K. B. Ucer

Control of p- and n-type conductivity in sputter deposition of undoped ZnO

Recent theoretical studies have concluded that the low formation enthalpies of intrinsic donor defects should preclude achievement of p-type conductivity in undoped ZnO grown in thermal equilibrium with a molecular oxygen reservoir. This letter demonstrates that reactive sputtering can produce intrinsic p-type ZnO, controlled by adjusting the oxygen partial pressure in the sputtering plasma. We report the properties of p–n homojunctions fabricated in this way, and characterize transport in the films by Hall measurements. Our finding of p-type conductivity in undoped ZnO grown with dissociated oxygen is qualitatively consistent with the effect of higher chemical potential of atomic oxygen reactant on defect formation enthalpies. This parallels to some degree the recent attention to nitrogen acceptor incorporation by means of dissociating nitrogen source gases.

Donor-acceptor pair luminescence of nitrogen-implanted ZnO single crystal

Donor-acceptor-pair sDAPd luminescence is a direct probe of the acceptors in ZnO. We report the near-surface doping of a ZnO single crystal by ion implantation with nitrogen and titanium. Secondary-ion-mass spectroscopy shows that the doping depth is approximately 80 nm sNd and 50 nm sTid. The DAP photoluminescence centered at 3.232 eV is observed from both the undoped and doped ZnO single-crystal samples. The luminescence spectrum of the nitrogen-doped sample shows enhancement of the DAP transition compared to the “pure” ZnO sample. The acceptor energy is calculated to be 177 meV, consistent with nitrogen as the acceptor in DAP luminescence. The DAP recombination lifetime is found to be ,5.5 ns. The temperature evolution of spectra shows the gradual transition from DAP luminescence to electron+ acceptor recombination luminescence at temperatures above 37 K. Our experimental results suggest that ion implantation is an effective way of doping nitrogen into ZnO.

Production and properties of p–n junctions in reactively sputtered ZnO

Abstract In order to develop electroluminescent and laser devices based on the ultraviolet exciton emission of ZnO, it will be important to fabricate good p–n junctions. As-grown ZnO is normally n-type because of intrinsic donor defects such as oxygen vacancies and zinc interstitials, or unintended hydrogen. Making p-type ZnO has been more difficult, possibly due to self-compensation by easily formed donor defects. In this work, we demonstrate that reactively sputtered, annealed ZnO films can be changed from n-type to moderate p-type by adjusting the oxygen/argon ratio in the sputtering plasma. We report the properties of p–n homojunctions fabricated in this way, and characterize transport in the films by the Hall measurements. Ohmic contacts were formed by deposition of Au/Al films. Our finding of p-type conductivity in apparently intrinsic ZnO formed by reactive sputtering is not inconsistent with calculated defect formation enthalpies if account is taken of the higher chemical potential of the dissociated oxygen reservoir represented by the sputter plasma, compared to the molecular oxygen reservoir assumed in the calculation of formation enthalpies. If hydrogen turns out to be the main compensating donor, the role of oxygen pressure in controlling incorporation of background hydrogen during sputtering may also be implicated.

Luminescence and Stimulated Emission in Zinc Oxide Nanoparticles, Films, and Crystals

ZnO has attracted attention as a candidate material for ultraviolet light-emitting devices. Its 3.37 -eV band gap is comparable to that of GaN, and single crystal substrates can be grown. Control of p-type conductivity in ZnO is under study in several laboratories including ours. We report streak camera measurements of time-resolved luminescence and stimulated emission excited in single crystal, film, and particle samples under excitation by 300 fs laser pulses at temperatures from 17 K to 295 K. We also describe p-n junctions formed by control of oxygen pressure in reactive sputtering of ZnO films, and results of introducing nitrogen during reactive sputtering.

Properties of ultrathin films of porous silicon

We have produced films of light emitting porous silicon (LEPSi) thinner than 1 μm, lifted them off the silicon wafer by an electropolishing step, and deposited them onto sapphire windows where they remain attached by van der Waals or electrostatic forces. Although free‐standing LEPSi films had been obtained before, our films are one order of magnitude thinner, luminesce strongly, and have excellent mechanical properties because of the sapphire substrate. The important steps in this procedure are discussed, and the structural, chemical, and optical properties of these films as measured using a variety of probes are reported. These films are semitransparent in the visible and thus make several new optical measurements possible. In particular, the results of photoinduced absorption measurements performed with 100 fs time resolution are presented.

