n-GaN Surface Research Articles

Investigation of atomic layer deposition methods of Al2O3 on n-GaN

In this work, three atomic layer deposition (ALD) approaches are used to deposit an Al2O3 gate insulator on n-GaN for application in vertical GaN power switches: thermal ALD (ThALD), plasma-enhanced ALD (PEALD), and their stacked combination. The latter is a novel method to yield the most ideal insulating layer. Also, the influence of an in situ NH3 or H2 plasma pre-treatment is studied. Planar MIS capacitors are used to investigate the electrical properties and robustness of the gate insulators. In vacuo x-ray photoelectron spectroscopy (XPS) is used to study the changes in chemical composition after every surface treatment. XPS shows that all plasma pre-treatments efficiently remove all carbon contamination from the surface, but only NH3 plasma is observed to additionally remove the native oxide from the n-GaN surface. The water precursor step in the ThALD process does not completely remove the CH3 ligands of the trimethylaluminum precursor step, which might electrically be associated with a reduced forward bias robustness. The O2 plasma step in the PEALD process is associated with the removal of carbon and a tremendous increase of the O content in the GaN surface region. Electrically, this strongly correlates to an enhanced forward bias robustness and an increased forward bias hysteresis, respectively. The ThALD/PEALD stack method mitigates the shortcomings of both ALD processes while maintaining its advantages. Electrical measurements indicate that the stack method alongside NH3 plasma pretreatment provides the best characteristics in terms of hysteresis, threshold voltage, forward bias robustness, and interface trap density of states.

Exploration of microstructural, chemical states and electrical features of the Au/Er2O3/n-GaN MIS diode with a Er2O3 interlayer

The study involved the formation of high-k erbium oxide (Er2O3) films on the n-GaN surface and evaluated their microstructural and chemical states by XRD, TEM and XPS approaches. By employing Er2O3 as an interlayer between the Au and n-GaN substrate, the metal/insulator/ semiconductor (MIS) diode is developed and explored its electrical features by I–V and C–V procedures. The electrical properties of the MIS diode demonstrate a better rectification nature with a lower leakage current compared with the Au/n-GaN Schottky diode (SD). The MIS diode's derived barrier height (BH) is higher than the SD, suggesting that Er2O3 plays a momentous role in altering the BH. The BH was determined using Cheung’s and Norde techniques, and the measurements showed a high degree of consistency, indicating the methods used are valid. The derived density of states (NSS) value for the MIS diode is beneath the SD, suggesting that the Er2O3 layer significantly contributes to the reduction of NSS. Under reverse bias, the PFE is a dominant current mechanism in the lower-bias, whereas the SE is the leading current mechanism in the higher-bias region of the SD. However, the PFE is the prevailing conduction mechanism in the lower-bias and higher-bias regions of the MIS diode. Outcomes of the investigation reveal that the Er2O3 thin films are favorable interfacial layers for fabricating MIS/MOS diodes.

Directive and coherent thermal emission of hybrid surface plasmon-phonon polaritons in n-GaN gratings of linear and radial shapes

Beaming and coherent thermal emission of the hybrid surface plasmon phonon polaritons (SPPhPs) was numerically and experimentally investigated employing the n-GaN surface relief gratings (SRGs) shaped in a linear and radial geometry. The polariton propagation losses were minimized numerically with the help of a rigorous coupled wave analysis model, while the spatial and temporal quality of selected mode radiation in a normal direction was maximized by fixing the grating period value at 17.5 µm and varying the grating filling factor from 75% to 25%. A set of optimal design linear and radial geometry SRG samples were fabricated in order to validate the emission characteristics of hybrid SPPhPs found by numerical modeling. We demonstrated that both efficient emission and beaming are possible to achieve through the excitation and interference of the same number but opposite sign hybrid polariton modes in n-GaN SRG.

Effect of plasma process on n-GaN surface probed with electrochemical short loop

Plasma etching treatments are important steps in GaN-based devices fabrication, but can create defects on GaN surfaces. These surface defects can strongly alter device performances. The main objective of this work is to characterize the impact of different plasma etching recipes using an innovative electrochemical short loop based on Mott-Schottky (MS) method. The effect of defects has been studied in terms of Fermi Level pinning. Barrier height has been identified as a relevant criteria to describe surface damage induced by plasma treatment. Probing Conventional LETI plasma etching process with the Mott-Schottky method demonstrates a good reproducibility of the electrochemical data and confirms the reliability of the developed method. Various electrochemical tests conducted on 3 plasmas recipes demonstrated that: 1) the RIE etching is damaging, 2) the optimized RIE (Steady A) is less damaging than the other RIE (Steady B) and 3) the RIE associated with ALE process shows the least damaging plasma recipe, as expected.

