Summary form only given. We report our studies on the 2D lateral diffusion behaviors in InGaN/GaN MQWs using optical techniques. Similar to previous observation of the giant ambipolar diffusion coefficient in GaAs-based n-i-p-i superlattices, we have observed giant ambipolar diffusion coefficient in large well-width InGaN MQWs due to the spatial charge separation by the large built-in piezoelectric field. With a well width of 62 /spl Aring/, a room-temperature ambipolar diffusion. coefficient of 2700/spl plusmn/500 cm/sup 2//s was measured.

Lateral diffusion behavior of two-dimensional carrier gas in InGaN/GaN multiple quantum wells was investigated using optical transient transmission measurements. A large ambipolar diffusion coefficient was observed, which was attributed to the enhancement by a strong piezoelectric field. This large ambipolar diffusion coefficient was found to increase with increased well width with a value on the order of 3000 cm2/s for a 62 Å well-width sample.

Taking advantage of the electric field-enhanced second-harmonic generation effect in bulk gallium nitride (GaN) and indium gallium nitride (InGaN) quantum wells, we demonstrated the piezoelectric field distribution mapping in bulk GaN and InGaN multiple-quantum-well (MQW) samples using scanning second-harmonic generation (SHG) microscopy. Scanning SHG microscopy and the accompanying third-harmonic generation (THG) microscopy of the bulk GaN sample were demonstrated using a femtosecond Cr:forsterite laser at a wavelength of 1230 nm. Taking advantage of the off-resonant electric field-enhanced SHG effect and the bandtail state-resonance THG effect, the second- and third-harmonic generation microscopic images obtained revealed the piezoelectric field and bandtail state distributions in a GaN sample. Combined with 720 nm wavelength excited two-photon fluorescence microscopy in the same sample, the increased defect density around the defect area was found to suppress bandedge photoluminescence, to increase yellow luminescence, to increase bandtail state density, and to decrease residue piezoelectric field intensity. Scanning SHG microscopy of the InGaN MQW sample was resonant excited with 800 nm femtosecond pulses from a Ti:sapphire laser in order to suppress SHG contribution from the bulk GaN substrate. Taking advantage of the strong piezoelectric field inside the InGaN quantum well, the wavelength resonant effect, and the electric field-enhanced SHG effect of InGaN quantum wells, resonant scanning SHG microscopy revealed the piezoelectric field distribution inside the wells. Combined with accompanying three-photon fluorescence microscopy from the bulk GaN substrate underneath the quantum wells, the direct correspondence between the piezoelectric field strength inside the quantum well and the substrate quality can be obtained. According to our study, the GaN substrate area with bright bandedge luminescence corresponds to the area with strong SHG signals indicating a higher stained-induced piezoelectric field. These scanning harmonic generation microscopies exhibit superior images of the piezoelectric field and defect state distributions in GaN and InGaN MQWs not available before. Combining with scanning multiphoton fluorescence microscopy, these techniques open new ways for the physical property study of this important material system and can provide interesting details that are not readily available by other microscopic techniques.

We demonstrate coherent optical control of phonon oscillations using a femtosecond control pulse. The experiments were performed in InGaN/GaN multiple quantum wells. Coherent acoustic phonon oscillations was initiated with an UV femtosecond pulse. The subsequent manipulation, including magnitude and phase, of the coherent acoustic phonon oscillations was achieved using another UV femtosecond pulse by controlling the pulse time delay and intensity.

Room temperature (RT) pulsed operation of blue nitride based multi-quantum well (MQW) laser diodes grown on c-plane sapphire substrates was achieved. Atmospheric pressure MOCVD was used to grow the active region of the device which consisted of a 10 pair In{sub 0.21}Ga{sub 0.79}N (2.5nm)/In{sub 0.07}Ga{sub 0.93}N (5nm) InGaN MQW. The threshold current density was reduced by a factor of 2 from 10 kA/cm{sup 2} for laser diodes grown on sapphire substrates to 4.8 kA/cm{sub 2} for laser diodes grown on lateral epitaxial overgrowth (LEO) GaN on sapphire. Lasing wavelengths as long as 425nm were obtained. LEDs with emission wavelengths as long as 500nm were obtained by increasing the Indium content. These results show that a reduction in nonradiative recombination from a reduced dislocation density leads to a higher internal quantum efficiency. Further research on GaN based laser diodes is needed to extend the wavelength to 490nm which is required for numerous bio-detection applications. The GaN blue lasers will be used to stimulate fluorescence in special dye molecules when the dyes are attached to specific molecules or microorganisms. Fluorescein is one commonly used dye molecule for chemical and biological warfare agent detection, and its optimal excitation wavelength is 490 nm. InGaN alloys can be used to reach this wavelength.

Etch damage of GaN was investigated using a quantum-well probe structure. A clear decrease in photoluminescence (PL) intensity was observed and was aggravated with increasing ion-beam voltage. The magnitude of decrease in PL intensity was much larger than expected, even greater than for GaAs subjected to similar etch conditions. Angle-dependent bombardment studies were carried out to investigate channeling as a damage mechanism in GaN. The large decrease in PL intensity observed near normal incidence or along the [0001] direction suggests that channeling is a damage mechanism for low-energy bombardment in GaN.

