Y.Z. Tong

Temperature dependence of Raman scattering in single crystal GaN films

Micro-Raman scattering from single crystal GaN films, both free-standing and attached to Al2O3 substrates, was performed over the temperature range from 78 to 800 K. These measurements reveal that the Raman phonon frequency decreases and the linewidth broadens with increasing temperature. This temperature dependence is well described by an empirical relationship which has proved to be effective for other semiconductors. The experiments also demonstrate that the strain from Al2O3 substrates compresses the epitaxial GaN in the c-axis direction.

Relationship of background carrier concentration and defects in GaN grown by metalorganic vapor phase epitaxy

Experimental results show that the background carrier concentrations in GaN films grown by metalorganic vapor phase epitaxy are related to defects. A thermal equilibrium method was used to calculate the background carrier concentration related to intrinsic defects in an ideal GaN crystal. The results show that the N vacancy concentration does not exceed 2×1017 cm−3 in GaN grown at temperatures ranging from 800 to 1500 K. It can be concluded that the N vacancy is one of the major sources of carriers when the carrier concentration n 2×1017 cm−3; this conclusion may lead to ways for further improving the quality of GaN films.

TWO LOCAL VIBRATIONAL MODES RELATED TO HYDROGEN IN GAN

We have observed two absorption bands located at around 1730 and 2960 cm−1 in the infrared (IR) absorption spectra from undoped GaN samples which are grown using low pressure metalorganic vapor phase epitaxy and irradiated by gamma ray and then exposed to a radio frequency hydrogen plasma. Proton implantation followed by gamma-ray irradiation of the GaN samples can also activate the IR band at around 1730 cm−1. Based on the experimental results, we tentatively ascribe the 1730 cm−1 band to the local vibrational modes of Ga–H complexes in the vicinity of N vacancies and the 2960 cm−1 band to those of either N–H complexes in the vicinity of Ga vacancies or C–H complexes.

InGaN/GaN MQW high brightness LED grown by MOCVD

Abstract In this work, InGaN/GaN multi-quantum wells (MQWs) high brightness light emitting diode (LED) was grown by metalorganic chemical deposition (MOCVD). Photoluminescence (PL), X-ray diffraction (XRD), cathodoluminescence (CL), transmission electron microscope (TEM) were performed on the LED wafers to investigate the effects of growth temperature, well width and In composition in the InGaN quantum well on the emitting wavelength of LED. It was found that the wavelength of InGaN MQW-LED shifted from about 400–470 nm due to the difference in the In composition introducing into the InGaN well layer at different growth temperature. It was also demonstrated that the wavelength can be changed from 470 nm to 504 nm only by changed the thickness of the InGaN quantum well layer. Combined with the CL image and TEM observation, it was concluded that the change from MQW structure to multi-quantum dot structure would be the cause of the shift from 470 to 504 nm by changed the InGaN well width.

Formation and dissolution of microcrystalline graphite in carbon-implanted GaN

Two sharp bands at ∼1350 and ∼1600 cm−1 were observed in the Raman spectra of carbon-implanted GaN after postimplantation annealing treatments. The intensities of these two bands increased while their full widths at half maximum decreased with increasing annealing temperature. The observation of these two bands indicates the formation of microcrystalline graphite in C-implanted GaN. Hall measurements demonstrated that some dispersed C in GaN acted as acceptors and played a role in reducing electron concentration and Hall mobility. The facts that in 1100 °C furnace annealing the intensities of these two Raman peaks decreased rapidly to zero and the resistivity increased by 3 orders of magnitude indicate the dissolution of microcrystalline graphite at this temperature.

