Ga 2p Core Level Research Articles

Molecular beam epitaxy of single-crystalline bixbyite (In1−xGax)2O3 films ( x≤0.18 ): Structural properties and consequences of compositional inhomogeneity

In this work, we show the heteroepitaxial growth of single-crystalline bixbyite (In$_{1-x}$Ga$_x$)$_2$O$_3$ films on (111)-oriented yttria-stabilized zirconia substrates using plasma-assisted molecular beam epitaxy under various growth conditions. A pure In$_2$O$_3$ buffer layer between the substrate and (In$_{1-x}$Ga$_x$)$_2$O$_3$ alloy is shown to result in smoother film surfaces and significantly improved crystallinity. Symmetric out-of-plane 2$\theta - \omega$ x-ray diffraction scans show a single (111) crystal orientation and transmission electron microscopy confirms the single-crystallinity up to $x = 0.18$ and only slight film quality deterioration with increasing Ga content. Partially relaxed layers are demonstrated via reciprocal space mapping with lattice parameters fitting well to Vegard's law. However, the Ga cations are not evenly distributed within the films containing nominally $x > 0.11$: inclusions with high Ga density up to $x = 0.50$ are observed within a "matrix" with $x \approx 0.08$. The cubic bixbyite phase is preserved, in both the "matrix" and the inclusions. Moreover, for $x \geq 0.11$, both the Raman phonon lines as well as the optical absorption onset remain nearly constant. Hard x-ray photoelectron spectroscopy measurements also indicate a widening of the band gap and exhibit similar saturation of the Ga 2p core level position for high Ga contents. This saturation behavior of the spectroscopic properties further supports the limited Ga incorporation into the "matrix" of the film.

Using a NiZn solid solution layer to produce high-barrier height Schottky contact to semipolar (20–21) n-type GaN

Formation of high barrier height Schottky contacts to semipolar (20–21) n-GaN was realized by using a NiZn solid solution (NiZn s.s.) layer. The X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) results exhibited the creation of Ni-oxides and N-gallide phases when the contact samples were annealed at 650 °C. The XPS Ga 2p core levels attained from the NiZn s.s./GaN interface underwent a shift toward lower energies upon annealing. STEM element mapping and XRD results illustrated Ga outdiffusion in the 650 °C-annealed sample. The current-voltage (I–V) plots of the samples revealed that the reverse leakage characteristics were improved with an increase in the annealing temperature from 0 to 650 °C. The ideality factors and Schottky barrier heights (SBHs) assessed by the I–V method were in the range 2.13–2.73 and 0.54–0.68 eV, respectively. With increasing annealing temperature, the ideality factor decreased, while the SBH increased. It was also shown that the barrier inhomogeneity and capacitance-voltage methods produced much larger SBHs of 0.61–1.54 eV than the I–V method. Based on the XRD, STEM, and XPS analyses, the dependence of the SBHs on the annealing temperature is described.

Impact of annealing temperature on band-alignment of PLD grown Ga2O3/Si (100) heterointerface

