The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021 cm−3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted.

A longest carrier lifetime of 33.2 µs was achieved by eliminating the Z1/2 center via thermal oxidation at 1400 °C for 48 h and subsequent surface passivation with a nitrided oxide on a 220-µm-thick n-type 4H-SiC epilayer. By deep-level elimination, photoluminescence (PL) in the infrared region (wavelength: 700–950 nm) was remarkably enhanced at locations of threading dislocations. A threading screw dislocation exhibited much stronger infrared PL than a threading edge dislocation. The present results indicate that carrier recombination at extended defects becomes pronounced through the elimination of the Z1/2 center in the epilayers.

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...

Photoluminescence decay dynamics in an InGaN/GaN/AlGaN single quantum well

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