P-type GaN Surface Research Articles

Effects of SiO2 cap annealing on MOS interfaces formed on Mg-doped p-type GaN surface

In this paper, we report the effects of 800 °C SiO2 cap annealing on the Al2O3/p-type GaN (p-GaN):Mg and SiO2/p-GaN:Mg interfaces formed at relatively low temperatures, as determined by X-ray photoelectron spectroscopy (XPS) and sub-bandgap-light-assisted capacitance–voltage (C–V) measurement. For the sample with capless annealing at 800 °C and subsequent HF treatment before the Al2O3/p-GaN interface formation by atomic layer deposition at 300 °C, its C–V characteristics indicated the existence of high-density midgap states. By SiO2 cap annealing and subsequent HF treatment to remove the cap layer, we found that the Al2O3/p-GaN interface showed a reduction in midgap state density. The same effect was confirmed at the SiO2/p-GaN interface. Taking this finding and XPS results together, we consider the possibility that SiO2 cap annealing at 800 °C and the subsequent HF treatment prior to the formation of the Al2O3/p-GaN and SiO2/p-GaN interfaces led to the reduction of interface disorder.

RGB monolithic GaInN-based μLED arrays connected via tunnel junctions

We report a 330 ppi monolithic RGB micro light-emitting diodes (μLED) array of blue, green and red GaInN-based LEDs stacked on the same wafer. Considering it is challenging to form ohmic electrodes on the plasma-etched p-type GaN surface, GaInN-based tunnel junctions were used to connect each LED, and anode electrodes for the blue and green LEDs were formed on n-type AlGaN. The fabricated stacked monolithic μLED arrays were tested at room temperature (approximately 26 °C) and DC. Each μLED emitted blue, green and red with peak wavelengths of 486, 514 and 604 nm at a current density of 50 A cm−2.

Interfacial Polarization of Thin Alq3, Gaq3, and Erq3 Films on GaN(0001).

This report presents results of research on electronic structure of three interfaces composed of organic layers of Alq3, Gaq3, or Erq3 deposited on GaN semiconductor. The formation of the interfaces and their characterization have been performed in situ under ultrahigh vacuum conditions. Thin layers have been vapor-deposited onto p-type GaN(0001) surfaces. Ultraviolet photoelectron spectroscopy (UPS) assisted by X-ray photoelectron spectroscopy (XPS) has been employed to construct the band energy diagrams of the substrate and interfaces. The highest occupied molecular orbitals (HOMOs) are found to be at 1.2, 1.7, and 2.2 eV for Alq3, Gaq3, and Erq3 layers, respectively. Alq3 layer does not change the position of the vacuum level of the substrate, in contrast to the other layers, which lower it by 0.8 eV (Gaq3) and 1.3 eV (Erq3). Interface dipoles at the phase boundaries are found to be −0.2, −0.9, −1.2 eV, respectively, for Alq3, Gaq3, Erq3 layers on GaN(0001) surfaces.

Interplay of intrinsic and extrinsic states in pinning and passivation of m-plane facets of GaN n-p-n junctions

Intrinsic and extrinsic pinning and passivation of m-plane cleavage facets of GaN n-p-n junctions were investigated by cross-sectional scanning tunneling microscopy and spectroscopy. On freshly cleaved and clean p-type GaN(101¯0) surfaces, the Fermi level is found to be extrinsically pinned by defect states, whereas n-type surfaces are intrinsically pinned by the empty surface state. For both types of doping, air exposure reduces the density of pinning states and shifts the pinning levels toward the band edges. These effects are assigned to water adsorption and dissociation, passivating intrinsic and extrinsic gap states. The revealed delicate interplay of intrinsic and extrinsic surface states at GaN(101¯0) surfaces is a critical factor for realizing flatband conditions at sidewall facets of nanowires exhibiting complex doping structures.

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.

Investigation of Fermi level pinning at semipolar (11–22) p-type GaN surfaces

Schottky barrier height (SBH; ΦB) and their dependence on the work function of metals (ΦM) at semipolar (11–22) p-GaN surfaces were investigated using Schottky diodes fabricated with different metals. The SBH increased with temperature, whereas the ideality factor decreased. This behavior was explained by means of the barrier inhomogeneity model, giving the mean barrier heights of 1.93–2.05eV for different metals. The S-parameter (dΦB/dΦM) was obtained to be 0.04. This small S-parameter implies that the surface Fermi level is nearly perfectly pinned at deep-level states (caused by vacancy-related and/or Mg-induced defects) located at 1.98eV above the valence band. This finding indicates that the surface modification is essentially required for the formation of high-quality ohmic and/or Schottky contacts.

