Sung-Jin Son

Stress relaxation in Si-doped GaN studied by Raman spectroscopy

We report the Si-doping-induced relaxation of residual stress in GaN epitaxial layers grown on (0001) sapphire substrate by the metalorganic vapor phase epitaxy technique. Micro-Raman spectroscopy is used to assess stress situation in the films with systematically modulated doping concentration from 4.0×1017 up to 1.6×1019 cm−3. As the Si-doping concentration increases, a monotonic decrease of the E2 phonon frequency is observed, which signifies gradual relaxation of the stress in the film. The layers are fully relaxed when electron concentration exceeds 1.6×1019 cm−3. The linear coefficient of shift in Raman frequency (ω) induced by the in-plane biaxial compressive stress (σ∥) is estimated to be Δω/Δσ∥=7.7 cm−1/GPa. We suggest that Si doping increases density of misfit dislocation, judging from linewidth of x-ray rocking curve.

High-quality GaN epilayer grown by newly designed horizontal counter-flow MOCVD reactor

We have fabricated a newly designed horizontal counter-flow reactor for growing high-quality III-V nitrides and characterized the GaN/sapphire(0 0 0 1) grown in it. The surface morphology of the film was featureless and smooth without any defects such as hillocks or truncated hexagonals. The measured background concentration and carrier mobility of the film 1.5 m thick are 4 × 1017/cm3 and 180 cm2/V s, respectively. The defect density measured by TEM is about 1 × 109/cm2 and the FWHM of DCX-ray curving is 336 arcsec, respectively. This crystallinity is similar to what was commonly obtained for GaN on sapphire until recently. The FWHM of the band-edge emission peak measured by PL at room temperature is typically around 14 and 4 meV for the main extonic peak(DBE) at 10 K. Except DBE at 3.490 eV, two minor structures are detected on the high-energy and low-energy shoulder of DBE at 3.498 eV(FE) and 3.483(ABE).

Characteristics of Mg-doped GaN epilayers grown with the variation of Mg incorporation

Abstract We have studied the growth characteristics of Mg-doped GaN epilayers grown by MOCVD with the variation of Cp 2 Mg flow rate. To optimize the p-type conductivity. We investigated the dependence of acceptor concentration on the dopant source (Cp 2 Mg) flow rate. The van der Pauw technique, double-crystal X-ray diffractometry (DCXRD) and photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties. As the incorporation of Mg in GaN epitaxy increases, the surface morphology and crystallinity of the layers become rough and worse because of the increase of lattice distortion due to the large difference of the atomic size between Ga and Mg. With the increase of Mg incorporation, the resistivity of the epilayers increases abruptly without discontinuity because of the increase of much Mg–H complex not cracked. So, it is possible to know that only the partial amount of Mg–H complex in the layers are unbound by annealing at a certain condition. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers first increases and then decreases from a certain Mg flow rate. As well as the results of hole concentration, the blue emission intensity of the layers in PL spectra at room temperature first increases and then decreases from a certain Mg flow rate. Therefore, it can be concluded that there is a limitation in Mg activation and the hole concentration decreases from this limit though the incorporation of Mg increases.

Mobility enhancement and yellow luminescence in Si-doped GaN grown by metalorganic chemical vapor deposition technique

Abstract We have studied the Si-doping characteristics in GaN on (0 0 · 1) sapphire substrates. The films were grown by horizontal metalorganic chemical vapor deposition technique at 100 Torr. Prior to Si-doping, achievement of optimum growth conditions was preceded in order to insure doping controllability. For undoped film, background carrier concentration was as low as 4 × 10 17 cm −3 . 10 K-photoluminescence (PL) in undoped film was dominated by the luminescence from two sharp free-excitons and one bound-exciton. n-Type doping with SiH 4 was carried out and electron concentration of up to 2 × 10 19 cm −3 was achieved. As the electron concentration increased, the activation efficiency of Si donor also increased. In spite of the increased carrier concentration, Hall mobility in the doped films was increased by a factor of up to two (220 cm 2 /V s) compared to the undoped films. The mobility behavior is attributed to a transport mechanism involving defect band conduction and does not correlate with the line width of the X-ray rocking curve. In addition, as the doping concentration increased, yellow luminescence occurring at around 2.2 eV gradually dominated PL spectra at 10 K and room temperature. Our PL study suggests that gallium vacancy is responsible for the yellow luminescence.

Characteristics of InxGa1-xN/GaN grown by LPMOVPE with the variation of growth temperature

Abstract We have studied the growth of In x Ga 1 − x N/GaN on (0 0 0 1) sapphire substrates. The films were grown in a horizontal MOVPE reactor at the reduced pressure of 100 Torr. In composition, x in In x Ga 1 − x N/GaN grown at 810°C determined by the DCXRD is 0.08 and 0.13 for 770°C. The FWHMs of the DCXRD for (0 0 0 2) diffraction from the In 0.08 Ga 0.92 N and In 0.13 Ga 0.87 N are 11 arcmin and 9 arcmin, respectively, whose difference is considered to be resulted from In dissociation. In addition, we confirmed that the thermal pit density of In 0.08 Ga 0.92 N was higher than that of In 0.13 Ga 0.87 N with the SEM observation. The carrier concentration and mobility of In 0.08 Ga 0.92 N are 3 × 10 18 /cm 3 and 47 cm 2 /V s, and those of In 0.13 Ga 0.87 N are 9 × 10 18 /cm 3 and 70 cm 2 /V s, respectively. In spite of the lower carrier concentration, the mobility of In 0.08 Ga 0.92 N is lower than that of In 0.13 Ga 0.87 N, which is considered to be due to the higher defect density of In 0.08 Ga 0.92 N than that of In 0.13 Ga 0.87 N as can be seen from the result of DCXRD. The FWHMs of band-edge emission peak measured by PL at room temperature were 110 meV at 395 nm for In 0.08 Ga 0.92 N and 120 meV at 410 nm for In 0.13 Ga 0.87 N. The In composition in InGaN increased as the growth temperature decreased, and In was still dissociated at 770°C. Some high-quality InGaN films were grown on GaN films compared with the recent results of others.

Growth and characterization of GaN epilayers grown at various flow rates of trimetylgallium during growth of nucleation layers

GaN epilayers have been grown on GaN nucleation layers as a function of the flow rate of TMG (trimetylgallium) decreases by deposition (MOCVD). The size of crystalline and amorphous GaN droplets became smaller as the flow rate of TMG (10.0, 8.0, 6.0, 4.0 SCCM, respectively) decreases during the growth of nucleation layers having the same thickness. The surface morphology and crystallinity of the epilayers characterized by DCXRD (double crystal X-ray diffractometer) depend on the TMG flow rate during growth of the GaN nucleation layers. The GaN epilayers grown on a nucleation layer at a rate of 8.0 SCCM exhibits the best crystallinity.

Effects of hillocks and post-growth annealing on electrical properties in GaN grown by metalorganic chemical vapor deposition

Abstract We have studied the effects of growth conditions of GaN buffer layer on the properties of GaN epilayer which was grown by metalorganic chemical vapor deposition technique. As nucleus size was increased by growth conditions of the GaN buffer layer, hillocks dominated surface morphology. The hillocks were detrimental to the electrical properties of GaN epilayer because they enhance the incorporation of electrically active impurities. From post-growth annealing, it was proposed that dislocations and/or carbon are the electron Hall mobility limiting compensators.