Growth Of Gallium Nitride Films Research Articles

Thermal Atomic Layer Deposition of Gallium Nitride Films Using Tris(dimethylamido)Gallium and Ammonia

Gallium nitride (GaN) is a III-V semiconductor material with a wide and direct bandgap, relatively high carrier mobility, and high breakdown voltage. GaN is suitable for applications in optoelectronic devices and power devices, including light-emitting diodes (LEDs) and high electron mobility transistors (HEMTs). Typically, epitaxial GaN layers are grown by metal-organic chemical vapor deposition (MOCVD) at high deposition temperatures. However, low-temperature growth of GaN is necessary for temperature-sensitive devices and substrates. Atomic layer deposition (ALD) is a technique that offers unique advantages, such as low process temperature, excellent conformality, and atomic-level thickness control.In this study, we investigated the thermal ALD process of GaN at a low temperature of 200°C. Tris(dimethylamido)gallium (TDMAGa) was selected as the Ga precursor because tris(dimethylamido)titanium and tris(dimethylamido)aluminum were successfully adopted in the low-temperature ALD process of TiN and AlN thin films [1]. The growth and properties of ALD GaN films were investigated. ALD GaN films exhibited a self-limiting reaction with a deposition rate of 1.34 Å/cycle at 200°C, with a refractive index of 2.07. The saturation times for TDMAGa and NH3 pulses were 5 s and 30 s, respectively. Deposition at 250°C showed a deposition rate higher than 3 Å/cycle due to the thermal decomposition of TDMAGa. The ALD GaN film deposited at 200°C was amorphous and nitrogen deficient. To improve the crystallinity and composition, the as-deposited GaN films were annealed in an NH3 atmosphere in the deposition chamber without air exposure. The effects of annealing on the crystallinity and composition of the films were investigated. Acknowledgments This work was supported by Samsung Display Co., Ltd.

Influence of nitrogen gas flow ratio on gallium nitride film growth using high-density convergent plasma sputtering device at room temperature

The influence of the nitrogen gas ratio on the gallium nitride (GaN) film growth has been investigated using a high-density convergent plasma sputtering device at room temperature. To obtain a highly reactive environment for GaN film growth, the convergent magnetic field lines assist the high-density plasma transport near the target surface. An orthogonal array in the design of experiments was used to effectively investigate GaN film growth conditions. To optimize the GaN film growth conditions, we focus on four sputtering parameters: (1) the N2 gas flow ratio, (2) the inflowing ion current to the target, (3) the process gas pressure, and (4) the target-substrate distance. The N2 gas flow ratio mainly characterizes the growth condition of c-axis oriented wurtzite-structure GaN films on an unheated glass substrate. The full width at half maximum of the rocking curve for the GaN(0002) peak position decreases to 3.9° in the growth condition with 100% nitrogen at room temperature.

Effect of Gallium Nitride Film Growth Conditions on Surface Segregation

The micro-and nanomorphology and local composition of gallium nitride (GaN) films produced by four different procedures have been studied with the aim of detecting autosegregation phenomena. As a result, a complex autosegregation picture has been demonstrated and discussed. Independent of the growth procedure, all of the gallium nitride films have nonstoichiometric chemical compositions (gallium deficiency), with a degree of nonstoichiometry ranging from ~0.30 to <0.10. We discuss the segregation mechanism, which presumably involves predominant selective diffusion of nitrogen atoms to the surface.

Non-polar GaN film growth on (0 1 0) gallium oxide substrate by metal organic chemical vapor deposition

Abstract

Amorphous carbon buffer layers for separating free gallium nitride films

The possibility of using amorphous diamond-like carbon (DLC) films for self-separation of gallium nitride (GaN) layers grown by hydride vapor-phase epitaxy has been analyzed. DLC films have been synthesized by plasma-enhanced chemical vapor deposition under low pressure on sapphire (Al2O3) substrates with a (0001) crystallographic orientation. The samples have been studied by the methods of Raman scattering and X-ray diffraction analysis. It is shown that thin DLC films affect only slightly the processes of nucleation and growth of gallium nitride films. Notably, the strength of the “GaN film–Al2O3” substrate interface decreases, which facilitates separation of the GaN layers.

