ALBERT

Notes: The p-type doping of ZnTe, CdTe, and Cd1−xZnxTe (CZT) using a nitrogen dc plasma source during growth by molecular beam epitaxy is demonstrated. For ZnTe, doping levels as high as 1020 cm−3 were achieved. In CZT alloys, a progressive decrease of the maximum doping level is observed for decreasing Zn content. Using pulse doping methods, a doping level of p≈3×1018 cm−3 is obtained for a 12% Zn CZT layer. For CdTe layers, the highest level achieved is p≈1017 cm−3. The progressive acceptor compensation phenomenon is discussed with emphasis on the role of the lattice distortion on the nitrogen incorporation mechanisms.

Notes: Nitrogen acceptors confined in a CdZnTe single quantum well, grown by molecular beam epitaxy, are investigated by different optical methods. The transitions related to nitrogen acceptors confined in the well are observed in the doping range between 1017 and 1018/cm3. The temperature and excitation intensity dependence of the nitrogen-related transitions in photoluminescence spectra indicates that these transitions correspond to the nitrogen acceptor bound excitons and to free electron to neutral nitrogen acceptor recombination. The binding energy of nitrogen acceptors confined in an 130-A(ring)-wide Cd0.96Zn0.04Te/Cd0.86Zn0.12Te structure is deduced to be 51.7±0.5 meV from this study. © 1996 American Institute of Physics.

Notes: We present a detailed study of the electronic properties of individual silicon nanocrystals (nc-Si) elaborated by low-pressure chemical vapor deposition on 1.2 nm thick SiO2 grown on Si (100). The combination of ultrathin oxide layers and highly doped substrates allows the imaging of the hemispherical dots by scanning tunneling microscopy. Spectroscopic studies of single dots are made by recording the I(V) curves on the Si nanocrystal accurately selected by a metallic tip. These I(V) curves exhibit Coulomb blockade and resonant tunneling effects. Coulomb pseudogaps between 0.15 and 0.2 V are measured for different dots. Capacitances between 0.2 and 1 aF and tunnel resistances around 5×109 Ω are deduced from the width and height of the staircases. The charging and confinement energies deduced from the I(V) curves are in good agreement with a modified orthodox model which includes the quantification of electronic levels. © 2001 American Institute of Physics.

Notes: We present a detailed study of p-type doping of Te-based compounds (CdTe, ZnTe) and alloys (CdMgTe, ZnMgTe, and CdZnTe) during molecular beam epitaxy, using nitrogen atoms produced by a direct current glow plasma source. Characterization includes capacitance–voltage, Hall effect, low temperature reflectivity and luminescence, double crystal x-ray diffraction (XRD), nuclear reaction analysis and secondary ion mass spectroscopy. Doping introduces shallow hydrogenic acceptors NTe, whose ionization energy was determined. For ZnTe, doping as high as p(approximate)1020 cm−3 can be obtained when (approximate)1.5×1020 cm−3 nitrogen atoms are incorporated in the layer. This doping level decreases considerably for CdZnTe and ZnMgTe alloys as the Zn content decreases. The highest concentration obtained for CdTe is p(approximate)1018 cm−3. For a CdTe layer with a doping level p(approximate)1017 cm−3 the hole mobility is μp=235 cm2/V s at 65 K. In ZnTe, incorporation of nitrogen atoms in NTe configurations induces a noticeable change of the lattice parameter, a change which was not observed in CdTe:N layers. The XRD pattern of ZnTe:N/ZnTe pseudosuperlattices allows measurement of the Zn–N bond distance dZn–N=2.16±0.05 Å. Study of the doping efficiency as a function of the growth conditions indicates that the compensation mechanism is related to the formation of nitrogen interstitial defects or complex defects involving metal vacancies, but no deep center has been detected in luminescence. A comparative study of various doped telluride materials indicates that presence of Zn atoms strongly enhances the solubility limit of NTe, hence the key role of Zn in the doping efficiency. Results are discussed on the basis of geometric and thermodynamic considerations. The first argument is that the Zn–Te distance is the closest fit to the metal–N bond distance, and this minimizes the elastic contribution to the energy of formation of the NTe acceptor. As a corollary, the doping efficiency decreases when the alloy lattice parameter increases. The second argument compares the formation enthalpies ΔHF of II3N2 nitride (II being Zn, Cd, or Mg) to that of the IITe compound. © 1998 American Institute of Physics.

Notes: The reduction of extended defects in ZnSe based II-VI heterostructures grown by molecular beam epitaxy on (001) GaAs is reported, using BeTe buffer layers as a novel approach. After defect selective chemical etching three different types of etch pits could be observed by optical microscopy. By the application of a thin BeTe buffer layer the density of paired Frank type stacking faults could be strongly reduced to values below 103 cm−2. The role of Se in the background pressure for the defect nucleation at the II-VI/GaAs interface is significant. It has been found that BeTe can form a smooth interface to GaAs and ZnSe which is reflected in high resolution x-ray diffraction data. © 1998 American Institute of Physics.

Notes: Beryllium chalcogenides have a much higher degree of covalency than other II–VI compounds. Be containing ZnSe based mixed crystals show a significant lattice hardening effect. In addition, they introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures due to their band gaps, lattice constants, and doping behavior. Therefore, these compounds seem to be very interesting materials for short wavelength laser diodes. Here, we report on the first fabrication of laser diodes based on the wide band gap II–VI semiconductor compound BeMgZnSe. The laser diodes emit at a wavelength of 507 nm under pulsed current injection at 77 K, with a threshold current of 80 mA, corresponding to 240 A/cm2. © 1997 American Institute of Physics.

Notes: The successful n-type doping of CdTe and CdZnTe alloys by using iodine as a dopant is reported. For CdTe, doping levels as high as 6.2×1018 cm−3 can be obtained with carrier mobility around 500 cm2 V−1 s−1 at room temperature. An ionization energy of 14.8 meV has been determined for the shallow donor by Hall effect measurements. In the case of Cd1−xZnxTe ternary alloys, only a slight decrease in the carrier density is observed when increasing the Zn concentration, by contrast to doping with indium. The doping level of 5×1017 cm−3 can be achieved for a Zn concentration x as high as 0.25, demonstrating the higher efficiency of iodine doping as compared to indium doping. © 1995 American Institute of Physics.

Notes: CdZnMgTe/CdZnTe superlattices were doped with nitrogen from a dc plasma source during their growth by molecular beam epitaxy. The samples were investigated by photoluminescence, double crystal x-ray diffraction, and secondary ion mass spectroscopy. A very strong intermixing across the quantum well interfaces was observed, producing a homogeneous alloy. The mixing is attributed to cation diffusion assisted by lattice defects generated by the simultaneous presence of Mg and N atoms. © 1997 American Institute of Physics.

Notes: We report p-type doping of tellurium-based compounds (CdTe, ZnTe) and alloys (CdMgTe, ZnMgTe, ZnCdTe) in molecular beam epitaxy using nitrogen atoms from a plasma source. The dominant shallow acceptor, as deduced from transport and optical measurements, has an ionization energy close to the effective mass value. A systematic decrease of the doping efficiency is observed as the Zn content decreases. We discuss the key role of the Zn content in the material in the framework of two factors: (i) local strain induced in the lattice by the presence of nitrogen and (ii) formation of compounds involving nitrogen atoms. © 1995 American Institute of Physics.