Erfan Baghani

Electron mobility limited by scattering from screened positively charged dislocation lines within indium nitride

In the present work, we address the open question of the contribution from threading dislocations to the problem of unintentional n-type conductivity exhibited by indium nitride through an examination of the effect that positively charged dislocation lines have on the transverse electron mobility within this material. Assuming that the threading dislocation lines within indium nitride act as a source for free electrons, the screening associated with the positively charged threading dislocation lines is evaluated. The impact this screening has on the dislocation limited electron mobility within this material is then considered. Our results indicate that one of the implications of attributing a donor character to the threading dislocation lines within indium nitride would be a strong non-uniformity in the free electron concentration in the plane of growth of this semiconductor. This contrasts dramatically with the case of gallium nitride.

Auger-Limited Carrier Recombination and Relaxation in CdSe Colloidal Quantum Wells.

Using time-resolved photoluminescence spectroscopy, we show that two-exciton Auger recombination dominates carrier recombination and cooling dynamics in CdSe nanoplatelets, or colloidal quantum wells. The electron-hole recombination rate depends only on the number of electron-hole pairs present in each nanoplatelet, and is consistent with a two-exciton recombination process over a wide range of exciton densities. The carrier relaxation rate within the conduction and valence bands also depends only on the number of electron-hole pairs present, apart from an initial rapid decay, and is consistent with the cooling rate being limited by reheating due to Auger recombination processes. These Auger-limited recombination and relaxation dynamics are qualitatively different from the carrier dynamics in either colloidal quantum dots or epitaxial quantum wells.

An enhancement in the low-field electron mobility associated with a ZnMgO/ZnO heterostructure: The role of a two-dimensional electron gas

We determine the role that a two-dimensional electron gas, formed at a ZnMgO/ZnO heterojunction, plays in shaping the corresponding temperature dependence of the low-field electron Hall mobility. This analysis is cast within the framework of the model of Shur et al. [M. Shur et al., J. Electron. Mater. 25, 777 (1996)], and the contributions to the mobility related to the ionized impurity, polar optical phonon, piezoelectric, and acoustic deformation potential scattering processes are considered, the overall mobility being determined through the application of Mathiessens rule. The best fit to the ZnMgO/ZnO experimental results of Makino et al. [T. Makino et al., Appl. Phys. Lett. 87, 022101 (2005)] is obtained by setting the free electron concentration to 3×1018u2009cm−3 and the ionized impurity concentration to 1017u2009cm−3, i.e., within the two-dimensional electron gas formed at the heterojunction, the free electron gas concentration is a factor of 30 times the corresponding ionized impurity concentration. Ho...

Occupation statistics of dislocations within uncompensated n-type wurtzite gallium nitride

Within the framework of a grand partition function approach, we develop a four-state model for the analysis of occupancy for dislocation defects within uncompensated n-type wurtzite gallium nitride and compare the obtained results with those determined using the energy minimization and free energy minimization approaches of Read [W. T. Read, Jr., Philos. Mag. 45, 775 (1954)]. The advantages of this particular formulation are its simplicity, the fact that we can now consider both p-type and n-type materials, and the fact that it allows for the consideration of more complex core structures. The sensitivity of the results to variations in the electron-electron interactions within a given dangling bond are considered and found to be relatively minor for the case of n-type doping.

Occupation statistics of the VGa – ON dislocations within n-type gallium nitride

In this paper, we apply a Gibbs factor formalism in order to determine the occupation statistics of the different states associated with VGa — ON dislocations within uncompensated n-type wurtzite gallium nitride. We compare our results with those obtained from the simulated annealing analysis of Leung et al. [K. Leung, A. F. Wright, and E. B. Stechel, Appl. Phys. Lett. 74, 2495 (1999)]. This comparison enables us to confirm the validity of a much simpler dislocation defect site occupation formalism that is valid at high dislocation densities and/or low free electron (low bulk doping) concentrations. It is also seen that our formulation provides a notable computational advantage over the energy minimization approach of You et al. [J. H. You, J.-Q. Lu, and H. T. Johnson, J. Appl. Phys. 99, 033706 (2006)].

Dislocation line charge screening within n-type gallium nitride

A revised electrostatic theory for the charged dislocation lines within n-type GaN is formulated, this formalism allowing for the screening of the charge trapped along the dislocation lines, by both free carriers and a partial ionization of the impurities within the space-charge region surrounding the dislocation lines. This goes beyond the abrupt space-charge region assumption of the Read model [W. T. Read, Jr., Philos. Mag. 45, 775 (1954)], where the only screening mechanism considered is a complete ionization of bulk donor atoms within the Read radius. In addition to determining the spatial distribution of the charge enveloping charged dislocation lines, this procedure also provides a solution to the electrostatic potential surrounding the dislocation lines. An iterative, self-consistent numerical approach to the solution of this problem is developed for the purposes of this analysis. A special limit for which the results of this model reduce to that of Read is indicated. The results obtained from our ...

