Semipolar GaN Substrates Research Articles

Tri-halide vapor-phase epitaxy of GaN using GaCl3 on polar, semipolar, and nonpolar substrates

Homoepitaxial tri-halide vapor-phase epitaxy (THVPE) growth on polar, semipolar, and nonpolar bulk GaN substrates was demonstrated using GaCl3 as the precursor. The influence of the surface orientation of the substrate on GaN growth by THVPE was compared with that observed for GaN grown by hydride vapor-phase epitaxy. The dependence of the GaN growth on the surface orientation of the substrate was confirmed; GaN could be grown on , , , , and but not on , , , or . This behavior was explained to be due to the changes in adsorption energy, the magnitudes of which were estimated by theoretical calculations.

Semipolar III–nitride light-emitting diodes with negligible efficiency droop up to ∼1 W

We demonstrate 1 mm2 blue light-emitting diodes with a negligible efficiency droop up to ∼1 W. LEDs with 12- to 14-nm-thick single quantum wells were grown by metalorganic chemical vapor deposition on a free-standing semipolar GaN substrate. Packaged devices showed an external quantum efficiency of 42.3% at 20 A/cm2 with a negligible efficiency droop up to 991 mW at 900 mA. At 900 mA, the thermal droop and hot/cold factor were 8.2% and 0.92, respectively. The adoption of a thick active region resulted in excellent optical and thermal performance characteristics that are suitable for high-power lighting applications.

Dynamic characteristics of 410 nm semipolar (202¯1¯) III-nitride laser diodes with a modulation bandwidth of over 5 GHz

The dynamic characteristics of III-nitride multi-quantum well laser diodes (LDs) emitting at 410 nm were investigated. LDs were grown on semipolar (202¯1¯) bulk GaN substrates and fabricated into devices with cavity lengths ranging from 900 nm to 1800 nm. A 3-dB bandwidth of 5 GHz and 5 Gbit/s direct modulation with on-off keying were demonstrated, which were limited by the bandwidth of the photodetector used for the measurements. The differential gain of the LDs was determined to be 2.5 ± 0.5 × 10−16 cm2 by comparing the slope efficiency for different cavity lengths. Analysis of the frequency response showed that the K-factor, the gain compression factor, and the intrinsic maximum bandwidth were 0.33 ns, 7.4 × 10−17 cm3, and 27 GHz, respectively.

High-speed 405-nm superluminescent diode (SLD) with 807-MHz modulation bandwidth.

III-nitride LEDs are fundamental components for visible-light communication (VLC). However, the modulation bandwidth is inherently limited by the relatively long carrier lifetime. In this letter, we present the 405 nm emitting superluminescent diode (SLD) with tilted facet design on semipolar GaN substrate, showing a broad emission of ~9 nm at 20 mW optical power. Owing to the fast recombination (τe<0.35 ns) through the amplified spontaneous emission, the SLD exhibits a significantly large 3-dB bandwidth of 807 MHz. A data rate of 1.3 Gbps with a bit-error rate of 2.9 × 10-3 was obtained using on-off keying modulation scheme, suggesting the SLD being a high-speed transmitter for VLC applications.

GaN quantum dot polarity determination by X-ray photoelectron diffraction

Growth of GaN quantum dots (QDs) on polar and semipolar GaN substrates is a promising technology for efficient nitride-based light emitting diodes (LED). The QDs crystal orientation typically repeats the polarity of the substrate. In case of non-polar or semipolar substrates, the polarity of QDs is not obvious. In this article, the polarity of GaN QDs and of underlying layers was investigated nondestructively by X-ray photoelectron diffraction (XPD). Polar and semipolar GaN/Al0.5Ga0.5N heterostructures were grown on the sapphire substrates with (0001) and (11¯00) orientations by molecular beam epitaxy (MBE). Polar angle dependence of N 1s core-level photoelectron intensities were measured from GaN QDs and compared with the corresponding experimental curves from free-standing GaN crystals. It is confirmed experimentally, that the crystalline orientation of polar (0001) GaN QDs follows the orientation of the (0001) sapphire substrate. In case of semipolar GaN QDs grown on (11¯00) sapphire substrate, the (112¯2) polarity of QDs was determined.