Etching damage and its recovery in n-GaN by reactive ion etching

Plasma-induced damage of n-type GaN in Cl2/CH4/Ar reactants and its recovery by the O2/CHF3 plasma in a reactive ion etching (RIE) system were studied by photoluminescence and Hall measurements. The mobility and optical properties of n-type GaN etched in Cl2/CH4/Ar were degraded gradually with increased radio frequency power. The effects of O2/CHF3 plasma treatment on the electrical and optical properties of n-type etched GaN were investigated by changing the ratio of O2/CHF3 flow rate. It was found that the damage caused by the conventional RIE process could be partly recovered by O2/CHF3 plasma treatment. The recovery mechanism was also discussed.

Occurrence of cubic GaN and strain relaxation in GaN buffer layers grown by low-pressure metalorganic vapor phase epitaxy on (0001) sapphire substrates

Investigations on GaN buffer layers grown by low-pressure metalorganic vapor phase epitaxy on (0001) sapphire substrates indicated that the mechanisms by way of which GaN buffer layers relax stresses introduced by the lattice mismatch and thermal expansion coefficient difference between GaN epilayer and sapphire substrate are related to both the crystallographic structure of GaN and thickness of the buffer layers. Beside forming misfit dislocations, mismatch-induced stresses can also be relaxed by forming stacking faults and microtwin boundaries parallel to (11-1) of GaN near the interface between GaN and sapphire substrate in cubic GaN buffer layers. It was found that, in cubic GaN buffer layers, there exists a critical thickness within which the stacking faults and/or microtwin boundaries parallel to (11-1) of GaN can be formed. This critical value is determined to be 50 nm.

Growth and doping characteristics of InGaN films grown by low pressure MOCVD

Abstract High quality InxGa1−xN films with x values up to 0.3 were grown by low pressure MOCVD. It was found that x values in InxGa1−xN films near exponentially decrease with growth temperature. The doping characteristics of InGaN films with Si and Zn were studied. At optimum growth conditions, the PL intensity of Zn doped InGaN film is about 30 times stronger than that of undoped InGaN film. The PL intensity of Si-doped InGaN film is strong as 13 times as that of undoped InGaN film.

Effects of oxidation by O2 plasma on formation of Ni∕Au ohmic contact to p-GaN

Oxidation of Ni∕Au (5nm∕10nm) contact to p-GaN layer was performed by O2 plasma in a reactive ion etching system. The structural characteristics of the Ni∕Au p-GaN for different oxidation time were investigated by x-ray diffraction (XRD) measurements, using an intense synchrotron x-ray source. The XRD measurements indicated that the grains of nickel oxide polycrystalline in the contact were grown continually when the oxidation time increased in 10min. However, Au showed amorphouslike and the intensities of Bragg diffraction peaks were hardly changed when oxidation time increased to 10min. The nickel oxide formed by O2 plasma without sequent thermal annealing did not reduce the specific contact resistance (ρc) to p-GaN, but it took an important role in lowering ρc followed by thermal annealing in N2 at 500°C for 10min. Optical transmission spectra confirmed that the nickel was easy to be oxidized and few interdiffusions occurred at the metal interface in O2 plasma ambient. Finally, the mechanism of oxidati...

Near-field spectroscopy of GaN blue light emission diode

The light emitting properties of GaN blue light diode has been characterized by near-field optical microscopy, near- field spectroscopy and conventional spectroscopy. Since the mechanism of the light mission from this material with high defect density is not yet fully understand, it is necessary to study the optical properties in conjunction with the nano-scale structure. The conventional spectroscopic methods are limited by the diffraction barrier, hence the information of the correlation of light emission and defects is not sufficient. By using near-field spectroscopy and near-field optical microscopy, we have studied the electro- emission spectrum of GaN blue diode, which is fabricated on sapphire substrate using low-pressure MOCVD epitaxy technique in our lab. The results how that the near-field spectroscopy can provide spatially resolved local spectrum of the samples surfaces with sub-wavelength resolution and hence provide a new technique to study the mechanism of light emission at nanometer scale. The dependence of light emission intensities vs. injection currents in the near- field spectra reveals the donor levels of the energy bands in GaN blue diode.