Cost-effective integration with existing silicon CMOS electronics has been one of the primary motivations for most of the emerging non-silicon devices. In this context, β-gallium oxide (Ga2O3) films were deposited on Si (100) substrate using pulsed laser deposition (PLD) technique in this research. After deposition, samples were further annealed at 600 °C and 800 °C under vacuum. X-ray diffractometer (XRD) was employed to observe the crystallinity variation due to the annealing. The crystallinity of samples degrades with annealing at 600 °C and an incremental improvement in crystallinity was again exhibited at 800 °C due to the possible rearrangement of the Ga and O atoms to their optimal sites. Further, x-ray photoelectron spectroscopy (XPS) was used for identification of elements and chemical composition. XPS results were also analyzed to locate the position of Ga 2p and Si 2p core levels and calculate the valance band offset (VBO) at Ga2O3/Si interface. Reflected electron energy loss spectroscopy (REELS) and ultraviolet photoelectron spectroscopy (UPS) were utilized to calculate the bandgap and work function, respectively. Moreover, valance band maxima and Fermi level position of the as-deposited and annealed samples were calculated using UPS. The bandgaps for the as-deposited, annealed samples at 600 °C and 800 °C were estimated to be 4.72 ± 0.05, 4.52 ± 0.05, and 4.48 ± 0.05 eV, respectively. Consequently, the work function of the sample increases with annealing due to increase in the oxygen vacancies and resulting bandgap narrowing. The band offsets (valance band, conduction band) were determined to be (3.35 ± 0.05 eV, 0.26 ± 0.02 eV), (3.55 ± 0.05 eV, 0.15 ± 0.02 eV) and (3.54 ± 0.05 eV, 0.17 ± 0.02 eV) for the as-deposited, annealed samples at 600 °C and 800 °C, respectively. This optimization of band alignment with annealing temperature shall be very useful for controlling the carrier transport at the interface for Ga2O3/Si based devices. Thus, a trade-off is to be performed between VBO and crystallinity of the Ga2O3/Si to operate the devices efficiently and reliably respectively.

Ag–Pd–Cu alloy reflector to improve the opto-electrical performance and electromigration resistance of near ultraviolet GaN-based light-emitting diode

We investigated the opto-electrical and electromigration properties of near ultraviolet light emitting diodes (UV-LEDs) fabricated with Ag–Pd–Cu (APC) and Ag only reflectors. It was shown that unlike Ag only sample, the APC sample revealed a smooth surface with hillocks when annealed at 600 °C. The 600 °C-annealed APC sample gave a reflectance of 84.2% at 400 nm, whereas the Ag sample had 69.0%. Both the samples exhibited ohmic behavior when annealed. The specific contact resistivity of the Ag and APC contacts annealed at 500 °C were estimated to be 2.59 × 10−4 and 1.85 × 10−4 Ωcm2, respectively. The X-ray photoemission spectroscopy Ga 2p core level results showed that for the annealed APC sample, the Ga 2p core level was shifted towards the lower binding-energies by 0.67 eV as compared to that of the as-deposited sample. Both UV-LEDs with the 500 °C-annealed Ag and APC reflectors gave the same forward voltage of 3.23 V at 20 mA. The UV-LED with the annealed APC reflector yielded 9.07% higher output at 100 mA than that with the annealed Ag reflector. The APC sample exhibited a longer median-time-to-failure (MTF) by a factor of 1.4 than the Ag sample. The activation energy for the electromigration of these samples was measured to be 0.58–0.7 eV. Electron back scatter diffraction (EBSD) and inverse pole figure (IPF) images revealed that the Ag sample was more <111>-textured than the APC sample. Based on the scanning electron microscopy, EBSD and IPF results, the better thermal and electromigration properties of the APC sample are described and discussed.

Improved Output Power Behavior of Ultraviolet GaN-Based Light-Emitting Diode By Using Agpdcu Reflector

