Freestanding GaN Wafers Research Articles

Characterization of Ga-face/Ga-face and N-face/N-face interfaces with antiparallel polarizations fabricated by surface-activated bonding of freestanding GaN wafers

Electrical properties of heterojunctions of group-III nitrides are largely sensitive to interface charges due to the discontinuity of polarizations. By means of surface-activated bonding of double-side polished freestanding GaN (0001) wafers, we fabricate Ga-face/Ga-face and N-face/N-face interfaces with antiparallel spontaneous polarizations, i.e. interfaces with the greatest discontinuity of polarizations, to investigate their electrical and nanostructural properties. Built-in potential of N-face/N-face interface is smaller than that of Ga-face/Ga-face interface after a post-bonding annealing at 600 °C. The difference in built-in potentials between the two antiparallel polarized interfaces is analyzed in the framework of charge-neutrality-level model with effects of antiparallel polarizations incorporated, and the density of interface states is roughly estimated. The leak is enhanced in both Ga-face/Ga-face and N-face/N-face interfaces by annealing at higher temperatures. Contribution of defects observed in the vicinity of bonding interfaces is suggested.

Characterization of Defects in GaN: Optical and Magnetic Resonance Techniques

GaN and its alloys with InN and AlN are of technological importance for a variety of optical, electronic, and optoelectronic devices due to its high thermal conductivity, wide band gap, high breakdown voltage and high saturation velocity. GaN-based devices now provide superior performance for a variety of high power, high frequency, high temperature, and optical applications. The major roadblock for the full realization of Nitride semiconductor potential is still the availability of affordable large-area and high-quality native substrates with controlled electrical properties. Despite the impressive accomplishments recently achieved by techniques such as hydride vapor phase epitaxy and ammonothermal for GaN growth, much more must be attained before establishing a fully satisfactory bulk growth method for this material. Recent results suggest that ammonothermal GaN wafers can be successfully used as seeds to grow thick freestanding GaN wafers by hydride vapor phase epitaxy. A brief review of defect-sensitive optical and paramagnetic spectroscopy techniques employed to evaluate structural, optical, and electronic properties of the state-of-the-art bulk and thick-film (quasi-bulk) Nitride substrates and homoepitaxial films is presented. Defects control the performance of devices and feeding back knowledge of defects to growth efforts is key to advancing technology.

Study on the stress and mechanism of self-separated GaN grown by Na-flux method

A 2 inch free-standing c-plane GaN wafer was fabricated through in situ self-separation using HVPE-seed crystal etching back (HCEB) by intentionally adjusting the nitrogen pressure in the Na-flux growth process of GaN. First, adjust the nitrogen pressure in the reactor to a lower level to facilitate HCEB to form a large number of voids at the interface between the c-plane HVPE seed and the c-plane Na-flux GaN. After regrowth of approximately 340 μm thick Na-flux GaN, self-separation was achieved during the cooling process. The free-standing GaN wafer was almost stress-free as a result of strain relief by the in situ self-separation process, which was confirmed by room-temperature Raman and low-temperature photoluminescence measurements. It is supposed that the HCEB process can be applied to fabricate high-quality free-standing GaN wafers in the future.

Lattice bow in thick, homoepitaxial GaN layers for vertical power devices

Lattice bow generated by 40 µm thick HVPE homoepitaxial layers on commercial free-standing, ammonothermal and HVPE GaN wafers was studied. While a change in lattice bow was measured for all wafers, the additional bow on the ammonothermal GaN wafers was minimal. The main driving force for the observed increase in the lattice bow for HVPE wafers was related to stress in the films generated by the elongation of dislocations via climb and generation of new dislocations at the homoepitaxial interface. Lattice bow is a crucial wafer parameter as it determines the variation of the offcut across the surface. If an offcut variation of 0.1° is allowed for desired control surface morphology, composition of alloys, and uniformity of doping on this surface, the measured bow on the two HVPE GaN wafers and one ammonothermal GaN wafer limits their uniformity-diameter to ~0.5″, 1″ and >4″, respectively.

Room-temperature infrared photoluminescence in GaN doped with various impurities

The steady-state infrared-photoluminescence spectra (IR-PL) emitted from about 400 μm thick, free-standing GaN wafers, grown by the ammono-thermal and hydride vapour-phase epitaxy GaN, and containing carbon, magnesium, manganese and iron doping have been examined. The room-temperature IR-PL spectra are correlated with pulsed-photo-ionization spectra using van Roosbroeck-Schockley approach for spectrum conversion. It has been revealed that iron and carbon dopants appear as the most efficient impurities for the room temperature of infra-red emission from GaN grown using different technologies.

