Improved performance of GaN based light emitting diodes with ex-situ sputtered AlN nucleation layers

The crystal quality, electrical and optical properties of GaN based light emitting diodes (LEDs) with ex-situ sputtered physical vapor deposition (PVD) aluminum nitride (AlN) nucleation layers were investigated. It was found that the crystal quality in terms of defect density and x-ray diffraction linewidth was greatly improved in comparison to LEDs with in-situ low temperature GaN nucleation layer. The light output power was 3.7% increased and the reverse bias voltage of leakage current was twice on LEDs with ex-situ PVD AlN nucleation layers. However, larger compressive strain was discovered in LEDs with ex-situ PVD AlN nucleation layers. The study shows the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics.

Solid state lighting based on GaN materials has evolved to be the most important general lighting technology recently because of its high conversion efficiency, rigid structure and long lifetime. 1,2rystal quality is always one of the most important factors which affect quantum efficiency in GaN based light-emitting diodes (LEDs).Due to the lack of natural GaN substrates, GaN-based LED structures are made typically on the (0001) c-plane sapphire substrates. 3,4The interfacial energy difference between sapphire and GaN could cause large amount of dislocations and low crystal quality of the subsequently grown GaN layer. 5The degraded quality of GaN layers presents intensive nonradiative recombination centers severely reducing the efficiency. 6For the purpose of decreasing the energy difference from lattice constant and thermal expansion coefficient mismatch between sapphire substrates and GaN, early pioneers applied in-situ low temperature AlN or GaN film to form the nucleation layer which provide a stable surface condition to grow the following bulk GaN. 7,8This two-step growth has made great success and become foundation in realization of commercial grade blue GaN-based LEDs.To further improve the GaN crystal quality, several growth methods, such as epitaxially lateral over-growth, 9,10 insertion of superlattice layers or SiN x nano-masks, 11,12 etc. had been proposed to reduce threading dislocation densities (TDDs).Among them, patterned sapphire substrates (PSS) are then widely used on GaN based LEDs because the light extraction efficiency can be largely enhanced due to fluctuated interfaces between GaN and sapphire and the TDDs can be well controlled to a certain level to achieve high internal quantum efficiency (IQE). 13n the other hand, recently developed GaN free-standing substrates can provide excellent crystal quality and benefits of homo-epitaxy in making excellent optoelectronics. 14However, high material costs of GaN substrates still hold back the popularity of utilizing the GaN substrates.More recently, exsitu sputtered AlN nucleation layer on PSS has been demonstrated as a cost-effective way to improve the crystal quality and electrical properties of GaN epitaxy layers for GaN based LEDs. 15The improvement could be due to the fact that ex-situ sputtered AlN nucleation layer has better coverage on sapphire substrates and could lower the lattice constant mismatch between sapphire and the GaN film.
Nevertheless, there still exists large difference of lattice constant and thermal expansion coefficient between GaN and sapphire substrate which leads to large strain in the LED structure and formation of cracks.7][18] Therefore, relationship of strain, crystal quality and electrical properties needs to be systematically studied.
In the present study, ex-situ AlN nucleation layer was deposited on 4-inch PSS by sputtered physical vapor deposition (PVD).Then, the GaN-based LED epitaxial structures were grown by metal organic chemical vapor deposition (MOCVD) followed by the process to form LED chips.The structural and optical characteristics of the as-grown wafers have been investigated and excellent crystal quality has been obtained for GaN films grown on the ex-situ sputtered AlN nucleation layer.On the other hand, the LED with AlN nucleation layer showed a higher strain within the structure.Finally, the improvement of electrical and optical properties can be observed at the LED samples with AlN nucleation layers.
The GaN based LED structures were grown on 4-inch (0001) PSS by MOCVD.Trimethylgallium (TMGa), trimethylaluminium (TMAl), trimethylindium(TMIn), and ammonia (NH 3 ) were used as precursors of chemical reactions for Ga, Al, In, and N sources, respectively.N-type and ptype doping sources were silane (SiH 4 ) and bicyclopentadienyl magnesium (Cp 2 Mg), respectively.From Fig. 1, for sample A, a conventional in-situ 25-nm-thick low temperature GaN (LT-GaN) nucleation layer was deposited at 550 • C on PSS in MOCVD and the sample B was deposited with an ex-situ 25-nm-thick PVD AlN nucleation layer.After that, the same GaN based LED structures grown on both samples consisted of a 4-µm-thick unintentionally doped GaN (u-GaN) layer, 3-µm-thick n-type GaN layer (n-doping = 1 x 10 19 cm −3 ), 900-Å-thick strain-release layer, nine pairs of InGaN/GaN MQWs with a 3-nm-thick un-doped well and a 12-nm-thick n-doping barriers as active regions, a 60-nm-thick p-type Al 0.1 Ga 0.9 N (p-doping = 8 x 10 19 cm −3 ) electron blocking The efficiency was defined as the photon numbers emitted from the sample divided by the excitation photon numbers. 19The strain distribution of the thin film structure was investigated by measuring depth resolved Raman spectra. 