Measurement of GaN/Ge(001) Heterojunction Valence Band Offset by X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset (VBO) at the GaN/Ge heterostructure interface. The VBO is directly determined to be 1.13 +/- 0.19 eV, according to the relationship between the conduction band offset Delta E-C and the valence band offset Delta E-V : Delta E-C = E-g(GaN) - E-g(Ge) - Delta E-V, and taking the room-temperature band-gaps as 3.4 and 0.67 eV for GaN and Ge, respectively. The conduction band offset is deduced to be 1.6 +/- 0.19 eV, which indicates a type-I band alignment for GaN/Ge. Accurate determination of the valence and conduction band offsets is important for the use of GaN/Ge based devices.

First principles study of N-N split interstitial in GaN nanowires

Atomic and electronic properties of N-N split interstitial in GaN nanowires have been investigated using first principles calculations. The formation energy calculations show that the N-N interstitial favors substituting an N atom at the surface of the nanowires. The interstitial induces localized states in the band gap of GaN nanowires.

Charge Separation in Wurtzite/Zinc-Blende Heterojunction GaN Nanowires

The electronic properties of wurtzite/zinc-blende (WZ/ZB) heterojunction GaN are investigated using first-principles methods. A small component of ZB stacking formed along the growth direction in the WZ GaN nanowires does not show a significant effect on the electronic property, whereas a charge separation of electrons and holes occurs along the directions perpendicular to the growth direction in the ZB stacking. The later case provides an efficient way to separate the charge through controlling crystal structure. These results have significant implications for most state of the art excitonic solar cells and the tuning region in tunable laser diodes.

The fabrication of GaN-based nanopillar light-emitting diodes

InGaN/GaN multiple quantum well-based light-emitting diode (LED) nanopillar arrays were fabricated using Ni self-assembled nanodots as etching mask. The Ni nanodots were fabricated with a density of 6 x 10(8)-1.5 x 10(9) cm(-2) and a dimension of 100-250 nm with varying Ni thickness and annealing duration time. Then LED nanopillar arrays with diameter of approximately 250 nm and height of 700 nm were fabricated by inductively coupled plasma etching. In comparison to the as-grown LED sample an enhancement by a factor of four of photoluminescence (PL) intensity is achieved for the nanopillars and a blueshift as well as a decrease in full width at half maximum of the PL peak are also observed. The method of additional chemical etching was used to remove the etching-induced damage. Then nano-LED devices were further completed using a planarization approach to deposit p-type electrode on the tips of nanopillars. The current-voltage curves of both nanopillars and planar LED devices are measured for comparison.

Abnormal photoabsorption in high resistance GaN epilayer

Unintentionally doped GaN epilayers are grown by the metalorganic chemical vapor deposition (MOCVD). Photovoltaic (PV) spectroscopy shows that there appears an abnormal photoabsorption in some undoped GaN films with high resistance. The peak energy of the absorption spectrum is smaller than the intrinsic energy band gap of GaN. This phenomenon may be related to exciton absorption. Then metal-semiconductor-metal (MSM) Schottky photodetectors are fabricated on these high resistance epilayers. The photo spectrum responses are different when the light individually irradiates each of the two electrodes with the photodetector which are differently biased. When the excitation light irradiates around the reverse biased Schottky junction, the responsivity is almost one order of magnitude larger than that around the forward biased junction. Furthermore, when the excitation light irradiates the reverse biased Schottky junction, the peak energy of the spectrum has a prominent red-shift compared with the peak energy of the spectrum measured with the excitation light irradiating the forward biased Schottky junction. The shift value is about 28 meV, and it is found to be insensitive to temperature. According to the analyses of the distribution of the electric field within the MSM device and the different dependences of the response on the electric field intensity between the free carriers and excitons, a reliable explanation for the different response among various areas is proposed.

The exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses

This paper studies the exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses by low temperature photoluminescence (PL) measurements With increasing cap layer thickness, the PL peak energy shifts to lower energy and the coupling strength between the exciton and longitudinal-optical (LO) phonon, described by Huang-Rhys factor, increases remarkably due to an enhancement of the internal electric field With increasing excitation intensity, the zero-phonon peak shows a blueshift and the Huang-Rhys factor decreases These results reveal that there is a large built-in electric field in the well layer and the exciton-LO phonon coupling is strongly affected by the thickness of the cap layer

STUDY ON PREPARATION OF GAN AND COSI2 EPITAXIAL-FILMS BY MASS ANALYZED LOW-ENERGY DUAL ION-BEAM EPITAXY

The Mass Analyzed Low Energy Dual Ion Beam Epitaxy (MALE-DIBE) system has been designed and constructed in our laboratory. We believe that the system, which was installed and came into full operation in 1988, is the first facility of this kind. With our system we have carried out studies, for the first time, on compound synthesis of GaN and CoSi2 epitaxial thin films. RHEED and AES results show that GaN films, which were deposited on Si and sapphire substrates, are monocrystalline and of good stoichiometry. To our knowledge, GaN film heteroepitaxially grown on Si. which is more lattice-mismatched than GaN on sapphire, has not been reported before by other authors. RBS and TEM investigations indicated a rather good crystallinity of CoSi2 with a distinct interface between CoSi2 and the Si substrate. The channelling minimum yield chi(min) from the Co profile is approximately 4%. The results showed that the DIBE system with simultaneous arrival of two beams at the target is particularly useful in the formation of novel compounds at a relatively low substrate temperature.

MBE of GaN with DC-plasma source for nitrogen activation

In this paper we report on the first results of epitaxial growth of GaN layers on GaAs (100) substrates using a modified MBE system, equipped with a DC-plasma source for nitrogen activation in configuration of reverse magnetron at ultra-low pressures.

