Regrown GaN Research Articles

Insulating behavior of interfaces in regrown Al0.23Ga0.77N/GaN double heterostructures on Al0.07Ga0.93N back‐barrier templates

AbstractAn important issue in multi‐step epitaxial growth processes are the electrical characteristics of regrown interfaces. Here, we present a study on interface quality and its effect onto insulating behavior and scattering effects in planar transistors. A double heterostructure with an AlGaN/GaN high electron mobility transistor on top of an AlGaN back‐barrier is used to assess electrical properties. Back‐barrier templates were exposed to air for several days and subsequently treated by wet and dry chemical etching. A strong impact on leakage behaviour of the regrown interface was observed. Lateral leakage currents as low as 1x10–8 A/mm are achieved with an optimized pre‐treatment. Furthermore, under optimized conditions, the regrown GaN channel thickness is varied to investigate the effect of different distances between the two‐dimensional electron gas (2DEG) and the regrown interface. Here, enhanced scattering mechanisms, probably due to ionized impurities, lead to a mobility drop of the 2DEG. A parasitic channel is observed, which indicates the presence of additional impurities at the regrown interface. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dual temperature process for reduction in regrowth interfacial charge in AlGaN/GaN HEMTs grown on GaN substrates

AbstractThe effects of growth temperature and Mg compensation doping on the structural and electrical properties of AlGaN/AlN/GaN high electron mobility transistor (HEMTs) structures grown on low threading dislocation density bulk GaN substrates by metalorganic chemical vapor deposition were investigated. The background electron concentration in the regrown GaN was found to decrease from 1.5x1018 cm‐3 to 2x1016 cm‐3 as the growth temperature was reduced from 1100 °C to 950 °C. A dual temperature process was then employed for growth of the GaN base layer and AlGaN/AlN/GaN top heterostructure in the HEMT along with Mg doping of the GaN base to compensate residual Si donors at the regrown interface. Using this approach, AlGaN/AlN/GaN HEMTs were produced that had a sheet carrier density as high as 1.1x1013 cm‐3 with a room temperature mobility of 1600 cm2/Vs. The incorporation of a thin AlN layer at the regrown interface was found to reduce the sheet carrier density of the overall structure (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved Performance of Near-Ultraviolet Light Emitting Diodes on Selectively Etched GaN Templates

An approach that dramatically improved the performance of near-ultraviolet InGaN-based light-emitting diodes (NUV-LEDs) was proposed using the selectively etched GaN (SE-GaN) template, which was fabricated via a combination of patterned sapphire substrate (PSS), defect selective etching, SiO2 deposition and chemical-mechanical planarization. The dislocation density of n-type GaN regrown on the SE-GaN template can be reduced to 1.3 10 cm 2 by concentrating threading dislocations (TDs) within undoped GaN and terminating the propagation of concentrated TDs. When operated with an injection current of 350 mA, the output power of the NUV-LED fabricated on SE-GaN template was 13% higher than that of NUV-LED fabricated on PSS. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3555075] All rights reserved.

InGaN light-emitting diodes with oblique sidewall facets formed by selective growth on SiO_2 patterned GaN film

In this study, GaN-based light-emitting diodes (LEDs) with naturally formed oblique sidewall facets (OSFs) were fabricated through a selective regrowth process. The SiO₂ mask layer was patterned on a heavily doped n-GaN template layer rather than on a sapphire substrate. As a result, the periphery of the LED included several OSFs around the regrown GaN mesa. While processing the device, dry etching was unnecessary for exposing the n-GaN underlying layer in order to form the n-type Ohmic contacts. This could be attributed to the fact that the n-GaN template layer with an electron concentration of around 8 × 10¹⁸/cm³ was exposed after the removal of the SiO₂ mask layer. With an injection current of 20 mA, GaN-based LEDs with OSFs exhibited a 21% enhancement in light output compared with those that have vertical sidewall facets. The enhancement is attributed to the fact that photons extracted from OSFs can reduce internal absorption loss.

Void shape control in GaN re-grown on hexagonally patterned mask-less GaN

We present the results of GaN re-growth on hexagonally patterned GaN templates. Sapphire was used as the original substrate and the samples were grown by metalorganic vapor phase epitaxy (MOVPE). The re-growth on the patterned templates results in the formation of voids at the GaN/sapphire interface. Our extensive scanning electron microscopy (SEM)-based experimental investigations show that the void shape can be controlled from nearly vertical to fully inclined configurations. It was found that the initial hexagon hole diameter plays a key role in determining the final profile of the void sidewalls. X-ray diffraction analysis of the GaN layers indicates that the layers with inclined sidewall voids have an improved crystalline quality. Knowledge of the void configurations in the GaN layers and a possibility to control their shape can help in enhancing light extraction from the light emitting structures.

