Sam Kyu Noh

Evolution of stress relaxation and yellow luminescence in GaN/sapphire by Si incorporation

We report a systematic study accomplished with a series of undoped and Si-doped GaN epilayers grown on sapphire (0001) with the carrier concentration of 4.0×1017−1.6×1019 cm−3 in order to investigate the evolution of stress relaxation and yellow luminescence by Si incorporation. As the Si doping becomes higher, the bound exciton peaks are gradually shifted to lower energy due to relaxation of the thermal residual stress with the linear coefficient of ΔE/Δσ∥=42 meV/GPa. The present results show that both the full width at half maximum of double-crystal x-ray diffractometry and the photoluminescence intensity ratio of the yellow luminescence to edge emission gradually increase as the Si incorporation becomes heavier. We suggest that the Si doping in GaN epilayers induces overall defects and gives rise to stress relaxation during the cool-down process, and the yellow luminescence may be originated from a complex of VGa and the Si-induced defect.

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.

Tunable continuous-wave terahertz generation/detection with compact 1.55 μm detuned dual-mode laser diode and InGaAs based photomixer

We demonstrate a tunable continuous-wave (CW) terahertz (THz) homodyne system with a novel detuned dual-mode laser diode (DML) and low-temperature-grown (LTG) InGaAs photomixers. The optical beat source with the detuned DML showed a beat frequency tuning range of 0.26 to over 1.07 THz. Log-spiral antenna integrated LTG InGaAs photomixers are used as THz wave generators and detectors. The CW THz radiation frequency was continuously tuned to over 1 THz. Our results clearly show the feasibility of a compact and fast scanning CW THz spectrometer consisting of a fiber-coupled detuned DML and photomixers operating in the 1.55-μm range.

Study of freshly excised brain tissues using terahertz imaging

We demonstrated that tumors in freshly excised whole brain tissue could be differentiated clearly from normal brain tissue using a reflection-type terahertz (THz) imaging system. THz binary images of brain tissues with tumors indicated that the tumor boundaries in the THz images corresponded well to those in visible images. Grey and white-matter regions were distinguishable owing to the different distribution of myelin in the brain tissue. THz images corresponded closely with magnetic resonance imaging (MRI) results. The MRI and hematoxylin and eosin-stained microscopic images were investigated to account for the intensity differences in the THz images for fresh and paraffin-embedded brain tissue. Our results indicated that the THz signals corresponded to the cell density when water was removed. Thus, THz imaging could be used as a tool for label-free and real-time imaging of brain tumors, which would be helpful for physicians to determine tumor margins during brain surgery.

Nanoscale InGaAs concave disks fabricated by heterogeneous droplet epitaxy

The detailed cross-sectional structure of InGaAs quantum dots fabricated by a heterogeneous droplet epitaxy method was investigated by means of cross-sectional transmission electron microscopy observation. It was confirmed that concave disks without any dislocations or wetting layer were formed at the upper part of the flat surface. This result was consistent with the change of photoluminescence intensity and peak position. The sizes of the disks were estimated to be 30 and 12 nm in lateral and vertical directions, respectively. From this estimation, the occurrence of a phase-separation effect is suggested.

Systematic study of different transitions in high operating temperature quantum dots in a well photodetectors

We report a systematic study of different transitions in quantum dots-in-a-well infrared photodetectors in order to optimize the signal to noise ratio of the detector. Bound to continuum transitions offer very high extraction probability for photoexcited electrons but poor absorption coefficient, while the bound to bound transitions have higher absorption but poorer extraction probability. Bound to quasibound transitions are optimum for intermediate values of electric fields with superior signal to noise ratio. The bound to quasibound device has the detectivity of 4×1011 cm Hz1/2 W−1 (3V, f/2 system) at 77 K and 7.4×108 cm Hz1/2 W−1 at 200 K, which is highest reported detectivity at 200 K for detector with long wave cutoff wavelength.

Demonstration of Bias-Controlled Algorithmic Tuning of Quantum Dots in a Well (DWELL) MidIR Detectors

The quantum-confined Stark effect in intersublevel transitions present in quantum-dots-in-a-well (DWELL) detectors gives rise to a midIR spectral response that is dependent upon the detectors operational bias. The spectral responses resulting from different biases exhibit spectral shifts, albeit with significant spectral overlap. A postprocessing algorithm was developed by Sakoglu that exploited this bias-dependent spectral diversity to predict the continuous and arbitrary tunability of the DWELL detector within certain limits. This paper focuses on the experimental demonstration of the DWELL-based spectral tuning algorithm. It is shown experimentally that it is possible to reconstruct the spectral content of a target electronically without using any dispersive optical elements for tuning, thereby demonstrating a DWELL-based algorithmic spectrometer. The effects of dark current, detector temperature, and bias selection on the tuning capability are also investigated experimentally.

Effects of high potential barrier on InAs quantum dots and wetting layer

Effects of a thin AlAs layer (1 nm) with different position on InAs quantum dots (QDs) and wetting layer have been investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR). The PL peak position of InAs QDs directly grown on the thin AlAs is blueshifted from that of InAs QDs grown on the GaAs layer by 171 meV mainly due to the high potential barrier and reduced dot size shown in the TEM image. As the additional GaAs layer (1 and 2 nm) is inserted on top of the AlAs layer, the PL peak position is systematically shifted toward longer wavelength with increase in the thickness. Temperature dependent PL of QD samples shows that a thin AlAs layer significantly influences the thermal activation energy. The wetting layer related peak in PR spectra is changed to lower energy with increase in the thickness of an additional GaAs layer, which is mainly caused by the reduction in the effects of the AlAs layer.

Optical fiber-coupled InGaAs-based terahertz time–domain spectroscopy system

The successful demonstration of an optical fiber-coupled terahertz time-domain spectroscopy (THz-TDS) system is described in this study. The terahertz output power of the emitter with two optical band rejection filters was 132 nW, which is an improvement of 70% over the output power without any filters. This improvement is due to the suppression of an optical modulated signal that is reverse-generated when an alternating current bias exceeding a certain threshold is applied to the emitter. Under the optimal alignment conditions, the terahertz detector in a fiber-coupled THz-TDS system clearly measured water vapor dips in the free space.

Resonant Tunneling Barriers in Quantum Dots-in-a-Well Infrared Photodetectors

The use of resonant tunneling (RT) barriers in the design of quantum dots-in-a-well (DWELL) infrared photodetectors is reported. The design of RT barriers for a variety of goals has been discussed. For simple DWELL designs, we demonstrate 2-3 orders-of-magnitude reduction in the dark current, with significant increase in the specific detectivity (D *) of the device. Two RT barriers are designed to selectively extract midwave and longwave components of the spectral response. We also report the use of RT barriers on strain-optimized quantum dots-in-a-double-well (DDWELL) structures to achieve very low dark current levels with peak D * of 2.9 ×1010 cm· Hz1/2 /W for a longwave infrared detection. Ability to select a particular wavelength in the spectral response is demonstrated with DDWELL architectures as well.