High-brightness semipolar (2021¯) blue InGaN/GaN superluminescent diodes for droop-free solid-state lighting and visible-light communications.

A high-brightness, droop-free, and speckle-free InGaN/GaN quantum well blue superluminescent diode (SLD) was demonstrated on a semipolar (2021¯) GaN substrate. The 447-nm emitting SLD has a broad spectral linewidth of 6.3 nm at an optical power of 123 mW. A peak optical power of 256 mW was achieved at 700 mA CW injection current. By combining YAG:Ce phosphor, SLD-generated white light shows a color-rendering index (CRI) of 68.9 and a correlated color temperature (CCT) of 4340 K. The measured frequency response of the SLD revealed a -3 dB bandwidth of 560 MHz, thus demonstrating the feasibility of the device for both solid-state lighting (SSL) and visible-light communication (VLC) applications.

Terahertz intersubband photodetectors based on semi-polar GaN/AlGaN heterostructures

Terahertz intersubband photodetectors are developed based on GaN/AlGaN quantum wells grown on a free-standing semi-polar (202¯1¯) GaN substrate. These quantum wells are nearly free of the polarization-induced internal electric fields that severely complicate the design of nitride intersubband devices on traditional c-plane substrates. As a result, a promising bound-to-quasi-bound THz photodetector design can be implemented. Pronounced photocurrent peaks at the design frequency near 10 THz are measured, covering frequencies that are fundamentally inaccessible to existing arsenide intersubband devices due to reststrahlen absorption. This materials system provides a favorable platform to utilize the intrinsic advantages of nitride semiconductors for THz optoelectronics.

Strain relaxation in semipolar (202¯1) InGaN grown by plasma assisted molecular beam epitaxy

Strain relaxation in semipolar (202¯1) InGaN layers grown by plasma assisted molecular beam epitaxy (PAMBE) was investigated with high-resolution X-ray diffraction (XRD) reciprocal space mapping, cathodoluminescence (CL), fluorescent light microscopy (FLM), and atomic force microscopy. We find that XRD detects lattice relaxation much later than its actual onset occurs. Other techniques used in this study allowed to detect local footprints of plastic relaxation before it was evidenced by XRD: at the initial stages of strain relaxation, we observed changes in layer morphology, i.e., formation of short trench line segments on the surface along the ⟨112¯0⟩ direction as well as dark lines in CL and FLM. The misfit dislocations formation and glide were observed in two slip systems: initially in basal slip system ⟨112¯0⟩{0001} and for larger amount of strain in non-basal, prismatic slip system ⟨112¯0⟩{11¯00}. Experimentally determined critical thickness for InGaN layers grown by PAMBE on semipolar (202¯1) bulk GaN substrates agrees well with literature data obtained with metalorganic vapor phase epitaxy and follows the Matthews-Blakeslee model prediction. We discuss the impact of substrate structural properties on the strain relaxation onset and mechanisms. We also describe the layer morphology and surface roughness evolution related to the increasing In content and strain relaxation of the semipolar (202¯1) InGaN layers.

Suppression of extended defects propagation in a laser diodes structure grown on (20-21) GaN

In this work, we fabricated green-light-emitting laser structures on a (20-21) semipolar GaN substrate. Using cathodoluminescence mapping, x-ray diffraction, and transmission electron microscopy, we revealed the formation of relaxation defects within InGaN waveguides and AlGaN claddings. The observed defects in the AlGaN layers are stripe-like and extend along the a axis, but in the InGaN layers they form a characteristic checkered pattern. We demonstrate that using the selective area growth method we can effectively suppress the formation of both types of defects, thus enabling the fabrication of defect-free green laser structures on semipolar GaN substrates.

