Plane GaN Substrates Research Articles

Using a Au/pitted a-plane GaN substrate to aggregate polar molecules for highly efficient surface-enhanced Raman scattering.

Searching for efficient surface-enhanced Raman scattering (SERS) substrates remains a challenge. In this study, we used metal-organic chemical vapor deposition to directly grow a pitted a-plane GaN thin film, subsequently covered by a thin Au layer (∼25nm), for use as a SERS substrate, without the need for any additional etching or lithography process. The SERS substrate containing these micrometer-sized pits provided a low limit of detection (∼10-9 M) for Rhodamine 6G (R6G), with a high enhancement factor (4.27 × 108) relative to normal Raman spectroscopy. Furthermore, Raman spectral mapping indicated that most of the R6G molecules were concentrated in the pits, enhancing the localization of the probe molecules for further analysis. The same molecular localization phenomenon was also effective for polar methylene blue but not for nonpolar paraffin. The molecular aggregation became more ambiguous upon increasing the thickness of the Au layer, suggesting that the polarity of the Ga and N atoms in the pits was responsible for the efficient aggregation of the polar R6G molecules, which could be potentially beneficial for biomedical detection.

Cathodoluminescence Investigation of Stacking Faults and Dislocations in the Edge Part of Seed‐Grown m‐Plane GaN Substrate

The extended defects in m‐plane seed‐grown GaN substrate have been investigated by cathodoluminescence (CL) and transmission electron microscopy (TEM). The presence of basal‐plane stacking faults (BSFs) has been confirmed in the edge part of seed growth. The luminescence features of BSFs are characterized by high‐resolution CL with monochromatic image and spatially resolved spectral analysis. Most stacking faults are intrinsic I1 BSFs with characteristic emission peak centered at 3.42 eV. There are a few intrinsic I2 BSFs which show varied emission energies of 3.33–3.38 eV. The motion of dislocations under electron beam irradiation has also been monitored and the correlation with stacking faults is discussed.

High-Speed Nonpolar InGaN/GaN Superluminescent Diode With 2.5 GHz Modulation Bandwidth

We demonstrate a superluminescent diode fabricated on a nonpolar ${m}$ -plane GaN substrate by employing a linearly tapered waveguide design. A high electrical −3dB modulation bandwidth ( $f_{\mathbf {3dB}}$ ) of 2.5 GHz at a current density of 30 kA/cm2 is achieved. The high modulation bandwidth is attributed to the shorter carrier recombination lifetime, the linear gain curve in the nonpolar ${m}$ -plane quantum wells, and the ability to operate at high current densities while effectively suppressing lasing. We derive a general expression for the −3dB bandwidth as a function of current density for SLDs using a similar approach to that for laser diodes. The −3dB bandwidth of a nonpolar superluminescent diode increases exponentially with current density. The experimental results are consistent with the derived expression for $f_{\mathbf {3dB}}$ vs . current density.

Thermal conductivity of GaN, GaN71 , and SiC from 150 K to 850 K

The thermal conductivity (\ensuremath{\Lambda}) of wide-band-gap semiconductors GaN and SiC is critical for their application in power devices and optoelectronics. Here, we report time-domain thermoreflectance measurements of \ensuremath{\Lambda} in GaN, $^{71}\mathrm{GaN}$, and SiC between 150 and 850 K. The samples include bulk $c$- and $m$-plane wurtzite GaN grown by hydride vapor phase epitaxy (HVPE) and ammonothermal methods; homoepitaxial natural isotope abundant GaN and isotopically enriched $^{71}\mathrm{GaN}$ layers with thickness of 6--12 \ensuremath{\mu}m grown on $c$-, $m$-, and $a$-plane GaN substrates grown by HVPE; and bulk crystals of 4H and 6H SiC. In low dislocation density ($l{10}^{7}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$) bulk and homoepitaxial GaN, \ensuremath{\Lambda} is insensitive to crystal orientation and doping concentration (for doping $l{10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$); $\mathrm{\ensuremath{\Lambda}}\ensuremath{\approx}200\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 300 K and $\ensuremath{\approx}50\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 850 K. In $^{71}\mathrm{GaN}$ epilayers at 300 K, \ensuremath{\Lambda} is $\ensuremath{\approx}15%$ higher than in GaN with natural isotope abundance. The measured temperature dependence of \ensuremath{\Lambda} in GaN is stronger than predicted by first-principles based solutions of the Boltzmann transport equation that include anharmonicity up to third order. This discrepancy between theory and experiment suggests possible significant contributions to the thermal resistivity from higher-order phonon scattering that involve interactions between more than three phonons. The measured \ensuremath{\Lambda} of 4H and 6H SiC is anisotropic, in good agreement with first-principles calculations, and larger than GaN by a factor of $\ensuremath{\approx}1.5$ in the temperature range $300lTl850\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This paper provides benchmark knowledge about the thermal conductivity in wide-band-gap semiconductors of GaN, $^{71}\mathrm{GaN}$, and SiC over a wide temperature range for their applications in power electronics and optoelectronics.