Pit assisted oxygen chemisorption on GaN surfaces.

A comprehensive analysis of oxygen chemisorption on epitaxial gallium nitride (GaN) films grown at different substrate temperatures via RF-molecular beam epitaxy was carried out. Photoemission (XPS and UPS) measurements were performed to investigate the nature of the surface oxide and corresponding changes in the electronic structure. It was observed that the growth of GaN films at lower temperatures leads to a lower amount of surface oxide and vice versa was observed for a higher temperature growth. The XPS core level (CL) and valence band maximum (VBM) positions shifted towards higher binding energies (BE) with oxide coverage and revealed a downward band bending. XPS valence band spectra were de-convoluted to understand the nature of the hybridization states. UPS analysis divulged higher values of electronic affinity and ionization energy for GaN films grown at a higher substrate temperature. The surface morphology and pit structure were probed via microscopic measurements (FESEM and AFM). FESEM and AFM analysis revealed that the film surface was covered with hexagonal pits, which played a significant role in oxygen chemisorption. The favourable energetics of the pits offered an ideal site for oxygen adsorption. Pit density and pit depth were observed to be important parameters that governed the surface oxide coverage. The contribution of surface oxide was increased with an increase in average pit density as well as pit depth.

Structural Inhomogeneities and Impurity Incorporation in Growth of High-Quality Ammonothermal GaN Substrates

Ammonothermal Gallium Nitride (GaN) substrates are the most promising substrates for homoepitaxial growth of GaN films having low dislocation density. Growth-induced structural inhomogeneities in these substrates were investigated by high-resolution X-ray topography (HR-XRT) and high-resolution X-ray diffraction (HR-XRD). A one-to-one correlation of defects was observed in photoluminescence imaging. From the HR-XRD intrinsic rocking curve widths were found to be 16 arcsecs indicating superior crystalline quality. The lattice constants were measured from symmetric and asymmetric reflections to be similar to bulk values indicting low sample strian. The true dislocation density, from the HR-XRT images, was observed to be of the order of 102 cm–2 in the samples, and the radius of curvature was greater than 600 m. Growth striations were observed in the a and m-pane samples and are attributed to inhomogeneity in impurity incorporation during growth. Photoluminescence imaging showed deep level luminescent centers along the growth striations.

Low-Temperature Growth of Crystalline Gallium Nitride Films Using Vibrational Excitation of Ammonia Molecules in Laser-Assisted Metalorganic Chemical Vapor Deposition

Low-temperature growth of crystalline gallium nitride (GaN) films on c-plane sapphire (α-Al2O3) substrates was achieved by laser-assisted metalorganic chemical vapor deposition (LMOCVD) and coupling laser energy into the chemical reactions. Trimethylgallium (TMGa) and ammonia (NH3) were used as precursors for the growth of GaN films. Through the resonant excitation of rotational–vibrational transition (1084.71 cm–1) of the NH-wagging mode (v2) in NH3 molecules using a wavelength-tunable CO2 laser tuned at 9.219 μm, highly c-axis oriented GaN films were deposited on sapphire at low substrate temperatures from 250 to 600 °C. GaN films with a large thickness of 12 μm were obtained within 1 h at a substrate temperature of 600 °C. The GaN films deposited by LMOCVD showed a higher degree of crystallinity, higher growth rate, and lower defect densities as compared to those synthesized by MOCVD without resonant excitation of NH3 molecules. This low-temperature synthesis technique opens a promising approach to gro...

