Ga 3d Core Level Research Articles

Effects of ammonia and oxygen plasma treatment on interface traps in AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors

A HfO2/Al2O3/AlN stack deposited in a plasma-enhanced atomic layer deposition system is used as the gate dielectric for GaN metal–insulator–semiconductor structures. Prior to the deposition, in situ remote plasma pretreatments (RPPs) with O2, NH3, and NH3–O2 (in sequence) were carried out. Frequency-dependent capacitance–voltage (C–V) measurements reveal that for all the RPP-treated samples, the interface trap density Dit was significantly reduced compared to the sample without RPP. In particular, the NH3-treated sample exhibits an extremely low Dit of ∼1011 cm−2·eV−1 from EC − 0.31 eV to EC − 0.46 eV. To characterize the interface traps at deeper levels, the transfer pulsed I–V measurement was employed further. In the energy range of EC − ET > 0.54 eV, the interface trap charge density Qit of various samples demonstrates the following order: NH3 RPP < NH3–O2 RPP < O2 RPP < without RPP. By adjusting multiple pulse widths, the Dit derived from the Qit result matches the C–V measurement precisely. Moreover, X-ray photoelectron spectroscopy analysis of the Ga 3d core-level indicates that NH3 RPP facilitates the conversion of Ga2O into GaN at high RF power of 2500 W, whereas O2 RPP mainly oxidizes GaN to relatively stable Ga2O3. Combined with the C–V and pulsed I–V measurements, it is confirmed that a strong positive correlation exists between Ga2O and the interface traps, rather than Ga2O3.

Suppression of current dispersion in AlGaN/GaN MISHFETs with in-situ AlN passivation layer

We demonstrate effective reduction of interface states and current collapse in AlGaN/GaN metal insulator semiconductor heterostructure field effect transistor (MISHFETs) with Al2O3 and in-situ AlN passivation layer. Here, first, a 3 nm-thick in-situ AlN layer is grown on AlGaN/GaN structure by using MOCVD at 1070 °C, and second a 7 nm-thick atomic layer deposition (ALD) Al2O3 layer is immediately deposited on the AlN layer. The X-ray photoelectron spectroscopy (XPS) analyses for the Ga 3d core level and multi-frequency capacitance–voltage (C–V) measurement show that the density of interface states originated from the Ga-O bonds, which is dramatically reduced with the in-situ AlN passivation layer. The AlGaN/GaN-based MISHFET with the Al2O3/in-situ AlN layers exhibits the low gate-lag effect (8%), remarkably less than that of the device with the typical GaN capping layer (21.5%). These results indicate that the Al2O3/in-situ AlN double passivation layer is very effective in suppressing the current collapse, and hence, it is found to be the promising method for the fabrication of AlGaN/GaN MISHFETs with enhanced performance.

Tuning of ferromagnetic behavior of GaN films by N ion implantation: An experimental and first principle-based study

Magnetic properties of N-ion implanted GaN films (150 nm) have been reported. It is found that GaN films grown by the MOCVD technique show strong room temperature ferromagnetic behavior, which can be tuned by implanting N-ions at different fluences (1 × 1015 to 5 × 1016 ions-cm−2). Presence of implanted N at interstitial sites of the GaN host matrix is indicated from the strain observed in GaN by analysis of XRD data. PL spectra show presence of different types of defects in the as deposited film and engineering of defects after N-ion implantation. XPS spectra of Ga 3d-core level and valence band reveal the bonding of implanted N with the host Ga and/or N. The origin of ferromagnetic behavior is ascribed to unpaired electrons created at N sites due to Ga vacancies. First principle-based calculations also confirm ferromagnetism due to Ga vacancies and the reduction of magnetic behavior in Ga deficient GaN with N-ion implantation at interstitial site. The systematic reduction in the saturation magnetic moment value after N-ion implantation is explained on the basis of pairing of the unpaired electrons due to the bond formation of interstitial N with Ga and N present in the host matrix.

