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 {101} facets, were achieved with a good selectivity. Si-doped GaN hexagonal pyramids delimited by vertical {100} facets and (0001) top facet were obtained for a high SiH4 flow rate in the vapor phase. We found that the GaN growth rates VR and VC, measured in the R <101> and C <0001> directions respectively, were drastically affected by the Mg and Si incorporation. By adjusting the Mg partial pressure in the growth chamber, the VR/VC 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} Al2O3. 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 〈100〉 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.

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.

p-doping of GaN by MOVPE

Mg has been widely used as p-doping species despite its intrinsic difficulties. It is nowadays well established that during the growth process of Mg doped GaN, atomic H is generated from the decomposition of NH3 and Mg-H complexes are formed in the layer. This has been for instance shown by the occurrence of LO mode in IR absorption, and by the observation of the Mg-H local vibration modes. This H passivation limits the electrical activity of Mg, therefore an activation process is required to get full activation of the Mg atoms. In the present study, bismethylcyclopentadienyl magnesium [(MeCp)2Mg] was used as precursor. However, this precursor reacts in the gas phase with NH3 to produce tiny solid particles as evidenced by a very bright diffuse emission visible along the laser beam used for reflectometry measurements. This simplest obvious product would be [(MeCp)Mg(NH2)]m(m≥2). To limit this drawback, Ga and Mg precursor lines have been separated. With proper in situ heat treatment, doping densities up to 1.5×1018 cm−3 have been obtained. PL spectra of lightly Mg doped samples (1016 cm−3) are dominated by shallow donor-acceptor pairs whereas for higher doping densities ( 1018 cm−3), the luminescence is dominated by a broad band in the 2.7-2.9 eV range. GaN LEDs were fabricated from Si doped (n-type) and Mg-doped (p-type) GaN, these LEDs emit in the blue-UV range.

Ultrawide-bandwidth, superluminescent light-emitting diodes using InAs quantum dots of tuned height

An ultrawide-bandwidth, superluminescent light-emitting diode (SLED) utilizing multiple layers of dots of tuned height is reported. Due to thermal effect, the superluminescent phenomenon is observed only under pulse-mode operation. The device exhibits a 3 dB bandwidth of 190 nm with central wavelength of 1020 nm under continuous-wave (cw) conditions. The maximum corresponding output power achieved in this device under cw and pulsed operation conditions are 0.54 mW and 17 mW, respectively.

Mode-locked pulses from mid-infrared quantum cascade lasers

In this study, we report the unequivocal demonstration of midinfrared mode-locked pulses from quantum cascade lasers. The train of short pulses was generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL). Pulses with duration of about 3 ps at full-width-at-half-maxima and energy of 0.5 pJ were characterized using a second-order interferometric autocorrelation technique based on a nonlinear quantum well infrared photodetector. The mode-locking dynamics in the QCLs was modeled based on the Maxwell-Bloch equations in an open two-level system. Our model reproduces the overall shape of the measured autocorrelation traces and predicts that the short pulses are accompanied by substantial wings as a result of strong spatial hole burning. The range of parameters where short mode-locked pulses can be formed is found.

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.