GaN Samples Research Articles

Raman spectroscopy as a depth sensor in cubic phase n-GaN

In this work, we investigate the elementary excitations of epitaxial n-doped GaN grown in its meta-stable cubic phase on GaAs substrate. The Raman spectra were obtained in backscattering geometry with the incoming and outgoing light polarizations parallel. We covered the whole spectrum of wavelengths of an argon laser (457–514.5 nm) in a wide range of Raman energy shift, with a minimum temperature of 10 K. We focused our attention on the nature of the Raman structures, in particular, the ones which Stokes shifts are between 550 and 4000 cm−1 whose natures were identified. The control of laser light penetration in the GaN samples together with electronic Raman cross-sections calculations lead us to assign the structures as plasmon-LO phonon coupled modes and density of states of phonons resulting from structural disorder.

Doping in bulk HVPE-GaN grown on native seeds – highly conductive and semi-insulating crystals

Results of gallium nitride crystallization on native seeds by Hydride Vapor Phase Epitaxy method are described. The seeds are high quality ammonothermal GaN crystals. Properties of unintentionally doped HVPE-GaN are briefly presented. A review on doping with donors and acceptors is prepared. Intentional incorporation of silicon or germanium is proposed in order to grow highly conductive HVPE-GaN. Carbon, iron, or manganese was introduced to increase the resistivity of crystallized material. GaN samples with different dopants are described in terms of their structural, optical, and electrical properties.

The influence of the substrate misorientation on the structural quality of GaN layers grown by HVPE

The paper describes a Transmission Electron Microscopy study on the structural quality of GaN samples grown by Hydride Vapor Phase Epitaxy (HVPE) on ammonothermal (Am) GaN seeds. The only difference between these samples is the crystallographic direction of misorientation of the Am GaN seeds for the further growth. The studied samples were 1° misoriented in two perpendicular crystallographic directions: the [1100] and the [1120]. Cross-section samples were studied and showed that growth of homo-epitaxial GaN on the substrate tilted toward the [1100] direction appeared to lead to lower defect density than those tilted toward the [1120] direction. This was associated with slow growth on {1011} planes that are oriented “edge-on” for the growth along [1120] direction leading to the formation of facets while the [1100] direction is devoid of such facets. The competition between slow and fast growth rates on particular planes would lead to the change of the crystal shape such as triangle or trapezoidal one. Our present study also found that cracks in the samples tilted toward the [1120] direction are formed and they are the origin for half loops. Moreover, dislocation propagation from these cracks is observed. Such cracks were not observed for the samples tilted toward the [1100] directions, probably due to more uniform growth. There were no dislocations at the sample surface. However, some parasitic growth occurred on the wafer edges for both samples. Cross-section microscopy found high dislocation density in these areas with and a majority of them are edge dislocations.

Overgrowth and strain investigation of (11\u201320) non-polar GaN on patterned templates on sapphire

Non-polar (11–20) GaN with significantly improved crystal quality has been achieved by means of overgrowth on regularly arrayed micro-rod templates on sapphire in comparison with standard non-polar GaN grown without any patterning processes on sapphire. Our overgrown GaN shows massively reduced linewidth of X-ray rocking curves with typical values of 270 arcsec along the [0001] direction and 380 arcsec along the [1–100] direction, which are among the best reports. Detailed X-ray measurements have been performed in order to investigate strain relaxation and in-plane strain distribution. The study has been compared with the standard non-polar GaN grown without any patterning processes and an extra non-polar GaN sample overgrown on a standard stripe-patterned template. The standard non-polar GaN grown without involving any patterning processes typically exhibits highly anisotropic in-plane strain distribution, while the overgrown GaN on our regularly arrayed micro-rod templates shows a highly isotropic in-plane strain distribution. Between them is the overgrown non-polar GaN on the stripe-patterned template. The results presented demonstrate the major advantages of using our regularly arrayed micro-rod templates for the overgrowth of non-polar GaN, leading to both high crystal quality and isotropic in-plane strain distribution, which is important for the further growth of any device structures.

Effect of carbon on the magnetic properties of Dy-implanted GaN films

In this paper, samples were prepared by co-implanting Dy and carbon ions into c-plane (0001) GaN films and a subsequent rapid thermal annealing at 800 °C. The room temperature magnetic properties of Dy and C co-implanted GaN samples were investigated by experiments and theoretical calculations. Dy and C co-implanted GaN samples had the larger room temperature magnetic moment than that of the Dy-implanted GaN. The calculated results showed that the spin polarization of C-2p electrons leads to the local magnetic moment due to the hybridization between Dy-4f electron states and C-2p electron, which explains the enhancement of magnetic moment in co-implanted GaN samples. Our results suggest that co-implantation is a potential approach to enhance the room temperature ferromagnetism.

