C-plane GaN Films Research Articles

Ablation threshold of GaN films for ultrashort laser pulses and the role of threading dislocations as damage precursors.

The laser-induced ablation threshold of c-plane GaN films upon exposure to ultrashort laser pulses was investigated for different wavelengths from the IR to the UV range and pulse widths between 0.34 and 10 ps. The one-pulse ablation threshold ranges between 0.15 and 3 J/cm2 and shows an increase with the wavelength and the pulse width, except for deep UV pulses. Based on a rate equation model, we attribute this behavior to the efficiency of seed carrier generation by interband absorption. In addition, the multi-pulse ablation threshold was analyzed. Accumulation effects are more prominent in case of IR than with UV pulses and are closely linked to damage precursors. By a thorough structural investigation, we demonstrate that threading dislocations, especially those with a screw component, significantly contribute to laser damage, since they provide a variety of dispersed states within the band gap.

Indium as a surfactant: Effects on growth morphology and background impurity in GaN films grown by ammonia-assisted molecular beam epitaxy

We report on the improvement of the surface morphology of c-plane GaN films grown at high growth rates (∼1 µm/h) using ammonia molecular beam epitaxy through a series of growth optimizations as well as the introduction of indium as a surfactant. The indium surfactant was expected to help with the adatom mobility and, thus, provide smoother growth surfaces. Through a combination of varying V/III ratios, In flux, and growth temperatures, an optimal condition for surface morphology, characterized by atomic force microscopy, was achieved. At higher Ga fluxes for fast growth rates (∼1 µm/h and beam equivalent pressures of ∼5 × 10−7 Torr), higher ammonia flows were necessary to preserve the surface morphology. In addition, indium was an effective surfactant—reducing the roughness and improving the overall surface morphology. However, excessive indium causes the surface morphology to degrade, potentially due to the enhancement of the Ga desorption from the surface as a result of the reaction of indium with ammonia for high indium fluxes. The indium surfactant also resulted in a reduction of background Si impurity concentrations in the film. These effects allow for the growth of thick drift layers with low background dopant concentrations for vertical GaN power devices.

Ion Implantation into Nonconventional GaN Structures

Despite more than two decades of intensive research, ion implantation in group III nitrides is still not established as a routine technique for doping and device processing. The main challenges to overcome are the complex defect accumulation processes, as well as the high post-implant annealing temperatures necessary for efficient dopant activation. This review summarises the contents of a plenary talk, given at the Applied Nuclear Physics Conference, Prague, 2021, and focuses on recent results, obtained at Instituto Superior Técnico (Lisbon, Portugal), on ion implantation into non-conventional GaN structures, such as non-polar thin films and nanowires. Interestingly, the damage accumulation is strongly influenced by the surface orientation of the samples, as well as their dimensionality. In particular, basal stacking faults are the dominant implantation defects in c-plane GaN films, while dislocation loops predominate in a-plane samples. Ion implantation into GaN nanowires, on the other hand, causes a much smaller density of extended defects compared to thin films. Finally, recent breakthroughs concerning dopant activation are briefly reviewed, focussing on optical doping with europium and electrical doping with magnesium.

Effect of carbon on the magnetic properties of Dy-implanted GaN films

In this paper, samples were prepared by co-implanting Dy and carbon ions into c-plane (0001) GaN films and a subsequent rapid thermal annealing at 800 °C. The room temperature magnetic properties of Dy and C co-implanted GaN samples were investigated by experiments and theoretical calculations. Dy and C co-implanted GaN samples had the larger room temperature magnetic moment than that of the Dy-implanted GaN. The calculated results showed that the spin polarization of C-2p electrons leads to the local magnetic moment due to the hybridization between Dy-4f electron states and C-2p electron, which explains the enhancement of magnetic moment in co-implanted GaN samples. Our results suggest that co-implantation is a potential approach to enhance the room temperature ferromagnetism.