Effect of ion irradiation on GaAs core-level electron binding energies and band structure

Electronic structure and chemical composition of GaAs-based semiconductors are considered to be well characterized by photoelectron spectroscopy and other surface-sensitive methods. However, effect of ion irradiation on electron binding energies and band structure of semiconductors can drastically distort results of diagnostics. This effect was studied by synchrotron-based XPS applied to n-GaAs wafer after 1250 eV - Ar+ ion exposure with fluence Q ~ 1 × 1015 ions/cm2 which is typical for preparation of atomically clean surfaces. Mechanical action of ions was shown to change the n-GaAs electron binding energies by the value of the bandgap width due to creation of defect states, and conversion of the conductivity type from n to p. The Ga3d and As3d core-level binding energies for p-GaAs and n-GaAs were measured in one experiment on the p-n plane structure formed due to irradiation: EB(p/n) = 19.3/20.4 eV and EB(p/n) = 41.3/42.4 eV. The p-layer nanothickness was determined and the band diagram of the p-n GaAs structure was constructed. The revealed effect may be comparable with core-level chemical shifts and should be taken into account to avoid mistakes in the XPS chemical composition diagnostic of GaAs-based semiconductors.

Swift heavy ion irradiation-induced modifications in the electrical and surface properties of β-Ga2O3

The electrical device characteristics of Ni/β-Ga2O3 vertical Schottky barrier diodes (SBDs) were measured in situ during the irradiation of 120 MeV Ag7+ swift heavy ions (SHIs). These devices exhibit SHI irradiation-induced degradation with 120 MeV Ag7+ ions in the ion fluence ranges of 1 × 1010 to 1 × 1012 ions/cm2. The height of the Schottky barrier is found to decrease from 1.11 to 0.93 eV, and the ideality factor increases from 1.16 to 2.06. These changes indicate the degradation of the device with SHI irradiation. A significant four orders increase is observed in the leakage current density from 4.04 × 10−8 to 1.98 × 10−4 A/cm2 at −1 V, and the series resistance also increases from 3.38 × 103 to 1.15 × 104 Ω. X-ray photoelectron spectroscopy measurements show that the Ga ions are present in divalent and trivalent states with the spectral features having the binding energies centered at 20.2 eV and 19.9 eV (Ga 3d core-levels) before and after ion irradiation. The O 2s peak shifts to 23.7 eV, and there is an increase in intensity and peak broadening due to the change in the trivalent to divalent state of Ga due to the irradiation. The O(I) peak appears at 530.7 eV in the pristine sample with the Ga–O bonding with the Ga3+ state in pure Ga2O3. Moreover, there is a significant change in the intensity and the peak width of O(II) centered at 533.0 eV after ion irradiation at the fluence of 1 × 1012 ions/cm2. This indicates that there is an increase in the surface adsorbed/lattice oxygen, resulting in GaO.

Intersubband Absorption in Gallium Arsenide Implanted with Silicon Negative Ions

Gallium arsenide (GaAs) implanted with silicon forming intersubband of SiGaAs is a promising material for making novel electronic and optoelectronic devices. This paper is focused on finding optimum fluence condition for formation of intersubband of SiGaAs in GaAs sample after implantation with 50[Formula: see text]keV silicon negative ions with fluences varying between [Formula: see text] and [Formula: see text] ions cm[Formula: see text]. The GaAs samples were investigated using X-ray photoelectron spectroscopy (XPS), UV-Vis.-NIR spectroscopy and X-ray diffraction (XRD) techniques. The X-ray photoelectron spectra for unimplanted sample showed peaks at binding energy of 18.74[Formula: see text]eV and 40.74[Formula: see text]eV indicating Ga3d and As3d core level, whereas the corresponding core level peaks for implanted sample were observed at binding energy of 19.25[Formula: see text]eV and 41.32[Formula: see text]eV. The shift in Ga3d and As3d core levels towards higher binding energy side in the implanted sample with respect to unimplanted sample were indicative of change in chemical state environment of Ga–As bond. The relative atomic percentage concentration of elemental composition measured using casa XPS software showed change in As/Ga ratio from 0.89 for unimplanted sample to 1.13 for sample implanted with the fluence of [Formula: see text] ion cm[Formula: see text]. The UV-Vis-NIR spectra showed absorption band between 1.365[Formula: see text]eV and 1.375[Formula: see text]eV due to the formation of intersubband of SiGaAs for fluences greater than [Formula: see text] ion cm[Formula: see text]. The GaAs crystallite size calculated using Brus formula was found to vary between 162[Formula: see text]nm and 540[Formula: see text]nm, respectively. The XRD spectra showed the presence of Bragg’s peak at 53.98∘ indicating (311) silicon reflection. The silicon crystallite size determined from full width at half maxima (FWHM) of (311) XRD peak was found to vary between 110[Formula: see text]nm and 161[Formula: see text]nm, respectively.

