GaN Decomposition Research Articles

WSiN cap for Ohmic contacts to n‐GaN with better morphology

A WSiN cap can improve the surface morphology of Ti/Al Ohmic contact to n-GaN significantly. This is because it is a conductive diffusion barrier which can help to suppress the decomposition of GaN. With WSiN cap, the edge is much sharper. However, the WSiN film may pick up Ar during the WSiN sputtering process, resulting in some surface morphology problem. This can be solved by lowering the Ar flow rate during sputtering. Auger electron spectroscopy results show that our best WSiN cap has a chemical composition of W0.62Si0.14N0.34. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Metal/GaN reaction chemistry and their electrical properties

AbstractWe investigated the reaction chemistry of metal contacts to GaN during annealing using X‐ray photoelectron spectroscopy (XPS). GaN decomposition was estimated, using XPS, to occur in N2 annealed Ni‐alloy contacts at 550 °C. The reaction was greatly accelerated by the catalytic effect of Au and Pt. The decomposition was correlated with the rapid degradation of electrical properties during annealing. The results suggest that high‐temperature applications may be critically limited by the degradation of metal contacts especially due to activated Ni reactivity in Ni‐alloyed contacts. Meanwhile, the thermal stability of Ni/Au contact greatly improves by suppressing the activated Ni reactivity, which is able to be obtained by forming preferential Ni–O bonding through annealing in air. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

CHARACTERIZATIONS OF LOW-FREQUENCY NOISE IN LASER-DEBONDED HVPE-GROWN GaN THIN FILMS

Low-frequency excess noise was measured on laser-debonded HVPE-grown GaN films. While the Schottky MSM devices, fabricated on 5 μm-thick films, demonstrated reduction in the noise level, the ohmic MSM devices showed significant increase in the low-frequency noise. To account for the data we conducted two-dimensional simulation of the temperature of the GaN layer as a function of time and distance from the GaN/sapphire interface. The simulation results show that the temperature at the GaN/sapphire interface may rise up to 1400 K, whereas the maximum temperature at the GaN surface was about 400 K at the. Based on the experimental data and the simulation results, it is postulated that the illumination of the GaN sample by high-power excimer laser led to the decomposition of GaN at the GaN/sapphire interface resulting in the generation of localized states at the interface. The same process may have led to some thermal annealing effect at the GaN surface. To provide experimental proof of the hypothesis, detailed low-frequency noise measurement on ohmic MSM devices fabricated on 20 μm-thick GaN films were performed. The results indicate similar noise level for both the debonded and the control devices.

GaN films and GaN‐based light emitting diodes grown on the sapphire substrates with high‐density nano‐craters formed in situ metalorganic vapor phase epitaxial reactor

A novel process has been developed to grow GaN films and GaN-based light emitting diodes on the sapphire substrates with high-density nano-craters formed on the surface of the substartes in situ metalorganic vapor phase epitaxial (MOCVD) reactor. The whole process is a single and integrated MOCVD run. Firstly a thin GaN layer is grown on the sapphire substrate, and then it is etched away by only flowing the H2 into the reactor at the higher temperature. It is found, amazingly, that the thermal decomposition of GaN can induce the chemical etching of sapphire, and result in the formation of high-density nano-craters on the surface of the substrates. Finally the device-quality GaN films could be grown on the etched sapphire substrate with the residual gallium droplets as nucleation sites. A preliminary result shows that the output power of GaN light-emitting diodes built on the in situ etched sapphire substrates could be increased by 30%. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation chemistry of high-density nanocraters on the surface of sapphire substrates with an in situ etching and growth mechanism of device-quality GaN films on the etched substrates

An efficient method has been investigated to grow GaN films with sapphire substrates being treated in situ metalorganic chemical vapor deposition reactor for a special effect rather than simple thermal cleaning. First, a thin GaN layer is grown on the sapphire substrate. And then it is almost etched away by thermal decomposition. It is found that the decomposition of GaN induces the decomposition of sapphire resulting in the formation of high-density nanocraters on its surface. Finally the device-quality GaN film is regrown on the etched substrate with residual gallium droplets as nucleation sites. The chemistry of the etching process and the mechanism of the final GaN growth process have been discussed. The distinct feature of this method is the in situ formation of high-density nanocraters on the surface of the substrate. A rough interface between the substrate and GaN can improve the efficiency of the light-emitting diode built on it greatly.