GaN-based ultraviolet UVLEDs have attracted a great deal of interest because of their applications in water and air purification and solid-state lighting. These applications have stimulated the technological development of high-efficiency UVLEDs. On the one hand, UVLEDs exhibit relatively lower external quantum efficiencies (EQEs) than visible LEDs. In particular, high-power UVLEDs should have high light extraction and heat dissipation efficiencies. For the flip-chip (FC) LEDs, the fabrication of high-performance p-type reflector with low contact resistance and high reflectivity is reqyired. Silver (Ag) is a commonly adopted p-type reflector for near UV LEDs but suffers from thermal instability, namely, agglomerated when annealed in air above 300 °C. In addition, electromigration (EM) is one of the major failure mechanisms of electronic interconnect under high current density (> ~105 A/cm2) stressing in microelectronics devices. In this talk, we investigated the opto-electrical properties, and electromigration reliability of a Ag-Pd-Cu alloy (APC) reflector for near UV (385 nm)-LEDs as a function of temperature and compared with those of Ag only reflector. SEM results revealed that the Ag samples were less stable than the APC samples upon annealing. The contact resistivities of the Ag and APC contacts annealed at 500 °C were measured to be 2.59 × 10– 4 and 1.85 × 10–4 Wcm2, respectively. The Ga 2p core level for the APC sample shifted towards the lower binding-energies by 0.67 eV as compared to that of the as-deposited sample. The forward I-V properties of near UV LEDs fabricated with the Ag and APC reflectors were observe to be similar each other. Notably, the UV LEDs with the APC reflector produced 9.04% higher output at 100 mA than those with the Ag reflector. To investigate the device reliability, the EM behaviour of Ag and APC films were also characterised. The APC samples exhibited better EM resistance than the Ag samples. The APC samples yielded higher activation energies of the EM failures than the Ag sample. The better EM behaviour of the APC samples will be described and discussed on the basis of XRD pole figure and electron back scatter diffraction, phi scan, and inverse pole figures (IPFs) examination results.

Improved Light Output Power of Ultraviolet GaN-Based Light-Emitting Diode by Using AgPdCu Reflector

We investigated the output performance of UV LEDs with Ag-Pd-Cu (APC) and Ag-only reflectors. After annealing at 600°C, the Ag-only layer surface was seriously roughened, while the APC sample revealed a smooth surface with hillocks. The 600 °C-annealed APC and Ag samples gave reflectance of 84.9% and 70.9%at 385 nm. The contact resistivities of the Ag and APC contacts annealed at 500 °C were measured to be 2.59×10– 4 and 1.85×10– 4 Ωcm2, respectively. For the annealed APC sample, the Ga 2p core level was shifted towards the lower binding-energies by 0.67 eV as compared to that of the as-deposited sample. The UV LEDs with the annealed APC reflector yielded 9.04% higher output at 100 mA than those with the Ag reflector. Based on the XRD pole figure and SEM results, the better thermal property of the APC sample is described in terms of thermal stress and surface energy.

X-ray photoelectron spectroscopy study of Ga nanodroplet on silica-terminated silicon surface for nanowire growth

In this paper the early stages of the self-catalyzed Vapor-Liquid-Solid (VLS) growth of GaAs nanowires on Si substrates by Molecular Beam Epitaxy (MBE) are studied. The interaction of Ga nanodroplets (NDs) with the silica overlayer is investigated by X-ray Photoemission Spectroscopy (XPS) and Atomic Force Microscopy (AFM). We show how Ga NDs drill the silica overlayer and make contact with bulk Si to allow GaAs nanowires (NWs) epitaxial growth by studying each of the three steps of the NW growth process sequentially: Ga NDs pre-deposition, post-deposition annealing to reach the NW growth temperature, and finally NW growth itself. The pre-deposition temperature allows to control the density and morphology of the NDs. A high enough annealing temperature enhances the reduction of SiOx by Ga oxidation and leads to the formation of holes which is seen by a new component in XPS spectra, assumed to be the consequence of the GaSi interaction. Finally, these nano-holes act as nucleation sites for the epitaxial GaAs VLS growth. During the GaAs growth, three different chemical environments of Ga are identified in Ga2p core level: metallic Ga due to the droplets at the top of the NWs, Ga oxide in contact with the SiOx overlayer and Ga arsenide from NWs.

The electronic structure of ε-Ga2O3

The electronic structure of ε-Ga2O3 thin films has been investigated by ab initio calculations and photoemission spectroscopy with UV, soft, and hard X-rays to probe the surface and bulk properties. The latter measurements reveal a peculiar satellite structure in the Ga 2p core level spectrum, absent at the surface, and a core-level broadening that can be attributed to photoelectron recoil. The photoemission experiments indicate that the energy separation between the valence band and the Fermi level is about 4.4 eV, a valence band maximum at the Γ point and an effective mass of the highest lying bands of – 4.2 free electron masses. The value of the bandgap compares well with that obtained by optical experiments and with that obtained by calculations performed using a hybrid density-functional, which also reproduce well the dispersion and density of states.