Regrowth of Semipolar GaN on Nanoporous GaN Template by Metal Organic Chemical Vapor Deposition

A nanoporous semipolar GaN template has been fabricated by photoenhanced electrochemical etching to obtain porous GaN with nanoscale pores. The surface morphology of regrown semipolar GaN on a nanoporous GaN template was enhanced by surface modification. Cross-sectional transmission electron microscopy (TEM) images showed reductions in the densities of dislocations and basal stacking faults at the regrown interface. Photoluminescence measurement also revealed that the crystallinity of regrown GaN was improved by reducing the density of defects. Our results suggest that the photoenhanced electrochemical etching and regrowth technique is promising for high-quality semipolar GaN growth with a reduced defect density on a sapphire substrate.

High quality GaN film overgrown on GaN nanorods array template by HVPE

The high density vertically aligned GaN nanorods array was fabricated by thermal evaporation of GaN powder with the assistance of HCl gas. The GaN nanorods array was used as template for GaN film growth by hydride vapor phase epitaxy (HVPE). The full width at half maximum values (FWHM) of high-resolution X-ray diffraction (HRXRD) rocking curves for the GaN film with GaN nanorods array template are 247 arc sec (002 reflection) and 308 arc sec (102 reflection), while those for the GaN film without GaN nanorods array template are 292 and 369 arc sec, respectively. This result indicates a significant reduction of dislocation density in the overgrown GaN film with GaN nanorods array template. Photoluminescence spectra measurements reveal the compressive strain relaxation and an improvement in the quality of the overgrown GaN film with GaN nanorods array template as compared to the regrown GaN film without GaN nanorods array template, which is consistent with the trend observed by HRXRD.

Selective Defect Blocking by Self-Assembled Silica Nanospheres for High Quality GaN Template

We demonstrate the use of self-assembled silica nanospheres as a mask to block the propagation of threading dislocations on a defect selective etched GaN template. The selective etched pits were generated on the latent dislocations of GaN template using the wet etching technique. The silica nanospheres (500 nm) were then coated on the selective etched GaN using a spin-coating method. The silica nanospheres were confined in the etched pits by capillary force and geometric shape. The GaN template was then regrown by epitaxial overgrowth using metallorganic chemical vapor deposition. As a result, the regrown GaN layer was significantly reduced from 5 × 10 9 to 3 × 10 7 cm ―2 in threading dislocation density and from 3 × 10 7 to 3 × 10 5 cm ―2 in etched pit density. These significant enhancements are attributable to selective defect blocking by self-assembled silica nanospheres.

Threading dislocations annihilation in regrown GaN film on nanoporous GaN template

AbstractRegrowth of GaN buffer layer on nanoporous GaN at different chamber temperature of 750 °C, 850 °C and 1000 °C are used to study the mechanism behind threading dislocations reduction in the GaN film subsequently overgrown. The growth of a 100 nm GaN buffer layer at 850 °C causes dislocations to bend into the underlying GaN and annihilated at the interface of GaN buffer/nanoporous GaN. Thermal treatment of nanoporous GaN in MOCVD growth chamber at 850 °C in NH3 and N2 ambient leads to the pinning of the threading dislocations at its surface edge steps by impurity –vacancy complexes of SiNx. The SiNx are formed by the impinging adatoms of NH3 which interact with Si dangling bonds at the subgrain boundaries of the nanoporous Si‐doped GaN. When the nanoporous GaN sample undergoes rapid thermal annealing in N2 ambient, no effective filling of these voids are observed and dislocations density is not substantial reduced. Regrowth of GaN at 850 °C re‐structured the nanoporous GaN, forming a SiNx complex at the voids or pores which pinned dislocation propagation. This explained the mechanism behind dislocation annihilation in regrown GaN on nanoporous GaN template with the use of additional interfacial GaN buffer layer. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydride vapor phase epitaxy growth of high-quality GaN film on in situ etched GaN template

In this paper, high-quality GaN film is grown by hydride vapor phase epitaxy (HVPE) on in situ etched porous GaN template, which can be prepared by in situ etching of GaN template with HCl gas at high temperature. High-resolution X-ray diffraction measurement shows that the crystalline quality of HVPE-GaN film on porous GaN template is better than that directly deposited on as-grown GaN template. Micro Raman measurement reveals the compressive strain relaxation in the regrown GaN film on porous GaN template, which coincides with the result of the photoluminescence spectra measurement.