Precise determination of surface band bending in Ga-polar n-GaN films by angular dependent X-Ray photoemission spectroscopy

We present a systematic study of surface band bending in Ga-polar n-GaN with different Si doping concentrations by angular dependent X-ray photoelectron spectroscopy (ADXPS). The binding energies of Ga 3d and N 1 s core levels in n-GaN films increase with increasing the emission angle, i. e., the probing depth, suggesting an upward surface band bending. By fitting the Ga 3d core level spectra at different emission angles and considering the integrated effect of electrostatic potential, the core level energy at the topmost surface layer is well corrected, therefore, the surface band bending is precisely evaluated. For moderately doped GaN, the electrostatic potential can be reflected by the simply linear potential approximation. However, for highly doped GaN samples, in which the photoelectron depth is comparable to the width of the space charge region, quadratic depletion approximation was used for the electrostatic potential to better understand the surface band bending effect. Our work improves the knowledge of surface band bending determination by ADXPS and also paves the way for studying the band bending effect in the interface of GaN based heterostructures.

Temperature-Dependent Electrical Characteristics of β-Ga2O3 Diodes with W Schottky Contacts up to 500°C

There is a rapidly growing need for high performance power switching semiconductors in applications such as renewable energy systems, smart grids, transportation electrification, electric and hybrid electric vehicles, industrial machinery control, electricity production, and military systems. Interest in even more efficient, lightweight, and reliable power converters, based on ultra-wide-bandgap (WBG) semiconductors has led to Ga2O3 drawing significant attention due to its large breakdown field of around 8 MV.cm-1. Switches fabricated from wide bandgap semiconductors such as Ga2O3 have a higher power density, better efficiency, and thermal tolerance compared to Si, which can reduce the need for expensive bulky cooling systems. A major drawback of Ga2O3 is its low thermal conductivity. Thermally stable Schottky contacts for these rectifiers are also needed to ensure stability in power conversion systems. In this study, we have measured the electrical characteristics of sputter-deposited W Schottky contacts with Au overlayers for reducing sheet resistance on n-type Ga2O3 before and after device operation up to 500°C. Assuming thermionic emission is dominant, the extracted barrier height decreases with measurement temperature from 0.97 eV (25°C) to 0.39 eV (500°C) while showing little change from its initial value of 0.97 eV after cooling down from each respective operation temperature. The room temperature value is comparable to that obtained by determining the energy difference between binding energy of the Ga 3d core level and the valence band of the Ga2O3 when W is present, 0.80 ± 0.2 eV in this case. The Richardson constant was 54.05 A.cm-2.K-2 for W and the effective Schottky barrier height at zero bias (eΦb0) was 0.92 eV from temperature-dependent current-voltage characteristics. The temperature coefficient for reverse breakdown voltage was 0.16 V/K for W/Au and 0.12 V/K for Ni/Au. The W-based contacts are more thermally stable than conventional Ni-based Schottkies on Ga2O3 but do have more reverse leakage current at higher operating temperatures.

