Philip Dawson

Structural, electronic and optical properties of m-plane (In,Ga)N/GaN quantum wells: Insights from experiment and atomistic theory

In this paper we present a detailed analysis of the structural, electronic, and optical properties of an

The nature of carrier localisation in polar and nonpolar InGaN/GaN quantum wells

m

The microstructure of non-polar a-plane (11 2¯0) InGaN quantum wells

-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.

Defect reduction in semi-polar (11-22) gallium nitride grown using epitaxial lateral overgrowth

In this paper, we compare and contrast the experimental data and the theoretical predictions of the low temperature optical properties of polar and nonpolar InGaN/GaN quantum well structures. In both types of structure, the optical properties at low temperatures are governed by the effects of carrier localisation. In polar structures, the effect of the in-built electric field leads to electrons being mainly localised at well width fluctuations, whereas holes are localised at regions within the quantum wells, where the random In distribution leads to local minima in potential energy. This leads to a system of independently localised electrons and holes. In nonpolar quantum wells, the nature of the hole localisation is essentially the same as the polar case but the electrons are now coulombically bound to the holes forming localised excitons. These localisation mechanisms are compatible with the large photoluminescence linewidths of the polar and nonpolar quantum wells as well as the different time scales and form of the radiative recombination decay curves.

Terahertz spectroscopy of shift currents resulting from asymmetric (110)-oriented GaAs/AlGaAs quantum wells

Atom probe tomography and quantitative scanning transmission electron microscopy are used to assess the composition of non-polar a-plane (11-20) InGaN quantum wells for applications in optoelectronics. The average quantum well composition measured by atom probe tomography and quantitative scanning transmission electron microscopy quantitatively agrees with measurements by X-ray diffraction. Atom probe tomography is further applied to study the distribution of indium atoms in non-polar a-plane (11-20) InGaN quantum wells. An inhomogeneous indium distribution is observed by frequency distribution analysis of the atom probe tomography measurements. The optical properties of non-polar (11-20) InGaN quantum wells with indium compositions varying from 7.9% to 20.6% are studied. In contrast to non-polar m-plane (1-100) InGaN quantum wells, the non-polar a-plane (11-20) InGaN quantum wells emit at longer emission wavelengths at the equivalent indium composition. The non-polar a-plane (11-20) quantum wells also show broader spectral linewidths. The longer emission wavelengths and broader spectral linewidths may be related to the observed inhomogeneous indium distribution.

High excitation carrier density recombination dynamics of InGaN/GaN quantum well structures: Possible relevance to efficiency droop

We report on the characterization of semi-polar (112) gallium nitride (GaN) films grown on m-plane (100) sapphire by an asymmetric epitaxial lateral overgrowth (ELOG) process first reported by de Mierry et al. [Appl. Phys. Lett. 94 (2009) 191903]. The overgrowth conditions were engineered to greatly enhance the growth rate along the [0001] direction, which combined with the inclination of the [0001] axis from the film surface at ~32°, allowing a low defect density wing to overgrow the highly defective window region and thus eliminating basal plane stacking faults (BSFs). By correlating cross-sectional transmission electron microscopy and cathodoluminescence data, we confirm that BSFs and dislocations are terminated by the coalescence boundary formed as a result of the overgrowth anisotropy. Low temperature photoluminescence spectra reveal a strong GaN emission at 3.485 eV associated with donor-bound exciton recombination and very small BSF-related emission at 3.425 eV. The intensity ratio between the GaN bound exciton and the BSF emission is ~220, which is four orders of magnitude greater than that of the semi-polar seed layer. Scanning capacitance microscopy data showed that almost the entire film is unintentionally n-type. The impurity incorporation rate is strongly dependent on which crystallographic planes are present during different stages of the ELOG process.

Terahertz cyclotron resonance spectroscopy of an AlGaN/GaN heterostructure using a high-field pulsed magnet and an asynchronous optical sampling technique

We report the observation and the study of an additional shift current tensor element in (110)-oriented GaAs quantum wells, which arises from an out-of-plane asymmetry of the quantum well structure. The current resulting from this tensor element is optically induced with 150 fs laser pulses and detected by measuring the simultaneously emitted terahertz radiation. This terahertz spectroscopy of shift currents is a powerful technique for symmetry investigations, which shows, for example, that our nominally symmetric (110)-oriented GaAs/AlGaAs quantum wells grown by molecular beam epitaxy are in reality asymmetric structures with different right and left interfaces.

The effect of high compressive strain on the operation of AlGaInP quantum-well lasers

We report on the optical properties of InGaN/GaN quantum well structures measured at 10 K as a function of excitation density. At high excitation power densities we observe a component in the spectra that decays more rapidly than the localised carrier emission observed for low excitation power densities. We attribute this component to recombination involving weakly localised or delocalised carriers. At the high excitation power densities there is a reduction in the recombination internal quantum efficiency, so called efficiency droop. These observations are compatible with the model whereby efficiency droop is explained in terms of the non radiative loss of delocalised carriers.

Resonant photoluminescence studies of carrier localisation in c-plane InGaN/GaN quantum well structures

The effective mass, sheet carrier concentration, and mobility of electrons within a two-dimensional electron gas in an AlGaN/GaN heterostructure were determined using a laboratory-based terahertz cyclotron resonance spectrometer. The ability to perform terahertz cyclotron resonance spectroscopy with magnetic fields of up to 31 T was enabled by combining a high-field pulsed magnet with a modified asynchronous optical sampling terahertz detection scheme. This scheme allowed around 100 transmitted terahertz waveforms to be recorded over the 14 ms magnetic field pulse duration. The sheet density and mobility were measured to be 8.0 × 1012 cm−2 and 9000 cm2 V−1 s−1 at 77 K. The in-plane electron effective mass at the band edge was determined to be 0.228 ± 0.002m0.

Room-temperature semiconductor modulators for free-space signal transmission with THz waves

In this paper we describe the properties of Ga/sub x/In/sub 1-x/P-(Al/sub y/Ga/sub 1-y/)/sub 0.52/In/sub 0.48/P strained quantum-well (QW) lasers at compressive strains of greater than 1%. Structures containing single 100-/spl Aring/ Ga/sub x/In/sub 1-x/P QWs of different compositions have been grown by low-pressure metal organic chemical vapor deposition (MOCVD) with the intention of studying the physical mechanisms which inhibit the operation of strained lasers at high values of compressive strain. In these lasers, we observe a monotonic increase in threshold current with increasing strain between 1% and 1.7%. We show that the increase in threshold current can be attributed to increased optical losses and we measure an increase in the optical mode loss from 10 to 45 cm/sup -1/ with increasing strain. Using transmission electron microscopy (TEM), we are able to link the increased optical losses at high strain with a strain-induced growth nonuniformity in the active region of the device similar to the Stranski-Krastanov growth mode, which results in the formation of islands in the active region on a 100-nm-length scale.