Gallium Nitride Films Research Articles

Measurement of sheet resistance of GaN films on a dielectric substrate

The sheet resistance of gallium nitride films deposited with metal organic chemical vapor deposition on sapphire is investigated in this paper. Two types of contactless measurement techniques are used for that purpose. The first one is a microwave dielectric resonator technique, widely applicable to the precise electrical characterization of resistive films. Due to a small exposure area, the profile of sheet resistance can be acquired across the whole semiconductor film. Another technique is based on a frequency domain capacitive approach, where the film under test functions as a resistive electrode. The method is supplemented with full-wave electromagnetic simulations as well as equivalent circuit representation. The capacitive method is sensitive mostly to the out-of-plane resistance of the layer. Consequently, the combination of both methods provides the tools for the comprehensive and quantitative evaluation of the quality of the semiconductor film.

Effect of thermal annealing in a-InxGa1–xN films prepared by reactive RF-sputtering

We deposited amorphous indium gallium nitride (a-InxGa1–xN) films at room temperature by reactive radio frequency sputtering with GaN and InN targets and investigated the change of their properties from thermal annealing at annealing temperatures, Ta, below 200 °C. A large change in the indium and nitrogen composition ratios was not observed by thermal annealing at a Ta below 200 °C. In the X-ray diffraction patterns of the films annealed at a Ta below 200 °C, no perceivable peaks assigned to crystalline InxGa1–xN were found. The photoconductivty, σp, increased with an increase in Ta. On the other hand, the increase of the dark conductivity, σd, was very small with an increase in Ta below 200 °C. As a result, the photosensitivity, σp/σd, increased from 252 to 2500 by thermal annealing at a Ta of 200 °C.

Properties of GaN-Related Epitaxial Thin Films Grown on Sapphire Substrates as Transparent Conducting Electrodes

Thin films of gallium nitride (GaN) and related nitride materials were prepared, and their properties as transparent conducting electrodes were investigated. GaN thin films were directly grown on sapphire single crystal substrates by the molecular beam epitaxy. Heavy doping of germanium was employed to reduce resistivity of the films, with sufficient reduction found to be possible while maintaining their epitaxial growth state. Optical transmission spectra of the films in the short wavelength region were slightly deteriorated by the heavy doping; however, this was successfully improved by growing GaN films under metal-rich conditions to increase the electron mobility and suppress unwanted increase of the carrier densities. In addition, the optical transmission spectra in the short wavelength region was improved also by alloying GaN with aluminum nitride, though the resistivities of these films were relatively higher than those of the unmodified GaN films. The prepared nitride thin films exhibited sufficiently suitable properties as transparent conducting electrodes for use in applications such as full-spectrum nitride-based solar cells.

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.

The magnetic field effect on optical properties of Sm-doped GaN thin films

We report the optical and magnetic properties of Sm doped wide-band-gap Gallium nitride (GaN) prepared by ion implantation. The photoluminescence (PL) intensity obviously increased as the increasing of annealing temperature from 700 to 1,100 °C. Magnetic hysteresis was observed at room temperature. Our study was focusing on the influence of magnetic field on optical property of Sm3+ doped GaN. It was discovered that the PL intensity of Sm3+ increases remarkably under an external magnetic field. The mechanism of the magnetic ordering-modulated PL peak intensity of Sm3+ doped GaN films was discussed. This may open up a suite of potential applications in producing magneto-optical 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.

Optical characteristics of nanocrystalline AlxGa1−xN thin films deposited by hollow cathode plasma-assisted atomic layer deposition