Simultaneous growth of GaN/AlGaN quantum wells on c-, a-, m-, and (20.1)-plane GaN bulk substrates obtained by the ammonothermal method: Structural studies

GaN/AlGaN quantum wells (QWs) were grown by metal–organic vapor phase epitaxy (MOVPE) on c-, a-, m-, and (20.1)-plane GaN substrates obtained by the ammonothermal method in the same MOVPE process, i.e. a process with growth parameters optimized for c-plane GaN templates. The structural properties of GaN/AlGaN QWs were carefully investigated by high angle annular dark field scanning transmission electron microscopy. Sharp GaN/AlGaN interfaces were seen for QWs grown on the c-, a-, and m-plane GaN substrates, but very rough interfaces with {1-100} and {1-101} facets were observed on the (20.1)-plane GaN substrate. In addition, the Al-rich region of AlGaN and GaN transition was identified for each of the GaN/AlGaN QW samples deposited in this process. The thickness and composition of this region varied with the crystallographic orientation of GaN substrates.

Reliability and degradation of InAlGaN semipolar plane green lasers with relaxed InGaN waveguide core

We demonstrate >400 h continuous-wave operation with 50% output power degradation for InAlGaN ridge laser diodes with InGaN waveguide relaxed via interfacial misfit dislocations. The lasers with emission wavelength of 503 nm were grown on free-standing plane GaN substrates. After >1000 h of operation, the misfit dislocations remained localized in the waveguide/cladding interfaces and no degradation occurred via their motion or growth toward the active region. No intensity degradation was detected for spontaneous emission devices without mesa elements, which implies that point defect migration from the surfaces, induced by ridge dry etching, is responsible for the ridge laser degradation.

Comparison of the physical, chemical and electrical properties of ALD Al2O3 on c‐ and m‐ plane GaN

AbstractThis study compares the physical, chemical and electrical properties of Al2O3 thin films deposited on gallium polar c‐ and nonpolar m ‐plane GaN substrates by atomic layer deposition (ALD). Correlations were sought between the film's structure, composition, and electrical properties. The thickness of the Al2O3 films was 19.2 nm as determined from a Si witness sample by spectroscopic ellipsometry. The gate dielectric was slightly aluminum‐rich (Al:O=1:1.3) as measured from X‐ray photoelectron spectroscopy (XPS) depth profile, and the oxide‐semiconductor interface carbon concentration was lower on c ‐plane GaN. The oxide's surface morphology was similar on both substrates, but was smoothest on c ‐plane GaN as determined by atomic force microscopy (AFM). Circular capacitors (50‐300 μm diameter) with Ni/Au (20/100 nm) metal contacts on top of the oxide were created by standard photolithography and e‐beam evaporation methods to form metal‐oxide‐semiconductor capacitors (MOSCAPs). The alumina deposited on c ‐plane GaN showed less hysteresis (0.15 V) than on m ‐plane GaN (0.24 V) in capacitance‐voltage (CV) characteristics, consistent with its better quality of this dielectric as evidenced by negligible carbon contamination and smooth oxide surface. These results demonstrate the promising potential of ALD Al2O3 on c ‐plane GaN, but further optimization of ALD is required to realize the best properties of Al2O3 on m ‐plane GaN. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of MOCVD GaN/AlGaN heterostructures grown on polar and non‐polar bulk GaN substrates

