Ultraviolet-B band lasers fabricated on highly relaxed thick Al_0.55Ga_0.45N films grown on various types of AlN wafers

Abstract

In this paper, we investigated the dependence of threshold power density on the Al0.55Ga0.45N underlying layer film thickness in ultraviolet-B band (UV-B) lasers on various AlN wafers (four types). We also prepared and compared AlN templates for AlN freestanding substrates, AlN films fabricated by metalorganic vapor phase epitaxy, and annealed sputtered AlN templates at high temperature. The initial growth of AlGaN became three-dimensional by inserting a homoepitaxial Ga-doped AlN layer between the AlN template and Al0.55Ga0.45N, before it shifted to two-dimensional growth. It is possible to reduce the dislocation in Al0.55Ga0.45N using this mode. The dependence of AlGaN film thickness and that of the AlN template on samples with an inserted homoepitaxial Ga-doped AlN layer were studied. Compared with Al0.55Ga0.45N having a thickness of 5 μm, there was almost no noticeable difference between the dark spot density characterized by cathodoluminescence and the threshold power density in UV-B lasers for the AlN template. Besides, the characteristics were noticeably different for the film thickness of Al0.55Ga0.45N. The threshold power density in UV-B laser and dark spot density were reduced by increasing the film thickness. Through the optimization of the crystal growth condition, the threshold power density in UV-B laser and dark spot density were reduced to 36 kW cm−2 and 7.5 × 108 cm−2, respectively.

High‐Temperature Annealing of AlGaN

“…Otherwise, AlGaN relaxation can lead to surface roughening and formation of misfit dislocations during growth. [12,13] Misfit dislocations can drastically increase the TDD by bending out of plane. Hence, providing low TDD Al x Ga 1Àx N buffer layers instead of low TDD AlN buffers is desirable for UVA and UVB LEDs.…”

The Impact of AlN Templates on Strain Relaxation Mechanisms during the MOVPE Growth of UVB‐LED Structures

“…A possible way to reduce the strong compressive strain in the nonpseudomorphic AlGaN would be the change of the growth mode to a 3D mode as it was shown for the development of low TDD AlN templates with increased AlN layer thickness [ 20,31–34 ] and for UVB‐LED structures [ 10,35 ] as well as recently for the deposition of Al 0.55 Ga 0.45 N layers for UVB laser structures. [ 9 ] However, here the surface needs to be smoothened before the MQW is grown, which again is a challenge.…”

“…This high mismatch as well as the not perfect alignment between the AlN nuclei formed at the start of heteroepitaxial growth generally causes threading dislocation densities (TDDs) much higher than 10 9 cm −2 . Since the group of Miyake [ 5–7 ] showed the possibility of decreasing the TDD of AlN/sapphire templates down to 2 × 10 8 cm −2 by high temperature annealing (HTA), many groups tried to grow UV‐LED structures on such low dislocation density templates and found this to be challenging, [ 8–10 ] especially for LED structures emitting in the UVB region (UVB‐LEDs). Here the Al‐mole fraction of the (In)AlGaN layers of the active region is below 40% making pseudomorphic growth on AlN templates impossible due to the high compressive strain.…”

High Crystallinity and Highly Relaxed Al0.60Ga0.40N Films Using Growth Mode Control Fabricated on a Sputtered AlN Template with High‐Temperature Annealing

“…Up to this point, this is thought to be in good agreement with our previous results. [39] However, the behavior of the reflectance with respect to film thickness is dependent on the amount of Ga doping in the AlN homoepitaxial layer. When the Ga-doping level is zero or low, the reflectance increases rapidly from the early growth stage of Al 0.60 Ga 0.40 N. In contrast, it was found that as the amount of Ga doping increased, the tendency of the decrease in reflectance became gradual.…”