III-Nitride p-down green (520 nm) light emitting diodes with near-ideal voltage drop

III-Nitride semiconductors are of great technological importance due to a range of applications in optoelectronics and electronics^1–12 and have been adopted widely across lighting and display applications. While the efficiency and power output of GaN-based light-emitting diodes in the violet/blue emission wavelength range have improved significantly over the past decades, emitters at longer wavelengths still show lower efficiency. For emitters designed for longer wavelength, the indium mole-fraction in (In,Ga)N quantum wells leads to challenges associated with the larger lattice mismatch, defects within the quantum wells, and higher polarization sheet charge density at the well-barrier interfaces – all of which contribute to degradation in device performance.^13–16 

The impact of the polarization dipole with quantum well is shown in the charge and energy band diagrams of a conventional light-emitting p-up structure, where the c-axis is oriented from N to P regions [Fig. 1(a)]. In conventional LEDs, the polarization dipole within the quantum well opposes the depletion field, and depletion barriers are formed on both sides of the well. These barriers block electron and hole injection into the quantum well and, therefore, could cause degradation in the electrical efficiency of the diode. LEDs with the PN junctions along the opposite polarity direction to this have the sense of the polarization fields inverted relative to the conventional Ga-polar LEDs.^17–20 In Fig. 1(b), the energy band and charge diagram of an LED structure with identical composition and doping values, but with the c-axis oriented from P to N, are shown. In this case, the polarization dipoles are in the same direction as the depletion field, and therefore, the electrostatic barriers to electron and hole injection at the edge of the quantum well are reduced. This reduction in electrostatic barriers can enable lower turn-on voltage and more efficient carrier injection into the well.^21–26 Figure 1(d) shows the J–V plot of the simulated green emitting LED in the p-up and p-down structures from Figs. 1(a) and 1(b) without the tunnel junction (TJ). The simulation was made using the Silvaco TCAD software with a single quantum [25% indium composition and 100% fixed polarization charge at the GaN/(In,Ga)N interfaces]. The predicted turn-on and forward voltage for the p-up structure shows higher voltage drop compared to the experimental green LED device due to the absence of compositional fluctuation for the (In,Ga)N quantum well and consideration of 100% polarization charge.^14,27 Considering similar electrostatics of the device simulation, the p down LED shows almost 0.8 V less forward voltage at 20 and 100 A/cm², which indicates the advantage of reduced polarization induced barrier for p-down devices compared to p-up devices. The energy band diagram for p-up and p-down LEDs operating at 20 A/cm² [Fig. 1(c)] shows lower electron and hole injection barrier for the p-down LED compared to p-up LED. This concept was demonstrated previously using a N-polar oriented p-up PN junction LED,^17,18,23,26 and the lower turn-on voltage in that work confirmed the prediction from the energy band diagram analysis. Since N-polar LEDs use an unconventional crystal polarity and have been reported to have challenges associated with higher point defect incorporation,^28,29 it would be preferable to have LEDs with the same sense of polarization relative to the PN diode, but with the growth along the Ga-polar direction. Such a p-down structure [shown in Fig. 1(b)] would enable such inverted polarity LEDs to be achieved. To overcome the high spreading resistance of the p-type layer, an n-type current spreading layer connected to it by an efficient tunnel junction is needed.^30–36 

Low-resistance inter-band tunnel junctions have been demonstrated for GaN LEDs,^37–43 but prior reports have demonstrated low resistance only for the N/active region/P/tunnel junction/N sequence. The opposite case (N/TJ/P/active region/N) showed significantly higher high turn-on voltage.^32,44,45 In this work, we show that the use of efficient tunnel junctions can enable p-down LEDs with low turn-on and forward operating voltage.

The epitaxial structure described here [Fig. 2(a)] was grown by plasma-assisted molecular beam epitaxy (PAMBE) on n-type (0001)-oriented GaN/sapphire templates with a dislocation density of $5×108$ cm⁻². The MBE growth process for the homojunction TJs used in this report is based on previously reported work on PN homojunction tunnel junctions.⁴⁶ Epitaxial growth was done on a Veeco Gen930 system with elemental Ga, In, Mg, and Si sources, and a radio frequency N₂ plasma source. Typical background chamber pressure was measured using an ion gauge to be lower than 10⁻¹⁰ Torr. Growth was carried out with a nominal N₂ gas flow rate of 2.5 sccm with an rf plasma power of 300 W supplied by Veeco Uni-bulb plasma source, corresponding to a background chamber pressure of 1.65 × 10⁻⁵ Torr under growth conditions. The N₂ gas (Matheson Tri Gas, ULSI 6N Purity) flow rate was controlled using a single channel flow controller (MKS Instruments) calibrated for N₂ gas with an ability to control up to 10 sccm flow. Effusion cells with elemental Ga (E-Science, 7N), In (Veeco, 7N), Si (Veeco, 6N), and Mg (Veeco, 7N) were used. GaN growth was carried out in Ga-stable conditions.^47–49 GaN and (In,Ga)N growth were done in metal-rich condition {$ΦGa>ΦN (GaN growth);ΦIn+Ga>ΦN In,GaN growth]}$ at a nominal growth rate of 0.3 μm/h, following a growth model developed earlier.^50,51 Mg and Si doping densities were estimated using secondary ion mass spectroscopy calibrations. Samples were pre-baked (400 °C, 1 h) in the load-lock chamber prior to growth. The sample was exposed to five cycles of Ga flux (⁠$1.7×10−7 Torr$⁠, 20 s cycle) at T_sub = 720 °C to desorb sub-oxides from the surface before the start of growth. Details of the growth condition, doping density, corresponding beam equivalent pressure (BEP) for Mg source (measured separately before growth), cell temperatures for Si source, and layer thickness of all the epitaxial layer grown are presented in Table I.

