Cathodoluminescence studies of exciton diffusion in gallium nitride

Gallium nitride is used as an example to show the possibilities of the application of a developed threedimensional mathematical model in the cathodoluminescence studies of direct-gap semiconductor materials using the time-of-flight procedure for obtaining quantitative information on their properties. The values of the diffusion coefficient in gallium nitride are determined from the results of its experimental studies.     

Effect of N isotopic mass on the photoluminescence and cathodoluminescence spectra of gallium nitride

GaN is a wurtzite-type semiconductor at ambient conditions whose natural composition consists of almost pure ¹⁴N (99.63% ¹⁴N and 0.37% ¹⁵N) and a mixture of 60.1% ⁶⁹Ga, and 39.9% ⁷¹Ga. We report a low-temperature photoluminescence and cathodoluminescence study of GaN thin films made from natural Ga and N, and from natural Ga and isotopically pure ¹⁵N. The contribution of the nitrogen vibrations to the bandgap renormalization by electron-phonon interaction has been estimated from the nitrogen isotopic mass coefficient of the bound exciton energy. The temperature dependence of the bandgap of GaN can be explained with the measured isotopic mass coefficients of Ga and N. We have estimated the aluminum and indium contribution to the bandgap renormalization in AlN and InN from the temperature dependence of the AlN and InN bandgap up to 300 K, assuming that the N contribution is similar to that found in GaN. The similar bandgap isotopic mass coefficients of C, N, and O, of Al, Si and P, of Zn, Ga and Ge, and of Cd and In suggests that elements of the same row of the periodic table have similar bandgap isotopic mass coefficients.Received: 5 December 2004, Published online: 20 July 2004PACS: 71.20.Nr Semiconductor compounds     

Luminescent properties of gallium nitride and gallium-aluminum nitride

Translated from Zhurnal Prikladnoii Spektroskopii, Vol. 55, No. 5, pp. 861–863, November, 1991.     

Electron-hole plasma generation in gallium nitride

The electron-hole plasma has been studied in both epitaxial and needle GaN. The threshold and the stability of the plasma are also discussed. The observed different behaviour of the samples we studied is interpreted in terms of growth technique.     

Ion implantation into gallium nitride

This comprehensive review is concerned with studies regarding ion implanted gallium nitride (GaN) and focuses on the improvements made in recent years. It is divided into three sections: (i) structural properties, (ii) optical properties and (iii) electrical properties. The first section includes X-ray diffraction (XRD), transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS), Rutherford Backscattering (RBS), emission channeling (EC) and perturbed γγ-angular correlation (PAC) measurements on GaN implanted with different ions and doses at different temperatures as a function of annealing temperature. The structural changes upon implantation and the respective recovery upon annealing will be discussed. Several standard and new annealing procedures will be presented and discussed. The second section describes mainly photoluminescence (PL) studies, however, the results will be discussed with respect to Raman and ellipsometry studies performed by other groups. We will show that the PL-signal is very sensitive to the processes occurring during implantation and annealing. The results of Hall and C–V measurements on implanted GaN are presented in Section 3. We show and discuss the difficulties in achieving electrical activation. However, optical and electrical properties are both a result of the structural changes upon implantation and annealing. Each section will be critically discussed with respect to the existing literature, and the main conclusions are drawn from the interplay of the results obtained from the different techniques used/reviewed.     

Electrical performance of gallium nitride nanocolumns

The electrical characterization of gallium nitride (GaN) nanocolumns with a length up to 1 μm and a diameter of about 30–80 nm grown on doped silicon is a challenge for nano analytics. To determine the conductivity of these nanocolumns, I–V characteristics were recorded by atomic force microscopy (AFM). To measure the conductivity of a single nanocolumn, a conductive AFM tip was placed at the top of the nanocolumn. The measured current/voltage characteristic of a single nanocolumn shows the typical performance of a Schottky contact, which is caused by the contact between the metallic AFM tip and the semiconductor material of the nanocolumn. The height of the Schottky barrier is dependent on the work function of the AFM tip metal used. The linear part of the curve was used to calculate the differential resistance, which was found to be about 13 Ω cm and slightly dependent on the diameter.     

Transient demonstration of exciton behaviours in solid state cathodoluminescence under different driving voltage

In the solid state cathodoluminescence (SSCL), organic materials were excited by hot electrons accelerated in silicon oxide (SiO₂) layer under alternating current (AC). In this paper exciton behaviours were analysed by using transient spectra under different driving voltages. The threshold voltages of SSCL and exciton ionization were obtained from the transient spectra. The recombination radiation occurred when the driving voltage went beyond the threshold voltage of exciton ionization. From the transient spectrum of two kinds of luminescence (exciton emission and recombination radiation), it was demonstrated that recombination radiation should benefit from the exciton ionization.     

Synthesis of aligned gallium nitride nanowire quasi-arrays

Self-aligned GaN nanowire quasi-arrays were synthesized on MgO crystal through a simple gas reaction method. They were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy and high-resolution transmission electron microscopy (HRTEM). FE-SEMimages showed that the product consisted of quasi-arrays of nanowires. XRD, EDX and HRTEM indicated that the nanowires were wurtzite GaN single crystals. Received: 19 June 2000 / Accepted: 21 June 2000 / Published online: 9 August 2000     

Catalytic synthesis of single-crystalline gallium nitride nanobelts

Single-crystalline gallium nitride nanobelts have been synthesized through the reaction of gallium vapor with flowing ammonia using nickel as a catalyst. The as-synthesized products were characterized using X-ray powder diffraction (XRD), scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected-area electron diffraction (SAED). XRD and SAED results revealed that the products are pure, single-crystalline GaN with hexagonal structure. The widths and thickness of the nanobelts ranged from 80 to 200 nm, and 10 to 30 nm, respectively. The lengths were up to several tens of micrometers. The nanobelts had smooth surface with no amorphous sheath, and a sharp-tip end. The growth mechanism of nanobelts was discussed.     