Structural and magnetic properties of Dy-implanted GaN films

GaN:Dy films were prepared by implanting Dy ions into c-plane (0001) GaN films and a subsequent rapid thermal annealing process. The structural and magnetic properties of samples were investigated by means of X-ray diffraction (XRD), Raman and Physical Property Measurement System (PPMS). The XRD and Raman studies showed that lattice damage and dislocation caused by ion implantation can be effectively recovered by rapid thermal annealing. The PPMS results showed ferromagnetism at room temperature. The average value of the moment per Dy ion (Ms/Dy) is increased after thermal annealing. The available vacancies and formation of bound magnetic poloron as a result of implantation and thermal annealing are assigned responsible for observed ferromagnetism.

Current mapping of nonpolar a-plane and polar c-plane GaN films by conductive atomic force microscopy

Nonpolar (11-20) a-plane GaN and polar (0001) c-plane GaN films have been grown by metal organic chemical vapor deposition on r-plane (1-102) and c-plane (0001) sapphire substrates, respectively. Conductive atomic force microscopy (C-AFM) has been used to investigate the local conductivity of the films. C-AFM shows enhanced current conduction within the etch pits of c-plane GaN and triangular pits of a-plane GaN. The results indicate that the off-axis planes are more electrically active than c-plane and a-plane. Surprisingly, the C-AFM values in triangular pit of the a-plane GaN are much smaller than that in etch pits of the c-plane GaN. The dislocations type related current leakage mechanism is revealed for polar c-plane and nonpolar a-plane GaN films.

Homogeneity improvement of N-polar InGaN/GaN multiple quantum wells by using c-plane sapphire substrate with off-cut-angle toward a-sapphire plane

To improve the homogeneity of the N-polar (−c-plane) InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy (MOVPE), the growth of GaN and MQW on two c-plane sapphire substrates with an off-cut angle of 0.8° toward the a-plane (sub-A) and the m-plane (sub-M) was performed. The effects of the off-cut direction on the structural properties and surface morphologies of −c-plane GaN films were elucidated. It was found that the step bunching and meandering of −c-plane GaN were significantly suppressed on sub-A. The spatial homogeneity of the −c-plane InGaN/GaN MQWs along the off-cut direction was observed in the submicrometer scale using microbeam X-ray diffraction. By inhibiting the step bunching of the GaN template using sub-A, the thickness homogeneity of the MQWs on sub-A has been significantly improved in comparison with that on sub-M.

Correlation of current–voltage–temperature analysis with deep level defects in epitaxial GaN films

The effect of temperature on the nature of metal-semiconductor system in a Au contact deposited on c-plane and a-plane GaN film was investigated by current–voltage (I–V) measurements. The I–V measurements have been obtained systematically at different temperatures ranging from room temperature (300 K) to low temperature (78 K). Photoluminescence measurements were obtained to investigate correlation between the growth conditions, the substrate used for the growth of GaN film, and the presence of deep level defects therein by equating with the yellow band luminescence. The resistance–voltage–temperature analysis indicates that a gradual shift of the nature of contact towards Schottky behavior takes place while moving from room temperature to low temperature. Additionally, memory effect like aberration is present at low temperature, which can be attributed to the presence of deep-level defects and carrier recombination therein.

