Properties of blue laser diodes based on ZnSe-related II-VI semiconductor heterostructures are reported. At 77 K, continuous-wave (cw) operation has been achieved for lasers emitting at wavelengths as short as 470.4 nm (2.635 eV), while pulsed laser emission has been observed up to ∼200 K, for samples with uncoated facets. Measured turn-on voltages for stimulated emission at 77 K were as low as 12 V for some of the laser diodes due to improved ohmic contacts. For some particular devices, differential quantum efficiencies as high as 36% per (uncoated) facet have been obtained at 77 K.

Integrated heterostructure devices (IHDs) comprised of II-VI materials in multi-layered structures for light emitting diode (LED) and laser diode (LD) applications are described. These IHDs combine a light emission multilayer structure (wide band gap II-VI layers) with an abrupt or graded heterostructure (comprised of narrow band gap II-VI layers) for improved ohmic contact to the upper p-type layer of the light emitting structure.

The recent success in obtaining blue-green diode lasers using ZnSe-based heterostructures has been mainly due to the successful p-doping of ZnSe using a N plasma source in an MBE system. P-type ZnSe can also be grown by MOCVD, using NH3 as the dopant source. Dopant concentrations of up to 1018 cm−3 have been obtained but net acceptor concentrations are in the range of only about 1015 cm−3 Activation of the remaining N has been achieved to some extent (NA -ND∼3*1016cm−3 ) by rapid thermal annealing. However, this has had limited success. We have used fast surface annealing by an excimer laser to activate the N. We have observed an increase in the ratio of Acceptor Bound Exciton peak intensity to Free Exciton peak intensity on annealing and this effect increases as we increase the laser power density from 10 to 30 MW/cm2. Electrical measurements ( C-V ) give a net acceptor concentration of ∼2*1016 cm−3 and thus confirm the increase in carrier concentration after annealing.

Transmission election microscopy studies show that nitrogen doping changes the misfit dislocation structure in ZnSe films and decreases the density of threading dislocations. There appears to be a critical N doping concentration of ∼ 1.5 × 1018/cm3 above which the density of threading dislocations increases again. Samples with high N doping concentrations (> 1019/cm3 ) also show compensation or decrease in the carrier density of the films. Our TEM observations show that N doping can produce low energy nucleation sites for the 60° misfit dislocations at or close to the ZnSe/GaAs interface.

A conduction band-to-acceptor (e-A°) peak at 2.7060 eV has been identified for the first time in 1.7 K PL spectra of Li-doped ZnSe/GaAs. Photoluminescence (PL) measurements as a function of temperature and excitation level have been employed to confirm the identification and to study the behavior of this peak. This peak was previously misidentified as the “R”-band characteristic of bulk material, which has been associated with transitions between preferentially paired interstitial Li donors and substitutional Li acceptors on Zn sites. We find no direct evidence of Li interstitials in the PL data. The splitting of the Li acceptor-bound exciton is dramatically reduced by removal of the GaAs substrate, which proves that the splitting is due to strain and not to the hole-hole exchange interaction. A P-doped ZnSe/GaAs sample exhibits a shallow P bound exciton peak nearly superimposed on the donor-bound excitons. It is distinguished by its strong LO phonon replica and low energy undulation wing, which are both unique characteristics of shallow acceptor-bound excitons. Transitions involving the conduction band to split P acceptor light and heavy hole levels have been detected for the first time in P-doped heteroepitaxial ZnSe. Using the temperature dependence of the (e-A°) peak positions, accurate binding energies of 89.1±0.6 and 114.4±0.4 meV have been obtained for shallow P and Li acceptor levels in unstrained ZnSe, respectively

The growth condition and the structural properties of short period ZnSe-GaAs superlattices grown by metalorganic vapor phase epitaxy (MOVPE) are described. First, the growth condition was investigated. Superlattices (SLs) grown at 450°C have better structural property than those grown at 500°C. The thickness fluctuation of each constituent layer was investigated by means of the X-ray diffraction analysis, and it was indicated that the fluctuation in the fabricated structure was within 1 and 2 monomolecular layers for GaAs and ZnSe, respectively. Further optimization for growing thinner layers successfully brought SL with 25Å well and 140Å barrier layers. X-ray diffraction analysis and transmission electron microscopy (TEM) observation showed its good structural properties; high crystalline quality, high degree of lateral uniformity, well defined interfaces and strong periodicity. Another SL with 100 periods of ZnSe (33Å) / GaAs (25Å), which have smaller layer thickness and much larger number of periods compared with the SL mentioned above, could be also fabricated and the crystalline quality was found to be excellent by X-ray diffraction measurement.

We have successfully grown ZnMnTe alloys by molecular beam epitaxy using GaAs as a substrate. Bulk MnTe has the wurtzite crystal structure but the structural phase of the material was confirmed to be zinc-blende by standard θ-2θ x-ray diffraction techniques. The composition was also determined using x-ray diffraction techniques. Manganese cancentration was also estimated from magnetization measurements taken as a function of temperature. Magneto-luminescence studies were performed at 1.4K on the acceptor-bound exciton in the semimagnetic semiconductor ZnMnTe alloys. As expected, the photoluminescence peak energy decreased with increasing magnetic field.

