p-GaN Growth Research Articles

Startified thermal degradation in blue InGaN quantum well structures: P-GaN growth temperature and its influence on quantum well optical properties

Thermal degradation in blue InGaN multi-quantum well (MQW) structures induced by varying high p-GaN growth temperature and the subsequent influence on the optical properties of MQW are investigated. The results indicate that higher p-GaN growth temperatures improve p-GaN layer contact quality, but may lead to an optical property degradation of InGaN MQWs. A stratified thermal degradation mechanism is observed. The degradation primarily affects the upper quantum barrier layers and results in indium desorption and re-evaporation, where elevated p-GaN growth temperatures may induce more indium segregation and decomposition than in lower layers of MQW. In extreme conditions, such as 1060℃, indium atoms in lower layers escape, causing partial lattice structure corruption and severe degradation in MQW structure, which is also more distinctive in the upper layers.

(Invited) Selective Area Growth, Etching, and Doping of GaN By MOCVD for Power Electronics

The lack of selective-area doping technique in GaN greatly hinders the demonstration of sophisticated device structures for high power application. We explored the possibility of using selective-area etching followed by selective area growth to achieve SAD. For the etching step, we explored the in-situ TBCl etching, where we achieved controlled etch rate and smooth trenches. The TBCl etching was proven to be a low-damage etching process compared to the conventional dry etching. Moreover, selective area growth of p-GaN in a patterned trench was analyzed by atom probe tomography (APT). We found Mg atoms cannot diffuse on the GaN surface, which led to an observed inverse proportionality between Mg concentration and local growth rate. However, it was discovered that Mg precursors, like Ga, exhibit inter-trench diffusion through the gas phase.

Formation and Optical Characteristics of GaN:Eu/GaN Nanowires for Applications in Light-Emitting Diodes

This paper reviews our recent research about the formation and optical characteristics of GaN:Eu/GaN nanowires (NWs) by metalorganic vapor phase epitaxy for application in GaN-based red light-emitting diodes (LEDs). Two types of GaN:Eu/GaN NWs with different configurations are introduced, core–shell and axial geometries. The configuration of GaN:Eu layers on GaN core NWs can be controlled by changing the growth conditions, and affects the properties of Eu luminescence in the GaN NWs. Next, the optimization of the p-GaN growth conditions is performed to allow to form the p-GaN shell layers on the NWs with the pedestal of the NWs free from radial overgrowth, resulting in efficient electrical isolation between top and bottom part of the NWs. Then, the fabrication process of the NW LEDs towards future possible realization of flexible devices is established, including an etch-back process of the PDMS membranes to expose the top p-GaN contact layers. Finally, a proto-type of p-GaN/GaN:Eu/n-GaN NW LEDs on sapphire substrates is fabricated to characterize the device properties. Sharp red luminescence at room temperature from Eu3+ ions is observed under current injection. These results would pave the way towards the realization of flexible light-emitting devices utilizing NW structures based on compound semiconductors.

Unveiling the influence of selective-area-regrowth interfaces on local electronic properties of GaN p-n junctions for efficient power devices

We report correlated nanoscale mapping of the structure, composition, and properties of regrown GaN p-n junctions to identify how etching and non-planar regrowth processes limit diode performance via the introduction of unintentional dopants and defect states. p-GaN was selectively regrown in n-GaN trenches with SiO2 masks of variable mask-to-trench-width ratio. Dilute Al layers were periodically introduced during regrowth as markers of the growth interface. Correlated nanoscale mapping of doping, conductivity, and dopant complexes was achieved with atom probe tomography (APT), scanning spreading resistance microscopy (SSRM), and cathodoluminescence (CL) spectroscopy, respectively. The Al marker layers, detected by APT, enabled reconstruction of the faceted growth interface and correlation of the dopant concentration with position and time. The p-GaN growth rate is proportional to the mask-to-trench width ratio while the dopant incorporation rate is invariant. At trench edges, magnesium incorporation is suppressed, and oxygen incorporation enhanced, due to preferential incorporation on the semi-polar growth surface, leading to compensation and less abrupt p-n junctions; the SiO2 mask is a source of oxygen. Residual etch damage below the regrowth interface induces n-type and p-type conductivity, creating leakage pathways. The non-uniform Mg incorporation is driven by crystal anisotropy and is thus inherent to non-planar regrowth, but can be mitigated by engineering the regrowth interface and process parameters. The unprecedented integration of spatially resolved mapping of dopants, impurities, conductivity, and carrier type is a powerful approach to discriminating distinct factors that limit the performance of regrown diodes, enabling the rational optimization of process and device design.

