Kohji Ohtsuka

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...

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...

AlGaN/GaN Heterostructure Field-Effect Transistors (HFETs) on Si Substrates for Large-Current Operation

We demonstrated AlGaN/GaN heterostructure field-effect transistors (HFETs) grown on 2 inch Si (111) substrates by metalorganic chemical vapor phase epitaxy (MOVPE). Using GaN/AlN multilayers, we successfully fabricated nonpitted and crack-free GaN films thicker than 1 µm on Si substrates. An electron mobility of 1200 cm2/Vs, a sheet carrier density of 4.5 ×1012 /cm2, and a sheet resistance of 1100 Ω/sq were obtained. Fabricated 60-mm-gate-width HFETs exhibited a maximum drain current of more than 10 A, an on-state resistance of 0.5 Ω, and a breakdown voltage of more than 350 V.

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.

Properties of GaN films deposited on Si(111) by radio-frequency-magnetron sputtering

GaN films have been deposited on Si(111) substrates by reactive rf-magnetron sputtering at nitrogen pressures from 0.08 to 2.70 Pa without intentionally heating the substrates. X-ray diffraction (XRD), spectroscopic ellipsometry (SE), and ex situ atomic-force microscopy (AFM) observations have been carried out. The XRD patterns indicate that the GaN films deposited at pressures lower than 1.10 Pa are polycrystalline films highly oriented with the (0001) plane preferred, while those deposited at ⩾1.10 Pa display mixed orientations or amorphous form. The pseudodielectric function e(E)=e1(E)+ie2(E) of the sputter-deposited GaN films has been measured by SE in the range between 1.50 and 5.00 eV at room temperature. The measured e(E) spectra are analyzed by taking into account the effects of surface roughness based on an effective medium model. The roughness thickness for the film deposited at 0.27 Pa is determined to be ∼17 A, which is comparable to the AFM rms value (∼11 A).

Optical properties of AlxGa1−xN alloy

The optical response of the AlxGa1−xN alloy has been analyzed using a simplified model of interband transitions (E≤10 eV). The present model reveals distinct structures at energies of the E0, E1, F1, and E0′ critical points. The Cauchy−Lorentz expression popularly used for the modeling of the three-dimensional M0 and saddle-point excitonic transitions does not satisfy the Kramers−Kronig requirements but does so if its form is properly modified. As a result, excellent agreement is achieved between the experimental and calculated e(E) spectra over the entire range of photon energies. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of the AlxGa1−xN alloy are also presented. The high-frequency and static dielectric constants of the AlxGa1−xN alloy are determined to be e∞(x)=5.27−1.07x and es(x)=9.28−1.45x, respectively.

Properties of radio-frequency-sputter-deposited GaN films in a nitrogen∕hydrogen mixed gas

GaN films have been deposited by reactive sputtering in nitrogen gas at pressures from 0.08 to 2.70 Pa with and without the addition of hydrogen gas. X-ray diffraction (XRD), Fourier transform infrared (FTIR), optical absorption, and photoluminescence (PL) spectroscopy have been used to characterize the sputter-deposited GaN films. The XRD pattern reveals that the GaN films deposited in nitrogen gas at pressures lower than 0.53 Pa are polycrystals with the (0001) texture (α-GaN), while those deposited at or above 1.07 Pa display mixed crystalline orientations or an amorphous-like nature. The GaN:H films deposited in nitrogen∕hydrogen mixed gas, on the other hand, show an amorphous or amorphous-like nature. The FTIR spectra indicate that the GaN:H films show peaks arising from hydrogen-related bonds at ∼1000 and ∼3200cm−1, in addition to the GaN absorption band at ∼555cm−1. The optical absorption spectra at 300 K indicate the fundamental absorption edges at ∼3.38 and ∼3.7eV for the highly oriented α-GaN an...