D. B. Eason

Characterization of homoepitaxial p-type ZnO grown by molecular beam epitaxy

An N-doped, p-type ZnO layer has been grown by molecular beam epitaxy on an Li-diffused, bulk, semi-insulating ZnO substrate. Hall-effect and conductivity measurements on the layer give: resistivity=4×101 Ω cm; hole mobility=2 cm2/V s; and hole concentration=9×1016 cm−3. Photoluminescence measurements in this N-doped layer show a much stronger peak near 3.32 eV (probably due to neutral acceptor bound excitons), than at 3.36 eV (neutral donor bound excitons), whereas the opposite is true in undoped ZnO. Calibrated, secondary-ion mass spectroscopy measurements show an N surface concentration of about 1019 cm−3 in the N-doped sample, but only about 1017 cm−3 in the undoped sample.

Production of Nitrogen Acceptors in ZnO by Thermal Annealing

Nitrogen acceptors are formed when undoped single crystals of zinc oxide (ZnO) grown by the chemical-vapor transport method are annealed in air or nitrogen atmosphere at temperatures between 600 and 900 °C. After an anneal, an induced near-edge absorption band causes the crystals to appear yellow. Also, the concentration of neutral shallow donors, as monitored by electron paramagnetic resonance (EPR), is significantly reduced. A photoinduced EPR signal due to neutral nitrogen acceptors is observed when the annealed crystals are exposed to laser light (e.g., 364, 442, 458, or 514 nm) at low temperature. The nitrogens are initially in the nonparamagnetic singly ionized state (N−) in an annealed crystal, because of the large number of shallow donors, and the light converts a portion of them to the paramagnetic neutral acceptor state (N0).

High‐brightness blue and green light‐emitting diodes

We report high‐brightness blue and green light‐emitting diodes (LEDs) based on II–VI heterostructures grown by molecular beam epitaxy on ZnSe substrates. The devices consist of a 2–3 μm thick layer of n‐type ZnSe:Cl, a ∼0.1 μm thick active region of Zn0.9Cd0.1Se (blue) or ZnTe0.1Se0.9 (green), and a 1.0 μm thick p‐type ZnSe:N layer. The blue LEDs produce 327 μW (10 mA, 3.2 V), with the light output sharply peaked at 489 nm, and exhibit an external quantum efficiency of 1.3%. The green LEDs produce 1.3 mW (10 mA, 3.2 V) peaked at 512 nm, corresponding to an external quantum efficiency of 5.3%. In terms of photometric units, the luminous performance (luminous efficiency) of the devices is 1.6 lm/W (blue) and 17 lm/W (green), respectively, when operated at 10 mA.

Polarized photoreflectance spectra of excitonic polaritons in a ZnO single crystal

Exciton–polariton structures in a high-quality bulk ZnO single crystal were resolved at 8 K by means of polarized photoreflectance (PR) and photoluminescence measurements. The energies of the PR resonances corresponded to those of the upper and lower exciton–polariton branches, where A, B, and C excitons couple simultaneously to an electromagnetic wave. Longitudinal-transverse splitting of ground-state exciton polaritons and resonances due to the first excited states of respective excitons were observed due to the large oscillator strength. Photoluminescence peaks due to the corresponding polarion branches were clearly resolved. The valence-band ordering was confirmed to be A-Γ9v, B-Γ7vu, and C-Γ7vl.

Molecular nitrogen (N2−) acceptors and isolated nitrogen (N−) acceptors in ZnO crystals

Electron paramagnetic resonance (EPR) has been used to investigate molecular nitrogen and isolated nitrogen acceptors in single crystals of ZnO. These samples were grown by the seeded chemical vapor transport method with N2 added to the gas stream. A five-line EPR spectrum is observed at low temperature in the as-grown bulk crystals and is assigned to N2− molecules substituting for oxygen. This structure arises from nearly equal hyperfine interactions with two nitrogen nuclei (14N, 99.63% abundant, I=1). The spin Hamiltonian parameters for the N2− center are g∥=2.0036, g⊥=1.9935, A∥=9.8 MHz, and A⊥=20.1 MHz, with the unique directions parallel to the c axis. Laser excitation at 9 K, with 325 or 442 nm light, eliminates the N2− spectrum (when the N2− convert to N20) and independently introduces the EPR spectrum due to isolated nitrogen acceptors (when N− acceptors convert to N0). Removing the laser light and warming to approximately 100 K restores the crystal to its preilluminated state. In separate experi...