Influence of growth temperature of p-GaN layer on the characteristics of InGaN/GaN blue light emitting diodes

Optimisation for p-GaN epitaxial growth temperature ranges from 920°C to 1020°C was successfully conducted on InGaN/GaN blue light-emitting diodes (LEDs). These InGaN/GaN blue LEDs have been grown on patterned sapphire substrates (PSS) via metal organic chemical vapour deposition (MOCVD). The significance of p-GaN growth temperature was studied in detail according to the structural and optoelectronic performances of InGaN/GaN blue react to depth mechanism of surface evolution to LEDs grown on PSS. From experimental analysis, increment in the epitaxial growth temperature of p-GaN layer caused the crystalline quality of GaN and structural properties slightly improve. Other than that, there are decrement of RMS roughness and PL FWHM due to the increment in the epitaxial growth temperature of p-GaN layer. However, the epitaxial growth temperature of p-GaN increase led to no improvement of optoelectronic properties of InGaN/GaN blue LEDs. The LEDs with higher epitaxial growth temperature of the p-GaN layer show poorer optoelectronic properties. Therefore, the optimised epitaxial growth temperature of the p-GaN layer is of significant importance in achieving highly efficient InGaN/GaN blue LEDs.Besides that, introducing PSS as the substrate did not demonstrate high impact to achieve better performance LEDs. Further improvement of the p-GaN layer crystalline and structural quality might be required for the optimisation of the optoelectronic properties of InGaN/GaN blue LEDs.

Morphology Control and Crystalline Quality of p-Type GaN Shells Grown on Coaxial GaInN/GaN Multiple Quantum Shell Nanowires.

The morphology and crystalline quality of p-GaN shells on coaxial GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) were investigated using metal-organic chemical vapor deposition. By varying the trimethylgallium (TMG) flow rate, Mg doping, and growth temperature, it was verified that the TMG supply and growth temperature were the dominant parameters in the control of the p-GaN shape on NWs. Specifically, a sufficiently high TMG supply enabled the formation of a pyramid-shaped NW structure with a uniform p-GaN shell. The ratio of the growth rate between the c- and m-planes on the NWs was calculated to be approximately 0.4545. High-angle annular dark-field scanning transmission electron microscopy characterization confirmed that no clear extended defects were present in the n-GaN core and MQS/p-GaN shells on the sidewall. Regarding the p-GaN shell above the c-plane MQS region, only a few screw dislocations and Frank-type partial dislocations appeared at the interface between the serpentine c-plane MQS and the p-GaN shell near the tips. This suggested that the crystalline quality of the MQS structure can trigger the formation of screw dislocations and Frank-type partial dislocations during the p-GaN growth. The growth mechanism of the p-GaN shell on NWs was also discussed. To inspect the electronic properties, a prototype of a micro light-emitting diode (LED) with a chip size of 50 × 50 μm2 was demonstrated in the NWs with optimal growth. By correlating the light output curve with the electroluminescence spectra, three different emission peaks (450, 470, and 510 nm) were assignable to the emission from the m-, r-, and c-planes, respectively.

(Invited) Selective Area Etching and Doping of GaN for High-Power Applications

Selective area doping (SAD) of gallium nitride (GaN), especially p-type doping, is desirable for high-power applications but yet challenging. The lack of this process greatly limits the power device design flexibility, so the reported device figure-of-merits are well below the theoretical calculation/simulation. Selective-area etching followed by regrowth is the approach we choose to overcome this obstacle. For the etching step, instead of adopting conventional Cl-based plasma etching process, we explored in-situ TBCl etching, studied the etching mechanism, and optimized the process to prepare smooth surface and trenches. Room-temperature PL, XPS, and electrical characterizations indicate that TBCl etching creates a low-defect surface. Moreover, selective area growth of p-GaN in a patterned trench was analyzed by atom probe tomography (APT). We found a reverse proportionality between the local growth rate and Mg doping concentration, and the development of fast-growing semi-polar facet from the sidewall. As a consequence, non-uniform Mg doping in non-planar growth is observed.