AbstractIn this work we present structural, optical and electrical properties of GaN epilayers and AlGaN/GaN MOCVD heterostructures obtained on polar and non‐polar 10x10 mm2, 1 and 1.5 inch bulk ammonothermal GaN substrates. The results were obtained based not only on a single‐point measurement but also on mapping across the sample area. In contactless electroreflectance (CER) spectra CER resonances related to optical transitions between excited states were clearly observed for GaN/AlGaN QWs grown on c ‐plane GaN substrates, whereas such features were not noted in the case of GaN/AlGaN QWs grown on m ‐plane GaN substrates. This experimental result clearly shows that the polarization‐related electric field leads to the quantum confinement of some extra states in the polar QW system. To check the advantage of homo‐epitaxy using ammonothermal GaN, high electron mobility transistors (HEMTs), and light emitting diodes (LEDs) structures were grown on (0001) Ga‐polarity, semi‐insulating and conductive GaN substrates respectively (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of GaN substrate off‐cut on properties of InGaN and AlGaN layers

AbstractThis paper summarizes the knowledge how off‐orientation involving lattice planes not parallel to the surface influences properties of AlGaN and InGaN layers. The following issues are presented: (i) morphology of GaN and InGaN layers, (ii) unit cell deformation in AlGaN and InGaN layers, (iii) cracking of AlGaN layers, and (iv) In incorporation into InGaN layers. Finally, laterally patterned on‐axis c ‐plane GaN substrates were successfully used to construct violet laser diodes as well as first multicolour laser diode arrays. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of GaN epilayers on c-, m-, a-, and (20.1)-plane GaN bulk substrates obtained by ammonothermal method

GaN epilayers were grown by metalorganic chemical vapor deposition (MOCVD) on c-, m-, a-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. The influence of (i) the surface preparation of substrates, (ii) MOCVD growth parameters, and (iii) the crystallographic orientation of substrates on the structural and optical properties of GaN epilayers was investigated and carefully analyzed. It was observed that the polishing of substrates and their misorientation have strong impact on the quality of GaN epilayers grown on these substrates. The MOCVD growth process was optimized for epilayers grown on m-plane GaN substrates. The best structural and optical properties were achieved for epilayers deposited at 1075 °C and the total reactor pressure of 50 mbar. These conditions were applied to grow GaN epilayers on substrates with other ( c-, a-, and (20.1)-plane) crystallographic orientations in the same MOCVD process. Particularly good optical properties were obtained for GaN epilayers deposited on polar and non-polar ( m- and a-plane) substrates, whereas slightly worse optical properties were observed for epilayers deposited on the semi-polar substrate. It therefore means that MOCVD growth conditions optimized for a given crystallographic direction ( m-plane direction in this case) work rather well also for other crystallographic directions.

Contactless electroreflectance of AlGaN/GaN heterostructures deposited on c-, a-, m-, and (20.1)-plane GaN bulk substrates grown by ammonothermal method

Room temperature contactless electroreflectance (CER) has been applied to study optical transitions and the distribution of the built-in electric field in AlGaN/GaN heterostructures grown on c-, a-, m-, and (20.1)-plane GaN substrates obtained by the ammonothermal method. It has been clearly shown that polarization effects in the AlGaN/GaN heterostructures grown on the c-plane lead to a strong built-in electric field in the AlGaN layer. The aforementioned field was determined to be ∼0.43 MV/cm from the period of Franz–Keldysh oscillations (FKOs). In addition, polarization effects lead to the formation of a two dimensional electron gas at the AlGaN/GaN interface, which screens the band bending modulation in the GaN buffer layer, and, therefore, GaN-related excitonic transitions are not observed for this heterostructure. Such features/effects are also not observed in the AlGaN/GaN heterostructures grown on nonpolar and semipolar GaN substrates because any strong polarization effects are not expected in this case. For these heterostructures, very strong and sharp GaN excitonic resonances are clearly visible in CER spectra. The resonances are very similar to the excitonic transitions observed for the GaN epilayers deposited on nonpolar and semipolar substrates. Moreover, there is a very weak AlGaN-resonance without FKO for nonpolar and semipolar heterostructures instead of the strong AlGaN-related FKO, which is typical of polar AlGaN/GaN heterostructures.