The oxidation of gallium nitride epilayers in dry oxygen

The oxidation of GaN epilayers in dry oxygen has been investigated. The GaN epilayers, about 1 7m thick, were grown on (0001) sapphire substrates by rapid thermal process/low pressure metalorganic chemical vapor deposition. Bulk /-2/ X-ray diffraction (XRD) data showed that the slight oxidation of GaN began to occur at 800 °C for 6 h. The oxide was identified as the monoclinic #-Ga₂O₃. The GaN epilayers were completely oxidized at 1050 °C for 4 h or at 1100 °C for 1 h. For all samples, the strongest oxide's peak is (11-3), and (30-6) followed. There is a rapid oxidation process in the initial stage of oxidation, and a relatively slow process followed when the temperature was over 1000 °C. The oxidation of two stages was limited by the rate of an interfacial reaction mechanism and by the diffusion mechanism, respectively. When the temperature reaches 1100 °C, the oxidation rate is very fast, which is considered as the results of the GaN decomposition at high temperature under atmosphere. The oxide layers were also observed by a scanning electron microscope, which shows a rough oxide surface and an expansion of the volume of 40%. The photoluminescence (PL) seriously influenced by the oxidation is also discussed.     

Fine structure of effective mass acceptors in gallium nitride

Magnetoluminescence of the exciton bound to a neutral acceptor was measured to investigate the electronic structure of a shallow acceptor center in GaN. The application of magnetic fields along different directions with respect to the crystal c axis allowed us to determine the symmetry of the ground (Gamma(9)) and the first excited state (Gamma(7)) of the acceptor. The observed Zeeman splitting pattern has axial symmetry but can be explained well only by assuming a significant reduction of the spin-orbit interaction for this acceptor state. Because of this reduction, the energy structure of the neutral acceptor is found to be very sensitive to any local, axial perturbation.     

Growth of gallium nitride epitaxial layers and applications

The lack of appropriate substrates has delayed the realisation of devices based on IIInitrides. Currently, the heteroepitaxial growth of GaN by metal organic vapour phase epitaxy (MOVPE) produces GaN layers which, despite huge densities of dislocations, allow the fabrication of highly efficient optoelectronic devices. Henceforth, a new technology in heteroepitaxy of GaN, the epitaxial lateral overgrowth (ELO) has produced GaN layers in which the density of dislocations has been reduced by several orders of magnitude. With the ELO, nitride based laser diodes (LDs) working at room temperature in cw mode with a lifetime of 10,000 hours have been demonstrated by Nichia. In addition to LDs, IIInitrides presently offer a wide range of applications in optoelectronics (high brightness light emitting diodes (LEDs), from amber to UV, solar blind detectors); in electronics, high temperature/high power field effect transistors (FETs). The development of molecular beam epitaxy (MBE) of nitrides has been hindered during several years by the lack of an efficient nitrogen source. This problem being solved, MBE has recently demonstrated state-of-theart quantum well and quantum dot heterostructures, and 2D electron gas heterostructures.     

The fundamental surface science of wurtzite gallium nitride

A review is presented that covers the experimental and theoretical literature relating to the preparation, electronic structure and chemical and physical properties of the surfaces of the wurtzite form of GaN. The discussion includes the adsorption of various chemical elements and of inorganic, organometallic and organic species. The focus is on work that contributes to a microscopic, atomistic understanding of GaN surfaces and interfaces, and the review concludes with an assessment of possible future directions.     

Temperature-controlled catalytic growth of one-dimensional gallium nitride nanostructures using a gallium organometallic precursor

Catalytic growth of 1-D GaN nanostructures is achieved at temperatures from 550 to 850 °C using NH₃ and gallium acetylacetonate. Structural characterization of the 1-D GaN nanostructures by HRTEM shows that straight GaN nanowires, needle-like nanowires (nanoneedles), and bamboo-shoot-like nanoneedles are synthesized at 750, 650, and 550 °C, respectively. In addition to selecting a proper catalyst, providing sufficient precursors has been demonstrated to be a crucial factor for the low-temperature growth of 1-D GaN nanostructures via the VLS mechanism. Possible mechanisms for forming nanoneedles at low temperatures are proposed. PACS 61.46.+w; 68.65.–k; 81.07.–b     

Anisotropic longitudinal electron diffusion coefficient in wurtzite gallium nitride

The longitudinal electron diffusion coefficient (D _l ) of wurtzite (WZ) gallium nitride (GaN) is calculated by an ensemble Monte Carlo (EMC) method. By using the power spectral density associated with velocity fluctuation, the relationship between D _l and electric field strength, frequency, doping concentration and temperature is presented. The anisotropic D _l of GaN impacted by anisotropy of the electronic dispersion is also investigated. It has been found that the D _l in Γ–A direction (c-direction) is larger than that in Γ–M direction (basal plane) in most cases. For lower electric field, the D _l keeps constant at first, then decreases with increasing frequency. However, for higher electric field, the D _l firstly approaches a peak value, then decreases with increasing frequency. When the frequency is zero, the D _l decreases with the increasing electric field, and then increases until a peak value. Finally, it decreases with increasing electric field again. When the temperature increases, the D _l decreases in both directions for increasing scattering rate. A comparison between our calculated diffusion coefficient and the mobility under low electric field by Einstein equation is presented.