Shunro Fuke

Selective etching of GaN polar surface in potassium hydroxide solution studied by x-ray photoelectron spectroscopy

Etching characteristics of nondoped GaN films with the polar surface in KOH solution have been investigated. It is confirmed that the continuous etching in KOH solution takes place only for the GaN films with N-face (−c) polarity independent of the deposition method and growth condition. It is found by x-ray photoelectron spectroscopy (XPS) analysis for the Ga face (+c) and N-face (−c) GaN films that the atomic composition of the +c surface is not changed before and after dipping in KOH solution and that on the other hand, the amount of oxygen (oxide) on the −c surface is significantly decreased by the etching. It is also found that the band bending increases by −0.4±0.2 and 0.6±0.2 eV for the +c and −c surfaces after etching, respectively. This is discussed in terms of the surface chemistry. Based on the XPS result, the selective etching of the GaN polar surface is pointed out to originate from bonding configuration of nitrogen at the surface.

Optical properties of hexagonal GaN

Single-crystalline hexagonal GaN (α-GaN) films have been grown on (0001) sapphire substrates by metalorganic chemical vapor deposition at 1040 °C. The complex dielectric functions, e(E)=e1(E)+ie2(E), of the epitaxial films have been measured by spectroscopic ellipsometry (SE) for E⊥c in the region between 1.5 and 5.0 eV at room temperature. Previously published ultraviolet SE spectra of α-GaN are examined by considering the effects of surface roughness using an analysis based on an effective medium model. Ex situ atomic force microscopy is used to assess independently surface flatness. By mathematically removing the effects of surface roughness, the most reliable e(E) values for α-GaN are presented in the 1.25–10 eV photon–energy range. Theoretical dispersion analysis suggests that the E0 structure could be characterized by a three-dimensional M0 critical point and the E1α (α=A,B,C) structures by two-dimensional M1 critical points. To facilitate design of various optoelectronic devices, dielectric-functio...

Growth mode and surface morphology of a GaN film deposited along the N-face polar direction on c-plane sapphire substrate

The dependence of polar direction of GaN film on growth conditions has been investigated by changing either the group-V/group-III ratio (V/III ratio) in supplying the source gas or the deposition rate. GaN films were deposited on a nitrided sapphire by two-step metalorganic chemical vapor deposition. The surface morphology changed from flat hexagonal to pyramidal hexagonal facet with the increase of V/III ratio. However, the polar direction of GaN on an optimized buffer layer of 20 nm thickness was N-face (−c) polarity, independent of both the V/III ratio and the deposition rate. The polarity of the GaN epitaxtial layer can be determined by that of an interface (nitrided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation density, and reduced the size of hexagonal facets. The nuclei, forming the favorable hexagonal facets of wurtzite GaN, should grow laterally along the {1010} directions to cover a room among the facet...

Dependence of impurity incorporation on the polar direction of GaN film growth

We have investigated the dependence of impurity incorporation on the polar direction of GaN growth by using secondary ion mass spectroscopy (SIMS). GaN films were deposited under conditions used for growing device-quality materials on sapphire substrates while controlling their polar direction. It was found that the polarity of the GaN film influences the incorporation of impurities. SIMS analysis has revealed that the impurities related to carbon, oxygen, and aluminum are more readily incorporated into N-face GaN films.

Analysis of the polar direction of GaN film growth by coaxial impact collision ion scattering spectroscopy

Nondestructive determination of the polarity of GaN has been achieved by the use of coaxial impact-collision ion scattering spectroscopy analysis. The polarity of a GaN film with a smooth surface on non-nitrided c-plane sapphire was identified (0001) (Ga face; +c). GaN films with a 20 nm buffer layer on nitrided sapphire had (0001) (N face; −c) polarity and a hexagonal faceted surface. The influence of both the buffer layer and of substrate nitridation on the polarity of wurtzite {0001} GaN films deposited by two-step metal organic chemical vapor deposition (MOCVD) has been investigated. The polarity of the buffer layer on a nitrided sapphire substrate was altered by varying its thickness or the annealing time. It was found that the polarity of the GaN film is determined by the polarity of the annealed buffer layer; MOCVD-GaN films on buffer layers with +c and −c polarity have either +c (smooth surface) or −c (hexagonal facet) polarity, respectively.