Magnesium acceptor in gallium nitride. I. Photoluminescence from Mg-doped GaN

Defect-related photoluminescence (PL) is analyzed in detail for $n$-type, $p$-type, and semi-insulating Mg-doped GaN grown by different techniques. The ultraviolet luminescence (UVL) band is the dominant PL band in conductive $n$-type and $p$-type GaN:Mg samples grown by hydride vapor phase epitaxy (HVPE) and molecular beam epitaxy. The UVL band in undoped and Mg-doped GaN samples is attributed to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor with the ionization energy of 223 meV. In semi-insulating GaN:Mg samples, very large shifts of the UVL band (up to 0.6 eV) are observed with variation of temperature or excitation intensity. The shifts are attributed to diagonal transitions, likely due to potential fluctuations or near-surface band bending. The blue luminescence $(\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}})$ band is observed only in GaN:Mg samples grown by HVPE or metalorganic chemical vapor deposition when the concentration of Mg exceeds ${10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. The $\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}}$ band is attributed to electron transitions from an unknown deep donor to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor. Basic properties of the observed PL are explained with a phenomenological model.

Magnesium acceptor in gallium nitride. II. Koopmans-tuned Heyd-Scuseria-Ernzerhof hybrid functional calculations of its dual nature and optical properties

The problem of magnesium acceptor in gallium nitride is that experimental photoluminescence measurements clearly reveal a shallow defect state, while most theoretical predictions favor a localized polaronic defect state. To resolve this contradiction, we calculate properties of magnesium acceptor using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, tuned to fulfill the generalized Koopmans condition. We test Koopmans tuning of HSE for defect calculations in GaN using two contrasting test cases: a deep state of gallium vacancy and a shallow state of magnesium acceptor. The obtained parametrization of HSE allows calculations of optical properties of acceptors using neutral defect-state eigenvalues, without relying on corrections due to charged defects in periodic supercells. Optical transitions and vibrational properties of $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ defect are analyzed to bring the dual (shallow and deep) nature of this defect into accord with experimental photoluminescence measurements of the ultraviolet band in Mg-doped GaN samples.

Fabrication of GaN single crystals at 700°C using Na-Li-Ca mixed flux system

GaN single crystals were grown in Na-Li-Ca flux system of which Li-Ca gradually replaces Na. X-ray powder diffraction analysis confirmed that the structure of GaN samples was wurtzite. As concentration of Li-Ca in flux increased, the shape of GaN crystals was changed from pyramidal, prism to platelets in sequence, and the color of them became transparent gradually. Among of them, the transparent prism crystals were grown first at 700°C when the concentration of Li-Ca was 31.6 mol% and 48.9 mol% in flux solution, respectively. Raman spectra implied that these crystals were stress-free and the transparent crystal grown in the high concentration of Li-Ca solution had high structural quality or low impurity concentrations.

Interface Engineering of Monolayer MoS2/GaN Hybrid Heterostructure: Modified Band Alignment for Photocatalytic Water Splitting Application by Nitridation Treatment.

Interface engineering is a key strategy to deal with the two-dimensional (2D)/three-dimensional (3D) hybrid heterostructure, since the properties of this atomic-layer-thick 2D material can easily be impacted by the substrate environment. In this work, the structural, electronic, and optical properties of the 2D/3D heterostructure of monolayer MoS2 on wurtzite GaN surface without and with nitridation interfacial layer are systematically investigated by first-principles calculation and experimental analysis. The nitridation interfacial layer can be introduced into the 2D/3D heterostructure by remote N2 plasma treatment to GaN sample surface prior to stacking monolayer MoS2 on top. The calculation results reveal that the 2D/3D integrated heterostructure is energetically favorable with a negative formation energy. Both interfaces demonstrate indirect band gap, which is a benefit for longer lifetime of the photoexcited carriers. Meanwhile, the conduction band edge and valence band edge of the MoS2 side increases after nitridation treatment. The modification to band alignment is then verified by X-ray photoelectron spectroscopy measurement on MoS2/GaN heterostructures constructed by a modified wet-transfer technique, which indicates that the MoS2/GaN heterostructure without nitridation shows a type-II alignment with a conduction band offset (CBO) of only 0.07 eV. However, by the deployment of interface nitridation, the band edges of MoS2 move upward for ∼0.5 eV as a result of the nitridized substrate property. The significantly increased CBO could lead to better electron accumulation capability at the GaN side. The nitridized 2D/3D heterostructure with effective interface treatment exhibits a clean band gap and substantial optical absorption ability and could be potentially used as practical photocatalyst for hydrogen generation by water splitting using solar energy.

