Im Taek Yoon

Deep level transient spectroscopy in plasma-assisted molecular beam epitaxy grown Al0.2Ga0.8N/GaN interface and the rapid thermal annealing effect

We investigated deep-level traps formed in Al0.2Ga0.8N/GaN heterostructures grown using plasma-assisted molecular beam epitaxy and by performing deep level transient spectroscopy (DLTS). Two electron traps with activation energies of Ec−150 meV and Ec−250 meV were observed, and their capture cross-sections (σT) were estimated to be 2.0×10−18 cm2 and 1.1×10−17 cm2, respectively. Different behaviors in the dependence of DLTS on filling pulse length confirm that the traps originated from N vacancies and dislocations. The amplitude of the dislocation-induced DLTS signal was reduced significantly by high-temperature rapid thermal annealing under N2 ambient after hydrogen treatment due to the reduction in dislocation density.

Characterization of ferromagnetic Ga1−xMnxN layers grown on sapphire (0001) substrates

Ga1−xMnxN epilayers with a well-ordered ferromagnetic structure were grown on sapphire (0001) substrates, using the plasma enhanced molecular-beam epitaxy technique. Ga1−xMnxN films were found to be homogeneous, and to exhibit n-type conductivity and ferromagnetic ordering with a Curie temperature (TC) above room temperature. This was confirmed by transmission electron microscopy, x-ray diffraction, and by magnetometry using a superconducting quantum interference device. The high-temperature (T=300 K) photoluminescence (PL) spectra exhibited Mn-related free-to-acceptor pair transitions for Ga1−xMnxN layers with x≈0.2% and x≈0.6%. A Mn acceptor level of a Ga1−xMnxN layer with low Mn content was found to be located around 330 meV above the top of the valence band, suggesting that Mn-bound holes in group-III nitrides exhibit effective-masslike behavior. The excitation and temperature-dependent PL provided convincing evidence of a band-edge exciton to acceptor transition.

Spin disorder scattering mechanism of ferromagnetic Ga1-xMnxAs layers on (100) GaAs substrates

The temperature-dependent Hall resistivity and carrier concentrations of Ga1−xMnxAs epilayers grown on (100) semi-insulating GaAs substrates by molecular beam epitaxy have been investigated in the temperature range of 10–300 K. A Ga1−xMnxAs sample with x≈4.4% shows typical insulator behavior and Ga1−xMnxAs samples with x≈2.2 and 3.7 % show typical metallic behavior. A model taking into account ionized impurity and spin disorder scattering mechanisms was used to portray properly the observed features of the temperature-dependent Hall resistivity data. The value of the p-d exchange energy was J=59.4±0.5 and 71.9±0.5 eV A3 for the samples with x≈2.2 and 3.7 %, respectively. Ionized impurity scattering dominates the entire temperature range, with a temperature-independent spin disorder scattering in the paramagnetic region. It was found that the spin disorder scattering mechanism had a strong temperature dependence on 1−T2 in the ferromagnetic region.

Deep level defects in Si-doped AlxGa1−xN films grown by molecular-beam epitaxy

The deep trap levels of AlxGa1−xN films with x in the range from 0 to 0.15 grown on c-plane sapphire substrates using rf-plasma-assisted molecular-beam epitaxy have been investigated by deep level transient spectroscopy measurements. Two distinct defect levels (denoted as Ei and Di) were observed. The origins of the Ei and the Di are associated with point defects such as the N vacancies and extended defects, such as the threading dislocations, respectively. According to Al content (x), the activation energy and capture cross section for the Di defect ranged from 0.19to0.41eV and 1.1–6.6×10−15cm2, respectively. The trap energy levels of Di defects in AlxGa1−xN were calculated and the values were nonlinear with Al content. The bowing parameter of AlxGa1−xN films was determined to be b=1.22.

