B. Jogai

Optically Pumped Ultraviolet Lasing from ZnO

Well formed, longitudinal optically-pumped lasing modes from vapor phase grown ZnO platelets have been observed. The lasing cavity was formed from as-grown crystal planes, and the lasing occurs at very low pump powers.

Similarities in the Bandedge and Deep-Centre Photoluminescence Mechanisms of ZnO and GaN

Abstract Several of the optical transitions in ZnO and GaN appear to derive from a similar origin and have considerable overlap in the energy regions where they occur. In particular the donor-acceptor pair transitions and the well known “yellow band” in GaN and the analogous “green band” in ZnO show remarkable similarities. Because of these similarities it is likely that the respective transitions in the two materials are defect related and share common mechanisms.

Time-resolved photoluminescence lifetime measurements of the Γ5 and Γ6 free excitons in ZnO

Time-resolved photoluminescence spectroscopy at 2 K was used to measure the radiative recombination lifetime of the allowed (Γ5) and forbidden (Γ6) free excitons in ZnO. The measurements were made on a sample containing internal strain, which altered the sample symmetry, and resulted in relaxed selection rules, allowing the Γ6 exciton to be observed. A radiative recombination lifetime of 259 ps was measured for the Γ5 exciton and 245 ps for the Γ6 exciton. The decay of the free excitons was of single-exponential form, and the decay times were obtained using a least-squares fit of the data.

Combined effects of screening and band gap renormalization on the energy of optical transitions in ZnO and GaN

The energy positions of the optical transitions in both GaN and ZnO were investigated when the samples were excited simultaneously with a HeCd laser and an Ar+ ion laser. The increased number of free electrons excited by the Ar+ ion laser will effectively screen both the free exciton and bound exciton transitions, resulting in a blueshift. The increased number of free electrons also produces many-body effects, which lead to a reduction of the band gap energy and thus a redshift. The resultant of screening and renormalization results in a redshift of the optical transitions in ZnO but a nearly vanishing shift in GaN.

Photoluminescence measurements from the two polar faces of ZnO

The crystal structure of ZnO is wurtzite and the stacking sequence of atomic layers along the “c” axis is not symmetric. As a result, a ZnO crystal surface that is normal to the c axis exposes one of two distinct polar faces, with (0001) being considered the O face and (0001) the Zn face. Photoluminescence (PL) measurements on the two faces reveal a striking difference. Two transitions are observed in PL that are dominant from the O face and barely observed in PL from the Zn face. These lines are identified as phonon replicas of a particular D0,X transition using energy separations, excitation dependence, and time-resolved PL measurements. In addition, PL emission from free excitons is found to be more intense from the O face than from the Zn face.

Three-dimensional strain field calculations in coupled InAs/GaAs quantum dots

A detailed calculation of the three-dimensional strain field in and around InAs/GaAs quantum dots is presented. The strain field is calculated by minimizing the elastic strain energy on a cubic grid. Surface boundary conditions are incorporated to enable the strain field at the surface of the cap layer to be examined in detail. This has important implications for the vertical and lateral ordering of subsequent layers of dots. Results are presented for a single dot as well as two and four coupled dots. It is found that the elastic energy density at the surface of the epilayer above a layer of seed dots exhibits strong minima directly above the seed dots, facilitating vertical ordering. An intriguing result is that, under certain conditions, satellite minima also occur at interstitial points, raising the possibility that a second layer of dots above the seed layer can have significantly more dots than the seed layer because of lateral ordering.

Effect of electromechanical coupling on the strain in AlGaN/GaN heterojunction field effect transistors

The strain in AlGaN/GaN heterojunction field-effect transistors (HFETs) is examined theoretically in the context of the fully coupled equation of state for piezoelectric materials. Using a simple analytical model, it is shown that, in the absence of a two-dimensional electron gas (2DEG), the out-of-plane strain obtained without electromechanical coupling is in error by about 30% for an Al fraction of 0.3. This result has consequences for the calculation of quantities that depend directly on the strain tensor. These quantities include the eigenstates and electrostatic potential in AlGaN/GaN heterostructures. It is shown that for an HFET, the electromechanical coupling is screened by the 2DEG. Results for the electromechanical model, including the 2DEG, indicate that the standard (decoupled) strain model is a reasonable approximation for HFET calculations. The analytical results are supported by a self-consistent Schrodinger–Poisson calculation that includes the fully coupled equation of state together with...

Strain Variation with Sample Thickness in GaN Grown by Hydride Vapor Phase Epitaxy

High quality GaN crystals can be grown on sapphire by hydride vapor phase epitaxy. The thermal expansion mismatch between sapphire and GaN produces strain in the GaN crystal as it is cooled from the growth temperature to room temperature. The strain is evidenced by shifts in the photoluminescence and reflectance line positions. By analyzing the surface strain as the crystal thickness is increased, the thickness required to obtain zero surface strain can be estimated. This structure might provide a lattice matched and thermally matched substrate for further epitaxial growth of GaN.

Identification of the Γ5 and Γ6 free excitons in GaN

The Γ5 and Γ6 free excitons have been identified in GaN from emission measurements. Another emission peak is also observed which we believe to be the longitudinal free exciton. These measurements along with electrical measurements, which show the sample to have very high peak mobility, attest to the high quality of the sample.

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.