GaN decomposition after high-energy ball milling and its transformation to BN hollow spheres in annealing samples doped with B

In this work, GaN powders doped with B and Si were treated by high-energy ball milling, and subsequently, annealing at different temperatures. Transmission electron microscopy (TEM) observations showed the in-situ GaN decomposition in the undoped GaN specimen milled for 5 h. TEM also revealed that the chemical decomposition of GaN was more affected in samples doped with B. Thermo-gravimetric analysis indicates that an undoped GaN specimen decomposes at 1120 °C, Si-doped GaN samples disintegrate at 1150 °C, and finally, B-doped GaN samples decompose at the lower temperature of 940 °C. TEM and SEM results show that the annealing treatment of the as-milled B-doped GaN samples at 1150 °C for 15 min leads to the GaN decomposition and BN hollow nanostructures formation under an N2 atmosphere. The synthesized product consists of mesoporous powders with variable particle sizes from 500 nm to 1 μm having variable cavities in the range from 60 nm to 800 nm.

Exploring possibilities of band gap measurement with off-axis EELS in TEM

A technique to measure the band gap of dielectric materials with high refractive index by means of energy electron loss spectroscopy (EELS) is presented. The technique relies on the use of a circular (Bessel) aperture and suppresses Cherenkov losses and surface-guided light modes by enforcing a momentum transfer selection. The technique also strongly suppresses the elastic zero loss peak, making the acquisition, interpretation and signal to noise ratio of low loss spectra considerably better, especially for excitations in the first few eV of the EELS spectrum. Simulations of the low loss inelastic electron scattering probabilities demonstrate the beneficial influence of the Bessel aperture in this setup even for high accelerating voltages. The importance of selecting the optimal experimental convergence and collection angles is highlighted. The effect of the created off-axis acquisition conditions on the selection of the transitions from valence to conduction bands is discussed in detail on a simplified isotropic two band model. This opens the opportunity for deliberately selecting certain transitions by carefully tuning the microscope parameters.The suggested approach is experimentally demonstrated and provides good signal to noise ratio and interpretable band gap signals on reference samples of diamond, GaN and AlN while offering spatial resolution in the nm range.

The origins and properties of intrinsic nonradiative recombination centers in wide bandgap GaN and AlGaN

The nonradiative lifetime (τNR) of the near-band-edge emission in various quality GaN samples is compared with the results of positron annihilation measurement, in order to identify the origin and to determine the capture-cross-section of the major intrinsic nonradiative recombination centers (NRCs). The room-temperature τNR of various n-type GaN samples increased with decreasing the concentration of divacancies composed of a Ga vacancy (VGa) and a N vacancy (VN), namely, VGaVN. The τNR value also increased with increasing the diffusion length of positrons, which is almost proportional to the inverse third root of the gross concentration of all point defects. The results indicate that major intrinsic NRC in n-type GaN is VGaVN. From the relationship between its concentration and τNR, its hole capture-cross-section is estimated to be about 7 × 10−14 cm2. Different from the case of 4H-SiC, the major NRCs in p-type and n-type GaN are different: the major NRCs in Mg-doped p-type GaN epilayers are assigned to multiple vacancies containing a VGa and two (or three) VNs, namely, VGa(VN)n (n = 2 or 3). The ion-implanted Mg-doped GaN films are found to contain larger size vacancy complexes such as (VGa)3(VN)3. In analogy with GaN, major NRCs in Al0.6Ga0.4N alloys are assigned to vacancy complexes containing an Al vacancy or a VGa.

