Cheol Koo Hahn

Selective formation of one- and two-dimensional arrayed InGaAs quantum dots using Ga2O3 thin film as a mask material

We report on the selective formation of InGaAs quantum dots (QDs) by molecular beam epitaxy. Nanoscale patterned Ga2O3 thin film deposited on the GaAs (100) substrate was employed as a mask material. Due to the enhanced migration effect of the group-III adatoms, such as Ga and In on Ga2O3 mask layer, the InGaAs QDs formed on the patterned substrate results in coalesced islands unlike those formed on the nonpatterned substrate. The estimation of the relative volume of the islands per unit area revealed that the desorption process as well as the migration of the Ga and In adatoms might occur on the Ga2O3 layer during the growth process, providing a good selective growth of self-assembled QDs.

Effects of nitride-based plasma pretreatment prior to SiNx passivation in AlGaN/GaN high-electron-mobility transistors on silicon substrates

The effects of nitride-based plasma pretreatment on the output characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) on silicon substrates are investigated. N2 and NH3 plasma pre-treatment methods are studied to overcome the RF dispersion phenomenon caused by nitrogen-vacancy (VN)-related defect reduction. It is found that the nitride-based plasma pretreatment is effective to overcome the RF dispersion in AlGaN/GaN HEMTs on Si. The NH3 plasma pretreatment markedly reduced RF dispersion from 63 to 1%. This is considered to be attributable to the reduction of the effective VN-related defect density and elimination of carbon/oxide residuals on the surface of AlGaN/GaN HEMTs. A NH3 plasma pretreatment prior to SiNx 100 nm passivation in the AlGaN/GaN HEMTs on Si markedly improves the total output power from 15 to 18.1 dBm under the operating conditions of VDS = 15 V/VGS = -1 V.

High-Voltage Schottky Barrier Diode on Silicon Substrate

New GaN Schottky barrier diodes (SBDs) on Si substrates are proposed to achieve a high-breakdown voltage. We have fabricated GaN SBDs using doped GaN/unintentionally doped (UID) GaN because doped GaN with the thickness of 200 nm is suitable for high-current operation. The 1-µm-deep mesa and low-temperature annealing of ohmic contacts suppress the leakage current of GaN SBDs. Annealing of Schottky contacts also improves the interface between a Schottky contact and GaN. Annealing of ohmic contacts at 670 °C yields the low leakage current of 2.8 nA through the surface and the buffer. When the anode–cathode distance is 5 µm, the fabricated GaN SBD successfully achieves a low forward voltage drop of 1.3 V at 100 A/cm2, low on-resistance of 4.00 mΩ cm2, and the low leakage current of 0.6 A/cm2 at -100 V. The measured breakdown voltage of GaN SBDs is approximately 400 V.

Comparative analysis of dark current between SiNx and polyimide surface passivation of an avalanche photodiode based on GaAs

In this paper, we present the effects of different surface passivation types, one with SiNx and the other with polyimide (PI), on the dark (leakage) current of a GaAs-based avalanche photodiode. We identified that the reverse dark current originates from the surface, and not from the bulk, showing the nearly linear dependence on perimeters of active-mesa (A-M) up to 90% of breakdown voltage (Vbr). From the theoretical results, total dark current consists of generation–recombination (G–R), shunt and tunneling components from a surface and the avalanche gain component from a bulk for both passivation types. Although the bulk component of avalanche gain⋅bulk current generates the breakdown process, it appears only near Vbr (12.7 V) because of a very small bulk current of a few fA in theory. For a surface current, SiNx passivation has values two to eight times lower than PI passivation. The different behaviors of surface current between passivation types could be theoretically explained by quantitative description of the current components.

Analysis of surface dark current dependent upon surface passivation in APD based on GaAs

In this paper, we investigated the dependence of reverse dark current on two types of surface passivation, one of which is polyimide and the other is SiNx, for InAs quantum dots/GaAs separate absorption, charge, multiplication avalanche photodiode (SACM APD). From the experimental results, we found that dark current was dominated by surface current, and not bulk current. It was also noted that SiNx passivation has a surface current that is lower by three to nine times in magnitude than that in polyimide passivation in the whole range of bias. To analyze the difference in dark current due to the passivation types, we propose the theoretical current components. This shows that the dark current of both passivation types is mainly composed of generation–recombination (G–R) and tunneling components, originating from the surface. However, each component has a different magnitude for passivation types, which can be explained by carrier concentration and trap density. The dependence of dark current on temperature shows the different behaviors between passivation types and supports a theoretical description of current components.

