The blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content. It emerges near a Mg concentration of 1x10(exp 19) cm(-3) and at higher concentrations dominates the room temperature PL spectrum. The excitation power dependence of the 2.8 eV band provides convincing evidence for its donor-acceptor (D-A) pair recombination character. It is suggested that the acceptor A is isolated Mg(Ga) while the spatially separated, deep donor (430 meV) D is attributed to a nearest-neighbor associate of a Mg(Ga) acceptor with a nitrogen vacancy, formed by self-compensation

The concentration p and the mobility mu of holes in metal-organic chemical vapor deposition (MOCVD) GaN:Mg layers were studied by room temperature Hall-effect measurements as a function of the Mg concentration N(A) in the range 3 x 10(exp 18) cm(exp-3) <= N(A) <= 1 x 10(exp 20) cm(exp -3). The hole density first increases with increasing N(A), reaches a maximum value p(max)~6*10(exp 17) cm(exp -3) at N(A)~2*10(exp 19) cm(exp - 3), decreases for larger N(A) values, and drops to very small values at N(A) 1 x 10(exp 20) cm(exp -3). The hole mobility decreases monotonically with increasing N(A) . The p(N(A)) data provide strong evidence for self-compensation, i.e., for a doping driven compensation of the Mg acceptor by intrinsic donor defects. This effect becomes significant when N(A) exceeds a value of 2 x 10(exp 19) cm(exp -3). A semiquantitative self-compensation model involving nitrogen vacancies is developed. It accounts satisfactorily for the measured p(N(A)) dependence and suggests that self-compensation limits the hole conductivity in bulklike MOCVD GaN:Mg layers grown near 1300 K to about 1.2 (omega cm)(exp -1))

The efficient room-temperature photoluminescence bands of wurtzite GaN, which are peaked in the red (1.8 eV), the yellow (2.2 eV), and the blue (2.8 eV) spectral range, have been studied as a function of doping (species and concentration) and excitation power density (PD). It is shown that the yellow and the blue band are induced by Si and Mg doping, respectively, while codoping with Si and Mg generates the red band. At high-doping levels, the yellow and the blue band reveal strong peak shifts to higher energy with increasing PD providing very strong evidence for their distant donor-acceptor (DA) pair recombination character. The deep centers involved in DA recombination having electrical activity opposite to that of the shallow level of the dopant, are suggested to arise from self-compensation and to be vacancy-dopant associates. Self-compensation is found to be weak in the case of Si doping, but significant for Mg doping. A recombination model is presented, which accounts for the ess ential properties of all three bands in deliberately doped GaN. These results also suggest that the yellow and the blue bands in nominally undoped GaN arise from distant DA pairs involving residual Si and Mg impurities, respectively, as well as their respective vacancy associates

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We report on high-quality short-period superlattices in the AlN/ GaN material system. Thanks to significant advances in the epitaxial growth, up to 40 superlattice periods with a total layer thickness of 120 nm could be grown without cracking problems. Given an intersubband transition energy on the order of 910 meV, these superlattices could be used as room temperature, narrow-band, photovoltaic detectors for wavelengths around 1.4 m. In photovoltaic operation, the full width at half maximum is as narrow as 90 meV, underlining the high quality of the interfaces and the single layers in our structures.Recently, there has been increasing interest in III-nitride semiconductors for fabrication of ultrahigh-speed intersubband ͑ISB͒ devices. 1 Because of the large conduction band discontinuity of nearly 2 eV between AlN and GaN, this material combination is suitable for ISB transitions with an energy difference of roughly 1 eV. 2 Since such a large photon energy covers the technologically interesting wavelength region around 1.3/ 1.55 m, passive or active modulators and photodetectors for telecommunication applications can be envisioned. It is expected that the ultrafast recovery time of such ISB transitions ͑150-370 fs, depending on the refer-ence͒ will eventually result in high-speed devices in the Ͼ10 GHz range. 3,4 Shortly after the demonstration of ISB absorption in both quantum well ͑QW͒ and quantum dot superlattices ͑SLs͒, 5,6 several prototypes of QW and quantum dot ISB detectors have been published. 7-9 However, all devices reported so far, and in particular the QW-based ISB photodetectors ͑QWIPs͒, had relatively broad detection peaks ͓full width at half maximum ͑FWHM͒ considerably larger than 100 meV for ISB energies in the 800 meV range͔. This broadening is mainly due to interfacial roughness and might be augmented by drift of the growth rate during the epitaxy. Recently, improved deposition techniques based on the use of In as a surfactant during epitaxial growth of AlGaN, GaN, and AlN layers have resulted in significantly smoother surfaces, and therefore a better interface quality. 10 In this letter, we present a series of QWIP samples with a nominally identical active region, i.e., the same QW and barrier layer thickness, and differing in the doping level in the QWs and the thickness of the cap layer only. All samples present extremely narrow ͑FWHM ϳ90 meV͒ TM-polarized absorption/detection spectra at an ISB transition wavelength around 1.4 m, which demonstrates both improved crystalline quality of the layers and interfaces and high reproducibility of the growth process. QWIP structures consist of 40 periods of 1.0-nm-thick Si-doped GaN QWs with 2.0-nm-thick AlN barriers, grown by plasma-assisted molecular-beam epitaxy ͑PAMBE͒ on 1m-thick AlN-on-sapphire substrates. Active nitrogen was provided by a radio-frequency plasma cell, and standard effusion cells were used for Ga, Al, Si, and In. The doping level in the GaN QWs was 5 ϫ 10 19 cm −3 for sample E728 and 1 ϫ 10 19 cm −3 for E739 and E740. Prior to th...

Oxygen doped GaN has been grown by metalorganic chemical vapor deposition using N2O as oxygen dopant source. The layers were deposited on 2" sapphire substrates from trimethylgallium and especially dried ammonia using nitrogen (N2) as carrier gas. Prior to the growth of the films, an AlN nucleation layer with a thickness of about 300 AA was grown using trimethylaluminum. The films were deposited at 1085 degrees C at a growth rate of 1.0 mu m/h and showed a specular, mirrorlike surface. Not intentionally doped layers have high resistivity (>20 kW/square). The gas phase concentration of the N2O was varied between 25 and 400 ppm with respect to the total gas volume. The doped layers were n-type with carrier concentrations in the range of 4*1016 cm-3 to 4*1018 cm-3 as measured by Hall effect. The observed carrier concentration increased with increasing N2O concentration. Low temperature photoluminescence experiments performed on the doped layers revealed besides free A and B exciton emissi on an exciton bound to a shallow donor. With increasing N2O concentration in the gas phase, the intensity of the donor bound exciton increased relative to that of the free excitons. These observations indicate that oxygen behaves as a shallow donor in GaN. This interpretation is supported by covalent radius and electronegativity arguments