Defect reduction of laterally regrown GaN on GaN/patterned sapphire substrates

Structural properties of GaN epilayers on wet-etched protruding and recess-patterned sapphire substrates (PSSs) have been investigated in detail using high-resolution double-crystal X-ray diffraction (DCXRD) and etch-pit density methods. The DCXRD results reveal various dislocation configurations on both types of PSSs. The etch pits of GaN on the recess PSS exhibit a regular distribution, i.e. less etch pits or threading dislocation density (TDD) onto the recess area than those onto the sapphire mesas. On the contrary, an irregular distribution is observed for the etch pits of GaN on the protruding PSS. A higher crystal quality of the GaN epilayer grown onto the recess PSS can be achieved as compared with that onto the protruding PSS. These data reflect that the GaN epilayer on the recess PSS could be a better template for the second epitaxial lateral overgrowth (ELOG) of GaN. As a result, the GaN epilayers after the ELOG process display the TDDs of around ∼10 6 cm −2.

Enhanced luminescence efficiency due to carrier localization in InGaN∕GaN heterostructures grown on nanoporous GaN templates

Low defect density GaN was achieved through dislocation annihilation by regrowing GaN on strain relaxed nanoporous GaN template formed by UV-enhanced electrochemical etching. The InGaN∕GaN single and multiple quantum wells grown on this nanoporous GaN template show enhanced indium incorporation due to strain relaxation. The step edges of regrown GaN on these nanoporous GaN act as effective nucleation sites for impinging indium atoms during growth. Evidence shows fluctuation in the quantum well width caused by indium segregation leading to carrier localization. A higher luminescence efficiency of InGaN∕GaN quantum wells is achieved through a combination of excitons localization, higher energy barrier for nonradiative recombination of carriers with dislocations and the reduction in defect density of the materials grown on the nanoporous GaN template.

Improved GaN film overgrown with a molybdenum nanoisland mask

We report the improved crystalline and optical quality of GaN film overgrown by hydride vapor phase epitaxy adopting a molybdenum (Mo) nanoisland mask (MNM). The MNM is fabricated following thermal annealing of the nanometer-thick Mo film deposited by electron-beam evaporation. The full width at half maximum values of high-resolution x-ray diffraction (HRXRD) rocking curves for the GaN film with MNM are 188arcsec (002 reflection) and 219arcsec (102 reflection), while those for the GaN film without MNM are 256 and 364arcsec, respectively. This result indicates a significant reduction of dislocation density in the overgrown GaN film with MNM. Photoluminescence spectra measurements reveal the compressive strain relaxation and improvement in the quality of the overgrown GaN film with MNM as compared to the regrown GaN film without MNM, which is consistent with the trend observed by HRXRD.

Reduction of Mg segregation in a metalorganic vapor phase epitaxial grown GaN layer by a low-temperature AlN interlayer

The redistribution behavior of Mg in a sequentially regrown GaN epilayer on a p-type doped GaN template was studied. All samples in this study were regrown by metalorganic vapor phase epitaxy on the sapphire substrates. A high density and a slow tail of Mg concentration were observed in a nominally undoped layer due to the surface segregation. We found that the insertion of a low-temperature (LT) AlN interlayer was effective to suppress the Mg redistribution in the GaN regrown layer. Analyzing the temperature dependence of the surface segregation, the activation energy of the Mg segregation was estimated to be 0.63eV in GaN and 2.47eV in a LT-AlN layer, respectively.

Annihilation of Threading Dislocations in Regrown GaN on Electrochemically Etched Nanoporous GaN Template with Optimization of Buffer Layer Growth

Nanoporous GaN template has been fabricated by electrochemical etching to give hexagonal pits with nano-scale pores of size 20-50 nm in the underlying grains. Electrochemical etching at The effect of GaN buffer layer grown at various temperatures from 650°C to 1015°C on these as-fabricated nano-pores templates are investigated by transmission electron microscopy. The buffer layer grown at the optimized temperature of 850°C partially fill up the pores and voids with annihilation of threading dislocations, serving as an excellent template for high-quality GaN growth. This phenomenon is, however not observed for the samples grown with other temperature buffer layers. The PL spectrum for the regrowth GaN on nanoporous GaN template also shows an enhancement of PL intensity for GaN peak compared to as-grown GaN template, which is indicative of its higher crystal quality. This makes it as a suitable template for subsequent device fabrication.

