Hong Joo Song

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.

Characterization of the morphology and optical properties of InAs∕AlAs quantum dots with a GaAs insertion layer

InAs self-assembled quantum dots by utilizing a thin GaAs insertion layer (IL) on a 1nm thick AlAs seed layer were grown on GaAs(100) substrates by using a molecular beam epitaxy technique. InAs quantum dots (QDs) were formed by varying the thickness of the GaAs IL from 1 to 9 ML (monolayer), and their morphological and optical properties were characterized by atomic force microscopy and photoluminescence (PL). As a result, when the GaAs IL was thicker than 5 ML, normal InAs QDs with an average diameter of 30nm and a density of 2×1010∕cm2 were formed, because the enhanced surface roughness due to the AlAs layer was leveled by the GaAs IL. However, when the thickness of the GaAs IL was decreased from 5 to 3 ML, the formed InAs QDs showed a bimodal size distribution, i.e., large dots with a lateral size of about 30nm and small dots with that of about 20nm. When the GaAs IL was below 1 ML, InAs QDs with an average diameter of less than 15nm and a high density of 1.5×1011∕cm2 were grown. Consequently, it was ...

High-voltage GaN SBD on Si substrate by suppressing metal spikes

We have successfully fabricated high-voltage GaN Schottky barrier diodes (SBDs) on Si substrate by suppressing metal spikes under ohmic contacts. The breakdown voltage of GaN SBDs is 450 V with superior device-to-deice uniformity. Metal spikes are suppressed by low-temperature annealing at 700 °C. The low contact resistance of 0.6 ohm-mm is also achieved due to ohmic contacts on the doped GaN. The diffusion of Ti/Al/Mo/Au into GaN is analyzed by Auger electron spectroscopy and scanning electron microscope. The depth and the number of metal spikes are proportional to the annealing temperature of ohmic contacts. Metal spikes in GaN power devices should be suppressed for the low power loss and the high breakdown voltage.

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 Optical Characterization of Passive Pixel with Sensitivity-Improved InGaAs/InP Phototransistors Considering Light-Dependent Shunt Resistance for Near Infrared Imaging Applications

In this study, a InGaAs/InP passive unit pixel with an optical-sensitivity-improved heterojunction phototransistor (HPT) for array imaging applications has been designed and characterized for low-light signal detection considering light-dependent shunt resistance. Using the proposed devices, a 1×256 highly sensitive linear array chip with suitable shunt resistance has been fabricated and characterized. The designed passive unit pixel consists of one photodetector and one select transistor with a collector–base terminal tied configuration for a wide dynamic range. We also present epitaxial structures and an equivalent model to optimize optical gain and shunt resistance characteristics. The device operation mechanism and experimental results are discussed. The experimental results show that our device has an optical sensitivity of 118 A/W, which is significantly higher than that of a conventional PIN photodetector with the same light-absorbing area. This high sensitivity originates from the optical gain-enhanced device structure. A typical optical gain is approximately 236, which means HPTs are 236-fold more sensitive than PIN photodetectors. The proposed HPT also has tens of kΩ shunt resistance with high optical sensitivity under low illumination, which is sufficient for effective signal conversion through a transimpedance amplifier circuit.

Red-emitting external cavity diode laser with high slope efficiency and narrow bandwidth

External cavity diode laser with broad-area laser diode is operated up to the output power of 160 mW at the injection current of 850 mA and the bandwidth of 80 pm at a wavelength of 648 nm in external cavity. High slope efficiency of output power and narrow bandwidth using broad-area laser (BAL) diode, the width of active layer in the slow axis is too broad to select a specific wavelength. In this paper, more efficient wavelength selection method is investigated by confirming the tendency of grating grooves and designing to set up the wavelength dispersion direction along the fast axis of a solitary laser diode (LD) geometrically. Thus, the tunable external cavity diode laser module by using BAL diode is designed with an overall size of 49 mm x 52 mm x 48.5 mm. From injection current in the range of 650-900 mA, ECDL showed excellent wavelength locking behavior without a non-shift of the peak wavelength. Here, the tuning range is 4 nm with maintaining the narrow bandwidth of 80 pm and up to the output power of 100 mW. A side-mode-suppression of 36.5dB is also achieved at the output power of 160 mW and the injection current of 850 mA.

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.