Low-temperature ECR-PEMOCVD deposition of high-quality crystalline gallium nitride films: A comparative study of intermediate layers for growth on amorphous glass substrates

A low temperature growth method based on an electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system (ECR-PEMOCVD) was proposed for the growth of GaN (Gallium nitride) films on ordinary amorphous soda-lime glass substrates. To alleviate the large lattice mismatch between GaN film and glass substrate and improve the heat dissipation performance for potential optoelctrical device application, five intermediate layers (Cu, Ni, Ti, Ag, and ITO) were deposited on the glass substrate before the growth of GaN. A comparative study was performed through structural analysis of the as-grown GaN films with various intermediate layers investigated by means of in-situ reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), and atomic force microscopy (AFM). The results indicate that the Ti intermediate layer has a great advantage over other intermediate layers in view of crystalline quality and smooth surface, therefore is more suitable and preferred for the potential application in optoelectronic devices.

Low-temperature growth of gallium nitride films by inductively coupled-plasma-enhanced reactive magnetron sputtering

Gallium nitride (GaN) films were grown on sapphire substrate by reactive magnetron sputtering. Inductively coupled-plasma (ICP) source was installed between the substrate holder and the sputtering target to increase the plasma density and the degree of ionization of nitrogen gas. Liquid Ga and Ar/N2 were used as the sputtering target and sputtering gases, respectively. X-ray diffraction measurements confirmed that the authors could grow high quality GaN crystallites at 500 °C. However, the crystalline GaN (0002) peak remained even by lowering the growth temperature down to 300 °C. The N:Ga ratio of the film grown at 500 °C was almost 1:1, and the nitrogen composition became higher toward the 1:1 N:Ga ratio with increasing the growth temperature. The high degree of ionization induced by ICP source was essential to the growth of high crystalline quality GaN films.

Epitaxial growth of GaN by radical-enhanced metalorganic chemical vapor deposition (REMOCVD) in the downflow of a very high frequency (VHF) N2/H2 excited plasma – effect of TMG flow rate and VHF power

Gallium nitride (GaN) films have been grown by using our newly developed Radical-Enhanced Metalorganic Chemical Vapor Deposition (REMOCVD) system. This system has three features: (1) application of very high frequency (60MHz) power in order to increase the plasma density, (2) introduction of H2 gas together with N2 gas in the plasma discharge region to generate not only nitrogen radicals but also active NHx molecules, and (3) radical supply under remote plasma arrangement with suppression of charged ions and photons by employing a Faraday cage. Using this new system, we have studied the effect of the trimethylgallium (TMG) source flow rate and of the plasma generation power on the GaN crystal quality by using scanning electron microscopy (SEM) and double crystal X-ray diffraction (XRD). We found that this REMOCVD allowed the growth of epitaxial GaN films of the wurtzite structure of (0001) orientation on sapphire substrates with a high growth rate of 0.42μm/h at a low temperature of 800°C. The present REMOCVD is a promising method for GaN growth at relatively low temperature and without using costly ammonia gas.

Growth of high c-orientated crystalline GaN films on amorphous Cu/glass substrates with low-temperature ECR-PEMOCVD

A low temperature growth method based on electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system (ECR-PEMOCVD) was proposed for the growth of gallium nitride (GaN) films on ordinary soda-lime glass substrates with sputtered Cu as intermediate layer (Cu/glass substrates). The influence of deposition temperature on the properties of the GaN films on Cu/glass substrates was systematically investigated by means of In-situ reflection high energy electron diffraction, X-ray diffraction, atomic force microscopy and photoluminescence spectra. With this method, high c-orientated crystalline GaN films with relatively smooth surface were achieved on amorphous Cu/glass substrate at an extremely low temperature of ~400 °C. The successfully growth of crystalline GaN films on amorphous Cu/glass substrates show great potential for significant improvements in the scalability and cost of GaN based devices, since the adverse effects with high temperature process for glass substrates can be effectively suppressed by this technique.