Gallium nitride (GaN), aluminum nitride (AlN), and AlxGa1−xN films have been deposited by hollow cathode plasma-assisted atomic layer deposition at 200 °C on c-plane sapphire and Si substrates. The dependence of film structure, absorption edge, and refractive index on postdeposition annealing were examined by x-ray diffraction, spectrophotometry, and spectroscopic ellipsometry measurements, respectively. Well-adhered, uniform, and polycrystalline wurtzite (hexagonal) GaN, AlN, and AlxGa1−xN films were prepared at low deposition temperature. As revealed by the x-ray diffraction analyses, crystallite sizes of the films were between 11.7 and 25.2 nm. The crystallite size of as-deposited GaN film increased from 11.7 to 12.1 and 14.4 nm when the annealing duration increased from 30 min to 2 h (800 °C). For all films, the average optical transmission was ∼85% in the visible (VIS) and near infrared spectrum. The refractive indices of AlN and AlxGa1−xN were lower compared to GaN thin films. The refractive index of as-deposited films decreased from 2.33 to 2.02 (λ = 550 nm) with the increased Al content x (0 ≤ x ≤ 1), while the extinction coefficients (k) were approximately zero in the VIS spectrum (>400 nm). Postdeposition annealing at 900 °C for 2 h considerably lowered the refractive index value of GaN films (2.33–1.92), indicating a significant phase change. The optical bandgap of as-deposited GaN film was found to be 3.95 eV, and it decreased to 3.90 eV for films annealed at 800 °C for 30 min and 2 h. On the other hand, this value increased to 4.1 eV for GaN films annealed at 900 °C for 2 h. This might be caused by Ga2O3 formation and following phase change. The optical bandgap value of as-deposited AlxGa1−xN films decreased from 5.75 to 5.25 eV when the x values decreased from 1 to 0.68. Furthermore, postdeposition annealing did not affect the bandgap of Al-rich films.

Investigation on the structural properties of GaN films grown on La0.3Sr1.7AlTaO6 substrates

Gallium nitride (GaN) films with excellent structural, electrical and optical properties have been epitaxially grown on La0.3Sr1.7AlTaO6 (LSAT) (111) substrates by radio-frequency molecular beam epitaxy at low temperature. The GaN films grown at 500 °C exhibits high crystalline quality with the (0002) and (10−12) full width at half maximum of 0.056° and 0.071°. There is a maximum of 1.1-nm-thick interfacial layer existing between the as-grown GaN and LSAT (111) substrate, and the as-grown about 300-nm-thick GaN films are almost fully relaxed only with a 0.0094% in-plane tensile strain. Hall and photoluminescence (PL) measurements also reveal outstanding electrical and optical properties of the as-grown GaN films on LSAT. This achievement brings the prospect for achieving highly-efficient GaN-based optoelectronic devices on LSAT (111) substrates.

Stress and Grain Boundary Properties of GaN Films Prepared by Pulsed Laser Deposition Technique

Polycrystalline gallium nitride films were successfully deposited on fused silica substrates by ablating a GaN target using pulsed Nd-YAG laser. Microstructural studies indicated an increase in the average crystallite size from ~8 nm to ~70 nm with the increase in substrate temperature from 300 K to 873 K during deposition. The films deposited here were nearly stoichiometric. XPS studies indicated two strong peaks located at ~1116.6 eV and ~395 eV for Ga2p3/2 and a N1s core-level peak, respectively. The films deposited at substrate temperature above 573 K are predominantly zinc blende in nature. PL spectra of the films deposited at higher temperatures were dominated by a strong peak at ~3.2 eV. FTIR spectra indicated a strong and broad absorption peak centered ~520 cm−1 with two shoulders at ~570 cm−1 and 584 cm−1. Characteristic Raman peak at ~531 cm−1 for the A1(TO) mode is observed for all the films. Grain boundary trap states varied between 3.1×1015 and 7×1015 m−2, while barrier height at the grain boundaries varied between 12.4 meV and 37.14 meV. Stress in the films decreased with the increase in substrate temperature.

