P. M. Raccah

Optoelectronic properties of Cd1−xZnxTe films grown by molecular beam epitaxy on GaAs substrates

Photoluminescence (PL) experiments were carried out at 300 and 12 K to investigate the electro‐optical properties of Cd1−xZnxTe grown by molecular beam epitaxy on GaAs substrates. The compositional dependence of the band‐gap energy was determined. It has a quadratic dependence on x. The near band edge PL spectra at 12 K show free and bound exciton lines for x=0 and 1 and only broadened bound exciton peaks for other compositions. The bound exciton broadenings are quantitatively explained based on the compositional fluctuations of the cations. The PL line shapes give indications of the high quality of the layers.

Study of the interface of undoped and p‐doped ZnSe with GaAs and AlAs

We have used electrolyte electroreflectance (EER) to characterize ZnSe/GaAs and ZnSe/AlAs interfaces. The great sensitivity of EER to interface space‐charge regions enabled us to detect both interface crossover transitions and transitions to triangular‐well interface states. The observation of these transitions provides the first unambiguous proof that the ZnSe/GaAs interface is type I and allowed us to determine the band offsets and band bendings, the diffusion lengths across each interface, and the amount of interdiffusion.

Study of mercury cadmium telluride epilayers grown by metalorganic vapor‐phase epitaxy

We report on the growth and characterization of HgCdTe epilayers grown by metalorganic vapor‐phase epitaxy. The transport properties indicate that it is possible to grow epilayers with mobilities comparable to those of the best bulk‐grown materials. Electrolyte electroreflectance studies have yielded depth profiles for the alloy composition, the strains, and the density of polarizable defects. These latter results indicate very clearly that the interface alloy/substrate is very sharp and does not extend over more than 2000 A. Moreover, in the bulk of the epilayers the strains are low and comparable to those found in high‐quality bulk single crystals.

Electrolyte electroreflectance study of the effects of anodization and of chemomechanical polish on Hg1−xCdxTe

We havw shown that electrolyte electroreflectance (EER) can be used as a sensitive probe to study the effects of processing agents on the near surface bulk. Our results indicate that Br2/methanol may be damaging Hg‐annealed materials to a depth of about 600 Angstroms, depleting them in Cd and leaving their surfaces covered with a Te‐rich layer. While it is possible to remove the damaged layer anodization‐dissolution steps, our results show that such a procedure must be terminated when the undamaged material has been reached. If it is continued, the difference between the rates at which the Cd and Hg diffuse through the oxide would again deplete the interface in Cd.

Minority carrier diffusion length measurements in CdTe by a photocurrent technique

A simple technique for measuring the minority carrier diffusion length in semiconductors is demonstrated. The technique does not require the knowledge of any parameter of the investigated material. It consists, basically, in measuring the photocurrent of a semiconductor‐electrolyte Schottky barrier, as a function of applied reverse bias. Weakly absorbed light is used, so that the light penetration depth is much larger than the Schottky barrier width. The diffusion length of n‐type CdTe doped at a concentration of 8×1014/cm3 is found to be 1.75 μm.

Study of the interface changes during operation of nCdTe-electrolyte solar cells

Previous work has shown that while nCdTe‐electrolyte solar cells are low cost and efficient, their photocurrent decays after a few hours of operation. We ave systematically analyzed the changes that take place at the interface. Auger spectroscopy measurements have shown out‐diffusion of Cd2+ and the formation of a Te layer at the surface. Electroreflectance (ER) and electroluminescence measurements have further shown that this process results in the formation of a p‐type region. The evolution of the ER spectra as the p‐type region develops was followed and fully analyzed. This technique offers the advantage of easy control and optimization of the p‐n junction growth parameter. The influence of the Te layer on the cell performance was determined from spectral response measurements.

Line shape of the optical dielectric function

A systematic study of the optical dielectric function by spectroscopic ellipsometry and electroreflectance has shown that the proper functional form for the Green’s function for an electron‐hole pair in GaAs or CdTe is primarily Gaussian, not Lorentzian as is commonly assumed, although it is primarily Lorentzian for Hg1−xCdxTe. The Lorentzian part of the broadening is shown to measure the alloy, impurity, and defect scattering.

High‐speed high‐current field switching of NbO2

The field‐switching properties of NbO2/NbO or NbO2/(reduced NbO2) devices have been investigated. Under an applied electric field they switch from a high (≳10 kΩ) to a low (∼10 Ω) resistance in times shorter than 0.7 ns. For pulse durations of several ns, the current carrying capability is higher than 80 A. The switching mechanism appears to proceed in two stages: the field‐assisted Poole‐Frenkel mechanism for low applied field and thermal runaway. However, the unique switching properties of NbO2 could also be understood by a small poloron‐conduction mechanism.

Electron beam electroreflectance studies of GaAs and CdTe surfaces

An electron beam electroreflectance (EBER) system has been built and used to investigate GaAs and CdTe sample surfaces. EBER permits one to go to low temperatures, to couple electroreflectance with high‐vacuum techniques and to study the effects of different surface treatments. Analysis shows that the EBER technique leads to only negligible thermal modulation of the sample for low‐energy electron beams such as that used here. The results of our measurements before and after sputtering with 2 keV O+2 ions clearly show that the measurements are very sensitive to the condition of sample surfaces. Analysis of the results shows the existence of distinct surface and bulk signals and gives detailed information about the surface before and after sputtering. Comparison of our EBER results with room‐temperature electrolyte‐electroreflectance results on both GaAs and CdTe confirms the analysis and establishes that electroreflectance is a surface‐sensitive technique.

Electroreflectance of ion‐implanted GaAs

A systematic investigation of the electrolyte electroreflectance of GaAs with implanted Be, Se, and Si ions has been made. Topographical maps of carrier concentration and of implant‐dependent modifications of the lineshapes have been obtained. The fluences for the implanted samples ranged from ∼1013 to 1015 ions/cm2. Using the E0 and E1 peaks, the lineshape variations have been mapped for the low‐ as well as high‐fluence samples. The evidence suggests that at high fluence the extreme structural damage, resulting from implantation, generates an amorphous layer which upon annealing recrystallizes by a mechanism similar to liquid phase epitaxy. Depending on the fluence of the implant, apparent peak shifts of as much as 80 meV have been observed. These are explained as definitely not due to gross changes in the E1 and E1+Δ1 transitions but rather either due to a modification of the simple parabolic band model or due to a definite folding of one of these structures with a third, probably of excitonic origin an...