Plasma and Annealing Treatments to Form Height-Barrier Ni-Based Schottky Contact to n-GaN

We investigated the combined effects of plasma and annealing treatments on the electrical and interfacial reactions of Ni/Au Schottky contacts on n-GaN for high-efficiency electronic and optoelectronic devices. Thermionic emission (TE)-based ideal current-voltage model showed that the Schottky barrier heights (SBHs) ranged from 0.68 to 0.92 eV and the ideality factors were in the range of 1.16–2.15. On the other hand, inhomogeneity model and capacitance-voltage characteristics gave similar SBHs in the range of 1.11–1.45 eV. X-ray photoemission spectroscopy (XPS) results displayed that annealing caused the Ga 2p core level to shift toward lower energies by 0.17–0.18 eV, while plasma treatment induced the Ga 2p core level to shift toward higher energies by 0.12 eV. The XPS Ni 2p core level results exhibited that annealing samples at 600°C gave rise to the formation of NiO and Ni2O3 in addition to Ni3Ga4 and Ni3Ga interfacial phases, which were confirmed by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) results. Based on the XPS, XRD, and STEM results, the annealing and plasma-treatment-induced increase in the SBHs is described and discussed.

(Invited) in-Vacuo Studies of MBE Grown Transition Metal Dichalcogenides

Here we present recent work on the synthesis and characterization of two-dimensional transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE). Bulk TMD crystals consist of two-dimensional layers bonded together by van der Waal interactions; the lack of primary bonds at the TMD interface allow for the growth of TMD heterostructures on a variety of substrates without the need to consider lattice constant mismatch or crystal structure of the substrate which we aim to demonstrate. TMDs are grown and characterized in an all in-vacuo ultra-high vacuum system which enables the simultaneous co-deposition of multiple transition metals and chalcogens allowing for the study of TMD ternary alloys and layered heterostructures followed by their compositional and intrinsic electronic characterization prior to atmospheric exposure. Presented will be a review of our work studying the interface chemistry between TMD and conventional semiconducting substrates, as well as our investigations of 2D-2D heterostructure grown by MBE. Figure 1 shows an example of in-vacuo x-ray photoelectron spectroscopy (XPS) carried out on GaAs(001) before an after the growth of monolayer MoSe2. The sample was initial passivated by a sulfur based treatment, and the presence of sulfur bonded to the surface can be observed in the As 2p and Ga 2p spectra. The asymmetry observed in the As 2p and Ga 2p core-levels suggest that some reactions with Se take place during the growth. The thermal stability of this interface will be investigated using in-situ UHV heating and XPS. In separate work we evaluated the valence band offsets using in-vacuo XPS and UPS (ultraviolet photoelectron spectroscopy). This is achieved by measuring core-level to valence band maximum separation for control samples as well as the core-level shifts in heterostructure. In-vacuo XPS analysis can also be used to reveal any interface reactors that may occur between the TMDs. It is found that for chalcogen deficient growths, metallic signatures can be detected. Of particular interest is the process structure relationships. Presented will be parametric studies showing impact of growth temperature and relative metal-chalcogen flux ratios on the electronic structure and interface chemistry in both the 2D-2D and 2D-3D systems. The all in-vacuo synthesis and analysis cluster tool allows these investigations to be carried out without air exposure which can induce substantial changes in the chemistry of these materials. Figure 1

Intrinsic electronic defects and multiple-atom processes in the oxidic semiconductor Ga2O3

We report on the electronic structure of gallium oxide (Ga2O3) single crystals as studied by resonant photoelectron spectroscopy (resPES). We identify intrinsic electronic defects that are formed by mixed-atomic valence states. We differentiate three coexisting defect states that differ in their electronic correlation energy and their spatial localization lengths. Their relative abundance is described by a fractional ionicity with covalent and ionic bonding contributions. For Ga2O3, our analyses of the resPES data enable us to derive two main aspects: first, experimental access is given to determine the ionicity based on the original concepts of Pauling and Phillips. Second, we report on multi-atomic energy loss processes in the Ga2p core level and X-ray absorption data. The two experimental findings can be explained consistently in the same context of mixed-atomic valence states and intrinsic electronic defects.

