Todd R. Hayes

Reactive ion etching of InP using CH4/H2 mixtures: Mechanisms of etching and anisotropy

Reactive ion etching of InP with CH4/H2 mixtures, a promising process for optoelectronic device fabrication, has been studied to understand the mechanisms of etching and anisotropy. Special attention has been paid to the polymer film that deposits on inert surfaces in the discharge; deposition rates have been used as a monitor of the discharge chemistry as well as for process optimization. Surface analysis shows that under etching conditions that maximize the InP etch rate while minimizing polymer deposition, the hydrocarbon coverage on the InP surface equals typical ‘‘adventitious’’ carbon levels, and the surface is significantly depleted of P. The etch rate here is limited by the flux to the surface of hydrocarbon reactants responsible for In desorption. The absence of a significant hydrocarbon film on the vertical‐etched surfaces under conditions of 8:1 anisotropy precludes a surface inhibitor mechanism of anisotropy, implicating instead energy deposition via ion bombardment as the major contributor to...

Damage to InP and InGaAsP surfaces resulting from CH4/H2 reactive ion etching

Structural and electrical damage imparted to InP and In0.72Ga0.28As0.6P0.4 (λg≂1.3 μm) surfaces during CH4/H2 reactive ion etching (RIE) have been examined. X‐ray photoelectron spectroscopy was used to monitor changes in the surface chemistry, Rutherford backscattering spectrometry was used to measure crystallographic damage, and current‐voltage and capacitance‐voltage measurements were made to examine electrically active damage and its depth. Two classes of damage are observed: crystallographic damage originating from preferential loss of P (As) and/or ion bombardment‐induced collision cascade mixing and, for p‐type material, hydrogen passivation of Zn acceptors. Etching at 13.6 MHz, 60–90 mTorr, 10% CH4/H2, and bias voltages of ∼300 V contains gross (≳1%) damage as measured by RBS to within 40 A and electrically active damage to within 200 A of the surface. This is a factor of 3–6 shallower than other RIE processes operated below 10 mT with comparable or higher bias voltages. Acceptor passivation of bot...

Electron impact ionization cross sections of SiF2

Absolute cross sections are measured for electron impact ionization and dissociative ionization of SiF2 from threshold to 200 eV. A fast (3 keV) neutral beam of SiF2 is formed by charge transfer neutralization of SiF+2 with Xe; it is primarily in the ground electronic state with about 10% in the metastable first excited electronic state (a 3B1). The absolute cross section for ionization of the ground state by 70 eV electrons to the parent SiF+2 is 1.38±0.18 A2. Formation of SiF+ is the major process with a cross section at 70 eV of 2.32±0.30 A2. The cross section at 70 eV for formation of the Si fragment ion is 0.48±0.08 A2. Ion pair production contributes a significant fraction of the positively charged fragment ions.

Passivation of acceptors in InP resulting from CH4/H2 reactive ion etching

Reactive ion etching of InP with CH4/H2 mixtures leads to hydrogen passivation of near‐surface Zn acceptors but not S donors. Secondary‐ion mass spectrometry (SIMS) measurements of CH4/D2 etched samples show deuterium diffuses to a depth of 2000 A in p‐InP (1.5×1018 cm−3) when etching at a rate of 520 A/min and a temperature of about 80 °C. Acceptor passivation occurs to the same depth. For n‐InP, no donor passivation is observed, even though SIMS shows deuterium diffusion to a depth of 7000 A. Annealing at 350 °C for 1 min restores carrier concentrations to near pre‐etched levels.

Absolute cross sections for electron-impact ionization and dissociative ionization of the SiF free radical

Absolute cross sections for electron‐impact ionization of the SiF free radical from threshold to 200 eV are presented for formation of the parent SiF+ ion and the fragment Si+ and F+ ions. A fast beam of SiF is prepared by charge transfer neutralization of an SiF+ beam. The radicals form in the ground electronic state and predominantly in their ground vibrational state, as shown by agreement of the measured ionization threshold with the ionization potential. The absolute cross section for SiF→SiF+ at 70 eV is 3.90±0.32 A2. The ratio of cross sections for formation of Si+ to that for SiF+ at 70 eV is 0.528±0.024; the ratio for formation of F+ to that of SiF+ is 0.060±0.008. The observed threshold energy for Si+ formation indicates the importance of ion pair formation SiF→Si++F−. Breaks in the cross section at 14.3 and 17 eV are assigned as dissociative ionization thresholds.

