Growth Of High-quality GaN Research Articles

Thick and high-quality GaN growth on GaAs (1 1 1) substrates for preparation of freestanding GaN

The growth of thick, high-quality GaN layers on GaAs (1 1 1)A substrates was investigated using a two-step growth process by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). The thickness and temperature ramping rate of the low-temperature (LT)-grown GaN buffer layer as well as the growth temperature of the subsequent thick GaN layer at high temperatures were found to be the key factors controlling the crystalline quality of the GaN layer on GaAs (1 1 1)A substrate. Growth of a 50-nm-thick LT-GaN buffer layer at 550°C and subsequent heating to 1000°C at a temperature ramping rate of 16.1°C/min afforded a 100-μm-thick high-quality GaN layer with a specular surface. The results indicate that a thick GaN layer can be successfully grown on a GaAs (1 1 1)A substrate at high temperatures and represent a promising technique for the preparation of freestanding GaN substrates.

Lattice Relaxation of AlN Buffer on Surface-Treated SiC in Molecular-Beam Epitaxy for Growth of High-Quality GaN

ABSTRACTThe effects of SiC surface treatment on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers grown on the buffer layers were studied. AlN buffer layers and GaN main layers were grown by plasma-assisted molecular-beam epitaxy on on-axis 6H-SiC (0001)Si substrates. High-temperature HCl-gas etching resulted in an atomically flat SiC surface with (√3×√3)R30° surface reconstruction, while HCl-gas etching followed by HF chemical treatment resulted in an atomically flat surface with (1×1) structure. The AlN layer grown on the (1×1) surface showed slower lattice relaxation. GaN grown on the AlN buffer layer exhibited a (0002) X-ray rocking curve of 70 arcsec and 107 cm−2 of screw-type dislocation density, which was superior than that of GaN grown on (√3×√3)R30° surface.

An Oxygen Doped Nucleation Layer for the Growth of High Optical Quality GaN on Sapphire

We present a new oxygen-doped AlN nucleation layer for the growth of high quality GaN on sapphire by metalorganic vapor phase epitaxy. We investigated in detail the influence of oxygen in the gas phase in a range from 0.5 to 10 ppm. Around 4 ppm we obtained very stable and reproducible process conditions. A further optimization of the nucleation thickness resulted in GaN layers showing smooth and featureless surfaces with excellent spectroscopic properties. The best values were 45 arcsec in XRD rocking curve of the (002) peak and 1.7 meV in low temperature photoluminescence (PL) of the donor bound exciton. No yellow luminescence even at room temperature could be detected. Because of the low FWHM in PL a lot of features near the bandgap of GaN could be resolved. We observed the donor and acceptor bound exciton as well as the free A, B and C exciton.

An Oxygen Doped Nucleation Layer for the Growth of High Optical Quality GaN on Sapphire

We present a new oxygen-doped AlN nucleation layer for the growth of high quality GaN on sapphire by metalorganic vapor phase epitaxy. We investigated in detail the influence of oxygen in the gas phase in a range from 0.5 to 10 ppm. Around 4 ppm we obtained very stable and reproducible process conditions. A further optimization of the nucleation thickness resulted in GaN layers showing smooth and featureless surfaces with excellent spectroscopic properties. The best values were 45 arcsec in XRD rocking curve of the (002) peak and 1.7 meV in low temperature photoluminescence (PL) of the donor bound exciton. No yellow luminescence even at room temperature could be detected. Because of the low FWHM in PL a lot of features near the bandgap of GaN could be resolved. We observed the donor and acceptor bound exciton as well as the free A, B and C exciton.

Effects of Substrate Pretreatment and Buffer Layers on GaN Epilayers Grown by Hydride Vapor Phase Epitaxy

We have studied the effects of substrate nitridation and buffer layers on the structural quality of GaN grown by HVPE. Our work shows that, at certain process conditions, both substrate nitridation and buffer layers can improve the crystalline quality of GaN grown by HVPE. This indicates that the two techniques are transferable to the HVPE growth of high quality GaN.

Effects of Substrate Pretreatment and Buffer Layers on GaN Epilayers Grown by Hydride Vapor Phase Epitaxy

We have studied the effects of substrate nitridation and buffer layers on the structural quality of GaN grown by HVPE. Our work shows that, at certain process conditions, both substrate nitridation and buffer layers can improve the crystalline quality of GaN grown by HVPE. This indicates that the two techniques are transferable to the HVPE growth of high quality GaN.

