eV For GaN Research Articles

Analysis of polar GaN surfaces with photoelectron and high resolution electron energy loss spectroscopy

The properties of Ga-face and N-face GaN surfaces were studied by X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED) as well as high resolution electron energy loss spectroscopy (HREELS). The Ga-face GaN was grown on 6H-SiC(0001) and the N-face GaN directly on α-Al2O3(0001) by plasma assisted molecular beam epitaxy (PAMBE) and in situ analysed by photoelectron spectroscopy. The Ga-face GaN surfaces show a 2 × 2 reconstruction measured by RHEED. From the analysis of the XPS spectra a valence band maximum located 2.9 eV below the Fermi level results. Furthermore, the UPS spectra exhibit two surface states. The N-face GaN presented a 1 × 1 surface with one surface state and a valence band maximum located 2.4 eV below EF. These values correspond to an upward band bending of about 0.9 eV for the N-face GaN()-1 × 1 and about 0.4 eV for the GaN(0001)-2 × 2 (Ga-face) results. The different surface band bending is further supported by the evaluation of the FWHM of the quasi-elastically reflected electrons in HREELS.

Highly mismatched crystalline and amorphous GaN1−xAsx alloys in the whole composition range

Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1−xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17<x<0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition, and sharp, well defined optical absorption edges. The band gap energy varies in a broad energy range from ∼3.4 eV in GaN to ∼0.8 eV at x∼0.85. The reduction in the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x>0.2, and to the upward movement of the valence band for alloys with x<0.2. The unique features of the band structure offer an opportunity of using GaN1−xAsx alloys for various types of solar power conversion devices.

Luminescence of Eu ions inAlxGa1−xNacross the entire alloy composition range

Photoluminescence (PL) and PL excitation (PLE) spectra of Eu-implanted AlxGa1-xN are obtained across the whole alloy composition range. The dominant D-5(0)-F-7(2) emission band broadens and then narrows as x increases from 0 to 1 while the peak shifts monotonically. This behavior is surprisingly similar to the broadening of excitons in a semiconductor alloy caused by composition fluctuations [E. F. Schubert et al., Phys. Rev. B 30, 813 (1984). PLE spectra reveal a steplike AlxGa1-xN band-edge absorption and two subgap bands X-1,X-2:X-1 peaks at 3.26 eV in GaN and shifts linearly to 3.54 eV in AlN. For x > 0.6, X-2 emerges approximately 1 eV higher in energy than X-1 and shifts in a similar way. We propose that X-1,X-2 involve creation of core-excitonic complexes of Eu emitting centers.

Recent knowledge of strength and dislocation mobility in wide band-gap semiconductors

Abstract The mechanical strengths and dislocation activities in wide band-gap semiconductors SiC, AlN, GaN, ZnO and ZnSe at elevated temperatures are summarized. From investigations by indentation hardness test and compressive deformation, activation energies for dislocation motion are evaluated to be 2–2.7, 0.7–1.2 and 0.5–0.7 eV in GaN, ZnO and ZnSe, respectively. Dislocations in II–VI compounds ZnO and ZnSe are essentially mobile.

When group-III nitrides go infrared: New properties and perspectives

Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN) or 0.2 μm (6.2 eV for AlN). The continuous range of bandgap energies now spans the near infrared, raising the possibility of new applications for group-III nitrides. In this article we present a detailed review of the physical properties of InN and related group III-nitride semiconductors. The electronic structure, carrier dynamics, optical transitions, defect physics, doping disparity, surface effects, and phonon structure will be discussed in the context of the InN bandgap re-evaluation. We will then describe the progress, perspectives, and challenges in the developments of new electronic and optoelectronic devices based on InGaN alloys. Advances in characterization and understanding of InN and InGaN nanostructures will also be reviewed in comparison to their thin film counterparts.