Oil-Immersion Raman Spectroscopy for c-Plane GaN on Si and Al2O3 Substrates

Introduction Nitride semiconductors are recognized as one of the most important materials for the optoelectronic devices and high-temperature high-frequency electronic devices, although the material quality is currently still insufficient to realize such great potential performance. In addition, residual stress inevitably exists in the epitaxially grown films due to the lattice mismatch and difference of thermal expansion coefficient. Raman spectroscopy has been performed to evaluate GaN, AlN, and AlGaN in terms of crystal quality, stress and material compositions [1]. In our previous study, we performed oil-immersion Raman spectroscopy for the excitation of in-active, transvers optical (TO) phonons, from (001) Si and SiGe [2,3]. Moreover, we evaluated anisotropic biaxial stress state in Si and SiGe mesa-shaped structure using the forbidden phonons [4,5]. In this work, we focused on the Raman measurements for hexagonal GaN and report the excitation of E1(TO) phonon, which is one mode in wurtzite crystal with C6 u symmetry [7]. The E1(TO) mode is generally Raman in-active under the backscattering geometry from c-plane GaN according to the Raman selection rule [7]. We performed oil-immersion Raman spectroscopy in order to obtain the forbidden mode in c-plane GaN. Experimental Procedure Figure 1(a) shows the schematic cross-sections of the samples. The samples are a 2.0-μm-thick c-plane GaN film with a 5-to 10-nm-thick nucleation layer on a the Al2O3 substrate and a 1.8-μm-thick c-plane GaN film with a 270-nm-thick buffer layer on a Si substrate. The Raman measurements were performed under the backscattering geometry from c-plane GaN and cleaved cross-section. The focal length of the spectroscope is 2 m, which allows us to obtain Raman spectra with a high-spectral resolution of approximately 0.1 cm- 1[6]. The wavelength of the excitation laser was 532 nm. The numerical aperture (NA) and refractive index of oil were 1.4 and 1.5, respectively. In comparison, dry lens with the NA of 0.7 was also used (Fig. 1(b)). Results and Discussion Figure 2 shows the Raman spectra of GaN on Si and GaN on Al2O3 measured from the cross-section. The Raman peaks appear at around 535 cm-1, 560 cm-1, 570 cm-1, which can be identified as A1(TO), E1(TO), and E2 high phonon modes respectively [1]. Each peak position is different because stress states in the GaN layers should be depending on the substrates, Si or Al2O3. The stress calculated with the E2 peaks are -350 MPa and 650 MPa in GaN layers on Si and Al2O3, respectively[8]. Figure 3 shows the Raman spectrum for GaN on Al2O3 measured from the top surface under the oil-immersion conditions. In comparison, the Raman spectrum obtained in the dry condition is also shown. The peak on the left side of the large E2 peak can be seen in the oil-immersion conditions, although there is no peak in the dry condition. This peak position was 559.7 cm- 1, which was almost the same as the peak of E1(TO) measured from the cross-section. As shown in Fig. 1(b), the aperture angle becomes significantly large in the oil-immersion condition, which leads to the generation of a z-component in the electrical field [2]. From the Raman selection rules, the z-polarization can excite E1(TO) mode [7]. Therefore, we concluded that E1(TO) can be detected even in the backscattering geometry from c-plane GaN by oil-immersion Raman spectroscopy. In oil-immersion Raman measurements, E2 high, A1(LO), and E1(TO) modes can be used for the detail evaluation of the GaN material properties, e.g., stress, material composition, and temperature. Conclusions We have demonstrated the excitation of E1(TO) from the c-plane GaN film by oil-immersion Raman spectroscopy, although E1(TO) is conventionally in-active under the backscattering geometry from c-plane GaN. Acknowledgements The authors thank Dr. Naomi Sawamoto for the TEM observation. This study was partially supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Young Scientists (B) (No. 25790049).

Domain matching epitaxy of GaN films on a novel langasite substrate: an in-plane epitaxial relationship analysis

We report the epitaxial growth of c-plane GaN films on a novel langasite (La3Ga5SiO14, LGS) substrate by plasma-assisted molecular beam epitaxy. The in-plane epitaxial relationship and the structural properties of GaN films on an LGS substrate were investigated using in situ reflective high energy electron diffraction (RHEED), high resolution X-ray diffraction (HR-XRD) and Raman spectroscopy. The in-plane epitaxial relationship between GaN and LGS determined using RHEED pattern was found to be GaN[100]//LGS[210] and GaN[110]//LGS[140]. HR-XRD results confirmed the exact epitaxial relationship, and showed that six reflection peaks of GaN(102) were shifted around 19° from those of LGS(102). Raman analysis revealed that a minute compressive strain still existed in the GaN film due to the very small lattice mismatch between GaN and LGS. The results obtained in this study demonstrate that the nearly lattice-matched LGS can be a promising and futuristic substrate material for the growth of GaN, and it is foreseen that our results could be a reference for the further development of high performance nitride-based devices.