On the presence of Ga2O sub-oxide in high-pressure water vapor annealed AlGaN surface by combined XPS and first-principles methods

We conducted X-ray photoelectron spectroscopy (XPS) and first-principles calculations based on density functional theory (DFT) to confirm the presence of Ga2O sub-oxide in high-pressure water vapor annealed AlGaN surface. We note that the Ga 3d XPS peak broadens and shifts towards higher binding energies, which suggests surface oxide formation. Deconvoluted Ga 3d XPS profiles between HPWVA-treated and reference samples reveal reasonable inclusion of Ga2O peak, suggesting formation of Ga2O sub-oxide. To theoretically confirm the presence of Ga2O, we calculated the Ga 3d core-level shift using initial state approximation. We obtained a 0.74 eV shift, in reasonable agreement with that of Ga2O. Moreover, based on the calculation of net charge on Ga using DFT, we also obtained a +1 oxidation state for Ga, indicating its existence in Ga2O form. By combining theory and experiment, therefore, we have explored the possibility of the formation of Ga2O sub-oxide, which may provide new avenues for obtaining highly stable operation in GaN-based devices.

Temperature-Dependent Electrical Characteristics of β-Ga2O3 Diodes with W Schottky Contacts up to 500°C

The development of thermally stable contacts capable of high temperature operation are necessary for Ga2O3 high power rectifiers. We have measured the electrical characteristics of sputter-deposited W Schottky contacts with Au overlayers for reducing sheet resistance on n-type Ga2O3 before and after device operation up to 500°C. Assuming thermionic emission is dominant, the extracted barrier height decreases with measurement temperature from 0.97 eV (25°C) to 0.39 eV (500°C) while showing little change from its initial value of 0.97 eV after cooling down from each respective operation temperature. The room temperature value is comparable to that obtained by determining the energy difference between binding energy of the Ga 3d core level and the valence band of the Ga2O3 when W is present, 0.80 ± 0.2 eV in this case. The Richardson constant was 54.05 A.cm−2.K−2 for W and the effective Schottky barrier height at zero bias (eΦb0) was 0.92 eV from temperature-dependent current-voltage characteristics. The temperature coefficient for reverse breakdown voltage was 0.16 V/K for W/Au and 0.12 V/K for Ni/Au. The W-based contacts are more thermally stable than conventional Ni-based Schottkies on Ga2O3 but do show evidence of Ga migration through the contact after 500°C device operation.

Ultrafast time-resolved extreme ultraviolet (XUV) photoelectron spectroscopy of hole transfer in a Zn/n-GaP Schottky junction.

The addition of a metal overlayer to a semiconductor photocatalyst is a frequently used synthetic route to passivate the surface and, via the formation of a Schottky barrier, to enhance catalytic activity of the photocatalyst material. While it is known that Schottky junctions decrease recombination by charge separation, measurements of the depletion region dynamics have remained elusive. Here, we use ultrafast pump-probe transient photoelectron spectroscopy to measure material-specific dynamics of the Zn/n-GaP(100) system. Through photoemission measurements the Schottky barrier height is determined to be 2.1 ± 0.1 eV at 10 monolayers of total Zn deposition. Transient photoemission measurements utilizing a 400 nm pump pulse show that, after excitation, holes are transferred from n-GaP(100) to the Zn overlayer within a few ps, as evidenced by shifts of the Zn 3d and Ga 3d core levels to higher binding energies. Within the timescale of the experiment (130 ps) no carrier recombination is observed in the junction. Furthermore, a long-lived surface photovoltage signal is observed at times >1 ms after photoexcitation. This work further exemplifies the potential of transient extreme ultraviolet photoelectron spectroscopy as a material-specific technique for the study of heterojunctions.

Theoretical and experimental investigation of the electronic properties of the wide band-gap transparent semiconductor MgGa2O4

We present an experimental and theoretical study of the electronic structure of the transparent conductive oxide semiconductor ${\mathrm{MgGa}}_{2}{\mathrm{O}}_{4}$. Its valence band and the core levels have been measured experimentally by angle-resolved photoemission spectroscopy and compared to theoretical ab initio density-functional calculations. The bands stem from oxygen orbitals and have a dispersion of about 0.6 eV and high effective masses, in agreement with the calculations. Angle-resolved measurements of the Ga 3d core levels indicate a sizable upward band bending, which in combination with an exact model potential yielded a quantitative estimate of the spatial extension of the depletion region, the effective Debye length in the material, and the extrinsic bulk carrier density.

