Youn Ho Park

Separation of Rashba and Dresselhaus spin-orbit interactions using crystal direction dependent transport measurements

The Rashba spin-orbit interaction effective field is always in the plane of the two-dimensional electron gas and perpendicular to the carrier wavevector but the direction of the Dresselhaus field depends on the crystal orientation. These two spin-orbit interaction parameters can be determined separately by measuring and analyzing the Shubnikov-de Haas oscillations for various crystal directions. In the InAs quantum well system investigated, the Dresselhaus term is just 5% of the Rashba term. The gate dependence of the oscillation patterns clearly shows that only the Rashba term is modulated by an external electric field.

Synthesis of p-type GaN nanowires

GaN has been utilized in optoelectronics for two decades. However, p-type doping still remains crucial for realization of high performance GaN optoelectronics. Though Mg has been used as a p-dopant, its efficiency is low due to the formation of Mg-H complexes and/or structural defects in the course of doping. As a potential alternative p-type dopant, Cu has been recognized as an acceptor impurity for GaN. Herein, we report the fabrication of Cu-doped GaN nanowires (Cu:GaN NWs) and their p-type characteristics. The NWs were grown vertically via a vapor-liquid-solid (VLS) mechanism using a Au/Ni catalyst. Electrical characterization using a nanowire-field effect transistor (NW-FET) showed that the NWs exhibited n-type characteristics. However, with further annealing, the NWs showed p-type characteristics. A homo-junction structure (consisting of annealed Cu:GaN NW/n-type GaN thin film) exhibited p-n junction characteristics. A hybrid organic light emitting diode (OLED) employing the annealed Cu:GaN NWs as a hole injection layer (HIL) also demonstrated current injected luminescence. These results suggest that Cu can be used as a p-type dopant for GaN NWs.

Observation of gate-controlled spin―orbit interaction using a ferromagnetic detector

From the Shubnikov-de Haas oscillation, estimation of the spin―orbit interaction parameter is possible only for very low temperature. An alternative method available for much wider temperature range is utilizing the ferromagnetic electrode which contacts the quantum well channel and reads the Fermi level of spin-up and -down subbands. The amount of spin-subband shift is controlled by a gate electric field and finally the spin―orbit interaction induced by Rashba effect is obtained near room temperature. At T = 150 K, a gate voltage shifts the spin subband potential and changes the spin―orbit interaction parameter by 23%.

Observation of Spin-Orbit Interaction Parameter Over a Wide Temperature Range Using Potentiometric Measurement

Spin-orbit interaction parameter (¿) can be obtained by measuring the Shubnikov-de Haas oscillation, but this method is valid at only very low temperature. The current induced spin polarization in a 2-D electron gas layer is measured by potentiometric geometry which measures spin dependent chemical potential shifts. Subsequently, a spin-orbit interaction parameter can be extracted up to T = 250 K. In an inverted In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As quantum well system, a of 5.65 × 10^-12 eVm and 3.85 × 10^-12 eVm are obtained at T = 1.8 and 250 K, respectively.

Electrical Spin Transport in n-Doped In 0.53 Ga 0.47 As Channels

Spin injection from a ferromagnet into an n-doped In 0.53 Ga 0.47 As channel was electrically detected by a ferromagnetic detector. At T = 20 K, using non-local and local spin-valve measurements, a non-local signal of 2 ㎶ and a local spin valve signal of 0.041% were observed when the bias current was 1 ㎃. The band calculation and Shubnikov-de Haas oscillation measurement in a bulk channel showed that the gate controlled spin-orbit interaction was not large enough to control the spin precession but it could be a worthy candidate for a logic device using spin accumulation and diffusion.