Growth of GaN layers on GaAs and GaP (111) and (001) substrates by molecular beam epitaxy

Films of GaN have been grown using a modified MBE technique in which the active nitrogen is supplied from an RF plasma source. Wurtzite films grown on (001) oriented GaAs substrates show highly defective, ordered polycrystalline growth with a columnar structure, the (0001) planes of the layers being parallel to the (001) planes of the GaAs substrate. Films grown using a coincident As flux, however, have a single crystal zinc-blende growth mode. They have better structural and optical properties. To improve the properties of the wurtzite films we have studied the growth of such films on (111) oriented GaAs and GaP substrates. The improved structural properties of such films, assessed using X-ray and TEM method, correlate with better low-temperature FL.

Cyclotron-resonance mass of two-dimensional electrons in GaN/AlxGa1-xN heterostructures

The electron cyclotron-resonance (CR) mass of quasi-two-dimensional electrons in GaN/AlxGa1-xN heterostructures is studied theoretically. The correction to the CR mass due to electron-phonon interaction is investigated, taking into account band nonparabolicity, the occupation effect, and the screening of the electron-phonon coupling. The dependence of the CR mass on the electron density and on the magnetic field strength is displayed in detail, and the calculated CR mass agrees well with a recent experiment. We found that the effective electron-phonon coupling strength in GaN heterostructures is reduced below the bulk value.

GaN m-i-n LED grown by MOVPE

Undoped and Zinc-doped GaN films have been grown using TMGa, DEZn and Ammonia by MOVPE. The GaN blue-green LEDs of m-i-n structure have been fabricated. They can be operated at forward bias less than 5 volts. The EL peak wavelengths was from 455 nm to 504 nm.

Properties of cubic GaN grown by MBE

We review our investigation of cubic GaN films on (001) GaAs, focusing on the structural, optical, and electrical properties of these films. Cubic GaN films grown epitaxially on GaAs suffer from the large lattice mismatch between these two materials in that they contain extremely high densities of structural defects. Surprisingly, the optical quality of these films does not seem much affected by the presence of defects, as intense photoluminescence is detected a? room temperature and above. Finally, the rather high background electron concentrations in our films is shown to be a consequence of contamination with O and not to be an intrinsic property of cubic phase GaN. (C) 1997 Elsevier Science S.A.

Donor acceptor pair in molecular beam epitaxy grown GaN

We have recently found evidence of new donor acceptor pair (DAP) luminescence in molecular beam epitaxy (MBE) grown films. A variety of nominally undoped samples have been studied by photoluminescence (PL) over a temperature range of 5-300 K. The samples show intensive luminescence al energies of 3.404-3.413 eV varying with different sample at 5 K, as well as a fairly strong (DX)-X-0 line at low temperature. We attribute the Line at 3.404-3.413 eV to DAP recombination which is over 0.1 eV different from the well known DAP caused by ME-doping in GaN. The DAP line shows fine structure. it even predominates in one particular sample. The peak position shifts to higher energy with temperature increasing from 5 up to 70 K, and as the excitation laser intensity increases. The data are consistent with DAP luminescence involving an acceptor level of about 90 meV (presumably carbon) above the valence band edge in GaN. It is much shallower than the acceptor level of 250 meV produced by the p-type dopant Mg which is commonly used at present. (C) 1997 Elsevier Science S.A.

Cyclotron resonance and quantum Hall effect studies of the two-dimensional electron gas confined at the GaN/AlGaN interface

We report on high magnetic fields (up to 40 T) cyclotron resonance, quantum Hall effect and Shubnikov-de-Hass measurements in high frequency transistors based on Si-doped GaN-AlGaN heterojunctions. A simple way of precise modelling of the cyclotron absorption in these heterojunctions is presented, We clearly establish two-dimensional electrons to be the dominant conducting carriers and determine precisely their in-plane effective mass to be 0.230 +/- 0.005 of the free electron effective mass. The increase of the effective mass with an increase of two-dimensional carrier density is observed and explained by the nonparabolicity effect. (C) 1997 American Institute of Physics.

Valence hole subbands and optical gain spectra of GaN/Ga1-xAlxN strained quantum wells

The valence hole subbands, TE and TM mode optical gains, transparency carrier density, and radiative current density of the zinc-blende GaN/Ga0.85Al0.15N strained quantum well (100 Angstrom well width) have been investigated using a 6 X 6 Hamiltonian model including the heavy hole, Light hole, and spin-orbit split-off bands. At the k = 0 point, it is found that the light hole strongly couples with the spin-orbit split-off hole, resulting in the so+lh hybrid states. The heavy hole does not couple with the light hole and the spin-orbit split-off hole. Optical transitions between the valence subbands and the conduction subbands obey the Delta n=0 selection rule. At the k not equal 0 points, there is strong band mixing among the heavy hole, light hole, and spin-orbit split-off hole. The optical transitions do not obey the Delta n=0 selection rule. The compressive strain in the GaN well region increases the energy separation between the so1+lh1 energy level and the hh1 energy level. Consequently, the compressive strain enhances the TE mode optical gain, and strongly depresses the TM mode optical gain. Even when the carrier density is as large as 10(19) cm(-3), there is no positive TM mode optical gain. The TE mode optical gain spectrum has a peak at around 3.26 eV. The transparency carrier density is 6.5 X 10(18) cm(-3), which is larger than that of GaAs quantum well. The compressive strain overall reduces the transparency carrier density. The J(rad) is 0.53 kA/cm(2) for the zero optical gain. The results obtained in this work will be useful in designing quantum well GaN laser diodes and detectors. (C) 1996 American Institute of Physics.