Femtosecond carrier dynamics in low‐temperature grown Ga0.51In0.49P

Pump‐probe spectroscopy is used to determine the dynamics of carriers in thin films of Ga0.51In0.49P grown at temperatures of 500, 300, and 200 °C. The carrier trapping time τtr, is found to be ∼0.2 ps for the 200 °C sample and ∼0.9 ps for the 300 °C sample. Annealing of the 200 °C sample at 450 °C for 10 min causes the optical response to slow appreciably (τtr∼0.7 ps), indicating that the point defects are directly responsible for the ultrashort optical response observed in this material.

Lifetime and Oscillator Strength of Excitonic Luminescence in Zinc Oxide

Time-dependent photoluminescence spectra of ZnO have been measured from 17 v K to 295 v K. Thermal de-trapping of bound excitons to free excitons is observed. Competition between rates of phonon-assisted broadening and of exciton-exciton inelastic collisions can be discerned as a function of temperature. The velocity of excitation transport across a crystal in resonance with shallow bound excitons 20 v meV below the A exciton was measured to be 3.3 2 10 8 v cm/s at 17 v K. The phenomenon of oscillator strength enhancement of bound or confined excitons is significant in several of the observations.

Self-trapped electron and transient defect absorption in niobate and tungstate crystals

Abstract We have measured transient optical absorption in LiNbO3, KNbO3, PbWO4, CdWO4, and ZnWO4 following band-gap excitation by 200-fs laser pulses and by 10-ns electron pulses. A strong infrared transient absorption band centered near 1 eV was found in a sample of stoichiometric LiNbO3. We tentatively attribute this infrared absorption band to intrinsic electron polarons, following observation and EPR characterization of a similar (persistent) band in Mg-doped, thermochemically reduced congruent LiNbO3 by Schirmer et al‥ Socalled blue-light induced infrared absorption in KNbO3 was investigated by time-resolved spectroscopy as well. In PbWO4, CdWO4, and ZnWO4, broad, transient visible and infrared absorption bands are observed. It is considered whether infrared absorption in PbWO4 may be partly due to self-trapped electrons already characterized by Laguta et al. and Hofstaetter et al. using EPR.

Time-of-flight study of bound exciton polariton dispersive propagation in ZnO

We report direct observations of polariton dispersive propagation at the donor-bound exciton (D0X) resonance in single-crystal ZnO. Time-resolved photoluminescence spectra are measured from both surfaces of a ZnO single crystal that is excited in a thin layer on one surface. When the excitation and the detector are at opposite sides of the crystal, the time-resolved luminescence spectrum in the streak camera display has a curved shape in the t versus plane, indicating dispersion in transport time across the crystal thickness. The group velocity at the donor-bound exciton resonance (3.36 eV) is measured to be 6 × 106 m s−1. The EHP (electron–hole plasma) relaxation time is about 36 ps, in agreement with published results. The polariton reflection at the crystal boundary is observed in the time domain, confirming the dispersion hypothesis. The low-temperature transmission spectrum shows that optical emission from the donor-bound exciton polariton is attenuated by 95% on traversing 0.5 mm crystal thickness due to reflections and polariton decay. From these data the D0X polariton lifetime is estimated as 37 ps. The temperature-dependent change of streak camera curvatures corresponds to thermal de-trapping of the donor-bound excitons.

Short-pulse excitation and spectroscopy of KNbO3, LiNbO3 and KTiOPO4

Abstract Light-induced optical absorption in KNbO3, LiNbO3, and KTiOPO4 (KTP) is of concern for the performance of these important nonlinear optical materials. Gray track formation in KTP and LiNbO3, and blue-light-induced infrared absorption in KNbO3 have been the focus of recent research on photo-damage. Picosecond growth and decay of optical absorption produced by ∼ 0.8 ps pulses of blue laser light are described in the present work on crystals of KNbO3, LiNbO3, and KTP in comparison to measurements made with electron pulses and X-ray irradiation.