Soufien Haffouz

Magnesium induced changes in the selective growth of GaN by metalorganic vapor phase epitaxy

Atmospheric pressure metalorganic vapor phase epitaxy is used to perform selective regrowth of GaN on a silicon nitride patterned mask, capping a GaN epitaxial layer deposited on (0001) sapphire substrate. The basic pattern is constituted by a 15 μm period hexagonal array of hexagonal openings in the mask, these openings being circumscribed into 10 μm diam circles. We investigate the effect of Mg incorporation on the growth anisotropy of the localized GaN islands by varying the ratio [Mg]/[Ga] of bis-methylcyclopentadienyl-magnesium and trimethylgallium partial pressures. Both undoped and Mg-doped GaN hexagonal pyramids, delimited by C (0001) and R (1 101) facets, are achieved with a good selectivity. It is found that the GaN growth rates VR and VC, measured in the R〈1101〉 and C 〈0001〉 directions respectively, are drastically affected by the Mg incorporation. By adjusting the Mg partial pressure in the growth chamber, the VR/VC ratio can be increased so that the delimiting top C facet does not vanish as...

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 1150 °C. The coalescence of these GaN islands yields a reduction of the total number of extended defects from the 1010–1011 cm−2 range usually obtained down to the low 109 cm−2 range for the best samples.

Luminescence and reflectivity studies of undoped, n- and p-doped GaN on (0001) sapphire

GaN grown by three different methods (MOVPE, GSMBE and HVPE) have been studied using temperature (T) dependent reflectivity and photoluminescence (PL). Both non intentionally doped (MOVPE, GSMBE and HVPE), n- and p-doped samples (MOVPE and GSMBE) have been investigated. Reflectivity is used to obtain intrinsic transition energies. These energies vary with the amount of strain in the crystal. Growth parameters influencing this strain state are discussed. Using MOVPE (T-g approximate to 1050 degrees C) and GSMBE (T-g approximate to 800 degrees C), it is possible to grow samples whose low temperature PL spectra are dominated by free and bound excitons and their phonon replica. Intentional n-type doping up to 10(20) cm(-3) is easily achieved with Si. For n much greater than 10(18) cm(-3), the spectra broaden and exhibit a blue shift, attributed to band filling. p-Type doping has been attempted using Mg, C and Ca. Ca doping led to compensated samples. C doping using CCl4 resulted in n-type samples, due to simultaneous oxygen incorporation in the layers; a strong enhancement of the 3.27 eV donor acceptor pair PL is also observed in this case. p-Type doping up to 10(18) cm(-3) has been achieved with Mg. With increasing densities, a deepening of the donor acceptor pair PL energy is observed. For high Mg doping, the spectra are dominated by a blue band in the 2.8 eV range, involving deep electron states

Effect of Magnesium and Silicon on the lateral overgrowth of GaN patterned substrates by Metal Organic Vapor Phase Epitaxy

Metalorganic vapor phase epitaxy was used to achieve selective regrowth of undoped, Mg- and Si-doped GaN on a silicon nitride patterned mask, capping a GaN epitaxial layer deposited on (0001) sapphire substrate. Hexagonal openings in the mask defined into 10 µm diameter circles separated by 5µm were used as a pattern for the present study. Uniform undoped and Mg-doped GaN hexagonal pyramids, delimited by C (0001) and R {1 01} facets, were achieved with a good selectivity. Si-doped GaN hexagonal pyramids delimited by vertical {1 00} facets and (0001) top facet were obtained for a high SiH 4 flow rate in the vapor phase. We found that the GaN growth rates V R and V C , measured in the R 01> and C directions respectively, were drastically affected by the Mg and Si incorporation. By adjusting the Mg partial pressure in the growth chamber, the V R /V C ratio can be increased. Hence, the delimiting top C facet do not vanish as usually observed in undoped GaN selective regrowth but conversely expands. On the other hand, under proper growth conditions, 20µm-high Si-doped GaN columns were obtained.