The trap states in lightly Mg-doped GaN grown by MOVPE on a freestanding GaN substrate

We investigated traps in lightly Mg-doped (2 × 1017 cm−3) p-GaN fabricated by metalorganic vapor phase epitaxy (MOVPE) on a freestanding GaN substrate and the subsequent post-growth annealing, using deep level transient spectroscopy. We identified four hole traps with energy levels of EV + 0.46, 0.88, 1.0, and 1.3 eV and one electron trap at EC − 0.57 eV in a p-type GaN layer uniformly doped with magnesium (Mg). The Arrhenius plot of hole traps with the highest concentration (∼3 × 1016 cm−3) located at EV + 0.88 eV corresponded to those of hole traps ascribed to carbon on nitrogen sites in n-type GaN samples grown by MOVPE. In fact, the range of the hole trap concentrations at EV + 0.88 eV was close to the carbon concentration detected by secondary ion mass spectroscopy. Moreover, the electron trap at EC − 0.57 eV was also identical to the dominant electron traps commonly observed in n-type GaN. Together, these results suggest that the trap states in the lightly Mg-doped GaN grown by MOVPE show a strong similarity to those in n-type GaN, which can be explained by the Fermi level close to the conduction band minimum in pristine MOVPE grown samples due to existing residual donors and Mg-hydrogen complexes.

Nano-indentation used to study pyramidal slip in GaN single crystals

The nucleation and structure of dislocations created by the nano-indentation of GaN samples with dislocation densities ≈103, 106 or 109 ⊥/cm2 were studied in the interest of learning how dislocations can be created to relieve the mismatch strain in ternary nitride films grown on (0001) oriented binary nitride substrates. Using transmission electron microscopy and stress analyses to assist in interpreting the nano-indentation data, we determined that the pop-ins in the indenter load vs. penetration depth curves are created by an avalanche process at stresses well above the typical yield stress. The process begins by the homogeneous formation of a basal plane screw dislocation that triggers the formation of pyramidal and other basal plane dislocations that relieve the excess stored elastic energy. It appears that pyramidal slip can occur on either the {1122} or {0111} planes, as there is little resistance to the cross slip of screw dislocations.

Role of film thickness on the structural and optical properties of GaN on Si (100) grown by hollow-cathode plasma-assisted atomic layer deposition

In this study, the authors report on the evolution of crystallinity, chemical composition, surface morphology, and optical properties of highly oriented (002) GaN films as a function of film thickness, ranging from 5.37 to 81.40 nm deposited on Si substrates via low-temperature hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD). GaN thin films were prepared using a sequential injection of triethyl gallium and N2/H2 plasma within the self-limited growth regime, i.e., ALD window at 200 °C. The grazing-incidence x-ray diffraction (GIXRD) analysis reveals that GaN films have a (002) preferential growth direction and the crystalline quality of GaN films was improved with the increase in thickness. GIXRD and atomic force microscopy confirmed the presence of GaN grains, and the grain size increases when the thickness of the GaN layer increases from 5.37 to 48.65 nm. From the x-ray photoelectron spectra, it was shown that the amount of oxygen incorporated in the GaN film decreases as the thickness increases. The spectroscopic ellipsometry analysis reveals that the optical film density and local crystallinity was improved with increasing film thickness to 48.65 nm, but further increase in the film thickness does not seem to improve these features. The optical band edge results suggested that bandgap widening is valid for all HCPA-ALD grown GaN samples. The overall results suggested that GaN films with thicknesses above 48.65 nm have different behavior compared to the thinner GaN films.

Detection of deep-level defects and reduced carrier concentration in Mg-ion-implanted GaN before high-temperature annealing

We report experimental results for the detection of deep-level defects in GaN after Mg ion implantation before high-temperature annealing. The n-type GaN samples were grown on GaN free-standing substrates by metalorganic vapor phase epitaxy. Mg ions were implanted at 50 keV with a small dosage of 1.5×1011 cm-2, which did not change the conduction type of the n-GaN. By depositing Al2O3 and a Ni/Au electrode onto the implanted n-GaN, metal-oxide-semiconductor (MOS) diodes were fabricated and tested. The measured capacitance–voltage (C–V) characteristics showed a particular behavior with a plateau region and a region with an anomalously steep slope. Fitting to the experimental C–V curves by simulation showed the existence of deep-level defects and a reduction of the carrier concentration near the GaN surface. By annealing at 800oC, the density of the deep-level defects was reduced and the carrier concentration partially recovered.

