Xinqiang Wang

Proposal and achievement of novel structure InN/GaN multiple quantum wells consisting of 1 ML and fractional monolayer InN wells inserted in GaN matrix

The authors propose and demonstrate the fabrication of InN∕GaN multiple quantum well (MQW) consisting of 1 ML and fractional monolayer InN well insertion in GaN matrix under In-polarity growth regime. Since the critical thickness of InN epitaxy on GaN is about 1 ML and the growth temperature for 1 ML InN insertion can be remarkably higher, the proposed MQW structure can avoid/reduce generation of misfit dislocation, resulting in higher quality MQW-structure nature in principle than former InN-based MQWs. The proposed InN∕GaN MQWs are potentially applicable to room temperature operating excitonic devices working in short-wavelength visible colors.

High-Electron-Mobility InN Layers Grown by Boundary-Temperature-Controlled Epitaxy

A boundary-temperature-controlled epitaxy, where the growth temperature of InN is controlled at its maximum, is used to obtain high-electron-mobility InN layers on sapphire substrates by molecular beam epitaxy. The Hall-effect measurement shows a recorded electron mobility of 3280 cm2 V-1 s-1 and a residual electron concentration of 1.47×1017 cm-3 at room temperature. The enhanced electron mobility and reduced residual electron concentration are mainly due to the reduction of threading dislocation density. The obtained Hall mobilities are in good agreement with the theoretical modelling by the ensemble Monte Carlo simulation.

Effect of epitaxial temperature on N-polar InN films grown by molecular beam epitaxy

Effect of growth temperature on 2.3μm thick N-polar InN films grown on GaN template at 440–620°C by plasma-assisted molecular beam epitaxy was investigated. We found different growth behaviors depending on the growth temperatures, which greatly influenced surface morphology. The surface showed dendritic morphologies at temperatures lower than 540°C while step-flow-like morphologies and spiral growth were clearly observed at temperatures higher than 540°C. Crystalline quality was improved with increasing growth temperature up to 540°C and was almost saturated at higher temperatures. A gradual blueshift of the emission peak was observed from 13K photoluminescence (PL) spectra with increasing growth temperature, which was due to the increased compressive strain in InN films. A Poisson ratio (ν=0.21±0.05) for InN and a slope of PL peak energy as a function of ec (δE∕δec=18.9eV) were obtained. Our results indicated that the epitaxial temperatures of 540–600°C were best to achieve high quality InN films with a ...

Systematic study on p-type doping control of InN with different Mg concentrations in both In and N polarities

p-type conductivity control of In- and N-polar InN layers grown on GaN templates with different Mg concentrations ([Mg]) was systematically investigated by using electrolyte-based capacitance-voltage (ECV) analyses. With increasing [Mg], p-type conduction was confirmed for [Mg]s from ∼1018to∼3×1019cm−3. The conduction was reversed to n type at [Mg]s above 1020cm−3, however, due to overdoping effects introducing shallow donors in InN. Further, it was found that charges at the interface states located within forbidden band of InN greatly affected the ECV measurements resulting in overestimation of net acceptor concentrations, and some calibration was necessary to estimate them.

Threading dislocations in In-polar InN films and their effects on surface morphology and electrical properties

In-polar InN films with atomically flat surface are grown on Ga-polar GaN templates by molecular beam epitaxy. Densities of threading dislocations with screw and edge components in these films are about 108 and low 1010cm−2, respectively. It is found that the screw-component threading dislocation is the dominant cause for macroscopic surface defects appearing as growth-spiral hillocks; their densities (their growth temperature dependences) are almost the same (similar) with each other. Further, it is shown that the residual electron concentration in InN is almost the same with the density of dangling bonds at the edge-component threading dislocations.

Growth and properties of Mg-doped In-polar InN films

Mg doping into In-polar InN layers for different Mg fluxes is performed on GaN templates by molecular beam epitaxy, and their electrical and optical properties are investigated. Mg concentration is linearly proportional to Mg-beam flux, indicating that the Mg-sticking coefficient is almost unity. With Mg doping, electron concentration decreases by the effect of carrier compensation, but it begins to increase with further increasing Mg flux because of Mg-related donorlike-defects formation. For the partially carrier-compensated Mg-doped InN, two photoluminescence peaks are observed; one is originated from free-to-acceptor emission with an acceptor activation energy of about 61meV and the other is similar to the conventional band-to-band emission.

Step-Flow Growth of In-Polar InN by Molecular Beam Epitaxy

The step-flow mode growth of an In-polar InN epilayer on a GaN template was achieved by molecular beam epitaxy. A uniform terrace structure was observed with a step height of one monolayer. The surface rms roughness was less than 1 nm over a 10×10 µm2 area. To obtain the step-flow mode growth, it was preferable to use a slightly In-rich growth condition in the In-polarity growth regime, a GaN template of low dislocation density, and a high epitaxial temperature. A typical electron concentration of bulk InN was 5–6×1017 cm-3 with a Hall mobility of 1400 cm2/(V s).

Polarity control of ZnO films grown on nitrided c-sapphire by molecular-beam epitaxy

The polarity of molecular-beam epitaxy grown ZnO films was controlled on nitrided c-sapphire substrate by modifying the interface between the ZnO buffer layer and the nitrided sapphire. The ZnO film grown on nitrided sapphire was proven to be Zn-polar while the O-polar one was obtained by using gallium predeposition on nitrided sapphire, which was confirmed by coaxial impact collision ion scattering spectroscopy and chemical etching effect. The Zn-polar ZnO film showed higher growth rate, slightly better quality, and different surface morphology in comparison to the O-polar one.

Phonon lifetimes and phonon decay in InN

We report on the Raman analysis of A1(LO) (longitudinal optical) and E2 phonon lifetimes in InN and their temperature dependence from 80 to 700 K. Our experimental results show that among the various possible decay channels, the A1(LO) phonon decays asymmetrically into a high energy and a low energy phonon, whereas the E2 phonon predominantly decays into three phonons. Possible decay channels of the A1(LO) phonon may involve combinations of transverse optical and acoustic phonons. Phonon lifetimes of 1.3 and 4 ps were measured at 80 K for the A1(LO) and the E2 phonons, respectively. This rather long A1(LO) phonon lifetime suggests that hot phonon effects will play a role in InN for carrier relaxation.

Hole mobility in Mg-doped p-type InN films

Mg-doped p-type InN layers with different thicknesses were grown under the same growth/doping conditions so that their net acceptor concentrations were almost the same (3–6)×1018cm−3, which were confirmed by electrolyte capacitance-voltage measurements. The conductivity of p-InN region embedded under high density surface electrons could be extracted through the slope of total sheet conductivity against thickness, which was about 8.1Ω−1cm−1. Then, corresponding hole mobility was determined to be about 17–36cm2∕Vs for the hole concentrations of about (1.4–3.0)×1018cm−3 obtained by providing the hole effective mass and Mg acceptor activation energy as 0.42m0 and 61meV, respectively.