Modification of interface properties of Au/n-GaN Schottky junction by rare-earth oxide Nd2O3 as an interlayer and its microstructural characterization

We demonstrate the impact of rare-earth Nd2O3 interlayer on the electrical properties of Au/n-GaN Schottky junction (SJ), the Au/Nd2O3/n-type GaN metal/interlayer/semiconductor (MIS) junctions were prepared and characterized. X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques were employed to analyze the structural properties of the deposited Nd2O3 films on the n-GaN surface. Results confirmed that the Nd2O3 layer was formed on the n-GaN surface. The MIS junction electrical results are compared with the Au/n-GaN Schottky junction (SJ) results. Electrical outcomes demonstrated that the MIS junction revealed a good rectification behaviuor with a higher barrier height (BH) (0.66 eV) than the SJ (0.59 eV), which indicates the BH is manipulated by the Nd2O3 interlayer. The I–V, Cheung's, ΨS-V and α(V)–V plots were employed to derive BH and determined that values were similar to one another, demonstrating that the techniques used here are consistent and valid. The capacitance-frequency (C-f) and conductance-frequency (G-f) properties of SJ and MIS junction are assessed in the range of 1 kHz-1 MHz and results demonstrate that the capacitance and conductance are sturdily reliant on frequency. The estimated interface state density (NSS) of the MIS junction is less than the SJ which shows that the Nd2O3 interlayer was responsible to reduces the NSS. Results demonstrate that ohmic and space charge limited conduction mechanisms in the forward bias of the SJ and MIS junctions. Experimental outcomes established that the Schottky emission directs the reverse current in the SJ. Though, the Poole-Frenkel emission ruled reverse current in the lower bias, while the Schottky emission governs reverse current in the higher bias region of the MIS junction. Findings demonstrate that the Nd2O3 layer could be appropriate for the development of MIS/MOS devices.

Obtaining Niobium Nitride on n-GaN by Surface Mediated Nitridation Technique

In this work the n-GaN(1000) surface is used as a source of nitrogen atoms in order to obtain niobium nitride film by a surface-mediated nitridation technique. To this end, the physical vapor deposition of the niobium film on GaN is followed by sample annealing at 1123 K. A thermally induced decomposition of GaN and interfacial mixing phenomena lead to the formation of a niobium nitride compound, which contains Nb from thin film and N atoms from the substrate. The processes allowed the obtaining of ordered NbNx films on GaN. Structural and chemical properties of both the GaN substrate and NbNx films were studied in-situ by surface-sensitive techniques, i.e., X-ray and UV photoelectron spectroscopies (XPS/UPS) and a low-energy electron diffraction (LEED). Then, the NbNx/GaN surface morphology was investigated ex-situ by scanning tunneling microscopy (STM).

Effects of FeOOH Nanofilms on Photoelectrochemical Reaction Using Gallium Nitride as Photoelectrodes

This article reports a rapid and low-cost method to fabricate FeOOH nanofilms decorated on GaN epitaxial films to carry out photoelectrochemical (PEC) reactions. In reality, n-GaN epitaxial films grown on a sapphire substrate contain dense surface defects due to lattice mismatch. Consequently, n-GaN photoelectrodes suffer from corrosion during PEC reactions. An active photocatalyst, FeOOH nanofilm, was synthesized via a facile hydrothermal technique and decorated on n-GaN to passivate surface defects and suppress the recombination of photogenerated carriers. The deposited FeOOH nanofilm on the n-GaN surface can enhance the carrier transfer efficiency and kinetics of PEC reactions, as confirmed through the findings of electrochemical impedance spectroscopy and linear sweep voltammetry. Moreover, band bending between the GaN photoelectrode and the electrolyte can be affected by decoration of the FeOOH nanofilm, which was evidenced by the Mott–Schottky plot. As a result, the photocurrent of FeOOH-decorated GaN photoelectrodes was approximately four times higher than the bare GaN photoelectrodes. In addition to the improved photocurrent, the FeOOH-decorated GaN photoelectrodes exhibited a corrosion-free surface feature after a stability test. Overall, the FeOOH nanofilm is an excellent photoelectrocatalyst to strengthen the performance of n-GaN in conducting PEC reactions while protecting it from photocorrosion in PEC experiment.