Studies of early stages of Mn/GaN(0001) interface formation using surface-sensitive techniques

Non-doped GaN(0001) crystals are used as substrates in this study, on which Mn films are vapour deposited in situ under ultrahigh vacuum (UHV). The early stages of the Mn/GaN(0001) interface formation at room temperature are determined by means of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). The electron affinity for the already cleaned GaN(0001)-(1 × 1) surface, achieved by thermal cleaning, is 3.5 eV. The binding energy (BE) of the Ga-3d core level line shifts from 20.3 eV to 19.8 eV and the Mn-3p state from 47.6 eV to 47.2 eV upon stepwise Mn deposition due to charge transfer, e.g. Schottky barrier (SB) formation. The splitting of the Mn-2p doublet amounts to 11.2 eV and the Mn-2p3/2 peak has a BE of 638.7 eV, these values corresponding to metallic manganese. The work function of the Mn films with the thickness of 1 nm is 3.4 eV and slightly decreases to 3.2 eV with further overlayer thickness. The SB height of the interface is determined to be 1.2 eV. The Mn films show no LEED patterns. The XPS results, generally, show the absence of any interfacial compound formation.

Electronic structure of the PLD grown mixed phase MoS2/GaN interface and its thermal annealing effect

We report the electronic structure of Molybdenum disulfide (MoS2) ultrathin 2D films grown by pulsed laser deposition (PLD) on top of GaN/c-Al2O3 (0001) substrates annealed up to 550 °C in an ultrahigh vacuum. Our X-ray photoemission spectroscopy (XPS) study shows that the grown films are mixed phase character with semiconducting 2H and metallic 1T phases. After ultrahigh vacuum (UHV) annealing, the 1T/2H phase ratio is significantly modified and film-substrate bonding becomes the leading factor influencing variation of mixed phase compositions. The semiconducting phase is partially transformed to metallic phase by thermal annealing; suggesting that the metallic phase observed here may indeed have more stability compared to the semiconducting phase. The notable enhancement of the 1T/2H ratio induces significant changes in Ga 3d core level spectra taken from bare GaN and MoS2/GaN sample. The impact of S and/or Mo atoms on the Ga core level spectra is further pronounced with the thermal annealing of grown films. The analysis shows that an enhancement of 1T metallic phase with thermal annealing in MoS2 layers is manifested by the occurrence of new spectral component in the Ga 3d core level spectra with the formation of Ga-S adlayer interaction through the Ga bonding in defect assisted GaN structure.

Electron band bending and surface sensitivity: X-ray photoelectron spectroscopy of polar GaN surfaces

The role of electron band bending and surface sensitivity in determining the core level binding energies by X-ray photoelectron spectroscopy is investigated. A dominating contribution of surface atomic layers to photoemission intensity is confirmed for normal photoemission. The energy of the photoelectron core level peak does not deviate from core level peak energies of electrons photoemitted from the surface atomic layers of the crystal. The higher surface sensitivity regime, achieved e.g. at off-normal photoelectron detection angle, can be used to study the surface potential barrier in just a few topmost atomic layers. In addition, it is demonstrated that core level binding energy measured by angle-resolved X-ray photoelectron spectroscopy reflect the electron attenuation anisotropy. In particular, core level binding energy changes with emission angle and correlates with the forward focusing directions in a crystal. This effect is demonstrated by measuring the polar angle dependence of Ga 3d core levels on clean GaN(0001) and GaN(0001¯) surfaces with a higher and a lower band bending, respectively. The effect is explained by variation of emission depth in a crystal for normal and off-normal photoelectron emission angles.

