Previous studies thus suggest that a higher IQE should be realized if GaN QWs exclusive of AlN from Al x Ga 1−x N alloy QWs are used as emitters. To reach the deep UV spectral range (200-300 nm wavelength), the GaN QW width must be very thin because the bandgap of GaN is 3.4 eV (365 nm). Ultrathin GaN QWs on the monolayer (ML) scale may also realize a strong in-plane polarization. [16] In conventional Al x Ga 1−x N/AlN (0001) QWs, the transvers magnetic (TM) polarization becomes dominant for x > 0.82, preventing light extraction from the (0001) surface. [17] On the other hand, GaN/AlN (0001) QWs exhibit strong emissions along the [0001] direction, even at a wavelength of 237 nm. [16] To fabricate such ultrathin GaN QWs embedded in Al(Ga)N, molecular beam epitaxy (MBE) appears to be better suited than metalorganic vapor phase epitaxy (MOVPE) because MBE can precisely control the thickness with the assistance of a reflection high-energy electron diffraction technique. In fact, MBE growth of high-quality, ultrathin GaN (as well as InN) QWs has been demonstrated frequently. [18][19][20][21][22][23] However, MOVPE growth is quite rare, [16,22] despite its technological importance. In this study, we demonstrate self-limiting of the GaN thickness to the ML scale during MOVPE growth, facilitating the fabrication of highly reproducible ultrathin GaN QWs. In addition to the conventional (0001) c-plane, we examine the (11  02) r-plane and reveal the higher radiative recombination probability of r-plane QWs.The substrate for c-plane growth at a pressure of 76 Torr was either sapphire (0001) or AlN (0001). Both provide similar results. Initially, an AlN layer (600-2500 nm) was grown at 1200 °C followed by a GaN single QW and a 15 nm thick AlN cap layer. The typical growth temperature and molar flow ratio between the group V and III sources (V/III ratio) for the GaN growth were 1150 °C and 109, respectively. The optical properties were assessed via photoluminescence (PL) spectroscopy. The excitation light source was the fourth harmonics of a Ti:Sapphire laser (a pulse width of 1.8 ps and a repetition frequency of 80 MHz), unless otherwise stated. The wavelength was 212.5 nm, which excited only the GaN QW layer. The excitation energy density per pulse was 7 nJ cm −2 . If the absorption coefficient is assumed to be 1 × 10 5 cm −1 , this energy density corresponds to an initial carrier density of 6 × 10 14 cm −3 in ultrathin QWs. PL was detected by a charge-coupled device (CCD) camera through a monochromator. Figure 1a shows the PL spectrum of a GaN/AlN (0001) QW at 6 K. The growth time of the GaN layer is 10 s. The peak GaN/AlN ultrathin quantum wells (QWs) emitting in the deep UV spectral range are fabricated by metalorganic vapor phase epitaxy. The GaN thickness is automatically limited to the monolayer (ML) scale due to the balance between crystallization and evaporation of Ga adatoms. This growth characteristic facilitates the fabrication of highly reproducible GaN ML QWs. The strong quantum confinement within the GaN ML QW...

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We fabricated sub-230-nm (far UV-C) light emitting diodes (LEDs) on a single-crystal AlN substrate. With 20 quantum well cycles implemented to enhance carrier injection into the active layers, over 1-mW output power (1.4 and 3.1 mW for 226- and 229-nm LEDs, respectively) was obtained under 100-mA operation. The maximum output power reached 21.1 mW for the single-chip 229-nm LED operating at 700 mA, without significant drooping. The forward voltage for both sub-230-nm LEDs operating at 100 mA was low (5.9 V) due to their low resistances and ideal Ohmic contacts between metal and semiconductor components. Additionally, wall plug efficiencies were 0.24% and 0.53% for the 226- and 229-nm LEDs, respectively. The lifetime of the 226-nm LED while operating at 25 °C reached over 1500 h and did not show current leakage, even after 1524 h. This long lifetime will be achieved by improving carrier injection due to many quantum wells, using a high-quality AlN substrate and achieving high wall plug efficiency.

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Low-temperature photoluminescence spectroscopy is performed for unintentionally doped and silicon-doped aluminum nitride (AlN) films grown on AlN substrates. Considering the positive electron–hole exchange interaction constant substantially changes the neutral silicon-donor bound exciton binding energy from 28.5 to 15.3 meV. The silicon-donor binding energy is also experimentally deduced as 64.8 meV from a two-electron transition, which is justified by a theoretical calculation considering the crystal anisotropy and electron–phonon coupling. The estimated binding energies are discussed with a previous theoretical result within the effective mass approximation and the experimentally known binding energy of elementary four-particle (three-particle) systems.