Dae Kon Oh

Low-threshold loss-coupled laser diode by new grating fabrication technique

In this letter, we introduce a novel fabrication method to make InGaAs absorptive first-order grating for loss-coupled distributed feedback laser diode (DFB-LD) using reactive ion etching. By this technique, it is possible to control duty rate of the first-order grating. A very-low-threshold current of 6 mA was obtained from our loss-coupled LD at the wavelength of 1.547 /spl mu/m, which was unusually high for a loss-coupled LD because of the excessive light absorption. In addition to the low-threshold current, it shows high SMSR and longitudinal single-mode yield as high as 50 dB and 90%, respectively.

Improvement of linewidth enhancement factor in 1.55-μm multiple-quantum-well laser diodes

The improvement of the linewidth enhancement factor in complex-coupled laser diode (CC-LD), or loss-coupled, was confirmed by measuring the spontaneous emission spectra below threshold from the sidewall of laser diodes. In addition, the serial resistance of the device was measured. The linewidth enhancement factor is improved by the presence of a light absorbing InGaAs grating for loss coupled distributed-feedback (DFB) laser diode (LD). We report the comparison of the linewidth enhancement factors of Fabry-Perot (FP) LD, conventional DFB-LD, and loss coupled DFB-LDs.

Cavity length dependence of high-speed 1.55-μm multiple-quantum-well laser diode characteristics

Spontaneous emission spectra below threshold were measured from the side wall of InGaAs QW laser diodes to extract laser design parameters such as cavity length dependence of gain, linewidth enhancement factor, and serial resistance. The threshold current varies according to the change of cavity length, and thus, the lasing peak shifts and the serial resistance changes. It is interesting that the linewidth enhancement factor, however, is not deteriorated by shortening cavity length. The short cavity length would rather improve the linewidth enhancement factor mainly by shifting the lasing peak to smaller wavelength side, where the linewidth enhancement factor is inherently low.

The contraction of lattice constant and the reduction of growth rate in p-InGaAs grown by organometallic vapor phase epitaxy

We have investigated the effect of diethylzinc (DEZn) on the lattice constant and the growth rate of InGaAs. Introducing DEZn for p-type doping induces the contraction of lattice constant and the reduction of growth rate compared to undoped InGaAs. Depletion of indium is responsible for these effects. These effects are reduced at lower growth temperatures or at lower growth pressures. From the observed effects of the growth temperatures and the growth pressures on the contraction of the lattice constant, it is concluded that depletion of indium occurs in the gas phase.

COMPARISON OF FE- AND P/N/P-DOPED INP AS A CURRENT BLOCKING LAYER FOR MONOLITHICALLY INTEGRATED INGAASP/INP LASER DIODES WITH PASSIVE WAVEGUIDES

We have fabricated InGaAsP/InP waveguide integrated laser diodes for the 1550 nm wavelength region. The passive waveguide was butt-coupled using the CH4/H2 reactive ion etching and metal-organic chemical vapor deposition regrowth techniques. We compared the performances of waveguide integrated laser diodes with different current blocking layers, i.e., Fe-doped InP and p/n/p-doped InP layers. On the whole, Fe-doped InP was better than p/n/p-doped InP as a current blocking layer. In particular, the reverse leakage current through the butt-coupling region with Fe-InP current blocking layer was approximately 99.9 µA, while that with p/n/p-InP was 193.7 µA. We also discussed the leakage current characteristics of waveguide integrated laser diodes with analyses of IdV/dI vs I curves.

Device performance of SI-PBH DFB-LD with undulation of SL-MQW layers

Strained layer multiple quantum well (SL-MQW) distributed feedback laser diodes (DFB-LD) have been widely used as single mode light sources for long-haul, high-speed optical telecommunication systems. Much work has been performed for thermal deformation of surface corrugations on substrates. In the growth of SL-MQW layers on first-order corrugation grating, one may observe the undulation of such layers. Such undulation deteriorates the device performance of DFB-LD. In this work, we observed that this undulation was dependent upon AsH/sub 3/ partial pressure and heat-up time before main growth, and was removed by controlling the partial pressure and the time. The influence of undulation on the device characteristics of semi-insulating planar-buried heterostructure (SI-PBH) DFB-LD is also discussed. The epilayers used in this study were grown on (100) InP substrates with a first-order corrugation grating using low-pressure metalorganic vapor phase epitaxy. The layer structure consists of compressively strained MQW (InGaAs(well)/lnGaAsP(barrier), 10 periods) as an active layer emitting at 1.55 /spl mu/m, and 1.24 /spl mu/m InGaAsP waveguide layers. The layer qualities of SL-MQW are analyzed using DXRD, TEM, and SEM. We fabricated the high speed DFB-LD with a highly resistive Fe-doped SI InP layer as a current blocking layer to minimize the parasitic capacitance.

