Ga-polar Surface Research Articles

Charge-Controlled Energy Optimization of the Reconstruction of Semiconductor Surfaces: sp3-sp2 Transformation of Stoichiometric GaN(0001) Surface to (4 × 4) Pattern.

It was demonstrated by ab initio calculations that energy optimization in the reconstruction of semiconductor surfaces is controlled by the global charge balance. The charge control was discovered during simulations of the influence of heavy doping in the GaN bulk, which changes sp3 to sp2 ratio in the reconstruction of stoichiometric GaN(0001), i.e., a Ga-polar surface. Thus, the reconstruction is not limited to the charge in the surface only; it can be affected by the charge in the bulk. The discovered new reconstruction of the GaN(0001) surface is (4 × 4), which is different from the previously reported (2 × 1) pattern. The undoped GaN reconstruction is surface charge controlled; accordingly, (3/8) top-layer Ga atoms remain in a standard position with sp3 hybridized bonding, while the remaining (5/8) top-layer Ga atoms are shifted into the plane of N atoms with sp2 hybridized bonding. The change in the charge balance caused by doping in the bulk leads to a change or disappearance of the reconstruction pattern.

Effects of UV/O3 and O2 plasma surface treatments on the band-bending of ultrathin ALD-Al2O3 coated Ga-polar GaN

The recently demonstrated semiconductor grafting approach allows one to create an abrupt, low interface-trap-density heterojunction between a high-quality p-type single-crystalline semiconductor (non-nitrides) with n-type GaN. However, due to the surface band-bending from GaN polarization, an energy barrier exists at the grafted heterojunction, which can impact the vertical charge carrier transport. Reducing the energy barrier height is essential for some advanced device development. In this work, we employed UV/O3 and O2 plasma to treat a Ga-polar GaN surface with/without an ultrathin (∼2 nm) ALD-Al2O3 coating and studied the effects of the treatments on surface band-bending. Through XPS measurements, it was found that the treatments can suppress the upward band-bending of the Ga-polar GaN by 0.11–0.39 eV. The XPS results also showed that UV/O3 treatment is an effective surface cleaning method with little surface modification, while O2 plasma causes a strong oxidation process that occurs inside the top layer GaN.

Investigations of Nanoscale Columnar AlxGa1-xN/AlN Heterostructures Grown on Silicon Substrates with Different Modifications of the Surface

The growth of nanoscale columnar AlxGa1-xN/AlN heterostructures on the surface of silicon substrates using plasma-activated nitrogen molecular-beam epitaxy was investigated in this work. Silicon substrates include atomic-smooth cSi substrate, Si substrate with a transition layer of porous silicon porSi/cSi and a hybrid substrate involving a silicon carbide layer grown with matched substitution of the atoms on the surface of porous silicon SiC/porSi/cSi. A complex analysis performed using a set of structural and spectroscopic techniques demonstrated that the epitaxial growth of the nuclear AlN layer on all types of the substrates in a N-enriched environment resulted in the formation of AlxGa1-xN/AlN heterostructures with a Ga-polar surface, which was realized only on the SiC/porSi/cSi substrate. The layer of AlxGa1-xN on cSi and porSi/cSi substrates was in the state of disordered alloy with an excess of gallium atom content. It was shown that a great difference in the lattice parameters of a substrate–film pair resulted not only in the appearance of a number of various defects but also in a considerable effect on the chemical process of the formation of the alloys, in our case, the AlxGa1-xN alloy. It was shown that nanoscale columns of AlxGa1-xN formed on SiC/porSi/cSi substrate were inclined relative to the c-axis, which was connected with the features of the formation of a SiC layer by the matched substitution of the atoms on the porous Si substrate, resulting in the formation of the inclined (111) SiC facets at the boundary of the (111) Si surface and pores in Si. Optical studies of the grown samples demonstrated that the optical band-to-band transition for the AlxGa1-xN alloy with Eg = 3.99 eVB was observed only for the heterostructure grown on the SiC/porSi/cSi substrate. A qualitative model is proposed to explain the difference in the formation of AlxGa1-xN layers on the substrates of cSi, porSi/cSi and SiC/porSi/cSi. The results obtained in our work demonstrate the availability of using SiC/porSi/cSi substrates for the integration of silicon technology and that used for the synthesis of nanoscale columnar AlxGa1-xN heterostructures using plasma-activated molecular-beam epitaxy with a nitrogen source.

