C. J. Sandroff

Dramatic enhancement in the gain of a GaAs/AlGaAs heterostructure bipolar transistor by surface chemical passivation

With a simple chemical treatment we have passivated nonradiative recombination centers at the periphery of a GaAs/AlGaAs heterostructurebipolar transistor, resulting in a 60‐fold increase in the current gain of the device at low collector currents. This large enhancement in gain was achieved by spin coatingthin films of Na2S9H2O onto the devices after their fabrication. We briefly discuss the passivation mechanism and the implications for other III‐V optoelectronic devices.

Nearly ideal electronic properties of sulfide coated GaAs surfaces

We have discovered that a class of inorganic sulfides [Li2S, (NH4)2S, Na2S⋅9H2O, etc.] imparts excellent electronic properties to GaAs surfaces. The surface recombination velocity at the interface between Na2S⋅9H2O and GaAs begins to approach that of the nearly ideal AlGaAs/GaAs interface. We propose the formation of a robust covalently bonded sulfide layer to explain the favorable electronic quality of such interfaces.

Effects of passivating ionic films on the photoluminescence properties of GaAs

The passivating effects of spin‐coated films of Na2S⋅9H2O on GaAs surfaces have been studied using room‐temperature photoluminescence (PL) and low‐temperature PL spectroscopy. After passivation, the 300 K PL efficiency is increased on both n‐ and p‐type material; improvements of up to 2800× are observed. The surface field and surface recombination‐related notch features in the free and bound exciton emission spectra at low temperature are eliminated, implying that the residual band bending under illumination is less than 0.15 V.

Electronic passivation of GaAs surfaces through the formation of arsenic—sulfur bonds

X‐ray photoelectron spectroscopy reveals that the remarkable electronic quality of GaAs/sulfide interfaces can be ascribed to the formation of AsxSy phases which grow on an oxide‐free GaAs surface. While one of these phases is akin to As2S3, another shows significant in‐plane S—S bonding. Raman experiments indicate that the band bending on this disulfide‐ terminated surface has been reduced to 0.12 eV.

Clusters in solution: growth and optical properties of layered semiconductors with hexagonal and honeycombed structures

Transmission electron microscopy and optical absorption were used to examine small clusters of the layered semiconductors, PbI2 and BiI3. In both systems, a layer of metal is sandwiched between two hexagonally closed‐packed layers of iodine. We describe a simple solution preparation which gives rise to clusters corresponding to single layer sandwiches, roughly 7 A thick, whose lateral dimensions vary from 12 to 90 A depending on the solvent and the nature of the growing cluster interface. The cluster size distributions are markedly different for PbI2 and BiI3 reflecting the different structure in the metal planes of these systems. PbI2 cluster sizes are determined by hexagonal symmetry, with cluster growth achieved by placement of lead atoms symmetrically about a smaller cluster. In BiI3, whose metal plane has a honeycombed structure like graphite, clusters grow to be much larger with their sizes determined by the closure of six‐membered rings. The optical absorption spectra of PbI2 and BiI3 can be quanti...

Raman scattering measurements of decreased barrier heights in GaAs following surface chemical passivation

We present Raman scattering data from GaAs samples whose surfaces had been treated with thin films of sodium sulfide nonahydride (Na2S⋅9H2O). Raman scattering provides a quantitative, contactless means of measuring the reduced barrier height associated with decreased density of GaAs surface states. For GaAs samples doped at levels of n≊1018 cm−3, the barrier height is reduced to 0.48±0.10 eV.

Near‐ideal transport in an AlGaAs/GaAs heterostructure bipolar transistor by Na2S⋅9H2O regrowth

The deposition of sodium sulfide nonahydrate (Na2S⋅9H2O) onto mesa AlGaAs/GaAs heterostructure bipolar transistors confers near‐ideal transport characteristics to the device structure. By reducing the GaAs surface recombination velocity, sulfide regrowth leads to current gain ( β) almost independent of collector current, and β>1 at collector current density below 5×10−7 A/cm−2. Furthermore, we obtain by passivation an emitter junction ideality factor of n=1.03.

Improvements in electrostatic discharge performance of InGaAsP semiconductor lasers by facet passivation

Chemically treating laser facets with aqueous sulfides can significantly improve the electrostatic discharge (ESD) performance of InGaAsP semiconductor lasers. Commercial lasers free of internal defects were subjected to forward-biased Human Body Model ESD stress pulses. Devices passivated with sulfides exhibited a mean ESD failure voltage more than 400% higher than that of the untreated control group. Subsequent accelerated aging experiments suggest that a thick layer of oxide covering the laser facets, largely removed by the sulfide treatment, is responsible for the low ESD failure voltage on untreated devices. This suggests that sulfide passivation followed by facet encapsulation in a robust dielectric could result in permanent protection against ESD failure. >

Investigation of finite size effects in a first order phase transition: High pressure Raman study of CdS microcrystallites

We have used Raman scattering to explore finite size effects in the phase transition of 300 A colloidal microcrystallites of CdS. By measuring the shifts in LO phonon frequencies as a function of pressure we conclude that these small microcrystallites undergo a well‐defined first order phase transition similar to the one occurring in bulk CdS. However, the phase transformation in the colloids exhibits a smaller hysteresis, which we attribute to efficient annealing of defects in the high surface area colloidal material.

Degenerate four‐wave mixing from layered semiconductor clusters in the quantum size regime

We report the first measurement of the third‐order nonlinear susceptibility χ(3) in layered semiconductor clusters exhibiting pronounced quantum size effects at room temperature. BiI3 clusters prepared in colloidal form in acetonitrile had a thickness of ≂7 A and lateral dimensions between 60 and 90 A. Using degenerate four‐wave mixing, we observed that the conjugate pulses from the small and the large gratings had comparable intensities, verifying the electronic origin of the nonlinearity. The nonlinear susceptibility was found to be 2.3×10−11 esu for a colloid with a cluster volume fraction of 10−5.