Inhomogeneity of barrier heights of transparent Ag/ITO Schottky contacts on n-type GaN annealed at different temperatures

We report on the formation of high barrier-height and transparent Ag/ITO Schottky contacts on n-GaN (nd = 5 × 1018 cm−3) for optoelectronic and transparent electronic devices. Calculations using the thermionic emission model-based current-voltage characteristics of the samples annealed at various temperatures showed small Schottky barrier heights (SBHs) of 0.31–0.37 eV and ideality factors of 1.84–2.19. Conventional activation energy plot showed greatly smaller Richardson constant than the theoretical value. To understand such abnormality, the modified Richardson plot, a model of lateral SBH variation with Gaussian distribution, and capacitance-voltage method were used, where their SBHs were estimated to be in the range 0.74–0.93 eV. Together with the temperature-dependent SBHs and ideality factors, these results imply that SBH behavior could be explained in terms of barrier inhomogeneity at the interfaces. The Ag/ITO samples annealed at 500 °C transmitted 80.9% at 560 nm. The X-ray photoemission spectroscopy (XPS) Ga 2p core level spectra from the interfaces of the samples shifted toward either higher or lower energies. Scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy mapping results revealed the outdiffusion of Ga atoms from n-GaN when annealed at 500 °C. Based on the electrical, XPS and STEM results, the annealing temperature dependence of the SBHs is described and discussed.

Interfacial reactions to form high-barrier-height ITO-based Schottky contacts on p-type GaN using a Ti interlayer

We report the development of high-barrier-height and transparent Ti/ITO Schottky contacts on p-GaN for optoelectronic and transparent electronic devices. The Schottky barrier heights (SBHs) and ideality factors estimated using the current–voltage characteristics were in the ranges of 0.36–0.39 eV and 1.74–2.07, respectively, depending on the annealing temperature. However, the barrier inhomogeneity and modified Richardson plot methods yielded much larger SBHs in the range of 0.82–1.18 eV. At 560 nm, the transmittance of the Ti/ITO samples was in the range of 47.4–89.9%. The Ga 2p core levels obtained from the X-ray photoemission spectroscopy (XPS) of the interface regions of the ITO/Ti/GaN samples shifted toward higher or lower energies, depending on the annealing temperature. The normalized N/Ga atomic ratio showed that N and Ga vacancies were formed at the p-GaN surface region at 300 and 500 °C, respectively. The XPS Ti 2p, N 1s, and O 1s core level results showed the formation of interfacial TiN and TiO2 phases at 300 and 500 °C, respectively. The elemental mapping results obtained using scanning transmission electron microscopy (STEM) demonstrated the outdiffusion of Ga atoms in the sample annealed at 500 °C. On the basis of XPS and STEM results, the dependence of the SBHs on the annealing temperature is described and discussed.

A Thermally Stable NiZn/Ta/Ni Scheme to Replace AuBe/Au Contacts in High-Efficiency AlGaInP-Based Light-Emitting Diodes

