M. J. Jou

High-transparency Ni/Au ohmic contact to p-type GaN

In this study, a very thin Ni/Au bilayer metal film was prepared by electron beam evaporation and thermal alloying to form ohmic contact on p-type GaN film. After thermal alloying, the current–voltage (I–V) characteristic of Ni/Au contact on p-type GaN film exhibited ohmic behavior. The Ni/Au contacts showed a specific contact resistance of 1.7×10−2 Ω cm2 at an alloying temperature of 450 °C. In addition, the light transmittance of the Ni/Au (2 nm/6 nm) bilayer on p-type GaN was measured to be around 85% at 470 nm. These results suggest that a suitable metallization technology for the fabrication of light emitting devices can be achieved.

Effects of thermal annealing on the indium tin oxide Schottky contacts of n-GaN

In this work indium tin oxide (ITO) films were prepared using electron beam evaporation to form Schottky contacts on n-type GaN films. The thermal stability of ITO on n-type GaN was also investigated by annealing the samples at various temperatures. In addition, current–voltage (I–V) measurements were taken to deduce the Schottky barrier heights. Owing to the large series resistance, the Norde method was used to plot the F(V)–V curves and the effective Schottky barrier heights were determined as well. The effective Schottky barrier heights were 0.68, 0.88, 0.94, and 0.95 eV for nonannealed, 400, 500, and 600 °C annealed samples, respectively. Results presented herein indicate that an increase of the barrier heights may be attributed to the formation of an interfacial layer at the ITO/GaN interface after annealing.

Inductively coupled plasma etching of GaN using Cl2/Ar and Cl2/N2 gases

This work investigates inductively coupled plasma (ICP) etching processes of GaN. Etching behaviors are also characterized by varying the ICP power, Cl2/Ar or Cl2/N2 mixing ratio, radio-frequency (rf) power, and chamber pressure. Experimental results indicate that the etching profiles are highly anisotropic over the range of etching conditions. Maximum etching rates of 8200 A/min in Cl2/Ar plasma and 8330 A/min in Cl2/N2 plasma are obtained as well. In addition, pressure, ICP power, Cl2/Ar(N2) flow ratio and rf power significantly influence etching rate and surface morphology. In particular, dc bias heavily influences the etching rates, suggesting that the ion-bombardment effect is an important factor of these etching processes.

Low-operation voltage of InGaN/GaN light-emitting diodes by using a Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN superlattice

Low-resistivity Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN strained-layer superlattices were grown. In these superlattices, the maximum hole concentration is 3/spl times/10/sup 18//cm/sup 3/ at room temperature. Hall-effect measurements indicate high conductivity of this structure in which the high activation efficiency is attributed to the strain-induced piezoelectric fields. This work also fabricated InGaN/GaN blue LEDs that consist of a Mg-doped Al/sub 0.15/Ga/sub 0.85/N/GaN SLs. Experimental results indicate that the LEDs can achieve a lower operation voltage of around 3 V, i.e., smaller than conventional devices which have an operation voltage of about 3.8 V.

Enhanced output power in an InGaN-GaN multiquantum-well light-emitting diode with an InGaN current-spreading layer

InGaN-GaN multiple quantum-well (MQW) light-emitting diodes (LEDs) with InGaN current-spreading layer were grown by metal-organic vapor-phase epitaxy (MOVPE) and their characteristics were evaluated by current-voltage (I-V), as well as output power measurements. Experimental results indicate that the LEDs exhibited a higher output power and a lower operation voltage than that of conventional LEDs. The external quantum efficiency of InGaN-GaN MQW LEDs for bare chips operated at injection current of 20 mA with InGaN current spreading layer near 5%. This is two times higher than that of conventional LEDs. This could be tentatively attributed to the better current-spreading effect resulting from Si-doped In/sub 0.18/Ga/sub 0.82/N wide potential well in which electron states are not quantized.

Indium tin oxide ohmic contact to highly doped n-GaN

Abstract The electrical characteristics of the indium tin oxide (ITO) contacts on n-GaN with various doping concentrations have been studied. Ohmic behavior was observed for ITO films on highly doped n-GaN ( n =1×10 19 cm −3 ) without thermal annealing and the measured specific contact resistance was 5.1×10 −4 Ω cm 2 . This result could be attributed to the formation of a tunneling junction on the heavily n-type GaN surface. However, as the thermal annealing was performed to the ITO/n-GaN ( n =1×10 19 cm −3 ) Ohmic contact, it exhibited Schottky characteristics. This result might be due to the microscopic interfacial reaction among In, Sn, O and GaN and their alloys which extend into GaN films, thereby influencing the electrical properties of ITO/n-GaN contacts.

Luminescence of an InGaN/GaN multiple quantum well light-emitting diode

Abstract In GaN/GaN multiple quantum wells (MQW) and blue MQW light-emitting diodes (LEDs) were grown by metal-organic vapor phase epitaxy (MOVPE). Band-gap narrowing of the PL spectra for the InGaN/GaN MQW LEDs can be observed at room temperature. In addition, the emission wavelength of EL and PL spectra for the MQW blue LEDs exhibit a blue-shift phenomenon while increasing the injection current and laser power, respectively. This luminescence behavior can tentatively be understood as a competition between a spectral red-shift mechanism of piezoelectricity-induced quantum-confined Stark effect (PQCSE) and a blue-shift mechanism of band-filling and charge screening effects.

Thermal stability study of Ni/Ta n-GaN Schottky contacts

The Schottky behavior of Ni/Ta and Ni contacts on n-GaN was investigated under various annealing conditions by current–voltage measurements. It is found that with the addition of Ta between the Ni layer and the GaN substrate the thermal stability properties of devices can be significantly improved. Experimental results indicate that a high quality Ni/Ta n-GaN Schottky diode with an ideality factor and barrier height of 1.16 and 1.24 eV, respectively, can be obtained under 1 h annealing, at 700 °C.

Crystal orientation dependence of optical gain in InGaN/GaN multiple-quantum-well structures

The net modal gain of the InGaN/GaN multiple quantum well has been measured by the variable excitation stripe length method for an optically pumped cavity along each crystal orientation on the (0001) plane. These results demonstrated the theoretical prediction of the fact that the maximum optical gain can be obtained at a [1210]-oriented edge-emitting laser cavity, which has been reported in the literature. “Crystal orientation” is confirmed to be a related parameter to the optical gain for a GaN-based strained structure.

Dependence of optical gain on direction of optically pumped cavity on (0001)-plane for InGaN/GaN multiple quantum well structure

Abstract In this work, we demonstrate the theoretical prediction about the dependence of the optical gain on the orientation of the laser cavity in (0001) plane for the strained InGaN/GaN multiple quantum well structure. The net modal gain of the InGaN/GaN multiple quantum well has been measured by variable excitation stripe length method for optically pumped cavity along each crystal orientation on (0001) plane. The measured optical gain for the cavity along the [ 1 2 1 0 ] direction is larger than any other oriented cavities. ‘Crystal orientation’ is confirmed to be a parameter related to the optical gain for GaN-based strained structure. The results reveal that the best cavity orientation of GaN-based edge-emitting laser is in [ 1 2 1 0 ] direction.