Growth of GaN based structures on Si(1 1 0) by molecular beam epitaxy

The growth of GaN based structures on Si(1 1 0) substrates by molecular beam epitaxy using ammonia as the nitrogen precursor is reported. The structural, optical and electrical properties of such structures are assessed and are quite similar to the ones obtained on Si(1 1 1) in-spite of the very different substrate surface symmetry. A threading dislocation density of 3.7×10⁹ cm⁻² is evaluated by transmission electron microscopy, which is in the low range of typical densities obtained on up to 2 μm thick GaN structures grown on Si(1 1 1). To assess the potential of such structure for device realization, AlGaN/GaN high electron mobility transistor and InGaN/GaN light emitting diode heterostructures were grown and their properties are compared with the ones obtained on Si(1 1 1).     

Ca3N2 as a flux for crystallization of GaN

In this work Ca₃N₂ was investigated as a potential flux for crystallization of GaN. Melting temperature of the potential flux at high N₂ pressure evaluated by thermal analysis as 1380 °C is in good agreement with the theoretical prediction. It is shown that Ca₃N₂ present in the liquid gallium in small amount (1 at%) dramatically accelerates synthesis of GaN from its constituents. On the other hand, it does not influence significantly the rate of GaN crystallization from solution in gallium in temperature gradient for both unseeded and seeded configurations. However the habit and color of the spontaneously grown GaN crystals change drastically. For 10 mol% Ca₃N₂ content in the liquid Ga it was found that the GaN thick layer and GaN crystals (identified by micro-Raman scattering measurements) were grown on the substrate. For growth from molten Ca₃N₂ (100%) with GaN source, the most important observations were (i) GaN source material was completely dissolved in the molten Ca₃N₂ flux and (ii) after experiment, GaN crystals were found on the sapphire substrate.     

Improvement of crystal quality in ammonothermal growth of bulk GaN

Several key improvements in crystal quality of bulk GaN grown by the ammonothermal method are presented. Full width at half maximum of (0 0 2) X-ray rocking curve was reduced to 53 and 62 arcsec for Ga-side and N-side, respectively. Transparent bulk GaN crystal was also demonstrated. Oxygen and sodium concentrations were reduced to mid-10¹⁸ and mid-10¹⁵ cm⁻³, respectively. We are currently searching for a growth condition that produces transparent bulk GaN with high structural quality and low impurities. Small-sized, semi-transparent GaN wafers were fabricated by slicing the grown bulk GaN crystals, which demonstrate the high feasibility of ammonothermal growth for production of GaN wafers.     

Bulk GaN crystals grown by HVPE

We succeeded in preparing very thick c-plane bulk gallium nitride (GaN) crystals grown by hydride vapor phase epitaxy. Growth of the bulk GaN crystals was performed on templates with 3 μm GaN layer grown by metal organic chemical vapor deposition on (0 0 0 1) sapphire substrates. Colorless freestanding bulk GaN crystals were obtained through self-separation processes. The crystal's diameter and thickness were about 52 and 5.8 mm, respectively. No surface pits were observed within an area of 46 mm diameter of the bulk GaN crystal. The dislocation density decreased with growth direction (from N-face side to Ga-face side) and ranged from 5.1×10⁶ cm⁻² near the N-face surface to 1.2×10⁶ cm⁻² near the Ga-face. A major impurity was Si, and other impurities (O, C, Cl, H, Fe, Ni and Cr) were near or below the detection limits by SIMS measurements.     

Growth and optical properties of high-density InN nanodots

High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10¹⁰ cm⁻² at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH₃ flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy (V_In) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E_a∼51 meV, indicating strong localization of carriers in the high density InN nanodots.     

Electrical properties and deep traps spectra in undoped M-plane GaN films prepared by standard MOCVD and by selective lateral overgrowth

Electrical properties, deep traps spectra and structural performance were studied for m-GaN films grown on m-SiC substrates by standard metalorganic chemical vapor deposition (MOCVD) and by MOCVD with lateral overgrowth (ELO) or sidewall lateral overgrowth (SELO). Standard MOCVD m-GaN films with a very high dislocation density over 10⁹ cm⁻² are semi-insulating n-type with the Fermi level pinned near E_c−0.7 eV when grown at high V/III ratio. For lower V/III they become more highly conducting, with the electrical properties still dominated by a high density (∼10¹⁶ cm⁻³) of E_c−0.6 eV electron traps. Lateral overgrowth that reduces the dislocation density by several orders of magnitude results in a marked increase in the uncompensated shallow donor density (∼10¹⁵ cm⁻³) and a substantial decrease of the density of major electron traps at E_c−0.25 and E_c−0.6 eV (down to about 10¹⁴ cm⁻³). Possible explanations are briefly discussed.     