In this study, cross-sectional transmission electron microscopy (XTEM) was used to investigate the defect structure which occurs at the interface between CdTe(001) and GaAs(001). The heterostructures were fabricated by molecular beam epitaxy on chemically etched and thermally deoxidized GaAs(001) substrates as well as GaAs(001) buffer layers grown in-situ by molecular beam epitaxy. This allowed for investigation of the GaAs surface preparation on the subsequent interfacial structure. The as-etched substrate led to a microscopically rough interface with the CdTe depositing in etch pits on the GaAs surface, while growth on the buffer layer led to a macroscopically flat interface. Further, growth was accomplished on different Te-induced surface reconstructions ((6×1) vs (2×1)) in an effort to understand the role of the precursor surface treatment on the subsequent interfacial structure. In this case growth on the (6×1) reconstruction led to the introduction of (111)-oriented inclusions at the interface, while the (2×1) reconstruction led to pure (001)-oriented growth. A mechanism for the formation of planar defects at the CdTe/GaAs(001) interface is described which is based on local misorientations of the CdTe and GaAs. Finally, preliminary results of ex-situ annealing experiments on the interfacial defect structure will be discussed.

There are two phases of CdS, wurtzite (hexagonal) and zincblende (cubic). To the best of our knowledge there is no report on the growth of large single crystal cubic CdS. Although there have been reports on the vapor deposition of cubic dominated CdS thin films, physical measurements were limited. Substrate material has been considered as the primary factor in attaining the cubic dominated CdS thin films. We report new results on CW Nd:YAG laser deposition of CdS thin films at various temperatures. X-ray diffraction patterns show that the films deposited at 200°C have a dominant cubic phase, those at 400°C being hexagonal. Optical transmission measurements reveal room temperature absorption edges of 515nm and 500nm for films deposited at 200°C and 400°C, respectively. Transmission electron microscopy further reveals differences in crystal structure of the two films. Raman spectra of the cubic film is similar to that of the hexagonal film.

The temperature dependence of photoluminescence in SrS singly and doubly doped with Eu2+, Sm3+ was studied with different excitation energies. Different energy transfer mechanisms, namely, phonon-assisted energy transfer and electron transfer through energy bands, depending on the excitation energy, are proposed to take place in SrS:Eu2+, Sm3+ thin films. We estimate that under UV light excitation 10% of Eu2+ emission originates from electronic transitions between Eu2+ and Sm3+ through the conduction and valance bands. In contrast, direct inter-ionic transitions take place for 2.84 eV excitation energy via a phonon assistance mechanism.

A new substrate material consisting of a buffer layer of a Ba-Si compound was developed by making use of the chemical reaction between BaF2 and Si. This substrate is very promising for the integration of IV-VI semiconductor materials with silicon. PbTe films of excellent quality, as determined by X-ray and Reflected High Energy Electron Diffraction spectra, have been deposited over (111)- and (100)-oriented silicon wafers of 3 inch diameter. These PbTe films are (100)-oriented irrespective of the Si orientation. X-ray photoelectron spectroscopy studies reveal very interesting chemistry at the interface between Ba-Si and Te. They suggest that BaTe may form between PbTe and Ba-Si at their interface. This interfacial region, which is of the order of only a few molecular layers, appears to be critical in the success of the deposition. Thermal cycling showed that the PbTe/BaSi/Si system is mechanically very stable. The possibility of a similar growth mechanism for the deposition of II-VI semiconductors such as CdTe is considered.

Using reflection high energy electron diffraction (RHEED), the heteroepitaxy of EuTe on PbTe (111) by molecular beam epitaxy (MBE) was investigated. The resulting EuTe (111) surfaces exhibit different surface reconstructions corresponding to a Te-stabilized or a Eu-stabilized surface. We have observed perfect 2D layer-by-layer heteroepitaxial growth and RHEED intensity oscillations only for a small range of growth conditions. Using such optimum conditions, we have fabricated strained PbTe/EuTe superlattices with superior structural perfection, as shown by high resolution x-ray diffraction.

From transmission experiments in the mid infrared region the optical interband transitions between electric subbands in the valence and conduction band of the PbTe wells embedded in Pb1−xEuxTe barriers (x = 2.6% to 3.0%) were determined and compared with calculations based on the envelope function approximation. The frequency dependence of the absorption constant α was calculated, taking the in-plane dispersion of the multiquantum well band structure into account. The frequency dependence of the refractive index n(ω) was derived from α(ω) using a Kramers-Kronig transformation and it exhibits several extrema which coincide with the onset of the various interband transitions. Experimentally the onset of these transitions was also observed in photoconductivity experiments. The changes in n(ω) are as large as 15 % and therefore important for the optical confinement in quantum well lasers grown with these materials.