INFLUENCES OF INITIAL NITRIDATION AND BUFFER LAYER DEPOSITION ON THE MORPHOLOGY OF A (0001) GAN LAYER GROWN ON SAPPHIRE SUBSTRATES

Hexagonal GaN is grown on (0001) sapphire substrates using an atmospheric pressure organometallic vapor phase epitaxy method. We investigate the influences of the initial treatment of sapphire substrate, such as initial nitridation and low-temperature GaN buffer layer deposition, upon the surface morphology and crystallinity. The thermal stability of grown GaN layers is also investigated using a thermal etching process in a H2 atmosphere in order to obtain the information concerning the surface polarity of GaN(0001) layers. When sapphire substrates are initially nitrided, highly crystalline GaN layers with large hexagonal facets are obtained and its surface appears to be (0001)N. On the other hand, the deposition of a thicker buffer layer on the nitrided sapphire substrates improves the surface morphology, and the surface polarity of the mirror surface appears to be (0001)Ga. The initial nitridation of sapphire substrates and the GaN buffer layer deposition are considered to be important processes from vi...

INFLUENCE OF THERMAL ANNEALING ON GAN BUFFER LAYERS AND THE PROPERTY OF SUBSEQUENT GAN LAYERS GROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION

We have investigated the influence of low-temperature buffer layer deposition conditions, such as thickness and thermal annealing time, on the properties of the high-temperature GaN growth layer. The surface morphology of the buffer layer after thermal annealing at 1040°C depends on both the thickness of the buffer layer and the annealing time. When a thick buffer layer was used, large trapezoidal growth nuclei were formed after annealing, which led to the poor crystallinity of the GaN growth layer. On the other hand, when a thin buffer layer or a fully annealed thick buffer layer was used, small growth nuclei having a relatively small misorientation were formed, which led to good crystallinity of GaN growth layer. Thus, the thermal annealing time must be optimized, taking the thickness of the buffer layer into the consideration.

Reduction of threading dislocations by InGaAs interlayer in GaAs layers grown on Si substrates

High-quality GaAs epilayers with dislocation densities of 1.2×106 cm−2 on (100)Si substrates have been obtained by insertion of an InGaAs strained interlayer combined with thermal cycle annealing instead of strained layer superlattices. All the layers were grown by low-pressure metalorganic vapor phase epitaxy. The threading dislocation density near the surface of 4 μm thick GaAs was measured by plan-view transmission electron microscopy. The threading dislocation density was found to be very sensitive to the In composition of the interlayer and the specifics of thermal cycle annealing.

Magneto-Optical Spectroscopy of Anatase TiO2 Doped with Co

Magneto-optical spectroscopy of a transparent ferromagnetic semiconductor, anatase TiO2 doped with Co, is carried out at room temperature. A large magneto-optical response with ferromagnetic field dependence is observed throughout from ultraviolet to visible range and increases with increasing Co content or carrier concentration. The magnitude of magnetic circular dichroism (MCD) per unit thickness has a peak around the absorption edge such a huge value of ~10400 deg/cm at 3.57 eV for a 10 mol% Co-doped specimen. Although the results are not sufficient to prove that the ferromagnetism is in the ordinary framework of diluted magnetic semiconductors, the coexistence of Co impurity and mobile carrier is shown to transform the band structure of host TiO2 to generate ferromagnetism.

Quantitative control and detection of heterovalent impurities in ZnO thin films grown by pulsed laser deposition

Impurities in ZnO specimens, including targets for pulsed laser deposition and thin films resulting from their use, were analyzed by secondary ion mass spectroscopy (SIMS). Negatively charged complex ions bound with oxygen (71Ga16O− and 14N16O−) were found to be the most reliable species with which to evaluate the Ga and N content of ZnO films by clarifying possible mass interference effects in SIMS analysis. Calibrations were carried out to determine the Ga concentration (CGa) and the nitrogen concentration (CN) by normalizing the signal intensities for 71Ga16O− and 14N16O−, respectively, to that for 70Zn16O−. Alternative ablation of pure single crystal and Ga-doped ceramic ZnO targets was found to be effective not only for achieving systematic control of the Ga concentration in ZnO:(Ga,N) films, but also for minimizing the contamination of undesired impurities from the sintered targets. The substrate temperature plays a decisive role in control of CN due to a thermally activated desorption process of N-...