Title : The influence of Al-Graded undoped-AlGaN cladding layers thickness on the operating voltages as well as on injection current of Ultraviolet B laser diode
Abstract:
Typical laser processing uses infrared light, however, due to the high energy absorption rates by metallic materials, demand for a portable short-wavelength deep ultraviolet (DUV) and ultraviolet-B (UVB) laser diodes (LDs) are increasing. UV LDs technology will find extensive application in medicine, recording, lighting, printing, nanolithography, fine machining, and painting. However, AlGaN-based UVB LEDs and LDs has a central issue of their low hole injection toward the multi-quantumwell (MQWs). Recently, Sato et al. successfully realized high current-injected UVB LD at 298 nm grown on sapphire, where multimodal laser spectrum was obtained over the threshold current density at 67 kA cm–2. We also attempted to opticallypumped UVB LD devices, where stimulated emission from the AlGaN DUV quantum wells with a very low-threshold exciting power density of 68 kW/cm2 was obtained. However, we used a different LD design than the one given. Based on good crystalline quality of AlGaN UVB LEDs and LDs, we re-design our electrically pumped UVB LD structure for better carrier and optical confiment (OC). Crystal growth and fabrication of ud-AlGaN cladding layer (CL) as well as Al-graded Mg-doped p-AlGaN hole sources layer (HSL) for UV LDs was attempted using LPMOVPE system. The relaxation ratio in the ud-AlGaN optical confinement layer (OCL) with respect to fully relaxed AlN template was enhanced up to 40%, as shown in the reciprocal space mapping (RSM). Next, the influence of udoped (ud)- AlGaN CL thickness on the operating volatges (Vfs) of UVB LDs were investigated, and quite low Vf of 10.3 V under 20 mA was obtained by using 200nm-thick ud-AlGaN CL in UVB LD. When the ud-AlGaN CL thickness were reduced from 400nm to 200nm using 2Fold-Quantum Wells and also set the Mg-level in the Al-graded p-AlGaN hole source layer (HSL) (Polarization effect), then the external-quantiumefficiency (EQE) and light power (L) on bare-wafer condition were improved up to 0.45% and 3.4 mW respectively. The current density of 1.28 kA/cm2 was estimated using 250 x 250 µm2 area under 800 mA current drive. Next the LD structure with ridged structure of area ~ 10µm x 700µm was processed including gold pad without mirror. Subsequently, we attempted for injected current density measurement using pulse measurement with pulse width of 150ns and number of pulse shot around 01. This time the current densities were drastically improved from 1.2k A/cm2 to 15.7 k A/cm2 by using the ridge width of 10 μm and length of 700 μm. Quite high current density is expected after reducing the width of ridge beyond 5μm and length beyond 400μm as well as using mirror like structure. Summary: In this paper, a design and crystal growth of ultraviolet-B semiconductor laser diode (UVB LD) that operates under current injection at room temperature and at a UVB-wavelength emission of 288nm was demonstrated. The LD was grown on the (0001) face of an AlN template on sapphire substrate. The measured emission wavelength was 288nm with a pulsed duration of 150 ns. A polarization-induced Al-graded cladding layer (undoped) was employed to achieve improved conductivity and hole injection. By using undoped AlgaN CL and Mg-doped p-AlGaN hole source layer (HSL), a low operation voltage of 10.3 V at a 20 mA was achieved. This time the current densities (CDs) were drastically improved from 1.2k A/cm2 to 15.7 k A/cm2 by using the ridge width of 10 μm and length of 700 μm under pulse operation with pulse width of 150ns