Comparative studies of nanoscale columnar AlxGa1-xN/AlN heterostructures grown by plasma-assisted molecular-beam epitaxy on cSi, porSi/cSi and SiC/porSi/cSi substrates

Problems of the growth of nanoscale columnar AlxGa1-xN/AlN heterostructures on hybrid substrates involving porous silicon and silicon carbide layers by molecular beam epitaxy technique with plasma-activated nitrogen are discussed in this study. The epitaxial growth of nanoscale columnar AlxGa1-xN/AlN heterostructure is shown to be specified by the layer of silicon carbide, which is formed by atomic substitution technique, and a porous silicon sublayer predetermines the oriented growth of SiC. The performed complex of structural-spectroscopic analysis demonstrated that epitaxial growth of the nuclear AlN layer on all types of the substrates in N-enriched conditions resulted in the formation of AlxGa1-xN/AlN heterostructures with Ga-polar surface. At the same time it was found that the layer of ordered AlxGa1-xN alloy was formed only on the hybrid SiC/porSi/cSi substrate. The layer of AlxGa1-xN on the substrates of cSi and porSi/cSi is present in the state of disordered alloy with an excess content of gallium atoms. Nanocolumns of AlxGa1-xN/AlN grown on the hybrid SiC/porSi/cSi substrate have two types of preferential azimuthal orientation that affects not only their structural and optical properties but also the value of elastic deformation in the heterostructure nanoscale layers. For the first time, azimuthal dependence of intensity of E1(TO) and E2high photon modes was detected in the Raman spectra. The observed periodicity angle in micro-Raman scattering coincides with the characteristic swivel of nanoscale columns of AlxGa1-xN/AlN around c-axis according to the data of XRD pole figure measurements. Results obtained in our work show promising capabilities in the use of SiC/porSi/сSi substrates for integration of silicon technology and technology of synthesis of the nanoscale columnar AlxGa1-xN heterostructures by molecular-beam epitaxy with plasma-activated nitrogen.

High current implementation of Cu/P-type GaN triboelectric nanogenerator

Traditional surface engineering, as a means of manufacturing triboelectric nanogenerator (TENG), is complex and expensive. The yield of traditional polymer process is low, which leads to the high cost and low stability of traditional TENGs and greatly limits their practical applications. Moreover, it is worth noting that with the miniaturization and integration of electronic devices, generators need to provide higher current in parallel circuits. In this study, we report the performance of the enhanced Cu/P-type GaN TENG contacts in centimeter scale. Considering the high surface mechanical strength and surface structure characteristics of GaN wafers, we propose using molten KOH to etch the Ga polar GaN surface to form more interface electrons and dangling bonds without destroying the surface structure. Our experimental results show that the generator performance has been drastically improved (the short circuit current increases from 9 to 80 μA, and the open circuit voltage increases from 8 to 29 V). The maximum load electric power density of ∼0.28 W/m2 was obtained. We also compared the open circuit current density with the reported different type TENGs based on Schottky contact at the centimeter-level. The Cu/P-type GaN TENGs achieved in this work exhibit excellent open circuit current density of ∼36 μA/cm2. Thus, we provide insight into surface engineering for future generation TENG devices.