Characterization of MOVPE grown AlN/GaN heterointerfaces by grazing incidence X‐ray reflectivity. Alloy formation at heterointerfaces

AlN/GaN heterostructures were grown by MOVPE and their heterointerfaces were characterized by grazing incidence X-ray reflectivity. The results revealed that alloy formation occurs at heterointerfaces even when conventional growth sequences and conditions are used. A lower growth temperature and a higher growth rate relieve the degree of alloy formation to some extent. A high density of pits was observed by atomic force microscopy on surfaces of AlN/GaN heterostructures with a large degree of alloy formation. These pits correspond to dislocation terminations and GaN decomposition followed by the incorporation of Ga atoms into the AlN layer probably occurs. Therefore, it is concluded that GaN decomposition during AlN growth is the primary cause of alloy formation at AlN/GaN heterointerfaces grown by MOVPE. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Excimer-Laser-Induced Oxidation of GaN Epilayer

The 248 nm excimer-laser-induced oxidation of GaN epilayers with assisting oxygen was investigated. The mechanism of laser-induced oxide formation was discussed. It was revealed that the laser-induced oxidation reaction of GaN occurred at 250 mJ/cm2 which was much lower than the threshold fluence for GaN decomposition or damage (above 600 mJ/cm2). It was postulated that laser light excitation of electron–hole pairs was the main mechanism responsible for the enhancement of the oxidation reaction of GaN. Using glancing-angle X-ray diffraction, the induced oxide was determined to be monoclinic β-Ga2O3. It was also found that with an increase in laser pulse number, the oxygen concentration in the oxide film increased to a value corresponding to Ga2O3. The morphology of the laser-induced oxide surface was smooth and uniform. The process has the potential for being applied in the fabrication of GaN-based electronic and optoelectronic devices.

GaN based laser diodes – epitaxial growth and device fabrication

Four selected material issues of group-III nitride layer structures grown by metalorganic vapor phase epitaxy and molecular beam epitaxy on basal plane sapphire are reviewed. 1) The decomposition of GaN under the impact of hydrogen gas was measured and described thermodynamically. 2) The nucleation and coalescence of GaN islands on a low-temperature nucleation layer was identified to be the key process for the formation of edge-type threading dislocations and heterogeneous stress. 3) The doping with magnesium was associated with pyramidal defects which formed upon a critical layer thickness and led to self-compensation of the acceptors. 4) The relaxation of plain tensile stress via cracks in bulk layers and superlattices based on AlGaN was investigated. The described studies allowed for a material optimization which finally led to the successful demonstration of a GaN based laser diode under pulsed current injection. The operation of the device is discussed in terms of the perfection of the crystallographically wet etched mirror facets and the lateral current spreading in p-type AlGaN cladding layers. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Freestanding GaN‐substrates and devices

Various physical aspects and potential applications of the laser-induced separation of GaN epilayers from their sapphire substrate are reviewed. The effect of short laser pulses on the thermal decomposition of GaN and possible applications of the laser-induced dissociation of GaN for fast etching of this material is discussed. Particular emphasis is placed on the defect-free delamination of large area GaN films with thicknesses ranging from 3 to 300 μm from sapphire substrates. The use of the resulting freestanding GaN films in device technology and homoepitaxy of III-nitrides are outlined. Specific examples are the flip-chip bonding of freestanding InGaN/GaN LEDs to a silicon submount and the production of pseudosubstrates for the homoepitaxy of high quality GaN epilayers. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermodynamics and Phase Stability in the Ga–N System

The decomposition of GaN powder was studied experimentally using two different customized thermogravimetric methods, dynamic oscillation TGA, and isothermal stepping TGA for a higher resolution of the decomposition start. A reproducible mass gain at slightly lower temperature suggests the equilibrium temperature to be at 1110±10 K under 1 bar of nitrogen. A CALPHAD type thermodynamic analysis of all available phase equilibrium and thermodynamic data is performed. This includes the determination of the absolute entropy of GaN, 30±4 J/mol K, based on a Debye- and Einstein-analysis of the experimental data on the heat capacity. An explicit equation for the fugacity–pressure relation of nitrogen, f( P), is developed, which is useful for the conversion of complex phase diagram calculation output. This is crucial because f can be several orders of magnitude higher than P for the high pressures encountered during GaN decomposition. Based on the consistent thermodynamic description, developed for the Ga–N system, all thermodynamic data and various phase diagrams are calculated. They indicate a good overall agreement between the different types of experimental data (calorimetric, vapor pressure, phase equilibrium). The high pressure part of the decomposition pressure of GaN is actually predicted from the thermodynamic model in good agreement with the experimental data.