In Situ Chemical Functionalization of Gallium Nitride with Phosphonic Acid Derivatives during Etching

In situ functionalization of polar (c plane) and nonpolar (a plane) gallium nitride (GaN) was performed by adding (3-bromopropyl) phosphonic acid or propyl phosphonic acid to a phosphoric acid etch. The target was to modulate the emission properties and oxide formation of GaN, which was explored through surface characterization with atomic force microscopy, X-ray photoelectron spectroscopy, photoluminescence (PL), inductively coupled plasma-mass spectrometry, and water contact angle. The use of (3-bromopropyl) phosphonic acid and propyl phosphonic acid in phosphoric acid demonstrated lower amounts of gallium oxide formation and greater hydrophobicity for both sample sets, while also improving PL emission of polar GaN samples. In addition to crystal orientation, growth-related factors such as defect density in bulk GaN versus thin GaN films residing on sapphire substrates were investigated as well as their responses to in situ functionalization. Thin nonpolar GaN layers were the most sensitive to etching treatments due in part to higher defect densities (stacking faults and threading dislocations), which accounts for large surface depressions. High-quality GaN (both free-standing bulk polar and bulk nonpolar) demonstrated increased sensitivity to oxide formation. Room-temperature PL stands out as an excellent technique to identify nonradiative recombination as observed in the spectra of heteroepitaxially grown GaN samples. The chemical methods applied to tune optical and physical properties of GaN provide a quantitative framework for future novel chemical and biochemical sensor development.

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.

Effects of Substrate Temperature on Properties of Amorphous InxGa1-xN Films Deposited by Reactive Radio Frequency Sputtering

Amorphous indium gallium nitride (a-InxGa1-xN) films were deposited at various substrate temperatures TS by simultaneous reactive rf-magnetron sputtering using GaN and InN targets. The nitrogen composition ratio increased with an increase in TS, and became almost the same as the stoichiometry ratio at a TS above 100 °C. In the X-ray diffraction patterns of the films deposited at a TS below 200 °C, no perceivable peaks assigned to crystalline InxGa1-xN were found. In those deposited at a TS above 300 °C, however, small broad peaks assigned to crystalline InxGa1-xN were observed. The dark conductivity σd increased with an increase in TS. On the other hand, the photoconductivty σp increased with an increase in TS below 200 °C and a large change in σp was not obtained at a TS above 300 °C. The photosensitivity σp/σd showed its maximum (1175) at a TS of 200 °C.

Electronic and optical characteristics of thin hexagonal gallium nitride films specified by surfaces

A computational procedure using density functional theory and generalized to the case of periodic finite-layered structures in the basis of valence orbitals with the inclusion of polarization functions is applied to calculation of the energy spectrum of electrons and curves for the optical absorption of thin hexagonal gallium-nitride films. It is shown that, to stabilize the atomically pure surface, it is necessary to place additional atoms onto it (those of gallium on one side of the film and those of nitrogen on the other side in the ratio: one additional atom to four surface ones, three of which form chemical bonds with the additional atom). In this case, the calculated curve for the optical absorption is close to the experimental one. It is also shown that the attachment of additional atoms to the atomically pure surface enhances its capability of emitting electrons.

Mechanical Properties of Nanoporous GaN and Its Application for Separation and Transfer of GaN Thin Films

Nanoporous (NP) gallium nitride (GaN) as a new class of GaN material has many interesting properties that the conventional GaN material does not have. In this paper, we focus on the mechanical properties of NP GaN, and the detailed physical mechanism of porous GaN in the application of liftoff. A decrease in elastic modulus and hardness was identified in NP GaN compared to the conventional GaN film. The promising application of NP GaN as release layers in the mechanical liftoff of GaN thin films and devices was systematically studied. A phase diagram was generated to correlate the initial NP GaN profiles with the as-overgrown morphologies of the NP structures. The fracture toughness of the NP GaN release layer was studied in terms of the voided-space-ratio. It is shown that the transformed morphologies and fracture toughness of the NP GaN layer after overgrowth strongly depends on the initial porosity of NP GaN templates. The mechanical separation and transfer of a GaN film over a 2 in. wafer was demonstrated, which proves that this technique is useful in practical applications.