Brillouin scattering study of ZnO

Polarized Brillouin scattering measurements were carried out for a bulk ZnO single crystal. The whole set of elastic stiffness constants was determined to be c11=19.0, c12=11.0, c13=9.0, c33=19.6, c44=3.9, and c66=4.0 in units of 1011 dyn/cm2. The relationships between photoelastic constants at wavelength of 514.5 nm were also obtained: p11, p13, p44, and p66=1.8, 2.3, 0.50, and 0.38, respectively, relative to p12.

Molecular‐beam epitaxy of ZnS using an elemental S source

The successful growth of ZnS using elemental sources of Zn and S is reported. This wide band gap semiconductor is difficult to grow by molecular‐beam epitaxy (MBE) using elemental sources because of the very high vapor pressure of S. As a consequence, a new type of effusion cell, designed for very low temperature operation, was developed. Techniques for the evaporation and containment of S in the MBE system are also described. ZnS films were deposited onto (100) GaP substrates, with and without ultraviolet radiation incident on the substrate during film growth. High quality epitaxy of semi‐insulating ZnS was achieved at temperatures as low as 150 °C by means of photoassisted MBE. N‐type ZnS:Cl films were also prepared using ZnCl2 as a solid dopant source. The electrical and optical properties of these epilayers are discussed.

Blue and green light-emitting diode structures grown by molecular beam epitaxy on ZnSe substrates

Abstract We report the first double-heterostructure blue, blue/green, and green light emitting devices (LEDs) grown by molecular beam epitaxy (MBE) on ZnSe substrates. The II–VI heterostructures were grown on (100) ZnSe substrates produced at Eagle-Picher Laboratory by the seeded physical vapor transport (SPVT™) process. The device structures consisted of a 2–3 μm thick layer of n-type ZnSe:Cl a 0.1 μm thick active region of multiple quantum well (MQW) Zn 0.9 Cd 0.1 Se (blue/green) or ZnTe 0.1 Se 0.9 (green), and a 1.0 μm thick p-type ZnSe:N layer. Deep blue emitting devices composed of Zn 0.9 Mg 0.1 S 0.15 Se 0.85 lattice-matched cladding layers and a MQW ZnSe active region were also fabricated. The diodes emit blue-to-green electroluminescence at 300 K with emission peaks occurring at 466 to 512 nm depending on the type of active region employed. The brightest devices are the green LEDs which produce 1.29 mW (10 mA, 4 V) peaked at 512 nm. This corresponds to an external efficiency of 3.2%. In terms of photometric units, the luminous performance of the devices is 13.6 lm/W (at 10 mA).

Integrated heterostructure devices composed of II–VI materials with Hg-based contact layers

Abstract Integrated heterostructure devices (IHDs) composed of II–VI materials in epitaxial multilayered structures for light-emitting diode and laser diode applications are described. These IHDs combine a light emission multilayer structure with an abrupt or graded heterostructure which includes Hg-based materials for improved ohmic contact to the upper p-type layer of the light emitting structure.

Quantum Dot Infrared Photodetectors: Photoresponse Enhancement Due to Potential Barriers

Potential barriers around quantum dots (QDs) play a key role in kinetics of photoelectrons. These barriers are always created, when electrons from dopants outside QDs fill the dots. Potential barriers suppress the capture processes of photoelectrons and increase the photoresponse. To directly investigate the effect of potential barriers on photoelectron kinetics, we fabricated several QD structures with different positions of dopants and various levels of doping. The potential barriers as a function of doping and dopant positions have been determined using nextnano3 software. We experimentally investigated the photoresponse to IR radiation as a function of the radiation frequency and voltage bias. We also measured the dark current in these QD structures. Our investigations show that the photoresponse increases ~30 times as the height of potential barriers changes from 30 to 130 meV.