Mg-enhanced lateral overgrowth of GaN on patterned GaN/sapphire substrate by selective Metal Organic Vapor Phase Epitaxy

Selective and lateral overgrowth by Metal Organics Vapour Phase Epitaxy (MOVPE) was carried out until coalescence to produce smooth and optically flat thick GaN layers. A GaN epitaxial layer is first grown using atmospheric pressure Metalorganic Vapour Phase Epitaxy on a {0001} Al 2O3. substrate. Then a 30Å silicon nitride dielectric film is deposited in-situ by reaction of silane and ammonia to form a selective mask. Afterwards, the openings and the figures in the dielectric films are achieved using standard photolithographic technology. Stripes openings in the mask, revealing free GaN surface, are aligned in the 〈1010〉 direction. Typical stripes spacing and width are 10 μm and 5 μm respectively. These patterned layers are further on used for epitaxial regrowth of GaN by MOVPE. The growth anisotropy and therefore the coalescence process is achieved by introducing (MeCp)2Mg in the vapour phase. A two-step process is reported which allows a dramatic reduction of threading dislocations density not only above the masked areas but also above the windows opened in the mask. With this process, very sharp bound exciton luminescence peaks are measured at low temperature in the overgrown GaN.

GaN-based solar-ultraviolet detection instrument

We report on the fabrication of a solar-UV monitoring system that uses GaN-based photodetectors. GaN photoconductors, p-n junction photodiodes, and Schottky barrier photodiodes have been fabricated and characterized as UV sensors. The best performances are obtained in Schottky photodiodes, which show a linear response, a flat responsivity of 100 mA/W, a visible rejection ratio higher than 10(3), and a noise-equivalent power of 1 nW/Hz(-1/2). Preliminary data on Al(x)Ga(1-x)N (x = 0.15, 0.22) detectors are also presented. Using GaN Schottky diodes, we fabricate and evaluate a complete solar-UV detection head.

Optimization of Si/N Treatment Time of Sapphire Surface and Its Effect on the MOVPE GaN Overlayers

We report that the exposure of the sapphire substrate, prior to the deposition of a GaN nucleation layer, under simultaneous silane and ammonia flows fundamentally influences the quality of GaN epilayers grown by Metal-Organic Vapor Phase Epitaxy (MOVPE). This step of the growth process will be referred to as “Si/N treatment”. An increase of the interfacial energy between the GaN nucleation layer and sapphire, induced by this Si/N treatment, changes the morphology (2D to 3D) of this nucleation layer and results in improved quality for GaN epilayers. For an optimized Si/N treatment time (30 s), a 300 K electron mobility as high as 650 cm2/Vs was obtained. Using symmetric (0002) rocking curves, we measured 180 arcsec full width at half maximum (FWHM) in ω-scan. 9 K photoluminescence spectra are dominated by a sharp peak (FWHM = 3.5 meV) corresponding to donor bound exciton recombination.

Analysis of the Visible and UV Electroluminescence in Homojunction GaN LED's

The electrical and electroluminescent properties of MOVPE GaN p-n homojunctions have been analyzed as a function of temperature and bias. Electroluminescence is observed for V>3 V under dc and ac conditions. The main emission at low T is a donor-acceptor transition involving shallow acceptors, though it disappears at higher T due to the ionization of the acceptors and compensation by ionized donors. Room temperature dc and ac electroluminescence spectra evolve under increasing bias from a blue-shifting visible band involving deep states at the p-type side of the p-n junction, to a band-to-band UV recombination at high bias. In agreement, the superlinear dependence of light-current characteristics at low current injection becomes linear when the defects are saturated. Time analysis of the spectra vs pulse duration and duty cycle allows the determination of the visible radiative recombination and relaxation times associated to the Mg-related deep states, which are found to behave as acceptors lying 0.55 eV above the valence band. A simple 3-level model is able to explain the visible emission, which involves the conduction band (or shallow donor) and those deep acceptors in the p-layer. Optimum UV/visible ratio emission requires intense and relatively long pulses, with a high duty cycle to impede visible recombination.

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.

Sub-bandgap optical absorption of MOVPE-GaN grown under controlled nucleation

Abstract Photothermal deflection spectroscopy (PDS) was used to measure the absorbance of GaN films grown from a 3D nucleation layer. A modeling of absorbance was compared with the experimental data. The results indicate the presence of a highly defective region (∼50 nm) near the substrate in the 3D nucleation template. During the subsequent growth, the films form by lateral overgrowth while the defects remain localized at the interface, leading to a two-layer system. The upper layer exhibits a much lower defect density compared to GaN grown on conventional buffer layers.