R. L. Jones

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.

Electrical Properties of Bulk ZnO

Abstract Large-diameter (2-inch), n -type ZnO boules grown by a new vapor-phase transport method were investigated by the temperature-dependent Hall-effect technique. The 300-K Hall carrier concentration and mobility were about 6 × 10 16 cm −3 and 205 cm 2 V −1 s −1 , respectively, and the peak mobility (at 50 K) was about 2000 cm 2 V −1 s −1 , comparable to the highest values reported in the past for ZnO. The dominant donor had a concentration of about 1 × 10 17 cm −3 and an energy of about 60 meV, close to the expected hydrogenic value, whereas the total acceptor concentration was much lower, about 2 × 10 15 cm −3 . Photoluminescence measurements confirm the high quality of the material.

Production and annealing of electron irradiation damage in ZnO

High-energy (>1.6 MeV) electrons create acceptors and donors in single-crystal ZnO. Greater damage is observed for irradiation in the [0001] direction (Zn face) than in the [0001] direction (O face). The major annealing stage occurs at about 300–325 °C, and is much sharper for defects produced by Zn-face irradiation, than for those resulting from O-face irradiation. The defects appear to have a chain character, rather than being simple, near-neighbor vacancy/interstitial Frenkel pairs. These experiments suggest that ZnO is significantly more “radiation hard” than Si, GaAs, or GaN, and should be useful for applications in high-irradiation environments, such as electronics in space satellites.

AlGaAs/GaAs double barrier diodes with high peak‐to‐valley current ratio

We report the largest peak‐to‐valley current (PVC) ratios to date from AlGaAs/GaAs double barrier (either alloy barrier or superlattice barrier) diodes. PVC ratios as high as 3.6 and 21.7 were obtained from an AlAs/GaAs superlattice barrier structure at 300 and 77 K, respectively. In an alloy barrier structure with x=0.42 (x=0.3), PVC ratios of 3.9 (2.2) and 14.3 (7.0) were observed at 300 and 77 K, respectively. We attribute these excellent results to a ‘‘two‐step’’ spacer layer incorporated in the devices studied which facilitated the growth of high material quality.

Optical properties of ZnO crystals containing internal strains

The way in which in-grown strain impacts the optical properties of crystals can be revealed when combined with annealing studies. During the annealing process strains may be relieved, and when they are, the intrinsic energy bands adjust to these changes. These changes are readily detected by reflection measurements. Energy changes in the intrinsic bands may also be reflected in the extrinsic optical transitions. In this paper we show how stress changes during the annealing process can be used to develop a model that explains the origin of emission lines at 3.2898 and 3.2176 eV.

Point Defect Characterization of GaN and ZnO

Abstract Point defects are created in bulk ZnO and epitaxial GaN by 1–2 MeV electron irradiation at 300 K, and are studied by temperature-dependent Hall effect, photoluminescence, and deep level transient spectroscopy measurements. The N vacancy is identified as a fairly shallow donor in GaN, whereas defect identifications in ZnO are uncertain at this time. Both materials, but especially ZnO, are quite resistant to displacement damage.

Growth of vertically self-organized InGaAs quantum dots with narrow inhomogeneous broadening

We have fabricated vertically self-organized multiple sets of In0.6Ga0.4As quantum dots (QDs) on GaAs (001) that combines the concepts of variable amount deposition and shape stabilization and size equalization of QDs. The inhomogeneous broadening of optical emission from these dots reached a record low value of 18.4 meV at a wavelength of ∼1185 nm (4 K). The seed layer and the second dot layer have essentially the same dot density of ∼250 μm−2 due to the high degree of dot vertical alignment. The deposition amount for the second dot layer was selected to be 9 monolayers, which resulted in dots with convergent lateral size (∼62 nm) and stabilized facets, close to {011}. The third layer, with the same amount of InGaAs as the second layer, had a dot density of ∼350 μm−2, an average lateral dot size of ∼71 nm, an average dot height of ∼11 nm, and shallower side facets close to {023}.

Photocurrent spectroscopy of InxGa1−xAs/GaAs multiple quantum wells

Photocurrent spectra of InxGa1−xAs/GaAs multiple quantum well structures grown by molecular beam epitaxy are studied in the presence of electric fields perpendicular to the heterointerface. Several Δn=0 allowed and Δn≠0 forbidden excitonic transitions are observed. Both negative and positive shifts of exciton transitions are found. Good agreement is found between the photocurrent observations and calculations using a multiband effective‐mass approach, taking into account the strain‐induced splitting.

Effect of high-temperature annealing on electrical and optical properties of undoped semi-insulating GaAs

A comprehensive characterization, including room temperature Hall effect, near infrared absorption, temperature dependent dark current and photocurrent (using 1.13 eV light), normalized thermally stimulated current (NTSC), photoluminescence at 4.2 K in both near band edge and deep level regions, and selective pair photoluminescence (SPL) at 2 K, has been carried out on undoped semi-insulating GaAs samples, cut from four wafers which were grown by the low pressure liquid encapsulated Czochralski technique and annealed by three different schedules: a 1100 °C anneal with either fast or slow cooling, or a 1000 °C standard anneal. The 1100 °C anneal clearly introduces higher concentrations of NTSC traps near 0.3 and 0.5 eV, a PL center at 0.8 eV, and acceptor centers, which are mainly due to the point defects and increase the resistivity. Slow cooling to some extent reduces all of these additional centers. The SPL measurements show changes in the relative intensities of C, Zn, and Si related emissions with changes in annealing conditions.A comprehensive characterization, including room temperature Hall effect, near infrared absorption, temperature dependent dark current and photocurrent (using 1.13 eV light), normalized thermally stimulated current (NTSC), photoluminescence at 4.2 K in both near band edge and deep level regions, and selective pair photoluminescence (SPL) at 2 K, has been carried out on undoped semi-insulating GaAs samples, cut from four wafers which were grown by the low pressure liquid encapsulated Czochralski technique and annealed by three different schedules: a 1100 °C anneal with either fast or slow cooling, or a 1000 °C standard anneal. The 1100 °C anneal clearly introduces higher concentrations of NTSC traps near 0.3 and 0.5 eV, a PL center at 0.8 eV, and acceptor centers, which are mainly due to the point defects and increase the resistivity. Slow cooling to some extent reduces all of these additional centers. The SPL measurements show changes in the relative intensities of C, Zn, and Si related emissions with cha...

Parametric study of AlAs/GaAs superlattice double‐barrier diodes

Quantum well diodes with barriers formed by thin, short‐period binary AlAs/GaAs superlattices were fabricated and found to have very high peak‐to‐valley current ratios. The effects of varying the AlAs and GaAs layers in the barriers are studied. The peak current density is found to decrease by orders of magnitude for monolayer increases in the AlAs layer thicknesses. Tunneling current peaks due to both resonance levels in the quantum well and resonance levels in the superlattice barriers are observed.