Selective Area Regrowth Produces Nonuniform Mg Doping Profiles in Nonplanar GaN p–n Junctions

Nonplanar GaN p–n junctions formed by selective area regrowth were analyzed using pulsed laser atom probe tomography. Dilute Al marker layers were used to map the evolution of the p-GaN growth interface, enabling extraction of time-varying growth rates for nonpolar, semipolar, and polar surfaces from the trench edge to the center, respectively. The Mg dopant concentration is facet-dependent and varies inversely with the growth rate for the semipolar facets that grow rapidly away from the trench sidewalls. The negligible growth on the vertical sidewall of the trench coincides with an order of magnitude higher Mg concentration and substantial clustering of likely inactive dopants. A high Mg concentration is also observed near the regrowth interface of polar and semipolar planes, which we attribute to etching damage. We conclude that device fabrication processes employing selective area regrowth on nonplanar interfaces should consider both the spatial and temporal dependencies of growth rate that lead to nonuniform doping and explore growth conditions that could reduce variations in growth rate when nonuniform doping would adversely affect device performance.

Effects of three-dimensional strain distribution on the performance of GaN-based light-emitting diodes on patterned sapphire substrates

Depth-resolved confocal Raman and micro photoluminescence (PL) are adopted to analyze GaN-based light-emitting diodes (LEDs) with an ex situ sputtered AlN nucleation layer on patterned sapphire substrates (PSSs). The strain on PSSs is measured by Raman spectral mapping, and the results demonstrate that the strain distribution within the LED structures strongly correlates to the pattern array of the PSS. Cone regions with less dislocations have lower strain than plane regions. The array pattern of the PSS’s surface can directly affect strain existing in the later crystal growth even at the position of the layer before the p-GaN growth. Moreover, the two-dimensional micro-PL mapping of the GaN-based LED samples on a PSS reveals a close relationship with the strain distribution on the sample surface. It is found that the strain as well as the PL intensity difference can be mitigated by proper pre-strained layers in order to realize high efficiency LEDs.

Enhancing the Hole-Injection Efficiency of a Light-Emitting Diode by Increasing Mg Doping in the p-AlGaN Electron-Blocking Layer

By increasing the Mg-doping level and hence the hole concentration in the p-AlGaN electron-blocking layer (EBL), particularly around the interface between the EBL and the top quantum barrier (QB), of a light-emitting diode (LED), the polarization field in this layer can be screened for reducing the potential barrier of hole and hence enhancing the hole-tunneling efficiency such that the overall LED emission efficiency is increased. The increase of Mg-doping level is implemented based on an Mg preflow growth technique, in which Mg source is supplied into the metalorganic chemical vapor deposition chamber for several minutes before the growth of p-AlGaN or p-GaN. It is found that by increasing the Mg-doping level by ~20 times near the interface between the EBL and the top QB, LED emission intensity can be enhanced by ~9.4 times. Based on a simulation study, we observe that the energy difference between the valence band edge and the quasi-Fermi level of hole in the EBL is reduced by increasing the Mg-doping level in this layer such that the total hole density in the quantum wells is increased for enhancing the LED emission efficiency.