Transmission electron microscopy studies of regrown GaN Ohmic contacts on patterned substrates for metal oxide semiconductor field effect transistor applications

Contact selected area regrowth of GaN was performed by metal organic chemical vapor deposition using a silicon nitride dielectric hard mask to define plasma etched recesses and to define source-drain regions. A low temperature regrowth process at 750–850°C was adopted to limit lateral overgrowth. High resolution electron microscopy images and selected area diffraction confirmed the regrowth selectivity and revealed that the low temperature regrown GaN is epitaxial and has a wurtzite crystal structure. I-V characteristics of the fabricated metal oxidesemiconductor field effect transistor show enhancement mode operation.

Dislocation annihilation in regrown GaN on nanoporous GaN template with optimization of buffer layer growth

Nanoporous GaN template has been fabricated by electrochemical etching to give hexagonal pits with nanoscale pores of size 20–50nm in the underlying grains. The effect of GaN buffer layer grown at various temperatures from 650to1015°C on these as-fabricated nanopores templates is investigated by transmission electron microscopy. The buffer layer grown at the optimized temperature of 850°C partially fill up the pores and voids with annihilation of threading dislocations, serving as an excellent template for high-quality GaN growth. This phenomenon is, however, not observed for the samples grown with other temperature buffer layers. Micro-Raman measurements show significant strain relaxation and improvement in the crystal quality of the overgrown GaN layer on nanoporous GaN template as compared to overgrown on conventional GaN template.

Deep levels in KOH etched and MOCVD regrown GaN p‐n junctions

Threading dislocations continue to challenge the functionality and reliability of GaN based devices. This work investigates the selective etching of threading dislocations in a GaN template grown by metalorganic chemical vapor deposition (MOCVD), and subsequent regrowth as a method to reduce the dislocation density. A 2.0 µm template layer was grown first, followed by either 2 second or 30 second etching of the GaN in KOH, followed by regrowth of 2 µm of GaN and 0.2 µm of p-type GaN. Atomic force microscopy (AFM) images showed that hexagonal pits were opened up by the KOH etch. Deep level transient spectroscopy (DLTS) measurements showed several traps present, including a dominant electron trap at 0.585 eV, with capture cross section of 1.0 × 10–15 cm2 and concentration of 1016/cm3. Another electron trap was also measured in the same range (0.536 eV, 5.7 × 10–16 cm2) with an order of magnitude lower concentration. From the DLTS measurements, the KOH dislocation etching did not significantly reduce the concentration of defects, or eliminate any specific traps. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A study of GaN regrowth on the micro‐facetted GaN template formed by in‐situ thermal etching

AbstractWe report a study of GaN regrowth on micro‐facetted GaN templates formed by in‐situ thermal etching in a low‐pressure metalorganic chemical vapor deposition system. First, the 1.5 µm‐thick GaN epilayers were grown on c‐plane sapphire substrates. This was followed by an in‐situ thermal etching process under hydrogen and ammonia ambient with the etching time being a parameter. The thermally etched GaN templates showed hexagonal GaN pyramids, which were aligned along the growth direction on the c‐plane substrate. The 3 µm‐thick GaN regrowth was performed on these micro‐facetted GaN templates. The surface of the overgrown GaN was atomically smooth. The full width at half maximum of (102) peak in the X‐ray rocking curve profile decreased from 9.5 to 6.5 arcmin as the thermal etching time increased from 0 to 45 min. Etch pit density measurements revealed that the pit density of regrown GaN decreased by about one order of magnitude, compared to that of the control sample. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication of GaN/Alumina/GaN Structure to Reduce Dislocations in GaN

GaN/Alumina/GaN structures were fabricated to reduce dislocation density in GaN. Alumina films were deposited on GaN templates by electron cyclotron resonance plasma sputtering. GaN was regrown on the alumina films by metalorganic vapor phase epitaxy. It was found that GaN on alumina selectively regrown from pinholes in alumina by annealing prior to GaN regrowth. Hardly any of the dislocations in the GaN template propagated through the interlayer. Most of dislocations were terminated or bent at the interface between the alumina and regrown GaN. On the other hand, new vertical dark lines and horizontal dislocations were generated in regrown layer. The vertical lines were found to correspond to an inversion domain from convergent beam electron diffraction analysis.