Molecular beam epitaxy of crystalline and amorphous GaN layers with high As content

We have studied the low-temperature growth of gallium nitride arsenide (GaN)As layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN_1−xAs_x alloys over a large composition range by growing the films much below the normal GaN growth temperatures with increasing the As₂ flux as well as Ga:N flux ratio. We found that alloys with high As content x>0.1 are amorphous and those with x<0.1 are crystalline. Optical absorption measurements reveal a continuous gradual decrease of band gap from ∼3.4 to ∼1.35 eV with increasing As content. The energy gap reaches its minimum of ∼1.35 eV at x∼0.6–0.7. The structural, optical and electrical properties of these crystalline/amorphous GaNAs layers were investigated. For x<0.3, the composition dependence of the band gap of the GaN_1−xAs_x alloys follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN_1−xAs_x alloys have short-range ordering that resembles random crystalline GaN_1−xAs_x alloys.     

Recent achievements in AMMONO-bulk method

In this paper we present progress made recently in the development of the growth of truly bulk GaN crystals by the ammonothermal method in basic environment. High quality 2-in c-plane GaN seeds are shown. Non-polar wafers can also be cut out from thick GaN crystals grown by ammonothermal method. Perfect crystallinity manifests in very narrow peaks in X-ray rocking curves (the full width at half maximum equals about 15 arcsec). GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines, which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving the truly non-polar character of such AMMONO-GaN substrates. Other challenges like homogenous insulating properties or high p-type conductivity have been also accomplished by means of ammonothermal method. Semi-insulating crystals of resistivity up to 10¹¹ Ω cm and p-type conductivity within hole concentration up to 10¹⁸ cm⁻³ are already available in diameters up to 1.5-in.     

Development of m-plane GaN anisotropic film properties during MOVPE growth on LiAlO2 substrates

The anisotropic film properties of m-plane GaN deposited by metal organic vapour phase epitaxy (MOVPE) on LiAlO₂ substrates are investigated. To study the development of layer properties during epitaxy, the total film thickness is varied between 0.2 and 1.7 μm. A surface roughening is observed caused by the increased size of hillock-like features. Additionally, small steps which are perfectly aligned in (1 1 −2 0) planes appear for samples with a thickness of ∼0.5 μm and above. Simultaneously, the X-ray rocking curve (XRC) full width at half maximum (FWHM) values become strongly dependent on incident X-ray beam direction beyond this critical thickness. Anisotropic in-plane compressive strain is initially present and gradually relaxes mainly in the [1 1 −2 0] direction when growing thicker films. Low-temperature photoluminescence (PL) spectra are dominated by the GaN near-band-edge peak and show only weak signal related to basal plane stacking faults (BSF). The measured background electron concentration is reduced from ∼10²⁰ to ∼10¹⁹ cm⁻³ for film thicknesses of 0.2 μm and ∼1 μm while the electron mobilities rise from ∼20 to ∼130 cm²/V s. The mobilities are significantly higher in [0 0 0 1] direction which we explain by the presence of extended planar defects in the prismatic plane. Such defects are assumed to be also the cause for the observed surface steps and anisotropic XRC broadening.     

Thick homoepitaxial GaN with low carrier concentration for high blocking voltage

High voltage GaN Schottky diodes require a thick blocking layer with an exceptionally low carrier concentration. To this aim, a metal organic chemical vapor deposition process was developed to create a (14 μm) thick stress-free homoepitaxial GaN film. Low temperature photoluminescence measurements are consistent with low donor background and low concentration of deep compensating centers. Capacitance–voltage measurements performed at 30 °C verified a low level of about 2×10¹⁵ cm⁻³ of n-type free carriers (unintentional doping), which enabled a breakdown voltage of about 500 V. A secondary ion mass spectrometry depth profile confirms the low concentration of background impurities and X-ray diffraction extracted a low dislocation density in the film. These results indicate that thick GaN films can be deposited with free carrier concentrations sufficiently low to enable high voltage rectifiers for power switching applications.     