Semiconductor nanoplatelets: a new colloidal system for low-threshold high-gain stimulated emission (Presentation Recording)

Quantum wells (QWs) are thin semiconductor layers than confine electrons and holes in one dimension. They are widely used for optoelectronic devices, particularly semiconductor lasers, but have so far been produced using expensive epitaxial crystal-growth techniques. This has motivated research into the use of colloidal semiconductor nanocrystals, which can be synthesized chemically at low cost, and can be processed in the solution phase. However, initial demonstrations of optical gain from colloidal nanocrystals involved high thresholds. Recently, colloidal synthesis methods have been developed for the production of thin, atomically flat semiconductor nanocrystals, known as nanoplatelets (NPLs). We investigated relaxation of high-energy carriers in colloidal CdSe NPLs, and found that the relaxation is characteristic of a QW system. Carrier cooling and relaxation on time scales from picoseconds to hundreds of picoseconds are dominated by Auger-type exciton-exciton interactions. The picosecond-scale cooling of hot carriers is much faster than the exciton recombination rate, as required for use of these NPLs as optical gain and lasing materials. We therefore investigated amplified spontaneous emission using close-packed films of NPLs. We observed thresholds that were more than 4 times lower than the best reported value for colloidal nanocrystals. Moreover, gain in these films is 4 times higher than gain reported for other colloidal nanocrystals, and saturates at pump fluences more than two orders of magnitude above the ASE threshold. We attribute this exceptional performance to large optical cross-sections, relatively slow Auger recombination rates, and narrow ensemble emission linewidths.

A transition in the nature of the occupancy of the dislocation lines within n-type wurtzite gallium nitride

Within the framework of the model of Read [Philos. Mag. 45, 775 (1954)], we examine the occupancy of the dislocation lines within n-type wurtzite gallium nitride. In particular, we examine the transition that occurs as the bulk doping concentration is increased, from the depletion limit to the non-depletion limit. We note that an abrupt transitional bulk doping concentration can be defined. The dependence of this transitional bulk doping concentration on the dislocation line density is then determined. We note that existing theoretical results on the occupation statistics of the threading dislocation lines within wurtzite gallium nitride also exhibit such a transition. Since these theoretical results assume different structures for the core of the threading dislocation lines, we conclude that this transition between the depletion and non-depletion domains should be a universal feature, holding true irrespective of the particular structure being assumed for the core of the threading dislocation lines. Cond...

Occupation statistics of the 5/7-atom dislocation core structure within n-type indium nitride

Recent density functional calculations by Kalesaki et al. [Appl. Phys. Lett. 98, 072103 (2011)] and by Takei and Nakayama [J. Cryst. Growth 311, 2767 (2009)] have shown that the 5/7-atom dislocation core structure possesses a donor character within intrinsic wurtzite InN. In the present work, we relax the assumption that wurtzite InN is intrinsic and obtain the occupation statistics of the different possible ionization states of the 5/7-atom dislocation defect sites as a function of the bulk doping concentration. An underlying Gibbs factor formalism, similar to that applied earlier to the problem of obtaining the occupation statistics of the dislocation defect sites within n-type gallium nitride, has been employed for the purposes of the present analysis. The occupation statistics results of this analysis suggest that dislocation line densities below 1010u2009cm−2 are necessary in order to achieve bulk free electron concentrations lower than 1017u2009cm−3 within wurtzite InN. Our occupation statistics results are shown to be consistent with existing electron mobility measurements from the literature.Recent density functional calculations by Kalesaki et al. [Appl. Phys. Lett. 98, 072103 (2011)] and by Takei and Nakayama [J. Cryst. Growth 311, 2767 (2009)] have shown that the 5/7-atom dislocation core structure possesses a donor character within intrinsic wurtzite InN. In the present work, we relax the assumption that wurtzite InN is intrinsic and obtain the occupation statistics of the different possible ionization states of the 5/7-atom dislocation defect sites as a function of the bulk doping concentration. An underlying Gibbs factor formalism, similar to that applied earlier to the problem of obtaining the occupation statistics of the dislocation defect sites within n-type gallium nitride, has been employed for the purposes of the present analysis. The occupation statistics results of this analysis suggest that dislocation line densities below 1010u2009cm−2 are necessary in order to achieve bulk free electron concentrations lower than 1017u2009cm−3 within wurtzite InN. Our occupation statistics results are...