Large third-order nonlinearity of nonpolar A-plane GaN film at 800 nm determined by Z-scan technology

We report an investigation on the optical third-order nonlinear property of the nonpolar A-plane GaN film. The film sample with a thickness of ~2 μm was grown on an r-plane sapphire substrate by metal-organic chemical vapor deposition system. By performing the Z-scan method combined with a mode-locked femtosecond Ti:sapphire laser (800 nm, 50 fs), the optical nonlinearity of the nonpolar A-plane GaN film was measured with the electric vector E of the laser beam being polarized parallel (//) and perpendicular (⊥) to the c axis of the film. The results show that both the third-order nonlinear absorption coefficient β and the nonlinear refractive index n2 of the sample film possess negative and large values, i.e. β// = −135 ± 29 cm/GW, n2// = −(4.0 ± 0.3) × 10−3 cm2/GW and β⊥ = −234 ± 29 cm/GW, n2⊥ = −(4.9 ± 0.4) × 10−3 cm2/GW, which are much larger than those of conventional C-plane GaN film, GaN bulk, and even the other oxide semiconductors.

Achieve high-quality GaN films on La0.3Sr1.7AlTaO6 (LSAT) substrates by low-temperature molecular beam epitaxy

The growth of c-plane GaN films on La0.3Sr1.7AlTaO6 (LSAT) (111) substrates has been carried out by molecular beam epitaxy (MBE) at various substrate temperatures. GaN grown at 750 °C shows poor crystallinity with an up to 8 nm thick interim layer between LSAT and GaN, mainly due to the severe interfacial reaction between the substrate and the film. On the contrary, an atomically abrupt interface has been observed when GaN was grown at a lower temperature of 500 °C. This low-temperature grown GaN exhibits high crystallinity with a very smooth surface and strong band-edge emission, which we attribute to the well-controlled oxygen atom evaporation from LSAT substrate surface thanks to the low temperature growth. On the one hand, it avoids the severe interfacial reaction that takes place in the case of high-temperature epitaxial growth; on the other hand, it conserves the crystalline structure of the substrate surface, and consequently takes good advantage of the small lattice mismatch between GaN and LSAT. Considering the advantages of LSAT over sapphire, and the device-ready film quality of GaN achieved on LSAT, this work exhibits a promising future for high-performance light emitting diodes on this unconventional substrate.

Structural, morphological and magnetic characteristics of Tb-implanted GaN and AlGaN films

Abstract Diluted-magnetic GaN:Tb and AlGaN:Tb films have been fabricated by implanting Tb + ions into c-plane (0 0 0 1) GaN and AlGaN films and a subsequent annealing process. The structural, morphological and magnetic characteristics of the samples have been investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and superconducting quantum interference device (SQUID), respectively. The XRD and AFM analyses show that the annealing process can effectively recover the crystalline degradation caused by implantation. According to the SQUID analysis, both the GaN:Tb and the AlGaN:Tb films exhibit clear room-temperature ferromagnetism. It is very interesting to find the saturation magnetization value of the AlGaN:Tb sample is almost two times that of GaN:Tb sample. The possible origin of the ferromagnetism of the samples was discussed briefly.

Growth regimes during homoepitaxial growth of GaN by ammonia molecular beam epitaxy

c-plane GaN films were grown by ammonia molecular beam epitaxy on metal-organic chemical vapor deposition templates for a wide range of NH3:Ga flux ratios and growth temperatures, and the resulting films were characterized using atomic force microscopy, reflection high-energy electron diffraction, and transmission electron microscopy. Three distinct nitrogen-rich growth regimes—unstable layer-by-layer, quasi-stable step flow, and dislocation-mediated pitting—were identified based on the growth mode and film properties. In addition, step flow growth was observed under conditions of gallium droplet accumulation. The results indicate the existence of two regimes for step-flow growth of GaN by ammonia MBE—both gallium-rich and nitrogen-rich. Growth mode instabilities and mound formation were observed and are discussed in the context of a step-edge energy barrier to adatom diffusion over a terrace.