Improvement of luster consistency between the p-Pad and the n-Pad of GaN-based light-emitting diodes via the under-etching process

For improvement of the light extraction efficiency of GaN-based lateral light-emitting diodes (LEDs), a p-GaN surface was textured through a low-temperature (850 °C) p-GaN growth process. However, the p-GaN texturing process caused luster inconsistency between the n-pad and the p-pad due to the roughness difference between the indium-tin oxide (ITO) and the n-GaN beneath the pads, which decreased the image recognition rate and accuracy during the wire bonding process for LED packaging. Therefore, an under-etching process was proposed to improve the luster consistency between the p-pad and the n-pad of GaN-based LEDs with a naturally textured p-GaN surface. The under-etching process decreased the roughness of the exposed n-GaN surface from 109 nm to 73.1 nm, which was similar to the roughness (74.8 nm) of the ITO surface. Optical microscopy showed that LEDs with a naturally textured p-GaN surface exhibited excellent luster consistency between the n-pad and the p-pad after the proposed under-etching process had been applied. Further analysis indicated that the LEDs with a naturally textured p-GaN surface showed no degradation of optical or the electrical performance after the proposed under-etching process had been applied. At a 20-mA injection current, the light output power of a LED with naturally a textured p-GaN surface was 8.7% higher than that of a LED with a smooth p-GaN surface.

Effect of p-GaN layer on the properties of InGaN/GaN green light-emitting diodes

Abstract

Efficiency-droop reduction in blue InGaN light-emitting diodes with low temperature p-type insertion layer

GaN based light emitting diodes with specially designed low temperature (LT) transition layers are investigated. Theoretical simulation shows that the LT-pGaN with LT-InGaN/GaN superlattice composite transition layer structure can effectively alleviate polarization field in the active layer and p-n junction region, consequently enhancing hole injection efficiency and electron blocking capability. Furthermore, the experimental results demonstrate that the low temperature (LT) transition layers can protect the active region from detrimental thermal annealing effect during high temperature p-GaN growth process. Finally, the reduced forward voltage and the enhanced output power about 42.2% were achieved at the injection current of 200mA.

Semipolar (202̄1) Single-Quantum-Well Red Light-Emitting Diodes with a Low Forward Voltage

We have demonstrated the InGaN/GaN single-quantum-well (SQW) red light-emitting diodes (LEDs) grown on the free-standing GaN (202̄1) substrate with a forward voltage as low as 2.8 V at 20 mA. A low p-GaN growth temperature is required to prevent the structure deterioration during the p-GaN growth. The reduction of the forward voltage was observed as the emission wavelength increased in the (202̄1) SQW LEDs, which is attributed to its reversed polarization-related electric field compared to the conventional c-plane LEDs.

Selectively grown photonic crystal structures for high efficiency InGaN emitting diodes using nanospherical-lens lithography

We report a low-cost and high-throughput process for the fabrication of two-dimensional SiO2 photonic crystal (PhC) by nanospherical-lens photolithography method to improve the light extraction of GaN-based light-emitting diodes (LEDs). The PhC structures were realized by the selective area growth of p-GaN using SiO2 nanodisks, which were patterned utilizing a self-assembled nanosphere as an optical lens. Without prejudice to the electrical properties of LEDs, the light output power (at 350 mA) of LEDs with the SiO2 and corresponding air-hole PhC was enhanced by 71.3% and 49.3%, respectively, compared to that without PhC. The LEDs with selectively grown PhC structures were found to exhibit partial compression strain release and reduced emission divergence. The finite-difference time-domain simulation was also performed to further reveal the emission characteristics of PhC LEDs.

The growth of heavily Mg-doped GaN thin film on Si substrate by molecular beam epitaxy

This paper reports the growth of p-GaN by molecular beam epitaxy. During growth, reflection high electron energy diffraction displayed streaky pattern. Hall Effect measurement indicated a hole concentration of 3.90 × 10 20 cm − 3 . Scanning electron microscopy and X-ray diffraction measurements revealed that p-GaN has high structural quality. Photoluminescence spectrum showed that band edge emission was found at 354.1 nm, significantly shifted from usual reported value, i.e. 364 nm. The shift was attributed to Burstein–Moss effect. In addition, a broad emission peak at 387.5 nm was also observed which was due to the transition from conduction band edge to Mg acceptor level. Moreover, the presence of 657 cm − 1 Raman peak also confirmed the heavy Mg-doped characteristic in p-GaN.