InGaN-based true green laser diodes on novel semi-polar [formula omitted] GaN substrates

The crystal quality and emission characteristics of InGaN-based laser diodes (LDs) with lattice-matched quaternary InAlGaN cladding layers on novel semi-polar { 2 0 2 ¯ 1 } plane GaN substrates were investigated. Highly homogeneous InGaN quantum wells (QWs) with a suppressed piezoelectric field can be realized on the { 2 0 2 ¯ 1 } plane even in the green spectral region. Optical polarization measurements revealed that [ 1 ¯ 0 1 4 ] oriented stripes are advantageous for LDs on { 2 0 2 ¯ 1 } planes. Gain-guided LDs on the novel { 2 0 2 ¯ 1 } plane exhibited the longest lasing wavelength at 533.6 nm under pulsed operation. Continuous-wave (cw) operation at the lasing wavelength of 523.3 nm at 115 mA was also demonstrated. The I th , J th , V th , and the slope efficiency were 110 mA, 9.2 kA/cm 2, 7.4 V, and 0.04 W/A, respectively. These results indicate that the green LDs on semi-polar { 2 0 2 ¯ 1 } plane GaN substrates are promising candidates for laser display applications.

Optical and structural properties of m ‐plane GaN substrates grown by ammonothermal method and GaN epilayers grown on these substrates

Abstractm ‐plane oriented GaN substrates of size 6 x 8 mm have been obtained by ammonothermal method and used to growing GaN epilayers by metalorganic chemical vapor deposition. The structural and optical properties have been determined for both m ‐plane GaN substrates and GaN epilayers deposited on these substrates. Excellent structural quality of m ‐plane GaN substrates has been confirmed by very narrow XRD peaks (the full‐width‐half‐maximum for the symmetrical (10.0) and asymmetrical (20.1) peaks are 16 and 19 arcsec, respectively) and the low concentration of threading dislocation density (∼ 104 cm‐2). In addition, these substrates exhibit quite good optical quality: the energy gap‐related transition is clearly observed at room temperature in photoluminescence and contactless electroreflectance spectra. The optical quality becomes excellent for GaN epilayers deposited on these substrates. In low temperature reflectance spectra very narrow intrinsic exciton lines which are very sensitive to the optical selection rules typical for the hexagonal symmetry were observed. In low temperature photoluminescence spectra a strong bound exciton line, relatively strong FXA and FXB lines and no emission band around 3.42 eV, which could be associated with the stacking faults, were observed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

InGaN‐based 518 and 488 nm laser diodes on c‐plane GaN substrate

AbstractWe succeeded in fabricating InGaN‐based laser diodes (LDs) with a wavelength of 518 and 488 nm under continuous wave (cw) operation. The both LDs structures were grown on conventional c‐plane GaN substrates by metal organic chemical vapor deposition (MOCVD). The threshold current and threshold voltage were 45 mA and 5.5 V at 518 nm, 30 mA and 4.5 V at 488 nm, respectively. The lifetime test of these LDs was carried out under high driving temperature up to 80 °C in cw operation. Lifetime was estimated to be over 5000 h with an optical output power of 5 mW at 80 °C in 515–518 nm LDs from 1000 h operation, and was estimated to be over 10,000 h with an output power of 60 mW at 60 °C in 488 nm LDs from 2000 h operation.

Epitaxial design of 475 nm InGaN laser diodes with reduced wavelength shift

AbstractWe have analyzed the quantum well stability of blue‐green emitting laser diodes grown on c ‐plane GaN substrates. Different parameters such as the material composition of the surrounding layers of the quantum well and the number of quantum wells were varied. We observed a strong influence of the barrier material composition (AlGaN, GaN, InGaN) on the quality of In rich quantum wells. Our preferred barrier material is GaN. In contrast InGaN barriers result in a higher defect density of 9.6 × 106 cm–2 in the quantum well whereas GaN barriers show a defect density of 1.8 × 106 cm–2. High indium content quantum wells with good crystalline quality were grown by improving the structure of the complete active region. Further enhancement could be achieved by using a double quantum well structure instead of a single quantum well structure. This offers the advantage that the wavelength shift is strongly reduced as a result of the lower current density in each quantum well. With our improvements we could realize laser diodes emitting at 475 nm with threshold current densities of 12 kA/cm2 and a slope efficiency of 500 mW/A. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High‐quality nonpolar m ‐plane GaN substrates grown by HVPE