Structural and device properties of DFB-LD with lateral undulation of SL-MQW

The InGaAsP/InGaAs strained-layer multiple quantum well (SL-MQW) structures for 1.3 /spl mu/m or 1.55 /spl mu/m distributed feedback laser diodes (DFB-LD) have been grown by low-pressure metalorganic vapor phase epitaxy. We observed the lateral undulation or deformation of SL-MQW grown on sawtooth-patterned substrates before the fabrication of DFB-LD, and it was dependent upon the initial growth conditions, AsH/sub 3/ partial pressure and heat-up time, prior to the 1st active layer growth. It is mainly due to the excess accumulation of strain in a given growth condition. The structural qualities of SL-MQW are analyzed in-depth using DCXRC, PL, TEM, SEM, and OM. We will discuss the effect of undulation or deformation on the device properties, such as I-V, I-L, differential quantum efficiency, internal loss, and characteristic temperature.

Structural characterization of distributed feedback laser diodes with undulation of strained-layer MQW

The InGaAsP/InGaAs strained-layer multiple quantum well (SL-MQW) structures for 1.55 /spl mu/m distributed feedback laser diodes have been grown by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The undulation or deformation of SL-MQW layers grown on the corrugated InP substrates has been observed, and it was dependent upon AsH/sub 3/ partial pressure and heat-up time prior to the 1st active layer growth. The structural qualities of SL-MQW are discussed in-depth using DCXRC, PL, TEM, and SEM.

Unstable characteristics of complex coupled laser diode with InGaAs absorptive grating

Recently, there have been many efforts on developing loss coupled (LC) laser diodes, one type of complex coupled (CC) DFB-LD, using a periodic light intensity change by an absorption medium adjacent to an active layer. In practice, the fabrication of LC DFB-LD is more stable and more reproducible compared to gain coupled (GC) DFB-LDs which use active layer etching and regrowth on it. However, the effects of the selection of an absorption layer on the characteristics of LC DFB-LD have not been analyzed enough yet. One of them is the dynamic property of LC DFB-LD depending on the doping type of the InGaAs absorbing layer. There was a first report on the abnormal lasing properties of LC DFB-LD in which the p-InGaAs absorptive grating was surrounded by p-type cladding and placed above an active layer, like most of LC DFB-LD structures. It was estimated that the saturation of the light absorption in the p-InGaAs layer caused the unstable lasing characteristics, because such phenomena were not found in n-InGaAs absorption gratings embedded in p-cladding layers, so called the conduction inverted type of LC DFB-LD. On the other hand, there was a theoretical report that the unstable lasing property resulted from the long carrier lifetime and the high differential gain in the absorption layer. Here we show first experimental evidence that the abnormal lasing property comes from the long carrier transport effect of p-doped absorption material rather than the combination between an absorptive layer and the adjacent layers like the conduction inverted type.

LP-MOCVD grown (InAs) m (GaAs) m short period superlattices on InP

Abstract(InAs)m(GaAs)m (1 ≤ m ≤ 12) short period superlattices (SPSs) have been grown on semi-insulated InP substrates with a 200 nm InP cap layer using low pressure metalorganic chemical vapor deposition (MOCVD). According to double crystal x-ray diffraction and transmission electron microscopy results, the critical layer thickness of (InAs)m(GaAs)m SPS was observed to be ∼30Å (m = 5). For the SPS below the critical layer thickness, mirror-like surface morphology was found without defects, and strong intensity Fourier transformed photoluminescence (FT-PL) spectra were also obtained at room temperature. The SPS with m = 4 showed a drastic improvement in photoluminescence intensity of order of two compared to an InGaAs ternary layer. However, the SPS with a large value of m (m ≥ 6), rough surface was observed with defects, with broad and weak FT-PL spectra. The surface morphology of SPS was greatly affected by the substrate orientation. The SPS with m = 5 was grown on two degree tilted substrate from (100) direction and showed poor surface morphology as compared to the one grown on (100) exact substrate Moreover, the SPS grown on a (111)B substrate showed a rough triangular pattern with Nomarski optical microscopy. In-situ thermal annealed SPS with m = 4 showed a 18 meV increase in PL peak energy compared to the as-grown sample due to phase separation resulting from thermal interdiffusion.