X. H. Wu

Role of threading dislocation structure on the x‐ray diffraction peak widths in epitaxial GaN films

In this letter we demonstrate that the anomalously low (002) x‐ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation (TD) geometry. Epitaxial GaN films were grown on c‐plane sapphire by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) in a horizontal flow reactor. Films were grown with (002) rocking curves (ω‐scans) widths as low as 40 arcsec and threading dislocation densities of ∼2×1010 cm−2. The threading dislocations in this film lie parallel to the [001] direction and within the limit of imaging statistics, all are pure edge with Burgers vectors parallel to the film/substrate interface. These TDs will not distort the (002) planes. However, distortion of asymmetric planes, such as (102), is predicted and confirmed in (102) rocking curve widths of 740 arcsec. These results are compared with films with (002) rocking curves of ∼270 arcsec and threading dislocation densities of ∼7×108 cm−2.

Defect structure of metal‐organic chemical vapor deposition‐grown epitaxial (0001) GaN/Al2O3

Defect structures were investigated by transmission electron microscopy for GaN/Al2O3 (0001) epilayers grown by metal‐organic chemical vapor deposition using a two‐step process. The defect structures, including threading dislocations, partial dislocation bounding stacking faults, and inversion domains, were analyzed by diffraction contrast, high‐resolution imaging, and convergent beam diffraction. GaN film growth was initiated at 600 °C with a nominal 20 nm nucleation layer. This was followed by high‐temperature growth at 1080 °C. The near‐interfacial region of the films consists of a mixture of cubic and hexagonal GaN, which is characterized by a high density of stacking faults bounded by Shockley and Frank partial dislocations. The near‐interfacial region shows a high density of inversion domains. Above ∼0.5 μm thickness, the film consists of isolated threading dislocations of either pure edge, mixed, or pure screw character with a total density of ∼7×108 cm−2. The threading dislocation reduction in the...

Homoepitaxial growth of GaN under Ga-stable and N-stable conditions by plasma-assisted molecular beam epitaxy

The structure, morphology, and optical properties of homoepitaxial GaN layers grown by molecular beam epitaxy on metalorganic chemical vapor deposition (MOCVD)-grown GaN “template” layers were investigated as a function of the group III/group V flux ratio during growth. GaN layers grown with a low III/V ratio (N-stable growth) displayed a faceted surface morphology and a tilted columnar structure with a high density of stacking faults. In contrast, films grown with a high III/V ratio (Ga-stable growth) displayed comparable structure to the underlying MOCVD-grown template. The transition from N-stable to Ga-stable growth modes was found to occur over a narrow range of Ga fluxes at a growth temperature of 650 °C. Evidence of Ga accumulation and step-flow growth was observed for films grown under Ga-stable conditions, leading to the formation of spiral growth features at the surface termination of mixed edge/screw dislocations. Photoluminescence measurements indicate that the deep-level (∼550 nm) emission is...

Structural origin of V-defects and correlation with localized excitonic centers in InGaN/GaN multiple quantum wells

In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the “V-defect”) initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {1011} planes) and an open hexagonal inverted pyramid which is defined by the six {1011} planes. Thus, in cross section this defect appears as an open “V”. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({1011} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the fol...

Structural evolution in epitaxial metalorganic chemical vapor deposition grown GaN films on sapphire

The structural evolution of epitaxial GaN layers grown on basal plane sapphire has been studied by atomic force microscopy (AFM), x‐ray diffraction, and transmission electron microscopy (TEM). High‐temperature growth (1050–1080 °C) on optimized nucleation layers leads to clear, specular films. AFM on the as‐grown surface shows evenly spaced monatomic steps indicative of layer by layer growth. AFM measurements show a step termination density of 1.7×108 cm−2 for 5 μm films. This value is in close agreement with TEM measurements of screw and mixed screw‐edge threading dislocation density. The total measured threading dislocation density in the 5 μm films is 7×108 cm−2.

Microstructure of GaN laterally overgrown by metalorganic chemical vapor deposition

Extended defect reduction in GaN grown by lateral epitaxial overgrowth (LEO) on large-area SiO2/GaN/Al2O3 wafers by low pressure metalorganic chemical vapor deposition is characterized using transmission electron microscopy and atomic force microscopy. The laterally overgrown GaN (LEO GaN) has a rectangular cross section with smooth (0001) and {1120} facets. The density of mixed-character and pure edge threading dislocations in the LEO GaN (<5×106 cm−2) is reduced by at least 3–4 orders of magnitude from that of bulk GaN (∼1010 cm−2). A small number of edge dislocations with line directions parallel to the basal plane are generated between the bulk-like overgrown GaN and the LEO GaN regions as well as at the intersection of adjacent merging LEO GaN stripes. The edge dislocations are most likely generated to accommodate the small misorientation between bulk-like GaN and LEO GaN regions as well as between adjacent single-crystal LEO GaN stripes.

Nucleation layer evolution in metal‐organic chemical vapor deposition grown GaN

The structure and morphology of low growth temperature GaN nucleation layers have been studied using atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). The nucleation layers were grown at 600 °C by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on c‐plane sapphire. The layers consist of predominantly cubic GaN (c‐GaN) with a high density of stacking faults and twins parallel to the film/substrate interface. The average grain size increases with increasing layer thickness and during the transition from low temperature (600 °C) to the high temperatures (1080 °C) necessary for the growth of device quality GaN. Upon heating to 1080 °C the nucleation layer partially converts to hexagonal GaN (h‐GaN) while retaining a high stacking fault density. The mixed cubic‐hexagonal character of the nucleation layer region is sustained after subsequent high‐temperature GaN growth.

Metalorganic chemical vapor deposition growth of high optical quality and high mobility GaN

Hall mobilities as high as 702 and 1230 cm2/Vs at 300 and 160K along with low dislocation densities of 4.0 × 108 cm-2 have been achieved in GaN films grown on sapphire by metalorganic chemical vapor deposition. High growth temperatures have been established to be crucial for optimal GaN film quality. Photoluminescence measurements revealed a low intensity of the deep defect band around 550 nm in films grown under optimized conditions.

The Effect of Growth Environment on the Morphological and Extended Defect Evolution in GaN Grown by Metalorganic Chemical Vapor Deposition

The evolution of morphology and associated extended defects in GaN thin films grown on sapphire by metalorganic chemical vapor deposition (MOCVD) are shown to depend strongly on the growth environment. For the commonly used two-step growth process, a change in growth parameter such as reactor pressure influences the initial high temperature (HT) GaN growth mechanism. By means of transmission electron microscopy (TEM), atomic force microscopy (AFM), and high resolution X-ray diffraction (HRXRD) measurements, it is shown that the initial density of HT islands on the nucleation layer (NL) and subsequently the threading dislocation density in the HT GaN film may be directly controlled by tailoring the initial HT GaN growth conditions.

MORPHOLOGY AND MICROSTRUCTURAL EVOLUTION IN THE EARLY STAGES OF HYDRIDE VAPOR PHASE EPITAXY OF GAN ON SAPPHIRE

The early stages of hydride vapor phase epitaxy (HVPE) of GaN on sapphire were studied using atomic force microscopy, field-emission scanning electron microscopy, cross-sectional transmission electron microscopy, and x-ray diffraction rocking curves. At the high growth rate used (∼33 nm/s), the films appear to be fully coalesced for growth periods as short as 1 s. A distinct surface and subsequent bulk transformation were observed, resulting in significantly smoother film surfaces and improved bulk morphology. The growth of thick (i.e., 300 μm) GaN films using HVPE offers a promising technique for the deposition of high-quality substrates for GaN homoepitaxy.