Influence of Ga vacancies, Mn and O impurities on the ferromagnetic properties of GaN micro- and nanostructures

We present a study of the influence of gallium vacancy (VGa) point defects on the ferromagnetic properties of GaN:Mn and GaN:Mn,O micro- and nanostructures. Results demonstrate that the generation of these point defects enhances the ferromagnetic signal of GaN:Mn microstructures, while incorporation of oxygen as an impurity inhibits this property. XPS measurements revealed that Mn impurities in ferromagnetic GaN:Mn samples mainly exhibit a valence state of 2+. Cathodoluminescence (CL) spectra from Mn-doped GaN samples displayed emissions centered at about 1.97 eV, attributed to transitions between the 4T1-6A1 states of the Mn2+ d orbitals, and emissions centered at 2.45 and 2.9 eV, associated with the presence of VGa. CL measurements also revealed a blue shift of the GaN band-edge emission generated by the expansion of the wurtzite lattice due to Mn incorporation, which was confirmed by XRD measurements. These latter measurements also revealed an amorphization of GaN:Mn due to the incorporation of oxygen as impurities. The GaN:Mn samples were synthesized by thermal evaporation of GaN and MnCO3 powders onto Ni0.8Cr0.2/Si(100) in a horizontal furnace operated at low vacuum. The residual air inside the system was used as a source of oxygen during the synthesis of Mn and O co-doped GaN nanostructures. Mn and O impurities were incorporated into the nanostructures at different concentrations by varying the growth temperature. Energy Dispersive Spectroscopy, XRD, and XPS measurements confirmed that the obtained samples predominantly consisted of GaN.

Eu–Mg defects and donor–acceptor pairs in GaN: photodissociation and the excitation transfer problem

We have investigated the temperature-dependent photoluminescence (TDPL) profiles of Eu3+ ions implanted in an HVPE-grown bulk GaN sample doped with Mg and of donor–acceptor pairs (DAP) involving the shallow Mg acceptor in GaN(Mg) (unimplanted) and GaN(Mg):Eu samples. Below 125 K, the TDPL of Eu3+ in GaN(Mg):Eu correlates with that of the DAP. Below 75 K, the intensity of Eu3+ emission saturates, indicating a limitation to the numbers of Eu–Mg defects available to receive excitation transferred from the host, while the DAP continues to increase, albeit more slowly in the implanted than the unimplanted sample. Prolonged exposure to UV light at low temperature results in the photodissociation of Eu–Mg defects in their Eu1(Mg) configuration, with a corresponding increase in shallow DAP emission and the emergence of emission from unassociated EuGa (Eu2) defects.

Analysis on Photo Emission and Absorbing Spectrum on GaN Sample by Finite Difference Time Domain Method

III-Nitride semiconductors are especially capable for both electronics and optical devices. The capability of the III-Nitride semiconductors as light emitters to extent the electromagnetic spectrum from deep ultraviolet light, throughout the whole visible region, and into the infrared part of the spectrum, is a significant characteristic, making this material indispensable for the areas of light emitting devices. The near and far field characteristics of the GaN samples are studied by affecting the finite-difference time domain (FDTD) technique. The far region spreading characteristics at diverse incident angles are also conferred. In addition, the spreading field would be concentrated and the transmission efficiency could be enhanced by the phase shift caused by the dielectric substrate. The intended of optoelectronic devices fictitious from III-Nitride materials is supported by acquaintance of refractive index and absorption coefficient of these materials.

Study of Nonradiative Recombination Centers in n-GaN Grown on LT-GaN and AlN Buffer Layer by Below-Gap Excitation

Nonradiative recombination (NRR) centers in n-type GaN samples grown by MOCVD technique on a LT-GaN buffer layer and aAlN buffer layer have been studied by two wavelength excited photoluminescence (TWEPL). The near band-edge photoluminescence (PL) intensity decreases due to the superposition of below-gap excitation (BGE) light of energies 0.93, 1.17 and 1.27 eV over above-gap excitation (AGE) light of energy 4.66 eV. The decrease in PL intensity due to the addition of the BGE has been explained by a two levels recombination model based on SRH statistics. It indicates the presence of a pair of NRR centers in both samples, which are activated by the BGE. The degree of quenching in PL intensity for the sample grown on LT-GaN buffer layer is stronger than the sample grown on AlN buffer layer for all BGE sources. This result implies that the use of the AlN buffer layer is more effective for reducing the NRR centers in n-GaN layers than the LT-GaN buffer layer. The dependence of PL quenching on the AGE density, the BGE density and temperature has been also investigated. The NRR parameters have been quantitatively determined by solving rate equations and fitting the simulated results with the experimental data.