A Novel Surface Preparation and Post-Etch Removal Technique for InGaAs Sidewalls
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ABSTRACT This paper describes in detail a surface preparation, and post-etch removal technique developed for InGaAs sidewalls. It illustrates the results demonstrating the effect of sidewall post-etch, surface preparation, and surface passivation on the performance of high speed InGaAs detectors. Dark current density for circular diodes with a diameter size varying between 10 and 100 ptm was measured at a reverse bias voltage of -5 V. The effectiveness of various surface
preparation techniques was studied by measuring the immediate improvement in dark current density, as well as its long-term stability. The benefits of this new technique compared to other techniques we have investigated include improved device characteristics, long-term stability, as well as a much less critical process to achieve optimum surface properties.
INTRODUCTION High speed InGaAs PIN diodes are commonly formed by mesa isolation [1,2] which results in large dark current, drift in characteristics, and reduced reliability. A surface treatment technique is described in this paper that removes the conductive oxide on an InGaAs etched mesa, and effectively passivates the InGaAs surface. For diodes with a diameter size varying between 10 and 100 pm, we have achieved a dark current density of 1.5x10
5
A/cm 2, at -5 V bias,
with a uniformity of 81% across a lx 1 cm 2 processed area. 317 Mat. Res. Soc. Symp. Proc. Vol. 477 01997 Materials Research Society
EXPERIMENT Three different techniques were used in forming the i-mesa. In the first approach, the sample was loaded into a PlasmaLab planar reactive ion etching (RIE) chamber. A 3000
A thick
Pt was used as an etch mask. The reactive gases were CH4 :H2:Ar at flow rates of 12.5, 25, and 12.5 sccm, respectively. An overpressure of 75 mTorr was used to ensure a positive slope mesa for conformal metalization in a later step. The RF power was 148W, resulting in a DC bias of 480V. Etch rates for InP and InGaAs were 840 and 420 A/min, respectively. An oxygen plasma was used to remove built-up polymer that results from the methane etching. The parameters for this cleaning step are 16 sccm 02, 200 mTorr pressure, and -200V DC bias. This technique causes damage to the InGaAs sidewall, resulting in an excessive leakage current. A novel sidewall surface treatment has been proposed in [3] that significantly reduces the dark current. In this approach, a 1:1 mixture of H2SO 4 :IPA is used to oxidize the damaged surface. TMAH is then used to remove this oxide. This sequence is repeated until no further reduction in dark current is observed. In a second technique, the sample was loaded into a chemically assisted ion beam etching (CAIBE) chamber. The operating parameters for CAIBE are TLub=260 0C, 3sccm C12 and 500 eV ion beam. The beam current density is estimated to be 0.6 mA/cm 2 from Faraday
cup measurements. A Pt(2000A) /Ti(300A) metal system was used as the mask. The Ti overlayer has a lower ion beam etch rate, 330A/min at 500 eV Ar ion beam (from Commonwealth Scientific Corporation), compared with Pt
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