III-nitride nanopyramid light emitting diodes grown by organometallic vapor phase epitaxy

Nanopyramid light emitting diodes (LEDs) have been synthesized by selective area organometallic vapor phase epitaxy. Self-organized porous anodic alumina is used to pattern the dielectric growth templates via reactive ion etching, eliminating the need for lithographic processes. (In,Ga)N quantum well growth occurs primarily on the six {11¯01} semipolar facets of each of the nanopyramids, while coherent (In,Ga)N quantum dots with heights of up to ∼20 nm are incorporated at the apex by controlling growth conditions. Transmission electron microscopy (TEM) indicates that the (In,Ga)N active regions of the nanopyramid heterostructures are completely dislocation-free. Temperature-dependent continuous-wave photoluminescence of nanopyramid heterostructures yields a peak emission wavelength of 617 nm and 605 nm at 300 K and 4 K, respectively. The peak emission energy varies with increasing temperature with a double S-shaped profile, which is attributed to either the presence of two types of InN-rich features within the nanopyramids or a contribution from the commonly observed yellow defect luminescence close to 300 K. TEM cross-sections reveal continuous planar defects in the (In,Ga)N quantum wells and GaN cladding layers grown at 650–780 °C, present in 38% of the nanopyramid heterostructures. Plan-view TEM of the planar defects confirms that these defects do not terminate within the nanopyramids. During the growth of p-GaN, the structure of the nanopyramid LEDs changed from pyramidal to a partially coalesced film as the thickness requirements for an undepleted p-GaN layer result in nanopyramid impingement. Continuous-wave electroluminescence of nanopyramid LEDs reveals a 45 nm redshift in comparison to a thin-film LED, suggesting higher InN incorporation in the nanopyramid LEDs. These results strongly encourage future investigations of III-nitride nanoheteroepitaxy as an approach for creating efficient long wavelength LEDs.

10.1007/s11453-008-2021-x

The effect of the composition of the carrier gas on anisotropy of p-GaN growth rates in side-wall metal-organic chemical vapor deposition was studied. p-GaN layers with a nominal thickness of ∼400 nm were grown on side-walls of GaAs mesa stripes formed preliminarily by selective-area epitaxy on Si3N4. It is shown that, if hydrogen is used as the carrier gas, the p-GaN growth occurs mainly in the lateral direction, so that the p-GaN layer is either absent or is thin at the top faces of mesa stripes; in contrast, if nitrogen is used as the carrier gas, growth in the normal (0001) direction is prevalent, so that a p-GaN layer is formed at all faces of the mesa stripe. The results of our study are indicative of a significant role of hydrogen in the process of epitaxial growth of GaN and can be used in the development of technology of devices with p-n junctions based on GaN and with the use of selective-area growth.

Effect of hydrogen on anisotropy of the p-GaN growth rate in the case of side-wall MOCVD

The effect of the composition of the carrier gas on anisotropy of p-GaN growth rates in side-wall metal-organic chemical vapor deposition was studied. p-GaN layers with a nominal thickness of ∼400 nm were grown on side-walls of GaAs mesa stripes formed preliminarily by selective-area epitaxy on Si3N4. It is shown that, if hydrogen is used as the carrier gas, the p-GaN growth occurs mainly in the lateral direction, so that the p-GaN layer is either absent or is thin at the top faces of mesa stripes; in contrast, if nitrogen is used as the carrier gas, growth in the normal (0001) direction is prevalent, so that a p-GaN layer is formed at all faces of the mesa stripe. The results of our study are indicative of a significant role of hydrogen in the process of epitaxial growth of GaN and can be used in the development of technology of devices with p-n junctions based on GaN and with the use of selective-area growth.

Surface Study of P-Type MBE Gallium Nitride Growth over CdSe Quantum Dots

The objective of our current work is to observe the effects of MBE p-GaN growth over II-VI semiconductor nanocrystals on an MOCVD n-GaN template. The starting material was approximately 2 microns of n-type GaN MOCVD material atop a sapphire wafer. CdSe quantum dots were applied through drop and spin casting. A p-type MBE GaN layer of 2500 A was grown using a nitrogen plasma source. MBE allowed precise growth while SIMS analysis showed the relatively low temperature associated with this method, versus MOCVD, preserved the quantum dots. The surface was investigated using atomic force microscopy in tapping mode for samples at various high/low substrate and magnesium cell temperatures.