Study of dry etched N-polar (Al)GaN surfaces obtained by inductively coupled plasma etching

We report the Cl-based inductively coupled plasma etching of N-polar Al(Ga)N layers obtained from layer transfer. It is found that debris appeared on the etched N-polar surface after exposing in air for a short period whereas the etched Al-/Ga-polar surface was clean and smooth. The debris can be completely self-vanished on the N-polar Al0.4Ga0.6N surface after exposing in air for a few hours but still remained on the N-polar GaN surface even after over 1 month. The surface chemical analysis results suggested that the debris is the result of Cl-related byproduct generated during the etching process. Byproducts like Al(Ga)Clx and its derivatives are believed to cover on the N-polar surface after the inductively coupled plasma etching and increase the etched surface roughness significantly. The formation and disappearance of debris are attributed to the formation of Al(Ga)Clx⋅ 6H2O crystals when Al(Ga)Clx absorbs moisture in the air and its spontaneous decomposition on the N-polar surface, respectively. Adding O2/SF6 in the process helps remove Al(Ga)Clx byproducts but at the cost of roughened surface/reduced etch rate. With an additional cleaning process after etching, an uniform and smooth N-polar GaN surface with a low root-mean-square surface roughness of 0.5–0.6 nm has been successfully obtained at a reasonable etch rate (∼150 nm/min). The results can provide valuable guidance for the fabrication of high-performance N-polar GaN devices.

Origin of Surface Barrier Temperature Dependence for the Polar GaN Surface

Combining the mature technology of gallium nitride (GaN) with emerging materials such as van der Waals crystals is presently an important path in applied science research. Up to now, a broad spectrum of optoelectronic devices including light emitters, photodetectors, and solar cells based on mentioned materials has been presented. Understanding and controlling the surface-related phenomena is a crucial aspect for proper electrical device operation. In particular, an insight into the impact of external conditions such as elevated temperature on technologically important parameters such as surface barrier is of great importance. In this study we propose the first spectroscopic investigation of the vulnerability of the surface barrier on temperature for the Ga-polar GaN surface. Studies performed in the 300–440 K temperature range using contactless electroreflectance (CER) enabled nondestructive probing of GaN surface states. The range of investigated materials was extended to graphene/GaN and h-BN/GaN hybrids together with GaAs as a reference in order to provide a comprehensive case study. We show that GaN surface densities of states revealed robust temperature dependence compared to the energy gap. The mechanism of temperature-induced carrier redistribution at the surface is proposed as an explanation.

Surface energy and surface stress of polar GaN(0001)

The polar wurtzite GaN(0001) surface has several proposed surface reconstructions whose relative stability is measured by their surface energy and surface stress. However, the polar nature of the surface, together with the slab geometry, fixed layers with pseudohydrogen termination used in calculations, leads to significant difficulties in accurately identifying the numerical value of surface quantities. We show a method to calculate the surface energy of the clean surfaces by separating the contributions due to the pseudohydrogen layer and its interaction with the slab. This yields a value of 368 meV/Å 2 for the sum of surface energies of the unreconstructed Ga-polar (0001) surface and the bulk-terminated N-polar (0001̄) surface, and 359–364 meV/Å 2 for the sum of surface energies of reconstructed Ga-polar surfaces and the bulk-terminated N-polar surface. In order to calculate surface stress, we propose a modified plane stress condition that is suitable for slabs with a passivated bottom layer. By detailed analysis of the stress for the reconstructed and unreconstructed slabs of different sizes, we show that the unreconstructed surface is in a state of compressive surface stress with Σxx<−0.65N∕m. This compressive stress is relieved through surface reconstructions. The method developed here is applicable for other polar materials.