On the stability of fluorine ions in AlGaN/GaN heterostructures: a molecular dynamics simulation study

AbstractFluorine ions can be effectively incorporated into AlGaN/GaN high electron mobility transistor (HEMT) structures, enabling the modulation of local potential and carrier density. The physical mechanism of fluorine incorporation in AlGaN/GaN heterojunctions is of fundamental importance to the stability of fluorine ions in AlGaN/GaN HEMTs. In this work, the molecular dynamic (MD) simulation method is used to calculate the potential energies of interstitial and substitutional fluorine atoms in AlGaN/GaN material system. Ziegler‐Biersack‐Littmark (ZBL), Lindhard‐Sorensen (L‐S) and Coulomb potential functions are applied in the MD simulation. The geometric lattice structures, spontaneous and piezoelectric polarizations, and temperature dependence are also included in the simulation. The activation energies associated with interstitial‐substitutional and interstitial‐interstitial diffusions are obtained. It is revealed that the fluorine ions are most likely located at the substitutional group‐III cation sites S(III) and the diffusion of fluorine ions should be dominated by S(III)‐interstitial process which exhibits an activation energy of 1.1 eV in Al0.25Ga0.75N and 1.4 eV in GaN in the presence of group‐III vacancies. It is expected that the removal of group‐III vacancies can significantly suppress the fluorine diffusion, which in turn, leads to excellent fluorine stability in III‐nitride materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduction of Mg segregation in a metalorganic vapor phase epitaxial grown GaN layer by a low-temperature AlN interlayer

The redistribution behavior of Mg in a sequentially regrown GaN epilayer on a p-type doped GaN template was studied. All samples in this study were regrown by metalorganic vapor phase epitaxy on the sapphire substrates. A high density and a slow tail of Mg concentration were observed in a nominally undoped layer due to the surface segregation. We found that the insertion of a low-temperature (LT) AlN interlayer was effective to suppress the Mg redistribution in the GaN regrown layer. Analyzing the temperature dependence of the surface segregation, the activation energy of the Mg segregation was estimated to be 0.63eV in GaN and 2.47eV in a LT-AlN layer, respectively.

Deep‐level luminescence at 1.93 eV in GaN prepared by ammonothermal growth

AbstractDeep‐level luminescence at 1.93 eV (red luminescence) was observed from GaN grown by the acidic ammonothermal method. Typical deep emission at 2.2 eV (yellow luminescence) was not observed. The intensity decreased linearly with increasing temperature, which is different from the red and yellow luminescence reported thus far. The time decay of the luminescence appeared partially exponential, i.e. with the both possibilities of a simple two‐level transition and donor–acceptor pairs. The origin of the red luminescence is probably different from the previously reported one [J. Appl. Phys. 97, 061301 (2005)]. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of the electronic fundamental transition of zincblende InN/InGaN quantum wells

In this work we calculate the transition energy from the first level of holes to the first level of electrons (1h–1e) for cubic InN/InxGa1-xN quantum wells. We employ the empirical tight binding approach with a sp3s* orbital basis, nearest neighbors interactions and the spin–orbit coupling, together with the surface Green function matching method. Our tight binding parameters give a value of 0.65eV for the InN gap and 3.3eV for GaN. For the InxGa1-xN alloy we use the virtual crystal approximation. We study the transition energy behavior varying the well width for concentrations x=0.2, 0.5, and 0.8. For the calculations, we use two values of the valence band offset: 30% and 50%. We take into account the strain in the well, due to the different lattice constants between the well and the barrier.

Evaluation of dislocation‐related defects in GaN using deep‐level transient spectroscopy

AbstractA method to extract the carrier capture kinetics of traps in the neutral region by isothermal deep‐level transient spectroscopy using two sets of bias pulses was applied to the traps with similar energy levels around Ec–0.6 eV in GaN grown by metalorganic chemical vapor deposition on GaN and sapphire substrates. The Ec–0.6 eV trap in GaN on GaN is found to be the isolated point defects. The Ec–0.6 eV trap in GaN on sapphire shows the logarithmic capture kinetics, expected for dislocation‐related defects. This is ascribed to the difference in dislocation density between GaN on GaN and sapphire substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Band offset measurements of the pulsed‐laser‐deposition‐grown Sc2O3 (111)/GaN (0001) heterostructure by X‐ray photoelectron spectroscopy

AbstractThe energy band alignment of the Sc2O3/GaN heterointerface has been studied by means of X‐ray photoelectron spectroscopy. The single‐crystalline Sc2O3 was grown epitaxially on GaN template using pulsed laser deposition. A valence band offset of 0.84 ± 0.05 eV has been obtained across the heterointerface by using Ga 3d and Sc 3p energy core levels as the references. Given the bandgaps of 3.42 and 6.3 eV for GaN and Sc2O3, respectively, the conduction band offset has been estimated to be 2.04 eV in this system. These large band offsets are desirable, especially for GaN‐based electronic devices, which are most suited for high‐temperature operations, where the carrier thermionic emission must be maintained at a sufficient low level. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Band gap bowing and electron localization of GaxIn1−xN

