 1.6 nm (ZnO), calculated using a total film energy model. Transmission electron microscopy (TEM) and Fourier-filtered imaging supported the bonding configuration and crystallinity of wurtzite BeO thin films on GaN and ZnO substrates.     

High breakdown voltage in AlGaN/GaN HEMTs using AlGaN/GaN/AlGaN quantum-well electron-blocking layers

In this paper, we numerically study an enhancement of breakdown voltage in AlGaN/GaN high-electron-mobility transistors (HEMTs) by using the AlGaN/GaN/AlGaN quantum-well (QW) electron-blocking layer (EBL) structure. This concept is based on the superior confinement of two-dimensional electron gases (2-DEGs) provided by the QW EBL, resulting in a significant improvement of breakdown voltage and a remarkable suppression of spilling electrons. The electron mobility of 2-DEG is hence enhanced as well. The dependence of thickness and composition of QW EBL on the device breakdown is also evaluated and discussed.     

Influence of the ratio of gate length to drain-to-source distance on the electron mobility in AlGaN/AlN/GaN heterostructure field-effect transistors

ABSTRACT: Using measured capacitance-voltage curves with different gate lengths and current-voltage characteristics at low drain-to-source voltage for the AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) of different drain-to-source distances, we found that the dominant scattering mechanism in the AlGaN/AlN/GaN HFETs is determined by the ratio of gate length to drain-to-source distance. For the devices with small ratio (here less than 1/2), the polarization Coulomb field scattering dominates the electron mobility. However, for the devices with large ratio (here more than 1/2), the LO phonon scattering and interface roughness scattering are dominant. The reason is closely related to the polarization Coulomb field scattering.     

Analyses of 2-DEG characteristics in GaN HEMT with AlN/GaN super-lattice as barrier layer grown by MOCVD

ABSTRACT: GaN-based high-electron mobility transistors (HEMTs) with AlN/GaN super-lattices (SLs) (4 to 10 periods) as barriers were prepared on (0001) sapphire substrates. An innovative method of calculating the concentration of two-dimensional electron gas (2-DEG) was brought up when AlN/GaN SLs were used as barriers. With this method, the energy band structure of AlN/GaN SLs was analyzed, and it was found that the concentration of 2-DEG is related to the thickness of AlN barrier and the thickness of the period; however, it is independent of the total thickness of the AlN/GaN SLs. In addition, we consider that the sheet carrier concentration in every SL period is equivalent and the 2-DEG concentration measured by Hall effect is the average value in one SL period. The calculation result fitted well with the experimental data. So, we proposed that our method can be conveniently applied to calculate the 2-DEG concentration of HEMT with the AlN/GaN SL barrier.     

Growth of GaN on nano-crystalline diamond substrates

In this study GaN has been grown on nano-crystalline diamond substrates utilizing metal-organic chemical vapour deposition (MOCVD). It is shown that the growth of closed GaN films onto synthetic diamond substrates is feasible, when applying the correct buffer layer and growth parameters. XRD measurements showed that the GaN formed is of wurzite structure and polycrystalline, but the high intensity of the (0002) diffraction peak indicates a preferential crystallite orientation. This preferred [0001] orientation was confirmed by SEM analysis. The optical quality of the deposited GaN layer was investigated using cathodoluminescence and showed a large yellow luminescence peak. This work comprises a first step in preparing heterogeneous layers and GaN devices with a diamond heat sink as a substrate, facilitating the thermal management of these devices.     

Growth of nitride crystals,BN, AlN and GaN by using a Na flux

Bulk crystals of BN, AlN and GaN were grown by means of Na flux. All these crystals were grown at a temperature of 800°C and a nitrogen pressure of 100 atm, relatively lower than that required by many flux and melt growth methods. High-quality GaN single crystals as large as 0.5 mm were obtained. Furthermore, the oriented GaN crystals were obtained by means of the seeded Na flux method with the addition of oriented AlN (0001) film in the growth ambient. The nucleation of bulk GaN was spatially confined on top of the AlN film and grown with the GaN [0001] axis parallel to the AlN [0001] axis. In addition, the h-BN polycrystals were confirmed by the h-BN (0002) peak of X-ray diffraction (XRD) at 2θ=26.700. A hexagonal grain with a size as large as 2 μm was observed by scanning electron microscopy (SEM). Likewise, AlN crystals were also obtained from Al wires.     