We developed NiZn/(Ta/)Ni ohmic contacts to replace expensive AuBe/Au contacts commonly used in high-efficiency AlGaInP-based light-emitting diodes (LEDs), and compared the electrical properties of the two contact types. Unlike the AuBe/Au (130 nm/100 nm) contact, the NiZn/Ta/Ni (130 nm/20 nm/100 nm) contact shows improved electrical properties after being annealed at 500°C, with a contact resistivity of 5.2 × 10−6 Ω cm2. LEDs with the NiZn/Ta/Ni contact exhibited a 4.4% higher output power (at 250 mW) than LEDs with the AuBe/Au contact. In contrast to the trend for the AuBe/Au contact, the Ga 2p core level for the NiZn/Ta/Ni contact shifted toward lower binding energies after being annealed at 500°C. Auger electron spectroscopy (AES) depth profiles showed that annealing the AuBe/Au samples caused the outdiffusion of both Be and P atoms into the metal contact, whereas in the NiZn/Ta/Ni samples, Zn atoms indiffused into the GaP layer. The annealing-induced electrical degradation and ohmic contact formation mechanisms are described and discussed on the basis of the results of x-ray photoemission spectroscopy and AES.

A tantalum diffusion barrier layer for improving the output performance of AlGaInP-based light-emitting diodes

We have investigated the effect of a Ta diffusion barrier layer on the electrical characteristics of AuBe/Au contacts on a p-GaP window layer for AlGaInP-based light-emitting diodes (LEDs). It was shown that after annealing at 500 °C, the AuBe/Ta/Au contacts exhibited nearly 2 orders of magnitude lower specific contact resistance (2.8 × 10−6 Ω·cm2) than the AuBe/Au contacts (1.0 × 10−4 Ω·cm2). The LEDs with and without the Ta diffusion barrier layer showed an external quantum efficiency of 14.03 and 13.5% at 50 mA, respectively. After annealing at 500 °C, the AuBe/Ta/Au contacts showed a higher reflectance (92.8% at 617 nm) than that of the AuBe/Au contacts (87.7%). X-ray photoemission spectroscopy (XPS) results showed that the Ga 2p core level for the annealed AuBe/Au samples shifted to higher binding energies, while this level shifted towards lower binding energies for the AuBe/Ta/Au samples. Depth profiles using Auger electron spectroscopy (AES) showed that annealing of the AuBe/Au samples caused the outdiffusion of both Be and P atoms into the metal contact, while for the AuBe/Ta/Au samples, the outdiffusion of Be atoms was blocked by the Ta barrier layer and more Be atoms were indiffused into GaP. The annealing-induced electrical degradation and ohmic contact formation are described and discussed based on the XPS and electrical results.

Thermally stable AuBe-based ohmic contacts to p-type GaP for AlGaInP-based light-emitting diode by using a tungsten barrier layer

We investigated how a tungsten diffusion barrier layer affected the electrical properties of AuBe/Au contacts to a p-GaP window layer (na = 5 × 1019 cm−3) for an AlGaInP-based light emitting diode. All of the as-deposited samples were ohmic. After annealing at 500 °C, the AuBe/Au contacts were electrically degraded with a specific contact resistivity of 1.0 × 10−4 Ωcm2. However, the electrical properties of the W-based contacts were improved, having a contact resistivity of 5.0 × 10−6 Ωcm2. The X-ray photoemission spectroscopy (XPS) results showed that the Ga 2p core level for the annealed AuBe/Au contacts shifted to the high binding-energy side. On the other hand, that for the AuBe/W/Au contacts shifted toward the lower binding-energy side. For the AuBe/Au contacts, both Be and P atoms were shown to be outdiffused into the metal contact after annealing. However, for the AuBe/W/Au contacts, the outdiffusion of Be atoms was prevented by the W barrier layer, and the Be atoms were indiffused into GaP. Based on the X-ray photoemission spectroscopy (XPS), Auger electron spectroscopy (AES), and electrical results, the annealing-induced electrical degradation and improvement are described and discussed.

