Nitride Semiconductor Heterostructures Research Articles

Nonlinear optical susceptibilities in InxGa1−xN/GaN hexagonal single quantum well under applied electric field

The hexagonal single quantum well fabricated using the nitride semiconductor heterostructures GaN/InxGa1−xN/GaN contains a built-in electric field as a result of lattice strain in this wurtzite structure. The eigenstates of the quantum well are obtained by solving the effective mass equation under applied electric field along the growth direction and opposite to it by the analytic Airy function approach. Analytic expressions for the nonlinear optical properties up to third-order are obtained by using the compact density matrix approach taking a quantum well containing two levels. The Stark shifted envelope functions yield large dipole matrix elements which enhance the optical susceptibilities. The internal and external electric fields significantly blue shift the susceptibilities. The susceptibilities are increased under electric field along the growth direction, while it decrease under the opposite direction.

Convergent beam electron-diffraction investigation of lattice mismatch and static disorder in GaAs/GaAs1−xNx intercalated GaAs/GaAs1−xNx:H heterostructures

Hydrogen incorporation in diluted nitride semiconductors dramatically modifies the electronic and structural properties of the crystal through the creation of nitrogen-hydrogen complexes. We report a convergent beam electron-diffraction characterization of diluted nitride semiconductor-heterostructures patterned at a sub-micron scale and selectively exposed to hydrogen. We present a method to determine separately perpendicular mismatch and static disorder in pristine and hydrogenated heterostructures. The roles of chemical composition and strain on static disorder have been separately assessed.

Microstructure and polarization fields in nitride semiconductors

The nitride semiconductors are widely used in high efficiency light emitting devices and are currently being considered for photovoltaic applications. The reduced symmetry in the wurtzite structure compared to cubic semiconductors results in the growth of large densities of crystalline defects and in the presence of strong spontaneous and piezoelectric polarization effects. A correlation between the microstructure and the polarization fields can be achieved with electron holography in the transmission electron microscope. Electron holograms thus obtained can provide energy band profiles with sub-nanometer spatial resolution. The phase of the electron beam is sensitive to the electrostatic potential, and a direct measurement of the latter can be achieved by making the electron beam signal that traverses the specimen interfere with a reference electron beam that travels through vacuum. This technique has been quite useful in probing the fields and charges at dislocations and at interfaces in semiconductors, and it is particularly useful to determine the piezoelectric effects in group III nitride semiconductor heterostructures. A review of applications to InGaN and AlGaN based heterostructures is presented in this paper.

Novel system for X-ray CTR scattering measurement on in-situ observation of OMVPE growth of nitride semiconductor heterostructures

A novel system for in-situ X-ray CTR scattering measurement on in-situ observation of group-III nitride semiconductor growth was constructed by setting an OMVPE reactor at the sample position of the X-ray diffractometer which was developed for in-situ measurement of growing semiconductor crystals. For in-situ observation of the growth of semiconductor, firstly, it is necessary to investigate the capabilities of the new system. In this work, after the safety check of the OMVPE reactor with thin Be windows, the capability of the new system as an X-ray diffractometer and a growth reactor of group-III nitride semiconductors was tested. After the GaInN layer was deposited successfully, the X-ray CTR scattering measurement was conducted at the growth temperature. The different CTR spectra measured at the growth temperature and at room-temperature after the growth were obtained. It indicates that the status of the crystal at growth conditions can be analyzed by using the new system. Therefore, the new system can be developed for in-situ observation of the OMVPE growth of the group-III nitride semiconductors.

Electrostatic energy profiles at nanometer‐scale in group III nitride semiconductors using electron holography

AbstractElectron holography in the transmission electron microscope can provide energy band profiles with sub‐nanometer spatial resolution. The phase of the electron beam is sensitive to the electrostatic potential, and a direct measurement of the latter can be achieved by making the electron beam signal that traverses the specimen interfere with a reference electron beam that travels through vacuum. This technique has been quite useful in probing the fields and charges at dislocations and at interfaces in semiconductors. This article presents a review of work done with the participation of the author in the past decade in the use of this technique to determine the piezoelectric effects in group III nitride semiconductor heterostructures.

Demonstration and analysis of reduced reverse-bias leakage current via design of nitride semiconductor heterostructures grown by molecular-beam epitaxy

An approach for reducing reverse-bias leakage currents in Schottky contacts formed to nitride semiconductor heterostructures grown by molecular-beam epitaxy is described, demonstrated, and analyzed. By incorporation of a GaN cap layer atop a conventional AlxGa1−xN∕GaN heterostructure field-effect transistor epitaxial layer structure, the direction of the electric field at the metal-semiconductor interface of a Schottky contact is reversed, resulting in a suppression of electron flow into conductive screw dislocations that are known to dominate reverse-bias leakage currents in nitride semiconductors grown by molecular-beam epitaxy. Analysis of temperature-dependent current-voltage characteristics indicates that, in structures incorporating a GaN cap layer, reverse-bias leakage currents are reduced by one to three orders of magnitude, with the mechanism for leakage current flow differing from that established previously for the more conventional structure due to the alteration in the electric field at the metal-semiconductor interface. Scanned probe measurements of local, nanoscale current distributions confirm directly that current flow via conductive dislocations is suppressed in structures incorporating the GaN cap layer.

Blue and Near-Ultraviolet Vertical-Cavity Surface-Emitting Lasers

AbstractVertical-cavity surface-emitting lasers (VCSELs) based on gallium nitride semiconductor heterostructures represent a contemporary focus of research, the aim of which is to develop a new class of planar microdevices in the blue, violet, and near-ultraviolet ranges. We review recent progress in this exploratory area, to highlight the challenges and scientific excitement associated with the efforts that are under way to create flexible, short-wavelength sources for applications expected to range from biomedical diagnostics to solid-state displays and lighting.

Polarization charges and polarization-induced barriers in AlxGa1−xN/GaN and InyGa1−yN/GaN heterostructures

Polarization charges are measured and the formation of large electrostatic barriers arising primarily as a consequence of the presence of polarization-induced charge densities is deduced from capacitance–voltage analysis of n-type AlxGa1−xN/GaN and InyGa1−yN/GaN heterostructures. In structures consisting of 5–10 nm AlxGa1−xN or InyGa1−yN surrounded by n-GaN, capacitance–voltage profiling studies combined with elementary electrostatic analysis yield experimental estimates of polarization charge densities, which are compared with values expected based on the combined effects of spontaneous and piezoelectric polarization. These results imply the existence of electrostatic barriers that are due primarily to the large polarization charge densities at each heterojunction interface and the resulting potential difference maintained across the thin AlxGa1−xN or InyGa1−yN layers. The electrostatic barriers formed in these structures are large in comparison to the heterojunction conduction-band offsets, demonstrating the utility of polarization-based engineering of electrostatic barriers in nitride semiconductor heterostructures.

Charge transport in low-dimensional nitride semiconductor heterostructures

Low-dimensional conducting channels in high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures have been modeled and the results are described in this paper. The Schroedinger's equation and the Poisson's equation have been solved self-consistently in order to obtain a relationship between the sheet carrier density and the applied gate voltage. The relationship is treated using a non-linear exponential fit that enables a more accurate analysis of the transport saturation region compared to other models used hitherto. The current–voltage ( I– V) characteristics have then been determined by a charge control analysis that utilizes the exponential charge–potential relationship. This identifies the saturation point of channel conduction without parameters extracted from experiments. The intrinsic non-linearity of transport identified by the simulation explains why such devices do not have a singular channel “pinch-off” voltage.