Reuse of ScAlMgO4 substrates utilized for halide vapor phase epitaxy of GaN

ScAlMgO4 (SCAM) substrates with a small lattice-mismatch to GaN and c-plane cleavability are promising for fabricating high-quality free-standing GaN wafers. To reduce the cost in the fabrication of free-standing GaN wafers, the reuse of a SCAM substrate is demonstrated. By cleaving a SCAM substrate which has been already utilized for the growth of a thick GaN film by halide vapor phase epitaxy, the atomically flat surface can be obtained. The threading dislocation density of a 320 μm thick GaN film grown on this cleaved SCAM substrate is 2.4 × 107 cm−2, which is almost the same as that on a new SCAM substrate. This result indicates that a SCAM substrate can be reused for GaN growth.

Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require AlxGa1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of AlxGa1-xN bulk crystals by any standard bulk growth techniques has not been developed so far.There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by any standard bulk growth techniques were not successful.Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA-MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite AlxGa1-xN wafers. Zinc-blende and wurtzite AlxGa1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and AlxGa1-xN wafers. At a thickness of ∼30µm, free-standing GaN and AlxGa1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and AlxGa1-xN wafers with thicknesses in the 30–100μm range may be used as substrates for further growth of GaN and AlxGa1-xN-based structures and devices.We have compared different RF nitrogen plasma sources for the growth of thick nitride AlxGa1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3µm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day.Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.

Critical issues for homoepitaxial GaN growth by molecular beam epitaxy on hydride vapor-phase epitaxy-grown GaN substrates

While the heteroepitaxial growth of gallium nitride-based materials and devices on substrates such as SiC, sapphire, and Si has been well-documented, the lack of a cost-effective source of bulk GaN crystals has hindered similar progress on homoepitaxy. Nevertheless, freestanding GaN wafers are becoming more widely available, and there is great interest in growing GaN films and devices on bulk GaN substrates, in order to take advantage of the greatly reduced density of threading dislocations, particularly for vertical devices. However, homoepitaxial GaN growth is far from a trivial task due to the reactivity and different chemical sensitivities of N-polar (0001) and Ga-polar (0001) GaN surfaces, which can affect the microstructure and concentrations of impurities in homoepitaxial GaN layers. In order to achieve high quality, high purity homoepitaxial GaN, it is necessary to investigate the effect of the ex situ wet chemical clean, the use of in situ cleaning procedures, the sensitivity of the GaN surface to thermal decomposition, and the effect of growth temperature. We review the current understanding of these issues with a focus on homoepitaxial growth of GaN by molecular beam epitaxy (MBE) on c-plane surfaces of freestanding GaN substrates grown by hydride vapor phase epitaxy (HVPE), as HVPE-grown substrates are most widely available. We demonstrate methods for obtaining homoepitaxial GaN layers by plasma-assisted MBE in which no additional threading dislocations are generated from the regrowth interface and impurity concentrations are greatly reduced.

Photo-electrochemical etching of free-standing GaN wafer surfaces grown by hydride vapor phase epitaxy

An investigation into the photo-electrochemical (PEC) etching of free-standing GaN wafers produced by hydride vapor phase epitaxial growth (HVPE) has found that etching is only possible with UV illumination in an acidic or basic electrolyte. Through photo-current measurement and X-ray analysis it was determined that lack of etching in a neutral electrolyte can be attributed to the formation of an oxide film on the GaN surface. Surface damage was also found to be a significant factor, with the etching rate and photo current density of surfaces treated by grinding and mechanical polishing being markedly less compared to a finely polished surface. Subsequent investigation of the luminescence and the etching characteristics of the intentionally-introduced scratches indicated that subsurface damage is difficult to remove from GaN by PEC etching due to the trapping of photo-excited carriers. A peculiar surface feature of concentric ring structures made up of alternating small and large pores was observed on the GaN surface along with small island regions, which is attributed to variations in the electronic properties of the GaN crystal that is created during HVPE growth.