Dislocation movement in GaN films

We demonstrate that significant dislocation movement occurs below the surface of heteroepitaxial c-plane GaN films during their growth by metalorganic vapor phase epitaxy. Dislocations move primarily by vacancy-assisted climb, which appears to be driven by the high in-plane biaxial stresses present during growth. Annealing low dislocation density (4.3×108 cm−2) GaN films promotes dislocation climb and thus reduces both dislocation densities and in-plane stresses (at high temperatures), independent of epilayer growth conditions.

Compensation effect of Mg-doped a- and c-plane GaN films grown by metalorganic vapor phase epitaxy

The electrical and optical properties of Mg-doped a- and c-plane GaN films grown by metalorganic vapor phase epitaxy were systematically investigated. The photoluminescence spectra of Mg-doped a- and c-plane GaN films exhibit strong emissions related to deep donors when Mg doping concentrations are above 1×10 20 cm −3 and 5×10 19 cm −3, respectively. The electrical properties also indicate the existence of compensating donors because the hole concentration decreases at such high Mg doping concentrations. In addition, we estimated the N D /N A compensation ratio of a- and c-plane GaN by variable-temperature Hall effect measurement. The obtained results indicate that the compensation effect of the Mg-doped a-plane GaN films is lower than that of the Mg-doped c-plane GaN films.

Structural and magnetic properties of GaN:Sm:Eu films fabricated by co-implantation method

Diluted-magnetic GaN:Sm:Eu films have been fabricated by co-implantation of Sm and Eu ions into c-plane(0001) GaN films and a subsequent annealing process. The structural, morphological and magnetic characteristics of the samples have been investigated by means of high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and superconducting quantum interference device (SQUID). The XRD and AFM analyses show that the annealing process can effectively recover the crystalline degradation caused by implantation. Compared with GaN:Sm films, more defects have been introduced into GaN:Sm:Eu films due to the Eu implantation process. According to the SQUID analysis, GaN:Sm:Eu films exhibit clear room-temperature ferromagnetism. Moreover, GaN:Sm:Eu films show a lower saturation magnetization (Ms) than GaN:Sm films.

Complete in-plane polarization anisotropy of the A exciton in unstrained A-plane GaN films

Using reflectance spectroscopy, the in-plane polarization behavior of unstrained C- and A-plane GaN films is experimentally investigated. While no in-plane polarization anisotropy is observed for all three band-gap related excitons (A, B, and C) in unstrained C-plane GaN films, the A exciton is completely linearly polarized perpendicular to the c axis in unstrained A-plane GaN films. However, the B and C excitons are only partially polarized. This observation is in excellent agreement with results based on band-structure calculations using the Bir-Pikus Hamiltonian for the wurtzite crystal structure.

Effect of anisotropic strain on phonons in a-plane and c-plane GaN layers

We have studied phonons in two types of anisotropically strained GaN films: c-plane GaN films grown on a-plane sapphire and a-plane GaN films grown on r-plane sapphire. The anisotropic strain in the films is determined by high-resolution X-ray diffraction (HRXRD) in different measuring geometries and the phonon parameters have been assessed by generalized infrared spectroscopic ellipsometry (GIRSE). The effect of strain anisotropy on GaN phonon frequencies is presented and the phonon deformation potentials a A 1 ( TO ) , b A 1 ( TO ) , c E 1 ( TO ) and c E 1 ( LO ) are determined.

Polarization anisotropy in GaN films for different nonpolar orientations studied by polarized photoreflectance spectroscopy

We use photoreflectance (PR) spectroscopy to study the electronic band structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in-plane strain. The exciton transition energies of an unstrained, high-quality C-plane GaN film are used to accurately determine the crystal-field and spin-orbit splitting energies. For films with a nonpolar orientation, the resonant features observed in the PR spectra exhibit a strong in-plane polarization anisotropy and different transition energies from the ones measured in the C-plane GaN film. The deformation potential D5 is accurately determined from four GaN films with a nonpolar orientation using the measured energies together with the polarization properties and out-of-plane strain.