Summary form only given. We report on our study of the carrier dynamics in InGaN/GaN MQWs by using femtosecond pump-probe transmission measurements. Our study indicates the existence of a strong barrier field, which is in balance with the strong piezoelectric field within the QWs. After photocarriers have been excited by optical pulses, the thermionically emitted electrons and holes were found to be driven by the barrier field toward different sides of the MQWs, which caused the electron/hole wavefunction separation. The space charge field induced by the separated electrons and holes will screen out part of the barrier field and increase the total electric field in the QWs. Due to the existing barrier field, a weak dependence of the thermionic emission time on the barrier width was observed. Our study suggests that the separated electron/hole wavefunctions might lead to decreased wavefunction overlaps and cause the increased carrier lifetime.

We have observed coherent acoustic-phonon oscillation in InGaN/GaN multiple-quantum wells. With femtosecond pulse excitation, photogenerated carriers screen out the strained-induced piezoelectric field and initiate the coherent acoustic-phonon oscillation. The resulted modulation of the piezoelectric field will then cause large optical property oscillation through the quantum-confined Stark effect.

The effects of Si-doping in barriers on both optical and structural properties of InGaN/InGaN multiquantum well (MQW) purplish-blue laser diode (LD) wafers were studied. Although evidence for effective bandgap inhomogeneity was found in both structures, the MQWs with the Si-doped barriers exhibited smaller apparent Stokes-like shifts and higher order satellite peak in the XRD pattern. The Si-doping enhanced the Coulomb screening of the piezoelectric field (F-PZ) in the wells. Formation of nanoscale islands found in the undoped quantum well (QW) structures was suppressed by doping-with Si in the barriers. In addition anomalous emergence of a secondary amplified spontaneous emission (ASE) peak at 3.05 eV, which was found in the undoped barrier MQWs and uncoupled with the primary one at 2.93 eV, was suppressed. Furthermore Si-doping reduced the threshold power density required to obtain the stimulated emission. In contrast to other III-V QWs, under our growth conditions, Si-doping was found to improve the structural and electronic homogeneity.

In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the “V-defect”) initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {101̄1} planes) and an open hexagonal inverted pyramid which is defined by the six {101̄1} planes. Thus, in cross section this defect appears as an open “V”. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({101̄1} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the following characteristics: (i) its intensity is correlated with the side-wall quantum wells; (ii) the temperature independence of the full width at half maximum strongly supports a localized exciton recombination process.

The MOCVD growth of InGaN / GaN multiple quantum well (MQW) structures for optoelectronic applications has been investigated. The structural and optical properties of the layers have been characterized by x-ray diffraction and photoluminescence. The effect of barrier and well dimensions on the optical properties have been examined; highest emission intensity and narrowest linewidth were obtained with thin wells (20-30 Å) and thick barriers (greater than 50 Å). By incorporating an MQW structure as the active region in a GaN p-n diode, high-brightness light emitting diodes (LEDs) have been produced. Under a forward current of 20 raA, these devices emit 2.2 mW of power corresponding to an external quantum efficiency of 4.5%. The emission spectrum peaks at 445 nm and exhibits a narrow linewidth of 28 nm. Under pulsed high current conditions, output power as high as 53 mW was realized and the peak emission wavelength shifted to 430 nm.

Room temperature (RT) pulsed operation of blue (420 nm) nitride based multi-quantum well (MQW) laser diodes grown on c-plane sapphire substrates with lifetimes exceeding 6 hours have been demonstrated. Threshold current densities as low as 12.7 kA/cm2 were observed for 10×1200 µm lasers with uncoated reactive ion etched (RIE) facets. The emission is strongly TE polarized and has a sharp transition in the far field pattern above threshold.This work was supported by: DARPA (Dr. Anis Husain, Program Manager), ARO, NSF, HP Labs and Philips. The authors would like to acknowledge Daniel Cohen, Evelyn Hu, Milan Minsky, James Speck, X. Wu, John Bowers, Kehl Sink of UCSB and Bruce Young of Philips Research Labs for useful inputs and discussions.

We have investigated spectroscopically the emergence of gain in InGaN/GaN quantum well diodes under high current injection (>kA/cm2). The spectral characteristics suggest that the electronic states responsible for blue laser action in this material are strongly influenced by the presence of microscopic crystalline disorder.

E-beam pumped lasers are attractive for Laser Cathode Ray Tubes (LCRT) in projection displays and a variety of applications typically associated with optically pumped lasers. For the first time an InGaN/GaN multiple quantum well (MQW) in-plane laser pumped by surface normal pulse and scanning electron beams was demonstrated. Pumping at room temperature (RT) and 80 K showed peak stimulated emission wavelengths of 402 and 409 nm with a full width half maximum (FWHM) of 0.6 nm and 1.2 nm, respectively. The threshold electron beam current densities have been estimated as 60 A/cm 2 for 35 keV electron energy at 80 K using scanning e-beam pumping and 200-300 A/cm 2 at RT using pulsed e-beam pumping with a maximum electron energy of 150 keV. At 80 K, light output of 150 mW was measured out of one facet at an e-beam current of 1.7 mA.