Design and fabrication of monolithic integrated optical array sensor and characterization of its single chip

Monolithic integrated optical array sensor is designed and fabricated. This array sensor is composed of vertical-cavity surface-emitting laser (VCSEL) and resonant cavity enhanced (RCE) photodiode. After the single chip is packaged, VCSEL and RCE photodiode are characterized independently. The VCSELs optical power of 0.3mW is achieved at 30mA with peak wavelength of 781nm. The full width at half-maximum (FWHM) of 1nm is achieved at 30mA. The RCE photodiodes responsivity of 0.92A/W is achieved at 780nm.

Improved spectral response of an InAs QD RC-SACM-APD with Ta2O5/SiO2 DBRs

We report the improvement in the spectral response of an InAs QD resonant-cavity separate absorption, charge, and multiplication avalanche photodetector (RC-SACM-APD) by increasing the quantum efficiency (QE) using dielectric Ta2O5/SiO2 top distributed Bragg reflectors (DBRs). The reflectivities of the top and bottom DBRs were numerically designed to be 70 % and 99.1 %, respectively, in order to maximize the QE. The spectral response characteristics of the InAs QD RC-SACM-APD with enhanced top DBRs was remarkably improved at 1098 nm, which is close to the target wavelength of 1100 nm, and its full width at half-maximum (FWHM) was 19 nm. The photoluminescence (PL) spectra revealed an intense and narrow single-mode peak at 1101 nm. The FWHM of the PL peak was as narrow as 8 nm and the difference in wavelength between the spectrum and the PL peak was as small as 3 nm.

Selective formation of InxGa1-xAs quantum dots by molecular beam epitaxy

We have investigated the selective formation of InxGa1-xAs quantum dots by molecular beam epitaxy. Particularly, we report on selectivity formed InxGa1-xAs QDs on the GaAs(100) substrate with fine- patterned oxide layers such as gallium oxide and silicon oxide, which are prepared with electron bema lithography technique. The electron beam lithography pre-patterned oxide layers were used to assemble the dots in a specific region. Both of the oxide layers served as mask materials were compared for the better selective growth of self-assembled QDs. The migration and the desorption of the In and Ga adatoms on the oxide layers are considered to be the most important factors for the selective formation of InxGa1-xAs quantum dots by molecular beam epitaxy. The influences of the mole fractions of indium, the growth temperature and the growth interruption times on the morphological transformation from 2D to 3D structures were discussed. Particularly, the 2D arrays of quantum dots selectively formed on GaAs substrates with pre-patterned Ga2O3 oxide layer may eventually be used for novel electronic or optical devices.

Fiber Bragg grating sensing system using a TO-can–based compact optical module for wavelength demodulation

Abstract. A combined scheme using the light source of a reflective semiconductor optical amplifier (RSOA) and an optical signal processing unit (OSPU) based on the compact TO-can package is fabricated and characterized for a fiber Bragg grating (FBG) sensing system. Due to the optical feedback behavior from the FBG sensor, the RSOA is self-injection locked and lasing occurs at the Bragg wavelength. Using the wavelength-dependent filter method, all of the components in the OSPU are compactly integrated on the TO-can package with a height of 17.6 mm and diameter of 6.0 mm. The wavelength demodulating output signals are based on the optical power difference, depending only on the wavelengths without the effect of input optical power variations. The sensitivity of the output signal to temperature shows 0.026  dB/°C. The entire FBG sensing system has an excellent linear response to temperatures controlled with an accuracy of ±0.3°C.

Process dependent output characteristics of resonant cavity type avalanche photodetector

In this work, we proposed resonant cavity (RC) type avalanche photodetector (APD) as one technical breakthrough in their wavelength performance and integration with surface light emitting devices for further applications. InAs self-assembled quantum dots (SAQDs) were used as absorbing layer for the 1 /spl mu/m light absorption and AlAs/GaAs quarter-wavelength DBRs were used to construct resonant cavity. Optimized epitaxial layer and device structures were calculated by impact ionization model and also the device fabrication processes were calibrated.