Mg-H Complexes Research Articles

P-doping of GaN by MOVPE

Mg has been widely used as p-doping species despite its intrinsic difficulties. It is nowadays well established that during the growth process of Mg doped GaN, atomic H is generated from the decomposition of NH3 and Mg-H complexes are formed in the layer. This has been for instance shown by the occurrence of LO mode in IR absorption, and by the observation of the Mg-H local vibration modes. This H passivation limits the electrical activity of Mg, therefore an activation process is required to get full activation of the Mg atoms. In the present study, bismethylcyclopentadienyl magnesium [(MeCp)2Mg] was used as precursor. However, this precursor reacts in the gas phase with NH3 to produce tiny solid particles as evidenced by a very bright diffuse emission visible along the laser beam used for reflectometry measurements. This simplest obvious product would be [(MeCp)Mg(NH2)]m(m≥2). To limit this drawback, Ga and Mg precursor lines have been separated. With proper in situ heat treatment, doping densities up to 1.5×1018 cm−3 have been obtained. PL spectra of lightly Mg doped samples (1016 cm−3) are dominated by shallow donor-acceptor pairs whereas for higher doping densities ( 1018 cm−3), the luminescence is dominated by a broad band in the 2.7-2.9 eV range. GaN LEDs were fabricated from Si doped (n-type) and Mg-doped (p-type) GaN, these LEDs emit in the blue-UV range.

Spectroscopic Identification of the Acceptor-Hydrogen Complex in Mg-Doped GaN Grown by MOCVD

ABSTRACTMg-doped GaN films grown by metalorganic chemical vapor deposition were characterized by variable-temperature Hall-effect measurements and Fourier-transform infrared absorption spectroscopy. As-grown, thermally activated, and deuterated Mg-doped GaN samples were investigated. The existence of Mg-H complexes in GaN is demonstrated with the observation of a local vibrational mode (LVM) at 3125 cm-1 (8 K). At 300 K this absorption line shifts to 3122 cm-1. The intensity of the LVM line is strongest in absorption spectra of as-grown GaN. Mg which is semi-insulating. Upon thermal activation, the intensity of the LVM line significantly decreases and an acceptor concentration of 2×1019cm-3 is derived from the Hall-effect data. After deuteration at 600°C the resistivity of the Mg-doped GaN increased by four orders of magnitude. A LVM line at 2321 cm-1 (8 K) appears in the absorption spectra which is consistent with the isotopie shift of the vibrational frequency when D is substituted for H.

Theoretical Studies Of Hydrogen Passivated Substitutional Magnesium Acceptor In Wurzite GaN

Infrared measurements on wurzite GaN codoped with Mg and H reveal strong absorption at 3125 cm−1. Theorectical work provides strong evidence for the H being antibonding to N. We have performed an ab-initio study of Mg-H complexes in wurzite GaN, using the Local Density Approximation on a large H-terminated cluster MgHGa25N26H42. We have investigated the physical properties of three neutral configurations along the c direction. In all configurations Mg sits in a gallium substitutional site. H is then located in the Mg-N bond centre (BC), in the antibonding site on nitrogen side (ABN) or in the antibonding position on magnesium side (ABMg). We found the lowest total energy configuration is hydrogen in the antibonding on the nitrogen site. The stretch mode in this configuration is calculated at 3277 cm−1 which agrees with experiment and previous LDA calculations and we predict an unreported infra red active wag mode at 1311 cm−1. The experimental isotopic shift with D is well reproduced. The BC and ABMg configurations are 0.5 and 3.7 eV higher, producing local modes at 3645 and 2144 cm−1, respectively. No wag modes appear for the BC and ABMg configurations.

Minority-carrier-enhanced reactivation of hydrogen-passivated Mg in GaN

The influence of minority-carrier injection on the reactivation of hydrogen passivated Mg in GaN at 175 °C has been investigated in p-n junction diodes. The dissociation of the neutral MgH complexes is greatly enhanced in the presence of minority carrier and the reactivation process follows second-order kinetics. Conventional annealing under zero-bias conditions does not produce Mg-H dissociation until temperatures ≥450 °C. These results provide an explanation for the e-beam-induced reactivation of Mg acceptors in hydrogenated GaN.

Reactivation of Acceptors and Trapping of Hydrogen in GaN/InGaN Double Heterostructures

ABSTRACTThe apparent thermal stability of hydrogen passivated Mg acceptors in GaN is a function of the annealing ambient employed, with H2 leading to a reactivation temperature approximately 150°C higher than N2. The dissociation of Mg-H complexes and the loss of hydrogen from GaN are sequential processes, with reactivation occurring at ≤700°C for annealing under N2, while significant concentrations of hydrogen remain in the crystal even at 900°C in implanted samples. The hydrogen is gettered to regions of highest defect density such as the InGaN layer in a GaN/InGaN double heterostructure. The addition of an accelerating potential for 2H+ ions in the plasma did not greatly affect the deuterium profiles.

Hydrogen in GaN: Novel aspects of a common impurity.

We have studied electronic structure, energetics, and migration of hydrogen and hydrogen complexes in GaN, based on first-principles total-energy calculations. Our calculations reveal a number of features very different from those exhibited by hydrogen in more traditional semiconductors such as Si or GaAs: a very large negative-$U$ effect ($U\ensuremath{\approx}2.4$ eV), the instability of the bond-center site, high energies for hydrogen molecules, and an unusual geometry for the Mg-H complex. All of these features are shown to be a consequence of distinctive properties of GaN, namely, the strongly ionic nature and the large bond strength of the Ga-N bond. We propose a simple model for the negative-$U$ behavior, which should be valid for H in any semiconductor.

