LED tunnel junctions spread the charge One of the limitations of conventional GaN-based LEDs is the low conductivity of p-type GaN:Mg used as the top contact layer. A large area p-type ohmic contact is required to spread current from the p-bonding pad across the junction area to prevent current crowding in the high-resistivity GaN:Mg. This electrode must be semi transparent for maximum light emission. A group at Chonbuk National University, Korea, have published a method to facilitate lateral current spreading by reducing lateral hole conduction (Appl. Phys. Lett. 2001 78(21) 3265). This involves using buried tunnel contact junctions on the p-side. A reverse biased tunnel junction (TJ) supplies holes to the p-type crystal above the active region by lateral electron currents. This allows the use of a low resistivity n-type GaN contact layer that further facilitates current spreading, and also the use of an n-type ohmic contact. Device fabrication is simplified as only n-type ohmic contacts are needed, resulting in a single metalization process. The TJ concept has been demonstrated for GaAs-based edge-emitters and VCSELs, but this is the first reported use for GaN-based LEDs. At 4.9 V (20 mA), the operating voltage of a TJ-LED is higher than that of a conventional GaN-based LED (4.5 V at 20 mA). This is as a result of a series resistance across the tunnel junction. However, the light output of the TJ-LED is twice that of a conventional LED. This is because there is no need for a large metal contact to spread current, and light emission occurs across the whole junction area because of the low resistivity n-type contact layer. shows the blue light emission at 5, 20, and 100 mA for (a) a conventional LED without a semi-transparent electrode and (b) a TJ-LED. At 100 mA operation, the emission is slightly more intense around the electrode. This is the result of current crowding in the n-type GaN contact layer. The uniformity of the radiance was further increased in the TJ-LED by increasing the doping level of the n-type GaN to improve its conductivity. NTT achieves 1 mW output from AlGaN-based UV-LED Toshio Nishida and colleagues from NTT Basic Research Laboratories in Japan have reported an AlGaN-based SQW UV-LED with an output power of 1 mW. Room temperature CW operation gives a peak wavelength of 341343 nm (Appl. Phys.Lett. 2001 78(25) 3927). The device is grown by MOCVD on a SiC substrate. The backside contact is provided by a conductive AlGaN (18% Al) buffer layer with a short period (3 nm) alloy superlattice giving transparent and conducting p-side cladding. The active regions of the structures consist of 2 nm thick Al0.06Ga0.94N SQWs with Al0.12Ga0.88N barriers. Current confinement is provided by 20 nm thick n- and p-type Al0.3Ga0.7N layers below and above the SQW structure. The shows the I-L curves for three different LED structures tested under RT, CW operation: the inset shows the band structures of the devices. LED (b), which does not have carrier confinement layers, begins to show saturation at 100 mA injection current. Without confinement, carriers are spilling over from the SQW to the other side of the p-n junction. LEDs (a) and (c) are carrier-confined and show near linear characteristics up to 150 mA, though slope efficiency for LED (c) is lower than LED (a). This is ascribed to the optical reabsorption in the upper QWs. LED (a) did not show saturation until an injection current of 400 mA was reached. At this point, the output power was 1 mW. This output power is almost 10 times at half the injection current than that previously reported by this group. High-power 980nm VCSEL arrays in demand Investigations into both edge-emitting laser diodes and VCSELs are being driven in part by demand for compact, high-power devices for use in materials processing, printing, medical applications and free-space data transmission. This paper by Miller and colleagues from the University of Ulm (Phot. Tech. Lett. 2001 13(3) 173) describes an array consisting of 19 high-power VCSELs operating at 980 nm. The devices (see ) were grown by MBE and contain C-doped upper Bragg reflectors made up of 30 pairs of Al0.9Ga0.1As/GaAs layers. The active region comprises three 8 nm thick In0.2Ga0.8As quantum wells. Above the p-type cladding, a 30 nm thick layer of AlAs is added, which is partially wet etched at 410C after mesa etching to form 50 m apertures, which are spaced 100 m apart. The total array size was 0.64 mm2. Backside light emission through the 150 m thick GaAs substrate is facilitated by an n-type DBR with only 20 Si-doped mirror pairs with the same composition as the