S.H. Groves

Flow visualization studies for optimization of OMVPE reactor design

Abstract Gas flow visualization studies have been performed in a vertical reactor tube incorporating a rotating disk susceptor. Smoke particles of TiO 2 transported in a stream of He gas are illuminated by a sheet of laser light. This technique permits detailed observation of gas flow patterns. To study the influence of the method of gas injection, the gas was injected through (1) a vertical pipe inlet coaxial with the tube, (2) four inlets tangential to the tube and (3) a radial inlet above a porous plug. The method of injection was found to have a critical effect on vortex formation, which is minimized by using the porous plug. Disk rotation can be effective in creating a uniform boundary layer at the disk surface. When the susceptor is heated, the flow is strongly perturbed by thermally induced convection, which can be reduced by lowering the operating pressure.

Double heterostructure Pb1‐xSnx Te–PbTe lasers with cw operation at 77 K

Diode lasers that operate cw at 77 K have been made from a Pb0.88Sn0.12 Te–PbTe double heterostructure grown by liquid phase epitaxy. Junction diffusion during growth has been prevented by the use of Tl‐doped PbTe substrates. The wavelength emitted in cw operation has been temperature tuned from 10.5 μm at 12 K to less than 8.2 μm at 80 K, a range of nearly 280 cm−1. The cw threshold current density increases from 1.6 × 103 A/cm2 at 12 K to 4.2 × 103 A/cm2 at 77 K. The measured cw power is 10 mW in four modes at 12 K (1.8 × 104 A/cm2) and 1.2 mW in a single mode at 77 K (7.5 × 103 A/cm2).

Impact ionization in (100), (110), and (111) oriented InP avalanche photodiodes

The impact ionization process in the 〈100〉, 〈110〉, and 〈111〉 crystallographic directions in InP has been investigated by analysis of photomultiplication and multiplication noise data from InP avalanche photodiodes. This is the first report of such measurements for (110)‐oriented InP and the first consistent investigation of impact ionization in the three principal crystallographic directions. Our measurements indicate that, unlike the reports for GaAs, no significant orientation dependence of the impact ionization coefficients exists in InP. Momentum‐randomizing collisions with phonons, which result in intervalley transfer of energetic electrons, are believed to be the reason for the lack of anisotropy in the electron impact ionization coefficients.

Ionization coefficients of electrons and holes in InP

The ionization coefficients of electrons and holes in InP have been determined from photomultiplication measurements on abrupt‐junction low‐leakage np+ InP avalanche photodiodes. The ionization rate of holes (u2009β) was found to be greater than that for electrons (α), the ratio varying with peak electric field Em from β/α=3.8 at Em=4.85×105 Vu2009cm−1 to β/α=2.7 at Em=6.37×105 Vu2009cm−1.

Optimization of four‐wave mixing conversion efficiency in the presence of nonlinear loss

We analyze the effect of nonlinear loss on four‐wave mixing (FWM) conversion efficiency. Maximum conversion efficiency is geometry independent and equal to e−2|χ(3)/Im{χ(3)}|2 when nonlinear loss dominates. Optimum device length and operating conditions are obtained and theoretical results are verified with a picosecond pulse FWM experiment.

Donor identification in liquid phase epitaxial indium phosphide

High resolution photoluminescence spectroscopy at 4.2 K and at a magnetic field of 9.7 T is employed to resolve and identify residual shallow donors in liquid phase epitaxial InP. Sulphur is found to be the dominant inadvertent contaminant in high purity material prepared from growth solutions which have undergone long baking treatments. Silicon is also observed in most samples, but at significantly lower levels than sulphur.

Liquid-phase epitaxial growth of InP and InGaAsP alloys☆

Abstract Liquid-phase epitaxy (LPE) has been used to grow high-quality layers of InP and InGaAsP alloys on InP substrates for a wide variety of device-development and physical-measurement applications. An apparatus featuring a transparent furnace and an atmosphere with controlled amounts of H 2 , PH 3 and H 2 O has proved to be flexible enough for diverse crystal growth requirements. Carrier concentration vs temperature measurements on LPE-grown InP and InGaAsP samples with N D - N A in the low 10 15 cm -3 range show that they contain a moderate concentration of deep donors. There is evidence that these deep donor states also account for a strong peak in the photoluminescence seen in LPE-grown samples. Substrate-orientation studies have shown that critically oriented substrates improve the morphology for growth of heavily Zn-doped InP.

Use of flow visualization and tracer gas studies for designing an InP/InGaAsP OMVPE reactor

Studies of gas flow using smoke particle visualization and a tracer gas measurement technique were used to provide design information about growth tubes for organometallic vapor phase epitaxy (OMVPE) of InP and InGaAsP alloys. It is shown that convective cells may form when a gas of a different density than that of the gas already flowing in the tube is rapidly switched into the reactor, even when the vent and run lines of the switching manifold are pressure and flow balanced. This effect, reported here for the first time, could severely degrade the material quality of InP/InGaAsP heterojunctions. This transient convection after gas switching is particularly acute for a vertical growth tube with a large cross-sectional area (≈ 80 cm2), such as the vertical reactor tube studied has a rectangular tube with a high aspect ratio (7.6/1.6) and a susceptor surface parallel to the gas flow. The cross-sectional area (≈ 10 cm2) is greatly reduced and for the same volumetric flow rate the gas velocities are increased, which eliminated convection, even for density changes considerably greater than those encountered in InP/InGaAsP growth. Susceptor rotation in this geometry is expected to average growth nonuniformities which result from gas phase depletion. At high rotation rates (> 200 rpm), the susceptor motion induces vortices which are likely to produce nonuniform growth and graded interfaces. However, at ≈ 100 rpm there is no significant perturbation of the gas flow with mean gas velocities of >15 cm/s. This rotation rate is sufficient to achieve considerable averaging at typical OMVPE growth rates.

Short‐pulse wavelength shifting by four wave mixing in passive InGaAsP/InP waveguides

We have demonstrated wavelength conversion in the 1.5 μm regime using the near band gap nonlinearity of semiconductor quantum wells. In a 7.5 mm long passive InGaAsP/InP single quantum well waveguide, a wavelength shift of 20 nm has been obtained with −11 dB conversion efficiency using picosecond pulses. We confirmed the ultrafast nature of the nonlinearity by measuring the conversion efficiency as a function of the frequency shift. An order of magnitude increase in the value of n2 close to the band gap is observed compared with the off‐resonance value.

LPE growth and characterization of InP photodiodes

Advances have been made in understanding high-purity LPE growth, and several important materials parameters have been determined from measurements on high performance InP APDs. In particular, for the former, sulfur contamination resulting from opening the growth system to air can occur via adsorption on and re-evaporation from graphite. Also the type of fused silica used in the growth tube affects the rate of approach to equilibrium during baking. From the study of high performance InP APDs with grown p-n junctions we have determined the hole diffusion length Lp ≈ 12 μm, appropriate to LPE material with ND−NA≈1 ×1016 cm-3, by fitting the calculated to measured photocurrent versus voltage and versus n-layer thickness. This is an order of magnitude higher than Lps determined from EBIC, a technique that is not well suited to these long diffusion lengths. Messurements of dark current versus dislocation etch pit density show no correlation between these parameters; this is in contrast to work reported by other authors on junctions formed by diffusion. Impact ionization measurements have been made for the electric field parallel to 〈100〉 and 〈110〉 directions, and excess noise versus multiplication data have also been analyzed for these directions as well as <111. We find that there is not a measurable orientational dependence of the ratio of hole to electron impact ionization coefficients.