H. Lahrèche

Stress control in GaN grown on silicon (111) by metalorganic vapor phase epitaxy

The strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy has been investigated. The insertion of AlN/GaN superlattices was found to decrease the stress sufficiently for avoiding crack formation in an overgrown thick (2.5u2002μm) GaN layer. X-ray diffraction and photoluminescence measurements are used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layers. A reduction of threading dislocation density is also observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed with a full width at half maximum of the bound exciton line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) x-ray rocking curve also attests the high crystalline quality of GaN on Si (111), when using these AlN/GaN superlattices.

Growth of high-quality GaN by low-pressure metal-organic vapour phase epitaxy (LP-MOVPE) from 3D islands and lateral overgrowth

Abstract In this work, we present a novel growth method to obtain high structural quality GaN films on sapphire by low-pressure metal-organic vapour phase epitaxy (LP-MOVPE). Our purpose is to achieve epitaxy by lateral overgrowth (ELO) starting from self-organised islands on a GaN layer. Classical ELO is carried out by coalescence of the crystal grown selectively through striped windows opened into a dielectric mask. This classical process implies the deposition and the etching of the mask (commonly SiO 2 or Si x N y ). The use of self-organised islands enables to suppress these two technological steps. Such islands are obtained after a growth mode change induced by silicon as chemical impurity. They present {1xa01xa00xa01} lateral facets and a top {0xa00xa00xa01} facet. Their densities and sizes depend on the silicon treatment and the growth temperature and can be controlled by these two factors. An overgrowth of optimised islands leads to a drastic improvement of the material quality. The defect analysis reveals the presence of only threading dislocations, with a density of 5×10 8 and 9×10 8 xa0cm −2 compared to 10 10 xa0cm −2 for classical layers.

The effect of the Si/N treatment of a nitridated sapphire surface on the growth mode of GaN in low-pressure metalorganic vapor phase epitaxy

In this letter, we studied the effect of the high-temperature Si/N treatment of the nitridated sapphire surface followed by the deposition of a low-temperature GaN nucleation layer on the growth mode of GaN in low-pressure metalorganic vapor phase epitaxy. It was shown that the nucleation layer, initially flat and continuous, converts to wide isolated truncated hexagonal islands having {1–101} facet planes and a top (0001) plane, after heating up to 1150u2009°C. The coalescence of these GaN islands yields a reduction of the total number of extended defects from the 1010–1011u2009cm−2 range usually obtained down to the low 109u2009cm−2 range for the best samples.

Optimisation of AlN and GaN growth by metalorganic vapour-phase epitaxy (MOVPE) on Si(111)

Abstract Single-crystal GaN thin films (500xa0nm) were grown by low-pressure metal-organic vapour-phase epitaxy (LP-MOVPE) on Si(1xa01xa01) substrates using AlN buffer layers. Depending on the AlN growth temperature, the growth mode of GaN can be either two- or three-dimensional (2D or 3D) and structural, electronic and optical properties of GaN layers are consequently changed. 2D growth leads to the best material with a full-width at half-maximum (FWHM) of the (0xa00xa00xa02) X-ray diffraction (XRD) line in rocking curve for GaN of about 656xa0arcsec, a dislocation density in the low 10 10 xa0cm −2 range and a surface root-mean-square (RMS) roughness as low as 0.3xa0nm. AlGaN/GaN two-dimensional electron gas (2DEG) were grown on such layers and mobility of μ =813xa0cm 2 /Vxa0s at 300xa0K and μ =2200xa0cm 2 /Vxa0s at 77xa0K were obtained.

Buffer free direct growth of GaN on 6H–SiC by metalorganic vapor phase epitaxy

The growth of GaN on 6H–SiC is three dimensional (3D) and results in the formation of large islands presenting hexagonal truncated shape with {1–101} lateral facets and a top {0001} facet. In this work, we present a three steps growth process that enables us to grow high quality mirrorlike GaN layers without using AlN buffer layers. During a first step, a thin 3D GaN layer is deposited at high temperature. This layer is smoothed under ammonia flow for several minutes when the growth is interrupted. The subsequent growth of GaN is two dimensional. 600 nm thick GaN films were grown. They were analyzed by high resolution x-ray diffraction, reflectivity, and photoluminescence. All the layers are under strong tensile biaxial strain. The correlation between residual tensile strain in GaN layers and their optical properties is reported for biaxial deformations exx ranging up to 0.37%.