Atomic force micrographs were obtained using a Bruker Dimension Icon atomic force microscope system. The atomic force microscope image [shown in Fig. 2(b)] shows step flow features with relatively low RMS roughness of approximately 1 nm (5 × 5 μm²). The surface shows smooth topography after the growth with some pitting on the surface that we attribute to the heavy doping of Si of the top-most GaN layer.^48,52 High-resolution XRD $ω−2θ$ scan (Bruker D8 Discover XRD system) shown in Fig. 2(c) shows thickness fringes suggesting sharp interfaces and suggests a reasonably good match with the simulated dynamical x-ray diffraction curve (LEPTOS XRD Simulation Software).

Device fabrication was carried out with patterning by direct-write optical lithography (Heidelberg MLA150). Light-emitting diodes with dimensions ranging from $10$ to $150 μm$ were patterned. Mesa isolation of the devices was done through inductively coupled plasma (ICP)-RIE etching using the BCl₃/Cl₂/Ar (5/50/5 sccm flow) etch chemistry at 40 /40 W power condition for 20 min with an etch rate of 35 nm/min. Bottom and top n-type Ohmic contacts (Al/Ni/Au with thicknesses 30/30/100 nm, respectively) were deposited using e-beam evaporation. On-wafer electrical characteristics were measured using an Agilent B1500 Semiconductor Parameter Analyzer. The electroluminescence characteristics were measured using a Top200 optical probe supplied by Instrument Systems coupled with a Keithley 2400 series source measure unit.

The dependence of current density on voltage for the p-down green LEDs (device sizes $150×150$⁠, $100×100, 50×50, 40×40$⁠, $30×30$⁠, $20×20,$ and $10×10 μm2$⁠) is shown in Fig. 3(a). A diode turn-on voltage of 2.35 V is extracted from the intercept of the linear portion of the J–V characteristics. The forward voltage was measured to be 2.42 V at 20 A/cm² and 2.75 V at 100 A/cm². At 20 A/cm², the quasi-Fermi levels inside the quantum well are separated by 2.27 eV {emission wavelength at 20 A/cm² is 548 nm [shown in Fig. 4(a)]}, and thus, the voltage drop at the diode region will be at least 2.27 V. Assuming no extra voltage drop from the diode region, the voltage drop at the tunnel junction is estimated to be 0.15 V at 20 A/cm² and 0.42 V at 100 A/cm². These values are in agreement with expected voltage losses as reported previously in heavily doped PAMBE-grown homojunction tunnel junction (<0.2 V at 20 A/cm² and <0.5 V at 100 A/cm²).⁴⁶ When compared with previous green LEDs, with or without TJs, these values are significantly lower.^{19,32,53–56} This confirms the hypothesis that the lowered barriers in the p-down LED can reduce the turn-on voltage compared to the conventional p-up case.⁴⁶ In comparison with previous p-down results, the more efficient tunnel junction leads to a low voltage drop and low series resistance. This homojunction TJ also offers advantages over polarization engineered TJs by eliminating possible absorption losses in heavily doped regions, and strain issues from lattice mismatch.^37,38,57,58 The higher series resistance for the larger device area is attributed to the spreading resistance.

Figure 4(a) shows the measured electroluminescence spectra for the p-down LEDs. Peak emission wavelength shifts from 548 nm at the lowest current density where emission was detectable at (23 A/cm²) in the EL measurement setup, to 518 nm at higher current densities at around 943 A/cm². The peak shift from 548 to 518 nm with increasing current injection can be explained by localized state filling and screening of built-in fields shown in Fig. 4(b).³² Figure 4(c) shows the measured voltage drop of previously reported III-nitride LEDs as a function of emission wavelength, together with the theoretical minimum voltage drop (⁠$Vf=hν/q$⁠).^{27,55,56,59–68} While previously reported blue LEDs are close to this limit, green LEDs have typically shown significantly higher voltage drop. The use of a p-down structure brings this voltage drop closer to the theoretical value, as predicted from theory.

In summary, we have demonstrated p-down green light-emitting diodes with excellent low-voltage operation. The low voltage drop measured here suggests that the use of well-engineered tunnel junctions can, indeed, enable very low voltage penalty in the green wavelength range. Furthermore, having an n-type region above the LED could have several benefits for higher level integration of other electronic devices, such as Schottky diodes and transistors. Finally, the design and demonstration of efficient tunneling-based p-down LEDs provides a framework to explore other designs including longer wavelength LEDs and novel multiple active region LEDs.

This material is based upon work supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Building Technologies Office Award No. DE-EE0009163. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the U.S. Government.