Sb Layers on p-GaN: UPS, XPS and LEED Study

The electronic structure of p-type GaN(0001) surfaces and its modi cation by antimony adsorption, and properties of Sb/GaN(0001) interface, are presented in this report. The studies were carried out in situ by ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and low-energy electron di raction. Thin Sb layers were deposited under ultrahigh vacuum conditions onto the substrate at room temperature. Electron a nity of clean p-GaN surface amounted to 3.0 eV. A small amount of Sb on GaN(0001) surface reduced the electron a nity to 1.9 eV. The work function of the Sb layer was equal to 4.4 eV. For the Schottky barrier height of the Sb/GaN interface, the value of 2.50 eV was obtained.

Adsorption of gallium on GaN(0001) surface in ammonia-rich conditions: A new effect associated with the Fermi level position

Density functional theory (DFT) calculations were used to study GaN(0001) surface covered with NH3 admolecules and NH2 radicals, corresponding to physical conditions during GaN growth by hydride vapor phase epitaxy (HVPE) and metalorganic vapor phase epitaxy (MOVPE). Using larger representation of the surface i.e. slabs of lateral size 4×4, the effect of the doping was examined. It is shown that for specific surface coverage the electron counting (EC) rule is fulfilled so that the pinning of the Fermi level by surface states and band bending disappears. In this case, according to Krukowski et al. (2013) [14], the doping of the semiconductor (n- or p-type) and the related Fermi level are extremely important for stability of the surface and the adsorption/desorption processes. The difference in adsorption energies of gallium atoms at n- and p-type GaN(0001) surface exceeds the energy gap. This effect is observed in a narrow range of surface coverage, therefore cannot be detected in the calculations using small systems i.e. 2×2 slabs. This new phenomenon may be crucial for the growth of GaN and the incorporation of dopants and impurities into semiconductor crystals.

Enhanced Light Output of InGaN-Based Light Emitting Diodes with Roughed p-Type GaN Surface by Using Ni Nanoporous Template

Roughened surfaces of light-emitting diodes (LEDs) provide substantial improvement in light extraction efficiency. By preparing the self-assemble nanoporous Ni template through rapid annealing of a thin Ni film, followed by a low damage dry etching process, a p-side-up LED with a roughened surface has been fabricated. Compared to a conventional LED with plane surface, the light output of LEDs with nanoporous p-GaN surface increases up to 71% and 36% at applied currents of 1 mA and 20 mA, respectively. Meanwhile, the electrical characteristics are not degraded obviously after surface roughening.

Nanometer sized Ni-dot/Ag/Pt structure for high reflectance of p-type contact metal in InGaN light emitting diodes

The Ni-dot/Ag/Pt layer, where Ni-dot layer is formed of nanometer sized Ni dots, has been used to improve the reflectivity from the surface of p-type GaN in a light emitting diode (LED). Comparing with Ni/Ag/Pt layer, where Ni layer is a thin film, the Ni-dot/Ag/Pt structure shows significantly improved reflectivity with stable contact resistivity. The optical output power and external quantum efficiency of InGaN LEDs with Ni-dot/Ag/Pt structure for p-metal have improved by 28% and 29%, respectively, over the results of Ni/Ag/Pt structure.

Ab initio determination of atomic structure and energy of surface states of bare and hydrogen covered GaN (0001) surface — Existence of the Surface States Stark Effect (SSSE)

