Photo-selective chemical etching of InAs and GaSb to manufacture microscopic mirrors

Abstract

1 IntroductionPhotochemical processing of materials, particularly insemiconductors, has been a relatively well-known techniqueused for some time with relatively few applications [1].However, devices demanding new processing techniques,suchasMEMS[2],Leds[3]andlaserswithintegratedlensesor gratings, etc. have renewed interest in this technique.The technique can be described as follows [4]: a semi-conductor is immersed in an electrolyte, forming a spacecharge region at the solution–semiconductor interface; at thesame time the semiconductor is illuminated with light (nor-mally a laser) of energy greater than the material’s band gap,creating electron-hole pairs. The electric field due to bandbending at the semiconductor surface attracts one type ofcarrier to the surface and repels the other type of carriertowardsthebulk.Normally,inn-typematerial,theholesgotothesurfaceandtheelectronstothebulk.Inthiscasetheexcessholes increase the oxidation state of the surface atoms,increasingtheetchingrate.Ifthereactionrateislimitedbythesupplyofphoto-generatedholesatthesurface,theetchingratecan be adjusted simply by changing the light intensity.Since the technique uses a beam of monochromaticphotons (laser) to locally control reactive interactions nearthe solid surface, it is particularly useful to performlocalized material deposition (writing), solid doping,alloying and/or, as in our case, material removing (etching)without masking assistance [5]. Due to these unique char-acteristics Laser-Assisted Chemical Etching (LACE) offersadvantageous distinctiveness that makes it preferred toother materials processing techniques [6].Of particular interest in this work is the photochemicaletching of semiconductors, also called LACE. This techniquehas been already used successfully to produce channels withapproximately parabolic cross section along GaAs substrates,creating a refraction index negative gradient inside a Graded-Index Separate Confinement Heterostructure (GRIN-SCH)laser structure. This effect was used to deviate the higher orderlateral modes out of the laser str ucture keeping only the centralone[7].Inthatworkthefeasibilityofanewtypeofhigh-powercoherent semiconductor laser with lateral emission was dem-onstrated. As far as we know this was the first time LACE wasused successfully to build structures in laser diodes thatsimultaneously improved their power output and coherence.In this work we show the results of applying LACE ton-type GaSb substrates to create cavities with nearly par-abolic cross sectioned profiles that could be used as micromirrors or micro lenses to concentrate or deviate lightbeams emitted from a surface emitting laser diode. Aparabolic profile of the mirror is best suited to obtain singlemode operation in an unstable resonator [8, 9].GaSb is an interesting material for the production oflasers and photodetectors in the near and mid infraredregion. However, the use of GaSb requires the developmentof some processing techniques such as etching andpolishing. Chemical processing of GaSb is particularly