Impact of high-dose gamma-ray irradiation on electrical characteristics of N-polar and Ga-polar GaN p–n diodes

We investigate the impact of high-dose gamma-ray irradiation on the electrical performance of Ga-polar and N-polar GaN-based p–n diodes grown by metalorganic chemical vapor deposition. We compare the current density–voltage (J–V), capacitance–voltage (C–V), and circular transfer length method characteristics of the p–n diodes fabricated on Ga-polar and N-polar orientations before and after irradiation. The relative turn-on voltage increases for the Ga-polar diodes with an increasing irradiation dose, while it increases initially and then starts to decrease for the N-polar diodes. The p-contact total resistance increases for Ga-polar and decreases for N-polar samples, which we attribute to the formation of point defects and additional Mg activation after irradiation. The J–V characteristics of most of the tested diodes recovered over time, suggesting the changes in the J–V characteristics are temporary and potentially due to metastable occupancy of traps after irradiation. X-ray photoelectron spectroscopy and photoluminescence measurements reveal the existence of different types of initial defects and surface electronic states on Ga-polar and N-polar samples. Gallium vacancies (VGa) are dominant defects in Ga-polar samples, while nitrogen vacancies (VN) are dominant in N-polar samples. The presence of a higher concentration of surface states on Ga-polar surfaces than N-polar surfaces was confirmed by calculating the band bending and the corresponding screening effect due to opposite polarization bound charge and ionized acceptors at the surface. The difference in surface stoichiometry in these two orientations is responsible for the different behavior in electrical characteristics after gamma-ray interactions.

Electrical properties of GaN metal-insulator-semiconductor field-effect transistors with Al2O3/GaN interfaces formed on vicinal Ga-polar and nonpolar surfaces

GaN metal-insulator-semiconductor field-effect transistors (MISFETs) with Ni/Al2O3/GaN gate stack structures formed on a vicinal Ga-polar (c-plane) or nonpolar (m-plane) surface were fabricated. Current–voltage characteristics and carrier transport properties of the channels in the MISFETs with and without post-metallization annealing (PMA) were systematically investigated. GaN layers grown on freestanding substrates with vicinal surfaces led to good transistor behaviors for the m-plane as well as the c-plane. It was found that PMA improves the electron mobility in the MIS channel (μEFF) for both the c- and m-planes, and that μEFF for the m-plane is slightly higher than that for the c-plane both with and without PMA. Temperature dependences of μEFF clarified that μEFF for the m-plane is dominantly limited by phonon scattering, while both Coulomb and phonon scatterings are dominant for the c-plane. Consequently, we achieved high performance for the GaN MISFETs fabricated on the m-plane because of the reduced numbers of Coulomb scattering centers at the Al2O3/GaN interface.

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.

Unusual step meandering due to Ehrlich-Schwoebel barrier in GaN epitaxy on the N-polar surface

The stability of the Nitrogen-polar (000-1) surface of single-crystal bulk GaN substrates is studied for layers grown by plasma-assisted molecular beam epitaxy (PAMBE) in Nitrogen-rich conditions at 730 °C. It is shown that smooth GaN layers with parallel atomic steps are obtained for substrates when the surface crystal miscut angle is larger than 2o, revealing a highly stable epitaxial growth regime on single crystals. A step meandering pattern is observed on layers grown on lower miscut angle substrates. The meandering periodicity is found to have an inverse dependence on growth rate and miscut angle. This is opposite to what is observed for epitaxy on the Ga-polar surface. Combining analytic modeling and kinetic Monte Carlo simulations, it is shown that the existence of an Ehrlich-Schwoebel Barrier (ESB) in the PAMBE growth of GaN in nitrogen-rich conditions on (000-1) GaN reproduces the experimentally observed periodicity of step meandering. Assuming that ESB height depends on interactions between diffusing adatoms, all experimental phenomena are reproduced.

Electrical characteristics of n-GaN Schottky contacts on cleaved surfaces of free-standing substrates: Metal work function dependence of Schottky barrier height

We report the electrical characteristics of Schottky contacts with nine different metals (Ag, Ti, Cr, W, Mo, Au, Pd, Ni, and Pt) formed on clean m-plane surfaces by cleaving freestanding GaN substrates, compared with these of contacts on Ga-polar c-plane n-GaN surfaces grown on GaN substrates. The n-values from the forward current–voltage (I–V) characteristics are as good as 1.02–1.18 and 1.02–1.09 for the m- and c-plane samples, respectively. We found that the reverse I–V curves of both samples can be explained by the thermionic field emission theory, and that the Schottky barrier height of the cleaved m-plane contacts shows a metal work function dependence.

