Louis A. Koszi

Pulsations and absorbing defects in (Al,Ga)As injection lasers

Forty‐seven strip‐buried‐heterostructure (Al,Ga)As lasers have been examined for pulsations and absorbing defects in the active volume. All 18 pulsating lasers were found to have defects while only one of the 29 nonpulsating lasers had a visible defect. These results show conclusively that absorbing defects can cause pulsations in double‐heterostructure injection lasers and support a recently published model for the origin of these pulsations. The presence of absorbing defects and pulsations also correlates with structure in the functional dependence of −I2 d2V/dI2 upon I above threshold. Lasers not exhibiting pulsations were aged, and the same correlation was found between the development of defects (particularly mirror darkening), the change in −I d2V/dI2, and the development of pulsations.

Pt/Ti/p‐In0.53Ga0.47As low‐resistance nonalloyed ohmic contact formed by rapid thermal processing

Very low resistance nonalloyed ohmic contacts of Pt/Ti to 1.5×1019 cm−3 Zn‐doped In0.53Ga0.47As have been formed by rapid thermal processing. These contacts were ohmic as deposited with a specific contact resistance value of 3.0×10−4 Ω cm2. Cross‐sectional transmission electron microscopy showed a very limited interfacial reacted layer (20 nm thick) between the Ti and the InGaAs as a result of heating at 450 °C for 30 s. The interfacial layer contained mostly InAs and a small portion of other five binary phases. Heating at 500 °C or higher temperatures resulted in an extensive interaction and degradation of the contact. The contact formed at 450 °C, 30 s exhibited tensile stress of 5.6×109 dyne cm−2 at the Ti/Pt bilayer, but the metal adhesion remained strong. Rapid thermal processing at 450 °C for 30 s decreased the specific contact resistance to a minimum with an extremely low value of 3.4×10−8 Ω cm2 (0.08 Ω mm), which is very close to the theoretical prediction.

Defect mechanisms in degradation of 1.3-μm wavelength channeled-substrate buried heterostructure lasers

Channeled-substrate buried heterostructure (CSBH) lasers which were purged from populations undergoing high reliability qualification have been studied in detail. Gradual and rapid degradation mechanisms leading to accelerated aging failure modes have been analyzed by transmission electron microscopy, convergent beam electron diffraction, electroluminescence, energy dispersive x-ray analysis, and chemical etching. The gradual degradation mode of CSBH lasers is characterized by (1) a gradual increase in room-temperature threshold current; (2) a decrease in external quantum efficiency, typically a drop in peak value of dL/dI greater than 25%; (3) a drop in forward voltage at low current, indicating a change in junction characteristics; (4) a large peak inI(dV/dI) below threshold (at around 3 mA); and (5) an enhancement in the peak in I2(d2V/dI2) at laser threshold. A defect mechanism associated with the gradual degradation begins with a nucleation of extrinsic dislocation loops along the V-groove {111} p-n–...

Integrated multijunction GaAs photodetector with high output voltage

Integrated structures consisting of two P‐I‐N double heterostructure GaAs photodetectors, series connected by means of a tunnel junction, have been fabricated by molecular beam epitaxy. Open‐circuit output voltages, external power efficiencies, and fill factors were 1.777 V, 33.4%, and 0.83, respectively, for antireflection‐coated cells excited by ∼5 mW optical power from a focused DH AlGaAs laser emitting at λ=0.815 μm.

Pt/Ti/p‐InGaAsP nonalloyed ohmic contact formed by rapid thermal processing

Nonalloyed ohmic contacts of Pt/Ti to 5×1018 cm−3 doped p‐InGaAsP (λg =1.3 μm) have been fabricated by rapid thermal processing of sputtered and e‐gun‐deposited metallizations. While the former as‐deposited had a rectifying characteristic, the latter showed ohmic behavior prior to any heat treatment, with a specific contact resistance of 4×10−3 Ω cm2. Rapid thermal processing at temperatures higher than 400 °C caused the formation of ohmic contacts for the sputtered metals also, but with the evaporated metals producing slightly lower contact resistance. The lowest specific contact resistance values of 3.6–5.5×10−4 Ω cm2 for evaporated and sputtered metallizations, respectively, were achieved in both cases as a result of heating at 450 °C for 30 s. These heating conditions produced only a limited reaction at the Ti/InGaAsP interface, which was sharper for the e‐gun‐deposited contact, but had a significant effect on the stresses in the Ti/Pt bilayer. In both the sputtered and electron gun evaporated samples...

