Wen-Yen Hwang

Broadly wavelength-tunable external cavity, mid-infrared quantum cascade lasers

Wavelength tuning over 120 nm for a grating-coupled 5.1-/spl mu/m quantum-cascade type-I laser was studied for temperature from 80 to 243 K (-30/spl deg/C). Both the Littman-Metcalf and first-order grating direct feedback cavity configurations were used with similar tuning results. The goal is to achieve broad tunability, and the result is a combined grating and temperature tuning of 245 nm, from 5.040 to 5.285 /spl mu/m. The laser was designed for predominantly single-mode or at most, two-longitudinal mode operation. The instrument-limited laser linewidth was less than the cavity longitudinal mode spacing. Stepping-motor control of the grating allowed 0.4-GHz wavelength increments (35 pm) to be realized with high reproducibility. A current-induced wavelength shift of /spl sim/2-3 GHz was observed, corresponding to an effective refractive index change of /spl sim/10/sup -3/. Analysis indicates that single-mode, continuously tunable operation is feasible with a more optimal device and cavity.

Carbon doping of GaAs and (In,Ga)As in solid source molecular beam epitaxy using carbon tetrabromide

We have grown C‐doped GaAs and (In,Ga)As epitaxial layers of device quality in a standard solid source molecular beam epitaxy system using carbon tetrabromide (CBr4) as the carbon source. Dopant incorporation was relatively efficient for both GaAs and (In,Ga)As, requiring a CBr4 beam pressure of about 1×10−6 Torr to achieve a hole density of 1.5×1020/cm3. For doping in the 1019/cm3 range, hole mobilities were comparable to or slightly higher than those of Be‐doped layers with the same carrier concentrations. Modulation‐doped structures grown immediately after heavily C‐doped GaAs layers exhibited reduced two‐dimensional electron gas mobility, but the mobility recovered to previous values within 24 h. (Al,Ga)As/GaAs heterojunction bipolar transistors (emitter size=25 μm×50 μm) with C‐doped bases (p=1.2×1019 cm−3) had common emitter small signal current gains averaging 86 at an emitter current density of 970 A/cm2. The relatively low gas load during growth, the lack of long‐term memory effect, and the accep...

Continuous-wave operation of a 5.2 μm quantum-cascade laser up to 210 K

Continuous-wave operation of a 5.2 μm-type I quantum-cascade laser with more than 5 mW of output power is reported at a heat sink temperature of 210 K (−63 °C). This temperature is within the range obtainable with thermal-electric coolers. The device was mounted epi-side down on a copper submount and exhibited a thermal resistance of ∼10 K/W at 210 K. Using the experimentally determined values for T0=136 K, J0=535 A/cm2 and Vop=8.1 V and the above thermal resistance, the maximum theoretical operating temperature was found to be 212 K, in close agreement with experiment. Thermal simulations show that by improving the device design and heat sinking, thermal resistance can be reduced to 8.8 K/W and the maximum cw operating temperature can be increased to 230 K.

Large area InAlAs/InGaAs metal semiconductor metal photodiode with very low dark current and its frequency response

An MBE grown InGaAs metal semiconductor metal (MSM) photodiode (PD) with an InAlAs barrier enhancement layer is reported that has very low dark current and high speed characteristics. The detector using Cr/Au Schottky metal fingers with 4m spacing on a large active area of 300×300m2 shows a low dark current of 38nA at 10V. This corresponds to a dark current density of 0.42pA/m2 and is, to our knowledge, the best dark current ever obtained from a large area InGaAs MSM PD. The device also shows a low capacitance of 0.8pF and a high 3dB bandwidth of 2.4GHz. By fitting the measured frequency response to a model consisting of both RC time and transit time limited responses, we show that the device has an RC time and a transit time limited 3dB bandwidth of 3.0 and 4.9GHz, respectively.

10 Gb/s operation of 1.3 /spl mu/m uncooled ridge-waveguide Be-doped DFB laser

We report 10 Gb/s direct modulation of 1.3 /spl mu/m-uncooled DFB. The lasers were optimized for 0 to 80/spl deg/C operation. Output power of 35 mW is reported at 25/spl deg/C for 100 mA. 8 GHz bandwidth was measured for entire temperature range.

High-performance mid-IR type-II interband cascade lasers

Summary form only given.The wavelengths spanning from 2 to 13 /spl mu/m include the absorption lines of most target gases and pollutants, which are currently the object of environmental sensing applications. A compact NIR semiconductor laser would be extremely useful for those applications. Here, we have used two InAs layers in the active region, so called W active region of the semiconductor laser.

Interband cascade lasers

We report the recent progress of interband cascade (IC) lasers based on InAs/Ga(In)Sb/AlSb type-II quantum wells. For the 4.5-micrometers IC lasers, the internal loss was 11.6 cm-1 and the internal quantum efficiency was 460% at 90 K. When mounted epi-side down on diamond, cw operation was observed with an external quantum efficiency (EQE) of 193%, a cw output power over 500 mW, and a threshold current density as low as 35 A/cm2 at 80 K. Dual-wavelength IC laser was also demonstrated. The device lased simultaneously at 4.482 and 4.568 micrometers . At 110 K, a peak output power of 150 mW per facet was achieved with 5-microsecond(s) pulses at 1-KHz repetition rate. The threshold current density, average EQE, and peak output power of a 0.4-mm long device were 119 A/cm2, 278%, and 150 mW per facet, respectively.

Molecular beam epitaxy of In0.74Ga0.26As on InP for low temperature TPV generator applications

The growth by molecular beam epitaxy of In0.74Ga0.26As is investigated because of its importance as a PV converter for a variety low temperature TPV system configurations. In this work, a linearly graded buffer layer is used to grow high quality In0.74Ga0.26As layers on a lattice mismatched InP substrate. The thickness of the buffer layer and the substrate temperature during the growth of the buffer and active layers were varied in order to optimize the active layer material quality. The resulting p+−i−n+ epitaxial layers were compared using double crystal x‐ray diffraction, spectral response, and current‐voltage measurements. A more conventional PV cell structure was also evaluated using current‐voltage measurements.

Mid-IR type-I and type-II quantum cascade lasers

In the past three years, we have been optimizing an alternative cascade configuration based on interband transitions in type-II InAs/InGaSb/AlSb quantum wells (QWs). We have achieved an internal quantum efficiency (IQE) of 580% and a CW output power of 53 mW were achieved at 80 K at 4.4 /spl mu/m. The maximum operation temperature for a 4.2 /spl mu/m type-II IC laser was 220 K with a T/sub 0/ of 285 K around 100 K. A 25-stage interband cascade (IC) laser with a W active region and a third hole quantum well for the suppression of leakage current has exhibited lasing up to 286 K at 3.5 /spl mu/m. We report our most recent results of type-II IC lasers. We also grew 4.6 and a 7.4 /spl mu/m type-I QC laser structures.