NiOx nanoparticles as active water splitting catalysts for the improved photostability of a n-GaN photoanode

In this article, we propose a scheme using the conformal coating of p-NiOx nanoparticles on the n-GaN photoanode to prevent the photoelectrodes from corrosion and thereby enhance conversion efficiency. Furthermore, the various weight ratios of NiOx (1:5 and 3:2) are explored to optimize the study. NiOx nanoparticles avoid corrosion on the n-GaN surface, decrease the solar water-splitting reaction overpotential, and enhance the photocurrent density at a working electrode applied bias of 1 V in the two-electrode system. Finally, in the stability test, the hydrogen evolution rate of NiOx (3:2) is more effective than that of pristine n-GaN even after 5 h. These results indicate that NiOx nanoparticle-decorated n-GaN gains higher photostability and hydrogen production performance in PEC cells.

Effect of surface stoichiometry on the non-alloyed ohmic contact to N-face n-GaN

This letter reports a systematic study about the effect of surface stoichiometry induced by inductively coupled plasma (ICP) etching on the ohmic contact to N-face n-GaN. It is found that N-face n-GaN surface has a very high activity towards oxygen adsorption. And it is indispensable to remove GaOx layer on the N-face n-GaN surface prior to the metal deposition to realize ohmic contact. Moreover, the radio frequency power of the ICP etching and the plasma species greatly affect the surface stoichiometry and the electrical property of the ohmic contact to N-face n-GaN. X-ray photoelectron spectroscopy was implemented to characterize the surface stoichiometry of N-face n-GaN after plasma treatment and reveal the mechanism behind the phenomena. As a result, a non-alloyed ohmic contact to N-face n-GaN has been successfully achieved in a reproducible way.

Performance-enhanced vertical LED using laser irradiation treatment to control wafer-level n-GaN protrusion arrays

A controllable, mask-free, and wafer-level surface texturing method is proposed, which is applied to n-GaN protrusion array using laser irradiation treatment targeting to achieve high-performance vertical light-emitting diodes. The size and density of the n-GaN protrusions could be modified by controlling the energy density and pulse number of laser irradiation. Measurement results of current-voltage and light output power (LOP) reveal that the textured surface structure increases the LOP up to 47.89% at an injection current of 350 mA without affecting the current spread, such an enhancement is attributed to a higher probability that light could escape from the textured n-GaN surface.

Improvement of Ti/Al ohmic contacts on N-face n-type GaN by using O2 plasma treatment

The effect of O2 plasma treatment on the electrical properties of Ti/Al (20/200 nm) contact to N (Nitrogen)-face n-GaN of vertical light-emitting diodes (VLEDs) has been investigated by varying the O2 plasma treatment power and time. Both photolithography and chemical lift-off methods are used to prepare the circular transfer length method (CTLM) patterns on the N-face n-GaN for measuring the contact and sheet resistances. Both total and sheet resistances are decreased with the increase in plasma treatment power up to 250 W. It is found that the Ti/Al-based Schottky contact layer transforms to ohmic characteristics after plasma treatment. GaN chemical bonds on the n-GaN surface are broken due to O2 plasma treatment; thus, resulting in formed GaO bonds which are interactively understood by using X-ray photoelectron spectroscopy characteristics. After plasma treatment, the forward voltage of VLED is not only reduced owing to decrease in total resistance but also both the light intensity distribution and the light-output-power are improved because of increase in current spreading through the contact layer.

Microstructural and interface properties of Au/SrTiO3 (STO)/n-GaN heterojunction with an e-beam evaporated high-k STO interlayer

This paper reviews the microstructural and electrical properties of Au/n-GaN metal/semiconductor (MS) diode with an e-beam evaporated SrTiO3(STO) as an insulating layer between the Au and n-GaN substrate. The microstructural properties of STO thin films are assessed by employing XRD and TEM approaches and the results reveal that the STO thin films are formed on n-GaN surface. Then, the Au/STO/n-GaN heterojunction (HJ) type Schottky diode is fabricated and measured its electrical characteristics by current-voltage (I–V) and capacitance-voltage (C–V) techniques. The HJ exhibits lower reverse leakage current compared to the MS diode. Higher barrier height is achieved for the HJ (0.86 eV) than the MS diode (0.67 eV) with ideality factors of 1.86 and 1.21. This implies that the barrier height is modified by the STO insulating layer. Using Z(V,T)i - Vd and ΨS-V plots, the barrier heights of MS diode and HJ are also evaluated and compared with one another. The derived interface state density (NSS) of HJ is lower than the MS diode, demonstrating that the STO layer plays a substantial role in the reduced NSS. Results suggest that the STO thin film is a promising high-k material for the development of new electronic device applications.