Density Functional Theory (DFT) calculations indicate that energetically stable structure of clean GaN(0001) surface posses (2 × 1) reconstruction, having every second row of Ga located near plane of N atoms, that gives rise to Ga-related dispersionless surface electronic state, already identified by angle resolved photoelectron spectroscopy (ARPES) measurements [S.S. Dhesi et al. Phys. Rev. B 56 (1997) 10271, L. Plucinski et al. Surf. Sci 507-10 (2002) 223, S. M. Widstrand et al. Surf. Sci. 584 (2005) 169]. The energy reduction in reconstruction proceeds via change of the hybridization of the occupied Ga surface states from sp 3 to sp 2, transforming the empty states to p z type. It is also shown that the electric subsurface field, modeled in new slab model which allows to simulate electric fields at the semiconductor surfaces [P. Kempisty et al., J. Appl. Phys. 106 (2009) 054901], strongly affects the energy of electronic states of GaN(0001) surfaces. The change of the field may shift the energy of surface states of bare and hydrogen covered GaN(0001) surface, by several eV with respect to the band states. The phenomenon, denoted as Surface States Stark Effect (SSSE), explains various band bending values, measured at differently doped n-type GaN(0001) surfaces. It is shown also that, for the adsorbate density up to one H atom for each Ga surface atom i.e. 1 monolayer coverage (1 ML), the hydrogen adatoms are located at the on-top positions, i.e. directly above Ga atoms. For these adsorbate densities, the H-related quantum surface state is located slightly below the valence band maximum (VBM) in the case of p-type GaN surface. For n-type GaN, the H-related surface state is located deeply in the valence band, about 2 eV below VBM. For higher, 1.25 ML hydrogen coverage, the two H adatoms create either surface attached H 2 ad-molecule (energetically stable) or triple bridge configuration is created (metastable). The H 2 ad-molecule is weekly attached to the surface, having the desorption energy barrier equal to 0.16 eV. For 1.25 ML coverage the DFT results were obtained for p-type GaN only. They show that in the ad-molecule case, a new surface electronic state arises which is located about 6.7 eV below VBM. In the case of the bridge configuration, the bridge related surface state is located closely to the conduction band minimum (CBM).

Optical Properties of the Partially Strain Relaxed InGaN/GaN Light-Emitting Diodes Induced by p-Type GaN Surface Texturing

Partial strain relaxation from the light-emitting diode (LED) with surface-textured p-GaN was observed. The textured device possesses less efficiency droop and a higher current level at the efficiency maximum, as compared with the planar one. The results suggest that surface roughening affects not only the external light extraction but also the internal quantum efficiency. Furthermore, the photoluminescent (PL) measurement at low temperature reveals that the percentage increment of the optical power of the textured LED over that of the planar LED becomes lower. In addition to the effect of frozen nonradiative defect states, the PL difference is related to the strain-correlated quantum-confined Stark effect.

Formation of Selective High Barrier Region by Inductively Coupled Plasma Treatment on GaN-Based Light-Emitting Diodes

By inductively coupled plasma (ICP) etching, a selective high barrier region (SHBR) was fabricated below the p-pad metal electrode for modifying the injection current distribution on p-type GaN of GaN-based light-emitting diodes (LEDs). Through the analysis of current noise power spectra, the samples with ICP etching treatment have excess nitrogen vacancies at the selectively etched surface of p-type GaN; thus, they have a lower hole concentration than the as-grown sample, resulting in a larger barrier height for carrier transport. With this SHBR, the light-output power for the LED chip measured at 20 mA was significantly increased by 12% as compared with that for the conventional LED chip. The light-output power increase could be attributed to a relative reduction in optical power absorption under the p-pad electrode and a higher density of current effectively injected into the active layer of the LED by the SHBR structure.

Effect of thermal annealing on the Ir/Ag contact to p-type GaN

In the present work, ohmic contacts of iridium(Ir)/silver(Ag) bi-layer contacts on p-type GaN have been fabricated. A metal Ag thin layer was selected as the capping layer due to its surface plasmon coupling and reflective characteristics at visible light emissions. The ohmic contacts were annealed at various temperatures to ascertain their thermal stability. The structural and electrical characteristics of the Ir/Ag contacts at 400 °C–700 °C were investigated, as thermally stable metal–semiconductor junction contacts are essential for high quality ohmic contact devices. The changes in the top surface morphology of the contacts on annealing were observed using scanning electron microscopy (SEM). Ohmic behavior and specific contact resistivity were determined using transmission line method (TLM). Current–voltage (I–V) measurements were carried out to verify the ohmic nature of the annealed contacts. All the as-deposited Ir/Ag contacts revealed non-linear I–V behaviors. However, the contacts become ohmic as a function of anneal temperature. The annealing induced improvement of the I–V behaviors of the Ir/Ag contacts could be related to the minimization of agglomeration of intermetallics and formation of intra-agglomerate voids. The Ir/Ag interlayer effectively prevents the void formation at the Ag–GaN interfaces. The remarkable contact resistance is attributed to the reduction of the oxide by Ir diffusion along with the formation of metal gallide phases at the p-type GaN surface region. The improved ohmic behaviors could be because of the inhomogeneous barrier formation at the contact scheme interface properties related to the breaking up of the IrO 2 thin layer. The electronic transport theory at the Ir/Ag and semiconductor interface with inhomogeneous Schottky barriers shows that the existence of the IrO 2 and IrO 2/GaN could lead to higher built in electric field at contact schemes of GaN interface.

