GaN-based Epilayers Research Articles

Microstructural characterization of AlxGa1−xN/GaN high electron mobility transistor layers on 200 mm Si(111) substrates

Realization of high crystal quality GaN layers on silicon substrates requires a great control over epitaxy as well as detailed characterization of the buried defects and propagating dislocations within the epilayers. In this Letter, we present the microstructural characterization of AlxGa1−xN/GaN high electron mobility transistor (HEMT) structures epitaxially grown on 200 mm Si (111) substrates with superlattice (SL) buffers. The HEMT stack is also composed of an intermediate thick AlxGa1−xN layer sandwiched between the short-period and long-period AlxGa1−xN/AlN SLs. Structural and morphological characteristics of the GaN-based epilayers are studied to assess the quality of the HEMT stack grown on such 200 mm diameter substrates. The detailed microstructural uniformity of the epilayers is addressed by the transmission electron microscopy (TEM) technique. In depth scanning TEM with electron energy loss spectroscopy (EELS) investigations have been carried out to probe the microstructural quality of the HEMT stack comprising of such short- and long-period AlxGa1−xN/AlN SLs. The EELS-plasmon maps are utilized to address the sharp interface characteristics of the SL layers as well as the top active p-GaN/AlxGa1−xN/GaN layers. This work highlights the capability of high-resolution TEM as a complementing characterization method to produce reliable AlxGa1−xN/GaN HEMTs on such a large diameter silicon substrate.

Study of GaN-Based Thermal Decomposition in Hydrogen Atmospheres for Substrate-Reclamation Processing.

This study investigates the thermal decomposition behavior of GaN-based epilayers on patterned sapphire substrates (GaN-epi/PSSs) in a quartz furnace tube under a hydrogen atmosphere. The GaN-epi/PSS was decomposed under different hydrogen flow rates at 1200 °C, confirming that the hydrogen flow rate influences the decomposition reaction of the GaN-based epilayer. The GaN was completely removed and the thermal decomposition process yielded gallium oxyhydroxide (GaO2H) nanostructures. When observed by transmission electron microscopy (TEM), the GaO2H nanostructures appeared as aggregates of many nanograins sized 2–5 nm. The orientation relationship, microstructure, and formation mechanism of the GaO2H nanostructures were also investigated.

Dislocation Reduction and Stress Relaxation of GaN and InGaN Multiple Quantum Wells with Improved Performance via Serpentine Channel Patterned Mask.

The existence of high threading dislocation density (TDD) in GaN-based epilayers is a long unsolved problem, which hinders further applications of defect-sensitive GaN-based devices. Multiple-modulation of epitaxial lateral overgrowth (ELOG) is used to achieve high-quality GaN template on a novel serpentine channel patterned sapphire substrate (SCPSS). The dislocation blocking brought by the serpentine channel patterned mask, coupled with repeated dislocation bending, can reduce the dislocation density to a yet-to-be-optimized level of ∼2 × 10(5) to 2 × 10(6) cm(-2). About 80% area utilization rate of GaN with low TDD and stress relaxation is obtained. The periodical variations of dislocation density, optical properties and residual stress in GaN-based epilayers on SCPSS are analyzed. The quantum efficiency of InGaN/GaN multiple quantum wells (MQWs) on it can be increased by 52% compared with the conventional sapphire substrate. The reduced nonradiative recombination centers, the enhanced carrier localization, and the suppressed quantum confined Stark effect, are the main determinants of improved luminous performance in MQWs on SCPSS. This developed ELOG on serpentine shaped mask needs no interruption and regrowth, which can be a promising candidate for the heteroepitaxy of semipolar/nonpolar GaN and GaAs with high quality.

Temperature-Dependent Characteristics of a GaN/InGaN/ZnO Heterojunction Bipolar Transistor

This work presents dc characteristics (gain and leakage current) of a GaN/InGaN/ZnO npn collector-up heterojunction bipolar transistor (HBT) measured at 300, 200, and 100 K. The GaN-based epilayers of HBT were grown by metallorganic chemical vapor deposition, and the n-ZnO film was then deposited on top of p-InGaN by sputtering. X-ray diffraction analysis demonstrates that annealing at for 60 s in ambient improved the quality of the ZnO film. Moreover, this study shows that the current gains from the Gummel plot increased as the temperature decreased because the leakage current of the collector current is not a strong function of temperature. However, gains from the measured common-emitter current–voltage characteristics increased slightly as the temperature decreased due to a reduction in the recombination current coupled with lower injection efficiency. These preliminary results show the potential of fabricating GaN/InGaN/ZnO HBTs without dry etching to reach the p-GaN layer, as required in the emitter-up geometry.

Improvement of the performance of GaN-based LEDs grown on sapphire substrates patterned by wet and ICP etching

Sapphire substrates patterned by a selective chemical wet and an inductively coupled plasma (ICP) etching technique was proposed to improve the performance of GaN-based light-emitting diodes (LEDs). GaN-based LEDs were fabricated on sapphire substrates through metal organic chemical vapor deposition (MOCVD). The LEDs fabricated on the patterned substrates exhibit improved device performance compared with the conventional LED fabricated on planar substrates when growth and device fabricating conditions were the same. The light output powers of the LEDs fabricated on wet-patterned and ICP-patterned substrates were about 37% and 17% higher than that of LEDs on planar substrates at an injection current of 20mA, respectively. The enhancement is attributable to the combination of the improvement of GaN-based epilayers quality and the improvement of the light extraction efficiency.

Enhancement of the light output power of InGaN/GaN light-emitting diodes grown on pyramidal patterned sapphire substrates in the micro- and nanoscale

Sapphire substrates were patterned by a chemical wet etching technique in the micro- and nanoscale to enhance the light output power of InGaN/GaN light-emitting diodes (LEDs). InGaN/GaN LEDs on a pyramidal patterned sapphire substrate in the microscale (MPSS) and pyramidal patterned sapphire substrate in the nanoscale (NPSS) were grown by metalorganic chemical vapor deposition. The characteristics of the LEDs fabricated on the MPSS and NPSS prepared by wet etching were studied and the light output powers of the LEDs fabricated on the MPSS and NPSS increased compared with that of the conventional LEDs fabricated on planar sapphire substrates. In comparison with the planar sapphire substrate, an enhancement in output power of about 29% and 48% is achieved with the MPSS and NPSS at an injection current of 20 mA, respectively. This significant enhancement is attributable to the improvement of the epitaxial quality of GaN-based epilayers and the improvement of the light extraction efficiency by patterned sapphire substrates. Additionally, the NPSS is more effective to enhance the light output power than the MPSS.

Light output improvement of InGaN ultraviolet light-emitting diodes by using wet-etched stripe-patterned sapphire substrates

GaN-based epilayers are grown on wet-etched stripe-patterned sapphire substrates, with stripes along the ⟨11−20⟩sapphire and ⟨1−100⟩sapphire directions, for 400nm ultraviolet light-emitting diodes (LEDs). The effects of the etching depth and stripe orientation on the structural and optical properties of the GaN layer as well as on the LEDs are investigated. Much better material quality and light output power are obtained when the GaN and the LEDs are grown on a 0.9μm deep patterned sapphire substrate with stripes along the ⟨1−100⟩sapphire direction. Stripe-orientation dependent growth modes accounting for the observed experimental results are proposed.