InGaN/AlGaN stress compensated superlattices coherently grown on semipolar () GaN substrates

The crystal growth and optical properties of semipolar () InGaN/AlGaN stress‐compensated superlattices (SCSLs) are investigated. X‐ray diffraction and transmission electron microscopy reveal that appropriately designed 125‐period In0.2Ga0.8N (1 nm)/Al0.2Ga0.8N (3 nm) SCSLs grow coherently on () GaN substrates. The refractive index of a fabricated SCSL is lower than that of GaN in the green spectral range; indicating that SCSLs have potential as the cladding layers in green laser diodes (LDs). Photoluminescence appears in the near ultraviolet (UV) range without significant in‐plane optical anisotropy because the two topmost valence bands are close together in coherent semipolar InGaN with an experimentally determined In composition of 28%. This nearly isotropic nature allows photo‐pumped lasing in a cavity with () cleaved mirrors; suggesting potential applications of SCSLs in UV LD active layers.Schematic of InGaN/AlGaN SCSLs on GaN substrates. Balanced tensile and compressive stresses in AlGaN and InGaN, respectively, realize coherent growth of thick SCSLs.

Polarity of GaN with polar {0001} and semipolar , , orientations by x-ray photoelectron diffraction

Abstract

(Invited) High Efficiency Green-Yellow Emission from InGaN/GaN Quantum Well Structures Grown on Overgrown Semi-Polar (11-22) GaN on Regularly Arrayed Micro-Rod Templates

Last two decades have seen major developments in the field of III-nitride materials and optoelectronics, which is mainly based on polar GaN on sapphire. So far, the growth of c-plane GaN on sapphire has almost approached its theoretical limits and the major limit to the performance of GaN-based LEDs is due to the growth on c-plane. Consequently, this polar orientation leads to a number of fundamental issues, such as efficiency droop, and green and yellow gaps in wavelength coverage. One clear way forward, meeting the fundamental challenges, is to grow along semi-polar/non-polar direction. Semi-polar orientations, in particular, (11-22), offer another major advantage, namely, it can also significantly enhance indium incorporation into GaN, which cannot be achieved using c-plan GaN and thus is extremely important for growth of long wavelength emitters, such as green and even better for yellow. However, the current challenge is due to the crystal quality of semi-/non-polar GaN, which is far from satisfactory. So far, the best performing semi-polar emitters are grown on semi-polar GaN substrates which are typically ~ 10x10 mm in size. Therefore, it is necessary to develop a cost-effective approach to developing large size semi-polar GaN templates with major improvement in crystal quality but still based on sapphire substrates. The Sheffield team has developed an overgrowth approach for 2" (11-22) semi-polar GaN with significantly enhanced crystal quality on nanorod templates fabricated by self-organised nickel nano-mask approach. Very recently, a new overgrowth approach based on optical lithography mask pattern technique has been developed, where regularly arrayed micro rod templates can be cost-effectively fabricated and the size of the micro rods can be well controlled. The new approach has led to further improvement in both crystalline quality and surface morphology. This has been confirmed by high resolution transmission microscopy (TEM) studies, demonstrating further significant reduction in both dislocation density and basal stacking fault density. InGaN/GaN multiple quantum well structures with high In compositions have been grown on such semi-polar GaN templates, and we have achieved strong emission at long wavelengths, even down to ~600 nm. For the green-yellow emission at 555 nm, we have obtained an internal quantum efficiency (IQE) of up to 14%, and 7-10% for the pure yellow emission at ~570 nm. Con-focal photoluminescence (PL), micro-PL, time-resolved PL (TRPL), and micro-TRPL have been performed

Properties of InGaN/GaN multiquantum wells grown on semipolar (20-21) substrates with different miscuts

