Thick GaN Crystals Research Articles

Investigation of different buffer layer impact on AlN/GaN/AlGaN HEMT using silicon carbide substrate for high-speed RF applications

The AlN/GaN heterostructure on the AlGaN back barrier with different buffer layer structures using a silicon carbide (SiC) substrate was investigated in this work. This study mainly focused on selecting a cost-effective buffer layer with a thickness of more than 1 μm and fewer defects for improving DC/RF performance. The extra epitaxial growth process steps can be avoided by choosing an appropriate, high-quality buffer material called β-Ga2O3 (β-gallium oxide). It has a good lattice match with AlGaN alloys and SiC. It leads to fabricating cost-effective, reliable HEMTs. The GaN buffer has a poor lattice match with SiC; the nucleation layer must be used between GaN and SiC for lattice match to avoid stress defects and dislocations, which helps to reduce buffer traps. The growth of thick GaN crystals leads to high costs and more defects. Creating a solid conduction band barrier with the help of a back-barrier (AlGaN) leads to a boost in the confinement of electrons in both devices. Still, the traps and defects in the GaN buffer do not effectively halt the leakage current compared to the β-Ga2O3 buffer. The T-shaped gate with the impact of LGD (gate to drain length) and LGS (gate to source length) scaling in lateral dimensions was also studied and the device performance was reported. The HEMT with β-Ga2O3 buffer delivers significant results in drain current (2.3 A/mm), transconductance (603 S/mm), and cut-off frequency (486 GHz) for the scaled lateral lengths LG (Gate Length) of 60 nm,LGD of 0.9 μm, and LGS of 0.4 μm. This excellent performance helps to use this HEMT device in cost-effective future high-speed RF applications.

High-rate OVPE-GaN crystal growth at a very high temperature of 1300 °C

Polycrystal formation is an issue to be resolved to grow thick GaN crystals by the oxide vapor phase epitaxy (OVPE) method. Since the high-temperature growth at 1250 °C was effective in suppressing the polycrystal formation in our previous study, we attempt further high-temperature OVPE-GaN crystal growth at 1300 °C. However, the GaN surface decomposition becomes severe at 1300 °C. The pre-growth epitaxy was employed to avoid surface decomposition and enable high-temperature growth. Nearly polycrystal-free growth of GaN crystal was achieved, and we obtained a 478 μm thick OVPE-GaN layer at 1300 °C with a further high growth rate of about 200 μm h−1.

Effect of additional N2O gas on the suppression of polycrystal formation and high-rate GaN crystal growth by OVPE method

One of the challenges in the oxide vapor phase epitaxy (OVPE) method is to suppress polycrystal formation for the high-rate growth of thick GaN crystals. In this study, we present the addition of N2O gas as the method to suppress the polycrystal formation in the OVPE-GaN growth. We confirmed that N2O gas could remove the Ga droplet, the origin of polycrystalline GaN. Results showed that the polycrystal density decreased by adding N2O gas. The decline in growth rate with the increase of N2O gas flow rate was smaller compared to that with H2O vapor, which was consistent with the fact that N2O would not affect directly the GaN growth reaction. Furthermore, we obtained a 512-µm-thick OVPE-GaN layer at the growth rate of 128 µm/h, which is the highest value for growing thick GaN layers in the OVPE method.

In-Situ Preparation of GaN Sacrificial Layers on Sapphire Substrate in MOVPE Reactor for Self-Separation of the Overgrown GaN Crystal

