Kwangwook Park

Enhancement of minority carrier lifetime of GaInP with lateral composition modulation structure grown by molecular beam epitaxy

We report the enhancement of the minority carrier lifetime of GaInP with a lateral composition modulated (LCM) structure grown using molecular beam epitaxy (MBE). The structural and optical properties of the grown samples are studied by transmission electron microscopy and photoluminescence, which reveal the formation of vertically aligned bright and dark slabs corresponding to Ga-rich and In-rich GaInP regions, respectively, with good crystal quality. With the decrease of V/III ratio during LCM GaInP growth, it is seen that the band gap of LCM GaInP is reduced, while the PL intensity remains high and is comparable to that of bulk GaInP. We also investigate the minority carrier lifetime of LCM structures made with different flux ratios. It is found that the minority carrier lifetime of LCM GaInP is ∼37 times larger than that of bulk GaInP material, due to the spatial separation of electrons and holes by In-rich and Ga-rich regions of the LCM GaInP, respectively. We further demonstrate that the minority ca...

Optical and structural properties of microcrystalline GaN on an amorphous substrate prepared by a combination of molecular beam epitaxy and metal–organic chemical vapor deposition

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the other hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances.

Unveiling interfaces between In-rich and Ga-rich GaInP vertical slabs of laterally composition modulated structures

We report changes at the interface between Ga-rich/In-rich GaInP vertical slabs in laterally composition modulated (LCM) GaInP as a function of the V/III ratio. The photoluminescence exhibits satellite peaks, indicating that the parasitic potential between the GaInP vertical slabs disappears as the V/III ratio decreases. However, a high V/III ratio leads to an abrupt interface, increasing the parasitic potential because of the phosphorus-amount-dependent diffusion of group-III atoms during growth. These results suggest that the V/III ratio is an important parameter that must be wisely chosen in designing optoelectronic devices incorporating LCM structure.

Robust optical properties of sandwiched lateral composition modulation GaInP structure grown by molecular beam epitaxy

Double-hetero structure lateral composition modulated (LCM) GaInP and sandwiched LCM GaInP having the same active layer thickness were grown and their optical properties were compared. Sandwiched LCM GaInP showed robust optical properties due to periodic potential nature of the LCM structure, and the periodicity was undistorted even for thickness far beyond the critical layer thickness. A thick LCM GaInP structure with undistorted potential that could preserve the properties of native LCM structure was possible by stacking thin LCM GaInP structures interspaced with strain compensating GaInP layers. The sandwiched structure could be beneficial in realizing the LCM structure embedded high efficiency solar cells.

Tailoring Heterovalent Interface Formation with Light

Integrating different semiconductor materials into an epitaxial device structure offers additional degrees of freedom to select for optimal material properties in each layer. However, interfaces between materials with different valences (i.e. III-V, II-VI and IV semiconductors) can be difficult to form with high quality. Using ZnSe/GaAs as a model system, we explore the use of ultraviolet (UV) illumination during heterovalent interface growth by molecular beam epitaxy as a way to modify the interface properties. We find that UV illumination alters the mixture of chemical bonds at the interface, permitting the formation of Ga-Se bonds that help to passivate the underlying GaAs layer. Illumination also helps to reduce defects in the ZnSe epilayer. These results suggest that moderate UV illumination during growth may be used as a way to improve the optical properties of both the GaAs and ZnSe layers on either side of the interface.

Directional Terahertz Radiation from GalnP Lateral Superlattice.

We devised directionally controllable THz emission sources based on lateral composition modulation (LCM) structures. LCM structures were composed of In-rich Ga0.47In0.53P and Ga-rich Ga0.51In0.49P layers whose period was in quantum scale of ~`5 nm. The inherent type II band alignment in these structures leads to electron-hole (e-h) separation and plays a key role in generating later- ally polarized dipole ensembles, thus concomitantly emitting enhanced transmissive THz waves as compared to bulk sample. On the other hand, in lateral geometry, changes in THz fields between LCM and bulk structures turned out to negligible since the vertical electronic diffusion was allowed in both samples.