p-type reflector. The GaAs substrate was coated with an anti-reflection layer and the device also employs a diamond heat spreader and copper heat sink. Each device provides 1.08 W of CW output at room temperature and 1.4 W at a heat sink temperature of 10C, corresponding to a total power density for the array of 1 kW/cm2. A beam divergence of 16 FWHM was also achieved, and the circularly symmetric far-field pattern allows for simple focusing of the beam with power densities of over 10 kW/cm2. Long-term testing has shown the device to be reliable for at least 10 000 h, with less than 1% degradation in output per 1000 h. Superlattices improve electrical properties The low doping efficiency in III-nitrides is an inherent property of a deep acceptor in wide bandgap materials. For the p-type dopant Mg in AlGaN, the activation energy increases with Al content, which leads to a decrease in the doping efficiency compared to bulk GaN. J Sheu et al. (Elec. Dev. Lett. 2001 22(4) 160) describe the incorporation of Mg-doped Al0.15Ga0.85As strained-layer superlattices (SLS) to improve p-dopant activation. Hall measurements showed a higher conductivity as a result of improved Mg activation efficiency, which was associated with strain-induced piezoelectric fields in the AlGaAs SLS. The authors (from Taiwan s National Central University and Epistar) applied the same AlGaN SLS design to a blue InGaN/GaN LED, and were able to re-duce the operating voltage from the 3.8 V typical of conventional devices to 3.0 V (at 20 mA). Grown on a sapphire substrate, the LED consisted of a nucleation layer and thick Si-doped GaN layer, followed by 5 In0.3Ga0.7N/GaN QWs (3 nm wells, 7 nm barriers). A layer of Mg-doped GaN was followed by the AlGaN/GaN 10-pair SLS (in the ratio 16 nm : 8 nm) and a NiAu transparent contact. Considerable improvements in the electrical properties were measured: the hole carrier concentration was 3 1018/cm3, or 30 times that of a bulk AlGaN layer. In addition, a very low contact resistance of 4 106 O.cm2 and series resistance of 18 O were obtained, compared to 40 O in an AlGaN device without the SLS. ELO-GaN improves laser facets The performance of violet laser diodes has been improved using epitaxial laterally overgrown (ELO) GaN as a substrate for the subsequent deposition of active layers. ELO-GaN is known to show lower dislocation density than GaN deposited directly onto sapphire substrates, leading to improved device lifetimes. Further increases in lifetime can be achieved if device operating currents are reduced. The operating current of a laser diode is directly related to the quality of the mirror facets. A paper by Tojyo and colleagues of Sony Shiroishi Semiconductor, Japan (Jap. J. Appl. Phys. Pt. One 2001 40(6A) 3206) describes a method of forming much improved laser facets on ELO-GaN. Photoluminescence measurements at 362 nm were made on the ELO-GaN layers. The photoluminescence intensity distribution was taken along a line perpendicular to the ELO stripes. The photoluminescence intensity in the ELO region was three times that of the seed region and 30% greater than the region where adjacent ELO wings coalesce. Since photoluminescence intensity is closely related to defect density, the wing regions should contain a relatively low number of defects. This was confirmed by comparing plan view TEM shots of the ELO wing regions and seed regions. AFM measurements of the cleaved facets of InGaN laser diode structures grown on ELO-GaN and sapphire substrates were also compared. The Ra roughness of the laser facet in the sapphire sample was 10 nm, which would cause mirror losses of between 23%. In the ELO sample the Ra roughness was 34 dB is over 24 years when operating at a frequency corresponding to a heatsink temperature of 50C. High-temperature operation of VCSELs In order for components such as VCSELs to be suitable for commercial applications, they must be capable of operating over a wide range of conditions. F Peters and H MacDougal of Gore Photonics repor-ted 850 nm oxide-confined VCSELs that retain sufficient bandwidth for 10 Gbit/s operation up to 150C (Phot. Tech. Lett. 2001 13(7) 645). Since the limiting factor for high-frequency operation is generally either poor thermal characteristics or parasitics, high-speed devices are designed with small mesas to limit parasitic capacitance. A semi-insulating substrate is utilized, and an air-bridge is used to reach the completely isolated top contact. Because the size of the top contact is very small, the resistance of the DBR mirror becomes significant; this was minimized by optimizing the doping in the DBR.