Sunayna Binte Bashar

Realization of Cu-Doped p-Type ZnO Thin Films by Molecular Beam Epitaxy.

Cu-doped p-type ZnO films are grown on c-sapphire substrates by plasma-assisted molecular beam epitaxy. Photoluminescence (PL) experiments reveal a shallow acceptor state at 0.15 eV above the valence band edge. Hall effect results indicate that a growth condition window is found for the formation of p-type ZnO thin films, and the best conductivity is achieved with a high hole concentration of 1.54 × 10(18) cm(-3), a low resistivity of 0.6 Ω cm, and a moderate mobility of 6.65 cm(2) V(-1) s(-1) at room temperature. Metal oxide semiconductor capacitor devices have been fabricated on the Cu-doped ZnO films, and the characteristics of capacitance-voltage measurements demonstrate that the Cu-doped ZnO thin films under proper growth conditions are p-type. Seebeck measurements on these Cu-doped ZnO samples lead to positive Seebeck coefficients and further confirm the p-type conductivity. Other measurements such as X-ray diffraction, X-ray photoelectron, Raman, and absorption spectroscopies are also performed to elucidate the structural and optical characteristics of the Cu-doped p-type ZnO films. The p-type conductivity is explained to originate from Cu substitution of Zn with a valency of +1 state. However, all p-type samples are converted to n-type over time, which is mostly due to the carrier compensation from extrinsic defects of ZnO.

An Sb-doped p-type ZnO nanowire based random laser diode.

An electrically pumped Sb-doped ZnO nanowire/Ga-doped ZnO p-n homojunction random laser is demonstrated. Catalyst-free Sb-doped ZnO nanowires were grown on a Ga-doped ZnO thin film on a Si substrate by chemical vapor deposition. The morphology of the as-grown titled nanowires was observed by scanning electron microscopy. X-ray photoelectron spectroscopy results indicated the incorporation of Sb dopants. Shallow acceptor states of Sb-doped nanowires were confirmed by photoluminescence measurements. Current-voltage measurements of ZnO nanowire structures assembled from p- and n-type materials showed a typical p-n diode characteristic with a threshold voltage of about 7.5 V. Very good photoresponse was observed in the UV region operated at 0 V and different reverse biases. Random lasing behavior with a low-threshold current of around 10 mA was demonstrated at room temperature. The output power was 170 nW at 30 mA.

Electrically driven deep ultraviolet MgZnO lasers at room temperature

Semiconductor lasers in the deep ultraviolet (UV) range have numerous potential applications ranging from water purification and medical diagnosis to high-density data storage and flexible displays. Nevertheless, very little success was achieved in the realization of electrically driven deep UV semiconductor lasers to date. In this paper, we report the fabrication and characterization of deep UV MgZnO semiconductor lasers. These lasers are operated with continuous current mode at room temperature and the shortest wavelength reaches 284 nm. The wide bandgap MgZnO thin films with various Mg mole fractions were grown on c-sapphire substrate using radio-frequency plasma assisted molecular beam epitaxy. Metal-semiconductor-metal (MSM) random laser devices were fabricated using lithography and metallization processes. Besides the demonstration of scalable emission wavelength, very low threshold current densities of 29~33 A/cm2 are achieved. Numerical modeling reveals that impact ionization process is responsible for the generation of hole carriers in the MgZnO MSM devices. The interaction of electrons and holes leads to radiative excitonic recombination and subsequent coherent random lasing.

Enhanced random lasing from distributed Bragg reflector assisted Au-ZnO nanowire Schottky diode

An electrically pumped ultraviolet random laser based on an Au-ZnO nanowire Schottky junction on top of a SiO2/SiNx distributed Bragg reflector (DBR) has been fabricated. Electrical characterization shows typical Schottky diode current-voltage characteristics. Evident random lasing behavior is observed from electroluminescence measurement at room temperature. In comparison with a reference device having similar nanowire morphology but no DBR, this laser demonstrates almost 1.8 times reduction in threshold current and 4 times enhancement in output power. The performance enhancement originates from the incorporation of the DBR structure, which provides high reflectivity in the designed wavelength range.

Electrically pumped random lasing based on Au-ZnO nanowire Schottky junction

Electrically pumped random lasing based on Au-ZnO nanowire Schottky junction diode is demonstrated. Good lasing behavior is achieved and excitonic recombination is responsible for lasing generation. It provides an alternative approach towards semiconductor random lasers.