AbstractRelatively large size (about 10 mm × 10 mm) m ‐plane GaN substrates are grown by hydride vapor phase epitaxy (HVPE). The high crystalline quality of the substrates was observed by X‐ray diffraction analysis. Typical full widths at half maximum values of X‐ray rocking curves measured for the (10$ \bar 1 $0) and (10$ \bar 1 $2) reflections were 25–35 arcsec and 30–40 arcsec, respectively. The defect density distribution was analyzed from the [10$ \bar 1 $0] direction by the cathodeluminescence method. The dark spot density decreased in the growth direction and could be as low as 2.5 × 105 cm–2. No stacking faults could be seen. The surface roughness after the lapping, polishing and cleaning was measured by atomic force microscopy and the RMS roughness was 0.072 nm. All of the phonon modes that the selection rules allow have been observed in polarized Raman measurements. High‐quality large‐size m ‐plane GaN substrates will enable the realization of commercial production of nonpolar devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved electroluminescence on nonpolar m ‐plane InGaN/GaN quantum wells LEDs

AbstractImproved nonpolar m ‐plane $\bar 1$ light emitting diodes (LEDs) with a thick InGaN multi‐quantum‐well (MQW) structure have been fabricated on low extended defect bulk m ‐plane GaN substrates using metal organic chemical vapor deposition (MOCVD). The peak wavelength of the electroluminescence emission from the packaged LEDs was 402 nm, which is in the blue‐violet region. The output power and EQE were 28 mW and 45.4%, respectively, at a pulsed driving current of 20 mA. With increasing current, the output power increased linearly, and fairly flat EQE was observed with increasing drive current. At 200 mA, the power and EQE were 250 mW and 41%, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

P‐type conduction in stacking‐fault‐free m ‐plane GaN

AbstractTransport measurements of p‐type m ‐plane GaN films grown on low extended‐defect density, free‐standing m ‐plane (10$ \bar 1 $0) GaN substrates are presented. No significant anisotropy in in‐plane mobility was found for hole concentrations between 2.45 × 1017 and 8.7 × 1018 cm–3. Since faulted, heteroepitaxial m ‐plane films showed significant anisotropy in electron and hole mobility a microstructural feature with anisotropic distribution (basal plane stacking faults) is discussed as a possible source of anisotropic scattering in non‐polar and semi‐polar films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparative study of GaN/AlGaN MQWs grown homoepitaxially on [formula omitted] and (0 0 0 1) GaN

Structural and optical properties of GaN/AlGaN multiple quantum wells (MQWs) grown by plasma-assisted molecular-beam epitaxy on ( 1 1 ̄ 0 0 ) plane free-standing GaN substrates and (0 0 0 1) GaN quasi-substrates have been compared. Atomic force microscopy studies indicate that the films and MQW structures grown on both substrates replicate the surface morphology of the substrates. MQWs with AlGaN barriers grown in the presence of In flux have stronger photoluminescence (PL) intensity than those with AlGaN barriers without In. X-ray diffraction spectra of MQWs grown on the (0 0 0 1) GaN substrates show larger number of superlattices peaks than those grown on ( 1 1 ̄ 0 0 ) substrates suggesting that the former have smoother interfaces. The PL spectra of MQWs deposited on (0 0 0 1) GaN substrates, where the growth is in a polar direction, exhibit a red-shift as well as a decrease in peak intensity with increase in well widths. Similar MQW structures on the ( 1 1 ̄ 0 0 ) GaN, on which the growth is in a non-polar direction, do not exhibit this phenomenon, which we attribute to the absence of internal electric fields in these structures. PL intensity of MQWs with a well width of 75 Å is 20 times stronger for those grown on the ( 1 1 ̄ 0 0 ) plane than on the (0 0 0 1) plane GaN substrate.