Electron‐impact ionization cross sections of the SiF3 free radical

Absolute cross sections for electron‐impact ionization of the SiF3 free radical from threshold to 200 eV are presented for formation of the parent SiF+3 ion and the fragment SiF+2, SiF+, and Si+ ions. A 3 keV beam of SiF3 is prepared by near‐resonant charge transfer of SiF+3 with 1,3,5‐trimethylbenzene. The beam contains only ground electronic state neutral radicals, but with as much as 1.5 eV of vibrational energy. The absolute cross section for formation of the parent ion at 70 eV is 0.67±0.09 A2. At 70 eV the formation of SiF+2 is the major process, having a cross section 2.51±0.02 times larger than that of the parent ion, while the SiF+ fragment has a cross section 1.47±0.08 times larger than the parent. Threshold measurements show that ion pair dissociation processes make a significant contribution to the formation of positively charged fragment ions.

Optical measurement of surface recombination in InGaAs quantum well mesa structures

Surface recombination of optically created electron‐hole plasma in InGaAs/InP quantum well mesa structures formed by chemical beam epitaxy followed by anisotropic plasma etching is observed optically by a picosecond pump‐probe method. The exponential carrier lifetime in 3.3‐μm‐diam structures is reduced from 31 ns as measured for large diameters to 5.5 ns. We ascribe this reduction to a surface recombination velocity of 1.2×104 cm/s. The surface recombination velocity is about two orders of magnitude smaller than those reported for bulk GaAs layers exposed to air.

Maskless laser interferometric monitoring of InP/InGaAsP heterostructure reactive ion etching

Infrared laser interferometry is used to measure etch rate, measure wafer temperature, and identify heterostructure layers in situ during reactive ion etching, with or without masked regions. Interference between reflections from the etching wafer surface, buried heterointerfaces, and polished wafer back allows etch rate monitoring and endpoint determination. Changes in the optical path length that occur as a wafer heats and cools upon processing also produce reflected intensity oscillations that allow determination of the process‐induced change in wafer temperature. We also show that λ=0.6238 μm light can be used to monitor optically thin heterostructure layers with enhanced depth resolution over infrared light.

Collector-up light-emitting charge injection transistors in n-InGaAs/InAlAs/p-InGaAs and n-InGaAs/InP/p-InGaAs heterostructures

The realization of collector‐up light‐emitting complementary charge injection transistors is reported. The devices have been implemented in molecular‐beam‐epitaxy‐grown n‐InGaAs/InAlAs/p‐InGaAs and n‐InGaAs/InP/p‐InGaAs heterostructures using a self‐aligned process for the collector stripe definition. Electrons, injected over the wide‐gap heterostructure barrier (InAlAs or InP) by the real‐space transfer (RST) process, luminesce in the low‐doped p‐type InGaAs active layer. An essential feature of present devices, besides their self‐aligned collector‐up configuration, is a relatively heavy doping of the n‐type emitter channel, with the sheet dopant concentration of 4×1012 cm−2. This ensures a higher uniformity of the electric field in the channel and provides a relief from RST instabilities at a high level of collector current (linear density ∼10 A/cm). Devices with InAlAs and InP barriers show rather different optical characteristics, mainly due to the different band lineups ΔEC/ΔEV in InGaAs/InAlAs and I...

SiO2 mask erosion and sidewall composition during CH4/H2 reactive ion etching of InGaAsP/InP

SiO2 mask erosion has been studied during CH4/H2 reactive ion etching of InGaAsP/InP double heterostructures. The amount of mesa mask narrowing at a pressure of 100 mT, normalized for an etch depth of 3.5 μm, is approximately 0.4–0.6 μm and decreases slightly with increasing self‐bias voltage. It is not strongly dependent on the sidewall angle of the mask or CH4 concentration. Mask residue deposits on the etched sidewall under conditions of relatively high CH4 concentration and low power density. Auger electron spectroscopic analysis of the sidewall shows that the deposit contains a significant amount of elemental Si, which suggests a mechanism for mask erosion in which SiO2 is reduced to Si in the hydrogen/hydrocarbon‐rich environment of the plasma.