Chih-Hung Chiou

Strain-Compensated GaAsN/InGaAs Superlattice Structure Solar Cells

In this work, we investigate the metal organic chemical vapor deposition (MOCVD) growth of GaAsN/InGaAs strain-compensated superlattice cells in view of their application in solar cells. The compressive strain in InGaAs layers is matched by the tensile strain in GaAsN layers, overcoming the lattice-mismatch limitation. GaAsN/InGaAs strained layer superlattice cells, lattice-matched to GaAs, are proposed to extend the long-wavelength absorption of the bottom cell in a cascade solar cell structure. A strain-compensated superlattice solar cell with 0.6 µm GaAsN/InGaAs incorporated in the intrinsic region of p–i–n GaAs cells was fabricated. Compared with the fabricated InGaAs and InGaNAs cells, it was found that the GaAsN/InGaAs superlattice cell can lower the band gap energy and extend the absorption the most, followed by the InGaAs cell. In addition, the efficiency of the GaAsN/InGaAs superlattice cell was 4.3%, which is comparable to that of the InGaNAs cell. The GaAsN/InGaAs strain-compensated superlattice structure shows many characteristics required to make it a candidate for the next-generation multijunction solar cells, which means this design can be used as the third junction of future-generation ultrahigh-efficiency three- and four-junction devices.

Single-mode InGaAs photonic crystal vertical-cavity surface-emitting lasers emitting at 1170 nm

We have made MOCVD-grown InGaAs photonic crystal vertical-cavity surface-emitting lasers (PhC-VCSELs) for fiber-optic applications. Multi-mode InGaAs VCSELs have achieved a maximum power of over 1 mW. Single-mode characteristics of 0.18 mW of the PhC-VCSELs have been made by using the combined AlOx oxide layer with proton-implantion for better current confinement.

Low-Threshold-Current-Density, Long-Wavelength, Highly Strained InGaAs Laser Grown by Metalorganic Chemical Vapor Deposition

A long-emission-wavelength laser with multiple InGaAs/GaAs quantum wells without a strain-compensated barrier was grown by metalorganic chemical vapor deposition (MOCVD). InGaAs quantum well (QW) broad-area laser diodes with an emission wavelength of up to 1214 nm were realized. A measured room-temperature threshold current density of only 173 A/cm2 for a 2-mm-cavity device and a transparency current density of 66 A/cm2 were obtained. The internal quantum efficiency and laser cavity loss were 67% and 6 cm-1, respectively.

Characteristics of InGa(N)As VCSELs for fiber optic applications

We report our results on InGaNAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) for fiber-optic applications in the 1.3 μm range. The epitaxial structures were grown on (100) GaAs substrates by MBE or MOCVD. The nitrogen composition of the InGaNAs/GaAs quantum-well (QW) active region is 0 to 0.02. Long-wavelength (up to 1.3 μm) room-temperature continuous-wave (RT CW) lasing operation was achieved for MBE and MOCVD-grown VCELs. For MOCVD-grown devices with n- and p-doped distributed Bragg reflectors (DBRs), a maximum optical output power of 0.74 mW was measured for In0.36Ga0.64N0.006As0.994/GaAs VCSELs. The MBE-grown devices were made with intracavity structure. Top-emitting multi-mode 1.3 μm In0.35Ga0.65N0.02As0.98/GaAs VCSELs with 1mW output power have been achieved under RT CW operation. Emission characteristics of InGaNAs/GaAs VCSELs were measured and analyzed.

Characteristics of GaAs-Based Long-Wavelength, Highly Strained InGaAs Quantum Well Vertical-Cavity Laser

Continuous-wave (CW) operation of highly strained double quantum well InGaAs/GaAs vertical-cavity surface emitting lasers (VCSELs) emitting at 1252 nm at room temperature have been realized by metal organic chemical vapor deposition (MOCVD). Our preliminary results reveal the relation between the output performance and gain-cavity detuning of the long wavelength InGaAs VCSELs. Compared with the quaternary GaInNAs VCSEL, it is found that the InGaAs VCSEL manifests better temperature stability.