OMVPE growth of P-type GaN using solution Cp2Mg

Bis(cyclopentadienyl)magnesium (Cp2Mg) is a common source for p-type doping in GaN and AlInGaP materials. It is a white crystalline solid with very low vapor pressure, leading to transport problems similar to solid trimethyindium (TMI). Some of these problems can be alleviated by a newly developed source-solution magnesocene, Cp2Mg, dissolved in a solvent that is essentially nonvolatile. In this paper, we report the growth and comparative results of Mg-doped GaN grown by OMVPE using solid and solution Cp2Mg. Using both sources, we optimized parameters to obtain high-quality GaN growth with hole concentrations up to 1 1018/cm3.

Effects of step-graded AlxGa1−xN interlayer on properties of GaN grown on Si(111) using ultrahigh vacuum chemical vapor deposition

We report the growth of high-quality GaN on a Si(111) substrate using a five step-graded AlxGa1−xN (x=0.87–0.07) interlayer between GaN epilayer and AlN buffer layer by ultrahigh vacuum chemical vapor deposition. The crack density and the surface roughness of the GaN layer grown on the graded AlxGa1−xN interlayer were substantially reduced, compared to those of GaN grown on an AlN buffer layer. Significant improvement in the structural and optical properties of the GaN layer was also achieved by the use of a graded interlayer. These results are attributed to the decrease of the lattice mismatch between GaN and AlN layer, and the reduction of the thermal stress by the graded interlayer.

Gas source molecular beam epitaxy of high quality AlxGa1−xN (0⩽x⩽1) on Si(111)

Layers of AlxGa1−xN, with 0⩽x⩽1, were grown on Si(111) substrates by gas source molecular beam epitaxy with ammonia. We show that the initial formation of the Si–N–Al interlayer between the Si substrate and the AlN layer, at a growth temperature of 1130–1190 K, results in very rapid transition to two-dimensional growth mode of AlN. The transition is essential for subsequent growth of high quality GaN, AlxGa1−xN, and AlGaN/GaN superlattices. The undoped GaN layers have a background electron concentration of (2–3)×1016 cm−3 and mobility up to (800±100) cm2/V s, for film thickness ∼2 μm. The lowest electron concentration in AlxGa1−xN, with 0.2<x<0.6, was ∼(2–3)×1016 cm−3 for 0.5–0.7-μm-thick film. Cathodoluminescence and optical reflectance spectroscopy were used to study optical properties of these AlxGa1−xN layers. We found that the band gap dependence on composition can be described as Eg(x)=3.42+1.21x+1.5x2. p–n junctions have been formed on crack-free layers of GaN with the use of Mg dopant. Light emitting diodes with peak emission wavelength at 3.23 eV have been demonstrated.

Growth of high-quality GaN on Si(111) substrate by ultrahigh vacuum chemical vapor deposition

An ultrahigh vacuum chemical vapor deposition (UHVCVD) system was employed to grow high-quality GaN on a Si (111) substrate using a thin AlN buffer layer. X-ray diffraction, high-resolution electron microscopy (HREM), and photoluminescence (PL) data indicate that a single crystalline GaN layer with a wurtzite structure was epitaxially grown on a silicon substrate. The full width at half maximum (FWHM) of the x-ray rocking curve for the GaN (0002) diffraction was 16.7 arc min. A cross-sectional HREM image showed an amorphous SiNx layer at the Si/AlN interface, as well as stacking faults and inversion domain boundaries in the GaN epilayer. An intense PL emission line, which is associated with the recombination of the donor bound exciton, was observed at 10 K PL spectra (FWHM=6.8 meV) and a strong band edge emission was obtained (FWHM=33 meV) as well, even at room temperature. These results indicate that high-quality GaN can be grown on Si (111) substrates using a UHVCVD growth method.