Watching GaN Nanowires Grow

We report real-time high temperature transmission electron microscopy observations of the growth of GaN nanowires via a self-catalytic vapor−liquid−solid (VLS) mechanism. High temperature thermal decomposition of GaN in a vacuum yields nanoscale Ga liquid droplets and gallium/nitrogen vapor species for the subsequent GaN nanowire nucleation and growth. This is the first direct observation of self-catalytic growth of nanowires via the VLS mechanism and suggests new strategies for synthesizing electronically pure single-crystalline semiconductor nanowires.

Rutherford backscattering analysis of GaN decomposition

The decomposition of GaN at temperatures ranging from 500 °C to 1100 °C has been studied by Rutherford backscattering (RBS), x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The development of a surface defect peak is a consequence of preferential N2 loss at elevated temperatures. Additionally, broadening of the defect peak at 1100 °C, corresponding to a damage depth of approximately 0.25 μm beneath the surface, can be attributed to the diffusion of defects from the interface. At such temperatures, severe roughening of the surface is observed through AFM scans, which also correlated well with the damage depths estimated from RBS spectra. Nevertheless, Ga droplet formation is not detected from our samples as verified by XPS. Our results show that GaN remains thermally stable in N2 up to 900 °C. At higher temperatures, significant decomposition occurs and gives rise to degradation to the structural and morphological properties of the film.

In situ measurements of GaN nucleation layer decompostion

GaN nucleation layer (NL) decomposition was measured using optical reflectance over a wide range of pressure P, temperature T, and H2/NH3 gas mixture. The GaN NLs show measurable decomposition above 800 °C and do not significantly roughen until above 960 °C. The NL decomposition rates increase with increasing P, increasing T, and decreasing NH3 flow. An activation energy EA of 2.68 eV was measured (from 820 to 960 °C) for NL decomposition and an EA of 2.62 eV was measured (from 900 to 1075 °C) for decomposition of thick, high-T bulk GaN films. Depending on P, the pre-exponential factor A0 was four to nine times larger for NL decomposition compared to bulk GaN decomposition. The EA measured for both NL and bulk GaN decomposition in mixed H2 and NH3 flows is similar to the EA for Ga desorption, suggesting that the rate-limiting step for both NL and bulk GaN decomposition is Ga desorption.

Crystal growth of gallium nitride and manganese nitride using an high pressure thermal gradient process

Standard, semiconductor-industry bulk crystal growth processes are virtually impossible for the production of GaN as this is prohibited by both the high melt temperature of GaN and thermal decomposition of the compound into Ga metal and diatomic nitrogen gas. In this study, a novel hydrostatic pressure system was employed to grow GaN crystals in a very high pressure ambient. The ultra-high pressure, high temperature process uses a solid-phase nitrogen source to form GaN crystals in a metal alloy melt. Using a thermal gradient diffusion process, in which nitrogen dissolves in the high temperature region of the metal melt and diffuses to the lower temperature, lower solubility region, high quality crystals up to ∼0.5 mm in size were formed, as determined by scanning electron microscopy, X-ray diffraction, and micro-Raman analysis.

Surface polarity dependence of decomposition and growth of GaN studied using in situ gravimetric monitoring

The influence of lattice polarity of wurzite GaN(0 0 0 1) on its decomposition rate was investigated using an in situ gravimetric monitoring (GM) method with freestanding GaN(0 0 0 1) substrates. At temperatures between 800°C and 850°C, the decomposition rate of GaN(0 0 0 1) was faster than that of GaN( 0 0 0 1 ̄ ). On the other hand, the decomposition rate of GaN( 0 0 0 1 ̄ ) was faster than that of GaN(0 0 0 1) at temperatures between 900°C and 950°C. The relation between the decomposition rate and the H 2 partial pressure ( P H 2 ) indicates that the rate-limiting reactions are N(surface)+ 3 2 H 2(g)→NH 3(g) at lower temperatures, but Ga(surface)+ 1 2 H 2(g)→GaH(g) at higher temperatures. In addition, we used the GM method to measure the growth rate of GaN(0 0 0 1) on freestanding GaN(0 0 0 1) substrates. In the low-temperature region, GaN( 0 0 0 1 ̄ ) grew faster than GaN(0 0 0 1), whereas GaN(0 0 0 1) grew faster than GaN( 0 0 0 1 ̄ ) in the high-temperature region.

Surface analysis of GaN decomposition

The decomposition of GaN at a range of temperatures has been studied by Rutherford backscattering (RBS), x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The development of a surface defect peak is a consequence of preferential N2 loss at elevated temperatures. Additionally, the emergence of a direct scattering peak approximately 0.25 µm beneath the surface at 1100 °C can be attributed to the buildup of extended defects. At such temperatures severe roughening of the surface is observed through AFM scans. Nevertheless, Ga droplet formation is not detected from our samples as verified by XPS. Our results show that GaN remains thermally stable in N2 up to 900 °C. At higher temperatures, significant decomposition occurs and gives rise to degradation of the structural and morphological properties of the film.