Role of surface energy on the morphology and optical properties of GaP micro & nano structures grown on polar and non-polar substrates

Role of surface energy on the morphology, crystalline quality, electronic structure and optical properties of GaP layer grown on Si (001), Si (111), Ge (111) and GaAs (001) is investigated. GaP layers are grown on four different substrates under identical growth kinetics by metal organic vapour phase epitaxy. The atomic force microscopy images show that GaP layer completely covers the surface of GaAs substrate. On the other hand, the surfaces of Si (001), Si (111), Ge (111) substrates are partially covered with crystallographically morphed GaP island type micro and nano-structures. Origin of these crystallographically morphed GaP island is explained by the theoretical calculation of surface energy of the layer and corresponding substrates respectively. The nature of GaP island type micro and nano-structures and layers are single crystalline with existence of rotational twins on Si and Ge (111) substrates which is confirmed by the phi, omega and omega/2theta scans of high resolution x-ray diffraction. The electronic valence band offsets between the GaP and substrates have been determined from the valence band spectra of ultraviolet photoelectron spectroscopy. The valence electron plasmon of GaP are investigated by studying the energy values of Ga (3d) core level along with loss peaks in the energy dependent photoelectron spectra. The peak observed within the range of 3–6eV from the Ga (3d) core level in the photoelectron spectra are associated to inter band transitions as their energy values are estimated from the pseudo dielectric function by the spectroscopic ellipsometry.

Electron band bending of polar, semipolar and non-polar GaN surfaces

The magnitudes of the surface band bending have been determined by X-ray photoelectron spectroscopy for polar, semipolar, and non-polar surfaces of wurtzite GaN crystals. All surfaces have been prepared from crystalline GaN samples grown by the hydride-vapour phase epitaxy and separated from sapphire substrates. The Ga 3d core level peak shifts have been used for band bending determination. Small band bending magnitudes and also relatively small difference between the band bendings of the surfaces with opposite polarity have been found. These results point to the presence of electron surface states of different amounts and types on surfaces of different polarity and confirm the important role of the electron surface states in compensation of the bound surface polarity charges in wurtzite GaN crystals.

Modification of band offsets of InGaZnO4/Si heterojunction through nitrogenation treatment

The effect of nitrogen on the band offset of sputtering-derived InZnGaO4 (IGZO)/Si heterostructures has been systematically investigated by x-ray photoelectron spectroscopy (XPS) measurements. Elemental analysis indicates that nitrogen has been successfully incorporated into the IGZO film. By using In 3d5/2, In 3d3/2 and Ga 3d core level (CL) XPS spectra as references, values of valence band offsets (ΔEv) of have been determined to be 2.56 ± 0.02 and 2.44 ± 0.02 eV for IGZO/Si and IGZO:N/Si heterojunctions, respectively. Using the experimental band gap of 3.59 and 3.50 eV of the IGZO/Si and IGZO:N/Si, the calculated values of conduction band offset (ΔEc) is 0.09 ± 0.01 and 0.06 ± 0.01 eV, respectively. The results indicate that nitrogen incorporation leads to the reduction in band gap and ΔEv and the slight effect on the ΔEc has also been detected.

Single Crystal Perovskites Analyzed Using X-ray Photoelectron Spectroscopy: 5. NdGaO3(110)

X-ray photoelectron spectroscopy (XPS) was used to analyze a commercially available NdGaO3 (110) bulk single crystal. XP spectra were obtained using incident monochromatic Al Kα radiation at 0.83401 nm. A survey spectrum together with Ga 2p, Nd 3d, O 1s, C 1s, Nd 4p3/2, Ga 3s, Nd 4d, Ga 3p, Nd 5s, O 2s, Ga 3d and Nd 5p core level spectra and the valence band are presented. The spectra indicate the principle core level photoelectron and Auger electron signals and show only minor carbon contamination. Making use of the O 1s, Nd 4d, Ga 3p lines and neglecting the components related to surface contaminants, XPS quantitative analysis reveals an altered stoichiometry of the air-exposed crystal surface of Nd1.13GaO2.17.

Band alignment between GaN and ZrO2 formed by atomic layer deposition

The band alignment between Ga-face GaN and atomic-layer-deposited ZrO2 was investigated using X-ray photoelectron spectroscopy (XPS). The dependence of Ga 3d and Zr 3d core-level positions on the take-off angles indicated upward band bending at GaN surface and potential gradient in ZrO2 layer. Based on angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity ΔEV of 1 ± 0.2 eV and conduction band discontinuity ΔEC of 1.2 ± 0.2 eV at ZrO2/GaN interface were determined by taking GaN surface band bending and potential gradient in ZrO2 layer into account.