Large spin accumulation and crystallographic dependence of spin transport in single crystal gallium nitride nanowires

Semiconductor spintronics is an alternative to conventional electronics that offers devices with high performance, low power and multiple functionality. Although a large number of devices with mesoscopic dimensions have been successfully demonstrated at low temperatures for decades, room-temperature operation still needs to go further. Here we study spin injection in single-crystal gallium nitride nanowires and report robust spin accumulation at room temperature with enhanced spin injection polarization of 9%. A large Overhauser coupling between the electron spin accumulation and the lattice nuclei is observed. Finally, our single-crystal gallium nitride samples have a trigonal cross-section defined by the (001), () and () planes. Using the Hanle effect, we show that the spin accumulation is significantly different for injection across the (001) and () (or ()) planes. This provides a technique for increasing room temperature spin injection in mesoscopic systems.

Spin accumulation at in-situ grown Fe/GaAs(100) Schottky barriers measured using the three- and four-terminal methods

We examined the spin accumulation in Fe/n-GaAs Schottky barriers to evaluate the accuracy of the three-terminal (3T) and four-terminal (4T) measurement geometries. A fully epitaxial Fe/n-GaAs junction was grown in situ using cluster molecular beam epitaxy without breaking the vacuum to exclude the formation of an oxide layer or surface roughness at the interface during intermixing. The spin resistance of the 4T nonlocal spin valve (ΔRNLSV = 0.71 Ω) was twice the value obtained using the 4T Hanle effect method (ΔR4TH = 0.35 Ω) at 10 K, as predicted theoretically, and this value remained constant over the temperature range examined, from 10 K to 77 K. The temperature-dependent spin lifetimes measured using the 3T and 4T Hanle effects exhibited similar behaviors. Although the spin resistance obtained using the 3T Hanle effect was enhanced compared with that obtained using the 4T effect, it was reasonable to conclude that the spin signals obtained from the 3T and 4T measurements originated from spin accumulat...

Detection of Rashba field using a rotational applied field

The intrinsic Rashba field in an InAs quantum well is observed using a potentiometric geometry in which a circular shaped permalloy electrode is utilized with a rotational applied field. From the ratio of the curve shift to the applied field and the amplitude of potentiometric signal, a Rashba field of 8.84 T can be estimated. This method is a very simple and accurate alternative to quantitatively observe the Rashba field.

Complementary spin transistor using a quantum well channel

In order to utilize the spin field effect transistor in logic applications, the development of two types of complementary transistors, which play roles of the n- and p-type conventional charge transistors, is an essential prerequisite. In this research, we demonstrate complementary spin transistors consisting of two types of devices, namely parallel and antiparallel spin transistors using InAs based quantum well channels and exchange-biased ferromagnetic electrodes. In these spin transistors, the magnetization directions of the source and drain electrodes are parallel or antiparallel, respectively, depending on the exchange bias field direction. Using this scheme, we also realize a complementary logic operation purely with spin transistors controlled by the gate voltage, without any additional n- or p-channel transistor.

Recrystallization of Ge thin film on SiO2 substrates using a two-step annealing process

The fabrication of high-quality crystalline germanium thin films (GeTF) on an amorphous SiO2 layer is crucial for the realization of high performance-, low cost III-V solar cells used in many applications. Herein, we report the growth of a high-quality crystalline GeTF on SiO2/Si substrates using an ultra-vacuum chemical vapor deposition (UHV-CVD) method. GeTF was grown on the SiO2 layer using a two-step growth and multi-annealing processes. The fabrication method involved the deposition of a 1st seeding layer, annealing, and deposition of a 2nd main layer followed by three times of cyclic annealing. The crystallization of the seeding layer having a thickness of less than 10 nm could be ascribed to the evolution of polycrystalline structures in the main layer. The cyclic annealing performed after the deposition of the main layer is also found to be crucial for the formation of single crystalline, high-quality Ge films on SiO2 substrates with <311> direction. The cyclic annealing results in a further reduction of the defects, thereby threading dislocations significantly to a density of ~ 5.311 × 107 cm−2. Electrical measurements using the van der Pauw method revealed that the GeTF exhibits p-type characteristics and a high mobility of 360.10 cm2/Vs at room temperature.