N-polar Layers Research Articles

III-Nitride Magnetron Sputter Epitaxy on Si: Controlling Morphology, Crystal Quality, and Polarity Using Al Seed Layers.

Group III-nitride semiconductors have been subject of intensive research, resulting in the maturing of the material system and adoption of III-nitrides in modern optoelectronics and power electronic devices. Defined film polarity is an important aspect of III-nitride epitaxy as the polarity affects the design of electronic devices. Magnetron sputtering is a novel approach for cost-effective epitaxy of III-nitrides nearing the technological maturity needed for device production; therefore, control of film polarity is an important technological milestone. In this study, we show the impact of Al seeding on the AlN/Si interface and resulting changes in crystal quality, film morphology, and polarity of GaN/AlN stacks grown by magnetron sputter epitaxy. X-ray diffraction measurements demonstrate the improvement of the crystal quality of the AlN and subsequently the GaN film by the Al seeding. Nanoscale structural and chemical investigations using scanning transmission electron microscopy reveal the inversion of the AlN film polarity. It is proposed that N-polar growth induced by Al seeding is related to the formation of a polycrystalline oxygen-rich AlN interlayer partially capped by an atomically thin Si-rich layer at the AlN/Si interface. Complementary aqueous KOH etch studies of GaN/AlN stacks demonstrate that purely metal-polar and N-polar layers can be grown on a macroscopic scale by controlling the amount of Al seeding.

Systematic oxygen impurity reduction in smooth N-polar GaN by chemical potential control

Process chemical potential control and dislocation reduction were implemented to control oxygen concentration in N-polar GaN layers grown on sapphire substrates via metal organic chemical vapor deposition (MOCVD). As process supersaturation was changed from ∼30 to 3400, the formation energy of the oxygen point defect increased, which resulted in a 25-fold decrease in oxygen incorporation. Reducing dislocations by approximately a factor of 4 (to ∼109 cm−3) allowed for further reduction of oxygen incorporation to the low-1017 cm−3 range. Smooth N-polar GaN layers with low oxygen content were achieved by a two-step process, whereas first a 1 µm thick smooth N-polar layer with high oxygen concentration was grown, followed by low oxygen concentration layer grown at high supersaturation.

Spontaneous Formation of Indium Clusters in InN Epilayers Grown by Molecular-Beam Epitaxy

We have studied the influence of growth conditions on the number of metallic indium clusters formed spontaneously in indium nitride (InN) layers grown by nitrogen plasma-assisted molecular-beam epitaxy (PAMBE). InN epilayers of N-and In-polarity were grown on c-sapphire substrates and GaN and AlN templates, respectively. N-polar layers were obtained in the standard PAMBE regime, while In-polar layers were grown using a three-stage regime including the stages of epitaxy with enhanced atomic migration and interruption of growth under nitrogen flow. A series of samples were prepared at various growth temperatures and relative In/N flow rates. Measurement of the magnetic-field dependences of the Hall-effect coefficient and its model approximation were used to determine the percentage content of In clusters in various InN layers and the minimum amount of such inclusions that can be achieved by varying the conditions of MBE growth.

Спонтанное формирование кластеров In в эпитаксиальных слоях InN, выращенных методом молекулярно-пучковой эпитаксии

AbstractWe have studied the influence of growth conditions on the number of metallic indium clusters formed spontaneously in indium nitride (InN) layers grown by nitrogen plasma-assisted molecular-beam epitaxy (PAMBE). InN epilayers of N-and In-polarity were grown on c-sapphire substrates and GaN and AlN templates, respectively. N-polar layers were obtained in the standard PAMBE regime, while In-polar layers were grown using a three-stage regime including the stages of epitaxy with enhanced atomic migration and interruption of growth under nitrogen flow. A series of samples were prepared at various growth temperatures and relative In/N flow rates. Measurement of the magnetic-field dependences of the Hall-effect coefficient and its model approximation were used to determine the percentage content of In clusters in various InN layers and the minimum amount of such inclusions that can be achieved by varying the conditions of MBE growth.

