p-GaN Growth Research Articles

Growth of p-GaN at low temperature and its properties as light emitting diodes

The p-type GaN(p-GaN) samples grown at low temperature 870—980℃ on sapphire su bstrate were prepared by the metal organic chemical vapor deposition technique(M OCVD), and their electrical properties were investigated. The p-GaN samples grow n below 900℃ show high-resistivity, and samples grown at above 900℃ have good conductivity. In addition, the electrical properties are also related with the d oping level and the growth condition of p-GaN. The low N-Ga mole ratio leads to poor conductivity, the high ratio leads to rough morphology. At last, we use the optimized p-GaN to fabricate the green-light emitting diodes.We found that when the growth temperature is lower, the luminescence intensity and reverse voltage is higher but the forward voltage increases slightly.

Improvement of green LED by growing p-GaN on In 0.25GaN/GaN MQWs at low temperature

The effect of temperature on the growth of p-GaN on In 0.25GaN/GaN multiple quantum well (MQW) in a green light-emitting diode (LED) was investigated. p-GaN was grown at a low temperature of 900 °C to prevent the thermal degradation of MQWs in the green LED. A green LED with low-temperature (LT) p-GaN did not show a blue-shift in its electroluminescence (EL) spectrum, which is frequently observed in InGaN/GaN MQW with a high indium content. X-ray diffraction, high-resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy analyses of the MQW showed that the improved structural quality of MQWs with a LT p-GaN can be attributed to the suppression of inter-diffusion and the re-evaporation of indium in the MQW layers. Current–voltage measurement of the pure green LEDs with a LT p-GaN showed a forward voltage of 3.6 V at 20 mA and the peak wavelength and full-width at half-maximum for the EL emission peak were 525 and 25 nm, respectively.

Effect of compressive strain relaxation in GaN blue light-emitting diodes with variation of n+-GaN thickness on its device performance

The effects of compressive strain relaxation with increasing n-GaN thickness on device performances of GaN blue light-emitting diodes (LEDs) were investigated. It was found that the compressive strain relaxation in LEDs with thicker n-GaN occurred more considerably, following by the growth of active layer and p-GaN, and generated many stacking faults right beneath the InGaN active layer, which might be related to a decrease of the LED output power. On the contrary, the LED photoluminescence intensity increased surprisingly with n-GaN thickness. It was understood that the compressive strain relaxation enhanced localized states in InGaN wells.

Mg fluctuation in p-GaN layers and its effects on InGaN/GaN blue light-emitting diodes dependent on p-GaN growth temperature

Correlation between material properties of bulk p-GaN layers grown on undoped GaN and device performance of InGaN/GaN blue light-emitting diodes (LEDs) as a function of p-GaN growth temperature were investigated. The p-GaN layers of both structures grown by metal-organic chemical-vapor deposition were heavily doped with Mg. As the growth temperature of the bulk p-GaN layer increased up to 1,080°C, NA-ND increased. However, above 1,110°C, NA-ND sharply decreased, while the fluctuation of Mg concentration ([Mg]) increased. At this time, a peculiar surface, which originated from inversion domain boundaries (IDBs), was clearly observed in the bulk p-GaN layer. The IDBs were not found in all LEDs because the p-GaN contact layer was relatively thin. The change in photoluminescence emission from the ultraviolet band to blue band is found to be associated with the fluctuation of [Mg] and IDBs in bulk p-GaN layers. The LED operating voltage and reverse voltage improved gradually up to the p-GaN contact-layer growth temperature of 1,080°C. However, the high growth temperature of 1,110°C, which could favor the formation of IDBs in the bulk p-GaN layer, yielded poorer reverse voltage and saturated output power of the LEDs.

Anomalous current–voltage characteristics of InGaN/GaN light-emitting diodes depending on Mg flow rate during p-GaN growth

The dependence of the forward operating voltage of In1−xGaxN/GaN light-emitting diodes (LEDs) on Mg flow rate during the growth of the p-GaN layer has been investigated. The hole concentration in the p-GaN layer increases with Mg flow rate up to a critical value and gradually decreases for higher flow rates. We find that the forward threshold voltage of the LEDs mainly depends on the Mg concentration in the p-GaN layer and is rather independent of hole concentration. A current flow mechanism involving the impurity acceptor band, instead of the usual valence band, is proposed to elucidate this anomalous behaviour of the I–V characteristics.

P-type GaN epitaxial layers and AlGaN/GaN heterostructures with high hole concentration and mobility grown by HVPE

ABSTRACTIn this paper we report p-GaN growth by hydride vapor phase epitaxy (HVPE) on sapphire substrates. Mg or Zn impurities were used for doping. Layer thickness ranged from 2 to 5 microns. For both impurities, as-grown GaN layers had p-type conductivity. Concentration NA-ND was varied from 1016 to 1018 cm−3. An annealing procedure at 750°C in argon ambient typically increased the concentration NA-ND in 1.5–3.5 times. For Mg doped GaN layers, room temperature hole mobility of 80 cm2V−1s−1 was measured by conventional Van Der Pau Hall effect technique for material having hole concentration of about 1x1018 cm−3. Initial results on highly electrically conducting p-type AlGaN/GaN heterostructures doped with Zn are also reported.

Investigation into the Role of Low-Temperature GaN in n-GaN/InGaN/p-GaN Double-Heterostructure Light-Emitting Diodes

The role of two-step low-temperature GaN (LT-GaN) layers was investigated by cathodoluminescence, high resolution double crystal X-ray diffraction, transmission electron microscopy, atomic force microscopy, and current-voltage measurements. It was shown that the introduction of the LT-GaN layer prevents In from evaporating from InGaN during the high-temperature growth of p-GaN. The trasmission electron microscopic (TEM) results showed that the LT-GaN hampers dislocation propagation from the InGaN active layer into the p-GaN, leading to reduction in the dislocation density in the p-GaN. The use of the two-step LT-GaN resulted in an increase in the output power of light-emitting diodes and a decrease in the operating forward voltage.

Large Schottky barriers for Ni/p-GaN contacts

Large Schottky barriers were measured for Ni contacts formed on low Mg-doped p-GaN. In order to improve leaky Schottky characteristics, low-Mg doping was examined. This provided atomically flat surfaces and a low dislocation density of 5.5×108 cm−2. The Schottky barrier height (qφB) as high as 2.4±0.2 eV and n values of 1.84±0.06 were obtained from current–voltage measurements. These results are in good agreement with the prediction that the sum of qφB of n and p types adds up to the band gap. In the capacitance–voltage measurements, a transient response of capacitance was observed. This indicates that the evaluation of deep levels close to the valence band is possible, which could result in improvement of p-GaN growth.