The impact of an intermediate temperature buffer on the growth of GaN on an AlN template by hydride vapor phase epitaxy

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×10⁷ cm⁻²) and a higher edge-dislocation density (8.157×10⁹ cm⁻²) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.     

Growth of hexagonal and cubic InN nanowires using MOCVD with different growth temperatures

We have performed a detailed investigation of the metal-organic chemical vapor deposition (MOCVD) growth and characterization of InN nanowires formed on Si(1 1 1) substrates under nitrogen rich conditions. The growth of InN nanowires has been demonstrated by using an ion beam sputtered (∼10 nm) Au seeding layer prior to the initiation of growth. We tried to vary the growth temperature and pressure in order to obtain an optimum growth condition for InN nanowires. The InN nanowires were grown on the Au+In solid solution droplets caused by annealing in a nitrogen ambient at 700 °C. By applying this technique, we have achieved the formation of InN nanowires that are relatively free of dislocations and stacking faults. Scanning electron microscopy (SEM) showed wires with diameters of 90–200 nm and lengths varying between 3 and 5 μm. Hexagonal and cubic structure is verified by high resolution X-ray diffraction (HR-XRD) spectrum. Raman measurements show that these wurtzite InN nanowires have sharp peaks E₂ (high) at 491 cm⁻¹ and A₁ (LO) at 591 cm⁻¹.     

Method for modulating the wafer bow of free-standing GaN substrates via inductively coupled plasma etching

The bowing curvature of the free-standing GaN substrate significantly decreased almost linearly from 0.67 to 0.056 m⁻¹ (i.e. the bowing radius increased from 1.5 to 17.8 m) with increase in inductively coupled plasma (ICP) etching time at the N-polar face, and eventually changed the bowing direction from convex to concave. Furthermore, the influences of the bowing curvature on the measured full width at half maximum (FWHM) of high-resolution X-ray diffraction (HRXRD) in (0 0 2) reflection were also deduced, which reduced from 176.8 to 88.8 arcsec with increase in ICP etching time. Decrease in the nonhomogeneous distribution of threading dislocations and point defects as well as V_Ga–O_N complex defects on removing the GaN layer from N-polar face, which removed large amount of defects, was one of the reasons that improved the bowing of the free-standing GaN substrate. Another reason was the high aspect ratio of needle-like GaN that appeared at the N-polar face after ICP etching, which released the compressive strain of the free-standing GaN substrate. By doing so, crack-free and extremely flat free-standing GaN substrates with a bowing radius of 17.8 m could be obtained.     

The influence of indium on the growth of GaN from solution under high pressure

The influence of significant fraction (10–50 mole%) indium in liquid gallium on GaN crystallization from a ternary Ga–In–N solution was analyzed. Crystallization experiments of GaN on GaN-sapphire templates from Ga–In solutions, at 1350–1450 °C, with prior to the growth seed wetting at 1500 °C, and 1.0 GPa N₂ pressure, without solid GaN source showed faster growth of GaN on the seed (by a factor of 1.5–2) than using pure gallium solvent. Nevertheless the new grown crystals were morphologically unstable. The instability was reduced by decrease of the wetting temperature down to 1100 °C or by omitting the wetting procedure entirely, which indicated that GaN dissolves much faster in Ga–In melt than in pure Ga and that the unstable growth was caused most likely by complete dissolution of GaN template before the growth. It was observed that the crystals grown on bulk GaN substrates did not show morphological instability observed for GaN-sapphire templates. The influence of indium on thermodynamic and thermal properties of the investigated system is discussed.     

Properties of Ge-doped,high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy

We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×10¹⁹ cm⁻³. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl₄ as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×10⁶ cm⁻². The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146 cm² V⁻¹ s⁻¹ for n=3.3×10¹⁸ and 3.35×10¹⁹ cm⁻³, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0 W cm⁻¹ K⁻¹. Thermal expansion coefficients along the a- and m-axes were approximately constant at 5.0×10⁻⁶ K⁻¹ in the measured doping concentration range.     