Single mode InGaAs photonic crystal vertical-cavity surface-emitting lasers

We have made MOCVD-grown InGaAs photonic crystal vertical-cavity surface-emitting lasers (PhC-VCSELs) for fiber-optic applications. Multi-mode InGaAs VCSELs have achieved a maximum power of over 1 mW. Single-mode characteristics of 0.18 mW of the PhC-VCSELs have been made by using the combined AlOx oxide layer with proton-implantion for better current confinement.

Temperature-dependent characteristics and burn-in performance of GaAs-based long-wavelength vertical-cavity surface-emitting lasers emitting at 1.26 µm

Continuous-wave operation of highly strained double quantum well vertical-cavity surface-emitting lasers (VCSELs) with active regions of GaInNAs and InGaAs emitting at 1.26 µm at room temperature has been realized by metal organic chemical vapour deposition. The proposed InGaAs VCSEL can operate at high temperatures and exhibits superior temperature stability. The degradation of output power of the 1.26 µm InGaAs VCSEL is approximately 2.8% after a 1000 h burn-in life test.

Effect of Annealing on Low-Threshold-Current Large-Wavelength InGaAs Quantum Well Vertical-Cavity Laser

An emission wavelength record emission wavelength of 1245 nm in double InGaAs/GaAs quantum wells without a strain-compensated barrier is obtained by metalorganic chemical vapor deposition (MOCVD). The effect of annealing on highly strained InGaAs quantum wells is analyzed using phtoluminanescence spectra. By comparing devices p-type distributed Bragg reflector (p-DBR) growth temperatures, the preliminary results indicate that a device with a low p-DBR growth temperature of 670°C exhibits a low threshold current and a high light output power. A threshold current of 6.7 mA in an InGaAs vertical-cavity surface emitting laser (VCSEL) in CW operation is realized with an emission spectrum up to 1.26 µm.

Growth of Highly Strained InGaAs Quantum Wells by Metalorganic Chemical Vapor Deposition with Application to Vertical-Cavity Surface-Emitting Laser

A series of highly strained InGaAs quantum wells (QWs) with GaAs barriers emitting at wavelength longer than 1.2 µm are grown on GaAs substrates by metalorganic chemical vapor deposition (MOCVD). The optimized windows of the V/III ratio of the InGaAs layer and the growth rate of the barrier are first investigated on thease highly strained QWs. By an appropriate choice of the growth conditions, we extend the room-temperature photoluminescence (PL) wavelength of InGaAs QWs to 1245 nm, which corresponds to an indium content of 42%. A GaAs-based InGaAs vertical-cavity surface-emitting laser (VCSEL) at an emission wavelength of 1.28 µm with a large detuning of 90 nm has been realized by the use of highly strained InGaAs QWs.

Long-wavelength VCSEL devices on GaAs substrates

Two approaches to realize the VCSEL devices based on GaAs substrates are investigated. The first approach utilizes InGaAs quantum wells with dilute nitride to extend the bandgap toward long wavelenegth. The second approach utilizes InAs/InGaAs quantum dots based on Stranski and Krastanov growth mode with confinement and strain combined to adjust the bandgap to 1.3 μm wavelength. High quality epitaxial layers with low threshold have been achieved with MBE and MOCVD. VCSEL performances that have been achieved are: Multimode operation at 1.303 μm with slope efficiency of 0.15 W/A (0.2 W/A), and maximum power of 1 mW (4 mW) for room temperature CW (pulse) operation have been achieved with MBE-grown In GaAaN active regions. Room temperature, CW single mode operation with SMSR > 40 dB at 1.303 μm has also been achieved with a slope efficiency of 0.17 W/A and maximum power of 0.75 mW also with MBE-grown InGaAaN active regions. In addition, MOCVD grown has also achieved a performance at 1.29 μm with slope efficiency, 0.066 W/A, and maximum power, 0.55 mW. VCSELs with 9 layers of quantum dots and all-semiconductor DBRs also achieved lasing at 1.3 μm.