Growth of Ga- and N-polar GaN layers on O face ZnO substrates by molecular beam epitaxy

Gallium nitride (GaN) epitaxial layers have been grown on O face (0001¯) zinc oxide (ZnO) substrates by ammonia source molecular beam epitaxy. By adjusting the growth temperature and the III/V ratio during the nucleation stage, GaN layers with Ga (0001) or N (0001¯) polarities have been obtained. We show that low growth temperatures (<550°C) and Ga-rich conditions lead to Ga-polar layers, whereas higher growth temperatures (>600°C) and N-rich conditions lead to N-polar layers. Furthermore, the formation of a zinc gallate (ZnGa2O4) interfacial layer between GaN and ZnO has been evidenced, which is responsible for the growth of Ga-polar GaN layers. The structural and optical properties of Ga- and N-polar GaN layers have been characterized and Ga-polar GaN layers exhibit higher crystal quality.

Role of nitridation on polarity and growth of InN by metal–organic vapor phase epitaxy

We report on metal–organic vapor phase epitaxy (MOVPE) of (0001)InN layers simultaneously grown on a-plane (112¯0) and c-plane (0001) sapphire substrates. The substrates were nitridated at temperatures from 500°C to 1050°C prior to the growth of c-plane InN layers. Nitridation determined the polarity, the crystallinity, and the surface morphology of the InN layers. Nitridation temperatures above 800°C lead to N-polar InN layers, while nitridation temperatures from 700°C to 750°C produce mixed-polar InN layers, and nitridation temperatures from 500°C to 650°C produce In-polar InN layers. The roughness and crystallinity of the InN layers are correlated with the changes of polarity. The incorporation of nitrogen into the nitridation layers at different nitridation temperatures was measured. A strong N–Al bond signal after nitridation is correlated with N-polarity layers after overgrowth.

Growth and characterization of In-polar and N-polar InAlN by metal organic chemical vapor deposition

In x Al 1 − x N layers, with 0.09≤x≤0.23, were grown on GaN on both the In-polar and N-polar orientations by metal organic chemical vapor deposition. The impact of growth conditions, including temperature and the group-III flow rates, on the surface morphology and indium mole fraction was investigated. In-polar layers had a smooth surface morphology characterized by mounds which decreased in size with increasing supersaturation during growth. Smooth N-polar InAlN was achieved through the use of vicinal sapphire substrates with misorientation angles of 3°–5° toward ⟨101¯0⟩GaN, and a trend of an increase in step bunching with decreasing supersaturation was observed for N-polar InAlN layers. The indium incorporation increased with decreasing growth temperature and increasing growth rate for both In-polar and N-polar layers. The indium incorporation was similar for both orientations on samples which were coloaded in our reactor.

GaN devices based on nanorods

Transmission and scanning electron microscopy are used to examine the role of an intermediate nanorod layer in reducing threading defect densities in GaN/(0001)sapphire. Films grown by molecular beam epitaxy under N-rich conditions showed Ga-polar nanorods growing out of a more compact N-polar layer. The nanorods sometimes contained extended threading defects, which were faulted dipoles lying on {10-10} planes with a displacement vector of ±1/2[0001], which act as sources for spiral growth. By overgrowing the nanorods under Ga-rich conditions, continuous epilayers were formed with threading defect densities down to 108 cm−2. In a second approach, nanorods produced by etching through a self-organised layer of Ni islands were overgrown by metal-organic chemical vapour deposition, to produce overlayers with defect densities down to 5*107 cm−2. In both cases, the mechanisms by which the nanorod layer reduces the threading defect density are identified.

Lattice polarity detection of InN by circular photogalvanic effect

We report an effective and nondestructive method based on circular photogalvanic effect (CPGE) to detect the lattice polarity of InN. Because of the lattice inversion between In- and N-polar InN, the energy band spin splitting is opposite for InN films with different polarities. Consequently under light irradiation with the same helicity, CPGE photocurrents in In- and N-polar layers will have opposite directions, thus the polarity can be detected. This method is demonstrated by our CPGE measurements in both n- and p-type InN films.