New buffer layer technique using underlying epitaxial AlN films for high-quality GaN growth

AbstractWe demonstrate high-quality epitaxial AlN films on C-plane sapphire as a new buffer layer technique for the growth of high-quality GaN. The obtained GaN films were atomically flat and the full width at half maximum (FWHM) values of the X-ray rocking curve (XRC) were 120arcsec and 510arcsec for (0004) and (10-10), respectively. XRC-FWHM values of the underlying AlN film were 90arcsec and 1800arcsec for (0004) and (10-10), respectively. The XRC results of the GaN indicated that tilted mosaicity of the GaN was much smaller than that of conventional GaN and that small tilted mosaicity coincided with small twisted mosaicity. It was characteristic that the twisted mosaicity drastically reduced from the AlN to the GaN. According to TEM results, it was clarified that the high-quality AlN layer had an effect in reducing the amount of screw-type threading dislocations and that the GaN/AlN interface played an important role in reducing the amount of edge-type dislocations.

Two-step growth of high-quality GaN by hydride vapor-phase epitaxy

The use of a low-temperature layer of GaN formed by hydride vapor-phase epitaxy (HVPE) as a template to grow high-quality HVPE films is demonstrated. Using layers formed by reacting GaCl and NH3 at 550 °C and annealed at a growth temperature of 1050 °C, thick films of GaN can be grown by HVPE with fewer than 108 dislocations per cm2. Dislocation densities measured by high-resolution x-ray diffraction, atomic-force microscopy step termination density and plan-view transmission electron miscroscopy reveal that ∼23 μm films have dislocation densities of ∼6×107 cm−2. Obtaining high-quality single-crystal character films was found to be dependent on several factors, most importantly, the rate of temperature increase to growth temperature and the layer thickness.

Interfacial Structure and Defects in GaN/AlGaN Heterojunction Epitaxially Grown on LiGa02 Substrate by Molecular Beam Epitaxy

Abstract The performance of III-Nitride based Light Emitting Diodes (LEDs), LASERs, GaN/AlGaN MODFETs (Modulation-doped Field Effect Transistors) and HEMTs (High Electron Mobility Transistors) have been improved dramatically over the past few years [1,2], despite the relatively poor material quality of GaN, as compared to GaAs, for example. The intrinsic properties of the materials system make it extremely well suited to both optoelectronic and microwave power transistor applications. Typically, GaN is grown on substrates such as GaAs, Al2O3 (sapphire) or SiC with large lattice mismatch. This has usually resulted in an extremely high defect density in the GaN layer. The growth of GaN on lithium gallate LiGaO2 (LGO) affords many advantages compared to all other available substrates. LGO offers the smallest average lattice mismatch of any available substrate for the Ill-nitrides. This facilitates the growth of high quality GaN directly on Lithium Gallate without the need for a defective buffer to decouple the strain associated with the large lattice mismatch of other substrates [3].

High Quality Epitaxial Growth of Hexagonal GaN on Al2O3(0001) and Cubic GaN on GaAs(100) by Molecular Beam Epitaxy

The growth of high quality GaN thin films of both hexagonal (h-GaN) and cubic phase (c-GaN) was achieved by molecular beam epitaxy. An ultra-thin amorphous GaN buffer layer was used to obtain a high quality h-GaN film on Al2O3(0001). Near-band edge emission (I2 and DAP) dominated the observed photoluminescence spectrum, and the full width at half maximum of the X-ray rocking curve was 9.8 arcmin. P-type conductivity was successfully achieved by Mg doping of h-GaN film with a hole concentration of 3 × 1017 cm–3 and a hole mobility of 27 cm2/Vs. With Si doping, it was found that the electron concentration could be controlled between 1019 and 1021 cm–3 by the Si cell temperature. For c-GaN growth on GaAs(100), a high phase purity was realized by using a newly introduced AlGaAs buffer layer technique. It was found that the formation of an AlGaN surface by nitridation of an AlGaAs buffer layer is essential to grow a highly pure cubic phase GaN film. An X-ray diffraction peak attributable to secondary hexagonal GaN could not be observed in reciprocal space area mapping and pole-figure measurements.

Optimized structural properties of wurtzite GaN on SiC(0001) grown by molecular beam epitaxy

We have investigated the optimal conditions for molecular beam epitaxial growth of high quality GaN on 6H-SiC(0001) substrates. The quality of these films is reflected both by the narrow x-ray peak widths as well as the excellent surface morphology. In this work, it is shown that increasing growth temperature leads to an improvement in bulk quality and lower x-ray peak widths for both symmetric and asymmetric reflections. We also note a marked improvement in surface morphology, from a columnar appearance to a two-dimensional surface, under extremely Ga-rich growth conditions.