GaN nanorods grown on Si (111) substrates and exciton localization

We have investigated exciton localization in binary GaN nanorods using micro- and time-resolved photoluminescence measurements. The temperature dependence of the photoluminescence has been measured, and several phonon replicas have been observed at the lower energy side of the exciton bound to basal stacking faults (I1). By analyzing the Huang-Rhys parameters as a function of temperature, deduced from the phonon replica intensities, we have found that the excitons are strongly localized in the lower energy tails. The lifetimes of the I1 and I2 transitions were measured to be < 100 ps due to enhanced surface recombination.PACS: 78.47.+p, 78.55.-m, 78.55.Cr, 78.66.-w, 78.66.Fd

Properties of room-temperature ferromagnetic semiconductor in manganese-doped bilayer graphene by chemical vapor deposition

We report a ferromagnetic graphene field-effect transistor with a band gap. The Mn-doped graphene has a coercive field (Hc) of 188 Oe and a remanent magnetization of 102 emu cm−3 at 10 K. The temperature-dependent conductivity indicates that the Mn-doped graphene has a band gap of 165 meV. A fabricated graphene FET revealed p-type semiconducting behavior, and the field effect mobility was determined to be approximately 2543 cm2 V−1 s−1 at room temperature.

Clarification of enhanced ferromagnetism in Be-codoped InMnP fabricated using Mn/InP:Be bilayers grown by molecular beam epitaxy

The p-type InMnP:Be epilayers were prepared by the sequential growth of Mn/InP:Be bilayers using molecular-beam-epitaxy and the subsequent in-situ annealing at 200–300 °C. In triple-axis x-ray diffraction patterns, the samples revealed a shoulder peak indicative of intrinsic InMnP. The ferromagnetic transition in InMnP:Be was observed to occur at the elevated temperature of ∼140 K, and the ferromagnetic spin-domains clearly appeared in magnetic force microscopy images. The improved ferromagnetic properties are attributed to the increased p–d hybridation due to high p-type conductivity of InMnP:Be (p ∼ 1020 cm−3). The results suggest that enhanced ferromagnetism can be effectively obtained from Be-codoped InMnP.

Optical properties of Mn-doped GaAs layers grown on (100) GaAs substrate

Mn-doped Ga1−xMnxAs epilayers grown on semi-insulating (100) GaAs substrates using the liquid phase epitaxy technique were investigated using photoluminescence and Hall effect measurements from 20 to 300 K. Transitions involving shallow Mn acceptors were identified through photoluminescence measurements and the ionization energy of Mn acceptor was determined to be 104.6 meV which is in good agreement with values calculated from a hydrogenic hole model including sp–d exchange contribution. Also, it was concluded that Ga1−xMnxAs alloys were obtained from samples with low Mn concentration (below x≈1%) and these layers have a simple impurity band merging to the valence band.

Magnetotransport properties of ferromagnetic Ga1−xMnxAs layers on a (100) GaAs substrate

The magnetotransport properties of ferromagnetic Ga1−xMnxAs epilayers with Mn mole fractions in the range of x≈2.2%–4.4% were investigated using Hall effect measurements. The temperature-dependent Hall carrier concentration for a metallic sample with x≈2.2% was analyzed assuming an activation energy from two acceptor levels. It was found that the two acceptor levels with activation energies of 129.4 and 31.6 meV at B=0Oe decreased to 87.6 and 30.7 meV, respectively, at B=5kOe. The decrease in acceptor activation energy from 129.6 to 87.6 meV was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for the increase in carrier concentration. From magnetic-field-dependent Hall resistance data, the Curie temperature was estimated to be TC=60 and 70 K for Ga1−xMnxAs samples with x≈2.2 and x≈4.4%, respectively. The magnetoresistance measurements confirmed that the anomalous Hall effect existed in these samples that showed metallic and insulating behavior, respectively.

Native hole traps of ferromagnetic Ga1-xmnxAs layers on (100) GaAs substrates

Dominant hole traps of ferromagnetic Ga1−xMnxAs and epilayers with an Mn mole fraction of x≈2.2% and 4.4% were identified employing deep-level transient spectroscopy. Three hole traps with binding energies of EA=0.38±0.01 eV at 140 K, EB=0.43±0.01 eV at 220 K, and EC=0.65±0.01 eV at 300 K above the top of the valence band were observed. Comparing with theoretical data of GaAs, it appears most likely that the trap with EA is associated with a gallium vacancy (VGa) or the arsenic antisite complex (GaAs+VAs), whereas the traps with EB and EC are associated with two charge states of arsenic antisite (AsGa) defect. The hole capture cross sections were determined as σp(A)=3.7×10−11, σp(B)=1.5×10−14, and σp(C)=1.1×10−14 cm2, respectively. The samples with x≈2.2% and x≈4.4% show typical behavior for metallic Ga1−xMnxAs and insulator Ga1−xMnxAs, respectively, through Hall measurements.