Frequency chirp under current modulaton in InGaAsP injection lasers

We have measured the frequency chirp in gain‐guided, weakly index‐guided, and strongly index‐guided InGaAsP lasers under direct‐current modulation. The measured chirp width is largest for gain‐guided and smallest for weakly index‐guided ridge waveguide lasers. The chirp width for 1.5‐μm InGaAsP lasers is about a factor of 2 larger than that for 1.3‐μm InGaAsP lasers of the same structure. The frequency chirping results from a modulaton of the carrier density which modulates the effective refractive index of the guided mode. The frequency chirping can introduce a limitation on the performance of single‐frequency injection laser sources for high bit‐rate digital transmission in fiber communication systems at 1.55 μm.

Microscopic studies of semiconductor lasers utilizing a combination of transmission electron microscopy, electroluminescence imaging, and focused ion beam sputtering

The microstructure of semiconductor laser diodes is studied using a combination of focused ion beam sputtering, electroluminescence imaging, and cross‐sectional transmission electron microscopy. Careful control of focused ion beam sputtering allows fabrication of high quality thin membranes for transmission electron microscope imaging, which can be located to submicron accuracy at a given position on the laser active stripe. By correlation with electroluminescence imaging, the membrane may then be positioned at an optically degraded region of the active stripe. In addition, imaging of the complete cross‐sectional laser structure, from substrate to surface contact layers is possible. The applications of these techniques to studies of laser degradation mechanisms are demonstrated and discussed.

A high‐efficiency GaAlAs double‐heterostructure photovoltaic detector

An antireflection‐coated GaAlAs double‐heterostructure photovoltaic detector is described whose extrinsic power conversion efficiency is 56% when used with a focused 8075‐μm wavelength laser beam. This, to our knowledge, is the highest photovoltaic efficiency yet reported. A partially reflective rear contact provides extrinsic quantum efficiencies exceeding 0.90 with a relatively thin active region. By restricting the diameter of this contact to 50 μm and using high dopings to reduce series resistance, open‐circuit voltages of 1.15 V and fill factors up to 0.84 are obtained. The series resistance was 3.0 Ω, with 1.0 Ω being attributable to the contacts.

Fabrication and performance characteristics of 1.55‐μm InGaAsP multiquantum well ridge guide lasers

We report the fabrication and performance characteristics of InGaAsP ridge waveguide lasers with multiquantum well (MQW) active layers emitting near 1.55 μm. The active region has four active wells (1.55 μm InGaAsP) and three barriers (1.3 μm InGaAsP). The thicknesses of the active wells and the barrier layers are ∼250 A. The 360‐μm‐long lasers have threshold currents in the range 60–80 mA at 30 °C, external differential quantum efficiency ∼25% at 30 °C, and T0∼70 K. The modulation bandwidths of the lasers are ∼1.5 GHz and they exhibit less frequency chirping than similar lasers with conventional double heterostructure (DH) active layer. Since frequency chirp limits the performance of high bit rate long haul fiber communication system at 1.55 μm, we believe MQW lasers offer an advantage over conventional DH lasers.

Bonding pad induced stresses in (Al,Ga)As double heterostructure lasers

We have studied Au bonding pad induced stresses in (AlGa)As double heterostructure lasers by photoluminescence, x‐ray Automatic Bragg Angle Control measurements, and infrared and optical microscopy. Examination of several devices which have thick (10–12 μm) plated Au bonding pads indicates that stresses approaching the fracture stress can exist in such devices. The high stress induces aggregates of microcracks along 〈100〉 directions in the epitaxial layers. Regions in the stripe where the cracks intersect are nonradiative, causing failure of the device.