Hydrogen in GaN

ABSTRACTHydrogen is implicated in the low doping efficiency of acceptors (e.g., Mg) in MOCVD-grown GaN by the following observations: the well-established role of hydrogen in the same phenomenon in other semiconductors; re-passivation of thermally activated Mg in GaN upon exposure to NH3 or H at temperatures > 400°C; correlation between Mg and H concentrations; and computational studies on the diffusivities of charged H in GaN and on the configuration and stability of the Mg-H complex in GaN. Strong experimental evidence for the Mg-H complex comes from variable-temperature Hall effect measurements which reveal that after hydrogenation of activated GaN:Mg the hole mobility increases as the hole concentration decreases, which is consistent with removal of ionized scattering centers and not with the introduction of separate compensating defects. However, experimental evidence is also accumulating to suggest that the Mg activation process involves the generation or transformation of electrically active defects.

Photoluminescence and Sims Studies of Hydrogen Passivation of Mg-Doped P-Type Gallium Nitride

ABSTRACTThe effects of hydrogen passivation in MOCVD grown Mg doped p-type GaN were studied using low temperature (5K) photoluminescence (PL) and secondary-ion-mass spectroscopy (SIMS). GaN films with different Mg doping level were annealed at 700° C in N2 ambient with different annealing times. The SIMS results indicate that the hydrogen concentration increases with increasing Mg doping level in the as-grown Mg:GaN film. After 20 minutes of annealing, most of the hydrogen escapes from the film. The 3.455 eV PL peak before annealing and the 3.446 eV peak after annealing found in the Mg doped samples were attributed to the exciton bound to the Mg-H complex and to the Mg acceptor, respectively. The shift of the bound exciton peak to higher energy (3.465 eV) in the lightly doped sample is due to an effective n-type compensation associated with an annealing-induced increase in the nitrogen vacancies. In heavily doped Mg:GaN, the decreases in the integrated PL intensity after 700° C annealing may be associated with the hydrogen depassivation of nonradiative recombination centers in the film. The increase of PL intensity in the lightly doped sample after annealing is attributed to the reduction of defects by the annealing process.

Reverse-bias annealing kinetics of Mg-H complexes in InP

The reactivation kinetics of hydrogen-passivated Mg acceptors in InP have been studied by annealing experiments carried out with different reverse biases. It is shown using a new analysis that the actual dissociation energy of the Mg-H complexes can be estimated even without applying a sufficient reverse bias to overcome retrapping of H at the dopant site. The dissociation process follows a first-order kinetics and the dissociation frequency and activation energy were estimated at various depths ranging from the surface up to 0.5 μm using an empirical analysis of the experimental data. A bias-independent dissociation energy of 1.40±0.08 eV was obtained when estimated at the surface. An apparently higher dissociation energy results when calculated in the bulk. This overestimation is pronounced for low-bias anneals and is explained as a result of retrapping. The concentration profile data is consistent with the positively charged state of the diffusing H in p-InP.

Deposition of highly resistive, undoped, and p-type, magnesium-doped gallium nitride films by modified gas source molecular beam epitaxy

Undoped, highly resistive (≳102 Ω cm) and Mg-doped, p-type (0.3 Ω cm) monocrystalline GaN films with a thickness of (4–5)×103 Å have been achieved via modified gas source molecular beam epitaxy on AIN buffer layers without the post-processing procedures of either electron beam irradiation or annealing necessary in chemical vapor deposition (CVD) derived films. The carrier concentrations and mobilities could not be accurately measured in the undoped films; values to 1018 cm−3 and 10 cm2/V s, respectively, have been achieved in the p-type films. These results indirectly support the hypothesis of Mg-H complexes as the cause of the difficulty in achieving highly conducting p-type materials using CVD techniques.

Dissociation energies of acceptor-hydrogen complexes in InP

The dissociation energies of Zn-H, Cd-H, and Mg-H complexes in p-InP have been determined from the reactivation kinetics of these passivated dopants in reverse-biased Au Schottky diodes. The reactivation process is first order under these conditions, yielding thermal dissociation energies of 1.20±0.10 eV for Zn acceptors, 1.40±0.10 eV for Cd acceptors, and 1.35±0.10 eV for Mg acceptors. These results are consistent with the model in which the hydrogen passivates the acceptor by attaching to a neighboring P atom, leaving the acceptor essentially threefold coordinated. They also indicate that acceptor-hydrogen retrapping during cool down after epitaxial growth of p-InP layers is the primary cause of apparently stable acceptor passivation seen in such layers.

Grain boundary segregation in Al-Zn-Mg alloys—Implications to stress corrosion cracking

Prior studies on grain boundary segregation in Al-Zn-Mg have shown that free Mg is present along the grain boundaries in these alloys under all heat treatment conditions. In this paper, a physical basis for the observed segregation profiles is provided through the vacancy-pump model developed by Anthony. The implications of the observed segregation to stress corrosion cracking (SCC) and other environmentally induced embrittling phenomena (where hydrogen could possibly play a dominant role) are discussed in terms of a possible Mg-H complex formation.