Plasma assisted molecular beam epitaxy of GaN with growth rates >2.6 µm/h

Plasma-assisted molecular beam epitaxial (PAMBE) growth of gallium nitride (GaN) was explored with a novel modification of a commercially available nitrogen plasma source. The modified nitrogen plasma source enabled a dramatic increase in the flux of active nitrogen and thus a significantly higher growth rate than has been previously reported. GaN films were grown using N2 gas flow rates between 1 and 8sccm while varying the plasma source's RF forward power from 200 to 600W. The highest growth rate, and therefore the highest active nitrogen flux achieved was ~2.65μm/h. For optimized growth conditions the surfaces displayed a clear step-terrace structure with an average RMS roughness (3µm×3µm) on the order of 1nm. Secondary ion mass spectroscopy (SIMS) impurity analysis demonstrates oxygen and hydrogen incorporation of 1×1016 and 5×1016 respectively, comparable to the metal organic chemical vapor deposition (MOCVD) grown template layer. Initial un-optimized electron mobility measurements of 1µm thick GaN layers have shown a peak mobility of ~705cm2/Vs for an electron concentration of ~3.5×1016cm−3. A revised universal growth diagram is proposed allowing the rapid determination of the metal flux needed to grow in a specific growth regime for any and all active nitrogen fluxes available.

Deposition and characteristics of GaN films on Ni metal substrate by ECR-PEMOCVD

Gallium nitride (GaN) films were deposited on Ni metal substrate using electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system. With this approach, highly c-oriented GaN films with smooth surface were obtained at an extremely low temperature of ~480 °C. The trimethyl gallium (TMGa) flux dependent structural, morphological, and optical characteristics of GaN films were investigated by X-ray diffraction analysis, reflection high energy electron diffraction, atomic force microscopy and photoluminescence analysis. The results indicate that it is feasible to deposit GaN films on Ni metal substrate under the proper deposition procedures. The high quality GaN films with high c-axis orientation and strong ultraviolet emission peak are successfully achieved under the optimized TMGa flux of 1.2 sccm. The GaN/Ni structure has great potential for the development of high power devices with excellent heat dissipation.

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.

Synthesis of l-cystine nanotubes by alkalinization of l-cysteine in the presence of gallium nitride

The alkaline oxidation of l-cysteine (l-Cys) was conducted in aqueous media forming the insoluble l-cystine dimer as confirmed by X-ray diffraction. In another experiment, a gallium nitride film was added to the l-Cys solution and the l-cystine crystals presented a preferential growth. The transmission electron microscopy images showed the formation of single- and multi-wall l-cystine nanotubes. The infrared and high resolution electron microscopy data suggested that the growth of the tubes resulted from the successive staking of l-cystine dimers onto the (001) family of planes, which was a consequence of the ionic assembly between l-cystine over a layer of l-Cys molecules previously grafted to the gallium nitride particles.

Effect of the Conduction Type of Si (111) Substrates on the Performance of GaN MQW LED Epitaxial Films

The present paper prepared a structural epitaxial film of gallium nitride multiple-quantum-well (GaN MQW) blue light-emitting diode (LED) on Si (111) substrates with different conduction types using the metal-organic chemical vapor deposition method. The method prevented the interdiffusion of GaN and Si to achieve high-quality film growth by introducing aluminium nitride (AlN)-interposed layer and rich-Gallium GaN layer with low V/III ratio double buffer layers. Surface analysis shows that the GaN LED epitaxial film on the Si (111) substrate with different conduction types presented an entirely different appearance. The surface roughness of all the samples was less than 3 nm. A much smoother surface of the epitaxial film on the N-type substrate had less roughness, whereas a layered stack surface of the epitaxial film on the P-type substrate had larger surface roughness. The full width at half maximum of the XRD Omega rocking curve with (002) and (102) planes of GaN film grown on the N-type substrate was less than that of the GaN film on the P-type substrate. Furthermore, the film was superior to the samples on the P-type substrate in terms of crystal quality. There was little difference in the peak position of the PL of the epitaxial film on the N-type substrate, but the peak position of the PL of the epitaxial film on the P-type substrate was long and had a large half-peak width. The tensile stress of the GaN film on the P-type substrate was higher. The above results show that the N-type Si (111) substrate with high resistivity is more suitable for the growth of GaN MQW LED epitaxial film.