(Invited) Synchrotron White-Beam X-Ray Topography Analysis of the Defect Structure of HVPE-GaN Substrates

For demanding GaN-based applications like laser diodes or high mobility transistors there is a need for substrate material with low defect densities, since threading dislocations act as centers of non-radiative recombination, increasing the leakage current, reducing the room temperature mobility and thereby limiting the efficiency, performance and lifetime of devices. Hydride vapor-phase epitaxy (HVPE) is the most common technique for the growth of large area freestanding GaN wafers with diameters up to 50 mm and a threading dislocation density below 107 cm−2. An alternative technique with great potential for the growth of GaN bulk crystals is the ammonothermal growth method. For this method GaN threading dislocation densities as low as 5 × 103 cm−2were reported [1]. We are interested examining significant differences in the defect structure of the GaN substrates which can be observed in samples obtained from different manufacturers or as a function of the growth technique. In this work we present a detailed analysis of defect structures of freestanding GaN (0001) crystals grown by HVPE and the ammonothermal method from different vendors. Synchrotron white-beam X-ray topography (SWXRT), high resolution X-ray diffraction (HRXRD) and microphotoluminescence (µPL) were used for this combined study. SWXRT in two principal X-ray topography geometries was used in the course of this analysis, namely the large-area back-reflection mode and section transmission mode. HRXRD reciprocal space mappings and in-plane measurements were used for mosaicity analyses. The combination of these methods allows a detailed statement of the nature, density and distribution of typical defects in GaN bulk crystals with a large field of view. As an example, Fig. 1 shows a large-area back-reflection topograph (0006 reflection) of a HVPE GaN substrate. The topograph shows dislocation free areas, surrounded by rather sharp lines, indicating good material quality with low defect density. These features are interpreted as dislocation cell walls, suggesting the formation of a cellular defect network within this GaN sample, analogous to what was commonly observed in LEC GaAs [2]. With the combination of the results from SWXRT, HRXRD and µPL a consistent model for different defect structures of the freestanding GaN samples grown by HVPE and the ammonothermal method is discussed. References P.J. R. Doradzinski, R. Dwilinski, J. Garczynski, L.P. Sierzputowski, Y. Kanbara, in Technology of Gallium Nitride Crystal Growth, edited by D.Ehrentraut, E. Meissner, and M. Bockowski, (Springer-Verlag, Heidelberg, ISBN 978-3-642-04828-9), 137-158, (2010). P.J. McNally, P.A.F. Herbert, T. Tuomi, M. Karilahti, and J.A. Higgins, J. Appl. Phys. (1996) 79, 8294. Figure 1

Optical properties of C-doped bulk GaN wafers grown by halide vapor phase epitaxy

Freestanding bulk C-doped GaN wafers grown by halide vapor phase epitaxy are studied by optical spectroscopy and electron microscopy. Significant changes of the near band gap (NBG) emission as well as an enhancement of yellow luminescence have been found with increasing C doping from 5 × 1016 cm−3 to 6 × 1017 cm−3. Cathodoluminescence mapping reveals hexagonal domain structures (pits) with high oxygen concentrations formed during the growth. NBG emission within the pits even at high C concentration is dominated by a rather broad line at ∼3.47 eV typical for n-type GaN. In the area without pits, quenching of the donor bound exciton (DBE) spectrum at moderate C doping levels of 1–2 × 1017 cm−3 is observed along with the appearance of two acceptor bound exciton lines typical for Mg-doped GaN. The DBE ionization due to local electric fields in compensated GaN may explain the transformation of the NBG emission.

Fabrication of free-standing GaN by using thermal decomposition of GaN

Free-standing GaN wafers were fabricated by hydride vapor phase epitaxy (HVPE) using an in-situ self-separation technique. Separation of GaN happened during the high-temperature GaN growth through thermal decomposition of the decomposable buffer layer (DBL). We optimized the growth condition of DBL, which affects not only the separation property but also the crystallinity of GaN. Free standing GaN revealed negligible residual stress, narrow (0002) omega rocking curve linewidth (67ʺ), and low etch pit density (6×106cm−2) as well. The carrier concentration of 4×1018cm−3 and mobility of 150cm2/Vs was observed from Hall effect measurement. These results showed the feasibility of the in-situ self-separation method to obtain high quality free-standing GaN wafer.

Nearly 4-Inch-Diameter Free-Standing GaN Wafer Fabricated by Hydride Vapor Phase Epitaxy with Pit-Inducing Buffer Layer

A free-standing GaN wafer was fabricated by depositing a GaN buffer that induced the formation of pits (hereafter, pit-inducing GaN buffer) on a low-temperature-grown GaN buffer on the sapphire substrate. A high-temperature-grown GaN layer was grown on the pit-inducing GaN buffer that induced the formation of pits on the high-temperature-grown GaN layer. The pit-inducing buffer suppresses crack formation in the thick GaN film thereby releasing growth stress. Thermal stress in GaN on a sapphire system is also discussed on the basis of calculations utilizing a bilayer model. We have succeeded in the fabrication of a nearly 4-in.-diameter free-standing GaN thick wafer with a pit-inducing GaN buffer by one-stop hydride vapor phase epitaxy, which will lead to a low-cost fabrication of free-standing GaN wafers.