Contactless electroreflectance studies of the Fermi level position at the air/GaN interface: Bistable nature of the Ga-polar surface

In this paper we show that the surface Fermi level of Ga-polar GaN exhibits a bistable behavior allowing it to be located at two distinct energetic positions at the air/GaN interface which is unusual for other III–V semiconductors such as GaAs or GaSb. To determine the Fermi level position at the air/GaN interface we perform contactless electroreflectance measurements on specially designed UD+ structures [GaN(undoped)/GaN(highly doped)/substrate] doped by Si and Mg. Analyzing the period of Franz-Keldysh oscillation we determine the built-in electric field in the undoped (U) layer. These studies coupled with numerical solutions of the Poisson equation allowed us to determine the position of the Fermi level at the air/GaN interface. We observe a change in the band bending correlated to different Fermi level positions in the doped (D+) layer. We show that depending on the doping type in the D+ layer the Fermi level at the air/GaN interface is located in the upper or lower singularity of surface density of states (SDOS) for Si or Mg doping of D+ layer, respectively. We support our findings with the density functional theory calculations of the SDOS and the dependence of the Fermi level position on the doping concentration in the bulk of a GaN slab.

Critical issues for homoepitaxial GaN growth by molecular beam epitaxy on hydride vapor-phase epitaxy-grown GaN substrates

While the heteroepitaxial growth of gallium nitride-based materials and devices on substrates such as SiC, sapphire, and Si has been well-documented, the lack of a cost-effective source of bulk GaN crystals has hindered similar progress on homoepitaxy. Nevertheless, freestanding GaN wafers are becoming more widely available, and there is great interest in growing GaN films and devices on bulk GaN substrates, in order to take advantage of the greatly reduced density of threading dislocations, particularly for vertical devices. However, homoepitaxial GaN growth is far from a trivial task due to the reactivity and different chemical sensitivities of N-polar (0001) and Ga-polar (0001) GaN surfaces, which can affect the microstructure and concentrations of impurities in homoepitaxial GaN layers. In order to achieve high quality, high purity homoepitaxial GaN, it is necessary to investigate the effect of the ex situ wet chemical clean, the use of in situ cleaning procedures, the sensitivity of the GaN surface to thermal decomposition, and the effect of growth temperature. We review the current understanding of these issues with a focus on homoepitaxial growth of GaN by molecular beam epitaxy (MBE) on c-plane surfaces of freestanding GaN substrates grown by hydride vapor phase epitaxy (HVPE), as HVPE-grown substrates are most widely available. We demonstrate methods for obtaining homoepitaxial GaN layers by plasma-assisted MBE in which no additional threading dislocations are generated from the regrowth interface and impurity concentrations are greatly reduced.

Sensitivity of Fermi level position at Ga-polar, N-polar, and nonpolar m-plane GaN surfaces to vacuum and air ambient

Ga-polar, N-polar, and nonpolar m-plane GaN UN+ structures have been examined in air and vacuum ambient by contactless electroreflectance (CER). This technique is very sensitive to the surface electric field that varies with the Fermi level position at the surface. For UN+ GaN structures [i.e., GaN (undoped)/GaN (n-type)/substrate], a homogeneous built-in electric field is expected in the undoped GaN layer that is manifested by Franz–Keldysh oscillation (FKO) in CER spectra. A clear change in FKO has been observed in CER spectra for N-polar and nonpolar m-plane structures when changing from air to vacuum ambient. This means that those surfaces are very sensitive to ambient atmosphere. In contrast to that, only a small change in FKO can be seen in the Ga-polar structure. This clearly shows that the ambient sensitivity of the Fermi level position at the GaN surface varies with the crystallographic orientation and is very high for N-polar and nonpolar m-plane surfaces. This feature of the N-polar and nonpolar m-plane surfaces can be very important for GaN-based devices grown on these crystallographic orientations and can be utilized in some of the devices, e.g., sensors.