The morphology and defect structures associated with thin NiSi2 films prepared on Si(100) were investigated. Facets were found to be a common defect structure at the interface unless stoichiometric co-deposition of Ni and Si was performed. Annealing thin, uniform NiSi2 layers prepared by stoichiometric co-deposition to temperatures ≥ 700°C yielded epitaxial films of high crystalline perfection. Films in which the deposition was off-stoichiometric exhibited a variety of defect structures. In addition to facets at the interface, networks of meandering micro-twins or stacking faults were observed in highly Ni-rich films, while pinholes were observed in Si-rich films. Facets at the interface were found to be eliminated by annealing at temperatures ≥ - 750°C, indicating that the planar NiSi2/Si(100) interface is energetically stable with respect to the formation of NiSi2(111) facets. Through manipulation of the film growth kinetics, NiSi2/Si(100) interfaces with arbitrary facet densities were produced. Partially faceted NiSi2/Si(100) interfaces represent ideal experimental systems for revealing the effects of Schottky barrier inhomogeneity on the electrical measurements of metal/semiconductor contacts.

It is shown that the presence of 8 × 1015 hydrogen atoms/cm2 in the CoSi2/Si (100) interface causes an increase in the Schottky barrier height of 120 meV, and that passivation of dopants in the substrate is not the cause of this change. The data is evidence that the position of the Fermi level in this interface is controlled by defect-related interface states. After hydrogenation the Schottky barrier height agrees with that predicted by theory for Fermi level pinning by virtual gap states of the silicon.

The transport of germanium through a co-deposited palladium-silicon layer on epitaxially grown GexSi1−x on Si(100) was studied. The Ge concentration in the UHV-CVD grown GexSi1−x film was x=.14 with a thickness of 155 nm. When the co-deposited layer is Pd rich with respect to Pd2Si, Ge was found to move through the Pd-Si layer and grow polycrystalline on the GexSi1−x. Ge transport and Pd2Si(111) grain growth ceased simultaneously. No transport was found for Si rich Pd-Si layers. Electrical characterization showed the contacts were ohmic with a specific contact resistivity between 6–12 μωQ-cm2 and an end resistance of.75 ω, resulting in a carrier concentration under the contact of 1.5×1014 cm−2. Hall effect measurements of the semiconductor gave n=5.77×1014 cm−2 and μ=36.96 cm−2/Vs. The sheet resistance of the contact layer was found to be 23.54 ω/square and the semiconductor was 237 ω/square.

Uniform, single crystal Si/NiSi2/Si(100) and Si/CoSi2/Si(100) structures were fabricated by molecular beam epitaxy and furnace annealing. A Si template technique and a post-growth anneal at ∼960°C led to high quality buried NiSi2 layers with thickness ∼1000Å. The conditions for fabricating high quality buried CoSi2 layers by the recently proposed allotaxy process were studied and discussed. The endotaxial growth of buried CoSi2 layers in Si(100) was also studied.

The structural and electrical properties of cobalt annealed on heteroepitaxial, strained-layer silicon-germanium were studied by transmission electron microscopy, four-point-probe and four-terminal resistor resistivity measurements, and junction diodes. The I50nm thick epitaxial p-Si0.87Ge0.13 was grown by UHV-CVD at 590°C. Cobalt was deposited by DC magnetron sputtering. The cobalt was rapid-thermally annealed at various temperatures in forming gas.

The cobalt film and the SiGe layer reacted, as illustrated by changes in their film thicknesses, and increased interfacial roughness. The roughness and reacted-film thickness increased with increasing temperature. The sheet resistances of the samples were dependant on the anneal temperature and time. Films formed by annealing at 700°C, 3 min, were 80nm thick, and had 2.8ω/sq sheet resistivity, corresponding to 20μω-cm resistivity. The contact was ohmic, with contact resistivity of l.6×10−4ω-cm2. SiGe/ Si heterojunction diodes contacted by the reacted film, containing Co, Si, and Ge, were compared to diodes contacted by aluminum.

Ballistic Electron Emission Microscopy (BEEM) was used to investigate the effects of a sulfide interfacial layer on the electronic properties of the Au/n-GaAs interface. Two diodes were simultaneously prepared in UHV by evaporating Au onto (100)GaAs substrates which were either treated with (NH4)2S or NH4OH etching solutions prior to Au deposition. Auger electron spectroscopy was used in situ to monitor the surface chemical composition. The effective Schottky barrier (SB) height was measured with BEEM at 10 different locations on the surface of each diode. The (NH4)2S treatment increased the average barrier height and reduced the spread in the values, as compared to the contact formed on the NH4OH treated GaAs.

A new surface passivation technique using P2S5/(NH4)2S on GaAs was investigated, and the results are compared with those of the (NH4)2Sx treatment. With this new surface treatment, the effective barrier heights for both Al- and Au—GaAs Schottky diodes were found to vary with the metal work functions, which is a clear evidence of the lower surface state density. Results of I—V measurements show that P2S5/(NH4)2S—passivated diodes have lower reverse leakage current and higher effective barrier height than those of the (NH4)2Sx -treated ones. Auger Electron Spectroscopy, X—ray photoelectron spectroscopy and Raman scattering measurements were done to characterize the surfaces including their compositions and surface band bending. In this paper, interpretations on this novel passivation effect is also provided.