Studies about wafer bow of freestanding GaN substrates grown by hydride vapor phase epitaxy

Free-standing GaN wafers grown by hydride vapor phase epitaxy are typically concavely bowed. In situ and ex situ curvature measurements indicate that some strain developing at the very beginning of the epitaxial process or even in the template grown by metalorganic vapor phase epitaxy may be the origin of this bow. It can be only partly released by etching the defective back-side of the samples indicating that the strong dislocation density gradient is not the only reason for strain in free-standing GaN.

Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals

We have studied the growth of zinc-blende GaN and AlxGa1−xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100μm in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (001) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1−xN wafers.

Chemical etchant dependence of surface structure and morphology on GaN(0001) substrates

We studied the procedure of cleaning GaN(0001) substrate surfaces by wet etching and subsequent annealing in ultrahigh vacuum for two different types of freestanding GaN wafers: hydride vapor phase epitaxy (HVPE) crystal and Na flux liquid phase epitaxy (LPE) crystal wafers. A flat surface containing GaN(0001)2 × 2 reconstruction was successfully achieved on both HVPE and LPE surfaces by etching in HF and subsequent annealing at ∼ 550 °C but was not achieved by etching in HCl, NaOH, and HNO 3.

Stress Adjustment and Bonding of H-Implanted 2 in. Freestanding GaN Wafer: The Concept of Double-Sided Splitting

Heterointegration of thin layers obtained from freestanding GaN wafers by the Smart-Cut process faces major challenges due to strong wafer bowing. The post-implantation bow was found to range between ∼40 and 60 μm for 2 in. wafers, prohibiting any bonding of H-implanted GaN. Here, we demonstrate that stress engineering by back-side implantation is an effective strategy to manipulate the bow to meet the criterion of long-range flatness. Based on our approach, high-quality bonding of a 2 in. freestanding GaN wafer onto sapphire was achieved. An efficient method to transfer thin layers from freestanding GaN is proposed.

III-V and III-Nitride Engineered Heterostructures: Wafer Bonding, Ion Slicing, and More

Wafer bonding in combination with ion slicing emerges as a nonpareil strategy by which bulk quality thin layers can be transferred onto different host materials. In the first part of this paper a successful application of this process to transfer 4-inch InP layer onto Si wafer is demonstrated. The use of SiO2 interlayer makes the fabricated heterostructures compatible with high temperature processes. In the second section we address the applicability of this process for 2-inch freestanding GaN wafers which exhibit a strong post-implantation bowing making any bonding exceedingly difficult. We describe the origin of this bow enhancement and present a novel strategy to manipulate it. Based on our approach a successful bonding of 2-inch free-standing GaN into sapphire handle wafers is achieved. Finally, by using a variety of experimental techniques, we explore the atomic processes and structural transformations involved in H ion-induced GaN splitting. A plausible mechanistic picture is presented.

Free‐standing zinc‐blende (cubic) GaN substrates grown by a molecular beam epitaxy process

AbstractIn this paper bulk, free‐standing, zinc‐blende (cubic) GaN wafers grown by plasma‐assisted molecular beam epitaxy (PA‐MBE) are described. GaN layers of up to 60 microns in thickness have been grown. Characterisation measurements confirm the cubic nature of the GaN crystals and show that the fraction of hexagonal material present is not more than ∼10%. Cubic (001)GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0001) c‐axis wurtzite GaN. Free standing GaN wafers grown by PA‐MBE make ideal lattice‐matched substrates for the growth of cubic GaN‐based structures for optoelectronic devices, and high power and high frequency electronic applications. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of free-standing zinc-blende (cubic) GaN layers and substrates

In this paper, we describe bulk, free-standing, zinc-blende (cubic) GaN wafers grown by plasma-assisted molecular beam epitaxy. We have grown GaN layers of up to 60 µm in thickness. We present the data from characterization measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the material that is hexagonal in nature is not more than about 10% in the best thick samples. Cubic (0 0 1) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0 0 0 1) c-axis wurtzite GaN. Therefore, the free-standing GaN wafers we have grown would make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high-power and high-frequency electronic applications.