Junlin Liu

The Hartman effect in graphene

We investigate theoretically the Hartman effect in quantum tunneling through single and double barriers in a single graphene layer. The numerical results indicate that the Hartman effect in graphene depends heavily on the incident angle and the energy of the carrier in the tunneling process through single and double barriers. We find that the Hartman effect disappears for normal incidence and appears when the incident angle and energy are larger than some critical values.

Roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well light-emitting diodes

The roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well (MQW) light-emitting diodes are investigated by numerical simulation. The simulation results show that V-shaped pits cannot only screen dislocations, but also play an important role on promoting hole injection into the MQWs. It is revealed that the injection of holes into the MQW via the sidewalls of the V-shaped pits is easier than via the flat region, due to the lower polarization charge densities in the sidewall structure with lower In concentration and {10–11}-oriented semi-polar facets.

Stability of Al/Ti/Au contacts to N-polar n-GaN of GaN based vertical light emitting diode on silicon substrate

The electrical characteristics of Al/Ti/Au contact to N-polar n-GaN on Si substrate are investigated. It was found that a pre-treatment to the surface with Ar plasma could significantly enhance the stability of Al/Ti/Au contact. Forward voltage of the pre-treated sample was stabilized at about 3.23 V upon 1000 h aging under 900 mA and room temperature. In contrast, forward voltage of the untreated sample increased from 3.52 V to 4 V after 24 h aging. Those differences between the Ar plasma treated sample and untreated sample were attributed to the increase of the VN concentrate near surface of n-GaN by the Ar plasma treatment.

Effects of AlN interlayer on growth of GaN-based LED on patterned silicon substrate

GaN-based light emitting diodes (LEDs) were grown on 1 mm × 1 mm patterned 2-inch and 6-inch Si (111) substrates by metal–organic vapour phase epitaxy (MOVPE). AlN interlayers with different thicknesses were introduced between the composition-graded AlGaN buffer layer and the GaN seed layer in different LED structures. The crystalline quality, wavelength uniformity and crack density of the 2-inch wafer were improved by increasing the AlN interlayer thickness. With a 30 nm AlN interlayer, a crack-free, smooth and reflective 6-inch LED wafer was grown, the full width at half maximum (FWHM) of the XRD rocking curves of GaN (002) and GaN (102) planes were 384 and 432 arcsec, respectively. The standard deviation of thickness and dominant wavelength were 0.03 μm (average thickness was 3.84 μm) and 1.52 nm (average dominant wavelength was 457.9 nm), respectively. The AlN interlayer can change the growth mode of the GaN seed layer and subsequent n-GaN, which changes the density of dislocation and the residual tensile stress of the GaN film. A smaller residual tensile stress in the GaN film can help to reduce bowing of the wafer, improve the wavelength uniformity, and suppress the generation of cracks.

Electroluminescence from the sidewall quantum wells in the V-shaped pits of InGaN light emitting diodes

InGaN/GaN multi-quantum well (MQW) light emitting diodes with heavily Mg doped and unintentionally doped (UID) low-temperature Al0.2Ga0.8N electron blocking layer (EBL) were investigated. Broad short-wavelength electroluminescence peak, which has strong relative intensity to the main emission, was found in the UID-EBL sample at cryogenic temperatures. Study suggests that the broad peak is emitted by the sidewall MQWs. This result indicates that the electroluminescence of sidewall MQWs, in which the carrier density is high enough, can be detected at cryogenic temperatures. The lineshape variation with current density reveals detailed information on the process of carrier injection into the sidewall MQWs.