Defect reduction in GaN/(0001)sapphire films grown by molecular beam epitaxy using nanocolumn intermediate layers

Transmission and scanning electron microscopies are used to examine the epitaxial lateral overgrowth of GaN on GaN nanocolumns grown on AlN/(0001)sapphire by molecular beam epitaxy. Initially, N-rich growth gave a bimodal morphology consisting of defect-free Ga-polar nanocolumns emanating from a compact, highly defective N-polar layer. Under subsequent Ga-rich conditions, the nanocolumns grew laterally to produce continuous Ga-polar overlayers. Threading dislocation (TD) densities in the overlayer were in the range of 108–109cm−2, up to two orders of magnitude less than in the N-polar underlayer. It is proposed that the change in polarity is a key factor controlling the reduction in TD density.

Al- and N-polar AlN layers grown on c-plane sapphire substrates by modified flow-modulation MOCVD

We report the growth of N- and Al-polar AlN layers on c-plane sapphire by flow-modulation MOCVD (FM-MOCVD) with some flow sequence modifications. Surface polarities were decided by pre-treatment for sapphire substrates prior to AlN seeding layer growth. Low-temperature nitridation (620 °C) was done for the N-polar samples, and no-nitridation was done for the Al-polar. To avoid strong vapor-phase reaction between TMA and NH 3 and to enhance the surface migration of Al atoms, FM-MOCVD technique was used. The flow sequence was modified for optimization of each N- and Al-polar growth. The surface features were completely different between the N- and Al-polar AlN layers in atomic force microscope images. The former had domed structures and the latter atomic steps. The dislocation densities were counted from plane-view transmission microscope images to be about 3×10 10 cm −2 for the N-polar layer and about 1×10 10 cm −2 for the Al-polar. Secondary-ion mass spectroscopy measurement revealed oxygen- and carbon-incorporation into the layers. Band-edge and impurity-related emissions were observed from only the Al-polar layer by room-temperature cathodoluminescence, whereas the N-polar one did not emit.

Application of Kelvin Probe Microscopy for Nitride Heterostructures

Kelvin probe microscopy is an experimental technique designed to investigate fluctuations of surface potential (work function per electron) related to distribution of electric charge or variations in composition. The paper describes principle and precision of measurements. The results obtained for group-III nitrides semiconductor heterostructures grown on c-plane sapphire by metal-organic vapour phase epitaxy are presented. The observations concerns defects: inversion domains for Gaand N-polar layers, threading dislocations and effects of spontaneous polarization leading to 2D carrier gas. To achieve insight in the evolution of defects, bevelled and cross-sectioned samples were investigated along with surface of “as grown” layers. Applicability of standard Kelvin probe microscopy method was also extended by investigating dependence of the surface potential on variable wavelength illumination, offering opportunity for spectroscopy of individual defects.

Selective photoetching and transmission electron microscopy studies of defects in heteroepitaxial GaN

Photoelectrochemical (PEC) etching has been used to study defects in heteroepitaxial GaN layers. In Ga-polar layers PEC etching reveals only dislocations in the form of filamentary etch features (whiskers). Transmission electron microscopy (TEM) confirmed a one-to-one correspondence between the whiskers and straight threading dislocations, which are mainly of edge and mixed type. In N-polar layers, apart from dislocations, inversion domains (IDs) also give rise to the formation of more complex etch features that also have been confirmed by TEM. IDs of nanometer diameter result in formation of whiskers similar to the dislocation-related ones. However, when the diameter of IDs exceeds a critical size (about 100 nm), crater-like deep etch features are formed during PEC etching. Based on the mechanism of PEC etching of GaN in aqueous KOH solutions, it is argued that inversion domain boundaries are electrically active defects.