Growth of GaN films on silicon (1 1 1) by thermal vapor deposition method: Optical functions and MSM UV photodetector applications

In this study, gallium nitride (GaN) films were grown on n-Si (111) substrate by thermal vapor deposition method in quartz tube furnace for different growth duration. Gallium metal mixed with GaN powder and aqueous ammonia (NH3) solutions were used as sources of Ga and N. Structural, elemental, and optical characterizations were carried out using various techniques in order to investigate the properties of the films. Scanning electron microscopy images showed that the films surface have self-textured morphology, which was introduced during the growth process. Moreover, further deposition resulted in the formation of heterogeneous film. X-ray diffraction (XRD) measurements reveal in all samples a typical diffraction pattern of hexagonal GaN wurtzite structure. Raman spectra demonstrated redshifts in E2-high with increasing deposition time due to tensile stress inside the GaN films, confirmed by XRD. The photoluminescence spectra of the films demonstrated strong near band edge emission at about 363nm. The fabricated GaN films based metal–semiconductor-metal (MSM) UV photodetector shows a contrast ratio of ∼240–40 at +5V and responsivity in the range of 0.28–0.01A/W for the UV photodetectors. This study shows the possibility of synthesizing GaN films on Si wafers at low-cost and has potential applications in UV photodetection.

Initial stages of the ion-beam assisted epitaxial GaN film growth on 6H-SiC(0001)

Ultra-thin gallium nitride (GaN) films were deposited using the ion-beam assisted molecular-beam epitaxy technique. The influence of the nitrogen ion to gallium atom flux ratio (I/A ratio) during the early stages of GaN nucleation and thin film growth directly, without a buffer layer on super-polished 6H-SiC(0001) substrates was studied. The deposition process was performed at a constant substrate temperature of 700°C by evaporation of Ga and irradiation with hyperthermal nitrogen ions from a constricted glow-discharge ion source. The hyperthermal nitrogen ion flux was kept constant and the kinetic energy of the ions did not exceed 25eV. The selection of different I/A ratios in the range from 0.8 to 3.2 was done by varying the Ga deposition rate between 5×1013 and 2×1014at.cm−2s−1. The crystalline surface structure during the GaN growth was monitored in situ by reflection high-energy electron diffraction. The surface topography of the films as well as the morphology of separated GaN islands on the substrate surface was examined after film growth using a scanning tunneling microscope without interruption of ultra-high vacuum. The results show, that the I/A ratio has a major impact on the properties of the resulting ultra-thin GaN films. The growth mode, the surface roughness, the degree of GaN coverage of the substrate and the polytype mixture depend notably on the I/A ratio.

Gas phase optical emission spectroscopy during remote plasma chemical vapour deposition of GaN and relation to the growth dynamics

A remote plasma chemical vapour deposition (RPCVD) system for the growth of gallium nitride (GaN) thin films is investigated using optical emission spectroscopy (OES). The intensities of the various excited species in pure nitrogen as well as nitrogen/hydrogen plasmas are correlated with GaN film growth characteristics. We show a correlation between the plasma source spectrum, the downstream spectrum where trimethylgallium is introduced and the GaN film quality. In particular, we investigate the addition of hydrogen, which greatly affects the gas phase species and the GaN film characteristics. OES is demonstrated to be a valuable monitoring tool in a RPCVD system for optimization of GaN growth.

Effect of the nitrogen ion energy on the MBE growth of thin gallium nitride films

The influence of the energy of bombarding nitrogen ions on the growth of thin gallium nitride (GaN) films under molecular beam epitaxy (MBE) conditions has been simulated using the method of balance kinetic equations. The dependence of the GaN film growth rate on the ion energy is determined and changes in the structure of films grown at different ion energies are explained. Theoretical estimates satisfactorily agree with the available experimental data.

The growth of GaN films by alternate source gas supply hot-mesh CVD method

Gallium nitride (GaN) films and Aluminium nitride (AlN) layers were deposited on SiC/Si (111) substrates by an alternating source gas supply or an intermittent supply of a source gas such as ammonia (NH 3), trimethylgallium (TMG) or trimethylaluminum (TMA) in a hot-mesh chemical vapor deposition (CVD) apparatus. The AlN layer was deposited as a buffer layer using NH 3 and TMA on a SiC layer grown by carbonization on Si substrates using propane (C 3H 8). GaN films were grown on an AlN layer by a reaction between NH x radicals generated on a ruthenium (Ru) coated tungsten (W)-mesh and TMG molecules. An alternating source gas supply or an intermittent supply of one of the source gases during the film growth are expected to be effective for the suppression of gas phase reactions and for the enhancement of precursor migration on the substrate surface. By the intermittent supply of alkylmetal gas only during the growth of the AlN layer, the defect generation in the GaN films was reduced. GaN film growth by intermittent supply on an AlN buffer layer, however, did not lead to the improvement of the film quality.