We describe surface patterning as a method to obtain discrete regions of different local miscut angles, δ, in semipolar (202¯1) GaN substrates. During patterning the region angle δ was varied between 0° and ±2° with respect to the growth plane. We chose two nonequivalent miscut directions: a 〈1¯21¯0〉 and c׳ 〈1¯014¯〉. On such patterned substrates InxGa1−xN/GaN quantum wells were grown by a Metalorganic Vapor Phase Epitaxy method. Angled regions were not planarized during growth and their initial miscut angles remained. We studied structural, morphological and optical properties of InxGa1−xN/GaN quantum wells as a function of δ. The intended In concentration, x, was about 13% on exact oriented (202¯1) planes. For miscut towards the a-direction, indium content decreases and luminescence energy increases with δ. For miscut towards c׳, we observe quite different behaviors, indium content and quantum well width are constant and luminescence energy increases with δ. We propose the explanation of these effects on the basis of sample morphology, In-content and built-in electric field. The obtained results are important for understanding the role of miscut (intentional and unintentional) in basic properties of semipolar InGaN/GaN quantum well structures. It is also worth to note that, the spectral width of luminescence was the smallest for regions miscut 2° towards c׳ direction, which can be important for semipolar optoelectronic devices.

High‐Power True Green Laser Diodes on Semipolar GaN Substrates

SUMMARYInGaN green laser diodes (LDs) on semipolar GaN substrates with output powers of over 100 mW in the spectral region beyond 530 nm are demonstrated. Wall plug efficiencies as high as 7.0–8.9% are realized in the wavelength range of 525–532 nm, exceeding those reported for c‐plane LDs. The lifetime at a case temperature of 55 °C was estimated to be over 5000 hours for an optical output power of 50 mW. These results suggest that InGaN green LDs on the plane are better suited as light sources for applications requiring wavelengths above 525 nm.

High-power blue laser diodes with indium tin oxide cladding on semipolar (202¯1¯) GaN substrates

We demonstrate a high power blue laser diode (LD) using indium tin oxide as a cladding layer on semipolar oriented GaN. These devices show peak output powers and external quantum efficiencies comparable to state-of-the-art commercial c-plane devices. Ridge waveguide LDs were fabricated on (202¯1¯) oriented GaN substrates using InGaN waveguiding layers and GaN cladding layers. At a lasing wavelength of 451 nm at room temperature, an output power of 2.52 W and an external quantum efficiency of 39% were measured from a single facet under a pulsed injection current of 2.34 A. The measured differential quantum efficiency was 50%.

Electron tomography of (In,Ga)N insertions in GaN nanocolumns grown on semi-polar (112̄2) GaN templates

We present results of scanning transmission electron tomography on GaN/(In,Ga)N/GaN nanocolumns (NCs) that grew uniformly inclined towards the patterned, semi-polar GaN(112̄2) substrate surface by molecular beam epitaxy. For the practical realization of the tomographic experiment, the nanocolumn axis has been aligned parallel to the rotation axis of the electron microscope goniometer. The tomographic reconstruction allows for the determination of the three-dimensional indium distribution inside the nanocolumns. This distribution is strongly interrelated with the nanocolumn morphology and faceting. The (In,Ga)N layer thickness and the indium concentration differ between crystallographically equivalent and non-equivalent facets. The largest thickness and the highest indium concentration are found at the nanocolumn apex parallel to the basal planes.

Recent Advances in Nonpolar and Semipolar InGaN Light-Emitting Diodes (LEDs).