GaN nanowire fabricated by selective wet-etching of GaN micro truncated-pyramid

We demonstrated the fabrication of GaN nanowire (NW) by selective etching of the GaN micro truncated-pyramid (μ-TP) in KOH solution. The GaN μ-TP, which consists of a (0001) Ga-polar top surface and six {11¯01} N-polar sidewalls, were grown on the patterned AlN/Si template in metal organic chemical vapor deposition (MOCVD) system. KOH solution can selectively etch the N-polar sidewalls while leave the Ga-polar top surface intact. Hexagonal-prism-structured GaN NW with (0001) top surface and {11¯00} sidewalls was obtained after adequate chemical etching. It was found that the three-dimensional geometry of the GaN NW is determined by the diameter of the (0001) top surface and the height of the GaN μ-TP. And the chemical etching mechanism of GaN μ-TP towards GaN NW in hydroxide solution was explained.

On the reliable analysis of indium mole fraction within InxGa1−xN quantum wells using atom probe tomography

Surface crystallography and polarity are shown to influence the detection probability of In, Ga, and N ions during atom probe tomography analysis of InxGa1−xN m-plane, c-plane, and (202¯1¯) quantum wells. A N deficit is observed in regions of the reconstruction generated from Ga-polar surfaces, and the probability of detecting group-III atoms is lower in InxGa1−xN quantum wells than in GaN barrier layers. Despite these artifacts, the detected In mole fraction is consistent throughout a given quantum well regardless of the crystal orientation of the quantum well or the evaporation surface from which the reconstruction was generated.

Growth of thick InGaN layers by tri-halide vapor phase epitaxy

This is the first study to report InGaN growth by tri-halide vapor phase epitaxy (THVPE) using InCl3 and GaCl3 generated by the reactions between metal sources (i.e., metallic indium and gallium) and gaseous chlorine. The influence of the surface orientation of the initial substrate on InGaN-THVPE growth was investigated using freestanding (0001) and GaN substrates. Only a N-polar InGaN epitaxial layer was obtained by THVPE because of the instability of GaCl3 adsorption toward nitrogen atoms on the Ga-polar surface. In addition, we investigated the influence of the group-III input partial pressure on the growth rate and solid composition of InGaN layers grown on a GaN substrate. The growth rate increased linearly with group-III concentration, and a maximum growth rate of 15.6 µm/h was achieved at PIII = 2.0 × 10−3 atm.

Understanding and controlling indium incorporation and surface segregation on InxGa1−xN surfaces: Anab initioapproach

The incorporation of In into the technologically relevant $(0001)$ (Ga-polar) and $(000\overline{1})$ (N-polar) surfaces of ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{N}$ is investigated using density functional theory. The cases of coherent pseudomorphic growth on GaN and on lattice-matched heterointerfaces are considered. For pseudomorphic growth on GaN, In incorporation into the ${0001}$ surface layers is limited to a tiny growth window corresponding to extreme In-rich growth conditions and at the In-rich/Ga-poor region of the metal chemical potentials. Lattice-matched growth, however, allows for a wider growth window. Surface phase diagrams are constructed as a function of growth conditions and reveal similarities between the two polar growth planes. However, a strong driving force is found for segregation of In atoms to the first III-N layer for Ga-polar growth, but not for N-polar growth. The former was found to be mainly due to chemical effects (stronger Ga-N as compared to In-N bonds), absent in the case of N-polar growth. Furthermore, finite-temperature calculations show that In incorporated into the first III-N layer is stable to $\ensuremath{\approx}150$ K higher temperatures in the N-polar surface than in the Ga-polar surface, indicating that for a given level of In incorporation, higher temperatures can be used for N-polar growth as compared to Ga-polar growth.