Yury Georgievich Shreter

Stacking Faults as Quantum Wells for Excitons in Wurtzite GaN

A model of the exciton bound to stacking faults (SF) in GaN is suggested. It is shown that SFs are potential wells (depth ≈120 meV and width ≈10 A) for electrons and potential barriers (height ≈60 meV and width ≈10 A) for holes. The binding energy of excitons at SF found from variational calculation is 45 meV. The 364 nm line in GaN photoluminescence at 4 K is attributed to excitons bound to SFs.

CATHODOLUMINESCENCE AND TRANSMISSION ELECTRON MICROSCOPY STUDY OF THE INFLUENCE OF CRYSTAL DEFECTS ON OPTICAL TRANSITIONS IN GAN

Defect related states and excitonic transitions in epitaxial GaN have been studied by combining cathodoluminescence and transmission electron microscopy. A series of deep levels with energies at about 2.4, 2.6 and 2.8 eV has been found by low temperature cathodoluminescence on free-standing 150 μm thick epitaxial GaN. These deep levels are characterised by a high recombination efficiency. They are radiative from 5 to 70 K and undergo a nonradiative transition at 70 K. These levels completely quench the near band edge and the conventional yellow emissions. We discuss the structural origin of these defects in terms of formation of VGa–SiGa and VGa–ON complexes. The consequences of our model with respect to non radiative transitions at threading dislocations are also presented. An excitonic transition at 3.41 eV close to the near band edge line on differently grown epitaxial GaN has been correlated to stacking faults. This line can be explained by a model based on the concept of excitons bound to SFs that form a quantum well of cubic material in the wurtzite lattice of the layer.

Light Emitting Diode with Charge Asymmetric Resonance Tunneling

We suggest a system of two wells connected with Charge Asymmetric Resonance Tunneling (CART) as a basic element of light emitting diode (LED) structure for semiconductors with different masses of electrons and holes. The system consists of an emitter of electrons, an emitter of holes and an active layer. The hole emitter is coupled with the active in such a way that holes can be freely supplied into the active layer without a barrier. The electron emitter is coupled to the active layer via a barrier. The barrier design uses the charge asymmetric resonance tunneling phenomenon which allows to make the barrier transparent for electrons and blocking for holes. Advantages of this design are: the increased capture efficiency of the electrons into the active layer due to direct resonance tunneling of the electrons from the electron emitter on bound electron level in the active quantum well, the suppression of electron leakage into the hole emitter, the elimination of the parasitic light generated outside the active layer, and the electron emitter acts also as a good current spreading layer. First results of experimental investigation and theoretical modeling of the CART LED devices are reported.

Efficiency droop in GaN LEDs at high current densities: Tunneling leakage currents and incomplete lateral carrier localization in InGaN/GaN quantum wells

The phenomenon of the emission efficiency droop of InGaN/GaN quantum wells (QWs) in light-emitting diode p-n structures is studied. The influence exerted by two basic processes on the emission efficiency is considered: tunnel injection into a QW and incomplete lateral carrier localization in compositional fluctuations of the band-gap width in InGaN. The sharp efficiency peak at low currents and the rapid efficiency droop with increasing current are due to tunneling leakage currents along extended defects, which appear as a result of a local increase in the electron hopping conductivity via the depletion n region and a corresponding local decrease in the height of the injection p barrier. A less sharp efficiency peak and a weak, nearly linear, decrease in efficiency with increasing current are caused by incomplete lateral carrier localization in the QW due to slowing-down of the carrier energy-relaxation rate and to the nonradiative recombination of mobile carriers.

Degradation and transient currents in III-nitride LEDs

Effect of degradation processes on transient currents in LEDs has been studied. It has been found that transient currents are several orders of magnitude higher than steady-state currents. The transient current time dependencies are non-exponential and show a distribution of relaxation times in the range of 1-100 microseconds. The charge associated with the transient currents is Q ~3x10-10 C which corresponds to high number of carrier traps Nt ~ 2x109 in the investigated chips. For one-year old chips an increase of charge and trap number by ~ 25% has been found compared to the fresh chips. Two probable reasons have been suggested to explain the observed increase of number of carrier traps: first one is related to increase of the number of trap sites at dislocations, and second one is a gradual phase separation process in quantum wells resulting in degradation of their quality.

Hopping transport in the space-charge region of p - n structures with InGaN/GaN QWs as a source of excess 1/ f noise and efficiency droop in LEDs

It is shown that the emission efficiency and the 1/f noise level in light-emitting diodes with InGaN/GaN quantum wells correlate with how the differential resistance of a diode varies with increasing current. Analysis of the results shows that hopping transport via defect states across the n-type part of the space-charge region results in limitation of the current by the tunneling resistance at intermediate currents and shunting of the n-type barrier at high currents. The increase in the average number of tunneling electrons suppresses the 1/f current noise at intermediate currents. The strong growth in the density of current noise at high currents, SJ ∝ J3, is attributed to a decrease in the average number of tunneling electrons as the n-type barrier decreases in height and width with increasing forward bias. The tunneling-recombination leakage current along extended defects grows faster than the tunneling injection current, which leads to emission efficiency droop.

On the laser lift-off of lightly doped micrometer-thick n -GaN films from substrates via the absorption of IR radiation in sapphire

The intense absorption of CO2 laser radiation in sapphire is used to separate GaN films from GaN templates on sapphire. Scanning of the sapphire substrate by the laser leads to the thermal dissociation of GaN at the GaN/sapphire interface and to the detachment of GaN films from the sapphire. The threshold density of the laser energy at which n-GaN started to dissociate is 1.6 ± 0.5 J/cm2. The mechanical-stress distribution and the surface morphology of GaN films and sapphire substrates before and after laser lift-off are studied by Raman spectroscopy, atomic-force microscopy, and scanning electron microscopy. A vertical Schottky diode with a forward current density of 100 A/cm2 at a voltage of 2 V and a maximum reverse voltage of 150 V is fabricated on the basis of a 9-μm-thick detached n-GaN film.

III-nitride efficient LEDs

A III-nitride blue LED structure based on the system of two wells with charge asymmetric resonance tunneling (CART), which allows enhancing the number of the electrons captured into the active region with the quantum well, was systematically studied. The barrier design uses the charge asymmetric resonance-tunneling phenomenon, which allows making the barrier transparent for electrons and blocking for holes. The growth and post-growth processes were optimized to achieve an efficient CART LED. The output power of 4 mW at the operating current of 20 mA has been achieved, corresponding to the external efficiency of 6%. Results presented in this report include the optimization of the quantum well growth parameters, bowing parameter for InGaN alloys grown on GaN, dry etching of III-nitride materials, Ohmic contacts to p- and n- type GaN, electrostatic discharge (ESD) problems related with the reliability of LEDs. The results presented include also modulation-technique LED characterization to tune the maximum radiative-recombination efficiency in accordance with the common operating current density.

Misfit dislocations and radiative efficiency of InxGa1-xN/GaN quantum wells

Abstract We report results of calculations of radiative efficiency of In x Ga 1− x N quantum wells embedded in wurtzite GaN epilayer. It was found that misfit dislocations with density up to ∼10 5–6 cm −1 could improve the quantum efficiency of the In x Ga 1− x N wells by more than 10 times because they reduce the quantum well built-in electric field. At higher densities, the misfit dislocations suppress the quantum efficiency of the wells since they produce an additional channel of nonradiative recombination.