BARRIER HEIGHT ENHANCED GaN SCHOTTKY DIODES USING A THIN AlN SURFACE LAYER

Gallium nitride (GaN) is a highly promising wide band gap semiconductor with applications in high power electronic and optoelectronic devices. Thin films of GaN are most commonly grown in the hexagonal wurtzite structure on sapphire substrates. Growth of GaN onto silicon substrates offers a very attractive opportunity to incorporate GaN devices onto silicon-based integrated circuits. Although direct epitaxial growth of GaN films on Si substrates is a difficult task (mainly due to the 17% lattice mismatch present), substantial progress in the crystal quality can be achieved using a buffer layer. A full characterization of the quality of the material needs to be assessed by a combination of different techniques. In this work, a thin AlN cap layer of 50 nm was incorporated in GaN Schottky diode to enhance the effective Schottky barrier height and reduces the dark current. A barrier height of 0.52 eV for normal GaN Schottky diode was increased to the effective barrier height of 0.63 eV. The resulting Schottky diodes show a dark current of as low as 6.3×10-5 A at 5 V bias, which is about two orders of magnitude lower than that of normal GaN (5.2×10-3 A at 5 V bias) Schottky diode.

Epitaxial Growth of Hexagonal GaN Films on SiC/Si Substrates by Hot-Mesh CVD Method

Gallium nitride (GaN) films were grown on SiC/Si(111) substrates by hot-mesh chemical vapor deposition (CVD) using trimethylgallium (TMG) and ammonia (NH3). A SiC buffer layer was formed by carbonization on the Si(111) substrates using propane (C3H8). GaN epitaxial films were grown on the SiC layer by the reaction between NHx radicals generated on a tungsten hot-mesh surface and TMG molecules. From the X-ray diffraction pattern, the GaN epitaxial films grown by the two- or three-step growth technique at the substrate temperatures of 600°C and 800°C to 1000°C and the hot-mesh temperature of 1200°C showed good crystallinity. Photoluminescence spectra of GaN films grown by the three step growth technique showed a strong near-band-edge emission and a weak yellow luminescence.

Growth and Properties of GaN/Si Heterojunction

Gallium nitride (GaN) is a highly promising wide band gap semiconductor with applications in high power electronic and optoelectronic devices. Thin films of GaN are most commonly grown in the hexagonal wurtzite structure on sapphire substrates. Growth of GaN onto silicon substrates offers a very attractive opportunity to incorporate GaN devices onto silicon based integrated circuits. Although direct epitaxial growth of GaN films on Si substrates is a difficult task (mainly due to the 17% lattice mismatch present), substantial progress in the crystal quality can be achieved using a buffer layer. A full characterization of the quality of the material needs to be assessed by a combination of different techniques. In this work, a detailed characterization study of GaN thin film grown on Si(111) with AlN buffer layer by low pressure metalorganic chemical vapor deposition (LP-MOCVD) was carried out. Post deposition analysis includes scanning electron microscopy (SEM), x-ray diffraction (XRD), Hall and infrared (IR) spectroscopy techniques. The IR spectra were compared to the calculated spectra generated with a damped single harmonic oscillator model. Through this method, a complete set of reststrahlen parameters (such as ε∞, S, wTO, γ) of the GaN epilayer were obtained. Our results show that the GaN film has a single crystalline structure. Current-voltage characteristics (I-V) of this GaN/Si heterojunction were measured at room temperature. Rectification behavior was observed for this anisotype heterojunction. The electrical characteristics of Ni Schottky barriers on this unintentionally doped n-type film were also investigated. The barrier height of Ni/GaN Schottky barriers has been determined to be 0.93 eV by I-V measurement.