Selective growth of high quality GaN on Si(111) substrates

We demonstrate selective growth of high-quality GaN by gas-source molecular beam epitaxy on Si(111) wafers patterned with SiO2. GaN was grown on wafers having two different buffer layers. The first buffer layer contains two AlGaN/GaN superlattices, separated by GaN spacer, grown on AlN, with a total thickness of 400 nm. The second is a thin AlN (1.5 nm) buffer layer. X-ray diffraction confirms (0001) growth orientation, smooth interfaces, and coherence lengths comparable to the layer thickness in both samples. In the case of the thin AlN buffer layer, the tensile stress measured by the E2 Raman line shift is attributed to the mismatch in the thermal expansion coefficients of GaN and Si. However, when the AlGaN/GaN superlattice buffer layer is grown first, a reduced stress is measured. High carrier concentrations (≈1018 cm−3) are seen in the GaN grown on the thin AlN buffer layer, which we attribute to the incorporation of silicon from the substrate during the growth process. The superlattice buffer layer is seen to inhibit this diffusion, resulting in a carrier concentration of <1017 cm−3 in the GaN.

MBE growth of high quality GaN on LiGaO 2 for high frequency, high power electronic applications

Lithium gallate is receiving ever increasing attention as a possible candidate for III-nitride material growth due to its superior lattice match. This paper summarizes the recent, promising results from material growths on lithium gallate including aluminum, gallium and indium-nitride alloys. Structural, optical and electrical diagnostics are presented. The use of this material could result in significant advantages for use in power electronic applications. Specifically, the present material quality is in some ways, superior to all other current substrate technologies since very high quality, extremely thin nitride material can be produced. The areas for which lithium gallate still lags other technologies is also discussed. Issues that directly effect electronic devices, such as overcoming the limited thermal conductivity via heat pipes or substrate removal, the possibilities of achieving higher p-type doping and higher indium concentrations through the use of low growth temperatures, obtaining single polarity material with nearly no mosaic spread in grain orientation, along with issues of substrate cost are discussed.

High Quality AlN and GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

We describe the growth of high quality AlN and GaN on Si(111) by gas source molecular beam epitaxy (GSMBE) with ammonia (NH3). The initial nucleation (at 1130−1190K) of an AlN monolayer with full substrate coverage resulted in a very rapid transition to two-dimensional (2D) growth mode of AlN. The rapid transition to the 2D growth mode of AlN is essential for the subsequent growth of high quality GaN, and complete elimination of cracking in thick ( > 2 μm) GaN layers. We show, using Raman scattering (RS) and photoluminescence (PL) measurements, that the tensile stress in the GaN is due to thermal expansion mismatch, is below the ultimate strength of breaking of GaN, and produces a sizable shift in the bandgap. We show that the GSMBE AlN and GaN layers grown on Si can be used as a substrate for subsequent deposition of thick AlN and GaN layers by hydride vapor phase epitaxy (HVPE).

Nitride Semiconductor Surfaces. Effect of Low-temperature Deposited Layer on the Growth of Group-III Nitrides on Sapphire.

Surface control by low-temperature deposited buffer layer enables the growth of high-quality GaN on a sapphire substrate the lattice mismatch of which is as large as 14%. With use of such highly mismatched systems, novel electronic devices such as new lighting source which replaces conventional light bulb, super-high-density optical storage systems, high-power and high-speed transistors and UV image sensors for remote sensing and flame detection will be realized. This paper describes the details of the mechanism of this surface control technology.

High quality GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia

We describe the growth of hexagonal GaN on Si(111) by gas source molecular beam epitaxy with ammonia. The initial deposition of Al, at 1130–1190 K, resulted in a very rapid transition to a two-dimensional growth mode of AlN. The rapid transition is essential for the subsequent growth of high quality GaN and AlGaN. This procedure also resulted in complete elimination of cracking in thick (>2 μm) GaN layers. For layers thicker than 1.5 μm, the full width at half maximum of the (0002) GaN diffraction peak was less than 14 arc sec. We show that a short period superlattice of AlGaN/GaN grown on the AlN buffer can be used to block defects propagating through GaN, resulting in good crystal and luminescence quality. At room temperature, the linewidth of the GaN exciton recombination peak was less than 40 meV, typical of the best samples grown on sapphire.