Animesh Banerjee

InGaN/GaN disk-in-nanowire white light emitting diodes on (001) silicon

High density (∼1011 cm−2) GaN nanowires and InGaN/GaN disk-in-nanowire heterostructures have been grown on (001) silicon substrates by plasma-assisted molecular beam epitaxy. The nanowires exhibit excellent uniformity in length and diameter and a broad emission is obtained by incorporating InGaN disks of varying composition along the length of the nanowires. Monolithic lighting emitting diodes were fabricated with appropriate n- and p-doping of contact layers. White light emission with chromaticity coordinates of x=0.29 and y=0.37 and a correlated color temperature of 5500–6500 K at an injection current of 50 A/cm2 is measured. The measured external quantum efficiency of the devices do not exhibit any rollover (droop) up to an injection current density of 400 A/cm2.

A InGaN/GaN quantum dot green (λ=524 nm) laser

The characteristics of self-organized InGaN/GaN quantum dot lasers are reported. The laser heterostructures were grown on c-plane GaN substrates by plasma-assisted molecular beam epitaxy and the laser facets were formed by focused ion beam etching with gallium. Emission above threshold is characterized by a peak at 524 nm (green) and linewidth of 0.7 nm. The lowest measured threshold current density is 1.2 kA/cm2 at 278 K. The slope and wall plug efficiencies are 0.74 W/A and ∼1.1%, respectively, at 1.3 kA/cm2. The value of T0=233 K in the temperature range of 260–300 K.

InGaN/GaN Quantum Dot Red

Lasers emitting in the 600 nm wavelength range have gained attention for a number of important applications, including optical information processing, plastic fiber communication systems, optical storage, and full color (RGB) laser displays and laser projectors. Visible lasers are currently realized with GaN-based heterostructures having InGaN/GaN quantum wells as the gain media. The performance of these devices, particularly at longer wavelengths, is limited by materials inhomogeneity and effects related to a large strain-induced polarization in the quantum wells. A laser emitting in the red (λ ~ 630 nm) has not been realized. Here, we demonstrate lasers which emit at 630 nm, the longest wavelength achieved with the nitride system, by incorporating InGaN/GaN self-organized quantum dots as the gain media. Strain relaxation during dot formation results in reduced polarization fields and consequently low threshold current density, Jth=2.5 kA/cm2, small blue shift of the emission peak, very weak temperature dependenc eof Jth (T0=236 K), and linearly TE polarized output.

(\lambda=630~{\rm nm})

Acceptor doping of GaN with Mg during plasma-assisted molecular beam epitaxy, under N-rich conditions and a relatively high growth temperature of 740 °C, was investigated. The p-doping level steadily increases with increasing Mg flux. The highest doping level achieved, determined from Hall measurements, is 2.1×1018 cm−3. The corresponding doping efficiency and hole mobility are ∼4.9% and 3.7 cm2/V s at room temperature. Cross-sectional transmission electron microscopy and photoluminescence measurements confirm good crystalline and optical quality of the Mg-doped layers. An InGaN/GaN quantum dot light emitting diode (λpeak=529 nm) with p-GaN contact layers grown under N-rich condition exhibits a low series resistance of 9.8 Ω.

Laser

The differential gain and coherent output characteristics of blue-emitting In0.18Ga0.82N/GaN quantum dot ridge waveguide lasers have been measured. The laser heterostructures were grown by molecular beam epitaxy. Injected carrier lifetimes in the quantum dots have been measured by temperature dependent and time resolved photoluminescence measurements. The radiative lifetime at 280 K is 480 ps. The threshold current densities at room temperature are 930 and 970 A/cm2 for pulsed and continuous wave bias operation, respectively. The measured differential gain is 2 × 10−16 cm2. The output slope and wall plug efficiency at 1050 A/cm2 under continuous wave operation are 0.4 W/A and 0.4%, respectively. The measured blue shift in the emission wavelength due to screening of the piezoelectric field with injection is as small as 4.4 nm.

