BAlGaN light emitting diode emitting at 350 nm

P. Milner^1,2, V.Z. Zubialevich¹, T. O’Connor^1,2, S.M. Singh^1,2, D. Singh¹, B. Corbett¹ and P.J. Parbrook^{1, 2}

¹ Tyndall National Institute, University College Cork, Lee Maltings, Dyke Parade, Cork, T12 R5CP, Ireland

² Department of Electrical and Electronic Engineering, University College Cork, Cork, T12 K8AF, Ireland

Email: peter.milner@tyndall.ie, p.parbrook@ucc.ie

Introduction: III-Nitride ultraviolet (UV) light sources are in increasing demand to replace mercury-based lamps owing to potentially higher efficiencies, smaller footprints, and tunable wavelengths [1]. While significant developments have been made over the past two decades, the performance of AlGaN-based light-emitting diodes (LEDs) still lags behind visible (In)GaN-based devices [2, 3]. Some of the major challenges are the crystalline quality of initial substrates/templates, light absorption within the LED heterostructure (due to p -GaN contact layers), lack of reflective contacts, and large intrinsic polarisation fields across the active region [4]. These fields originate from a combination of the ionicity of the bonds in the wurtzite lattice and the lattice mismatch in the crystal, which result in a separation of the electron and hole wavefunctions in the quantum wells (QWs), along with a redshift in the emission spectrum. This effect is more commonly known as the quantum confined Stark effect (QCSE) and is detrimental to the internal quantum efficiency (IQE) of the device.

Wurtzite boron nitride is an ultra-wide indirect bandgap material with a bandgap of ~6.7 eV, while the bandgap estimate for a direct transition is ~13 eV [5, 6]. The relatively small BN lattice constant of a ≈ 2.5 Å allows for the possibility of lattice-matching between well and barrier/buffer material [7]. Therefore, the addition of small amounts of BN to the (Al)GaN QWs has the potential to negate the polarisation field across the well and reduce the QCSE, thereby increasing the IQE. However, boron-containing QWs have to overcome many practical challenges in order to demonstrate experimentally their potential. In this Letter, we report, to our knowledge, the first UV LED with B-containing QWs.

Growth and fabrication: A BAlGaN MQW UVA LED was grown by metalorganic chemical vapor deposition (MOCVD) in an AIXTRON 3×2” close coupled showerhead reactor. A 2-inch c -plane sapphire wafer with 25 nm sputtered AlN (Kyma Technologies) was used as the substrate, onto which a 200 nm AlN connecting layer was grown. This was followed by 400 nm u -Al_0.35Ga_0.65N, 1.8 μm Al_0.350.22Ga_0.650.78N (upper 0.9 μmn -doped) graded composition layer, five QW/QB B_x (Al_0.06Ga_0.94)_1-x N (3.5 nm)/Al_0.16Ga_0.84N (10 nm), 30 nm Al_0.35Ga_0.65N electron blocking layer (top 20 nm p -type doped), 50 nmp ‑Al_0.350.00Ga_0.651.00N, and 10 nm p -GaN contact layer. The growth temperature of the active region was 1160°C. Trimethylgallium, trimethylaluminum, triethylboron (TEB), and ammonia were used as the main precursors. The TEB/III ratio for the QW was 1%. Disilane and bis-cyclopentadienyl magnesium were used as the n - and p -type dopants, respectively. A reference LED was also grown, with the exact same structure as above, but with no TEB flowing during QW growth.

After growth, both samples were processed into individual LEDs. First, a surface clean consisting of buffered oxide etchant, HCl:DI 1:1 and 45% KOH at 100°C was carried out to prepare the p -GaN surface for processing. Pd was evaporated as the p -metal, followed by the formation of 100 μm diameter circular mesas by inductively-coupled plasma etching with a Cl₂-based plasma. Finally, Ti/Al/Ti/Au was evaporated to form the n -metal.

Effects of boron incorporation: Test MQW stacks consisting of B_x (Al_0.05Ga_0.95)_1-x N/Al_0.16Ga_0.84N with different TEB/III ratios were grown by MOCVD, and a transmission electron microscopy (TEM) image revealed the presence of nanopipe-shaped voids in the active region for a low TEB/III ratio (< 1%). This effect has been attributed to boron surface segregation during growth [8]. It is proposed that boron acts as a surfactant, reducing the surface energy of the semipolar and nonpolar planes to below that of polar c -plane. In between the nanopipes, the QWs are undisturbed and show high uniformity. For the sample prepared with a higher TEB/III ratio (3%), the QWs disappear completely due to a high concentration of voids. Hence, for this LED experiment the TEB/III was set to 1%, using a QW growth temperature of 1160°C.