GaN layers on sapphire substrates were prepared by using metal organic vapor phase epitaxy (MOVPE) combined with an in-situ H2 etching process for the purpose of later self-separation of thick GaN crystals produced by hydride vapor phase epitaxy (HVPE) on such substrates. The etching process results in deep pits and long voids that formed on the surface and along the lower interface between GaN and sapphire, respectively. The pits, which were investigated by SEM analysis, can be modified in their aspect ratio and density by controlling the etching parameters. Using a proper set of in-situ etching parameters, a seed layer with internal voids can be prepared, which is suitable for HVPE overgrowth and the self-separation process. The quality of the in-situ-etched seed GaN layer and overgrown GaN crystal were characterized by X-ray diffraction (XRD) and defect selective etching (DSE). With the aid of atomic force microscopy (AFM) in tapping mode, the interface morphology of the separated GaN crystal was analyzed. The crystal quality of the separated HVPE-GaN crystal is comparable to the crystal grown on untreated GaN MOVPE-seed, which did not separate from the sapphire substrate. The introduced technique to promote the crystal separation during the HVPE process has no obvious drawback on the quality of the grown GaN crystals. Using this technique, the self-separation occurs more gently due to a weakened interface between GaN/sapphire. The conventional separation from an untreated seed by pure thermomechanical action results in higher mechanical forces on the crystal and consequently much higher risk of crystal breakage.

High-rate OVPE-GaN growth by the suppression of polycrystal formation with additional H2O vapor in a high-temperature condition

The oxide vapor phase epitaxy (OVPE) method enables the fabrication of low-resistivity GaN crystals. However, polycrystal formation is detrimental to the growth of thick GaN crystals at the high growth rate by the OVPE method. In this study, we focused on H2O additive under the high-rate and high-temperature growth condition to suppress generation of Ga droplet, the origin of polycrystalline GaN. The polycrystal density dramatically decreased by adding H2O, which can convert Ga droplet into Ga2O vapor. We obtained a 431 μm thick OVPE-GaN layer at 108 μm h−1, which is the highest value in GaN crystals grown by the OVPE method.

Effect of methane additive on GaN growth using the OVPE method

The oxide vapor phase epitaxy method is expected to be a useful technique for bulk GaN growth, because it allows long-term growth without producing a solid byproduct. However, thick GaN crystals have not been realized due to the growth inhibition caused by polycrystal formation resulting from high H2O partial pressure. In this study, we formed GaN crystals with CH4 gas to decrease the H2O partial pressure in a growth zone by the reaction of CH4 with H2O to produce CO and H2. As a result, H2O partial pressure decreased with increasing CH4 flow rate, and GaN layers could be grown without decrease of growth rate or degradation of the crystalline qualities at a flow rate of 50–100 sccm of CH4 gas. Furthermore, we obtained high crystalline 400-um thick GaN crystals after a growth period of 10 h.

Investigation of the Effect of Electron-Beam Irradiation on the Defect Structure of Laterally Overgrown GaN Films via the Induced-Current and Cathodoluminescence Methods

The low-energy electron irradiation effect on the defect structure on epitaxial laterally overgrown (ELOG) films and thick GaN crystals grown via the hydride epitaxy method is studied using a scanning electron microscope by the induced-current and cathodoluminescence methods. Electron-beam irradiation is carried out at room temperature. In this case, the electron-beam irradiation effect on dislocation segments located in the basal plane is studied in ELOG GaN films. At the same time, dislocations are introduced into GaN crystals by indentation at room temperature. It is found that the behavior of growth dislocations and those introduced by indentation is different under electron-beam irradiation.

Defect sensitive etching of nitrides: appraisal of methods

AbstractDefect selective etching methods used for structural characterization of nitrides (GaN, AlGaN, InGaN and AlN) single crystals and epitaxial layers are summarized. The mechanism of surface reactions during orthodox chemical etching and photoetching is briefly described. Examples of defects which can be revealed mainly in bulk and epitaxial GaN using different etching systems are shown together with schematic models of defects. Special attention is given to selective etching and analysis of complex electrically active non‐homogeneities in bulk and HVPE‐grown GaN, which show up as areas of different etch rate, i.e. different carrier concentration. These defects are clearly visible after photo‐etching on both Ga‐polar surface and on any non‐polar cleavage plane. Ultra high selectivity of electroless photoetching on electrically active defects is explained by separation of photocurrent‐potential plots of areas with different carrier concentration. Formation of nano‐pillars on dislocations during photoetching is explained by recombinative properties of linear defects for photo‐generated carriers. Special applications of defect‐selective etching methods for (i) nano‐patterning of GaN surface for preparing highly efficient platforms for surface enhanced Raman spectroscopy (SERS) measurements and (ii) overgrowth of deep etch pits for reduction of dislocation density during HVPE growth of thick GaN crystals is also demonstrated. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN boules grown by high rate HVPE