20The excitation source was a 532 nm laser light and the scattered light was collected by a 100x, NA0.7 objective lens.To perform depth-resolved Raman measurement, the sample was mounted on a z-axis piezo stage to vary the laser focusing depth.Since the phonon frequency shift was linearly proportional to both the biaxial and uniaxial strain in GaN, 21,22 the local strain can be evaluated by analyzing the Raman peak position.The fabricated LEDs had dimension of 26 x 30 mil. 2 The light output power-current-voltage (L-I-V) characteristics of LEDs were analyzed by a probe station and an integrated sphere instrument under CW operation at room temperature.Fig. 2 shows the surface morphology of GaN grown on PSS with different nucleation layers for growth time of 10 minutes and 25 minutes, respectively.With the in-situ LT-GaN nucleation layer in sample A [Fig. 2(a) and 2(b)], the growth of GaN was not only on the spacing area of c-plane region but also on the slope of cones.On the other hand, GaN layers of sample B with the ex-situ PVD AlN nucleation layer [Fig.2(c) and 2(d)] have less GaN grains on the cone region.Moreover, Fig. 2(c) and 2(d) also show better crystalline edges forming perfect hexagonal-shape stacks.Therefore, ex-situ PVD AlN nucleation layer can offer a better coverage condition to enhance c-plane region growth and suppress cone region lateral growth. 23,24 the forward bias voltages were 3.87 and 3.85 V for sample A and sample B, respectively.The light output power (L OP ) of sample B was 3.7 % higher than sample A. Furthermore, the forward voltage of sample A was slightly higher than sample B indicating that the resistance caused by defects could also be decreased while the crystal quality was improved.Fig. 4(b) shows the reversed bias leakage current.The reversed bias leakage current for sample A started at -6V, and that for sample B started at -14V.Moreover, the breakdown voltages of sample A and B were -51V and -59V, respectively.It is well-known that reversed bias leakage current closely depends on the dislocation density so that the better crystal quality would result in the lower leakage current.The L-I-V results again demonstrated the benefits of using the ex-situ PVD AlN nucleation layer.Figs.5(a) and 5(b) show the PL emission peak and spectra FWHM of sample A and B as a function of excitation power, respectively.At the lower excitation power, the FWHM of PL emission peak decreased with the blue-shift of PL emission peak because the QCSE was gradually screened and the MQWs became flatter with increasing the carrier density.At the higher excitation power, the QCSE was completely screened and the Burstein-Moss effect started to dominate the blue-shift of PL emission peak. 25Meanwhile, large amount of carriers were pushed to higher energy states due to the fact that all states close to the conduction band are populated, so the FWHM of PL emission peak increased.The energy shift of PL emission peak of sample A and B before the FWHM raising were 8.0 and 15.5 meV, respectively.The energy shift result indicates that higher compressive strain could exist in the LED with the ex-situ PVD AlN nucleation layer.For the purpose of understanding strain distribution within the LED structure, the depth resolved confocal Raman spectroscopy was measured.The result in Fig. 5(c) indicates that both samples were under compressive strain, because the E 2 high Raman peak was higher than that of unstrained GaN bulk occurred at 567.6 cm −1 . 21,22The Raman peak positions were 569.10 and 569.7 cm −1 for sample A and B, respectively.uncertainty in the curve-fitting.There was partial strain release during the growth of GaN in sample A due to the generation of more defects so that the strain around the surface was lower than that in sample B. On the other hand, compressive strain in sample B was relatively high due to the more coherent growth by using the ex-situ PVD AlN nucleation layer.Despite a larger strain existed in sample B, it still possessed higher light output power and reverse breakdown voltage because of the better crystal quality.Crystal quality of GaN layers was greatly improved with ex-situ PVD AlN nucleation layers owing to the fact that PVD AlN nucleation layer can suppress crystal growth on the cone area of PSS and effectively reduce the dislocation density of GaN layers compared with GaN layers on in-situ LT GaN nucleation layers.In addition, the L OP of LEDs with ex-situ PVD AlN nucleation layers was increased 3.7% with a lower forward bias voltage.Furthermore, the voltage of reverse bias leakage current and breakdown voltage were significantly improved.Finally, larger compressive strain was found in LEDs with ex-situ PVD AlN nucleation layers through PL and depth resolved Raman measurement.Nevertheless, LEDs with ex-situ PVD AlN nucleation layers still possessed better performance on optical and electrical properties because of the excellent crystal quality in LEDs with ex-situ PVD AlN nucleation layers.

FIG. 1 .
FIG. 1. Schematic diagram of LED structures with an in-situ LT GaN nucleation layer (sample A) and an ex-situ PVD AlN nucleation layer (sample B).The diameter, space, and height of cone on PSS were 2.8, 0.2, and 1.8 µm, respectively.

Fig. 3 FIG. 3 .
FIG. 2. SEM top view images of GaN epitaxial layer grown on PSS with an in-situ LT GaN nucleation layer after (a) 10 minutes and (b) 25 minutes growth, and that with an ex-situ PVD AlN nucleation layer after (c) 10 minutes and (d) 25 minutes.

Fig. 5 (
FIG. 5. (a) and (b) Power dependent PL peak wavelength and FWHM as a function of excitation power for sample A and B, respectively.(c) The E 2 high Raman peak of sample A and sample B, respectively.(d) The Raman peak frequency shifts of the GaN E 2 high phonon modes of sample A and B are plotted as a function of the depth of epitaxial layer from the p-GaN surface toward the sapphire substrate.