The III-nitrides have attracted much attention because of their applicability in optoelectronic devices, whose emission wavelengths range from green to ultraviolet light due to their wide band gap. However, conventional c-plane GaN-based devices are influenced significantly by spontaneous and piezoelectric polarization effects, which could pose a limitation for increased luminous efficiency as a result of the quantum confined stark effect. Since the early 2000s, many groups have tried to solve these problems by examining the growth of GaN on non- or semipolar surface planes. High power non- and semipolar LEDs can be realized by the growth of a thick active layer. In addition, it is expected that it is possible to grow nonpolar InGaN LEDs with high quality p-GaN layers due to lower hole activation energy, and also long-wavelength semipolar InGaN LEDs because of the capacity for high indium incorporation in the quantum wells (QWs). However, non- and semipolar structures grown on sapphire substrate usually contain a high density of basal stacking faults and threading dislocations. For this reason, the growth of non- and semipolar GaN-based LEDs on a sapphire substrate has been attempted through the introduction of defect reduction techniques such as epitaxial lateral overgrowth, patterned sapphire substrate and re-growth techniques on a porous GaN layer, etc. Also, some researchers have grown high quality non- and semipolar GaN-based LEDs using non- and semipolar freestanding GaN substrates. In this review paper, we introduce and discuss recent progress in the development of non- and semipolar GaN-based LEDs and freestanding GaN substrates.

High-power low-droop violet semipolar (303¯1¯) InGaN/GaN light-emitting diodes with thick active layer design

Devices grown on nonpolar and semipolar planes of GaN offer key performance advantages over devices grown on the conventional c-plane, including reduced polarization fields. This allows for a wider design space on semipolar planes for light emitting diodes (LEDs) to address the problem of efficiency droop at high current densities. LED structures with very thick (10–100 nm) InGaN single-quantum-well/double heterostructure active regions were grown using conventional metal organic chemical vapor deposition on semipolar (303¯1¯) free-standing GaN substrates and processed and packaged using conventional techniques. Simulated band diagrams showed reduced polarization fields on the (303¯1¯) plane. The calculated critical thickness for misfit dislocation formation is higher on the (303¯1¯) plane than on other semipolar planes, such as (202¯1¯), allowing for thicker active regions than our previous work to further reduce droop. The higher critical thickness was confirmed with defect characterization via cathodoluminescence. A trend is demonstrated in lower efficiency droop for devices with thicker active regions. Thermal droop characteristics of these devices are also presented. These observed results were utilized to demonstrate over 1 W of output power at a current density of 1 kA/cm2 from a single 0.1 mm2 LED device.

InGaN based green laser diodes on semipolar GaN substrate

This paper reviews the development of the InGaN-based green laser diodes on semipolar GaN substrates, especially focusing on plane. The reduction of piezoelectric fields in InGaN quantum wells on the planes was investigated by a small blue shift in electroluminescence peaks, and high crystal quality was confirmed by clear interfaces in a transmission electron microscopy image, narrower FWHM in electroluminescence peaks, and smaller localization energy in time resolved PL results, as compared with the other planes. These physical characteristics of the leads to better laser properties: lower threshold current densities, higher output powers of over 100 mW in the spectral region beyond 530 nm, and higher wall plug efficiencies as high as 7.0–8.9% in the wavelength range of 525–532 nm, compared to those of the other planes. Estimated lifetimes were over 5000 h at 50 mW and 2000 h at 70 mW under cw operation with auto power control at a case temperature of 55 °C. The plane is the promising candidate for InGaN-based true green laser diodes.

Selective-area growth of GaN on non- and semi-polar bulk GaN substrates

We carried out the selective-area growth of GaN and fabricated InGaN/GaN MQWs on non- and semi-polar bulk GaN substrates by MOVPE. The differences in the GaN structures and the In incorporation of InGaN/GaN MQWs grown on non- and semi-polar GaN substrates were investigated. In the case of selective-area growth, different GaN structures were obtained on GaN, GaN, and GaN substrates. A repeating pattern of and facets appeared on GaN. Then, we fabricated InGaN/GaN MQWs on the facet structures on GaN. The emission properties characterized by cathodoluminescence were different for and facets. On the other hand, for InGaN/GaN MQWs on non- and semi-polar GaN substrates, steps along the a-axis were observed by AFM. In particular on GaN, undulations and undulation bunching appeared. Photoluminescence characterization indicated that In incorporation increased with the off-angle from the m-plane and also depended on the polarity.