Chunmei Jiao

One-step growth of ZnO from film to vertically well-aligned nanorods and the morphology-dependent Raman scattering

We observed a transition from film to vertically well-aligned nanorods for ZnO grown on sapphire (0001) substrates by metalorganic chemical vapor deposition. A growth mechanism was proposed to explain such a transition. Vertically well-aligned homogeneous nanorods with average diameters of similar to 30, 45, 60, and 70 nm were grown with the c-axis orientation. Raman scattering showed that the E-2 (high) mode shifted to high frequency with the decrease of nanorod diameters, which revealed the dependence of nanorod diameters on the stress state. This dependence suggests a stress-driven diameter-controlled mechanism for ZnO nanorod arrays grown on sapphire (0001) substrates. (c) 2005 American Institute of Physics.

Determination of MgO/AlN heterojunction band offsets by x-ray photoelectron spectroscopy

MgO is a promising gate dielectric and surface passivation film for GaN/AlGaN transistors, but little is known of the band offsets in the MgO/AlN system. X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (Delta E-v) of MgO/AlN heterostructures. A value of Delta E-v=0.22 +/- 0.08 eV was obtained. Given the experimental band gap of 7.83 eV for MgO, a type-I heterojunction with a conduction band offset of similar to 1.45 eV is found. The accurate determination of the valence and conduction band offsets is important for use of III-N alloys based electronic devices.

Valence band offset of ZnO∕4H-SiC heterojunction measured by x-ray photoelectron spectroscopy

The valence band offset (VBO) of the wurtzite ZnO∕4H-SiC heterojunction is directly determined to be 1.61±0.23eV by x-ray photoelectron spectroscopy. The conduction band offset is deduced to be 1.50±0.23eV from the known VBO value, which indicates a type-II band alignment for this heterojunction. The experimental VBO value is confirmed and in good agreement with the calculated value based on the transitive property of heterojunctions between ZnO, SiC, and GaN.

Rapid thermal annealing properties of ZnO films grown using methanol as oxidant

ZnO thin films were grown by metal-organic chemical vapour deposition using methanol as oxidant. Rapid thermal annealing (RTA) was performed in an ambient of one atmosphere oxygen at 900 degrees C for 60 s. The RTA properties of the films have been characterized using scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, photoluminescence spectra and Hall measurement. The grains of the film were well coalesced and the surface became denser after RTA. The full-width at half maximum of rocking curves was only 496 arcsec. The ZnO films were also proved to have good optical quality. The Hall mobility increased to 43.2 cm(2) V-1 s(-1) while the electron concentration decreased to 6.6 x 10(16) cm(-3). It is found that methanol is a potential oxidant for ZnO growth and the quality of ZnO film can be improved substantially through RTA.

Investigation of cracks in GaN films grown by combined hydride and metal organic vapor-phase epitaxial method

Cracks appeared in GaN epitaxial layers which were grown by a novel method combining metal organic vapor-phase epitaxy (MOCVD) and hydride vapor-phase epitaxy (HVPE) in one chamber. The origin of cracks in a 22-μm thick GaN film was fully investigated by high-resolution X-ray diffraction (XRD), micro-Raman spectra, and scanning electron microscopy (SEM). Many cracks under the surface were first observed by SEM after etching for 10 min. By investigating the cross section of the sample with high-resolution micro-Raman spectra, the distribution of the stress along the depth was determined. From the interface of the film/substrate to the top surface of the film, several turnings were found. A large compressive stress existed at the interface. The stress went down as the detecting area was moved up from the interface to the overlayer, and it was maintained at a large value for a long depth area. Then it went down again, and it finally increased near the top surface. The cross-section of the film was observed after cleaving and etching for 2 min. It was found that the crystal quality of the healed part was nearly the same as the uncracked region. This indicated that cracking occurred in the growth, when the tensile stress accumulated and reached the critical value. Moreover, the cracks would heal because of high lateral growth rate.

Thermal diffusion of nitrogen into ZnO film deposited on InN/sapphire substrate by metal organic chemical vapor deposition

ZnO film with high crystal quality was prepared on InN/sapphire substrate by metal organic chemical vapor deposition. The diffusion of nitrogen (N) into ZnO film was investigated via Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and low-temperature photoluminescence (LT-PL). AES revealed that some N atoms out-diffused into ZnO film after a rapid thermal annealing (RTA) process, while most of the In atoms remained in InN layers, which was confirmed by XPS. LT-PL spectra at 10 K further confirmed that N atoms diffused into the upper ZnO film and acted as acceptors after RTA. It might be an attractive way to obtain high-quality p-type ZnO:N on InN films by this thermal diffusion doping technique.

Effects of V/III ratio on a-plane GaN epilayers with an InGaN interlayer

The effects of V/III growth flux ratio on a-plane GaN films grown on r-plane sapphire substrates with an InGaN interlayer are investigated. The surface morphology, crystalline quality, strain states, and density of basal stacking faults were found to depend heavily upon the V/III ratio. With decreasing V/III ratio, the surface morphology and crystal quality first improved and then deteriorated, and the density of the basal-plane stacking faults also first decreased and then increased. The optimal V/III ratio growth condition for the best surface morphology and crystalline quality and the smallest basal-plane stacking fault density of a-GaN films are found. We also found that the formation of basal-plane stacking faults is an effective way to release strain.

Using different carrier gases to control AlN film stress and the effect on morphology, structural properties and optical properties

On the metalorganic chemical vapour deposition growth of AlN, by adjusting H-2+N-2 mixture gas components, we can gradually control island dimension. During the Volmer - Weber growth, the 2-dimensional coalescence of the islands induces an intrinsic tensile stress. Then, this process can control the in-plane stress: with the N-2 content increasing from 0 to 3 slm, the in-plane stress gradually changes from 1.5 GPa tensile stress to - 1.2GPa compressive stress. Especially, with the 0.5 slm N-2 + 2.5 slm H-2 mixture gas, the in-plane stress is only 0.1 GPa, which is close to the complete relaxation state. Under this condition, this sample has good crystal and optical qualities.

Study of the one dimensional electron gas arrays confined by steps in vicinal GaN/AlGaN heterointerfaces

One dimensional electron gas (1DEG) arrays in vicinal GaN/AlGaN heterostructures have been studied. The steps at the interface would lead to the lateral barriers and limit the electron movement perpendicular to such steps. Through a self-consistent Schrodinger-Poisson approach, the electron energy levels and wave functions were calculated. It was found that when the total electron density was increased, the lateral barriers were lowered due to the screening effects by the electrons, and the electron gas became more two-dimension like. The calculated 1DEG densities were compared to the experimental values and good agreements were found. Moreover, we found that a higher doping density is more beneficial to form 1-D like electron gas arrays.

Anisotropic scattering effect of the inclined misfit dislocation on the two-dimensional electron gas in Al(In)GaN/GaN heterostructures

In this paper, a theory is developed to study the anisotropic scattering effect of the inclined misfit dislocation on the two-dimensional electron gas in Al(In)GaN/GaN heterostructures. The inclined misfit dislocation, which differs from the well-known vertical threading dislocation, has a remarkable tilt angle from the vertical. The predicted electron mobility shows a remarkable anisotropy. It has a maximum mobility value along the direction perpendicular to the projection of the inclined dislocation line, and a minimum mobility value along the direction parallel to the projection. The degree of the anisotropic scattering effect will be even greater with the increase of the tilt angle.