Conditions for seeded growth of GaN crystals by the Na flux method

Conditions for seeded growth of GaN crystals by the Na flux method were investigated. Seeded growth in the Na flux could be achieved under temperature–N 2 pressure conditions between the condition of GaN formation without seeding and the condition of GaN decomposition. GaN single crystals with a maximum area of 3.0×1.5 mm 2 and thickness of 1.0 mm were grown from seeds at 850 °C and 2 MPa of N 2 for 200 h. The crystal growth occurred selectively on the (0001) Ga-polar surface of the platelet seed crystals. The growth rate in the c direction was higher than those of other directions and increased with growth temperature and N 2 pressure. The maximum growth rate in the c direction was about 4 μm/h at 850 °C and 2 MPa of N 2.

Growth and decomposition of bulk GaN: role of the ammonia/nitrogen ratio

Gallium nitride crystals grown via vapor-phase transport processes that incorporate ammonia as the only source of nitrogen below atmospheric pressures experience significant surface roughening and the eventual cessation of growth. Investigations of these phenomena in this research, and in the context of the discovery of a non-ceasing process route to larger GaN crystals, showed that the RMS values of the surface roughness of single crystal GaN (0 0 0 1) films exposed to pure ammonia flowing at 60 sccm for 2 h at 1130°C increased from the as-received value of 3.7–6.8 nm, 21.4 and 32.6 nm at 100, 430 and 760 Torr, respectively. Quadrupole mass spectrometry revealed that the concentrations of H 2 and N 2 measurably increased at pressures above 400 Torr. The primary reason for the increased roughness above 430 Torr was the enhanced etching of GaN via reaction with atomic and molecular hydrogen derived from the dissociation of the ammonia. At lower pressures, the decomposition of the GaN via the formation and evaporation of N 2 and Ga increased in importance relative to etching for enhancing surface roughness. Dilution with nitrogen reduced the amount of hydrogen generated from the dissociation of the ammonia. The GaN surface annealed at 1130°C and 430 Torr in ammonia diluted with 33 vol% N 2 maintained the smoothest surface with a nominal RMS value of 10.4 nm. Mixtures with higher and lower percentages of N 2 showed enhanced roughness under the same conditions. Use of this optimum gas mixture also allowed the seeded growth of a 1.5×1.5×2.0 mm 3 GaN crystal and a 2.3×1.8×0.3 mm 3 thick platelet with neither observable decomposition nor cessation of the growth over periods of 36 and 48 h, respectively.

A Study of the Decomposition of GaN during Annealing over a Wide Range of Temperatures

ABSTRACTAnnealing experiments were carried out on gallium nitride layers, which were grown on sapphire through Metal Organic Chemical Vapor Deposition (MOCVD). Rutherford Backscattering Spectrometry (RBS) was performed on as-grown and annealed GaN samples using a 2 MeV proton beam to study the stoichiometric changes in the near-surface region (750 nm) with depth resolution better than 50 nm. No decomposition was measured for temperatures o up to 800 °C. Decomposition in the near-surface region increased rapidly with a further increase o of temperature, resulting in a near-amorphous surface-region for annealing at 1100 °C. The depth profiles of nitrogen and incorporated oxygen in the decomposed GaN are extracted from the nanoscale RBS data for different annealing temperatures. The surface roughness of the GaN layers observed by atomic force microscopy (AFM) is consistent with RBS decomposition measurements. We describe the range of annealing conditions under which negligible decomposition of GaN is observed, which is important in assessing optimal thermal processing conditions of GaN for both conventional and nanoscale optoelectronic devices.

Influence of Polarity on Surface Reaction between GaN{0001} and Hydrogen

The influence of polarity on GaN decomposition has been investigated by an in situ gravimetric monitoring (GM) method using freestanding GaN(0001). The decomposition rate of the GaN was measured as a function of P in the temperature ranging from 800 to 950 °C. In the low-temperature region, the decomposition rate of GaN(0001) is faster than that of GaN(), where the decomposition rates of both surfaces are proportional to P3/2. On the other hand, the decomposition rate of GaN() is faster than that of GaN(0001) in the high-temperature region. In this case, the decomposition rates of both surfaces are proportional to P1/2. These results indicate that the rate-limiting reactions of GaN decomposition can be written as follows: N(surface) + 3/2H2(g) → NH3(g) at lower temperatures, and Ga(surface) + 1/2H2(g) → GaH(g) at higher temperatures.