The effect of silicon doping in the barrier on the electroluminescence of InGaN/GaN multiple quantum well light emitting diodes

InGaN/GaN multiple quantum well (MQW) light emitting diodes were grown on silicon substrate by metal organic chemical vapor deposition. A different barrier was heavily doped with silicon based on the same structure. Temperature dependent electroluminescence was performed on the devices. The results reveal that heavily doping the barrier distant from the n-type layer with silicon causes two emission peaks. As the doped barrier gets closer to n-type layer, the energy gap between the two peaks becomes narrower. Silicon doped in the barrier is believed to generate p-n junction built-in field from the doped barrier towards p-type layer. This field compensates the piezoelectric field in the well(s) between the doped barrier and p-type layer. It results in higher emission energy of this (these) well(s). When the doped barrier gets closer to the n-type layer, the compensation is less significant.

Hole injection from the sidewall of V-shaped pits into c-plane multiple quantum wells in InGaN light emitting diodes

The role which the V-shaped pits (V-pits) play in InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) has been proposed to enable the formation of sidewall MQWs, whose higher bandgap than that of the c-plane MQWs is considered to act as an energy barrier to prevent carriers from reaching the dislocations. Here, with increasing proportion of current flowing via the V-pits, the emission of the c-plane MQWs broadens across the short-wavelength band and shows a blueshift successively. This phenomenon is attributed to hole injection from the sidewall of V-pits into the c-plane MQWs, which is a new discovery in the injection mechanism of InGaN/GaN MQW LEDs.

A new interpretation for performance improvement of high-efficiency vertical blue light-emitting diodes by InGaN/GaN superlattices

The effect of InGaN/GaN superlattices(SLs) on quantum efficiency and forward voltage of vertical blue InGaN/GaN multiple quantum well(MQW)light-emitting diodes(LED) grown on Si substrate has been experimentally and theoretically investigated. We have prepared two LED samples, in which the 30 and 45 periods of SLs are inserted between MQW active layers and n-GaN layer, respectively. Electroluminescence measurement shows that the LED with 45 periods of SLs has higher quantum efficiency but lower forward voltage. It is observed that V-shaped pits grow up in size with an increase in SLs period number by means of scan transmission electron microscope and secondary ion mass spectrometry. Further numerical simulations confirm that the performance improvement of LED by SLs is mainly ascribed to enhancing hole injection from the V-shaped pits.

Temperature-dependent electroluminescence from InGaN/GaN green light-emitting diodes on silicon with different quantum-well structures

Temperature-dependent electroluminescence from InGaN/GaN light-emitting diodes (LEDs) grown on Si (111) are investigated. With the increase of current density, internal quantum efficiencies (IQEs) firstly rise accompanied by full width at half maximum (FWHM) shrinkage and then IQEs droop combined with FWHM broadening are presented. With the decline of temperature, both the maximum of IQEs accompanying the minimum of FWHM shift towards the direction of low current density. Moreover, the maximum of IQEs shift faster than the minimum of FWHM for single quantum well LED. Furthermore, it was found that the quantum well close to n-GaN has priority to radiate in small current injection, especially at low temperature (100 K) for multiple quantum wells LED.

Effects of thickness ratio of InGaN to GaN in superlattice strain relief layer on the optoelectrical properties of InGaN-based green LEDs grown on Si substrates

InGaN-based multiple quantum well (MQW) green light-emitting diodes with a InGaN/GaN superlattice as a strain relief layer (SSRL) were grown on Si(111) substrates by metal organic chemical vapor deposition. The influences of the thickness ratio of InGaN to GaN in SSRL on the optoelectrical properties have been investigated. Electrical measurements show that the sample with a higher thickness ratio has a lower series resistance. This is mainly ascribed to the improvement of carrier vertical transport due to the thinner GaN in SSRL. However, it is found that the leakage current increases with the thickness ratio from 1:1 to 2.5:1, which could be attributed to the larger density of small size V-pits forming at the first few QW pairs. Compared with the smaller thickness ratio, the sample with a higher thickness ratio of InGaN to GaN in SSRL is found to exhibit larger strain relaxation (about 33.7%), but the electroluminescence measurement exhibits inferior emission efficiency. Carrier leakage via the small V-...