60 GHz Double Deck T-Gate AlN/GaN/AlGaN HEMT for V-Band Satellites

Silicon - The influence of double deck T-gate on LG = 0.2 μm AlN/GaN/AlGaN HEMT is analysed in this paper. The T-gate supported with Silicon Nitride provides a tremendous...     

Indium droplet formation in InGaN thin films with single and double heterojunctions prepared by MOCVD

Indium gallium nitride (InGaN) samples with single heterojunction (SH) and double heterojunction (DH) were prepared using metal-organic chemical vapor deposition. SH has a layer of InGaN thin film (thicknesses, 25, 50, 100, and 200 nm) grown on an uGaN film (thickness, 2 μm). The DH samples are distinguished by DH uGaN film (thickness, 120 nm) grown on the InGaN layer. Reciprocal space mapping measurements reveal that the DH samples are fully strained with different thicknesses, whereas the strain in the SH samples are significantly relaxed with the increasing thickness of the InGaN film. Scanning electron microscopy results show that the surface roughness of the sample increases when the sample is relaxed. High-resolution transmission electron microscopy images of the structure of indium droplets in the DH sample indicate that the thickness of the InGaN layer decreases with the density of indium droplets. The formation of these droplets is attributed to the insufficient kinetic energy of indium atom to react with the elements of group V, resulting to aggregation. The gallium atoms in the GaN thin film will not be uniformly replaced by indium atoms; the InGaN thin film has an uneven distribution of indium atoms and the quality of the epitaxial layer is degraded.     

Diamond overgrown InAlN/GaN HEMT

In this work the technology and characterization of nanocrystalline diamond (NCD) films directly grown on InAlN/GaN HEMTs is presented. Optimization of GaN based HEMT process steps including metallization stacks is discussed. A fully processed InAlN/GaN HEMT structure with 7 nm barrier has been overgrown in a temperature range of 750 °C to 800 °C with a 500 nm thick nanocrystalline diamond film in a Hot Filament CVD system. First results of semi-enhancement mode of DC and RF HEMT operation are reported. The grown NCD films were characterized by SEM, TEM, and Raman spectroscopy. Although no direct thermal conductivity measurements are conducted yet; the performed experiments shows the compatibility of growing high quality NCD films, several microns thick, on InAlN/GaN HEMTs as a potential material for heat extraction purposes.     

Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer

ABSTRACT: We investigated the effect of gallium oxide hydroxide (GaOOH) nanorod arrays (NRAs) on the light extraction of InGaN/GaN MQW blue light-emitting diodes (LEDs). The GaOOH NRAs were prepared on the indium tin oxide electrode (ITO) layer of LEDs by the electrochemical deposition method. The GaOOH NRAs with preferred orientations were grown on the ITO surface by sputtering a thin antimony-doped tin oxide (ATO) seed layer, which enhances heterogeneous reactions. The surface density and coverage were also efficiently controlled by the different growth voltages. For the LEDs with GaOOH NRAs grown at -2 V, the light output power was increased by 22% without suffering from any serious electrical degradation and wavelength shift, compared to the conventional LEDs.     

Effect of Eu-implantation and annealing on the GaN quantum dots excitonic recombination

Undoped self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer layers were submitted to thermal annealing treatments. Changes in the balance between the quantum confinement, strain state of the stacked heterostructures and quantum confined Stark effect lead to the observation of GaN QD excitonic recombination above and below the bulk GaN bandgap. In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence. For samples implanted with high fluence, a broad emission band at 2.7 eV was tentatively assigned to the emission of large blurred GaN QD present in the damage region of the implanted SL. This emission band is absent in the SL structures implanted with lower fluence and hence lower defect level. In both cases, high energy emission bands at approx. 3.9 eV suggest the presence of smaller dots for which the photoluminescence intensity was seen to be constant with increasing temperatures. Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent. Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.