Characterization of bulk GaN crystals grown from solution at near atmospheric pressure

The properties of GaN crystals grown from solution at temperatures ranging from 780 to 810 °C and near atmospheric pressure ∼0.14 MPa, have been investigated using low temperature X-band (∼9.5 GHz) electron paramagnetic resonance spectroscopy, micro-Raman spectroscopy, photoluminescense spectroscopy, and photoluminescence imaging. Our samples are spontaneously nucleated thin platelets of approximate dimensions of 2×2×0.025 mm³, or samples grown on both polycrystalline and single crystal HVPE large-area (∼3×8×0.5 mm³) seeds. Electron paramagnetic resonance spectra consists of a single Lorentzian line with axial symmetry about the c-axis, with approximate g-values, g_∥=1.951 and g_⊥=1.948 and a peak-to-peak linewidth of∼4.0 G. This resonance has been previously assigned to shallow impurity donors/conduction electrons in GaN and attributed to Si- and/or O impurities. Room temperature photoluminescence and photoluminescence imaging data from both Ga- and N-faces show different dominant emission bands, suggesting different incorporation of impurities and/or native defects. Raman scattering and X-ray diffraction show moderate to good crystalline quality.     

Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H₂ and NH₃, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10⁸ cm⁻².     

GaP:Mg layers grown on GaN by MOCVD

We have investigated the growth of magnesium-doped GaP (GaP:Mg) layers on GaN by metalorganic chemical vapor deposition. The hole carrier concentration increased linearly from 0.8×10¹⁸ to 4.2×10¹⁸ cm⁻³ as the Bis(cyclopentadienyl) magnesium (Cp₂Mg) mole flow rate increased from 1.2×10⁻⁷ to 3.6×10⁻⁷ mol/min. However, the hole carrier concentration decreased when the CP₂Mg mole flow rate was further increased. The double crystal X-ray diffraction (DCXRD) rocking curves showed that the GaP:Mg layers were single crystalline at low CP₂Mg molar flow. However, the GaP:Mg layers became polycrystalline if the CP₂Mg molar flow was too high. The decrease in hole carrier concentration at high CP₂Mg molar flow was due to crystal quality deterioration of the GaP layer, which also resulted in the low hole mobility of the GaP:Mg layer.     

Mg doping behavior of MOVPE InxGa1−xN (x∼0.4)

The Mg doping behavior of MOVPE indium gallium nitride (InGaN), such as secondary ion mass spectrometry (SIMS) Mg profile, crystalline quality and n–p conversion of the films are described and discussed in this paper. The SIMS analysis reveals that the memory effect of Cp₂Mg as a doping source deteriorates the controllability of Mg doping level and profile, especially for thin (−0.4 μm) InGaN. The high residual donors (10¹⁹–10²⁰ cm⁻³) in InGaN with In content from 0.05 to 0.37 can be compensated by Mg doping and p-type conduction is obtained for those with In content up to 0.2. It is found that a higher Cp₂Mg flow rate is needed to get p-type conduction in InGaN with a higher In content x; for example, Cp₂Mg/(TEG+TMI)≈0.5% for x=0 (GaN), ≈2% for x=0.05 and ≈4% for x=0.2. Such a high Cp₂Mg flow rate is needed due to the high residual donor concentration (10¹⁹–10²⁰ cm⁻³) of InGaN films and the low activation efficiency of Mg. The crystalline quality of InGaN is deteriorated with increasing In content as well as Mg doping level. To achieve a p-type InGaN with a lower Mg doping, it is essential to improve the crystalline quality of non-doped InGaN. For this purpose, the use of a thicker GaN interlayer is effective.     

Thermogravimetric analysis and microstructural observations on the formation of GaN from the reaction between Ga2O3 and NH3

Thermogravimetric analysis (TGA) and microstructural observations were carried to investigate the nitridation mechanism of β-Ga₂O₃ powder to GaN under an NH₃/Ar atmosphere. Non-isothermal TGA showed that nitridation of β-Ga₂O₃ starts at ∼650 °C, followed by decomposition of GaN at ∼1100 °C. Isothermal TGA showed that nitridation follows linear kinetics in the temperature range 800–1000 °C. At an early stage of nitridation, small GaN particles (∼5 nm) are deposited on the β-Ga₂O₃ crystal surface and they increase with time. We proposed a mechanism for the nitridation of Ga₂O₃ by NH₃ whereby nitridation of β-Ga₂O₃ proceeds via the intermediate vapor species Ga₂O(g).