Study of electronic properties on the n-GaN (0001) surface with points defects

The influence of defects on the surface of the semiconductor is irreversible. The influence of intrinsic point defects on the electronic properties of n-doped GaN (0001) surface is studied based on the first principles. The results show that, the N interstitial defect (Ni) and Ga Vacancy (VGa) are the more easily formed in the case of Si doping. The defect level generated by an appropriate amount of defects contributes to the transition of electrons, thereby improving the n-type conductivity characteristics. In particular, the Ga vacancy makes the work function drop significantly, which promotes the emission of electrons. However, once the defects inside the material exceed a certain level, any defects will play a counterproductive role. This paper could provide some guidance for the preparation of n-GaN optoelectronic devices.

Accurate surface band bending determination on Ga-polar n-type GaN films by fitting x-ray valence band photoemission spectrum

The surface band bending in Ga-polar n-type GaN surfaces, as well as the effect of Si doping levels and in situ Ar+ ion processing on band bending, was systematically investigated. To precisely determine the valence band maximum (VBM) of GaN beyond instrumental and material surface environments by XPS, a valence band feature fitting procedure based on photoemission spectra and theoretical densities of states has been developed. Poisson calculation with quadratic depletion approximation on surface potential has been used to model the band bending and further correct the VBM energy. Then, the actual surface band bending was correctly evaluated. Upward band bending of 1.55 ± 0.03 eV with highly Si doped n-GaN, which is about 0.88 eV higher than that of the moderately doped sample, was found. After in situ Ar+ plasma treatment, the varying degree of band bending was observed distinctly depending on the Si doping density. The surface components associated with the Ga/N ratio and Ga–O bonding concentration on the n-GaN surface have been used to evaluate the contribution to surface band bending.

Influence of reactive-ion-etching depth on interface properties in Al2O3/n-GaN MOS diodes

Influence of etching depth on the interface properties in Al2O3/n-GaN MOS diodes has been investigated. The n-GaN surface was etched by inductively coupled reactive-ion-etching (RIE) with shallow (20 nm) and deep etching (1200 nm) depth. The resulting Al2O3/n-GaN interface properties were then evaluated by the capacitance/conductance–voltage (C/G–V) and current–voltage (I–V) characteristics measurements. It was found that: (i) the interface state density of the shallow-etched MOS diode is almost the same as that of the un-etched reference sample, (ii) the deep-etched MOS diode showed around five times higher interface state density than the shallow-etched MOS diode, (iii) the increased interface state density with deep etching was not recovered by Tetra-Methyl-Ammonium Hydroxide (TMAH) treatment nor by post annealing, and (iv) the deep-etched MOS diode indicated about five orders of magnitude higher leakage current, which could be attributed to the lowering of the effective barrier height in the Fowler–Nordheim type conduction. The results obtained in this study indicate that improved RIE process has to be developed for the fabrication of GaN MOS FETs that require deep etching such as vertical trench-type FETs.

High-Power GaN-Based Vertical Light-Emitting Diodes on 4-Inch Silicon Substrate.

We demonstrate high-power GaN-based vertical light-emitting diodes (LEDs) (VLEDs) on a 4-inch silicon substrate and flip-chip LEDs on a sapphire substrate. The GaN-based VLEDs were transferred onto the silicon substrate by using the Au–In eutectic bonding technique in combination with the laser lift-off (LLO) process. The silicon substrate with high thermal conductivity can provide a satisfactory path for heat dissipation of VLEDs. The nitrogen polar n-GaN surface was textured by KOH solution, which not only improved light extract efficiency (LEE) but also broke down Fabry–Pérot interference in VLEDs. As a result, a near Lambertian emission pattern was obtained in a VLED. To improve current spreading, the ring-shaped n-electrode was uniformly distributed over the entire VLED. Our combined numerical and experimental results revealed that the VLED exhibited superior heat dissipation and current spreading performance over a flip-chip LED (FCLED). As a result, under 350 mA injection current, the forward voltage of the VLED was 0.36 V lower than that of the FCLED, while the light output power (LOP) of the VLED was 3.7% higher than that of the FCLED. The LOP of the FCLED saturated at 1280 mA, but the light output saturation did not appear in the VLED.