Mg doping of GaN grown by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions

We report the direct measurement of spin transport characteristics in a GaN spin valve, with a relatively defect-free single GaN nanowire (NW) as the channel and FeCo/MgO as the tunnel barrier spin contact. Hanle spin precession and non-local transport measurements are made in an unintentionally doped nanowire spin valves. Spin diffusion length and spin lifetime values of 260 nm and 100 ps, respectively, are derived. Appropriate control measurements have been made to verify spin injection, transport, and detection.We report the direct measurement of spin transport characteristics in a GaN spin valve, with a relatively defect-free single GaN nanowire (NW) as the channel and FeCo/MgO as the tunnel barrier spin contact. Hanle spin precession and non-local transport measurements are made in an unintentionally doped nanowire spin valves. Spin diffusion length and spin lifetime values of 260 nm and 100 ps, respectively, are derived. Appropriate control measurements have been made to verify spin injection, transport, and detection.

Continuous-wave operation and differential gain of InGaN/GaN quantum dot ridge waveguide lasers (λ = 420 nm) on c-plane GaN substrate

A spatial potential trap is formed in a 6.0-μm Al(Ga)N nanowire by varying the Al composition along its length during epitaxial growth. The polariton emission characteristics of a dielectric microcavity with the single nanowire embedded in-plane have been studied at room temperature. Excitation is provided at the Al(Ga)N end of the nanowire, and polariton emission is observed from the lowest bandgap GaN region within the potential trap. Comparison of the results with those measured in an identical microcavity with a uniform GaN nanowire and having an identical exciton–photon detuning suggests evaporative cooling of the polaritons as they are transported into the trap in the Al(Ga)N nanowire. Measurement of the spectral characteristics of the polariton emission, their momentum distribution, first-order spatial coherence, and time-resolved measurements of polariton cooling provides strong evidence of the formation of a near-equilibrium Bose–Einstein condensate in the GaN region of the nanowire at room temperature. In contrast, the condensate formed in the uniform GaN nanowire–dielectric microcavity without the spatial potential trap is only in self-equilibrium.

Room temperature single GaN nanowire spin valves with FeCo/MgO tunnel contacts

We report small-signal modulation bandwidth and differential gain measurements of a ridge waveguide In0.4Ga0.6N/GaN quantum dot laser grown by molecular beam epitaxy. The laser peak emission is at λ = 630 nm. The −3 dB bandwidth of an 800 μm long device was measured to be 2.4 GHz at 250 mA under pulsed biasing, demonstrating the possibility of high-speed operation of these devices. The differential gain was measured to be 5.3 × 10−17 cm2, and a gain compression factor of 2.87 × 10−17 cm3 is also derived from the small-signal modulation response.

Polariton Bose–Einstein condensate at room temperature in an Al(Ga)N nanowire–dielectric microcavity with a spatial potential trap

We have derived the Auger recombination coefficients, as a function of temperature, for In0.4Ga0.6N/GaN self-organized quantum dots from large-signal modulation measurements made on lasers in which the quantum dots form the gain media. The value of Ca = 1.3 ±0.2 × 10−31 cm6 s−1 at room temperature and the coefficient decreases with increase of temperature.

Small-signal modulation and differential gain of red-emitting (λ = 630 nm) InGaN/GaN quantum dot lasers

The barrier height of Schottky diodes made on InxGa1−xN nanowires have been determined from capacitance-voltage measurements. The nanowires were grown undoped on n-type (001) silicon substrates by plasma-assisted molecular beam epitaxy. The length, diameter and density of the nanowires are ∼1 μm, 20 nm, and 1×1011 cm−2. The Schottky contact was made on the top surface of the nanowires with Pt after planarizing with parylene. The measured barrier height ΦB varies from 1.4 eV (GaN) to 0.44 eV (In0.5Ga0.5N) and agrees well with the ideal barrier heights in the Schottky limit.