The band gap bowing and the electron localization of GaxIn1−xN are calculated using both the local density approximation (LDA) and screened-exchange local density functional (sX-LDA) methods. The calculated sX-LDA band gaps are in good agreement with the experimentally observed values, with errors of −0.26 and 0.09eV for bulk GaN and InN, respectively. The LDA band gap errors are 1.33 and 0.81eV for GaN and InN, in order. In contrast to the gap itself, the band gap bowing parameter is found to be very similar in sX-LDA and LDA. We identify the localization of hole states in GaxIn1−xN alloys along In–N–In chains. The predicted localization is stronger in sX-LDA.

Determination of acceptor concentration in GaN from photoluminescence

The concentration of the acceptor responsible for the yellow luminescence (YL) band at about 2.2eV in GaN is determined from photoluminescence. The YL band intensity increases linearly with excitation power density and partially saturates above some critical value. The dependence is quantitatively described within a phenomenological model accounting for recombination statistics in GaN layer and saturation of acceptors with photogenerated holes. The incomplete saturation of the YL intensity at high excitation intensities is explained by gradual saturation of acceptors at different distances from the sample surface. The identity of deep and shallow acceptors in GaN is discussed.

Photoluminescence Studies of In-Doped GaN:Mg Films

Photoluminescence (PL) studies of In-doped GaN:Mg films revealed that the Mg-related emission at 3.1 eV is enhanced by more than one order of magnitude on the shoulder of the broad band centered at 2.8 eV for GaN:Mg after an optimal In concentration was added into the films. This enhancement of the 3.1 eV band is believed to be associated with the reduction in the number of self-compensation centers. A slow decay in PL intensity evolution was also observed, which may be ascribed to a local energy barrier that impedes carriers that relax into the valence band. The temperature dependences of the decay time constants were measured and a barrier energy as high as ∼103±7 meV was obtained for In-doped GaN:Mg as compared with 69±8 meV for GaN:Mg.

Relationship between structure characteristic and blue luminescence in unintentional doped GaN layers

Properties of some GaN layers with different blue luminescence relative intensity were investigated by photoluminescence, Rutherford backscattering/channeling and double crystal X-ray diffraction, respectively. The surface minimum yields χ min of Rutherford backscattering/channeling and the FWHM of double crystal X-ray diffraction obviously increase with the increase of the intensity ratio of the blue luminescence to the band-edge emission. The blue luminescence is due to some intrinsic defects that can largely deteriorate the crystalline quality of GaN films. The luminescence mechanism of the blue luminescence in unintentional doped GaN films is different from that of the blue luminescence about 2.9 eV in GaN:Mg films.

Analysis and control of excess leakage currents in nitride-based Schottky diodes based on thin surface barrier model

Using a rigorous computer simulation program for current transport through a Schottky barrier with an arbitrary potential profile, the leakage current mechanism in GaN and AlGaN Schottky diodes was investigated on the basis of the thin surface barrier (TSB) model recently proposed by the authors’ group. Computer simulation assuming various possible defect density distributions was carried out to reproduce the measured temperature dependent current voltage (I–V)-temperature characteristics of the GaN and AlGaN Schottky diodes which showed excessive reverse leakage. By assuming exponentially decaying distributions from surface for defect donors with energy depth of 0.25 eV for GaN and 0.37 eV for Al0.15Ga0.85N, I–V curves measured by our group as well as reported in the literatures were almost completely reproduced both in forward and reverse direction over a wide temperature range. The defect donors are proposed to be N vacancies or their related complexes that are formed during metal deposition. The result confirms the validity of the TSB model. From the viewpoint of the TSB model, attempts were also made to suppress leakage currents. It was found that a low-energy electrochemical metal deposition process and a metal–insulator–semiconductor Schottky structure using an ultrathin Al2O3 film by electron cyclotron resonance oxidation of Al film were remarkably effective in reducing excess leakage currents due to reduction of defect deep donors.