Effects of pre-annealed ITO film on the electrical characteristics of high-reflectance Ni/Ag/Ni/Au contacts to p-type GaN

In this study, a Ni/Ag/Ni/Au multilayer with first Ni layer of 0.5nm was first optimized for high reflectivity (92.3%), low specific contact resistance (2.1×10−3Ωcm2) and good attachment strength to p-type GaN. To further decrease the contact resistance, the p-type GaN surface was previously treated with pre-annealed indium-tin-oxide (ITO) film before deposition of the Ni/Ag/Ni/Au multilayer, and resulted in a lower specific contact resistance of 1.9×10−4Ωcm2. The X-ray photoelectron spectroscopy results indicated that Ga 2p core level of the p-type GaN surface with the pre-annealed ITO film had a lower binding energy, leading to a reduction in the contact resistance. Furthermore, GaN-based flip-chip light-emitting diodes (LEDs) with and without the pre-annealed ITO film were fabricated. The average forward voltage of the flip-chip LEDs fabricated with the pre-annealed ITO film is 3.22V at an injection current density of 35A/cm2, which is much lower than that (3.49V) of flip-chip LEDs without the pre-annealed ITO film. These results reveal that the proposed approach is effectively to fabricate high quality p-type contacts toward high power GaN-based LEDs.

Formation of low resistance Ti/Al-based ohmic contacts on (11–22) semipolar n-type GaN

The electrical properties of Ti/Al-based ohmic contacts to (0001) c-plane and (11–22) semipolar n-type GaN were investigated as a function of annealing temperature. The electrical properties of both c-plane Ti/Al/Au and semi-polar Ti/(Ta)/Al/Au contacts became improved upon annealing at 600 °C for 1 min. The specific contact resistances of the 600 °C-annealed c-plane Ti/Al/Au, semi-polar Ti/Al/Au, and semipolar Ti/Ta/Al/Au contacts were 3.2 × 10−4, 1.5 × 10−4, and 4.8 × 10−5 Ωcm2, respectively. The X-ray photoemission spectroscopy (XPS) results showed that the Ga 2p core level for the c-plane samples experienced a smaller shift toward the conduction band than that for the semipolar samples. The XPS depth profile results exhibited that the semipolar samples contained more interfacial oxygen than the c-plane samples. Oxygen was present in the form of Al-oxide at the interface region, but as oxygen atoms in the GaN surface region. On the basis of the electrical and XPS results, the annealing-induced improvement of the electrical properties was described in terms of the formation of donor-like defects.

Reducing forward bias voltage of InGaN/GaN-based light emitting diodes by using (In)GaN contact layer

The electrical properties of GaN-based light emitting diodes (LEDs) fabricated with indium–tin-oxide (ITO) p-contacts were investigated as functions of the thickness of the (In)GaN contact layers and the In content. The LEDs with the GaN contact layers showed lower forward voltages (in the range of 3.48–3.03 V) than the LEDs with ITO-only contacts (4.2 V); the forward voltages of the LEDs decreased with increasing contact layer thickness (from 1–4 nm) and increasing In content. However, the output power linearly decreased with increasing In content, whereas that of the GaN contact layer LEDs became saturated at a thickness of 2 nm. The X-ray photoemission spectroscopy (XPS) Ga 2p core level for the samples with the contact layers was shifted toward lower binding energies by 0.11–0.22 eV compared with that of the sample without the contact layer. However, the energy shift decreased with increasing In content. Unlike the contact-layer samples, the sample without the contact layer experienced outdiffusion of N atoms. Based on the XPS and atomic force microscopy results, the contact-layer-induced electrical improvement was described and discussed.

Electronic and surface properties of Ga-doped In2O3 ceramics

The limit of solubility of Ga2O3 in the cubic bixbyite In2O3 phase was established by X-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepared at 1250°C. Density functional theory calculations suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga2O3 as compared to In2O3. However both diffuse reflectance spectroscopy and valence band X-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga+ surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addition photoemission reveals a broadening of the valence band edge. Core X-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chemically shifted structure in Ga 2p core XPS associated with Ga+. However experiments on ion bombarded Ga2O3, where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga+ at the surface, show that the chemical shift between Ga+ and Ga3+ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chemically shifted structure associated with Ga+ is not inconsistent with the proposal that band gap narrowing is associated with lone pair states at surfaces and interfaces.