Enhanced Light Output in Roughened GaN-Based Light-Emitting Diodes Using Electrodeless Photoelectrochemical Etching

We have demonstrated enhanced output power from roughened GaN-based light-emitting diodes (LEDs) by using electrodeless photoelectrochemical etching with a chopped source (ELPEC-CS etching). It was found that the 20-mA output power of the ELPEC-CS treated LED (with roughened surfaces on the top p-type and bottom n-type GaN surface as well as the mesa sidewall) was 1.41 and 2.57 times as high as those LEDs with a roughened p-type GaN surface and a conventional surface, respectively. The light output pattern of the ELPEC-CS treated LED was five times greater than the conventional LED at 0deg which was caused by the roughened GaN surface that improved the light extraction efficiency of the LED

Reduction of ohmic contact on p-GaN with surface treatment using Cl2 inductively coupled plasma

A reduction of Ni/Au ohmic contact on p-type GaN is obtained with surface treatment on GaN films using Cl2 inductively coupled plasma (ICP). X-ray photoelectron spectroscopy (XPS) shows the modifying of the surface atomic ratio after the Cl2 ICP treatment. The atomic ratio of nitrogen to gallium becomes larger after the Cl2 ICP treats samples for 5 s. It suggests that GaClx and/or GaOx is formed and then removed in the boiling HCl solution. The Ga vacancies at the p-type GaN surface are therefore produced and act as acceptors for holes. It leads to the reduction of the contact resistance through the decrease of the resistivity for the conduction of holes.

Electronic Properties of GaN (0001) – Dielectric Interfaces

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

Low resistivity contacts to p‐type GaN by plasma treatment

A N2-based plasma has been applied to a p-type GaN surface in order to improve contact resistivity. It has been found that the dc bias voltage of the plasma, which dictates the energies of incident ions, plays an important role in the effectiveness of the treatment. A reduction of contact resistivity has been achieved with a plasma of −75 V dc bias voltage. Excessive ion energies result in a deterioration of the contacts, as a result of plasma damage. The result is supported by XPS data, where a decrease of contact resistivity is linked to a shift of the binding energy of the Ga 3d core level towards lower energies. Such a shift indicates a reduction of the surface band bending coincident with an decrease of the surface barrier height, giving rise to a decrease contact resistance. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Band offset measurements of the Si3N4/GaN (0001) interface

X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy were used to measure electronic states as Si3N4 was deposited on clean GaN (0001) surfaces. The n-type (2×1018) and p-type (1×1017) GaN surfaces were atomically cleaned in NH3 at 860 °C, and the n-and p-type surfaces showed upward band bending of ∼0.2±0.1 eV and downward band bending of 1.1±0.1 eV, respectively, both with an electron affinity of 3.1±0.1 eV. Layers of Si (∼0.2 nm) were deposited on the clean GaN and nitrided using an electron cyclotron resonance N2 plasma at 300 °C and subsequently annealed at 650 °C for densification into a Si3N4 film. Surface analysis was performed after each step in the process, and yielded a valence band offset of 0.5±0.1 eV. Both interfaces exhibited type II band alignment where the valence band maximum of GaN lies below that of the Si3N4 valence band. The conduction band offset was deduced to be 2.4±0.1 eV, and a change of the interface dipole of 1.1±0.1 eV was observed for Si3N4/GaN interface formation.

Effects of KrF excimer laser irradiation on metal contacts to n-type and p-type GaN

Electrical properties of metal contacts on laser-irradiated n-type and p-type GaN surfaces were investigated using current–voltage, capacitance–voltage, and synchrotron radiation photoemission spectroscopy. After the irradiation of a KrF excimer laser pulse (600 mJ/cm2 at 248 nm for 38 ns) onto Si-doped GaN, a nonalloyed Ti/Al metallization formed an ohmic contact with the specific contact resistivity of 1.7×10−6 Ω cm2. The laser irradiation decomposed GaN into metallic Ga and nitrogen gas. The decomposed metallic Ga reacted with oxygen in air to form a Ga oxide layer with the thickness of ∼40 Å, producing a large number of N vacancies near the surface. The formation of a degenerated n-type GaN layer resulted in the low contact resistivity. For Mg-doped GaN, the laser irradiation increased the effective acceptor concentration. Simultaneously, the activation efficiency of Mg dopants was enhanced by the photon-assisted breaking of Mg–H bonds and/or the removal of hydrogen atoms in the presence of oxygen, producing the p-type GaN with an increased hole concentration. As a result, the contact resistivity of an oxidized Ni/Au contact could be reduced from 1.3×10−3 to 3.6×10−4 Ω cm2.