Optical properties of Mn4+ ions in GaN:Mn codoped with Mg acceptors

The optical properties of Mn–Mg-codoped epitaxial GaN were studied. Addition of Mg acceptors quenches the weak manganese-related photoluminescence (PL) band at 1.3eV in GaN:Mn and a series of sharp PL peaks are observed at 1eV in codoped epilayers. The change in PL spectra indicates that the Mg addition stabilizes the Mn4+ charge state by decreasing the Fermi level. The 1eV PL peaks are tentatively attributed to intracenter transitions involving Mn4+ ions. Spin-allowed 3d-shell T24–T14 transitions and their phonon replicas are involved. The relative intensities of the sharp peaks are strongly dependent on the excitation wavelength, indicating that the optically active Mn4+ centers involved in the separate peaks are different. The temperature dependence of the PL spectrum suggests the presence of at least three distinct Mn4+ complex centers.

Electronic Properties of GaN (0001) – Dielectric Interfaces

The characteristics of clean n- and p-type GaN (0001) surfaces and the interface between this surface and SiO 2, Si 3 N 4, and HfO 2 have been investigated. Layers of SiO 2, Si3 N 4, or HfO 2 were carefully deposited to limit the reaction between the film and clean GaN surfaces. After stepwise deposition, the electronic states were measured with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). A valence band offset (VBO) of 2.0±0.2 eV with a conduction band offset (CBO) of 3.6±0.2 eV was determined for the GaN / SiO 2 interface. The large band offsets suggest SiO 2 is an excellent candidate for passivation of GaN . For the GaN / Si 3 N 4, interface, type II band alignment was observed with a VBO of -0.5±0.2 eV and a CBO of 2.4±0.2 eV . While Si3 N 4 should passivate n-type GaN surfaces, it may not be appropriate for p-type GaN surfaces. A VBO of 0.3±0.2 eV with a CBO of 2.1±0.2 eV was determined for the annealed GaN / HfO 2 interface. An instability was observed in the HfO 2 film, with energy bands shifting ~0.4 eV during a 650°C densification anneal. The electron affinity measurements via UPS were 3.0, 1.1, 1.8, and 2.9±0.1 eV for GaN , SiO 2, Si 3 N 4, and HfO 2 surfaces, respectively. The deduced band alignments were compared to the predictions of the electron affinity model and deviations were attributed to a change of the interface dipole. Interface dipoles contributed 1.6, 1.1 and 2.0±0.2 eV to the band alignment of the GaN / SiO 2, GaN / Si 3 N 4, and GaN / HfO 2 interfaces, respectively. It was noted that the existence of Ga-O bonding at the heterojunction could significantly affect the interface dipole, and consequently the band alignment in relation to the GaN .

Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures

The influence of AlGaN and GaN cap layer thickness on Hall sheet carrier density and mobility was investigated for Al0.32Ga0.68N/GaN and GaN/Al0.32Ga0.68N/GaN heterostructures deposited on sapphire substrates. The sheet carrier density was found to increase and saturate with the AlGaN layer thickness, while for the GaN-capped structures it decreased and saturated with the GaN cap layer thickness. A relatively close fit was achieved between the measured data and two-dimensional electron gas densities predicted from simulations of the band diagrams. The simulations also indicated the presence of a two-dimensional hole gas at the upper interface of GaN/AlGaN/GaN structures with sufficiently thick GaN cap layers. A surface Fermi-level pinning position of 1.7 eV for AlGaN and 0.9–1.0 eV for GaN, and an interface polarization charge density of 1.6×1013–1.7×1013 cm−2, were extracted from the simulations.

Structural Defect-Related Photoluminescence in GaN

ABSTRACTBroad, low temperature photoluminescence (PL) peaks near 3.4–3.42 eV in GaN have previously been associated with basal-plane stacking faults. Recently, we observed unusually sharp and highly structured PL peaks in this region in high quality bulk GaN grown from a Na/Ga flux, some of which display characteristic shifts and narrowing as a function of excitation power. Here, we study these peaks as a function of excitation intensity and crystal polarity, and compare them to those observed in GaN grown on off-axis SiC or sapphire (0001) substrates by metalorganic chemical vapor deposition (MOCVD). In the off-axis material on either substrate, similar peaks are observed to those in the bulk samples. In addition, a low energy peak near 3.21 eV is observed, which does not occur in the bulk material.