R. N. Bhargava

Doped nanocrystals of semiconductors - a new class of luminescent materials

Abstract We report on the unique luminescent properties of nanocrystals of Mn-doped ZnS with varying sizes from 30 to 70A˚prepared at room temperature. These nanosize quantized particles yield the best external photoluminescent quantum efficiency of about 18% at room temperature and luminescent decay time at least five orders of magnitude faster than the corresponding Mn 2+ radiative transition in the bulk crystals. These luminescent measurements also suggest that the efficiency increases with decreasing size of the nanocrystalline particles. These novel properties may be attributed to electron-hole localization and hybridization of the s-p host states with d-electrons of the Mn impurity.

Shallow acceptors and p‐type ZnSe

Shallow acceptors have been incorporated in ZnSe by liquid‐phase epitaxy using Bi as a solvent. Epilayers with Li, Na, N, and P as dopants were proven to be p type by establishing the position of the Fermi level by photocapacitance and photoconductivity measurements, and by measuring the potential drop at biased Schottky barriers.

Materials growth and its impact on devices from wide band gap II–VI compounds

Abstract The progress made in the materials preparation and characterization of ZnSe in the last few years is reviewed. The recent results on MOVPE and MBE grown ZnSe show promise of achieving a p-n junction by reliable control of the quality of the material and the dopant incorporation. The impact of superlattices and multiquantum-well structures on the injection devices, electron beam pumped lasers, passivation of III–V compounds by II–VI compounds and nonlinear optical devices is briefly discussed.

Effect of lattice mismatch in ZnSe epilayers grown on GaAs by molecular beam epitaxy

We report a detailed study of the effect of lattice mismatch on ZnSe epilayers grown on 〈001〉 GaAs by molecular beam epitaxy using photoluminescence (PL), x‐ray diffraction, and transmission electron microscopy (TEM) techniques. We find that our samples are of high quality, exhibiting sharp and strong bound excitons, and that these bound excitons shift to higher energies due to tetragonal distortion as the thickness of the ZnSe epilayer is systematically reduced from ∼1 to 0.1 μm. Fairly good agreement is found between PL and x‐ray data for the total strain relaxation as a function of layer thickness. TEM measurements are also used to estimate an inelastic component of the strain relaxation in the layers.

High‐purity ZnSe grown by liquid phase epitaxy

A study was performed to establish the origin and nature of background compensating impurities in undoped ZnSe layers grown by liquid phase epitaxy on ZnSe substrate wafers in a low‐contamination‐level environment. The width of bound exciton lines in low‐temperature photoluminescence spectra was used to define the quality of the material, and the energy of the lines was used to identify these low‐level impurities. The sharpest spectra occurred in layers grown rapidly on a previously grown buffer layer indicating the importance of impurity outdiffusion from the substrate into the growing layer. The sharpness of these bound exciton lines indicates that the total concentration of electrically active impurities (NA+ND) is <1017/cc, an estimate which is confirmed by mass spectroscopy.

Thermally stimulated current measurements and their correlation with efficiency and degradation in GAP LED'S

Thermally stimulated current (TSC) measurements have been performed on p‐n junctions in GaP grown by double liquid phase epitaxy. Several deep states have been observed in both the n and p sides of the junction, and a correlation has been established between the presence and concentration of some of these traps and the quantum efficiency of the corresponding light‐emitting diode (LED). It has also been observed that the degradation of the LED by heating under forward bias leads to the appearance or increase of the intensity of some peaks in the TSC spectrum.

Mechanism for efficient blue second‐harmonic generation in periodically segmented waveguides

We present a theoretical investigation of the mechanisms responsible for the recently reported efficient type I blue second‐harmonic generation in periodically segmented KTiOPO4:Rb,Tl waveguides. In these guides, grating‐assisted phase matching of second‐harmonic light between 390 and 480 nm has been achieved. 5‐mm‐long guides give output powers on the order of 1 mW at around 425 nm with fundamental powers in the 50–100 mW range. We show that such high efficiencies can be expected from strongly perturbed guides through terms representing the modulation of phase mismatch and mode size in addition to the nonlinear susceptibility and refractive index modulations.

Shallow N acceptor in N+‐implanted ZnSe

A new bound exciton line IN1 at 2.7916 eV and its longitudinal phonon replicas have been observed in N+‐ion‐implanted ZnSe. Compared with the data of unimplanted samples, the pair bands are more intense and shift towards high energy by about 13 meV. These results provide confirmation that nitrogen is incorporated into ZnSe as a shallow acceptor.

Compact blue lasers in the near future

Abstract A short wavelength laser offers unique oppurtunities in high density optical recording as well as in laser printing. To achieve a compact blue laser, the current effort worldwide is primarily concentrated on achieving well-conducting p-type ZnSe and fabricating quantum well heterostructures so as to achieve a low threshold laser at room temperature. The recent milestone “an injection blue laser below room temperature” gives us confidence that indeed we are on the right path. In photopumped lasers in various II-VI heterostructures, thresholds at room temperature comparable to the theoretical limit have been reported. This not only reinforces that indeed a room temperature injection laser is possible but when combined with earlier electron-beam pumped laser results, a scanned compact laser is also feasible. In recent years several breakthroughs have demostrated that an infrared emitting GaAs laser can be used to generate blue light through efficient second harmonic generation (SHG) in certain non-linear optical materials. Recent exciting results on SHG of GaAs lasers in KTP grating waveguides resulted in a blue laser with output power in the range of several milliwatts. Alternative schemes such as upconversion lasers and SHG in III–V quantum wells structures are presented. In summary, various efforts to achieve compact blue lasers and their availability in the near future are presented.

Extended x‐ray absorption fine structure studies of diffused copper impurities in ZnSe

We have performed the first extended x‐ray absorption fine structure investigation of the local environment around dilute copper impurities diffused into the II‐VI semiconductor ZnSe. An average first nearest neighbor distance of 2.32±0.06 A is obtained. Our results indicate a rather large distortion of the local environment surrounding the copper impurities and/or displacement of these impurity atoms from simple substitutional sites (CuZn) and high symmetry interstitial sites (CuI).