Green InGaN Light-Emitting Diodes Grown on Silicon (111) by Metalorganic Vapor Phase Epitaxy

We report on the growth and characterization of green InGaN light-emitting diodes (LEDs) grown on Si (111) substrates using metalorganic vapor phase epitaxy. A single InGaN quantum well active layer has been used. The optical qualities of InGaN on Si(111) and the p–doping efficiency of GaN are discussed. The turn-on voltage of the LED is 6.8 V and the operating voltage is 10.7 V at 20 mA. Electroluminescence of the LEDs starts at a forward bias of 3.5 V. The electroluminescence peaks at 508 nm, with a full-width at half maximum of 52 nm. An optical output power of 6 µW (in ~ 8π/5 sr) was achieved for an applied current of 20 mA.

Phase separation in metalorganic vapor-phase epitaxy AlxGa(1−x)N films deposited on 6H–SiC

The microstructure of AlxGa(1−x)N films deposited on 6H–SiC substrates by metalorganic vapor-phase epitaxy is studied by transmission electron microscopy and energy-dispersive x-ray analysis. It is shown that in the first step of the growth (three-dimensional), the deposit is constituted of a thin Al-rich AlGaN wetting layer covering the substrate surmounted by pure GaN islands. In a second step, a homogeneous AlxGa(1−x)N film, with nominal Al concentration, is deposited and smoothing is observed. The results of the high-resolution transmission electron microscopy about the strain state of the different parts of the deposit indicate that the driving force for the phase separation phenomena is the decrease of the interfacial energy due to the low mismatch between Al-rich AlGaN and 6H–SiC (1% for pure AlN).

Growth of GaN on (111) Si: a route towards self-supported GaN

Abstract Crack-free GaN/Si(1xa01xa01) thin layers (0.5xa0μm) were grown by metal organic vapour phase epitaxy, using either an AlN buffer layer or (AlN/GaN) strained superlattices. High-resolution X-ray diffraction exhibited a full-width at half-maximum as low as 630xa0arcsec for rocking curve scan on (0xa00xa00xa02) line. Low temperature PL spectra of GaN/Si(1xa01xa01) show unambiguously a state of tensile biaxial strain for all of the layers. These GaN layers were then lifted-off by wet chemical etching from their original silicon substrate and Van der Waals bonded on sapphire or quartz substrates. The ability of growing thick GaN (15–100xa0μm) by halide vapour phase epitaxy on such templates has been successfully achieved, marking a step towards self-supported GaN substrates.

Low pressure MOVPE grown AlGaN for UV photodetector applications

Abstract The low pressure Metalorganic Vapor Phase Epitaxy (LP-MOVPE) growth conditions of AlGaN epilayers on c -oriented sapphire have been optimized with a view to application to UV photodetectors both on GaN and AlN nucleation layers, for aluminium mole fractions lying typically in the range 0–25%. Good structural, electrical and optical properties were obtained for AlGaN alloys on (0001) oriented sapphire substrates, for both undoped and n-type doped epilayers. A typical full width at half maximum (FWHM) of 670–700 arc-s is measured for the (0002) X-ray double diffraction peak in the ω -configuration of 1 μm-thick AlGaN epilayers grown on a GaN nucleation layer, and a typical electron mobility of 50–90 cm 2 V −1 s −1 is measured at T =300 K on 10 18 cm −3 n-type doped AlGaN epilayers. The low temperature photoluminescence ( T =9 K) performed on non intentionally doped AlGaN epilayers with low Al contents (5 and 10%) exhibits reproducibly a sharp exciton-related peak, associated with two phonon replica and a total absence of low photon energy transitions. Optical transmission as well as absorption coefficient measurements using the photothermal deflection spectroscopy (PDS) clearly show that the variation of the energy gap of AlGaN with the aluminium concentration is linear. Solar-blind AlGaN-based photoconductors and Schottky barrier photodiodes with good operating characteristics have been fabricated with these materials.

Near-Band Gap Selective Photoluminescence in Wurtzite GaN

Selective photoluminescence (SPL) experiments are performed at low temperature on n-type GaN layers obtained by epitaxial lateral overgrowth on sapphire. A significant impurity electronic Raman scattering (ERS) effect is observed when excited in the spectral range of excited states of A and B free excitons, giving an accurate determination of the transition energies to the n = 2 states and a straightforward estimation of the A and B Rydberg. From the donor transition energy measured on the ERS spectra we deduce a 30.5 meV binding energy for the main residual donor in our epitaxial layers. In addition, SPL experiments have been used to analyse the neutral donor bound exciton (I2) spectra. It is shown that a laser tuned near the A free exciton induces strong and narrow resonant peaks superimposed to I2. It is concluded that such effect is due to the selective excitation of inhomogeneously broadened I2 excited states. Up to five rotational I2 excited states are identified.