C. W. Litton

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.

Role of Near-Surface States in Ohmic-Schottky Conversion of Au Contacts to ZnO

A conversion from ohmic to rectifying behavior is observed for Au contacts on atomically ordered polar ZnO surfaces following remote, room-temperature oxygen plasma treatment. This transition is accompanied by reduction of the “green” deep level cathodoluminescence emission, suppression of the hydrogen donor-bound exciton photoluminescence and a ∼0.75eV increase in n-type band bending observed via x-ray photoemission. These results demonstrate that the contact type conversion involves more than one mechanism, specifically, removal of the adsorbate-induced accumulation layer plus lowered tunneling due to reduction of near-surface donor density and defect-assisted hopping transport.

Remote hydrogen plasma doping of single crystal ZnO

We demonstrate that remote plasma hydrogenation can increase electron concentrations in ZnO single crystals by more than an order of magnitude. We investigated the effects of this treatment on Hall concentration and mobility as well as on the bound exciton emission peak I4 for a variety of ZnO single crystals–bulk air annealed, Li doped, and epitaxially grown on sapphire. Hydrogen increases I4 intensity in conducting samples annealed at 500 and 600 °C and partially restores emission in the I4 range for Li-diffused ZnO. Hydrogenation increases carrier concentration significantly for the semi-insulating Li doped and epitaxial thin film samples. These results indicate a strong link between the incorporation of hydrogen, increased donor-bound exciton PL emission, and increased n-type conductivity.

Growth of InSb and InAs1−xSbx on GaAs by molecular beam epitaxy

InSb and InAs1−xSbx epitaxial layers have been successfully grown on (100)GaAs substrates by molecular beam epitaxy. Remarkably good morphologies were obtained despite the large lattice mismatch (14%) between InSb and GaAs. Room‐temperature electron mobilities as high as 57 000 cm2/V s were measured in InSb layers of about 5 μm thick with ND−NA∼1.6×1016 cm−3. The substrate temperature and Sb/In flux ratio were found to critically influence the quality of InSb epilayers. By employing dimeric instead of tetrameric sources, the composition of the InAs1−xSbx films was observed to be relatively independent of substrate temperature. Electron mobilities of 20 000 and 8800 cm2/V s, at 300 and 77 K, respectively, were obtained for a 1.6‐μm‐thick InAs1−xSbx (x=0.67) layer.

Optical Properties of GaN Grown on ZnO by Reactive Molecular Beam Epitaxy

High quality wurtzite GaN epilayers have been grown on ZnO(0001) substrates by reactive molecular beam epitaxy. Photoluminescence and reflectivity measurements point to high quality presumably due to the near match of both the crystal lattice parameter and the stacking order between GaN and ZnO. In addition, the good films lack the characteristic yellow photoluminescence band. Any misorientation of the GaN epilayer planes with respect to the ZnO substrate is not detectable with polarized reflectivity. The x-ray double crystal diffraction measurements indicate this misorientation is much smaller than those for GaN epilayers on SiC and Al2O3 .

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.

Direct evidence for the site of substitutional carbon impurity in GaAs

Direct evidence that substitutional carbon in GaAs is predominantly on the As sublattice is obtained from Fourier transform infrared spectroscopy absorption measurements of the carbon‐induced localized vibrational mode (LVM). The previously reported LVM absorption band of carbon is measured under high resolution conditions and is found to be the near superposition of at least four bands. It is shown by comparison with similar measurements of silicon‐doped GaAs and by physical arguments that the only satisfactory explanation is that the bands arise from carbon on As sites with different nearest‐neighbor configurations of the two Ga isotopes. There is no experimental indication of carbon on the Ga sublattice. These are the first observations of such shifts in LVM spectra for simple substitutional impurity defects in semiconductors.

High-Quality, Melt-Grown ZnO Single Crystals

High-quality, melt-grown ZnO crystals are reported. The reflection and emission spectra of the melt-grown samples are compared with the same spectra from high-quality, vapor-grown ZnO crystals. We isolate the reflection and emission spectra predominantly related to the intrinsic properties associated with the wurtzite structure of the crystals. The quality of the crystals is reflected in the spectral reproduction of the intrinsic properties of the crystals. Both the ground state and the n=2 state of the free excitons associated with the A, B, and C valence bands of the crystals are spectrally observed in reflection. Assuming a hydrogenic character for the free excitons, the binding energy of these excitons associated with all three valence bands was determined. For the intrinsic emission spectra, attention was focused on the A-band free excitons and related optical parameters. Both the reflection and emission spectra for the melt-grown material compared very closely with the same spectra observed from hig...

Determination of interfacial quality of GaAs‐GaAlAs multi‐quantum well structures using photoluminescence spectroscopy

Well size fluctuations have been observed in high quality GaAs‐Ga0.75Al0.25As multi‐quantum well structures having very sharp photoluminescence transitions. The effect of well size fluctuations appears as multiple peaks in both the heavy hole free exciton and the heavy hole donor bound exciton transitions. The observed energy separation of the peaks corresponds to what would be expected for a change in well thickness of 1/2 monolayer. The observed linewidths and the well size fluctuations suggest that these are interlayer rather than intralayer fluctuations. These results are different from earlier reported work. The very narrow emission lines reflect the excellent structural quality of the layers. A model to explain the effective half‐monolayer well size fluctuations is proposed.