V. Diadiuk

Gallium phosphide microlenses by mass transport

Arrays of high quality refractive microlenses have been formed in GaP substrates by mesa etching followed by a heat treatment in which the multistep mesas were smoothed due to surface energy minimization. A smooth lens surface and an accurately controlled lens profile have been obtained. Microlenses of 130 μm diameter and 200 μm focal length have been used to collimate the outputs of GaInAsP/InP and GaAs/GaAlAs diode lasers and have yielded a nearly diffraction‐limited beam divergence of 0.68°.

Avalanche multiplication and noise characteristics of low‐dark‐current GaInAsP/InP avalanche photodetectors

High‐performance avalanche photodiodes responding out to 1.25 μm have been fabricated in inverted‐mesa n+‐InP/n‐GaInAsP/n‐InP/p+‐InP structures. Uniform avalanche gains of 700, dark current densities of 3×10−6 A/cm2 at M=10, and an excess‐noise factor of ∼3, also at M=10, have been measured. The low dark current results from the placement of the p‐n junction in the InP and from the use of a new passivation technique. Pulse‐response rise times, measured with an avalanche gain of 40 and limited by the rise time of the mode‐locked Nd:YAG laser pulse, were less than 160 psec.

Q switching of low-threshold buried-heterostructure diode lasers at 10 GHz

Buried‐heterostructure actively Q‐switched diode lasers have been made with threshold currents as low as 14 mA. The lasers operate continuously at room temperature. Modulation has been observed at rates up to 10.5 GHz. Evidence of several modes of Q switching has been obtained.

Low dark-current, high gain GaInAs/InP avalanche photodetectors

High performance inverted-mesa GaInAsP/InP avalanche photodiodes responding out to 1.25 μm have been fabricated. Uniform avalanche gains M , of 700 dark-current densities of 3 \times 10^{-6} A/cm2at M = 10 , and an excess noise factor of ∼3 at M = 10 have been achieved by placing the p-n junction in the InP and using a new passivation technique. Pulse-response rise times of less than 160 ps, limited by the rise time of the mode-locked Nd:YAG laser pulse, were measured with an avalanche gain of 40.

Large‐numerical‐aperture InP lenslets by mass transport

Lenslets with diameters up to 130 μm and numerical apertures of 0.39–0.75 have been formed in InP substrates by using mass transport to smooth out chemically etched multilevel mesa structures. The lenslets show a smooth surface with an accurately controlled profile (i.e., curvatures) and are capable of forming clear images. Some lenslets (with a diameter of approximately 67 μm) have been used to collimate the output of a buried‐heterostructure diode laser (of 1.3 μm wavelength) and resulted in a nearly diffraction‐limited beam divergence of 1.4°.

Lateral photodetectors on semi‐insulating InGaAs and InP

A new type of high‐performance lateral PIN photodector has been formed by alloying p‐type and n‐type metallic contacts onto semi‐insulating InGaAs or InP without a separate junction fabrication step. Low dark current (<1 nA), high external quantum efficiency (40% at λ=1.24 μm, without antireflection coating), and high speed (full width at half‐maximum <50 ps) have been obtained with reverse bias of 10 V. These characteristics, plus the simplicity of fabrication and the planar lateral configuration, make these devices attractive for monolithic integration with field‐effect transistors and for photodetector arrays.

Diffusion length of moles in n‐InP

By measuring the photocurrent as a function of reverse bias for InP photodiodes with a range of junction depths, the hole diffusion length Lp of epitaxial n‐type InP (n∼1.5×1016 cm−3) was determined to be approximately 12 μm. This value of Lp is an order of magnitude larger than that determined by the electron beam induced current and surface photovoltage techniques. Reasons for these discrepancies, which involve geometrical and material considerations, respectively, are discussed.

Surface passivation techniques for InP and InGaAsP p-n junction structures

Surface passivation techniques developed for InP and InGaAsP avalanche photodiodes have resulted in reductions of dark current as large as four orders of magnitude, to values as low as 1.6 × 10^-6A/cm²at 0.9V<inf>b</inf>. Devices consisting entirely of InP have been passivated with plasma-deposited Si<inf>3</inf>N<inf>4</inf>, and those with a InGaAsP layer but with the p-n junction in InP have been passivated with polyimide. Neither of these techniques successfully reduces dark currents in devices with the p-n junction in the InGaAsP, but a film of photoresist sprayed with SF<inf>6</inf>as the propellant has given excellent results.

Planar guarded avalanche diodes in InP fabricated by ion implantation

Planar guarded avalanche diodes in InP have been fabricated using a double‐ion‐implantation technique. Silicon was selectively implanted into an n‐type epitaxial layer to increase the concentration portion of the diode, and beryllium was implanted to form the p‐n junction. When appropriately reverse biased, these diodes exhibited uniform avalanche photocurrent gain in the central portion, where the electric field is maximum. With a 1‐kΩ load, photoresponse gains as high as 20 were measured, although values of 6 to 8 were more typical. At present, the external photoresponse gain with a fixed load is limited by the shunting effect of the diode’s differential resistance.

External‐cavity coherent operation of InGaAsP buried‐heterostructure laser array

Coherent operation of a monolithic linear array of InGaAsP buried‐heterostructure lasers operating at λ=1.3 μm has been acheived by means of a spatial filter in an external cavity. An array of mass‐transported InP microlenses was used to collimate the beams of the individual laser elements and couple the laser array output to the external cavity. The coherent array output exhibited a narrow (3.2 mrad), three‐lobe far‐field pattern with ∼65% of the energy concentrated in the central peak.