Free-standing and ultrathin inorganic light-emitting diode array

We report on the fabrication and characteristics of an individually addressable gallium nitride (GaN) microdisk light-emitting diode (LED) array in free-standing and ultrathin form. A high-quality GaN microdisk array with n-GaN, InGaN/GaN quantum wells and p-GaN layers was epitaxially grown on graphene microdots patterned on SiO2/Si substrates. Due to the weak attachment of the graphene microdots to the growth substrate, a microdisk array coated with a polyimide layer was easily separated from the substrate using mechanical or chemical methods to form an ultrathin free-standing film. Individually addressable microdisk LEDs were created by forming thin metal contacts on the p-GaN and n-GaN surfaces in a crossbar configuration. Each microdisk LED that comprised an ultrahigh resolution array of 2500 pixels per inch was found to be uniquely addressable. The devices in free-standing form exhibited stable electrical and optoelectronic characteristics under extreme bending conditions and continuous operation mode despite the absence of a heat dissipating substrate. These results present promising approaches for the fabrication of high-quality inorganic semiconductor devices for ultrahigh resolution and high-performance flexible applications.

Research on photoelectric properties of n-GaN (0001) surface with point defects via first-principles

In order to reveal the influence of n-type doping and point defects on the electronic and optical properties of GaN (0001) surface, the pristine and defective n-GaN (0001) surface models are established. The formation energy, work function, atomic structure, electronic and optical properties of these surfaces are discussed by first-principles calculations. The results show that the defects have the strong tendency to be close to the surface position, and the N interstitial defect (Ni) and the Ga vacancy (VGa) are easily formed. The work function of the surface with VGa drops the greatest, and the conductivity of n-type increases. In addition, the absorption coefficient of the surface with various defects is smaller than that of the pristine n-type surface. The appropriate amount of Ga vacancies can increase the probability of electron emission, but it is not conducive to the absorption of photons.

Mapping of damage induced by neutral beam etching on GaN surfaces using scanning internal photoemission microscopy

We applied the method of scanning internal photoemission microscopy (SIPM) to characterize selectively neutral-beam-etched damage, which is much finer than that associated with inductively coupled plasma etching and ion implantation. We used a neutral beam to selectively etch the MOCVD-grown n-GaN surface using Cl2 gas, and formed Ni Schottky electrodes so that the etching patterns were included in the electrode areas. SIPM measurements were conducted with laser beams whose photon energies were below the bandgap (λ = 405, 517, 660 nm). Photocurrent was detected at each wavelength and a two-dimensional image of photoyield (Y) was obtained; an image of Schottky barrier height (qΦB) was also obtained according to the Fowler plot. We clearly observed etching patterns in the Y maps of all the wavelengths. SIPM can sensitively visualize surface damage because etched regions can be compared with unetched regions on the same electrode. In the qΦB map, we found that qΦB in the etched region increased slightly by 0.1 eV. These results indicate that SIPM is effective for mapping the variation created by etching with high sensitivity.

Structural, Chemical and Electrical Properties of Au/La2O3/n-GaN MIS Junction with a High-k Lanthanum Oxide Insulating Layer

This paper demonstrates the role of high-k La2O3 on the electrical performance of the Au/n-GaN Schottky junction (SJ) as an insulating layer between the Au and n-GaN films. First, the La2O3 is deposited on a n-type GaN surface by e-beam technique and analysed for its structural and chemical properties with x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) approaches. XRD and XPS results confirmed the growth of La2O3 on the n-GaN surface. Then, the Au/La2O3/n-GaN metal/insulator/semiconductor (MIS) junction is fabricated and analysed for its electrical properties and compared with the SJ electrical results. The MIS junction exhibits a good rectifying nature with a low leakage current compared to the SJ. Experimental findings reveal a higher barrier height obtained for the MIS junction than the SJ, suggesting that the barrier height is altered by the La2O3 insulating layer. Also, the barrier heights are estimated by Cheung’s, Norde functions and ΨS–V plot, and the values are nearly matched with each other, indicating the techniques used here are valid. The extracted interface state density (NSS) of MIS junction is lower than the SJ, implying the La2O3 insulating layer plays a vital role in the decreased NSS. Experimental findings confirmed that the Schottky emission governs the reverse current in SJ. However, for the MIS junction, the Poole–Frenkel and Schottky emissions are the dominant current conduction mechanisms in the lower and higher bias regions.