AbstractThe most promising approach for GaN substrate fabrication on the base of HVPE consists in the growth of thick GaN crystals and subsequent cutting of these boules into slices followed by conventional surface preparation. This work focusses on a HVPE GaN boule growth process optimized to highest growth rate and the use of a commercially available vertical HVPE reactor from AIXTRON in order to develop a technology of high productivity to enable low‐cost substrates. To this end, GaN crystals up to thicknesses of 6.3 mm have been grown on 2 inch GaN‐on‐sapphire templates produced by MOVPE. The rate in HVPE growth was 450 µm/h and the material properties of the crystals have been evaluated. No degradation of the material quality was found by comparison of structural, optical and transport properties with published data of the state of the art of bulk GaN. Dislocation densities as low as 6x105 cm‐2, high electron mobility of 960 cm2/Vs for a carrier density of 1x1016 cm‐3, low background impurity incorporation of oxygen and silicon, luminescence with well resolved excitonic emission, and a high value of thermal conductivity of 294±44 W/mK were determined. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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 10 11 Ω cm and p-type conductivity within hole concentration up to 10 18 cm −3 are already available in diameters up to 1.5-in.

Dislocation reduction in GaN crystal by advanced-DEEP

Extremely low-dislocation density in GaN crystal was realized by a new method taking advantage of a polarity difference. Dot patterned layer and stripe patterned layer with periodicity were formed on GaAs initial substrates, respectively. Thick GaN crystals were grown on them by hydride vapor phase epitaxy (HVPE). In most area GaN was grown with Ga-polarity but only on the patterned layer GaN with N-polarity was grown. We call this area with N-polarity a core. At the core the GaN crystal always forms concave shape because of lower growth rate of N-polarity. So, inverse pyramidal pits or V-shaped valleys were formed placing cores at the bottom. The position of the pits and valleys was fixed by the cores during the growth. As the growth proceeds, dislocations existing on the facet plane of pit or V-shaped valley propagate to the core. Thus, dislocation density was drastically reduced. Treading dislocation density lower than 6×10 3 cm −2 was obtained between the cores. This process is improved process of dislocation elimination by the epitaxial-growth with inverse-pyramidal pits (DEEP) already we have proposed, so we call this new process as advanced-DEEP (A-DEEP).

Crystallization of GaN by HVPE on pressure grown seeds

Growth of GaN under pressure from solution in gallium results in almost dislocation free plate-like crystals but with size limited to approx. 1 cm (lateral) and 100 μm (thickness). On the other hand, deposition of GaN by HVPE on the pressure grown substrates allows stable crystallization with rates of a few hundreds μm/h. The crystals with a thickness exceeding 2 mm are grown in this way. However, in these thick GaN crystals grown directly on almost dislocation free substrates quite high number of dislocations appears if the crystal thickness exceeds certain critical value probably due to a small lattice mismatch between GaN grown by HVPE and the heavily doped pressure grown substrates Since the critical thickness for defect generation is of the order of 100 μm, almost dislocation free layers (density below 10 4 cm -2 ) thinner than 100 μm were grown. The pressure grown substrates were then removed by mechanical polishing, or conductivity sensitive electrochemical etching (for strongly n-type substrates). Then the HVPE low dislocation density GaN platelets were used as substrates for the growth of a few mm thick bulk GaN crystals.